**Meet the editor**

Dr. Chandraleka Saravanan is currently working as an assistant professor at the Department of Chemistry, Urumu Dhanalakshmi College, Tiruchirappalli, Tamil Nadu, India. She has experience in the fields of coordination chemistry and bioinorganic chemistry. Her current research focuses on metal complex synthesis, structural characterization, and their utility in pharma-

cological and biological applications. She has published 20 research papers and 1 book on *Transition Metal Complexes*: *Biological Activity*. She guided eight MPhil candidates and participated in summer school on materials science. She has presented 16 research papers on national and international conferences/seminars. She is an editorial board member of national and international journals and an examiner committee member of MSc, MPhil, and PhD in Chemistry.

Contents

**Preface VII**

**Limiting 9**

Chao-Qun Ye

**Analogues 75**

Aijian Wang and Wei Zhao

Chapter 1 **Introductory Chapter: An Outline of Chemical Reagents and**

Chapter 2 **Porphyrin and Phthalocyanine Covalently Functionalized**

Chapter 3 **Sol-Gel Processes of Functional Powders and Films 31**

**Designs and Characterizations 51**

Chapter 4 **The Effect of Substituent on Molecules That Contain a Triple**

Chapter 5 **Theoretical Investigations of Mechanisms for the Reactions of**

Zheng-Feng Zhang, Ling-Hsuan Liu and Ming-Der Su

**Graphene and Carbon Nanotube Nanohybrids for Optical**

**Bond Between Arsenic and Group 13 Elements: Theoretical**

Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su and Ming‐Der Su

**Seven-Member Ring N-Heterocyclic Carbene and Its Heavier**

**Reactions in Inorganic Synthesis 1** Chandraleka Saravanan and Bhaskar Biswas

## Contents

#### **Preface XI**



Chapter 4 **The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group 13 Elements: Theoretical Designs and Characterizations 51** Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su and Ming‐Der Su

Chapter 5 **Theoretical Investigations of Mechanisms for the Reactions of Seven-Member Ring N-Heterocyclic Carbene and Its Heavier Analogues 75** Zheng-Feng Zhang, Ling-Hsuan Liu and Ming-Der Su

Preface

than others.

A chemical reagent is a molecule or compound, which helps to bring a chemical change dur‐ ing mixing with another molecule or is added to see if a reaction occurs or not. Although the terms reactant and reagent are often used interchangeably, a reactant is more specifically a "substance that is consumed in the course of a chemical reaction." Before entering the main context of the write-up, it is better to present few sentences about the synthetic aspect in inorganic chemistry. Inorganic chemists have long been enthralled by the magnificent colors of inorganic compounds. From the historical perspective, in the early 15,000 BC, cave paint‐ ers used iron oxides for the sources of yellows and reds, aluminum silicates for greens, and manganese oxide for browns. By 3000 BC, Egyptian and Syrian artisans and jewelers were staining metal surfaces with inorganic salts derived from ground minerals and alloys. The colors obtained mostly depended on the types of transition metals contained in the minerals and on their combinations. One of the first synthetic pigments, Egyptian Blue, CaCuSi4O10, was prepared by heating malachite/sand (quartz or silica) mixtures to a temperature of 800– 900°C. Later in the fifteenth to eighteenth centuries, metal-containing pigments, such as cop‐ per carbonate and the brilliant ferric ferrocyanide coordination complex, Prussian Blue, Fe4[Fe(CN)6]3, were synthesized for use in the textile industry. Iron oxides, lead chromates, and the ubiquitous white pigment, titanium dioxide, are just a few of the many inorganic compounds that are still important in this industry today. The existence of ionic forces in‐ cluding some weak forces primarily accounts on the distinction of inorganic compounds

A purely inorganic compound refers to a class of compound that does not contain a carbon to hydrogen (C-H) bond. Not all, but most inorganic compounds contain a metal. Actually, metal ions with labile reaction centers are generally used as geometry setters. For this rea‐ son, inorganic compounds have an overwhelming amount of applications and practical uses in the real world. Thus, in the recent spurt in research and advance academic framework, inorganic compounds are an exciting area that have important implications for the rational design of functional materials to be used as catalysts, conducting materials, molecular mag‐ netic materials, nonlinear optical materials, liquid crystals, chemical sensors, and molecular recognitions. Syntheses of molecules and materials are a very important aspect behind the development of science, especially in chemistry. The attempts to synthesize new molecules and materials have always depended on the experience and extrapolation of the existing knowledge to new situations and new synthesis of molecules. The strategies adopted in syn‐ theses of molecules and materials have undergone an enormous expansion in recent times, and research in inorganic synthesis has opened up a new vista in this direction. This book describes an overview of chemical reagents used in inorganic chemical reactions for the syn‐ thesis of different compounds including coordination, transition metal, organometallic, clus‐

## Preface

A chemical reagent is a molecule or compound, which helps to bring a chemical change dur‐ ing mixing with another molecule or is added to see if a reaction occurs or not. Although the terms reactant and reagent are often used interchangeably, a reactant is more specifically a "substance that is consumed in the course of a chemical reaction." Before entering the main context of the write-up, it is better to present few sentences about the synthetic aspect in inorganic chemistry. Inorganic chemists have long been enthralled by the magnificent colors of inorganic compounds. From the historical perspective, in the early 15,000 BC, cave paint‐ ers used iron oxides for the sources of yellows and reds, aluminum silicates for greens, and manganese oxide for browns. By 3000 BC, Egyptian and Syrian artisans and jewelers were staining metal surfaces with inorganic salts derived from ground minerals and alloys. The colors obtained mostly depended on the types of transition metals contained in the minerals and on their combinations. One of the first synthetic pigments, Egyptian Blue, CaCuSi4O10, was prepared by heating malachite/sand (quartz or silica) mixtures to a temperature of 800– 900°C. Later in the fifteenth to eighteenth centuries, metal-containing pigments, such as cop‐ per carbonate and the brilliant ferric ferrocyanide coordination complex, Prussian Blue, Fe4[Fe(CN)6]3, were synthesized for use in the textile industry. Iron oxides, lead chromates, and the ubiquitous white pigment, titanium dioxide, are just a few of the many inorganic compounds that are still important in this industry today. The existence of ionic forces in‐ cluding some weak forces primarily accounts on the distinction of inorganic compounds than others.

A purely inorganic compound refers to a class of compound that does not contain a carbon to hydrogen (C-H) bond. Not all, but most inorganic compounds contain a metal. Actually, metal ions with labile reaction centers are generally used as geometry setters. For this rea‐ son, inorganic compounds have an overwhelming amount of applications and practical uses in the real world. Thus, in the recent spurt in research and advance academic framework, inorganic compounds are an exciting area that have important implications for the rational design of functional materials to be used as catalysts, conducting materials, molecular mag‐ netic materials, nonlinear optical materials, liquid crystals, chemical sensors, and molecular recognitions. Syntheses of molecules and materials are a very important aspect behind the development of science, especially in chemistry. The attempts to synthesize new molecules and materials have always depended on the experience and extrapolation of the existing knowledge to new situations and new synthesis of molecules. The strategies adopted in syn‐ theses of molecules and materials have undergone an enormous expansion in recent times, and research in inorganic synthesis has opened up a new vista in this direction. This book describes an overview of chemical reagents used in inorganic chemical reactions for the syn‐ thesis of different compounds including coordination, transition metal, organometallic, clus‐

ter, bioinorganic, and solid-state compounds. It will be helpful for the graduate students, teachers, researchers, and chemistry professionals who are interested to fortify and expand their knowledge about sol-gel preparation and application, porphyrin and phthalocyanine, carbon nanotube nanohybrids, triple bond between arsenic and group 13 elements, and Nheterocyclic carbene and its heavier analogues.

The book comprises a total of five chapters from multiple contributors around the world including China, India, and Taiwan. I am grateful to all the contributors and leading experts for the submission of their stimulating and inclusive chapters in the preparation of the edit‐ ed volume to bring the book on *Chemical Reactions in Inorganic Chemistry*. I offer our special thanks and appreciation to Ms. Romina Rovan, Publishing Process Manager, for her encour‐ agement and help and IntechOpen publisher in bringing out the book in the present form. I am also indebted to the Secretary and Principal of Urumu Dhanalakshmi College for their concern, efforts, and support in the task of publishing this volume.

#### **Dr. Chandraleka Saravanan** Assistant Professor Department of Chemistry Urumu Dhanalakshmi College Kattur, Tiruchirappalli Tamil Nadu, India

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: An Outline of Chemical Reagents**

A substance that is used in a chemical reaction to detect, measure, examine, or produce other substances is known as chemical reagent. A reagent in chemical science is a "substance or compound that is added to a system in order to bring a chemical reaction or is added to check whether a reaction is occurred or not." Such a reaction is used to confirm the detection of the presence of another substance. Although the terms "reactant and reagent" are often used interchangeably, a reactant is more specifically a "substance that is consumed in the course of a chemical reaction." Commonly, the involvement of solvents and catalysts in the course of a chemical reaction is not considered as reactants. Small organic molecules and metal salts/ compounds in organic chemistry play a significant role in different organic reactions of labo-

Grade of chemical reagents remains very important in inorganic synthesis. Purity of a chemical agent can also be expressed in terms of the grade. Chemical reagents in scientific community are commonly found as spectroscopic grade, analytical grade (A.R. grade), synthetic grade, and laboratory grade (L.R. grade). Depending on the nature of uses, specially qualitative or quantitative, one can select the grade of reagents. Whatever the chemical substances we commonly use, a minimum quality must be maintained, and the purity of the chemical substances is determined by few organizations of international standard like ASTM International.

Inorganic chemistry, which is serving as a fundamental branch of chemistry, deals with the synthesis and behavior of inorganic and organometallic compounds. This field covers all chemical compounds consisting of most of the chemical bonding comprising ionic, covalent, coordinate, μ-bonding, δ-bonding, H-bonding, weak interactions, and so on. The important aspects of chemical bonding in inorganic compounds reflect their physicochemical properties

**Introductory Chapter: An Outline of Chemical** 

**Reagents and Reactions in Inorganic Synthesis**

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.76536

**and Reactions in Inorganic Synthesis**

Chandraleka Saravanan and Bhaskar Biswas

Chandraleka Saravanan and Bhaskar Biswas

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.76536

ratory and industrial importance [1].

**1. Introduction**

#### **Introductory Chapter: An Outline of Chemical Reagents and Reactions in Inorganic Synthesis Introductory Chapter: An Outline of Chemical Reagents and Reactions in Inorganic Synthesis**

DOI: 10.5772/intechopen.76536

Chandraleka Saravanan and Bhaskar Biswas

Additional information is available at the end of the chapter Chandraleka Saravanan and Bhaskar BiswasAdditional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.76536

#### **1. Introduction**

ter, bioinorganic, and solid-state compounds. It will be helpful for the graduate students, teachers, researchers, and chemistry professionals who are interested to fortify and expand their knowledge about sol-gel preparation and application, porphyrin and phthalocyanine, carbon nanotube nanohybrids, triple bond between arsenic and group 13 elements, and N-

The book comprises a total of five chapters from multiple contributors around the world including China, India, and Taiwan. I am grateful to all the contributors and leading experts for the submission of their stimulating and inclusive chapters in the preparation of the edit‐ ed volume to bring the book on *Chemical Reactions in Inorganic Chemistry*. I offer our special thanks and appreciation to Ms. Romina Rovan, Publishing Process Manager, for her encour‐ agement and help and IntechOpen publisher in bringing out the book in the present form. I am also indebted to the Secretary and Principal of Urumu Dhanalakshmi College for their

**Dr. Chandraleka Saravanan**

Assistant Professor Department of Chemistry Urumu Dhanalakshmi College

> Kattur, Tiruchirappalli Tamil Nadu, India

heterocyclic carbene and its heavier analogues.

VIII Preface

concern, efforts, and support in the task of publishing this volume.

A substance that is used in a chemical reaction to detect, measure, examine, or produce other substances is known as chemical reagent. A reagent in chemical science is a "substance or compound that is added to a system in order to bring a chemical reaction or is added to check whether a reaction is occurred or not." Such a reaction is used to confirm the detection of the presence of another substance. Although the terms "reactant and reagent" are often used interchangeably, a reactant is more specifically a "substance that is consumed in the course of a chemical reaction." Commonly, the involvement of solvents and catalysts in the course of a chemical reaction is not considered as reactants. Small organic molecules and metal salts/ compounds in organic chemistry play a significant role in different organic reactions of laboratory and industrial importance [1].

Grade of chemical reagents remains very important in inorganic synthesis. Purity of a chemical agent can also be expressed in terms of the grade. Chemical reagents in scientific community are commonly found as spectroscopic grade, analytical grade (A.R. grade), synthetic grade, and laboratory grade (L.R. grade). Depending on the nature of uses, specially qualitative or quantitative, one can select the grade of reagents. Whatever the chemical substances we commonly use, a minimum quality must be maintained, and the purity of the chemical substances is determined by few organizations of international standard like ASTM International.

Inorganic chemistry, which is serving as a fundamental branch of chemistry, deals with the synthesis and behavior of inorganic and organometallic compounds. This field covers all chemical compounds consisting of most of the chemical bonding comprising ionic, covalent, coordinate, μ-bonding, δ-bonding, H-bonding, weak interactions, and so on. The important aspects of chemical bonding in inorganic compounds reflect their physicochemical properties

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and the materialistic application in industrial catalysis, therapeutic agents, optical materials, magnetic materials, and conducting materials [2]. This chapter describes an overview of chemical regents using inorganic chemical reactions for the synthesis of different compounds including coordination, organometallic, cluster, bioinorganic, solid state compounds, and so on.

#### **2. Inorganic reaction types**

A chemical reaction is basically a process in which an integral chemical change of a reactant is taken place during the conversion of reactant to products. In the chemical reactions, electronic motion involves to head a bond making and breaking. Different types of chemical reactions in inorganic chemistry may be classified more than a single way. Only some of the fundamental broad inorganic chemical reactions are categorized in the following.

#### **2.1. Combination reactions**

This is a type of a chemical reaction in which two or more reactants react themselves to produce a product. Elemental sulfur reacts with molecular oxygen to form sulfur dioxide is an example of combination reaction.

$$\text{S}\_{\text{}}\text{(s)} \text{+} \text{O}\_{\text{}}\text{(g)} \rightarrow \text{SO}\_{\text{}}\text{(g)}\tag{1}$$

compounds, supramolecules, coordination polymers, catalysts, magnetic compounds, and

Introductory Chapter: An Outline of Chemical Reagents and Reactions in Inorganic Synthesis

Coordination compounds cover a wide fundamental area in inorganic chemistry and primarily deal with coordination bonding between a ligand and a metal ion. Classical coordination compounds feature metals bound to "lone pairs" of electrons residing on the main

large numbers of ligands with different coordination motifs are available. Most of the metal complexes contain principally 3D metal ions with different ligand backbones. A. Werner, the founder of coordination chemistry showed different classes of coordination molecules. He also classified the ligands as classical/Werner type and non-classical/non-Werner type ligands. Commonly, in coordination compounds ligands are dominated by the elements belonging to groups 13–17 [4]. On the other hand, Schiff bases, a very common and widely used ligand systems, named after H. Schiff, are the condensation products between primary amines and aldehydes or ketones (RCH=NR′, where R and R′ represent alkyl and aryl substituents), which

Compounds with metal ions from group 4 to 11 are considered transition metal compounds. Compounds with a metal from group 3 or 12 are sometimes also incorporated into this group but often classified as main group compounds. Transition metal compounds also show rich coordination geometry, varying from coordination number 3 to 12, although tetrahedral, square planar, square pyramidal/trigonal bipyramid, and octahedral geometry for coordination geometries are commonly seen. A large number of transition elements are primarily responsible for carrying out different bio-functions in living systems like iron in heme proteins, zinc in carbonic anhydrase enzyme in respiration, Fe & Mo in nitrogen fixation, Mn in oxygen evolving complex in photosynthesis, Co in vitamin B12, Cu in hemocyanin and differ-

Usually, organometallic compounds are considered as a class of compound which contains at least one M─C bond which is exclusively covalent in character [8]. The metal (M) in these species can either be a main group element or a transition metal. Operationally, the definition of an organometallic compound is also more relaxed to include highly lipophilic complexes such as metal carbonyls, metal nitrosyls, and even metal alkoxides. These compounds have

Cluster compounds can be classified as metallic cluster, non-metallic cluster, and metal complex cluster. Clusters can be found in all classes of chemical compounds. According to the

drawn considerable interest mainly as homogeneous and heterogeneous catalysts.

, and CN−

. In modern coordination compounds,

http://dx.doi.org/10.5772/intechopen.76536

3

, Cl−

others [3]. A brief discussion on some classes of inorganic compounds is presented.

O, NH3

are used for the preparation of complex compounds [5].

**3.1. Coordination compounds**

group atoms of ligands such as H<sup>2</sup>

**3.2. Transition metal compounds**

ent metallo-enzymes, and so on [6, 7].

**3.3. Organometallic compounds**

**3.4. Cluster compounds**

#### **2.2. Decomposition reactions**

Often, decomposition reactions are considered as an opposite type of combination reactions. A chemical entity in the decomposition reaction breaks down into two or more substances. Decomposition usually results from electrolysis or heating. An example of a decomposition reaction is the breakdown of mercury(II) oxide into its component elements.

$$2\text{HgO}\,(\text{s})\text{+ heat} \rightarrow 2\text{Hg}\,(\text{l})\text{+O}\_2\,(\text{g})\tag{2}$$

#### **2.3. Displacement reactions**

A displacement reaction is occurred during the replacement of an atom or ion with another atom or ion in a compound. An example of a single displacement reaction is the displacement of copper ion in a copper sulfate solution by zinc metal ion to form zinc sulfate.

$$\text{Zn} \text{ (s)} \text{ + CuSO}\_4 \text{(aq)} \rightarrow \text{Cu} \text{ (s)} \text{ + ZnSO}\_4 \text{(aq)}\tag{3}$$

#### **3. Classification of inorganic compounds**

Inorganic chemistry includes different types of compounds comprising coordination compounds, simple metal compounds, organometallic compounds, cluster molecules, biological compounds, supramolecules, coordination polymers, catalysts, magnetic compounds, and others [3]. A brief discussion on some classes of inorganic compounds is presented.

#### **3.1. Coordination compounds**

and the materialistic application in industrial catalysis, therapeutic agents, optical materials, magnetic materials, and conducting materials [2]. This chapter describes an overview of chemical regents using inorganic chemical reactions for the synthesis of different compounds including coordination, organometallic, cluster, bioinorganic, solid state compounds, and so on.

A chemical reaction is basically a process in which an integral chemical change of a reactant is taken place during the conversion of reactant to products. In the chemical reactions, electronic motion involves to head a bond making and breaking. Different types of chemical reactions in inorganic chemistry may be classified more than a single way. Only some of the fundamental

This is a type of a chemical reaction in which two or more reactants react themselves to produce a product. Elemental sulfur reacts with molecular oxygen to form sulfur dioxide is an

S (s) + O<sup>2</sup> (g) → SO<sup>2</sup> (g) (1)

Often, decomposition reactions are considered as an opposite type of combination reactions. A chemical entity in the decomposition reaction breaks down into two or more substances. Decomposition usually results from electrolysis or heating. An example of a decomposition

2HgO (s) + heat → 2Hg (l) + O<sup>2</sup> (g) (2)

A displacement reaction is occurred during the replacement of an atom or ion with another atom or ion in a compound. An example of a single displacement reaction is the displacement

Zn (s) + CuSO<sup>4</sup> (aq) → Cu (s) + ZnSO<sup>4</sup> (aq) (3)

Inorganic chemistry includes different types of compounds comprising coordination compounds, simple metal compounds, organometallic compounds, cluster molecules, biological

reaction is the breakdown of mercury(II) oxide into its component elements.

of copper ion in a copper sulfate solution by zinc metal ion to form zinc sulfate.

broad inorganic chemical reactions are categorized in the following.

**2. Inorganic reaction types**

2 Chemical Reactions in Inorganic Chemistry

**2.1. Combination reactions**

example of combination reaction.

**2.2. Decomposition reactions**

**2.3. Displacement reactions**

**3. Classification of inorganic compounds**

Coordination compounds cover a wide fundamental area in inorganic chemistry and primarily deal with coordination bonding between a ligand and a metal ion. Classical coordination compounds feature metals bound to "lone pairs" of electrons residing on the main group atoms of ligands such as H<sup>2</sup> O, NH3 , Cl− , and CN− . In modern coordination compounds, large numbers of ligands with different coordination motifs are available. Most of the metal complexes contain principally 3D metal ions with different ligand backbones. A. Werner, the founder of coordination chemistry showed different classes of coordination molecules. He also classified the ligands as classical/Werner type and non-classical/non-Werner type ligands. Commonly, in coordination compounds ligands are dominated by the elements belonging to groups 13–17 [4]. On the other hand, Schiff bases, a very common and widely used ligand systems, named after H. Schiff, are the condensation products between primary amines and aldehydes or ketones (RCH=NR′, where R and R′ represent alkyl and aryl substituents), which are used for the preparation of complex compounds [5].

#### **3.2. Transition metal compounds**

Compounds with metal ions from group 4 to 11 are considered transition metal compounds. Compounds with a metal from group 3 or 12 are sometimes also incorporated into this group but often classified as main group compounds. Transition metal compounds also show rich coordination geometry, varying from coordination number 3 to 12, although tetrahedral, square planar, square pyramidal/trigonal bipyramid, and octahedral geometry for coordination geometries are commonly seen. A large number of transition elements are primarily responsible for carrying out different bio-functions in living systems like iron in heme proteins, zinc in carbonic anhydrase enzyme in respiration, Fe & Mo in nitrogen fixation, Mn in oxygen evolving complex in photosynthesis, Co in vitamin B12, Cu in hemocyanin and different metallo-enzymes, and so on [6, 7].

#### **3.3. Organometallic compounds**

Usually, organometallic compounds are considered as a class of compound which contains at least one M─C bond which is exclusively covalent in character [8]. The metal (M) in these species can either be a main group element or a transition metal. Operationally, the definition of an organometallic compound is also more relaxed to include highly lipophilic complexes such as metal carbonyls, metal nitrosyls, and even metal alkoxides. These compounds have drawn considerable interest mainly as homogeneous and heterogeneous catalysts.

#### **3.4. Cluster compounds**

Cluster compounds can be classified as metallic cluster, non-metallic cluster, and metal complex cluster. Clusters can be found in all classes of chemical compounds. According to the commonly accepted definition, a cluster consists minimally of a triangular set of atoms that are directly bonded to each other, but metal-metal bonded di-/tri-/polymetallic complexes are highly relevant to the area. Clusters occur in "pure" inorganic systems, organometallic chemistry, main group chemistry, and bioinorganic chemistry. The distinction between very large clusters and bulk solids is increasingly blurred. Few examples are Fe3 (CO)12, B10H14, [Mo<sup>6</sup> Cl14]2−, and 4Fe─4S.

solvents like water, ethanol, methanol, dimethyl sulfoxide, and some nonpolar solvents like

Introductory Chapter: An Outline of Chemical Reagents and Reactions in Inorganic Synthesis

http://dx.doi.org/10.5772/intechopen.76536

5

Melting point determination for any compound helps to identify the level of reactivity in solid state for any synthesized ligands and metal complexes. It also helps to predict the molecular

Elemental analysis remains an important method to study the nature of elements (especially C, H, N, and O) exist in compounds. The analysis on the abundance of carbon, nitrogen and hydrogen in a compound also helps to determine the class of compounds in the scientific literature.

Various spectroscopic methods are available in the determination of proposed structures for inorganic complexes. Among these, elemental analyses, FT-IR, UV-Vis, NMR, EPR, mass spectrometry, and thermogravimetric analysis are the most common techniques and widely used globally. But, the most authenticated technique in the determination of detail structural geometry is single crystal X-ray diffraction study. This particular analytical tool helps to

The UV-Vis spectra of all the complexes are recorded in spectrophotometer using different

Magnetic susceptibility measurements of the complexes in the solid state are determined by

IR spectral analysis is one of the most widely used analytical tools, which is utilized to assign different functional chromophores in the molecule. Incorporation of ligand with metal ions

locate the atomic position in a 3D structure with an absolute precise manner.

the Gouy balance at room temperature using metal as the calibrant.

The molar conductivity is measured with a conductivity meter.

**5. Structural characterization of metal complexes**

hexane, ethylacetate, benzene, etc.

composition of the complexes.

**5.1. Electronic spectroscopy**

**5.2. Magnetic susceptibility**

**5.3. Molar conductivity**

**5.4. Infrared spectroscopy (IR)**

solvents in the wave range of 200–1100 nm.

**4.2. Melting point**

**4.3. CHN analysis**

#### **3.5. Bioinorganic compounds**

Bioinorganic compounds are one of the most significant classes of a compound in chemical science, which are not only integrally related to the basic processes of nature but also provided significant insights into exploring the chemistry in living world [9]. It is a branch of chemistry, which deals with the interface between chemistry and biology. Starting from DNA to RNA to photosynthesis to oxygen transport and storage to nitrogen fixation, each and every parts of the living world are engrossed with the bioinorganic compounds. Traditional bioinorganic chemistry focuses on electron- and energy-transfer in proteins relevant to respiration. Medicinal inorganic chemistry includes the study of both non-essential and essential elements with applications to diagnosis and therapies.

When bioorganic molecules or drugs are bound to metal ions, there is a drastic change in their biomimetic properties, therapeutic effects, and pharmacological properties. One such remarkable molecule is amino acid, and they form complexes with metal atoms and exhibit significant biological and enzymatic activities. Essential metal ions and their complexes are found to have antitumor, antimicrobial, cytotoxic, and anti-HIV activity [10]. It was, therefore, considered worthwhile to study the complexation and to determine the biological activity of these new complexes.

#### **3.6. Solid state compounds**

This important area focuses on structure [11], bonding, and the physical properties [12] of materials. In practice, solid state inorganic chemistry uses techniques such as crystallography to gain an understanding of the structure and structural aspects that result from collective interactions between the subunits of the solid. Metals and their alloys or intermetallic derivatives are included in solid state chemistry. Related fields are condensed matter physics, mineralogy, and materials science. Examples are silicon chips, zeolites, and YBa<sup>2</sup> Cu3 O7 .

#### **4. Characterization of inorganic compounds**

#### **4.1. Solubility test**

Solubility is one of the basic parameters that help to understand the properties of a compound. The synthesized compounds are generally tested for their solubility in various polar solvents like water, ethanol, methanol, dimethyl sulfoxide, and some nonpolar solvents like hexane, ethylacetate, benzene, etc.

#### **4.2. Melting point**

commonly accepted definition, a cluster consists minimally of a triangular set of atoms that are directly bonded to each other, but metal-metal bonded di-/tri-/polymetallic complexes are highly relevant to the area. Clusters occur in "pure" inorganic systems, organometallic chemistry, main group chemistry, and bioinorganic chemistry. The distinction between very

Bioinorganic compounds are one of the most significant classes of a compound in chemical science, which are not only integrally related to the basic processes of nature but also provided significant insights into exploring the chemistry in living world [9]. It is a branch of chemistry, which deals with the interface between chemistry and biology. Starting from DNA to RNA to photosynthesis to oxygen transport and storage to nitrogen fixation, each and every parts of the living world are engrossed with the bioinorganic compounds. Traditional bioinorganic chemistry focuses on electron- and energy-transfer in proteins relevant to respiration. Medicinal inorganic chemistry includes the study of both non-essential and essential

When bioorganic molecules or drugs are bound to metal ions, there is a drastic change in their biomimetic properties, therapeutic effects, and pharmacological properties. One such remarkable molecule is amino acid, and they form complexes with metal atoms and exhibit significant biological and enzymatic activities. Essential metal ions and their complexes are found to have antitumor, antimicrobial, cytotoxic, and anti-HIV activity [10]. It was, therefore, considered worthwhile to study the complexation and to determine the biological activity of

This important area focuses on structure [11], bonding, and the physical properties [12] of materials. In practice, solid state inorganic chemistry uses techniques such as crystallography to gain an understanding of the structure and structural aspects that result from collective interactions between the subunits of the solid. Metals and their alloys or intermetallic derivatives are included in solid state chemistry. Related fields are condensed matter physics, min-

Solubility is one of the basic parameters that help to understand the properties of a compound. The synthesized compounds are generally tested for their solubility in various polar

eralogy, and materials science. Examples are silicon chips, zeolites, and YBa<sup>2</sup>

(CO)12, B10H14,

Cu3 O7 .

large clusters and bulk solids is increasingly blurred. Few examples are Fe3

[Mo<sup>6</sup> Cl14]

2−, and 4Fe─4S.

elements with applications to diagnosis and therapies.

**4. Characterization of inorganic compounds**

**3.5. Bioinorganic compounds**

4 Chemical Reactions in Inorganic Chemistry

these new complexes.

**4.1. Solubility test**

**3.6. Solid state compounds**

Melting point determination for any compound helps to identify the level of reactivity in solid state for any synthesized ligands and metal complexes. It also helps to predict the molecular composition of the complexes.

#### **4.3. CHN analysis**

Elemental analysis remains an important method to study the nature of elements (especially C, H, N, and O) exist in compounds. The analysis on the abundance of carbon, nitrogen and hydrogen in a compound also helps to determine the class of compounds in the scientific literature.

#### **5. Structural characterization of metal complexes**

Various spectroscopic methods are available in the determination of proposed structures for inorganic complexes. Among these, elemental analyses, FT-IR, UV-Vis, NMR, EPR, mass spectrometry, and thermogravimetric analysis are the most common techniques and widely used globally. But, the most authenticated technique in the determination of detail structural geometry is single crystal X-ray diffraction study. This particular analytical tool helps to locate the atomic position in a 3D structure with an absolute precise manner.

#### **5.1. Electronic spectroscopy**

The UV-Vis spectra of all the complexes are recorded in spectrophotometer using different solvents in the wave range of 200–1100 nm.

#### **5.2. Magnetic susceptibility**

Magnetic susceptibility measurements of the complexes in the solid state are determined by the Gouy balance at room temperature using metal as the calibrant.

#### **5.3. Molar conductivity**

The molar conductivity is measured with a conductivity meter.

#### **5.4. Infrared spectroscopy (IR)**

IR spectral analysis is one of the most widely used analytical tools, which is utilized to assign different functional chromophores in the molecule. Incorporation of ligand with metal ions can be easily detected to draw a simple comparison between IR spectra of free ligand and compound. The principle involved during this spectral analysis is to examine the changes in stretching modes.

**References**

Engineering News. 2006;**84**:35

Industrial Research. 1982;**41**:501-506

in man. Clinical Practice. 1982;**9**:20-22

Wiley-VCH; 1992. ISBN: 3527281649

Science Books; 1994. ISBN: 0-935702-73-3

ed. Wiley-VCH; 1991. ISBN: 3-527-28389-7

studies. Inorganica Chimica Acta. 1988;**151**:201-208

ISBN: 3-527-29311-6

[1] IUPAC. Compendium of chemical terminology. In: McNaught AD, Wilkinson A, editors.

Introductory Chapter: An Outline of Chemical Reagents and Reactions in Inorganic Synthesis

http://dx.doi.org/10.5772/intechopen.76536

7

[2] McCoy M, Reisch M, Tullo AH. Facts & figures of the chemical industry. Chemical and

[3] Lehn JM. Supramolecular Chemistry: Concepts and Perspectives. Weinheim: VCH; 1995.

[4] Greenwood NN, Earnshaw A. Chemistry of the Elements. 2nd ed. Butterworth-

[5] Dhar DN, Taploo CL. Schiff bases and their applications. Journal of Scientific and

[6] Paterson JW. Citation classic – The pharmacodynamics and metabolism of propranolol

[7] Parashar RK, Sharma RC, Kumar A, et al. Stability studies in relation to IR data of some Schiff base complexes of transition metals and their biological and pharmacological

[8] Elschenbroich C, Salzer A. Organometallics: A Concise Introduction. 2nd ed. Weinheim:

[9] Lippard SJ, Berg JM. Principles of Bioinorganic Chemistry. Mill Valley, CA: University

[10] Li R, George LK, Fred CE, Chen X, Gong B, Dominguez JN, Davidson E, Kurzban G, Miller RE, Nuzum EO, Rosenthal PJ, McKerrow JH. In vitro antimalarial activity of chal-

[12] Wilkins RG. Kinetics and Mechanism of Reactions of Transition Metal Complexes. 2nd

cones and their derivatives. Journal of Medicinal Chemistry. 1995;**38**:5031-5037

[11] Wells AF. Structural Inorganic Chemistry. Oxford: Clarendon Press; 1984

The Gold Book. 2nd ed. Cambridge, UK: Royal Society of Chemistry; 1997

Heinemann; 2nd ed. Oxford, Boston: Pergamon Press; 1997

#### **5.5. 1 H NMR spectroscopy**

Proton nuclear magnetic resonance is probably the best analytical method to determine the geometry and structural backbone for a chemical species. Further, the involvement of the ligand with metal ions in the formation of metal complexes can also be predicted with this technique. The chemical shifts associated with the location of H-atom in the skeletal of ligand remain very important in different applications in material sciences such as proton transfer mechanism and metal organic framework.

#### **5.6. ESR spectroscopy**

Electron spin resonance spectral technique is one of the fundamental techniques in detecting the spin state of metal ion or free ligand in inorganic compounds. It actually helps to determine the important aspects of chemical bonding along with significant illumination on structural features for metal complexes. Not only the structural part, but also the generation of ligand centered radical in different organic transformations of laboratory and industrial significance during the investigation of catalytic pathways can also be defined with this particular analytical technique. ESR measurements for the inorganic complexes at different temperatures and in different phases bring additional importance to this technique.

#### **5.7. Thermal analysis**

The thermal behavior of the synthesized complexes has been studied to establish different decomposition processes and to confirm the proposed stoichiometry. The thermal behavior of synthesized complexes was characterized on the basis of TGA/DTG and DTA method. Thermal analysis plays an important role in studying the stability, melting point, structure, and decomposition properties of the metal complexes.

#### **5.8. Single crystal X-ray diffraction analysis**

The most authenticated way to determine the 3D structure of inorganic complexes is single crystal X-ray diffraction study. It helps to locate the perfect atomic position in a molecule.

#### **Author details**

Chandraleka Saravanan<sup>1</sup> and Bhaskar Biswas<sup>2</sup> \*

\*Address all correspondence to: icbbiswas@gmail.com


#### **References**

can be easily detected to draw a simple comparison between IR spectra of free ligand and compound. The principle involved during this spectral analysis is to examine the changes in

Proton nuclear magnetic resonance is probably the best analytical method to determine the geometry and structural backbone for a chemical species. Further, the involvement of the ligand with metal ions in the formation of metal complexes can also be predicted with this technique. The chemical shifts associated with the location of H-atom in the skeletal of ligand remain very important in different applications in material sciences such as proton transfer

Electron spin resonance spectral technique is one of the fundamental techniques in detecting the spin state of metal ion or free ligand in inorganic compounds. It actually helps to determine the important aspects of chemical bonding along with significant illumination on structural features for metal complexes. Not only the structural part, but also the generation of ligand centered radical in different organic transformations of laboratory and industrial significance during the investigation of catalytic pathways can also be defined with this particular analytical technique. ESR measurements for the inorganic complexes at different tem-

The thermal behavior of the synthesized complexes has been studied to establish different decomposition processes and to confirm the proposed stoichiometry. The thermal behavior of synthesized complexes was characterized on the basis of TGA/DTG and DTA method. Thermal analysis plays an important role in studying the stability, melting point, structure,

The most authenticated way to determine the 3D structure of inorganic complexes is single crystal X-ray diffraction study. It helps to locate the perfect atomic position in a molecule.

\*

peratures and in different phases bring additional importance to this technique.

stretching modes.

**5.6. ESR spectroscopy**

**5.7. Thermal analysis**

**Author details**

Chandraleka Saravanan<sup>1</sup>

**H NMR spectroscopy**

6 Chemical Reactions in Inorganic Chemistry

mechanism and metal organic framework.

and decomposition properties of the metal complexes.

\*Address all correspondence to: icbbiswas@gmail.com

and Bhaskar Biswas<sup>2</sup>

2 Department of Chemistry, Surendranath College, Kolkata, India

1 Department of Chemistry, Urumu Dhanalakshmi College, Tiruchirppalli, India

**5.8. Single crystal X-ray diffraction analysis**

**5.5. 1**


**Chapter 2**

**Provisional chapter**

**Porphyrin and Phthalocyanine Covalently**

**Porphyrin and Phthalocyanine Covalently** 

**Nanohybrids for Optical Limiting**

**Nanohybrids for Optical Limiting**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Aijian Wang and Wei Zhao

**Abstract**

**1. Introduction**

Aijian Wang and Wei Zhao

http://dx.doi.org/10.5772/intechopen.69587

**Functionalized Graphene and Carbon Nanotube**

**Functionalized Graphene and Carbon Nanotube** 

DOI: 10.5772/intechopen.69587

Optical limiters are smart materials that follow passive approaches to provide laser protection, which are useful for the protection of human eyes, optical elements, and optical sensors from intense laser pulses. Many functional materials have been widely investigated with the view to realize practical passive optical limiting application. However, preparation of the required nonlinear optical active materials for optical limiters still presents a significant chemical challenge. In particular, this chapter gives emphasis to the nonlinear properties modulation of porphyrin and phthalocyanine covalently functionalized graphene and carbon nanotubes nanohybrids for the function of optical power limiting aiming the achievement of effective systems through the appropriate combination and modulation of several structural components. The nonlinear optical mechanisms observed in inorganic-organic nanohybrids, i.e., nonlinear scattering, nonlinear absorption, nonlinear refraction, and others, are discussed in conjunction with the influence of the materials properties and the laser source on the optical limiting performances.

**Keywords:** porphyrin, phthalocyanine, graphene, carbon nanotubes, optical limiting

Nonlinear optics, which investigates the interaction of intense pulse field with materials, is a relatively promising field in photoelectronics and photonics with many fundamental scientific and technological potential [1–3]. Select the topic "nonlinear optics" in the ISI Web of Science and I found 44,486 papers in January, 2017 (**Figure 1**). This is because nonlinear optics impacts a wide range of technical fields, including X-rays, quantum optics,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Provisional chapter**

#### **Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube Nanohybrids for Optical Limiting Functionalized Graphene and Carbon Nanotube Nanohybrids for Optical Limiting**

**Porphyrin and Phthalocyanine Covalently** 

DOI: 10.5772/intechopen.69587

Aijian Wang and Wei Zhao Aijian Wang and Wei Zhao Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69587

#### **Abstract**

Optical limiters are smart materials that follow passive approaches to provide laser protection, which are useful for the protection of human eyes, optical elements, and optical sensors from intense laser pulses. Many functional materials have been widely investigated with the view to realize practical passive optical limiting application. However, preparation of the required nonlinear optical active materials for optical limiters still presents a significant chemical challenge. In particular, this chapter gives emphasis to the nonlinear properties modulation of porphyrin and phthalocyanine covalently functionalized graphene and carbon nanotubes nanohybrids for the function of optical power limiting aiming the achievement of effective systems through the appropriate combination and modulation of several structural components. The nonlinear optical mechanisms observed in inorganic-organic nanohybrids, i.e., nonlinear scattering, nonlinear absorption, nonlinear refraction, and others, are discussed in conjunction with the influence of the materials properties and the laser source on the optical limiting performances.

**Keywords:** porphyrin, phthalocyanine, graphene, carbon nanotubes, optical limiting

#### **1. Introduction**

Nonlinear optics, which investigates the interaction of intense pulse field with materials, is a relatively promising field in photoelectronics and photonics with many fundamental scientific and technological potential [1–3]. Select the topic "nonlinear optics" in the ISI Web of Science and I found 44,486 papers in January, 2017 (**Figure 1**). This is because nonlinear optics impacts a wide range of technical fields, including X-rays, quantum optics,

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

**Figure 1.** Publications statistics about nonlinear optics, graphene, and carbon nanotubes from Web of Science (Thomson-Reuters) database.

has been published related to the study of these novel forms of carbon-based nanomaterials as highlighted in some prestigious reviews of graphene and carbon nanotubes with the view to realize practical passive optical limiting application [12–14]. The data about carbon nanomaterials have been swelled up so quickly that a single word of graphene and carbon nanotubes on web of science has resulted in thousands of publications statistics; the results are shown in **Figure 1**, which can help spur on the rapid development of carbon-based nanoscience. Porphyrins and phthalocyanines are attractive target materials for optical limiting applications due to the fact that they have a highly conjugated two-dimensional 18π-electron system resulting in a small HOMO-LUMO energy difference [15]. They also exhibit other additional advantages, i.e., exceptional stability, versatility, and processability features. The architectural flexibility of porphyrins and phthalocyanines can alter the electronic structure of the macrocyclic core, and thus, they allow the fine-tuning of the nonlinear optical performances [16]. Novel nonlinear optical materials may not be obtained from simple monomeric porphyrins or phthalocyanines, but be produced when strong electronic interactions between the highly delocalized π-electron systems and graphene/carbon nanotubes are induced by connecting them in a single system. Over times, the number of publications concerning the investigations and applications of porphyrins and phthalocyanines-functionalized graphene/carbon nanotubes has been an interesting observation. While phthalocyanine-functionalized graphene/carbon nanotubes have elicited wide interest in this area, porphyrins-functionalized graphene/carbon nanotubes have also increasingly attracted more attention (**Figure 3**). However, preparation of the required nonlinear optical active materials for optical limiters still presents a significant chemical challenge. Far from giving an exhaustive description of all the work that have been done in this area. This chapter will not explore application in detail, but only lay special stress on describing the recent achievements on the optical limiting properties of porphyrin and phthalocyanine covalently functionalized graphene/carbon nanotube nanohybrids. A great

**Figure 2.** Trend of the light intensity (output fluence) transmitted by an ideal optical limiter versus the incoming light

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube...

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11

intensity (input fluence) [9].

deal of information about nonlinear optics is now available on the ISI Web of Science.

optical communications, fiber optics, ultrafast lasers, spectroscopy, photorefractivity, liquid crystals, polymers, semiconductors, organics, switching, ultraviolet, telecommunications, and signal processing [4, 5]. To further advance the performance of optical materials and devices, researchers have sought for lots of novel materials with improved nonlinear optical properties, including inorganic semiconductors, organic molecules, polymeric systems, and other nanomaterials [6, 7]. The latter, specifically, are of great interest because they present high nonlinear optical properties combined with versatility of available routes of synthesis, used to alter and optimize nanostructure to maximize nonlinear responses and other properties [8]. The rapid development of nanoscience and nanotechnology provides many unique opportunities for nonlinear optics. A growing number of nanomaterials, including graphene, carbon nanotubes, and their derivatives have been shown to possess remarkable nonlinear optical properties [9, 10], which promotes the design and fabrication of nanoscale optoelectronic and photonic materials. Materials that exhibit nonlinear optical performances are useful because they allow manipulation of the fundamental properties of laser light beams, and are hence of great technological importance in areas such as photonic switching, optical computing, and other optical data processing systems. It is thus of critical importance to explore innovative nonlinear optical materials required for the practical applications.

Basically, optical limiting, an important application of nonlinear optics, is useful for the protection of human eyes, optical elements, and optical sensors from intense laser pulses and is of great interest to the private industry and military [11]. As shown in **Figure 2**, ideal optical limiters have a linear transmittance at low input fluence, while above the threshold intensity the transmittance becomes constant. Over the past several decades, a huge pile of scientific reports

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube... http://dx.doi.org/10.5772/intechopen.69587 11

**Figure 2.** Trend of the light intensity (output fluence) transmitted by an ideal optical limiter versus the incoming light intensity (input fluence) [9].

optical communications, fiber optics, ultrafast lasers, spectroscopy, photorefractivity, liquid crystals, polymers, semiconductors, organics, switching, ultraviolet, telecommunications, and signal processing [4, 5]. To further advance the performance of optical materials and devices, researchers have sought for lots of novel materials with improved nonlinear optical properties, including inorganic semiconductors, organic molecules, polymeric systems, and other nanomaterials [6, 7]. The latter, specifically, are of great interest because they present high nonlinear optical properties combined with versatility of available routes of synthesis, used to alter and optimize nanostructure to maximize nonlinear responses and other properties [8]. The rapid development of nanoscience and nanotechnology provides many unique opportunities for nonlinear optics. A growing number of nanomaterials, including graphene, carbon nanotubes, and their derivatives have been shown to possess remarkable nonlinear optical properties [9, 10], which promotes the design and fabrication of nanoscale optoelectronic and photonic materials. Materials that exhibit nonlinear optical performances are useful because they allow manipulation of the fundamental properties of laser light beams, and are hence of great technological importance in areas such as photonic switching, optical computing, and other optical data processing systems. It is thus of critical importance to explore innovative nonlinear optical materials required for the practical applications.

**Figure 1.** Publications statistics about nonlinear optics, graphene, and carbon nanotubes from Web of Science (Thomson-

Reuters) database.

10 Chemical Reactions in Inorganic Chemistry

Basically, optical limiting, an important application of nonlinear optics, is useful for the protection of human eyes, optical elements, and optical sensors from intense laser pulses and is of great interest to the private industry and military [11]. As shown in **Figure 2**, ideal optical limiters have a linear transmittance at low input fluence, while above the threshold intensity the transmittance becomes constant. Over the past several decades, a huge pile of scientific reports has been published related to the study of these novel forms of carbon-based nanomaterials as highlighted in some prestigious reviews of graphene and carbon nanotubes with the view to realize practical passive optical limiting application [12–14]. The data about carbon nanomaterials have been swelled up so quickly that a single word of graphene and carbon nanotubes on web of science has resulted in thousands of publications statistics; the results are shown in **Figure 1**, which can help spur on the rapid development of carbon-based nanoscience. Porphyrins and phthalocyanines are attractive target materials for optical limiting applications due to the fact that they have a highly conjugated two-dimensional 18π-electron system resulting in a small HOMO-LUMO energy difference [15]. They also exhibit other additional advantages, i.e., exceptional stability, versatility, and processability features. The architectural flexibility of porphyrins and phthalocyanines can alter the electronic structure of the macrocyclic core, and thus, they allow the fine-tuning of the nonlinear optical performances [16]. Novel nonlinear optical materials may not be obtained from simple monomeric porphyrins or phthalocyanines, but be produced when strong electronic interactions between the highly delocalized π-electron systems and graphene/carbon nanotubes are induced by connecting them in a single system. Over times, the number of publications concerning the investigations and applications of porphyrins and phthalocyanines-functionalized graphene/carbon nanotubes has been an interesting observation. While phthalocyanine-functionalized graphene/carbon nanotubes have elicited wide interest in this area, porphyrins-functionalized graphene/carbon nanotubes have also increasingly attracted more attention (**Figure 3**). However, preparation of the required nonlinear optical active materials for optical limiters still presents a significant chemical challenge. Far from giving an exhaustive description of all the work that have been done in this area. This chapter will not explore application in detail, but only lay special stress on describing the recent achievements on the optical limiting properties of porphyrin and phthalocyanine covalently functionalized graphene/carbon nanotube nanohybrids. A great deal of information about nonlinear optics is now available on the ISI Web of Science.

**Figure 3.** Publications statistics about phthalocyanine-functionalized graphene (Pc-G), porphyrin-functionalized graphene (TPP-G), phthalocyanine-functionalized carbon nanotubes (Pc-C), and porphyrin-functionalized carbon nanotubes (TPP-C) from Web of Science (Thomson-Reuters) database.

through the samples as a function of the sample position along the axis provides important information about the imaginary part (the intensity-dependent absorption coefficient) and the real part (change in refractive index) of the third-order nonlinear susceptibility. Since the sample experiences different incident laser energy at each position as it moves along the z direction, any nonlinearity in transmission can be revealed by this measurement. The energy of the input (laser energy reaching the sample) and output (laser energy exiting the sample) laser pulses was monitored simultaneously by two energy detectors (Rjp-765 energy probe), which were linked to a computer by a general-purpose interface bus (GPIB) interface. To measure the nonlinear absorption of the samples, the Z-dependent transmittance was measured by using the open-aperture Z-scan method. In this technique, the aperture is absent, and so the total transmitted energy is detected, from which the nature of the absorptive optical nonlinearity can be determined alone. Division of the Z-scan curve obtained with an aperture by that without an aperture gives a curve with nonlinear absorption effectively eliminated and affords the nonlinear refraction response. The optical limiting measurements were performed when the sample was located at focal point by varying the input energy and recording the output energy. Both the incident and the transmitted energies were measured simultaneously

**Figure 4.** Experimental setup of the Z-scan technique with pulse trains, used to characterize the sample's nonlinear

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube...

http://dx.doi.org/10.5772/intechopen.69587

13

optical response [3].

by two pyroelectric detectors with Laser Probe Rjp-765 Energy Ratio meter.

**and carbon nanotubes nanohybrids for optical limiting**

Graphene is the name given to a two-dimensional (2D) sheet of sp2

**3.1. Porphyrin and phthalocyanine covalently functionalized graphene**

**3. Porphyrin and phthalocyanine covalently functionalized graphene** 

in a hexagonal lattice, with a carbon-carbon distance of 0.142 nm. Long-range π-conjugation in graphene yields extraordinary thermal, physical, chemical, mechanical, and electrical properties, such as ultrafast carrier dynamics, superlative mechanical strength, large specific surface area, and high electrical and thermal conductivity, among others [19]. Thanks to these


### **2. Experimental setup**

With the aim to characterize the nonlinear optical absorption and optical limiting properties of a sample, the Z-can technique, developed by Sheik-Bahae et al. [17, 18], can be used, which is widely used in optical nonlinearity, mainly due to the simplicity of the experimental setup and the easy interpretation of the results. The major advantage of this technique is that the magnitude and the sign of the nonlinearity can be provided simultaneously. The Z-scan is a nonlinear optical experiment in which the irradiance (or fluence) is continuously varied from the linear regime of the sample to an irradiance high enough to induce nonlinear effects and then back to the linear regime. Because of the light-induced lens-like effect, the sample has a tendency to recollimate or defocus the incident beam, depending on its *z* position with respect to the focal plane. This can be accomplished by translating the sample along the z-axis of a focused Gaussian beam, the results being shown in **Figure 4**. In order to avoid cumulative thermal effects, data were collected in single shot mode.

In the Z-scan experiment, a laser pulse is focused to a minimum waist at the focal point along the propagation direction (z-axis) of this pulse. By moving the sample along the z-axis, the input pulse intensity is varied in the sample. When the sample is placed at the focal point, nonlinear process can be observed because the pulse intensity is high. The pulse intensity within the sample is decreased when the sample is moved away from the focal point, producing no nonlinear process. The consecutive recording of the relative pulse intensity transmitted Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube... http://dx.doi.org/10.5772/intechopen.69587 13

**Figure 4.** Experimental setup of the Z-scan technique with pulse trains, used to characterize the sample's nonlinear optical response [3].

through the samples as a function of the sample position along the axis provides important information about the imaginary part (the intensity-dependent absorption coefficient) and the real part (change in refractive index) of the third-order nonlinear susceptibility. Since the sample experiences different incident laser energy at each position as it moves along the z direction, any nonlinearity in transmission can be revealed by this measurement. The energy of the input (laser energy reaching the sample) and output (laser energy exiting the sample) laser pulses was monitored simultaneously by two energy detectors (Rjp-765 energy probe), which were linked to a computer by a general-purpose interface bus (GPIB) interface. To measure the nonlinear absorption of the samples, the Z-dependent transmittance was measured by using the open-aperture Z-scan method. In this technique, the aperture is absent, and so the total transmitted energy is detected, from which the nature of the absorptive optical nonlinearity can be determined alone. Division of the Z-scan curve obtained with an aperture by that without an aperture gives a curve with nonlinear absorption effectively eliminated and affords the nonlinear refraction response. The optical limiting measurements were performed when the sample was located at focal point by varying the input energy and recording the output energy. Both the incident and the transmitted energies were measured simultaneously by two pyroelectric detectors with Laser Probe Rjp-765 Energy Ratio meter.

#### **3. Porphyrin and phthalocyanine covalently functionalized graphene and carbon nanotubes nanohybrids for optical limiting**

#### **3.1. Porphyrin and phthalocyanine covalently functionalized graphene**

**2. Experimental setup**

12 Chemical Reactions in Inorganic Chemistry

from Web of Science (Thomson-Reuters) database.

single shot mode.

With the aim to characterize the nonlinear optical absorption and optical limiting properties of a sample, the Z-can technique, developed by Sheik-Bahae et al. [17, 18], can be used, which is widely used in optical nonlinearity, mainly due to the simplicity of the experimental setup and the easy interpretation of the results. The major advantage of this technique is that the magnitude and the sign of the nonlinearity can be provided simultaneously. The Z-scan is a nonlinear optical experiment in which the irradiance (or fluence) is continuously varied from the linear regime of the sample to an irradiance high enough to induce nonlinear effects and then back to the linear regime. Because of the light-induced lens-like effect, the sample has a tendency to recollimate or defocus the incident beam, depending on its *z* position with respect to the focal plane. This can be accomplished by translating the sample along the z-axis of a focused Gaussian beam, the results being shown in **Figure 4**. In order to avoid cumulative thermal effects, data were collected in

**Figure 3.** Publications statistics about phthalocyanine-functionalized graphene (Pc-G), porphyrin-functionalized graphene (TPP-G), phthalocyanine-functionalized carbon nanotubes (Pc-C), and porphyrin-functionalized carbon nanotubes (TPP-C)

In the Z-scan experiment, a laser pulse is focused to a minimum waist at the focal point along the propagation direction (z-axis) of this pulse. By moving the sample along the z-axis, the input pulse intensity is varied in the sample. When the sample is placed at the focal point, nonlinear process can be observed because the pulse intensity is high. The pulse intensity within the sample is decreased when the sample is moved away from the focal point, producing no nonlinear process. The consecutive recording of the relative pulse intensity transmitted

Graphene is the name given to a two-dimensional (2D) sheet of sp2 -hybridized carbon packed in a hexagonal lattice, with a carbon-carbon distance of 0.142 nm. Long-range π-conjugation in graphene yields extraordinary thermal, physical, chemical, mechanical, and electrical properties, such as ultrafast carrier dynamics, superlative mechanical strength, large specific surface area, and high electrical and thermal conductivity, among others [19]. Thanks to these extraordinary and superior properties, graphene has long been the interest of many theoretical studies and more recently became an exciting area for experimentalists with many commercial applications, including gas and energy storage, as well as micro- and optoelectronics [20]. Despite the great application potential, it is worth mentioning that pristine graphene has no appreciable solubility in most solvents. However, the solubility and processability are the primary problems for many prospective applications of graphene-based materials. To improve the dispersibility of graphene in common organic solvents and change its photophysical properties, a variety of methods for the graphene surface modification have been developed [19]. These graphene-based nanohybrids with different forms have different nonlinear optical properties and photophysical processes. In most cases, when organic molecules are covalently attached on the graphene surface, its extended aromatic character is perturbed, providing the opportunities to control its optoelectronic properties. Indeed, it is expected that the combination of graphene and optoelectronically active porphyrin and phthalocyanine molecules would afford species that possess not only the intrinsic properties of porphyrin and phthalocyanine but also some functions arising from the mutual π interaction between graphene and porphyrin/phthalocyanine; multifunctional nanometer-scale materials with enhanced optical limiting responses compared with that of the individual components may thereby be generated. The nonlinear optical properties of graphene-based materials have been extensively investigated, and this area has been reviewed [5, 8, 9, 13, 16]. The discussion here will be limited to selected highlights of our studies and more recent developments.

method that involves a 1,3-dipolar cycloaddition to the RGO surface using 4-hydroxybenzaldehyde, followed by nucleophilic substitution with an appropriate porphyrin. The interaction between RGO and the porphyrins was followed by fluorescence spectroscopy, and significant fluorescence quenching was observed for both nanohybrids, indicating the presence of efficient electron/energy transfer. When compared to RGO and porphyrins (TPP 1 and TPP 2), both nanohybrids display better nonlinear optical performances (**Figure 6**), implying a synergistic effect between two components due to the covalent linkage. Effective combination of the

**Scheme 1.** Chemical routes to functionalize RGO with porphyrin molecules: preparation of RGO-TPP 1 (Route 1) and

[25].

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube...

, GO, and GO and TPP-NH<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.69587

blend, and (b)

15

**Figure 5.** Open-aperture Z-scan curves of (a) Graphene-TPP, TPP-NH2

, GO, and GO and blend C60(OH)<sup>x</sup>

Graphene-C60, C60(OH)<sup>x</sup>

RGO-TPP 2 (Route 2) [26].

The electronic structure of porphyrin and phthalocyanine molecules can be tailored by altering the peripheral and axial functionalities or by either metal substitution at the central binding site, thus affording great versatility in controlling their optoelectronic properties [21–23]. For the nonlinear optical and optical limiting applications, Xu et al. designed and synthesized a novel soluble graphene nanohybrid material (TPP-NHCO-SPFGraphene) covalently functionalized with metal-free porphyrin (TPP-NH2 ) via an amidation reaction [24]. Attachment of the metalfree porphyrin TPP-NH<sup>2</sup> improves the solubility and dispersion stability of the TPP-NHCO-SPFGraphene nanohybrids in organic solvents. For the donor-acceptor nanohybrids, the fluorescence of the metal free porphyrin TPP-NH2 was effectively quenched due to a possible electron transfer process. The effective intramolecular energy quenching may also be facilitated by a through-bond mechanism, originating from the direct linkage mode of the two moieties by the amide bond. A superior optical limiting performance, better than the benchmark material C60, the control samples, and the individual components (TPP-NH2 and graphene oxide (GO)), is observed. Similarly, the preparation and nonlinear optical properties of two novel graphene nanohybrid materials covalently functionalized with porphyrin and C60 were reported by Tian et al. [25]. The results displayed that covalently functionalizing graphene with porphyrin and C60 can improve the nonlinear optical performance in the nanosecond regime (**Figure 5**).

Very recently, porphyrin-reduced graphene oxide (RGO-TPP) nanohybrids with a push-pull motif have been satisfactorily prepared by our group following the Prato protocol, via a 1,3-dipolar cycloaddition reaction of appropriate formyl derivatives with sarcosine [26]. Stepwise and "one-pot" procedures have been explored (**Scheme 1**): A straightforward Prato reaction (i.e., a 1,3-dipolar cycloaddition) with sarcosine and a formyl-containing porphyrin, and a stepwise

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube... http://dx.doi.org/10.5772/intechopen.69587 15

extraordinary and superior properties, graphene has long been the interest of many theoretical studies and more recently became an exciting area for experimentalists with many commercial applications, including gas and energy storage, as well as micro- and optoelectronics [20]. Despite the great application potential, it is worth mentioning that pristine graphene has no appreciable solubility in most solvents. However, the solubility and processability are the primary problems for many prospective applications of graphene-based materials. To improve the dispersibility of graphene in common organic solvents and change its photophysical properties, a variety of methods for the graphene surface modification have been developed [19]. These graphene-based nanohybrids with different forms have different nonlinear optical properties and photophysical processes. In most cases, when organic molecules are covalently attached on the graphene surface, its extended aromatic character is perturbed, providing the opportunities to control its optoelectronic properties. Indeed, it is expected that the combination of graphene and optoelectronically active porphyrin and phthalocyanine molecules would afford species that possess not only the intrinsic properties of porphyrin and phthalocyanine but also some functions arising from the mutual π interaction between graphene and porphyrin/phthalocyanine; multifunctional nanometer-scale materials with enhanced optical limiting responses compared with that of the individual components may thereby be generated. The nonlinear optical properties of graphene-based materials have been extensively investigated, and this area has been reviewed [5, 8, 9, 13, 16]. The discussion here will be limited to selected highlights of our studies and more recent developments.

The electronic structure of porphyrin and phthalocyanine molecules can be tailored by altering the peripheral and axial functionalities or by either metal substitution at the central binding site, thus affording great versatility in controlling their optoelectronic properties [21–23]. For the nonlinear optical and optical limiting applications, Xu et al. designed and synthesized a novel soluble graphene nanohybrid material (TPP-NHCO-SPFGraphene) covalently functionalized

SPFGraphene nanohybrids in organic solvents. For the donor-acceptor nanohybrids, the

electron transfer process. The effective intramolecular energy quenching may also be facilitated by a through-bond mechanism, originating from the direct linkage mode of the two moieties by the amide bond. A superior optical limiting performance, better than the benchmark material

is observed. Similarly, the preparation and nonlinear optical properties of two novel graphene nanohybrid materials covalently functionalized with porphyrin and C60 were reported by Tian et al. [25]. The results displayed that covalently functionalizing graphene with porphyrin and

Very recently, porphyrin-reduced graphene oxide (RGO-TPP) nanohybrids with a push-pull motif have been satisfactorily prepared by our group following the Prato protocol, via a 1,3-dipolar cycloaddition reaction of appropriate formyl derivatives with sarcosine [26]. Stepwise and "one-pot" procedures have been explored (**Scheme 1**): A straightforward Prato reaction (i.e., a 1,3-dipolar cycloaddition) with sarcosine and a formyl-containing porphyrin, and a stepwise

C60 can improve the nonlinear optical performance in the nanosecond regime (**Figure 5**).

) via an amidation reaction [24]. Attachment of the metal-

was effectively quenched due to a possible

and graphene oxide (GO)),

improves the solubility and dispersion stability of the TPP-NHCO-

with metal-free porphyrin (TPP-NH2

fluorescence of the metal free porphyrin TPP-NH2

C60, the control samples, and the individual components (TPP-NH2

free porphyrin TPP-NH<sup>2</sup>

14 Chemical Reactions in Inorganic Chemistry

**Figure 5.** Open-aperture Z-scan curves of (a) Graphene-TPP, TPP-NH2 , GO, and GO and TPP-NH<sup>2</sup> blend, and (b) Graphene-C60, C60(OH)<sup>x</sup> , GO, and GO and blend C60(OH)<sup>x</sup> [25].

method that involves a 1,3-dipolar cycloaddition to the RGO surface using 4-hydroxybenzaldehyde, followed by nucleophilic substitution with an appropriate porphyrin. The interaction between RGO and the porphyrins was followed by fluorescence spectroscopy, and significant fluorescence quenching was observed for both nanohybrids, indicating the presence of efficient electron/energy transfer. When compared to RGO and porphyrins (TPP 1 and TPP 2), both nanohybrids display better nonlinear optical performances (**Figure 6**), implying a synergistic effect between two components due to the covalent linkage. Effective combination of the

**Scheme 1.** Chemical routes to functionalize RGO with porphyrin molecules: preparation of RGO-TPP 1 (Route 1) and RGO-TPP 2 (Route 2) [26].

**Figure 6.** (a) Open-aperture Z-scan curves and (b) Optical limiting performances of RGO, TPP 2, TPP 1, RGO-TPP 1, and RGO-TPP 2 [26].

different nonlinear optical mechanisms, i.e., reverse saturable absorption, saturable absorption, excited absorption, nonlinear scattering, and photo-induced energy/electron transfer in the RGO-TPP nanohybrids leads to the improved nonlinear optical (including optical limiting) performances for 4 ns pulses at 532 nm. In the nanohybrid system, the nonlinear scattering originating from the graphene moieties can largely enhance the damage threshold of the nanohybrids. The similar accumulation effect leading to an increased nonlinear optical performance was also confirmed in RGO-TPP nanohybrid systems prepared by two synthetic routes that involve functionalization of the RGO by means of diazonium chemistry [27].

"Axial-bonding" at porphyrins may provide an approach for the preparation of arrays with tunable photoelectronic properties by changing the π-orbital interactions [28]. Axial substituents in porphyrins can favorably influence the nonlinear optical properties due to the presence of a dipole moment that is perpendicular to the macrocycle in the axially functionalized porphyrins. SnTPP and phosphorus-cored porphyrin (PTPP) (**Figure 7**) are wellknown axial-bonding building blocks, the Sn-OH and P-Cl bonds being reactive toward carboxylates and aryloxides, respectively. By using the reactive activity of hydroxyl and carboxyl groups on the basal planes of GO, as shown in **Figure 7**, Wang et al. synthesized two novel graphene nanohybrids incorporating axially functionalized porphyrins (GO-PTPP and GO-SnTPP) [29]. Their nonlinear optical properties and optical limiting performance have been studied by using the Z-scan technique at 532 nm with 4 ns and 21 ps laser pulses. Both nanohybrids exhibited enhanced nonlinear optical and optical limiting properties in comparison to the individual GO and porphyrins in both the nanosecond and picoseconds regimes, due to an effective combination of different nonlinear optical mechanisms, i.e., nonlinear scattering and/or two-photon absorption with reverse saturable absorption, and photo-induced electron or energy transfer from the electron-donor porphyrin moiety to the acceptor graphene.

Very recently, in order to optimize the RGO dispersion and enhance interfacial bonding, we reported two covalent functionalization approaches for the fabrication of RGO-TPP nanohybrid materials (**Scheme 2**) [30]. These are based on the initial covalent introduction of phenol groups onto the RGO surfaces by 1,3-dipolar cycloaddition or a diazotization reaction, and subsequent nucleophilic substitution at dichloro(5,10,15,20-tetraphenylporphyrinato)tin(IV) (SnTPP). The

porphyrin-functionalized RGO nanohybrids exhibited superior nonlinear optical performance to a RGO suspension at 532 nm under both picosecond and nanosecond regimes (**Figure 8**), due to a combination of mechanisms, while significant differences in their nonlinear optical

**Figure 7.** Synthetic routes of GO-PTPP and GO-SnTPP, and open-aperture Z-scan curves of GO, SnTPP, GO-SnTPP,

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PTPP, and GO-PTPP at 532 nm with (a) 4 ns and (b) 21 ps pulse duration [29].

**Scheme 2.** Syntheses of RGO-SnTPP 1 and RGO-SnTPP 2 [30].

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube... http://dx.doi.org/10.5772/intechopen.69587 17

**Figure 7.** Synthetic routes of GO-PTPP and GO-SnTPP, and open-aperture Z-scan curves of GO, SnTPP, GO-SnTPP, PTPP, and GO-PTPP at 532 nm with (a) 4 ns and (b) 21 ps pulse duration [29].

**Scheme 2.** Syntheses of RGO-SnTPP 1 and RGO-SnTPP 2 [30].

different nonlinear optical mechanisms, i.e., reverse saturable absorption, saturable absorption, excited absorption, nonlinear scattering, and photo-induced energy/electron transfer in the RGO-TPP nanohybrids leads to the improved nonlinear optical (including optical limiting) performances for 4 ns pulses at 532 nm. In the nanohybrid system, the nonlinear scattering originating from the graphene moieties can largely enhance the damage threshold of the nanohybrids. The similar accumulation effect leading to an increased nonlinear optical performance was also confirmed in RGO-TPP nanohybrid systems prepared by two synthetic routes that

**Figure 6.** (a) Open-aperture Z-scan curves and (b) Optical limiting performances of RGO, TPP 2, TPP 1, RGO-TPP 1, and

"Axial-bonding" at porphyrins may provide an approach for the preparation of arrays with tunable photoelectronic properties by changing the π-orbital interactions [28]. Axial substituents in porphyrins can favorably influence the nonlinear optical properties due to the presence of a dipole moment that is perpendicular to the macrocycle in the axially functionalized porphyrins. SnTPP and phosphorus-cored porphyrin (PTPP) (**Figure 7**) are wellknown axial-bonding building blocks, the Sn-OH and P-Cl bonds being reactive toward carboxylates and aryloxides, respectively. By using the reactive activity of hydroxyl and carboxyl groups on the basal planes of GO, as shown in **Figure 7**, Wang et al. synthesized two novel graphene nanohybrids incorporating axially functionalized porphyrins (GO-PTPP and GO-SnTPP) [29]. Their nonlinear optical properties and optical limiting performance have been studied by using the Z-scan technique at 532 nm with 4 ns and 21 ps laser pulses. Both nanohybrids exhibited enhanced nonlinear optical and optical limiting properties in comparison to the individual GO and porphyrins in both the nanosecond and picoseconds regimes, due to an effective combination of different nonlinear optical mechanisms, i.e., nonlinear scattering and/or two-photon absorption with reverse saturable absorption, and photo-induced electron or energy transfer from the electron-donor porphyrin moiety to the

Very recently, in order to optimize the RGO dispersion and enhance interfacial bonding, we reported two covalent functionalization approaches for the fabrication of RGO-TPP nanohybrid materials (**Scheme 2**) [30]. These are based on the initial covalent introduction of phenol groups onto the RGO surfaces by 1,3-dipolar cycloaddition or a diazotization reaction, and subsequent nucleophilic substitution at dichloro(5,10,15,20-tetraphenylporphyrinato)tin(IV) (SnTPP). The

involve functionalization of the RGO by means of diazonium chemistry [27].

acceptor graphene.

RGO-TPP 2 [26].

16 Chemical Reactions in Inorganic Chemistry

porphyrin-functionalized RGO nanohybrids exhibited superior nonlinear optical performance to a RGO suspension at 532 nm under both picosecond and nanosecond regimes (**Figure 8**), due to a combination of mechanisms, while significant differences in their nonlinear optical

Song et al. demonstrated the surface functionalization of RGO with zinc phthalocyanine (ZnPc), based on the initial covalent linkage of ZnPc to GO and subsequent *in situ* reduction of GO moiety to RGO during mild thermal treatment in dimethylformamide solvent [33]. The results display that the new nanohybrids allow the formation of stable colloidal solutions in various polar solvents such as dimethylsulfoxide, increasing the manipulation and processing of RGO. The nonlinear optical properties of the RGO-ZnPc nanohybrid were investigated by using the Z-scan technique. At a concentration of 0.13 mg/L, the RGO-ZnPc nanohybrid showed much larger nonlinear absorption coefficient β and better optical limiting performance than those of individual GO, ZnPc, and GP-ZnPc nanohybrids, ascribed to a

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube...

carbon clusters, and excited state absorption from numerous localized sp<sup>2</sup>

figurations in RGO moiety, reverse saturable absorption resulting from ZnPc moiety, and the contribution of improved photo-induced electron/energy transfer from ZnPc to RGO. Indeed, an improved photo-induced electron/energy transfer process with more efficient fluorescence quenching and energy release is observed in RGO-ZnPc than that of GO-ZnPc. In addition, the effect of peripheral substituents attached to phthalocyanines on the third-order nonlinear optical properties of graphene oxide-zinc(II) phthalocyanine hybrids, and the effects of central metals on the photophysical and nonlinear optical properties of reduced graphene oxide-

**3.2. Porphyrin and phthalocyanine covalently functionalized carbon nanotubes**

Carbon nanotubes (CNTs) are structures from the fullerene family consisting of a honey-

Typically, CNTs are divided into two distinct types, both with high structural perfection: single-walled carbon nanotubes (SWCNTs) that are made from one graphene sheet rolled as a tube and multiwalled carbon nanotubes (MWCNTs) that are obtained when several stacked graphitic shells are built to give concentric cylinders or graphene sheets are rolled [36]. CNTs have attractive mechanical, electrical, and thermal properties, which have found many potential applications in the field of nanoscience and nanotechnology. CNTs exhibit remarkable optical limiting properties on intense laser beams with ultrafast recovery time and broad bandwidth operation [37]. The advantage of CNTs-based nanomaterials manifests themselves in tailorable chemical properties by combining functional materials. Most importantly, the one-dimensional nanostructure of CNTs acts a favorable host for functional materials, forming versatile optical limiting nanocomposites [38]. It is therefore that CNTs are becoming a key component toward the development of nonlinear photonic devices. However, a CNTs system alone is not sufficient to fulfill all of the specifications of optical limiting. Indeed, various systems need to be coupled to extend the spectral and temporal ranges of effective optical

A CNT possesses one of the most extensive delocalized π-electron systems, which has been observed to be effective in optical limiting material and extends outside the range of many other optical limiting materials [39]. Covalently functionalized CNTs with porphyrins naturally pursued. For instance, Liu et al. reported the preparation and optical limiting

bonded carbon atoms rolled seamlessly into itself to form a cylinder.

domains, saturable absorption

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con-

19

combination of two-photon absorption resulting from the sp3

metal(II) phthalocyanine hybrids were also reported [34, 35].

from the sp<sup>2</sup>

comb nanosheet of sp<sup>2</sup>

limiting.

**Figure 8.** Left: Open-aperture Z-scan traces of RGO, RGO-SnTPP 1, and RGO-SnTPP 2 in dimethylsulfoxide (DMSO), obtained under 21 ps, 532 nm laser excitation. Right: Open-aperture Z-scan traces of RGO, RGO-SnTPP 1, and RGO-SnTPP 2 in DMSO, obtained under 4 ns, 532 nm laser excitation [30].

responses were observed, highlighting the influence on photophysical properties of the degree of functionalization and the synthetic approach employed. We also found that the two-photon absorption coefficients β2 values increase on proceeding from the components RGO and SnTPP to the nanohybrids, and that the β2 values of the nanohybrids under ns conditions differ significantly from that of the RGO/SnTPP blend, though being mindful of the errors associated with these measurements. Photo-induced electron or energy transfer mechanisms play an important role in the superior optical limiting performance. While nonlinear scattering dominates the optical limiting response, the photo-induced electron or energy transfer facilitates the deactivation of the nanohybrids, leading to energy dissipation via the nonradiative decay and thus, the effective heat accumulation in the nanohybrids or heat transfer from RGO to the adjacent solvent.

As porphyrin analogs, the structurally similar and biologically phthalocyanine are also effective optical limiters. Zhu et al. reported the synthesis and optical limiting performances of a soluble GO covalently functionalized with zinc phthalocyanine (GO-PcZn) by an amidation reaction [31]. A superior nonlinear optical effect and broadband optical limiting performance, better than GO and PcZn, are observed for GO-PcZn at both 532 and 1064 nm, suggesting a remarkable accumulation effect as a result of the covalent link between GO and PcZn. Similarly, a soluble GO axially substituted gallium phthalocyanine nanohybrid material (GO-PcGa) was prepared and designed by Li et al. [32]. At the same level of concentration of 0.1 g/L, this material displays much better optical limiting than the other three samples (GO, PcGa, and the benchmark optical limiting material C60) at both 532 and 1064 nm. The improved optical limiting performance was assigned to the effective combination of the different optical limiting mechanisms, i.e., reverse saturable absorption in PcGa, and nonlinear scattering and two-photon absorption in GO. For PcGa and C60, only reverse saturable absorption dominates at 532 nm, and no obvious nonlinear scattering was observed. Although the origin of the enhancement of the optical limiting effect for GO-PcGa at 1064 nm is not clear yet, the PcGa should certainly play an important but unknown role in this system due to the fact that GO-PcGa possesses much better broadband nonlinear optical and optical limiting performances than GO and PcGa alone.

Song et al. demonstrated the surface functionalization of RGO with zinc phthalocyanine (ZnPc), based on the initial covalent linkage of ZnPc to GO and subsequent *in situ* reduction of GO moiety to RGO during mild thermal treatment in dimethylformamide solvent [33]. The results display that the new nanohybrids allow the formation of stable colloidal solutions in various polar solvents such as dimethylsulfoxide, increasing the manipulation and processing of RGO. The nonlinear optical properties of the RGO-ZnPc nanohybrid were investigated by using the Z-scan technique. At a concentration of 0.13 mg/L, the RGO-ZnPc nanohybrid showed much larger nonlinear absorption coefficient β and better optical limiting performance than those of individual GO, ZnPc, and GP-ZnPc nanohybrids, ascribed to a combination of two-photon absorption resulting from the sp3 domains, saturable absorption from the sp<sup>2</sup> carbon clusters, and excited state absorption from numerous localized sp<sup>2</sup> configurations in RGO moiety, reverse saturable absorption resulting from ZnPc moiety, and the contribution of improved photo-induced electron/energy transfer from ZnPc to RGO. Indeed, an improved photo-induced electron/energy transfer process with more efficient fluorescence quenching and energy release is observed in RGO-ZnPc than that of GO-ZnPc. In addition, the effect of peripheral substituents attached to phthalocyanines on the third-order nonlinear optical properties of graphene oxide-zinc(II) phthalocyanine hybrids, and the effects of central metals on the photophysical and nonlinear optical properties of reduced graphene oxidemetal(II) phthalocyanine hybrids were also reported [34, 35].

#### **3.2. Porphyrin and phthalocyanine covalently functionalized carbon nanotubes**

responses were observed, highlighting the influence on photophysical properties of the degree of functionalization and the synthetic approach employed. We also found that the two-photon

**Figure 8.** Left: Open-aperture Z-scan traces of RGO, RGO-SnTPP 1, and RGO-SnTPP 2 in dimethylsulfoxide (DMSO), obtained under 21 ps, 532 nm laser excitation. Right: Open-aperture Z-scan traces of RGO, RGO-SnTPP 1, and RGO-

cantly from that of the RGO/SnTPP blend, though being mindful of the errors associated with these measurements. Photo-induced electron or energy transfer mechanisms play an important role in the superior optical limiting performance. While nonlinear scattering dominates the optical limiting response, the photo-induced electron or energy transfer facilitates the deactivation of the nanohybrids, leading to energy dissipation via the nonradiative decay and thus, the effective heat accumulation in the nanohybrids or heat transfer from RGO to the adjacent solvent. As porphyrin analogs, the structurally similar and biologically phthalocyanine are also effective optical limiters. Zhu et al. reported the synthesis and optical limiting performances of a soluble GO covalently functionalized with zinc phthalocyanine (GO-PcZn) by an amidation reaction [31]. A superior nonlinear optical effect and broadband optical limiting performance, better than GO and PcZn, are observed for GO-PcZn at both 532 and 1064 nm, suggesting a remarkable accumulation effect as a result of the covalent link between GO and PcZn. Similarly, a soluble GO axially substituted gallium phthalocyanine nanohybrid material (GO-PcGa) was prepared and designed by Li et al. [32]. At the same level of concentration of 0.1 g/L, this material displays much better optical limiting than the other three samples (GO, PcGa, and the benchmark optical limiting material C60) at both 532 and 1064 nm. The improved optical limiting performance was assigned to the effective combination of the different optical limiting mechanisms, i.e., reverse saturable absorption in PcGa, and nonlinear scattering and two-photon absorption in GO. For PcGa and C60, only reverse saturable absorption dominates at 532 nm, and no obvious nonlinear scattering was observed. Although the origin of the enhancement of the optical limiting effect for GO-PcGa at 1064 nm is not clear yet, the PcGa should certainly play an important but unknown role in this system due to the fact that GO-PcGa possesses much better broadband nonlinear optical and optical limiting

values increase on proceeding from the components RGO and SnTPP

values of the nanohybrids under ns conditions differ signifi-

absorption coefficients β2

18 Chemical Reactions in Inorganic Chemistry

to the nanohybrids, and that the β2

SnTPP 2 in DMSO, obtained under 4 ns, 532 nm laser excitation [30].

performances than GO and PcGa alone.

Carbon nanotubes (CNTs) are structures from the fullerene family consisting of a honeycomb nanosheet of sp<sup>2</sup> bonded carbon atoms rolled seamlessly into itself to form a cylinder. Typically, CNTs are divided into two distinct types, both with high structural perfection: single-walled carbon nanotubes (SWCNTs) that are made from one graphene sheet rolled as a tube and multiwalled carbon nanotubes (MWCNTs) that are obtained when several stacked graphitic shells are built to give concentric cylinders or graphene sheets are rolled [36]. CNTs have attractive mechanical, electrical, and thermal properties, which have found many potential applications in the field of nanoscience and nanotechnology. CNTs exhibit remarkable optical limiting properties on intense laser beams with ultrafast recovery time and broad bandwidth operation [37]. The advantage of CNTs-based nanomaterials manifests themselves in tailorable chemical properties by combining functional materials. Most importantly, the one-dimensional nanostructure of CNTs acts a favorable host for functional materials, forming versatile optical limiting nanocomposites [38]. It is therefore that CNTs are becoming a key component toward the development of nonlinear photonic devices. However, a CNTs system alone is not sufficient to fulfill all of the specifications of optical limiting. Indeed, various systems need to be coupled to extend the spectral and temporal ranges of effective optical limiting.

A CNT possesses one of the most extensive delocalized π-electron systems, which has been observed to be effective in optical limiting material and extends outside the range of many other optical limiting materials [39]. Covalently functionalized CNTs with porphyrins naturally pursued. For instance, Liu et al. reported the preparation and optical limiting performances of three covalently functionalized SWCNTs with porphyrins [40]. The optical limiting properties were determined by Z-scan experiments with linearly polarized 5 ns pulses at 532 nm generated from a frequency doubled Q-switched Nd:YAG laser. The structures of the porphyrin-functionalized SWCNTs are illustrated in **Figure 9**. The optical limiting performances of the three SWCNTs-based nanohybrids are better than not only the individual porphyrins, SWCNTs, and C60 but also the blended system of SWCNTs and porphyrins. The optical limiting thresholds approximately 70, 100, and 150 mJ/cm2 were determined for I, II, and III, respectively, all of which are much smaller than those of C60 (300 mJ/cm2 ) and SWCNTs (250 mJ/cm2 ).

Recently, Liu et al. reported the preparation of tetraphenylporphyrin covalently functionalized MWCNTs (MWCNT-TPP) with diameter ranges of <10, 10–30, and 40–60 nm by the reaction of MWCNTs with *in situ* generated porphyrin diazonium compounds (**Scheme 3**) [41]. A considerable quenching of the fluorescence intensity was found in the photoluminescence spectrum due to the fact that the unique direct linkage mode facilitates the effective photoinduced electron transfer between the excited porphyrin moiety and the extended π-system of MWCNTs. As laser pulse width increased from 5.6 to 11.7 ns, an obvious enhancement in the optical nonlinearities of MWCNT-TPP nanohybrid materials was observed (**Figure 10**), due to the improved efficiency of photo-induced electron transfer from porphyrin to MWCNTs with the increasing of pulse width.

Nanohybrids MWCNT-TPP and MWCNT-AIBN were prepared by radical polymerization (**Scheme 4**) [42]. The open-aperture Z-scan technique at 532 nm with 4 ns laser pulses was used to investigate the optical limiting performances; the results suggested that these nanohybrids exhibited generically good nonlinear optical properties. Comparison between the optical limiting performances of nanohybrids MWCNT-TPP and MWCNT-AIBN and that of TPP, MWCNTs, and mixture of both was effectuated. All samples displayed that the output fluence increased linearly with increasing input fluence at low laser pulse and deviations from linearity with further increase in input fluence pointed to the occurrence of optical limiting response. The order of the decreases in output fluence was MWCNT-TPP > TPP > MWCNT-AIBN > MWCNTs, implying that the optical limiting effect of MWCNT-TPP was superior to that of the MWCNT-AIBN and to that of the individual porphyrin as well as MWCNTs. As shown in **Figure 11**, at the wavelength used, the mechanism of

increased optical limiting may be ascribed to reverse saturable absorption, nonlinear scattering, and photo-induced electron/energy transfer between the MWCNTs and the porphyrin moieties. The covalent attachment of porphyrins onto the surfaces of MWCNTs is expected to facilitate the fabrication of devices requiring the use of CNTs in optical

**Figure 10.** Transmittance valleys (Tmin) of Z-scan curves of MWCNT-TPP (II) with different pulse width of 5.6, 5.9, 7.0,

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Two porphyrin covalently functionalized MWCNTs nanohybrids, MWCNT-ZnTPP and MWCNT-TPP, have been prepared directly from pristine MWCNTs through 1,3-dipolar cycloaddition reactions (**Scheme 5**), and the respective optical limiting properties of the materials were compared to those of zinc and free-base meso-tetraphenylporphyrins (ZnTPP and TPP) [43]. The nanohybrids displayed improved optical limiting ability. At the highest

limiting applications.

9.2, and 11.7 ns [41].

**Scheme 3.** Synthetic routes of MWCNT-TPP [41].

**Figure 9.** Structures of porphyrins covalently functionalized SWCNTs I, II, and III [40].

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube... http://dx.doi.org/10.5772/intechopen.69587 21

**Scheme 3.** Synthetic routes of MWCNT-TPP [41].

performances of three covalently functionalized SWCNTs with porphyrins [40]. The optical limiting properties were determined by Z-scan experiments with linearly polarized 5 ns pulses at 532 nm generated from a frequency doubled Q-switched Nd:YAG laser. The structures of the porphyrin-functionalized SWCNTs are illustrated in **Figure 9**. The optical limiting performances of the three SWCNTs-based nanohybrids are better than not only the individual porphyrins, SWCNTs, and C60 but also the blended system of SWCNTs and porphyrins. The optical limiting thresholds approximately 70, 100, and 150 mJ/cm2

determined for I, II, and III, respectively, all of which are much smaller than those of C60 (300

Recently, Liu et al. reported the preparation of tetraphenylporphyrin covalently functionalized MWCNTs (MWCNT-TPP) with diameter ranges of <10, 10–30, and 40–60 nm by the reaction of MWCNTs with *in situ* generated porphyrin diazonium compounds (**Scheme 3**) [41]. A considerable quenching of the fluorescence intensity was found in the photoluminescence spectrum due to the fact that the unique direct linkage mode facilitates the effective photoinduced electron transfer between the excited porphyrin moiety and the extended π-system of MWCNTs. As laser pulse width increased from 5.6 to 11.7 ns, an obvious enhancement in the optical nonlinearities of MWCNT-TPP nanohybrid materials was observed (**Figure 10**), due to the improved efficiency of photo-induced electron transfer from porphyrin to MWCNTs with

Nanohybrids MWCNT-TPP and MWCNT-AIBN were prepared by radical polymerization (**Scheme 4**) [42]. The open-aperture Z-scan technique at 532 nm with 4 ns laser pulses was used to investigate the optical limiting performances; the results suggested that these nanohybrids exhibited generically good nonlinear optical properties. Comparison between the optical limiting performances of nanohybrids MWCNT-TPP and MWCNT-AIBN and that of TPP, MWCNTs, and mixture of both was effectuated. All samples displayed that the output fluence increased linearly with increasing input fluence at low laser pulse and deviations from linearity with further increase in input fluence pointed to the occurrence of optical limiting response. The order of the decreases in output fluence was MWCNT-TPP > TPP > MWCNT-AIBN > MWCNTs, implying that the optical limiting effect of MWCNT-TPP was superior to that of the MWCNT-AIBN and to that of the individual porphyrin as well as MWCNTs. As shown in **Figure 11**, at the wavelength used, the mechanism of

).

**Figure 9.** Structures of porphyrins covalently functionalized SWCNTs I, II, and III [40].

mJ/cm2

) and SWCNTs (250 mJ/cm2

20 Chemical Reactions in Inorganic Chemistry

the increasing of pulse width.

were

**Figure 10.** Transmittance valleys (Tmin) of Z-scan curves of MWCNT-TPP (II) with different pulse width of 5.6, 5.9, 7.0, 9.2, and 11.7 ns [41].

increased optical limiting may be ascribed to reverse saturable absorption, nonlinear scattering, and photo-induced electron/energy transfer between the MWCNTs and the porphyrin moieties. The covalent attachment of porphyrins onto the surfaces of MWCNTs is expected to facilitate the fabrication of devices requiring the use of CNTs in optical limiting applications.

Two porphyrin covalently functionalized MWCNTs nanohybrids, MWCNT-ZnTPP and MWCNT-TPP, have been prepared directly from pristine MWCNTs through 1,3-dipolar cycloaddition reactions (**Scheme 5**), and the respective optical limiting properties of the materials were compared to those of zinc and free-base meso-tetraphenylporphyrins (ZnTPP and TPP) [43]. The nanohybrids displayed improved optical limiting ability. At the highest

valley positions for MWCNT-TPP and MWCNT-ZnTPP are larger than those for the other compounds in this study. This clearly indicated that the porphyrin-functionalized MWCNT nanohybrids possess larger nonlinear refraction coefficients than the individual components,

**Scheme 5.** Preparation of MWCNT-ZnTPP and MWCNT-TPP hybrid materials through a 1,3-dipolar cycloaddition

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Wang et al. synthesized axially coordinated metal-porphyrin-functionalized multiwalled carbon nanotubes nanohybrids via two different synthetic approaches (**Scheme 6**, a one-pot 1,3-dipolar cycloaddition reaction and a stepwise approach that involved 1,3-dipolar cycloaddition followed by nucleophilic substitution) [44], namely MWCNT-SnTPP 1 and MWCNT-SnTPP 2. The attachment of the tin porphyrin as an electron-donor onto the surface of the MWCNTs resulted in an increased electron absorption than its parent compound in the whole spectral region and an efficient fluorescence quenching. The resultant materials exhibit significant reverse saturable absorption or saturable absorption when nanosecond or picosecond pulses, respectively, are employed. A combination of the outstanding properties of MWCNTs and the chemically attached metal-porphyrins should be responsible for the improvement in the nanosecond

**Figure 12.** Normalized closed-aperture Z-scan curves of TPP, ZnTPP, MWCNT-TPP, and MWCNT-ZnTPP in N,N-

dimethylformamide (DMF) with (a) 4 ns and (b) 21 ps pulse durations [43].

presumably originating from their covalent linkage.

reaction [43].

**Scheme 4.** Synthetic routes of MWCNT-TPP and MWCNT-AIBN [42].

input fluence used in the experiments (1.65 J/cm2 ), the output fluences were determined to be 0.90, 0.83, 0.75, 0.40, and 0.27 J/cm2 for TPP, ZnTPP, MWCNTs, MWCNT-ZnTPP, and MWCNT-TPP, respectively. This clearly suggested that MWCNT-ZnTPP and MWCNT-TPP possessed the best optical limiting effect. In addition, the closed-aperture Z-scan measurements were also performed at 532 nm with both nanosecond and picosecond pulses for TPP, ZnTPP, MWCNT-TPP, and MWCNT-ZnTPP. As shown in **Figure 12**, all samples exhibit a self-defocusing effect, corresponding to a negative nonlinearity as revealed by the peak-valley signature, and the differences between the normalized transmittance values and

**Figure 11.** Possible mechanisms for the enhanced optical limiting effect of MWCNT-TPP [42].

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube... http://dx.doi.org/10.5772/intechopen.69587 23

**Scheme 5.** Preparation of MWCNT-ZnTPP and MWCNT-TPP hybrid materials through a 1,3-dipolar cycloaddition reaction [43].

valley positions for MWCNT-TPP and MWCNT-ZnTPP are larger than those for the other compounds in this study. This clearly indicated that the porphyrin-functionalized MWCNT nanohybrids possess larger nonlinear refraction coefficients than the individual components, presumably originating from their covalent linkage.

Wang et al. synthesized axially coordinated metal-porphyrin-functionalized multiwalled carbon nanotubes nanohybrids via two different synthetic approaches (**Scheme 6**, a one-pot 1,3-dipolar cycloaddition reaction and a stepwise approach that involved 1,3-dipolar cycloaddition followed by nucleophilic substitution) [44], namely MWCNT-SnTPP 1 and MWCNT-SnTPP 2. The attachment of the tin porphyrin as an electron-donor onto the surface of the MWCNTs resulted in an increased electron absorption than its parent compound in the whole spectral region and an efficient fluorescence quenching. The resultant materials exhibit significant reverse saturable absorption or saturable absorption when nanosecond or picosecond pulses, respectively, are employed. A combination of the outstanding properties of MWCNTs and the chemically attached metal-porphyrins should be responsible for the improvement in the nanosecond

input fluence used in the experiments (1.65 J/cm2

**Scheme 4.** Synthetic routes of MWCNT-TPP and MWCNT-AIBN [42].

and MWCNT-TPP, respectively. This clearly suggested that MWCNT-ZnTPP and MWCNT-TPP possessed the best optical limiting effect. In addition, the closed-aperture Z-scan measurements were also performed at 532 nm with both nanosecond and picosecond pulses for TPP, ZnTPP, MWCNT-TPP, and MWCNT-ZnTPP. As shown in **Figure 12**, all samples exhibit a self-defocusing effect, corresponding to a negative nonlinearity as revealed by the peak-valley signature, and the differences between the normalized transmittance values and

**Figure 11.** Possible mechanisms for the enhanced optical limiting effect of MWCNT-TPP [42].

to be 0.90, 0.83, 0.75, 0.40, and 0.27 J/cm2

22 Chemical Reactions in Inorganic Chemistry

), the output fluences were determined

for TPP, ZnTPP, MWCNTs, MWCNT-ZnTPP,

**Figure 12.** Normalized closed-aperture Z-scan curves of TPP, ZnTPP, MWCNT-TPP, and MWCNT-ZnTPP in N,Ndimethylformamide (DMF) with (a) 4 ns and (b) 21 ps pulse durations [43].

highest triplet quantum yield was observed for the nanocomposite in DMSO, and thus the best optical limiting properties can be expected when compared to the other systems investigated.

Porphyrin and Phthalocyanine Covalently Functionalized Graphene and Carbon Nanotube...

http://dx.doi.org/10.5772/intechopen.69587

25

Many functional materials have been widely investigated with the view to realize practical passive optical limiting application. However, preparation of the required nonlinear optical active materials for optical limiters still presents a significant chemical challenge. As we mentioned above, the excellent chemical activity of graphene and CNTs provides a broad platform for various functional counterparts, forming multi-component, multifunctional hybrid composites with wider spatial and temporal responses for optical limiting, which are suitable candidates to satisfy the requirements for the next generation of photonic components. The porphyrins and phthalocyanines covalently functionalized graphene and carbon nanotubes nanohybrids described in this work are the product of recent efforts spent in designing effective nanomaterial structures for optical limiting applications. The nanohybrids exhibited a strong optical limiting behavior, and the incorporation of porphyrins and phthalocyanines significantly increased the nonlinear optical performances of graphene and CNTs. The nonlinear optical mechanisms observed in inorganic-organic nanohybrids, i.e., nonlinear scattering, nonlinear absorption, nonlinear refraction, and others, are discussed in conjunction with the influence of the materials properties and the laser source on the optical limiting

Generally speaking, covalent functionalization of graphene and CNTs materials results in better optical limiting effect than noncovalent functionalization. This is because the covalent link can more effectively improve photo-induced energy or electron transfer from the functional moiety to the graphene and CNTs. In most of such nanohybrids, it is being attached importance to the photo-induced energy or electron transfer from functional moieties to graphene or CNTs, which is considered playing an important role on improving optical limiting performance. However, the data are very fragmentary, a detailed understanding of the factors affecting the nonlinear response is still necessary, and further work should be devoted to this objective. Multifunctional optical limiters are expected to have broad application range for various laser sources. The plasticity and flexibility of graphene and CNTs allow one to design and fabricate a range of structures to meet different demands. Graphene- and CNTs-based nanomaterials are still in their infancy, current demonstrations only show modest results, and while the very high third-order susceptibility combined with the fast optical response is promising, significant developments will need to be made before optical limiting by graphene- and CNTsbased nanomaterials can become a reality in a realistic commercial application. Looking toward the future use of graphene- and CNTs-based nanomaterials, it may be that in expanding on the already studied synthetic procedures lies a key to unlock a realm of opportunities for these materials. From a practical application of view, many efforts have been currently invested into the research of graphene- and CNTs-based nanocomposites in an attempt to allow for the fabrication of solid-state films or embedded solid-state thin films, which is very significant but challenging, and will undoubtly open a way to design and fabricate the industrially applicable

**4. Summary and outlook**

performances.

optical limiters.

**Scheme 6.** Preparation of MWCNT-SnTPP 1 and MWCNT-SnTPP 2 [44].

regime nonlinear absorption. It is well-known that nonlinear absorption performance is strongly affected by the pulse width. Indeed, the behaviors of MWCNT-SnTPP 1 and MWCNT-SnTPP 2 were quite different at the ns and ps time scales. Both nanohybrids exhibit saturable absorption performance in the ps regime, while reverse saturable absorption was observed in the ns regime. This change was ascribed to the result of transitions from the first excited state to higher excited states arising from the temporally longer pulse width under ns conditions.

CNTs in combination with Pcs were also investigated as optical limiting materials. Several reports appeared on the preparation and nonlinear optical properties of covalently functionalized CNTs with Pcs. For instance, the nonlinear optical properties of a novel unsymmetrically substituted metal-free phthalocyanine covalently functionalized MWCNTs nanohybrid (PcH2 - MWCNTs) were investigated by using the Z-scan technique at 532 nm in the ns time scale [45]. Strong nonlinear scattering was observed for this material at higher intensities, which evidently comes from the MWCNTs moiety. However, no obvious nonlinear scattering was observed from the pristine PcH<sup>2</sup> solution, indicating that the optical limiting effect totally originates from reverse saturable absorption. The nonlinear optical response of PcH<sup>2</sup> is ascribed to reverse saturable absorption, while that of PcH<sup>2</sup> -MWCNTs is due to both reverse saturable absorption and nonlinear scattering. Both nonlinear mechanisms may be conflicting for optical limiting, suppressing the whole nonlinear response of PcH<sup>2</sup> -MWCNTs. However, an improved nonlinear absorption was observed for indium phthalocyanine-SWCNTs composite in DMSO, when compared to the nonlinked phthalocyanine counterpart [46]. The optical properties of this composite were found to display high sensitivity toward the change of solvent matrix. Due to quenching by the counterionic DMF species, the triplet quantum yield values of the materials in DMF were found to be lower than those of their values in DMSO. In other words, solvents that can generate resonance-stabilized counterions as shown for DMF may tend to lower the inherent optical limiting effect of materials. However, the longest-lived triplet lifetimes with a highest triplet quantum yield was observed for the nanocomposite in DMSO, and thus the best optical limiting properties can be expected when compared to the other systems investigated.

#### **4. Summary and outlook**

regime nonlinear absorption. It is well-known that nonlinear absorption performance is strongly affected by the pulse width. Indeed, the behaviors of MWCNT-SnTPP 1 and MWCNT-SnTPP 2 were quite different at the ns and ps time scales. Both nanohybrids exhibit saturable absorption performance in the ps regime, while reverse saturable absorption was observed in the ns regime. This change was ascribed to the result of transitions from the first excited state to higher excited

CNTs in combination with Pcs were also investigated as optical limiting materials. Several reports appeared on the preparation and nonlinear optical properties of covalently functionalized CNTs with Pcs. For instance, the nonlinear optical properties of a novel unsymmetrically substituted metal-free phthalocyanine covalently functionalized MWCNTs nanohybrid (PcH2

MWCNTs) were investigated by using the Z-scan technique at 532 nm in the ns time scale [45]. Strong nonlinear scattering was observed for this material at higher intensities, which evidently comes from the MWCNTs moiety. However, no obvious nonlinear scattering was

absorption and nonlinear scattering. Both nonlinear mechanisms may be conflicting for optical

nonlinear absorption was observed for indium phthalocyanine-SWCNTs composite in DMSO, when compared to the nonlinked phthalocyanine counterpart [46]. The optical properties of this composite were found to display high sensitivity toward the change of solvent matrix. Due to quenching by the counterionic DMF species, the triplet quantum yield values of the materials in DMF were found to be lower than those of their values in DMSO. In other words, solvents that can generate resonance-stabilized counterions as shown for DMF may tend to lower the inherent optical limiting effect of materials. However, the longest-lived triplet lifetimes with a

nates from reverse saturable absorption. The nonlinear optical response of PcH<sup>2</sup>

solution, indicating that the optical limiting effect totally origi-




is ascribed

states arising from the temporally longer pulse width under ns conditions.

**Scheme 6.** Preparation of MWCNT-SnTPP 1 and MWCNT-SnTPP 2 [44].

observed from the pristine PcH<sup>2</sup>

24 Chemical Reactions in Inorganic Chemistry

to reverse saturable absorption, while that of PcH<sup>2</sup>

limiting, suppressing the whole nonlinear response of PcH<sup>2</sup>

Many functional materials have been widely investigated with the view to realize practical passive optical limiting application. However, preparation of the required nonlinear optical active materials for optical limiters still presents a significant chemical challenge. As we mentioned above, the excellent chemical activity of graphene and CNTs provides a broad platform for various functional counterparts, forming multi-component, multifunctional hybrid composites with wider spatial and temporal responses for optical limiting, which are suitable candidates to satisfy the requirements for the next generation of photonic components. The porphyrins and phthalocyanines covalently functionalized graphene and carbon nanotubes nanohybrids described in this work are the product of recent efforts spent in designing effective nanomaterial structures for optical limiting applications. The nanohybrids exhibited a strong optical limiting behavior, and the incorporation of porphyrins and phthalocyanines significantly increased the nonlinear optical performances of graphene and CNTs. The nonlinear optical mechanisms observed in inorganic-organic nanohybrids, i.e., nonlinear scattering, nonlinear absorption, nonlinear refraction, and others, are discussed in conjunction with the influence of the materials properties and the laser source on the optical limiting performances.

Generally speaking, covalent functionalization of graphene and CNTs materials results in better optical limiting effect than noncovalent functionalization. This is because the covalent link can more effectively improve photo-induced energy or electron transfer from the functional moiety to the graphene and CNTs. In most of such nanohybrids, it is being attached importance to the photo-induced energy or electron transfer from functional moieties to graphene or CNTs, which is considered playing an important role on improving optical limiting performance. However, the data are very fragmentary, a detailed understanding of the factors affecting the nonlinear response is still necessary, and further work should be devoted to this objective. Multifunctional optical limiters are expected to have broad application range for various laser sources. The plasticity and flexibility of graphene and CNTs allow one to design and fabricate a range of structures to meet different demands. Graphene- and CNTs-based nanomaterials are still in their infancy, current demonstrations only show modest results, and while the very high third-order susceptibility combined with the fast optical response is promising, significant developments will need to be made before optical limiting by graphene- and CNTsbased nanomaterials can become a reality in a realistic commercial application. Looking toward the future use of graphene- and CNTs-based nanomaterials, it may be that in expanding on the already studied synthetic procedures lies a key to unlock a realm of opportunities for these materials. From a practical application of view, many efforts have been currently invested into the research of graphene- and CNTs-based nanocomposites in an attempt to allow for the fabrication of solid-state films or embedded solid-state thin films, which is very significant but challenging, and will undoubtly open a way to design and fabricate the industrially applicable optical limiters.

## **Acknowledgements**

This research was supported financially by the National Natural Science Foundation of China (51506077, 51602130), the Natural Science Foundation of Jiangsu Province (BK20150488), the Natural Science Foundation of Jiangsu High School (15KJB430007, 15KJB610003), the China Postdoctoral Foundation (2016M601733), and Research Foundation of Jiangsu University (13JDG066, 15JDG156).

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### **Author details**

Aijian Wang1 \* and Wei Zhao<sup>2</sup>

\*Address all correspondence to: wajujs@ujs.edu.cn

1 School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, PR China

2 School of Energy and Power Engineering, Jiangsu University, Zhenjiang, PR China

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**Chapter 3**

**Provisional chapter**

**Sol-Gel Processes of Functional Powders and Films**

The key principles of sol-gel process and its characteristics are outlined and its major control parameters are summarized. Different samples of functional powders and films with magnetic, optical, and dielectric properties prepared by the sol-gel method are described. To determine the relationship between microstructure and properties, the effects of preparation conditions on the size and microstructure and electric properties, dielectric prop-

**Keywords:** sol-gel, magnetic, optical properties, dielectric, CCTO, multiferroics, hexagonal

Sol-gel processing is a well-recognized, cost-competitive, bottom-up synthesis technique, which is used in the field of material science and ceramic engineering for the generation of oxide nanopowder and composite NPs from a sol followed by a gel formation [1]. Functionality refers to typical chemicophysical properties owned by materials, which makes them possible to be exploited in technological applications, as in electrical conductors, thermoelectrics, ferroelectrics, magnets, and transparent optical devices. The sol-gel process of functionality opens promising applications in many areas, such as optics, electronics, ionics, mechanics, energy, environment, biology, medicine, for example as membranes and separation devices, fuel and solar cells, catalysts, sensors, and functional smart coatings. Sol-gel-derived coatings provide a wide range of applications, such as corrosion protective, hydrophilic, hydrophobic, antireflective, and antifog coatings; migration barriers against liquid and volatile compounds; antibacterial modification of textiles and water-repellent antistatic textiles; and

erties, optical properties, and magnetic properties are analyzed.

**Sol-Gel Processes of Functional Powders and Films**

DOI: 10.5772/intechopen.69588

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69588

Chao-Qun Ye

**Abstract**

ferrite, YFeO3

self-cleaning coatings [2].

**1. Introduction**

Chao-Qun Ye

**Provisional chapter**

## **Sol-Gel Processes of Functional Powders and Films**

**Sol-Gel Processes of Functional Powders and Films**

DOI: 10.5772/intechopen.69588

#### Chao-Qun Ye Chao-Qun Ye Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69588

#### **Abstract**

The key principles of sol-gel process and its characteristics are outlined and its major control parameters are summarized. Different samples of functional powders and films with magnetic, optical, and dielectric properties prepared by the sol-gel method are described. To determine the relationship between microstructure and properties, the effects of preparation conditions on the size and microstructure and electric properties, dielectric properties, optical properties, and magnetic properties are analyzed.

**Keywords:** sol-gel, magnetic, optical properties, dielectric, CCTO, multiferroics, hexagonal ferrite, YFeO3

#### **1. Introduction**

Sol-gel processing is a well-recognized, cost-competitive, bottom-up synthesis technique, which is used in the field of material science and ceramic engineering for the generation of oxide nanopowder and composite NPs from a sol followed by a gel formation [1]. Functionality refers to typical chemicophysical properties owned by materials, which makes them possible to be exploited in technological applications, as in electrical conductors, thermoelectrics, ferroelectrics, magnets, and transparent optical devices. The sol-gel process of functionality opens promising applications in many areas, such as optics, electronics, ionics, mechanics, energy, environment, biology, medicine, for example as membranes and separation devices, fuel and solar cells, catalysts, sensors, and functional smart coatings. Sol-gel-derived coatings provide a wide range of applications, such as corrosion protective, hydrophilic, hydrophobic, antireflective, and antifog coatings; migration barriers against liquid and volatile compounds; antibacterial modification of textiles and water-repellent antistatic textiles; and self-cleaning coatings [2].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

The sol-gel method initial study can be traced back to 1846, when Ebelmen find SiCl<sup>4</sup> mixed with ethanol in the wet air hydrolysis and formation of gel. The process of making tofu in ancient China may be the first method to use sol-gel technology in a purposeful and effective way; the soybean protein solution was flocculated into a solid bean curd (gel). Until the end of 1930s, W. Geffeken can conduct oxide film preparation through metal alkoxide hydrolysis and gelation. In 1971 in Germany, H. Dislieh reported sol by the hydrolysis of metal alkoxides, after gelation and retreatment in 923–973 K and high pressure resulting in the formation of SiO2 -B2 O3 -Al2 O3 -Na<sup>2</sup> O-K2 O multicomponent glass, which aroused great interest and attention in the field of material science. In 1975, B. E. Yoldas and M. Yamane dried the gel to prepare monolithic ceramic materials and porous transparent alumina films. Since 1980s, sol-gel technology has been successfully applied in glass, oxide coatings, and functional ceramic powders, especially composite oxide such as high critical temperature superconducting oxide material synthesis which is difficult to prepare by traditional method.

#### **2. Key principles of sol-gel process and its characteristics**

Sol is a system of tiny solid particles suspended in the liquid phase and constantly showing Brownian movement. Because the Gibbs free energy of the interface atoms is higher than that of the internal atoms, the sol is thermodynamically unstable. The particles tend to agglomerate spontaneously, reaching a low surface energy state. If the process is reversible, it is called flocculation; if not reversible, it is called gelation. The gel is a kind of solid feature of the colloidal system composed of fine particles in three-dimensional network structure and a continuous medium consisting of dispersed phase. The transition process to the gel can be briefly described as the formation of the polymer or the particle aggregates formed by the condensation reaction into a small particle cluster and gradually connected to a three-dimensional network structure. Therefore, the gelation process can be regarded as a small particle cluster connected to each other as a continuous solid network.

Sol-gel processes are mainly based on hydrolysis and condensation reactions of metal alkoxides, the molecular precursors that develop an oxide network in a liquid medium [3].

The structures of oxide materials change from nanoparticles to gel according to the reaction rate and the subsequent drying and post-treatment [2, 4–6].

Accurate control of the sol-gel reaction parameters can be designed to produce many forms of certain properties of the materials in the form of glass, fiber, ceramic powder, and thin film (**Figure 1**).

According to the types of precursors, the sol-gel process can be divided into two types: organic and inorganic. The organic route is that the sol-gel is prepared by metal alkoxides which can be considered as a two-step inorganic polymerization. The first step is the hydrolysis of alkoxide ligands to yield hydroxylated metal centers [7]:

$$\text{M(OR)}\_{\text{n}} + \text{H}\_{2}\text{O}^{\cdot} = \text{M(OR)}\_{\text{n-1}}\text{OH} + \text{ROH} \tag{1}$$

The second step is the condensation of the hydroxylated species to form oxypolymers, involving an oxylation reaction which creates oxygen bridges and expels XOH species as

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 33

M − OH + M − OX = M − O − M + XOH (X = H or R) (2)

The macroscopic properties of the material are determined by its microstructure, and the

Generally speaking, the sintering properties of the powder vary in different preparation methods. While using large surface area and high surface activity of monodisperse ultrafine

**3. Major control parameters of sol-gel process**

**Figure 1.** Schematic diagram of the sol-gel process.

microstructure is determined by the conditions of the material.

follows [7]:

**Figure 1.** Schematic diagram of the sol-gel process.

The sol-gel method initial study can be traced back to 1846, when Ebelmen find SiCl<sup>4</sup>

in the formation of SiO2

32 Chemical Reactions in Inorganic Chemistry

method.

(**Figure 1**).


connected to each other as a continuous solid network.

rate and the subsequent drying and post-treatment [2, 4–6].

oxide ligands to yield hydroxylated metal centers [7]:

M (OR)

**2. Key principles of sol-gel process and its characteristics**

with ethanol in the wet air hydrolysis and formation of gel. The process of making tofu in ancient China may be the first method to use sol-gel technology in a purposeful and effective way; the soybean protein solution was flocculated into a solid bean curd (gel). Until the end of 1930s, W. Geffeken can conduct oxide film preparation through metal alkoxide hydrolysis and gelation. In 1971 in Germany, H. Dislieh reported sol by the hydrolysis of metal alkoxides, after gelation and retreatment in 923–973 K and high pressure resulting

interest and attention in the field of material science. In 1975, B. E. Yoldas and M. Yamane dried the gel to prepare monolithic ceramic materials and porous transparent alumina films. Since 1980s, sol-gel technology has been successfully applied in glass, oxide coatings, and functional ceramic powders, especially composite oxide such as high critical temperature superconducting oxide material synthesis which is difficult to prepare by traditional

Sol is a system of tiny solid particles suspended in the liquid phase and constantly showing Brownian movement. Because the Gibbs free energy of the interface atoms is higher than that of the internal atoms, the sol is thermodynamically unstable. The particles tend to agglomerate spontaneously, reaching a low surface energy state. If the process is reversible, it is called flocculation; if not reversible, it is called gelation. The gel is a kind of solid feature of the colloidal system composed of fine particles in three-dimensional network structure and a continuous medium consisting of dispersed phase. The transition process to the gel can be briefly described as the formation of the polymer or the particle aggregates formed by the condensation reaction into a small particle cluster and gradually connected to a three-dimensional network structure. Therefore, the gelation process can be regarded as a small particle cluster

Sol-gel processes are mainly based on hydrolysis and condensation reactions of metal alkox-

The structures of oxide materials change from nanoparticles to gel according to the reaction

Accurate control of the sol-gel reaction parameters can be designed to produce many forms of certain properties of the materials in the form of glass, fiber, ceramic powder, and thin film

According to the types of precursors, the sol-gel process can be divided into two types: organic and inorganic. The organic route is that the sol-gel is prepared by metal alkoxides which can be considered as a two-step inorganic polymerization. The first step is the hydrolysis of alk-

*<sup>n</sup>* + H2 O = M (OR)

ides, the molecular precursors that develop an oxide network in a liquid medium [3].

mixed

O multicomponent glass, which aroused great

*<sup>n</sup>*−<sup>1</sup> OH + ROH (1)

The second step is the condensation of the hydroxylated species to form oxypolymers, involving an oxylation reaction which creates oxygen bridges and expels XOH species as follows [7]:

$$\text{M}-\text{OH}^{\cdot} + \text{M}-\text{OX}=\text{M}-\text{O}-\text{M} + \text{XOH}\text{ (X}=\text{H} \text{or R)}\tag{2}$$

#### **3. Major control parameters of sol-gel process**

The macroscopic properties of the material are determined by its microstructure, and the microstructure is determined by the conditions of the material.

Generally speaking, the sintering properties of the powder vary in different preparation methods. While using large surface area and high surface activity of monodisperse ultrafine ceramic powder, due to short diffusion distance and only need lower sintering temperature and activation energy.

Factors that influence the properties of final products, acquired by sol-gel technique, include hydrolysis ratio, acidity of the hydrolyzing agent, gelation condition, drying condition and procedures and types of solvent.

#### **3.1. Chemical compositions of the precursors**

The nature of the precursor can significantly affect the sol-gel reaction kinetics and final products.

#### **3.2. Hydrolysis ratio**

The hydrolysis ratio, as defined by Eq. (3), is the main external parameter and has been shown to be important in controlling the hydrolysis and condensation of metal alkoxides [5]:

$$h \quad = \ \lbrack \mathcal{H}\_2 \mathcal{O} \rbrack / \lbrack \mathcal{M}(\mathcal{OR})\_z \rbrack \tag{3}$$

hydrolysis under the condition of alkali catalysis. Acid or base catalysis is not the only catalyst. Some nucleophiles, such as NaF or 2-methylpyridine, also have the ability to improve

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 35

Zinc oxide (ZnO) nanoparticles were prepared and synthesized via sol-gel method, using citric acid as a precursor. The annealing temperature was fixed at 600°C. The impact of pH on the particle size was investigated. Three different pH (3.0, 5.0, and 1.01) for the precursor were chosen, and it was revealed that the Zn NPs size increased with the alkalinity of the precursor [8]. The pH value of the sol-gel solution will change when added with acids and alkalis to catalyze the reactions. Experimental results show that the pH value and hydrolysis ratio have

Generally, increasing temperature results in both hydrolysis and condensation rate increase

The sol-gel process provides samples with high purity, homogeneity, and structure of easy control. The most important advantages of the sol-gel process in the preparation of functional

(i) In all stages, the temperatures required are low and close to room temperature. Then, thermal decomposition of organic material and any entrapped species is minimized

(ii) As the organometallic precursors for different metals are miscible, the homogeneous sol solutions are easily achieved [2]. Since the sol-gel is initiated by the reaction of the solution, the materials are very uniform and easy to modify, which is crucial for controlling

(iii) Precursors such as metal alkoxides and mixed alkyl/alkoxides are easily purified by common techniques (e.g., distillation or sublimation), which lead to high-purity products [2].

(iv) The chemical conditions are mild in sol-gel process. Hydrolysis and condensation are

(vi) Colloid particle size and pore size, porosity, and chemistry of the final product can be optimized by chemical treatment of the precursors, controlled rates of hydrolysis, and

(v) Highly porous and nanocrystalline materials can be synthesized by this method [2].

a synergistic effect on the morphologies of the products.

**4. Advantages and shortages of sol-gel processing**

leading to high purity and stoichiometry [2].

the physical and chemical properties of the material.

catalyzed by acid or alkali under mild pH conditions [2, 9].

the reaction rate.

**3.5. Reaction temperature**

materials are as follows:

by condensation [2].

**3.4. pH value**

[6, 7].

where [H<sup>2</sup> O] and [M(OR)*<sup>z</sup>* ] are the molar numbers for the water and metal alkoxide, respectively; *z* is the valance charge of metal M.

The lower amount of water added is easy to form the product of low crosslinking, and the viscosity of the sol is increased; the higher amount of water added is easy to form a highly cross-linked product, and the viscosity is decreased. Therefore, the amount of water has an important effect on the structure of the product and the viscosity and gelation time. When *h* < 1, water molecules are not sufficient for complete hydrolysis, and thus gelation or precipitation cannot occur. When 1 < *h* < *z*, chain oxypolymers can form. When *h* > *z*, cross-linked oxypolymers can be obtained and nanoparticles or gels can be formed. The hydrolysis ratio also has a great influence on the hydrolysis and condensation rate. When *h* is low (< ~1), the hydrolysis rate is low, so the sol-gel reaction rate including hydrolysis and condensation is also slow. When *h* is high (< ~7), the hydrolysis rate is high, but the condensation rate is low, the result being a slow sol-gel reaction. When *h* is intermediate (1 < *h* < 7), the sol-gel reaction is the fastest because of high rates of both hydrolysis and condensation [7].

#### **3.3. Catalysts**

Both acids and bases can catalyze sol-gel reactions. The hydrolysis and condensation reactions can be explained by SN2 nucleophilic substitution reaction mechanism. Take alcohol as an example, it is not easy to hydrolyze silicon alkoxide, and its hydrolysis and polymerization are usually catalyzed by acid or alkali. The inorganic acid can make the partially negatively charged alkoxy matrix, so that it is easy to break away from the silicon atom. Alkali provides nucleophilic hydroxyl OH in alkali-catalyzed hydrolysis, and Si—OH loss of protons, thereby accelerating polymerization. In the presence of excess water, because of the acid-catalyzed hydrolysis reaction, Si(OH) can be formed. The rate of polymerization is faster than that of hydrolysis under the condition of alkali catalysis. Acid or base catalysis is not the only catalyst. Some nucleophiles, such as NaF or 2-methylpyridine, also have the ability to improve the reaction rate.

#### **3.4. pH value**

ceramic powder, due to short diffusion distance and only need lower sintering temperature

Factors that influence the properties of final products, acquired by sol-gel technique, include hydrolysis ratio, acidity of the hydrolyzing agent, gelation condition, drying condition and

The nature of the precursor can significantly affect the sol-gel reaction kinetics and final

The hydrolysis ratio, as defined by Eq. (3), is the main external parameter and has been shown

*h* = [ H2 O]/[ M(OR )*<sup>z</sup>* ] (3)

The lower amount of water added is easy to form the product of low crosslinking, and the viscosity of the sol is increased; the higher amount of water added is easy to form a highly cross-linked product, and the viscosity is decreased. Therefore, the amount of water has an important effect on the structure of the product and the viscosity and gelation time. When *h* < 1, water molecules are not sufficient for complete hydrolysis, and thus gelation or precipitation cannot occur. When 1 < *h* < *z*, chain oxypolymers can form. When *h* > *z*, cross-linked oxypolymers can be obtained and nanoparticles or gels can be formed. The hydrolysis ratio also has a great influence on the hydrolysis and condensation rate. When *h* is low (< ~1), the hydrolysis rate is low, so the sol-gel reaction rate including hydrolysis and condensation is also slow. When *h* is high (< ~7), the hydrolysis rate is high, but the condensation rate is low, the result being a slow sol-gel reaction. When *h* is intermediate (1 < *h* < 7), the sol-gel reaction

Both acids and bases can catalyze sol-gel reactions. The hydrolysis and condensation reactions can be explained by SN2 nucleophilic substitution reaction mechanism. Take alcohol as an example, it is not easy to hydrolyze silicon alkoxide, and its hydrolysis and polymerization are usually catalyzed by acid or alkali. The inorganic acid can make the partially negatively charged alkoxy matrix, so that it is easy to break away from the silicon atom. Alkali provides nucleophilic hydroxyl OH in alkali-catalyzed hydrolysis, and Si—OH loss of protons, thereby accelerating polymerization. In the presence of excess water, because of the acid-catalyzed hydrolysis reaction, Si(OH) can be formed. The rate of polymerization is faster than that of

is the fastest because of high rates of both hydrolysis and condensation [7].

] are the molar numbers for the water and metal alkoxide, respec-

to be important in controlling the hydrolysis and condensation of metal alkoxides [5]:

and activation energy.

34 Chemical Reactions in Inorganic Chemistry

products.

where [H<sup>2</sup>

**3.3. Catalysts**

**3.2. Hydrolysis ratio**

procedures and types of solvent.

O] and [M(OR)*<sup>z</sup>*

tively; *z* is the valance charge of metal M.

**3.1. Chemical compositions of the precursors**

Zinc oxide (ZnO) nanoparticles were prepared and synthesized via sol-gel method, using citric acid as a precursor. The annealing temperature was fixed at 600°C. The impact of pH on the particle size was investigated. Three different pH (3.0, 5.0, and 1.01) for the precursor were chosen, and it was revealed that the Zn NPs size increased with the alkalinity of the precursor [8]. The pH value of the sol-gel solution will change when added with acids and alkalis to catalyze the reactions. Experimental results show that the pH value and hydrolysis ratio have a synergistic effect on the morphologies of the products.

#### **3.5. Reaction temperature**

Generally, increasing temperature results in both hydrolysis and condensation rate increase [6, 7].

#### **4. Advantages and shortages of sol-gel processing**

The sol-gel process provides samples with high purity, homogeneity, and structure of easy control. The most important advantages of the sol-gel process in the preparation of functional materials are as follows:


**Figure 2** shows the room temperature hysteresis loop and magnetization temperature curve of h-YFO powder. As can be seen from **Figure 2**, h-YFO has a weak magnetism, and the hysteresis loop is narrow and small, which belongs to the soft magnet. The remanent magnetization (Mr) and coercive field (H) are 0.11 memu g-1 and 0.19 kOe, respectively. The

about 100 Oe and 26.0 memu g-1 and 150 Oe and 70 memu g-1 at room temperature, respec-

The magnetization of h-YFO is very small and the coercive field is much larger, so it is possible to exclude the influence of impurities on the magnetic properties of the samples, which

**Figure 3** shows the h-YFO magnetization-temperature relationship curve (M-T) and its first derivative curve (dM/dT-T), measured at the external magnetic field strength of H = 1000 Oe using temperature mode. It can be seen from the figure that the magnetization of h-YFO is

which indicates that h-YFO has a complex magnetic phase transition in this temperature range. **Figure 4** shows the hysteresis loop and the magnetization temperature curve of o-YFO powder at room temperature. Compared to h-YFO, the hysteresis loop of o-YFO is wide which belongs to hard magnet, and the magnetization and coercive field of o-YFO is 0.72 memu g-1 and 24.5 kOe respectively which is much larger than h-YFO. It is also shown that the influence of impurities on the magnetic properties is small. From the extension direction of the loop

It is worth noting that **Figure 4** llustrations for the hysteresis curve of o-YFO on a small magnetic field (<10 kOe) that shows a peculiar shape with a sudden change magnetization in H = 0 kOe, which is considered to be the inherent characteristic of o-YFO. The causes of the abnor-

tion with the external magnetic field, the DM antisymmetric interaction, and the anisotropy

hysteresis loop are produced by the interaction of the same iron ions, the interac-

Fe<sup>5</sup>

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588

to about zero in the temperature range of 750 K to 877 K,

O12 and γ-Fe<sup>2</sup>

is of a slight distortion,

O3 are 37

coercivity and saturation magnetization of possible impurities Y<sup>3</sup>

is characterizing the magnetic properties of h-YFO.

line, o-YFO has a certain antiferromagnetic property.

effect of the magnetic crystal. The adjacent two iron ions in YFeO<sup>3</sup>

gradually reduced from 0.72 memu g-1

**Figure 2.** M-H hysteresis curve of h-YFO powder.

tively [12, 13].

mal YFeO3


The negative factors of sol-gel processing include high precursor cost, overall high cost, and environmental problems associated with the disposal of large quantities of organic by-products. A common theme out of the negative comments was wet chemical processing using alkoxides as precursors [11].

The sol-gel process is more complex in terms of the difficulty in phase control, which is the different chemical and crystal morphology formation at different temperatures. The morphology is relatively simple, generally, spherical particles.

Gelation, drying, and heat treatment take a lot of time to prepare the sample. In the stage of drying and heat treatment, the sample has a great weight loss and residual stress, film prone to cracking and objectively restricting the thickness of thin film and result in film residual porosity.

#### **5. Sol-gel processing of functional powder materials**

#### **5.1. Multiferroic properties of powder materials**

First, Y(NO<sup>3</sup> ) 3 ·5H<sup>2</sup> O and Fe(NO<sup>3</sup> ) 3 ·9H2 O were dissolved in deionized water in the molar ratio of *n*(Y):*n*(Fe) = 1:1, then citric acid and metal cations were added in the molar ratio of 1.5:1 to dissolve them completely and for the full complexation with metal ions. The acrylamide monomer is added in the ratio of 9:1 to the cation. The mixture at 358 K temperature is heated to polymerize to form a gel at 393 K. The dry gel is crushed into fine powder with a mortar, placed in a muffle furnace with different calcination temperature for 10 h, with the furnace cooling to room temperature to get fine powders of YFeO<sup>3</sup> . Vibrating sample magnetometer (VSM, Model 7300, Lake Shore) and MPMS magnetic measurement system (SQUID, Quantum, Design) were used at room temperature to measure the temperature curve of the magnetic hysteresis loop. The powder samples and a small amount of ethanol were mixed after grinding and were compressed at a molding pressure of about 300Mpa in the muffle furnace sintering at 923 K and 1173 K for 2 h. Silver paste dried at 353 K was used as the electrode. The sample thickness was about 1.5 mm, and the diameter was about 15 mm. The capacitance and loss were measured by Agilent 4980E LCR analyzer and TH 2617 capacitance measuring instrument, and the temperature and magnetic field were controlled by ET 9000 low-temperature measuring system.

**Figure 2** shows the room temperature hysteresis loop and magnetization temperature curve of h-YFO powder. As can be seen from **Figure 2**, h-YFO has a weak magnetism, and the hysteresis loop is narrow and small, which belongs to the soft magnet. The remanent magnetization (Mr) and coercive field (H) are 0.11 memu g-1 and 0.19 kOe, respectively. The coercivity and saturation magnetization of possible impurities Y<sup>3</sup> Fe<sup>5</sup> O12 and γ-Fe<sup>2</sup> O3 are about 100 Oe and 26.0 memu g-1 and 150 Oe and 70 memu g-1 at room temperature, respectively [12, 13].

The magnetization of h-YFO is very small and the coercive field is much larger, so it is possible to exclude the influence of impurities on the magnetic properties of the samples, which is characterizing the magnetic properties of h-YFO.

**Figure 3** shows the h-YFO magnetization-temperature relationship curve (M-T) and its first derivative curve (dM/dT-T), measured at the external magnetic field strength of H = 1000 Oe using temperature mode. It can be seen from the figure that the magnetization of h-YFO is gradually reduced from 0.72 memu g-1 to about zero in the temperature range of 750 K to 877 K, which indicates that h-YFO has a complex magnetic phase transition in this temperature range.

**Figure 4** shows the hysteresis loop and the magnetization temperature curve of o-YFO powder at room temperature. Compared to h-YFO, the hysteresis loop of o-YFO is wide which belongs to hard magnet, and the magnetization and coercive field of o-YFO is 0.72 memu g-1 and 24.5 kOe respectively which is much larger than h-YFO. It is also shown that the influence of impurities on the magnetic properties is small. From the extension direction of the loop line, o-YFO has a certain antiferromagnetic property.

It is worth noting that **Figure 4** llustrations for the hysteresis curve of o-YFO on a small magnetic field (<10 kOe) that shows a peculiar shape with a sudden change magnetization in H = 0 kOe, which is considered to be the inherent characteristic of o-YFO. The causes of the abnormal YFeO3 hysteresis loop are produced by the interaction of the same iron ions, the interaction with the external magnetic field, the DM antisymmetric interaction, and the anisotropy effect of the magnetic crystal. The adjacent two iron ions in YFeO<sup>3</sup> is of a slight distortion,

**Figure 2.** M-H hysteresis curve of h-YFO powder.

(vii) Incorporating several components in a single step or in two steps [2].

and composite materials.

36 Chemical Reactions in Inorganic Chemistry

using alkoxides as precursors [11].

the performance of the material is expanded.

(x) Suitable for large-scale industrial production.

ogy is relatively simple, generally, spherical particles.

**5.1. Multiferroic properties of powder materials**

O and Fe(NO<sup>3</sup>

room temperature to get fine powders of YFeO<sup>3</sup>

First, Y(NO<sup>3</sup>

) 3 ·5H<sup>2</sup>

**5. Sol-gel processing of functional powder materials**

) 3 ·9H2

(viii)Production of samples in different physical forms [2, 10]. Starting from the same raw material, changing the process can get different products, such as fiber, powder, or film

(ix) Treating temperature is low, the inorganic materials can be synthesized under the condition of low temperature at about 600°C, the composition and the structure of the product are uniform, the grain size is small, the activity of the material particles is increased, and

The negative factors of sol-gel processing include high precursor cost, overall high cost, and environmental problems associated with the disposal of large quantities of organic by-products. A common theme out of the negative comments was wet chemical processing

The sol-gel process is more complex in terms of the difficulty in phase control, which is the different chemical and crystal morphology formation at different temperatures. The morphol-

Gelation, drying, and heat treatment take a lot of time to prepare the sample. In the stage of drying and heat treatment, the sample has a great weight loss and residual stress, film prone to cracking and objectively restricting the thickness of thin film and result in film residual porosity.

of *n*(Y):*n*(Fe) = 1:1, then citric acid and metal cations were added in the molar ratio of 1.5:1 to dissolve them completely and for the full complexation with metal ions. The acrylamide monomer is added in the ratio of 9:1 to the cation. The mixture at 358 K temperature is heated to polymerize to form a gel at 393 K. The dry gel is crushed into fine powder with a mortar, placed in a muffle furnace with different calcination temperature for 10 h, with the furnace cooling to

7300, Lake Shore) and MPMS magnetic measurement system (SQUID, Quantum, Design) were used at room temperature to measure the temperature curve of the magnetic hysteresis loop. The powder samples and a small amount of ethanol were mixed after grinding and were compressed at a molding pressure of about 300Mpa in the muffle furnace sintering at 923 K and 1173 K for 2 h. Silver paste dried at 353 K was used as the electrode. The sample thickness was about 1.5 mm, and the diameter was about 15 mm. The capacitance and loss were measured by Agilent 4980E LCR analyzer and TH 2617 capacitance measuring instrument, and the temperature and magnetic field were controlled by ET 9000 low-temperature measuring system.

O were dissolved in deionized water in the molar ratio

. Vibrating sample magnetometer (VSM, Model

**Figure 3.** M-T curve of h-YFO powder and its derivative curve.

increase in the electric field frequency, the relaxation peak moves to the high temperature and the peak value decreases gradually. At the same time, the dielectric loss curve also has the corresponding loss peak, the loss peak moves to the high temperature with the increase in the frequency, and the peak value of the loss increases. The second dielec-

peak at 297 K, and the dielectric peak does not change with the frequency shows that the peak is not caused by the thermal activation, but by a structural phase transition. The third anomalous dielectric is at above 340 K where exists a strong frequency dispersion and the loss increases obviously. It is because the thermal activation mobility of charge carrier increases with the temperature rises, which result in the increase of conductivity. At 300 kHz, the dielectric loss was relatively stable, the main reason was that no electric charge was generated with the change of external electric field. It can be seen

showed a dielectric anomaly

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 39

have a trend of decline in the

tric anomaly appeared between 250 and340 K and h-YFeO<sup>3</sup>

**Figure 5.** M-T curve of o-YFO powder and its derivative curve.

from the figure that *εr* and dielectric loss tan *δ* of o-YFeO3

**Figure 6.** Temperature dependence of dielectric constant *εr* of h-YFO.

highest temperature.

**Figure 4.** M-H hysteresis curve of o-YFO powder. Inset: M-H hysteresis curve of o-YFO at lower magnetization field.

rather than a strict spin antiparallelly which reveals overall magnetic anisotropy. When the external magnetic field is too small to overcome the anisotropic magnetic field of the magnetic material, the magnetic behavior will be different.

**Figure 5** shows the magnetization-temperature curve (M-T) and its first derivative curve (dM/ dT-T) for o-YFO, measured in the same way those for h-YFO. The magnetic properties of o-YFO become smaller in the range of 540 K till 645 K, the magnetization is 0, which is similar to the Nair temperature TN = 640 K of the orthorhombic phase YFO, which indicates that the magnetic phase transition process is from antiferromagnetic to paramagnetic.

**Figures 6** and **7** show the *εr* and dielectric loss tan *δ* of h-YFO variation with temperature in the range of 1 kHz to 300 kHz. It can be seen from the figure that the dielectric constant of h-YFO in the whole frequency range is relatively low, about 10–30, and the dielectric constant and dielectric loss of the sample decrease slightly with the increase in the measurement frequency. In the whole temperature range, there are three obvious dielectric anomalies. The first dielectric anomaly was observed in 125–250 K; the *εr*-*T* curve shows the dielectric relaxation and frequency dispersion characteristics. With the

**Figure 5.** M-T curve of o-YFO powder and its derivative curve.

rather than a strict spin antiparallelly which reveals overall magnetic anisotropy. When the external magnetic field is too small to overcome the anisotropic magnetic field of the magnetic

**Figure 4.** M-H hysteresis curve of o-YFO powder. Inset: M-H hysteresis curve of o-YFO at lower magnetization field.

**Figure 5** shows the magnetization-temperature curve (M-T) and its first derivative curve (dM/ dT-T) for o-YFO, measured in the same way those for h-YFO. The magnetic properties of o-YFO become smaller in the range of 540 K till 645 K, the magnetization is 0, which is similar to the Nair temperature TN = 640 K of the orthorhombic phase YFO, which indicates that the

**Figures 6** and **7** show the *εr* and dielectric loss tan *δ* of h-YFO variation with temperature in the range of 1 kHz to 300 kHz. It can be seen from the figure that the dielectric constant of h-YFO in the whole frequency range is relatively low, about 10–30, and the dielectric constant and dielectric loss of the sample decrease slightly with the increase in the measurement frequency. In the whole temperature range, there are three obvious dielectric anomalies. The first dielectric anomaly was observed in 125–250 K; the *εr*-*T* curve shows the dielectric relaxation and frequency dispersion characteristics. With the

magnetic phase transition process is from antiferromagnetic to paramagnetic.

material, the magnetic behavior will be different.

**Figure 3.** M-T curve of h-YFO powder and its derivative curve.

38 Chemical Reactions in Inorganic Chemistry

increase in the electric field frequency, the relaxation peak moves to the high temperature and the peak value decreases gradually. At the same time, the dielectric loss curve also has the corresponding loss peak, the loss peak moves to the high temperature with the increase in the frequency, and the peak value of the loss increases. The second dielectric anomaly appeared between 250 and340 K and h-YFeO<sup>3</sup> showed a dielectric anomaly peak at 297 K, and the dielectric peak does not change with the frequency shows that the peak is not caused by the thermal activation, but by a structural phase transition. The third anomalous dielectric is at above 340 K where exists a strong frequency dispersion and the loss increases obviously. It is because the thermal activation mobility of charge carrier increases with the temperature rises, which result in the increase of conductivity. At 300 kHz, the dielectric loss was relatively stable, the main reason was that no electric charge was generated with the change of external electric field. It can be seen from the figure that *εr* and dielectric loss tan *δ* of o-YFeO3 have a trend of decline in the highest temperature.

**Figure 6.** Temperature dependence of dielectric constant *εr* of h-YFO.

**Figure 7.** Temperature dependence of dielectric loss tan *δ* of h-YFO.

from the valence band to the conduction band. Energy band gap of samples can be evaluated

tion method. The raw materials were used in analytical grade (AR). Lanthanum nitrate

dissolved in 100 mL diluted water in a beaker under the constant stirrer at 60°C until the

Afterward, the homogeneous sol was heated at 120°C for 2.5 h. The sol color and viscosity changed and got ignited for further heating automatically. The solid dry gel was calcinated at 600°C in air for 5 h. The UV-Vis absorbance was carried out using Perkin Elmer

0.1 and 0.2) NPs is shown in (**Figure 10**). The band gap of semiconductor materials can be

(*αhν* )*<sup>m</sup>* = *K*(*hν* − *Eg*) (4)

where *α* is the absorption coefficient, *K* is a constant, m is an integer whose value depends on the nature of transition. The value of m is 2, 1/2, 2/3, and 1/3 for acceptable direct transition, permitted indirect transition, forbidden direct transition, and forbidden indirect transition,

indicate that the band gap increases with increasing Sr and Ni concentrations. The increase in the band gap could be attributed to the Burstein-Moss effect with an obvious blue shift of the

versus photon energy (*hν*) for direct transitions to determine the band

, (*x* = 0.0, 0.1 and 0.2) NPs are displayed in **Figure 11**, which

), and nickel nitrate hexahydrate (Ni(NO<sup>3</sup>

H8 O7

The optical absorption spectra as a function of wavelength of (La(1−*<sup>x</sup>*)

estimated by UV-Vis absorption spectroscopy by the Tauc relation given below:

O), ferric nitrate nonahydrate (Fe(NO<sup>3</sup>

, (*x* = 0.0, 0.1 and 0.2) NPs were synthesized by sol-gel combus-

)2 ·6H2

) was used as fuel. All chemicals were completely

)3 ·9H2

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 41

) was slowly added to adjust the pH to 7.

Sr*x* )(Fe(1−*<sup>x</sup>*) O), strontium

, (*x* = 0.0,

O) were used as oxidiz-

Ni*<sup>x</sup>* )O<sup>3</sup>

from the absorption spectra.

Ni*<sup>x</sup>* )O<sup>3</sup>

)2

ing agents, whereas citric acid (C<sup>6</sup>

)3 ·6H2

**Figure 9.** Temperature dependence of dielectric loss tan *δ* of o-YFO.

homogeneous sol was formed. Ammonia (NH<sup>3</sup>

)(Fe(1−*<sup>x</sup>*)

hexahydrate (La(NO<sup>3</sup>

spectrophotometer [14].

respectively [14]. The plots of (*αhν*)<sup>2</sup>

gap of the (La(1−*<sup>x</sup>*)

absorption edge [14].

Sr*x* )(Fe(1−*<sup>x</sup>*)

Ni*<sup>x</sup>* )O<sup>3</sup>

nitrate (Sr(NO<sup>3</sup>

(La(1−*<sup>x</sup>*) Sr*<sup>x</sup>*

**Figure 8.** Temperature dependence of dielectric constant *εr* of o-YFO.

**Figures 8** and **9** show the dielectric constant and dielectric loss tan *δ r* variation with temperature at different frequency from 1 kHz to 300 kHz of o-YFeO<sup>3</sup> . As can be seen from the figure, o-YFeO<sup>3</sup> has two obvious dielectric anomalies in the whole range of temperature. The *εr*-*T* curve of o-YFeO<sup>3</sup> shows the dielectric relaxation and frequency dispersion phenomenon in the range of 125–340 K. With the increase in electric field frequency, the dielectric constant decreased, the dielectric loss increased, and the dielectric peak shifted to high temperature with the longer dielectric platform afterward. After 340 K, there are second obvious dielectric anomalies, showing a strong frequency dispersion, which is consistent with the change of h-YFeO3 . The dielectric relaxation behavior of o-YFeO<sup>3</sup> is the result of the decrease of dielectric constant.

#### **5.2. Optical properties of powder materials**

For semiconductor nanomaterials, the optical absorption is closely related to the band gap. The short wavelength cutoff in the transmission is due to the fundamental band gap excitation

**Figure 9.** Temperature dependence of dielectric loss tan *δ* of o-YFO.

**Figures 8** and **9** show the dielectric constant and dielectric loss tan *δ r* variation with tem-

in the range of 125–340 K. With the increase in electric field frequency, the dielectric constant decreased, the dielectric loss increased, and the dielectric peak shifted to high temperature with the longer dielectric platform afterward. After 340 K, there are second obvious dielectric anomalies, showing a strong frequency dispersion, which is consistent with the change of

For semiconductor nanomaterials, the optical absorption is closely related to the band gap. The short wavelength cutoff in the transmission is due to the fundamental band gap excitation

has two obvious dielectric anomalies in the whole range of temperature. The

shows the dielectric relaxation and frequency dispersion phenomenon

. As can be seen from the

is the result of the decrease of dielec-

perature at different frequency from 1 kHz to 300 kHz of o-YFeO<sup>3</sup>

**Figure 8.** Temperature dependence of dielectric constant *εr* of o-YFO.

**Figure 7.** Temperature dependence of dielectric loss tan *δ* of h-YFO.

40 Chemical Reactions in Inorganic Chemistry

. The dielectric relaxation behavior of o-YFeO<sup>3</sup>

**5.2. Optical properties of powder materials**

figure, o-YFeO<sup>3</sup>

h-YFeO3

tric constant.

*εr*-*T* curve of o-YFeO<sup>3</sup>

from the valence band to the conduction band. Energy band gap of samples can be evaluated from the absorption spectra.

(La(1−*<sup>x</sup>*) Sr*<sup>x</sup>* )(Fe(1−*<sup>x</sup>*) Ni*<sup>x</sup>* )O<sup>3</sup> , (*x* = 0.0, 0.1 and 0.2) NPs were synthesized by sol-gel combustion method. The raw materials were used in analytical grade (AR). Lanthanum nitrate hexahydrate (La(NO<sup>3</sup> )3 ·6H2 O), ferric nitrate nonahydrate (Fe(NO<sup>3</sup> )3 ·9H2 O), strontium nitrate (Sr(NO<sup>3</sup> )2 ), and nickel nitrate hexahydrate (Ni(NO<sup>3</sup> )2 ·6H2 O) were used as oxidizing agents, whereas citric acid (C<sup>6</sup> H8 O7 ) was used as fuel. All chemicals were completely dissolved in 100 mL diluted water in a beaker under the constant stirrer at 60°C until the homogeneous sol was formed. Ammonia (NH<sup>3</sup> ) was slowly added to adjust the pH to 7. Afterward, the homogeneous sol was heated at 120°C for 2.5 h. The sol color and viscosity changed and got ignited for further heating automatically. The solid dry gel was calcinated at 600°C in air for 5 h. The UV-Vis absorbance was carried out using Perkin Elmer spectrophotometer [14].

The optical absorption spectra as a function of wavelength of (La(1−*<sup>x</sup>*) Sr*x* )(Fe(1−*<sup>x</sup>*) Ni*<sup>x</sup>* )O<sup>3</sup> , (*x* = 0.0, 0.1 and 0.2) NPs is shown in (**Figure 10**). The band gap of semiconductor materials can be estimated by UV-Vis absorption spectroscopy by the Tauc relation given below:

$$(\alpha l \nu \text{ })^{m} = \text{ } K \text{[} h \nu - \text{Eg]}\tag{4}$$

where *α* is the absorption coefficient, *K* is a constant, m is an integer whose value depends on the nature of transition. The value of m is 2, 1/2, 2/3, and 1/3 for acceptable direct transition, permitted indirect transition, forbidden direct transition, and forbidden indirect transition, respectively [14].

The plots of (*αhν*)<sup>2</sup> versus photon energy (*hν*) for direct transitions to determine the band gap of the (La(1−*<sup>x</sup>*) Sr*x* )(Fe(1−*<sup>x</sup>*) Ni*<sup>x</sup>* )O<sup>3</sup> , (*x* = 0.0, 0.1 and 0.2) NPs are displayed in **Figure 11**, which indicate that the band gap increases with increasing Sr and Ni concentrations. The increase in the band gap could be attributed to the Burstein-Moss effect with an obvious blue shift of the absorption edge [14].

conduction band above the Fermi level. Based on the Pauli's exclusion principle this excitation into occupied states is prohibited, resulting in the increase in the optical band gap [14].

Sol-gel-derived coating film is suitable for substrates with large areas. Preferred crystal orientation can be given depending on the conditions of preparation. The techniques can be applied to accomplishing a large variety of functional films with optical, electronic, magnetic,

In sol-gel process, the key film-forming method mainly includes the dip-coating method, the rotating method, the sputtering method, and so on. The dip-coating method is elevating impregnated substrate from uniform solution, after gelling process, to become gel, and the film with nanoparticles can be obtained after a certain temperature, and the film thickness can be controlled by the number of pulling speed or spin coating. The rotation method is a method of liquid film coating on substrate, the film thickness reduces with the increase in the substrate rotation speed. Sputtering is a method of forming film by spraying the solution from one to several stationary sputtering guns onto a preheated substrate which runs at a

O12 (CCTO) ceramic has an extremely high dielectric con-

for single crystals) and moderately lower dielectric loss

)2 ·4H2

)4

O) were dissolved in methoxyethanol. Subsequently, the solution was

3 (no. 204),

CH2 OH)

O) and copper

O) in the ratio

O), and tetrabutyl titanate

) was used to introduce Zr elements

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 43

)2 ·4H2

H8 O7 ·H2

) as the introduction of Zr doping agents for the

Ti4O12) with a certain dose of dilute nitric acid, and then a

OCH2

**6.1. Dielectric properties of the perovskite-related oxide CCTO film**

as dopingagents in the sol. Stoichiometric calcium nitrate (Ca(NO<sup>3</sup>

stoichiometric amount of tetrabutyl titanate was added. Citric acid (C<sup>6</sup>

(~0.1) at low frequencies with centrosymmetric bcc structure (space group Im¯

lattice parameter *a* = 0.7391 nm and *Z* = 2). And it retains its high dielectric constant from 100 to 600 K. Hence, it is capable of potential industrial application for miniaturizing electronic

O), copper nitrate (Cu(NO<sup>3</sup>

as solvent; dilute nitric acid as stabilizer; acetic acid as chelating agent; and citric acid as col-

heated and stirred to be fully clarified and was cooled to room temperature (select appropri-

of 1:1.5 of sum of the metal ions to the number of moles of citric acid was placed into the solution as complexing agent. By mixing completely, the transparent homogeneous solution of concentration 0.15 mol/L was formed. The solution was aged for 24 h to prepare the powder. Then it was calcined at 450°C for pyrogenic decomposition, and a brown, black powder was obtained. As-prepared powder was made into bulk samples, 10 mm in diameter and 2 mm

) 4

) were selected as solutes; ethylene glycol monomethyl ether (CH<sup>3</sup>

Ti4

)2 ·4H2

loidal complexing agent. Zirconium nitrate (Zr(NO<sup>3</sup>

**6. Sol-gel processes of functional film**

chemical, and mechanical functions.

Perovskite-related oxide CaCu<sup>3</sup>

stant (~10,000 for bulk and ~105

certain speed.

devices [15].

H9 )4

nitrate(Ca(NO<sup>3</sup>

(Ti(OC<sup>4</sup>

Calcium nitrate (Ca(NO<sup>3</sup>

)2 ·4H2

preparation of Zr-doped CaCu3

ate amount of zirconium nitrate (Zr(NO<sup>3</sup>

**Figure 10.** UV-Vis absorption spectrum of the NPs of (La(1−*<sup>x</sup>*) Sr*x* )(Fe(1−*<sup>x</sup>*) Ni *<sup>x</sup>* )O<sup>3</sup> , (*x* = 0.0, 0.1 and 0.2).

**Figure 11.** Plot of (*αhν*)<sup>2</sup> versus *hν* for NPs of (La(1−*<sup>x</sup>*) Sr*x* )(Fe(1−*<sup>x</sup>*) Ni*<sup>x</sup>* )O<sup>3</sup> , (*x* = 0.0, 0.1 and 0.2) NPs.

With the increase in the concentration of doping, the donor states push the Fermi level higher in energy, which shift close to the conduction band above the occupied donor state. In the degenerate semiconductor, only an electron can be excited from the valence band into conduction band above the Fermi level. Based on the Pauli's exclusion principle this excitation into occupied states is prohibited, resulting in the increase in the optical band gap [14].

### **6. Sol-gel processes of functional film**

With the increase in the concentration of doping, the donor states push the Fermi level higher in energy, which shift close to the conduction band above the occupied donor state. In the degenerate semiconductor, only an electron can be excited from the valence band into

, (*x* = 0.0, 0.1 and 0.2) NPs.

Sr*x* )(Fe(1−*<sup>x</sup>*) Ni*<sup>x</sup>* )O<sup>3</sup>

Sr*x* )(Fe(1−*<sup>x</sup>*) Ni *<sup>x</sup>* )O<sup>3</sup>

, (*x* = 0.0, 0.1 and 0.2).

**Figure 10.** UV-Vis absorption spectrum of the NPs of (La(1−*<sup>x</sup>*)

42 Chemical Reactions in Inorganic Chemistry

versus *hν* for NPs of (La(1−*<sup>x</sup>*)

**Figure 11.** Plot of (*αhν*)<sup>2</sup>

Sol-gel-derived coating film is suitable for substrates with large areas. Preferred crystal orientation can be given depending on the conditions of preparation. The techniques can be applied to accomplishing a large variety of functional films with optical, electronic, magnetic, chemical, and mechanical functions.

In sol-gel process, the key film-forming method mainly includes the dip-coating method, the rotating method, the sputtering method, and so on. The dip-coating method is elevating impregnated substrate from uniform solution, after gelling process, to become gel, and the film with nanoparticles can be obtained after a certain temperature, and the film thickness can be controlled by the number of pulling speed or spin coating. The rotation method is a method of liquid film coating on substrate, the film thickness reduces with the increase in the substrate rotation speed. Sputtering is a method of forming film by spraying the solution from one to several stationary sputtering guns onto a preheated substrate which runs at a certain speed.

#### **6.1. Dielectric properties of the perovskite-related oxide CCTO film**

Perovskite-related oxide CaCu<sup>3</sup> Ti4 O12 (CCTO) ceramic has an extremely high dielectric constant (~10,000 for bulk and ~105 for single crystals) and moderately lower dielectric loss (~0.1) at low frequencies with centrosymmetric bcc structure (space group Im¯ 3 (no. 204), lattice parameter *a* = 0.7391 nm and *Z* = 2). And it retains its high dielectric constant from 100 to 600 K. Hence, it is capable of potential industrial application for miniaturizing electronic devices [15].

Calcium nitrate (Ca(NO<sup>3</sup> )2 ·4H2 O), copper nitrate (Cu(NO<sup>3</sup> )2 ·4H2 O), and tetrabutyl titanate (Ti(OC<sup>4</sup> H9 )4 ) were selected as solutes; ethylene glycol monomethyl ether (CH<sup>3</sup> OCH2 CH2 OH) as solvent; dilute nitric acid as stabilizer; acetic acid as chelating agent; and citric acid as colloidal complexing agent. Zirconium nitrate (Zr(NO<sup>3</sup> ) 4 ) was used to introduce Zr elements as dopingagents in the sol. Stoichiometric calcium nitrate (Ca(NO<sup>3</sup> )2 ·4H2 O) and copper nitrate(Ca(NO<sup>3</sup> ) 2 ·4H2 O) were dissolved in methoxyethanol. Subsequently, the solution was heated and stirred to be fully clarified and was cooled to room temperature (select appropriate amount of zirconium nitrate (Zr(NO<sup>3</sup> )4 ) as the introduction of Zr doping agents for the preparation of Zr-doped CaCu3 Ti4O12) with a certain dose of dilute nitric acid, and then a stoichiometric amount of tetrabutyl titanate was added. Citric acid (C<sup>6</sup> H8 O7 ·H2 O) in the ratio of 1:1.5 of sum of the metal ions to the number of moles of citric acid was placed into the solution as complexing agent. By mixing completely, the transparent homogeneous solution of concentration 0.15 mol/L was formed. The solution was aged for 24 h to prepare the powder. Then it was calcined at 450°C for pyrogenic decomposition, and a brown, black powder was obtained. As-prepared powder was made into bulk samples, 10 mm in diameter and 2 mm in thickness at 20 MPa pressure. Samples were sintered at 1000°C in silicon carbide furnace for 2 h and dense polycrystalline ceramic flakes were formed. The furnace heating rate was 100°C/h and the cooling rate was 150°C/h.

likely to replace titanium ions at B site belonging to the perovskite ABO<sup>3</sup>

O12 and CaCu3

O12 and CaCu3

were studied and are illustrated in **Figure 13**. The dielectric temperature spectra of CaCu3

with temperature is relatively flat in the range of 100–350 K. As the temperature increases, the

Zr0.05Ti3.95O12 is larger than that of CaCu3

the temperature was increased from 420 K to 450 K. This sudden increase in dielectric permittivity observed at 100 kHz may be assigned to the space charge/interfacial polarization effects. **Figures 14** and **15** show the frequency dependence of dielectric permittivity and loss at room

increases in the range of 1 KHz to 1000 KHz, the dielectric constant of CCTO-based ceramics decreased. However, the dielectric loss increased significantly with increasing frequency. The dielectric loss of the test samples in the lower frequency at room temperature are relatively closed and nearly keep constant. With increasing frequency, the dielectric constants

the CCTO-based ceramics possess large dielectric constant in low-frequency ranges, which is attributed to the interfacial polarization at the interfaces between the two components.

As shown in **Figures 3** and **4**, each sample has similar dielectric behavior at room temperature. The dielectric constant displays a steep decrease at a certain frequency, and at the same time the dielectric loss shows a relaxation peak. There is a clear relaxation characteristic and the relaxation is closely related to the measurement frequency, which shows a typical Debye

Zr0.05Ti3.95O12 ceramics were measured at 100 kHz. Dielectric properties' variation

Zr0.05Ti3.95O12 is narrower than that of CaCu<sup>3</sup>

The temperature dependence of dielectric properties of CaCu3

Ti4

Ti4

**Figure 13.** Temperature-dependent dielectric permittivities at 100 kHz for CaCu<sup>3</sup>

in case of the occurrence of solid solution.

and CaCu3

thermal stabilities of CaCu3

temperature for pure CaCu3

Ti4

O12 and CaCu3

of pure CaCu3

relaxation.

the peak width of CaCu<sup>3</sup>

stable temperature zone of CaCu3

regime, the dielectric permittivity of CaCu<sup>3</sup>

structural ceramics

Zr0.05Ti3.95O12

O12, whereas

Ti4 O12 45

Ti4

O12 and CaCu3

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588

Ti4

O12. In the low frequency

Zr0.05Ti3.95O12 deteriorate. The dielectric property

Ti4

Zr0.05Ti3.95O12 decrease rapidly at low-frequency ranges. Both

Ti4

O12 and CaCu3

Zr0.05Ti3.95O12.

Zr0.05Ti3.95O12 increased from 50,000 to 500,000 when

Zr0.05Ti3.95O12, respectively. As the frequency

The densities of samples were measured by Archimedes method. The crystalline phases were identified by powder X-ray diffraction analysis installation of a spectrum analyzer with CuKα radiation (*λ* = 0.154056 nm) (XRD, D/max-IIIC) in a wide range of 2*θ* (5° ≤ 2*θ* ≤ 85°). The samples' microstructure were characterized by SEM (Philips XL30 ESEM-TMP). For the electrical characterization, the ceramics were cut into small pieces of 5 × 5 mm, and gold electrodes with 3 mm radii were sputtered at the center on both sides of each sample. The dielectric properties and impedance analyses were carried out using the precision LCR meter (Agilent 4284A) in the frequency range from 20 Hz to 1 MHz over the temperature range of 150–550 K. The measurement accuracy of the instrument is better than 5%. The dielectric permittivity was evaluated using the standard relation *εr* = *C* ·*d*/(*ε*<sup>0</sup> · *A*), where *C* is capacitance, *d* is the thickness of the sample, *ε*0 = 8.854 × 10–12 F/m, and *A* is the effective area of the sample. Dielectric strength measurements were carried out as per the procedure outlined in ASTM D149.

The CaCu3 Zr0.05Ti3.95O12 was confirmed by X-ray diffraction. The X-ray diffraction patterns of pure CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 are shown in **Figure 12**. It shows the room temperature XRD patterns obtained for the CaCu<sup>3</sup> Zr0.05Ti3.95O12 nanocrystalline powder compared well with the ICDD data (01-075-1149). The samples reveal quite similar diffraction profiles and diffraction peaks corresponding to oxide CaCu<sup>3</sup> Ti4 O12 phase (JCPDS No. 75-2188) with bcc cubic perovskite-related structure. All the XRD patterns of the main diffraction peaks can be attributed to the CCTO phase. It can determine the crystalline CCTO phase generated. Two curves can be found similar from the figure, which indicate that the content of zirconium has almost no effect on the crystal phase. From the point of view of the ionic radius of Ca2+, Cu2+, and Ti4+, the ionic radius of Ca2+, Cu2+, and Ti4+ are l00 pm, 73 pm, and 60.5 pm, respectively, whereas the ionic radius of Zr4+ is 79 pm. Through the solution rule of thumb, zirconium is

**Figure 12.** The XRD diffraction patterns for CaCu<sup>3</sup> Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 powder prepared by the sol-gel process.

likely to replace titanium ions at B site belonging to the perovskite ABO<sup>3</sup> structural ceramics in case of the occurrence of solid solution.

in thickness at 20 MPa pressure. Samples were sintered at 1000°C in silicon carbide furnace for 2 h and dense polycrystalline ceramic flakes were formed. The furnace heating rate was

The densities of samples were measured by Archimedes method. The crystalline phases were identified by powder X-ray diffraction analysis installation of a spectrum analyzer with CuKα radiation (*λ* = 0.154056 nm) (XRD, D/max-IIIC) in a wide range of 2*θ* (5° ≤ 2*θ* ≤ 85°). The samples' microstructure were characterized by SEM (Philips XL30 ESEM-TMP). For the electrical characterization, the ceramics were cut into small pieces of 5 × 5 mm, and gold electrodes with 3 mm radii were sputtered at the center on both sides of each sample. The dielectric properties and impedance analyses were carried out using the precision LCR meter (Agilent 4284A) in the frequency range from 20 Hz to 1 MHz over the temperature range of 150–550 K. The measurement accuracy of the instrument is better than 5%. The dielectric permittivity was evalu-

the sample, *ε*0 = 8.854 × 10–12 F/m, and *A* is the effective area of the sample. Dielectric strength

with the ICDD data (01-075-1149). The samples reveal quite similar diffraction profiles and

cubic perovskite-related structure. All the XRD patterns of the main diffraction peaks can be attributed to the CCTO phase. It can determine the crystalline CCTO phase generated. Two curves can be found similar from the figure, which indicate that the content of zirconium has almost no effect on the crystal phase. From the point of view of the ionic radius of Ca2+, Cu2+, and Ti4+, the ionic radius of Ca2+, Cu2+, and Ti4+ are l00 pm, 73 pm, and 60.5 pm, respectively, whereas the ionic radius of Zr4+ is 79 pm. Through the solution rule of thumb, zirconium is

Zr0.05Ti3.95O12 was confirmed by X-ray diffraction. The X-ray diffraction patterns of

Ti4

Zr0.05Ti3.95O12 are shown in **Figure 12**. It shows the room tempera-

· *A*), where *C* is capacitance, *d* is the thickness of

Zr0.05Ti3.95O12 nanocrystalline powder compared well

O12 phase (JCPDS No. 75-2188) with bcc

Zr0.05Ti3.95O12 powder prepared by the sol-gel process.

= *C* ·*d*/(*ε*<sup>0</sup>

measurements were carried out as per the procedure outlined in ASTM D149.

Ti4

O12 and CaCu3

100°C/h and the cooling rate was 150°C/h.

44 Chemical Reactions in Inorganic Chemistry

ated using the standard relation *εr*

O12 and CaCu3

diffraction peaks corresponding to oxide CaCu<sup>3</sup>

ture XRD patterns obtained for the CaCu<sup>3</sup>

**Figure 12.** The XRD diffraction patterns for CaCu<sup>3</sup>

Ti4

The CaCu3

pure CaCu3

The temperature dependence of dielectric properties of CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 were studied and are illustrated in **Figure 13**. The dielectric temperature spectra of CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 ceramics were measured at 100 kHz. Dielectric properties' variation with temperature is relatively flat in the range of 100–350 K. As the temperature increases, the thermal stabilities of CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 deteriorate. The dielectric property stable temperature zone of CaCu3 Zr0.05Ti3.95O12 is narrower than that of CaCu<sup>3</sup> Ti4 O12, whereas the peak width of CaCu<sup>3</sup> Zr0.05Ti3.95O12 is larger than that of CaCu3 Ti4 O12. In the low frequency regime, the dielectric permittivity of CaCu<sup>3</sup> Zr0.05Ti3.95O12 increased from 50,000 to 500,000 when the temperature was increased from 420 K to 450 K. This sudden increase in dielectric permittivity observed at 100 kHz may be assigned to the space charge/interfacial polarization effects.

**Figures 14** and **15** show the frequency dependence of dielectric permittivity and loss at room temperature for pure CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12, respectively. As the frequency increases in the range of 1 KHz to 1000 KHz, the dielectric constant of CCTO-based ceramics decreased. However, the dielectric loss increased significantly with increasing frequency. The dielectric loss of the test samples in the lower frequency at room temperature are relatively closed and nearly keep constant. With increasing frequency, the dielectric constants of pure CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 decrease rapidly at low-frequency ranges. Both the CCTO-based ceramics possess large dielectric constant in low-frequency ranges, which is attributed to the interfacial polarization at the interfaces between the two components.

As shown in **Figures 3** and **4**, each sample has similar dielectric behavior at room temperature. The dielectric constant displays a steep decrease at a certain frequency, and at the same time the dielectric loss shows a relaxation peak. There is a clear relaxation characteristic and the relaxation is closely related to the measurement frequency, which shows a typical Debye relaxation.

**Figure 13.** Temperature-dependent dielectric permittivities at 100 kHz for CaCu<sup>3</sup> Ti4 O12 and CaCu3 Zr0.05Ti3.95O12.

**Figure 14.** Frequency dependence of room temperature dielectric permittivities for CaCu<sup>3</sup> Ti4 O12 and CaCu3 Zr0.05Ti3.95O12.

phase was obtained which can be confirmed from XRD and SEM photographs. The powder prepared by sol-gel process helped in the uniform mixing of all the elements in nanoscale, and corresponding nanocrystals were formed. From the SEM photographs, it is evident that

grain boundary effect can make a major contribution to the high dielectric constant of poly-

Iron-based semiconducting oxide magnetic nanoparticles, for instance spinel ferrites with

prominent family in the magnetic materials due to their high electrical resistivity, high value of magnetic permeability, and use in advanced technologies like microwave devices, magnetic information storage devices, and biomedical applications [16]. Ferrites are one of the

> )2 ·6H2

most effective photocatalysts for the ejection of toxic dyes from the industrial sewage.

O, (Co(NO<sup>3</sup>

) were used as starting materials for the preparation of Co(1−*<sup>x</sup>*)

Zr0.05Ti3.95O12 are composed of many grain, the

Ti4

O12; (b) CaCu<sup>3</sup>

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 47

Zr0.05Ti3.95O12.

(where M2+ = Mn2+, Co2+, Ni2+, Cu2+,Zn2+, etc.), have become a

O, (Mg(NO<sup>3</sup>

)2 ·6H2

O, and citric acid

(*x* = 0.0,

Mg*<sup>x</sup>* Fe2 O4

prepared samples of CaCu3

general formula M2+(Fe3+)<sup>2</sup>

Analytical grade Fe(NO<sup>3</sup>

(C<sup>6</sup> H8 O7

crystalline CCTO-based ceramics.

**6.2. Optical properties of magnetic films**

Ti4

**Figure 16.** SEM images of CCTO-based ceramics sintered at 1120°C for 2 h. (a) CaCu<sup>3</sup>

O4

)3 ·9H2

O12 and CaCu3

**Figure 15.** The room temperature dielectric losses for CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 at various frequencies.

It is obvious that all of these samples exhibit the giant permittivity at low frequency. As the frequency increases, permittivity has a sudden drop in the frequency range below 100 kHz; and then becomes flat. The permittivity increased with decreasing frequency which signaled the possibility of the presence of interfacial polarization. It is probably that the charge carriers may be blocked at an electrode interface when migrated under the influence of an electric field, which causes interfacial polarization. At the same time, the frequency response of dielectric properties of CCTO-based ceramics is closely linked to the polycrystalline microstructure. CCTO ceramics are of very high permittivity, the performance is more obvious especially in the condition of lower frequency, which happens on account of the famous Maxwell-Wagner effect.

It can be seen from the SEM photographs in **Figure 16** that the powder particles are homogeneous and dense, the particle sizes are less than 100 nm estimated from the figure. Crystalline

**Figure 16.** SEM images of CCTO-based ceramics sintered at 1120°C for 2 h. (a) CaCu<sup>3</sup> Ti4 O12; (b) CaCu<sup>3</sup> Zr0.05Ti3.95O12.

phase was obtained which can be confirmed from XRD and SEM photographs. The powder prepared by sol-gel process helped in the uniform mixing of all the elements in nanoscale, and corresponding nanocrystals were formed. From the SEM photographs, it is evident that prepared samples of CaCu3 Ti4 O12 and CaCu3 Zr0.05Ti3.95O12 are composed of many grain, the grain boundary effect can make a major contribution to the high dielectric constant of polycrystalline CCTO-based ceramics.

#### **6.2. Optical properties of magnetic films**

It is obvious that all of these samples exhibit the giant permittivity at low frequency. As the frequency increases, permittivity has a sudden drop in the frequency range below 100 kHz; and then becomes flat. The permittivity increased with decreasing frequency which signaled the possibility of the presence of interfacial polarization. It is probably that the charge carriers may be blocked at an electrode interface when migrated under the influence of an electric field, which causes interfacial polarization. At the same time, the frequency response of dielectric properties of CCTO-based ceramics is closely linked to the polycrystalline microstructure. CCTO ceramics are of very high permittivity, the performance is more obvious especially in the condition of lower frequency, which happens on account of the famous Maxwell-Wagner effect. It can be seen from the SEM photographs in **Figure 16** that the powder particles are homogeneous and dense, the particle sizes are less than 100 nm estimated from the figure. Crystalline

Ti4

O12 and CaCu3

Ti4

Zr0.05Ti3.95O12 at various frequencies.

O12 and CaCu3

Zr0.05Ti3.95O12.

**Figure 14.** Frequency dependence of room temperature dielectric permittivities for CaCu<sup>3</sup>

**Figure 15.** The room temperature dielectric losses for CaCu3

46 Chemical Reactions in Inorganic Chemistry

Iron-based semiconducting oxide magnetic nanoparticles, for instance spinel ferrites with general formula M2+(Fe3+)<sup>2</sup> O4 (where M2+ = Mn2+, Co2+, Ni2+, Cu2+,Zn2+, etc.), have become a prominent family in the magnetic materials due to their high electrical resistivity, high value of magnetic permeability, and use in advanced technologies like microwave devices, magnetic information storage devices, and biomedical applications [16]. Ferrites are one of the most effective photocatalysts for the ejection of toxic dyes from the industrial sewage.

Analytical grade Fe(NO<sup>3</sup> )3 ·9H2 O, (Co(NO<sup>3</sup> )2 ·6H2 O, (Mg(NO<sup>3</sup> )2 ·6H2 O, and citric acid (C<sup>6</sup> H8 O7 ) were used as starting materials for the preparation of Co(1−*<sup>x</sup>*) Mg*<sup>x</sup>* Fe2 O4 (*x* = 0.0, 0.3, 0.6, 0.9 and 1.0) ferrites. The molar ratio between metal nitrates and citric acid was adjusted to 1:2. After stoichiometric calculations and weighing, the appropriate quantity of all metal nitrates and citric acid were dissolved in 50 mL deionized water to form a mixed solution. A little quantity of ammonia was included dropwise to maintain the pH value at 7.0. The aqueous solution was heated at 95°C, stirred using a magnetic stirrer, and a viscous solution was obtained. With gel formation, the temperature was raised to 300°C which led to a self-sustaining combustion process followed by the formation of a loose powder. The powder was calcined at 600°C for 6 h with a heating rate of 8.5°C/min resulting in the formation of the final spinel structure samples. Energy band gaps of powder samples were determined using diffused reflection accessory attached with UV-Vis spectrophotometer.

**Author details**

Address all correspondence to: ycq1998@gmail.com

Henan, People's Republic of China

10.1007/978-94-017-9780-1

10.1016/0025-5408(89)90225-0

York: AIP; 2016. p. 1787

Wiley; 2007

2004;**13**:149-164

School of Biological and Chemical Engineering, Nanyang Institute of Technology, Nanyang,

Sol-Gel Processes of Functional Powders and Films http://dx.doi.org/10.5772/intechopen.69588 49

[1] Bhushan, B. Encyclopedia of Nanotechnology, Dordrecht: springer Nature; 2016. DOI:

[2] Amiri S, Rahimi A. Hybrid nanocomposite coating by sol–gel method: A review. Iranian

[3] Rahimi A. Inorganic and organometallic polymers: A review. Iranian Polymer Journal.

[4] Rabinovich EM, Klein LC. Sol–Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes. New Jersey, USA: Noyes Publications; 1988. DOI:

[5] Kessler VG, Spijksma GI, Seisenbaeva GA, Hakansson S, Blank DHA, Bouwmeester HJM. New insight in the role of modifying ligands in the sol–gel processing of metal alkoxide precursors: A possibility to approach new classes of materials. Journal of Sol-

[6] Livage J, Henry M, Sanchez C. Sol–gel chemistry of transition metal oxides. Progress in

[7] Liu C, Shaw L. Nanoparticulate materials and core/shell structures derived from wet chemistry methods. In: Bharat Bhushan, Editors. Encyclopedia of Nanotechnology. Dordrecht: Springer Netherlands; 2015. p. 1-21. DOI: 10.1007/978-94-007-6178-0\_100906-1

[8] Lee K, Guan BH, Zaid HM, Soleimani H, Ching DLC. Impact of pH on zinc oxide particle size by using sol-gel process. In: 4th International Conference on Fundamental and Applied Sciences (ICFAS 2016); 15-17 August 2016; Kuala Lumpur, MALAYSIA. New

[9] Kickelbick G. Hybrid Materials: Synthesis, Characterization and Applications. Weinheim:

[10] Xiong J, Das SN, Shin B, Kar JP, Choi JH, Myoung JM. Biomimetic hierarchical ZnO structure with super hydrophobic and antireflective properties. Journal of Colloid and

Interface Science. 2010;**350**:344-347. DOI: 10.1016/j.jcis.2010.06.053

Gel Science and Technology. 2006;**40**:163-179. DOI: 10.1007/s10971-006-9209-6

Solid State Chemistry. 1988;**18**:259-341. DOI: 10.1016/0079-6786(88)90005-2

Polymer Journal. 2016;**25**:6. DOI: 10.1007/s13726-016-0440-x

Chao-Qun Ye

**References**

UV-visible spectra were utilized to determine the band gap energies (*Eg*) of all the samples. Diffuse reflection spectra of Mg substituted CoFe<sup>2</sup> O4 powder samples were recorded as shown in **Figure 17**. It shows that the *Eg* values were decreased tremendously from 2.4 eV to 1.8 eV with Mg substitution increase that ended in the visible range of e.m. spectra [16].

**Figure 17.** UV-visible diffuse reflectance spectra of Co(1−*<sup>x</sup>*) Mg*<sup>x</sup>* Fe2 O4 (a) *x* = 0.0, (b) *x* = 0.3, (c) *x* = 0.6, (d) *x* = 0.9, and (e) *x* = 1.0.

#### **7. Conclusion**

The preparation methods of the thin films or powders and the heat treatment conditions have a crucial influence on the grain growth of the film. It affects not only the grain size of the film but also the surface morphology of the film, which affects the dielectric properties of the material. Microstructure has effects on the dielectric properties, optical properties, and magnetic properties of samples. Sol-gel method is a reasonable way to prepare function materials. Polycrystalline and nanocrystalline materials can be prepared by this process.

### **Author details**

#### Chao-Qun Ye

0.3, 0.6, 0.9 and 1.0) ferrites. The molar ratio between metal nitrates and citric acid was adjusted to 1:2. After stoichiometric calculations and weighing, the appropriate quantity of all metal nitrates and citric acid were dissolved in 50 mL deionized water to form a mixed solution. A little quantity of ammonia was included dropwise to maintain the pH value at 7.0. The aqueous solution was heated at 95°C, stirred using a magnetic stirrer, and a viscous solution was obtained. With gel formation, the temperature was raised to 300°C which led to a self-sustaining combustion process followed by the formation of a loose powder. The powder was calcined at 600°C for 6 h with a heating rate of 8.5°C/min resulting in the formation of the final spinel structure samples. Energy band gaps of powder samples were determined using diffused reflection accessory attached with UV-Vis

UV-visible spectra were utilized to determine the band gap energies (*Eg*) of all the samples.

in **Figure 17**. It shows that the *Eg* values were decreased tremendously from 2.4 eV to 1.8 eV

The preparation methods of the thin films or powders and the heat treatment conditions have a crucial influence on the grain growth of the film. It affects not only the grain size of the film but also the surface morphology of the film, which affects the dielectric properties of the material. Microstructure has effects on the dielectric properties, optical properties, and magnetic properties of samples. Sol-gel method is a reasonable way to prepare function materials.

Mg*<sup>x</sup>* Fe2 O4

Polycrystalline and nanocrystalline materials can be prepared by this process.

with Mg substitution increase that ended in the visible range of e.m. spectra [16].

O4

powder samples were recorded as shown

(a) *x* = 0.0, (b) *x* = 0.3, (c) *x* = 0.6, (d) *x* = 0.9, and

spectrophotometer.

48 Chemical Reactions in Inorganic Chemistry

**7. Conclusion**

(e) *x* = 1.0.

Diffuse reflection spectra of Mg substituted CoFe<sup>2</sup>

**Figure 17.** UV-visible diffuse reflectance spectra of Co(1−*<sup>x</sup>*)

Address all correspondence to: ycq1998@gmail.com

School of Biological and Chemical Engineering, Nanyang Institute of Technology, Nanyang, Henan, People's Republic of China

#### **References**


[11] Uhlmann DR. The future of sol-gel science and technology. Journal of Sol-Gel Science and Technology. 1997;**8**(1-3):1083-1091. DOI: 10.1007/BF02436988

**Chapter 4**

**Provisional chapter**

**The Effect of Substituent on Molecules That Contain a**

**Triple Bond Between Arsenic and Group 13 Elements:** 

**The Effect of Substituent on Molecules That Contain a** 

DOI: 10.5772/intechopen.69586

**Triple Bond Between Arsenic and Group 13 Elements:**

The effect of substitution on the potential energy surfaces of RE13≡AsR (E13 = group 13

(M06‐2X/Def2‐TZVP,B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp). The computa‐ tional studies demonstrate that all triply bonded RE13≡AsR species prefer to adopt a bent geometry that is consistent with the valence electron model. The theoretical studies also demonstrate that RE13≡AsR molecules with smaller substituents are kinetically unstable, with respect to the intramolecular rearrangements. However, triply bonded R′E13≡AsR′

occupy the lowest minimum on the singlet potential energy surface, and they are both kinetically and thermodynamically stable. That is to say, the electronic and steric effects of bulky substituents play an important role in making molecules that feature an E13≡As

**Keywords:** arsenic, group 13 elements, triple bond, density functional theory, multiple bond

In the past two decades, studies that have been performed by many synthetic chemists have successfully synthesized and characterized homonuclear heavy alkyne‐like RE14≡E14R (E14 = Si, Ge, Sn, and Pb) molecules [1–23]. Recently, heteronuclear ethyne‐like compounds, RC≡E14R, have also been experimentally studied [24, 25, 26] and theoretically predicted [27, 28, 29].

)2

, Si*i*PrDis2

) is determined using density functional theory

, and NHC) are found to

, and SiH<sup>3</sup>

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Theoretical Designs and Characterizations**

**Theoretical Designs and Characterizations**

Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su and

species with bulkier substituents (R′ = SiMe(Si*t*Bu<sup>3</sup>

triple bond as viable synthetic target.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su

http://dx.doi.org/10.5772/intechopen.69586

elements; R = F, OH, H, CH<sup>3</sup>

Ming‐Der Su

and Ming‐Der Su

**Abstract**

**1. Introduction**


**Provisional chapter**

#### **The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group 13 Elements: Theoretical Designs and Characterizations Triple Bond Between Arsenic and Group 13 Elements: Theoretical Designs and Characterizations**

**The Effect of Substituent on Molecules That Contain a** 

DOI: 10.5772/intechopen.69586

Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su and Ming‐Der Su and Ming‐Der Su Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Jia‐Syun Lu, Ming‐Chung Yang, Shih‐Hao Su

http://dx.doi.org/10.5772/intechopen.69586

#### **Abstract**

[11] Uhlmann DR. The future of sol-gel science and technology. Journal of Sol-Gel Science

[12] Nguyet DTT, Duong NP, Satoh T, et al. Temperature-dependent magnetic properties of yttrium iron garnet nanoparticles prepared by citrate sol-gel. Journal of Alloys and

[13] Kishimoto M, Minagawa M, Yanagihara H, et al. Synthesis and magnetic properties

[14] Pankove IJ, Aigrain P. Optical absorption of arsenic-doped degenerate germanium.

[15] Qun YC, Ding L. Dielectric properties of Zr-doped CCTO based ceramics prepared via sol-gel method. Advanced Materials Research. 2012;**528**:121-125. DOI: 10.4028/www.sci-

[16] Assi H, Atiq S, Rammay SM, Alzayed NS, Saleem M, Riaz S, Naseem S. Substituted Mg-Co-nanoferrite: Recyclable magnetic photocatalyst for the reduction of methylene blue and degradation of toxic dyes. Journal of Materials Science: Materials in Electronics.

Journal of Magnetism and Magnetic Materials. 2012;**324**(7):1285-1289. DOI: 10.1016/j.

particles for medical applications using hysteresis-loss heating.

and Technology. 1997;**8**(1-3):1083-1091. DOI: 10.1007/BF02436988

Compounds. 2012;**541**(22):18-22. DOI: 10.1016/j.jallcom.2012.06.122

Physical Review. 1962;**126**(3):956-962. DOI: 10.1103/PhysRev.126.956

2017;**28**(2):2250-2256. DOI: 10.1007/s10854-016-5795-4

of platelet γ-Fe<sup>2</sup>

50 Chemical Reactions in Inorganic Chemistry

jmmm.2011.11.036

entific.net/AMR.528.121

O3

The effect of substitution on the potential energy surfaces of RE13≡AsR (E13 = group 13 elements; R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ) is determined using density functional theory (M06‐2X/Def2‐TZVP,B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp). The computa‐ tional studies demonstrate that all triply bonded RE13≡AsR species prefer to adopt a bent geometry that is consistent with the valence electron model. The theoretical studies also demonstrate that RE13≡AsR molecules with smaller substituents are kinetically unstable, with respect to the intramolecular rearrangements. However, triply bonded R′E13≡AsR′ species with bulkier substituents (R′ = SiMe(Si*t*Bu<sup>3</sup> )2 , Si*i*PrDis2 , and NHC) are found to occupy the lowest minimum on the singlet potential energy surface, and they are both kinetically and thermodynamically stable. That is to say, the electronic and steric effects of bulky substituents play an important role in making molecules that feature an E13≡As triple bond as viable synthetic target.

**Keywords:** arsenic, group 13 elements, triple bond, density functional theory, multiple bond

#### **1. Introduction**

In the past two decades, studies that have been performed by many synthetic chemists have successfully synthesized and characterized homonuclear heavy alkyne‐like RE14≡E14R (E14 = Si, Ge, Sn, and Pb) molecules [1–23]. Recently, heteronuclear ethyne‐like compounds, RC≡E14R, have also been experimentally studied [24, 25, 26] and theoretically predicted [27, 28, 29].

> Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

However, from the valence electron viewpoint, RE13≡E15R (E13 = group 13 elements and E15 = group 15 elements) is isoelectronic with the RE14≡E14R species. Therefore, triply bonded RE13≡E15R is the next synthetic challenge. To the best of the authors' knowledge, only R2 BN molecules that contain a B≡N triple bond have been experimentally demonstrated to exist [30–40].

**R F OH H CH3 SiH3 B≡As (Å)** 1.901 1.892 1.837 1.839 1.814

∠**R–B–As (°)** 177.2 179.5 178.1 175.1 175.3

∠**B–As–R (°)** 93.03 92.73 81.22 94.69 68.92

∠**R–B–As–R (°)** 180.0 179.8 180.0 179.8 148.7

<sup>1</sup> 0.354 0.184 −0.017 −0.007 0.037

<sup>2</sup> 0.243 0.080 −0.152 −0.073 −0.085

**BE (kcal mol**−1**)**<sup>3</sup> 63.56 56.97 114.7 94.39 79.90

**WBI**<sup>4</sup> 1.800 1.830 2.141 2.027 2.204

3 BE = E(triplet state for R–B) + E(triplet state for R–As) − E(singlet state for RB≡AsR).

Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

4 The Wiberg bond index (WBI) for the B–As bond: see [45, 46].

*Q*B

*Q*As

1 The natural charge density on the B atom. 2 The natural charge density on the As atom.

(1.898) (1.888) (1.835) (1.839) (1.820) [1.908] [1.906] [1.849] [1.861] [1.839]

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

http://dx.doi.org/10.5772/intechopen.69586

53

(177.8) (179.5) (174.6) (175.1) (172.4) [177.0] [179.1] [177.5] [174.3] [174.8]

(92.71) (92.21) (89.39) (94.69) (68.98) [92.39] [92.95] [78.37] [96.15] [72.25]

(180.0) (180.0) (180.0) (179.8) (180.0) [180.0] [176.2] [179.0] [176.3] [179.4]

(0.262) (0.108) (−0.028) (−0.057) (0.036) [0.232] [0.070] [−0.106] [−0.160] [−0.407]

(0.255) (0.097) (−0.134) (−0.040) (−0.017) [0.238] [0.086] [0.034] [−0.035] [0.030]

(63.34) (60.28) (120.1) (137.6) (74.75) [57.45] [55.28] [113.7] [132.6] [73.79]

(1.813) (1.823) (2.158) (2.029) (2.168) [1.835] [1.836] [2.135] [2.041] [2.185]

**Table 1.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*B and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RB≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/

### **2. Theoretical methods**

This chapter reports the possible existence of triply bonded RE13≡AsR molecules, from the viewpoint of the effect of substituents, using density functional theories (DFT): M06‐2X/ Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp for small substituents and B3LYP/LANL2DZ+dp//RHF/3‐21G\* for large substituents. It is hoped that this theoreti‐ cal study will stimulate further research into the synthetic chemistry of triply bonded RE13≡AsR species.

#### **3. Results and discussion**

#### **3.1. Small ligands on substituted RE13**≡**AsR**

The effect of the electronegativity of six types of small substituents (R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ) on the stability of the triply bonded RE13≡AsR molecules is determined using the three DFT methods. The molecular properties (geometrical parameters, singlet‐triplet energy split‐ ting, natural charge densities, binding energies (BE), and the Wiberg Bond Index (WBI)) are all listed in **Tables 1**–**5**. The reaction profiles for the unimolecular rearrangement reactions for the RE13≡AsR compounds are also given in **Figures 1**–**5**.

There are three noteworthy features of **Tables 1**–**5** and **Figures 1**–**5**.


The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ... http://dx.doi.org/10.5772/intechopen.69586 53


1 The natural charge density on the B atom.

However, from the valence electron viewpoint, RE13≡E15R (E13 = group 13 elements and E15 = group 15 elements) is isoelectronic with the RE14≡E14R species. Therefore, triply bonded RE13≡E15R is the next synthetic challenge. To the best of the authors' knowledge, only R2

molecules that contain a B≡N triple bond have been experimentally demonstrated to exist

This chapter reports the possible existence of triply bonded RE13≡AsR molecules, from the viewpoint of the effect of substituents, using density functional theories (DFT): M06‐2X/ Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp for small substituents and B3LYP/LANL2DZ+dp//RHF/3‐21G\* for large substituents. It is hoped that this theoreti‐ cal study will stimulate further research into the synthetic chemistry of triply bonded

The effect of the electronegativity of six types of small substituents (R = F, OH, H, CH<sup>3</sup>

) on the stability of the triply bonded RE13≡AsR molecules is determined using the three DFT methods. The molecular properties (geometrical parameters, singlet‐triplet energy split‐ ting, natural charge densities, binding energies (BE), and the Wiberg Bond Index (WBI)) are all listed in **Tables 1**–**5**. The reaction profiles for the unimolecular rearrangement reactions for

**1.** From the tables, the three DFT calculations show that the triple bond distances (*Å*) for B≡As, Al≡As, Ga≡As, In≡As, and Tl≡As are estimated to be 1.835–1.908 (**Table 1**), 2.218– 2.358 (**Table 2**), 2.239–2.364 (**Table 3**), 2.404–2.546 (**Table 4**), and 2.426–2.570 (**Table 5**). As previously mentioned, no experimental values for these triple bond lengths have been

**2.** In **Tables 1**–**5**, these DFT computations all demonstrate that the triply bonded RE13≡AsR molecules favor a bent structure, rather than a linear structure. This is explained by the bonding model, as shown in **Figure 6**. Because there is a significant difference between the sizes of the valence s and p atomic orbitals in the As atom, hybrid orbitals between the va‐ lence *s* and *p* orbitals are not easily formed (the so‐called orbital non‐hybridization effect or the inert s‐pair effect) [41–44]. Therefore, RE13≡AsR molecules that have a heavier As center are predicted to favor a bent angle ∠E13–As–R (close to 90°). The DFT computational data

[30–40].

**2. Theoretical methods**

52 Chemical Reactions in Inorganic Chemistry

**3. Results and discussion**

**3.1. Small ligands on substituted RE13**≡**AsR**

the RE13≡AsR compounds are also given in **Figures 1**–**5**.

There are three noteworthy features of **Tables 1**–**5** and **Figures 1**–**5**.

reported, so these computational data are a prediction.

that are shown in **Tables 1**–**5** confirm this prediction.

RE13≡AsR species.

SiH<sup>3</sup>

BN

, and

2 The natural charge density on the As atom.

3 BE = E(triplet state for R–B) + E(triplet state for R–As) − E(singlet state for RB≡AsR).

4 The Wiberg bond index (WBI) for the B–As bond: see [45, 46].

**Table 1.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*B and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RB≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/ Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.


**3.2. Large ligands on substituted R′E13**≡**AsR′**

, Si*i*PrDis2

)2

(R′In≡AsR′), and **14** and **15** (R′Tl≡AsR′).

1 The natural charge density on the Ga atom. 2 The natural charge density on the As atom.

4 The Wiberg bond index (WBI) for the Ga–As bond: see [45, 46].

(R′= SiMe(Si*t*Bu<sup>3</sup>

*Q*Ga

*Q*As

Bulky substituents are used to determine the possible existence of triply bonded R′E13≡AsR′

natural bond orbital (NBO) [45, 46], and the natural resonance theory (NRT) [47, 48, 49] analy‐ ses of R′E13≡AsR′ are computed at the B3LYP/LANL2DZ+dp//RHF/3‐21G\* level of theory, and the results are shown in **Tables 6**, **7** (R′B≡AsR′), **8**, **9** (R′Al≡AsR′), **10**, **11** (R′Ga≡AsR′), **12**, **13**

> (2.319) (2.314) (2.224) (2.243) (2.242) [2.364] [2.364] [2.263] [2.285] [2.270

> (178.5) (177.4) (178.6) (173.6) (179.1) [179.3] [176.2] [179.1] [171.1] [179.2]

> (94.36) (94.54) (79.18) (99.37) (73.64) [91.81] [93.68] [80.30] [100.4] [76.86]

> (180.0) (178.1) (179.1) (178.4) (175.6) [173.1] [177.4] [178.2] [174.5] [178.1]

> (0.554) (0.410) (0.215) (0.260) (0.241) [0.706] [0.474] [0.435] [0.295] [0.174]

(0.154) (0.023) (−0.262) (−0.151) (−0.222) [0.133] [0.006] [−0.184] [−0.246] [−0.284]

(30.61) (25.96) (71.91) (58.12) (51.77) [27.65] [90.75] [65.14] [50.32] [62.24]

(1.486) (1.503) (1.717) (1.652) (1.596) [1.487] [1.495] [1.707] [1.668] [1.615]

**Table 3.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*Ga and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RGa≡AsR using the M06‐2X/Def2‐TZVP,

**R F OH H CH3 SiH3 Ga≡As (Å)** 2.261 2.339 2.239 2.330 2.243

∠**R–Ga–As (°)** 179.5 173.2 176.2 169.9 168.5

∠**Ga–As–R (°)** 92.80 93.16 76.00 103.0 93.43

∠**R–Ga–As–R (°)** 180.0 175.6 179.6 175.7 173.5

<sup>1</sup> 0.7067 0.592 0.310 0.4451 0.3352

<sup>2</sup> 0.0899 −0.047 −0.374 −0.256 −0.3697

**BE (kcal mol**−1**)**<sup>3</sup> 28.56 23.82 67.79 53.57 49.26

**WBI**<sup>4</sup> 1.476 1.498 1.691 1.648 1.646

B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

3 BE = E(triplet state for R–Ga) + E(triplet state for R–As) − E(singlet state for RGa≡AsR).

, and NHC; (**Scheme 1**)) molecules. The molecular properties, the

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The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

1 The natural charge density on the Al atom.

2 The natural charge density on the As atom.

3 BE = E(triplet state for R–Al) + E(triplet state for R–As) − E(singlet state for RAl≡AsR).

4 The Wiberg bond index (WBI) for the Al–As bond: see [45, 46].

**Table 2.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*Al and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RAl≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

**3.** In terms of the stability of the RE13≡AsR species, the three DFT computations are used to study the energy surfaces for the RE13≡AsR systems, and the theoretical results are shown in **Figures 1**–**5**. These figures show three local minima (i.e., R2 E13=As, RE13≡AsR, and E13=AsR2 ) and two saddle points that connect them. It is seen that regardless of the type of small substituent, triply bonded RE13≡AsR molecules are unstable on the potential energy surfaces, so they easily undergo a 1,2‐migration reaction to produce the most stable doubly bonded isomers. There is strong theoretical evidence that there is no possibility of observ‐ ing triply bonded RE13≡AsR compounds in transient intermediates or even in a matrix.

## **3.2. Large ligands on substituted R′E13**≡**AsR′**

Bulky substituents are used to determine the possible existence of triply bonded R′E13≡AsR′ (R′= SiMe(Si*t*Bu<sup>3</sup> )2 , Si*i*PrDis2 , and NHC; (**Scheme 1**)) molecules. The molecular properties, the natural bond orbital (NBO) [45, 46], and the natural resonance theory (NRT) [47, 48, 49] analy‐ ses of R′E13≡AsR′ are computed at the B3LYP/LANL2DZ+dp//RHF/3‐21G\* level of theory, and the results are shown in **Tables 6**, **7** (R′B≡AsR′), **8**, **9** (R′Al≡AsR′), **10**, **11** (R′Ga≡AsR′), **12**, **13** (R′In≡AsR′), and **14** and **15** (R′Tl≡AsR′).


1 The natural charge density on the Ga atom.

**3.** In terms of the stability of the RE13≡AsR species, the three DFT computations are used to study the energy surfaces for the RE13≡AsR systems, and the theoretical results are shown

**Table 2.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*Al and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RAl≡AsR using the M06‐2X/Def2‐TZVP,

small substituent, triply bonded RE13≡AsR molecules are unstable on the potential energy surfaces, so they easily undergo a 1,2‐migration reaction to produce the most stable doubly bonded isomers. There is strong theoretical evidence that there is no possibility of observ‐ ing triply bonded RE13≡AsR compounds in transient intermediates or even in a matrix.

) and two saddle points that connect them. It is seen that regardless of the type of

E13=As, RE13≡AsR, and

in **Figures 1**–**5**. These figures show three local minima (i.e., R2

3 BE = E(triplet state for R–Al) + E(triplet state for R–As) − E(singlet state for RAl≡AsR).

**R F OH H CH3 SiH3 Al≡As (Å)** 2.327 2.321 2.218 2.253 2.227

∠**R–Al–As (°)** 178.6 174.4 172.5 172.8 168.4

∠**Al–As–R (°)** 93.07 91.08 66.95 98.77 91.93

∠**R–Al–As–R (°)** 180.0 180.0 180.0 174.2 174.7

<sup>1</sup> 0.555 0.4574 0.2401 0.293 0.291

<sup>2</sup> 0.158 0.015 −0.276 −0.170 −0.262

**BE (kcal mol**−1**)**<sup>3</sup> 33.90 28.23 71.86 56.47 53.22

**WBI**<sup>4</sup> 1.532 1.523 1.714 1.649 1.647

B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

(2.325) (2.323) (2.221) (2.256) (2.236) [2.355] [2.358] [2.269] [2.285] [2.292]

(179.5) (174.3) (172.2) (172.0) (167.3) [178.8] [173.9] [177.5] [171.1] [173.7]

(93.51) (92.45) (67.45) (100.7) (95.83) [90.64] [90.97] [75.97] [100.5] [90.36]

(179.8) (178.5) (179.6) (176.8) (175.7) [180.0] [179.0] [178.0] [174.5] [176.8]

(0.530) (0.443) (0.234) (0.280) (0.313) [0.784] [0.540] [0.504] [0.353] [0.245]

(0.142) (−0.007) (−0.246) (−0.156) (−0.209) [0.056] [−0.032] [−0.209] [−0.284] [−0.290]

(38.90) (31.24) (77.42) (60.57) (54.98) [33.89] [25.68] [69.27] [52.63] [67.74]

(1.567) (1.553) (1.742) (1.679) (1.675) [1.557] [1.545] [1.714] [1.690] [1.550]

E13=AsR2

1 The natural charge density on the Al atom. 2 The natural charge density on the As atom.

54 Chemical Reactions in Inorganic Chemistry

4 The Wiberg bond index (WBI) for the Al–As bond: see [45, 46].

*Q*Al

*Q*As

2 The natural charge density on the As atom.

3 BE = E(triplet state for R–Ga) + E(triplet state for R–As) − E(singlet state for RGa≡AsR).

4 The Wiberg bond index (WBI) for the Ga–As bond: see [45, 46].

**Table 3.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*Ga and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RGa≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.


**R F OH H CH3 SiH3 Tl≡As (Å)** 2.535 2.531 2.426 2.446 2.431

∠**R–Tl–As (°)** 179.9 178.2 180.0 176.6 176.5

∠**Tl–As–R (°)** 91.49 94.88 84.22 97.14 90.08

∠**R–Tl–As–R (°)** 180.0 175.5 173.0 178.0 179.2

<sup>1</sup> 0.736 0.640 0.3883 0.482 0.3051

<sup>2</sup> 0.190 0.035 −0.4169 −0.251 −0.3290

**BE (kcal mol**−1**)**<sup>3</sup> 13.48 10.36 50.28 38.25 29.93

**WBI**<sup>4</sup> 1.109 1.148 1.456 1.382 1.409

3 BE = E(triplet state for R–Tl) + E(triplet state for R–As) − E(singlet state for RTl≡AsR).

Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

*Q*Tl

*Q*As

1 The natural charge density on the Tl atom. 2 The natural charge density on the As atom.

4 The Wiberg bond index (WBI) for the Tl–As bond, see [45, 46].

(2.533) (2.536) (2.428) (2.450) (2.432) [2.558] [2.570] [2.429] [2.459] [2.433]

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(179.9) (175.8) (179.5) (175.0) (173.4) [179.2] [177.0] [179.5] [173.8] [177.7]

(93.64) (96.73) (84.51) (99.33) (93.68) [92.21] [96.20] [84.07] [99.33] [89.37]

(179.3) (176.7) (178.1) (178.2) (178.5) [180.0] [172.9] [179.6] [177.6] [177.2]

(0.656) (0.538) (0.352) (0.428) (0.382) [0.817] [0.549] [0.472] [0.361] [0.244]

(0.163) (0.013) (−0.351) (−0.208) (−0.291) [0.139] [0.021] [−0.204] [−0.273] [−0.336]

(16.73) (13.88) (55.13) (43.44) (30.60) [15.13] [8.720] [49.40] [37.22] [45.10]

(1.143) (1.174) (1.492) (1.416) (1.407) [1.168] [1.175] [1.484] [1.413] [1.411]

**Table 5.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*Tl and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RTl≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/

1 The natural charge density on the In atom.

2 The natural charge density on the As atom.

3 BE = E(triplet state for R–In) + E(triplet state for R–As) − E(singlet state for RIn≡AsR).

4 The Wiberg bond index (WBI) for the In–As bond, see [45, 46].

**Table 4.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*In and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RIn≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/ Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

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1 The natural charge density on the Tl atom.

**R F OH H CH3 SiH3 In≡As (Å)** 2.511 2.512 2.412 2.431 2.411

∠**R–In–As (°)** 179.9 178.8 179.3 173.6 170.9

∠**In–As–R (°)** 92.32 95.31 81.43 99.72 93.85

∠**R–In–As–R (°)** 180.0 169.3 177.3 174.7 177.1

<sup>1</sup> 1.288 1.233 1.012 1.144 0.8840

<sup>2</sup> 0.138 0.036 −0.624 −0.388 −0.767

**BE (kcal mol**−1**)**<sup>3</sup> 22.14 18.30 55.63 53.87 57.82

**WBI**<sup>4</sup> 1.536 1.551 1.773 1.719 1.726

3 BE = E(triplet state for R–In) + E(triplet state for R–As) − E(singlet state for RIn≡AsR).

Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

*Q*In

56 Chemical Reactions in Inorganic Chemistry

*Q*As

1 The natural charge density on the In atom. 2 The natural charge density on the As atom.

4 The Wiberg bond index (WBI) for the In–As bond, see [45, 46].

(2.495) (2.497) (2.399) (2.418) (2.404) [2.535] [2.546] [2.432] [2.459] [2.444]

(179.9) (176.9) (179.9) (173.3) (168.4) [177.8] [175.2] [179.8] [172.5] [167.4]

(93.86) (96.11) (82.67) (100.4) (99.59) [91.08] [94.22] [82.28] [100.5] [102.0]

(180.0) (166.8) (175.9) (173.0) (177.4) [180.0] [163.8] [179.6] [179.8] [178.2]

(1.196) (1.123) (0.912) (1.037) (0.7881) [1.343] [1.287] [1.076] [1.121] [0.9682]

(0.146) (0.047) (−0.571) (−0.335) (−0.703) [0.077] [−0.005] [−0.591] [−0.367] [−0.748]

(19.72) (20.13) (60.95) (50.24) (57.34) [24.06] [16.22] [57.18] [53.36] [54.39]

(1.546) (1.554) (1.798) (1.738) (1.749) [1.572] [1.562] [1.780] [1.729] [1.710]

**Table 4.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*In and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RIn≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/

2 The natural charge density on the As atom.

3 BE = E(triplet state for R–Tl) + E(triplet state for R–As) − E(singlet state for RTl≡AsR).

4 The Wiberg bond index (WBI) for the Tl–As bond, see [45, 46].

**Table 5.** The main geometrical parameters, the singlet‐triplet energy splitting (Δ*E*ST), the natural charge densities (*Q*Tl and *Q*As), the binding energies (BE), and the Wiberg Bond Index (WBI) for RTl≡AsR using the M06‐2X/Def2‐TZVP, B3PW91/ Def2‐TZVP (in round brackets), and B3LYP/LANL2DZ+dp (in square brackets) levels of theory.

**Figure 1.** The relative Gibbs free energies for RB ≡ AsR (R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

**Figure 2.** The relative Gibbs free energies for RAl≡AsR (R = F, OH, H, CH<sup>3</sup>

calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

, and SiH<sup>3</sup>

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**Figure 2.** The relative Gibbs free energies for RAl≡AsR (R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

**Figure 1.** The relative Gibbs free energies for RB ≡ AsR (R = F, OH, H, CH<sup>3</sup>

58 Chemical Reactions in Inorganic Chemistry

calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

, and SiH<sup>3</sup>

**Figure 3.** The relative Gibbs free energies for RGa ≡ AsR (R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

**Figure 4.** The relative Gibbs free energies for RIn≡AsR (R = F, OH, H, CH<sup>3</sup>

calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

, and SiH<sup>3</sup>

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**Figure 4.** The relative Gibbs free energies for RIn≡AsR (R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

**Figure 3.** The relative Gibbs free energies for RGa ≡ AsR (R = F, OH, H, CH<sup>3</sup>

60 Chemical Reactions in Inorganic Chemistry

are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

, and SiH<sup>3</sup>

The results in **Tables 6**–**15** allow three conclusions to be drawn.

crease the bond order for the E13≡As triple bond length.

bonding in (Si*i*PrDis2

B≡As–Si*i*PrDis2

between the B–Si*i*PrDis2

**Scheme 1.** Three bulky ligands: SiMe(Si*t*Bu<sup>3</sup>

explained by bonding model (II), which is shown in **Figure 6**.

–B≡As–Si*i*PrDis2

these R'E13≡AsR' molecules have a weak E13≡As triple bond.

) 2 , Si*i*PrDis2

and the As–Si*i*PrDis2

**1.** The calculations that are shown in **Tables 6** (B), **8** (Al), **10** (Ga), **12** (In), and **14** (Tl) show that the computed E13≡As triple bond distances (*Å*) for these bulkily substituted species (R′E13≡AsR′) are estimated to be 1.821–1.837 (B≡As), 2.257–2.307 (Al≡As), 2.252–2.316 (Ga≡As), 2.430–2.482 (In≡As), and 2.565–2.653 (Tl≡As). The values for the WBO that are shown in **Tables 6**–**10** (for bulky ligands) are obviously greater than those that are shown in **Tables 1**–**5** (for smaller ligands). These WBO values show that bulkier substituents in‐

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

**2.** Similarly to the results for small ligands, the computational results show that R'E13≡AsR' species that feature large substituents all adopt a bent conformation. This phenomenon is

**3.** The NBO values that are shown in **Tables 7** (B≡As), **9** (Al≡As), **11** (Ga≡As), **13** (In≡As), and **15** (Tl≡As) show that the acetylene‐like R'E13≡AsR' compounds feature a weak triple bond. For example, the B3LYP/LANL2DZ+dp data for the NBO analyses of the B≡As π

B + 0.8089(4s4p1.00)As, provide strong evidence that the predominant bonding interaction

tion, whereby boron's electron deficiency and π bond polarity are partially balanced by the donation of the arsenic lone pair into the empty boron *p* orbital to develop a hybrid π bond. The polarization analyses using the NBO model again demonstrate the presence of the B≡As π bonding orbital, 34.58% of which is composed of natural B orbitals and 65.42% of which is natural As orbitals. **Table 7** also shows that the B≡As triple bond in (Si*i*PrDis2

character (36.74%), but a greater double bond character (57.2%), because the ionic part of the NRT bond order (0.53) is shorter than its covalent part (1.71). The same theoretical observations are also seen for the other two differently substituted R'B≡AsR' compounds, as shown in **Table 7**, and in the data for the other R'E13≡BiR' compounds that is shown in **Tables 9 (Al)**, **11 (Ga)**, **13 (In)**, and **15 (Tl)**. These computational data demonstrate that

) has a shorter single bond character (6.04%) and a shorter triple bond

, and N‐heterocyclic carbine.

), which shows that NBO(B≡As) = 0.5880(2s2p99.99)

units results from 2p(B) ← 4p(As) dona‐

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**Figure 5.** The relative Gibbs free energies for RTl≡AsR (R = F, OH, H, CH<sup>3</sup> , and SiH<sup>3</sup> ). All energies are in kcal/mol and are calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

The results in **Tables 6**–**15** allow three conclusions to be drawn.


**Scheme 1.** Three bulky ligands: SiMe(Si*t*Bu<sup>3</sup> )2 , Si*i*PrDis2 , and N‐heterocyclic carbine.

**Figure 5.** The relative Gibbs free energies for RTl≡AsR (R = F, OH, H, CH<sup>3</sup>

62 Chemical Reactions in Inorganic Chemistry

calculated at the M06‐2X/Def2‐TZVP, B3PW91/Def2‐TZVP, and B3LYP/LANL2DZ+dp levels of theory.

, and SiH<sup>3</sup>


1 The natural charge density on the central B atom.

2 The natural charge density on the central As atom.

3 BE = E (triplet state for B–R′) + E (triplet state for As–R′) − E (singlet state for R′B≡AsR′).

4 The Wiberg bond index (WBI) for the B–As bond.

5 Δ*H*<sup>1</sup> = E (:B=AsR′2 ) − E (R′B≡AsR′); see Scheme 2.

6 Δ*H*<sup>2</sup> = E (R′2 B=As:) − E (R′B≡AsR′); see Scheme 2.

**Table 6.** The geometrical parameters, natural charge densities (*Q*B and *Q*As), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′B≡AsR′ at the B3LYP/LANL2DZ+dp// RHF/3‐21G\* Level of Theory.

**R′Al**≡**AsR′ WBI NBO analysis NRT analysis**

**Al**≡**As (Å)** 2.285 2.257 2.307 ∠**R′–Al–As (°)** 179.4 176.0 174.5 ∠**Al–As–R′ (°)** 116.4 118.7 113.0 ∠**R′–Al–As–R′ (°)** 176.1 170.6 176.4

<sup>1</sup> 0.3771 0.3120 0.4392

<sup>2</sup> −0.5579 −0.4907 −0.3144

**Wiberg BO**<sup>4</sup> 2.171 2.184 2.185

3 BE = E(triplet state for Al–R′) + E(triplet state for As–R′) − E(singlet state for R′Al≡AsR′).

<sup>3</sup> 44.64 54.23 34.53

π = 1.92 π : 0.4437 Al

π = 1.91 π : 0.4383 Al

π = 1.94 π : 0.4408 Al

LANL2DZ+dp level of theory for R′Al≡AsR′ compounds that have large substituents.

(sp1.59) + 0.8614 As (sp1.14)

**Table 8.** The geometrical parameters, natural charge densities (*Q*Al and *Q*As), binding energies (BE), the HOMO‐LUMO energy gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Al≡AsR′ at the B3LYP/LANL2DZ+dp//

> (sp99.99) + 0.8962 As (sp99.99)

(sp1.84) + 0.8685 As (sp1.06)

(sp99.99) + 0.8988 As (sp99.99)

(sp0.99) + 0.8122 As (sp10.87)

(sp90.78) + 0.8976 As (sp99.99)

**Table 9.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/

)<sup>2</sup> 2.21 σ = 1.92 σ : 0.5080 Al

R′ = Si*i*PrDis2 2.29 σ = 1.92 σ : 0.4956 Al

R′ = NHC 2.36 σ = 1.87 σ : 0.5834 Al

R′ = SiMe(Si*t*Bu<sup>3</sup>

*Q*Al

*Q*As

5 Δ*H*<sup>1</sup>

6 Δ*H*<sup>2</sup>

**ΔE**ST **(kcal mol**−1**)**

= E(:Al=AsR′2

RHF/3‐21G\* level of theory.

= E(R′2

**R′ SiMe(Si***t***Bu3**

1 The natural charge density on the central Al atom. 2 The natural charge density on the central As atom.

4 The Wiberg bond index (WBI) for the Al–As bond.

) − E(R′Al≡AsR′); see Scheme 2.

Al=As:) – E(R′Al≡AsR′); see Scheme 2.

**Occupancy hybridization Polarization total/covalent/**

**)2 Si***i***PrDis2 NHC**

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

25.81% (Al) 74.19% (As)

19.69% (Al) 80.31% (As)

24.57% (Al) 75.43% (As)

19.21% (Al) 80.79% (As)

34.04% (Al) 65.96% (As)

19.43% (Al) 80.57% (As) **ionic**

**Resonance weight**

Al=As : 70.32% Al≡As : 23.17%

Al≡As : 57.55% Al≡As : 37.93%

Al=As : 74.90% Al=As : 18.49%

2.24/1.66/0.58 Al–As : 6.51%

http://dx.doi.org/10.5772/intechopen.69586

65

2.27/1.73/0.54 Al–As: 4.52%

2.30/1.59/0.71 Al–As : 6.61%


**Table 7.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/ LANL2DZ+dp level of theory for R′B≡AsR′ compounds that have large substituents.

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ... http://dx.doi.org/10.5772/intechopen.69586 65


1 The natural charge density on the central Al atom.

2 The natural charge density on the central As atom.

3 BE = E(triplet state for Al–R′) + E(triplet state for As–R′) − E(singlet state for R′Al≡AsR′).

4 The Wiberg bond index (WBI) for the Al–As bond.

5 Δ*H*<sup>1</sup> = E(:Al=AsR′2 ) − E(R′Al≡AsR′); see Scheme 2.

6 Δ*H*<sup>2</sup> = E(R′2 Al=As:) – E(R′Al≡AsR′); see Scheme 2.

**R′B≡AsR′ WBI NBO analysis NRT analysis**

**<sup>B</sup>**≡**As (Å)** 1.837 1.821 1.819 ∠**R′–B–As (°)** 177.2 172.9 174.5 ∠**B–As–R′ (°)** 128.2 121.6 111.2 ∠**R′–B–As–R′ (°)** 179.6 177.4 171.5

<sup>1</sup> −0.280 −0.397 −0.205

<sup>2</sup> −0.228 −0.134 0.061 **ΔE**ST **(kcal mol**−1**)**<sup>3</sup> 94.42 75.22 83.64 **Wiberg BO**<sup>4</sup> 2.327 2.395 2.254

3 BE = E (triplet state for B–R′) + E (triplet state for As–R′) − E (singlet state for R′B≡AsR′).

π = 1.94 π : 0.5941 B

π = 1.94 π : 0.5880

π = 1.94 π : 0.5899

LANL2DZ+dp level of theory for R′B≡AsR′ compounds that have large substituents.

2.31 σ = 1.98 σ : 0.6627 B

R′ = Si*i*PrDis2 2.27 σ = 1.98 σ : 0.6630 B

R′ = NHC 2.26 σ = 1.98 σ : 0.6918 B

R′ = SiMe(Si*t*Bu<sup>3</sup>

*Q*B

*Q*As

5 Δ*H*<sup>1</sup>

6 Δ*H*<sup>2</sup>

)2

= E (:B=AsR′2

RHF/3‐21G\* Level of Theory.

= E (R′2

**R′ SiMe(Si***t***Bu3**

64 Chemical Reactions in Inorganic Chemistry

1 The natural charge density on the central B atom. 2 The natural charge density on the central As atom.

4 The Wiberg bond index (WBI) for the B–As bond.

) − E (R′B≡AsR′); see Scheme 2.

B=As:) − E (R′B≡AsR′); see Scheme 2.

**Occupancy hybridization Polarization total/covalent/**

**)2 Si***i***PrDis2 NHC**

43.91% (B) 56.09% (As)

35.29% (B) 64.71% (As)

43.96% (B) 56.04% (As)

34.58% (B) 65.42% (As)

47.86% (B) 52.14% (As)

34.80% (B) 65.20% (As)

(sp1.46) + 0.7489 As (sp1.07)

**Table 6.** The geometrical parameters, natural charge densities (*Q*B and *Q*As), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′B≡AsR′ at the B3LYP/LANL2DZ+dp//

> (sp1.00) + 0.8044 As (sp99.99)

> (sp1.54) + 0.7486 As (sp1.22)

> (sp0.90) + 0.7221 As (sp2.66)

**Table 7.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/

B (sp99.99) + 0.8075 As (sp99.99)

B (sp99.99) + 0.8089 As (sp99.99)

**ionic**

**Resonance weight**

B=As: 60.70% B≡As: 33.62%

B=As : 57.2% B=As : 36.74%

B=As : 69.13% B≡As : 23.82%

2.35/1.66/0.69 B–As: 5.68%

2.24/1.71/0.53 B–As: 6.04%

2.23/1.52/0.71 B–As : 7.05%

**Table 8.** The geometrical parameters, natural charge densities (*Q*Al and *Q*As), binding energies (BE), the HOMO‐LUMO energy gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Al≡AsR′ at the B3LYP/LANL2DZ+dp// RHF/3‐21G\* level of theory.


**Table 9.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/ LANL2DZ+dp level of theory for R′Al≡AsR′ compounds that have large substituents.


1 The natural charge density on the central Ga atom.

2 The natural charge density on the central As atom.

3 BE = E(triplet state for Ga–R′) + E(triplet state for As–R′) − E(singlet state for R′Ga≡AsR′).

4 The Wiberg bond index (WBI) for the Ga–As bond.

5 Δ*H*<sup>1</sup> = E(:Ga=AsR′2 ) − E(R′Ga≡AsR′); see Scheme 2.

6 Δ*H*<sup>2</sup> = E(R′2 Ga=As:) − E(R′Ga≡AsR′); see Scheme 2.

**Table 10.** The geometrical parameters, natural charge densities (*Q*Ga and *Q*As), binding energies (BE), the HOMO‐LUMO energy gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Ga≡AsR′ at the B3LYP/LANL2DZ+dp// RHF/3‐21G\* level of theory.

**R′ SiMe(Si***t***Bu3**

1 The natural charge density on the central In atom. 2 The natural charge density on the central As atom.

4 The Wiberg bond index (WBI) for the In–As bond.

) – E(R′In≡AsR′); see Scheme 2.

In=As:) − E(R′In≡AsR′); see Scheme 2.

*Q*In

R′ = SiMe(Si*t*Bu<sup>3</sup>

)2

*Q*As

5 Δ*H*<sup>1</sup>

6 Δ*H*<sup>2</sup>

**ΔE**ST **(kcal mol**−1**)**

= E(:In=AsR′2

RHF/3‐21G\* Level of Theory.

= E(R′2

**)2 Si***i***PrDis2 NHC**

**In**≡**As (Å)** 2.446 2.430 2.482 ∠**R′–In–As (°)** 155.9 168.4 171.3 ∠**In–As–R′ (°)** 127.8 120.3 110.8 ∠**R′–In–As–R′ (°)** 173.9 162.0 168.3

LANL2DZ+dp level of theory for R′Ga≡AsR′ compounds that have large substituents.

**R′Ga**≡**AsR′ WBI NBO analysis NRT analysis**

2.19 σ = 1.90 σ : 0.5320 Ga

R′ = Si*i*PrDis2 2.25 σ = 1.91 σ : 0.5386 Ga

R′ = NHC 2.33 σ = 1.85 σ : 0.6076 Ga

**Occupancy Hybridization Polarization total/covalent/**

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

28.30% (Ga) 71.70% (As)

19.95% (Ga) 80.05% (As)

29.01% (Ga) 70.99% (As)

19.29% (Ga) 80.71% (As)

36.92% (Ga) 63.08% (As)

19.09% (Ga) 80.91% (As)

17.54% π = 1.93 π : 0.4370

24.84% π = 1.92 π : 0.4392

38.67% π = 1.93 π : 0.4467

(sp1.52) + 0.8468 As (sp1.32)

(sp1.49) + 0.8426 As (sp1.46)

(sp0.98) + 0.7942 As (sp12.06)

**Table 11.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/

Ga (sp82.50) + 0.8995 As (sp99.99)

Ga (sp99.99) + 0.8984 As (sp99.99)

Ga (sp99.99) + 0.8947 As (sp99.99)

**ionic**

**Resonance weight**

67

Ga=As : 56.61% Ga=As :

Ga=As : 68.13% Ga=As :

Ga=As : 75.34% Ga=As :

2.27/1.62/0.65 Ga–As : 4.72%

http://dx.doi.org/10.5772/intechopen.69586

2.31/1.64/0.67 Ga–As : 7.03%

2.14/1.71/0.43 Ga–As : 7.12%

<sup>1</sup> 0.874 0.880 1.021

<sup>2</sup> −0.783 −0.822 −0.359

**Wiberg BO**<sup>4</sup> 2.174 2.271 2.141

3 BE = E(triplet state for In–R′) + E(triplet state for As–R′) − E(singlet state for R′In≡AsR′).

<sup>3</sup> 41.5 45.2 35.7

**Table 12.** The geometrical parameters, natural charge densities (*Q*In and *Q*As), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′In≡AsR′ at the B3LYP/LANL2DZ+dp//

**Figure 6.** The bonding models (I) and (II) for the triply bonded RE13≡AsR molecule.

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ... http://dx.doi.org/10.5772/intechopen.69586 67


**Table 11.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/ LANL2DZ+dp level of theory for R′Ga≡AsR′ compounds that have large substituents.


1 The natural charge density on the central In atom.

**R′ SiMe(Si***t***Bu3**

66 Chemical Reactions in Inorganic Chemistry

1 The natural charge density on the central Ga atom. 2 The natural charge density on the central As atom.

4 The Wiberg bond index (WBI) for the Ga–As bond.

) − E(R′Ga≡AsR′); see Scheme 2.

Ga=As:) − E(R′Ga≡AsR′); see Scheme 2.

*Q*Ga

*Q*As

5 Δ*H*<sup>1</sup>

6 Δ*H*<sup>2</sup>

= E(:Ga=AsR′2

RHF/3‐21G\* level of theory.

= E(R′2

**)2 Si***i***PrDis2 NHC**

**Ga**≡**As (Å)** 2.274 2.252 2.316 ∠**R′–Ga–As (°)** 178.8 178.1 171.9 ∠**Ga–As–R′ (°)** 119.3 122.8 114.5 ∠**R′–Ga–As–R′ (°)** 176.6 171.0 176.5

<sup>1</sup> 0.1760 0.09195 0.2133

<sup>2</sup> −0.4683 −0.3978 −0.2257 **ΔE**ST **(kcal mol**−1**)**<sup>3</sup> 40.67 31.52 33.97 **Wiberg BO**<sup>4</sup> 2.125 2.174 2.154

**Table 10.** The geometrical parameters, natural charge densities (*Q*Ga and *Q*As), binding energies (BE), the HOMO‐LUMO energy gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Ga≡AsR′ at the B3LYP/LANL2DZ+dp//

3 BE = E(triplet state for Ga–R′) + E(triplet state for As–R′) − E(singlet state for R′Ga≡AsR′).

**Figure 6.** The bonding models (I) and (II) for the triply bonded RE13≡AsR molecule.

2 The natural charge density on the central As atom.

3 BE = E(triplet state for In–R′) + E(triplet state for As–R′) − E(singlet state for R′In≡AsR′).

4 The Wiberg bond index (WBI) for the In–As bond.

5 Δ*H*<sup>1</sup> = E(:In=AsR′2 ) – E(R′In≡AsR′); see Scheme 2.

6 Δ*H*<sup>2</sup> = E(R′2 In=As:) − E(R′In≡AsR′); see Scheme 2.

**Table 12.** The geometrical parameters, natural charge densities (*Q*In and *Q*As), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′In≡AsR′ at the B3LYP/LANL2DZ+dp// RHF/3‐21G\* Level of Theory.


**Table 13.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/ LANL2DZ+dp level of theory for R′In≡AsR′ compounds that have large substituents.


**4. Conclusion**

R′ = SiMe(Si*t*Bu<sup>3</sup>

)2

This study of the effect of substituents on the possibility of the existence of triply bonded

**Table 15.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/

**1.** The theoretical observations provide strong evidence that bonding mode (B) is dominant in the triply bonded RE13≡BiR species, because their structures are bent due to electron transfer (denoted by arrows in **Figure 1**) and the relativistic effect, which increases stability.

**2.** The theoretical evidence shows that both the electronic and the steric effects of substituents are crucial to rendering the E13≡As triple bond synthetically accessible. However, this theo‐ retical study shows that these E13≡As triple bonds are weak. They are not as strong as the traditional C≡C triple bond. The results of this theoretical study show that triply bonded R′E13≡AsR′ molecules that feature bulky substituents are more stable because bulky sub‐ stituents not only protect the central E13≡As triple bond because there is large steric hin‐

RE13≡AsR allows the following conclusions to be drawn (**Scheme 2**):

LANL2DZ+dp level of theory for R′Tl≡AsR′ compounds that have large substituents.

**R′Tl**≡**AsR′ WBI NBO analysis NRT analysis**

π = 1.79 π : 0.3968 Tl

π = 1.94 π : 0.3315

π = 1.97 π : 0.4726

2.15 σ = 1.74 σ : 0.5404 Tl

R′ = Si*i*PrDis2 2.21 σ = 1.90 σ : 0.3627

R′ = NHC 2.11 σ = 1.97 σ : 0.7814 Tl

**Occupancy Hybridization Polarization total/covalent/**

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

29.20% (Tl) 70.80% (As)

15.74% (Tl) 84.26% (As)

13.16% (Tl) 86.84% (As)

10.99% (Tl) 89.01% (As)

61.06% (Tl) 38.94% (As)

22.34% (Tl) 77.66% (As)

(sp1.51) + 0.8414 As (sp2.20)

(sp4.25) + 0.9179 As (sp1.51)

Tl (sp38.20) + 0.9318 As (sp1.44)

Tl (sp99.99) + 0.9435 As (sp1.00)

(sp0.07) + 0.6240 As (sp52.63)

Tl (sp57.71) + 0.8812 As (sp17.96)

**ionic**

**Resonance weight**

69

Tl=As : 57.27% Tl=As : 36.62%

Tl=As : 66.48% Tl=As : 26.51%

Tl=As : 75.51% Tl=As : 17.78%

2.24/1.68/0.56 Tl–As : 6.11%

http://dx.doi.org/10.5772/intechopen.69586

2.16/1.73/0.43 Tl–As : 7.01%

2.14/1.71/0.43 Tl–As : 6.71%

drance but also prohibit polymerization reactions.

1 The natural charge density on the central Tl atom.

2 The natural charge density on the central As atom.

3 BE = E(triplet state for Tl–R′) + E(triplet state for As–R′) − E(singlet state for R′Tl≡AsR′).

4 The Wiberg bond index (WBI) for the Tl–As bond.

5 Δ*H*<sup>1</sup> = E(:Tl=AsR′2 ) − E(R′Tl≡AsR′); see Scheme 2.

6 Δ*H*<sup>2</sup> = E(R′2 Tl=As:) – E(R′Tl≡AsR′); see Scheme 2.

**Table 14.** The geometrical parameters, natural charge densities (*Q*Tl and *Q*As), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Tl≡AsR′ at the B3LYP/LANL2DZ+dp// RHF/3‐21G\* Level of Theory.

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ... http://dx.doi.org/10.5772/intechopen.69586 69


**Table 15.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/ LANL2DZ+dp level of theory for R′Tl≡AsR′ compounds that have large substituents.

#### **4. Conclusion**

**R′ SiMe(Si***t***Bu3**

1 The natural charge density on the central Tl atom. 2 The natural charge density on the central As atom.

4 The Wiberg bond index (WBI) for the Tl–As bond.

) − E(R′Tl≡AsR′); see Scheme 2.

Tl=As:) – E(R′Tl≡AsR′); see Scheme 2.

**Q***Tl*

R′ = SiMe(Si*t*Bu<sup>3</sup>

)2

68 Chemical Reactions in Inorganic Chemistry

*Q*As

5 Δ*H*<sup>1</sup>

6 Δ*H*<sup>2</sup>

= E(:Tl=AsR′2

RHF/3‐21G\* Level of Theory.

= E(R′2

**)2 Si***i***PrDis2 NHC**

**Tl**≡**As (Å)** 2.615 2.565 2.653 ∠**R′–Tl–As (°)** 176.9 177.6 178.7 ∠**Tl–As–R′ (°)** 127.7 121.8 108.0 ∠**R′–Tl–As–R′ (°)** 172.2 170.4 175.2

LANL2DZ+dp level of theory for R′In≡AsR′ compounds that have large substituents.

**R′In**≡**AsR′ WBI NBO analysis NRT analysis**

π = 1.85 π : 0.4411 In

π = 1.83 π : 0.3873

π = 1.94 π : 0.4805

1.50 σ = 1.87 σ : 0.4940 In

R′ = Si*i*PrDis2 1.48 σ = 1.87 σ : 0.4854 In

R′ = NHC 1.33 σ = 1.80 σ : 0.5709 In

**Occupancy Hybridization Polarization total/covalent/**

24.41% (In) 75.59% (As)

19.45% (In) 80.55% (As)

23.56% (In) 76.44% (As)

15.00% (In) 85.00% (As)

32.60% (In) 67.40% (As)

23.09% (In) 76.91% (As)

(sp1.58) + 0.8695 As (sp1.28)

(sp2.80) + 0.8975 As (sp4.33)

(sp1.71) + 0.8743 As (sp1.26)

(sp1.07) + 0.8210 As (sp8.66)

**Table 13.** Selected results for the natural bond orbital (NBO) and natural resonance theory (NRT) analyses at the B3LYP/

In (sp37.19) + 0.8770 As (sp14.95)

In (sp99.99) + 0.9220 As (sp1.00)

**ionic**

**Resonance weight**

In=As :55.2 % In=As : 39.0%

In=As : 56.29% In=As : 37.70%

In=As : 78.30% In=As : 13.98%

2.31/1.55/0.76 In–As : 5.78%

2.18/1.62/0.56 In–As : 6.01%

2.21/1.48/0.73 In–As : 7.72%

<sup>1</sup> 0.310 0.246 0.262

<sup>2</sup> −0.462 −0.440 −0.313 **ΔE**ST **(kcal mol**−1**)**<sup>3</sup> 45.07 32.71 34.83 **Wiberg BO**<sup>4</sup> 2.157 2.214 2.209

**Table 14.** The geometrical parameters, natural charge densities (*Q*Tl and *Q*As), Binding Energies (BE), the HOMO‐LUMO Energy Gaps, the Wiberg Bond Index (WBI), and some reaction enthalpies for R′Tl≡AsR′ at the B3LYP/LANL2DZ+dp//

3 BE = E(triplet state for Tl–R′) + E(triplet state for As–R′) − E(singlet state for R′Tl≡AsR′).

This study of the effect of substituents on the possibility of the existence of triply bonded RE13≡AsR allows the following conclusions to be drawn (**Scheme 2**):


[5] Power PP. Synthesis and some reactivity studies of germanium, tin and lead analogues

The Effect of Substituent on Molecules That Contain a Triple Bond Between Arsenic and Group ...

http://dx.doi.org/10.5772/intechopen.69586

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**Scheme 2.** The predicted structure for the triply bonded RE13≡AsR molecules based on the present theoretical computations.

#### **Acknowledgements**

The authors are grateful to the National Center for High‐Performance Computing of Taiwan in providing huge computing resources to facilitate this research. They also thank the Ministry of Science and Technology of Taiwan for the financial support.

#### **Author details**

Jia‐Syun Lu<sup>1</sup> , Ming‐Chung Yang<sup>1</sup> , Shih‐Hao Su<sup>1</sup> and Ming‐Der Su1,2\*

\*Address all correspondence to: midesu@mail.ncyu.edu.tw

1 Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan

2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan

#### **References**


**Acknowledgements**

70 Chemical Reactions in Inorganic Chemistry

**Author details**

Kaohsiung, Taiwan

**References**

Jia‐Syun Lu<sup>1</sup>

The authors are grateful to the National Center for High‐Performance Computing of Taiwan in providing huge computing resources to facilitate this research. They also thank the Ministry

**Scheme 2.** The predicted structure for the triply bonded RE13≡AsR molecules based on the present theoretical computations.

and Ming‐Der Su1,2\*

, Shih‐Hao Su<sup>1</sup>

1 Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan

2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,

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[4] Power PP. Silicon, germanium, tin and lead analogues of acetylenes. Chemical Comm‐

of Science and Technology of Taiwan for the financial support.

\*Address all correspondence to: midesu@mail.ncyu.edu.tw

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**Chapter 5**

**Provisional chapter**

**Theoretical Investigations of Mechanisms for the**

**Theoretical Investigations of Mechanisms for the** 

DOI: 10.5772/intechopen.75227

The potential energy surfaces for the chemical reactions of group 14 carbenes were studied using density functional theory (B3LYP/LANL2DZ + dp). Five group 14 carbene species containing a seven-member ring, **7-Rea-E**, where E = C, Si, Ge, Sn and Pb, were chosen as model reactants for this work. Three types of chemical reactions (water addition, imine cycloaddition and dimerization) were used to study the reactivity of these **7-Rea-E** molecules. Present theoretical investigations suggest that the relative reactivity of carbenes decreases in the order: **7-Rea-C** > **7-Rea-Si** > **7-Rea-Ge** > **7-Rea-Sn** > **7-Rea-Pb**. That is, the heavier the group 14 atom (E), the more stable its corresponding **7-Rea-E** compound to chemical reaction. This study's theoretical findings suggest that all of the seven-member **7-Rea-E** should be readily synthesized and isolated at room temperature, since they are quite inert to chemical reaction, except for reaction with moisture. Furthermore, the group 14 **7-Rea-E** singlet-triplet energy splitting, as described in the configuration-mixing model of Pross and Shaik, can be used as a diagnostic tool to predict their reactivity. The results obtained allow a number of predictions to be made. **Keywords:** N-heterocyclic carbenes, seven-member carbene, group 14 elements, density

Since the first synthesis and isolation of thermally stable carbenes, imidazol-2-ylidenes, by Arduengo and co-workers [1, 2], the chemistry of N-heterocyclic carbenes (NHCs) has been the subject of intense research and has been studied both experimentally and theoretically during the last two decades [3–23]. Indeed, the isolation of stable NHCs has led to a renaissance for the nucleophilic

functional theory, imine cycloaddition, dimerization, water addition

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Reactions of Seven-Member Ring N-Heterocyclic**

**Reactions of Seven-Member Ring N-Heterocyclic** 

**Carbene and Its Heavier Analogues**

**Carbene and Its Heavier Analogues**

Zheng-Feng Zhang, Ling-Hsuan Liu and

Zheng-Feng Zhang, Ling-Hsuan Liu and

http://dx.doi.org/10.5772/intechopen.75227

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Ming-Der Su

**Abstract**

**1. Introduction**

Ming-Der Su

#### **Theoretical Investigations of Mechanisms for the Reactions of Seven-Member Ring N-Heterocyclic Carbene and Its Heavier Analogues Theoretical Investigations of Mechanisms for the Reactions of Seven-Member Ring N-Heterocyclic Carbene and Its Heavier Analogues**

DOI: 10.5772/intechopen.75227

Zheng-Feng Zhang, Ling-Hsuan Liu and Ming-Der Su Zheng-Feng Zhang, Ling-Hsuan Liu and Ming-Der Su

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75227

#### **Abstract**

The potential energy surfaces for the chemical reactions of group 14 carbenes were studied using density functional theory (B3LYP/LANL2DZ + dp). Five group 14 carbene species containing a seven-member ring, **7-Rea-E**, where E = C, Si, Ge, Sn and Pb, were chosen as model reactants for this work. Three types of chemical reactions (water addition, imine cycloaddition and dimerization) were used to study the reactivity of these **7-Rea-E** molecules. Present theoretical investigations suggest that the relative reactivity of carbenes decreases in the order: **7-Rea-C** > **7-Rea-Si** > **7-Rea-Ge** > **7-Rea-Sn** > **7-Rea-Pb**. That is, the heavier the group 14 atom (E), the more stable its corresponding **7-Rea-E** compound to chemical reaction. This study's theoretical findings suggest that all of the seven-member **7-Rea-E** should be readily synthesized and isolated at room temperature, since they are quite inert to chemical reaction, except for reaction with moisture. Furthermore, the group 14 **7-Rea-E** singlet-triplet energy splitting, as described in the configuration-mixing model of Pross and Shaik, can be used as a diagnostic tool to predict their reactivity. The results obtained allow a number of predictions to be made.

**Keywords:** N-heterocyclic carbenes, seven-member carbene, group 14 elements, density functional theory, imine cycloaddition, dimerization, water addition

#### **1. Introduction**

Since the first synthesis and isolation of thermally stable carbenes, imidazol-2-ylidenes, by Arduengo and co-workers [1, 2], the chemistry of N-heterocyclic carbenes (NHCs) has been the subject of intense research and has been studied both experimentally and theoretically during the last two decades [3–23]. Indeed, the isolation of stable NHCs has led to a renaissance for the nucleophilic

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

carbenes. In particular, NHCs were found to be strong, two-electron σ-donors and to be highly air- and moisture-stable substituents, which allowed the preparation of organometallic catalysts of enormous utility in organic synthesis [24–34]. Nevertheless, although a broad range of geometrical structures of NHCs have been synthesized and structurally characterized, their architectures are still limited to the three- [25], four- [36, 37], five- [35–40], and six-membered rings [41–47].

(2)

77

(3)

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It is hoped that this theoretical study will (i) clarify the reaction mechanism and determine the possible transition-state structures and the relative energetics for Eqs. (1)–(3), (ii) examine the thermodynamics of the seven-member carbene analogues' reactions with various substrate molecules and (iii) establish general trends and predictions for the chemical reactions

Geometries were fully optimized using the B3LYP level of theory, which is implemented by the Gaussian 03 program package [51]. Moreover, the LANLADZ basis sets were utilized in this study. The LANL2DZ basis sets contain the pseudo-relativistic effective core potentials on the group 14 elements modeled, using the double-zeta (DZ) basis sets augmented by a set of d-type polarization functions [52], in which the d exponents used for C, Si, Ge, Sn and Pb were 0.587, 0.296, 0.246, 0.186 and 0.179, respectively. As a result, B3LYP/LANL2DZ + dp represents the B3LYP computations. It is noteworthy that the model reactant (**7-Ring-E**; E = group 14 element) has a total of 604 (180 electrons) basis functions for LANL2DZ + dp sets. Frequency computations were executed on all of the stationary points in order to make sure whether they are the minimum points (no imaginary frequencies) or the transition states (only one imaginary frequency). Therefore, the relative energies were corrected by using the zero-point energies (ZPE, not scaled).

Also, the relative free energies (ΔG) at 298 K were computed at the same level of theory.

In order to verify that the computational method used in this work is suitable for discussion of the potential energy surfaces for the related chemical reactions (Eqs. (1)–(3)), the geometry

(R = 2,4,6-trimethylbenzene)

of seven-member-ring heavy NHCs.

**2. Computational details**

**3. Results and discussion**

**3.1. Geometric structures of 7-Mes**

Recently, because of the elegant studies performed by Cavell, Dervisi, Fallis and many coworkers, thermally stabilized molecules that possess the seven-member ring unit, in which the carbene center is flanked by two nitrogen atoms, have been synthesized and structurally characterized [48–50]; one is 1,3-bis(2,4,6-trimethylphenyl)-4,5,6,7-tetrahydro-3H-[1,3]diazepine (**7-Mes**), a novel seven-member carbene system and the other is the first stable 1,3-bis(2,6-dimethylphenyl)- 4,5,6,7-tetrahydro-3H-[1,3]diazepine (**7-Xyl**) to have been isolated, again by taking advantage of bulky substituent groups. As a result, carbene systems that feature a seven-member ring are no longer imaginary species. However, attempts to isolate other heavy carbene analogues containing the heavy group 14 elements (such as Si, Ge, Sn, and Pb) have all been unsuccessful, until now.

The objective of this work is, therefore, to enlarge upon previous experimental approaches by studying the mechanism and reactivity of seven-member heavy carbene systems, wherein the carbene center (E) is represented by group 14 atoms from carbon to lead. To the best of the authors' knowledge, neither experimental nor theoretical studies have yet been performed on seven-member-rings that possess main group elements, let alone a systematic theoretical study of the elemental effects on the reactivity of such species. In consequence, a study of three kinds of typical reactions for seven-member carbene analogues (E¨ = C, Si, Ge, Sn and Pb), Eqs. (1)–(3), using the density functional theory (DFT), is represented as follows:

Theoretical Investigations of Mechanisms for the Reactions of Seven-Member Ring N… http://dx.doi.org/10.5772/intechopen.75227 77

(R = 2,4,6-trimethylbenzene)

carbenes. In particular, NHCs were found to be strong, two-electron σ-donors and to be highly air- and moisture-stable substituents, which allowed the preparation of organometallic catalysts of enormous utility in organic synthesis [24–34]. Nevertheless, although a broad range of geometrical structures of NHCs have been synthesized and structurally characterized, their architectures are

Recently, because of the elegant studies performed by Cavell, Dervisi, Fallis and many coworkers, thermally stabilized molecules that possess the seven-member ring unit, in which the carbene center is flanked by two nitrogen atoms, have been synthesized and structurally characterized [48–50]; one is 1,3-bis(2,4,6-trimethylphenyl)-4,5,6,7-tetrahydro-3H-[1,3]diazepine (**7-Mes**), a novel seven-member carbene system and the other is the first stable 1,3-bis(2,6-dimethylphenyl)- 4,5,6,7-tetrahydro-3H-[1,3]diazepine (**7-Xyl**) to have been isolated, again by taking advantage of bulky substituent groups. As a result, carbene systems that feature a seven-member ring are no longer imaginary species. However, attempts to isolate other heavy carbene analogues containing the heavy group 14 elements (such as Si, Ge, Sn, and Pb) have all been unsuccessful, until now. The objective of this work is, therefore, to enlarge upon previous experimental approaches by studying the mechanism and reactivity of seven-member heavy carbene systems, wherein the carbene center (E) is represented by group 14 atoms from carbon to lead. To the best of the authors' knowledge, neither experimental nor theoretical studies have yet been performed on seven-member-rings that possess main group elements, let alone a systematic theoretical study of the elemental effects on the reactivity of such species. In consequence, a study of three kinds of typical reactions for seven-member carbene analogues (E¨ = C, Si, Ge, Sn and

Pb), Eqs. (1)–(3), using the density functional theory (DFT), is represented as follows:

still limited to the three- [25], four- [36, 37], five- [35–40], and six-membered rings [41–47].

76 Chemical Reactions in Inorganic Chemistry

It is hoped that this theoretical study will (i) clarify the reaction mechanism and determine the possible transition-state structures and the relative energetics for Eqs. (1)–(3), (ii) examine the thermodynamics of the seven-member carbene analogues' reactions with various substrate molecules and (iii) establish general trends and predictions for the chemical reactions of seven-member-ring heavy NHCs.

#### **2. Computational details**

Geometries were fully optimized using the B3LYP level of theory, which is implemented by the Gaussian 03 program package [51]. Moreover, the LANLADZ basis sets were utilized in this study. The LANL2DZ basis sets contain the pseudo-relativistic effective core potentials on the group 14 elements modeled, using the double-zeta (DZ) basis sets augmented by a set of d-type polarization functions [52], in which the d exponents used for C, Si, Ge, Sn and Pb were 0.587, 0.296, 0.246, 0.186 and 0.179, respectively. As a result, B3LYP/LANL2DZ + dp represents the B3LYP computations. It is noteworthy that the model reactant (**7-Ring-E**; E = group 14 element) has a total of 604 (180 electrons) basis functions for LANL2DZ + dp sets. Frequency computations were executed on all of the stationary points in order to make sure whether they are the minimum points (no imaginary frequencies) or the transition states (only one imaginary frequency). Therefore, the relative energies were corrected by using the zero-point energies (ZPE, not scaled). Also, the relative free energies (ΔG) at 298 K were computed at the same level of theory.

#### **3. Results and discussion**

(1)

#### **3.1. Geometric structures of 7-Mes**

In order to verify that the computational method used in this work is suitable for discussion of the potential energy surfaces for the related chemical reactions (Eqs. (1)–(3)), the geometry of free seven-member N-heterocyclic carbene, **7-Mes** is firstly considered. Since the structure of **7-Mes** has been determined by X-ray crystal analysis, its geometry optimization was first performed, in order to calibrate the B3LYP/LANL2DZ + dp calculations. As shown in **Figure 1**, the calculated average C-N and N-C (ring) distances are 1.356 and 1.493 Å, respectively. These values are very close to those of 1.348 and 1.493 Å, observed for the X-ray crystal structure [48–50]. In addition, the calculated ∠N-C-N and average ∠C-N-C (ring) angles are estimated to be 118.2° and 128.6°, which values are in good agreement with the experimental values (116.6° and 128.8° [48–50], respectively). These results strongly suggest that the B3LYP/ LANL2DZ + dp calculations are reliable enough for the present purpose.

The selected geometrical parameters of **7-Rea-E** for the singlet and triplet states are detailed in **Figure 1** and compared with some available experimental data [48–50]. The calculated molecular orbitals (MOs) for the **7-Rea-E** molecules are given in **Figure 2**. Several important

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79

Firstly, as seen in **Figure 1**, regardless of whether it is a singlet or triplet of neutral **7-Rea-E** compounds, the E▬N bond length shows a monotonic increase in the period, from C toward Pb. For instance, for the singlet neutral group 14 **7-Rea-E**, the E▬N distances increase in the order: **7-Rea-C** (1.356 Å) <**7-Rea-Si** (1.751 Å) <**7-Rea-Ge** (1.867 Å) <**7-Rea-Sn** (2.042 Å) <**7-Rea-Pb** (2.123 Å). The same phenomenon is observed for the triplet neutral group 14 **7-Rea-E**, as given in **Figure 1**. However, irrespective of its multiplicity, the bond angle, ∠NEN, decreases uniformly as the central atom, E, progresses from C to Pb. For example, the bond angle, ∠NEN, decreases in the order: 118.2° (**7-Rea-C**) > 109.9° (**7-Rea-Si**) > 101.5° (**7-Rea-Ge**) > 97.25° (**7-Rea-Sn**) > 93.11° (**7-Rea-Pb**)

**Figure 2.** Calculated frontier molecular orbitals for the **7-Rea-E** (E = C, Si, Ge, Sn, and Pb) species. For more information

see the text.

conclusions can be drawn from these figures.

#### **3.2. Geometric and electronic structures of 7-Rea-E**

Other reactants for seven-member heavy carbene analogues (**7-Rea-E**) were also examined at both singlet and triplet states, which were calculated at the B3LYP/LANL2DZ + dp level.

**Figure 1.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of the reactants **7-Rea-E** (E = C, Si, Ge, Sn, and Pb) at the singlet and triplet states. The experimental values (see ref. [51]) are in parenthesis. Hydrogens are omitted for clarity.

The selected geometrical parameters of **7-Rea-E** for the singlet and triplet states are detailed in **Figure 1** and compared with some available experimental data [48–50]. The calculated molecular orbitals (MOs) for the **7-Rea-E** molecules are given in **Figure 2**. Several important conclusions can be drawn from these figures.

of free seven-member N-heterocyclic carbene, **7-Mes** is firstly considered. Since the structure of **7-Mes** has been determined by X-ray crystal analysis, its geometry optimization was first performed, in order to calibrate the B3LYP/LANL2DZ + dp calculations. As shown in **Figure 1**, the calculated average C-N and N-C (ring) distances are 1.356 and 1.493 Å, respectively. These values are very close to those of 1.348 and 1.493 Å, observed for the X-ray crystal structure [48–50]. In addition, the calculated ∠N-C-N and average ∠C-N-C (ring) angles are estimated to be 118.2° and 128.6°, which values are in good agreement with the experimental values (116.6° and 128.8° [48–50], respectively). These results strongly suggest that the B3LYP/

Other reactants for seven-member heavy carbene analogues (**7-Rea-E**) were also examined at both singlet and triplet states, which were calculated at the B3LYP/LANL2DZ + dp level.

**Figure 1.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of the reactants **7-Rea-E** (E = C, Si, Ge, Sn, and Pb) at the singlet and triplet states. The experimental values (see ref. [51]) are in parenthesis. Hydrogens are omitted for

LANL2DZ + dp calculations are reliable enough for the present purpose.

**3.2. Geometric and electronic structures of 7-Rea-E**

78 Chemical Reactions in Inorganic Chemistry

clarity.

Firstly, as seen in **Figure 1**, regardless of whether it is a singlet or triplet of neutral **7-Rea-E** compounds, the E▬N bond length shows a monotonic increase in the period, from C toward Pb. For instance, for the singlet neutral group 14 **7-Rea-E**, the E▬N distances increase in the order: **7-Rea-C** (1.356 Å) <**7-Rea-Si** (1.751 Å) <**7-Rea-Ge** (1.867 Å) <**7-Rea-Sn** (2.042 Å) <**7-Rea-Pb** (2.123 Å). The same phenomenon is observed for the triplet neutral group 14 **7-Rea-E**, as given in **Figure 1**.

However, irrespective of its multiplicity, the bond angle, ∠NEN, decreases uniformly as the central atom, E, progresses from C to Pb. For example, the bond angle, ∠NEN, decreases in the order: 118.2° (**7-Rea-C**) > 109.9° (**7-Rea-Si**) > 101.5° (**7-Rea-Ge**) > 97.25° (**7-Rea-Sn**) > 93.11° (**7-Rea-Pb**)

**Figure 2.** Calculated frontier molecular orbitals for the **7-Rea-E** (E = C, Si, Ge, Sn, and Pb) species. For more information see the text.

and 123.0° (**7-Rea-C**) > 107.2° (**7-Rea-Si**) > 96.18° (**7-Rea-Ge**) > 89.09° (**7-Rea-Sn**) > 85.64° (**7-Rea-Pb**), at the singlet and triplet states, respectively. Accordingly, it is apparent that the angles at the group 14 elements generally decrease as atomic number increases (from C to Pb). This can be attributed to the relativistic effect [54–57]. As a result, the heavier group 14 elements have a stronger tendency to maintain the (ns)2 (np)3 valence electron configuration. Subsequently, the ∠NEN angle monotonously decreases from **7-Rea-C** to **7-Rea-Pb**, for both singlet and triplet states.

**3.3. Geometries and energetics of 7-Rea-E+ H2**

be drawn from these results.

to O▬H addition.

1

2

3

**E** + H2

tion of a hydrogen atom to the E center.

**System Δ***E***‡2 (kcal mol−1) Δ***H***<sup>3</sup>**

points see **Figure 3**. The Gibbs free energies are given in the square bracket.

The reaction enthalpy of the product, relative to the corresponding reactants.

O) → Transition state (**7-TS-E-1**) → Addition product (**7-pro-E-1**)1

The activation energy of the transition state, relative to the corresponding reactants.

**7-Rea-C** 8.51 [21.1] −51.3 [−41.4] **7-Rea-Si** 21.1 [30.2] −19.4 [−8.96] **7-Rea-Ge** 33.7 [44.2] −12.2 [−0.105] **7-Rea-Sn** 37.0 [48.2] −7.50 [+3.89] **7-Rea-Pb** 56.5 [66.4] +29.0 [+39.0]

**O**

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81

The mechanisms, which proceed via Eq. (1), are now considered, focusing on the transition states, as well as on the addition products themselves. That is, the addition mechanisms may be thought to proceed as follows: reactants (**7-Rea-E**+ **H2O**) → transition state (**7-TS-E-1**) → product (**7-Pro-E-1**). The optimized geometries, calculated at the B3LYP/LANL2DZ + dp level of theory, involving **7-TS-E-1** and **7-Pro-E-1** are detailed in **Figure 3**. The corresponding relative energies at the B3LYP level of theory are given in **Table 1**. There are several important conclusions that can

Firstly, in relation to the O-H bond addition reaction path, the transition state for each **7-Rea-E** case (**7-TS-C-1**, **7-Rea-Si-1**, **7-TS-Ge-1**, **7-TS-Sn-1**, and **7-TS-Pb-1**) has been located at the DFT level of theory. As can be seen in **Figure 3**, all five transition state structures show the same three-center pattern, involving E, oxygen and hydrogen atoms. The transition state vectors indicate an addition process, primarily with O▬H bond stretching, accompanied by migra-

However, in the transition state, there is a trend for the stretching O▬H′ bond to become longer and for the forming E▬H′ bond length to increase, as E increases in atomic weight. For instance, the breaking O▬H′ bond lengths (Å) are 1.241 (**7-TS-C-1**), 1.303 (**7-TS-Si-1**), 1.419 (**7-TS-Ge-1**), 1.496 (**7-TS-Sn-1**) and 1.656 (**7-TS-Pb-1**), respectively, whereas the forming E▬O bond lengths (Å) are 0.682 (**7-TS-C-1**), 0.459 (**7-TS-Si-1**), 0.413 (**7-TS-Ge-1**), 0.284 (**7-TS-Sn-1**) and 0.357 (**7-TS-Pb-1**) longer than that in the (7-Rea-E)(H)(OH) addition product. That is to say, the O-H bond addition happens earlier along the reaction coordinate for **7-Rea-E** with lighter carbene centers. In other words, the E▬H′ and E▬O bond lengths in the transition structure are more product-like for E = Sn and Pb, and more reactant-like for E = C and Si. On the basis of the Hammond postulate [59], **7-TS-Sn-1** and **7-TS-Pb-1** should have the highest activation barriers and **7-TS-C-1** and **7-TS-Si-1** the smallest. Additionally, the barrier height (Gibbs free energy) for the O▬H addition reaction decreases in the order (kcal/mol): **7-TS-Pb-1** (+66) >**7-TS-Sn-1** (+48) >**7-TS-Ge-1** (+44) >**7-TS-Si-1** (+30) >**7-TS-C-1** (+21). These values strongly suggest that the heavier the atomic number of the E center, the greater is the barrier

All were calculated at the B3LYP/LANL2DZ + dp level of theory. The B3LYP optimized structures of the stationary

**Table 1.** Relative energies for the addition reaction of the singlet group 14 **7-Rea-E** species with water: Reactants (**7-Rea-**

.

 **(kcal mol−1)**

Secondly, in order to gain a greater understanding of the nature of the chemical bonding in the series of **7-Rea-E** reactants, the valence molecular orbitals based on the B3LYP/LANL2DZ + dp calculations are represented in **Figure 2**. Basically, there are two important key orbitals, which determine both the chemical and physical properties off the traditional carbene systems, i.e., the nonbonding σ (sp<sup>2</sup> ) orbital and the unoccupied *p*-π orbital, both of which are strongly located on the carbonic center [53].

Note that the nature of the σ and the *p*-π orbitals in **7-Rea-E** are quite similar to those encountered in traditional group 14 divalent compounds [53]. Nevertheless, their energy levels no longer exist on the HOMO and LUMO, respectively, as is already known for the CH2 system [53]. As seen in **Figure 2**, the substitution of a single E atom at the **7-Rea-E** center decreases the energy of the σ orbital, on going from **7-Rea-C** to **7-Rea-Pb**. Likewise, this substitution also decreases the energy of the unoccupied *p*-π orbital. The reason for such molecular orbital locations may be attributed to the "orbital non-hybridization effect", also known as the "inert s-pair effect", as discussed earlier [54–57].

Thirdly, the other interesting feature is the singlet-triplet splitting ΔEst (= Etriplet-Esinglet). From **Figure 2**, it is apparent that the magnitude of the difference in energy between the occupied σ (sp<sup>2</sup> ) and unoccupied *p*-π for the seven-member-ring heavy NHCs becomes larger along the series from **7-Rea-C** to **7-Rea-Pb**. The DFT calculations indicate that the ΔEst for the **7-Rea-E** species increases in the order: 107 (**7-Rea-C**) < 109 (**7-Rea-Si**) < 114 < (**7-Rea-Ge**) < 123 (**7-Rea-Sn**) 124 (**7-Rea-Pb**) kcal/mol, respectively. In other words, the heavier the central group 14 atom (E), the larger is the ΔEst of the seven-member **7-Rea-E** molecule. Again, as discussed earlier, the reason for this difference can be traced directly to electronic factors. In consequence, the theoretical findings reveal that the electronic perturbation effect, wherein the symmetry of frontier orbitals changes, should play a decisive role in determining the energy ordering of the frontier orbitals. This, in turn, affects the magnitude of ΔEst for the seven-member carbene analogues. It should be noted that the stability of the seven-member-ring heavy NHCs is determined by the singlet-triplet energy separations in **7-Rea-E** . If ΔEst is small, the carbene-type structures are unstable and subsequently easily undergo chemical reaction (such as reaction with solvents, etc.). As already mentioned, the B3LYP calculations demonstrate that the group 14 **7-Rea-E** species have comparatively large ΔEst (> 107 kcal/mol). Accordingly, these molecules should be stable enough to be detected experimentally. The supporting evidence comes from the fact that, so far, one NHC compound with a seven-member ring (i.e., **7-Rea-C**) has been synthesized and characterized [48–50]. Also, the synthesis and structure of a dialkylplumbylene with a similar substitution pattern was reported recently in which the lead atom is part of a seven-member ring [58].

Finally, since the DFT calculations indicate that the **7-Rea-E** (E = C, Si, Ge, Sn and Pb) molecules all possess a singlet ground state, then all related chemical reactions (Eqs. (1)–(3)) should proceed on the singlet surface. This study shall, therefore, herewith focus on the singlet surface.

#### **3.3. Geometries and energetics of 7-Rea-E+ H2 O**

and 123.0° (**7-Rea-C**) > 107.2° (**7-Rea-Si**) > 96.18° (**7-Rea-Ge**) > 89.09° (**7-Rea-Sn**) > 85.64° (**7-Rea-Pb**), at the singlet and triplet states, respectively. Accordingly, it is apparent that the angles at the group 14 elements generally decrease as atomic number increases (from C to Pb). This can be attributed to the relativistic effect [54–57]. As a result, the heavier group 14

Subsequently, the ∠NEN angle monotonously decreases from **7-Rea-C** to **7-Rea-Pb**, for both

Secondly, in order to gain a greater understanding of the nature of the chemical bonding in the series of **7-Rea-E** reactants, the valence molecular orbitals based on the B3LYP/LANL2DZ + dp calculations are represented in **Figure 2**. Basically, there are two important key orbitals, which determine both the chemical and physical properties off the traditional carbene systems, i.e.,

Note that the nature of the σ and the *p*-π orbitals in **7-Rea-E** are quite similar to those encountered in traditional group 14 divalent compounds [53]. Nevertheless, their energy levels no longer exist on the HOMO and LUMO, respectively, as is already known for the CH2 system [53]. As seen in **Figure 2**, the substitution of a single E atom at the **7-Rea-E** center decreases the energy of the σ orbital, on going from **7-Rea-C** to **7-Rea-Pb**. Likewise, this substitution also decreases the energy of the unoccupied *p*-π orbital. The reason for such molecular orbital locations may be attributed to the "orbital non-hybridization effect", also known as the "inert

Thirdly, the other interesting feature is the singlet-triplet splitting ΔEst (= Etriplet-Esinglet). From **Figure 2**, it is apparent that the magnitude of the difference in energy between the occupied

tern was reported recently in which the lead atom is part of a seven-member ring [58].

Finally, since the DFT calculations indicate that the **7-Rea-E** (E = C, Si, Ge, Sn and Pb) molecules all possess a singlet ground state, then all related chemical reactions (Eqs. (1)–(3)) should proceed on the singlet surface. This study shall, therefore, herewith focus on the singlet surface.

) and unoccupied *p*-π for the seven-member-ring heavy NHCs becomes larger along the series from **7-Rea-C** to **7-Rea-Pb**. The DFT calculations indicate that the ΔEst for the **7-Rea-E** species increases in the order: 107 (**7-Rea-C**) < 109 (**7-Rea-Si**) < 114 < (**7-Rea-Ge**) < 123 (**7-Rea-Sn**) 124 (**7-Rea-Pb**) kcal/mol, respectively. In other words, the heavier the central group 14 atom (E), the larger is the ΔEst of the seven-member **7-Rea-E** molecule. Again, as discussed earlier, the reason for this difference can be traced directly to electronic factors. In consequence, the theoretical findings reveal that the electronic perturbation effect, wherein the symmetry of frontier orbitals changes, should play a decisive role in determining the energy ordering of the frontier orbitals. This, in turn, affects the magnitude of ΔEst for the seven-member carbene analogues. It should be noted that the stability of the seven-member-ring heavy NHCs is determined by the singlet-triplet energy separations in **7-Rea-E** . If ΔEst is small, the carbene-type structures are unstable and subsequently easily undergo chemical reaction (such as reaction with solvents, etc.). As already mentioned, the B3LYP calculations demonstrate that the group 14 **7-Rea-E** species have comparatively large ΔEst (> 107 kcal/mol). Accordingly, these molecules should be stable enough to be detected experimentally. The supporting evidence comes from the fact that, so far, one NHC compound with a seven-member ring (i.e., **7-Rea-C**) has been synthesized and characterized [48–50]. Also, the synthesis and structure of a dialkylplumbylene with a similar substitution pat-

(np)3

) orbital and the unoccupied *p*-π orbital, both of which are strongly

valence electron configuration.

elements have a stronger tendency to maintain the (ns)2

singlet and triplet states.

80 Chemical Reactions in Inorganic Chemistry

the nonbonding σ (sp<sup>2</sup>

σ (sp<sup>2</sup>

located on the carbonic center [53].

s-pair effect", as discussed earlier [54–57].

The mechanisms, which proceed via Eq. (1), are now considered, focusing on the transition states, as well as on the addition products themselves. That is, the addition mechanisms may be thought to proceed as follows: reactants (**7-Rea-E**+ **H2O**) → transition state (**7-TS-E-1**) → product (**7-Pro-E-1**). The optimized geometries, calculated at the B3LYP/LANL2DZ + dp level of theory, involving **7-TS-E-1** and **7-Pro-E-1** are detailed in **Figure 3**. The corresponding relative energies at the B3LYP level of theory are given in **Table 1**. There are several important conclusions that can be drawn from these results.

Firstly, in relation to the O-H bond addition reaction path, the transition state for each **7-Rea-E** case (**7-TS-C-1**, **7-Rea-Si-1**, **7-TS-Ge-1**, **7-TS-Sn-1**, and **7-TS-Pb-1**) has been located at the DFT level of theory. As can be seen in **Figure 3**, all five transition state structures show the same three-center pattern, involving E, oxygen and hydrogen atoms. The transition state vectors indicate an addition process, primarily with O▬H bond stretching, accompanied by migration of a hydrogen atom to the E center.

However, in the transition state, there is a trend for the stretching O▬H′ bond to become longer and for the forming E▬H′ bond length to increase, as E increases in atomic weight. For instance, the breaking O▬H′ bond lengths (Å) are 1.241 (**7-TS-C-1**), 1.303 (**7-TS-Si-1**), 1.419 (**7-TS-Ge-1**), 1.496 (**7-TS-Sn-1**) and 1.656 (**7-TS-Pb-1**), respectively, whereas the forming E▬O bond lengths (Å) are 0.682 (**7-TS-C-1**), 0.459 (**7-TS-Si-1**), 0.413 (**7-TS-Ge-1**), 0.284 (**7-TS-Sn-1**) and 0.357 (**7-TS-Pb-1**) longer than that in the (7-Rea-E)(H)(OH) addition product. That is to say, the O-H bond addition happens earlier along the reaction coordinate for **7-Rea-E** with lighter carbene centers. In other words, the E▬H′ and E▬O bond lengths in the transition structure are more product-like for E = Sn and Pb, and more reactant-like for E = C and Si. On the basis of the Hammond postulate [59], **7-TS-Sn-1** and **7-TS-Pb-1** should have the highest activation barriers and **7-TS-C-1** and **7-TS-Si-1** the smallest. Additionally, the barrier height (Gibbs free energy) for the O▬H addition reaction decreases in the order (kcal/mol): **7-TS-Pb-1** (+66) >**7-TS-Sn-1** (+48) >**7-TS-Ge-1** (+44) >**7-TS-Si-1** (+30) >**7-TS-C-1** (+21). These values strongly suggest that the heavier the atomic number of the E center, the greater is the barrier to O▬H addition.


1 All were calculated at the B3LYP/LANL2DZ + dp level of theory. The B3LYP optimized structures of the stationary points see **Figure 3**. The Gibbs free energies are given in the square bracket.

2 The activation energy of the transition state, relative to the corresponding reactants.

3 The reaction enthalpy of the product, relative to the corresponding reactants.

**Table 1.** Relative energies for the addition reaction of the singlet group 14 **7-Rea-E** species with water: Reactants (**7-Rea-E** + H2 O) → Transition state (**7-TS-E-1**) → Addition product (**7-pro-E-1**)1 .

seven-member heavy carbene analogues (such as **7-Rea-Sn** and **7-Rea-Pb**) are moisture-stable compounds. Unfortunately, to the best of the authors' knowledge, no experiments relating to the effect of moisture on **7-Rea-E** molecules have been reported. Therefore, this is a prediction. Thirdly, all of the above DFT results can be rationalized on the basis of the configurationmixing (CM) model of Pross and Shaik [60–63]. According to this model, the stabilization of an addition transition state depends on the singlet-triplet splitting, ΔEst (= Etriplet-Esinglet), of the reactant group 14 **7-Rea-E**; i.e., a smaller ΔEst results in a more stable transition state, a lower activation energy, a faster addition reaction and a more exothermic reaction. It is necessary to emphasize the importance of the status of the triplet states of the group 14 **7-Rea-E** reactants. Since two new covalent bonds must be formed in the addition product, 7-Rea-E(OH)(H) (**7-Pro-E-1**), i.e., the E-ΟH and E-H bonds (**Figure 3**), the bond-prepared **7-Rea-E** state thus must have at least two open shells and the lowest state of this type is the triplet state. Therefore, from the standpoint of valence-bonding [60–63], the bonding in the product can be considered to be between the triplet **7-Rea-E** state and the two doublet radicals (overall singlet), the OH radical and the hydrogen atom. This is much the same as bonding in the water molecule.

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83

O is considered to be between two doublet hydrogen atoms and a triplet oxygen

, ΔH) for group 14 seven-member **7-Rea-E** spe-

atom [60–63]. As indicated earlier, the DFT results suggest an increasing trend in ΔEst for the **7-Rea-E** reactant, along the group 14 family. This result is in accordance with the trends in

cies, as already discussed. These results strongly support the previous predictions that the smaller the ΔEst of the group 14 seven-member **7-Rea-E**, the lower is the barrier height and, in turn, the faster is the water addition reaction and the more exothermic is the reaction.

The cycloaddition reactions, which proceed via Eq. (2), are now considered. To ensure consistency with previous work, the following reaction mechanism is used to examine the cycloaddition reaction of group 14 **7-Rea-E** molecules with imine: reactants (**7-Rea-E** + PhHC〓NPh) → transition state (**7-TS-E-2**) → cycloaddition product (**7-Pro-E-2**). The geometries and energetics of the systems (E = C, Si, Ge, Sn and Pb) were calculated using the B3LYP/LANL2DZ + dp level of theory. Selected geometrical parameters and the relative energies of stationary points for the above mechanism are detailed in **Figure 4** and **Table 2**, respectively. The major conclusions to be drawn

Firstly, a seven-member **7-Rea-E** species and imine (PhHC〓NPh) are predicted to undergo a [1 + 2] cycloaddition to produce a cycloaddition product. From **Figure 4**, all of these transition states (i.e., **7-TS-C-2**, **7-TS-Si-2**, **7-TS-Ge-2**, **7-TS-Sn-2** and **7-TS-Pb-2**) proceed in a three-center pattern that involves the carbon, nitrogen and the group 14, E, atoms. The B3LYP computational normal modes associated with the single imaginary frequency are consistent with the C〓N activation process, primarily the C〓N bond stretching with the migration of an E atom to the double bond. It should be noted that these characteristic three-centered cyclic transition states are quite analogous to the mechanisms observed for the cycloaddition reactions of

Secondly, the DFT results demonstrate that the larger the ∠N1EN2 bond angle, the more reactant-like is the transition state structure, as shown in **Figure 4** [54–57]. Comparison of the C1▬N′ bond length in the TS structures with the corresponding distance in imine (1.283 Å)

Namely, H2

singlet carbenes [64].

activation energy and reaction enthalpy (ΔE**‡**

**3.4. Geometries and energetics of 7-Rea-E+ PhHC**〓**NPh**

from the current study can be summarized as follows.

**Figure 3.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of transition states (**7-TS-E-1**), and water addition products (**7-pro-E-1**) of **7-Rea-E** (E = C, Si, Ge, Sn, and Pb). The relative energies for each species see **Table 1**. Hydrogens are omitted for clarity.

Secondly, as **Figure 3** shows, the order of the E-O bond length for the water addition product increases as the atomic weight of the carbene atom, E: **7-Pro-C-1** (1.407 Å) <**7-Pro-Si-1** (1.672 Å) <**7-Pro-Ge-1** (1.780 Å) <**7-Pro-Sn-1** (1.946 Å) <**7-Pro-Pb-1** (2.018 Å). To the authors' best knowledge, experimental structures for such compounds have not yet been identified [48–50]. As already mentioned, a group 14 **7-Rea-E** with a lighter atomic weight carbene E atom reaches the transition state relatively early, whereas one with a carbene atom, E, of a heavier atomic weight arrives relatively late. For example, the order of Gibbs free enthalpy follows the same trend as the activation energy (kcal/mol): **7-Pro-C-1** (−41) <**7-Pro-Si-1** (−9.0) <**7-Pro-Ge-1** (−0.11) <**7-Pro-Sn-1** (+3.9) <**7-Pro-Pb-1** (+39). Note that the free energies of **7-Pro-C-1** and **7-Pro-Si-1** are lower than those of their corresponding starting materials, whereas the free energies of **7-Pro-Sn-1** and **7-Pro-Pb-1** are higher than those of their corresponding starting materials. Consequently, the theoretical findings strongly indicate that the seven-member carbene (**7-Rea-C**) and silylene (**7-Rea-Si**) should be greatly damaged in moist environments, while the seven-member heavy carbene analogues (such as **7-Rea-Sn** and **7-Rea-Pb**) are moisture-stable compounds. Unfortunately, to the best of the authors' knowledge, no experiments relating to the effect of moisture on **7-Rea-E** molecules have been reported. Therefore, this is a prediction.

Thirdly, all of the above DFT results can be rationalized on the basis of the configurationmixing (CM) model of Pross and Shaik [60–63]. According to this model, the stabilization of an addition transition state depends on the singlet-triplet splitting, ΔEst (= Etriplet-Esinglet), of the reactant group 14 **7-Rea-E**; i.e., a smaller ΔEst results in a more stable transition state, a lower activation energy, a faster addition reaction and a more exothermic reaction. It is necessary to emphasize the importance of the status of the triplet states of the group 14 **7-Rea-E** reactants. Since two new covalent bonds must be formed in the addition product, 7-Rea-E(OH)(H) (**7-Pro-E-1**), i.e., the E-ΟH and E-H bonds (**Figure 3**), the bond-prepared **7-Rea-E** state thus must have at least two open shells and the lowest state of this type is the triplet state. Therefore, from the standpoint of valence-bonding [60–63], the bonding in the product can be considered to be between the triplet **7-Rea-E** state and the two doublet radicals (overall singlet), the OH radical and the hydrogen atom. This is much the same as bonding in the water molecule. Namely, H2 O is considered to be between two doublet hydrogen atoms and a triplet oxygen atom [60–63]. As indicated earlier, the DFT results suggest an increasing trend in ΔEst for the **7-Rea-E** reactant, along the group 14 family. This result is in accordance with the trends in activation energy and reaction enthalpy (ΔE**‡** , ΔH) for group 14 seven-member **7-Rea-E** species, as already discussed. These results strongly support the previous predictions that the smaller the ΔEst of the group 14 seven-member **7-Rea-E**, the lower is the barrier height and, in turn, the faster is the water addition reaction and the more exothermic is the reaction.

#### **3.4. Geometries and energetics of 7-Rea-E+ PhHC**〓**NPh**

Secondly, as **Figure 3** shows, the order of the E-O bond length for the water addition product increases as the atomic weight of the carbene atom, E: **7-Pro-C-1** (1.407 Å) <**7-Pro-Si-1** (1.672 Å) <**7-Pro-Ge-1** (1.780 Å) <**7-Pro-Sn-1** (1.946 Å) <**7-Pro-Pb-1** (2.018 Å). To the authors' best knowledge, experimental structures for such compounds have not yet been identified [48–50]. As already mentioned, a group 14 **7-Rea-E** with a lighter atomic weight carbene E atom reaches the transition state relatively early, whereas one with a carbene atom, E, of a heavier atomic weight arrives relatively late. For example, the order of Gibbs free enthalpy follows the same trend as the activation energy (kcal/mol): **7-Pro-C-1** (−41) <**7-Pro-Si-1** (−9.0) <**7-Pro-Ge-1** (−0.11) <**7-Pro-Sn-1** (+3.9) <**7-Pro-Pb-1** (+39). Note that the free energies of **7-Pro-C-1** and **7-Pro-Si-1** are lower than those of their corresponding starting materials, whereas the free energies of **7-Pro-Sn-1** and **7-Pro-Pb-1** are higher than those of their corresponding starting materials. Consequently, the theoretical findings strongly indicate that the seven-member carbene (**7-Rea-C**) and silylene (**7-Rea-Si**) should be greatly damaged in moist environments, while the

**Figure 3.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of transition states (**7-TS-E-1**), and water addition products (**7-pro-E-1**) of **7-Rea-E** (E = C, Si, Ge, Sn, and Pb). The relative energies for each species see **Table 1**. Hydrogens

are omitted for clarity.

82 Chemical Reactions in Inorganic Chemistry

The cycloaddition reactions, which proceed via Eq. (2), are now considered. To ensure consistency with previous work, the following reaction mechanism is used to examine the cycloaddition reaction of group 14 **7-Rea-E** molecules with imine: reactants (**7-Rea-E** + PhHC〓NPh) → transition state (**7-TS-E-2**) → cycloaddition product (**7-Pro-E-2**). The geometries and energetics of the systems (E = C, Si, Ge, Sn and Pb) were calculated using the B3LYP/LANL2DZ + dp level of theory. Selected geometrical parameters and the relative energies of stationary points for the above mechanism are detailed in **Figure 4** and **Table 2**, respectively. The major conclusions to be drawn from the current study can be summarized as follows.

Firstly, a seven-member **7-Rea-E** species and imine (PhHC〓NPh) are predicted to undergo a [1 + 2] cycloaddition to produce a cycloaddition product. From **Figure 4**, all of these transition states (i.e., **7-TS-C-2**, **7-TS-Si-2**, **7-TS-Ge-2**, **7-TS-Sn-2** and **7-TS-Pb-2**) proceed in a three-center pattern that involves the carbon, nitrogen and the group 14, E, atoms. The B3LYP computational normal modes associated with the single imaginary frequency are consistent with the C〓N activation process, primarily the C〓N bond stretching with the migration of an E atom to the double bond. It should be noted that these characteristic three-centered cyclic transition states are quite analogous to the mechanisms observed for the cycloaddition reactions of singlet carbenes [64].

Secondly, the DFT results demonstrate that the larger the ∠N1EN2 bond angle, the more reactant-like is the transition state structure, as shown in **Figure 4** [54–57]. Comparison of the C1▬N′ bond length in the TS structures with the corresponding distance in imine (1.283 Å)

shows that the C1▬N′ bond distance in the transition structures is stretched by 5.9, 9.0, 10, 11 and 14% for **7-TS-C-2**, **7-TS-Si-2**, **7-TS-Ge-2**, **7-TS-Sn-2** and **7-TS-Pb-2**, respectively. All of the theoretical data indicate that the structures of the TSs (**7-TS-E-2**) are as expected from Hammond's postulate [59]. Namely, the [1 + 2] cycloaddition for the seven-member **7-Rea-E** that possesses lighter group 14 elements, E, has lower activation energy, since its TS resembles the corresponding reactants more closely than the final products. From **Table 2**, it is easily seen that the Gibbs free activation energy (kcal/mol) of the **7-TS-E-2** follows the same trend as the ΔEst in the seven-member **7-Rea-E** system, i.e., **7-TS-C-2** (+15) <**7-TS-Si-2** (+19) <**7-TS-Ge-2** (+27) <**7-TS-Sn-2** (+30) <**7-TS-Pb-2** (+52). That is to say, according to the CM model [60–63], it is predicted that seven-member **7-Rea-E** molecules with a lighter atomic weight E atom have a larger ∠N1EN2 bond angle, a smaller ΔEst and experience easier cycloaddition to imine.

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Thirdly, as can be seen in **Table 3**, the Gibbs free energy of the final cycloproducts, relative to their corresponding reactants, increases in the order: **7-Pro-C-2** (−21) <**7-Pro-Si-2** (+11) <**7-Pro-Ge-2** (+12) <**7-Pro-Sn-2** (+25) <**7-Pro-Pb-2** (+53), indicating that the reactions of sevenmember **7-Rea-E** molecules with heavier group 14 atoms (E) are endothermic. It should also be noted that the order of the reaction enthalpy follows the same trend as the ∠N1EN2 bond angle and the ΔEst. In other words, the model calculations demonstrate that the values of ΔEst can be used as a criterion to predict the reactivity of seven-member **7-Rea-E**

The dimerization reaction of the seven-member **7-Rea-E** molecule was examined in this work in order to understand more about their kinetic stability. Selected geometrical parameters along the pathway, shown in Eq. (3) and computed at the B3LYP/LANL2DZ + dp level, are collected in **Figure 5**. The relative energies at the same level of theory are listed in **Table 3**.

Firstly, as predicted, a double bond between the two group 14 E atoms should be formed during the dimerization reaction of two **7-Rea-E** molecules. However, repeated attempts to determine the transition state for the concerted dimerization of two **7-Rea-E** compounds, using the B3LYP/LANL2DZ + dp level of theory, are always unsuccessful. That is to say, no transition states exist on the B3LYP surface for such dimerization reactions. Further, the B3LYP calculations demonstrate that all five dimers (i.e., **7-Pro-C-3**, **7-Pro-Si-3**, **7-Pro-Ge-3**, **7-Pro-Sn-3** and **7-Pro-Pb-3**) do not have the imaginary frequency. Therefore, they can be considered as true minima on the potential energy surfaces. As stated earlier, unfortunately, because of a lack of experimental and data on such species, geometrical information studied in this work should be

Secondly, as demonstrated in **Figure 4**, the E〓E bond length in the dimer molecule was calculated to decrease in the order: 1.436 Å (**7-Pro-C-3**) < 2.491 Å (**7-Pro-Si-3**) < 2.602 Å (**7-Pro-Ge-3**) < 2.941 Å (**7-Pro-Sn-3**) < 3.493 Å (**7-Pro-Pb-3**), which correlates with the atomic size of the main group 14 element, E, as it changes from carbon to lead. It must be stressed that the computed E = E double bond lengths are somewhat larger than the experimental values (1.356 [65, 66], 2.139–2.360 [67, 68], 2.212–2.509 [69, 70], 2.601–2.961 [71, 72] and 2.990–3.537 [73, 74] Å, for C=C, Si = Si, Ge = Ge, Sn = Sn and Pb = Pb, respectively). The former

**3.5. Geometries and electronic structures of dimerization reactions**

**Figure 5** and **Table 3** have several noteworthy features.

considered as predictions for future investigations.

compounds.

**Figure 4.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of transition states (**7-TS-E-2**), and cycloaddition products (**7-pro-E-2**) of **7-Rea-E** (E = C, Si, Ge, Sn, and Pb) and imine. The relative energies for each species see **Table 2**. Hydrogens are omitted for clarity.


1 All were calculated at the B3LYP/LANL2DZ + dp level of theory. The B3LYP optimized structures of the stationary points see **Figure 4**. The Gibbs free energies are given in the square bracket.

2 The activation energy of the transition state, relative to the corresponding reactants.

3 The reaction enthalpy of the product, relative to the corresponding reactants.

**Table 2.** Relative energies for the cycloaddition reaction of the singlet group 14 **7-Rea-E** species with imine: Reactants (**7-Rea-E** + PhHC=NPh) → transition state (**7-TS-E-2**) → addition product (**7-pro-E-2**) 1 .

shows that the C1▬N′ bond distance in the transition structures is stretched by 5.9, 9.0, 10, 11 and 14% for **7-TS-C-2**, **7-TS-Si-2**, **7-TS-Ge-2**, **7-TS-Sn-2** and **7-TS-Pb-2**, respectively. All of the theoretical data indicate that the structures of the TSs (**7-TS-E-2**) are as expected from Hammond's postulate [59]. Namely, the [1 + 2] cycloaddition for the seven-member **7-Rea-E** that possesses lighter group 14 elements, E, has lower activation energy, since its TS resembles the corresponding reactants more closely than the final products. From **Table 2**, it is easily seen that the Gibbs free activation energy (kcal/mol) of the **7-TS-E-2** follows the same trend as the ΔEst in the seven-member **7-Rea-E** system, i.e., **7-TS-C-2** (+15) <**7-TS-Si-2** (+19) <**7-TS-Ge-2** (+27) <**7-TS-Sn-2** (+30) <**7-TS-Pb-2** (+52). That is to say, according to the CM model [60–63], it is predicted that seven-member **7-Rea-E** molecules with a lighter atomic weight E atom have a larger ∠N1EN2 bond angle, a smaller ΔEst and experience easier cycloaddition to imine.

Thirdly, as can be seen in **Table 3**, the Gibbs free energy of the final cycloproducts, relative to their corresponding reactants, increases in the order: **7-Pro-C-2** (−21) <**7-Pro-Si-2** (+11) <**7-Pro-Ge-2** (+12) <**7-Pro-Sn-2** (+25) <**7-Pro-Pb-2** (+53), indicating that the reactions of sevenmember **7-Rea-E** molecules with heavier group 14 atoms (E) are endothermic. It should also be noted that the order of the reaction enthalpy follows the same trend as the ∠N1EN2 bond angle and the ΔEst. In other words, the model calculations demonstrate that the values of ΔEst can be used as a criterion to predict the reactivity of seven-member **7-Rea-E** compounds.

#### **3.5. Geometries and electronic structures of dimerization reactions**

**Figure 4.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of transition states (**7-TS-E-2**), and cycloaddition products (**7-pro-E-2**) of **7-Rea-E** (E = C, Si, Ge, Sn, and Pb) and imine. The relative energies for each species see **Table 2**.

All were calculated at the B3LYP/LANL2DZ + dp level of theory. The B3LYP optimized structures of the stationary

**Table 2.** Relative energies for the cycloaddition reaction of the singlet group 14 **7-Rea-E** species with imine: Reactants

 **(kcal mol−1)**

1 .

**System Δ***E***‡2 (kcal mol−1) Δ***H***<sup>3</sup>**

points see **Figure 4**. The Gibbs free energies are given in the square bracket.

The reaction enthalpy of the product, relative to the corresponding reactants.

The activation energy of the transition state, relative to the corresponding reactants.

(**7-Rea-E** + PhHC=NPh) → transition state (**7-TS-E-2**) → addition product (**7-pro-E-2**)

**7-Rea-C** 9.62 [14.9] −27.0 [−20.5] **7-Rea-Si** 12.4 [18.9] +4.85 [+11.2] **7-Rea-Ge** 20.9 [26.7] +5.10 [+12.1] **7-Rea-Sn** 25.5 [30.1] +19.6 [+25.2] **7-Rea-Pb** 47.5 [51.7] +48.3 [+52.8]

Hydrogens are omitted for clarity.

84 Chemical Reactions in Inorganic Chemistry

1

2

3

The dimerization reaction of the seven-member **7-Rea-E** molecule was examined in this work in order to understand more about their kinetic stability. Selected geometrical parameters along the pathway, shown in Eq. (3) and computed at the B3LYP/LANL2DZ + dp level, are collected in **Figure 5**. The relative energies at the same level of theory are listed in **Table 3**. **Figure 5** and **Table 3** have several noteworthy features.

Firstly, as predicted, a double bond between the two group 14 E atoms should be formed during the dimerization reaction of two **7-Rea-E** molecules. However, repeated attempts to determine the transition state for the concerted dimerization of two **7-Rea-E** compounds, using the B3LYP/LANL2DZ + dp level of theory, are always unsuccessful. That is to say, no transition states exist on the B3LYP surface for such dimerization reactions. Further, the B3LYP calculations demonstrate that all five dimers (i.e., **7-Pro-C-3**, **7-Pro-Si-3**, **7-Pro-Ge-3**, **7-Pro-Sn-3** and **7-Pro-Pb-3**) do not have the imaginary frequency. Therefore, they can be considered as true minima on the potential energy surfaces. As stated earlier, unfortunately, because of a lack of experimental and data on such species, geometrical information studied in this work should be considered as predictions for future investigations.

Secondly, as demonstrated in **Figure 4**, the E〓E bond length in the dimer molecule was calculated to decrease in the order: 1.436 Å (**7-Pro-C-3**) < 2.491 Å (**7-Pro-Si-3**) < 2.602 Å (**7-Pro-Ge-3**) < 2.941 Å (**7-Pro-Sn-3**) < 3.493 Å (**7-Pro-Pb-3**), which correlates with the atomic size of the main group 14 element, E, as it changes from carbon to lead. It must be stressed that the computed E = E double bond lengths are somewhat larger than the experimental values (1.356 [65, 66], 2.139–2.360 [67, 68], 2.212–2.509 [69, 70], 2.601–2.961 [71, 72] and 2.990–3.537 [73, 74] Å, for C=C, Si = Si, Ge = Ge, Sn = Sn and Pb = Pb, respectively). The former

computed values might be attributable to steric effects, which cause the dimer formed by seven-member N-heterocyclic carbene analogues to have a longer double bond distance than the less substituted dimetallene itself. However, the DFT calculations show that the energy of the final product (dimers), relative to its corresponding reactants, is 33 (**7-Pro-C-3**), 38 (**7-Pro-Si-3**), 43 (**7-Pro-Ge-3**), 49 (**7-Pro-Sn-3**) and 54 (**7-Pro-Pb-3**) kcal/mol. Also, as seen in **Table 3**, the difference in the values of ΔG between the reactants and the dimer are 46, 58.4, 55, 57 and 68.2 kcal/mol, for carbon, silicon, germanium, tin and lead species, respectively. Consequently, the computational results predict that the dimerization reaction should not occur during the formation of the seven-member **7-Rea-E** (E = C, Si, Ge, Sn and Pb) species

Theoretical Investigations of Mechanisms for the Reactions of Seven-Member Ring N…

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87

This study uses the B3LYP level of theory to study the mechanisms and reactivity for three types of chemical reactions of carbene analogues, **7-Rea-E** (E = C, Si, Ge, Sn and Pb), which possess a seven-member-ring. It should be noted that this study provides the first theoretical demonstration of the reaction trajectory and the first theoretical estimation of the activation energy and the reaction enthalpy for these chemical processes. In relation to the aforementioned five seven-member heavy carbine systems and their related chemical reactions, as

**1.** With regard to both the activation barrier and the reaction enthalpy, based on the model calculations presented here, it is concluded that the seven-member **7-Rea-E** reactivity order is as follows: **7-Rea-C** > **7-Rea-Si** > **7-Rea-Ge** > **7-Rea-Sn** > **7-Rea-Pb**. In other words, a **7-Rea-E** molecule possessing a group 14 atom, E, of lower atomic number accelerates chemical reactions, whereas a **7-Rea-E** compound bearing a group 14 atom, E, of higher atomic number hinders reactions. From this conclusion, it is predicted that the heavy carbene analogues with a seven-membered ring can be synthesized and isolated. Indeed, it was recently reported that the stable carbon(II) and lead(II) compounds bearing a seven-membered ring, the so-called dialkylcarbene (**7-Rea-C**) and dialkylplumbylene, have been synthesized and

**2.** This theoretical work suggests that seven-member cyclic **7-Rea-E** compounds that feature a lighter group 14 element, E (such as **7-Rea-C** and **7-Rea-Si**) should be moisture-sensitive, whereas the seven-member heavy carbene analogues (such as **7-Rea-Sn** and **7-Rea-Pb**) are

**3.** This theoretical work reveals that the [1 + 2] imine cycloadditions of seven-member **7-Rea-E** molecules produces the final cycloadduct in a concerted manner. That is, these cycloadditions proceed stereospecifically and result in cycloproducts that retain their

**4.** These theoretical investigations strongly suggest that the dimerization reaction should not occur during the formation of the seven-member **7-Rea-E** (E = C, Si, Ge, Sn and Pb) species. Indeed, this theoretical conclusion is in good agreement with the available experimental

studied in this paper, the following conclusions can be drawn:

structurally characterized [48–50, 58].

moisture-stable compounds.

stereochemistry.

observations [48–50].

at room temperature.

**4. Conclusion**

**Figure 5.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of the dimer products of **7-Rea-E**(E = C, Si, Ge, Sn, and Pb). The relative energies for each species see **Table 3**. Hydrogens are omitted for clarity.


1 All were calculated at the B3LYP/LANL2DZ + dp level of theory. The B3LYP optimized structures of the stationary points see **Figure 5**.

2 The reaction enthalpy of the product, relative to the corresponding reactants.

3 The Gibbs free enthalpy of the product, relative to the corresponding reactants.

**Table 3.** Relative energies for the dimerization reaction of the singlet group 14 **7-Rea-E** species: Reactants (2 **7-Rea-E**) → dimerization product (**7-pro-E-3**) 1 .

computed values might be attributable to steric effects, which cause the dimer formed by seven-member N-heterocyclic carbene analogues to have a longer double bond distance than the less substituted dimetallene itself. However, the DFT calculations show that the energy of the final product (dimers), relative to its corresponding reactants, is 33 (**7-Pro-C-3**), 38 (**7-Pro-Si-3**), 43 (**7-Pro-Ge-3**), 49 (**7-Pro-Sn-3**) and 54 (**7-Pro-Pb-3**) kcal/mol. Also, as seen in **Table 3**, the difference in the values of ΔG between the reactants and the dimer are 46, 58.4, 55, 57 and 68.2 kcal/mol, for carbon, silicon, germanium, tin and lead species, respectively. Consequently, the computational results predict that the dimerization reaction should not occur during the formation of the seven-member **7-Rea-E** (E = C, Si, Ge, Sn and Pb) species at room temperature.

#### **4. Conclusion**

**Figure 5.** B3LYP/LANL2DZ + dp optimized geometries (in Å and deg) of the dimer products of **7-Rea-E**(E = C, Si, Ge, Sn,

All were calculated at the B3LYP/LANL2DZ + dp level of theory. The B3LYP optimized structures of the stationary

**Table 3.** Relative energies for the dimerization reaction of the singlet group 14 **7-Rea-E** species: Reactants (2 **7-Rea-E**)

 **(kcal mol−1)**

 **(kcal mol−1) Δ***G***<sup>3</sup>**

and Pb). The relative energies for each species see **Table 3**. Hydrogens are omitted for clarity.

**7-Rea-C** +9.31 +16.3 **7-Rea-Si** +13.1 +21.0 **7-Rea-Ge** +20.7 +28.7 **7-Rea-Sn** +23.0 +31.2 **7-Rea-Pb** +44.1 +56.0

The reaction enthalpy of the product, relative to the corresponding reactants.

1 .

The Gibbs free enthalpy of the product, relative to the corresponding reactants.

**System Δ***H***<sup>2</sup>**

86 Chemical Reactions in Inorganic Chemistry

→ dimerization product (**7-pro-E-3**)

1

2

3

points see **Figure 5**.

This study uses the B3LYP level of theory to study the mechanisms and reactivity for three types of chemical reactions of carbene analogues, **7-Rea-E** (E = C, Si, Ge, Sn and Pb), which possess a seven-member-ring. It should be noted that this study provides the first theoretical demonstration of the reaction trajectory and the first theoretical estimation of the activation energy and the reaction enthalpy for these chemical processes. In relation to the aforementioned five seven-member heavy carbine systems and their related chemical reactions, as studied in this paper, the following conclusions can be drawn:


**5.** This study shows that knowledge of the singlet-triplet splitting (ΔEst) of the seven-member carbene analogues **7-Rea-E** is of great importance in achieving a deeper understanding of their reactivity, since it is the driving force for the related reactions.

[6] Krahulic KE, Tuononen HM, Parvez M, Roesler R. Isolation of free Phenylide-like Carbanions with N-heterocyclic Carbene frameworks. Journal of the American Chemical

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89

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**6.** This study demonstrates that the heavier the atomic weight of group 14 elements, E, involved in the seven-member **7-Rea-E**, the larger is the ΔEst, the higher is the activation barrier and the smaller is the enthalpy of its final product. Consequently, based on these theoretical conclusions, it is easily predicted that the heavier seven-member **7-Rea-E** (such as E = Si, Ge, Sn and Pb) should be readily synthesized and isolated at room temperature.

#### **Acknowledgements**

The authors are grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time and to the Ministry of Science and Technology of Taiwan for the financial support.

#### **Author details**

Zheng-Feng Zhang1 , Ling-Hsuan Liu1 and Ming-Der Su1,2\*

\*Address all correspondence to: midesu@mail.ncyu.edu.tw

1 Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan

2 Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan

#### **References**


[6] Krahulic KE, Tuononen HM, Parvez M, Roesler R. Isolation of free Phenylide-like Carbanions with N-heterocyclic Carbene frameworks. Journal of the American Chemical Society. 2009;**131**:5858-5865

**5.** This study shows that knowledge of the singlet-triplet splitting (ΔEst) of the seven-member carbene analogues **7-Rea-E** is of great importance in achieving a deeper understanding

**6.** This study demonstrates that the heavier the atomic weight of group 14 elements, E, involved in the seven-member **7-Rea-E**, the larger is the ΔEst, the higher is the activation barrier and the smaller is the enthalpy of its final product. Consequently, based on these theoretical conclusions, it is easily predicted that the heavier seven-member **7-Rea-E** (such as E = Si, Ge, Sn and Pb) should be readily synthesized and isolated at room temperature.

The authors are grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time and to the Ministry of Science and Technology of

and Ming-Der Su1,2\*

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C(CH2 )]2


## *Edited by Saravanan Chandraleka*

The book "Chemical Reactions in Inorganic Chemistry" describes an overview of chemical reagents used in inorganic chemical reactions for the synthesis of different compounds including coordination, transition metal, organometallic, cluster, bioinorganic, and solid-state compounds. This book will be helpful for the graduate students, teachers, and researchers, and chemistry professionals who are interested to fortify and expand their knowledge about sol-gel preparation and application, porphyrin and phthalocyanine, carbon nanotube nanohybrids, triple bond between arsenic and group 13 elements, and N-heterocyclic carbene and its heavier analogues. It comprises a total of five chapters from multiple contributors around the world including China, India, and Taiwan.

Published in London, UK © 2018 IntechOpen © Nikolay Tsuguliev / iStock

Chemical Reactions in Inorganic Chemistry

Chemical Reactions in

Inorganic Chemistry