Preface

The Schiff base is a well-known organic compound category and a typical organic ligand for metal complexes in both organic and inorganic chemistry. However, development in material and life sciences using Schiff base compounds is still ongoing. This book provides a comprehensive overview of Schiff base chemistry.

Each chapter presents various viewpoints of Schiff base in current developments: overview and synthesis of organic chemistry; metal complexes with structures, aromatic ligand, bioactive core, cytotoxic activity as inorganic chemistry, chemosensors for heavy metal ions and catalytic applications as applied chemistry. The information summarized in this book will be useful for all readers who are interested in Schiff base and their metal complexes.

From where is such a variety? Indeed, imine or azomethine (C∙N) of Schiff base is merely a functional group, which is usually synthesized from aldehyde and (primary) amine. However, introducing extended groups into aldehyde or amine is easy. Therefore, as a lone pair donating group (Lewis base) coordinating with a metal ion to yield metal complexes, Schiff base (ligands) and their metal complexes can obtain different functions or properties. For example, azobenzene containing salen-type metal complexes have been used as photo-functional dyes dispersing into polymer films. Besides coordination to metal ions as a rigid chelate ligand, introducing an azobenzene group and strong pi-conjugated planar moiety plays an essential role in polarized light-induced control of the molecular orientation, which was also investigated with DFT calculation theoretically. As for material science use, the stability of Schiff base ligands has advantages. We used redox active Schiff base metal complexes (potentially exhibiting catalytic function) for electron transfer mediator to a metalloenzyme. In this case, the reactivity of the Schiff base metal complex plays an important role. As for catalytic or biochemical reactions towards assisting life science, reactivity should be considered during molecular design.

At glance, stability and reactivity are opposite requirements, though Schiff base (ligands) can realize both by molecular design. Thus, Schiff base compounds have wide applications and are at the frontier of chemistry.

> **Takashiro Akitsu** Department of Chemistry, Tokyo University of Science, Tokyo, Japan

**1**

**Chapter 1**

**Abstract**

**1. Introduction**

*Shilpa Laxman Sangle*

Introduction to Schiff Base

synthesis, catalysts, pigments and dyes, polymer stabilizers, etc.

many important catalysts, such as Jacobsen's catalyst (**Figure 1**).

296), which is key in the photoreception mechanism.

metal complexes, formation mechanism of Schiff base

When any primary amine reacts with an aldehyde or a ketone under specific conditions, Schiff bases are formed. The general structure of Schiff base is R2C∙NR′ and is considered as a subclass of imines which is commonly used as a synonym for azomethine. The first imines were prepared in the nineteenth century by a classical method that involves condensation of a carbonyl compound with the help of amine under the distillation of azeotropic and to remove water formed in the system, molecular sieves are used. Later many ways of synthesis of Schiff bases are invented. Schiff bases exhibit a wide range of biological activities and are commonly used for industrial purposes. These are the most widely used as intermediates in organic

**Keywords:** Schiff bases, biological activity of Schiff bases, azomethine, Schiff base

In the year 1864, Hugo Schiff was the first to synthesize Schiff's base under azeotropic distillation by using aldehyde or ketone and primary amine. They can be considered a sub-class of imines with the general structure R1R2C∙NR′ (R′ ≠ H) [1–5]. Depending on their structure, they can be considered as either secondary aldimines or secondary ketimines. When these compounds are being used as ligands to form coordination complexes with metal ions, the term Schiff base is applied. Corrin complexes occur naturally, but the majority of artificial Schiff bases are used to form

Schiff bases are imines in which R3 is an alkyl or aryl group (not hydrogen). R1 and R2 may be hydrogen. Schiff bases have a wide range of biological properties such as antimicrobial, anticancer, and antiviral. Inhibition of amyloid-β aggregation is achieved by Schiff bases [6]. They are common enzymatic intermediates where an aldehyde or ketone of a cofactor or substrate reversibly reacts with the terminal group of a lysine residue. Lysine residue forms a Schiff base with the common enzyme cofactor pyridoxal phosphate (PLP) and is transaldiminated to the substrate(s) [7]. Similarly, the cofactor retinal forms a Schiff base in human rhodopsin (via Lysine

In coordination chemistry, Schiff bases are common ligands. The ligands are derived from aromatic aldehydes and alkyl diamines [8]. The imine nitrogen is
