**Physicochemical Properties and Catalytic Applications of Iron Porphyrazines and Phthalocyanines**

Tomasz Koczorowski, Wojciech Szczolko and Tomasz Goslinski

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/68071

#### **Abstract**

Porphyrazines and phthalocyanines belong to porphyrinoids, which are macrocyclic compounds consisting of four pyrrole or indole rings, respectively. The aromatic rings of porphyrazines and phthalocyanines are fused together by azamethine bridges (meso nitrogen atoms) in place of methine bridges present in porphyrins. The physicochemical properties of these macrocycles can be modified in two ways. The first is by substitu‐ tion of metal cation in the core, whereas the second relies on peripheral modification with various substituents. Porphyrazines and phthalocyanines can be modified inside the macrocyclic core with various transition metal cations, including iron(II/III), which impacts their electrochemical properties and influences potential applications in redox reactions. Due to their unique optical and electrochemical properties, porphyrazines and phthalocyanines found many potential and practical applications in medicine and technology. They were mainly researched as photosensitizers in photodynamic therapy, as sensors in biomedical and analytical applications or as building blocks for materials chemistry. This chapter presents physicochemical properties and catalytic applications of iron porphyrazines and phthalocyanines. The first part summarizes the influence of peripheral and axial substituents of iron(II/III) porphyrazines and phthalocyanines on their spectral properties, whereas the second focuses on the electrochemical properties of these molecules. The third part covers the activity of selected iron(II/III) porphyr‐ azines and phthalocyanines of potential value for diverse applications including cata‐ lytic reactions.

**Keywords:** catalytic properties, electrochemistry, iron, porphyrazines, phthalocyanines

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## **1. Introduction to porphyrinoids**

Porphyrinoids are macrocyclic compounds consisting of four pyrrole rings usually linked together through methine or azamethine bridges. Porphyrins (Ps) are planar and aromatic with nominally 22π‐electrons of which 18π‐electrons are engaged in a conjugative path. Porphyrins can be substituted at the peripheral β and methine meso positions. Chlorins and bacterichlorins possess similar structure to porphyrins, and they are defined as dihydro or tetrahydro derivatives of porphyrins. Corrole macrocycle constitutes an 18π‐electron system with the characteristic feature being the lack of a methine bridge between the A and D pyr‐ role rings. In addition, corolles carry inside the macrocyclic core three NH protons, which is different from porphyrins and chlorins, which carry two NH protons. It is worth noting that structurally related corrins with the key natural product cobalamine (vitamin B12) are not aromatic and contain only one NH proton inside the macrocyclic core. Porphyrazines (Pzs) and phthalocyanines (Pcs) commonly known as tetraazaporphyrins belong to synthetic por‐ phyrinoids. Methine bridges are replaced by azamethine (with nitrogen atoms) as the most notable feature of their structure. In addition, Pcs are tetrabenzo tetraazaporphyrins, which have annulated benzene rings in comparison with the Pz core. Formally, in phthalocyanines, unlike in porphyrazines, the azamethine bridges combine four indole instead of pyrrole rings, respectively (**Figure 1**). The derivatives of Pcs can be obtained by substitution of fused benzo‐ rings at peripheral (2,3 or β) and nonperipheral positions (1,4 or α). The tetraazaporphyrins possess unique physicochemical properties due to the presence of a conjugated system of

**Figure 1.** Structures of porphyrins (a), chlorins (b), corroles (c), porphyrazines (d), and phthalocyanines (e).

π electrons, bulky periphery and an ability to coordinate various metal cations inside macro‐ cyclic core. The chelation reaction of the inner NH protons with various metal ions leads to metal chelates, which is a common feature for all porphyrinoids [1–6].

**1. Introduction to porphyrinoids**

102 Recent Progress in Organometallic Chemistry

Porphyrinoids are macrocyclic compounds consisting of four pyrrole rings usually linked together through methine or azamethine bridges. Porphyrins (Ps) are planar and aromatic with nominally 22π‐electrons of which 18π‐electrons are engaged in a conjugative path. Porphyrins can be substituted at the peripheral β and methine meso positions. Chlorins and bacterichlorins possess similar structure to porphyrins, and they are defined as dihydro or tetrahydro derivatives of porphyrins. Corrole macrocycle constitutes an 18π‐electron system with the characteristic feature being the lack of a methine bridge between the A and D pyr‐ role rings. In addition, corolles carry inside the macrocyclic core three NH protons, which is different from porphyrins and chlorins, which carry two NH protons. It is worth noting that structurally related corrins with the key natural product cobalamine (vitamin B12) are not aromatic and contain only one NH proton inside the macrocyclic core. Porphyrazines (Pzs) and phthalocyanines (Pcs) commonly known as tetraazaporphyrins belong to synthetic por‐ phyrinoids. Methine bridges are replaced by azamethine (with nitrogen atoms) as the most notable feature of their structure. In addition, Pcs are tetrabenzo tetraazaporphyrins, which have annulated benzene rings in comparison with the Pz core. Formally, in phthalocyanines, unlike in porphyrazines, the azamethine bridges combine four indole instead of pyrrole rings, respectively (**Figure 1**). The derivatives of Pcs can be obtained by substitution of fused benzo‐ rings at peripheral (2,3 or β) and nonperipheral positions (1,4 or α). The tetraazaporphyrins possess unique physicochemical properties due to the presence of a conjugated system of

**Figure 1.** Structures of porphyrins (a), chlorins (b), corroles (c), porphyrazines (d), and phthalocyanines (e).

Many natural porphyrins and chlorins, for example, heme in hemoglobin and chlorophyll, reveal various vital functions and are responsible for many biochemical processes. Nowadays, porphyrinoids possess potential applications in science and technology, for example, syn‐ thetic derivatives of porphyrins as well as phthalocyanines found many applications in the dye industry and revealed potential for medicine (photodynamic therapy and photodynamic diagnosis) and technology (artificial enzymes, catalysts). Many tetraazaporphyrins have also been studied as analytical indicators, structural elements in materials chemistry, in optical data drivers and microchips, as well as photovoltaic cells. In addition, there is a possibility to utilize porphyrazines and phthalocyanines as catalysts in various organic synthesis reac‐ tions due to their ability to coordinate transition metal cations inside macrocyclic core or in the periphery. Lately, tetraazaporphyrins have also been considered as building blocks in nanotechnology due to their self‐assembly and self‐organization ability (**Figure 2**) [3, 6–19].

Tetraazaporphyrins can be modified using two approaches. The first relies on the introduc‐ tion of various alkyl and/or aryl substituents with sulfur, nitrogen or oxygen atoms into por‐ phyrazine β peripheral positions of pyrrole rings and phthalocyanine α nonperipheral and/ or β peripheral positions of indole rings. The second concerns removal or exchange of cen‐ tral metal cation present in macrocyclic core. By using all of these modification approaches, there is a possibility to obtain macrocyclic compounds of altered physicochemical properties, for example, extended thermal and photochemical stability, increased solubility in organic solvents, improved luminescence and spectroscopic, magnetic, electrochemical properties, photoconductivity and surface activity [1, 2]. Incorporation of iron(II/III) cations into the por‐ phyrinoid core allowed the application of these compounds as catalysts in redox reactions. There is a great interest in the catalytic properties of iron(II/III) tetraazaporphyrins, which dates back to the 1980s. These macrocyclic systems were considered as potential electron

**Figure 2.** The main practical and potential applications of tetraazaporphyrins, M—metal ion.

and/or molecule carriers. For example, iron(III) octaphenylporphyrazine pyridine adduct developed by Stuzhin was found to be a molecular oxygen carrier [20]. Theoretical calcula‐ tions using density functional theory (DFT) and experimental studies indicated that there are significant differences between metalated tetraazaporphyrins and porphyrins. The dif‐ ference in the core size and shape of the macrocycle has a substantial effect on the electronic structure and properties of the overall system. DFT calculations indicated on differences in bond lengths between pyrrole/indole nitrogen atoms and coordinated iron(II) cation in por‐ phyrins, phthalocyanines and porphyrazines, which were 1.98; 1.93 and 1.90 Å, respectively. The smaller coordination cavity results in a stronger ligand field in Pzs than in porphyrins. However, the benzo annulation in phthalocyanines produces a surprisingly strong destabiliz‐ ing effect on the metal‐macrocycle bonding [21, 22]. The calculations also showed how the dif‐ ferences in porphyrinoid (Ps, Pcs and Pzs) structures influence the axial ligand coordination of pyridine and CO to the iron(II) complexes [22].
