**3. Catalytic activity and electrochemical properties of iron porphyrazines and phthalocyanines**

The presence of iron(II/III) cation in the coordination center of porphyrazine and phtha‐ locyanine macrocycles determines the possibility of using them as catalysts of the oxida‐ tion‐reduction reactions. Research studies carried out for several years showed that the iron tetraazaporphyrins are efficient catalysts as compared to the structurally similar porphyrin compounds. Porphyrinoid catalysts, also called biomimetic catalysts, are also more effec‐ tive in carrying out the oxidation reactions of organic compounds, in comparison with other catalysts. It is related to the increased influence of the electron‐donor effect of the ferric cat‐ ion, which is conjugated to the π‐electron system of the macrocyclic ring. The advantages of iron(II/III) porphyrazines and phthalocyanines as catalysts include high selectivity, mild and environmentally friendly reaction conditions and low energy consumption during cataly‐ sis [41]. In an early 1990s, Fitzgerald et al. provided various studies indicating significant differences in physicochemical properties of Ps, Pcs and Pzs possessing the same periph‐ eral substituents and iron(III) cation inside a macrocyclic core [42]. It was suggested that porphyrazines are stronger σ‐donors and π‐acceptors than porphyrins. The electrochemical studies indicated that similarly to phthalocyanines, porphyrazines have positively shifted redox potential of 400 mV in comparison with their porphyrin analogues. Moreover, Pzs are more soluble in organic solvents than structurally relevant Pcs and can split the d orbitals of coordinated metal to a greater extent than Ps. In conclusion, it was suggested that, due to the high solubility in organic solvents, accompanied by coordination of metal ions with unusual spin states, and positively shifted redox potentials, Pzs can be considered as more efficient catalysts in comparison with Ps and Pcs [42]. Taking all this into account, iron(II/III) tetraaza‐ porphyrins became an object of intense studies aimed at obtaining macrostructures with increased catalytic abilities. For instance, large structures like porphyrin‐phthalocyanine pen‐ tads composed of five fused macrocyclic compounds **12** were even synthesized by Kobayashi et al. (**Figure 6**) [43]. Noteworthy is that the bimetallic Fe‐Cu complexes, like metal‐linked face‐to‐face porphyrazine dimer **13** with an increased strong metal‐metal spin coupling, were obtained by Barrett, Hoffman and their coworkers [44].

Iron(II/III) porphyrazines and phthalocyanines are active in redox reactions and, therefore, reveal high electrochemical activity. This feature was confirmed by cyclic voltammetry (CV) and square wave voltammetry (SWV) studies, which show, in most cases, four reversible or quasi‐reversible oxidation and reduction peaks. The origin of the two peaks was attrib‐ uted to reactions associated with the presence of iron cation, whereas the other two are the

pentyloxy substituents in the periphery, which results in its high solubility in organic sol‐ vents. The investigation of the redox properties of **9** in electrochemical study indicated that its oxidized and reduced mixed‐valence complexes were stable [38]. Another of two tetraazapor‐ phyrin macrocycles conjugated peripherally by 4,4′‐[1,1′‐methylenebis‐(naphtalene‐2,1‐diyl)] bis(oxy)diphthalo‐nitrile linker constitutes homonuclear (Fe‐Fe) **10** and heteronuclear (Fe‐Cu) **11** ball‐type compounds (**Figure 5**). In both compounds, two phthalocyanine units were rig‐ idly bound at two sides with four linking arms to form intramolecular cofacial coupling in which the splitting of the classical monophthalocyanine redox processes was observed in a cyclic voltammetry study. Moreover, the nature of the metal centers affected the distance between two Pc units in both ball‐type complexes and thus the extent of the mentioned intra‐

**Figure 5.** Structure of side‐by‐side dimer **9** and homonuclear and heteronulear dimers **10** and **11**.

**3. Catalytic activity and electrochemical properties of iron porphyrazines** 

The presence of iron(II/III) cation in the coordination center of porphyrazine and phtha‐ locyanine macrocycles determines the possibility of using them as catalysts of the oxida‐ tion‐reduction reactions. Research studies carried out for several years showed that the iron tetraazaporphyrins are efficient catalysts as compared to the structurally similar porphyrin compounds. Porphyrinoid catalysts, also called biomimetic catalysts, are also more effec‐ tive in carrying out the oxidation reactions of organic compounds, in comparison with other catalysts. It is related to the increased influence of the electron‐donor effect of the ferric cat‐ ion, which is conjugated to the π‐electron system of the macrocyclic ring. The advantages of iron(II/III) porphyrazines and phthalocyanines as catalysts include high selectivity, mild and environmentally friendly reaction conditions and low energy consumption during cataly‐ sis [41]. In an early 1990s, Fitzgerald et al. provided various studies indicating significant differences in physicochemical properties of Ps, Pcs and Pzs possessing the same periph‐ eral substituents and iron(III) cation inside a macrocyclic core [42]. It was suggested that porphyrazines are stronger σ‐donors and π‐acceptors than porphyrins. The electrochemical

molecular cofacial interactions between them [39, 40].

**and phthalocyanines**

108 Recent Progress in Organometallic Chemistry

**Figure 6.** Structures of pentad **12**, bimetallic complex **13**, iron(II) phthalocyanine **14** and porphyrazine **15**, iron(II) naphthalocyanine **16**, iron(III) porphyrazine **17** and iron(III) phthalocyanines **18** and **19**.

result of the electronic processes within the macrocyclic ring [45]. However, there are some exceptions to this rule. For example, in the CV study performed in organic solvents for iron(II) phthalocyanine **14** (**Figure 6**), the presence of six oxidation‐reduction processes was observed. Two of them were identified as reversible and irreversible reductions, while the other two were found to be quasi‐reversible oxidation reactions [46]. In contrast to other iron porphyrazines, cyclic voltammetry study of water‐soluble iron(II) porphyrazine **15** revealed no peaks observed for the FeII/FeI couple, probably as a result of slow heterogeneous electron transfer kinetics for this couple. Moreover, replacing of water in the coordination sphere of **15** by N‐donor ligands increases the extinction coefficients of both the Q and Soret bands with a blue shift of the first one [47].

Electrochemical properties of iron(II/III) porphyrazine and phthalocyanine complexes are influenced by the periphery of the macrocycle, which can lead to an increase or a decrease of their electrochemical activity. An increase in activity is related to the presence of the periph‐ eral substituents with lone pairs of electrons or π‐electron systems, which are able to increase the coupling of electrons around the macrocycle. The decrease in activity is observed in the presence of electrochemically inactive substituents, for example, *tert*‐butyl groups, as was found for iron(II) naphthalocyanine **16** (**Figure 6**) [48].

An axial coordination of molecules to the central metal ion can cause a shift of the oxidation potential of the macrocycle or a split of peaks belonging to oxidation process. The rationale for this may be connected with the coordination of solvent molecules to Fe3+ cation in the center of the oxidized macrocyclic compound [49]. It is known from the literature that there are dif‐ ferences in the values of oxidation‐reduction potentials of iron phthalocyanines, when one or two solvent molecules are attached or released from the iron(II/III) cation [50].

Electrochemical studies with iron porphyrazines and phthalocyanines were also carried out using the modified electrodes with tetraazaporphyrins deposited on their surface. One example is the use of iron(III) porphyrazine **17** (**Figure 6**) as an azide and nitrate(III) anions sensor [51]. This porphyrazine was deposited onto a matrix with PVC, and the selective elec‐ trode membrane revealed a shorter electrode response time, greater tolerance within a wide range of pH at high analyte concentration and a high selectivity toward the targeted ions [51]. Another example is iron(III) phthalocyanine with four peripheral coumarin **18** or chromone **19** substituents (**Figure 6**) deposited by the electropolymerization on the Pt‐working elec‐ trodes and applied as electron mediators in the electrocatalytic oxidation of nitrates(III) [52].

Studies concerning catalytic properties of iron porphyrazines and phthalocyanines have been conducted over the last 20 years, and they concerned mostly the potential applications in oxidation reactions of linear and cyclic alkenes as well as photocatalytic degradation of organic dyes. However, unsubstituted iron(III) phthalocyanine was widely used to catalyze the reaction of both the incorporation of amino substituents and the hydroxylation of aryl and alkyl molecules [53, 54]. Moreover, this compound was also used as a catalyst in the oxygen reduction reactions and revealed good stability for potential use in fuel cells or bat‐ teries [55, 56]. Some studies assessed the ability of iron(II/III) tetraazaporphyrins and their dimers in decomposition and removal of organic pollutants from industrial wastes. So far the most successfully applied photocatalytic reaction was the degradation of Rhodamine B, which was considered as a model compound in studies on environmental contamination with organic substances. The most commonly used catalysts applied were symmetrical iron sul‐ fanylporphyrazines **20**, **21** and **22**, as well as unsymmetrical **23** (**Figure 7**). They were used either as homogeneous or heterogeneous catalysts, after deposition on the carrier, which was very often ion exchange resin (e.g., Amberlite CG400). It was shown that the deposition of the catalyst on this kind of support increases the efficiency of catalytic reactions due to an increase in the concentration of catalysts on the resin surface. Therefore, the molecules of the substrate are present in high concentration in the vicinity of the catalyst, facilitating the oxidation and reduction reactions. In addition, it was shown that the iron(II) porphyrazine—resin system is active even in the dark, without activation of macrocycle by irradiation with proper wave‐ length [57]. Noteworthy, some oxygen atom donors (OAD) like molecular oxygen and hydro‐ gen peroxide were added to the reaction mixture to form an active oxygen species, which allowed the oxidative degradation of substrates. The highest reaction rate and yield were achieved with H2 O2 [58]. It was also noticed that the modification of peripheral groups of the macrocycle by introducing hydroxymethyl substituents demonstrated an increased solubility of the catalyst in water and in organic solvents [41]. In order to improve the catalytic proper‐ ties, the structure of the macrocycle was equipped with electron donor methyl moieties [59].

result of the electronic processes within the macrocyclic ring [45]. However, there are some exceptions to this rule. For example, in the CV study performed in organic solvents for iron(II) phthalocyanine **14** (**Figure 6**), the presence of six oxidation‐reduction processes was observed. Two of them were identified as reversible and irreversible reductions, while the other two were found to be quasi‐reversible oxidation reactions [46]. In contrast to other iron porphyrazines, cyclic voltammetry study of water‐soluble iron(II) porphyrazine **15** revealed

transfer kinetics for this couple. Moreover, replacing of water in the coordination sphere of **15** by N‐donor ligands increases the extinction coefficients of both the Q and Soret bands

Electrochemical properties of iron(II/III) porphyrazine and phthalocyanine complexes are influenced by the periphery of the macrocycle, which can lead to an increase or a decrease of their electrochemical activity. An increase in activity is related to the presence of the periph‐ eral substituents with lone pairs of electrons or π‐electron systems, which are able to increase the coupling of electrons around the macrocycle. The decrease in activity is observed in the presence of electrochemically inactive substituents, for example, *tert*‐butyl groups, as was

An axial coordination of molecules to the central metal ion can cause a shift of the oxidation potential of the macrocycle or a split of peaks belonging to oxidation process. The rationale for this may be connected with the coordination of solvent molecules to Fe3+ cation in the center of the oxidized macrocyclic compound [49]. It is known from the literature that there are dif‐ ferences in the values of oxidation‐reduction potentials of iron phthalocyanines, when one or

Electrochemical studies with iron porphyrazines and phthalocyanines were also carried out using the modified electrodes with tetraazaporphyrins deposited on their surface. One example is the use of iron(III) porphyrazine **17** (**Figure 6**) as an azide and nitrate(III) anions sensor [51]. This porphyrazine was deposited onto a matrix with PVC, and the selective elec‐ trode membrane revealed a shorter electrode response time, greater tolerance within a wide range of pH at high analyte concentration and a high selectivity toward the targeted ions [51]. Another example is iron(III) phthalocyanine with four peripheral coumarin **18** or chromone **19** substituents (**Figure 6**) deposited by the electropolymerization on the Pt‐working elec‐ trodes and applied as electron mediators in the electrocatalytic oxidation of nitrates(III) [52]. Studies concerning catalytic properties of iron porphyrazines and phthalocyanines have been conducted over the last 20 years, and they concerned mostly the potential applications in oxidation reactions of linear and cyclic alkenes as well as photocatalytic degradation of organic dyes. However, unsubstituted iron(III) phthalocyanine was widely used to catalyze the reaction of both the incorporation of amino substituents and the hydroxylation of aryl and alkyl molecules [53, 54]. Moreover, this compound was also used as a catalyst in the oxygen reduction reactions and revealed good stability for potential use in fuel cells or bat‐ teries [55, 56]. Some studies assessed the ability of iron(II/III) tetraazaporphyrins and their dimers in decomposition and removal of organic pollutants from industrial wastes. So far the most successfully applied photocatalytic reaction was the degradation of Rhodamine B,

two solvent molecules are attached or released from the iron(II/III) cation [50].

couple, probably as a result of slow heterogeneous electron

no peaks observed for the FeII/FeI

110 Recent Progress in Organometallic Chemistry

with a blue shift of the first one [47].

found for iron(II) naphthalocyanine **16** (**Figure 6**) [48].

Another sulfur iron(II) porphyrazine **24** (**Figure 7**) was used in the catalytic oxidation reac‐ tion of organic compound, that is, X3B dye (*Reactive Brilliant Red*). The reaction was provided with hydrogen peroxide as OAD and with simultaneous exposure to light. The catalyst was active in a broad temperature and pH range, with the best yield at pH = 2, and at the higher

**Figure 7.** Chemical structure of iron(II) sulfanylporphyrazines **20**–**24**.

temperature. Lower catalytic ability was observed in the absence of light. It has been shown that the use of H2 O2 as a source of oxygen resulted in the production of hydroxyl radicals as reactive oxygen species responsible for degradation of the substrate in the catalytic reaction. This process was called catalytic wet hydrogen peroxide oxidation (CWPO) and has potential for treatment of waste water [60]. As a result of research by Su et al. [61] and Theodoridis et al. [62], it was found that for the application of H2 O2 as a commonly used oxygen atom donor in oxidation reactions catalyzed by iron(II) porphyrazine results in the hydroperoxo complex of iron(III) Pz formation. According to the reaction conditions there are two competitive redox routes: heterolysis (involving transfer of 2 electrons) and homolysis (1 electron transfer) lead‐ ing to O–O bond cleavage (**Figure 8**). At acidic conditions, proton is utilized in heterolysis of O–O bond and transient high‐valence iron‐centered oxidizing species Pz●+FeIV=O is generated as the reactive oxygen species (ROS). Of note is that the electronic structure of N4‐ligand complexes allows for the stabilization of transient high‐valent intermediates. For this reason, high‐valence state iron species are often identified as ROS in biomimetic catalysis. On the other hand, homolysis of O–O bond in neutral and alkaline pH conditions leads to hydroxyl radical species formation as ROS, whereas the hydroperoxo complex of porphyrazine is trans‐ formed to porphyrazine radical FeIII=O● [61].

Another important objective for the application of iron phthalocyanines and porphyrazines as catalysts of the oxidation reactions of organic compounds is their use in chemical synthesis, which leads to new derivatives without using classical synthetic routes. The presence of Fe(II/ III) tetraazaporphyrins with the use of suitable oxygen donors permits one or two electron oxidation reactions. As the result, various derivatives containing epoxy groups or hydroxyl, carbonyl and carboxyl substituents can be obtained. In the study aiming to assess catalytic properties of iron porphyrazines and phthalocyanines, cyclohexane was considered as a reference compound. In various studies, there were applied iron(II) phthalocyanine deriva‐ tives **25**–**27** and iron(III) phthalocyanine **28** as catalysts (**Figure 9**) and also diverse sources of oxygen: *tert*‐butylhydroperoxide (TBHP), hydrogen peroxide, chloro‐peroxybenzoic acid (m‐CPBA), molecular oxygen and oxone. It was concluded that with an increase in tempera‐ ture and catalyst concentration, an increase in reaction yield was observed. The source of

**Figure 8.** The heterolytic and homolytic mechanisms of O–O bond cleavage in the hydroperoxo complex of iron(III) porphyrazine following [61].

Physicochemical Properties and Catalytic Applications of Iron Porphyrazines and Phthalocyanines http://dx.doi.org/10.5772/68071 113

**Figure 9.** Structures of iron(II/III) phthalocyanine derivatives **25**–**30**, and cyclic tetramer **31**.

temperature. Lower catalytic ability was observed in the absence of light. It has been shown

reactive oxygen species responsible for degradation of the substrate in the catalytic reaction. This process was called catalytic wet hydrogen peroxide oxidation (CWPO) and has potential for treatment of waste water [60]. As a result of research by Su et al. [61] and Theodoridis et al.

O2

oxidation reactions catalyzed by iron(II) porphyrazine results in the hydroperoxo complex of iron(III) Pz formation. According to the reaction conditions there are two competitive redox routes: heterolysis (involving transfer of 2 electrons) and homolysis (1 electron transfer) lead‐ ing to O–O bond cleavage (**Figure 8**). At acidic conditions, proton is utilized in heterolysis of O–O bond and transient high‐valence iron‐centered oxidizing species Pz●+FeIV=O is generated as the reactive oxygen species (ROS). Of note is that the electronic structure of N4‐ligand complexes allows for the stabilization of transient high‐valent intermediates. For this reason, high‐valence state iron species are often identified as ROS in biomimetic catalysis. On the other hand, homolysis of O–O bond in neutral and alkaline pH conditions leads to hydroxyl radical species formation as ROS, whereas the hydroperoxo complex of porphyrazine is trans‐

Another important objective for the application of iron phthalocyanines and porphyrazines as catalysts of the oxidation reactions of organic compounds is their use in chemical synthesis, which leads to new derivatives without using classical synthetic routes. The presence of Fe(II/ III) tetraazaporphyrins with the use of suitable oxygen donors permits one or two electron oxidation reactions. As the result, various derivatives containing epoxy groups or hydroxyl, carbonyl and carboxyl substituents can be obtained. In the study aiming to assess catalytic properties of iron porphyrazines and phthalocyanines, cyclohexane was considered as a reference compound. In various studies, there were applied iron(II) phthalocyanine deriva‐ tives **25**–**27** and iron(III) phthalocyanine **28** as catalysts (**Figure 9**) and also diverse sources of oxygen: *tert*‐butylhydroperoxide (TBHP), hydrogen peroxide, chloro‐peroxybenzoic acid (m‐CPBA), molecular oxygen and oxone. It was concluded that with an increase in tempera‐ ture and catalyst concentration, an increase in reaction yield was observed. The source of

**Figure 8.** The heterolytic and homolytic mechanisms of O–O bond cleavage in the hydroperoxo complex of iron(III)

as a source of oxygen resulted in the production of hydroxyl radicals as

as a commonly used oxygen atom donor in

that the use of H2

O2

112 Recent Progress in Organometallic Chemistry

[62], it was found that for the application of H2

formed to porphyrazine radical FeIII=O● [61].

porphyrazine following [61].

oxygen influenced the structures of obtained products. In all experiments monitored by the UV‐Vis spectra, the degradation of the catalysts, manifested by a decrease of their Q‐band absorption, was observed. However, in most cases, the catalytic processes proceeded, even after catalyst degradation, which can indicate the influence of other oxidation mechanisms, for example, Fenton reaction [63–66].

Lately performed study with iron(II) phthalocyanine, **29**, **30**, and the cyclic tetramer consist‐ ing of iron(III) phthalocyanine linked to 3,4,9,10‐perylenetetracarboxylate, tentatively named FePPOP **31** (**Figure 9**) indicate the possibility to use these compounds toward catalytic oxida‐ tion reaction of benzyl alcohol. In the case of tetramer, it was found that **31** exhibited high stability and also exhibited the large turnover number of the reaction reaching the value of 960 [67–69]. However, in the initial phase of the experiment, the catalyst **31** acted much slower in comparison with the other compounds [69]. In addition, heterogeneous catalysts consisting of unsubstituted iron(II) phthalocyanines deposited on the electrodes by electropolymeriza‐ tion were applied in oxidation reactions of organic compounds. Such reactions were carried out in two kinds of systems: (i) phenolic resin/unsubstituted iron(II) phthalocyanine or (ii) the phenolic resin/structurally branched iron(II) phthalocyanine [70, 71]. In another study, various thiol derivatives (e.g., 2‐mercaptoethanol) were applied as substrates in electrooxida‐ tion reaction toward disulfides. Transparent indium tin oxide electrodes were modified with iron(II) tetraaminophthalocyanine [72, 73].

#### **4. Summary**

The iron(II/III) porphyrazines and phthalocyanines have interesting electrochemical proper‐ ties, which were demonstrated in many valuable studies performed during the last 30 years. Moreover, many applications of these macrocycles were presented in medicine, in biomedical and analytical fields, in materials chemistry as well as in chemical synthesis. It clarifies why catalytic abilities of iron(II/III) tetraazaporphyrins became an object of intense studies. This chapter aimed to summarize the influence of peripheral substituents of iron(II/III) porphyr‐ azines and phthalocyanines on their spectral and electrochemical properties. Electrochemical properties of iron(II/III) porphyrazine and phthalocyanine complexes are significantly influ‐ enced by the periphery of the macrocycle, which can lead to an increase or a decrease of their electrochemical activity. Similarly, an axial coordination of molecules to the central metal ion causes a shift of the oxidation potential of the macrocycle or splits peaks belonging to oxidation processes. Selected studies on iron(II/III) porphyrazines and phthalocyanines were found not only to present their interesting physicochemical features but also further perspec‐ tive applications, and thus, they were discussed in more detail. What is of immense value for further applications of these molecules in materials chemistry and nanotechnology is that some macrocycles demonstrated an ability to form coordination assemblies alone or with nanostructures, including fullerenes, and molecular wires. Especially interesting are binu‐ clear complexes based on iron(II/III) porphyrazine and phthalocyanine bridged by oxygen, nitrogen or carbon atoms. Interesting modification of classical redox processes was observed in novel potential molecular quantum‐dot cellular automata cells in which phthalocyanines were connected "side‐by‐side" or by forming ball‐type dimers in which there were utilized sophisticated linkers binding two phthalocyanine units at two sides rigidly with four linking arms. Porphyrinoid catalysts also have the designation by biomimetic catalysts, this being because they are more effective in carrying out the oxidation reactions of organic compounds to other catalysts. It is related to the increased electron‐donor effect of the ferric cation, which is conjugated to the π‐electron system of the macrocyclic ring. The advantages of iron(II/III) porphyrazines and phthalocyanines as catalysts include high selectivity, mild and environ‐ mentally friendly reaction conditions and low energy consumption during catalysis. Studies of catalytic properties or iron(II/III) Pzs and Pcs concerned mostly with their potential applica‐ tions in oxidation reactions of linear and cyclic alkenes as well as photocatalytic degradation of organic dyes. Some studies assessed the ability of iron(II/III) tetraazaporphyrins and their dimers in decomposition and removal of organic pollutants from industrial wastes. A huge area for further application of these macrocycles results from the electrochemical studies in which iron Pzs and Pcs were deposited on the surface of electrodes and further applied as selective anions sensors. To sum up, iron(II/III) tetraazaporphyrins appear to present many interesting perspectives for biomedical and technological applications.
