**2. Physicochemical properties of iron porphyrazines and phthalocyanines**

## **2.1. Low solubility and tendency to form aggregates hampering utilization**

Applications of porphyrazines and phthalocyanines in science and technology are limited by their low solubility in water and organic solvents and their tendency to form aggregates. These unwelcome features are the result of their likelihood to molecular interactions based on π‐π stacking. Unfavorable common feature of iron(II/III) porphyrazines and phthalocyanines to form aggregates is mainly related to their conjugated, extended system of π‐electrons and an ability of iron cation to coordinate compounds with heteroatoms. Annulation of a porphyrazine macrocyclic system with four benzene rings leads to a phthalocyanine with four indole rings of enhanced aggregation properties. Most of the iron(II/III) tetraazaporphyrins form two types of aggregates: *J*‐type aggregates (*head to tail*) or *H*‐type aggregates (*face to face*) [23, 24]. In addition, it was also observed that an incorporation of bulky substituents into iron(II) porphyrazine ring, which resulted in the separation of molecules at the distance of ca. 11 Å in the X‐ray structure, did not prevent its tendency to form aggregates in solution [25].

Generally, the unsubstituted tetraazaporphyrins possess low solubility. For this reason, the most effective method applied for increasing their solubility is peripheral functionalization. Peripheral functionalization of these compounds with ester groups is able to increase their sol‐ ubility in many organic solvents. For example, magnesium(II) porphyrazine with 4‐hydroxy‐ butylthio substituents was subjected to esterification reaction with 4‐biphenylcarboxylic acid and further metalated with Fe2+ salt toward **1** (**Figure 3**). Unlike the parent demetalated por‐ phyrazine, the metalated porphyrazines functionalized with eight ester groups were soluble in common organic solvents, such as chloroform, dichloromethane, tetrahydrofuran, acetone, and toluene and were insoluble in water and *n*‐hexane [23].

Peripheral functionalization of iron(II/III) porphyrazines and phthalocyanines with halo‐ gen electron withdrawing groups (like ‐F or ‐CF<sup>3</sup> ) was found to improve their solubility in polar solvents like methanol or ethanol, ionization potential and their stability in catalytic

Physicochemical Properties and Catalytic Applications of Iron Porphyrazines and Phthalocyanines http://dx.doi.org/10.5772/68071 105

**Figure 3.** Structures of porphyrazine **1** and phthalocyanines **2** and **3**.

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

**2. Physicochemical properties of iron porphyrazines and phthalocyanines**

Applications of porphyrazines and phthalocyanines in science and technology are limited by their low solubility in water and organic solvents and their tendency to form aggregates. These unwelcome features are the result of their likelihood to molecular interactions based on π‐π stacking. Unfavorable common feature of iron(II/III) porphyrazines and phthalocyanines to form aggregates is mainly related to their conjugated, extended system of π‐electrons and an ability of iron cation to coordinate compounds with heteroatoms. Annulation of a porphyrazine macrocyclic system with four benzene rings leads to a phthalocyanine with four indole rings of enhanced aggregation properties. Most of the iron(II/III) tetraazaporphyrins form two types of aggregates: *J*‐type aggregates (*head to tail*) or *H*‐type aggregates (*face to face*) [23, 24]. In addition, it was also observed that an incorporation of bulky substituents into iron(II) porphyrazine ring, which resulted in the separation of molecules at the distance of ca. 11 Å in the X‐ray structure,

Generally, the unsubstituted tetraazaporphyrins possess low solubility. For this reason, the most effective method applied for increasing their solubility is peripheral functionalization. Peripheral functionalization of these compounds with ester groups is able to increase their sol‐ ubility in many organic solvents. For example, magnesium(II) porphyrazine with 4‐hydroxy‐ butylthio substituents was subjected to esterification reaction with 4‐biphenylcarboxylic acid and further metalated with Fe2+ salt toward **1** (**Figure 3**). Unlike the parent demetalated por‐ phyrazine, the metalated porphyrazines functionalized with eight ester groups were soluble in common organic solvents, such as chloroform, dichloromethane, tetrahydrofuran, acetone,

Peripheral functionalization of iron(II/III) porphyrazines and phthalocyanines with halo‐

polar solvents like methanol or ethanol, ionization potential and their stability in catalytic

) was found to improve their solubility in

**2.1. Low solubility and tendency to form aggregates hampering utilization**

did not prevent its tendency to form aggregates in solution [25].

and toluene and were insoluble in water and *n*‐hexane [23].

gen electron withdrawing groups (like ‐F or ‐CF<sup>3</sup>

of pyridine and CO to the iron(II) complexes [22].

104 Recent Progress in Organometallic Chemistry

oxidation reactions [24, 26]. For example, iron(II) phthalocyanine with peripheral 4‐fluoro‐ phenoxy groups **2** (**Figure 3**), which was synthesized using microwave‐assisted cyclotetra‐ merization (much faster in comparison to classical method), revealed very good solubility in various organic solvents, thus allowing solvatochromic measurements. Unfortunately, this phthalocyanine formed *H*‐aggregates as compared to demetalated phthalocyanine and other metalated phthalocyanines (CuPc, Li<sup>2</sup> Pc), especially in polar solvents [24].

Over the years, many methods have been developed in order to obtain soluble tetraazapor‐ phyrins and to utilize them in aqueous media. This was a big challenge because as presented above these macrocycles are known for their aggregation properties. For this reason, a study was performed aiming to incorporate macrocycles into larger structures like β‐cyclodextrines (β‐CDs). For example, such complexes of β‐CDs and iron(II) phthalocyanine **3** with peripheral quaternary pyridinium salt substituents were obtained in Kobayashi group (**Figure 3**) [27]. It is worth noting that the quaternization reaction was performed to enhance water solubil‐ ity. The addition of β‐CD to the aqueous solution of **3** disturbed the monomer‐dimer equi‐ librium, as was seen by an increase in the monomer band and a decrease in the dimer band absorbance in the UV‐Vis spectrum. This inclusion complex was deposited on a glassy carbon electrode. In the cyclic voltammetry studies in aqueous solution, **3** revealed very good ability toward oxygen reduction reaction (ORR) at low potential values. It indicates that the "host" compound (β‐cyclodextrine) has no influence on physicochemical properties of the "guest" compound (macrocycle).

#### **2.2. Advanced physicochemical features**

Iron cation coordinated inside a macrocyclic core of porphyrazines and phthalocyanines can be involved in redox reactions and influence their electrochemical properties. By changing the valence of central iron(II/III) metal cation in tetraazaporphyrins, it is possible to transfer electrons on diverse molecules. This feature concerns also axially coordinated compounds, which form enhanced complexes and can be divided into two types. To the first group belong small ions or molecules with heteroatoms in their structure, like pyridine, pyrazine or hydroxyl and bisulfate anions. The obtained complexes are formally named as the axial complexes. To the second group belong dimers with single atom bridging groups between two iron macrocycles. In both cases, the obtained molecules have modified optical and elec‐ trochemical properties [28, 29].

A coordination of iron cation with proper ligand results in the formation of five‐ or six‐coordi‐ nated macrocyclic complexes, which were subjected to broad study by Stuzhin et al. [28]. The coordination of iron(II) tetraazaporphyrins with axial ligands leads to the oxidation of iron(II) to iron(III). In this way, iron(II) octaphenylporphyrazine coordinated axially with F− , Cl− , Br<sup>−</sup> , I− and HSO4 − anions was transformed to five‐coordinated iron(III) complex **4** (**Figure 4**) [28, 30]. The spin state of the iron in this type of complex, which was evidenced for chloride complex, depended mainly on the nature of both the macrocyclic and axial ligands. In addition, when iron porphyrazine was coordinated with pyridine, the macrocyclic ligand adopted an interme‐ diate structure between common porphyrins and phthalocyanines [30]. Iron(II) porphyrazines reversibly bind a variety of neutral ligands such as THF, nitrogenous bases, and carbon mon‐ oxide. This fact can be related to the higher π acidity of the porphyrazine ligand as compared to the porphyrin ligand. Iron(II) porphyrazine demonstrates no affinity for molecular oxygen, which can be a result of positively shifted III/II redox couple potential [31].

Another group of complexes, six‐coordinated iron(II) complexes called bisaxial complexes also constitute a large group of compounds. DMSO, pyridine and pyrazine are one of the most often utilized molecules for coordination of iron cation, thus forming adducts as it was studied for iron(II) tetrakis(thiadiazole)porphyrazine **5** in Ercolani and Stuzhin groups (**Figure 4**) [32]. Coordinated bidentate molecules, for example, pyrazine, can be used as link‐ ers between two or more macrocycles forming bridged complexes. What is more, the axial ligation of metalated Pz derivative with chloride influenced the Q‐band absorption, which was the result of change in the symmetry from D4h to C4v [23]. FT‐IR studies of iron(II) porphy‐ razine complex **5** axially coordinated with DMSO showed that two Fe‐O coordination bonds

**Figure 4.** Chemical structures of iron(III) octaphenylporphyrazine **4**, iron(II) tetrakis(thiadiazole)porphyrazine **5**, crystal structure of **6**, phthalocyanine trinuclear molecular wire **7** and iron(III) porphyrazine **8**.

were formed. However, in phthalocyanines, analogical process was based on Fe‐S coordina‐ tion bond formation. It indicates that porphyrazine **5** is stronger π‐acceptor [32].

complexes. To the second group belong dimers with single atom bridging groups between two iron macrocycles. In both cases, the obtained molecules have modified optical and elec‐

A coordination of iron cation with proper ligand results in the formation of five‐ or six‐coordi‐ nated macrocyclic complexes, which were subjected to broad study by Stuzhin et al. [28]. The coordination of iron(II) tetraazaporphyrins with axial ligands leads to the oxidation of iron(II)

The spin state of the iron in this type of complex, which was evidenced for chloride complex, depended mainly on the nature of both the macrocyclic and axial ligands. In addition, when iron porphyrazine was coordinated with pyridine, the macrocyclic ligand adopted an interme‐ diate structure between common porphyrins and phthalocyanines [30]. Iron(II) porphyrazines reversibly bind a variety of neutral ligands such as THF, nitrogenous bases, and carbon mon‐ oxide. This fact can be related to the higher π acidity of the porphyrazine ligand as compared to the porphyrin ligand. Iron(II) porphyrazine demonstrates no affinity for molecular oxygen,

Another group of complexes, six‐coordinated iron(II) complexes called bisaxial complexes also constitute a large group of compounds. DMSO, pyridine and pyrazine are one of the most often utilized molecules for coordination of iron cation, thus forming adducts as it was studied for iron(II) tetrakis(thiadiazole)porphyrazine **5** in Ercolani and Stuzhin groups (**Figure 4**) [32]. Coordinated bidentate molecules, for example, pyrazine, can be used as link‐ ers between two or more macrocycles forming bridged complexes. What is more, the axial ligation of metalated Pz derivative with chloride influenced the Q‐band absorption, which was the result of change in the symmetry from D4h to C4v [23]. FT‐IR studies of iron(II) porphy‐ razine complex **5** axially coordinated with DMSO showed that two Fe‐O coordination bonds

**Figure 4.** Chemical structures of iron(III) octaphenylporphyrazine **4**, iron(II) tetrakis(thiadiazole)porphyrazine **5**, crystal

structure of **6**, phthalocyanine trinuclear molecular wire **7** and iron(III) porphyrazine **8**.

anions was transformed to five‐coordinated iron(III) complex **4** (**Figure 4**) [28, 30].

, Cl− , Br<sup>−</sup> ,

to iron(III). In this way, iron(II) octaphenylporphyrazine coordinated axially with F−

which can be a result of positively shifted III/II redox couple potential [31].

trochemical properties [28, 29].

106 Recent Progress in Organometallic Chemistry

I−

and HSO4

−

Iron(II) phthalocyanines demonstrate the ability to form coordination assemblies with large structures like neutral and negatively charged fullerenes. An interesting example is crystal of **6** (**Figure 4**) obtained by cocrystallization from *n*‐hexane and composed of fullerenes C60 and unsubstituted iron(II) phthalocyanines axially ligated with pyridines. In the structure of **6,** there are π‐π stacking interactions, which do not affect the geometry of iron(II) phtha‐ locyanine [33]. Thus, metal phthalocyanines can be involved simultaneously in molecular complexes with fullerenes as coordination assemblies and with the addition of axial ligands. Another example is trinuclear complex **7** (**Figure 4**) involving two isocyanoferrocene ligands axially coordinated to iron(II) phthalocyanine. The structure **7** forms a molecular wire between iron cation from phthalocyanine and two iron ions from ferrocenes. However, there is weak electronic communication present between two iron centers of the ferrocene ligands despite a relatively large distance (11.5 Å) [34].

An improvement in synthetic methods from the late 1980s allowed the obtaining of iron(II/III) porphyrazine and phthalocyanine complexes able to form dimers of macrocycles bridged by oxygen, nitrogen or carbon atoms (μ‐oxo, μ‐nitrido and μ‐carbido dimers, respectively). Electrochemical studies demonstrated that macrocyclic ligand can influence the redox behavior of the binuclear complex. According to Colomban et al., dimer consisting of two porphyrazines possesses intermediate properties between corresponding porphyrin and phthalocyanine dimers [29]. This statement is based on the observation that the values of half‐waved oxidation potentials of porphyrazine dimer were in the middle between similar phthalocyanine and porphyrin potentials, and for this reason, oxidation potentials in por‐ phyrazine‐based complexes are closer to phthalocyanines than porphyrins. Cyclic voltam‐ metry of monomeric iron(III) porphyrazine axially ligated with 2‐chloroethoxy substituent **8** (**Figure 4**) and its μ‐oxo dimer demonstrated in DMF/TBAP three and six redox processes, respectively. Thus, the redox behavior of μ‐oxoporphyrazine dimer differs significantly not only from that of the small ring system of μ‐oxoporphyrin dimer, but also from that of the large‐ring based μ‐oxo dimer of phthalocyanine [35]. Compound **8** was also used for thin film formation utilizing Langmuir‐Schaefer (LS) technique (horizontal lifting) following the study presented by Garramone et al. [36]. In aqueous solution, the μ‐oxo dimer was formed as a predominant component (prevalent molecular building block) of LS films. The obtained LS‐Fe films showed remarkable changes in the UV‐Vis spectra, which are consistent with a significant μ‐oxo dimer to monomer conversion [36]. The studies on μ‐oxo iron phthalocya‐ nine dimer demonstrated that according to the reaction conditions, it is possible to obtain bent or linear Fe‐O‐Fe structures (with Fe‐O‐Fe angle up to 300°). These two forms were identified by FT‐IR signals from Fe‐O antisymmetric stretching vibration. Moreover, it was possible to transform bent into linear form by applying the following reaction conditions: (i) by adding of 2‐propylamine to compound suspended in chloronaphthalene or (ii) by the mixing the compound in the saturated H2 SO4 solution in the presence of oxygen [37].

In comparison with well‐known classic alone atop the other dimer structures, there is an example of significantly different "side‐by‐side" dimer structure **9** (**Figure 5**), which was con‐ sidered as a novel molecular QCA (*quantum-dot cellular automata*) cell [38]. This dimer has

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

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‐ molecular cofacial interactions between them [39, 40].
