Metalloporphyrins macrocyclic

Scheme 1 General structural formula of a metalloporphyrin macrocycle with indicated positions available for peripheral substitution...

Effects of Peripheral Substituents of the Metalloporphyrin Macrocycle on the Photovoltaic Cell Performance... 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

Pharmaceutical discoveries are principally made by thoughtful structural variation on a lead compound which has been found, by chance or design, to have a certain amount of the desired activity. It is clear that with metalloporphyrins, there are additional structural variations to be had. In the first place, it is possible to vary the metal. It was realized at an early stage that inserting and varying the metal would modify PDT activity.61 The possibility also exists of structural changes in axial ligands in those metalloporphyrins which possess them. This structural variation occurs in the space immediately above and below the macrocycle, which is a space not readily accessible to controlled variation in metal-free compounds of this series. 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

As is known, the triplet and singlet tetrapyrrolic pigment states are caused by excitation of delocalized ir electrons of the heteroaromatic macrocycle (1 ). The most favourable structure of dimeric complexes for metal 1oporphyrin TTA seems to be a sandwich dimer as in the case when molecular orientation of plane to plane electronic shells reaches the maximum overlap. Quantum chemistry calculations of metalloporphyrin dimers indicates that for the dimeric emission process the 2 state is split into states of higher energy (SJ) and lower energy ( 2). In the case of a sandwich dimer of 04 symmetry, the following states are seen ... 

Coordinated transition metal redox-active macrocycles, 39 108-124 ammonium cation, 39 128-133 crown ether and bis crown ether ligands containing bipyridyl transition metal recognition sites, 39 111 crown ether dithiocarbamate and dithiolene complexes, 39 123-124 metalloporphyrin crown ether compounds, 39 108-109... 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

The ligand field of the macrocycle is also modified by ring distortion.154 The metal to alkylated nitrogen distance is 2.33 A (Fe) to 2.53 A (Zn) which is considerably longer than the normal value of 2.0-2.1 A for the first row transition metalloporphyrins in a high spin state, while the extension is small for the other three M—Npor bonds. The reduced and distorted ligand field of the macrocycle makes it possible for even the Co complexes as well as other metals to take a high spin state. 

The soluble metalloporphyrin-containing polymers are formed by the copolymerization of MM A or 4-VP with macrocyclic MCM — an interaction product of acrylic acid chloride with tetra-p-aminophenylporphyrinate acetate manganese [96]. Copolymers obtained by the radical copolymerization of acryloyl derivatives of cobalt phthalocyanine with 9-vinylcarbazole [97] should also be mentioned. 

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