Iron porphyrin complexes properties

Haem peroxidases are globular proteins with an iron-porphyrin complex as a prosthetic group. These enzymes are widespread among prokaryotes and eukaryotes. They catalyze the oxidation of substrates by organic peroxides or hydrogen peroxide. During the past decades, considerable scientific effort has been put into elucidation ofthe mechanisms of reactions catalyzed by these enzymes. Pulse radiolysis technique has made an important contribution by providing information on the redox states of the enzymes and their interconversion, as well as on the properties ofthe free radical intermediates involved [23]. 

Since the porphyrin ring neutralizes only two of the positive charges of the metal, Fe porphyrins must have an associated counterion. This counterion may be directly bound to the metal, as in five-coordinate high-, intermediate-, and low-spin Fe porphyrins, or may exist as a separate species that forms an ion-pair with the iron porphyrin complex, as in all six-coordinate complexes of Fe porphyrins with neutral ligands. We will survey the structures, properties, and reactions of Fe porphyrins of each of the possible spin states. 

The influence of pyridine binding and spin state on the spectroscopic properties and electrochemistry of aryl iron porphyrin complexes has been studied. The reduction of styrene to ethylbenzene and 2,3-diphenylbutane catalysed by [(TPP)FeCl] is proposed to proceed via a o-Fe-CHMeRi intermediate. The phthalocyanine anion Na2[FePc] reacts with ROCBr to give Na(PcFeC R] (R = Ph, Pr), and similar alkynyl anions have been prepared with a hemin-like iron centre.55... 

It is noteworthy that siroheme is present in the enzymes responsible for catalyzing two out of only three known six-electron processes, and accordingly it is of great interest to try and identify any feature in siroheme that makes it particularly suitable for the mediation of multielectron transfer. A comparison of octaethylporphyrin, octaethylchlorin and octaethylisobacteriochlorin complexes of iron shows that redox potentials and vco of Fe(P)L(CO) and Fe(P)L(CO)2 were nearly independent of the porphyrin. The property that was most dependent upon the macrocycle structure was the potential for ring-based oxidation which increased in the order OEiBC < OEC < OEP.734... 

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