Octaethylporphyrin redox properties

Synthesis. Homometallic pz dimers of Ru (227a) and Os (227b) were prepared, in almost quantitative yield, by vacuum pyrolysis of the monomeric Ru(py)2 or Os(py)2 octaethylporphyrazines (226a and 226b). Heterometallic pz dimers, composed of octaethylporphyrazine and octaethylporphyrin were prepared by the same route that initially produces a mixture of two homometallic (16% each) and the heterometallic dimer (68%), Ru (228a) or Os (229b). The redox properties of the three products are significantly different, by 0.4-0.6 V, so the heterometallic dimer can be separated from the homometallic dimers by redox titrations. 

Arnold, D.P., G.A. Heath, and D.A. James (1998). Conjugated dimers of nickeljll) octaethylporphyrin linked by extended meso,meso-aikynyl bridges. II. Redox properties and electronic spectra of electrogenerated anions and dianions. New J. Chem. 22, 1377-1387. 

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... 

The oxidative reactivities of silver(ii) and silver(m) have been compared. The first- and second-order terms in [Ag ] in various reactions have been assessed and an attempt has been made to resolve the apparent reverse of reactivity between the bi-and ter-valent silver ions, depending on the substrate. A reaction scheme for the oxidation of water has been presented and the data are consistent with a reactive dimeric form of Ag. The hydrolysis constant of Ag is also considered to be less than 31 mol . The properties of octaethylporphyrins show the Agi complex to be stable,and cyclic voltammetric studies show reversibility in the +3<- +2 redox steps. In the gold(m) (as AuCl4 ) oxidation of methionine, there is stereospecificity in the reaction, the product being the corresponding sulphoxide with no evidence for a sulphone. The reaction is biphasic, with an initial complex formation which is too rapid for study by conventional techniques followed by a slower intramolecular redox reaction leading to gold(i). The rate of this latter process is dependent on the methionine concentration. The oxidation also takes place when the substrate is part of a peptide chain. 

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