Reduction reactions of water soluble cyano-cobalt(III)-porphyrins: Metal versus ligand centered processes

Abstract

Reduction reactions of dicyano-cobalt(III)-porphyrins [potential in vivo cyanide scavenger drugs] were studied by radiolytic and electrochemical methods using the water soluble tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP). For [(CN) 2Co IIITPPS] −, reduction occurs stepwise to the Co II, Co I, and finally to the phlorin anion. This behavior is similar to that of the cobalt porphyrins in the absence of cyanide, except that the cyanide ligand shifts the reduction potentials to much more negative values. On the other hand, under radiolytic conditions, [(CN) 2Co IIITMPyP] − is reduced on the porphyrin macrocycle by one electron to give the Co III π-radical anion, which disproportionates into the initial complex and the two-electron ring reduced Co III phlorin. The radical anion is also formed by intramolecular electron transfer subsequent to the reaction of Co IITMPyP and cyanide. The results are compared with the chemistry of Vitamin B-12.