Electrochemistry of nickel(II) porphyrins and chlorins

The electron-transfer mechanisms of five nickel(II) porphyrins and a nickel(II) chlorin were investigated in nonaqueous media. The potentials for oxidation or reduction were monitored by cyclic voltammetry in up to 11 solvents containing either TBAP or TBA(PF₆) as supporting electrolyte. ESR, IR, and electronic absorption spectroscopy were used to monitor the reaction products at both high and low temperature, and these data were used to postulate a reaction mechanism for each of the six investigated complexes. Three difference oxidation mechanisms were found to occur depending upon the nature of the porphyrin or chlorin ring system. In contrast, all of the complexes were reduced by a similar mechanism, which involved formation of anion radicals. Unexpectedly, the three investigated tetraalkylporphyrins showed no evidence for Ni(III) formation as is the case for unsubstituted (TPP)Ni.