Photochemistry of manganese porphyrins. Part 6. Oxidation–reduction equilibria of manganese( III ) porphyrins in aqueous solution

Water-soluble manganese(III) porphyrins are oxidised in alkaline aqueous solution to the corresponding manganese(IV) porphyrins which, from magnetic moment measurements, appear to exist in solution as µ-oxo-dimers. Midpoint potentials and rate constants for oxidation of the manganese(III) porphyrins have been measured for a series of oxidants and throughout the range 9 < pH < 14, but the overall electronic charge on the metalloporphyrin had little effect upon either parameter. The midpoint potentials for the MnIII/IV couple are strongly dependent upon pH and increase with decreasing pH. Although manganese(IV) porphyrins are mild oxidants at pH 14, they should be capable of oxidising water to molecular oxygen in neutral solution. With hypochlorite as oxidant, a second oxidation step is possible and the final product is believed to be a manganese(V) oxo-porphyrin.