Photochemistry of manganese porphyrins. Part 3.—Interconversion of mnII/MnIII

Abstract

Water-soluble Mn^III porphyrins can be reduced by dithionite in aqueous solution. The rate of reduction is independent of pH but shows a marked dependence upon the nature of the water-solubilising group. The reaction mechanism involves outer-sphere attack upon the porphyrin ring periphery by a sulphoxylate radical anion (SO[graphic omitted]) to form a Mn^III porphyrin π-radical anion which rapidly rearranges to a Mn^II porphyrin.

The Mn^II porphyrin is oxidised by molecular oxygen via an axial inner-sphere mechanism. The rate of oxidation is independent of the water-solubilising group provided that intermolecular complexation does not occur. A similar oxidation can be achieved with quinones, the rate of oxidation being considerably faster for benzo-1,4-quinone than for oxygen, and this reaction is enhanced by irradiation with red light.