Kinetics of the Oxidation of Dithionite (S2O4 2−) by Manganese (III)-Hematoporphyrin

Abstract Manganese porphyrins were first prepared by Zaleski over sixty years ago,1 and can exist in the plus two, three, and four oxidation States. Calvin2 has suggested that the manganese required for the oxygen evolution step in plant photosynthesis is bound in a porphyrin type ligand. The synthetic, electrochemical and theoretical studies on manganese porphyrins have been recently reviewed in detail by Boucher.3 The kinetics of oxidation of Cr2+ and V2+ by Mn(III)-tetrapyridylporphin have been studied in acid solution,4 the main result being that such reactions are halide catalyzed, but much less so than their Fe(III) counterparts. The dithionite ion (S2O4 2−) is the most common reductant used to produce Mn(II) porphyrins in basic solution,5 presumably by the overall stoichiometry: We report the kinetics of oxidation of dithionite by Mn(III) hematoporphyrin with a view of establishing the mechanism of such metalloporphyrin reduction and to further explore the chemistry of dithionite.