Decay of high-valent manganese porphyrins in aqueous solution and catalysed formation of oxygen

Abstract

Manganese(III) porphyrins (Mn^IIIP) are easily oxidised to the corresponding Mn^IVP in alkaline aqueous solution. At pH < 5 the oxidation product is a Mn^IIIP π-radical cation. These oxidised metalloporphyrins have limited stability in water and they revert to the original Mn^IIIP upon standing in the dark. The rate and mechanism of this inherent reduction process depends upon pH, with lower pH giving the higher rates. The inherent reduction appears to involve disproportionation and rearrangement of the Mn^IVP but it does not lead to formation of molecular O₂. Addition of colloidal RuO₂·2H₂O, a good O₂-evolving catalyst, has a pronounced effect upon the reduction process. The oxidised metalloporphyrin is bound to the catalyst particles by electrostatic forces, and at pH < 11 the bound material decays more slowly than the free compound. For 8 < pH < 11, decay of the bound metalloporphyrin involves oxidation of water to O₂, but the yield of O₂ is much less than the stoichiometric value.