Chemistry of a high-oxidation-level manganese porphyrin in aqueous solution

Abstract

Manganese(III) tetrakis(N-methyl-4-pyridiniumyl)porphyrin (Mn/sup III/P/sup +/) (chloride salt) and other water-soluble manganese(III) porphyrins undergo facile one-electron electrochemical or chemical oxidation in alkaline solution. Best available evidence indicates that the final oxidized species is a manganese(IV) /mu/-oxo dimer, PMn/sup IV/-O-Mn/sup IV/P/sup 2+/. This same species is also produced by the reaction of manganese(II) porphyrin and oxygen. The Mn(IV) /mu/-oxo dimer has limited stability in water returning to 90-94% of the original Mn(III) porphyrin. The rate of this reaction is pH dependent with faster rates at lower pH. Oxygen is not produced during this reduction process. Rather, the reaction involves an unusual disproportionation in which a small percentage of the porphyrin macrocycles supply multiple electrons to rescue the remainder of the oxidized dimer. It was also found that the manganese(IV) dimer reacts rapidly with water-soluble olefins as it also does in aprotic solvent. A mechanism for the disproportionation reaction is discussed with a rate-determining step involving rearrangement of charge within the symmetric dimer to one with both oxidation equivalents on one metalloporphyrin unit, viz., PMn/sup IV/-O-Mn/sup III/P/sup +/ or PMn/sup III/-O-Mn/sup III/P/sup 2+/. This species undergoes nucleophilic attach by water or hydroxide, producing an isoporphyrin or bilirubin type molecule that has many olefinicmore » bonds capable of reaction with remaining Mn(IV) /mu/-oxo dimer. Since coordination by OH/sup /minus// to the manganese center favors the higher manganese(IV) oxidation level, the pH dependence of the disproportionation can be explained by rearrangement within the dimer to a porphyrin-centered oxidation site. 24 references, 3 figures.« less