Mechanistic Studies on the Oxidation of Glyoxylic and Pyruvic Acid by a [Mn4O6]4+ Core in Aqueous Media: Kinetics of Oxo-Bridge Protonation

Abstract

In aqueous media (pH 2.5–6.0), the MnIV tetramer [Mn4(μ-O)6(bipy)6]4+ (14+; bipy = 2,2′-bipyridine) oxidizes both glyoxylic and pyruvic acid to formic and acetic acid, respectively, under formation of CO2. Kinetics studies suggest that the species 14+, its oxo-bridge protonated form [1H]5+, i.e., [Mn4(μ-O)5(μ-OH)(bipy)6]5+, the reducing acids (RH) and their conjugate bases (R−) all take part in the reaction. The oxo-bridge protonated oxidant [1H]5+ was found to react much faster than 14+. Thereby, the gem-diol forms of the α-oxo acids (especially in the case of glyoxylic acid) are the possible reductants. A one-electron/one-proton electroprotic mechanism operates in the rate-determining step.