Mechanistic Studies on the Oxidation of Glyoxylic and Pyruvic Acid by a [Mn_4O_6]~(4+) Core in Aqueous Media: Kinetics of Oxo-Bridge Protonation

摘要

In aqueous media (pH 2.5-6.0),the Mn~(IV)tetramer [Mn_4(mu-O)_6(bipy)_6]~(4+) (1~(4+); bipy = 2,2'-bipyridine) oxidizes both glyoxylic and pyruvic acid to formic and acetic acid,respectively,under formation of CO_2.Kinetics studies suggest that the species 1~(4+),its oxo-bridge protonated form [1H]~(5+) ,i.e.,[Mn_4(mu-O)_5(mu-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 1~(4+).Thereby,the gem-diol forms of the alpha-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.