Mechanistic studies on the oxidation of glyoxylic and pyruvic acids by a {Mn3O4}4+core in aqueous media

Abstract

In aqueous media, the MnIV trimer [MnIV3(μ-O)4(phen)4(H2O)2]4+ (1, phen = 1,10-phenanthroline) equilibrates with its deprotonated from [Mn3(μ-O)4(phen)4(H2O)(OH)]3+ (2). Among the several synthetic multinuclear oxo- and/or carboxylato-bridged manganese complexes known to date containing metal-bound water, to the best of our knowledge, 1 is one of the rare examples that deprotonates (1⇌2 + H+; pKa = 4.00 (±0.15) at 25.0°C, I = 1.0 mol dm−3, maintained with NaNO3) at physiological pH. In aqueous media (pH 2–4), 1 oxidizes both glyoxylic and pyruvic acids to formic and acetic acid, respectively, along with the formation of CO2, the end manganese state being MnII. Kinetic studies suggest that the species 1, its deprotonated form 2, the reducing acids (HA), and their conjugate bases (A−) all take part in the reaction. The oxidant 1 is found to be more reactive than its conjugate base 2, and HA reacts faster than A− in reducing 1 or 2. The gem-diol form of the α-oxo acids (especially for glyoxylic acid) is the possible reducing species. The MnIV3 to MnII transition in the present observation proceeds through the intermediate generation of the spectrally characterized mixed-valent MnIIIMnIV dimer that quickly collapses to MnII. The observed rates of glyoxylic or pyruvic acid oxidation do not depend on the variation of 1,10-phenanthroline content of the solution, indicating the absence of any phen-releasing preequilibrium of the title complex in solution. The reactions rates were found to be lowered in media enriched with D2O in comparison to that in H2O and a rate-limiting one electron one proton (1e, 1H+) electroprotic mechanism is proposed. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 323–335, 2010