Mechanistic Studies on the Oxidation of Glyoxylic and Pyruvic Acid by a [Mn 4 O 6 ] 4+ Core in Aqueous Media: Kinetics of Oxo‐Bridge Protonation

In aqueous media (pH 2.5–6.0), the Mn IV tetramer [Mn 4 ( μ ‐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 [ 1 H] 5+ , i.e. , [Mn 4 ( 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 [ 1 H] 5+ was found to react much faster than 1 4+ . 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.