Mechanistic studies on the oxidation of pyruvic acid by an oxo-bridged diiron(III,III) complex in aqueous acidic media

In aqueous solution [Fe2(μ-O)(phen)4(H2O)2]4+ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate bases [Fe2(μ-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(μ-O)(phen)4(OH)2]2+ (3). In the presence of excess phen and in the pH range 2.5–5.5, the dimer quantitatively oxidizes pyruvic acid to acetic acid and carbon dioxide, the end iron species being ferroin, [Fe(phen)3]2+. The observed reaction rate shows a bell-shaped curve as pH increases, but is independent of added phen. Kinetic analysis shows that (3) is non-reactive and (1) has much higher reactivity than (2) in oxidizing pyruvic acid. The basicity of the bridging oxygen increases with deprotonation of the aqua ligands. The reaction rate decreases significantly in media enriched with D2O in comparison to that in H2O, with a greater retardation at higher pH, suggesting the occurrence of proton coupled electron transfer (PCET; 1e, 1H+), which possibly drags the energetically unfavorable reaction to completion in presence of excess phen.

[Fe2(μ-O)(phen)4(H2O)2]4+ (1) equilibrates with [Fe2(μ-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(μ-O)(phen)4(OH)2]2+ (3) in aqueous solution (phen = 1,10-phenanthroline). In presence of excess phen, an aqueous solution of 1 oxidizes pyruvic acid to acetic acid and carbon dioxide; itself being reduced to two moles of [Fe(phen)3]2+. The reactive species are 1, 2 and the pyruvate anion (Py−). The reaction proceeds through a rate limiting 1e, 1H+ electroprotic mechanism.Figure optionsDownload as PowerPoint slide