Kinetics of oxidation of glyoxylic acid by [ethylenebis(biguanide)]silver(III) in aqueous media

Abstract

The complex cation, [ethylenebis(biguanide)]silver(III), [Ag(H 2L)] 3+, and its conjugate bases, [Ag(HL)] 2+ and [AgL] + oxidise glyoxylic acid, CHOCO 2H quantitatively to formic acid and carbon dioxide, themselves being reduced to Ag +. Free ethylenebis(biguanide) was recovered in near-quantitative yield. In the investigated pH range (1.00–7.00), the reaction is first order in both [complex] and total glyoxylate, [HGX] T. The reaction proceeds through four parallel paths: [Ag(H 2L)] 3+ – HGX ( k 1), [Ag(H 2L)] 3+ – GX − ( k 2), [Ag(HL)] 2+ – GX − ( k 3), and [AgL] + – GX − ( k 4). The respective rate constants (in 10 4 M −1 s −1) are, respectively, 7.34, 112, 40.7 and 19.2 at 25.0 °C and I=1.0 M (NaNO 3). Formation of adducts by the reducing species with the metal complex via hydrogen-bonding or electrostriction may be proposed for the low-energy pathway for electron-transfer. A substantial rate retardation in presence of the radical scavenger acrylonitrile suggests successive one-electron changes, Ag III → Ag II → Ag I.