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

摘要

The complex cation, [ethylenebis(biguanide)]silver(III), [Ag(H2L)](3+), and its conjugate bases, [Ag(HL)](2+) and [AgL](+) oxidise glyoxylic acid, CHOCO2H 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(H2L)](3+) - HGX (k(1)), [Ag(H2L)](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 degreesC and 1 = 1.0 M (NaNO3). 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. (C) 2003 Elsevier Ltd. All rights reserved.