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

Abstract

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO2H and CO2, together with a colourless solution of the Ag+ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H2L)]3+ and one of its conjugate bases, [Ag(HL)]2+, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py−) as the reactive reducing species. Ag+ was found to be catalytically inactive. At 25.0°C, I=1.0moldm−3, rate constants for the reactions [Ag(H2L)]3++HPy (k1), [Ag(H2L)]3++Py− (k2), [Ag(HL)]2++HPy (k3) and [Ag(HL)]2++Py− (k4) arek1=(94±6)×10−5dm3mol−1s−1, (k2Ka+k3Ka1)= (1.3±0.1)×10−5s−1 and k4=(58±4)×10−5dm3mol−1s−1, respectively, where Ka1is the first acid dissociation constant of the [Ag(H2L)]3+ and Ka is for pyruvic acid. A comparison between the k1 and k4 values is indicative of the judgement that k2≫k3. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.