Kinetics and Mechanism for Oxidation of L-Proline by Bis(hydrogen periodato)argentate(III) Complex Anion

Oxidation of L‐proline by bis(hydrogen periodato)argentate(III) complex anion, [Ag(HIO6)2]5−, has been studied in aqueous alkaline medium in the temperature range of 25–40 °C by use of conventional spectrophotometry. Under the conditions for kinetic measurements, the oxidation results in decarboxylation of proline, giving rise to γ‐aminobutyrate as identified by mass spectrometry. The oxidation kinetics is first order with respect to the silver(III) and proline concentrations; the second‐order rate constants, decreasing with increasing [periodate], are essentially independent of [OH−]. The kinetic results were interpreted in terms of a reaction mechanism which involves a pre‐equilibrium between [Ag(HIO6)2]5− and [Ag(HIO6)(H2O)(OH)]2−, a mono‐periodate coordinated silver(III) complex. Both Ag(III) complexes are reduced parallelly by the fully deprotonated form of proline in rate‐determining steps (described by k1 for the former Ag(III) and k2 for the latter one). The determined rate constants and their associated activation parameters are k1(25 °C) = (1.87±0.04) L·mol−1·s−1, ΔH1≠= (45±4) kJ· mol−1, ΔS1≠= (−90±13) J·K−1·mol−1 and k2(25 °C) = (3.2±0.5) L·mol−1·s−1, ΔH2≠= (34±2) kJ·mol−1, ΔS2≠= (−122±10) J·K−1·mol−1. This is also the first time to find that [Ag(HIO6)2]5− is also reactive toward a particular reductant and shows a lower reactivity than [Ag(HIO6)(OH)(H2O)]2−.