Kinetics of the uncatalysed and silver(I)-catalysed oxidation of hydrogen peroxide by [ethylenebis(biguanide)]silver(III) ion in acid perchlorate media

Abstract

In aqueous acid media [ethylenebis(biguanide)]silver(III) oxidises H₂O₂ to O₂, itself being reduced to Ag^I and ethylenebis(biguanide): a 1 : 1 stoicheiometry has been established. The reaction is catalysed by Ag⁺ and the empirical rate law is –d[complex]/dt=(k_uncat+k_cat[Ag⁺])[H₂O₂][complex]/[H⁺]. At 30 °C and I= 1.0 mol dm^–3, k_uncat=(6.5 ± 0.5)× 10^–5 s^–l and k_cat=(9 ± 1)× 10^–4 dm³ mol^–1 s^–1. For k_uncat, ΔH^‡= 31 ± I2 kJ mol^–1 and ΔS^‡=–222 ± 37 J K^–1mol^–1, while for k_cat the values are 41 ± 16 kJ mol^–1 and –167 ± 53 J K^–1 mol^–1 respectively. The inverse proton dependence of the rate arises from deprotonation of H₂O₂ facilitated by its axial co-ordination to the complex, forming an adduct; catalysis by Ag^I arises through complexation of Ag¹ with this adduct. The probable mechanism and plausible role of any intermediate silver(II) species are discussed.