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 /= 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.