Kinetics of the uncatalysed and silver(I)-catalysed oxidation of hydrogen peroxide by [ethylenebis(biguanide)]silver(III) ion in acid perchlorate media
In aqueous acid media [ethylenebis(biguanide)]silver(III) oxidises H2O2 to O2, itself being reduced to AgI 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=(kuncat+kcat[Ag+])[H2O2][complex]/[H+]. At 30 °C and I= 1.0 mol dm–3, kuncat=(6.5 ± 0.5)× 10–5 s–l and kcat=(9 ± 1)× 10–4 dm3 mol–1 s–1. For kuncat, ΔH‡= 31 ± I2 kJ mol–1 and ΔS‡=–222 ± 37 J K–1mol–1, while for kcat 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 AgI arises through complexation of Ag1 with this adduct. The probable mechanism and plausible role of any intermediate silver(II) species are discussed.