Kinetics of silver(I)-catalysed oxidation of hydrazinium ion by the [ethylenebis(biguanide)]silver(III) cation

Abstract

Silver(I) strongly catalyses the quantitative oxidation of hydrazine to dinitrogen by the complex [ethylenebis(biguanide)]silver(III), [Ag(H₂L)]³⁺. In the range pH 2–3 the reaction follows the rate law (i)–d[complex]/dt=k⁰= 2K₁k₁[Ag⁺]²[N₂H₅⁺]/[H⁺](i), where k⁰ is the observed zero-order rate constant for large excess of Ag⁺, H⁺, and N₂H₅⁺ over the complex. Values of 1 O⁴K₁k₁ were 2.22 ± 0.16, 4.1 ± 0.25, 7.17 ± 0.5, and 12.5 ± 0.8 dm³ mol^–1 s^–1 at 15,20, 25, and 30°C. Corresponding ΔH^‡ and ΔS^‡ values are 82 ± 8 kJ mol^–1 and –8 ± 2J K^–1 mol^–1 respectively. Under experimental conditions where Ag^II is detectable in concentrations less than about 10^–6 mol dm^–3, the whole reaction course is e.p.r. silent. No evidence for the formation of any radical intermediate is available. Apparently, a Ag^I–Ag⁰ catalytic cycle is involved wherein Ag⁺ slowly oxidises hydrazine of [Ag(N₂H₄)]⁺, with use of the complex as an oxidant which rapidly regenerates Ag⁺ from Ag⁰₂ formed as an intermediate.