Stopped-flow kinetic analysis of the oxidation of semicarbazide by hexachloroiridate(IV)

Abstract

A kinetic analysis of the oxidation of semicarbazide (SEM) by the single-electron oxidant [IrCl₆]²⁻ has been carried out by stopped-flow spectrometric techniques. The reaction proved to be first order each in [IrCl₆ ²⁻] and [SEM]_tot, leading to overall second-order kinetics. The variation in the observed second-order rate constant k′ with pH was explored over the pH range of 0–7.11. Spectrophotometric titration revealed a stoichiometry of Δ[IrCl₆ ²⁻]/Δ[SEM]_tot = 4:1 for the redox reaction. On the basis of the rate law, the redox stoichiometry, and the rapid scan spectra, a reaction mechanism is proposed which involves parallel attacks of [IrCl₆]²⁻ on both H₂NCONHNH₃ ⁺ and H₂NCONHNH₂ as rate-determining steps, followed by several rapid reactions. The rate expression, derived from the reaction mechanism, was utilized to simulate the k′–pH profile yielding a virtually perfect fit and indicating that the reaction path involving H₂NCONHNH₃ ⁺ does not make a significant contribution to the overall rate. The reaction between [IrCl₆]²⁻ and H₂NCONHNH₂ was further studied as a function of both temperature and ionic strength. From the temperature dependence, activation parameters were obtained as: ∆H ^‡₂  = 34.9 ± 1.5 kJ mol⁻¹ and ∆S ^‡₂  = −78 ± 5 J K⁻¹ mol⁻¹. The observed ionic strength dependence suggests that the rate-determining step is between [IrCl₆]²⁻ and a neutral species of SEM. Hence, both the temperature and ionic strength dependency studies are in good agreement with the proposed reaction mechanism, in which the rate-determining step involves an outer sphere electron transfer.