Anti-oxidative reactivity and mechanism of formic hydrazide, acethydrazide and butyrohydrazide towards reduction of single electron oxidant iridium(IV)

Abstract

Reduction of a well-known single electron oxidant [IrCl6]2– by aliphatic hydrazides, viz. formic hydrazide, acethydrazide and butyrohydrazide, was investigated by use of several techniques including 1H NMR and rapid scan spectroscopies. It was demonstrated that the redox reactions exclusively follow second-order kinetics over a wide pH range; the observed second-order rate constants k' versus pH profiles have been established. When the features revealed by the rapid scan spectra, the reaction stoichiometry and the oxidation products were considered together, a reaction mechanism was proposed involving three parallel rate-determining steps in which [IrCl6]2– reacted with all the three protolytic forms of the hydrazides. Rate constants of the rate-determining steps and the dissociation constants of hydrazides at 25.0 °C and 1.0 M ionic strength were derived from the kinetic data. Activation parameters were determined for the reactions of [IrCl6]2– with the neutral forms of the hydrazides, and an outer-sphere electron transfer mode was implied. The reactivity of each protolytic form of the hydrazides was elucidated and its distribution versus pH was calculated. The reactivity of the protolytic forms was compared with that of the corresponding forms of aryl hydrazides, revealing that both the neutral and enolate forms of aliphatic hydrazides are less reactive than those of the aryl hydrazides towards reduction of [IrCl6]2–. It was concluded that aryl hydrazides are more important than aliphatic ones in the development of new hydrazide-based antioxidants in the future. Additionally, the protolysis equilibria of these aliphatic hydrazides in aqueous solution and their redox properties have been characterized and the dissociation constants in the case of butyrohydrazide are reported for the first time.