Kinetics and mechanism of oxidation of trichloro-oxobis(triphenylphosphine oxide)molybdenum(V) by nitrate in dichloromethane

Abstract

The kinetics of the redox reaction between [Et₄N][NO₃] and [MoOCl₃(OPPh₃)₂] have been studied in CH₂Cl₂ solution at temperatures from 3·2 to 25 °C. This reaction proceeds in three observable steps when [NO₃^–] > [Mo^V]. The first step involes co-ordination of the nitrate ion via an S_N1 (limiting) mechanism involving loss of a loss of a triphenylphosphine oxide molecule; at 25 °C, k_obs.= 40 ± 1 s^–1 and the corresponding activation parameters are ΔH^‡= 9·7 ± 0·5 kcal mol^–1 and ΔS^‡=–18·4 ± 1·7 cal K^–1 mol^–1. The data obtained for the second and third steps strongly suggest that they involve intramolecular substitution at molybdenum(V) and inter-molecular substitution at molybdenum(VI) centres, respectively. Rapid non-rate-determining electron transfer from molybdenum(V) to nitrate is proposed to occur between these two latter steps, k_e.t. 1 s^–1 at 25 °C, giving a dioxomolybdenum(VI) complex and nitrogen dioxide. The kinetic results imply that a specific orientation of the nitrato-group with respect to the oxomolybdenum(V) centre is necessary prior to rapid electron transfer.