Kinetics and mechanism of halide-substitution reactions of trichloro-oxobis(triphenylphosphine oxide)molybdenum(V)

Abstract

A detailed study of the substitution reactions of chloride and bromide ions with trichloro-oxobis(triphenyl-phosphine oxide)molybdenum(V), [MoOCl₃(OPPh₃)₂], [MoOCl₃(OPPh₃)₂]+ X^–→[MoOCl₃X(OPPh₃)]^–+ Ph₃PO, has been accomplished in dichloromethane solution over the temperature range 1–25 °C. The rate of substitution was found to be independent of halide-ion concentration and, for constant [X^–](X = Cl or Br), to be inversely dependent on [Ph₃PO]. These data are consistent with an S_N1 (limiting) mechanism, with essentially identical values for the activation parameters for both chloride and bromide substitution: ΔH^‡= 10·6 ± 0·8 and 11·4 ± 0·2 kcal moh^–1 and ΔS^‡=–15·4 ± 2·9 and –12·5 ± 0·8 cal K^–1 mol^–1 at 298 K, respectively. For both reactions the rate-determining step is considered to involve loss of the Ph₃PO ligand trans to the oxo-group in [MoOCl₃(OPPh₃)₂]. Subsequent to halide substitution, exchange of the substituted halide with the equatorial Ph₃PO occurs. The reactions of Cl^– with [MoOCl₃Br(OPPh₃)] and of several nucleophiles with [MoOCl₄]^– in dichloromethane solution at 25 °C have been studied and the data obtained shown to provide support for the proposed mechanism of halide substitution of [MoOCl₃(OPPh₃)₂].