Kinetics and mechanism of reduction of gold(III) complexes by dimethyl sulfide 

Abstract

The reactions between trans-[Au(CN)₂X₂]^- (X = Cl or Br) and Me₂S have been studied by conventional and high-pressure stopped-flow spectrophotometry in acidic aqueous solution containing 10% (w/w) methanol. The overall stoichiometry Au^III∶Me₂S is 1∶(1.0 ± 0.1) in agreement with the reaction: trans-[Au(CN)₂X₂]^- + Me₂S + H₂O → [Au(CN)₂]^- + Me₂SO + 2H⁺ + 2X^-. Initial rapid substitution processes result in the formation of a pre-equilibrium between transient gold(III) complexes, which are reduced to [Au(CN)₂]^- in a subsequent slower redox process. Complexes trans-[Au(CN)₂X(Me₂S)] with an asymmetric electron distribution along the X–Au–S axis are reduced rapidly via an intermolecular process, in which Me₂S attacks a co-ordinated halide. The complex trans-[Au(CN)₂(Me₂S)₂], on the other hand, undergoes slow reduction to gold(I) involving a water molecule. The rapid halide-mediated oxidation of thioethers implies that reduction of metal ions in biological systems by such moieties should be favoured in extracellular environments, where the chloride concentrations are high.

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