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

Abstract

The reactions between trans-[Au(CN) ₂ X ₂ ] ^- (X = Cl or Br) and Me ₂ S 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 ∶Me ₂ S is 1∶(1.0 ± 0.1) in agreement with the reaction: trans-[Au(CN) ₂ X ₂ ] ^- + Me ₂ S + H ₂ O → [Au(CN) ₂ ] ^- + Me ₂ SO + 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) ₂ X(Me ₂ S)] with an asymmetric electron distribution along the X–Au–S axis are reduced rapidly via an intermolecular process, in which Me ₂ S attacks a co-ordinated halide. The complex trans-[Au(CN) ₂ (Me ₂ S) 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.