Reduction of gold(III) to gold(I) by dialkyl sulphides. Evidence for an atom-transfer redox process

Abstract

A kinetic study of the reduction of gold(III) to gold(I) by dialkyl sulphides, in aqueous mathanol, is reported. The reaction takes place in two steps. In the first step, substitution equilibria which are strongly dependent on the bulkiness of the entering sulphide are set up. In the second step, the gold(III) complexes react with an extra molecule of sulphide to give gold(I) species and sulphoxide; very likely the reductant attacks the substrate at a chlorine and not at the metal centre, the halogen is then transferred from gold to sulphur leaving an electron pair on the metal. The reactivity of the gold(III) substrates increases with increasing substitution {[AuCl₄]^–[AuCl₃(SR₂)] < [AuCl₂(SR₂)₂]⁺} paralleling the increasing positive charge on the complex; on the other hand that of the sulphides parallels their basicity (SMe₂ < SEt₂ < SPrⁱ₂) indicating that in the redox step polar effects are more important than steric ones. The reaction rate also exhibits a marked solvent dependence, and increases with increasing percentage of water in the reaction medium, the relationship of log k_1,2 to solvent polarity being linear.