Mechanism of complex formation between [AuCl4]- and l-methionine

Abstract

The kinetics of the reaction between the tetrachloroaurate(III) ion and l-methionine (l-Met) (0.1 M HClO4, pH 1.0-2.5) have been studied spectrophotometrically using a stopped-flow technique at different temperatures. Initially, the fast substitution reaction was ascribed to the formation of the short-lived square-planar Au(III)-(l-Met) that was followed by the replacement of a Cl- ligand and a subsequent, slower reduction to Au(I)-(l-Met). This is an intermolecular process, involving attack on the [AuCl4]- complex by an outer-sphere l-methionine. The activation parameters (ΔH≠ and ΔS≠) for substitution and reduction were determined. IR spectroscopy indicates that l-methionine acts as a bidentate ligand, most likely coordinating via the S and N atoms, while 1H and 13C NMR data indicate methionine sulfoxide as the final product. Finally, the components of the reaction were treated thermally in order to investigate the solid phase synthesis of the resulting complex.