Spectroscopic and Electrochemical Studies of the Interaction of Some Gold(III) Complexes with Biologically Relevant Thiones

The interaction of gold(III) complexes, [Au(cis-DACH)Cl₂]Cl and [Au(cis-DACH)₂]Cl₃ complexes (DACH = cis-1,2-diaminocyclohexane), with ¹³C, ¹⁵N-enriched thiourea (Tu) and 1,3-diazinane-2-thione ligands was investigated. The progress of these reactions was monitored by NMR (¹H, ¹³C, and ¹⁵N) and UV–vis spectroscopy as well as square wave stripping voltammetry. The kinetic studies of the substitution reactions between the above-mentioned complexes with thiones in aqueous solutions containing 30 mM KCl, which is used to suppress the hydrolysis of the chloride complexes, were conducted. These reactions were followed under pseudo–first-order conditions as functions of ligand concentration, pH, and temperature. The activation parameters (ΔH^#, ΔS^#) were calculated from Eyring plots, and the negative values of ΔS^≠ lend support for an associative mechanism. The kinetic data also indicated a relatively higher reactivity of [Au(cis-DACH)Cl₂]Cl than that of [Au(cis-DACH)₂]Cl₃ toward the thiones.