Gold oxidative dissolution by (thioamide)–I2 adducts

Abstract

Elemental gold powder is easily oxidised under mild reaction conditions with 1 : 1 I₂-adducts of the thioamides 3-methyl-benzothiazole-2-thione (mbtt) and 1-methyl-1H-benzimidazole-2(3H)-thione (mbit) with separation of the ionic complexes [Au^I(mbtt)₂]I₃ (1) and [(mtbiH)₂](AuI₄)I₃ (2) [mtbiH = the imido protonated form of 2-(methylthio)benzimidazole] (gold oxidation yield >78% and 45%, respectively). The X-ray crystal structure of 1 shows the Au(I) centre linearly bound by two neutral ligands via the sulphur atom, whereas the X-ray crystal structure of 2 shows the Au(III) centre surrounded by four iodides in a square-planar arrangement. The oxidising/complexing properties of the mbtt·I₂ and mbit·I₂ adducts were interpreted considering the S-donor to I₂ interaction that leads to a charge separation between the I_b and I_t iodine atoms in the fragment S–I_b–I_t. The oxidation of gold in a non-aqueous solvent by thioamide–I₂ adducts could be a promising alternative process to the numerous hydrometallurgical ones for the recovery of gold from secondary sources.