Gold oxidative dissolution by (thioamide)–I2 adducts

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 [AuI(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 Ib and It iodine atoms in the fragment S–Ib–It. 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.