Charge transfer complexes of dithioxamides with dihalogens as powerful reagents in the dissolution of noble metals

Commonly involved in the recovery/refining processes of noble metals (NMs), coordination chemistry is now required by new legislation to face requirements in the selection of ligands that must combine effectiveness with low environmental impact, in order to balance sustainability with economic development for conventional applications and for the recovery of NMs from secondary sources thus helping to convert Trash in Resource. In this paper, we review the properties of dihalogen/cyclic-dithioxamide adducts as a case-study to show how suitable complexes can provide innovation in the recovery processes of NMs from secondary sources and in the gold etching process in improving the reliability of microelectronic devices. These adducts, which do not show cytotoxicity, are capable of dissolving metal palladium and even gold in a one-step reaction and under mild conditions. In particular, Me₂dazdt·2I₂ (Me₂dazdt = N,N'-dimethyl-perhydrodiazepine-2,3-dithione) has proved to be the most effective in practical applications. It has been used in the palladium recovery from model spent three way catalysts (TWC). It selectively dissolved palladium, almost quantitatively, under mild conditions, even in a complex system such as an exhaust catalytic converter, a ceramic material that has undergone severe thermal and chemical stresses. Quite satisfactory results have also been obtained in gold recovery from selected WEEE (waste from electrical and electronic equipments) scrap and from deprocessing procedures for the failure analysis of microelectronic devices.