In our research, elemental Au is dissolved into organic solutions of coordinating ligands, mainly heterocyclic thiols, using mild oxidants. Recently, we reported a highly efficient dissolution method, where elemental Au was quantitatively dissolved when combined with 4-pyridinethiol (4-PSH), octasulfur and hydrogen peroxide. The following mechanistic steps were experimentally verified: isomerization of 4-PSH to pyridine-4-thione (4-PS); coordination of 4-PS with Au0 centers; oxidation of Au0 thione to AuI simultaneously with the oxidation of free 4-PS into several S-based organic side-products and elemental sulfur (Figure 1). Combined experimental and computational investigations revealed that the preference of 4-PSH ligand is based on its good coordinative ability with Au0 along with the low oxidation potential of the generated bisligand intermediate species. As a result of one-electron oxidation, AuI coordinates two neutral 4-PS ligands and SO42- anion balances the overall charge of two adjacent AuI centers. A solid state structure of 1 [bis(pyridine-4-thione)Au]2SO4 illustrates linear coordination sphere of AuI and the weak aurophilic interaction (3.1314 Å). The dissolution is selective towards metals with stable oxidation state +1 offering the possibility to recover Au from e.g. Au/Pd/Pt mixtures.

Ligand-assisted dissolution of gold in organic solvents

2019 Fall National Meeting

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