Noble Metal Corrosion: Halogen Bonded Iodocarbenium Iodides Dissolve Elemental Gold-Direct Access to Gold-Carbene Complexes

Iodine‐Catalysed Dissolution of Elemental Gold in Ethanol

Gold is a scarce element in the Earth's crust but indispensable in modern electronic devices. New, sustainable methods of gold recycling are essential to meet the growing eco‐social demand of gold. Here, we describe a simple, inexpensive, and environmentally benign dissolution of gold under mild conditions. Gold dissolves quantitatively in ethanol using 2‐mercaptobenzimidazole as a ligand in the presence of a catalytic amount of iodine. Mechanistically, the dissolution of gold begins when I₂ oxidizes Au⁰ and forms a [Au^II₂]⁻ species, which undergoes subsequent ligand‐exchange reactions and forms a stable bis‐ligand Au^I complex. H₂O₂ oxidizes free iodide and regenerated I₂ returns back to the catalytic cycle. Addition of a reductant to the reaction mixture precipitates gold quantitatively and partially regenerates the ligand. We anticipate our work will open a new pathway to more sustainable metal recycling with the utilization of just catalytic amounts of reagents and green solvents.

Crystal structures of the gold NHC complex bis-(4-bromo-1,3-di-ethyl-imidazol-2-yl-idene)gold(I) iodide and its 1:1 adduct with trans-bis-(4-bromo-1,3-diethyl-imidazol-2-yl-idene)di-iodido-gold(III) iodide

The first title compound, [Au(C7H11BrN2)2]I, crystallizes in the space group P without imposed symmetry. The cations and anions are linked to form chains by Br⋯I⋯Br halogen-bond linkages. The second title compound, [Au(C7H11BrN2)2][AuI2(C7H11BrN2)2]I2, is an adduct of the first and its formally I2-oxidized AuIII analogue. It also crystallizes in space group P , whereby both gold atoms occupy inversion centres. The extended structure is a reticular layer involving Br⋯I⋯Br and I⋯I⋯Au linkages.

Cooperative Ligands in Dissolution of Gold

Development of new, environmentally benign dissolution methods for metallic gold is driven by needs in the circular economy. Gold is widely used in consumer electronics, but sustainable and selective dissolution methods for Au are scarce. Herein, we describe a quantitative dissolution of gold in organic solution under mild conditions by using hydrogen peroxide as an oxidant. In the dissolution reaction, two thiol ligands, pyridine-4-thiol and 2-mercaptobenzimidazole, work in a cooperative manner. The mechanistic investigations suggest that two pyridine-4-thiol molecules form a complex with Au0 that can be oxidized, whereas the role of inexpensive 2-mercaptobenzimidazole is to stabilize the formed AuI species through a ligand exchange process. Under optimized conditions, the reaction proceeds vigorously and gold dissolves quantitatively in two hours. The demonstrated ligand-exchange mechanism with two thiols allows to drastically reduce the thiol consumption and may lead to even more effective gold dissolution methods in the future.

Oxidative Dissolution of Metals in Organic Solvents

Dissolution of metals in organic solvents is relevant to various application fields, such as metal extraction from ores or secondary resources, surface etching or polishing of metals, direct synthesis of organometallic compounds, and separation of metals from other compounds. Organic solvents for dissolution of metals can offer a solution when aqueous systems fail, such as separation of metals from metal oxides, because both the metal and metal oxide could codissolve in aqueous acidic solutions. This review critically discusses organic media (conventional molecular organic solvents, ionic liquids, deep-eutectic solvents and supercritical carbon dioxide) for oxidative dissolution of metals in different application areas. The reaction mechanisms of dissolution processes are discussed for various lixiviant systems which generally consist of oxidizing agents, chelating agents, and solvents. Different oxidizing agents for dissolution of metals are reviewed such as halogens, halogenated organics, donor-acceptor electron-transfer systems, polyhalide ionic liquids, and others. Both chemical and electrochemical processes are included. The review can guide researchers to develop more efficient, economic, and environmentally friendly processes for dissolution of metals in their elemental state.