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.

Reactions of N-heterocyclic carbene-based chalcogenoureas with halogens: a diverse range of outcomes

We have investigated the reactions of chalcogenoureas derived from N-heterocyclic carbenes, referred to here as [E(NHC)], with halogens. Depending on the structure of the chalcogenourea and the identity of the halogen, a diverse range of reactivity was observed and a corresponding range of structures was obtained. Cyclic voltammetry was carried out to characterise the oxidation and reduction potentials of these [E(NHC)] species; selenoureas were found to be easier to oxidise than the corresponding thioureas. In some cases, a correlation was found between the oxidation potential of these compounds and the electronic properties of the corresponding NHC. The reactivity of these chalcogenoureas with different halogenating reagents (Br₂, SO₂Cl₂, I₂) was then investigated, and products were characterised using NMR spectroscopy and single-crystal X-ray diffraction. X-ray analyses elucidated the solid-state coordination types of the obtained products, showing that a variety of possible adducts can be obtained. In some cases, we were able to extrapolate a structure/activity correlation to explain the observed trends in reactivity and oxidation potentials.

Charge-Transfer Adducts of Chalcogenourea Derivatives of N-Heterocyclic Carbenes with Iodine Monochloride

In this communication, we investigate the reaction between seleno- and thiourea, derived from N-heterocyclic carbenes, with the interhalogen iodine monochloride. The formation of all three products was confirmed by NMR spectroscopy, while single-crystal X-ray analyses were able to establish a charge-transfer coordination type, which showed a linear Se/S-I-Cl arrangement, for all adducts formed. Based on a detailed crystallographic analysis, we can deduce the zwitterionic character of these compounds.

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.

Oxidant/complexing properties of the methimazole (MeImHS)/iodine system towards palladium and gold metals. Crystal structure of the complex cation [PdII(MeImHS)4]2+ balanced by a tetraiodide/iodide mixture

The oxidative dissolution of palladium is easily and safely achievable both in dichloromethane and water by the methimazole/I₂ mixture.

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

A common method to dissolve elemental gold involves the combination of an oxidant with a Lewis base that coordinates to the gold surface, thus lowering the metal's redox potential. Herein we report the usage of organic iodide salts, which provide both oxidative power and a coordinating ligand, to dissolve gold under formation of organo-gold complexes. The obtained products were identified as Au^III complexes, all featuring Au-C bonds, as shown by X-ray single-crystal analysis, and can be isolated in good yields. Additionally, our method provides direct access to N-heterocyclic carbene (NHC-type) complexes and avoids costly organometallic precursors. The investigated complexes show dynamic behavior in acetonitrile and in the case of the NHC(-type) complexes, the involved species could be identified as a monocarbene [AuI₃ (carbene)] and biscarbene complex [AuI₂ (carbene)₂ ]⁺ .

Transition metal mediated formation of dicationic diselenides stabilised by N-heterocyclic carbenes: designed synthesis

Designed syntheses of dicationic diselenides have been developed by (i) reduction of dihaloselones (routes A and B) and (ii) by oxidation of selone adducts of metal halides (route C).