Cooperative Ligands in Dissolution of Gold

Abstract

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.

Iodine‐Catalysed Dissolution of Elemental Gold in Ethanol

“…As seen from the dissolution curve (Figure 3), complete dissolution is reached at 13 h. In general, it is worth to notice that dissolution of gold in organic solution is faster than optimized cyanide leaching [54] but it is very much depending on the ligand, ligand concentration and reaction conditions. Here as the reaction is based on just catalytic amount of I 2 , the dissolution rate is lower (0.15 mg h −1 ) than in previous examples with marked excess of 4‐PS [54] (12.9 mg h −1 ) but only slightly lower than in the recently reported bi‐ligand systems [59] (1 mg h −1 ).…”

“…First, I 2 oxidizes Au 0 to Au I by forming [AuI 2 ] − species (Figure 2, 1 ) identified from the ESI‐HRMS spectra (Figure S8). 1 , a labile complex, readily undergoes a ligand exchange reaction, and one iodide ligand is substituted with a S‐ bonding 2‐MBI molecule in its dominant thione tautomeric form [54, 59, 60, 61] . The formation of a neutral mixed‐ligand species (Figure 2, 2 ) is a fingerprint of ligand exchange reaction and observed by ESI‐HRMS as [ M −H] − (Figure S9).…”

“…[54,59,60,61] The formation of a neutral mixed-ligand species (Figure 2, 2) is a fingerprint of ligand exchange reaction and observed by ESI-HRMS as [MÀ H] À (Figure S9). After the second iodide is replaced with yet another 2-MBI molecule, relatively stable complex 3 [59,60,61] (Figure 2, 3) is formed and detected by ESI-HRMS in positive and negative mode as [M] + (Figure S13) and [MÀ 2 H] À (Figure S10), respectively. Both, OH À or SO 4 2À that are produced by redox reactions between H 2 O 2 and I À or 2-MBI, could serve as a counterion for 3.…”

“…1, a labile complex, readily undergoes a ligand exchange reaction, and one iodide ligand is substituted with a S-bonding 2-MBI molecule in its dominant thione tautomeric form. [54,59,60,61] The formation of a neutral mixed-ligand species (Figure 2, 2) is a fingerprint of ligand exchange reaction and observed by ESI-HRMS as [MÀ H] À (Figure S9). After the second iodide is replaced with yet another 2-MBI molecule, relatively stable complex 3 [59,60,61] (Figure 2, 3) is formed and detected by ESI-HRMS in positive and negative mode as [M] + (Figure S13) and [MÀ 2 H] À (Figure S10), respectively.…”

Iodine‐Catalysed Dissolution of Elemental Gold in Ethanol

“…To further improve the approach, we have recently developed a novel strategy based on the ligand exchange on Au I cation. By introducing inexpensive 2‐mercaptobenzimidazole (2‐MBI), a safe compound used in medicinal applications, [55–58] the total amount of thiols is dramatically reduced (Figure 1, d) [59] . When dealing with complex metal mixtures, sequential dissolution seems like a realistic solution.…”