Pyridinethiol‐Assisted Dissolution of Elemental Gold in Organic Solutions

Abstract

Dissolution of elemental gold in organic solutions is ac ontemporary approach to lower the environmental burden associated with gold recycling.Herein, we describe fundamental studies on ah ighly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine-4-thiol (4-PSH) as ar eactive ligand and hydrogen peroxidea sa no xidant. Dissolution of Au proceeds through several elementary steps:i somerization of 4-PSH to pyridine-4-thione (4-PS), coordination with Au 0 ,and then oxidation of the Au 0 thione species to Au I simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is aA u I complex bearing two 4-PS ligands and SO 4 2À as ac ounterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Cooperative Ligands in Dissolution of Gold

“…Also, the number of signals (equivalent carbons) in the 13 C NMR spectrum further implies that 1 is present in a C 2 ‐symmetric thione form; if otherwise, the absence of a NH group would destroy the symmetry of the molecule. A similar tendency was observed earlier for 4‐PS [18] . Unavoidably, aromatic thiols are easily oxidized in the DMF‐ d 7 /H 2 O 2 solution, and accordingly, a new singlet appears at δ9.5 in the aromatic region of the spectrum, indicating the formation 1H ‐benzimidazole (Figure 2, 7 , 7 %), an oxidized derivative of 1 .…”

“…The thione form of 4‐PS coordinates to the Au 0 surface, and the formed bisliganded Au 0 species assist the one‐electron oxidation and dissolution of Au I . The in situ formed sulfate anion balances the charge [18] …”

Evaluation of Simple Amides in the Selective Recovery of Gold from Secondary Sources by Solvent Extraction

“…This process forms, through heap leaching, solutions of the water-soluble Au(CN)2complex, which is recovered from solution by precipitation, adsorption, and reduction. 14 The environmental and safety concerns over cyanidation has led to recent studies on alternative leaching and separation methods, 15 including the use of various combinations of oxidizing agents and organic solvents, [16][17][18][19] oxidative mechanochemistry, 20 , electrochemical dissolution, 21 and Metal-Organic Framework (MOF) -polymer composites. [22][23][24][25] .…”

Pyridinethiol‐Assisted Dissolution of Elemental Gold in Organic Solutions

“…According to the calculations,t he significant lowering of the oxidation potential of bisligand Au 0 species is caused by delocalization of the 6s 1 electron on the ligands,t hus allowing the one- Step IV:F urther oxidation of S 8 to sulfuric acid. [30] Angewandte Chemie Zuschriften electron oxidation of the ligand instead of the metal. This theoretical insight further supports the experimental results, which showed that Pd and Pt were not dissolved with pyridinethiol ligands as their dissolution would require twoelectron oxidation (see Section S5.2).…”

“…Step I: Isomerization of pyridine-4-thioltop yridine-4-thione and coordination to the Au 0 surface. Step II/III:Oxidation of Au 0 to Au I and simultaneous oxidation of free pyridine-4-thione.Step IV:F urther oxidation of S 8 to sulfuric acid [30].…”