Gold precipitation by diamides - ICP-OES and NMR data

The efficient separation of gold and other metals from their ores and composite secondary sources such as electronic waste is necessary to realising circularity in metal supply chains. Processes based on precipitation are becoming increasingly popular and their development is reliant on designing and understanding chemical recognition to achieve selectivity. Here we show that the simple tertiary diamide PhC(O)NMe(CH2CH2)NMeC(O)Ph (L) precipitates gold selectively and directly from aqueous acidic solutions as the ion pair [HL][AuCl4], including from aqua regia solutions of electronic waste. The X-ray crystal structure of the precipitate displays an infinite chain of diamide cations, formed through an intermolecular proton chelate, interleaved with tetrachloridoaurate. Gold is released quantitatively from the isolated precipitate as HAuCl4 on contact with water, enabling ligand recycling. The diamide is highly selective, with its addition to a mixture of 29 metals in 2 M HCl resulting in 70% gold uptake and minimal removal of all other metals. Increasing the HCl concentration to 6 M allows complete collection of gold, iron, tin, and platinum, demonstrating that L is an adaptable selective metal precipitant controlled by just one variable. Thus, this discovery could be exploited in metal refining and recycling processes due to its tuneable selectivity under different metal leaching conditions, the avoidance of organic solvents inherent to biphasic extraction, and the straightforward recycling of the precipitant.