Metal-Assisted Deprotonation as a Key Step in Selective Copper Extraction: A Theoretical and Experimental Study

The growing demand for copper, together with the environmental limitations of conventional recovery methods, has intensified the search for extractants capable of operating directly in acidic mining solutions. In this work, a combined experimental-theoretical approach is presented to understand the coordination and extraction behaviour of Cu²⁺, Ni²⁺, Co²⁺ and Cd²⁺ ions with the ligand HDDMP (4-hexyl-dithiocarboxylate-5-hydroxy-3-methyl-1-phenylpyrazole). Experimental solvent-extraction tests show that copper forms stable coordination complexes even under highly acidic conditions (pH ≈ 0), unlike Ni²⁺, Co²⁺ and Cd²⁺, which require higher pH values for efficient extraction. DFT calculations reveal that Cu²⁺ promotes a spontaneous, low-barrier deprotonation-coordination process that is exergonic and electronically stabilised through strong Cu-S orbital interactions. This mechanism explains the exceptional selectivity of HDDMP towards copper, in which the copper ion acts simultaneously as both a coordinating centre and a deprotonating agent. These findings provide a molecular basis for designing new extractants suited to hydrometallurgical environments, offering direct industrial relevance for acidic copper-recovery circuits, minimising reagent consumption and improving selectivity in solvent-extraction processes widely used in mining operations.