Targeted ligand design using a combined computational and experimental approach to unlock new metal separations

As societal demand for metals grows, sustainable routes to their efficient recovery from primary ores and secondary waste streams have become increasingly important. In this regard, understanding the chemical detail of metal separations is implicit to designing more efficient and environmentally benign processes. Herein, we report a combined computational and experimental approach to targeted ligand design by modification of a simple diamide (L), previously reported as a selective precipitant for Au. This afforded three new reagents through the addition of electron-donating (LOMe) and withdrawing (LCl) groups, and the inversion of the amide linkage (Linv). Computational studies confirmed the expected trends in protonation of the amide and subsequent metal uptake experiments showed all ligands act as selective precipitants for Au at low acid concentrations in the presence of an excess of ligand. Significantly however, the use of stoichiometric amounts of LOMe and LCl resulted in a switch in selectivity from square-planar (AuCl4-) to tetrahedral metalates (GaCl4- and FeCl4-), with precipitation suppressed for Linv. Computational modelling rationalised this reversal in metalate selectivity with LOMe and LCl on thermodynamic grounds, while Hirshfeld surfaces, NCI plots and QTAIM analyses highlighted halogen bonding as the most important structure directing interaction. Importantly, preliminary studies with LOMe suggested a viable pathway to the separation of Ga and Fe from industrially generated waste streams, which could be exploited in the recycling of zinc residues and end-of-life GaN light-emitting diodes.