Anion selectivity in zwitterionic amide-functionalised metal salt extractants

Amide‐functionalised salen ligands capable of extracting metal salts have been synthesised and characterised. Single‐crystal X‐ray structure determinations of complexes of NiSO4, [Ni(L)(SO4)], confirm that the ionophores are in a zwitterionic form with NiII bound in the deprotonated salen moiety and the SO42− ion associated with protonated pendant N′‐amidopiperazine groups. Treatment of [Ni(L)(SO4)] with base removes the protons from the pendant amido–amine group resulting in loss of the SO42− ion and formation of metal‐only complexes of type [Ni(L−2H)], which have been characterized by single‐crystal X‐ray diffraction. Three of the ligands with solubilities suitable for solvent extraction studies show loading and stripping pH‐profiles that are suitable for the recovery of CuSO4 or CuCl2 from industrial leach solutions. The copper‐only complexes, [Cu(L−2H)], are selective for Cl− over SO42− in both solvent extraction and bulk liquid membrane transport experiments and were found to bind Cl− in two steps via the formation of a 1:1:1 [Cu(L−H)Cl] assembly, followed by a 1:1:2 [Cu(L)Cl2] assembly as the pH of the aqueous phase is lowered. The anion transport selectivity was evaluated for a number of other mono‐charged anions and interestingly the ligands were found to display a preference for the Br− ion. To probe the influence of the Hofmeister bias on the selectivity of anion complexation, single‐phase potentiometric titration experiments were employed to investigate the binding of SO42− and Cl− by one of the copper only complexes, [Cu(L−2H)] in 95 %/5 % MeOH/water. Under these conditions selectivity was reversed (SO42−>Cl−) confirming that the Hofmeister bias, which reflects the relative hydration energies of the anions, dominates the selectivity of anion extraction from aqueous media into CHCl3