title: Anion selectivity in zwitterionic amide-functionalised metal salt extractants.
abstract: 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 Ni(II) bound in the deprotonated salen moiety and the SO4(2-) 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 SO4(2-) 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 SO4(2-) 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 SO4(2-) and Cl- by one of the copper only complexes, [Cu(L-2H)] in 95 %/5 % MeOH/water. Under these conditions selectivity was reversed (SO4(2-)>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.
authors.affiliation: School of Chemistry, University of Edinburgh, Edinburgh, EH9 3JJ, UK.
authors.forename: Stuart G
authors.initials: SG
authors.lastname: Galbraith
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
authors.forename: Li
authors.initials: L
authors.lastname: Li
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Blake
authors.forename: Claire
authors.initials: C
authors.lastname: Wilson
authors.forename: Simon R
authors.initials: SR
authors.lastname: Collinson
authors.forename: Leonard F
authors.initials: LF
authors.lastname: Lindoy
authors.forename: Paul G
authors.initials: PG
authors.lastname: Plieger
authors.forename: Martin
authors.initials: M
authors.lastname: Schröder
authors.forename: Peter A
authors.initials: PA
authors.lastname: Tasker