• Plieger, Paul G.
• Lindoy, Leonard F.
• Collinson, Simon R.
• Blake, Alexander J.
• Tasker, Peter A.
• Galbraith, Stuart G.
• Li, Li
• Schröder, Martin
• Wang, Qiang
• Wilson, Claire

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^-IIbound in the deprotonated salen moiety and the SO₄^2- ion associated with protonated pendant N'-amidopiperazine groups. Treatment of [<i>N'</i>(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)Cl-₂] 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.

Topics

• impedance spectroscopy
• phase
• x-ray diffraction
• experiment
• copper
• amine
• solvent extraction
• titration