Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 13, Issue 21, Pages 6091-6107Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200601874
Keywords
anions; copper; extractive metallurgy; hydrogen bonds; nickel; receptors; solvent extraction
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Funding
- Engineering and Physical Sciences Research Council [GR/S87713/01, EP/C528964/1] Funding Source: researchfish
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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 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)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 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.
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