DOI: 10.1002/chem.200601874 Anion Selectivity in Zwitterionic Amide-Functionalised Metal Salt Extractants Stuart G. Galbraith, [a] Qiang Wang, [b] Li Li, [b] Alexander J. Blake, [b] Claire Wilson, [b] Simon R. Collinson, [b] LeonardF. Lindoy, [c] PaulG. Plieger, [d] Martin Schrçder,* [b] and PeterA. Tasker* [a] Introduction Cation-exchange reagents such as the phenolic oximes [1,2] are well suited to solvent extraction processes used for the recoveryofbasemetalsfromtheiroxidicoresafterleaching with sulfuric acid, and such processes now account for around 30% of the world)s copper production. [3] Different types of hydrometallurgical flowsheets are needed for metal-recoveryfromsulfideoresinordertoexploitarange of very efficient leaching processes that have been devel- oped recently by the mining industry. [4] Extraction of a metal salt to achieve the unit operations of concentration andseparation(Figure1)hasbeensuggestedasanapproach that will result in better materials balances, particularly from pregnant leach solutions, which contain high concen- trationsofmetalsalts. [2,5,6] Transportofametalsaltacrossa flowsheetbymeansofawater-immisciblesolventcanbefa- cilitated through the use of zwitterionic heterotopic ligands Abstract: Amide-functionalised salen ligands capable of extracting metal saltshavebeensynthesisedandcharac- terised. Single-crystal X-ray structure determinations of complexes of NiSO 4 , [Ni(L)ACHTUNGTRENNUNG(SO 4 )], confirm that the iono- phores are in a zwitterionic form with Ni II bound in the deprotonated salen moiety and the SO 4 2 ion associated with protonated pendant N’-amidopi- perazine groups. Treatment of [Ni(L)- ACHTUNGTRENNUNG(SO 4 )] with base removes the protons from the pendant amido–amine group resulting in loss of the SO 4 2 ion and formation of metal-only complexes of type [NiACHTUNGTRENNUNG(L2H)], which have been characterized by single-crystal X-ray diffraction. Three of the ligands with solubilities suitable for solvent extrac- tionstudiesshowloadingandstripping pH-profilesthataresuitableforthere- coveryofCuSO 4 orCuCl 2 from indus- trial leach solutions. The copper-only complexes, [CuACHTUNGTRENNUNG(L2H)], are selective for Cl  over SO 4 2 in both solvent ex- traction and bulk liquid membrane transport experiments and were found tobindCl  intwostepsviatheforma- tion of a 1:1:1 [CuACHTUNGTRENNUNG(LH)Cl] assembly, followed by a 1:1:2 [Cu(L)Cl 2 ] assem- bly as the pH of the aqueous phase is lowered. The anion transport selectivi- tywasevaluatedforanumberofother mono-charged anions and interestingly the ligands were found to display a preference for the Br  ion. To probe theinfluenceoftheHofmeisterbiason the selectivity of anion complexation, single-phase potentiometric titration experiments were employed to investi- gate the binding of SO 4 2 and Cl  by oneofthecopperonlycomplexes,[Cu- ACHTUNGTRENNUNG(L2H)] in 95%/5% MeOH/water. Under these conditions selectivity was reversed (SO 4 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 intoCHCl 3 . Keywords: anions · copper · extrac- tive metallurgy · hydrogen bonds · nickel · receptors · solvent extraction [a] Dr.S.G.Galbraith,Prof.P.A.Tasker SchoolofChemistry,UniversityofEdinburgh Edinburgh,EH93JJ(UK) Fax:(+ 44)131-650-6543 [b]Dr.Q.Wang,Dr.L.Li,Prof.A.J.Blake,Dr.C.Wilson, Dr.S.R.Collinson,Prof.M.Schrçder SchoolofChemistry,TheUniversityofNottingham UniversityPark,Nottingham,NG72RD(UK) Fax:(+ 44)115-951-3563 [c] Prof.L.F.Lindoy CentreforHeavyMetalResearch,SchoolofChemistryF11 UniversityofSydney,N.S.W.(Australia) [d] Dr.P.G.Plieger InstituteofFundamentalSciences,MasseyUniversity PrivateBag11222,PalmerstonNorth(NewZealand) SupportinginformationforthisarticleisavailableontheWWW underhttp://www.chemistry.orgorfromtheauthor. Chem. Eur. J. 2007, 13,6091–6107 #2007Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim 6091 FULL PAPER