Cation and anion selectivity of zwitterionic salicylaldoxime metal salt extractants

Abstract

3-Dialkylaminomethyl substituted salicylaldoximes are efficient metal salt extractants, and, in contrast to related “salen”-based reagents, are sufficiently stable to acid hydrolysis to allow commercial application in base metal recovery. Crystal structures show that metal salts are bound by a zwitterionic form of the reagents, with copper(II) nitrate, tetrafluoroborate and trifluoroacetate forming [Cu(L)₂X₂] assemblies in a tritopic arrangement with a trans-disposition of the anions outwith the coordination sphere. Copper(II) chloride, bromide and zinc(II) chloride form 1:1 assemblies, [Cu(L)X₂], with the halides in the inner coordination sphere of the metal, leading to high chloride selectivity and very good mass transport efficiencies of CuCl₂. Introduction of the anion-binding sites into the salicylaldoxime extractants changes their cation selectivities; the ligands co-extract small amounts of Fe^III along with Cu^II from mixed metal aqueous feed solutions, an issue which will need to be addressed prior to industrial application.