Anion-selective receptors based on dinuclear copper(II) and nickel(II) cage complexes of bis-salicylaldimines

The selectivity, anion uptake and exchangeability of anion-binding by metal salt extractants of the form [M2L2](4+) have been assessed by the method of anion exchange chromatography in biphasic systems. The order of sulfate-, nitrate-, and chloride-uptake into the solid copper(II) complex as of the dioxime pro-ligand N,N'-dimethyl-N,N'-hexamethylenedi(3-hydroxyiminomethyl-2-hydroxy-5-tert-butylbenzylamine (L-1) is 56, 42, and 16%, respectively, consistent with the relative magnitudes of formation constants for the inclusion complexes, [A subset of Cu2L21](n+) where A = anion, found in UV-vis titration studies in a single phase. X-ray structural determination of the bis-benzylimine pro-ligand, N,N'-dimethyl-N,N'-hexamethylenedi-(3-benzyliminomehyl-2-hydroxy-5-tert-butylbenzylamine), nickel(II) sulfato complex [SO4 subset of Ni2L22]SO4 reveals the nickel atoms to have a significant tetrahedral distortion, providing more favourable sulfate-alkylammonium interactions within the cage.