Host–guest assembly of ligand systems for metal ion complexation; synergistic solvent extraction of copper(ii) ions by N3O2-donor macrocycles and carboxylic or phosphinic acids

Abstract

Host–guest formation between N₃O₂-donor macrocyclic hosts and lipophilic organic acid guests (that are themselves potential metal-ion ligands/extractants) has been investigated and the implications of formation of such ligand assemblies for the binding of metal ions have been probed. The formation of 2 ∶ 1 (organic acid ∶ macrocycle) assemblies has been demonstrated using NMR titration experiments in deuterochloroform for 4-tert-butylbenzoic, palmitic, phenylphosphinic, diphenylphosphinic and salicylic acids, with evidence for the additional formation of 1 ∶ 1 intermediates in some cases. Molecular adducts of this type have been used in solvent extraction (water/chloroform) experiments to define the effect of assembly formation on metal ion binding. From entropy considerations it was anticipated that the formation of particular host–guest species of the above type might result in enhanced metal ion binding (and hence enhanced metal ion extraction) – a consequence of the fact that the components of the coordination sphere are, at least in part, assembled for complex formation. Using a range of assemblies of the above type, enhanced (synergistic) extraction of Cu(II) ions was confirmed in all cases. Adducts of 1,12,15-triaza-3,4:9,10-dibenzo-5,8-dioxacycloheptadecane and 1,12,15-triaza-3,4:9,10-bis(4′-tert-butylbenzo)-5,8-dioxacycloheptadecane with 4-tert-butylbenzoic acid and salicylic acid have been isolated and their solid-state structures (and 1 ∶ 2 stoichiometries) confirmed by X-ray diffraction.