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

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(4ʹ-tertbutylbenzo)-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.