Highly selective copper(II) ion receptors: tetraazacrown ethers bearing two 8-hydroxyquinoline side arms

Four 8-hydroxyquinoline-containing tetraazacrown ethers were studied for their potential application as highly selective Cu2+ receptors. Equilibrium constants for complexation of these tetraazacrown ethers with H+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ were determined in aqueous solution at 25°C by potentiometry. The most stable complexes were formed with Cu2+. The UV–Vis spectra of two of the ligands were studied in the presence and absence of the metal ions. The Cu2+ complexes exhibited unique absorption peaks at 655 nm in aqueous acetic acid buffer solution (pH 4.76). Thus, these systems demonstrated copper(II) ion selectivity. Three of the ligands showed both 1:1 and 1:2 complexation with Cu2+ (L:Cu2+). In competitive transport experiments using bulk liquid membranes, Cu2+ was transported selectively from a mixture containing all of the above metal ions.