Di-ionizable calix[4]arene-1,3-crown-4 ligands with an enlarged crown ring: synthesis and metal ion extraction

Di-ionizable calix[4]arene-1,3-crown-4 ligands with an enlarged crown ring were synthesized for comparison as metal ion extractants with analogues having identical pendent acidic groups, but a “normal” crown ring. The ligand conformation and regioselectivity are verified by NMR spectroscopy. Competitive solvent extraction of hard alkali metal cations and alkaline earth metal cations from aqueous solutions into chloroform by these new ligands were performed. Single species solvent extractions were conducted with intermediate Pb2+ and soft Hg2+. Comparison of the results with those from di-ionizable calix[4]arene-1,3-crown-4 ligands having “normal” crown rings provides insight into the effects of enlarging the crown ring as a function of the metal ion species being extracted and the identity of the pendent ionizable groups. Enlargement of the crown ring produced the largest increase in extraction efficiency with soft Hg2+.