Synthesis of new proton-ionizable macrocyclic ligands
Calixarene is a scaffold for the construction of new metal ion receptors. Introduction of a crown ether loop at the lower rim of calixarenes not only increases the cation binding ability of the parent calixarene, but allows control of the selectivity through the modulation of the crown ether ring size. However, research on 1,2-bridged calixcrowns is rare. Known members are limited. In general, they showed low binding ability and selectivity toward metal cations. In this dissertation, five series of di-ionizable calixarene-1,2-crown ethers in the cone conformation were prepared as potential metal ion extractants. The conformations and regioselectivities of the new ligands were verified by 1H and 13C NMR spectroscopy. With the variation of proton-ionizable groups, which include oxyacetic acid and N-(X)sulfonyl oxyacetamide units with X = methyl, phenyl, 4-nitrophenyl, and trifluoromethyl, ¡°tunable¡± acidity of the ligand is obtained. Competitive solvent extraction of alkaline earth metal cations reveals that di-ionizable calixarene-1,2-crown compounds are effective extractants with high selectivity for Ba2+. To our knowledge, this is the first example of a calixarene-1,2-crown ether ligand is efficient and selective in metal cation separations. To explore the influence of additional cation-¦Ð interactions between the aromatic pendant on lariat ether and the bound metal ion, one series of proton-ionizable dibenzo-16-crown-5 ethers with a sterically very bulky, 9-phenanthryl group was synthesized. The ligands are found to be effective extractants with high selectivity for Na+ in competitive solvent extraction of alkali metal cations. The Na+/K+ selectivity is three times larger than the corresponding values of those geminal aliphatic, proton-ionizable dibenzo-16-crown-5 compounds reported in literature.