Solvent extraction of alkaline-earth ions by dicarboxylated calix[4]arenes

The extraction of the alkaline-earth metal ions by 25,27-dicarboxymethyl-26,28-dimethoxy-5,11,17,23-tetra-tert-butylcalix[4]arene ((LH2)-H-I) and 25,27-dicarboxymethyl-26,28-dimethoxyethoxy-5,11,17,23-tetra-tert-butylcalix[4]arene ((LH2)-H-II) in 1,2-dichloroethane is shown to be pH dependent. The analysis of the extraction equilibrium results indicates that the extraction process takes place via the formation of (ML2H2)-H-I (M2+ = Ca2+, Sr2+ and Ba2+), (ML2H2)-H-II (M2+ = Ca2+ and Sr2+) and MLII (M2+ = Ba2+) complexes. Both ligands show a high selectivity for calcium. The existence of the ML2H2 complexes is supported by H-1-NME and FAB(+) experiments on the corresponding Ca2+ complexes. H-1-NMR investigations show that the calcium ion is located outside the calixarene cavity. Competitive extraction of Mg, Ca, Sr and Ba suggests the formation of mixed complexes. (C) 1998 Elsevier Science Ltd. All rights reserved.