SYNTHESIS, COMPLEXATION, AND MEMBRANE-TRANSPORT STUDIES OF 1,3-ALTERNATE CALIX[4]ARENE-CROWN-6 CONFORMERS - A NEW CLASS OF CESIUM SELECTIVEIONOPHORES

1,3-Dialkoxycalix[3]arene-crown-6 ionophores (4a-d) are obtained in th e fixed 1,3-alternate conformation in 63-85% yield by the reaction of the corresponding 1,3-dialkoxycalix[4]arenes 3a-d with pentaethylene g lycol ditosylate in CH3CN in the presence of Cs2CO3. The corresponding cone conformer of the diisopropyl derivative (6) was synthesized via selective demethylation of the 1,3-dimethoxycalix-crown (2a) and subse quent dialkylation. Extraction experiments with alkali picrates reveal a strong preference of ligands 4 for Cs+. Thermodynamic parameters ob tained for the complexation of 4a show that the high stability constan t in MeOH (log beta = 6.4 +/- 0.4) is reflected mainly in the large -D elta H-c value of 50.2 +/- 0.2 kJ . mol(-1). The entropy of complexati on (T Delta S-c = -15 kJ . mol(-1)), less negative than for other crow n ethers, is explained in terms of preorganization of 4a. The X-ray st ructure of the cesium picrate . 4a complex shows clearly that the cati on is positioned between the two aromatic rings with short Cs-C distan ces of 3.49 and 3.69 Angstrom, respectively. H-1 MMR spectroscopy conf irms this type of structure in solution. Ligands 4 incorporated in sup ported liquid membranes transport Cs+ cations with a high preference o ver Na+. By the application of an anion gradient, traces of Cs+ can be removed (greater than or equal to 99.8%) from acidic solutions (pH = 0) that contain 4 M of NaNO3.