Synthesis, Complexation and Membrane Transport Studies of 1,3-Alternate Calix[4]arene-crown-6 Conformers: A New Class of Cesium Selective Ionophores

1,3-Dialkoxycalix[3]arene-crown-6 ionophores (4a-d) are obtained in the fixed 1,3-alternate conformation in 63-85% yield by the reaction of the corresponding 1,3-dialkoxycalix[4]arenes 3a-d with pentaethylene glycol 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 subsequent dialkylation. Extraction experiments with alkali picrates reveal a strong preference of ligands 4 for Cs+. Thermodynamic parameters obtained for the complexation of 4a show that the high stability constant in MeOH (log beta = 6.4 +/- 0.4) is reflected mainly in the large -Delta H-c value of 50.2 +/- 0.2 kJ . mol(-1). The entropy of complexation (T Delta S-c = -15 kJ . mol(-1)), less negative than for other crown ethers, is explained in terms of preorganization of 4a. The X-ray structure of the cesium picrate . 4a complex shows clearly that the cation is positioned between the two aromatic rings with short Cs-C distances of 3.49 and 3.69 Angstrom, respectively. H-1 MMR spectroscopy confirms this type of structure in solution. Ligands 4 incorporated in supported liquid membranes transport Cs+ cations with a high preference over 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.