THERMODYNAMIC STUDIES ON ALKALI-METAL COMPLEXES OF CALIX[4]-CROWNS-5 - INFLUENCE OF THE CONFORMATION

The extracting and complexing properties of the three possible stereoi somers of the 1,3-diisopropoxy p-H calix[4]-crowns-5: 1,3-alternate, 1 a, partial cone, 1b, and cone, 1c, have been studied towards alkali me tal and Sr2+ cations. The properties of two related conformationally m obile p-tert-butyl and p-H analogues (2a and 2b) have also been invest igated for comparison purpose. Within the alkali series, all the compo unds are selective for K+ and Rb+ in extraction and for K+ in complexa tion. Their efficiency and their selectivity have been found to decrea se according to the sequence: 1a>1b>2b>1c. In methanol, the 1,3-altern ate derivative 1a is at least 15 times more selective to K+ with respe ct to Na+ than its partial cone couterpart 1b which is itself 10 times more selective than the natural antibiotic valinomycin, the best K+ i onophore known to date. The stabilisation of the K+ and Rb+ complexes is enthalpy driven, and the better performance of the 1,3-alternate de rivative in comparison to its partial cone counterpart originates from a more favourable (or less unfavourable) entropy change overcoming a lower enthalpy change which can be related to the better preorganisati on of the former ligand. Neither extraction nor complexation of Sr2+ h , been shown with these ligands, whereas Ag+ is well complexed.