Complexation of Alkali Metal Cations by Calix[4]-Crowns-5: Influence of the Conformation

The extracting and complexing properties of the three possible stereoisomers of the 1,3-diisopropoxy p-H calix[4]-crowns-5: 1,3-alternate, 1a, partial cone, 1b, and cone, 1c, have been studied towards alkali metal and Sr(2+) cations. The properties of two related conformationally mobile p-tert-butyl and p-H analogues (2a and 2b) have also been investigated for comparison purpose. Within the alkali series, all the compounds are selective for K(+) and Rb(+) in extraction and for K(+) in complexation. Their efficiency and their selectivity have been found to decrease according to the sequence: 1a>1b>2b>1c. In methanol, the 1,3-alternate derivative 1a is at least 15 times more selective to K(+) with respect to Na(+) than its partial cone couterpart 1b which is itself 10 times more selective than the natural antibiotic valinomycin, the best K(+) ionophore known to date. The stabilisation of the K(+) and Rb(+) complexes is enthalpy driven, and the better performance of the 1,3-alternate derivative 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 preorganisation of the former ligand. Neither extraction nor complexation of Sr(2+) h, been shown with these ligands, whereas Ag(+) is well complexed.