Synthesis, crystal structures, cation-binding properties and the influence of intramolecular C-H•••O interactions on the complexation behaviour of a family of cone p-tert-butylcalix[4]arene-crown-5 compounds

A family of cone p-tert-butylcalix[4]arene-crown-5 compounds with various substituents (H, COCH3, CH2CO2C2H5 and CH2CO2H) appended at the opposite phenolic oxygen atoms have been synthesised to evaluate their efficiency and selectivity towards different alkali and alkaline-earth metal ions, and also to ascertain the role of the appended side-arms in the complexation process. The selectivity of these ionophores towards Na+, K+, Mg2+, and Ca2+ has been evaluated with an aqueous solution containing an equimolar mixture of these ions. The concentration of metal ion in the extract (organic phase) has been estimated by ion chromatographic assay. Among these ions, K+ shows the highest selectivity in all cases except one, where the two phenolic oxygen atoms contain COCH3 substituents. All the ionophores show poor selectivity towards Mg2+ and Ca2+. Association constants (Ka) for the binding of Na+ and K+ to these ionophores have been determined spectrophotometrically. K-a (7.2 x 10(7)) is highest for the binding of K+ to the ionophore with CH2CO2C2H5 substituents. The molecular structures of four of the ionophores and four of the metal complexes have been established by single-crystal X-ray crystallography. Analysis of the structures revealed that in case of the ionophore with two COCH3 substituents, the C-H center dot center dot center dot O interactions form an eight-membered zigzag ring almost perpendicular to the crown ring, which prevents entry of the metal ions into the calix-crown cavity. The ionophore with CH2CO2C2H5 substituents, where no such interaction is observed, forms metal complexes easily and exhibits the highest association constant. H-1 and C-13 NMR studies have also been carried out to investigate the conformational behaviour of these ionophores and their metal complexes in solution. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)