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

Abstract

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. Ka (7.2 × 107) 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...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. ¹H and ¹³C NMR studies have also been carried out to investigate the conformational behaviour of these ionophores and their metal complexes in solution.