Synthesis, crystal structures and competitive binding property of a family of functionalized calix[4]arene ionophores

A family of calix-crown hybrid molecules incorporating crown-4, crown-5 and crown-6 moieties as ionophore has been synthesized to evaluate their competitive complexing and extracting ability towards different alkali and alkaline earth metal ions. Selectivity of these ionophores towards Li+, Na+, K+, Ca2+, Mg2+ and Sr2+ has been evaluated with aqueous solution containing equimolar mixture of these ions. The concentration of metal ion in the extract (organic phase) has been estimated by ion chromatographic assay. Two of the ionophores show strong complexation with Na+ and K+, whereas the third ionophore exhibits no complexation with any metal ions used in this study. Association constants (Ka) for the binding of Na+ and K+ with two of these ionophores have been determined spectrophotometrically. Molecular structures of the Na+ and K+ complexes of these ionophores have been established by single crystal X-ray study. 1H NMR study of the ionophores and their complexes has also been carried out to investigate conformational behavior of the ionophores and their metal complexes in solution. Results have been discussed in light of various factors contribute to determine ion-selectivity.