Synthesis, crystal structures and competitive binding property of a family of calix[4]arene-biscrown-5/6 and their application in extraction of alkali metal ions from sea bittern

Abstract

A family of calix[4]arene compounds containing crown moieties at both the upper and lower rims of the calix unit with different substituents attached to the nitrogen atom of the aza-crown moiety have been synthesized, characterized and their competitive binding property towards Li+, Na+, K+, Mg2+, Ca2+ and Sr2+ in aqueous media has been investigated. These ionophores exhibit high selectivity towards K+, assessed by two-phase extraction method followed by ion chromatographic assay of the metal ions in the extract. Isothermal calorimetric titration for complex formation with K+ was carried out to evaluate stoichiometry, binding constant and other thermodynamic parameters (ΔH, ΔS and ΔG). For structural elucidation in solution, detail 1H NMR study was carried out and K+ complexes were isolated for solid state characterization. Molecular structures of two of the ionophores and K+-complex of one of the ionophores has been established by single crystal X-ray study. 1H NMR, ITC and single crystal XRD studies suggest 1:1 complex formation with encapsulation of K+ in the crown-6 ring at the upper rim. These ionophores have been applied for extraction of metal ions from bittern.