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

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.

Graphical abstractCompetitive metal ion binding property of a family of calix-crown hybrid ionophore has been reported. 1H NMR, ITC and crystal structure studies were carried out for structure elucidation and to determine thermodynamic parameters for complexation. The ionophores were applied for extraction of K+ from a natural source such as sea bittern.Figure optionsDownload full-size imageDownload as PowerPoint slide