Carbazole diester strapped calix[4]pyrrole: Synthesis, anion binding and self-assembly based on ion pair recognition

Abstract

A new carbazole strapped calix[4]pyrrole 1 has been synthesized and ion binding properties was investigated using 1H NMR, isothermal titration calorimetry and single crystal X-ray diffraction analyses. Compared to the parent octamethyl calix[4]pyrrole 2, receptor 1 showed enhanced binding towards halide anions. Solid state structural analysis revealed that receptor 1 is capable of binding various halide salts showing different binding modes depending on the bound ion pairs. Interestingly, complexes of tetrabutyammonium (TBA+), tetraethylammonium (TEA+) and tetramethylammonium (TMA+) salts of chloride anion displayed three distinctly different binding geometries in the solid state. As a result, the linear supramolecular polymers seen in the crystal lattices are also dramatically different for these three chloride complexes of 1. Combination of 1H NMR and solid state studies revealed that binding of anion in one condition do not translates to another completely. Receptor 1 also found to form 1/1 complexes with CsCl, CsBr and CsI salts and 1D coordination polymer chains are stabilized in the solid state. Overall, construction of four different 1D linear chains are described for chloride anion based on ion pair recognition. In addition, the solid state structure of ion pair complex with CsI salt is unprecedented in the calix[4]pyrrole chemistry. Moreover, under solid-to-liquid extraction condition, receptor 1 is able to solubilise CsF and CsCl ion pairs in chloroform in which these salts are otherwise insoluble.