Calix[4]Arene Anion Receptors Bearing 2,2,2-Trifluoroethanol Groups at the Upper Rim

Several di- and tetrafunctionalized anion receptors have been synthesized by attaching 2,2,2-trifluoroethanol binding groups at the upper rim of cone calix[4]arenes using two different synthetic procedures. The best results were obtained by treating calix[4]arene formyl derivatives with trifluoromethyltrimethylsilane and tetrabutylammonium fluoride in dry THF. The bis-trifluoroethanol calix[4]arene receptors are able to bind anions showing selectivity for carboxylates and dihydrogenphosphate but at a lower efficiency compared to analogous receptors bearing urea, sulfonamide or activated amide binding groups. The conformational properties of the free ligands and their acetate complexes have been investigated by Dynamic (1)H NMR, Molecular Modeling and in one case, by X-ray crystallography. Calix[4]arenes bearing cation coordinating groups at the lower rim and 2,2,2-trifluoroethanol moieties at the upper rim behave as ditopic receptors, since they bind simultaneously cation and anion and extracts ion pairs in organic media. In one case evidence was obtained that coordination of sodium metal ion at the lower rim enhances the binding of acetate anion at the upper rim (positive allosteric effect).