Bile acid-based cyclic bisbenzimidazolium receptors for anion recognition: highly improved receptors for fluoride and chloride ions.

The anion binding properties of bile acid-based cyclic bisbenzimidazolium receptors 6-8 bridged with m-xylene, p-xylene, and 2,6-dimethylpyridine have been studied. Receptors 6 and 7 exhibit much higher binding affinity for fluoride and chloride ions, respectively, as compared to the imidazolium receptors 1 and 2. Receptor 8, however, shows high selectivity but very low binding affinity for anions due to the presence of pyridyl nitrogen. The single-crystal X-ray structure of imidazolium receptor 10-(Br)2 containing pyridyl spacer reveals the binding pattern.