Comparison of pillar[5]arene and calix[4]arene anion receptor ability in aqueous media

Recognition ability of both cationic pillar[5]arene and calix[4]arene has been studied in aqueous media. Anion complexation can be evaluated from their ability to complex their counterions as well as an added external organic anion. DOSY NMR experiments and fluorescence quenching show that pillararenes have a larger ability for including their own counterions than calixarenes irrespective of the anion (tetrafluoroborate or chloride or bromide) and the structure of the cationic moiety (trimethylammonium or methylimidazolium). Counterion complexation shows a picture where four to five positive charges of the pillar[5]arene are neutralised, meanwhile only one positive charge of the calixarene is neutralised for a 1mM solution of the macrocycle. Irrespective of the smaller net positive charge in the pillar[5]arene, its binding ability for organic anions (toluenesulfonate or hydroxybenzoate) is larger than for calix[4]arene allowing a better accommodation of the guest in its cavity. The larger separation between the cationic groups of the receptor and its electron-rich aromatic region improves the anion recognition ability for pillar[5]arene.