Cucurbiturils as fluorophilic receptors

Abstract

<div><p>We report the formation of strong inclusion complexes between the macrocyclic host cucurbit[7]uril (CB7) and more than 20 perfluorinated compounds (PFCs) in aqueous solution. The binding constants and structures of the complexes were determined through a combination of dye displacement titrations, ¹⁹F as well as ¹H NMR spectroscopy, and quantum chemical calculations. The high affinities, for example 1.2 × 10⁷ M^− 1 for perfluorohexane and 1.1 × 10⁸ M^− 1 for perfluoro(methylcylohexane), are driven, among others, by the low polarisability of the cucurbituril cavity, which favours the binding of non-polarisable guests such as PFCs. The complexation-induced chemical shifts of the ¹⁹F resonances are downfield in cases where a conformational flexibility allows the (partial) interconversion of <i>gauche</i> conformations (favoured in water) to <i>anti</i> conformations (favoured inside the cavity of CB7). Only for conformationally rigid substrates, for example SF₆ and (CF₃)₃COH, upfield shifts prevail. These are attributed to the low polarisability of the inner cavity, similar to the upfield shifts observed for ¹⁹F resonances upon going from highly polarisable to less polarisable solvents. Cucurbituril homologues (CB5, CB6 and CB8) were also studied and, among those, evidence for the formation of inclusion complexes was observed for CB6 as host and CF₄, SF₆ and CF₃CH₂OH as guests. Besides contributing to the fundamental understanding of fluorophilic interactions, the results have implications for PFC separation and for the efficient removal of PFCs from and the sensing of PFCs in aqueous solution.</p></div