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The cation-π interaction and the hydrophobic effect are important intermolecular forces in chemistry and play major roles in controlling recognition in biological systems. We compared their relative contributions to the binding of molecular "dumbbell" guests in synthetic container hosts in water. The guests offered direct, intramolecular competition between trimethylammonium groups, -N+(CH3)3, and tert-butyl groups, -C(CH3)3, for the internal surfaces (aromatic panels) of the containers. In contrast with previous studies, the container molecules consistently preferred binding to the uncharged tert-butyl groups, regardless of the presence of anionic, cationic, or zwitterionic groups on the container peripheries. This preference is determined by solvation of the polar trimethylammonium group in water, which outcompetes the attraction between the positive charge and the π-surfaces in the container. The synthetic container complexes provide a direct measure of the relative strengths of cation-π interactions and desolvation in water. Interactions with the uncharged tert-butyl group are more than 12 kJ mol-1 more favorable than the cation-π interactions with the trimethylammonium group in these cavitand complexes.

We report the synthesis and characterization of a new water-soluble cavitand 1. The container features 2-aminobenzimidazole panels at the "rim" and pyridiniums at the "feet". In the solid state, a single-crystal X-ray structure of the organic-soluble precursor 2 showed a stable vase form. The structure is stabilized by hydrogen-bonded bridges between adjacent panels through solvents and ions. In aqueous solution, binding of hydrophobic and amphiphilic guest molecules to 1 was investigated using 1H NMR. Alkanes, alcohols, acids, diols, and diacids formed 1:1 host-guest complexes, and the guest conformations were deduced from characteristic chemical shift changes. In the presence of [Pd(ethylenediamine)(H2O)2·2NO3], cavitand 1 formed a complex incorporating two metals. The metal-coordinated cavitand also bound hydrophobic linear alkanes and difluorobenzene isomers in aqueous medium. The metallo-cavitand showed shape and size selectivity and was used to separate o-difluorobenzene from its isomers as observed by 19F NMR spectroscopy. The primary amino function of the cavitands offers possibilities for further elaboration to covalent clusters of these container compounds.