Encapsulation of Ion Pairs in Extended, Self-Assembled Structures

Encapsulation of ion pairs in small spaces that are isolated from the medium is expected to result in amplified interactions between the ions. Yet, sequestration of ion pairs in self-assembled capsules is complicated by competition of the acids and bases for binding directly to the assembly components. We describe here a hydrogen bonded capsule 1.2(8).1 that accommodates two gamma-picolines and two acids as ion pairs. The supramolecular structure of the discrete 14-component assembly is characterized by NMR spectroscopy. The structure reveals the acids in the tapered ends of the capsule and gamma-picoliniums near the glycoluril spacers in the capsules center. Similar acid base ion pairs are also obtained with 4-ethylpyridine, gamma-picoline with difluoroacetic acid, and gamma-picoline with trifluoromethanesulfonic acid. The H-1 NMR spectrum of the gamma-picoline/trifluoroacetic acid ion pair shows a signal at delta = 18.7 ppm, indicating the acidic proton is in contact with both the picoline nitrogen and the trifluoroacetate oxygen. Further details about the unusual structures of ion pairs in small spaces are reported.