Attractive Interactions between Heteroallenes and the Cucurbituril Portal

In this paper, we report on the noteworthy attractive interaction between organic azides and the portal carbonyls of cucurbiturils. Five homologous bis-alpha,omega-azidoethylammonium alkanes were prepared, where the number of methylene groups between the ammonium groups ranges from 4 to 8. Their interactions with cucurbit[6]uril were studied by NMR spectroscopy, IR spectroscopy, X-ray crystallography, and computational methods. Remarkably, while the distance between the portal plane and most atoms at the guest end groups increases progressively with the molecular size, the beta-nitrogen atoms maintain a constant distance from the portal plane in all homologues, pointing at a strong attractive interaction between the azide group and the portal. Both crystallography and NMR support a specific electrostatic interaction between the carbonyl and the azide beta-nitrogen, which stabilizes the canonical resonance form with positive charge on the beta-nitrogen and negative charge on the gamma-nitrogen. Quantum computational analyses strongly support electrostatics, in the form of orthogonal dipole-dipole interaction, as the main driver for this attraction. The alternative mechanism of n -> pi* orbital delocalization does not seem to play a significant role in this interaction. The computational studies also indicate that the interaction is not limited to azides, but generalizes to other isoelectronic heteroallene functions, such as isocyanate and isothiocyanate. This essentially unexploited attractive interaction could be more broadly utilized as a tool not only in relation to cucurbituril chemistry, but also for the design of novel supramolecular architectures.