N-(4-Phenylbutyl)azacalix[3.1.1.1]arene: Self-Inclusion of the Side Chain through CH•••π Interactions

Azacalixarene 1, with a relatively long chain substituent, can exhibit both inter-and intramolecular CH center dot center dot center dot pi interactions. In the crystalline state, the 4-phenybutyl group is included in the molecular cavity, allowing interactions between the methylene H atoms and the aromatic rings of the azacalix skeleton. In addition, similar edge-to-face interactions occur between the terminal phenyl ring and that of an adjacent molecule, thus forming a polymeric, supramolecular array of azacalixarene molecules. In solution, the 4-phenylbutyl arm adopts two orientations, one included in the calix cavity, one not, giving rise to isomers describable as exo and endo conformers. Characterization of the exo/endo equilibrium as a function of temperature by H-1 NMR spectroscopy enabled estimation of the enthalpy difference favoring inclusion attributed to the intracavity CH center dot center dot center dot pi interactions.