Solid-state NMR and diffraction studies of a tunable p-tert-butylcalix[4] arene guest structures

The majority of p-tert-butylcalix[4]arene inclusion compounds are 4-fold symmetric in the solid state, with the guest molecular and the host symmetry axes approximately aligned. In contrast, nitrobenzene (and related guests) induces a permanent symmetry-reducing distortion of the p-tert-butylcalix[4]arene compound and occupies a position in which the guest molecular axis is no longer aligned with that of the host. These compounds have been characterized by single-crystal X-ray diffraction as well as C-13 CP-MAS and H-2 NMR in the solid state. Introduction of propane as a second, minority guest in sufficient quantities induces the alignment of the nitrobenzene molecular axis with the host C-4 symmetry axis. Nitrobenzene-d(5) guest dynamics in the symmetric and asymmetric structures reveal a much stronger host guest interaction in the latter. The nature of the asymmetry is due to a cooperative effect rather than any intrinsic property of the individual p-tert-butylcalix[4]arene.guest units. In general, this work give initial insight into the suitability of the p-tert-butylcalixarene framework for crystal engineering and illustrates the close connection between dynamics and lattice symmetry and structure.