Weak intermolecular interactions and molecular recognition:: Structure and dynamics of the benzene and pyridine p-tert-butylcalix[4]arene inclusions?

The host cavity of 1:1 host-guest compounds of p-tert-butylcalix [4] arene is well suited for the study of weak interactions in the solid state, as the motional freedom of the guests tests the weak intermolecular interactions in a very direct way. Benzene and pyridine are guests with a controlled number of similar (size, shape) as well contrasting (dipole moment, lone electron pair) properties that allow a meaningful comparative investigation of the structural and dynamic features. The 150 K single-crystal X-ray diffraction studies for the two host-guest compounds show that in both cases the guests occupy essentially the same orientation in the host cavity, with pyridine situated 0.11 Angstrom deeper into the cavity than benzene. The complementarity of the diffraction and solid-state NMR techniques is illustrated, in particular, by incorporating the pyridine structural information obtained from H-2 NMR studies into the diffraction data, thus resolving the ambiguity of the nitrogen atom position. Despite similar structural environments, the guests exhibit quite different dynamic behavior. Variable temperature 2H NMR spectra of the perdeuterated pyridine and benzene guests are interpreted in terms of specific motional models; benzene undergoes in-plane rotation followed by reorientation about the compound's 4-fold axis of symmetry. in contrast, pyridine reorients about the pyridine C-2 molecular symmetry axis (rather than in-plane rotation), followed by guest reorientation about the compound's C-4 axis of symmetry. A significant point is-that the pyridine nitrogen has definite orientations in the cavity that cannot be explained by any specific directional electrostatic interactions between the host and guest. Both the dynamically averaged N-15 NMR chemical shift tensor components and the absence of short contacts rule out a C-H ... N hydrogen bonding interaction of the host to the guest. Despite the fact that the pyridine molecule is tightly docked in the host cavity, intermolecular interactions must be ascribed strictly to steric interactions acting in concert rather than specific directional interactions. Both guests are oriented in the host cavity such that the aromatic plane minimizes rather than maximizes contact with the host CH3 groups. This result questions the ability to ascribe structural features in the solid state to isolated directional interactions, such as the importance and role of CHhost...pi(guest) interactions which have been suggested many times as having a stabilizing influence on this system.