Eric B. Brouwer

Two newly identified structures arise upon the crystallisation of the p-tert-butylcalix[4]arene host molecule from tetradecane: the guest-free, self-included host structure, and a 1:1 host–guest structure in which the paraffin guest is anchored in the bowl-shaped host cavity and intercalates layers of host molecules in a lamellar structure reminiscent of clays.

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 13C CP-MAS and 2H 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 C4 symmetry axis. Nitrobenzene-d5 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[4]arene framework for crystal engineering and illustrates the close connection between dynamics and lattice symmetry and structure.

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 Å 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 2H 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 C2 molecular symmetry axis (rather than in-plane rotation), followed by guest reorientation about the compound's C4 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 15N 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···πguest interactions which have been suggested many times as having a stabilizing influence on this system.

Depending on the details of the technique employed, desolvation of tBC inclusion compounds leads to either a dense, guest-free polymorph, or a guest-free low density polymorph, the latter also having distinct high and low temperature forms.