Cucurbit[n]urils (n=5-8): A Comprehensive Solid State Study

Cucurbit[n]urils (CB [n], n = 5-8) have been prepared, separated, and purified, and the structure of their solid state assemblies has been addressed. A number of general features were identified which are of interest to understand some peculiar properties of cucurbiturils (solubility, aggregation, assembly, transformation to porous crystals, influence of air humidity). CB[5], CB[6], and CB[8] were isolated as hydrate crystals whose structures were found to show a strong tendency of the macrocycles to interpenetrate. A self-closing effect was rationalized in terms of multiple weak CH center dot center dot center dot O interactions between the macrocycles, the degree of which is discussed in detail. Solid state cross polarization magic angle spinning (CP-MAS) C-13 NMR spectra obtained at 900 MHz were correlated with the crystal structures. An odd even effect in the crystallinity of thermally treated CB samples (CB [5] and CB[7] amorphous, CB [6] and CB [8] crystalline) was observed, which is reflected in differences in water solubility (CB [5] and CB [7] are water-soluble, whereas CB [6] and CB [8] are only very scarcely so). This may be explained by a less efficient self-association for CB [5] and CB [7] as compared with CB [6] and CB [8], which is reflected in the number of inter-cucurbituril CH center dot center dot center dot O interactions per cucurbituril. This leads to a more favorable solvation for the CBs having an odd symmetry, whereas those with even symmetry prefer to self-associate in a manner ultimately leading to crystallization. We also propose an explanation for the presence of anions (Cl-) inside some cucurbituril materials, whose cavity is often considered hydrophobic. Furthermore, it is demonstrated that large quantities of the very stable microporous CB [6] crystals (which have the R (3) over bar channel structure) can be obtained very easily by a simple thermal treatment of the hexagonal crystals of CB [6] (P6/mmm structure) obtained directly from the initial reaction used to synthesize the various CB [n]. The micromorphology of the CB [n] powders was established using scanning electron microscopy (SEM), and the tendency of each material to absorb water from the atmosphere was demonstrated by thermogravimetric analyses (TGA).