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

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Abstract

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···O interactions between the macrocycles, the degree of which is discussed in detail. Solid state cross polarization magic angle spinning (CP-MAS) 13C 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···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 R3̅ 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).

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