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···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]...