Cucurbit[n]urils (n = 5鈥?): A Comprehensive Solid State Study

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• 作者：David Bardelang ; Konstantin A. Udachin ; Donald M. Leek ; James C. Margeson ; Gordon Chan ; Christopher I. Ratcliffe ; John A. Ripmeester
• 刊名：Crystal Growth & Design
• 出版年：2011
• 出版时间：December 7, 2011
• 年：2011
• 卷：11
• 期：12
• 页码：5598-5614
• 全文大小：1704K
• 年卷期：v.11,no.12(December 7, 2011)
• ISSN：1528-7505

文摘

Cucurbit[n]urils (CB[n], n = 5鈥?) 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路路路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鈥揺ven 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路路路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^{鈥?/sup>) 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).}