Comparison of inclusion properties between p-tert-butylcalix[4]arene and p-tert-butylthiacalix[4]arene towards primary alcohols in crystals

Powdery crystals of p-tert-butylcalix[4]arene (1), when suspended in primary alcohols with C1-C7 carbon chains, absorb the alcohols to form 1 : 2, 1 : 1 and 2 : 1 (host : guest) inclusion crystals with C1, C2-C4 and C5-C7 alcohols, respectively, whereas powdery crystals of compound 2 absorb only ethanol by the same treatment. In competitive experiments, the crystals of compound 1 preferentially absorb propanol and hexanol among the alcohols that form 1 : 1 and 2 : 1 inclusion crystals, respectively, but the selectivities are inferior to the selectivity of compound 2 towards ethanol. These differences in inclusion properties between compounds 1 and 2 are attributed to the difference in the crystal packing of the inclusion crystals. X-ray analysis reveals that compound 1 constructs a bilayer structure with the aid of a network of the intermolecular CH-π interaction between a methylene group of a host molecule and a benzene ring of an adjacent host molecule. The bilayers are laminated in two different manners depending on the size of guest compounds. A small alcohol is included into the cavity of a host molecule to form 1 : 1 inclusion crystals, whereas a large alcohol is included into a molecular capsule constructed by two host molecules gathered in a head-to-head manner to form 2 : 1 inclusion crystals. The inclusion crystals with the same packing structure have almost the same spaces to accommodate guest molecules, regardless of the guest size, producing good receptivity of compound 1 towards alcohols. On the other hand, compound 2, which lacks methylene bridges, forms a CH-π interaction with the terminal methyl group of an alcohol molecule included into its cavity, and the alcohol molecule forms hydrogen bonds with the hydroxyl groups of an adjacent host molecule to construct a columnar structure. The difference in stability of the columnar structure among the alcohols causes the high inclusion selectivity of compound 2 towards ethanol.