Powdery crystals of p-tert-butylcalixij4]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. CrystEngCommThis journal is
The development of separation materials for hard-to-separate molecular mixtures is highly desired from environmental and economic perspectives. Although the crystal of p-tert-butylthiacalix[4]arene exhibits high guest selectivity in inclusion from a mixture of molecules with similar sizes and shapes, it cannot include molecules larger than its calix cavity. To extend its guest inclusivity, we designed and synthesized an open-chain host, [3,3'-thiobis(5-tert-butyl-2-hydroxybenzene)-1,1'-diyl]diacetic acid (4). The competitive inclusion among toluidine isomers using compound 4 gave inclusion crystals containing the p-isomer in 1:1 (host/guest) ratio, with lesser amounts of other isomers and/or solvent molecules. The isomer selectivity varied between 66% and 97% depending on the solvent employed. X-ray analysis of inclusion crystals 4·p-toluidine·MeCN and 4·p-toluidine·(o-toluidine) revealed that compound 4 includes p-toluidine by forming macrocyclic 2:2 inclusion complex(es) and that its higher-order structure has vacant spaces, in which molecules other than p-toluidine are included. Compound 4 was then transformed into monopropyl ester 5 to fill the vacant spaces with propyl moieties. Compound 5 included p-toluidine with high selectivity (∼96%) without the coinclusion of other molecules, regardless of the solvent employed.
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