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
A crystal of p-tert-butylcalix[4]arene (1) has selectively included regioisomers of eight different disubstituted benzenes, giving inclusion crystals classified into four types: 2:1 (host/guest) with p-isomer and 1:1 with o-, m-, and p-isomers. X-ray crystallographic analysis of 1 2·p-xylene, 1·o-cresol, 1·m-dichlorobenzene, and 1·p-chlorotoluene, which were chosen as representatives for the individual types of inclusion crystals, revealed that the electrostatic properties and steric bulk of the substituents in the rigid guest molecules are efficiently reflected in the interaction with the host molecules, which construct flexible bilayer-based packing structures in the inclusion crystals. This is considered as a principal origin for the high selectivity and wide applicability achieved in the inclusion of aromatic regioisomers with crystals of compound 1.
p-tert-Butylcalix[4]arene (1) crystals can selectively include one regioisomer from a mixture of the three regioisomers for eight kinds of disubstituted benzenes. In this study, the mechanisms for the guest selectivity in the inclusion of nitrotoluene, xylene, and cresol isomers, which were chosen as representative guests showing different guest selectivity, were investigated in detail. In the competitive inclusion of three regioisomers of nitrotoluene, xylene, and cresol, crystals of compound 1 selectively include pnitrotoluene, p-xylene, and o-cresol, respectively. Time course of the change in isomer selectivity in the competitive inclusion experiment and comparison of thermogravimetric analysis profiles of the inclusion crystals with each of the isomers revealed that the selectivity for p-nitrotoluene and o-cresol was achieved under kinetic control, whereas the selectivity for p-xylene was achieved under thermodynamic control. X-ray crystallography of the inclusion crystals revealed that p-nitrotoluene and o-cresol form type A (host/ guest = 1:1) inclusion crystals, in which a guest molecule is included in the cavity of a host molecule. On the other hand, o-and mnitrotoluene and m-and p-cresol form thermodynamically stable type B (host/guest = 2:1) inclusion crystals, in which a guest molecule is included in a capsule constructed by gathering of two host molecules in a head to head manner. The selectivity for pnitrotoluene and o-cresol could be explained by the effective host−guest interaction during the formation of type A inclusion crystals, which tends to be kinetically favored over the formation of type B inclusion crystals. X-ray structure analysis and Hirshfeld surface analysis of the inclusion crystals revealed that the inclusion of xylene isomers in the crystals of compound 1 resulted in the formation of type B inclusion crystals for all the three isomers. The CH−π interaction between the two methyl groups of the p-isomer and the benzene rings of the two host molecules encapsulating the isomer is the most effective. Therefore, the inclusion crystal with p-xylene is the most stable among the three isomers, resulting in p-isomer selectivity. In this way, the crystals of compound 1 precisely distinguish different disubstituted benzene isomers under kinetic or thermodynamic control by the virtue of various host−guest interactions with the guest substituents in the cavities of type A and type B inclusion crystals.
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