Mechanistic Consideration for the Selective Inclusion of Disubstituted Benzene Isomers with p-tert-Butylcalix[4]arene Crystals

Abstract

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 p-nitrotoluene, 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 m-nitrotoluene 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 p-nitrotoluene 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.