Switching of Guest Selectivity for the Inclusion of Regioisomers of Monosubstituted Phenols with Crystals of p-tert-Butylcalix[4]arene

Abstract

To develop an inclusion method for hard-to-separate organic molecules with wide guest scopes, we examined the control of guest selectivity via external factors such as solvents, temperature, and additives for the competitive inclusion of monosubstituted phenol isomers with crystals of p-tert-butylcalix[4]arene 1. Guest selectivity was thus switched between two isomers in the inclusion of cresol, fluorophenol, and bromophenol and between three isomers in the inclusion of chlorophenol. The mechanism of guest selectivity in chlorophenol inclusion was mainly determined using X-ray crystal structural analysis and a comparison of the thermal stabilities and the formation rates of inclusion crystals with each isomer. Under kinetic control, the inclusion from decane at a low temperature selectively yielded type-A (host/guest = 1:1) inclusion crystals of p-isomer (1·p-chlorophenol) in which the host and guest molecules exhibited CH−π and OH–π interactions. In aqueous solutions at high temperatures, where water molecules inhibited the inclusion of m-isomer, thermally stable type-B (host/guest = 2:1) crystals of o-isomer (12·o-chlorophenol), in which a guest molecule was included in a capsule constructed by two host molecules, selectively formed. Furthermore, the inclusion from decane in the presence of methanol produced a co-inclusion crystal of type-C (host/guest/additive = 2:1:1) in which a 1:1 complex of m-isomer and methanol, connected via an intermolecular hydrogen bond, was selectively included by the distorted capsule of two host molecules to form 12·m-chlorophenol·CH3OH.