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 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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“…40 The strong peak at 746 cm −1 is ascribed to the C−H vibration of the benzene ring. 41 A broad band is seen in the UV−vis absorption spectrum (Figure 1F), displaying that Fe-MIL-88 is a potential broad-spectrum fluorescence quencher, which is in accordance with a previous report. 42 All the above characterization results show that Fe-MIL-88 is an octahedral crystal structure containing Fe, N, O, and C. fluorescence dye-marked ssDNA probe can be absorbed to the surface of the MOF materials through π−π stacking and electrostatic and hydrogen bonding.…”

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

“…Table presents the selectivity values. In the inclusion of cresol isomers from decane at 0 °C, crystals containing o -isomer formed with low selectivity (entry 1), even though high selectivity was observed in the inclusion using neat guests. , At higher temperatures, the preferentially included isomer changed to p -isomer, albeit with a low selectivity. Next, we investigated the competitive inclusion of halogenated phenol isomers because p -isomer selectivity was observed for the competitive inclusion of chlorophenol isomers in the neat condition .…”

“…We previously reported that the crystal of 1 2 •mnitrotoluene includes a guest molecule in the sliding-capsule structure. 79 We considered the structure of the co-inclusion crystal of m-chlorophenol as an example. One m-chlorophenol molecule is located in the cavity of a host molecule, which exhibits a CH−π interaction between an aryl hydrogen atom of the guest molecule and a host benzene ring and CH−halogen interactions between the chloro group of the guest molecule and a tert-butyl group of the host molecule (Figures 2b and S9).…”

“…Recently, we have found that crystals of compound 1 selectively include one isomer from a mixture of eight disubstituted benzene isomers, while the development of their separation method using host molecules is an attractive subject. , In this system, type-A crystals, with a host–guest ratio of 1:1, and type-B crystals, with a host–guest ratio of 2:1, are generated according to the guest structure; ,,, the former is easy to generate kinetically, while the latter is thermodynamically stable . If we can switch the formation of type-A and type-B crystals, in which different isomers are selectively included, respectively, by varying external factors such as temperature and solvents, we propose a new method for switching guest selectivity.…”

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“…C-H⋯π is a class of non-covalent interactions found in different molecular systems including organic molecules, 42 proteins, 43 nucleic acids, 44 protein-ligand complexes 45 and supramolecular assemblies. 46,47 Similarly, the concept of C-H⋯O interaction has recently gained much interest. It is found to be important for stabilizing nucleic acids, 48 protein structures, 49 protein-DNA recognition 50 and drug design, 51 and even as a determinant of compound conformation.…”