Crystalline Cyclodextrin Inclusion Compounds Formed with Aromatic Guests:  Guest-Dependent Stoichiometries and Hydration-Sensitive Crystal Structures

A series of solid inclusion complexes (ICs) containing the aromatic guests aniline, benzene, ethylbenzene, phenol, p -xylene, styrene, and toluene were formed with host γ-cyclodextrin (γ-CD). IC stoichiometry was observed to depend on the nature of the included aromatic guest. The molar ratio of styrene, aniline, and phenol guests to γ-CD host was ∼2:1 in their individual IC crystals, whereas ethylbenzene, p -xylene, and toluene guests formed ∼1:1 inclusion complexes with γ-CD. Thermogravimetric analysis showed that the thermal stabilities of these volatile aromatic guest molecules increased due to guest−host interactions once they were included in their γ-CD-ICs. X-ray diffraction (WAXD) observations performed on the aromatic guest−CD-IC crystals showed that all of them have channel-type crystalline structures. Moreover, it was observed that the presence of guest molecules inside the γ-CD cavities stabilized the channel structure of stacked γ-CDs. However, a solid-phase transition from tetragonal to hexagonal packing of γ-CD stacks was observed upon dehydration of the styrene−, toluene−, p -xylene−, benzene−, and ethylbenzene−γ-CD-IC crystals. We have also discovered that the solid-state transformation between tetragonal and hexagonal channel crystal structures is reversible upon the desorption or sorption of water. In the cases of aniline− and phenol−γ-CD-ICs, no solid-phase transition from tetragonal packing to hexagonal packing was observed upon dehydration. Instead, the tetragonal packing structure was stable even after vacuum-drying at elevated temperatures for an extended period. The stability of the tetragonal packing in aniline− and phenol−γ-CD-ICs was attributed to the presence of hydrogen bonding between the included guests and neighboring host γ-CD molecules.