Conformations of p-tert-Butylcalix[8]arene in Solvated Crystal Structures

Calixarenes are notable materials that have been used for a number of purposes, ranging from ionic recognition through to a variety of biological applications; hence, understanding the factors that control their conformations is important. Herein, we examined the conformations of p-tert-butylcalix[8]arene (TBC8) to determine the relationship between molecular structure, crystal packing, and the crystallization solvent (DMF, chloroform, ethyl acetate, acetonitrile, pyridine, and DMSO), with X-ray diffractometry revealing various macrocycle-to-solvent-molecule ratios (from 1:1 to 2:17). Two types of crystals with different structures were formed with DMF as the solvate. Pleated-loop conformations were observed for crystal structures solvated with DMF, chloroform, ethyl acetate, and acetonitrile, a chairlike conformation was determined for pyridine, and a hitherto unknown loop-chair conformation was observed for DMSO. Each asymmetric unit contains eight intra- and intermolecular hydrogen bonds. The most stable crystals form chain-packing motifs created by pleat-looped macrocycles, with tert-butyl groups replacing characteristic solvent molecule positions in the higher-ratio crystal structures. Density functional theory calculations located three TBC8 conformations, with the pleated-loop and chair-loop conformations corresponding to the lowest and highest energies, respectively.