Winged-Cone Conformation in Hexa-p-tert-butylcalix[6]arene Driven by the Unusually Strong Guest Encapsulation

Hexa-p-tert-butylcalix[6]arene (1) is believed to adopt a winged conformation in a solution, featured by four phenyl rings perpendicular to the calix basis and two others at 1,4-positions lying down. However, there is some controversy on the occurrence of this conformation because it has never been found in the solid state of calix[6]arenes, regardless of the substitution pattern at lower and upper rims. Here, we have observed the winged-cone conformation for the first time in a solvate form of 1 with dimethyl sulfoxide (DMSO), dimethylformamide, and pyridine. The DMSO molecule is strongly encapsulated into 1 through two OH···O hydrogen bonds with both flattened phenolic moieties, one lp_(S)···π and four CH···π interactions with the four perpendicular phenyl rings. This host-guest complex has energy lower by 23.4 kcal mol^-1 than the isolated species. In addition, another DMSO solvate form with 1,2,3-alternate conformation was also obtained in this study, and its structure is compared with that of the precedent one. A detailed density functional theory study has also been carried out to understand the energetic relationships among cone conformers, intramolecular hydrogen-bonding patterns, and DMSO encapsulation.