Inclusion of Amine Isomers with Open-Chain Hosts Having a Partial Structure of p-tert-Butylthiacalixarene

Porous materials, which can capture a specific compound from a hard-to-separate molecular mixture, are strongly desired for practical separation and purification processes. Aiming to develop such materials, we have investigated the performance of our original host compounds, [3,3'-thiobis(5-tert-butyl-2-hydroxybenzene)-1,1'-diyl]diacetic acid (2) and its monopropyl ester (3), in discriminating among regio- or stereoisomers of three groups of amines, 2-, 3-, and 4-methylpyridine, 2-, 6-, and 8-methylquinoline, and cis- and trans-4-cyclohexanamine. Diacid 2 selectively included 4-methylpyridine in hexane and 3-methylpyridine in toluene in competitive inclusion among the three regioisomers. Mechanistic studies revealed that the inclusions of 3- and 4-methylpyridine are favored under kinetic and thermodynamic control, respectively. Solvent-dependent switching in guest selectivity was also observed in competitive inclusion among the methylquinoline isomers with diacid 2, whereas trans-4-methylcyclohexanamine was selectively included over the cis-isomer by monoester 3, as well as diacid 2, regardless of the solvent employed. X-ray crystallographic analysis of the resulting inclusion crystals suggests that the wide guest scope of the host compounds originates from their flexible ability to form complexes with amines.

Unraveling the Sorption Mechanism of CO2 in a Molecular Crystal without Intrinsic Porosity

The facile uptake of CO₂ gas in a nonporous molecular crystal constituted by long molecules with carbazole and ethynylphenyl moieties was reported in experiments recently. Herein, the mechanism of gas uptake by this crystal is elucidated using atomistic molecular simulations. The uptake of CO₂ is shown to be facilitated by (i) the capacity of the crystal to expand in volume because of weak intermolecular interactions, (ii) the parallel orientation of the long molecules in the crystal, and (iii) the ability of the molecule to marginally bend, yet not lose crystallinity because of the anchoring of the terminal carbazole groups. The retention of crystallinity upon sorption and desorption cycles is also demonstrated. At high enough pressures, near-neighbor CO₂ molecules sorbed in the crystal are found to be oriented parallel to each other.

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.