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

Abstract

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.

Insights into molecular recognition from the crystal structures of p-tert-butylcalix[6]arene complexed with different solvents

“…Here, only the structures with the pinched-cone conformation are discussed. The crystal structures with winged-cone (Martins et al, 2017) and 1,2,3-alternate conformations (Wolfgong et al, 1996) have been reported previously. To describe clearly the interactions of the solvent molecules with the calixarene structure, the phenyl rings of the calixarene were labelled as A-and B-type rings (Fig.…”

“…acetone or dimethyl sulfoxide (DMSO)] can disrupt the intramolecular hydrogen bonding, the calixarene assumes a distorted 1,2,3-alternate conformation. However, pyridine (Malinska, 2021) and DMSO (Martins et al, 2017), which are both molecules that can form hydrogen bonds, crystallize with p-tert-butylcalix[6]arene (TBC6) in a winged-cone conformation. This conformation exists only in the metastable crystal structure at room temperature, and dissolves to form crystal structures with lower host-guest ratios or pinched-cone conformations.…”

Temperature- and Solvent-Induced Crystal-Form Transformations of the Pyridine@p-tert-butylcalix[6]arene Host–Guest System

“…Different conformations were also suggested by Molins et al; i.e., winged conformations that feature four benzene rings oriented perpendicularly to the molecular base, with the two at the 1,4-positions lying entirely flat, whereas the 1,2,3-alternating conformation has three hydroxyl groups in the upper-rim and the rest of the hydroxyl groups in the lower-rim. Interestingly, low-temperature solution NMR spectroscopy identified the predominant conformation to be the winged cone . The energy differences between the winged-cone and the 1,2,3-alternating and basic conformations of p - tert -butylcalix­[6]­arene (TBC6, Scheme ) are 50 and 66 kJ mol –1 , respectively .…”

“…The energy differences between the winged-cone and the 1,2,3-alternating and basic conformations of p - tert -butylcalix­[6]­arene (TBC6, Scheme ) are 50 and 66 kJ mol –1 , respectively . The crystal formed with dimethyl sulfoxide (DMSO) was found to have a 1,2,3-alternating conformation, and new structures with a winged-cone conformations were crystallized recently with DMSO, dimethylformamide (DMF), and pyridine . The latter one have a layered structure with alternating solvent and TBC6 layers.…”

Exploring the structural landscape of 2-(thiophen-2-yl)-1,3-benzothiazole: high-Z′ packing polymorphism and cocrystallization with calix[4]tube

“…Their versatility is in part associated with possible functionalization in the upper and lower rims, increasing the selectivity for specific guests (Atwood et al, 2002;Martins et al, 2016). Likewise, methylene bridges enable different conformations of calix [n]arenes, resulting in the possibility of a wide range of guest molecules being included (Ř ezanková et al, 2017;Martins et al, 2017). Furthermore, calix[n]arenes show several biological properties, such as antiviral (Motornaya, et al, 2006), anticancer (Dings et al, 2012;Pelizzaro-Rocha et al, 2013) and antimicrobials (Mourer et al, 2006;Lamartine et al, 2002).…”