Preferential Crystallization of tert-Butyl-calix[6]arene Chlorobenzene Solvate from a Solvent Mixture

Abstract

Calixarenes can be functionalized with relative ease on their upper and lower rims, i.e., the para positions of benzene rings and OH groups, respectively. Chemical structure adjustment allows for changes in the size and properties of the calixarene cavity; however, it is difficult to predict the separation product because both kinetic and thermodynamic factors are important in crystal formation. To explore the preferential crystallization of calixarene solvates, tert-butyl-calix[6]arene (TBC6) was crystallized from a solvent mixture [benzene, pyridine, dichloromethane, cyclohexane, toluene, methyl acetate, tetrahydrofuran, chlorobenzene (PhCl), anisole, n-heptane, acetonitrile, and ethyl acetate]. TBC6 was selectively crystallized as the PhCl solvate. The PhCl molecule was disordered over two positions with a chlorine atom rotated by ∼120°, suggesting that dichlorobenzene (PhCl2) would fit into the binding pocket of calixarenes. Due to their different sizes (oPhCl2 < mPhCl2 < pPhCl2), the dichlorobenzene derivatives affected the formation of TBC6 dimers and thereby the crystal packing. Therefore, the solvent size is the most important factor for determining crystal packing, and solvent preference for the TBC6 macrocycle follows the order of PhCl > oPhCl2 ≈ mPhCl2 ≈ pPhCl2. These results suggest that TBC6 can be used to recover chlorobenzene from a solvent mixture via selective crystallization.