Host Compound DED: Guest Accommodation Type, Noncovalent Host···Guest Interactions, Crystal Densities, and Thermal Stabilities Ensure High Host Selectivities for 4-Methylanisole in Anisole and Methylanisole Mixtures

Abstract

trans-9,10-Dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid (DED), dimethyl trans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylate (H1), and trans-α,α,α′,α′-tetraphenyl-9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanol (H2) formed complexes with anisole (ANI) and 2-, 3-, and 4-methylanisole (2-, 3-, and 4-MA) in single-solvent recrystallization experiments. DED was observed to possess an overwhelming selectivity for 4-MA when recrystallized from equimolar binary, ternary, and quaternary mixtures (82.2–99.3%) and nonequimolar binary solutions where this guest species was present. It was therefore demonstrated that DED may serve as an efficient host compound for the separation and purification of many of the mixtures employed in this investigation; extremely high K values (K is a measure of the selectivity of the host compound) were calculated in many instances (137.9, 190.7). However, the selectivity behavior of H1 and H2 in analogous experimental conditions could not be established owing to the extremely slow (months) crystal growth process from these mixtures. Single-crystal X-ray diffraction (SCXRD) experiments revealed that 4-MA interacted with DED via two short stabilizing (host)C–O···π(guest) contacts (3.2782(17), 3.3780(17) Å, 145, 138°). This interaction type was absent in 2(DED)·3-MA and weaker in 2(DED)·2-MA (and only one interaction of this kind was present in the latter complex). Additionally, the crystal density of 2(DED)·4-MA (1.335 g·cm–1) was greater than those of 2(DED)·3(ANI) (1.277 g·cm–1), 2(DED)·2-MA (1.257 g·cm–1), and 2(DED)·3-MA (1.283 g·cm–1), alluding to a tighter packing in the complex with the preferred guest species owing to the shorter contacts in this crystal. The higher density was also explained by a significantly shorter intermolecular (host)π···π(host) interaction in the 4-MA-containing complex. Furthermore, 4-MA experienced more constricted channel accommodation compared with ANI, 2- and 3-MA. Finally, 2(DED)·4-MA possessed the greater thermal stability (the onset of the guest release process, Ton = 101.8 °C) compared with 2(DED)·3(ANI) (unstable at room temperature), 2(DED)·2-MA (Ton = 90.4 °C), and 2(DED)·3-MA (Ton = 85.3 °C).