Hydrogen-bonded host frameworks with tunable cavities: structural characterization and inclusion-based separations of molecular isomers
Single crystal X-ray diffraction reveals that inclusion compounds formed from G2 MDS and G2TMBDS hosts (MDS = mesitylenedisulfonate; TMBDS = 2,4,5,6-tetramethylbenzenedisulfonate) and various isomeric methylated benzene guests crystallize in polar space groups with guests occupying one- dimensional channels between sheets of hydrogen-bonded guanidinium ions and sulfonate moieties. The inclusion selectivity for isomeric mixtures of trimethylbenzenes and tetramethylbenzene guests by these host frameworks has been examined. The organic residues of the MDS and TMBDS pillars mimic the isomeric trimethylbenzenes and tetramethylbenzenes, respectively, suggesting an avenue to a separations protocol based on molecular recognition during inclusion and subsequent crystallization. Pairwise competition experiments, in which inclusion compounds are crystallized from solutions containing a mixture of two molecular isomers, map the inclusion selectivity of a particular host as a function of guest content in solution. The single crystal structures suggest that guest inclusion selectivity is governed by the ability of a host to achieve efficient packing with the guest molecules and its ability to allow efficient guest-guest packing in the one-dimensional channels. Consequently, the more compliant G2 MDS host exhibits higher selectivities for guest inclusion. Overall, the selectivity patterns appear to reflect size and shape compatibility between the host and guest, suggesting opportunities for separations of isomers based on straightforward crystallization processes.