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abstracts
Porous organic molecular crystals are of particular interest to crystal engineers because of their potential applications in small molecule storage, separation and catalysis. Compared to network solids, molecular solids present advantages for processing related to their solubility and ease of derivatization. Our research group recently established the microporosity of a carboxylated triphenylbenzene crystal structure, which retains crystallinity even after solvent evaporation. The extrinsically porous structure of this compound is largely directed by two intermolecular interactions: aromatic stacking, and hydrogen bonding in the familiar R22(8) motif. We have synthesized new derivatives bearing various functional groups to probe their steric and electronic effects on the molecular packing and the surface polarity of the pores. The structures of two solvated quasi-polymorphs of the nitro-substituted derivative have been determined using single-crystal X-ray diffraction methods. These structures provide insight into the interplay between the two important synthons, while exhibiting different catenation modes of hexagonal hydrogen-bonded sheets. In both packings, the nitro functional group points towards the interior of solvent-filled channels, suggesting that the installation of other functional groups at the same position is a viable method for tailoring the interactions between guest molecules and the host framework.
abstracts
Porous molecular crystals that retain their structure in the absence of trapped solvent molecules are rare, given the flexibility of most non-covalent interactions. The crystal structure of microporous 1,3,5-tris(4-carboxyphenyl)benzene (tcpb) is notable for its large void volume and thermal stability, which stems from a complex polycatenation of its hydrogen-bonded network. Our group is exploring the crystallography of derivatives of this model compound with an eye towards tuning the dimensions and polarity of its pore structure. In one synthetic pathway, tritolylarenes are prepared as intermediates. We have discovered that even these molecules, which have no hydrogen-bond forming groups, tend towards complex crystal packings that exhibit disorder, aperiodicity, and solvent-filled voids. Additional exploration of co-crystals of these propeller shaped entities produced a pseudohexagonal columnar structure assembled from π-stacked helices. These helices enclose channels containing disordered tetrafluoroborate counterions, suggesting the possibility of ion exchange properties.
