Chiral discrimination in low-density hydrogen-bonded frameworks

The chiral organization in the host frameworks of 16 crystalline inclusion compounds, assembled from guanidinium ions and racemic mixtures of various substituted 4,4′-biphenyldisulfonates or 4,4′-binaphthyl- disulfonate, was examined with single-crystal X-ray diffraction. These inclusion compounds adopt lamellar architectures as a consequence of hydrogen-bonded sheets consisting of complementary sulfonate moieties (S) and guanidinium (G) ions arranged in a quasihexagonal motif, which has proven pervasive in numerous other guanidinium organodisulfonates. The GS sheets were connected by the chiral organosulfonate "pillars," which support inclusion cavities occupied by the guests. Only one of the 16 compounds crystallized as a conglomerate of enantiomorphs. Of the remaining 15 compounds, 5 crystallized as racemates, and 10 crystallized as pseudoracemates in which each crystal contained equal amounts of crystallographically disordered left- and right-handed pillars. This preference for pseudoracemates, which is unusually high for molecular crystals, can be attributed to the unique structure of the guanidinium-sulfonate network, which enforces large separations between the chiral pillars so that short-range chiral discrimination energies that would favor homo- or heterochirality are absent along certain crystal directions. The ability to examine chiral organization in well-defined subunits with hierarchical dimensionality - one-dimensional (1D) ribbons, two-dimensional (2D) layers, three-dimensional (3D) lattice - permits examination of both short- and long-range exchange of chiral information in the crystal lattices. The structural trends suggest that close-packing interactions within 1D guanidinium-sulfonate ribbons promote either homochiral or heterochiral organization whereas permanent electric dipoles associated with biphenyl pillars having large dipole moments are important for the long-range transmission of chirality, consistent with previous theories.