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‘-binaphthyldisulfonate, 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.

本研究通过单晶X射线衍射（single-crystal X-ray diffraction），对16种结晶包合化合物（crystalline inclusion compounds）主体骨架中的手性组织（chiral organization）进行了系统表征。该系列包合化合物由胍离子（guanidinium ions）与多种取代4,4'-联苯二磺酸盐或4,4'-联萘二磺酸盐的外消旋混合物（racemic mixtures）组装得到。由于互补磺酸盐基团（S）与胍离子（G）构成的氢键片层呈现准六边形基元排布——该基元已被证实广泛存在于诸多其他胍基有机二磺酸盐体系中，此类包合化合物均呈现层状构筑。该GS片层由手性有机磺酸盐“支柱”连接，这些支柱形成了容纳客体分子的包合空腔。

16种化合物中仅1种以对映异构体（enantiomorphs）聚集体的形式结晶；剩余15种里，5种为外消旋体（racemates），10种为假外消旋体（pseudoracemates）——此类假外消旋体的单晶体中含有等量晶体学无序的左手与右手支柱。这种对假外消旋体的偏好（在分子晶体中极为罕见）可归因于胍-磺酸盐网络的独特结构：该网络迫使手性支柱之间保持较大间距，使得在部分晶体方向上，原本会倾向于同手性（homochirality）或异手性（heterochirality）的短程手性识别能垒不复存在。

本研究可在具有分级维度的明确亚单元——一维（1D）条带、二维（2D）层状结构、三维（3D）晶格——中探究手性组织，借此实现对晶体晶格内手性信息短程与长程传递的考察。结构趋势表明，一维胍-磺酸盐条带内的紧密堆积相互作用（close-packing interactions）会促进同手性或异手性组织；而带有大偶极矩的联苯支柱所携带的永久电偶极矩（electric dipoles）则对手性的长程传递至关重要，这与既往理论相符。