Promoting a desired conformational preference of an aromatic amide in various crystals by rational design of intramolecular hydrogen bonding

Although the conformational mobility of flexible ligands provides a way to generate structural diversity in the crystalline structure, strategies are needed to promote a conformational preference based on rational design. Herein, two methoxy groups were introduced onto N,N'-bis(4-pyridyl)isophthalamide (BPI) in order to gain a conformational preference by intramolecular hydrogen bonding. Therefore, we synthesized 4,6-dimethoxy-N,N'-bis(4-pyridyl)isophthalamide (DMBPI), and further prepared organic crystals and coordination polymers of DMBPI. Single crystal X-ray diffraction revealed that all DMBPI molecules in these crystals adopt anti,anti-conformations owing to the intramolecular-hydrogen-bond formation. In contrast, the diverse conformations of BPI molecules in the 27 BPI-containing crystals are categorized into three conformation types. The remarkable difference in conformational distribution may be attributed to that the energy gaps between the different conformations of DMBPI are significantly greater than those of BPI on the basis of density functional theory calculations. These results demonstrate that introduction of a suitable substituent to form intramolecular hydrogen bonds is an effective and rational strategy for achieving an expected conformation of molecules in crystals, which may facilitate the design and controllable assembly of functional materials.