Conformational effect of 2,6-bis(imidazol-1-yl)pyridine on the self-assembly of 1D coordination chains: spontaneous resolution, supramolecular isomerism, and structural transformation

The achiral 2,6-bis(imidazol-1-yl)pyridine (L) was used as the ditopic organic tecton for the formation of coordination polymers with Zn(II) ions. Hydrothermal reaction between L and ZnX2 (X=Br, Cl) afforded spontaneous resolved double helical motifs in ZnLCl2.0.5H2O (1) and ZnLBr2.0.25H2O (2). In the homochiral crystals of 1a and 2a, the helices are of M-helicity, whereas, in 1b and 2b, they are of P-helicity. In contrast, solvothermal reaction between L and ZnCl2 in dried DMF afforded achiral ZnLCl2 (3a), which exhibits a zigzag polymeric motif. An achiral polymorph 3b which contains 21 helical chains was obtained in wet DMF. The formation of different 1D motifs was related to the conformations of L. All these compounds were characterized by infrared spectroscopy, elemental analyses, and single-crystal X-ray diffraction. As revealed by thermal gravimetric analysis and powder X-ray diffraction study, the homochiral motif in 1 was stable even upon removal of guest water molecules. Contrastingly, structural transformation from 3a or 3b to 1 is possible upon hydration.