A Case Study of Zn^II-bmb Meso-Helical Coordination Polymers upon the Spacer Angles and Lengths of Dicarboxylate Coligands

Abstract

To systematically explore the influence of the spacer angles and lengths of dicarboxylate coligands on building ZnII-bmb [1,4-bis(2-methylbenzimidazol-1-ylmethyl)benzene] meso-helical coordination polymers, we synthesized seven metal−organic frameworks {[Zn(bmb)0.5(o-bdc)(H2O)]·0.5(bmb)·H2O}n (1), {[Zn(bmb)(2,6-pydc)]·2H2O}n (2), [Zn(bmb)(m-bdc)]n (3), [Zn(bmb)0.5(p-bdc)·H2O]n (4), [Zn(bmb)(fum)]n (5), {[Zn(bmb)(suc)]·1.5H2O}n (6), and {[Zn(bmb)(glu)]·H2O}n (7) (o-H2bdc = 1,2-benzenedicarboxylic acid, 2,6-H2pydc = 2,6-pyridinedicarboxylic acid, m-H2bdc = 1,3-benzenedicarboxylic acid, p-H2bdc = 1,4-benzenedicarboxylic acid, H2fum = fumaric acid, H2suc = succinic acid, and H2glu = glutaric acid) through rationally varying the organic acid linkers. Structural analyses reveal that complexes 1−7 display diverse helical characters under the influence of dicarboxylates with different spacers. In complexes 1, 2, 3, and 7, the bmb ligands adopt trans-conformation bridging ZnII ions to form rare meso-helices, which are further extended and decorated by different dicarboxylate coligands. Complex 4 exhibits a fascinating Borromean topology with three parallelly interlocked 63-hcb networks, in which p-bdc2- and bmb with trans-conformation are linked by ZnII ions to form a charming meso-helical chain. Complex 5 displays a 3-fold interpenetrated 3D diamonded network containing ZnII/bmb/fum left- and right-handed helical microchannels. Complex 6 features unusual interweaved meso-helices constructed by cis- and trans-conformational bmb, which is the second example of tetra-flexural helix with four spiral shafts in one helical chain. In addition, the suc2- anions further link ZnII-bmb meso-helices to form a 3D framework. These results indicate that the spacers of dicarboxylate coligands and the conformations of bmb have important effects on the meso-helical characters and ultimate frameworks. Moreover, the investigation of photoluminescence properties of 1−7 reveals that the helical feature plays an important role in reducing photoluminescent intensities.