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 Zn II -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)(H 2 O)]·0.5(bmb)·H 2 O} n ( 1 ), {[Zn(bmb)(2,6-pydc)]·2H 2 O} n ( 2 ), [Zn(bmb)(m-bdc)] n ( 3 ), [Zn(bmb) 0.5 (p-bdc)·H 2 O] n ( 4 ), [Zn(bmb)(fum)] n ( 5 ), {[Zn(bmb)(suc)]·1.5H 2 O} n ( 6 ), and {[Zn(bmb)(glu)]·H 2 O} n ( 7 ) (o-H 2 bdc = 1,2-benzenedicarboxylic acid, 2,6-H 2 pydc = 2,6-pyridinedicarboxylic acid, m-H 2 bdc = 1,3-benzenedicarboxylic acid, p-H 2 bdc = 1,4-benzenedicarboxylic acid, H 2 fum = fumaric acid, H 2 suc = succinic acid, and H 2 glu = 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 Zn II 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 6 3 -hcb networks, in which p-bdc 2- and bmb with trans -conformation are linked by Zn II ions to form a charming meso-helical chain. Complex 5 displays a 3-fold interpenetrated 3D diamonded network containing Zn II /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 suc 2- anions further link Zn II -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.