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

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)(H2O)]center dot 0.5(bmb)center dot H2O}(n) (1), {[Zn(bmb)(2,6-pydc)]center dot 2H(2)O}(n) (2), [Zn(bmb)(m-bdc)](n) (3), [Zn(bmb)(0.5)(p-bdc)center dot H2O](n) (4), [Zn(bmb)(fum)](n) (5), {[Zn(bmb)(suc)]center dot 1.5H(2)O}(n) (6), and {[Zn(bmb)(glu)]center dot H2O}(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 add, H(2)fum = fumaric acid, H(2)suc = succinic acid, and H(2)glu glutaric add) 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 transconformation 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.