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Construction of Three-Dimensional Metal−Organic Frameworks with Helical Character through Coordinative and Supramolecular Interactions

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journal contribution
posted on 04.03.2009, 00:00 by Jian-Qiao Chen, Yue-Peng Cai, Hua-Cai Fang, Zheng-Yuan Zhou, Xu-Lin Zhan, Gang Zhao, Zhong Zhang
Self-assemblies of the semirigid dipolar ligand 1,4-bis(benzimidazol-1-ylmethyl)benzene (L) with zinc and cobalt salts have led to six new complexes, namely, {[M(L)(DMF)2(H2O)2]·2(NO3)}n [M = Zn (1) and Co (2)], {[M(L)2Cl2]·DMF·S}n [M = Zn(3), S = C2H5OH; Co(4), S = CH3OH + H2O], [Zn(L)2]·(ClO4)2·4DMF·3H2O]n (5), and {[Co(L)2(DMF)2](ClO4)2}n (6), respectively. Their structures were determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and thermogravimetric analyses. Compounds 1 and 2 are isomorphous and present one-dimensional chain-like structures. It is interesting that three-dimensional (3D) organic−inorganic hybrid frameworks containing meso-helical chains (P + M) have been observed in the solid-state of 1 and 2, in which meso-helical chains are alternately trapped by the cooperative association of coordination interactions as well as hydrogen bonds. With the replacement of ZnII/CoII nitrate salts with ZnCl2 or CoCl2 in the above reactions, the resulting complexes 3 and 4 contain P-helical and M-helical chains with 21 screw axis but crystallize as a racemate. Through π···π stacking interactions between two well-overlapping benzimidazoleyl rings from two adjacent chains, the 3D racemic supramolecular network is further assembled. However, reaction of M(ClO4)2 (M = Zn(5), Co(6)) and L yields two high dimensional compounds 5 and 6 with helical character, in which compound 5 has a two-dimensional (2D) (4,4) wave-like layer constructed by vertical crossing of the left-(M) and right-(P)-handed helical chains with the central ZnII ions as the hinge nodes, and similarly, the final 3D supramolecular structure is assembled via weak π···π interactions between two central benzene rings from two adjacent ligands L of two neighboring 2D layers. Compound 6 presents a four-connected 3-fold interpenetrated 3D diamondoid network containing meso-helical chains along two mutually perpendicular directions, respectively. Furthermore, the photoluminescent properties of compounds 1, 3, and 5 were studied.