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

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)]center dot 2(NO3)}(n) [M = Zn (1) and Co (2)], {[M(L)(2)Cl-2] center dot DMF center dot S}(n) [M = Zn(3), S = C2H5OH; Co(4), S = CH3OH + H2O], [Zn(L)(2)]center dot(ClO4)(2)center dot 4DMF center dot 3H(2)O](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 (31)) 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 Zn-II/Co-II nitrate salts with ZnCl2 or CoCl2 in the above reactions, the resulting complexes 3 and 4 contain P-helical and M-helical chains with 2(1) screw axis but crystallize as a racemate. Through pi center dot center dot center dot pi 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 (21)) (4,4) wave-like layer constructed by vertical crossing of the left-(M) and right-(P)-handed helical chains with the central Zn-II ions as the hinge nodes, and similarly, the final 3D supramolecular structure is assembled via weak pi center dot center dot center dot pi 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.