New Inorganic/Organic Coordination Polymers Generated from Bidentate Schiff-Base Ligands

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Abstract
The coordination chemistry of the long conjugated bidentate Schiff-base ligands 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (L1) and 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (L2) with cadmium and cobalt nitrate hydrates is investigated. Four new coordination polymers are prepared by solution reactions and fully characterized by infrared spectroscopy, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction. [Cd(NO3)2(L1)1.5·0.5(L1)]n (1; monoclinic, P21/c; a = 7.7729(16) Å, b = 19.049(4) Å, c = 17.865(4) Å, β = 93.13(3)°, Z = 4) is obtained by combination of L1 with Cd(NO3)2·4H2O in a benzene/methanol or THF/methanol mixed-solvent system. The structure features two-dimensional brick wall sheets that are cross-linked by weak noncovalent π−π interactions (alternating face-to-face stacking of coordinated and uncoordinated L1 molecules) to generate a novel three-dimensional network. [Co(NO3)2(L1)1.5·H2O]n (2; orthorhombic, Ccca; a = 19.031(4) Å, b = 33.627(7) Å, c = 14.299(3) Å, Z = 4) is generated from the reaction of L1 with Co(NO3)2·6H2O in a benzene/ethanol mixed-solvent system. It forms with a unique three-dimensional framework that can be considered a new polymeric motif based on the 1:1.5 metal-to-ligand composition M(L)1.5. The Cd(II) and Co(II) centers in 1 and 2, which lie in seven-coordinate environments, generate two new types of building blocks. The topologies of these two new building blocks are distinctly different from the common “T-shaped” building block generated from the same {MN3O4} coordination environment reported previously. Cd(L2)2(NO3)2 (3) and Co(L2)2(NO3)2 (4) are obtained by combination of L2 with Cd(NO3)2·4H2O and Co(NO3)2·6H2O, respectively. Compounds 3 and 4 are isostructural, crystallizing in the monoclinic space group P21/n, with a = 8.5802(17) Å, b = 17.506(4) Å, c = 10.443(2) Å, β = 96.59(3)°, and Z = 2 for 3 and a = 8.5283(17) Å, b = 17.408(4) Å, c = 10.229(2) Å, β = 97.05(3)°, and Z = 2 for 4. 3 and 4 adopt a novel one-dimensional chain structural motif, consisting of M2(L2)2 (M = Cd, Co) ringlike units. O···H−C hydrogen-bonding interactions in both 3 and 4 play a significant role in aligning the polymer strands in the solid state.

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