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

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.