Synthesis, crystal structure and characterization of a three-dimensional Cd^II coordination polymer constructed from...

Bifunctional organic ligands are very popular for the design of coordination polymers because they allow the formation of a great diversity of structures. In the title coordination polymer, the new bifunctional inversion-symmetric ligand 2,5-bis(1H-1,2,4-triazol-1-yl)terephthalic acid (abbreviated as H₂bttpa) links Cd^II cations, giving rise to the three-dimensional Cd^II coordination polymer catena-poly[diaqua[μ₄-2,5-bis(1H-1,2,4-triazol-1-yl)terephthalato-κ⁴O¹:O⁴:N⁴:N^4']cadmium(II)], [Cd(C₁₂H₆N₆O₄)(H₂O)₂]_n or [Cd(bttpa)(H₂O)₂]_n. The asymmetric unit consists of half a Cd^II cation, half a bttpa^2- ligand and one coordinated water molecule. The Cd^II cation is located on a twofold axis and is hexacoordinated in a distorted octahedral environment of four O and two N atoms. Four different bttpa^2- ligands contribute to this coordination, with two carboxylate O atoms in trans positions and two triazole N atoms in cis positions. Two aqua ligands in cis positions complete the coordination sphere. The fully deprotonated bttpa^2- ligand sits about a crystallographic centre of inversion and links two Cd^II cations to form a chain in a μ₂-terephthalato-κ²O¹:O⁴ bridge. This chain extends in the other two directions via the triazole heterocycles, producing a three-dimensional framework. O-H...O hydrogen bonds and weak C-H...N interactions stabilize the three-dimensional crystal structure. The FT-IR spectrum, X-ray powder pattern, thermogravimetric behaviour and solid-state photoluminescence of the title polymer have been investigated. The photoluminescence is enhanced and red-shifted with respect to the uncoordinated ligand.