Synthesis, crystal structure and characterization of a three-dimensional CdII coordination polymer constructed from 2,5-bis(1H-1,2,4-triazol-1-yl)-terephthalate

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 H(2)bttpa) links Cd-II cations, giving rise to the three-dimensional Cd-II coordination polymer catena-poly[diaqua[mu(4)-2,5-bis(1H-1,2,4-triazol-1-yl) terephthalato-kappa O-4(1):O-4:N-4: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 mu(2)-terephthalato-kappa O-2(1):O-4 bridge. This chain extends in the other two directions via the triazole heterocycles, producing a three-dimensional framework. O-H center dot center dot center dot O hydrogen bonds and weak C-H center dot center dot center dot 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.