Syntheses, crystal structures and properties of metal complexes with 4,4′,5,5′-tetracarboxyl-2,2′-biimidazole

Three novel coordination polymers have been synthesized via hydrothermal reactions at different temperatures, namely [Cu2(H2tcbi)(H2O)4] (1), [Cd3(H3tcbi)2(H2O)6]·2H2O (2) and [Ca(H4tcbi)(H2O)2] (3) (H6tcbi = 4,4′,5,5′-tetracarboxyl-2,2′-biimidazole). This is the first report on complexes of the Hntcbi ligand. Single crystal X-ray analyses reveal that they have rich structural chemistry, ranging from one-dimensional (1), to two-dimensional (2 and 3). Three different coordination modes of the H6tcbi ligand, μ2, μ3 and μ4, have been observed. The coordination geometries of the metal sites vary from five-coordinate square pyramidal, six-coordinate octahedral to seven-coordinate pentagonal bipyramidal. The H6tcbi ligand in complexes 1, 2 and 3 is tetra-deprotonated to H2tcbi4−, tri-deprotonated to H3tcbi3− and bi-deprotonated to H4tcbi2−, and coordinates to the metal ions as a tetradentate, pentadentate and pentadentate ligand respectively, resulting in the formation of 1D, 2D and 2D coordination polymers. Photoluminscent investigation of complexes 2 and 3 shows intense blue and green fluorescent emissions respectively.