Diversity of coordination architecture of metal 4,5-dicarboxyimidazole.

Seven complexes of metal 4,5-dicarboxyimidazole acid (H3dcbi), namely, [Cd(H2dcbi)2(H2O)3].H2O (1.alpha), [Cd(H2dcbi)2(H2O)2].2H2O (1.beta), [Cd(H2dcbi)2(H2O)2].2H2O (1.gamma), [Cd(H2dcbi)2(H2O)2] (2), [Cd(Hdcbi)(H2O)] (3), [Cd5(Hdcbi)2(dcbi)2(H2O)].XH2O (4), [Cd2(Hdcbi)(C2O4)] (5), [Ag5(Hdcbi)2(CN)] (6), and [Mn(Hdcbi)(H2O)] (7), have been hydro(solvo)thermally synthesized by fine control over synthetic conditions such as stoichiometry, solvent, and pH value. X-ray single-crystal structural analyses reveal that they have rich structural chemistry ranging from mononuclear (1), one-dimensional (2), and two-dimensional (3 and 7) to three-dimensional (4-6), among which 1 crystallizes in three types (alpha, beta, and gamma) of polymorphs. Seven coordination modes of H(n)dcbi ranging from monodentate to mu5 have been observed, among which four modes are found first. The coordination geometries of the Cd(II) sites vary from five-coordinate trigonal bipyramid and square pyramid, six-coordinate octahedron to seven-coordinate pentagonal bipyramid. Analyses of the synthetic conditions and structures of the Cd(II) complexes show that the influences of the solvent and the metal-to-ligand molar ratio are very important to the products and coordination modes of H(n)dcbi (n =0, 1, 2). Studies of the coordination modes of H(n)dcbi and the structures of the Cd(II) complexes also reveal that the singly deprotonated H2dcbi generally coordinates in the monodentate imidazole-N or N,O-chelate mode to result in mononuclear structures, the doubly deprotonated Hdcbi coordinates in the mu2, mu3, or mu4 mode to generate one-dimensional or two-dimensional structures, and the triply deprotonated dcbi can coordinate in the mu5 mode to form three-dimensional structures. The cyanide was in situ formed via C-C bond cleavage of acetonitrile during the preparation of 6, which adopts a rare mu4-kC,kC:kN,kN mode to bridge four Ag(I) ions. The microporous three-dimensional framework of 4 is maintained after the removal of the guest molecules. Compounds 1-5 show strong violet emissions with maxima around 380 nm, tentatively attributed to the ligand-centered transition.