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·α), [Cd(H2dcbi)2(H2O)2]·2H2O (1·β), [Cd(H2dcbi)2(H2O)2]·2H2O (1·γ), [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 (α, β, and γ) of polymorphs. Seven coordination modes of Hndcbi ranging from monodentate to μ5
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
Hndcbi (n =0, 1, 2). Studies of the coordination modes of Hndcbi 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 μ2, μ3, or μ4 mode to
generate one-dimensional or two-dimensional structures, and the triply deprotonated dcbi can coordinate in the μ5
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 μ4-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.