T. Premkumar

Preparation and Thermal behaviour of Transition Metal Complexes of 4,5-Imidazoledicarboxylic Acid

Some new transition metal imidazolehydrogendicarboxylate hydrates of empirical formula M(Himdc)2·nH2O (H2imdc=4,5-imidazoledicarboxylic acid), where n=2 for M=Mn, Ni, Zn, Cd and n=3 for M=Co, have been prepared in aqueous solution. The compounds have been characterized by analytical, electronic and IR spectroscopic, thermal analysis and X-ray powder diffraction studies. Electronic spectroscopic data suggest that the Co and Ni compounds are of spin free (high spin) type with octahedral geometry. For these compounds, the IR bands in the region 1750-1710 cm-1 has been assigned to stretching vibrations of the non-ionized carboxylic group, confirming that the ligand is monoionized. IR spectra also suggest the unidentate co-ordination behaviour of carboxylate (vasy =1570 and vsym=1390 cm-1) groups of the imidazoledicarboxylate monoanion. The thermal behaviour of these compounds has been studied by simultaneous TG-DTA techniques. All of these compounds are dihydrates except cobalt which is a trihydrate. Thermal decomposition studies show that they lose two water molecules endothermally in the range 200-270°C to give their anhydrous compounds, indicating that these water molecules are coordinated to the metal. The anhydrous compounds further decompose exothermally in the range 300-620°C to leave the respective metal oxides via the metal oxalate intermediates. Whereas the manganese compound undergoes pyrolytic cleavage in a single step to give the manganese carbonate as the final residue. Isomorphic nature of these compounds is evident from XRD data. Six-coordination for the metal atoms has been proposed based on the thermal analysis, visible and IR spectroscopic results.

Crystal structures of two Ca(II) complexes with imidazole-4,5-dicarboxylate and water ligands

The structure of compound I: poly-diaqua(μ-imidazole-4,5-dicarboxylato-N,O; -O′; -O′′, -O′′′) calcium(II) monohydrate [Ca(C5H2N2O4)(H2O)2·H2O] is built of molecular sheets in which imidazole-4,5-dicarboxylate ligands bridge the metal ions using both carboxylate groups, each bidentate. Ca(II) is coordinated by six oxygen atoms and one hetero-ring nitrogen atom distributed at the apices of a capped tetragonal bipyramid. The basal plane of the pyramid is formed by two carboxylate oxygen atoms [d(Ca–O2 = 2.374(1) Å, d(Ca–O4) = 2.412(1) Å] and two water oxygen atoms [d(Ca–O5) = 2.384(1) Å, d(Ca–O6) = 2.455(1) Å], the capped position is occupied by the carboxylate oxygen atom O3 [d(Ca–O3) = 2.325(1) Å], the hetero-ring nitrogen atom [d(Ca–N2) = 2.523(1) Å] and the carboxylate oxygen atom O4 [d(Ca–O2) = 2.412(1) Å] form the apices of the prism. The solvation water molecule plays a significant role in a framework of hydrogen bonds responsible for the stability of the crystal. The structure of compound II: trans-tetraquadi(H-imidazole-4,5-dicarboxylato-N,O) calcium(II) monohydrate, [Ca(C5H3N2O4)2(H2O)4·H2O] consists of monomers in which the Ca(II) ion is located on a centre of symmetry. The coordination around the Ca(II) is a strongly deformed pentagonal bipyramidal with the imidazole-4,5-dicarboxylate (4,5-IDA) ligands in the trans arrangement forming a dihedral angle of 68.3°. An imidazole-ring nitrogen atom [d(Ca–N) = 2.632(2) Å] and one carboxylate O atom [d(Ca–O) = 2.531(2) Å] from each ligand coordinate to the metal ion. The coordination is completed by four water oxygen atoms [d(Ca–O) = 2.393(2) Å] and [d(Ca–O) = 2.367(2) Å]. The coordinated water molecules act as hydrogen bond donors and acceptors to the unbonded carboxylate oxygen atoms in adjacent monomers giving rise to a three-dimensional molecular network.

Molecular sheets in the crystal structure of a Ba(II) complex with imidazole-4,5-dicarboxylate and water ligands

The structure of poly-tetraaquabis(μ-Himidazole-4,5-dicarboxylato-N,O;-O′)barium(II) dihydrate, Ba(C5H3N2O4)2(H2O)4·2H2O is built of molecular sheets in which singly-deprotonated imidazole-4,5-dicarboxylate [H(4,5-IDA)] bridges metal ions using its N, O bonding moiety and one oxygen atom of its second carboxylate group. Each barium(II) is coordinated by N, O bonding moieties of two ligands, two carboxylate oxygen atoms of two other ligands and four waters. The coordination number of Ba(II) is ten, and the coordination polyhedron contains fourteen faces. A network of hydrogen bonds is responsible for the stability of the crystal.

Crystal Structure of a Lanthanum(III) Complex with Pyrazine-2-Carboxylate and Water Ligands

The structure of (aqua-O)di(pyrazine-2-carboxylate-N,O-μ-O)mono(pyrazine-2-carboxylate-N,O)lanthanum (III) dihydrate contains La(III) ions, each coordinated by three pyrazine-2-carboxylate (PYR) ligands via their N,O bonding moieties and a water molecule forming an La(PYR)3·(H2O) structural unit. In addition, each La(III) ion is coordinated by two carboxylate oxygen atoms each belonging to a ligand chelated to a different adjacent metal ion, giving rise to a bridging system. Three bridging paths are observed: one links the structural units into pairs, the others link the pairs into molecular sheets. The carboxylate group of the third ligand coordinates only one carboxylate oxygen atom leaving the second unbonded to the metal ion. The coordination number of the La(III) ion is nine. Solvation and coordinated water molecules participate in a network of hydrogen bonds which also contributes to the stability of the molecular arrangement.