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.