Thermoanalytical and spectral properties of new rare-earth metal 2-pyrazinecarboxylate hydrates

New 4f-trivalent lanthanide 2-pyrazinecarboxylate hydrates of composition Ln(pyzCOO)₃·3.5H₂O, where Ln=La, Ce, Pr, Nd, Sm and Dy have been isolated from aqueous solution containing the respective metal nitrates and 2-pyrazinecarboxylic acid (HpyzCOO), in 1:3 molar ratios. The complexes have been characterized by analytical, electronic and IR spectroscopic, thermal analysis and X-ray diffraction studies. The OCO ^- group of 2-pyrazinecarboxylate bridges in both mono- and bidentate fashion (v_as(COO)=1612 and 1578 cm^-1; v _s(COO)=1423 and 1388 cm^-1) as evidenced by IR data. The IR spectra further indicate that coordination takes place through the carboxyl group and the nearest hetero-ring nitrogen atom. The bonding parameters β, b^1/2, %δ and η have been calculated from the electronic spectral (hypersensitive) bands of Pr(III) and Nd(III) complexes. Both IR and thermal data reveal the presence of a coordinated water and two and a half lattice water molecules in the complexes. It is further substantiated by single crystal X-ray study for the corresponding lanthanum compound. Based on the results obtained, nine coordination is proposed for the lanthanide ions. Simultaneous TG-DTA of the complexes show that they are thermally stable up to 150°C and undergo endothermic (∼155 and 250°C) followed by exothermic (∼480°C) decompositions to give the respective metal oxides as the final product. The compounds are found to be isostructural as evidenced from X-ray powder patterns.