Thermoanalytical study of selected transition bivalent metal complexes with 5-carbaldehyde-4-methylimidazole

Abstract

To compare thermal stability of Co(II), Zn(II), and Cd(II) complexes with 4-CHO-5-MeIm, the two compounds of formula [MnL2(NO3)2] and [NiL3](NO3)2 have been prepared and structurally characterized. Elemental analysis and spectroscopic studies have confirmed a bidentate fashion of coordination of the ligand to Mn(II) and Ni(II) ions. IR and Raman spectra indicate that there are different coordination modes of the NO3− in compounds: non-coordinated and coordinated. The decomposition process of the studied complexes in nitrogen and argon (Ni(II) complex) atmosphere proceeds in three main stages, except Zn(II) complex, in temperature range 353–1163 K. The final products of decomposition are CoO, MnO, Cd, ZnN4, NiN3. In addition, we have to admit that the different coordination mode of the NO3− ions in complexes: non-coordinated (in the (1), (4), and (5)) and coordinated (in the (2) and (3)) correlate with its thermal behavior. Thus, temperature ranges of its decompositions are observed: below 533 K and above 533 K, respectively. In Co(II), Mn(II), and Cd(II) complexes the fragments of N-donor atom-containing ligands decompose in the last stages, contrary to Zn(II) and Ni(II) compounds, in which metal ion surrounded by N atoms remains until the end. The course of pyrolysis and molecular structure of the complexes lead to the same conclusion about the strength of metal–ligand bonds. On the basis of obtained results, it is concluded that the thermal stability of the studied compounds follows the order: (1) < (5) < (2) < (3) < (4).