Thermoanalytical study of imidazole-substituted coordination compounds: Cu(II)- and Zn(II)-complexes of bis(1-methylimidazol-2-yl)ketone

Four copper(II) and three zinc(II) coordination compounds with bis(1-methylimidazol-2-yl)ketone (BIK) of general formula X(BIK)2Y2 (X = Cu(II), Zn(II), while Y = Cl, Br, NO3 or ClO4) were synthesized and characterized by elemental analysis and FTIR spectroscopies to be compared with the literature data. As a follow-up of our previous thermoanalytical studies on imidazole-substituted coordination compounds, the thermal behaviour of the synthesized Cu(II) and Zn(II) BIK complexes was investigated using thermogravimetry, where three consecutive releasing steps were ascribed to a complex decomposition process. All the complexes investigated showed the same reaction mechanisms, identified on the basis of the percentages of mass loss calculated from the TG curves. The decomposition mechanisms were confirmed by EGA analysis, performed by coupling the TG analyzer to a MS spectrometer. In particular, the first step is ascribed to the release of two anions, followed by the loss of four methyl groups (side chains) and two bridge-carbonyl groups. The residual tetra-imidazole copper(II) or zinc(II) compound decomposes in a final step to give the metal(II) oxide as the final residue. Both the initial decomposition temperatures and the kinetic rate constants associated to the first decomposition step indicated a higher stability of the Cu(BIK)2Y2 complexes with respect to the corresponding Zn ones. As far as the effect of the presence of the anion on the thermal stability is concerned, it can be demonstrated that both the perchlorate Cu(II) and Zn(II) complexes have the lower thermal stability (lower E values), while the thermal stabilities of the bromide, chloride and nitrate Cu(II) and Zn(II) complexes are substantially comparable. Finally, the model mechanism that shows the best fit between theoretical and reconstructed g(α) vs. α dependencies for the first decomposition step is, as showed in a previous paper for the analogues Mn(II) complexes, the three-dimensional diffusion model (D3). Selection of the best kinetic triplet associated to the first decomposition step was successfully validated by comparing the experimental and reconstructed portion of the TG curves of Cu(BIK)2(NO3)2 within the corresponding temperature interval.