Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) transition metals isonicotinate complexes: Thermal behavior in N2 and air atmospheres and spectroscopic characterization

Solid-state M(IN)2·nH2O complexes, where M stands for bivalent transition metals (Mn, Fe, Co, Ni, Cu and Zn), IN is isonicotinate and n = 0.5 to 4.0 H2O, were synthesized. Characterization and thermal behavior of the compounds were performed employing elemental analysis (EA), complexometric titration with EDTA, powder X-ray diffraction (PXRD), infrared spectroscopy (FTIR), simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) in dynamic dry air and nitrogen atmospheres, differential scanning calorimetry (DSC) and TG-DSC coupled to FTIR. The thermal behavior of isonicotinic acid and its sodium salt was also investigated in both atmospheres. The dehydration of these compounds occurs in a single step in both atmospheres. In air atmosphere, the thermal decomposition of the anhydrous compounds also occurs in a single step, except for the copper compound where two steps are observed. In N2 the thermal decomposition of the anhydrous compounds occurs in two consecutive steps, except for iron compound, where three steps are observed. The main gaseous products of thermal decomposition/pyrolysis of the compounds were identified as CO, CO2 and Pyridine. Mn, Co, Cu and Zn compounds show a physical transformation process in DSC curves. The ligand coordinates through the pyridine nitrogen atom to the metal and for the Zn compound, the carboxylate group also participates in the coordination. The IR absorption profile of hydrated and dehydrated compounds suggest that there is a probable change in the coordination mode of the ligand upon dehydration. This change needs to be further investigated, once it is not possible to ensure only with infrared spectroscopy data.