Structural characterization and thermal behaviour of some azomethine compounds derived from pyridoxal and 4-aminoantipyrine

Eight azomethine-type compounds (1–8) derived from pyridoxal (3-hydroxy-5-(hydroxymethyl)-2-methylpyridine-4-carbaldehyde) and 4-aminoantipyrine, respectively, were prepared and thoroughly characterized from structural and thermal perspective. The structures of the compounds were studied based on IR, 1H and 13C NMR spectroscopy and electrospray ionization mass spectrometry. The formation of the azomethine derivatives is confirmed by the occurence of signals typical for the imine bond. Compounds 6 and 8 feature a high crystallinity, and their structures were resolved by single-crystal X-ray diffraction. The thermal behaviour of all the compounds was studied by non-isothermal chemiluminescence in static air atmosphere at different heating rates. The pattern of the oxidation process is described. Moreover, thermo-gravimetric and differential scanning calorimetry experiments were conducted in order to assess the thermal oxidation stability. The onset oxidation temperatures show a high thermal stability for all the tested azomethine compounds. The crystallinity degree was comparatively evaluated on the basis of melting enthalpy values and discussed in connection with the molecular structure. Compounds 6–8 show the higher thermal stability and crystallinity degree.