Thermal stability and crystallographic characterization of mononuclear Ni(II) and heterodinuclear Ni(II)-Co(II) complexes of the reduced ONNO type Schiff base

Abstract In this study, the ONNO type Schiff base ligand N, N’-bis(2-hydroxyphenylidene)-1,3-propanediamine (LH 2 ) was reduced and converted into a tetradentate phenolic amine ligand, N, N′-bis(2-hydroxybenzyl)-1,3-diaminopropane (L H H 2 ). First, a mononuclear [NiL H Cl(DMF)] 2 (DMF) complex (I) was obtained with the reduced ligand in dimethylformamide (DMF). CoCl 2 was then added to the complex I to give the heterodinuclear [NiL H CoCl 2 (DMF) 2 ] complex (II) . The complexes were characterized by elemental analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and single-crystal X-ray diffraction (XRD). The XRD analysis of complex I revealed that one phenolic oxygen persisted in its coordinated phenol form, while the charge of Ni(II) was observed to be neutralized by a chloride ion and a phenolate ion. The Ni(II) center ion in the complex exhibited octahedral coordination, being surrounded by two phenolic oxygens, two aminic nitrogens, an oxygen from a DMF molecule, and a chloride ion. In complex II , the Ni(II) ion was coordinated by two phenolic oxygens, two aminic nitrogen, and two DMF oxygens. The Co(II) ion was found to have a tetrahedral coordination sphere with two phenolate oxygens and two chloride ions. Thermogravimetric analysis demonstrated distinct thermal behaviors for mononuclear and heterodinuclear complexes. The mononuclear complex exhibited stability up to 210 °C and an exothermic reaction at 510 °C in an oxygen atmosphere, resembling energetic materials. The heterodinuclear complex showed higher thermal stability and a distinct decomposition pathway, influenced by Co(II) coordination. These findings highlight the significance of heterometallic coordination in thermal properties and controlled decomposition applications.