Synthesis, characterization and sodium chloride catalytic effects of diaminomaleonitrile derivative schiff base and metal complexes

In this thesis, a novel Schiff base ligand (L) was synthesized and characterized, and the corresponding transition metal complexes (Mn(II), Co(II), Ni(II), Zn(II), Fe(II), Cu(II)) were prepared and characterized to investigate their catalytic properties in the production of chlorine dioxide (ClO₂) from sodium chlorite (NaClO₂). Known Mn(III) complexes were resynthesized based on the ligands GL1 and GL2, as previously reported in the literature [24]. Comparative theoretical analyses were performed using experimentally obtained data for both the synthesized ligands and their metal complexes. The primary objective of this study was to understand the mechanistic pathways underlying the catalytic potential of Schiff base complexes in environmentally benign oxidant applications such as ClO₂. Kinetic measurements were conducted using UV-Vis spectroscopy in buffered media (pH = 1, 2, 4, 6) containing sodium chlorite and the respective catalysts. The reaction kinetics were monitored at the characteristic absorption wavelength of chlorine dioxide at 360 nm. The results demonstrated that ClO₂ production was highly efficient in strongly acidic buffers (pH = 1 and 2) even in the absence of metal complexes, whereas in moderately acidic conditions (pH = 4 and 6), the production was significantly limited, indicating that ClO₂ generation is more favorable under strong acidic environments. It was observed that the GL1-Mn(III) complex advanced ClO₂ production up to a certain extent at pH = 1 and subsequently stabilized at an equilibrium state. This behavior was attributed to the ability of Mn(III) to form high-valent Mn(IV)-oxo intermediates via d-orbital electron transitions, thereby facilitating chlorine dioxide formation through an oxygen atom transfer (OAT) mechanism. Although our research group had previously shown that the GL2-Mn(III) complex efficiently oxidized aldehydes to carboxylic acids at high pH levels (pH 6–7), in the current study it was found to act merely as a stabilizer in ClO₂ production kinetics, without demonstrating catalytic activity. This observation led to the interpretation that catalysts appearing "inactive" under certain kinetic conditions may become catalytically active in the presence of appropriate substrates. Additionally, kinetic evaluations of newly synthesized Co(II), Ni(II), Cu(II), Fe(II), Mn(II), and Zn(II) complexes revealed low catalytic activity across the pH range of 1 to 6. Initially considered as potential inhibitors, these complexes were later understood to act as stabilizers in the absence of substrate, maintaining the stability of NaClO₂ in the reaction medium. The study also involved theoretical calculations (DFT, TD-DFT) and spectroscopic analyses (FTIR, UV-Vis, NMR) to characterize the structural and electronic properties of the complexes. These analyses enabled the establishment of correlations between experimental observations and theoretical predictions, particularly shedding light at the molecular level on the roles of Mn(III)-based complexes within the catalytic cycle.