Oxovanadium(IV) complexes of the pyridoxal Schiff bases: Synthesis, experimental and theoretical characterizations, QTAIM analysis and antioxidant activity

Two vanadyl complexes of the pyridoxal Schiff bases have been newly synthesized and characterized by several experimental methods, where the 4,4'-[1,4-butanediylbis-[(E)-nitrilomethylidyne])bis[5-hydroxy-6-methyl-3-pyridinemethanol] and trans-4,4'-[1,2-cyclohexanediylbis-[(E)-nitrilo-methylidyne]]bis[5-hydroxy-6-methyl-3-pyridinemethanol] Schiff base were used. Geometry optimization, assignment of the IR vibrational frequencies and the natural bond orbital (NBO) analysis of the complexes have been calculated by employing the density functional theory (DFT) approaches. Deprotonated form of the Schiff bases (L2-) acts as a tetradentate N2O2 ligand, which coordinates to the V(IV) via two phenolate oxygens and two imine nitrogens. In the square-pyramidal geometry of the [VO(L)], the apical position is occupied by an oxo ligand. The DFT-calculated vibrational frequencies show a good consistency with the corresponding experimental values, confirming suitability of the optimized geometries for the complexes. Characteristics of the bonding interactions have been explored using the quantum theory of atoms in molecule (QTAIM) analysis. The complex formation results in decrease in strength of the C-N bond of the azomethine group and increase in the strength of the C-O bonds of the phenolate group. High-energy gaps approve stability of the complexes. Both of the complexes show significant radical scavenging activities against the ABTS and DPPH radicals, even higher than the BHA.