Synthesis, characterization, DFT and antibacterial studies of a novel vitamin B6 Schiff base and its Cu(II) and Zn(II) complexes

•A Schiff base of the pyridoxal and its Cu(II) and Zn(II) complexes were synthesized.•DFT approaches were used to explore geometry and spectral properties of the compounds.•The Schiff base acts as a N2O2 tetradantate ligand in square planar complexes.•Two phenolate oxygens and two azomethine nitrogens are coordinated to metal ions.•Two deep eutectic solvents were employed to synthesize the compounds in high yield. In this project, condensation reaction of the pyridoxal and 3,4-diaminopyridine resulted in a novel pyridoxal Schiff base (H2L). Synthesis and characterization by several experimental methods have been reported for the H2L species as well as its Cu(II) and Zn(II) complexes. Structural parameters, tautomerism, spectral properties and frontier orbitals of three synthesized compounds have been explored by using the density functional theory (DFT) approaches. In the square geometries for both of the complexes, the dianionic form of the Schiff base, L2- species, acts as a tetradentate ligand, coordinates to the Cu2+ and Zn2+ metal ions via two azomethine nitrogens and two phenolate oxygens. Good agreement between the experimental values and DFT-predicted IR wavenumbers and NMR chemical shifts demonstrated validity of the optimized geometries for the studied species. Also, the Atoms In Molecules (AIM) analysis was employed to characterize bonding interactions. Use of two choline chloride based eutectic solvents, as green and environmentally friendly media, resulted in high yield synthesis of the investigated species in three cycles. In addition, among the synthesized compounds, the H2L Schiff base shows great antibacterial activities. [Display omitted]