Synthesis, characterization, DFT calculation, and biological activity of a new Schiff base ligand and its ZnO and Co3O4 nano-metal oxide complexes

A novel heterocyclic Schiff base ligand, {2,2'-((4-chloro-1,3-phenylene)-bis(oxy))bis-(N'-((E)-(1 H-benzo[1-3]-triazole-1-yl)methylene)acetohydrazide)}, was synthesized and coordinated with Co(II) and Zn(II) chlorides to yield two metal complexes. The ligand and complexes were analyzed using FT-IR, ¹H and ¹³C NMR, UV-visible spectroscopy, and mass spectrometry, which provided spectral shifts typical of coordination involving the imine nitrogen and amide carbonyl groups. DFT calculations (B3LYP/LanL2DZ) showed that complexation decreased the energy gap between HOMO and LUMO from 4.26 eV (free ligand) to 3.18 eV and 2.66 eV for Zn(II) and Co(II) complexes, respectively, showing increased chemical reactivity. Similarly, the electrophilicity index increased to 33.81 eV for the Zn complex and 40.97 eV for the Co complex, indicating increased electron-accepting ability and potential biological activity. Thermal decomposition of Zn and Co complexes yielded ZnO NPs (mean crystallite size 35.6 nm) and Co₃O₄ NPs (33.8 nm), as evidenced by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Antibacterial activity against Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis) and Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) was found to show that ZnO NPs had the largest inhibition zones (up to 28 mm), followed by metal complexes [Zn(L)]Cl₂ and [Co(L)]Cl₂, whereas the free ligand had very poor activity (only inhibition zones of 11-13 mm).