Synthesis, characterization, biological evaluation, DFT and molecular docking studies of (Z)-2-((2-bromo-4-chlorophenyl)imino)methyl)-4-chlorophenol and its Co(ii), Ni(ii), Cu(ii), and Zn(ii) complexes

An equimolar reaction of 5-chlorosalicylaldehyde and 2-bromo-4-chloroaniline yielded the Schiff base (Z)-2-((2-bromo-4-chlorophenyl)imino)methyl)-4-chlorophenol) (HL), which was used for complexation to Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ metal salts. Elemental and thermogravimetric analyses, conductivity measurements, powder X-ray diffraction, nuclear magnetic resonance (¹H and ¹³C), infrared, ultraviolet-visible, energy dispersive X-ray-scanning electron and mass spectroscopies confirm the Schiff base structure and show mono-nuclear homoleptic complexes of the type ML₂ for all metal salts used. The crystal structures of Ni²⁺ and Cu²⁺ complexes revealed a perfect square planar geometry around the metal ions, with the ligand acting as bidentate through oxygen and nitrogen atoms of the phenolic and azomethine groups, respectively. The antimicrobial potential of the compounds was evaluated on some selected pathogenic bacteria consisting of Gram-positive (Staphylococcus aureus and Bacillus subtilis) and Gram-negative (Klebsiella pneumonioe and Pseudomonas aeruginosa) strains using an in vitro assay. Antioxidant activity was evaluated using the DPPH assay. The complexes showed enhanced activity over the free Schiff base ligand in all the assays. Toxicity studies on WISH-ATCC-CCL-25, human epithelial amnion (normal liver cell lines), and MRC-5-ATCC-CCL-171 (normal human lung fibroblast cell lines) revealed that at lower concentrations, the complexes did not affect the cell lines. A computational study was deployed to investigate the electronic properties of the ligands and the complexes relating to their stability, reactivity, and biological potential. The computational data corroborated sufficiently with the experimental findings. Molecular docking studies demonstrated the compounds' mechanism of action and identified potential binding sites consistent with the in vitro assays. Hirshfeld surface analysis was also performed on selected compounds to reveal qualitative and quantitative intermolecular interactions within the topology crystal network of crystal structures.