Synthesis, Characterization, Biological Evaluation, DFT Calculations, and Molecular Docking Study of Transition Metal...

The escalating global challenge of antibiotic resistance amid rising oxidative stress emphasizes the critical necessity for innovative antimicrobial strategies. This study, reported synthesis of Schiff base ligand, (Z)-2-[(4-nitrobenzylidene)amino]phenol (HL), derived from condensation between 2-aminophenol and 4-nitrobenzaldehyde, and it Co(II), Ni(II), Cu(II), and Zn(II) complexes. The compounds were characterized using NMR, FTIR, UV-Vis, MS, TGA, SEM-EDX, and elemental (CHN) analysis. In addition, solid-state structure of HL was obtained using single-crystal x-ray diffraction. The characterizations show that the ligand coordinated to the metal ions through the oxygen and nitrogen atoms of phenolic and imine moieties, resulting in a homoleptic mononuclear complexes of the form ML₂, in which, M represents cobalt, nickel, copper, or zinc, and L stands for the ligand. Stability studies using time-dependent UV-Vis spectroscopy in a 5% DMSO solution showcases the stability of the complexes with constant stability (K) as 3.76 × 10³, 3.51 × 10³, 3.30 × 10³, and 2.88 × 10³ for NiL₂, CuL₂, CoL₂, and ZnL₂, respectively. Antibacterial and antioxidant activities were evaluated against selected bacteria and DPPH radical, with the complexes outperforming the ligand. Notably, Ni(II) complex showed superior activity, with MIC value as low as 3.9 µg/mL against Bacillus subtilis and IC₅₀ values of 2.3 µM on DPPH radical. DFT calculations at the B3LYP/Def2TZVP level supported the experimental results and provided additional insight into the geometry of the complexes. Molecular docking further validated the biological study results, providing insights into the mechanisms of action and binding affinities against the receptor.