SCHIFF BASE COMPLEXES OF CO(II) AND NI(II) DERIVED FROM 2-HYDROXY-5-NITROBENZALDEHYDE AND 8-AMINOQUINOLINE: SYNTHESIS, CHARACTERIZATION AND ANTIBACTERIAL STUDIES

Abstract

A Schiff base ligand, (E)-4-nitro-2-((quinolin-8-ylimino)methyl)phenol (C₁₆H₁₁N₃O₃), was synthesized through condensation of 2-hydroxy-5-nitrobenzaldehyde with 8-aminoquinoline in ethanolic medium. The ligand was subsequently coordinated with cobalt(II) and nickel(II) ions to form the corresponding metal complexes. The synthesized ligand and its complexes were characterized using elemental analysis, FT-IR spectroscopy, molar conductivity measurements, and magnetic susceptibility studies. Spectral analysis revealed coordination of the ligand to the metal ions via the azomethine nitrogen and phenolic oxygen atoms, resulting in stable complexes with octahedral geometry around the metal centers. Molar conductivity measurements in DMSO indicated the non-electrolytic nature of the complexes, while magnetic susceptibility values confirmed their paramagnetic behavior. The antibacterial activities of the ligand and its metal complexes were evaluated in vitro against Staphylococcus aureus (ATCC 29213) and Escherichia coli (ATCC 25922) using the agar diffusion method. The results demonstrated that the metal complexes exhibited significantly enhanced antibacterial activity compared with the free ligand. Among the synthesized compounds, the Co(II) complex showed the highest activity, with minimum inhibitory concentration (MIC) values of 8 µg mL⁻¹ against E. coli and 10 µg mL⁻¹ against S. aureus. In contrast, the Ni(II) complex showed moderate activity. The improved biological activity of the complexes is attributed to the chelation effect, which increases lipophilicity and facilitates penetration through the bacterial cell membrane. Although the activity of the synthesized complexes was lower than that of the standard antibiotic tetracycline, the results suggest that the compounds may serve as promising scaffolds for the development of new antimicrobial agents.