Synthesis and Evaluation of Schiff Base Complexes of Copper and Nickel as Potential Antimicrobial Agents

Schiff base metal complexes represent a structurally diverse class of coordination compounds that have attracted significant attention for their antimicrobial, antifungal, and antitumour properties. In the context of rising antimicrobial resistance in Nigeria, the development of novel metal-based antimicrobial agents from locally designed ligand systems is an important research contribution. This study synthesised novel Schiff base ligands by condensation of salicylaldehyde and 4-aminobenzoic acid derivatives and prepared their copper(II) and nickel(II) complexes. Ligands and complexes were characterised by elemental analysis, melting point determination, molar conductance, magnetic susceptibility measurements, infrared spectroscopy, electronic spectra, thermogravimetric analysis, and X-ray powder diffraction. In vitro antimicrobial activity was evaluated against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Candida albicans, and Aspergillus niger using the agar disc diffusion method, with minimum inhibitory concentrations determined by broth microdilution. Spectroscopic data confirmed coordination through the azomethine nitrogen and phenolic oxygen of the Schiff base with square planar geometry for copper complexes and octahedral geometry for nickel complexes. All metal complexes demonstrated significantly enhanced antimicrobial activity compared to the free ligands, consistent with the chelation enhancement effect. The copper complex of the 4-aminobenzoic acid-derived Schiff base showed minimum inhibitory concentrations of 3.13 mg/mL against Staphylococcus aureus and 6.25 mg/mL against Candida albicans. These results position these novel complexes as promising leads for further pharmacological investigation as antimicrobial agents.

Keywords: Schiff base complexes, copper, nickel, antimicrobial activity, coordination chemistry