Synthesis, characterization and antibacterial activities of indole-derived ligand and its Ni (II), Co (II) and Cr (II) complexes

Objective: Schiff bases derived from indole frameworks have gained significant attention in medicinal and coordination chemistry due to their versatile donor sites and biological relevance. Their metal complexes often exhibit enhanced pharmacological properties through chelation, which improves lipophilicity and cellular permeability. However, despite their potential, only a limited number of studies have explored the antimicrobial properties of indole-based Schiff base complexes, especially those incorporating Ni(II), Co(II), and Cr(II). Therefore, this research focuses on synthesizing, characterizing, and evaluating new complexes of these metals to help bridge this gap. Methods: In this study, a Schiff base ligand, (E)-1-(5-bromoindolin-3-yl)-N-(2-(piperidin-1-yl) ethyl) methanimine (L1), was synthesized from 5-bromoindole-3-carbaldehyde and 1-(2-aminoethyl)piperidine in an ethanolic medium. Its Cr (II), Co (II), and Ni (II) complexes were prepared by refluxing the ligand with the corresponding metal chlorides. The compounds were characterized using FTIR, UV–Vis spectroscopy, molar conductance, and magnetic susceptibility measurements. Their antimicrobial and antifungal activities were evaluated against Staphylococcus aureus, Klebsiella pneumoniae, Aspergillus niger, and Candida albicans. Results: Spectroscopic and magnetic data confirmed octahedral coordination involving the indole and piperidine. The metal complexes showed higher thermal stability and molar conductance values indicative of [ML]Cl₂ electrolytic structures. Antimicrobial assays revealed improved activity of the complexes compared to the free ligand, particularly the Cr (II) and Ni (II) complexes, which exhibited notable inhibitions against S. aureus and A. niger. Conclusion: The study demonstrates that complexation of indole-based Schiff bases with transition metals enhances their physicochemical and biological properties. These findings support the potential of such metal complexes as promising antimicrobial agents and a foundation for designing therapeutic compounds.