Synthesis, structural characterization, and antimicrobial assessment of transition metal complexes derived from a Mannich base of 5-chloro-2-mercapto-oxazole

A novel Mannich base ligand (L) was synthesized by combining amine, formaldehyde, and 5-chloro-2-mercapto-oxazole, and characterized by FTIR, CHNS analysis, ¹H-NMR, ¹³C-NMR, and UV-Vis spectroscopy. Its coordination behavior with Mn(II), Cr(III), Ni(II), and Co(II) ions yielded new metal complexes. The spectroscopic, molar conductivity, and magnetic moment measurements were employed to characterize the complexes and to deduce their coordination mode and geometry. Using spectroscopic methods, molar conductivity, and magnetic moment measurements, it was found that the ligand acts as a bidentate donor through nitrogen and sulfur atoms, and the resulting complexes are non-electrolytic. Infrared data further supported the coordination mode, while electronic spectral analysis suggested octahedral with (Cr(III), Mn(II), and Ni(II) complexes, while tetrahedral with Co(II) complex. The in vitro antimicrobial activity of the synthesized ligand and its complexes was evaluated against two bacterial strains (Staphylococcus aureus and Escherichia coli) and two fungal strains (Penicillium spp. and Aspergillus flavus), using ampicillin and fluconazole as reference drugs. The metal complexes exhibited enhanced antibacterial and antifungal activity compared to the free ligand, demonstrating their potential as promising antimicrobial agents. KEY WORDS: 5-chloro-2-mercapto-oxazole, Formaldehyde, Mannich base ligand, Mannich base complexes, Coordination complexes, Antimicrobial Bull. Chem. Soc. Ethiop. 2026, 40(1), 199-210. DOI: https://dx.doi.org/10.4314/bcse.v40i1.16