Synthesis, Structural Characterization, and Antimicrobial Activity of Fe(III), Co(II), and Ni(II) Complexes with Quercetin and 2-Aminothiazole

Abstract

Mixed-ligand complexes of quercetin (QUE) and 2-aminothiazole (2-AMT) with Fe(III), Co(II), and Ni(II) ions were synthesized via a solvothermal reflux route using a 1:1:1 metal-to-ligand ratio. The complexes, formulated as [M(QUE)(2-AMT)(H₂O)₂] (M = Fe, Co, Ni), were characterized by elemental analysis, UV–Visible Spectroscopy, and FT–IR spectroscopy. Spectral data confirmed bidentate coordination of QUE through its carbonyl and hydroxyl oxygen atoms and of 2-AMT via the endocyclic nitrogen, consistent with octahedral geometry around the metal centers. The complexes exhibited low molar conductivities (15–25 Ω⁻¹ cm² mol⁻¹), indicating their non-electrolytic nature and neutral stoichiometry. Antimicrobial activity was assessed against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Aspergillus niger using the agar well diffusion method. All complexes showed enhanced activity compared to the free ligands, with the Co(II) complex displaying the highest potency (22 mm inhibition zone against P. aeruginosa, MIC = 75 μg/mL). The observed enhancement is attributed to increased lipophilicity and metal–ligand synergism that facilitates microbial membrane penetration. These findings underscore the potential of quercetin–2-aminothiazole coordination frameworks as promising scaffolds for metal-based antimicrobial drug design.