Synthesis, Characterization, Quantum Chemical Studies, and Biological Activity of an Oxadiazole–Hydrazone Ligand Derived from 3-Ethyl-5-Methylpyridine and Its Metal Complexes

Abstract

A new 1,2,3-oxadiazole-based hydrazone ligand, (Z)-4-(5-(2-((3-ethyl-5-methylpyridin-2-yl)methylene)hydrazinyl)-1,2,3-oxadiazol-2-yl)benzene-1,2,3-triol (L), and its Cr(III), Fe(III), and Ni(II) complexes were successfully synthesized and characterized by FT-IR, 1H-NMR, mass spectrometry, magnetic susceptibility, and molar conductivity measurements. The spectral data suggested tetrahedral geometries for the Cr(III) and Fe(III) complexes, while the Ni(II) complex exhibited a square-planar geometry. The electronic properties of the ligand were investigated using density functional theory (DFT). The calculated HOMO-LUMO energy gap (4.60 eV) indicated good molecular stability and moderate chemical reactivity. The antibacterial activity of the ligand and its metal complexes was evaluated against Serratia and Pseudomonas by the agar well diffusion method. The metal complexes showed higher antibacterial activity than the free ligand, with the Cr(III) complex displaying the strongest inhibitory effect. The combined experimental and theoretical results suggest that coordination with transition metal ions enhances both the electronic properties and the biological activity of the synthesized ligand