Synthesis, Spectroscopic Characterization, Crystal Structure of the Ligand N’-[(E)-(4-Hydroxy-3-Methoxybenzylidene)] Pyridine-3-Carbohydrazide : Antibacterial and Antioxidant Activities of Ligand and Its Zinc(II) Complex

Abstract

The hydrazone Schiff-base ligand N’-[(E)-(4-hydroxy-3-methoxybenzylidene)]pyridine-3-carbohydrazide (H₂L) was synthesized by condensing paravanillin with nicotinohydrazide and was characterised using infrared, nuclear magnetic resonance, ultraviolet–visible, and single-crystal X-ray diffraction techniques. The ligand crystallised in the monoclinic system with the P2₁/c space group and unit-cell parameters a = 7.9079(2) Å, b = 27.5162(6) Å, c = 6.9736(2) Å, and β = 110.495(3)°. A zinc(II) complex was subsequently prepared using zinc acetate and examined by infrared and ultraviolet–visible spectroscopy and conductivity measurements. The complex was classified as neutral, and its conductivity values remained stable over the 15-day measurement interval. Antibacterial activity was evaluated against Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, and Pseudomonas aeruginosa. The free ligand produced inhibition zones of 6–7 mm against susceptible strains, whereas the zinc complex produced zones of 14 mm against E. coli, 12 mm against S. aureus, and 10 mm against E. faecalis. Activity against P. aeruginosa was not demonstrated. Antioxidant activity was assessed by the DPPH radical-scavenging assay. The zinc(II) complex showed a lower IC₅₀ value than the free ligand, 0.22 versus 0.58 mg/mL, indicating greater radical-scavenging activity, although its activity remained lower than that of ascorbic acid, with an IC₅₀ of 0.10 mg/mL. Overall, zinc coordination was associated with increased antibacterial and DPPH radical-scavenging activities under the reported experimental conditions.