Synthesis, Spectroscopic Characterization, and Crystal Structure of a Mononuclear Mn(II) Complex Derived from the N'-(4-hydroxy-3-methoxybenzylidene) Isonicotinohydrazide Ligand: Evaluation of the Antioxidant Activity of the Ligand and Its Complex

Abstract

A mononuclear manganese(II) complex of N′-(4-hydroxy-3-methoxybenzylidene)isonicotinohydrazide (H₂L) was synthesised and characterised to examine its structural features and DPPH radical-scavenging activity relative to the free ligand. The ligand was prepared by condensation of isonicotinic acid hydrazide with para-vanillin in ethanol, and the metal complex was obtained by reaction with manganese(II) chloride in methanol. Characterisation involved elemental analysis, infrared and electronic spectroscopy, molar conductivity measurements and single-crystal X-ray diffraction, with nuclear magnetic resonance spectroscopy also used for the ligand. Crystallographic analysis showed that the manganese(II) centre was coordinated by two pyridine nitrogen atoms from neutral ligand molecules and four oxygen atoms from water molecules, resulting in a distorted octahedral MnN₂O₄ environment. Spectroscopic findings were consistent with coordination through the pyridine nitrogen atoms. The radical-scavenging activity of both compounds was evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Inhibition increased with concentration within the investigated range. The ligand exhibited inhibition values of 21.13 ± 0.17% to 53.62 ± 0.23%, whereas the complex exhibited values of 33.26 ± 0.34% to 61.66 ± 0.23%. The reported IC₅₀ values were 459 μM for the ligand, 310 μM for the complex and 493.36 μM for Trolox. Under the reported assay conditions, the complex exhibited greater DPPH radical-scavenging activity than the free ligand. Further stability and toxicity studies are necessary to assess the broader significance of these observations.