Synthesis, Characterization and Antimicrobial Studies of a Hydrazide Hydrazone Ligand Derived from 2-Pyridinecarboxaldehyde and 4-Hydroxybenzohydrazide and its Ni(II) and Cu(II) Complexes

Abstract

A hydrazide hydrazone ligand was synthesized via the condensation of 2-pyridinecarboxaldehyde and 4hydroxybenzohydrazide. Its corresponding Ni(II) and Cu(II) complexes were prepared and characterized through elemental analysis, molar conductance, magnetic susceptibility, FT-IR, and UV-Vis spectroscopic techniques. Analytical and spectroscopic data confirmed the formation of neutral complexes with the general formulae [Ni(HL)Cl2(H2O)] and [Cu(L)Cl], where HL is the neutral ligand and L- is its deprotonated form. The FTIR results revealed that the ligand coordinates in a tridentate manner. The electronic spectra and magnetic moment (3.42 BM) suggested an octahedral geometry for the Ni(II) complex, while the data for the Cu(II) complex were consistent with a square planar geometry, despite an anomalous magnetic moment (3.11 BM). The molar conductance values in DMSO (~15 S cm² mol⁻¹) confirmed the neutral nature of both complexes. The compounds were evaluated for their in vitro antimicrobial activity against Gram-negative (Escherichia coli), Gram-positive (Staphylococcus aureus) bacteria, and fungal strains (Aspergillus niger, Candida albicans). The bioactivity trend, Cu(II) > Ni(II) > Ligand, was established. The Cu(II) complex demonstrated superior, broad-spectrum efficacy, exhibiting inhibition zones up to 26 mm and an activity index of 93% against E. coli compared to the standard drug streptomycin. This significant enhancement is attributed to the complex's square planar geometry, anhydrous nature, and potential for redox cycling, underscoring the critical role of metal ion coordination in advancing antimicrobial chemotherapeutics.