Eco-friendly, Synthesis, Characterization, and Antimicrobial Studies of Transition Metal Complexes of Schiff Base Ligand Derived from 1H-indole-3-carbaldehyde with 4-nitrobenzoydrazide

Abstract

Rapid, efficient, clean and environmentally benign exclusive synthesis of Schiff base ligand and their transition metal complexes have been developed using condensation of 1H-indole-3-carbaldehyde with 4-nitrobenzoydrazide efficiently in an alcoholic suspension medium using alkali catalyst with excellent yields under microwave irradiation. The present work, microwave assisted green synthesis of novel Schiff base ligand (E)-N\'-((1H-indol-3-yl)methylene)-4-nitrobenzohydrazide and its transition metal complexes of Co(II), Ni(II), Cu(II), Zn(II),and Ag(I) were prepared under microwave irradiation as a green approach method. The novel Schiff base ligand was known by melting point and thin layer chromatography. Characterization was by elemental analysis, Infrared spectra, 1HNMR spectra and mass spectroscopy. The metal complexes were recognized by melting point, thin layer chromatography and their distinct colour. Metal complexes were characterized by Infrared spectra, UV-visible spectra and thermogravimetric analysis. The synthesized Schiff base ligand and its metal complexes were screened for their antibacterial activity against bacterial species Escherichia coli, Staphylococcus aurous and Salmonella Typhi.

Introduction

Schiff base ligands are widely studied in coordination chemistry due to their easy synthesis, availability, electronic properties, good solubility, and ability to form stable complexes with transition metals. Ligands containing O and N donor atoms, such as those derived from 1H-indole-3-carbaldehyde and 4-nitrobenzoydrazide, are particularly important for catalysis, magnetism, biological applications, and material chemistry. Microwave-assisted synthesis offers advantages such as shorter reaction times, higher yields, and environmentally friendly processing.

Materials and Methods

• Ligand Synthesis: Condensation of 1H-indole-3-carbaldehyde with 4-nitrobenzoydrazide under microwave irradiation produced yellow crystals, purified by recrystallization.

• Metal Complexes: Prepared by reacting metal nitrates (Co, Ni, Cu, Zn, Ag) with the ligand in a 1:2 metal-to-ligand ratio under microwave irradiation.

• Characterization Techniques: Elemental analysis (C, H, N), IR, 1H-NMR, mass spectrometry, UV-Vis spectroscopy, and thermogravimetric analysis (TGA) were performed to confirm structures and stability.

Results and Discussion

• Physical Properties: Ligand and metal complexes are colored, solid, thermally stable, and soluble in DMF and DMSO. Yields were high (87–94%), and melting points were sharp.

• Spectral Analysis:

  • IR: Characteristic bands confirmed azomethine (C=N) groups, aromatic structures, N-H stretches, and metal-ligand (M-N) bonds.

  • 1H-NMR: Peaks corresponded to azomethine, indole, CO-NH, and aromatic protons.

  • Mass Spectra: Molecular ion peak matched the ligand's calculated molecular weight.

  • UV-Vis: Electronic transitions confirmed octahedral geometry of Ni(II) and Cu(II) complexes.

• Thermal Analysis (TGA): Metal complexes decomposed in three stages, first losing coordinated water molecules, followed by ligand decomposition, leaving metal oxides as residue.

Antimicrobial Activity

• Both the Schiff base ligand and its metal complexes were tested against E. coli, S. aureus, and S. typhi.

• Metal complexes showed enhanced antibacterial activity compared to the free ligand, attributed to chelation and azomethine moiety.

• Ni(II) and Zn(II) were most effective against E. coli, Cu(II) against S. aureus, and Co(II) and Zn(II) against S. typhi.

Conclusion

Metal complexes with Co(II), Ni(II), Cu(II) ,Zn(II), and Ag(I) ions were synthesized using the resynthesized ligand (E)-N\'-((1H-indol-3-yl)methylene) -4-nitrobenzohydrazide. The ligand (L) and its metal complexes were analyzed using TLC, melting point determination, FT-IR spectroscopy, 1H-NMR spectroscopy, UV-Vis spectroscopy, and thermo gravimetric analysis. Complexes exhibit octahedral stereochemistry based on UV-Vis spectroscopy. When a metal ion is chelated with a ligand, its polarity will be reduced to a greater extent due to the overlap of ligand orbital and the partial sharing of the positive charge of the metal ion with donor groups. Furthermore, the chelation process increases the delocalization of the ?-electrons over the whole chelate ring, which results in an increase in the lipophilicity of the metal complexes.

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