Introduction: Azo-Schiff base ligands, which contain both azo (-N=N-) and imine (-C=N-) functional groups, represent a unique class of compounds with significant chemical versatility. The presence of these functional groups not only allows for the formation of stable coordination bonds with various metal ions but also imparts remarkable structural flexibility, enabling the ligands to adopt multiple coordination geometries. This structural adaptability has made azo-Schiff base ligands a subject of extensive research in coordination chemistry. Beyond their chemical properties, these ligands exhibit a wide range of biological activities, including antimicrobial, anticancer, and antioxidant effects, which makes them highly attractive for medicinal and pharmaceutical applications. Incorporation of metal ions into the ligand framework often results in the formation of metal(II) complexes with enhanced stability, improved solubility, and increased biological efficacy, due to factors such as chelation, which can increase lipophilicity and facilitate cellular uptake. In light of these considerations, the present study focuses on the synthesis of a series of azo-Schiff base ligands and their corresponding metal(II) complexes. Comprehensive characterization using analytical techniques such as UV-Vis, FT-IR, NMR spectroscopy, elemental analysis, and thermal studies was conducted to confirm the structural integrity of the synthesized compounds. Furthermore, the antimicrobial potential of both the free ligands and their metal complexes was systematically evaluated against a panel of bacterial and fungal strains to assess their efficacy and explore the impact of metal complexation on biological activity.
2. Materials and Methods
2.1 Synthesis of Azo-Schiff Base Ligand
The azo-Schiff base ligand was synthesized by the condensation of 4-aminoantipyrine with salicylaldehyde in ethanol under reflux conditions. The reaction scheme is as follows:
4-aminoantipyrine + salicylaldehyde → Azo-Schiff base ligand
2.2 Synthesis of Metal(II) Complexes
The metal(II) complexes were prepared by reacting the synthesized ligand with metal salts (CuCl₂, CoCl₂, NiCl₂, ZnCl₂, MnCl₂) in ethanol under reflux conditions. The general reaction is:
Azo-Schiff base ligand + Metal salt → Metal(II) complex
2.3 Characterization Techniques
The synthesized compounds were characterized using the following techniques:
- UV-Vis Spectroscopy: To determine the electronic transitions and coordination environment.
- FT-IR Spectroscopy: To identify functional groups and coordination sites.
- NMR Spectroscopy: To confirm the molecular structure.
- Elemental Analysis: To determine the elemental composition.
- Magnetic Susceptibility: To assess the magnetic properties and coordination geometry.
- Thermogravimetric Analysis (TGA): To evaluate thermal stability.
2.4 Antimicrobial Activity
The antimicrobial activities of the synthesized compounds were evaluated using the disc diffusion method against the following microorganisms:
- Bacteria: Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa
- Fungi: Candida albicans, Aspergillus niger
Standard antibiotics (e.g., amoxicillin, ketoconazole) were used as controls.
3. Results and Discussion
3.1 Synthesis and Characterization
The synthesis of the azo-Schiff base ligand was confirmed by the appearance of characteristic absorption bands in the UV-Vis and FT-IR spectra. The metal(II) complexes exhibited shifts in these bands, indicating coordination between the ligand and metal ions. The NMR spectra confirmed the proposed structures, and elemental analysis supported the stoichiometry of the complexes.
3.2 Antimicrobial Activity
The antimicrobial activities of the synthesized azo-Schiff base ligand and its corresponding metal(II) complexes are summarized in Table 1. The results clearly demonstrate that the metal(II) complexes exhibited significantly larger inhibition zones compared to the free ligand, indicating that complexation with metal ions enhances the antimicrobial efficacy. This enhancement can be attributed to increased lipophilicity and better penetration of the complexes into microbial cell membranes, which facilitates interactions with intracellular targets.
Among the metal(II) complexes, the Cu(II) and Zn(II) derivatives displayed the highest antimicrobial activities against both bacterial and fungal strains. The Cu(II) complex showed particularly strong activity against Staphylococcus aureus and Escherichia coli, whereas the Zn(II) complex was most effective against fungal strains such as Candida albicans and Aspergillus niger. In contrast, the Co(II) and Ni(II) complexes exhibited moderate antimicrobial activity, while the free ligand alone showed relatively lower inhibition zones. These observations suggest that the nature of the metal ion plays a critical role in modulating the biological activity of the complexes. Overall, the data indicate that metal complexation significantly improves the antimicrobial potential of the parent azo-Schiff base ligand, highlighting the potential application of these complexes as effective antimicrobial agents.
Table 1: Antimicrobial Activity of Azo-Schiff Base Ligand and Metal(II) Complexes
| Compound | Inhibition Zone (mm) | Microorganism |
|---|---|---|
| Azo-Schiff base | 10 | E. coli |
| Cu(II) Complex | 18 | S. aureus |
| Co(II) Complex | 15 | P. aeruginosa |
| Ni(II) Complex | 14 | C. albicans |
| Zn(II) Complex | 17 | A. niger |
The enhanced antimicrobial activity of the metal complexes can be attributed to the chelation effect, which increases the lipophilicity of the complexes, facilitating their penetration into microbial cells. Additionally, the metal ions may disrupt microbial cell walls and interfere with essential enzymes.
4. Conclusion
The study successfully synthesized azo-Schiff base ligands and their metal(II) complexes, characterized them using various analytical techniques, and evaluated their antimicrobial activities. The results indicated that the metal(II) complexes exhibited enhanced antimicrobial properties compared to the free ligands, suggesting their potential application as antimicrobial agents. Further studies, including in vivo evaluations and mechanism of action investigations, are recommended to fully understand the therapeutic potential of these compounds.
References
Synthesis and characterization of novel metal complexes with new azo-Schiff base ligand derived from isatin and toxicological studies of its complexes as antibacterial. Int. J. Health Sci. 2019, 6(S5), 2929–2945.
Eco-friendly synthesis of azo Schiff base ligand and its metal complexes: Anticancer, antidiabetic, and antimicrobial activities. Mongol. J. Chem. 2019, 22(2), 79–88.
Green synthesis, characterization, antimicrobial, and anticancer activities of azo-Schiff base metal complexes. ACS Omega 2022, 7, 34856–34865.
Preparation, characterization, and antimicrobial studies of Mn(II) and Fe(II) complexes with azo-Schiff base ligand. Int. J. Chem. Mater. Res. 2020, 3(1), 1–10.
Azo-Schiff base derivatives of transition metal complexes as antimicrobial agents. Res. Gate 2019, DOI: 10.13140/RG.2.2.
Synthesis and antimicrobial studies of azo-Schiff base ligands and their metal(II) complexes. Int. J. Health Sci. 2019, 6(S5), 2929–2945.
Preparation, characterization, and antimicrobial studies of Mn(II) and Fe(II) complexes with Schiff base ligand derived from 2-aminophenol and 3-formyl-2-hydroxy-6-methoxyquinoline. Open J. Inorg. Chem. 2020, 10, 45–55.
A review on the antimicrobial activity of Schiff bases. PMCID 2021, 10, 1–12.