Syntheses and Photophysical Properties of Schiff Base Ni(II) Complexes: Application for Sustainable Antibacterial Activity and Cytotoxicity

Five new imine-linked ligands (H2L1, H4L2, and L3–L5) were synthesize in two step reactions and final products were fully characterized with spectroscopic techniques. The binding behavior of these ligands was evaluated with the library of metal ions. However, substantial photophysical modulations were observed with the coordination of [Ni2+] ion to the ligand pocket. The fact that Ni2+ induced remarkable modulation in photophysical studies encouraged us to prepare the nickel complexes of respective ligands. The resultant coordination complexes offered interesting properties like tunable coordination geometry, variable stereochemistry, and possibility of electrostatic interactions which are important for biomolecular recognition. These properties of coordination complexes encouraged us to evaluate the antimicrobial activity of nickel complexes. All nickel complexes have shown appreciable antibacterial activity against Staphylococcus aureus and methicillin-resistant S. aureus (MRSA). The SEM imaging studies were performed to know the mechanism of cell death, and results revealed that the cell wall inhibition is the main reason for bacterial cell death. The cytotoxicity of these complexes and their respective ligands for human cell lines was established with the HeLa cell, and observations affirm 85–90% cell viability after 24 h.