Co(II), Ni(II), Cu(II) and Zn(II) complexes of Schiff base ligands: synthesis, characterization, DFT, in vitro antimicrobial activity and molecular docking studies

Three Schiff base ligands [H2L1-H2L3] containing nitrogen/oxygen donor atoms and their Co(II), Ni(II), Cu(II) and Zn(II) complexes were synthesized by stirring metal acetates with Schiff base ligands obtained from condensation reaction of 2-amino-6-chloro-4-nitrophenol with 5-chloro salicylaldehyde/3,5-dibromo salicylaldehyde/3-methoxy-5-nitro salicylaldehyde. The structural traits of the synthesized compounds were done by using elemental analysis, spectroscopic techniques (UV-Vis, H-1 and C-13 NMR, FT-IR), mass spectrometry and some physical studies (XRD, TGA). According to spectral data, ligands behave as a tridentate (ONO) and formed complexes with octahedral geometry. The thermogravimetric analysis revealed that metal complexes decay in multi-steps leaving metal oxide as an end product. Powder XRD study suggested crystalline nature of the compounds. The energy gap (HOMO-LUMO) and molecular electrostatic potential calculation were computed by using DFT/B3LYP/6-31G** basis set. Derived ligands and complexes were explored for in vitro antimicrobial potential toward two gram-positive bacteria, two gram-negative bacteria, i.e., S. aureus, B. subtilis, P. aeruginosa, E. coli, and two fungal strains, i.e., A. niger, C. albicans, through serial dilution method taking ciprofloxacin and fluconazole as standard. The investigated results showed that complexes are more potent than free Schiff base ligands. The Cu(L-2)(H2O)(3) (0.0115 mu mol/mL) and Zn(L-2)(H2O)(3) (0.0115 mu mol/mL) complexes were found to be more active among all the investigated compounds. Additionally, molecular docking studies were also performed for some compounds in the active site of DNA Gyrase enzyme (PDB code: 1AJ6), suggesting good hydrophobic interactions of compounds with the enzyme.