Synthesis, characterization, molecular modelling and bioactivity studies of cobalt, nickel, copper and zinc complexes with tetradentate schiff bases

Abstract

The search for new active antimicrobial compounds is of growing interest since the current antibiotics remain insufficient to tackle the challenge of increasing emergence and spread of antimicrobial resistance. Inorganic compounds particularly metal complexes have played an important role in the development of new metal-based drugs. In this regard, there is a strong need to synthesize new compounds that possess good chemical reactivity and are comprehensively studied with the help of theoretical calculation. In this study, four types of Schiff base ligands have been synthesized by the condensation between 2,4-dihydroxyacetophenone and appropriate amines in a 2:1 molar ratio. Mononuclear Ni(II), Cu(II), Co(II) and Zn(II) metal complexes were prepared and characterized by elemental analysis, molar conductivity, Ultraviolet-Visible, infrared and nuclear magnetic resonance spectroscopies as well as single crystal X-ray diffraction analysis. Spectral data revealed that the ligand acted as O,N,N,O tetradentate chelating to the metal ion through phenolate oxygen and azomethine nitrogen atoms. The geometry of the ligands and the metal complexes were optimized by Density Functional Theory (DFT) using Gaussian 09 software package at theoretical level B3LYP/6-311G++(d,p) for ligands and B3LYP/GEN for metal complexes. The experimental results were corroborated by the calculations data. All compounds were subjected to antibacterial activities against two gram-positive and two gram-negative bacterial strains. All the compounds showed broad spectrum of antibacterial activities. Co(II) complexes were generally active against both gram-positive and gram-negative bacteria with minimum inhibitory concentration (MIC) values ranging from 0.02 to 5 mg/mL. The results showed that metal chelation contributes to significant antibacterial activities. The antioxidant potentials of the compounds were evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. The ligands had higher antioxidant potentials compared to metal complexes. However, some phenylene-containing complexes exhibited excellent scavenging activity with their SC50 values ranging from 11.3 to 79.9 µg/mL.