Nickel(II) complexes of 2-hydroxy-benzaldehyde-thiosemicarbazone: Catalase inhibition, antioxidant, DFT, and chemical activities

The mixed ligand nickel(II) complexes with an ONS coordination mode were synthesized by reacting N-methyl- and N-phenyl-thiosemicarbazone ligands derived from 3-bromo-5-chloro-2-hydroxy-benzaldehyde with triphenylphosphine and characterized. In the molecular structure of the complexes (1 and 2), the nickel center has a distorted square planar environment via ONSP atoms. The antioxidant capacities by CUPRAC, free radical scavenging activities by DPPH, and the catalase enzyme (CAT) inhibitory activities of the synthesized compounds with ONS coordination mode and previously prepared complexes with ONN mode were investigated. The CUPRAC studies showed that the ligands and their complexes containing ONS donors provide much more activity than those having ONN donors. According to the results, 1 was found to have the most effective CAT activity. Furthermore, Density Functional Theory (DFT) studies were conducted at the CAM-B3LYP/cc-pVDZ/LANL2DZ(Ni) level to gain deeper insights into the structural, chemical, spectroscopic, and electronic properties of the compounds. In general, there was an acceptable agreement between the experimental and theoretical structural and spectroscopic results. The energy gap between the highest occupied molecular orbitals (HOMO) and lowest unoccopied (LUMO) was found 6.119 and 6.007 eV for 1 and 2, respectively. Computational studies have compared the differences in chemical activity between the ONS and ONN systems and supported the higher activity of ONS complexes.