Impact of the functionalized tetrazole ring on the electrochemical behavior and biological activities of novel nickel (II) complexes with a series of tetrazole derivatives

Abstract

Six new nickel (II) complexes with a series of tetrazole derivatives functionalized by different substituents {5-(2-(1-Benzyltetrazol-5-yl) phenyl)-2-ethyl-4-methylthiazole (L1), 1-Benzyl-5-(2-(1-methylpyrrol-2-yl)phenyl) tetrazole (L2), 5-(2-(1-Pivalyltetrazol-5-yl)phenyl)-2-ethyl-4-methylthiazole (L3), 5-(2-(1-Methylpyrrol-2-yl) phenyl)-1-pivalyltetrazole (L4), 2-Ethyl-4-methyl-5-(2-(1 methyltetrazol-5-yl) phenyl) thiazole (L5) and 1-Methyl-5-(2-(1-methylpyrrol-2-yl)phenyl) tetrazole (L6)} have been synthesized and characterized by analytical and spectral methods. The data clearly indicated that the nickel (II) complexes are coordinated to the monodentate tetrazole derivatives via nitrogen (N3) atom of the tetrazole ring. The octahedral geometry is observed for all the complexes. The thermogravimetric analysis revealed the presence of coordinated and hydrated water molecules in the coordination sphere. The DFT calculations performed on both the ligands and the complexes allowed to optimize the structures, the stability and to explain the electrochemical behavior and the biological activities of the nickel (II) complexes.The study of the substituents effects on the redox properties of the ligands and their nickel (II) complexes were discussed via cyclic voltammograms. Electron donating substituents shift the reduction potentials toward negative values, while anodic shift of the oxidation potentials is manifested by the substituents having an electron withdrawing effect.The in vitro antimicrobial activities of the ligands and their corresponding nickel (II) complexes have been evaluated against four bacterial and two fungal strains.L1–L6 ligands and their nickel (II) complexes showed a better activity against Gram-positive bacteria. The reactivity of the complexes against Streptococcus group D varied from very sensitive to extremely sensitive activity. No activity has been recorded for the six ligands against the tested fungal strains but their complexes showed extremely sensitive antifungal activity against these strains. The electron donating effect of the substituents was found to weaken the antimicrobial activity of the complexes.The antioxidant activity of the compounds has been evaluated using the DPPH method. The results indicated that the ligands and their nickel (II) complexes exhibited a better activity against the radical DPPH in comparison to the used reference. Electron withdrawing effect of the substituents on the ligands enhanced the antioxidant activity of the ligands and their complexes.