Binuclear Cu(II), Ni(II) and Zn(II) complexes of hydrazone Schiff bases: Synthesis, spectroscopy, DFT calculations, and SOD mimetic activity

Two new salicylaldimine-based N-heterocyclic hydrazone ligands, N'-[(2-hydroxyphenyl) methylene]-2-(2-phenyl-1H-benzimidazol-1-yl)acetohydrazone (H2L1) and 2-[2-(3-chlorobenzyl)-1H-benzimidazol-1-yl]-N'-[(2-(hydroxyimino)-1-ethylpropylidene]aceto hydrazone (H2L2) and their homo-binuclear nickel(II), copper(II) and zinc(II) complexes have been synthesized and their structures were identified by using spectroscopic techniques and elemental analysis. The two ligands in both the solid state and solution state show tautomerism, but their complexes don't show tautomerism due to the deprotonation and metal ion(s) binding the ligand in the enol-imine-enol form. The stoichiometries of the complexes were also confirmed by thermal analysis with formation of the corresponding metal oxides. TD-DFT calculations have also been performed to correlate geometrical parameters with the spectral properties of compounds and to assign vibrations and electronic transitions. Evaluations of the mimetic catalytic activities of the complexes for the superoxide dismutase (SOD) enzyme were also performed through in vitro tests. All of the complexes studied exhibit activity and the Cu(II) complex (3b) shows the highest activity with the lowest IC50 value (2.50 ± 0.60 µM).