Synthesis, characterization, crystal structure, antioxidant, and cytotoxicity studies of Ni(II) complexes derived from 2-(benzyloxy)benzoylhydrazone and different aldehydes

In this work, four new aroylhydrazone-based Schiff base ligands (HL1-HL4) have been synthesized by the condensation of 2-(benzyloxy)benzoylhydrazine with four different aldehydes, namely cinnamaldehyde, 4-methylbenzaldehyde, 4-methoxybenzaldehyde, 4-allyloxybenzaldehyde. The correspondent bischelated Ni(II) complexes [Ni(L1)₂-Ni(L4)₂] were prepared in-situ by refluxing an ethanolic solution of 2-(benzyloxy)benzoylhydrazine with the aldehydes mentioned above in the presence of Ni(II) acetate tetrahydrate. The ligands and complexes were characterized by FT-IR, ¹H NMR, mass, and UV–visible spectroscopic techniques. The molecular structures of ligands HL1, HL2, and complex Ni(L3)₂ were determined by single crystal X-ray diffraction technique. In ligand HL1, the benzohydrazide moiety forms dihedral angles of 15.48 and 38.78° with the phenyl rings of the molecule. In contrast, the two independent molecules detected in the crystal of ligand HL2 slightly differ in their conformation, due to a different orientation of the benzyl ring that makes a dihedral angle of 29.54 and 49.25° with the benzohydrazide moiety. The metal ion in complex Ni(L3)₂ exhibits a distorted square-planar geometry with trans-configuration of the deprotonated benzohydrazide ligands coordinating Ni(II) through the azomethine-N and carbonylate-O atoms. The antioxidant activity of the ligands and complexes was evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. The ligands showed a low level of radical scavenging activity compared to the standard ascorbic acid, whereas the complexes did not show any activity. Moreover, the cytotoxicity of the synthesized compounds was tested in vitro against brine shrimp, and the results showed a promising cytotoxic effect of ligands and complexes.