New Ni(II) complexes involving symmetrical bidentate N,O-donor Schiff base ligands: synthesis at ambient temperature, crystal structures, electrochemical study, antioxidant and cytotoxic activities

A new series of Ni(II) complexes, [Ni(L1)2] (1), [Ni(L2)2] (2), [Ni(L3)2] (3), and [Ni(L4)2] (4), were synthesized at ambient temperature. The bidentate Schiff base ligands HL1−4 have been obtained by the condensation reaction of 2-hydroxybenzaldehyde, 5-bromo-2-hydroxybenzaldehyde, 3-methoxy-2-hydroxy-benzaldehyde, and 4-methoxy-2-hydroxy-benzaldehyde, respectively, with 2-methoxyethylamine. The newly synthesized complexes were characterized by elemental analyses, FT-IR and UV–vis spectroscopy. The crystal structures of mononuclear Ni(II) complexes 2 and 3 were determined by the single-crystal X-ray diffraction technique. Electrochemical properties of the complexes were investigated in acetonitrile. The antioxidant properties of the Schiff base ligands and complexes were evaluated by two in vitro tests, DPPH radical scavenging and reducing power. The compounds were screened for their in vitro anticancer potential using gastric cancer cell lines by MTT assay. All ligands and complexes showed considerable cytotoxic activity against cancer cell lines (IC50 = 0.2516–5.468 μg·mL−1). The most promising result was achieved for complex 1 with the best IC50 value of 0.2516 μg·mL−1. It was found that the proliferation rate of MKN-45 cells decreased after treatment with the complexes in a dose-dependent way.