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

<p>A new series of Ni(II) complexes, [Ni(L¹)₂] (<b>1</b>), [Ni(L²)₂] (<b>2</b>), [Ni(L³)₂] (<b>3</b>), and [Ni(L⁴)₂] (<b>4</b>), 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 <b>2</b> and <b>3</b> 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 <i>in vitro</i> tests, DPPH radical scavenging and reducing power. The compounds were screened for their <i>in vitro</i> 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 <b>1</b> 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.