Synthesis, structural characterization, DFT calculation, and antitumor activity of a new Co(II) complex based on 2-((2′-Carboxybenzyl)oxy)benzoic acid and 2,2′-bipyridine ligands

In this work, a new Co(II) complex, abbreviated as [Co(L)(bipy)(H₂O)₂]_n (1) was prepared using cobalt(II) acetate tetrahydrate, 2-((2'-carboxybenzyl)oxy)benzoic acid (H₂L), and 2,2'-bipyridine (bipy) ligands in a mixture solution of ethanol and water (v: v = 3: 1). The structure of the complex (1) was analyzed by elemental analysis (EA), infrared (IR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, thermogravimetric analysis (TG), and single-crystal X-ray diffraction techniques. The complex (1) crystallizes in the monoclinic space group C2/c, featuring a distorted octahedral [CoO₄N₂] coordination sphere, by two oxygen atoms from the carboxylate groups of two 2-((2'-carboxybenzyl)oxy)benzoate (L) ligands, two oxygen atoms from coordinated water molecules, and two nitrogen atoms (N1 and N2) from a single 2,2'-bipyridine (bipy) ligand. Bridging carboxylate ligands (L) form a 1D chain, which extends into a 2D layer via π-π interactions of the 2,2'-bipyridine ligands (bipy). The DFT calculations of the complex (1) indicates that the HOMO is predominantly distributed on the oxygen and nitrogen atoms bonded to Co(II) ion, but the LUMO is mainly localized around distributed in the six-membered carbon ring adjacent to the Co(II) ion. Electrostatic potential calculation of the complex (1) shows that the regions with higher electrostatic potential are mainly located on the aromatic ring, whilst the lower electrostatic potential regions are primarily located near the oxygen and nitrogen atoms. The electrochemical behavior of the complex (1) was investigated in acetonitrile and 1 mol·L⁻¹ sulfuric acid. In ACN/TBATFB, the complex (1) exhibits a well-defined, predominantly reversible Co(III)/Co(II) redox couple, indicating good electrochemical stability, whereas in 1 mol·L⁻¹ H₂SO₄, a significantly different redox response with enhanced anodic currents and a prominent oxidation peak is observed, arising from the synergistic effect of ligand protonation-oxidation processes and the metal-centered Co(III)/Co(II) redox couple under acidic conditions. The cytotoxicity of the complex (1) and CoCl₂ was evaluated against L02, PANC-1, and MCF7 cell lines using the MTT assay. The complex (1) exhibited lower IC₅₀ values and higher selectivity for cancer cells than CoCl₂, attributable to improved cellular uptake and ROS-mediated oxidative stress, whereas free Co²⁺, which stimulated cell proliferation at low concentrations, the complex (1) showed no growth-inducing effect, highlighting its potential as a cobalt-based anticancer agent.