Synthesis, Characterization, and Biological Activities of Novel Vanadium(IV) and Cobalt(II) Complexes

Herein, we report novel Co(II) and V(IV) complexes synthesized from an (E)-2-(((2-((2-hydroxyethyl)amino)quinolin-3-yl)methylene)amino)ethan-1-ol ligand (L), cobalt(II) chloride hexahydrate, and vanadyl(IV) sulfate in methanolic solutions. The ligand and the complexes were characterized by ¹H NMR spectroscopy,¹³C NMR spectroscopy, UV-visible spectroscopy, fluorescence spectroscopy, FT-IR spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), mass spectroscopy (MS), thermal analysis, and molar conductance. The FT-IR spectral data showed that the ligand adopted a tridentate fashion when binding with the metal ions via the nitrogen atoms of the imine (C=N) and amine (N-H), and the oxygen atom of the hydroxyl group (O-H). The PXRD and SEM results indicated that the complexes are amorphous in nature. The density functional theory (DFT) calculated absorption and IR spectra agree very well with the corresponding experimental results. The antibacterial activities of the free ligand and its complexes were evaluated using a paper disk diffusion method. The complexes have better percent activitiy index than the free ligand. The cobalt complex exhibited a more recognizable antibacterial activity than the vanadium complex, specifically against Pseudomonas aeruginosa with a mean inhibition zone of 18.62 ± 0.19 mm, when compared with the positive control, ciprofloxacin, with a mean inhibition zone of 22.98 ± 0.08 mm at the same concentration. Furthermore, the antioxidant activities of the free ligand and its metal complexes were also determined in vitro using 2,2-diphenyl-1-picrylhydrazyl. The ligand exhibited less in vitro antioxidant activity than its transition metal complexes, in which the cobalt complex has a better antioxidant activity with half-inhibitory concentrations (IC₅₀ of 16.01 μg/mL) than the ligand and the vanadium complex. Quantum molecular descriptors from the DFT calculations further support the experimental results. Molecular docking analysis also shed more light on the biological activities of the novel cobalt and vanadium complexes.