Synthesis and characterisation of novel oxovanadium(IV) Schiff base complexes: A study of their electronic spectral properties, peroxide binding affinities, DFT-calculated geometries and spectra, and cytotoxicity towards human carcinoma cells.

Abstract

A series of five, tetradentate Schiff-base ligands were synthesised and chelated to vanadyl to form oxovanadium(IV) complexes. The ligands, 4,4’-{benzene-1,2-diylbis[nitrilo(1E)phen-1-yl-1ylidene]}- dibenzene-1,3-diol (H2L1), 4,4’-{ethane-1,2-diylbis[nitrilo(1E)phenyl-1-yl-1-ylidene]}dibenzene-1,3-diol (H2L2), 4,4’-{propane-1,2-diylbis[nitrilo(1E)phen-1-yl-1-ylidene]}dibenzene-1,3-diol (H2L3), 4,4’-{(2- hydroxypropane-1,3-diyl)bis[nitrilo(1E)phen-1-yl-1-ylidene]}dibenzene-1,3-diol (H2L4) and 4,4’-{2,2- dimethylpropane-1,3-diyl)bis-[nitrilo(1E)phen-1-yl-1-ylidene]}-dibenzene-1,3-diol (H2L5), characterised by TOF-MS, IR, electronic absorption, 1H and 13C NMR spectroscopy. The ligand H2L5 was also characterised by XRD. The ligands were shown to have a bis-zwitterionic structure in the solid state, and possibly also in solution. Complexes were characterised by Elemental Analysis, TOF-MS, IR, electronic absorption spectra, EPR and 51V NMR spectroscopy. They form mononuclear complexes, with one ligand binding a single vanadyl ion. EPR spectroscopy was performed on both the powdered form and solutions of the complexes. All the complexes displayed axial symmetry, with increasing distortion from an ideal square pyramidal geometry as the size and bulk of the central chelate ring was increased. Isotropic g0 values suggest solvent interaction with the vanadium ion for the coordinating solvent DMSO. Additional distortion on the coordination geometry, presumably from the benzyl groups of the compounds, causes the isotropic hyperfine coupling constants to be greater than expected. Furthermore, the ability of the complexes to bind peroxide species was investigated by following the addition of H2O2 to the complexes using 51V NMR spectroscopy to observe shielding changes at the vanadium nucleus, and 1H NMR spectroscopy to monitor the bulk magnetic susceptibility, via a modified Evan’s NMR method. Similar experiments were done with sodium hydroxide for comparison. As expected, the oxoperoxovanadium(V) complexes were more stable than their progenitor oxovanadium(IV) complexes. Additionally, increasing the distortion from the ideal pseudo square-pyramidal coordination geometry for the vanadyl ion resulted in a greater increase in the apparent stability of the peroxocomplexes. This latter effect is further enhanced by the addition of a hydrogen-bonding group in close proximity to the vanadium nucleus. DFT calculations of the optimized geometries, natural bond orbitals, electronic absorption and infra-red frequencies were performed for both the ligands and the complexes; nuclear magnetic resonance calculations were performed for the ligands as well. The B3LYP/6-311G (d,p) and B3LYP/LANL2DZ level of theories were used for the ligands and complexes respectively, except for electronic transitions, which were calculated using TD-SCF methods for both ligands and complexes. Calculated and experimental results were compared where possible, and showed reasonable agreement for all calculations performed. The exception to this was for the NMR calculations for the ligands, which were poorly simulated. Finally, the in vitro biological activity of the complexes was evaluated for cytotoxicity against the human tumour cell lines: A549, U251, TK-10 and HT29, via an MTT assay. All complexes showed promising anticancer activity, as evidenced by their low IC50 values for the cell lines A549, U251 and TK-10, which are in general, lower than that observed for cisplatin. They did, however, express negligible activity against the HT29 colon adenocarcinoma cell line; showing an apparent selectivity for certain cell lines. These oxovanadium(IV) complexes, thus warrant further evaluation as chemotherapeutic agents.