Oxovanadium(IV) complexes based on S-alkyl-thiosemicarbazidato ligands. Synthesis, characterization, electrochemical, and antioxidant studies

Reaction of VOSO4 with 2-hydroxy-napthaldeyde-S-R-thiosemicarbazones (R: methyl, ethyl, propyl or allyl) and salicyl aldehyde yielded five-coordinate oxovanadium(IV) complexes having a N-1,N-4-diarylidene-S-R-thiosemicarbazidato structures. The compounds were characterized by elemental analysis, magnetic measurements, electronic, infrared, H-1-NMR, and electron paramagnetic resonance (EPR) spectra. The X-band EPR signals were recorded from powder forms and also in solution. All the complexes have a single asymmetric line shape and theoretical fit studies prove the presence of axial symmetry around the paramagnetic vanadium ions. A computer simulation of the EPR spectrum of each complex was carried out to derive the related EPR parameters. Cyclic voltammograms of the complexes exhibited two metal-based reversible redox peaks around 500 and -800 mV corresponding to one electron oxidation/reduction of (VO)-O-IV/(VO)-O-V and (VO)-O-IV/(VO)-O-III, respectively. The reductive response in the 50-350 mV region was assigned to ligand reduction. Antioxidant activities of the compounds were determined with CUPric Reducing Antioxidant Capacity, 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid, and 1,1-diphenyl-2-picrylhydrazyl assays. The test results indicated that the antioxidant capacity of the compounds increases with the carbon number of saturated hydrocarbon chain on sulfur atom.