Studies directed toward the prediction of the oxidative reactivity of vanadium peroxo complexes in water, Correlations between the nature of the ligands and V-51-NMR chemical shifts

The V-51-NMR spectroscopic behavior in water of a series of vanadium peroxo complexes was investigated. Monoperoxo vanadium complexes containing bidentate ligands in which the two binding sites are either oxygen atoms (O boolean AND O ligands) or one oxygen and one nitrogen atom (N boolean AND O ligands) and diperoxo vanadium complexes containing variously substituted pyridines or anilines were examined. A direct dependence of the magnetic shielding of the metal, measured by its V-51-NMR chemical shift, on the electronic character of the ligand was observed in all cases. For diperoxo complexes Hammett-type correlations between the V-51-NMR chemical shifts and the a values of the substituents in the pyridine or aniline rings were established. For monoperoxo complexes, a Ramsey-type correlation between the chemical shifts and the energy of the ligand-to-metal charge transfer electronic transitions was found thus suggesting that the electron donating ability of the ligands and the energy gap HOMO-LUMO of the peroxo complexes are linearly correlated. All these observations indicate that peroxo vanadium complexes containing strongly electron donating ligands should be weak electrophilic oxidants and poor one-electron accepters. Preliminary data concerning the reactivity of a series of peroxo vanadium species in the simple self-decomposition reaction confirm this prediction.