51V-NMR investigation on the formation of peroxo vanadium complexes in aqueous solution: Some novel observations

Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H₂O₂ to NH₄VO₃. The ⁵¹V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range 0–4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by ⁵¹V-NMR spectroscopy. Thus, different species, e.g., VO(O₂)L, [VO(O₂)L₂]⁻, [VO(O₂)₂L]²⁻ as a function of the pH has been detected and the corresponding association constants have been measured.