V-51 NMR AND EPR STUDY OF THE MECHANISTIC DETAILS OF OXIDATION WITH VO(O-2)(PIC)(H2O)(2)

Using V-51 NMR, it was shown that peroxo complex VO(O-2) (Pic) (H2O)(2) (Pic = picolinic acid) exists in CH3CN, MeOH and H2O in the forms of VO(O-2) (Pic)H2O.CH3CN, VO(O-2) (Pie)H2O.MeOH and VO(O-2) (Pic) (H2O)(2), respectively. To elucidate the nature of reactivity of vanadium peroxo complexes of VO(O-2) (Pic)H2O.S family towards hydrocarbons their interactions with spin trap 3,3,5,5-tetramethyl-pyrroline-N-oxide (TMPO) were investigated by EPR. Only the peroxo complex VO(O-2) (Pic) H2O CH3CN was reactive towards the hydrocarbons and the spin trap. This complex can abstract the beta-hydrogen atom from TMPO (V-V reduces to V-IV by this reaction). Such hydrogen atom abstraction was not observed for the other radical species studied (free radicals OH.(HO2.), RO(.)(RO(2)(.)) and superoxo complex Pd-3(OAc)(5)O-2(.)). The latter species form with TMPO a spin adduct by addition to the double bond. The data provide evidence, that the diradical species V-IV-O-O-. are active particles of oxidation by VO(O-2) (Pic)H2O.CH3CN. Deactivation of VO(O-2) (Pic)H2O.CH3CN upon substitution of CH3CN by stronger sigma-donor ligands (e.g., H2O, MeOH, DMF) may be caused by prevention of electron transfer from the peroxo group to the metal to give V-IV-O-O-.. Vanadium(V) superoxo complex V-V(O-2(.-)) detected by EPR in a solution of VO(O-2) (Pic) (H2O)(2) in 30% H2O2 was inert towards alkenes and spin traps.