51V NMR and EPR study of the mechanistic details of oxidation with VO(O2) (Pic) (H2O)2

Abstract

Using ⁵¹V NMR, it was shown that peroxo complex VO(O₂) (Pic) (H₂O)₂ (Pic = picolinic acid) exists in CH₃CN, MeOH and H₂O in the forms of VO(O₂) (Pic) H₂O·CH₃CN, VO(O₂) (Pic) H₂O·MeOH and VO(O₂) (Pic) (H₂O)₂, respectively. To elucidate the nature of reactivity of vanadium peroxo complexes Of VO(O₂) (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(O₂) (Pic) H₂O·CH₃CN was reactive towards the hydrocarbons and the spin trap. This complex can abstract the β-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•(RO₂•) and superoxo complex Pd₃(OAc)₅O₂•). 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(O₂) (Pic) H₂O·CH₃CN.

Deactivation of VO(O₂) (Pic) H₂O·CH₃CN upon substitution of CH₃CN by stronger σ-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(O₂^.−) detected by EPR in a solution of VO(O₂) (Pic) (H₂O)₂ in 30% H₂O₂was inert towards alkenes and spin traps.