Synthesis and Reactivity of Oxo-Peroxo-Vanadium(V) Bipyridine Compounds

The vanadium(IV) compound [VIVO(OH)(tBu2bpy)2]BF4 (VIVO(OH)) (tBu2bpy = 4,4′-di-tert-butylbipyridine) is slowly oxidized by O2 in ethereal solvents to give the oxo-peroxo compound [VVO(O2)(tBu2bpy)2]BF4 (VVO(O2)) in excellent yield. This and related compounds were fully characterized by NMR, IR, and optical spectroscopies; mass spectrometry; elemental analyses; and an X-ray crystal structure of the 4,4′-dimethylbipyridine analog, [VVO(O2)(Me2bpy)2]BF4. Monitoring the reaction of VIVO(OH) with O2 in THF/acetonitrile mixtures by 1H NMR and optical spectroscopies surprisingly shows that the initial product is the cis-dioxo compound [VV(O)2(tBu2bpy)2]BF4 (VVO2), which then converts to VVO(O2). Reaction of VIVO(OH) with 18O2 gives ca. 60% triply 18O labeled VVO(O2). The mechanism of formation of VVO(O2) is complex and may occur via initial reduction of O2 at vanadium(IV) to give a superoxo-vanadium(V) intermediate, autoxidation of the THF solvent, or both. That VVO2 is generated first appears to be due to the ability of VIVO(OH) to act as a hydrogen atom donor. For instance, VIVO(OH) reacts with VVO(O2) to give VVO2. VVO(O2) is also slowly reduced to VIVO(OH) by the organic hydrogen atom donors hydroquinone and TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol) as well as by triphenylphosphine. Notably, the peroxo complex VVO(O2) is much less reactive with these substrates than the analogous dioxo compound VVO2.