Elucidating the Protonation Site of Vanadium Peroxide Complexes and the Implications for Biomimetic Catalysis

Coordination complexes of vanadium(5+) played a key role in understanding the structure and mechanism of vanadium-dependent haloperoxidases, particularly the effects of protonation on peroxide coordination to dioxovanadium(5+) species, and in the activation of the peroxo−oxovanadium(5+) complex for substrate oxidation. There has been no spectroscopic evidence that could test the presence of a hydroxo intermediate in a catalytically active oxovanadium(5+) complex. Herein we report the use of the pre-edge transition in X-ray absorption spectroscopy as a spectroscopic signature for VO bonding. Displacement of oxo donors with hydrogen peroxide or chloride donors dramatically decreases the pre-edge intensity, confirming that the source of the intense pre-edge feature is closely related to the π-bonding associated with the VO. Protonation of a catalytically active tripodal amine oxovanadium(5+) complex has no affect on the pre-edge intensity and, therefore, rules out the possibility of a hydroxo intermediate in the catalytic cycle.