Reactivity of mu-peroxo-bridged dimeric vanadate in bromoperoxidation.
Diglycyl triperoxodivanadate [V2O2(O2)3(Gly H)2(H2O)2], a synthetic compound with mu-peroxo-bridge derived from H2O2 and vanadate, oxidized bromide to a bromination-competent intermediate in phosphate buffer and physiological pH. This is in contrast to the requirement of acid medium with H2O2 as the oxidant. Addition of its solid to bromide solution instantly produced a 262-nm-absorbing compound that converted phenol red (a trap) to its 592-nm-absorbing bromo-derivative. The high bromination activity was lost on dissolving this compound in water and the solution showed the presence of peroxovanadates (mono and di) and vanadates (V1 and oligomeric V10) in 51V-NMR spectrum. Of these, diperoxovanadate and vanadate together supported slow bromination activity by a second set of reactions including bromide-assisted reductive formation of vanadyl. Bromination activity dependent on vanadyl was sensitive to oxidation by excess H2O2 and to complexation by EDTA, whereas that of triperoxodivanadate was relatively insensitive. Vanadyl and diperoxovanadate are capable of forming a mu-peroxo-bridged complex that is essentially similar to the synthetic vanadate dimer used in the present experiments. It appears that a mu-peroxo-intermediate is the proximal oxidant of bromide in vanadium-catalyzed bromoperoxidation.