Activation of H₂O₂ over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates

Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POMs), (Bu4N)2[W5O18Zr(H2O)3] (1) and (Bu4N)6[{W5O18Zr(μ-OH)}2] (2), have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 2 are hydrolytically stable catalyze epoxidation C═C bonds in unfunctionalized alkenes α,β-unsaturated ketones, well sulfoxidation thioethers. Monomer is more active than dimer 2. Acid additives greatly accelerate oxygenation reactions increase oxidant utilization efficiency up >99%. Product distributions indicative a heterolytic oxygen transfer that involves electrophilic oxidizing species formed upon interaction Zr-POM H2O2. The with H2O2 resulting peroxo derivatives investigated by UV–vis, FTIR, Raman spectroscopy, HR-ESI-MS, combined HPLC-ICP-atomic emission spectroscopy techniques. between an 17O-enriched dimer, (Bu4N)6[{W5O18Zr(μ-OCH3)}2] (2′), was also analyzed 17O NMR spectroscopy. Combining these experimental studies DFT calculations suggested existence dimeric [(μ-η2:η2-O2){ZrW5O18}2]6– monomeric Zr-hydroperoxo [W5O18Zr(η2-OOH)]3– Zr-peroxo [HW5O18Zr(η2-O2)]3– species. Reactivity revealed inert toward but able atoms thioethers, while intermediate capable epoxidizing bonds. analysis reaction identifies real corresponding α-oxygen substrate rate-determining step. showed protonation significantly reduces free-energy barrier key oxygen-transfer step because greater electrophilicity catalyst hampers approach alkene substrates due steric repulsions reducing its reactivity. improved performance relative Ti(IV) Nb(V) catalysts respectively flexible coordination environment low tendency form energy deep-well low-reactive intermediates.