Search

Zr-monosubstituted Lindqvist- type polyoxometalates (Zr-POMs), (Bu4N)(2)[W5O18Zr(H2O)(3)] (1) and (Bu4N)(6)[{W5O18Zr(mu-OH)}(2)] (2), have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 and 2 are hydrolytically stable and catalyze the epoxidation of C.C bonds in unfunctionalized alkenes and alpha,beta-unsaturated ketones, as well as sulfoxidation of thioethers. Monomer 1 is more active than dimer 2. Acid additives greatly accelerate the oxygenation reactions and increase oxidant utilization efficiency up to >99%. Product distributions are indicative of a heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon the interaction of Zr-POM and H2O2. The interaction of 1 and 2 with H2O2 and the resulting peroxo derivatives have been investigated by UV-vis, FTIR, Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICP-atomic emission spectroscopy techniques. The interaction between an O-17-enriched dimer, (Bu4N)(6)[{W5O18Zr(mu-OCH3)}2] (2'), and H2O2 was also analyzed by O-17 NMR spectroscopy. Combining these experimental studies with DFT calculations suggested the existence of dimeric peroxo species [(mu-eta(2):eta(2)-O-2){ZrW5O18}(2)](6-) as well as monomeric Zr-hydroperoxo [W5O18Zr(eta(2)-OOH)](3-) and Zr-peroxo [HW5O18Zr(eta(2)-O-2)](3-) species. Reactivity studies revealed that the dimeric peroxo is inert toward alkenes but is able to transfer oxygen atoms to thioethers, while the monomeric peroxo intermediate is capable of epoxidizing C.C bonds. DFT analysis of the reaction mechanism identifies the monomeric Zr-hydroperoxo intermediate as the real epoxidizing species and the corresponding alpha-oxygen transfer to the substrate as the rate-determining step. The