Formation, Reactivity, and Catalytic Behavior of a Keggin Polyoxometalate/Bipyridine Hybrid in the Epoxidation of Cyclooctene with H2O2

When the (2,2 '-Hbpy)(3)[PW12O40] (a precatalyst) reacts withH(2)O(2) undercatalytic conditions, it forms two oxidized species, (2,2 '-bpyHO)(3)[PW12O40] (an inactive byproduct) and[WO-(O-2)(2)(2,2 '-bpy)] (an activator). Thelatter then activates H2O2 to form the activespecies that catalyzes the epoxidation reaction. H2O2 also operates as a "conservative agent" whosepresence in the catalytic system is essential to keeping the catalystfrom deactivating irreversibly. The present study further explores the behavior of polyoxometalate-basedhybrid compounds as catalysts for liquid-phase cyclooctene epoxidationwith H2O2. Precisely, it unveils the natureof the relevant active species derived from the hybrid based on Kegginpolyoxometalate (POM) and bipyridines (bpy) of formula (2,2 '-Hbpy)(3)[PW12O40] (1). Whereas(i) it is generally accepted that the catalytic oxidation of organicsubstrates by H2O2 involving Keggin HPAs proceedsvia an oxygen transfer route from a peroxo intermediate and (ii) thecatalytically active peroxo species is commonly postulated to be thepolyperoxotungstate {PO4[W-(O)-(O-2)(2)](4)}(3-) complex (PW4), we showthat the studied epoxidation reaction seems to be more sophisticatedthan commonly reported. During the catalytic epoxidation, 1 underwent a partial transformation into two oxidized species, 2 and 3. Compound 3 correspondingto 2,2 '-bipyridinium oxodiperoxotungstate of formula [WO-(O-2)(2)(2,2 '-bpy)] was shown to be the main speciesresponsible for the selective epoxidation of cyclooctene since 2 (in which the POM is associated with a protonated mono-N-oxidederivative of 2,2 '-bpy of formula (2,2 '-HbpyO)(3)[PW12O40]) exhibited no activity. The structuresof 1, 2, and 3 were solvedby single-crystal X-ray diffraction and were independently synthesized.The speciation of 1 was monitored under catalytic conditionsby H-1 and H-1 DOSY NMR spectroscopies, wherethe formation in situ of 2 and 3 was revealed. A reaction mechanism is proposed that highlightsthe pivotal, yet often underestimated, role of H2O2 in the reached catalytic performances. The active speciesresponsible for the oxygen transfer to cyclooctene is a hydroperoxideintermediate species that is formed by the interaction between theanionic structure of the catalyst and H2O2.The latter operates as a "conservative agent" whosepresence in the catalytic system is required to prevent the catalystsfrom deactivating irreversibly.