Mechanistic Insights into Alkene Epoxidation with H2O2 by Ti- and other TM-Containing Polyoxometalates: Role of the Metal Nature and Coordination Environment

摘要

The oxidation of alkenes by H2O2 catalyzed by Ti(IV)-containing polyoxometalates (POMs) asnmodels of Ti single-site catalysts has been investigated at DFT level and has been compared with othernearly transition-metal-substituted polyoxometalates. We have studied in detail the reaction mechanism ofnthe C2H4 epoxidation with H2O2 mediated by two different POMs, the Ti-monosubstituted Keggin-type POMn[PTi(OH)W11O39]4- and the Ti-disubstituted sandwich-type POM [Ti2(OH)2As2W19O67(H2O)]8-. These speciesnexhibit well-defined 6- and 5-coordinated titanium environments. For both species, the reaction proceedsnthrough a two-step mechanism: (i) the Ti-OH groups activate H2O2 with a moderate energy barrier yieldingneither Ti-hydroperoxo (TiIV-OOH) or Ti-peroxo (TiIV-OO) intermediate, and (ii) the less stable but more reactivenTi-hydroperoxo species transfers oxygen to alkene to form the epoxide, this latter step being the ratedeterminingnstep. The higher activity of the sandwich anion was attributed to the absence of dimer formation,nand its higher selectivity to the larger energy cost of homolytic O-O bond breaking in the hydroperoxonintermediate. We also propose several requisites to improve the efficiency of Ti-containing catalysts, includingnflexible and 5-fold (or lower) coordinated Ti environments, as well as reagent-accessible Ti sites. Calculationsnon other TM-containing Keggin-type POMs [PTM(OH)W11O39]4- (TM ) Zr(IV), V(V), Nb(V), Mo(VI), W(VI),nand Re(VII)) showed that when we move from the left to the right in the periodic table the formation of thenepoxide via peroxo intermediate becomes competitive because of the higher mixing between the orbitalsnof the TM and the O-O unit.