Mechanistic Insights into Alkene Epoxidation with H₂O₂by Ti- and other TM-Containing Polyoxometalates: Role of the Metal Nature and Coordination Environment

Abstract

The oxidation of alkenes by H(2)O(2) catalyzed by Ti(IV)-containing polyoxometalates (POMs) as models of Ti single-site catalysts has been investigated at DFT level and has been compared with other early transition-metal-substituted polyoxometalates. We have studied in detail the reaction mechanism of the C(2)H(4) epoxidation with H(2)O(2) mediated by two different POMs, the Ti-monosubstituted Keggin-type POM [PTi(OH)W(11)O(39)](4-) and the Ti-disubstituted sandwich-type POM [Ti(2)(OH)(2)As(2)W(19)O(67)(H(2)O)](8-). These species exhibit well-defined 6- and 5-coordinated titanium environments. For both species, the reaction proceeds through a two-step mechanism: (i) the Ti-OH groups activate H(2)O(2) with a moderate energy barrier yielding either Ti-hydroperoxo (Ti(IV)-OOH) or Ti-peroxo (Ti(IV)-OO) intermediate, and (ii) the less stable but more reactive Ti-hydroperoxo species transfers oxygen to alkene to form the epoxide, this latter step being the rate-determining step. The higher activity of the sandwich anion was attributed to the absence of dimer formation, and its higher selectivity to the larger energy cost of homolytic O-O bond breaking in the hydroperoxo intermediate. We also propose several requisites to improve the efficiency of Ti-containing catalysts, including flexible and 5-fold (or lower) coordinated Ti environments, as well as reagent-accessible Ti sites. Calculations on other TM-containing Keggin-type POMs [PTM(OH)W(11)O(39)](4-) (TM = Zr(IV), V(V), Nb(V), Mo(VI), W(VI), and Re(VII)) showed that when we move from the left to the right in the periodic table the formation of the epoxide via peroxo intermediate becomes competitive because of the higher mixing between the orbitals of the TM and the O-O unit.

Strengths, Weaknesses, Opportunities and Threats: Computational Studies of Mn- and Fe-Catalyzed Epoxidations

“…In the case of vanadium, this was first systematically studied by Vandichel et al [104], whereas the case of titanium was tackled earlier by Sever and Root [111]. Some theoretical studies on titanium-catalyzed epoxidations also focused on this metal's ability to efficiently catalyze the epoxidation of olefins when immobilized into a heterogeneous matrix, such as silicates [112] or polyoxometalates [113,114]. These theoretical studies follow the same trend observed in the case of manganese-and iron-catalyzed epoxidations, with a predominance of DFT, with B3LYP as the preferred density function.…”

Epoxidation of Alkenes with H2O2 Catalyzed by Dititanium‐Containing 19‐Tungstodiarsenate(III): Experimental and Theoretical Studies

“…In a previous contribution, [11] we have theoretically analyzed in detail alkene epoxidation with H 2 O 2 catalyzed by nonprotonated model anion 1 by means of DFT methods. The results agree well with the proposed two-step mechanism, in which the Ti centres are responsible for the catalytic activity.…”

“…It is possible to understand this small discrepancy with regards to the geometry of the transition state for ethene insertion. [11] The ethene substrate attacks the hydroperoxo moiety in a spiro orientation (i.e., perpendicular to the TiOO plane). Thus, the cis-stilbene can accommodate the two phenyl substituents far away from the POM structure, thereby minimizing the steric repulsion.…”

“…POMs are useful molecular models for studying mechanisms of H 2 O 2 -based liquid-phase selective oxidations. [6] Kholdeeva et al have demonstrated that the catalytic performance of tetra-n-butyl ammonium (TBA) salt of Timonosubstituted Keggin-type heteropolytungstate, (TBA) 4 -[PTi(OH)W 11 O 39 ] (Ti-POM), in the oxidation of alkenes, thioethers and alkylphenols with H 2 O 2 is very similar to that of hydrophilic mesoporous titanium-silicate catalysts. [6,7] It has been established that hexacoordinate isolated Ti centres catalyze H 2 O 2 -based oxidations predominantly through homolytic mechanisms but are not capable of a heterolytic oxygen atom transfer.…”

Highly selective detection of cellular guanine and xanthine by polyoxometalate modified 3D graphene foam

“…Thus, suitable nanomaterial loaded onto 3DGF may lead to new electrochemical biosensors due to synergistic effect of anchored functional nanomaterial and highly conductive 3DGF matrix. In the last decades, polyoxometalates (POMs) have shown diverse structures and applications in many fields such as electronics, catalysis, and materials design [41][42][43][44][45][46][47][48][49][50][51][52], and much attention has been paid to synthesize POM-based carbon-materials [53][54][55][56]. Especially, the cyclic voltammetry (CV) or amperometry studies demonstrated that POMs ameliorated electrochemical sensors have fast response and good catalytic activity for the redox of hydrogen peroxide [57,58], dopamine [59,60], ascorbic acid [61][62][63], L-cysteine [64], and even glucose [65].…”