Isostructural Molecular and Surface Mimics of the Active Sites of the Industrial WO₃/SiO₂ Metathesis Catalysts

Abstract

In classical, ill-defined WO 3 /SiO 2 alkene metathesis catalysts, isolated, silica-supported tungsten oxo alkylidene species are suspected to represent the catalytically active species. Here, we report the synthesis and the characterization of molecular and supported isostructural mimics of these active sites. The good activities at room temperature of these well-defined oxo alkylidene complexes indicate that the lower activity of WO 3 /SiO 2 , even after preactivation, is likely due to the difficulty to generate active alkylidene sites from W oxo species in the classical system.

Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

“…While o ‐alkoxybenzylidene initiating ligands have been widely used for Ru‐based catalysts (so‐called Hoveyda−Grubbs catalysts, Figure ), they are much less employed in Schrock ‐type systems. The first examples of 5‐coordinated Mo and W complexes bearing the pendant alkoxy group attached to the alkylidene ligand were synthesized in the 1990s, with only few additional reports being published since then, mostly focusing on structure rather than catalytic performance …”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[519][520][521][522][523][524][525][526] Supporteda lkylidynes were also shownt o be active metathesis catalystp recursors. Preliminaryr eports appeared in early2 000s, [541,542] andt he applicationo fm odern characterization techniques eventually allowedf or precise controlofthe structures of thesesurface speciesand gave rise to theg enerationo fr emarkablya ctive well-defined silica-supportedMo- [543][544][545][546][547][548][549] andW-imido-alkylidenes [550][551][552][553][554][555] andmore recently W-oxo-alkylidenes [556][557][558][559][560] (Section 7.3.2.4).I nt he 1970sa nd 1980st he groups of Yermakov [561][562][563][564] andI wasawa [565,566] were thef irst to useo rganometallicp recursors (Mo(C 3 H 5 ) 4 and W(C 4 H 7 ) 4 )t op repare well-defined supported Mo and Woxides relevant to industrial systems and discriminate the activities of different oxidation states.T hese studies suggested that neither fully oxidized Mo VI /W VI nor highly reduced Mo II /W II species were responsible for the metathesis activity and that active sites likely arose from the oxidation state + IV.Recent advances in describing the redox chemistry of well-defined supported W-oxo sites in relation to alkene metathesis are discussed in Section 7.3.2.5. [441] Supportedt ungstenh ydridesr epresent as pecial classo fi nsitug enerated olefin m...…”

“…Thec orresponding well-defined oxo-alkylidene surface complex, (SiO)W(=O)(=CHtBu)-(OHMT) (Scheme 43 a), [556] is significantly more active and stable (TOF 3min 280 min À1 at 0.02 mol %; cis-4-nonene; batch). [558] DFT calculations suggest that oxo-alkylidenes are better catalysts than imido-alkylidenes because deactivation by b-H transfer from the SP metallacycle (Scheme 35) is energetically disfavored by the oxo ligand. [557] ( SiO)W( = O)( = CHtBu)(dAdPO) is more active than all the previously reported silica-supported imidoalkylidene catalysts (TOF 3min 325 min À1 ;0 .02 mol %; cis-4nonene;batch) and allows acatalyst loading as low as 50 ppm.…”

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

“…However, it displays a low initial activity (Turnover frequency (TOF) = 12 min −1 ) in metathesis of cis oct-4-ene at 80°C. This low activity has been explained by the low thermal stability of this complex in absence of phosphine ligands [34].…”

“…Indeed, the coordination environment of 22 More recently, Schrock et al synthesized new oxo tungsten complexes bearing various ligands and studied their grafting on silica dehydroxylated at 700°C. Ligands such as 2,6-mesitylphenoxy, 2,6-diadamantyl-methylphenoxy, thio-2,6-masitylphenoxy, or tris(tert-butoxy)siloxy were used and the corresponding carbenes were synthesized (Figure 25) [34,[89][90][91]. 2 ], is formed by protonation of the carbene moiety by the silanol group [90].…”