Silica-Supported Tungsten Neosilyl Oxo Precatalysts: Impact of the Podality on Activity and Stability in Olefin Metathesis

Abstract

In order to establish structure−reactivity relationships in propylene metathesis as a function of the podality of tungsten oxo species bearing neosilyl ligands, we targeted the parent tris alkyl [(SiO)WOR 3 ] and bis alkyl oxo [(SiO) 2 WOR 2 ] derivatives prone to carbene formation. Thus, [WO(CH 2 SiMe 3 ) 3 Cl] (1) was grafted onto silica dehydroxylated at 700°C (SiO 2−700 ), proceeding via W−Cl cleavage to yield well-defined monopodal species [(SiO)-WO(CH 2 SiMe 3 ) 3 ] (2a) along with HCl release. On the other hand, the corresponding bipodal species [(SiO) 2 WO(CH 2 SiMe 3 ) 2 ] (2b) was obtained on SiO 2−200 by release of both HCl and TMS. The formation of these species were demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1 H, 13 C, 29 Si, and 17 O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. Materials 2a and 2b proved to lead to stable and efficient supported tungsten oxo catalysts for propene metathesis under dynamic conditions at 80°C. The symmetric bipodal precatalyst (expressed as [W(E)(CHR)(X)(Y)] (X = Y, E = spectator ligand)) showed somewhat higher activity than the asymmetric (X ≠ Y) counterparts. ■ INTRODUCTIONTungsten oxide supported on silica is an efficient catalyst for olefin metathesis used in industrial processes since the 1960s on a large-scale in Phillips' neohexene process and ABB Lummus' olefins conversion technology (OCT). 1 This heterogeneous catalyst is classically prepared by incipient wetness impregnation of ammonium metatungstate followed by calcination. 2 Different surface species, difficult to distinguish, are inevitably generated upon the calcination step despite modification of the preparation procedure. 3−8 The nature of the active site still needs to be determined, but WO 3 crystallites can be excluded as an active species. 1,9 The catalytic activity is attributed to a bipodal isolated tungsten(VI) center bearing an oxo ligand and carbene fragment. 10,11 Such a surface species exists only in very low concentration in the commercialized catalyst. Other tungsten species are formed during catalyst preparation and pretreatment in the presence of water, which can enhance the mobility of WO 3 on the silica support and result in poor dispersion. In order to avoid water in the preparation steps, we have developed an anhydrous route for the preparation of supported tungsten oxo metathesis (pre)-catalysts by surface organometallic chemistry. The very first well-defined silica-supported tungsten oxo species active in olefin metathesis was prepared by grafting of [OW-(CH 2 tBu 3 ) 4 ] or [OWCl(CH 2 tBu 3 ) 3 ] complexes, both affording tungsten oxo tris-neopenyl surface species grafted on silica through a single W−O−Si bond. 12,13 As expected, this material catalyzes propylene self-metathesis after in situ generation of the tungsten carbene moiety at 80°C. The striking discovery was that this catalyst showed an enhanced stability with tim...

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[514] Anumber of mechanisms for the formation of alkylidenes from surface oxo species have been proposed. [519][520][521][522][523][524][525][526] Supporteda lkylidynes were also shownt o be active metathesis catalystp recursors. Thef irst attempts to applyo rganometallicp recursorsi nt he preparationo fs upported metathesis catalysts date back to the1 970s with theu se of WMe 6 [517] ando ther perhydrocarbyl complexes [518] that likely generate surface alkylidenesi nsitu.…”

“…[521,522] Thes ame monopodal surface species is formed when W(=O)-(CH 2 tBu) 3 Cl is grafted onto SiO . [524,525] Thep erformance of both neosilyl systems ( SiO) x W( = O)(CH 2 SiMe 3 ) 4Àx in the metathesis of propene under flow conditions at 80 8 8C( TON 18 000 and 24 000 after 25 hf or x = 1a nd 2, respectively) is significantly better than that of (SiO)W(=O)(CH 2 CMe 3 ) 3 (TON 6000 after 25 h). [524,525] Thep erformance of both neosilyl systems ( SiO) x W( = O)(CH 2 SiMe 3 ) 4Àx in the metathesis of propene under flow conditions at 80 8 8C( TON 18 000 and 24 000 after 25 hf or x = 1a nd 2, respectively) is significantly better than that of (SiO)W(=O)(CH 2 CMe 3 ) 3 (TON 6000 after 25 h).…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…Die ersten Versuche zur Anwendung metallorganischer Vorstufen bei der Herstellung von Trägerkatalysatoren für Metathesen gehen auf die 1970er Jahre zurück, wobei WMe 6 und andere Perhydrocarbylkomplexe verwendet wurden, die wahrscheinlich Oberflächenalkylidene in situ erzeugen. Dieser Ansatz wurde kürzlich mit fortgeschrittenen Charakterisierungsverfahren neu untersucht (Abschnitt 7.3.2.2) . Ferner wurde gezeigt, dass auch trägerfixierte Alkylidine Vorstufen von aktiven Metathesekatalysatoren sind .…”

“…Dagegen liefert Verankern des isoelektronischen W(=O)(CH 2 SiMe 3 ) 3 Cl auf SiO 2‐700 und SiO 2‐200 einfach bzw. doppelt verankerte Oberflächenkomplexe (Schema ) . Beide Neosilylsysteme (≡SiO) x W(=O)(CH 2 SiMe 3 ) 4− x sind in der Propenmetathese unter Strömungsbedingungen bei 80 °C (18 000 und 24 000 TON nach 25 h für x= 1 bzw.…”

Well-defined silica supported bipodal molybdenum oxo alkyl complexes: a model of the active sites of industrial olefin metathesis catalysts

“…27 Bearing in mind that the ancillary oxo ligand has a striking effect in the stability compared to the imido counterpart, [28][29][30] we have studied olefin metathesis catalysts based on supported group VI neutral oxo species for several years. [29][30][31][32][33][34][35][36][37] Previous investigations on the silica-supported W-oxo system have evidenced that the bipodal [(SiO)2WO(CH2SiMe3)2] 33 pre-catalyst can exhibit superior activity in propylene self-metathesis than the monopodal [(SiO)W(=O)(CH2SiMe3)3] counterpart, 34 as well as the isoelectronic monopodal [(SiO)W(=O)(CH2SiMe3)2OAr] species. 31 Herein, we present the preparation and characterisation of silica-supported bipodal Mo-oxo alkyl species that can be in-situ transformed into highly active olefin metathesis catalyst after contacting with olefinic substrates.…”