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 [OW-(CH 2 tBu 3 ) 4 ] or [OWCl(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...