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

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)WOR3] and bis alkyl oxo [(= SiO)(2)WOR2] derivatives prone to carbene formation. Thus, [WO(CH2SiMe3)(3)Cl] (1) was grafted onto silica dehydroxylated at 700 degrees C (SiO2-700), proceeding via W-Cl cleavage to yield well-defined monopodal species [(= SiO)(2)WO(CH2SiMe3)(3)] (2a) along with HCl release. On the other hand, the corresponding bipodal species [(= SiO)(2)WO(CH2SiMe3)(2)] (2b) was obtained on SiO2-200 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 H-1, C-13, Si-29, and O-17), 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 degrees 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 not equal Y) counterparts.