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)2WOR2] derivatives prone to carbene formation. Thus,
[WO­(CH2SiMe3)3Cl] (1) was grafted onto silica dehydroxylated at 700 °C (SiO2–700), proceeding via W–Cl cleavage to yield
well-defined monopodal species [(SiO)­WO­(CH2SiMe3)3] (2a) along with HCl release. On
the other hand, the corresponding bipodal species [(SiO)2WO­(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 1H, 13C, 29Si, and 17O), 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.