Magnitude and consequences of OR ligand σ-donation on alkene metathesis activity in d0 silica supported (SiO)W(NAr)(CHtBu)(OR) catalysts

Abstract

Well-defined silica supported W catalysts with the general formula [(SiO)W(NAr)(CHtBu)(OR)] (OR = OtBu_F9, OtBu_F6, OtBu_F3, OtBu and OSi(OtBu)₃), prepared by grafting bis-alkoxide complexes [W(NAr)(CHtBu)(OR)₂] on silica dehydroxylated at 700 °C (SiO₂₍₇₀₀₎), display unexpectedly high activity in comparison to their Mo homologues. In this series, the activity of the self-metathesis of cis-4-nonene increases as a function of the OR ligand as follows: OtBu < OtBu_F3 < OSi(OtBu)₃ < OtBu_F6 < OtBu_F9. In addition, the ratio of the two parent metallacyclobutane intermediates, trigonal bipyramidal (TBP)/square pyramidal (SP), which were formed by the metathesis of ethylene and observed by solid-state NMR, follows the same order: OtBu < OtBu_F3 < OSi(OtBu)₃ < OtBu_F6 < OtBu_F9. This provides clear evidence of the decreasing σ-donating ability of the OR ligand with an increasing number of fluorine atoms and the positioning of a siloxy ligand in between OtBu_F3 and OtBu_F6. This study provides the first detailed structure–activity relationship analysis of a series of well-defined heterogeneous catalysts, showing that weaker σ-donor OR ligands lead to higher activity, and that the surface siloxy ligand is a rather small and weak σ-donor ligand overall, thus providing highly active yet stable catalysts.