Magnitude and consequences of OR ligand σ-donation on alkene metathesis activity in d0 silica supported (SiO)W(NAr)(CHtBu)(OR) catalysts†
Well-defined silica supported W catalysts with the general formula [(SiO)W(NAr)(CHtBu)(OR)] (OR = OtBuF9, OtBuF6, OtBuF3, 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 < OtBuF3 < OSi(OtBu)3 < OtBuF6 < OtBuF9. 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 < OtBuF3 < OSi(OtBu)3 < OtBuF6 < OtBuF9. 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 OtBuF3 and OtBuF6. 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.