Detailed structural investigation of the grafting of [Ta(=CHtBu)(CH ₂tBu)₃] and [Cp*TaMe₄] on silica partially dehydroxylated at 700 °C and the activity of the grafted complexes toward alkane metathesis

The reaction of [Ta(=CHtBu)(CH₂tBu)₃] or [Cp*Ta(CH₃)₄] with a silica partially dehydroxylated at 700 °C gives the corresponding monosiloxy surface complexes [(≡SiO)Ta(=CHtBu)(CH₂tBu)₂] and [(≡SiO) Ta(CH₃)₃Cp*] by eliminating a σ-bonded ligand as the corresponding alkane (H-CH₂tBu or H-CH₃). EXAFS data show that an adjacent siloxane bridge of the surface plays the role of an extra surface ligand, which most likely stabilizes these complexes as in [(≡SiO)Ta(=CHtBu)(CH₂tBu)₂(≡SiOSi≡)] (1a′) and [(≡SiO)Ta(CH₃)₃ Cp*(≡SiOSi≡)] (2a′). In the case of [(≡SiO)Ta(=CHtBu) (CH₂tBu)₂(≡SiOSi≡)], the structure is further stabilized by an additional interaction: a C-H agostic bond as evidenced by the small J coupling constant for the carbenic C-H (J_C-H = 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [(≡SiO)Ta(=CHtBu)- (CH₂tBU)₂(≡SiOSi≡)] (1a′) as a catalyst precursor and the inactivity of the surface complex [(≡SiO)Ta-(CH ₃)₃CP*(≡SiOSi≡)] (2a′) show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.