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The reaction of [Ta(dCHtBu)(CH2tBu)3] or [Cp*Ta(CH3)4] with a silica partially dehydroxylated at 700 °C gives the corresponding monosiloxy surface complexes [(tSiO)Ta(dCHtBu)(CH2tBu)2] and [(tSiO)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 [(tSiO)Ta(dCHtBu)(CH2tBu)2(tSiOSit)] (1a′) and [(tSiO)Ta(CH3)3Cp*(tSiOSit)] (2a′). In the case of [(tSiO)Ta(dCHtBu)(CH2tBu)2(tSiOSit)], 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 (JC-H ) 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [(tSiO)Ta(dCHtBu)(CH2tBu)2(tSiOSit)] (1a′) as a catalyst precursor and the inactivity of the surface complex [(tSiO)Ta(CH3)3Cp*(tSiOSit)] (2a′) show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.

Detailed structural characterization of the grafting of [Ta(=CHtBu)(CH2tBu)3] and [Cp*TaMe4] on silica partially dehydroxylated at 700oC and the activity of the grafted complexes toward alkane metathesis