| Abstract
| - The reaction of [Ta(CHtBu)(CH2tBu)3] or [Cp*Ta(CH3)4] with a silica partially dehydroxylatedat 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)3Cp*(⋮SiOSi⋮)] (2a‘). In the case of [(⋮SiO)Ta(CHtBu)(CH2tBu)2(⋮SiOSi⋮)], the structureis further stabilized by an additional interaction: a C−H agostic bond as evidenced by the small J couplingconstant for the carbenic C−H (JC-H = 80 Hz), which was measured by J-resolved 2D solid-state NMRspectroscopy. 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(CH3)3Cp*(⋮SiOSi⋮)] (2a‘) show that the active site is required to be highly electrophilic and probablyinvolves a metallacyclobutane intermediate.
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