Detailed Structural Investigation of the Grafting of [Ta(CHtBu)(CH2 tBu)3] and [CpTaMe4] on Silica Partially Dehydroxylated at 700 °C and the Activity of the Grafted Complexes toward Alkane Metathesis

• Published in: Journal of the American Chemical Society 2004-10, Vol.126 (41), p.13391-13399
• Main Authors: Le Roux, Erwan, Chabanas, Mathieu, Baudouin, Anne, de Mallmann, Aimery, Copéret, Christophe, Quadrelli, E. Alessandra, Thivolle-Cazat, Jean, Basset, Jean-Marie, Lukens, Wayne, Lesage, Anne, Emsley, Lyndon, Sunley, Glenn J
• Format: Article
• Language: English
• Subjects: Catalysis; Chemical reactivity; Chemical Sciences; Chemistry; Coordination chemistry; Exact sciences and technology; Organic chemistry; Reactivity and mechanisms
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja046486r

The reaction of [Ta(CHtBu)(CH2 tBu)3] or [Cp*Ta(CH3)4] with a silica partially dehydroxylated at 700 °C gives the corresponding monosiloxy surface complexes [(⋮SiO)Ta(CHtBu)(CH2 tBu)2] and [(⋮SiO)Ta(CH3)3Cp*] by eliminating a σ-bonded ligand as the corresponding alkane (H−CH2 tBu 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)(CH2 tBu)2(⋮SiOSi⋮)] (1a‘) and [(⋮SiO)Ta(CH3)3Cp*(⋮SiOSi⋮)] (2a‘). In the case of [(⋮SiO)Ta(CHtBu)(CH2 tBu)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)(CH2 tBu)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 probably involves a metallacyclobutane intermediate.