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

Erwan Le Roux; Mathieu Chabanas; Anne Baudouin; Aimery de Mallmann; Christophe Copéret; E Alessandra Quadrelli; Jean Thivolle-Cazat; Jean-Marie Basset; Wayne Lukens; Anne Lesage; Lyndon Emsley; Glenn J Sunley

doi:10.1021/ja046486r

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

J Am Chem Soc. 2004 Oct 20;126(41):13391-9. doi: 10.1021/ja046486r.

Authors

Erwan Le Roux¹, Mathieu Chabanas, Anne Baudouin, Aimery de Mallmann, Christophe Copéret, E Alessandra Quadrelli, Jean Thivolle-Cazat, Jean-Marie Basset, Wayne Lukens, Anne Lesage, Lyndon Emsley, Glenn J Sunley

Affiliation

¹ Laboratoire de Chimie Organométallique de Surface, UMR 9986 CNRS/ESCPE Lyon, Villeurbanne, France.

PMID: 15479095
DOI: 10.1021/ja046486r

Abstract

The reaction of [Ta(=CHtBu)(CH2tBu)3] or [Cp*Ta(CH3)4] with a silica partially dehydroxylated at 700 degrees C gives the corresponding monosiloxy surface complexes [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2] and [([triple bond]SiO)Ta(CH3)3Cp*] by eliminating a sigma-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 [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])] (1a') and [([triple bond]SiO)Ta(CH3)3Cp*([triple bond]SiOSi[triple bond])] (2a'). In the case of [(SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])], 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 [([triple bond]SiO)Ta(=CHtBu)(CH2tBu)2([triple bond]SiOSi[triple bond])] (1a') as a catalyst precursor and the inactivity of the surface complex [([triple bond]SiO)Ta(CH3)3Cp*([triple bond]SiOSi[triple bond])] (2a') show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.