Detailed Structural Investigation of the Grafting of [Ta(CHtBu)(CH₂tBu)₃] and [Cp*TaMe₄] on Silica Partially Dehydroxylated at 700 °C and the Activity of the Grafted Complexes toward Alkane Metathesis

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.

Silica‐Alumina‐Supported, Tungsten‐Based Heterogeneous Alkane Metathesis Catalyst: Is it Closer to a Silica‐ or an Alumina‐Supported System?

“…In addition, as observed previously, linear alkanes are favoured over branched ones independently of the catalysts used (2 or 3), and the selectivities of higher homologues are as follows: C n+1 > C n+2 @ C n+3 . The observed selectivities are very close to those observed for alumina supported Wbased systems and are fully consistent with the model proposed earlier (Scheme 3), [10,11] which is based on olefin metathesis intermediates and which predicts that the favoured pathway for the formation of higher homologues involves a [1,3]-disubstituted metallacyclobutane rather than a [1,2]-disubstituted one (Scheme 3). [24] The higher selectivity of linear versus branched compounds is also consistent with this www.asc.wiley-vch.de model since the latter would involve the less reactive disubstituted carbenes.…”

“…[1,2] Since the early 1990s, we have developed a series of oxidesupported metal hydrides [3][4][5][6] for the low temperature alkane transformation. [7][8][9] In contrast to hydrogenolysis, alkane metathesis allows both lower and higher homologues to be obtained, and we have shown recently through a combination of structure-reactivity [10] and kinetic [11] studies that the key carbon-carbon cleavage and formation process involves olefin metathesis. Other approaches have been investigated to carry out alkane metathesis.…”

A Well-Defined, Silica-Supported Tungsten Imido Alkylidene Olefin Metathesis Catalyst

“…In addition, the presence of an O atom at 2.43(1) Å and a Si at 2.96(1) Å suggests the presence of a siloxane bridge close to the W center. 7,22 DFT periodic calculations using VASP 23,24 on ( SiO)W(=N-2,6-di-iPr-C 6 H 3 )(=CHtBu)(CH 2 tBu) supported on a cristobalite (110) surface as a model for silica, 2q, 14 confirm the metal-ligand bond lengths obtained by EXAFS (Table 1 and Figure S3). Moreover, they show that the surface complex has the usual pseudo-tetrahedral syn structure, the anti isomer lying 5.7 kcal mol -1 above.…”

Strategies to Immobilize Well‐Defined Olefin Metathesis Catalysts: Supported Homogeneous Catalysis vs. Surface Organometallic Chemistry

“…} can also be used as catalyst precursors not only in alkyne [89] but also in alkane metathesis {e.g., [117][118][119] a reaction, which also involves olefin metathesis as a key carbon-carbon bond formation step. [120] …”