Synthesis, Characterization, and Reactivity, in the C−H Bond Activation of Cycloalkanes, of a Silica-Supported Tantalum(III) Monohydride Complex:  (⋮SiO)₂Ta^III−H

Abstract

Reaction of Ta[CH2C(CH3)3]3[CHC(CH3)3] (1) with a silica dehydroxylated at 500 °C has been previously reported to produce a mixture of ⋮SiOTa[CH2C(CH3)3]2[CHC(CH3)3] (2a) and (⋮SiO)2Ta[CH2C(CH3)3][CHC(CH3)3] (2b). Treatment of these two surface organometallic complexes under 1 atm of hydrogen up to 200 °C leads to the formation of a surface tantalum(III) monohydride: (⋮SiO)2TaIIIH (3) as a major product and surface Si−H groups. 3 has been characterized in the following way: it reversibly exchanges with deuterium to give the corresponding [Ta]−D species; 3 reacts with CH3I to give methane; 3 reacts quantitatively with di(tert-butyl)ketone to form the corresponding tantalum di(tert-butyl)methoxide; 3 reacts with D2O to give a mixture of HD and D2 (2 ± 0.2 mol equiv per tantalum); and 3 activates the C−H bond of cycloalkanes (C5 to C8) at room temperature to form the corresponding surface tantalum(III) monoalkyls (⋮SiO)2Ta-alkyl, with liberation of 1 ± 0.1 equiv of hydrogen. The surface tantalum-alkyls are transformed under oxygen into the corresponding (⋮SiO)2TaV(O)(O-alkyl). Quantitative determinations on the last three reactions are consistent with the formulation of 3 as a bis(siloxy)tantalum(III) monohydride (⋮SiO)2TaIIIH as the major surface species. Ta LIII-edge EXAFS studies of 3 confirm that two σ-bonded surface Si−O groups are attached to tantalum (Ta−O 1.89 Å). However, additional evidence is provided for a third surface oxygen (most probably siloxane) interacting weakly with the tantalum center (Ta···O 2.63 Å). A mechanism for the simultaneous formation of (⋮SiO)2TaIIIH 3 and Si−H groups from 2a is proposed, which involves a hydride transfer from tantalum to a neighbor silicon atom and the transfer of an oxygen from the same silicon atom to the more oxophilic tantalum.

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

“…[6] Similarly, the band at 2270 cm À1 is not affected by this treatment, and can be assigned to a n (SiÀH) . [3][4][5] Additionally, the band at 2961 cm À1 associated with the residual alkyl ligands (5 %, Figure 1 c) are modified, while a weak band at 2214 cm…”

“…Moreover, in contrast to the tungsten hydride supported on alumina, [6] the IR spectrum of this supported tungsten hydride on silica-alumina, [W À H/SiO 2 -Al 2 O 3 ] SA (3) show several tungsten hydride bands, residual surface alkyl species as well as both Si À H and Al À H species. These latter hydride species are probably formed by the reaction of tungsten hydride intermediates with adjacent ( Si À O À Si ) and ( Si À O À Al s ) bridges, [3][4][5] thus providing bisgrafted species, [( SiO) 2 WH n ] and [( SiO)A C H T U N G T R E N N U N G (Al s O)WH n ] along with ( Si À H) and (Al s À H). The absence of SiH 2 species (not observed in the IR spectrum) [4] suggests the absence of formation of trisgrafted species.…”

“Hydro‐metathesis” of Olefins: A Catalytic Reaction Using a Bifunctional Single‐Site Tantalum Hydride Catalyst Supported on Fibrous Silica (KCC‐1) Nanospheres

“…Any new reaction related to this challenge is important. In 1991, we discovered that the highly electrophilic earlytransition-metal hydride [(SiO) 3 Zr-H] supported on silica [1][2][3][4] could activate the C À H and C À C bonds of alkanes or polyolefins and could also catalyze the hydrogenolysis of these hydrocarbons into a range of gasolines.[5] Later, in 1997, we found that the highly electrophilic silica-supported tantalum hydride [(SiO) 2 TaH] [6,7] could transform any light alkane into its lower and higher homologues by both cleavage and formation of C À H and C À C bonds. We called this new catalytic reaction "alkane metathesis" by analogy to "olefin metathesis" [Eq.…”

“…Also remarkable is the regenerative ability of the catalyst: after using TaH/KCC-1 in a 56 hour reaction it was regenerated by treatment with hydrogen (3 mL min À1 ) at 150 8C for 12 hours, after which it was used in another propene hydro-metathesis under identical reaction conditions and demonstrated slightly better activity (Figure 3 a, dotted lines), selectivity (see Figure S1 in the Supporting Information), and stability. The slight increase in the catalytic activity after regeneration could result from the fact that the total elimination of alkyl groups from TaNp/KCC-1 under H 2 requires heating up to 250 8C instead of 150 8C with Degussa silica [6] or MCM-41; [9] under these conditions the regeneration could help to remove more Np groups and produce a little bit more TaH than the first run.…”

“…The remaining elementary steps are the same as those in alkane metathesis. Intermediates 2 and 3 undergo an a-hydrogen abstraction to form the Ta/carbene/ hydrides 4 and 5, respectively, which can react with the incoming alkene from the gas phase to produce four different tantallacyclobutanes (6)(7)(8)(9). [15] These tantallacyclobutanes later cleave metathetically to yield higher alkenes and the hydrido tantallacarbenes 10 and 11.…”