Synthesis, characterization, and reactivity, in the C-H bond activation of cycloalkanes, of a silica-supported tantalum(III) monohydride complex: (≡SiO)Ta-H
Journal of the American Chemical Society, ISSN: 0002-7863, Vol: 118, Issue: 19, Page: 4595-4602
1996
- 140Citations
- 46Captures
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Article Description
Reaction of Ta[CHC(CH)][=CHC(CH )] (1) with a silica dehydroxylated at 500 °C has been previously reported to produce a mixture of ≡SiOTa[CHC(CH)] [=CHC(CH)] (2a) and (=SiO)Ta[CHC(CH)][=CHC-(CH )] (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)Ta (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 CHI to give methane; 3 reacts quantitatively with di(tert-butyl)ketone to form the corresponding tantalum di(tert-butyl)methoxide; 3 reacts with DO to give a mixture of HD and D (2 ± 0.2 mol equiv per tantalum); and 3 activates the C-H bond of cycloalkanes (C to C) at room temperature to form the corresponding surface tantalum(III) monoalkyls (≡SiO)Ta-alkyl, with liberation of 1 ± 0.1 equiv of hydrogen. The surface tantalum-alkyls are transformed under oxygen into the corresponding (≡SiO)Ta(=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)TaH as the major surface species. Ta L-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)TaH 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.
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