Reaction of Ta[CH2C(CH3)3]3[=CHC(CH 3)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-(CH 3)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)2TaIII (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)2TaIIH 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.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of the American Chemical Society|
|State||Published - Dec 1 1996|
ASJC Scopus subject areas
- Colloid and Surface Chemistry