Synthesis, characterization, and reactivity, in the C-H bond activation of cycloalkanes, of a silica-supported tantalum(III) monohydride complex: (≡SiO)₂Ta^II-H

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)₂Ta^III (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)₂Ta^V(=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)₂Ta^IIH as the major surface species. Ta L_III-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)₂Ta^IIIH 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.