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

Reaction of Ta[CH 2 C(CH 3 ) 3 ] 3 [ CHC(CH 3 ) 3 ] ( 1 ) with a silica dehydroxylated at 500 °C has been previously reported to produce a mixture of ⋮SiOTa[CH 2 C(CH 3 ) 3 ] 2 [ CHC(CH 3 ) 3 ] ( 2a ) and (⋮SiO) 2 Ta[CH 2 C(CH 3 ) 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) 2 Ta III H ( 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 CH 3 I to give methane; 3 reacts quantitatively with di( tert -butyl)ketone to form the corresponding tantalum di( tert -butyl)methoxide; 3 reacts with D 2 O to give a mixture of HD and D 2 (2 ± 0.2 mol equiv per tantalum); and 3 activates the C−H bond of cycloalkanes (C 5 to C 8 ) at room temperature to form the corresponding surface tantalum(III) monoalkyls (⋮SiO) 2 Ta-alkyl, with liberation of 1 ± 0.1 equiv of hydrogen. The surface tantalum-alkyls are transformed under oxygen into the corresponding (⋮SiO) 2 Ta 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) 2 Ta III H 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) 2 Ta III H 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.