Tantalum hydrides supported on MCM-41 mesoporous silica: Activation of methane and thermal evolution of the tantalum-methyl species

The Ta(=CHtBu)(CH₂tBu)₃ complex 1 reacts with the OH groups of a MCM-41 mesoporous silica dehydroxylated at 500°C to form the monosiloxy surface species [(≡SiO)Ta(=CHtBu)(CH₂tBu) ₂] 2, with evolution of 1 equiv per Ta of neopentane. Complex 2 leads to a mixture of supported tantalum hydrides [(≡SiO)₂Ta(H) _x] (x = 1, 3), 3, by treatment under hydrogen at 150°C. These surface complexes were characterized by the combined use of several techniques such as IR and EXAFS spectroscopies as well as ¹H MAS, ¹³C CP/MAS, 2D ¹H-¹³C HETCOR, and J-resolved solid-state NMR and mass balance analysis. The surface tantalum hydrides evolve reversibly to the monohydride species (≡SiO)₂Ta-H by heating at 150°C under vacuum; they lead progressively to the complete formation of the supported trisiloxy tantalum complex (≡SiO)₃Ta by heating under hydrogen (600 Torr) up to 500°C. They can activate at 150°C the C-H bond of CH₄ to form first the surface tantalum methyl species [(≡SiO)₂Ta-(CH₃)_x] with liberation of H₂. The initially rapid decrease of the v(Ta-H) bands followed by a slower rate indicates the presence of a distribution of Ta-H sites of various reactivity. The combined use of ¹³C CP/MAS solid-state NMR and 100% ¹³C-labeled methane affords the observation of methylidene and methylidyne species on a few tantalum sites, which indicates the occurrence of an α-H elimination process. In parallel, a progressive transfer of methyl groups from tantalum to neighboring siloxane bridges was also evidenced, which grows with temperature; this process is reasonably accompanied by the formation of the trisiloxy tantalum complex (≡SiO)₃Ta.