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

The Ta(=CHtBu)(CH(2)tBU)(3) Complex 1 reacts with the OH groups of a MCM-41 mesoporous silica dehydroxylated at 500 degrees C to form the monosiloxy surface species [(equivalent to SiO)Ta(=CHtBu)(CH(2)tBu)(2)] 2, with evolution of 1 equiv per Ta of neopentane. Complex 2 leads to a mixture of supported tantalum hydrides [(equivalent to SiO)(2)Ta(H)(x)] (x = 1, 3) 3, by treatment under hydrogen at 150 degrees C. These surface complexes were characterized by the combined use of several techniques such as IR and EXAFS spectroscopies as well as H-1 MAS, C-13 CP/MAS, 2D H-1-C-13 HETCOR, and J-resolved solid-state NMR and mass balance analysis. The surface tantalum hydrides evolve reversibly to the monohydride species (equivalent to SiO)(2)Ta-H by heating at 150 degrees C under vacuum; they lead progressively to the complete formation of the supported trisiloxy tantalum complex (equivalent to SiO)(3)Ta by heating under hydrogen (600 Torr) up to 500 degrees C. They can activate at 150 degrees C the C-H bond Of CH4 to form first the surface tantalum methyl species [(=SiO)(2)Ta(CH3)(x)] with liberation of H-2. 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 C-13 CP/MAS solid-state NMR and 100% C-13-labeled methane affords the observation of methylidene and methylidyne species on a few tantalum sites, which indicates the occurrence of an alpha-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 (equivalent to SiO)(3)Ta.