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

Abstract

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. © 2006 American Chemical Society.