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 ν(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.