Abstract
The well-defined single-site silica-supported tungsten complex [( Si-O-) W(Me)(5)], 1, is an excellent precatalyst for alkane metathesis. The unique structure of 1 allows the synthesis of unprecedented tungsten hydrido methyl surface complexes via a controlled hydrogenolysis. Specifically, in the presence of molecular hydrogen, 1 is quickly transformed at -78 degrees C into a partially alkylated tungsten hydride, 4, as characterized by H-1 solid-state NMR and IR spectroscopies. Species 4, upon warming to 150 degrees C, displays the highest catalytic activity for propane metathesis yet reported. DFT calculations using model systems support the formation of [( Si-O-)WH3(Me)(2)], as the predominant species at -78 degrees C following several elementary steps of hydrogen addition (by sigma-bond metathesis or alpha-hydrogen transfer). Rearrangement of 4 occuring between -78 degrees C and room temperature leads to the formation of an unique methylidene tungsten hydride [( Si-O-)WH3(=CH2)], as determined by solid-state H-1 and C-13 NMR spectroscopies and supported by DFT. Thus for the first time, a coordination sphere that incorporates both carbene and hydride functionalities has been observed.