Alkane Metathesis with the Tantalum Methylidene [( SiO)Ta(=CH2)Me-2]/[( SiO)(2)Ta(=CH2)Me] Generated from Well-Defined Surface Organometallic Complex [( SiO)(TaMe4)-Me-V]

By grafting TaMe5 on Aerosil(700), a stable, well-defined, silica-supported tetramethyl tantalum(V) complex, [( SiO)TaMe4], is obtained on the silica surface. After thermal treatment at 150 degrees C, the complex is transformed into two surface tantalum methylidenes, [( SiO)(2)Ta(-CH2)Me] and [( SiO)Ta(-CH2)Me-2], which are active in alkane metathesis and comparable to the previously reported [( SiO)(2)TaHx]. Here we present the first experimental study to isolate and identify a surface tantalum carbene as the intermediate in alkane metathesis. A systematic experimental study reveals a new reasonable pathway for this reaction.