Intermolecular Activation of Hydrocarbon C−H Bonds Initiated by the Tungsten Hydrocarbyl Hydrido Complexes Cp*W(NO)(R)(H)(PMe₃) (R = Alkyl, Aryl)

trans-Cp*W(NO)(CH₂EMe₃)(H)(PMe₃) (E = C, Si) complexes can be prepared by the hydrogenation at 1 atm of the appropriate Cp*W(NO)(CH₂EMe₃)₂ precursor in the presence of a slight excess of PMe₃. (Our designation of a particular geometrical isomer as cis or trans in this family of complexes indicates the relative positions of the hydrocarbyl and the hydrido ligands in the base of a four-legged piano-stool molecular structure.) The use of D₂ in place of H₂ during these syntheses affords the corresponding trans-Cp*W(NO)(CH₂EMe₃)(D)(PMe₃) (E = C, Si) complexes. The cis-Cp*W(NO)(CH₂EMe₃)(H)(PMe₃) isomers are obtainable by C−H bond-activation reactions of the trans precursors. Thus, activation of SiMe₄ by trans-Cp*W(NO)(CH₂CMe₃)(H)(PMe₃) under ambient conditions produces cis-Cp*W(NO)(CH₂SiMe₃)(H)(PMe₃). Similarly, activations of C₆H₆ and C₆D₆ at 20−27 °C by the trans-Cp*W(NO)(CH₂EMe₃)(H)(PMe₃) complexes produce cis-Cp*W(NO)(C₆H₅)(H)(PMe₃) and cis-Cp*W(NO)(C₆D₅)(D)(PMe₃), respectively, and the solid-state molecular structure of the latter complex has been established by a single-crystal X-ray crystallographic analysis. Kinetic, mechanistic, and theoretical investigations of these benzene C−H activation processes are consistent with initial trans to cis isomerization of the reactants followed by intramolecular reductive elimination of EMe₄ to form the 16-electron Cp*W(NO)(PMe₃) intermediate. Subsequent oxidative addition of the incoming benzene substrate to this coordinatively unsaturated intermediate produces the final cis hydrido phenyl complex. These single C−H activation processes are the requisite first steps in the development of these organometallic complexes as catalysts for the selective functionalization of hydrocarbons. All new complexes have been characterized by conventional spectroscopic methods.