Intermolecular C–H Activations of Hydrocarbons Initiated by Cp*M(NO)(CH2CMe3)(η3-CH2CHCHPh) Complexes (M = Mo, W)

Thermolysis of Cp*W­(NO)­(CH2CMe3)­(η3-CH2CHCHPh) (1) at 55 °C leads to the loss of neopentane and the formation of the 16e η2-allene intermediate complex Cp*W­(NO)­(η2-CH2CCHPh) (A), which has been isolated as its 18e PMe3 adduct (2). Further support for the existence of the allene intermediate A is provided by the thermolysis of 1 in cyclohexene, which affords Cp*W­(NO)­(H)­(η3-CH2C­(3-cyclohexenyl)­CHPh) (3) as the principal organometallic product. In the presence of n-heptane, n-octane, or n-pentane, A effects C–H activations of the hydrocarbons exclusively at their terminal carbons and forms 18e Cp*W­(NO)­(n-alkyl)­(η3-CH2CHCHPh) complexes (4–6). Similarly, treatments of 1 with mesitylene, methylcyclohexane, and ethylcyclohexane all lead to the corresponding primary sp3 C–H activation products (7–9). Complex mixtures of organometallic products result when 1 is thermolyzed in p-xylene and toluene, reflecting the occurrence of both aryl and benzylic C–H activations. Interestingly, the aryl C–H activations do not afford the expected Cp*W­(NO)­(aryl)­(η3-CH2CHCHPh) products but rather their Cp*W­(NO)­(H)­[η3-CH­(aryl)­CHCHPh] isomers resulting from aryl–H exchange. The thermal chemistry of the molybdenum analogue of 1, namely Cp*Mo­(NO)­(CH2CMe3)­(η3-CH2CHCHPh) (14), has also been investigated, and it turns out to be much more limited in scope. When 14 is heated at 35 °C in neat mesitylene for 22 h, it results in conversion to the mesitylene-activated product Cp*Mo­(NO)­(CH2C6H3-3,5-Me2)­(η3-CH2CHCHPh) (15) in low yield, but thermolyses of 14 in other hydrocarbons do not produce tractable organometallic materials. The results of DFT calculations on the model reaction of CpW­(NO)­(η2-CH2CCHMe) with propane confirm that the rate-determining step is the cleavage of a propane C–H bond and that the lower energy anti conformers favor terminal activation by 11.5 kJ/mol. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by X-ray crystallographic analyses.