Intermolecular C-H activation of hydrocarbons by tungsten alkylidene complexes: An experimental and computational mechanistic study

Cp*W(NO)(CH2CMe3)(2) (1) and Cp*W(NO)(CH2CMe3)(CH2C6H5) (2) under moderate conditions (70 degreesC, 40 h) generate the reactive complexes Cp*W(NO)(=CHCMe3) (A) and Cp*W(NO)(=CHC6H5) (B), respectively, which activate hydrocarbon solvents via the addition of C-H across the M=C bond. The alpha -deuterated derivative Cp*W(NO)(CD2CMe3)(2) (1-d(4)) undergoes intramolecular H/D exchange within the neopentyl ligands, consistent with the formation of a-neopentane complexes prior to neopentane elimination. The thermolysis of 1 in a 1:1 molar mixture of tetramethylsilane-h(12) and tetramethylsilane-d(12) (70 degreesC, 40 h) yields an intermolecular KIE of 1.07(4):1. Thermolysis of I and 2 in 1:1 benzene/benzene-d(6) yields intermolecular KIEs of 1.03(5):1 and 1.17(19):1, respectively. The KIE values are inconsistent with rate-determining C-H bond addition to the M=C linkage and indicate that coordination of the substrate to the metal center is the discriminating factor in alkane and arene intermolecular competitions. The complexes Cp*W(NO)(CH2CMe3)(C6D5) (5-d(5)) and CP*W(NO)(CH2C6H5)(C6D5) (6-d(5)) convert to the respective H/D scrambled products Cp*W(NO)(CHDsynCMe3)(C6D4H1) (5'-d(5)) and Cp*W(NO)(CHDsynC6H5)(C6D4H1) (6'-d(5)) under thermolytic conditions, consistent with the occurrence of reversible aromatic sp(2) C-H bond cleavage. The results suggest that the previously reported discrimination between the aryl and benzyl products of toluene activation by A and B originates from coordination of toluene to the metal center in two distinct modes. Supporting DFT calculations on the activation of toluene by CpW(NO)(=CH2) (C) indicate that aromatic sp(2) C-H bond activation proceeds through a pi -arene complex, while benzylic sp(3) C-H bond activation proceeds through a eta (2)(C,H),sigma -phenylmethane complex. The principal factor behind the preferential formation of the aryl products appears to be the relative energies of formation of these intermediates.