Intermolecular C−H Activation of Hydrocarbons by Tungsten Alkylidene Complexes: An Experimental and Computational Mechanistic Study

Cp*W(NO)(CH 2 CMe 3 ) 2 ( 1 ) and Cp*W(NO)(CH 2 CMe 3 )(CH 2 C 6 H 5 ) ( 2 ) under moderate conditions (70 °C, 40 h) generate the reactive complexes Cp*W(NO)( CHCMe 3 ) ( A ) and Cp*W(NO)( CHC 6 H 5 ) ( B ), respectively, which activate hydrocarbon solvents via the addition of C−H across the M C bond. The α-deuterated derivative Cp*W(NO)(CD 2 CMe 3 ) 2 ( 1- d 4 ) undergoes intramolecular H/D exchange within the neopentyl ligands, consistent with the formation of σ-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 °C, 40 h) yields an intermolecular KIE of 1.07(4):1. Thermolysis of 1 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)(CH 2 CMe 3 )(C 6 D 5 ) ( 5- d 5 ) and Cp*W(NO)(CH 2 C 6 H 5 )(C 6 D 5 ) ( 6- d 5 ) convert to the respective H/D scrambled products Cp*W(NO)(CHD syn CMe 3 )(C 6 D 4 H 1 ) ( 5 ‘ - d 5 ) and Cp*W(NO)(CHD syn C 6 H 5 )(C 6 D 4 H 1 ) ( 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)( CH 2 ) ( C ) indicate that aromatic sp 2 C−H bond activation proceeds through a π-arene complex, while benzylic sp 3 C−H bond activation proceeds through a η 2 ( C, H ),σ-phenylmethane complex. The principal factor behind the preferential formation of the aryl products appears to be the relative energies of formation of these intermediates.