Theoretical Studies of the sp² versus sp³ C−H Bond Activation Chemistry of 2-Picoline by (C₅Me₅)₂An(CH₃)₂ Complexes (An = Th, U)

Abstract

The C−H bond activation chemistry of (C 5 Me 5 ) 2 Th(CH 3 ) 2 and (C 5 Me 5 ) 2 U(CH 3 ) 2 with 2-picoline (2-methylpyridine) is examined with use of density functional techniques. In particular, the differences between insertion into the ortho ring sp 2 C−H bond and the methyl sp 3 C−H bond are explored. The energies to form the η 2 -( N, C )-pyridyl products resulting from the activation of the aromatic ring sp 2 C−H bond are calculated as thermodynamic products for both thorium and uranium systems with similar reaction energies of −15.8 kcal/mol. The products corresponding to insertion into the methyl sp 3 C−H bond are found to be higher in energy by 3.5 and 5.4 kcal/mol for Th and U, respectively. In the transition states the actinide atom mediates the hydrogen migration from 2-picoline to the leaving methyl group by forming an agostic five-centered C−H complex. The relative activation energies between sp 2 and sp 3 C−H bond activation differ slightly between Th, Δ E ⧧ (sp 2 ) > Δ E ⧧ (sp 3 ) and U, Δ E ⧧ (sp 2 ) < Δ E ⧧ (sp 3 ). These results are in agreement with the experimental observations that the sp 2 insertion product is the thermodynamic product in both cases, but that the sp 3 insertion product is the kinetic product in the case of Th.