Theoretical studies of the sp(2) versus sp(3) C-H bond activation chemistry of 2-picoline by (C5Me5)(2)An(CH3)(2) complexes (An = Th, U)

摘要

The C-H bond activation chemistry of (C5Me5)(2)Th(CH3)(2) and (C5Me5)(2)U(CH3)(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 eta(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, Delta E-double dagger(sp(2)) > Delta E-double dagger(sp(3)) and U, Delta E-double dagger(sp(2)) < Delta E-double dagger(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.