Computational Studies of Intramolecular Carbon−Heteroatom Bond Activation of N-Aryl Heterocyclic Carbene Ligands

Density functional theory calculations have been performed on intramolecular C(aryl)−X bond activation reactions in model N-heterocyclic carbene (NHC) complexes of the type Ru(NHC)(PH 3 ) 2 (CO), where NHC = 1-(C 6 H 4 -2-X)imidazol-2-ylidene (I( o- C 6 H 4 X), X = H, CH 3, F, OH, NH 2, OCH 3, and CF 3 ). In all cases C(aryl)−X activation is found to be thermodynamically favored, and the largest barrier to reaction is computed to be +21.3 kcal/mol when X = CH 3 . As C(aryl)−CH 3 bond activation has been observed experimentally for a Ru−NHC complex (Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.; Richards, S. P.; Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124, 4944), this suggests that a wide range of heteroatom-substituted N-aryl NHC ligands may be susceptible to intramolecular bond activation and potential ligand degradation. The computed exothermicity of C(aryl)−X activation follows the trend X = NH 2 < CH 3 < H < OH ≈ OCH 3 < CF 3 < F, while the barriers vary as X = H < F < OH ≈ OCH 3 < CF 3 < NH 2 < CH 3 . Both series reflect the promotion of C(aryl)−X activation by the formation of stronger Ru−X bonds in the product. However, the ability of heteroatom ligands to stabilize the Ru(0) reactants through chelation can disfavor C(aryl)−X cleavage and explains the low exothermicity and high barrier associated with C(aryl)−NH 2 activation. For X = OCH 3 C(aryl)−O bond activation was found to be favored kinetically over O−C(alkyl) activation, although the latter process yields an extremely stable aryloxide product. The arrangement of coligands around the metal can significantly affect C(aryl)−X bond activation, and when X is a π-donor, this process is promoted by a trans CO ligand. These insights suggest not only possible ways to control unwanted C(aryl)−X activation in heteroatom-subtituted N-aryl NHC ligands but also factors that may promote such reactions in catalytic processes where this step is desirable.