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

Abstract

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)(PH3)2(CO), where NHC = 1-(C6H4-2-X)imidazol-2-ylidene (I(o-C6H4X), X = H, CH3, F, OH, NH2, OCH3, and CF3). 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 = CH3. As C(aryl)−CH3 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 = NH2 < CH3< H < OH ≈ OCH3 < CF3 < F, while the barriers vary as X = H < F < OH ≈ OCH3 < CF3 < NH2< CH3. Both series reflect the pr...