Computational Study of C−C Activation of 1,3-Dimesitylimidazol-2-ylidene (IMes) at Ruthenium: The Role of Ligand Bulk in Accessing Reactive Intermediates

Abstract

Density functional theory calculations have been employed to model phosphine substitution in Ru(PPh 3 ) 3 (CO)(H) 2 to form Ru(IMes)(PPh 3 ) 2 (CO)(H) 2 ( 1 mono ) and Ru(IMes) 2 (PPh 3 )(CO)(H) 2 ( 1 bis ), as well as the novel C(aryl)−C(sp 3 ) intramolecular bond activation of the IMes ligand in 1 bis . The computed ligand exchange energies show that 1 bis is unstable with respect to displacement of IMes by PPh 3 and will thus re-form 1 mono over time. PPh 3 /IMes substitution also leads to a significant labilization of the PPh 3 ligand trans to hydride, a result of increasing steric encumbrance upon the introduction of the bulky IMes ligands. The energetics of intramolecular C−C and C−H activation have been computed for both 16e Ru(IMes) n (PPh 3 ) 3- n (CO) and 14e Ru(IMes) n (PPh 3 ) 2- n (CO) species ( n = 1 or 2) and indicate that the introduction of a second IMes ligand does not significantly promote the actual C−C activation step. Instead the need to have two IMes ligands present in the metal coordination sphere before C−C activation can occur is linked to the promotion of PPh 3 loss in 1 bis, which makes the formation of unsaturated species such as Ru(IMes) 2 (CO) particularly accessible.