Computational studies on intramolecular C-H and C-C bond activation in ruthenium N-heterocyclic carbene complexes

Ru(CO)(IMes)(L)(PPh3)(H)2 species (IMes = bis-1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene) are capable of undergoing both intramol. C-C and C-H bond activation processes. When L = PPh3, dehydrogenation leads to C-H activation at one o-Me group. In contrast, when L = IMes, thermolysis in the presence of PPh3 leads to C-C bond activation. We shall present the results of d. functional calcns. on these processes. Calcns. using PH3 and imidazol-2-ylidene model ligands show phosphine/NHC substitution does not significantly alter the energetics of oxidative addn. Two competitive pathways for C-H activation have been located: either H2 loss followed by C-H bond oxidative addn. at Ru(0), or phosphine loss followed by C-H bond activation via a metathesis step. C-C bond activation is always kinetically less accessible, suggesting that the full steric bulk of the ligands is important in directing these various bond activation processes. The results of hybrid QM/MM calcns. addressing this point will be discussed.