Ability of N-Heterocyclic Carbene Ligands to Promote Intermolecular Oxidative Addition Reactions at Unsaturated Ruthenium Centers

We report the results of density functional calculations on the reactivity of a series of coordinatively unsaturated mixed phosphine/N-heterocyclic carbene complexes of ruthenium of the type Ru(CO)(IR)_3-n(PH ₃)_n, where n = 1-3 and R = H (1,3-imidazol-2-ylidene) and R = Me (1,3-dimethylimidazol-2-ylidene). The oxidative addition reactions of H₂ and CH₄ and the C-C bond activation of C ₂H₆ have been studied. For all three processes, substitution of PH₃ by IH results in minimal changes in the reaction energetics. In all cases H₂ oxidative addition is barrierless and is downhill by around 120 kJ/mol. With CH₄ activation barriers of around 75 kJ/mol are computed and the reaction is approximately thermoneutral. With C₂H₆ activation barriers increase to around 260 kJ/mol and the reaction is disfavored by about + 35 kJ/mol. Introduction of the IMe ligand disfavors oxidative addition, especially for the C₂H₆ reaction, and this trend is linked to increased steric bulk of the IMe ligand compared to IH. Computed Ru-PH₃ and Ru-IR bond strengths and CO stretching frequencies indicate that PH₃/IR substitution does create a more electron-rich metal center, and yet this does not facilitate oxidative addition with these Ru species. A fragment analysis reveals that, as expected, PH₃/IH substitution enhances the Lewis basicity of the metal reactant. However, a more important effect is a reduction in Lewis acidity, and this factor lies behind the similar reaction energetics computed for analogous PH₃- and IH-containing species.