Reactivity of a (Bis-NHC)tricarbonylruthenium(0) Complex with Methyl Triflate and Methyl Iodide. Formation of Methyl- and Acetylruthenium(II) Derivatives: Experimental Results and Mechanistic DFT Calculations

The ruthenium(0) complex [Ru{kappa C-2(2)-MeIm-(CH2)(3)ImMe}(CO)(3)] (1), Mehn(CH2)(3)ImMe = 1,3-bis(3-methylimidazol-2-yliden-1-yl)propane, which contains a chelating bis(N-heterocyclic carbene) ligand, reacts with MeOTf at room temperature to give the ionic ruthenium(II) methyl derivative [RuMe{kappa C-2(2)-MeIm(CH2)(3)ImMe}-(CO)(3)]OTf ([2]OTf), whereas an analogous reaction of 1 with MeI renders the neutral ruthenium(II) acetyl derivative [RuI{C(O)Me}{kappa C-2(2)-MeIm(CH2)(3)ImMe}(CO)(2)] (3), in which the iodide and acetyl ligands occupy mutually trans coordination sites. The fact that [2]OTf reacts with [Et4N]I to give 3 evidences the participation of the cationic species 2(+) in the synthesis of 3. The mechanisms of these reactions in THF solution have been modeled by DFT calculations. They have shown that 2(+) can be made from complex 1 and either MeOTf or MeI. In both cases, two plausible reaction pathways have been identified. They start with a rate-determining S(N)2 substitution process in which the metal atom of 1 attacks the C atom of MeI or MeOTf, displacing the corresponding anion to give, depending on how compound 1 approaches MeI or MeOTf, 2(+) or a less stable isomeric species 2(+) that is easily transformed into 2(+). A subsequent CO migratory insertion in 2(+) leads to an unsaturated (pentacoordinated) acetyl intermediate that easily adds the iodide anion to end in 3. DFT calculations have also shown that the reaction of 1 with MeOTf to give [2]OTf is thermodynamically more favorable than that of 1 with MeI to give [2]I due to the resonance stabilization and greater solvation energy of the triflate anion. These two effects are also responsible for the fact that the incorporation of the triflate anion with 2(+) to give a putative triflate complex analogous to 3 is thermodynamically disfavored, whereas the incorporation of the iodide anion with 2(+) to give 3 is thermodynamically favored.