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{κ2C2-MeIm­(CH2)3ImMe}­(CO)3] (1), MeIm­(CH2)3ImMe = 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{κ2C2-MeIm­(CH2)3ImMe}­(CO)3]­OTf ([2]­OTf), whereas an analogous reaction of 1 with
MeI renders the neutral ruthenium­(II) acetyl derivative [RuI­{C­(O)­Me}­{κ2C2-MeIm­(CH2)3ImMe}­(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 SN2 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.