Migratory insertion of acetylene in N-heterocyclic carbene complexes of ruthenium: Formation of (ruthenocenylmethyl)imidazolium salts

With the parent acetylene and [RuCp(IPr)(CH3CN)(2)]PF6 (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) an unusual C-C coupling process takes place involving three acetylene molecules and migration of the NHC ligand to give the formal [2 + 2 + 1] cycloaddition product [RuCp(eta(4)-C5H5-eta(1)-CH-IPr)]PF6. This complex undergoes a facile 1,2-H shift to afford the (ruthenocenylmethyl)imidazolium salt [RuCp(eta(5)-C5H4-CH2-IPr)]PF6. The same reaction takes place with [RuCp(SIPr)(CH3CN)(2)]PF6 (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene), giving [RuCp(eta(5)-C5H4-CH2-SIPr)]PF6. A conceivable mechanism for this reaction sequence is established by means of DFT/B3LYP calculations. The key step is the facile insertion of acetylene into the Ru-C bond of the NHC ligand, requiring merely 4.1 kcal/mol with 1,3-dimethylimidazol-2-ylidene but 17.2 kcal/mol with 1,3-diphenylimidazol-2-ylidene as model NHC ligands.