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(η⁴-C₅H₅-η¹-CH-IPr)]PF₆. This complex undergoes a facile 1,2-H shift to afford the (ruthenocenylmethyl)imidazolium salt [RuCp(η⁵-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(η⁵-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.