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.

以母体乙炔与[RuCp(IPr)(CH₃CN)₂]PF₆（其中IPr=1,3-双(2,6-二异丙基苯基)咪唑-2-亚基）为反应物时，会发生罕见的涉及三分子乙炔的碳-碳偶联过程，并伴随氮杂环卡宾（N-Heterocyclic Carbene，NHC）配体的迁移，最终生成形式上符合[2+2+1]环加成的产物[RuCp(η⁴-C₅H₅-η¹-CH-IPr)]PF₆。该配合物可发生快速的1,2-氢迁移反应，得到(二茂钌基甲基)咪唑鎓盐[RuCp(η⁵-C₅H₄-CH₂-IPr)]PF₆。以[RuCp(SIPr)(CH₃CN)₂]PF₆（其中SIPr=1,3-双(2,6-二异丙基苯基)-4,5-二氢咪唑-2-亚基）为反应物时可发生相同反应，得到产物[RuCp(η⁵-C₅H₄-CH₂-SIPr)]PF₆。通过密度泛函理论（DFT）/B3LYP计算，阐明了该反应序列的可行机理。该反应的关键步骤为乙炔向NHC配体的Ru-C键发生易于进行的插入反应：以1,3-二甲基咪唑-2-亚基作为模型NHC配体时，该步骤的活化能仅为4.1 kcal/mol；而以1,3-二苯基咪唑-2-亚基作为模型配体时，活化能则高达17.2 kcal/mol。