Low-Coordinate NHC-Cobalt(0)-Olefin Complexes: Synthesis, Structure, and Their Reactions with Hydrosilanes

The one-pot reaction of CoCl₂ with 1 or 2 equiv of N-heterocyclic carbene (NHC) ligands, vinyltrimethylsilane (vtms), and 2 equiv of reducing reagents (KC₈ or Na(Hg)) proved an effective protocol for the synthesis of three-coordinate NHC-cobalt(0)-olefin complexes, and by which the new low-coordinate cobalt(0) complexes [(NHC)Co(vtms)₂] (NHC = 1,3-di(2',6'-diethylphenyl)-4,5-(CH₂)₄-imidazol-2-ylidene (cyIDep), 6; 1,3-diadamantylimidazol-2-ylidene (IAd), 7) and [(NHC)₂Co(vtms)] (NHC = 1-(2',4',6'-trimethylphenyl)-3-cyclohexylimidazol-2-ylidene (IMesCy), 8; 1,3-dicyclohexylimidazol-2-ylidene (ICy), 9) were prepared in moderate yields and further characterized by spectroscopic methods. The NHC-cobalt(0)-vtms complexes can serve as low-coordinate NHC-cobalt(0) precursors to react with hydrosilanes. From the reactions of Ph₂SiH₂ with the corresponding NHC-cobalt(0)-vtms complexes, the cobalt silyl complexes [(cyIDep)(H)Co(μ-η²:η²-H₂SiPh₂)(μ-η²-HSiPh₂)₂Co(H)(cyIDep)] (10), [(NHC)Co(μ-η²-SiHPh₂)₂Co(NHC)] (NHC = IPr, 13; IAd, 14), and [(IMesCy)₂Co(H)(SiHPh₂)] (15) were synthesized. Further reactivity studies established the thermal decomposition of 10 to give [(cyIDep)Co(μ-η²-HSiPh₂)₂Co(cyIDep)] (11), H₂, and Ph₂SiH₂, the reaction of 11 with Bu^tNC (4.5 equiv) to afford [(cyIDep)(Bu^tNC)Co(μ-η²-HSiPh₂)₂Co(CNBu^t)₃] (12) and cyIDep, as well as the conversion of 15 to a silyl-functionalized NHC-cobalt(II) complex 16 upon eliminating H₂. These cobalt silyl complexes have been characterized by single-crystal X-ray diffraction studies, NMR, absorption, and infrared spectroscopies, as well as elemental analyses. The diversified reaction outcomes demonstrate the rich reaction chemistry of low-coordinate NHC-cobalt(0) complexes.