Four-coordinate disilyl cobalt(Ⅱ) complexes with NHC ligation:Synthesis, characterization, and reactivity

Silyl cobalt species are putative intermediates in cobalt-catalyzed transformations of hydrosilanes.However, their reactivity has remained poorly understood. Reported here is the investigation on four-coordinate disilyl Co(Ⅱ) complexes with N-hetereocyclic carbene ligation. The reactions of[(ICy)2Co(vtms)](ICy = 1,3-dicyclohexylimidazol-2-ylidene, vtms = vinyltrimethylsilane) with primary and secondary hydrosilanes(3 equiv.) furnish the four-coordinate disilyl complexes [trans-(ICy)2Co(SiHRR’)2](SiHRR’ = SiH2Mes, 1; SiH2Ph, 2; SiH2Cy, 3; SiHPh2, 4; SiHEt2, 5) in moderate to good yields. The structures of 1, 2 and 4 were established by single-crystal X-ray diffraction. Solution magnetic susceptibility measurement and EPR spectroscopy indicate their low-spin nature(S = 1/2). Reactivity studies on 4 led to the establishment of the conversions of 4 to the disilyl dihydride Co(Ⅲ) complex[K(THF)][(ICy)2Co(H)2(SiHPh2)2]n(6) and the fluorosilyl Co(Ⅱ) complex [(ICy)2Co(THF)(SiFPh2)][BF4](7)when 4 was treated with excess amount of K and AgBF4, respectively, in THF. These conversions hint at the high activity of low-valent and high-valent disilyl cobalt species [trans-(ICy)2Co(SiHPh2)2]1-and[trans-(ICy)2Co(SiHPh2)2]2+. Complex 4 is reactive toward terminal alkynes, but inert toward alkenes and internal alkynes. The reactions of 4 with terminal alkynes CyC≡CH and Me3SiC≡CH(3 equiv.) yield the Co(Ⅱ) complexes [(ICy)2Co(C≡CCy)2](8) and [(ICy)2Co(C≡CSiMe3)((SiMe3)C=CH2)](9), respectively, along with H2SiPh2and alkynylsilanes RC≡CSiHPh2(R = Cy, SiMe3), whereas the reaction with 4-CF3C6H4C≡CH(3 equiv.) produce [(ICy)2Co(C≡CAr)((Ar)C=CH(SiHPh2)C=CHAr)](Ar = 4-CF3C6H4)(10) and H2SiPh2.These reactions are proposed to involve σ-bond metathesis reactions between alkyne C(sp)-H bonds and Co-Si bonds in 4. Complexes 6–10 have been characterized by NMR spectroscopy, X-ray diffraction study,and elemental analysis.