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

Abstract

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(SiHRRʹ)2] (SiHRRʹ = 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 CyCCH and Me3SiCCH (3 equiv.) yield the Co(II) complexes [(ICy)2Co(CCCy)2] (8) and [(ICy)2Co(CCSiMe3)((SiMe3)CCH2)] (9), respectively, along with H2SiPh2 and alkynylsilanes RCCSiHPh2 (R = Cy, SiMe3), whereas the reaction with 4-CF3C6H4CCH (3 equiv.) produce [(ICy)2Co(CCAr)((Ar)CCH(SiHPh2)CCHAr)] (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.