Si−H Bond Activation of Alkynylsilanes by Group 4 Metallocene Complexes

The reactivity of variously substituted alkynylsilanes toward selected group 4 metallocene complexes was investigated. Reactions of the alkynylsilanes R¹C₂SiR²₂H (R¹ = SiMe₃, R² = Me, 1; R¹ = SiMe₃, R² = Ph, 2; R¹ = SiMe₂H, R² = Me, 5) with Cp₂TiMe₂ (Cp = η⁵-cyclopentadienyl) resulted, upon methyl group transfer to the silyl group, in the previously described titanocene alkyne complexes Cp₂Ti(R¹C₂SiR²₂R³) (R¹ = Me₃Si, R² = R³ = Me, 3; R¹ = HMe₂Si, R² = R³ = Me, 6) or the unreported complex 4 (R¹ = Me₃Si, R² = Ph, R³ = Me). The Cp₂TiCl₂/n-BuLi system yielded alkyne complexes 6 and 7 (R¹ = HMe₂Si, R² = Me, R³ = H); no alkyl group transfer was detected. On the other hand, reactions utilizing the Cp₂ZrCl₂/n-BuLi system afforded inseparable mixtures; however, complexes of the type Cp₂Zr[R¹C₂SiMe₂(n-Bu)] (R¹ = Me₃Si, 8; R¹ = HMe₂Si, 9) were detected. Cp₂Hf(n-Bu)₂ reacted with the alkynylsilanes in a diverse way, depending on the substituents of the alkyne substrate. The reaction with an excess of alkyne 1 (R¹ = Me₃Si, R² = Me) afforded only an intractable mixture, which contained Me₃SiC₂SiMe₂(n-Bu) (10). Hafnacyclopentadienes 13−15 as precedented product types were obtained when alkyne 12 (R¹ = Ph, R² = Me) was used. In sharp contrast, the symmetrically substituted alkynes 5 (R¹ = HMe₂Si, R² = Me) and H₂PhSiC₂SiPhH₂ (18) yielded the hitherto unknown Si-containing metallacycles 16 and 19. A reaction mechanism leading to these products was proposed and subsequently supported by DFT calculations. In addition, the reduction of Cp₂HfCl₂ with magnesium in THF in the presence of alkynylsilanes was shown to be an alternative route to compounds 14−16 and 19. Presumably due to steric reasons, alkyne 1 could not form any of the product types described above. Nevertheless, it was utilized for the preparation of the PMe₃-stabilized hafnocene alkyne complex 11.