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.