Synthesis and Reactivity of η⁵-Silolyl, η⁵-Germolyl, and η⁵-Germole Dianion Complexes of Zirconium and Hafnium

Reaction of 2 equiv of Li[C₄Me₄GeSiMe₃] with Cp*HfMe₂Cl produced the first transition metal complex of a germole dianion, [Cp*(η⁵-C₄Me₄Ge)HfMe₂Li(THF)]₂ (1), via the apparent elimination of Me₃SiCl, along with C₄Me₄Ge(SiMe₃)₂ as the final Me₃Si-containing product. Compound 1 adopts a dimeric structure in which one Li atom is sandwiched in an η⁵-fashion between two germole rings, while the other Li atom is coordinated by both germanium atoms. Reaction of 1 with an excess of Me₃SiCl resulted in loss of the germole ligand as C₄Me₄Ge(SiMe₃)₂, while 2 equiv of Me₃SiOSO₂CF₃ reacted with 1 to give the new germolyl complex Cp*[η⁵-C₄Me₄GeSiMe₃]HfMe₂ (2). Yet a different process results from treatment of 1 with CH₃CH₂OSO₂CF₃, involving migration of a methyl group from hafnium to germanium to produce Cp*(η⁴-C₄Me₄GeMeEt)HfMe (3). Reaction of 2 with H₂ gave CH₄ and Me₃SiH as the result of σ-bond metathesis involving the germole-bound trimethylsilyl group and (presumably) an intermediate hafnium hydride species. Similarly, the reaction of 2 with PhSiH₃ gave PhMeSiH₂ and Me₃SiH. Compound 2 also reacted with MeI to produce C₄Me₄Ge(Me)SiMe₃, while the reaction with (Et₂O)LiCH₂Ph gave 1 and Me₃SiCH₂Ph. Compound 2 did not react cleanly with various small molecules (CO, CN(2,6-Me₂C₆H₃), trimethylsilylacetylene, and benzophenone), nor with the methide abstraction reagents B(C₆F₅)₃ and [Ph₃C][B(C₆F₅)₄]. In addition, reaction of 2 with these abstraction reagents in the presence of 1-hexene or cyclohexene did not result in the formation of a polymer. The germole C₄Me₄Ge(H)CMe₃ was prepared via reaction of C₄Me₄GeCl₂ with 1.5 equiv of Me₃CLi, followed by treatment with LiAlH₄. This germole was cleanly deprotonated by ⁿBuLi in THF to give the new germole anion Li[C₄Me₄GeCMe₃] as a THF solvate. This anion reacted with Cp*HfMe₂Cl to give the product of methyl migration from hafnium to germanium, Cp*[η⁴-C₄Me₄Ge(Me)CMe₃]HfMe (4). Analogously, Li[C₄Me₄GePh] reacted with Cp*HfMe₂Cl to give Cp^*[η⁴-C₄Me₄Ge(Me)Ph]HfMe (5). Treatment of MgBr₂(Et₂O) with 2 equiv of K[C₄Me₄SiSiMe₃] in THF resulted in formation of Mg[η¹-C₄Me₄SiSiMe₃]₂(THF) (6). Reaction of 6 with Cp*ZrCl₃ gave quantitative formation of Cp*[η⁵-C₄Me₄SiSiMe₃]ZrCl₂ (7), while the reaction of 6 with Cp*HfCl₃ provided the previously reported complex Cp*[η⁵-C₄Me₄SiSiMe₃]HfCl₂ (8) in quantitative yield.