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.