Chemistry of new electrophilic metal ALKYL compounds

The reaction of the neutral carborane C₂B₉H₁₃ with Cp*M(CH₃)₃ (M = Zr (a), Hf (b); Cp* = η⁵-C₅Me₅) yields Cp(C₂B₉H₁₁)M(CH₃)]_n (3a, b). Complexes 3a, b form THF adducts Cp*(C₂B₉H₁₁)M(CH₃)(THF) 4, insert 2-butyne to yield Cp*(C₂B₉H₁₁)M{C(Me=CMe₂} 5, and undergo methane elimination upon thermolysis to yield methylene-bridged complexes Cp*(C₂B₉H₁₁)M]₂(μ-CH₂) (6). These chemical studies, and companion structural and theoretical studies establish that 3a, b are neutral analogues of the cationic Cp₂M(R)⁺ species (1; Cp = η⁵-C₅H₅) and Cp₂M(R)(L)⁺ (2) which are believed to be active in Cp₂MX₂-based Ziegler catalysts. Despite the lower metal charge, 3–6 exhibit characteristic “electrophilic metal alkyl” properties including agostic M-H-C and M-H-B interactions, high insertion and intramolecular C-H activation reactivity, and high ethylene polymerization and propene oligomerization activity. These observations suggest that the key requirement for high insertion/polymerization activity in metallocene systems is high metal unsaturation (i.e. two empty metal-centered orbitals) rather than charge.