Formation and Structures of Hafnocene Complexes in MAO- and AlBuⁱ₃/CPh₃[B(C₆F₅)₄]-Activated Systems

The formation of cationic species relevant to olefin polymerization based on (SBI)HfCl₂, Me₂C(C₅H₄)(Flu)HfCl₂, Ph₂C(C₅H₄)(Flu)HfCl₂, and L′HfCl₂ activated by MAO, AlMe₃/CPh₃[B(C₆F₅)₄], and AlBuⁱ₃/CPh₃[B(C₆F₅)₄] (SBI = rac-Me₂Si(Ind)₂; L′ = C₂H₄(Flu)(5,6-C₃H₆-2-MeInd)) was studied by ¹H, ¹³C, and ¹⁹F NMR spectroscopy. Thermally stable heterobinuclear intermediates of the type [LHf(μ-Me)₂AlMe₂]⁺[MeMAO]⁻ and [LHf(μ-Me)₂AlMe₂]⁺[B(C₆F₅)₄]⁻ were identified when using MAO and AlMe₃/CPh₃[B(C₆F₅)₄] as activators, respectively. The stability of these species explains the low productivity of hafnocene catalysts in the presence of AlMe₃-containing activators, compared to zirconocenes. By contrast, in the ternary systems LHfCl₂/AlBuⁱ₃/CPh₃[B(C₆F₅)₄] hydride species were detected that must be responsible for the formation of the highly active sites in olefin polymerization. The ionic hydrido species differ significantly in stability. The formation of the mixed-alkyl complex L′Hf(Me)CH₂SiMe₃ proceeds with surprisingly high diastereoselectivity; the sterically more hindered isomer is produced preferentially. It reacts with CPh₃[B(C₆F₅)₄] to afford the ion pair [L′Hf-CH₂SiMe₃]⁺[B(C₆F₅)₄]⁻ as two diastereomers that exist in dynamic equilibrium. The rates of site epimerization of this ion pair indicate only small energy differences between the two isomers.