Synthesis and Reactivity of a Sterically Crowded Tris(pyrazolyl)borate Hafnium Benzyl Complex

The reaction of Hf(CH₂Ph)₄ and Tl[Tp^Ms*] (Tp^Ms* = HB(3-mesitylpyrazolyl)₂(5-mesitylpyrazolyl)⁻) yields Tp^Ms*Hf(CH₂Ph)₃ (1), bibenzyl, and Tl⁰ via ligand exchange followed by thermal decomposition of Tl[CH₂Ph]. Compound 1 reacts with [H(OEt₂)₂][B(C₆F₅)₄] to form Tp^Ms*Hf(CH₂Ph)₂(OEt₂)⁺ (2, 100%) and with [Ph₃C][B(C₆F₅)₄] or B(C₆F₅)₃ to form an unstable species characterized by NMR as Tp^Ms*Hf(CH₂Ph)₂⁺. 1/[Ph₃C][B(C₆F₅)₄] behaves similarly to Tp^Ms*HfCl₃/MAO in olefin polymerization, producing high molecular weight, linear PE, and ethylene/1-hexene copolymer with good hexene incorporation. These results are consistent with the proposal that similar cationic Tp^Ms*Hf(X)R⁺ species (X = Cl or R) are active species in these catalysts. Ethylene polymerization by 1/[Ph₃C][B(C₆F₅)₄] at low temperature proceeds without significant chain transfer, enabling the synthesis of bromo benzyl double-end-capped linear PE (PhCH₂(CH₂CH₂)_nBr).