Unusual Reactivity of Tris(pyrazolyl)borate Zirconium Benzyl Complexes

The synthesis and reactivity of [Tp*Zr(CH₂Ph)₂][B(C₆F₅)₄] (2, Tp* = HB(3,5-Me₂pz)₃, pz = pyrazolyl) have been explored to probe the possible role of Tp‘MR₂⁺ species in group 4 metal Tp‘MCl₃/MAO olefin polymerization catalysts (Tp‘ = generic tris(pyrazolyl)borate). The reaction of Tp*Zr(CH₂Ph)₃ (1) with [Ph₃C][B(C₆F₅)₄] in CD₂Cl₂ at −60 °C yields 2. 2 rearranges rapidly to [{(PhCH₂)(H)B(μ-Me₂pz)₂}Zr(η²-Me₂pz)(CH₂Ph)][B(C₆F₅)₄] (3) at 0 °C. Both 2 and 3 are highly active for ethylene polymerization and alkyne insertion. Reaction of 2 with excess 2-butyne yields the double insertion product [Tp*Zr(CH₂Ph)(CMeCMeCMeCMeCH₂Ph)][B(C₆F₅)₄] (4). Reaction of 3 with excess 2-butyne yields [{(PhCH₂)(H)B(μ-Me₂pz)₂}Zr(Cp*)(η²-Me₂pz)][B(C₆F₅)₄] (6, Cp* = C₅Me₅) via three successive 2-butyne insertions, intramolecular insertion, chain walking, and β-Cp* elimination.