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.