Zirconium phosphinimide complexes: Synthesis, structure, and deactivation pathways in ethylene polymerization catalysis

Article abstract of DOI:10.1021/om010433q

Zirconium phosphinimide complexes of the form CpZr(NP-t-Bu₃)Cl₂ (1) and Cp*Zr(NPR₃)-Cl₂ (R = i-Pr (2), t-Bu (3)) were readily prepared under ambient conditions via the reaction of [CpZrCl₃]_n or Cp*ZrCl₃ with the appropriate trialkylphosphinimide lithium salt (R₃PNLi). A series of derivatives were readily obtained via alkylation or arylation of the above dihalide precursors. These included CpZr(NP-t-Bu₃)Me₂ (4), Cp*Zr(NPR₃)Me₂ (R = i-Pr (5), t-Bu (6)), CpZr(NP-t-Bu₃)Ph₂ (7), Cp*Zr(NPR₃)Ph₂ (R = i-Pr (8), t-Bu (9)), CpZr(NP-t-Bu₃)Bn₂ (10), CpZr(NP-t-Bu₃)(CH₂SiMe₃)₂ (11), Cp*Zr(NPR₃)(allyl)₂ (R = i-Pr (12), t-Bu (13)), Cp*Zr(NPR₃)(Cp)Cl (R = i-Pr (14), t-Bu (15)), and Cp*Zr(NPR₃)(CH₂C(CH₃)C(CH₃) CH₂) (R = i-Pr (16), t-Bu (17)). Reaction of 17 with the borane B(C₆F₅)₃ yielded the zwitterionic and cationic complexes Cp*Zr(NP-t-Bu₃)(CH₂C(CH₃)C(CH₃ )CH₂B(C₆F₅)₃) (18) and Cp*Zr(NP-t-Bu₃)(THF)-(CH₂C(CH₃)C(CH₃ )CH₂B(C₆F₅) ₃) (19). A number of the above compounds were screened for their potential as catalyst precursors in ethylene polymerization. In general, upon activation with methylaluminoxane, the resulting catalysts exhibit low activity. Efforts to understand the deactivation pathway for these zirconium catalysts involved investigating the interactions of catalyst precursors with activators. For example, reaction of 4 with the borane B(C₆F₅)₃ leads to aryl group transfer and formation of catalytically inactive CpZr(NP-t-Bu₃)(C₆F₅)₂ (20). Interactions with MAO were modeled via reaction with AlMe₃. The Zr clusters (Cp*Zr)₄(μ-Cl)₅(Cl) (μ-CH)₂ (21) and (Cp*Zr)₅(μ-Cl)₆(μ-CH)₃ (22) were two of the products that were characterized from these reactions. The isolation of 21 and 22 infers that aryl for methyl exchange, ligand abstraction, and C-H bond activation may be catalyst deactivation pathways.