Intramolecular Ion−Ion Interactions in Zwitterionic Metallocene Olefin Polymerization Catalysts Derived from “Tucked-In” Catalyst Precursors and the Highly Electrophilic Boranes XB(C₆F₅)₂ (X = H, C₆F₅)

The reactions of so called “tuck-in” permethyl zirconocene compounds Cp*(η⁵-η¹-C₅Me₄CH₂)ZrX (X = Cl (1a), C₆H₅ (1b), CH₃ (1c)) with the highly electrophilic boranes HB(C₆F₅)₂ and B(C₆F₅)₃ are described. The products are zwitterionic olefin polymerization catalysts. Reactions with 1a and 1b yielded single products cleanly, but reactions with tuck-in methyl starting material 1c gave mixtures. Spectroscopic and structural studies showed that the electrophilic zirconium center in the product zwitterions was stabilized by a variety of mechanisms. In the products of reaction between 1a and 1b with HB(C₆F₅)₂, Cp*[η⁵,η¹-C₅Me₄CH₂B(C₆F₅)₂(μ-H)]ZrX (X = Cl (2a), 74%), C₆H₅ (2b, 62%)), the metal is chelated by a pendant hydridoborate moiety. Chloride product 2a was characterized crystallographically. In the reaction of B(C₆F₅)₃ with 1a, the fluxional zwitterionic product Cp*[η⁵-C₅Me₄CH₂B(C₆F₅)₃]ZrCl (3a, 84%) is stabilized by a weak donor interaction between one of the ortho fluorine atoms of the −CH₂B^-(C₆F₅)₃ counterion and the zirconium center (Zr−F = 2.267(5) Å). In the product of the reaction between 1b and B(C₆F₅)₃, Cp*[η⁵-C₅Me₄CH₂B(C₆F₅)₃]ZrC₆H₅ (3b, 82%), a similar ortho-fluorine interaction was found in a yellow kinetic product (y-3b), which converted upon heating gently to a thermodynamic orange polymorph (o-3b) in which the zirconium center is compensated via an agostic interaction from an ortho C−H bond of the phenyl group and an interaction between the methylene group of the −CH₂B^-(C₆F₅)₃ counteranion. These compounds were both characterized by X-ray crystallography. Zwitterion o-3b reacts with H₂ to form the zwitterionic hydride Cp*[η⁵-C₅Me₄CH₂B(C₆F₅)₃]ZrH (4, 77%), characterized by NMR spectroscopy and X-ray crystallography to reveal a return to the ortho-fluorine mode of stabilization. Compounds 2a, 3a, o-3b, and 4 were all found to be active ethlyene polymerization catalysts; the chloride derivatives required minimal amounts of methylaluminoxane (MAO) to alkylate the zirconium center. Polymerization data are discussed in light of the structural findings for the catalysts employed.