Intramolecular Ion−Ion Interactions in Zwitterionic Metallocene Olefin Polymerization Catalysts Derived from “Tucked-In” Catalyst Precursors and the Highly Electrophilic Boranes XB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> (X = H, C<sub>6</sub>F<sub>5</sub>)

The reactions of so called “tuck-in” permethyl zirconocene compounds Cp*(η<sup>5</sup>-η<sup>1</sup>-C<sub>5</sub>Me<sub>4</sub>CH<sub>2</sub>)ZrX (X = Cl (<b>1a</b>), C<sub>6</sub>H<sub>5</sub> (<b>1b</b>), CH<sub>3</sub> (<b>1c</b>)) with the highly electrophilic boranes HB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> are described. The products are zwitterionic olefin polymerization catalysts. Reactions with <b>1a</b> and <b>1b</b> yielded single products cleanly, but reactions with tuck-in methyl starting material <b>1c</b> 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 <b>1a</b> and <b>1b</b> with HB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>, Cp*[η<sup>5</sup>,η<sup>1</sup>-C<sub>5</sub>Me<sub>4</sub>CH<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>(μ-H)]ZrX (X = Cl (<b>2a</b>), 74%), C<sub>6</sub>H<sub>5</sub> (<b>2b</b>, 62%)), the metal is chelated by a pendant hydridoborate moiety. Chloride product <b>2a</b> was characterized crystallographically. In the reaction of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> with <b>1a</b>, the fluxional zwitterionic product Cp*[η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>CH<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]ZrCl (<b>3a</b>, 84%) is stabilized by a weak donor interaction between one of the <i>ortho</i> fluorine atoms of the −CH<sub>2</sub>B<sup>-</sup>(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> counterion and the zirconium center (Zr−F = 2.267(5) Å). In the product of the reaction between <b>1b</b> and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, Cp*[η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>CH<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]ZrC<sub>6</sub>H<sub>5</sub> (<b>3b</b>, 82%), a similar <i>ortho</i>-fluorine interaction was found in a yellow kinetic product (<b>y-3b</b>), which converted upon heating gently to a thermodynamic orange polymorph (<b>o-3b</b>) in which the zirconium center is compensated via an agostic interaction from an <i>ortho</i> C−H bond of the phenyl group and an interaction between the methylene group of the −CH<sub>2</sub>B<sup>-</sup>(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> counteranion. These compounds were both characterized by X-ray crystallography. Zwitterion <b>o-3b</b> reacts with H<sub>2</sub> to form the zwitterionic hydride Cp*[η<sup>5</sup>-C<sub>5</sub>Me<sub>4</sub>CH<sub>2</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]ZrH (<b>4</b>, 77%), characterized by NMR spectroscopy and X-ray crystallography to reveal a return to the <i>ortho</i>-fluorine mode of stabilization. Compounds <b>2a</b>, <b>3a</b>, <b>o-3b</b>, and <b>4</b> 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.