Zirconocenes as models for homogeneous Ziegler-Natta olefin polymerization catalysts
<p>Using density functional theory, we studied the fundamental steps of olefin polymerization for zwitterionic and cationic Group IV ansa-zirconocenes and a neutral ansa- yttrocene. Complexes [H<sub>2</sub>E(C<sub>5</sub>H<sub>4</sub>)<sub>2</s...
| Summary: | <p>Using density functional theory, we studied the fundamental steps of olefin
polymerization for zwitterionic and cationic Group IV ansa-zirconocenes and a neutral ansa-
yttrocene. Complexes [H<sub>2</sub>E(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe]<sup>n</sup> (n = 0: E = BH<sub>2</sub> (1), BF<sub>2</sub> (2), AlH<sub>2</sub>(3); n = +: E = CH<sub>2</sub>(4), SiH<sub>2</sub>(5)) and
H<sub>2</sub>Si(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>YMe were used as computational models. The largest
differences among these three classes of compounds were the strength of olefin binding and the
stability of the β-agostic alkyl intermediate towards β-hydrogen elimination. We investigated
the effect of solvent on the reaction energetics for land 5. We found that in benzene the
energetics became very similar except that a higher olefin insertion barrier was
calculated for 1. The calculated anion affinity of [CH<sub>3</sub>BF<sub>3</sub>]<sup>-</sup> was weaker towards 1 than 5. The
calculated olefin binding depended primarily on the charge of the ansa linker, and the olefin
insertion barrier was found to decrease steadily in the following order: [H<sub>2</sub>C(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe]<sup>+</sup> > [F<sub>2</sub>B(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe] ≈ [H<sub>2</sub>B(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe] > [H<sub>2</sub>Si(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe]<sup>+</sup> >
[H<sub>2</sub>Al(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe].</p>
<p>We prepared ansa-zirconocene dicarbonyl complexes Me<sub>2</sub>ECp<sub>2</sub>Zr(CO)<sub>2</sub> (E = Si, C), and
t-butyl substituted complexes (t-BuCp)<sub>2</sub>Zr(CO)<sub>2</sub>, Me<sub>2</sub>E(t-BuCp)<sub>2</sub>Zr(CO)<sub>2</sub> (E = Si, C),
(Me<sub>2</sub>Si)<sub>2</sub>(t-BuCp)<sub>2</sub>Zr(CO)<sub>2</sub> as well as analogous zirconocene complexes. Both the reduction
potentials and carbonyl stretching frequencies follow the same order: Me<sub>2</sub>SiCp<sub>2</sub>ZrCl<sub>2</sub>>
Me<sub>2</sub>CCp<sub>2</sub>ZrCl<sub>2</sub>> Cp<sub>2</sub>ZrCl<sub>2</sub>> (Me<sub>2</sub>Si)<sub>2</sub>Cp<sub>2</sub>ZrCl<sub>2</sub>. This ordering is a result of both the donating
abilities of the cyclopentadienyl substituents and the orientation of the cyclopentadiene rings.
Additionally, we prepared a series of analogous cationic zirconocene complexes
[LZrOCMe<sub>3</sub>][MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] (L = CP<sub>2</sub>, Me<sub>2</sub>SiCp<sub>2</sub>, Me<sub>2</sub>CCP<sub>2</sub>, (Me<sub>2</sub>Si)<sub>2</sub>Cp<sub>2</sub>) and studied the kinetics of anion dissociation. We found that the enthalpy of anion dissociation increased from 10.3 kcal•mol<sup>-1</sup> to 17.6 kcal•mol<sup>-1</sup> as exposure of the zirconium center increased.</p>
<p>We also prepared series of zirconocene complexes bearing 2,2-dimethyl-2-sila-4-pentenyl substituents (and methyl-substituted olefin variants). Methide abstraction with B(C<sub>6</sub>F<sub>5</sub>) results in reversible coordination of the tethered olefin to the cationic zirconium center. The kinetics of olefin dissociation have been examined using NMR methods, and the effects of ligand variation for unlinked, singly [SiMe<sub>2</sub>]-linked and doubly [SiMe<sub>2</sub>]-linked bis(cyclopentadienyl) arrangements has been compared (ΔG‡ for olefin dissociation varies from 12.8 to 15.6 kcal•mol<sup>-1</sup>). Methide abstraction from 1,2-(SiMe<sub>2</sub>)<sub>2</sub>(η<sup>5</sup>-C<sub>5</sub>H<sub>3</sub>)<sub>2</sub>Zr(CH<sub>3</sub>)-(CH<sub>2</sub>CMe<sub>2</sub>CH<sub>2</sub>CH = CH<sub>2</sub>) results in rapid β-allyl elimination with loss of isobutene yielding the allyl cation [{1,2-(SiMe<sub>2</sub>)<sub>2</sub>(η<sup>5</sup>-C<sub>5</sub>H<sub>3</sub>)<sub>2</sub>Zr(η<sup>3</sup>-CH<sub>2</sub>CH=CH<sub>2</sub>)]<sup>+</sup>.</p>
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