Computational Assessment of Counterion Effect of Borate Anions on Ethylene Polymerization by Zirconocene and Hafnocene Catalysts

The olefin polymerization performance of metallocene catalysts strongly depends on the nature of the counteranion provided by the activator system. The relative effect of two borate anions [B­(C₆F₅)₄]⁻ and [B­(C₁₀F₇)₄]⁻ (D_P^– and D_N^–, respectively) have been quantified through computational studies. The free energies for model initiation, propagation, and termination steps of ethylene polymerization catalyzed by (SBI)­MCl₂ for M = Zr and Hf have been calculated, (SBI = Me₂Si­(1-Ind)₂). Initially, the cationic catalyst active species forms an inner-sphere ion pair [SBIMMe]⁺[D_P/_N]⁻ with M---F bonding that is weak enough to be displaced by either incoming monomer or a β-C–H agostic interaction from the growing polymer chain. Calculations indicate that formation of an ion-pair complex is highly exergonic in all cases (ca. −25 kcal/mol). Formation of a monomer π-complex where ethylene is bonded to the inner coordination sphere of M is always downhill with reference to the corresponding outer-sphere isomers. Similarly, formation of the transition states for monomer insertion (TS1 and TS2) is higher for D_P^– versus D_N^–. These differences shed light on the observed performance of metallocene catalysts as a function of changing the counterion used.