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]− (DP– and DN–, 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]+[DP/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 DP– versus
DN–. These differences shed light on
the observed performance of metallocene catalysts as a function of
changing the counterion used.