As can be seen from Table 14, for all three systems, in general, the effect of
solvation is to stabilize the transition state species involved, relative to the
reactants, i.e., the L 2 Zr-iBu
+ MeB(C 6 F 5 ) 3
À close-contact ion pair plus propylene.
To estimate the importance of the anion’s charge distribution in the differential
solvation of the species involved, the site epimerization of system 2 in the presence
of three different anions was investigated. In addition to the already mentioned
MeB(C 6 F 5 ) 3
À , the site epimerization was studied with an anion developed by
Marks, the fluoro-tris-perfluoro-biphenylaluminate, FAl(Bif f ) 3
À anion [168, 169],
and also with a model representing methylated MAO, MAO-Me
À [171, 172]. The
reaction profiles in the gas phase and in toluene (marked COSMO) are shown in
Fig. 22. The energies of the species are reported in Table 15. Marks’ anion is much
bulkier than MeB(C 6 F 5 ) 3
À and its negative charge is more concentrated on the
fluorine, which bridges with the zirconocenium cation. The model for the
methylated MAO also has the negative charge more concentrated in the part that
interacts with the cation, compared to MeB(C 6 F 5 ) 3
À , while its steric bulk is lower
Table 14 Dependence of the relative energies of the species involved in the site epimerization on
the nature of the ligand system
Catalyst
Gas phase
Solution (toluene)
β-Agostic product
TS SE
Ion pair
β-Agostic product
TS SE
Ion pair
1
À5.3
À2.0
À23.9
À9.9
À5.9
À25.6
2
À4.3
À1.7
À23.0
À9.9
À6.5
À25.5
3
À5.9
À0.6
À22.9
À11.3
À4.8
À20.9
The counter-anion is MeB(C 6 F 5 ) 3
À
. All energies are relative to the respective parent close-contact
ion-pair precursor and propylene. Relative energies in kcal/mol
Fig. 21 Ion pairing in the regular polymerization mechanism in system 2, after the diffusioncontrolled anion reorganization. Plot shows relative energy (in kcal/mol) versus anion–Zr distance
(in A ˚ )
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