most favorable transition state for system 3 (TS3 away/anti) is 5.0 kcal/mol below
the reference energy point. It should be noted that when the counter-ion is taken into
account, both transition states would be destabilized because in the reference state
the anion and cation interact closely.
Due to the different bulks of the substituents, it is likely that the ion-pair energy
is, however, weaker in system 3. On the other hand, the anion does not play any
important role in stabilizing one particular conformation of the diastereomeric
cations versus the other. Therefore, enantio-face selectivity is hardly affected by
including the anion in the picture and the anion can be disregarded in the
calculations. The influence of toluene solvation, mimicking the polymerization
medium, has been estimated by calculating the solvation energies of the
gas-phase optimized geometries. The relative energies of the species involved in
the insertion process are reported in [146]. Unsurprisingly, for all the different
ligand systems, solvation effects are comparable for the four competing conformational channels. ΔΔE ranges between 0.1 and 0.2 kcal/mol. Thus, differential
solvation plays a minimal role.
From the above calculation and discussion, two conclusions can be drawn regarding
the origin and factors impacting the enantioselectivity of the syndiospecific catalyst
systems:
1. The enantioselectivity does not emanate from a particular molecular symmetry
of the catalyst structure but is tightly related to the steric arrangement of the
organic ligand that engulfs the transition metal and its direct interaction with the
growing polymer chain, which dictates its conformation and final spatial orientation (further ensured through a-H–Zr agnostic interaction). The combination of
the steric arrangement of the ligand and the particular spatial orientation of the
polymer chain (directing the coordination mode of the propylene) creates an
overall repulsive steric force that “blocks” a large space in and around the
coordination sphere of the transition metal center. The remaining “free” and
accessible space forms a shape-selective “chiral pocket” that can only accept
monomers of a certain shape, i.e., unique π-face that fits in that pocket easily
without much repulsive interaction.
2. Any new substituent(s) at proper position(s) that could enhance the shapeselectivity of the chiral pocket via enforcing and stabilizing the spatial orientation
of the growing polymer chain, regardless of its impact on the symmetry, would
enhance the enantioselectivity of the final catalyst, be it of C s or C 1 symmetry
(see below). The substitution(s) at position 3(6) or 3,6 of the fluorenyl group, just
beneath the growing polymer chain, provide exactly that enhancing effect.
4.3 Importance of the Frontal Substituents
To emphasize the importance of fluorenyl’s frontal substituents at positions 3 and
6 and to confirm the findings of the computational calculations presented in
Sect. 3.2, two new syndioselective catalysts systems are introduced whose ligands
76
A. Razavi
the reference energy point. It should be noted that when the counter-ion is taken into
account, both transition states would be destabilized because in the reference state
the anion and cation interact closely.
Due to the different bulks of the substituents, it is likely that the ion-pair energy
is, however, weaker in system 3. On the other hand, the anion does not play any
important role in stabilizing one particular conformation of the diastereomeric
cations versus the other. Therefore, enantio-face selectivity is hardly affected by
including the anion in the picture and the anion can be disregarded in the
calculations. The influence of toluene solvation, mimicking the polymerization
medium, has been estimated by calculating the solvation energies of the
gas-phase optimized geometries. The relative energies of the species involved in
the insertion process are reported in [146]. Unsurprisingly, for all the different
ligand systems, solvation effects are comparable for the four competing conformational channels. ΔΔE ranges between 0.1 and 0.2 kcal/mol. Thus, differential
solvation plays a minimal role.
From the above calculation and discussion, two conclusions can be drawn regarding
the origin and factors impacting the enantioselectivity of the syndiospecific catalyst
systems:
1. The enantioselectivity does not emanate from a particular molecular symmetry
of the catalyst structure but is tightly related to the steric arrangement of the
organic ligand that engulfs the transition metal and its direct interaction with the
growing polymer chain, which dictates its conformation and final spatial orientation (further ensured through a-H–Zr agnostic interaction). The combination of
the steric arrangement of the ligand and the particular spatial orientation of the
polymer chain (directing the coordination mode of the propylene) creates an
overall repulsive steric force that “blocks” a large space in and around the
coordination sphere of the transition metal center. The remaining “free” and
accessible space forms a shape-selective “chiral pocket” that can only accept
monomers of a certain shape, i.e., unique π-face that fits in that pocket easily
without much repulsive interaction.
2. Any new substituent(s) at proper position(s) that could enhance the shapeselectivity of the chiral pocket via enforcing and stabilizing the spatial orientation
of the growing polymer chain, regardless of its impact on the symmetry, would
enhance the enantioselectivity of the final catalyst, be it of C s or C 1 symmetry
(see below). The substitution(s) at position 3(6) or 3,6 of the fluorenyl group, just
beneath the growing polymer chain, provide exactly that enhancing effect.
4.3 Importance of the Frontal Substituents
To emphasize the importance of fluorenyl’s frontal substituents at positions 3 and
6 and to confirm the findings of the computational calculations presented in
Sect. 3.2, two new syndioselective catalysts systems are introduced whose ligands
76
A. Razavi