Counteranion Effects on the Zirconocene Polymerization Catalyst

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Counteranion Effects on the Zirconocene Polymerization Catalyst Olefin Complex from QM/MM Molecular Dynamics Simulations Christopher N. Rowley† and Tom K. Woo* Centre for Catalysis Research and Innovation, Department of Chemistry, D’Iorio Hall, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

bS Supporting Information ABSTRACT: We have used QM/MM molecular dynamics simulations of the olefin-bound borate ion pair of the mainstay zirconocene catalyst ([Cp2Zr(C2H5)(C2H4)]þ [CH3B(C6F5)3]
) to examine the effects of the counteranion on the active catalyst. Free energy perturbation/molecular dynamics calculations show that the R-agostic configuration is stabilized by 1 kcal mol
1 when the counteranion is present, which we attribute to the larger counteranion
dipole interaction of the more polar R-agostic structure.

S

ingle-site group IV metallocene catalysts are of major importance in olefin polymerization chemistry. These catalysts have demonstrated excellent catalytic activity1 and have allowed exceptional control of polymer microstructure.2 While the metallocene is the species directly involved in the polymerization process, a cocatalyst plays an essential role in the catalytic cycle. The active, cationic form of the metallocene is formed through a reaction with a cocatalyst, such as tris(pentafluorophenyl)borane (B(C6F5)3), which abstracts an anionic ligand from the neutral precatalyst to activate the catalyst (Figure 1a). After activation, the olefin substrate can coordinate to the metal to form a metal
olefin complex (Figure 1b). From this complex, the olefin can undergo insertion into the σ(M
C) bond, extending the length of the polymer chain by one unit (Figure 1c). Alternatively, the growing alkyl chain can be terminated through β-hydrogen transfer to the coordinated olefin (Figure 1d). It had originally been assumed that the cationic active catalyst and the anionic cocatalyst dissociate after initiation; however, more recently it has been demonstrated that these species remain in close proximity in the nonpolar solvents most commonly used for these catalysts.3,4 This ion pairing has been demonstrated to affect polymerization kinetics5,6 and stereochemistry,7,8 which has sparked interest in better understanding counteranion effects on the catalytic cycle. Of particular interest is the report by Marks and co-workers that using bulky, more weakly interacting cocatalysts with the precatalyst Me2C(Cp)-(Flu)ZrMe2 led to higher polymer molecular weights.9 A potential explanation for this is that the counteranion biases the barriers of the competing insertion and β-hydrogen transfer mechanisms to favor propagation over termination (Figure 1c,d, respectively). While there has been some progress toward characterizing ion pairs, a comprehensive description is still lacking. Single-crystal X-ray structures of metallocene
methyltris(pentafluorophenyl) borate ion pairs are generally inner-sphere pairs, where the r 2011 American Chemical Society

Figure 1. Catalyst activation and polymerization reaction mechanisms for the zirconocene olefin polymerization catalyst (A = cocatalyst).

methyl ligand of the borate bridges with the cation.10 Brintzinger and co-workers reported a solution NMR study of alkylzirconocene
borate ion pairs.11 Although the unsaturated metallocene formed a methyl-bridged complex with the counteranion, Lewis bases such as THF displaced the counteranion from the metal so that the counteranion is in the outer coordination sphere. In these outer-sphere ion pairs, the counteranion has only noncovalent interactions with the cation, allowing a freer range of orientation. Subsequent solution NMR studies by Marks el al. on the ion pair [Me2SiCp2Zr(Me)THF]þ [CH3B(C6F5)3]
concluded that the counteranion was preferentially oriented adjacent to the neutral THF ligand with the B
CH3 bond axis directed away from the cation.4,12 Although they are central to the polymerization activity of these catalysts, olefin-coordinated ion pairs have not been observed experimentally, as olefins coordinate to the metal relatively weakly. Received: December 20, 2010 Published: March 22, 2011 2071

dx.doi.org/10.1021/om101188t | Organometallics 2011, 30, 2071–2074

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Figure 3. Zirconocene
borate ion pair in an R-agostic configuration (1A-IP). Figure 2. Structural parameters characterizing the isomerization from 1B-IP to 1A-IP during a 50 ps MD simulation. The Zr
CR
Cβ and average Zr
CR
HR angles are plotted on the primary axis, while the Zr
Hag distance is plotted on the secondary axis.

Because of this, we set out to characterize a representative olefin-coordinated metallocene
borate pair using computer modeling. Quantum chemistry has been used extensively to study the structure and reactivity of olefin
metallocene complexes, although most of these studies have modeled the catalyst as the bare cation, without its counteranion.13 More recently, there have been several computational studies that have included a counteranion in the model.14,15 The conventional “static” DFT approach of examining only potential energy optimized structures has limitations for modeling ion pairs in solution, as NMR studies indicate that there is significant configurational freedom in the ion pair orientation and that this orientation can readily adjust to changes in the solvent or the metal coordination sphere.6,18,19 Molecular dynamics (MD) simulations provide an obvious strategy for modeling the solution structures of these fluxional ion pairs. Significantly, MD allows free energy profiles of complex processes to be calculated without resorting to the simplistic harmonic oscillator approximation. Cavallo and co-workers have reported MD simulations of metallocene ion pairs,16 although the MM force fields used to represent the metallocene cannot model changes in the electronic structure induced by the counteranion. QM/MM MD simulations, where the metallocene was represented using DFT, were restricted to short time scales.15 In this communication, we have used QM/MM MD simulations to study the archetypical zirconocene
olefin complex (Cp2Zr(C2H5)(C2H4)þ) and its borate ion pair ([Cp2Zr(C2H5)(C2H4)]þ[CH3B(C6F5)3]
) at 300 K in an explicit pentane solvent, where the counteranion and solvent were represented using MM and the entire metallocene was represented using DFT.17 To compare the solution structures and dynamics of the cation-only (1) and ion pair complexes (1-IP), we have run 50 ps MD simulations of these complexes at 300 K in a periodic solvent box of pentane. The simulations were initiated from the ‘static’ minimum-energy geometries, where a β-agostic interaction was present between the Zr and a Cβ
H bond of the ethyl ligand (1B).17 Although there is a significant degree of fluxional behavior, the MD simulation of 1 remains close to the minimumenergy structure throughout the simulation, in a geometry

Figure 4. Free energy profile of the Zr
CR
Cβ angle bending corresponding to R-agostic to β-agostic isomerization of the cationonly (1) and ion pair systems (1-IP).

consistent with an agostic interaction between the Zr and a Cβ
H bond.

In contrast to 1, 1-IP undergoes a significant rearrangement during the MD simulation. After 20 ps of the simulation, the complex underwent an isomerization from the β-agostic configuration (1B-IP) to an R-agostic configuration (1A-IP, Figure 3). In this new geometry, the β-agostic interaction is absent, as the ethyl ligand is extended away from the metal. Instead, transient R-agostic interactions occur between the CR
HR bonds of the ethyl ligand and the metal, where the average Zr
CR
H angle decreases to 100°. This isomerization is surprising, as “static” calculations on the bare cation system predict 1A to be 2.7 kcal mol
1 higher in energy than 1B. The isomerization of 1B-IP to 1A-IP begins with a significant fluctuation in the Zr
CR
Cβ angle to 95° (Figure 2). This coincides with an increase in the Zr
Hag distance due to the 2072

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Figure 5. Dipole moments of 1A and 1B. Dipole moment vectors are indicated by blue arrows.

breaking of the β-agostic interaction. The complex remains in a transient state where the β-agostic interaction is broken but the Zr
CR
Cβ angle remains near 85° for roughly 90 fs before the Zr
CR
Cβ angle and the Zr
Hag distance increase sharply to the ranges they hold in the 1A-IP configuration. The average Zr
CR
HR angle decreases gradually during this period, corresponding to the formation of the R-agostic interactions. The isomerization process occurs rapidly, completing within 300 fs, although this is 1 order of magnitude longer than the transition periods of bond breaking/forming organometallic reactions examined in our previous studies.18 To quantify the apparent stabilization of the R-agostic configuration in the presence of the counteranion, we calculated the free energy profile of the Zr
CR
Cβ angle in 1 and 1-IP (Figure 4), which is a coordinate for the isomerization.19 This profile was calculated using the WHAM method, which constructs a free energy profile using a series of MD simulations to rigorously incorporate the broad range of zirconocene, counteranion, and solvent orientations.20 There is a minimum near 85° on both the cation-only and ion pair profiles, consistent with a strong β-agostic interaction. In the cationonly system, there is a barrier to isomerization of 2.8 kcal mol
1, with the transition state located at 104°. The minimum of the R-agostic configuration (1A) is broad, lying approximately 1.6 kcal mol
1 higher than 1B through the range of 115
130°. When the counteranion is present (1-IP), the later sections of the profile are systematically stabilized with respect to 1, with a barrier of 1.9 kcal mol
1, and the R-agostic configuration is now only 1.0 kcal mol
1 less stable than the β-agostic configuration. This is consistent with our preliminary MD simulations, which showed that the zirconocene will more readily isomerize to the R-agostic configuration when the counteranion is present. To determine the origin of this stabilization, we initially examined the effect of the counteranion on bond orders and charges of the zirconocene, although surprisingly these properties change only slightly in the presence of the counteranion.17 A simpler explanation is apparent in the dipole moments of the 1A and 1B configurations, which extend from the Zr center toward the neutral ethylene ligand, in the opposite direction of the anionic ethyl ligand (Figure 5). The dipole moment of 1A is notably larger than that of 1B (2.54 and 2.30 D, respectively), due to the greater separation of the anionic ethyl ligand from the Zr(IV) center. As the ion pair has a stronger dipole
anion interaction in the 1A configuration than in the less polar 1B configuration, the R-agostic bonding mode is stabilized in the presence of the counteranion. The magnitude of this stabilization is consistent with the strength of interaction of a simple electrostatic model where a point charge interacts with a point dipole at the average separation of the ion pairs (7.5 Å), which increases by

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1.5 kcal mol
1 when the magnitude of the dipole is increased to 2.54 D. As the relative energies of these two states are small, other environmental effects such as solvent
solute interactions or steric interactions between the counteranion could also contribute. By the Curtin
Hammett principle, stabilization of 1A over 1B should not itself affect the relative rates of propagation, as these are solely determined by their relevant transition states. However, the transition state for propagation still has an Ragostic interaction, while that for chain transfer is much more reminiscent of a β-agostic complex. Should the counteranioninduced bias toward 1A persist at those two transition states, then the counteranion effect would also favor formation of a higher MW polymer.21 Verification of this hypothesis will require more extensive simulations. Although these calculations are highly computationally demanding, this type of QM/MM MD simulations will be needed to rigorously investigate “environmental” effects such as this in the future. In this study, extended QM/MM MD simulations were used to examine the effect of a borate counteranion on a model olefin
zirconocene complex relevant to olefin polymerization catalysis. The presence of the counteranion was found to stabilize the R-agostic configuration of the zirconocene relative to the β-agostic configuration, allowing more facile isomerization to the R-agostic form. We attribute this stabilization to a greater dipole
counteranion interaction in the more polar R-agostic configuration. While the current discussion of counteranion effects has focused on ion pair dissociation energies, catalyst epimerization, and steric effects,5,7,9 our simulations suggest that counteranions can influence the relative rates of complete pathways by favoring more polar structures.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text, tables, and a figure giving details of the calculations, bonding analysis, and a description of our QM/MM code and an MPEG animation of the isomerization. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: (613) 562-5800 ext. 6145. Fax: (613) 562-5170. Present Addresses †

Current address: Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, IL.

’ ACKNOWLEDGMENT We thank the NSERC and the Canada Research Chairs program for funding as well as CFI, the Ontario Research Fund, and IBM for providing computing resources. We thank the reviewers of this paper for helpful suggestions. ’ REFERENCES (1) (a) Mohring, P. C.; Coville, N. J. J. Organomet. Chem. 1994, 479, 1–29. (b) Brintzinger, H. H.; Fischer, D.; Muelhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. 1995, 34, 1143–70. (c) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253–1345. 2073

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(18) (a) Rowley, C. N.; Woo, T. K. J. Chem. Phys. 2007, 126, 024110/1–024110/8. (b) Rowley, C. N.; Woo, T. K. Organometallics 2008, 27, 6405–6407. (c) Rowley, C. N.; Woo, T. K. J. Am. Chem. Soc. 2008, 130, 7218–7219. (19) We chose to calculate this energy difference using molecular dynamics, as optimized snapshots of the ion pair showed a distribution of relative energies of the two configurations, ranging from
0.5 to 3.5 kcal mol
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counteranion interaction was varied by increasing the steric bulk of the counteranion, could test this hypothesis.

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