Intrinsic Ancillary Ligand Effects in Cationic Zirconium Polymerization

5 Intrinsic Ancillary Ligand Effects Downloaded by PENNSYLVANIA STATE UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch005

in Cationic Zirconium Polymerization Catalysts David E. Richardson Department of Chemistry, University of Florida, Gainesville, FL 32611-7200

The intrinsic electrophilicity of zirconocenium polymerization catalysts has been obtained by determining the gas-phase rates of reaction of var -ious catalyst ions with dihydrogen and unsaturated hydrocarbons. The expected decrease in electrophilicity when cyclopentadienyl (Cp) is replaced with more electron-donating Cp derivatives is observed. Thus, the increased polymerization activity often observed for complexes with more electron-donating ligands is probably a result of increased rate of initiation due to lower ion pair binding energy, decreased termination rates due to inhibition of β-elimination, or both.

Electrophilic Group 4 Metallocene Polymerization Catalysts Homogeneous and supported alkene polymerization catalysts based on group 4 metallocenes have recently moved from the basic research laboratory to practical operation in the production of polymer in large-scale plants (I). The fundamental active site i n these catalysts is now widely accepted to be a cationic complex of the general type [ L M R ] , where L is typically a cyclopen tadienyl (Cp) derivative and R represents methyl or the growing polymer chain (2). The cation is formed by reaction of a catalyst precursor with a strong Lewis acid such as an aluminum alkyl or B ( C F ) (2). Polymerization of an alkene is based on the repetitive insertion of monomer into the M - R bond to form a long-chain saturated polyalkene. A n example of a typical titanium metallocene/ trimethylaluminum catalyst is shown in Scheme I. Scheme I greatly simplifies the true nature of the catalyst systems, which in practice can be a complex mixture of a metallocene precursor, activating +

2

6

5

3

© 1997 American Chemical Society

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

79

Downloaded by PENNSYLVANIA STATE UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch005

80

E L E C T R O N TRANSFER REACTIONS

new chain growth

Scheme I. Lewis acid, alkylating agents, supports, and other agents that promote efficient polymerization or control molecular weight (e.g., Hg). The cation itself is not "bare", but instead is solvated and ion-paired to varying extents depending on the solvent used, the ancillary ligands on the metallocene, and the type of counterions present. The co-catalysts, such as methylalumoxane (MAO) (3), are not always well-characterized. Thus, the activity of a given metallocene can be varied extensively by altering many aspects of the catalyst system. For exam ple, a Lewis basic solvent such as acetonitrile can effectively eliminate poly-



5.

RICHARDSON

81

Ligand Effects in Zr Polymerization Catalysts

merization activity i n an otherwise active catalyst (4). Therefore, typical poly merizations are run in a nonpolar aprotic solvent such as toluene (or the mono mer itself for higher alkenes), and the anionic counterion is chosen to be as unreactive and noncoordinating as possible (5). However, as shown by recent work (6), even under these conditions, substantial ion-pairing must be over come before monomer can be activated and inserted. One of the attractive features of these catalysts is the possibility for "tun ing" the catalyst for production of polymers with a variety of desirable proper ties, which can include molecular weight, stereochemistry, and copolymerization with other monomers. This tuning is often accomplished by variation of the ancillary ligand set i n the metallocene catalyst precursor. The catalytic activity of the 14-electron complex cation [ L M R ] arises from its electrondeficient nature, which encourages binding and activation of the monomer toward insertion. Thus, the incorporation of electron-donating ligands would be expected to decrease reactivity of the catalyst in a reaction in which it acts as an electrophile (as in Scheme I). However, just the opposite is observed in many instances. Both permethylcyclopentadienyl (Cp*) and indenyl (Ind) are well-known to be more electron releasing than C p itself (7), yet they can lead to more active catalysts in some cases (8, 9). Systematic studies of alkene poly merization by substituted bis(indenyl) zirconium(IV) catalysts showed that electron-withdrawing substituents (e.g., X = C l , F) lead to reduced activity compared to X = H (JO, II). In these latter examples (10, II), electronic effects are assumed to be important factors because the site of substitution is remote from the metal center. These observations illustrate one of the more confusing aspects of predict ing changes in activity of the catalysts as the ancillary ligands are varied. The explanations in the literature for relative catalyst activity focus on the rates of initiation, propagation, and termination steps as well as the strength of the ion pairing of the cation with the counterion, all of which can be affected in differ ent ways by ancillary ligand substitution. A method that examines intrinsic electrophilicity of the active species in these catalysts could clarify the origins of the activity trends, and we turned to gas-phase methods for this reason. We have previously examined the gas-phase ion-molecule reactions of the 14-electron cation C p Z r C H J by using Fourier transform mass spectrometric methods (12-14). In these studies, the intrinsic reactivity of the cation could be probed without interference due to ion pairing or solvation. In general, all of the observed gas-phase reaction pathways can be explained by using a known mech anism in solution (either insertion, deinsertion, β-Η shift, or C - H activation). To assess the intrinsic electrophilicity of zirconocenium cations as a func tion of ancillary ligands, we decided to determine the kinetic influence of dif ferent C p substituents on the rates of some of the previously observed reac tions (eq 1-3). +

2

2

[L Zr-CH ] + H ->[L Zr-H] + C H 2

3

+

2

2

+

4

t

x


(1)

82


[L Zr-CH ] + C H ->[L Zr-C H ] + H 2

[L Zr-CH ]+ + 2

3

3

+

2

4

2

3

5

+

(2)

k

2

H C=C(CH ) -> 2

3

2

[L Zr-H C-C(CH )=CH ] + C H 2


2

2

3

2

+

4

fc

(3)

3

Equation 1 represents the most straightforward and sterically undemanding reaction in the group. Hydrogenolysis (eq 1) and allylic C - H activation of isobutene (eq 3) at the Z r - M e bond proceed via 4-center, 4-electron transition states (Scheme II). Reaction with ethylene is initiated by insertion followed by β-hydride shift and C - H activation of the allylic hydrogen to eliminate dihydrogen (Scheme II).

Parameterization of Electronic Effects for Cyclopentadienyl Ligands Ancillary ligand effects on reactivity at a metal center can be modeled as a combination of electronic and steric effects, and this approach has been widely used to rationalize the variations in reactivity in group 4 polymerization cata lysts (2). Attempts to probe electronic effects by analysis of polymerization activity have been instructive but indirect approaches to separating these effects. We believe that the rates of gas-phase reactions (eqs 1-3) are more direct measures of electronic effects.

(+H2.D2)

Scheme II.


5.

RICHARDSON

Ligand Effects in Zr Polymenzation

Catalysts

83

At this point, it is useful to review our understanding of gas-phase sub stituent effects in C p derivatives. We have applied the technique of gas-phase electron-transfer equilibria (ETE) to determine the ionization free energies for the reaction L L ' M —» L L ' M + e~, where L and L ' are Cp derivatives and M is either Fe, Ru, or N i {15-19). By assigning ligand parameters y to C p (y = 0) and C p * (y = -1), it is possible to derive a set of parameters for a variety of Cp derivatives (Table I). It is assumed in the derivation of the parameters that ligand effects are additive for L and L ' . Negative values of y indicate a ten dency to decrease the metallocene ionization energy relative to Cp, whereas positive values increase the ionization energy. The general trends are not unex pected and largely coincide with other measures of electron-donating and electron-withdrawing tendencies of substituents, but the values of y are spe cific for metallocenes in the gas phase and are based on thermal equilibrium reactions. In addition, fused ring ligands such as Ind and fluorenyl (Flu) are readily parameterized. +

L

L

L


L

L

°By definition.


84


Rates and Mechanisms of Reactions ofL ZrCH£ with Dihydrogen and Alkenes


2

Kinetics for the reactions in eqs 1-3 were determined for the cations (1-5) shown in Chart I {20a, 20b). Besides the parent cation 1, Ind- and Flu-substi tuted complexes (2, 3, and 5) were examined along with complex 4, which fea tures a silyl linker between the Cp ligands. Unfortunately, it has not been possible to determine the reaction kinetics for complexes bearing alkyl substituents because the alkyl group is rapidly C - H activated intramolecularly as shown in eq 4 for the permethylated cation.

+ CH

4

(4)

Σγ=-2.0

The experimental methods for obtaining kinetic data by using Fourier transform ion cyclotron resonance mass spectrometry were similar to those described previously (20c). The observed ion-molecule reaction pathways were modeled as a series of pseudo-first-order elementary steps. In addition to

π* —CHa

-CH

3

1

2

Σγ = 0

Σγ = -0.4

2yeff - ?

Σγ=-1.3

3

Σγ=-0.8

Chart L


5.

RICHARDSON


85

the reaction with the substrate to produce the desired product ion, reaction of the methyl cation with background water (~ 1 0 Torr) forms the metallocene hydroxide ion ( [ L Z r O H ] , see Scheme II), and reaction of various cations with the neutral dimethyl parent compound produces dimer ions (i.e., binuclear Zr complex ions). These alternate pathways were incorporated into the full kinetic model used to fit the data. The resulting differential equations were solved to yield an analytical solution describing the time dependence of the intensity for each product ion and reactant ion, and the time dependence of ion intensities was fit to the model by optimizing the rate constants simultaneously. The sec ond-order rate constants for the reactions in eqs 1-3 are plotted vs. summed γ parameters in Figure 1. A n effective value of Σγ for 4 was chosen (+0.16) to give the best fit to the lines derived from fits to the Ind and C p complexes. Rates for the reactions of 5 were immeasureably slow, and the best fit lines for the measured rate constants were used to predict the rate constants for 5 shown in Figure 1. From Figure 1 it is clear that more electron-donating ligands substantially retard the reactions in eqs 1-3. In addition, the general trends are the same for -9

+


2

collisional limits

H2C=C(CH ) 3

-1.5

0.0

-0.5

-1.0

2

Σγ Figure 1. Plot ofhg(k) vs. Σγ/or k (filled circles), k (filled squares), and k (filled triangles). The solid lines represent the best fit to the available parameters. The dashed lines are the Langevin collisional limits for the second-order rate constants (upper line for C 1 > 2 > 3 > 5 . The hydrogenolysis reaction is the most sensitive to the value of Σγ.


Rehtionship of Intrinsic Reactivity to Solution Kinetics A simplified mechanism is shown in Scheme H I for the solution polymeriza tion of a 1-alkene by a cationic zirconocene catalyst. Actual initiation of the chain reaction can only occur once the solvent-separated ion pair is trapped by a monomer insertion. Propagation is terminated in this scheme by a β-eh'mination step, and the zirconocene hydride is assumed to be rapidly deactivated. In actual catalytic systems, hydride intermediates can contribute to propagation by reacting further with monomer, but dimerization of the hydride complexes may be one mechanism of catalyst inactivation (21). In the limiting case where fc.jX"] > > fc [monomer], the rate of overall propagation (R , which is monomer consumption rate) is proportional to ini

p

(V ^ p r o p ^ t e r m '

w

h

e

r

e

ip

K

=

a

n

d

K

ip> ™ > * W k

a

n

d

term

k

a

r

e

t

h

e

i

o

n

pairing equilibrium constant and rate constants for initiation, propagation, and termination, respectively. Thus, when ion pairing is faster than reaction with monomer, the overall rate of polymerization depends on the equilibrium con stant for ion-pair separation and the rates of initiation, propagation, and termi nation. Any attempt to ascertain the separate effects of modification of ancillary ligands on each of these steps by measuring overall catalyst activity will be dif ficult at best.

Scheme III.



5.

RICHARDSON


87

The gas-phase data provide an intrinsic order of electrophilicity as a func tion of the ancillary ligand set. It is not surprising that all three reactions stud ied follow the same trend 4 > 1 > 2 > 3 > 5 , since each reaction has a ratedetermining step that is controlled by a 4-center, 4-electron transition state (i.e., either insertion, deinsertion, or C - H activation). Scheme IV illustrates the relationship between the potential energy sur faces for the gas-phase reactions and the condensed-phase reactions using the ethylene as an example substrate. The C p ancillary ligands are left off the structures for simplicity. The energies where shown are based on a combina tion of theory (22) and known thermodynamic quantities for hydrocarbons. The remaining details are only qualitative. The observed gas-phase reaction pro-

Gas Phase Zr+Me + =

Zr—H +

Scheme IV.



88


duces the allyl complex and dihydrogen (eq 2). Other gas-phase pathways are shown that are not observed. One of these, the ehmination of propylene (i.e., β-elimination), is not observed because the transition state for that near-thermoneutral reaction is higher than that of the observed path. Although the insertion of a second ethylene into the Zr-propyl bond is substantially down hill, the bimolecular nature of the reaction means that it will not compete with the unimolecular ehmination of dihydrogen. To connect the gas-phase potential energy with that for solution, we intro duce the solvation energies for each species along the various pathways. A borate anion is shown as the weakly coordinating counterion, but the ion-pair binding energy even in the case of borate ions is substantial (>10 kcal/mol). Solvation energies for coordinatively saturated metallocenium ions are on the order of 20-40 kcal/mol in polar solvents depending on the C p substituents (14-17). For typical nonpolar solvents used in polymerization studies (i.e., ben zene or toluene) the range would be expected to be ~ 15-30 kcal/mol. To that must be added the binding energy for specific inner-sphere solvation (even the "nonbasic" toluene probably coordinates via a C - H bond to the 14-electron cation with a significant binding energy). Thus, although ethylene is predicted to bind to the cation with an exoergicity of - 2 3 kcal/mol, in solution it is assumed that the solvent and ethylene are roughly equal in donor strength and the energies of the solvent (S) and ethylene adducts in Scheme IV are shown as equal. On the basis of the order of electrophilicity observed i n the gas-phase reactions of eqs 1-3, we suggest that the order of fc is most likely 4 > 1 > 2 > 3 > 5. Assuming that the solvation energies of all intermediates and transi tion states are constant for a given ancillary ligand set except for the 14-elec tron methylzirconium cation, the trends observed in the gas-phase kinetics will also be observed in solution. For example, consider the insertion of ethylene into the Z r - M e bond (i.e., initiation). The transition state energy for this reac tion can influence the rate of insertion in the gas phase even though it lies below the energy of the initial reactants. Significant kinetic barriers due to negative activation energies are commonly encountered in ion-molecule reac tions. Although seemingly irrational to solution chemists, a negative barrier can slow a gas-phase reaction, and the effect can be modeled for metal complex ion-molecule reactions by statistical models such as R R K M (23). To convert to the solution phase, all species (including activated com plexes) are solvated. For the 14-electron Z r - M e , some additional solvation energy results from inner-sphere solvation, but the other species are assumed to have equal solvation energies. Therefore, although the initiation reaction in solution is less exoergic than the gas-phase reaction, the trend in the rate as a function of ancillary ligands should be the same in both phases. If indeed the propagation reaction rate (controlled by & in Scheme III) decreases with more electron-donating ligands such as Ind, then the observed increase in the activity for the Ind catalyst must arise from the ion-pair equilibprop

+

prop


5.

RICHARDSON


89

rium constant, initiation rate, or termination rate in Scheme III. The most likely candidate is the ion-pair binding constant. Bulkier ligands may reduce the binding constant for the counterion to the cationic catalyst and thereby accelerate the polymerization by increasing the equilibrium amount of the sol vent-separated cation.


Summary The intrinsic electrophilicity of zirconocenium polymerization catalysts has been obtained by determining the gas-phase rates of reaction of various cata lyst ions with dihydrogen and unsaturated hydrocarbons. The expected decrease in electrophilicity when Cp is replaced with more electron-donating C p derivatives is observed. Thus, the increased polymerization activity often observed for complexes with more electron-donating ligands may result from an increase in the rate of initiation (due to lower ion-pair binding energy), a decrease in the termination rate (inhibition of β-elimination), or both.

Acknowledgments I express gratitude to Henry Taube, my doctoral advisor (Stanford University, 1976-1980). It is hoped that the work described here illustrates the insights that can be achieved through a Taube-inspired application of thermodynamics and kinetics to fundamental problems in chemistry. I also thank co-workers and collaborators who have made this research possible, especially N . George Alameddin, Matthew F. Ryan, Allen R. Siedle (3M), and John R. Eyler. Other co-workers who have contributed to the work in gas-phase organometallic chemistry are listed i n the references. This work was supported by a grant from the National Science Foundation (CHE9311614).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Haggin, J. Chem. Eng. News 1995, 73(18), 7. Mohring,P.C.;Coville,N.J.J.Organomet. Chem. 1994, 479, 1. Sinn, H.; Kaminsky, W.; Vollmer, H.; Woldt, R. Angew. Chem. 1980, 92, 396. Jordan, R. F. Adv. Organomet. Chem. 1991, 32, 325. Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015 and refer -ences therein. Deck, P. Α.; Marks,T.J.J.Am. Chem. Soc. 1995, 117, 6128. Gassman,P.G.;Winter,C.H.J.Am. Chem. Soc. 1988, 110, 6130. Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani, T. J. Am. Chem. Soc. 1992, 114, 1025. Giannetti, E.; Nicoletti, G. M.; Mazzocchi, R. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2117. Lee, I.-K.; Gauthier, W. J.; Ball, J. M.; Iyengar, B.; Collins, S. Organometallics 1992, 11, 2115.


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11. Piccolrovazzi, N.; Pino, P.; Consiglio, G.; Sironi, Α.; Moert, M . Organometallics 1990, 9, 3098. 12. Christ, C. S.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. Soc. 1988, 110, 4038. 13. Christ, C. S.; Eyler, J. R. Richardson, D. E. J. Am. Chem. Soc. 1990, 112, 596. 14. Christ, C. S.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. Soc. 1990, 112, 4778. 15. Richardson, D. E. In Organometallic Ion Chemistry; Freiser, B. S., Ed.; Kluwer Academic Publishers: Dordrecht, 1996; Ch. 8. 16. Richardson, D. E.; Ryan, M . F.; Khan, Md. N.I.; Maxwell, K. A. J. Am. Chem. Soc. 1992, 114, 10482. 17. Ryan, M . F.; Eyler, J. R.; Richardson, D. E. J. Am. Chem. Soc. 1992, 114, 8611. 18. Ryan, M . F.; Richardson, D. E.; Lichtenberger, D. L.; Gruhn, N. Organometallics 1994, 13, 1190. 19. Ryan, M . F.; Siedle, A. R.; Burk, M . J.; Richardson, D. E. Organometallics 1992, 11, 4231. 20. (a) Alameddin, N. G.; Ryan, M . F.; Eyler, J. R.; Siedle, A. R.; Richardson, D. E. Organometallics 1995, 14, 5005; (b) Richardson, D. E.; Alameddin, N. G.; Ryan, M . F.; Eyler, J. R.; Hayes, T.; Siedle,A.R.J.Am. Chem. Soc. in press; (c) Sharpe, P.; Richardson, D. E. Coord. Chem. Rev. 1989, 93, 59. 21. Stehling, U.; Diebold, J.; Kirsten, R.; Röll, W. Brintzinger, H.-H.; Jüngling, S.; Mül -haupt, R.; Langhauser, F. Organometallics 1994, 13, 964. 22. Woo,T.K.;Fan, L. Ziegler, T. Organometallics 1994, 13, 2252. 23. Richardson, D. E.; Eyler, J. R. Chem. Phys. 1993, 176, 457.


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Intrinsic Ancillary Ligand Effects in Cationic Zirconium Polymerization

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