Organometallics 2010, 29, 1541–1550 DOI: 10.1021/om900843h
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Comparative Theoretical Study on Homopolymerization of r-Olefins by Bis(cyclopentadienyl) Zirconocene and Hafnocene: Elemental Propagation and Termination Reactions between Monomers and Metals Anniina Laine,† Mikko Linnolahti,† Tapani A. Pakkanen,† John R. Severn,‡ Esa Kokko,‡ and Anneli Pakkanen*,‡ †
Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland, and ‡ Borealis Polymers Oy, R&D, P.O. Box 330 FI-06101, Porvoo, Finland Received September 30, 2009
A comparative quantum chemical study has been performed to shed light on the fundamental differences between hafnocenes and zirconocenes concerning reactions between the metals and R-olefin monomers, namely, ethene, propene, 1-butene, and 1-hexene. Analogous species along the R-olefin polymerization pathways were studied for bis(cyclopentadienyl) zirconocene and hafnocene, taking into account the structural variations of the first two monomer insertion steps and of the competitive chain-termination reactions. The results were analyzed as a function of both the metal and the monomer, the metal showing more distinct differences. The most notable difference in the reactions of the zirconocene and hafnocene can be seen in β-hydrogen transfer to metal, activation energies for which are significantly higher for the hafnocene. 1. Introduction Group 4 metallocenes, zirconocenes and hafnocenes in particular, are excellent catalysts for polymerization of ethene and higher R-olefins. Modification of the ligand structure of the metallocenes provides an elegant way for rational tailoring of polymers with desired microstructure. In addition to the ligand structure, the central metal itself has a distinct role in the polymerization process, zirconocenes providing higher activities while the hafnocenes produce greater molecular weights.1,2 Less apparent is the effect of the monomer size, reactivities of the monomers decreasing with growing chain length, whereas the molecular weights are more or less independent of the chain length of the monomer.3 The available experimental data also suggest that the preferred pathways for chain terminations are dependent on both the metal and the monomer.4 The reasons behind the different behaviors of zirconocenes and hafnocenes are not yet fully understood, in part due to most of the experimental and theoretical work having been focused around zirconocenes.1 The question of what makes them behave so differently in olefin polymerization is particularly intriguing in light of the structural and electronic similarities of the metals.5 The difference in polymerization
behavior has been suggested to originate from the weaker σ-bond between zirconium and the polymer chain,6 facilitating not only the chain propagation but also the release of the chain. Nonetheless, the situation is likely to be more complex, as indicated by the different reactivities of the metallocenes against trimethylaluminum.7 The various steps in the olefin polymerization process have been extensively studied by computational techniques, mainly using ethene and propene as a monomer.8 Moreover, the comparative computational studies on the effects of the metal have revealed the different reactivities of the two metallocenes.7,9 Due to the complexity of the whole
*Corresponding author. E-mail:
[email protected]. (1) Alt, H. G.; K€ oppl, A. Chem. Rev. 2000, 100, 1205. (2) See e.g.: (a) Mallin, D. T.; Rausch, M. D.; Chien, J. D. W. Polym. Bull. 1988, 20, 421. (b) Silveira, F.; Simplicio, L. M. T.; Rocha, Z.; Santos, J. H. Z. Appl. Catal., A 2008, 344, 98. (c) Wahner, U. M.; Brull, R.; Pasch, H.; Raubenheimer, H. G.; Sanderson, R. Angew. Makromol. Chem. 1999, 270, 49. (3) See e.g.: Grumel, W.; Brull, R.; Pasch, H.; Raubenheimer, H. G.; Sanderson, R.; Wahner, U. M. Macromol. Mater. Eng. 2001, 286, 480. (4) (a) See e.g.: Resconi, L.; Camurati, I.; Malizia, F. Macromol. Chem. Phys. 2006, 207, 24. (b) D'Agnillo, L.; Soares, J. B. P.; Penlidis, A. Macromol. Chem. Phys. 1998, 199, 955. (5) Greenwood, N. N.; Earnshaw, A. Chemistry of Elements; Butterworth Heineman: Great Britain, 2001.
(6) Simoes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629. (7) Busico, V.; Cipullo, R.; Pellechia, R.; Talarico, G.; Razavi, A. Macromolecules 2009, 42, 1789. (8) See e.g.: (a) Rappe, A. K.; Skiff, W. M.; Casewit, C. J. Chem. Rev. 2000, 100, 1435. (b) Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2009, 28, 2609. (c) Borrelli, M.; Busico, V.; Cipullo, R.; Ronca, S. Macromolecules 2003, 36, 8171. (d) Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2007, 26, 2024. (e) Correa, A.; Talarico, G.; Cavallo, L. Kinet. Catal. 2006, 47, 170. (f) Borrelli, M.; Busico, V.; Cipullo, R.; Ronca, S.; Budzelaar, P. H. M. Macromolecules 2002, 35, 2835. (g) Mercandelli, P.; Sironi, A.; Resconi, L.; Camurati, I. J. Organomet. Chem. 2007, 692, 4784. (h) Margl, P.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1998, 120, 5517. (i) Resconi, L.; Camurati, I.; Sudmeijer, O. Top. Catal. 1999, 7, 145. (j) Jenssen, V. R.; Koley, D.; Jagadeesh, M. N.; Thiel, W. Macromolecules 2005, 38, 10266. (k) Karttunen, V. A.; Linnolahti, M.; Pakkanen, T. A.; Severn, J. R.; Kokko, E.; Maaranen, J.; Pitk€anen, P.; Pakkanen, A. J. Organomet. Chem. 2008, 693, 3915. (l) Cruz, V. L.; Ramos, J.; Martinez, S.; Munoz-Escalona, A.; Martinez-Salazar, J. Organometallics 2005, 24, 5095. (m) Guerra, G.; Longo, P.; Cavallo, L.; Corradini, P.; Resconi, L. J. Am. Chem. Soc. 1997, 119, 4394. (n) Pilme, J.; Busico, V.; Cossi, M.; Talarico, G. J. Organomet. Chem. 2007, 692, 4227. (9) (a) Yoshida, T.; Koga, N.; Morokuma, K. Organometallics 1995, 14, 766. (b) Gruber-Woelfler, H.; Flock, M.; Sassmannshausen, J.; Khinast, J. G. Organometallics 2008, 27, 5196. (c) Talarico, G.; Budzelaar, P. H. M. Organometallics 2008, 27, 4098. (d) Silanes, I.; Mercero, J. M.; Ulgade, J. M. Organometallics 2006, 25, 4483. (e) Correa, A.; Talarico, G.; Cavallo, L. J. Organomet. Chem. 2007, 692, 4519. (f) Talarico, G.; Budzelaar, P. H. M. Organometallics 2008, 27, 4098.
r 2010 American Chemical Society
Published on Web 03/18/2010
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Figure 1. Schematic presentation of the ethene polymerization pathway.
polymerization system, involving components such as the catalyst, cocatalyst, and solvent, approximations have been necessary for theoretical treatment of the system. Even with the approximations neglecting the effects of cocatalyst and solvent, precise theoretical description of the polymerization process is a challenge. This is due to the various regio- and stereostructural variations of the growing polymer chain, numerous alternative insertion and termination routes, and conformational isomerism. The present study is motivated by the distinctly different polymerization behaviors of zirconocenes and hafnocenes.1,2,9 Despite the potential contribution of many factors due to the complexity of the system, one may ask if there are fundamental differences in the reactions between olefin monomers and the two classes of metallocenes. To single out the effect of the metal on reactions with olefins, a comparative quantum chemical study on homopolymerization of ethene, propene, 1-butene, and 1-hexene is reported. (10) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (11) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (12) Linnolahti, M.; Hirva, P.; Pakkanen, T. A. J. Comput. Chem. 2001, 22, 51. (13) Karttunen, V. A.; Linnolahti, M.; Pakkanen, T. A.; Maaranen, J.; Pitk€ anen, P. Theor. Chem. Acc. 2007, 118, 899. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
2. Computational Details The catalytic intermediate structures were fully optimized with the hybrid density functional B3LYP method.10 For Zr and Hf, Los Alamos ECP (LANL2DZ)11 was employed. A standard 6-31G* basis set was applied for all other elements. The described methodology has been previously demonstrated to apply for both zirconocenes12 and hafnocenes.13 Harmonic frequencies were calculated at all optimized geometries to verify their nature at the potential energy surface as either a minimum or a transition state and also to obtain Gibbs free energies. Throughout the text, Gibbs free energies are given at T = 298.15 K and p = 0.1 MPa. All calculations were carried out with the Gaussian03 program package.14
3. Results and Discussion 3.1. Polymerization Pathways. A schematic presentation of the considered polymerization pathways for the first two olefin insertions is given in Figure 1. The scheme follows the typical approach for the R-olefin polymerization reactions,15 the propagation proceeding according to the modified Cossee-Arlman mechanism.16 The starting point for the polymerization process is the generation of the monomethyl cationic form of the metallocene (1). The intermediate (1) reacts with the incoming monomer, thus forming a π-complex (2), which proceeds through an R-agostic transition state (TS) (3) to form a β-agostic intermediate (4). From the second π-complex (5) the reaction proceeds either via chain propagation (6) or via chain termination by β-hydrogen transfer to monomer (7). The β-agostic intermediate (4) can also lead directly to an alternative termination reaction, β-hydrogen transfer to metal (8). Also chain termination by reaction with H2 (9), hydrogenolysis (10), was studied. Numbering of the reaction intermediates, as described in Figure 1, is adopted for naming of all catalytic intermediates (15) Krentsel, B. A.; Kissin, Y. V.; Kleiner, V. J.; Stotskaya, L. L. Polymers and Copolymers of Higher R-Olefins; Hanser Publishers: G€ottingen, 1997. (16) (a) Cossee, P. J. Catal. 1964, 3, 80. (b) Arlman, E. J. J. Catal. 1964, 3, 89. (c) Arlman, E. J.; Cossee, P. J. Catal. 1964, 3, 99.
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Table 1. Gibbs Free Energies [kJ/mol] Relative to the Methyl Cation for the Reaction Intermediates and Activation Energies for Insertion of the First Monomer 1,2-insertion
Zr/et Hf/et Zr/pro Hf/pro Zr/1-but Hf/1-but Zr/1-hex Hf/1-hex
2,1-insertion
(2a) π-complex
(3a) TS
(4a) β-product
activation energy (2a f 3a)
(2b) π-complex
(3b) TS
(4b) β-product
activation energy (2b f 3b)
-27.9 -23.0 -35.0 -31.9 -36.0 -34.9 -40.0 -37.6
16.5 19.9 30.4 33.8 28.2 31.2 24.9 28.0
-67.4 -56.9 -43.0 -32.9 -42.3 -32.0 -42.6 -32.6
44.4 42.9 65.4 65.7 64.2 66.1 64.9 65.6
-32.8 -27.6 -35.3 -31.4 -38.1 -32.5
50.9 56.8 47.6 54.2 54.8 59.0
-34.9 -23.8 -38.0 -26.9 -38.0 -25.1
83.7 84.4 82.9 85.6 92.9 91.5
Figure 2. Considered combinations of stereoregular and regioregular insertions.
discussed below. The focus of the study being on the effects of metal rather than the effects of ligands, the studies were carried out for metallocenes with unsubstituted Cp ligands. Introducing a monomer with a prochiral carbon significantly increases the number of alternative routes for insertion and termination in terms of (1) polymer regiostructure (primary 1,2-insertion or secondary 2,1-insertion), (2) polymer stereostructure (R or S configuration of a carbon), and (3) rotational freedom of the growing polymer chain, giving rise to conformational isomers.17 Combinations of (1) and (2) basically produce 16 alternatives at the point of second olefin insertion. The number of combinations was reduced by symmetry considerations; two of the four (R,S) stereostructures of the first monomer insertion are equivalent, leaving eight alternatives at the point of second olefin insertion (Figure 2). Each of these was taken into account, together with the paths leading there. The (R,S) nomenclature was adopted for all reaction intermediates. The starting point for description of the growing polymer chain was alltrans configuration, as taking into account all the conformational possibilities would have not been practical. The calculations for chain transfer reactions, β-hydrogen transfer to monomer, β-hydrogen transfer to metal, and hydrogenolysis were carried out after the first monomer insertion. Exactly the same approach was employed for both zirconocenes and hafnocenes, allowing their comparison, which was the focus of the study. (17) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253.
3.2. Insertion of the First Monomer. The energetics of the first monomer insertion (steps 1-4 in Figure 1) are summarized in Table 1. In terms of the relative stability of the methyl cation with respect to the dichloride precursor, there is an energy difference of 8.5 kJ/mol in favor of the zirconocene. The reaction for coordination of the monomer (2a and 2b) is exothermic by 20-40 kJ/mol. Among the monomers, π-coordination is the least exothermic for ethene and, among metals, for hafnium. In terms of activation energy, insertion of ethene is preferred over higher monomers by about 20 kJ/ mol, as is also primary insertion over secondary insertion, whereas the metal has practically no influence. Concerning the β-agostic product, the reaction is systematically more exothermic for the zirconocene than for the hafnocene and also more exothermic with shorter monomers. 3.3. Chain Propagation. Here we concentrate on the insertion of the second monomer, which, unlike insertion of the first monomer, contains the basic elements involved in chain propagation. Throughout the report, the illustrations and relevant bond lengths are given for catalyst intermediates along the 1-butene polymerization pathway, as it involves all relevant structural features, i.e., prochiral carbon and alkyl side chain. a. Enantioface Selectivity. The stereostructure of the polymer is mainly guided by two factors: by catalyst ligand structure, i.e., enantiomorphic site control mechanism, and by the chirality of the last inserted monomer, i.e., chain end control mechanism.17 The barrier for monomer insertion at a specified orientation is affected by the steric requirements of the chain, which eventually are controlled by the ligand structure of the catalyst.18 Previous computational studies on catalyst stereoselectivity-ligand structure relationship have reproduced the experimental findings of the sitecontrolling effect of the fluorenyl- and indenyl-based catalysts in differentiating between the monomers.8b-d In this respect, the hereby studied Cp ligand has no preference for the R- vs S-insertion of the incoming monomer,19 making chain end control the dominant mechanism in this particular case. In this context, we concentrate on the 1,2-1,2 insertion route. Configuration of the coordinated monomer (R, S) gives rise to two routes for the reaction to proceed. In addition, insertion to the 1,2 chain end can proceed through two distinct R-agostic transition states. This is due to the CH2 hydrogens of the R-carbon, both of which can stabilize the transition state by agostic interactions to the metal. This, in turn, determines the alignment of the polymer chain, further (18) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (19) Ewen, J. A. J. Mol. Catal. A 1998, 128, 103.
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Figure 3. Alternative transition states for the second primary insertions of 1-butene. For energy comparisons [kJ/mol], configuration 6a is set as a reference within each system. The incoming monomer is illustrated by green color.
determining the energetically favored orientation of the monomer.17 The resulting four transition states are depicted in Figure 3. The energetically favored route is through 6a, where the chain, in this particular case, takes up a monomer in R-configuration, leading to a system of the least steric repulsion. The rotation of the chain (6b) results in increased steric repulsion between the growing polymer chain and the monomer. The energy difference between transition states 6a and 6b is about 10-20 kJ/mol and is not very sensitive to either metal or monomer. Alternatively, the chain can take up a monomer in S-configuration (6c). As the Cp ligand has no preference for R- or S-insertion, the energy differences between 6a and 6c are equal for the case of propene, because the β-carbon of the chain is bound to two identical methyl groups, and marginal for the cases of 1-butene and 1-hexene, where the β-carbon of the chain is bound to methyl and ethyl, and methyl and butyl, respectively. The fourth alternative (6d) is a product of rotation of chain from 6c, analogous to the 6a vs 6b comparison in the case of R-configuration of the monomer. b. Regioselectivity. Besides two consecutive primary insertions, focusing on the R- and S-insertions and thereby on different enantioface selectivities of zirconocenes and hafnocenes, we performed a comparison of primary and secondary insertion for the metallocenes to shed light on possible differences in regioselectivity. The starting points for the study were the β-agostic products formed after the first insertion (Figure 4), for which the second olefins were brought in the R-configuration (Figure 5). Competing Insertions to Primary Chain End. The energy differences between the transition states for primary (6a) and secondary (6e) insertions at the primary chain end are shown in Figure 5a for each monomer. Primary insertion is favored over the secondary insertion, in line with previous computational studies for a number of catalysts and monomers.8b-g As shown in Figure 5a, the energy differences are practically independent of the monomer. However, the metal has a systematic effect, the energy gaps between the transition states being higher for the hafnocene. This indicates that the secondary insertion is more likely for the zirconocenes.
Figure 4. Starting points for the comparison of primary and secondary insertion. Top: β-Agostic chain end after primary insertion (4a). Bottom: β-Agostic chain end after secondary insertion (4b). The β-carbon is illustrated by red color.
Competing Insertions to Secondary Chain End. Concerning the secondary chain end, the two hydrogens of the β-carbon give rise to two isomers with β-agostic interactions to the metal. The isomer where the β-agostic hydrogen directs the methyl group, representing the growing polymer chain, to the trans position with respect to the alkyl side chain is favored in energy (4b, see Figure 4, bottom). Complex 4b was the initial structure for studying the insertion reaction taking place at a secondary chain end. There are two
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Figure 5. Energy differences between transition states for primary and secondary insertion for (a) chain end after primary insertion (b) chain end after secondary insertion.
characteristic differences between the primary and secondary chain end: (1) In the primary chain end, the alkyl side is attached to the β-carbon; whereas in the secondary chain end, it is attached to the R-carbon, and therefore (2) there is a secondary R-carbon and tertiary β-carbon in the primary chain end, and vice versa in the secondary chain end. The energy differences between the transition states for primary (6f) and secondary (6g) insertions at the secondary chain end are shown in Figure 5b for each monomer. This, together with comparison to the corresponding reactions to the primary chain end (Figure 5a), depicts the overall differences between the hafnocene and zirconocene catalysts with regard to regioselectivity: The zirconocene is less selective than the hafnocene in taking up a monomer, irrespective of whether the chain end is primary or secondary. Also, the secondary chain end is less effective in discriminating between the primary and secondary insertions. The result of the less selective nature of the 2,1 chain end is in agreement with previous theoretical studies.8e-g c. Formation of the π-Complex and Activation Energies. While the transition states for monomer insertion can be precisely described, within the selected approximation and methodology, the situation is less straightforward for the preceding π-complex with complex structural freedom. This complicates the determination of the activation energies. From the point of view of the purposes of the paper, i.e., comparison between the zirconocene and the hafnocene, this is less of an obstacle; the issue is to be consistent, comparing analogous structures. There are a number of ways the incoming olefin can coordinate to the metal, all of which fit in between two
Figure 6. Primary nonagostic (5a) and agostic (5f) 1-butene π-complexes formed after first primary insertion, their relevant bond lengths [A˚], and stabilities relative to the preceding β-agostic intermediate (4a).
limiting cases: nonagostic and agostic π-complexes (Figure 6). In agostic π-complexes, there is a β-agostic interaction from hydrogen to the metal. In nonagostic π-complexes, the agostic interaction has made room for the incoming monomer. The π-complexes shown in Figure 6 were used as starting points for subsequent steps due to their closest resemblance with the following transition states for chain propagation (nonagostic π-complex) and chain termination (agostic π-complex in chain transfer to monomer). In other words, proceeding from these particular π-complexes to the subsequent transition state requires the least amount of chain rearrangement. One should note that the selection is mostly technical. Unlike the zirconocene with no clear preference for the type of π-complex, the hafnocene has a clear preference for nonagostic π-complexes. This is practically independent of the size of the monomer (Figure 6). This has the consequence of the hafnocene preferring π-complex preceding chain propagation, whereas the zirconocene has almost equal preference for a π-complex preceding insertion and a π-complex preceding chain termination (chain transfer to monomer).
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Table 2. Gibbs Free Energies [kJ/mol] Relative to the β-Agostic Intermediate (4a) for the Reaction Intermediates and Activation Energies for Primary and Secondary Insertion of the Second Monomer 1,2-insertion
Zr/et Hf/et Zr/pro Hf/pro Zr/1-but Hf/1-but Zr/1-hex Hf/1-hex
2,1-insertion
5a π-complex
6a TS
11a β- product
6a-5a activation energy
5e π-complex
6e TS
11b β- product
6e-5e activation energy
13.4 10.4 21.6 16.8 17.8 9.5 17.4 15.3
36.9 34.4 62.6 57.1 64.3 58.4 15.3 56.9
-51.0 -48.9 -22.6 -21.1 -13.2 -13.1 -14.9 -12.7
23.5 24.0 40.9 40.2 46.5 48.9 43.6 41.6
15.8 10.1 14.2 9.5 12.6 9.0
70.6 70.2 73.1 70.2 71.1 69.1
-22.7 -23.4 -12.6 -10.8 -24.3 -19.7
54.9 60.1 58.9 60.7 58.6 60.1
Figure 7. Activation energies [kJ/mol] for competing insertions to the primary chain end: (a) primary vs (b) secondary insertion.
The activation energies together with relative stabilities of reaction intermediates for monomer insertion are given in Table 2 for primary and secondary insertion to primary chain end in the R-configuration, the activation energies being further illustrated in Figure 7. In line with experiments,17 the barrier for the primary insertion is clearly lower than for the secondary. Beyond ethene, the barriers are not strongly influenced by the monomer, and for primary insertion, not systematically influenced by the metal either. The barriers for secondary insertion are slightly but systematically higher for hafnocenes. In the case of ethene, the lower insertion barrier in comparison to those of higher monomers8e-g has been reported previously. The effect of the metal on the stability of the β-agostic product is marginal. The monomer, instead, has a strong influence, the product being particularly stable in the case of ethene. 3.4. Chain Termination. Three chain-termination reactions were studied: β-hydrogen transfer to metal, β-hydrogen transfer to monomer, and hydrogenolysis. Termination via β-methyl transfer to metal was excluded from the study, as it only applies for propene polymerization and has been
experimentally shown to not be a likely termination route for metallocenes with unsubstituted Cp ligands.20 Similar to the chain-propagation studies, the focus was on reactions after the first monomer insertion. Hence, the starting points for the study were the two β-agostic catalytic intermediates formed after primary and secondary insertion of the first monomer (Figure 4, 4a and 4b). The elemental structural features of the reaction site of the growing polymer chain, i.e., the alkyl side chain and the different chemical environment of the primary and secondary chain ends (see Section 3.3b), are already present in the systems. Moreover, the approach makes the competitive steps of monomer insertion and chain termination comparable, as will be discussed in Section 3.5. β-Hydrogen Transfer to Metal. The barriers for β-hydrogen transfer to metal are shown in Figure 8 for the zirconocene and the hafnocene as a function of monomer and for both primary and secondary chain ends. Relative stabilities of the reaction intermediates involved in the reaction are reported in Table 3. Concerning primary chain end, the termination barriers are practically independent of the metal and, beyond ethene, also independent of the monomer. The decrease in reaction barriers when moving from ethene to propene has been reported before.8i The somewhat higher barriers for ethene apparently originate from an elemental structural difference: with ethene, the migrating β-hydrogen belongs to a secondary carbon, whereas the β-carbon is tertiary for the longer olefins. The feasibility of the β-hydrogen with monomers higher than ethene is also supported by the thermodynamically more favored reaction products (see 12a in Table 3). Concerning the influence of the metal, the hafnocene produces more stable products than the zirconocene. The chain end turns out to play a decisive role in discriminating the zirconocene and the hafnocene. After primary insertion, the barriers for β-hydrogen transfer to metal are practically equal for the metallocenes, but they are significantly lower for the zirconocene after secondary insertion (Figure 8). In the case of the zirconocene, the termination barriers decrease as a function of the size of the monomer, whereas the barriers of the hafnocene are less sensitive toward the monomer. Furthermore, there is evidence for the zirconocenes to deactivate after 2,1 insertion.4 The reason behind the significantly different behavior of the two metallocenes is illustrated in Figure 8, showing the transition-state structures, which are distinctly different (20) (a) Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani, T. J. Am. Chem. Soc. 1992, 114, 1025. (b) Resconi, L.; Abis, L.; Franciscono, G. Macromolecules 1992, 25, 6814.
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Figure 8. Activation energy barriers [kJ/mol] and relevant bond lengths [A˚] for β-hydrogen transfer to metal taking place after both (a) primary (transition state (8a)) and (b) secondary (transition state (8b)) insertion.
(8a and 8b). After 2,1 insertion, the alkyl side chain is attached to the R-carbon of the polymer backbone, which is located closer to the reaction center. From here, a hydrogen bound to the alkyl side chain is able to provide an additional agostic interaction to the metal in the transition state but, curiously, only in the case of the zirconocene. The agostic interaction seems to be more beneficial for zirconium, as was already found in the case of the π-complexes
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(Section 3.3c), being in accordance with the general trends of the strengths of agostic interactions decreasing when moving down a transition metal triad.21,22 On the other hand, the lengths of the M-C bonds decrease when moving from Zr down to Hf, making the transition state with only one agostic interaction, but shorter M-C bond, more favorable for the hafnocene (see Figure 8). In short, in the case of the zirconocene, an extra agostic interaction stabilizes the transition state for β-hydrogen transfer to the metal, resulting in significantly lower termination barrier for the zirconocene. β-Hydrogen Transfer to Monomer. Here we focus on the activation energies for the termination reactions taking place on the primary chain end, in analogy with the propagation study reported in Section 3.3c. The role of the secondary chain end will be addressed, in terms of transition state heights, in Section 3.5, where the competitive insertions and terminations are investigated. Termination by β-hydrogen transfer to monomer was initiated from the agostic π-complex, as described in Section 3.3c, the π-complex with the closest resemblance to the subsequent transition state. Two different transition states have been reported for β-hydrogen transfer to monomer,23 the transition states differing in spatial requirements around the metal. The transition state 7a, with the transferring β-hydrogen interacting with the metal, was employed here, as it has been shown to be the preferred pathway for both metallocenes.9f Relative stabilities of the reaction intermediates involved in the reaction are reported in Table 3. The barriers for chain termination are illustrated for both metals in Figure 9 as a function of the monomer. The termination barriers are not much influenced by the metal, but are lower for ethene than for higher monomers. In comparison to β-hydrogen transfer to metal, the barriers for β-hydrogen transfer to monomer are higher. Note, however, that the products of β-hydrogen transfer to the metal lie high in energy in comparison to the products of β-hydrogen transfer to monomer. Hydrogenolysis. Termination of the chain growth by reaction with molecular hydrogen was studied for both primary and secondary chain ends. Termination barriers, as a function of metal and monomer, are illustrated in Figure 10. As expected, the barriers are low, about 10 kJ/mol, which is in accordance with experiments17 and with previous calculations24 of molecular hydrogen being efficient in terminating the chain growth. Notwithstanding the small barriers, there is a systematic difference between the metals, the barriers being higher for the zirconocene. 3.5. Monomer Insertion vs Chain Termination. The rate of the polymerization is ultimately determined as a ratio between insertions and terminations. Even though this is out of the scope of the present approach, because of the complexity of the system as a whole, comparisons between the alternative routes for chain propagation or chain termination, and their dependence on the metal, are of interest. A practical approach is to compare two alternative routes between the same reactants, i.e., olefin uptake, leading to either chain (21) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. 2007, 104, 6908. (22) Han, Y.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 5939. (23) Talarico, G.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2006, 128, 4524. (24) (a) Petitjean, L.; Pattou, D.; Ruiz-Lopez, M. F. Tetrahedron 2001, 57, 2769. (b) Budzelaar, P. H. M.; Coussens, B. B.; Friedrichs, N. J. Organomet. Chem. 2007, 4473–4480.
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Table 3. Gibbs Free Energies [kJ/mol] relative to the β-agostic structures (4a and 4b) for the reaction intermediates for β-hydrogen transfer to metal and β-hydrogen transfer to monomer elimination from 1,2-end
Zr/et Hf/et Zr/pro Hf/pro Zr/1-but Hf/1-but Zr/1-hex Hf/1-hex
elimination from 2,1-end
4a β- product
8a TS
12a product
4b β- product
8b TS
12b product
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
38.5 37.7 26.7 25.3 27.6 23.9 29.1 26.7
23.8 19.1 -5.3 -11.8 -0.7 -5.2 -0.3 -9.9
0.0 0.0 0.0 0.0 0.0 0.0
23.5 33.8 21.4 32.5 16.4 34.0
-2.6 -1.0 -0.6 -1.5 -6.2 -8.0
transfer to monomer from 1,2 end
Zr/et Hf/et Zr/pro Hf/pro Zr/1-but Hf/1-but Zr/1-hex Hf/1-hex
transfer to monomer from 2,1 end
5f π-complex
7a TS
13a products
5g π-complex
7b TS
13b products
17.5 26.3 18.2 27.9 19.2 29.4 18.3 29.3
69.0 76.2 85.7 94.9 89.8 92.9 86.6 90.4
-13.6 -14.2 -4.2 6.1 -2.1 7.5 1.0 13.3
16.5 32.9 25.2 34.1 20.0 24.4
86.0 96.7 87.4 96.3 89.7 96.0
4.8 11.4 5.5 14.7 3.7 10.6
Figure 10. Activation energy barriers [kJ/mol] for hydrogenolysis, taking place after (a) primary and (b) secondary insertion.
Figure 9. Activation energy barriers [kJ/mol] for β-hydrogen transfer to monomer, taking place after (a) primary (7a) and (b) secondary (7b) insertion.
growth or chain termination by hydrogen transfer to monomer. Here we approach the problem by comparing the energy levels of the transition states for olefin insertion and β-hydrogen transfer to monomer, taking into consideration both primary and secondary chain ends. The energy differences between the transition states for propagation and termination are illustrated in Figure 11 for both metallocenes as a function of the monomer, the combinations of two consecutive olefin insertions giving rise to
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the differences being higher for the hafnocenes. The difference decreases when shifting from the primary to the secondary chain end, resulting from a more difficult insertion for the secondary chain end, bringing the energies of the two transition states closer to each other. In the case of 1-hexene, the zirconocene systems have almost equal preference for the insertion and termination transition states. Curiously, in the case of 1-butene, the zirconocene systems actually prefer the termination transition state over insertion. Concerning the influence of the incoming monomer (Figure 11, a vs b and c vs d), secondary insertion leads to lowering of the differences between the transition state heights for propagation and termination. This is relatively independent of the metal. One should keep in mind, as demonstrated in Section 3.4, that the termination barriers for β-hydrogen transfer to monomer are higher than for β-hydrogen transfer to metal, in particular for propene, 1-butene, and 1-hexene. Comparison to primary insertion (see Section 3.3c) shows that primary insertion, yielding regioregular chain growth, is a more viable route, a prerequisite of the polymerization process. The situation markedly changes upon moving from the primary to the secondary insertion. As shown in Section 3.4, in the case of the zirconocene, chain transfer to metal is much more likely to take place after secondary insertion, making the reaction favorable over chain propagation. The effect not being detected for the analogous hafnocene, one may ask if this is in relationship with the experimental observations1,2 that hafnocenes generally produce higher molecular weight polymers than zirconocenes and that zirconocenes typically tend to deactivate after 2,1 insertion. One should keep in mind, though, that the experimental polymerization catalysis is a very complex process involving numerous influential factors. Nonetheless, within the scope of the present study, with focus on the fundamental differences in the reactions between the two metallocenes and olefin monomers, the primary difference in the behavior of the zirconocene and hafnocene can be seen in the activation energies for β-hydrogen transfer to the metal, which are significantly higher for the hafnocene.
4. Conclusions
Figure 11. Comparison of the transition states of the insertion and termination by β-hydrogen transfer: (a) 1,2 insertion vs termination to primary chain end; (b) 2,1 insertion vs termination to primary chain end; (c) 1,2 insertion vs termination to secondary chain end; and (d) 2,1 insertion vs termination to secondary chain end.
four combinations: primary-primary, primary-secondary, secondary-primary, and secondary-secondary. Concerning the influence of the chain end (Figure 11, a vs c and b vs d) the heights of the transition states after primary insertion are systematically lower for propagation than for termination,
A quantum chemical study on homopolymerization of ethene, propene, 1-butene, and 1-hexene by a bis(cyclopentadienyl) zirconocene and hafnocene was reported. The focus of the study was on comparison of the elementary reactions between the metallocenes and the olefin monomers, leading to chain propagation and chain termination. Aiming at discriminating the fundamental differences between the two metals, a systematic study was carried out, taking into consideration the reactions affecting stereo- and regiochemistry, the role of chain end for both propagation and termination, and the competition between chain growth and chain termination, ultimately deciding the molecular weight of the polymer. Concerning chain growth, the hafnocene appears to be somewhat better in terms of regiocontrol. However, the most distinct differences between Zr and Hf originate from the reactivity of the chain end, in particular in the case of chain transfer to the metal. The origin of the dissimilarity can be traced to stronger M-C bonds but weaker M-H agostic interactions in hafnocenes, culminated in a preference of
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different transition states for chain transfer to the metal by the two metallocenes, thereby making the elimination barrier after 2,1 insertion distinctly lower for the zirconocene. Likewise, due to the variations in the bond strengths, the hafnocene prefers a π-complex with no agostic interactions, i.e., the one preceding chain propagation, whereas the zirconocene has no such preference.
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Acknowledgment. M.L. gratefully acknowledges funding from the Academy of Finland (project 130815). Supporting Information Available: A listing of Cartesian coordinates and absolute energies of all reported molecular structures. This material is available free of charge via the Internet at http://pubs.acs.org.
