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Organometallics 2009, 28, 2609–2618

2609

Stereoregularity, Regioselectivity, and Dormancy in Polymerizations Catalyzed by C1-Symmetric Fluorenyl-Based Metallocenes. A Theoretical Study Based on Density Functional Theory Simone Tomasi,† Abbas Razavi,§ and Tom Ziegler*,† Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta, Canada T2N1N4, and Total Petrochemicals Research Feluy, Zone Industrielle C, B-7181 Seneffe (Feluy), Belgium ReceiVed January 19, 2009

Cs-Symmetric propylene polymerization catalysts 1 with a bridged cyclopentadienyl and fluorenyl architecture are known to produce syndiotactic polymers. On the other hand, related C1-symmetric catalysts, such as 2, that are obtained from 1 by the introduction of a bulky substituent (tert-butyl) on the cyclopentadienyl ring afford isotactic polymers. In this study we employ DFT calculations in order to analyze several aspects of olefin polymerizations catalyzed by the fluorenyl-based C1-symmetric zirconocene 2. Modeling of the propagation in naked cationic systems, disregarding the noncoordinating counterion, yields information on the factors that affect streoselectivity (and ultimately stereoregularity), regioselectivity, and reactivity of the “crowded” site of the zirconocene relative to the “open” one. Several hypotheses are investigated, with the aim to rationalize the experimental observation that 2 affords isotactic polymers whereas 1 gives rise to syndiotactic polymers. We provide in addition an analysis of the stability of dormant species 5 produced from 2,1 propylene mis-insertions. For this task, the need to include explicitly the counterion in the modeling seems to be inevitable. Comparative studies of the energetics of β-H elimination to the metal or β-H transfer to the monomer, relative to insertion into a Zr-secondary C bond, indicate that dormant species 5 are prone to β-H elimination. Introduction The first stereospecific olefin polymerization catalyst bearing a bridged cyclopentadienyl and fluorenyl architecture1 was the Cs-symmetric system 1, shown in Scheme 1. This catalyst has proven to be highly efficient in the polymerization of syndiotactic polypropylene (sPP). Subsequent experimental2-9 and computational10-20 studies by several groups have established the relation between the structure of metallocene catalysts and the stereochemistry of the polyolefin. It has thus become possible rationally and systematically to introduce alterations to the metallocene structure that has allowed for the production of polymers with microstructures possessing desired physicomechanical characteristics. One of the major breakthroughs in the early 1990s has been the discovery that breaking the Cs catalyst symmetry allows for the synthesis of polymers with completely different microstruc* Corresponding author. E-mail: [email protected]. † University of Calgary. § Total Petrochemicals Research Feluy. (1) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc. 1988, 110, 6255. (2) Razavi, A.; Ferrara, J. J. J. Organomet. Chem. 1992, 435, 299. (3) Kaminski, W.; Kulper, H.; Britzinger, H. H.; Wild, F. R. W. P. Angew. Chem., Int. Ed. Engl. 1985, 24, 507. (4) Ewen. J. A.; Jones, R. L.; Curtis, S., Cheng, H. N. Catalytic Olefin Polymerization; Keii T., Soga, K., Eds.; Elsevier: New York, 1990; p 439. (5) Ewen, J. A.; Elder, M. J.; Jones, R. L.; Haspeslagh, L.; Atwood, J. L. Makromol. Chem., Makromol. Symp. 1991, 48-49, 253. (6) Britzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (7) Scheirs, J., Kaminski, W., Eds. Metallocene-Based Polyolefins; Wiley: Chichester, 2000; Vols. 1, 2. (8) Alt, H. G.; Ko¨ppl, A. Chem. ReV. 2000, 100, 1205. (9) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100, 1253.

tures, where the microstructures of the new polymers depend strongly on the size of the groups employed for the structural modification of the Cs systems to produce C1 catalysts.20Using a relatively small group allows one to obtain hemi-isotactic polymers, that is, polymers in which the stereogenic carbons are alternately of random and constant chirality. These systems are very useful for the study of the fine details of the polymerization mechanism.21 However, it is from a practical point of view of more interest that an isotactic polyolefin is obtained if the group used to introduce C1 symmetry is large enough. Isotactic polymers can also be synthesized by indenylbased C2-symmetric catalysts as well. However, indenyl systems produce small undesirable fractions of atactic polymers (see Figure 1). (10) Corradini, P.; Guerra, G.; Vacatello, M.; Villani, V. Gazz. Chim. Ital. 1988, 118, 173. (11) Cavallo, L.; Guerra, G.; Oliva, L.; Vacatello, M.; Corradini, P. Polym. Commun. 1989, 30, 16. (12) Venditto, V.; Guerra, G.; M.; Corradini, P.; Fusco, R. Polymer 1990, 31, 530. (13) Cavallo, L.; Guerra, G.; Vacatello, M.; Corradini, P. Macromolecules 1991, 24, 1784. (14) Castonguay, L. A.; Rappe´, A. K. J. Am. Chem. Soc. 1992, 114, 5832. (15) Hart, J. R.; Rappe´, A. K. J. Am. Chem. Soc. 1993, 115, 6159. (16) Rappe´, A. K.; Skiff, W. M.; Casewit, C. J. Chem. ReV. 2000, 100, 1435. (17) Yoshida, T.; Koga, N.; Morokuma, K. Organometallics 1996, 15, 766. (18) Schaverien, C. J.; Ernst, R.; Terlouw, T.; Schut, P.; Sudmeijer, O.; Budzelaar, P. H. M. J. Mol. Catal. A: Chem. 1998, 128, 245. (19) van der Leek, V.; Angermund, K.; Reffke, M.; Kleinschmidt, R.; Goretzki, R.; Fink, G. Chem.-Eur. J. 1997, 3, 585. (20) Angermund, K.; Fink, G.; Jensen, V. R.; Kleinschmidt, R. Macromol. Rapid Commun. 2000, 21, 91. (21) Farina, M.; Di Silvestro, G.; Sozzani, P. Macromolecules 1993, 26, 946.

10.1021/om900044s CCC: $40.75  2009 American Chemical Society Publication on Web 04/02/2009

2610 Organometallics, Vol. 28, No. 8, 2009 Scheme 1. Cs-Symmetric Catalysts for the Generation of Syndiotactic Polypropylene

Catalysts containing additional modifications have been introduced with time, aiming at increasing the stereo- and regioselectivity. One such system is 2 developed by Razavi and Thewald.22 This system will be the subject of the present study (see Scheme 2). On the basis of experience gained for other C1-symmetric systems with a bulky substituent,6,17 it was argued that the two bulky tert-butyl groups on one side of 2 make that site unavailable for coordination of the growing polymer chain, while the methyl group in the 2-position of the Cp ring creates steric interactions that force coordination of the incoming olefin (usually propylene) with the desired regiochemistry. The usual alternation of coordination site during the migratory insertion mechanism would therefore be interrupted, and a site epimerization event would follow each insertion. Another different hypothesis has been brought forward by Bercaw and Miller, who have studied a C1-symmetric system derived from the original Cp-Flu system of Ewen and Razavi similar to 2.23 These authors presented experimental results suggesting that at least for their system site epimerization does not occur at every step and that the “crowded” site can accommodate the polymer chain, and therefore is also reactive, although to a lesser extent and with lower stereoselectivity. It is therefore of obvious interest to assess whether a strict site epimerization mechanism is actually in place. If this is not the case, one would further like to establish how reactive the catalyst is with the growing chain situated in the “crowded” site. We shall finally in the first part of our study attempt to explain why C1-symmetric catalysts such as 2 afford isotactic polymers whereas Cs-symmetric catalysts such as 1 give rise to syndiotactic polymers. The 2,1 mis-insertion of a propylene monomer is a common regioerror in polymerization catalyzed by Cp-Flu systems. The resulting species from such a regioerror has a secondary

Figure 1 Scheme 2. C1-Symmetric Catalyst for the Generation of Isotactic Polypropylene

Tomasi et al.

R-carbon attached to the metal. Such a conformation might inhibit further propagation and lead to dormancy. We investigate in the second part of the study the susceptibility of the potentially dormant site toward insertion as well as termination through β-hydrogen elimination and transfer of β-hydrogen to monomer. The second part of the study supplements a previous investigation24 where we have compared the rate of termination and propagation in polymerization catalyzed by Cp-Flu systems in order to explain why these catalysts afford high molecular weight polymers in ethylene/propylene homopolymerization compared to low molecular weight polymers in ethylene/ propylene copolymerization.

Computational Details Density functional theory (DFT) calculations on the systems of interest were carried out with the program ADF,25,26 version 2005.01,27 using the Perdew-Wang exchange-correlation functional (PW91).28 Double-ζ STO basis sets with a polarization function were employed for the H, B, C, F, and Cl atoms, while for the Zr atom a triple-ζ STO basis set with one p-type polarization function was employed. The 1s electrons of B, C, and F as well as the 1s-2p electrons of Cl and the 1s-3d electrons of Zr were treated as frozen core. First-order scalar relativistic corrections were applied to the systems studied.29-31 All optimizations were carried out in the gas phase without any symmetry constraint. For the insertion of propylene and ethylene into the Zr-alkyl bond, approximate transition states were located through reaction coordinate studies in which all degrees of freedom were minimized, while keeping fixed the C-C separation in the bond being formed. The reaction coordinate chosen to study β-H transfer reactions is the C-H distance in the bond being broken minus the C-H distance in the bond being formed. Using a linear combination of internal coordinates as leading coordinate in those reactions where there is concerted bond cleavage and bond formation ensures a smooth reaction profile, without discontinuities at the transition state. All stationary points were then fully optimized as minima or transition states, starting from the constrained geometries. The anion plays a fundamental role in determining the reaction rate of metallocene-catalyzed polymerizations of olefins, because of the high activation energy required to extract the anion from the ion pair to free a metal site for monomer uptake. However, the magnitude of the effect is the same for the correct insertion and for mis-insertions (causing stereoerrors or regioerrors); there(22) (a) Razavi, A.; Thewald, U. Coord. Chem. ReV. 2006, 250, 155. (b) Razavi, A.; Bellia, V.; Baekelmans, D.; Slawinsky, M.; Sirol, S.; Peters, L.; Thewald, U. Kinet. Catal. 2006, 47, 257. (c) Razavi, A.; Bellia, V.; De Brauwer, Y.; Hortmann, K.; Peters, L.; Sirol, S.; Van Belle, S.; Thewald, U. Macromol. Chem. Phys. 2004, 205, 347. (d) Razavi, A.; Verecke, D.; Peters, L.; DenDauw, K.; Nafpliotis, L.; Atwood, J. L. Proceedings of the International Symposium “40 years Ziegler Catalysts”; Fink, ; Muelhaupt; , Britzinger, Eds.; Springer Verlag: Berlin, 1995; p 112. (23) Miller, S. E.; Bercaw, J. E. Organometallics 2006, 25, 3576. (24) Wang, D.; Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2008, 27, 2861. (25) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931. (26) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391. (27) ADF2005.01, SCM; Theoretical Chemistry, Vrije Universiteit, Amsterdam: The Netherlands (http://www.scm.com). (28) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (29) Snijders, J. G.; Baerends, E. J.; Ros, P. Mol. Phys. 1979, 38, 1909. (30) Boerrigter, P. M.; Baerends, E. J.; Snijders, J. G. Chem. Phys. 1988, 122, 357. (31) Ziegler, T.; Tschinke, V.; Baerends, E. J.; Sijders, J. G.; Ravenek, W. J. Phys. Chem. 1989, 93, 3050.

C1-Symmetric Fluorenyl-Based Metallocenes Scheme 3. Naked Cationic Alkyl Zirconocenes

Organometallics, Vol. 28, No. 8, 2009 2611 Table 1. Relative Energies (kcal/mol) of Cationic Zr-Alkyl Complexes 3a-5b n-Pr 3 i-Bu 4 s-Bu 5

fore, the anion can be disregarded in the study of regio- and stereoselectivity. While the anion can be omitted in calculations for the prediction of stereoselectivity and regioselectivity, it must be included when computing absolute rates of polymerization, because often the ratedetermining step is the formation of the olefin π-complex, in which the anion is displaced by the olefin. In comparing propagation with termination processes that involve species different from the olefin π-complex, the olefin uptake is an important process that needs to be described, and therefore the anion must be included in the description. Because of the large size of the ion pair, it has been necessary to resort to a hybrid QM/MM description. Hybrid quantum-mechanical (QM) and molecular-mechanical (MM) models (QM/MM) have been applied to the geometry optimizations that included the anion, using the IMOMM scheme by Morokuma and Maseras,32 as implemented in ADF by Woo et al.33 The part of the anion more closely in contact with the cation has been described at the QM level, while the bulky fluorinated side groups of MeB(C6F5)3- were described using a modified SYBYL/TRIPOS 5.2 force field.34 Cl atoms were used to cap the QM system, in order to mimic the electron-withdrawing effect of the perfluorinated arylic groups. Reactions considered in the presence of the anion are olefin (ethylene) uptake, insertion, β-H transfer, and β-H elimination to Zr. The reaction coordinate employed in the ethylene uptake TS searches was the distance of the methide bridge to the Zr center minus the distance of the midpoint of the CdC double bond to Zr. For insertion and β-H transfer, the reaction coordinate of choice is the same used in the absence of the anion. For β-H eliminations, the reaction coordinate was the C-H distance of the bond being broken minus the Zr-H distance of the bond being formed. All energies in the following discussion are given in kcal/ mol relative to the parent reactants, as defined case by case in the text.

Discussion Cationic Zr-Alkyl Complexes. In the real system, the two front coordination sites of the catalytically active species are occupied by the alkyl group (the growing polymer chain) and the counterion. This is commonly accepted to be the resting state of the polymerization. However, in the study of stereoand regioselectivity it is convenient to use naked cationic alkyl complexes to set the reference energy. In this kind of complexes the counterion, which otherwise would occupy the vacant coordination site, is not considered. Naked cationic alkylzirconocenes 3a-5b (see Scheme 3) are divided into two main groups, depending on which coordination site is taken by the alkyl chain. The “open” coordination site is the one on the side of the 2-methyl substituent of the Cp ring, whereas the “crowded” site is that of the 4-t-Bu substituent of the Cp ring. The alkyl chains that have been considered are n-propyl (3a, 3b), i-butyl (4a, 4b), and s-butyl (5a, 5b). Here 3a and 3b are obtained by inserting ethylene into the Zr-Me bond of the (32) Morokuma, K.; Maseras, F. J. Comput. Chem. 1995, 117, 5179. (33) Woo, T. K.; Cavallo, L.; Ziegler, T. Theor. Chem. Acc. 1998, 100, 307. (34) Clark, M.; Cramer, R. D. I.; van Opdenbosch, N. J. Comput. Chem. 1989, 10, 982.

“open” site a

“crowded” site b

0.0 0.0 0.0

3.3 2.3 5.1

catalyst. They are models for the linear chain obtained in the homopolymerization of ethylene. The species 4a and 4b are the result of 1,2 propylene insertion onto the Zr-Me bond and model the ideal polypropylene chain (that is, without regioerrors). Systems 5a and 5b are the result of 2,1-propylene insertion and therefore are a model for the occasional regioerror in propylene homopolymerization. In all cases, it has been found that the naked alkyl complexes are more stable when the alkyl chain occupies the “open” coordination site. The “crowded” site is less available because of the presence of two tert-butyl groups at the front of the catalyst. Therefore, alkyl-zirconocenes in which the alkyl chain is in the “crowded” site have positive energies (see Table 1). The extent to which coordination to the open site is preferred depends on the steric requirements of the side chain: a primary alkyl chain differentiates less between the two coordination sites than a secondary alkyl. Ethylene and Propylene π-Complexes and Corresponding Insertion Transition States. Energies of the π-complexes (Table 2) and of the corresponding insertion transition states (Table 3) are reported relative to the parent Zr-alkyl cationic complex and the olefin to be inserted. The relative energies reported in Table 2 should not be considered complexation energies, since the complexation process involves substituting the counteranion with the olefin, rather than bringing the olefin in contact with the naked cation. Olefin complexation in real systems is an endothermic process, because even a poorly coordinating anion such as MeB(C6F5)3- interacts more strongly with the Zr center than an olefin (as shown in the last section). The different alkyl chains employed in the cationic Zr-alkyl complexes (n-propyl, i-butyl, or s-butyl) are the result of the insertion of a monomer unit of ethylene, propylene (with correct regiochemistry), or propylene (with wrong regiochemistry) into the Zr-Me bond of the activated catalyst, respectively. The species discussed in this study therefore refer to the second insertion of a monomer unit, which can be considered to be representative of the polymerization process. The possible olefin π-complexes can differ in the type of olefin (ethylene or propylene), the coordination site taken by the alkyl chain (“open” or “crowded”), and, in the case of propylene, the regiochemistry of coordination (2,1 or 1,2). Furthermore for each combination multiple conformations are possible, as determined by the orientation of the alkyl chain with respect to the fluorenyl group (ch-away or ch-toward) and the orientation of the alkyl chain with respect to the olefin substituent (syn or anti). As in the π-complexes of 1c previously studied by us,35 the Zr center of 2 is asymmetric, because it is coordinated to four different ligands. The chirality of the metal center is assigned using the Cahn-Ingold-Prelog rules,36,37 as extended to chiral metallocenes by Stanley and Baird38 and (35) Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2007, 26, 2024– 2036. (36) Cahn, R. S.; Ingold, C.; Prelog, V. Angew. Chem., Int. Ed. Engl. 1966, 5, 385. (37) Prelog, V.; Helmchem, G. Angew. Chem., Int. Ed. Engl. 1982, 21, 567. (38) Stanley, K.; Baird, M. C. J. Am. Chem. Soc. 1975, 97, 6598.

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Tomasi et al. Table 2. Olefin π-Complexesa olefin

parent Zr-R+

alkyl chain R

R coord site

R vs Flu orientation

R vs olefin orientationb

1,2 propylene

2,1 propylene

ethylene

4a 4a 4a 4a 4b 4b 4b 4b 5a 5a 5a 5a 5b 5b 5b 5b 3a 3a 3a 3a 3b 3b 3b 3b

i-Bu

open open open open crowded crowded crowded crowded open open open open crowded crowded crowded crowded open open open open crowded crowded crowded crowded

away away toward toward away away toward toward away away toward toward away away toward toward away away toward toward away away toward toward

anti syn anti syn anti syn anti syn anti syn anti syn anti syn anti syn anti syn anti syn anti syn anti syn

-10.5 (re) -9.2 (si) -4.9 (si) -7.0 (re) -5.6 (si) -4.5 (re) -6.9 (re) -8.7 (si) -8.2 (re) -6.9 (si) -4.1 (si) -2.7 (re) -2.2 (si) -3.0 (re) -8.8 (re) -6.9 (si) -12.2 (re) -9.9 (si) -6.1 (si) -8.3 (re) -8.6 (si) -5.3 (re) -12.4 (re) -11.2 (si)

-8.2 (re) -6.8 (si) -3.5 (si) -4.7 (re) -2.1 (si) -2.8 (re) -8.0 (re) -6.2 (si) -4.8 (re) -5.1 (si) 3.0 (si) 1.3 (re) -1.8 (si) -4.0 (re) -1.4 (re) -5.6 (si) -8.1 (re) -7.3 (si) -3.9 (si) -4.0 (re) -6.0 (si) -6.8 (re) -10.5 (re) -8.1 (si)

-6.7

s-Bu

n-Pr

-5.1 -0.6 -4.0 -6.2 -1.3 -0.4 -7.3 -6.8 -6.1 -4.3 -8.6

a For each complexation mode two rotamers have been optimized, but only the more stable is reported. Energies (in kcal/mol) are relative to the isolated naked cationic alkyl complex in its most stable conformation + olefin at infinite distance. b Does not apply to ethylene complexes.

Table 3. Insertion Transition States from the Lowest π-Complex for Each Reaction Pathway (open vs crowded and re vs si)a olefin parent Zr-R+

alkyl chain R

R coord site

R vs Flu orientation

R vs olefin orientationb

1,2 propylene

2,1 propylene

4a 4a 4b 4b 5a 5a 5b 5b 5b 3a 3a 3b 3b

i-Bu

open open crowded crowded open open crowded crowded crowded open open crowded crowded

away away toward toward away away away toward toward away away toward toward

anti syn anti syn anti syn syn anti syn anti syn anti syn

-6.1 (re) -3.6 (si) -5.1 (re) 0.9 (si) -4.5 (re) -2.6 (si)

2.5 (re) 0.5 (si) 1.9 (re) -4.0 (si) 5.8 (re) 6.9 (si) 11.1 (re)

s-Bu

n-Pr

-6.3 (re) -0.5 (si) -9.6 (re) -6.2 (si) -9.1 (re) -3.8 (si)

-2.6c (si) 4.0 (re) 3.0c (si) -4.6 (re) 1.0c (si)

ethylene d

d

-4.1 -4.4

d

d

a Energies (in kcal/mol) are relative to the isolated naked cationic alkyl complex in its most stable conformation + olefin at infinite distance. b :Does not apply to ethylene complexes. c Approximate TS (reaction coordinate frozen). Full optimization of the TS could not be achieved. d No TS was found on the PES.

Schlo¨gl.39 Additionally, and differently from Cs-symmetric systems, the substituted Cp ligand is not symmetric. Therefore, there are four diasteroisomeric π-complex configurations, constituting two pairs of enantiomers, depending on whether the re or the si face of the substituted Cp ligand is complexing Zr (see Scheme 4). The four diasteromers are denominated (R)-re, (S)-si (first enantiomer pair) and (R)-si, (S)-re (second enantiomer pair). The enantiomers in each pair have the same energy and reactivity, the only difference being that they are mirror images and therefore produce opposite absolute stereochemistries upon inserting propylene. Depending on the configuration of the system, one of the two faces of propylene, also termed re or si,40 is inserted preferentially. The coordination site taken by the chain and the orientation of the methyl group of propylene relative to the chain determine the absolute configuration of the stereogenic center generated during the insertion. In (R)-re (39) Schlo¨gl, K. Top. Sterochem. 1966, 1, 39. (40) Hanson, K. R. J. Am. Chem. Soc. 1966, 88, 2731.

Scheme 4. Diasteromeric Propylene π-Complexes of 2

complexes, where the chain occupies the “open” site, propylene inserts preferentially in a 1,2-re fashion. The (S)-si complexes

C1-Symmetric Fluorenyl-Based Metallocenes Scheme 5. Schematic Representation of the Possible Conformations of 1,2 Propylene π-Complexes of a C1-Symmetric Zirconocenea

a The ligand backbone is omitted for clarity. The configuration or the Zr atom is R; that is, the fluorenyl moiety is assumed to be below and the Cp above the plane formed by Zr, propylene, and the alkyl chain.

are identical except for the fact that they are mirror images, so while the chain still occupies the “open” site, propylene inserts preferentially in a 1,2-si fashion. Similarly, the (S)-re and (R)si enantiomers (where the chain occupies the “crowded” site) have identical reactivity, except for the opposite stereochemical outcome in the insertion of propylene (see following sections for details). Using a racemic catalyst has no effect on the physicomechanical properties of the polymer, because these depend on the stereoregularity and not the absolute configuration of the individual stereogenic centers. In other words, it does not matter whether an isotactic polymer has only stereogenic centers with R or S absolute configuration (the very definition of R and S chirality is lost in an infinitely long polymer chain), as long as they are all of the same kind. For this reason C1symmetric catalysts do not need to be enantiomerically pure, but can indeed be used as racemic mixtures. Therefore, in order to describe completely the system, it is necessary to study only one configuration from each enantiomeric pair. In the following discussion, the Zr center is restricted to having the R configuration. Furthermore, in order to avoid confusion with the enantiotopic face of propylene involved in the insertion, no further reference will be made to the enantiotopic face used by the Cp ligand. Instead, the π-complexes will be differentiated on the basis of the coordination site taken by the alkyl chain (“open” or “crowded”). A description of all the conformations possible for 1,2-propylene π-complexes of the system under investigation is provided in Scheme 5. For clarity the ligand backbone is omitted, but information on which is the “open” and “crowded” coordination site is retained with appropriate labels. Multiple rotamers of each configuration can be optimized, if rotational degrees of freedom of the alkyl chain further away from the Zr center are taken into account. In the present study two rotamers have been optimized for each main conformation, corresponding to a 180 degree rotation of the CR-Cβ bond. Overall, two rotamers for each of the 60 main conformations considered (16 propylene π-complexes and 4 ethylene π-complexes for each type of alkyl chain) have been optimized. The π-complex energies reported in Table 2 correspond to the most stable of each pair of rotamers. The relative stability of the olefin π-complexes of cationic alkyl complexes 3a-5b is the result of a more complex interplay of factors than previously observed for the similar Cs-symmetric system 1c. For the parent Cs-symmetric system 1c the methyl

Organometallics, Vol. 28, No. 8, 2009 2613

group of propylene and the polymer chain are in an anti arrangement in the most favorable conformation, while the chain points away from the fluorenyl ligand34 (from now on, this conformation will be called chain-away/anti). In the present, more complex system other factors play an important role. When the alkyl chain occupies the “open” site, the preferred π-complex conformation is determined by the same factors described for 1c. However, when the alkyl chain occupies the “crowded” site, a strong repulsive interaction with the tert-butyl group of the Cp ring forces the chain in a chain-toward arrangement. This arrangement is preferred over the usually more stable chainaway one, even though this entails paying a (smaller) energy penalty for the unfavorable interaction with the tert-butyl group on the fluorenyl ligand. This can clearly be seen in Table 2, where for all π-complexes of 3b, 4b, and 5b the preferred conformation is always of the chain-toward type. In general, 2,1-propylene complexes are less stable than the corresponding 1,2-propylene complexes. Because of the increased steric repulsion between the Me group of propylene and the ligands, in 2,1-propylene complexes the propylene molecule is forced further away from the Zr center. When the chain sits in the “crowded” site (propylene coordinated to the “open” site), the additional repulsive interaction of the Me substituent of propylene with the Me substituent on the Cp ring further distorts the π-complex. Because of the numerous additional repulsive interactions present in 2,1-propylene complexes, the commonly accepted model for predicting relative stabilities is not readily applicable to these π-complexes. In complexes in which the alkyl chain occupies the “open” position it is likely that a secondary chain experiences a stronger repulsive interaction with the methyl group on the Cp ring compared to when the alkyl chain is primary. However, this interaction seems to be weaker compared to the repulsive interaction with the tert-butyl group on the Cp ring found when the chain is in the “crowded” position, as shown in Table 1. When the chain occupies the “open” site, the chain-away/ anti arrangement is preferred, as expected. Overall the stability of the π-complexes reflects the balance between the stabilizing interaction involving the π-system of the olefin and the empty d orbital of Zr on one side and the mutually repulsive interactions involving the alkyl chain, the olefin, and the olefin and the catalyst ligand system. For ethylene and propylene (in both coordination modes), the relative energies of the resulting π-complexes are more negative for the linear n-propyl chain and become progressively less favorable with increasing bulk in the case of i-butyl and s-butyl. It is not computationally convenient nor necessary to investigate the insertion process for all the π-complexes shown in Table 2. While the stability order of the complexes is not necessarily the same for the transition states, it is quite likely that the transition states corresponding to the less stable complexes will be too high in energy to play an important role. Therefore no transition state searches have been started from those π-complexes, which generate products that can also be obtained from other more stable π-complexes. In the following sections, each aspect of selectivity (enantioselectivity, regioselectivity, and site selectivity) is analyzed, on the basis of the energies of the π-complexes and related transition states reported in Tables 2 and 3. Enantioselectivity of the Propylene Insertion Step. The widely accepted mechanism of enantiotopic site control of stereochemistry in propylene polymerizations is based on the so-called “growing chain orientation effect”, which was originally introduced by Corradini et al.13 as deduced from MM

2614 Organometallics, Vol. 28, No. 8, 2009

Tomasi et al.

Scheme 6. Competing Insertion and β-H Transfer Pathways

investigations on [rac-(Me2Si)Ind2-Zr-R]+ systems and supported and confirmed later by QM/MM studies of Casewit and Rappe´,14 Morokuma et al.,17 and Moscardi et al.41 In the fourcenter insertion-TS, the growing chain can point “up” or “down”, which correspond to the “away” and “toward” labels adopted in this paper. The orientation of the chain is driven by minimization of steric interactions with the catalyst’s backbone and with the methyl group of the incoming propylene molecule. For the same reason, the propylene molecule adopts preferentially an anti orientation with respect to the chain. This model, originally developed for C2-symmetric catalysts, can easily be extended to C1-symmetric compounds, like the one investigated here, even though additional interactions have to be considered, as mentioned in the section on π-complexes. In addition to the interactions previously rationalized by other groups, in the present system interactions involving the methyl substituent in the 2-position of the Cp ring must be taken into account. It is reasonable to assume that when an olefin is coordinated to the “open” site (that is, on the same side of the Me substituent of the Cp ring), minimization of steric interactions will further favor a 1,2 coordination mode compared to a 2,1 mode, as indeed confirmed by the results shown in Table 2. As can be seen in Table 3, in the dominant, repeated 1,2 insertion mode, 1,2-re propylene insertion is favored over 1,2si no matter whether the alkyl chain occupies the “open” or the “crowded” coordination site (the case of propylene homopolymerization with correct regiochemistry is represented by the 1,2 propylene insertion in 4a and 4b). In the former case, the 1,2-re ch-away/anti pathway is preferred over the 1,2-si chaway/syn by 2.5 kcal/mol. When the alkyl chain occupies the “crowded” coordination site, 1,2-re insertion is achieved preferentially via a ch-toward/anti conformation. The TS having such a conformation is more stable by 6.0 kcal/mol, because the competing ch-toward/syn insertion TS is strained by the repulsion between the alkyl chain and the propylene. Interestingly, while the system in which the alkyl chain occupies the “crowded” site is less reactive (the insertion TS is 1.0 kcal/mol higher, see section on site selectivity), its enantioselectivity is higher, and its stereochemical outcome is the same as for the other site. This means that propylene enchainment through repeated 1,2 insertion events gives rise to a stereoregular isotactic polymer even if an imperfect site epimerizationcontrolled mechanism is in place. Therefore experimentally it is difficult to tell from the analysis of the pentad distribution whether a perfect site epimerization-controlled mechanism is in place. (41) Moscardi, G.; Resconi, L.; Cavallo, L. Organometallics 2001, 20, 1918.

Similar results are found for the 1,2 insertion of propylene following the insertion of ethylene, modeled by the insertion into Zr-nPr instead of Zr-iBu (systems 3a and 3b). The chaway/anti pathway is preferred over the 1,2-si ch-away/syn by 3.4 kcal/mol when the alkyl chain occupies the “open” position. When the n-propyl chain is in the “crowded” site, the system is selective toward the same stereochemical outcome, and the more selective (the TS for the 1,2-re (ch-toward/syn) pathway is lower in energy than that for the 1,2-si (ch-toward/anti) pathway by 5.3 kcal/mol). The insertion with 1,2 regiochemistry into the secondary C-Zr bond of Zr-sBu (systems 5a and 5b) also favors the stereochemistry previously observed for the insertion into the primary carbon. However, it is less selective: when the s-butyl chain occupies the “open” position, the TS for the 1,2-re (chaway/anti) pathway is lower in energy than the competing TS (1,2-si ch-away/syn) by 1.9 kcal/mol. The selectivity increases to 5.8 kcal/mol when the alkyl chain occupies the “crowded” position (1,2-re ch-toward/syn TS preferred over 1,2-si ch-tow/ anti TS). The reaction is also slower. The reason for it being slower is twofold: the reaction has a smaller rate constant because of the higher barrier, and Zr-sBu complexes are much less common than Zr-iBu complexes (as a consequence of the fact that 1,2 insertion is vastly dominant over 2,1 insertion). The 2,1 insertion into the primary C-Zr bond of Zr-iBu has a stereoselectivity opposite that observed for the 1,2 insertion. When the s-butyl chain is in the “open” site, the 2,1si ch-away/syn pathway is preferred over the 2,1-re ch-away/ anti by 2.0 kcal/mol, while when it is in the “crowded” site, the selectivity is 5.9 kcal/mol in favor of the 2,1-si ch-toward/ syn over the 2,1-re ch-toward/anti. This would lead not only to tail-to-tail enchainment but also to the creation of a stereogenic center directly bound to Zr, instead of one atom removed. Although in the preferred 2,1 channel propylene inserts exposing the si face instead of the re one, the reaction is of less importance because the 2,1 π-complexes and their respective insertion transition states are higher in energy than the corresponding species with 1,2 regiochemistry. Similar results are obtained for the 2,1 insertion into the primary C-Zr bond of Zr-nPr: the stereoselectivity is opposite that observed for the 1,2 insertion. In this case the selectivity for the wrong stereochemistry is 1.0 kcal/mol when the n-propyl chain is in the “open” coordination site and 5.6 kcal/mol when it is in the “crowded” site. Insertion in a 2,1 fashion into the secondary C-Zr bond of Zr-sBu is slowest for a combination of all the reasons mentioned above (Zr-iBu complexes are much more common than Zr-sBu complexes, and 2,1 π-complexes and their respec-

C1-Symmetric Fluorenyl-Based Metallocenes

tive insertion transition states are higher in energy than the corresponding species with 1,2 regiochemistry). These findings, together with the analysis presented above, clearly rationalize the experimental data, showing for this system high (almost complete) stereoselectivity of propylene enchainment. Chain end control (that is, the influence of the configuration of the previously created stereocenter on enantioselectivity) has been investigated for the parent Cs-symmetric, syndiospecific system 1c.34 It was found that the contribution of chain end control to enantioselectivity is minimal, and therefore this aspect has not been further investigated for the present system. Regioselectivity of the Propylene Insertion Step. The results for the cationic model catalyst reported in Table 3 clearly reveal the high preference for inserting propylene with 1,2 regiochemistry, rather than 2,1. The TS energies for the insertion of propylene into the Zr-iBu bond (indicating previous 1,2 propylene insertion) are -6.1 kcal/mol for the 1,2-re pathway and 0.5 kcal/mol for the 2,1-si pathway when the i-Bu group occupies the “open” site, a difference of 6.6 kcal/mol in favor of the 1,2-re insertion path. When the i-Bu group occupies the “crowded” site, the difference is 1.1 kcal/mol in favor of the 1,2 insertion path (-5.1 kcal/mol for the 1,2-re TS compared to -4.0 kcal/mol for the 2,1-si TS). The effect of a previous regioerror is captured by the data on the insertion into the Zr-sBu bond. Also in this case, propylene insertion with 1,2 regiochemistry is preferred when the alkyl chain occupies the “open” site (by 10.3 kcal/mol) as well as when it occupies the “crowded” site (by 3.7 kcal/mol). The insertion of propylene after an ethylene unit (insertion into the Zr-nPr bond) is qualitatively similar to the insertion into the Zr-iBu bond, as could be expected by the fact that in both cases the alkyl group is primary. The energy difference in favor of 1,2 insertion when the alkyl group is in the “open” site is 12.6 kcal/mol (-9.6 kcal/mol for the 1,2-re TS compared to 3.0 kcal/mol for the 2,1-si TS), whereas when it occupies the “crowded” site, the difference in favor of the 1,2 insertion is 4.5 kcal/mol (-9.1 kcal/mol for the 1,2-re TS compared to -4.6 kcal/mol for the 2,1-si TS). From the computed TS energy differences it can be concluded that the 1,2 path should be largely predominant, which is in agreement with the experimentally determined high regioselectivity for isotactic propylene homopolymerization. However, the available data suggest that the regioselectivity is somewhat lowered if the system reacts while the chain occupies the “crowded” site. Reactivity of the “Open” Site Compared to the “Crowded” Site. The prototypical Cs-symmetric systems, in which the migratory insertion mechanism results in perfect alternation of the coordination site of the growing chain, are known to produce syndiotactic polymers. The reactivity at the two sites is identical, except for the fact that they produce opposite stereochemistries upon insertion of an R-olefin (because the system is Cssymmetric). However, C1-symmetric catalysts such as 2 produce isotactic polymers. C1-Symmetric systems are different because the reactivities of the two sites are different, and in principle they can produce either stereochemistry upon insertion of an R-olefin. If the two sites produce opposite stereochemistries, or if they produce preferentially the same stereochemistry but with different selectivities, it is desirable for the formation of an isotactic polymer that only one of the sites (the more stereoselective one) react. It has been suggested by previous research6,17,22 that for C1-symmetric systems with a bulky substituent a site epimerization event occurs at each catalytic cycle, as steric repulsion forces the growing chain out of the

Organometallics, Vol. 28, No. 8, 2009 2615

crowded site. The effect of having a site epimerization event for each catalytic cycle is that the insertion occurs every time at the same site. An alternative hypothesis has been brought forward by Angermund et al.20 and by Miller and Bercaw,23 suggesting that reactivity and stereoselectivity are reduced when the polymer chain occupies the “crowded” site, but not completely inhibited. This means that when the migratory insertion brings the alkyl chain into the “crowded” site, sometimes the system does not undergo site epimerization, but a normal migratory insertion occurs instead. However, it is not clear how often this can happen. As can be seen from the transition state energies reported in Table 3, the reactivity of the system when the alkyl chain occupies the “crowded” position is not as low as one might think, suggesting that the system might react also in this situation, rather than following a perfect site epimerization-controlled mechanism. The 1,2 propylene insertion into the Zr-iBu bond is easier by 1.0 kcal/mol when the chain is in the “open” site compared to when it is in the “crowded” one, and passing to Zr-nPr, the difference decreases to only 0.5 kcal/mol. The reactivity of the two sites could not be compared for the insertion of ethylene, because the insertion of ethylene into Zr-nPr or Zr-iBu was found to proceed essentially free of any barrier on the potential energy surface. Olefin insertion into the Zr-sBu bond is easier when the chain is in the “crowded” site (by 1.8 kcal/mol for 1,2 propylene insertion and by 0.3 kcal/mol for ethylene insertion), although this is a rather unlikely event since the catalyst produces very few regioerrors. Despite the rather unexpected low reactivity difference between the two sites, this is not sufficient to demonstrate that insertion when the chain is in the “crowded” site is actually non-negligible. To have a full picture of the reactivity in this situation, it would be necessary to compare how the rate of olefin uptake, followed by migratory insertion, compares to that of site epimerization. This of course requires including the anion in the model; therefore a naked cation model cannot give this information. The different stereoselectivity of Cs- and C1-symmetric systems can be rationalized on the basis of current computational data as follows. If a strict site epimerization control is in place, insertion occurs only when the chain is in the “open” site; therefore stereoselectivity is the same for all insertions and an isotactic polymer is formed. If the site epimerization control is imperfect, insertion can also occur (to a lesser extent) when the chain occupies the “crowded” site. It has been demonstrated that the less reactive site produces the same stereochemistry as the more reactive one (with better selectivity); therefore also in this case an isotactic polymer is formed. Cs-Symmetric systems are different, because the two sites have opposite stereoselectivities and identical reactivity; perfect migratory insertion produces syndiotactic polymers and skipped insertions (site epimerizations) are the odd, unwanted event. Since the “crowded” site can also contribute to the formation of an isotactic polymer, it is difficult to unequivocally establish with experiments whether a site epimerization-controlled mechanism is strictly in place or not for 2. Determining the reactivity difference of two sites that both contribute to form an isotactic polymer, but with different selectivities, is beyond the scope of the analysis of the pentad distribution and certainly constitutes a difficult experimental challenge. Also in this case, the “crowded” site produces the same stereochemistry as the “open” one with higher selectivity. These unexpected results suggest that the “crowded” site can bring a minor contribution to the catalyst’s overall reactivity and that the site epimerization-controlled mechanism could indeed be

2616 Organometallics, Vol. 28, No. 8, 2009

defective. Since the “crowded” site can also contribute to the formation of an isotactic polymer, it is difficult to unequivocally establish with experiments whether a site epimerizationcontrolled mechanism is strictly in place or not. 2,1 Mis-insertion and Dormancy in Polymerization Catalyzed by Cp-Fu-Based Systems. We have up to now discussed the stereochemistry of Cp-Flu-based catalysts. We shall in the second part concentrate on an important aspect of their regiochemistry, namely, the formation of a (potentially) dormant species as a result of a 2,1 mis-insertion.42 We investigate in the second part of the study the susceptibility of the (potentially) dormant site toward insertion as well as termination through β-hydrogen elimination and transfer of β-hydrogen (Scheme 5) to monomer. In order to compare the rates of propagation and of β-H elimination, the anion must be included explicitly. The reason for this is that the β-H elimination occurs at the resting state when a H atom (formally, a hydride) is transferred from the β-C to the metal. The β-H elimination therefore is in competition with the olefin uptake or the insertion, depending on which of the two is the rate-limiting step of the polymerization. The β-H elimination, as well as the transfer to the monomer (that is, the β-H transfer), and propagation itself have been investigated using the usual C1-symmetric system 2c and MeB(C6F5)3- as the anion. The uptake and insertion of ethylene was studied only for a secondary Zr-sBu bond. Ethylene was chosen as the monomer, rather than propylene, because of the less complex stereochemistry and because its rate of insertion is faster than that of propylene. If side reactions are competitive with the insertion of ethylene, they will be even more so compared to the insertion of propylene. This is especially true for the β-H elimination, which is a unimolecular process whose rate is not affected by the nature of the olefin. The uptake and insertion processes have been studied for two different configurations, in which the s-butyl group occupies either the “open” or the “crowded” coordination site. For each configuration, a single conformation of the alkyl group was chosen, based on the geometry of the most stable naked cationic complex 5a or 5b, reoptimized in the presence of the anion. In the following discussion the reference state corresponds to the close contact ion pair, having the alkyl chain in the open or crowded site as appropriate, and ethylene at infinite distance (when needed for the uptake, insertion, and β-H transfer processes). The close contact ion pair is 3.7 kcal/mol more stable when the sBu chain occupies the crowded coordination site. This is a complete reversal of what was found for the naked cationic alkyl complexes, where 5a (chain in open coordination site) is more stable than 5b (chain in the crowded coordination site) by 5.1 kcal/mol (see Table 1). This is likely due to the balance between the stabilizing electrostatic interaction involving Zr and the anion and steric repulsion involving the alkyl chain and the ligands. When the anion is in the open coordination site, it can come into closer contact with the Zr center. The extra electrostatic stabilization compensates the increased steric repulsions caused by placing the chain in the crowded site. Uptake of ethylene can in principle occur in either a cis or trans fashion, relative to the anion being displaced. The cis (42) Froese, R. D. J. Olefin Polymerization using Homogeneous Group IV Metallocenes. In Computational Modeling for Homogeneous and Enzymatic Catalysis. A Knowledge-Base for Designing Efficient Catalysts; Morokuma, K., Musaev, D. G., Eds.; Wiley-VCH: Weinheim, 2008; Chapter 7, p 149.

Tomasi et al. Table 4. Energies (in kcal/mol) of Species in the Uptake and Insertion of Ethylene, Compared to β-H Transfer and β-H Elimination to Zra

“open” ch/away “crowded” ch/toward

cis trans cis trans

uptake TS

πcomplex

insertion TS

β-H transfer TS

β-H elimin TS

15.0

4.3

18.1

13.1

15.9

b

19.5

7.8

10.7

18.2 15.4 19.2

14.6

b

a

The reference state corresponds to the appropriate close-contact ion-pair (plus ethylene for the uptake, insertion, and β-H transfer processes). b Direct reaction without formation of an intermediate π-complex.

uptake channel leaves the alkyl chain in the same coordination site, whereas the trans channel pushes the alkyl chain to the other coordination site, at the same time as the anion is forced out of the first coordination shell. In agreement with previous Car-Parrinello QM/MM simulations,43 ethylene uptake is easier through a cis approach for both configurations, whereas no stable trans π-complexes have been found (see Table 4). In the case of the cis approach, a stable π-complex is instead obtained, which can eventually proceed to insertion (see Scheme 7) or to β-H transfer to the monomer. In the case of the trans approaches however, exploration of the PES has shown that only transfer of a β-H to the monomer is accessible, which occurs without prior formation of a π-complex. The relative energies of all species found in the uptake, insertion, β-H transfer, and β-H elimination are shown in Table 4. The energy barriers for the cis uptake of ethylene into the Zr-sBu+X- resting state (close-contact ion pair) are 15.0 kcal/ mol (alkyl chain on “open” site) and 19.5 kcal/mol (alkyl chain on “crowded” site). Formation of the resulting cis π-complex is endothermic by 4.3 kcal/mol in the former case and by 7.8 kcal/mol in the latter. The endothermicity of the ethylene complexation, which is not correctly reproduced using a naked cation, is due to the fact that the ion-pairing energy is more stabilizing than the π-complexation energy. The entropy penalty due to the loss of the translational and rotational degrees of freedom (the anion remains at a relatively short distance so it cannot be assumed that it is fully free to translate and rotate) is estimated to further destabilize the π-complex 10-12 kcal/mol at room temperature44 and increases as the temperature grows. The energy barriers for the insertion of ethylene into the Zr-sBu bond following cis ethylene uptake are 18.1 kcal/mol (alkyl chain in the “open” site) and 10.7 kcal/mol (alkyl chain in the “crowded” site). Interestingly, the nature of the rate-limiting step (uptake or insertion) depends on the site occupied by the alkyl chain. When the chain is in the “open” site, the barriers for the uptake and the insertion of ethylene along the cis approach are close in energy to those found in the previously mentioned Car-Parrinello QM/MM dynamics study of olefin polymerization catalyzed by the prototypical zirconocene,43 the insertion being the slower of the two processes. However, when the chain occupies the “crowded” site, the uptake is the ratelimiting step of propagation. Again, the difference in reactivity of the two sites does not seem to be enough to rule out completely the possibility that also the “crowded” site may react. Furthermore, the picture of olefin reactivity in the presence of the counterion is qualitatively consistent with the results presented in the first part of this paper, according to which there (43) Yang, S.-Y.; Ziegler, T. Organometallics 2006, 25, 887. (44) Woo, T.; Blo¨chl, P. E.; Ziegler, T. J. Phys. Chem. A 2000, 104, 121.

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Scheme 7. Cis and Trans Anion Displacement by Ethylene and Subsequent Insertion

Scheme 8. Mechanism of the β-H Transfer for Cis and Trans Approach of the Olefin

Scheme 9. Mechanism of β-Elimination from the Ion-Pair Resting State, to Give a Metal Hydride and 1-Butene

is only a modest barrier for the insertion of ethylene into the Zr-sBu bond once the π-complex has been formed. The energy barriers obtained for the insertion process in the absence of the anion are 2.1 kcal/mol for the “open”, chain away arrangement and 2.9 kcal/mol for the “crowded”, chain toward arrangement (see Tables 2 and 3). Transfer of a β-H to the monomer is directly competing with insertion. Entropic contributions in the β-H transfer to the monomer are virtually the same as in the insertion because the two reactions have the same molecularity. In the cis approach the β-H involved in the transfer is residing on the C1 methyl group of the sBu chain (see Scheme 8). One of the β-H’s of the methyl group is involved in a strong agostic interaction with the Zr center, and therefore that C-H bond is weakened. In the trans approach, the β-H involved in the transfer belongs to the methylene group of the sBu chain. One of the β-H’s of the methylene group can form a weaker agostic interaction with the Zr center, and therefore that can also somewhat facilitate the transfer. The energy barriers for the β-H transfer to ethylene in Zr-sBu cis π-complexes are 13.1 kcal/mol (alkyl chain on “open” site) and 15.4 kcal/mol (alkyl chain on “crowded” site). The energy

barriers for the direct β-H transfer to ethylene in the trans approach are 18.2 kcal/mol (alkyl chain on “open” site) and 19.2 kcal/mol (alkyl chain on “crowded” site). The β-H transfer following the cis approach is more facile than following the trans approach, regardless of which coordination site is taken by the alkyl chain. The activation energy of the β-H transfer from the cis “open” π-complex is 5.0 kcal/mol lower than that of the insertion and 1.9 kcal/mol lower than the activation energy of the uptake reverse process. The activation energy of the β-H transfer from the cis “crowded” π-complex is 4.7 kcal/mol higher than the activation energy of the corresponding insertion, but still 4.1 kcal/mol lower than that of the uptake reverse process. It is worth noting that for the “crowded” case, cis ethylene uptake is slower than both insertion and β-H transfer to monomer. The elimination of a β-H from the resting state is in direct competition with the uptake of ethylene. The general reaction, which gives a metal hydride and a macro-olefin termination product, is represented in Scheme 9. The entropic factor disfavoring the ethylene uptake does not affect the β-H elimination; therefore this termination reaction becomes increasingly more important with increasing temper-

2618 Organometallics, Vol. 28, No. 8, 2009

ature. The energy barriers for the β-H elimination in the resting state (close-contact ion pair) are 15.9 kcal/mol (alkyl chain on “open” site) and 14.6 kcal/mol (alkyl chain on “crowded” site). It is worth recalling that the olefin uptake is a bimolecular process, which is further penalized relative to the unimolecular elimination by up to 12 kcal/mol44 at room temperature (and more at higher temperatures), due to entropic factors. Elimination of a β-H from the resting state following a regioisomeric misinsertion is surprisingly facile, and when the secondary alkyl chain occupies the crowded coordination site, it is preferred over ethylene uptake even before considering the entropic contribution to the free energy. Elimination of a β-H is therefore preferred over ethylene uptake when the previous propylene insertion produced a regioerror, no matter what is the monomer being taken up or the polymerization temperature. We conclude that formation of a regioerror almost inevitably leads to chain termination and therefore must be avoided at all costs.

Conclusions Several aspects of olefin polymerizations catalyzed by C1symmetric zirconocene 2 have been considered. Studies on cationic models have allowed us to gain an understanding of the stereoselectivity and regioselectivity of the polymerization process, as well as of the different reactivity of the two coordination sites. Different dispositions of the alkyl chain representing the growing polymer relative to the substituents on the Cp-Flu ligand and on the olefin are found to be preferred energetically, depending on the coordination site taken by the alkyl chain. The complex interplay of interactions between these elements is at the base of the observed regioselectivity and stereoselectivity. Regioisomeric 2,1 mis-insertion is in all cases slower that 1,2 insertion. Both sites favor the same stereochemical outcome, differently from Cs-symmetric systems such as 1, where the two sites produce opposite stereochemistries. Another element of difference from Cs-symmetric systems is

Tomasi et al.

that although both sites favor the same stereochemistry, their selectivities and reactivities are different. In Cs-symmetric systems the two sites have the same reactivity and selectivity because they are the mirror image of each other. The results obtained from the cationic model was used to rationalize why polymers produced by 1 are syndiotactic whereas 2 affords isotactic polymers. Since both sites produce the same stereochemical outcome, both can contribute to the formation of an isotactic polymer, and therefore it is difficult to assess whether a site epimerization-controlled mechanism, as proposed by Razavi and Thewald, is strictly in place. The relatively low difference in reactivity found in some cases for the two sites suggests that a defective site epimerization-controlled mechanism might be in place, as proposed independently by Angermund et al. and by Bercaw and Miller. Further work comparing site epimerization to olefin uptake and insertion could provide more conclusive evidence. In the second part of the study we discussed the species resulting from a 2,1 mis-insertion of a propylene monomer. Such a species has a secondary R-carbon attached to the metal and might delay further propagation, thus leading to a dormant state in which the insertion of an olefin monomer is slower than other side reactions. A full model system formed of 4 and MeB(C6F5)3- as a counterion was used to compare the uptake of ethylene, followed by insertion or β-H transfer, with β-H elimination. It was indeed shown that the s-butyl chain is prone to β-H elimination, which occurs faster than olefin uptake and monomer insertion or transfer of β-H to the monomer.

Acknowledgment. This work has been supported financially by Total. T.Z. thanks the Canadian government for a Canada Research Chair in theoretical inorganic chemistry. Most of the calculations required for this research have been carried out using the WestGrid computing resources. OM900044S