Titanium Complexes in the Early Stages of Olefin Polymerization

Nov 2, 2010 - University of Opole, Faculty of Chemistry, Oleska 48, 45-052 Opole, Poland. Received May 14, 2010. The behaviors of three structurally s...
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Organometallics 2010, 29, 6196–6200 DOI: 10.1021/om100470d

Structural Flexibility of Bis(phenoxyimine) Titanium Complexes in the Early Stages of Olefin Polymerization Process: A DFT Study Zygmunt Flisak* and Patrycja Suchorska University of Opole, Faculty of Chemistry, Oleska 48, 45-052 Opole, Poland Received May 14, 2010

The behaviors of three structurally similar salan- and phenoxyimine-based catalysts activated by perfluorophenylborate were compared in the early stages of ethylene polymerization. It was found that moderate modification of the ligand structure can dramatically reduce the interactions between the cationic active site and the counteranion and, as a result, decrease the theoretically calculated upper bound to the ion separation barrier from 15 to 2 kcal/mol. The interactions between the ions in the ion pair have further repercussions on the structure of the active sites (octahedral vs square pyramid), transition states and thus the insertion barriers.

1. Introduction The last six decades have witnessed an enormous development in industrially important catalytic processes, including low-pressure olefin polymerization. The original discovery of heterogeneous titanium-based catalysts by Ziegler and Natta in the 1950s was later augmented and supplemented by other significant breakthroughs: the introduction of homogeneous, metallocene-based systems in the 1980s and;more recently;the development of the post-metallocene class of catalysts, which comprise bis(phenoxyimine) systems, abbreviated as FI.1-3 The FI catalysts, invented within the so-called ligandoriented catalyst design approach,4 are based on earlytransition-metal complexes with electronically flexible ligands.5 It is believed that these nonsymmetric bidentate ligands that contain the nitrogen and oxygen atoms of markedly different electronic properties assist in attaining considerable catalytic activities in the process of polymerization by accepting electrons from the coordinated olefin and releasing them when necessary.5 This mechanism is confirmed computationally by variations in the metal-nitrogen bond length that take place in the course of polymerization, while the metal-oxygen bond length remains practically intact.6 In contrast, relatively little is known about the structural flexibility of FI ligands and its influence on the properties of *To whom correspondence should be addressed. E-mail: zgf@uni. opole.pl. (1) Matsui, S.; Tohi, Y.; Mitani, M.; Saito, J.; Makio, H.; Tanaka, H.; Nitabaru, M.; Nakano, T.; Fujita, T. Chem. Lett. 1999, 10, 1065. (2) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Tanaka, H.; Fujita, T. Chem. Lett. 1999, 12, 1263. (3) Makio, H.; Fujita, T. Acc. Chem. Res. 2009, 42, 1532. (4) Kawai, K.; Fujita, T. Top. Organomet. Chem. 2009, 26, 3. (5) Matsugi, T.; Fujita, T. Chem. Soc. Rev. 2008, 37, 1264. (6) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa, N.; Takagi, Y.; Tsuru, K.; Nitabaru, M.; Nakano, T.; Tanaka, H.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2001, 123, 6847. (7) Strauch, J.; Warren, T. H.; Erker, G.; Fr€ ohlich, R.; Saarenketo, P. Inorg. Chim. Acta 2000, 300-302, 810. pubs.acs.org/Organometallics

Published on Web 11/02/2010

the catalysts based on them. The isomerism of FI complexes has been studied extensively, both experimentally and theoretically,6-11 and it was demonstrated that this phenomenon affects directly the catalytic activity;12 the models applied take into account idealized structures with bare cationic active sites. It is well established that, after activation with an organoaluminum cocatalyst, the resulting cationic active site is not fully exposed for the insertion of the alkene monomer and different competing species (including the counteranion) may severely impede the polymerization process.13 Additionally, the energies required to separate the ion pair and make the active site available to the olefin fall in a wide range.14 Finally, there is strong evidence that the possible ion pairs may adapt various structures, differing not only in the position of the counteranion and its distance from the transition metal atom but also the geometry of the cationic active site itself.15 For example, it may form either an octahedron (as in the original precursor before an activation) or a distorted square pyramid. Despite the fact that this behavior has recently been observed for the salanbased systems,15 we believe it is not specific for that particular kind of catalyst and it is very likely to be encountered in the FI catalysts, supposedly to a greater extent. Moreover, mutual interconversion between the geometrical isomers may occur even at the stage of the precursor for relatively rigid salan-based complexes (see ref 16); therefore, it should (8) Cherian, A. E.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2005, 38, 6259. (9) Davidson, M. G.; Johnson, A. L.; Jones, M. D.; Lunn, M. D.; Mahon, M. F. Eur. J. Inorg. Chem. 2006, 4449. (10) P€arssinen, A.; Luhtanen, T.; Klinga, M.; Pakkanen, T.; Leskel€a, M.; Repo, T. Organometallics 2007, 26, 3690. (11) Flisak, Z. J. Mol. Catal. A 2010, 316, 83. (12) P€arssinen, A.; Luhtanen, T.; Pakkanen, T.; Leskel€ a, M.; Repo, T. Eur. J. Inorg. Chem. 2010, 266. (13) Vanka, K.; Ziegler, T. Organometallics 2001, 20, 905. (14) Xu, Z.; Vanka, K.; Firman, T.; Michalak, A.; Zurek, E.; Zhu, C.; Ziegler, T. Organometallics 2002, 21, 2444. (15) Ciancaleoni, G.; Fraldi, N.; Budzelaar, P. H. M.; Busico, V.; Macchioni, A. Dalton Trans. 2009, 41, 8824. (16) Meppelder, G.-J. M.; Fan, H.-T.; Spaniol, T. P.; Okuda, J. Organometallics 2009, 28, 5159. r 2010 American Chemical Society

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be present in more flexible FI species. All these profound structural changes must in turn act on the energetic profiles of the polymerization process, but their final effect seems to be unclear at present. The dilemmas discussed above motivated us to assess the structural flexibility of the active sites based on titanium bis(phenoxyimine) complexes along the entire polymerization reaction path;from the isolated ion pair resulting from the activation of the precursor with B(C6F5)3, through the ethylene uptake, to the insertion transition state;and its influence on the thermodynamics and kinetics of the polymerization. The FI-based catalysts will be compared with a structurally more rigid salan system developed by Kol.17 It should be mentioned that the electronic properties of the imine nitrogen atom in the FI ligand and the amine nitrogen atom in the salan molecule are markedly different. Therefore, a tetradentate [ONNO] ligand with two imine nitrogen atoms would be more appropriate as a reference molecule. Unfortunately, it is difficult to devise such a structure suitable for calculations. For example, salen ligands, which are posssible candidates, have too little flexibility and predominantly form octahedral complexes, in which the donor atoms occupy the vertices of the base.

2. Computational Details DFT calculations were carried out by using the ADF 2009.01 program,18-22 and the entire molecule was treated quantum mechanically, without the QM/MM approximation. The functional applied was made up of the exchange correction by Becke23 and the correlation correction by Perdew24 with the Vosko, Wilk, and Nusair parametrization of the electron gas.25 A valence triple-ζ Slater-type orbital basis set was applied to the transition-metal atom (the core definition used in the frozen core approximation extended up to 2p) and the double-ζ basis set augmented with a single polarization function for the C, H, N, and O atoms. The molecular density and the Coulomb and exchange potentials were fitted with an auxiliary s, p, d, f, and g set of Slater-type orbital functions26 centered on each nucleus. The geometry convergence criteria were 1.0  10-4 au for energy and 1.0  10-3 au A˚ for gradients. The integration parameter was set at 5.0. Analytical frequencies were calculated for each ion pair, cationic active site, and the insertion transition state, but the energies reported are without the zero-point corrections. The separation of counteranions was calculated by linear transit with a step of 0.1 A˚, where the reaction coordinate was the distance between titanium and the methyl group of the counteranion (RC1). Linear transit for the olefin uptake was carried out using the differential reaction coordinate described in ref 27 with a step of 0.03 A˚ for ion pairs (RC2). For the cationic active sites, the reaction coordinate was the distance between the midpoint on the ethylene carbon-carbon (17) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000, 122, 10706. (18) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931. (19) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41. (20) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322. (21) Te Velde, G.; Baerends, E. J. Phys. Rev. B 1991, 44, 7888. (22) Fonseca Guerra, C.; Snijders, J. G.; Te Velde, G.; Baerends, E. J. Theor. Chim. Acta 1998, 99, 391. (23) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (24) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (25) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (26) Krijn, J.; Baerends, E. J., Fit Functions in the HFS Method; Technical Report; Department of Theoretical Chemistry, Free University: Amsterdam, The Netherlands, 1984. (27) Vanka, K.; Xu, Z.; Ziegler, T. Organometallics 2004, 21, 2900.

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Figure 1. Phenoxyimine (left) and salan (right) ligands. double bond and the titanium atom (titanium-olefin distance, RC20 ) with a step of 0.05 A˚. The reaction coordinate in the transition state search, following ethylene uptake and counteranion displacement associated with it, was the distance between the carbon of the methyl group attached to the titanium and the carbon atom of ethylene (RC3). The results of the linear transit served as a starting point for the transition state optimization. The QM/MM procedure applied in one case includes a validated model of the counteranion and is described elsewhere.14 The solvent applied in the COSMO model28 was toluene (ε = 2.38). Single-point calculations were performed on the selected structures fully optimized in the gas phase.

3. Results and Discussion Post-metallocene catalysts are different from typical metallocenes, not only in terms of geometry of the starting material (octahedral vs tetrahedral coordination of the central atom) but also in terms of less conspicuous but more important aspects of the mechanisms in the early stages of polymerization.15 Restricting our analysis to the apparently similar titanium complexes with FI and salan ligands, we have compared the behavior of three selected systems, mainly in activation; for certain systems also the first event of ethylene insertion was taken into account. These were (see Figure 1): 1. Ti complex with the salan ligand and two methyl groups attached to the Ti atom, I 2. Ti complex with two FI ligands, R = CH3, and two methyl groups attached to the Ti atom, II 3. Ti complex similar to II, but with R = Ph, III All the calculations were carried out for the most stable isomer of the FI species, which is N,N-cis-O,O-trans.11 For the tetradentate salan complexes, fewer isomers are possible, due to the constraints imposed by the bridge connecting the nitrogen atoms. Thermodynamically, the most stable isomer is also N,N-cis-O,O-trans.16 3.1. Counteranion Binding Strength. The first indication of the differences in the affinity of the FI- and salan-based cationic active sites toward the [MeB(C6F5)3]- counteranion is the distance between the transition-metal atom and the methyl group of the counteranion (Table 1). Our calculations (28) Klamt, A.; Schuurmann, G. J. Chem. Soc., Perkin Trans. 1993, 799.

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Table 1. Counteranion Distances in the Ion Pairs (A˚) ion pair

Ti-Ccounterion

I

II

III

2.461

2.488

2.492

Figure 3. Schematic representation of the counteranion separation from the octahedral ion pair (left), which leads to two isomeric distorted-square-pyramidal cationic active sites (right): (purple) titanium; (gray) carbon; (blue) nitrogen; (red) oxygen.

Figure 2. Counteranion separation from the I (empty dots), II (filled dots), and III (filled triangles) systems. All energies are relative to the isolated substrate: i.e., ion pair. For a description of the RC1 reaction coordinate (Ti-C distance), expressed in A˚, see the Computational Details.

show a slightly shorter Ti-C bond for I, which means that the counterion binds more strongly to the salan complex than to the FI-based species. A comparison of the counteranion separation profiles for these ion pairs obtained by linear transit along the RC1 reaction coordinate;see Figure 2;leads to similar conclusions. The process requires 15 kcal/mol in the case of I, and ca. 8 kcal/mol for II, it is almost barrierless for III. However, different intermediates are observed in the course of separation for these ion pairs. In the salan-based species, I, the methyl group of the counteranion is first displaced to 4.66 A˚ and the energy maximum is reached on the separation profile. Further displacement brings one of the fluorine atoms close to the titanium atom; this process coordinatively saturates the transition-metal atom, stabilizes the system by almost 9 kcal/mol, and causes discontinuity in the energy curve. However, the most fundamental observation is that the titanium retains octahedral coordination throughout the process, with either a free coordination site or a fluorine atom in one of the basal positions. The behavior of the phenoxyimine-based ion pair II is similar. Interestingly, when an alternative reaction coordinate, i.e. Ti-B distance, is selected for II, the formation of the Ti-F contacts is prevented and a distorted square pyramid (the cis isomer; see Figure 3) is formed very early in the course of separation. However, if the III ion pair is taken into account, gradual increase in the N-Ti-N angle from 87° to 136° throughout the separation process is very apparent. As a result, the titanium atom becomes five-coordinated and the distortedsquare-pyramidal geometry of the complex is adapted (the trans isomer). Such a rearrangement displaces the counteranion and;most of all;prevents any Ti-F contacts, unlike in the I and II ion pairs. It should be stressed that there exist two isomeric square-pyramidal complexes: cis and trans (see

Figure 3). The trans form is produced as a result of counteranion separation from the III ion pair. In our opinion, the steric hindrance of phenyl substituents prevents the formation of the cis isomer, unlike in the II ion pair. On the other hand, in the isolated cationic active sites, titanium can retain octahedral geometry with a free coordination site in one position, or rearrange to a distorted square pyramid (or equivalent distorted trigonal bipyramid). Computationally, such structures have already been reported, at least for the phenoxyamine active sites.29 Our calculations indicate that, for the salan-based species (derived from I), the square pyramid is favored by 10.9 kcal/mol, whereas for the FI species (the II system) we did not manage to optimize the octahedral species at all, probably due to a lack of a bridge which would otherwise keep the nitrogen atoms cis to each other and stabilize the octahedral geometry. In this case, the trans isomer of the square pyramid is favored by 3.4 kcal/mol. Before, it has been demonstrated that the square pyramid is also preferred for the zirconium analogue of I.15 It should be stressed that in both (I and II) squarepyramidal active sites, the original N,N-cis-O,O-trans arrangement is lost and either salan or FI molecules become the basal ligands. This phenomenon might increase the propensity of the cationic active sites toward the formation of the other geometrical isomer (N,N-cis-O,O-cis) in the presence of the sixth ligand, such as ethylene or the counteranion, which coordinating to the cationic active site trans to the methyl group recreates an octahedral geometry. On the other hand, if such an incoming ligand approaches the basal plane and takes the position trans to the nitrogen atom, the original N,N-cis-O,O-trans geometry might be retained. The energy difference between the ions separated to infinity for the I and II ion pairs equals 65.6 and 50.2 kcal/ mol, respectively. The presence of solvent stabilizes the ions and decreases both energy differences to 30.8 and 17.5 kcal/ mol. The values for I are consistent with the results obtained for the zirconium salan complex studied before.30 All these facts further reinforce our hypothesis that the salan-based (29) Saito, J.; Suzuki, Y.; Makio, H.; Tanaka, H.; Onda, M.; Fujita, T. Macromolecules 2006, 39, 4023. (30) Flisak, Z.; Ziegler, T. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15338.

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Table 2. Ethylene Insertion Barriers (kcal/mol) insertion barrier isomer N,N-cis-O,O-trans N,N-trans,O,O-cis N,N-cis-O,O-cis a

Figure 4. Olefin uptake energetic profile for the cationic FI (filled dots) and salan (empty dots) systems. All energies are relative to isolated substrates: i.e., active site and olefin. For a description of the RC20 reaction coordinate (olefin distance), expressed in A˚, see Computational Details.

catalytic system binds the counteranion more strongly than its FI counterpart. We have also rerun the separation linear transit for the FI species using the QM/MM regime. In this case, the upper bound to the separation barrier was slightly lower, but;to our surprise;the whole process was exoenergetic. We suppose that the reason for such a result is the inadequate description of Ti-F interactions in the QM/MM model, which leads to an energetically favorable rearrangement of the cation to a square pyramid as if it were isolated from the counteranion. 3.2. Formation of the π-Complexes. To obtain qualitative results concerning the olefin uptake to the active sites, we first performed our calculations on the isolated FI- and salan-based cationic active sites using the RC20 reaction coordinate. It is well-known that the formation of the π-complex for the FI-based system is either endoenergetic or slightly exoenergetic.31 Taking into account the entropic contribution, it is unlikely that any of these complexes form at room temperature. Our calculations yield similar results for the FI-derived active site II; however, the salan-based active site I binds ethylene more strongly;see Figure 4. It is worth mentioning that the geometry of the FI complex reverts from a distorted square pyramid, preferred for the bare cationic site, to an octahedron during the uptake simulation run. Next, we calculated the energy profile for the ethylene uptake to the FI system with the counteranion present, following the RC2 reaction coordinate. It is not surprising that the olefin cannot bind to form a π-complex, as in the case of the bare cationic species. Interestingly, the active site undergoes rearrangement to the distorted-square-planar geometry very early in the reaction path and the counteranion is displaced quite rapidly. We believe that this is caused by the disrupted Ti-F interactions (cf. counteranion separation process described in the previous section) brought about by the presence of olefin which separates both ions. Furthermore, the cis isomer of the distorted square pyramid is formed. (31) Yakimanskii, A. V.; Ivanchev, S. S. Dokl. Phys. Chem. 2006, 410, 269.

ethylene position

no counteranion

counteranion present

trans to N trans to O trans to N trans to O

4.4 18.0 13.7 12.5

6.9a 19.4a 17.1 11.0a

“Brute force” method of optimization; see the text for details.

3.3. Olefin Insertion. Proceeding along the reaction path for II, we changed the reaction coordinate from RC2 to RC3 and located the approximate ethylene insertion transition state. It was then fully optimized to the transition state, whose energy is 17.1 kcal/mol higher than the energy of the isolated ion pair and ethylene. It should be reiterated here that the ligand pattern in the insertion transition state is now N,N-cis-O,O-cis as a result of the rearrangement occurring in the olefin uptake phase. Due to this rearrangement, we were unable to optimize the insertion transition state corresponding to the initial N,Ncis-O,O-trans coordination mode by applying the steps described above: i.e., performing ethylene uptake along the RC2 reaction coordinate and then following the RC3 reaction coordinate toward the insertion transition state. However, the “brute force” method of optimization, i.e. starting from the reasonable initial geometry with the counteranion located at an arbitrary position ca. 5 A˚ from the titanium atom, followed by the linear transit along RC3 and the final optimization without any constraints, yielded the desired structure, whose energy was only 6.9 kcal/mol higher with respect to isolated substrates. Irrespective of the counteranion presence, there are identical trends in the insertion barriers, e.g. the N,N-cis-O,O-trans transition state has always the lowest energy. We suppose that the insertion transition state that retains the ion pair’s ligand arrangement, although having much lower energy than its counterpart, is not easily accessible for the FI system in the course of the reaction. Obviously, this statement does not apply in the case of the salan system, where the -CH2CH2- bridge attached to both nitrogen atoms might facilitate retention of the original octahedral geometry in the neutral species. However, the presence of this bridge is not sufficient to block the isomerization between octahedral and square-pyramidal species, which was demonstrated in section 3.1 for the cationic active sites. Therefore, we suppose that other modifications of the FI ligands that might hinder the isomerization mentioned above would probably make this particular transition state more feasible. The energetic barriers reported in Table 2 indicate that the counteranion has little influence on the ethylene insertion catalyzed by a selected phenoxyimine system derived from II.

4. Concluding Remarks The influence of the counteranion on the overall thermodynamics and kinetics of the coordinative olefin polymerization has already been reported in many theoretical works. Our calculations add a certain refinement to the description of the initial stages of the process and demonstrate that the counteranion binding strength can vary in a great range for apparently similar active sites. The three examples of titanium complexes discussed in this work explain how the joint

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effect of structural flexibility of phenoxyimine ligands and the substituents within their framework suppresses the barrier of counterion separation. We believe that this barrier can be further reduced by inclusion of the solvent in the computational model.30 In such a situation, the presence of the counteranion itself does not exert much influence on the energetic profiles of the polymerization process and the insertion becomes the rate-limiting step. In contrast, the counteranion mode of binding and separation controls the isomerism of the resulting active sites at early stages of the polymerization process and thus affects the insertion barriers indirectly. The results of our calculations are consistent with the experimental results; indeed, the activity of a certain tetradentate bridged system turned out to be lower than that of its unbridged analogue.12 We suppose that the former has higher affinity to the counteranion and thus a higher barrier of separation. The warning against tracing analogies between markedly different classes of coordinative olefin polymerization catalysts (such as the classical systems, metallocenes and (32) Talarico, G.; Busico, V.; Cavallo, L. J. Am. Chem. Soc. 2003, 125, 7172.

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post-metallocenes), formulated earlier by other authors,15 has to be reiterated here. Theoretical studies of phenoxyiminebased systems require careful analysis of an illusive interplay of electronic, steric, and structural effects.32 Under no circumstances do we suggest that the counteranion be neglected in future theoretical studies on olefin polymerization. The literature reports many examples where its presence seriously influences energetic profiles of this process. The unprecedented activities of phenoxyimine catalysts can probably be attributed to many factors: perhaps a weak interaction between the active site and the counteranion is just one of them?

Acknowledgment. This work was supported by the Polish Ministry of Science and Higher Education (Grant No. N N205 267835). The Wroclaw Supercomputing and Networking Centre as well as the Academic Computer Centre CYFRONET AGH (Grant No. MNiSW/SGI3700/UOpolski/126/2006) are acknowledged for a generous allotment of computer time. We thank Alexander Yakimansky for supplying Cartesian coordinates of selected π-complexes, Krystyna Czaja for advice on experimental and industrial aspects of catalysis and the reviewers for careful analysis of the manuscript.