Organometallics 2009, 28, 5889–5895 DOI: 10.1021/om900534w
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Theoretical Study on a Multicenter Model Based on Different Metal Oxidation States for the Bis(imino)pyridine Iron Catalysts in Ethylene Polymerization Vı´ ctor L. Cruz,*,† Javier Ramos,† Javier Martı´ nez-Salazar,† Soledad Gutierrez-Oliva,‡ and Alejandro Toro-Labbe‡ †
GEMPPO, Departamento de Fı´sica Macromolecular, Instituto de Estructura de la Materia, CSIC, Serrano 113bis, 28006-Madrid, Spain, and ‡Laboratorio de Quı´mica Te orica Computacional, Facultad de Quı´mica, Pontificia Universidad de Chile, Casilla 306, Correo 22, Santiago, Chile Received June 19, 2009
We have analyzed by DFT methods the reactivity shown by the bis(imino)pyridine catalysts based on the oxidation states, II or III, of the Fe atom. Although it has been proposed that both Fe-based species could be involved in the olefin polymerization process, there is some controversy over which of them is the more active one. Previous theoretical studies based on conceptual DFT descriptors showed that the Fe(III) cationic species was more active than the Fe(II) one. The results obtained in the present work, based on the analysis of energy profiles for the ethylene insertion and β-hydrogen chain transfer steps calculated from DFT, confirm these observations, showing that unlike the Fe(II) species, the Fe(III) oxidation state would be more active in olefin polymerization and would yield polymers with a low molecular weight fraction. This result is in agreement with experimental observations showing the characteristic bimodal polymer molecular weight distribution obtained with these catalysts. 1. Introduction The discovery of highly active bis(imino)pyridine iron complexes by Gibson’s and Brookhart’s research groups1 symbolizes a striking success in the development of new postmetallocene catalysts for the polymerization of olefins. However, some aspects of the complex behavior of these systems still remain obscure, as it has been recently revealed by several authors.2,3-5 For example, the nature and oxidation state of the active species during the polymerization process is nowadays a matter of open debate. Some authors have suggested combining M€ ossbauer and electron paramagnetic resonance (EPR) experiments so that the LFe(II)X2 (where L represents bis(imino)pyridine ligands and X are halogen atoms) precursor species are oxidized by cocatalyst (MAO) to form high-spin Fe(III) as the sole active *Corresponding author. E-mail:
[email protected]. Phone: +34915616800 (3115). Fax: +34915855413. (1) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Novel olefin polymerization catalysts based on iron and cobalt. Chem. Commun. 1998, 7, 849–850. (2) Britovsek, G. J. P.; Clentsmith, G. K. B.; Gibson, V. C.; Goodgame, D. M. L.; McTavish, S. J.; Pankhurst, Q. A. The nature of the active site in bis(imino)pyridine iron ethylene polymerization catalysts. Catal. Commun. 2002, 3 (5), 207–211. (3) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. Bis(imino)pyridine iron(II) alkyl cations for olefin polymerization. J. Am. Chem. Soc. 2005, 127 (27), 9660–9661. (4) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. Reduction of (diiminopyridine)iron: Evidence for a noncationic polymerization pathway? Organometallics 2005, 24 (26), 6298–6300. (5) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. Metal versus ligand alkylation in the reactivity of the (bis-iminopyridinato)Fe catalyst. J. Am. Chem. Soc. 2005, 127 (37), 13019–13029. r 2009 American Chemical Society
species.2 On the contrary, Bryliakov et al. have reported that there are no EPR signals attributable to the Fe(III) species upon activation of LFe(II)X2 and LFe(III)X3 with MAO.6 Consequently, these authors claimed that the Fe(II) is the only active species. On the other hand, Bouwkamp et al.3 showed that a high-spin MAO-free complex, [LFe(II)CH2SiMe2CH2SiMe3]+[MeB(C6F5)3]-, which does not contain Fe(III) at all, was active in the ethylene polymerization, although is less productive and yields higher molecular weight polymer than its counterpart high-spin LFe(II)Cl2/ MAO catalyst. Recently, G€ orl et al. have confirmed that, under the same conditions, a Fe(III) precursor is more active than a Fe(II) one (cf. catalyst 15 and 16 in ref 7). By means of electron-spray ionization tandem mass spectroscopy, a cationic complex, [LFe(II)-Me]+, was identified as the active species after MAO activation.8 However, very recently, it has been proposed on the basis of conceptual DFT descriptors than both Fe(II) and Fe(III) high-spin cationic species can coexist in the polymerization process.9 (6) Bryliakov, K. P.; Semikolenova, N. V.; Zudin, V. N.; Zakharov, V. A.; Talsi, E. P. Ferrous rather than ferric species are the active sites in bis(imino)pyridine iron ethylene polymerization catalysts. Catal. Commun. 2004, 5 (1), 45–48. (7) Gorl, C.; Alt, H. G. Influence of the para-substitution in bis(arylimino)pyridine iron complexes on the catalytic oligomerization and polymerization of ethylene. J. Organomet. Chem. 2007, 692, 4580–4592. (8) Castro, P. M.; Lahtinen, P.; Axenov, K.; Viidanoja, J.; Kotiaho, T.; Leskela, M.; Repo, T. Activation of 2,6-bis(imino)pyridine iron(II) chloride complexes by methylaluminoxane: An electrospray ionization tandem mass spectrometry investigation. Organometallics 2005, 24 (15), 3664–3670. (9) Martinez, J.; Cruz, V.; Ramos, J.; Gutierrez-Oliva, S.; MartinezSalazar, J.; Toro-Labbe, A. On the nature of the active site in bis(imino)pyridyl iron, a catalyst for olefin polymerization. J. Phys. Chem. C 2008, 112 (13), 5023–5028. Published on Web 09/28/2009
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Apart from the electronic configuration of the active species, there is some experimental evidence that is not fully understood. In particular, the broad and bimodal molecular weight distributions (MWD) observed in ethylene polymerization cannot be easily attributable to a unique active species. Furthermore, the low MW fraction is experimentally observed to be formed at the beginning of the polymerization process.1,10,11 There are two suitable explanations for the broad and bimodal molecular weight distributions. First, the propagation and chain transfer rates may change during the course of the polymerization, and second, there might be two different species operating at the same time. The first possibility was adopted by Gibson on the base of the time-dependent competency between chain transfer to aluminum and β-hydrogen transfer to monomer, producing saturated and vinyl-end polymer chains, respectively.11 At the beginning of the reaction, when the AlR3 concentration is high, the rate of the chain transfer to aluminum dominates the size of the chains, yielding a low-MW fraction. In order to produce a second fraction (high-MW fraction), the concentration of AlR3 should rapidly decrease and at longer times the ratio between the rates of the two chain transfer reactions should be reversed. However, several experimental observations strongly disagreed with this assumption.10,12 The second hypothesis is adopted by several other authors as more suitable on the basis of spectroscopic3,4,13,14 and kinetic data.12,14 Different active species are formed at the beginning of the polymerization process, depending on the reaction conditions. Some of these species are metastable and rapidly transform into the active species for olefin polymerization in the presence of the MAO cocatalyst. The nature of this active species remains a source of debate.2-4,13,15 Some authors have proposed stable Fe complexes with low electronic density on the metal atom as in Fe(III)2 or Fe complexes with reduced ligands.4 (10) Li, L. D.; Wang, Q. Synthesis of polyethylene with bimodal molecular weight by supported iron-based catalyst. J. Polym. Sci., Part A: Polym. Chem. 2004, 42 (22), 5662–5669. (11) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Stromberg, S.; White, A. J. P.; Williams, D. J. Iron and cobalt ethylene polymerization catalysts bearing 2,6-bis(imino)pyridyl ligands: Synthesis, structures, and polymerization studies. J. Am. Chem. Soc. 1999, 121 (38), 8728–8740. (12) Kissin, Y. V.; Qian, C. T.; Xie, G. Y.; Chen, Y. F. Multi-center nature of ethylene polymerization catalysts based on 2,6-bis(imino)pyridyl complexes of iron and cobalt. J. Polym. Sci., Part A: Polym. Chem. 2006, 44 (21), 6159–6170. (13) Scott, J.; Gambarotta, S.; Korobkov, I.; Knijnenburg, Q.; de Bruin, B.; Budzelaar, P. H. M. Formation of a paramagnetic Al complex and extrusion of Fe during the reaction of (diiminepyridine)Fe with AIR3 (R=Me, Et). J. Am. Chem. Soc. 2005, 127 (49), 17204–17206. (14) Talsi, E. P.; Babushkin, D. E.; Semikolenova, N. V.; Zudin, V. N.; Panchenko, V. N.; Zakharov, V. A. Polymerization of ethylene catalyzed by iron complex bearing 2,6-bis(imine)pyridyl ligand: H-1 and H-2 NMR monitoring of ferrous species formed via catalyst activation with AlMe3, MAO, AlMe3/B(C6F5) (3) and AlMe3/CPh3(C6F5) (4). Macromol. Chem. Phys. 2001, 202 (10), 2046–2051. (15) Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P.; Babushkin, D. E.; Sobolev, A. P.; Echevskaya, L. G.; Khysniyarov, M. M. Study of the ethylene polymerization over homogeneous and supported catalysts based on 2,6-bis(imino)pyridyl complexes of Fe(II) and Co(II). J. Mol. Catal A: Chem. 2002, 182 (1), 283–294. (16) Cruz, V. L.; Martinez, J.; Martinez-Salazar, J.; Ramos, J.; Reyes, M. L.; Toro-Labbe, A.; Gutierrez-Giliva, S. QSAR model for ethylene polymerization catalyzed by supported bis(imino)pyridine iron complexes. Polymer 2007, 48 (26), 7672–7678.
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There are a number of theoretical works devoted to the study of the reactivity of iron-based homogeneous catalysts for olefin polymerization.16-24 Most of them take into consideration the formation of a cationic monoalkyl complex as the active species, where mechanistic peculiarities of the reactivity of these centers were studied. From the mechanistic point of view, Ziegler et al.17,21 showed the preferred mode of ethylene coordination to the metal center is through the axial position, further confirmed by our own calculations taking into account ethylene and higher R-olefins.22,23 With respect to the electronic configuration, several authors found that high-spin states are preferred for the olefin polymerization reaction.5,6,19 Our previous work9 showed the ability of the Fe(III) species to perform ethylene polymerization even more easily than the Fe(II) complex on the basis of conceptual DFT descriptors. The different reactivity shown by the two cationic Fe(III) and Fe(II) complexes as depicted by their dual descriptors induces one to think in a different way with respect to the alkyl chain transfer reactions. On the basis of this result, we have carried out a theoretical study of the reactivity of both compounds for the ethylene insertion as well as the β-H transfer to monomer reaction, which has been postulated as the main termination reaction process.17 These observations have been confirmed by a recent paper,25 which analyzed the reactivity of different active species by DFT theoretical models, taking into account different oxidation states and different multiplicities. They concluded that the Fe(III) species is the active one that yields an ethylene oligomer. They also found that the Fe(II)-based catalyst is unable to polymerize because the energy barrier of the β-hydrogen transfer to monomer process is lower than the insertion barrier. In this paper we examine the reactivity of these species by calculation of the stationary points for the insertion as well as for the β-H transfer to monomer termination reaction along (17) Deng, L. Q.; Margl, P.; Ziegler, T. Mechanistic aspects of ethylene polymerization by iron(II)-bisimine pyridine catalysts: A combined density functional theory and molecular mechanics study. J. Am. Chem. Soc. 1999, 121 (27), 6479–6487. (18) Griffiths, E. A. H.; Britovsek, G. J. P.; Gibson, V. C.; Gould, I. R. Highly active ethylene polymerisation catalysts based on iron: an ab initio study. Chem. Commun. 1999, 14, 1333–1334. (19) Khoroshun, D. V.; Musaev, D. G.; Vreven, T.; Morokuma, K. Theoretical study on bis(imino)pyridyl-Fe(II) olefin poly- and oligomerization catalysts. Dominance of different spin states in propagation and beta-hydride transfer pathways. Organometallics 2001, 20 (10), 2007–2026. (20) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar, P. H. M. The electronic structure of (diiminopyridine)cobalt(I) complexes. Eur. J. Inorg. Chem. 2004, 6, 1204–1211. (21) Margl, P.; Deng, L. Q.; Ziegler, T. Cobalt(II) imino pyridine assisted ethylene polymerization: A quantum-mechanical/molecularmechanical density functional theory investigation. Organometallics 1999, 18 (26), 5701–5708. (22) Ramos, J.; Cruz, V.; Munoz-Escalona, A.; Martinez, S.; MartinezSalazar, J. Computational studies of the Brookhart’s type catalysts for ethylene polymerisation. Part 2: ethylene insertion and chain transfer mechanisms. Polymer 2003, 44 (7), 2169–2176. (23) Ramos, J.; Cruz, V.; Munoz-Escalona, A.; Martinez-Salazar, J. A computational study of iron-based Gibson-Brookhart catalysts for the copolymerisation of ethylene and 1-hexene. Polymer 2002, 43 (13), 3635–3645. (24) Tellmann, K. F.; Humphries, M. J.; Rzepa, H. S.; Gibson, V. C. Experimental and computational study of beta-H transfer between cobalt(I) alkyl complexes and 1-alkenes. Organometallics 2004, 23 (23), 5503–5513. (25) Raucoules, R.; de Bruin, T.; Raybaud, P.; Adamo, C. Evidence for the iron(III) oxidation state in bis(imino)pyridine catalysts. A density functional theory study. Organometallics 2008, 27 (14), 3368– 3377.
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the polymerization process using well-established DFT models. We mainly focus on the insertion and termination pathways in the second monomer complexation, which is considered an adequate model to simulate the polymerization process where the growing polymer chain is approximated by a propyl substituent.19
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Scheme 1
2. Computational Methods Geometries and energies were obtained by the BP86 method implemented in the Amsterdam Density Functional (ADF) package.26 This method comprises local density approximations according to the Vosko-Wilk-Nusair functional,27 adding nonlocal corrections to exchange according to Perdew28 and correlation according to Becke.29 Innermost atomic shells for all atoms were treated within the frozen core approximation to minimize computational effort. Outermost shells were described using a triple-ζ basis set plus a polarization function on the iron atom, and a double-ζ basis set, augmented with a polarization function, was used for the remaining atoms. Spin-unrestricted calculations have been performed to take into account the high-spin states considered for each species. The approximate reaction paths were evaluated by a linear synchronous transit (LST) calculation. The reaction coordinate for the insertion reaction was taken as the distance between the CR atom and the nearest ethylene C atom. For the β-hydrogen transfer to monomer reaction, we selected the distance from one β-hydrogen to the nearest ethylene C atom. The structure of the different stationary points was optimized without restrictions to local minima for reactants and products and to a saddle point for the transition state (TS). The nature of the different stationary points was confirmed by subsequent analytical frequency calculation. The ADF output files are available upon request to the authors. All calculations were performed without solvation corrections.
3. Results and Discussion In the present section we provide a description of the energy profiles calculated for the ethylene insertion and β-H transfer to monomer reactions in two bis(imino)pyridine Fe(II) and Fe(III) complexes as a model of the active species during the polymerization process. As it is well accepted, the insertion and transfer energy profiles can disclose information on the activity of the catalytic species. Furthermore, the assessment of the insertion versus termination competitive processes yields suitable information about the molecular weight (MW) of the polymer chains that can be obtained with each of the catalytically active species mentioned above. All the calculations were performed with the high-spin electronic configuration of each complex, i.e., S=5/2 for the Fe(III) complex and S=2 for the Fe(II) complex. Experimental evidence3 and our previous results regarding the reactivity (26) SCM ADF Software, 2008.01; SCM: Amsterdam, 2008. (27) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent electron liquid correlation energies for local spin-density calculations-a critical analysis. Can. J. Phys. 1980, 58 (8), 1200–1211. (28) Perdew, J. P.; Yue, W. Accurate and simple density functional for the electronic exchange energy-generalized gradient approximation. Phys. Rev. B 1986, 33 (12), 8800–8802. (29) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A 1988, 38 (6), 3098– 3100.
Table 1. Bond Energy (see ref 26) (in au) and Relative Energies (in kcal/mol) for π-Complexes and Transition States for Ethylene Insertion along Frontside (FS) and Backside (BS) Paths Fe(II) (FS)a Fe(III) (FS)a Fe(II) (BS)b Fe(III) (BS)b π-complex -15.7382 TS insertion -15.7156 14.2 ΔEc
-15.3875 -15.3703 10.8
-15.7342 -15.7077 16.6
-15.3817 -15.3646 10.7
a Frontside insertion path. b Backside β-H transfer path. c ΔE = ETS Eπ-complex in kcal/mol.
of different electronic states9 support the election of these spin states for both compounds. The reader is directed to the previous paper for more details about the implication of the spin in these systems. For the Fe(III) catalyst we have selected the dicationic monoalkyl since it has been established that this species is more stable than the cationic dialkyl one.25 3.1. Ethylene Insertion. Axial monomer approximation and insertion is considered the most favored ethylene insertion pathway, according to previous theoretical evidence17,23 (see Scheme 1). There are two possibilities for the monomer insertion through the axial pathway, regarding the relative conformation of the growing chain with respect to the incoming ethylene. The so-called frontside (syn) insertion occurs when the Cβ is oriented cis with respect to the incoming monomer. In the backside (anti) insertion the Cβ is positioned trans with respect to the ethylene molecule (see Scheme 1). Polymerization transfer reactions are possible only through the frontside pathway.17,23 We have calculated the energy profiles for both frontside and backside ethylene insertion paths for the two oxidation states of the bis(imino)pyridine iron catalyst along the highspin potential energy surface (PES). In Table 1, the electronic energies obtained for reactants and transition states for these processes are shown. As can be observed, the π-complex energies are slightly lower in the case of the frontside path for both catalysts by 2-3 kcal/mol. The insertion barrier corresponding to the frontside insertion (14.2 kcal/mol) is lower by 2.4 kcal/mol than the backside path (16.6 kcal/mol) for the Fe(II) case. This is an important variation with respect to the ethylene insertion process in the lowspin PES, in which the only permitted monomer insertion pathway is the backside one.17,21,23,25 However, the frontside and the backside insertion barriers are very similar for the Fe(III) catalyst (10.8 vs 10.7 kcal/mol, as shown in Table 1). Taking into account these results, we will focus only on the frontside insertion and β-H transfer mechanisms to discuss the different reactivity shown by both active species. The electronic energy profiles for ethylene insertion following the frontside path into both complexes are depicted in Figure 1.
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Figure 1. Relative bond energy (see ref 26) profile corresponding to the stationary points found for the ethylene insertion along the frontside path and termination reactions. Energies are given in kcal/mol.
Figure 2. Optimized structure for cationic iron(II) (a) and dicationic iron(III) alkyl ethylene π-complexes. Distances are given in A˚.
The monomer complexation energy can be calculated as the difference between the π-complex energy and the sum of the individual ethylene and cationic catalyst energies. As can be observed in Figure 1, the ethylene coordination is more favorable in the Fe(III) active species (-9.9 vs -6.5 kcal/mol). The geometries of both π-complexes show some differences in the coordination of the ethylene molecule to the central metal atom. We have considered that the monomer coordinates in the axial position in both cases, as was stated in the previous section. In the Fe(III) active species the ethylene molecule seems to be more weakly coordinated to the metal center than in the Fe(II) active species, as can be seen from the metal-monomer distances (see Figure 2). In addition to this, the olefin double bond is weaker in the Fe(II) case, as can be deduced from the larger C1-C2 bond distance (1.381 A˚ vs 1.357 A˚ in Figure 2). All these geometrical parameters suggest that the Fe(II) catalyst species allows a stronger interaction with the incoming ethylene monomer, which is more activated in this case. Figure 3 shows the main geometrical parameters obtained for both insertion transition states. Both transition states are slightly stabilized by an R-agostic interaction (Fe-H distances 2.101 and 2.115 A˚), and they are confirmed by the
presence of only one negative frequency. The distances between the atoms forming the four sides of the typical four-membered planar ring in this insertion reaction are longer for the Fe(III) catalyst. The transition state for the insertion reaction catalyzed by the Fe(III) species is more advanced than in the Fe(II) case. This observation is in agreement with the resulting activation energies, taking into account the Hammond postulate, so that the more the transition state resembles the reactant, the lower the activation energy. The resulting growing polymer chain is located in the axial position, as seen in Figure 4. Remarkably, these products do not show the characteristic γ-agostic interaction ubiquitous in other single-site catalysts. This can be interpreted as a high-spin complex case in which all d-orbitals are halfoccupied. Thus the charge transfer from the C-H bond is less favorable. The structure of the products is very similar in both cases, in view of the geometrical parameters shown in Figure 4. The main difference between the two catalyst systems is the longer Fe-C1 bond distance in the Fe(III) case. On the other hand, the energy barrier for ethylene insertion is higher in the case of the Fe(II) active species by ca. 3.4 kcal/mol, as can be seen in Figure 1. Taking into account
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Figure 3. Optimized structure for cationic iron(II) (a) and dicationic iron(III) transition states of the ethylene insertion. Distances are given in A˚.
Figure 4. Optimized structure for the cationic iron(II) (a) and dicationic iron(III) direct product of the ethylene insertion. Distances are given in A˚.
Figure 5. Geometry of the β-hydrogen transfer to monomer transition state for the Fe(II) and Fe(III) active species. Distances are given in A˚.
the insertion barriers for both active species, the Fe(III) catalytic species seems to be more active in ethylene polymerization than Fe(II). This observation is in agreement with the result obtained in our previous paper regarding the reactivity of these species measured through conceptual DFT descriptors.9 In addition, the resulting insertion products are, as expected, more stable than the reactants by 16.6 and 15.1 kcal/mol for Fe(II) and Fe(III), respectively (Figure 1). 3.2. β-Hydrogen Transfer Termination Reaction (BHT). The reaction profiles corresponding to the BHT to ethylene
reaction for the two catalysts are also depicted in Figure 1. The reactant is the same π-complex considered in the insertion reaction. In this case, a hydrogen atom in β-position belonging to the alkyl chain interacts with the nearest carbon atom of the ethylene monomer aided by the Fe atom. The resulting products are an ethyl chain bonded to the metal center ready to begin a new polymer chain and a vinylterminated polyolefin molecule, respectively. The energy profile shows that the H transfer to monomer is much more favored for the Fe(III) catalyst. The energy difference between termination and insertion barriers is
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Table 2. Thermal Analysis of the Different Stationary Points Found in the Insertion and BHT Processes Eo (au)a -15.7382 -15.7156 -15.7015 -15.7650 -15.7409 -15.3875 -15.3703 -15.3605 -15.4125 -15.3960
Fe(II)-π complex Fe(II)-TS insertion Fe(II)-TS BHT Fe(II)-insertion product Fe(II)-BHT product Fe(III)- π complex Fe(III)-TS insertion Fe(III)-TS BHT Fe(III)- insertion product Fe(III)- BHT product a
EZPE (au)
Etot (kcal mol-1)
Stot (kcal mol-1 K-1)
Gcorr (kcal mol-1)
0.5880 0.5874 0.5822 0.5905 0.5868 0.5880 0.5869 0.5827 0.5902 0.5867
392.02 390.83 387.35 392.70 390.81 392.88 391.26 387.52 392.43 391.86
0.22 0.22 0.21 0.22 0.22 0.23 0.22 0.21 0.21 0.23
326.36 325.32 324.02 328.91 326.63 325.59 325.78 328.08 329.40 324.72
G (au) -14.6303 -14.6098 -14.6028 -14.6503 -14.6336 -14.2806 -14.2642 -14.2597 -14.2974 -14.2918
Bond energy (see ref 26).
6.1 kcal/mol for the Fe(III) complex and 8.8 kcal/mol for the Fe(II) catalyst. In addition, from a thermodynamics point of view the BHT reaction is more exothermic for Fe(III) than for Fe(II) active species (-2.4 vs -5.8 kcal/mol). Therefore, the expected molecular weight that can be obtained with the Fe(III) active species should be substantially lower than that obtained with the Fe(II) ones. The geometry of the transition state corresponding to the BHT termination reaction shows the migrating β-H atom halfway between the Cβ and one of the monomer carbon atoms (Figure 5). At the same time, the hybridization of the C atoms implied in the process is changing from sp3 to sp2 in both alpha and beta alkyl chain C atoms and from sp2 to sp3 in both ethylene C atoms (see Figure 5). The main difference between the Fe(III) and Fe(II) transition state geometries for the termination reaction is located mainly in the position of the migrating H atom with respect to both the donor and acceptor C atoms, as can be seen in Figure 5. 3.3. Thermochemical Calculations. In Table 2, we show the main thermochemical data for the π-complexes, transition states, and products obtained from the frequency calculations in the harmonic approximation. It is worthwhile to mention that the differences between insertion and termination Gibbs free energy barriers are 4.3 and 2.8 kcal/mol for the Fe(II) and Fe(III) active species, respectively. These differences are almost half of those found for the electronic energy barriers (see discussion above). We can estimate the probability ratio of monomer insertion to termination reaction according to the Boltzmann statistics. The probability of monomer insertion is proportional to the free energy difference between the insertion transition state and the π-complex reactant.
Pins = exp
-ΔG#ins þ ΔG0π-complex
!
RT
The probability for the termination process can be equally estimated as the corresponding difference between β-H transfer to monomer transition state and π-complex reactant free energies.
Pterm = exp
-ΔG#term þ ΔG0π-complex
!
RT
Both processes share the same reactant structure, so the probability ratio of insertion to termination reaction can be calculated as
-ΔG#ins þ ΔGterm Pins=term = exp RT
!
where Pins/term is the number of insertions per termination reaction and ΔG# is the Gibbs free energy associated with
the energy barrier for the insertion and/or termination processes. This number should be considered as an approximation of the average superior bound molecular weight of the produced polymer for each catalytically active species, because other termination mechanisms, such as chain transfer to aluminum, have not been taken into account in our calculations. The probability ratio of monomer insertion to chain termination is 1410 for the Fe(II) species and 115 for the Fe(III) case. These values would correspond to molecular weights of ca. 40 000 and 3200 g/mol for the Fe(II) and Fe(III) catalysts, respectively. These values are in good agreement with the experimental molecular weights obtained with this catalyst, which are in the ranges 104-105 and 103-104 g/mol for the high and low molecular weight fractions (see Table 3 in ref 12 and Table 3 in ref 14).
4. Conclusions The results presented in this work confirm the differences found in reactivity between bis(imino)pyridine catalysts based on the Fe atom in different oxidation states. The observation made in our previous work concerning the superior reactivity of the Fe(III) compound against the Fe(II) complex for nucleophilic attack based on conceptual reactivity indexes has been confirmed by the DFT calculations of the energy profiles for both ethylene insertion and bimolecular β-hydrogen transfer reactions presented in this work. Thus, the Fe(III)-based catalyst presents a more exothermic olefin complexation process and a lower insertion barrier than the Fe(II) analogue. From this point of view, the former catalyst seems to be more active in olefin polymerization. On the other hand, the polymerization termination process by β-hydrogen transfer to monomer is easier for the Fe(III) than for the Fe(II) active species. Furthermore, the difference between termination and insertion energy barriers is lower in the Fe(III) case, indicating that lower MW is expected with this active species. A bimodal MW distribution would be expected should the two species coexist in the reaction media. In addition, the development of the low-MW fraction would be earlier than the high-MW fraction along the polymerization time. Moreover, the Fe(III) species, if present in the reaction medium, would be rapidly reduced to Fe(II), as suggested by Talsi.30 The overall behavior suggested by the calculations coincides with most of the experimental observations reported, among (30) Talsi, E. P.; Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.; Bochmann, M. Key intermediates in metallocene- and postmetallocene-catalyzed polymerization. Kinet. Catal. 2007, 48 (4), 490– 504.
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others, by Kissin.12 Although, so far, we are not able to give a suitable theoretical explanation for the formation of Fe(III) species from L2Fe(II)Cl2 precursors, we cannot rule out the coexistence of the two oxidation states in the polymerization vessel, mainly at the beginning of the reaction. Gibson2 gave some spectroscopic support for the existence of the Fe(III) oxidation state in the bis(imino)pyridine catalyst. This catalyst species will probably reduce rapidly to Fe(II) in the reductive polymerization reaction media. The redox Fe(III)/ Fe(II) equilibrium seems to be consistent with some experiments recently carried out in our group showing an increase in polymer MW upon addition of small amounts of H2 to the reaction vessel. The acceleration of the reduction of Fe(III) species to Fe(II) in the presence of H2 as a reductor agent, giving rise to higher MW, can be a plausible explanation for this behavior. In this paper we give further theoretical support to the idea that Fe(III) species are very competent in ethylene polymerization, and the reactivity behavior of the cationic Fe(III)
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complex, shown by the theoretical calculations, can explain several experimental findings.
Acknowledgment. We gratefully acknowledge financial support from the Ministerio de Ciencia e Innovaci on, Spain (Grant MAT2006-0400), CSIC (2005CL0049), and the Comunidad de Madrid (S-0505/PPQ-0328). J.R. thanks CSIC for financial support through an I3P tenure track. The authors also acknowledge Centro Tecnico de Informatica (CTI-CSIC, Madrid, Spain) and Centro de Supercomputaci on de Galicia (CESGA, Santiago de Compostela, Spain) for the use of their computational resources. Repsol-YPF is also acknowledged for useful discussions concerning molecular structure and experimental information. Supporting Information Available: XYZ coordinate files for every species described in the text . This material is available free of charge via the Internet at http://pubs.acs.org.
