pubs.acs.org/Organometallics Published on Web 11/02/2010 r 2010 American Chemical Society

6196 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 byperfluorophenylborate were compared in the early stages of ethylene polymerization. It was foundthat moderate modification of the ligand structure can dramatically reduce the interactions betweenthe cationic active site and the counteranion and, as a result, decrease the theoretically calculatedupper bound to the ion separation barrier from 15 to 2 kcal/mol. The interactions between the ions inthe ion pair have further repercussions on the structure of the active sites (octahedral vs squarepyramid), transition states and thus the insertion barriers.

1. Introduction

The last six decades have witnessed an enormous devel-opment in industrially important catalytic processes, includ-ing low-pressure olefin polymerization. The original dis-covery of heterogeneous titanium-based catalysts by Zieglerand Natta in the 1950s was later augmented and supplemen-ted by other significant breakthroughs: the introductionof homogeneous, metallocene-based systems in the 1980sand;more recently;the development of the post-metallo-cene class of catalysts, which comprise bis(phenoxyimine)systems, abbreviated as FI.1-3

The FI catalysts, invented within the so-called ligand-oriented catalyst design approach,4 are based on early-transition-metal complexes with electronically flexibleligands.5 It is believed that these nonsymmetric bidentateligands that contain the nitrogen and oxygen atoms ofmarkedly different electronic properties assist in attainingconsiderable catalytic activities in the process of polymeri-zation by accepting electrons from the coordinated olefinand releasing them when necessary.5 This mechanism is con-firmed computationally by variations in the metal-nitrogenbond length that take place in the course of polymerization,while the metal-oxygen bond length remains practicallyintact.6

In contrast, relatively little is known about the structuralflexibility of FI ligands and its influence on the properties of

the catalysts based on them. The isomerism of FI complexeshas been studied extensively, both experimentally andtheoretically,6-11 and it was demonstrated that this phenom-enon affects directly the catalytic activity;12 the modelsapplied take into account idealized structures with barecationic active sites. It is well established that, after activa-tion with an organoaluminum cocatalyst, the resulting cat-ionic active site is not fully exposed for the insertion of thealkene monomer and different competing species (includingthe counteranion) may severely impede the polymerizationprocess.13 Additionally, the energies required to separate theion pair and make the active site available to the olefin fallin a wide range.14 Finally, there is strong evidence that thepossible ion pairs may adapt various structures, differingnot only in the position of the counteranion and its distancefrom the transition metal atom but also the geometry of thecationic active site itself.15 For example, it may form eitheran octahedron (as in the original precursor before anactivation) or a distorted square pyramid. Despite the factthat this behavior has recently been observed for the salan-based systems,15 we believe it is not specific for that parti-cular kind of catalyst and it is very likely to be encountered inthe FI catalysts, supposedly to a greater extent. Moreover,mutual interconversion between the geometrical isomersmay occur even at the stage of the precursor for relativelyrigid salan-based complexes (see ref 16); therefore, it should

*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.

(8) Cherian, A. E.; Lobkovsky, E. B.; Coates, G.W.Macromolecules2005, 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.

Article Organometallics, Vol. 29, No. 23, 2010 6197

be present in more flexible FI species. All these profoundstructural changes must in turn act on the energetic profilesof the polymerization process, but their final effect seems tobe unclear at present.The dilemmas discussed above motivated us to assess the

structural flexibility of the active sites based on titaniumbis(phenoxyimine) complexes along the entire polymeriza-tion reaction path;from the isolated ion pair resulting fromthe activation of the precursor with B(C6F5)3, through theethylene uptake, to the insertion transition state;and itsinfluence on the thermodynamics and kinetics of the poly-merization. The FI-based catalysts will be compared with astructurally more rigid salan system developed by Kol.17 Itshould be mentioned that the electronic properties of theimine nitrogen atom in the FI ligand and the amine nitrogenatom in the salan molecule are markedly different. There-fore, a tetradentate [ONNO] ligand with two imine nitrogenatoms would be more appropriate as a reference molecule.Unfortunately, it is difficult to devise such a structuresuitable for calculations. For example, salen ligands, whichare posssible candidates, have too little flexibility and pre-dominantly form octahedral complexes, in which the donoratoms occupy the vertices of the base.

2. Computational Details

DFT calculations were carried out by using the ADF 2009.01program,18-22 and the entire molecule was treated quantummechanically, without the QM/MM approximation. The func-tional applied was made up of the exchange correction byBecke23 and the correlation correction by Perdew24 with theVosko, Wilk, and Nusair parametrization of the electron gas.25

A valence triple-ζ Slater-type orbital basis set was applied to thetransition-metal atom (the core definition used in the frozen coreapproximation extended up to 2p) and the double-ζ basis setaugmented with a single polarization function for the C, H, N,and O atoms. The molecular density and the Coulomb andexchange potentials were fitted with an auxiliary s, p, d, f, and gset 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 integrationparameter was set at 5.0. Analytical frequencies were calculatedfor each ion pair, cationic active site, and the insertion transitionstate, but the energies reported are without the zero-pointcorrections. The separation of counteranions was calculatedby linear transit with a step of 0.1 A, where the reaction co-ordinate was the distance between titanium and the methylgroup of the counteranion (RC1). Linear transit for the olefinuptake was carried out using the differential reaction coordi-nate described in ref 27 with a step of 0.03 A for ion pairs (RC2).For the cationic active sites, the reaction coordinate was thedistance between the midpoint on the ethylene carbon-carbon

double bond and the titanium atom (titanium-olefin distance,RC20) with a step of 0.05 A. The reaction coordinate in thetransition state search, following ethylene uptake and counter-anion displacement associated with it, was the distance betweenthe carbon of the methyl group attached to the titanium andthe carbon atom of ethylene (RC3). The results of the lineartransit served as a starting point for the transition state optimi-zation.

The QM/MM procedure applied in one case includes avalidated model of the counteranion and is described else-where.14

The solvent applied in the COSMOmodel28 was toluene (ε=2.38). Single-point calculations were performed on the selectedstructures fully optimized in the gas phase.

3. Results and Discussion

Post-metallocene catalysts are different from typical me-tallocenes, not only in terms of geometry of the startingmaterial (octahedral vs tetrahedral coordination of thecentral atom) but also in terms of less conspicuous but moreimportant aspects of the mechanisms in the early stages ofpolymerization.15 Restricting our analysis to the apparentlysimilar titanium complexes with FI and salan ligands, wehave compared the behavior of three selected systems,mainly in activation; for certain systems also the first eventof ethylene insertion was taken into account. These were (seeFigure 1):1. Ti complex with the salan ligand and two methyl groups

attached to the Ti atom, I2. Ti complex with two FI ligands, R = CH3, and two

methyl groups attached to the Ti atom, II3. Ti complex similar to II, but with R = Ph, III

All the calculations were carried out for the most stableisomer of the FI species, which is N,N-cis-O,O-trans.11 Forthe tetradentate salan complexes, fewer isomers are possible,due to the constraints imposed by the bridge connecting thenitrogen atoms. Thermodynamically, the most stable isomeris also N,N-cis-O,O-trans.16

3.1. Counteranion Binding Strength.The first indication ofthe differences in the affinity of the FI- and salan-basedcationic active sites toward the [MeB(C6F5)3]

- counteranionis the distance between the transition-metal atom and themethyl groupof the counteranion (Table 1).Our calculations

Figure 1. Phenoxyimine (left) and salan (right) ligands.

(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) FonsecaGuerra, C.; Snijders, J. G.; TeVelde, 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.

(28) Klamt, A.; Schuurmann, G. J. Chem. Soc., Perkin Trans. 1993,799.

6198 Organometallics, Vol. 29, No. 23, 2010 Flisak and Suchorska

show a slightly shorter Ti-C bond for I, which means thatthe counterion binds more strongly to the salan complexthan to the FI-based species.

A comparison of the counteranion separation profiles forthese ion pairs obtained by linear transit along the RC1reaction coordinate;see Figure 2;leads to similar conclu-sions. 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 separa-tion for these ion pairs. In the salan-based species, I, themethyl group of the counteranion is first displaced to 4.66 Aand the energy maximum is reached on the separationprofile. Further displacement brings one of the fluorineatoms close to the titanium atom; this process coordinativelysaturates the transition-metal atom, stabilizes the system byalmost 9 kcal/mol, and causes discontinuity in the energycurve. However, the most fundamental observation is thatthe titanium retains octahedral coordination throughout theprocess, with either a free coordination site or a fluorineatom in one of the basal positions. The behavior of thephenoxyimine-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-Fcontacts is prevented and a distorted square pyramid (the cisisomer; see Figure 3) is formed very early in the course ofseparation.

However, if the III ion pair is taken into account, gradualincrease in the N-Ti-N angle from 87� to 136� throughoutthe separation process is very apparent. As a result, thetitanium atom becomes five-coordinated and the distorted-square-pyramidal geometry of the complex is adapted (thetrans isomer). Such a rearrangement displaces the counter-anion and;most of all;prevents any Ti-F contacts, unlikein the I and II ion pairs. It should be stressed that there existtwo isomeric square-pyramidal complexes: cis and trans (see

Figure 3). The trans form is produced as a result of counter-anion separation from the III ion pair. In our opinion, thesteric hindrance of phenyl substituents prevents the forma-tion 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 coordi-nation site in one position, or rearrange to a distorted squarepyramid (or equivalent distorted trigonal bipyramid). Com-putationally, such structures have already been reported, atleast for the phenoxyamine active sites.29 Our calculationsindicate that, for the salan-based species (derived from I),the square pyramid is favored by 10.9 kcal/mol, whereas forthe FI species (the II system) we did not manage to optimizethe octahedral species at all, probably due to a lack of abridge which would otherwise keep the nitrogen atoms cis toeach other and stabilize the octahedral geometry. In thiscase, the trans isomer of the square pyramid is favored by3.4 kcal/mol. Before, it has been demonstrated that thesquare pyramid is also preferred for the zirconium analogueof I.15

It should be stressed that in both (I and II) square-pyramidal active sites, the original N,N-cis-O,O-transarrangement is lost and either salan or FI molecules becomethe basal ligands. This phenomenon might increase thepropensity of the cationic active sites toward the formationof the other geometrical isomer (N,N-cis-O,O-cis) in thepresence of the sixth ligand, such as ethylene or the counter-anion, which coordinating to the cationic active site trans tothe methyl group recreates an octahedral geometry. On theother hand, if such an incoming ligand approaches the basalplane and takes the position trans to the nitrogen atom, theoriginal N,N-cis-O,O-trans geometry might be retained.

The energy difference between the ions separated toinfinity for the I and II ion pairs equals 65.6 and 50.2 kcal/mol, respectively. The presence of solvent stabilizes the ionsand decreases both energy differences to 30.8 and 17.5 kcal/mol. The values for I are consistent with the results obtainedfor the zirconium salan complex studied before.30 All thesefacts further reinforce our hypothesis that the salan-based

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

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

ion pair

I II III

Ti-Ccounterion 2.461 2.488 2.492

Figure 2. Counteranion separation from the I (empty dots),II (filled dots), and III (filled triangles) systems. All energies arerelative to the isolated substrate: i.e., ion pair. For a descriptionof theRC1 reaction coordinate (Ti-Cdistance), expressed in A,see the Computational Details.

(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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catalytic system binds the counteranion more strongly thanits FI counterpart.

We have also rerun the separation linear transit for the FIspecies using the QM/MM regime. In this case, the upperbound to the separation barrier was slightly lower, but;toour surprise;the whole process was exoenergetic. We sup-pose that the reason for such a result is the inadequatedescription of Ti-F interactions in the QM/MM model,which leads to an energetically favorable rearrangement ofthe cation to a square pyramid as if it were isolated from thecounteranion.3.2. Formation of the π-Complexes. To obtain qualitative

results concerning the olefin uptake to the active sites, wefirst performed our calculations on the isolated FI- andsalan-based cationic active sites using the RC20 reactioncoordinate. It iswell-known that the formation of theπ-com-plex for the FI-based system is either endoenergetic or slightlyexoenergetic.31 Taking into account the entropic contribution,it is unlikely that any of these complexes form at roomtemperature. Our calculations yield similar results for theFI-derived active site II; however, the salan-based activesite I binds ethylene more strongly;see Figure 4. It is worthmentioning that the geometry of the FI complex reverts from adistorted square pyramid, preferred for the bare cationic site,to an octahedron during the uptake simulation run.

Next, we calculated the energy profile for the ethyleneuptake to the FI system with the counteranion present,following the RC2 reaction coordinate. It is not surprisingthat the olefin cannot bind to form a π-complex, as in thecase of the bare cationic species. Interestingly, the active siteundergoes rearrangement to the distorted-square-planargeometry very early in the reaction path and the counteran-ion is displaced quite rapidly. We believe that this is causedby the disrupted Ti-F interactions (cf. counteranion separa-tion process described in the previous section) brought aboutby the presence of olefin which separates both ions. Further-more, the cis isomer of the distorted square pyramid isformed.

3.3. Olefin Insertion. Proceeding along the reaction pathfor II, we changed the reaction coordinate fromRC2 to RC3and located the approximate ethylene insertion transitionstate. It was then fully optimized to the transition state,whose energy is 17.1 kcal/mol higher than the energy of theisolated ion pair and ethylene. It should be reiterated herethat the ligand pattern in the insertion transition state is nowN,N-cis-O,O-cis as a result of the rearrangement occurringin the olefin uptake phase.

Due to this rearrangement, we were unable to optimize theinsertion transition state corresponding to the initial N,N-cis-O,O-trans coordination mode by applying the stepsdescribed above: i.e., performing ethylene uptake along theRC2 reaction coordinate and then following the RC3 reac-tion coordinate toward the insertion transition state. How-ever, the “brute force” method of optimization, i.e. startingfrom the reasonable initial geometry with the counteranionlocated at an arbitrary position ca. 5 A from the titaniumatom, followed by the linear transit along RC3 and the finaloptimization without any constraints, yielded the desiredstructure, whose energy was only 6.9 kcal/mol higher withrespect to isolated substrates. Irrespective of the counter-anion presence, there are identical trends in the insertionbarriers, e.g. the N,N-cis-O,O-trans transition state has alwaysthe lowest energy.

We suppose that the insertion transition state that retainsthe ion pair’s ligand arrangement, although having muchlower energy than its counterpart, is not easily accessible forthe FI system in the course of the reaction. Obviously, thisstatement does not apply in the case of the salan system,where the -CH2CH2- bridge attached to both nitrogenatoms might facilitate retention of the original octahedralgeometry in the neutral species.However, the presence of thisbridge is not sufficient to block the isomerization betweenoctahedral and square-pyramidal species, which was demon-strated in section 3.1 for the cationic active sites. Therefore,we suppose that other modifications of the FI ligands thatmight hinder the isomerization mentioned above wouldprobably make this particular transition state more feasible.

The energetic barriers reported in Table 2 indicate that thecounteranion has little influence on the ethylene insertioncatalyzed by a selected phenoxyimine systemderived from II.

4. Concluding Remarks

The influence of the counteranion on the overall thermo-dynamics and kinetics of the coordinative olefin polymeri-zation has already been reported in many theoretical works.Our calculations add a certain refinement to the descriptionof the initial stages of the process and demonstrate that thecounteranion binding strength can vary in a great range forapparently similar active sites. The three examples of tita-nium complexes discussed in this work explain how the joint

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

Table 2. Ethylene Insertion Barriers (kcal/mol)

insertion barrier

isomerethyleneposition

nocounteranion

counteranionpresent

N,N-cis-O,O-trans trans to N 4.4 6.9a

N,N-trans,O,O-cis trans to O 18.0 19.4a

N,N-cis-O,O-cis trans to N 13.7 17.1trans to O 12.5 11.0a

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

(31) Yakimanskii, A. V.; Ivanchev, S. S.Dokl. Phys. Chem. 2006, 410,269.

6200 Organometallics, Vol. 29, No. 23, 2010 Flisak and Suchorska

effect of structural flexibility of phenoxyimine ligands andthe substituents within their framework suppresses the bar-rier of counterion separation.We believe that this barrier canbe further reduced by inclusion of the solvent in the com-putational model.30 In such a situation, the presence ofthe counteranion itself does not exert much influence onthe energetic profiles of the polymerization process and theinsertion becomes the rate-limiting step. In contrast, thecounteranion mode of binding and separation controlsthe isomerism of the resulting active sites at early stages ofthe polymerization process and thus affects the insertionbarriers indirectly.The results of our calculations are consistent with the

experimental results; indeed, the activity of a certain tetra-dentate bridged system turned out to be lower than that of itsunbridged analogue.12 We suppose that the former hashigher affinity to the counteranion and thus a higher barrierof separation.The warning against tracing analogies between markedly

different classes of coordinative olefin polymerizationcatalysts (such as the classical systems, metallocenes and

post-metallocenes), formulated earlier by other authors,15 hasto be reiterated here. Theoretical studies of phenoxyimine-based systems require careful analysis of an illusive interplayof electronic, steric, and structural effects.32

Under no circumstances do we suggest that the counter-anion be neglected in future theoretical studies on olefinpolymerization. The literature reports many examples whereits presence seriously influences energetic profiles of thisprocess. The unprecedented activities of phenoxyimine cat-alysts can probably be attributed to many factors: perhapsa weak interaction between the active site and the counter-anion is just one of them?

Acknowledgment. This work was supported by thePolish Ministry of Science and Higher Education(Grant No. N N205 267835). The Wroclaw Supercom-puting and Networking Centre as well as the AcademicComputer Centre CYFRONET AGH (Grant No.MNiSW/SGI3700/UOpolski/126/2006) are acknowledgedfor a generous allotment of computer time. We thankAlexander Yakimansky for supplying Cartesian coordi-nates of selected π-complexes, Krystyna Czaja for adviceon experimental and industrial aspects of catalysis and thereviewers for careful analysis of the manuscript.

(32) Talarico, G.; Busico, V.; Cavallo, L. J. Am. Chem. Soc. 2003,125, 7172.