Formation of octameric methylaluminoxanes by hydrolysis of trimethylaluminum and the mechanisms of catalyst activation in single-site α-olefin polymerization catalysis.

CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402298

Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single-Site a-Olefin Polymerization Catalysis Janne T. Hirvi,[a] Manfred Bochmann,[b] John R. Severn,[c] and Mikko Linnolahti*[a] Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n(AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n = 8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically

active sites. Catalyst activation can take place either by Lewisacidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2 + cation. Thermodynamics suggest that activation through AlMe2 + transfer is the dominant mechanism because sites that are able to release AlMe2 + are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.

1. Introduction Methylaluminoxane (MAO) is the archetypical co-catalyst in center of the catalyst and a negative partial charge is delocalsingle-site a-olefin polymerization catalysis. Its main task is to ized over the spatial area of the MAO co-catalyst, thereby leadactivate the precatalyst, typically a Group 4 metallocene coming to weak cation–anion interactions and facile separation of plex, to enable its reaction with olefin monomers, resulting in the charges by the incoming monomer to initiate the polymerthe formation of polyolefins.[1] In spite of intensive research ization process. Two general mechanisms have been proposed ever since its discovery,[2] the structural characteristics of MAO, for the formation of the ion pair: 1) abstraction of either Me and hence the mechanism of catalyst activation, is not fully unor Cl from the catalyst by a Lewis-acidic site of MAO,[3] and derstood. It is generally thought that MAOs are in dynamic 2) transfer of an AlMe2 + end group from MAO to the cataequilibrium, which may contribute to the difficulty of its struclyst[8, 9] (Figure 1). To validate the proposed mechanisms, it is [3–7] tural characterization. The lack of understanding of the essential to understand the structural features of MAOs, in parstructure and performance of MAO translates into major costs, ticular regarding its active sites. because a large excess of MAO relative to catalyst is required in the process.[3] It is assumed that the reaction between the catalyst and the co-catalyst leads to the formation of a cation–anion pair, in which a positive partial charge is localized on the metal Figure 1. Proposed general mechanisms of catalyst activation. [a] Dr. J. T. Hirvi, Prof. M. Linnolahti Department of Chemistry, University of Eastern Finland Joensuu Campus, 80101 Joensuu (Finland) E-mail: [email protected] [b] Prof. M. Bochmann Wolfson Materials and Catalysis Centre School of Chemistry, University of East Anglia Earlham Road, Norwich, NR4 7TJ (United Kingdom) [c] Dr. J. R. Severn DSM Ahead, Urmonderban 22 6160 MD Geleen (The Netherlands) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402298.

2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

In the absence of a full structural characterization, various structural alternatives have been proposed for MAO.[6, 10–24] Early proposals included chains and rings with three-coordinate Al and two-coordinate O atoms, but a crystal-structure analysis of [Al7O6Me16] showed the preference for four-coordinate Al and three-coordinate O atoms.[25] The preferred coordination numbers are attained by cage formation. However, the problem with the cages is the strain associated with cage closure, for which four-membered Al2O2 rings are required. The strain is particularly high in the presence of adjacent fourmembered rings, making small cage structures unfavorable. ChemPhysChem 0000, 00, 1 – 12

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CHEMPHYSCHEM ARTICLES The smallest cage with Al/O/Me stoichiometry of 1:1:1 obeying the “isolated square rule”, in analogy with the “isolated pentagon rule” familiar for the fullerenes, is octahedral (MeAlO)12, which was calculated to be the most stable MAO cage by Ziegler et al. in 2001.[18] This, together with the wide acceptance of cage-like structures, was supported by the synthesis of (tBuAlO)n cages in the 1990s.[26–29] The structural analogy between Me- and tBu-substituted aluminoxanes was later questioned on the basis of steric considerations.[22] More recent computational studies have suggested that extended structures, such as nanotubes, are thermodynamically favored forms of cages, which is due to the larger relative proportion of less strained six-membered Al3O3 rings in comparison to the Al2O2 rings.[22, 24] A study by de Bruin and co-workers suggests the minimum in armchair configuration lies at (MeAlO)18.[24] The structures with Al/O/Me = 1:1:1 stoichiometry do not take into account association of trimethylaluminum (TMA) into the (MeAlO)n core. The presence of TMA is known to be essential for catalyst activation, and the concentration of TMA has been shown to control both catalyst activity and polymer molecular weight.[30] Various structure proposals for MAOs involving TMA association have been reported, showing its substantial impact on the structural characteristics. TMA association has been shown to result in partial opening of the cage structures, thereby reducing the strain associated with the cage closure.[31, 32] Size estimates of MAO show significant variation, depending on the method of measurement.[2, 33, 34] Based on a combination of chemical analysis, NMR spectroscopy, and small-angle neutron scattering, we recently suggested an average molecular weight for MAO of 1800 100 g mol1 and an aggregate of about 30 Al atoms.[8] Given the various approximations involved, this is in good agreement with an estimate given by Collins, McIndoe, and co-workers, based on mass spectrometric studies, which suggested a molecular formula of (MeAlO)23(AlMe3)7 for the primary component.[35] Such compositions, involving associated TMA, are supported by the empirical formula of [Me1.4–1.5AlO0.80–0.75]n reported on the basis of proton NMR spectroscopic studies.[4] TMA association to MAO is also well explained by the most recent computational studies on the preparation of MAO by hydrolysis of TMA.[31, 32, 36, 37] Reaction between TMA and water has been shown to initiate a series of oligomerization reactions yielding MAOs. The MAOs have a general formula of (MeAlO)n(AlMe3)m where n describes the degree of oligomerization and m describes the number of monomeric TMA molecules associated with the MAO core. Here, we provide a detailed computational description of the formation of MAO oligomers, (MeAlO)n(AlMe3)m, up to n = 8, thus reaching the lower limits of its experimental size estimates.[8, 11, 12, 33, 35, 38, 39] The study, involving thousands of calculations, allows us to identify the types of sites likely present in MAO as well as the sites potentially responsible for its cocatalytic activity, as demonstrated in the second section of this paper by mechanistic investigations of the catalyst activation and ethene polymerization processes.


www.chemphyschem.org 2. Results and Discussion 2.1 Modeling MAO Formation Prior work has demonstrated that the thermodynamics of TMA hydrolysis, yielding MAOs of general formula (MeAlO)n(AlMe3)m,[31, 36, 37] can be described by a network of reactions involving three elementary species: TMA, water, and methane.[32] In terms of the elementary species, the total reaction to produce the MAOs is given in Equation (1): 0:5ðn þ mÞ Al2 Me6 þ n H2 O ! ðMeAlOÞn ðAlMe3 Þm þ 2n CH4 ð1Þ

where n is the degree of MAO oligomerization and m is the number of associated TMA monomers. Based on the total reaction, the reaction energies and relative stabilities of the MAOs can be calculated for each MAO composition. Repeating the study for all combinations of n and m would, hence, basically solve the composition of the MAO mixture, at a specified temperature, by establishing the relative abundance of each individual species. However, this is not trivial because the number of isomers for both reaction intermediates and MAO products rapidly increases as a function of size, with each isomer furthermore having separate reaction pathways. An approach taken previously to deal with the isomer problem has been to omit the irrelevant species by setting a window of 50 kJ mol1 from the local global minimum isomer. This WINDOW approach was then applied for the study of MAOs up to tetramers (n = 4), and showed that the total reaction is strongly exergonic and that up to four TMA monomers become associated into the (MeAlO)n core. The highest relative stability was obtained for a tetramer with four bound TMAs, that is, (MeAlO)4(AlMe3)4, which makes a ring structure (for details, see Ref. [32]). Advancing beyond the tetramers, we first repeated the WINDOW approach described above for MAO pentamers (n = 5). Calculations involving the elementary species were carried out by considering reactions taking place between every atom/bond and in all spatial orientations of the incoming species for each isomer. The resulting global minimum structures for n = 1–5 are illustrated in Figure 2, and the reaction energies, Gibbs energies, and relative stabilities are given in Table 1. Comparison of total energies and Gibbs energies suggests that MAO oligomerization reactions are largely driven by entropy due to methane formation, because the total energy considerations lead to a preference for monomeric (MeAlO)(AlMe3)3. Nevertheless, within each degree of oligomerization, the Gibbs energies point towards incorporation of less TMA than indicated by the total energies. However, only in the case of trimer, the total energies and Gibbs energies gave a different composition within the degree of oligomerization. Upon addition of TMA, the pentamers show a transition from cages to a sheet structure (when m = 3), similar to the tetramers. Due to the presence of a three-coordinate aluminum, the species continue to react with further TMA to yield ChemPhysChem 0000, 00, 1 – 12

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Figure 2. Global minimum isomers of (MeAlO)n(AlMe3)m, where n = 1–5, based on Gibbs energies. The MAOs for n = 1–4 are reproduced from Ref. [32]. Species that do not bind TMA are not shown but their energies are given in Table 1. For greater clarity, schematic pictures of the thermodynamically favored MAOs for each n value are given at the bottom.

(MeAlO)5(AlMe3)4 as the thermodynamically favored product among the pentameric MAOs. Note, the five-coordinate aluminum in the middle of the sheet, bound to four oxygen atoms and one methyl group, and four-coordinate oxygen atoms at the edges of the sheets bound to AlMe2 end groups (the latter turn out to be of critical importance regarding cocatalytic ac 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

tivity, as will be demonstrated later). Further addition of TMA, although thermodynamically unfavorable, would eventually lead to a pentameric ring structure similar to that of the thermodynamically favored tetrameric ring, consisting solely of four-coordinate aluminum and three-coordinate oxygen atoms. Entropy, however, disfavors association of a fifth TMA moleChemPhysChem 0000, 00, 1 – 12

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Table 1. Energies (DrE and DrG in kJ mol1) and relative stabilities [D(DrE) and D(DrG) in kJ mol1n1] for the formation of the global minimum isomers of MAOs up to pentamers through reaction 0.5(n+m) Al2Me6 + n H2O!(MeAlO)n(AlMe3)m + 2n CH4. n

m

Formula

DrE

D(DrE)[a]

DrG

D(DrG)[a]

1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 5

0 1 2 3 4 0 1 2 3 4 5 0 1 2 3 4 0 1 2 3 4 5 0 1 2 3 4 5

AlMeO Al2Me4O Al3Me7O Al4Me10O Al5Me13O Al2Me2O2 Al3Me5O2 Al4Me8O2 Al5Me11O2 Al6Me14O2 Al7Me17O2 Al3Me3O3 Al4Me6O3 Al5Me9O3 Al6Me12O3 Al7Me15O3 Al4Me4O4 Al5Me7O4 Al6Me10O4 Al7Me13O4 Al8Me16O4 Al9Me19O4 Al5Me5O5 Al6Me8O5 Al7Me11O5 Al8Me14O5 Al9Me17O5 Al10Me20O5

30.7 240.9 362.2 430.2 412.0 375.5 528.0 735.6 796.6 849.4 824.1 792.3 964.5 1083.7 1213.3 1244.3 1265.5 1375.9 1503.1 1630.7 1671.9 1664.7 1605.6 1850.5 1919.6 2007.4 2091.6 2088.6

448.7 177.1 55.8 12.2 6.0 230.2 154.0 50.2 19.7 6.7 5.9 153.9 96.5 56.7 13.5 3.2 101.6 74.0 42.2 10.3 0.0 1.8 96.9 47.9 34.1 16.5 0.3 0.3

20.9 297.5 373.1 381.7 363.2 439.8 587.4 756.5 769.7 776.5 750.1 863.7 985.4 1100.2 1179.7 1177.6 1305.8 1391.6 1491.4 1592.3 1605.8 1562.4 1656.0 1865.1 1908.9 1978.2 2004.5 1957.1

380.6 104.0 28.3 19.7 38.2 181.5 107.8 23.2 16.6 13.2 26.4 113.6 73.0 34.7 8.2 8.9 75.0 53.6 28.6 3.4 0.0 10.9 70.3 28.4 19.7 5.8 0.6 10.0

[a] Relative to (MeAlO)4(AlMe3)4.

reproducing the stability orders as a function of both n and m, and in 16 out of 20 n/m combinations, providing exactly the same isomer as the much more laborious WINDOW approach. As a conclusion, the BOTTOM approach does not guarantee that the global isomer is found—indeed, for larger MAOs that would become increasingly unlikely—but it succeeds in providing a picture of the overall energetics and structural features of the MAOs. We therefore continued with the BOTTOM approach, employing this method to calculate MAO structures and reaction profiles up to octamers (n = 8). The resulting lowest energy structures are illustrated in Figure 4 and the reaction energies and Gibbs energies for their formation together with relative stabilities are given in Table 2. Similar to the pentamers, we found that, upon association of TMA, the hexamers, heptamers and octamers also undergo structural transition from cages to sheets. In terms of Gibbs energies, the stabilities of these structures improve until addition of four (n = 6–7) or five (n = 8) AlMe3 units. In the case of the heptamer, total energy considerations suggest association of one extra TMA. A clear structural pattern emerges (m = 4), the first members of which were already seen in n = 4–5: molecular sheets composed of five-coordinate aluminum atoms and four-coordinate oxygen atoms. It turns out, however, that the discussed sheets only persist as the thermodynamically favored structures for oligomers in which n = 5–7, reaching their minimum energy at n = 6. Further enlargement of the structural motif eventually, at n = 8, leads to opening of the structure by association of a fifth AlMe3 unit, leading to another structural family: molecular sheets with four-coordinate aluminum and three-coordinate oxygen atoms (see the schematic pictures of n = 7 vs. n = 8 in Figure 4). Regarding thermodynamic stabilities, the (MeAlO)8(AlMe3)5 sheet is 1.8 kJ mol1n1 higher in Gibbs energy than the lowest energy compositions found so far, that is, (MeAlO)4(AlMe3)4 and (MeAlO)6(AlMe3)4. However, this being the first member of a series of sheet structures based on four-coordinate Al atoms,

cule, which is required for ring formation, thus resulting in transition from rings to sheets when moving up from tetramers to pentamers. Advancing beyond the pentamers with the WINDOW approach would be very laborious due to the rapidly increasing number of isomers. To illustrate the problem, completing the work up to the pentamers required nearly 5000 calculations, all at the MP2 level of theory. Before moving up, we therefore considered an approximation, in which only the reactions of the lowest energy isomers at each stage are followed, thus omitting the very real possibility that a higher energy species at one step can lead to a lower energy species at a subsequent step. The validity of this BOTTOM approach in comparison to the more thorough WINDOW approach can be evaluated from Figure 3. It became apparent Figure 3. Comparison of the relative stabilities of MAOs, (MeAlO)n(AlMe3)m, where n = 1–5, obtained by the that the BOTTOM approach is WINDOW (solid line) and BOTTOM (dashed line) approaches. Not shown: n = 1, m = 0 for which notably successful, in each case D(DrG) = 380.6 kJ mol1n1. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. Located lowest energy isomers of (MeAlO)n(AlMe3)m, where n = 6–8, based on Gibbs energies. Species that do not bind TMA are not shown but their energies are given in Table 2. For greater clarity, schematic pictures of the thermodynamically favored MAOs for each n value are given at the bottom.



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Table 2. Energies (DrE and DrG in kJ mol1) and relative stabilities [D(DrE) and D(DrG) in kJ mol1n1] for the formation of the located lowest energy isomers of MAOs for n = 6–8 through reaction 0.5(n+m) Al2Me6 + n H2O! (MeAlO)n(AlMe3)m + 2n CH4. n

m

Formula

DrE

D(DrE)[a]

DrG

D(DrG)[a]

6 6 6 6 6 6 7 7 7 7 7 7 8 8 8 8 8 8 8

0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 6

Al6Me6O6 Al7Me9O6 Al8Me12O6 Al9Me15O6 Al10Me18O6 Al11Me21O6 Al7Me7O7 Al8Me10O7 Al9Me13O7 Al10Me16O7 Al11Me19O7 Al12Me22O7 Al8Me8O8 Al9Me11O8 Al10Me14O8 Al11Me17O8 Al12Me20O8 Al13Me23O8 Al14Me26O8

2141.4 2261.9 2329.8 2406.3 2498.6 2477.8 2471.1 2693.0 2773.7 2821.9 2882.0 2919.5 3005.7 3083.3 3200.4 3232.5 3287.4 3337.4 3327.1

61.1 41.0 29.7 16.9 1.6 5.0 65.0 33.3 21.7 14.9 6.3 0.9 42.3 32.6 17.9 13.9 7.1 0.8 2.1

2172.7 2268.2 2300.5 2323.7 2408.7 2325.0 2510.1 2694.3 2734.1 2759.4 2792.2 2789.7 3030.9 3084.0 3156.0 3145.2 3183.0 3197.4 3142.3

39.4 23.4 18.0 14.2 0.0 14.0 42.9 16.6 10.9 7.3 2.6 2.9 22.6 16.0 7.0 8.3 3.6 1.8 8.7

[a] Relative to (MeAlO)4(AlMe3)4.

it is probable the minimum in energy will be found above n = 8, presumably facing competition there from the cage structures, which become less strained as a function of n, as shown in Table 3. The relative stabilities of TMA-free and TMA-associated cages are tabulated separately to illustrate the strong stabilization of the MAOs upon TMA association. For comparison, we have included the octahedral (MeAlO)12 cage calculated by Ziegler et al.,[18] which is analogous to the (tBuAlO)12 cage synthesized by Barron et al.[26] The comparison reveals that all of the sheets are lower in Gibbs energy, and suggests that if cage structures are dominant in the MAO mixture, they are likely to contain associated TMA and/or are larger than n = 12. Nevertheless, larger nanotubular cages, both TMA-free[24] and TMAassociated,[31] have been shown to be more stable than (MeAlO)12. A more thorough investigation of the larger MAO structures is thus necessary to address properly the possible location of the sheet-to-cage transition.

Table 3. Relative stabilities [D(DrG) in kJ mol1n1] of TMA-free cages, TMA-associated cages, and TMA-associated sheets. The stabilities are given relative to (MeAlO)4(AlMe3)4. (AlOMe)n cages

(AlOMe)n(AlMe3)m cages

(AlOMe)n(AlMe3)m sheets

4 6 8

75.0 39.4 22.6

28.6 14.2 7.0

3.4 0.0 3.6[a] 1.8[b]

12[c]

4.0

n

[a] For the sheet including five-coordinate Al atoms (n = 8, m = 4). [b] For the sheet with solely four-coordinate Al atoms (n = 8, m = 5). [c] From Ref. [18].


2.2 MAO as Catalyst Activator We then examined the two catalyst activation mechanisms[3, 8, 9] (see Figure 1), using model MAO co-catalysts selected based on the above study, in combination with previously reported results. The starting point of the model selection was to take into account that the systems under study, although reaching the lower limits of experimental size estimates of the MAO, are unlikely to reach the actual size domains dominant in the MAO. Therefore, the model selection needed to focus on potential active sites. Regarding such potential active sites, a previous study on ligand affinities[32] (i.e. relative energies for the reaction MAO+Ligand ![MAOLigand]) suggests that potential co-catalyst candidates are those containing associated TMA, but are not fully saturated by TMA so that they reduce the reactivity of the MAO. On the other hand, a study of anionization potentials[8] (i.e. relative energies for the reaction [Me2AlMAO]!AlMe2 + +MAO) suggests that the most probable site undergoing AlMe2 + dissociation is the point at which the AlMe2 moiety[40] is bound to a four-coordinate oxygen and to a bridging methyl group. The model co-catalysts shown in Figure 5 were chosen 1) to make the two activation mecha-

Figure 5. Model MAO molecules for catalyst activation studies: (MeAlO)8(AlMe3) (left) and (MeAlO)8(AlMe3)2 (right). Active Al sites (left three-coordinate and right four-coordinate) are colored black.

nisms directly comparable through TMA association/dissociation, 2) to acknowledge the possibility that the real MAO mixture may be predominantly composed of cages and related structures, and 3) to note that the most stable species are likely not the most reactive ones, which is one probable reason for the required excess of MAO in the process. As a conclusion, the purpose of the model systems is not to be representative of the actual structure of the MAO. Their purpose, in this particular experiment, is to represent the sites and chemical environments likely found in the MAO that are possibly responsible for its performance as catalyst activator. Regarding Lewis-acidic activation, our results show that the three-coordinate aluminum of (MeAlO)8(AlMe3) is capable of abstracting a methyl group from the Cp2ZrMe2 precatalyst. For reaction Cp2ZrMe2 + (MeAlO)8(AlMe3)!Cp2ZrMe–Me-(MeAlO)8(AlMe3), DrE = 166.4 kJ mol1 and DrG = 87.9 kJ mol1. The abstracted methyl group is clearly located closer to the aluminum, as illustrated in Figure 6. The length of the newly formed AlC bond is 2.08 , which is 0.12 longer than the original AlC bond, with the elongation being due to persistence of interaction with the metal cation. The broken bond has a ZrC distance of 2.47 compared with ZrC = 2.29 in the neutral ChemPhysChem 0000, 00, 1 – 12

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Figure 6. Lewis-acidic activation of Cp2ZrMe2 by (MeAlO)8(AlMe3). A schematic picture with relevant bond distances is given below.

Cp2ZrMe2 precatalyst. Under such circumstances, initiation of the olefin polymerization process upon introduction of an olefin into the system appears feasible. Before exploring the interactions with olefins, however, it is worthwhile examining the mechanism of AlMe2 + cleavage from the (MeAlO)8(AlMe3)2 model co-catalyst, because, as it turns out, the two mechanisms are interconnected. The energy profiles for AlMe2 + cleavage from the MAO by the Cp2ZrMe2 precatalyst are shown in Figure 7, and the optimized structures

www.chemphyschem.org of the intermediate species are illustrated in Figure 8 (B–G). The energies are given relative to the neutral, noninteracting Cp2ZrMe2 and MAO (A), and are also tabulated in Table 4. The reaction starts with Cp2ZrMe2–MAO coordination (B), which breaks an AlC bond to the bridging methyl group in MAO, while at the same time aluminum regains four-coordination by partial abstraction of the methyl group of the metallocene. Entropy is lost in the complex formation, making Gibbs energies systematically higher than total energies thereafter. Bending of the Zr–Me–Al moiety costs energy, but leads, through a shallow transition state (C*), to the precursor for the AlMe2 + cleavage (D). The AlMe2 + cleavage (E*) yields the heterobinuclear complex [Cp2Zr(m-Me)2AlMe2] + (F) with increased cation– anion separation. The heterobinuclear complex has been experimentally observed at high Al/Zr ratios that are typical for active catalyst systems.[41] The shortest distance between the two ions is 3.07 (from bridging methyl group of the heterobinuclear complex to MAO oxygen). The mechanism can be described as an associative interchange, in which methyl as nucleophile displaces the MAO ligand trans to it. The last step in Figure 7, dissociation of AlMe3 from F, leads to a contact ion pair (G), in which the metallocene and the MAO share one methyl group. Note that, in this particular case, the contact ion pair is precisely the same as the product for Lewis acidic activation (see Figure 6), which is due to TMA association/dissociation equilibrium through reaction (MeAlO)8(AlMe3) + 0.5 Al2Me6$(MeAlO)8(AlMe3)2. The equilibrium of the reaction is strongly on the side of the product with DrE = 117.1 kJ mol1 and DrG = 72.5 kJ mol1. Based on the TMA

Figure 7. Energy profiles for AlMe2 + cleavage from (MeAlO)8(AlMe3)2 by the Cp2ZrMe2 precatalyst. The energies are given relative to the neutral, noninteracting Cp2ZrMe2 and MAO (A).



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www.chemphyschem.org Table 4. Energies (DE and DG in kJ mol1) for the intermediates for AlMe2 + cleavage, catalyst activation, and subsequent initiation of ethene polymerization by the Cp2ZrMe2/(MeAlO)8(AlMe3)2 catalyst. Intermediate

DE[a]

DG[a]

A B C* D E* F G H* I J* K L M N

0 97.9 39.9 49.7 3.4 39.7 49.3 28.6 7.2 13.1 59.7 115.8 154.9 162.2

0 21.0 36.0 31.4 80.3 26.4 15.4 149.4 86.1 107.3 46.8 3.3 50.9 59.9

[a] Relative to the neutral, noninteracting Cp2ZrMe2 and MAO (A).

Figure 8. Intermediates for AlMe2 + cleavage, catalyst activation, and subsequent initiation of ethene polymerization by the Cp2ZrMe2/(MeAlO)8(AlMe3)2 catalyst.

hydrolysis study reported above, this can be generalized to propose that sites responsible for Lewis-acidic activation are not abundant in MAO, because they tend to react with TMA, 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

thus making sites prone to AlMe2 + cleavage more abundant. Therefore, whereas from the point of view of overall thermodynamics it is irrelevant at which step the TMA association/dissociation takes place, with both mechanisms leading to the same product (G), the thermodynamics of MAO formation suggests the path involving AlMe2 + cleavage is dominant. The next questions to be answered are: 1) how does AlMe3 dissociation from the heterobinuclear complex (F) take place, and 2) does the system polymerize ethene. Energy profiles for the actual catalyst activation and subsequent initiation of ethene polymerization by the Cp2ZrMe2/(MeAlO)8(AlMe3)2 catalyst are shown in Figure 9 and are tabulated in Table 4. The optimized structures of the intermediates are illustrated in Figure 8 (H*–N). The two questions are linked by the incoming ethene monomer displacing the AlMe3 from F by an associative interchange mechanism,[42] leading to the actual catalyst activation, which thus involves the monomer. The transition state (H*) for the AlMe3 to ethene interchange is the rate-limiting step, with DE* = 28.6 kJ mol1 and DG* = 149.4 kJ mol1 with respect to the neutral, noninteracting Cp2ZrMe2 and MAO (A). The large difference in energy and Gibbs energy, in addition to the previously noted difference due to complex formation, originates from the high entropy of ethene monomer. The high barrier for catalyst activation is consistent with experimental findings that only a small fraction of catalyst in the resting state is ever activated.[43] Next, the displaced AlMe3 can either bind back to the MAO or dissociate in the form of the dimer, Al2Me6. Entropy favors the latter, leading to an ethene p-complex (I) as seen in some studies where co-catalysts are omitted. At this stage, MAO is no longer directly involved in the process and stays nearby as the counterion (ZrO = 4.97 ). From here, ethene insertion can take place smoothly, in line with experiments.[44, 45] The insertion takes place through the four-centered transition-state stabilized by an a-agostic interaction (J*), in accordance with the Brookhart–Green mechanism,[46] leading to a g-agostic propyl insertion product (K). This is followed by rotation of the propyl chain, which makes ChemPhysChem 0000, 00, 1 – 12

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Figure 9. Energy profiles for catalyst activation and subsequent initiation of ethene polymerization by the Cp2ZrMe2/(MeAlO)8(AlMe3)2 catalyst. Interaction between metallocene and MAO through the oxygen or methyl group in MAO is indicated by [O-MAO] or [Me-MAO] . The energies are given relative to the neutral, noninteracting Cp2ZrMe2 and MAO (A).

room for the MAO to bind back to the metal through one of its oxygen atoms (L). Rotation of the MAO to replace the ZrO interaction (2.31 ) with a ZrMe interaction can yield the bagostic propyl product (M) with ZrMe distance of 2.58 . However, the ZrMe interaction is strong enough to completely replace the b-agostic interaction, thereby shortening the ZrMe distance from 2.58 to 2.47 . The resulting species (N), without agostic interactions, is the thermodynamically favored propyl product. It lies 7.3 kJ mol1 and 9.0 kJ mol1 lower in energy and Gibbs energy, respectively, than the b-agostic product. Likewise, the nonagostic product has been reported in previous computations to be preferred for a Cp2ZrPr + – [Cl(MeAlO)4(AlMe3)2] ion pair[32] and for a Cp2ZrEt + –MeB(C6F5)3 ion pair.[47] This is also in line with experiments, which suggest that the agostic product becomes favorable for the less nucleophilic B(C6F5)4 counteranion.[48] The nonagostic product (N) is 59.9 kJ mol1 lower in Gibbs energy than the neutral, noninteracting Cp2ZrMe2 and MAO. The overall process is thus clearly exergonic, demonstrating the feasibility of ethene polymerization by the described catalyst system. The resting state, either M or N, provides the starting points for insertion of the next olefin and continuation of chain growth.

3. Conclusions By following the thermodynamics of TMA hydrolysis pathways in detail, with the whole process involving thousands of calcu 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

lations at the MP2 level of theory, we have described the formation of MAOs of general formula (MeAlO)n(AlMe3)m up to n = 8. Associated TMA is shown to be of crucial importance with respect to both the structural features and the cocatalytic properties of MAO. Depending on the degree of oligomerization, it is thermodynamically preferable for the MAOs to bind three to five TMAs, leading to compositions in that are consistent with the empirical formula of MAO. The structural preferences of the MAOs show distinct variations as a function of both the degree of oligomerization, n, and the number of associated TMAs, m. Association of TMA tends to open up cage structures, eventually leading to rings (n = 3–4), sheets with five-coordinate aluminum (n = 5–7), and sheets with four-coordinate aluminum (n = 8). The driving force for cage opening is the release of strain, making TMA association thermodynamically preferable. It is plausible that cage-like structures become thermodynamically favorable for larger MAO oligomers because ring-strain decreases as a function of size. Two octameric MAOs involving associated TMA, (MeAlO)8(AlMe3), and (MeAlO)8(AlMe3)2 were selected for the study of Cp2ZrMe2 precatalyst activation; the former to model the Lewis-acidic activation and the latter to model activation through AlMe2 + cleavage from MAO. Although both activation mechanisms were shown to be feasible by the model co-catalysts, the TMA association/dissociation equilibrium in MAO formation suggests that the AlMe2 + cleavage mechanism dominates because of the higher concentration of active species. ChemPhysChem 0000, 00, 1 – 12

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CHEMPHYSCHEM ARTICLES The rate-limiting step of catalyst activation is the displacement of AlMe3 in the heterobinuclear [Cp2Zr(m-Me)2AlMe2] + complex by ethene in an associative interchange-type process. The catalyst system was further shown to allow polymerization of ethene with realistic energy barriers. Overall, the results suggest the catalyst activity may be directly related to the ability to cleave AlMe2 + from MAO and to cleave AlMe3 from the catalyst complex.

Computational Methods TMA dimer, Al2Me6, involves bridging five-coordinate methyl groups and electron-deficient three-centered two-electron bonds.[42] Due to association of TMA into the (MeAlO)n core, MAO contains similar bonding environments. The electronic structure of TMA, and hence of MAO, greatly complicates the theoretical treatment by giving rise to dispersive interactions. For instance, the widely employed B3LYP method[49] completely fails, suggesting erroneously that TMA is a monomer rather than a dimer[50] at ambient temperatures.[24, 31] Correlated ab initio methods are necessary to properly account for the dispersive interactions in MAO,[51] although the M06 series of functionals[52] has recently been shown to provide a cost-effective alternative.[24] Throughout this work we employed the RI-MP2/def-TZVP method,[53–55] which has been used also in prior closely related work, because it has been shown to provide energetics that are in very good agreement with those calculated at the CCSD(T)/def2-QZVPP level of theory.[31] The calculations were carried out by TURBOMOLE version 6.3.[56, 57] All calculations were performed on molecules in the gas phase. The effect of using toluene as a solvent was very small as was verified for the initial steps of TMA hydrolysis, producing the monomeric MAOs. The reported MAOs and zirconocene-MAO complexes were fully optimized, followed by calculation of harmonic vibrational frequencies to confirm them as true local minima or transition states in the potential energy surfaces as well as to obtain Gibbs energies, which were calculated at T = 298.15 K and p = 1 atm. No scaling factors were applied.

Acknowledgements This work was supported by the European Commission (Grant NMP4-SL-2010-246274). The computations were made possible by use of the Finnish Grid infrastructure resources. Keywords: ab initio calculations · ion pairs · metallocenes · reaction mechanisms · structure elucidation [1] [2] [3] [4] [5] [6] [7]

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CHEMPHYSCHEM ARTICLES [47] I. E. Nifant’ev, L. Y. Ustynyuk, D. N. Laikov, Organometallics 2001, 20, 5375 – 5393. [48] F. Song, S. J. Lancaster, R. D. Cannon, M. Schormann, S. M. Humphrey, C. Zuccaccia, A. Macchioni, M. Bochmann, Organometallics 2005, 24, 1315 – 1328. [49] A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652. [50] M. B. Smith, J. Organomet. Chem. 1972, 46, 31 – 49. [51] B. G. Willis, K. F. Jensen, J. Phys. Chem. A 1998, 102, 2613 – 2623. [52] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215 – 241. [53] F. Weigend, M. Hser, Theor. Chem. Acc. 1997, 97, 331 – 340. [54] C. Httig, A. Hellweg, A. Kçhn, Phys. Chem. Chem. Phys. 2006, 8, 1159 – 1169.


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Received: May 5, 2014 Published online on && &&, 2014


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ARTICLES J. T. Hirvi, M. Bochmann, J. R. Severn, M. Linnolahti* && – && Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single-Site a-Olefin Polymerization Catalysis

Modeling MAOs: Hydrolysis of trimethylaluminum forms methylaluminoxanes with general formula (AlOMe)n(AlMe3)m. The process is computationally followed until the formation of octamer-


ic structures. By using model co-catalysts, associated trimethylaluminum is shown to be responsible for the cocatalytic activity of the methylaluminoxanes.


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Molecular weight control in organochromium olefin polymerization catalysis by hemilabile ligand-metal interactions.

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