Mechanistic DFT Study on Ethylene Trimerization of Chromium

May 15, 2014 - ... and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The ..... ethylene together with a possible π*...
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Mechanistic DFT Study on Ethylene Trimerization of Chromium Catalysts Supported by a Versatile Pyrrole Ligand System Yun Yang,†,‡ Zhen Liu,*,† Ruihua Cheng,† Xuelian He,† and Boping Liu*,† †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, People’s Republic of China ‡ Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands S Supporting Information *

ABSTRACT: To elucidate fundamental mechanistic aspects of the landmark Chevron−Phillips ethylene trimerization system, a detailed theoretical study has been carried out by DFT methods on an aluminum pyrrolyl chromium catalyst. Reaction pathways for selective ethylene oligomerization have been successfully located on the basis of the metallacycle mechanism. Consistent with experimental results, for the model system ethylene trimerization was proven to be energetically preferred in comparison to ethylene dimerization or further ring expansion toward the formation of higher α-olefins. The Cr(I/III) redox couple was found to be the most likely for the catalytic ethylene trimerization. A careful electronic configuration analysis has been conducted, and the ground state of all active species involved in the catalytic cycle is identified to be S = 3/2 except for the bare active species, which favors a high spin state of S = 5/2. The role of a pendant chlorine functionality has been investigated as well. Variable Cr−Cl bond distance and NBO charge analysis of every intermediate clearly exhibit the hemilabile behavior of the chlorine. This unique hemilability is considered to be a key factor for the selectivity toward 1-hexene formation.

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INTRODUCTION Despite the rapid expansion of the trimerization catalyst family,1−3 the Chevron−Phillips trimerization system4−6 is still the sole example of a successfully commercialized ethylene trimerization process. This prominent system consists of chromium(III) tris(2-ethylhexanoate), 2,5-dimethylpyrrole, triethylaluminum (TEA), and diethylaluminum chloride (DEAC) in a molar ratio of 1:3.3:10.8:7.8 and annually produces ca. 47000 tons of 1-hexene,7 which is mainly used as a comonomer in the production of linear low-density polyethylene (LLDPE). Due to the importance of the Chevron−Phillips trimerization catalyst, various experimental8−13 and theoretical14,15 investigations have been conducted in recent years, in order to clarify the fundamental aspects of this system. These research works mainly concerned issues such as the oxidation state(s) of chromium responsible for selectivity and the role of the ancillary ligand system during the catalysis. Gambarotta and coworkers have isolated Cr(I) species stabilized by a carbazole ligand (1; Scheme 1).9 This complex performed as a selfactivating, single-site ethylene trimerization catalyst and displayed a catalytic behavior resembling that of the Chevron−Phillips trimerization catalyst. Later, the same group reported a well-defined bimetallic Cr(II) complex (2; Scheme 1) obtained from the combination of CrCl2(THF)2, tetramethylpyrrole and trimethylaluminum (TMA)ingredients similar to those of the Chevron−Phillips system.8 This Cr(II) precursor (2) was capable of selectively trimerizing ethylene without the assistance of any cocatalysts. Both systems © XXXX American Chemical Society

Scheme 1. Isolated Single-Component Trimerization Catalysts (1, 2) and Computational Molecular Models for Active Species in the Chevron−Phillips Trimerization System (3, 4)

were proposed to follow the Cr(I/III) redox cycle for trimerization. Subsequent theoretical studies14 using a model species of the Chevron−Phillips trimerization system (3; Scheme 1) supported the assumption that the Cr(I/III) redox couple is involved in ethylene trimerization. This assumption has been further supported by an electron Received: March 21, 2014

A

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study regarding the same Chevron−Phillips trimerization system.8 For this catalytic Cr/pyrrole system, Gambarotta and co-workers proposed that the active species is formed by heterolytic dissociation of a welldefined dimeric chromium complex (2; Scheme 1) in the presence of ethylene. The active site consists of one chromium center η5coordinated by the pyrrole ring and a pendant AlMe2Cl group in which the aluminum is σ-bonded to the pyrrolate nitrogen and a chlorine is σ-bonded to chromium. Since 2,5-dimethylpyrrole is considered to be the pyrrole ligand providing the most efficient Chevron−Phillips trimerization system,3 to simplify the study and to get closer to the realistic system in our study 2,5-dimethylpyrrole was utilized instead of the tetramethylpyrrole used by Gambarotta and coworkers. To investigate the role of the metal oxidation state, neutral Cr(I) model A and cationic Cr(II) model B were both envisaged, which will be involved in the Cr(I/III) and Cr(II/IV) redox cycles, respectively. For the cationic catalyst model B, no discrete counterions were taken into account in the present calculations, since it is uncertain whether the inclusion of a counterion would afford a more appropriate model and both experimental and theoretical evidence showed that the formation of dissociated ion-pair complexes was favorable for an efficient trimerization to proceed.16,17 Computational Details. All of the DFT calculations were performed using the Gaussian 09 program package.18 A variety of density functionals (B3LYP, O3LYP, BLYP, OLYP, BP86, PBEPBE) in combination with different basis sets (BS1, LANL2TZ for Cr and 6311G(d,p) for other elements; BS2, Ahlrichs triple-ζ split-valence basis set TZVP) were tested by conducting the geometry optimization of model A under all possible spin multiplicities and comparing their corresponding free energies (Figure S1, Supporting Information). For the current system, despite the different extents of energy gap, all of the evaluated methods showed the same preference in predicting the ground state. Since the TZVP basis set showed higher efficiency in the test and pure functionals had previously been found (i) to be less affected by spin contamination19 and (ii) to typically provide a fairly good geometrical description of the chromium species,20 PBE/TZVP was selected to perform a geometry optimization of 2 in Scheme 1. The calculated bond parameters (see Table S1 in the Supporting Information) showed good agreement with the reported single-crystal data,8 further proving that PBE/TZVP is suitable for the present study. Geometry optimizations for all species along the reaction pathway were carried out at the PBE/TZVP level without any symmetry constraints. All optimized structures were verified to be minima (no imaginary frequencies) or transition states (one imaginary frequency) on the free energy surface with analytical frequency calculations. The transition states were confirmed by intrinsic reaction coordinate (IRC) calculations at the PBE/TZVP level and were further verified by full IRC at the PBE/SV level. Full IRC calculations allowed displaying the direct connection between transition states and their corresponding reactants and products. All energies reported refer to Gibbs free energy corrections to the total electronic energies at room temperature (298.15 K) and ambient pressure (0.1 MPa).

paramagnetic resonance (EPR) investigation on the same system.10 Despite these supporting data for a Cr(I/III) redox system, the Cr(II/IV) redox cycle simply cannot be excluded. An early DFT study showed the feasibility of starting ethylene trimerization with a divalent chromium complex (4; Scheme 1),15 raising the question of which of the two redox cycles would be more possible and accessible for the Chevron− Phillips system. Therefore, it is essential to compare reaction pathways following both Cr(I/III) and Cr(II/IV) cycles under the same conditions to reach a conclusion on the oxidation states. Another mechanistic debate in the Chevron−Phillips system is the role of the ancillary ligand. van Rensburg suggested that the pyrrole undergoes ring slippage between η5and σ-coordinating modes on the minimum-energy pathway and the ring slippage efficiently assisted ethylene trimerization. In another theoretical study by Budzelaar, the pyrrole ring was found to be solely η5-coordinated to the chromium center along the entire reaction pathway, while the pendant chlorine features a hemilabile manner instead.14 The switchable association/ dissociation interaction between chlorine and chromium was considered to play a key role in achieving high selectivity. Herein, we revisit the landmark Chevron−Phillips ethylene trimerization system with theoretical approaches, attempting to clarify the mechanistic debates on the metal oxidation states and the role of the ligands. To get insight into the redox state of the metal during the process, reaction pathways for both the Cr(I/III) and the Cr(II/IV) redox cycles have been completely identified by DFT methods. Taking the possible occurrence of spin crossover into account, the ground state of every species involved in the catalytic cycle was verified. The role of the ligand was investigated in detail as well.

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COMPUTATIONAL METHODS

Molecular Model. The molecular models A and B (Scheme 2) in the present study were designed on the basis of a previous modeling

Scheme 2. Illustration of Molecular Models: Neutral Cr(I) Model A and Cationic Cr(II) Model B

Figure 1. Optimized structures of 1A in the gas phase, in toluene, and in methylcyclohexane. The corresponding free energies and selected bond distances (Å) are shown as well. The spin multiplicity of 1A is a quartet. B

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workers.22 Natural population analyses were performed with the Gaussian NBO program.23

Considering the fact that the real trimerization reaction was performed under homogeneous solution conditions, in addition to the gas-phase calculations, the polarizable continuum model (PCM) was utilized to perform solvation corrections in the present study as well. Toluene and methylcyclohexane, as feasible solvents used in the experimental systems, were both tested for the optimization of the mono-olefin structure 1A (Figure 1). A bond-parameter analysis of 1A shows that there is no distinguishable structural difference between the species in the gas phase and in solution. The free energy of 1A lies about 2 kcal/mol lower in solution than in the gas phase. However, the influence of toluene differs very little from that of methylcyclohexane, which is consistent with the findings in previous DFT studies on the Cr/pyrrole system.8 This finding is not surprising, since toluene and methylcyclohexane feature similar polarities and solvation corrections from PCM do not take into account possible stronger interactions such as coordination of aromatic toluene to the Cr(I) species, which might remarkably stabilize or even poison an ethylene trimerization system.9 Given the similar solvation effects of toluene and methylcyclohexane on the present model system, the whole reaction pathway was only relocated under the assumption that the system is in toluene solution. The electronic configurations of chromium in every intermediate and transition state were carefully examined. The geometry optimization was performed for each species under all possible spin states (S = 1/2, 3/2, 5/2 for Cr(I), S = 0, 1, 2 for Cr(II), S = 1/2, 3/2 for Cr(III), and S = 0, 1 for Cr(IV)). The corresponding free energies were compared to determine the resting states of the complexes. The calculated spin S* was carefully monitored during the calculation. As shown in Table 1, the amount of spin contamination (S*2 − S(S + 1))

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RESULTS AND DISCUSSION Reaction Pathway. The full catalytic cycles (Scheme 3) for the selective oligomerization of ethylene were calculated on the basis of the metallacycle mechanism proposed by Briggs.24 Following coordination of a second ethylene molecule to the vacant sites of the low-valent chromium in 1X, the corresponding chromacyclopentane25−29 3X is formed via oxidative addition. Subsequent coordination and insertion of the third ethylene results in the formation of the chromacycloheptane25−29 5X. Subsequently, 1-hexene is generated via reductive elimination from the relatively unstable seven-membered chromacycle 5X and released via uptake of ethylene to regenerate 1X. Possible side reactions can occur, including 1-butene elimination from the chromacyclopentane species 3X (ethylene dimerization) and further ring expansion of the chromacycloheptane 5X via the coordination and insertion of subsequent ethylene molecules (ethylene tetramerization or nonselective oligomerization).30 Obviously, selective 1-hexene formation can only be achieved when the reaction rates of the two competitive steps are distinguishably lower than the rate of the trimerization process. In addition, special attention was paid to locate possible pathways for metallacycle collapse. For other ethylene oligomerization catalysts, the generation of short-chain α-olefins out of the metallacycle intermediates was found to undergo either a concerted intramolecular β-hydrogen transfer to the α′-carbon of the metallacycle or a stepwise β-hydrogen transfer to the metal center followed by reductive elimination.14,15,31−40 In the present study, both possibilities were taken into account and calculated. The energetically preferred routes, a stepwise pathway for 1-butene formation and a concerted route for 1hexene production, are depicted in the free energy diagrams (Figures 2 and 3). During the whole reaction cycle, the chromium oxidation states switch between Crn and Crn+2. Free Energy Profiles. The entire reaction pathways for both model A (Cr(I/III)) and model B (Cr(II/IV)) were first calculated under gas-phase conditions. Then the PCM model was utilized to conduct the solvation correction for the relocation of the free energy surfaces under the toluene

Table 1. Calculated Spin Contamination (S*2 − S(S + 1)) Relative to S(S + 1) for Species under Different Spin Statesa [S* − S(S + 1)]/S(S + 1), % 2

a

S=1

S = 3/2

S=2

S = 5/2

0.04−0.40

0.02−0.78

0−0.03

0−0.01

S* = calculated spin state.

in the present study is within 10% of S(S + 1) and thus can be safely neglected, as suggested by Perdew and co-workers.21 For clarity reasons, throughout this work the free energy profiles will only depict the species in the ground state and their corresponding free energies. For catalytic reactions involving intermediates with different ground states, which is also known as the spin crossover phenomenon, all minimum energy crossing points (MECPs) on the pathway were located using the methodology developed by Harvey and co-

Scheme 3. Proposed Catalytic Cycles Based on the Metallacycle Mechanism by Briggs24

C

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Figure 2. Calculated free energy diagram for model A. Energetic barriers are indicated in italics, and heat absorption energies are underlined. Blue, red, and green lines illustrate free energy surfaces of ethylene di-, tri-, and tetramerization, respectively. The orange point represents the MECP, and the corresponding free energy is shown in boldface. The ground state of 0A is a sextet, and the ground state of other intermediates and transition states is a quartet.

Figure 3. Calculated free energy diagram for model B. Energetic barriers are indicated in italics, and heat absorption energies are underlined. Pink, red, and green lines illustrate free energy surfaces of ethylene trimerization under a quintet state, trimerization under a triplet state, and tetramerization under a triplet state, respectively. Orange points represent MECPs, and the corresponding free energies are shown in boldface. The ground state of 0B, 1B, 6B, and 7B is a quintet, and the ground state of other intermediates and transition states is a triplet.

solution conditions. Figures 2 and 3 exhibit the free energy profiles of possible reaction routes on models A and B on the

basis of the metallacycle mechanism in the toluene solution, respectively. D

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Figure 4. Optimized geometries and selected bond distances of intermediates for model A.

coupling (ΔG⧧ = 8.4 kcal/mol, ΔGrxn = −5.4 kcal/mol) and the final reductive elimination (ΔG⧧ = 9.4 kcal/mol, ΔGrxn = −5.8 kcal/mol) steps are both very easy, indicating that even the low oxidation state Cr(I) is relatively stable in this system. The ratedetermining step is the formation of chromacycloheptane, which requires the reaction to overcome an energetic barrier of 19.0 kcal/mol. The height of the barrier arises from the difficulty of the third ethylene in coordinating to the chromium center. The chromacyclopentane intermediate is stabilized by coordination of a chlorine to the chromium site, which hampers coordination of an incoming ethylene. The coordination of ethylene being the rate-determining step implies a first-order reaction rate dependence with respect to ethylene concentration. This finding shows good agreement with the previous theoretical results obtained for the Cr/pyrrole system.15 Spin state analysis revealed that the ground state of the bare Cr(I) model A (Scheme 2) without the coordination of any ethylene molecules is a sextet, while the rest of the species involved in pathway A showed a preference for the quartet state, which indicated the possibility of spin crossover. Similar proposals of spin crossover during the interaction of the first coordinating monomer and the active species have been reported for both Cr-41,42 and Ti-catalyzed43 olefin oligomerization systems. The occurrence of this sextet−quartet spin crossover depends on the relative rates of the spin flip and ethylene uptake steps, since bare model A can hardly exist in the system when the

Pathways of ethylene dimerization, trimerization, and tetramerization for model A were all successfully located (Figure 2). In the calculated ethylene dimerization pathway, 1butene is generated in a stepwise manner through the chromium butenyl hydride intermediate 11A. The highly unstable 11A (ΔGrxn = 16.0 kcal/mol) shows a tendency to reversibly form the original chromacyclopentane 3A. As a result, formation of 1-butene requires conquering an effective barrier of 30.7 kcal/mol (ΔGTS-11A‑12A − ΔG3A). On the other hand, chromacycloheptane 5A, as the product of ring expansion from 3A, is much more stable, lying 20.4 kcal/mol lower in free energy than 11A. On comparison of the free energies of activation for the formation of the 1-butene-coordinating species 12A (30.7 kcal/mol) and chromacycloheptane 5A (19.0 kcal/mol), 3A more likely undergoes further ring expansion rather than ring opening and releasing 1-butene. In contrast to the stepwise β-hydrogen transfer for dimerization, 1-hexene generation from 5A follows a concerted route, which is assisted by a β-hydrogen agostic interaction (see TS-5A-6A in Figure 4). In light of the free activation energy (ΔG5A‑6A − ΔG8A‑9A = −13.0 kcal/mol) of the reaction and the stability of the final product (ΔG7A − ΔG9A = −6.3 kcal/mol), this step is explicitly favorable in comparison to further ring expansion toward chromacyclononane 9A. Thus, model A appears to be a suitable catalyst model for the Cr/pyrrole ethylene trimerization system. In the trimerization cycle, the first oxidative E

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Figure 5. Charges of Cl and Cr−Cl distances in each intermediate and transition state along the trimerization reaction pathway.

their Cr(II) and Cr(IV) analogues indicates the better balance between oxidation states could be one possible reason for the lower effective barrier of pathway A. Geometric Aspects. Along the whole reaction pathway of model A (Figure 4), the aluminum atom of the AlMe2Cl group is consistently σ-bound to the pyrrolate nitrogen and the chromium center is η5-coordinated by the pyrrole ring. The 13e Cr(I) starting species is highly unsaturated, with only the chlorine and pyrrole firmly bound to the Cr center. Thus, it could easily accept electron donors such as an ethylene molecule to form the corresponding monoethylene-coordinated species 1A. The chlorine and pyrrole remain strongly bonded to chromium in 1A. In the meantime, π donation of ethylene together with a possible π* back-donation from chromium to the coordinated ethylene contributes to an elongated C−C double bond (1.45 Å vs 1.33 Å in free ethylene). Coordination of a second ethylene molecule to chromium is accompanied by a dissociation of chlorine from chromium, with the Cr−Cl distance increasing from 2.44 Å in 1A to 3.57 Å in 2A. The length of the C−C bonds in both coordinated ethylene molecules in 2A (1.40 Å) is still considerably longer than the C−C double bond in free ethylene, which is again a result of the Cr−ethylene interaction. Upon formation of the chromacyclopentane 3A, the chlorine coordinates to chromium again (Cr−Cl distance 2.46 Å). Coordination of an additional ethylene to the chromacyclopentane again results in displacement of the chlorine. However, the Cr−ethylene interaction in 4A is much weaker than that in 2A, which can be seen from the difference in the Cr−Cethylene bond lengths (2.15 Å in 2A vs 2.38 Å in 4A) and the C−C bond distances in the coordinated ethylene molecules (1.40 Å in 2A vs 1.36 Å in 4A). The less effective interaction of the ethylene in 4A in comparison to 2A is probably due to the increasing steric constraint in the chromium coordination sphere. After the insertion of the third ethylene, the resulting chromacycloheptane species 5A exhibits a similarly short Cr− Cl distance (2.45 Å) as its analoguethe chromacyclopentane 3A. The 1-hexene−chromium species 6A is generated from the chromacycloheptane 5A via a concerted β-H transfer and subsequently undergoes a structural reconfiguration to yield the more stable final product 7A with an unfolded 1-hexene. The chlorine remains bound to chromium in both 6A and 7A. It is noteworthy that, in the transition state TS-5A-6A, the four atoms involved in the β-H transfer (α′-C, β-C, Cr, and β-H) practically reside in the same plane. This structure is impossible

uptake of ethylene is rapid enough. Nevertheless, this spin crossover would not affect the reaction rate, even if it happens, since the crossing point is located after a saddle point.44 For the Cr(II) model B only ethylene trimerization and tetramerization routes were successfully located (Figure 3). Although an optimized geometry for the resulting 1-butenecoordinating species was obtained, location of crucial transition states for the dimerization pathway failed for both the stepwise and concerted routes. In the trimerization pathway, collapse of chromacycloheptane 5B to release 1-hexene is very easy and occurs without any significant energy barrier (1.6 kcal/mol). In contrast, the relatively high activation energy barrier (ΔG⧧ = 38.4 kcal/mol) for ring expansion from chromacycloheptane 5B to chromacyclononane 9B indicates that further growth of the seven-membered ring is very unlikely to occur. With ethylene trimerization as the only feasible reaction pathway, model B also appears to be a reasonable catalyst model. In model B the oxidation state of chromium shuttles between Cr(II) and Cr(IV) along the pathway. Reduction from Cr(IV) to Cr(II) (ΔG⧧ = 1.6 kcal/mol) is much easier than oxidizing Cr(II) to Cr(IV) (ΔG⧧ = 6.2 kcal/mol), which makes sense since Cr(II) is more commonly found than the higher oxidation state Cr(IV) in alkylchromium complexes.14 Active species involved in the catalytic cycle show different preferences for their spin state in comparison to the ground state.45 All monoolefin-coordinating species favor a high-spin state (S = 2), and all other species are more stable in the triplet state (S = 1). Spin crossover first occurs during the coordination of a second ethylene molecule to chromium, where the ground state of chromium shifts from quintet to triplet. The second MECP was located on the pathway of reductive elimination to generate 1hexene, where the spin state of the metal shifts back to the quintet state. As was observed for model A, the rate-determining step for model B is again the formation of the metallacycloheptane. Since the monoethylene-coordinating species 1B is the most stable intermediate before TS-4B-5B, all steps before TS-4B5B are rapid pre-equilibria. Therefore, the effective activation energy for model B is 31.4 kcal/mol (ΔG⧧ = ΔGTS-4B‑5B − ΔG1B), which is apparently much higher than that for model A (ΔG⧧ = 19.0 kcal/mol). With regard to the activation energy of the rate-determining step, model A proves to be a more promising active site model for the Chevron−Phillips ethylene trimerization system than model B. The fact that the stabilities of Cr(I) and Cr(III) species are more comparable than those of F

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controlling the delicate electronic balance in the active species is crucial for the selectivity toward 1-hexene formation. To investigate the importance of the hemilabile Cr···Cl interaction on the catalytic behavior of the system, model C with an AlMe3 group instead of an AlMe2Cl moiety attached to the pyrrolate nitrogen was studied by relocating the whole reaction pathway. Hemilabile behavior similar to that for model A was found for the less electron donating methyl group as well. An α-agostic interaction between chromium and one of the methyl hydrogens was revealed while the dangling methyl group approached the chromium center. For instance, the αH−Cr distance in 3C was found to be around 2 Å (Figure 6).

for the chromacyclopentane due to geometric constraints from the smaller ring size. In the five-membered-ring species, the βH is forced to lie outside the plane containing Cr, α′-C, and βC during the hydrogen transfer, adopting a distortedtetrahedral geometry. As a result of this unique configuration, if it happens at all, formation of 1-butene proceeds in a two-step route via a chromium−hydride intermediate rather than via the concerted process. The same phenomenon has been reported in previous DFT studies on Ti- and Ta-catalyzed ethylene trimerization systems.31,33 Influence of Hemilability. The hemilabile character of the pendant chlorine is evident from the varying Cr−Cl distances along the reaction pathway (Figures 4 and 5). It should be noted that for every intermediate two initial structures were used in the input file, one with chromium bound by chlorine and the other with chlorine displaced from the chromium center. In all cases both starting structures resulted in the same final optimized geometry. Throughout the trimerization pathway, chlorine was found to bind to chromium in all 13e intermediates, while in the 15e intermediates dissociation of chlorine is favored. This result is consistent with Budzelaar’s finding on another model system for the Phillips trimerization catalyst, showing that the accessibility of chlorine toward the chromium center is dependent on the numbers of electrons in the coordination sphere of chromium.14 Further examination of NBO charges demonstrates that the charge on the chlorine atom is strongly related to the Cr−Cl distance (Figure 5). Elongation of the Cr−Cl distance results in a drop of the chlorine charge. Therefore, the hemilability appears to be mainly but not exclusively a consequence of electronic effects. Steric constraints might also play a role in adjusting the Cr−Cl distance, but the influence is not dominant. For instance, 2A and 3A have similar steric hindrance in the coordinating sphere but feature notably different Cr−Cl distances. The hemilability of the chlorine is believed to play a key role in the selectivity of this system. A similar hemilabile character of the ancillary ligand was manifested in the Cp-arene titanium-catalyzed ethylene trimerization system, in which the selectivity is induced by the association−dissociation behavior of the pendant arene moiety.32,33,35,37 However, as revealed in a DFT study on the Cr/pyrrole system by van Rensburg and co-workers, the coordination sphere of chromium might also be effectively compensated by switching between η5- and σ-coordinating modes of the pyrrole ligand (4; Scheme 1).15 These alternative bonding modes differ from the invariable η5-coordinating mode of pyrrole observed in our system. The inconsistent behavior of pyrrole moieties arises from the differences in structure of the catalyst and model investigated in our study and in the study by van Rensburg and co-workers.15 In the present study the pyrrolate nitrogen is tightly σ-bonded to the aluminum of the AlMe2Cl moiety, which prevents the ring slippage of the pyrrole throughout the whole reaction pathway. Since the molecular model design in the present study has been based on the crystal structure of a single-component catalyst that served as a model for the Chevron−Phillips system,8 we consider that model A should feature a close resemblance to the real system. Nevertheless, at the current stage it is difficult to draw a definite conclusion that model A is preferred over the model with a hemilabile pyrrolate ligand in van Rensburg’s study.15 Further experiments need to be carried out to provide solid proof of the nature of the active site. Despite the different roles granted to the ligand in dictating the valence electrons, it is clear that

Figure 6. Optimized geometry of 3C at the PBE/TZVP level of theory.

As expected, model C also showed the potential to promote selective ethylene trimerization. The chromacyclopentane 3C preferred further ring expansion rather than yielding 1-butene via β-H transfer, and the activation energy for the chromacyclononane formation was found to be clearly higher than that for the 1-hexene formation (ΔGTS‑8C‑9C − ΔGTS‑5C‑6C = 12.6 kcal/mol). This result is identical with the observation that high 1-hexene selectivity can be achieved even in some halogen-free Phillips trimerization systems.5 Considering that TEA is employed as an activator in the real catalytic system, one of the α-Hs of the ethyl might be crucial for providing the hemilabile character of ethyl which is responsible for 1-hexene selectivity. Moreover, the activation energy of every reaction step for model C proved to be approximately 3 kcal/mol lower than that for model A. Although this decline in activation energy might seem insignificant, in the perspective of catalyst design it provides an implication of a potential trimerization system comprising model C. The enhanced reactivity might in part arise from the lower charge on Cr in model C (Figure 7), which again emphasizes the importance of the reduction of chromium to a low oxidation state.

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CONCLUSIONS Detailed computational investigations were carried out on a model system related to the commercial Chevron−Phillips ethylene trimerization system. Calculations conducted by DFT methods at the PBE/TZVP level of theory provided some essential insight into the mechanistic aspects of this industrial landmark system. The complete Gibbs free energy surfaces G

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Figure 7. Cr charge on each intermediate involved in the trimerization pathway for models A and C.

have been successfully located for both Cr(I/III) and Cr(II/IV) redox cycles and both proved to be capable of promoting selective ethylene trimerization. However, in terms of the effective activation energy of the trimerization reaction, 19.0 kcal/mol for pathway A and 31.4 kcal/mol for pathway B, the Cr(I/III) redox couple is considered to be a more promising model for the catalytic system. The geometries of the active species were optimized under all possible spin states, and the corresponding free energies were obtained. For pathway A involving Cr(I) and Cr(III) species, there is a possibility of spin crossover occurring during the starting species entering the catalytic cycle, although the crossover would not affect the rate of the relevant trimerization reaction. The pendant chlorine in the ligand has associating/dissociating interactions with the chromium center, and this hemilability is proposed to be a key factor for 1-hexene selectivity.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Figures, a table, text, and an xyz file giving calculated bond parameters of 2 in Scheme 1 at the PBE/TZVP level of theory, free energies of model A obtained by the combinations of different density functionals, and Cartesian coordinates of optimized geometry for all species mentioned in the present study. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail for Z.L.: [email protected]. *E-mail for B.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Duchateau for many useful suggestions. We also thank the China Scholarship Council, the Natural Science Foundation of China (No. 21004020, No. 21174037, No. 21304033), and the Eindhoven University of Technology for financial support.

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REFERENCES

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dx.doi.org/10.1021/om500306a | Organometallics XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/om500306a | Organometallics XXXX, XXX, XXX−XXX