Role of the Counteranion in the Reaction Mechanism of Propylene

5 days ago - We studied the stereoselective propylene polymerization catalyzed by the ion pair (IP) active species of (pyridylamido)hafnium(IV) comple...
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Role of the Counteranion in the Reaction Mechanism of Propylene Polymerization Catalyzed by a (Pyridylamido)hafnium(IV) Complex K. Matsumoto,† M. Takayanagi,†,‡,§ S. K. Sankaran,†,§ N. Koga,§,∥ and M. Nagaoka*,§,∥,⊥ †

Graduate School of Information Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan The Center for Data Science Education and Research, Shiga University, 1-1-1 Banba, Hikone, Shiga 522-8522, Japan § Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, Honmachi, Kawaguchi 332-0012, Japan ∥ Graduate School of Informatics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan ⊥ Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyodai Katsura, Kyoto 615-8520, Japan ‡

S Supporting Information *

ABSTRACT: We studied the stereoselective propylene polymerization catalyzed by the ion pair (IP) active species of (pyridylamido)hafnium(IV) complex, the catalyst. In particular, we focused on the role of the counteranion (CA) for the reaction mechanism of propylene insertion because it had never been investigated from the molecular point of view. First, we searched for a variety of IP configurations by the molecular dynamics (MD) method and quantum chemical calculation, and located the transition states (TSs) of propylene insertion. By comparing the CA-bound catalysts with the isolated ones, it was revealed that the manner of energetically favorable insertion is interestingly altered due to the location of the CA. Next, we showed that the CA is essential to determine the coordination structures of the Hf atom at transition state. Therefore, it was concluded that the role of the CA is critical to accurately control the polymerization mechanism. Finally, we proposed a synthesis experiment which would be advantageous to increase the stereoselectivity of the catalyst on the basis of the present polymerization mechanism. screening technologies.9−11 This complex belongs to a new family of homogeneous hafnium catalysts with nonconventional structure and can generate highly isotactic polypropylene with high molecular weight at high temperature. In particular, since its application to chain shuttling polymerization (CSA)11−13 is a novel polymerization method to efficiently synthesize an olefin block copolymer, its reaction mechanism has been investigated in detail.14−24 As in the case of metallocene complexes, the (pyridylamido)hafnium(IV) complex 1 also requires activation by a cocatalyst (Scheme 1). When 1 is activated by tris(pentafluorophenyl)borane [B(C6F5)3] as a cocatalyst, one of the Me groups connecting to the Hf atom is abstracted, and the cation 2 and anion 3 are generated (Scheme 1). Afterward, olefin insertion into the Hf−Caryl bond of 2 occurs15,16 to form an active species called “monomer-inserted active species”, i.e., in the present case, “propylene-inserted active species”. It is true that there are eight isomeric propylene-inserted active species.15,16 However, 4 in Scheme 1 is known to be the kinetically most favorable according to the previous study.15 Rosa et al.20 investigated the origin of stereoselectivity in propylene polymerization catalyzed by the propylene-inserted active species. On the basis of their analysis, they concluded that

1. INTRODUCTION Organometallic complexes hold prominent positions in the field of catalysts for olefin polymerization. Among such complexes, a family of metallocene catalysts has been wellstudied since the discovery of the remarkable cocatalyst effect of methylaluminoxane (MAO) by Kaminsky and Sinn.1,2 Detailed studies revealed that the active species are the ion pairs (IPs) generated from metallocene complexes through the activations by cocatalysts.3,4 Thus, the active species has welldefined chemical structures derived from the metallocene complexes, which allows us to control the microstructures of product polymers.4−6 For example, in the case of propylene polymerization, ansa-C2 and ansa-Cs symmetric metallocene complexes provide isotactic and syndiotactic polypropylene, respectively, while ansa-C1 symmetric metallocene complexes do a wide variety of polymers such as isotactic, hemiisotactic, and syndiotactic polypropylene. Therefore, metallocene complexes have paved the way to control the physical and chemical properties of the product polymers through catalyst designing. Since the 1990s, such organometallic complexes with ligands different from metallocenes have been developed to realize novel olefin polymerization methods for the synthesis of useful polyolefins.7,8 These catalysts are collectively called nonmetallocene catalysts. Under such circumstances, the (pyridylamido)hafnium(IV) complex, one among them, has been discovered by Dow and Symyx with high-throughput © XXXX American Chemical Society

Received: October 14, 2017

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DOI: 10.1021/acs.organomet.7b00767 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 1. Activation of (Pyridylamido)hafnium(IV) Complex by [B(C6F5)3], and the Following Propylene Insertion into the Hf−Caryl Bond

various IP configurations by executing the MD. Next, using them as initial configurations, we obtained numerous locally stable configurations of the IP by DFT calculations. Then we searched for transition states (TSs) for propylene insertion and studied what kind of insertion manner is favorable. As a result, we found that the favorable insertion manner is altered with change in the location of the CA. Then, we investigated the structural features of the energetically favorable TSs and found that the coordination structure of the Hf atom can play an important role. Finally, on the basis of the results obtained, we would like to suggest an experimental approach that can identify what insertion manner should increase the stereoselectivity.

the propylene-inserted active species has two active sites, with one being favorable for the propylene insertion. They also showed that the si-face of propylene is selected due to the active site structure, which is the origin of the stereoselectivity. As in the case of the metallocene catalyst, the active species of the (pyridylamido)hafnium(IV) complex is closely connected to the counteranion (CA).15,17 At present, much work has been done on modeling the IP active species of metallocene.25−32 However, in the case of the (pyridylamido)hafnium(IV) complex, the insertion mechanism by the IP active species has not been investigated to date, although the ligand modification was investigated with IP.15,17 In the previous study,15 it was pointed out that the anion plays a necessary role for monomer insertion into the Hf−Caryl bond and, therefore, should be explicitly included in the quantum mechanical (QM) treatment, i.e., quantum chemical (QC) calculation. In addition, recently, we also revealed that,22 at the stage of 2 and 3 just after the activation by [B(C6F5)3], the occupation of the active site of 2 by the anion 3 affects directly the activity of the catalyst.21 Thus, it is natural that the anion also has a decisive influence on the propylene polymerization and the origin of the stereoselectivity. In this paper, therefore, we theoretically investigate the mechanism of propylene polymerization reaction and the origin of the stereoselectivity with the explicit anion 3. For these purposes, we need some appropriate theoretical methods which can properly describe the process of breaking and recombination of chemical bonds. In fact, we basically adopted density functional theory (DFT) for QM calculation. However, it simply demands huge computational cost to obtain many varieties of locally stable IP configurations. Thus, we also adopted the molecular dynamics (MD) method to sample those configurations with significantly less computational cost. First, we developed molecular mechanics (MM) force field parameters for the active species of the (pyridylamido)hafnium(IV) complex based on the previous studies22,29and sampled

2. COMPUTATIONAL DETAILS 2.1. Model of Activated (Pyridylamido)hafnium(IV) Complex. In this study, for the sake of simplicity and reducing calculation cost, we adopt hereafter the simplest conjugated molecule “ethylene”inserted active species 5 (Figure 1) as a model of monomer-inserted active species. The effect of the removed Me group will be briefly checked in subsection 3.1. In addition, the previous study20 showed that the selection of the propylene enantioface is directly dictated by the active site’s structure, rather than by the orientation of the polymer chain. Thus, although 5 does not have the polymeryl group but Me

Figure 1. Ethylene-inserted active species adopted as a computational model of monomer-inserted active species. B

DOI: 10.1021/acs.organomet.7b00767 Organometallics XXXX, XXX, XXX−XXX

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Organometallics group, we consider that it would be sufficient and reliable enough as a model for the present purpose. 2.2. QM Calculations. All QM calculations were performed by DFT with the M06 functional, which is known to be appropriate for transition metals,33 by using Gaussian 09.34 The basis set LANL2DZ was chosen with the associated effective core potential and additional f orbitals on the Hf atom, and 6-31G(d,p), on all other atoms, respectively. The coefficients for the additional f orbitals were used according to the previous research.35 2.3. Development of MM Force Field Parameters for Ethylene-Inserted Active Species. In order to sample various IP configurations, we first developed MM force field parameters for the ethylene-inserted active species 5 on the basis of those for the cation 2 we have reported previously22 and those in GAFF 1.4.36 For more details of the force field development, parameters, and reproducibility, see Supporting Information. 2.4. Sampling of Locally Stable IP Configurations. We performed MD calculations of the IP of 5 and 3 in a vacuum by using AMBER 12.37 First, an NVT (constant number of molecules, volume, and temperature) MD simulation was performed for 1 ns at 800 K. The temperature was regulated by the weak coupling algorithm with a time constant 1 ps. In this calculation, we imposed harmonic constraints on the position of the anion to efficiently sample the IP configurations (see Supporting Information). We obtained 1000 configurations from the MD trajectory and classified them into 20 representative configurations by a K-means clustering algorithm implemented in the ptraj program in AmberTools12. We performed structural optimization by DFT calculation starting from these representative configurations and obtained 20 locally stable configurations (see Supporting Information).

or si-face, leading to eight insertion manners as a whole. We name them as shown in Chart 1. For example, when the propylene is inserted via the trans coordination in the 1,2insertion with the re-face, the insertion manner is called trans1,2-re. We assumed that these eight manners of insertion are essential, ignoring others. It is because some typical QM calculations showed that other such insertions would be high in potential energy, for example, one in which propylene is inserted via the trans coordination with the anion also in the trans site. In addition, in the case of the present catalyst, it is known that site epimerization occurs once the propylene is inserted.20 This means, therefore, that the alternation of reaction sites does not occur; that is, it would be reasonable that the insertion manner with a lower energy TS is always preferred. Then, to investigate which insertion manner is the most favorable, we studied 20 TSs for each one from the IP configurations obtained in the subsection 2.4. In Table 1, potential energies ETS and Gibbs free energies GTS in the lowest-energy path TSs of each insertion manner are summarized. Hereafter, all energies for the ion pair are shown as relative values with respect to the potential energy at the TS of trans-1,2-re taken as the origin. In general, it can be said that 1,2-insertion is more favorable than 2,1-insertion in both cis and trans insertions not only in ETS but also in GTS. This result is consistent with the experimental observation that the 1,2-insertion is the major insertion manner.10,14,23 Therefore, we will focus on 1,2insertion in the following discussion. More precisely, it is understood in Table 1 that the most favorable insertion manner is trans-1,2-re in both ETS and GTS. Whereas, in the previous theoretical study where the CA was not included,20 it was reported that the insertion reaction with the same propylene orientation as cis-1,2-re provides the most stable TS, which we have been presently reconfirmed (see Supporting Information). Thus, it can be understood, on the basis of the DFT calculation with the same level of theory, that the energetically favorable insertion manner is rather strongly altered by including the CA. In addition, it is worth mentioning that the same tendency was obtained when we additionally performed a set of calculations of 1,2-insertion TSs corresponding to those in Table 1 with propylene-inserted active species (see Supporting Information). The potential energy ETS and Gibbs free energy GTS of 1,2insertions in Table 1 can be interpreted by considering the cation−anion interaction and molecular vibration. In TSs of cis insertions, the Me−Hf bond changes its direction and prevents the cation−anion interaction (Chart 1). Whereas, in the TSs of the trans insertions, the cation−anion interaction is not prevented (Chart 1). In fact, the bond angle Npyr−Hf−CMe is 117.92° at the optimized structure of the isolated cation, and 121.52° on average for four trans insertion TSs in Table 1. However, for the four cis insertion TSs, the bond angle is 165.67° on average. Thus, the cation interacts with the anion more strongly in trans insertions. In fact, this is clearly shown by cation−anion interaction energy ΔETS = ETS − EcationTS − EanionTS, where ETS, EcationTS, and EanionTS are potential energies of the IP, cation, and anion at the structures in each TS, respectively. As shown in Table 2, ΔETS of the cis insertion is at least 20 kcal/mol less negative than that of the trans insertion. As a result, trans insertions are in general more favorable than cis insertions with larger stabilization energy (Table 1). Furthermore, we also revealed that the strong cation−anion

3. RESULTS AND DISCUSSION 3.1. Energetically Favorable Reaction Path in Propylene Polymerization Catalyzed by the IP. In the first subsection, we will discuss the mechanism of propylene insertion reaction catalyzed by the IP. Due to the rigid structure of the pincer ligand and the direction of the Me group, the cation has only two coordination sites (Figure 2), so

Figure 2. Two possible coordination sites of the anion and propylene.

that we define trans and cis as those words to distinguish the two coordinations relative to the pyridine nitrogen. Thus, we consider two patterns according to the relative positions of the anion and propylene. In the first pattern, the anion is in the trans site, and the propylene is inserted via the cis coordination. In the second pattern, the anion is in the cis site, and the propylene is inserted via the trans coordination. We call these patterns cis and trans insertions, respectively (for a detailed discussion on the coordination sites, see Supporting Information). In addition, there are four possible orientations of propylene in each pattern, that is, 1,2- or 2,1-insertion with propylene’s reC

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Organometallics Chart 1. Eight Insertion Manners of Monomer Propylene in Trans and Cis Insertions

Table 1. Potential Energies (ETS) and Gibbs Free Energies (GTS) at the Lowest-Energy TSs among 20 TSs of Each Insertiona trans-1,2-re TS

trans-1,2-si

cis-1,2-re

Table 3. Gibbs Free Energy (GTS) and Its Five Components ETS, FvibTS, FtraTS, FrotTS and PV in the Lowest-Energy TSs of 1,2-Insertionsa

cis-1,2-si

E GTSa,b

0.0 579.3 trans-2,1-re

1.7 581.2 trans-2,1-si

3.7 580.8 cis-2,1-re

4.2 580.8 cis-2,1-si

ETS GTSa,b

6.7 586.3

11.0 589.3

6.7 583.2

10.9 585.4

TSb

E FvibTSc FtraTSc FrotTSc PVc GTSb

a

The unit is kcal/mol. The potential energy of the most stable TS for trans-1,2-re is set to the origin (0.0 kcal/mol). bTemperature and pressure are 20 °C and 2.04 atm, which are the same as those in the experimental condition.24

ΔETSb

trans-2,1-si

cis-2,1-re

cis-2,1-si

−81.1

−79.5

−55.9

−56.6

trans-1,2-si

cis-1,2-re

cis-1,2-si

0.0 602.4 −12.6 −11.1 0.6 579.3

1.7 602.6 −12.6 −11.1 0.6 581.2

3.7 600.3 −12.6 −11.2 0.6 580.8

4.2 599.8 −12.6 −11.2 0.6 580.8

Temperature and pressure are 20 °C and 2.04 atm.24 bThe unit is kcal/mol. The potential energy of the most stable TS for trans-1,2-re is set to the origin (0.0 kcal/mol). cFvibTS, FtraTS, and FrotTS are those energy contributions from vibration, translation, and rotation, and P and V are pressure and volume of the system, respectively. a

Table 2. Cation−Anion Interaction Energy (ΔETS) in the Lowest-Energy TSs of 1,2-Insertionsa trans-2,1-re

trans-1,2-re

From the discussion above, trans insertion is stabilized by cation−anion interaction, but destabilized by the vibrational contribution. As a whole, trans-1,2-re is more stable than cis insertions in GTS although trans-1,2-si is less stable than cis insertions (Table 1). According to the energetics we discussed above, it can be shown that the free energy differences of the 1,2-insertion in Table 1 are consistent with the stereoselectivity observed in an experiment. In the previous experimental study,24 it was reported that a related catalyst gives polypropylene with meso pentad fraction [m4] = 0.860 at 20 °C and 2.04 atm. In addition, the mechanism of stereoselectivity is enantiomorphic site control; that is, a specific prochiral face of propylene is selected by the structure of the active site. We can estimate from [m4] = 0.860 that one prochiral face of propylene is selected with a probability of p = 0.970 (see Supporting Information). Then, the probability p was estimated from the Gibbs free energies in Table 1, assuming that 1,2-insertions are dominant, and the sequential insertion reactions are mutually independent. On these assumptions, it is theoretically conjectured that the re-face of propylene is selected with a probability of p = 0.904 at 20 °C, 2.04 atm (see Supporting Information). This estimation is consistent with the experimentally estimated value p = 0.970. 3.2. Hexacoordinated Structure Induced by the Counteranion in trans Insertion. In this subsection, we

The unit is kcal/mol. bΔETS is defined as ΔETS = ETS − EcationTS − EanionTS where ETS, EcationTS, and EanionTS are potential energies of the IP, cation, and anion at the structure in each TS. The basis set superposition error was corrected by counterpoise method.38 a

interaction in trans insertion affects the free energy of activation ΔG‡ by changing the Hf’s coordination structure. In the next subsection 3.2, we will discuss the structures of the TSs for trans insertion, revealing how the CA influences the reaction mechanism. In contrast, the vibrational contribution destabilizes trans insertion. In Table 3, GTS is decomposed into five components as GTS = ETS + FvibTS + FtraTS + FrotTS + PV, where FvibTS, FtraTS, and FrotTS are contributions from vibration, translation, and rotation, and P and V are the pressure and volume of the system, respectively. It is clearly shown that the vibrational component in trans insertion is about 2 kcal/mol larger than that in the cis one, although FtraTS and PV are obviously the same, and FrotTS is within the margin of 0.1 kcal/mol. This is because the strong cation−anion interaction in general increases the vibrational frequencies of ion pairs, leading to an increase in the zero-point energy and decrease in the entropy. D

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Organometallics will discuss the structures of the TSs for trans insertion, showing how the CA affects the reaction. Figure 3 illustrates a

more distorted octahedral-like structure. Then, we investigated the relationship between θ and the free energy of activation ΔG‡, i.e., the Gibbs free energy difference between the TS and the corresponding reactant. As shown in Figure 5, θ and ΔG‡

Figure 3. Stereoimage of the typical TS for trans insertion without the CA. The cation and propylene are drawn with red and yellow sticks, respectively. The orange, cyan, and blue spheres represent Hf, C, and N atoms, respectively. Hydrogen atoms are not shown.

Figure 5. Correlations between the angle θ and the free energy of activation ΔG‡.

stereoimage of the typical TS for trans insertion without CA. In the case without the anion, the Hf atom has a pentacoordinated and trigonal bipyramidal-like structure. In contrast, in the case with the CA, the Hf atom has a different coordination structure. Figure 4 illustrates stereoimages of the most stable TSs for

show relatively good correlation with each negative correlation coefficient, i.e., r = −0.788 and −0.848 for trans-1,2-re and trans-1,2-si, respectively. It means that the propylene insertion reaction occurs more easily as the Hf atom takes a more distorted octahedral-like structure (for the reason why they have a negative correlation, see Supporting Information). From the discussion above, it is clear that the hexacoordinated structure induced by the CA is important. Therefore, we conclude that the CA is necessary to accurately describe the mechanism of the polymerization reaction. 3.3. Suggestion of Synthesis Experiment to Increase the Stereoselectivity in Propylene Polymerization. In this subsection, we will suggest a synthesis experiment which would possibly accomplish higher stereoselectivity. As discussed in the previous subsection, the magnitude relationship of the free energy differences in Table 1 is consistent with the stereoselectivity observed experimentally, and the trans-1,2-re is the most favorable manner of insertion. Thus, if we would be able to stabilize the TS for trans-1,2-re and to destabilize the others, the stereoselectivity should be greatly enhanced. Such a situation would be realized by introducing a Me group into the α position to the Hf atom (Figures 6 and 7). As shown in Figure 6, because the Me group causes steric hindrance against the anion in the trans site, the TSs for cis-1,2-re and cis-1,2-si are destabilized more relative to the trans-1,2-re and trans-1,2-si. In addition, as shown in Figure 7, the Me group also destabilizes the TS for trans-1,2-si more than that for trans-1,2-re due to the

Figure 4. (a) Stereoimage of the most stable TS for trans-1,2-re and (b) that for trans-1,2-si. The cation, anion, and propylene are drawn with red, blue, and yellow sticks, respectively. The orange, cyan, blue, and pink spheres represent Hf, C, N, and B atoms, respectively. Hydrogen atoms are not shown.

trans-1,2-re and trans-1,2-si insertions. The direction of the Me group connecting to the Hf atom changes due to the coordination of the anion. As a result, the Hf atom has the hexacoordinated, distorted octahedral-like structure. Therefore, we consider that the CA is essential in the coordination structure of the Hf atom. To show the relation between the coordination structure and the reactivity, we chose the angle θ (N1−Hf−C1) as an index to describe the coordination structure of the Hf atom (Figure 4) because θ becomes close to 180° as the Hf atom becomes a

Figure 6. Me group causes steric hindrance against the anion in trans site. As a result, the cis insertion should be destabilized against the trans insertion. E

DOI: 10.1021/acs.organomet.7b00767 Organometallics XXXX, XXX, XXX−XXX

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The “ligand” modification by monomer insertion is a characteristic feature of the (pyridylamido)hafnium(IV) complex. Our suggestion indicates that such a characteristic could possibly be utilized to increase the stereoselectivity. In other words, it should be also an advantage that the stereoselectivity can be increased just by pretreating the reactant with cis-2butene without exchanging the organometallic complex itself. Figure 7. Me group in α position has steric hindrance with the monomeric propylene. As a result, the TS of trans-1,2-si should be destabilized against that of trans-1,2-re.

4. CONCLUDING REMARKS In this paper, we investigated microscopically the mechanism of a propylene polymerization reaction catalyzed by the ion pair (IP) active species of the (pyridylamido)hafnium(IV) complex. First, we revealed that the monomer insertion reaction via the trans coordination has the most favorable TS due to the location of the counteranion (CA). Second, we studied the possible TS structures and found that the Hf atom shows a hexacoordinated, distorted octahedral-like structure due to the CA coordination. Then, we defined the angle θ (N1−Hf−C1) as an index to describe the coordination structure of the Hf atom, and it turned out that the angle θ and the free energy of activation ΔG‡ have a good negative correlation. It means that the propylene insertion reaction occurs more easily, as the Hf atom shows a more distorted octahedral-like characteristic. Thus, it is clear that the influence of the CA on the coordination structure of the Hf atom is significant for the reaction mechanism. We can conclude that the CA has a specific role to accurately describe the reaction mechanism, and therefore, is necessary to theoretically study the catalytic activity of the (pyridylamido)hafnium(IV) complex whatever characteristic of the reaction mechanism we concern. Finally, we suggested a synthesis experiment which indicates that the ligand modification could be possibly utilized to increase the stereoselectivity by using cis-2-butene. In addition, since this method could increase the stereoselectivity without exchanging the organometallic complex itself, it might be advantageous from a practical point of view.

steric hindrance against the propylene. Hence, the TSs other than that for trans-1,2-re are destabilized more, leading to the increase of stereoselectivity. The above proposed experiment could also be a test of our assumed reaction mechanism for the stereoselective propylene polymerization catalyzed by the IP active species of the (pyridylamido)hafnium(IV) complex. If the Me group introduction would have no influence on the stereoselectivity, it would mean that the present mechanism should not stand and any other reaction mechanism, such as insertion via cis coordination, might be dominant. Finally, we would like to discuss a synthesis method to introduce the Me group into the α position to the Hf atom with the desired stereochemistry. The method we propose is to generate cis-2-butene-inserted active species. As shown in Figure 8, there are two isomeric cis-2-butene-inserted active

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Two possible isomeric cis-2-butene-inserted active species, and the Gibbs free energies of TSs (GTS) for each isomer. The unit is kcal/mol, and GTS for 6a is set to the origin (0.0 kcal/mol). GTS was calculated under the condition 20 °C and 2.04 atm, which is the same as the experimental one.24

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00767. The force field parameters and its developments, the IP configurations and sampling, the results of quantum chemical calculation without the counteranion, the way to estimate the probability of stereoselectivity, and the reason why the angle θ (N1−Hf−C1) and the free energy of activation ΔG‡ have a negative correlation (PDF) Cartesian coordinates of the structures discussed (ZIP)

species 6a and 6b, and our QM calculations showed that the TS leading to 6a has a lower Gibbs free energy. Thus, we can suggest that the isomer 6a having the Me group at the α position with the desired stereochemistry should be more feasible. It is true that, to synthesize cis-2-butene-inserted active species, we have to pretreat the cation 2 with cis-2-butene. However, according to the previous study,15,16,29 olefin insertion into the Hf−Caryl bond is slower than an elongation reaction by monomer-inserted active species. Thus, for a sufficient amount of cis-2-butene-inserted active species, cation 2 has to react with continuously supplied cis-2-butene. In such a situation, the problem is that cis-2-butene-inserted active species could rapidly generate poly(cis-2-butene) and precipitate. To avoid this problem, it could be effective to enhance the termination reaction by continuously supplying hydrogen at the same time.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./fax: +81-52-7895623. ORCID

M. Takayanagi: 0000-0002-2458-0180 M. Nagaoka: 0000-0002-1735-7319 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.organomet.7b00767 Organometallics XXXX, XXX, XXX−XXX

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(21) Cueny, E. S.; Johnson, H. C.; Anding, B. J.; Landis, C. R. J. Am. Chem. Soc. 2017, 139 (34), 11903−11912. (22) Matsumoto, K.; Sandhya, K. S.; Takayanagi, M.; Koga, N.; Nagaoka, M. Organometallics 2016, 35 (24), 4099−4105. (23) Zhou, Z.; Stevens, J. C.; Klosin, J.; Qiu, X.; Redwine, D.; Cong, R.; Taha, A.; Mason, J.; Winniford, B.; Chauvel, P.; Montan, N. Macromolecules 2009, 42 (6), 2291−2294. (24) Domski, G. J.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2007, 40 (9), 3510−3513. (25) Rappé, A. K.; Skiff, W. M.; Casewit, C. J. Chem. Rev. 2000, 100 (4), 1435−1456. (26) Correa, A.; Cavallo, L. J. Am. Chem. Soc. 2006, 128 (33), 10952−10959. (27) Sandhya, K. S.; Koga, N.; Nagaoka, M. Bull. Chem. Soc. Jpn. 2016, 89 (9), 1093−1105. (28) Laine, A.; Coussens, B. B.; Hirvi, J. T.; Berthoud, A.; Friederichs, N.; Severn, J. R.; Linnolahti, M. Organometallics 2015, 34 (11), 2415− 2421. (29) Rowley, C. N.; Woo, T. K. Organometallics 2011, 30 (8), 2071− 2074. (30) Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2007, 26 (8), 2024−2036. (31) Motta, A.; Fragalà, I. L.; Marks, T. J. J. Am. Chem. Soc. 2007, 129 (23), 7327−7338. (32) Motta, A.; Fragalà, I. L.; Marks, T. J. J. Am. Chem. Soc. 2008, 130 (49), 16533−16546. (33) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120 (1−3), 215−241. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (35) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208 (1−2), 111−114. (36) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. J. Comput. Chem. 2004, 25 (9), 1157−1174. (37) Case, D. A.; Darden, T. A.; Cheatham, T. E.; Simmerling, C. L.; Wang, J.; Duke, R. E.; Luo, R.; Walker, R. C.; Zhang, W.; Merz, K. M.; Roberts, B.; Hayik, S.; Roitberg, A.; Seabra, G.; Swails, J.; Goetz, A. W.; Kolossváry, I.; Wong, K. F.; Paesani, F.; Vanicek, J.; Wolf, R. M.; Liu, J.; Wu, X.; Brozell, S. R.; Steinbrecher, T.; Gohlke, H.; Cai, Q.; Ye, X.; Wang, J.; Hsieh, M. J.; Cui, G.; Roe, D. R.; Mathews, D. H.; Seetin, M. G.; Salomon-Ferrer, R.; Sagui, C.; Babin, V.; Luchko, T.; Gusarov, S.; Kovalenko, A.; Kollman, P. A. AMBER 12; University of California: San Francisco, 2012. (38) Boys, S. F.; Bernardi, F. D. Mol. Phys. 1970, 19 (4), 553−566.

ACKNOWLEDGMENTS This work was supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science Technology Agency (JST); by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan; and also by the MEXT programs “Elements Strategy Initiative for Catalysts and Batteries (ESICB)” and “Priority Issue 5 on Post-K Computer” (Development of new fundamental technologies for highefficiency energy creation, conversion/storage and use). The calculations were partially performed using several computing systems at the Information Technology Center in Nagoya University.

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REFERENCES

(1) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99− 149. (2) Sinn, H.; Kaminsky, W.; Vollmer, H. J.; Woldt, R. Angew. Chem., Int. Ed. Engl. 1980, 19 (5), 390−392. (3) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143−1170. (4) Alt, H. G.; Alexander, K. Chem. Rev. 2000, 100 (4), 1205−1222. (5) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100 (4), 1253−1345. (6) Osakada, K. Organometallic Reactions and Polymerization; Carpenter, B., Ceroni, P., Kirchner, B., Landfester, K., Leszczynski, J., Luh, T. Y., Perlt, E., Polfer, N. C., Salzer, R., Eds.; Lecture Notes in Chemistry Series Vol. 85; Springer-Verlag: Berlin, Heidelberg, 2014. (7) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103 (1), 283− 315. (8) Baier, M. C.; Zuideveld, M. A.; Mecking, S. Angew. Chem., Int. Ed. 2014, 53 (37), 9722−9744. (9) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M.; Lund, C.; Murphy, V.; Shoemaker, J. A. W.; Tracht, U.; Turner, H.; Zhang, J.; Uno, T.; Rosen, R. K.; Stevens, J. C. J. Am. Chem. Soc. 2003, 125 (14), 4306−4317. (10) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen, R. K.; Stevens, J. C.; Alfano, F.; Busico, V.; Cipullo, R.; Talarico, G. Angew. Chem., Int. Ed. 2006, 45 (20), 3278−3283. (11) Chum, P. S.; Swogger, K. W. Prog. Polym. Sci. 2008, 33 (8), 797−819. (12) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312 (5774), 714−719. (13) Wenzel, T. T.; Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L. Top. Organomet. Chem. 2009, 26, 65−104. (14) Frazier, K. A.; Froese, R. D.; He, Y.; Klosin, J.; Theriault, C. N.; Vosejpka, P. C.; Zhou, Z.; Abboud, K. A. Organometallics 2011, 30 (12), 3318−3329. (15) Zuccaccia, C.; Busico, V.; Cipullo, R.; Talarico, G.; Froese, R. D. J.; Vosejpka, P. C.; Hustad, P. D.; Macchioni, A. Organometallics 2009, 28 (18), 5445−5458. (16) Froese, R. D. J.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. J. Am. Chem. Soc. 2007, 129 (25), 7831−7840. (17) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. J. Am. Chem. Soc. 2008, 130 (31), 10354−10368. (18) Busico, V.; Cipullo, R.; Pellecchia, R.; Rongo, L.; Talarico, G.; Macchioni, A.; Zuccaccia, C.; Froese, R. D. J.; Hustad, P. D. Macromolecules 2009, 42 (13), 4369−4373. (19) Niu, A.; Stellbrink, J.; Allgaier, J.; Richter, D.; Hartmann, R.; Domski, G. J.; Coates, G. W.; Fetters, L. J. Macromolecules 2009, 42 (4), 1083−1090. (20) De Rosa, C.; Di Girolamo, R.; Talarico, G.; Chimiche, S.; Federico, N.; Cintia, V. ACS Catal. 2016, 6 (6), 3767−3770. G

DOI: 10.1021/acs.organomet.7b00767 Organometallics XXXX, XXX, XXX−XXX