An Active Site Opening Mechanism in a (Pyridylamide)hafnium(IV) Ion

Dec 15, 2016 - Thus, we suggest the design of pincer type complexes with the possibility of AASO taken into account. In addition, if we attempt to mak...
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An Active Site Opening Mechanism in a (Pyridylamide)hafnium(IV) Ion Pair Catalyst: An Associative Mechanism Kentaro Matsumoto,† Karakkadparambil Sankaran Sandhya,†,‡ Masayoshi Takayanagi,†,‡ Nobuaki Koga,†,‡ and Masataka Nagaoka*,†,‡,§ †

Graduate School of Information Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Honmachi, Kawaguchi 332-0012, Japan § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University Katsura, Kyoto 615-8520, Japan ‡

S Supporting Information *

ABSTRACT: We report a molecular dynamics (MD) study on the ion pair [{N−,N,C−}HfMe][MeB(C6F5)3], which is the active species of a (pyridylamide)hafnium(IV) complex for olefin polymerization. The present simulation model system is composed of an ion pair in explicit organic solvent (heptane) with and without ethylene monomers. MD simulations revealed an active site opening (ASO) mechanism, i.e., the associative active site opening (AASO), where the counteranion dissociates from the active site associatively with the monomer coordination to the active site. AASO also explains consistently the experimental fact that the activity of the catalyst increases when the counteranion has no Me groups. Further we show that AASO is the major mechanism of ASO by using the replica exchange molecular dynamics (REMD) method. In addition, we discuss two important factors for the AASO: one is the structural condition of the ion pair, and the other is the net stabilization energy. Finally, we conclude that both the ASO and the monomer coordination should be taken into account to accurately design the behavior of the ion pair and predict the activity.

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INTRODUCTION

After the metallocene catalysts, we enter the era of metal complexes with other ligand motifs, summarized as nonmetallocenes.24 These catalysts are important not only in industry but also in developing new possibilities of polyolefins.25 An example of such a catalyst is the (pyridylamide)hafnium(IV) complex 126 (Figure 1). In particular, its recent application in the novel polymerization method called chain shuttling polymerization27 (CSP) has accelerated the study of the reaction mechanism with 1.28−33 The group 4 metal complex 1 needs activation by a cocatalyst. When B(C6F5)3 (2) is used as a cocatalyst, a Me group abstraction leads to the ion pair 3 (Figure 1). After that, olefin insertion into the Hf−Caryl bond occurs, and the genuine active species called a “monomer-inserted catalyst” is generated.29,30 However, experimental evidence indicates a direct interaction between the vacant coordination site on the Hf atom and the borate anion 4 or active site occupation by the anion (Figure 2).29,31 Therefore, to initiate the polymerization reaction, the active site should be opened by anion dissociation. This means that the microscopic behavior of anion 4 around the active site is important in the catalytic activity. In fact, some

One of the most important discoveries in industrial production of polyolefins has been that of the Ziegler−Natta catalysts. Since their discovery in the 1950s, they have been improved through several generations.1 Also in the 1950s, by Breslow and Newburg2 and by Natta, Pino, and their co-workers,3 it was shown that metallocene complexes catalyze ethylene polymerization under conditions similar to those applied to the heterogeneous Ziegler−Natta catalyst. However, metallocene complexes were applicable only to ethylene polymerization.4 This limitation was overcome by a discovery by Kaminsky and Sinn. They discovered that metallocene activated by methylalumoxane has high activity and is capable of polymerizing propylene and higher olefins.4,5 Their catalytic system is called a Kaminsky catalyst today. Detailed studies on the Kaminsky catalyst have revealed that the active species is a cation generated from the metallocene complex.4,6 Thus, the active species of the Kaminsky catalyst have well-defined chemical structures, which allow the control of the physical properties of the product polymers by designing the catalyst.6,7 Therefore, a variety of Kaminsky catalysts have been developed,6,7 and much work has been done on modeling the ion pair active species.8−23 © XXXX American Chemical Society

Received: October 20, 2016

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of the ion pair in explicit organic solvent (heptane), but the active site is always occupied by the anion. Thus, we focus on the monomer coordination to the active site. By including the stabilization energy coming from the monomer coordination, we reveal the ASO mechanism associated with the monomer coordination. In this article, we call this mechanism the associative active site opening (AASO) mechanism. Then we confirm that the AASO is the major mechanism in ASO process by using the replica exchange molecular dynamics (REMD) method.35 Finally, we discuss the important factors for the AASO.

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RESULTS AND DISCUSSION 2.1. Structure of the Ion Pair. We first investigated the ion pair structure by DFT calculations in a vacuum using the M06 functional, which is one of the appropriate functionals for organometallic systems,36 with the basis sets LANL2DZ with additional f orbitals37 on the Hf atom and 6-31G(d,p) on all other atoms. We selected a stereoisomer of the cation according to previous research by NMR.31 As a result, we obtained two important insights. The first insight is the attractive interactions in the ion pair. The interactions exist between the Hf atom of the cation and F atoms of the perfluorophenyl groups and between the Hf atom and H atoms of the borate Me group. These interactions enable the anion to interact with the cation in different manners, leading to different structures of the ion pair. Typically, we show three structures in Figure 3. First, the anion coordinates to the active site only with its Me group (Figure 3a). Second, the anion coordinates with its Me group and an F atom (Figure 3b). Third, the anion coordinates only with its F atoms (Figure 3c). The second insight is that there are two relative positions of the cation and anion (Figure 4). In the first position, the anion is located above the active site as for the three structures in Figure 3. Thus, the active site is completely closed (Figure 4a; the Npyr−Hf−B angle is around 180°). In the second position, the anion is on the side of the cation and the active site is open (Figure 4b; the Npyr−Hf−B angle is around 60°). We call these two structures “top structure” and “side structure”, respectively. In terms of the ion-pair concept of transition-metal complexes, they are classified as the inner-sphere ion pair (ISIP) and the outer-sphere ion pair (OSIP), respectively.38 Our DFT calculations showed that the top structure is 15.3 kcal/mol

Figure 1. Activation of a (pyridylamide)hafnium(IV) complex by Me group abstraction.

Figure 2. Stereoview of a typical ion pair structure appearing in our MD simulation.

experiments showed that the activity depends on the choice of cocatalyst.32,34 For these reasons, we focused on the behaviors of the ion pair 3, especially its active site opening (ASO) process. As the ASO is a dynamic process, we need an analysis focusing on its microscopic dynamics. However, experiments cannot directly observe it. Thus, we use a theoretical method, the molecular dynamics (MD) simulation technique, to investigate the dynamics of the ion pair 3. First, we investigate the ion pair structure and the interaction between the cation and anion by density functional theory (DFT) calculations. Next we develop the force fields for the ion pair on the basis of the DFT calculation results. Then, we conduct MD simulations

Figure 3. Three typical structures of the ion pair with different coordination manners. B

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Figure 4. Typical ion pair structures with different relative positions of the cation and anion: (a) top structure; (b) side structure; (c) example of the relative positions ignored.

more stable than the side structure on average (see the Supporting Information for more details). Thus, most of the ion pairs in solution have the top structure and the active site is completely closed. We assumed that these two relative positions are essential for the ASO and ignored other relative positions in the following. For an example of such ignored positions, the anion can be located at a different side of the cation (Figure 4c; the Npyr−Hf−B angle is around 300°). This position is ignorable because the lack of a direct interaction with the Hf atom will make this position unstable. In addition, this position is hard to investigate on the time scale of our simulation, since the Me group on the Hf atom hinders the anion in the top structure from migrating to this side of the cation. 2.2. Force Fields To Describe the Ion Pair Structure and Dynamics. From the results in the previous section, we realized that the interactions between the cation and anion are of importance to accurately simulate the ion pair’s behavior. To this end, we developed the force field parameter set of the Hf cation by fitting to DFT calculations, and then 6−12 LennardJones (LJ) interaction parameters between the Hf atom and borate anion atoms were modified to reproduce the interaction energy of the ion pair. The obtained force field parameters were able to correctly simulate the interaction energy and structural dynamics of the ion pair. For more details of the force field developments, parameters, and reproducibility, see the Supporting Information. In all the following simulations, we adopt these developed force fields. 2.3. Behavior of the Ion Pair in Inert Organic Solvent. Next, we investigated the behavior of the ion pair in an explicit inert organic solvent, heptane, by a 1.2 μs MD simulation. Here the model system was composed of 1 ion pair and 140 heptane solvent molecules. The thermodynamic conditions were 400 K and 1.4 MPa, which are the reported experimental conditions.39 During the simulation, the anion changed its coordination manner to the active site. In Figure 5, we show the angle θ (Hf−B−C) along the MD trajectory as an indicator of the coordination manner. When θ is in the ranges 0−20, 20−50, and 50−180°, the anion coordinates to the Hf atom through its Me group (as in Figure 3a), through its Me group and F atom (as in Figure 3b) and through the F atoms (as in Figure 3c), respectively. The ratios of the three regions during the MD simulation are 1.6%, 79.4%, and 19.0%, respectively. Although the ion pair changed its coordination manner, it always kept the top structure. Thus, on the time scale of this simulation, the top structure is very stable in inert organic solvent. Thus, the active site is completely closed and we could not observe the ASO.

Figure 5. Definition of angle θ (left) and its value along the MD trajectory (right).

2.4. Coordination of Olefin Monomer to the Active Site. In the experimental situations for the polymerization, olefin monomers also exist in the system. Since the monomers have double bonds, they coordinate strongly to the Hf atom by donating their π electrons. Thus, we made a prediction that the monomer coordination competes with the anion coordination to the Hf atom. In the present study, we adopted ethylene as the simplest model of olefin monomer. We conducted DFT calculations to investigate the coordination interaction between an ethylene molecule and the Hf atom without the anion. The calculation showed that the ethylene coordination results in 22.3 kcal/mol stabilization. In the previous section, our DFT calculations showed that the top structure is 15.3 kcal/mol more stable than the side structure on average. Thus, if the ion pair is in the side structure and the ethylene coordinates to the active site, it would result in 7.0 kcal/mol net stabilization. Since the simulations are conducted at the finite temperature 400 K, it is true that the energy compensation between them may be not so simple. However, the consideration implies that the ethylene coordination can assist the ASO. To accurately describe the ethylene coordination energy in MD simulations, we developed force fields by modifying the LJ interaction parameters between the Hf atom and the carbon atoms in the ethylene, as we have done for those between the Hf atom and borate anion atoms. For more details of the force field developments, see the Supporting Information. In the next section, we show the result of our MD simulation with these parameters. 2.5. Associative Active Site Opening (AASO) Mechanism. Next, we investigated the behavior of the ion pair in the presence of monomer ethylene in explicit solvent by 1.2 μs MD simulation. Here the model system with ethylene was composed of 1 ion pair, 70 ethylene molecules, and 140 C

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Organometallics heptane solvent molecules. The thermodynamic conditions were 400 K and 1.4 MPa, which are the reported experimental conditions.39 As we expected, we observed ion pair structure changes between the top structure and the side structure. This occurred five times over 1.2 μs. In particular, these processes occurred in a characteristic way, as shown in Figure 6. In the

Figure 7. Energy changes of cation−anion and cation−ethylene nonbonding interactions. The energy is averaged over time intervals of 40 ps.

6 Lennard−Jones potential, whose parameters were modified to reproduce the metal−ligand interactions (see Tables S1 and S3 in the Supporting Information). In Figure 7, the dissociation occurs between the time period of 0.1 and 0.2 ns. It can be clearly understood that the energy compensation occurs between two interactions during the dissociation process. Another important point of AASO is that the anion coordinates to the Hf atom through only its F atoms just before the ASO occurs. Thus, we expect that the dissociation occurs more easily when the anion has no Me groups. This is consistent with the experimental observation that the polymerization proceeds at lower temperature and shorter time if the Hf complex is activated by [CPh3]+[B(C6F5)4]−.29,31 Therefore, AASO reasonably explains this experimental fact from a dynamic and molecular point of view. 2.6. AASO as the Major Mechanism of ASO. It is true that the present results of MD simulations suggested that AASO is the important mechanism for the ASO. However, AASO occurred only five times in the 1.2 μs MD simulation. Thus, it is not obvious whether the AASO is the major mechanism of the ASO at the thermal equilibrium and if any other different mechanisms could exist as the major ASO mechanism. To answer this question, we investigated the distribution of the ion pair structure at the thermal equilibrium in the presence or absence of ethylene molecules by using the REMD method,35 which is able to provide global equilibrium information by sampling over many local structures. Along the two reaction coordinates, the interatomic distance r between the Hf and B atoms and the dihedral angle ϕ (C−N−Hf−B) (Figure 8), we have determined the free energy surfaces as shown in Figure 9.

Figure 6. Schematic representation of the associative ASO with the existence of ethylene monomers.

initial structure, the borate anion coordinates to the Hf atom with its Me group and F atom (Figure 6a). As we have seen in the model without ethylene (Figure 5), the anion changes its coordination manner and coordinates to the Hf atom through only its F atoms (Figure 6b). The active site is still closed by the borate anion in these top structures. Then, we observed that the borate anion slides on the surface of the cation with the coordination remaining to the Hf atom (Figure 6c). This sliding is enabled by the planar shapes of the pyridylamide ligand and the perfluorophenyl group and the fact that they are perpendicular to each other. As a result of this anion sliding, the ion pair yields enough coordination space at the active site. Then, a monomeric ethylene can come into the active site to coordinate to the Hf atom. Finally, the dissociation of the anion from the active site occurs associatively with the ethylene coordination, and the side structure appears (Figure 6d). In the above mechanism, the ASO and the ethylene coordination occur in an associative way. Thus, we call this mechanism the associative active site opening (AASO) mechanism, which is energetically favorable since the stabilization energy of the ethylene coordination compensates the destabilization energy coming from the structural change from the top structure to the side structure. In fact, the energy changes of cation-anion and cation-ethylene nonbonding interactions along the trajectory are shown in Figure 7. Here the nonbonding interaction energy indicates the sum of the intermolecular Coulombic interaction energy and the intermolecular van der Waals interaction energy represented by 12−

Figure 8. Definition of distance r and dihedral angle ϕ (C−N−Hf−B). D

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Figure 9. Free energy surfaces along r and ϕ for the model (a) with ethylene and (b) without ethylene. The free energy at the most stable point in region A is set to the origin (0.00 kcal/mol).

In the model with ethylene, we divided the free energy surface into four regions (Figure 9a) that are defined as in Table 1. By analyzing the structures in each region, we found

We also estimated the activation free energy barriers from A to B, B to C, and C to D from the plots in Figure 9. As shown in Table 3, the energy barriers from region A to B are 4.22 and

Table 1. Definitions of the Four Regions A−D

Table 3. Activation Free Energies Calculated from the Free Energy Profiles in Figure 9

region

r (Å)

ϕ (deg)

A B C D

3.6−4.8 4.8−5.8 4.8−5.8 5.8−9.3

110−220 135−225 50−135 20−80

transition

with ethylenea

without ethylenea

from A to B from B to C from C to D

4.22 3.10 1.23

5.02 2.76

a

Values are given in units of kcal/mol. Calculations were performed by using bins with size of (2.0°, 0.05 Å).

that the four structures in parts a−d of Figure 6 can be projected onto the regions denoted A−D, respectively. In this model, the dissociated structure, which corresponds to the region D, is realized. In contrast, in the model without ethylene, we divided the free energy surface into three regions (Figure 9b). The three regions are defined in the same way as in Figure 9a. While the most remarkable fact is that the regions A−C still exist, region D representing the dissociated structure has disappeared. This result shows that, although the ion pair can fluctuate in its temperature among the three structures (Figure 6a−c), the ion pair could hardly dissociate from the active site without ethylene. Next, we searched the most stable points in each region to estimate the relative stability of the regions. The free energies become larger in the order A < B < C (Table 2). In the case without ethylene, the free energy in region D is too high to estimate from this simulation. However, in the case with ethylene, region D is greatly stabilized. The free energy in region D is 0.97 kcal/mol, which is more stable than those in regions B and C.

5.02 kcal/mol with and without ethylene, respectively. They are high in comparison to that from B to C (3.10 and 2.76 kcal/ mol) and that from C to D (1.23 kcal/mol with ethylene). Thus, it is suggested that the slowest step in AASO is the transition from A to B: that is, the dissociation of the borate Me group from the active site. Finally we focus on how the process of the ASO is described on the free energy surface. In the model with ethylene, there is a path from region A to region D on the free energy surface in Figure 9a, indicating that the structural changes could occur sequentially from A to B, from B to C, and from C to D. This result is consistent with the AASO process shown in Figure 6. In fact, there is no other path that is able to connect among them. It is, therefore, concluded that there is no other way except the most favorable path of the ASO by AASO under the existence of ethylene molecules. 2.7. Two Important Factors for AASO. Because the enhancement of the ASO should lead to more efficient catalyst activation and polymerization, it is meaningful to consider what is important for AASO. From the above discussion, we can point out two important factors. One factor is the structure of the ion pair. For AASO, the catalyst should have a structure which allows a monomer to approach the active site as in Figure 6c. In our case, the planar structure of the ligand, a feature of pincer type ligands, and the coordination manner of the anion enables the anion sliding on the cation and results in AASO. This insight leads us to expect that AASO can widely occur in ion pairs of pincer complexes. Thus, we suggest the design of pincer type complexes with the possibility of AASO taken into account. In addition, if we attempt to make AASO occur in other systems, we should design a structure of the cation which allows a motion of the

Table 2. Free Energies at the Most Stable Points in Each Region region

with ethylenea

without ethylenea

A B C D

0.00 1.67 3.77 0.97

0.00 2.26 3.59

a

Values are given in units of kcal/mol. The free energy at the most stable point in region A is set to the origin (0.00 kcal/mol). Calculations were performed by using bins with size (2.0°, 0.05 Å). E

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To correctly represent the intermolecular interactions of cation− anion and cation−monomer, we also modified several parameters of the LJ 12−6 interactions involving the Hf atom. A similar approach has already been applied to a zirconocene catalyst by Cavallo et al.10 They fitted force field parameters to DFT geometries and energies of two stable ion pair structures, and succeeded in describing the interionic interactions. See the Supporting Information for more details. We prepared two MD simulation models. The first is composed of the ion pair 3 and 140 heptane solvent molecules. The second is composed of the ion pair 3, 140 heptane solvent molecules, and 60 ethylene monomers on the basis of the density of ethylene molecules in the previous study.44 MD Simulation Procedure. All of the MD simulations were executed using pmemd in AMBER12 under periodic boundary conditions at constant temperature and pressure. A weak-coupling algorithm with a time constant of 1 ps was applied to control the temperature and pressure. The integration time step was 1 fs, and the SHAKE algorithm was used to constrain the bond distances including hydrogen atoms. We followed the same following procedure for both models. First, we allocated solvent molecules around the ion pair and performed a molecular mechanics optimization. Then, we equilibrated the system by two MD calculations, one for 50 ps at 400 K and 100 MPa, followed by another for 1 ns at 400 K and 1.4 MPa. Finally, a production MD calculation was executed for 1.2 μs at 400 K and 1.4 MPa, which are the reported experimental conditions of CSP.39 REMD Simulation Procedure. We performed three REMD simulations35 by using pmemd in AMBER 12 for both models with and without ethylene molecules. The simulations consisted of 34 replicas with temperatures ranging from 392 to 800 K. The system was set up as follows. First, we picked up one configuration from the production MD trajectory. Next, we prepared 34 replicas and equilibrated each replica by a 1 ns MD calculation under constant temperature and volume conditions. Then, we performed production REMD simulations. The replica exchange was attempted every 1 ps. Each REMD simulation was run for 50000 attempts (in total, 50 ns of MD trajectory). Our force fields for the cation are designed for the Hf catalyst in which the Me group on the Hf atom and iBuPh group are in a trans relationship. However, in some configurations of the production REMD trajectories, they were occasionally in a cis relationship. Thus, we ignored such configurations for analyses. The ignored configurations are about 10% of the whole REMD trajectories.

anion to yield enough space on the active site for monomers to locate at. The other factor is the net stabilization energy obtained by AASO. As we have already mentioned, our DFT calculations showed that there is 7.0 kcal/mol net stabilization if the ion pair has the side structure and the monomer coordinates to the active site. The enhancement of the ASO comes from this net stabilization energy. From this discussion, we suppose that the ion pair dissociation is not necessarily enhanced even if the interaction between a catalyst and an anion is weakened. This is because, if the interaction between a catalyst and a monomer is also weakened, the net stabilization energy will not be large enough for ASSO. Conversely, even if the cation−anion interaction itself is strong, the dissociation can be enhanced by increasing the stabilization energy coming from the cation− monomer interaction.

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CONCLUDING REMARKS We studied the behavior of the ion pair 3 generated from (pyridylamide)hafnium(IV) complex 1. First, by the MD simulation method, we revealed the active site opening (ASO) mechanism which occurs associatively with the monomer coordination. We call this mechanism the associative active site opening (AASO) mechanism. From a dynamic and molecular point of view, this mechanism also explained the experimental fact that the activity of the catalyst increases if the counteranion has no Me groups. Next, by the REMD method, we revealed that AASO is the major mechanism for the ASO at the thermal equilibrium. Finally, we discussed two important factors for AASO. One is the structure of the cation and anion. The other is the net stabilization energy. Our study showed that the monomer coordination plays an important role in the ASO processes of ion pairs. Thus, it is suggested that the behavior of the ion pair and the monomer coordination cannot be considered as independent processes. Finally, it is concluded that we should take monomer coordination into account in studying the behavior of the ion pair of organometallic complexes, especially pincer type complexes. In this paper, we studied the behavior of the catalyst before the genuine active species called “monomer-inserted catalyst” is generated. The following polymerization process catalyzed by the genuine active species is our future work. In general, as the polymerization reaction proceeds, a polymer chain is elongated and can interact with the metal via an agostic interaction. Thus, it would be necessary to take the agostic interaction into consideration.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00804. Computational details (PDF) MD trajectory of AASO (MPG)

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COMPUTATIONAL DETAILS

MD Simulation Models. We used general AMBER force field (GAFF)40 version 1.4 in AMBER 1241 for heptane and ethylene molecules. As an anion, we adopted [MeB(C6F5)3]− because we could investigate the effect of both the Me and perfluorophenyl groups in the anion and we could use force field parameters developed in previous research.13 However, there is no force field available for the cationic Hf catalyst in the ion pair 3. Thus, we developed the force field by modifying the parameters in GAFF. Parameters of bonded interaction terms (bond, angle, and dihedral angle terms) including the Hf atoms were newly developed, and some parameters were modified by fitting to DFT calculations. We selected a stereoisomer of the catalyst according to previous research.31 The atomic charges were determined by DFT calculations with the Mertz−Singh−Kollman method.42,43 See the Supporting Information for more details.

AUTHOR INFORMATION

Corresponding Author

*M.N.: e-mail, [email protected]; tel/fax, +81-52789-5623. ORCID

Masataka Nagaoka: 0000-0002-1735-7319 Notes

The authors declare no competing financial interest.

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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 F

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(30) Froese, R. D. J.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. J. Am. Chem. Soc. 2007, 129 (25), 7831−7840. (31) 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. (32) 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. (33) 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. (34) Domski, G. J.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2007, 40 (9), 3510−3513. (35) Sugita, Y.; Okamoto, Y. Chem. Phys. Lett. 1999, 314, 141−151. (36) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (37) 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. (38) Macchioni, A. Chem. Rev. 2005, 105 (6), 2039−2073. (39) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312 (5774), 714−719. (40) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. J. Comput. Chem. 2004, 25 (9), 1157−1174. (41) 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. (42) Besler, B. H.; Merz, K. M.; Kollman, P. A. J. Comput. Chem. 1990, 11 (4), 431−439. (43) Singh, U. C.; Kollman, P. A. J. Comput. Chem. 1984, 5 (2), 129− 145. (44) Ahmadi, M.; Nasresfahani, A. Macromol. Theory Simul. 2015, 24, 311−321.

Research from the Ministry of Education, Culture, Sport, Science and Technology (MEXT) in Japan, and also by the MEXT programs “Elements Strategy Initiative for Catalysts and Batteries (ESICB)” and FLAGSHIP2020 within the priority study5 (Development of new fundamental technologies for high-efficiency energy creation, conversion/storage and use). The calculations were partially performed using several computing systems at the Information Technology Center at Nagoya University.

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DOI: 10.1021/acs.organomet.6b00804 Organometallics XXXX, XXX, XXX−XXX