BCSJ Award Article

BCSJ Award Article

Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C2 Symmetric [SiH2(Ind)2ZrCH3]+ and [SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3]¹ Karakkadparambil Sankaran Sandhya,1,2 Nobuaki Koga,*1,2 and Masataka Nagaoka*1,2 1

Graduate School of Information Science, Nagoya University, Nagoya 464-8601

Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Honmachi, Kawaguchi 332-0012 2

E-mail: [email protected], [email protected] Received: April 5, 2016; Accepted: May 25, 2016; Web Released: June 1, 2016

Nobuaki Koga Nobuaki Koga is a Professor in the Department of Complex Systems Science, Graduate School of Information Science at Nagoya University. He received his Doctor of Engineering degree from Kyoto University in 1987. He was a Research Associate at the Institute for Molecular Science from 1986 to 1993 and an Associate Professor at Nagoya University from 1993 to 1998. Since 1998, he has been a Professor at Nagoya University. His research interests include chemical reactivity and excited-state properties of organic and organometallic compounds.

Masataka Nagaoka Masataka Nagaoka is a Professor in the Department of Complex Systems Science, Graduate School of Information Science at Nagoya University. He received his Doctor of Engineering degree from Kyoto University in 1987. His current research interests focus on controlling complex chemical reactions in the “molecular aggregation state”. He is now the project leader of the Japan Science and Technology Agency (JST), Strategic Basic Research Programs (CREST) (2013–2019) and a PI of the national project “Elements Strategy Initiative for Catalysts & Batteries (ESICB)” (2012–2018).

Abstract The [SiH2(Ind)2ZrCH3]+ (Ind = indenyl) catalyzed stereoregularity of propylene polymerization mechanism has been investigated at M06 level of theory. Four different approaches of propylene to the reactive catalyst lead to four isomeric products due to the C2 symmetry of bridged Ind ligand of the catalyst. Consequently, various possibilities of propylene attack as well as orientation of polymer chain yield numerous stereoisomers. The calculations of the first and second insertions with various conformers clarified the most favorable reaction pathway and showed that isotactic propagation is more favorable (3.5 kcal mol¹1) than syndiotactic propagation. The structures of resting state catalysts displayed various agostic interactions of the CH bond with the Zr center which stabilize the catalytic systems and play important roles in determining the favorable reaction pathway. The influence of counter anion Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

[CH3B(C6F5)3]¹ on the reactivity of the catalyst was also studied. The results also confirm that the trans orientation of the counter anion with respect to propylene is more favorable than its cis orientation and clarify the most favorable reaction pathway in the first and second insertion. Because agostic interactions are involved in various aspects, AIM analysis has been used to find the bonding nature of agostic interactions as well as ion-pair bonds. The overall results suggest that rigidity of ansa-zirconocene, unique structure of C2 symmetric ansa ligand, influence of [CH3B(C6F5)3]¹ and β agostic interaction may restrict the attack of propylene only to isotactic polymerization and not to syndiotactic polymerization.

© 2016 The Chemical Society of Japan | 1093

1. Introduction Ansa metallocenes have been a subject of great interest due to the stereospecificity and stereoregularity for α olefin polymerization catalyzed by them.1­4 The rigidity as well as chirality of bridged ligands probably gives exceptional performance in these state-of-the-art catalysts.5 This concept has been best exemplified in the [Me2Si(Ind)2]ZrCl2 (Ind = indenyl) and [Me2C(Flu)(Cp)]ZrCl2 (Cp = C5H5, Flu = fluorenyl) singlesite catalysts. The former affords isotactic polymers,6,7 whereas the latter produces syndiotactic polymers8 from propylene. Brintzinger et al. synthesized a large number of ansa metallocenes, in which C2 symmetric metallocenes produce isotactic polymers and Cs symmetric metallocenes yield syndiotactic polymers.9­12 Stereoregularity of these catalysts was explained using a symmetry rule by Ewen,13 which was originally formulated for Ziegler­Natta catalysts. Further, C1 symmetric metallocenes usually afford isotactic, syndiotactic, atactic or semi-isotactic polymers depending upon the polymerization condition and ligand substitution pattern. Fink put forward a “universal model” which is capable of predicting the tacticity for any metallocenes by means of difference in energy gap between (re and si) conformers of propylene coordination with ansa metallocenes.14 This model successfully explains polymerization behavior for any C1 symmetric metallocenes by noting the substitution pattern in the ligand. Kaminsky pointed out that the angle between the bridged ligand and its various substitutions can play a pivotal role for activity and stereospecificity.15 This was verified by the experimental result that Et(2-Ind)(2-Ph-1-Ind)ZrCl2 (Et = C2H4) gave polypropylenes with higher isotacticity16 ([mmmm] = 74%) than anti C2 symmetric Me2Si(2-Ph-1-Ind)2ZrCl2 ([mmmm] = 58%). A large amount of theoretical work has been devoted to study the general reaction mechanism for π complex formation, propagation, and termination with group 4 metallocene catalysts (Scheme 1).1,17­27 It is well known that in situ an activated catalyst must be formed from a parent catalyst using a cocatalyst to initiate the propagation steps, and as shown in Scheme 1 this activated cationic complex catalyzes polymerization. The most widely accepted Cossee­Arlman mechanism28,29 has been used in discussion of reaction steps, while a modified Green­Rooney mechanism30 also has come into play when αCH agostic interaction with metal occurs prior to

H2Si

Zr

Cl

tio

+ CH2CHCH3

n

Zr

H2Si

Zr

Zr

H2 Si

CH4

H

Polymer end

+

Te H rm in a

H2 Si

π complex formation

CH3 CH2CHCH3

H

n

Pro -t im H2Si pag e s atio n

H

Zr

Cl H2Si

Zr Cl

Scheme 1. General mechanism catalyzed polymerization.

of

ansa-zirconocene-

1094 | Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

insertion. In our earlier studies with ab initio MO method on ethylene insertion with [H2Si(C5H4)2ZrMe]+ a stable βCH agostic interaction was observed in the product, whereas αCH agostic interaction occurs in the transition state.31 These agostic interactions have been experimentally and theoretically examined. Erker experimentally investigated the stereochemistry for first propylene insertion in [rac-Me2Si(Ind)2]Zr(®-C4H6)B(C6F5)3 and observed that α agostic metal­ CH interaction could be a reason for the isoselective nature of propylene attack.32 The αCH agostic interaction might decrease the activation energy for insertion and give more space for propylene attack.33 However, analytical tools such as NMR and X-ray techniques have provided considerable proof for the formation of βCH agostic structures in the resting states.34,35 It is noteworthy that Baird et al. experimentally investigated with the help of theoretical calculations the existence of α agostic interaction in [Cp2TiCH2CHMetBu]+ and proposed that the interaction occurs possibly due to steric effects. It supports the involvement of chain end control in the setereoselectivity.36 Various agostic interactions could arise due to the fact that agostic products can interchange structures and sometimes this may cause stereoerrors via site epimerisation.23,24,37 Our group also theoretically demonstrated that [H2Si(C5H4)2ZrCH2CH2R]+ shows kinetically favorable γ agostic interaction which in turn isomerizes into a stable β agostic product.19 Talarico and Budzelaar performed excellent statistical analysis of Me2Si(3-Me-C5H3)2Zr tBu+, [2,2¤-bis(2-Ind)biphenyl]Zr tBu+, and “ONNO” type catalyst [1,2-(2-O-3-tBu-C6H3CH2NMe)2-C2H4]Zr tBu+, to quantify the stereo and regiocontrol in propylene polymerization.38 They found that monomer-chain and ligand-chain interactions are dominant for the stereopreference of catalyst and sterecontrol of growing chain and olefin. The studies of the isotacticity of polymethylmethacrylate (PMMA) on C2 symmetric zirconocene revealed that steric interaction between growing chain and Ind ligand induces the stereoselectivity.39,40 Recently, Maron et al. have observed that the syn configuration of ansa-zirconocene, [(Ph(H)C(3,6t Bu2Flu)(3-tBu-5-Me-C5H2))ZrMe]+ always prefers 1,2 insertion due to less steric hindrance of ligand with the monomer.41 A few interesting theoretical studies of the stereochemistry and stereocontrol of syndiotactic polymers on Cs symmetric Me2C(Cp)(Flu)Zr+ and related substituted systems have been conducted by several groups.41­44 However, we notice that the reaction mechanism including the stereochemistry of isotactic polymerization catalyzed by C2 symmetric [Me2Si(Ind)2]ZrCl2 is hardly discussed in the literature.45­47 To the best of our knowledge, the pathway of first insertions to second insertions through proper agostic resting states in propagation is not appropriately discussed anywhere due to the various possibilities of the insertion products. It remains unclear, (i) how and from which insertion products the agostic resting catalysts are originally generated, (ii) why so many resting catalysts or epimers are formed during the reaction and how the system avoids the stereoirregularities, (iii) what is the role of counter anion (CA) such as [CH3B(C6F5)3]¹ in the stereochemistry of polymer and, (iv) what could be the correct complete pathway from initial reactants to second insertion products. We propose to dig more deeply using theoretical approaches in order to study the stereochemistry and reaction mechanism that is pre© 2016 The Chemical Society of Japan

valent for the propagation. We also assume that our proposal can give a new direction for the transferability of reaction pathway to other C2 symmetric ansa catalysts. Inclusion of [CH3B(C6F5)3]¹ counter anion in the reaction mechanism can further scrutinize the real situation in terms of energetics. Further, nowadays density functional theory methods are widely accepted and hence are a routine tool for understanding catalytic reaction mechanism which helps to design new catalysts with improved performance. Bearing these facts and the importance of stereochemical aspects of polymerization in mind, in the present study, we addressed the above questions by applying competent computational methods. 2. Computational Details All calculations were done with Gaussian 09 version D 0.1 suite of program using M06 level of theory.48 The effective core potential (ECP) was used with double-² valence basis set (LANL2DZ) with added f polarization functions for Zr.49 All other atoms use 6-31++G(d,p) basis set.50 Minimum energy structures for reactants, products, and intermediates were confirmed by noting positive eigenvalues of force constant matrix. Transition states (TSs) having an imaginary vibrational mode were also confirmed in frequency calculations. Further, the nature of selected TSs was verified by the intrinsic reaction coordinate (IRC) calculations. Involvement of [CH3B(C6F5)3]¹ in calculations takes a long time for optimization and frequency calculations. Hence, we used smaller 6-31G(d,p)51 and LANL2DZ basis sets in optimization runs keeping the same level of theory. The energy of the model system with the counter ion was improved by single-point energy calculation with triple-zeta-valence basis set in conjunction with two sets of polarization and diffuse basis functions (def2_TZVPP)52 for all atoms. The single point energy calculations at the same level of theory were performed to take solvent effect into account. The SMD method53 was applied using experimental toluene solvent (¾ = 2.3741). Solvent-effect-corrected relative free energy was calculated by summation of gas-phase free energy at a temperature of 298.15 K and a pressure of 1.00 atm (¦Ggas) and solvation energy (¦Gsolv). Gas-phase entropy term has a significant contribution in the propylene association steps and solvation effects slightly change the energy profile for the reactions. However, they do not change preference of the reaction pathways. For instance, the energy profile based on the gas-phase enthalpy values is in qualitative agreement with that drawn by using the solvent-effect corrected free energies. Before doing all calculations, in order to examine the effect of density functionals on the calculated-energy barriers in the cases of zirconocene catalyst, eight different functionals (B3LYP-3D, B3LYP-3D, M06L, M06, B3LYP, TPSS, ωB97XD, and M05-2X) with the same basis set (6-31++G(d,p)) were carefully chosen. The activation-energy barrier in gas phase was compared among different functionals for the first insertion of approach A (the Supporting Information). We have taken M06 as the best functional because it gave the average of eight functionals. Moreover, the M06 level was successfully used for dispersion correction and long-range correction in organic molecules and organometallics complexes.54,55 Atoms-in-molecules (AIM) analysis was conducted on the intermediate catalysts and ion pairs of first insertion products using AIMALL software.56 The Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

wave function was created from the Gaussian 09 for calculating bond critical points between non-bonded atoms. 3. Results and Discussion Spaleck et al. synthesized C2 symmetric SiH2(Ind)2ZrCl2 for isotactic propylene polymerization.5 They have also applied Si(CH3)2(Ind)2ZrCl2 to the isotactic polymerization of polypropylene.7 We chose the former metallocene as a parent catalyst for studying the stereochemistry behavior during the first and second insertion. The active catalyst for the present investigation is SiH2(Ind)2ZrCH3+ because the parent catalyst is not a reactive species for polymerization and methylaluminoxane (MAO) or borane is used as a co-catalyst to generate the active catalyst. Due to the unknown structure of MAO, we decided to use [CH3B(C6F5)3]¹ as CA. Coordination of CA may limit the stereochemistry and the use of this gives realistic explanation of the reaction, which will be discussed in the later section. In the present study, we use the S form of SiH2(Ind)2ZrCH3+ (based on the notation given by Kaminsky for [Et(Ind)]ZrCl2)15 of possible mirror image R and S forms. Four possible insertion modes of propylene to this active intermediate would produce four different conformational intermediates based on the C2 symmetry of bis(Ind) ligand and the orientation of propylene. The propylene π complexes with the SiH2(Ind)2ZrCH3+ for two 1,2 insertions (or primary insertions) and two 2,1 insertions (or secondary insertions) are depicted in Figure 1. The same situation was seen when CA is located at cis position or trans position with respect to propylene (Scheme 4 shown later). In the A and C approaches, the propylene CH3 group, which is located closer to the CH3 ligand, is pointing up to the upper Ind and down to the lower Ind, respectively. Similarly, the same terminology is applied to the D and B approaches (Figure 1). In order to reduce the complexity in naming of various species, we used simple notation and thereby avoided re or si notation of prochial propylene which was reported by various authors.1,25,42,57 Four different approaches of propylene generally lead to four different products as shown in Figure 2. In the succeeding section, we will explain all possible approaches of propylene for the formation of isotactic, syndiotactic, and atactic polymers. In terms of energetics we will explain the most favorable pathway which leads to the isotactic products. Though syndiotactic products are also possible in the second and later insertion in the D approach as shown later, it is dormant due to a high-energy barrier in the first insertion. It is clear that the first insertion in the A and C approach should give the same product because there are only achiral carbon atoms in the products. The kinetic products, however, are different in the conformations of isobutyl ligand. In the cases of the first insertions in the B and D approaches, there is a chiral α carbon at the

CH3 H2Si

Zr

A

CH3 H2Si

Zr

C

CH3 H2Si

Zr

CH 3 H2Si

B

Zr

D

Figure 1. Four different conformers of intermediate π complex. © 2016 The Chemical Society of Japan | 1095

CH3 H2Si

Zr

H2Si

Zr B

CH3

A

H2Si CH3 H2Si

Zr

CH3

D

Zr

Zr

H2Si

C

CH3

Figure 2. Four possible modes in the first insertion of propylene which lead to four different products. The β carbon atom in the products in the A and C approaches is stereochemically the same, except the conformation. M

M

M

M

M

M

M

M

M

M

M

M

A/C 1,2 insertion insertion 1,2 2,1 B,D A,C B 2,1 insertion insertion 1,2 2,1 B,D A,C

M

M

M

M

M

M

M

M

M

M

M

M

D 2,1 insertion insertion 1,2 2,1 B,D A,C

Scheme 2. Different propylene second insertion products of A, C, B, and D conformers. The chain end CH3 group (represented by sphere) purely comes from the activated SiH2(Ind)2ZrCH3+.

growing ligand; α carbon in B is labelled with R configuration, whereas in D it is done with S configuration according to Cahn­Ingold­Prelog rules for chiral carbon.58 Therefore their products are different (Figure 2). Enantio- and Regioselectivity Based on the Energetics. Previously, Pakkanen et al. demonstrated different possibilities of 1,2 and 2,1 propylene attack to Cp2MCH3+ (M = Zr, Hf ) based on the regiostructures as well as stereostructures.59 As a result, four products are formed in each combination of the two successive insertion steps which are 1,2 and 1,2, 1,2 and 2,1, 2,1 and 1,2, and 2,1 and 2,1 insertions (Scheme 2). There are other different possibilities in propylene attack, the propylene CH3 group being up or down. They indicate many possibilities of insertions that are similar to a fission reaction (hence, we propose them as “fission chain reaction” type). Here, we will concentrate on the energetics of 1,2 and 1,2 insertion and 2,1 and 2,1 insertion rather than giving importance to 1,4 or 2,3 products formed in the second insertion (the Supporting Information). Therefore, the two successive insertions presented are 1,2 and 1,2 insertions and 2,1 and 2,1 insertions. By taking into account the orientation of propylene CH3 group, the former includes the combinations of insertions, A-A, A-C, C-A, and C-C and the latter does B-B, B-D, D-B, and D-D. We call the reactions starting with the A approach (A-A and A-C) the A pathway, those starting with the C approach (C-A and C-C) the C pathway, those starting with the B 1096 | Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

approach (B-B and B-D) the B pathway, and those starting with the D approach (D-B and D-D) the D pathway. Many reports are focused only on the trans position between the CH3 group of propylene and the growing polymer chain to explain stereoregularity and stereospecificity. Apparently, NMR studies reveal that the chain end isomerization of growing polymer was the reason for stereoerror formation, meaning that other possibilities of propylene attack give misinsertions.60 Hence, we assume that the skeleton carbon chain which represents polymer is in the same plane (Scheme 2) and its two CH3 groups should be up or down (shown in red) in identifying the syndiotactic and isotactic products. Therefore, first and second insertions are not enough in the cases of the A and C approaches to judge the tacticity, since resting intermediates formed after second insertions contain only one chiral carbon. Hence, a third insertion has been studied before generalization of this assumption. In the B and D approaches, the products of second insertions have two chiral carbons so that we can distinguish isotactic and syndiotactic polymerization. Regarding the “site epimerization mechanism or back skip mechanism”, errors in stereoselectivity of syndiotactic propylene polymerization catalyzed by [Me2C(Flu)(Cp)ZrCl2] were successfully explained.25,61,62 In contrast, an “alternating mechanism” 63 explained by Miller and Bercaw for ansa[Me2C(3-tert-butyl-Cp)(Flu)ZrCl2] operates through a less stereoselective site in the transition state due to the lack of polymer chain flipping for the formation of isotactic products.64 Both mechanisms are shown in the scheme in the later section. The bulky tert-butyl substituents could prohibit the flipping of polymer chain in the latter mechanism. However, both mechanisms are competitive in the different polymerization conditions. α, β and γ agostic interactions are also a decisive factor to enhance the enantioface selectivity and regioselectivity. Moreover, chain end control of growing polymer is also an important factor to increase the enantioface selectivity of stereocenter of polymers. Figure 3 represents the overall solvent-effect-corrected free energy profile for the A and C pathways and Figure 4 represents all TSs involved in these pathways. Because the two reaction pathways are merged or exchanged after the products are formed in the first insertions, they are shown in a single figure and also we used symbol “a” and “c” as the first letter in the names of intermediates and TSs depending on the orientation of propylene CH3 group. The reactant π complexes ar1 in the A pathway and cr1 in the C pathway are more stable by 13.90 and 15.04 kcal mol¹1, respectively, than initial active catalyst + propylene, Int. However, ar1 is slightly less stable than cr1 by 1.14 kcal mol¹1 due to more steric interaction of the propylene CH3 group with the six-membered ring of the upper Ind. The reactions from these reactants pass through TS1A and TS1C with activation energies of 7.79 and 9.22 kcal mol¹1, respectively, and lead to the products ap1 and cp1. These products can be distinguished by noting the conformation of the isobutyl ligand. They bear γCH agostic interaction with the Zr­ H distance of 2.278 ¡ in ap1 and 2.273 ¡ in cp1 and are more stable by 13.89 and 14.84 kcal mol¹1, respectively, than Int. Now, we turn to the second insertion. Interestingly we traced out three different resting catalysts, ap1-r2u, cp1, and ap1, prior to approach of propylene for the next insertion. The structures of © 2016 The Chemical Society of Japan

TSβ-n 12.78

Int

TSγ β

0.00 -5.82 Relative

TS1C

(kcal mol–1)

TS1A

ΔG

ar1

-7.95

ap1-r2u

ap1

-5.36

a1β

-13.89

-6.10 -13.89 -13.90

ap1

-13.89

-15.04

cp1

cp1

-14.84

-14.84

-11.74

-16.95

cTS2d

ap1

-12.26 -13.37

aTS2d aTS2u

-16.94

cr2d

ar2d

cr1

ap1-r2u -5.36

cTS2u -17.96

-18.41 -21.45 cr2u

ar2u -21.87

-26.17 -26.74 c -27.23 p2d

cp2u

ap2d ap2u

-27.70

ar2d

cr2d

ap2d

ap2u

cp2d

Figure 3. Solvent-effect-corrected free energy profile for the first and second insertions of propylene according to the A and C pathways. “a” and “c” mean that the orientation of CH3 of propylene is up and down, respectively (Figure 1), whereas “d” and “u” indicate that the projection of growing chain is down or up. Red lines indicate the pathways for the first insertion and isomerization of the first insertion products. Green lines represent the second insertion step. Dashed lines represent the second insertion for favorable path from ap1 and cp1. See Figure 4 for the transition-state structures. In the wireframe structures the Zr atom is shown by an orange sphere and agostic interaction is denoted by an orange line.

these resting catalysts were determined by removing propylene from the reactant π complexes, ar2d, ar2u, cr2d, and cr2u, for the second insertion. cp1 is thus obtained from cr2d and ar2d, whereas ap1 is formed from cr2u. ap1-r2u is formed from ar2u. One of the resting catalysts, ap1-r2u, is less stable than the others due to the lack of any agostic interaction, its energy being ¹5.36 kcal mol¹1 relative to Int. Other resting catalysts, cp1 and ap1, with γCH agostic interaction are, however, more stable by 14.84 and 13.89 kcal mol¹1 than Int. It is notable that in cp1 the β H atom in the isobutyl growing chain is projected up and propylene approaches in either A or C fashion to lead to ar2d and cr2d, respectively. In the case of resting catalyst ap1 the β H atom is projected down. The pathways from the resting catalysts are shown in green colored lines in the energy profile. Because growing chain in cp1 and ap1 can change its conformation very easily, it is impossible to definitely connect the products in the first insertion and the resting catalysts formed by the removal of propylene from the reactants in the second insertion (rectangular pink box in Figure 3). Change in conformation is due to the search of the more stable agostic resting catalysts. This clearly indicates that agostic interaction plays a key role for selecting the next favorable insertion path. AIM analysis also gave valid evidence for the various agostic interactions that is explained in the last section. We observed that conformation change from ap1 to ap1-r2u is not easy since it has to pass through two different transition states for C­C rotation Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

(TSγβ and TSβ¹n) with activation barriers of 5.94 and 29.74 kcal mol¹1. This path is unfavorable and ignored. We only present in Figure 3 the results for the primary insertion in the second insertion as shown before. The secondary insertion to the primary first insertion products which leads to the formation of 1,4 products and hence atactic products or stereoerrors during the polymerization requires an activation barrier of 10.30 and 16.88 kcal mol¹1. Compared to activation barriers to primary insertion shown from now on, these barriers are much higher. The energy profile for these insertions can be seen in the Supporting Information. Each second insertion has its transition state and thus there are four TSs in the second insertion, aTS2d, cTS2d, aTS2u, and cTS2u. The four different propylene reactant complexes can be distinguished from each other by noting whether the projection of growing chain is down or up and whether propylene CH3 group is up or down. “a”, “c”, “d”, and “u” in “aTS2d” and “cTS2u” denote these orientations. Figure 4 shows that an α agostic interaction exists in TSs of the second insertion as well as the first insertion, which is in agreement with the “transition state α agostic” mechanism.33 The Zr­H agostic distance lies in between 2.0 and 2.3 ¡ for all TSs. This mechanism has some features of Cossee­Arlman and modified Green­Rooney mechanisms. Two new C­C and Zr­C bonds are forming in the products at the expense of breaking of the Zr­C bond and the C­C π bond. The products of all growing chains have γCH © 2016 The Chemical Society of Japan | 1097

TS1C

aTS2d

aTS2u

2 1.409

2.079

2.28 2.299

2.041

2.307

cTS2d

46 1 .1

1.146 2.2 55

1.422

2.305 2 .2 77

2.086

1 2.32

2.302

2.333

47 1.1 1.410

2.314

2.086

16

2.312

2.296

1.151

1.4

TS1A

2.176 2.298

2

97 2. 1

8 1.41

2.311

1. 1 3

9 1.41

2.201

2.297

2 1.13 2.1 76

2.335

cTS2u

Figure 4. Transition states for the first and second primary insertion in the pathway A and C. Bond distances are in the unit of ¡. Hydrogen atoms of Ind have been omitted for clarity. down up R H2Si

M

down

Scheme 3. A general representation of sterically favorable TS for isotactic product formation.

agostic interactions similar to the products in the first insertions. An activation barrier of 3.49 kcal mol¹1 for propylene insertion of cr2u through cTS2u is the smallest, whereas that of 8.50 kcal mol¹1 for insertion of ar2u through aTS2u is the largest. Activation energies of 6.15 and 5.20 kcal mol¹1 for insertion through aTS2d and cTS2d are intermediate. Here, we can see that the activation energy for the insertion through cTS2u is the smallest and in addition cTS2u is the most stable among four TSs, to show that the insertion through this TS is the most favorable (dashed green line in Figure 3). In this stable TS the propylene CH3 group is down to be far from the six-membered ring of the upper Ind and furthermore the growing polymer chain is up, so that the polymer chain locates far from the propylene CH3 group. The sequence of Ind six-membered ring­ polymer end­propylene CH3 group is down­up­down, being sterically favorable as shown in Scheme 3. This result is consistent with the trend demonstrated in Borrelli et al.46 model and Corradini et al.65 model and our studies also confirmed the same results,19,47 although Borrelli et al. missed one more possible transition state due to chirality of Ind. They have assumed that all other possibilities lead to misorientation of propylene except aTS2u. In the cases of the primary insertion of A and C, tacticity is not distinguished in the second insertion products due to bearing only one chiral carbon atom. However, it is reasonably expected that the above result, in which one of the insertions is more favorable than the others, is not dependent on the length of the polymer end except the methyl group in the initial catalyst and therefore the stereochemistry remains the same in the subsequent polymerization steps so that isotactic polymerization is realized. We carried out the calculations for the third insertion with the same down­up­down sequence of the polymer end, propylene CH3 group, and the six-membered ring of Ind (cr3u, cp3u, and cTS3u in the Supporting Information) to 1098 | Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

obtain an energy profile similar to that for the second insertion through cTS2u. This supports the above discussion. Figure 5 illustrates the energy profile for the first and second insertions in the B pathway. Figure 6 represents, the isomerization of γ to β agostic structure and finally to the structure with α agostic interaction for the first insertion product from B. The structures of the corresponding TSs are shown in Figure 7. The reactant π complex in the first insertion br1 is stabilized by 13.27 kcal mol¹1 from Int. The insertion from br1 passes through transition-state TS1B to lead to the product with 1methyl propyl ligand, bp1, in which the α carbon atom of the polymer end is chiral. bp1 is less stable than br1. Interestingly, TS1B is less stable compared with TS1A and TS1C because of the steric interaction of propylene CH3 group with the Ind and its energy is the same as that of Int. Resting catalysts between the first insertion products and the second insertion reactants are obtained by removing propylene from the second insertion reactants br2d-i, br2d-s, br2u-s, and br2u-i. br2d-i and br2d-s are formed from two energetically different resting catalysts, bα1 and bp1. Nonetheless, the calculations for br2u-s and br2u-i produce the same species, bβ1. Because the resting catalysts differ in the agostic interactions, “α” and “β” in suffix of these compound names denote the kind of agostic interactions. bβ1, br2u-s, and br2u-i have βCH agostic interaction between the methyl group in α position and Zr. Both bα1 and br2d-i exhibit αCH agostic interaction and their Zr­H distances are 2.215 and 2.160 ¡, respectively. However, br2d-s has no α agostic interaction (Zr­H, 2.735 ¡), to indicate that it is formed purely from bα1. On analyzing bp1 it appears that γCH agostic interaction exists with a distance of 2.244 ¡. Change in the conformation around the Zr­Cα and Cα­Cβ bond in bp1 with γ agostic interaction simply leads to bβ1 with β agostic interaction, whereas the structure change from bp1 to bα1 with α agostic interaction consists of two successive conformation changes. Figure 6 displays this structure change of γ to α agostic structure via β agostic structure bβ¹i through two TSs. The reaction of bp1 to bβ¹i through TSbγβ¹i requires an activation energy of 6.43 kcal mol¹1, whereas that of bβ¹i to bα1 requires a significantly high activation barrier (21.22 kcal mol¹1). This clearly indicates that high energy is requisite to break the β agostic interaction in bβ¹i66 and shift to less stable α agostic interaction. Hence, such path may not be preferable and the other low-energy paths through bp1 and bβ1 are followed to accomplish the second secondary insertion. bTS2d-i and bTS2d-s with the polymer end being down are formed from br2d-i and br2d-s with an activation barrier of 2.51 © 2016 The Chemical Society of Japan

bTS2d-s 3.29

Int

TS1B

0.00

0.00

bTS2u-s

bα1

1.88

0.70

2 .2 15

Relative

ΔG

bTS2d-i

(kcal mol–1)

-6.62

-9.37

bβ1

bp1

2. 24 4

-13.27

-11.63

br1

bp1 -11.63

-9.12

br2d-s br2d-i

bTS2 u-i -8.54

bp2u-s -16.52

-14.57

br2u-s 2 17 2.

bp2d-s

br2u-i -18.46

2.213

-18.34

1 2.18

bp2u-i bp2d-i -23.48

0 2.16

2 2.12

br2d-i

-19.33

07 2.2

br2u-s

br2d-s

Figure 5. Solvent-effect-corrected free energy profile for the first insertion according to the B approach and the syndiotactic and isotactic second insertion. Black line indicates the first insertion pathway. The carbon atoms marked with a star are the chiral carbon atoms. Dashed line paths represent the isotactic formation and solid line paths represent syndiotactic formation. Red and green lines indicate growing chain is up or down as indicated by “u” and “d” in compound names.

TSbβ α-i 2.6

73

32 2.2 1.15 2.232 0

7.15 2.545

bα1 0.70

TSbγ β-i -5.21

58 2. 1

bp1 -11.63

bβ-i -14.07

Figure 6. Solvent-corrected free energy profile for the transition of γ agostic interaction to β and eventually to α agostic interaction in the first insertion product for B approach.

and 12.40 kcal mol¹1, respectively. In Figure 5 for the energy profile, both pathways are indicated by green lines after the first insertion. Likewise, bTS2u-i and bTS2u-s with the polymer end being up have reactants br2u-i and br2u-s and products bp2u-i and bp2u-s as shown by red lines in Figure 5. The corresponding activation barriers are 9.91 and 18.39 kcal mol¹1, respectively. Comparing these four different pathways, two of which are for the formation of isotactic polymer chain and the rest of which are for the formation of syndiotactic polymer chain, bTS2d-s and bTS2u-s have higher energy than bTS2d-i and bTS2u-i. According to Sason Shaik,67 the rate-determining step would be the highest energy transition state in each pathway. Thus the formation of the syndiotactic polymer chain through bTS2d-s and bTS2u-s is less favorable. Comparing two isotactic forBull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

mation pathways, bp1 ¼ bα1 ¼ br2d-i ¼ bTS2d-i ¼ bp2d-i and bp1 ¼ bβ1 ¼ br2u-i ¼ bTS2u-i ¼ bp2u-i, the latter path is more favorable according to Sason Shaik,67 of which the ratedetermining step is br2u-i ¼ bTS2u-i. All the kinetic products in the second insertion show γ or δ agostic interaction in the growing polymer chain. Though no products showed any α or β agostic interactions, conformation change in the polymer end may arise stable β agostic structure. We suspect that α agostic interaction may not occur in the third and later insertion products because all structures of kinetic products in the second insertion are stabilized by γ or δ agostic interaction. Moreover, steric repulsion between Ind and growing chain increases with increase in the chain length if the polymer end utilizes α agostic interaction. Concerning four different TSs formed in the third pathway (i.e., B), α agostic interaction in bTS2d-i and bTS2d-s assists the second insertion, whereas β agostic interaction does in bTS2u-i and bTS2u-s (Figure 7). This is different from the primary insertion TSs in Figure 4 in which only an α agostic interaction contributes. This is because when the polymer end is up as in bTS2u-i and bTS2u-s the methyl group on the α carbon atom is close to the Zr atom. It should be noted that in the secondary insertion TSs the sp2 carbon atom bearing the methyl group in propylene (called first chiral carbon atom hereafter) is deformed to tetrahedral and the sp3 α carbon atom in the polymer chain (called second chiral carbon atom) is in a deformed square pyramidal or trigonal bipyramidal shape to realize change in the chemical bonds through four-centered TSs. Configuration for the polymer which is syndiotactic or isotactic is clearly predicted in the transition state. If the first and second chiral carbon atoms have the same stereochemistry, then the second insertion product is © 2016 The Chemical Society of Japan | 1099

84

1.3

2 .1 3 3

bTS2d-s

2.374

1.125

1.524 2 .1 47

2 .1 1 2

bTS2u-i

2.394

16

0 2 .4 4

2.176

1.4

bTS2d-i

1.525

1.120

2.527

2.510

2.0 64

2.284

1 1.41

TS1B

2.378

2.484

2.078

63

1. 1 61

1.1

96 1.3

89

2 2.33

2 .3

2.323

96 2.286

1.4 05

2.169

2.1

34 1.1

bTS2u-s

Figure 7. Transition states for the first and second insertion in the B pathway. Hydrogen atoms of Ind have been omitted for clarity. Bond distances are in ¡.

isotactic. As a matter of fact, the two chiral carbon atoms in bTS2d-i and bTS2u-i have the same stereochemistry (Figure 7). In general, for isotactic polymer product formation, all chiral carbon atoms in the polymer chain must have the same stereochemistry as that of TS1B. The isotactic and syndiotactic polymer chains can be obtained by passing through the D pathway. However, this pathway is much higher in energy than the other pathways and thus its detailed explanations including its overall energy profile and optimized transition states are given in the Supporting Information. For the D approach, even though the syndiotactic formation is easy in the second insertion, the first insertion is difficult compared to the A/C and B pathways. The D pathway is generally less favorable than the B pathway mostly due to larger steric interaction with the six-membered ring of Ind in the former pathway compared to smaller steric interaction with the five-membered ring of Ind in the latter pathway. The energetics of the profiles reveal that the low energy isotactic pathway is formed from the primary insertion of propylene. However, in this case also, the orientations of the growing polymer and the CH3 group of propylene should have less steric interaction i.e., they are in the trans position. This process should propagate continuously for isotactic product formation. Overall mechanism can operate without site epimerization or back-skip mechanism, because this requires tail flipping that we did not consider. Reaction Mechanism of cis and trans Approach of [CH3B(C6F5)3]¹ on the Various Stereoisomers. Concerning the activation of metallocenes, it was seen that co-catalysts such as methyl aluminoxane (MAO),6 fluorinated boranes, aluminates, and salts of borates68 coordinate with the metallocenes in the form of [ML2-P]+A¹ (L = ligand, M = metal, P = polymer, and A = co-catalyst) for triggering the propagation step. In addition to that, Marks et al. reported that very large sterically congested co-catalyst (tris(2,2¤,2¤¤-nonafluorobiphenyl)fluoroaluminate) (PBA)57 alters stereoselectivity and activity of polymerization. In the present study, we investigated the first and second insertions with CA in cis and trans positions with respect to coordinating propylene (Scheme 4). The profiles of solvent-corrected relative Gibbs free energies for the first insertions with CA are summarized in Figure 8, in which the reaction pathways are identified by the orientation of CA with respect to propylene. At first, we consider the innersphere ion pair IP1 ([SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3]¹) as an initial catalyst. In IP1, CA strongly coordinates to the Zr atom through the CH3 group of CA with a distance of 4.09 ¡ between 1100 | Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

CA

CA

H2Si

Zr

P

Al ter

cis

CA

CA

P

P

C - trans attack of propylene

Zr

P

A - cis attack of propylene

Zr

H2Si

Zr

H2Si P

H2Si

CA

H2Si

Zr

na ti

ve

m

ec ha ni sm

Back skip mechanism

CA

H2Si

Zr

P

trans

Scheme 4. Mechanism of first and second insertion of A/C approach in the presence of CA in cis and trans position. Back skip and alternative mechanisms are also denoted.

the B atom and the Zr atom. This IP1 should change into two different outer-sphere ion pairs IP2 and IP3 for trans and cis position of CA with respect to propylene. Both structures are determined by removing propylene from the reactant propylene complex R1 in the pathways with cis and trans orientation of CA (called cis and trans pathways hereafter). In fact, both structures are connected to each other due to the numerous possibilities of the reorientation of CA without change from outer-sphere to inner-sphere ion pair. Structural change from IP2 to IP3 could be one of the favorable movements for attaining a stable isomeric structure. Finding the most stable outer-sphere cis or trans orientation is rather difficult and hence such structures can be visible only in molecular dynamics simulation.69 During the change from IP1, the B­Zr distance is increased to 6.64 ¡ in IP2. It is elongated to a lesser extent to 6.03 ¡ in IP3. This shows that the interaction between the ions in IP2 with trans orientation of CA is weaker as seen in the energy difference between the outer-sphere ion pairs in the energy profiles in Figure 8 that are plotted with respect to IP2. IP2 and IP3 are 37.86 and 31.29 kcal mol¹1 less stable than IP1, respectively, IP3 being more stable than IP2 by 6.58 kcal mol¹1. Also, this clearly meant that the change in coordination of CA from inner-sphere to outer-sphere is rather difficult. All these observations clearly point out that ion pairs occur as a stable species in the medium when the reaction proceeds. In fact this kind of strong interaction was experimentally observed in [Me2Si(Cp)2ZrCH3][MeB(C6F5)3], ion pair of [Cp2ZrMe]+ and [MeB(C6F5)3]¹, and [rac-Et(Ind)2ZrCH3]­ [FPBA] (FPBA = tris(2,2¤,2¤¤-nonafluorobiphenyl)fluoroalumi© 2016 The Chemical Society of Japan

Relative ΔG (kcal mol–1) IP2 0.00

TS1 9.36

TS1 7.79 2.59

1.82 R1 -6.29 -7.59 -7.83 -8.85

P1

-1.95

-2.56

IP3 -6.58

-5.44

-4.59

-3.00

-7.52

P1 -11.68 -12.53 -14.95 -16.76

R1

-10.02

-12.83 -14.25

-10.20

-14.46 -14.74

Figure 8. Solvent-effect corrected free energy profile for stereoisomeric first insertion reactions with trans (left) and cis (right) orientation of CA with respect to propylene. Green, pink, blue, and red lines represent A, B, C, and D approaches, respectively.

5.950

trans-TS1A

cis-TS1A

trans-TS1C

2.217 2.3 12 1.128 34 2. 1

5.961

2.29 2.3 4 12

20 1.4

2.194

0

2.290 1. 4 20

2.341

1. 1 3

6.014

06

2.233

66 2.2

2 13

10 2.341

2.2

1.408

1.

2.228

3

6.165

34 2.1 93

1.4

Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

1 .1

2.27

nate) by Marks et al.70 Moreover, Correa and Cavallo observed a short distance of 2.55 ¡ between the Me group of borate and the Zr atom in [H2Si(Cp)2ZrMe][MeB(C6F5)3] and [Me2Si(Cp)2ZrMe][MeB(C6F5)3] during the MD simulations.71 A strong electrostatic interaction encountered between CA and the resting catalyst, which requires high energy to separate, would make anion displacement the rate-determining step in the whole reaction pathway. Recently, Linnolahti et al. also observed that [B(C6F5)4]¹ displacement from [rac-SiMe2bis(1-(2-Me-4-PhInd))ZrMe2]+ and [rac-SiMe2-bis(1-(2-Me4,5-BenzInd))ZrMe2]+ is the rate-determining step compared to the propagation step. Moreover, they observed that innersphere ion pair is the most stable structure compared to the outer-sphere ion pair.72 Owing to the weak interaction of CA in trans position, all reactants are less stabilized by propylene coordination in the range of 6.29 to 8.85 kcal mol¹1 compared with those in the cis approach (12.83­14.74 kcal mol¹1 relative to IP2). However, the products P1 in the trans pathway are stabilized more significantly than the products in the cis pathway. This is basically due to the weak direct interaction of the CH3 group of CA with the vacant coordination site of Zr in the products for the cis pathway. In the cis products the vacant coordination site directs opposite to CA, whereas in the products of the trans pathway CA can interact with this site (Scheme 4). The distance between the B atom of CA and the Zr atom lies in between 5.9 and 6.2 ¡ in the transition states (Figure 9) for the first insertions of the A and C approach with CA in both positions, to show that at the TSs the CA does not strongly interact with the Zr atom, CA being in the outer coordination sphere. However, the interaction of CA affects the relatively weak agostic interaction as we observed that the α agostic Zr­H distance is slightly increased (maximum of 0.07 ¡) in the presence of CA. It is also noted that the CH3 group of CA in these TSs is projecting towards the CH3­Zr fragment when CA is in the trans position and towards the CH2 group of propylene when CA is in the cis position.

4 2 .1 3

cis-TS1C

Figure 9. Transition states with CA in cis and trans position in the first insertion in A and C pathways.

It is shown that the first insertion in the trans pathway is more favorable than that in the cis pathway. In the A approach of propylene in the cis pathways, an activation barrier (11.46 kcal mol¹1) is found to be twice as large as that in the trans pathway (5.03 kcal mol¹1), whereas for the C approach of propylene an activation barrier in the trans pathway is even less, i.e., 3.41 kcal mol¹1, which is roughly three times lower as compared to that in the cis pathway. This C approach in the trans pathway is the most favorable among the paths in Figure 8. These results indicate that a non-negligible interaction may take place between CA and the Zr complex to affect the favorable reaction pathway. This observation is just opposite to that in QM/MM simulations performed by Ziegler et al. where the cis uptake of propylene is easier on Cs symmetric [Me2C(Flu)(Cp)ZrR][MeB(C6F5)3] and related systems.42 However, they could not find the stable trans π ethylene complexes and hence they have concluded that the trans position is not accessible for CA. Car­Parrinello QM/MM simulations © 2016 The Chemical Society of Japan | 1101

than R2, is larger than those for the A and C approaches (4.76 and 2.33 kcal mol¹1, respectively). Therefore, in the trans pathways the C approach is more favorable than the other approaches. In the reactants and resting catalysts CA in the trans position is in the outer-sphere. However, the product of the A approach shows very strong electrostatic interaction, forming the strong ion pair. Its stabilization energy compared to IP1 is 16.42 kcal mol¹1. The A approach seems to be unfavorable when CA is in trans position. Comparing all the paths in the second insertion, the most favorable path follows the C approach with CA in trans position. If the polymer propagation proceeds with this insertion mode repeatedly, isotactic products would be yielded. In order to clarify the whole mechanism of stereoselective polymerization with CA, it is necessary to investigate how CA would move and change in coordination mode in the propagation steps. DFT calculations, however, are too time-consuming to reply these questions and other simulation methods such as MD technique is necessary. Such a study to investigate the polymerization mechanism with CA using molecular simulation is now in progress. We have also investigated another different trans orientation of CA in A, B, C and D approaches, in order to analyze the effect of CA on the energetics and to find other pathways with a lower activation energy. Surprisingly, the activation barrier for the first insertion for all conformers in the pathways thus found were decreased significantly to 0.88 (A), 1.49 (C) 2.34 (B), and 22.50 kcal mol¹1 (D). Propylene uptake for the second insertion shows an easy process for activation as well. For instance, the activation energies are 2.90, 2.34, and 6.32 kcal mol¹1 for the formation of second insertion product of the A, C, and B approaches, respectively. This clearly indicates that orientation of CA can alter the activation barrier.74 We also believe that there are many other possible orientations of CA with which the reaction could proceed in the most favorable way. Note that the first insertion in the A approach of this alternative mechanism is more exergonic by 72.57 to 23.08 kcal mol¹1 compared to that of the C approach. Accordingly, the C pathway is followed rather than the A pathway, being the same as the results in Figures 8 and 10. A detailed solvent corrected relative Gibbs free energy profile is shown in Tables S3 and S4 in the Supporting Information.

also gave the same observation in [Cp2ZrR(μ-Me)B(C6F5)3] (R = Me, Pr) for syndiospecific polymerization.73 However, a short CP­PAW dynamic simulation into Cp2ZrEt­MeB(C6H5)3 showed that the favorable approach of ethylene is from trans position.27 These studies suggested that the nature of ansa ligand and the presence or absence of CA can change significantly the reaction pathway of the insertion and stereochemistry of the system. The steric hindrance of three bulky phenyl groups and strong electrostatic interaction between CA and the Zr complex could determine the favorable reaction pathways. We have analyzed the second insertion paths with CA being in cis and trans positions and their resting catalysts prior to second insertion. The D approach for the second insertion with CA was not studied due to the high-energy barrier as compared to others based on the investigation of the first part of this work. Our energetic results are summarized in Figure 9. In these calculations the structures of the resting catalysts ReC1 connecting the first and second insertion steps were determined by removing the propylene molecule from the reactant complex R2 in the second insertion. Though the resting catalysts for A and C approaches with CA in cis position are stabilized by the inner-sphere ion-pair contact, in the resting catalysts for B approach γ agostic interaction prohibits CA from strongly interacting with the Zr catalyst, not to stabilize the resting catalyst. This indicates that agostic interaction dominates over ion-pair interaction. Then, the system is likely to choose the B approach for the subsequent reaction to occur, because the second insertion for the A and C approaches is difficult due to the large stability of these resting catalysts. For the B approach, less energy is required by anion displacement for the reaction to proceed. π-Coordination of propylene to Zr in P1 of the A and C approaches is, however, possible without passing through inner-sphere resting states. It is also noted that the first insertion product P1 shows an agostic interaction, where CA appears to be in the outer-sphere. Agostic interactions are strong enough for expelling the strong ion-pair contact and CA may not need large energy to dissociate. Interestingly, an activation barrier for the insertion in the B approach of propylene is lower than that for A or C approaches when CA is in trans position. But an activation energy of 7.65 kcal mol¹1 relative to ReC1, a resting state being more stable

R2

TS2 1.33

-1.57

IP2 0.00 Relative ΔG (kcal mol–1)

R2

ReC1 -7.96

-9.95 -13.07 -13.97

-4.64 -5.82

TS2 1.01

P2 -14.31 -16.41 -18.69

IP3

ReC1

-6.58

-6.63 -10.17 -11.17

-6.57

-11.34 -13.90

-11.57 P2 -21.29 -21.97

-34.03 -40.10 -54.29

Figure 10. Solvent-effect-corrected free energy profile for the A, B, and C pathways in the second insertion with CA being cis (left) and trans (right) position with respect to propylene. Green, pink, and blue lines represent A, B, and C approaches, respectively. 1102 | Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

© 2016 The Chemical Society of Japan

88 00 0. 0.01 00

0.0284

(a)

0.0275

(b) Figure 11. Molecular graphs of representative agostic resting catalysts ((a) ap1 in A approach and (b) ReC1 in A approach with CA in trans position). μ values for (3, ¹1) bond critical points shown in red sphere are represented. μ values shown in blue color are for agostic bond and others are for inter- and intramolecular interactions. Very weak secondary interactions with less than μ = 0.009 are omitted.

We have not considered all the stereochemical aspects in this section as we investigated the stereochemistry without CA. In fact, we analyzed only the favorable isotactic products rather than syndiotactic products based on the observations obtained in the first part of this work. It is also possible that some stereochemical intermediates may not exist due to the steric interaction between CA and catalyst. This argument was supported by AIM analysis where we can see the numerous secondary interactions (details of AIM study can be seen in the next section and in the Supporting Information). QTAIM Analysis on the Intermediate Species. “Quantum theory of atoms in molecule” (QTAIM) has been widely used to study the structural and bonding information of molecules involving hydrogen bonds, dihydrogen bonds, etc. Popelier and Logothetis were the first to propose the characteristic β agostic interactions in the Cl2Ti­alkyl complexes using QTAIM theory.75 They analyzed the electron density distribution or electron density μ(r) in terms of bond critical points and found the value of electron density to be 0.0402 and 0.0530 a.u. at the bond critical point between the agostic CH bond and the Ti atom for Cl2TiCH2CH3+ and Cl2TiCH2CH2CH3+, respectively. Seifert et al. investigated the influence of agostic interactions in the polymerization process of TiR2C2H5 (R = CN and OCH3) and they observed that Ti£H­C interaction is dominant over Ti­Cβ interaction in the first insertion step and vice versa in the second insertion step.76 We characterized the agostic interaction in the presence and absence of CA. A representative set of two structures in Figure 11 shows a clear indication of a weak interaction between the metal center and the agostic hydrogen atom. The Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

values of electron density at the bond critical points lie in the range of 0.025­0.038 a.u. for some of the resting catalysts with or without CA, whereas other resting structures (in the absence of CA) did not show any type of agostic interactions (It is noteworthy that the ranges are at higher compared to the hydrogen bonds (0.002­0.035 a.u.))(Supporting Information). Positive values of total energy density, Hc (sum of kinetic energy density and potential energy density), at the bond critical point of agostic interactions indicate that all are closedshell contact. In the cases of resting catalysts of ion-pair complexes, various interesting secondary interactions apart from that of agostic interactions are observed. Moreover, we also observed intramolecular interactions exist between the hydrogen atoms in the polymer chain and those of Ind ligands as shown in Figure 11. However, the strength of these interactions is very feeble (0.010 to 0.009) compared to the agostic interactions. All these observations indicate that intermolecular interactions between CA and Ind as well as intramolecular interactions between Ind and polymer chain of catalyst plays a vital role for enhancing isotacticity of products formation. 4. Conclusion In the present work, we explored the stereochemical aspect of C2 symmetric [SiH2(Ind)2ZrCH3]+ catalyzed propylene polymerization based on the energetics with reliable DFT method. We investigated the various reaction pathways of the first and second insertion of propylene to clarify which pathway is more favorable and what factor determines the selectivity. In the cases of the primary insertion (pathways A and C), we cannot distinguish tacticity in the products of the first and second insertion. However, we clarified that the most favorable reaction pathway in the second insertion is pathway C. If the insertions with the same stereochemistry as this most favorable pathway repeatedly occur, isotactic polymerization is realized. The reason why this reaction pathway is the most favorable is that the steric hindrance among the Ind ligand, the polymer end, and the propylene methyl group is the smallest as previously discussed in the literature. In one of the secondary insertion pathways (B) the isotactic polymerization is more favorable than the syndiotactic polymerization. In another secondary insertion pathway (D), different from other three pathways the syndiotactic polymerization is more favorable than the isotactic polymerization. However, because of larger steric hindrance these secondary insertion pathways (B and D) are less favorable than the primary insertion pathways (A and C). In general, enantioface selectivity, regioselectivity, and growing chain conformation are decisive factors for the formation of isotactic products. We observed that Ind ligand and growing chain end cause these selectivities. Moreover, Zr­Cα and Cα­Cγ rotations lead to α, β, and γ agostic interactions in the resting catalysts prior to second insertion and subsequently influence the transition-state energy that increases or decreases the activation barrier. In addition, we studied the reaction mechanism in the presence of [MeB(C6F5)3]¹ by taking into account two possibilities of orientation of this counter anion, one of them being trans orientation with respect to the coordinating propylene and the other being cis orientation. The difference between two orientations appears even in the first insertion. The trans orientation of [MeB(C6F5)3]¹ is more favored than cis orientation and we © 2016 The Chemical Society of Japan | 1103

identified the most favorable reaction pathways in both the first and second insertions to be the C approach with the counter anion in trans position, although we found that the ion pair between the catalyst and the counter anion is strongly bound (inner-sphere ion pair) and that the dissociation of this ion pair could be rate-determining. Although we identified the most favorable pathway in the present model system, the reaction system is very fluxional and has many possibilities in conformations, coordination modes, and so on. Investigation of these possibilities with DFT methods is impossible and other methods such as MD simulation are necessary. Such a study using molecular simulation techniques is in progress. We found in the current study that agostic interactions play an important role in polymerization because it prohibits a strong ion-pair formation that could obstruct further propylene attack. We analyzed the bonding nature of this agostic interaction with the AIM analysis. Moreover, the analysis has given a clear indication of a very weak interaction between the Ind ligand and the hydrogen atoms of growing polymer chain and this interaction would limit the reaction pathway in the case with [MeB(C6F5)3]¹. This work has been supported by JST-CREST, Japan. Supporting Information Details about the unfavorable D pathway, optimized structures for the third insertion, activation barrier and complexation energy for the first insertion with different functionals, the energy profile for atactic second insertion, the results of the AIM analysis, and solvent-corrected relative Gibbs free energies for the first and second insertions with the different orientation of counter anion in the trans approach. This material is available on http://dx.doi.org/10.1246/bcsj.20160119. References 1 L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100, 1253. 2 W. Kaminsky, Catal. Today 2000, 62, 23. 3 W. Kaminsky, A. Funck, H. Hähnsen, Dalton Trans. 2009, 8803. 4 W. Kaminsky, J. Chem. Soc., Dalton Trans. 1998, 1413. 5 W. A. Herrmann, J. Rohrmann, E. Herdtweck, W. Spaleck, A. Winter, Angew. Chem., Int. Ed. Engl. 1989, 28, 1511. 6 W. Kaminsky, A. Laban, Appl. Catal., A 2001, 222, 47. 7 W. Spaleck, F. Kueber, A. Winter, J. Rohrmann, B. Bachmann, M. Antberg, V. Dolle, E. F. Paulus, Organometallics 1994, 13, 954. 8 J. A. Ewen, R. L. Jones, A. Razavi, J. D. Ferrara, J. Am. Chem. Soc. 1988, 110, 6255. 9 A. Schäfer, E. Karl, L. Zsolnai, G. Huttner, H.-H. Brintzinger, J. Organomet. Chem. 1987, 328, 87. 10 F. R. W. P. Wild, M. Wasiucionek, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1985, 288, 63. 11 F. R. W. P. Wild, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1982, 232, 233. 12 U. Stehling, J. Diebold, R. Kirsten, W. Roell, H. H. Brintzinger, S. Juengling, R. Muelhaupt, F. Langhauser, Organometallics 1994, 13, 964. 13 J. A. Ewen, J. Mol. Catal. A: Chem. 1998, 128, 103. 14 Y. van der Leek, K. Angermund, M. Reffke, R. 1104 | Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

Kleinschmidt, R. Goretzki, G. Fink, Chem.®Eur. J. 1997, 3, 585. 15 W. Kaminsky, J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3911. 16 M. Finze, S. E. Reybuck, R. M. Waymouth, Macromolecules 2003, 36, 9325. 17 G. W. Coates, Chem. Rev. 2000, 100, 1223. 18 S. Sakai, J. Phys. Chem. 1994, 98, 12053. 19 T. Yoshida, N. Koga, K. Morokuma, Organometallics 1995, 14, 746. 20 F. Bernardi, A. Bottoni, G. P. Miscione, Organometallics 1998, 17, 16. 21 K. Vanka, Z. Xu, M. Seth, T. Ziegler, Top. Catal. 2005, 34, 143. 22 S. Sakai, Y. Kojima, J. Organomet. Chem. 2009, 694, 3276. 23 V. A. Karttunen, M. Linnolahti, T. A. Pakkanen, J. R. Severn, E. Kokko, J. Maaranen, P. Pitkänen, A. Pakkanen, J. Organomet. Chem. 2008, 693, 3915. 24 V. A. Karttunen, M. Linnolahti, T. A. Pakkanen, J. R. Severn, E. Kokko, J. Maaranen, P. Pitkänen, Organometallics 2008, 27, 3390. 25 G. Guerra, P. Longo, L. Cavallo, P. Corradini, L. Resconi, J. Am. Chem. Soc. 1997, 119, 4394. 26 I. Silanes, J. M. Ugalde, Organometallics 2005, 24, 3233. 27 M. S. W. Chan, T. Ziegler, Organometallics 2000, 19, 5182. 28 P. Cossee, Tetrahedron Lett. 1960, 1, 12. 29 E. J. Arlman, P. Cossee, J. Catal. 1964, 3, 99. 30 K. J. Ivin, J. J. Rooney, C. D. Stewart, M. L. H. Green, R. Mahtab, J. Chem. Soc., Chem. Commun. 1978, 604. 31 H. Kawamura-Kuribayashi, N. Koga, K. Morokuma, J. Am. Chem. Soc. 1992, 114, 8687. 32 G. Erker, Acc. Chem. Res. 2001, 34, 309. 33 R. H. Grubbs, G. W. Coates, Acc. Chem. Res. 1996, 29, 85. 34 M. Brookhart, M. L. H. Green, G. Parkin, Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6908. 35 W. E. Piers, J. E. Bercaw, J. Am. Chem. Soc. 1990, 112, 9406. 36 A. F. Dunlop-Brière, M. C. Baird, P. H. M. Budzelaar, J. Am. Chem. Soc. 2013, 135, 17514. 37 M. Graf, K. Angermund, G. Fink, W. Thiel, V. R. Jensen, J. Organomet. Chem. 2006, 691, 4367. 38 G. Talarico, P. H. M. Budzelaar, Organometallics 2014, 33, 5974. 39 L. Caporaso, L. Cavallo, Macromolecules 2008, 41, 3439. 40 Y. Zhang, Y. Ning, L. Caporaso, L. Cavallo, E. Y.-X. Chen, J. Am. Chem. Soc. 2010, 132, 2695. 41 L. Castro, E. Kirillov, O. Miserque, A. Welle, L. Haspeslagh, J.-F. Carpentier, L. Maron, ACS Catal. 2015, 5, 416. 42 S. Tomasi, A. Razavi, T. Ziegler, Organometallics 2007, 26, 2024. 43 S. Tomasi, A. Razavi, T. Ziegler, Organometallics 2009, 28, 2609. 44 F. J. Gómez, R. M. Waymouth, Macromolecules 2002, 35, 3358. 45 M. K. Leclerc, H. H. Brintzinger, J. Am. Chem. Soc. 1995, 117, 1651. 46 M. Borrelli, V. Busico, R. Cipullo, S. Ronca, P. H. M. Budzelaar, Macromolecules 2003, 36, 8171. 47 T. Yoshida, N. Koga, K. Morokuma, Organometallics 1996, 15, 766. 48 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.

© 2016 The Chemical Society of Japan

Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, M. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09 (Revision D.01), Gaussian, Inc., Wallingford CT, 2009. 49 P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270. 50 T. Clark, J. Chandrasekhar, G. W. Spitznagel, P. V. R. Schleyer, J. Comput. Chem. 1983, 4, 294. 51 R. Ditchfield, W. J. Hehre, J. A. Pople, J. Chem. Phys. 1971, 54, 724. 52 F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297. 53 A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2009, 113, 6378. 54 Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215. 55 K. Remya, C. H. Suresh, J. Comput. Chem. 2013, 34, 1341. 56 T. A. Keith, AIMAll, Version 15.05.18, TK Gristmill Software, 2015. 57 Y.-X. Chen, C. L. Stern, T. J. Marks, J. Am. Chem. Soc. 1997, 119, 2582. 58 R. S. Cahn, C. Ingold, V. Prelog, Angew. Chem., Int. Ed. Engl. 1966, 5, 385. 59 A. Laine, M. Linnolahti, T. A. Pakkanen, J. R. Severn, E.

Bull. Chem. Soc. Jpn. 2016, 89, 1093–1105 | doi:10.1246/bcsj.20160119

Kokko, A. Pakkanen, Organometallics 2010, 29, 1541. 60 M. K. Leclerc, H. H. Brintzinger, J. Am. Chem. Soc. 1995, 117, 1651. 61 G. Guerra, L. Cavallo, G. Moscardi, M. Vacatello, P. Corradini, Macromolecules 1996, 29, 4834. 62 M.-C. Chen, T. J. Marks, J. Am. Chem. Soc. 2001, 123, 11803. 63 M. Farina, G. Di Silvestro, P. Sozzani, Macromolecules 1993, 26, 946. 64 S. A. Miller, J. E. Bercaw, Organometallics 2006, 25, 3576. 65 L. Cavallo, G. Guerra, M. Vacatello, P. Corradini, Macromolecules 1991, 24, 1784. 66 V. R. Jensen, D. Koley, M. N. Jagadeesh, W. Thiel, Macromolecules 2005, 38, 10266. 67 S. Kozuch, S. Shaik, Acc. Chem. Res. 2011, 44, 101. 68 E. Y.-X. Chen, T. J. Marks, Chem. Rev. 2000, 100, 1391. 69 K. S. Sandhya, C. Barberot, M. Takayanagi, N. Koga, M. Nagaoka, to be submitted. 70 C. Zuccaccia, N. G. Stahl, A. Macchioni, M.-C. Chen, J. A. Roberts, T. J. Marks, J. Am. Chem. Soc. 2004, 126, 1448. 71 A. Correa, L. Cavallo, J. Am. Chem. Soc. 2006, 128, 10952. 72 A. Laine, B. B. Coussens, J. T. Hirvi, A. Berthoud, N. Friederichs, J. R. Severn, M. Linnolahti, Organometallics 2015, 34, 2415. 73 S.-Y. Yang, T. Ziegler, Organometallics 2006, 25, 887. 74 E. Clot, Eur. J. Inorg. Chem. 2009, 2319. 75 P. L. A. Popelier, G. Logothetis, J. Organomet. Chem. 1998, 555, 101. 76 H. A. De Abreu, W. B. De Almeida, H. A. Duarte, G. Fischer, T. Heine, G. Merino, G. Seifert, THEOCHEM 2006, 762, 9.

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