Elucidating the Key Role of PhosphineâSulfonate Ligands in

Research Article pubs.acs.org/acscatalysis

Elucidating the Key Role of Phosphine−Sulfonate Ligands in Palladium-Catalyzed Ethylene Polymerization: Effect of Ligand Structure on the Molecular Weight and Linearity of Polyethylene Ryo Nakano,† Lung Wa Chung,‡ Yumiko Watanabe,§ Yoshishige Okuno,§ Yoshikuni Okumura,∥ Shingo Ito,† Keiji Morokuma,*,⊥ and Kyoko Nozaki*,† †

Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, People’s Republic of China § Computational Science and Technology Information Center, Showa Denko K.K., 1-1-1 Ohnodai, Midori-ku, Chiba, Chiba 267-0056, Japan ∥ Institute for Advanced and Core Technology, Showa Denko K.K., 2 Nakanosu, Oita, Oita 870-1809, Japan ⊥ Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano Nishihiraki-cho, Sakyo, Kyoto 606-8103, Japan S Supporting Information *

ABSTRACT: The mechanism of linear polyethylene formation catalyzed by palladium/phosphine−sulfonate and the effect of the ligand structure on the catalytic performance, such as linearity and molecular weight of the polyethylene, were reinvestigated theoretically and experimentally. We used dispersion-corrected density functional theory (DFT-D3) to study the entire mechanism of polyethylene formation from (R2PC6H4SO3)PdMe(2,6-lutidine) (R = Me, t-Bu) and elucidated the key steps that determine the molecular weight and linearity of the polyethylene. The alkylpalladium ethylene complex is the key intermediate for both linear propagation and β-hydride elimination from the growing polymer chain. On the basis of the key species, the effects of substituents on the phosphorus atom (R = t-Bu, i-Pr, Cy, Men, Ph, 2-MeOC6H4, biAr) were further investigated theoretically to explain the experimental results in a comprehensive manner. Thus, the experimental trend of molecular weights of polyethylene could be correlated to the ΔΔG⧧ value between (i) the transition state of linear propagation and (ii) the transition state of the path for ethylene dissociation leading to β-hydride elimination. Moreover, the experimental behavior of the catalysts under varied ethylene pressure was well explained by our computation on the small set of key species elucidated from the entire mechanism. In our additional experimental investigations, [o-Ani2PC6H4SO3]PdH[P(t-Bu)3] catalyzed a hydrogen/ deuterium exchange reaction between ethylene and MeOD. The deuterium incorporation from MeOD into the main chain of polyethylene, therefore, can be explained by the incorporation of deuterated ethylene formed by a small amount of Pd−H species. These insights into the palladium/phosphine−sulfonate system provide a comprehensive understanding of how the phosphine−sulfonate ligands function to produce linear polyethylene. KEYWORDS: phosphine sulfonate, palladium catalysts, ethylene polymerization, linearity, molecular weight, suppression of β-hydride elimination compatibility, especially with polar materials.3 Although earlytransition-metal catalysts used in the industrial production of polyolefins are heavily deactivated by polar functional groups,

1. INTRODUCTION Polyolefins such as polyethylene and polypropylene comprise one of the largest classes of chemicals today.1,2 A longstanding challenge in polyolefin synthesis is the development of efficient methods to introduce polar functional groups into the polymer structure. These polar groups greatly improve material properties such as adhesion, dye retention, printability, and © XXXX American Chemical Society

Received: March 30, 2016 Revised: July 5, 2016

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DOI: 10.1021/acscatal.6b00911 ACS Catal. 2016, 6, 6101−6113

Research Article

ACS Catalysis recently emerged late-transition-metal complexes have been revealed to act as catalysts for the copolymerization of polar monomers directly with ethylene.4,5 Using α-diimine ligated palladium complexes, coordination−insertion copolymerization of ethylene with methyl acrylate to produce highly branched copolymers was first accomplished by Brookhart and coworkers in 1996.6 In 2002, Drent et al. reported the synthesis of linear copolymers of ethylene and alkyl acrylates using palladium/phosphine−sulfonate catalysts.7 Subsequently, the palladium/phosphine−sulfonate system has been proven to tolerate a variety of functional groups,8 which successfully enabled the unique copolymerizations of ethylene with polar monomers such as acrylates,7,9 acrylic acid,10 acrylonitrile,11 vinyl halides,12 vinyl ethers,13 vinyl acetate,14 and allyl monomers.15 The differences in the polymer microstructures of polyethylenes prepared by palladium/α-diimine and palladium/ phosphine−sulfonate catalysts are noteworthy (Figure 1).

The observation of 1-octene isomerization into internal octenes by Claverie and co-workers also supported the chain-walking reactions by palladium/phosphine−sulfonate catalysts.23 From a theoretical viewpoint, in contrast, Ziegler and co-workers reported that the β-hydride elimination is slower in comparison to the polymer-chain growth in the catalyst system.24 In order to consolidate this discrepancy between the experimental and computational insights, we previously studied the mechanism of linear polyethylene formation. 25 On the basis of the experimental observation of long-alkyl intermediates during polymerization and the detailed theoretical calculations on the mechanism of both linear and branched propagation pathways, we proposed that the chain propagation dominates over βhydride elimination in the presence of sufficient ethylene pressure. Since our mechanistic studies in 2009, further mechanistic understanding of the palladium/phosphine−sulfonate system has been accumulated. The mechanism of cis/trans isomerization, which is the key step prior to the ethylene insertion reaction, was examined by Jordan and co-workers.26 After the isolation of cis and trans isomers of alkylpalladium complexes and kinetic control experiments, they concluded that the isomerization process does not require neutral ligands such as pyridine and 2,6-lutidine, as suggested in our previous computational study. The influence of the fourth monodentate ligand was scrutinized by Mecking and co-workers.27 Their study revealed little effect of the fourth ligands on the polymer microstructure and the ceiling of activity by modification of such ligands. Moreover, they established an effective access to a fourth-ligand free active species by in situ activation of the sodium chloride adduct Na[(R2PC6H4SO3)PdMeCl] with AgBF4. The steric effect from the substituents on the phosphorus atom plays an important role in modulating the molecular weight of the obtained polyethylene (Figure 2), although the

Figure 1. Differences in polyethylene microstructures obtained by palladium/α-diimine and palladium/phosphine−sulfonate catalysts.

Palladium/α-diimine catalysts are susceptible to repetitive βhydride elimination and reinsertion reactions, commonly referred to as chain-walking,16 and generally form highly branched polyolefins. On the other hand, phosphine−sulfonate ligated palladium complexes produce highly linear polyethylene in which the linear versus branched ratio in a main chain is usually more than 1000. In addition to palladium and nickel17 catalysts bearing phosphine−sulfonate ligands, group 10 metal catalysts that can achieve a comparable level of linear selectivity are still limited. Nickel/salicylaldiminate18 and SHOP-type nickel/R2PC6H4O complexes19 with elaborate catalyst tuning produced highly linear polyethylene. Recently, we also developed palladium complexes bearing bis(phosphine monoxide) ligands (BPMO)20 or imidazo[1,5-a]quinolin-9-olate-1ylidene (IzQO)21 ligands, which produce almost linear polyethylene with minimum branching. The unique linear selectivity in polyethylene formation by the palladium/phosphine−sulfonate system prompted researchers to study the reaction mechanism of ethylene polymerization both experimentally22,23 and theoretically.24,25 From the experimental side, Jordan and co-workers reported their attempt to copolymerize 6-chloro-1-hexene with ethylene using palladium/phosphine−sulfonate catalysts. However, the microstructure analysis of the obtained polymers suggested the occurrence of β-chloride elimination from the ω-chloroalkylpalladium species, which implied that chain-walking also proceeded with the palladium/phosphine−sulfonate system.22

Figure 2. Substituents on the phosphorus atom effective in increasing the molecular weight of polyethylene.

exact role of the substituents remains unclear. Systematic variations of the substituent on the phosphorus atom were reported by Claverie,28 Jordan,29 Mecking,27c,30 Nozaki,31 Rieger,32 and their co-workers, independently. Generally, the molecular weight of polyethylene was increased by the steric protection of the axial orientation of the palladium center. The highest molecular weight of polyethylene of more than 50000 was accomplished with catalysts bearing Men (menthyl)31a or 6102


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depth computational study on the entire mechanism of ethylene polymerization using dispersion-corrected density functional theory (DFT-D3) and a realistic catalyst model was performed. The study allowed us to determine the key steps/factors affecting the molecular weight and microstructure of polyethylene. In the second part, the key steps in the first part were further examined both experimentally and theoretically with various substituents on the phosphorus atom. The effect of the substituents was reasonably explained by the relative energy levels of the key species. In the third part, a new plausible mechanism of D incorporation into the polyethylene chain from MeOD was proposed, on the basis of control experiments and mechanistic considerations.

biaryl (2-[2′,6′-(MeO)2C6H3]C6H4) substituents on the phosphorus atom (Figure 2). This trend has been interpreted on the basis of the inhibition of the associative chain-transfer process by the steric protection, which enhances the molecular weight of polyethylene, in parallel to the aforementioned palladium/α-diimine complexes in which the blocking of the axial positions effectively hampers the chain transfer, leading to the production of polyethylene of high molecular weights (Scheme 1).33,34 Our previous computational studies,25 which Scheme 1. Protection of the Axial Orientation of a Palladium Center Bearing an α-Diimine Ligand, Leading to the Suppression of Chain Transfer and the Production of HighMolecular-Weight Polyethylene

2. RESULTS AND DISCUSSION 2.1. Entire Free Energy Surface with Improved Computational Methods and Models. In this computational study, several points were improved from our previous computations.25 (1) Instead of the hybrid functional B3LYP, the dispersioncorrected functional B3LYP-D3 was employed for better estimation of the coordination energy.37 The solvent effect (toluene) was also included for all of the species by using the SMD method.38 (2) Several new transition states and intermediates involved in the β-hydride elimination step were identified. (3) In our previous study,25 the palladium/phosphine−sulfonate complexes were simplified by replacement of the substituents on the phosphorus atom by Me groups and 2,6lutidine by pyridine, as a model for comprehensive calculation. In addition to the (Me2PC6H4SO3)Pd(n-propyl)(2,6-lutidine) complex, we also carried out calculations with a realistic model of the complex (t-Bu2PC6H4SO3)Pd(n-propyl)(2,6-lutidine). The results of the theoretical calculation for polyethylene formation by (R2PC6H4SO3)Pd(n-propyl)(2,6-lutidine) complexes (R = Me, t-Bu) are summarized in Figures 3 and 4, respectively. In the following discussion, the intermediates are numbered according to our previous report25 for ease of comparison. Due to the electronic asymmetry of the phosphine−sulfonate ligands which consist of a strong donor motif (phosphine) and a weak donor motif (sulfonate), all the intermediates and the transition states have a pair of cis and trans isomers such as 10 and 10′. Substituents on the phosphorus atom are shown as subscripts such as 10Me and 10t‑Bu, when discrimination is required. The ethylenecoordinated intermediate 10 was set as the starting complex according to our previous paper. Grimme’s dispersion correction (D3) relatively stabilized the intermediates and transition states bearing neutral ligands, such as 2,6-lutidine and ethylene (see Scheme S1 and Table S1 in the Supporting Information for a comparison). On the basis of Claverie’s experimental observation of high activity for ethylene homopolymerization,28d t-Bu substituents highly destabilized resting intermediates 8t‑Bu and 10t‑Bu by a combination of high steric repulsion and electron-donating ability. In the following sections, the mechanisms of linear propagation, β-hydride elimination, and subsequent branch formation and chain transfer are discussed. 2.1.1. Linear Propagation. Linear polyethylene formation preferentially proceeds along ethylene-coordinated intermediate 10, pentacoordinated cis/trans isomerization via TS(1010′), and then ethylene insertion via TS(10′-11′) (blue

focused on highly simplified model complexes bearing Me- or Ph-substituted phosphine−sulfonate ligands, were unsuccessful in providing a mechanistic rationale for the steric influence. Recently, Rezabal and co-workers studied the steric effect of Me-, Ph-, and ferrocenyl (Fc)-substituted phosphine−sulfonate ligands.35 They explained the steric effect on the chain length of polyethylene obtained by probing a pyridine dissociation step prior to β-hydride elimination. In addition, deuterium-labeling experiments suggested the involvement of reversible β-hydride elimination during ethylene polymerization,36 which was not in accordance with the results of our previous computation.25 Nozaki and co-workers observed deuterium incorporation into the main chain of polyethylene from MeOD as the deuterium source, when (Cy2PC6H4SO3)PdMe(2,6-lutidine) or [(o-Ani)2PC6H4SO3]PdMe(2,6-lutidine) was heated in a mixture of MeOD and toluene under ethylene pressure (Scheme 2).36 The proposed Scheme 2. Deuterium Incorporation into the Main Chain of Polyethylene from MeOD

mechanism for the deuterium incorporation was as follows: the palladium hydride species formed after β-hydride elimination from the Pd alkyl species can undergo hydride/deuteride exchange at the palladium center and return to the polymerization cycle. This proposed mechanism including a reversible β-hydride elimination reaction during ethylene polymerization was not compatible with our previous conclusion based on mechanistic studies claiming that the chain propagation dominates over β-hydride elimination in the presence of ethylene pressure. In the present study, we report a comprehensive mechanistic rationale for polyethylene formation by the palladium/ phosphine−sulfonate system, which can explain the aforementioned newly reported observations. In the first part, an in6103


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Figure 3. Free energy profile of palladium/dimethylphosphine sulfonate catalyst (B3LYP-D3/6-31G*, lanl2dz for palladium) in toluene.

Figure 4. Free energy profile of palladium/di-tert-butylphosphine sulfonate catalyst (B3LYP-D3/6-31G*, lanl2dz for palladium) in toluene.

pathway in Figures 3 and 4), as discussed in our previous work.25 2.1.2. β-Hydride Elimination. 2.1.2.1. Formation of cis-P− Pd−H Species 12a′ and 12′. In the dispersion-corrected free energy surfaces, the most favorable pathway for β-hydride elimination proceeds along the formation of β-agostic intermediate 9′ and TS(9′-12a′) (Figures 3 and 4). For this route, the rates of all the paths for the formation of intermediate 9′ are much slower than that of the following βhydride elimination via TS(9′-12a′). Three possible types of formation pathways were found for the formation of intermediate 9′: (1) direct cis/trans isomerization from β-agostic intermediate 9 to 9′ via TS(99′), (2) the fourth ligand associated cis/trans isomerization via TS(8-8med)/TS(8med-8′) or TS(10-10′) and subsequent associative ligand exchange between the β-C−H bond and ethylene or 2,6-lutidine (via TS(8′-9′) or TS(9′-10′), respectively), and (3) the fourth ligand associated cis/trans isomerization, ligand dissociation to three-coordinated intermediate 9NA′ (9′ with no agostic interaction), and following isomerization to 9′. The contribution of pathway 1 via TS(99′) should be negligible, since it requires much higher activation energy than pathways 2 and 3 (Figures 3 and 4). For pathway 2, the key dissociation steps via TS(8′-9′) and TS(9′-10′) are associative ligand exchange reactions in which the fourth neutral ligand (2,6-lutidine or ethylene) is replaced by a β-agostic interaction via five-coordinated transition states. Therefore, the steric hindrance on the axial orientation of the palladium center provided by substituents on the phosphorus atom can retard these processes. The acceleration effect with

2,6-lutidine is comparable with that of ethylene, considering that the concentration of ethylene is much higher than that of 2,6-lutidine under sufficient ethylene pressure. The mole fraction of ethylene in toluene under 3.0 MPa of ethylene pressure at 90 °C was reported to be 0.20.39 Then, under typical conditions (0.10 mmol/L of catalyst in toluene and 3.0 MPa of ethylene pressure at 80 °C), the ratio between ethylene and 2,6-lutidine (=catalyst) in the solvent mixture reaches more than 2400. When the activity difference between ethylene and 2,6-lutidiene is ignored, the ratio corresponds to ca. 5 kcal/mol difference. Thus, TS(9′-10′) is the key transition state before βhydride elimination under a high ethylene pressure.40 The contribution of pathway 3, which is discussed in this work for the first time, depends upon the steric pressure from substituents on the phosphorus atom. With the less hindered Me-substituted ligand, the relative free energy of intermediate 9NA′Me is higher than that of TS(9′-10′)Me by ca. 2 kcal/mol, and then the contribution of the type 3 pathway should be negligible under sufficient ethylene pressure. On the other hand, with the more sterically demanding t-Bu-substituted ligand, three-coordinated intermediate 9NA′t‑Bu is more stable than TS(9′-10′)t‑Bu by ca. 5 kcal/mol, suggesting that this pathway can contribute even under high pressure of ethylene. 2.1.2.2. Formation of trans-P−Pd−H Species 12a and 12. Other β-hydride elimination/reinsertion pathways via TS(912a), TS(9a-12), TS(12-13a), and TS(12a-13) were then discussed for the first time. The difference between 12a and 12 lies in the orientation of the coordinating propylene (see Figures S28 and S30 in the Supporting Information), and 12a is more easily accessible. The thermodynamic instability of 6104


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Figure 5. Possible pathways for chain-transfer reaction: (a) R = Me; (b) R = t-Bu.

tendency indicates that, once the β-hydride elimination does take place, there is a better chance of methyl branch formation if it does not proceed to chain transfer. 2.1.3.2. Linear Propagation and Methyl-Branch Formation from trans-P−Pd−H Species 12 or 12a. Due to the thermodynamic inaccessibility of 12 and 12a (trans-P−Pd−H species), further reactions from those species are less likely to happen. The possible pathways from 12 and 12a should be discussed separately, since we could not find the direct interconversion transition states between intermediates 12 and 12a. When the ethylene is placed coplanar with the palladium plane, reinsertion of the olefin easily proceeds to reform 9 and 9a. From the more accessible intermediate 12a, further conversion to methyl branches or chain transfer is less plausible. The isomerization to branched intermediate 13 via TS(12a-13) required much greater activation energy in comparison to the pathway to the linear intermediate 9 via TS(9-12a) by more than 5 kcal/mol for both of the substituents on the phosphorus atom. For an associative chain-transfer reaction, five-coordinated intermediate 18Me was the intermediate for the transition between 12Me and 17Me (Figure 5a). On the other hand, with t-Bu substituents, only the direct transition state TS(12a-17)t‑Bu was found for the transition between 12t‑Bu and 17t‑Bu (Figure 5a). Given that the dissociation of each alkene from 18Me requires 1−2 kcal/ mol activation energy, we treated both 18Me and TS(12a17)t‑Bu as the barrier for the associative chain-transfer reaction. While the associative chain-transfer reaction via 18Me seems feasible with the Me-substituted phosphine−sulfonate ligand (Figure 5a), the chain-transfer reactions via TS(12a-17)t‑Bu or via 19 (14-electron intermediate) are highly hindered and inaccessible with the t-Bu-substituted ligand (Figure 5b). From the less accessible intermediate 12, on the other hand, methyl branch formation via TS(12-13a) is faster than the preceding β-hydride elimination via TS(9a-12), suggesting that isomerization via 12 cannot be excluded under low pressure of ethylene. 2.1.4. Consideration I: Linearity of the Polyethylene. In order to summarize the first part, the key transition states for linear propagation, β-hydride elimination (BHE), subsequent

intermediates 12a and 12 can be attributed to the strong trans influence from the phosphine donor. The activation barriers of TS(9-12a) and TS(9a-12) for forming the trans-P−Pd−H species are higher than that for the main pathway of polyethylene formation via TS(10′-11′) (shown in blue) by 4−6 kcal/mol due to the thermodynamic instability of the products and are comparable to that for TS(9′-10′) (which is the rate-determining step for the formation of the cis-P−Pd−H species). Therefore, the formation of the trans-P−Pd−H species should be less plausible, unless under extremely low concentration of ethylene. 2.1.3. Possible Pathways after β-Hydride Elimination. 2.1.3.1. Linear Propagation, Chain Transfer, and MethylBranch Formation from cis-P−Pd−H Species 12a′ or 12′. There are three possible pathways from intermediates 12a′ and 12′ (cis-P−Pd−H species), which are formed after β-hydride elimination from 9′ or 9a′ and in fast equilibrium via TS(12a′12′): reinsertion to linear-chain formation (the left side of Figures 3 and 4), methyl-branch formation (the right side of Figures 3 and 4), and chain-transfer reactions (Figure 5). When the three possibilities are compared, the chain-transfer reactions are more favored than the other two pathways in the cases of both the Me- and t-Bu-substituted phosphine−sulfonate ligands. The Me-substituted palladium/phosphine−sulfonate favors an associative chain transfer via five-coordinated TS(12′17′)Me, while the sterically crowded t-Bu-substituted palladium/phosphine−sulfonate favors a dissociative chain transfer via the three-coordinated intermediate 19′t‑Bu (Figure 5). Here, the selectivity of linear chain propagation versus branch formation after β-hydride elimination is discussed. Since the β-hydride elimination/reinsertion (via TS(9′-12a′)/TS(12′-13′)) and propylene rotation (via TS(12a′-12′)) are relatively fast, the recoordination of ethylene or 2,6-lutidine toward the β-agostic intermediate 9′ (linear) or 13′ (branched) determines the selectivity between linear repropagation and branch formation. In the cases of both Me and t-Bu substituents on the phosphorus atom, branch formations via TS(13′-14′) and TS(13′-20′) are slightly lower in energy than the corresponding linear pathways via TS(9′-10′) and TS(8′-9′). The relative stabilities of intermediates 9′ and 13′ (13′ is more stable than 9′ by ca. 3 kcal/mol) agree with this trend. This 6105


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Scheme 3. Selected Key Intermediates and Transition States for Linear Propagation, β-Elimination, Chain-Transfer Reactions, and Methyl-Branch Formation

When the branch formation via trans-P−PD−hydride species 12 is negligible, the linearity should reflect the relative rate of linear propagation via TS(10′-11′) versus frequency of the entry to the BHE valley, since branch formation and chaintransfer reactions would be dominant over linear chain propagation from the BHE valley involving 9′. Therefore, in the presence of enough ethylene which would diminish the role of 2,6-lutidine-mediated TS(8′-9′) and ethylene-dissociated 9NA′, the linearity should be in principle proportional to ΔΔG⧧ between TS(10′-11′) and TS(9′-10′). Under such conditions, the linearity of polyethylene does not depend on the ethylene concentration, since both pathways start from the same intermediate 10′. On the other hand, when the contribution of the 2,6-lutidine-mediated transition state and spontaneous ethylene dissociation to enter the BHE valley becomes significant, the ratios of the methyl-branch formation and chain-transfer reactions may be affected by the ethylene pressure. 2.1.5. Consideration II: Molecular Weight of the Polyethylene. The molecular weight of polyethylene is determined by the frequency of entering the BHE valley and the ratio of the subsequent olefin reinsertion vs chain transfer. The favorable mechanism of chain transfer depends on the size of the substituent on the phosphorus atom, such as Me or t-Bu (vide infra); some catalysts favor an ethylene-mediated associative chain transfer via TS(12′-17′), and others favor a dissociative chain transfer via 19′. If the chain-transfer reactions dominate over the linear and branch propagation pathways from the BHE valley, the molecular weight of the obtained polyethylene without methyl branches is simultaneously determined by the linearity of the obtained polyethylene in the presence of the

methyl-branch formation, and chain-transfer reactions are shown in Scheme 3. The linear chain of polyethylene is formed by the linear PE formation cycle involving intermediates 9, 10, 10′, and 11′ (left cycle). Both of the side reactions, methyl-branch formation and chain-transfer reaction, arise from the BHE valley involving intermediates 9′, 9a′, 12′, 12a′, 13′, and 13a′. Due to the electronic asymmetry of the phosphine−sulfonate ligands, entrance to the BHE valley is facilitated by a fourth neutral ligand such as ethylene via TS(10′-9′) or 2,6-lutidine via TS(8′-9′). In the case of a t-Bu substituent on the phosphorus atom, spontaneous ethylene dissociation via 9NA′ should be considered for the entry to the BHE valley as well. After entry into the BHE valley, methylbranch formation (right) and chain-transfer reactions (middle) are feasible pathways. The catalyst goes back to the linear PE formation cycle after methyl-branch formation (right), and intermediate 17′ formed after chain-transfer reactions (middle) can also reinitiate the linear PE formation. In addition to the pathways from the BHE valley, branch forming via transP−PD−hydride species 12 (upper pathway) cannot be excluded, especially with the t-Bu substituent on the phosphorus atom. Here the linearity of polyethylene prepared by a catalyst is defined as linearity = ln(DP of linear chain in polyethylene) DP of polyethylene = ln 1(=chain end per chain) + branches per chain

where DP is the degree of polymerization. 6106


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Table 1. Effect of Substituents on the Phosphorus Atom on Ethylene Polymerization Activity, Molecular Weight, and Microstructure of Polyethylenea

catalyst entry

conditions

result of polymerization

[cat.] (mM)

ethylene (MPa)

temp (°C)

activityb (kg mol−1 h−1)

R

L

1f 2

t-Bu t-Bu

lut lut

0.10 0.10

3.0 1.0

80 80

1860 663

3f 4

i-Pr i-Pr

lut lut

0.10 0.10

3.0 1.0

80 80

641 497

5f 6 7g

Cy Cy Cy

lut lut lut

0.10 0.10 0.10

3.0 1.0 3.0

80 80 80

1150 280 121

8f 9

Men Men

lut lut

0.10 0.10

3.0 1.0

80 80

205 123

10 11 12h 13 14 15h

Ph Ph Ph o-Ani o-Ani o-Ani

lut lut NaCl lut lut NaCl

0.10 0.10 0.046 0.10 0.10 0.039

3.0 1.0 0.5 3.0 1.0 0.5

80 80 80 80 80 80

533 167 1820 631 229 2750

16i 17h

biAr biAr

py NaCl

0.050 0.034

2.1 0.5

85 80

1040 5330

PDIc

methyl branches on 1000Cd

methyl branches per chain

linearitye

6.2 4.5

4.1 6.2

1.0 1.2

0.48 0.38

5.1 4.8

6.7 8.4

2.7 2.6

0 0

0 0

5.5 5.7

11 12 12

2.4 2.8 2.9

0 0 0

0 0 0

6.0 6.1 6.1

170 29

1.5 3.7

0 0

0 0

8.7 7.0

5.0 5.1 1.5j 14 13 12j

2.2 2.1 2.2 2.1 2.6 2.1

0 0 0.6 0.55 0.83 3

0 0 0.06 0.53 0.79 2.5

5.2 5.2 3.9 5.8 5.6 4.8

3.9 2.8

n.r. 0

n.r. 0

(9.0) 6.9

Mnc (103)

230k 29j

A mixture of catalyst 1 (10 μmol) in toluene (100 mL) was stirred under an ethylene atmosphere (3.0−1.0 MPa) in a 300 mL autoclave for 1.0 h at 80 °C, unless otherwise noted. Abbreviations: Cy, cyclohexyl; Men, (−)-menthyl; o-Ani, 2-MeOC6H4; biAr, 2-[2′,6′-(MeO)2C6H3]C6H4; lut, 2,6lutidine; py, pyridine; n.r., not reported. bBased on isolated yields after precipitation with methanol. cMolecular weights determined by size-exclusion chromatography (SEC) using polystyrene standards and corrected by universal calibration. dThe number of methyl branches determined by quantitative 13C NMR analyses. eln(DP of linear chain in polyethylene) = ln(MW of polyethylene) − ln(1 + branches per chain) − ln(MW of ethylene). See also section 2.1 for the definition. Values in parentheses represent that no data for the branch ratio were available. fFrom ref 31. g9.0 equiv of additional lutidine (90 μmol) was added. hFrom ref 27c. iFrom ref 28d. jMolecular weights determined by SEC using polyethylene standards. kMolecular weights determined by light scattering analysis. a

catalysts. On the other hand, catalysts that can form methyl branches from the BHE valley can form higher molecular weight polyethylene in comparison to that estimated from their linearity. 2.2. Effect of Various Substituents on the Phosphorus Atom on the Key Steps. In order to confirm the theoretical results obtained in section 2.1, the effects of the substituents present on the phosphorus atom on the molecular weight and linearity of the obtained polyethylene were experimentally examined with varying ethylene pressure. The observed substituent trend in the experimental study was further studied theoretically by computing the energetics for the key intermediates and transition states depicted in Scheme 3. 2.2.1. Experimental Part. The experimental data of ethylene polymerization with palladium/phosphine−sulfonate catalysts bearing the alkyl substituents t-Bu, i-Pr, cyclohexyl (Cy), and menthyl (Men) and aryl substituents Ph, 2-MeOC6H4 (o-Ani), and 2-[2′,6′-(MeO)2C6H3]C6H4 (biAr) on the phosphorus atom are summarized in Table 1. The results in entries 1, 3, 5, 7, 8, 12, and 15−17 were taken from the literature (see the footnotes in Table 1 for the data sources). In addition to the

reported results, we performed the polymerization reaction under 1.0 MPa of ethylene pressure, in order to probe the influence of ethylene pressure on the architecture of the obtained polyethylene. In Table 1, entries 1 and 2, with t-Bu substituents on the phosphorus atom, a slight decrease in molecular weight was observed upon lowering the ethylene pressure. Interestingly, a slight increase in the branches per chain was observed under a higher pressure of ethylene (0.48 under 3.0 MPa of ethylene; 0.38 under 1.0 MPa of ethylene). On the other hand, the catalysts bearing i-Pr (entries 3 and 4) and Cy (entries 5−7) substituents on the phosphorus atom showed little response to ethylene pressure, in terms of the molecular weight and microstructure of polyethylene without methyl branches. It is noteworthy that the presence of an additional 9 equiv of 2,6lutidine which corresponds to a 10 times higher concentration of 2,6-lutidine did not change the structure of polyethylene obtained using the palladium catalyst bearing the Cy substituent on the phosphorus atom (compare entries 5 and 7). Given that the increase of 2,6-lutidine concentration corresponds to a relative decrease of ethylene concentration, 6107


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on the phosphorus atom did not form methyl-branched polyethylene even under low ethylene pressure. The Phsubstituted catalysts exhibited trace branch formation under low ethylene pressure (0.5 MPa), but the branch number per chain was as low as 0.06. These experimentally observed features of each substituent were studied theoretically by evaluating the energetics of the key species shown in Scheme 3. 2.2.2. Theoretical Part. The results of the computational study on the key transition states and intermediates listed in Scheme 3 are summarized in Table 2. In this theoretical study, the propagating chain end is modeled by n-pentyl instead of npropyl for a better estimation of the steric bulkiness from a wide range of substituents on the phosphorus atom at positions farther away from the palladium center. For a linear PE formation cycle, the ethylene insertion reaction via TS(10′-11′) was the turnover-limiting step with all of the examined substituents, not the cis/trans isomerization via TS(10−10′). After entry to the BHE valley, all the substituents examined seem to favor branch formation or chain transfer in comparison to linear repropagation, which means that the entry to the BHE valley should result in the decrease of linearity by methylbranch formation or chain termination (compare the rows of entry to BHE valley, chain transfer, and branch formation in Table 2). In the following sections, the effect of the substituents present on the phosphorus atom on the microstructure of polyethylene classified in Figure 6 is discussed on the basis of the mechanistic features obtained from Table 2. 2.2.2.1. Substituent Effect on the Linearity. Although the computational methods employed here do not reach a quantitative level, the observed experimental linearity of polyethylene (see the columns of linearity in Table 1) correlated well with the calculated ΔΔG⧧ values between TS(10′-11′) and TS(9′-10′) (see the row of linearity in Table 2); the computational trend of linearity is in excellent agreement with the experimental trend.

the effect of 2,6-lutidine on the microstructure of polyethylene may be negligible under the experimental conditions in Table 1 (3.0−0.5 MPa of ethylene with 0.10 mM of catalyst). Using Men substituents on the phosphorus atom (entries 8 and 9), which can dramatically increase the molecular weight of polyethylene,31a the molecular weight and linearity of the obtained polyethylene were decreased by lowering the ethylene pressure, whereas no detectable methyl branch was formed in both cases. In the case of ligands bearing aryl substituents, plentiful experimental data were provided by many researchers, and even fourth ligand free conditions were established by Mecking and co-workers.27b With Ph substituents on the phosphorus atom (Table 1, entries 10−12), the reported molecular weights and calculated linearity differed in the studies by different authors, whereas our experiments (entries 10 and 11) suggest no dependence on the ethylene pressure. At 5 bar of ethylene pressure (entry 12), low linearity and formation of a slight amount of methyl branches were observed by Mecking and coworkers,27c which suggests the contribution of non-ethylenemediated pathways for methyl-branch formation. The introduction of o-Ani substituents on the phosphorus atom (entries 13−15) increased the methyl branches in polyethylene even under a high pressure of ethylene. The branch-forming nature was pronounced under lower pressures of ethylene, which led to the decrease in the linearity of polyethylene but not the molecular weight. The biAr substituent (entries 16 and 17), which effectively increases the molecular weight of polyethylene, showed an effect similar to that of the Men substituent; the molecular weight and linearity of the obtained polyethylene were decreased by lowering the ethylene pressure. In a summary of the experimental study in Table 1, the substituents on the phosphorus atom are classified according to the linearity and the experimental response to varied ethylene pressure (Figure 6). The experimental order of linearity was Ph

experimental: Ph ≈ t ‐Bu < o‐Ani ∼ i‐Pr < Cy < Men ≈ biAr computed: Ph < t ‐Bu < o‐Ani < i‐Pr < Cy < Men < biAr

2.2.2.2. Substituent Effect on the Molecular Weight Dependence on Ethylene Pressure. The molecular weight dependence on the ethylene pressure used in the polymerization, which varied with the substituent on the phosphorus atom (see also Table 1 and Figure 6), could be explained by the free energy difference between the pathways for the entrance to the BHE valley, namely ethylene-associated TS(9′-10′) and spontaneously dissociated 9NA′, and the free energy difference between the associative and dissociative chain transfers. For the associative chain transfer, we found not only TS(12′-17′) for the conversion between 12′ and 17′ but also the pentacoordinated intermediate 18′ structurally similar to TS(12′-17′). Given that the dissociation of each alkene from 18′ requires 1− 2 kcal/mol activation energy, we treated both TS(12′-17′) and 18′ as the barriers for the associative chain-transfer reaction, just as we treated 19′ as the barrier for the dissociative chain transfer. (see also the description about 18Me in section 2.1.3.2). The catalysts that showed no response to varied ethylene concentration (R = i-Pr, Cy, Ph, o-Ani) favor the ethyleneassociated TS(9′-10′) for entrance to the BHE valley and 18′

Figure 6. Classification of the substituents on the phosphorus atom based on the experimental response to varied ethylene pressure.

≈ t-Bu < o-Ani ≈ i-Pr < Cy < Men < biAr. In terms of the molecular weight dependence on the ethylene pressure, i-Pr-, Cy-, Ph-, and o-Ani-substituted catalysts showed no significant dependence on the ethylene pressure around 1.0−3.0 MPa. On the other hand, the molecular weights of polyethylene formed by the catalysts bearing t-Bu, Men, and biAr substituents on the phosphorus atom were affected by the ethylene pressure even at 1.0 MPa. For the branch-forming tendency, only o-Ani and tBu substituents can facilitate the formation of an observable degree of branches even under a high pressure of ethylene, whereas catalysts bearing i-Pr, Cy, Men, and biAr substituents 6108


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Table 2. ΔG, ΔG⧧, and ΔΔG⧧ Values of the Key Transition States and Intermediates from the Corresponding 10 (kcal/mol)a

a

B3LYP-D3/6-31G* and lanl2dz for palladium in toluene. The propagating chain end is modeled by n-pentyl with all-anti configuration, instead of n-propyl for a better estimation of steric bulk at a position farther from the substituents on the phosphorus atom. Abbreviations: E, ethylene; BHE, βhydride elimination. Legend: (*) 18′; (**) coordination of ipso carbon was observed.

14′ to 13NA′. Actually, in the case of the t-Bu substituent, a slight increase in the methyl branches per chain was experimentally observed under higher pressures of ethylene. 2.2.2.3.2. i-Pr, Cy, and Ph. The catalysts bearing i-Pr, Cy, and Ph substituents on the phosphorus atom produced highly linear polyethylene and showed no response to variations in ethylene pressure (entries 3−7 and 10−12 in Table 1). On the basis of the computation in Table 2, the formation of trans-P− Pd−H species via TS(9a-12) and entry to the BHE valley via 9NA′ are less likely, since those required a larger activation energy in comparison to the linear propagation via TS(10′-11′) and entry to the BHE valley via TS(9′-10′). From the BHE valley, these substituents facilitate both associative chain transfer via TS(12′-17′) and dissociative path via 19′ over the branch-forming pathway via TS(13′-14′) and 13NA′. Hence, little branch formation occurs from the BHE valley. 2.2.2.3.3. o-Ani. The catalyst bearing o-Ani substituents on the phosphorus atom can produce slightly branched polyethylene even under high ethylene pressure, and the branches per chain was increased at lower ethylene pressures (entries 13−15 in Table 1). From the computational results in Table 2, a characteristic of this catalyst is the low-energy TS(9a-12), comparable to TS(9′-10′), the favored entry to the BHE valley. Therefore, the branch formation with the catalyst possibly occurs via the trans-P−Pd−H species 12 (zeroth order to ethylene pressure) and not via the BHE valley through 9′/9a′ and 12′/12a′. This explanation is consistent with the experimental observation of increased branches per chain at lowered ethylene pressure. 2.2.2.3.4. Men and biAr. In the case of Men and biAr substituents, while the formation of methyl branches was not experimentally detectable (entries 8, 9, 16, and 17 in Table 1),

for chain transfer, rather than dissociative 9NA′ and 19′ (see the rows of linearity dependence on ethylene pressure, associative vs dissociative chain transfer in Table 2). Therefore, all the steps that determine the molecular weight of polymers, namely chain propagation via TS(10′-11′), the entrance to the BHE valley via TS(9′-10′), and the chain transfer via 18′, should show a first-order dependence on ethylene concentration, and thereby the effect of ethylene activity is canceled. On the other hand, the catalysts that produce lower molecular weight polyethylene at decreased ethylene pressure (R = t-Bu, Men, biAr) apparently favor dissociative entry to the BHE valley via 9NA′ rather than TS(9′-10′) with associated ethylene (see the row of linearity dependence on ethylene pressure in Table 2). Therefore, at a lower ethylene pressure that decelerates the linear propagation cycle, the frequency of chain transfer can be pronounced. 2.2.2.3. Substituent Effect on Methyl-Branch Formation. The trend of each catalyst to form methyl branches is discussed separately on the basis of the classification in Figure 6. 2.2.2.3.1. t-Bu. The catalyst bearing t-Bu substituents on the phosphorus atom produced slightly branched polyethylene even under high ethylene pressures (entries 1 and 2 in Table 1). On the basis of the computation in Table 2, the formation of trans-P−Pd−H species via TS(9a-12) is less likely, since it required a larger activation energy than the linear propagation via TS(10′-11′) and the entry to the BHE valley via TS(9′-10′) or via 9NA′. From the BHE valley, ethylene coordination to 13NA′ to give 14′ proceeds with little activation barrier and is in competition with associative and dissociative chain transfers. Therefore, a higher concentration of ethylene may increase the branches per chain, due to a relative decrease in the rate of dissociative chain transfer via 19′ and reverse conversion from 6109


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ACS Catalysis the computational results suggest that methyl-branch formation after β-hydride elimination is feasible under high pressures of ethylene. This discrepancy may arise from some overlooked factors in the small set of calculations, such as conformational sampling or entropic effects, or uncovered advent effects from these huge substituents on the phosphorus atom toward chaintransfer process. 2.2.3. Summary of the Substituent Effect on the Polyethylene Formation. In section 2.2, the effect of the substituents present on the phosphorus atom of the palladium/ phosphine−sulfonate catalysts on the mechanism of linear polyethylene formation was experimentally and theoretically studied. Experimentally, several polymerization reactions were carried out under varied ethylene pressures, in order to investigate the mechanism of linear polyethylene formation. Theoretically, the key transition states and intermediates in Scheme 3 were examined with various substituents on the phosphorus atom. The experimental behaviors of each catalyst, including linearity, tendency to form methyl branches, and response to varied ethylene pressure, were qualitatively described by the relative free energies of the key species. Therefore, the key species proposed in Scheme 3 are very likely to constitute the main catalytic pathways for the linear propagation, β-hydride elimination, chain transfer, and methyl-branch-forming reactions. 2.3. Mechanism of Deuterium Incorporation from MeOD into Polyethylene: H/D Exchange Reaction of 1Alkenes Catalyzed by Palladium Hydride Species. One of the questions remaining in our proposed mechanism in Scheme 2 is the mechanism of deuterium incorporation into the main chain of polyethylene from MeOD used as a deuterium source (Scheme 2).36 Although reversible β-hydride elimination and H/D exchange reactions during polymerization were proposed in the previous paper,36 our computation and experimental observation in sections 2.1 and 2.2 suggest that, once β-hydride elimination occurred, the chain transfer or the branch formation dominates over the linear propagation. In this part, we propose an alternative mechanism of deuterium incorporation, on the basis of the experimental observation of a hydrogen/deuterium exchange reaction of 1-alkenes catalyzed by palladium hydride species. When a solution of [o-Ani2PC6H4SO3]PdH[P(t-Bu)3]41 (22H) in benzene was treated with atmospheric pressure of ethylene and MeOD, catalytic deuteration of ethylene was confirmed by quantitative NMR analyses (Scheme 4). On the

Scheme 5. Plausible Mechanism of Deuterium Incorporation from MeOD into Ethylene42

23 and the following C−C bond rotation and β-hydride (not deuteride) elimination affords 25, having deuterated ethylene. Finally, an ethylene exchange reaction releases the deuterated ethylene and regenerates the starting complex 17′-H. This proposal also leads to an alternative mechanism of deuterium incorporation into polyethylene from MeOD, which is compatible with the mechanism of linear polyethylene formation detailed in sections 2.1 and 2.2. The palladium− hydride species 17′ formed after β-elimination and chain transfer may be able to catalyze deuteration of ethylene as proposed in Scheme 4, before 23 and 24 reinitiate linear polyethylene formation. Therefore, the deuterium inclusion into polyethylene from MeOD can be possible via the formation and incorporation of deuterated ethylene into linear polyethylene formation. When 1-eicosene was used as the substrate for the hydrogen/ deuterium exchange reaction, the C1 position of the 1-alkene was slowly but selectively deuterated (Scheme 6). The Scheme 6. Deuteration of C1 Position of 1-Alkene by a Pd/ Phosphine−Sulfonate Complex

Scheme 4. Deuteration of Ethylene by a Palladium/ Phosphine−Sulfonate Complex selectivity for the C1 position reached ca. 30-fold, which could originate from the free energy surface of the BHE valley. We propose that the insertion of propylene into palladium hydride species 12a′ or 12′ favors the 2,1-fashion to give 13′, because 13′ is more stable than 9′ by 2.1 kcal/mol mainly due to steric effects (Scheme 7). In addition, the 2,1-insertion via TS(12′-13′) is lower in energy than the 1,2-insertion via TS(9′-12a′) by 1.0−1.5 kcal/mol for the (R2PC6H4SO3)PdPr complexes (R = Me, t-Bu) (Figures 3 and 4).

basis of the partial deuteration of the palladium hydride species (Scheme 4), as previously observed by Mecking and coworkers,12b we propose a plausible mechanism for the deuteration of ethylene (Scheme 5). The starting palladium hydride complex 22-H undergoes a hydride/deuterium exchange reaction with MeOD,12b and the phosphine ligand in 22-D can be replaced by the excess amount of alkenes present. Once 17′-D was formed, insertion of ethylene to give

3. CONCLUSION In this study, we theoretically and experimentally investigated the mechanism of linear polyethylene formation by palladium/ 6110


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ACS Catalysis Scheme 7. Plausible Explanation for the C1 Selectivity

Scheme 8. Role of the Phosphine−Sulfonate Ligand on Palladium-Catalyzed Linear Polyethylene Formation

phosphine−sulfonate catalysts. In the first part, dispersioncorrected DFT studies on the entire mechanism of polyethylene formation starting from (R2PC6H4SO3)PdMe(2,6lutidene) (R = Me, t-Bu) were carried out. Alkylpalladium ethylene complex 10′ is the key intermediate for both linear propagation and β-hydride elimination from the growing polymer chain: ethylene insertion to 10′ for linear propagation takes place via TS(10′-11′), and β-hydride elimination proceeds from 9′, which is formed from 10′ either associatively via five-coordinated TS(9′-10′) or dissociatively via threecoordinated intermediate 9NA′. Once β-hydride elimination took place via TS(9′-12a′), associative and dissociative chaintransfer reactions (via TS(12′-17′) and 19′, respectively) dominate over the branch formation via TS(13′-14′) and TS(14′-15′), for both Me- and t-Bu-substituted phosphine−sulfonate ligands. In section 2.2, the effects of various substituents on the phosphorus atom (R = Me, t-Bu, i-Pr, Cy, Men, Ph, o-Ani, biAr) were investigated theoretically and experimentally. The effect of the substituents on the phosphorus atom was classified by experimentally observed linearity and the response to varied ethylene pressure. The obtained relative free energies of the key species proposed in section 2.1 reasonably explained the experimental behavior of the corresponding catalysts (Figure 6). The linearity of polyethylene was correlated to the relative activation energy difference between TS(10′-11′) for ethylene insertion and TS(9′-10′) for ethylene dissociation, both after cis/trans isomerization. The mechanism of entering the BHE valley leading to the chain-transfer reaction and branch formation differed by steric repulsion from substituents on the phosphorus atom. The catalysts that produce constant molecular weights of polyethylene under varied ethylene pressures (i-Pr, Cy, Ph, and o-Ani substituents) were computationally suggested to favor the ethylene-associated entry to the BHE valley via TS(10′-11′) and undergo associative chain transfer. In contrast, the catalysts that showed a dependence on the ethylene pressures (t-Bu, Men, and biAr substituents) favored entry into the BHE valley by the spontaneous dissociation of ethylene via 9NA′. Furthermore, the tendency of slight methyl-branch formation of t-Bu- and o-Anisubstituted catalysts was also explained by the energies of a small set of key transition states. In section 2.3, we found that [o-Ani2PC6H4SO3]PdH[P(tBu) 3] catalyzed the C-1 selective hydrogen/deuterium exchange reaction of 1-alkenes using MeOD as the deuterium source. On the basis of this observation, a new mechanism has been proposed for the incorporation of deuterium into ethylene and eventual incorporation of deuterated ethylene, formed by a small amount of Pd−H species, into the main chain of polyethylene. The correspondence of computations and experiments with various phosphine−sulfonate ligands shows how the unsymmetric nature of the phosphine−sulfonate ligands affects linear polyethylene formation with a group 10 metal center (Scheme 8). The combination of a strong and sterically bulky σ donor

and a weak σ donor promotes linear polyethylene formation after cis/trans isomerization, suppressing (1) the formation of trans-P−Pd−H species by a strong trans influence from the phosphine donor (electronic inhibition) and (2) the formation of cis-P−Pd−H species by retarding the prior ethylene dissociation (steric inhibition). In the case of the related palladium/αdiimine catalysts, steric coverage of the axial orientation of the palladium center was proposed to hinder the associative chaintransfer process (Scheme 1).33 On the other hand, in the case of the palladium/phosphine−sulfonate catalysts, steric bulkiness provided by substituents on the phosphorus atom can retard not only the associative chain-transfer process but also ethylene dissociation via TS(9′-10′) prior to the β-hydride elimination reaction. The importance of the ethylene dissociation transition state for linear polyethylene formation was recently proposed by Jensen in the case of nickel/ salicylaldiminate catalysts.40 Our computation revealed that the transition states can be controlled by steric inhibition, and the linearity of polyethylene can be an experimental indicator of the transition states. The occurrence of the dissociative ethylene dissociation process via 9NA′, when bulky Men- and biAr-type substituents are present on the phosphorus atom, represents an intrinsic limitation of the efforts to block associative ligand exchange processes but also indicates the possibility of variability of molecular weights of polyethylene and ethylene/ polar monomer copolymers by tuning reaction conditions. We believe that the insights obtained here can be the basis for further in silico modification of the phosphine−sulfonate ligands and can also help in designing brand-new unsymmetric ligands for olefin polymerization. Sufficient electronic dissymmetry as well as construction of a steric environment around the strong donor is essential for linear polymerization of 6111


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(8) For a review, see: Nakamura, A.; Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K. Acc. Chem. Res. 2013, 46, 1438− 1449. (9) (a) Guironnet, D.; Roesle, P.; Rünzi, T.; Göttker-Schnetmann, I.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 422−423. (b) Guironnet, D.; Caparaso, L.; Neuwald, B.; Göttker-Schnetmann, I.; Cavaro, L.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 4418−4426. (10) (a) Rünzi, T.; Fröhlich, D.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 17690−17691. (b) Daigle, J.-C.; Piche, L.; Claverie, J. P. Macromolecules 2011, 44, 1760−1762. (11) (a) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 8948−8949. (b) Nozaki, K.; Kusumoto, S.; Noda, S.; Kochi, T.; Chung, L. W.; Morokuma, K. J. Am. Chem. Soc. 2010, 132, 16030−16042. (12) (a) Weng, W.; Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 15450−15451. (b) Leicht, H.; Göttker-Schnetmann, I.; Mecking, S. Angew. Chem., Int. Ed. 2013, 52, 3963−3966. (13) Luo, S.; Vela, J.; Lief, G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 8946−8947. (14) Ito, S.; Munakata, K.; Nakamura, A.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14606−14607. (15) (a) Ito, S.; Kanazawa, M.; Munakata, K.; Kuroda, J.; Okumura, Y.; Nozaki, K. J. Am. Chem. Soc. 2011, 133, 1232−1235. (b) Jian, Z.; Mecking, S. Angew. Chem., Int. Ed. 2015, 54, 15845−15849. (16) (a) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Science 1999, 283, 2059−2062. (b) Guo, L.; Dai, S.; Sui, X.; Chen, C. ACS Catal. 2016, 6, 428−441. (17) (a) Nowack, R. J.; Hearley, A. K.; Rieger, B. Z. Anorg. Allg. Chem. 2005, 631, 2775−2781. (b) Guironnet, D.; Rünzi, T.; GöttkerSchnetmann, I.; Mecking, S. Chem. Commun. 2008, 40, 4965−4967. (c) Zhou, X.; Bontemps, S.; Jordan, R. F. Organometallics 2008, 27, 4821−4824. (d) Noda, S.; Kochi, T.; Nozaki, K. Organometallics 2009, 28, 656−658. (e) Zhang, D.; Wang, J.; Yue, Q. J. Organomet. Chem. 2010, 695, 903−908. (f) Perrotin, P.; McCahill, J. S. J.; Wu, G.; Scott, S. L. Chem. Commun. 2011, 47, 6948−6950. (g) Ito, S.; Ota, Y.; Nozaki, K. Dalton Trans. 2012, 41, 13807−13809. (18) (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460−462. (b) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217−3219. (c) Jenkins, J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827− 5842. (d) Bastero, A.; Göttker-Schnetmann, I.; Röhr, C.; Mecking, S. Adv. Synth. Catal. 2007, 349, 2307−2316. (e) Berkefeld, A.; Drexler, M.; Möller, H. M.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 12613− 12622. (f) Berkefeld, A.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 1565−1574. (g) Soshnikov, I. E.; Semikolenova, N. V.; Zakharov, V. A.; Möller, H. M.; Ö lscher, F.; Osichow, A.; Göttker-Schnettmann, I.; Mecking, S.; Talsi, E. P.; Bryliakov, K. P. Chem. - Eur. J. 2013, 19, 11409−11417. (h) Osichow, A.; Göttker-Schnetmann, I.; Mecking, S. Organometallics 2013, 32, 5239−5242. (i) Osichow, A.; Rabe, C.; Vogtt, K.; Narayanan, T.; Harnau, L.; Drechsler, M.; Ballauff, M.; Mecking, S. J. Am. Chem. Soc. 2013, 135, 11645−11650. (19) (a) Kuhn, P.; Sémeril, D.; Matt, D.; Chetcuti, M. J.; Lutz, P. Dalton Trans. 2007, 515−528. (b) Shimizu, F.; Shin, S.; Tanna, A.; Goromaru, S.; Matsubara, Y. WO 2010/050256, 2010. (20) (a) Carrow, B. P.; Nozaki, K. J. Am. Chem. Soc. 2012, 134, 8802−8805. (b) Mitsushige, Y.; Carrow, B. P.; Ito, S.; Nozaki, K. Chem. Sci. 2016, 7, 737−744. (c) Sui, X.; Dai, S.; Chen, C. ACS Catal. 2015, 5, 5932−5937. (21) (a) Nakano, R.; Nozaki, K. J. Am. Chem. Soc. 2015, 137, 10934− 10937. See also: (b) Tao, W.-j; Nakano, R.; Ito, S.; Nozaki, K. Angew. Chem., Int. Ed. 2016, 55, 2835−2839. (22) Vela, J.; Lief, G. R.; Shen, Z.; Jordan, R. F. Organometallics 2007, 26, 6624−6635. (23) Skupov, K. M.; Piche, L.; Claverie, J. P. Macromolecules 2008, 41, 2309−2310. (24) Haras, A.; Anderson, G. D. W.; Michalak, A.; Rieger, B.; Ziegler, T. Organometallics 2006, 25, 4491−4497.

simple olefins, which is shared in the design of our recent works on palladium/BPMO20 and palladium/IzQO21 catalysts.

4. COMPUTATIONAL METHODS All calculations were performed using the Gaussian 09 packages.43 The DFT (B3LYP-D3) methods chosen were used primarily with 6-31G* basis sets44 for the light atoms and Lanl2DZ basis sets and effective core potential for palladium.45 All local minima and saddle points were confirmed by their vibrational frequency calculations (with zero and one imaginary frequencies, respectively). The saddle points found were confirmed to be the correct ones by visualizing the vibrational mode of the imaginary frequency with GaussView. To include solvation effects (toluene as the solvent), the SMD solvation model38 was used.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00911. Experimental procedures and full characterization data of polymers (PDF) Cartesian coordinates of optimized species and comparison of computational methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for K.M.: [email protected]. *E-mail for K.N.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by CREST, JST. The theoretical calculations were performed using computational resources provided by the Research Center for Computational Science, National Institutes of Natural Sciences, Okazaki, Japan. R.N. is grateful to the Japan Society for the Promotion of Science (JSPS) for a Research Fellowship for Young Scientists.

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REFERENCES

(1) (a) Jeremic, D. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2014. (b) Gahleitner, M.; Paulik, C. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2014. (2) (a) Boor, J., Jr. Ziegler-Natta Catalysts and Polymerizations; Academic Press: New York, 1979. (b) McMillan, F. M. The Chain Straighteners; The MacMillan Press: London, 1979. (3) (a) Boaen, N. K.; Hillmyer, M. A. Chem. Soc. Rev. 2005, 34, 267− 275. (b) Chung, T. C. Functionalization of Polyolefins; Academic Press: London, 2002. (4) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479−1493. (5) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169−1203. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283−315. (c) Sen, A.; Borkar, S. J. Organomet. Chem. 2007, 692, 3291−3299. (d) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215−5244. (e) Ito, S.; Nozaki, K. Chem. Rec. 2010, 10, 315−325. (f) Mu, H.; Pan, L.; Song, D.; Li, Y. Chem. Rev. 2015, 115, 12091−12137. (6) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267−268. (b) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888−899. (7) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 744−745. 6112


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ACS Catalysis

ethylene coordination to 23 with TS(13′-14′), a comparison between the rows of associative chain transfer via TS(12′-17′) or 18′ and ethylene coordination via TS(13′-14′) in Table 2 could be informative for the staying period of 17′-H in the H/D exchange cycle. The catalyst bearing an o-Ani-substituted ligand easily proceeds to branch formation from the BHE valley, since the chain transfer via 18′ is slightly favored over ethylene coordination via TS(13′-14′) by 3.1 kcal/mol. On the other hand, in the case of a Cy-substituted ligand, the preference to the chain transfer via 18′ over ethylene coordination via TS(13′-14′) is more pronounced by 6.0 kcal/mol in comparison to that of the o-Ani substituted ligand by 3.1 kcal/mol. Therefore, the palladium hydride species with Cy substituents on the phosphorus atom can stay longer in the H/D exchange cycle in comparison to that with o-Ani substituents, leading to the more efficient deuteration of ethylene. (43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, M. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2009. (44) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724−728. (45) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.

(25) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088−14100. (26) (a) Conley, M. P.; Jordan, R. F. Angew. Chem., Int. Ed. 2011, 50, 3744−3746. (b) Zhou, X.; Lau, K.-C.; Petro, B. J.; Jordan, R. F. Organometallics 2014, 33, 7209−7214. (27) (a) Friedberger, T.; Wucher, P.; Mecking, S. J. Am. Chem. Soc. 2012, 134, 1010−1018. (b) Neuwald, B.; Ö lscher, F.; GöttkerSchnetmann, I.; Mecking, S. Organometallics 2012, 31, 3128−3137. (c) Neuwald, B.; Falivene, L.; Caporaso, L.; Cavallo, L.; Mecking, S. Chem. - Eur. J. 2013, 19, 17773−17788. (28) (a) Skupov, K. M.; Marella, P. R.; Simard, M.; Yap, G. P. A.; Allen, N.; Conner, D.; Goodall, B. L.; Claverie, J. P. Macromol. Rapid Commun. 2007, 28, 2033−2038. (b) Skupov, K. M.; Hobbs, J.; Marella, P.; Conner, D.; Golisz, S.; Goodall, B. L.; Claverie, J. P. Macromolecules 2009, 42, 6953−6963. (c) Piche, L.; Daigle, J.-C.; Poli, R.; Claverie, J. P. Eur. J. Inorg. Chem. 2010, 4595−4601. (d) Piche, L.; Daigle, J.-C.; Rehse, G.; Claverie, J. P. Chem. - Eur. J. 2012, 18, 3277− 3285. (29) (a) Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2010, 132, 52−53. (b) Shen, Z.; Jordan, R. F. Macromolecules 2010, 43, 8706−8708. (c) Contrella, N. D.; Jordan, R. F. Organometallics 2014, 33, 7199− 7208. (30) Jian, Z.; Wucher, P.; Mecking, S. Organometallics 2014, 33, 2879−2888. (31) (a) Ota, Y.; Ito, S.; Kuroda, J.; Okumura, Y.; Nozaki, K. J. Am. Chem. Soc. 2014, 136, 11898−11901. See also: (b) Ota, Y.; Ito, S.; Kobayashi, M.; Kitade, S.; Sakata, K.; Tayano, T.; Nozaki, K. Angew. Chem., Int. Ed. 2016, 55, 7505−7509. (32) (a) Anselment, T. M. J.; Anderson, C. E.; Rieger, B.; Boeddinghaus, M. B.; Fässler, T. F. Dalton Trans. 2011, 40, 8304− 8313. (b) Anselment, T. M. J.; Wichmann, C.; Anderson, C. E.; Herdtweck, E.; Rieger, B. Organometallics 2011, 30, 6602−6611. (33) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414−6415. (b) Camacho, D. H.; Salo, E. V.; Ziller, J. W.; Guan, Z. Angew. Chem., Int. Ed. 2004, 43, 1821−1825. (c) Popeney, C. S.; Levins, C. M.; Guan, Z. Organometallics 2011, 30, 2432−2452. (d) Allen, K. E.; Campos, J.; Daugulis, O.; Brookhart, M. ACS Catal. 2015, 5, 456−464. (e) Dai, S. Y.; Sui, X. L.; Chen, C. L. Angew. Chem. Int. Ed. 2015, 54, 9948−9953. (34) (a) Musaev, D. G.; Svensson, M.; Morokuma, K.; Stromberg, S.; Zetterberg, K.; Siegbahn, P. E. M. Organometallics 1997, 16, 1933− 1945. (b) Froese, R. D. J.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1998, 120, 1581−1587. (35) Rezabal, E.; Asua, J. M.; Ugalde, J. M. Organometallics 2015, 34, 373−380. (36) Kanazawa, M.; Ito, S.; Nozaki, K. Organometallics 2011, 30, 6049−6052. (37) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. (38) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378−6396. (39) Sato, Y.; Hosaka, N.; Inomata, H.; Kanaka, K. Fluid Phase Equilib. 2013, 344, 112−116. (40) The importance of the ethylene-dissociation TS was proposed in related nickel catalysts. See: Heyndrickx, W.; Occhipinti, G.; Jensen, V. R. Chem. - Eur. J. 2014, 20, 7962−7978. (41) Rünzi, T.; Tritschler, U.; Roesle, P.; Göttker-Schnetmann, I.; Möller, H. M.; Caporaso, L.; Poater, A.; Cavallo, L.; Mecking, S. Organometallics 2012, 31, 8388−8406. (42) According to the proposed mechanism for the formation of deuterated ethylene, the higher D incorporation ratio by Cysubstituted palladium/phosphine−sulfonate in comparison to that by the o-Ani-substituted species can be explained as follows. Since the molecular weights of polyethylene formed by these catalysts are comparable, the frequency to generate 17′-H should be similar between these catalysts. Then the degree of deuteration will be determined by the rate of H/D exchange reaction and the staying period of 17′-H in the H/D exchange cycle. Because of the similarities of the alkene exchange TS(25-17′) with TS(12′-17′)/18′ and the 6113


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