Alkyl Effects on the Chain Initiation Efficiency of Olefin Polymerization

Organometallics , 2016, 35 (6), pp 913–920 ... In contrast, 1 with CH2SiMe3 displays the best chain initiation ability, and 3 with η3-allyl gives m...
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Alkyl Effects on the Chain Initiation Efficiency of Olefin Polymerization by Cationic Half-Sandwich Scandium Catalysts: A DFT Study Xiaohui Kang,†,‡ Guangli Zhou,† Xingbao Wang,† Jingping Qu,† Zhaomin Hou,*,†,§ and Yi Luo*,† †

State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ CAS Key Lab of Separation Sciences for Analytical Chemistry, National Chromatographic R&A Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), Dalian 116023, People’s Republic of China § Organometallic Chemistry Laboratory and Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: The effect of alkyls on the chain initiation efficiency of ethylene, propene, 1-hexene, styrene, butadiene, and isoprene polymerizations catalyzed by the half-sandwich cationic rare-earth-metal alkyl complexes [(η5-C5Me5)ScR]+ (R = CH2SiMe3, 1; R = o-CH2C6H4NMe2, 2; R = η3-C3H5, 3) has been studied by using a DFT approach. It has been found that 2 with the largest sterically demanding aminobenzyl group results in the lowest initiation efficiency and thus longest induction period among the three catalysts investigated. In contrast, 1 with CH2SiMe3 displays the best chain initiation ability, and 3 with η3-allyl gives moderate chain initiation activity, mainly due to the most stable resulting coordination complex. Species 1 and 3 have better regioselectivity in the chain initiation of styrene polymerization than species 2. In addition, species 1′ ([(η5-C5Me5)Sc(CH2SiMe3)THF]+) with a THF ligand has better chain initiation efficiency in styrene and isoprene polymerizations than species 2 but is reasonably worse than the analogue 1 without a THF ligand.

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INTRODUCTION High-performance poly(olefins), which play an irreplaceable role in industrial production and in our daily lives, have attracted considerable interest in academia and industry. The development of highly active olefin polymerization catalysts has become a primary impetus behind the evolution of new polymers. So far, numerous well-defined catalysts with high activity and high selectivity for polymerization based on group 4 and late-transition-metal complexes1−3 have been synthesized successfully, which has effectively promoted the design and development of novel polymer materials. In the last decades, cationic rare-earth alkyl complexes have been reported to be a new family of olefin polymerization catalysts4−19 and have afforded a series of novel polymers that were hard to achieve previously. As we know, the polymerization performances, such as activity, selectivity, molecular weight, and weight population, are mainly governed by the structure of the catalyst. A cationic rare-earth-metal monoalkyl catalyst for olefin polymerization generally consists of four parts (Chart 1), viz., metal center (M), ancillary ligands (L), alkyl (R), and an external neutral Lewis base (X) such as THF. All of these parts affect the polymerization performance. Numerous experimental studies have confirmed that the metal center has a significant effect on the activity. Gade et al. © XXXX American Chemical Society

Chart 1. General Catalyst Structure of Olefin Polymerization

reported a series of cationic rare-earth-metal alkyl complexes [Ln(iPr-trisox)(CH2SiMe3)]2+ (Ln = Sc, Y, Tm, etc.). Among these species, the dicationic Sc alkyl species showed extremely high activity and isoselectivity toward the polymerization of 1hexene. However, other metal analogues showed rather low or no activity.9,10 The origin of the aforementioned activity difference between Sc and Y complexes has been clarified in our previous theoretical work.20 An apparent dependence of the stereoselectivity on the atom radius of the metal center was also observed. The complexes [Ln(η 5 -C 5 Me 4 SiMe 3 ){(μReceived: February 3, 2016

A

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Figure 1. Geometric structures (distances in Å and angles in deg) of cationic half-sandwich scandium alkyl species 1−3.

mechanism concerning such alkyl effects on the induction period or chain-end microstructures at the chain initiation stage has remained unclear. Numerous computational studies22−39 have demonstrated that DFT calculations as a powerful tool could be applied successfully to investigate the mechanism of various olefin polymerizations. Therefore, in the present work, we have conducted systematic computational studies on the chain initiation of ethylene, propene, 1-hexene, styrene, butadiene, and isoprene polymerization mediated by the half-sandwich cationic rare-earth-metal Sc catalysts [(η 5 -C 5 Me 5 )Sc(CH2SiMe3)(THF)n]+ (n = 0, 1; n = 1, 1′), [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (2), and [(η5-C5Me5)Sc(η3-C3H5)]+ (3). The main purpose of this study is to clarify the influence of the three alkyls and THF on the chain initiation of olefin polymerization. We hope that this work will provide some theoretical insights into the aforementioned polymerization systems.

Me)2(AlMe2)}2] (Ln = Y, La, Nd, Gd, Lu) activated by [Ph3C][B(C6F5)4] were applied successfully to 1,3-butadiene polymerization in the presence of [AliBu3]. The results showed that mainly cis-1,4 polybutadiene was obtained with Gd species, while the La analogue gave high trans-1,4 content.11 It is well-known that the striking effect of ancillary ligands on regio- and stereoselectivity has also been proven. During the 3,4-selective polymerization of isoprene, the replacement of ancillary ligands (L) PhNSNdipp (NSNdipp = [S(NC6H4iPr22,6)2]) with Ph2NPNdipp in the Lu-centered complex LLu(CH2SiMe3)2(THF) led to a dramatic shift in selectivity from isotacticity (mmmm = 99%) to syndiotacticity (rr = 66%).12 In the case of the mono(cyclopentadienyl) scandium dialkyl complexes Cp′Sc(CH2SiMe3)2(THF) (Cp′ = C5H5, C5MeH4, C5Me4H, C5Me5, C5Me4SiMe3, C5Me4C6H4OMe) serving as precursors for isoprene polymerization reported by Hou et al., less sterically demanding complexes with C5H5 and C5MeH4 ligands showed high cis-1,4 selectivity, the ether side arm containing complex yielded trans-1,4 polyisoprene as a major product, and other more sterically demanding complexes preferred the formation of 3,4-polyisoprene.13 While numerous studies have been carried out to explore the effects of ancillary ligands and metal centers on polymerization performance, the correlation between the alkyls or Lewis base and polymerization properties has remained almost unexplored. Hou et al. found that the THF-containing half-sandwich scandium complex [(η5-C5Me4SiMe3)Sc(CH2SiMe3)2THF] as a precursor could give a cyclopolymer of 1,6-heptadiene (HPD) with lower molecular weight and rather broad molecular weight distribution in comparison with the THF-free complex [(η5C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2].14 In addition, a halfsandwich scandium complex such as [(η5-C5Me4SiMe3)Sc(CH2SiMe3)2THF], in the presence of a proper activator, showed excellent activity and selectivity toward the (co)polymerization of a wide range of olefins, such as (co)polymerization of styrene with ethylene, 1,3-dienes, and other olefins.13,15−18 The aforementioned THF-free aminobenzyl complex in combination with an activator showed unprecedented activity for the copolymerization of 1-hexene with dicyclopentadiene.19 In this context, the CH2SiMe3 alkyl species requires a Lewis base such as THF to stabilize highly unsaturated Lewis acidic centers. In contrast, half-sandwich species with the −CH2C6H4NMe2-o ligand and π-η3-allyl alkyls21 make the metal center relatively saturated and an external Lewis base such as THF is not required. Although some effects of the alkyl and THF have been found in the aforementioned polymerization experiments, the molecular

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COMPUTATIONAL DETAILS

All calculations were performed with the Gaussian 09 program.40 The DFT method of B3PW9141−43 was used for geometry optimizations. The 6-31G* basis set was used for C, H, O, and N atoms, and the Sc and Si atoms were treated with the Stuttgart/Dresden effective core potential (ECP) and the associated basis sets.44,45 In the Stuttgart/ Dresden ECP used in this study, the most inner 10 electrons of Si and Sc are included in the core. The 4 valence electrons of Si atom and 11 valence electrons of Sc were treated with optimized basis sets: viz., (4s4p)/[2s2p] for Si and (8s7p6d1f)/[6s5p3d1f] for Sc, respectively. The basis set for the Sc atom contains one f-polarization function with an exponent of 0.27. The Si basis set was augmented with one dpolarization function (exponent of 0.28).46 Each optimized structure was analyzed by harmonic vibrational frequencies obtained at the same level and characterized as a minimum (Nimag = 0) or a transition state (Nimag = 1). The frontier orbital analysis was performed with 631+G** for C, H, O, and N atoms, and the basis sets for Sc and Si atoms are same as those for geometrical optimization. Considering that the solvation effect of weakly polar toluene as a typical solvent for olefin polymerization is insignificant and its coordination to Sc is weak relative to monomer coordination,35 the effect of solvent was not considered in this work. The same strategy was also adopted in the studies of olefin polymerization catalyzed by rare-earth-metal complexes.20,32−35

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RESULTS AND DISCUSSION Structures of Cationic Active Species 1−3. The three bare cationic scandium alkyl species [(η 5 -C 5 Me 5 )Sc(CH2SiMe3)]+ (1), [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (2), and [(η5-C5Me5)Sc(η3-C3H5)]+ (3) have been optimized. B

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Organometallics Geometrically, species 1 shows a β-agostic Si−C···Sc interaction, as suggested by the Sc···C2 distance of 2.42 Å, Si−C2 contact of 2.01 Å, and Sc−C3−Si angle of 91.1°. Species 2 indicates strong interactions of the Sc atom with benzyl and amino groups, as suggested by the Sc−C1 distance of 2.20 Å and Sc−N distance of 2.29 Å. In addition, two phenyl C atoms were found to interact with the metal Sc center, indicated by the Sc···C2 distance of 2.47 Å and Sc···C3 contact of 2.50 Å. Species 3 shows a symmetrical geometry with a C2 axis passing through the Sc and C2 atoms (Figure 1). On the basis of the optimized structures of 1−3, the chain initiation efficiencies of a series of olefins, including ethylene, propene, 1-hexene, styrene, butadiene, and isoprene, have been investigated, respectively. Chain Initiation of Ethylene Polymerization by Cationic Species 1−3. Monoolefin polymerizations generally follow the Cossee−Arlman mechanism,37,38 namely, the olefin initially approaches the metal center to form a π complex (C), and then the reaction proceeds via a four-center transition state (TS), leading to the insertion product (P). The ethylene (e) insertion also follows this mechanism. At first, the ethylene coordinates to the Sc atom of species 1−3 and the π complexes C1e, C2e, and C3e are thus formed, respectively. As shown in Figure 2, the complex C3e is the most easily accessible and

(−5.769 eV) and 2 (−5.197 eV); (b) the species 3 is more electron-deficient and is a stronger Lewis acid, as suggested by the greater chemical hardness (3.113 eV for 3 vs 3.035 eV for 1 and 2.999 eV for 2). Geometrically (Figure 2), the interaction between the chelating aminobenzyl group and metal Sc atom in species 2 could be viewed as two parts: η 3 -π-allyl and −NMe 2 interactions with the metal Sc center. These two kinds of interactions could hamper the ethylene coordination and subsequent insertion, thus leading to the instability of complex C2e and the high energy barrier at the chain initiation stage. Even so, this process remains easily accessible at room temperature. The lesser steric congestion of species 3 makes it easier for ethylene to coordinate with the Sc atom to produce the stable complex C3e. However, it is more difficult for ethylene to insert into the Sc−π-allyl bond of species 3 than into the Sc−η1-alkyl bond of species 1. This could account for the higher energy barrier of 10.7 kcal/mol in the case of species 3. Chain Initiation of α-Olefin Polymerization by Cationic Species 1−3. As we all know, the insertion of an α-olefin can adopt two manners, viz., 1,2-insertion (primary insertion with re and si faces, respectively) and 2,1-insertion (secondary insertion with re and si faces, respectively), because of the existence of an alkyl group (methyl in propylene and nbutyl in 1-hexene).34 Herein, we investigate in detail the chain initiation of propene and 1-hexene polymerization. At the chain initiation stage, four insertion fashions of propene (p), viz., 1,2-si, 1,2-re, 2,1-si, and 2,1-re (Table S3 in the Supporting Information and Table 1), have been Table 1. Computed Relative Free Energies for the Chain Initiation of Propylene Polymerization Catalyzed by Species 1−3a ΔG (kcal/mol) species 1 2

Figure 2. Computed energy profiles for the chain initiation of ethylene polymerization catalyzed by cationic species 1−3. Free energies are relative to the energy sum of 1/2/3 and ethylene.

3

insertion mode 1,2-re 2,1-re 1,2-re 2,1-re 1,2-si 2,1-re

C −2.8 −2.6 3.0 4.6 −7.6 −9.1

(C112p) (C121p) (C212p) (C221p) (C312p′) (C321p)

TS 7.7 9.8 19.1 22.0 8.1 11.7

(TS112p) (TS121p) (TS212p) (TS221p) (TS312p′) (TS321p)

P −4.4 −2.9 3.8 9.9 −4.3 −2.6

(P112p) (P121p) (P212p) (P221p) (P312p′) (P321p)

a

Energies are relative to the energy sum of 1/2/3 and propylene. Only favorable insertion modes are given; see Table S3 in the Supporting Information for more details.

stable. However, it needs to overcome a barrier of 10.7 kcal/ mol to reach TS3e. In contrast, the species 1 catalyzed reaction has only an energy barrier of 7.2 kcal/mol, which is more kinetically favorable than those by species 2 (16.8 kcal/mol) and species 3 (10.7 kcal/mol). Although a cationic species with a less sterically demanding alkyl group may be thermodynamically more difficult to form (such as 1), such a species is more active and more easily coordinates with the olefin monomer in the chain initiation step because of greater electron deficiency and less steric effect at the metal center. With the growth of the polymer chain, the cationic species could be stabilized. On the basis of the frontier orbital analysis (Tables S1 and S2 in the Supporting Information), the stability of complex C3e could be ascribed to the following two aspects: (a) the HOMO of ethylene is closer in energy (−7.619 eV) to the LUMO of the species 3 (−6.095 eV), suggesting that ethylene coordinates more easily with the species 3 in comparison with species 1

considered for the species 1−3, respectively. It is found that 1,2-insertion (energy barriers of 10.5 kcal/mol and exergonic by 4.4 kcal/mol for re and si faces) in the case of 1 is more favorable both kinetically and thermodynamically than the 2,1insertion mode (energy barriers of 12.4 kcal/mol and exergonic by 2.9 kcal/mol for re and si faces). Like the case for species 1, 1,2-insertion is also more favorable in the case of species 2 and 3 in comparison to the 2,1-manner. Among these insertion reactions, the preference for 1,2-insertion in species 3 is more distinct, where the barrier difference between 1,2- and 2,1insertion is 5.2 kcal/mol (1.9 kcal/mol for species 1 and 2.9 kcal/mol for species 2). To elucidate the origin of the kinetic preference for 1,2-insertion, energy decomposition analyses20,36 of two transition states TS112p and TS121p have been carried out. It is found that the interaction energies ΔEint between 1 C

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Organometallics and propene moieties in TS112p and TS121p are −32.9 and −28.9 kcal/mol, respectively, which could completely offset the unfavorable total deformation energy of the two moieties, ΔEdef (23.8 kcal/mol for TS112p and 21.5 kcal/mol for TS121p). Therefore, the ΔETS value (−32.9 + 23.8 = −9.1 kcal/mol) obtained for TS112p is lower than that (−28.9 + 21.5 = −7.4 kcal/mol) for TS121p. Therefore, the decreased stability of TS121p is mainly due to the weaker interaction between the propylene moiety and its contacting part. In accordance with the discussion above, we conclude that the alkyls of the cationic species investigated have no effect on the regioselectivity of propene insertion at the chain initiation stage. To further explore the effect of the three alkyls, we made a comparison of the favorable 1,2-insertion of propene by species 1−3. As shown in Figure 3, the energy profiles of propylene

In total, species 1 in comparison with species 2 and 3 could be a better catalyst for the chain initiation of α-olefin polymerization, which is in agreement with experimental observations.4 Chain Initiation of Styrene Polymerization by Cationic Species 1−3. As in the case of α-olefin, four insertion manners, viz., 1,2-si, 1,2-re, 2,1-si, and 2,1 re insertions, have been also calculated (Table S5 in the Supporting Information). The most favorable insertion modes are collected in Table 2. As Table 2. Computed Relative Free Energies for the Chain Initiation of Styrene Polymerization by Species 1−3a ΔG (kcal/mol) species 1 2 3

insertion mode 1,2-si 2,1-si 1,2-si 2,1-si 1,2-re 2,1-si

C −4.5 −0.6 2.6 1.7 −11.4 −10.9

TS 1

(C 12s′) (C121s′) (C212s′) (C221s′) (C312s) (C321s′)

10.8 5.5 22.5 22.3 12.0 4.9

P 1

(TS 12s′) (TS121s′) (TS212s′) (TS221s′) (TS312s) (TS321s′)

−0.6 −10.1 6.9 9.3 −7.2 −11.9

(P112s′) (P121s′) (P212s′) (P221s′) (P312s) (P321s′)

a Energies are relative to the energy sum of 1/2/3 and styrene. In the complex labeling shown in parentheses, “s” and “s′” represent re and si faces, respectively.

shown in this table, in the case of species 1, 2,1-si insertion is more kinetically and thermodynamically favorable. The kinetic preference of 2,1-si insertion is rooted in an η3-coordination fashion (Sc atom interacting with one benzylic and two phenylic carbon atoms; Figure 4) in transition state TS21s′. In the case of species 2, the si-face insertion (energy barriers of ca. 22 kcal/mol) is more kinetically favorable than re-face insertion (energy barriers of 24−28 kcal/mol). The 1,2-si and 2,1-si insertions overcome almost the same energy barriers (about 22 kcal/mol, Table 2). This suggests that the two si-face insertions are kinetically competitive, resulting in no regioselectivity at the chain initiation stage. To explore the origin of the above phenomena, energy decomposition analyses of the two insertion transition states (TS212s′ and TS221s′) have been carried out. It has been found that the interaction energies ΔEint between 2 and styrene moieties in TS212s′ and TS221s′ are −35.5 and −30.7 kcal/mol, respectively, which could partially offset the unfavorable total deformation energy of the two moieties, ΔEdef (45.1 kcal/mol for TS212s′ and 40.0 kcal/mol for TS221s′). Therefore, the ΔETS value (−35.5 + 45.1 = 9.7 kcal/ mol) obtained for TS212s′ is similar to that (−30.7 + 40.0 = 9.3 kcal/mol) for TS221s′. Unlike the case of 1, where the structure of TS121s′ (Figure 4) shows an η3 coordination between the metal and the inserting styrene, the structure of TS221s′ has no such η3 coordination interaction (3.99 Å for the Sc···C7 contact) due to the repulsion between the −NMe2 group and the phenyl ring of the inserting styrene (Figure 4). The absence of an η3 coordination interaction could explain that the 2,1-si insertion no longer has kinetic priority in the case of 2. Therefore, the 2-mediated chain initiation has almost no regioselectivity. Computed free energy profiles for the chain initiation of styrene polymerization mediated by cationic species 1−3 are given in Figure 5. In addition, the species 3 induced chain initiation is shown in Table 2. It is found that the 2,1-si insertion overcomes the lower energy barrier by 15.9 kcal/mol and is more exergonic by 11.9 kcal/mol than that of the 1,2-mode (energy barrier of 23.4

Figure 3. Computed free energy profiles for the chain initiation of propene polymerization catalyzed by species 1−3. Energies are relative to the energy sum of 1/2/3 and propylene.

insertion are similar to those of ethylene. Namely, the coordination of the propylene with species 3 also gives the most stable complex C312p′, which is similar to C3e. Subsequent insertion proceeds via the transition state TS312p′ with an energy barrier of 15.7 kcal/mol, leading to the insertion product P312p′. Apparently, the energy of product P312p′ (−4.3 kcal/mol) is higher than that of complex C312p′ (−7.6 kcal/ mol), which suggests that the process is thermodynamically unfavorable. In comparison, the insertion in species 1 just overcomes an energy barrier of 10.5 kcal/mol and is exergonic by 4.4 kcal/mol. The path involving pecies 2 needs to surmount a 19.1 kcal/mol free energy barrier and is endergonic by 3.8 kcal/mol, which is relatively difficult to occur. Therefore, the alkyls have a significant effect on the chain initiation efficiency and thus the induction period. In addition, it may be difficult for species 2 and 3 to initiate the polymerization of propylene. Likewise, the chain initiation (Table S4 and Figure S1 in the Supporting Information) of the 1-hexene polymerization shows a trend similar to that of propylene: (1) the 1,2regioselectivity is more favorable than 2,1-insertion; (2) the superiority of 1,2-insertion is the most remarkable in the case of 3; (3) the energy barrier of the chain initiation by species 2 is the highest, which results in the longest induction period. D

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Figure 4. Geometric structures (distances in Å) for transition states TS121s′, TS221s′, TS312s, TS321s′, and TS321s.

TS312s and TS321s′ has also been performed. The results show that the interaction energies ΔEint between species 3 and styrene moieties in TS312s and in TS321s′ are −48.1 and −43.20 kcal/mol, respectively, which can entirely offset the unfavorable term ΔEdef (total deformation energy: 47.9 kcal/mol for TS312s and 34.5 kcal/mol for TS321s′). It is obvious that ΔETS (−48.1 + 47.9 = −0.2 kcal/mol) for TS312s is higher than that (−43.2 + 34.5 = −8.7 kcal/mol) for TS321s′. Therefore, the decreased stability of TS312s is mainly ascribed to the greater steric repulsion between species 3 and the styrene moiety. On the basis of the discussion above, it is concluded that species 1 and 3 have better 2,1-regioselectivity in chain initiation of styrene polymerization than species 2. In contrast, the complex C321s′ is more stable than complexes C121s′ and C221s′, which is consistent with the aforementioned monoolefin polymerizations. The 1-mediated 2,1-insertion occurs with a 6.1 kcal/mol energy barrier and is exoergic by 10.1 kcal/mol. However, an energy barrier of 15.9 kcal/mol and an energy release of 11.9 kcal/mol are found for the case of species 3. The aforementioned processes are accessible at room temperature; however, the process involving 2 is not easy. Therefore, the alkyl in 2 may reduce the chain initiation efficiency and result in a long induction period. Chain Initiation of 1,3-Diene Polymerization by Cationic Species 1−3. Numerous theoretical studies30−34,36 on 1,3-dienes polymerization have been carried out. In this context, the chain growth mainly follows the π-allyl insertion mechanism proposed by Taube and co-workers.39 Therefore, the intermediates of chain propagation should mainly adopt a π-allyl coordinated style. However, from a theoretical computation angle, we conducted a detailed investigation not only on trans-1,4 and cis-1,4 insertion but also on 1,2-insertion of butadiene and 4,3-insertion of isoprene. It has been found that the 1,2-insertion of butadiene and 4,3-insertion of isoprene

Figure 5. Computed free energy profiles for the chain initiation of styrene polymerization mediated by cationic species 1−3. Energies are relative to the energy sum of 1/2/3 and styrene.

kcal/mol and energy release of 7.2 kcal/mol). This suggests that the initiation of styrene polymerization both kinetically and energetically prefers 2,1- to 1,2-insertion. Structurally (Figure 4), a remarkable difference is found in the four transition states of styrene insertion by species 3. The η3 fashion exists in three transition states, viz., TS312s, TS321s′, and TS321s. However, this interaction is absent in transition state TS312s′ of the 1,2-si manner, which accounts for the highest energy barrier of 25.2 kcal/mol. TS321s′ shows a stronger interaction between the Sc atom and two benzylic C atoms (Sc−C6 distance of 2.55 Å and Sc−C7 distance of 2.63 Å) in comparison to those in TS312s (2.84 and 2.67 Å) and TS321s (2.54 and 2.90 Å). The greater steric congestion between the Cp* ring and the phenyl observed in TS321s could result in the higher energy barrier (24.4 kcal/mol). To further detemine the reason for the stability of TS321s′, an energy decomposition analysis20,36 of E

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Organometallics Table 3. Computed Energy Profilesa for the Chain Initiation of Butadiene Polymerization by Species 1−3 ΔG (kcal/mol) species

insertion mode

1

trans-1,4 → supine-syn cis-1,4 → prone-anti trans-1,4 → supine-syn cis-1,4 → prone-anti trans-1,4 → supine-syn cis-1,4 → prone-anti

2 3

C −2.2 −1.6 6.5 6.0 −8.8 −4.3

TS

(C114b) (C114b′) (C214b) (C214b′) (C314b) (C314b′)

4.6 2.4 19.2 17.4 6.8 0.2

(TS114b) (TS114b′) (T214b) (TS214b′) (TS314b) (TS314b′)

P −19.6 −20.1 −5.2 −7.0 −21.7 −21.2

(P114b) (P114b′) (P214b) (P214b′) (P314b) (P314b′)

a

Free energies are relative to the energy sum of 1/2/3 and trans-butadiene. Just the more favorable insertion modes are given; see Table S6 in the Supporting Information for others. trans-1,4 → supine-syn represents the 1,4-insertion of trans-butadiene leading to supine syn-butenyl, and “b” and “b′” represent trans and cis monomers, respectively.

are the kinetically and thermodynamically unfavorable (Tables S6 and S7 in the Supporting Information). Therefore, a discussion is given only for trans-1,4 insertion and cis-1,4 insertion of butadiene (isoprene) leading to syn-butenyl (prenyl) and anti-butenyl (prenyl), respectively. In addition, there are two coordination modes30 of anti- or syn-butenyl (prenyl) with the metal relative to the Cp* ligand, viz., supine and prone. Insertion of Butadiene. As shown in Table 3, the insertion of trans- and cis-butadiene into the Sc−C bond of species 1 have been optimized. The cis-butadiene (b′) insertion commences from the coordinated complex C114b′ and proceeds via the transition state TS114b′, leading to the prone anti-butenyl product P114b′. This process has an energy barrier of 4.0 kcal/ mol and is exergonic by 20.1 kcal/mol. In comparison, the generation of the anti-butenyl product P114b′ is more kinetically favorable than that of the syn isomer P114b (energy barrier of 6.8 kcal/mol). This leads us to conclude that the chain propagation occurs on the basis of the anti-butenyl complex. The same selectivity is observed in the case of species 2 and 3, and the cis1,4 insertion by 3 possesses the more remarkable kinetic preference, as suggested by the greater energy barrier difference (11.1 kcal/mol vs 2.8 and 1.8 kcal/mol for 1 and 2 cases, respectively). Subsequently, the most favorable cis-1,4 → prone-anti path was chosen to conduct the following comparison. As shown in Figure 6, the chain initiation mediated by species 3 (barrier of 4.5 kcal/mol) shows activity similar to that of species 1. In addition, the energy barrier in the species 2 case is slightly higher but is more easily surmounted at room temperature. Importantly, this process is exoergic by 7.0 kcal/mol, which is different from other olefin chain initiations catalyzed by species 2. In this sense, species 1 and 3 could be better chain initiation agents for the polymerization of 1,3-dienes, as reported in experimental works.4 Possibly, species 2 can initiate the butadiene polymerization but may need a longer induction period. Insertion of Isoprene. Numerous experimental investigations on isoprene polymerization have been conducted, while its related computational studies remain scarce. To get a better understanding of the isoprene polymerization, we have also conducted a systematic study on the chain initiation of isoprene polymerization by species 1−3. It is found that all of the three species show good cis-1,4 regioselectivity in the chain initiation of isoprene polymerization, which is same as that found for the butadiene case. Figure 7 indicates that the polymerization of isoprene is more difficult to initiate than that of butadiene in the case of the crowded species 2. In comparison, species 1 and 3 show high activity and similar

Figure 6. Computed energy profiles for the chain initiation of butadiene polymerization catalyzed by cationic species 1−3. Free energies are relative to the energy sum of 1/2/3 and trans-butadiene.

selectivity in the chain initiation of isoprene polymerization, which is consistent with experiments.12,13 Effect of THF on the Chain Initiation Efficiency of Olefins Polymerization. Considering the possibility of THF coordination with species 1, we have further modeled the species 1′ ([(η5-C5Me5)Sc(CH2SiMe3)THF]+; Figure S2 in the Supporting Information) with THF coordination to mediate the chain initiation of the polymerization of olefins investigated. As shown in Figure 8, the energy barriers for chain initiation mediated by species 1′ are apparently higher than that by species 1 (15.4 vs 7.2 kcal/mol for ethylene, 20.9 vs 10.5 kcal/ mol for propene, 19.6 vs 12.3 kcal/mol for 1-hexene, 17.5 vs 6.1 kcal/mol for styrene, 18.0 vs 4.0 kcal/mol for cis-butadiene, and 18.1 vs 3.7 kcal/mol for cis-isoprene). This result suggests that the Lewis base THF can decrease the chain initiation efficiency. However, the activities of chain initiation in ethylene/styrene/ isoprene polymerization by species 1′ are still higher than those by species 2, as suggested by the insertion free energy barriers (15.4 vs. 16.8 kcal/mol for ethylene, 17.5 vs 22.3 kcal/mol for styrene, 18.1 vs 20.4 kcal/mol for isoprene; see Table S8 in the Supporting Information and Figure 8). It is noteworthy that the separation of THF from the active site via monomer (styrene) coordination is endergonic by 20.2 kcal/mol. Considering our previous result that the separation of F

DOI: 10.1021/acs.organomet.6b00081 Organometallics XXXX, XXX, XXX−XXX

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and the closest frontier molecular orbital energy between 3 and monomers. All species 1 mediated processes are the most kinetically favorable. Because of the greater hindrance of the aminobenzyl group in 2, the coordination and insertion of olefin are the most difficult, thus leading to a longer induction period in comparison with the other two species. The alkyls have no effect on the regioselectivity at the chain initiation stage of monoolefin polymerization. However, species 3 with an allyl group prefers a cis fashion of the dienes for insertion reactions, and species 2 results in bad regioselectivity in the chain initiation of styrene polymerization, which could be rooted in the steric hindrance of the −NMe2 group. In addition, it is found that the coordination of the Lewis base THF decreases the activity of chain initiation, and species 1′ still shows higher chain initiation efficiency than species 2 in ethylene, styrene, and isoprene polymerizations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00081. Relative free energies of stationary points involved in the chain initiation starting with species 1, 1′, 2, and 3, frontier orbital information for these species and all related monomers, and imaginary frequencies of the TSs (PDF) Optimized Cartesian coordinates together with energies (au) (XYZ)

Figure 7. Computed energy profiles for the chain initiation of isoprene polymerization catalyzed by cationic species 1−3. Free energies are relative to the energy sum of 1/2/3 and trans-isoprene.

toluene solvent from the active site via monomer coordination is an obviously exergonic process,35 the toluene solvent could not detach THF from the metal center. Therefore, the pathway where THF detaches first and the chain initiation follows the same pathway as that of 1 is unlikely to occur at the chain initiation stage with respect to the decreased energy requirement (barrier of 17.5 kcal/mol in the case of styrene) for 1′mediated chain initiation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Z.H.: [email protected]. *E-mail for Y.L.: [email protected].

CONCLUSION We have computationally studied the chain initiation of ethylene, propene, 1-hexene, styrene, butadiene, and isoprene polymerizations by the cationic species [(η5-C5Me5)Sc(CH2SiMe3)(THF)n]+ (1, n = 0; 1′, n = 1), [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (2), and [(η5-C5Me5)Sc(η3-C3H5)]+ (3). At first, it was found that the decreased steric hindrance of species 3 provides the most stable coordination complexes, which could also be ascribed to the greatest chemical hardness

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (Nos. 21174023, 21429201, 21231003) and a Grant-in-Aid for Scientific Research (S) from

Figure 8. Free energy barriers for the chain initiation of various olefin polymerizations catalyzed by cationic species 1, 1′, 2, and 3. Free energies are relative to the energy sum of 1/1′/2/3 and their corresponding monomers. The energy barriers are those for the most favorable pathway of the corresponding monomer. G

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(35) Luo, Y.; Luo, Y. J.; Qu, J.; Hou, Z. Organometallics 2011, 30, 2908−2919. (36) Kang, X.; Luo, Y.; Zhou, G.; Wang, X.; Yu, X.; Hou, Z.; Qu, J. Macromolecules 2014, 47, 4596−4606. (37) Cossee, P. J. Catal. 1964, 3, 80−88. (38) Arlman, E. J. J. Catal. 1964, 3, 89−98. (39) Taube, R.; Gehrke, J. P.; Radeglia, R. J. Organomet. Chem. 1985, 291, 101−115. (40) Frisch, M. et al. Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009. (41) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (42) Perdew, J. P.; Wang, Y. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13244−13249. (43) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16533−16539. (44) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866−872. (45) Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1989, 90, 1730− 1734. (46) Höllwarth, A.; Böhme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 237−240.

the JSPS (No. 26220802). The authors also thank the RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of the Dalian University of Technology for computational resources.

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DOI: 10.1021/acs.organomet.6b00081 Organometallics XXXX, XXX, XXX−XXX