Theoretical Investigations of Isoprene Polymerization Catalyzed by

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Theoretical Investigations of Isoprene Polymerization Catalyzed by Cationic Half-Sandwich Scandium Complexes Bearing a Coordinative Side Arm Guangli Zhou,† Xiaohui Kang,†,‡ Xingbao Wang,† Zhaomin Hou,*,†,§ and Yi Luo*,† †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China College of Pharmacy, Dalian Medical University, Dalian, Liaoning 116044, China § Organometallic Chemistry Laboratory and RIKEN Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡

S Supporting Information *

ABSTRACT: Density functional theory studies have been conducted for isoprene polymerization catalyzed by the cationic half-sandwich scandium alkyl species containing a methoxy side arm [(C5Me4C6H4OMe-o)Sc(CH2SiMe3)]+ (1) and that containing a phosphine oxide side arm [{C5Me4SiMe2CH2P(O)Ph2}Sc(CH2SiMe3)]+ (2). It has been found that trans-1,4-polymerization of isoprene by species 1 prefers an insertion− isomerization mechanism: (i) an insertion of cis-isoprene into the metal−alkyl bond to give η3-π-anti-form, (ii) anti-syn isomerization of the resulting 1,2disubstituted allyl complex to yield a syn-allyl form, (iii) repetitive insertion of cis-isoprene into the metal−syn-allyl bond and subsequent anti−syn isomerization. The resulting η3-π-syn-allyl species is suitable for more kinetically favorable cis-monomer insertion. The stability of the key transition state involved in the most feasible pathway could be ascribed to the smaller deformation of cis-isoprene and stronger interaction between the cis-isoprene moiety and the remaining metal complex. The origin of experimentally observed inertness of 2 toward isoprene polymerization is that the steric hindrance derived from the crowding of η3-π-syn-allyl species hampers the insertion of the incoming isoprene monomer. The modeling of 2-mediated chain propagation also has a high energy barrier and is endergonic. To corroborate the steric effect on the kinetic and thermodynamic aspects, various analogue complexes with smaller hindrance have been computationally modeled on the basis of 2. Expectedly, lower energy barrier and favorable thermodynamics are found for the monomer insertion mediated by these complexes with less steric hindrance around the metal center.

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INTRODUCTION The stereospecific polymerization of conjugated dienes is an attractive research subject and has drawn much attention from both academic and industrial researchers since the 1950s because of the limited supply of natural rubber and the increasing demands for high-performance synthetic materials.1−4 In particular, polyisoprenes with versatile microstructures show different chemical, physical, and mechanical properties. Great achievements have been made in cis-1,4selective5−11 and 3,4-selective12−20 polymerization of isoprene; those resulting polymers are important rubbers used for tires and other elastic materials. Comparatively, trans-1,4-polyisoprene, which is a thermoplastic crystalline polymer with high hardness, high tensile strength, good abrasion resistance, and low heat buildup for myriad applications such as medical materials and electrical insulated materials, has received less attention.21−30 Notably, previous studies on the trans-1,4polymerization of isoprene were mainly based on group 4 and late transition metals such as Ti, Cr, Fe, Co, and Ni in combination with aluminum additives.21 However, the activity © XXXX American Chemical Society

and selectivity remained to be improved. Rare earth alkyl complexes have been reported to act as efficient catalysts for the trans-1,4-polymerization of isoprene,22−30 some of which showed excellent performance with no need for any aluminum additives.24,27,28 In this context, ligand modulation has been proven to be an efficient strategy to improve catalytic performance. It is known that side-arm-substituted cyclopentadienyls, as monoanionic ancillary ligands, can mimic constrained geometry configuration, thus accomplishing solvent-free, highly stable precatalysts with well-defined reaction sites. Hou and co-workers reported that the halfsandwich complex [(C5Me4C6H4OMe-o)Sc(CH2SiMe3)2] (A) with a coordinative ether side arm, in combination with an equivalent of [Ph3C][B(C6F5)4], was effective for the trans-1,4polymerization of isoprene, but its analogue [C5Me4SiMe2CH2P(O)Ph2]Sc(CH2SiMe3)2 (B) having a phosphine oxide side arm was inert toward isoprene polymerReceived: December 8, 2017

A

DOI: 10.1021/acs.organomet.7b00876 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics ization under the same conditions (Scheme 1).30 These two systems provided a good model to comparably investigate the

calculations on the catalytic synthesis of trans-1,4-polyisoprene by rare-earth-metal complexes are scarce.41,45 Based on previous computational studies,41,45 there could be two possible pathways for producing trans-1,4-polyisoprene (Scheme 2): (a) successive insertion mechanism, an insertion of trans-isoprene into metal−alkyl bond to form η3-π-syn-allyl structure and successive trans-isoprene insertion into metal− syn-allyl bond; and (b) insertion−isomerization mechanism, an insertion of cis-isoprene into the metal−alkyl bond to give η3-πanti-form, which undergoes anti−syn isomerization yielding a syn-allyl form, then repeatedly undergoes the subsequent cisisoprene insertion into the metal−syn-allyl bond and anti−syn isomerization. Encouraged by these theoretical results, we became interested in clarification of the mechanism of the A-mediated trans-1,4-specific polymerization of isoprene and the origin of inertness of the B-involved system (Scheme 1). It is found that the trans-1,4-specific polymerization of isoprene favorably follows an insertion−isomerization mechanism, and both steric and electronic factors govern the stereoselectivity. The bulky sterics could account for the inertness of B. We hope that the current results could add better understanding to the selective polymerization mediated by the catalysts with a coordinative side arm and hopefully offer some hints for design of more efficient homogeneous rare-earth-metal polymerization catalysts.

Scheme 1. Performance of Scandium Complexes (A or B) with Different Coordinative Side Arms for Isoprene Polymerization

origin of selectivity and activity discrepancy with respect to different coordinative side arms. Considering the great importance of this field, the in-depth mechanistic understanding of this polymerization process is in great demand. Numerous computational studies have been widely and successfully applied to investigate the mechanisms of various olefin polymerizations.31−39 However, there are few examples of theoretical work on isoprene polymerization catalyzed by rare-earth-metal complexes.40−45 Maron et al. conducted computational studies on the cis-1,4-polymerization of isoprene catalyzed by [Cp*ScR]+ as well as a mechanistic survey on ethylene−isoprene copolymerization activity of [Cp*(BH4)LnR].40,41 Cui et al. computationally rationalized the excellent 3,4-isospecific selectivity of isoprene polymerization catalyzed by the NSN-bidentate β-diimidosulfonate-ligated rare-earthmetal complexes19 and the formation of copolymers containing 3,4-isoprene and cis-1,4-butadiene units by yttrium cationic species.42 We reported the first isospecific 3,4-polymerization of isoprene by cationic rare-earth-metal alkyl species resulting from a binuclear precursor13 and the highly selective cis-1,4polymerization of butadiene catalyzed by a cationic rare-earthmetal complex bearing an ancillary PNP ligand.43 The effect of alkyls on the chain initiation efficiency of olefin polymerizations catalyzed by the half-sandwich cationic rare-earth-metal alkyl complexes had also been clarified.44 Nevertheless, theoretical

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COMPUTATIONAL DETAILS

The B3PW91 functional46−48 implemented in the Gaussian 09 software package49 was used for geometrical optimization and subsequent frequency calculations without any symmetry or geometrical constraints. In these calculations, the 6-31G(d) basis set was used for C, H, O, and P atoms, and the Si, Sc, Y, and La atoms were treated by Stuttgart/Dresden effective core potential (ECP) and the associated basis sets.50,51 One d-polarization function with an exponent of 0.284 was augmented for the basis set of the Si atom.52 Such basis sets are referred to as BSI. To obtain more accurate energies, singlepoint energy calculations were performed with a larger basis set (BSII), viz., 6-311+G(d,p) for C, H, O, P, and Si atoms, and the Stuttgart/ Dresden ECP and associated basis sets for metal atoms. The solvation energies were calculated on the basis of gas-phase optimized geometries by using the CPCM solvation model, and toluene was taken as the solvent (ε = 2.37).53 The free energies in solution are reported, unless otherwise specified, which are derived from these single-point energies, including Gibbs free energy corrections obtained in the gas phase. To reduce the overestimation of the entropy contribution derived from the gas-phase model, corrections for free energies were made by −2.6 kcal/mol for 2:1 transformations.54−59 The three-dimensional molecular structures are visualized by CYLView.60

Scheme 2. Two Possible Pathways for Isoprene Polymerization To Afford trans-1,4-Polyisoprene Catalyzed by Rare-EarthMetal Complex

B


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Organometallics

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RESULTS AND DISCUSSION The structures of optimized bare cationic species 1 derived from A and 2 derived from B are shown in Figure 1. As shown

As shown in Figure 2, along with the coordination−insertion pathway, trans-isoprene insertion occurs via 1C1 → 1TS1 → 1P1 (path a), while the cis-monomer goes through 2C1 → 2TS1 → 2P1 (path b). The latter has a lower energy barrier (19.2 vs 17.3 kcal/mol), and such a favorable si-face insertion has also been reported in previous theoretical study on cationic systems.44 However, the product 2P1 with the η3-π-anti-allyl moiety is less stable than 1P1 (−12.2 vs −16.4 kcal/mol). This is in line with the previous finding that cis-isoprene insertion was inclined to give a kinetic product and the trans-isoprene insertion tends to yield a thermodynamic one.40 On the other hand, 2P1 could undergo anti−syn isomerization to give thermodynamically more stable 1P1 via transition state TS1iso featuring allyl group rotation around the C2−C3 bond. Such an anti−syn isomerization process could be driven by thermodynamics and overcomes a moderate energy barrier (20.1 kcal/ mol for TS1iso, Figure 2), which makes it feasible. To further access the trans-1,4-polyisoprene, the chain propagation is calculated based on the more stable η3-π-synallyl species 1P1. As shown in Figure 2, along with path a, successive insertion of trans-isoprene into 1P1 goes through 1TS2 with an energy barrier of 26.1 (9.7 + 16.4 = 26.1) kcal/ mol, yielding 1P2 with a trans-1,4-unit. On the other hand, starting with 1P1 along with path b featuring cis-isoprene insertion and subsequent isomerization (Figure 2), the insertion transition state 2TS2 has an energy barrier of 24.5 (8.1 + 16.4 = 24.5) kcal/mol. This insertion gives η3-π-antistructure 2P2 (−11.3 kcal/mol), which feasibly undergoes anti−syn isomerization to yield thermodynamically more stable η3-π-syn-allyl species 1P2 (−17.9 kcal/mol). Therefore, the insertion−isomerization pathway (path b) is more kinetically favorable than the successive trans-isoprene insertion (path a) during the chain propagation. It is noted that the structure of

Figure 1. Geometry structures (distances in Å) of optimized cationic scandium alkyl species 1 and 2.

in this figure, 1 and 2 have a β-Si−C agostic interaction,61 as manifested by the Sc−Cγ1 contacts (2.46 Å in 1 and 2.51 Å in 2) and the elongated Si−Cγ1 bond length (1.99 Å in 1 and 1.98 Å in 2) compared to the usual Si−Cγ2 and Si−Cγ3 distances (1.89 Å). It is noted that there is a stronger interaction between Sc and O in 2 (2.02 Å) than in 1 (2.20 Å). Meanwhile, the coordination environment in 2 is more crowded at the metal center than that in 1. Mechanism of trans-1,4-Polymerization of Isoprene Catalyzed by (C5Me4C6H4OMe-o)Sc(CH2SiMe3)]+ (1). To investigate the initiation step of isoprene polymerization, four coordination modes of monomer (cis-re, cis-si, trans-re, and trans-si) are considered for 1-mediated insertion reactions.19,44 The relatively more favorable modes are shown in Figure 2, and the others are shown in Table S1 (see Supporting Information).

Figure 2. Computed free energy profiles for forming trans-1,4-polyisoprene. The energies are relative to species 1 and two molecules of transisoprene. The important corresponding 3D structures (distances in Å) are shown in Figure S1 in the Supporting Information. C


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Organometallics

Figure 3. Computed energy profiles for the formation of cis-1,4-polyisoprene (path c) and 3,4-polyisoprene (path d or e) catalyzed by 1. The energies are relative to species 1 and two molecules of trans-isoprene.

TS2iso shows a backbiting coordination of a CC double bond of growing chain to the metal center, as manifested by the Sc− C contacts (Sc−C2 = 2.89 Å, Sc−C3 = 2.64 Å, Figure S1). This could account for that TS2iso is much lower in energy than TS1iso, indicating an easier anti−syn isomerization in chain propagation than that in the chain initiation. To corroborate such an insertion−isomerization mechanism, density functionals with dispersion corrections have also been used for the calculation of the chain propagation step, representing a polymerization process. As expected, all the results suggest that the pathway of cis-monomer insertion and subsequent anti−syn isomerization is more favorable (Figure S2 in the Supporting Information). It is noteworthy that a 2P1-based monomer coordination complex is kinetically unfavorable to undergo direct insertion (Figure 3) or anti−syn isomerization (Figure S3). To elucidate the origin of the kinetic preference of cismonomer insertion (path b in Figure 2), energy decomposition analyses62,63 have been carried out for the key transition states, viz., 1TS1, 2TS1, 1TS2, and 2TS2. For this purpose, the transition state (TS) structures can be divided into two fragments, viz., the monomer moiety (F1) and the remaining active species (F2). The energies of the fragments at the geometries they have in the transition states are evaluated via single-point calculations. Such single-point energies of the fragments and the energy of the transition state are used to estimate the interaction energy ΔEint. These energies, together with the energy of the respective fragments in their optimal geometry, allow for the estimation of the deformation energies of the two fragments, ΔEdef(F1) and ΔEdef(F2). That is, for instance, the deformation energy of fragment F1 is evaluated as its energy difference between its geometry in the TS structure and its optimized structure. The similar is true for the fragment F2. Such a deformation energy is required to reach the TS. As the energy of TS, ΔEtotal, is evaluated with respect to the energy of the two separated fragments, the relation ΔEtotal = ΔEint + ΔEdef(F1) + ΔEdef(F2) holds. As shown in Table 1, there is a stronger interaction between the two fragments in 2TS1 (−29.2 kcal/ mol) for cis-isoprene insertion. This and the relatively smaller

Table 1. Energy Decomposition (ΔE, kcal/mol) of the Key Transition States

a

TS

ΔEint

ΔEdef(F1)

ΔEdef(F2)

ΔEdefa

ΔEtotal

1TS1 2TS1 1TS2 2TS2 3TS2 5TS2

−27.5 −29.2 −23.0 −24.0 −22.8 −25.0

8.2 6.6 14.8 8.3 14.4 14.5

23.6 21.5 17.3 19.6 21.0 23.4

31.8 28.1 32.1 27.9 35.4 37.9

4.3 −1.1 9.1 3.9 12.6 12.9

ΔEdef = ΔEdef(F1) + ΔEdef(F2).

deformation energy (28.1 vs 31.8 kcal/mol) make 2TS1 have a total energy lower than that with 1TS1 (−1.1 vs 4.3 kcal/mol). Therefore, both stronger interaction and smaller geometrical deformation could account for the stability of 2TS1 in comparison with 1TS1. At the chain propagation stage, the geometrical deformation of isoprene (F1) mainly accounts for the less stability of 1TS2 compared with that of 2TS2 (Table 1). While the steric repulsion plays an important role in the destabilization of the transition state for trans-isoprene insertion (1TS2), the role of orbital and electrostatic interactions could not be excluded, as suggested by the weaker interaction between trans-isoprene and the metal center (interaction energy of −23.0 for the trans case vs −24.0 for the cis case, Table 1). These factors eventually make path b more favorable (Figure 2). For better understanding of the origin of trans-1,4-selective polymerization of isoprene, the formation of cis-1,4- and 3,4polymers has been calculated. Because the cis-isoprene insertion process is kinetically favorable, it is reasonable to model the insertion of an incoming monomer into the η3-π-anti-allyl species 2P1. As shown in Figure 3, continuous insertion of cisisoprene into 2P1 could give the cis-1,4-sequence (path c: 3C2 → 3TS2 → 3P2, C−C bond formation between C4′ atom of the incoming cis-monomer and the C1 atom of the allyl) or 3,4unit (path d: 4C2 → 4TS2 → 4P2, C−C bond formation between C4′ atom of the incoming cis-monomer and C3 atom of the allyl). Certainly, if trans-isoprene inserts into more stable 1P1, the 3,4-unit could be generated (path e: 5C2 → 5TS2 → D


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Figure 4. Computed energy profiles for the isoprene insertion mediated by [(C5Me4SiMe2CH2P(O)Ph2)Sc(CH2SiMe3)]+ (2). Path a2: transisoprene insertion; path b2: cis-isoprene insertion. Some H atoms were omitted for clarity. The energies are relative to species 2 and two molecules of trans-isoprene.

condition30 (Scheme 1). To elucidate the origin of inertness of 2 toward isoprene polymerization, the 2-mediated isoprene insertions have been calculated. As shown in Figure 4, the transisoprene coordinated complex 1C12 is significantly lower in energy than complex 2C12 with a cis-isoprene moiety (8.6 vs 20.1 kcal/mol). This is in contrast to the case of 1, where the coordination of cis-isoprene to the metal center of 1 is slightly more favorable compared with that of the trans-isomer (Figure 2), possibly due to the steric effect resulting in a different coordination manner (η2 for 1C12 vs η4 for 2C12; see optimized structures in Figure S4 in the Supporting Information). To see the stability discrepancy between complexes 2C12 and 1C12, energy decomposition analysis, as methodologically illustrated above, has been performed. The results indicate that the deformation energy of cationic species in 2C12 is larger than that in 1C12 (17.3 vs 11.6 kcal/mol). However, such a larger deformation energy could not be compensated by the favorable interaction energy ΔEint in 2C12 (−14.0 kcal/mol). Even though there is a weaker interaction (ΔEint = −11.8 kcal/mol) between the cationic species and trans-isoprene moiety in 1C12, much smaller deformation of the cationic species (11.6 kcal/mol) led to more stable complex 1C12. The deformation of monomer moieties is similar in the two complexes (1.3 kcal/mol for 1C12 and 0.9 kcal/mol for 2C12). It is noted that, although 1C12 is more stable than other adducts involved in chain initiation (Figures 2 and 4), its corresponding transition state 1TS12 has an energy barrier relatively higher than that for 1TS1 and 2TS1 (Figure 2). This could be interpreted as the steric interaction increased more significantly in 1TS12 in the catalyst 2 system compared with the case of 1 due to the bulky phosphine oxide group of 2. As shown in Figure 4, in the chain initiation step (the first monomer insertion), the trans-isoprene insertion along with path a2 is more favorable both thermodynamically and kinetically than the cis-monomer insertion along with path b2. This trans-monomer insertion leads to η3-π-syn-allyl species 1P12, which could be used for modeling the chain propagation

5P2). However, these pathways are kinetically unfavorable compared with the formation of the trans-1,4-sequence (29.4− 34.3 vs 24.5 kcal/mol, Figures 3 and 2). These results are in good agreement with the experimental observation that trans1,4-polyisoprene was obtained as a major product.30 Other possible insertion modes are also found to be kinetically less favorable (Table S2 in the Supporting Information). It is noteworthy that the C−C bond lengths of the allyl moiety of the five transition structures (TS2, Figures 2 and 3) have different features. If the C3 is involved in the C−C formation giving the 3,4-unit, the bonds between C1, C2, and C3 are more likely to be delocalized. If the C1 is involved in the C−C formation giving the 1,4-unit, the length of the C1−C2 bond is slightly longer and more like a single bond compared with the C2−C3 bond (see Figure S1 and optimized coordinates in the Supporting Information). To cast light on the origin of stereoselectivity, the energy decomposition analysis of the key transition states 3TS2 (cis1,4-unit) and 5TS2 (3,4-unit) was also carried out for a comparison with the case of trans-1,4-unit formation (Table 1). A comparison of 5TS2 with 2TS2 indicates that the interaction energies ΔEint are similar to each other (−25.0 vs −24.0 kcal/ mol), whereas the larger deformation energy in the 5TS2 case (37.9 vs 27.9 kcal/mol, especially caused by monomer deformation) could mainly contribute to the destabilization of 5TS2. The same is true for the case of 3TS2 (Table 1). To sum up, the smaller deformation of the monomer and thus more favorable kinetics for the formation of the trans-1,4 sequence in comparison with the cis-1,4- and 3,4-unit formations and feasible anti−syn isomerization during the insertion of the cis-monomer are most likely to be the major factors governing the stereoselectivity. Origin of Inertness of [{C5Me4SiMe2CH2P(O)Ph2}-Sc(CH2SiMe3)]+ (2) toward Isoprene Polymerization. When the methoxy phenyl side arm of 1 is replaced by a phosphine oxide group SiMe2CH2P(O)Ph2, it could give 2. In this case, no polyisoprene was obtained under the same experimental E


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Organometallics (the second monomer insertion). However, during the chain propagation, neither trans-isoprene nor cis-isoprene feasibly achieves an insertion process due to the energy barrier being as high as 34.7 kcal/mol and the thermodynamic feature (endergonic by more than 2.7 kcal/mol). It is noteworthy that, although the insertion transition states were computationally modeled, the monomer moiety in 1C22 and 2C22 is too far to coordinate with the metal center (Figure S4). This suggests that the sterics in 1P12 with a syn-allyl chain-end hampered the continuous isoprene coordination and insertion and may provide a better understanding to the alternating rather than block copolymerization of isoprene and 1,5hexadiene.30 These results could account for the experimentally observed inertness of 2 toward isoprene homopolymerization. In addition, the energies in Figure 4 suggest that the isoprene monoinsertion product (1P12) is expected to be a future synthetic target mediated by cation 2. The optimized key structures of 1TS22 (Figure 4) and 2TS22 (see Figure S4 in the Supporting Information) indicate repulsions between the polymer chain and the phenyl group of the side arm. To corroborate that the steric hindrance of the ancillary ligand hampered the continuous monomer insertion, the modification of species 2 has been computationally modeled to obtain an analogous species with smaller steric hindrance. For this purpose, various modifications on the substituents of phosphine oxide ligand (2-a, 2-b), the metal center (2-c, 2-d), as well as the cyclopentadiene ligand (2-e, 2-f, 2-g) were considered, as shown in Figure 5.

Table 2. Computed Free Energies (kcal/mol) in Solution for the Chain Propagation of Isoprene Polymerization Catalyzed by Cationic Species 2 and the Analogues 2-i (i = a−g) species

P1a

C2

TS2

P2a

ΔG⧧

ΔG(P2−P1)b

2 2-a 2-b 2-c 2-d 2-e 2-f 2-g

−15.9 −14.6 −15.8 −16.2 −15.9 −17.4 −16.0 −17.3

−9.2 −0.1 −1.0 −1.4 −0.9 −1.7 −4.8 −6.6

18.8 14.4 12.0 11.9 11.9 11.9 7.4 5.8

−13.2 −15.2 −14.1 −18.5 −19.2 −17.8 −19.9 −24.9

34.7 29.0 27.8 28.1 27.8 29.3 23.4 23.1

2.7 −0.6 1.7 −2.3 −3.3 −0.4 −3.9 −7.6

P1 and P2 denote the insertion products of the first and the second monomer, respectively. bΔG(P2−P1) = G(P2) − G(P1), where G(P2) represents the free energy of the second monomer insertion product, and G(P1) denotes the free energy sum of P1 and the corresponding monomer. a

the metal with a larger ion radius such as Y and La (energy barriers of 28.1 kcal/mol for 2-c and 27.8 kcal/mol for 2-d). The values shown in Table 2 also demonstrate that the larger metal center induced thermodynamically a more stable insertion product. What’s more, the smaller cyclopentadienyl ancillary ligand (Cp) in the cationic scandium alkyl species also reduces the olefin insertion barrier (e.g., 29.3 kcal/mol for 2-e vs 34.7 kcal/mol for 2). These results suggest that the steric hindrance played a key role in the isoprene polymerization. Therefore, the activity of the phosphine oxide side arm catalyst could be increased by using a smaller PO- or Cp-based ligand and/or larger metal in size. Certainly, while increasing the catalytic activity via modification of the ligand or metal size, the selectivity and thermal stability of catalysts might be affected.

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CONCLUSION The polymerization of isoprene catalyzed by the cationic halfsandwich scandium alkyl species [(C5Me4C6H4OMe-o)Sc(CH2SiMe3)]+ (1) with a methoxy side arm has been comprehensively studied utilizing density functional theory. It is found that the trans-1,4-specific polymerization prefers an insertion−isomerization mechanism: (i) an insertion of cisisoprene into the metal−alkyl bond to give η3-π-anti-form, (ii) anti−syn isomerization of allyl complex yielding a syn-allyl form, (iii) repetitive cis-isoprene insertion into the metal−syn-allyl bond and anti−syn isomerization. Along with the strong coordination capability of an oxygen atom to the Sc metal, the formation of stable η3-π-syn-allyl insertion product is more kinetically favorable for cis-isoprene insertion. The stability of the key transition state featuring cis-isoprene insertion into synallyl species involved in the most feasible pathway could be ascribed to the smaller deformation of cis-isoprene and stronger interaction between the monomer and remaining metal complex. In the aspect of stereoselectivity, the smaller deformation of monomer and therefore more favorable kinetics for the formation of trans-1,4-sequence compared with other types of enchainment as well as feasible anti−syn isomerization in cis-isoprene insertion are believed to be the major factors accounting for the stereospecific trans-1,4-polymerization of isoprene by 1 with a methoxy side arm. The anti−syn isomerization event could add better understanding to the rare-earth-metal-catalyzed diene polymerizations. The formations of cis-1,4- and 3,4-polymers were also calculated and

Figure 5. 2-based cationic rare-earth-alkyl species with smaller steric hindrance.

The calculated free energies for chain propagation of isoprene polymerization catalyzed by cationic species 2 and the analogous 2-i (i = a−g) are shown in Table 2, whereas those for chain initiation via the most favorable insertion modes are shown in Table S3. As shown in Table 2, the free energy barriers reduce gradually with smaller substituents on the phosphine oxide ligand (PO ligand, from 34.7 kcal/mol for 2 to 29.0 kcal/mol for 2-a and to 27.8 kcal/mol for 2-b). Additionally, the catalyst activity could be improved by using F


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Organometallics

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found to be kinetically unfavorable compared with the formation of the trans-1,4-sequence observed experimentally. In addition, an analysis of the energy profile disclosed that the origin of the inertness of an analogous complex [(C5Me4SiMe2CH2P(O)Ph2)-Sc(CH2SiMe3)]+ (2) toward isoprene polymerization could be the steric hindrance in the resulting η3-π-syn-allyl species, which made the insertion of the second monomer infeasible due to high energy barrier and endergonic character. The computational modifications of the ancillary ligand and metal center of 2 indicate that smaller steric hindrance induced higher activity and therefore demonstrated again the important role of sterics in the activity of 2-mediated isoprene polymerization.

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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.7b00876. Optimized structures of key stationary points, Gibbs freeenergies in solution for other possible insertion manners (PDF) Optimized Cartesian coordinates of all stationary points together with their single-point energy in solution and the imaginary frequencies (cm−1) of transition states (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhaomin Hou: 0000-0003-2841-5120 Yi Luo: 0000-0001-6390-8639 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Nos. 21174023, 21429201, 21674014). Y.L. thanks the Fundamental Research Funds for the Central Universities (DUT2016TB08). The authors also thank RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of Dalian University of Technology for part of computational resources.

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REFERENCES

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Theoretical Investigations of Isoprene Polymerization Catalyzed by

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