DFT Studies on Isoprene/Ethylene ... - Wiley Online Library

FULL PAPER DOI: 10.1002/cjoc.201600673

DFT Studies on Isoprene/Ethylene Copolymerization Catalyzed by Cationic Scandium Complexes Bearing Different Ancillary Ligands Xiaojie Hu,a Xiaohui Kang,c Guangli Zhou,a Xingbao Wang,a Zhaomin Hou,*,a,b and Yi Luo*,a a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, China b Organometallic Chemistry Laboratory and Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan c College of Pharmacy, Dalian Medical University, Dalian, Liaoning 116044, China The isoprene/ethylene copolymerization catalyzed by cationic rare earth metal complexes [(5-C5Me5)Sc(CH2SiMe3)]＋ (A) had afforded alternating isoprene-ethylene copolymer with rich 3,4-polyisoprene microstructures, whereas no isoprene-ethylene copolymer was observed by using analogous [(PNPPh)Sc(CH2SiMe3)]＋ (B) under the same conditions. Theoretical calculations in this work have revealed that, in the case of A, successive 3,4-insertion of isoprene resulted in a noncovalent interaction between the C＝C double bond of penultimate unit and the metal center, suppressing the further insertion of monomers due to higher energy barrier and endergonic character. On the other hand, the ethylene pre-inserted species with alkyl active site is more suitable for the subsequent kinetically and thermodynamically favorable isoprene insertion and copolymerization is therefore realized. In the case of B, the experimentally observed cis-1,4-specific homopolymerization of isoprene was the outcome of both kinetic and thermodynamic control. And, the unfavorable ethylene insertion into the isoprene pre-inserted species with allyl active site could account for the experimental finding that no isoprene-ethylene copolymer was obtained. These computational results are expected to provide some hints for the design of rare earth copolymerization catalysts. Keywords

DFT, ethylene/isoprene copolymerization, rare earth metal

Introduction The development of single-site monoolefin/diene copolymerization catalysts to precisely control the microstructure of the copolymer products can create new opportunities for the preparation of novel polymer materials with desirable properties in industrial applications, and has therefore received significant interest.[1-3] Further post-functionalization of the remaining C＝C double bonds of the diene unit in such copolymers can introduce a polar group or reactive site into the copolymer backbone, affording a broad range of new functionalized copolymers with improved properties including solubility, dying, acidity, and surfactivity.[3] The amount and the configuration (E or Z) of the unsaturated groups are key factors that require to be controlled during the polymerization reaction.[4,5] The copolymerization of monoolefin and conjugated dienes is highly challenging since each monomer typically requires different polymerization conditions.[6-8]

Most catalysts designed for monoolefin homopolymerization are inefficient toward conjugated diene polymerization, and vice versa. This is possibly originated from the involvement of two distinctly different active sites. In the field of monoolefin/diene copolymerization, many efforts have been made for the copolymerization of butadiene with ethylene.[6-12] However, there are few examples about the copolymerization of isoprene with ethylene.[2,3,12a,13] Hou and coworkers[2] reported ethylene/isoprene copolymerization reactions catalyzed by cationic halfsandwich scandium alkyl complexes bearing various cyclopentadienyl ligands generated from the dialkyl precursors. These cationic half-sandwich scandium alkyl catalyst systems can exhibit cis-1,4- or 3,4-selective copolymerization of isoprene with ethylene and yield the corresponding random or alternating copolymer, which is different from the ansa-neodymocene allyl catalyst yielding the isoprene-rich (76%) isoprene-

* E-mail: [email protected], [email protected]; Tel.: 0086-0411-84986192 Received October 13, 2016; accepted January 5, 2017; published online April 28, 2017. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc. 201600673 or from the author. In Memory of Professor Enze Min. Chin. J. Chem. 2017, 35, 723—732

© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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ethylene copolymer with predominant trans-1,4-polyisoprene microstructures.[13] Notably, it was stated that the catalyst system of (5-C5Me5)Sc(CH2SiMe3)2(THF) (A')/ [Ph3C][B(C6F5)4] afforded for the first time alternating isoprene-ethylene copolymers with 3,4-rich polyisoprene microstructures (Scheme 1).[2] In contrast, the use of the analogous non-metallocene Sc or Y complexes bearing an ancillary (phosphinophenyl) amido (PNPPh)[14] or amidinate ligand,[15] which had shown excellent regio- and stereoselectivity for the homopolymerization of isoprene, yielded only a mixture of the homopolymers under the same conditions, whereas an isoprene-ethylene copolymer product was not observed.[1a,14] The catalyst system of (PNPPh)Sc(CH2SiMe3)2(B')/[Ph3C][B(C6F5)4] had shown excellent cis1,4-selectivity for the homopolymerization of isoprene.[14] These two systems provided a good opportunity to comparably investigate the origin of monoolefin/diene copolymerization catalyzed by rare earth metal complexes. However, as far as we are aware, the mechanism of copolymerization of isoprene with ethylene has not been reported previously, although such mechanisms are fundamentally crucial for development of new copolymerization catalysts. Scheme 1 The (co)polymerization of ethylene and isoprene catalyzed by scandium complexes

Sc Me3SiH2C A' (

THF CH2SiMe3

5-C5Me5)Sc(CH2SiMe3)2(THF)

[Ph3C][B(C6F5)4]

n

Chlorobenzene, r.t.

3,4-isoprene-ethylene alternating copolymer

N +

Ph2P

Sc

Me3SiH2C

PPh2 CH2SiMe3

B' (PNPPh)Sc(CH2SiMe3)2 [Ph3C][B(C6F5)4] Chlorobenzene, r.t.

no copolymer, but a mixture of homopolymers

Numerous computational studies have been widely and successfully conducted to investigate the mechanism of olefin polymerization or other reactions catalyzed by group 4 and late transition-metal complexes.[12,16-24] Our group computationally studied the styrene/ethylene copolymerization catalyzed by the cationic half-sandwich scandium alkyl species (η5-C5Me5)Sc＋ (CH2SiMe3) and trans-1,4-specific polymerization of isoprene catalyzed by La-Al binuclear complexes.[25] Maron and coworkers computationally investigated olefin/butadiene copolymerization catalyzed by a hemi724

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lanthanidocene and found that the copolymerization is mainly under thermodynamic control.[12a] Perrin et al. recently reported a relationship of ligand-structure/activity/polymer-microstructure according to DFT calculations of ethylene-butadiene copolymerization catalyzed by neodymocene complexes.[12b] Encouraged by these results, in the present work, we have conducted computational studies on the isoprene/ethylene copolymerization mediated by the half-sandwich cationic Sc ＋ complexes [(5-C5Me5)Sc(CH2SiMe3)] (A) generated in situ from A'/[Ph3C][B(C6F5)4] and the non-metallo＋ cene Sc complexes [(PNPPh)Sc(CH2SiMe3)] (B) generated in situ from B'/[Ph3C][B(C6F5)4] (Scheme 1). The purpose of this work is to reveal the origin of isoprene-ethylene copolymerization catalyzed by cationic species A and why no copolymer was obtained in the case of B. We hope that this work will contribute to the development of more efficient and selective catalysts for the copolymerization of isoprene and ethylene.

Computational Methodology All calculations were performed with Gaussian 09 program.[26] The B3PW91 functional[27-29] was used for geometry optimizations. The 6-31G* basis set was used for C, H, and N atoms, and the Sc, P, and Si atoms were treated by the Stuttgart/Dresden effective core potential (ECP) and the associated basis sets.[30] 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, 5 valence electrons of P atom, and 11 valence electrons of Sc were treated by the optimized basis sets, viz., (4s4p)/[2s2p] for Si atom, (4s4p1d)/[2s2p1d] for P atom, and (8s7p6d1f)/ [6s5p3d1f] for Sc, respectively. One d-polarization function was augmented for the basis sets of Si (exponent of 0.284) and P (exponent of 0.387) atoms, respectively.[31] Each optimized structure was subsequently analyzed by harmonic vibrational frequencies at the same level of theory and was characterized as a minimum (Nimag＝0) or a transition state (Nimag＝1). To obtain more reliable energy, single-point energy calculations were carried out on the basis of optimized geometries. In such single-point calculations, the M06 functional[32] together with the larger basis set 6-311＋G** for C, H, and N atoms and the Stuttgart/Dresden ECP as well as associated basis sets for remaining atoms was utilized. The chlorobenzene solvation effect was considered in such single-point calculations by using the SMD model.[33] Actually, a strategy[34] of B3PW91 optimization in solution (SMD model) plus D3BJ correction has been tested for crucial steps in this study and the result is in line with that derived from M06(SMD)// B3PW91 (Table S1). For some important stationary points, calculations at the levels of M06(SMD)// B3PW91-D3BJ and MP2//B3PW91 have been also carried out for energy comparisons.


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DFT Studies on Lsoprene/Ethylene Copolymerization

Results and Discussion Structures of cationic active species A and B Two cationic scandium alkyl species [(5-C5Me5)Sc＋＋ (CH2SiMe3)] (A) and [(PNPPh)Sc(CH2SiMe3)] (B) have been optimized. It is noteworthy that, in the case of A, the effect of THF was not considered here since THF was previously proposed to dissociate from metal center during olefin polymerizations catalyzed by rare earth metallocene complexes.[35] Geometrically, A shows a β-agostic Si－C…Sc interaction as suggested by the Sc…C2 distance of 2.41 Å, Si－C2 contact of 2.02 Å, and Sc－C1－Si angle of 90.7°. Similarly, B also shows a β-agostic Si－C…Sc interaction as manifested by the Sc…C2 distance of 2.60 Å, Si－C2 contact of 1.97 Å, and Sc － C1 － Si angle of 95.8° (Figure 1). The β-agostic Si－C…Sc interaction in B is weaker than that in A, as suggested by the Sc…C2 distances (2.60 vs. 2.41 Å). On the basis of the optimized structures of A and B, the copolymerization of isoprene with ethylene has been investigated, respectively.

performed according to the experimental information. That is, species A afforded almost alternating isoprene-ethylene sequences with 3,4-rich polyisoprene microstructures and species B gave cis-1,4-polyisoprene (96.5%) as a homopolymer under the same conditions.[2,14] The main pathways to be calculated are shown in Scheme 2c. It is generally considered that the polymerization of conjugated diene catalyzed by transition metal complexes follows η3--allyl insertion mechanism.[38] Based on species IP (Scheme 2b), the subsequent isoprene insertion into C1 and C3 sites of metal-allyl may give rise to 1,4- and 3,4-polyisoprene, respectively, and the subsequent ethylene insertion into two reactive sites C1 and C3 sites of metal−allyl may yield cis-1,4-I-alt-E unit and 3,4-I-alt-E copolymer, respectively. Scheme 2 The Cossée-Arlman mechanism and reaction landscape of the possible insertions during ethylene/isoprene copolymerization in the presence of species A and B

Figure 1 Geometry structures (distances in Å and angles in degree) of cationic scandium alkyl species A and B. (All H atoms were omitted for clarity.)

Reaction landscape of isoprene/ethylene copolymerization The computations were performed according to the Cossée-Arlman mechanism.[36,37] As shown in Scheme 2a (taking ethylene insertion as an example), one monomer initially approaches the metal center to form π-complex (C) in a η-π coordination mode (cis-η4-π or trans-η4-π for isoprene and η2-π for ethylene). The subsequent insertion of the coordinating monomer into the Sc － CH2SiMe bond of active species gives corresponding insertion product (P) via a four-center transition state (TS). There are two types of active site: alkyl (typically yielded by ethylene insertion, such as EP, Scheme 2b) and -allyl (generated by isoprene insertion, such as IP). It is noted that the initial species A and B could be viewed to have alkyl active sites. In order to investigate the mechanism of ethylene/isoprene copolymerization, the computations were Chin. J. Chem. 2017, 35, 723—732

Insertion of the ethylene has no regio- or stereo-selectivity due to the absence of substituents on two carbon atoms. However, according to the stereoisomers, the insertion of isoprene has trans-1,4 or cis-1,4 coordination manners. Moreover, depending on the stereochemistry, isoprene insertion could occur at re-face or si-face.[19b] Therefore, four different modes were considered for isoprene insertion in the chain initiation step (Scheme 3). It is noted that the 4,1-insertion is kinetically less favorable compared with corresponding 1,4-insertion manner (see Table S2). In the first part of this study, calculations have been performed to clarify stereoselectivity of isoprene insertion catalyzed by species A. As shown in Figure 2, isoprene insertion is exergonic by 9.8－18.4 kcal/mol. The calculated activation barrier for the si-face insertion of cis-isoprene is lower than those for other three insertion modes (ATS1cis-re, ATS1trans-si, and ATS1trans-re). Such a



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situation was also reported in a previous theoretical study on cationic systems.[35b] In view of the result that the formation of AP1cis-si is more kinetically favorable compared with AP1trans-si, the allyl complexes AP1cis-si could be more suitable for chain propagation.[39] It is noteworthy that AP1trans-si is thermodynamically more stable than AP1cis-si, which could account for the formation of trans-1,4 unit observed experimentally.[2] Scheme 3 Four different modes for isoprene coordination to metal center of active species

To elucidate the origin of the kinetic preference of TS1cis-si, the energy decomposition analyses of ATS1cis-si, A TS1cis-re, ATS1trans-si, and ATS1trans-re were performed by the distortion/interaction theory.[40] In the distortion/ interaction theory, the activation energy (ETS) is divided into two main components: the deformation (Edef) and interaction (Eint) energies. As depicted in Figure 3, each transition state was divided into two fragments (F1 and F2). The energies of the fragments ＋ [(5-C5Me5)Sc(CH2SiMe3)] (F1) and isoprene (F2) in the four TSs geometries were evaluated by single-point calculations. The single-point energies of TSs and their corresponding fragments were used to estimate the inteaction energy Eint. The energy difference between the distorted fragment and its corresponding optimized ground state structure is the deformation energy [i.e., Edef(F1)/Edef(F2)]. The relationship ETS ＝Eint ＋ Edef holds. And the calculated results are summarized A

22.8

2.29 3

13.6

4

13.1 A TS1

2 3

4

7 1.41 2 .1

2.23 A TS1tr ans-si (13.6 kcal/mol)

1.4 A 1 C

1

2.36

2

A TS1tr ans-r e (22.8 kcal/mol)

Sc Si C

1

-9.8

3 2.46 4

2

2.64

4 2.5

1

4

1.3 8

A + trans-isoprene

1

2.3 5

20.9

2.1 0.0

At the chain initiation stage of ethylene/isoprene copolymerization, competitive insertions of ethylene and isoprene into the cationic species A are computed. Then, the competitive insertions of two monomers into each preinserted catalytic species, viz., AP1E (ethylenepreinserted species) and AP1I (isoprene-preinserted species) were also calculated for modelling chain propagation.

2.2 0

8.1 5.8

Isoprene/ethylene copolymerization catalyzed by cationic species A

1.4 0

Gsol (kcal/mol) trans-isoprene (re) trans-isoprene (si) cis-isoprene (re) cis-isoprene (si)

in Table 1. A comparison of the deformation/interaction energies indicates that there is almost no difference in Eint values for these four transition states (Table 1), and the Edef(total) value of 8.9 kcal/mol in ATS1cis-si is significantly lower than those in other TSs (15.8, 14.2, and 20.4 kcal/mol). Therefore, the ETS value of 8.6 kcal/mol for ATS1cis-si is lower than those for the others (1.6, 5.1, and 3.2 kcal/mol). These results suggest that the insertion of isoprene into species A in cis-1,4-si fashion needs a lower deformation energies in comparison with other three modes. This could be the dominant factor governing the stereoselectivity of isoprene insertion. For the stereoselectivity of isoprene insertion catalyzed by species B, the free energy profiles are shown in Figure 4. Like the case of species A, the si-face insertion of cis-isoprene is the most kinetically favorable insertion mode. The energy decomposition analyses of B TS1cis-si, BTS1cis-re, BTS1trans-si, and BTS1trans-re are summarized in Table 2. The Edef(total) value of 7.9 kcal/mol in BTS1cis-si is also significantly lower than those in other TSs. It is also found that the deformation, especially for that caused by isoprene, is the dominant factor determining the stereoselectivity of isoprene insertion.

2 3

1.40 2.29

-16.4 -18.0 -18.4

A 1

P

A TS1cis-si (13.1 kcal/mol)

A TS1cis-r e (20.9 kcal/mol)

Figure 2 Computed energy profiles for isoprene insertion into the Sc-CH2SiMe bond of species A through different coordination modes. Free energies are relative to the energy sum of trans-isoprene and A. (All H atoms were omitted for clarity.) 726



Chin. J. Chem. 2017, 35, 723—732


Chain initiation

F1

C3 C2

F2

Si A

TS1cis-si

Figure 3 The fragments [(5-C5Me5)Sc(CH2SiMe3)]＋(F1) and isoprene (F2) in ATS1cis-si geometry. Table 1 Energy decomposition analyses (kcal/mol) for the key transition states involved in A-mediated isoprene insertion

cis-si

17.5

A

1

cis-re

TS

2.8

Edef(total) ETS a

6.1

8.6

8.9

17.5

2.2

13.6

15.8

1.6

A

TS1trans-si 19.3

3.8

10.4

14.2

5.1

A

TS1trans-re 17.2

6.8

13.6

20.4

3.2

TS

a

ETS ＝ Eint ＋ Edef(total), where Edef(total) ＝ Edef(F1) ＋ Edef(F2). Table 2 Energy decomposition analyses (kcal/mol) for the key transition states involved in B-mediated isoprene insertion Eint Edef(F1) Edef(F2)

TSs

Edef(total) ETS a

B

1

TS

cis-si

16.9

4.8

3.1

7.9

9.0

B

TS1cis-re

16.7

7.2

4.4

11.6

5.1

B

1

TS

trans-si

16.8

6.0

10.1

16.0

0.8

B

TS1trans-re

19.0

7.7

9.4

17.1

1.9

a

ETS ＝ Eint ＋ Edef(total), where Edef(total) ＝ Edef(F1) ＋ Edef(F2).

7.8

Active species

(Gj‡ ‒ Gi‡)/ (kcal•mol1)

r1

A

0.4

A 1

P

cis-si A-I 2 PE

16.0 14.3

2.49

10.1 TS1

－

－

0.8

－

2.59×101

0.5

4.30×101

－

2

1

2.82

4

2 3

1.11×101 b

r2＝

1.4 0

2.25 4

-16.4 -16.4 -17.8 B 1 P

1 2 3

B

TS1tr ans-si (16.1 kcal/mol)

1

2.43

B TS1cis-r e (14.3 kcl/mol)

Sc Si P Ph N C

1

3 2.3

-15.1

2.3 6

3 4

2.56

B TS1cis-si (10.1 kcal/mol)

1.6 C

B 1

2.2 3

B + trans-isoprene

5.09×101

16.1

1.6

0.0

r1·r2

r1＝kEE/kEI, ki/kj＝exp((Gj‡ ‒ rGi‡)/RT), T＝298 K. kII/kIE, ki/kj＝exp((rGj‡ ‒ rGi‡)/RT), T＝298 K.

B

7.5

r2

a

2.19

Gsol (kcal/mol) trans-isoprene (re) trans-isoprene (si) cis-isoprene (re) cis-isoprene (si)

Table 3 The difference of insertion barriers between ethylene and isoprene and kinetic statistic model based on reactivity ratios r1 a and r2 b.

1.37

Eint Edef(F1) Edef(F2)

1

2.25

TSs A

3

2. 28

2

1.40

C1

2.2 1

C4

1.36

Sc

In the ethylene/isoprene copolymerization, the competitive insertions of ethylene and isoprene into Sc–CH2SiMe bond of species A are calculated. As shown in Figure 5, although the cis-1,4-si coordination of isoprene is less favorable than that of ethylene, the subsequent isoprene insertion requires slightly lower energy barrier (13.1 kcal/mol) compared to that (13.5 kcal/mol) of ethylene insertion. Thermodynamically, isoprene insertion is also more significantly favorable than ethylene insertion. It is noted that the initial species A can be viewed as an alkyl active site and therefore serves as the propagating species of ethylene insertion. The calculated activation barriers of this step were employed for copolymerization kinetic mode analysis based on Markov chain statistics.[12b] In this model, ethylene and isoprene reactivity ratios are defined as r1＝ kEE/kEI and r2＝kII/kIE, respectively. Ratio of kinetic constants are computed according to Eyring equation ki/kj＝ exp((Gj‡－Gi‡)/RT) evaluated at T＝298 K. The calculated reactivity ratios (r1＝kEE/kEI, Table 3, Entry 1) is less than 1, which means that the insertion of isoprene

4

B TS1tr ans-r e (16.0 kcal/mol)

Figure 4 Computed energy profiles for isoprene insertion into the Sc-CH2SiMe bond of species B in different insertion modes. Free energies are relative to the energy sum of trans-isoprene and B. (All H atoms were omitted for clarity.) Chin. J. Chem. 2017, 35, 723—732



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2.88 2.77

13.5 13.1 A TS1

2.13

1

2

2

8 1.5

A TS1E (13.5 kcal/mol)

A 1 C E (2.8 kcl/mol)

A 1

C

1

1.55

2

2.33

2.8

A 1 P E (-2.3 kcal/mol)

Sc Si C

-2.3

A + trans-isoprene or ethylene

7 2.6

-9.8

The 1st monomer insertion

3 2

4

3.11

A 1

P

A 1 C cis-si (8.1 kcal/mol)

2.46 3

2.80

1

2.67

1.3 8

0.0

2.33

1

1.3 9

Gsol (kcal/mol) cis-isoprene (si) ethylene 8.1

4

2.45

2

1

1

2.37

2 3

2.31

4

2.54 A TS1cis-si (13.1 kcal/mol)

A 1 P cis-si (-9.8 kcal/mol)

Figure 5 Computed energy profiles for the chain initiation of ethylene with isoprene polymerization catalyzed by cationic species A. Free energies are relative to the energy sum of isolated free reactants.

into Sc‒alkyl complex A is more favored than ethylene insertion. Chain propagation For the chain propagation, the coordination of ethylene to Sc‒-allyl complex AP1cis-si is more stable (–10.5 kcal/mol) than that of isoprene (–5.2 kcal/mol, Figure 6), suggesting that ethylene complexation is more favorable than isoprene complexation. The insertion of ethylene into AP1cis-si overcomes a slightly lower energy barrier (8.6 kcal/mol) than that (9.4 kcal/mol) for isoprene insertion. The calculated reactivity ratios (r2＝ kII/kIE, Table 3, Entry 2) is also less than 1, i.e. kII＜kIE, which indicates that the insertion of ethylene into Sc–allyl complex AP1cis-si is favorable compared with the isoprene insertion. Meanwhile, the insertion of the second monomer into Sc‒alkyl complex AP1E was also calculated. The energy profiles (Figure S2) indicate kinetic priority for isoprene insertion, yielding -allyl chain end, suggesting again the copolymerization feature. The copolymers produced by species A had mostly alternating isoprene-ethylene sequences with 3,4-rich polyisoprene microstructures, which implies that kEE＜ kEI and kII＜kIE. Actually, the calculated r1＝kEE/kEI and r2＝kII/kIE are less than 1, suggesting a copolymerization event. Along with this way, the insertion of the third monomer into A-IP2E with alkyl active site was further calculated for better understanding of such a copolymerization. As shown in Figure 6, like the case of species A (Figure 5), the insertion of isoprene into A-IP2E has slightly lower energy barrier (11.4 kcal/mol)[41] compared to ethylene insertion (11.9 kcal/mol). The calculated reactivity ratio r1＝kEE/kEI is also less than 1, i.e. kEE＜kEI, which also means that the insertion of isoprene into the propagating species (Sc‒alky site) is more favorable than ethylene insertion. According to copoly728


merization statistics, r1•r2 tends to be 0, (Table 3) in propagation, which demonstrates again an alternating character with a predominance of 3,4-I-alt-E unit. Thus, like the species AP1cis-si with allyl active site, the resulting A-IEP3I could be suitable for effective ethylene insertion, and the chain could be propagated. This is in good agreement with the experimental observation that the copolymer was produced by species A. Considering the experimental finding that the copolymerization activity was much higher than that of homopolymerization of isoprene, successive insertion of isoprene into Sc‒-allyl complex A-IP2I has been also calculated. The result indicates that the insertion of either ethylene or isoprene into the A-IP2I needs to overcome a relatively higher energy barrier (25.8 kcal/mol for ethylene insertion and 37.3 kcal/mol for isoprene insertion) and is a significantly endergonic process (Figure S3). This suggests that the ethylene/isoprene copolymerization is more favorable than isoprene homopolymerization. A structural analysis could give us better understanding of this result. In the 3,4-insertion intermediate A-IP2I, there is a backbiting coordination of the C＝C double bond of penultimate unit (marked in red, Figure S3) to the metal center. A-IP2I is actually the ligand-to-metal encapsulated thermodynamically stable product, which has a relative energy of 25.7 kcal/mol. The backbiting coordination still exists in A-IITS3E and A-II TS3I (Figure S3). The removal of backbiting coordination of the C＝C double bond of the penultimate unit to the metal center being the rate-determining step of the conjugated diene polymerizations with the CpTiCl3-MAO catalyst was previously reported.[42] The noncovalent interaction between the C＝C double bond of penultimate unit and the metal center in complex A-I 2 P I is disadvantageous for further insertion of monomers into allyl complex A-IP2I, which could be the rate-determining step but might be not impossible.


Chin. J. Chem. 2017, 35, 723—732

DFT Studies on Lsoprene/Ethylene Copolymerization Cp*

Gsol (kcal/mol) cis-isoprene ethylene Cp* Sc 1

3

2

A 1

P

4

Sc

1

A-I 2

C

Cp*

Sc

2

3

1

4

R

1 2

Sc

3

3 4

2

2

1

1

P

2

Sc 3 4

P

4

R R' =

-15.3

E

A-I 2

R The 2nd monomer insertion

Cp*

Cp*

-25.7 1 2

-3.4 TS3

A-II

-9.5

A-I 2

Cp*

R' -3.1

4

R'

4

3 4

2

3

1

2

1

1

2

3

R -14.5

1

3

R = CH2SiMe3

1

C Cp*

Sc

Sc

A-II 3

R

4

R

2

3 4

4

Cp*

1 2

Sc

1

Cp*

3 2

Cp*

Sc 2

Sc

-1.9 A-I TS2

-10.5

cis-si

Cp*

R -0.4

1

R -5.2 -9.8

1

3 4

2 2

Cp*

2

Sc

1

1

1

1 2

2

R'

2

Sc

R'

-23.6 A-II 3

P

1 I

1 2

2

E

Cp*

R' Both kinetically and thermodynamically unsuitable for the 3rd monomer insertion

2

Sc

1

-36.5

3 4 1

A-II 3

P

I

2

R' The 3rd monomer insertion

Figure 6 Computed energy profiles for the chain propagation of the monomer polymerization catalyzed by cationic species A. Free energies are relative to the energy sum of the isolated free reactants (energy of trans-isoprene was used).

Isoprene/ethylene copolymerization catalyzed by cationic species B Chain initiation The free energy profiles for B-mediated monomer insertion has been also calculated (Figure 7) to find the reason why species B could not produce copolymer but homopolymer instead. As shown in Figure 7, like the case of A, the ethylene coordination to B is also more favorable than isoprene. However, the free energy barrier (10.1 kcal/mol) for cis-isoprene insertion is lower than that (15.7 kcal/mol) for ethylene insertion. Furthermore, the isoprene insertion product is more stable (–17.8 vs. –5.4 kcal/mol). Therefore, the isoprene insertion into alkyl complex B forming -allyl complex BP1cis-si is more favorable in both kinetics and thermodynamics than ethylene insertion yielding alkyl complex BP1E. This result suggests that the EI sequence is more likely to form than the EE sequence at the chain initiation stage. Chain propagation At the chain propagation stage, although the difference in the insertion energy barriers between ethylene and isoprene is lower than that for insertion of the first monomer, the insertion of ethylene into Sc‒-allyl complex BP1cis-si is endergonic by 1.4 kcal/mol (Figure 8). Such an endergonic character is also suggested by the M06(SMD)//B3PW91-D3BJ calculation (endergonic by 2.6 kcal/mol, Table S3) and MP2//B3PW91 calculation (endergonic by 3.4 kcal/mol, Figure S4). More importantly, MP2 results indicate that the copolymerization is both kinetically and thermodynamically less favorable than homopolymerization of isoprene (Figure S4). This could be one reason why isoprene/ethylene copolymerization was not observed in the case of B. However, the process of isoprene inserChin. J. Chem. 2017, 35, 723—732

tion into Sc‒-allyl complex BP1cis-si needs to overcome a barrier of 24.4 kcal/mol and is exergonic by 2.2 kcal/mol. It is obvious that such an insertion process is more thermodynamically favorable in comparison with ethylene insertion. This is in agreement with the experimental observation that cis-1,4-polyisoprene as a homopolymer was obtained in the case of B. To corroborate this result, the insertion of the third isoprene molecule into allyl complex B-IP2I was also further calculated. The activation energy barrier of isoprene insertion is 17.5 kcal/mol, which is lower than that computed for the second isoprene insertion into BP1cis-si. This reaction is also exergonic by 6.3 kcal/mol. Both kinetically and thermodynamically, the second and third isoprene insertions are feasible. This is in good agreement with the experimental observation of the formation of cis-1,4 polyisoprene. In addition, the third ethylene insertion into allyl complex B-IP2I was also investigated. According to our calculations, the insertion of ethylene into the allylic system is less thermodynamically favorable compared to the isoprene insertion. The corresponding MP2 results still show that the process of ethylene insertion into Sc‒-allyl species B-IP2I is both kinetically and thermodynamically less favorable than isoprene insertion (Figure S4). This could account for the experimental observation that no ethylene/isoprene copolymer was obtained in system B.

Conclusions The isoprene/ethylene copolymerization catalyzed ＋ by cationic species [(5-C5Me5)Sc(CH2SiMe3)] (A) and ＋ Ph [(PNP )Sc(CH2 SiMe3 )] (B) has been theoretically



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1

1

2.2 9

2.81

2

2.13 1.39

2

3.5 B 1

C

B + trans-isoprene or ethylene

1.56

Sc Si P Ph N C

B 1

B

TS1E (15.7 kcal/mol)

C E (3.5 kcal/mol)

B 1

P E (-5.4 kcal/mol)

-5.4

1

2

2.67 2.82 2.81

B 1

P

The 1st monomer insertion

3 4

B 1 C cis-si (7.8 kcal/mol)

2.49 1

2

2

.82 4

3

2.34

1.36

2.6 8

-17.8

2.19

0.0

1

1.55

2

32 2.

TS1

7.8

75 2.

10.1 B

2.17

15.7

Gsol (kcal/mol) cis-isoprene (si) ethylene

2

1 4

B TS1cis-si (10.1 kcal/mol)

2.41

3

2.39

B 1 P cis-si (-17.8 kcal/mol)

Figure 7 Computed energy profile for the chain initiation catalyzed by cationic species B. Free energies are relative to the energy sum of isolated free reactants (All H atoms were omitted for clarity). PNPPh

Gsol (kcal/mol) cis-isoprene ethylene

2 3 4

PNPPh

4

B-I

-1.8

R

2

B-I 2

C

B 1

P

2 3

cis-si

PNP

3

2

1

3

PNP R

2

1

Sc 4 3

4

PNPPh

2

R

4 3

1

2

Sc

R

3

1 2

1 1

-11.3 B-II 3 C

B-I 2 P E

PNPPh

2

-20.0 B-I 2 P I

-2.5 TS3

B-II

-16.4

Sc 2

4

1

3

1

2

4

R' = nd

The 2

monomer insertion

4

P

2

3 1

3

1

B-II 3

Sc

2

2

R' -24.3

1

4

R

2

PNPPh

3 4

Sc

1 3

1

R'

R'

R = CH2SiMe3

4

PNPPh

PNPPh Sc

2

2

1

1

4

3

-11.2

2

4

4

R'

1

3

R' -1.4

2 1

4

Ph

R

1

4

Ph

Sc

Sc

Sc

2

2 1

2

1

TS2

Sc

3

PNPPh -17.8

PNPPh

PNPPh

-7.0

Sc

1

5.9

4

1

PNPPh

4

6.6

3

1

3

R

2 3

1

1

2

Sc

2

Sc

3 4

E

-26.3 B-II 3 P I

R'

R rd

The 3

monomer insertion

Figure 8 Computed energy profiles (kcal/mol) for the chain propagation of ethylene with isoprene polymerization catalyzed by cationic species B. Free energies are relative to the energy sum of the isolated free reactants (energy of trans-isoprene was used).

studied, respectively. To the best of our knowledge, it is the first example of computational study on isoprene/ ethylene copolymerization by using realistic model. In the both systems, the coordination of cis-isoprene in si-face manner is more favorable than other possible fashions, and the deformation is the dominant factor governing the stereoselectivity. In the case of A, alternative kinetic priority for the insertion of two monomers could achieve copolymerization, as suggested by the kinetic model analysis. In addition, lower coordination energy of ethylene and more stable isoprene insertion product could also thermodynamically account for the 730


observed copolymerization event. However, successive isoprene insertion leads to a backbiting coordination of the C＝C double bond of penultimate unit to the metal center in the resulting 3,4-insertion intermediate, a ligand-to-metal encapsulated thermodynamically stable product, which makes the further insertion of monomers into Sc‒-allyl complexes less favorable due to higher energy barrier and endergonic characters. In the case of B, DFT calculations suggest that the cis-1,4-insertion of isoprene has significant priority in thermodynamics compared with ethylene insertion. Higher-level (MP2) calculations suggest both kinetic and thermodynamic


Chin. J. Chem. 2017, 35, 723—732


priority for isoprene insertion. The unfavorable ethylene insertion into the isoprene pre-inserted species with allyl active site could account for the experimental finding that no copolymer was obtained in the case of B. This study could add better understanding to the ethylene/ isoprene copolymerization mechanism, and could be helpful for designing effective rare earth copolymerization catalysts.

Supporting Information The computed energy profile for the insertion of ethylene or isoprene into AP1trans-si, the insertion of ethylene or isoprene into alkyl complex AP1E, and the insertion of ethylene or isoprene into A-IP2I, the 3D structures of A-IP2I, A-IITS3E, and A-IITS3I, tables providing the computed relative free energies of the crucial steps tested at a strategy of B3PW91 optimization in solvent (SMD model) plus D3BJ correction and the stationary points BP1I and BP2E at the M06(SMD)//B3PW91-D3BJ level, the figure providing the computed relative free energies of some important stationary points at MP2// B3PW91 level, the optimized Cartesian coordinates together with energies, and the imaginary frequencies of TSs. This material is available free of charge via the Internet at http://dx.doi.org/.

Acknowledgement This work was partly supported by the NSFC (Nos. 21174023, 21429201, 21674014) and a Grant-in-Aid for Scientific Research (S) from the JSPS (No. 26220802). The authors also thank RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of Dalian University of Technology for computational resources.

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needs to overcome a free energy barrier of as high as 36.1 kcal/mol (see Figure S1). However, the insertion of ethylene into AP1cis-si, of which the formation is more kinetically favorable compared with A 1 P trans-si, overcomes a free energy barrier of 8.6 kcal/mol (see Figure 6). These results suggest that the species AP1cis-si rather than AP1trans-si could be involved in the insertion of incoming monomer. [40] (a) Kitaura, K.; Morokuma, K. Int. J. Quantum Chem. 1976, 10, 325; (b) Pan, Y.; Xu, X.; Wei, N.-N.; Hao, C.; Zhu, X.; He, G. RSC Adv.

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