Computational Analyses of the Effect of Lewis Bases on Styrene

Dec 3, 2015 - Organometallics , 2015, 34 (23), pp 5540–5548 ... rare-earth metal complexes [(η5-C5Me5)Sc(CH2SiMe3)(THF)n]+ (n = 0 (A), 1 (thfA)), ...
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Computational Analyses of the Effect of Lewis Bases on Styrene Polymerization Catalyzed by Cationic Scandium Half-Sandwich Complexes Xiaohui Kang,† Atsushi Yamamoto,‡ Masayoshi Nishiura,‡ Yi Luo,*,† and Zhaomin Hou*,†,‡ †

State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ Organometallic Chemistry Laboratory and Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: The styrene polymerizations catalyzed by cationic half-sandwich rare-earth metal complexes [(η5-C5Me5)Sc(CH2SiMe3)(THF)n]+ (n = 0 (A), 1 (thfA)), [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (B), and [(η5-C5Me5)Sc(C6H4OMe-o)]+ (C) have been computationally studied. It has been found that THF as an external Lewis base has no effect on the regioselectivity in the chain initiation step. However, it can make activity lower toward styrene insertion. THF is computationally proposed to move away from the Sc center during chain propagation and thus has no effects on stereoselectivity. Aminobenzyl as an internal Lewis base in B results in no regioselectivity at the chain initiation stage and has no effect on syndioselectivity during chain propagation. The internal Lewis base anisyl induces high-isotactic chain-end microstructure. The discrepancy in chain-end microstructures induced by aminobenzyl and anisyl groups could be ascribed to the different coordination capability of oxygen and nitrogen atoms to Sc metal. The size of the metal-involved ring in the bare cationic species plays an important role in the control of chain-end microstructure of the resulting polystyrene.

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for syndiospecific polymerization of styrene.3g A series of novel constrained geometry configuration (CGC) rare-earth metal complexes (2,6-Me2C6H3N-8-quinoline)Sc(CH2SiMe3)2(THF),3k (C5Me4-C5H4N)Ln(η3-C3H5)2 (Ln = Sc and Lu),3o and (RCH2-Py)Ln(CH2SiMe3)2(THF)n (R = fluorenyl, Py = pyridyl)3p displayed high activity and syndiotacticity in styrene polymerization. We have recently found that a cationic Sc complex with an anisyl group could initiate styrene polymerization.4 In this context, the rare-earth metal complexes as precursors often bear a THF ligand and have various alkyls such as CH2SiMe3, aminobenzyl, and anisyl.3−5 While their polymerization activity and selectivity attracted considerable interest, the effects of the THF ligand and the alkyl groups on the chain initiation efficacy and chainend microstructure have received much less attention, although such an effect is crucial not only for a better understanding of the chain initiation efficiency and analysis of chain-end microstructure but also for the design of more efficient precursors.

INTRODUCTION Stereospecific synthesis of polystyrene is of great importance for meeting various applications. Syndio-PS (syndiotactic polystyrene) discovered by Ishihara and co-workers in 1986 currently plays an irreplaceable role in industrial production and in our daily lives because of its high crystallization rate, good electrical properties, and excellent heat and chemical resistance.1 Numerous homogeneous group 4 and late transition metal single-site catalysts have been synthesized and applied successfully to the syndiospecific styrene polymerization.2 In the recent decade, novel rare-earth metal catalysts have been developed for highly efficient and selective (co)polymerization of styrene.3 The allyl ansa-lanthanidocene [Flu-C(CH3)2Cp]Ln(η3-C3H5)(THF) (Flu = fluorenyl, Ln = Y, La, Sm, and Nd) system synthesized by Carpentier and coworkers3b provided syndiotactic polystyrene without any cocatalyst. The scandium half-sandwich complex (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) with 1 equiv of [Ph3C][B(C6F5)4] exhibited excellent activity and selectivity for syndiospecific homo- and co- polymerization of styrene with ethylene.3c Along the same lines, bis(dimethylaminobenzyl) complexes (Dtp)-Sc(CH2C6H4N(CH3)2-o)2 (Dtp = 2,5bitBuC4H2N) with [Ph3C][B(C6F5)4] also showed good results © XXXX American Chemical Society

Received: August 21, 2015

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DOI: 10.1021/acs.organomet.5b00719 Organometallics XXXX, XXX, XXX−XXX

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along the reaction pathway. Considering that the solvation effect of toluene as an often-used solvent in such polymerization systems is insignificant and its coordination to Sc is weak relative to styrene coordination,7d the effect of solvent was not considered in this work. Such a strategy was also previously adopted in the studies on styrene polymerization catalyzed by rare-earth metal complexes.7a−c

A large number of computational studies have been successfully carried out for styrene polymerization catalyzed by group 4 and late transition metals.6 Gavallo et al. have computationally rationalized syndiospecific chain-end stereocontrol of styrene polymerization by cationic CpTiP+ (P = polymer) species and have further explored the possible role of active species with different oxidation states of titanium.6a,b Won Ho Jo et al. conducted DFT studies on stereoselectivity of styrene polymerization promoted by ansa-metallocene Si(CH3)2Cp2Ti(CH3)2. They found that primary insertion was favorable in the initiation step but led to a stable product, which prevents further primary insertion. The secondary insertion was therefore more favorable during chain propagation due to repulsion between the two phenyl rings of the preinserted styrene and the incoming unit.6d,e These calculations have provided valuable information for the design and development of homogeneous transition metal polymerization catalysts. In comparison, the theoretical studies7 on styrene polymerization catalyzed by rare-earth metal complexes are relatively less. Maron et al. computationally accounted for the origins of activity and stereoselectivity of styrene polymerization initiated by the complex [Flu-X(CH3)2Cp]Eu(R)(THF) (Flu = fluorenyl; X = C, Si; R = allyl, CH2SiMe3).7a,b We previously reported a comprehensive QM/MM study on scandiumcatalyzed syndiospecific copolymerization of styrene and ethylene.7d However, the effect of THF as a Lewis base on the polymerization performance has remained unclear.7e Attracted by the high activity of the scandium half-sandwich complex toward styrene polymerization, we have conducted computational studies on styrene polymerization catalyzed by cationic scandium complexes (η5-C5Me5)Sc(CH2SiMe3)(THF) n + (n = 0, A; n = 1, thf A), (η 5 -C 5 Me 5 )Sc(CH2C6H4NMe2-o)+ (B), and (η5-C5Me5)Sc(C6H4OMe-o)]+ (C, Chart 1), addressing the effect of Lewis bases such as THF,

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RESULTS AND DISCUSSION Effect of THF as a Lewis Base. To investigate the effect of THF on styrene polymerization, the cationic active species (η5C5Me5)Sc(CH2SiMe3)(THF)n+ with (n = 1, thfA) and without (n = 0, A) THF has been considered. As shown in Figure 1,

Figure 1. Optimized structures (distances in Å and angles in deg) of species A and thfA. The numbers in black denote bond lengths or angles, and those in blue represent atomic NBO charges. All H atoms are omitted for clarity.

both optimized structures of A and thfA show strong agostic interactions. The coordination of THF leads to a smaller positive NBO charge on the Sc center (1.46 in thfA vs 1.59 in A) and a relatively weaker agostic interaction suggested by a longer Sc−C2 distance (2.51 Å in thfA vs 2.42 Å in A) and a larger Sc−C1−Si angle (93.0° in thfA vs 91.1° in A). At the chain initiation stage, the four insertion modes of styrene, viz., 1,2-si, 1,2-re, 2,1-si, and 2,1-re (Figure 2), have been considered for the species A and thfA, respectively. It is noted that the Sc atom in A can be viewed as a pseudochiral center, and the styrene coordination to the Sc center leads to a pair of diastereomers (see 2,1-insertion TSs in Figure 2), which have different free energy barriers to achieve the insertion process. What we emphasize here is that we are not suggesting absolute enantioface selectivity in the styrene insertion, but rather relative enantioface selectivity to form syndiotactic polystyrene. In the initial insertion of styrene into the Sc−alkyl bond, eight possible transition states (TS) (four pairs of pseudoenantiomers) were actually considered as shown in Figure S1 (see Supporting Information, SI). In the case of the pair of pseudoenantiomers TS21‑si and TS*21‑re as an example, the initial insertion of styrene into the Sc−alkyl bond will afford TS21‑si and TS*21‑re in the ratio of 50:50, because the scandium catalyst is not chiral. Therefore, in Figure 2, the other pseudoenantiomers (TS*12 and TS*21) are omitted for simplification. It can be proposed that the syndioselectivity is under chain-end control,6b although there is no direct experimental evidence for this system at this moment. The 2,1-si and 2,1-re insertions at the same side of the Sc−CH2− SiMe3 plane lead to different chirality of the end-groups, and the chain-initiation-induced stereocenter will then control the emerging stereocenter for the growing polymer chain, resulting in a sydiotactic polymer. Therefore, differentiating the re- from

Chart 1. Cationic Scandium Alkyl Species Initiating Styrene Polymerization

aminobenzyl, and anisyl on chain-end microstructure. The result showing different behaviors of these Lewis bases is reported in this work.

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COMPUTATIONAL DETAILS

All calculations were performed with the Gaussian 09 program.8 The DFT method of B3PW919 was used for geometry optimizations. The 6-31G* basis set was used for C, H, O, and N atoms, and the Stuttgart/Dresden effective core potential (ECP) together with associated basis sets10 was utilized for Si and Sc atoms. In the Stuttgart/Dresden ECP, the innermost 10 electrons of Si and Sc atoms were included in the core. The four valence electrons of the Si atom and 11 valence electrons of the Sc atom were treated by the optimized basis sets, viz., (4s4p)/[2s2p] for Si and (8s7p6d1f)/[6s5p3d1f] for Sc. Each optimized structure was analyzed by harmonic vibration frequencies obtained at the same level and characterized as a minimum (Nimag = 0) or a transition state (Nimag = 1). The energy profiles were described by computed Gibbs free energy (ΔG, 298.15 K, 1 atm). The stationary points reported are their lowest conformations in energy B

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Figure 2. Four possible transition states (TS) and corresponding free energy barriers (in kcal/mol) for insertion of styrene into the Sc−R (alkyl) bond of A or thfA.

Figure 3. Optimized structures (distances in Å) of transition states TS21‑si and thfTS21‑si. The Cp* ligand is represented by a wireframe form, and all H atoms are omitted for clarity.

the si-faces at the same side of the Sc−CH2−SiMe3 plane is computationally (static calculation) helpful for modeling the stereoselectivity in the chain propagation. The intermediate derived from most kinetically and thermodynamically favorable chain initiation reaction pathways will be considered for insertion of the second monomer. The computed insertion free energy barriers indicate that the 2,1-si-insertion transition state, TS21‑si, has the lowest energy for both the A and thfA cases (6.1 and 17.5 kcal/mol, respectively). The 2,1-si-insertion product of each case is also lowest in energy among the four insertion products (see Table S1 in the SI). Therefore, in the cases of both A without THF and thfA with THF, the 2,1-siinsertion pattern is more favorable both kinetically and thermodynamically. This result suggests that the Lewis base THF has no effect on the regioselectivity at the chain initiation stage. However, the presence of THF results in lower activity, as suggested by the free energy barriers (17.5 vs 6.1 kcal/mol, Figure 2). The structures of two favorable 2,1-insertion TSs, TS21‑si for the A case and thfTS21‑si for the thfA case, are shown in Figure 3. As indicated in this figure, the average distance between Sc and C5, C6, and C7 atoms in TS21‑si (2.56 Å) is smaller than that in thf TS21‑si (2.91 Å). In addition, the structure of TS21‑si shows a Si−C2···Sc agostic interaction, which is absent in thfTS21‑si (Figure 3) due to the presence of THF. These geometrical

features could account for the relative stability of TS21‑si and thus the higher activity of A in comparison with thfA. At the stage of chain propagation, the subsequent 2,1-reinsertion of styrene into the metal−C bond of P21‑si or thfP21‑si could give a syndiotactic sequence, while the successive 2,1-siinsertions could produce an isotactic polymer. Both products P21‑si and thfP21‑si have a chiral growing chain. Therefore, there are two possible sites in P21‑si and thfP21‑si (site I and site II, Figure 4) for the coordination of the incoming styrene

Figure 4. Products P21‑si and

thf

P21‑si of 2,1-si-insertion.

molecule (St). As shown in Table 1, in the case of A, the 2,1-re-insertion mode at the site II, yielding a syndiotactic sequence, has the lowest energy barrier (13.6 kcal/mol) among the insertion modes and is exergonic by 6.3 kcal/mol. In the case of thfA, however, the 2,1-re-insertion leading to syndiotactic structure has a barrier of as high as 34.7 kcal/mol, which is C

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Table 1. Relative Free Energies (ΔG, in kcal/mol) of Stationary Points Involved in Chain Propagation Based on P21‑si and thf P21‑si system site I

A

site II thf

A

site II

insertion fashions

Coo

TS

P

ΔG⧧ (kcal/mol)

2,1-si 2,1-re 2,1-si 2,1-re 2,1-si 2,1-re

13.6 7.6 1.7 11.7(Coosyn) 19.6 24.4(thfCoosyn)

15.8 24.6 24.9 13.6(TSsyn) 38.9 34.7(thfTSsyn)

−7.1 5.5 −5.2 −6.3(Psyn) 10.0 8.0(thfPsyn)

15.8 24.6 24.9 13.6 38.9 34.7

kinetically infeasible at low or room temperature. It is noted that the strong coordination of THF makes site I of thfP21‑si inaccessible. Therefore, the THF-free cationic species such as P21‑si could be the true active species during the chain propagation with respect to the syndiotactic selectivity observed experimentally, although THF might attach to the metal center at the chain initiation stage. Effect of Aminobenzyl as an Internal Lewis Base. In order to avoid the coordination of THF, a Cp*-ligated bis(aminobenzyl) scandium complex was synthesized and used as a precursor for styrene polymerization. This drove us to wonder whether the corresponding cationic active species [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (B) has a different effect on the chain-end microstructure. The optimized structure of B indicates strong binding between the metal and aminobenzyl, as suggested by the Sc−C1 bond length of 2.19 Å and the Sc···X distances of 2.3−2.5 Å (X = C2, C3, and N atoms, Figure 5). This is understandable in light of the electron-deficient metal center.

In the chain initiation, as shown in Figure 5, the monomer could access from the vacant site. Like the case of A, four insertion modes, viz., 1,2-si-, 1,2-re-, 2,1-si-, and 2,1-reinsertions, have also been calculated. The results indicate that the si-face insertion (energy barriers of ca. 22 kcal/mol) is more kinetically favorable than the re-face (energy barriers of 24−28 kcal/mol; see Table S2). The 1,2-si- and 2,1-si-insertions have almost the same energy barriers (22.5 and 22.3 kcal/mol). This suggests that the two si-face insertions are kinetically competitive, resulting in no regioselectivity at the chain initiation stage. To have a better understanding of such competitive insertions, the energy decomposition analyses of the two insertion transition states (TSB12‑si and TSB21‑si) are carried out. It has been found that the interaction energies ΔEint between B and styrene moieties in TSB12‑si and TSB21‑si are −35.5 and −30.7 kcal/mol, respectively. This could partly offset the unfavorable total deformation energy of the two moieties, ΔEdef (45.1 kcal/mol for TSB12‑si and 40.0 kcal/mol for TSB21‑si). Therefore, the ΔETS (−35.5 + 45.1 = 9.7 kcal/mol) obtained for TSB12‑si is similar to that (−30.7 + 40.0 = 9.3 kcal/ mol) for TSB21‑si. Unlike TS21‑si (Figure 3), showing a η3coordination structure, TSB21‑si has no such η3-coordination interaction (3.99 Å of Sc···C7 contact) due to the repulsion between the −NMe2 group and the phenyl ring of inserting styrene (Figure 6). The absence of the η3-coordination interaction could destabilize TSB21‑si, and thus the 2,1-siinsertion no longer has kinetic priority in the case of B. These results could account for the loss of regioselectivity in Bmediated chain initiation. Like the case of A, the 2,1-insertion mode was considered for insertion of the second monomer into the Sc−C bond of PB21‑si since the initial alkyl was far away from the metal center. There

Figure 5. Optimized structure (distances in Å) of species B.

Figure 6. Optimized structures (distances in Å) for transition states TSB12‑si and TSB21‑si. The Cp* ligand is represented by a wireframe form, and all H atoms are omitted for clarity. D

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Figure 7. Optimized structures for complexes CooBiso1, CooBsyn1, CooBiso2, and CooBsyn2. The energies (ΔG, in kcal/mol) are relative to the energy sum of PB21‑si and one molecule of styrene.

decomposition of TSBiso2 and TSBsyn2 have been analyzed (Figure 9). The average Sc−Ci (i = 3, 4, and 5) distance in TSBsyn2 (2.49 Å) is shorter than that in TSBiso2 (3.11 Å), which suggests that the metal−styrene binding is tighter in TSBsyn2 than that in TSBiso2. Energy decomposition analysis of TSBsyn2 and TSBIso2 indicates that the interaction energies ΔEint between styrene moieties and their interacting part are −26.8 and −39.3 kcal/mol for TSBiso2 and TSBsyn2, respectively. These energies could partly offset the unfavorable value ΔEdef (total deformation energy, 40.5 kcal/mol for TSBiso2 and 38.9 kcal/ mol for TSBsyn2). The ΔETS (−26.8 + 40.5 = 13.7 kcal/mol) obtained for TSBiso2 is higher than that (−39.3 + 38.9 = −0.4 kcal/mol) for TSBsyn2. Therefore, the lower stability of TSBiso2 is mainly due to the weaker interaction between the styrene moiety and its contacting part. These results suggest that both geometrical and electronic features account for the stability of TSBsyn2. Effect of Anisyl as an Internal Lewis Base. It was reported that the reaction of [(η5-C5Me5)Sc(CH2C6H4NMe2o)2] with [C6H5NMe2H][B(C6F5)4] in the presence of anisole yielded cationic half-sandwich scandium anisyl species C (Figure 10), being capable of styrene insertion.4 Considering this, the C-initiated styrene polymerization has also been calculated to investigate the effect of anisyl as an internal Lewis base on the chain-end microstructure. As shown in Figure 10, the optimized structure C indicates a strong interaction between Sc and the O atom of the anisyl group, as suggested by the Sc···O distance of 2.16 Å. Four insertion modes, viz., 1,2si, 1,2-re, 2,1-si, and 2,1-re, have also been calculated for C. The result indicates that the 2,1-re-insertion has the lowest energy barrier in comparison with other insertion modes (8 vs 13 kcal/ mol), and the corresponding 2,1-re-insertion product PC21‑re is also the most stable (exergonic by 22 kcal/mol for PC21‑re and by 9−19 kcal/mol for others, Table S5). The 2,1-re-insertion is therefore more favorable both kinetically and thermodynamically. It is noteworthy that all four insertion products retain the Sc···O interaction. The 2,1-re-insertion product PC21‑re (Figure 11) was therefore used for further calculation of the preferred 2,1-insertion of the next monomer to investigate the influence of anisyl on the chain-end microstructure. Like the A and B cases, the second styrene molecule could approach the metal center from site I or site II (Figure 11). Considering this and the possibilities of re- and si-face insertions, four coordination complexes, viz., CooC iso1, CooCsyn1, CooCiso2, and CooCsyn2, have been optimized for the 2,1-insertions (Figure 12 and Table S6). As suggested by their relative energies shown in Figure 12, the complexes CooCiso1 and CooCsyn1 formed by styrene access from site I are significantly less stable than CooCiso2 and CooCsyn2 generated

are still two sites (site I and site II) for styrene coordination to the metal center of PB21‑si. Four possible coordination complexes have therefore been optimized. As shown in Figure 7, with the coordination of styrene from site II, the Sc···N interaction disappeared. On the basis of structure PB21‑si, the subsequent re-face insertion could give a syndiotactic sequence and si-face insertion could yield an isoselective unit. Complexes CooBiso2 (1.8 kcal/mol) and CooBsyn2 (4.0 kcal/mol) are more stable in energy than complexes Cooiso1 (10.3 kcal/mol) and Coosyn1 (8.7 kcal/mol). CooBiso2, with si-coordination of styrene, and CooBsyn2, with re-coordination of the monomer, could lead to isotactic and syndiotactic sequences, respectively. Therefore, the complexes CooBiso2 and CooBsyn2 were considered for calculations of further insertion reactions to investigate the stereoselectivity. The calculated relative energies of insertion TSs (TSBiso2 and TSBsyn2) and products (PBiso2 and PBsyn2) are shown in Figure 8. The process CooBsyn2 → TSBsyn2 → PBsyn2 leading to the

Figure 8. Relative energies (ΔG, in kcal/mol) of stationary points involved in chain propagation starting with CooBiso2 and CooBsyn2. The energies are relative to the energy sum of PB21‑si and one molecule of styrene.

syndiotactic unit is both kinetically and thermodynamically favorable compared with the process of CooBiso2 → TSBiso2 → PBiso2, yielding an isotactic sequence. One may raise the question of whether the 1,2-si- and 2,1-siinsertions are kinetically competitive in the B-mediated chain initiation and both PB21‑si and PB12‑si should be considered for 1,2- and 2,1-insertions of the incoming monomer. In fact, all of the 1,2-insertion patterns are kinetically less favorable in comparison with the aforementioned favorable process of CooBsyn2 → TSBsyn2 → PBsyn2 leading to a syndiotactic unit (see Table S3 for the PB21‑si case and Table S4 for the PB12‑si case). To elucidate the origin of the significant preference in kinetics for syndioselectivity, the structure and energy E

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Figure 9. Optimized structures (distances in Å) for transition states TSBsyn2 and TSBiso2. The Cp* ligand and aminobenzyl are represented by a wireframe form, and all H atoms are omitted for clarity.

six-membered ring constructed by Sc, O, and four C atoms and the Sc···O interaction is retained (Figure 12). These features could account for their stabilities. In other words, the lower stability of CooCiso1 and CooCsyn1 could be ascribed to the loss of the strong Sc···O interaction. This is in contrast to the situation of the B system, where the coordination complexes with the analogous Sc···N interaction are less stable than that without the same interaction (Figure 7). This is understandable since the removal of a Sc···O bonding interaction requires more energy than the corresponding Sc···N interaction. In view of the relative stabilities of the complexes shown in Figure 12, CooCiso2 and CooCsyn2 are considered for subsequent insertion processes, respectively. The calculated energy profiles for the insertion processes CooCiso2 → TSCiso2 → PCiso2 and CooCsyn2 → TSCsyn2 → PCsyn2 are shown in Figure 13. As indicated in this figure, the CooBiso2 overcomes transition state TSCiso2 with an energy barrier of 26.6 kcal/mol, leading to isospecific insertion product PCiso2. This process is both kinetically and thermodynamically more favorable than the process of CooCsyn2 → TSCsyn2 → PCsyn2 leading to a syndiospecific unit (barrier of 26.6 vs 32.5 kcal/mol and endergonic by 0.9 vs 18.5 kcal/mol, Figure 13). One may care that the free energy barrier of 26.6 kcal/mol is a little bit higher with respect to this polymerization system. It could be ascribed to the entropy effect. In fact, the corresponding enthalpy barrier is 14.5 kcal/mol for TSCiso2, which is still lower than that for TSCsyn2 (18.2 kcal/mol). These results suggest

Figure 10. Optimized structures (distances in Å) for species C.

Figure 11. Optimized geometries of PC21‑re.

by styrene access from site II. A geometrical comparison of these complexes indicates that the latter two complexes have a

Figure 12. Optimized structures for complexes CooCiso1, CooCsyn1, CooCiso2, and CooCsyn2. Free energies (ΔG, in kcal/mol) are relative to the energy sum of species PC21‑re and one molecule of styrene. F

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Figure 15. Optimized geometries of PCiso2.

On the basis of more stable complexes CooCiso3 and CooCsyn3, two insertion processes, CooCiso3 → TSCiso3 → PCiso3 and CooCsyn3 → TSCsyn3 → PCsyn3, have been calculated, and the energy profiles are shown in Figure 17. As shown in this figure, the process CooCsyn3 → TSCsyn3 → PCsyn3 leading to a syndiospecific unit has significant kinetic preference compared with the other process giving an isospecific unit (barrier of 21.1 vs 35.5 kcal/mol). This situation is the same as that for the chain propagation mediated by species A and B since the alkyl groups have moved away from the metal center. As a whole, the strong interaction between Sc and O atoms of the anisyl group is prone to produce an isotactic chain-end microstructure, which is different from the aminobenzyl case, giving a syndiotactic chain-end microstructure. The different chain-end microstructure induced by anisyl and aminobenzyl is certainly due to the different electronegativity of O and N atoms and thus their metal-binding ability. Such a difference could account for the Sc···O interaction being retained but the Sc···N interaction disappearing during the insertion of the second monomer. Geometrically, however, species B has a methylene group connecting with Sc, which is absent in species C (Chart 1). This led us to wonder whether such a structural discrepancy also plays a role in the chain-end microstructure. For this purpose, the cationic species (η5C 5 Me 5 )Sc(CH 2 C 6 H 4 OMe-o) + (D) and (η 5 -C 5 Me 5 )Sc(C6H4NMe2-o)]+ (E, Figure 18 and Figure S2) have been computationally modeled for styrene polymerization. In the chain initiation step, the D-mediated 1,2-si- and 2,1-siinsertions overcome similar energy barriers (18.3 and 19.3 kcal/ mol, respectively, Table S8). The former is endergonic by 5.6 kcal/mol, while the latter is exoergic by 7.4 kcal/mol. Therefore, the 2,1-si-insertion is more thermodynamically favorable than 1,2-si-insertion. The 2,1-re-insertion is also less favorable both thermodynamically and kinetically compared with 2,1-si-insertion (Table S8). The E-mediated 2,1-re-

Figure 13. Calculated energy profiles (ΔG, in kcal/mol) for 2,1insertions of the second styrene molecule on the basis of the PC21‑re structure. Free energies are relative to the energy sum of species PC21‑re and one molecule of styrene.

that the anisyl group could lead to an isospecific chain-end microstructure, which is different from the aforementioned effect of aminobenzyl. To elucidate the origin of the kinetic preference for such an unexpected chain-end microstructure, the energies and geometries of TSCiso2 and TSCsyn2 have been investigated. During the energy decomposition analysis, the PC21‑re and styrene moieties in these TSs are defined as fragments F1 and F2, respectively. The interaction energy (ΔEint) between F1 and F2 and their total deformation energy (ΔEdef) are calculated to be −31.8 and 45.4 kcal/mol for TSCiso2 and −30.1 and 50.5 kcal/ mol for TSCsyn2, respectively. Therefore, the total energy of TSCsyn2 (ΔETS = 50.5 − 30.1 = 20.4 kcal/mol) is higher than that of TSCiso2 (ΔETS = 45.4 − 31.8 = 13.6 kcal/mol). It is obvious that the larger deformation energy destabilized TSCsyn2 (50.5 vs 45.4 kcal/mol). Such a larger deformation is mainly caused by the repulsive interaction between the ancillary ligand Cp* and the phenyl group of the styrene moiety. To further access the stereoselectivity at the chain propagation stage, the insertion of the third monomer into PCiso2 was also investigated. Like insertion of the second monomer, two coordination sites (site I and site II) and four coordination complexes have been considered (Figures 15 and 16 and Table S7). As shown in Figure 16, the coordination at site I led to more stable complexes without the Sc···O interaction. The approach of the monomer from site II gave two less stable complexes retaining the Sc···O interaction, possibly due to the unstable eight-membered structure.

Figure 14. Optimized structures (distances in Å) for transition states TSCiso2 and TSCsyn2. The Cp* ligand is represented by a wireframe form, and all H atoms are omitted for clarity. G

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Figure 16. Optimized structures for complexes CooCiso3, CooCsyn3, CooCIso4, and CooCsyn4 with a PCiso2 moiety. The energies (ΔG, in kcal/mol) are relative to PCIso2 and one molecule of styrene.

Sc(CH2SiMe3)(THF)n]+ (n = 0 (A), 1 (thfA)), [(η5-C5Me5)Sc(CH2C6H4NMe2-o)]+ (B), and [(η5-C5Me5)Sc(C6H4OMeo)]+ (C). In the chain initiation step, THF as an external Lewis base has no effect on the regioselectivity, but results in lower activity toward styrene insertion. Species A kinetically prefers syndioselectivity, as observed experimentally, and the THF in thf A tends to move away from the Sc center during chain propagation and thus has no effect on the stereoselectivity. The aminobenzyl group as an internal Lewis base in B induced no regioselectivity in the chain initiation step and also has no influence on the stereoselectivity, giving a syndiotactic chainend microstructure. In contrast, the stronger internal Lewisbase anisyl group is prone to produce a highly isotactic chainend microstructure because of the retained Sc···O interaction during the insertion of the second monomer. The computational modeling of the analogues of B and C indicates that the benzylic methylene as a linker between the metal and the phenyl group could regulate the binding between the metal and Lewis base atoms and therefore play an important role in the control of the chain-end microstructure of the resulting polystyrene.

Figure 17. Relative energies (ΔG, in kcal/mol) of stationary points involved in chain propagation starting with CooCiso3 and CooCsyn3. The energies are relative to PCiso2 and one molecule of styrene.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 18. Cationic scandium alkyl species D and E.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00719. Tables providing the relative free energies of stationary points involved in the chain initiation starting with species A, thfA, B, C, D, and E and in chain propagation starting with PB21‑si, PB12‑si, PC21‑re, PCiso2, PD21‑si, and PE21‑re, the optimized Cartesian coordinates, total energies, and the imaginary frequencies of TSs (PDF) Molecular model (XYZ)

insertion is the most favorable mode in both kinetics and thermodynamics among all insertion modes (Table S10). On the basis of the energetically favorable 2,1-si-insertion product PD2,1‑si, the syndiospecific insertion of the incoming monomer, during which the Sc···O interaction is disrupted, has significant priority in the kinetics (17.9 vs 27−43 kcal/mol, Table S9 and Figure S3). Therefore, the chain-end microstructure is similar to the case of species B, with an aminobenzyl group. In the case of E, on the basis of the favorable 2,1-re-insertion product PE2,1‑re, the syndiospecific insertion of the incoming monomer, during which the Sc···N interaction is disrupted, is also kinetically favorable (18.7 vs 23−31 kcal/mol, Table S11 and Figure S4). These results suggest that the methylene “linker”, viz., the metal-involved four- or five-membered ring, in the cationic species also plays an important role in the control of the chain-end microstructure.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Luo). *E-mail: [email protected] (Z. Hou). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Nos. 21174023, 21137001, 21429201). The authors also thank RICC (RIKEN Integrated Cluster of Clusters) and the Network and

CONCLUSION We have computationally investigated the chain initiation and propagation during styrene polymerization catalyzed by cationic half-sandwich rare-earth metal complexes [(η5-C5Me5)H

DOI: 10.1021/acs.organomet.5b00719 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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Information Center of Dalian University of Technology for part of the computational resources.

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DOI: 10.1021/acs.organomet.5b00719 Organometallics XXXX, XXX, XXX−XXX