Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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DFT Studies on the Polymerization of Functionalized Styrenes Catalyzed by Rare-Earth-Metal Complexes: Factors Affecting C−H Activation Relevant to Step-Growth Polymerization Yanan Zhao,† Gen Luo,† Xingbao Wang,† Xiaohui Kang,*,‡ Dongmei Cui,§ 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 § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ∥ Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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‡
S Supporting Information *
ABSTRACT: The polymerization mechanism of functionalized styrene derivatives, viz., p-NMe2 (pMNS)-, o-/p-OCH3 (o/pMOS)-, and p-SCH3 (pMTS)-substituted styrene, catalyzed by cationic rare-earth-metal catalysts has been comparatively studied through density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it is found that large steric hindrance as a main factor prevents oMOS from undergoing o-C−H activation relevant to step-growth polymerization, while the nonoccurrence of C−H activation of pMNS and pMTS can be explained by the relatively less dispersion of charge distribution in the four-center transition states. In addition, the electron-donating pyridine side arm weakened the interaction between pMOS and the metal center and meanwhile increased the steric hindrance, preventing the C−H activation. Therefore, the simultaneous occurrence of C−H activation (step-growth polymerization) and CC insertion (chain-growth polymerization) are affected by multiple factors such as the coordination ability of the heteroatom of monomers, steric hindrance, and the electron-donating ability of the ancillary ligand.
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INTRODUCTION
can provide the unique microstructure of polymers, but the few suitable organometallic catalysts limit the development of this field. Therefore, the quest for efficient catalyst systems for such extraordinary transformations remains challenging, and mechanistic studies are of substantial importance. Methoxystyrenes (MOSs) are commercially available, wellknown styrene derivatives. Hou et al. had reported that cationic half-sandwich rare-earth-metal alkyl complexes [(C5Me5)Ln(o-NMe2CH2C6H4)]+ (Ln = Sc (A+), Y (B+)) could serve as efficient catalysts for the chain-growth (co)polymerization of a wide range of monomers (including styrene).3−9 In 2016, they further found that, for the first time, A+ or B+ could catalyze simultaneous chain-growth and stepgrowth polymerization (proceeded by the C−H polyaddition of anisole units to vinyl groups) of para-MOS (pMOS) and meta-MOS (mMOS), producing a novel family of polymers containing unique alternating anisole−ethylene sequences
Functionalized polystyrenes with improved surface properties are highly desired materials for industrial applications.1 In comparison with postmodification, the direct (co)polymerization of styrene derivatives containing polar groups is a more convenient and controllable method to produce functionalized polystyrenes. The reported catalysts for functionalized styrene (co)polymerization are mainly based on hard Lewis acidic transition metals that are swiftly poisoned by the Lewis basic polar atoms, accompanied by a series of problems such as low activity, low molecular weight, and low incorporation of polar monomers. Moreover, these catalysts were generally used in the chain-growth polymerization of functionalized styrene, which proceeds by the reaction of growing chain ends and monomers. However, an alternative polymerization style, stepgrowth polymerization occurring by the reactions among chain-end groups, monomers, and the resulting oligomers and polymers, still remains a largely unexplored area.2 The simultaneous chain-growth and step-growth polymerizations © XXXX American Chemical Society
Received: July 27, 2018
A
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Scheme 1. Polymerizations of Various Functionalized Styrene Derivatives by Cationic Species A+, B+, and C+: (a) MonomerDependent Polymerization; (b) Catalyst-Dependent Polymerization
(Scheme 1).10 In contrast, the Hou and Cui groups reported that the polymerization of ortho-MOS (oMOS) by cationic half-sandwich rare-earth-metal complexes occurred exclusively in a chain-growth fashion by continuous CC insertion.10,11 Actually, in the context of polymerization of functionalized styrenes catalyzed by metal complexes, it had been reported that species A+ could also catalyze the chain-growth polymerization of p-dimethylaminostyrene (pMNS).12 Cui et al. found that the constrained-geometry catalyst [(Py-CH2-Flu)Y(CH2SiMe3)(THF)]+ (C+) was effective for chain-growth polymerizations of p/o/mMOS.13 In addition, they also confirmed that complex C+ showed remarkably high activity toward chain-growth polymerization of polar p-methylthiostyrene (pMTS), leading to perfect syndiotactic poly(MTS) with narrow molecular weight distributions.14 In such systems,11−14 unlike the A+- or B+-mediated polymerization of pMOS,10 no products of step-growth polymerization were observed. These findings invoked our interests in the mechanism of various polymerization reactions: viz., step-growth and chain-growth polymerizations. Many computational studies have been successfully conducted to investigate the mechanism of styrene polymerization catalyzed by rare-earth-metal complexes.15−24 These calculations provided a large amount of valuable information on the design of homogeneous metal catalysts for chain-growth polymerization of olefins. In contrast, the computational investigations on cationic rare-earth-metal-catalyzed heteroatom-containing olefin polymerization have been reported more recently.11,25−27 These theoretical studies were limited to chain-growth polymerization mechanisms, and the step-growth polymerization mechanisms have remained largely unexplored. Attracted by the unique catalytic performance of rare-earthmetal complexes toward the polymerization of various functionalized styrene derivatives, we selected six different polymerization processes as the computational models (Scheme 1) and conducted systematic studies on the catalytic chain-growth and step-growth polymerization mechanisms of pMOS, pMNS, pMTS, and oMOS, with the purpose of elucidating the origin of different polymerization manners. It is
expected that this work could provide theoretical information for the design of step-growth polymerization catalysts.
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COMPUTATIONAL DETAILS
All calculations were performed with the Gaussian 09 program.28 The B3PW91 hybrid exchange-correlation functional was utilized for geometry optimization.29−31 Each optimized structure was subsequently analyzed by harmonic vibration frequencies for characterization of a minimum (Nimag = 0) or a transition state (Nimag = 1) and provision of thermodynamic data. The transition state structures are shown to connect the reactant and product on either side via intrinsic reaction coordinate (IRC) following. The 6-31G* basis set was considered for C, H, O, and N atoms, and the LANL2DZ basis set was used for the Si atom. The Sc and Y atoms were treated by the Stuttgart/Dresden effective core potential (ECP) and the associated basis sets.32,33 The basis sets of Si and Y were augmented with one d (exponent of 0.284)34 and f polarization function (exponent of 0.835),35 respectively. This basis set is denoted as “BSI”. Such a computational strategy has been widely used for the study of transition-metal-containing systems.21−23,36 To obtain more reliable relative energies, the single-point calculations of optimized structures were carried out at the level of B3PW91-D3 (B3PW91 with Grimme’s DFT-D3 correction)37,38/BSII, taking into account the solvation effect of toluene with the SMD39 solvation model. In the BSII, the 6311+G(d,p) basis set was used for nonmetal atoms, while the basis sets together with associated pseudopotentials for Sc and Y atoms are the same as those in geometry optimization. Therefore, unless otherwise mentioned, the free energy (ΔG, 298.15 K, 1 atm) in solution, which was used for the description of energy profiles, was obtained from the solvation single-point calculation and the gas-phase Gibbs free energy correction. The 3D molecular structures displayed in this paper were drawn by using CYLview.40
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RESULTS AND DISCUSSION C−H Activation and CC Insertion of Various Monomers. To investigate the effect of the heteroatom of monomers on the polymerization, the three monomers pMOS, pMNS, and pMTS were first considered for the calculations of C−H activation and CC insertion reactions, where the optimized bare cationic species A+ was used as the uniform initial active species (Scheme 1a). B
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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Organometallics As in the previous study,21 four insertion fashions of pMOS, viz., 1,2-si, 1,2-re, 2,1-si, and 2,1-re, have been calculated in the chain initiation step (see Figure S1 in the Supporting Information). The computational results indicate that the 2,1-si insertion overcomes the lowest energy barrier (TS1ApMOSCC, 17.8 kcal/mol) and is the most exergonic (relative energy of −5.0 kcal/mol) among the four insertion manners. This suggests that 2,1-si insertion of pMOS is both kinetically and thermodynamically more favorable than the other three modes. Therefore, during the first monomer insertions, the 2,1-si model of the CC insertion reaction was selected for a comparison with C−H activation. In the C−H activation process (Figure 1), the coordination of pMOS via its
Figure 2. Computed energy profiles for A+-mediated C−H (black curve) activation and CC insertion (red curve) of pMNS. Free energies are relative to isolated reactants.
in Figure 3, the C−H activation starts with the formation of the complex C1ApMTSC−H, which goes through TS1ApMTSC−H to
Figure 1. Computed energy profiles for A+-mediated C−H activation (black curve) and CC insertion (red curve) of pMOS (distances in Å). Free energies are relative to isolated reactants.
oxygen atom to the Sc center of A+ forms the complex C1ApMOSC−H with a relative energy of −7.7 kcal/mol, which surmounts an energy barrier (TS1ApMOSC−H) of 17.9 kcal/mol to give P1ApMOSC−H with a relative energy of −6.0 kcal/mol. In comparison, the almost identical energy barriers (17.8 and 17.9 kcal/mol) for CC insertion (C1ApMOSCC → TS1ApMOSCC → P1ApMOSCC) and C−H activation (C1ApMOSC−H → TS1ApMOSC−H → P1ApMOSC−H) suggest that these two processes are kinetically competitive. It is noteworthy that, however, the stronger coordination ability of the oxygen atom of pMOS in comparison with its CC double bond (interaction energy of −23.4 vs −20.0 kcal/mol) makes its corresponding complex more stable (−7.7 vs −3.3 kcal/mol, Figure 1). This and the slightly thermodynamic advantage of the product (−6.0 vs −5.0 kcal/mol) could make the C−H activation more favorable than CC insertion. Therefore, the first molecule pMOS is more likely to undergo a C−H activation reaction. The energy profiles of CC insertion (2,1-si) and C−H activation of pMNS have also been investigated. As shown in Figure 2, the CC insertion reaction has an energy barrier of 17.3 kcal/mol, which is more kinetically favorable than that (23.2 kcal/mol) for C−H activation. This theoretical result is in line with the experimental observation that only the successive CC insertion (chain-growth polymerization) product was obtained.12 Similarly, for a comparison, the C−H activation and CC insertion reactions have been further calculated for the monomer pMTS, which could not undergo the C−H activation by cationic species A+ in experiments.14 As shown
Figure 3. Computed energy profiles for A+-mediated C−H activation (black curve) and CC insertion (red curve) of pMTS. Free energies are relative to isolated reactants.
give P1ApMTSC−H. This process has an energy barrier of 21.4 kcal/mol and is exergonic by 5.1 kcal/mol. In contrast, the complex C1ApMTSCC of the CC insertion reaction (2,1-si) is higher in energy than C1ApMTSC−H (−2.6 vs −7.7 kcal/mol), suggesting the stronger coordination ability of a sulfur atom in comparison with a CC double bond. However, the corresponding energy barrier (TS1ApMTSC−H, 13.6 kcal/mol) for vinyl insertion is apparently lower than that (TS1ApMTSC−H, 21.4 kcal/mol) for C−H activation. Meanwhile, the products of the two processes are almost isoenergetic. In order to further compare the probabilities of CC insertion and C−H activation, an analysis of Boltzmann statistics was carried out. As our previous study,21 the nCC/nC−H values are used to represent the population ratios of the coordination complexes C1ApMTSCC and C1ApMTSC−H. Moreover, on the basis of the value of nCC/nC−H and the free energy barriers, the probability ratio of CC insertion and C−H activation by species A+, PCC/PC−H, was calculated to be 95, suggesting that the probability of CC insertion could be about 99.0% at the chain initiation stage. To explore the origin of the co-occurrence of chain-growth and step-growth polymerizations in the pMOS case, the continuous reaction of the second molecule of pMOS was also calculated. It is noteworthy that the 2,1-re insertion of styrene into the species with an anisyl alkyl was found to be the most C
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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Organometallics favorable manner, as reported in our previous work.22 The 2,1re insertion manner was therefore considered here for the second molecule of pMOS. As shown in Figure 4, the CC
insight into this difference, energy decomposition41,42 and natural bond orbital (NBO) charge analyses were carried out for the corresponding three transition states (TS1ApMOSC−H, TS1ApMNSC−H, and TS1ApMTSC−H) featuring C−H activation (Figure 5). The energies of the monomer moiety and the
Figure 4. Computed energy profiles for A+-mediated C−H activation (black curve) and CC insertion (red curve) of pMOS. Free energies are relative to isolated reactants.
insertion occurs through the pathway C2ApMOSCC → TS2ApMOSCC → P2ApMOSCC. This process has an energy barrier of 18.3 kcal/mol and is exergonic by 4.5 kcal/mol. However, the C−H activation reaction has a higher energy barrier (TS2ApMOSC−H, 26.5 kcal/mol) and is endergonic by 5.6 kcal/mol. Therefore, the CC insertion reaction of the second pMOS is more favorable in both kinetics and thermodynamics than C−H activation. As aforementioned previously, in the chain-initiation step (reaction of the first molecule of pMOS), CC insertion and C−H activation are kinetically competitive, with a thermodynamic priority of C−H activation. This situation stimulated us to calculate the subsequent reaction of the third molecule. The result indicates that the C−H activation is more favorable in kinetics than C C insertion (13.9 vs 16.2 kcal/mol; Figure S2). The calculated energy profiles for the reactions of first three molecules (Figures 1 and 4 and Figure S2) demonstrated an alternative priority for the CC insertion and C−H activation of pMOS. This could account for the experimentally observed simultaneous step-growth and chain-growth polymerization of pMOS. Meanwhile, the CC insertion and C−H activation of the second molecules of pMNS and pMTS have also been investigated, respectively, during which only the 2,1-re fashion was considered because of the experimentally observed syndiotactic polymers.12,14 In the case of pMNS, the results show that the CC insertion is both kinetically and thermodynamically more favorable than C−H activation (Figure S3). The same is true for the case of pMTS (Figure S4). Therefore, it is possible for both C−H activation and C C insertion to simultaneously take place in the pMOS case, while the CC insertion occurs exclusively in the cases of pMNS and pMTS. These results are consistent with previous experimental observations.12,14 For a better understanding of the heteroatom effect, it is necessary to make a systematic comparison of the C−H activation of pMOS, pMNS, and pMTS by species A+. In the chain-initiation step, energy barriers of C−H activation in pMOS, pMNS, and pMTS are 17.9, 23.2, and 21.4 kcal/mol, respectively. The energy barrier of the pMOS case is distinctly lower than those of the pMNS and pMTS cases. To get deep
Figure 5. (a) Energy (kcal/mol) decomposition of TS1ApMOSC−H, TS1ApMNSC−H, and TS1ApMTSC−H. (b) Geometric structures of TS1ApMOSC−H, TS1ApMNSC−H, and TS1ApMTSC−H. NBO atomic charges are shown in blue in parentheses. The S value denotes the average square error of charge (S = ∑(|Qx| − |Q|)2/n, n = 4) of Sc, C1, C2, and H atoms, where |Q| represents unsigned average charge and Qx denotes the charge on each atom included.
remaining metal complex (two fragments) in the TS geometries were evaluated via single-point calculations. Such single-point energies of the fragments and the energy of TS were used to estimate the interaction energy ΔEint. These energies, together with the energies of the respective fragments in their optimal geometry, allow for the estimation of the deformation energies of the two fragments, ΔEdef(cat.) and ΔEdef(mono.). As the energy of the TS, ΔETS, is evaluated with respect to the energy of the two separated fragments, the relation ΔETS = ΔEint + ΔEdef(cat.) + ΔEdef(mono.) holds. As shown in Figure 5a, in the case of TS1ApMOSC−H, the total deformation energy ΔEdef is 54.3 kcal/mol, which could be partially balanced out by its ΔEint value (−40.2 kcal/mol), leading to a ΔETS value of 14.1 kcal/mol. In contrast, a larger total deformation energy (ΔEdef = 70.4 kcal/mol) in the TS1ApMNSC−H case can hardly be compensated by a stronger interaction energy of −49.9 kcal/mol, thus producing a higher ΔETS value (20.5 kcal/mol). Therefore, the greater steric repulsion between the metal center and monomer moiety in TS1ApMNSC−H could account for the lower stability of TS1ApMNSC−H in comparison with TS1ApMOSC−H. Similarly, there is still a larger total deformation energy (57.4 kcal/mol) in the TS1ApMTSC−H case, which results in a larger ΔETS value (18.4 kcal/mol) in comparison with TS1ApMOSC−H. As a D
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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Figure 6. Computed energy profiles for C−H activation (black curve) and CC insertion (red curve) polymerization of the first and second molecules of oMOS by cationic species A+. Free energies are relative to the energy sum of species A+ and corresponding monomers.
consequence, the lower stabilities of TS1ApMTSC−H and TS1ApMNSC−H are mainly due to steric hindrance inducing greater deformations of monomer moieties and the remaining metal complexes. In order to further explore the reasons for the stability of TS1ApMOSC−H, we carried out NBO charge analysis. As shown in Figure 5b, the NBO charges of Sc (1.41, 1.48, 1.27) and H (0.24, 0.23, 0.23) atoms are all positive in the TSs, which suggest proton transfer events in such C−H activations featuring σ-bond metathesis.43 As we know, charge dispersion is closely related to the stability of a structure.44 In C−H activation transition states, the four-membered Sc−C1−H−C2 units are the most important moieties related to chemical changes. To understand the stability of the structures, the average square errors (S) of Sc, C1, H, and C2 atoms in transition states TS1ApMOSC−H, TS1ApMNSC−H, and TS1ApMTSC−H were considered in this study to estimate the degree of charge dispersion. In general, the smaller the value of S, the more stable the structure. In contrast, the S value in TS1ApMOSC−H is 0.198, which is smaller than those (S = 0.228 and S = 0.203) for TS1ApMNSC−H and TS1ApMTSC−H, accounting for the greater stability of TS1ApMOSC−H in comparison with TS1ApMNSC−H and TS1ApMTSC−H. Effect of Methoxyl Substituent Position on C−H Activation and CC Insertion. Unlike pMOS, the oMOS polymerization mediated by A+ occurred exclusively in a chaingrowth fashion via continuous CC insertions.10,11 This phenomenon drove us to further calculate the CC insertion and C−H activation of oMOS. As shown in Figure 6, the C−H activation starts with the complex C1AoMOSC−H and then goes through TS1AoMOSC−H with an energy barrier of 20.0 kcal/mol. However, the CC insertion reaction along the path C1AoMOSCC → TS1AoMOSCC → P1AoMOSCC has a lower energy barrier (11.9 kcal/mol), suggesting a favorable CC insertion event. The more stable CC insertion product P1AoMOSCC should be considered for the subsequent reaction of the second molecule. As for the first molecule of oMOS, the second molecule favorably undergoes CC insertion both kinetically and thermodynamically (Figure 6). This result is in good agreement with the experimentally observed exclusive chain-growth event.10,11
For a better understanding of the effect of methoxyl substitution position, similar energy decomposition analyses were performed for transition states TS1AoMOSC−H and TS1AoMOSCC. The results indicate that the greater deformation energy of the oMOS moiety in TS1AoMOSC−H made this C−H activation TS less stable than TS1AoMOSCC (Figure S5). For a comparison, the energy decomposition result of TS1ApMOSC−H is also included in Figure S5, which indicates that the pMOS moiety has a smaller deformation energy in comparison with that in TS1AoMOSC−H (40.0 vs 43.0 kcal/mol). In a structural comparison of TS1AoMOSC−H and TS1ApMOSC−H (Figure 7), it is found that the dihedral angle ∠C5C6C7C8
Figure 7. Geometric parameters (distances in Å) of TS1AoMOSC−H, TS1ApMOSC−H, TS1AoMOSCC, and TS1ApMOSCC.
(23.3°) involving OMe and the dihedral angle ∠C11C8C9C10 (54.4°) involving CC in TS1AoMOSC−H are obviously larger than those (∠C5C6C7C8 = 8.7° and ∠C9C10C11C12 = 8.1°) in TS1ApMOSC−H. This suggests that the deformations of the vinyl group and OMe of oMOS make the C−H activation of oMOS relatively unfavorable. However, in the case of CC E
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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Figure 8. Computed energy profiles for C−H activation (black curve) and CC insertion (red curve) of the first and second pMOS molecules by cationic species C+. Free energies are relative to the energy sum of species C+ and corresponding monomers.
insertion of oMOS, the coordination of the OMe group endowed the TS with stability and therefore the CC insertion reaction was favorable in comparison with the case of pMOS (see transition structures in Figure 7). In this sense, the coordination of a heteroatom could be capable of promoting a CC insertion reaction (olefin polymerization). Such a promotion effect of the heteroatom coordination was also observed in rare-earth-metal-catalyzed polymerization of ethercontaining α-olefins,25 which is contrast to group 4 metal catalyzed polymerization systems, where the promotion effect of a heteroatom coordination was not reported for the analogous monomers.45−49 C−H Activation and CC Insertion of pMOS by Different Catalysts. Attracted by the discrepancy of pMOS polymerization by species A+ (Scheme 1a) and C+ (Scheme 1b),10,13 we further investigated the effects of the different ligands on C−H activation and CC insertion. Previous experimental works reported that only the CC insertion reaction of pMOS occurred by cationic species C+.13 Our previous study indicates that C+-mediated styrene insertion favorably follows the alternative manner of 2,1-re → 2,1-si → 2,1-re... to achieve a syndiotactic sequence.24 On the basis of this result, the CC insertion processes of the first four molecules of pMOS have been calculated (Figure 8 and Figure S6). It is found that the first four monomer insertions overcome lower energy barriers (13.4/8.3/14.0/19.4 kcal/ mol) and release greater energies (−14.4/−21.0/−6.4/−5.0 kcal/mol) in comparison with C−H activation reactions (energy barriers of 18.2/19.1/23.1/21.9 kcal/mol and energy releases of −6.8/−8.7/3.4/−2.5 kcal/mol). Therefore, the CC insertions mediated by species C+ are both kinetically and thermodynamically more favorable than C−H activations, which is consistent with the experimental observation. In contrast, the experimental10 and calculation (Figure S7) results showed that species B+ (analogous to A+, Scheme 1b) can also catalyze simultaneous CC insertion and C−H activation of pMOS at the chain propagation stage.
The ligand-induced chemoselectivity drove us to further investigate the effect of the ancillary ligand of the yttrium complexes (B+ and C+) on C−H activation and CC insertion (Scheme 1b). For this purpose, similar energy decomposition analyses of transition states TS4CpMOSCC and TS4CpMOSC−H involved in the C+-mediated processes (Figure S6) have also been carried out (Figure S8). In TS4CpMOSCC, the total deformation energy ΔEdef of the two fragments (monomer and the remaining metal complex) is 50.2 kcal/mol, which could partially offset the ΔEint value (−24.5 kcal/mol), leading to a ΔETS value of 25.7 kcal/mol. In the case of TS4CpMOSC−H for C−H activation, although the interaction between the corresponding two fragments is stronger (ΔEint = −37.3 kcal/mol), it could not offset the larger total deformation energy (ΔEdef = 68.6 kcal/mol) and eventually leads to a higher ΔETS value (31.3 kcal/mol) in comparison with TS4CpMOSCC for CC bond insertion (Figure S8). Therefore, the greater steric hindrance makes pMOS deform more significantly for C−H activation, which accounts for the lower stability of TS4 CpMOS C−H in comparison with TS4CpMOSCC. This could be the main reason for the unfavorable C−H activation. A comparison of TS4CpMOSC−H with TS4BpMOSC−H involved in B+-initiated C−H activation indicates that the interaction between the corresponding two fragments is stronger in the latter case (interaction energy of −45.9 vs −37.3 kcal/mol, Figure S8). This contributes to the stability of TS4BpMOSC−H and thus accounts for relatively feasible C−H activation in B+initiated processes. For a better understanding of the stronger interaction in the TS4BpMOSC−H case, the analyses of geometry and charge population of these two transition states have been carried out. As shown in Figure 9, the Y−O (2.41 Å) and Y− C1 (2.46 Å) distances in TS4BpMOSC−H are shorter than those (d(Y−O) = 2.43 Å, d(Y−C1) = 2.52 Å) in TS4CpMOSC−H, which is consistent with a stronger interaction in the TS4BpMOSC−H case. In addition, the NBO charges on the Y (1.59), O (−0.57), and C1 (−0.46) atoms of TS4CpMOSC−H are less positive or less negative than those (Y, 1.79; O, 0.58; C1, F
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for X.K.:
[email protected]. *E-mail for Y.L.:
[email protected]. ORCID
Gen Luo: 0000-0002-5297-6756 Dongmei Cui: 0000-0001-8372-5987 Zhaomin Hou: 0000-0003-2841-5120 Yi Luo: 0000-0001-6390-8639 Notes
The authors declare no competing financial interest. Figure 9. Geometric structures (distances in Å) of TS4CpMOSC−H and TS4BpMOSC−H. The values in black denote bond lengths, and those in blue represent atomic NBO charges.
ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the NSFC (No. 21429201, 21674014, 21704011). X.K. thanks the State Key Laboratory of Fine Chemicals for a Research Grant (KF 1713). Y.L. and G.L. thank the Fundamental Research Funds for the Central Universities (DUT2016TB08, DUT18GJ201, DUT18RC(3) 002). The RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of Dalian University of Technology are acknowledged for part of the computational resources.
0.51) in TS4BpMOSC−H, respectively. Therefore, the pyridine side arm of species C+ as a strong electron donor decreases the Lewis acidity of the metal Y center, thus weakening the coordination ability between pMOS and the Y center.
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CONCLUSION The polymerization mechanism of polar functional group substituted styrenes catalyzed by cationic rare-earth-metal alkyl complexes has been comparatively studied through DFT calculations. The origin of various polymerization manners, viz. step growth via C−H activation and chain growth through vinyl insertion, has been disclosed. Having achieved an agreement between theory and experiment, the following conclusions can be drawn. (1) Smaller steric hindrance and greater average charge distribution in the four-center transition state are beneficial for C−H activation relevant to step-growth polymerization; (2) The C−H activation reaction can hardly occur in ortho-substituted styrene mainly because of the large steric hindrance of the substituted functional group next to the vinyl group. Instead, the coordination of the heteroatom of the functional group could facilitate the insertion of its neighboring vinyl group, resulting in a relatively detrimental effect on C−H activation. (3) The catalyst with a coordinative side arm is adverse to C−H activation. This is due to the steric hindrance around the metal center and electron-donating nature of the side arm, which destabilized the C−H activation transition state. The current theoretical results provide useful information for the development of a C−H activation based polymerization system of polar group substituted styrene and the control of polymer microstructure.
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ASSOCIATED CONTENT
S Supporting Information *
(PDF file). (XYZ file). The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00532. Computed energy profiles for chain initiation or propagation by cationic species A+, B+, and C+ and energy decomposition analyses of some key stationary points (PDF) Optimized Cartesian coordinates of all stationary points together with their single-point energies (au) in solution and the imaginary frequencies (cm−1) of transition states (XYZ) G
DOI: 10.1021/acs.organomet.8b00532 Organometallics XXXX, XXX, XXX−XXX
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