Mechanistic Understanding of Dinuclear Cobalt (III) Complex

Article pubs.acs.org/Macromolecules

Mechanistic Understanding of Dinuclear Cobalt(III) Complex Mediated Highly Enantioselective Copolymerization of mesoEpoxides with CO2 Ye Liu, Wei-Min Ren, Chuang Liu, Song Fu, Meng Wang, Ke-Ke He, Rong-Rong Li, Rong Zhang, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: The desymmetrization copolymerization of meso-epoxides with CO2 using chiral catalysts or reagents is regarded as a valuable strategy for the synthesis of optically active polycarbonates with main-chain chirality. The present study demonstrates that the biphenol-linked dinuclear Co(III) complexes show unprecedented activity, enantioselectivity, and broad substrate scope for coupling CO2 with various mesoepoxides to afford the corresponding isotactic polycarbonates. This investigation also focuses on the mechanistic understanding on the origin of enantioselectivity and highly catalytic activity in the enantiopure dinuclear Co(III) complex mediated copolymerization process, using cyclohexene oxide as a model monomer of meso-epoxides. The kinetic study by in situ infrared spectroscopy revealed a first-order dependence on the catalyst concentration in the systems of biphenol-linked dinuclear Co(III) complex alone or in the presence of an ionic cocatalyst. An intramolecular bimetallic cooperation mechanism was proposed to be predominantly responsible for the copolymerization process, wherein alternating chain growth and dissociation take turns between two Co(III) ions from the inside cleft of the catalyst by the nucleophilic attack of the growing carboxylate species at one metal center toward the activated epoxide at the other. Density functional theory calculations suggested that in the two diastereoisomers of the biphenol-linked dinuclear Co(III) complexes the matched configuration was (S,S,S,S,S)- rather than (S,S,R,S,S)-conformer for CO2/meso-epoxide copolymerization to give the corresponding polycarbonate with S,S-configuration. The addition of an ionic cocatalyst with bulky cation significantly improves both the catalytic activity and enantioselectivity, while the presence of a coordination Lewis base caused dramatically a change in the chiral induction orientation.

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stereoregular catalysts for coupling CO2 with epoxides,18 we became interested in the preparation of crystalline CO2-based polycarbonates by stereospecific polymerization catalysis. Initially, we focused on the enantioselective copolymerization of CO2 and racemic propylene oxide, a much-studied and easily accessible epoxide. However, highly stereoregular poly(propylene carbonate)s even with >99% isotacticity and nearly 100% head-to-tail linkages did not show any crystallizable behavior.19 This unexpected result draw our attention toward the synthesis of isotactic polycarbonates by chiral metal complex mediated desymmetrization copolymerization of alicyclic mesoepoxides with CO2, though in our subsequent studies two highly isotactic CO2 copolymers from enantiopure epichlorohydrin and phenyl glycidyl ether were found to be crystallizable.20 The first attempt for asymmetric CO2/meso-epoxide coupling (Scheme 1) was disclosed by Nozaki and co-workers using equivalent Et2Zn and (S)-α,α-diphenyl(pyrrolidin-2-yl)methanol mixture as cata-

INTRODUCTION The use of carbon dioxide as a safe and cheap C1 feedstock in the production of degradable polycarbonates by the alternating copolymerization with epoxides has attracted much attention over the past decade due to the economic and environmental benefits arising from the utilization of renewable source.1 This process represents a much greener alternative to the commonly practiced use of 1,2-diols and the highly toxic carbonylation reagent, phosgene. Numerous catalyst systems have been developed for this transformation.2−15 Particularly, significant progress in the design of well-defined metal catalysts resulted in greatly improved catalytic activity, selectivity for copolymer formation and stereochemistry control, and importantly a better understanding of the mechanistic aspects.9−16 These studies also led to producing various CO2-based polycarbonates from not only alicyclic and aliphatic epoxide monomers but also the epoxides with an electron-withdrawing group.17 Nevertheless, most of the CO2-based polycarbonates are known to be amorphous, and their low thermal deformation resistance makes them difficult to utilize as structural materials. Pursuant to our own efforts toward the development of highly © XXXX American Chemical Society

Received: September 15, 2014 Revised: October 21, 2014

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dx.doi.org/10.1021/ma5019186 | Macromolecules XXXX, XXX, XXX−XXX

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selectivity of 96% ee was achieved by performing the polymerization at −25 °C with (S)-2-methyltetrahydrofuran as a chiral induction agent. Although this complex is highly enantioselective at low temperature, its catalytic activity (TOF less than 3 h−1 at −25 °C) is not satisfied, and the molecular weight of the resultant copolymers is low. More recently, we developed a chiral catalyst system based on the biphenol-linked dinuclear Co(III) complexes 1, which exhibits excellent activity, unprecedented enantioselectivity, and molecular-weight control for the alternating copolymerization of CO2 with both CHO and cyclopentene oxide (CPO, a less reactive epoxide) under mild reaction conditions.26 Unexpectedly, reducing the steric hindrance of the ortho-substituents from tert-butyl groups to methyl groups or hydrogen atoms significantly improves the ee to 99% while maintaining a TOF greater than 180 h−1 for CO2/ CPO copolymerization at 25 °C. Moreover, this catalyst system was found to be highly enantioselective in copolymerizing CO2 with meso-3,5-dioxaepoxides, affording typical semicrystalline polymers, possessing Tms of 179−257 °C, dependent on the substitute groups at 4-position of the meso-epoxides.27 Herein, we communicate studies aimed at the mechanistic interpretation of CO2/meso-epoxide coupling process mediated by biaryl-linked dinuclear cobalt(III) complexes, particularly with focus on the effect of biaryl linker, cocatalyst, and the ortho-substituents of the ligand on the configuration preference and enantioselectivity of the resultant copolymers.

Scheme 1

lyst, producing the corresponding polycarbonates with moderate enantioselectivity.21 Interestingly, a nonlinear relationship between the enantiopurity of the ligand and the enantiomeric excess (ee) of the hydrolyzed polycarbonate was found in their subsequent study, suggesting a bimetallic mechanism.21c Soon thereafter, Coates’ research group reported that C1-symmetric imine-oxazoline-ligated zinc bis(trimethylsilyl)amido complexes were more active in catalyzing CO2/meso-epoxide copolymerization.22 Notably, the authors were the first to note the existence of both the glass transition temperature (Tg) and melting point (Tm) in their synthesized poly(cyclohexene carbonate) (PCHC) with 86% isotacticity. Recently, enantiopure dinuclear aluminum complexes of β-ketoiminate or aminoalkoxide, in conjunction with a bulky Lewis base as catalyst activator, were investigated for copolymerizing CO2 with meso-epoxides to afford the copolymers with moderate enantioselectivity (60−80% ee).23 In 2006, we described binary catalyst systems consisted of a chiral mononuclear Co(III)−Salen complex and bis(triphenylphosphine)iminium chloride (PPNCl) for the desymmetrization copolymerization of alicyclic cyclohexene oxide (CHO) and CO2 at ambient temperature, affording PCHC with a low ee of 38%.24 The subsequent study found that asymmetric Co(III)−Salen complexes containing a bulk adamantyl orthosubstituent on one side of the ligand and a tert-butyl group on the other showed an improved enantioselectivity.25 The highest

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RESULTS AND DISCUSSION Enantioselective Copolymerization of CO2 and Various meso-Epoxides. The optically active polycarbonates with chiral (R,R)- or (S,S)-trans-1,2-diol units in the main chain can be produced by desymmetrization copolymerization of CO2 with meso-epoxides. This methodology was first demonstrated by Nozaki and co-workers in 1999,21a and significant progress in this field has been made in recent years.22−28 Most studies have been focused on the formation of polycarbonates from mesocyclohexene oxide. Compared with the diversity of terminal epoxides, CO2 copolymers from meso-epoxide have historically received far less attention as a consequence of the lack of highly

Chart 1

B


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Table 1. Asymmetric Copolymerization of CHO with CO2 Catalyzed by Cobalt Complexesa

entry

catalyst/cocatalyst

time (h)

TOFb (h−1)

carbonate linkagesc (%)

Mnd (kg/mol)

PDId

eee (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21f 22f 23f 24g

(S,S,S,S)-1a (S,S,S,S)-1b (S,S,S,S)-1c (S,S,S,S)-1d (S,S)-4a (S,S,S,S)-5 (S,S,S,S,S)-2 (S,S,R,S,S)-2 (S,S,S,S)-1a/PPN-DNP (S,S,S,S)-1b/PPN-DNP (S,S,S,S)-1c/PPN-DNP (S,S,S,S)-1d/PPN-DNP (S,S)-4a/PPN-DNP (S,S,S,S)-5/PPN-DNP (S,S,S,S,S)-2/PPN-DNP (S,S,R,S,S)-2/PPN-DNP (S,S,R,S,S)-3a/ PPN-DNP (S,S,R,S,S)-3b/PPN-DNP (S,S,S,S,S)-3b/PPN-DNP (S,S,R,S,S)-3c/PPN-DNP (S,S,S,S)-1a/PPN-DNP (S,S,S,S)-1b/PPN-DNP (S,S,S,S)-1c/PPN-DNP (S,S,S,S)-1c/PPN-DNP

2 2 2 10 24 24 24 24 0.25 0.25 0.25 2 3 2 5 1 4 1 4 0.5 1 1 1 6

194 189 200 17 6 10 14 11 1269 1356 1409 173 101 198 56 395 71 483 99 758 1000 1000 1000 166

>99 >99 >99 >99 90 91 98 87 >99 >99 >99 >99 >99 >99 >99 65 68 94 99 92 >99 >99 >99 >99

20.5 19.7 21.1 8.5 5.9 6.4 7.1 6.8 16.7 11.5 18.9 12.9 12.2 13.7 10.4 12.0 12.7 18.7 14.0 13.8 59.0 56.9 58.3 58.2

1.25 1.19 1.21 1.11 1.24 1.20 1.19 1.22 1.21 1.18 1.23 1.22 1.18 1.21 1.15 1.21 1.11 1.21 1.18 1.15 1.20 1.18 1.23 1.21

33 (S,S) 67 (S,S) 71 (S,S) 5 (S,S) 39 (R,R) 41 (R,R) 38 (R,R) 4 (S,S) 47 (S,S) 77 (S,S) 81 (S,S) 12 (S,S) 38 (R,R) 36 (R,R) 58 (R,R) 7(R,R) 20 (R,R) 4 (R,R) 39 (R,R) 4 (R,R) 71 (S,S) 90 (S,S) 92 (S,S) 98 (S,S)

The reaction was performed in neat CHO (3.0 mL, except entries 21−24 in toluene solution of CHO) in 20 mL autoclave at 25 °C and 1.5−2.0 MPa CO2 pressure. Catalyst/CHO = 1/1000 (molar ratio). Catalyst/PPN-DNP = 1/2 (molar ratio) for entries 9−24, except entry 13 of 1/1. DNP = 2,4-dinitrophenoxide. The polymer selectivity is >99% based on 1H NMR spectroscopy. bTurnover frequency (TOF) = moles of product (polycarbonates)/mole of catalyst per hour. cDetermined by using 1H NMR spectroscopy. dDetermined by gel permeation chromatography in THF, calibrated with polystyrene. eMeasured by hydrolyzing the polymer and analyzing the resulting diol by chiral GC. fThe reaction was carried out in toluene solution of CHO (CHO/toluene = 1/2, volume ratio) at 25 °C. gThe reaction was carried out in toluene solution of CHO (CHO/toluene = 1/2, volume ratio) at 0 °C. a

of highly isotactic polycarbonates from various meso-epoxides demonstrates that the biphenol-linked dinuclear Co(III) complex is a rare privileged chiral catalysts for this enantioselective copolymerization. Systematic Study on Effects of Co(III) Complex Structure and Cocatalyst. Since cyclohexene oxide (CHO) has relatively high reactivity in copolymerizing with CO2 catalyzed by both mono- and dinuclear Co(III) complexes in the absence or presence of any nucleophilic cocatalyst, it was chosen as a model monomer of meso-epoxides for testing the catalytic activity and enantioselectivity of various enantiopure Co(III) complexes. In a previous study, we have demonstrated that the dinuclear complexes (S,S,S,S)-1a, 1b, and 1c alone could operate at high efficiency in catalyzing this asymmetric copolymerization with TOFs (turnover frequency) of about 200 h−1 at 25 °C (Table 1, entries 1−3).26 The resultant copolymers show enantioselectivities of 33−71% but possess perfectly alternating nature with >99% carbonate linkages. The use of (S,S,S,S)-1d with bulky adamantyl groups on the phenolate ortho position showed a low activity of 17 h−1 and poor enantioselectivity of 5% ee in catalyzing this reaction under the same conditions. Surprisingly, the use of these catalyst with an S,S,S,S-configuration predominantly provides the product with S,S-configuration. The result is distinct from the mononuclear Co(III) complex (S,S)-4a and a bimetallic cobalt complex

active and enantioselective catalyst as well as the lower reactivity associated with meso-epoxides due to their large steric hindrance. Recent investigation showed that stereospecific CO2 copolymers from meso-epoxides are easier to crystallize than that from the terminal epoxides. Particularly, stereoregularity plays the critical role on the crystallization for this kind of polymeric material.29 Therefore, the development of highly efficient catalysts for enantioselective copolymerization of CO2 and meso-epoxides is of great significance. More recently, we developed a highly active and enantioselective catalyst systems based on the biphenollinked dinuclear Co(III) complex 1b or 1c for the desymmetrization copolymerization of CO2 with meso-CPO or CHO, affording the corresponding polycarbonate with more than 99% carbonate unit and ≥98% enantioselectivity.26 In the present investigation, the scope with respect to various challenge meso-epoxide substrates was then explored under the mild reaction conditions: 0.1 mol % of (S,S,S,S)-1b or 1c, 1.5−2.0 MPa CO2 pressure, and 0−25 °C. Surprisingly, all tested meso-epoxides, including the simplest meso-epoxide, cis-2,3-epoxybutane, showed high reactivities in copolymerizing with CO2 to produce the corresponding polycarbonates with complete alternating structure and ≥99% enantioselectivity. The TOFs are in the range of 120− 1400 h−1, dependent on the structure of meso-epoxides (Supporting Information, Table S1). Among them, CHO is more reactive meso-epoxide. The wide generality for the synthesis C


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Table 2. Enantioselective CO2/CHO Copolymerization at Various Conditionsa

entry

catalyst/cocatalyst

catalyst/cocatalyst (molar ratio)

time (h)

concb (%)

Mnc (kg/mol)

PDIc

eed (%)

1 2 3 4 5 6 7 8 9 10

(S,S,S,S)-1a/PPN-DNP (S,S,S,S)-1a/PPN-DNP (S,S,S,S)-1a/PPN-DNP (S,S,S,S)-1a/PPN-DNP (S,S,S,S)-1a/PPN−PF6 (S,S,S,S)-1e/PPN-DNP (S,S,S,S)-1a/PPN-OAc (S,S,S,S)-1a/PPN-F (S,S,S,S)-1a/PPN-Cl (S,S,S,S)-1a/nBu4N-DNP

1/1 1/1 1/2 1/5 1/2 1/1 1/2 1/2 1/2 1/2

0.33 0.42 0.33 0.33 0.33 0.33 0.50 0.50 0.50 0.50

38.9 41.9 42.0 37.3 23.9 16.8 41.0 32.6 31.4 24.8

16.7 18.2 18.7 15.8 10.6 8.9 16.5 11.7 11.0 10.1

1.20 1.14 1.17 1.18 1.21 1.16 1.24 1.22 1.20 1.22

46 (S,S) 46 (S,S) 47 (S,S) 46 (S,S) 46 (S,S) 45 (S,S) 46 (S,S) 48 (S,S) 46 (S,S) 42 (S,S)

a The reaction was performed in neat CHO (3.0 mL, 30 mmol) in 20 mL autoclave at 25 °C and a 2.0 MPa CO2 pressure; catalyst/CHO = 1/1000 (molar ratio); DNP = 2,4-dinitrophenoxide. The polymer selectivity and carbonate linkages of the resulted polycarbonates are >99% based on 1H NMR spectroscopy. bThe conversion of epoxide, determined by using 1H NMR spectroscopy. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene. dMeasured by hydrolyzing the polymer and analyzing the resulting diol by chiral GC, and the (S,S)-diol is the major enantiomer.

(S,S,S,S)-5 bearing a flexible linker alone as catalyst for CO2/ CHO copolymerization, affording the copolymers with R,Rconfiguration excess (entries 5 and 6). Although the binaphthollinked dinuclear Co(III) complex (S,S,S,S,S)-2 alone exhibited a relatively low activity in catalyzing this reaction, the resultant polycarbonate has an enantioselectivity of 38% ee for R,Rconfiguration excess (entry 7). Similar activity was also observed in the system of (S,S,R,S,S)-2 as catalyst; however, a neglectable enantioselectivity of 4% ee was found in the resultant polymers, in which ether linkage content of 13% was randomly dispersed in the polycarbonate chains (entry 8). The great increases in catalytic activity for all tested Co(III) complexes were realized by the addition of an ionic cocatalyst PPN−DNP (DNP = 2,4-dinitrophenoxide) (Table 1, entries 9− 16). For the catalyst systems based on (S,S,S,S)-1a−1d the product enantioselectivity was also improved to a certain extent (vide inf ra). The screening results also revealed that the steric hindrance of the phenolate ortho-substituents strongly influenced the catalytic activity and product enantioselectivity. The bulky substituents on the aromatic rings were not the best choice for the biphenol-linked dinuclear Co(III) catalyst systems. The product enantioselectivity decreased with increasing size of the substituents in the phenolate ortho positions (ee: 81% > 77% > 47% > 12%; size: H < Me < tBu < adamantyl) in the presence of PPN−DNP (entries 9−12). Recently, Coates and co-workers have developed the binaphthol-linked dinuclear cobalt complex 2 in conjunction with an ionic ammonium salt as cocatalyst for highly enantioselective homopolymerization of terminal epoxides and also performed mechanistic study extensively.30 Density functional theory (DFT) calculations suggested that the absolute stereochemistry of the binaphthol linker determined the enantiomer preference in the polymerization, and the interaction between the Salen ligand and the growing polymer chain was a fundamental aspect of enantioselectivity; especially, the matched configuration of (S,S,R,S,S) was more effective than (S,S,S,S,S) for epoxide homopolymerization. However, when the binaphtholbridged dinuclear cobalt catalyst 2 was applied to asymmetric CO2/CHO coupling, the matched configuration of (S,S,R,S,S) was less effective than (S,S,S,S,S) (Table 1, entries 7 and 8). Additionally, in contrast with the high enantioselectivity in

biphenol-bridged dinuclear cobalt catalyst 1 mediated asymmetric CO2/meso-epoxide coupling, the binaphthol-bridged analogue 2 is less efficient for the same reaction. Surprisingly, the addition of PPN−DNP led to a dramatic increase in catalyst activity of (S,S,R,S,S)-2, but the resultant polymer had a very low enantioselectivity of 7% ee for R,R-configuration excess and a reduced carbonate unit content of 65% (entry 16). However, (S,S,S,S,S)-2/PPN−DNP catalyst system can provide the copolymers with 58% ee, although the activity decreased to a certain extent compared with (S,S,R,S,S)-catalyst (entry 15). For comparison purposes, the dinuclear cobalt complexes 3a−c based on chiral biphenol with variable length to link the diaryl groups were also designed for the asymmetric copolymerization CHO with CO2 (for detailed synthesis procedures, see the Supporting Information). As the tunable linking bridge at the 6 and 6′ positions on the biphenol can restrict its free rotation, the cobalt complexes 3a−c have the single enantiomeric axial chirality compared with 1a−d based on biphenol linker. We discovered that the enantiopure complexes (S,S,R,S,S)-3a complex with C4 linking bridge in conjunction with PPN− DNP (DNP = 2,4-dinitrophenoxide) could catalyze CO2/CHO copolymerization, with a TOF of about 71 h−1 at ambient temperature; the produced copolymers had an enantioselectivity of 20% ee with R,R-configuration (entry 17). An increase in the length of linking bridge between the diaryl groups, the TOFs were improved largely (entries 18 and 20). The catalyst system based on (S,S,R,S,S)-3b with C6 linking bridge exhibited an enhanced activity of 483 h−1, while that on (S,S,R,S,S)-3c with C8 linking bridge was 758 h−1. Unfortunately, the enantioselectivities of both systems was very low (4% ee for R,R-configuration). It was discovered that the catalyst with a S,S,S,S,S-configuration was beneficial for enantioselectivity control. With the use of (S,S,S,S,S)-3b/PPN−DNP as catalyst, the enantioselectivity was increased to 39% ee with R,R-configuration (entry 19). Interestingly, both systems based on (S,S,S,S,S)-3 and (S,S,R,S,S)-3 predominantly provided the copolymers with R,R-configuration excess, which is the similar to the dinuclear cobalt complex 2 based on binaphthol linker. The further improvement in product enantioselectivity was discovered in the dilute solution of toluene (Table 1, entries 21− 24). For example, the copolymerization was carried out in D


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toluene solution of CHO (CHO/toluene = 1/2, volume ratio) at 25 °C, the enantioselectivity increased to 71% and 90% for dinuclear Co(III) 1a and 1b catalyst systems, respectively. Importantly, when the copolymerization proceeded at 0 °C and in the presence of the organic solvent toluene, the resultant poly(cyclohexene carbonate) possesses a high enantiopurity up to 98% ee. Although mononuclear Co(III)−Salen 4a alone could catalyze the copolymerization of CO2 with epoxides at high catalyst loadings, the activity was not satisfactory. Nevertheless, the addition of a nucleophilic cocatalyst, such as an ionic quaternary ammonium salt (e.g., PPN−DNP), significantly increased the catalytic activity. In this binary catalyst system, the nucleophilic cocatalyst was demonstrated to function as an initiator.13 Another role of the cocatalyst was suggested to stabilize the active Co(III) against decomposition to the inactive Co(II) complex by alternating chain growth and dissociation of propagating carboxylate species derived from the nucleophilic anions at both sides of the Co(III)−Salen plane.14d ESI-MS study and control experiments demonstrated an important polymerization feature of the binary catalyst systems based on mononuclear Co(III)−Salen, in which the dissociation of the propagating carboxylate from the metal center is a much faster process than propagation, and the free propagating carboxylate can also act as a nucleophile for attack at a cobalt-coordinated epoxide during the copolymerization. The reaction rate increases with the concentration of both Co(III) complex and the nucleophilic cocatalyst; moreover, the number of polymer chains is proportional to the cocatalyst concentration. For the biphenol-linked dinuclear Co(III) complex catalyzed asymmetric copolymerization of meso-epoxide with CO2, the presence of a nucleophilic ionic cocatalyst was found to greatly increase both the catalytic activity and product enantioselectivity (Table 1, entries 1−4 versus 9−12). In the presence of PPN− DNP (DNP = 2,4-dinitrophenoxide), complex (S,S,S,S)-1b or 1c exhibits unprecedented enantioselectivity and activity in copolymerizing CO2 and various meso-epoxides at mild conditions. Surprisingly, being different with the catalyst systems based on mononuclear Co(III)−Salen 4a, the cocatalyst loading beyond 1 equiv has little effect on the reaction rate and copolymer molecular weight (Table 2, entries 1−4). For example, an alteration of the cocatalyst loading from 1 equiv increasing to 2 equiv only results in a slight increase in the catalytic activity, while further increasing to 5 equiv of PPN− DNP, a slight decrease in reaction rate was observed. The resultant copolymers from various cocatalyst loadings all exhibit perfectly monomodal distributions. Moreover, even excess PPN−DNP was added into the reaction mixture at various time points; the obtained polymers also show narrow monomodal distributions rather than bimodal distributions (Figure 1). It is worthwhile noting here parenthetically that at a close conversion the resultant copolymers from various cocatalyst loadings have the very close molecular weight. The results indicate that the propagating polymer chain transfer caused by the excess PPN−DNP does not easily appear.31 Furthermore, some control experiments were performed by changing the ionic cocatalyst or the axial counterions of dinuclear Co(III) complex 1 (entries 5−10). It was found that the substitution of no nuclephilic PF6− for 2,4-dinitrophenoxide ion in cocatalyst had no influence on the copolymer enantioselectivity but caused a decrease in the reaction rate (entry 5). A similar situation also occurred in the catalyst system of PPN− DNP in combination with dinuclear Co(III) complex 1e with no

Figure 1. GPC traces of CO2/CHO copolymers obtained at various conditions. (A) The reaction was carried out with [(S,S,S,S)-1a]/ [PPN−DNP]/[CHO] = 1/2/1000 (molar ratio) at 25 °C and 2.0 MPa CO2 pressure, CHO/toluene = 1/4 (volume ratio) for 0.5 h, and (B) then excess 3 equiv of PPN−DNP was added into the (A) system and further reacted 1.5 h. (C) The reaction was carried out in the presence of 2 equiv of PPN−DNP at the same conditions for 2 h.

nuclephilic PF6− or BF4− (entry 6). The replacement of 2,4dinitrophenoxide ion by other nucleophilic anions, such as OAc−, F−, and Cl−, did not result in any observable change in the catalytic activity and enantioselectivity (entries 7−9). However, in comparison with the ionic cocatalyst with the bulky and hydrophobic PPN+ cation, the nBu4N+ was less effective in enantioselective control (entry 10).13b Kinetic Studies of CO2/CHO Copolymerization. In order to better assess the mechanistic aspects of the copolymerization reaction, we first performed the kinetic study of this process as a function of catalyst loading by in situ FTIR spectroscopy with a probe fitted to a modified stainless steel Parr reactor. As is easily observed, the intense absorbance for the asymmetric v(CO) vibration of polycarbonates appears at ∼1750 cm−1.9b,32 CHO was chosen as a model monomer of meso-epoxides due to its higher reactivity in copolymerizing with CO2 catalyzed by various Co(III) complexes in the absence of any nucleophilic cocatalyst. The initial reaction rates resulted from these experiments are listed in Table S2 of the Supporting Information along with their corresponding catalyst loadings. We discovered that the initial rate is directly proportional to catalyst concentration for the three catalyst systems (Figure 2). However, the effect of catalyst concentration on the initial rate is significantly different for these systems. A reaction order of 1.05 was obtained from dinuclear Co(III) complex 1a mediated CO2/CHO copolymerization, implying the overall reaction pathway is consistent with the first-order dependence on catalyst concentration.15b,32,33 This result also suggests that an intramolecular bimetallic mechanism predominantly determines the copolymerization process. Contrarily, with the mononuclear Co(III) complex 4a as catalyst, a reaction order of 1.49 of catalyst concentration was observed, indicating the complexity of the copolymerization. This demonstrates a considerable contribution of the bimetallic initiation process in course of the reaction along with the monometallic rate-determining step.9e Similarly, a reaction order of 1.44 of catalyst concentration was found in the system of dinuclear Co(III) complex 5 bearing a flexible linker alone as catalyst. It should be pointed out that even at a relatively low CHO concentration for the systems regarding dinuclear E


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Figure 2. (A) Representative three-dimensional stack plot of the IR spectra collected every 1 min during CO2/CHO copolymerization. (B) 1a, (C) 4a, and (D) 5 describe the logarithmic plots of the initial rate versus catalyst concentration.

Figure 3. Molecular structure of dinuclear SalenAl(III)Cl (hydrogen atoms and uncoordinated solvent omitted for clarity; carbon atoms are unlabeled). Thermal ellipsoids are at the 30% probability level. Left plot: (R,R,R,R,R)-SalenAl(III)Cl with a Al−Al distance of 7.89 Å and endo phenol−phenol dihedral angle of 134.9°. Right plot: (R,R,S,R,R)-SalenAl(III)Cl with a Al−Al distance of 7.67 Å and endo phenol−phenol dihedral angle of 114.9°.

Co(III) catalyst 1a,34 the initial rate is about 350 and 42 times faster than that with 4a and 5 alone as catalyst, respectively. In order to get a more reasonable understanding of the polymerization mechanism, the kinetic study of this process as a function of catalyst loading for both dinuclear Co(III) complex 1a and mononuclear Co(III)−Salen 4a was performed in the presence of 1 equiv of PPN−DNP cocatalyst by in situ FTIR spectroscopy (for detailed procedures, see Supporting Information, Table S3, Figures S11 and S12). From the logarithmic plots of the initial rate versus catalyst concentration described in Figure S12, with equivalent PPN−DNP as cocatalyst, the reaction orders on catalyst concentration for dinuclear Co(III) complex 1a and mononuclear Co(III) complex 4a are 1.03 and

1.47, respectively. In comparison with the bimolecular process regarding binary 4a/PPN−DNP system, in which the nucleophilic attack of cocatalyst or the propagating carboxylate species at the activated epoxide by its coordination to the metal center of complex 4a, an intramolecular bimetallic cooperation mechanism between the two cobalt centers of dinuclear Co(III) complex 1a predominantly determines the copolymer chain growth process. Structural Features of Biaryl-Linked Dinuclear Cobalt(III) Complexes. The great discrepancies of chiral induction preference and enantioselectivity for dinuclear cobalt(III) complexes 1 and 2 (or 3) with the same configuration in catalyzing CO2/meso-epoxide copolymerization drove us to F


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less open catalytic cleft in comparison with the complex 1 with biphenol linker. Based on the difference of the steric hindrance around the Co(III) centers provided by the biaryl bridge and the chiral cyclohexyldiamine skeletons and the ortho-R groups of the Salen moieties, the biaryl-linked dinuclear Co(III) complex space could be divided into two separate regions, inside and outside clefts (Scheme 2). It is quite possible that the two clefts show

probe into their structural features, particularly with focus on the differences of the Co−Co distance and dihedral angle between the two Salen planes. Although much effort was placed in attempts to clarify the solid-state structure of these dinuclear Co(III) complexes without any bound coordination agent or solvent, we failed to isolate their crystals. Fortunately, we succeeded in obtaining the X-ray crystal structure of a biphenollinked dinuclear Al(III) complex bearing chloride counterions (Figure 3). The biphenol-linked dinuclear Al(III) complex with (R,R,R,R)-configuration has two diastereoisomers with R- or Sbiphenol stereochemistry, (R,R,R,R,R) and (R,R,S,R,R)-conformers, though it originates from an achiral biphenol linker.35 The formation of diastereoisomers should be ascribed to the coordination of the ligand to metallic ion, confining the free rotation of C−C bond of the biphenol linker. Figure 3 shows the structures of the two diastereoisomers with hydrogen atoms omitted for clarity. Both of the aluminum ions adopt a five-coordinate, distorted square-pyramidal geometry, and the two chlorides are connected to the metallic ions in a synarrangement. The Al−Al distance in the (R,R,R,R,R)-conformer is 7.89 Å and the endo phenol−phenol dihedral angle is 134.9°, while in the (R,R,S,R,R)-conformer the corresponding values are 7.67 Å and 114.9°, respectively. This result indicates that (R,R,R,R,R)-conformer has a more open cleft. A similar discrepancy was also observed in the previously reported (S,S,S,S,S)- and (S,S,R,S,S)-diastereoisomers of dinuclear cobalt(III) complex 2 with four bound pyridines, in which the (S,S,S,S,S)-conformer had a more open cleft than the (S,S,R,S,S)conformer, with a Co−Co distance of 6.94 Å and an endo naphthol−naphthol dihedral angle of 90°, compared to 6.45 Å and 79°, respectively, for the latter isomer.30b Furthermore, the optimized structures of three dinuclear Co(III) complexes 6a, 6b, and 6c bearing chloride counterions (the same ligand structures with complexes 1a, 1b, and 1d) were obtained by density functional theory (DFT) calculations (Supporting Information, Figure S13). Similar to the corresponding dinuclear Al(III) complex without any bound solvents, these dinuclear Co(III) complexes all have large endo phenol−phenol dihedral angles of 136.2°−145.4° and Co−Co distance of 8.09−8.33 Å. For comparison purposes, DFT calculations were performed for identifying the structures of (S,S,S,S,S)- and (S,S,R,S,S)diastereoisomers of binaphthol-linked dinuclear cobalt(III) complex 7 (the same ligand structures with complex 2) without any coordinating agents (Supporting Information, Figure S14). Compared to biphenol-linked dinuclear Co(III) complex 6 bearing chloride counterions, complex 7 with a binaphthol bridge has a less open cleft, with a Co−Co distance of 6.41−6.65 Å and an endo naphthol−naphthol dihedral angle of 86.8°−88.8°. Especially, the structures of dinuclear cobalt(III) complex 8 (the same ligand structures with complex 3) with chiral biphenol linking bridge were also carried out for molecular geometry optimization at identical conditions (Supporting Information, Figure S15). As the tunable linking bridge not only confines the conformational rotation but also significantly minimizes the Co− Co distance and an endo phenol−phenol dihedral angle to a reasonable extent. For example, from C4 to C8, DFT calculations indicated that the complexes 8a−c covered a wide range of Co− Co distance (5.93−6.80 Å) and an endo phenol−phenol dihedral angle (68.2°−95.1°), similar to complex 7 with the binaphthol linker. As a result, we can tentatively ascribe the relatively low enantioselectivity and activity of complexes 2 and 3 mediated CHO/CO2 copolymerization to the large steric hindrance in the

Scheme 2

different chiral induction circumstances for the CO2/mesoepoxide asymmetric coupling and thus probably causing significant discrepancy in the enantioselectivity and reaction rate. Because of the larger hindrance of the inside cleft, the coupling occurred in this region probably exhibit higher enantioselectivity than that in the outside cleft. Such an “asymmetric activation” strategy is a powerful platform for discovering new and efficient chiral catalyst systems.36 The chiral metal complexes with a flexible biphenol moiety have been demonstrated to be effective catalyst systems for various reactions.37,38 In previous kinetic study by in situ infrared spectroscopy revealed a first-order dependence on the catalyst concentration in the systems of biphenol-linked dinuclear Co(III) complex 1 alone or in the presence of an ionic cocatalyst. Moreover, because of a possible ideal Co−Co distance of 6−8 Å and a changeable endo aryl−aryl dihedral angle of 79°−135° associated with the biaryl-linked bimetallic Co(III) complexes 1, 2 (or 3), the CO2/meso-epoxide copolymerization occurred in the inside cleft is more favorable for an intramolecular bimetallic mechanism. Contrarily, the copolymer chain propagation in the outside cleft of dinuclear Co(III) catalyst predominantly concerns a monometallic mechanism or intermolecular bimetallic mechanism at high catalyst loadings, as similar to the previously reported mononuclear Co(III)−Salen catalyst 4 systems. Origin of Enantioselectivity of the Copolymer Chain Growth Step. For dinuclear Co(III) complexes 1a−1d mediated CO2/meso-epoxide copolymerization, we could not rule out the existence of a monometallic mechanism regarding polymer chain growth occurred in the outside cleft (Scheme 2), particularly in the high catalyst loading. However, from the G


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Figure 4. Diastereoisomers of (S,S,S,S)-1f and their transition states in catalyzing asymmetric copolymerization CPO with CO2.

Figure 5. Four possible transition states for the ring-opening of a cyclopentene oxide (CPO) molecule by an adjacent cobalt bound carbonate group. (a) TS-1: ring-opening at the (R)-C−O bond of CPO activated by (S,S,S,S,S)-1f; (b) TS-2: ring-opening at the (S)-C−O bond of CPO activated by (S,S,S,S,S)-1f; (c) TS-3: ring-opening at the (R)-C−O bond of CPO activated by (S,S,R,S,S)-1f; (d) TS-4: ring-opening at the (S)-C−O bond of CPO activated by (S,S,R,S,S)-1f. The energy was given in kcal/mol, and toluene was employed as a solvent.

remarkable difference of the initial rate for 1a and 4a based on the above kinetic studies, it can be reasonably assumed that the polycarbonate chain propagation predominantly proceeds in the inside cleft of the catalyst, associated with an intramolecular bimetallic mechanism. Unfortunately, since the two diastereoisomers with R- or S-biphenol stereochemistry in the dinuclear trivalent metal complexes 1 originated from an achiral biphenol

linker, the separation of the two conformers proven to be very difficult. To gain insight into the origin of asymmetric induction of the biphenol-linked dinuclear Co(III) complex 1 for the ringopening of meso-epoxides in the inside cleft (Figure 4), DFT calculations were performed for simulating the possible chaingrowth processes in (S,S,S,S,S)- and (S,S,R,S,S)-conformers. In order to simplify the calculations, the substituents on the 3- and H


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Table 3. Effect of Lewis Base on the Enantioselectivity of (S,S,S,S)-1a Mediated CO2/CHO Copolymerizationa entry

Lewis base

(S,S,S,S)-1a/base (molar ratio)

time (h)

TOFb (h−1)

Mnc (kg/mol)

PDIc

eed (%)

1 2 3 4 5 6 7 8

N-MeIm N-MeIm DMAP MTBD H-TeIm M-TeIm P-TeIm

1/− 1/2 1/1 1/2 1/2 1/2 1/2 1/2

2 4 4 4 4 4 4 4

194 46 88 71 60 28 89 115

20.5 5.7 9.2 6.7 6.2 2.8 8.6 12.4

1.25 1.12 1.13 1.12 1.18 1.15 1.11 1.17

33 (S,S) 32 (R,R) 3 (S,S) 6 (R,R) 4 (S,S) 2 (S,S) 11 (S,S) 28 (S,S)

All reactions were performed in neat CHO (3.0 mL, 30 mmol) in 20 mL autoclave at 25 °C and a 2.0 MPa CO2 pressure. (S,S,S,S)-1a/CHO = 1/ 1000 (molar ratio). The polymer selectivity and carbonate linkages of the resulted polycarbonates are >99% based on 1H NMR spectroscopy. b Turnover frequency (TOF) = moles of product (polycarbonates)/mole of catalyst per hour. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene. dMeasured by hydrolyzing the polymer and analyzing the resulting diol by chiral GC. a

outside cleft regarding the monometallic mechanism. The former path was more effective in this polymerization process and thus predominantly determines the enantioselectivity of the resultant copolymers. However, this situation might be changed by the addition of a coordination agent. As a consequence, Nmethylimidazole (N-MeIm) was first investigated at various loadings. When 2 equiv of N-MeIm was added into the system of dinuclear cobalt(III) complex (S,S,S,S)-1a as catalyst, the catalytic activity greatly decreased to 46 from 194 h−1. Surprisingly, the chiral induction orientation for the resultant copolymers changed dramatically from S,S-configuration (33% ee) to R,R-configuration (32% ee) (Table 3, entries 1 and 2), being close to the value (39% ee for R,R-configuration) of the polycarbonates resulted from mononuclear cobalt(III) complex (S,S)-4a mediated CO2/CHO copolymerization (Table 1, entry 5). It was found that by using 1 equiv N-MeIm loading, the resultant copolymers have a very low ee of 3% for S,Sconfiguration (Table 3, entry 3). A similar tendency was also observed in the replacement of N-MeIm with N,N-dimethyl-4aminopyridine (DMAP) (Table 3, entry 4). Furthermore, various sterically hindered Lewis bases, such as 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD) and 2-substituted-Nmethylimidazole (H-TeIm, M-TeIm, P-TeIm) (Scheme 3),

5-phenolate position were removed and cyclopentene oxide (CPO) was chosen as a model monomer. In this model, a CPO molecule was coordinated to one of the Co centers inside the catalytic cleft of 1f and ring-opened by a methyl carbonate (CH3OCO2−) bound to the adjacent Co center, while two 2,4dinitrophenoxide ions were placed in the exo positions as counterions (Figure 5). Four transition states (TSs) shown in Figure 5 display the different possibilities for ring-opening at the (R)-C−O or (S)-C−O bond of CPO activated by (S,S,S,S,S)- or (S,S,R,S,S)-1f. Two transition states (TS-3 and TS-4) corresponding to the ring-opening at the (R)-C−O and (S)-C−O bond of CPO coordinated to (S,S,R,S,S)-conformer are 5.9 and 6.8 kcal/mol, higher in energy than the favored TS-1 (ΔG = 0 kcal/mol) and TS-2 (ΔG = 4.3 kcal/mol) relative to (S,S,S,S,S)conformer, respectively. This result suggests that in the two diastereoisomers, (S,S,S,S,S)-conformer is more effective in catalyzing CO2/CPO copolymerization than the corresponding (S,S,R,S,S)-conformer. The calculations also showed that the dinuclear Co(III) catalyst with a S,S,S,S,S-configuration predominantly resulted in the formation of polycarbonates with S,Sconfiguration (TS-1), which was in agreement with the experimental findings. Furthermore, DFT calculations were also applied to the asymmetric copolymerization of CO2 with CHO with the same catalyst system (Supporting Information, Figure S16). As similar to the CO2/CPO copolymerization, the calculations also showed that the favored transition state corresponded to the ringopening occurring at (R)-C−O bond of CHO activated by (S,S,S,S,S)-1f. However, the energy difference between TS-1 (ΔG = 0 kcal/mol) and TS-2 (ΔG = 0.7 kcal/mol) relative to the nucleophilic ring-opening at (R)-C−O and (S)-C−O bonds of CHO activated by (S,S,S,S,S)-1f is 0.7 kcal/mol, significantly lower than that found in CPO system (4.3 kcal/mol). This provides a reasonable explanation of the experimental result that the biphenol-linked dinuclear Co(III) complexes were found to be more effective in enantioselectively catalyzing CO2/CPO copolymerization, in comparison with for the desymmetrization coupling of CO2 and CHO at the same conditions. This phenomenon was also found in the SalenCr(III)Cl-catalyzed asymmetric ring-opening of meso-epoxides using azidotrimethylsilane as a nucleophile.39 Effect of Lewis Base on the Catalyst Activity and Enantioselectivity. As described in Scheme 2, there are two possible polymer chain propagation paths for biaryl-linked dinuclear cobalt(III) complex mediated CO2/CHO copolymerization. One occurs in the inside cleft concerning an intramolecular bimetallic mechanism, and another path appears in the

Scheme 3

were all tested in 2 equiv for the effects on the catalytic activity and product enantioselectivity (Table 3, entries 5−8). All tested Lewis bases have negative influences on both the catalytic activity and product enantioselectivity; nevertheless, the extents are in inverse proportion to their steric hindrances. These results make us tentatively assume that the addition of a Lewis base seems to block polymer chain propagation path regarding the intramolecular bimetallic mechanism in the inside cleft of (S,S,S,S)-1a and thereby lead to the copolymerization predominantly proceeding in the monometallic mechanism just like the (S,S)4a system. To shed light on effects of Lewis bases on (S,S,S,S)-1a mediated CO2/CHO copolymerization, we set out to investigate the bindings of N-MeIm to 1a and mononuclear SalenCo(III)PF6 (4b) in a solution by means of electrospray ionization mass I


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Figure 6. Difference coordination mode of N-MeIm and P-TeIm to mononuclear Co(III) complex 4b.

Figure 7. Possible coordination modes of 2 equiv of Lewis base to dinuclear Co(III) complex.

spectrometry (ESI-MS) and nuclear magnetic resonance (1H NMR). The positive ion ESI-MS spectra of 4b/N-MeIm and 4b/ P-TeIm mixtures in different molar ratios are illustrated in Figure 6. When the ratio of N-MeIm to Co(III) was 1:1, the spectrum mainly contains the species at m/z 603.3 and 767.4, corresponding to [4b−PF6−]+ and [4b−PF6− + 2N-MeIm]+ cations, respectively. With the increase of N-MeIm, the species at m/z 603.3 disappears completely, and only the species at m/z 767.4 was observed. However, the species at m/z 685 corresponding to [4b−PF6− + N-MeIm]+ cation was not detected at various N-MeIm loadings. These results indicate that Co(III) preferably binds two molecules of N-MeIm to form six-coordinated complex ions. The 1H NMR spectrum can be also clearly assigned as the coordination of 2 equiv of N-MeIm to one Co(III) center (Supporting Information, Figure S19). As for sterically hindered base P-TeIm, three species at m/z 353.2, 603.3, and 955.5 were observed with even a molar ratio of 1:3 (Co(III) complex/P-TeIm), assigned to [P-TeIm + H]+, [4b− PF6−]+, and [4b−PF6−+P-TeIm]+ cations, respectively. The species at m/z 1307 corresponding to [4b−PF6− + 2P-TeIm]+

cation was not detected. Therefore, we can reasonably confirm that only one molecule of P-TeIm binds Co(III) to form a fivecoordinated complex ion due to its large steric hindrance. The coordination of Lewis bases to dinuclear Co(III) complex 1a is very complicated. For example, when two molecules of Lewis bases were bound to 1a, it is impossible to confirm the accurate coordination positions of the two bound base molecules. There are four possible modes for the coordination of Lewis base to the dinuclear Co(III) complex (Figure 7); however, probably one or two of them are predominant. Among these coordination modes, only mode A regarding two molecules of base bound to different Co(III) centers in the outside cleft is favor of polymer chain propagation occurring in the inside cleft in accordance with an intramolecular bimetallic mechanism. The other modes B, C, and D are beneficial for a monometallic mechanism. When 2 equiv of N-MeIm was added into the catalyst system based on (S,S,S,S)-1a, the more favorable coordination is the mode B regarding two molecules of bases bound to one Co(III) center, based on the study on the bindings of N-MeIm to mononuclear Co(III) complex 4b. If this J


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Table 4. Effect of N-MeIm on the Enantioselectivity of Dinuclear Co(III) Complex Mediated CO2/CHO Copolymerization in the Presence of PPN−DNPa entry

catalyst

catalyst/N-MeIm (molar ratio)

temp (°C)

time (h)

TOFb (h−1)

Mnc (kg/mol)

PDIc

eed (%)

1 2 3 4e 5e 6e 7f 8 9 10 11 12

(S,S,S,S)-1b (S,S,S,S)-1b (S,S,S,S)-1b (S,S,S,S)-1b (S,S,S,S)-1b (S,S,S,S)-1b (S,S,R,S,S)-2 (S,S,R,S,S)-2 (S,S,R,S,S)-2 (S,S,S,S,S)-2 (S,S,S,S,S)-2 (S,S,S,S,S)-2

1/− 1/1 1/2 1/− 1/1 1/2 1/− 1/1 1/2 1/− 1/1 1/2

25 25 25 0 0 0 25 25 25 25 25 25

0.25 1.5 6 6 24 48 1 6 24 5 24 48

1356 260 31 166 15

11.5 10.4 5.4 55.4 9.8

1.18 1.20 1.25 1.22 1.22

77 (S,S) 20 (S,S) 9 (R,R) 96 (S,S) 36 (S,S)

395 49 20 56 9

12.0 9.1 5.9 10.4 6.1

1.21 1.17 1.20 1.15 1.12

7 (R,R) 6 (S,S) 11 (S,S) 58 (R,R) 54 (R,R)

a

All reactions were performed in neat CHO (3.0 mL, except entries 4−6 in toluene solution of CHO) in 20 mL autoclave at 2.0 MPa CO2 pressure. Catalyst/PPN-DNP/CHO = 1/2/1000 (molar ratio), DNP = 2,4-dinitrophenoxide. The polymer selectivity and carbonate linkages of the resulted polycarbonates are >99% based on 1H NMR spectroscopy. bTurnover frequency (TOF) = moles of product (polycarbonates)/mole of catalyst per hour. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene. dMeasured by hydrolyzing the polymer and analyzing the resulting diol by chiral GC. eThe reaction was carried out in toluene solution of CHO (CHO/toluene = 1/2, volume ratio). fThe carbonate linkages content of the copolymers is 65%.

Scheme 4

Scheme 5. Intramolecular Bimetallic Mechanism for Enantioselective Polymer Chain Growth from Inside the Cleft of Dinuclear Co(III) Catalyst

ratiocination is right, (S,S,S,S)-1a with two molecules of N-MeIm bound its one Co(III) ion corresponds to mononuclear Co(III)Salen complex 4a, in which polymer chain growth only involves a

monometallic mechanism. Therefore, the great decrease in catalytic activity and complete configuration inversion of the resultant copolymer are not strange. K


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When sterically hindered Lewis base P-TeIm was applied, the situation should be different from N-MeIm or DMAP. The steric hindrance between P-TeIm and tert-butyl groups of (S,S,S,S)-1a likely induced the formation of the mode A regarding two molecules of bases bound to different Co(III) centers in the outside cleft, which is beneficial for the copolymerization occurring in the inside cleft, the more enantioselective site. However, we did not observe the improvement in the enantioselectivity. This might be ascribed to the unstable coordination of P-TeIm to Co(III) ion and thus easily being displaced by epoxides or the growing polymer chains. Moreover, it was found that in the presence of an ionic cocatalyst PPN−DNP the addition of N-MeIm also significantly decreased both the catalytic activity and enantioselectivity of dinuclear Co(III) complex (S,S,S,S)-1b (Table 4, entries 1−6). For example, the catalytic activity from 1356 h−1 decreased to 260 and 31 h−1 for the addition of 1 and 2 equiv of N-MeIm, respectively. Notably, the presence of 2 equiv of N-MeIm resulted in the enantioselectivity from 77% ee for S,Sconfiguration changing to 9% ee for R,R-configuration. With the use of dinuclear Co(III) complex (S,S,R,S,S)-2, the addition of N-MeIm also led to an obvious configuration inversion of the resultant copolymer, in companion with the significant decrease in reaction rate (entries 7−9). Surprisingly, in the catalyst systems based on (S,S,S,S,S)-2, the presence of N-MeIm only caused a slight decrease in the enantioselectivity, though the great loss in the catalytic activity was similar to the former catalyst systems (entries 10−12). Comprehensive Understanding of Enantioselective Copolymerization Process. On the basis of the studies above-mentioned, we can give a comprehensive mechanism understanding of (S,S,S,S)-1 mediated enantioselective copolymerization of CO2 and meso-epoxides. The biphenol-linked Co(III) complexes (S,S,S,S)-1 are the mixture of two diastereoisomers with R- or S-biphenol stereochemistry, (S,S,R,S,S) and (S,S,S,S,S)-conformers, though they originate from an achiral biphenol linker. The matched configuration of (S,S,S,S,S) was more effective than (S,S,R,S,S) for enantioselectively catalyzing this asymmetric copolymerization (Scheme 4). The initiation is triggered by one of the two nucleophilic anions of the bimetallic cobalt catalyst from the inside cleft, and chain-growth step predominantly involves an intramolecular bimetallic cooperation mechanism, wherein alternating chain growth and dissociation of propagating carboxylate species takes turn between two Co(III) ions from the inside cleft of dinuclear Co(III) catalysts by the nucleophilic attack of the growing carboxylate species at one metal center toward the activated epoxide at the other (Scheme 5). Both the absolute stereochemistry of the biphenol linker and cyclohexyl diamine skeletons determine the enantiomer preference in the copolymerization. Another path for initiation and chain growth concerns a monometallic mechanism or intermolecular bimetallic mechanism occurred in the outside cleft of catalyst, as similar to the mononuclear Co(III) catalyst 4a, in which the cyclohexyldiamine skeleton determines the enantiomer preference, providing the copolymers with the opposite configuration. Since the former is predominantly responsible for the copolymer formation and the inside cleft is the more enantioselective site, it determines the product configuration and enantioselectivity. The coordination environments around the cobalt ions could be changed by the addition of a nucleophilic cocatalyst. The presence of an ionic quaternary ammonium salt greatly increases both the catalytic activity and product enantioselectivity of the

biphenol-linked Co(III) complexes (S,S,S,S)-1a−1d for CO2/ meso-epoxides copolymerization. The initiation is triggered by the nucleophilic anion from the inside cleft of the catalyst, and the chain-growth step predominantly involves an intramolecular bimetallic cooperation mechanism. The initiator originates from the anion of the ionic cocatalyst or one of the two nucleophilic anions of the bimetallic cobalt catalyst. Only one anion per catalyst system can initiate a copolymer chain in the absence of chain-transfer agents (such as water, methanol, etc.), while exo ligands are incapable of initiation or initiating this copolymerization in a very low rate from the outside cleft of the catalyst. It seems to be very difficult at the employed condition to cause the chain transfer by the excess ionic cocatalyst or exo ligands. The cation of the ionic cocatalyst has a positive effect on enantioselective attack of the growing carboxylate species at one metal center toward the activated epoxide at the other through Coulombic interaction during the intramolecular chain growth. When the coordination Lewis base such as N-MeIm was added into the system regarding (S,S,S,S)-1, two molecules of N-MeIm are more favorable for binding one metal ion of dinuclear Co(III) complex, thus completely inhibiting copolymer chain growth by the intramolecular bimetallic mechanism. Since the binding of Lewis bases will significantly change the coordination environments of epoxide to cobalt ion, leading to the copolymerization with CO2 only involves a monometallic mechanism. As a result, the great decrease in catalytic activity and complete configuration inversion of the resultant copolymers are foreseeable. For the biphenol-linked Co(III) complexes (S,S,S,S)-1 mediated CO2/CHO copolymerization, an improvement in product enantioselectivity was discovered in the dilute solution of toluene (Table 1, entries 21−24). As previously described, there are two possible copolymerization paths with different enantiomer preferences. One involves the intramolecular bimetallic cooperation mechanism only occurring in the inside cleft of the dinuclear Co(III) catalyst, and the other concerns a monometallic mechanism appearing in the outside cleft of catalyst. The decrease in CHO concentration in the copolymerization system has a negative effect on the reaction rate of both two copolymerization paths. However, the extents are different, and obviously the latter path has the more significant influence. As a result, the copolymerization predominantly occurring in the more enantioselective site is beneficial for improving the product enantioselectivity. In the biphenol-linked Co(III) complexes (S,S,S,S)-1 mediated CO2/CHO copolymerization, the screening results revealed that the steric hindrance of the phenolate ortho-substituents strongly influenced the catalytic activity and product enantioselectivity. The bulky substituents on the aromatic rings were not the best choice for the biphenol-linked dinuclear Co(III) catalyst systems. The product enantioselectivity decreased with increasing size of the substituents in the phenolate ortho positions in the absence or presence of PPN−DNP (Table 1, entries 1−4 and 9− 12). A reasonable explanation is that the large steric hindrance of the bulky groups such as adamantyl significantly blocks the coordination of epoxide from the inside cleft of the dinuclear cobalt catalyst, leading to the great decrease in both the catalytic activity and enantioselectivity.

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CONCLUSION In summary, we have demonstrated that the biphenol-linked Co(III) complexes (S,S,S,S)-1b or 1c are privileged chiral catalysts for highly active and enantioselective copolymerization L


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Notes

of CO2 and various meso-epoxides, affording the corresponding polycarbonate with more than 99% carbonate unit content and ≥98% enantioselectivity. In the two diastereoisomers with R- or S-biphenol stereochemistry of the biphenol-linked dinuclear Co(III) complexes (S,S,S,S)-1, the matched configuration of (S,S,S,S,S) was more effective than (S,S,R,S,S) in enantioselectively catalyzing this asymmetric copolymerization, wherein the ring-opening predominantly occurred at the (R)-C−O bond of meso-epoxide to afford the corresponding copolymers with S,Sconfiguration. The mechanistic study revealed that there are two possible paths for initiation and chain growth in the biaryl-linked dinuclear Co(III) complex mediated CO2/meso-epoxide copolymerization: the more enantioselective site in the inside cleft of the catalyst regarding an intramolecular bimetallic mechanism and the less enantioselective site in the outside cleft involving a monometallic mechanism. The two sites show different chiral induction circumstances for the CO2/meso-epoxide asymmetric coupling, thus causing significant discrepancy in the enantiomer preference and reaction rate. The addition of an ionic cocatalyst significantly increases both the catalytic activity and enantioselectivity of the biaryl-linked dinuclear Co(III) complexes. Contrarily, the presence of a coordination Lewis base such as N-MeIm has a great negative influence on the copolymerization process. The use of an organic solvent not only benefits the complete conversion for achieving the polycarbonates with high molecular weight but also improves the product enantioselectivity to a certain extent by greatly inhibiting the copolymerization occurring at the outside cleft of the catalyst regarding a monometallic mechanism for polymer chain growth. Further efforts will focus on developing new dinuclear catalyst systems that exhibit higher activity and excellent enantioselectivity for this enantioselective copolymerization under mild conditions and exploring the synthesis of various stereoregular polycarbonates with novel structure and properties.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, Grant 21134002, 21104007) and Program for Changjiang Scholars and Innovative Research Team in University (IRT13008). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (T2011056) from the Ministry of Education of China.

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Copolymerization Reactions Monitored by in Situ FTIR Spectroscopy. In a typical experiment, a 100 mL stainless steel Parr autoclave reactor, modified with a ZnSeW AR window to allow for the use of an ASI ReactIR 45 system equipped with a MCT detector and 30 bounce DiCOMP in situ probe, is heated to the desired temperature. In this manner, a single 256-scan background spectrum was collected. The catalyst was dissolved in an epoxide/toluene mixture and then injected into the reactor via the injection port. The reactor was pressurized to 2.0 MPa CO2 as the FTIR probe began collecting scans. The infrared spectrometer was set up to collect one spectrum every 1 min over a certain period. Profiles of the absorbance at ∼1750 cm−1 v(CO) corresponding to the copolymer with time were recorded and used to provide the initial reaction rate for analysis (Note: catalyst loading varied with the experiment as described in the Results and Discussion section and the Supporting Information.)

S Supporting Information *

General experimental procedures and characterizations of catalyst, organic bases, and copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Mechanistic Understanding of Dinuclear Cobalt (III) Complex

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