Mechanism of fixation of CO₂ with an epoxide catalyzed by ZnBr₂ and a choline chloride co-catalyst: a DFT study

Abstract

To explore the reason for the high activity of the cycloaddition reaction of PO (propylene oxide) with CO₂ catalyzed by ZnBr₂/CH (choline chloride) co-catalyst, the mechanism has been constructed using a DFT (density functional theory) method. The combination of CH and ZnBr₂ will generate three new stable complexes, i.e., [Ch]₂[ZnBr₂Cl₂], Ch⁺ZnBr₂Cl⁻, and Ch⁺ZnBrCl₂⁻. The latter two are derived from the dissociation of [Ch]₂[ZnBr₂Cl₂]. The detailed mechanism of a coupling reaction catalyzed by the more stable complex Ch⁺ZnBrCl₂⁻ is explored. It has been elucidated that the attack from the Zn complex and the Br⁻ anion is the major factor in promoting the cleavage of the C–O bond of PO. Finally, the performance of [Ch]₂[ZnBr₂Cl₂] is also investigated, providing less activity, indicating that it should dissociate to gain better catalytic effect.

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1 Introduction

The fixation and utilization of carbon dioxide (CO₂) have received increasing attention in light of the growing problems of the greenhouse effect and depletion of the fossil fuels.^1,2 Emission of CO₂ into the atmosphere has already resulted in global warming and climate change, while CO₂ also serves as an abundant, cheap, nontoxic C1 raw material. In the past two decades, various pathways of CO₂ utilization have been explored. Among them, the synthesis of five-membered cyclic carbonates via the coupling reaction of CO₂ with epoxides is one of the most promising methodologies.³ However, the thermodynamic stability and kinetic inertness of CO₂ are roadblocks that are in the way of utilizing it as a raw material.⁴ Exploring efficient catalysts is a promising method to overcome this obstacle. Diverse catalysts have been successfully developed, such as, alkali metal salts,^5,6 metal oxides,^7,8 transition-metal complexes,^9–11 ionic liquids (ILs),^12–14 and others.^15–17 Most of the catalysts suffer from low activity, harsh reaction conditions, the need of organic co-solvents, or a combination of these.

In recent years, ILs have received much more attention because of their unique properties, including low melting temperature, undetectable vapor pressure, and special solubility for many organic or inorganic compounds, and especially due to their environmental friendliness and feasibility for design.¹⁸ Moreover, numerous experiments have proven that the involvement of Lewis acids will dramatically enhance the catalytic activity. Zinc halides are one of the most popular co-catalysts that have appeared in the IL-based catalyst system. This bicomponent catalyst system is robust, air stable, cheap, and free of organic solvent. Some composite catalyst systems have been reported, which have been widely applied in the processes of a regiospecific Fischer indole reaction,¹⁹ the protection of carbonyls,²⁰ Diels–Alder reactions,²¹ and the fixation of CO₂.^22,23 Zhang et al. firstly investigated the activity of a ZnBr₂/CH bicomponent catalyst for the synthesis of cyclic carbonates.²⁴ The turnover frequency (TOF) of the ZnBr₂/CH bicomponent catalyst of 494 is higher than that of ZnBr₂ of 5 and that of CH of a trace amount, which is similar to other bicomponent catalysts reported in the previous literature.^22,25 Although the TOFs are not high enough, the catalytic activity is enhanced by introducing the zinc halide.

To our best knowledge, detailed mechanisms for the coupling of CO₂ with PO catalyzed by C(2,4,6)mimCl,²⁶ DMimBr Me₂PO₄ + ZnBr₂,²⁷ TBABr,^2,28 TEA(Br–Cl),² LiBr,²⁹ TBD·HBr,³⁰ KI/glycerol,³¹ and TBAI/pyrogallol³² have been thoroughly studied. Moreover, the mechanism for the fixation of CO₂ with PO mediated by ZnCl₂/[BMIM]Cl has also been explored by Xia et al.³³ The conclusions can not be totally transferred to the ZnBr₂/CH bicomponent system, since the anions of ZnBr₂ and CH are different and the Cl⁻ anions are in excess. In a mixture of ZnBr₂ and CH with a molar ratio of 1 : 5, lots of Cl⁻ anions are available. What is the most stable configuration of Zn complex? What is the real main catalytic active species for the title reaction? Will the Zn complex exist in the catalytic system as a three-fold coordination complex or a four-fold coordination complex?

To elucidate the above questions, we investigated the mechanism for the coupling reaction of CO₂ with an epoxide using the M06 method.³⁴ After the computational methods section, the mechanism investigations for the coupling reaction catalyzed by ZnBr₂ or CH are firstly discussed, respectively; in the next section, the main catalytic active species are analyzed; subsequently, the role of the composite catalyst is explored and its activity is compared with a single component catalyst, with a conclusion section at the end. It is expected that our work will provide valuable clues for further theoretical and experimental studies for the synthesis of more powerful catalysts.

2 Computational details

The Gaussian 09 program was employed to perform all the electronic calculations.³⁵ All electronic structures of stationary points including the reactants, products, intermediates, and transition states were optimized using the M06 method.³⁴ On the basis of previous experiences,^26,30,41 the hydrogen bond is an essential factor for stabilizing the structures. So the noncovalent interactions should be carefully considered. On the other hand, a metal atom is included in the catalytic system. Considering the abovementioned two factors, the M06 functional becomes the final choice because the M06 functional is recommended by Truhlar et al. for application in organometallic and inorganometallic chemistry and for noncovalent interactions.³⁴ Owing to the inclusion of the hydrogen atoms and the Zn atom, the basis set of 6-31+G(d,p) was employed for all atoms to obtain a high ratio of performance to cost, which is also a popular basis set for investigating the IL-based catalytic system and mechanism of other organic reactions.^33,36,37 Four criteria are employed in the Gaussian program to judge the completion of optimization. They are the maximum remaining force on one atom of the system as well as the average (RMS, root mean square) force on all atoms, and the maximum displacement, that is, the maximum structural change of one coordinate as well as the average (RMS) change over all structural parameters in the last two iterations. The default RMS force criterion is 3 × 10⁻⁴. To ascertain the stability of the Zn complexes, the dissociation energies were refined at the M06/6-311+G(d,p) level based on the M06/6-31+G(d,p) optimized geometries. To identify the nature of the stationary points and to obtain the ZPE (zero-point energy) correction, the vibrational frequency was calculated at the same level. The number of imaginary frequency was employed to identify the minimum or the transition state, i.e., NIMAG = 1 for a saddle point or NIMAG = 0 for a minimum. The MEP (minimum-energy path) was constructed starting from the saddle point geometry and going downhill to the reactant and product using the intrinsic reaction coordinate (IRC) theory.³⁸ Along this energy path, the reaction coordinate s is defined as the distance from the saddle point, with s > 0 referring to the product side. The abovementioned calculations were performed in the gas-phase state. Finally, the critical barrier heights involved in routes 1–5 (ring-opening and ring-closure steps) were corrected at the M06/6-311+G(d,p) level using a self-consistent reaction field (SCRF) method in the polarizable continuum model (PCM)^39,40 on the basis of the gas-phase geometries.

3 Results and discussion

3.1 Uncatalyzed reaction

The coupling reaction of CO₂ with PO without any catalyst has been substantially investigated in previous studies.^26,31,41 It has been identified that the CO₂ insertion and C–O bond rupture of PO are completed in a concerted mechanism with a high barrier height. For comparison with the results in this work, the preferential route is calculated at the M06/6-31+G(d,p) level. The corresponding results are plotted in Fig. S1 of the ESI.† At the M06/6-31+G(d,p) level, the barrier height is 55.33 kcal mol⁻¹, which is difficult to overcome.

3.2 ZnBr₂/ZnCl₂ catalyst or CH catalyst

Before exploring the mechanism of binary catalysts, the coupling reaction of CO₂ with PO catalyzed by a single catalyst is investigated to compare the activity and to discover the different features between the catalysts. As described in literature,³³ the reaction becomes a stepwise mechanism with two steps. The cleavage of the C–O bond is activated by electrophilic attack from the Zn atom of ZnCl₂ on the O atom of PO. The CO₂ insertion and ring-closure are completed in the second step, which is the rate-determining step with a barrier height of 54.59 kcal mol⁻¹ at the M06/6-31+G(d,p) level. When the catalyst is changed from ZnCl₂ to ZnBr₂, the barrier height of the rate-determining step is increased by 5.01 kcal mol⁻¹. The potential energy surface profiles are given in Fig. 1, and the optimized geometries for all involved species are shown in Fig. S2.† Neither ZnCl₂ nor ZnBr₂ has any reactivity for the coupling reaction of CO₂ with PO, which is consistent with the experimental measurements.^23,24

Fig. 1 Potential energy profiles of the route catalyzed by ZnBr₂ (–) or ZnCl₂ (---).

In addition to ZnBr₂, the effect of sole CH is also explored to confirm its co-catalytic role. The mechanism of CO₂ with PO catalyzed by a single HETEAB (2-hydroxyl-ethyl-triethylammonium bromide) catalyst has been studied in our previous work.⁴² CH and HETEAB have a similar structure and the same functional group indicating the same mechanism. Different chain lengths and anions between the two catalysts will not modify the mechanism, although they will have a little effect on the barrier height. As depicted in Fig. 2 and Fig. S3,† the most favorable pathway follows a three-step mechanism, that is, ring-opening, CO₂ insertion, and ring-closure, with barrier heights of 25.63, 0.76, and 22.71 kcal mol⁻¹, respectively. The barrier height of the rate-determining step (25.63 kcal mol⁻¹) is much lower than that of the ZnBr₂-mediated coupling reaction. This is contradictory to the experimental result that the catalytic activity of CH is lower than that of ZnBr₂ with trace product yield at 110 °C.²⁴ However, when the temperature is increased to 125 °C the conversion catalyzed by CH is improved to 45.1%.⁴³ The catalytic activity of CH is greatly affected by the temperature, and has great possibility to be better than ZnBr₂ at a slightly higher temperature.

Fig. 2 Potential energy profile of the most favorable route catalyzed by CH (choline chloride).

3.3 Zn-complex

To elucidate the role of ZnBr₂/CH in a cycloaddition reaction, the active Zn species that exists in the reaction system is firstly explored. Three possible chlorozincate clusters ZnCl₂, ZnCl₃⁻, and ZnCl₄²⁻ have been considered in Xia’s work.³³ The ZnCl₃⁻ is the most suitable one to be the actual catalytic component, as it is difficult to decompose into other species. The bromozincate clusters ZnBr₂, ZnBr₃⁻, and ZnBr₄²⁻ are expected to have a similar stability sequence. However, the formations of the Zn complex are more complicated owing to the different anions between ZnBr₂ and CH and the excess Cl⁻ anions that exist in the title reaction. Sparked by the testified method used by Xia et al., the possible Zn complexes and their dissociation energies are shown in Table 1. To refine the reaction energy, the single-point energy calculation is performed at the M06/6-311+G(d,p) level on the basis of the M06/6-31+G(d,p) optimized structure. The dissociation of ZnBr₂Cl₂²⁻ into ZnBrCl₂⁻ and Br⁻ is exothermic by 50.78 kcal mol⁻¹, suggesting the stability of the ZnBrCl₂⁻ anion. Moreover, it is difficult to further dissociate ZnBrCl₂⁻ to ZnCl₂ and Br⁻ with large endothermic energies.

Table 1 The possible dissociation forms of the Zn complexes and the corresponding dissociation energies (ΔG, kcal mol⁻¹) calculated at the M06/6-31+G(d,p) level and refined at the M06/6-311+G(d,p) level

Dissociation forms	M06/6-31+G(d,p)	M06/6-311+G(d,p)
ZnBr2 + Cl^− → ZnBr2Cl^−	−53.38	−52.56
ZnBr2Cl^− + Cl^− → ZnBr2Cl2^2−	45.32	46.50
ZnBr2Cl2^2− → ZnBrCl2^− + Br^−	−44.56	−50.78
ZnBrCl2^− → ZnCl2 + Br^−	54.69	49.31
ChCl + ZnBr2 → Ch^+ZnBr2Cl^−	−33.62	−28.29
Ch^+ZnBr2Cl^− + ChCl → [Ch]2[ZnBr2Cl2]	−33.63	−21.74
[Ch]2[ZnBr2Cl2] → Ch^+ZnBrCl2^− + ChBr	36.53	21.71
Ch^+ZnBrCl2^− → ZnCl2 + ChBr	31.46	31.66

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It is in no doubt that the stability of bromo-chlorozincate clusters will be affected by the Ch⁺ cation. Due to the different anions in CH and ZnBr₂, there are more possible situations. Three structures Ch⁺ZnBr₂Cl⁻, Ch⁺ZnBrCl₂⁻, and [Ch]₂[ZnBr₂Cl₂] were optimized at the M06/6-31+G(d,p) level and their energies were refined at the M06/6-311+G(d,p) level. The CH and ZnBr₂ will form Ch⁺ZnBr₂Cl⁻ spontaneously with a formation energy of −28.29 kcal mol⁻¹. Next, the Ch⁺ZnBr₂Cl⁻ will react with excess CH to produce the more stable [Ch]₂[ZnBr₂Cl₂], which is an exothermic process with a formation energy of −21.74 kcal mol⁻¹. However, the interaction between [Ch]₂[ZnBr₂Cl₂] and PO will make the [Ch]₂[ZnBr₂Cl₂] dissociate into other species. On one hand is that [Ch]₂[ZnBr₂Cl₂] will dissociate into Ch⁺ZnBr₂Cl⁻ and ChCl, which needs 21.74 kcal mol⁻¹ dissociation energy. On the other hand is that it will dissociate into Ch⁺ZnBrCl₂⁻ and ChBr with a lesser dissociation energy of 21.71 kcal mol⁻¹, which is a more favorable route. The major parameters of the optimized geometries for ZnBr₂, ZnBr₂Cl₂²⁻, and [Ch]₂[ZnBr₂Cl₂] are shown in Fig. 3. Due to the interactions between the Ch⁺ cation and ZnX₄²⁻, the Zn–Br bonds are elongated, while the Zn–Cl bonds are shortened. To get a high ratio of computational performance to cost, Ch⁺, Br⁻, and ZnBrCl₂⁻ will be taken into account as the essential catalysts. In addition, the activity of catalyst [Ch]₂[ZnBr₂Cl₂] is also considered to compare with those of Ch⁺, Br⁻, and ZnBrCl₂⁻.

Fig. 3 Optimized geometries for ZnBr₂, ZnBr₂Cl₂²⁻, and [Ch]₂[ZnBr₂Cl₂].

3.4 Ch⁺, Br⁻, and ZnBrCl₂⁻ catalysts

According to experience obtained from the literature and our previous work,^33,41,44 a three-step mechanism is more favorable than a two-step mechanism. Neither activating the C atom of CO₂ nor activating the O atom of CO₂ is easy to complete because of the stability of CO₂. Thus, only a three-step mechanism will be considered in the following study. In the three-step mechanism, the oxygen atom and carbon atom of the epoxide are firstly activated by the Lewis acid and halide anion, respectively; subsequently, the CO₂ reacts with the activated ring-opening substrate; finally, the cyclic carbonate is formed by subsequent intramolecular cyclization and the catalyst is regenerated.^33,44 The oxygen atom can be motivated by the Ch⁺, ZnBrCl₂⁻, or both of them. Four possible pathways are located on the basis of the different species employed to activate PO. Both the substituted and non-substituted carbon atom will be attacked, so another corresponding four routes are easily found by extension. For clarity, only the most favorable route of each kind will be discussed in detail.

Route 1. Since the Lewis acid can activate the oxygen atom of PO, it is easy to conject that more acidity is helpful to promote the ring-opening. Both ZnBrCl₂⁻ and Ch⁺ are employed to attack the O atom of PO, which is also the mechanism proposed by Zhang et al.²⁴ The optimized geometries for the first transition state are plotted in Fig. 4 and the potential energy profiles are shown in Fig. 5. The other structures are shown in Fig. S4.† The ChBr, ZnBrCl₂⁻, and PO firstly form a trimolecule complex 1-a with a relative energy of −64.84 kcal mol⁻¹. Hydrogen bonds are formed to stabilize the PO and ZnBrCl₂⁻ with the distances of O₁–H⋯Cl, C₁–H⋯O₂, and C₂–H⋯O₂ being 2.32, 2.20, and 2.58 Å, respectively. Taking the complex 1-a as a starting point, the nucleophilic attack of the Br⁻ anion on the C atom of PO occurs associated with the synergetic attack from both the Zn atom of ZnBrCl₂⁻ and the H atom of the hydroxyl group on the O atom of PO via TS1-1. Overcoming the barrier height of 30.31 kcal mol⁻¹, the intermediate 1-b is generated. In 1-b, not only ZnBrCl₂⁻ but also the –OH group of Ch⁺ have interactions with the O atom of PO. Next, the CO₂ is introduced into the reaction to form intermediate 1-c by electrostatic attraction between the O₂ atom and the C₃ atom of CO₂. Subsequently, the attack of CO₂ on PO results in the formation of another intermediate 1-d by surmounting a barrier height of 6.09 kcal mol⁻¹. Through the steric adjustment of the configuration, 1-d is converted into configuration 1-e which is a little more stable. In the following step, complex 1-e will convert into product-like intermediate 1-f via TS1-3. The imaginary vibration of TS1-3 corresponds to the shrinking of the C₄–O₃ bond, which results in the formation of PC. Finally, the catalysts are easily released and the catalytic cycle is completed. The ring-opening step is the rate-determining step in route 1 with a barrier height of 30.31 kcal mol⁻¹, which is lower than the routes catalyzed by single catalyst ZnBr₂ but higher than CH. However, the barrier height is still not easy enough to surmount at 110 °C. Are there any other favorable routes or competitive routes? Since the ring-opening step promoted by both Ch⁺ and ZnBrCl₂⁻ is not good enough, could one of them have a better performance? So the possibility of the O atom of PO being attacked by a single species, Ch⁺ or ZnBrCl₂⁻, is explored. Three other routes are discovered and the potential energy profiles are also exhibited in Fig. 5. All of them are a three-step mechanism with ring-opening, CO₂ insertion, and ring-closure. The main difference is the ring-opening step, thus, only the first step is discussed in the following routes. The optimized geometries of the first transition states are shown in Fig. 4 and the other structures are shown in Fig. S4.†

Fig. 4 Optimized geometries for the first transition states included in routes 1(TS1-1), 2(TS2-1), 3(TS3-1), and 4(TS4-1) of the cycloaddition reaction catalyzed by Ch⁺, Br⁻, and ZnBrCl₂⁻.

Fig. 5 Potential energy profiles of routes 1–3 (---) and 4 (–) for the cycloaddition reaction catalyzed by Ch⁺, Br⁻, and ZnBrCl₂⁻. TSm-n (n = 1, 2, or 3) denotes the first, second or third transition state involved in route m, and m-n (n = a–f) denotes the intermediate in route m.

Route 2. We speculate that PO is only activated by the hydroxyl group of Ch⁺. As shown in Fig. 4, the ring-opening of PO in route 2 is motivated by the cooperation of electrophilic attack from the hydroxyl group and nucleophilic attack from the Br⁻ anion via TS2-1. The ZnBrCl₂⁻ is coordinated to the O atom of the hydroxyl group, which will make the acidity of the hydroxyl group greater. Thus, the ring-opening step is promoted, and the energy barrier is reduced to 23.42 kcal mol⁻¹. Moreover, there is a proton transfer process in route 2, which is a common appearance for hydroxyl-functionalized ILs.⁴¹ The rate-determining step is the ring-closure step for route 2 with a barrier height of 25.03 kcal mol⁻¹. So the ring-opening and ring-closure are two competitive steps.

Route 3. In route 2, Ch⁺ is taken as the vital catalytic species and the co-catalyst ZnBrCl₂⁻ is utilized to stabilize the Ch⁺. Thus, it is easy to reverse them. The ZnBrCl₂⁻ is coordinated to the O atom of PO to promote the ring-opening. While the Ch⁺ plays two roles: one is that the atoms H₁, H₂, and H₃ of Ch⁺ are exerted to stabilize the ZnBrCl₂⁻via hydrogen bonds; the other is that the interaction between Ch⁺ and ZnBrCl₂⁻ will make the ZnBrCl₂⁻ a better electrophilic species. Similar to route 2, the ring-opening and ring-closure steps for route 3 are also competitive steps with the closer barrier heights of 23.47 and 24.63 kcal mol⁻¹.

Route 4. Except that the H atoms from the CH₃ group will form the hydrogen interaction with ZnBrCl₂⁻, the –OH group may play the same role with a better result. The O atom of PO is attacked by the ZnBrCl₂⁻ and the H atom in the –OH group is coordinated to the Cl₁ atom of ZnBrCl₂⁻via a hydrogen bond to form route 4. The barrier heights of the ring-opening, CO₂ insertion, and ring-closure steps are 17.73, 5.76, and 14.75 kcal mol⁻¹, respectively. Compared with routes 1–3, route 4 is the most favorable one with the lowest barrier height. To obtain more reliable results, the key barrier heights were corrected at the M06/6-311+G(d,p) level using a PCM model. The corresponding results are listed in Table 2. The barrier heights mostly decreased, especially for the ring-opening step of route 1. However, the most favorable route is still route 4, which is the same as in the above conclusion.

Table 2 The barrier heights (ΔE, kcal mol⁻¹) for the first and third elementary steps involved in routes 1–4

Route	M06/6-31+G(d,p)		M06/6-311+G(d,p) (PCM)
	Ring-opening	Ring-closure	Ring-opening	Ring-closure
1	30.31	10.38	19.49	15.67
2	23.42	25.03	21.97	22.25
3	23.47	24.63	17.36	20.60
4	17.73	14.75	15.55	18.44

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We hope to find valuable clues that are helpful for proposing mechanisms in future and for designing new catalytic systems with better activity. A combination of ZnBrCl₂⁻ and Ch⁺ has more acidity than just one of them, which looks to be a better electrophilic reagent. However, the bulk structure hinders the close interaction between catalyst and PO. The distance of Zn–O(PO) is 2.05 Å in TS1-1 (route 1). The same distance in TS4-1 is 2.01 Å. Shorter distances between catalyst and substrate will give a stronger interaction that is helpful for improving the catalytic performance. As compared with Ch⁺, ZnBrCl₂⁻ is a better electrophilic reagent, so the most favorable route is where ZnBrCl₂⁻ is employed as the main catalyst and Ch⁺ is utilized to stabilize it. Designing ILs with more acidity will be helpful for improving the positive charge of the Zn center of ZnBrCl₂⁻via the weak interactions between them. Consequently, it is profitable to improve the catalytic ability of ZnBrCl₂⁻.

3.5 Ch⁺, Br⁻, ZnBrCl₂⁻, and Ch⁺ catalysts

Considering that there are abundant catalysts in the real system, they are omitted in the above calculation. How will they affect the reaction barrier heights? To elucidate the puzzle, the coupling reaction of CO₂ with PO catalyzed by Ch⁺, Br⁻, ZnBrCl₂⁻, and Ch⁺ following the mechanisms of routes 3 and 4 is investigated at the M06/6-31+G(d,p) level. Then, the barrier heights are corrected at the M06/6-311+G(d,p) level using the PCM model. The optimized geometries for the intermediates and transition states are shown in Fig. S5.† As shown in Table 3, the barrier heights are varied, however, the barrier heights of route 3′ are higher than that of route 4′. The favorable route determined using the catalysts Ch⁺, Br⁻, and ZnBrCl₂⁻ could be expected to be reliable.

Table 3 The barrier heights (ΔE, kcal mol⁻¹) for the first and third elementary steps involved in routes 3, 4, 3′, 4′, and 5 calculated at the M06/6-311+G(d,p) (PCM)//M06/6-31+G(d,p) level

Route	The essential catalyst	Ring-opening	Ring-closure
3	Ch^+, Br^−, and ZnBrCl2^−	17.36	20.60
3′	Ch^+, Br^−, ZnBrCl2^−, and Ch^+	16.06	20.86
4	Ch^+, Br^−, and ZnBrCl2^−	15.55	18.44
4′	Ch^+, Br^−, ZnBrCl2^−, and Ch^+	12.11	16.89
5	[Ch]2[ZnBr2Cl2]	27.06	12.47

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3.6 [Ch]₂[ZnBr₂Cl₂] catalyst

The formation of [Ch]₂[ZnBr₂Cl₂] is exothermic indicating a spontaneous process. There is a possibility that it does not dissociate. Up to now, no theoretical study has been reported for the mechanism of the coupling reaction of CO₂ with PO catalyzed by [Ch]₂[ZnBr₂Cl₂]. On the basis of experience obtained from Section 3.4 and other literature,^44,45 the most favorable route is that the ZnBr₂Cl₂²⁻ is coordinated to the O atom of PO as the electrophilic species and two Ch⁺ cations are utilized to stabilize the ZnBr₂Cl₂²⁻. The excess Cl⁻ anions that exist in the reaction system will jointly assist the ring-opening by attacking the C atom of PO. According to this conjecture, a route with a similar mechanism to route 4 should be located. The difference is that the ring-opening of PO is activated by a tetra-coordinated Zn complex not a tri-coordinated one. It is unfortunate that one Zn–Cl bond is elongated in the process of forming a transition state. As a result, the ring-opening step is still activated by a tri-coordinated Zn complex rather than a tetra-coordinated Zn complex.

Alternatively, the ring-opening can be promoted by the synergetic effects of the tetra-coordinated Zn complex and the IL. As shown in Fig. 6 and Fig. S6,† the O is attacked by the –OH group of the IL, and simultaneously the C atom of PO is motivated by the Br atom of ZnBr₂Cl₂²⁻ leading to an energy-rich substrate. The two following steps are the CO₂ insertion and the ring-closure. The ring-opening step is the rate-determining step with a barrier height of 25.34 kcal mol⁻¹, which is higher than that of route 4. As shown in Table 3, the same situation occurs after the barrier heights are corrected at the M06/6-311+G(d,p) (PCM) level. So the catalytic components Ch⁺, Br⁻, and ZnBrCl₂⁻ will facilitate the reaction much easier.

Fig. 6 Potential energy profiles of route 4 (–) for the cycloaddition reaction catalyzed by Ch⁺, Br⁻, and ZnBrCl₂⁻ and route 5 (---) catalyzed by [Ch]₂[ZnBr₂Cl₂].

4 Conclusion

The mechanism of the coupling reaction of CO₂ with PO catalyzed by ZnBr₂/CH is studied using a DFT method. The theoretical study clearly indicates the reason why the cycloaddition of CO₂ with PO will be more favorable in the presence of ZnBr₂/CH compared with a single catalyst of ZnBr₂ or CH. It has been identified that using solely ZnBr₂ almost has no catalytic effect on the cycloaddition reaction with a much higher barrier height. While single component catalyst CH presented moderate catalytic activity for the rate-determining step. In the composite catalyst, the real catalytic components are Ch⁺, ZnBrCl₂⁻, and Br⁻ rather than [Ch]₂[ZnBr₂Cl₂]. The ring-opening is promoted by the electrostatic interaction between the Zn atom of ZnBrCl₂⁻ and the O atom of PO, and simultaneous nucleophilic attack from the Br⁻ anion of ChBr. In the whole catalytic process, Ch⁺ is employed to stabilize the Zn complex and Br⁻ anion via hydrogen bonds, although Ch⁺ does not directly have an interaction with PO. Moreover, the interaction between the –OH group of Ch⁺ and the Zn complex will make the latter become a stronger electrophilic species, which will facilitate the rupture of the C–O bond of PO. These cooperative effects reduce the barrier height. The calculated results elucidate the mechanism of composite catalyst ZnBr₂/CH, which is not consistent with the experimental assumption.

Acknowledgements

We thank the State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences for providing computational resources. This work was financially supported by the National Natural Science Foundation of China (21376063, 21476061), Program for He’nan Innovative Research Team in University (15IRTSTHN005), Natural Science Foundation of He’nan Province of China (134300510008, 144300510032, 142300410120), Science Foundation of Henan Province (14A150034), and Foundation for University Key Teachers from the He’nan Educational Committee.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05544j

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