All the DFT calculations throughout this work were carried out in the Gaussian09 software package (Frisch et al., 2009). The geometrical structures of all reactants, intermediates and products were directly optimized in solution phase. In order to obtain accurate calculated results, we employ the method of functional ωB97X-D (Chai and Head-Gordon, 2008) combined with a large basis set 6-31+G^* (Rassolov et al., 2001), denoted as ωB97X-D/6-31+G^*. The solvent effect is described by the SMD (Dolg et al., 1987; Andrae et al., 1990) model with the value of dielectric constant being 20.8. The frequency analysis is performed at the same computational level to identify the minima or transition states on potential energy surfaces. The calculation conditions for the reaction temperature and pressure were chosen as 353 K and 1 standard atmosphere pressure, which was as same to the experimental conditions. The high temperature was considered to make a significant contribution of TΔS in Gibbs free energy and the units for energies are in kcal/mol. More details about the optimized structures are provided in Supplementary Material.

Epichlorohydrin (0.925 g, 10 mmol) and AFIL (0.38 g, 0.1 mmol) were put into a 50 mL stainless steel autoclave equipped with a magnetic stirrer. The reaction was carried out under atmospheric pressure of CO₂ at 80°C. After the completion of reaction, the autoclave was cooled to room temperature. In order to remove the ionic liquid, Et₂O (50 mL) was added to the reaction mixture and filtered. The filtrate was subsequently analyzed by gas chromatography to determine yield using dodecane as internal standard. The product was purified by chromatography on silica gel and characterized structurally by NMR spectroscopy.

3-Chloro-1,2-propylene carbonate: Colorless liquid, ¹H NMR (400 MHz, CDCl₃, TMS) δ 5.01–4.95(m, 1H), 4.57 (t, J = 8.0 Hz, 1H), 4.39–4.35 (m, 1H), 3.81–3.77 (m, 1H), 3.72–3.68 (m, 1H). ¹³C NMR (100 MHz, CDCl₃, TMS) δ 154.24, 74.33, 66.99, 43.74.

The structures and synthetic procedures of ionic liquid catalysts for the cycloaddition of CO₂ and epichlorohydrin were shown in Figure 1.

To investigate the intramolecular synergistic catalytic system of imidazolium and amino functional group in AFIL, BnBimBr, DEBA and AFIL were applied to catalyze the cycloaddition of CO₂ and epichlorohydrin. The synergistic catalytic effect of organic base and ionic liquid for the conversion of CO₂ was shown in Table 1. When BnBimBr was used alone, a moderate yield of 50% of 3-chloro-1,2-propylenecarbonate was obtained after 3 h under atmospheric CO₂ pressure (entry 1, Table 1). If DEBA was used alone, a poor yield of 14% was obtained (entry 2, Table 1). However, the AFIL gave excellent yield of 85% (entry 3, Table 1), indicating the intramolecular synergistic catalytic system of AFIL for accelerating the cycloaddition reaction.

Reaction conditions: Epichlorohydrin (10 mmol), CO₂ (0.1 MPa), catalyst (0.1 mmol), 80°C, 3 h.

The optimized experimental conditions such as reaction time, reaction temperature and catalyst amount on the reaction have been systemically investigated. The effect of different reaction times on the product yield was shown in Figure 2. The yield of the 3-chloro-1,2-propylenecarbonate increased with raising reaction time from 1 to 6 h. After 3 h, the growth trend of the yield slowed down obviously. When the reaction time was 6 h, the yield could reach 97%.

Reaction condition: Epichlorohydrin (10 mmol), CO₂ (0.1 MPa), AFIL (0.1 mmol), 80°C, GC yield.

The effect of reaction temperatures was listed in Figure 3. The yield increased significantly with the temperature from 50 to 80°C. When the reaction temperature higher than 80°C, the yield barely increased.

Reaction condition: Epichlorohydrin (10 mmol), CO₂ (0.1 MPa), AFIL (0.1 mmol), 6 h, GC yield. The effect of catalyst amount on the reaction was also investigated (Figure 4). In the absence of any catalyst, the reaction did not proceed. However, in the presence of the catalyst, the reaction took place immediately. When the catalyst amount increased from 0.1 to 0.5 mol%, the yield increased from 71 to 94%. The highest yield of 3-chloro-1,2-propylenecarbonate was obtained under the amount of AFIL increased to 1 mol%. Further increasing the amount of AFIL to 2 mol%, the yield was not increasing anymore.

Reaction condition: Epichlorohydrin (10 mmol), CO₂ (0.1 MPa), 80°C, 6 h, GC yield.

The synthesized AFIL possesses the dual functions of both ionic liquids and organic bases. The previous experimental and theoretical studies (Elageed et al., 2016; Ji et al., 2018) have revealed that the ionic liquids having imidazolium rings could assist the reaction of ring-opening of epoxides by forming hydrogen bonding interaction with epoxides. On the other hand, it has been reported that the yields of products for CO₂ conversion catalyzed by the tertiary amines such as DBU (Heldebrant et al., 2005) and DABCO (Ji et al., 2018) increased remarkably in the presence of water. In the reactions of CO₂ and epoxides catalyzed by ionic liquids, the appropriate amount of water would facilitate the formation of bicarbonate salts effectively. The reason may be that CO₂ can dissolve in H₂O to form carbonic acid (H₂CO₃) which dissociates as bicarbonate (HCO3-) and proton. The tertiary amine groups in ionic liquids possess strong basicity so that they can prompt the dissociation of H₂CO₃ and increase the dissolution of CO₂ in solution. In the first step, we should consider the protonation state of amine group of AFIL in solution because the chemical equilibrium between AFIL and H₂CO₃ might exist in the solution phase.

The calculated DFT results in Figure 5 indicate that the proton abstraction from H₂CO₃ by AFIL, denoted as Int-a1, occurs easily to form the complex Int2-a2, releasing an energy of 14.1 kcal/mol. The process of proton transfer from carbonic acid to Int-a1 has no barrier and the formed complex Int2-a2 is quite stable, which demonstrates that the ionic liquid AFIL exists in a protonated state in solution. The releasing of HCO3- from the complex Int2-a2 needs only 3.5 kcal/mol and the protonated Int-a3 is ready for the subsequent catalytic reaction.

The previous studies (Elageed et al., 2016; Luo et al., 2016; Huang et al., 2017; Ji et al., 2018) supported that the imidazolium rings of ionic liquids could form hydrogen bonding interaction with epoxides. In AFIL, the moiety of 2-buthylimidazole can assist the ring-opening process of epoxides through hydrogen-bonding interaction with them. Figures 6A,B show the calculated energy profile for the ring-opening reaction of epichlorohydrin as well as the corresponding optimized structures. The protonated Int-a3 in Figure 5 and epichlorohydrin form the stable complex Int-b1 through the bridged oxygen atom of epichlorohydrin. The calculated energy of Int-b1 is −14.9 kcal/mol, which is stable than the isolated Int-a3 and epichlorohydrin by 4.3 kcal/mol. The distance of hydrogen bonding interaction in Int-b1 is 2.08 Å. In addition, we notice that the Br⁻ stays 2.18 Å away from protonated amine group and forms a weak hydrogen bonding interaction, with the distance being 2.18 Å.

Next, the Br⁻ attacks the carbon atom of epichlorohydrin via the transition state TS-b2, with a modest energy barrier of 26.6 kcal/mol. If a count ion HCO3- is considered in this step, the calculated barrier would be 27.7 kcal/mol and the influence of count ion on barrier is not obvious. With the help of hydrogen bonding interaction of imidazole ring, the O-C bond cleaves to give rise to the ring-opening intermediate Int-b3. We note in Int-b3 that the Br⁻ stabilized by amine group forms covalent bond with the carbon atom of cleaved O-C bond. The oxygen atom carries negative charge, which is stabilized by the acid C-H bond of imidazolium ring. Such an oxy anion species is very active so that the addition reaction could occur by the electrophilic attack of free CO₂. It is noticed that the optimized structure of Int-b3 in Figure 6 shows that the ring-opening product is embedded in the catalytic cavity formed by the surrounded ionic liquid molecule. The direct addition of CO₂ to the oxy anion species in the complex Int-b3 would encounter a high energy barrier due to the steric effect of surrounded molecule. By taking the thermal dynamics of complexes in solution into account, we propose a reaction pathway for the CO₂ addition. It is possible that the complex Int-b3 could dissociate into the isolated oxy anion species Int-b4 and the ionic liquid Int-b5 by breaking the H-bond interaction, with absorbing an energy of 3.0 kcal/mol. Then, the direct electrophilic attack of CO₂ to Int-b4 yields the stable addition product Int-b6. This process of CO₂ addition is extremely exothermic by 18.3 kcal/mol. The DFT scan calculation (Supplementary Material) shows that the energy profile of CO₂ attack is downhill in energy, without an obvious barrier. For the whole ring-opening process, the O-C bond cleavage in the epichlorohydrin of Int-b1 is the rate-determining step, with the calculated barrier of 26.6 kcal/mol.

The final step is the intramolecular ring-closing initiated from the substrate Int-b6. Figure 7 shows the calculated energy profile for the ring-closing process. As shown in TS-b7, one of the oxygen atoms in the carboxyl COO group attacks the terminal carbon atom to form a new O-C bond, while the C-Br bond is being broken at the same time. In the optimized TS-b7, the distances of O-C and C-Br atoms are 1.93 and 2.49 Å, respectively, which indicates that it follows a typical S_N2 mechanism. The five-member-ring complex Int-b8 is 4.0 kcal/mol lower than that of Int-b6 in energy. In the structure of Int-b8, the Br⁻ has to stay far away from the cyclic carbonate, since the carbonate itself contains a few atoms such as the oxygen atoms that carry negative charges.

We notice that in the structure of Int-b1 of Figure 6, the Br⁻ anion is stabilized by the N-H bond of amine group. Later, it assists the ring-opening in TS-b2 by attacking the carbon atom and forms the C-Br bond with the carbon atom in Int-b3. After the ring-closing via TS-b7, it is evident that the Br⁻ is released again through the cleavage of the covalent C-Br bond. Our DFT calculation shows that if the free Br⁻ reforms the complex Int-a3 with the Int-b3 and yields the final cylic carbonate Pro9, this process is exothermic in thermodynamics and the energy is lower than that of Int-b8 by 8.3 kcal/mol. The barrier of rate-determining step for the ring-closing process is 23.2 kcal/mol, lower than that step of ring-opening by 3.4 kcal/mol.

Finally, the whole mechanism of CO₂ conversion catalyzed by AFIL is summarized in Scheme 1. The protonated ionic liquid Int-a3 catalyzes the ring-opening of Int-b1 to the ring-opening intermediate Int-b3. The direct addition of CO₂ to the released Int-b4 yields Int-b6. The species Int-b6 undergoes the ring-closing to yield Int-b8. The Int-b8 gives rise to the final product Pro9 by releasing the Br⁻ who reforms the catalytic Int-a3 with Int-b5.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.