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DFT Study on the Formation Mechanism of Normal and Abnormal N-Heterocyclic Carbene-Carbon Dioxide Adducts from the Reaction of an Imidazolium-Based Ionic Liquid with CO

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Mengmeng Dong, Jun Gao, Chengbu Liu, and Dongju Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07191 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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The Journal of Physical Chemistry

DFT Study on the Formation Mechanism of Normal and Abnormal N-Heterocyclic Carbene-Carbon Dioxide Adducts from the Reaction of an Imidazolium-Based Ionic Liquid with CO2

Mengmeng Dong,† Jun Gao,‡ Chengbu Liu,† and Dongju Zhang*,†

†

Institute of Theoretical Chemistry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China

‡

Hubei Key Laboratory of Agricultural Bioinformatics, College of Informatics, Huazhong Agricultural University, Wuhan 430070, P. R. China

Corresponding Author: Email: [email protected]

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ABSTRACT

To illustrate the formation mechanism of normal and abnormal N-heterocyclic carbene-carbon dioxide adducts (NHC-CO2 and aNHC-CO2), we implement density functional theory calculations on the reactions of two imidazolium-based ionic liquids ([C2C1Im][OAc] and [C2C1Im][CH3SO3]) with CO2. The reaction of [C2C1Im][OAc] with CO2 is mimicked using the gas phase model, implicit solvent model, and combined explicit-implicit solvent model. In gas phase, the calculated barriers at 125℃ and 10 MPa are 12.1 kcal/mol for the formation of NHC-CO2 and 22.5 kcal/mol for the formation of aNHC-CO2, and the difference is significant (10.4 kcal/mol). However, the difference becomes less important (1.5 kcal/mol) as the solvation effect is considered more realistically using the combined explicit-implicit solvent model, rationalizing the experimental observation of aNHC-CO2 adduct in [C2C1Im][OAc]-CO2 system. The anion of ionic liquid is shown to play a substantial role, which can adjust the reactivity of imidazolium cation towards CO2: replacing the basic [OAc]- anion with a less basic [CH3SO3]- anion the reaction becomes very difficult, as indicated by high free energy barriers involved (41.4 kcal/mol for the formation of NHC-CO2 and 39.2 kcal/mol for the formation of aNHC-CO2). This is in agreement with the fact that neither NHC-CO2 or aNHC-CO2 is formed in [C2C1Im][CH3SO3]-CO2 system, emphasizing the important dependence of the reactivity on the basicity of the anion of imidazolium-based ionic liquids for the formation of NHC- and aNHC-CO2 adducts.

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■ INTRODUCTION Imidazolium-based ionic liquids, consisting of the imidazolium cation and specific organic/inorganic anions such as acetate, sulfonate, halogen, triflate and tetrafluoroborate, have a wide variety of applications, due to their remarkable physicochemical properties, for instance, fine solubility, good thermal stability, low melting point, high electric conductivity, negligible vapor pressure.1-3 Imidazolium acetates, as one of the most widely used ionic liquids, have recently showed promising applications in many areas, including electrochemistry,4,5 organic synthesis,6,7 functional materials,8,9 clean energy,10,11 and separation and extraction science.12,13 As a kind of widely applied ionic liquids, imidazolium acetates are generally considered as the important sources of N-heterocyclic carbenes (NHCs),14-18 which are extensively used as organocatalysts19-22 and transition metal ligands23-25 in molecular chemistry for a number of transformations. More importantly, in recent years, imidazolium acetate ionic liquids have shown promising application prospect in absorption of CO2,26 since the pioneer work of Brennecke et al.,13 a vital milestone for the usage of ionic liquids in gas adsorption field.

Scheme 1. Normal (a) and abnormal (b and c) N-heterocyclic carbene-carbon dioxide adducts observed by Hollócaki et al.33 from the reaction of [C2C1Im][OAc] with CO2.

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The most widely accepted mechanism of CO2 absorption in imidazolium acetate ionic liquids involves a Brønsted acid/base solvation mechanism where the acetate anion is proposed to play a substantial role, acting as a Lewis base to react with acidic CO2.27 However, Rogers et al.28 and several other groups29-32 showed the formation of the N-heterocyclic carbene-carbon dioxide (NHC-CO2) adduct (structure (a) in Scheme 1) in the system of imidazolium acetate ionic liquids with CO2, indicating that the imidazolium cation also contribute to CO2 absorption with C2 position being the reactive site of the reaction. More

recently,

Hollócaki

and

co-workers33

reported

the

reaction

of

1-ethyl-3-methylimidazolium acetate ([C2C1Im][OAc]) ionic liquid with CO2 at 125℃ and 10 MPa. Interestingly, except for the normal NHC-CO2 adduct at C2 position (Scheme 1a), the other two abnormal NHC-CO2 (aNHC-CO2) adducts were also observed (structures (b) and (c) in Scheme 1), where the carbene center is located at C4 or C5 position of the imidazolium ring.34 However, under the same experimental conditions, neither NHC-CO2 nor aNHC-CO2 was observed when the basic acetate anion of [C2C1Im][OAc] was replaced by the less basic methanesulfonate [CH3SO3]- anion. Thus Hollócaki33 believed that the anion of imidazolium-based ionic liquids plays a crucial role for the formation of NHC-CO2 and aNHC-CO2 adducts: the acetate, acting as a much stronger Lewis base than methanesulfonate can promote the reaction. Furthermore, by performing static density functional theory (DFT)35-37

calculations

and

ab

initio

molecular

dynamics

simulations

on

the

[C2C1Im][OAc]-CO2 system, Hollócaki et al.33 proposed the formation mechanism of both NHC- and aNHC-CO2 adducts. As shown in Scheme 2, the addition of CO2 on C2 position with the abstraction of C2-proton by acetate anion results in the NHC-CO2 adduct, while two 4


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aNHC-CO2 adducts were obtained via anionic imidazolium-2,4-dicarboxylate (Scheme 2d) and anionic imidazolium-2,5-dicarboxylate (Scheme 2e), respectively.

Scheme 2. Formation of normal (a) and abnormal (b and c) N-heterocyclic carbene-carbon dioxide adducts proposed by Hollócaki et al.33 Density functional theory (DFT) calculations have been extensively applied to the study of the interaction of ILs with CO2.38-40 In this work, we present a further DFT computational study on the reaction of imidazolium-based ionic liquids with CO2. Through calculations at the molecular level, we present a new understanding of the formation mechanism of NHCand aNHC-CO2 adducts, which emphasizes that the reactivities of the imidazolium cation at C2, C4, and C5 position towards CO2 are closely relevant to their polar microenvironment induced by ionic liquids. In addition, we also show the different reactivities of two imidazolium-based ionic liquids [C2C1Im][OAc] and [C2C1Im][CH3SO3] towards CO2 to understand the significance of basicity of the anion in imidazolium-based ionic liquids for the formation of NHC- and aNHC-CO2 adducts. As far as we know, no such a topic has been addressed in published works.

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■ COMPUTATIONAL DETAILS The present study uses the cluster model to describe the reactions of ionic liquids [C2C1Im][OAc] and [C2C1Im][CH3SO3] with CO2. One ionic pair of each ionic liquid is used to study their respective reactivities towards CO2 to form NHC- and aNHC-CO2 adducts. The special attention is paid to the [C2C1Im][OAc]-CO2 system, for which a second ionic pair is also introduced into the system to mimic the polar microenvironment around the reactive site. DFT calculations were performed employing the B97D41 functional with the standard 6-311+G(d,p) basis set, as implemented in the Gaussian 09 software package.42 The B97D functional has been confirmed to achieve efficient and reliable performance for describing the systems involving nonbonding interactions.41 All geometries were first fully optimized in the gas phase without any symmetry constraints, and then fully re-optimized by considering solvation effects of the ionic liquid using the polarizable continuum model (PCM).43 The solvent used is 1,2-dichloroethane (1,2-DCE), which has similar dielectric properties with ILs.44,45 The electronic energies were corrected to Gibbs free energies at the experimental temperature and pressure (398.15 K and 10 MPa) using the calculated harmonic frequencies. To reduce the overestimation of entropy contributions to Gibbs free energies for steps involving multicomponent change, the initial complex of the ionic pair(s) with CO2 were chosen as the zero energy reference point.46 Transition states located have been checked by performing intrinsic reaction coordinate (IRC)47,48 calculations to confirm that each of them actually connects to the forward and reverse minima. To ascertain the reliability of the B97D functional for describing the present ionic liquid system, single-point solvation Gibbs free energy calculations were also carried out for several key processes using the more accurate 6


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wB97X-D49 functional, which includes both exchange and dispersion corrections and plays a good performance in both bond changes and weak interactions.50,51 Natural bond orbital (NBO) analyses52-54 at the B97D/6-311+G(d,p) level were performed to assign the atomic charges. ■

RESULTS AND DISCUSSION

Structures of the imidazolium cation and clusters of ionic pairs of [C2C1Im][OAc]. To understand the formation mechanism of NHC- and aNHC-CO2 adducts in the [C2C1Im][OAc]-CO2 system,

we

first

calculated

the

structures

of

an

isolated

1-ethyl-3-methylimidazolium cation and the ion-pair clusters of [C2C1Im][OAc], including the monomer, dimer, and trimer of the ionic pair (Figure 1). For the isolated cation (a), the C2−H unit is shown to carry the larger positive charge (0.516 e) than the C4−H (0.242 e) and C5−H (0.243 e) units although these three C−H bonds have identical bond length (1.081 Å), indicating that the C2−H unit is more acidic and its proton is more easily to be extracted by an alkaline substance. Figure 1 shows the three stable geometries of a single [C2C1Im][OAc] ionic pair (b, c and d), where the negative oxygen of acetate anion interacts via strong H-bonding with the C2−H, C4−H and C5−H units, as indicated by the calculated much shorter C − H L O H-bond distances (1.628, 1.729 and 1.752 Å) than the sum of van der Waals atomic radius of oxygen and hydrogen (~2.27 Å).55 Structure b with the anion approaching the C2−H unit is calculated to be the energetically most favor, which is more stable by ~8-9 kcal/mol in Gibbs free energy than structures c and d with the anion approaching the C5−H and C4−H unit,

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respectively. These results provide support for the claim that the C2−H proton is the more acidic.

Figure 1. Optimized geometries at the B97D/6-311+G(d,p) level for an isolated 1-ethyl-3-methylimidazolium cation (a) and the ion-pair clusters of [C2C1Im][OAc], including the monomer (b, c and d), dimer (e and f), and the trimer (g and h), with selected structural parameters (bond lengths, blue, in Å) and NBO charges (red, in e). The values in parentheses refer to are calculated relative Gibbs free energies (in kcal/mol).

The present work also calculated the structures of double and triple ionic pairs, referring to the published papers of the larger-sized imidazolium-based ionic-pair clusters.56-60 Two typical stable structures (e and f) of the dimer of the ionic pair are presented in Figure 1. 8


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There are two different H-bonding patterns: the strong H-bonding interaction between the H atom on imidazolium ring and the negative oxygen of carboxylate and the weak H-bonding interaction between the N-methylene hydrogen atom and the carbonyl oxygen atom. Comparing with the single ionic pair, it is found that the C2/C4 − H L O H-bond distances in these two structures become longer, and more importantly, the difference between C2−H and C4−H distances becomes smaller (0.002 Å in the dimer of the ionic pair vs 0.021 Å in the single ionic pair). These observations are again confirmed in two trimer structures (g and

h) of the ionic pair, as shown in Figure 1. Moreover, both in structures g and h there exist a bifurcate H-bond, where the negative oxygen of carboxylate on one hand forms a strong H-bond with C2/C4−H atom and on the other hand shows weak H-bonding interaction with the N-methylene hydrogen atom. These results emphasize that a single ionic pair that ignores the effect of adjacent ions is not a suitable model for describing the structure and property, and in particularly, the chemical reactivity of a bulk ionic liquid phase. As will be seen in the following sections, the calculated results using a single ionic pair can not rationalize the formation of aNHC-CO2 adducts from the reaction of [C2C1Im][OAc] with CO2. However, by considering the presence of the second ionic pair, we are capable of explaining the competitive formation of NHC- and aNHC-CO2 adducts.

Reaction of an isolated [C2C1Im][OAc] ionic pair with CO2. Figure 2 displays the calculated energy profiles in gas phase for the formation of NHC-CO2 adduct (pathway I) and aNHC-CO2 adduct (pathway II) from the reaction of an isolated [C2C1Im][OAc] ionic pair with CO2. Two stable geometries of the initial supramolecular complex of the ionic pair with CO2 are denoted as 1 and 2, which are precursors forming the NHC- and aNHC-CO2 adducts, 9



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respectively. Along pathway I, the reaction proceeds via transition state TS1, where the acetate anion acts as a nucleophile, extracting the C2−H proton while the CO2 molecule electrophilically attacks on C2 atom of the imidazolium cation, leading to structure 3, a H-bond complex between the NHC-CO2 adduct and an acetic acid molecule. The reaction is calculated to be endothermic by 2.3 kcal/mol with a barrier of 12.1 kcal/mol in Gibbs free energy. In contrast, along with pathway II, the aNHC-CO2 adduct is formed via transition state TS2, a structurally very similar counterpart to TS1 with an overall barrier of 22.5 kcal/mol and an endothermic effect of 9.9 kcal/mol. The values in parentheses in Figure 2 are corresponding electronic energies, which are in good agreement with those obtained using the same B97D functional with the larger aug-cc-pvTZ61 basis set by Hollócaki and co-workers.33 Based on the theoretical results, it seems that the formation of aNHC-CO2 adduct is both kinetically and thermodynamically much less favorable than that of NHC-CO2 adduct (barriers 22.5 vs. 12.1 kcal/mol, and the exothermicities 2.3 vs 9.9 kcal/mol). The results can not interpret the formation of aNHC-CO2 adduct observed in the experiment.33 This may be due to ignoring the effect of the solvent on the reactivity. It should be noted that paths I and II involve intrinsically similar mechanism, where the C-H bond rupture and the C-C bond couple occurs concertedly. We failed to find a step-by-step mechanism with the proton transfer from the cation to the acetate anion prior the carboxylate formation, even using the more accurate M0662 functional with the larger 6-311++G(d,p) basis set. This indicates that the presence of CO2 molecule can promote imidazolium cation donating its acidic proton to a neighboring acetate anion and forming a

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carboxylate compound with the CO2 molecule. The present result is in good agreement with the very recent report by Yan and co-workers.63

Figure 2. Calculated Gibbs free energy profiles for the formation of NHC-CO2 and aNHC-CO2 adducts from the reaction of an isolated [C2C1Im][OAc] ion pair with CO2 in the gas phase. Values in parentheses denote electronic energies, and selected bond distances in schematic geometries are given in Å.

To account for the solvation effect of [C2C1Im][OAc] ionic liquid, the implicit solvent PCM calculations with 1,2-DCE as solvent were firstly carried out. The results are presented in Figure 3, where pathway I′ and II′ refer to the formation of NHC- and aNHC-CO2 adducts, respectively. It is found that the structures of the binding complexes and transition states are very similar to those in the gas phase with only small differences in bond distances. With inclusion of the solvation effect, the calculated barriers for the formation of NHC- and aNHC-CO2 adducts added to 22.7 vs 28.4 kcal/mol from 12.1 vs 22.5 kcal/mol, and two pathways (I′ and II′) involve the almost same exothermicity, endothermic by 5.4 kcal/mol. 11



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The higher barriers in 1,2-DCE are attributed to the smaller stabilization of the polar 1,2-DCE for the less polar transition states (TS1 and TS2) than for the corresponding more polar initial complexes (1 and 2), as indicated by calculated dipole moments shown in Table 1, 6.0 D for

TS1 vs 9.9 D for structure 1, and 8.5 D for TS2 vs 13.6 D for structure 2. By considering solvation effect using the implicit solvent model, the barrier difference of two pathways forming NHC- and aNHC-CO2 adducts becomes smaller: reduced to 5.7 kcal/mol (Figure 3) from 10.4 kcal/mol (Figure 2).

Figure 3. Calculated Gibbs free energy profiles for the formation of NHC-CO2 and aNHC-CO2 adducts from the reaction of an isolated [C2C1Im][OAc] ion pair with CO2 in 1,2-dichloroethane. Values in parentheses denote electronic energies, and selected bond distances in schematic geometries are given in Å.

It should be noted that the PCM implicit solvent model calculation only provides limited information on the solvation effect due to the absence of typical specific interactions between 12


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solvent and reactive center. In the following sections, the reactions will be mimicked using the hybrid explicit-implicit solvent model, where an explicit ionic pair is introduced into the reactive center and then the calculations are carried out using the implicit solvent model.

Table 1. Calculated dipole moments of compounds in Figure 2 reaction pathway

compound

dipole moment (Debye)

1

9.9

TS1

6.0

2

13.6

TS2

8.5

path I path II

Effect of a second [C2C1Im][OAc] ionic pair on the reactivity. With the presence of a second ionic pair at the reactive site, the present calculations explored two possible reaction models, as denoted by Figure 4a and 4b, where the second ionic pair (the structure in gray) occurs at the reactive site as an integral part (Figure 4a) or the separate cation and anion (Figure 4b). The intrinsic reaction mechanism is found to remain unchanged because the second ionic pair does not directly participate in the reaction. However, it is noted that throughout the reaction, both the cation and anion of the second ionic pair show weak H-bonding and electrostatic interaction with the species at the reactive site, which change the microenvironment of the reactive site and thereby have a significant effect on the reactivity.

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Figure 4. Calculated Gibbs free energy profiles for the formation of NHC-CO2 and aNHC-CO2 adducts from the reaction of two [C2C1Im][OAc] ionic pairs with CO2 in 1,2-dichloroethane. Values in parentheses denote electronic energies, and selected bond distances in schematic geometries are given in Å.

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Figure 4a shows the calculated results with the integral presence of the second ionic pair. The initial geometry of each supramolecular structure is obtained based on a combination of chemical and electrostatic senses, then fully optimized, and finally characterized through vibration analysis. This method is similar to that in the work by Bessac and Maseras.56 The calculated free energy barriers are 27.4 kcal/mol for the formation of NHC-CO2 adduct (pathway Ia) and 32.1 kcal/mol for that of aNHC-CO2 adduct (pathway IIa). These two barriers obtained using the combined model of implicit and explicit solvent model are again higher than those from the gas-phase calculations (Figure 2), and are also higher than those obtained by the PCM implicit solvent calculations (Figure 3). Obviously, the presence of the explicit ionic pair significantly affects the reaction. The transition states involved in the reaction are of the less ionic character than the corresponding reactant-like intermediates, as a result, are less stabilized by the strongly polar ionic pair. (Figure 5 shows the schematic structures involved in Figure 4a) More importantly, the barrier difference of two pathways forming NHC- and aNHC-CO2 adducts is reduced to 4.7 kcal/mol from 10.4 kcal/mol obtained using the gas-phase model (Figure 2) and from 5.7 kcal/mol calculated by employing the PCM solvent model (Figure 3).

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Figure 5. Optimized structures with selected bond distances (in Å) for all schematic structures involved in Figure 4a.

Alternatively, when the second ionic pair is separately introduced into the reactive site, the relevant results are shown in Figure 4b. The barrier involved in pathway Ib to form NHC-CO2 adduct is 29.1 kcal/mol, which is not remarkably different from that in pathway IIb to form aNHC-CO2 adduct, 30.6 kcal/mol. Moreover, the formation of aNHC-CO2 adduct is even thermodynamically more favorable than that of aNHC-CO2 adduct. Figure 6 summarizes the calculated barrier difference forming NHC- and aNHC-CO2 adducts using different theoretical models. The barrier difference varies from 10.7 kcal/mol from the gas phase model to 5.7 kcal/mol from the PCM solvent model, and to 4.7 and 1.5 kcal/mol from the combined explicit-implicit solvent model. These results imply that the C2−H and C4/5−H units are of intrinsically similar reactivities towards the reaction with CO2, 16


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being good agreement with the formation of both NHC- and aNHC-CO2 adducts from the reaction of [C2C1Im][OAc] with CO2. To verify the validity of the results above based on the B97D calculations, we also calculated the energies of several key processes involving transition states TS1 and TS2,

TS1′and TS2′, TS3a and TS4a, and TS3b and TS4b with wB97X-D functional. The results are shown in Table S1 and Figure S1 in the Supporting Information. It is found that the calculated relative energy differences are in good agreement with those obtained using B97D functional. For example, for pathways I′ and II′ (Figure 3), the calculated barrier differences are 5.7 kcal/mol using B97D functional, and 6.6 kcal/mol employing wB97X-D functional, confirming the reliable performance of B97D functional for describing the present ionic liquid system. Based on the discussion above, the solvation effect of [C2C1Im][OAc] ionic liquid considered by the hybrid explicit-implicit solvent model calculations, illustrates reasonably the competitive formation of NHC- and aNHC-CO2 adducts.

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Figure 6. Barrier differences for the formations of NHC-CO2 and aNHC-CO2 adducts with different theoretical models calculated at the B97D/6-311+G(d,p) level. “n” denotes as the number of ionic pair.

The anion effect of ionic liquid on the reactivity. From the discussion above for the reaction of [C2C1Im][OAc] with CO2, it is clear that the anion of ionic liquid works as a Brønsted base, extracting the proton of C2/4/5−H positions in imidazolium ring. Thus, the basicity of anion is expected to play an important role for the formation of the NHC- and aNHC-CO2 adducts. As indicated by the experiment of Hollócaki,33 replacing the basic [OAc]- anion with a less basic anion [CH3SO3]-, neither NHC-CO2 nor aNHC-CO2 was observed in the reaction of [C2C1Im][CH3SO3] with CO2 under the same experimental conditions (125℃ and 10 MPa). In order to interpret this phenomenon, the calculations were also implemented for the reaction of [C2C1Im][CH3SO3] with CO2 using the PCM solvent model with 1,2-DCE as solvent. The results are shown in Figure 7. It is found that the free energy barrier is 41.4 kcal/mol for the formation of NHC-CO2 adduct (pathway IA) and 39.2 kcal/mol for the formation of aNHC-CO2 adduct (pathway IIA). These free energy barriers are 18


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much higher than those involved in the reaction of [C2C1Im][OAc] with CO2 discussed above, rationalizing the experimental finding of Hollócaki33 that NHC- and aNHC-CO2 adducts were not observed in [C2C1Im][CH3SO3]-CO2 system. This fact indicates that the basicity of the anion is mainly responsible for the formation of two adducts.

Figure 7. Calculated Gibbs free energy profiles for the formation of NHC-CO2 and aNHC-CO2 adducts from the reaction of an isolated [C2C1Im][CH3SO3] ion pair with CO2 in 1,2-dichloroethane. Values in parentheses denote electronic energies, and selected bond distances in schematic geometries are given in Å.

CONCLUSIONS

In summary, DFT calculations have been carried out to illustrate the formation mechanism of NHC- and aNHC-CO2 adducts from the reactions of two imidazolium-based ionic liquids ([C2C1Im][OAc] and [C2C1Im][CH3SO3]) with CO2. The reactions are shown to proceed via a concerted mechanism, where the anion of ionic liquid serves as a Brønsted base, extracting 19



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the C−H proton on the imidazolium ring while CO2 electrophilically attacks the imidazolium carbon atom. Using the combined explicit-implicit solvent model, the barrier involved for the formation of aNHC-CO2 adduct is found to be comparable with that involved for the formation of NHC-CO2 adduct. The basicity of ionic liquid anion significantly affects the reactivity: replacing the basic [OAc]- anion with a less basic anion [CH3SO3]-, the formation of both NHC-CO2 and aNHC-CO2 adducts become very difficult as indicated by calculated high free energy barriers (41.4 and 39.2 kcal/mol). These results rationalizes the experimental observations of the competitive formation of NHC- and aNHC-CO2 adducts in [C2C1Im][OAc]-CO2

system

as

well

as

the

absence

of

such

adducts

in

[C2C1Im][CH3SO3]-CO2 system.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

.

Comparison of calculated relative Gibbs free energies using the B97D and wB97X-D functionals (Table S1 and Figure S1); Cartesian coordinates of all stationary points located in this work

■ AUTHOR INFORMATION Corresponding Author *

D. J. Zhang. E-mail: [email protected].

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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support from National Natural Science Foundation of China (Nos. 21433006 and 21773139).

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