A computational study on the chemical fixation of carbon dioxide with 2-aminobenzonitrile catalyzed by 1-butyl-3-methyl imidazolium hydroxide ionic liquids

Computational and Theoretical Chemistry 978 (2011) 47–56

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A computational study on the chemical fixation of carbon dioxide with 2-aminobenzonitrile catalyzed by 1-butyl-3-methyl imidazolium hydroxide ionic liquids Ying Ren, Ting-Ting Meng, Jianfeng Jia, Hai-Shun Wu ⇑ School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China

a r t i c l e

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Article history: Received 5 May 2011 Received in revised form 25 September 2011 Accepted 25 September 2011 Available online 14 October 2011 Keywords: Carbon dioxide 2-Aminobenzonitrile [Bmim]OH DFT calculations NHC

a b s t r a c t The mechanism of chemical fixation of carbon dioxide with 2-aminobenzonitrile catalyzed by 1-butyl3-methyl imidazolium hydroxide ([Bmim]OH) ionic liquid has been extensively investigated by the density functional theory (DFT) calculations. The purpose is to show the detailed reaction mechanism, and in particular to better understand the role of [Bmim]OH played in the reaction as a catalyst. Three mechanistic pathways are proposed and evaluated. Our calculations indicate that the real activate catalyst is the N-heterocyclic carbine (NHC). In the favored pathway, it is found that the attack of C3 atom of NHC to C1 atom, the C1–O2 bond cleavage, and the C1–N2 bond formation have close activation free energies. Therefore, each of them can determine the reaction rate with variation in the reaction conditions. Furthermore, it is shown that the scCO2 solution does not change significantly the potential energy surface profile. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Carbon dioxide (CO2) is one of the most important greenhouse gases, which has resulted in serious environmental problems and global warming effects. CO2 is also an attractive C1 building block in organic synthesis as it is highly abundant, cheap, nontoxic, and nonflammable [1–3]. Hence, CO2 fixation has attracted much interest in view of the sustainable chemistry and ‘‘green chemistry’’ concepts [4,5]. One of the promising methodologies for CO2 chemical fixation is the reaction of CO2 with 2-aminobenzonitriles to form quinazoline-2,4(1H,3H)-diones. Quinazoline-2,4(1H,3H)diones have emerged as preeminent classes of organic compounds, which hold applications due to the wide range of biological properties [6–9]. During the past few decades, numerous synthetic methodologies have been developed for the preparation of quinazoline-2,4(1H,3H)diones, i.e. via reaction of anthranilic acid with urea [10], anthranilamide with phosgene [11], and anthranilic acid with potassium cyanate or chlorosulfonyl isocyanate [12]. However, much effort has been often limited by the requirement for specialized reagents, and operational complexity due to the use of either toxic or cumbersome reagents like phosgene. A few efforts have been made to

⇑ Corresponding author. Tel.: +86 357 2052468; fax: +86 357 2051375. E-mail address: [email protected] (H.-S. Wu). 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.09.032

replace toxic reagents using incorporation of carbon dioxide into quinazoline-2,4(1H,3H)-diones derivatives [13–17]. Nevertheless, most of these studies have the following drawbacks: high catalyst loading, lower substrate compatibility, and the use of non-recyclable homogeneous base, which limits their application. Thus, considering the economical value of the quinazoline-2,4(1H,3H)-diones derivatives still there is need to develop a truly catalytic and environmentally viable protocol which can minimize the number of unit operations and waste streams [13]. With increasing interest in the green chemistry concept, ionic liquids have received much attention in organic synthesis. Ionic liquids have been the subject of considerable current interest as benign reaction media in organic synthesis because of their unique properties of nonvolatility, nonflammability, recyclability and ability to dissolve a wide range of materials, among others [18–23]. Today ionic liquids have marched far beyond this border, showing their significant role in controlling the reaction as catalysts [24–29]. Recently, Patil et al. [30] reported a facile protocol for the synthesis of quinazoline-2,4(1H,3H)-diones from CO2 and 2-aminobenzonitriles using 1-butyl-3-methyl imidazolium hydroxide ([Bmim]OH) as an efficient catalyst. Also, they proposed a possible reaction mechanism (see Scheme 1), which is closely related to that of the 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) catalyzed reactions [15]. Nonetheless, the comprehensive understanding of the reaction mechanism by experimental methods presents several challenges. The most significant challenge is to isolate or trap reaction intermediates.

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reaction temperature of 393.15 K and pressure of 30 atm. The natural bond orbital (NBO) [43,44] analysis was carried out on some structures in order to observe the bond order and charge changes in the process of the reactions. The binding energy of selected complex species was computed while taking into account the well-known basis set superposition error (BSSE) by means of the counterpoise approximation method of Boys et al. [45,46]. For evaluating the solvent effects, single-point self-consistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM) [47,48] were performed on the optimized gasphase geometries for all intermediates and transition states at the B3PW91/6-311G(d,p) level. A GEPOL cavity with an average area of the tesserae of 0.4 Å2 has been used in our calculations. The values of the dielectric constant (e = 1.49) and density (q = 0.817 g/cm3) of the scCO2 solution were employed to simulate carbon dioxide as solvent [49]. 3. Results and discussion Scheme 1. Proposed Path 1 mechanism for the chemical fixation of CO2 with 2aminobenzonitrile in the presence of [Bmim]OH.

In the past few years, there have been few theoretical studies using ionic liquids as catalyst for the organic transformations. For example, Sun and Zhang reported the mechanism of the cycloaddition reaction of carbon dioxide with propylene oxide catalyzed by alkylmethylimidazolium chlorine ionic liquids at the B3PW91/631G(d,p) level [31]. To our knowledge, no theoretical study has been performed to understand the mechanism of [Bmim]OH catalyzed chemical fixation of carbon dioxide with 2-aminobenzonitriles. Many mechanistic details of the reaction process remain ambiguous. The structural and energetic details about how these intermediates transform to each other are still unclear. Moreover, is [Bmim]+ indeed the catalytically active species? How many possible pathways does the reaction have? Which pathway is more favorable? Which step is the rate-determining step in the title reaction? To answer the questions raised above and gain insight into the mechanism of the whole catalytic cycle, we have studied the mechanism of [Bmim]OH catalyzed chemical fixation of CO2 with 2-aminobenzonitrile using the B3PW91 density functional method. These results may have valuable implications for the development of new more effective catalyst systems for the chemical fixations. 2. Computational details All calculations were performed with the Gaussian 03 program package [32]. The geometries of the reactants, products, intermediates, and transition states were fully optimized at the B3PW91 level of theory [33,34]. The 6-311G(d,p) split-valence polarized basis set was used, which contains polarization functions on heavy atoms and on hydrogen atoms [35–37]. The harmonic vibrational frequencies were also calculated at the same level to characterize the nature of the stationary points as true minima with no imaginary frequency or transition states with only one imaginary frequency. Calculations of intrinsic reaction coordinate (IRC) [38] were followed at the same level. The B3PW91 functional has been proven to give more reliable intermolecular interaction energy than other functionals, such as the most popular B3LYP functional, for complexes involving strongly bound ionic hydrogen bonds [31,39]. To check the reliability of our computational scheme, further single-point MP2/6-311G(d,p) calculations were performed on some key species optimized at the B3PW91/6-311G(d,p) level [40–42]. The calculated energy results are presented in Table S1 (Supporting Information). To match with the experimental conditions in ref 30, all thermodynamic data reported in this paper were calculated under an actual

To find out the most favorable pathway, we have calculated the free energy profiles for the reaction occurring both in the absence of and assisted by the [Bmim]OH catalyst. 3.1. The uncatalyzed chemical fixation Before investigating the mechanism of the [Bmim]OH catalyzed reaction, it is worth analyzing the chemical fixation of CO2 with 2aminobenzonitrile in the absence of [Bmim]OH. Fig. 1 shows the free energy profile with the optimized structures of intermediates and transition states involved in the reaction. The geometrical structures of reactants and product are presented in the Supporting Information. First, a bimolecular complex between CO2 and 2-aminobenzonitrile, denoted as IM1, is formed via hydrogen bond, as indicated by Ccarbondioxide–H2-aminobenzonitrile distance (2.337 Å) and small binding energy (1.8 kcal/mol). From IM1, CO2 addition takes place via the transition state, TS1, in which one of the N–H (1.278 Å) bond in 2-aminobenzonitrile is breaking and the C–N (1.652 Å) and O–H (1.272 Å) bonds between CO2 and 2-aminobenzonitrile are forming. The free energy profile in Fig. 1 shows that the process is endoergic by 18.0 kcal/mol and needs to overcome the free energy barrier of 53.6 kcal/mol. Overcoming the activation barrier, IM2 is generated with the well-formed C–N (1.376 Å) and O–H (0.965 Å) bonds. Then, IM2 transforms to IM3 via nucleophilic attack of O atom to C atom and simultaneous H atom migration from O atom to N atom, as demonstrated by TS2 with a barrier of 39.9 kcal/mol. At this transition state, the O–H bond (1.610 Å) is practically broken, whereas two new bonds start to form, the C–O (2.413 Å) and N–H (1.100 Å) bonds. The imaginary frequency is 751i cm 1 and the normal mode mainly corresponds to largeamplitude motions of H, O, and N in the desired directions. Finally, IM3 can be converted into quinazoline-2,4(1H,3H)-dione via transition state TS3 with a higher barrier of 75.3 kcal/mol. Quinazoline2,4(1H,3H)-dione is more stable than the initial reactants by 9.0 kcal/mol, which indicates that the chemical fixation is thermodynamically favorable. However, the overall barrier from reactants to product is calculated to be as high as 90.6 kcal/mol in gas phase and 89.8 kcal/mol in scCO2 solution, which may account for the observed difficulty of the chemical fixation in the absence of catalyst [Bmim]OH. 3.2. The [Bmim]OH catalyzed reaction mechanism In this section, we turn to the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH. We postulate

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Fig. 1. Gibbs free energy profile (kcal/mol, in gas phase and in solution) with the optimized geometries of stationary points (intermediates, transition states, and product) for the chemical fixation of CO2 with 2-aminobenzonitrile in the absence of [Bmim]OH. Bond distances are in angstroms.

three mechanisms for the reaction in the presence of [Bmim]OH. All of the relative free energies of stationary points are referenced to the free energies of CO2 + 2-aminobenzonitrile + [Bmim]OH. In the following sections, the geometrical structures and energies of all intermediates and transition states involved in Paths 1–3 are discussed in detail. Path 1: let us first discuss the Path 1 mechanism proposed by Patil and co-workers. The structures of the stationary points involved in Path 1 are shown in Fig. 2, and the free energy profile is depicted in Fig. 3. In the first step, 2-aminobenzonitrile and [Bmim]OH interact via hydrogen bond to form a supermolecule complex 1, which is more stable than the isolated reactants by 6.5 kcal/mol. Taking the most stable complex 1 as the starting point, the H1 is deprived by OH to form a 2-aminobenzonitrile anion-H2O complex 2 via the transition state TS(1/2). In TS(1/2), the forming O1–H1 bond is shortened to 1.223 Å and the two breaking N1–H1 and O1–H3 bonds are elongated to 1.261 and 1.681 Å, respectively. It is implied that the 2-aminobenzonitrile is activated by breaking the N1–H1 bond and pushing more electron density to the N1 atom. These interesting geometrical features provide conditions for the later attack of CO2 to 2-aminobenzonitrile. The energy barrier from 1 to TS(1/2) is 10.8 kcal/mol, indicating that the proton migration proceeds easily. When CO2 enters into the reaction system, a new complex 3 is produced. As shown by the NBO analysis, in TS(2/3), the attack of C1 atom of CO2 to N1 atom of 2-aminobenzonitrile is caused by a strong electrostatic attraction between electropositive C1 (0.493e) atom and electronegative N1 ( 0.501e) atom. The imaginary frequency is 158i cm 1, associated with the formation of C1– N1 bond. Then 3 can be converted into the six-membered-ring intermediate 4, in which the C2–O2 bond forms and H3 atom transfers from C3 atom to N2 atom. The nucleophilic attack of O2 atom to C2 atom increases the charge of C2 atom and decreases the charge of N2 atom, which causes the transfer of H3 atom of [Bmim]+ to N2 atom. This step proceeds through transition state TS(3/4) with a barrier of 15.9 kcal/mol. In the following step, the ring-opening of intermediate 4 takes place through the cleavage of C1–O2 bond leading to intermediate 5. In TS(4/5), the C1–O2 bond distance is elongated relative to that in 4 (2.203 vs

1.359 Å), which implies that the interaction between carbon and oxygen atoms begins to weaken. The imaginary vibration mode indicates that the cleavage of the C1–O2 bond is accompanied by a simultaneous transfer of H2 atom from N1 atom to O2 atom. The transfer of H2 atom can be attributed to the larger electronegativity of O2 atom compared to N1 atom, as revealed by the charges of the O2 and N1 atoms ( 0.568e vs 0.058e). The process (4 ? 5) is predicted to be endoergic by 10.9 kcal/mol with a high energy barrier of 68.6 kcal/mol. On the basis of the conformation of the intermediate 5, the migration of H3 atom to C3 atom occurs; as a result, the C3–H3 and C1–N2 bonds form and N3–H3 bond breaks, to produce a new six-membered-ring intermediate 6, which is 25.5 kcal/mol more stable than the intermediate 5. It should be noted that the migration of H3 atom before the formation of the C1–N2 bond can effectively increase the negative charge on N2 atom. The Wiberg bond indices of C1–N2 and C3–H3 are 0.0557 and 0.6109 in TS(5/6) with NBO analysis, which also indicated that the C3–H3 bond is earlier formed than C1–N2 bond. A similar analysis has been put forward to account for the Markovnikov addition of imidazole to vinyl acetate catalyzed by [Bmim]OH, where the N1–C8 bonds are earlier formed than H1–C7 bonds [50]. From the free energy profile in Fig. 3, it can be seen that the step (5 ? 6) needs to overcome the low free energy barrier of 13.1 kcal/mol. Subsequently, the H2 atom is migrated from O2 atom to N2 atom forming the species 7. The transition state TS(6/7) has been located. IRC calculations indicate that TS(6/7) connects 6 and 7. The migration step is predicted to be exoergic by 17.1 kcal/mol and has a moderate free energy barrier of 23.8 kcal/mol. From 7, the final step proceeds through TS(7/8), which involves a migration of H1 atom from H2O to N1 atom and the attack of O1 atom to the C3 atom simultaneously, to give product-like intermediate 8. At this intermediate, the H1 atom is linked to the N1 atom (1.025 Å), while the O1 atom is linked to the C3 atom (1.424 Å). In addition, 8 is found to be more stable than the initial reactants by 8.3 kcal/mol. Finally, the product quinazoline-2,4(1H,3H)-dione is directly generated by [Bmim]OH dissociation in 8. The process (7 ? 8) is calculated to be moderately endoergic, by 10.1 kcal/ mol, with an energy barrier of 13.4 kcal/mol.

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Fig. 2. Optimized structures with bond distances (in Å) for the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH along Path 1.

As shown in Fig. 3, it can be seen that the chemical fixation in Path 1 now involves seven-step mechanism and the rate-determining step is the cleavage of C1–O2 bond. It is found that H-bonding interaction exists throughout all intermediates and transition states, and in most cases it occurs between H atom of [Bmim]+ and N atom of 2-aminobenzonitrile due to relatively stronger Lewis acidity of H atom. Thus, H-bonding is significant for the stabilization of the structures involved in Path 1. However, the overall barrier from 1 to 8 is extremely high, 88.5 kcal/mol, which is only

lower than that the uncatalyzed chemical fixation by 2.1 kcal/mol. Therefore, the overall barrier along Path 1 is too high to be energetically feasible. Path 2: As mentioned above, the Path 1 mechanism proposed by Patil and co-workers is energetically unfeasible. Thus, more reasonable mechanism should be explored. Kuhn et al. have found that NHCs–CO2 adducts can be synthesized by NHCs and CO2 directly [51]. Inspired by this idea, a new Path 2 mechanism is proposed. We think that N-heterocyclic carbine (NHC) is the

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Fig. 3. Gibbs free energy profile (kcal/mol, in gas phase and in solution) for the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH along Path 1 at the B3PW91/6-311G(d,p) level.

Scheme 2. Proposed Path 2 mechanism for the chemical fixation of CO2 with 2aminobenzonitrile in the presence of [Bmim]OH.

catalytically active species and plays an important role in catalytic cycle. Scheme 2 shows the new mechanism we proposed. The structures involved in this pathway are given in Fig. 4, and the free energy profile is reported in Fig. 5. Initially, CO2 can combine with one molecule of [Bmim]OH to form 9, which is more stable than the initial reactants by 0.5 kcal/mol. In 9, the H atom of [Bmim]+ is deprived by OH to form the NHC and H2O. Next, the C3 atom of NHC attacks the C1 atom of CO2, resulting in the CO2 activation. This scene is very clear by observing the vivid transition vectors corresponding to the imaginary frequency of TS(9/10) (159i cm 1). Then 2-aminobenzonitrile enters into the reaction system, a Lewis acid–base adduct 10 is generated, in which the mode of CO2 is very similar to the characterized adduct IM1a reported by Huang et al. [52]. The donor–acceptor effect between [Bmim]+ and CO2 weakens the

C–O double bonds of the CO2 moiety, as shown by the elongated C1–O2 and C1–O3 bonds (1.160 and 1.158 Å in 9 vs 1.236 and 1.242 Å in 10), and results in a net charge of 0.560e on the CO2 moiety. To continue with the next step of the catalytic cycle, the H1 atom is migrated from N1 atom to N2 atom via TS(10/11). This is because the N2 atom is more electronegative than the N1 atom, as revealed by the charges of these two N atoms ( 0.386e vs 0.071e). In 11, the increased charge on the C2 atom induces the later nucleophilic attack of O2 atom to C2 atom. Starting with intermediate 11, the nucleophilic O2 atom attacks the electrophilic C2 atom and the N1 atom approaches the electrophilic C1 atom, forming a six-membered-ring intermediate 12. The imaginary vibration mode of the transition state TS(11/12) for this step shows that the attack of O2 atom leads to the formation of C2– O2 and C1–N1 bonds. The formation of C2–O2 bond causes the extension of C1–O2 bond from double to single bond and hence results in the change of hybridization state of C1 atom. In 12, the C2– O2 and C1–N1 bond distances are 1.345 and 1.493 Å, which are 0.574 and 1.250 Å shorter than those in TS(11/12), respectively; while the C1–O2 bond distance is elongated to 1.563 Å. The energy barrier from 11 to TS(11/12) is 21.0 kcal/mol with exothermicity of 20.4 kcal/mol. The increased negative charge on C1 atom disfavors for stabilizing 12. As a consequence, the breaking of C1–O2 bond can effectively reduce the negative charge on C1 atom via TS(12/ 13), resulting in the formation of intermediate 13. As shown in Fig. 5, this step (12 ? 13) is endoergic by 1.0 kcal/mol and has a low free energy barrier of 6.8 kcal/mol. Once intermediate 13 is formed, it can further convert to the more stable intermediate 14 which is lower in free energy than 13 by 13.6 kcal/mol. It is noted that the intermediates 13 and 14 are in a fast equilibrium, indicated by the low free energy barrier of 6.3 kcal/mol and the more exoergic property. The next step of the catalytic cycle is to produce the complex 15 via TS(14/15). Interestingly, the energy barrier from 14 to TS(14/15) is only 0.5 kcal/mol. And IRC calculations reveal that TS(14/15) indeed connects 14 and 15. In order to identify the relative stability of 14 and TS(14/15), we have performed more reliable single-point energy calculations on them at the MP2/6-311G(d,p) level. The MP2-calculated Gibbs free energies show intermediate 14 is more stable than TS(14/15) by 0.7 kcal/mol in gas phase and 2.4 kcal/mol in scCO2 solution. With these results it is safe to

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Fig. 4. Optimized structures with bond distances (in Å) for the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH) along Path 2 and TS(4/12) of Path 3.

say that 14 represents a very shallow minimum on the potential energy surface. As shown in Fig. 5, the overall barrier from 9 to 15 is 61.4 kcal/ mol, which is relatively lower than that in Path 1. However, the overall barrier along Path 2 is still too high. Therefore, the corresponding pathway cannot be responsible for the formation of the quinazoline-2,4(1H,3H)-dione. Path 3: On the basis of the above discussions, it can be found that the catalytic effect of NHC is significant in Path 2 mechanism,

but the overall barrier remains high. Combining the results in Paths 1 and 2, it is noted that the free energy barriers are moderate from 1 to 4 and from 12 to 15. One may ask whether the reaction could take place via the more favorable path than Paths 1 and 2. Thus, we have considered Path 3 mechanism that proceeds along the step 4 ? TS(4/12) ? 12 (Scheme 3). And the transition state TS(4/12), which directly connects 4 and 12, has been located. For simplification, here we only discuss the key structures and draw the free energy profile along Path 3 (Fig. 6).

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Fig. 5. Gibbs free energy profile (kcal/mol, in gas phase and in solution) for the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH along Path 2 at the B3PW91/6-311G(d,p) level.

Scheme 3. Proposed Path 3 mechanism for the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH.

Before analyzing the TS(4/12), the intermediate 4 is very interesting and it is necessary to discuss it again. As depicted in Fig. 2, the C3–H3 bond distance in 4 is elongated relative to that in 3 (2.023 vs 1.106 Å). The Wiberg bond index of C3–H3 bond changes from 0.8201 in 3 to 0.1038 in 4. These data indicate that the C3–H3 bond is partly broken. But there is still existing weak interaction between C3 atom and H3 atom, as indicated by the binding energy value (11.8 kcal/mol). However, the activation mode can favor the attack of the C3 atom to the C1 atom via TS(4/12), due to the strong nucleophilicity of NHC and the strong electrophilicity of C1 atom. As a result, the C3–H3 bond breaks and the C1–C3 bond forms. The imaginary frequency of TS(4/12) is 230i cm 1, and the corresponding transition vector is mainly associated with the C1–C3

bond stretching motion. The free energy profile in Fig. 6 clearly shows that the process (4 ? 12) is endoergic by 15.3 kcal/mol with a moderate free energy barrier of 22.1 kcal/mol. From Fig. 6, it is clearly seen that the reaction in Path 3 mechanism now involves seven-step mechanism and the overall barrier is 42.5 kcal/mol, which is easily overcome at the experimental temperature of 120 °C [30] and is in contrast with the uncatalyzed reaction involving three steps with the overall barrier as high as 90.6 kcal/mol. This fact demonstrates that the barrier to be surmounted has been reduced remarkably. Therefore, it is concluded that Path 3 mechanism is more favorable than Paths 1 and 2 mechanisms. On the basis of the constructed catalytic cycle in Fig. 6, it is important to note that the attack of C3 atom to C1 atom from 4 to

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Fig. 6. Gibbs free energy profile (kcal/mol, in gas phase and in solution) for the chemical fixation of CO2 with 2-aminobenzonitrile in the presence of [Bmim]OH along Path 3 at the B3PW91/6-311G(d,p) level.

12, the cleavage of C1–O2 bond from 12 to 13, and the formation of C1–N2 bond from 13 to 14 have close activation free energies (42.0–42.5 kcal/mol). Therefore, each of them can determine the reaction rate with variation in the reaction conditions (temperature, pressure, and solvent). It is noted that Path 3 is partly different from the mechanism studied by Patil et al. Our calculations propose that the OH deprives H1 atom of 2-aminobenzonitrile to strengthen its nucleophilic ability and the [Bmim]+ activates the intermediate 3 and then nucleophilic cyclization of 3 into 4, while in the description of Patil et al. [30], the NHC activation by C1 atom was not proposed. We ascribe this fact to the high nucleophilicity of NHC. Moreover, it is noteworthy that H-bonding interactions also play an important role in stabilizing these structures and taking the reaction to the energetically favorable pathway. To shed light on the mechanism of this process and fully understand the origins of this critical regiochemical preference, we have analyzed the frontier molecular orbital (FMO) [53,54] of 12. As illustrated in Fig. 7, it can be observed that the lone-pair sp2 orbital of C3 atom of NHC is r-bonding with a vacant sp3 orbital of C1 atom in the HOMO-1. This conclusion is consistent with the electronic structure of NHC [55,56]. Solvent Effects: The above calculations are conducted in gas phase. To examine the solvent effects of the scCO2 solution, we have performed SCRF calculations on the species involved in Paths 1, 2, and 3. The data are given in parentheses of the corresponding free energy profiles in Figs. 3, 5 and 6. Comparison of energies between gas phase and solvent phase reveals some interesting insights into the whole catalytic cycle. From Figs. 3, 5 and 6, it

can be seen that all of reaction species are slightly more stable in scCO2 solution than those in gas phase in the range of 2.0– 5.9 kcal/mol. In addition, the free energies of activation and reaction in scCO2 solution are comparable with the data in gas phase. These results indicate that the scCO2 solution can stabilize species involved in catalytic cycle to some extent, but cannot change their relative values in activation and reaction. Comparing the energetics of Paths 1, 2, and 3, it is clearly seen that Path 3 is favorable both thermodynamically and kinetically in scCO2 solution. The reaction key steps are still the same in solution, and only slight decrease in the energy of barriers is observed. In the step of nucleophilic attack of NHC to C1 atom, the activation free energies are 42.0 (in gas phase) and 40.7 (in scCO2) kcal/mol. For the cleavage of C1–O2 bond step, the activation free energies are 42.0 (in gas phase) and 38.9 (in scCO2) kcal/mol. As the formation of C1–N2 bond step, the activation free energies are 42.5 (in gas phase) and 39.5 (in scCO2) kcal/mol. On the basis of these data, it is concluded that the energy barriers change insignificantly. And the three steps still can determine the reaction rate of the entire reaction in scCO2 solution with variation in the reaction conditions (temperature, pressure, and solvent) in Path 3. Therefore, it is indicated that the scCO2 solution has a minor effect on the thermodynamics of each step. So the high reaction yield should be ascribed to factors such as the rapid diffusion, the high miscibility of 2-aminobenzonitrile, and especially the high concentration of CO2 molecules. Noteworthily, compared with the energy barrier of 89.8 kcal/ mol in the uncatalyzed chemical fixation, each step in Path 3 has a moderate barrier that can be easily overcome under experimental conditions. It is therefore summarized that scCO2 as solution can stabilize species of the catalytic cycle, but cannot change the relative free energies.

4. Conclusions

Fig. 7. Structure of HOMO-1 of intermediate 12.

At the B3PW91/6-311G(d,p) density functional level of theory, the entire catalytic cycle of the chemical fixation of carbon dioxide with 2-aminobenzonitrile catalyzed by [Bmim]OH has been investigated. All species involved in the catalytic cycle have been fully characterized to be energy minimum structures for the intermediates or saddle point structures for the transition states. The solvent effects of scCO2 have been considered by using the PCM model. By comparing the energetics of Paths 1, 2, and 3, it can be seen that Path 3 is a feasible reaction route for the formation of quinazoline-2,4(1H,3H)-dione. The title reaction in the presence of [Bmim]OH is exothermic and exoergic. On the basis of the constructed

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catalytic cycle in Fig. 6, it is found that the attack of C3 atom to C1 atom from 4 to 12, the cleavage of C1–O2 bond from 12 to 13, and the formation of C1–N2 bond from 13 to 14 have close activation free energies. Therefore, each of them can determine the reaction rate with variation in the reaction conditions. Our study indicates that the real active species is NHC, which plays a critical role in catalytic cycle. The NHC activates the C1 atom and then facilitates the cleavage of C1–O2 bond in reaction. It is also noted that scCO2 as solution does not change the potential energy surface compared to that found in gas phase. The overall reaction free energy is calculated to be 9.1 kcal/mol in scCO2 solution. The present theoretical results provide a clear profile for the detailed reaction mechanism and help us to understand the intrinsic properties of the reaction. Acknowledgment This work was supported by the Natural Science Foundations of China (20871077 and 21031003).

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