Insight into the Varying Reactivity of Different Catalysts for CO2 Cycloaddition into Styrene Oxide: An Experimental and DFT Study

The cycloaddition of CO2 into epoxides to form cyclic carbonates is a highly sought-after reaction for its potential to both reduce and use CO2, which is a greenhouse gas. In this paper, we present experimental and theoretical studies and a mechanistic approach for three catalytic systems. First, as Lewis base catalysts, imidazole and its derivatives, then as a Lewis acid catalyst, ZnI2 alone, and after that, the combined system of ZnI2 and imidazole. In the former, we aimed to discover the reasons for the varied reactivities of five Lewis base catalysts. Furthermore, we succeeded in reproducing the experimental results and trends using DFT. To add, we emphasized the importance of non-covalent interactions and their role in reactivity. In our case, the presence of a hydrogen bond was a key factor in decreasing the reactivity of some catalysts, thus leading to lower conversion rates. Finally, mechanistically understanding this 100% atom economy reaction can aid experimental chemists in designing better and more efficient catalytic systems.


Introduction
Climate change, which is being exacerbated by current global warming, has emerged as one of the century's most pressing concerns. In the scientific community, we can retrace and find the first published evidence from Callendar in 1938 who demonstrated that the climate was warming and CO 2 levels were rising [1]. Carbon dioxide (CO 2 ) is a greenhouse gas that absorbs infrared radiation that is emitted from Earth and that happen to cool its surface after a day in the Sun. This radiation, which is supposed to escape the atmosphere, is trapped by CO 2 [2]. In consequence, dire and sometimes irreversible effects are observed all around the world [3]. Many efforts have been dedicated to mitigate this global issue over the last decades. However, it was only until the late 1970s that the idea of Carbon dioxide Capture and Storage (CCS) was proposed as a solution to tackle the CO 2 issue [4,5]. Furthermore, the Intergovernmental Panel on Climate Change (IPCC) issued a report in 2005 that discussed this specific issue and laid a substantial part of the framework for scientists today [6]. Since then, CO 2 was considered as a carbon source for chemical reactions, consequently, several chemical routes for CCS have been proposed and studied, among which was the use of carbon dioxide as a reactant in organic synthesis, and the incorporation of CO 2 ultimately forming organic carbonates [7][8][9]. In this paper, we will be focusing on the latter, more specifically, the conversion of epoxides into cyclic carbonates via CO 2 cycloaddition (Scheme 1). The purpose of this study is not only to unravel the mechanism of the CO cycloaddition, but also to try to gain some insight into the reasons associated with the increase in activity, and in doing so, optimizing this catalytic reaction using quantum calculations.

Experimental Results
The catalytic activity of these compounds for the cycloaddition of CO2 were measured in solvent-free conditions with Styrene Oxide (SO) as the epoxide. SO is a stable well-known substrate often used in catalytic studies [23,24]. Various imidazole derivate or ZnI2 or their combination were used in 1.0 mol% vs. epoxide at 100 °C, during 18 h under 2.0 MPa of CO2. Styrene oxide conversion and cyclic Styrene Carbonate (SC) yield were determined by 1 H NMR spectroscopy using an internal standard. All the main experimental data are compiled in Table 1. This conversion has become quite popular over the years for the conversion of captured CO 2 (from, for example, waste combustion, flue gases or in the atmosphere) into useful products. Indeed, cyclic carbonates have been shown to have numerous applications, and they can be used as solvents for organic reactions [10], as monomers for polymerization [11][12][13][14], as well as reactive agents in numerous synthetic pathways [15][16][17]. This reaction can be accelerated with the use of some catalysts, the first of which was a polymer-supported nanogold catalyst [18]. Numerous parallels between these various catalytic systems have been discovered, which has led researchers to the conclusion that the catalyst must adhere to certain criteria in order to effectively catalyze the reaction. The catalytic system must contain for example, a nucleophile/Lewis base that should be a good leaving group such as halides or N-heterocycles [19,20]. It should also include Lewis acid sites that can be found in metals for example [19,21], or from a theoretical point of view, Brønsted acid sites able to donate protons, which are the simplest Lewis acids. Lastly, it is important to mention that there are reports in the importance of hydrogen bond donor sites [22]. In this article we will study Imidazole (HIm) and four of its derivatives: 1-Methylimidazole (1M-Im), 2-Methylimidazole (2M-HIm), Benzimidazole (Bz-HIm) and 2-Iodo-1-Methylimidazole (2I-1M-Im) as Lewis Base catalysts ( Figure 1). this paper, we will be focusing on the latter, more specifically, the conversion of epoxides into cyclic carbonates via CO2 cycloaddition (Scheme 1 This conversion has become quite popular over the years for the conversion of captured CO2 (from, for example, waste combustion, flue gases or in the atmosphere) into useful products. Indeed, cyclic carbonates have been shown to have numerous applications, and they can be used as solvents for organic reactions [10], as monomers for polymerization [11][12][13][14], as well as reactive agents in numerous synthetic pathways [15][16][17]. This reaction can be accelerated with the use of some catalysts, the first of which was a polymer-supported nanogold catalyst [18]. Numerous parallels between these various catalytic systems have been discovered, which has led researchers to the conclusion that the catalyst must adhere to certain criteria in order to effectively catalyze the reaction. The catalytic system must contain for example, a nucleophile/Lewis base that should be a good leaving group such as halides or N-heterocycles [19,20]. It should also include Lewis acid sites that can be found in metals for example [19,21], or from a theoretical point of view, Brønsted acid sites able to donate protons, which are the simplest Lewis acids. Lastly, it is important to mention that there are reports in the importance of hydrogen bond donor sites [22]. In this article we will study Imidazole (HIm) and four of its derivatives: 1-Methylimidazole (1M-Im), 2-Methylimidazole (2M-HIm), Benzimidazole (Bz-HIm) and 2-Iodo-1-Methylimidazole (2I-1M-Im) as Lewis Base catalysts ( Figure 1). The purpose of this study is not only to unravel the mechanism of the CO2 cycloaddition, but also to try to gain some insight into the reasons associated with the increase in activity, and in doing so, optimizing this catalytic reaction using quantum calculations.

Experimental Results
The catalytic activity of these compounds for the cycloaddition of CO2 were measured in solvent-free conditions with Styrene Oxide (SO) as the epoxide. SO is a stable well-known substrate often used in catalytic studies [23,24]. Various imidazole derivates or ZnI2 or their combination were used in 1.0 mol% vs. epoxide at 100 °C, during 18 h under 2.0 MPa of CO2. Styrene oxide conversion and cyclic Styrene Carbonate (SC) yield were determined by 1 H NMR spectroscopy using an internal standard. All the main experimental data are compiled in Table 1. The purpose of this study is not only to unravel the mechanism of the CO 2 cycloaddition, but also to try to gain some insight into the reasons associated with the increase in activity, and in doing so, optimizing this catalytic reaction using quantum calculations.

Experimental Results
The catalytic activity of these compounds for the cycloaddition of CO 2 were measured in solvent-free conditions with Styrene Oxide (SO) as the epoxide. SO is a stable well-known substrate often used in catalytic studies [23,24]. Various imidazole derivates or ZnI 2 or their combination were used in 1.0 mol% vs. epoxide at 100 • C, during 18 h under 2.0 MPa of CO 2 . Styrene oxide conversion and cyclic Styrene Carbonate (SC) yield were determined by 1 H NMR spectroscopy using an internal standard. All the main experimental data are compiled in Table 1.
All imidazole and its derivatives (entries 1-5) are able to catalyze the reaction between CO 2 and styrene oxide to selectively form cyclic styrene carbonate, even if moderate yields are obtained (31-53%), except for 2-Iodo-1-Methylimidazole (entry 5). In the case of ZnI 2 (entry 6), a low carbonate yield was obtained despite a high epoxide conversion. This lack of selectivity is largely improved when combining the zinc salt to any of the imidazole compounds in equimolar ratios, as for example HIm (entry 7) leading to almost complete SO conversion and 98% SC yield. To compare our results with experimental data, styrene oxide was also chosen as the epoxide for our theoretical calculations. This trend was also noticed when combining ZnBr 2 as Lewis acid with HIm as Lewis base [25]. The use of Zn complexes to catalyze the insertion of CO 2 into epoxides, in particular with salphen ligand, has been the object of several studies in the past [26][27][28].

Theoretical Results and Discussion
To compare our results with experimental data, styrene oxide was chosen as the epoxide for the theoretical calculations. The results will be divided into three main parts. In the first section, the reaction mechanism of imidazole and its derivatives without the presence of ZnI 2 will be discussed. An insightful discussion on the catalysts performance is included through the pathway with a Lewis Base (denoted LB throughout the article). In the second and third sections, the complete reaction pathway pertaining to the ZnI 2 catalyzed and Lewis acid (ZnI 2 ) + Lewis base (HIm) catalyzed reactions are discussed (denoted LA in this section). Theoretical calculations and fruitful results using the NCI (Non Covalent Index) are presented. At last, a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions are provided.

Lewis Base Mechanism
The pathway we propose is a two-step concerted mechanism (Scheme 2), similar to the one presented by Roshan et al. [20]. The first step is the attack of the nucleophile (catalyst) on the less-substituted carbon of the epoxide, and the opening of the latter. Simultaneously, the epoxide oxygen attacks the electrophilic carbon of CO 2 leading to Intermediate 1. The second and final step is an SN2 reaction where the free oxygen plays the role of the nucleophile closing the ring, and releasing the catalyst which then plays the role of a leaving group to produce SC. Materials (Figures S1-S4). Following this, the free energy diagram showing a comparative view of the different pathways for all the different catalysts in kcal/mol is presented in Figure 3. Table 2 shows the activation energies for the 1st step and the 2nd step as EA1 and EA2, respectively for all the catalysts.

Scheme 2.
Proposed reaction mechanism for the Lewis base catalysed cycloaddition of CO2 into SO to form SC. Imidazole is taken as representative example for all the Lewis base catalysts of this study.

Scheme 2.
Proposed reaction mechanism for the Lewis base catalysed cycloaddition of CO 2 into SO to form SC. Imidazole is taken as representative example for all the Lewis base catalysts of this study.
To explain the regioslectivity of the first step, we have computed free energy diagrams for both possible pathways. The results are presented in Figure 2. As shown in this figure, both activation energies pertaining to the Least Substituted (LS) pathway being 30.33 kcal/mol and 28.09 kcal/mol for E A1LS and E A2LS , respectively, are slightly less than those for the More Substituted (MS) pathway, which are 32.96 kcal/mol and 30.06 kcal/mol for E A1MS and E A2MS , respectively. Given that the rate determining step, the LS pathway is slightly more favorable than the MS pathway. However, the difference of less than 2 kcal/mol implies that both pathways are in competition. Similar results are seen in all the other LB catalysts, and their free energy diagrams can be found in the Supplementary Materials ( Figures S1-S4). Following this, the free energy diagram showing a comparative view of the different pathways for all the different catalysts in kcal/mol is presented in Figure 3. Table 2 shows the activation energies for the 1st step and the 2nd step as E A1 and E A2 , respectively for all the catalysts.  Table 2 shows the activation energies for the 1st step and the 2nd step as EA2, respectively for all the catalysts. Scheme 2. Proposed reaction mechanism for the Lewis base catalysed cycloaddition of CO2 in form SC. Imidazole is taken as representative example for all the Lewis base catalysts of this stu   Materials ( Figures S1-S4). Following this, the free energy diagram showing a comparative view of the different pathways for all the different catalysts in kcal/mol is presented in Figure 3. Table 2 shows the activation energies for the 1st step and the 2nd step as EA1 and EA2, respectively for all the catalysts.

Scheme 2.
Proposed reaction mechanism for the Lewis base catalysed cycloaddition of CO2 into SO to form SC. Imidazole is taken as representative example for all the Lewis base catalysts of this study.
The first thing to note is the small differences in E A1 , the range being 2 kcal/mol. From this, and taking into consideration a margin of error for the calculations, it can be concluded that the first step is nearly equivalent for all LBs. To put it more clearly, varying the catalyst does not markedly change the energy of the first transition state.
In contrast, the differences in E A2 are more prominent, with a range of more than 13 kcal/mol. This is largely due to the free energy of the intermediate I1. Interestingly, I1 of the yellow curve belonging to the 2M-HIm LB, is considerably less stable than those of HIm, 1M-Im, and Bz-HIm. Additionally, the intermediate pertaining to the 2I-1M-Im pathway is the highest in energy by far and therefore the most reactive among the intermediates. While analyzing the optimized structures of all intermediates, we noticed that the reason for the considerable differences in stability lies in the intermolecular interactions between the atom/substituent on carbon 2 of the imidazole catalyst (the carbon atom between the two nitrogen atoms), and the free nucleophilic oxygen. Consequently, we calculated the intramolecular interactions using the Non-Covalent Interaction (NCI) method [29] in this particular region.
The Reduced Density Gradient (RDG) calculated within the NCI methodology is a great tool to characterize the nature of the Non-Covalent Interaction in a particular region. To help read the scatter graphs, a color-coded legend is represented in Figure 4 [30]. The RDG (y-axis) is a dimensionless quantity that approaches 0 when there are covalent and/or non-covalent interactions. For a clearer picture, we used a range of sign (λ 2 )ρ that describes only weak interactions such as hydrogen bonds, Van der Waals interactions and steric repulsion ( Figure 4). In the case of an absence of a distinguished peak with an RDG value close to 0, as for example in Figure 5, we can assume that no NCI are present. Figure 5 shows the optimized structure and the RDG scatter graph for a single imidazole molecule. This graph allows further comparison with the catalyst structures that are related. Furthermore, in Figures 6-8, we show the surfaces of the intramolecular interactions of I1 HIm, 2M-HIm and 2I-1M-Im, respectively, as well as their RDG scatter graph. The NCI graph and results for I1 1M-Im and Bz-HIm are present in the Supporting Information (Figures S5 and S6). The first thing to note is the small differences in EA1, the range being 2 kcal/m this, and taking into consideration a margin of error for the calculations, it concluded that the first step is nearly equivalent for all LBs. To put it more clearly, the catalyst does not markedly change the energy of the first transition state.
In contrast, the differences in EA2 are more prominent, with a range of more kcal/mol. This is largely due to the free energy of the intermediate I1. Interesting the yellow curve belonging to the 2M-HIm LB, is considerably less stable than HIm, 1M-Im, and Bz-HIm. Additionally, the intermediate pertaining to the 2 pathway is the highest in energy by far and therefore the most reactive am intermediates. While analyzing the optimized structures of all intermediates, we that the reason for the considerable differences in stability lies in the interm interactions between the atom/substituent on carbon 2 of the imidazole cata carbon atom between the two nitrogen atoms), and the free nucleophilic Consequently, we calculated the intramolecular interactions using the Non-C Interaction (NCI) method [29] in this particular region.
The Reduced Density Gradient (RDG) calculated within the NCI methodo great tool to characterize the nature of the Non-Covalent Interaction in a particula To help read the scatter graphs, a color-coded legend is represented in Figure 4 [ RDG (y-axis) is a dimensionless quantity that approaches 0 when there are covalen non-covalent interactions. For a clearer picture, we used a range of sign (λ describes only weak interactions such as hydrogen bonds, Van der Waals interact steric repulsion (Figure 4). In the case of an absence of a distinguished peak with value close to 0, as for example in Figure 5, we can assume that no NCI are presen 5 shows the optimized structure and the RDG scatter graph for a single im molecule. This graph allows further comparison with the catalyst structures related. Furthermore, in Figures 6-8, we show the surfaces of the intram interactions of I1 HIm, 2M-HIm and 2I-1M-Im, respectively, as well as their RDG graph. The NCI graph and results for I1 1M-Im and Bz-HIm are present in the Sup Information (Figures S5 and S6).
When comparing Figure 5a with Figure 6a, we can see a clear blue pe extrapolation with a sign (λ2)ρ < −0.05 a.u. indicating the presence of a strong h bond represented by the green surface in Figure 6b, which represents the NCI of    Regarding Figure 7, which represents I1 2M-HIm, we can see a blue peak just below sign (λ2)ρ = −0.03 a.u. which similar to I1 HIm, shows the presence of a hydrogen bond (Figure 7a). However, from the sign (λ2)ρ value, we can deduce that the strength of tha H-bond is weaker than that in Figure 6a. It can also be seen that the presence of a methy group in position 2 of the Lewis base catalyst results in a weaker interaction.
Moreover, this accounts for a more reactive intermediate and a smaller EA2. Finally Figure 8a shows complete eradication of the H-bond as it can be shown with an absence of any blue peak. Instead, we can see a green peak at sign (λ2)ρ slightly higher than −0.01 a.u. demonstrating a Van der Waals interaction. In addition, a light green peak is clearly observed around 0.008 a.u. which is indicative in an NCI context of some steric effects The visual representation of these interactions is shown as a green surface in Figure 8b The trend of yield (%) is inversely proportional to the strength of the non-covalen interactions present in Intermediate 1 and to EA2.    Regarding Figure 7, which represents I1 2M-HIm, we can see a blue peak just below sign (λ2)ρ = −0.03 a.u. which similar to I1 HIm, shows the presence of a hydrogen bond ( Figure 7a). However, from the sign (λ2)ρ value, we can deduce that the strength of that H-bond is weaker than that in Figure 6a. It can also be seen that the presence of a methyl group in position 2 of the Lewis base catalyst results in a weaker interaction.
Moreover, this accounts for a more reactive intermediate and a smaller EA2. Finally, Figure 8a shows complete eradication of the H-bond as it can be shown with an absence of any blue peak. Instead, we can see a green peak at sign (λ2)ρ slightly higher than −0.01 a.u. demonstrating a Van der Waals interaction. In addition, a light green peak is clearly observed around 0.008 a.u. which is indicative in an NCI context of some steric effects. The visual representation of these interactions is shown as a green surface in Figure 8b. The trend of yield (%) is inversely proportional to the strength of the non-covalent interactions present in Intermediate 1 and to EA2.

Lewis Acid Mechanism
The LA catalyzed pathway follows a one-step concerted mechanism. The opening of the epoxide ring and the cycloaddition of the carbon dioxide are performed simultaneously. The free energy diagram, along with the structure of the transition state are presented in Figure 9. The activation energy (noted EA3 here) for this transformation is higher than those of the LB-catalyzed mechanism with a value of 48 kcal/mol. This high barrier gives rise to the possibility of competitive reactions and can explain the low yield of 21% obtained experimentally (Table 1). When comparing Figure 5a with Figure 6a, we can see a clear blue peak upon extrapolation with a sign (λ 2 )ρ < −0.05 a.u. indicating the presence of a strong hydrogen bond represented by the green surface in Figure 6b, which represents the NCI of I1 HIm.
Regarding Figure 7, which represents I1 2M-HIm, we can see a blue peak just below sign (λ 2 )ρ = −0.03 a.u. which similar to I1 HIm, shows the presence of a hydrogen bond (Figure 7a). However, from the sign (λ 2 )ρ value, we can deduce that the strength of that H-bond is weaker than that in Figure 6a. It can also be seen that the presence of a methyl group in position 2 of the Lewis base catalyst results in a weaker interaction.
Moreover, this accounts for a more reactive intermediate and a smaller E A2 . Finally, Figure 8a shows complete eradication of the H-bond as it can be shown with an absence of any blue peak. Instead, we can see a green peak at sign (λ 2 )ρ slightly higher than −0.01 a.u. demonstrating a Van der Waals interaction. In addition, a light green peak is clearly observed around 0.008 a.u. which is indicative in an NCI context of some steric effects. The visual representation of these interactions is shown as a green surface in Figure 8b. The trend of yield (%) is inversely proportional to the strength of the non-covalent interactions present in Intermediate 1 and to E A2 .

Lewis Acid Mechanism
The LA catalyzed pathway follows a one-step concerted mechanism. The opening of the epoxide ring and the cycloaddition of the carbon dioxide are performed simultaneously. The free energy diagram, along with the structure of the transition state are presented in Figure 9. The activation energy (noted E A3 here) for this transformation is higher than those of the LB-catalyzed mechanism with a value of 48 kcal/mol. This high barrier gives rise to the possibility of competitive reactions and can explain the low yield of 21% obtained experimentally (Table 1).

Lewis Acid Mechanism in Presence of ZnI 2
The third part of the discussion will be around the effect of the addition of ZnI 2 as a Lewis acid co-catalyst for this reaction. The results in Table 1 indicate a 45% yield with HIm alone, a 21% yield with ZnI 2 as the sole catalyst, and a 98% yield when both are combined. The higher yield in entry 7 has led us to conclude that the mechanism involves a synergistic effect of both catalysts rather than a cumulative effect.

Lewis Acid Mechanism in Presence of ZnI2
The third part of the discussion will be around the effect of the addition of ZnI2 as a Lewis acid co-catalyst for this reaction. The results in Table 1 indicate a 45% yield with HIm alone, a 21% yield with ZnI2 as the sole catalyst, and a 98% yield when both are combined. The higher yield in entry 7 has led us to conclude that the mechanism involves a synergistic effect of both catalysts rather than a cumulative effect.
The first suggestion would be that Zinc (II) complexes are usually four-coordinate complexes and take a tetrahedral or distorted tetrahedral geometry [31][32][33]. Some studies also show the possibility of a five-coordinate complex with a trigonal bipyramidal geometry; however, these instances are rare in the literature, and require certain conditions to be met [34][35][36][37]. Another possibility we ruled out is a polynuclear Zn catalyst because no condition is met for such structure, ZnI2′s catalytic concentration being only 1.0 mol%. In order to predict ZnI2′s dissociation, we calculated theoretically the Zn-I Bond Dissociation Energy (BDE) using the same level of theory as precedent. Results showed a BDE of dissociating the first iodide ion to be 192.75 kcal/mol. This high BDE value revealed that both iodide ions would remain attached to the zinc atom. Indeed, several studies indicate the robustness of Zn-I bonds upon addition of N-heterocycles [38][39][40][41]. Taking all of this into consideration, we propose a mononuclear, tetrahedral ZnI2(SO)2 complex presented in Figure 10. We can see in the latter that this Zn atom adopts a tetrahedral geometry ionically bonded to two iodide ions and linked with two styrene oxide with coordination bonds. Each of these SO ligands interact with each other via an NCI π-π interactions (represented with the green surface). The first suggestion would be that Zinc (II) complexes are usually four-coordinate complexes and take a tetrahedral or distorted tetrahedral geometry [31][32][33]. Some studies also show the possibility of a five-coordinate complex with a trigonal bipyramidal geometry; however, these instances are rare in the literature, and require certain conditions to be met [34][35][36][37]. Another possibility we ruled out is a polynuclear Zn catalyst because no condition is met for such structure, ZnI 2 s catalytic concentration being only 1.0 mol%. In order to predict ZnI 2 s dissociation, we calculated theoretically the Zn-I Bond Dissociation Energy (BDE) using the same level of theory as precedent. Results showed a BDE of dissociating the first iodide ion to be 192.75 kcal/mol. This high BDE value revealed that both iodide ions would remain attached to the zinc atom. Indeed, several studies indicate the robustness of Zn-I bonds upon addition of N-heterocycles [38][39][40][41]. Taking all of this into consideration, we propose a mononuclear, tetrahedral ZnI 2 (SO) 2 complex presented in Figure 10. We can see in the latter that this Zn atom adopts a tetrahedral geometry ionically bonded to two iodide ions and linked with two styrene oxide with coordination bonds. Each of these SO ligands interact with each other via an NCI π-π interactions (represented with the green surface). Using this structure, we propose a mechanistic pathway for the transformation of the SO into SC (Scheme 3). The first step being the attack of the imidazole on the epoxide, opening the epoxide and forming Intermediate 2 (I2). In the following step, the CO2 inserts itself via a four-membered cycle transition state (TS4). This leads to a penta-coordinated Figure 10. Proposed tetrahedral ZnI 2 (SO) 2 complex with a π-π interactions (green).
Using this structure, we propose a mechanistic pathway for the transformation of the SO into SC (Scheme 3). The first step being the attack of the imidazole on the epoxide, opening the epoxide and forming Intermediate 2 (I2). In the following step, the CO 2 inserts itself via a four-membered cycle transition state (TS4). This leads to a penta-coordinated trigonal bipyramidal intermediate (I3). After that, ring closing occurs in an SN 2 type mechanism where the free oxygen acts as a nucleophile and the Imidazolium as the leaving group leading to Intermediate 4 (I4). The final step is the substitution of the newly formed SC by a SO to regenerate the catalyst. Using this structure, we propose a mechanistic pathway for the transformation of the SO into SC (Scheme 3). The first step being the attack of the imidazole on the epoxide, opening the epoxide and forming Intermediate 2 (I2). In the following step, the CO2 inserts itself via a four-membered cycle transition state (TS4). This leads to a penta-coordinated trigonal bipyramidal intermediate (I3). After that, ring closing occurs in an SN2 type mechanism where the free oxygen acts as a nucleophile and the Imidazolium as the leaving group leading to Intermediate 4 (I4). The final step is the substitution of the newly formed SC by a SO to regenerate the catalyst. Scheme 3. Proposed reaction mechanism for the HIm/ZnI2 catalyzed cycloaddition. Figure 11 shows the free energy diagram for a potential mechanism of the LA + LB catalyzed pathway. It is important to note the relatively lower activation energy of the epoxide ring opening step (EA4 = 8.64 kcal/mol) compared to the LB catalyzed mechanism (EA1HIm = 30.33 kcal/mol). The relatively low energies of TS2 and TS3 (EA5 = 5.26 kcal/mol) Scheme 3. Proposed reaction mechanism for the HIm/ZnI 2 catalyzed cycloaddition. Figure 11 shows the free energy diagram for a potential mechanism of the LA + LB catalyzed pathway. It is important to note the relatively lower activation energy of the epoxide ring opening step (E A4 = 8.64 kcal/mol) compared to the LB catalyzed mechanism (E A1HIm = 30.33 kcal/mol). The relatively low energies of TS2 and TS3 (E A5 = 5.26 kcal/mol) are also noteworthy. The rate-determining step in this reaction is the ring closure with an activation energy of E A6 = 25.72 kcal/mol. Comparing this value with the activation energies of the rate-determining steps of the previous catalytic systems, E A6 is the lowest. These theoretical results are in accordance with the experimental results when looking at entries 1, 6, and 7 (Table 1). are also noteworthy. The rate-determining step in this reaction is the ring closure with an activation energy of EA6 = 25.72 kcal/mol. Comparing this value with the activation energies of the rate-determining steps of the previous catalytic systems, EA6 is the lowest. These theoretical results are in accordance with the experimental results when looking at entries 1, 6, and 7 (Table 1).
All geometry optimizations and energy calculations were performed using Density Functional Theory (DFT). The functional used in this work is ωB97X-D [42], mainly because it is a range-separated functional, which can capture both short and long-range interactions. The basis set chosen for all the calculations was LANL2DZ [43][44][45], which is a widely used basis set for systems containing heavy atoms, with a double equality and a relativistic effective core potential for iodide. All calculations have been performed in gas phase. Frequencies were computed in order to verify that no imaginary frequencies are present for the reactants, intermediates, and products. All transition states used in this work have only one imaginary frequency. Intrinsic Reaction Coordinate (IRC) calculations were performed on all transition states with the same level of theory to make sure that the transition structure leads to the sought-after reactants and products. Furthermore, all the free energies were calculated at 298.15 K, 1 atm. All the optimized structures were calculated using Gaussian09 [46] and Gaussian16 [47] quantum packages.
In all the free energy diagrams, the reactants are taken as reference with a ∆G = 0 kcal/mol. Concerning the characterization and visualization of the non-covalent interactions, the approach used was that which was introduced by Johnson et al. [29] in 2010. Finally, the visualization of the RDG scatter graphs in a mapped color code representing the NCIs was made possible by Multiwfn [48] using the Independent Gradient Model [30].

General Considerations
All chemicals and some solvents were purchased from Sigma-Aldrich ® (St. Louis, MO, USA) and used as received, in particular styrene oxide (97% purity), imidazole (99% purity) and ZnI 2 (98% purity). Carbon dioxide (99.995 % purity) was supplied by Air Liquide. 1 H NMR spectra in CDCl 3 were acquired on a Bruker Ascend™ 400 spectrometer (Bruker Corporation ® , Billerica, MA, USA) at 298 K and referenced to the solvent signals.

Catalytic Tests
All the catalytic tests were performed in a 40 mL stainless steel reactor equipped with a thermocouple and a magnetic stirrer. In a typical reaction, 0.2 mmol catalyst and 20 mmol of styrene oxide were placed into the reactor before closing, introducing 2.0 MPa of CO 2 and heating. After 18 h, the autoclave was cooled in an ice bath and slowly depressurized. Acetone was added to recover the reaction media and 1 mmol of 1,3,5-Trimethoxybenzene was added as internal standard for further 1 H NMR analysis.

Conclusions
In conclusion, the mechanism of a CO 2 cycloaddition into epoxides to form cyclic carbonates is an important reaction that is being studied intensively. In this paper and in order to contribute to this growing field, we performed an extensive catalyst study experimentally paired with theoretical results to propose and discuss the mechanistic aspect on the use of catalyst for such a reaction. We portrayed first a mechanism for the Lewis Based-catalysed reaction that is in accordance with the experimental results, the key factor being the stability of the first intermediate. We noted that this stability was dependent on the intramolecular interactions (H-bond) between the catalyst and the nucleophilic oxygen of the intermediate. We also showed that this non covalent interaction acts as a lock preventing the nucleophile from performing a dorsal attack for a ring closure.
In the second part of this study we proposed a reaction pathway for the Lewis acidcatalyzed mechanism, we found that it has the highest activation energy among all of the catalytic systems. This agrees well with the experimental results, a high E A implies that competitive reactions might take place, explaining the poor yield. Lastly, we presented theoretical calculations for a plausible mechanism of the HIm/ZnI 2 -catalyzed reaction. The synergistic effect of imidazole and Zinc Iodide is reflected by the system performance both in experimental and computational results.