How Does CO2 React with Styrene Oxide in Co-MOF-74 and Mg-MOF

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How Does CO React with Styrene Oxide in Co-MOF-74 and MgMOF-74? Catalytic Mechanisms Proposed by QM/MM Calculations Kai Xu, Adhitya Mangala Putra Moeljadi, Binh Khanh Mai, and Hajime Hirao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09790 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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The Journal of Physical Chemistry

How Does CO2 React with Styrene Oxide in Co-MOF-74 and Mg-MOF-74? Catalytic Mechanisms Proposed by QM/MM Calculations Kai Xu,1,= Adhitya Mangala Putra Moeljadi,1,= Binh Khanh Mai,2 and Hajime Hirao*,1,2 1

Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China 2

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371

=

These authors contributed equally to this work.

*[email protected]; Tel.: +852-3442-7096

Abstract: We report a QM/MM computational study of the cycloaddition between carbon dioxide (CO2) and styrene oxide in two different metal–organic frameworks (MOFs), Co-MOF-74 and Mg-MOF-74, to obtain atomic-level insights into the catalytic mechanism. The results suggest that both reactions begin by forming Lewis acid–base pairs between the epoxide and an open metal site and between CO2 and a phenolate moiety in the linker. Consequently, higher electrophilic and nucleophilic reactivities are conferred on the epoxide and CO2 (CO2(A)), respectively, thereby facilitating the initial ring opening of the epoxide moiety. The ring-opening process is followed by the adsorption of a second CO2 molecule (CO2(B)), which is necessary for the subsequent ring closure to occur. In the case of Co-MOF-74, the binding of CO2(B) to the alkoxide oxygen increases the flexibility of the substrate moiety, enabling the cyclization pathway in which CO2(A) is incorporated into the final product. In contrast, the carbonate intermediate in the Mg-MOF-74-catalyzed reaction undergoes an intramolecular nucleophilic attack to form a 5-membered cyclic product. During this step, CO2(A) behaves essentially as a co-catalyst and is released back into the framework upon product formation. Our QM/MM results also suggest that the Lewis acid site has somewhat different coordination geometries in Co-MOF-74 and Mg-MOF-74 during the final ring-closure step.

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Introduction The over-reliance on fossil fuels such as coal, oil, and natural gas to sustain the comfortable lifestyle of humankind has been fraught with several adverse consequences. Among others, there has been a continued increase in the atmospheric concentration of carbon dioxide (CO2, a greenhouse gas) in the past few centuries since the time of industrial revolution,1 and the steeper increase seen in the last decades has arguably contributed to global climate change. To mitigate this problem, the strategy of carbon capture and storage (CCS) has been pursued with the goal of sequestering captured CO2.2 For example, the applicability of porous adsorbent materials such as metal–organic frameworks (MOFs) has been examined in this context.3-8 However, if CO2 (a renewable and inexpensive C1 feedstock) is converted efficiently to other products, it should provide benefits compared with mere separation and sequestration. Known as carbon capture and utilization (CCU), this concept potentially allows for production of valuable chemicals or fuels, whilst reducing CO2 emissions and alleviating the depletion of fossil fuels.9-19 In general, chemical conversion of CO2 under mild conditions is a difficult challenge because of its high kinetic inertness and thermodynamic stability,11 and thus overcoming this problem has been an important goal of catalysis research. Among the several known catalytic conversions of CO2, cycloaddition of CO2 to epoxides has garnered considerable attention (Scheme 1).20-26 This reaction may enable efficient fixation of CO2. Furthermore, five-membered cyclic carbonate products have broad utility as polar aprotic solvents, electrolytes in lithium ion batteries, precursors of polymers, synthetic intermediates, etc.27,28 Today, one of the major challenges in the development of catalysts, either homogeneous or heterogeneous, for the synthesis of cyclic carbonates from CO2 and epoxides is to make the reaction condition milder, because the use of harsh reaction conditions entails emissions of CO2, which dampens the intended goal of CCU. In addition, many of the existing catalytic systems require the use of a co-catalyst, which adds complications to experiments. Despite these challenges, competent homogeneous catalysts have recently emerged.23,25 Furthermore, encouraging results have been reported for heterogeneous


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catalytic systems.23,29-35 Compared with homogeneous catalysts, heterogeneous catalysts generally have the advantage of easy catalyst separation.

Scheme 1. Synthesis of cyclic carbonates from CO2 and epoxides.

In recent years, several attractive features of MOFs36-43 have prompted researchers to examine the potential of MOFs as heterogeneous catalysts for the coupling of CO2 and epoxides.44-46 For example, M-MOF-74 (M = metal) materials, Co-MOF-74 and Mg-MOF-74 in particular, have been shown to exhibit catalytic activity in this reaction (Figure 1).47,48 Interestingly, the catalytic cycloaddition of CO2 to styrene oxide using these M-MOF-74 materials as catalysts yielded 4phenyl-1,3-dioxolan-2-one (styrene carbonate) in the absence of a co-catalyst and under relatively mild conditions (2.0 MPa, 373 K). In many other catalytic systems, a nucleophilic species (e.g., a halide ion included in a quaternary ammonium salt) from a co-catalyst is often responsible for the ring opening of the epoxide substrate. The unnecessity of a co-catalyst in the M-MOF-74-catalyzed cycloaddition adds value to these catalytic systems and alludes to the possibility that one of the two oxygen atoms in CO2 somehow acts as a nucleophile during the reaction. In fact, Ahn et al proposed a mechanism illustrated in Scheme 2, in which the ring-opening process uses CO2 itself and is facilitated by the involvement of Lewis acid and Lewis base centers existing within a MOF.47-49 It is highly likely that the open metal (Co2+ or Mg2+) sites in the MOFs act as Lewis acid sites. Existence of Lewis base sites in Mg-MOF-74 has also been verified experimentally, although it is still unclear which part of the framework acts as Lewis base centers.49 Ahn et al suggested that the oxygen atoms around the Lewis acidic metal site may play this role.47,48 The proposed mechanism in



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Scheme 2 also suggests that the ring-opening step is followed by a nucleophilic attack of the alkoxy oxygen on the carbon atom of CO2, to form the 5-membered cyclic product.

Figure 1. Portion of M-MOF-74.

R R

O

R

R

CA CB

O

C

O

O

O

O

O

O

O

O

+

O LB

LA

+

O LB

LB LA

LB LA

LA

Scheme 2. Proposed reaction mechanism for the conversion of CO2 and epoxide to cyclic carbonate. LA and LB denote Lewis acid and Lewis base sites, respectively.49

Computational chemistry can serve as a useful tool for studying mechanisms of important reactions like catalytic cycloaddition between CO2 and epoxides. Indeed, density functional theory (DFT) calculations have been applied to several homogeneous catalytic reactions of this sort, and valuable mechanistic insights have been obtained.50-53 However, as far as we are aware, no computational study has been conducted for MOF-catalyzed cycloaddition reactions, and thus underlying reaction mechanisms remain largely uncertain. Following on from our recent efforts to study MOF systems computationally, through which we have demonstrated the capability of 4 ACS Paragon Plus Environment

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multiscale quantum mechanics and molecular mechanics (QM/MM) techniques in investigating MOF systems,54-57 we herein explore the molecular mechanism of the cycloaddition reaction between CO2 and styrene oxide catalyzed by Co-MOF-74 and Mg-MOF-74. The specific goals of this study are (1) to identify which Lewis base site is responsible for the catalytic activity; (2) to clarify which carbon atom of styrene oxide receives a nucleophilic attack by a nucleophile and what acts as a nucleophile; (3) to determine how many reaction steps are involved in the coupling reaction; (4) to discern whether the reaction follows the previously proposed mechanism or some other mechanism; and (5) to examine if there is any distinction in the catalytic machinery between Co-MOF-74 and Mg-MOF-74.

Computational Method Multiscale QM/MM models were built from crystal structures of Co-MOF-74 and Mg-MOF74.58,59 The two-layer ONIOM(QM:MM) scheme implemented in Gaussian 09 software was used for QM/MM calculations.60-63 For all geometry optimization and vibrational frequency calculations, the B3LYP DFT functional was used in conjunction with the LANL2DZ effective core potential basis set (for Co) and the 6-31G* basis set (for the other atoms) to describe the QM atoms.64-68 Thermal corrections to Gibbs free energy (Gcorr) for 1 atm and 298.15 K were included in the reported energies. For MM atoms, the Universal Force Field (UFF) parameter set was used with restrained electrostatic potential (RESP) atomic charges, which were determined by applying DFT calculations to Co-MOF-74 and Mg-MOF-74 models (for details, see the Supporting Information).69,70 Single-point energy calculations at the ONIOM(B3LYP-D3BJ/6-311G(2d,p):UFF) level were subsequently performed on optimized geometries.71,72 The mechanical embedding (ME) scheme was used in all ONIOM calculations. The choice of ME instead of electronic embedding was based on the short distances between point charges and the QM atoms around the QM–MM boundary. As shown in Figure 2, the QM region in the multiscale model is located at the center of the system and contains six partial organic linkers and three metal ions, resulting in a total charge of -4. In both 5 ACS Paragon Plus Environment


Co-MOF-74 and Mg-MOF-74 models, the QM region is comprised of 105 atoms including 3 metal (Co2+ or Mg2+ ions). Before starting QM/MM calculations, the following three different spin states were examined for Co-MOF-74: (i) the ferromagnetically coupled high-spin state in which each of the three Co2+ ions has the quartet spin state (2S+1 = 10); (ii) the antiferromagnetically coupled high-spin state in which the central Co2+ ions has three β electrons (2S+1 = 4); (iii) the ferromagnetically coupled low-spin state in which each of the three Co2+ ions has the doublet spin state (2S+1 = 4). In our previous DFT study of several coordination compounds, we showed that the low-spin state is more stable in several tetracoordinate Co2+ complexes.73 In the current system, the high-spin electronic configurations (i and ii) were much more stable than the low-spin state (iii) by ~30 kcal/mol (see Table S1). States i and ii were almost comparable in stability, with the latter being slightly more stable by 0.7 kcal/mol (Table S1). As a single metal site in the MOF acts as a Lewis acid site, both of these states offer a high-spin (quartet) Co2+ center to the substrate. In this respect, these two high-spin states are essentially the same. To simplify the computational treatment, the ferromagnetically coupled state (i), which is easier to handle, was used in the subsequent calculations.

Figure 2. (a) Multiscale model used in this study for describing Co-MOF-74 and Mg-MOF-74. Optimized QM atoms are shown in ball-and-stick style. (b) Schematic of the QM region, indicating connectivity to different types of functional groups across QM/MM boundaries.


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Results and Discussion QM/MM calculations were performed on the MOF models to probe the catalytic conversion of CO2 and styrene oxide to cyclic carbonates. The reaction pathway begins with the coordination of styrene oxide to an open metal site of M-MOF-74 (M = Co, Mg) and the activation of CO2 at a Lewis base site to form a reactant complex (RC). Although there are many potential Lewis base sites around the styrene oxide substrate because of the multitopic nature of the dobdc4- ligand and the existence of many dobdc4- ligands in the system, only the oxygen atom from the deprotonated hydroxyl group in the phenolate moiety, illustrated in Scheme 3, was deemed likely to act as a Lewis base site. The oxygen atoms coordinating to the metal center are unlikely to play this role because the simultaneous activation of substrate molecules in a manner depicted in Scheme 2 is sterically unfeasible.

Scheme 3. Possible Lewis base sites for the activation of CO2 around an open metal site in MMOF-74.

Depending on which carbon atom (CA or CB in Scheme 2) of styrene oxide is attacked by the activated CO2 molecule (CO2(A)) in the initial epoxide ring-opening process via an SN2-type nucleophilic reaction, two possible pathways can be envisaged (Paths A and B, see Scheme 4). In Path A, the tertiary carbon atom CA is attacked by CO2(A), leading to a primary alkoxide intermediate, whereas Path B involves the attack of CO2(A) on the secondary carbon atom CB, 7 ACS Paragon Plus Environment


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resulting in the formation of a secondary alkoxide intermediate. In the next step, a second CO2 molecule (CO2(B)) is introduced and reacts with the alkoxide species to form a carbonate intermediate. Finally, a ring-closure event completes the formation of the cyclic carbonate product, releasing the CO2(A) molecule. It should be noted that the first (CO2(A)) molecule is usually assumed to receive a nucleophilic attack at the carbon atom by the alkoxy oxygen atom originating from epoxide (Scheme 2). In the reactions catalyzed by Co-MOF-74 and Mg-MOF-74, we rule out this possibility for a steric reason (vide infra). Instead, we argue for the involvement of two molecules of CO2 (CO2(A) and CO2(B)) in the formation of one cyclic carbonate product. As schematically shown in Scheme 4, our mechanism consists of three major consecutive steps in both pathways: ring opening of styrene oxide, carbonate formation, and ring closure (cyclization). Essentially the same cyclic carbonate product can be produced via Paths A and B.

(b) Path B

(a) Path A O

O O

O

Ph O O

O

C O (A)

Ph O

ring opening

M

Ph

O

O

C O (A)

CO2 adsorption

Ph

ring opening

M

CO2 adsorption O

O

Ph

O

C O

Ph Ph

O

CO2

O

O M

M O

cyclization

C

C

O

O CO2

Ph O C

O

O

O (B)

O

O

M

M

O (B)

O

C O Ph O O C O

C carbonate formation

cyclization

O

Ph O

carbonate formation

O C O

M

M

Scheme 4. Two pathways for the styrene carbonate formation involving nucleophilic attack on (a) CA (Path A) and (b) CB (Path B).


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Figure 3 shows the QM/MM-calculated energy profiles for the first two steps of M-MOF-74 catalyzed cycloaddition between CO2 and styrene oxide, which begins with the exergonic adsorption of styrene oxide and CO2 onto M-MOF-74. The substrate unbound states for Co-MOF74 and Mg-MOF-74 are denoted RC0 and RC0’, respectively. In the ring-opening steps of both MOF-catalyzed reactions, the attack of the activated CO2 molecule (CO2(A)) on CB (Path B) was calculated as having lower energy barriers (Co-MOF-74: 17.8 kcal/mol, Mg-MOF-74: 21.9 kcal/mol) than the attack on CA (Co-MOF-74: 24.6 kcal/mol, Mg-MOF-74: 24.1 kcal/mol). Indeed, close inspection of the transition states in the two pathways (TS1A and TS1B) obtained for the reaction in Co-MOF-74 (Figure 4) reveals that the C–O bond being formed between the styrene oxide and the CO2 is longer in TS1A (1.95Å) than in TS1B (1.92Å). The breaking C–O bond in styrene oxide is shorter in TS1A (1.93Å) than in TS1B (2.05Å). These values suggest that the transition state is somewhat earlier in Path B, consistent with the lower barrier seen in this pathway. The somewhat larger atomic charges were observed for the cobalt site, which may explain the lower barrier observed for Co-MOF-74. This ring-opening process results in the formation of an alkoxy intermediate Int1A in Path A or Int1B in Path B, and at this stage, the CO2 moiety retains a bond with the phenolate oxygen (see Figure 3).



Figure 3. Free energy profiles (in kcal/mol) for the adsorption and subsequent two steps in the initial stage of the reaction between CO2 and styrene oxide in (a) Co-MOF-74 and (b) Mg-MOF-74, as determined at the ONIOM(B3LYP-D3BJ/6-311G(2d,p):UFF)//ONIOM(B3LYP/LANL2DZ,631G(d):UFF) level.


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Figure 4. Transition states for ring opening in (a) Path A and (b) Path B of the reaction within CoMOF-74.

The lower barrier for the attack of CO2 on CB of styrene oxide observed in our calculations is consistent with the well-documented observations that an SN2-type nucleophilic reaction with epoxide tends to occur at a less hindered carbon atom.21,29,74 Our results also show that the phenolate oxygen in the linker, as a Lewis base center, can activate CO2(A) such that it can participate in the nucleophilic attack. In many other catalytic systems for the cycloaddition of CO2 to epoxides, a nucleophilic species (e.g., a halide ion in a co-catalyst) acts as a nucleophile in the ring-opening step. By contrast, it has been experimentally shown that the M-MOF-74-catalyzed cycloaddition reactions do not require the use of a co-catalyst.47-49 According to our QM/MM results, the reason a co-catalyst is not needed here is that the phenolate oxygen acts as a base to activate CO2(A), and the activated CO2(A) can attack the styrene oxide as a nucleophile. A direct nucleophilic attack of the phenolate oxygen on styrene oxide is sterically not permitted. As a result of the formation of Int1A and Int1B, the alkoxy oxygen within these intermediates takes on an anionic character, as illustrated in Scheme 2. In the subsequent step, it has been assumed conventionally that this anionic oxygen attacks the carbon atom of CO2(A), which is somewhat electrophilic. However, in our mechanism (Scheme 4 and Figure 3), another CO2 molecule (CO2(B)) is introduced to the reaction center to form a weakly bound complex Int2A or Int2B, and CO2(B) then binds covalently to this anionic alkoxy oxygen of the substrate to form 11 ACS Paragon Plus Environment


carbonate-type intermediate Int3A or Int3B. Subsequently, there is a minor structural reorganization leading to Int4A or Int4B. Thus, unlike what the conventional mechanism suggests (Scheme 2), we think that CO2(B) must be introduced to the reaction for the subsequent ring closure to occur. The reason is that if there is a direct attack of the alkoxy oxygen on CO2(A) from the rather inflexible substrate structures of Int1A or Int1B, either the strong coordination bond between the metal center and the alkoxy group or the covalent bond between CO2 and the phenolate oxygen must be cleaved. The C–O distance in Int1A and Int1B is >4.5 Å, as can be seen in Figure 5.

Figure 5. C–O distances (in Å) relevant to the direct cyclization in (a) Int1A and (b) Int1B.

The binding of CO2(B) to the alkoxy oxygen is found to have only a small barrier and is thus facile (Figure 3). During this binding process, a relatively unstable intermediate (Int3B’) was obtained in the reaction in Mg-MOF-74; nevertheless, the barrier for the formation of Int3B’ from Int2B’ is only 7.7 kcal/mol and can thus easily be surpassed. This step is followed by an exergonic reorientation of the carbonate functional group to form Int4A or Int4B, during which the coordination pattern changes in a manner that stabilizes the system. This reorientation step proceeds somewhat differently in Path A of the reaction in Mg-MOF-74; in this case, the reorientation occurs


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directly without the formation of Int3A’, probably because the relatively high stability of Int4A’ than the other corresponding Int4 species. Figure 6 shows the QM/MM-calculated free energy profiles for the final ring-closure step in the reactions in Co-MOF-74 and Mg-MOF-74. In this particular step, one of the two oxygen atoms from CO2(B), namely, the oxygen that is unbound to the Lewis acid center and thus exposed Int4A or Int4B, attacks the carbon bound to CO2(A) via TS4A or TS4B (Figure 7) in an SN2 mechanism to form cyclic carbonate intermediate Int5A or Int5B. As compared with Int3A and Int3B, Int4A and Int4B have relatively flexible structures, rendering the cyclization step well feasible. If we look at the relative energies of RC and Int4 species in the reaction in Co-MOF-74 (Figure 3a), we notice that Int4 species have positive relative energies, probably because of geometric strain; this suggests that the equilibrium between these two species is shifted in favor of RC. In this situation, the third cyclization step, which involves less stable transition states than in the first step, should become the rate-liming step of the reaction (Figure 6a). TS4A and TS4B have similar relative energies of 26.9 and 26.5 kcal/mol, indicating that these two pathways could compete with each other but Path B is slightly more favored. Close inspection of the transition states in the ring-closure step in Figure 7 clearly shows that this step is viewed as an SN2 reaction, in which the first CO2(A) acts as a leaving group. Our proposed mechanism of the ring-closure step is therefore distinctly different from the mechanism involving one CO2 molecule (Scheme 2) especially in terms of which atoms act as a nucleophile or an electrophile. We also obtained analogous cyclization pathways for the reaction in Mg-MOF-74 (Figure 6b), but in this case, more stable pathways were found as discussed below. Finally, there is a desorption step that releases both the cyclic carbonate product and the remaining CO2 molecule from the framework and completes the catalytic cycle. However, the complete desorption of both organic carbonate and CO2 is highly endergonic. Therefore, it is more likely that at least CO2 remains in the MOF for the next round of catalytic reaction.



Figure 6. Free energy profiles (in kcal/mol) for the ring-closure and desorption steps in the reaction between of CO2 and styrene oxide in (a) Co-MOF-74 and (b) Mg-MOF-74, as determined at the ONIOM(B3LYP-D3BJ/6-311G(2d,p):UFF)//ONIOM(B3LYP/LANL2DZ,6-31G(d):UFF) level.

Figure 7. Transition states for ring closure in the reaction within Co-MOF-74: (a) Path A and (b) Path B.

Interestingly, we also found an alternative route for the final ring-closure step in both Co-MOF74 and Mg-MOF-74 catalyzed reactions. In this route, the carbonate intermediate Int4A or Int4B, featuring a very different coordination environment, is formed prior to the 5-membered ring formation (Scheme 5). Specifically, there are shifts in the coordination sites of the substrate and a 14 ACS Paragon Plus Environment

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carbonate ligand from the linker. Thus, the coordinating carboxylate group is displaced by the migrating carbonate group, resulting in the formation of a pentacoordinate intermediate Int4Ax or Int4Bx (Scheme 5).

O C O

O Ph

O

C O

O

O

ligand rearrangement

O O O

O M

M

O O

O

O O

O M

O

M O

O O

O

O

O

O

M

O

C

O

O

O O

O

O

C O

Ph O

M

O

O

O

O

O

O

Scheme 5. Ligand rearrangement at the metal center in Path B of M-MOF-74. The displaced carbonate ligand is highlighted in blue.

In the case of Co-MOF-74, the Int4Ax and Int4Bx intermediates turned out to be much less stable than Int4A and Int4B (Figure 8a), thus indicating that this ligand reorganization is unlikely to occur in this MOF. In addition, the transition states (TS4Ax and TS4Bx) for the ring closure from Int4Ax and Int4Bx are seen to be very unstable. These results suggest that the cyclization step in Co-MOF-74 should proceed through the more easily accessible pathway that does not involve the ligand rearrangement (Figure 6a). By contrast, a strikingly different trend was observed for the reaction in Mg-MOF-74 (see Figure 8b). Compared to the alternative carbonate intermediate species in Co-MOF-74, the Int4Ax’ and Int4Bx’ intermediates in Mg-MOF-74 were calculated as highly stable. In particular, Int4Bx’ in the favorable pathway (Path B) has a relative energy of -7.7 kcal/mol (Figure 8b), which is much lower than that of Int4B (5.0 kcal/mol, Figure 6b). Therefore, there should be a structural reorganization around the metal center in the last cyclization step of the reaction in Mg-MOF-74. The negative value of the energy of Int4Bx’ suggests that during the first two steps from RC, the equilibrium is in favor of Int4Bx’, and once is Int4Bx’ formed, going back 15 ACS Paragon Plus Environment


to RC is difficult because TS1B’ is very unstable (21.9 kcal/mol) relative to Int4Bx’ (-7.7 kcal/mol). Moreover, cyclization in Path B through ligand arrangement has a relatively low barrier (17.4 kcal/mol, Figure 6b). These results suggest that the rate-determining step of the Mg-MOF-74 catalyzed reaction is the first ring-opening step. The energy gap of the first transition states is relatively large here (2.2 kcal/mol) compared with the energy gap of the cyclization transition states in the Co-MOF-74 reaction (0.4 kcal/mol). Therefore, Path B should be the major pathway in the Mg-MOF-74 catalyzed cycloaddition.

Figure 8. Free energy profiles (in kcal/mol) for the alternative ring-closure step and desorption in the reaction between CO2 and styrene oxide in (a) Co-MOF-74 and (b) Mg-MOF-74, as determined at

the

ONIOM(B3LYP-D3BJ/6-311G(2d,p):UFF)//ONIOM(B3LYP/LANL2DZ,6-31G(d):UFF)

level.

The contrasting behavior of Co-MOF-74 and Mg-MOF-74 in the ligand rearrangement event can be rationalized in terms of the different coordination modes adopted in these two systems. Thus, the greater flexibility around the Mg2+ site should be associated with the fact that d-orbitals are not used for coordination bonding. In other words, the coordination environment around the Mg2+ center is less directional than that around the Co2+ site, allowing more malleable ligand coordination in the former. This hypothesis is corroborated by additional calculations of coordination 16 ACS Paragon Plus Environment

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rearrangement energy of Co-MOF-74 and Mg-MOF-74 (see Tables S3 and S4). The coordination rearrangement energy was calculated by removing the substrates, styrene oxide and CO2, (see Scheme S1) from the optimized structure of Int4 species and performing single-point energy calculations at the ONIOM(B3LYP/6-311G(2d,p):UFF) level. The coordination rearrangement energy can thus be interpreted as the energy change of a MOF, arising from the displacement of a carboxylate ligand by a migrating carbonate substrate. The data show a clear tendency that the ligand rearrangement can take place much more easily in Mg-MOF-74 than in Co-MOF-74. As discussed above, the substrate moiety of Int1A or Int1B have rather inflexible geometries, which precludes the direct attack of the alkoxy oxygen on CO2(A). Furthermore, the adsorption of CO2(B) is required for ring closure to take place. After the inclusion of CO2(B), Int4A and Int4B gain some flexibility. In addition, as shown in Figure S12, the C(CO2(A))–O(alkoxy) distances in Int4A and Int4B are shorter than 4.0 Å in Co-MOF-74. The direct attack of the alkoxy oxygen on CO2(A) in Int4A and Int4B, therefore, becomes feasible. As shown in Figure 9, both Int4A and Int4B can undergo the direct ring-closure step via TS7A and TS7B to form five-membered cyclic ring intermediates Int8A and Int8B, respectively. Interestingly, the barriers for these new direct pathways are lower than those which we obtained above (Figure 6). In particular, TS7B is 3.4 kcal/mol lower in energy than TS4B. Int8B could be converted to the final product without any barrier while in Path A, Int8A could be converted to a more stable conformer Int9A. The release of the final product in Path A has a barrier of 10.1 kcal/mol via TS9A. The different barrier heights in the product release in Paths A and B are caused by the different C–O bond lengths between CO2(A) carbon atom and the phenolate oxygen atom in the ligand in Int8A and Int8B. As shown in Figure S13, this C–O distance is 1.45 Å in Int9A, which is shorter than in Int8B by 0.16 Å. Thus, it is harder to cleave the C–O bond in Int9A. In general, in Co-MOF-74, these new direct pathways are calculated as the most favorable pathways, and Path B is favored over Path A by 2.1 kcal/mol.



Figure 9. Free energy profile (in kcal/mol) for the direct ring-closure step and desorption in the reaction between CO2 and styrene oxide in Co-MOF-74, as determined at the ONIOM(B3LYPD3BJ/6-311G(2d,p):UFF)//ONIOM(B3LYP/LANL2DZ,6-31G(d):UFF) level.

A full mechanistic scheme for the reactions occurring in Co-MOF-74 and Mg-MOF-74, as derived from QM/MM calculations, is given in Figure 10. These two MOF-catalyzed reactions share considerable similarities in the mechanism. In both cases, the reactions consist of three major steps, and the first and third steps have high barriers. In both Co-MOF-74 and Mg-MOF-74, the rate-limiting step in Path B is significantly more stable than Path A, and thus Path B should be the predominant pathway in both cases. Nevertheless, several distinctions can also be noticed. First, the final cyclization step proceeds through the aforementioned ligand rearrangement in the case of MgMOF-74. Second, in the reaction in Co-MOF-74, the ring-closure step is calculated as the ratedetermining step, whereas in the case of Mg-MOF-74, the ring-opening step should be the ratedetermining step. Moreover, CO2(A) is incorporated into the final product in the case of Co-MOF74, but CO2(B) is incorporated in the case of Mg-MOF-74. Despite these distinctions, according to the calculation results, the rate-determining barriers are comparable in the reactions in Co-MOF-74


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and Mg-MOF-74, which is consistent with the similar catalytic activity observed experimentally for the two MOFs.47,48

Figure 10. QM/MM-derived mechanistic scheme (with relative energies in kcal/mol) of the most plausible pathways for the cycloaddition between styrene oxide and CO2 in Co-MOF-74 and MgMOF-74. Transition states in the rate-determining steps are highlighted in yellow.



Conclusions In this work, we applied ONIOM(B3LYP:UFF) QM/MM calculations to the cycloaddition reactions between styrene oxide and CO2 catalyzed by Co-MOF-74 and Mg-MOF-74, with the aim of gaining detailed mechanistic insights. Our calculations showed that the phenolate oxygen, which is situated near the Lewis acidic metal site but not directly bound to the metal center, can act as the Lewis base site. Comparisons of two possible pathways for ring opening showed that the attack of a nucleophile takes place preferentially at the secondary carbon atom (CB) of the epoxide. Here, the oxygen atom of a CO2 molecule (CO2(A)), which is activated by the phenolate oxygen acts as a nucleophile. In both MOF-catalyzed reactions, the cyclic carbonate formation reaction consists of three major steps: (1) SN2-type ring opening of styrene oxide by CO2(A); (2) binding of a second CO2 molecule (CO2(B)) to the alkoxy oxygen to form a carbonate intermediate; and (3) ring closure (cyclization) to form a 5-membered cyclic product. Thus, in our mechanism, two molecules of CO2 are involved in the formation of one molecule of styrene carbonate. In the case of Co-MOF-74, CO2(A) is incorporated into the final product while in the case of Mg-MOF-74, it is CO2(B) that is eventually incorporated into the final product, with CO2(A) acting merely as a “co-catalyst” for the reaction. Our results also suggest that the rate-determining step and the final ring-closure step are somewhat different in the two systems. Thus, in the reactions in Co-MOF-74 and Mg-MOF-74, the ring-closure step and the ring-opening step, respectively, constitute the rate-determining steps of the reactions. In addition, before the final-ring closure step, rearrangement of ligands occurs around the Mg2+ center in Mg-MOF-74, to give a more stable intermediate. The same ligand rearrangement is unlikely to occur in the case of Co-MOF-74, because d-orbitals are involved in the coordination bonds to make the coordination environment more directional and less flexible.

Supporting information Generation of RESP charges, Energy data and XYZ coordinates of optimized geometries (QM region only) are available in the Supporting Information.


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Acknowledgments H.H. is grateful for financial support from JST-PRESTO (JPMJPR141B) and City University of Hong Kong (7200534 and 9610369).

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How Does CO2 React with Styrene Oxide in Co-MOF-74 and Mg-MOF

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