Excess-electron-induced C–C bond formation in transformation of carbon dioxide

Abstract

This study presents a new fixation method of CO₂ through excess-electron-induced C–C bond formation using quantum chemical method. Because of active CO₂⁻˙ with a distinct radical character at the carbon center, two divalent anion complexes [O₂C–C₆H₆–CO₂]²⁻ (cis-II and trans-II) are obtained via C–C bond formation between the carbon atom of C₆H₆ and the carbon atom of CO₂ in the process of CO₂⁻˙ radical attacking on benzene molecule. Furthermore, the transformations of cis-II and trans-II are predicted. We found that the more favorable transformation is for cis-II. It can produce terephthalic and one H₂ molecule via two H-atom elimination with the energy barrier of 35.70 kcal mol⁻¹. Furthermore, we found that the formed hydrogen bond complex CO₂–HCN did not reduce the energy barrier; however, it could reduce the energy of the transition state with respect to that of the reactant, due to it dispersing the charge of benzene ring.

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1. Introduction

The study of the transformation of carbon dioxide is currently one of the most active research areas in the scientific community,^1–5 because the potentially devastating effects of steadily increasing concentrations of CO₂ are threatening the sustainable development of our society. Recently, a number of studies have shown that the use of CO₂ can not only generate useful organic compounds from a nontoxic, abundant, and economical carbon resource,^1–11 but also to a certain extent, reduce its atmospheric concentration. However, only a few reaction processes utilize CO₂ as a raw material because CO₂ is the most oxidized state of carbon and highly stable. Therefore, much effort needs to be made regarding the transformation of CO₂.^7,10 In recent years, various strategies of promoting the reactivity of organic materials toward CO₂ have been proposed, especially with regard to chemical transformation. The first proposal concentrates on coordinating CO₂ to transition-metal complexes^10–14 wherein cleavage of the transformed CO₂ moiety from the metal center hinders catalytic application. Second, CO₂ reacts with unsaturated hydrocarbons, for example, the oxidative coupling of CO₂ with olefins and alkynes.^15–29 The third one concerns insertion of CO₂ into M–X bonds (M = metal, X = C, O and H).^30–41 When a CO₂ molecule inserts into the M–C bond of a compound, a carboxylate species would be formed.^30–35 By inserting into the M–H bond with suitable catalysts, the synthesis of formic acid from the less toxic and more abundant CO₂ is feasible, and the insertion of CO₂ into M–O bonds results in the formation of carbonate species.^36–41 Another charming reaction strategy is the photocatalytic and electrochemical reduction of CO₂.^42,43 Despite many studies reporting the transformation of CO₂ by various physical and chemical means, it must be noted that most of reports still go back to the initial concepts of transition metal catalysis proposed by the research groups of Inoue, Musco, Hoberg, Walther and Behr et al. For example, about three decades ago, Hoberg and Walther started examining the oxidative coupling of carbon dioxide (1 bar) and olefins catalyzed by nickel(0) complexes or iron(0) complexes.^44,45 The research groups of Inoue and Musco were the first to report the reaction between carbon dioxide and dienes catalyzed by palladium(0) complexes.^46,47 Behr and co-workers improved the original synthetic procedures of Inoue and Musco using palladium(II) acetate or acetylacetonate complexes and phosphines in acetonitrile as the solvent, which led to a significant increase in the yield of the lactones.⁴⁸ However, no new breakthrough in the transformation of CO₂ has been developed. Therefore, it is significant that new transformation methods for using CO₂ as a starting material emerge in the near future.

The most prevalent obstacle of CO₂ usage is that it is a weak electrophile. It is difficult to achieve C–C bond formation when reacting with organic molecules. Luckily, the assistance from transition metals may be a good approach to address this problem.^10–43 Herein, we present idea that free electron assisting provides an alternative approach to achieve the transformation reactions (see Scheme 1). As shown in Scheme 1, first, CO₂ captures a free electron to form active CO₂⁻˙ with a distinct radical character at the carbon center. Then, the active CO₂⁻˙ reacts with an organic molecule H-CR to form intermediate electron–molecule compounds via C–C bond formation. Finally, one product would be obtained through dissociating an electron. In recent years, chemical reactions via free electron assisting have been widely investigated. For example, upon electron attachment, the van der Waals complex of azabenzene and CO₂ (ref. 49 and 50) would generate a covalent bond between the nitrogen atom and the carbon atom of CO₂ through an extended π-orbital conjugation over all the moieties of the complex. Swiderek et al. have demonstrated a new strategy to synthesize C₂H₅NH₂ from C₂H₄ and NH₃ by low-energy free electrons induction,⁵¹ which relied on the electrostatic attraction caused by the soft ionization of one of the reaction partners. Another important instance was that an excess electron induced the hydrogen-bonded complex, NH₃⋯HCl, to form the ionic salt,⁵² first forming a dipole-bound anion of NH₄Cl, and then a deformed Rydberg radical NH₄, polarized by a chloride anion, Cl⁻. For electron impact catalytic reaction, the Sajeev group reported a new elementary reaction mechanism of an electron and a molecule in a metastable compound,⁵³ which strongly relates to bond breaking and bond formation.

Scheme 1 One potential reaction route under an excess electron action.

Schröder et al. stated that the geometrically bent radical anion of CO₂ constitutes a new type of highly reactive ion, and therefore it can be regarded as an activated CO₂ unit.⁵⁴ Although in a real gas phase experiment it is difficult to use CO₂⁻˙ as a reactant, due to its short lifetime. Sajeev and Davis proposed a good strategy wherein the formation of the weak molecular complex [CO₂·HCN]⁻˙ helps the CO₂⁻˙ moiety have longer lifetime and permanent reactivity.⁵⁵ Beyer's group presented CO₂⁻˙ species that are concurrently solvated and stabilized by water ligands forming CO₂⁻˙(H₂O)_n^56–59 and performed their reactions with CH₃SSCH₃ to achieve cleavage of the disulfide bond in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry.

It is useful to understand the reaction of active CO₂⁻˙ and the benzene molecule to investigate other reactions of long-lived CO₂⁻˙ species in weak molecular complexes. CO₂⁻˙ attacks C₆H₆, which leads to the formation of a new C–C chemical bond. This is the first step to fix and transform CO₂ by electron impact. Furthermore, because of the attack from CO₂⁻˙, the aromaticity of C₆H₆ ring is broken in the electron–molecule compound [C₆H₆–CO₂]⁻˙. Then, a second CO₂⁻˙ would further attach to the electron–molecule compound [C₆H₆–CO₂]⁻˙ to form the divalent anion complexes cis-II, trans-II and the hydrogen bond complex I (see Scheme 2).

Scheme 2 Possible anion complexes formed by CO₂⁻˙ attaching to C₆H₆.

In this study, we investigated the reaction between CO₂⁻˙ and C₆H₆ to form the divalent anion complexes, cis-II, trans-II and hydrogen bond complex, I, via C–C bond formation (see Scheme 2). Furthermore, transformations of cis-II and trans-II are predicted; we found that the most likely transformation was that cis-II transforms to terephthalic and one H₂ molecule via H elimination with an energy barrier of 35.70 kcal mol⁻¹. Subsequently, investigations showed that the long-lived CO₂⁻˙ moiety in the weak molecular complex of [CO₂·HCN]⁻˙ also had permanent reactivity to induce C–C bond formation with the energy barrier of 36.35 kcal mol⁻¹. However, we found that it could reduce the energy of the transition state with respect to that of the reactant, due to it dispersing the charge on benzene ring. Therefore, we hope that this study is useful for further theoretical and experimental studies regarding the transformation of CO₂.

2. Computational details

All the calculations were performed with the GAUSSIAN 09 programs.⁶⁰ The geometrical optimizations of all the intermediates and transition states were performed at the second-order Møller-Plesset perturbation theory (MP2) level with the augmented correlation consistent basis set aug-cc-PVDZ. All studied structures are checked to be closed-shell structures, except for [C₆H₆–CO₂]⁻˙ and CO₂⁻˙. Frequency calculations at the same level were performed to confirm each stationary point to be either a minimum or a transition structure (TS). Energy-minimum geometries had only real frequencies, whereas TS geometries had an imaginary frequency corresponding to the relevant reaction coordinate. In several cases wherein TS are not easily confirmed by animation of their vibrations, intrinsic reaction coordinate (IRC)^61,62 calculations were performed to confirm the connection of each TS to its corresponding reactant and product. Moreover, zero-point energy (ZPE) is also evaluated with a scale factor of 1. Furthermore, high-level (CCSD(t)/aug-cc-PVDZ) energy calculation on the optimized structures is carried out to obtain the single point energy. The charge distributions of complexes were obtained by natural bond orbital (NBO)^63–65 analyses at the MP2/aug-cc-PVDZ level.

3. Results and discussion

3.1 Formation of electron–molecule complex [C₆H₆–CO₂]⁻˙

A metastable electron–molecule compound [C₆H₆–CO₂]⁻˙ is obtained in the process of CO₂⁻˙ attacking the benzene molecule. The barrier of C–C bond formation is only 7.22 kcal mol⁻¹, due to CO₂⁻˙ exhibiting radical character. Fig. 1 shows the optimized reactant, TS and product structures. Fig. 2 lists HOMOs of [C₆H₆–CO₂]⁻˙ and TS_{[C6H6–CO2]⁻˙} to roughly illustrate the formation of [C₆H₆–CO₂]⁻˙. From Fig. 2, the electron density transfers from the rich C₆H₆ ring to the C atom with the positive charge in CO₂⁻˙; moreover, the electron density of O atoms in CO₂⁻˙ gives a feedback on to the C₆H₆ ring (see Fig. 2a). The ensuing charge transfer therefore enhances the interaction between C₆H₆ and CO₂⁻˙. In the electron–molecule [C₆H₆–CO₂]⁻˙, the bond length of C1–C2 is 1.618 Å, which is longer than the usual C–C bond (∼1.55 Å). The formation of the new C1–C2 bond is a key step to fix and transform CO₂. Because the conjugation of the C₆H₆ ring is destroyed as a result of the CO₂⁻˙, the C₆H₆ ring becomes reactive. The second CO₂⁻˙ could be further attached to electron–molecule compound [C₆H₆–CO₂]⁻˙ to form the anion complexes cis-II and trans-II with no energy barriers (this issue will be discussed in the next section).

Fig. 1 PES profile for the process of CO₂⁻˙ attaching to C₆H₆.

Fig. 2 HOMOs of [C₆H₆–CO₂]⁻˙ and TS_{[C6H6–CO2]⁻˙} showing charge transfers to and fro from the new C–C bond.

The geometrical parameters of the optimized reactant, TS and product structures are listed in Fig. 1. In the reactant CO₂⁻˙, the bend angle of O–C–O is 136° and the bond lengths of C–O are 1.251 Å, whereas in the product structure, [C₆H₆–CO₂]⁻˙, the bend angle of O–C–O shrinks to 131° and the bond lengths of C–O extend to 1.261 Å.^49,50 This indicates that the newly formed C1–C2 bond further weakens the C–O bond and activates the C₆H₆ ring. From Fig. 1, we found that the C1–C2–C3 bond angle of 109.7° in TS_{[C6H6–CO2]⁻˙} is much smaller than 135.7° in [C₆H₆–CO₂]⁻˙, this indicates that the process that produced [C₆H₆–CO₂]⁻˙ has a tendency towards forming an extended π-conjugated structure.

Fig. 1 shows that the formation of [C₆H₆–CO₂]⁻˙ may be a reversible reaction. However, the formation of the C1–C2 bond provides an activation of the C₆H₆ ring, and the second CO₂⁻˙ easily attacks the C₆H₆ ring to form the divalent anion complexes [O₂C–C₆H₆–CO₂]²⁻ (cis-II and trans-II) with no energy barrier. This can be considered as the combination between two radical ions (see Scheme 3 for two possible mechanisms of the formation of the radical ion, [C₆H₆–CO₂]⁻˙). Thus, for the step of formation of [C₆H₆–CO₂]⁻˙, the shift of the equilibrium is promoted to the product by depleting the [C₆H₆–CO₂]⁻˙ (see Scheme 4).

Scheme 3 The stable radical ions.

Scheme 4 The shift of the equilibrium.

3.2 Fixation of the second CO₂⁻˙ to form divalent anion complexes

Due to the activation of C₆H₆, the second CO₂⁻˙ can easily fix the electron–molecule compound [C₆H₆–CO₂]⁻˙ to form divalent anion complexes [O₂C–C₆H₆–CO₂]²⁻ (see Scheme 2). Three possible anion complexes cis-II, trans-II and hydrogen bond complex I are shown in Fig. 3. In the divalent anions [O₂C–C₆H₆–CO₂]²⁻, the C1–C2 bond is reduced by 0.034–0.047 Å, which indicates that the strength of C1–C2 bonds is enhanced. As shown in Fig. 4, the second CO₂⁻˙ easily attacks the C₆H₆ ring to form the divalent anion complexes cis-II and trans-II with no energy barrier. However, comparing with the reactants, the energies of cis-II and trans-II only decrease by 1.96 and 2.21 kcal mol⁻¹, respectively. However, the formation of hydrogen bond complex I is very energetically unfavorable; this formation step is a strong endothermic reaction requiring the energy of 54.00 kcal mol⁻¹. Therefore, we concluded that the reaction of the second CO₂⁻˙ attacking C₆H₆ ring may follow a pathway producing cis-II or trans-II, with the former process being energetically more favorable.

Fig. 3 Optimized structures and their geometrical parameters. Bond lengths are in Angstroms and angles are in degrees.

Fig. 4 Energy profiles for the CO₂ transformation reaction.

Predicting feasible transformation routes is important to the study of CO₂ transformation. Thus, we calculated the possible transformations of cis-II and trans-II. Fig. 3 depicts the evolution of the structures, including the reactants, TS and product structures. Although the formation of hydrogen bond complex I is unfavorable in energy terms, we still considered it to compare with the transformations of cis-II and trans-II. (1) Supposing that hydrogen bond complex I may transform to OCOH⁻ and ph-COO⁻. In hydrogen bond complex I, due to the formation of a hydrogen bond, the C2–H1 bond length is lengthened to 1.156 Å, which is longer by 0.042 Å than that in the electron–molecule compound [C₆H₆–CO₂]⁻˙ (1.114 Å). This indicates that the C2–H1 bond has been activated. In TS-I, the C2–H1 bond length is 1.481 Å and the C2–C1 bond length is shortened to 1.561 Å. Interestingly, the C1–C2–C3 bond angle is approximately thought of as the dihedral between the two planes of C₆H₆ and CO₂⁻˙. It is found that this is 163.2°, 172.6° and 180° for the reactant, TS and product structures, respectively. This indicates that π₆⁶ of C₆H₆ and π₃⁵ of CO₂⁻˙ become more and more coupled to form an extended π-conjugated network from the reactant to product structure. (2) For the transformation of trans-II, the reaction starts with H-transfer from atom C2 to atom O1 to form an intermediate trans-II-a. Then, the second H-transfer process occurs to form the product [HOOC-ph-COOH]²⁻ (terephthalic acid with two negative charges). As shown in Fig. 3, although the second CO₂⁻˙ easily fixes onto the C₆H₆ ring with no energy barrier, the C2–H1 bond has not been activated, which is different from hydrogen bond complex I. With the evolution of the structures from the reactant to product, the C1–C2 and C3–C4 bond lengths become shorter and shorter. In the product [HOOC-ph-COOH]²⁻, their bond lengths decrease to 1.400 Å, which would exhibit double bond character. For the change of C1–C2–C3 (C2–C3–C4) bond angle, it is important to understand the formation of the product. It is found that both C1–C2–C3 and C2–C3–C4 bond angles increase with the evolution of the structures, even though C1–C2–C3 (or C2–C3–C4) has a small decrease in the transition state structure TS2-trans-II (or TS1-trans-II). These indicate that an extended π-conjugated network was formed by the coupling of one π₆⁶ of C₆H₆ and two π₃⁵ of CO₂⁻˙. (3) As shown in Fig. 3, cis-II can transform to one ⁻OOC-ph-COO⁻ and one H₂ molecule, and this plays a significant role in the transformation of CO₂, because it generates not only a useful chemical, but also a clean energy source of H₂. In the transformation of cis-II, the reactant cis-II with C_2v symmetry strengthens the C1–C2 and C3–C4 bond; they are 1.578 Å, which is shorter than that in the electron–molecule compound [C₆H₆–CO₂]⁻˙ (1.618 Å). As in trans-II, the C2–H1 bond has also not been activated, whereas in TS-cis-II, the C1–C2 and C3–C4 bonds of 1.554 Å slightly decrease by 0.024 Å. It can be noted that the H1–H2 distance is only 1.020 Å, which is much shorter than that in cis-II (3.394 Å). As in trans-II and hydrogen bond complex I, the change in C1–C2–C3 (C2–C3–C4) bond angle indicates that the formation of an extended π-conjugated network promotes the evolution of the structures and stabilizes the product complexes.

The transformation energy profile is presented in Fig. 4. As mentioned above, the formation of hydrogen bond complex I is very energetically unfavorable; therefore, we do not discuss its transformation here. For complex trans-II, in its transformation to the product it undergoes two transition states TS1-trans-II and TS2-trans-II. In TS1-trans-II (TS2-trans-II), the H-transfer process from C2 (C3) to O1 (O2) occurs via a four-membered ring C–H–O–C. However, if the H-transfer process occurs, the energy barriers of TS1-trans-II and TS2-trans-II are high (47.01 and 59.45 kcal mol⁻¹, respectively) and too high to achieve H-transfer. For cis-II, its transformation to the product only undergoes one transition state TS-cis-II (1635i cm⁻¹), which corresponds to the vibration of H elimination with an energy barrier of 35.70 kcal mol⁻¹. Compared to the transformation of trans-II, we found that the process of the formation of one H₂ by H elimination is easier than the H-transfer process. In a word, the transformation process of cis-II seems to be a more favorable route for the transformation of C₆H₆ and CO₂.

A theoretical analysis of the charge transfer process has been performed. Fig. 5 depicts the charges of the benzene skeleton along with the transformation from reactant to product. The charges of the benzene skeleton increased with respect to I and cis-II and decreased with respect to trans-II. The decreased charge decreases the stabilities of intermediates and transition states for trans-II. In contrast, the increased charge enhances the stabilities of intermediates and transition states for I and cis-II. Compared with TS-I and TS-cis-II, the charges of the benzene skeleton in TS1-trans-II and TS2-trans-II are very negative, and large energy barriers have to be overcome to achieve H transfer. Thus, the H elimination/transfer is easier in I and cis-II, than in trans-II. As Ramos and Fernandes stated,⁶⁶ the charge density of the transition structures is more widely distributed than that of reactants, which favors the reaction to be catalyzed in hydrophobic environments.

Fig. 5 The charges of benzene skeleton along with the transformation from the reactant to the product.

3.3 HCN assistance promoting the transformation

Sajeev stated that the activated CO₂ moieties in the weak molecular complexes are long-lived and reactive.⁵⁵ In this study, we further investigated the reaction for cis-II transformation between the benzene molecule and the long-lived CO₂⁻˙ moiety in the weak molecular complex of [CO₂·HCN]⁻˙. Fig. 6 and 7 depict the process of CO₂ attaching to C₆H₆ and the process of the formation of H₂ under HCN molecule assistance, respectively. First, comparing the process without HCN molecule assistance, the activation energy for the process of CO₂ attaching to C₆H₆ increases by 3.9 kcal mol⁻¹ from 7.22 to 11.12 kcal mol⁻¹, this is because the weak molecular complex of CO₂·HCN slightly decreases the radical character of the C atom due to the dispersion effect via hydrogen bond interaction. For the same reason, the product [C₆H₆–CO₂–HCN]⁻ has a lower energy of 4.27 kcal mol⁻¹ than the reactants. In the process of H elimination, we calculated that the energy barrier is 36.35 kcal mol⁻¹, which is very close to the 35.70 kcal mol⁻¹ without HCN molecule assistance. Thus, this shows that the formed hydrogen bond complex CO₂–HCN did not reduce the energy barrier for the H elimination process. However, the reduction of the energy of TS_{[NCH–2OC–C6H6–CO2–HCN]}²⁻ (18.57 kcal mol⁻¹) with respect to that of the reactant is much larger than TS-cis-II (33.49 kcal mol⁻¹) without HCN molecule assistance. This is due to the dispersion effect of HCN via hydrogen bond interaction. From Fig. 8, the NBO charge of the benzene skeleton of TS_{[NCH–2OC–C6H6–CO2–HCN]}²⁻ (−0.484 |e|) is lower than −0.52 |e| in TS-cis-II without HCN molecule assistance. Clearly, the NBO charge of the benzene skeleton is dispersed to enhance the stability of TS_{[NCH–2OC–C6H6–CO2–HCN]}²⁻. This point may be significant for investigating other relevant reactions.

Fig. 6 PES profile for the process of CO₂⁻˙ attaching to C₆H₆ via HCN assistance.

Fig. 7 Energy profiles for the CO₂ transformation reaction via HCN assistance.

Fig. 8 Comparing the charge of the benzene skeleton with and without HCN assistance.

4. Conclusion

In this study, we have revealed a new fixation method of CO₂ through excess-electron-induced C–C bond formation using a quantum chemical method. First, by excess electron induction, two CO₂ molecules are fixed on a benzene molecule to form two divalent anion complexes [O₂C–C₆H₆–CO₂]²⁻ (cis-II and trans-II) via C–C bond formation between the carbon atom of C₆H₆ and the carbon atom of CO₂, due to the distinct radical character at the carbon center in the excess electron species CO₂⁻˙. Furthermore, the transformations of cis-II and trans-II have been predicted. We found that the more favorable transformation of cis-II is to produce terephthalic and one H₂ molecule via two H-atom eliminations with an energy barrier of 35.70 kcal mol⁻¹. Furthermore, we found that the long-lived CO₂⁻˙ moiety in the weak molecular complex [CO₂·HCN]⁻˙ also had permanent reactivity to induce C–C bond formation; however, it did not reduce the energy barrier. However, it could reduce the energy of the transition state with respect to that of the reactant, due to its ability to disperse the charge of the benzene ring. As we expect, this study may encourage further related research into CO₂ transformation.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 21303065).

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