Jing-Feng Maa,
Fang Ma*a,
Zhong-Jun Zhoub and
Ying Tao Liuc
aSchool of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, P. R. China. E-mail: mafangchem@foxmail.com
bInstitute of Theoretical Chemistry, Jilin University, Changchun, 130023, P. R. China
cDepartment of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750012, P. R. China
First published on 2nd September 2016
This work reports the first example of boron-doped lithium clusters, BLin, activating CO2. The investigation shows that a kind of novel BLin–CO2 (n = 2–6) complex (In) with bidentate double oxygen M(η2-O2C) coordination is obtained through the bimetal 2Li of BLin clusters binding to the two O atoms of CO2, and the structural integrities of BLin clusters are not destroyed. We find that the partial negative charge transfer from BLin to the π* orbital of CO2 leads to weakening of the CO bonds of CO2 and an active CO2 moiety, except for when n = 2. Further, we perform reactions between In (n = 3–6) and benzene to elucidate a novel alternative approach to direct carboxylation by inserting CO2 into C–H bonds. We find that the carboxylation of the C–H bond of benzene can be achieved through the transition states (TS) of C–C bond formation and H-atom-transfer from C to O via two H2O molecules acting as a H-transfer tunnel. Comparing the transition states of H-direct transfer and one H2O molecule assisting H transfer, two H2O molecules assisting H-transfer as a tunnel is shown to lower the barrier, due to this long H-bond bridge effectively easing the rotation of the dihedral angle between the C6H6 and CO2 moiety planes. Considering the whole free energy profile, BLi5 and BLi6 clusters are more feasible for the carboxylation of C–H bonds.
Studies on clusters are of considerable interest in research due to their potential application in catalytic processes, chemisorptions, materials science. In particular, recently, cluster-catalyzed oxidation of CO to CO2 has attracted more and more attentions. Bernhardt investigated the mechanism and energetic of the low-temperature oxidation of CO to CO2 catalyzed by small free palladium clusters Pdx+ (x = 2–7).23 The CO oxidation on subnanometer gold clusters (Au16–Au18, Au20, and Au27–Au35) has been systematically studied by Zeng' group.24–26 Ramasubramaniam theoretically verified the catalytic activity of ultrasmall Pt nanoclusters through CO oxidation reaction.27 These studies show that clusters indeed make CO be active through electron back-donation to the CO antibonding orbital. However, the activation of CO2 mediated by clusters is regrettably scarce.28
In this work, in order to explore cluster assisting the activation of CO2, we selected boron-doped lithium clusters BLin (n = 2–6) to activate CO2 and further investigate the transformation of it. The lithium–boron system has been extensively studied,29–31 because of the practical importance of Li–B alloys as anode materials for the production of lithium batteries. Moreover, the advantages of complex In were concerned. First, in the complex In, BLin exhibits super-alkali atom characteristics, which provides the large charge transfer into the π* orbital of CO2, and the structural integrities of BLin cluster. Second, BLin cluster is a representative mononuclear cluster, in which the nuclear B atom is trapped into the Li alliance. The BLin–CO2 complex with a bidentate double oxygen M(η2-O2C) coordination provides a novel coordination modes to activate CO2. Third, BLin cluster only contains the main group metal Li atoms and the nonmetal B atom, and does not contain any transition metals.
A key piece of information in metal-catalyzed CO2 reactions is the large reactivity of metal–CO2 complexes. Generally, the coordination of CO2 to the metals which makes CO2 be active has several basic modes. A single metal atom monodentate coordination M(η1-CO2) or bidentate coordination M(η2-CO2) are most common, corresponding to structures (A) and (B) in Scheme 1, respectively.32–34 A bidentate double oxygen M(η2-O2C) coordination is also seen in the anionic monometal Mg complexes [XMgCO2] (X = Cl, Br, OH)35. Here, we present novel BLin–CO2 complexes with a bidentate double oxygen M(η2-O2C) coordination (C), where bimetal 2Li of BLin clusters bind to O of CO2, to activate CO2 (the mode D in Scheme 1). Further, we theoretically calculated the reactions between the BLin–CO2 complexes and benzene molecule, to elucidate the role of BLin clusters in the CO2 transformation. Although this work is a theoretical design, In provides a idea theoretical model to study the novel mechanism of activation CO2. We anticipate that our results guide the catalyst community to discover additional cluster catalyst in the transformation of CO2.
BLin cluster is a representative mononuclear cluster, in which the nuclear B atom is trapped into the Li alliance. Therefore, the interaction between CO2 and BLin cluster in BLin–O2C complex adopts the interaction between large electronegativity O and large ionizing potential Li. However, the BLi2–O2C complex not only adopts Li–O interaction, but also contains B–C interaction, because the nuclear B atom is exposed due to no enough Li atom. So the BLi2 acting on CO2 affords a stable BLi2–O2C complex, where the C atom of CO2 is not active. In contrast to BLi2–O2C, BLin–O2C (n = 3–6) do not contains B–C interaction, which leads to possess a potential active site (C position).
From Fig. 1, we found that the bidentate double oxygen coordination leads to a bent CO2 moiety as a result of the partial negative charge transfer into the π* orbital of CO2,44,45 and that the structural integrities of BLin clusters are not destroyed. When compared to neutral CO2, the C–O bonds are obviously elongated by 0.034–0.090 Å in the BLin–O2C complexes, and the O–C–O angels become to be 120–140° in the bent CO2 moiety from the 180° in linear CO2. Therefore, from the structural view point, CO2 has been activated by BLin cluster. Further, the unscaled harmonic vibrational frequencies and intensities are calculated to demonstrate the activation of CO2. The antisymmetric CO2 stretching (vAS), the CO2 symmetric stretch (vS) and the CO2 bend (vB) are listed in Fig. 1. In bare CO2, vAS, vS and vB is found at 2222, 1216 and 713 cm−1, respectively. Obviously, the antisymmetric CO2 stretching in the BLin–O2C complex becomes weaker than that in the bare CO2, in which a redshift of vAS is 240–700 cm−1. The symmetric stretch and the CO2 bend become stronger than those in the bare CO2, and their blueshift of vAS and vB is 110–610 cm−1 and 23–154 cm−1. This analysis then suggests that the action of BLin cluster on CO2 leads to a weakening of the CO bonds.
Here, the advantages of complex BLin were concerned. Comparing with BHn and AlLin clusters, BLin not only provides the large charge transfer into the π* orbital of CO2, but also holds the structural integrities of BLin clusters. First, we calculated the vertical ionization potential to describe the capabilities of the charge transfer of BLin (3.633–4.534 eV), BHn (6.859–13.128 eV) and AlLin (3.367–5.317 eV), and found that the capabilities of charge transfer of BHn is weakest. Second, although AlLin has the large capabilities of the charge transfer; it is difficult to maintain the structural integrities, due to weak Al–Li bond.
Comparing with the antisymmetric CO2 stretching in other structure with carbene character, it is significant to understand the weakening of the CO bonds. While isolated CO2− by itself is not stable in the gas phase with respect to electron autodetachment (EA = −0.6 eV), its antisymmetric CO2 stretching is measured at 1658 cm−1 in a Ne matrix.46 In CO2-solvated M(CO2)− complexes (M = Au, Ag),47 corresponding to partial charge transfers ranging from 0.5 to 0.9 |e|, intermediate values for vAS (1865–1680 cm−1) are found. Further, a considerably more redshited vAS (1128 cm−1) is observed in [ClMgCO2]−.35 Therefore, the calculated large redshited vAS (1506–1964 cm−1) suggests that the bent CO2 moiety in the BLin–O2C complexes may also exhibit carbene character. A superficial scheme explains the carbene character of the bent CO2 moiety in the BLin–O2C complexes (see Scheme 2).
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Scheme 2 The carbene character of the bent CO2 moiety in the BLin–O2C complexes through the partial negative charge transfer into the π* orbital of CO2. |
Natural bond orbital analyses were carried out to understand the charge transfer of BLin–O2C complexes. The partial negative charge transfer into the π* orbital of CO2, decreases the energies of the lowest empty orbitals (LUMOs) or the highest single occupation orbitals (SOMOs) in the BLin–O2C complexes. As listed in Fig. 1, the SOMOs of BLi2–O2C, BLi4–O2C and BLi6–O2C is −0.28142, −0.25438 and −0.19137 a.u., respectively; and the LUMOs of BLi3–O2C and BLi5–O2C BLi7–O2C is −0.15327, and −0.01241 a.u. Comparing with the high energy LUMO of CO2 (0.05419 a.u.), which indicates that the electrophilicities of the BLin–O2C complexes are increased. Moreover, the low energies of the lowest empty orbitals in the BLin–O2C complexes is close to that of unstable radical anion CO2−˙ with high reactive activity. Therefore, it is expected that the BLin–O2C complexes exhibited high reactive activity, and would react with some molecules.
At the beginning of the reaction, one CO2 molecule approaches BLin, and forms slightly stable BLin–O2C complexes In with a potential active site at the C atom of CO2. Then, In would attack at benzene molecule to produce intermediates IIn via the C–C bond formation between the active site C atom and the carbon atom of C6H6. Further, Li-bond complexes are obtained through H transfer. The free energy profile is shown in Fig. 2, and the transition states and important intermediates are depicted in Fig. 3 and 4.
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Fig. 2 Free energy profile of the catalytic cycle under two H2O assisting. Blue, red, black and green lines represents n = 3, 4, 5 and 6, respectively. |
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Fig. 3 Optimized structures of C–C formation TS1 and H-transfer TS2 through two H2O molecules acting as an H-transfer tunnel. Bond lengths are in Å, and frequencies are in cm−1. |
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Fig. 4 Optimized structures of intermediates II and products. Bond lengths are in Å, and angles are in degree. |
For the slightly stable complexes In (n = 3–6), they have lower energies of 12.69(n = 3), 28.46(n = 4), 4.29(n = 5) and 24.05(n = 6) kcal mol−1 (see Fig. 2), respectively, than the total energies of BLin and CO2. The decreased energies for n = 3 and 5 is smaller than for n = 4 and 6.
As discussion above, the partial negative charge transfer into the π* orbital of CO2, leads to the high reactive activity of the bent CO2 moiety in the complexes In with a potential active site at the C atom of CO2. Consequently, the intermediates IIn are obtained via the C–C bond formation between the active site C atom and the carbon atom of C6H6. The barrier for the C–C formation transition state TS1 is 5.92, 24.00, 9.01 and 9.42 kcal mol−1 for n = 3, 4, 5 and 6 (see Fig. 1, the molecular coordinates of TS1 are listed in the ESI†), respectively. In TS1, the C–C distance is 2.104, 2.153, 2.103 and 2.152 Å (see Fig. 3), respectively. For the obtained IIn, they have lower energies (9.42(n = 3), 4.44(n = 5) and 4.55 kcal mol−1 (n = 6)) than the total energies of In and C6H6, except for II4. The energy of II4 is higher by 12.11 kcal mol−1 than the total energies of I4 and C6H6. Therefore, these indicate that intermediates (II3, II5 and II6) are the resting states in the reaction process.
In intermediates IIn (see Fig. 4), the lengths of the new formed C–C bond are 1.549–1.562 Å, which are close to ordinary C–C bond length. Thus these reactions provide an alternative approach of C–C bond formation, which offers a new idea to employ CO2 as an attractive carbon source in organic transformations. We also found that the bond length of C–H is longer (∼0.02 Å) than 1.086 Å of C6H6. This indicates that the C–H bond is slightly active due to the C–C formation. On the other hand, the CCC angles (135.2–139.7°) can approximately be looked upon as the dihedral angles between the C6H6 ring and the plane of CO2, which are much larger than normal 109.5° of C-sp3.
To achieve direct C–H carboxylation through inserting CO2 into C–H bond, the H-atom-transfer from C to O is required. As shown in Scheme 3, if II are the start point, the more straightforward pathway is the direct transfer of the H atom from C to the O of CO2. However, the H-direct transfer transition states are only found for II3 and II5 (see Fig. 5), yet for II4 and II6, we have not obtained. This reason can be understood by the structure characters of II. Comparing the structures of II3 and II5 with II4 and II6, we found that the lengths of the new formed C–C bonds in II3 (1.555 Å) and II5 (1.562 Å), are slightly longer than those in II4 (1.549 Å) and II6 (1.549 Å), and the dihedral angles in II3 (139.7°) and II5 (139.5°) are also slightly larger than those in II4 (135.2°) and II6 (135.3°). Therefore, the new formed C–C bonds in II3 and II5 are able to easily rotate in comparison with those in II4 and II6. However, the predicted barriers for the H-direct transfer transition states are very large (up to 76.38 and 43.57 kcal mol−1 for II3 and II5, respectively), which can be attributed to the large rotation of the dihedral angle between the C6H6 and CO2 moiety planes. The large barriers are too high to accomplish under mild conditions. Therefore, an alternative H transfer route should be found. By understanding the reason of the large rotation barriers, we present that employing extra H2O molecule can act as an H-transfer tunnel and decreases the rotation barriers. Comparing with the H-direct transfer route, we find that the extra H2O molecule acting as an H-transfer tunnel indeed decreases the barriers for H transfer. As shown in Table 1, when the H transfer is mediated by one H2O molecule acting as an H-transfer tunnel, the barrier decreases to 64.13 and 36.54 kcal mol−1 for II3 and II5, respectively. The barrier is only 14.15 and 21.90 kcal mol−1 for II4 and II6, respectively. Their structures of transition states are depicted in Fig. 5. Further, when the H transfer is mediated by two H2O molecules acting as an H-transfer tunnel, the barrier decreases to 49.34, 5.81, 22.16 and 6.38 kcal mol−1 for II3, II4, II5 and II6, respectively. The molecular coordinates of different H-transfer transition states are listed in the ESI.† Here, we mainly depict the H transfer transition states TS2 through two H2O molecules acting as an H-transfer tunnel (see Fig. 2). In TS2 through two H2O molecules acting as an H-transfer tunnel (see Fig. 3), the H-bond bridge is long enough to effectively ease the rotation of the dihedral angle between the C6H6 and CO2 moiety planes. Comparing the H-direct transfer transition states and one H2O molecule assisting H-transfer transition states, we found that the barrier would decrease 7–15 kcal mol−1 per additional one H2O (see Table 1), which is consistent with the 10–16 kcal mol−1 barrier to internal rotation of the dihedral previously reported.48–51 This is because the rotation of the dihedral angle between the C6H6 and CO2 moiety planes decreases 20–40° (see Fig. 6), which leads to an added impetus to form an extended conjugated π system through the couple of πC6H6 and πCO2. The added impetus can promote to the H-atom-transfer from C to O.
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Fig. 5 Optimized structures of the H-direct transfer transition states and one H2O molecule assisting H-transfer transition states. Bond lengths are in Å, and frequencies are in cm−1. |
n | Direct H transfer | H transfer by one H2O assisting | H transfer by two H2O assisting |
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3 | 76.38 | 64.13 | 49.34 |
4 | No found TS | 14.15 | 5.81 |
5 | 43.57 | 36.54 | 22.16 |
6 | No found TS | 21.90 | 6.38 |
Through H-atom-transfer from C to O, the product of the Li-bond complex P is obtained. In P, the C–C bond lengths are ∼1.49 Å, and the dihedral angles between the C6H6 and CO2 moiety planes are close to 180°. Thus, the carboxylation of C–H bond is achieved through C–C bond formation and the H-atom-transfer from C to O via two H2O molecules acting as an H-transfer tunnel.
The mechanism of the reaction between complex BLin–CO2 and C6H6 can be described as follow. First, BLin–O2C complex with the bidentate double oxygen M(η2-O2C) coordination possesses a potential active site at the C atom of CO2 moiety. This is because the partial negative charge transfer from BLin moiety into the π* orbital of CO2 moiety, leads to the carbene character of the bent CO2 moiety in the BLin–O2C complex. Then, the C–C bond is formed between the active site C atom and the carbon atom of C6H6 through radical attack. Finally, the H transfer is performed with the help of two H2O molecules acting as an H-transfer tunnel. Here, we found that two H2O molecules assisting long H-bond bridge lowers the barrier of H-transfer transition state through effectively easing the rotation of the dihedral angle between the C6H6 and CO2 moiety planes. After comparing all relative free energies shown in Fig. 1, we can find that the C–C bond formation is more favorable for n = 3 and 5 than n = 4 and 6. But for the step of H-atom-transfer from C to O, the barriers for n = 4 and 6 are lower than those for n = 3 and 5. Therefore, we can conclude that BLi5 (the barrier of 22.16 kcal mol−1) and BLi6 (9.52 kcal mol−1) clusters are more feasible to the carboxylation of C–H bond of benzene through the two steps of the C–C bond formation and two H2O molecules assisting H transfer.
Footnote |
† Electronic supplementary information (ESI) available: The structures and their energies between two coordination modes and molecular coordinates of key transition states. See DOI: 10.1039/c6ra15152c |
This journal is © The Royal Society of Chemistry 2016 |