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Article

Reaction Mechanisms of CO2 Reduction to Formaldehyde Catalyzed by Hourglass Ru, Fe, and Os Complexes: A Density Functional Theory Study

by
Chunhua Dong
1,2,3,
Mingsong Ji
1,
Xinzheng Yang
1,*,
Jiannian Yao
2 and
Hui Chen
2,*
1
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
3
College of Chemistry, Chemical Engineering and Material, Handan Key Laboratory of Organic Small Molecule Materials, Handan University, Handan 056005, China
*
Authors to whom correspondence should be addressed.
Catalysts 2017, 7(1), 5; https://doi.org/10.3390/catal7010005
Submission received: 23 September 2016 / Revised: 15 December 2016 / Accepted: 21 December 2016 / Published: 27 December 2016
(This article belongs to the Special Issue Ruthenium Catalysts)
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Abstract

:
The reaction mechanisms for the reduction of carbon dioxide to formaldehyde catalyzed by bis(tricyclopentylphosphine) metal complexes, [RuH2(H2)(PCyp3)2] (1Ru), [FeH2(H2)(PCyp3)2] (1Fe) and [OsH4(PCyp3)2] (1Os), were studied computationally by using the density functional theory (DFT). 1Ru is a recently reported highly efficient catalyst for this reaction. 1Fe and 1Os are two analogues of 1Ru with the Ru atom replaced by Fe and Os, respectively. The total free energy barriers of the reactions catalyzed by 1Ru, 1Fe and 1Os are 24.2, 24.0 and 29.0 kcal/mol, respectively. With a barrier close to the experimentally observed Ru complex, the newly proposed iron complex is a potential low-cost catalyst for the reduction of carbon dioxide to formaldehyde under mild conditions. The electronic structures of intermediates and transition states in these reactions were analyzed by using the natural bond orbital theory.

Graphical Abstract

1. Introduction

As an abundant and non-toxic C1-building block, carbon dioxide can be reduced to various chemicals, such as carbon monoxide [1], methanol [2], formaldehyde [3], acetals [4,5], formic acid [5], formate [6,7], formamides [8], methylamines [8], formamidines [8], imines [3] and methane [9]. Recently, Beller and co-workers [10] reported the methylation of aromatic C–H bonds using CO2 and H2 with the assistance of a ruthenium triphos catalyst. In addition to the experimental studies, there are some theoretical studies on catalytic reduction of CO2 in recent years. Pidko [11] used a bis-N-heterocyclic carbene ruthenium CNC-pincer as catalyst and studied the mechanism of CO2 hydrogenation to formates by DFT method. Haunschild [12] reported the catalytic reduction of carbon dioxide to methanol by using (Triphos)Ru(TMM) as catalyst. Musashi and Sakaki [13] reported a theoretical study of cis-RuH2(PH3)4-catalyzed hydrogenation of CO2 into formic acid.
As mild reducing agents, boron-containing compounds have been widely used in carbon dioxide reduction [14,15]. Hazari and co-workers [16] reported the allene carboxylation with CO2 using a PSiP pincer ligand supported palladium complex as the catalyst in the hydroboration of CO2. Maron and co-workers [17,18] reported the phosphine–borane-mediated hydroboration of CO2 to methanol using an organocatalyst 1-Bcat-2-PPh2–C6H4 (Bcat = catecholboryl). Their mechanistic study showed that the simultaneous activation of both the reducing agent and CO2 plays an important role in the catalytic reaction. Bouhadir and co-workers [19] reported the hydroboration of CO2 to methoxyboranes with the ambiphilic phosphine–borane derivatives as catalysts. They also demonstrated that the formaldehyde adducts are indeed the actual catalyst in the reaction. Wegner and co-workers [20] reported a selective reduction of CO2 to methanol using Li2[1,2-C6H4(BH3)2] as the catalyst with the presence of pinacolborane. Their work shows a novel transition-metal-free mode in which the aromaticity plays an important role in the bidentate activation process.
Given the above progress, we can see that the reduction of carbon dioxide to formic acid and its derivatives has been well studied. However, as a key step in the production of methanol from CO2 and H2, the catalytic reduction of CO2 to formaldehyde is rarely reported. Huang et al. [21] studied the mechanistic details of nickel pincer-catalyzed reduction of CO2 to a methanol derivative with catecholborane (HBcat), and found that formaldehyde is an inevitable intermediate, although it was not observed in the experiment. Hazari and co-workers [22] reported a CO2 reduction reaction using a nickel η3-cyclooctenyl complex supported by tridentate PSiP pincer ligand as the precatalyst, and detected the characteristic peak (8.72 ppm) of formaldehyde in a 1H-NMR spectrum. However, the formaldehyde was mixed with miscellaneous unverifiable products without a clear yield. Hill and co-workers [23] reported a selective reductive hydroboration of CO2 to a methanol equivalent, CH3OBpin, using B(C6F5)3-activated alkaline earth compounds as catalysts. They also proposed a mechanism with the formation of formaldehyde as a byproduct. Tzschucke and co-workers [24] reported an electrocatalytic reduction of CO2 to formic acid and found an occasional formation of formaldehyde using bipyridine iridium complexes with pentamethylcyclopentadienyl ligands as the catalysts. In the above reactions, formaldehyde was only observed as byproducts with extremely low yields.
In 2014, Bontemps and co-workers [3] reported the first unambiguous detection of formaldehyde from the pinacolborane reduction of CO2 with a yield of 22% using the dihydride bis(dihydrogen) bis(tricyclopentylphosphine) hourglass ruthenium complex [RuH2(H2)2(PCyp3)2] (Ru-1cyp) as the catalyst precursor at room temperature in 24 h. Although the selectivity and yield of formaldehyde are not ideal enough, the controllable generation of formaldehyde by reducing CO2 is still a significant breakthrough. However, detailed mechanism of the above reaction, such as the structures of rate-limiting states, is still missing. In this paper, we report a density functional theory (DFT) study of the reaction mechanisms of the reduction of CO2 to formaldehyde catalyzed by [RuH2(H2)(PCyp3)2] (1Ru) and its Fe and Os analogues, [FeH2(H2)(PCyp3)2] (1Fe) and [OsH4(PCyp3)2] (1Os). The potentials of 1Fe and 1Os as catalysts for the reaction were predicted accordingly. The relations between the electronic structures and catalytic properties were analyzed using the natural bond orbital (NBO) theory [25].

2. Results and Discussion

2.1. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Ru Complex

Scheme 1 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru. Figure 1 shows the corresponding free energy profile. Figure 2 shows the optimized structures of transition states in the catalytic cycle.
At the beginning of the reaction, the dissociation of H2 from Ru-1cyp forms the 10.5 kcal/mol less stable catalyst [RuH2(H2)(PCyp3)2] (1Ru), which could attract a HBpin molecule and form a 15.8 kcal/mol more stable intermediate 1Ru-HBpin. The exchange of CO2 with HBpin in 1Ru-HBpin for the formation of 2Ru is 14.8 kcal/mol uphill. Next, 3Ru is formed through transition state TS2,3-Ru with a 5.5 kcal/mol barrier for the transfer of a hydride from Ru to the carbon in CO2. The newly formed formate group in 3Ru could easily rearrange through TS3,4-Ru and TS4,5-Ru for the formation of a more stable intermediate 5Ru. Then, a HBpin molecule attacks 5Ru and forms a 2.6 kcal/mol less stable intermediate 6Ru with a weak interaction between O and B. Next, 7Ru is formed quickly through TS6,7-Ru with the stretching of the B–H bond and the formation of the O–B and Ru–H bonds. A pinBOCHO molecule is formed with the cleavage of the Ru–O bond through transition state TS7,8-Ru, which is 12.2 kcal/mol higher than 7Ru in free energy. The dissociation of pinBOCHO from 8Ru for the regeneration of 1Ru is 1.1 kcal/mol downhill. Then the dissociated pinBOCHO molecule can come back to 1Ru and take a hydride from Ru to its carbonyl carbon for the formation of 10Ru (ΔG° = −8.3 kcal/mol). After a structural relaxation, 10Ru transforms to a 5.9 kcal/mol more stable intermediate 11Ru. Then a formaldehyde molecule forms with the formation of the Ru–OBpin bond and the breaking of the Ru–OCH2OBpin and C–OBpin bonds through transition state TS11,12-Ru, which is 3.8 kcal/mol higher than 11Ru in free energy. The release of formaldehyde from 12Ru for the formation of 13Ru is 3.0 kcal/mol downhill. Next, another HBpin molecule approaches 13Ru and forms a much more stable intermediate 15Ru with the formation of the B–O bond through TS14,15-Ru. The Ru–O and H–B bonds in 15Ru could break easily with the formation of a Ru–H bond through TS15,16-Ru, which is only 4.5 kcal/mol higher than 15Ru in free energy. Then the dissociation of pinBOBpin from 16Ru regenerates 1Ru and completes the catalytic cycle. It is worth noting that HBpin can also attack 10Ru and form a stable acetal compound (pinBO)2CH2, which was observed in the experiment.
By comparing all relative free energies shown in Figure 1, we can conclude that 1Ru-HBpin and TS9,10-Ru are the rate-determining states of the catalytic reaction with a total free energy barrier of 24.2 kcal/mol, which agrees well with the observed experimental reaction rate.
Compared with the mechanism proposed by Huang et al. for the reduction reaction of CO2 to a methanol derivative with catecholborane (HBcat) catalyzed by a pincer nickel complex [21], the catalytic reduction of carbon dioxide to formaldehyde reactions catalyzed by hourglass ruthenium complex and pincer nickel complex have similar but not exactly the same pathways. There are two oxygen atoms as possible bonding sites for HBpin in the nickel formate. However, there is only one oxygen atom as the bonding site for HBpin in the ruthenium formate (5Ru to 6Ru in Scheme 1). Our calculations indicate that the six-membered ring structure is less likely to be formed in the hourglass ruthenium complexes.

2.2. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Fe Complex

Scheme 2 shows the whole catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by the newly proposed Fe complex 1Fe. Figure 3 and Figure 4 show the corresponding free energy profile and optimized structures of key transition states, respectively. The reactions catalyzed by 1Fe and 1Ru have similar pathways but different rate-determining states.
Similar to the reaction catalyzed by 1Ru, a −22.0 kcal/mol more stable intermediate 1Fe-HBpin is formed at the beginning of the reaction catalyzed by 1Fe. The exchange of CO2 with HBpin in 1Fe-HBpin and the formation of 2Fe is 16.4 kcal/mol uphill. Next, 3Fe is formed quickly through transition state TS2,3-Fe for the formation of C–H bond and the rearrangement of hydrogen atoms. The rearrangement of the newly formed formate group and hydrogen atoms in 3Fe forms a slightly more stable intermediate 4Fe. After a structural relaxation, 4Fe transforms to a 2.0 kcal/mol more stable intermediate 5Fe, which attracts a HBpin molecule for the formation of a 3.9 kcal/mol more stable intermediate 6Fe with strong Fe–H and O–B interactions. Then, 7Fe is formed through a transition state TS6,7-Fe with a free energy barrier of 20.8 kcal/mol with the breaking of the Fe–O and B–H bonds and the formation of the O–B bond. 7Fe is 12.3 kcal/mol less stable than 6Fe with a weak interaction between H and B. The dissociation of pinBOCHO from 7Fe for the regeneration of 1Fe is 3.6 kcal/mol downhill. Next, the dissociated pinBOCHO molecule comes back to 1Fe and forms a Fe–O bond for the formation of a 7.3 kcal/mol more stable intermediate 8Fe. After a structural relaxation, 8Fe transforms to a 3.1 kcal/mol more stable intermediate 9Fe. Then a formaldehyde molecule is formed with the formation of the Fe–OBpin bond and the breaking of the Fe–OCH2OBpin and C–OBpin bonds through transition state TS9,10-Fe. The dissociation of formaldehyde from 10Fe leaves an 8.6 kcal/mol more stable intermediate 11Fe. Then, another HBpin molecule approaches 11Fe and forms a 10.9 kcal/mol more stable intermediate 12Fe with the formation of a B–O bond. The dissociation of newly formed pinBOBpin molecule from 12Fe for the regeneration of the catalyst 1Fe is 9.5 kcal/mol uphill.
By comparing all relative free energies shown in Figure 3, we can conclude that 1Fe-HBpin and TS6,7-Fe are the rate-determining states of the catalytic reaction with a total free energy barrier of 24.0 kcal/mol, which is 0.2 kcal/mol lower than the free energy barrier of the reaction catalyzed by 1Ru. Therefore, 1Fe is a potential low-cost and high efficiency catalyst for the reduction of CO2 to formaldehyde.
Sabo-Etienne and Bontemps [26] recently reported the catalytic reduction of CO2 to bis(boryl)acetal and methoxyborane, with Fe(H)2(dmpe)2 used as catalyst precursor, and pinacolborane used as the reductant. For 1Ru, only one oxygen atom of CO2 is abstracted, and CO2 transforms into HCHO. For Fe(H)2(dmpe)2, one oxygen atom of CO2 is abstracted at the reduction step, then the second oxygen atom is removed and CO2 is transformed to methylene. This illustrates that iron complexes can efficiently catalyze the reduction of CO2.

2.3. The Mechanism for the Reduction of Carbon Dioxide to Formaldehyde Catalyzed by the Os Complex

The reaction catalyzed by 1Os also has similar pathways to the reaction catalyzed by 1Ru (see Scheme S1 on catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Os). Figure 5 shows the calculated relative free energies in the reaction catalyzed by 1Os. Figure 6 shows the optimized structures of Os-1cyp, 1Os, 4Os, TS7,8-Os and 8Os, in which the distances between the two hydrogen atoms (marked in Figure 6) are longer than those in corresponding Ru complexes. As shown in Figure 5, the rate-determining states in the reaction catalyzed by the osmium complex are 1Os-HBpin and TS2,3-Os with a free energy barrier of 29.0 kcal/mol, which is 4.8 kcal/mol higher than the free energy barrier of the reaction catalyzed by 1Ru.Therefore, 1Os could also catalyze the reduction of carbon dioxide to formaldehyde under harsher conditions.

2.4. Structures and Reactivity

According to our mechanistic study, the catalytic reaction of 2HBpin + CO2 → HCHO + pinBOBpin has five stages, CO2 insertion (S1), σ-Bond metathesis (S2), pinBOCHO insertion (S3), HCHO elimination (S4) and σ-Bond metathesis (S5).
(S1) CO2 Insertion. The reaction in S1 is [M]–H + CO2 → [M]–OOCH. The most significant structural change in S1 is the cleavage of M–H bond. Table 1 lists the natural population analysis (NPA) charges of metal atoms (e), the Wiberg bond indices (WBIs) of the bonds, and the relative free energies between intermediates and transition states/intermediates (ΔG‡/ΔG°) in stages. We can see that 2Fe has the most negative NPA charge, the smallest WBI of the M–H bond and the lowest barrier for the cleavage of M–H bond among these three intermediates in S1.
(S2) σ-Bond Metathesis. The reaction in S2 is [M]–OOCH + HBpin → [M]–H + pinBOCHO. The most important structural change in S2 is the cleavage of M–O bond. We can see TS7,8-Os has the least NPA charge, the smallest WBI of the M–O bond and the lowest barrier for the cleavage of M–O bond among these three transition states in S2.
(S3) pinBOCHO Insertion. The reaction in S3 is [M]–H + pinBOCHO → [M]–O–CH2–OBpin. The most important structural change in S3 is the M–H bond cleavage. 9Ru has a more negative NPA charge, a smaller WBI of the M–H bond and a lower barrier for the cleavage of M–H bond than 9Os in S3. The Fe complexes cannot be compared with the Ru and Os complexes because they have a different pathway in S3. Scheme 1 shows that 1Ru and pinBOCHO first generate the intermediate 9Ru, then form the intermediate 10Ru through TS9,10-Ru, in which a Ru–O bond is formed. However, as shown in Scheme 2, 1Fe combines directly with pinBOCHO and transforms to intermediate 8Fe, in which an Fe–O bond is formed.
(S4) HCHO Elimination. The reaction in S4 is [M]–O–CH2–OBpin → HCHO + [M]–OBpin. The most important structural change in S4 is the cleavage of the M–O bond. TS9,10-Fe has the lowest barrier and the smallest WBI of the M–O bond among these three transition states in S4. There is no obvious relation between the energy barriers and the NPA charges of metal atoms in S4.
(S5) σ-Bond Metathesis. The reaction in S5 is [M]–OBpin + HBpin → [M]–H + pinBOBpin. The most important structural change in S5 is the cleavage of the H–B bond. 15Os has a smaller WBI of the H–B bond and a lower barrier for the cleavage of H–B bond than 15Ru in S5. There is no obvious relation between the energy barriers and the NPA charges of metal atoms in S5. The Fe complexes cannot be compared with the Ru and Os complexes because they have a different pathway in S5.
Overall, the above comparison of the important elementary steps of the reactions catalyzed by different metal complexes indicate that the smaller corresponding WBIs of bonds, and the lower energy barriers. We believe the reaction energy barriers are influenced by many factors. Due to the structural complexity of the transition metal complexes, the reaction energy barrier is not only decided by the electronic effect, but also relates to the sizes of metals, the binding strengths between metals and ligands, and the steric effects.

3. Computational Details

All DFT calculations were performed using the Gaussian 09 suite of programs [27] for the ωB97X-D functional [28,29] with the Stuttgart relativistic effective core potential and associated valence basis sets for Ru (ECP28MWB, (8s7p6d2f1g)/[6s5p3d2f1g]) and Os (ECP60MWB, (8s7p6d2f1g)/[6s5p3d2f1g]) [30,31], all-electron 6-31++G(d,p) basis set for Fe, the atoms coordinated to metal atoms and the atoms in the reactant, and the 6-31G(d) basis set for all other atoms [32,33,34]. The ωB97X-D functional was also used in the previous theoretical modeling of hydrogenation of carbon dioxide by Fe complex [35] and the hydrogenation of dimethyl carbonate by Ru and Fe complexes [36]. The calculation results in those studies are in agreement with the experimental observations. In addition, high-level ab initio coupled cluster calibration study shows that ωB97X-D performs well in barrier calculation of σ-bond activation promoted by Ru [37] and Fe [38] complexes. All structures reported in this paper were fully optimized with solvent effect corrections using the cavity-dispersion-solvent-structure terms in Truhlar and co-workers’ SMD solvation model for benzene (ε = 2.2706) [39]. Tables and an xyz file giving solvent effect corrected absolute free energies, electronic energies and atomic coordinates of all optimized structures are given in Supporting Information. The thermal corrections were calculated at 298.15 K and 1 atm pressure with harmonic approximation. An ultrafine integration grid (99, 590) was used for numerical integrations. The ground states of intermediates and transition states were confirmed as singlets through a comparison with the optimized high-spin analogues. Calculating the harmonic vibrational frequencies for optimized structures and noting the number of imaginary frequencies (IFs) confirmed the nature of all intermediates (no IF) and transition state structures (only one IF for each transition state). The latter were also confirmed to connect reactants and products by intrinsic reaction coordinate (IRC) calculations. The 3D molecular structure figures displayed in this paper were drawn by using the JIMP2 molecular visualizing and manipulating program [40]. The NPA charges [41] and Wiberg indices [42] were obtained by using the NBO 3.1 program.

4. Conclusions

In summary, the mechanistic insights of the reduction of carbon dioxide to formaldehyde catalyzed by Ru, Fe and Os complexes are investigated by using DFT. The formation of C–H and Ru–O bonds (TS9,10-Ru), the cleavage of Fe–O bond and the formation of O–B bond (TS6,7-Fe), and the formation of C–H bond (TS2,3-Os) are the rate-determining steps in the reactions catalyzed by the Ru, Fe, and Os complexes with total free energy barriers of 24.2 (1Ru-HBpin → TS9,10-Ru), 24.0 (1Fe-HBpin → TS6,7-Fe) and 29.0 (1Os-HBpin → TS2,3-Os) kcal/mol, respectively. Such barriers indicate that 1Fe is a potential low-cost catalyst for the reduction of carbon dioxide to formaldehyde under mild conditions. With all of our computational studies, we expect to predict the effects of different metals on the reaction and provide useful information for the development of base metal complexes for the conversion and utilization of carbon dioxide.

Supplementary Materials

Additional Supporting Information (Tables and an xyz file giving solvent effect corrected absolute free energies, electronic energies and atomic coordinates of all optimized structures; Scheme on catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Os) are available online at www.mdpi.com/2073-4344/7/1/5/s1.

Acknowledgments

X.Y. acknowledges financial support from the 100-Talent Program of the Chinese Academy of Sciences (CAS), and the National Natural Science Foundation of China (NSFC, 21373228, 21673250). H.C. is supported by the NSFC (21290194, 21473215). C.D. is grateful for the financial support from Natural Science Program of Handan University (16217).

Author Contributions

Xinzheng Yang and Chunhua Dong conceived and designed the computations; Chunhua Dong performed the computations; Chunhua Dong, Mingsong Ji, Hui Chen and Jiannian Yao analyzed the data; Chunhua Dong and Xinzheng Yang wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru.
Figure 1. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru.
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Figure 2. Optimized structures of TS2,3-Ru (513i cm−1), TS3,4-Ru (170i cm−1), TS4,5-Ru (176i cm−1), TS6,7-Ru (127i cm−1), TS7,8-Ru (209i cm−1),TS9,10-Ru (363i cm−1), TS11,12-Ru (159i cm−1), TS14,15-Ru (67i cm−1) and TS15,16-Ru (97i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity.
Figure 2. Optimized structures of TS2,3-Ru (513i cm−1), TS3,4-Ru (170i cm−1), TS4,5-Ru (176i cm−1), TS6,7-Ru (127i cm−1), TS7,8-Ru (209i cm−1),TS9,10-Ru (363i cm−1), TS11,12-Ru (159i cm−1), TS14,15-Ru (67i cm−1) and TS15,16-Ru (97i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity.
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Catalysts 07 00005 sch001 550
Scheme 1. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru.
Scheme 1. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Ru.
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Catalysts 07 00005 sch002 550
Scheme 2. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe.
Scheme 2. Catalytic cycle for the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe.
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Figure 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe.
Figure 3. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Fe.
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Figure 4. Optimized structures of TS2,3-Fe (460i cm−1), TS3,4-Fe (78i cm−1), TS6,7-Fe (191i cm−1) and TS9,10-Fe (86i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity.
Figure 4. Optimized structures of TS2,3-Fe (460i cm−1), TS3,4-Fe (78i cm−1), TS6,7-Fe (191i cm−1) and TS9,10-Fe (86i cm−1). Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity.
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Figure 5. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Os.
Figure 5. Free energies profile of the reduction of carbon dioxide to formaldehyde catalyzed by 1Os.
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Figure 6. Optimized structures of Os-1cyp, 1Os, 4Os, TS7,8-Os (239i cm−1) and 8Os. Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity.
Figure 6. Optimized structures of Os-1cyp, 1Os, 4Os, TS7,8-Os (239i cm−1) and 8Os. Bond lengths are in Å. The cyclopentyl and pinacol groups are not shown for clarity.
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Table
Table 1. The natural population analysis (NPA) charges of metal atoms (e), the Wiberg bond indices (WBIs) of the bonds, the relative free energies between intermediates and transition states/intermediates (ΔG‡/ΔG°).
Table 1. The natural population analysis (NPA) charges of metal atoms (e), the Wiberg bond indices (WBIs) of the bonds, the relative free energies between intermediates and transition states/intermediates (ΔG‡/ΔG°).
StagesComplexeseBondWBIΔG‡/ΔG° (kcal/mol)
Stage 12Fe−1.892M–H bond0.58760.4
TS2,3-Fe−1.7820.5420
2Ru−1.5380.62735.5
TS2,3-Ru−1.4390.4674
2Os−1.5400.68879.3
TS2,3-Os−1.5180.5094
Stage 26Fe−1.209M–O bond0.351720.8
TS6,7-Fe−0.9740.0287
7Ru−1.4110.270412.2
TS7,8-Ru−1.1040.0275
7Os−1.4270.299510.0
TS7,8-Os−1.1110.0271
Stage 31Fe−1.280M–H bond0.7154−7.3
8Fe−1.2790.2182
9Ru−1.8670.66610.5
TS9,10-Ru−1.8500.5757
9Os−1.8320.72061.0
TS9,10-Os−1.8080.6002
Stage 49Fe−0.890M–O bond0.43543.1
TS9,10-Fe−0.8400.2609
11Ru−1.1900.25833.8
TS11,12-Ru−1.1840.2616
11Os−1.1770.40494.7
TS11,12-Os−1.1930.3061
Stage 511Fe−0.778//−10.9
12Fe−1.290H–B bond0.5236
15Ru−1.4780.53254.5
TS15,16-Ru−1.7950.0863
15Os−1.4830.48983.7
TS15,16-Os−1.7420.0958

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Dong, C.; Ji, M.; Yang, X.; Yao, J.; Chen, H. Reaction Mechanisms of CO2 Reduction to Formaldehyde Catalyzed by Hourglass Ru, Fe, and Os Complexes: A Density Functional Theory Study. Catalysts 2017, 7, 5. https://doi.org/10.3390/catal7010005

AMA Style

Dong C, Ji M, Yang X, Yao J, Chen H. Reaction Mechanisms of CO2 Reduction to Formaldehyde Catalyzed by Hourglass Ru, Fe, and Os Complexes: A Density Functional Theory Study. Catalysts. 2017; 7(1):5. https://doi.org/10.3390/catal7010005

Chicago/Turabian Style

Dong, Chunhua, Mingsong Ji, Xinzheng Yang, Jiannian Yao, and Hui Chen. 2017. "Reaction Mechanisms of CO2 Reduction to Formaldehyde Catalyzed by Hourglass Ru, Fe, and Os Complexes: A Density Functional Theory Study" Catalysts 7, no. 1: 5. https://doi.org/10.3390/catal7010005

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