Article pubs.acs.org/IC
Computational Design of Cobalt Catalysts for Hydrogenation of Carbon Dioxide and Dehydrogenation of Formic Acid Hongyu Ge,†,‡ Yuanyuan Jing,† and Xinzheng Yang*,† †
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, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: A series of cobalt complexes with acylmethylpyridinol and aliphatic PNP pincer ligands are proposed based on the active site structure of [Fe]-hydrogenase. Density functional theory calculations indicate that the total free energy barriers of the hydrogenation of CO2 and dehydrogenation of formic acid catalyzed by these Co complexes are as low as 23.1 kcal/mol in water. The acylmethylpyridinol ligand plays a significant role in the cleavage of H2 by forming a strong Co−Hδ−···Hδ+−O dihydrogen bond in a fashion of frustrated Lewis pairs.
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TON values of 156 h−1 and 788, respectively, for hydrogenation of CO2. Yang7f,g computationally predicted similar iron and cobalt pincer complexes for hydrogenation of CO2. Gonsalvi and co-worker7h reported two PNP pincer iron complexes with quantitative yields and TONs near 1000 at 80 °C. The most active base metal catalysts so far for hydrogenation of CO2 are a series of iron PNP complexes reported by Bernskoetter and co-workers.7i They have reached a TOF near 200 000 h−1 at 80 °C and 69 atm with the help of Lewis acid cocatalysts. We can see that most of those reported base metal catalysts either require rigorous reaction conditions or have rather low catalytic activities. Therefore, the development of high efficiency base metal catalysts for hydrogenation of CO2 and dehydrogenation of formic acid under mild reaction conditions is highly desirable and remains a challenge. In our recent study, we have computationally designed a series of PNP iron pincer complexes based on the active site structure of [Fe]-hydrogenase for reversible base free hydrogenation of CO2 and acceptorless dehydrogenation of formic acid.7g However, the computationally predicted lowest free energy barrier is 23.9 kcal/mol in THF, which is slightly too high for a reaction at the room temperature. In order to find out more efficient catalysts, here we proposed a series of cobalt complexes based on the active site structure of [Fe]hydrogenase and computationally examined their catalytic activities using the density functional theory (DFT). Our calculation results indicate that the newly designed Co complexes are promising to catalyze base free hydrogenation of CO2 and acceptorless dehydrogenation of formic acid.
INTRODUCTION Carbon dioxide is one of the primary greenhouse gases in Earth’s atmosphere and has a close relation to the global climate change. The conversion and utilization of CO2 as an abundant, inexpensive, and nontoxic carbon source for the synthesis of valuable chemicals has attracted increasing attention in recent years.1 An ideal way for the conversion of CO2 is the catalytic hydrogenation of CO2 to methanol,2 which usually contains three cascade catalytic cycles, the hydrogenation of CO2 to formic acid, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol. In addition to its potential application in hydrogen storage, formic acid is also widely used in synthetic chemistry and industrial processes.3 In 1970, Haynes et al. reported the first catalytic hydrogenation of CO2 reaction.4 Since then, steady progresses have been achieved in homogeneously catalytic hydrogenation of CO2 using noble transition metals, such as rhodium, ruthenium, platinum, and iridium.3c,5 Recently, Pidko and co-workers6 developed a series of PNP pincer ruthenium complexes for hydrogenation of CO2 and achieved unprecedented turnover frequencies (TOFs) near 1 100 000 h−1 at 120 °C and 30 bar of H2. In contrast to noble metals, only a few base metal catalysts are reported so far for the hydrogenation of CO2.7 In these base metal catalysts, the PNP pincer complexes are shown high activities. Beller and co-workers7a−c reported a series of tetraphos iron complexes and achieved a TOF value of 255 h−1 at 100 °C and 100 atm pressure. Then, they synthesized a similar tetraphos cobalt complex7d and achieved a TON of 3877 at 120 °C. Milstein and co-workers7e reported an aromatic iron PNP pincer complex, which achieved TOF and © XXXX American Chemical Society
Received: July 21, 2016
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DOI: 10.1021/acs.inorgchem.6b01723 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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COMPUTATIONAL DETAILS
All DFT calculations in this study were performed by using the Gaussian 09 suite of programs8 for the M06 functional9 with the allelectron 6-31++G(d,p) basis set10 for all atoms. All structures studied in this paper were fully optimized in water (ε = 78.3553) by using the integral equation formalism polarizable continuum model (IEFPCM)11 with SMD12 atomic radii solvent effect corrections. An ultrafine integration grid (99,590) was used for numerical integrations. The ground states of intermediates and transition states were confirmed as singlets through comparison with the optimized highspin analogues. Thermal corrections were calculated within the harmonic potential approximation on optimized structures under T = 298.15 K and 1 atm pressure. Unless otherwise noted, the energies reported in the text are Gibbs free energies with solvent effect corrections. Calculating the harmonic vibrational frequencies for optimized structures and noting the number of imaginary frequency (IF) confirmed the nature of intermediate (no IF) and transition state (only one IF) structures. 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.13
Figure 2. Optimized structures of 1Co and 1Co′. Bond lengths are in angstroms. Isopropyl groups are omitted for clarity.
difference between 1Co and 1Co′ is the directions of the N2−H1 and Co−C1 bonds. These two bonds are at the same and opposite sides of the Co−P−N2 plane in 1Co and 1Co′, respectively. Calculation results indicate that 1Co is 3.3 kcal/mol more stable than 1Co′. Therefore, 1Co was adopted as the catalyst for further study. Scheme 1 is the catalytic cycle for the hydrogenation of CO2 catalyzed by 1Co. Figure 3 shows the corresponding free energy
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RESULTS AND DISCUSSION Figure 1 shows the crystal structure of the active site of [Fe]hydrogenase (A),14 recently reported PNP pincer Fe catalysts
Scheme 1. Mechanism of the Hydrogenation of CO2 Catalyzed by 1Co
Figure 1. (A) Observed crystal structure of the active site of [Fe]hydrogenase. (B) Recently reported aliphatic PNP iron catalysts. (C) Computationally designed iron complexes for hydrogenation of CO2. (D) Newly constructed cobalt complexes.
for hydrogenation and dehydrogenation reactions (C),7g and the newly constructed cobalt complexes (D). Similar to our previously proposed iron complexes, the newly designed cobalt complexes are also built with acylmethylpyridinol and aliphatic PNP pincer ligands, but are monocations. The ortho oxygen atom in the acylmethylpyridinol ligand could assist the cleavage of H2 by forming a Co−Hδ−···Hδ+−O dihydrogen bond.15 The aliphatic PNP pincer ligand could increase the electron density at the metal center and stabilize the complex. The vacant position in the five-coordinated cobalt complex could be filled by a H2 molecule for further reaction. More importantly, the synthetic routes of the acylmethylpyridinol and PNP pincer ligands are mature.16 In order to find out the catalytic activities of the newly proposed cobalt complexes, we have examined the mechanism of the hydrogenation of CO2 catalyzed by the simplest complex 1Co (R1 = R2 = H, R3 = N−H). Figure 2 shows the optimized structures of 1Co and its isomer 1Co′. The primary structural
Figure 3. Free energy profile for the hydrogenation of CO2 catalyzed by 1Co in water under T = 298.15 K and 1 atm pressure. B
DOI: 10.1021/acs.inorgchem.6b01723 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
release of HCOOH from 5 for the regeneration of the catalyst 1Co is 5.3 kcal/mol downhill. It is worth to note that the formation of HCOOH from H2 and CO2 is about 1 kcal/mol exothermic in water.5c Although additional base is usually required to drive the hydrogenation of CO2 in organic solvent, the formation of HCOOH might be base-free in water, especially when the acylmethylpyridinol ligand is acting as the external base for the cleavage of H2. According to the principle of microscopic reversibility, our newly designed catalyst could act as an efficient catalyst for the dehydrogenation of formic acid in organic solvents with similar barriers. In addition to the formation of 5, the formate group in 4 could rotate easily and bond to Co with one of its oxygen atoms through transition state TS4,4′ (Figure 4) for the formation of a much more stable intermediate 4′ (Figure 4). TS4,4′ is a 7-centered transition state with Co−P distances of 2.357 Å. Because of the strong Fe−O interaction (2.077 Å), 4′ is 10.4 and 3.3 kcal/mol more stable than 4 and 1Co, respectively. Therefore, 4′ is the resting state of the catalytic reaction. The reaction from 4′ back to 4 is still easy because TS4,4′ is only 12.7 kcal/mol higher than 4′. The total free energy barrier of this reaction is 23.7 kcal/mol (4′ → TS3,4), which is accessible under mild conditions. Since most of the experimental CO2 hydrogenation reactions took place in organic solvents, we also calculated the relative free energies of 4′ and TS3,4 using THF as the solvent. The free energy barriers of the reactions catalyzed by 1Fe, a previously proposed neutral analogue of 1Co, are also calculated. The relative free energies of 4′ and TS3,4 in water and THF are listed in Table 1. We can see that the free energy barriers are
profile. Figure 4 shows the optimized structures of key intermediates and transition states in the reaction. At the
Table 1. Influence of Solvents on the Free Energy Barrier ΔG (kcal/mol)
Figure 4. Optimized structures of 2, 3, TS3,4 (757.84i cm−1), TS4,5 (623.94i cm−1), TS4,4′ (86.00i cm−1), and 4′. Bond lengths are in angstroms. Isopropyl groups are omitted for clarity.
beginning of the reaction, a H2 molecule approaches 1Co and forms a slightly less stable intermediate 2 through transition state TS1,2. The barrier for H2 cleavage is only 3.6 kcal/mol (2 → TS2,3) with the assistance of the ortho oxygen atom of the acylmethylpyridinol ligand in a fashion of frustrated Lewis pairs (FLPs). The distance between Hδ− and Hδ+ in 3 is only 1.476 Å, which is much shorter than the H···H distance range of 1.7− 2.2 Å in most metal dihydrogen bonds reported so far.17 The Co−Hδ−···Hδ+ and Hδ−···Hδ+−O angles in 3 are 104.7° and 166.7°, respectively, which are consistent with the typical M−H σ-bond interactions in most of the observed metal dihydrogen bonds. Similar strong metal dihydrogen bonds were reported recently both in theory and in experiment.15,17,18After the cleavage of H2, a CO2 molecule approaches 3 and forms a formate anion by taking the hydride directly from Co through transition state TS3,4, which is 20.4 kcal/mol higher than 1Co in free energy. Similar to our previous studies of the CO2 hydrogenation reactions catalyzed by PNP pincer complexes,7f,g this CO2 insertion process is the rate-determining step in the whole reaction. The cleavage of H2 has a rather low barrier because of the FLP nature of the metal center and the acylmethylpyridinol ligand. After the formation of 4, the hydroxyl proton in acylmethylpyridinol transfers to one of the oxygen atoms in the newly formed formate anion through transition state TS4,5 and forms a formic acid molecule. The
ΔΔG (kcal/mol)
catalyst
solv.
4′Co/Fe
TS3,4‑Co/Fe
4′Co/Fe → TS3,4‑Co/Fe
1Co 1Fe 1Co 1Fe
water water THF THF
−3.3 −8.8 −2.8 −7.1
20.4 20.6 18.1 18.1
23.7 29.4 20.9 25.2
influenced by solvent significantly. 1Co has much lower free energy barriers than 1Fe both in water and in THF. TS3,4‑Co is only 20.9 kcal/mol higher than 4′Co in THF. Such a low barrier indicates that 1Co is a highly active catalyst under mild conditions. Aiming at the design of cobalt complexes with higher catalytic activities, we have investigated the influence of substituents in the acylmethylpyridinol ligand to the total free energy barrier of the reaction. As listed in Table 2, 21 cobalt complexes are constructed by replacing the hydrogen atoms at the meta (R1) and pare (R2) positions of the acylmethylpyridinol ligand using methyl, hydroxyl, and other functional groups. Since 4′ and TS3,4 are the resting states of the reaction, we calculated the free energies of the analogues of 4′ and TS3,4 relative to the corresponding analogues of 1, and the free energy differences between the analogues 4′ (or 1) and TS3,4. In addition, we also examined the cobalt complex with a PCP pincer ligand (1t, R1 = R2 = H, R3 = C−H). 1t is a neutral complex with a free energy barrier of 26.6 kcal/mol for hydrogenation of CO2. Among all cobalt complexes listed in Table 2, 1g (R1 = Me, R2 = OH, R3 = N−H) has the lowest free energy barrier of 23.1 kcal/mol, which is 0.6 kcal/mol lower than that of 1Co and 6.3 kcal/mol lower than 1Fe in water. We C
DOI: 10.1021/acs.inorgchem.6b01723 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Table 2. Influence of Substituents on the Total Free Energy Barriers
1Co 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m 1n 1o 1p 1q 1r 1s 1t 1u
R1
R2
R3
4′
TS3,4
total barriers
H H H H Me Me Me Me H H H H H Cl F NH2 NO2 CN OH H H
H Me OH OMe H Me OH OMe Cl F NH2 NO2 CN H H H H H H H H
N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H N−H C−H N−CH3
−3.3 −3.2 −2.7 −3.6 −3.1 −6.0 −3.6 −5.9 −0.9 −0.8 −4.7 −0.8 −0.1 1.3 0.2 −3.4 8.9 6.2 −2.9 −5.0 2.4
20.4 21.1 21.6 22.2 20.1 19.0 19.5 18.2 23.0 23.5 19.1 25.8 24.2 25.2 24.6 20.0 39.2 33.4 21.0 21.6 28.2
23.7 24.3 24.3 25.8 23.2 25.0 23.1 24.1 23.9 24.3 23.8 26.6 24.3 25.2a 24.6a 23.4 39.2a 33.4a 23.9 26.6 28.2a
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01723 Evaluation of density functionals and atomic coordinates of all optimized structures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the 100-Talent Program of the Chinese Academy of Sciences and the National Natural Science Foundation of China (21373228, 21673250).
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REFERENCES
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a
The total barriers calculated as the relative free energies between 1 and TS3,4 because 1 is more stable than 4′ in those reactions.
can see that the free energy barriers are influenced by the electron-donating ability of the substituents significantly. Generally, the catalysts with electron-donating groups have higher activities. Therefore, an appropriate adjustment of the functional groups on the pyridinol ligand could greatly improve the catalytic activity.
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ASSOCIATED CONTENT
S Supporting Information *
ΔG (kcal/mol)
catalyst
Article
CONCLUSIONS
In summary, a series of cobalt monocationic complexes with acylmethylpyridinol and PNP ligands are proposed and examined computationally by using the DFT method. Calculation results indicate that the novel designed cobalt complexes are promising to catalyze hydrogenation of carbon dioxide and dehydrogenation of formic acid. The calculated total free energy barrier of the reaction catalyzed by 1Co in water is 23.7 kcal/mol, which is 5.7 kcal/mol lower than the reaction catalyzed by 1Fe. Among all cobalt complexes we examined, 1g is the most active one with a free energy barrier of 23.1 kcal/mol in water. With THF as the solvent, the free energy barrier of the reaction catalyzed by 1Co is only 20.9 kcal/ mol. The acylmethylpyridinol ligand plays a significant role in the cleavage of H2 by forming a Co−Hδ−···Hδ+−O dihydrogen bond in a fashion of FLP. Our computational design and mechanistic study not only provide promising catalysts for lowcost and high efficiency hydrogenation of CO 2 and dehydrogenation of formic acid but also reveal the importance of metal dihydrogen bonds in hydrogen activation. Further design of non-noble metal catalysts based on the active site structures of [FeFe]- and [NiFe]-hydrogenases are underway. D
DOI: 10.1021/acs.inorgchem.6b01723 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01723 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01723 Inorg. Chem. XXXX, XXX, XXX−XXX
