Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Mechanistic Insights into Iridium Catalyzed Disproportionation of Formic Acid to CO2 and Methanol: A DFT Study Xiuli Yan†,‡ and Xinzheng Yang*,†,‡ †
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, CAS Research/Education Center for Excellence in Molecular Sciences, 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: The disproportionation of formic acid to methanol catalyzed by a half-sandwich iridium complex, [Cp*Ir(bpy-Me)OH2]2+, was computationally investigated by using density functional theory. A newly proposed mechanism features three interrelated catalytic cycles, the dehydrogenation of formic acid to CO2 and H2, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol. Methanol assisted proton transfer and direct C−O bond cleavage after hydroxyl deprotonation in two competitive pathways for the formation of formaldehyde are the rate-determining steps in the whole catalytic reaction. Calculation results indicate that the formation of formaldehyde from methanediol through direct cleavage of a C−O bond after hydroxyl deprotonation has a free energy barrier of 25.9 kcal/mol, which is 1.9 kcal/mol more favorable than methanol assisted proton transfer.
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INTRODUCTION The conception of ‘‘hydrogen economy” provides a promising solution to the energy crisis with hydrogen as an efficient, clean, and renewable energy carrier.1 However, the safe storage and transportation of H2 is still a big challenge in the development of hydrogen economy.2 In recent years, methanol has attracted considerable attention as a potential hydrogen storage candidate because of its high hydrogen content (12.6 wt %) and easy transportation.3 It can also be used as a feedstock for the production of formaldehyde, dimethyl ether, acetic acid, and synthetic fuels.4 An ideal way for methanol production is the direct hydrogenation of CO2, but it usually has rather low efficiency even under rigorous reaction conditions (200−300 °C and 50−100 atm).3d,5 The disproportionation of formic acid (Scheme 1) to methanol is an attractive alternative because of the easy formation of formic acid from CO2. In this reaction, formic acid not only acts as the hydrogen provider for the releasing CO2 but also accepts hydrogen for the production of methanol. Although steady progress has been achieved in the development of low-cost and high efficiency catalysts for the dehydrogenation of formic acid (Scheme 1),2a,d,6 only a
few catalysts have been reported so far for the disproportionation of formic acid. Therefore, the development of homogeneous catalysts for this promising transformation is still highly challenging. In 2013, Goldberg and co-workers7 reported the disproportionation of formic acid to methanol, water, and carbon dioxide catalyzed by a half-sandwich iridium complex [Cp*Ir(bpy)OH2][OTf]2 (Cp* = pentamethylcyclopentadienide, bpy = 2,2′-bipyridine) for the first time. They achieved a turnover number (TON) of 156 and a turnover frequency (TOF) of 6.5 h−1 at 80 °C. However, only a rather low MeOH selectivity of 12% was obtained. Soon after that, Cantat and co-workers8 significantly improved the methanol yield of formic acid disproportionation to 50.2% using a ruthenium catalyst, the precursor [Ru(COD)(methylallyl)2], an additional triphos (CH3C(CH2PPh2)3) ligand, and MSA (methanesulfonic acid). They achieved the highest selectivity for FA disproportionation to MeOH in a THF solution at 150 °C for 1 h. However, both of these reported catalysts contain precious metals, such as Ir and Ru. As such, the design of high efficiency and low-cost base metal catalysts for this transformation is highly desirable. In 2015, Neary and Parkin9 reported a molybdenum hydride complex, CpMo(CO)3H, for disproportionation of formic acid and achieved a selectivity of 21% in benzene at 100 °C. In 2016, Kubiak and co-workers10 prepared and evaluated a series of [Cp*IrIII (R-bpy)Cl]Cl (R-bpy = 4,4′-di-R-2,2′-bipyridine; R = CF3, H, Me, tBu, OMe) complexes for catalytic formic acid disproportionation. They found the most proficient complex in terms
Scheme 1. Deydrogenation (1), Hydrogenation (2, 3), and Disproportionation (4) of Formic Acid for the Production of Methanol
Received: December 29, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
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complex [Cp*Ir(bpy-Me)OH2]2+ (1) using density functional theory (DFT), and proposed a detailed reaction mechanism with three catalytic cycles, the dehydrogenation of formic acid to CO2 and H2, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol.
of methanol selectivity was the unsubstituted bipyridine complex (R = H). However, most catalysts reported relied on high temperature and low MeOH selectivity. Very recently, Himeda and co-workers11 achieved a TON of 1314 and methanol selectivity up to 47.1% for the disproportionation of formic acid (50−60 °C) using an iridium complex, [Cp*Ir(bpy-Me)OH2]SO4 (bpy-Me = 5,5′-dimethyl-2,2′-bipyridine) (Scheme 2) as the
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RESULTS AND DISCUSSION Dehydrogenation of Formic Acid to H2 and CO2. At the beginning of the reaction, 1 releases a H2O molecule in the solvent and forms a 5.3 kcal/mol more stable complex 2 (Figure 1, Scheme 3). Then the coordination of a formic acid molecule to the vacant position in 2 forms intermediate 3. The hydroxyl proton in 3 could easily dissociate in the solvent for the formation of 4′ (Figure 2) with a strong Ir−O bond (2.179 Å). The newly formed formate anion in 4′ then dissociates and comes back to 2 for the formation of intermediate 4 with a weak Ir··· HCO2 interaction. 4′ has a relative free energy of −5.9 kcal/mol, which is 0.6 kcal/mol more stable than 2. The hydride in the formate anion in 4 can easily be transferred to Ir through transition state TS4,5 (Figure 2, ΔG⧧ = 8.8 kcal/mol). Then the newly formed CO2 molecule dissociates from 5 without a barrier. Intermediate 6 can easily take a proton in the solvent and form a 4.6 kcal/mol less stable dihydrogen complex 7. The release of a H2 molecule from 7 for the regeneration of 2 is a 5.7 kcal/mol downhill step. Since 4′ is the most stable complex in Cycle 1, it could be considered as the rate-determining intermediate of this catalytic cycle. According to the energy span model, 4′ and TS4,5 are the rate-determining states in Cycle 1 with a free energy barrier of 14.7 kcal/mol (4′ → TS4,5). Such a low barrier indicates a fast dehydrogenation of formic acid. In addition, because 1 with a H2O molecule on Ir is not in the catalytic cycle, it could be considered as the precatalyst. Hydrogenation of Formic Acid to Formaldehyde and Water. The hydrogenation of formic acid to formaldehyde and water contains two catalytic cycles, the hydrogenation of formic acid to methanediol and the decomposion of methanediol to formaldehyde and water. At the beginning of Cycle 2, a H2 molecule fills the vacant position in 2 and forms a 5.7 kcal/mol less stable intermediate 7. Then a formate anion takes a proton from the H2 in 7 to its unsaturated oxygen atom via TS8,9
Scheme 2. Disproportionation of Formic Acid to Methanol Catalyzed by 1
catalyst in acidic solvent and 5.2 MPa pressure of H2. Although some postulated mechanisms have been proposed,11,12 detailed reaction pathways, especially the information on key transition states and intermediates of those catalytic reactions, are still not clear. In this study, we computationally investigated the disproportionation of formic acid catalyzed by a half-sandwich iridium
Figure 1. Free energy profile for the dehydrogenation of formic acid to H2 and CO2.
Scheme 3. Predicted Catalytic Cycle for the Dehydrogenation of Formic Acid to H2 and CO2 (Cycle 1)
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DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
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Figure 2. Optimized structures of 1, 2, 4′, 6, 7, and TS4,5 (645i cm−1) in Cycle 1. Bond lengths are in Å.
Figure 3. Free energy profile for the hydrogenation of formic acid to formaldehyde and water.
(Figure 4, ΔG⧧ = 14.4 kcal/mol). The release of a newly formed formic acid molecule forms a stable intermediate 6. Then the formic acid molecule comes back to intermediate 6 and forms a methanediol molecule by taking the hydride on Ir to its carbon atom via TS6,10 (Figure 4, ΔG⧧ = 17.2 kcal/mol) and a solvent proton to its unsaturated oxygen atom. The methanediol molecule could dissociate from 10 without a barrier and come back to 2 for the formation of 11 with a strong Ir−O interaction (2.372 Å). As shown in Scheme 4, there are two possible pathways, methanol assisted proton transfer (Path 1) and direct hydroxyl deprotonation (Path 2) for C−O bond cleavage and the formations of formaldehyde and water molecules after the formation of 11. In Path 1, an extra methanol molecule acts as a proton transfer shuttle between two hydroxyl groups in methanediol for the formation of water and formaldehyde molecules simultaneouly via TS12,13 (Figure 4, ΔG⧧ = 21.9 kcal/mol). In Path 2, methanediol’s hydroxyl group far away from Ir could be deprotonated in the solvent with a moderate relative free energy of 17.9 kcal/mol and form 10′, which is a 2.2 kcal/mol more stable isomer of 10. Then a HCHO molecule could easily be formed through direct C−O bond cleavage (TS10′,14, Figure 4), which is only 2.2 kcal/mol higher than 10′ in free energy. Then the hydroxyl group in 14 can easily catch a solvent proton, and dissociate from 14 as a water molecule. Although methanediol is experimentally more favorable
than formaldehyde and water,13 the hydrogenation of formaldehyde to stable methanol is much easier than the conversion of formaldehyde and water to methanediol. In addition to methanol, we also considered the possibility of using a water molecule as a proton transfer shuttle for the formation of formaldehyde. However, our calculation results indicate that the energy barrier of water assisted proton transfer is 3.0 kcal/mol higher than TS12,13. By comparing the relative free energies shown in Figure 3, we find that the barrier of direct C−O cleavage in Path 2 is 1.9 kcal/mol lower than the barrier of the methanol assisted proton transfer pathway. It should also be noted that the methanol assisted proton transfer pathway has a free energy of 21.4 kcal/mol after considering the −0.5 kcal/mol free energy correction with the experimental concentration of H2SO4 in the solvent. According to the energy span model, 4′ and TS10′,14 are the rate-determining states in the Cycle 2 with a total free energy barrier of 25.9 kcal/mol. It is worth noting that the calculation of the relative free energy of 10′ is based on the solvent free energy of proton. So the energy barrier of Path 2 may vary in different solvents. Hydrogenation of Formaldehyde to Methanol. Similar to the hydrogenation of formic acid, Cycle 3 also begins with the formation of 6 through a proton transfer from the H2 to the unsaturated oxygen atom of a formate anion and the dissociation C
DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
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Figure 4. Optimized structures of TS8,9 (737i cm−1), TS6,10 (477i cm−1), 10, 11, TS12,13 (376i cm−1), and TS10′,14 (235i cm−1) in Cycle 2. Bond lengths are in Å.
Scheme 4. Predicted Catalytic Cycle with Methanol Assisted Proton Transfer (Path 1, Blue) and Direct Deprotonation (Path 2, Green) Pathways for the Hydrogenation of Formic Acid to Formaldehyde and Water (Cycle 2)
a formaldehyde molecule could also fill the vacant position in 2 and form an unstable structure 17, which is 5.1 kcal/mol less stable than 2 (Figure 6). It is worth noting that there is an equilibrium that exists between FA/MeOH and methyl formate/H2O under acidic conditions,11 and the direct hydrogenation of methyl formate may also give methanol.3d,g,14 By comparing all relative free energies in the above three catalytic cycles, we can conclude that the C−O bond cleavage for the formation of a formaldehyde molecule is the rate-determining
of a newly formed formic acid molecule (Scheme 5). Then a formaldehyde molecule takes a hydride on Ir to its carbon atom via TS15,16 (Figure 5, ΔG⧧ = 5.7 kcal/mol). Intermediate 16 can easily catch a proton from the solvent and release a methanol for the regeneration of 2. The rate-determining step in this Cycle 3 is the proton transfer from the Ir atom to the unsaturated oxygen atom of formate anion with a free energy barrier of 20.3 kcal/mol (4′ → TS8,9). 18 is 14.9 kcal/mol more stable than 2 with a strong Ir−O bond (2.286 Å). Instead of 18, D
DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
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Figure 5. Optimized structures of TS15,16 (351i cm−1), 17, and 18 in Cycle 3. Bond lengths are in Å.
methanol proposed by Himeda are provided in the Supporting Information. In Himeda’s mechanism, [HC(OH)2]+ cation can be generated from formic acid and the solvent proton, which may come from the cleavage of H2. Then the [HC(OH)2]+ cation takes the hydride from Ir for the formation of formaldehyde and water. As shown in Figure 7, the transition state for hydride transfer from Ir to the [HC(OH)2]+ cation has a relative free energy of 29.1 kcal/mol. Because of the easy formation of stable structure 4′, the total free energy barrier of the catalytic cycle shown in Scheme 6 is 35.0 kcal/mol (4′ → TS19,20), which is too high to account for the observed reaction rates (Figure 8).Our mechanism features competitive methanol assisted proton transfer and hydroxyl deprotonation pathways for C−O bond cleavage and formaldehyde formation is a more reasonable one to explain the observed catalytic reactions.
Figure 6. Free energy profile for the hydrogenation of formaldehyde to methanol.
step with a free energy barrier of 25.9 kcal/mol (4′ → TS10′,14) in the whole reaction. Although the methanol assisted proton transfer pathway is 1.9 kcal/mol less favorable than the direct hydroxyl deprotonation pathway, we still consider these two pathways are competitive because of the uncertainty of the solvent free energy of proton. Evaluation of Himeda’s Mechanism. In order to find out all possible reaction pathways, we also evaluated the catalytic cycle proposed by Himeda and co-workers11 using the same computational method. Since the formation of methanediol is a key step for the cleavage of the C−O bond and formation of formaldehyde, we primarily discussed a plausible reaction pathway that contains direct hydride transfer for the hydrogenation of formic acid to methanediol (Scheme 6). Further computational details for the hydrogenation of formaldehyde to
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CONCLUSIONS In summary, our computational study of the disproportionation of formic acid to methanol catalyzed by half-sandwich iridium complex [Cp*Ir(bpy-Me)OH2]2+ reveals a mechanism that contains three catalytic cycles: the dehydrogenation of formic acid to CO2 and H2, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol. Our newly proposed mechanism features two competitive pathways, methanol assisted proton transfer and direct deprotonation of hydroxyl in methanediol, for the formation of formaldehyde. With a value of −272.2 kcal/mol for the solvent free energy of proton, the computed total free E
DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
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Scheme 6. Himeda’s Mechanism for the Hydrogenation of Formic Acid to Formaldehyde and Water with Direct Hydride Transfer
mechanistic insights of the disproportionation of formic acid to methanol catalyzed by half-sandwich iridium complexes but also points the way to facilitating the cleavage of a strong C−O bond, in which the proton transfer between two hydroxyl groups in methanediol plays a crucial role. Further mechanistic study and computational design of base metal complexes as potential catalysts for the hydrogenation of formic acid to methanol are underway.
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COMPUTATIONAL DETAILS
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ASSOCIATED CONTENT
All DFT calculations in this study were performed using the Gaussian 09 suite of programs15 for the M06 functional16 in conjunction with the all-electron 6-31++G(d,p) basis set for H, C, N, O atoms. The Stuttgart relativistic effective core potential basis set was used for Ir (ECP60MWB).17 An ultrafine grid (99 590) was used for numerical integrations. Seven other widely used density functionals, including ωB97X-D,18 B3PW91,19 HSE06,20 PBEh1PBE,21 TPSSh,22 M06L,16 and TPSS,22 are used to evaluate the dependency of calculated total free energy barriers to density functionals. The M06 functional was selected for this study because its result, 25.9 kcal/mol, matches well with the observed reaction rate at 50−60 °C. The difference between the calculated total free energy barriers by using those density functionals is less than 3.2 kcal/mol. All structures were fully optimized in water with solvent effect corrected by using the integral equation formalism polarizable continuum model (IEFPCM)23 and the SMD radii.24 Thermal corrections were computed within the harmonic potential approximation on optimized structures at 298.15 K and 1 atm pressure. Unless otherwise noted, the energies reported in the text are solvent corrected free energies. An experimental value of −272.2 kcal/mol was used to calculate the solvent free energy of proton Gsol(H+).25,26 The number of imaginary frequency (IF) confirmed the nature of intermediate (no IF) and transition state (only one IF). All transition states were also confirmed to connect corresponding reactants and products by intrinsic reaction coordinate (IRC) calculations. The 3D molecular structure figures displayed in the text were drawn by using the JIMP2 molecular visualizing and manipulating program.27
Figure 7. Computed relative free energies of postulated intermediates and transition states in the mechanism proposed by Himeda for the hydrogenation of formic acid to formaldehyde and water.
Figure 8. Optimized structures of TS19,20 (400i cm−1) and 20. Bond lengths are in Å.
energy barrier of direct C−O bond cleavage for formation of formaldehyde after the deprotonation of hydroxyl is 25.9 kcal/mol (4′ → TS10′,14), which matches well with the observed reaction rate at the temperature of 50−60 °C.11 The free energy barrier of the methanol assisted proton transfer pathway for the formation of formaldehyde is only 1.9 kcal/mol higher than the direct cleavage of the C−O bond. A water molecule is 3.0 kcal/mol less favorable than methanol as the proton transfer shuttle for the formation of formaldehyde. We also examined a previously proposed mechanism with direct proton transfer to formic acid for the formation of [HC(OH)2]+ cation and hydride transfer to [HC(OH)2]+ for the formation of methanediol using the same computational method and obtained a rather high barrier of 35.0 kcal/mol, which is too high to account for the observed reaction rate. Our computational study not only reveals the
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00913. Evalution of density functionals and the mechanism of the hydrogenation of formaldehyde to methanol proposed by Himeda and co-workers (PDF) Atomic coordinates with solvent corrected absolute free energies and electronic energies of all optimized structures described in the text (XYZ) F
DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xinzheng Yang: 0000-0002-2036-1220 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 (21673250).
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DOI: 10.1021/acs.organomet.7b00913 Organometallics XXXX, XXX, XXX−XXX
