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Theoretical investigation on the ruthenium catalyzed dehydrogenation of formic acid and ligand effect
Accepted Manuscript Title: Theoretical Investigation on the Ruthenium Catalyzed Dehydrogenation of Formic Acid and Ligand Effect Author: Jia Zhou PII: DOI: Reference:
S0926-860X(16)30043-6 http://dx.doi.org/doi:10.1016/j.apcata.2016.01.043 APCATA 15758
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
6-12-2015 19-1-2016 30-1-2016
Please cite this article as: Jia Zhou, Theoretical Investigation on the Ruthenium Catalyzed Dehydrogenation of Formic Acid and Ligand Effect, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.01.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Theoretical Investigation on the Ruthenium Catalyzed Dehydrogenation of Formic Acid and Ligand Effect Jia Zhou* [email protected] Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China *
Corresponding author. Tel.: +86-451-8640-2522.
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Graphical abstract
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Highlights Dehydrogenation of formic acid catalyzed by Ru complexes has been studied based on first-principles calculations. P(CH2CH2PPh2)3RuH+ shows a significantly lower reaction barrier (14.3 kcal/mol) than its iron analogue (18.9 kcal/mol), and the latter was investigated by both experiments and theoretical calculations. Isopropyl group, a more electron-donating group than phenyl group, results in an even lower reaction barrier of P(CH2CH2CH2PiPr2)3RuH+ (12.9 kcal/mol) than P(CH2CH2PPh2)3RuH+, indicating a more positive role the ligand could play in the performance of the catalysts.
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Abstract Density functional theory (DFT) calculations were used to study the mechanisms of dehydrogenation of formic acid catalyzed by two Ru complexes, including P(CH2CH2PPh2)3RuH+ (P(CH2CH2PPh2)3 = tris[2-(diphenylphosphino)ethyl]phosphine) and
P(CH2CH2CH2PiPr2)3RuH+
(P(CH2CH2CH2PiPr2)3
=
tris[3-
(diisopropylphosphino)propyl]phosphine). Two competing catalytic cycles (I and II) have been explored. In cycle I, the catalytic reaction starts with a direct hydride transfer from HCOO- to a Ru center, releasing CO2 in the first place, followed by H2 production, while in cycle II, neutral formic acid approaches Ru catalyst to produce H2 molecule prior to CO2 generation via β-hydride elimination. The computational results show that cycle I is more accessible than cycle II, regardless of the ligands surrounding Ru center. The calculated overall Gibbs free energy barriers for formic acid dehydrogenation catalyzed by P(CH2CH2PPh2)3RuH+ (14.3 kcal/mol) is significantly lower than its iron analogue in the previous experimental and theoretical studies (18.9 kcal/mol) at the same level of theory. More interestingly, isopropyl group, a more electron-donating group than phenyl group, leads to an even lower reaction barrier of P(CH2CH2CH2PiPr2)3RuH+ (12.9 kcal/mol) than P(CH2CH2PPh2)3RuH+, indicating a more positive role the ligand could potentially play in the performance of the catalysts. Our results pave a new way to design more efficient catalysts for formic acid dehydrogenation. Keywords: density functional theory (DFT); ruthenium complexes; dehydrogenation; formic acid; ligand effect
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Introduction Energy storage is a hot global issue, and molecular hydrogen is one of the ideal candidates for this purpose due to its high energy density.1-3 Fossil fuels, on the other hand, are becoming more scarce, resulting in dramatic price rises. Renewable energy, which comes from natural resources such as sunlight, wind, and tides, are thus being pursued. Biomass (plant material) is a renewable energy source because the energy it contains comes from the sun.4 It is one of the best ways to store solar energy as fuel,5 because of a much higher energy density compared to batteries or mechanical storage devices. Among currently available biomass materials, formic acid (HCOOH) is thought to be a good organic hydrogen storage molecule, due to its high hydrogen content (ca. 4.4 wt%),6 and could be produced by hydrothermal oxidation of model biomass materials (glucose, starch and cellulose).7 To this end, it has drawn great attention to study the dehydrogenation of formic acid, and many different heterogeneous and homogeneous catalyst systems have since been explored. In recent years, a variety of transition metals, such as Fe,8-11 Ru,12-16 Co,17, 18 and Ir,19, 20 accompanied with the stabilizing ligands have been utilized to catalyze the dehydrogenation of formic acid. The stabilizing ligands are usually polydentate phosphines, which are used for controlling the stereochemistry of coordination complexes and solubilizing metal catalysts.21 Among those transition metal catalysts, Beller and co-workers in particular developed a highly active iron based catalyst for hydrogen production from formic acid using
P(CH2CH2PPh2)3FeH+
(P(CH2CH2PPh2)3
=
tris[2-
(diphenylphosphino)ethyl]phosphine) in 2011.8 This iron catalyst has an impressive turnover frequency of up to 9425 h-1, and a turnover number of more than 93000 at 80 ºC.
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Shortly after, a theoretical study was carried out to elucidate the mechanism of the iron catalyzed formic acid dehydrogenation, pointing out β-hydrides elimination pathway is the most possible pathway, with the reaction barrier only 20.5 kcal/mol.11 In parallel, ruthenium is also widely investigated for formic acid dehydrogenation from both the experimental perspective12-14 and the theoretical perspective.15, 16 For instance, Gonsalvi and co-workers studied formic acid dehydrogenation catalyzed by Ru complexes bearing the polydentate tripodal ligands 1,1,1-tris-(diphenylphosphinomethyl)ethane (triphos) and tris-[2-(diphenylphosphino)ethyl]-amine (NP3). By combining the experimental studies and DFT calculations, they were able to clarify the mechanism of the catalytic reaction, which showed superior performances with a TON of 10000 after 6 h using 0.01 mol% of the catalyst.12, 15 Given that several different stabilizing ligands have been used in the studies of formic acid dehydrogenation combining with iron and ruthenium,8, 12 it would be worth figuring out the relationship between stabilizing ligands and the catalysis. Encouraged by previous theoretical study on the P(CH2CH2PPh2)3FeH+ system,10, 11 we would like to explore how P(CH2CH2PPh2)3RuH+, analogous to P(CH2CH2PPh2)3FeH+, will perform for formic acid dehydrogenation, and in addition to the direct comparison of metal center, if the stabilizing ligands, coordinating to the transition metal center, could impact the catalytic efficiency and how much extent the impact could possibly be. In this paper, we report a thorough theoretical study of dehydrogenation of formic acid catalyzed by P(CH2CH2PPh2)3RuH+ and P(CH2CH2CH2PiPr2)3RuH+. A detailed reaction mechanism with key transition states is explored based upon DFT calculations, as well as the influence of the different stabilizing ligands. We are aiming to develop new formic acid dehydrogenation catalysts with high efficiency with the aid of computational
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chemistry methods. Methods All DFT calculations were carried out with the Gaussian09 suite of ab initio programs.22 All structures were fully optimized by the M0623 functional in the gas phase. The Los Alamos basis set24 and the associated effective core potential (ECP) was used for Ru atom, and an all-electron 6-31G(d) basis set was used for all the other atoms. This basis set combination is referred to as BS1. An “ultrafine” integration grid of 99 radial shells and 590 angular points per shell was used throughout. The harmonic vibrational frequency calculations were performed to ensure that either a minimum (no imaginary frequency) or first-order saddle point (only one imaginary frequency) was obtained. Intrinsic reaction coordinate (IRC)25 calculations were used to confirm the connectivity between transition structures and minima. To further refine the energies obtained from the M06/BS1 calculations, we carried out single-point energy calculations for all of the structures (including HCOOH, CO2 and H2) with a larger basis set (BS2) in THF corresponding to the experimental reaction conditions,8 using the SMD26 solvation model. BS2 utilizes the triple ζ valence LANL2TZ plus f function basis set27, 28 on Ru atom, and 6-311++G(d,p) basis set on the others. Unless otherwise stated, the energies reported in this paper are Gibbs free energies under 298.15 K and 1 atm with solvent effect corrections. Given the complexity of the systems in the current study, the above techniques are a reasonable compromise to make the computations more tractable. The Cartesian coordinates of the optimized structures along with energies are reported in the Supporting Information.
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Results and Discussion Based on previous study on iron catalyzed dehydrogenation of formic acid,8, 11 two competing catalytic cycles (I and II) of dehydrogenation of formic acid catalyzed by Ru complexes are proposed, as displayed in Scheme 1. In cycle I, the catalytic reaction begins with the coordination of HCOO- to Ru catalyst, while in cycle II, neutral formic acid molecule directly coordinate with Ru catalyst. The Ru catalysts, including P(CH2CH2PPh2)3RuH+ (short as P(PPh2)3RuH+), and P(CH2CH2CH2PiPr2)3RuH+ (short as P(P’iPr2)3RuH+), each have two isomers. Figure 1 shows the optimized structures of P(PPh2)3RuH+ (1_transPh and 1_cisPh), and P(P’iPr2)3RuH+ (1_transiPr and 1_cisiPr). The structure of P(PPh2)3RuH+ is analogous to the Fe catalyst in the previous literatures,8, 11 while P(P’iPr2)3RuH+ could be produced from P(P’iPr2)3RuCl+.29 The molecular structure of 1_transPh and 1_transiPr is a distorted trigonal bipyramidal geometry30-33 with three equatorial phosphorus ligands, and the remaining phosphorus (labeled as Pc in Figure 1) and the hydride (labeled as H in Figure 1 ) being in the axial coordination site (the H-Ru-Pc angle is 173.4º and 179.5º for 1_transPh and 1_transiPr, respectively). The Ru-H distance of 1_transiPr (1.67 Å) is a little shorter than that of 1_transPh (1.69 Å). Different from 1_transPh and 1_transiPr with the hydride (H) trans to the Pc atom, the structure of 1_cisPh and 1_cisiPr contains a cishydride towards to the Pc atom. Both of them exhibit a distorted square-pyramidal geometry with three equatorial phosphorus ligands and the hydride making up the base and the Pc atom at the apex. In cis structures, the Ru–H bond distance is shorter (0.2-0.3 Å) than its respective trans counterpart, indicating a stronger Ru-H bond. In fact, the trans structure is higher in energy than its cis counterpart (8.0/6.0 kcal/mol for
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P(PPh2)3RuH+/P(P’iPr2)3RuH+), and the barrier for mutual exchange is 4.1/5.1 kcal/mol (from trans to cis) for P(PPh2)3RuH+/P(P’iPr2)3RuH+, as shown in Figure 2. These energetics suggest that both trans and cis isomers could be involved in the catalytic reactions, and thus have to be considered. Next, we are going to investigate the two catalytic cycles (I and II) of P(PPh2)3RuH+ and P(P’iPr2)3RuH+, respectively. Figure 3 shows the Gibbs energy profile for cycle I of P(PPh2)3RuH+ (See all the optimized structures in the Supporting Information). There are two ways for HCOO- to react with P(PPh2)3RuH+, forming either Ru-O bond or Ru-H bond, and the according products are 6_cisPh and 2_cisPh, respectively. Both processes are exothermic and spontaneous, with 6_cisPh ca. 7.9 kcal/mol lower in energy than 2_cisPh due to stronger Ru-O bond, but only 2_cisPh leads to more stable product afterwards. A similar phenomenon is also observed in the iron catalyzed dehydrogenation of formic acid.11 In this cycle, only the pathway starting from 1_cisPh is shown, because for trans case, 2_transPh and TS2trans_3Ph are not able to be located after numerous attempts. Moreover, 6_cisPh is lower in energy than 6_transPh (ca. 5.3 kcal/mol). Although it fails to optimize 2_transPh and TS2trans_3Ph structures, the hydrogen transfer from HCOO- to P(PPh2)3RuH+ for both trans and cis cases would result in the same products, neutral 3Ph + CO2. In neutral 3Ph, two hydrides occupy both the trans and cis position of Ru complex, and the H-H distance is ca. 2.26 Å, making it a clear dihydride structure. The mutual exchange between dihydride and dihydrogen ligands have been studied in the previous study,34 and we also investigate this possibility in our case. It turns out that only the cis hydride could move to the trans position, making the new H-H distance ca. 0.85 Å in 4_transPh, while the opposite could not (4_cisPh is not stable). The barrier for this process
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is ca. 11.7 kcal/mol. The H-H distance in 4_transPh (0.85 Å) is just a little longer than that in free H2 molecule (0.74 Å) at the same level, indicating 4_transPh a more dihydrogen structure. The empty cis position of 4_transPh provides a space for free proton (H+) attacking to become 5_transPh. However, there is another one-step route to arrive at 5_transPh for 3Ph via the protonation by a proton from a formic acid molecule (TS3_5transPh), and this route requires much less energy (8.5 kcal/mol) than the route via TS3_4transPh (11.7 kcal/mol). This route was also investigated for Fe-catalyzed formic acid dehydrogenation, and the corresponding barrier between 3Ph for 5_transPh is almost thermoneutral.10 Compared with 4_transPh, the H-H distance (0.83 Å) in the trans position of 5_transPh becomes a little shorter. Dissociation of dihydrogen ligands from 5_transPh requires 12.3 kcal/mol, regenerating 1_cisPh along with H2 molecule. 5_transPh is a distorted octahedral structure with the hydride and dihydrogen ligands in mutually cis coordination sites. A similar structure has been studied by others,29 and fast exchange between the hydrido and dihydrogen ligands has been found at room temperature. As shown in the inset of Figure 3, 5_transPh and 5_cisPh are relatively close in energy (less than 10 kcal/mol apart) and the barrier between them is fairly low. Dissociation of dihydrogen ligands from 5_cisPh reproduces 1_transPh along with H2 molecule. After comparing all the relative energies shown in Figure 3, the total Gibbs energy barrier is 14.3 kcal/mol (3Ph → TS5trans_1cisPh), which is far less than Fe-catalyzed formic acid dehydrogenation (18.9 kcal/mol).10 On the other hand, P(PPh2)3RuH+ can also react with neutral HCOOH to generate CO2 and H2 (cycle II), as shown in Figure 4 for the corresponding Gibbs energy profiles. Both trans (Figure 4a) and cis (Figure 4b) P(PPh2)3RuH+ are found to be able to be react
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with neutral HCOOH, despite the mutual exchange between the two isomers. In both reactions, the first step is the coordination of a formic acid molecule to the vacant position of P(PPh2)3RuH+. In the intermediate 7_transPh/7_cisPh, there is a strong Ru-Hδ···Hδ+-O interaction, with the Hδ-···Hδ+ distance 1.48/1.55 Å, comparable to the previous study on iron.11 The Hδ+ approaches to the Hδ- to generate 8_transPh/8_cisPh with dihydrogen ligand. In cis case, structure 8_cisPh is thermoneutral to TS7cis_8cisPh, and the latter is thus not shown on the potential energy surface (PES). After the dihydrogen ligand’s dissociation, both 8_transPh and 8_cisPh arrive at the same structure, 9Ph, possessing two Ru-O bonds (2.30 and 2.24 Å for trans and cis position). 9Ph is the most stable species on both trans and cis PESs. There are two pathways for CO2 generation from structure 9Ph: from trans or cis site of Ru center, as seen in 12_transPh or 12_cisPh. Generally, three steps are involved from 9Ph to 12_cisPh: Ru-O elongation (TS9_10cisPh), OH flip-flopping (TS10cis_11cisPh), and H transfer (TS11cis_12cisPh). For trans case, however Ru-O elongation (TS9_10transPh) and the according intermediate 10_transPh are not stable, making 9Ph directly to connect 11_transPh via TS9_11transPh. From structure 9Ph, it would be much easier for it to release CO2 to get back to 1_cisPh, overcoming a 16.0 kcal/mol barrier (TS10cis_11cisPh) than to 1_transPh with 19.4 kcal/mol barrier (TS11trans_12transPh). Both of them though are remarkably higher than the overall barrier (14.3 kcal/mol) in cycle I. As discussed previously, a variety of transition metal complexes were explored to do formic acid dehydrogenation, such as Fe,8-11 Ru,12-16 Co,17, 18 and Ir.19, 20 In addition to metal itself, we also wonder how the dentate ligands play a role to the catalytic reaction. Isopropyl group is a more electron-donating group than phenyl group. Take 1_cisPh and 1_cisiPr for example, and Ru Mulliken charge is -0.69 |e| and -0.83 |e|, respectively.
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P(CH2CH2CH2PiPr2)3RuH+ (short as P(P’iPr2)3RuH+), derived from the previous experimental study,29 is thought to be a potential candidate for formic acid dehydrogenation. Figure 5 shows the Gibbs energy profile for cycle I of P(P’iPr2)3RuH+. Despite a generally similar reaction route, there are a few differences between the PESs of P(PPh2)3RuH+ and that of P(P’iPr2)3RuH+. In the latter case, we were able to locate the 2_transiPr and TS2trans_3iPr, but they both lie above their respective cis counterparts. Both trans and cis P(P’iPr2)3RuH+ end up with neutral 3iPr + CO2 after reacting with HCOO-. We were able to locate the TS3_4transiPr, but it is lower in energy than 4_transiPr by 1.6 kcal/mol. TS3_5transiPr still lies lower in energy than 4_transiPr, as in P(PPh2)3RuH+, and becomes the rate-determining step of cycle I for P(P’iPr2)3RuH+ (12.9 kcal/mol relative to 3iPr), surpassing TS5trans_1cisiPr. Figure 6 shows the Gibbs energy profiles for cycle II of P(P’iPr2)3RuH+. Generally speaking, the PESs for cycle II of P(P’iPr2)3RuH+ are similar to those of P(PPh2)3RuH+. Major difference however exists for the PES prior to 9iPr. The barrier for the process from 1_transPh/1_cisPh to 9Ph, namely H2 production, is only 6.8 kcal/mol (8_transPh → TS8trans_9Ph; or 8.1 kcal/mol if considering 1_cisPh) and 10.8 kcal/mol (1_cisPh → TS8cis_9Ph), significantly lower than the following CO2 generation, as discussed previously. By contrast, the H2 production in P(P’iPr2)3RuH+ case requires more energy, 23.7 kcal/mol for 1_transiPr (1_cisiPr → TS8trans_9iPr; keep in mind that 1_cisiPr is more stable than 1_transiPr by 6 kcal/mol) and 26.4 kcal/mol for 1_cisiPr (1_cisiPr → TS8cis_9iPr). As for CO2 generation, the activation energy is 18.3 kcal/mol (9iPr → TS11trans_12transiPr) and 14.8 kcal/mol (9iPr → TS11cis_12cisiPr) for returning to 1_transiPr and 1_cisiPr, respectively. When comparing all the relative energies, it should be pointed out that for H2 production, it is a
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little easier for 1_cisiPr to convert to 1_transiPr in the first place (small barrier 11.1 kcal/mol as seen in Figure 2b), followed by passing TS8trans_9iPr than for 1_cisiPr to directly overcome TS8cis_9iPr. When reaching H2 production intermediate 9iPr, it is much easier for it to convert back to 1_cisiPr than to 1_transiPr. Therefore, the total Gibbs energy barrier for cycle II of P(P’iPr2)3RuH+ is 23.7 kcal/mol, with the key detailed pathway being 1_cisiPr to 1_transiPr to TS8trans_9iPr to 9iPr to TS11cis_12cisiPr to 1_cisiPr. We might notice in the above discussions, the starting points for cycle I and cycle II are different: HCOO- (somewhat like catalyst) and HCOOH in cycle I, while HCOOH only in cycle II. Formic acid is a week acid and thus mostly in the neutral form in solvent. We are aware that formic acid to some extent tends to exist as hydrogen-bonded dimers rather than individual molecules and counter anion in the solution is omitted in our computation, but our sophisticate model would be adequate to make a direct comparison of different metal centers and dentate ligands. Now we can conclude that for P(PPh2)3RuH+ and P(P’iPr2)3RuH+, the most likely pathway is that the Ru catalyst reacts with formate anion (HCOO-) in the first place, followed by CO2 generation and H2 production, and either TS5trans_1cisPh or TS3_5transiPr is the rate-determining step. For both Ru catalysts, cycle I is more accessible than cycle II. However, the difference of the reaction barriers of the two cycles is quite different: 1.7 kcal/mol for P(PPh2)3RuH+, and 10.8 kcal/mol for P(P’iPr2)3RuH+, indicating cycle II plays an even less role in P(P’iPr2)3RuH+ catalyzed formic acid dehydrogenation than in P(PPh2)3RuH+. Nevertheless, both Ru complexes show a better performance on formic acid dehydrogenation than the iron complex ((PPh2)3FeH+,) used in the previous studies.8, 10, 11
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Conclusions In summary, we have performed a detailed computational study on the dehydrogenation mechanisms of formic acid catalyzed by P(PPh2)3RuH+ and P(P’iPr2)3RuH+. Similar to the iron catalyzed formic acid dehydrogenation, two competing catalytic cycles (I and II) of Ru catalysts were investigated, and the results show the cycle I is more accessible no matter what the dentate ligand is. Both trans and cis isomer of P(PPh2)3RuH+ and P(P’iPr2)3RuH+ were investigated for catalyzing the formic acid dehydrogenation. Generally speaking, the cis isomer of Ru catalysts is more stable than its trans isomer, and is thus believed to dominate the whole catalytic reaction. P(PPh2)3RuH+ edges its iron analogue by 4.6 kcal/mol on formic acid dehydrogenation, suggesting a better impact of Ru over Fe. Furthermore, the reaction barrier of P(P’iPr2)3RuH+ is lower by 1.4 kcal/mol than that of P(PPh2)3RuH+, indicating isopropyl group is superior to phenyl group in this regard. These findings might be of great importance in the future choice of tailored ligands around metal centers (e.g. ruthenium) for efficient catalysts for formic acid dehydrogenation.
Notes: The authors declare no competing financial interest.
Acknowledgements This research is supported by the Fundamental Research Funds for the Central Universities of China (Grant No. AUGA5710013115). Computer time made available by the National Supercomputing Center of China in Shenzhen (Shenzhen Cloud Computing
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Center) is gratefully acknowledged. The author also thanks Prof. Ozerov and Dr. Chun-I Lee at Texas A&M University for helpful comments.
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D. Feller, The role of databases in support of computational chemistry calculations, J. Comput. Chem., 1996, 17, 1571-1586. K. L. Schuchardt, B. T. Didier, T. Elsethagen, L. Sun, V. Gurumoorthi, J. Chase, J. Li and T. L. Windus, Basis Set Exchange: A Community Database for Computational Sciences, J. Chem. Inf. Model., 2007, 47, 1045-1052. M. M. Bhadbhade, L. D. Field, R. Gilbert-Wilson, R. W. Guest and P. Jensen, Ruthenium Hydride Complexes of the Hindered Phosphine Ligand Tris(3diisopropylphosphinopropyl)phosphine, Inorg. Chem., 2011, 50, 6220-6228. I. E. Rachidi, O. Eisenstein and Y. Jean, A theoretical study of the possible structures of d6 ML5 complexes, New J. Chem. , 1990, 14, 671-677. J. F. Riehl, Y. Jean, O. Eisenstein and M. Pelissier, Theoretical study of the structures of electron-deficient d6 ML5 complexes. Importance of a π-donating ligand, Organometallics, 1992, 11, 729-737. M. Oliván, O. Eisenstein and K. G. Caulton, New Access to Vinylidenes from Ruthenium Polyhydrides, Organometallics, 1997, 16, 2227-2229. W. H. Lam, S. Shimada, A. S. Batsanov, Z. Lin, T. B. Marder, J. A. Cowan, J. A. K. Howard, S. A. Mason and G. J. McIntyre, Accurate Molecular Structures of 16-Electron Rhodium Hydrido Boryl Complexes: Low-Temperature SingleCrystal X-ray and Neutron Diffraction and Computational Studies of [(PR3)2RhHCl(Boryl)] (Boryl = Bpin, Bcat), Organometallics, 2003, 22, 45574568. G. R. Haynes, R. L. Martin and P. J. Hay, Theoretical investigations of classical and nonclassical structures of MH7L2 polyhydride complexes of rhenium and technetium, J. Am. Chem. Soc., 1992, 114, 28-36.
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Figure Captions
(a)
(b)
(c) (d) Ph Ph Figure 1. Optimized structures of 1_trans (a), 1_cis (b), 1_transiPr (c), and 1_cisiPr (d). All hydrogen atoms, except for that bonds with Ru atom, are omitted for clarity. (Gray: Carbon; Orange: Phosphorus; Green: Ruthenium; White: Hydrogen)
19
(a) (b) Figure 2. Gibbs energy profiles for trans and cis inter-conversion of two Ru catalysts: P(PPh2)3RuH+ (a) and P(PiPr2)3RuH+ (b).
20
Figure 3. Gibbs energy profiles for cycle I of P(PPh2)3RuH+.
21
(a)
(b) Figure 4. Gibbs energy profiles for cycle II of P(PPh2)3RuH+.
22
Figure 5. Gibbs energy profiles for cycle I of P(PiPr2)3RuH+.
23
(a)
(b) Figure 6. Gibbs energy profiles for cycle II of P(PiPr2)3RuH+.
24
Scheme 1. Proposed mechanism for the Ru-catalyzed dehydrogenation of formic acid, including two competing cycles I and II.
25
S0926-860X(16)30043-6 http://dx.doi.org/doi:10.1016/j.apcata.2016.01.043 APCATA 15758
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
6-12-2015 19-1-2016 30-1-2016
Please cite this article as: Jia Zhou, Theoretical Investigation on the Ruthenium Catalyzed Dehydrogenation of Formic Acid and Ligand Effect, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.01.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Theoretical Investigation on the Ruthenium Catalyzed Dehydrogenation of Formic Acid and Ligand Effect Jia Zhou* [email protected] Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China *
Corresponding author. Tel.: +86-451-8640-2522.
1
Graphical abstract
2
Highlights Dehydrogenation of formic acid catalyzed by Ru complexes has been studied based on first-principles calculations. P(CH2CH2PPh2)3RuH+ shows a significantly lower reaction barrier (14.3 kcal/mol) than its iron analogue (18.9 kcal/mol), and the latter was investigated by both experiments and theoretical calculations. Isopropyl group, a more electron-donating group than phenyl group, results in an even lower reaction barrier of P(CH2CH2CH2PiPr2)3RuH+ (12.9 kcal/mol) than P(CH2CH2PPh2)3RuH+, indicating a more positive role the ligand could play in the performance of the catalysts.
3
Abstract Density functional theory (DFT) calculations were used to study the mechanisms of dehydrogenation of formic acid catalyzed by two Ru complexes, including P(CH2CH2PPh2)3RuH+ (P(CH2CH2PPh2)3 = tris[2-(diphenylphosphino)ethyl]phosphine) and
P(CH2CH2CH2PiPr2)3RuH+
(P(CH2CH2CH2PiPr2)3
=
tris[3-
(diisopropylphosphino)propyl]phosphine). Two competing catalytic cycles (I and II) have been explored. In cycle I, the catalytic reaction starts with a direct hydride transfer from HCOO- to a Ru center, releasing CO2 in the first place, followed by H2 production, while in cycle II, neutral formic acid approaches Ru catalyst to produce H2 molecule prior to CO2 generation via β-hydride elimination. The computational results show that cycle I is more accessible than cycle II, regardless of the ligands surrounding Ru center. The calculated overall Gibbs free energy barriers for formic acid dehydrogenation catalyzed by P(CH2CH2PPh2)3RuH+ (14.3 kcal/mol) is significantly lower than its iron analogue in the previous experimental and theoretical studies (18.9 kcal/mol) at the same level of theory. More interestingly, isopropyl group, a more electron-donating group than phenyl group, leads to an even lower reaction barrier of P(CH2CH2CH2PiPr2)3RuH+ (12.9 kcal/mol) than P(CH2CH2PPh2)3RuH+, indicating a more positive role the ligand could potentially play in the performance of the catalysts. Our results pave a new way to design more efficient catalysts for formic acid dehydrogenation. Keywords: density functional theory (DFT); ruthenium complexes; dehydrogenation; formic acid; ligand effect
4
Introduction Energy storage is a hot global issue, and molecular hydrogen is one of the ideal candidates for this purpose due to its high energy density.1-3 Fossil fuels, on the other hand, are becoming more scarce, resulting in dramatic price rises. Renewable energy, which comes from natural resources such as sunlight, wind, and tides, are thus being pursued. Biomass (plant material) is a renewable energy source because the energy it contains comes from the sun.4 It is one of the best ways to store solar energy as fuel,5 because of a much higher energy density compared to batteries or mechanical storage devices. Among currently available biomass materials, formic acid (HCOOH) is thought to be a good organic hydrogen storage molecule, due to its high hydrogen content (ca. 4.4 wt%),6 and could be produced by hydrothermal oxidation of model biomass materials (glucose, starch and cellulose).7 To this end, it has drawn great attention to study the dehydrogenation of formic acid, and many different heterogeneous and homogeneous catalyst systems have since been explored. In recent years, a variety of transition metals, such as Fe,8-11 Ru,12-16 Co,17, 18 and Ir,19, 20 accompanied with the stabilizing ligands have been utilized to catalyze the dehydrogenation of formic acid. The stabilizing ligands are usually polydentate phosphines, which are used for controlling the stereochemistry of coordination complexes and solubilizing metal catalysts.21 Among those transition metal catalysts, Beller and co-workers in particular developed a highly active iron based catalyst for hydrogen production from formic acid using
P(CH2CH2PPh2)3FeH+
(P(CH2CH2PPh2)3
=
tris[2-
(diphenylphosphino)ethyl]phosphine) in 2011.8 This iron catalyst has an impressive turnover frequency of up to 9425 h-1, and a turnover number of more than 93000 at 80 ºC.
5
Shortly after, a theoretical study was carried out to elucidate the mechanism of the iron catalyzed formic acid dehydrogenation, pointing out β-hydrides elimination pathway is the most possible pathway, with the reaction barrier only 20.5 kcal/mol.11 In parallel, ruthenium is also widely investigated for formic acid dehydrogenation from both the experimental perspective12-14 and the theoretical perspective.15, 16 For instance, Gonsalvi and co-workers studied formic acid dehydrogenation catalyzed by Ru complexes bearing the polydentate tripodal ligands 1,1,1-tris-(diphenylphosphinomethyl)ethane (triphos) and tris-[2-(diphenylphosphino)ethyl]-amine (NP3). By combining the experimental studies and DFT calculations, they were able to clarify the mechanism of the catalytic reaction, which showed superior performances with a TON of 10000 after 6 h using 0.01 mol% of the catalyst.12, 15 Given that several different stabilizing ligands have been used in the studies of formic acid dehydrogenation combining with iron and ruthenium,8, 12 it would be worth figuring out the relationship between stabilizing ligands and the catalysis. Encouraged by previous theoretical study on the P(CH2CH2PPh2)3FeH+ system,10, 11 we would like to explore how P(CH2CH2PPh2)3RuH+, analogous to P(CH2CH2PPh2)3FeH+, will perform for formic acid dehydrogenation, and in addition to the direct comparison of metal center, if the stabilizing ligands, coordinating to the transition metal center, could impact the catalytic efficiency and how much extent the impact could possibly be. In this paper, we report a thorough theoretical study of dehydrogenation of formic acid catalyzed by P(CH2CH2PPh2)3RuH+ and P(CH2CH2CH2PiPr2)3RuH+. A detailed reaction mechanism with key transition states is explored based upon DFT calculations, as well as the influence of the different stabilizing ligands. We are aiming to develop new formic acid dehydrogenation catalysts with high efficiency with the aid of computational
6
chemistry methods. Methods All DFT calculations were carried out with the Gaussian09 suite of ab initio programs.22 All structures were fully optimized by the M0623 functional in the gas phase. The Los Alamos basis set24 and the associated effective core potential (ECP) was used for Ru atom, and an all-electron 6-31G(d) basis set was used for all the other atoms. This basis set combination is referred to as BS1. An “ultrafine” integration grid of 99 radial shells and 590 angular points per shell was used throughout. The harmonic vibrational frequency calculations were performed to ensure that either a minimum (no imaginary frequency) or first-order saddle point (only one imaginary frequency) was obtained. Intrinsic reaction coordinate (IRC)25 calculations were used to confirm the connectivity between transition structures and minima. To further refine the energies obtained from the M06/BS1 calculations, we carried out single-point energy calculations for all of the structures (including HCOOH, CO2 and H2) with a larger basis set (BS2) in THF corresponding to the experimental reaction conditions,8 using the SMD26 solvation model. BS2 utilizes the triple ζ valence LANL2TZ plus f function basis set27, 28 on Ru atom, and 6-311++G(d,p) basis set on the others. Unless otherwise stated, the energies reported in this paper are Gibbs free energies under 298.15 K and 1 atm with solvent effect corrections. Given the complexity of the systems in the current study, the above techniques are a reasonable compromise to make the computations more tractable. The Cartesian coordinates of the optimized structures along with energies are reported in the Supporting Information.
7
Results and Discussion Based on previous study on iron catalyzed dehydrogenation of formic acid,8, 11 two competing catalytic cycles (I and II) of dehydrogenation of formic acid catalyzed by Ru complexes are proposed, as displayed in Scheme 1. In cycle I, the catalytic reaction begins with the coordination of HCOO- to Ru catalyst, while in cycle II, neutral formic acid molecule directly coordinate with Ru catalyst. The Ru catalysts, including P(CH2CH2PPh2)3RuH+ (short as P(PPh2)3RuH+), and P(CH2CH2CH2PiPr2)3RuH+ (short as P(P’iPr2)3RuH+), each have two isomers. Figure 1 shows the optimized structures of P(PPh2)3RuH+ (1_transPh and 1_cisPh), and P(P’iPr2)3RuH+ (1_transiPr and 1_cisiPr). The structure of P(PPh2)3RuH+ is analogous to the Fe catalyst in the previous literatures,8, 11 while P(P’iPr2)3RuH+ could be produced from P(P’iPr2)3RuCl+.29 The molecular structure of 1_transPh and 1_transiPr is a distorted trigonal bipyramidal geometry30-33 with three equatorial phosphorus ligands, and the remaining phosphorus (labeled as Pc in Figure 1) and the hydride (labeled as H in Figure 1 ) being in the axial coordination site (the H-Ru-Pc angle is 173.4º and 179.5º for 1_transPh and 1_transiPr, respectively). The Ru-H distance of 1_transiPr (1.67 Å) is a little shorter than that of 1_transPh (1.69 Å). Different from 1_transPh and 1_transiPr with the hydride (H) trans to the Pc atom, the structure of 1_cisPh and 1_cisiPr contains a cishydride towards to the Pc atom. Both of them exhibit a distorted square-pyramidal geometry with three equatorial phosphorus ligands and the hydride making up the base and the Pc atom at the apex. In cis structures, the Ru–H bond distance is shorter (0.2-0.3 Å) than its respective trans counterpart, indicating a stronger Ru-H bond. In fact, the trans structure is higher in energy than its cis counterpart (8.0/6.0 kcal/mol for
8
P(PPh2)3RuH+/P(P’iPr2)3RuH+), and the barrier for mutual exchange is 4.1/5.1 kcal/mol (from trans to cis) for P(PPh2)3RuH+/P(P’iPr2)3RuH+, as shown in Figure 2. These energetics suggest that both trans and cis isomers could be involved in the catalytic reactions, and thus have to be considered. Next, we are going to investigate the two catalytic cycles (I and II) of P(PPh2)3RuH+ and P(P’iPr2)3RuH+, respectively. Figure 3 shows the Gibbs energy profile for cycle I of P(PPh2)3RuH+ (See all the optimized structures in the Supporting Information). There are two ways for HCOO- to react with P(PPh2)3RuH+, forming either Ru-O bond or Ru-H bond, and the according products are 6_cisPh and 2_cisPh, respectively. Both processes are exothermic and spontaneous, with 6_cisPh ca. 7.9 kcal/mol lower in energy than 2_cisPh due to stronger Ru-O bond, but only 2_cisPh leads to more stable product afterwards. A similar phenomenon is also observed in the iron catalyzed dehydrogenation of formic acid.11 In this cycle, only the pathway starting from 1_cisPh is shown, because for trans case, 2_transPh and TS2trans_3Ph are not able to be located after numerous attempts. Moreover, 6_cisPh is lower in energy than 6_transPh (ca. 5.3 kcal/mol). Although it fails to optimize 2_transPh and TS2trans_3Ph structures, the hydrogen transfer from HCOO- to P(PPh2)3RuH+ for both trans and cis cases would result in the same products, neutral 3Ph + CO2. In neutral 3Ph, two hydrides occupy both the trans and cis position of Ru complex, and the H-H distance is ca. 2.26 Å, making it a clear dihydride structure. The mutual exchange between dihydride and dihydrogen ligands have been studied in the previous study,34 and we also investigate this possibility in our case. It turns out that only the cis hydride could move to the trans position, making the new H-H distance ca. 0.85 Å in 4_transPh, while the opposite could not (4_cisPh is not stable). The barrier for this process
9
is ca. 11.7 kcal/mol. The H-H distance in 4_transPh (0.85 Å) is just a little longer than that in free H2 molecule (0.74 Å) at the same level, indicating 4_transPh a more dihydrogen structure. The empty cis position of 4_transPh provides a space for free proton (H+) attacking to become 5_transPh. However, there is another one-step route to arrive at 5_transPh for 3Ph via the protonation by a proton from a formic acid molecule (TS3_5transPh), and this route requires much less energy (8.5 kcal/mol) than the route via TS3_4transPh (11.7 kcal/mol). This route was also investigated for Fe-catalyzed formic acid dehydrogenation, and the corresponding barrier between 3Ph for 5_transPh is almost thermoneutral.10 Compared with 4_transPh, the H-H distance (0.83 Å) in the trans position of 5_transPh becomes a little shorter. Dissociation of dihydrogen ligands from 5_transPh requires 12.3 kcal/mol, regenerating 1_cisPh along with H2 molecule. 5_transPh is a distorted octahedral structure with the hydride and dihydrogen ligands in mutually cis coordination sites. A similar structure has been studied by others,29 and fast exchange between the hydrido and dihydrogen ligands has been found at room temperature. As shown in the inset of Figure 3, 5_transPh and 5_cisPh are relatively close in energy (less than 10 kcal/mol apart) and the barrier between them is fairly low. Dissociation of dihydrogen ligands from 5_cisPh reproduces 1_transPh along with H2 molecule. After comparing all the relative energies shown in Figure 3, the total Gibbs energy barrier is 14.3 kcal/mol (3Ph → TS5trans_1cisPh), which is far less than Fe-catalyzed formic acid dehydrogenation (18.9 kcal/mol).10 On the other hand, P(PPh2)3RuH+ can also react with neutral HCOOH to generate CO2 and H2 (cycle II), as shown in Figure 4 for the corresponding Gibbs energy profiles. Both trans (Figure 4a) and cis (Figure 4b) P(PPh2)3RuH+ are found to be able to be react
10
with neutral HCOOH, despite the mutual exchange between the two isomers. In both reactions, the first step is the coordination of a formic acid molecule to the vacant position of P(PPh2)3RuH+. In the intermediate 7_transPh/7_cisPh, there is a strong Ru-Hδ···Hδ+-O interaction, with the Hδ-···Hδ+ distance 1.48/1.55 Å, comparable to the previous study on iron.11 The Hδ+ approaches to the Hδ- to generate 8_transPh/8_cisPh with dihydrogen ligand. In cis case, structure 8_cisPh is thermoneutral to TS7cis_8cisPh, and the latter is thus not shown on the potential energy surface (PES). After the dihydrogen ligand’s dissociation, both 8_transPh and 8_cisPh arrive at the same structure, 9Ph, possessing two Ru-O bonds (2.30 and 2.24 Å for trans and cis position). 9Ph is the most stable species on both trans and cis PESs. There are two pathways for CO2 generation from structure 9Ph: from trans or cis site of Ru center, as seen in 12_transPh or 12_cisPh. Generally, three steps are involved from 9Ph to 12_cisPh: Ru-O elongation (TS9_10cisPh), OH flip-flopping (TS10cis_11cisPh), and H transfer (TS11cis_12cisPh). For trans case, however Ru-O elongation (TS9_10transPh) and the according intermediate 10_transPh are not stable, making 9Ph directly to connect 11_transPh via TS9_11transPh. From structure 9Ph, it would be much easier for it to release CO2 to get back to 1_cisPh, overcoming a 16.0 kcal/mol barrier (TS10cis_11cisPh) than to 1_transPh with 19.4 kcal/mol barrier (TS11trans_12transPh). Both of them though are remarkably higher than the overall barrier (14.3 kcal/mol) in cycle I. As discussed previously, a variety of transition metal complexes were explored to do formic acid dehydrogenation, such as Fe,8-11 Ru,12-16 Co,17, 18 and Ir.19, 20 In addition to metal itself, we also wonder how the dentate ligands play a role to the catalytic reaction. Isopropyl group is a more electron-donating group than phenyl group. Take 1_cisPh and 1_cisiPr for example, and Ru Mulliken charge is -0.69 |e| and -0.83 |e|, respectively.
11
P(CH2CH2CH2PiPr2)3RuH+ (short as P(P’iPr2)3RuH+), derived from the previous experimental study,29 is thought to be a potential candidate for formic acid dehydrogenation. Figure 5 shows the Gibbs energy profile for cycle I of P(P’iPr2)3RuH+. Despite a generally similar reaction route, there are a few differences between the PESs of P(PPh2)3RuH+ and that of P(P’iPr2)3RuH+. In the latter case, we were able to locate the 2_transiPr and TS2trans_3iPr, but they both lie above their respective cis counterparts. Both trans and cis P(P’iPr2)3RuH+ end up with neutral 3iPr + CO2 after reacting with HCOO-. We were able to locate the TS3_4transiPr, but it is lower in energy than 4_transiPr by 1.6 kcal/mol. TS3_5transiPr still lies lower in energy than 4_transiPr, as in P(PPh2)3RuH+, and becomes the rate-determining step of cycle I for P(P’iPr2)3RuH+ (12.9 kcal/mol relative to 3iPr), surpassing TS5trans_1cisiPr. Figure 6 shows the Gibbs energy profiles for cycle II of P(P’iPr2)3RuH+. Generally speaking, the PESs for cycle II of P(P’iPr2)3RuH+ are similar to those of P(PPh2)3RuH+. Major difference however exists for the PES prior to 9iPr. The barrier for the process from 1_transPh/1_cisPh to 9Ph, namely H2 production, is only 6.8 kcal/mol (8_transPh → TS8trans_9Ph; or 8.1 kcal/mol if considering 1_cisPh) and 10.8 kcal/mol (1_cisPh → TS8cis_9Ph), significantly lower than the following CO2 generation, as discussed previously. By contrast, the H2 production in P(P’iPr2)3RuH+ case requires more energy, 23.7 kcal/mol for 1_transiPr (1_cisiPr → TS8trans_9iPr; keep in mind that 1_cisiPr is more stable than 1_transiPr by 6 kcal/mol) and 26.4 kcal/mol for 1_cisiPr (1_cisiPr → TS8cis_9iPr). As for CO2 generation, the activation energy is 18.3 kcal/mol (9iPr → TS11trans_12transiPr) and 14.8 kcal/mol (9iPr → TS11cis_12cisiPr) for returning to 1_transiPr and 1_cisiPr, respectively. When comparing all the relative energies, it should be pointed out that for H2 production, it is a
12
little easier for 1_cisiPr to convert to 1_transiPr in the first place (small barrier 11.1 kcal/mol as seen in Figure 2b), followed by passing TS8trans_9iPr than for 1_cisiPr to directly overcome TS8cis_9iPr. When reaching H2 production intermediate 9iPr, it is much easier for it to convert back to 1_cisiPr than to 1_transiPr. Therefore, the total Gibbs energy barrier for cycle II of P(P’iPr2)3RuH+ is 23.7 kcal/mol, with the key detailed pathway being 1_cisiPr to 1_transiPr to TS8trans_9iPr to 9iPr to TS11cis_12cisiPr to 1_cisiPr. We might notice in the above discussions, the starting points for cycle I and cycle II are different: HCOO- (somewhat like catalyst) and HCOOH in cycle I, while HCOOH only in cycle II. Formic acid is a week acid and thus mostly in the neutral form in solvent. We are aware that formic acid to some extent tends to exist as hydrogen-bonded dimers rather than individual molecules and counter anion in the solution is omitted in our computation, but our sophisticate model would be adequate to make a direct comparison of different metal centers and dentate ligands. Now we can conclude that for P(PPh2)3RuH+ and P(P’iPr2)3RuH+, the most likely pathway is that the Ru catalyst reacts with formate anion (HCOO-) in the first place, followed by CO2 generation and H2 production, and either TS5trans_1cisPh or TS3_5transiPr is the rate-determining step. For both Ru catalysts, cycle I is more accessible than cycle II. However, the difference of the reaction barriers of the two cycles is quite different: 1.7 kcal/mol for P(PPh2)3RuH+, and 10.8 kcal/mol for P(P’iPr2)3RuH+, indicating cycle II plays an even less role in P(P’iPr2)3RuH+ catalyzed formic acid dehydrogenation than in P(PPh2)3RuH+. Nevertheless, both Ru complexes show a better performance on formic acid dehydrogenation than the iron complex ((PPh2)3FeH+,) used in the previous studies.8, 10, 11
13
Conclusions In summary, we have performed a detailed computational study on the dehydrogenation mechanisms of formic acid catalyzed by P(PPh2)3RuH+ and P(P’iPr2)3RuH+. Similar to the iron catalyzed formic acid dehydrogenation, two competing catalytic cycles (I and II) of Ru catalysts were investigated, and the results show the cycle I is more accessible no matter what the dentate ligand is. Both trans and cis isomer of P(PPh2)3RuH+ and P(P’iPr2)3RuH+ were investigated for catalyzing the formic acid dehydrogenation. Generally speaking, the cis isomer of Ru catalysts is more stable than its trans isomer, and is thus believed to dominate the whole catalytic reaction. P(PPh2)3RuH+ edges its iron analogue by 4.6 kcal/mol on formic acid dehydrogenation, suggesting a better impact of Ru over Fe. Furthermore, the reaction barrier of P(P’iPr2)3RuH+ is lower by 1.4 kcal/mol than that of P(PPh2)3RuH+, indicating isopropyl group is superior to phenyl group in this regard. These findings might be of great importance in the future choice of tailored ligands around metal centers (e.g. ruthenium) for efficient catalysts for formic acid dehydrogenation.
Notes: The authors declare no competing financial interest.
Acknowledgements This research is supported by the Fundamental Research Funds for the Central Universities of China (Grant No. AUGA5710013115). Computer time made available by the National Supercomputing Center of China in Shenzhen (Shenzhen Cloud Computing
14
Center) is gratefully acknowledged. The author also thanks Prof. Ozerov and Dr. Chun-I Lee at Texas A&M University for helpful comments.
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Figure Captions
(a)
(b)
(c) (d) Ph Ph Figure 1. Optimized structures of 1_trans (a), 1_cis (b), 1_transiPr (c), and 1_cisiPr (d). All hydrogen atoms, except for that bonds with Ru atom, are omitted for clarity. (Gray: Carbon; Orange: Phosphorus; Green: Ruthenium; White: Hydrogen)
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(a) (b) Figure 2. Gibbs energy profiles for trans and cis inter-conversion of two Ru catalysts: P(PPh2)3RuH+ (a) and P(PiPr2)3RuH+ (b).
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Figure 3. Gibbs energy profiles for cycle I of P(PPh2)3RuH+.
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(a)
(b) Figure 4. Gibbs energy profiles for cycle II of P(PPh2)3RuH+.
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Figure 5. Gibbs energy profiles for cycle I of P(PiPr2)3RuH+.
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(a)
(b) Figure 6. Gibbs energy profiles for cycle II of P(PiPr2)3RuH+.
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Scheme 1. Proposed mechanism for the Ru-catalyzed dehydrogenation of formic acid, including two competing cycles I and II.
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