DOI: 10.1002/cssc.201403458
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Amine-Free Reversible Hydrogen Storage in Formate Salts Catalyzed by Ruthenium Pincer Complex without pH Control or Solvent Change Jotheeswari Kothandaraman, Miklos Czaun,* Alain Goeppert, Ralf Haiges, John-Paul Jones, Robert B. May, G. K. Surya Prakash,* and George A. Olah*[a] Due to the intermittent nature of most renewable energy sources, such as solar and wind, energy storage is increasingly required. Since electricity is difficult to store, hydrogen obtained by electrochemical water splitting has been proposed as an energy carrier. However, the handling and transportation of hydrogen in large quantities is in itself a challenge. We therefore present here a method for hydrogen storage based on a CO2(HCO3¢)/H2 and formate equilibrium. This amine-free and
efficient reversible system (> 90 % yield in both directions) is catalyzed by well-defined and commercially available Ru pincer complexes. The formate dehydrogenation was triggered by simple pressure swing without requiring external pH control or the change of either the solvent or the catalyst. Up to six hydrogenation–dehydrogenation cycles were performed and the catalyst performance remained steady with high selectivity (CO free H2/CO2 mixture was produced).
Introduction Growing energy demand, diminishing fossil fuel reserves and increasing CO2 concentration in the atmosphere are paving the way to an increasing reliance on renewable energy sources such as solar, wind, etc. However, the intermittent and fluctuating nature of most of these energy sources is a drawback for their widespread utilization. A practical, large-scale technology for the storage of energy is therefore a significant challenge that has to be solved. In this context, hydrogen is considered as a possible energy storage medium with its only combustion product being water, an environmentally benign material.[1] Hydrogen can be produced by electrochemical water splitting using renewable energy sources. Unfortunately, its low volumetric energy density and high flammability renders its handling and storage very difficult. Thus, there is a need to store hydrogen safely and efficiently. One of the proposed ways is to react it chemically with CO2, an abundant, cheap, non-toxic C1 raw material, which can be captured from a variety of sources.[2] Hydrogenation of CO2 produces many valuable chemicals; for example, CO2 can be reduced to formic acid (FA) or formate salts,[3] formate esters,[4] methane[5] or methanol.[6] Among the CO2 hydrogenation products, formic acid and HCOONa/H2O can generate 4.4 wt % and 2.3 wt % H2, respectively. Formic acid and its salts are non-flam[a] J. Kothandaraman, Dr. M. Czaun, Dr. A. Goeppert, Prof. Dr. R. Haiges, J.-P. Jones, R. B. May, Prof. Dr. G. K. S. Prakash, Prof. Dr. G. A. Olah Loker Hydrocarbon Research Institute and Department of Chemistry University of Southern California University Park, Los Angeles, CA 90089-1661 (USA) Fax: (+ 1) 213-740-5087 E-mail: [email protected] [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403458.
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mable and non-toxic systems, making them safe and convenient H2 storage media. In the presence of an appropriate and convenient catalyst, formic acid and its salts, can be selectively decomposed to hydrogen and CO2 and a carbon neutral cycle can be envisioned by recycling the CO2 back to formic acid (FA) and its salts. FA is also used in dyeing industries, for tanning leathers and in synthetic chemistry; in addition, FA is a food preservative, and formate salts are used as deicing agents.[7] Many iron-,[8] cobalt-,[9] ruthenium-,[10] iridium-,[11] and rhodium-based[12] homogenous catalysts were explored for the hydrogenation of carbon dioxide to formate salts. The highest TON (3 500 000 at 120 8C) and TOF (150 000 h¢1 at 200 8C) were demonstrated by Nozaki et al. using an iridium PNP-pincer catalyst.[11a] On the other hand, numerous ruthenium-,[13] iridium-,[14] iron-,[15] and rhodium-containing[16] complexes were investigated as homogenous catalysts for the dehydrogenation of formic acid. Particularly, Himeda et al. achieved the highest TOF of 228 000 h¢1 at 90 8C for the decomposition of formic acid using a [{Ir(Cp*)(Cl)}2(thbpym)]2 + catalyst (Cp* = pentamethylcyclopentadienide, thbpym = 4,4’,6,6’-tetrahydroxy-2,2’-bipyrimidine).[17] Achieving dehydrogenation of formate (or formic acid) and hydrogenation of CO2 in one pot by altering variables such as temperature, pressure, solvent, or pH is desired for building a hydrogen-storage battery: a charge–discharge device that uses formate as a storage medium.[18] By changing the pH of the reaction mixture, Himeda et al.[17, 19] and Fukuzumi et al.,[20] independently demonstrated reversible hydrogen storage under mild conditions using iridium complexes as catalysts. Beller et al. studied the influence of pressure change on the bicarbonate–formate equilibrium (H2 + HCO3¢ÐHCO2¢ + H2O) to reversibly store hydrogen using [RuCl2(benzene)]2/1,1-bis(di-
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Full Papers phenylphosphino)methane (dppm) as catalyst precursors. In addition to pressure change, this process also demands solvent change to reverse the reaction, which could make its practical application challenging.[21] In another example, Joû and Laurenczy used [RuCl2(mtppms)2]2 as a catalyst precursor with an excess of mtppms ligand (mtppms = meta sodium diphenylphophinobenzene-3-sulfonate).[22] Although Figure 1. Ru-PNP pincer catalysts. the hydrogenation step gave an excellent yield (at a H2 pressure of 100 bar), the dehydrogenation of formate was significantly slower (TOF ~ 1–2 h¢1 at 83 8C at 1 bar) and led to only 40–50 % conversion (based on the NMR experiment). In addition, formic acid amine adducts (FAA) were used to store hydrogen. However, for the long run, it is necessary to prevent the amine vapors from entering the fuel cell, where hydrogen was utilized as a fuel.[23] Herein, we present for the first time an amine-free and efficient reversible hydrogen-storage system that can hydrogenate CO2 or bicarbonate to formate and also generate hydrogen from formate (> 90 % yield in both directions) by simply varying the pressure under relatively mild conditions without any need for an external pH control or change in solvent or catalyst Scheme 1. Carbon neutral cycle using formate as a hydrogen carrier. to reverse the reaction. For instance, carbon dioxide or bicarbonate can be reduced at higher hydrogen pressure to a formate salt using ruthenium-based aliphatic PNP pincer complex 1. The resulting formate salt can then be dehydrogenated to liberate the stored hydrogen at a lower pressure using the same catalyst 1 (Scheme 1, Figure 1).
Results and Discussion Characterization of the active catalytic species In the present study, a trans-diScheme 2. CO2 insertion to dihydride complex 1. Species observed by NMR in a) CDCl3 and b) THF-d8. hydride ruthenium complex with an aliphatic PNP-ligand backbone (1) was used as a pre-catalyst for CO2 hydrogenation. sponding hydride signal appeared as a triplet of doublet at When complex 1 was treated with CO2 (15 bar, RT, THF, 1 h), ¢17.2 ppm (2JH¢P = 19.1 Hz). Interestingly, a four-bond coupling between the hydride and the formate proton was noticed in we observed mostly unreacted starting material (complex 1) the 1H NMR spectrum (d, 4JH¢H = 2.7 Hz). Phosphorous signal at and some amount of hydridochloro complex (2) by 1H NMR in 55.6 ppm (31P NMR), and carbon signals at 173.3 ppm (formate) CDCl3 (catalyst had a good solubility in CDCl3) (Figure S2). The Cl ligand in complex 2 most probably originated from CDCl3 and 204.7 ppm (CO) in the 13C NMR were assigned to complex 3. Similarly, for complex 2, the hydride signal was observed as solvent. However, upon heating complex 1 in THF at 60 8C for a triplet at ¢15.5 ppm (2JH¢P = 19.6 Hz) in the 1H NMR, the an hour under 15 bar CO2 atmosphere, the CO2 insertion product, a formate complex (3), could be detected by 1H, 31P, and phosphorous signal at 52.4 ppm in the 31P NMR and CO signal 13 at 206.2 ppm in the 13C NMR. The formate complex (3) slowly C NMR (Scheme 2 a and Figures S3–S5). Analysis of the gas reacted with the CDCl3 in the J. Young NMR tube replacing the mixture after the reaction showed the presence of H2 in addihydride by Cl and formed a formatochloro complex (4) and tion to CO2. 1H NMR analysis of the reaction mixture in CDCl3 CDHCl2 (Figures S6 and S7). confirmed primarily the formation of the formate complex (3) Single crystal of complex 3 was grown by slow diffusion of along with some amount of 2. The formate proton of compentane into the above NMR sample (in CDCl3) and subjected plex 3 was observed at 8.2 ppm in the 1H NMR and the correChemSusChem 2015, 8, 1442 – 1451
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Figure 2. a) Single crystal X-ray structure of complexes a) 3, b) 6, and c) 5. ORTEP diagrams plotted at 50 % probability level. Selected hydrogen atoms have been omitted for clarity. Selected bond lengths (æ) and bond angles (8): a) (Left) 3 Ru1-H1 = 1.66(3), O2-Ru1 = 2.2323(18), Ru1-P1 = 2.3193(8), Ru1P2 = 2.3268(8), Ru1-N1 = 2.195(2), Ru1-C29 = 1.849(3), C29-O1 = 1.149(3), O2-C30 = 1.240(3), C30-O3 = 1.249(3), C30-H30 = 0.95, O2-C30-O3 = 128.3(3), O3-C30H30 = 115.8. b) (Center) 5 Ru1-Cl1 = 2.4211(7), Ru1-Cl2 = 2.4008(7), Ru1-N1 = 2.181(2), Ru1-P1 = 2.3456(7), Ru1-P2 = 2.3311(7), Ru1-C29 = 1.850(3), C29O1 = 1.142(4), Cl2-Ru1-Cl1 = 174.52(3), N1-Ru1-Cl1 = 86.72(6), Cl2-Ru1-N1 = 88.39(6), O1-C29-Ru1 = 176.4(3). c) (Right) 6 Ru1-H1 = 1.53, Ru1-O2 = 2.2702(12), O2C30 = 1.211(2), C29-O1 = 1.1577(19), C29-Ru1 = 1.8413(16), C30-O3 = 1.262(2), C30-O4 = 1.366(2), O4-H3 = 1.1225, Ru1-P1 = 2.3118(4), Ru1-P2 = 2.3272(4), Ru1N1 = 2.1983(12), C29-O1 = 1.1577(19), C29-Ru1-H1 = 86.3, P1-Ru1-P2 = 162.173(14), O2-Ru1-H1 = 176.7, C29-Ru1-O2 = 96.89(6) and O1-C29-Ru1 = 174.73(14).
to X-ray diffraction for structural characterization. Complex 3 crystallized as distorted octahedrons with a P1-Ru1-P2 bond angle of 163.70(2)8, along with RuCl2(PNP)CO complex, 5 (impurity from the starting material, 1). A bond length of 2.04(3) æ for N¢H···O¢C (N···O = 2.831(3) æ) and a bond angle of 157(3)8 for N¢H···O, both characteristic of hydrogen bonding, were observed between the NH (N1H) group of the ligand and the non-coordinated formate oxygen (O3) in complex 3 (see Figure 2). The hydrogen bonding of N1H with the formate oxygen (O3) was further confirmed by the deshielding of the NH proton of complex 3 to 8.6 ppm in the 1H NMR. Hazari et al. observed similar kind of hydrogen bonding for the Ir-PNP complexes with the formate ligand (N¢H···O¢C bond distance and N¢H···O bond angle in Ir-PNP complexes were 1.925(9) æ and 149.8(5)8, respectively).[11b] Due to a weak intramolecular hydrogen bond with a Ru1¢ Cl1···H1¢N1 distance of 2.78(2) æ (Cl1···N1 = 3.161(2) æ) in complex 5, the chloride ligand Cl1 syn to the N¢H is moved towards nitrogen N1, thus compressing the Cl1¢Ru1¢Cl2 angle from the ideal value of 1808 to 174.51(3)8. The N1¢Ru1¢Cl1 and N1¢H1···Cl1 bond angles are found as 86.71(6)8 and 105.6(1)8. For complex 5, the CO signal appeared as a triplet (2JC¢P = 11.06 Hz) at 204.2 ppm in the 13C NMR, and the phosphorous signal as a singlet at 46.9 ppm in the 31P NMR. The formatochloro complex (4) was identified as a singlet at 48.8 ppm in the 31P NMR and the formate and CO signals were identified as singlet and triplet at 174 ppm and 203.4 ppm (2JC¢P = 11.15 Hz), respectively, in the 13C NMR. As a result of strong hydrogen bonding, the NH proton was deshielded and observed at 9.2 ppm, and the formate proton overlapped with the aromatic protons.[24] We also isolated a ruthenium bicarbonate species (6) in the solid state and the obtained crystals were subjected to single crystal X-ray diffraction. The bicarbonate complex (6) also showed hydrogen bonding of the NH hydrogen (H2) with the bicarbonate oxygen (O3) with a N¢H···O¢ C bond distance of 1.968(1) æ (N···O = 2.844(2) æ) and the N¢ H¢O bond angle of 161.37(9)8. Even though the bicarbonate ChemSusChem 2015, 8, 1442 – 1451
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species (6) was well identified in the solid-state by X-ray study, we were not able to identify the carbonate moiety and the NH proton signals by 13C and 1H NMR, respectively, since it was only present in a minor concentration. In addition, NMR measurements taken in a non-chlorinated solvent, such as [D8]THF, resulted in the formate complex 3 as a major species, which remained stable in the J. Young NMR tube even after 3 days unlike in the chlorinated solvents. (Scheme 2 b; Figures S8–S11) (3, 9.19 (¢NH, t, 1 H), 7.93 (¢OOCH, d, 1 H), 7.74–7.62 (¢ArH, m, 8 H), 7.28–1.18 (¢ArH, m, 12 H), 3.39–3.24 (¢CH2, m, 2 H), 2.99– 2.91 (¢CH2, m, 2 H), 2.58–2.48 (¢CH2, m, 2 H), 2.35–2.22 (¢CH2, m, 2 H), ¢17.2 (Ru¢H, m, 1 H). DFT calculations for the formation of ruthenium formate complex 3 from trans-dihydride ruthenium complex 1’ by N-H assisted CO2 insertion were performed (Figure 3).[25] In the first step, the nucleophilic hydride of the trans-dihydride complex (1’) attacks CO2 and forms an H-bound ruthenium-formate intermediate B (+ 34.4 kJ mol¢1), through the TS1’/B transition state. Similar to Hazari and Crabtree’s CO2 insertion in iridiumPNP complex, N-H hydrogen bonding, evident from Ru¢H1¢C bond angle (155.018) in TS1’/B transition state, brings CO2 closer to Ru and facilitates its insertion across the Ru-H1 bond.[11b] The intermediate B undergoes rearrangement and forms an Obound ruthenium formate complex 3 (product), through an Oand H-bound ruthenium formate transition-state TSB/3.[15b, 26] The activation barrier for the formation of 3 from B is 34.7 kJ mol¢1. The transition states and intermediates that we calculated for the insertion of CO2 to Ru¢H bond are similar to the ones proposed for the Ir complexes with aliphatic and aromatic PNP backbones by Ahlquist[27] and Hazari et al.[11b] Overall, the formation of 3 from 1’ is exergonic by ¢42.6 kJ mol¢1 and a stable O-bound Ru-formate complex (3) is formed via an H-bound Ru-formate complex (B).
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Full Papers yield (90 %) for hydrogenation of sodium bicarbonate without the need to add any external carbon dioxide (Table 1, entry 7). The hydridochloro complex 2 was also studied for the hydrogenation of sodium bicarbonate (Table 1, entry 8, TON = 1150), which showed similar activity as catalyst 1 (TON = 1125). When DMF was used as a co-solvent, instead of THF, a comparable yield was obtained (Table 1, entry 11). In order to understand the role of the N-H moiety on the catalytic activity,[28] the N-Me analogue of catalyst 2, a ruthenium PN(Me)P-pincer complex (7, Figure 1) was prepared according to literature procedures[29] and studied for the hydrogena(Table 1, tion of NaHCO3 entry 12). Contrary to the aliphatic PNP pincer rutheniumcatalyzed reactions such as hydrogenation of cyclic carbonaFigure 3. DFT calculations at the B3LYP/cc-PVDZ level of theory for the CO2 insertion in the ruthenium dihydride tes,[29b] dehydrogenation of complex 1’. The relative free energies were given in kJ mol¢1. methanol,[30] dehydrogenation of ammonia boranes,[31] and hydroHydrogenation of CO2, bicarbonates and carbonates [32] genation of esters, where the N-H moiety of the PNP pincer Hydrogenation of CO2, bicarbonates and carbonates to formate ligand was shown to have an important role, the catalytic activity of 7 for the sodium bicarbonate hydrogenation was not were studied using three different Ru-pincer catalysts (1, 2 and 7, Table 1). When alkali hydroxides, such as, NaOH, KOH and Table 1. Hydrogenation of carbonates, bicarbonates and CO2 in the presence of a catalyst. LiOH were heated under CO2/H2 Solvent Formate yield[b] TON TOF[c] t[a] Entry Catalyst Base p(H2)/p(CO2) T pressure in the presence of [%] [h¢1] [bar] [8C] [min] precursor trans-dihydride ruthenium complex 1, the respective formate 93 1160 232 1 1 NaOH 30/30 76 300 THF/H2O salts were formed (Table 1, en83 1038 455 2 1 KOH 30/30 75 137 THF/H2O tries 1–3). Sodium formate is 80 1000 2000 3 1 LiOH 30/30 75 30 THF/H2O 4 1 Na2CO3 38/12 79 63 THF/H2O 87 2164 2061 formed in a higher yield (93 %) CO 38/12 85 83 THF/H O 70 1750 1265 5 1 K 2 3 2 than the potassium and lithium 38/12 83 300 THF/H2O 15 250 –[d] 6 1 CaCO3 formates (83 % and 80 %, respec7 1 NaHCO3 40/0 80 40 THF/H2O 90 1125 1688 tively) under the same reaction 80 72 THF/H2O 92 1150 958 8 2 NaHCO3 40/0 70 120 THF/H2O 94 1175 588 9 1 NaHCO3 40/0 condition. Similarly, carbonates [e] 40/0 70 < 30 THF/H O 75 938 > 2000 10 1 KHCO 3 2 such as Na2CO3 and K2CO3 were 72 300 DMF/H2O 98 1225 245 11 1 NaHCO3 40/0 converted to their correspond12 7 NaHCO3 40/0 73 76 THF/H2O 77 963 760 ing formate salts in good to 10 775 1096 13 7 NaOH 61/20 70 590 1,4-Dioxane/H2O 84 moderate yields at high H2/CO2 [a] Time at which no more change in pressure was observed. [b] NMR yields were calculated after evaporating pressure (Table 1, entries 4 and the solvents (THF/H2O mixture was evaporated in rotovap and the DMF/H2O mixture was evaporated in lyophilizer) and by using THF and DMF as internal standard with a relaxation delay of 10 s and the error in the 5). The calcium formate yield yield calculation is < 10 %. [c] TOF was calculated using t. [d] Unable to calculate the TOF because of the low was only 15 % with CaCO3 even reaction rate and the 10 % yield obtained was only after heating the reaction mixture for 5 h. [e] Substrate was in the presence of external CO2, hydrogenated before the autoclave reaches the set temperature; for all experiments, 10 mmol catalyst, which could be due to the low 12.5 mmol base, organic solvent (5 mL) ,and H2O (10 mL) were used except, entry 1, 2 and 13; for entries 1 and 2, 20 mmol catalyst, 25 mmol base, organic solvent (6 mL), and H2O (13 mL) were used; 2 mmol catalyst, solubility of CaCO3 in the reac20 mmol base, organic solvent (20 mL) and H2O (10 mL) were used for entry 13; p(H2)/p(CO2) were given at RT; tion mixture (Table 1, entry 6). yields were calculated with respect to the base. Catalyst 1 also showed high ChemSusChem 2015, 8, 1442 – 1451
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Figure 4. Single crystal X-ray structure of complex 2 and 7. ORTEP diagrams plotted at 50 % probability level. Crystals of complex 2 also contains ~ 10 % of Cl substituted in hydride and CO position. Selected hydrogen atoms have been omitted for clarity. Selected bond length (æ) and bond angle (8): a) (left) (2) Cl1Ru1 = 2.4979(6), N1-H1N = 0.85(3), P2-Ru1 = 2.3142(6), N1-Ru1 = 2.1736(18), P1-Ru1 = 2.3142(6), Ru1-H1 = 1.629(18), C29-O1 = 1.172(9), C29-Ru1 = 1.826(5), Cl1Ru1-H1 = 175.8(12), P1-Ru1-H1 = 84.4(11), P2-Ru1-H1 = 91.7(11), P2-Ru1-P1 = 164.71(2), N1-Ru1-Cl1 = 85.52(5). b) (right) (7) Ru1-H1 = 1.60(3), P1-Ru1 = 2.3341(5), P2-Ru1 = 2.2942(4), Cl1-Ru1 = 2.5089(4), N1-Ru1 = 2.2271(15), C1-O1 = 1.147(2), C1-Ru1 = 1.8398(19), C6-N1 = 1.498(2), P1-Ru1-H1 = 89.2(9), Cl1-Ru1H1 = 173.9(9), N1-Ru1-H1 = 87.2(9), N1-Ru1-Cl1 = 86.75(4), P2-Ru1-P1 = 165.446(17), O1-C1-Ru1 = 175.23(17).
significantly affected by the absence of N-H moiety as reflected by the observed TON (963 at 73 8C, compared to the corresponding value of 1175 for catalyst 1 at 70 8C). Hanson and coworkers made similar observations in Co-based aliphatic PNP catalyzed reactions such as hydrogenation of olefins, and dehydrogenation of alcohol.[33] X-ray diffraction of chlorohydrido ruthenium complexes 2 and 7 were carried out to understand how the methyl substitution at the N atom of the PNP ligand affects the bond angles and bond distances. Selected bond lengths and bond angles are given in Figure 4. Complexes 2 and 7 crystallized by slow diffusion of pentane to a dichloromethane solutions of complex 2 and 7, respectively. In complexes 2 and 7, the RuII center shows a distorted octahedron geometry with P2-Ru1-P1 bond angles of 164.71(2)8 and 165.446(17)8, respectively. Similar to the IrH2Cl(iPr2PCH2CH2)2NH complex,[34] in complex 2 a weak intramolecular H-bonding was observed between Cl1 and N1H with the Ru¢Cl···H¢N bond distance of 2.77(2) æ (Cl···N = 3.180(2) æ), which was further supported by the bending of chloride ligand towards the nitrogen (N1-Ru-Cl1 bond angle = 85.52 (5)8). We also observed that the presence of a methyl substituent on the N atom of the PNP ligand caused a significant change (lengthening) in the Ru¢N bond distance of complex 7 (2.2271(15) æ) compared to the ¢N¢H-containing PNP ruthenium complex 2 (2.1736(18) æ). Dehydrogenation of formate salts Next, the Ru-PNP pincer catalysts 1, 2 and 7 used for CO2 and HCO3¢ hydrogenation, were also studied in the formate dehydrogenation reaction. Different formate salts such as lithium formate, sodium formate, potassium formate, and cesium formate were tested as substrates for the dehydrogenation reaction in dioxane/H2O mixtures (Table 2, Figure 5). A relatively ChemSusChem 2015, 8, 1442 – 1451
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Figure 5. Effect of temperature, solvent and counter cation on the reaction rate. Entries 1-8 are from Table 2.
high boiling solvent, 1,4-dioxane, was used in place of THF in order to minimize solvent loss. Dioxane was tested previously (Table 1, entry 13) as a co-solvent for the hydrogenation step using catalyst 7, where we were able to achieve a high TON of 10 775. The gas evolved in the course of the dehydrogenation reaction was measured with a gas burette. GC analysis of the collected gas showed a H2/CO2 gas mixture, with no detectable amount of CO. This CO-free H2/CO2 gas mixture could therefore be directly used in hydrogen fuel cell applications. Both catalysts 1 and 2 displayed the same catalytic activity for the sodium formate dehydrogenation with an initial TOF of 286 h¢1 at 69 8C (entry 1–2, Table 2). Increasing the water content in the reaction mixture by two fold decreased the catalyst activity due to the low solubility of the catalyst in the aqueous media (entry 3, Table 2). Interestingly, ruthenium PN(Me)P-pincer complex 7 was more
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Full Papers tion of H2 and CO2 from sodium formate (Figure 6). Thus, in H2 yield TOF TON T Initial TOF Entry Catalyst Substrate t Volume of H2 Figure 6, we show the result of [mL] [h¢1] [%] [8C] [h¢1] precursor [min] a prototype of a H2-based charge–discharge device operat1 1 HCOONa 270 490 100 69 286 222 1000 2 2 HCOONa 300 485 99 69 286 198 990 ed by pressure swing. In addi3[b] 2 HCOONa 420 189 39 70 98 55 388 tion, in a practical system, the 4 7 HCOONa 240 490 100 68 430 250 1000 energy required for both the hy5 1 HCOOK 420 450 92 70 290 132 925 drogenation and the dehydro6 1 HCOOK 600 372 76 61 172 76 763 7 1 HCOOLi 180 490 100 71 437 333 1000 genation step can be obtained 8 1 HCOOCs 420 424 87 70 234 124 869 from waste energy sources and 663 994 9 1 HCOONa 90 487 99.4 84 820[c] solar thermal energy. The best 7 HCOONa 1440 243 50 70 735 208 5000 10[d] volumetric storage capacity re[a] Reaction conditions: Catalyst = 20 mmol, substrate = 20 mmol, total volume of the solvent = 30 mL (20 mL ported in the literature for the 1,4-dioxane + 10 mL H2O). [b] Total volume of the solvent = 30 mL (10 mL 1,4-dioxane + 20 mL H2O). Initial amine-free pressure swing hyTOF = TOF in first 2 h of the reaction. The error in the yield calculation is < 10 %. [c] TOF in first 1 h. [d] 2 mmol of catalyst was used. drogen-storage system was 16.4 L H2 per L of the solution, which is comparable to the present system, where we obtained 16.4 L H2 per L of the solution active than catalysts 1 and 2 for formate dehydrogenation (Table S3).[21] The former work required, however, solvent with an initial TOF of 430 h¢1(entry 4, Table 2). Among the difchange to reverse the reaction. The pH of the reaction mixture ferent formate salts examined (entry 1, 5, 7 and 8, Table 2), lithchanged from 10.77 to 7.86 during the hydrogenation of ium and sodium formates gave 100 % yield in 5 h with an inisodium bicarbonate to sodium formate. Under the reaction tial TOF of 437 and 286 h¢1, respectively, at 70 8C. Combining the hydrogenation and dehydrogenation studies (Table 1 and conditions, the pH change occurs spontaneously and no exterTable 2), it is evident that the sodium bicarbonate/sodium fornal pH control is necessary to reverse the reaction. The activamate system is suitable for reversible hydrogen storage. tion energy for the formate decomposition calculated from the In order to demonstrate the reversibility of the CO2 hydrogeArrhenius equation over the temperature range of 60 to 85 8C nation system in the presence of catalyst 1, the dehydrogenawas 80.7 kJ mol¢1 (Figure S14). tion of the in-situ-formed formate was attempted in the same pot (Figure 6). Sodium bicarbonate was hydrogenated to Consecutive CO2 hydrogenation (charge)/formate dehydrosodium formate in THF/H2O mixture at a H2 pressure of genation (discharge) cycles ~ 53 bar at 80 8C. After cooling the reaction mixture to RT and releasing the remaining hydrogen (however, in a practical inThe hydrogenation of CO2 to formate was performed in the dustrial unit, the excess hydrogen can be recycled rather than presence of NaOH and catalyst 1 at 70 8C and 75 bar of a 3:1 vented), the reaction mixture was heated to 60 8C, which remixture of H2 and CO2. The dehydrogenation was studied at sulted in a continuous pressure increase, indicating the genera70 8C and atmospheric pressure. Six hydrogenation–dehydrogenation cycles were performed, during which the catalyst did not show any significant loss in activity (Figure 7), as shown by the hydrogen yield during the dehydrogenation steps. In all six cycles of this amine-free system, there was no detectable amount of CO in the gas mixture, potentially enabling this catalyst to be applied in a hydrogen battery. A combined TON > 11 500 was obtained from all six hydrogenation–dehydrogenation cycles. Table 2. Dehydrogenation of formate salts.[a]
Mechanistic insight
Figure 6. Pressure vs. time plot for a) Hydrogenation of NaHCO3 to formate under higher H2 pressure (53 bar) in the presence of catalyst 1 at 80 8C. b) Dehydrogenation of the in situ formed formate at 60 8C at lower pressure. Reaction conditions: Catalyst 1 = 20 mmol, NaHCO3 = 20 mmol, Total volume of the solvent = 17 mL (5 mL THF + 12 mL H2O).
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Two possible pathways for the hydrogenation of CO2 and the dehydrogenation of formate are proposed (Scheme 3). The first step is the insertion of CO2 to the Ru¢H bond in trans-dihydride complex 1’(or 1) by nucleophilic attack of the hydride on the carbon atom of CO2 to form ruthenium formate complex 3. In the second step, formate decoordinates, forming a 16electron complex (8). Then, a hydrogen molecule coordinates to the ruthenium center to give 9. Subsequently, the base abstracts a proton from the dihydrogen complex 9, to form water and trans-dihydride complex 1’ (Pathway I, Scheme 3).
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Full Papers hydrogenation through pathway II conspicuously inaccessible, but the reaction can possibly take place via pathway I. This is supported by the fact that instead of complete inhibition very similar performance was observed using complex 2 and 7, respectively (Table 1). Deuterium labelling studies on formate and water were carried out to understand the isotopic effects on the reaction rate of dehydrogenation (Table 3). The deuterium kinetic isotopic effect (KIE) was high with DCOONa (DCOONa/H2O, KIE = 2.2, Table 3, entry 3) compared to D2O solvent (HCOONa/D2O KIE = 1.7, Table 3, entry 2), showing that the b-hydride elimination, that is, the decarboxylation step is the rate-limiting step for the sodium formate decomposition.[15c, 35] Figure 7. Multiple CO2 hydrogenation/formate dehydrogenation cycles in the presence of catalyst 1 at 70 8C for 5 h. Hydrogenation and dehydrogenation steps were performed at 75 bar (3:1) H2/CO2 and atmospheric pressure, respectively. Percentage (%) yields were calculated from the amount of H2 generated in the dehydrogenation step based on the initial base (NaOH) content. In a separate experiment, the formate yield calculated after the first CO2 hydrogenation step was 93 %. Catalyst 1 = 20 mmol, NaOH = 22 mmol, Total volume of the solvent = 30 mL (20 mL dioxane + 10 mL H2O).
Hazari et al. and Ahlquist proposed a similar kind of mechanism (pathway I, Scheme 3) for the IrIII trihydride complex with PNP ligand for the CO2 hydrogenation.[11b, 27] On the other hand, the second pathway goes via an amido complex 10, which is formed by the deprotonation of the NH group in the PNP ligand of complex 3. Then, amido complex 10 heterolytically cleaves H2 by metal-ligand cooperation to regenerate trans-dihydride 1’ (Pathway II, Scheme 3). Pathway II involves an amido intermediate 10, which is similar to the mechanism proposed for the transfer hydrogenation of ketones using IrH3[(iPr2PCH2CH2)2NH].[33] The presence of a N-Me moiety in complex 7 would make the hydrogenation of CO2/formate de-
Table 3. Deuterium kinetic isotopic effect study.[a] Entry
Substrate/co-solvent
Reaction rate [10¢5 m s¢1]
KIE
1 2 3 4
HCOONa/H2O HCOONa/D2O DCOONa/H2O DCOONa/D2O
8.232 4.844 3.792 3.079
1 1.7 2.2 2.7
[a] Reaction conditions: Catalyst 1 = 10 mmol, substrate = 5 mmol, total volume of the solvent = 7.5 mL (5 mL 1,4-dioxane + 2.5 mL H2O or D2O); for entries 2 and 4, dry dioxane was used.
NMR study on formate decomposition in the presence of catalyst 1
The catalyst 1 and sodium formate in [D8]THF and D2O mixture were heated to 60 8C for 1.5 h in a J. Young NMR tube. Continuous gas evolution was observed throughout the experiment. Due to the reversible nature of the reaction, NMR analysis of the reaction mixture showed the formate species 3, with the partial deuteration at NH, ¢ OOCH and Ru¢H (Figure S15 and S16). When the same experiment was conducted in H2O instead of D2O, hydridoformate complex 3 (¢17.2 ppm) was the only major species that was identified in the 1 H and 31P NMR of the reaction mixture (Figure S17–S19). Thus, complex 3 is the catalyst’s resting state and this further supports the assertion that the decarboxylation step (from 3 to 1’) is the rate-determining step in the formate decomposition process.[15c] In addition to 3, there were also minor amount of unidentified species in the 1 H NMR at ¢17.5 ppm (t, J = 19.4 Hz), ¢16.1 ppm (t, J = 19.9 Hz) and ¢13.1 ppm (t, J = 18.5 Hz). Scheme 3. Proposed catalytic cycles for the hydrogenation/dehydrogenation reactions. rds = rate-determining step for the dehydrogenation.
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Full Papers Conclusions For the first-time, a practical, efficient, (yields were ~ 90 % in both direction) amine-free system was developed that can hydrogenate CO2(/HCO3¢) to formate without external pH control or solvent change, and generate hydrogen from the in-situformed formate in one pot. This system was demonstrated by simply varying the pressure, using well-defined commercially available ruthenium catalysts. Continuous performance and high selectivity in all six CO2 hydrogenation/formate dehydrogenation cycles further display the practicality of this system. Unlike other ruthenium-based catalyst systems, where the N-H moiety plays a critical role by metal-ligand bifunctional catalysis, we have presented here the first example of a ruthenium system, where the catalytic activity was independent of the NH moiety of the PNP pincer ligand. This was proven by similar activities, under our reaction conditions, for the Ru catalysts with the PNP pincer ligand containing N-Me (7) and N-H (2) moieties.
Experimental Section Materials and Instrumentation Sodium bicarbonate (Mallinckrodt, 99.7 %), sodium hydroxide (Mallinckrodt, 98.8 %), sodium carbonate (Mallinckrodt, 99.9 %) potassium hydroxide (Mallinckrodt, 88 %), potassium bicarbonate (Baker analyzed reagent, > 99.9 %), potassium carbonate (Baker analyzed reagent, > 99.9 %), lithium hydroxide (Aldrich, 98 %), calcium carbonate (Fisher scientific, > 99.9 %), HCOOLi·H2O (Aldrich, 98 %), HCOONa (Mallinckrodt, 99.8 %), HCOOK (Aldrich, 99 %), HCOOCs (Aldrich, 98 %) tetrahydrofuran (EMD, 99.9 %), dimethyl formamide (EMD, 99.8 %), dichloromethane (Macron, 99.5 %), pentane (EMD, 98 %), CDCl3 (Cambridge Isotopes lab, 99.8 %), dioxane (Mallinckrodt, 99 %), H2 gas (Gilmore, ultra high pure grade), CO2 gas (Gilmore, instrument grade), 3:1 H2/CO2 gas (Airgas, grade = certified standard-spec), 1:1 H2/CO2 gas (Airgas, grade = certified standardspec) were used as received without any purification. Carbonylhydrido(tetrahydroborato)[bis(2-diphenylphosphinoethyl) amino]ruthenium(II) (1) (Strem, 98 %), Carbonylchlorohydrido{bis[2 (diphenylphosphinoethyl)amino}ruthenium(II) (2) (Strem, 98 %) and, Carbonyldihydridotris(triphenylphosphine) ruthenium(II) (Alfa Aesar, 98 %), diphenylphosphine (Aldrich, 98 %) and Me-N(CH2CH2Cl)2.HCl (Alfa Aesar, 98 %) were stored in a glove box and handled under argon atmosphere. 1 H, 31P and 13C were recorded on 400 and 500 MHz Varian NMR spectrometers. 1H NMR chemical shifts were determined relative to internal standard, TMS or residual proton signal of D2O (HOD). 31P NMR chemical shifts were determined relative to external standard, H3PO4. 13C NMR chemical shifts were determined relative to 13C signal of CDCl3 solvent. The gas mixture was analyzed using a Thermo gas chromatograph (column: Supelco, Carboxen 1010 plot, 30 m Õ 0.53 mm) equipped with a TCD detector (CO detection limit: 0.099 v/v %).
Catalysis CO2 insertion to Complex 1: Catalyst 1 (0.042 mmol) was added to the degassed anhydrous THF (5 mL) in a 125 mL Monel Parr reactor (autoclave) equipped with a thermocouple and piezoelectric pressure transducer (Figure S1). The autoclave was assembled in a glove box under nitrogen atmosphere and the internal pressure ChemSusChem 2015, 8, 1442 – 1451
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and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at RT (or 60 8C) under CO2 pressure (15 bar) for 1 h. After venting the autoclave, the reaction mixture was transferred to a round bottom flask. The solvent, THF, was evaporated using a rotavap, giving a yellow solid, which was analyzed by 1H NMR. CO2 insertion to Complex 1 in a J. Young NMR tube: 6 mg of catalyst 1 was transferred to the J. Young NMR tube using 1 g [D8]THF in a N2 atmosphere. The N2 atmosphere in the NMR tube was replaced with CO2 by performing three freeze-pump-thaw cycles. The NMR tube was heated to 60 8C under 1 atm of CO2 for 30 min in an oil bath. A pale yellow solution was obtained, which was analyzed by 1H, 31P, and 13C NMR. General procedure for the hydrogenation of CO2 in the presence of NaOH/KOH and catalyst: To the degassed mixture of THF (6 mL) and water (13 mL), catalyst 1 (20 mmol) and NaOH/KOH (25 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and filled with 30 bar H2 and 30 bar CO2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at room temperature for 30 min and heated at 75 8C for 5 h. After completion of the reaction, solvents were evaporated using a rotovap and a white solid was obtained, which was analyzed by 1H and 13C NMR spectroscopy. a) HCOONa yield (using NaOH/76 8C) = 93 %. b) HCOOK yield (using KOH/75 8C) = 83 %. General procedure for the hydrogenation of carbonate in the presence of a catalyst: To the degassed mixture of THF (5 mL) and water (10 mL), catalyst 1 (10 mmol) and carbonate (12.5 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and filled with 50 bar 3:1 H2 :CO2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at room temperature for 30 min and heated to the set temperature 85 8C for 5 h. After completion of the reaction, solvents were evaporated using a rotovap and a white solid was obtained, which was analyzed by 1H and 13 C NMR spectroscopy. a) HCOONa yield (using Na2CO3/79 8C) = 87 %. b) HCOOK yield (using K2CO3/85 8C) = 70 %. c) (HCOO)2Ca yield (using CaCO3/83 8C) = 10 %. General procedure for the hydrogenation of bicarbonate in the presence of a catalyst: To the degassed mixture of THF or DMF (5 mL) and water (10 mL), catalyst (10 mmol) and bicarbonate (12.5 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a nitrogen glove box under nitrogen atmosphere and filled with 40 bar H2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at room temperature for 30 min and heated to the set temperature (70 or 80 8C) for 5 h. After completion of the reaction, solvents were evaporated using a rotovap and a white solid was obtained, which was analyzed by 1 H and 13C NMR spectroscopy (Figures S9 and S10). a) Catalyst 1: HCOONa yield using: NaHCO3 (in THF)/80 8C = 90 %; NaHCO3 (in THF)/70 8C = 94 %; NaHCO3 (in DMF)/72 8C = 98 %. HCOOK yield using: KHCO3 (in THF)/70 8C = 75 %. b) Catalyst 2: HCOONa yield using: NaHCO3 (in THF)/80 8C = 92 %. c) Catalyst 7: HCOONa yield using: NaHCO3 (in THF)/73 8C = 77 %.
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Full Papers General procedure for the formate dehydrogenation in the presence of a catalyst: To the degassed mixture of dioxane (20 mL) and water (10 mL), catalyst (20 mmol) and formate (20 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and heated to the set temperature (60 or 70 8C) until no gas evolution was observed. The gas evolved in the course of the reaction was measured with a burette and the gas mixture was analyzed by GC. a) HCOONa decomposition:H2 yield using catalyst 1 = 100 % at 69 8C; H2 yield using catalyst 2 = 99.4 % at 69 8C; H2 yield using catalyst 7 = 100 % at 68 8C. b) HCOOK decomposition:H2 yield using catalyst 1 = 92 % at 70 8C; H2 yield using catalyst 1 = 76 % at 61 8C. c) HCOOLi decomposition: H2 yield using catalyst 1 = 100 % at 71 8C. d) HCOOCs decomposition: H2 yield using catalyst 1 = 87 % at 70 8C.
Hydrogenation of CO2 : To the degassed mixture of dioxane (20 mL) and water (10 mL), catalyst (20 mmol) and NaOH (22 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and filled with 75 bar 3:1 H2 :CO2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at RT for 30 min and heated to 70 8C for 5 h. Then, the reaction mixture was cooled to RT and the mixture was degased by stirring at RT under atmospheric pressure for 15 min. Dehydrogenation of sodium formate: The above mixture was heated in the same autoclave to 70 8C for 5 h. The gas evolved in the course of the reaction was measured with a burette and the gas mixture was analyzed by GC. The same hydrogenation/dehydrogenation cycle was repeated six times. NMR study on formate decomposition in the presence of catalyst 1 and [D8]THF/D2O mixture: 6 mg of catalyst 1 and 13.6 mg of HCOONa were transferred to J. Young tube using 0.8 mL of [D8]THF in N2 atmosphere. To this mixture, 0.5 mL D2O was added. At room temperature, the catalyst is only partially soluble in [D8]THF and H2O mixture (Figure S15). Then the NMR tube was heated to 60 8C for 1.5 h in a oil bath, gas evolution was observed (Note: While heating, the J. Young tube was connected to nitrogen through a needle using a Schlenk line manifold in order to avoid pressure build up). 1H and 31P NMR spectra showed formation of species 3, with partial deuteration at NH, ¢OOCH and Ru¢H (Figure S16). NMR study on formate decomposition in the presence of catalyst 1 and [D8]THF/H2O mixture: 6 mg of catalyst 1 and 13.6 mg of HCOONa were transferred to J. Young tube using 0.8 mL of [D8]THF in N2 atmosphere. To this mixture, 0.1 mL H2O was added. At room temperature, the catalyst is only partially soluble in [D8]THF and H2O mixture (Figure S17 A). Then the NMR tube was heated to 60 8C for 10 min in an oil bath. Gas evolution was observed (Note: While heating, the J. Young tube was connected to nitrogen through a needle using a Schlenk line manifold in order to avoid pressure build up). At this stage, only trace amount of sodium formate was left. However, a significant amount of catalyst 1 remained unreacted (Figure S17 B). Therefore, an additional www.chemsuschem.org
Acknowledgements We are grateful to Loker Hydrocarbon Research Institute and United States Department of Energy for financial support.
Consecutive CO2 hydrogenation/formate dehydrogenation cycles
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13.6 mg of sodium formate was added and the NMR tube was heated at 60 8C for an additional 10 min. More of the dihydride complex 1 was converted to hydrido formate complex 3 with continuous gas evolution (¢17.2 ppm) (Figure S17 C). Hydridochloro species 2 was originated from the small amount of dichloro impurity 5 in the starting material. Subsequently, the NMR tube was heated until the gas evolution completely stopped (~ 20 min). A pale yellow solution was obtained. 1H NMR spectrum (Figure S17 D and S18) showed almost complete conversion of 1 to 3, and this was further confirmed by the 31P NMR. (Figure S19) Small amount of non-identified hydride species at ¢17.5 ppm (t, J = 19.4 Hz), ¢16.1 ppm (t, J = 19.9 Hz) and ¢13.1 ppm (t, J = 18.5 Hz) were also observed by 1H NMR.
Keywords: carbon-neutral cycle · energy hydrogenation · PNP ligand · ruthenium
storage
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[1] Hydrogen as a Future Energy Carrier (Eds.: A. Zìttel, A. Borgschulte, L. Schlapbach), Wiley-VCH, Weinheim, Germany, 2008. [2] a) A. Goeppert, M. Czaun, R. B. May, G. K. S. Prakash, G. A. Olah, S. R. Narayanan, J. Am. Chem. Soc. 2011, 133, 20164 – 20167; b) A. Goeppert, M. Czaun, G. K. S. Prakash, G. A. Olah, Energy Environ. Sci. 2012, 5, 7833; c) S. Meth, A. Goeppert, G. K. S. Prakash, G. A. Olah, Energy Fuels 2012, 26, 3082 – 3090; d) A. Goeppert, M. Czaun, J. P. Jones, G. K. S. Prakash, G. A. Olah, Chem. Soc. Rev. 2014, 43, 7995 – 8048; e) M. Czaun, A. Goeppert, R. B. May, D. Peltier, H. Zhang, G. K. S. Prakash, G. A. Olah, J. CO2 Util. 2013, 1, 1 – 7. [3] a) D. K. K. Motokura, A. Miyaji, T. Baba, Org. Lett. 2012, 14, 2642 – 2645; b) W. Leitner, Angew. Chem. Int. Ed. Engl. 1995, 34, 2207 – 2221; Angew. Chem. 1995, 107, 2391 – 2405; c) S. Moret, P. J. Dyson, G. Laurenczy, Nat. Commun. 2014, 5, 1 – 7. [4] C. Federsel, A. Boddien, R. Jackstell, R. Jennerjahn, P. J. Dyson, R. Scopelliti, G. Laurenczy, M. Beller, Angew. Chem. Int. Ed. 2010, 49, 9777 – 9780; Angew. Chem. 2010, 122, 9971 – 9974. [5] S. Park, D. Bezier, M. Brookhart, J. Am. Chem. Soc. 2012, 134, 11404 – 11407. [6] a) G. A. Olah, G. K. S. Prakash, A. Goeppert, J. Am. Chem. Soc. 2011, 133, 12881 – 12898; b) C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122 – 18125; c) L. C. Grabow, M. Mavrikakis, ACS Catal. 2011, 1, 365 – 384; d) G. A. Olah, A. Goeppert, G. K. S. Prakash, Beyond Oil and Gas: The Methanol Economy, 2nd ed., Wiley-VCH, Weinheim, Germany, 2009; e) G. A. Olah, A. Goeppert, G. K. S. Prakash, J. Org. Chem. 2009, 74, 487 – 498. [7] a) W. Reutemann, H. Kieczka in Formic acid—Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Wienheim, Germany, 2011; b) http:// www.intermediates.basf.com/chemicals/formic-acid/de-icing. [8] a) C. Ziebart, C. Federsel, P. Anbarasan, R. Jackstell, W. Baumann, A. Spannenberg, M. Beller, J. Am. Chem. Soc. 2012, 134, 20701 – 20704; b) R. Langer, Y. Diskin-Posner, G. Leitus, L. J. Shimon, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed. 2011, 50, 9948 – 9952; Angew. Chem. 2011, 123, 10122 – 10126. [9] a) M. S. Jeletic, M. T. Mock, A. M. Appel, J. C. Linehan, J. Am. Chem. Soc. 2013, 135, 11533 – 11536; b) C. Federsel, C. Ziebart, R. Jackstell, W. Baumann, M. Beller, Chem. Eur. J. 2012, 18, 72 – 75. [10] a) J. Z. Zhang, Z. Li, H. Wang, C. Y. Wang, J. Mol. Catal. A 1996, 112, 9 – 14; b) C. A. Huff, M. S. Sanford, ACS Catal. 2013, 3, 2412 – 2416; c) H. Hayashi, S. Ogo, S. Fukuzumi, Chem. Commun. 2004, 2714 – 2715; d) P. Munshi, A. D. Main, J. C. Linehan, C.-C. Tai, P. G. Jessop, J. Am. Chem. Soc. 2002, 131, 14168; e) C. Federsel, R. Jackstell, A. Boddien, G. Laurenczy, M. Beller, ChemSusChem 2010, 3, 1048 – 1050.
1450
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers [11] a) R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131, 14168 – 14169; b) T. J. Schmeier, G. E. Dobereiner, R. H. Crabtree, N. Hazari, J. Am. Chem. Soc. 2011, 133, 9274 – 9277; c) A. Azua, S. Sanz, E. Peris, Chem. Eur. J. 2011, 17, 3963 – 3967; d) W.-H. Wang, J. F. Hull, J. T. Muckerman, E. Fujita, Y. Himeda, Energy Environ. Sci. 2012, 5, 7923 – 7926. [12] a) F. Gassner, W. Leitner, J. Chem. Soc. Chem. Commun. 1993, 1465 – 1466; b) F. Zou, J. M. Cole, T. G. J. Jones, L. Jiang, Appl. Organomet. Chem. 2012, 26, 546 – 549. [13] a) M. Czaun, A. Goeppert, R. May, R. Haiges, G. K. S. Prakash, G. A. Olah, ChemSusChem 2011, 4, 1241 – 1248; b) A. Boddien, B. Loges, H. Junge, M. Beller, ChemSusChem 2008, 1, 751 – 758; c) D. J. Morris, G. J. Clarkson, M. Wills, Organometallics 2009, 28, 4133 – 4140; d) M. Czaun, A. Goeppert, J. Kothandaraman, R. B. May, R. Haiges, G. K. S. Prakash, G. A. Olah, ACS Catal. 2014, 4, 311 – 320; e) B. Loges, A. Boddien, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3962 – 3965; Angew. Chem. 2008, 120, 4026 – 4029; f) C. Fellay, P. J. Dyson, G. Laurenczy, Angew. Chem. Int. Ed. 2008, 47, 3966 – 3968; Angew. Chem. 2008, 120, 4030 – 4032; g) A. Boddien, B. Loges, H. Junge, F. Grtner, J. R. Noyes, M. Beller, Adv. Synth. Catal. 2009, 351, 2517 – 2520. [14] a) S. Fukuzumi, T. Kobayashi, T. Suenobu, J. Am. Chem. Soc. 2010, 132, 1496 – 1497; b) Y. Himeda, Green Chem. 2009, 11, 2018; c) S. Oldenhof, B. de Bruin, M. Lutz, M. A. Siegler, F. W. Patureau, J. I. van der Vlugt, J. N. Reek, Chem. Eur. J. 2013, 19, 11507 – 10511; d) Y. Manaka, W.-H. Wang, Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y. Himeda, Catal. Sci. Technol. 2014, 4, 34 – 37. [15] a) A. Boddien, D. Mellmann, F. Gartner, R. Jackstell, H. Junge, P. J. Dyson, G. Laurenczy, R. Ludwig, M. Beller, Science 2011, 333, 1733 – 1736; b) T. Zell, B. Butschke, Y. Ben-David, D. Milstein, Chem. Eur. J. 2013, 19, 8068 – 8072; c) E. A. Bielinski, P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C. Wurtele, W. H. Bernskoetter, N. Hazari, S. Schneider, J. Am. Chem. Soc. 2014, 136, 10234 – 10237. [16] S. Fukuzumi, T. Kobayashi, T. Suenobu, ChemSusChem 2008, 1, 827 – 834. [17] J. F. Hull, Y. Himeda, W. H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, J. T. Muckerman, E. Fujita, Nat. Chem. 2012, 4, 383 – 388. [18] a) B. Zaidman, H. Wiener, Y. Sasson, Int. J. Hydrogen Energy 1986, 11, 341 – 347; b) S. Enthaler, J. von Langermann, T. Schmidt, Energy Environ. Sci. 2010, 3, 1207 – 1217. [19] E. Fujita, J. T. Muckerman, Y. Himeda, Biochim. Biophys. Acta Bioenerg. 2013, 1827, 1031 – 1038. [20] Y. Maenaka, T. Suenobu, S. Fukuzumi, Energy Environ. Sci. 2012, 5, 7360 – 7367.
ChemSusChem 2015, 8, 1442 – 1451
www.chemsuschem.org
[21] A. Boddien, F. Gartner, C. Federsel, P. Sponholz, D. Mellmann, R. Jackstell, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 6411 – 6414; Angew. Chem. 2011, 123, 6535 – 6538. [22] G. Papp, J. Csorba, G. Laurenczy, F. Joo, Angew. Chem. Int. Ed. 2011, 50, 10433 – 10435; Angew. Chem. 2011, 123, 10617 – 10619. [23] a) A. Boddien, C. Federsel, P. Sponholz, D. Mellmann, R. Jackstell, H. Junge, G. Laurenczy, M. Beller, Energy Environ. Sci. 2012, 5, 8907 – 8911; b) S. F. Hsu, S. Rommel, P. Eversfield, K. Muller, E. Klemm, W. R. Thiel, B. Plietker, Angew. Chem. Int. Ed. 2014, 53, 7074 – 7078; Angew. Chem. 2014, 126, 7194 – 7198; c) W. Leitner, E. Dinjus, F. Gartner, J. Organomet. Chem. 1994, 475, 257 – 266; d) K. Sordakis, M. Beller, G. Laurenczy, ChemCatChem 2014, 6, 96 – 99. [24] Complex 4 was tentatively assigned as formatochloro complex. Although the formate carbon was clearly identified by 13CNMR, the formate proton overlapped with the aromatic proton. Presence of chloride ligand in the complex 4 was further concluded by the formation of CHDCl2 in the reaction mixture from CDCl3. [25] Gaussian 09, Revision A.02, M. J. Frisch et al., see Supporting Information. [26] X. Yang, ACS Catal. 2011, 1, 849 – 854. [27] M. S. G. Ahlquist, J. Mol. Catal. A 2010, 324, 3 – 8. [28] a) S. D. Phillips, J. A. Fuentes, M. L. Clarke, Chem. Eur. J. 2010, 16, 8002 – 8005; b) B. Zhao, Z. Han, K. Ding, Angew. Chem. Int. Ed. 2013, 52, 4744 – 4788; Angew. Chem. 2013, 125, 4844 – 4889. [29] a) D. S. McGuinness, P. Wasserscheid, D. H. Morgan, J. T. Dixon, Organometallics 2005, 24, 552 – 556; b) Z. Han, L. Rong, J. Wu, L. Zhang, Z. Wang, K. Ding, Angew. Chem. Int. Ed. 2012, 51, 13041 – 13045; Angew. Chem. 2012, 124, 13218 – 13222. [30] M. Nielsen, E. Alberico, W. Baumann, H. J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85 – 89. [31] M. Kß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. Int. Ed. 2009, 48, 905 – 907; Angew. Chem. 2009, 121, 922 – 924. [32] W. Kuriyama, T. Matsumoto, Y. Ino, O. Ogata, US. Patent 0237814A1, 2011. [33] G. Zhang, K. V. Vasudevan, B. L. Scott, S. K. Hanson, J. Am. Chem. Soc. 2013, 135, 8668 – 8681. [34] Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J. Lough, K. Abdur-Rashid, Organometallics 2006, 25, 4113 – 4117. [35] W. H. Wang, S. Xu, Y. Manaka, Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y. Himeda, ChemSusChem 2014, 7, 1976 – 1983. Received: January 2, 2015 Published online on March 30, 2015
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Full Papers
Amine-Free Reversible Hydrogen Storage in Formate Salts Catalyzed by Ruthenium Pincer Complex without pH Control or Solvent Change Jotheeswari Kothandaraman, Miklos Czaun,* Alain Goeppert, Ralf Haiges, John-Paul Jones, Robert B. May, G. K. Surya Prakash,* and George A. Olah*[a] Due to the intermittent nature of most renewable energy sources, such as solar and wind, energy storage is increasingly required. Since electricity is difficult to store, hydrogen obtained by electrochemical water splitting has been proposed as an energy carrier. However, the handling and transportation of hydrogen in large quantities is in itself a challenge. We therefore present here a method for hydrogen storage based on a CO2(HCO3¢)/H2 and formate equilibrium. This amine-free and
efficient reversible system (> 90 % yield in both directions) is catalyzed by well-defined and commercially available Ru pincer complexes. The formate dehydrogenation was triggered by simple pressure swing without requiring external pH control or the change of either the solvent or the catalyst. Up to six hydrogenation–dehydrogenation cycles were performed and the catalyst performance remained steady with high selectivity (CO free H2/CO2 mixture was produced).
Introduction Growing energy demand, diminishing fossil fuel reserves and increasing CO2 concentration in the atmosphere are paving the way to an increasing reliance on renewable energy sources such as solar, wind, etc. However, the intermittent and fluctuating nature of most of these energy sources is a drawback for their widespread utilization. A practical, large-scale technology for the storage of energy is therefore a significant challenge that has to be solved. In this context, hydrogen is considered as a possible energy storage medium with its only combustion product being water, an environmentally benign material.[1] Hydrogen can be produced by electrochemical water splitting using renewable energy sources. Unfortunately, its low volumetric energy density and high flammability renders its handling and storage very difficult. Thus, there is a need to store hydrogen safely and efficiently. One of the proposed ways is to react it chemically with CO2, an abundant, cheap, non-toxic C1 raw material, which can be captured from a variety of sources.[2] Hydrogenation of CO2 produces many valuable chemicals; for example, CO2 can be reduced to formic acid (FA) or formate salts,[3] formate esters,[4] methane[5] or methanol.[6] Among the CO2 hydrogenation products, formic acid and HCOONa/H2O can generate 4.4 wt % and 2.3 wt % H2, respectively. Formic acid and its salts are non-flam[a] J. Kothandaraman, Dr. M. Czaun, Dr. A. Goeppert, Prof. Dr. R. Haiges, J.-P. Jones, R. B. May, Prof. Dr. G. K. S. Prakash, Prof. Dr. G. A. Olah Loker Hydrocarbon Research Institute and Department of Chemistry University of Southern California University Park, Los Angeles, CA 90089-1661 (USA) Fax: (+ 1) 213-740-5087 E-mail: [email protected] [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403458.
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mable and non-toxic systems, making them safe and convenient H2 storage media. In the presence of an appropriate and convenient catalyst, formic acid and its salts, can be selectively decomposed to hydrogen and CO2 and a carbon neutral cycle can be envisioned by recycling the CO2 back to formic acid (FA) and its salts. FA is also used in dyeing industries, for tanning leathers and in synthetic chemistry; in addition, FA is a food preservative, and formate salts are used as deicing agents.[7] Many iron-,[8] cobalt-,[9] ruthenium-,[10] iridium-,[11] and rhodium-based[12] homogenous catalysts were explored for the hydrogenation of carbon dioxide to formate salts. The highest TON (3 500 000 at 120 8C) and TOF (150 000 h¢1 at 200 8C) were demonstrated by Nozaki et al. using an iridium PNP-pincer catalyst.[11a] On the other hand, numerous ruthenium-,[13] iridium-,[14] iron-,[15] and rhodium-containing[16] complexes were investigated as homogenous catalysts for the dehydrogenation of formic acid. Particularly, Himeda et al. achieved the highest TOF of 228 000 h¢1 at 90 8C for the decomposition of formic acid using a [{Ir(Cp*)(Cl)}2(thbpym)]2 + catalyst (Cp* = pentamethylcyclopentadienide, thbpym = 4,4’,6,6’-tetrahydroxy-2,2’-bipyrimidine).[17] Achieving dehydrogenation of formate (or formic acid) and hydrogenation of CO2 in one pot by altering variables such as temperature, pressure, solvent, or pH is desired for building a hydrogen-storage battery: a charge–discharge device that uses formate as a storage medium.[18] By changing the pH of the reaction mixture, Himeda et al.[17, 19] and Fukuzumi et al.,[20] independently demonstrated reversible hydrogen storage under mild conditions using iridium complexes as catalysts. Beller et al. studied the influence of pressure change on the bicarbonate–formate equilibrium (H2 + HCO3¢ÐHCO2¢ + H2O) to reversibly store hydrogen using [RuCl2(benzene)]2/1,1-bis(di-
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Full Papers phenylphosphino)methane (dppm) as catalyst precursors. In addition to pressure change, this process also demands solvent change to reverse the reaction, which could make its practical application challenging.[21] In another example, Joû and Laurenczy used [RuCl2(mtppms)2]2 as a catalyst precursor with an excess of mtppms ligand (mtppms = meta sodium diphenylphophinobenzene-3-sulfonate).[22] Although Figure 1. Ru-PNP pincer catalysts. the hydrogenation step gave an excellent yield (at a H2 pressure of 100 bar), the dehydrogenation of formate was significantly slower (TOF ~ 1–2 h¢1 at 83 8C at 1 bar) and led to only 40–50 % conversion (based on the NMR experiment). In addition, formic acid amine adducts (FAA) were used to store hydrogen. However, for the long run, it is necessary to prevent the amine vapors from entering the fuel cell, where hydrogen was utilized as a fuel.[23] Herein, we present for the first time an amine-free and efficient reversible hydrogen-storage system that can hydrogenate CO2 or bicarbonate to formate and also generate hydrogen from formate (> 90 % yield in both directions) by simply varying the pressure under relatively mild conditions without any need for an external pH control or change in solvent or catalyst Scheme 1. Carbon neutral cycle using formate as a hydrogen carrier. to reverse the reaction. For instance, carbon dioxide or bicarbonate can be reduced at higher hydrogen pressure to a formate salt using ruthenium-based aliphatic PNP pincer complex 1. The resulting formate salt can then be dehydrogenated to liberate the stored hydrogen at a lower pressure using the same catalyst 1 (Scheme 1, Figure 1).
Results and Discussion Characterization of the active catalytic species In the present study, a trans-diScheme 2. CO2 insertion to dihydride complex 1. Species observed by NMR in a) CDCl3 and b) THF-d8. hydride ruthenium complex with an aliphatic PNP-ligand backbone (1) was used as a pre-catalyst for CO2 hydrogenation. sponding hydride signal appeared as a triplet of doublet at When complex 1 was treated with CO2 (15 bar, RT, THF, 1 h), ¢17.2 ppm (2JH¢P = 19.1 Hz). Interestingly, a four-bond coupling between the hydride and the formate proton was noticed in we observed mostly unreacted starting material (complex 1) the 1H NMR spectrum (d, 4JH¢H = 2.7 Hz). Phosphorous signal at and some amount of hydridochloro complex (2) by 1H NMR in 55.6 ppm (31P NMR), and carbon signals at 173.3 ppm (formate) CDCl3 (catalyst had a good solubility in CDCl3) (Figure S2). The Cl ligand in complex 2 most probably originated from CDCl3 and 204.7 ppm (CO) in the 13C NMR were assigned to complex 3. Similarly, for complex 2, the hydride signal was observed as solvent. However, upon heating complex 1 in THF at 60 8C for a triplet at ¢15.5 ppm (2JH¢P = 19.6 Hz) in the 1H NMR, the an hour under 15 bar CO2 atmosphere, the CO2 insertion product, a formate complex (3), could be detected by 1H, 31P, and phosphorous signal at 52.4 ppm in the 31P NMR and CO signal 13 at 206.2 ppm in the 13C NMR. The formate complex (3) slowly C NMR (Scheme 2 a and Figures S3–S5). Analysis of the gas reacted with the CDCl3 in the J. Young NMR tube replacing the mixture after the reaction showed the presence of H2 in addihydride by Cl and formed a formatochloro complex (4) and tion to CO2. 1H NMR analysis of the reaction mixture in CDCl3 CDHCl2 (Figures S6 and S7). confirmed primarily the formation of the formate complex (3) Single crystal of complex 3 was grown by slow diffusion of along with some amount of 2. The formate proton of compentane into the above NMR sample (in CDCl3) and subjected plex 3 was observed at 8.2 ppm in the 1H NMR and the correChemSusChem 2015, 8, 1442 – 1451
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Figure 2. a) Single crystal X-ray structure of complexes a) 3, b) 6, and c) 5. ORTEP diagrams plotted at 50 % probability level. Selected hydrogen atoms have been omitted for clarity. Selected bond lengths (æ) and bond angles (8): a) (Left) 3 Ru1-H1 = 1.66(3), O2-Ru1 = 2.2323(18), Ru1-P1 = 2.3193(8), Ru1P2 = 2.3268(8), Ru1-N1 = 2.195(2), Ru1-C29 = 1.849(3), C29-O1 = 1.149(3), O2-C30 = 1.240(3), C30-O3 = 1.249(3), C30-H30 = 0.95, O2-C30-O3 = 128.3(3), O3-C30H30 = 115.8. b) (Center) 5 Ru1-Cl1 = 2.4211(7), Ru1-Cl2 = 2.4008(7), Ru1-N1 = 2.181(2), Ru1-P1 = 2.3456(7), Ru1-P2 = 2.3311(7), Ru1-C29 = 1.850(3), C29O1 = 1.142(4), Cl2-Ru1-Cl1 = 174.52(3), N1-Ru1-Cl1 = 86.72(6), Cl2-Ru1-N1 = 88.39(6), O1-C29-Ru1 = 176.4(3). c) (Right) 6 Ru1-H1 = 1.53, Ru1-O2 = 2.2702(12), O2C30 = 1.211(2), C29-O1 = 1.1577(19), C29-Ru1 = 1.8413(16), C30-O3 = 1.262(2), C30-O4 = 1.366(2), O4-H3 = 1.1225, Ru1-P1 = 2.3118(4), Ru1-P2 = 2.3272(4), Ru1N1 = 2.1983(12), C29-O1 = 1.1577(19), C29-Ru1-H1 = 86.3, P1-Ru1-P2 = 162.173(14), O2-Ru1-H1 = 176.7, C29-Ru1-O2 = 96.89(6) and O1-C29-Ru1 = 174.73(14).
to X-ray diffraction for structural characterization. Complex 3 crystallized as distorted octahedrons with a P1-Ru1-P2 bond angle of 163.70(2)8, along with RuCl2(PNP)CO complex, 5 (impurity from the starting material, 1). A bond length of 2.04(3) æ for N¢H···O¢C (N···O = 2.831(3) æ) and a bond angle of 157(3)8 for N¢H···O, both characteristic of hydrogen bonding, were observed between the NH (N1H) group of the ligand and the non-coordinated formate oxygen (O3) in complex 3 (see Figure 2). The hydrogen bonding of N1H with the formate oxygen (O3) was further confirmed by the deshielding of the NH proton of complex 3 to 8.6 ppm in the 1H NMR. Hazari et al. observed similar kind of hydrogen bonding for the Ir-PNP complexes with the formate ligand (N¢H···O¢C bond distance and N¢H···O bond angle in Ir-PNP complexes were 1.925(9) æ and 149.8(5)8, respectively).[11b] Due to a weak intramolecular hydrogen bond with a Ru1¢ Cl1···H1¢N1 distance of 2.78(2) æ (Cl1···N1 = 3.161(2) æ) in complex 5, the chloride ligand Cl1 syn to the N¢H is moved towards nitrogen N1, thus compressing the Cl1¢Ru1¢Cl2 angle from the ideal value of 1808 to 174.51(3)8. The N1¢Ru1¢Cl1 and N1¢H1···Cl1 bond angles are found as 86.71(6)8 and 105.6(1)8. For complex 5, the CO signal appeared as a triplet (2JC¢P = 11.06 Hz) at 204.2 ppm in the 13C NMR, and the phosphorous signal as a singlet at 46.9 ppm in the 31P NMR. The formatochloro complex (4) was identified as a singlet at 48.8 ppm in the 31P NMR and the formate and CO signals were identified as singlet and triplet at 174 ppm and 203.4 ppm (2JC¢P = 11.15 Hz), respectively, in the 13C NMR. As a result of strong hydrogen bonding, the NH proton was deshielded and observed at 9.2 ppm, and the formate proton overlapped with the aromatic protons.[24] We also isolated a ruthenium bicarbonate species (6) in the solid state and the obtained crystals were subjected to single crystal X-ray diffraction. The bicarbonate complex (6) also showed hydrogen bonding of the NH hydrogen (H2) with the bicarbonate oxygen (O3) with a N¢H···O¢ C bond distance of 1.968(1) æ (N···O = 2.844(2) æ) and the N¢ H¢O bond angle of 161.37(9)8. Even though the bicarbonate ChemSusChem 2015, 8, 1442 – 1451
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species (6) was well identified in the solid-state by X-ray study, we were not able to identify the carbonate moiety and the NH proton signals by 13C and 1H NMR, respectively, since it was only present in a minor concentration. In addition, NMR measurements taken in a non-chlorinated solvent, such as [D8]THF, resulted in the formate complex 3 as a major species, which remained stable in the J. Young NMR tube even after 3 days unlike in the chlorinated solvents. (Scheme 2 b; Figures S8–S11) (3, 9.19 (¢NH, t, 1 H), 7.93 (¢OOCH, d, 1 H), 7.74–7.62 (¢ArH, m, 8 H), 7.28–1.18 (¢ArH, m, 12 H), 3.39–3.24 (¢CH2, m, 2 H), 2.99– 2.91 (¢CH2, m, 2 H), 2.58–2.48 (¢CH2, m, 2 H), 2.35–2.22 (¢CH2, m, 2 H), ¢17.2 (Ru¢H, m, 1 H). DFT calculations for the formation of ruthenium formate complex 3 from trans-dihydride ruthenium complex 1’ by N-H assisted CO2 insertion were performed (Figure 3).[25] In the first step, the nucleophilic hydride of the trans-dihydride complex (1’) attacks CO2 and forms an H-bound ruthenium-formate intermediate B (+ 34.4 kJ mol¢1), through the TS1’/B transition state. Similar to Hazari and Crabtree’s CO2 insertion in iridiumPNP complex, N-H hydrogen bonding, evident from Ru¢H1¢C bond angle (155.018) in TS1’/B transition state, brings CO2 closer to Ru and facilitates its insertion across the Ru-H1 bond.[11b] The intermediate B undergoes rearrangement and forms an Obound ruthenium formate complex 3 (product), through an Oand H-bound ruthenium formate transition-state TSB/3.[15b, 26] The activation barrier for the formation of 3 from B is 34.7 kJ mol¢1. The transition states and intermediates that we calculated for the insertion of CO2 to Ru¢H bond are similar to the ones proposed for the Ir complexes with aliphatic and aromatic PNP backbones by Ahlquist[27] and Hazari et al.[11b] Overall, the formation of 3 from 1’ is exergonic by ¢42.6 kJ mol¢1 and a stable O-bound Ru-formate complex (3) is formed via an H-bound Ru-formate complex (B).
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Full Papers yield (90 %) for hydrogenation of sodium bicarbonate without the need to add any external carbon dioxide (Table 1, entry 7). The hydridochloro complex 2 was also studied for the hydrogenation of sodium bicarbonate (Table 1, entry 8, TON = 1150), which showed similar activity as catalyst 1 (TON = 1125). When DMF was used as a co-solvent, instead of THF, a comparable yield was obtained (Table 1, entry 11). In order to understand the role of the N-H moiety on the catalytic activity,[28] the N-Me analogue of catalyst 2, a ruthenium PN(Me)P-pincer complex (7, Figure 1) was prepared according to literature procedures[29] and studied for the hydrogena(Table 1, tion of NaHCO3 entry 12). Contrary to the aliphatic PNP pincer rutheniumcatalyzed reactions such as hydrogenation of cyclic carbonaFigure 3. DFT calculations at the B3LYP/cc-PVDZ level of theory for the CO2 insertion in the ruthenium dihydride tes,[29b] dehydrogenation of complex 1’. The relative free energies were given in kJ mol¢1. methanol,[30] dehydrogenation of ammonia boranes,[31] and hydroHydrogenation of CO2, bicarbonates and carbonates [32] genation of esters, where the N-H moiety of the PNP pincer Hydrogenation of CO2, bicarbonates and carbonates to formate ligand was shown to have an important role, the catalytic activity of 7 for the sodium bicarbonate hydrogenation was not were studied using three different Ru-pincer catalysts (1, 2 and 7, Table 1). When alkali hydroxides, such as, NaOH, KOH and Table 1. Hydrogenation of carbonates, bicarbonates and CO2 in the presence of a catalyst. LiOH were heated under CO2/H2 Solvent Formate yield[b] TON TOF[c] t[a] Entry Catalyst Base p(H2)/p(CO2) T pressure in the presence of [%] [h¢1] [bar] [8C] [min] precursor trans-dihydride ruthenium complex 1, the respective formate 93 1160 232 1 1 NaOH 30/30 76 300 THF/H2O salts were formed (Table 1, en83 1038 455 2 1 KOH 30/30 75 137 THF/H2O tries 1–3). Sodium formate is 80 1000 2000 3 1 LiOH 30/30 75 30 THF/H2O 4 1 Na2CO3 38/12 79 63 THF/H2O 87 2164 2061 formed in a higher yield (93 %) CO 38/12 85 83 THF/H O 70 1750 1265 5 1 K 2 3 2 than the potassium and lithium 38/12 83 300 THF/H2O 15 250 –[d] 6 1 CaCO3 formates (83 % and 80 %, respec7 1 NaHCO3 40/0 80 40 THF/H2O 90 1125 1688 tively) under the same reaction 80 72 THF/H2O 92 1150 958 8 2 NaHCO3 40/0 70 120 THF/H2O 94 1175 588 9 1 NaHCO3 40/0 condition. Similarly, carbonates [e] 40/0 70 < 30 THF/H O 75 938 > 2000 10 1 KHCO 3 2 such as Na2CO3 and K2CO3 were 72 300 DMF/H2O 98 1225 245 11 1 NaHCO3 40/0 converted to their correspond12 7 NaHCO3 40/0 73 76 THF/H2O 77 963 760 ing formate salts in good to 10 775 1096 13 7 NaOH 61/20 70 590 1,4-Dioxane/H2O 84 moderate yields at high H2/CO2 [a] Time at which no more change in pressure was observed. [b] NMR yields were calculated after evaporating pressure (Table 1, entries 4 and the solvents (THF/H2O mixture was evaporated in rotovap and the DMF/H2O mixture was evaporated in lyophilizer) and by using THF and DMF as internal standard with a relaxation delay of 10 s and the error in the 5). The calcium formate yield yield calculation is < 10 %. [c] TOF was calculated using t. [d] Unable to calculate the TOF because of the low was only 15 % with CaCO3 even reaction rate and the 10 % yield obtained was only after heating the reaction mixture for 5 h. [e] Substrate was in the presence of external CO2, hydrogenated before the autoclave reaches the set temperature; for all experiments, 10 mmol catalyst, which could be due to the low 12.5 mmol base, organic solvent (5 mL) ,and H2O (10 mL) were used except, entry 1, 2 and 13; for entries 1 and 2, 20 mmol catalyst, 25 mmol base, organic solvent (6 mL), and H2O (13 mL) were used; 2 mmol catalyst, solubility of CaCO3 in the reac20 mmol base, organic solvent (20 mL) and H2O (10 mL) were used for entry 13; p(H2)/p(CO2) were given at RT; tion mixture (Table 1, entry 6). yields were calculated with respect to the base. Catalyst 1 also showed high ChemSusChem 2015, 8, 1442 – 1451
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Figure 4. Single crystal X-ray structure of complex 2 and 7. ORTEP diagrams plotted at 50 % probability level. Crystals of complex 2 also contains ~ 10 % of Cl substituted in hydride and CO position. Selected hydrogen atoms have been omitted for clarity. Selected bond length (æ) and bond angle (8): a) (left) (2) Cl1Ru1 = 2.4979(6), N1-H1N = 0.85(3), P2-Ru1 = 2.3142(6), N1-Ru1 = 2.1736(18), P1-Ru1 = 2.3142(6), Ru1-H1 = 1.629(18), C29-O1 = 1.172(9), C29-Ru1 = 1.826(5), Cl1Ru1-H1 = 175.8(12), P1-Ru1-H1 = 84.4(11), P2-Ru1-H1 = 91.7(11), P2-Ru1-P1 = 164.71(2), N1-Ru1-Cl1 = 85.52(5). b) (right) (7) Ru1-H1 = 1.60(3), P1-Ru1 = 2.3341(5), P2-Ru1 = 2.2942(4), Cl1-Ru1 = 2.5089(4), N1-Ru1 = 2.2271(15), C1-O1 = 1.147(2), C1-Ru1 = 1.8398(19), C6-N1 = 1.498(2), P1-Ru1-H1 = 89.2(9), Cl1-Ru1H1 = 173.9(9), N1-Ru1-H1 = 87.2(9), N1-Ru1-Cl1 = 86.75(4), P2-Ru1-P1 = 165.446(17), O1-C1-Ru1 = 175.23(17).
significantly affected by the absence of N-H moiety as reflected by the observed TON (963 at 73 8C, compared to the corresponding value of 1175 for catalyst 1 at 70 8C). Hanson and coworkers made similar observations in Co-based aliphatic PNP catalyzed reactions such as hydrogenation of olefins, and dehydrogenation of alcohol.[33] X-ray diffraction of chlorohydrido ruthenium complexes 2 and 7 were carried out to understand how the methyl substitution at the N atom of the PNP ligand affects the bond angles and bond distances. Selected bond lengths and bond angles are given in Figure 4. Complexes 2 and 7 crystallized by slow diffusion of pentane to a dichloromethane solutions of complex 2 and 7, respectively. In complexes 2 and 7, the RuII center shows a distorted octahedron geometry with P2-Ru1-P1 bond angles of 164.71(2)8 and 165.446(17)8, respectively. Similar to the IrH2Cl(iPr2PCH2CH2)2NH complex,[34] in complex 2 a weak intramolecular H-bonding was observed between Cl1 and N1H with the Ru¢Cl···H¢N bond distance of 2.77(2) æ (Cl···N = 3.180(2) æ), which was further supported by the bending of chloride ligand towards the nitrogen (N1-Ru-Cl1 bond angle = 85.52 (5)8). We also observed that the presence of a methyl substituent on the N atom of the PNP ligand caused a significant change (lengthening) in the Ru¢N bond distance of complex 7 (2.2271(15) æ) compared to the ¢N¢H-containing PNP ruthenium complex 2 (2.1736(18) æ). Dehydrogenation of formate salts Next, the Ru-PNP pincer catalysts 1, 2 and 7 used for CO2 and HCO3¢ hydrogenation, were also studied in the formate dehydrogenation reaction. Different formate salts such as lithium formate, sodium formate, potassium formate, and cesium formate were tested as substrates for the dehydrogenation reaction in dioxane/H2O mixtures (Table 2, Figure 5). A relatively ChemSusChem 2015, 8, 1442 – 1451
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Figure 5. Effect of temperature, solvent and counter cation on the reaction rate. Entries 1-8 are from Table 2.
high boiling solvent, 1,4-dioxane, was used in place of THF in order to minimize solvent loss. Dioxane was tested previously (Table 1, entry 13) as a co-solvent for the hydrogenation step using catalyst 7, where we were able to achieve a high TON of 10 775. The gas evolved in the course of the dehydrogenation reaction was measured with a gas burette. GC analysis of the collected gas showed a H2/CO2 gas mixture, with no detectable amount of CO. This CO-free H2/CO2 gas mixture could therefore be directly used in hydrogen fuel cell applications. Both catalysts 1 and 2 displayed the same catalytic activity for the sodium formate dehydrogenation with an initial TOF of 286 h¢1 at 69 8C (entry 1–2, Table 2). Increasing the water content in the reaction mixture by two fold decreased the catalyst activity due to the low solubility of the catalyst in the aqueous media (entry 3, Table 2). Interestingly, ruthenium PN(Me)P-pincer complex 7 was more
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Full Papers tion of H2 and CO2 from sodium formate (Figure 6). Thus, in H2 yield TOF TON T Initial TOF Entry Catalyst Substrate t Volume of H2 Figure 6, we show the result of [mL] [h¢1] [%] [8C] [h¢1] precursor [min] a prototype of a H2-based charge–discharge device operat1 1 HCOONa 270 490 100 69 286 222 1000 2 2 HCOONa 300 485 99 69 286 198 990 ed by pressure swing. In addi3[b] 2 HCOONa 420 189 39 70 98 55 388 tion, in a practical system, the 4 7 HCOONa 240 490 100 68 430 250 1000 energy required for both the hy5 1 HCOOK 420 450 92 70 290 132 925 drogenation and the dehydro6 1 HCOOK 600 372 76 61 172 76 763 7 1 HCOOLi 180 490 100 71 437 333 1000 genation step can be obtained 8 1 HCOOCs 420 424 87 70 234 124 869 from waste energy sources and 663 994 9 1 HCOONa 90 487 99.4 84 820[c] solar thermal energy. The best 7 HCOONa 1440 243 50 70 735 208 5000 10[d] volumetric storage capacity re[a] Reaction conditions: Catalyst = 20 mmol, substrate = 20 mmol, total volume of the solvent = 30 mL (20 mL ported in the literature for the 1,4-dioxane + 10 mL H2O). [b] Total volume of the solvent = 30 mL (10 mL 1,4-dioxane + 20 mL H2O). Initial amine-free pressure swing hyTOF = TOF in first 2 h of the reaction. The error in the yield calculation is < 10 %. [c] TOF in first 1 h. [d] 2 mmol of catalyst was used. drogen-storage system was 16.4 L H2 per L of the solution, which is comparable to the present system, where we obtained 16.4 L H2 per L of the solution active than catalysts 1 and 2 for formate dehydrogenation (Table S3).[21] The former work required, however, solvent with an initial TOF of 430 h¢1(entry 4, Table 2). Among the difchange to reverse the reaction. The pH of the reaction mixture ferent formate salts examined (entry 1, 5, 7 and 8, Table 2), lithchanged from 10.77 to 7.86 during the hydrogenation of ium and sodium formates gave 100 % yield in 5 h with an inisodium bicarbonate to sodium formate. Under the reaction tial TOF of 437 and 286 h¢1, respectively, at 70 8C. Combining the hydrogenation and dehydrogenation studies (Table 1 and conditions, the pH change occurs spontaneously and no exterTable 2), it is evident that the sodium bicarbonate/sodium fornal pH control is necessary to reverse the reaction. The activamate system is suitable for reversible hydrogen storage. tion energy for the formate decomposition calculated from the In order to demonstrate the reversibility of the CO2 hydrogeArrhenius equation over the temperature range of 60 to 85 8C nation system in the presence of catalyst 1, the dehydrogenawas 80.7 kJ mol¢1 (Figure S14). tion of the in-situ-formed formate was attempted in the same pot (Figure 6). Sodium bicarbonate was hydrogenated to Consecutive CO2 hydrogenation (charge)/formate dehydrosodium formate in THF/H2O mixture at a H2 pressure of genation (discharge) cycles ~ 53 bar at 80 8C. After cooling the reaction mixture to RT and releasing the remaining hydrogen (however, in a practical inThe hydrogenation of CO2 to formate was performed in the dustrial unit, the excess hydrogen can be recycled rather than presence of NaOH and catalyst 1 at 70 8C and 75 bar of a 3:1 vented), the reaction mixture was heated to 60 8C, which remixture of H2 and CO2. The dehydrogenation was studied at sulted in a continuous pressure increase, indicating the genera70 8C and atmospheric pressure. Six hydrogenation–dehydrogenation cycles were performed, during which the catalyst did not show any significant loss in activity (Figure 7), as shown by the hydrogen yield during the dehydrogenation steps. In all six cycles of this amine-free system, there was no detectable amount of CO in the gas mixture, potentially enabling this catalyst to be applied in a hydrogen battery. A combined TON > 11 500 was obtained from all six hydrogenation–dehydrogenation cycles. Table 2. Dehydrogenation of formate salts.[a]
Mechanistic insight
Figure 6. Pressure vs. time plot for a) Hydrogenation of NaHCO3 to formate under higher H2 pressure (53 bar) in the presence of catalyst 1 at 80 8C. b) Dehydrogenation of the in situ formed formate at 60 8C at lower pressure. Reaction conditions: Catalyst 1 = 20 mmol, NaHCO3 = 20 mmol, Total volume of the solvent = 17 mL (5 mL THF + 12 mL H2O).
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Two possible pathways for the hydrogenation of CO2 and the dehydrogenation of formate are proposed (Scheme 3). The first step is the insertion of CO2 to the Ru¢H bond in trans-dihydride complex 1’(or 1) by nucleophilic attack of the hydride on the carbon atom of CO2 to form ruthenium formate complex 3. In the second step, formate decoordinates, forming a 16electron complex (8). Then, a hydrogen molecule coordinates to the ruthenium center to give 9. Subsequently, the base abstracts a proton from the dihydrogen complex 9, to form water and trans-dihydride complex 1’ (Pathway I, Scheme 3).
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Full Papers hydrogenation through pathway II conspicuously inaccessible, but the reaction can possibly take place via pathway I. This is supported by the fact that instead of complete inhibition very similar performance was observed using complex 2 and 7, respectively (Table 1). Deuterium labelling studies on formate and water were carried out to understand the isotopic effects on the reaction rate of dehydrogenation (Table 3). The deuterium kinetic isotopic effect (KIE) was high with DCOONa (DCOONa/H2O, KIE = 2.2, Table 3, entry 3) compared to D2O solvent (HCOONa/D2O KIE = 1.7, Table 3, entry 2), showing that the b-hydride elimination, that is, the decarboxylation step is the rate-limiting step for the sodium formate decomposition.[15c, 35] Figure 7. Multiple CO2 hydrogenation/formate dehydrogenation cycles in the presence of catalyst 1 at 70 8C for 5 h. Hydrogenation and dehydrogenation steps were performed at 75 bar (3:1) H2/CO2 and atmospheric pressure, respectively. Percentage (%) yields were calculated from the amount of H2 generated in the dehydrogenation step based on the initial base (NaOH) content. In a separate experiment, the formate yield calculated after the first CO2 hydrogenation step was 93 %. Catalyst 1 = 20 mmol, NaOH = 22 mmol, Total volume of the solvent = 30 mL (20 mL dioxane + 10 mL H2O).
Hazari et al. and Ahlquist proposed a similar kind of mechanism (pathway I, Scheme 3) for the IrIII trihydride complex with PNP ligand for the CO2 hydrogenation.[11b, 27] On the other hand, the second pathway goes via an amido complex 10, which is formed by the deprotonation of the NH group in the PNP ligand of complex 3. Then, amido complex 10 heterolytically cleaves H2 by metal-ligand cooperation to regenerate trans-dihydride 1’ (Pathway II, Scheme 3). Pathway II involves an amido intermediate 10, which is similar to the mechanism proposed for the transfer hydrogenation of ketones using IrH3[(iPr2PCH2CH2)2NH].[33] The presence of a N-Me moiety in complex 7 would make the hydrogenation of CO2/formate de-
Table 3. Deuterium kinetic isotopic effect study.[a] Entry
Substrate/co-solvent
Reaction rate [10¢5 m s¢1]
KIE
1 2 3 4
HCOONa/H2O HCOONa/D2O DCOONa/H2O DCOONa/D2O
8.232 4.844 3.792 3.079
1 1.7 2.2 2.7
[a] Reaction conditions: Catalyst 1 = 10 mmol, substrate = 5 mmol, total volume of the solvent = 7.5 mL (5 mL 1,4-dioxane + 2.5 mL H2O or D2O); for entries 2 and 4, dry dioxane was used.
NMR study on formate decomposition in the presence of catalyst 1
The catalyst 1 and sodium formate in [D8]THF and D2O mixture were heated to 60 8C for 1.5 h in a J. Young NMR tube. Continuous gas evolution was observed throughout the experiment. Due to the reversible nature of the reaction, NMR analysis of the reaction mixture showed the formate species 3, with the partial deuteration at NH, ¢ OOCH and Ru¢H (Figure S15 and S16). When the same experiment was conducted in H2O instead of D2O, hydridoformate complex 3 (¢17.2 ppm) was the only major species that was identified in the 1 H and 31P NMR of the reaction mixture (Figure S17–S19). Thus, complex 3 is the catalyst’s resting state and this further supports the assertion that the decarboxylation step (from 3 to 1’) is the rate-determining step in the formate decomposition process.[15c] In addition to 3, there were also minor amount of unidentified species in the 1 H NMR at ¢17.5 ppm (t, J = 19.4 Hz), ¢16.1 ppm (t, J = 19.9 Hz) and ¢13.1 ppm (t, J = 18.5 Hz). Scheme 3. Proposed catalytic cycles for the hydrogenation/dehydrogenation reactions. rds = rate-determining step for the dehydrogenation.
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Full Papers Conclusions For the first-time, a practical, efficient, (yields were ~ 90 % in both direction) amine-free system was developed that can hydrogenate CO2(/HCO3¢) to formate without external pH control or solvent change, and generate hydrogen from the in-situformed formate in one pot. This system was demonstrated by simply varying the pressure, using well-defined commercially available ruthenium catalysts. Continuous performance and high selectivity in all six CO2 hydrogenation/formate dehydrogenation cycles further display the practicality of this system. Unlike other ruthenium-based catalyst systems, where the N-H moiety plays a critical role by metal-ligand bifunctional catalysis, we have presented here the first example of a ruthenium system, where the catalytic activity was independent of the NH moiety of the PNP pincer ligand. This was proven by similar activities, under our reaction conditions, for the Ru catalysts with the PNP pincer ligand containing N-Me (7) and N-H (2) moieties.
Experimental Section Materials and Instrumentation Sodium bicarbonate (Mallinckrodt, 99.7 %), sodium hydroxide (Mallinckrodt, 98.8 %), sodium carbonate (Mallinckrodt, 99.9 %) potassium hydroxide (Mallinckrodt, 88 %), potassium bicarbonate (Baker analyzed reagent, > 99.9 %), potassium carbonate (Baker analyzed reagent, > 99.9 %), lithium hydroxide (Aldrich, 98 %), calcium carbonate (Fisher scientific, > 99.9 %), HCOOLi·H2O (Aldrich, 98 %), HCOONa (Mallinckrodt, 99.8 %), HCOOK (Aldrich, 99 %), HCOOCs (Aldrich, 98 %) tetrahydrofuran (EMD, 99.9 %), dimethyl formamide (EMD, 99.8 %), dichloromethane (Macron, 99.5 %), pentane (EMD, 98 %), CDCl3 (Cambridge Isotopes lab, 99.8 %), dioxane (Mallinckrodt, 99 %), H2 gas (Gilmore, ultra high pure grade), CO2 gas (Gilmore, instrument grade), 3:1 H2/CO2 gas (Airgas, grade = certified standard-spec), 1:1 H2/CO2 gas (Airgas, grade = certified standardspec) were used as received without any purification. Carbonylhydrido(tetrahydroborato)[bis(2-diphenylphosphinoethyl) amino]ruthenium(II) (1) (Strem, 98 %), Carbonylchlorohydrido{bis[2 (diphenylphosphinoethyl)amino}ruthenium(II) (2) (Strem, 98 %) and, Carbonyldihydridotris(triphenylphosphine) ruthenium(II) (Alfa Aesar, 98 %), diphenylphosphine (Aldrich, 98 %) and Me-N(CH2CH2Cl)2.HCl (Alfa Aesar, 98 %) were stored in a glove box and handled under argon atmosphere. 1 H, 31P and 13C were recorded on 400 and 500 MHz Varian NMR spectrometers. 1H NMR chemical shifts were determined relative to internal standard, TMS or residual proton signal of D2O (HOD). 31P NMR chemical shifts were determined relative to external standard, H3PO4. 13C NMR chemical shifts were determined relative to 13C signal of CDCl3 solvent. The gas mixture was analyzed using a Thermo gas chromatograph (column: Supelco, Carboxen 1010 plot, 30 m Õ 0.53 mm) equipped with a TCD detector (CO detection limit: 0.099 v/v %).
Catalysis CO2 insertion to Complex 1: Catalyst 1 (0.042 mmol) was added to the degassed anhydrous THF (5 mL) in a 125 mL Monel Parr reactor (autoclave) equipped with a thermocouple and piezoelectric pressure transducer (Figure S1). The autoclave was assembled in a glove box under nitrogen atmosphere and the internal pressure ChemSusChem 2015, 8, 1442 – 1451
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and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at RT (or 60 8C) under CO2 pressure (15 bar) for 1 h. After venting the autoclave, the reaction mixture was transferred to a round bottom flask. The solvent, THF, was evaporated using a rotavap, giving a yellow solid, which was analyzed by 1H NMR. CO2 insertion to Complex 1 in a J. Young NMR tube: 6 mg of catalyst 1 was transferred to the J. Young NMR tube using 1 g [D8]THF in a N2 atmosphere. The N2 atmosphere in the NMR tube was replaced with CO2 by performing three freeze-pump-thaw cycles. The NMR tube was heated to 60 8C under 1 atm of CO2 for 30 min in an oil bath. A pale yellow solution was obtained, which was analyzed by 1H, 31P, and 13C NMR. General procedure for the hydrogenation of CO2 in the presence of NaOH/KOH and catalyst: To the degassed mixture of THF (6 mL) and water (13 mL), catalyst 1 (20 mmol) and NaOH/KOH (25 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and filled with 30 bar H2 and 30 bar CO2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at room temperature for 30 min and heated at 75 8C for 5 h. After completion of the reaction, solvents were evaporated using a rotovap and a white solid was obtained, which was analyzed by 1H and 13C NMR spectroscopy. a) HCOONa yield (using NaOH/76 8C) = 93 %. b) HCOOK yield (using KOH/75 8C) = 83 %. General procedure for the hydrogenation of carbonate in the presence of a catalyst: To the degassed mixture of THF (5 mL) and water (10 mL), catalyst 1 (10 mmol) and carbonate (12.5 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and filled with 50 bar 3:1 H2 :CO2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at room temperature for 30 min and heated to the set temperature 85 8C for 5 h. After completion of the reaction, solvents were evaporated using a rotovap and a white solid was obtained, which was analyzed by 1H and 13 C NMR spectroscopy. a) HCOONa yield (using Na2CO3/79 8C) = 87 %. b) HCOOK yield (using K2CO3/85 8C) = 70 %. c) (HCOO)2Ca yield (using CaCO3/83 8C) = 10 %. General procedure for the hydrogenation of bicarbonate in the presence of a catalyst: To the degassed mixture of THF or DMF (5 mL) and water (10 mL), catalyst (10 mmol) and bicarbonate (12.5 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a nitrogen glove box under nitrogen atmosphere and filled with 40 bar H2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at room temperature for 30 min and heated to the set temperature (70 or 80 8C) for 5 h. After completion of the reaction, solvents were evaporated using a rotovap and a white solid was obtained, which was analyzed by 1 H and 13C NMR spectroscopy (Figures S9 and S10). a) Catalyst 1: HCOONa yield using: NaHCO3 (in THF)/80 8C = 90 %; NaHCO3 (in THF)/70 8C = 94 %; NaHCO3 (in DMF)/72 8C = 98 %. HCOOK yield using: KHCO3 (in THF)/70 8C = 75 %. b) Catalyst 2: HCOONa yield using: NaHCO3 (in THF)/80 8C = 92 %. c) Catalyst 7: HCOONa yield using: NaHCO3 (in THF)/73 8C = 77 %.
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Full Papers General procedure for the formate dehydrogenation in the presence of a catalyst: To the degassed mixture of dioxane (20 mL) and water (10 mL), catalyst (20 mmol) and formate (20 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and heated to the set temperature (60 or 70 8C) until no gas evolution was observed. The gas evolved in the course of the reaction was measured with a burette and the gas mixture was analyzed by GC. a) HCOONa decomposition:H2 yield using catalyst 1 = 100 % at 69 8C; H2 yield using catalyst 2 = 99.4 % at 69 8C; H2 yield using catalyst 7 = 100 % at 68 8C. b) HCOOK decomposition:H2 yield using catalyst 1 = 92 % at 70 8C; H2 yield using catalyst 1 = 76 % at 61 8C. c) HCOOLi decomposition: H2 yield using catalyst 1 = 100 % at 71 8C. d) HCOOCs decomposition: H2 yield using catalyst 1 = 87 % at 70 8C.
Hydrogenation of CO2 : To the degassed mixture of dioxane (20 mL) and water (10 mL), catalyst (20 mmol) and NaOH (22 mmol) were added in a 125 mL Monel Parr reactor (autoclave). The autoclave was assembled in a glove box under nitrogen atmosphere and filled with 75 bar 3:1 H2 :CO2. The internal pressure and temperature of the reactor were monitored using the Lab view 8.6 software. The reaction mixture was stirred at RT for 30 min and heated to 70 8C for 5 h. Then, the reaction mixture was cooled to RT and the mixture was degased by stirring at RT under atmospheric pressure for 15 min. Dehydrogenation of sodium formate: The above mixture was heated in the same autoclave to 70 8C for 5 h. The gas evolved in the course of the reaction was measured with a burette and the gas mixture was analyzed by GC. The same hydrogenation/dehydrogenation cycle was repeated six times. NMR study on formate decomposition in the presence of catalyst 1 and [D8]THF/D2O mixture: 6 mg of catalyst 1 and 13.6 mg of HCOONa were transferred to J. Young tube using 0.8 mL of [D8]THF in N2 atmosphere. To this mixture, 0.5 mL D2O was added. At room temperature, the catalyst is only partially soluble in [D8]THF and H2O mixture (Figure S15). Then the NMR tube was heated to 60 8C for 1.5 h in a oil bath, gas evolution was observed (Note: While heating, the J. Young tube was connected to nitrogen through a needle using a Schlenk line manifold in order to avoid pressure build up). 1H and 31P NMR spectra showed formation of species 3, with partial deuteration at NH, ¢OOCH and Ru¢H (Figure S16). NMR study on formate decomposition in the presence of catalyst 1 and [D8]THF/H2O mixture: 6 mg of catalyst 1 and 13.6 mg of HCOONa were transferred to J. Young tube using 0.8 mL of [D8]THF in N2 atmosphere. To this mixture, 0.1 mL H2O was added. At room temperature, the catalyst is only partially soluble in [D8]THF and H2O mixture (Figure S17 A). Then the NMR tube was heated to 60 8C for 10 min in an oil bath. Gas evolution was observed (Note: While heating, the J. Young tube was connected to nitrogen through a needle using a Schlenk line manifold in order to avoid pressure build up). At this stage, only trace amount of sodium formate was left. However, a significant amount of catalyst 1 remained unreacted (Figure S17 B). Therefore, an additional www.chemsuschem.org
Acknowledgements We are grateful to Loker Hydrocarbon Research Institute and United States Department of Energy for financial support.
Consecutive CO2 hydrogenation/formate dehydrogenation cycles
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13.6 mg of sodium formate was added and the NMR tube was heated at 60 8C for an additional 10 min. More of the dihydride complex 1 was converted to hydrido formate complex 3 with continuous gas evolution (¢17.2 ppm) (Figure S17 C). Hydridochloro species 2 was originated from the small amount of dichloro impurity 5 in the starting material. Subsequently, the NMR tube was heated until the gas evolution completely stopped (~ 20 min). A pale yellow solution was obtained. 1H NMR spectrum (Figure S17 D and S18) showed almost complete conversion of 1 to 3, and this was further confirmed by the 31P NMR. (Figure S19) Small amount of non-identified hydride species at ¢17.5 ppm (t, J = 19.4 Hz), ¢16.1 ppm (t, J = 19.9 Hz) and ¢13.1 ppm (t, J = 18.5 Hz) were also observed by 1H NMR.
Keywords: carbon-neutral cycle · energy hydrogenation · PNP ligand · ruthenium
storage
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[1] Hydrogen as a Future Energy Carrier (Eds.: A. Zìttel, A. Borgschulte, L. Schlapbach), Wiley-VCH, Weinheim, Germany, 2008. [2] a) A. Goeppert, M. Czaun, R. B. May, G. K. S. Prakash, G. A. Olah, S. R. Narayanan, J. Am. Chem. Soc. 2011, 133, 20164 – 20167; b) A. Goeppert, M. Czaun, G. K. S. Prakash, G. A. Olah, Energy Environ. Sci. 2012, 5, 7833; c) S. Meth, A. Goeppert, G. K. S. Prakash, G. A. Olah, Energy Fuels 2012, 26, 3082 – 3090; d) A. Goeppert, M. Czaun, J. P. Jones, G. K. S. Prakash, G. A. Olah, Chem. Soc. Rev. 2014, 43, 7995 – 8048; e) M. Czaun, A. Goeppert, R. B. May, D. Peltier, H. Zhang, G. K. S. Prakash, G. A. Olah, J. CO2 Util. 2013, 1, 1 – 7. [3] a) D. K. K. Motokura, A. Miyaji, T. Baba, Org. Lett. 2012, 14, 2642 – 2645; b) W. Leitner, Angew. Chem. Int. Ed. Engl. 1995, 34, 2207 – 2221; Angew. Chem. 1995, 107, 2391 – 2405; c) S. Moret, P. J. Dyson, G. Laurenczy, Nat. Commun. 2014, 5, 1 – 7. [4] C. Federsel, A. Boddien, R. Jackstell, R. Jennerjahn, P. J. Dyson, R. Scopelliti, G. Laurenczy, M. Beller, Angew. Chem. Int. Ed. 2010, 49, 9777 – 9780; Angew. Chem. 2010, 122, 9971 – 9974. [5] S. Park, D. Bezier, M. Brookhart, J. Am. Chem. Soc. 2012, 134, 11404 – 11407. [6] a) G. A. Olah, G. K. S. Prakash, A. Goeppert, J. Am. Chem. Soc. 2011, 133, 12881 – 12898; b) C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122 – 18125; c) L. C. Grabow, M. Mavrikakis, ACS Catal. 2011, 1, 365 – 384; d) G. A. Olah, A. Goeppert, G. K. S. Prakash, Beyond Oil and Gas: The Methanol Economy, 2nd ed., Wiley-VCH, Weinheim, Germany, 2009; e) G. A. Olah, A. Goeppert, G. K. S. Prakash, J. Org. Chem. 2009, 74, 487 – 498. [7] a) W. Reutemann, H. Kieczka in Formic acid—Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Wienheim, Germany, 2011; b) http:// www.intermediates.basf.com/chemicals/formic-acid/de-icing. [8] a) C. Ziebart, C. Federsel, P. Anbarasan, R. Jackstell, W. Baumann, A. Spannenberg, M. Beller, J. Am. Chem. Soc. 2012, 134, 20701 – 20704; b) R. Langer, Y. Diskin-Posner, G. Leitus, L. J. Shimon, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed. 2011, 50, 9948 – 9952; Angew. Chem. 2011, 123, 10122 – 10126. [9] a) M. S. Jeletic, M. T. Mock, A. M. Appel, J. C. Linehan, J. Am. Chem. Soc. 2013, 135, 11533 – 11536; b) C. Federsel, C. Ziebart, R. Jackstell, W. Baumann, M. Beller, Chem. Eur. J. 2012, 18, 72 – 75. [10] a) J. Z. Zhang, Z. Li, H. Wang, C. Y. Wang, J. Mol. Catal. A 1996, 112, 9 – 14; b) C. A. Huff, M. S. Sanford, ACS Catal. 2013, 3, 2412 – 2416; c) H. Hayashi, S. Ogo, S. Fukuzumi, Chem. Commun. 2004, 2714 – 2715; d) P. Munshi, A. D. Main, J. C. Linehan, C.-C. Tai, P. G. Jessop, J. Am. Chem. Soc. 2002, 131, 14168; e) C. Federsel, R. Jackstell, A. Boddien, G. Laurenczy, M. Beller, ChemSusChem 2010, 3, 1048 – 1050.
1450
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers [11] a) R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131, 14168 – 14169; b) T. J. Schmeier, G. E. Dobereiner, R. H. Crabtree, N. Hazari, J. Am. Chem. Soc. 2011, 133, 9274 – 9277; c) A. Azua, S. Sanz, E. Peris, Chem. Eur. J. 2011, 17, 3963 – 3967; d) W.-H. Wang, J. F. Hull, J. T. Muckerman, E. Fujita, Y. Himeda, Energy Environ. Sci. 2012, 5, 7923 – 7926. [12] a) F. Gassner, W. Leitner, J. Chem. Soc. Chem. Commun. 1993, 1465 – 1466; b) F. Zou, J. M. Cole, T. G. J. Jones, L. Jiang, Appl. Organomet. Chem. 2012, 26, 546 – 549. [13] a) M. Czaun, A. Goeppert, R. May, R. Haiges, G. K. S. Prakash, G. A. Olah, ChemSusChem 2011, 4, 1241 – 1248; b) A. Boddien, B. Loges, H. Junge, M. Beller, ChemSusChem 2008, 1, 751 – 758; c) D. J. Morris, G. J. Clarkson, M. Wills, Organometallics 2009, 28, 4133 – 4140; d) M. Czaun, A. Goeppert, J. Kothandaraman, R. B. May, R. Haiges, G. K. S. Prakash, G. A. Olah, ACS Catal. 2014, 4, 311 – 320; e) B. Loges, A. Boddien, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3962 – 3965; Angew. Chem. 2008, 120, 4026 – 4029; f) C. Fellay, P. J. Dyson, G. Laurenczy, Angew. Chem. Int. Ed. 2008, 47, 3966 – 3968; Angew. Chem. 2008, 120, 4030 – 4032; g) A. Boddien, B. Loges, H. Junge, F. Grtner, J. R. Noyes, M. Beller, Adv. Synth. Catal. 2009, 351, 2517 – 2520. [14] a) S. Fukuzumi, T. Kobayashi, T. Suenobu, J. Am. Chem. Soc. 2010, 132, 1496 – 1497; b) Y. Himeda, Green Chem. 2009, 11, 2018; c) S. Oldenhof, B. de Bruin, M. Lutz, M. A. Siegler, F. W. Patureau, J. I. van der Vlugt, J. N. Reek, Chem. Eur. J. 2013, 19, 11507 – 10511; d) Y. Manaka, W.-H. Wang, Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y. Himeda, Catal. Sci. Technol. 2014, 4, 34 – 37. [15] a) A. Boddien, D. Mellmann, F. Gartner, R. Jackstell, H. Junge, P. J. Dyson, G. Laurenczy, R. Ludwig, M. Beller, Science 2011, 333, 1733 – 1736; b) T. Zell, B. Butschke, Y. Ben-David, D. Milstein, Chem. Eur. J. 2013, 19, 8068 – 8072; c) E. A. Bielinski, P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C. Wurtele, W. H. Bernskoetter, N. Hazari, S. Schneider, J. Am. Chem. Soc. 2014, 136, 10234 – 10237. [16] S. Fukuzumi, T. Kobayashi, T. Suenobu, ChemSusChem 2008, 1, 827 – 834. [17] J. F. Hull, Y. Himeda, W. H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, J. T. Muckerman, E. Fujita, Nat. Chem. 2012, 4, 383 – 388. [18] a) B. Zaidman, H. Wiener, Y. Sasson, Int. J. Hydrogen Energy 1986, 11, 341 – 347; b) S. Enthaler, J. von Langermann, T. Schmidt, Energy Environ. Sci. 2010, 3, 1207 – 1217. [19] E. Fujita, J. T. Muckerman, Y. Himeda, Biochim. Biophys. Acta Bioenerg. 2013, 1827, 1031 – 1038. [20] Y. Maenaka, T. Suenobu, S. Fukuzumi, Energy Environ. Sci. 2012, 5, 7360 – 7367.
ChemSusChem 2015, 8, 1442 – 1451
www.chemsuschem.org
[21] A. Boddien, F. Gartner, C. Federsel, P. Sponholz, D. Mellmann, R. Jackstell, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 6411 – 6414; Angew. Chem. 2011, 123, 6535 – 6538. [22] G. Papp, J. Csorba, G. Laurenczy, F. Joo, Angew. Chem. Int. Ed. 2011, 50, 10433 – 10435; Angew. Chem. 2011, 123, 10617 – 10619. [23] a) A. Boddien, C. Federsel, P. Sponholz, D. Mellmann, R. Jackstell, H. Junge, G. Laurenczy, M. Beller, Energy Environ. Sci. 2012, 5, 8907 – 8911; b) S. F. Hsu, S. Rommel, P. Eversfield, K. Muller, E. Klemm, W. R. Thiel, B. Plietker, Angew. Chem. Int. Ed. 2014, 53, 7074 – 7078; Angew. Chem. 2014, 126, 7194 – 7198; c) W. Leitner, E. Dinjus, F. Gartner, J. Organomet. Chem. 1994, 475, 257 – 266; d) K. Sordakis, M. Beller, G. Laurenczy, ChemCatChem 2014, 6, 96 – 99. [24] Complex 4 was tentatively assigned as formatochloro complex. Although the formate carbon was clearly identified by 13CNMR, the formate proton overlapped with the aromatic proton. Presence of chloride ligand in the complex 4 was further concluded by the formation of CHDCl2 in the reaction mixture from CDCl3. [25] Gaussian 09, Revision A.02, M. J. Frisch et al., see Supporting Information. [26] X. Yang, ACS Catal. 2011, 1, 849 – 854. [27] M. S. G. Ahlquist, J. Mol. Catal. A 2010, 324, 3 – 8. [28] a) S. D. Phillips, J. A. Fuentes, M. L. Clarke, Chem. Eur. J. 2010, 16, 8002 – 8005; b) B. Zhao, Z. Han, K. Ding, Angew. Chem. Int. Ed. 2013, 52, 4744 – 4788; Angew. Chem. 2013, 125, 4844 – 4889. [29] a) D. S. McGuinness, P. Wasserscheid, D. H. Morgan, J. T. Dixon, Organometallics 2005, 24, 552 – 556; b) Z. Han, L. Rong, J. Wu, L. Zhang, Z. Wang, K. Ding, Angew. Chem. Int. Ed. 2012, 51, 13041 – 13045; Angew. Chem. 2012, 124, 13218 – 13222. [30] M. Nielsen, E. Alberico, W. Baumann, H. J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85 – 89. [31] M. Kß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. Int. Ed. 2009, 48, 905 – 907; Angew. Chem. 2009, 121, 922 – 924. [32] W. Kuriyama, T. Matsumoto, Y. Ino, O. Ogata, US. Patent 0237814A1, 2011. [33] G. Zhang, K. V. Vasudevan, B. L. Scott, S. K. Hanson, J. Am. Chem. Soc. 2013, 135, 8668 – 8681. [34] Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J. Lough, K. Abdur-Rashid, Organometallics 2006, 25, 4113 – 4117. [35] W. H. Wang, S. Xu, Y. Manaka, Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y. Himeda, ChemSusChem 2014, 7, 1976 – 1983. Received: January 2, 2015 Published online on March 30, 2015
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