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Catalytic Conversion of CO2 to Formate with Renewable Hydrogen Donors: An Ambient-Pressure and H2-Independent Strategy Abhishek Kumar, Shrivats Semwal, and Joyanta Choudhury ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04430 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019
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ACS Catalysis
Catalytic Conversion of CO2 to Formate with Renewable Hydrogen Donors: An Ambient-Pressure and H2-Independent Strategy Abhishek Kumar, Shrivats Semwal, and Joyanta Choudhury* Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal 462 066, India ABSTRACT: Catalytic conversion of CO2 via ‘transfer hydrogenation’ using renewable non-H2 compounds (such as biomass-derived (poly)alcohols) to produce valuable energy-relevant chemicals, is a promising alternative strategy to the traditionally employed ‘hydrogenation’ of CO2 with gaseous H2. However the CO2-transfer hydrogenation has been explored exceptionally less, and limited but encouraging success has been achieved in recent time by applying high pressure (upto 50 atm) of CO2 gas. For safe and simple operation, ambient-pressure protocols are desirable, and toward this end suitable catalysts are required. Aiming to this goal, herein we report an efficient Ir–aNHC catalyst (aNHC = an abnormal NHC ligand) to achieve ambient-pressure CO2-transfer hydrogenation catalysis for generating formate salt (HCO2−) at the turnover frequency (TOF) value of 90 h−1 in 12 h of reaction at 150 °C using glycerol as hydrogen source. KEYWORDS . Carbon dioxide, formic acid, iridium, transfer hydrogenation, glycerol
INTRODUCTION The modern society is passing through a crucial phase in terms of the serious challenge posed by the current >400 ppm CO2 concentration in the atmosphere. The 2018 IPCC (Intergovernmental Panel on Climate Change) special report recorded the following caution: “Global net human-caused emissions of carbon dioxide (CO2) would need to fall by about 45 percent from 2010 levels by 2030, reaching ‘net zero’ around 2050. This means that any remaining emissions would need to be balanced by removing CO2 from the air.”1 The researchers around the world have been tackling this challenge through diversified approaches ranging from inventing renewable non-carbon-based energy options to developing various catalytic CO2-conversion technologies. In this context, catalytic hydrogenation of CO2 to formic acid/formate salts (and other product like methanol) with different transition-metal catalysts has received significant recent attention from sustainable energy-storage/delivery perspective.2 In parallel to the high-pressure technologies, nowadays extensive efforts are also being put forth to develop ambient-pressure CO2-hydrogenation catalysts.3 However, a CO2-hydrogenation protocol would become sustainable if the required dihydrogen gas (H2) is generated from water-electrolysis using renewable solar or wind energy. Unfortunately, at present, commercial bulk H2 gas is produced by the steam reforming of natural gas (CH4) which is non-renewable. On the other hand, production of H2 from renewable biomass is currently less-efficient, expensive and leads to emission of CO2 as well.4 Considering the above facts, a viable option is utilizing a renewable but non-H2 source, such as biomass-
derived alcohol (isopropanol) or polyol (glycerol) or sugar (glucose) as hydride fossil fuel Hydrogenation, H2(g) CO2
catalyst
HCO2
Transfer Hydrogenation, non-H2 source
current status CO2: high pressure
biomass
this work CO2: ambient pressure
Figure 1. Comparison of hydrogenation and transferhydrogenation protocols for conversion of CO2 to formate (HCO2−), and the key finding of the present work.
donor to CO2 (Figure 1).5 Interestingly, this alternate “transfer hydrogenation”6 option with CO2 (and HCO3− also) is indeed being tested at the laboratory set-up using various homogeneous catalysts for last few years.7 However all the protocols reported so far require very high pressure (up to 50 atm) (and temperature, upto even 200 °C) to achieve reasonable turnover frequency (TOF). For example, Peris’s Ir-catalyst showed TOF upto 36 h−1 (in 75 h) at 50 atm of CO2 pressure and 200 °C using isopropanol as non-H2 source.7a Similarly, in a very recent work, the TOF of 44 h−1 (in 24 h) was reported by Voutchkova-Kostal, achieved via transfer hydrogenation using glycerol as hydrogen donor with a Ru-catalyst at 26 atm CO2 pressure and 180 °C.7c Close to these processes, earlier Beller reported an elegant catalytic CO2-transfer hydrogenation process with methanol as hydrogen donor, but in this protocol primarily methanol was found to be converted to formate via consecutive dehydrogenation
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steps.7d Despite these discrete indisputable advances mentioned above, for future practical applications, there is a need for efficient catalysts for this promising alternate CO2-to-formate transfer hydrogenation protocol which can be operated under ambient pressure of CO2 and at low temperatures. Notably, for safe and simple operation, ambient-pressure technology is desirable. Toward this goal, herein we report an ambient-pressure, CO2-transfer hydrogenation catalyst which generates formate salt (HCO2−) at the TOF value of 90 h−1 in 12 h of reaction at 150 °C using glycerol as hydrogen source. This new Ircatalyst was recently discovered by our group and was found to be highly efficient in ambient-pressure CO2hydrogenation with H2 as well.3a The catalyst features a hybrid anionic amido-type and an imidazolylidene-based strongly σ-donating abnormal N-heterocyclic carbene (aNHC) backbone, which probably facilitated hydride delivery efficiently under ambient conditions, and also provided the required structural robustness and stability.
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glycerol (4 mL) as the hydride-transfer alcohol at a temperature of 150 °C. Notably, [Ir]pyd was inferior than [Ir]imd under optimized conditions as well using Table 1. Optimization of the Catalytic Protocol cat. = x mol, y M base/H2O (4 mL), T 0C, t = 1 h HCO2
CO2 (bubbling at atm P)
OH H donor = Me (4 mL)
#
cat./μmol
aq. base/M
Me , Me
OH
OH , HO
H− donor
OH
T
TOF
/°C
(1 h)
(A) catalyst identity 1
[Ir]pyd/1.0
2
[Ir]imd/1.0
3
[Ru]pyd/1.0
4
[Ru]imd/1.0
KOH/1.0
iPrOH
110
8±1
KOH/1.0
iPrOH
110
14±1
KOH/1.0
iPrOH
110
2±1
KOH/1.0
iPrOH
110
3±2
(B) base identity and strength M
I N
N
=
[Ir]pyd
N
M
I N
N
N N
Ir
M
=
Ir
M
[Ir]imd
Ru [Ru]pyd
KOH/1.0
iPrOH
110
14±1
KHCO3/1.0
iPrOH
110
17±1
K2CO3/1.0
iPrOH
110
26±2
K2CO3/0.5
iPrOH
110
16±1
9
[Ir]imd/1.0
K2CO3/2.0
iPrOH
110
24±2
10
[Ir]imd/0.50
K2CO3/1.0
iPrOH
110
17±1
11
[Ir]imd/0.25
K2CO3/1.0
iPrOH
110
21±1
12
[Ir]imd/0.15
K2CO3/1.0
iPrOH
110
32±2
13
[Ir]imd/0.15
K2CO3/1.0
MeOH
110
23±1
14
[Ir]imd/0.15
K2CO3/1.0
Glycerol
110
51±1
5
[Ir]imd/1.0
6
[Ir]imd/1.0
7
[Ir]imd/1.0
8
[Ir]imd/1.0
Ru [Ru]imd
Figure 2. Catalysts studied in this work: [Ir]pyd, [Ru]pyd, [Ir]imd and [Ru]imd.3a,8
RESULTS AND DISCUSSION Our investigation initiated with testing the performance of the catalysts [Ir]pyd, [Ru]pyd, [Ir]imd and [Ru]imd as shown in Figure 2,3a,8 in which two Ir and two Ru complexes of benzimidazolato-tethered two different NHC backbones – pyridinylidene-based aNHC and imidazolylidene-based aNHC were utilized. A rapid screening method was followed by running the CO2transfer hydrogenation catalysis for just 1 h through bubbling CO2 gas (50 mL/min9) into a mixture of H2O/hydrogen donor alcohol (8 mL, 1:1 v/v) at ambient pressure in presence of an inorganic base. As evident from entries 1-4, Table 1, [Ir]imd emerged as the best catalyst in terms of TOF obtained in 1 h screening time. It was evident that the stronger σ-donor ability of the imidazolylidene-based aNHC ligand as compared to the pyridinylidene-based aNHC benefited toward the efficiency.10 With the best catalyst, [Ir]imd in hand, similar rapid methodology was next applied to optimize the identity and strength of the base employed (entries 5-9, Table 1), catalyst loading (10-12), identity of the hydrogen donor (13-14) and the reaction temperature (15-18). Thus the optimized reaction conditions for the present ambient-pressure CO2-transfer hydrogenation to formate salt were found to employ 0.15 µmol of [Ir]imd as catalyst in presence of 1.0 M aqueous K2CO3 (4 mL) as base and
(C) catalyst loading
(D) hydrogen donor
(E) temperature 15
[Ir]imd/0.15
K2CO3/1.0
iPrOH
130
47±2
16
[Ir]imd/0.15
K2CO3/1.0
iPrOH
150
87±3
17
[Ir]imd/0.15
K2CO3/1.0
Glycerol
130
72±2
18
[Ir]imd/0.15
K2CO3/1.0
Glycerol
150
149±2
19
[Ir]pyd/0.15
K2CO3/1.0
iPrOH
150
31±2
20
[Ir]pyd/0.15
K2CO3/1.0
Glycerol
150
70±3
Figure 3. (A) Relevant sections of the 1H NMR spectra recorded at different time intervals for a catalytic transfer hydrogenation reaction of CO2 with glycerol using [Ir]imd as
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ACS Catalysis catalyst. The peak at 8.45 ppm corresponds to the HCO2− proton and the peak at 0.35 ppm is due to the standard ((CH3)3SiCH2CH2CH2SO3Na). Reaction conditions: [Ir]imd = 0.15 μmol, glycerol = 8 mL, aq. K2CO3 (1 M, 8 mL), 150 °C. (B) Corresponding reaction profile plot of the same reaction. See SI for experimental procedure.
both iPrOH and glycerol (entries 19-20, Table 1). Under the optimized conditions, the potential of the catalyst [Ir]imd was evaluated by performing the transfer hydrogenation reaction using glycerol for longer time. The 1H NMR spectral stacked plot (Figure 3A) obtained by monitoring the reaction at different time intervals, reflected the gradual increase of formate resonance peak at 8.45 ppm (HCO2−) with respect to the standard peak at 0.35 ppm ((CH3)3SiCH2CH2CH2SO3Na). The corresponding reaction profile plot as shown in Figure 3B clearly demonstrated an excellent performance of the system yielding 149.4 µmol of formate and exhibiting a TOF value of 90 h−1in 12 h of total reaction time . The nearly constant slope of the plot indicated little sign of saturation of the catalytic efficiency during the tested period of the reaction. It was notable that while we observed acetone as the only by-product in the reaction with isopropanol, the catalytic reaction with glycerol led to several byproducts such as pyruvaldehyde, dihydroxyacetone, and 1,2-propanediol.7,11 However, lactate was not observed in our case. Finally, the unreacted CO2 gas coming out from the reactor was trapped in 1 M NaOH solution as NaHCO3 which could again be utilized in our catalysis for formate formation (see SI for details). Next, a few control experiments were conducted to investigate some of the mechanistic issues related to the catalysis with [Ir]imd. At first, under optimized conditions (entry 18, Table 1), a catalytic reaction using iPrOH as hydrogen donor was run but without introducing CO2(g) into the reactor. No formate was formed in this reaction, suggesting that transfer hydrogenation of aq. K2CO3 (1 M) was not facile and hence CO32− was not the potential substrate for the present reaction (Figure 4A). The pH of this reaction solution was 12.1, as expected. Similarly, when the same reaction as above with CO2 in the presence of aq. KOH (1 M) was performed but without the catalyst [Ir]imd, again formate was not formed; however we observed bicarbonate (HCO3−) in the reaction solution as confirmed by the corresponding 1H and 13C{1H} NMR spectral signals at 2.13 ppm (with reference to 4.79 ppm for HOD in D2O) and 161.9 ppm respectively (Figure 4B).12 The same observation of bicarbonate formation was also confirmed when aq. K2CO3 (1 M) was used as the base. The presence of bicarbonate in the reaction mixture was not unexpected because in aqueous base, employed during CO2-transfer hydrogenation, CO2 gas dissolves and exhibits pH-dependent acid-dissociation equilibria as shown in equation 1 (Figure 4C).3a,c,e As the pH of the reaction solution was found to be 8.5, existence of bicarbonate (HCO3−) in the solution was obvious. This fact raised the question whether HCO3− was the actual substrate or it was also a substrate along with CO2 for the
present transfer hydrogenation catalysis with [Ir]imd. To probe this query, a catalytic reaction was carried out with
(C)
pKa1 = 6.35
CO2 + H2O
OH O C + H O
pKa2 = 10.33
O O C + 2H O
(1)
Figure 4. (A) Partial 1H NMR spectral profile (in D2O) for a catalytic transfer hydrogenation of aq. K2CO3 (1 M) as the sole substrate in iPrOH without CO2. (B) Partial 1H NMR spectral profile (in D2O) for a standard catalytic reaction of CO2 + aq. K2CO3 in iPrOH but in the absence of catalyst. Inset shows the corresponding 13C{1H} NMR spectral profile. (C) Acid-dissociation equilibria for aq. CO2 system. (D) Comparison of catalytic activity with CO2+ aq. K2CO3 (1 M) versus aq. KHCO3 (1 M) as substrates under standard conditions. Average values of two catalytic runs were used for the plot. (E) Monitoring the pH and yield of formate of the standard catalytic reaction for initial 1 h. Average values of two catalytic runs were used for the activity plot. 1H NMR spectral detection of Ir–H using (F) glycerol and (G) iPrOH as H-donor.
1.0 M aq. KHCO3 solution (4 mL) and glycerol (4 mL) under optimized conditions but in the absence of CO2. The formation of formate product (HCO2−) was observed, but the extent of product formation with time was found to be substantially lower than that of the standard catalytic reaction conducted with CO2(g) and 1.0 M aq. K2CO3 solution, as evident from the respective TON values at different time for initial few hours shown in Figure 4D. This result suggested that CO2 is the substrate of the present transfer hydrogenation reaction and HCO3− could also react in parallel and simultaneously. However, the actual pathway is not clear at present and in-depth computational study may provide insight related to the
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ACS Catalysis respective activation barriers. To check how fast the aciddissociation equilibria were established toward bicarbonate (HCO3−) during the standard reaction with CO2(g) and 1.0 M aq. K2CO3 solution, the pH and yield of formate of a set of reactions were checked after terminating the reactions at different time intervals (upto the initial 1 h only). The results, depicted in Figure 4E, clearly showed that the yield of formate increased with time and the pH sharply decreased from 11.6 to 9.3 within the first 15 min and then became almost constant at ~9.5, clearly indicating the attainment of the equilibrium in favor of HCO3− quite early in the reaction. Next, for this transfer hydrogenation catalysis, generation of Ir–H intermediate species was confirmed by 1H NMR spectroscopy via conducting a stoichiometric reaction of the [Ir]imd complex with glycerol in the presence of K2CO3 (1 M in 100 μL D2O) in CD3CN (500 μL) solvent (Figure 4F). The characteristic peak at −14.8 ppm confirmed the formation of Ir–H, in line with our previous observation. Similarly, the same Ir–H species could also be generated from iPrOH as hydrogen source by treating a mixture of [Ir]imd and D2O (200 µL)/iPrOH (200 µL) in the presence of K2CO3 (1 M), as confirmed by the appearance of the characteristic signal at −14.8 ppm in 1H NMR spectroscopy (Figure 4G).
I
N
Cp*
Cp* ROH RO Ir base/H2O N N N N N N N (I) imd [Ir] HCOO ROH base/H2O O Cp* O Ir CO2 H (a) N Ir
ROH = iPrOH, glycerol
e id dr ion hy at - min i el
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
N
N (III)
(b)
N OH
Ir
Cp* N
N OH O C O
H
N (II)
N
Figure 5. Plausible catalytic cycle for the present CO2transfer hydrogenation protocol.
Based on the above information, a plausible catalytic cycle for the present CO2-transfer hydrogenation reaction with glycerol (or isopropanol) was suggested (Figure 5). The complex [Ir]imd forms the Ir-alkoxy intermediate I from the alcohol in the presence of base followed by generation of the Ir–H species II via β-hydride elimination route. The later species then delivers its hydride ligand to the substrates CO2 (path a) as well as HCO3− (path b), to produce Ir–formato species III, which provides formate and regenerates the Ir-alkoxy intermediate upon reacting with alcohol and aqueous KOH solution to continue the cycle. While we detected the hydride intermediate II from control experiments as mentioned above, we could not detect any alkoxy (I) or formato (III) intermediate in similar control experiments. In fact, attempts to capture the alkoxy (I) and formato (III) intermediates via two independent stoichiometric
reactions of [Ir]imd with NaOiPr and HCO2Na respectively, led to the facile generation of the Ir–H species (II) (see SI). This might be probably due to fast β-hydride elimination (from I) and CO2 deinsertion (from III) respectively, in the absence of any CO2 or bicarbonate substrate in the medium. Finally, the hydride-delivery reactivity of the hydride intermediate II was verified by a control stoichiometric reaction of the hydride species with aqueous solution of bicarbonate (HCO3−), which showed the generation of formate (HCO2−) in the medium (see SI). The poor activity of the relatively less σ-donor pyridinylidene-based aNHC13 complex [Ir]pyd compared to the more σ-donor imidazolylidene-based aNHC-complex [Ir]imd,10 suggested that the hydride delivery step could be crucial in this catalysis and the stronger σ-donor aNHC ligand might have favored this step significantly by making the Ir–H species more hydridic.14 This trend is in line with the CO2-hydrogenation catalysis also, as recently reported by us.3a We had also shown computationally the stronger hydride donor ability of the Ir–H species generated from [Ir]imd as compared to [Ir]pyd.3a That hydride transfer is crucial in this type of reactions and electron-donating ligands facilitate the overall reaction, was also supported by some other relevant works.3b,c,f,15 CONCLUSION In summary, this work demonstrated the development of an efficient ambient-pressure CO2-transfer hydrogenation catalyst based on the [Cp*Ir(aNHC–benzimidazolato)] platform. The transfer hydrogenation activity with glycerol and also isopropanol as renewable alcohols was explored at ambient pressure and 150 °C leading to high TOF value in relatively quicker time in comparison to the reported high-pressure catalysts available till date. While we believe that the dimethylimidazolydene-based strong σ-donor abnormal NHC ligand backbone partnering with anionic benzimidazolato donor within the catalyst’s structure probably facilitated the hydride delivery step efficiently, resulting in high catalytic activity under ambient pressure conditions, an in-depth mechanistic investigation including computational studies will be required to further improvement of this promising design.
ASSOCIATED CONTENT Experimental details, spectra (PDF); CIF file CCDC 1876974 ([Ru]imd). This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected].
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT J.C. thanks DST-SERB (grant no. EMR/2016/003002) and IISER Bhopal for generous financial support. A.K. thanks IISER Bhopal for Integrated PhD fellowship. Doctoral fellowship to S.S. from UGC is gratefully acknowledged. We
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ACS Catalysis thank the reviewers for their valuable comments and suggestions.
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(5) (a) Ding, G.; Su, J.; Zhang, C.; Tang, K.; Yang, L.; Lin, H. Coupling Glucose Dehydrogenation with CO2 Hydrogenation by Hydrogen Transfer in Aqueous Media at Room Temperature. ChemSusChem 2018, 11, 2029–2034. (b) Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. Well-Defined Iron Catalysts for the Acceptorless Reversible DehydrogenationHydrogenation of Alcohols and Ketones. ACS Catal. 2014, 4, 3994−4003. (c) Neilsen, M.; Kammer, A.; Cozzula, D.; Junge, H.; Gladiali, S.; Beller, M. Efficient Hydrogen Production from Alcohols under Mild Reaction Conditions. Angew. Chem. Int. Ed.2011, 50, 9593–9597. (d) Kobayashi, H.; Fukuoka, A. Synthesis and Utilisation of Sugar Compounds Derived from Lignocellulosic Biomass. Green Chem. 2013, 15, 1740-1763. (6) (a) Wang, D.; Astruc, D. The Golden Age of Transfer Hydrogenation. Chem. Rev. 2015, 115, 6621-6686. (b) Lu, Z.; Cherepakhin, V.; Demianets, I.; Lauridsen, P. J.; Williams, T. J. Iridium-Based Hydride Transfer Catalysts: From Hydrogen Storage to Fine Chemicals. Chem. Commun. 2018, 54, 7711-7724. (7) (a) Azua, A.; Sanz, S.; Peris, E. Water-Soluble IrIII NHeterocyclic Carbene Based Catalysts for the Reduction of CO2 to Formate by Transfer Hydrogenation and the Deuteration of Aryl Amines in Water. Chem. Eur. J. 2011, 17, 3963–3967. (b) Sanz, S.; Benı´tez, M.; Peris, E. A New Approach to the Reduction of Carbon Dioxide: CO2 Reduction to Formate by Transfer Hydrogenation in iPrOH. Organometallics 2010, 29, 275–277. (c) Heltzel, J. M.; Finn, M.; Ainembabazi, D.; Wang, K.; Voutchkova-Kostal, A. M. Transfer Hydrogenation of Carbon Dioxide and Bicarbonate from Glycerol under Aqueous Conditions. Chem. Commun. 2018, 54, 6184-6187. (d) Liu, Q.; Wu, L.; Gülak, S.; Rockstroh, N.; Jackstell, R.; Beller, M. Towards a Sustainable Synthesis of Formate Salts: Combined Catalytic Methanol Dehydrogenation and Bicarbonate Hydrogenation. Angew. Chem. Int. Ed. 2014, 53, 7085–7088. (e) Dibendetto, A.; Stufano, P.; Nocito, F.; Aresta, M. RuII-Mediated Hydrogen Transfer from Aqueous Glycerol to CO2: From Waste to ValueAdded Products. ChemSusChem 2011, 4, 1311 – 1315. (8) For [Ir]imd, see: reference 3a. For [Ir]pyd, see: (a) Semwal, S.; Choudhury, J. Molecular Coordination-Switch in a New Role: Controlling Highly Selective Catalytic Hydrogenation with Switchability Function. ACS Catal. 2016, 6, 2424−2428. For [Ru]pyd, see: (b) Semwal, S.; Choudhury, J. Switch in Catalyst State: Single Bifunctional Bi-state Catalyst for Two Different Reactions. Angew. Chem. Int. Ed. 2017, 56, 5556 –5560. [Ru]imd is a new complex and the details are provided in SI. (9) The flow-rate of CO2(g) (50 mL/min) was optimized with respect to the yield of formate. For details, see SI. (10) The differential pulse voltammetry (DPV) showed that the IrIII/IrIV redox potential value was ~56 mV lower for [Ir]imd than that for [Ir]pyd, suggesting that the imidazolylidene-based aNHC ligand (in [Ir]imd) is more σ-donor than the pyridinylidene-based ligand (in [Ir]pyd). For details, see SI. (11) Finn, M.; Ridenour, J. A.; Heltzel, J.; Cahill, C.; Voutchkova-Kostal, A. Next-Generation Water-Soluble Homogeneous Catalysts for Conversion of Glycerol to Lactic Acid. Organometallics 2018, 37, 1400−1409. (12) (a) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities. J. Org. Chem. 1997, 62, 7512–7515. (b) Moret, S.; Dyson, P. J.; Laurenczy, G. Direct, in situ Determination of pH and Solute Concentrations in Formic Acid Dehydrogenation and CO2 Hydrogenation in Pressurised Aqueous Solutions using 1H and 13C NMR spectroscopy. Dalton Trans. 2013, 42, 4353–4356. (13) (a) Iglesias, M.; Albrecht, M. Expanding the Family of Mesoionic Complexes: Donor Properties and Catalytic Impact of Palladated Isoxazolylidenes. Dalton Trans. 2010, 39, 5213–5215. (b) Huynh, H. V. Electronic Properties of N ‑ Heterocyclic
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Carbenes and Their Experimental Determination. Chem. Rev. 2018, 118, 9457–9492. (14) (a) Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M. Thermodynamic Hydricity of Transition Metal Hydrides. Chem. Rev. 2016, 116, 8655−8692. (b) Raebiger, J. W.; Miedaner, A.; Curtis, C. J.; Miller, S. M.; Anderson, O. P.; DuBois, D. L. Using Ligand Bite Angles To Control the Hydricity of Palladium Diphosphine Complexes. J. Am. Chem. Soc. 2004, 126, 5502−5514. (c) Semwal, S.; Mukkatt, I.; Thenarukandiyil, R.; Choudhury, J. Small “Yaw” Angles, Large “Bite” Angles and an Electron-Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride Transfer Activity. Chem. Eur. J. 2017, 23, 13051–13057. (15) Onishi, N.; Xu, S.; Manaka, Y.; Suna, Y.; Wang, W-H.; Muckerman, J. T.; Fujita, E.; Himeda, Y. CO2 Hydrogenation Catalyzed by Iridium Complexes with a Proton-Responsive Ligand. Inorg. Chem. 2015, 54, 5114−5123.
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