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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Mechanistic Insights into Ruthenium-Pincer-Catalyzed AmineAssisted Homogeneous Hydrogenation of CO2 to Methanol Sayan Kar, Raktim Sen, Jotheeswari Kothandaraman, Alain Goeppert, Ryan Chowdhury, Socrates B. Munoz, Ralf Haiges, and G. K. Surya Prakash* Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, United States

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S Supporting Information *

ABSTRACT: Amine-assisted homogeneous hydrogenation of CO2 to methanol is one of the most effective approaches to integrate CO2 capture with its subsequent conversion to CH3OH. The hydrogenation typically proceeds in two steps. In the first step the amine is formylated via an in situ formed alkylammonium formate salt (with consumption of 1 equiv of H2). In the second step the generated formamide is further hydrogenated with 2 more equiv of H2 to CH3OH while regenerating the amine. In the present study, we investigated the effect of molecular structure of the ruthenium pincer catalysts and the amines that are critical for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNPR(CO)] (R = Ph/iPr/Cy/t-Bu) for both amine formylation and formamide hydrogenation, only catalyst Ru-Macho (R = Ph) provided a high methanol yield after both steps were performed simultaneously in one pot. Among various amines, only (di/ poly)amines were effective in assisting Ru-Macho for methanol formation. A catalyst deactivation pathway was identified, involving the formation of ruthenium biscarbonyl monohydride cationic complexes [RuHPNPR(CO)2]+, whose structures were unambiguously characterized and whose reactivities were studied. These reactivities were found to be ligand-dependent, and a trend could be established. With Ru-Macho, the biscarbonyl species could be converted back to the active species through CO dissociation under the reaction conditions. The Ru-Macho biscarbonyl complex was therefore able to catalyze the hydrogenation of in situ formed formamides to methanol. Complex Ru-Macho-BH was also highly effective for this conversion and remained active even after 10 days of continuous reaction, achieving a maximum turnover number (TON) of 9900.

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require a chemical carrier.6,7 In this regard, liquid organic hydrogen carriers (LOHCs) are particularly attractive because they can carry a high wt % of H2 and could more easily replace fossil fuels by using existing and similar infrastructures.8−11 To address both of the aforementioned problems in one elegant solution, along with our late Nobel Laureate colleague, Prof. George A. Olah, we have proposed the concept of the Methanol Economy,12−15 where CO2 captured from the atmosphere is combined with H2 to produce methanol. Methanol is a convenient liquid that can act as a fuel or produce H2 on demand through reforming.16−19 Methanol is also an excellent energy carrier because of its liquid nature, high H2 content, and easy synthesis. More than 75 million metric tons of methanol are produced annually worldwide20 for various applications including as a chemical feedstock and as a fuel. A number of countries including China have also successfully implemented methanol as a transportation fuel. However, most CH3OH is presently still synthesized from natural gas or coal through syngas (CO + H2), not CO2. The recycling of CO2, captured from concentrated emission sources

INTRODUCTION The accumulation of anthropogenic carbon dioxide in the environment due to excessive burning of fossil fuels is a massive problem with multifarious and disastrous ramifications including higher Earth surface temperature, increased climate irregularity, and ocean acidification. Unless we adopt necessary measures to invest in and utilize alternative energy sources including solar, wind, and geothermal as well as nuclear energy, the Intergovernmental Panel on Climate Change (IPCC) predicts an increase in global temperature of up to 4.8 °C over the next century.1 Besides the pressing CO2 emission problem, our reliance on fossil fuels should also be reduced due to their finite nature. However, it should be noted here that, per se, we do not have an energy problem; the Sun still provides us with ample energy in the form of sunlight. In fact, the amount of energy the Earth receives from sunlight in a day exceeds our energy requirement for an entire year. What we lack is a way to properly harness this energy, store it, and transport it to the points of useessentially an energy-storage problem. One way to store the Sun’s energy is in the form of hydrogen through water splitting.2−5 However, hydrogen is a flammable, highly diffusible gas with a very low volumetric energy density. Therefore, for most practical applications, H2 itself would © XXXX American Chemical Society

Received: November 28, 2018

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DOI: 10.1021/jacs.8b12763 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society or directly from air, to produce methanol would be more sustainable and desirable, offering at the same time an alternative to its proposed sequestration in underground rock formations.21−23 Methanol synthesis from CO2 is traditionally carried out at high temperatures and pressures with heterogeneous catalysts such as Cu/ZnO/Al2O3 and modifications thereof.24,25 Recently, however, less harsh conditions have been reported, with both homogeneous and heterogeneous catalysts to access low-temperature methanol synthesis from CO2.26−33 In this regard, homogeneous catalysis holds certain advantages by enabling researchers to systematically investigate the catalyst’s molecular structure in order to improve its efficiency and selectivity toward methanol formation. The homogeneous reduction of gaseous CO2 by molecular H2 to methanol has been challenging due to the kinetic and thermodynamic stability of CO2. In 1995, Tominaga et al. used Ru4(CO)12 to convert CO2 to methanol via the formation of CO, although high temperature (>200 °C) was required and concomitant formation of CO and CH4 was observed.34,35 Huff and Sanford in 2011 demonstrated an elegant sequential reduction of CO2 to methanol, via the formation of formic acid and methyl formate as intermediates, using three different catalysts for three distinct steps in the same pot.36 More recently, Leitner and co-workers have used (triphos)Ru(TMM) (TMM = trimethylenemethane) complex as a catalyst precursor in the presence of NHTf2 to obtain methanol with a TON of 769.37,38 Beller and co-workers reported a similar system with cobalt-based homogeneous catalysts in 2017.39 In 2015, Sanford and co-workers demonstrated the hydrogenation of CO2 to methanol in the presence of dimethylamine proceeding through the formation of a formamide intermediate (Scheme 1). 40 The ability of an amine to assist in the CO 2

capture and conversion, bypassing the energy-intensive CO2 desorption and compression steps. In 2015, our group demonstrated the feasibility of such a one-pot capture and conversion by hydrogenating CO2 directly captured from air into methanol.41 Similarly, Milstein and co-workers described the sequential capture of CO2 by ethanolamines and conversion to CH3OH by Milstein’s PNN catalyst.42 The easy recyclability of the catalyst as well as the amine through the use of a biphasic system has also been disclosed recently by our group.43 Other studies employing earth-abundant metalbased complexes as catalysts have been published by us and others.44,45 A list of selected reports of amine-assisted CO2-toCH3OH systems is provided in Table 1. However, despite the immense practical utility of the amineassisted CO2-to-methanol process, a systematic study describing the influence of catalyst/amine molecular structure on methanol production had not yet been undertaken. Given that the highest TON reported to date for this process is only ∼9000,46 there is clearly a need to improve upon the catalytic efficiency by a judicious choice of catalyst/amine pairing to make the process economically viable. In this study, we explore in detail the relationship between catalyst/amine molecular structure and methanol yield.

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RESULTS AND DISCUSSION Effect of the Catalyst’s Molecular Structure. In previous reports, our group and others demonstrated the ability of Ru-Macho-BH (C-1; Table 2) to convert CO2 into CH 3 OH in the presence of an amine (Me 2 NH or pentaethylenehexamine (PEHA)).40,41 To probe for the effect of structural changes in the ligand framework on methanol yield, we screened several RuPNPR pincer complexes with variable substitutions in the phosphine ligands (R = Ph (C-2), i-Pr (C-3), Cy (C-4), t-Bu (C-5)) (Table 2). To our surprise, the methanol yield dropped drastically as the R groups were changed from Ph to i-Pr/Cy/t-Bu. When the reaction was performed with PEHA (5.1 mmol) in 10 mL of triglyme and C-1 (10 μmol) as the catalyst under 75 bar of CO2/3H2 mixture, the formation of 10.5 mmol of CH3OH was observed (TON = 1050) after 40 h at 145 °C along with 8.0 mmol of formamide and 1.2 mmol of formate (entry 1). Catalyst C-2 (Ru-Macho) formed a similar amount of CH3OH under similar reaction conditions, although an additional base (K3PO4) was required for its initial activation (entry 2). All

Scheme 1. Amine-Assisted CO2 Hydrogenation to CH3OH

hydrogenation to methanol opened up the vista for integration of CO2 capture with conversion to methanol. Considering that amines are widely used as CO2 scrubbing agents, amineassisted CO2-to-methanol systems are ideal for integrated CO2

Table 1. Selected Examples of Reported Amine-Assisted CO2-to-Methanol Systems

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Journal of the American Chemical Society Table 2. Effect of Catalyst Molecular Structures on CH3OH Yielda

entry d

1 2 3 4e 5 6 7 8 9

catalyst

formateb (mmol)

formamideb (mmol)

methanolb (mmol)

COc (%)

TONformate+formamide

TONCH3OH

C-1 C-2 C-3 C-3 C-4 C-5 C-6 C-7 C-8

1.2 1.6 1.1 2.3 1.0 1.6 1.3 0.4 0.7

8.0 8.1 22.6 28.4 14.7 17.5 7.0 18.4 11.0

10.5 10.4 3.2 0 0.5 0 6.8 0 0

0.21 0.22 0 0 0 0 0.1 0 0

920 970 2370 3070 1570 1910 830 1880 1170

1050 1040 320 0 50 0 680 0 0

a Reaction conditions: PEHA (5.1 mmol), cat. (10 μmol), K3PO4 (1 mmol), triglyme (10 mL), CO2/3H2 (75 bar), 145 °C, 40 h. bYields were determined from 1H NMR spectra with 1,3,5-trimethoxybenzene (TMB) as an internal standard. cCO detection limit −0.099%. dIn the absence of K3PO4. eT = 125 °C. TONCH3OH = mol of CH3OH formed per mol of cat. Yield calculation error ±5%. See Supporting Information for details.

Table 3. Formamide Reduction by Ru Pincer Complexesa

entry

catalyst

time (min)

formamideb (%)

amineb (%)

CH3OHb (%)

TONCH3OH

1 2 3 4

C-2 C-3 C-4 C-5

440 110 270 480

26 26 18 86

67 79 80 4

70 79 87 5

1400 1580 1740 100

a Reaction conditions: N-formylpiperidine (20 mmol), H2 (20 bar), catalyst (10 μmol), K3PO4 (1 mmol), triglyme (10 mL). bYields were determined from 1H NMR spectra with TMB as an internal standard. Reaction times were determined based on cessation of pressure decrease. Yield calculation error ±5%.

synthesis (entry 4). Catalyst C-4, with its Cy group attached to phosphorus atoms, afforded a meager 0.5 mmol of methanol (TON = 50) and 14.7 mmol of formamide (entry 5). Similarly, catalyst C-5, Ru-PNPtBu, led to 17.5 mmol of formamide products but no CH3OH (entry 6). The inability of catalysts C-3, C-4, and C-5 and the exclusive ability of C-1 and C-2 for effective CO2-to-CH3OH conversion is somewhat surprising. Ding and co-workers, in their study on N-formylation of amines with CO2 and H2 catalyzed by ruthenium pincer complexes, reported similar activities for C2−C-5 in the N-formylation of morpholine.47 Similarly, we found in this study that, under a H2 pressure of 20 bar, C-2, C3, and C-4 (but not C-5, probably due to its bulky t-Bu groups) are able to catalyze very efficiently the hydrogenation

subsequent catalysts from C-3 to C-8 were screened in the presence of K3PO4 (1 mmol). When the P substituent was changed from Ph to i-Pr (complex C-3, RuHClPNPiPr(CO)), a stark decrease in CH3OH formation was observed with only 3.2 mmol of CH3OH obtained (TON = 320) (entry 3). Instead, a large accumulation of formamide products (22.6 mmol, 80% with respect to (wrt) the amine content) was noticed, indicating that the formamide-reduction step to CH3OH is the most challenging under these reaction conditions. At a lower temperature of 125 °C, in the presence of C-3, an even greater amount of formamide products (93%) was detected after 40 h, but no appreciable amount of methanol was observed as analyzed through 1H or 13C NMR, indicating the elevated temperature required for the methanol C


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Journal of the American Chemical Society of N-formylpiperidine (F-1) to methanol and the corresponding amine (Table 3).48 In fact, RuPNPiPr (C-3) and RuPNPCy (C-4) were even more active than RuPNPPh (C-2), as observed from the completion time of the hydrogenation reactions. The fact that the CH3OH formation is not dependent on the effectiveness of the parent catalysts to catalyze the two steps of this sequential reaction could indicate the formation of deactivating catalytic intermediate(s) during the reaction. We will explore these intermediates later while discussing the reaction mechanism. It also should be noted here that, although in this Article we concern ourselves with ruthenium pincer complexes, very similar observations were reported by us and Bernskoetter et al. with regards to iron pincer complexes.31,41,49,50 Going back to Table 2, among other ruthenium complexes, the dichloride pincer NHC complex C-6, with a PNPPh pincer ligand and NHC as the spectator ligand (as opposed to CO in other complexes), was able to reduce CO2 to CH3OH in the presence of PEHA (entry 7). After 40 h, 6.8 mmol of CH3OH (TON = 680) was observed through 1H NMR. A continuous pressure drop inside the reaction vessel until the reaction termination indicated the active nature of the catalyst throughout the reaction. However, with both Milstein’s PNN pincer complex (C-7) and the PNP acridine pincer complex C8, only intermediate formamide products were observed (18.4 and 11.0 mmol, respectively) with no observable methanol formation through 1H and 13C NMR analysis. Thus, it seems that the presence of PNPPh as the pincer ligand is essential to obtain good methanol yields directly from CO2 via an amineassisted process. In contrast, other structural features such as varying spectator ligand do not influence methanol yield to the same degree. Effect of the Amine’s Molecular Structure. Subsequently, the effect of different amine’s molecular structure on methanol formation was investigated in the presence of RuMacho-BH (C-1). The amine plays multiple roles during the catalytic reaction. First, it helps to dissolve CO2 in the organic solution, effectively increasing the CO2 concentration in the solution. Second, during the initial CO2 hydrogenation to form alkylammonium formate salts, the amine assists in the detachment of the formate ligand from the ruthenium center (Figure 1, top). In the absence of amine, no CO2 hydrogenation takes place (even to formic acid) as the catalyst is kinetically trapped in the formate form (C-1B) (Figure 1, bottom, eq 1). Similarly, in the absence of amine, formamide

reduction by CO2/3H2 gas mixture is not viable because of the formation of C-1B, which is unable to revert to the dihydride species (C-1A) under the reaction conditions (Figure 1, bottom, eq 2). Third, the amine forms formamide from the ammonium formate salt via condensation reaction, which is a crucial reaction step for obtaining methanol (Scheme 1). To our surprise, all the monoamines screened in this study (n-/s-/i-/t-BuNH2, n-/s-/i-Bu2NH; see Supporting Information) for the amine-assisted CO2-to-methanol synthesis in the presence of catalyst C-1 produced only traces of CH3OH and gave almost exclusively formamide and formate products (Table S2). On the other hand, when diamines were employed for the reaction (because the polyamine PEHA is known to be able to assist in CH3OH production), varying CH3OH yields were obtained (Figure 2). The nature of primary or secondary

Figure 2. Methanol and formamide yields with different amines. Reaction conditions: C-1 (10 μmol), triglyme (10 mL), CO2/3H2 (75 bar), 145 °C, 20 h. Amine functionality content = 30.6 mmol (PEHA 5.1 mmol, DETA (9) 10.2 mmol, all diamines 15.3 mmol). Yields were determined by 1H NMR with TMB as an internal standard. With piperidine, 20 mmol was used. Yield calculation error ±5%.

amine had a very slight effect on methanol formation, with secondary amines being marginally more efficient. With ethylenediamine (1), containing two primary amino groups, 4.3 mmol of CH3OH was observed after 20 h, whereas an 18% higher yield was obtained (5.1 mmol of methanol) in the case of N,N′-dimethylethylenediamine (2) containing two secondary amino groups (Figure 2). N-Methylethylenediamine 3, with one primary and one secondary amino group, provided a CH3OH yield that was in between those of 1 and 2. Surprisingly, in the presence of mixed primary−tertiary and secondary−tertiary amines, 4 and 5, respectively, no CH3OH was produced, but only formamide intermediates were observed. The effect of the presence of hydroxyl groups was then explored using N-(2-hydroxyethyl)ethylenediamine (6) and N,N′-bis(2-hydroxyethyl)ethylenediamine (7) for the

Figure 1. (Top) Interconversion of C-1A and C-1B in the presence of amine and H2; (bottom) failed hydrogenations in the absence of amine. D


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[RuHPNPPh(CO)2]+ (C-1D) (Figure 3) was assigned to the catalytic resting state. Finally, the cationic structure was confirmed through single-crystal X-ray diffraction, and the presence of bicarbonate as the counteranion was observed (Figure 4). The second carbonyl ligand is surmised to come

reaction. A rapid decrease in reaction pressure was observed during the initial hours of these experiments; and 3.7 and 4.8 mmol CH3OH were obtained after 20 h with 6 and 7, respectively. In comparison, the polyamines diethylenetriamine (DETA) (9) and PEHA (10) provided 5.8 and 6.2 mmol of methanol formation, respectively. In the case of piperidine (8), 92% of the amino groups were formylated after 20 h (18.4 mmol), along with only 3% of methanol formation. At this point, few comments regarding the correlation between amine structure and methanol formation deserve mentioning. The 1,2-diamines or polyamines with a 1,2diamine substructure, where both amine functional groups are either primary or secondary, provide the best methanol yields (as in the cases of 1, 2, 3, 9, and 10). The hydroxyl groups present in the amine, in the cases of 6 and 7, have a deactivating effect, providing somewhat inferior methanol yield. More interestingly, when one of the amino groups of the diamine was tertiary, as in 4 and 5, methanol formation completely subsided. Thus, 1,2-diamines with primary/ secondary amines were the unique structural motifs that were able to assist in the formation of methanol in high yields. The reason for this correlation between amine structure and methanol formation was investigated further through mechanistic studies presented hereafter. Mechanistic Investigations. To get a better understanding of the reaction system, catalytic species present in the reaction were monitored through various spectroscopic techniques. Catalysts C-1−C-5 are reported in the literature to form ruthenium formate species (similar to C-1B) in the presence of H2 and CO2 (see Figure 1). However, under the conditions of our mechanistic investigation (12 mg of C-1, 10 mg of PEHA in tetrahydrofuran (THF-d8), 145 °C for 40 h), a completely different species was observed in the solution. The 1 H spectra showed the presence of a triplet (J = 16.4 Hz) at −6.3 ppm (Figure S4A) along with a minor triplet at −5.6 ppm in a 20:1 ratio. The strong low field shift of this hydride peak signifies a strong trans effect from the opposite axial ligand. The corresponding 31P peak was observed at 58.5 ppm (major) and 57.4 ppm (minor), while in the 13C NMR spectra, two different carbonyl peaks for the major isomer were observed at 200.2 (t; J = 10.5 Hz) and 191.9 ppm in a 1:1 ratio (Figure S4). The coupling between the carbonyl ligands and the hydride was further observed from the proton-coupled 13C NMR (Figure 3). Similarly, in the attenuated total reflection infrared (ATR-IR) spectra, two different CO stretches were observed at 2052 and 1964 cm−1. In the electrospray ionization mass spectrometry (ESI-MS), the molecular fragment [RuHPNPPh(CO)2]+ corresponding to m/z = 600.08 was also observed. Thus, the cat ionic stru ct ure of

Figure 3. Observation of two different carbonyl peaks in (top, 1H decoupled; bottom, 1H coupled).

Figure 4. Single-crystal X-ray structure of the cation in C-1D (left) and C-5D (right). ORTEP diagrams plotted at 50% probability level. Selected hydrogen atoms have been omitted for clarity.

from the in situ generated CO gas. Indeed, the presence of CO gas was confirmed through ATR-IR analysis of the reaction gas mixture inside the reactor (Figure S6). A correlation between the presence of CH3OH and CO gas in the reaction mixture was observed (Table 2), indicating that the CH3OH formation pathway is the primary source for CO formation. However, when an experiment was conducted in the presence of 2 mmol of 13CH3OH, no enhanced carbonyl peak of the biscarbonyl complex C-1D was observed by 13C NMR. This indicates that CH3OH decarbonylation is unlikely to be the pathway for the generation of CO. Rather, it is most likely that the CO forms through the decomposition of in situ generated formaldehyde intermediate during the reaction.51−54 Furthermore, from 1H nuclear Overhauser effect (NOE) spectra, no spatial correlation between the hydride peak of the major isomer and the N−H peak was observed. Thus, the structure of anti-C-1D, where the Ru−H and N−H are trans to each other, was assigned to the major isomer. Notably, the same anti isomer was also observed in the single X-ray crystal structure. Thus, the other minor peak in the 1H and 31P was assigned to the other isomer, syn-C-1D (Figure 5).

Figure 5. Structures of anti-C-1D (major) and syn-C-1D (minor).

When the mechanistic studies were conducted with catalysts C-3 (R = i-Pr), C-4 (R = Cy), and C-5 (R = t-Bu) instead of C-1, similar biscarbonyl species (C-3D, C-4D, and C-5D, respectively) were observed in the reaction mixture, along with minute amounts of CH3OH and CO in the gas mixture. The presence of similar pincer biscarbonyl iron complexes has been reported recently by Hazari and co-workers.49 At the same time, they reported the catalytic activity of this biscarbonyl complex for N-formylation reaction. In our study, the crystal structures of the biscarbonyl complexes showed a higher bond length for the newly formed axial Ru−CO bond, similar to the previously reported, aforementioned iron complex. A compi-

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formamide reduction completely stopped. Thus, the biscarbonyl complex is unable to catalyze the formamide reduction step by itself and acts as a deactivated catalytic species under the reaction conditions. The lability of the axial carbonyl group in the biscarbonyl complex can be expected to follow a trend based on the substitution on the P atoms of the ligand. In theory, as the electron-donating ability of the PNP ligand increases with substitutions (t-Bu > i-Pr > Ph), the electron density at the metal center increases. As a result, the metal−carbonyl backbonding also increases in an attempt to diffuse the high electron density from the Ru center. Due to this increased back-bonding, the metal carbonyl bond gets stronger as the ligands become increasingly electron-donating (t-Bu > i-Pr > Ph), resulting in decreased lability of the second carbonyl ligand (lability: t-Bu < i-Pr < Ph). This decrease in lability can be conveniently monitored through CO stretching frequencies of the carbonyl ligands, as the CO bond strength decreases as metal carbonyl bond strengthens. Thus, while in the parent monocarbonyl complexes C-2, C-3, and C-5 the equatorial CO stretching frequencies were found to be similar (Table S3), in the ATR-IR spectra of the biscarbonyl complexes, the axial CO stretches showed increasing frequencies as the ligand was changed from t-Bu to i-Pr to Ph (Figure 7). This signifies that,

lation of the metal carbonyl bond lengths of manganese, iron, and ruthenium biscarbonyl complexes is shown in Table S4. The second axial carbonyl ligand in these (iron and ruthenium) biscarbonyl complexes is surmised to be labile,55,56 and depending on CO and H2 pressure can detach from the metal center to form dihydride species (Figures 6 and S7). The

Figure 6. Proposed routes of conversion of ruthenium biscarbonyl to dihydride species.

formation of the dihydride species should be favored at high H2 pressure and low CO pressure. The two plausible routes for the formation of the dihydride catalytic species C-XA from the biscarbonyl species (C-XD) are shown in Figure 6. Accordingly, when the biscarbonyl complex C-4D was treated with 60 bar H2 for 40 h at 145 °C, the two CO stretches of the biscarbonyl complex (2033 and 1966 cm−1) disappeared in the ATR-IR spectra. Instead, a new carbonyl peak was observed at 1912 cm−1 corresponding to the monocarbonyl dihydride species (C-4A) (Figure S7). The biscarbonyl complexes with R = Ph (C-1D) and i-Pr (C-3D) were able to catalyze the hydrogenation of formamide (N,N′-Bisformyl-N,N′-dimethylethylenediamine (F-2)) in pure H2 at a pressure of 60 bar (Table 4), proceeding through the formation of the dihydride complex (C-XA). However, when a CO pressure was introduced in the system (5 bar, R = Ph), the formation of dihydride species was inhibited and the Table 4. Hydrogenation of Formamides Using Biscarbonyl Complexesa

Figure 7. CO stretching frequencies of biscarbonyl complexes as observed in ATR-IR spectroscopy.

as the ligand is changed from PNPtBu to PNPiPr to PNPPh, the bond strength of the second axial CO and the metal center decreases (hence, stronger CO stretching frequency), and thus its lability increases. On the basis of these observations, a mechanistic explanation for the drastic effects of ligand substitution as observed in Table 2 can be provided. The main deactivating pathway for the catalytic species is through biscarbonyl monohydride ruthenium species (C-XD), which forms in the presence of in situ generated CO gas. The CO concentration continuously increases during the reaction along with increasing CH3OH production. The second axial carbonyl ligand in these biscarbonyl complexes displays a higher lability than the parent equatorial one. While the biscarbonyl complexes are able to catalyze the hydrogenation of CO2 to formate salts or formamides, they are unable to catalyze the formamide hydrogenation to methanol and amine. In the case

Reaction conditions: N−CHO (10 mmol), cat. (10 μmol), triglyme (10 mL), H2 (60 bar), 145 °C, 20 h. Percentage values in the table represent CH3OH yields as observed by 1H NMR; theoretical yield = 10 mmol. Yield calculation error ±5%. a

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Figure 8. Proposed mechanistic cycle for amine-assisted CO2 hydrogenation to methanol.

of PNPPh pincer ligand, the axial CO ligand of the biscarbonyl complex is most labile, due to the electronic effect of the ligand. As a result, even in the presence of 0.1% CO, it can revert back to the dihydride species, which catalyzes the formamide reduction. Thus, with catalyst C-2, a continuous methanol production was observed for 40 h (Table 2, entry 2). On the other hand, for Ru-PNPiPr (C-3), the lability of axial CO is lower, meaning that, if the CO concentration increases above a certain level, the biscarbonyl species is unable to revert back to dihydride, stopping the methanol formation. For RuPNPtBu (C-5), the threshold CO level is even lower, and the formamide hydrogenation stops even before any visible CH3OH formation through 1H NMR.

Having explored how the reaction mechanism is dependent on the catalyst’s molecular structure, the effect of amine structure was explored next. As mentioned earlier, none of the primary and secondary monoamines screened were able to effectively produce methanol under the reaction conditions with C-1, and the reduction stopped at the formamide stage. To probe the reason behind this, the hydrogenation of two representative amides, N,N′-bisformyl-N,N′-dimethylethylenediamine (F-2) and N-formylpiperidine (F-1), was tried using the biscarbonyl complexes C-1D and C-3D. We observed that, while the biscarbonyl complex with R = i-Pr (C-3D) was effective for the hydrogenation of both formamides, R = Ph (C-1D) selectively catalyzed the hydrogenation of the diamide only, all the way to diamine (Table 4). The reason behind this G


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Journal of the American Chemical Society observed selectivity only with C-1D is not clear, and further experimental and theoretical investigations are needed for its elucidation. However, the absence of CH3OH with monoamines can be surmised to be due to the inability of C-1D to catalyze the hydrogenation of their corresponding formamides. Taking these observations together, a plausible mechanism for the sequential CO2 hydrogenation to CH3OH can be proposed as depicted in Figure 8. Needless to say, detailed investigations dedicated to separate mechanistic steps need to be undertaken in the future to properly understand each individual reaction step. According to our proposed mechanism, in the first stage, in the presence of a base and H2, complex C-2 to C-5 can form the dihydride species C-XA (C1, Ru-Macho-BH can form the dihydride species even in the absence of a base under thermal activation; hence, catalysis with C-1 does not require an activating base). The dihydride species, in the presence of CO2, forms the formate complex (C-XB) through CO2 insertion into the Ru−H bond.57 Subsequently, in the presence of an amine, the formate ligand gets detached from the complex to form a pentacoordinated species C-XF along with an alkylammonium formate salt. In the presence of H2 gas, the pentacoordiated species can form back the dihydride species (C-XA) via C-XE to complete the catalytic cycle. Importantly, during this catalytic cycle to produce alkylammonium formate salts from CO2, the N−H moiety of PNP ligand does not actively take part in the reaction mechanism. Catalyst RuHClPNMePPh(CO) (C-9), in which the N−H was replaced with N−Me, was also able to form formate salts at a rate similar to C-2.58,59 Next, the alkylammonium formate salt forms the formamide product through a condensation reaction. The formamides are amenable to hydrogenation under the pressure of hydrogen, and after the first hydrogenation by the dihydride species (CXA), a hemiaminal is produced, along with the ruthenium amido complex C-XC, which forms back dihydride complex CXA in the presence of H2.60,61 The hemiaminal quickly decomposes to formaldehyde and amine under the reaction conditions as its presence was not detected in the reaction mixture. Formaldehyde gets further hydrogenated quickly by C-XA to produce methanol.62 Also, due to formaldehyde decomposition,52,53 minute amounts of CO are produced in the system, which coordinates with the amido complex C-XC to form the catalytic resting state C-XD. As mentioned earlier, the biscarbonyl complex is eventually reverted to the dihydride complex (C-XA) in the active catalytic cycle (see Figure 6). Notably, in this second formamide-reduction step, N−H moiety does actively take part in the reaction mechanism and the N−Me analogue (C-9) does not produce any CH3OH.41 Hence, amido complex CX-C is postulated to participate at this stage. Further Optimization of the Reaction Conditions. Catalyst Amount. As the catalyst screening study in the first section revealed that C-1 was the most efficient catalyst for CO2 reduction to CH3OH under our conditions, we decided to further investigate the extent of its catalytic efficiency. In this regard, TON and the amount of formed CH3OH both represent important characteristics of the system. Although very high TONs can sometimes be achieved for many reactions using extremely low catalyst loading, the corresponding product yield is generally found to be lowered. Similar results were also obtained with C-1 for CH3OH formation; i.e., the highest TON and lowest methanol yield were obtained with the lowest catalyst loading of 2.5 μmol (Figure 9A).

Figure 9. (A) TONs (y-axis) with μmols of C-1 (x-axis). TON = mol of CH3OH formed per mol of cat. (B) Amounts of formate, formamide, and methanol (y-axis) with μmol of C-1 (x-axis). Reaction conditions: PEHA (5.1 mmol), C-1 (specified amount), triglyme (10 mL), CO2/3H2 (75 bar), 145 °C, 40 h.

CH3OH yields increased significantly as the catalyst amount was increased from 2.5 to 5 and 10 μmol but increased moderately at catalyst loadings above 10 μmol (Figure 9B). For example, a 150% increase in the amount of catalyst used from 10 to 25 μmol resulted in an increase of only 24% in methanol formation. Notably, even with a catalytic concentration as low as 11 ppm wrt CO2 (1 μmol, 0.6 mg), catalyst C1 was found to be active for >10 days. After 244 h, a total of 9.9 mmol of CH3OH was observed with a TON of 9900. The amount of formamide intermediate was found to decrease going from 5 to 15 μmol catalyst loading and remained steady afterward. A steady amount of formate of ∼1 mmol was also observed in all the reactions (Figure 9B). Solvent Amount. The amount of triglyme used as solvent was also found to influence the product distribution to a large extent (Figure 10). Upon halving the solvent amount to 5 mL, the reaction rate was slower for CH3OH formation, and an increased accumulation of formamide products was observed. The amount of CH3OH formed decreased by 40% to 6.34 mmol. When the solvent amount was increased to 20 mL, 13.1 mmol of methanol was formed, along with less formamide intermediates. Thus, an increased amount of solvent had a positive impact on methanol yield. This effect is likely due to the limited solubility of the formamide intermediates in triglyme, which negatively affected the methanol production in cases where lower amounts of solvent were used. H


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Journal of the American Chemical Society Notes

The authors declare no competing financial interest. Crystallographic data (excluding structure factors) for the structures in this Article have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, U.K.. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC-1878618−1878619 (Fax: +44-1223-336-033; E-Mail: [email protected]; http://www.ccdc.cam.ac.uk).

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ACKNOWLEDGMENTS Support of our work by the Loker Hydrocarbon Research Institute, USC, is gratefully acknowledged. S.K. thanks Carolyn C. Franklin and Morris S. Smith Foundations for providing endowed Graduate Fellowships.

Figure 10. Effect of triglyme amount on product yields. Reaction condition: PEHA (5.1 mmol), C-1 (10 μmol), solvent (5/10/20 mL), CO2/3H2 (75 bar), 145 °C, 40 h.

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CONCLUSION In conclusion, correlations between methanol yield with catalyst and amine molecular structure have been established for the process of amine-assisted methanol synthesis through CO2 hydrogenation. Among various ruthenium pincer hydrogenation catalysts, complexes with the PNPPh ligand were most efficient in methanol production (C-1/C-2/C-6). The observed reactivity and efficiency were ascribed to the high lability of the axial carbonyl ligand in the in situ formed ruthenium biscarbonyl deactivated complex. The high lability of axial CO ligand was conveniently monitored through ATRIR spectroscopy. Among various amines, diamines or polyamines with primary/secondary diamine units were most efficient in CH3OH production. The reason for the high methanol yields using these amine structures was due to the ability of the aforementioned biscarbonyl complex of C-1 (C1D) to selectively catalyze the hydrogenation of the corresponding formamides of diamine/polyamines. On the basis of these results, Ru-Macho-BH and PEHA were selected for a prolonged reaction, where the catalyst was found to be active for >10 days, and a TON of 9900 was achieved. A main deactivation pathway has been elucidated and a liganddependent reactivity profile has been identified for several catalysts. With these new findings, our next focus in this context is toward developing second-generation pincer complexes with improved CO2 hydrogenation to methanol turnover numbers, and the future findings will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b12763. General information and experimental details (PDF) Crystallographic data (CIF) Crystallographic data (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Ralf Haiges: 0000-0003-4151-3593 G. K. Surya Prakash: 0000-0002-6350-8325 I


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