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Hydrogen Activation with Ru-PN3P Pincer Complexes for the Conversion of C1 Feedstocks
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  • Matthew D. Morton
    Matthew D. Morton
    Department of Chemistry, Imperial College London, Molecular Sciences Research Hub, White City Campus, 82 Wood Lane, London W12 0BZ, United Kingdom
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  • Boon Ying Tay
    Boon Ying Tay
    Institute of Sustainability for Chemicals, Energy and Environment (ICSE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road Jurong Island, Singapore 627833, Republic of Singapore
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  • Justin J.Q. Mah
    Justin J.Q. Mah
    Institute of Sustainability for Chemicals, Energy and Environment (ICSE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road Jurong Island, Singapore 627833, Republic of Singapore
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    Orcidhttps://orcid.org/0000-0002-1521-8753
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    Andrew J.P. White
    Department of Chemistry, Imperial College London, Molecular Sciences Research Hub, White City Campus, 82 Wood Lane, London W12 0BZ, United Kingdom
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  • James D. Nobbs*
    James D. Nobbs
    Institute of Sustainability for Chemicals, Energy and Environment (ICSE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road Jurong Island, Singapore 627833, Republic of Singapore
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    Martin van Meurs
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  • George J.P. Britovsek*
    George J.P. Britovsek
    Department of Chemistry, Imperial College London, Molecular Sciences Research Hub, White City Campus, 82 Wood Lane, London W12 0BZ, United Kingdom
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Inorganic Chemistry

Cite this: Inorg. Chem. 2024, 63, 7, 3393–3401
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https://doi.org/10.1021/acs.inorgchem.3c04001
Published February 8, 2024

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Abstract

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The hydrogenation of C1 feedstocks (CO and CO2) has been investigated using ruthenium complexes [RuHCl(CO)(PN3P)] as the catalyst. PN3P pincer ligands containing amines in the linker between the central pyridine donor and the phosphorus donors with bulky substituents (tert-butyl (1) or TMPhos (2)) are required to obtain mononuclear single-site catalysts that can be activated by the addition of KOtBu to generate stable five-coordinate complexes [RuH(CO)(PN3P–H)], whereby the pincer ligand has been deprotonated. Activation of hydrogen takes place via heterolytic cleavage to generate [RuH2(CO)(PN3P)], but in the presence of CO, coordination of CO occurs preferentially to give [RuH(CO)2(PN3P–H)]. This complex can be protonated to give the cationic complex [RuH(CO)2(PN3P)]+, but it is unable to activate H2 heterolytically. In the case of the less coordinating CO2, both ruthenium complexes 1 and 2 are highly efficient as CO2 hydrogenation catalysts in the presence of a base (DBU), which in the case of the TMPhos ligand results in a TON of 30,000 for the formation of formate.

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Synopsis

Hydrogenation of C1 feedstocks (CO and CO2) has been investigated using ruthenium complexes [RuHCl(CO)(PN3P)] as the catalyst precursor. PN3P ligands with bulky substituents are required to obtain mononuclear single-site hydrogenation catalysts. In the presence of CO, coordination of CO occurs preferentially to give [RuH(CO)2(PN3P−H)], which is unable to activate H2. In the case of the less coordinating CO2, the ruthenium complexes are highly efficient as CO2 hydrogenation catalysts.

Introduction

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The efficient production of green hydrogen and its conversion to renewable liquid energy carriers such as formic acid or methanol, so-called power-to-liquids (P2L) processes, are gaining increased interest due to the urgent need for sustainable fuels. (1−3) Green hydrogen refers to H2 obtained from a renewable source, for example, through water electrolysis using renewable electricity or from the gasification of biomass. Hydrogenations of C1 feedstocks such as CO or CO2 are important in this context as this can give access to a range of green chemicals, including HCO2H, MeOH, and hydrocarbons. (4−8) The hydrogenation of CO is currently applied in several large-scale chemical processes such as the Fischer–Tropsch process and the production of MeOH (110 Mt per annum), (9) all based on heterogeneous catalysis. Likewise, the industrial hydrogenation of CO2 to CH4 (10) or MeOH (11) also utilizes heterogeneous catalysis. Although at a much smaller scale of deployment as compared to that of CO, CO2 is becoming an increasingly important feedstock. The synthesis of MeOH appears to be the most attractive, exemplified by the 5,000 tons per annum plant operated by CRI in Iceland and other demonstration plants. (11,12) With advances in the efficiencies of electrolyzers and hydrogenation technologies, CO2 hydrogenation to green methanol or dimethyl ether (DME) is likely to become increasingly important in the near future. Longer term, this could result in a MeOH-based economy as originally envisaged by Asinger in 1986, (13) and later also by Olah et al. (14)
Efficient homogeneous systems capable of hydrogenation reactions of these C1 feedstocks have long remained elusive, despite the heterogeneous systems being well established, in some cases for more than a century. However, recently there have been some exciting discoveries of molecular catalysts that can reduce CO to MeOH via formamide or formate intermediates. (15−17) These discoveries have occurred in tandem with significant advances in homogeneous hydrogenation of CO2 to MeOH in the past decade. (18)
The catalytic hydrogenation of CO and CO2 feedstocks both require heterolytic cleavage of H2 to generate hydride and proton donors. Pincer ligands of the types PCNCP and PN3P such as those shown in Scheme 1 and similar PNN systems, (19) as well as the related MACHO ligand types, (20−24) have featured prominently in this area as they can be reversibly protonated and deprotonated. (25−30) This has made these ligands particularly attractive for hydrogenations using H2 activation reactions via metal–ligand cooperative (MLC) behavior, (31−34) although recent discoveries suggests that this MLC behavior may not be essential for all catalyst systems. (23) Here, we present our studies on the synthesis of a series of ruthenium(II) complexes featuring PN3P-R ligands (where R = tBu and TMPhos (35)) and their reactivity toward H2 and CO, as well as their application in the hydrogenation of CO and CO2.

Scheme 1

Scheme 1. Reversible Deprotonation and H2 Activation through Metal Ligand Cooperativity in MACHO, PCNCP, and PN3P Pincer Complexes
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Results and Discussion

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Synthesis and reactivity of PN3P Ru complexes

[RuHCl(CO)(PN3P-tBu)] (1) was prepared by mixing a PN3P-tBu ligand with [RuHCl(CO)(PPh3)3] in THF according to the procedure described by Huang et al. and shown in Scheme 2. (36) The 1H NMR spectrum (d4-MeOH) shows a distinctive hydride signal at −24.1 ppm (triplet, 2JHP = 18 Hz) and a doublet in the 31P NMR spectrum at 134 ppm (2JPH = 18 Hz). In d6-benzene or CDCl3 instead of d4-MeOH, the NH protons are observed as a broad singlet. The IR spectrum shows a CO stretch at 1936 cm–1, and the Ru–H stretch is seen at 2113 cm–1. All data is shown in the Supporting Information and is consistent with that reported by Huang and co-workers. (36)

Scheme 2

Scheme 2. Synthesis of Ru-PN3P Complexes 1 and 2
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An alternative bulky phosphine 2,2,6,6-tetramethylphosphinane (TMPhos), recently developed by some of us, (35) was also investigated as it provides a similar but subtly different stereoelectronic environment compared to PtBu2 groups. The new ligand PN3P-TMPhos was obtained in 47% yield from 2,6-diaminopyridine and chloro-TMPhos (1-chloro-2,2,6,6-tetramethylphosphinane). The reaction with PN3P-TMPhos proceeds in a similar fashion as above resulting in [RuHCl(CO)(PN3P-TMPhos)] (2) (Scheme 2). Interestingly, the hydride signal appears much further downfield in this case at −14.4 ppm (2JHP = 20 Hz, d3-MeCN), together with a significantly lower Ru–H stretching frequency at 2070 cm–1. The 31P NMR signal at 124 ppm and the carbonyl stretch at 1932 cm–1 are comparable to those of [RuHCl(CO)(PN3P-tBu)] (1).
Similar reactions using the related PN3P-iPr and -Ph ligands were also attempted but were largely unsuccessful. The reaction of PN3P-iPr with [RuHCl(CO)(PPh3)3] in refluxing THF led to a mixture of the desired complex [RuHCl(CO)(PN3P-iPr)] and presumably the cationic complex [RuH(CO)(PPh3)(PN3P-iPr)]Cl, as well as other unidentified byproducts (see Figures S23 and S24). The reaction between PN3P-Ph and [RuHCl(CO)(PPh3)3] gave the cationic complex [RuH(CO)(PPh3)(PN3P-Ph)]Cl with some unidentified species (Figures S26 and S27). Similar issues in the synthesis of Ru complexes with the PN3P-Ph ligand have been reported, and a dinuclear Ru complex with a bridging PN3P ligand was isolated. (26) Kirchner and co-workers synthesized the [(PN3P)RuCl2] complexes from [RuCl2(PPh3)3], which avoided the CO ligand. (37)
Treatment of [RuHCl(CO)(PN3P-tBu)] (1) with KOtBu results in the formation of the five-coordinate complex [RuH(CO)(PN3P-tBu–H)] (3) (Scheme 3). The 1H NMR spectrum of [RuH(CO)(PN3P-tBu–H)] in d6-benzene shows a loss of symmetry in the aromatic region, and the hydride signal is shifted upfield to −25.9 ppm (2JHP = 16 Hz) (Figure S5). The 31P{1H} NMR spectrum shows two doublets at 128.2 and 130.7 ppm (2JPP = 220 Hz). The IR spectrum shows a single CO stretch at 1885 cm–1, which is 50 cm–1 lower than that for [RuHCl(CO)(PN3P-tBu)] (1), suggesting greater back-bonding to CO. Similarly, [RuH(CO)(PN3P-TMPhos–H)] (4) can be generated from [RuHCl(CO)(PN3P-TMPhos)] (2) and 1 equiv of KOtBu. The protons in the dearomatized pyridine ring see an upfield shift from 7.16 and 6.72 ppm to 6.83 and 5.77 ppm, respectively, while the hydride signal shifts slightly upfield from −14.4 to −14.5 ppm (2JHP = 19 Hz). The 31P{1H} NMR spectrum shows a broad singlet at 122.1 ppm in contrast to the two doublets observed for the tBu complex 3, which suggests fluxional behavior in CD3CN, similar to that observed for the CO-coordinated complex 5 (vide infra). We found that complex 4 is thermally unstable and gradually decomposes in solution.

Scheme 3

Scheme 3. Reaction of Complexes 1 and 2 with KOtBu and Subsequent Reactivity
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The reaction of [RuH(CO)(PN3P-tBu–H)] (3) with CO (1 bar) at room temperature (RT) in C6D6 resulted immediately in a color change from red to pink/orange and the formation of a dicarbonyl complex [RuH(CO)2(PN3P-tBu–H)] (5) (Scheme 3). Complex 5 is also obtained from the reaction between PN3P-tBu and [Ru3(CO)12] in toluene at 110 °C. In contrast, the reaction of PN3P-iPr with [Ru3(CO)12] in refluxing toluene resulted in complicated mixtures of products (see Figure S25). The 1H NMR spectrum in C6D6 of [RuH(CO)2(PN3P-tBu–H)] (5) is similar to the spectrum of [RuH(CO)(PN3P-tBu–H)] (3) (see Figure S32), but the signals for the aromatic protons in 3 and 5 position and the tert-butyl groups are all broader. The hydride appears as a triplet at −6.1 ppm (2JHP = 19 Hz), significantly downfield from −25.9 ppm in starting monocarbonyl complex 3, probably due to the trans effect of the second carbonyl ligand. The 31P{1H} NMR spectrum (C6D6) shows two doublets at 135.2 and 140.9 ppm (2JPP = 190 Hz), which are also broadened. There are two ν(CO) absorptions at 1989 and 1940 cm–1 in the IR spectrum.
The dicarbonyl complex [RuH(CO)2(PN3P-tBu-H)] (5) shows fluxional behavior in solution at RT, which was investigated by variable temperature 1H NMR spectroscopy in d8-toluene (see Figure 1). At 213 K, the broad peaks are better resolved although not completely sharp. Increasing the temperature results in further broadening and coalescence of the tert-butyl groups at 373 K. The 31P NMR spectra show two sharp doublets at low temperatures and two broad doublets at higher temperatures, but no coalescence is observed up to 373 K.

Figure 1

Figure 1. Variable temperature 1H NMR spectra of [RuH(CO)2(PN3P-tBu–H)] (5) in d8-toluene (hydride signal has been omitted).

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At all measured temperatures, the 1H NMR spectra show the pyridine proton in the 4 position as a triplet at 6.7 ppm and the hydride as a triplet at −6 ppm, and both are sharp signals. The fluxionality is proposed to involve proton transfer between the imine and amine moieties of the ligand, interconverting between the two enantiomeric forms of the complex, as shown in Scheme 4. Noteworthy, there was no fluxional behavior reported for the related complex [RuH(CO)(PPh3)(PN3P-Ph–H)]. (26)

Scheme 4

Scheme 4. Equilibrium between the Two [RuH(CO)2(PN3P-tBu–H)] (5) Isomers (Protons in Blue Show Exchange Behavior)
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No evidence for an intermediate was observed, for example, a Ru(0) complex [Ru(CO)2(PN3P-tBu)], which would be structurally similar to the [Ru(CO)2(PONOP-tBu)] complex reported by Milstein et al. (38) The 1H and 31P NMR spectra of [RuH(CO)2(PN3P-tBu–H)] (5) were also measured in d4-methanol, where complete coalescence for the tBu signals is observed at RT. This indicates that the fluxional process most likely involves proton exchange with methanol facilitating the proton transfer from one side of the ligand to the other. The free energy of activation (ΔG‡) for the exchange process in d8-toluene was determined by NMR as 73 kJmol–1 (see the Supporting Information for details).
Protonation of [RuH(CO)2(PN3P-tBu–H)] (5) with a strong acid [H(OEt2)2][B(C6H3(CF3)2)4] (HBArF) in THF results in the cationic complex [RuH(CO)2(PN3P-tBu)]+ (6) in good yield (78%). The 1H NMR spectrum (C6D5Cl) is consistent with the rearomatization of the complex, resulting in a symmetric complex with only one signal for the protons in 3 and 5 position and a singlet in the 31P{1H} NMR spectrum at 142.7 ppm. No fluxional behavior was seen for this complex. The CO stretches in the IR spectrum are at 2017 and 1986 cm–1, which are 40 cm–1 higher than for the neutral complex [RuH(CO)2(PN3P-tBu–H)] (5). The cationic complex 6 was further characterized by 13C NMR spectroscopy, mass spectrometry, elemental analysis, and SC-XRD (see the Supporting Information and Figure 2). The octahedral complex shows different Ru–C bond lengths, whereby the axial Ru–C bond trans to the hydride ligand is significantly longer at 1.976(8) Å, compared to the equatorial Ru–C bond at 1.878(8) Å. Related cationic complexes [RuH(CO)2(PNP-R)]+ with a different PNP-pincer-type ligand that features a central NH donor have been reported. (20,39) In all cases, the Ru–C bond length of the axial CO ligand is significantly longer than that of the equatorial CO ligand by approximately 0.1 Å due to the trans hydride ligand.

Figure 2

Figure 2. Molecular structure of the cationic complex [RuH(CO)2(PN3P-tBu)]+ (6). The exact position of the hydride ligand could not be determined (Supporting Information). The anion [B(C6H3(CF3)2)4]− has been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru(1)–C(28) 1.878(8), Ru(1)–C(27) 1.976(8), Ru(1)–N(1) 2.115(4), Ru(1)–P(8) 2.345(3), Ru(1)–P(10) 2.357(3); C(28)–Ru(1)–C(27) 95.6(4), P(8)–Ru(1)–P(10) 156.02(15), and C(28)–Ru(1)–N(1) 175.1(4).

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Hydrogen and CO Activation

The hydrogenation of CO with homogeneous transition metal-based catalysts to generate MeOH and further homologation products such as ethylene glycol is generally believed to proceed via the reaction sequence shown in Scheme 5. Formyl and hydroxymethyl complexes are generally invoked as intermediates in these reactions. (40−42) Transition metal hydrides have been used by Bercaw et al. in homogeneous syngas conversion in combination with metal carbonyl complexes based on group 7 metals Re (43) and Mn. (44)

Scheme 5

Scheme 5. Proposed Reaction Sequence for the Hydrogenation of CO to MeOH and Higher Alcohols
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The reactivity of [RuH(CO)(PN3P-tBu–H)] (3) toward H2 was explored in a series of stoichiometric reactions. The reaction between 3 and H2 (5 bar) was monitored by 1H and 31P NMR spectroscopy in C6D6. Immediately after the addition of H2, a new triplet is observed in the 1H NMR spectrum at −5.3 ppm (2JHP = 19 Hz) and a new set of aromatic signals appears as a triplet and a doublet in a 1:2 ratio. A singlet is observed in the 31P{1H} NMR spectrum at 156.2 ppm. However, this reaction is slow, as noted by Huang et al., (45) and only approximately 1% conversion was observed after 1 h, but after 4 days at RT under H2, this increased to 10% (see Figures S19 and S20). Based on these observations, we assign the new signals to the symmetric trans-dihydride complex [Ru(H)2(CO)(PN3P-tBu)] (7). Heating to 60 °C for 30 min resulted in decomposition of [RuH(CO)(PN3P-tBu–H)] (3) rather than further reaction with H2 to generate more 7. Repeating the experiment above with D2 instead of H2 resulted in a gradual decrease in the intensity of the hydride and the NH signals relative to the aromatic proton signals. This indicates that the H2 activation is reversible and there is an equilibrium between the two complexes in solution under H2, as indicated in Scheme 6. A direct synthesis of complex [Ru(H)2(CO)(PN3P-tBu)] (7) was attempted by reacting complex [RuHCl(CO)(PN3P-tBu)] (1) with NaBEt3H in toluene, as reported for the related [Ru(H)2(CO)(PONOP-tBu)] complex, (38) but the major product observed in this case was the five-coordinate complex [RuH(CO)(PN3P-tBu–H)] (3). It is possible that complex 7 was formed as an intermediate but reverts immediately back to complex 3 in the absence of excess H2. The reaction of the dicarbonyl complex [RuH(CO)2(PN3P-tBu–H)] (5) with H2 (6 bar, in d8-toluene) was unsuccessful, and no reaction was observed during 48 h at RT or after 30 min at 60 °C.

Scheme 6

Scheme 6. Reactivity of Complex 3 toward H2 in the Absence and Presence of CO
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We subsequently explored the possibility of using a dual catalyst system, where one complex activates H2 and the other activates CO. The first system combines the ability of [RuH(CO)(PN3P-tBu–H)] (3) to activate H2 and subsequently engage in intermolecular hydride transfer to other carbonyl complexes. Initially, we chose the Ru-bipy complex [Ru(bipy)2(CO)2](B(C6F5)4)2 (8) as a previous work by Tanaka and co-workers has shown that the formation of the formyl complex [Ru(bipy)2(CO)(CHO)]+ can be achieved by stoichiometric borohydride reduction. (46) In an attempt to obtain a similar reduction using H2 instead of NaBH4, we reacted [Ru(bipy)2(CO)2](B(C6F5)4)2 (8) with [RuH(CO)(PN3P-tBu–H)] (3) in the presence of H2 (4 bar) in C6D5Cl at RT according to Scheme 7.

Scheme 7

Scheme 7. Hydride Transfer to Generate Metal Formyl Intermediates and Subsequent Decarbonylation
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The reaction of [Ru(bipy)2(CO)2](B(C6F5)4)2 (8) and [RuH(CO)(PN3P-tBu–H)] (3) with H2 leads to an instant conversion to the formyl complex [Ru(bipy)2(CO)(CHO)]+ (9) as indicated by the characteristic singlet at 13.9 ppm (47) together with the cationic Ru complex 10 (see Figure S33). This signal disappears over the course of several hours due to decarbonylation to form [Ru(bipy)2(CO)(H)]+ (11) evidenced by the emergence of the Ru-hydride signal at −11.4 ppm (48) and [RuH(CO)2(PN3P-tBu)]+ (6) at −6.7 ppm. The thermal instability of the formyl complex [Ru(bipy)2(CO)(CHO)]+ (9) has been noted previously. (47) Another experiment was performed by reacting [Ru(bipy)2(CO)2](B(C6F5)4)2 (8) and [RuH(CO)(PN3P-tBu–H)] (3) with syngas (CO/H2 = 3/1), which formed [RuH(CO)2(PN3P-tBu–H)] (5) instantaneously, precluding any further heterolytic cleavage of H2.
Inspired by Dombek’s early observations on the formation of MeOH and ethylene glycol from [Ru3(CO)12] and syngas, (49) albeit under forcing conditions, we attempted the hydrogenation of [Ru3(CO)12] in the presence of [RuH(CO)(PN3P-tBu–H)] (3) at RT (Scheme 8). Upon introduction of H2 (1 bar), fast H2 activation to give [RuH2(CO)(PN3P-tBu)] (7) was followed by a slow hydride and CO exchange with [Ru3(CO)12] to generate the known anionic complexes [Ru3H(CO)11]− and [RuH(CO)4]−. (50,51) It is possible that this exchange also involves metal formyl species as intermediates. Before H2 addition, some CO exchange was observed to form small amounts of [RuH(CO)2(PN3P-tBu–H)] (5) (see Figure S34).

Scheme 8

Scheme 8. Hydride Transfer to [Ru3(CO)12]
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From these stoichiometric reactivity studies, it can be concluded that pincer complexes such as [RuH(CO)(PN3P-tBu–H)] (3) are able to activate H2 to give [Ru(H)2(CO)(PN3P-tBu)] (7), but no hydride transfer to the coordinated carbonyl ligand is observed. The activated complex 7 is able to transfer hydride equivalents to other metal carbonyl complexes to generate metal formyl intermediates, but these tend to be unstable at higher temperatures. More importantly, when using syngas, H2 activation becomes inhibited due to the preferential coordination of CO. Higher partial pressures of H2 may be needed to overcome this and displace coordinated CO. The hydrogenation of CO2 however should proceed as long as no CO is present as one of the reaction products.

Hydrogenation of CO2 to Formate

Huang and co-workers have shown previously that Ru complex 1 is an efficient catalyst for H2 generation from the decomposition of HCO2H (TON > 1 million). (45) Since equimolar CO2 is also generated, the reverse reaction, i.e., hydrogenation of CO2, would be needed for HCO2H to be feasible as a renewable H2/energy carrier. However, direct conversion of CO2 to formic acid is endergonic and therefore a base or high pressures are usually employed to drive the forward reaction. (6,7,52) Pidko and co-workers have demonstrated that related Ru-PNP-pincer catalysts are highly efficient in the base-assisted hydrogenation of CO2 to formates with reported mol DBU-formate/(mol cat x h) (TOFs) in excess of 1 million h–1 at exceptionally low Ru loadings (<0.2 μmol). (25)
We have investigated complexes 1 and 2 as catalysts for the CO2 hydrogenation reaction, and the results are shown in Table 1. In a typical experiment, the precatalyst, dissolved in the reaction solvent dimethylformamide (DMF), was introduced into a batch reactor together with the base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). After heating to the reaction temperature (90 °C), a mixture of CO2/H2 (7.5/7.5 bar) was introduced into the reactor and maintained at this pressure throughout the reaction. In the absence of any catalyst (entry 1), negligible gas consumption was observed, although a small amount of white solid was formed, likely to be DBU-bicarbonate [(DBU)H]+[HCO3]− resulting from trace amounts of moisture in the system. (53) Gratifyingly, both catalysts 1 and 2 were highly efficient in the DBU-assisted hydrogenation of CO2 to formate. Quantitative conversion of DBU was observed within 1 h at catalyst loadings of >5 μmol (entries 2–5). At a catalyst loading of 14.2 μmol, catalyst 1 and 2 gave TONs of 4400 and 4700, respectively, reflecting >93% conversion of DBU. Even when the catalyst loading was lowered to 5.7 μmol, full conversion of DBU was observed for both complexes 1 and 2 equating to TONs of 11,800 (entries 4 and 5).
Table 1. Base-Assisted Hydrogenation of CO2 to Formate Using Complexes 1 and 2b
entrycatalyst[Ru] μmolconversiona (%)TONb
1blank000
2114.2934400
3214.2>994700
415.7>9911,800
525.7>9911,800
612.8102300
722.810023,600
811.40.3100
921.46430,300
a

conversion = DBU-formate/DBU × 100.

b

TON = mol DBU-formate/mol cat.

c

Conditions: CO2/H2 (1:1) = 15 bar, temp. = 90 °C, reaction time = 1 h, DBU = 10 mL (66.9 mmol), solvent = DMF (35 mL).

Since the reaction is limited by the amount of DBU, we lowered the catalyst loading further in order to differentiate between the performances of the two catalysts. At a lower catalyst loading of 2.8 μmol, using complex 1, conversion of DBU decreased to 10% (entry 6), whereas with complex 2, quantitative conversion of base was still achieved, with a TON of 23,600 (entry 7). At an even lower catalyst loading of 1.4 μmol, complex 1 was essentially inactive (entry 8), whereas complex 2 achieved 64% conversion of DBU and a TON of 30,300. Thus, both catalysts are highly efficient in the hydrogenation of CO2 to DBU-formate with TONs > 30,000 h–1 for complex 2. At lower catalyst loadings, both complexes appear to be susceptible to deactivation, but catalyst 2 appears to be more robust in that respect.

Conclusions

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We have shown that mononuclear Ru complexes with PN3P-R pincer-type ligands can be prepared cleanly, provided that sterically bulky R groups are used such as tert-butyl or TMPhos. Smaller R groups such as iPr and Ph lead to multiple products. Both six-coordinate complexes [RuClH(CO)(PN3P-R)] (1 and 2) react with KOtBu to form the five-coordinate complexes [RuH(CO)(PN3P-R–H)] (3 and 4). In the case of R = tBu, reactivity studies with CO have shown that a dicarbonyl complex [RuH(CO)2(PN3P-tBu–H)] (5) is formed, which shows temperature-dependent dynamic behavior in solution and which can be protonated to give the cationic complex [RuH(CO)2(PN3P-tBu)]+ (6). Dihydrogen is reversibly activated by complex 3 to give the dihydride complex [RuH2(CO)(PN3P-tBu)] (7). Attempts to use this dihydride complex as a hydride donor with other ruthenium carbonyl complexes led to unstable formyl complexes, which readily decarbonylated to give the stable cationic complex 6. From these studies, it can be concluded that the conversion of syngas with these Ru-based PN3P catalysts suffers from CO poisoning of the H2 activation mechanism in these systems, precluding the hydrogenation of CO to take place.
The hydrogenation of CO2 on the other hand takes place readily, and both complexes 1 and 2 are highly active catalysts for the hydrogenation of CO2 to formate in the presence of the base DBU. TONs in excess of 30,000 have been achieved with complex 2, which appears to show better stability compared to complex 1 at lower catalyst loadings. Further catalytic evaluations of these intriguing pincer complexes are underway.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c04001.

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  • Corresponding Authors
  • Authors
    • Matthew D. Morton - Department of Chemistry, Imperial College London, Molecular Sciences Research Hub, White City Campus, 82 Wood Lane, London W12 0BZ, United Kingdom
    • Boon Ying Tay - Institute of Sustainability for Chemicals, Energy and Environment (ICSE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road Jurong Island, Singapore 627833, Republic of Singapore
    • Justin J.Q. Mah - Institute of Sustainability for Chemicals, Energy and Environment (ICSE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road Jurong Island, Singapore 627833, Republic of Singapore;  Orcidhttps://orcid.org/0000-0002-1521-8753
    • Andrew J.P. White - Department of Chemistry, Imperial College London, Molecular Sciences Research Hub, White City Campus, 82 Wood Lane, London W12 0BZ, United Kingdom
    • Martin van Meurs - Institute of Sustainability for Chemicals, Energy and Environment (ICSE2), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road Jurong Island, Singapore 627833, Republic of Singapore;  Orcidhttps://orcid.org/0000-0002-8816-3458
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We gratefully acknowledge BP and the Agency for Science, Technology & Research (A*STAR) for funding this work (grant C231218005). We thank Solvay for a generous donation of ClPtBu2 and Johnson Matthey for the donation of RuCl3.

References

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This article references 53 other publications.

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    ACS Catalysis (2015), 5 (4), 2416-2422CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    This paper describes a novel approach to CO2 hydrogenation, in which CO2 capture with aminoethanols at low pressure is coupled with hydrogenation of the captured product, oxazolidinone, directly to MeOH. In particular, (2-methylamino)ethanol or valinol captures CO2 at 1-3 bar in the presence of catalytic Cs2CO3 to give the corresponding oxazolidinones in up to 65-70 and 90-95% yields, resp. Efficient hydrogenation of oxazolidinones was achieved using PNN pincer Ru catalysts to give the corresponding aminoethanol (up to 95-100% yield) and MeOH (up to 78-92% yield). We also have shown that both CO2 capture and oxazolidinone hydrogenation can be performed in the same reaction mixt. using a simple protocol that avoids intermediate isolation or purifn. steps. For example, CO2 can be captured by valinol at 1 bar with Cs2CO3 catalyst followed by 4-isopropyl-2-oxazolidinone hydrogenation in the presence of a bipy-based pincer Ru catalyst to produce MeOH in 50% yield after two steps.
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  20. 20
    Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S. Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160– 3170,  DOI: 10.1021/jacs.8b12763
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    20
    Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol
    Kar, Sayan; Sen, Raktim; Kothandaraman, Jotheeswari; Goeppert, Alain; Chowdhury, Ryan; Munoz, Socrates B.; Haiges, Ralf; Prakash, G. K. Surya
    Journal of the American Chemical Society (2019), 141 (7), 3160-3170CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    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 mol. structure of the ruthenium pincer catalysts and the amines that are crit. for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNPR(CO)] (R = Ph/i-Pr/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 dissocn. 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 max. turnover no. (TON) of 9900.
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  21. 21
    Rezayee, N. M.; Huff, C. A.; Sanford, M. S. Tandem amine and ruthenium-catalyzed hydrogenation of CO2 to methanol. J. Am. Chem. Soc. 2015, 137, 1028– 1031,  DOI: 10.1021/ja511329m
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    21
    Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol
    Rezayee, Nomaan M.; Huff, Chelsea A.; Sanford, Melanie S.
    Journal of the American Chemical Society (2015), 137 (3), 1028-1031CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    This Communication describes the hydrogenation of carbon dioxide to methanol via tandem catalysis with dimethylamine and a homogeneous ruthenium complex. Unlike previous examples with homogeneous catalysts, this CO2-to-CH3OH process proceeds under basic reaction conditions. The dimethylamine is proposed to play a dual role in this system. It reacts directly with CO2 to produce dimethylammonium dimethylcarbamate, and it also intercepts the intermediate formic acid to generate DMF. With the appropriate selection of catalyst and reaction conditions, >95% conversion of CO2 was achieved to form a mixt. of CH3OH and DMF.
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  22. 22
    Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778– 781,  DOI: 10.1021/jacs.5b12354
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    22
    Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst
    Kothandaraman, Jotheeswari; Goeppert, Alain; Czaun, Miklos; Olah, George A.; Prakash, G. K. Surya
    Journal of the American Chemical Society (2016), 138 (3), 778-781CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A highly efficient homogeneous catalyst system for the prodn. of CH3OH from CO2 using pentaethylenehexamine and Ru-Macho-BH (1) at 125-165 °C in an ethereal solvent has been developed (initial turnover frequency = 70 h-1 at 145 °C). Ease of sepn. of CH3OH is demonstrated by simple distn. from the reaction mixt. The robustness of the catalytic system was shown by recycling the catalyst over five runs without significant loss of activity (turnover no. > 2000). Various sources of CO2 can be used for this reaction including air, despite its low CO2 concn. (400 ppm). For the first time, we have demonstrated that CO2 captured from air can be directly converted to CH3OH in 79% yield using a homogeneous catalytic system.
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  23. 23
    Curley, J. B.; Hert, C.; Bernskoetter, W. H.; Hazari, N.; Mercado, B. Q. Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation. Inorg. Chem. 2022, 61, 643– 656,  DOI: 10.1021/acs.inorgchem.1c03372
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    23
    Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation
    Curley, Julia B.; Hert, Clayton; Bernskoetter, Wesley H.; Hazari, Nilay; Mercado, Brandon Q.
    Inorganic Chemistry (2022), 61 (1), 643-656CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A novel pincer ligand, iPrPNPhP [PhN(CH2CH2PiPr2)2], which is an analog of the versatile MACHO ligand, iPrPNHP [HN(CH2CH2PiPr2)2], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-contg. complexes were isolated and characterized by NMR and IR spectroscopies, as well as x-ray diffraction. Comparisons to previously published analogs ligated by iPrPNHP and iPrPNMeP [CH3N(CH2CH2PiPr2)2] illustrate that there are large changes in the coordination chem. that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the iPrPNPhP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by iPrPNHP form the anti isomer and complexes supported by iPrPNMeP form a mixt. of syn and anti isomers. Authors evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of iPrPNRP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO2) hydrogenation to formate. The iPrPNPhP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO2 hydrogenation, the iPrPNPhP-ligated species is again the most active under authors optimal conditions, and they report some of the highest turnover frequencies for homogeneous catalysts. Exptl. and theor. insights into the turnover-limiting step of catalysis provide a basis for the obsd. trends in catalytic activity. Addnl., the stability of complexes enabled to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, they have developed highly active catalysts for formic acid dehydrogenation and CO2 hydrogenation and also provided a framework for future catalyst development.
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    Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S. Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580– 1583,  DOI: 10.1021/jacs.7b12183
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    24
    Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy
    Kar, Sayan; Sen, Raktim; Goeppert, Alain; Prakash, G. K. Surya
    Journal of the American Chemical Society (2018), 140 (5), 1580-1583CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Herein we report an efficient and recyclable system for tandem CO2 capture and hydrogenation to methanol. After capture in an aq. amine soln., CO2 is hydrogenated in high yield to CH3OH (>90%) in a biphasic 2-MTHF/water system, which also allows for easy sepn. and recycling of the amine and catalyst for multiple reaction cycles. Between cycles, the produced methanol can be conveniently removed in vacuo. Employing this strategy, catalyst Ru-MACHO-BH and polyamine PEHA were recycled three times with 87% of the methanol productivity of the first cycle retained, along with 95% of catalyst activity after four cycles. CO2 from dil. sources such as air can also be converted to CH3OH using this route. We postulate that the CO2 capture and hydrogenation to methanol system presented here could be an important step toward the implementation of the carbon neutral methanol economy concept.
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    Filonenko, G. A.; van Putten, R.; Schulpen, E. N.; Hensen, E. J. M.; Pidko, E. A. Highly Efficient Reversible Hydrogenation of Carbon Dioxide to Formates Using a Ruthenium PNP-Pincer Catalyst. ChemCatChem. 2014, 6, 1526– 1530,  DOI: 10.1002/cctc.201402119
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    25
    Highly efficient reversible hydrogenation of carbon dioxide to formates using a ruthenium PNP-pincer catalyst
    Filonenko, Georgy A.; van Putten, Robbert; Schulpen, Erik N.; Hensen, Emiel J. M.; Pidko, Evgeny A.
    ChemCatChem (2014), 6 (6), 1526-1530CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The use of hydrogen as a fuel requires both safe and robust technologies for its storage and transportation. Formic acid (FA) produced by the catalytic hydrogenation of CO2 is recognized as a potential intermediate H2 carrier. Herein, we present the development of a formate-based H2 storage system that employs a Ru PNP-pincer catalyst. The high stability of this system allows cyclic operation with an exceptionally fast loading and liberation of H2. Kinetic studies highlight the crucial role of the base promoter, which controls the rate-detg. step in FA dehydrogenation and defines the total H2 capacity attainable from the hydrogenation of CO2. The reported findings show promise for the development of practical technologies that use formic acid as a hydrogen carrier.
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    Guan, C.; Pan, Y.; Ang, E. P. L.; Hu, J.; Yao, C.; Huang, M.-H.; Li, H.; Lai, Z.; Huang, K.-W. Conversion of CO2 from air into formate using amines and phosphorus-nitrogen PN3P-Ru(II) pincer complexes. Green Chem. 2018, 20, 4201– 4205,  DOI: 10.1039/C8GC02186D
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    Conversion of CO2 from air into formate using amines and phosphorus-nitrogen PN3P-Ru(II) pincer complexes
    Guan, Chao; Pan, Yupeng; Ang, Eleanor Pei Ling; Hu, Jinsong; Yao, Changguang; Huang, Mei-Hui; Li, Huaifeng; Lai, Zhiping; Huang, Kuo-Wei
    Green Chemistry (2018), 20 (18), 4201-4205CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Well-defined ruthenium(II) PN3P pincer complexes were developed for the hydrogenation of carbon dioxide. Excellent product selectivity and catalytic activity with TOF (turnover frequency) and TON (turnover no.) up to 13 000 h-1 and 33 000, resp., in a THF/H2O biphasic system were achieved. Notably, effective conversion of carbon dioxide from air into formate was conducted in the presence of an amine, allowing easy product sepn. and catalyst recycling.
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    Li, H.; Goncalves, T. P.; Zhao, Q.; Gong, D.; Lai, Z.; Wang, Z.; Zheng, J.; Huang, K. W. Diverse catalytic reactivity of a dearomatized PN3P*-nickel hydride pincer complex towards CO2 reduction. Chem. Commun. 2018, 54, 11395– 11398,  DOI: 10.1039/C8CC05948A
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    Diverse catalytic reactivity of a dearomatized PN3P*-nickel hydride pincer complex towards CO2 reduction
    Li, Huaifeng; Goncalves, Theo P.; Zhao, Qianyi; Gong, Dirong; Lai, Zhiping; Wang, Zhixiang; Zheng, Junrong; Huang, Kuo-Wei
    Chemical Communications (Cambridge, United Kingdom) (2018), 54 (81), 11395-11398CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A dearomatized PN3P*-nickel hydride complex has been prepd. using an oxidative addn. process. The first nickel-catalyzed hydrosilylation of CO2 to methanol has been achieved, with unprecedented turnover nos. Selective methylation and formylation of amines with CO2 were demonstrated by such a PN3P*-nickel hydride complex, highlighting its versatile functions in CO2 redn.
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    Tanaka, R.; Yamashita, M.; Nozaki, K. Catalytic hydrogenation of carbon dioxide using Ir(III)-pincer complexes. J. Am. Chem. Soc. 2009, 131, 14168– 14169,  DOI: 10.1021/ja903574e
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    Catalytic Hydrogenation of Carbon Dioxide Using Ir(III)-Pincer Complexes
    Tanaka, Ryo; Yamashita, Makoto; Nozaki, Kyoko
    Journal of the American Chemical Society (2009), 131 (40), 14168-14169CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Catalytic hydrogenation of carbon dioxide in aq. potassium hydroxide was performed using a newly synthesized isopropyl-substituted PNP-pincer iridium trihydride complex as catalyst. Potassium formate was obtained with turnover no. up to 3,500,000 and turnover frequency of 150,000 h-1.
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    Tanaka, R.; Yamashita, M.; Chung, L. W.; Morokuma, K.; Nozaki, K. Mechanistic Studies on the Reversible Hydrogenation of Carbon Dioxide Catalyzed by an Ir-PNP Complex. Organometallics 2011, 30, 6742– 6750,  DOI: 10.1021/om2010172
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    Mechanistic Studies on the Reversible Hydrogenation of Carbon Dioxide Catalyzed by an Ir-PNP Complex
    Tanaka, Ryo; Yamashita, Makoto; Chung, Lung Wa; Morokuma, Keiji; Nozaki, Kyoko
    Organometallics (2011), 30 (24), 6742-6750CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The PNP-ligated iridium(III) trihydride complex 1 exhibited the highest catalytic activity for hydrogenation of carbon dioxide in aq. KOH. The catalytic hydrogenation can be tuned to be a reversible process with the same catalyst at the expense of the activity, when triethanolamine was used as a base. Theor. studies on the hydrogenation of carbon dioxide using DFT calcns. suggested two competing reaction pathways: either the deprotonative dearomatization step or the hydrogenolysis step as the rate-detg. step. The results nicely explain our exptl. observations that the catalytic cycle is dependent on both the strength of the base and hydrogen pressure.
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    Pan, Y.; Guan, C.; Li, H.; Chakraborty, P.; Zhou, C.; Huang, K. W. CO2 hydrogenation by phosphorus-nitrogen PN3P-pincer iridium hydride complexes: elucidation of the deactivation pathway. Dalton Trans 2019, 48, 12812– 12816,  DOI: 10.1039/C9DT01319A
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    CO2 hydrogenation by phosphorus-nitrogen PN3P-pincer iridium hydride complexes: elucidation of the deactivation pathway
    Pan, Yupeng; Guan, Chao; Li, Huaifeng; Chakraborty, Priyanka; Zhou, Chunhui; Huang, Kuo-Wei
    Dalton Transactions (2019), 48 (34), 12812-12816CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    PN3P-Ir pincer hydride complexes were synthesized and characterized as catalysts and key intermediates in the direct hydrogenation of CO2 to formate under mild conditions. The formation of a dearomatized PN3P*-Ir(I)-CO species was identified as a plausible key process accountable for the loss of catalytic activity in the CO2 hydrogenation.
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    Khusnutdinova, J. R.; Milstein, D. Metal-ligand cooperation. Angew. Chem., Int. Ed. 2015, 54, 12236– 12273,  DOI: 10.1002/anie.201503873
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    Metal-Ligand Cooperation
    Khusnutdinova, Julia R.; Milstein, David
    Angewandte Chemie, International Edition (2015), 54 (42), 12236-12273CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review; metal-ligand cooperation (MLC) has become an important concept in catalysis by transition metal complexes both in synthetic and biol. systems. MLC implies that both the metal and the ligand are directly involved in bond activation processes, by contrast to "classical" transition metal catalysis where the ligand (e.g. phosphine) acts as a spectator, while all key transformations occur at the metal center. In this Review, we will discuss examples of MLC in which 1) both the metal and the ligand are chem. modified during bond activation and 2) bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligand is not directly bound to the metal (e.g. via tautomerization). The role of MLC in enabling effective catalysis as well as in catalyst deactivation reactions will be discussed.
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    Alig, L.; Fritz, M.; Schneider, S. First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands. Chem. Rev. 2019, 119, 2681– 2751,  DOI: 10.1021/acs.chemrev.8b00555
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    First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands
    Alig, Lukas; Fritz, Maximilian; Schneider, Sven
    Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2681-2751CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodol. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.
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    Gunanathan, C.; Milstein, D. Bond activation and catalysis by ruthenium pincer complexes. Chem. Rev. 2014, 114, 12024– 12087,  DOI: 10.1021/cr5002782
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    Bond Activation and Catalysis by Ruthenium Pincer Complexes
    Gunanathan, Chidambaram; Milstein, David
    Chemical Reviews (Washington, DC, United States) (2014), 114 (24), 12024-12087CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The focus of this Review is limited to a summary of key developments in the area of bond activation and catalysis by defined ruthenium pincer complexes.
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    Mathis, C. L.; Geary, J.; Ardon, Y.; Reese, M. S.; Philliber, M. A.; VanderLinden, R. T.; Saouma, C. T. Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition. J. Am. Chem. Soc. 2019, 141, 14317– 14328,  DOI: 10.1021/jacs.9b06760
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    Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition
    Mathis, Cheryl L.; Geary, Jackson; Ardon, Yotam; Reese, Maxwell S.; Philliber, Mallory A.; VanderLinden, Ryan T.; Saouma, Caroline T.
    Journal of the American Chemical Society (2019), 141 (36), 14317-14328CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is crit. Herein we describe a thorough thermochem. anal. of the (de)hydrogenation catalyst, (PNP)Ru-Cl (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concn. of MeOH produced is independent of catalyst concn., consistent with a deactivation pathway that is dependent on methanol concn., not equivalency. Our temp.-dependent equil. studies of the dearomatized congener, (*PNP)Ru, with various H-X species (to give (PNP)Ru-X; X = H, OH, OMe, OCHO, OC(O)NMe2) reveal that formic acid equil. is approx. temp.-independent; relative to H2, it is more favored at elevated temps. We also measure the hydricity of (PNP)Ru-H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal-ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will pos. impact rational catalyst design.
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    Nobbs, J. D.; Sugiarto, S.; See, X. Y.; Cheong, C. B.; Aitipamula, S.; Stubbs, L. P.; van Meurs, M. Tetramethylphosphinane as a new secondary phosphine synthon. Nat. Commun. 2023, 6, 85,  DOI: 10.1038/s42004-023-00876-8
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    He, L.-P.; Chen, T.; Xue, D.-X.; Eddaoudi, M.; Huang, K.-W. Efficient transfer hydrogenation reaction Catalyzed by a dearomatized PN3P ruthenium pincer complex under base-free Conditions. J. Organomet. Chem. 2012, 700, 202– 206,  DOI: 10.1016/j.jorganchem.2011.10.017
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    Efficient transfer hydrogenation reaction catalyzed by a dearomatized PN3P ruthenium pincer complex under base-free conditions
    He, Li-Peng; Chen, Tao; Xue, Dong-Xu; Eddaoudi, Mohamed; Huang, Kuo-Wei
    Journal of Organometallic Chemistry (2012), 700 (), 202-206CODEN: JORCAI; ISSN:0022-328X. (Elsevier B.V.)
    A dearomatized complex [RuH(PN3P)(CO)] (PN3P = N,N'-bis(di-tert-butylphosphino)-2,6-diaminopyridine) (I) was prepd. by reaction of the arom. complex [RuH(Cl)(PN3P)(CO)] with t-BuOK in THF. Further treatment of I with formic acid led to the formation of a rearomatized complex (II). These new complexes were fully characterized and the mol. structure of complex II was further confirmed by X-ray crystallog. In complex II, a distorted square-pyramidal geometry around the ruthenium center was obsd., with the CO ligand trans to the pyridinic nitrogen atom and the hydride located in the apical position. The dearomatized complex I displays efficient catalytic activity for hydrogen transfer of ketones in isopropanol.
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  37. 37
    Benito-Garagorri, D.; Becker, E.; Wiedermann, J.; Lackner, W.; Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Achiral and Chiral Transition Metal Complexes with Modularly Designed Tridentate PNP Pincer-Type Ligands Based on N-Heterocyclic Diamines. Organometallics 2006, 25, 1900– 1913,  DOI: 10.1021/om0600644
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    37
    Achiral and Chiral Transition Metal Complexes with Modularly Designed Tridentate PNP Pincer-Type Ligands Based on N-Heterocyclic Diamines
    Benito-Garagorri, David; Becker, Eva; Wiedermann, Julia; Lackner, Wolfgang; Pollak, Martin; Mereiter, Kurt; Kisala, Joanna; Kirchner, Karl
    Organometallics (2006), 25 (8), 1900-1913CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The synthesis and characterization of Mo, Fe, Ru, Ni, Pd, and Pt complexes contg. new achiral and chiral PNP pincer-type ligands based on the N-heterocyclic diamines 2,6-diaminopyridine, N,N'-di-10-undecenyl-2,6-diaminopyridine, N,N'-dihexyl-2,6-diaminopyridine, and 2,6-diamino-4-phenyl-1,3,5-triazine are reported. The new PNP ligands were prepd. conveniently in high yield by treatment of the resp. N-heterocyclic diamines with 2 equiv of a variety of achiral and chiral R2PCl compds. in the presence of base. Mo PNP complexes [Mo(PNP)(CO)3PNP] were obtained by treatment of [Mo(CO)3(MeCN)3] with 1 equiv of the resp. PNP ligand. They react with I2 to give novel seven-coordinate pincer complexes [Mo(PNP)(CO)3I]+ and [Mo(PNP)(CO)2(MeCN)I]+ depending of whether the reaction is carried out in CH2Cl2 or MeCN. With [Fe(H2O)6](BF4)2 and 1 equiv of PNP ligand in MeCN dicationic complexes [Fe(PNP)(MeCN)3](BF4)2 were obtained. The cis and trans dichloride complexes [Ru(PNP)(PPh3)Cl2] were prepd. by a ligand exchange reaction of [RuCl2(PPh3)3] with a stoichiometric amt. of the resp. PNP ligand. Cationic PNP complexes of Ni(II), [Ni(PNP)Br]Br, were synthesized by the reaction of [NiBr2(DME)] with 1 equiv of PNP ligand. In similar fashion, treatment of [M(COD)X2] (M = Pd, Pt; X = Cl, Br) with 1 equiv of PNP ligand yields the cationic square-planar complexes [M(PNP)X]X. If the reaction is carried out in the presence of the halide scavenger KCF3SO3, complexes [M(PNP)X]CF3SO3 were obtained, which are better sol. in nonpolar solvents than the analogous halide compds. X-ray structures of representative Mo, Fe, Ru, Ni, and Pd PNP complexes were detd. Finally, the use of the Pd complexes as catalysts for the Suzuki-Miyaura coupling of some aryl bromides and Ph boronic acid was examd.
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  38. 38
    Salem, H.; Shimon, L. J. W.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.; Milstein, D. Formation of Stable trans-Dihydride Ruthenium(II) and 16-Electron Ruthenium(0) Complexes Based on Phosphinite PONOP Pincer Ligands. Reactivity toward Water and Electrophiles. Organometallics 2009, 28, 4791– 4806,  DOI: 10.1021/om9004077
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    38
    Formation of Stable trans-Dihydride Ruthenium(II) and 16-Electron Ruthenium(0) Complexes Based on Phosphinite PONOP Pincer Ligands. Reactivity toward Water and Electrophiles
    Salem, Hiyam; Shimon, Linda J. W.; Diskin-Posner, Yael; Leitus, Gregory; Ben-David, Yehoshoa; Milstein, David
    Organometallics (2009), 28 (16), 4791-4806CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The synthesis of a series of new ruthenium complexes based on the new PONOP ligands (1) and (10) (C5H3N-1,3-(OPR2)2: 1, R = iPr; 10, R = tBu) is presented, including the stable trans-dihydride complexes (iPr-PONOP)Ru(H)2(PPh3) (4) and (tBu-PONOP)Ru(H)2(CO) (12) and the stable Ru(0) complexes (R-PONOP)Ru(CO)2 (6, R = iPr; 15, R = tBu). A surprisingly stable 16-electron Ru(0) complex (13) was formed by deprotonation of 12 with KOtBu. Complex 13 reacts with H2 to afford the cis-dihydride complex 12a, which isomerized to the trans-dihydride 12. Complex 13 reacted with CO to afford the satd. Ru(0) complex 15. Reaction of complex 12 with water led to hydrolysis of the phosphinite PONOP ligand and rearrangement to a dimeric product (14). Reaction of the trans-dihydride complex 4 with the electrophiles PhCOCl, MeI, and MeOTf led to abstraction of one of the hydride ligands, forming the monohydride complexes (iPr-PONOP)Ru(H)(PPh3)(X) (X = Cl (2), I (8a), OTf (8b)) together with benzaldehyde in the case of 2. Similarly, 12 afforded the monohydride complexes (tBu-PONOP)Ru(H)(CO)(X) (X = Cl (11), OTf (17), I (18)). Reaction of the Ru(0) complexes 6 and 15 with water resulted in hydrolysis of the O-P bond and formation of the zwitterionic complexes 7 and 16. Treatment of 2 and 11 with MeOTf or MeI resulted in abstraction of the chloride ligand rather than the hydride, forming complexes 8a,b and 17, 18, resp. Addnl. syntheses of complexes based on ligands 1 and 10 are presented.
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  39. 39
    Ogata, O.; Nara, H.; Fujiwhara, M.; Matsumura, K.; Kayaki, Y. N-Monomethylation of Aromatic Amines with Methanol via PN(H)P-Pincer Ru Catalysts. Org. Lett. 2018, 20, 3866– 3870,  DOI: 10.1021/acs.orglett.8b01449
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    39
    N-Monomethylation of Aromatic Amines with Methanol via PNHP-Pincer Ru Catalysts
    Ogata, Osamu; Nara, Hideki; Fujiwhara, Mitsuhiko; Matsumura, Kazuhiko; Kayaki, Yoshihito
    Organic Letters (2018), 20 (13), 3866-3870CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    In the presence of 0.02-0.1 mol% ruthenium PNP pincer complex I and in the presence of KOt-Bu at 150°, aryl amines underwent chemoselective and green N-methylation with methanol to give N-methylarylamines. I underwent ionization and assocn. of carbon monoxide to form cationic dicarbonyl pincer ruthenium complexes which also acted as effective catalysts for chemoselective methylation.
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  40. 40
    Herrmann, W. A. Organometallic Aspects of the Fischer–Tropsch Synthesis. Angew. Chem., Int. Ed. 1982, 21, 117– 130,  DOI: 10.1002/anie.198201171
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    Nelson, G. O.; Sumner, C. E. Synthesis and reactivity of pentamethylcyclopentadienylruthenium formyl and α-hydroxy methyl complexes. Organometallics 1986, 5, 1983– 1990,  DOI: 10.1021/om00141a009
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    Synthesis and reactivity of pentamethylcyclopentadienylruthenium formyl and α-hydroxy complexes
    Nelson, Gregory O.; Sumner, Charles E.
    Organometallics (1986), 5 (10), 1983-90CODEN: ORGND7; ISSN:0276-7333.
    (η-C5Me5)Ru(CO)2CH2OH (I, Cp = cyclopentadienyl), (η-C5Me5)Ru(CO)2CHO (II), and (η-C5Me5)Ru(CO)(PMe2Ph)CHO (III) were synthesized and studied as models for intermediates in the redn. of CO to org. oxygenates by transition metal catalysts. I was prepd. by NaBH3CN redn. of (η-C5Me5)Ru(CO)3+ BF4- (IV). II was synthesized by redn. of IV with [Ph3PCuH]6, but it could not be isolated in pure form. Pure cryst. III was isolated from the redn. of (η-C5Me5)Ru(CO)2(PMe2Ph)+ I- with NaBH4 in THF/H2O. Formyl complexes II and III decompd. by a radical chain mechanism. The intermediate formed from the decompn. of III underwent electron transfer with (η-C5R5)Ru(CO)2I (R = H, Me). An x-ray structure of III was completed.
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    Nelson, G. O. (Pentamethylcyclopentadienyl)ruthenium compounds. Synthesis and characterization of (η5-C5Me5)Ru(CO)2CH2OH. Organometallics 1983, 2, 1474– 1475,  DOI: 10.1021/om50004a046
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    42
    (Pentamethylcyclopentadienyl)ruthenium compounds. Synthesis and characterization of (η-C5Me5)Ru(CO)2CH2OH
    Nelson, Gregory O.
    Organometallics (1983), 2 (10), 1474-5CODEN: ORGND7; ISSN:0276-7333.
    Treatment of (η5-C5Me5)Ru(CO)2I (C5H5 = cyclopentadienyl) in CH2Cl2 with AgBF4 under 60 psi CO produces 82% [(η-C5Me5)Ru(CO)3][BF1], which on redn. with excess NaBH3CN in MeOH gave a mixt. of (η-C5Me2)Ru(CO)2H, (η-C5Me2)Ru(CO)2CH2OMe and (η-C5Me5)Ru(CO)2CH2OH (I) . A soln. of I in THF under 5000 psi CO at 80° does not react. Reaction of I with CF3CON(SiMe3)2 gave (η-C5Me2)Ru(CO)2CH2OSiMe3.
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  43. 43
    Teets, T. S.; Labinger, J. A.; Bercaw, J. E. A Thermodynamic Analysis of Rhenium(I)–Formyl C–H Bond Formation via Base-Assisted Heterolytic H2 Cleavage in the Secondary Coordination Sphere. Organometallics 2013, 32, 5530– 5545,  DOI: 10.1021/om400810v
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    43
    A Thermodynamic Analysis of Rhenium(I)-Formyl C-H Bond Formation via Base-Assisted Heterolytic H2 Cleavage in the Secondary Coordination Sphere
    Teets, Thomas S.; Labinger, Jay A.; Bercaw, John E.
    Organometallics (2013), 32 (19), 5530-5545CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    Conversion of synthesis gas, a mixt. of CO and H, into value-added Cn≥2 products requires both C-H and C-C bond-forming events. The authors' group has developed mol. complexes, based on Group 7 (Mn and Re) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, the authors explore a new mode of H2 activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H2 to form a formyl C-H bond at a Re-bound carbonyl. Cationic Re(I) complexes [ReI(P∼B:-κ1-P)(CO)5]+1, where P∼B: is a phosphine ligand with a tethered strong base, were prepd. and characterized; measurement of their protonation equil. demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepd. in 50% to 95% yields by hydride delivery to the parent pentacarbonyl complexes, and several of the free-base formyl complexes can be protonated, generating observable [ReI(P∼BH-κ1-P)(CHO)(CO)4]+1 complexes. Intramol. H bonding is evident for one of the complexes, providing addnl. stabilization to the protonated formyl complex. By measuring both the hydricity of the formyl, ΔG°H-, and its pKa, the overall free energy of H2 cleavage is calcd. from an appropriate cycle and is thermodynamically uphill in all cases (in the best case by only ∼8 kcal/mol), although significantly dependent upon the properties of the supporting ligand.
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  44. 44
    Elowe, P. R.; West, N. M.; Labinger, J. A.; Bercaw, J. E. Transformations of Group 7 Carbonyl Complexes: Possible Intermediates in a Homogeneous Syngas Conversion Scheme. Organometallics 2009, 28, 6218– 6227,  DOI: 10.1021/om900804j
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    44
    Transformations of Group 7 carbonyl complexes: possible intermediates in a homogeneous syngas conversion scheme
    Elowe, Paul R.; West, Nathan M.; Labinger, Jay A.; Bercaw, John E.
    Organometallics (2009), 28 (21), 6218-6227CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    Redn. of Group VIIB metal phosphine carbonyls by lithium triethylhydroborate gave access to formyl complexes, which were isolated and characterized as borane-stabilized adducts. A variety of C-H and C-C bond forming reactions of Group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C2+ compds. Reaction of [(PPh3)2M(CO)4][BF4] with LiBHEt3 gave neutral formyl complexes [(PPh3)2(CO)3M(CHO)] (2a,b; M = Mn, Re). Attempted silylation of 2b by Me3SiOTf gave unexpected product, [(PPh3)2Re(CO)3(CHO·BF3)] (4b), as a result of fluoride abstraction from BF4- anion and trifluoroborane coordination. Reaction of 2a,b with BX3 gave the stabilized formyl complexes directly, [(PPh3)2(CO)3M(CHO·BX3)] (4a,b, X = F, M = Mn, Re; 5a,b, X = C6F5, M = Mn, Re). The metal formyl complexes 2 are substantially stabilized by coordination to boranes BX3 in the form of novel boroxycarbene complexes M(CO)3(PPh3)2(CHOBX3), but these boron-stabilized carbenes 4 and 5 do not react with hydride sources to undergo further redn. to metal alkyls. The related manganese methoxycarbene cations [Mn(CO)5-x(PPh3)x(CHOMe)]+ (x = 1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atm. of CO. The resulting acyls, cis- and trans-Mn(PPh3)(CO)4(C(O)CH2OMe), can be alkylated to form the cationic carbene complex [Mn(PPh3)(CO)4(C(OR)CH2OMe)]+, which undergoes a 1,2-hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or di-Et ether adducts of [Mn(PPh3)(CO)4]+. The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh3)(CO)4(CH2COOMe) and is protonolyzed to yield Me acetate and [Mn(PPh3)(CO)4]+; addn. of L (L = PPh3, CO) to the manganese cation regenerates [Mn(PPh3)(CO)4(L)]+. Since the original formyl complex can be obtained by the reaction of [Mn(PPh3)(CO)5]+ with [PtH(dmpe)2]+, which in turn can be generated from H2, this set of transformations amts. to a stoichiometric cycle for selectively converting H2 and CO into a C2 compd. under mild conditions.
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  45. 45
    Pan, Y.; Pan, C.; Zhang, Y.; Li, H.; Min, S.; Guo, X.; Zheng, B.; Chen, H.; Anders, A.; Lai, Z.; Zheng, J.; Huang, K. Selective Hydrogen Generation from Formic Acid with Well-Defined Complexes of Ruthenium and Phosphorus-Nitrogen PN3 -Pincer Ligand. Chem. - Asian J. 2016, 11, 1357– 1360,  DOI: 10.1002/asia.201600169
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    45
    Selective Hydrogen Generation from Formic Acid with Well-Defined Complexes of Ruthenium and Phosphorus-Nitrogen PN3-Pincer Ligand
    Pan, Yupeng; Pan, Cheng-Ling; Zhang, Yufan; Li, Huaifeng; Min, Shixiong; Guo, Xunmun; Zheng, Bin; Chen, Hailong; Anders, Addison; Lai, Zhiping; Zheng, Junrong; Huang, Kuo-Wei
    Chemistry - An Asian Journal (2016), 11 (9), 1357-1360CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
    An unsym. protonated PN3-pincer complex in which ruthenium is coordinated by one nitrogen and two phosphorus atoms was employed for the selective generation of hydrogen from formic acid. Mechanistic studies suggest that the imine arm participates in the formic acid activation/deprotonation step. A long life time of 150 h with a turnover no. over 1 million was achieved.
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  46. 46
    Ooyama, D.; Tomon, T.; Tsuge, K.; Tanaka, K. Structural and spectroscopic characterization of ruthenium(II) complexes with methyl, formyl, and acetyl groups as model species in multi-step CO2 reduction. J. Organomet. Chem. 2001, 619, 299– 304,  DOI: 10.1016/S0022-328X(00)00705-1
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    46
    Structural and spectroscopic characterization of ruthenium(II) complexes with methyl, formyl, and acetyl groups as model species in multi-step CO2 reduction
    Ooyama, Dai; Tomon, Takashi; Tsuge, Kiyoshi; Tanaka, Koji
    Journal of Organometallic Chemistry (2001), 619 (2), 299-304CODEN: JORCAI; ISSN:0022-328X. (Elsevier Science S.A.)
    The mol. structures of Ru(II) complexes with Me, formyl, and acetyl groups [Ru(bpy)2(CO)L]+ (L = CH3, C(O)H and C(O)CH3) were examd. from the viewpoint of active species in multi-step redn. of CO2 on Ru. The Me complex was prepd. by the reaction of [Ru(bpy)2(OH2)2]2+ with trimethylsilyl acetylene and fully characterized by IR, Raman, 13C NMR and single-crystal x-ray crystallog. Disorder of the Ru-CO and Ru-C(O)H bonds in the crystal structure of the formyl complex made it difficult to det. the bond parameters of the two groups accurately, but the mol. structure of the analogous acetyl complex, which was obtained by the reaction of [Ru(bpy)2(CO3)] with propiolic acid, was detd. by x-ray anal. The Ru-carbonyl (Ru-C-O) bond angles of the Me and acetyl complex with 174(1) and 175.5(5)°, resp., are in the ranges of those of previously characterized [Ru(bpy)2(CO)L]n+ (L = CO2, C(O)OH, CO and CH2OH). However, the Ru-CH3 and Ru-C(O)CH3 bond distances showed unusual relation against the stretching frequency in the Raman spectra.
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    Toyohara, K.; Nagao, H.; Mizukawa, T.; Tanaka, K. Ruthenium Formyl Complexes as the Branch Point in Two- and Multi-Electron Reductions of CO2. Inorg. Chem. 1995, 34, 5399– 5400,  DOI: 10.1021/ic00126a003
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    Ruthenium Formyl Complexes as the Branch Point in Two- and Multi-Electron Reductions of CO2
    Toyohara, Kiyotsuna; Nagao, Hirotaka; Mizukawa, Tetsunori; Tanaka, Koji
    Inorganic Chemistry (1995), 34 (22), 5399-400CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Both [Ru(bpy)2(CO)(CHO)]+ and [Ru(bpy)(trpy)(CHO)]+ (bpy = 2,2'-bipyridine; trpy = 2,2',6',2''-terpyridine) were characterized and the latter is the key-intermediate of four- and six-electron redn. of CO2 producing HCHO and CH3OH. Higher reactivity of these formyl complexes than the corresponding hydrides reveals a new pathway of HCOOH formation in electrochem. CO2 redn.
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    Kelly, J. M.; Vos, J. G. cis-[Ru(bpy)2(CO)H]+: A Possible Intermediate in the Photochemical Production of H2 from Water Catalyzed by [Ru(bpy)3]2+?. Angew. Chem., Int. Ed. 1982, 21 (8), 628– 629,  DOI: 10.1002/anie.198206281
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    Dombek, B. D. Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts. J. Am. Chem. Soc. 1980, 102, 6855– 6857,  DOI: 10.1021/ja00542a036
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    Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts
    Dombek, B. Duane
    Journal of the American Chemical Society (1980), 102 (22), 6855-7CODEN: JACSAT; ISSN:0002-7863.
    Solns. of Ru3(CO)12 in carboxylic acids catalyze the hydrogenation of CO at low pressures (100 to 340 atm) to give MeOH (obtained as its ester), with smaller amts. of ethylene glycol diester. At 340 atm and 260° a combined rate to these products of 8.3 × 10-3 turnovers s-1 was obsd. in HOAc solvent. Similar rates to MeOH are obtainable in other polar solvents, but ethylene glycol is not obsd. under these conditions except in the presence of carboxylic acids. A reaction scheme is proposed in which the function of the carboxylic acid is to assist in converting a coordinated formaldehyde intermediate into a glycol precursor.
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    Walker, H. W.; Ford, P. C. Synthesis and characterization of [PPN][HRu(CO)4] and a convenient route to [PPN][HOs(CO)4]. J. Organomet. Chem. 1981, 214, C43– C44,  DOI: 10.1016/S0022-328X(81)80019-8
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    Synthesis and characterization of [PPN][HRu(CO)4] and a convenient route to [PPN][HOs(CO)4]
    Walker, Howard W.; Ford, Peter C.
    Journal of Organometallic Chemistry (1981), 214 (3), C43-C44CODEN: JORCAI; ISSN:0022-328X.
    [PPN][HM(CO)4] (M = Ru, Os; PPN = bis(triphenylphosphine)iminium were prepd. in 60% (M = Ru) yield by treating Na2M(CO)4 with PPN+Cl- in a min. amt. of MeOH at -196° followed by filtration of -78°.
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    Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Süss, G. The triruthenium cluster anion [Ru3H(CO)11]−: preparation, structure, and fluxionality. J. Chem. Soc., Dalton Trans. 1979, 9, 1356– 1361,  DOI: 10.1039/DT9790001356
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    Guntermann, N.; Franciò, G.; Leitner, W. Hydrogenation of CO2 to formic acid in biphasic systems using aqueous solutions of amino acids as the product phase. Green Chem. 2022, 24, 8069– 8075,  DOI: 10.1039/D2GC02598A
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    Hydrogenation of CO2 to formic acid in biphasic systems using aqueous solutions of amino acids as the product phase
    Guntermann, Nils; Francio, Giancarlo; Leitner, Walter
    Green Chemistry (2022), 24 (20), 8069-8075CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Carbon capture and utilization is considered a promising approach for introducing CO2 into the chem. value chain, esp. in combination with bioenergy applications (BECCU). We report here on the catalytic hydrogenation of CO2 to formic acid in a biphasic reaction system using aq. solns. of amino acids as the product phase and possible capture solns. for biogenic CO2. The mol. structure of the ruthenium catalyst and the catalyst phase were matched through a combined design process identifying n-dodecanol (lauryl alc.) as the preferred "green" solvent. A total turnover no. (TON) of over 100 000 mol HCOOH per mol of catalyst (46 582 g HCOOH per g of Ru) with minimal contamination of the aq. phase with metal or org. solvent was obtained. The resulting aq. solns. attained almost quant. conversions with up to 0.94 mol formic acid per mol amino acid (ca. 108 g HCOOH per kg). Such solns. may find use directly, or after upgrading, in agricultural applications without the need for energy intensive and costly isolation of pure formic acid.
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  53. 53
    Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.; Liotta, C. L. The reaction of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with carbon dioxide. J. Org. Chem. 2005, 70, 5335– 5338,  DOI: 10.1021/jo0503759
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    53
    The Reaction of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) with Carbon Dioxide
    Heldebrant, David J.; Jessop, Philip G.; Thomas, Colin A.; Eckert, Charles A.; Liotta, Charles L.
    Journal of Organic Chemistry (2005), 70 (13), 5335-5338CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    Amidines have been reported to react with CO2 to form a stable and isolable zwitterionic adduct but previous studies were performed in the presence of at least some water. However, spectroscopy of the reaction between DBU and CO2 detects the rapid formation of the bicarbonate salt of DBU when wet DBU is exposed to CO2 and does not indicate that an isolable zwitterionic adduct between DBU and CO2 forms either in the presence or the absence of water.
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Cite this: Inorg. Chem. 2024, 63, 7, 3393–3401
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Published February 8, 2024

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  • Abstract

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    Scheme 1

    Scheme 1. Reversible Deprotonation and H2 Activation through Metal Ligand Cooperativity in MACHO, PCNCP, and PN3P Pincer Complexes
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    Scheme 2

    Scheme 2. Synthesis of Ru-PN3P Complexes 1 and 2
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    Scheme 3

    Scheme 3. Reaction of Complexes 1 and 2 with KOtBu and Subsequent Reactivity
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    Figure 1

    Figure 1. Variable temperature 1H NMR spectra of [RuH(CO)2(PN3P-tBu–H)] (5) in d8-toluene (hydride signal has been omitted).

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    Scheme 4

    Scheme 4. Equilibrium between the Two [RuH(CO)2(PN3P-tBu–H)] (5) Isomers (Protons in Blue Show Exchange Behavior)
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    Figure 2

    Figure 2. Molecular structure of the cationic complex [RuH(CO)2(PN3P-tBu)]+ (6). The exact position of the hydride ligand could not be determined (Supporting Information). The anion [B(C6H3(CF3)2)4]− has been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru(1)–C(28) 1.878(8), Ru(1)–C(27) 1.976(8), Ru(1)–N(1) 2.115(4), Ru(1)–P(8) 2.345(3), Ru(1)–P(10) 2.357(3); C(28)–Ru(1)–C(27) 95.6(4), P(8)–Ru(1)–P(10) 156.02(15), and C(28)–Ru(1)–N(1) 175.1(4).

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    Scheme 5

    Scheme 5. Proposed Reaction Sequence for the Hydrogenation of CO to MeOH and Higher Alcohols
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    Scheme 6

    Scheme 6. Reactivity of Complex 3 toward H2 in the Absence and Presence of CO
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    Scheme 7

    Scheme 7. Hydride Transfer to Generate Metal Formyl Intermediates and Subsequent Decarbonylation
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    Scheme 8

    Scheme 8. Hydride Transfer to [Ru3(CO)12]
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  • References

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      Rohmann, K.; Kothe, J.; Haenel, M. W.; Englert, U.; Holscher, M.; Leitner, W. Hydrogenation of CO2 to Formic Acid with a Highly Active Ruthenium Acriphos Complex in DMSO and DMSO/Water. Angew. Chem., Int. Ed. 2016, 55, 8966– 8969,  DOI: 10.1002/anie.201603878
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      The novel [Ru(Acriphos)(PPh3)(Cl)(BzO)] [1; Acriphos = 4,5-bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO2 to give formic acid in DMSO and DMSO/H2O without the need for amine bases as co-reagents. Turnover nos. (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h-1 were achieved, thus rendering 1 among the most active catalysts for CO2 hydrogenations under additive-free conditions reported to date. The thermodn. stabilization of the reaction product by the reaction medium, through H bonds between formic acid and clusters of solvent or H2O, were rationalized by DFT calcns. The relatively low final concn. of formic acid obtained exptl. under catalytic conditions (0.33 mol L-1) is limited by product-dependent catalyst inhibition rather than thermodn. limits, and could be overcome by addn. of small amts. of acetate buffer, leading to a max. concn. of free formic acid of 1.27 mol L-1, which corresponds to optimized values of TON = 16 × 103 and TOFavg ≈103 h-1.
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      A review. Climate change and the unsustainability of fossil fuels are calling for cleaner energies such as methanol as a fuel. Methanol is one of the simplest mols. for energy storage and is utilized to generate a wide range of products. Since methanol can be produced from biomass, numerous countries could produce and utilize biomethanol. Here, we review methanol prodn. processes, techno-economy, and environmental viability. Lignocellulosic biomass with a high cellulose and hemicellulose content is highly suitable for gasification-based biomethanol prodn. Compared to fossil fuels, the combustion of biomethanol reduces nitrogen oxide emissions by up to 80%, carbon dioxide emissions by up to 95%, and eliminates sulfur oxide emission. The cost and yield of biomethanol largely depend on feedstock characteristics, initial investment, and plant location. The use of biomethanol as complementary fuel with diesel, natural gas, and di-Me ether is beneficial in terms of fuel economy, thermal efficiency, and redn. in greenhouse gas emissions.
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      Fundamentals and applications of photocatalytic CO2 methanation
      Ulmer Ulrich; Dingle Thomas; Duchesne Paul N; Morris Robert H; Tavasoli Alexandra; Wood Thomas; Ozin Geoffrey A; Dingle Thomas; Tavasoli Alexandra
      Nature communications (2019), 10 (1), 3169 ISSN:.
      The extraction and combustion of fossil natural gas, consisting primarily of methane, generates vast amounts of greenhouse gases that contribute to climate change. However, as a result of recent research efforts, "solar methane" can now be produced through the photocatalytic conversion of carbon dioxide and water to methane and oxygen. This approach could play an integral role in realizing a sustainable energy economy by closing the carbon cycle and enabling the efficient storage and transportation of intermittent solar energy within the chemical bonds of methane molecules. In this article, we explore the latest research and development activities involving the light-assisted conversion of carbon dioxide to methane.
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      Bowker, M. Methanol Synthesis from CO2 Hydrogenation. ChemCatChem. 2019, 11, 4238– 4246,  DOI: 10.1002/cctc.201900401
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      Methanol synthesis from CO2 hydrogenation
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      A review. In the future we will be phasing out the use of fossil fuels in favor of more sustainable forms of energy, esp. solar derived forms such as hydroelec., wind and photovoltaic. However, due to the variable nature of the latter sources which depend on time of day, and season of the year, we also need to have a way of storing such energy at peak prodn. times for use in times of low prodn. One way to do this is to convert such energy into chem. energy, and the principal way considered at present is the prodn. of hydrogen. Although this may be achieved directly in the future via photocatalytic water splitting, at present it is electrolytic prodn. which dominates thinking. In turn, it may well be important to store this hydrogen in an energy dense liq. form such as methanol or ammonia. In this brief review it is emphasized that CO2 is the microscopic carbon source for current industrial methanol synthesis, operating through the surface formate intermediate, although when using CO in the feed, it is CO which is hydrogenated at the global scale. However, methanol can be produced from pure CO2 and hydrogen using conventional and novel types of catalysts. Examples of such processes, and of a demonstrator plant in construction, are given, which utilize CO2 (which would otherwise enter the atm. directly) and hydrogen which can be produced in a sustainable manner. This is a fast-evolving area of science and new ideas and processes will be developed in the near future.
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      Catalytic Homogeneous Hydrogenation of CO to Methanol via Formamide
      Kar, Sayan; Goeppert, Alain; Prakash, G. K. Surya
      Journal of the American Chemical Society (2019), 141 (32), 12518-12521CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A novel amine-assisted route for low temp. homogeneous hydrogenation of CO to methanol is described. The reaction proceeds through the formation of formamide intermediates. The first amine carbonylation part is catalyzed by K3PO4. Subsequently, the formamides are hydrogenated in situ to methanol in the presence of a com. available ruthenium pincer complex as a catalyst. Under optimized reaction conditions, CO (up to 10 bar) was directly converted to methanol in high yield and selectivity in the presence of H2 (70 bar) and diethylenetriamine. A max. TON of 539 was achieved using the catalyst Ru-Macho-BH. The high yield, selectivity, and TONs obtained for methanol prodn. at low reaction temp. (145 °C) could make this process an attractive alternative over the traditional high temp. heterogeneous catalysis.
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      Alcohol-Assisted Hydrogenation of Carbon Monoxide to Methanol Using Molecular Manganese Catalysts
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      Alc.-assisted hydrogenation of carbon monoxide (CO) to methanol was achieved using homogeneous mol. complexes. The mol. manganese complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] ([HN(C2H4PiPr2)2] = MACHO-iPr) revealed the best performance, reaching up to turnover no. = 4023 and turnover frequency 857 h-1 in EtOH/toluene as solvent under optimized conditions (T = 150°C, p(CO/H2) = 5/50 bar, t = 8-12 h). Control expts. affirmed that the reaction proceeds via formate ester as the intermediate, whereby a catalytic amt. of base was found to be sufficient to mediate its formation from CO and the alc. in situ. Selectivity for methanol formation reached >99% with no accumulation of the formate ester. The reaction was demonstrated to work with methanol as the alc. component, resulting in a reactive system that allows catalytic "breeding" of methanol without any coreagents.
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      Ryabchuk, P.; Stier, K.; Junge, K.; Checinski, M. P.; Beller, M. Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol. J. Am. Chem. Soc. 2019, 141, 16923– 16929,  DOI: 10.1021/jacs.9b08990
      17
      Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol
      Ryabchuk, Pavel; Stier, Kenta; Junge, Kathrin; Checinski, Marek P.; Beller, Matthias
      Journal of the American Chemical Society (2019), 141 (42), 16923-16929CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Methanol synthesis from syngas (CO/H2 mixts.) is one of the largest man-made chem. processes with annual prodn. reaching 100 million tons. The current industrial method proceeds at high temps. (200-300°C) and pressures (50-100 atm) using a copper-zinc-based heterogeneous catalyst. In contrast, here, we report a molecularly defined manganese catalyst that allows for low-temp./low-pressure (120-150°C, 50 bar) carbon monoxide hydrogenation to methanol. This new approach was evaluated and optimized by quantum mech. simulations virtual high-throughput screenings. Crucial for this achievement is the use of amine-based promoters, which capture carbon monoxide to give formamide intermediates, which then undergo manganese-catalyzed hydrogenolysis, regenerating the promoter. Following this conceptually new approach, high selectivity toward methanol and catalyst turnover nos. (up to 3170) was achieved. The proposed general catalytic cycle for methanol synthesis is supported by model studies and detailed spectroscopic investigations.
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    18. 18
      Sen, R.; Goeppert, A.; Prakash, G. K. S. Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy. Angew. Chem., Int. Ed. 2022, 61, e202207278  DOI: 10.1002/anie.202207278
      18
      Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy
      Sen, Raktim; Goeppert, Alain; Surya Prakash, G. K.
      Angewandte Chemie, International Edition (2022), 61 (42), e202207278CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The traditional economy based on carbon-intensive fuels and materials has led to an exponential rise in anthropogenic CO2 emissions. Outpacing the natural carbon cycle, atm. CO2 levels increased by 50% since the pre-industrial age and can be directly linked to global warming. Being at the core of the proposed methanol economy pioneered by the late George A. Olah, the chem. recycling of CO2 to produce methanol, a green fuel and feedstock, is a prime channel to achieve carbon neutrality. In this direction, homogeneous catalytic systems have lately been a major focus for methanol synthesis from CO2, CO and their derivs. as potential low-temp. alternatives to the com. processes. This Review provides an account of this rapidly growing field over the past decade, since its resurgence in 2011. Based on the crit. assessment of the progress thus far, the present key challenges in this field have been highlighted and potential directions have been suggested for practically viable applications.
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    19. 19
      Khusnutdinova, J. R.; Garg, J. A.; Milstein, D. Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol. ACS Catal. 2015, 5, 2416– 2422,  DOI: 10.1021/acscatal.5b00194
      19
      Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol
      Khusnutdinova, Julia R.; Garg, Jai Anand; Milstein, David
      ACS Catalysis (2015), 5 (4), 2416-2422CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      This paper describes a novel approach to CO2 hydrogenation, in which CO2 capture with aminoethanols at low pressure is coupled with hydrogenation of the captured product, oxazolidinone, directly to MeOH. In particular, (2-methylamino)ethanol or valinol captures CO2 at 1-3 bar in the presence of catalytic Cs2CO3 to give the corresponding oxazolidinones in up to 65-70 and 90-95% yields, resp. Efficient hydrogenation of oxazolidinones was achieved using PNN pincer Ru catalysts to give the corresponding aminoethanol (up to 95-100% yield) and MeOH (up to 78-92% yield). We also have shown that both CO2 capture and oxazolidinone hydrogenation can be performed in the same reaction mixt. using a simple protocol that avoids intermediate isolation or purifn. steps. For example, CO2 can be captured by valinol at 1 bar with Cs2CO3 catalyst followed by 4-isopropyl-2-oxazolidinone hydrogenation in the presence of a bipy-based pincer Ru catalyst to produce MeOH in 50% yield after two steps.
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    20. 20
      Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S. Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160– 3170,  DOI: 10.1021/jacs.8b12763
      20
      Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol
      Kar, Sayan; Sen, Raktim; Kothandaraman, Jotheeswari; Goeppert, Alain; Chowdhury, Ryan; Munoz, Socrates B.; Haiges, Ralf; Prakash, G. K. Surya
      Journal of the American Chemical Society (2019), 141 (7), 3160-3170CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      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 mol. structure of the ruthenium pincer catalysts and the amines that are crit. for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNPR(CO)] (R = Ph/i-Pr/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 dissocn. 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 max. turnover no. (TON) of 9900.
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    21. 21
      Rezayee, N. M.; Huff, C. A.; Sanford, M. S. Tandem amine and ruthenium-catalyzed hydrogenation of CO2 to methanol. J. Am. Chem. Soc. 2015, 137, 1028– 1031,  DOI: 10.1021/ja511329m
      21
      Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol
      Rezayee, Nomaan M.; Huff, Chelsea A.; Sanford, Melanie S.
      Journal of the American Chemical Society (2015), 137 (3), 1028-1031CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      This Communication describes the hydrogenation of carbon dioxide to methanol via tandem catalysis with dimethylamine and a homogeneous ruthenium complex. Unlike previous examples with homogeneous catalysts, this CO2-to-CH3OH process proceeds under basic reaction conditions. The dimethylamine is proposed to play a dual role in this system. It reacts directly with CO2 to produce dimethylammonium dimethylcarbamate, and it also intercepts the intermediate formic acid to generate DMF. With the appropriate selection of catalyst and reaction conditions, >95% conversion of CO2 was achieved to form a mixt. of CH3OH and DMF.
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    22. 22
      Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778– 781,  DOI: 10.1021/jacs.5b12354
      22
      Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst
      Kothandaraman, Jotheeswari; Goeppert, Alain; Czaun, Miklos; Olah, George A.; Prakash, G. K. Surya
      Journal of the American Chemical Society (2016), 138 (3), 778-781CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A highly efficient homogeneous catalyst system for the prodn. of CH3OH from CO2 using pentaethylenehexamine and Ru-Macho-BH (1) at 125-165 °C in an ethereal solvent has been developed (initial turnover frequency = 70 h-1 at 145 °C). Ease of sepn. of CH3OH is demonstrated by simple distn. from the reaction mixt. The robustness of the catalytic system was shown by recycling the catalyst over five runs without significant loss of activity (turnover no. > 2000). Various sources of CO2 can be used for this reaction including air, despite its low CO2 concn. (400 ppm). For the first time, we have demonstrated that CO2 captured from air can be directly converted to CH3OH in 79% yield using a homogeneous catalytic system.
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    23. 23
      Curley, J. B.; Hert, C.; Bernskoetter, W. H.; Hazari, N.; Mercado, B. Q. Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation. Inorg. Chem. 2022, 61, 643– 656,  DOI: 10.1021/acs.inorgchem.1c03372
      23
      Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation
      Curley, Julia B.; Hert, Clayton; Bernskoetter, Wesley H.; Hazari, Nilay; Mercado, Brandon Q.
      Inorganic Chemistry (2022), 61 (1), 643-656CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A novel pincer ligand, iPrPNPhP [PhN(CH2CH2PiPr2)2], which is an analog of the versatile MACHO ligand, iPrPNHP [HN(CH2CH2PiPr2)2], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-contg. complexes were isolated and characterized by NMR and IR spectroscopies, as well as x-ray diffraction. Comparisons to previously published analogs ligated by iPrPNHP and iPrPNMeP [CH3N(CH2CH2PiPr2)2] illustrate that there are large changes in the coordination chem. that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the iPrPNPhP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by iPrPNHP form the anti isomer and complexes supported by iPrPNMeP form a mixt. of syn and anti isomers. Authors evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of iPrPNRP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO2) hydrogenation to formate. The iPrPNPhP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO2 hydrogenation, the iPrPNPhP-ligated species is again the most active under authors optimal conditions, and they report some of the highest turnover frequencies for homogeneous catalysts. Exptl. and theor. insights into the turnover-limiting step of catalysis provide a basis for the obsd. trends in catalytic activity. Addnl., the stability of complexes enabled to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, they have developed highly active catalysts for formic acid dehydrogenation and CO2 hydrogenation and also provided a framework for future catalyst development.
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    24. 24
      Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S. Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580– 1583,  DOI: 10.1021/jacs.7b12183
      24
      Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy
      Kar, Sayan; Sen, Raktim; Goeppert, Alain; Prakash, G. K. Surya
      Journal of the American Chemical Society (2018), 140 (5), 1580-1583CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Herein we report an efficient and recyclable system for tandem CO2 capture and hydrogenation to methanol. After capture in an aq. amine soln., CO2 is hydrogenated in high yield to CH3OH (>90%) in a biphasic 2-MTHF/water system, which also allows for easy sepn. and recycling of the amine and catalyst for multiple reaction cycles. Between cycles, the produced methanol can be conveniently removed in vacuo. Employing this strategy, catalyst Ru-MACHO-BH and polyamine PEHA were recycled three times with 87% of the methanol productivity of the first cycle retained, along with 95% of catalyst activity after four cycles. CO2 from dil. sources such as air can also be converted to CH3OH using this route. We postulate that the CO2 capture and hydrogenation to methanol system presented here could be an important step toward the implementation of the carbon neutral methanol economy concept.
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    25. 25
      Filonenko, G. A.; van Putten, R.; Schulpen, E. N.; Hensen, E. J. M.; Pidko, E. A. Highly Efficient Reversible Hydrogenation of Carbon Dioxide to Formates Using a Ruthenium PNP-Pincer Catalyst. ChemCatChem. 2014, 6, 1526– 1530,  DOI: 10.1002/cctc.201402119
      25
      Highly efficient reversible hydrogenation of carbon dioxide to formates using a ruthenium PNP-pincer catalyst
      Filonenko, Georgy A.; van Putten, Robbert; Schulpen, Erik N.; Hensen, Emiel J. M.; Pidko, Evgeny A.
      ChemCatChem (2014), 6 (6), 1526-1530CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The use of hydrogen as a fuel requires both safe and robust technologies for its storage and transportation. Formic acid (FA) produced by the catalytic hydrogenation of CO2 is recognized as a potential intermediate H2 carrier. Herein, we present the development of a formate-based H2 storage system that employs a Ru PNP-pincer catalyst. The high stability of this system allows cyclic operation with an exceptionally fast loading and liberation of H2. Kinetic studies highlight the crucial role of the base promoter, which controls the rate-detg. step in FA dehydrogenation and defines the total H2 capacity attainable from the hydrogenation of CO2. The reported findings show promise for the development of practical technologies that use formic acid as a hydrogen carrier.
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    26. 26
      Guan, C.; Pan, Y.; Ang, E. P. L.; Hu, J.; Yao, C.; Huang, M.-H.; Li, H.; Lai, Z.; Huang, K.-W. Conversion of CO2 from air into formate using amines and phosphorus-nitrogen PN3P-Ru(II) pincer complexes. Green Chem. 2018, 20, 4201– 4205,  DOI: 10.1039/C8GC02186D
      26
      Conversion of CO2 from air into formate using amines and phosphorus-nitrogen PN3P-Ru(II) pincer complexes
      Guan, Chao; Pan, Yupeng; Ang, Eleanor Pei Ling; Hu, Jinsong; Yao, Changguang; Huang, Mei-Hui; Li, Huaifeng; Lai, Zhiping; Huang, Kuo-Wei
      Green Chemistry (2018), 20 (18), 4201-4205CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Well-defined ruthenium(II) PN3P pincer complexes were developed for the hydrogenation of carbon dioxide. Excellent product selectivity and catalytic activity with TOF (turnover frequency) and TON (turnover no.) up to 13 000 h-1 and 33 000, resp., in a THF/H2O biphasic system were achieved. Notably, effective conversion of carbon dioxide from air into formate was conducted in the presence of an amine, allowing easy product sepn. and catalyst recycling.
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    27. 27
      Li, H.; Goncalves, T. P.; Zhao, Q.; Gong, D.; Lai, Z.; Wang, Z.; Zheng, J.; Huang, K. W. Diverse catalytic reactivity of a dearomatized PN3P*-nickel hydride pincer complex towards CO2 reduction. Chem. Commun. 2018, 54, 11395– 11398,  DOI: 10.1039/C8CC05948A
      27
      Diverse catalytic reactivity of a dearomatized PN3P*-nickel hydride pincer complex towards CO2 reduction
      Li, Huaifeng; Goncalves, Theo P.; Zhao, Qianyi; Gong, Dirong; Lai, Zhiping; Wang, Zhixiang; Zheng, Junrong; Huang, Kuo-Wei
      Chemical Communications (Cambridge, United Kingdom) (2018), 54 (81), 11395-11398CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A dearomatized PN3P*-nickel hydride complex has been prepd. using an oxidative addn. process. The first nickel-catalyzed hydrosilylation of CO2 to methanol has been achieved, with unprecedented turnover nos. Selective methylation and formylation of amines with CO2 were demonstrated by such a PN3P*-nickel hydride complex, highlighting its versatile functions in CO2 redn.
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    28. 28
      Tanaka, R.; Yamashita, M.; Nozaki, K. Catalytic hydrogenation of carbon dioxide using Ir(III)-pincer complexes. J. Am. Chem. Soc. 2009, 131, 14168– 14169,  DOI: 10.1021/ja903574e
      28
      Catalytic Hydrogenation of Carbon Dioxide Using Ir(III)-Pincer Complexes
      Tanaka, Ryo; Yamashita, Makoto; Nozaki, Kyoko
      Journal of the American Chemical Society (2009), 131 (40), 14168-14169CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Catalytic hydrogenation of carbon dioxide in aq. potassium hydroxide was performed using a newly synthesized isopropyl-substituted PNP-pincer iridium trihydride complex as catalyst. Potassium formate was obtained with turnover no. up to 3,500,000 and turnover frequency of 150,000 h-1.
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    29. 29
      Tanaka, R.; Yamashita, M.; Chung, L. W.; Morokuma, K.; Nozaki, K. Mechanistic Studies on the Reversible Hydrogenation of Carbon Dioxide Catalyzed by an Ir-PNP Complex. Organometallics 2011, 30, 6742– 6750,  DOI: 10.1021/om2010172
      29
      Mechanistic Studies on the Reversible Hydrogenation of Carbon Dioxide Catalyzed by an Ir-PNP Complex
      Tanaka, Ryo; Yamashita, Makoto; Chung, Lung Wa; Morokuma, Keiji; Nozaki, Kyoko
      Organometallics (2011), 30 (24), 6742-6750CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The PNP-ligated iridium(III) trihydride complex 1 exhibited the highest catalytic activity for hydrogenation of carbon dioxide in aq. KOH. The catalytic hydrogenation can be tuned to be a reversible process with the same catalyst at the expense of the activity, when triethanolamine was used as a base. Theor. studies on the hydrogenation of carbon dioxide using DFT calcns. suggested two competing reaction pathways: either the deprotonative dearomatization step or the hydrogenolysis step as the rate-detg. step. The results nicely explain our exptl. observations that the catalytic cycle is dependent on both the strength of the base and hydrogen pressure.
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    30. 30
      Pan, Y.; Guan, C.; Li, H.; Chakraborty, P.; Zhou, C.; Huang, K. W. CO2 hydrogenation by phosphorus-nitrogen PN3P-pincer iridium hydride complexes: elucidation of the deactivation pathway. Dalton Trans 2019, 48, 12812– 12816,  DOI: 10.1039/C9DT01319A
      30
      CO2 hydrogenation by phosphorus-nitrogen PN3P-pincer iridium hydride complexes: elucidation of the deactivation pathway
      Pan, Yupeng; Guan, Chao; Li, Huaifeng; Chakraborty, Priyanka; Zhou, Chunhui; Huang, Kuo-Wei
      Dalton Transactions (2019), 48 (34), 12812-12816CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      PN3P-Ir pincer hydride complexes were synthesized and characterized as catalysts and key intermediates in the direct hydrogenation of CO2 to formate under mild conditions. The formation of a dearomatized PN3P*-Ir(I)-CO species was identified as a plausible key process accountable for the loss of catalytic activity in the CO2 hydrogenation.
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    31. 31
      Khusnutdinova, J. R.; Milstein, D. Metal-ligand cooperation. Angew. Chem., Int. Ed. 2015, 54, 12236– 12273,  DOI: 10.1002/anie.201503873
      31
      Metal-Ligand Cooperation
      Khusnutdinova, Julia R.; Milstein, David
      Angewandte Chemie, International Edition (2015), 54 (42), 12236-12273CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review; metal-ligand cooperation (MLC) has become an important concept in catalysis by transition metal complexes both in synthetic and biol. systems. MLC implies that both the metal and the ligand are directly involved in bond activation processes, by contrast to "classical" transition metal catalysis where the ligand (e.g. phosphine) acts as a spectator, while all key transformations occur at the metal center. In this Review, we will discuss examples of MLC in which 1) both the metal and the ligand are chem. modified during bond activation and 2) bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligand is not directly bound to the metal (e.g. via tautomerization). The role of MLC in enabling effective catalysis as well as in catalyst deactivation reactions will be discussed.
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    32. 32
      Alig, L.; Fritz, M.; Schneider, S. First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands. Chem. Rev. 2019, 119, 2681– 2751,  DOI: 10.1021/acs.chemrev.8b00555
      32
      First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands
      Alig, Lukas; Fritz, Maximilian; Schneider, Sven
      Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2681-2751CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodol. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.
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    33. 33
      Gunanathan, C.; Milstein, D. Bond activation and catalysis by ruthenium pincer complexes. Chem. Rev. 2014, 114, 12024– 12087,  DOI: 10.1021/cr5002782
      33
      Bond Activation and Catalysis by Ruthenium Pincer Complexes
      Gunanathan, Chidambaram; Milstein, David
      Chemical Reviews (Washington, DC, United States) (2014), 114 (24), 12024-12087CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The focus of this Review is limited to a summary of key developments in the area of bond activation and catalysis by defined ruthenium pincer complexes.
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    34. 34
      Mathis, C. L.; Geary, J.; Ardon, Y.; Reese, M. S.; Philliber, M. A.; VanderLinden, R. T.; Saouma, C. T. Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition. J. Am. Chem. Soc. 2019, 141, 14317– 14328,  DOI: 10.1021/jacs.9b06760
      34
      Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition
      Mathis, Cheryl L.; Geary, Jackson; Ardon, Yotam; Reese, Maxwell S.; Philliber, Mallory A.; VanderLinden, Ryan T.; Saouma, Caroline T.
      Journal of the American Chemical Society (2019), 141 (36), 14317-14328CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is crit. Herein we describe a thorough thermochem. anal. of the (de)hydrogenation catalyst, (PNP)Ru-Cl (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concn. of MeOH produced is independent of catalyst concn., consistent with a deactivation pathway that is dependent on methanol concn., not equivalency. Our temp.-dependent equil. studies of the dearomatized congener, (*PNP)Ru, with various H-X species (to give (PNP)Ru-X; X = H, OH, OMe, OCHO, OC(O)NMe2) reveal that formic acid equil. is approx. temp.-independent; relative to H2, it is more favored at elevated temps. We also measure the hydricity of (PNP)Ru-H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal-ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will pos. impact rational catalyst design.
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    35. 35
      Nobbs, J. D.; Sugiarto, S.; See, X. Y.; Cheong, C. B.; Aitipamula, S.; Stubbs, L. P.; van Meurs, M. Tetramethylphosphinane as a new secondary phosphine synthon. Nat. Commun. 2023, 6, 85,  DOI: 10.1038/s42004-023-00876-8
      There is no corresponding record for this reference.
    36. 36
      He, L.-P.; Chen, T.; Xue, D.-X.; Eddaoudi, M.; Huang, K.-W. Efficient transfer hydrogenation reaction Catalyzed by a dearomatized PN3P ruthenium pincer complex under base-free Conditions. J. Organomet. Chem. 2012, 700, 202– 206,  DOI: 10.1016/j.jorganchem.2011.10.017
      36
      Efficient transfer hydrogenation reaction catalyzed by a dearomatized PN3P ruthenium pincer complex under base-free conditions
      He, Li-Peng; Chen, Tao; Xue, Dong-Xu; Eddaoudi, Mohamed; Huang, Kuo-Wei
      Journal of Organometallic Chemistry (2012), 700 (), 202-206CODEN: JORCAI; ISSN:0022-328X. (Elsevier B.V.)
      A dearomatized complex [RuH(PN3P)(CO)] (PN3P = N,N'-bis(di-tert-butylphosphino)-2,6-diaminopyridine) (I) was prepd. by reaction of the arom. complex [RuH(Cl)(PN3P)(CO)] with t-BuOK in THF. Further treatment of I with formic acid led to the formation of a rearomatized complex (II). These new complexes were fully characterized and the mol. structure of complex II was further confirmed by X-ray crystallog. In complex II, a distorted square-pyramidal geometry around the ruthenium center was obsd., with the CO ligand trans to the pyridinic nitrogen atom and the hydride located in the apical position. The dearomatized complex I displays efficient catalytic activity for hydrogen transfer of ketones in isopropanol.
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    37. 37
      Benito-Garagorri, D.; Becker, E.; Wiedermann, J.; Lackner, W.; Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Achiral and Chiral Transition Metal Complexes with Modularly Designed Tridentate PNP Pincer-Type Ligands Based on N-Heterocyclic Diamines. Organometallics 2006, 25, 1900– 1913,  DOI: 10.1021/om0600644
      37
      Achiral and Chiral Transition Metal Complexes with Modularly Designed Tridentate PNP Pincer-Type Ligands Based on N-Heterocyclic Diamines
      Benito-Garagorri, David; Becker, Eva; Wiedermann, Julia; Lackner, Wolfgang; Pollak, Martin; Mereiter, Kurt; Kisala, Joanna; Kirchner, Karl
      Organometallics (2006), 25 (8), 1900-1913CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The synthesis and characterization of Mo, Fe, Ru, Ni, Pd, and Pt complexes contg. new achiral and chiral PNP pincer-type ligands based on the N-heterocyclic diamines 2,6-diaminopyridine, N,N'-di-10-undecenyl-2,6-diaminopyridine, N,N'-dihexyl-2,6-diaminopyridine, and 2,6-diamino-4-phenyl-1,3,5-triazine are reported. The new PNP ligands were prepd. conveniently in high yield by treatment of the resp. N-heterocyclic diamines with 2 equiv of a variety of achiral and chiral R2PCl compds. in the presence of base. Mo PNP complexes [Mo(PNP)(CO)3PNP] were obtained by treatment of [Mo(CO)3(MeCN)3] with 1 equiv of the resp. PNP ligand. They react with I2 to give novel seven-coordinate pincer complexes [Mo(PNP)(CO)3I]+ and [Mo(PNP)(CO)2(MeCN)I]+ depending of whether the reaction is carried out in CH2Cl2 or MeCN. With [Fe(H2O)6](BF4)2 and 1 equiv of PNP ligand in MeCN dicationic complexes [Fe(PNP)(MeCN)3](BF4)2 were obtained. The cis and trans dichloride complexes [Ru(PNP)(PPh3)Cl2] were prepd. by a ligand exchange reaction of [RuCl2(PPh3)3] with a stoichiometric amt. of the resp. PNP ligand. Cationic PNP complexes of Ni(II), [Ni(PNP)Br]Br, were synthesized by the reaction of [NiBr2(DME)] with 1 equiv of PNP ligand. In similar fashion, treatment of [M(COD)X2] (M = Pd, Pt; X = Cl, Br) with 1 equiv of PNP ligand yields the cationic square-planar complexes [M(PNP)X]X. If the reaction is carried out in the presence of the halide scavenger KCF3SO3, complexes [M(PNP)X]CF3SO3 were obtained, which are better sol. in nonpolar solvents than the analogous halide compds. X-ray structures of representative Mo, Fe, Ru, Ni, and Pd PNP complexes were detd. Finally, the use of the Pd complexes as catalysts for the Suzuki-Miyaura coupling of some aryl bromides and Ph boronic acid was examd.
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    38. 38
      Salem, H.; Shimon, L. J. W.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.; Milstein, D. Formation of Stable trans-Dihydride Ruthenium(II) and 16-Electron Ruthenium(0) Complexes Based on Phosphinite PONOP Pincer Ligands. Reactivity toward Water and Electrophiles. Organometallics 2009, 28, 4791– 4806,  DOI: 10.1021/om9004077
      38
      Formation of Stable trans-Dihydride Ruthenium(II) and 16-Electron Ruthenium(0) Complexes Based on Phosphinite PONOP Pincer Ligands. Reactivity toward Water and Electrophiles
      Salem, Hiyam; Shimon, Linda J. W.; Diskin-Posner, Yael; Leitus, Gregory; Ben-David, Yehoshoa; Milstein, David
      Organometallics (2009), 28 (16), 4791-4806CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The synthesis of a series of new ruthenium complexes based on the new PONOP ligands (1) and (10) (C5H3N-1,3-(OPR2)2: 1, R = iPr; 10, R = tBu) is presented, including the stable trans-dihydride complexes (iPr-PONOP)Ru(H)2(PPh3) (4) and (tBu-PONOP)Ru(H)2(CO) (12) and the stable Ru(0) complexes (R-PONOP)Ru(CO)2 (6, R = iPr; 15, R = tBu). A surprisingly stable 16-electron Ru(0) complex (13) was formed by deprotonation of 12 with KOtBu. Complex 13 reacts with H2 to afford the cis-dihydride complex 12a, which isomerized to the trans-dihydride 12. Complex 13 reacted with CO to afford the satd. Ru(0) complex 15. Reaction of complex 12 with water led to hydrolysis of the phosphinite PONOP ligand and rearrangement to a dimeric product (14). Reaction of the trans-dihydride complex 4 with the electrophiles PhCOCl, MeI, and MeOTf led to abstraction of one of the hydride ligands, forming the monohydride complexes (iPr-PONOP)Ru(H)(PPh3)(X) (X = Cl (2), I (8a), OTf (8b)) together with benzaldehyde in the case of 2. Similarly, 12 afforded the monohydride complexes (tBu-PONOP)Ru(H)(CO)(X) (X = Cl (11), OTf (17), I (18)). Reaction of the Ru(0) complexes 6 and 15 with water resulted in hydrolysis of the O-P bond and formation of the zwitterionic complexes 7 and 16. Treatment of 2 and 11 with MeOTf or MeI resulted in abstraction of the chloride ligand rather than the hydride, forming complexes 8a,b and 17, 18, resp. Addnl. syntheses of complexes based on ligands 1 and 10 are presented.
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    39. 39
      Ogata, O.; Nara, H.; Fujiwhara, M.; Matsumura, K.; Kayaki, Y. N-Monomethylation of Aromatic Amines with Methanol via PN(H)P-Pincer Ru Catalysts. Org. Lett. 2018, 20, 3866– 3870,  DOI: 10.1021/acs.orglett.8b01449
      39
      N-Monomethylation of Aromatic Amines with Methanol via PNHP-Pincer Ru Catalysts
      Ogata, Osamu; Nara, Hideki; Fujiwhara, Mitsuhiko; Matsumura, Kazuhiko; Kayaki, Yoshihito
      Organic Letters (2018), 20 (13), 3866-3870CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      In the presence of 0.02-0.1 mol% ruthenium PNP pincer complex I and in the presence of KOt-Bu at 150°, aryl amines underwent chemoselective and green N-methylation with methanol to give N-methylarylamines. I underwent ionization and assocn. of carbon monoxide to form cationic dicarbonyl pincer ruthenium complexes which also acted as effective catalysts for chemoselective methylation.
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    40. 40
      Herrmann, W. A. Organometallic Aspects of the Fischer–Tropsch Synthesis. Angew. Chem., Int. Ed. 1982, 21, 117– 130,  DOI: 10.1002/anie.198201171
      There is no corresponding record for this reference.
    41. 41
      Nelson, G. O.; Sumner, C. E. Synthesis and reactivity of pentamethylcyclopentadienylruthenium formyl and α-hydroxy methyl complexes. Organometallics 1986, 5, 1983– 1990,  DOI: 10.1021/om00141a009
      41
      Synthesis and reactivity of pentamethylcyclopentadienylruthenium formyl and α-hydroxy complexes
      Nelson, Gregory O.; Sumner, Charles E.
      Organometallics (1986), 5 (10), 1983-90CODEN: ORGND7; ISSN:0276-7333.
      (η-C5Me5)Ru(CO)2CH2OH (I, Cp = cyclopentadienyl), (η-C5Me5)Ru(CO)2CHO (II), and (η-C5Me5)Ru(CO)(PMe2Ph)CHO (III) were synthesized and studied as models for intermediates in the redn. of CO to org. oxygenates by transition metal catalysts. I was prepd. by NaBH3CN redn. of (η-C5Me5)Ru(CO)3+ BF4- (IV). II was synthesized by redn. of IV with [Ph3PCuH]6, but it could not be isolated in pure form. Pure cryst. III was isolated from the redn. of (η-C5Me5)Ru(CO)2(PMe2Ph)+ I- with NaBH4 in THF/H2O. Formyl complexes II and III decompd. by a radical chain mechanism. The intermediate formed from the decompn. of III underwent electron transfer with (η-C5R5)Ru(CO)2I (R = H, Me). An x-ray structure of III was completed.
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    42. 42
      Nelson, G. O. (Pentamethylcyclopentadienyl)ruthenium compounds. Synthesis and characterization of (η5-C5Me5)Ru(CO)2CH2OH. Organometallics 1983, 2, 1474– 1475,  DOI: 10.1021/om50004a046
      42
      (Pentamethylcyclopentadienyl)ruthenium compounds. Synthesis and characterization of (η-C5Me5)Ru(CO)2CH2OH
      Nelson, Gregory O.
      Organometallics (1983), 2 (10), 1474-5CODEN: ORGND7; ISSN:0276-7333.
      Treatment of (η5-C5Me5)Ru(CO)2I (C5H5 = cyclopentadienyl) in CH2Cl2 with AgBF4 under 60 psi CO produces 82% [(η-C5Me5)Ru(CO)3][BF1], which on redn. with excess NaBH3CN in MeOH gave a mixt. of (η-C5Me2)Ru(CO)2H, (η-C5Me2)Ru(CO)2CH2OMe and (η-C5Me5)Ru(CO)2CH2OH (I) . A soln. of I in THF under 5000 psi CO at 80° does not react. Reaction of I with CF3CON(SiMe3)2 gave (η-C5Me2)Ru(CO)2CH2OSiMe3.
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    43. 43
      Teets, T. S.; Labinger, J. A.; Bercaw, J. E. A Thermodynamic Analysis of Rhenium(I)–Formyl C–H Bond Formation via Base-Assisted Heterolytic H2 Cleavage in the Secondary Coordination Sphere. Organometallics 2013, 32, 5530– 5545,  DOI: 10.1021/om400810v
      43
      A Thermodynamic Analysis of Rhenium(I)-Formyl C-H Bond Formation via Base-Assisted Heterolytic H2 Cleavage in the Secondary Coordination Sphere
      Teets, Thomas S.; Labinger, Jay A.; Bercaw, John E.
      Organometallics (2013), 32 (19), 5530-5545CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      Conversion of synthesis gas, a mixt. of CO and H, into value-added Cn≥2 products requires both C-H and C-C bond-forming events. The authors' group has developed mol. complexes, based on Group 7 (Mn and Re) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, the authors explore a new mode of H2 activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H2 to form a formyl C-H bond at a Re-bound carbonyl. Cationic Re(I) complexes [ReI(P∼B:-κ1-P)(CO)5]+1, where P∼B: is a phosphine ligand with a tethered strong base, were prepd. and characterized; measurement of their protonation equil. demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepd. in 50% to 95% yields by hydride delivery to the parent pentacarbonyl complexes, and several of the free-base formyl complexes can be protonated, generating observable [ReI(P∼BH-κ1-P)(CHO)(CO)4]+1 complexes. Intramol. H bonding is evident for one of the complexes, providing addnl. stabilization to the protonated formyl complex. By measuring both the hydricity of the formyl, ΔG°H-, and its pKa, the overall free energy of H2 cleavage is calcd. from an appropriate cycle and is thermodynamically uphill in all cases (in the best case by only ∼8 kcal/mol), although significantly dependent upon the properties of the supporting ligand.
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    44. 44
      Elowe, P. R.; West, N. M.; Labinger, J. A.; Bercaw, J. E. Transformations of Group 7 Carbonyl Complexes: Possible Intermediates in a Homogeneous Syngas Conversion Scheme. Organometallics 2009, 28, 6218– 6227,  DOI: 10.1021/om900804j
      44
      Transformations of Group 7 carbonyl complexes: possible intermediates in a homogeneous syngas conversion scheme
      Elowe, Paul R.; West, Nathan M.; Labinger, Jay A.; Bercaw, John E.
      Organometallics (2009), 28 (21), 6218-6227CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      Redn. of Group VIIB metal phosphine carbonyls by lithium triethylhydroborate gave access to formyl complexes, which were isolated and characterized as borane-stabilized adducts. A variety of C-H and C-C bond forming reactions of Group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C2+ compds. Reaction of [(PPh3)2M(CO)4][BF4] with LiBHEt3 gave neutral formyl complexes [(PPh3)2(CO)3M(CHO)] (2a,b; M = Mn, Re). Attempted silylation of 2b by Me3SiOTf gave unexpected product, [(PPh3)2Re(CO)3(CHO·BF3)] (4b), as a result of fluoride abstraction from BF4- anion and trifluoroborane coordination. Reaction of 2a,b with BX3 gave the stabilized formyl complexes directly, [(PPh3)2(CO)3M(CHO·BX3)] (4a,b, X = F, M = Mn, Re; 5a,b, X = C6F5, M = Mn, Re). The metal formyl complexes 2 are substantially stabilized by coordination to boranes BX3 in the form of novel boroxycarbene complexes M(CO)3(PPh3)2(CHOBX3), but these boron-stabilized carbenes 4 and 5 do not react with hydride sources to undergo further redn. to metal alkyls. The related manganese methoxycarbene cations [Mn(CO)5-x(PPh3)x(CHOMe)]+ (x = 1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atm. of CO. The resulting acyls, cis- and trans-Mn(PPh3)(CO)4(C(O)CH2OMe), can be alkylated to form the cationic carbene complex [Mn(PPh3)(CO)4(C(OR)CH2OMe)]+, which undergoes a 1,2-hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or di-Et ether adducts of [Mn(PPh3)(CO)4]+. The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh3)(CO)4(CH2COOMe) and is protonolyzed to yield Me acetate and [Mn(PPh3)(CO)4]+; addn. of L (L = PPh3, CO) to the manganese cation regenerates [Mn(PPh3)(CO)4(L)]+. Since the original formyl complex can be obtained by the reaction of [Mn(PPh3)(CO)5]+ with [PtH(dmpe)2]+, which in turn can be generated from H2, this set of transformations amts. to a stoichiometric cycle for selectively converting H2 and CO into a C2 compd. under mild conditions.
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    45. 45
      Pan, Y.; Pan, C.; Zhang, Y.; Li, H.; Min, S.; Guo, X.; Zheng, B.; Chen, H.; Anders, A.; Lai, Z.; Zheng, J.; Huang, K. Selective Hydrogen Generation from Formic Acid with Well-Defined Complexes of Ruthenium and Phosphorus-Nitrogen PN3 -Pincer Ligand. Chem. - Asian J. 2016, 11, 1357– 1360,  DOI: 10.1002/asia.201600169
      45
      Selective Hydrogen Generation from Formic Acid with Well-Defined Complexes of Ruthenium and Phosphorus-Nitrogen PN3-Pincer Ligand
      Pan, Yupeng; Pan, Cheng-Ling; Zhang, Yufan; Li, Huaifeng; Min, Shixiong; Guo, Xunmun; Zheng, Bin; Chen, Hailong; Anders, Addison; Lai, Zhiping; Zheng, Junrong; Huang, Kuo-Wei
      Chemistry - An Asian Journal (2016), 11 (9), 1357-1360CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An unsym. protonated PN3-pincer complex in which ruthenium is coordinated by one nitrogen and two phosphorus atoms was employed for the selective generation of hydrogen from formic acid. Mechanistic studies suggest that the imine arm participates in the formic acid activation/deprotonation step. A long life time of 150 h with a turnover no. over 1 million was achieved.
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    46. 46
      Ooyama, D.; Tomon, T.; Tsuge, K.; Tanaka, K. Structural and spectroscopic characterization of ruthenium(II) complexes with methyl, formyl, and acetyl groups as model species in multi-step CO2 reduction. J. Organomet. Chem. 2001, 619, 299– 304,  DOI: 10.1016/S0022-328X(00)00705-1
      46
      Structural and spectroscopic characterization of ruthenium(II) complexes with methyl, formyl, and acetyl groups as model species in multi-step CO2 reduction
      Ooyama, Dai; Tomon, Takashi; Tsuge, Kiyoshi; Tanaka, Koji
      Journal of Organometallic Chemistry (2001), 619 (2), 299-304CODEN: JORCAI; ISSN:0022-328X. (Elsevier Science S.A.)
      The mol. structures of Ru(II) complexes with Me, formyl, and acetyl groups [Ru(bpy)2(CO)L]+ (L = CH3, C(O)H and C(O)CH3) were examd. from the viewpoint of active species in multi-step redn. of CO2 on Ru. The Me complex was prepd. by the reaction of [Ru(bpy)2(OH2)2]2+ with trimethylsilyl acetylene and fully characterized by IR, Raman, 13C NMR and single-crystal x-ray crystallog. Disorder of the Ru-CO and Ru-C(O)H bonds in the crystal structure of the formyl complex made it difficult to det. the bond parameters of the two groups accurately, but the mol. structure of the analogous acetyl complex, which was obtained by the reaction of [Ru(bpy)2(CO3)] with propiolic acid, was detd. by x-ray anal. The Ru-carbonyl (Ru-C-O) bond angles of the Me and acetyl complex with 174(1) and 175.5(5)°, resp., are in the ranges of those of previously characterized [Ru(bpy)2(CO)L]n+ (L = CO2, C(O)OH, CO and CH2OH). However, the Ru-CH3 and Ru-C(O)CH3 bond distances showed unusual relation against the stretching frequency in the Raman spectra.
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    47. 47
      Toyohara, K.; Nagao, H.; Mizukawa, T.; Tanaka, K. Ruthenium Formyl Complexes as the Branch Point in Two- and Multi-Electron Reductions of CO2. Inorg. Chem. 1995, 34, 5399– 5400,  DOI: 10.1021/ic00126a003
      47
      Ruthenium Formyl Complexes as the Branch Point in Two- and Multi-Electron Reductions of CO2
      Toyohara, Kiyotsuna; Nagao, Hirotaka; Mizukawa, Tetsunori; Tanaka, Koji
      Inorganic Chemistry (1995), 34 (22), 5399-400CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Both [Ru(bpy)2(CO)(CHO)]+ and [Ru(bpy)(trpy)(CHO)]+ (bpy = 2,2'-bipyridine; trpy = 2,2',6',2''-terpyridine) were characterized and the latter is the key-intermediate of four- and six-electron redn. of CO2 producing HCHO and CH3OH. Higher reactivity of these formyl complexes than the corresponding hydrides reveals a new pathway of HCOOH formation in electrochem. CO2 redn.
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    48. 48
      Kelly, J. M.; Vos, J. G. cis-[Ru(bpy)2(CO)H]+: A Possible Intermediate in the Photochemical Production of H2 from Water Catalyzed by [Ru(bpy)3]2+?. Angew. Chem., Int. Ed. 1982, 21 (8), 628– 629,  DOI: 10.1002/anie.198206281
      There is no corresponding record for this reference.
    49. 49
      Dombek, B. D. Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts. J. Am. Chem. Soc. 1980, 102, 6855– 6857,  DOI: 10.1021/ja00542a036
      49
      Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts
      Dombek, B. Duane
      Journal of the American Chemical Society (1980), 102 (22), 6855-7CODEN: JACSAT; ISSN:0002-7863.
      Solns. of Ru3(CO)12 in carboxylic acids catalyze the hydrogenation of CO at low pressures (100 to 340 atm) to give MeOH (obtained as its ester), with smaller amts. of ethylene glycol diester. At 340 atm and 260° a combined rate to these products of 8.3 × 10-3 turnovers s-1 was obsd. in HOAc solvent. Similar rates to MeOH are obtainable in other polar solvents, but ethylene glycol is not obsd. under these conditions except in the presence of carboxylic acids. A reaction scheme is proposed in which the function of the carboxylic acid is to assist in converting a coordinated formaldehyde intermediate into a glycol precursor.
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    50. 50
      Walker, H. W.; Ford, P. C. Synthesis and characterization of [PPN][HRu(CO)4] and a convenient route to [PPN][HOs(CO)4]. J. Organomet. Chem. 1981, 214, C43– C44,  DOI: 10.1016/S0022-328X(81)80019-8
      50
      Synthesis and characterization of [PPN][HRu(CO)4] and a convenient route to [PPN][HOs(CO)4]
      Walker, Howard W.; Ford, Peter C.
      Journal of Organometallic Chemistry (1981), 214 (3), C43-C44CODEN: JORCAI; ISSN:0022-328X.
      [PPN][HM(CO)4] (M = Ru, Os; PPN = bis(triphenylphosphine)iminium were prepd. in 60% (M = Ru) yield by treating Na2M(CO)4 with PPN+Cl- in a min. amt. of MeOH at -196° followed by filtration of -78°.
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    51. 51
      Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Süss, G. The triruthenium cluster anion [Ru3H(CO)11]−: preparation, structure, and fluxionality. J. Chem. Soc., Dalton Trans. 1979, 9, 1356– 1361,  DOI: 10.1039/DT9790001356
      There is no corresponding record for this reference.
    52. 52
      Guntermann, N.; Franciò, G.; Leitner, W. Hydrogenation of CO2 to formic acid in biphasic systems using aqueous solutions of amino acids as the product phase. Green Chem. 2022, 24, 8069– 8075,  DOI: 10.1039/D2GC02598A
      52
      Hydrogenation of CO2 to formic acid in biphasic systems using aqueous solutions of amino acids as the product phase
      Guntermann, Nils; Francio, Giancarlo; Leitner, Walter
      Green Chemistry (2022), 24 (20), 8069-8075CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Carbon capture and utilization is considered a promising approach for introducing CO2 into the chem. value chain, esp. in combination with bioenergy applications (BECCU). We report here on the catalytic hydrogenation of CO2 to formic acid in a biphasic reaction system using aq. solns. of amino acids as the product phase and possible capture solns. for biogenic CO2. The mol. structure of the ruthenium catalyst and the catalyst phase were matched through a combined design process identifying n-dodecanol (lauryl alc.) as the preferred "green" solvent. A total turnover no. (TON) of over 100 000 mol HCOOH per mol of catalyst (46 582 g HCOOH per g of Ru) with minimal contamination of the aq. phase with metal or org. solvent was obtained. The resulting aq. solns. attained almost quant. conversions with up to 0.94 mol formic acid per mol amino acid (ca. 108 g HCOOH per kg). Such solns. may find use directly, or after upgrading, in agricultural applications without the need for energy intensive and costly isolation of pure formic acid.
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    53. 53
      Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.; Liotta, C. L. The reaction of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with carbon dioxide. J. Org. Chem. 2005, 70, 5335– 5338,  DOI: 10.1021/jo0503759
      53
      The Reaction of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) with Carbon Dioxide
      Heldebrant, David J.; Jessop, Philip G.; Thomas, Colin A.; Eckert, Charles A.; Liotta, Charles L.
      Journal of Organic Chemistry (2005), 70 (13), 5335-5338CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      Amidines have been reported to react with CO2 to form a stable and isolable zwitterionic adduct but previous studies were performed in the presence of at least some water. However, spectroscopy of the reaction between DBU and CO2 detects the rapid formation of the bicarbonate salt of DBU when wet DBU is exposed to CO2 and does not indicate that an isolable zwitterionic adduct between DBU and CO2 forms either in the presence or the absence of water.
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  • Supporting Information

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