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Cite This: Acc. Chem. Res. 2018, 51, 1858−1869

Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations Weiping Liu,† Basudev Sahoo,† Kathrin Junge, and Matthias Beller*

Acc. Chem. Res. 2018.51:1858-1869. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 08/21/18. For personal use only.

Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany CONSPECTUS: Catalytic hydrogenation using molecular hydrogen represents a green and practical approach for reductions of all kinds of organic chemicals. Traditionally, in the majority of these processes the presence of transition metal catalysts is required. In this regard, noble-metal-based catalysts have largely been implemented, such as the application of iridium, palladium, rhodium, ruthenium, and others. Recently, the employment of earth-abundant 3d metals has emerged to replace the utilization of scarce noble metals because of their availability, lower cost, and often reduced toxicity. In this respect, several cobalt complexes, in the form of either molecularly well-defined or in situ-formed complexes, are receiving increasing attention from the scientific community. Importantly, the stability and reactivity of the complexes have greatly been supported by multidentate ligands under steric and/or electronic influences. For instance, tridentate or tetradentate phosphine ligands indirectly tune the reactivity of the metal center to accelerate the overall process, whereas direct participation of the ligand in pincer-type complexes through ligand−metal cooperation regulates the elementary steps in the catalytic cycle. In this Account, we emphasize specifically the advancements in cobalt-catalyzed hydrogenations using molecular hydrogen accomplished in our group. A variety of substrate classes ranging from simple molecules (e.g., carbon dioxide) to complex compounds were explored under the mild and efficient catalytic conditions. Notable examples include the reduction of carbon dioxide to afford either formates using a Co(BF4)2·6H2O/Tetraphos catalyst system or methanol employing a Co(acac)3/ Triphos complex in the presence of HNTf2. As interesting examples of the synthesis of fine chemicals, cobalt-promoted hydrogenations of nitriles to primary amines and reductive alkylations of indoles using carboxylic acids as alkylating agents are highlighted. Moreover, highly selective hydrogenations of N-heteroarenes under additive-free conditions were possible by the application of specific cobalt complexes. More recently, a set of carboxylic esters could be hydrogenated to the corresponding alcohols with high efficiency by the use of a well-defined cobalt−PNP pincer catalyst. In particular, the decent reactivity of cobalt catalysts enabled high selectivity and functional group tolerance to be achieved. Throughout our studies, it was found that the pairing of a suitable cobalt precursor and an appropriate tridentate or tetradentate phosphine ligand plays a crucial role harnessing the desired reactivity, while other monodentate and bidentate phosphine ligands showed no reactivity in these investigations. Our developments could provide supervisory information for the future exploration of cobalt-catalyzed hydrogenation reactions and other types of reactions involving cobalt catalysis. Furthermore, relevant contributions from other groups, remaining challenges, and future perspectives in this research area are also presented.

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INTRODUCTION Catalytic hydrogenations constitute a prime technology for the conversion of fine and bulk chemicals to their counterparts with reduced redox state in a green and sustainable manner.1 From an ecological perspective, advantageous over several transfer hydrogenating reagents, molecular hydrogen represents the most atom-efficient and clean reductant. To mediate and control such hydrogenations, transition-metal-based catalysts have played a critical role and allowed for numerous applications in industry and academic laboratories.2 Despite the overwhelming success of noble-metal-based catalysts in such processes, their limited availability and tedious extraction from the earth’s crust, and therefore their higher cost, encouraged us and other research groups to find suitable replacements. In this regard, growing attention has been devoted to the development of catalysts based on first-row transition metals (e.g., manganese, iron, cobalt, nickel, etc.) © 2018 American Chemical Society

because of their availability, lower cost, and often reduced toxicity.3 Indeed, over the last two decades, first-row basemetal hydrogenation catalysts, in the form of molecularly welldefined complexes or in combination with suitable ligands, have remarkably contributed to the advancement of sustainable synthesis.4 While rhodium and iridium as its 4d and 5d congeners, respectively, have a profound impact on hydrogenations, the 3d metal cobalt has been investigated to a much lesser extent. On the other hand, it is much less expensive but more available because of its production as a byproduct of copper and nickel mining. Similar to other first-row base metals, the corresponding catalysts lack predictability and control over their reactivity due to easier accessibility of various spin states and competitive Received: June 4, 2018 Published: August 9, 2018 1858

DOI: 10.1021/acs.accounts.8b00262 Acc. Chem. Res. 2018, 51, 1858−1869

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Accounts of Chemical Research single electron transfer processes.3b,5 Hence, for the development of general Co hydrogenation catalysts, the design of suitable ligand systems was and is a crucial issue.3b,4,5 In particular, cobalt−pincer complexes with unique steric and electronic properties were successfully established in organometallic chemistry.4,6 Intriguingly, some of these cobalt complexes exhibited excellent activity in catalytic hydrogenation reactions.4 In addition these well-designed catalytic complex systems, which are formed in situ from cobalt precursors and a multidentate ligands (e.g., Triphos, Tetraphos, etc.), have also been demonstrated to catalyze important hydrogenation processes.4 Nowadays, regardless of the scientific and technical challenges associated with this technology, cobalt catalysts are considered to be competitive systems for the conversion of readily available feedstocks and biorenewables, such as carbon dioxide, esters, and amides. In this Account, we discuss the recent achievements in this area in general, and apart from our works, relevant other progress will also be described.

Table 1. Cobalt-Catalyzed Hydrogenation of Sodium Bicarbonate and CO2a

entry

product

PH2/PCO2 [bar]

T [°C]

yield [%]

TON

1 2b 3 4c 5

HCO2Na HCO2Na HCO2Me HCO2Me DMF

60/0 60/0 60/30 60/30 60/30

80 120 100 100 100

94 71 83 80 73

645 3877 427 392 1308

Co(BF4)2·6H2O (28 × 10−6 mol) and L1 (28 × 10−6 mol), 20 h; NEt3 (2.0 mL) for HCO2Me; dimethylamine (0.05 mol) for DMF. HCO2Na: yields based on 1H NMR signals of HCO2Na using THF as an internal standard. HCO2Me and DMF: yields calculated by GC analysis on the basis of molproduct/molbase. bCatalyst loading: 3.49 × 10−6 mol. c[Co(H2)PP3]+BPh4− was used as the catalyst. a

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CO2/HCO3− HYDROGENATION Recently, the utilization of CO2 as a renewable and nontoxic carbon resource has attracted significant interest for the synthesis of energy-storage and value-added chemicals.7 Depending on the catalyst and reaction conditions, hydrogenation of CO2 can lead to the production of formic acid, methanol, or methane. Important research activities utilizing precious-metal catalysts for the synthesis of formates started in the 1990s.8 For instance, Nozaki and co-workers developed an Ir−pincer catalyst for hydrogenation of CO2 to formate for which the turnover frequency (TOF) and turnover number (TON) reached up to 150 000 h−1 and 3 500 000, respectively.9 Iinspired by these works, in 2012 our group accomplished the first hydrogenation of bicarbonates and CO2 in high yields and TONs using a well-defined cobalt dihydride catalyst.10 Notably, only the combination of Co(BF4)2·6H2O and Tetraphos (L1, PP3) led to catalytic turnover, while other kinds of phosphine ligands exhibited no activity. For example, the hydrogenation of sodium bicarbonate to sodium formate was achieved in excellent yield (94%) with a TON of 645 under mild conditions (Table 1, entry 1). Increasing the temperature to 120 °C improved the TON to 3877 (Table 1, entry 2). Gratifyingly, the hydrogenation of CO2 in the presence of methanol or dimethylamine provided methyl formate and N,N-dimethylformamide (DMF) in high yields and TONs (Table 1, entries 3 and 5). In mechanistic investigations, the preformed monohydride complex [Co(H)PP3] (Co-1) showed no catalytic activity in these hydrogenation reactions. However, three cobalt dihydrogen complexes that were prepared from Co-1 by addition of triflic acid and a stabilizing anion source (Scheme 1) exhibited the desired reactivity. For example, the complex [Co(H2)PP3]+BPh4− (Co-2) remained as an active catalyst showing comparable efficiency (Table 1, entry 4).10 In high-pressure 1H NMR experiments on a mixture of Co(BF4)2·6H2O and PP3 in [D6]acetone in the presence of H2 (25 bar) at 80 °C, a hydride signal appeared at δH = −10.86 ppm, resembling that of [Co(H2)PP3]+BPh4− (δH = −11.05 ppm) rather than [Co(H)PP3] (δH = −9.93 ppm) (Figure 1). On the basis of these results, an in situ-generated dihydrogen cobalt(II)−PP3 complex was presumed to be the active species in this process.10 It is worth mentioning that Heinekey and co-

Scheme 1. Preparation of the Monohydride Complex [Co(H)PP3] (Co-1) and Dihydrogen Complexes Co-2−Co4

workers reported the above-mentioned complexes as cobalt dihydride complexes11 rather than dihydrogen complexes.12 Meanwhile, Bernskoetter and co-workers reported that the cationic complex [(iPrPNP)Co(CO)2]+Cl− (Co-6) successfully catalyzes the hydrogenation of CO2 to formate, providing TONs of up to 29 000 at 45 °C and 69 bar H2 with the assistance of lithium triflate (Scheme 2a).13 Later on, Milstein and co-workers demonstrated the selective N-formylation of various amines affording formamides via CO2 hydrogenation by employing another Co−PNP complex, Co-7 (Scheme 2b).14 Methanol is of major importance for today’s chemical industry. In addition, it has wider potential for a so-called methanol economy.15 To date, it is mainly produced from fossil fuels.16 Hence, it is highly compelling to explore new, more sustainable strategies for its production.17 In 2017, the first homogeneous cobalt-catalyzed hydrogenation of CO2 to methanol was developed by our group.18 In that study, [Co(acac)3]/Triphos (L2)/HNTf2 was found to be the best combination to afford methanol with TONs of up to 50 under comparably mild conditions (100 °C, 70 bar H2) (Scheme 3). The presence of HNTf2 as a cocatalyst was crucial for the desired reactivity. In an investigation of the mechanism of this process, an inner-sphere mechanism was initially proposed on the basis of kinetic studies. However, the presence of an induction period in the kinetic profile motivated us to conduct in situ highpressure NMR experiments (Figure 2). They showed that the initially formed cobalt−phosphine species was transformed into several new catalytically active complexes. Importantly, the 1859


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Accounts of Chemical Research

Figure 1. (left) 1H NMR spectra of [Co(H)PP3] in [D8]THF and [Co(H2)PP3]+BPh4− in [D6]acetone. (right) High-pressure 1H NMR spectra of the in situ-generated catalyst solution in [D6]acetone. Reproduced with permission from ref 10. Copyright 2011 John Wiley and Sons).

Scheme 2. Co−PNP Complex-Catalyzed Hydrogenation of CO2 to Formates and Formamides

Figure 2. Concentrations of methanol, ethyl formate, and formic acid with respect to time for hydrogenation of CO2 to methanol under the catalysis of [Co(acac)3]/Triphos/HNTf2. Reproduced with permission from ref 18. Copyright 2017 John Wiley and Sons.

Scheme 3. Co(acac)3/Triphos/HNTf2-Catalyzed Hydrogenation of CO2 to Methanol

Scheme 4. Proposed Pathway for Formation of the Active Catalyst from [Co(acac)3]/Triphos

two species [Co(acac)2(Triphos)]+ ([Co-8]+) and [Co(acac)(Triphos)]+ ([Co-9]+) were detected by high-resolution electrospray ionization mass spectrometry (ESI-MS) after 1 h, Co-8 was no longer detected after 16 h.18 As depicted in Scheme 4, the formation of the active catalyst is initiated by the coordination of Triphos to Co(acac)3, generating [Co(acac)2(Triphos)]+. After successive elimination of the remaining acac ligands, the active cationic cobalt− Triphos species was obtained, which is stabilized by NTf2−. The observed induction period of this catalysis may be attributed to slow removal of the acac ligands.18 More recently, Schieweck and Klankermayer19 described a tailored cobalt/Triphos catalyst system for the selective formation of dialkoxymethane ethers from CO2 and H2 (Scheme 5). It is noteworthy that the catalyst TON for the

formation of dimethoxymethane (DMM) could be enhanced by modifying the ligand skeleton.

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NITRILE HYDROGENATION Among the numerous known protocols for the synthesis of primary amines, the catalytic hydrogenation of nitriles provides an atom-economical and practical procedure.20 The first welldefined cobalt catalyst for such reductions was developed by Milstein and co-workers in 2015 (Scheme 6a).21 Later on, the group of Fout reported that bis(carbene)-ligated cobalt−pincer complexes Co-11 and Co-12 promoted the hydrogenation of 1860


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Accounts of Chemical Research Scheme 5. Cobalt-Catalyzed Transformation of CO2/H2 with Modified Triphos

Scheme 7. Cobalt-Catalyzed Hydrogenation of Benzonitrile with Different Ligands

Scheme 6. Cobalt-Catalyzed Nitrile Hydrogenation: (a) Milstein’s Protocol Using Complex Co-10; (b) Fout’s Protocol Using Complexes Co-11 and Co-12

product in excellent yield. Moreover, diamines were obtained in good yields via hydrogenation of terephthalodinitrile and isophthalodinitrile. The Co(acac)3/L4 system was also successfully applied to more challenging aliphatic nitriles to furnish the corresponding primary amines with good to excellent efficiency (Scheme 8).23 Scheme 8. Cobalt/Tetraphos-Catalyzed Hydrogenation of Nitriles to Primary Amines

various nitriles to primary amines at low pressure (4 bar H2, 115 °C) in the presence of different additives (NaHBEt3 and Lewis acid) (Scheme 6b).22 Besides, our group has demonstrated the Co(acac)3/ Tetraphos (L4)-catalyzed hydrogenation of a broad range of (hetero)aromatic and aliphatic nitriles to primary amines.23 The activity of the catalytic system for the hydrogenation of benzonitrile was highly sensitive to the ligand structure. Among different Tetraphos (L1, L3 and L4) and Triphos (L2) ligands, L1 and L2 led to no formation of benzyl amine, while L3 showed very low activity. Interestingly, ligand L4 with cyclohexyl substituents at the phosphorus centers promoted the hydrogenation of benzonitrile to benzylamine in quantitative yield (Scheme 7). Various benzonitriles with electron-donating and electronwithdrawing groups were successfully converted to the corresponding primary amines in high yields. Notably, the reducible amide group was tolerated under the standard conditions, providing exclusively the nitrile hydrogenation

To gain more insight into the Co(acac)3/L4 system, kinetic studies were conducted at 80 and 100 °C (Figure 3). A clear induction period was detected at 80 °C but not at 100 °C. The phenomenon is ascribed to the difficult replacement of acac by L4 at lower temperature. The strong temperature dependence of the formation of the active Co/L4 catalytic species could also be observed by ESI-MS analysis.23 The accepted mechanism of nitrile hydrogenation consists of initial hydrogenation of the nitrile to the imine and subsequent reduction of the imine to the primary amine. In this respect, it is worth mentioning that the first Co−PNP complex (Co-13)catalyzed imine hydrogenation was described by Hanson and co-workers as early as 2012 (Scheme 9a).24 Later on, Wolf and von Wangelin disclosed that the homoleptic arene cobalt complex Co-14 is also able to hydrogenate imines to amines (Scheme 9b).25 1861


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Scheme 10. Cobalt-Catalyzed Hydrogenation of Carboxylic Acids

reactivity was observed in the absence of the acid additive. Usually, the acid additive provides a weakly coordinating counteranion, which is able to stabilize the active species, as well as the optimal reaction medium pKa for the hydrogenation process to take place. Prompted by this observation, various Lewis and Brønsted acids were screened, and Al(OTf)3 was found to be the best additive (Figure 4). After further optimization, the desired C-

Figure 3. Yield/time kinetic profiles for the hydrogenation of benzonitrile with Co(acac)3/L4 at 80 °C (blue line) and 100 °C (red line). Reproduced with permission from ref 23. Copyright 2017 John Wiley and Sons.

Scheme 9. Cobalt-Catalyzed Hydrogenation of Imines to Amines: (a) Hanson’s Protocol Using Complex Co-13; (b) Protocol of Wolf and von Wangelin Using Complex Co-14

Figure 4. Screening of additives in the cobalt-catalyzed reductive C− H alkylation. Reproduced with permission from ref 29. Copyright 2017 The Royal Society of Chemistry.

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alkylation product was obtained in good yield (89%) when methyl cyclopentyl ether (MCPE) was used as the solvent at 160 °C under 30 bar H2. Under the standard conditions, both aromatic and aliphatic carboxylic acids afforded the corresponding C3-alkylated indoles in good yields. Moreover, N-protected indoles also underwent a C3-alkylation process to provide the corresponding C-alkylated indoles. Hydroxyl, amino, and amido substituents are well-tolerated under these conditions (Scheme 11).29 Furthermore, this Co(acac)3/Triphos/Al(OTf)3 catalytic system was extended to reductive C3-alkenylations of indoles using carboxylic acids (Scheme 12). As a mechanistic hypothesis for this process, the carboxylic acid 5 is slowly reduced to give aldehyde 10 or the corresponding hemiacetal, which is subsequently condensed with the indole at C-3 to generate alkylated indole 11. In the next step, dehydration of 11 affords alkenylated indole 9, which upon further hydrogenation results in alkylated indole product 8 (Scheme 13).29 Of note, it is not clear at present whether the cobalt catalyst remains homogeneous or becomes a heterogeneous or nanoparticle catalyst when the temperatures is as high as 160 °C.30 As depicted in Scheme 13, the slowly formed aldehyde 10 is immediately condensed with nucleophilic indole 7, thereby

CARBOXYLIC ACID HYDROGENATION AND C-ALKYLATION Among the variety of methods applied for the synthesis of primary alcohols, direct hydrogenation of carboxylic acids provides an unusual but straightforward and atom-efficient route. The difficulty of direct hydrogenation of carboxylic acids is attributed to the lower electrophilicity of carboxylic acids or carboxylates compared with the corresponding esters and the strong binding affinity to the catalyst. The homogeneous hydrogenation of carboxylic acids to alcohols was initially accomplished by using either a ruthenium/Triphos system26 or an iridium/bipyridine system.27 Interestingly, the use of Co(BF4)2·6H2O and L2 by Elsevier, de Bruin, and co-workers in 2015 represented a remarkable step forward in carboxylic acid hydrogenation.28 The described catalyst system operates at relatively low temperature (100 °C, 80 bar H2) with TONs of up to 8000 (Scheme 10). Inspired by the latter development, we accomplished the first cobalt-catalyzed reductive alkylation and alkenylation of indoles with carboxylic acids as alkylating agents and H2 as the reductant.29 The Co(acac)3/L2-catalyzed hydrogenative Calkylation of 2-methyl-1H-indole with carboxylic acids was highly dependent on the presence of an acid cocatalyst. No 1862


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Accounts of Chemical Research Scheme 11. Co(acac)3/Triphos/Al(OTf)3-Catalyzed C3Alkylation of Indoles

Scheme 13. Plausible Mechanism for the Co(acac)3/ Triphos/Al(OTf)3-Catalyzed Hydrogenative Alkylation of Indoles with Carboxylic Acids

Scheme 14. Cobalt-Catalyzed Hydrogenations of Ketones to Alcohols: (a, b) Hanson’s Reaction Conditions Using Complexes Co-13 and Co-15; (c) Reaction Conditions of Wolf and von Wangelin Using Complex Co-14; (d) Kempe’s Reaction Conditions Using Complex Co-16; (e) Li’s Reaction Conditions Using Chiral Complex Co-17 Scheme 12. Co(acac)3/Triphos/Al(OTf)3-Catalyzed C3Alkenylation of Indoles

suppressing further reduction to the alcohol. The cobaltcatalyzed hydrogenation of aldehydes or ketones with H2 to afford alcohols has been well-documented independently by Hanson and co-workers (Scheme 14a,b),24,31 Wolf, von Wangelin, and co-workers (Scheme 14c),25 and the group of Kempe (Scheme 14d).32 Significantly, the first enantioselective hydrogenation of ketones catalyzed by chiral PNNO−cobalt complexes with molecular hydrogen was accomplished by Li and co-workers (Scheme 14e).33

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ESTER HYDROGENATION The hydrogenation of carboxylic acids or esters constitutes an efficient and direct approach to obtain the corresponding

alcohols (vide supra). In 2006, Milstein and co-workers disclosed an interesting Ru−PNP complex for this trans1863


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Accounts of Chemical Research formation.34 Nearly 10 years later, the same group developed Co-10, the first molecularly defined Co−PNN pincer catalyst that successfully promoted the hydrogenation of aliphatic esters to corresponding alcohols, although aromatic and fluorinated esters remained inactive (Scheme 15a).35 More

Scheme 16. Cobalt-Catalyzed Hydrogenation of Esters to Alcohols by Complex Co-18

Scheme 15. Cobalt-Catalyzed Hydrogenation of Esters to Alcohols: (a) Milstein’s Reaction Conditions Using Complex Co-10; (b) Jones’s Reaction Conditions Using Complex Co-15

recently, Jones and co-workers extended the scope to aryl esters by using cationic Co−PNP pincer catalyst Co-15,36 which was originally synthesized by the group of Hanson in 2012 (Scheme 15b).24 This year, our group also explored ester hydrogenations by using a set of aliphatic Co−PNP pincer catalysts.37 After optimizations, the Co−PNP pincer complex Co-18 (Figure 5)

In their original work, Milstein and co-workers discussed the involvement of Co(I) hydrides as catalytically active species in this type of reaction.35 Hence, the hydrogenation of methyl benzoate was performed in the presence of [(PNP)CoICl] (Co-19) and delivered benzyl alcohol in 89% yield (Scheme 17a). Additionally, in sharp contrast to manganese−pincer and iron−pincer complexes, ligation of a π-accepting CO ligand in the cobalt−pincer complex showed a detrimental effect on the reaction outcome (Scheme 17a). This could be explained by the difficulty of forming Co(I) species due to the strong πaccepting ability of CO.35 Furthermore, a comparison study of standard complex Co-18 and N-methylated complex Co-23 for the hydrogenation of methyl benzoate and methyl n-octanoate led to the proposal of an outer-sphere mechanism through ligand−metal cooperation (Scheme 17b). Nevertheless, DFT calculations on this reaction mechanism could not completely exclude the possibility of an inner-sphere mechanism.37 Aside from the remarkable success of cobalt−pincer complexes, again the Co(BF4)2·6H2O/L2 system successfully promoted the hydrogenation of various aliphatic and aromatic carboxylic acid esters to their alcohols in moderate to excellent yields.28

Figure 5. X-ray structure of complex Co-18. H atoms have been omitted for clarity, and the thermal ellipsoids are drawn at 30% probability. Reproduced with permission from ref 37. Copyright 2017 John Wiley and Sons.

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AMIDE/IMIDE REDUCTION The selective catalytic (mono)reduction of cyclic imides, including phthalimides and succinimides, represents an attractive transformation in organic synthesis because the resulting isoindolines and γ-lactams are heavily featured in pharmaceuticals and agrochemicals. However, in the case of phthalimides, such methods are often accompanied by arene ring hydrogenation as well as over-reduction and even competitive C−N bond cleavage. In this respect, the development of a Ru(acac)3/L2 catalyst in the presence of methanesulfonic acid (MSA) as a cocatalyst is worth noting. It promoted the reductive alkoxylation and amination of cyclic

was identified as the most active precatalyst. Catalyst poisoning tests in the presence of mercury or triphenylphosphine did not show any influence on the reactivity, indicating a homogeneous pathway in these reactions. The hydrogenation of a broad range of aliphatic and (hetero)aromatic esters was possible under the optimal conditions (Scheme 16). Previously unexplored fluorinated carboxylic acid esters and a variety of substrates with electron-rich or electron-poor substituents, including the hydrogenation-sensitive alkene group and Nheteroarenes, were successfully converted to the corresponding alcohols.37 1864


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Accounts of Chemical Research Scheme 17. Cobalt-Catalyzed Hydrogenation of Esters to Alcohols: (a) Control Experiments Using Complexes Co-19 to Co-22; (b) Comparison of the Reactivities of Complexes Co-18 and Co-23

Scheme 18. Catalytic Reductive Alkoxylation of Cyclic Imides by the Cobalt/Triphos System

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N-HETEROARENE HYDROGENATION The interest in selective hydrogenation of N-heteroarenes to the completely or partially reduced N-heterocycles stems from their prevalence in numerous natural products, pharmaceuticals, agrochemicals, and materials. However, because of the resonance stabilization, the hydrogenation of N-heterocyclic compounds, compared with that of polar functional groups such as nitriles, esters, and amides, still constitutes a scientific and technical challenge. The majority of known homogeneous transition-metal-catalyzed methods for such reduction processes rely on noble-metal-based catalysts and often require additional cocatalysts/additives for substrate or catalyst activation. Recently, Jones and co-workers demonstrated that cationic cobalt pincer complex Co-15 successfully promotes the reversible (de)hydrogenation of N-heteroarenes (Scheme 20).40 Complementary to that work, our group reported a different catalyst system composed of Co(BF4)2·6H2O and L3 that efficiently catalyzed the selective hydrogenation of quinoline (Scheme 21).41 Importantly, the structural feature of L3 is crucial for the desired reactivity, since other Triphos or Tetraphos derivatives (L1, L2, and L4) did not catalyze the reaction (Scheme 21a). In that study, the cationic complex Co26 was also prepared from a 1:1 mixture of Co(BF4)2·6H2O and L3 in THF, and the fluoride ligand was derived from the counterion BF4− (Figure 7). Analogous to the in situ-formed catalyst system, complex Co-26 promoted the reaction with comparable efficiency (Scheme 21b). A variety of substituted N-heteroarenes were successfully hydrogenated to the corresponding partially reduced N-

imides in an efficient manner.38 Following that work, the pairing of Co(BF4)2·6H2O and L2 avoided the additional acid cocatalyst and allowed the reductive alkoxylation reactions to be performed under milder conditions.39 Various substituted symmetrical and asymmetrical phthalimides and succinimides were converted to the corresponding hydroxyl-substituted isoindolines and γ-lactams via selective reductive alkoxylation (Scheme 18). Particularly interesting is the excellent regioselectivity observed for some nonsymmetric substrates. The kinetic analysis based on the reaction of Nmethylphthalimide (15a) at three different hydrogen pressures (10, 20, and 30 bar) revealed the strong dependence of the reaction rate on the initial hydrogen pressure (Figure 6a). However, the reaction with the hemiaminal intermediate of Nmethylphthalimide (17a) at a hydrogen pressure of 20 bar was almost 4 times faster compared with that of 15a (Figure 6b). This observation indicated that reduction of the phthalimide to the corresponding hemiaminal intermediate is likely the ratelimiting step of the reaction. Furthermore, the cobalt species [Co(L2)2(H)2] (Co-24) detected by ESI-MS was presumed to serve as the active catalyst. An additional species present in the ESI-MS spectra, [Co(L2)(CH3CN)(16a)] (Co-25), showed that the final product likely forms a resting state intermediate. Hence, the plausible mechanism for this reaction involves initial reduction of the amide carbonyl to the hemiaminal (17a) followed by protonation of the so-formed hydroxyl group. Displacement of the protonated hydroxyl group by the alcohol releases the end product, albeit passing through a resting state (Scheme 19). 1865


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Scheme 20. Cobalt-Catalyzed Hydrogenation of Quinolines by Complex Co-15

Scheme 21. Cobalt-Catalyzed Hydrogenation of Quinoline: (a) Variation of the Phosphine Ligand; (b) Comparison with the Reactivity of Molecular Complex Co-26

Figure 6. (a) Yield/time profiles for the formation of product 16a from 15a under different pressures of H2: (A) 30 bar, (B) 20 bar, (C) 10 bar. (b) Yield/time profile for the formation of product 16a from 17a under 20 bar H2. Reproduced with permission from ref 39. Copyright 2017 The Royal Society of Chemistry.

Scheme 19. Plausible Mechanism for Reductive Methoxylation of Phthalimide by the Cobalt/Triphos System Figure 7. X-ray structure of the cation of the complex [CoF(L3)](BF4) (Co-26).41 The BF4− counterion and H atoms have been omitted for clarity, and the thermal ellipsoids are drawn at 30% probability. Reproduced with permission from ref 41. Copyright 2017 John Wiley and Sons.

esters, and amides) remained stable. The potential of the developed methodology was further showcased in the synthesis of biologically active (±)-galipinine (Scheme 22).41

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CONCLUSION AND OUTLOOK For some time homogeneous cobalt catalysts have been considered “old-fashioned” and were associated with the original hydroformylation process and hydrogenation reactions, which often need drastic conditions (>150 °C; > 100 bar) to take place. In contrast, in nature cobalt is the active

heterocyclic compounds in moderate to excellent yields with good selectivity (Scheme 22). Under the employed conditions, excellent functional group tolerance was noticed. For instance, other reducible functional groups (e.g., alkene, alkyne, Cl, Br, 1866


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feedstocks and renewables to platform chemicals and energy storage materials. For example, nitriles, carboxylic acid esters, and even challenging carboxylic acids have been efficiently hydrogenated to fundamentally important primary amines and alcohols, respectively. In addition, the reported cobalt catalysts enabled the conversion of abundant carbon dioxide to formic acid and its derivatives or even methanol as energy-storage materials. Thus, the utilization of cobalt-based complexes is clearly enlarging the chemical toolbox of catalysts. In this respect, what are the main challenges in the future? Obviously, it would be interesting to develop more active Co catalysts for hydroformylations and other C−C coupling reactions. Furthermore, the activities of the complexes described herein have to be improved by 2−4 orders of magnitude to be of significant industrial relevance in, e.g., methanol synthesis. Apart from that, an increased fundamental understanding of the individual catalytic steps and their control is also desired. Academically, asymmetric hydrogenations and related transformations are interesting, too. Because of all these challenges, the future looks bright for cobalt. The reader might consider participating in that.

Scheme 22. Selective Hydrogenation of N-Heteroarenes by the Cobalt/Tetraphos System

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weiping Liu: 0000-0002-1064-7276 Basudev Sahoo: 0000-0002-9746-9555 Matthias Beller: 0000-0001-5709-0965 Author Contributions †

W.L. and B.S. contributed equally.

Notes

The authors declare no competing financial interest. center of coenzymes called cobalamins (e.g., Vitamin B12), which work under mild conditions in a very selective manner. Hence, it is an essential trace mineral for animals and also a micronutrient for bacteria, algae, and fungi. In recent years, we have experienced significant advancements in hydrogenation chemistry with ligand-supported cobalt catalysts in the form of either molecularly well-defined complexes or ones formed in situ from a mixture of the cobalt precursor and a suitable ligand. In this Account, we have summarized specifically most of our achievements in this area but also important contributions from other groups. Compared with the vast number of ligand libraries available for noble-metal catalysis, the situation for cobalt is still different. To date, only privileged multidentate ligands such as pincer, Triphos, and Tetraphos derivatives induce significant activity for important hydrogenation reactions. Interestingly, the presence of CO ligands, which are essential for controlling the reactivity of Ru−pincer and Fe−pincer catalysts, inhibits the activity of related cobalt−pincer-based catalysts. Furthermore, the desired reactivity exhibited by the cobalt/Triphos and cobalt/Tetraphos systems is not trivial; rather, it is highly dependent on the cobalt precursor as well as the structural skeleton of the utilized multidentate ligand. Moreover, the presence of additives plays a critical role in precatalyst activation. Therefore, the development of new and improved cobalt catalysts with suitable ligand environments remains difficult. Nevertheless, cobalt catalysts developed in recent years have enabled the conversion of readily available

Biographies Weiping Liu received his M.Sc degree in 2012 under the supervision of Prof. Zhiping Li at Renmin University of China. In 2016 he obtained his Ph.D. degree from Georg-August-Universität Göttingen under the supervision of Prof. Lutz Ackermann. After a short postdoctoral stay in the same group, he joined the group of Prof. Matthias Beller (LIKAT) as a postdoctoral research fellow in 2017. Basudev Sahoo received his B.Sc. degree in chemistry from the University of Calcutta in India in 2009 and his M.Sc. degree from the Indian Institute of Technology Kanpur in 2011. In 2015 he obtained his Ph.D. degree from Westfälische Wilhelms-Universität Münster under the supervision of Prof. Frank Glorius. In 2015 he joined the group of Prof. Matthias Beller (LIKAT) as a postdoctoral research fellow. Kathrin Junge received her Ph.D. degree in chemistry from the University of Rostock in 1997, working with Prof. E. Popowski. After a postdoctoral position in the Max-Planck group of Uwe Rosenthal, she joined the group led by Matthias Beller in 2000. Since 2008 she has been the group leader for homogeneous redox catalysis at LIKAT. Her current main interest is the development of environmentally benign and efficient catalytic reactions based on nonprecious metals. Matthias Beller studied chemistry at the University of Göttingen in Germany, where he completed his Ph.D. thesis in 1989 in the group of L.-F. Tietze. After spending a year with K. B. Sharpless at MIT, he worked in industry from 1991 to 1995 and then moved to the Technical University of Munich as a professor of inorganic chemistry. 1867


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In 1998, he relocated to Rostock to head the Institute for Organic Catalysis, which in 2006 became the Leibniz-Institute for Catalysis. He has won several awards, including the Leibniz Prize of the DFG, the German Federal Cross of Merit, the Paul Rylander Award of the Organic Reaction Catalysis Society of the USA, and the Karl Ziegler Prize. Besides, he received the first European Prize for Sustainable Chemistry and was appointed as an honorary doctor of the University of Antwerp and the University of Rennes.

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ACKNOWLEDGMENTS The authors thank the State of Mecklenburg-Western Pomerania for financial support and those who contributed to the original reports discussed in this Account.

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REFERENCES

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