Article pubs.acs.org/IC
Catalytic Formylation of Primary and Secondary Amines with CO2 and H2 Using Abundant-Metal Catalysts Mohammad A. Affan and Philip G. Jessop* Department of Chemistry, Queen’s University, 90 Bader Lane, Chernoff Hall, Kingston, Ontario K7L 3N6, Canada S Supporting Information *
ABSTRACT: Catalytic hydrogenation of CO2 is an efficient and selective way to prepare formic acid derivatives, but most of the highly active catalysts used for this purpose require precious metals. In this study, in situ abundant-metal complexes have been evaluated as potential catalysts for CO2 hydrogenation to prepare formamides, including N-formylmorpholine, 2-ethylhexylformamide, and dimethylformamide, from the corresponding amines. From these initial screening results, the most active catalysts for these reactions were found to be MX2/dmpe in situ catalysts (M = Fe(II), Ni(II); X = Cl−, CH3CO2−, acac−; dmpe = 1,2bis(dimethylphosphino)ethane) in DMSO. The optimal reaction conditions were found to be 100−135 °C and a total pressure of 100 bar. Morpholine was formylated with a TON value of up to 18000, which is the highest TON for the hydrogenation of CO2 to formamides using any abundant-metal−phosphine complex. With an appropriate selection of catalyst and reaction conditions, >90−98% conversion of amine to formamide could be achieved.
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INTRODUCTION Carbon dioxide (CO2) is an abundant, stable, and inexpensive carbon feedstock of negligible toxicity. It is particularly useful as a replacement for toxic carbonyl-introducing reagents such as carbon monoxide and phosgene.1 One of the most challenging aspects of using CO2 as a source of carbon or even carbonyl, however, is its thermodynamic and kinetic stability. For the activation of CO2 to take place, therefore, a certain amount of energy must be supplied or a highly reactive reagent must be used. Use of an efficient catalyst makes it possible to overcome the kinetic barriers.2 One of the earliest examples was of the conversion of carbon dioxide and secondary amines to formamides with Raney nickel catalyst, producing up to 2 mol of amide/mol of Ni, reported by Farlow and Adkins.3 In the 1970s, studies on the catalytic homogeneous transformation of CO2 to formic acid and its derivatives were conducted by Inoue et al.4 and Trzebiatowska et al.5 Since then, many transition-metal complexes have been synthesized and evaluated as catalysts in various homogeneous catalytic transformations of carbon dioxide.6−8 However, in most cases, rare, expensive, and often toxic metals such as ruthenium, rhodium, and iridium complexes have been used as active catalytic systems for these transformations.9−11 In comparison to precious-metal catalysts, earth-abundant first-row transition metals, especially Mn, Fe, Co, Ni, and Cu, make effective catalysts for other large-scale chemical processes, and we see no reason why they should not be effective for CO2 hydrogenation as well.12 In 2003, the Jessop group screened catalysts for the hydrogenation of CO2 to formic acid and found active catalysts containing Cr, Fe, Co, Ni, Mo, W, Nb, and In. The most active catalyst was NiCl2(dcpe), with a TON value of 4400.13 Haynes et al.14 tested the catalytic activity of (Ph3P)3CuCl for CO2 hydrogenation in the presence of © 2017 American Chemical Society
dimethylamine to form N,N-dimethylformamide (DMF); however, the TON, which varied up to 900, remained far lower than that obtained using second- or third-row transition metal complexes, such as phosphine complexes of Rh, Ru, Ir, Pd, and Pt. In 2010, Federsel et al.15 reported CO2 hydrogenation using an efficient iron catalyst system for the reduction of carbon dioxide and bicarbonates to generate formates, alkyl formates, and formamides. A combination of Fe(BF4)2·6H2O as an iron source and the tetradentate ligand PP3 (P(CH2CH2PPh2)3) led to the formation of well-defined iron hydride complexes, [FeH(PP3)]BF4 and [FeH(H2)(PP3)]BF4. These complexes were utilized as catalysts for the hydrogenation of CO2 at elevated temperatures and pressures, affording formates, alkyl formates, and formamides in high yields, with TON values of up to 727 for dimethylformamide and a TON value of 373 for N-formylpiperidine. Federsel et al.16 then reported a cobalt complex catalyst for the hydrogenation of CO2 to make DMF with a TON value of up to 1254. In the same year, Ziebart et al.17 prepared a [Fe(P3P)F]BF4 (P3P = (C6H4PPh2)3P) complex which was catalytically active for the hydrogenation of CO2 to DMF and diethylformamide from the corresponding amines. Maximum TON values of 5104 for DMF and 2114 for diethylformamide were observed. Very recently, Zhang et al.18 reported that a combination of an Fe PNP complex with a Lewis acid cocatalyst could produce formic acid with a TON value of up to 59000. Despite the great progress made so far, abundant-metal catalysts still lag behind precious-metal catalysts in terms of TONs and TOFs. Herein, we report the catalytic activity of Received: May 17, 2017 Published: June 6, 2017 7301
DOI: 10.1021/acs.inorgchem.7b01242 Inorg. Chem. 2017, 56, 7301−7305
Article
Inorganic Chemistry Fe(II) and Ni(II) dmpe complexes for the hydrogenation of CO2 in the presence of amines to generate the corresponding formamides with high TON.
Table 1. Catalytic Formylation of Morpholine Using CO2 and H2a
RESULTS AND DISCUSSION Formylation of Morpholine. For our initial screening, 17 different non-precious-metal salts were screened in 40 combinations with phosphine ligands (top row in Figure 1)
Metal:Ligand:Amine = 1:2:20000 FeCl2 2000 NiCl2 2500 NiCl2·6H2O 1600 Ni(OAc)2·4H2O 14600 Ni(acac)2 18000 Metal:Ligand:Amine = 1:2:100000 Ni(acac)2 16000
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entry 1 2 3 4 5 6
metal precursor
TON
yield (%) 10 13 8 73 90 16
Reaction conditions: metal salt (0.30 μmol), dmpe (0.60 μmol), and morpholine (6.08 mmol) in 5 mL of DMSO; T = 100 °C; t = 21 h; CO2 pressure 60 bar, H2 added until total pressure of 100 bar; stirring rate 450 min−1. The TON value is an average of the results from two trials. Yield and TON were determined by NMR spectroscopy. a
Supporting Information). The hydride donor strength, which may be important for the attack on the CO2 carbon atom, is influenced by the metal, the ligand substituents, and the size of the chelate bite angle of the diphosphine ligands. We have considered four diphosphine ligands such as dmpe (107°), depe (115°), dmpp (121°), and dppe (125°) in this work. The most effective of these diphosphines, dmpe, has the smallest bite angle, combined with small and electron-donating substituents, which should combine to create high hydride donor strength and minimal steric encumbrance. The hydricity of [HNi(diphosphine)2]+ has been reported19 to be greater for dmpe than for dppe or dmpp. The higher catalytic activities of 3d metal dmpe/dppe/dppp complexes in comparison to 3d metal PPh 3 complexes parallel the results obtained with [RuCl2(dppe)2] and [RuCl2(PPh3)4].20 Formylation of 2-Ethylhexylamine. Another series of experiments, for the formylation of 2-ethylhexylamine, was performed using catalysts prepared in situ from 3d metal salts and added phosphines. The synthesis of 2-ethylhexylformamide was attempted by reacting CO2 with H2 in the presence of 2ethylhexylamine using a metal salt plus dmpe ligand as catalyst precursor in DMSO heated at 130−135 °C in an oil bath (Scheme 2). The reaction was initiated at a CO2 pressure of 60
Figure 1. Structures of the ligands used in this work.
for the catalytic hydrogenation of CO2 to formamides. The catalysts were prepared in situ from 3d metal salts and added ligands, which were allowed to react with each other in dry DMSO in the presence of morpholine under CO2 pressure (10−15 bar) at room temperature for 10−15 min. The mixture was then heated to 100 °C in an oil bath for 20−30 min, after which CO2 (45−50 bar) was introduced into the system and then pressurized with H2 to make a total pressure of 100 bar (Scheme 1). Scheme 1. Formylation of Morpholine
Scheme 2. Formylation of 2-Ethylhexylamine
Out of the 17 metal salts screened, 13 metal salts showed catalytic activity toward the preparation of N-formylmorpholine. The preliminary screening results for the formylation of morpholine using 3d transition-metal salts and phosphine ligands are summarized in Table S1 in the Supporting Information. Of the catalytically active metals (Fe, Co, Ni, Cu, and Zn), the most active were Ni(II), Fe(II,III), and Co(III). The best screening salt/ligand combinations for the formylation of morpholine using Fe(II)/Ni(II) precursors and dmpe ligand were retested at larger metal to amine ratios. TON values varying from 1600 to 18000 were obtained (Table 1). Blank tests with FeCl2 in the absence of a phosphine ligand were performed, and no formylation reaction occurred. Similarly, blank tests with triphenylphosphine or dmpe without any metal salt did not lead to any formylation products. To clarify the effect of ligand choice on the formylation of morpholine, dimethylamine, and 2-ethylhexylamine, several monodentate and bidentate phosphine and hemilabile ligands were investigated (Figure 1). Among these, dmpe displayed higher activities for the formylation reaction of morpholine in comparison to the other phosphine ligands (see Table S1 in the
bar followed by pressurization to a total of 100 bar with H2 (Scheme 2). In this case, the reaction could be carried out at a higher temperature (ca. 130−135 °C), because 2-ethylhexylamine has a boiling point of 165 °C. This increase in reaction temperature also leads to higher TON values. Having found Fe(II)/Ni(II)/dmpe to be the best catalyst for formylation of morpholine (Table S1 in the Supporting Information), we proceeded to examine the catalytic activity of the same catalysts for the formylation of 2-ethylhexylamine with CO2 and H2 (Table S2 in the Supporting Information). The best of the screening results for the formylation of 27302
DOI: 10.1021/acs.inorgchem.7b01242 Inorg. Chem. 2017, 56, 7301−7305
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Inorganic Chemistry
formic acid and clusters of DMSO provide the driving force for high equilibrium yields of formic acid. Formylation of Dimethylamine. Eleven different abundant-metal precursors were screened for the catalytic hydrogenation of CO2 to DMF (Scheme 3) using the same
ethylhexylamine using Fe(II)/Ni(II)/dmpe in situ catalyst are summarized in Table 2. These results indicate that the synthesis Table 2. Catalytic Formylation of 2-Ethylhexylamine using CO2 and H2 cntry 1 2 3 4 5 6 7
metal precursor
TON
yield (%)
Metal:Ligand:Amine = 1:2:100a FeCl2 58 Fe(OAc)2 58 Co(OAc)2·4H2O 40 NiCl2 32 Ni(OAc)2·4H2O 70 Metal:Ligand:Amine = 1:2:2000b FeCl2 1580 Ni(OAc)2·4H2O 1610
Scheme 3. Formylation of Dimethylamine
58 58 40 32 70 79 81
procedures as those for formylation of morpholine, although, for convenience, dimethylammonium dimethylcarbamate was used instead of free dimethylamine. Out of the 11 systems screened, 8 metal complexes showed catalytic activity toward the preparation of DMF. The preliminary screening results for the formylation of dimethylamine are shown in Table S3 in the Supporting Information, and the best of the screening results are summarized in Table 4. Again, we find that the best catalysts are formed from Ni(II) salts, especially if those salts are halide-free.
a
Reaction conditions unless specified otherwise: metal salt (30.5 μmol), dmpe (61.0 μmol), 2-ethylhexylamine (3.05 mmol), and DMSO (0.5 mL); T = 135 °C; t = 21 h; CO2 pressure 60 bar, and H2 added until total pressure 100 bar; stirring rate 450 min−1. The TON value is an average of the results from two trials. bReaction conditions: metal salt (0.36 μmol), dmpe (0.72 μmol), and 2-ethylhexylamine (0.72 mmol) in 5 mL of DMSO.
of 2-ethylhexylformamide could be achieved using abundant 3d metal phosphine complexes as catalyst with CO2, H2, and 2ethylhexylamine as the starting materials. The formation of white solid carbamate was not observed upon opening the reaction vessel during the hydrogenation of CO2 at 130−135 °C for 21 h. In the synthesis of 2-ethylhexylformamide, no trace of free formic acid in the reaction was detected by FTIR or 1H NMR spectroscopy. No formamides were found to have been formed when AlCl3, MnBr2, CuI, ZnBr2, and InCl3 were used as metal precursors. We have also studied the catalytic hydrogenation of CO2 with morpholine and 2-ethylhexylamine under solvent-free conditions (Table 3).
Table 4. Catalyst Screening for Formylation of Dimethylaminea entry 1 2 3 4 5 6 7 8
TON
9
amine
with DMSO
without DMSO
FeCl2 Ni(OAc)2·4H2O FeCl2 Ni(OAc)2·4H2O
morpholinea morpholinea 2-ethylhexylamineb 2-ethylhexylamineb
52 70 54 70
35 50 36 48
ligand
TON a
Table 3. Solvent Effect on the Hydrogenation of CO2 to Formamides metal salt
metal precursor
Metal Salt:Ligand:NHMe2 = 1:2:200 FeCl2 none none dmpe Fe(OAc)2 dmpe Fe(BF4)2·6H2O dmpe NiBr2·xH2O dmpe Ni(OAc)2·4H2O dmpe Ni(acac)2 dmpe Cu(OAc)2 dmpe Metal Salt:Ligand:NHMe2 = 1:2:2000b Ni(OAc)2·4H2O dmpe
0 0 108 78 92 142 138 86 632
a
Reaction conditions unless specified otherwise: metal salt (39.1 μmol), dmpe (78.2 μmol), dimethylammonium dimethylcarbamate (3.91 mmol) in 0.5 mL of DMSO; T = 100 °C; t = 21 h; CO2 pressure 60 bar, H2 added until total pressure 100 bar; stirring rate 450 min−1. The TON value is an average of the results from two trials. bReaction conditions: metal salt (8.0 μmol), dmpe (16.0 μmol), and dimethylammonium dimethylcarbamate (16.0 mmol) in 5 mL of DMSO.
a
Reaction conditions unless specified otherwise: metal:amine = 1:100; metal salt (57.2 μmol), dmpe (114.4 μmol), morpholine (5.72 mmol), DMSO (0.5 mL); T = 100 °C; t = 21 h; CO2 pressure 60 bar and H2 added until total pressure 100 bar. The TON value is an average of the results from two trials. bReaction conditions: metal salt (30 μmol), dmpe (61.0 μmol), 2-ethylhexylamine (3.05 mmol), DMSO (0.5 mL), T = 135 °C.
To understand these catalysts better, we attempted to isolate, characterize, and test the catalytic activity of Ni(II) complexes containing both acetylacetonate and dmpe ligands. Unfortunately, every attempt failed, suggesting that such complexes may be unstable. We did obtain and crystallographically characterize new Ni(II) complexes containing either acetylacetonate or DMPE ligands but never both. None of these new complexes were particularly active for CO2 hydrogenation. Because they are not relevant to the goals of the present work, these new complexes will be reported separately. To determine whether the actual catalyst might be heterogeneous, we looked for signs of precipitated metal at the end of each reaction (there was none) and performed a hot
It is clear from the results that DMSO played an important role for the efficient catalytic hydrogenation of CO2 to formamides, allowing a TON value higher than that observed without added solvent. The action of DMSO as a hydrogen bond acceptor in this reaction was reported by Moret and coworkers.21 Recently, Rohmann et al.22 investigated the role of DMSO solvent for the synthesis of formic acid by DFT calculations. It was found that the hydrogen bonds between 7303
DOI: 10.1021/acs.inorgchem.7b01242 Inorg. Chem. 2017, 56, 7301−7305
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Inorganic Chemistry filtration test during one formylation of morpholine. After 21 h, the hot reaction mixture was filtered through diatomaceous earth. The filtrate was found to be catalytically active, suggesting that the catalyst is homogeneous. In summary, the in situ catalytic activity of complexes prepared from combinations of an abundant-metal salt and a phosphine ligand has been investigated for the catalytic formylation of morpholine, 2-ethylhexylamine, and dimethylamine using CO2 and H2. Fe(II) and Ni(II) phosphine complexes showed good catalytic activity. The catalytic formylation of morpholine with CO2 and H2 has been found to exhibit a TON value of 18000. To the best of our knowledge, this is the highest TON reported using any abundant-metal catalyst for formylation using CO2. These studies of CO2 hydrogenation are continuing, with current efforts focused on the synthesis, structural identification, and testing of isolated Ni catalyst precursors. So far, repeated attempts at isolating Ni species simultaneously containing both acac and dmpe ligands have been unsuccessful.
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Characterization of Products. All of the synthesized formamide products are known, and their characterization data are in good agreement with those reported in the literature.16,20,23 Hot Filtration Test. This test relies on a comparison of catalytic activity before and after filtering the active catalyst solution.24,25 In this experiment, in a glovebox under a N2 atmosphere, precatalyst Ni(acac)2 (0.019 mmol), dmpe (0.058 mmol), morpholine (9.70 mmol), and 1 mL of DMSO were placed in a glass vial within a 31 mL steel pressure vessel. This vessel was closed, removed from the glovebox, connected to a CO2 line, and flushed with CO2 three times to replace most of the nitrogen. The reagents were stirred and allowed to react with each other in dry DMSO under CO2 pressure (10 bar) at room temperature for 10−15 min. The mixture was heated to 100 °C in an oil bath for 20−30 min and then pressurized to 60 bar of CO2. H2 was added until the total pressure was 100 bar. The reaction was allowed to proceed for 21 h. After this time, the vessel was cooled to about 50 °C and cleaned to remove residual heating oil. Inside a glovebox, the vessel was depressurized slowly and opened, and the hot solution was passed through a short column of diatomaceous earth in a fritted-glass filter. DMSO (1 mL) was used to rinse the filter. A small portion of the filtrate was tested by NMR spectroscopy to confirm the formylation. The remainder of the filtrate was transferred into a new glass vial in the steel vessel, fresh morpholine (9.70 mmol) was added, the vessel was closed, and the hydrogenation of CO2 was resumed in the same way mentioned above. After 21 h, the vessel was cooled in an ice bath and depressurized, and the reaction vessel was opened under air. The filtration process was done through a Celite filter again. Finally, the filtrate was analyzed by 1H NMR spectroscopy with cyclohexane as an internal standard, confirming the formylation of the second batch of morpholine.
EXPERIMENTAL METHODS
General Considerations. All metal precursors and ligands were purchased from Sigma-Aldrich, Alfa Aesar, and Strem Chemicals and used without further purification, unless otherwise specified. All manipulations involving air- and moisture-sensitive compounds were carried out using high vacuum and standard Schlenk techniques or in a glovebox under N2. The organic solvents were distilled under argon after drying over an appropriate drying agent. All solvents were degassed by freeze−pump−thaw cycles before use. CO2 (research grade, 4.8) and H2 (ultrahigh purity, 5.0) were supplied by Praxair (Canada) and used without further purification. All reagents for the catalytic reactions were loaded into a 31 or 160 mL high-pressure reaction vessel in the glovebox. Aliquots were removed from the crude reaction mixtures immediately after depressurization following the specified reaction time and analyzed by a 400 MHz Brüker Advance NMR spectrometer equipped with an autosampler. Chemical shifts are indicated in ppm relative to tetramethylsilane (TMS). FT-IR spectra were recorded as KBr disks or in CHCl3 as a solvent with a Bruker FTIR spectrophotometer. Standard Procedure for the Hydrogenation of Carbon Dioxide. The catalysts were prepared in situ from 3d metal(II) salts and added phosphines. The reactions were performed in glass vials within a 31 or 160 mL Parr vessel. The gas pressure was measured by a pressure gauge, and a burst disk was installed for safety. The reaction vessel (31 or 160 mL) was loaded with a glass vial containing a stir bar, 57 μmol of the metal salt, monophosphine ligand (228.8 μmol), diphosphine (114.2 μmol), and 0.5 mL of morpholine in 0.5 mL of DMSO under an inert atmosphere in a nitrogen-filled glovebox. The vessel was closed, removed from the glovebox, connected to a CO2 line, and flushed three times with CO2 to remove the nitrogen. The reagents were then allowed to react with each other in dry DMSO under CO2 pressure (10−15 bar) at room temperature for 10−15 min. This time delay was intended to allow ligand/metal salt reactions to begin before the introduction of H2. The mixture was then heated to 100 °C in an oil bath for 20−30 min, and then the system was pressurized with 60 bar of CO2. H2 was added until the total pressure was 100 bar. The 20−30 min delay is required in order for the vessel to attain the desired temperature before the final gas addition. The reaction was then allowed to proceed for 21 h. After this time, the vessel was cooled in an ice bath and depressurized. The crude product was filtered through diatomaceous earth and analyzed by NMR spectroscopy using the appropriate deuterated solvents, with cyclohexane as an internal standard. The determination of the extent of conversion of dimethylamine to DMF was not possible due to the volatility of the unreacted amine.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01242. Preliminary screening data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for P.G.J.:
[email protected]. ORCID
Philip G. Jessop: 0000-0002-5323-5095 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Sciences and Engineering Research Council, CREATE/413798-2012, and the Canada Research Chairs Program for financial support.
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REFERENCES
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