An Efficient Ruthenium Catalyst Bearing Tetradentate Ligand for Hydrogenations of Carbon Dioxide

Article abstract of DOI:10.1002/cjoc.201800278

A ruthenium complex with a tetradentate bipyridine ligand was proved to be a highly efficient catalyst for the conversions of CO₂. Turnover numbers up to 300 000, 9800, and 2100 were achieved for the hydrogenations of CO₂ to formamides, formamides to methanol and amines, and the direct hydrogenation of CO₂ to methanol, respectively.

Full text of DOI:10.1002/cjoc.201800278

An Efficient Ruthenium Catalyst Bearing Tetradentate Ligand for
Hydrogenations of Carbon Dioxide
Feng-Hua Zhang,‡,a Chong Liu,‡,a Wei Li,a Gui-Long Tian,a Jian-Hua Xiea and Qi-Lin Zhou*,a,b
ABSTRACT A ruthenium complex with a tetradentate bipyridine ligand was proved to be a highly efficient catalyst for the conversions of CO2. Turnover
numbers up to 300,000, 9800, and 2100 were achieved for the hydrogenations of CO2 to formamides, formamides to methanol and amines, and the
direct hydrogenation of CO2 to methanol, respectively.
KEYWORDS carbon dioxide, methanol, formamide, ruthenium, hydrogenation
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday.
Introduction
Carbon dioxide (CO2) is an abundant, inexpensive, and
renewable carbon resource, and catalytic conversions of CO2 to
useful organic compounds have gained considerable attentions
over the past decades.1 One research focus has been
transition-metal-catalyzed hydrogenation of CO2 to formic acid,
formates, formamides, methanol, and other reduced species.2
Among these reactions, the hydrogenation of CO2 to methanol,
which is a versatile raw material and a potential alternative fuel,3
is an attractive, albeit challenging, goal.
The biggest challenge for the direct reaction of CO2 with H2 to
produce methanol is to find a catalyst that is active for the two
key steps: hydrogenation of the very stable CO2 molecule to
formic acid or its derivatives and subsequent reduction of formic
acid or the derivatives to methanol. Various heterogeneous
catalysts for this conversion have been reported, including a
Cu/Zn-based multicomponent catalyst; however, harsh conditions
(>250 °C, >50 atm H2) are usually required, and the selectivity is
low.4 The search for homogeneous catalysts for the hydrogenation
of CO2 to methanol was begun in 1993 by Sasai et al.5 Recently,
significant effort has been made to convert CO2 to methanol by
using Ru catalysts with tridentate ligands.6 For example, Sanford
Results and Discussion
et al.7 designed a cascade system involving a Ru catalyst (1) with a
PNN ligand and RuCl(OAc)(PMe3)4 and Sc(OTf)3 as co-catalysts to
reduce CO2 to methanol with a TON of 21 (Figure 1).8 Leitner et al.
realized the hydrogenation of CO2 to methanol in the presence of
bis(trifluoromethane)sulfonamide (HNTf2) and a Ru catalyst with a
triphos ligand (2) and obtained TONs up to 221. Sanford et al.9
reported that a Ru catalyst with a PNP ligand (3) efficiently
converted CO2 to methanol in the presence of Me2NH. In a
one-pot, two-step process, TONs as high as 3600 were achieved
by Ding et al.10 in the hydrogenation of CO2 to methanol with
Ru-PNP catalyst 4.
Recently we developed new Ru catalyst 5, which contains a
tetradentate bipyridine ligand and efficiently catalyzes the
hydrogenation of carboxylic esters and lactones to alcohols.11
Further evaluation of catalyst 5 as part of our ongoing research on
CO2 chemistry12 revealed that it is also a highly efficient catalyst
for diverse conversions of CO2 via hydrogenation. We herein
report that 5 catalyzes hydrogenation of CO2 to formamides,
hydrogenation of formamides to methanol and amines, and direct
hydrogenation of CO2 to methanol with TONs of up to 300,000,
9800, and 2100, respectively.
Catalyst 5 was tested for the conversion of CO2 to formamides
in the presence of amines under hydrogenation conditions.13
After systematically optimizing the reaction conditions, we found
that hydrogenation of CO2 in iPrOH under 80 atm of 1:1 CO2/H2 in
t
the presence of Me2NH using BuOK as a base at 90 °C for 30 h
afforded DMF with TON up to 300,000 and TOF up to 10000 h-1
(for details, see Supporting Information). Under these optimal
reaction conditions, an array of primary and secondary amines
reacted with CO2 to produce the corresponding formamides in
good to excellent yields (65-88%) at a catalyst loading of 0.01 mol%
(Table 1). This reaction also provided a green method for the
protection of amines.
Encouraged by these results, we attempted to convert
formamides to methanol and amines,6 a process that could
provide an indirect way to convert CO2 to methanol, as well as a
useful way to remove an N-formyl protecting group. We found
that the hydrogenation of formamides required a higher reaction
temperature to complete the reaction (for details, see Supporting
Information). Under optimized reaction conditions (1 mol% 5 in
iPrOH at 110 °C under 40 atm of H2 in the presence of 1 mol%
tBuOK), formamides bearing various amine groups were
converted to methanol and amines in excellent yields (Table 2).
Figure 1 Selected homogeneous catalysts for the hydrogenation of CO2 to
methanol
a State Key Laboratory and Institute of Elemento-organic Chemistry, College of
Chemistry, Nankai University, Tianjin 300071, China
*E-mail: qlzhou@nankai.edu.cn
‡ These authors contributed equally to this work.
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, China
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Table 1 Hydrogenation of CO2 to formamides in the presence of aminesa
Increasing the reaction temperature to 170 °C at the same CO2
pressure dramatically improved the yield of methanol to 42%
(entry 4). That is, hydrogenation of CO2 to methanol in the
presence of Me2NH was possible when the CO2 pressure was kept
low.
a
Table 3 Catalytic hydrogenation of DMF to CH3OH in the presence of CO2
a
(General reaction conditions: alkylamine (10 mmol), catalyst 5 (0.01
Yield of CH3OH
t
i
Entry
PCO2 (atm)
Temp. (°C)
Conv. (%)b
(%)b
mol%), BuOK (0.1 mol%), CO2 (40 atm), H2 (40 atm), PrOH (5 mL), 30 h,
GC yield using n-dodecane as internal standard.) b (GC yield in parentheses
at 0.0001 mol% of catalyst 5.)
1
2
3
4
40
5
110
110
110
170
<2
13
24
0
8
The hydrogenation could also be performed at a catalyst
loading of only 0.01 mol%, producing methanol and amines in
good to high yields (55-98%). The best result was obtained for the
hydrogenation of N-formylmorpholine: the TON reached 9800.14
2.5
2.5
16
42
55
a (General reaction conditions: catalyst 5 (0.01 mol%), BuOK (0.1 mol%),
t
H2 (40 atm), PrOH (5 mL), 30 h.) b (GC yield using n-dodecane as internal
i
Table 2 Catalytic hydrogenation of formamides to methanol and aminesa
standard.)
The hydrogenation of CO2 to methanol under 2.5 atm of CO2
at 170 °C for 30 h afforded a mixture of methanol (TON = 47) and
DMF (TON = 341) (Table 4, entry 1). Extending the reaction time
to 48 h increased the TON for methanol to 122 (entry 2).
Considering that the conversion of CO2 to formamide requires a
lower temperature and the hydrogenation of formamide to
methanol requires a higher temperature, we also performed the
reaction at two temperatures, that is, first at 90 °C for 24 h and
then at 170 °C for 24 h, and this procedure resulted in a
substantial increase in conversion, from 35% to 92% (TON 450 for
methanol) (entry 3).15 When the hydrogen pressure was increased
from 40 to 50 atm, the TON for methanol improved to 600 (entry
4). Lowering the catalyst loading to 0.04 mol% and extending the
reaction time to 96 h (48 h at 90 °C and 48 h at 170 °C) increased
the TON for methanol to 836 (entry 5). We were delighted to find
that further lowering the catalyst loading to 0.02mol% and
extending the reaction time to 120 h (48 h at 90 °C and 72 h at
170 °C) improved the TON for methanol to 2100 (entry 6).
Table 4 Catalytic hydrogenation of CO2 to CH3OH in the presence of
Me2NHa
a (General reaction conditions: formamide (2 mmol), catalyst 5 (0.1 mol%),
i
tBuOK (1 mol%), H2 (40 atm), PrOH (5 mL), 110 °C, 30 h.) b (GC yield using
n-dodecane as internal standard.) c (GC yield in parentheses at 0.01 mol%
of complex 5.)
Time (h)
Conv. of CO2
(%)b
TONb
Entry 5 (mol%)
at 90 °C
at 170 °C
CH3OH DMF
Because 5 efficiently catalyzed both the conversion of CO2 to
DMF and the reduction of DMF to methanol, the Me2NH
appeared to be an appropriate vehicle for the conversion of CO2
to methanol. Thus, we hydrogenated CO2 in the presence of
1
2
0.08
0.08
0.08
0.08
0.04
0.02
0
30
48
24
24
48
72
31
36
92
94
89
84
47
341
334
700
570
0
122
450
600
3
24
24
48
48
i
4c
5c
6c
Me2NH under 80 atm of 1:1 CO2/H2 in PrOH at 110 °C for 30 h.
However, no methanol was obtained, and DMF was the major
product (80% yield). We hypothesized that the high CO2 pressure
hindered the hydrogenation of DMF to methanol. To test this
hypothesis, we performed the hydrogenation of DMF at various
CO2 pressures and found that lowering the pressure of CO2
favored conversion of DMF to methanol (Table 3). Specifically,
when the hydrogenation of CO2 was carried out at a CO2 pressure
of 2.5 atm at 110 °C for 30 h, 24% conversion of DMF was
observed, and methanol was obtained in 16% yield (entries 13).
836 1400
2100 2070
a (General reaction conditions: Cat. 5/tBuOK = 1/10, Me2NH (10 mmol),
CO2 (2.5 atm, 10 mmol), H2 (40 atm), iPrOH (5 mL), 30 h.) b (Determined by
GC analysis using n-dodecane as internal standard.) c (H2 (50 atm).)
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Conclusions
[3] (a) Olah, G. A. Angew. Chem., Int. Ed., 2005, 44, 2636; (b) Olah, G. A.;
Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol
Economy; Wiley-VCH: Weinheim, Germany, 2006, pp.168—256.
[4] (a) W. Wang, S. Wang, X. Ma and J. Gong, Chem. Soc. Rev. 2011, 40,
3703; (b) Y.-N. Li, R. Ma, L.-N. He and Z.-F. Diao, Catal. Sci. Technol.,
2014, 4, 1498.
In conclusion, ruthenium complex 5, which has a tetradentate
bipyridine ligand developed by our group, was proved to be a
highly efficient catalyst for conversions of CO2. The tetradentate
bipyridine ligand can improve the stability of the ruthenium
catalyst, which is critical for the hydrogenations of CO2 in high
TONs. Hydrogenation of CO2 to formamides, hydrogenation of
formamides to methanol and amines, and direct conversion of
CO2 to methanol were achieved with TONs up to 300,000, 9800,
and 2100, respectively. Further studies of other CO2 conversions
catalyzed by 5 are currently underway in our laboratory.
[5] Tominaga, K.; Sasaki, Y.; Kawai, M.; Watanabe, T.; Saito, M. J. Chem.
Soc. Chem. Commun., 1993, 629.
[6] (a) Balaraman, E.; Gnanaprakasam, B.; Shimon, L. J. W.; Milstein, D. J.
Am. Chem. Soc., 2010, 132, 16756; (b) Balaraman, E.; Gunanathan, C.;
Zhang, J.; Shimon, L. J. W.; Milstein, D. Nat. Chem., 2011, 3, 609; (c)
Balaraman, E.; Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed.,
2011, 50, 11702; (d) Han, Z.; Rong, L.; Wu, J.; Zhang, L.; Wang, Z.;
Ding, K.-L. Angew. Chem., Int. Ed., 2012, 51, 13041; (e) Spasyuk, D.;
Smith, S.; Gusev, D. G. Angew. Chem., Int. Ed., 2012, 51, 2772; (f)
Khusnutdinova, J. R.; Garg, J. A.; Milstein, D. ACS Catal., 2015, 5, 2416.
(g) Miura, T.; Naruto, M.; Toda, K.; Shimomura, T.; Saito, S. Scientific
Reports, 2017, 7, 1.
Experimental
General Procedure for Direct Hydrogenation of CO2 to
CH3OH. In a glove box, a 50 mL hydrogenation autoclave was
t
charged with complex 5 (4.6 mg, 0.008 mmol), BuOK (9.0 mg,
0.08 mmol). The autoclave was moved out and the solution of
Me2NH (5 mL, 2 M, 10 mmol, 1250 equiv relative to Ru) in dry
iPrOH was added through the injection port. The autoclave was
tightened and purged three times with argon and three times
with CO2 before it was finally charged with CO2 to 2.5 atm then by
H2 to 42.5 atm total initial pressure. The reaction was then heated
using the temperature ramp (90 °C to 170 °C). After the
appropriate reaction time, the reaction mixture was allowed to
cool to room temperature. The pressure was carefully released
and the reaction mixture was diluted by diethyl ether (5 mL). The
n-dodecane was added to the reaction mixture as an internal
standard. An aliquot portion of the mixture was taken and
submitted to GC analysis (Agilent CP7405, 25 m × 0.32 mm × 0.25
m) using a flame ionization detector (FID) operating at 250 °C.
Injector temperature was set at 230 oC. The carrier gas was
[7] Huff, C. A.; Sanford, M. S. J. Am. Chem. Soc. 2011, 133, 18122.
[8] Wesselbaum, S.; vom Stein, T.; Klankermayer, J.; Leitner, W. Angew.
Chem. Int. Ed. 2012, 51, 7499.
[9] Rezayee, N. M.; Huff, C. A.; Sanford, M. S. J. Am. Chem. Soc., 2015,
137, 1028.
[10] Zhang, L.; Han, Z.; Zhao, X.; Wang, Z.; Ding, K.-L. Angew. Chem., Int.
Ed., 2015, 54, 6186.
[11] Li, W.; Xie, J.-H.; Yuan, M.-L.; Zhou, Q.-L. Green Chem., 2014, 16,
4081.
[12] Liu, C.; Xie, J.-H.; Tian, G.-L.; Li, W.; Zhou, Q.-L. Chem. Sci., 2015, 6,
2928.
[13] Selected papers for the conversions of CO2 to formic acid, formates,
and formamides, see: (a) Jessop, P.; Ikariya, G. T.; Noyori, R. Nature,
1994, 368, 231; (b) Jessop, P. G.; Hsiao, Y.; Ikariya, T.; Noyori, R. J.
Am. Chem. Soc., 1996, 118, 344; (c) Munshi, P.; Main, A. D.; Linehan,
J. C.; Tai, C. C.; Jessop, P. G. J. Am. Chem. Soc., 2002, 124, 7963; (d)
Tanaka, R.; Yamashita, M.; Nozaki, . J. Am. Chem. Soc., 2009, 131,
14168; (e) Federsel, C.; Boddien, A.; Jackstell, R.; Jennerjahn, R.;
Dyson, P. J.; Scopelliti, R.; Laurenczy, G.; Beller, M. Angew. Chem.,
Int. Ed., 2010, 49, 9777; (f) Schmeier, T. J.; Dobereiner, G. E.;
Crabtree, R. H.; Hazari, N. J. Am. Chem. Soc., 2011, 133, 9274; (g)
Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J.; Ben-David, W.
Y.; Milstein, D. Angew. Chem., Int. Ed., 2011, 50, 9948; (h) Ziebart, C.;
Federsel, C.; Anbarasan, P.; Jackstell, R.; Baumann, W.; Spannenberg,
A.; Beller, M. J. Am. Chem. Soc., 2012, 134, 20701; (i) Hull, J. F.;
Himeda, Y.; Wang, W.-H.; Hashiguchi, B.; Periana, R.; Jzalda, S D.;
Muckerman, J. T.; Fujita, E. Nat. Chem., 2012, 4, 383; (j)
Wesselbaum, S.; Hintermair, U.; Leitner, W. Angew. Chem., Int. Ed.,
2012, 51, 8585; (k) Moret, S.; Dyson, P. J.; Laurenczy, G.; Nat.
Commun., 2014, 5, 4017.
nitrogen with
a flow rate of 1.0 mL/min. The following
temperature program was used in the analysis: 50 °C-3 °C/min to
90 °C -5 °C/min to 150 °C-25 °C/min to 300 °C (10 min). Using
these conditions, the following retention times were observed:
CH3OH (4.843 min), n-dodecane (12.158 min), DMF (15.694 min)
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
We thank the National Natural Science Foundation of China,
(No. 21790332, 21532003) and the “111” project (B06005) of the
Ministry of Education of China for financial support.
References
[1] (a) Gibson, D. H. Chem. Rev., 1996, 96, 2063; (b) M. Aresta, In
Carbon Dioxide as Chemical Feedstock, Wiley-VCH, Weinheim, 2010,
pp. 9—12.; (c) Aresta, M.; Dibenedetto, A.; Angelini, A. Chem. Rev., [14] For other highly efficient catalytic hydrogenation of amides, see: (a)
2014, 114, 1709.
John, J. M.; Bergens, S. H. Angew. Chem., Int. Ed., 2011, 50, 10377.
(b) Shi, L.-Y; Tan, X.-F; Long, J.; Xiong, X.; Yang, S.; Xue, P.; Lv, H.;
Zhang, X.-M Chem.-Eur. J. 2016, 23, 546.
[2] For selected reviews on the hydrogenation of CO2 to formic acid,
formamides, formates, methanol, see: (a) Jessop, P. G.; Ikariya, T.;
Noyori, R. Chem. Rev., 1995, 95, 259; (b) Jessop, P. G.; o , F.; Tai, C. [15] This temperature ramp strategy has also been used by Sanford, et
C. Coord. Chem. Rev., 2004, 248, 2425; (c) Ma, R.-Z.; Li, Y.-Q.; Zhou,
H.-Y.; Wang, J.-C. Chin. J. Org. Chem., 2009, 29, 1843; (d) Federsel, C.;
Jackstell, R.; Beller, M. Angew. Chem., Int. Ed., 2010, 49, 6254; (e) Li,
Y.; Junge, K.; Beller, M. ChemCatChem, 2013, 5, 1072; (f) Wang,
W.-H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E. Chem.
Rev., 2015, 115, 12936.
al., see Ref. 9.
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Revised manuscript received: XXXX, 2017
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Entry for the Table of Contents
Page No.
An Efficient Ruthenium Catalyst Bearing
Tetradentate Ligand for Hydrogenations
of Carbon Dioxide
A ruthenium catalyst with tetradentate bipyridine ligand gave TONs up to 300,000, 9800, and 2100
for the hydrogenations of CO2 to formamides, formamides to methanol and amines, and the direct
hydrogenation of CO2 to methanol.
Feng-Hua Zhang, Chong Liu, Wei Li,
Gui-Long Tian, Jian-Hua Xie and Qi-Lin
Zhou*
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