Hydrogenation of carbon dioxide to formate catalyzed by a copper/1,8-diazabicyclo[5.4.0]undec-7-ene system

Article abstract of DOI:10.1002/adsc.201500043

Hydrogenation of carbon dioxide to formate was achieved using copper (Cu) catalysts in the presence of strong organic bases including amidines and guanidines. Specifically, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) proved to be effective for the transformation of a 1:1 mixture of hydrogen and carbon dioxide into its formate salt under increased pressure in the presence of various Cu(I) and Cu(II) salts at 100°C. A novel complex derived from copper iodide and DBU equally promoted the same reaction, indicating that DBU-Cu species are involved as real catalysts in this hydrogenation.

Full text of DOI:10.1002/adsc.201500043

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DOI: 10.1002/adsc.201500043
Very Important Publication
Hydrogenation of Carbon Dioxide to Formate Catalyzed by
a Copper/1,8-Diazabicyclo[5.4.0]undec-7-ene System
Ryo Watari,a,* Yoshihito Kayaki,b Shin-ichi Hirano,a Norio Matsumoto,a
and Takao Ikariyab,*
a
Bioengineering Sector, Environmental Science Research Laboratory, Central Research Institute of Electric Power
Industry, 1646 Abiko, Abiko-shi, Chiba 270-1194, Japan
Fax : (+81)-4-7183-3347; phone: (+81)-4-7182-1181; e-mail: r-watari@criepi.denken.or.jp
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1
b
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Fax : (+81)-3-5734-2637; phone: (+81)-3-5734-2636; e-mail: tikariya@apc.titech.ac.jp
Received: January 18, 2015; Revised: March 4, 2015; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500043.
Abstract: Hydrogenation of carbon dioxide to for-
mate was achieved using copper (Cu) catalysts in
the presence of strong organic bases including ami-
dines and guanidines. Specifically, 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) proved to be effective
for the transformation of a 1:1 mixture of hydrogen
and carbon dioxide into its formate salt under in-
creased pressure in the presence of various Cu(I)
and Cu(II) salts at 1008C. A novel complex derived
from copper iodide and DBU equally promoted the
same reaction, indicating that DBU–Cu species are
involved as real catalysts in this hydrogenation.
drogenation could substitute conventional methods
using carbon monoxide, and formate has possible use
as a hydrogen energy carrier.[2]
Since the pioneering work by Inoue et al. in 1976,[3]
the catalytic hydrogenation of CO2 has been mainly
achieved by a range of homogeneous Ru, Rh, and Ir
complexes.[4] Recently, Nozaki and co-workers report-
ed that a maximum turnover number (TON) of
3,500,000 was obtained with a PNP-pincer Ir com-
plex.[5] Compared to precious metal catalysts, general-
ly abundant and inexpensive first-row transition
metals, especially Mn, Fe, Co, Ni, and Cu, are advan-
tageous for large-scale chemical processes.[6] In this
context, combinatorial catalyst screening by Jessopꢀs
group revealed that Fe, Co, and Ni with phosphine li-
gands exhibit catalytic activity in CO2 hydrogena-
tion.[7] More recently, well-defined Fe[8] and Co[9] com-
plexes have emerged as efficient catalysts for formic
acid synthesis. The TON has reached up to 9,400
from the results of appropriate ligand design, such as
Keywords: carbon dioxide; copper; 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU); formates; hydrogena-
tion
Carbon dioxide (CO2) has drawn much attention as tetradentate phosphine,[8a,c,9a] PNP-pincer,[8b] and
an abundant, inexpensive and low-toxic carbon feed- proton responsive bipyridine ligands.[9b]
stock for chemical synthesis. The amount of CO2 uti-
Cu complex catalysts have not been thoroughly in-
lized annually in chemical manufacturing, including vestigated in CO2 hydrogenation, whereas the hydro-
urea production, hovers around 120 million tons, in genation of C=O bonds of aldehydes and ketones has
contrast to the 24,000 million tons of CO2 released by contributed to long-standing Cu hydride chemistry.[10]
anthropogenic emission.[1] This disparity indicates that Haynes et al. tested the catalytic activity of CuCl in
CO2 transformation is not highly utilized in the chem- CO2 hydrogenation in the presence of dimethylamine
ical industry. The catalytic hydrogenation of CO2 to to form N,N-dimethylformamide (DMF); however,
À
formate (HCO2 ), described in Scheme 1, is promising the TON was lower than that obtained using second-
for the large-scale fixation of CO2, because such a hy- or third-row transition metal complexes such as Rh,
Ir, Pd, and Pt bearing phosphine ligands.[11] Aside
from several attempts to combine Cu with other
metals in homogeneous and heterogeneous sys-
tems,[12,13] monometallic Cu catalysts have been re-
Scheme 1. Catalytic hydrogenation of CO2 to formate.
garded as unsuitable for CO2 hydrogenation. Thus,
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Ryo Watari et al.
Table 1. Catalytic hydrogenation of CO2 to formate.[a]
Entry
Catalyst
Yield [%][b]
TON[c]
1
2
3
4
5
6
7
8
(IPr)Cu(O-t-C4H9)
Cu(OAc)2·H2O/dppbz[d]
Cu(OAc)2·H2O
Cu(OAc)2
4
5
21
25
103
96
101
88
91
82
53
66
61
-
21
20
19
18
18
17
11
14
12
<1
<1
<1
<1
Cu(OAc)
Figure 1. Organic bases used in Table 2.
Cu(NO3)2·2.4H2O
Cu(OCH3)2
Cu(OTf)2
CuCl
CuBr
CuI
none
Fe(OAc)2
Co(OAc)2
Table 2. Cu-catalyzed hydrogenation of CO2 with organic
bases (see Figure 1).[a]
9
10
11
12
13
14
15
Entry
Base
Yield [%][b]
TON[c]
1
2
3
4
5
6
7
DBU
DBN
21
7
8
103
37
42
2
<1
2
MTBD
TBD
N(C2H5)3
quinuclidine
DMAP
Ni(OAc)2·4H2O
9
40
[a]
<1
<1
<1
<1
<1
<1
Reaction conditions: catalyst (0.02 mmol), DBU
(10 mmol), 1,4-dioxane (5.0 mL), CO2 (20 atm), H2
(20 atm), 1008C, 21 h.
[b]
Molar ratio of the product/initial DBU determined by
[a]
Reaction conditions: Cu(OAc)2·H2O (0.02 mmol), base
(10 mmol), 1,4-dioxane (5.0 mL), CO2 (20 atm), H2
(20 atm), 1008C, 21 h.
1H NMR.
[c]
[d]
TON: turnover number.
Using 3 equiv. to Cu.
[b]
[c]
Molar ratio of the product/initial base determined by
1H NMR.
TON: turnover number.
the potential ability of Cu complexes to be useful in
H2 activation as well as CO2 insertion into Cu hydride
species prompted us to use Cu complexes for the cat-
AHCTUNGTREGUNfNN ormation, Table S1). The positive results using
alytic synthesis of formic acid using H2 gas. Herein we Cu(OAc)2·H2O encouraged us to test other Cu(II)
disclose the catalytic ability of Cu complexes for the and Cu(I) salts as catalysts. As listed in entries 4–11
hydrogenation of CO2 in the presence of strong or- in Table 1, Cu salts with various anions (acetate, ni-
ganic bases.
trate, methoxide, trifluoromethanesulfonate, and hal-
Initially, the catalytic hydrogenation of CO2 using ides) exhibited higher hydrogenation activities than
Cu catalysts was examined by addition of 1,8-diazabi- the above-mentioned Cu/IPr or Cu/phosphine cata-
cyclo[5.4.0]undec-7-ene (DBU) as a base.[14] The reac- lysts. Among them, copper acetates proved to be opti-
tion was conducted with a base/Cu catalyst ratio of mal Cu sources regardless of the Cu(II) or Cu(I) pre-
500 in 1,4-dioxane under 20 atm of both CO2 and H2 cursors. Other first-row transition metal salts, such as
at 1008C for 21 h. Table 1 summarizes the catalytic Fe(OAc)2, Co(OAc)2, and Ni(OAc)2·4H2O, had no
activity represented by the formate yield based on the effect under the current reaction conditions (en-
initial molar amount of DBU, as well as the TON. An tries 13–15).
N-heterocyclic carbene complex (IPr)Cu(O-t-C4H9)
[IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-yli-
The Cu-catalyzed CO2 hydrogenation was markedly
influenced by the choice of base (Figure 1). As sum-
dene], which serves as a catalyst in hydrosilylation[15] marized in Table 2, the presence of a related amidine
and hydroboration[16] of CO2, slightly promotes the base exhibiting comparable basicity to DBU, 1,5-di-
+
À
À
CO2 hydrogenation to give [DBU H] [HCO2]
ACHTUNGTRENaNGNU zabicyclo[4.3.0]non-5-ene (DBN), reduced the cata-
with a TON of 21 (entry 1). A combination of lytic efficiency (entry 2). The use of more basic guani-
Cu(OAc)2·H2O and 1,2-bis(diphenylphosphino)ben- dines, also resulted in lower TONs (entries 3 and 4).
zene (dppbz), which is an efficient catalytic system for These results indicate that the catalytic activity is not
the hydrosilylation of CO2,[17] showed comparable ac- strongly associated with the pKa of the added base.[18]
tivity (entry 2). In contrast, the use of Cu(OAc)2·H2O Other nitrogen bases, triethylamine, quinuclidine,[19]
even without carbene or phosphine ligands considera- and DMAP, did not contribute to formate production
bly promoted theÀ CO2 hydrogenation to produce (entries 5–7). Based on these experimental results,
+
À
[DBU H] [HCO2] with a TON of 103 (entry 3). Var- DBU was employed as the most effective base for hy-
ious aprotic solvents such as THF, diglyme, DMAc, drogenation of CO2 in the following experiments.
and 1,4-dioxane can be used in the reaction; however,
Further improvement in the catalytic efficiency was
1,4-dioxane is the preferred solvent (Supporting In- successfully achieved using the Cu(OAc)2·H2O/DBU
2
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Hydrogenation of Carbon Dioxide to Formate
Table 3. Hydrogenation of CO2 with the Cu(OAc)2·H2O/
To elucidate a possible coordination of DBU to Cu
species in the catalytic reaction,[21] CuI was treated
with 3 molar amounts of DBU in CH3CN at room
temperature. From the crude product in THF/diethyl
ether, (dbu)2CuI was successfully isolated in 88%
yield as colorless crystals, which were identified by
NMR and elemental analysis. The 13C NMR signal of
the amidine carbon in coordinated DBU (163.0 ppm)
was clearly shifted to a lower magnetic field com-
pared with that in free DBU (159.7 ppm) in C6D6, in-
dicating that the amidine unit maintains a rigid coor-
dination in solution. In addition, a preliminary X-ray
crystallographic analysis[22] revealed that the mononu-
clear Cu complex has a trigonal planar structure bear-
ing iodine and two DBU molecules where the sp2 ni-
trogen atoms are attached to the Cu center (see the
Supporting Information). It should be noted that the
catalytic use of (dbu)2CuI showed a comparable activ-
ity (TON=64) to CuI under the standard reaction
conditions mentioned above. DBU acts as a support-
DBU system.[a]
Entry P(CO2/H2) [atm] T [8C] t [h] Yield [%][b] TON[c]
1
2
3
4
20/20
20/20
20/20
20/20
30/30
40/40
30/30
100
100
80
120
100
100
100
21
116 33
21
103
167
26
21
21
21
21
21
5
13
32
81
5
163
165
60
6
34
7[d]
60 (54)[e]
[a]
Reaction conditions: Cu(OAc)2·H2O (0.02 mmol), DBU
(10 mmol), 1,4-dioxane (5.0 mL).
Molar ratio of the product/initial DBU determined by
1H NMR.
TON: turnover number.
Cu(OAc)2·H2O (0.05 mmol), DBU (5 mmol).
Isolated yield.
[b]
[c]
[d]
[e]
system. As shown in Table 3, TONs reached 167 after ing ligand to the Cu catalyst during the reaction,
prolonging the reaction time to 116 h (entries 1 and which accommodates the remarkable positive effect
2). While the optimal temperature was found to be on CO2 hydrogenation.[23]
1008C at a pressure ratio of CO2/H2 =20/20 atm,
In summary, we have newly developed the Cu-cata-
a temperature over 1008C resulted in lower catalytic lyzed hydrogenation of CO2 to formate. Contrary to
activity, possibly due to deposition of catalytically in- other first-row transition metal complexes bearing so-
active copper metals during the hydrogenation (en- phisticated ligands with fascinating catalytic activity,
tries 3 and 4). The TON increased to 165 under com- the Cu/DBU system promotes the formation of
bined pressures above 60 aÀtm (entries 5 and 6). The formic acid salt without the addition of extra ligands.
+
À
yield of [DBU H] [HCO2] increased to 60% when Particularly, our understanding of the new DBU-coor-
the reaction was conducted at a higher catalyst load- dinated complex strongly supports that it has a dual
ing (DBU/Cu=100; entry 7). The product can be iso- role of DBU to stabilize the Cu catalyst under the re-
lated with a 54% yield as hygroscopic white granules action conditions as well as to trap formic acid as
grown from the reaction mixture at room tempera- a base, this opens up the availability of Cu complexes
ture.
for hydrogenation. To develop more efficient Cu cata-
In the proposed hydrogenation mechanism de- lysts, mechanistic investigations of the hydrogenation
scribed in Figure 2, a copper hydride species 1 is gen- system and new designs of Cu-based complexes are
erated from Cu precursors under H2. Subsequent CO2 currently in progress in our laboratories.
À
insertion into the Cu H bond furnishes the Cu for-
mate complex 2, as previously reported.[15–17,20] The Cu
hydride 1 is regenerated by the activation of H2 with
Experimental Section
the concomitant release of formic acid through the
aid of the base. The superior catalytic performance
using DBU implies that the catalytic cycle should be
driven by the facile formation of thermodynamically
General Procedures for Hydrogenation
Distilled 1,4-dioxane (2.0 mL) was added to Cu(OAc)2·H2O
(4.0 mg, 0.02 mmol) in an oven-dried 20-mL Schlenk tube
+
À
À
stable [DBU H] [HCO2] and that DBU can pro-
mote the heterolytic splitting of H2 bound to the Cu under an argon atmosphere and ultrasonically agitated for
20 min to form a blue solution. Distilled DBU (1.52 g,
10 mmol) was added to the resulting solution. The solution
was transferred into a 50-mL autoclave equipped with
a glass insert and a magnetic stirring bar under argon, fol-
lowed by addition of distilled 1,4-dioxane (3.0 mL). The au-
toclave was filled with CO2 (30 atm) and H2 (30 atm) at
room temperature, then warmed to 1008C and stirred vigo-
rously for 21 h. After the reaction, the autoclave was cooled
in an ice bath and the pressure was carefully vented. An ali-
quot of the resulting pale yellow solution was measured by
1H NMR at a relaxation delay of 10 s in CD3OD to confirm
center to afford 1.
Figure 2. Plausible mechanism for the hydrogenation of CO2
catalyzed by the Cu–DBU complex.
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Ryo Watari et al.
+
À
À
the formation of [DBU H] [HCO2] . The yield was deter-
mined based on the relative integration of the signal at
8.56 ppm (1H) to that at 1.15 ppm due to the methyl groups
of 2-propanol used as an internal standard. The analytically
pure product was isolated by crystallization from the reac-
tion mixture at room temperature and washed with distilled
THF (3 mLꢂ3) under an argon atmosphere to give hygro-
scopic white platelet crystals. 1H NMR (399.78 MHz,
CDCl3): d=1.56–1.63 (m, 6H), 1.89 (quintet, J=5.8 Hz,
2H), 2.71–2.72 (m, 2H), 3.29–3.37 (m, 6H), 8.65 (s, 1H),
12.06 (brs, 1H); 13C{1H} NMR (100.53 MHz, CDCl3): d=
19.7, 24.1, 27.0, 29.0, 32.1, 38.2, 48.5, 54.0, 165.6, 168.6.
These spectral data are identical to those of an authentic
sample prepared by the following procedure: In a glove-
box, a mixture of distilled formic acid (1.0 g, 22 mmol) and
distilled diethyl ether (5 mL) was added dropwise to a mix-
ture of distilled DBU (3.2 g, 21 mmol) and distilled diethyl
ether (20 mL), to form a white precipitate. After stirring for
1 h, the resulting mixture was filtered and the residue was
washed with distilled diethyl ether (20 mL), then dried
erman, Y. Himeda, E. Fujita, Inorg. Chem. 2013, 52,
12576–12586; c) M. S. Jeletic, M. T. Mock, A. M.
Appel, J. C. Linehan, J. Am. Chem. Soc. 2013, 135,
11533–11536.
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1989, 111, 8818–8823; b) J.-X. Chen, J. F. Daeuble,
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[12] As reported by Denise and Sneeden, the Cu-containing
bimetallic complex Pd2Cu2Cl6(Ph2PCH2PPh2)3 acts as
a catalyst for CO2 hydrogenation yielding HCO2C2H5,
whereas the Cu complexes [(CuCl)2(Ph2PCH2PPh2)]2
and [Cu3Cl2(Ph2PCH2PPh2)3]Cl exhibited no catalytic
activity: B. Denise, R. P. A. Sneeden, J. Organomet.
Chem. 1981, 221, 111–116.
+
À
À
under vacuum to give [DBU H] [HCO2] as a white hygro-
scopic solid; yield: 4.1 g (21 mmol, 98%).
Acknowledgements
[13] As reported by Tsangꢀs group and Hanꢀs group, Cu-
containing ZnO heterogeneous catalysts promoted CO2
hydrogenation to give HCO2CH3 and DMF:
a) K. M. K. Yu, C. M. Y. Yeung, S. C. Tsang, J. Am.
Chem. Soc. 2007, 129, 6360–6361; b) J. Liu, C. Guo, Z.
Zhang, T. Jiang, H. Liu, J. Song, H. Fan, B. Han, Chem.
Commun. 2010, 46, 5770–5772.
[14] Very recently, a marked effect of modified DBU on the
Rh-catalyzed hydrogenation of CO2 was reported: Y.-
N. Li, L.-N. He, X.-D. Lang, X.-F. Liu, S. Zhang, RSC
Adv. 2014, 4, 49995–50002.
This work was financially supported by JSPS KAKENHI
(Grant Numbers 22225004, 24350079, and 26620143).
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Hydrogenation of Carbon Dioxide to Formate
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rahedron Lett. 2014, 55, 3878–3880.
Adv. Synth. Catal. 0000, 000, 0 – 0
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COMMUNICATIONS
6 Hydrogenation of Carbon Dioxide to Formate Catalyzed by
a Copper/1,8-Diazabicyclo[5.4.0]undec-7-ene System
Adv. Synth. Catal. 2015, 357, 1 – 6
Ryo Watari,* Yoshihito Kayaki, Shin-ichi Hirano,
Norio Matsumoto, Takao Ikariya*
6
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ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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