Carbon dioxide hydrogenation: Efficient catalysis by an NHC-amidate Pd(II) complex

Article abstract of DOI:10.1016/j.tetlet.2017.07.040

The utilization of carbon dioxide as a carbon source has long been a challenge in modern organic chemistry due to its low reactivity, yet high abundance. Herein we demonstrate the highly efficient hydrogenation of carbon dioxide into formic acid in the presence of an NHC-amidate Pd(II) complex. Excellent turnover number was observed when the catalyst was used under heterolytic conditions. This catalytic system provides a new and efficient carbon dioxide hydrogenation method.

Full text of DOI:10.1016/j.tetlet.2017.07.040

Accepted Manuscript
Carbon dioxide hydrogenation: Efficient catalysis by an NHC-amidate Pd(II)
complex
Nima Zargari, Erica Jung, Joo-Ho Lee, Kyung Woon Jung
PII:
S0040-4039(17)30886-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.07.040
TETL 49122
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
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8 July 2017
10 July 2017
Please cite this article as: Zargari, N., Jung, E., Lee, J-H., Jung, K.W., Carbon dioxide hydrogenation: Efficient
catalysis by an NHC-amidate Pd(II) complex, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.
2017.07.040
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Carbon dioxide hydrogenation: Efficient
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catalysis by an NHC-amidate Pd(II) complex
Nima Zargari, Erica Jung, Joo-Ho Lee, Kyung Woon Jung

1
Tetrahedron Letters
Carbon dioxide hydrogenation: Efficient catalysis by an NHC-amidate Pd(II) complex
Nima Zargari, Erica Jung, Joo-Ho Lee, Kyung Woon Jung*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089-1062 United States
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The utilization of carbon dioxide as a carbon source has long been a challenge in modern
organic chemistry due to its low reactivity, yet high abundance. Herein we demonstrate the
highly efficient hydrogenation of carbon dioxide into formic acid in the presence of an NHC-
amidate Pd(II) complex. Excellent turnover number was observed when the catalyst was used
under heterolytic conditions. This catalytic system provides a new and efficient carbon dioxide
hydrogenation method.
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
Carbon dioxide
Hydrogenation
Formic acid
Palladium catalysis
Being an undesirable side product of the industrial revolution
and the driving force of the destruction of biomass, carbon
dioxide is a greenhouse gas which is difficult to activate and
react due to its thermodynamic stability.1 Moreover, since there
are exhaustible resources available, it is of paramount importance
to utilize carbon dioxide as a C1-building block not only to
reduce the amount of greenhouse gasses, but also to recycle
carbon dioxide as a basic raw material into chemical feedstock
products or alternative fuel sources.2 Various works have been
performed in order to utilize carbon dioxide by either direct
hydrogenation, other activation reactions, or by direct carbon
in direct formic acid fuel cells (DFAFC). DFAFC are considered
to be an alternative to methanol and hydrogen fuel cells due to
their ease of refueling, efficiency, and safety. This allows
DFAFC to potentially be utilized in common consumer
electronics and automobiles.11
dioxide
capture
and
storage.3
Employing
N,N’-
ethylenebis(salicylimine) (Salen) and metal catalysts, carbon
dioxide has been copolymerized with epoxides producing
polycarbonates,4 while cyclic carbonate and polycarbonate
formation was achieved by Group 3, 4, and 5 metal complexes.5
The most sought out reaction of carbon dioxide is the direct
hydrogenation of the gas into valuable products such as
methanol, formamides, and formic acid.6 Transition metals,
including early transition metals, have excessively been utilized
to catalyze reactions with carbon dioxide and attain these
products.7 Using supercritical carbon dioxide and a ruthenium
catalyst, carbon dioxide has been selectively converted into
methanol.8 Other reaction conditions employ carbon dioxide at
much lower pressures yet still produce methanol in the presence
of copper, zinc, and aluminum co-catalysts.9 In the presence of
ammonia and primary and secondary alkyl amines, the direct
carbonylation of carbon dioxide may take place in order to
produce formamides or methylamines.10 Performing the reactions
in supercritical carbon dioxide significantly increased the yields
of these products, compared to reactions using liquid solvents.
A plethora of studies have been found which selectively
hydrogenate carbon dioxide into formic acid, under varying
solvents, metal catalysts, and bases. This is of great value since
several studies have employed formic acid as a hydrogen source
Figure 1. Structure of our NHC-amidate Pd(II) complex
Notably, recent studies have employed the use of NHC
ligands in order to reduce carbon dioxide into either methanol or
formic acid.12 Herein, we report the development of a set of
reaction conditions which selectively hydrogenate carbon dioxide
into formic acid utilizing our NHC-amidate Pd(II) complex 1
(Figure 1).13 Our initial reaction conditions (Scheme 1) closely
emulated that of Takezaki et. al. studies14 where a palladium
source was utilized in the presence of a base, producing formic
acid as the major product. When an equal pressure (10 atm) of
carbon dioxide and hydrogen gas were stirred at 160 °C for three
hours in the presence of 1 (5 μmol) in an alkaline aqueous
solution, 32.2 μmol (6.4 TON) of formic acid were produced as
the major product, while methanol and formaldehyde were
produced in trace amounts (less than 0.5 μmol).
Scheme 1. Initial reaction conditions utilizing complex 1

2
Tetrahedron
In light of this promising result, further optimization was
Varying the base added to the reaction mixture was then
tested. As expected, in the absence of any base, no reaction took
place (Table 2, entry 1). KOH not only stood out from other
group one alkaline bases (Table 2, entries 2 - 6), but also from
group two alkaline bases and organic bases as well. The major
product of the reaction using calcium hydroxide was methanol in
low yield (Table 2, entry 7), while silver trifluoroacetate
provided formaldehyde as the major product (Table 2, entry 8).
Potassium carbonate selectively produced formic acid in
moderate yields (Table 2, entry 9). Amine bases such as
triethylamine and ammonium hydroxide failed to produce any
significant result or produced trace amounts of both formic acid
or methanol, respectively (Table 2, entries 10 and 11).
investigated to obtain the highest yield for formic acid. Firstly,
since there is the possibility of over reduction of carbon dioxide
to form methanol or formaldehyde, variation of ratio between
CO2 and H2 was tested. When the same equivalents (10 atm) of
CO2 and H2 were used for the reaction, trace amount of methanol
(0.1 TON) and formaldehyde (0.04 TON) were formed as well as
6 TON of formic acid (Scheme 1). We found the optimal
conditions were the use of three equivalents of CO2 to H2
(Scheme 2), where the over reduced products were not detected.
Due to this high chemoselectivity, the yield of formic acid
increased, doubling to 12 TON.
Table 2. Effects of varying basea)
HCOOH
MeOH
HCHO
Entry
Base
(TON)b)
(TON)b)
(TON)b)
1
2
3
4
5
6
7
8
9
-
0
1
6
57
16
1
0
0
3
0
0
0
1
0
1
0
1
0
0
0
1
0
0
0
0
0
0
0
1
0
0
0
Scheme 2. Optimal ratio of carbon dioxide and hydrogen
With this ratio, when the reaction temperature was lowered,
the formation of formic acid was significantly decreased (Table 1,
entries 1 and 2). Only 1 TON of the acid was produced at 60 °C
and 5 TON at 100 °C. On the contrary, when the reaction
temperature was increased to 200 °C, the production of formic
acid was dramatically increased to 57 TON.
LiOH
NaOH
KOH
RbOH
CsOH
Ca(OH)2
AgTFA
K2CO3
Et3N
Table 1. Effects of varying temperaturea)
10
11
HCOOH
Entry
Temperature (°C)
Time (hrs.)
NH4OH
1
(TON)b)
a Reaction conditions: 1.7 mmol of varying base and 5 μmol complex 1 were
dissolved in 1.0 mL H2O. The reaction solution was then placed in a stainless
steel reactor and charged with 6.8 atm (800 μmol) hydrogen and 20 atm
(2,400 μmol) carbon dioxide gas. The mixture was heated at 200 C for 3
hours. b Yields were calculated using DMSO as an internal NMR standard.
1
2
3
4
60
3
3
3
3
1
5
12
57
100
160
200
a Reaction conditions: 1.7 mmol KOH and 5 μmol complex 1 were dissolved
in 1.0 mL H2O. The reaction solution was then placed in a stainless steel
reactor and charged with 6.8 atm (800 μmol) hydrogen and 20 atm (2,400
μmol) carbon dioxide gas. b Yields were calculated using DMSO as an
internal NMR standard.
We then compared our NHC-amidate Pd(II) complex 1 with
other palladium sources, which are commercially available and
widely used (Table 3). Critically, no reaction took place in the
absence of a palladium source (Table 3, entry 1). Palladium(II)
chloride produced the lowest yield of formic acid (Table 3, entry
3), while bis(acetonitrile)dichloropalladium (II) produced the
relatively high yield of formic acid (Table 3, entry 4).
Palladium(II) acetate was slightly less active, producing 43 TON
of formic acid (Table 3, entry 5).
We then examined time dependence of this transformation
(Figure 2). The reaction reached the highest turnover number
after three hours, while half of the product amount was produced
in two hours. Having the reaction run for an additional hour (4
hrs.) drastically decreased the amount of the acid. This result
might stem from the lower concentration of CO2 or potassium
carbonate, which could form by the reaction of CO2 with
potassium hydroxide (vide infra). After 4 hours (4 – 6 hrs.), the
interconversion between formic acid and H2 and CO2 equilibrated
to about 34 TON of formic acid. This interconversion was
previously reported by other Pd-NHC catalysts and other metal
catalysts such as rhodium and ruthenium.15
Table 3. Effects of palladium catalystsa)
Entry
Pd Source
HCOOH (TON)b)
1
2
3
4
5
-
1
0
57
7
49
43
PdCl2
Pd(MeCN)2Cl2
Pd(OAc)2
a Reaction conditions: 1.7 mmol KOH and 5 μmol of a palladium source were
dissolved in 1.0 mL H2O. The reaction solution was then placed in a stainless
steel reactor and charged with 6.8 atm (800 μmol) hydrogen and 20 atm
(2,400 μmol) carbon dioxide gas. The mixture was heated at 200 C for 3
hours. b Yields were calculated using DMSO as an internal NMR standard.
60
50
40
30
20
10
0
Furthermore, in order to improve yields, higher pressures of
carbon dioxide and hydrogen were applied under homogeneous
and heterogeneous conditions (Table 4). Increasing the pressure
of carbon dioxide to 68 atm and hydrogen to 41 atm yielded
almost twice formic acid with trace amounts of methanol (Table
4, entry 1). These pressures were the maximal amounts that could
be applied to the reactor system, while using the same amount of
the solvent. Interestingly, when NHC-amidate Pd(II) complex 1
was employed as a heterogeneous catalyst, a high turnover of
0
1
2
3
4
5
6
Time (hours)
Figure 2. Time study performed under standard reaction
conditions from 1-6 hours

3
formic acid (150 TON) was selectively produced in the absence
of any solvent (Table 4, entry 2). Though reported otherwise,16
no product was formed when no catalyst was used (Table 4, entry
3). This discrepancy between prior art may be due to the
differences in reaction scale or time. Comparatively, other
palladium(II) sources furnished only 24 TON of formic acid
under these heterolytic conditions.
other palladium salts. In addition, the heterolytic catalysis using
our NHC-amidate Pd(II) catalyst 1 gave rise to the optimal
conditions, yielding formic acid selectively and efficiently. This
is of great importance in the sense of converting greenhouse
gases into possible alternative fuel sources.
Acknowledgments
Table 4. Effects of varying palladium sources under
homogeneous and heterolytic conditionsa)
HCOOH
We acknowledge the Hydrocarbon Research Foundation for
generous financial support.
Entry
Pd Source
H2O (mL)
(TON)b)
References and notes
1
2
3
1
1
-
1.0
0.0
0.0
95
150
0
1.
2.
3.
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O
N
N
N
Pd
Me
Cl
O
CH3
+ K2CO3
- KCl
13. Lee, J. H.; Yoo, K. S.; Park, C. P.; Olsen, J. M.; Sakaguchi, S.;
Prakash, G. K. S.; Mathew, T.; Jung, K. W. Adv. Synth. Catal.
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CO2 + 2KOH
O
N
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N
N
Pd
KO
O
Me
H2O +
O
OH-
O
CH3
OK
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-O
O
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2
O
N
KO
O
N
O
H2
N
H
Pd
Me
O
CH3
O
H
3
Scheme 3. Proposed mechanism of CO2 hydrogenation by 1
Supplementary Material
Supplementary material that may be helpful in the review process
should be prepared and provided as a separate electronic file.
That file can then be transformed into PDF format and submitted
In summary, we have developed a set of reaction conditions
which selectively hydrogenate carbon dioxide gas into formic
acid, which can be utilized in alternative fuel sources, such as
DFAFC. Notably, our NHC-amidate Pd(II) complex outcompetes

4
Tetrahedron
along with the manuscript and graphic files to the appropriate
editorial office.

5
Highlights



Selective hydrogenation of carbon dioxide into
formic acid
Efficient heterolytic catalysis using our NHC-
amidate Pd(II) catalyst
Actual hydrogenation substrate: potassium carbonate
formed from CO2 and KOH