Iron-catalyzed hydrogenation of bicarbonates and carbon dioxide to formates

Article abstract of DOI:10.1002/cssc.201403234

The catalytic hydrogenation of carbon dioxide and bicarbonate to formate has been explored extensively. The vast majority of the known active catalyst systems are based on precious metals. Herein, we describe an effective, phosphine-free, airand moisture-

Full text of DOI:10.1002/cssc.201403234

DOI: 10.1002/cssc.201403234
Communications
Iron-Catalyzed Hydrogenation of Bicarbonates and Carbon
Dioxide to Formates
Fengxiang Zhu,[a] Ling Zhu-Ge,[a] Guangfu Yang,*[a] and Shaolin Zhou*[a, b]
The catalytic hydrogenation of carbon dioxide and bicarbonate
to formate has been explored extensively. The vast majority of
the known active catalyst systems are based on precious
metals. Herein, we describe an effective, phosphine-free, air-
and moisture-tolerant catalyst system based on Knçlker’s iron
complex for the hydrogenation of bicarbonate and carbon di-
oxide to formate. The catalyst system can hydrogenate bicar-
bonate at remarkably low hydrogen pressures (1–5 bar).
dioxide and bicarbonates with TON of up to 727.[8] Soon after,
the same group developed the most active and productive
iron-based catalyst system reported so far. The system,
Fe(BF4)2/tris(2-diphenylphosphine)phenyl)phosphine), can hy-
drogenate bicarbonate to formate with TON of up to 7546 and
carbon dioxide to DMF with TON of up to 5104.[9] Finally, Mil-
stein and co-workers reported an iron–pincer complex, trans-
[(tBu-PNP)FeH2], capable of reducing carbon dioxide and bicar-
bonate at remarkably low pressures (initial H2 pressure 6.2 bar)
with high TONs.[10]
Carbon dioxide is an attractive C1 source compared to other
molecules, such as carbon monoxide, because it is easily and
abundantly available, relatively nontoxic, and renewable. How-
ever, utilizing carbon dioxide still poses a challenge owing to
its thermodynamic stability. In the past decades, the homoge-
neous catalytic hydrogenation of carbon dioxide and its deriva-
tives has been widely investigated.[1] An overwhelming majori-
ty of the catalyst systems known to be active for this reaction
is based on precious metals such as rhodium,[2] ruthenium,[3]
iridium,[4] and palladium.[5] The relative toxicity, limited availa-
bility, and high price of these metals have resulted in a search
for alternatives that are more environmentally benign and sus-
tainable. Iron-based catalysts are especially attractive in this
context because of the abundant availability of the central
metal; however, there have been relatively few studies in the
past.
Although significant progress has been made in the iron-cat-
alyzed hydrogenation of carbon dioxide and bicarbonate, as
described above, challenges still remain. All of the mentioned
highly active iron catalyst systems contain at least two phos-
phine donors, which are often air- and moisture-sensitive and
tedious to prepare. Therefore, from an environmental as well
as a practical standpoint, the development of easily available,
air- and moisture-tolerant iron catalysts for this transformation
is still highly desirable.
A pioneering work by Guan and Casey in 2007 described
a bifunctional catalyst, Knçlker’s iron complex, as an efficient
hydrogen catalyst.[11] Its unique catalytic behavior and easy
access have lead to a number of applications in the last
6 years.[12] Although Knçlker’s iron complex is an active hydro-
gen catalyst, it is air- and light-sensitive. In sharp contrast, its
precursor, the tricarbonyl (cyclopentadienone)iron complex, is
air- and moisture-stable but not an active hydrogen catalyst.
However, it is capable of generating Knçlker’s iron complex
in situ by selective mono-decarbonylation/protonation. By uti-
lizing this property, several groups have recently successfully
applied the tricarbonyl (cyclopentadienone)iron complex or an-
alogues thereof as precatalysts for various redox transforma-
tions.[13] As part of our ongoing efforts to explore the catalytic
properties of Knçlker’s iron complex we present here, for the
first time, this complex as a highly active catalyst for the hydro-
genation of bicarbonate and carbon dioxide to formate.
Based on our previous work on the hydrogenation of alde-
hydes and ketones catalyzed by Knçlker’s iron complex,[14] we
started by investigating the hydrogenation of sodium bicar-
bonate using in situ-generated catalysts. Initially, iron complex
1a was selected as precatalyst for this investigation, and the
results are listed in Table 1. As expected the reaction did not
show any conversion in the absence of iron catalyst (entry 1).
The desired product, sodium formate, was formed with a TON
of 26 in the presence of complex 1a (entry 2). To further im-
prove the reaction, we evaluated the influence of critical reac-
tion parameters such as solvent, temperature, and hydrogen
pressure. The highest activity in a survey of several solvents
was achieved when using ethanol as co-solvent (TON of 447;
Although several achievements in iron-catalyzed reductions
have been made recently,[6] there are only few reports on the
iron-catalyzed hydrogenation of carbon dioxide to formic acid
or its derivatives. In 2003, Jessop and co-workers reported
a combination of FeCl3 or Fe(OAc)2 with bidentate phosphines
for the hydrogenation of carbon dioxide to formic acid with
turnover number (TON) of up to 113.[7] In 2010, Beller, Laurenc-
zy, and co-workers reported an iron complex [generated in situ
from Fe(BF4)2 and
a
tetradentate phosphine ligand,
P(CH2CH2PPh2)3); (PP3)] capable of the hydrogenation of carbon
[a] F. Zhu, L. Zhu-Ge, Prof. Dr. G. Yang, Prof. Dr. S. Zhou
Key Laboratory of Pesticide & Chemical Biology
Ministry of Education, College of Chemistry
Central China Normal University (CCNU)
152 Luoyu Road
Wuhan, Hubei 430079 (PR China)
E-mail: gfyang@mail.ccnu.edu.cn
szhou@mail.ccnu.edu.cn
Homepage: http://klpcb.ccnu.edu.cn/index.html
[b] Prof. Dr. S. Zhou
CCNU–uOttawa Joint Research Centre
Wuhan (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201403234.
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Table 1. Iron-catalyzed hydrogenation of sodium bicarbonate to sodium
formate.[a]
Table 2. Variation of iron complexes of 1b–g for hydrogenation of
sodium
bicarbonate
to
sodium
formate.[a]
Entry
Solvent
T
[8C]
p(H2)[b]
[bar]
Yield[c]
[%]
TON[d]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
THF/H2O
THF/H2O
120
120
120
120
120
100
140
120
120
120
120
120
120
120
120
120
30
30
30
30
30
30
30
1
0
2.6
10.5
1.3
44.7
0
0
26
105
13
447
0
2
iPrOH/H2O
MeOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
Entry
Fe catalyst
Yield[b]
[%]
TON[c]
0.2
4.7
47
2
5
10.4
16.3
22.9
38.2
39.4
31.8
33.8
32.5
104
163
229
382
394
318
338
325
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1 f
44.7
33.5
35.4
12.2
23.2
4.7
447
335
354
122
232
47
10
20
40
60
80
100
1g
0
0
[a] General reaction conditions: NaHCO3 (3 mmol), Fe complex
(0.003 mmol), EtOH/H2O (1.5 mL/3 mL), 1208C, 30 bar H2. [b] Determined
by 1H NMR analysis using DMF as an internal standard. [c] Mol of sodium
formate per mol of catalyst. TMS=trimethylsilyl.
[a] Reaction conditions: NaHCO3 (3 mol), Fe complex (0.003 mol). [b] Pres-
sure at RT. [c] Determined by 1H NMR analysis using DMF as an internal
standard. [d] Mol of sodium formate per mol of catalyst.
entry 5). The catalyst system also showed temperature-de-
pendence. For example, when the reaction was performed at
1008C and 1408C, the TONs dropped to 0 and 2, respectively
(entries 6 and 7). Variation of the hydrogen pressure had a sig-
nificant influence on the activity of the catalyst (entries 8–16).
Notably, at low hydrogen pressures (1 to 5 bar) the catalyst
system still showed good activities (entries 8–10).
Table 3. Iron-catalyzed
hydrogenation
of
carbon
dioxide.[a]
Entry
p(H2)[b]
p(CO2)[b]
[bar]
Yield[c]
[%]
TON[d]
[bar]
1
2
3
4
5[e]
40
50
60
60
30
20
10
20
30
20
<1
<1
<1
<1
–
–
–
–
307
Next, a series of analogues of complex 1a were tested for
the hydrogenation of sodium bicarbonate to sodium formate.
Compared to 1a, other iron complexes showed lower activities
(Table 2, entries 2–6). Complex 1g, bearing a tetraphenyl cyclo-
pentadienone ancillary ligand, did not yield any product at all.
Having established our optimal reaction conditions for the
hydrogenation of sodium bicarbonate, we wondered if this
catalyst system could be applied to the hydrogenation of
carbon dioxide in the presence of sodium hydroxide (Table 3).
A key requirement for this transformation is that Knçlker’s iron
complex is generated in the presence of a large excess of
carbon dioxide. Initial studies suggested that this may be prob-
lematic. For example, the iron-catalyzed hydrogenation of
carbon dioxide in the presence of sodium hydroxide only yield-
ed a trace amount of formate. Considering the Hieber base re-
action taking place under basic conditions, we speculate that
the pH value of the reaction solution may play an important
role for the catalyst system. Indeed, detailed measurements of
the pH of the reaction systems confirmed this speculation. For
example, the pH value of the hydrogenation of bicarbonate
with high activity is close to 8.80; however, the pH value of hy-
drogenation of carbon dioxide with no activity is around 7.45.
Because the in situ generation of Knçlker’s iron complex by
the Hieber base reaction is inhibited by the presence of
carbon dioxide, alternatively, sodium hydroxide was first trans-
30.7
[a] Reaction conditions: NaOH (3 mmol), Fe complex (0.003 mmol), EtOH/
H2O (1.5 mL/3.0 mL) [b] pressure at RT. [c] Determined by NMR analysis
using DMF as an internal standard. [d] Mol of sodium formate per mol of
catalyst. [e] NaOH (3 mmol) was first transformed to sodium bicarbonate
in the presence of CO2, then Fe complex (0.003 mmol) was added to the
solution and pressurized to 30 bar hydrogen.
formed to sodium bicarbonate in the presence of carbon diox-
ide, then complex 1a was added to the solution and pressur-
ized to 30 bar hydrogen. Gratifyingly, a TON of 307 was ach-
ieved with this slightly modified procedure.
Based on previous work by the groups of Knçlker,[15]
Casey,[16] and our own findings, we propose the following
mechanism (Scheme 1): Hydroxide adds to one carbon monox-
ide of the precatalyst 1a and the hydride complex 2 is formed
with the release of carbon dioxide by a Hieber base reaction.
After protonation, Knçlker’s iron complex 3 is formed and sub-
sequent insertion of carbon dioxide into the FeÀH bond forms
the corresponding iron formate complex 4. Formic acid is liber-
ated from 4 to form the 16-electron iron(0) species 5, which re-
generates Knçlker’s iron complex 3 by oxidative addition of
hydrogen.
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Acknowledgements
This work has been funded by the National Science Foundation
of China (21372091), the Research Fund for Doctoral Program of
Higher Education of China (20130144120003), the Science and
Technology
Department
of
Hubei
Provence,
China
(2012FFB03002), and Central China Normal University.
Keywords: carbon dioxide
·
formates
·
homogeneous
catalysis · hydrogenations · iron
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Scheme 1. Plausible mechanism for the hydrogenation of carbon dioxide
catalyzed by complex 1a.
In conclusion, we describe an effective, phosphine-free, air-
and moisture-tolerant iron-based catalyst system for the hydro-
genation of bicarbonate and carbon dioxide to formate. Nota-
bly, it is capable of hydrogenating bicarbonate under remarka-
bly low hydrogen pressures (1–5 bar). Further studies on the
application of this catalytic system to other synthetically inter-
esting substrates are ongoing in our laboratory and will be re-
ported in due course.
Experimental Section
General procedure for catalytic hydrogenation of sodium bicar-
bonate: First, 20–50 mL solutions of 1a in ethanol (0.002m) and
Na2CO3 (1m) were prepared. A 30 mL autoclave was charged with
the catalyst 1a (0.003 mmol) dissolved in 1.5 mL ethanol and
NaHCO3 (3 mmol) dissolved in 3 mL H2O. Once sealed, the auto-
clave was purged 3 times with hydrogen, then pressurized to
30 bar and heated at 1208C for 24 h. After reaction, the autoclave
was cooled to RT and depressurized. The solvent was evaporated
under reduced pressure and the yield was measured through
1H NMR spectroscopy in D2O with DMF as an internal standard.
The results shown in Table 1 and Table 2 were obtained by per-
forming each reaction at least three times to confirm its reproduci-
bility.
General procedure for catalytic hydrogenation of CO2: A 30 mL
autoclave was charged with NaOH (3 mol) dissolved in 3 mL H2O.
The autoclave was pressurized at ambient temperature with CO2 to
20 bar and the solution was stirred at RT for 1 h. After release of
CO2, the catalyst 1a (0.003 mmol) was added to the autoclave.
Once sealed, the autoclave was purged 3 times with hydrogen,
then pressurized to 30 bar and heated at 1208C for 24 h. After re-
action, the autoclave was cooled to RT and depressurized. The sol-
vent was evaporated under reduced pressure and the yield was an-
alyzed by 1H NMR spectroscopy in D2O with DMF as an internal
standard.
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Received: November 6, 2014
Published online on && &&, 0000
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F. Zhu, L. Zhu-Ge, G. Yang,* S. Zhou*
&& – &&
Iron-Catalyzed Hydrogenation of
Bicarbonates and Carbon Dioxide to
Formates
Carbon dioxide is an attractive C1
source because it is easily and abun-
dantly available, relatively nontoxic, and
renewable. An effective, phosphine-free,
air- and moisture-tolerant catalyst
for the hydrogenation of bicarbonate
and carbon dioxide to formate is de-
scribed. The catalyst system hydrogen-
ates bicarbonate at remarkably low hy-
drogen pressures.
system based on Knçlker’s iron complex
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