Well-defined iron catalyst for improved hydrogenation of carbon dioxide and bicarbonate

Article abstract of DOI:10.1021/ja307924a

The most efficient, stable, and easy-to-synthesize non-noble metal catalyst system for the reduction of CO₂ and bicarbonates is presented. In the presence of the iron(II)-fluoro-tris(2-(diphenylphosphino)phenyl)phosphino] tetrafluoroborate complex 3, the hydrogenation of bicarbonates proceeds in good yields with high catalyst productivity and activity (TON > 7500, TOF > 750). High-pressure NMR studies of the hydrogenation of carbon dioxide demonstrate that the corresponding iron-hydridodihydrogen complex 4 is crucial in the catalytic cycle.

Full text of DOI:10.1021/ja307924a

Article
pubs.acs.org/JACS
Well-Defined Iron Catalyst for Improved Hydrogenation of Carbon
Dioxide and Bicarbonate
Carolin Ziebart, Christopher Federsel, Pazhamalai Anbarasan, Ralf Jackstell, Wolfgang Baumann,
Anke Spannenberg, and Matthias Beller*
Leibniz-Institute for Catalysis e.V. at the University of Rostock, Albert Einstein-Strasse 29a, 18059, Rostock, Germany
*
S Supporting Information
ABSTRACT: The most efficient, stable, and easy-to-synthesize non-noble metal catalyst system for the
reduction of CO2 and bicarbonates is presented. In the presence of the iron(II)-fluoro-tris(2-
(
diphenylphosphino)phenyl)phosphino]tetrafluoroborate complex 3, the hydrogenation of bicarbonates
proceeds in good yields with high catalyst productivity and activity (TON > 7500, TOF > 750). High-
pressure NMR studies of the hydrogenation of carbon dioxide demonstrate that the corresponding iron-
hydridodihydrogen complex 4 is crucial in the catalytic cycle.
INTRODUCTION
compared with noble metal-based complexes. Although some
10
■
recent catalyst improvement was reported by us as well as
The usage and storage of CO is of fundamental interest for
chemistry and energy technologies. Clearly, refinement of
2
11
1
Milstein and co-workers, basically, all the non-noble metal
12
catalyst systems show still lower activity.
carbon dioxide to more valuable chemical products will not
solve the global problem of increasing CO2 emissions.
Nevertheless, CO2 constitutes already today an important
C1-source for the chemical industry, for example, for the
production of urea. It is likely its use in the future will
significantly increase as a result of the lower toxicity and
abundant availability in comparison with the widely employed
In addition to carbon dioxide, the reduction of bicarbonate is
of considerable interest, too, because CO can be easily trapped
2
from waste streams in basic solution. On the basis of this
principle, recently we developed a reversible energy storage
system in which bicarbonate is hydrogenated to sodium
formate and the latter releases hydrogen on demand to form
13
2
again bicarbonate without loosing CO2. Unfortunately, as a
catalyst a special ruthenium complex has to be applied for this
process. In fact, only a few reports using iron or cobalt catalysts
CO. In addition, with respect to energy storage, interesting
applications for carbon dioxide are hydrogen storage and
3
release technologies. Here, CO is reversibly transformed with
2
10,11
for the hydrogenation of bicarbonate are known.
hydrogen into valuable secondary energy carriers, such as
methane, methanol, or formic acid (Scheme 1).
In continuation of our work to develop “real” iron-catalyzed
14
hydrogenation/dehydrogenation reactions, here, we present
the most active and productive Fe catalyst for the reduction of
carbon dioxide and bicarbonates known to date.
Scheme 1. Hydrogenation of CO and Selected Products
2
RESULTS AND DISCUSSION
■
Synthesis of Tetradentate Ligands. Molecular-defined
iron complexes with tri- and tetradentate ligands show
remarkable behavior in catalytic hydrogenations and hydro-
Notably, the first reduction step of carbon dioxide to formic
acid proceeds under comparably mild conditions and does not
produce any water as a side product, which lowers the energy
efficiency in the cases of methanol and methane. Since the
15
silylations. Parallel to other groups, we also initiated a
program to synthesize iron complexes of multidentate
phosphorus ligands. More specifically, we had the idea to use
tris(2-(diarylphosphino)aryl)phosphines as ligands.
4
pioneering work of Graf and Leitner, Jessop and Noyori and
5
6
Although the preparation of most multidentate ligands
involves several reaction steps and tedious purification,
advantageously the respective phenyl ligand 2 is synthesized
in a one-pot reaction! By improving a known literature
́
co-workers, as well as Joo, various organometallic catalysts
have been developed in the past decade. Recently, high catalyst
turnover numbers for this hydrogenation were achieved by the
7
8
groups of Nozaki, and Himeda. Moreover, interesting process
16
9
protocol, 1,2-bromochlorobenzene is converted with magne-
improvements were reported by Schaub and Paciello at BASF.
Despite an increasing scientific interest in the hydrogenation of
carbon dioxide, the use of biorelevant metals such as iron,
nickel or cobalt as catalysts has still scarcely been investigated
Received: September 3, 2012
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja307924a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
sium into the corresponding aryne reagent in minutes. In situ
reaction with potassium diphenylphosphide and final quench-
ing of the metalated 2-diphenylphosphinophenyl with PCl3
gave the desired ligand 2 straightforward in 42% isolated yield
Table 1. Iron-Catalyzed Hydrogenation of Sodium
Bicarbonate and Sodium Methylcarbonate to Sodium
a
Formate
b
c
entry
substrate
T [°C]
PH2 [bar]
yield [%]
TON
(
Scheme 2). The structure of the ligand was confirmed by
common analytical methods and X-ray crystallography
Supporting Information Figure S1).
1
2
3
4
5
6
NaHCO3
80
100
60
60
60
60
60
85
30
60
60
60
60
60
60
60
60
86
80
47
15
85
52
83
77
77
84
48
85
80
71
1597
1485
873
NaHCO3
(
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
120
100
100
100
100
100
80
279
Scheme 2. Synthesis of the Tetradentate Ligand 2
1579
966
d
7
1541
7546
3850
1600
914
e
8
f
9
g
10
11
12
h
i
80
80
1619
1524
1352
j
Synthesis of Iron Complexes. Next, we set out to prepare
13
NaHCO
NaCO Me
3
80
1
4
100
iron complexes of 2. Although reaction of Fe(acac) with 2 did
3
3
a
not result in any defined complex, to our delight, the
combination of 2 and Fe(BF ) ·6H O gave the corresponding
Reaction conditions, entries 1−13: 40 mmol of substrate, 0.021 mmol
b
of Fe(BF ) ·6H O, 0.021 mmol of 2, 40 mL of MeOH, 20 h. Pressure
4 2 2
c
4
2
2
1
at room temperature. Entries 1−13: yields based on the ratio of H
complex 3 {[FeF(2)]BF }. Crystals suitable for X-ray analysis
4
d
NMR signals of sodium formate to THF (internal standard). 0.021
were grown from a solution of 3 in THF. The coordination
geometry at the iron center is distorted trigonal bipyramidal,
and selected bond lengths and angles are given below in Figure
e
mmol of the defined complex 3 (Figure 1). 49 mmol of substrate,
0
f
.005 mmol of catalyst loading of 3, 30 mL of MeOH. Reaction time,
g
h
5
h; TOF, 770. 0.021 mmol of Fe(acac) . 0.021 mmol of FeCl2.
3
1. Interestingly, this complex exhibits one fluorine ligand
i
j
0
.021 mmol of Fe(acac) . 0.021 mmol of FeF .
2
2
for any iron-catalyzed hydrogenation of carbon dioxide and its
derivatives. It is important to note that the catalyst is stable to
>
100 °C, which is an advantage to the previously known more
10b
sensitive ethyl-bridged ligand. At higher and lower temper-
atures, the yield of formate decreased (Table 1, entries 3 and
4
). Hence, for further optimization, we decided to choose 100
°
C as the standard temperature. Variation of the hydrogen
pressure had no significant influence on the activity of the
catalyst, but it does affect the equilibrium of bicarbonate and
formate (Figure 2). Pressure higher than 60 bar resulted in
Figure 1. Molecular structure of the complex 3 {[FeF(2)]BF4}.
Thermal ellipsoids correspond to 30% probability. Hydrogen atoms as
−
well as the counterion BF4 are omitted for clarity. Selected bond
lengths (Å) and angles (°): F1−Fe1, 1.849(2); P1−Fe1, 2.2892(9);
P2−Fe1, 2.1915(9); P3−Fe1, 2.2672(9); P4−Fe1, 2.2921(9). F1−
Fe1−P1, 97.11(6); P1−Fe1−P2, 84.18(3); P1−Fe1−P3, 120.06(3);
P1−Fe1−P4, 116.95(3); P3−Fe1−P4, 119.54(3).
derived from the original BF4− anion. Although different metal
17
complexes of ligand 2 have been analytically characterized, to
the best of our knowledge, no catalytic experiments have yet
been reported.
Optimization of the Reaction Conditions the Bicar-
bonate Reduction. In initial catalytic experiments on the
hydrogenation of sodium bicarbonate to sodium formate, either
a combination of Fe(BF ) ·6H O and the ligand 2 or the
defined complex 3 were used. Similar results were obtained
when comparing the in situ-generated catalyst with the defined
complex, demonstrating the immediate formation of the same
active catalyst species (Table 1, entry 2 vs entry 7). In both
cases, good yields (80−83%) and with respect to iron high
turnover numbers (around 1500) are achieved. Already, these
results represent the best catalyst productivities ever reported
Figure 2. Yield of formate as a function of pressure.
4
2
2
slightly increased yields (Table 1, entry 5), whereas only 52% of
formate was obtained at 30 bar hydrogen pressure (Table 1,
entry 6). The improved efficiency of the iron complex 3 was
confirmed by lowering the catalyst loading to 0.01 mol % Fe,
which still resulted in good yields and an excellent TON of
>7500 (Table 1, entry 8). With respect to the iron precursor, it
is noteworthy that apart from iron tetrafluoroborate, very
B
dx.doi.org/10.1021/ja307924a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
inexpensive Fe(acac) , Fe(acac) , FeF and even simple FeCl
2
Scheme 3. Postulated Mechanism for the Iron-Catalyzed
Hydrogenation of Carbon Dioxide
3
2
2,
showed significant catalytic reactivity (Table 1, entries 10−13).
Next, we studied the hydrogenation of sodium methylcar-
bonate (Table 1, entry 14) which might be an intermediate in
the hydrogenation of carbon dioxide to methyl formate. For
this purpose, CO was passed into a solution of sodium
2
methanolate in methanol to give sodium methylcarbonate.
Hydrogenation of the resulting white precipitate gave 71% yield
of sodium formate.
Hydrogenation of Carbon Dioxide. The hydrogenation
of carbon dioxide in methanol was also investigated in the
presence of amines. Without any further optimization the direct
hydrogenation of CO in the presence of triethylamine led to a
2
mixture of formic acid and methyl formate with a conversion of
carbon dioxide of around 15% (Table 2, entry 1). Addition of
a
Table 2. Iron-Catalyzed Hydrogenation of Carbon Dioxide
PH2
yield product yield HCOOH
/CO
2
c
d
b
e
entry
1
product
[%]
[%]
[bar]
TON
HCO Me
7.0
0
7.9
30/30
30/30
30/30
70/30
30/30
1692
1897
2329
5104
2114
2
f
2
HCO Me
16.7
2.9
2
3
DMF
70
g
4
DMF
74
39
7.7
5
HCONEt2
9.1
a
Reaction conditions: 0.014 mmol of Fe(BF ) ·6H O, 0.014 mmol of
4
2
2
2
, 20 mL of MeOH, 159 mmol of CO , 100 °C, 20 h. Entries 1−2: 36
2
°C), a second hydride signal (−12.10 ppm) appeared, together
with three phosphorus signals (135.8 ppm, rel. intensity 1, 86.2
ppm, rel. intensity 2, 82.8 ppm, rel. intensity 1) at the expense
of the resonances of complex 4. However, no formation of
formic acid or formate is observed in the proton NMR after
several hours. We explain this observation with equilibrium
between complexes 4 and 5, in which the hydrogen ligand is
exchanged either with solvent molecules or carbon dioxide.
Only after addition of base (triethylamine) did we immediately
observe a product signal, δ(1H) 8.95 ppm, and the complete
disappearance of complex 4. Apparently, the presence of base is
necessary to transform 4, which constitutes the resting state in
the catalytic cycle, into a more active reduction catalyst 6.
Subsequent insertion of carbon dioxide into the Fe−H bond
forms the corresponding iron formate complex. Final
protonation of 7 and coordination of hydrogen closes the
catalytic cycle, and formic acid and methyl formate can be
detected by NMR. Detailed information on the NMR
experiments under pressure is compiled in the Supporting
Information.
mmol of NEt . Entries 3−5: 40 mmol of dialkylamine, 5 mL of THF.
3
b
c
Pressure at room temperature. Entries 1−2: GC-yield based on CO
2
with diglyme as internal standard. Entries 3−5: GC-yield based on the
dialkylamine with diglyme as internal standard. HPLC-yield of formic
acid based on CO with benzoic acid as internal standard. Millimoles
of converted CO /mmol catalyst. Addition of 2.9 mL of water. 80
d
e
2
f
g
2
mmol of HNMe (2 M in MeOH).
2
water suppressed the formation of methyl formate (Table 2,
entry 2). When using dialkylamines as the base, the formation
of formamides is observed. As an example, dimethylamine led
to 74% yield of dimethylformamyde (DMF) (with respect to
the amine) and 7.7% of formic acid, which corresponds to a
turnover number >5100 (Table 2, entries 3 and 4). In
18
12c
agreement with the work of Sugita, Inoue,
and our
10b
previous reported iron system, longer alkyl chains resulted in
lower yields. Nevertheless, the achieved turnover numbers are
at least 1 order of magnitude higher in comparison with any
1
0−12
previously reported iron system.
Proposed Mechanism. To understand the mechanism of
these Fe-catalyzed hydrogenations in more detail, in situ
nuclear magnetic resonance (NMR) measurements of different
catalytic experiments were performed. As shown in Scheme 3,
in the catalytic cycle an initial formation of an iron-
hydridodihydrogen complex 4 is proposed. Indeed, in situ
NMR spectroscopy of a solution of 3 under 23 bar hydrogen
pressure showed that complex 4 is formed from the Fe−F
complex 3 at 80 °C within a few hours. Notably, complex 3 is
paramagnetic and has therefore been identified using MS and
X-ray, Figure 1), whereas 4 is characterized by two 31P NMR
CONCLUSION
■
In conclusion, we present the most active and productive iron-
based catalyst system for hydrogenation of bicarbonates and
carbon dioxide to formates and formamides. The key to success
is the tetradentate phosphorus ligand 2, which is easily
accessible in a one-pot procedure. The resulting iron
complexes, such as 3, are air- and temperature-stable and give
similar catalytic results compared with the in situ generated
catalysts. It can be expected that this novel iron complex will
also be of value for other catalytic reductions, such as
signals (142.5 ppm, rel. intensity 1, and 87.0 ppm, rel. intensity
1
20
3
, JP,P = 37 Hz) and one broad H signal at −9.18 ppm,
hydrogenation of nitroarenes and carbonyl compounds.
representing both the hydride and the dihydrogen ligand.
Complex 4 behaves similarly to its known analogue with the
tris[2-(diphenylphosphino)ethyl]phosphine (tetraphos) li-
ASSOCIATED CONTENT
Supporting Information
Further information for ligand synthesis, reaction parameters
and mechanistic investigations are included in the Supporting
■
*
S
1
9
gand, (see the Supporting Information). When CO was
2
added to the reaction solution (up to 30 bar total pressure at 80
C
dx.doi.org/10.1021/ja307924a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Information. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
E-mail: Matthias.Beller@catalysis.de
■
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Notes
The authors declare no competing financial interest.
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Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Science 2011, 333,
ACKNOWLEDGMENTS
This work was funded by BMBF, Bundesministerium fu
Bildung und Forschung (01RC 1006H).
■
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