Iron-catalyzed hydrogen production from formic acid

Article abstract of DOI:10.1021/ja100925n

Hydrogen represents a clean energy source, which can be efficiently used in fuel cells generating electricity with water as the only byproduct. However, hydrogen generation from renewables under mild conditions and efficient hydrogen storage in a safe and reversible manner constitute important challenges. In this respect formic acid (HCO₂H) represents a convenient hydrogen storage material, because it is one of the major products from biomass and can undergo selective decomposition to hydrogen and carbon dioxide in the presence of suitable catalysts. Here, the first light-driven iron-based catalytic system for hydrogen generation from formic acid is reported. By application of a catalyst formed in situ from inexpensive Fe₃(CO)₁₂, 2,2′:6′2′′-terpyridine or 1,10-phenanthroline, and triphenylphosphine, hydrogen generation is possible under visible light irradiation and ambient temperature. Depending on the kind of N-ligands significant catalyst turnover numbers (>100) and turnover frequencies (up to 200 h^-1) are observed, which are the highest known to date for nonprecious metal catalyzed hydrogen generation from formic acid. NMR, IR studies, and DFT calculations of iron complexes, which are formed under reaction conditions, confirm that PPh₃ plays an active role in the catalytic cycle and that N-ligands enhance the stability of the system. It is shown that the reaction mechanism includes iron hydride species which are generated exclusively under irradiation with visible light.

Full text of DOI:10.1021/ja100925n

Published on Web 06/15/2010
Iron-Catalyzed Hydrogen Production from Formic Acid
Albert Boddien,† Bjo¨rn Loges,† Felix Ga¨rtner,† Christian Torborg,† Koichi Fumino,‡
Henrik Junge,† Ralf Ludwig,*,†,‡ and Matthias Beller*,†
Leibniz - Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock, Albert Einstein Str. 29a, Rostock,
18059, Germany and UniVersita¨t Rostock, Institut fu¨r Chemie, Abteilung Physikalische Chemie,
Dr. Lorenz Weg 1, Rostock, 18059, Germany
Received September 17, 2009; E-mail: matthias.beller@catalysis.de; ralf.ludwig@uni-rostock.de
Abstract: Hydrogen represents a clean energy source, which can be efficiently used in fuel cells generating
electricity with water as the only byproduct. However, hydrogen generation from renewables under mild
conditions and efficient hydrogen storage in a safe and reversible manner constitute important challenges.
In this respect formic acid (HCO2H) represents a convenient hydrogen storage material, because it is one
of the major products from biomass and can undergo selective decomposition to hydrogen and carbon
dioxide in the presence of suitable catalysts. Here, the first light-driven iron-based catalytic system for
hydrogen generation from formic acid is reported. By application of a catalyst formed in situ from inexpensive
Fe3(CO)12, 2,2′:6′2′′-terpyridine or 1,10-phenanthroline, and triphenylphosphine, hydrogen generation is
possible under visible light irradiation and ambient temperature. Depending on the kind of N-ligands
significant catalyst turnover numbers (>100) and turnover frequencies (up to 200 h-1) are observed, which
are the highest known to date for nonprecious metal catalyzed hydrogen generation from formic acid. NMR,
IR studies, and DFT calculations of iron complexes, which are formed under reaction conditions, confirm
that PPh3 plays an active role in the catalytic cycle and that N-ligands enhance the stability of the system.
It is shown that the reaction mechanism includes iron hydride species which are generated exclusively
under irradiation with visible light.
Introduction
greenhouse gas CO2, recycling is possible through photosyn-
thesis in nature or technical processes in industry (CCS).3
Hydrogen has attracted considerable attention as an alternative
clean energy vector. So far, hydrogen production via steam
reforming and coal gasification is based to >95% on limited
fossil resources such as coal and oil. On a mid- to long-term
basis, there is an increasing demand for alternative technologies
to generate hydrogen in a more sustainable manner.1 The
development of improved technologies for hydrogen generation
and hydrogen storage in a safe and reversible manner is a
prerequisite for the utilization of H2 as transportation fuel.2 Even
though hydrogen generation from biomass like methanol,
ethanol, or formic acid also involves the emission of the
Notably, natural iron-based hydrogenases are known as highly
efficient catalysts to produce hydrogen or utilize it as an energy
source at ambient conditions. In this respect biomimetic
hydrogen production, particularly employing sunlight and easily
available non-noble metal catalysts, is a major challenge for
science.4 While iron-based enzymatic redox processes have been
extensively studied in the past decades, only a few reduction
reactions applying synthetic iron complexes, especially light-
involving processes, are known today. This is in clear contradic-
tion to the numerous examples of reduction processes in nature.5
Among the different hydrogen storage materials, formic acid
(4.4 wt % hydrogen) has recently received considerable atten-
tion.6 HCO2H is one of the major products formed in biomass
† Leibniz - Institut fu¨r Katalyse e.V. an der Universita¨t Rostock.
‡ Universita¨t Rostock, Institut fu¨r Chemie.
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8924 J. AM. CHEM. SOC. 2010, 132, 8924–8934
10.1021/ja100925n  2010 American Chemical Society

Iron-Catalyzed Hydrogen Production from Formic Acid
A R T I C L E S
15 min and full conversion.14 In 2008, Fukuzumi and co-workers
investigated [Rh(Cp*)(bipy)-(H2O)](SO4) and similar com-
plexes for the hydrogen generation from aqueous formic acid
solutions. They demonstrated that formic acid decomposition
occurs via formate and a hydride complex.15 More recently,
this group demonstrated that heteronuclear iridium-ruthenium
complexes are highly active catalysts for hydrogen generation
Figure 1. Hydrogen generation from biomass via formic acid as an
intermediate compound.
in an aqueous solution under ambient conditions giving a TOF
15b
of ∼426 h-1
.
Himeda et al. focused on iridium complexes
for hydrogen generation from formic acid/sodium formate in
aqueous solution and achieved an initial TOF of 14 000 h-1 at
90 °C.16 Our group and Laurenczy et al. independently
demonstrated that hydrogen generation is also possible under
relatively mild conditions using ruthenium phosphine com-
plexes.17 We identified several ruthenium phosphine complexes
which are capable of generating hydrogen from formic acid
amine adducts selectively at room temperature.18 The catalyst
activity is strongly influenced by the nature and the concentration
of amine in solution, which is not consumed during the reaction
and can easily be recovered from the reaction solution after full
conversion.19 An active catalyst system containing N,N-dim-
ethylhexylamine, [RuCl2(benzene)]2, and 1,2-bis(diphenylphos-
phino)ethane (dppe) was investigated in both batch and con-
tinuous mode and reached at room temperature a turnover
number (TON ) mol of H2/mol of catalyst) of more than
260 000 with a TOF of 900 h-1, which is the highest activity
for hydrogen generation from formic acid.20 Most recently, we
Figure 2. Hydrogen storage via carbon dioxide-formic acid conversion.
Scheme 1. Formic Acid Decomposition Pathways and Their
Thermodynamic Properties9c,10
processing such as fermentation, pyrolysis, and supercritical
reactions and can undergo selective decomposition to hydrogen
and carbon dioxide only in the presence of a catalyst (Figure
1).7 In addition to hydrogen generation, a sustainable and
reversible energy storage cycle can be envisioned by storage
of hydrogen in formic acid and release from it (Figure 2). Here,
carbon dioxide is converted to formic acid or formate derivatives
either electrochemically8 or by catalytic hydrogenation.9 The
resulting products are liquid at ambient conditions and can thus
be handled, stored, and transported easily.
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(Scheme 1). For the subsequent conversion of hydrogen into
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been reported in recent years. In 1998, Puddephat et al. studied
the binuclear ruthenium phosphine complex [Ru2(µ-CO)(CO)4(µ-
dppm)2] for selective hydrogen generation from formic acid.
They achieved with their system at room temperature a turnover
frequency (TOF ) mol H2/mol catalyst ·h-1) of ∼500 h-1 after
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J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 8925

A R T I C L E S
Boddien et al.
showed that ruthenium phosphine catalyst systems can be
triggered and accelerated via irradiation with visible light.21
An actual goal in organometallic catalysis is the replacement
of noble metal-based catalysts, such as ruthenium, iridium,
palladium, and rhodium, with nonprecious metal catalysts such
as iron compounds.22,23 Until now, no homogeneous non-noble
metal catalyst system is known for selectiVe hydrogen generation
from formic acid under ambient conditions.24,25 Herein, we
report that simple iron carbonyl phosphine complexes allow for
this transformation in the presence of visible light.
Results and Discussion
Catalyst Design. We started our investigation with an initial
catalyst testing of numerous non-noble metal precursors in
combination with different phosphorus and nitrogen containing
ligands. For this purpose we applied the convenient method of
Jessop et al. who used a visual dye based assay screening to
discover potential catalysts suitable for hydrogenation of carbon
dioxide to formic acid.26 We adopted this method, using pressure
tubes with valves as assays and the acid-base indicator
bromthymol blue to identify promising candidates, for hydrogen
generation from formic acid (see Supporting Information).27 To
ensure that the indicator does not influence catalysis it was added
after the reaction was finished. The color change of the indicator
from yellow to blue takes place at a ratio of 2.08 to 1.47 of
formic acid/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in DMSO.
In general, the reactions were performed with 60 µmol of
catalyst in DMSO in 1 mL of formic acid/DBU mixture (5:3)
at 60-120 °C for 24-72 h under an inert gas atmosphere
(argon). Organometallic precursors including Cr, Mn, Fe, Co,
Ni, Cu, and Mo-compounds together with different P- and
N-ligands were tested, and suitable candidates identified. All
reactions which showed color changes were tested again using
a procedure with a quantitative and qualitative determination
of the evolved gases. In such a standard experiment a double-
walled thermostatically controlled reaction vessel was evacuated,
heated, and purged with argon 10 times to remove any other
gases. The metal complexes (60 µmol of metal) and ligands
Figure 3. TON of HCO2H decomposition using 20 µmol of Fe3(CO)12/
PPh3/tpy (Fe/PPh3/tpy 1:1:1) in 5 mL of 5HCO2H·2NEt3 + 1 mL of DMF,
ambient pressure, reaction time 3 h, light source (Xenon lamp 300 W, 385
nm cutoff via hot mirror).
(mostly 1 equiv) were added either as powders in a Teflon
crucible and 1 mL of solvent or from a freshly prepared stock
solution (1 mL). A 5 mL aliquot of 5HCO2H·2NEt3 (FA/TEA)
was placed in the vessel, and the desired temperature was kept
constant. The reactions were started after equilibration for at
least 30 min. The volume of evolved gases was quantitatively
measured using automatic gas burets.18,28 In addition, gases were
qualitatively and quantitatively determined by GC (gas chro-
matograph HP6890N, carboxen 1000, TCD, external calibra-
tion). The conversion of formic acid in general did not exceed
10%.
The best catalyst system identified was triirondodecacarbonyl
(Fe3(CO)12) in the presence of triphenylphosphine (PPh3), 2,2′:
6′2′′-terpyridine (tpy), and dimethylformamide (DMF). This
system was capable of generating hydrogen from formic acid
amine adducts at temperatures above 100 °C, whereas significant
gas evolution occurred at 120 °C (Figure 3, blue column).
Unfortunately, the selectivity of the thermal reaction is very
low (H2 or CO2/CO 1:5), showing that dehydration is favored
at higher temperatures.
However, by testing the in situ generated catalyst system
under visible light irradiation hydrogen generation even occurred
at room temperature (Figure 3)! The light source which was
used for irradiation was a 300 W PerkinElmer Cermax PE300BF
Xenon Arc lamp. An A2-005 hot mirror was employed as a
filter to remove the UV portion (<385 nm) and IR portion (>750
nm) of the light spectrum. Irradiation of the solution with visible
light exclusively led to dehydrogenation, and only traces of CO
were detected via GC analysis. The catalyst activity increased
gradually between 25-60 °C and leveled off at ∼80 °C (Figure
3, red column).
Additionally we tested also other metal carbonyl complexes
in low oxidation states to determine whether similar effects upon
irradiation can be observed. All reactions shown in Table 1 were
performed at ambient temperature (40 °C), in DMF, and in the
presence of 60 µmol of metal following our standard protocol.
In cases where significant gas evolution occurred, the gas
mixtures contained H2:CO2 (1:1) and only traces of CO.
Reactions with molybdenumhexacarbonyl (Mo(CO)6) and
dimanganesedecacarbonyl (Mn2(CO)10) gave only slight activity
(21) Loges, B.; Boddien, A.; Noyes, J. R.; Baumann, W.; Junge, H.; Beller,
M. Chem. Commun. 2009, 28, 4185.
(22) For recent reviews and highlights on iron catalysis, see: (a) Plietker,
B., Ed. in Iron Catalysis in Organic Chemistry; Wiley-VCH: Wein-
heim, 2008. (b) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem.,
Int. Ed. 2008, 47, 3317. (c) Gaillard, S.; Renaud, J.-L. ChemSusChem
2008, 1, 505. (d) Bolm, C.; Legros, J.; Paith, J. L.; Zani, L. Chem.
ReV. 2004, 104, 6217. (e) Correa, A.; Manchen˜o, O. G.; Bolm, C.
Chem. Soc. ReV. 2008, 37, 1108. (f) Sherry, B. D.; Fu¨rstner, A. Acc.
Chem. Res. 2008, 41, 1500. (g) Fu¨rstner, A. Angew. Chem., Int. Ed.
2009, 48, 1364. (h) Bullock, R. M. Angew. Chem., Int. Ed. 2007, 46,
7360.
(23) Selected examples from our group: (a) Anilkumar, G.; Bitterlich, B.;
Gelalcha, F. G.; Tse, M. K.; Beller, M. Chem. Commun. 2007, 289.
(b) Gelalcha, F. G.; Bitterlich, B.; Anilkumar, G.; Tse, M. K.; Beller,
M. Angew. Chem., Int. Ed. 2007, 46, 7293. (c) Shi, F.; Tse, M. K.;
Pohl, M.-M.; Bru¨ckner, A.; Zhang, S.; Beller, M. Angew. Chem., Int.
Ed 2007, 46, 8866. (d) Kischel, J.; Jovel, I.; Mertins, K.; Zapf, A.;
Beller, M. Org. Lett. 2006, 8, 19. (e) Iovel, I.; Mertins, K.; Kischel,
J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 3913.
(24) In this regard the current work of Wills et al. should be noted, who
demonstrated recently the decomposition of formic acid in the presence
of Fe(II) and Fe(III) catalysts at 120 °C with a turnover number (TON)
of 3.
(25) Morris, D. J.; Clarkson, G. J.; Wills, M. Organometallics 2009, 28,
4133.
(26) Tai, C.-C.; Chang, T; Roller, B.; Jessop, P. G. Inorg. Chem. 2003,
42, 7340.
(27) Usage of the described pressure tube: (a) Hamid, M. H. S. A.; Allen,
C. L.; Lamb, G. W.; Maxwell, A. C.; Maytum, H. C.; Watson, A. J. A.;
Williams, J. M. J J. Am. Chem. Soc. 2009, 131, 1766.
(28) Loges, B.; Junge, H.; Spilker, B.; Fischer, C.; Beller, M. Chem. Ing.
Tech. 2007, 79, 741.
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8926 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Iron-Catalyzed Hydrogen Production from Formic Acid
A R T I C L E S
Table 1. Activity of Different Non-Noble Metal Carbonyl
Complexesa
entry
metal precursor
V2h/mL
V3h/mL
TON2h
TON3h
1
Mo(CO)6
Mn2(CO)10
Cr(CO)6
Co2(CO)8
Fe(CO)5
3.5
5.9
1.3
1.6
32
44
50
29
44
4.4
6.2
1.5
1.8
41
59
68
38
56
1.2
2.0
-
1.5
2.1
-
2
3b
4b
5
-
-
11
15
17
9.8
15
1.6
14
20
23
13
19
1.8
6
7
8
9
Fe2(CO)9
Fe3(CO)12
Fe(CO)3COT
Fe(CO)3BDAc
[CpFe(CO)2]2
10
4.7
5.3
a Reaction conditions: 60 µmol of [M], PPh3, tpy (Fe/PPh3/tpy 1:1:1),
1 mL DMF, 5 mL 5HCO2H·2NEt3, light irradiation via 300 W xenon
lamp (385 nm cutoff via hot mirror), reaction time 3 h, gas measured
via automatic gas buret and analyzed via GC (H2/CO2 1:1). b Neither H2
nor CO2 detected. c BDA ) benzylideneacetone; the complex was
synthesized according to Lewis or Brookhart.29
Figure 4. UV/vis spectrum of Fe3(CO)12 + 3 equiv of PPh3 + 3 equiv of
tpy (Fe/PPh3/tpy 1:1:1) in DMF and 5HCO2H·2NEt3 after 2 h of visible
light irradiation.
To distinguish between photoinduced and photoassisted
reactions we performed experiments where light is turned on
and off during constant time intervals (Figure 5). To minimize
temperature effects during these switches we performed the
reaction at 60 °C. The gas volume was quantified via gas buret,
and the hydrogen content was analyzed online using a hydrogen
sensor (Hach Ultra Analytics GmbH) and GC after the reaction.
During the first 5 min in the absence of light, only a slight
volume increase is observed, caused by interferences of the
equilibrium during the start. Then, irradiation with visible light
generated, through a photoinduced effect, an active iron catalyst
system and hydrogen production started. The average reaction
rate within the first and the second light period was 1 mL ·min-1
Table 2. Dependence of Catalytic Activity on the Ligand
Combination PPh3, tpy, and the Presence of Aminea
entry
ligand a
ligand b
V3h [mL]
TON3h
1
2
tpy
-
tpy
tpy
-
1.9
16
-
PPh3
PPh3
PPh3
5.4
3
68 (131)b
5.6
23 (44)b
1.9
4c
a Reaction conditions: 20 µmol of Fe3(CO)12, PPh3, tpy (Fe/PPh3/tpy
1:1:1), 1 mL DMF, 5 mL 5HCO2H·2NEt3, 40 °C, light irradiation via
300 W xenon lamp (385 nm cutoff via hot mirror), 3 h reaction time,
gas measured via automatic gas buret and analyzed via GC (H2/CO2
1:1). b 24 h. c Without amine.
and decreased during the following switches to 0.5 mL ·min-1
.
(Table 1, entries 1,2) under ambient conditions. In the presence
of chromiumhexacarbonyl (Cr(CO)6) and dicobaltoctacarbonyl
(Co2(CO)8) no activity at all is observed (Table 1, entries 3, 4).
In addition to Fe3(CO)12 other iron carbonyl complexes showed
significant activity. For example, ironpentacarbonyl (Fe(CO)5)
and irontricarbonylcyclooctatetraene (Fe(CO)3COT) showed
comparable activity (Table 1, entries 5, 8). Higher activity is
provided by diironnonacarbonyl (Fe2(CO)9) and irontricarbon-
ylbenzylidenacetone (Fe(CO)3BDA), whereas the highest TON
of 23 after 3 h is achieved with Fe3(CO)12 as a precatalyst (Table
1, entries 6, 7, 9). To identify the influence of the different
ligands on the catalytic system we performed reactions at 40
°C under irradiation with only one type of ligand present.
Moreover, the reaction was performed without amine present
(Table 2).
While Fe3(CO)12 with 3 equiv of tpy (Fe/tpy 1:1) showed
almost no activity, significant hydrogen evolution occurred with
3 equiv of PPh3 (Fe/PPh3 1:1) (Table 2, entries 1, 2). However,
in the presence of a catalyst system containing Fe/tpy/PPh3 (ratio
1:1:1) under irradiation a significant higher activity is obtained,
resulting in a TON of 44 (Table 2, entry 3) after 24 h. The
absence of amine resulted in very low activity (Table 2, entry
4).
In the first 5 min of each light period, the reaction rate reached
a maximum which is explained by an initial volume increase
due to an initial temperature increase, according to a small heat
transfer from the lamp. This is not the case under constant
irradiation in standard experiments. The equilibrium is reached
within a short time by the thermostat. Figure 5 shows clearly
that without irradiation no gas evolution is observed; therefore
light is essential to generate an active catalyst. MoreoVer, when
light irradiation is stopped, gas eVolution breaks down im-
mediately, which unambiguously indicates that light not only
is responsible for generating an actiVe system but also driVes
the catalytic cycle! Therefore, we assume a photoassisted
hydrogen generation.
Ligand Effects: Influence of Different P- and N-Ligands. To
improve the catalyst system a variety of phosphine and phosphite
ligands with different electronic and steric properties have been
investigated for their ability to promote hydrogen generation
from formic acid in the presence of Fe3(CO)12 and 3 equiv of
tpy (Fe/tpy 1:1) in DMF at 40 or 60 °C (Figure 6).
Among the phosphine ligands tested at 40 °C, bidentate
ligands like 1,2-bis(diphenylphosphino)methane (dppm) and 1,2-
bis(diphenylphosphino)ethane (dppe) showed only very low
activity. Standard ligands such as PPh3 or PCy3 provided higher
activity. Hence, a number of monodentate P-ligands were tested
at 60 °C. Tris(4-fluoromethylphenyl)phosphine (1), tris(4-
methoxyphenyl)phosphine (3), and tris(2-furyl)phosphine (4)
Light Effects. As shown above hydrogen generation from
formic acid in the presence of iron carbonyl complexes strongly
depends on light irradiation. UV/vis experiments (Lambda 5,
Perkin-Elmer) of the reaction solution (Fe3(CO)12/3tpy/3PPh3
in 5HCO2H·2NEt3 and DMF) revealed absorbance at 276.6,
283.8, and 320.2 nm in the UV region and 555.6 nm in the
visible light region, which is shown in Figure 4.
(29) (a) Domingos, A. J. P.; Howell, J. A. S.; Johnson, B. F. G.; Lewis, J.
Inorg. Synth. 1976, 16, 103. (b) Brookhart, M.; Nelson, G. O. J.
Organomet. Chem. 1979, 164, 193.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 8927

A R T I C L E S
Boddien et al.
Figure 5. The light switch experiment was performed with Fe3(CO)12 (20 µmol) + 3 equiv of PPh3 + 3 equiv of tpy (Fe/PPh3/tpy 1:1:1) in DMF (1 mL)
and 5HCO2H·2NEt3 (5 mL). After a 15 min induction period, the light source (300 W xenon lamp; 385 nm cutoff via hot mirror) was switched on and off
every 30 min. Gas measured via gas buret and analyzed by GC and online via H2-sensor (H2/CO2 1:1).
Figure 6. Influence of different phosphine and phosphite ligands on the
Figure 7. Influence of different nitrogen containing ligands on the activity
(TON) of H2 generation from HCO2H using 20 µmol of Fe3(CO)12/PPh3/
ligand (Fe/PPh3/ligand 1:1:1) in 5 mL of 5HCO2H·2NEt3 + 1 mL of DMF,
ambient pressure, reaction time 3 h, light irradiation via 300 W xenon lamp
(385 nm cutoff via hot mirror).
activity (TON) of H2 generation from HCO2H using 20 µmol of Fe3(CO)12/
P-ligand/tpy (Fe/P/tpy 1:1:1) in 5 mL of 5HCO2H·2NEt3, 1 mL of DMF,
ambient pressure, reaction time 3 h, light irradiation via 300 W xenon lamp
(385 nm cutoff via hot mirror).
showed significant activity. On the other hand tris(pentafluo-
rophenyl)phosphine (2), tris(2-tolyl)phosphine (5), and phosphite
ligands triphenylphosphite (6) and tris(3,5-tertbutyl)phosphite
(7) gave only negligible activities. In addition, we also examined
different N-ligands, e.g. pyridine (pyr), bipyridine (bipy),
terpyridine, and phenanthroline derivatives, to obtain a stable
and active catalyst system (Figure 7).
Applying bipyridine or pyridine resulted in a slightly lower
activity. However, varying the amount of pyridine from Fe/
pyridine ) 1:1 to Fe/pyridine 1:3 had no influence on the
catalyst activity. Interestingly, usage of 2 equiv of bipy (Fe/
bipy 1:2) or 1 equiv of pyr/bipy (Fe/pyridine/bipy 1:1:1)
provided similar turnover numbers. A significant influence on
the catalyst activity is observed using different terpyridine and
phenanthroline ligands at 60 °C. For example, 4′,4′′′′-(1,4-
phenylene)bis(2,2′:6′,2′′-terpyridine) (8), bearing two terpyridine
moieties being connected via a phenyl bridge, inhibited catalysis.
Also 4,4′,4′′-(tri-tert-butyl)-2,2′:6′,2′′-terpyridine (9), 4′-(4-
methylphenyl)-2,2′:6′,2′′-terpyridine (10), and 4′-(chloro)-2,2′:
6′,2′′-terpyridine (11) gave lower activities compared to terpy-
ridine itself. Interestingly, when changing the substituent in the
6,6′′-positions of terpyridine from H to bromine, phenyl, or
2-methylphenyl groups, an increase of activity is observed. 6,6′′-
(Bromo)-2,2′:6′,2′′-terpyridine (12) and 6,6′′-(phenyl)-2,2′:6′,2′′-
terpyridine (13) provided the highest activity with a turnover
frequency (TOF) of 200 h-1 for 13 compared to 84 h-1 for
terpyridine. However, the catalyst system derived from terpy-
ridine provided a longer time stability. Among the tested
phenanthroline coligands 1,10-phenanthroline (phen) (15) and
4,7-diphenyl-1,10-phenanthroline (16) led to high activity which
was comparable to the terpyridine system after 3 h, whereas in
the presence of 3,5,6,8-tetrabromo-1,10-phenathroline (17) a
considerable lower activity and stability were observed. After
9
8928 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Iron-Catalyzed Hydrogen Production from Formic Acid
A R T I C L E S
Figure 9, the 31P NMR spectra of both the in situ catalyst and
[Fe(CO)3(PPh3)2] under the same reaction conditions
(5HCO2H·2NEt3; irradiation) are shown.
The 31P NMR spectrum of the in situ mixture without
irradiation (FA/TEA present) exhibited two main singlets at 70.6
and 82.5 ppm (ratio 7/1) after the ligands were introduced, which
indicated that PPh3 is coordinated to the metal center forming
two complexes. Additionally, small singlets at 25.58 and -5.4
ppm, which correspond to impurities of triphenylphosphineoxide
and free triphenylphosphine, respectively, are observed.
[Fe(CO)3(PPh3)2] in DMF with FA/TEA showed only one
singlet at 82.7 ppm, which corresponds to two equally bonded
triphenylphosphine ligands (Figure 9A).31 Thus, formation of
[Fe(CO)3(PPh3)2] in the in situ catalyst mixture is confirmed
by 31P NMR. After irradiation of this solution for 10 min two
singlets at 70.6 and 82.5 ppm and free PPh3 at -5.4 ppm (3/
1/2) are observed in the 31P NMR spectrum. At the same time
in the 1H NMR spectrum a signal is formed at -8.9 ppm, which
proved the formation of an iron hydride species. This proton
shift is within the same range as those reported for other
monomolecular Fe-H species.32
Figure 8. Comparison of the catalyst activity of the in situ generated
catalyst from Fe3(CO)12/PPh3/tpy and [Fe(CO)3(PPh3)2] for H2 generation
from HCO2H using 20 µmol of Fe3(CO)12/PPh3/tpy (Fe/PPh3/tpy 1:1:1) and
60 µmol of [Fe(CO)3(PPh3)2] in 5 mL of 5HCO2H·2NEt3 + 1 mL of DMF,
ambient pressure, 60 °C, reaction time 3 h, light irradiation via 300 W
xenon lamp (385 nm cutoff via hot mirror).
The reaction solution containing [Fe(CO)3(PPh3)2] showed
four singlets in the 31P spectrum after 10 min of irradiation.
The signal at 82.7 decreased, and two new complexes are formed
with signals at 80.6 and 70.8 ppm (49/10/1). The signal at 70.8
ppm corresponds to a hydride signal at -8.8 (s) ppm, whereas
the signal at 80.6 corresponds to a triplet at -9.1 ppm, which
cannot be observed in the in situ catalyst mixture in the presence
of terpyridine (Figure 9B). It is important to note that all hydride
signals are only observed when the reaction solutions were
irradiated! After 30 min of irradiation the ratio of the signals at
82.5 to 70.6 ppm of the in situ catalyst mixture reached 1:1
(Figure 9C) and the ratio of bonded phosphine to free PPh3
decreased from 5:1 (Figure 9A) to 2:1 (Figure 9C). Irradiating
the [Fe(CO)3(PPh3)2] solution for 30 min led to a ratio of the
signals at 82.7, 80.6, and 70.8 ppm of 15/6/1. Here, the signal
of bonded triphenylphosphine to free PPh3 decreased during 30
min of irradiation from 17:1 (Figure 9A) to 3:2 (Figure 9C).
The ratio of hydride signals at -9.1 (t) and -8.8 (s) ppm were
in the same range like the phosphorus signals 80.6 and 70.8
ppm (Figure 9, C). From the NMR studies it can be concluded
that, in both reaction mixtures, besides other complexes,
[Fe(CO)3(PPh3)2] is formed as a major iron carbonyl species,
which is capable of selective dehydrogenation of formic acid.
Next, we compared both catalyst solutions by IR spectros-
copy. FTIR measurements were performed with a Vertex-70-
FTIR spectrometer (Bruker). In Figure 10 the IR spectrum of
[Fe(CO)3(PPh3)2] (1.5 mM) in DMF is shown.
24 h 15 gave a turnover number of 126 underlining the stability
of these catalyst systems. Notably, this is the highest catalyst
productiVity known for any homogeneous hydrogen generation
from formic acid with non-noble metal complexes. In summary,
it was found that both P- and N-ligands have a significant
influence on the activity and stability of the catalytic system.
Within the tested P-ligands PPh3 gave the best activity; a direct
dependence of the activity on the electronic and steric properties
of the N- and P-ligands was not observed. Among the
investigated N-ligands 6,6′′-substituted terpyridine ligands led
to high activity but lower stability. Phenanthroline ligands
provided a longer stability.
Spectroscopic Insight and Computational Analysis of
Iron-Catalyzed Hydrogen Generation from Formic Acid. To
identify active catalyst species, MS and HRMS studies applying
ESI-TOF (see Supporting Information) of the reaction mixture
containing Fe3(CO)12, 3 equiv of PPh3, and 3 equiv of tpy (Fe/
P/tpy 1:1:1) were performed. Here, [HFe(CO)(PPh3)(tpy)]- (580
m/z) and [Fe(CO)3(PPh3)2] (664 m/z) were detected as the two
main iron species. To isolate these complexes [Fe(CO)3(PPh3)2]
was synthesized according to a modified literature protocol,30
while attempts to prepare [Fe(CO)(PPh3)(tpy)] and [HFe(CO)-
(PPh3)(tpy)]- failed. Next, we tested the defined complex
[Fe(CO)3(PPh3)2] for hydrogen generation (Figure 8) and
compared the catalyst activity to the in situ system with
Fe3(CO)12/PPh3/tpy.
Notably, the reaction rates for the in situ catalyst system and
the defined complex were comparable within the first hour
confirming the same active species. Then, the molecular defined
precatalyst [Fe(CO)3(PPh3)2] is rapidly deactivated, which is not
the case for the in situ catalyst mixture, where terpyridine is
included. Apparently, in the in situ catalyst mixture terpyridine
is needed only for stabilizing purposes.
[Fe(CO)3(PPh3)2] showed four contributions V(CO) at ap-
proximately 1883, 1939, 1970, and 2048 cm-1, whereas in other
nonpolar solvents only one contribution V(CO) from 188132a
to 188732b cm-1 is observed. The intensities of the absorption
bands are almost independent of the temperature.
For comparison of the in situ catalyst and the
[Fe(CO)3(PPh3)2] complex under reaction conditions we took
Both catalyst systems were also studied by NMR and IR
spectroscopy. 1H and 31P NMR studies (295 K, 300 MHz) were
performed in DMF-d7 simulating the reaction conditions. In
(31) Venanzi et al. reported in 31P NMR experiments a singlet at 82.3 ppm
for [Fe(CO)3(PPh3)2]. Holderegger, R.; Venanzi, L. M. HelV. Chim.
Acta 1979, 62, 2154.
(32) (a) Albertin, G.; Antoniutti, S.; Bortoluzzi, M. Inorg. Chem. 2004,
43, 1328. (b) Ricci, J. S.; Koetzle, T. F.; Bautista, M. T.; Hofstede,
T. M.; Morris, R. H.; Sawyer, J. F. J. Am. Chem. Soc. 1989, 111,
8823. (c) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris,
R. H.; Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc.
1991, 113, 4876.
(30) (a) Sowa, J. R.; Zanotti, V.; Facchin, G.; Angelici, R. J J. Am. Chem.
Soc. 1991, 113, 9185. For another procedure, see also: (b) Manuel,
T. A. Inorg. Chem. 1963, 2, 854. (c) Garringer, A. M.; Hesse, A. J.;
Magers, R. J.; Pugh, K. R.; O’Reilly, S. A.; Wilson, A. M.
Organometallics 2009, 28, 6841.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 8929

A R T I C L E S
Boddien et al.
Figure 9. 31P NMR spectra of the different catalyst solutions. 3 mM of the in situ catalyst (Fe3(CO)12 + 3 equiv of PPh3 + 3 equiv of tpy) and accordingly
3 mM [Fe(CO)3(PPh3)2] in 10 mL of DMF; ambient pressure, 40 °C, reaction time 3 h, light irradiation via 300 W xenon lamp (385 nm cutoff via Hot
Mirror). (A) 0.2 mL of 5HCO2H·2NEt3 added. (B) 0.2 mL of 5HCO2H·2NEt3 added and 10 min irradiation. (C) 0.2 mL of 5HCO2H·2NEt3 added 30 min
irradiation.
Figure 10. FT-IR spectra of 1.5 mM [Fe(CO)3(PPh3)2] in DMF at different temperatures. Background (DMF) subtracted. Observed absorption bands were
shifted for clarity reasons.
samples from the reaction solution at defined reaction times.
To obtain better quality IR spectra we decreased the amount of
FA/TEA from 5 mL (standard reaction) to 0.2 mL and increased
the volume of the solvent DMF from 1 to 10 mL. Except for
the signal at 1883 cm-1, all bands in the IR spectrum of the
catalyst solution with [Fe(CO)3(PPh3)2] decreased dramatically
under irradiation (Figure 11A). The band at 1883 cm-1 declined
comparably less and was still seen after 180 min of irradiation.
The contributions at approximately 2048, 1970, and 1943 cm-1
are more significant than the band at 1883 cm-1 in the in situ
catalyst solution containing Fe3(CO)12/PPh3/tpy without irradia-
tion. After 30 min of constant irradiation the band at 1883 cm-1
increased, whereas the other bands decreased slowly. Notably,
the diminishment of the bands at 2048, 1970, and 1943 cm-1
was significantly smaller in the in situ catalyst solution compared
to the catalysts solution with [Fe(CO)3(PPh3)2], where after 180
min of irradiation only a band at 1883 cm-1 is observed.
To explain these spectroscopic results, we performed DFT
calculations for [Fe(CO)3(PPh3)2]. Geometry optimizations have
been carried out using the Gaussian 03 program package.33 We
used the B3PW91 gradient corrected hybrid density functional34,35
to calculate the structures and vibrational frequencies of the
complexes. No imaginary frequencies were found indicating that
all complexes and fragments represent at least local minimum
structures on the potential energy surface. For all complexes
the calculations have been performed with 6-31G* and 6-311G**
(33) Frisch, M. J. et al. Gaussian 03, revision D.01; Gaussian Inc.:
Wallingford, CT, 2004.
(34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(35) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244.
9
8930 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Iron-Catalyzed Hydrogen Production from Formic Acid
A R T I C L E S
Figure 11. FT-IR spectra corresponding to different irradiation times of (A) 1.5 mM of [Fe(CO)3(PPh3)2] and (B) 3 mM Fe3(CO)12/3 equiv of PPh3/3 equiv
of tpy (Fe/PPh3/tpy 1:1:1) in 10 mL of DMF, 0.2 mL of 5HCO2H·2NEt3 added, reaction temperature 40 °C, light irradiation via 300 W xenon lamp (385
nm cutoff via hot mirror). Background (DMF) subtracted. Observed absorption bands were shifted for clarity reasons.
basis sets implemented in Gaussian 03. All calculations have
been carried out on High Performance Computer Cluster in
Rostock.
we calculated the singlet (S) as well as the triplet states (T)
(see Supporting Information).38
For the complexes 2Fe and 3Fe the triplet states were lower
in energy compared to the singlet state. Therefore, the dissocia-
tion energies are discussed with respect to complexes which
are the lowest in energy. The dissociation of one carbonyl or
phosphine ligand lead to the metal fragments 3bFe and 4bFe
in the electronic triplet state, respectively. For both complexes
1Fe and 2Fe the dissociation energies of the Fe-CO bonds are
lower than that for the iron pentacarbonyl complex. The
displacement of one CO ligand in 1Fe requires ∼10 kcal mol-1
more energy than the removal of the same ligand from complex
2Fe. However, the same trend is found for the dissociation of
PPh3 from both complexes. This behavior is nicely reflected in
the measured infrared spectra (Figure 11). After irradiation
complex 1Fe is still present whereas 2Fe lost its CO ligands.
For all complexes the dissociation energy values are higher for
CO compared to PPh3. For the most stable complex 1Fe this
energy difference is ∼4 kcal ·mol-1. Thus, the phosphine ligand
will dissociate from the metal center first. The calculated
frequencies are given in Table 4 for species 1Fe and 2Fe. For
complex 1Fe with all CO ligands in an equatorial position we
obtained mainly one intensive vibrational mode at ∼1990 cm-1.
Complex 2Fe yields three vibrational bands at approximately
2000, 2030, and 2090 cm-1, respectively. Two contributions
are significantly red-shifted versus the 1Fe modes. These
findings allow for an interpretation of the measured IR spectra
before and after irradiation. Before irradiation, both complexes
1Fe and 2Fe are present. After irradiation, mainly the ligands
of complex 2Fe dissociate which is in agreement with the lower
Figure 12 shows the optimized geometries at B3PW91/6-
31G* of the complexes 18VE 1Fe and 2Fe and the 16VE
fragments 3aFe, 3bFe, 4aFe, and 4bFe which are formed after
dissociation of the ligands PPh3 or CO from the former
complexes. The metal fragments have been calculated at the
singlet and triplet electronic states. The calculated geometries
of complex [Fe(CO)3(PPh3)2] (1Fe) are comparable to those of
Krapp et al.36 obtained for the complex [Fe(CO)3(PMe3)2] by
using BP86 and CCSD(T) levels of theory. In principle the bond
lengths Fe-CO and Fe-P indicate the bond dissociation
energies (BDE) of the ligands.
Table 3 gives the BDEs at the B3PW91/6-31G* and
B3PW91/6-311G** levels of theory of the Fe-PPh3 and
Fe-CO bonds of the complexes 1Fe and 2Fe, respectively.
For an evaluation of our method we also calculated the
optimized geometries for the iron pentacarbonyl complex (5Fe)
and its fragments (6aFe, 6bFe) and the corresponding bond
dissociation energies (see Supporting Information). The results
for both basis sets, 42.8 kcal mol-1 for 6-31G* and 41.2
kcal·mol-1 for 6-311G**, are in very good agreement with the
experimental bond dissociation energies for Fe(CO)5 (41 ( 2
kcal mol-1).37 The dissociation energies have been considered
as the differences between the total energies of the complexes
and the separated fragments. The dissociation energies were
corrected for the zero-point energies (ZPE). For the complexes
and fragments 3aFe, 3bFe, 4aFe, 4bFe, 5Fe, and 6aFe, 6bFe
(38) [Fe(CO)5] (5Fe) Etot in kcal ·mol-1: -1830.043, -1830.288; ZPE
/kcal·mol-1: 26.6, 26.9; Bond dissociation energies D0 /kcal·mol-1
(CO): 42.78, 41.20. [Fe(CO)4](s) (6aFe) Etot:-1716.710, -1716.924;
ZPE: 20.8, 20.8. [Fe(CO)4](T) (6bFe) Etot: -1716.704, 1716.913; ZPE:
20.4, 20.1.
(36) Krapp, A.; Pandey, K. K.; Frenking, G. J. Am. Chem. Soc. 2007, 129,
7596.
(37) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc. 1984,
106, 3905.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 8931

A R T I C L E S
Boddien et al.
Figure 12. Optimized geometries at B3PW91/6-31G* and B3PW/6-311G** (B3PW/6-311G** values are underlined) of the calculated compounds. Calculated
total energies Etot and zero-point energies ZPE are given in kcal mol-1. Distances in Å, angles in deg. The symmetry is given below each structure. Hydrogen
atoms are omitted for clarity reasons.
band at ∼2047 cm-1 we have no explanation yet. It also
Table 3. Bond Dissociation Energies D0 of 1Fe and 2Fe at the
B3PW91/6-31G* and B3PW/6-311G**a Levels of Theory
disappears after radiation and could be assigned to 2Fe.
molecule
diss.
D0 / kcal·mol-1
However, compared to the other vibrational bands it appears
too sharp to be related to CO vibrational modes.
1Fe
Catalytic Cycle. Based on the above studies, we propose
[HFe(CO)3(PPh3)]- as the active iron catalyst for the dehydro-
genation of formic acid; however additional active iron species
cannot be fully excluded. Apparently, the addition of terpyridine
and similar N-ligands prevents rapid deactivation of the catalyst
system by formation of stabilized iron complexes, which in turn
can form [HFe(CO)3(PPh3)]- back. In this respect it is interesting
to note that the IR studies confirmed less CO dissociation in
the presence of terpyridine. Thus, it is likely that catalyst
deactivation proceeds via CO dissociation, which is an irrevers-
ible process.
In Scheme 2, the proposed overall catalytic cycle for the iron-
catalyzed hydrogen generation from formic acid is shown. The
initial precatalyst, either [Fe(CO)3(PPh3)2] or the in situ mixture
of Fe3(CO)12/terpyridine/triphenylphosphine, is activated by
irradiation with visible light to give stable mononuclear iron
hydride species 18. According to the calculated dissociation
energies one PPh3 ligand dissociates from the complex to
provide a free coordination site for a formate anion. Subsequent
protonation via formic acid or ammonium formate and ligand
exchange form the corresponding iron formate complex 19.
Release of hydrogen and recoordination of PPh3 lead to the
(CO)
32.53
30.97
28.43
29.45
(PPh3)
2Fe
(CO)
24.14
22.86
20.04
21.34
(PPh3)
a B3PW91/6-311G** values are underlined.
Table 4. Calculated Vibrational Frequencies ν(CO) at the
B3PW91/6-31G* and B3PW/6-311G**a Levels of Theory for 1Fe
and 2Fe
molecule
ν(CO) /cm-1
ν(CO) /cm-1
ν(CO) cm-1
1Fe
2062 (0.6)
2058 (0.3)
2090 (1051)
2089 (856)
1995 (907)
1986 (933)
2034 (323)
2023 (441)
1991 (894)
1985 (938)
2007 (781)
1994 (971)
2Fe
a B3PW/6-311G** values are underlined.
bond dissociation energies for both ligands in this species. The
calculated vibrational modes are ∼4% larger than the measured
ones, which is in agreement with the average scaling factor of
0.96 given for this level of theory.39 For the measured vibrational
(39) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. 2007, 11683.
9
8932 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Iron-Catalyzed Hydrogen Production from Formic Acid
A R T I C L E S
Scheme 2. Proposed Catalytic Cycle for
[Fe(CO)3(PPh3)2]-Catalyzed Hydrogen Generation from Formic
Acid under Light Assistance
The use of organometallic iron complexes for catalytic
reductions is yet in its infancy.41 Apart from hydrogen genera-
tion, the reported catalyst systems are of interest for such
processes, too. Due to the importance of biomimetic reductions
both in biological systems and on an industrial scale, further
catalyst improvements are highly desirable. In further studies
we are focusing on these aspects.
Experimental Section
6,6′′-(Phenyl)-2,2′:6′,2′′-terpyridine. General, experimental pro-
cedure: A pressure tube equipped with a stirring bar was filled with
6.6′′-dibromo-2,2′:6′,2′′-terpyridine (0.1 mmol, 39.1 mg), phenyl-
boronic acid (0.22 mmol, 26.8 mg), sodium carbonate (0.44 mmol,
47.0 mg), and [Pd(PPh3)4] (0.005 mmol, 5.8 mg). The tube was
evacuated and filled with argon. Then, dry dioxane (1 mL) and
degassed water (0.2 mL) were added. The tube was closed with a
Teflon screwing cap and placed in an oil bath (T ) 100 °C). After
12 h the reaction was cooled to room temperature, and CH2Cl2 (5
mL) and water (5 mL) were added. The water layer was extracted
an additional two times with CH2Cl2. The organic layers were
combined and concentrated. Purification via column chromatogra-
phy (eluent: cyclohexane 20:ethyl acetate 1:triethylamine 0.2)
yielded the phenyl-substituted terpyridine (0.095 mmol, 36.6 mg,
95%) as a white, crystalline solid. 1H NMR (CDCl3): δ ) 8.66 (1,
d, J ) 7.8 Hz), 8.63 (2, dd, J ) 7.8, 0.9 Hz), 8.12-8.09 (8, m),
7.97 (4, t, J ) 15.6 Hz), 7.87 (3, t, J ) 7.8 Hz), 7.73 (5, dd, J )
7.8, 0.9 Hz), 7.48-7.35 (6,7 m) ppm; 13C NMR (CDCl3): δ ) 156.5
(Cq), 155.6 (Cq), 155.2 (Cq), 139.3 (Cq), 138.2 (CH), 137.8 (CH),
129.1 (CH), 128.8 (CH), 127.0 (CH), 121.4 (CH), 120.5 CH), 119.6
(CH) ppm. FTIR (ATR) 3052.6, 2922, 1564, 1427, 1265.7, 1089.7,
1022.6, 988.5, 803.3, 760.3, 693.8, 634.9 cm-1; HRMS (ESI-TOF)
m/z calcd for C27H20N3 + H 386.1652, found 386.1653.
active iron complex 20. Release of carbon dioxide through a
ꢀ-hydride elimination of the coordinated formate regenerates
the active catalyst 18.
It should be noted that the release of hydrogen from the metal
center is likely also influenced by light. Photoelimination
reactions of molecular hydrogen from iron and ruthenium
hydride species are well-known.40 More specifically it was
shown that the exited states of those metal hydride species are
highly labile with respect to loss of H2.
6,6′′-(2-Methylphenyl)-2,2′:6′,2′′-terpyridine. General, experi-
mental procedure: A pressure tube equipped with a stirring bar was
filled with 6.6′′-dibromo-2,2′:6′,2′′-terpyridine (0.1 mmol, 39.1 mg),
2-methylphenylboronic acid (0.22 mmol, 29.9 mg), sodium carbon-
ate (0.44 mmol, 47.0 mg), and [Pd(PPh3)4] (0.005 mmol, 5.8 mg).
The tube was evacuated and filled with argon. Then, dry dioxane
(1 mL) and degassed water (0.2 mL) were added. The tube was
closed with a Teflon screwing cap and placed in an oil bath (T )
100 °C). After 12 h the reaction was cooled to room temperature,
and CH2Cl2 (5 mL) and water (5 mL) were added. The water layer
was extracted an additional two times with CH2Cl2. The organic
layers were combined and concentrated. Purification via column
chromatography (eluent: cyclohexane 20:ethyl acetate 1:triethy-
lamine 0.2) yielded the 2-methylphenyl-substituted terpyridine
(0.080 mmol, 33.1 mg, 80%) as a white, crystalline solid. 1H NMR
(CDCl3): δ ) 8.54 (2, dd, J ) 7.9, 1.0 Hz), 8.46 (1, d, J ) 7.8
Hz), 7.88-7.81 (3,6 m), 7.46-7.43 (4, m), 7.38 (3, dd, J ) 7.7,
1.0 Hz), 7.27-7.22 (7,8,9 m), 2.43 (s, CH3) ppm; 13C NMR
(CDCl3): δ ) 159.4 (Cq), 155.3 (Cq), 155.3 (Cq), 140.3 (Cq), 138.2
(CH), 138.0 (CH), 137.3 (CH), 136.2 (Cq), 131.0 (CH), 129.9 (CH),
128.4 (CH), 126.0 (CH), 124.2 (CH), 121.4 (CH), 119.0 (CH), 20.8
(CH3) ppm. FTIR (ATR) 3052.6, 2922, 1564, 1427, 1265.7, 1155.6,
Conclusions
In summary, we present for the first time a light-driven iron
catalyst capable of generating hydrogen from formic acid under
ambient conditions. By studying the effects of various P- and
N-ligands in the presence of metal carbonyl complexes, we
identified active in situ iron catalysts consisting of Fe3(CO)12/
PPh3/tpy or Fe3(CO)12/PPh3/phen. It is demonstrated that visible
light is needed for both, creating an active system and driving
the catalytic cycle. Hence, hydrogen evolution can be easily
triggered by switching on and off the light source.
An overall catalyst turnover number of 126 is observed with
the inexpensive and convenient combination of Fe3(CO)12/PPh3/
1,10-phenanthroline. Moreover, applying 6,6′′-(phenyl)-2,2′:
6,2′′-terpyridine and PPh3 as ligands results in a TOF of 200
h-1 at 60 °C. To the best of our knowledge, these are the
highest activity and productivity results for a nonprecious
metal based catalyst system which is capable of selectively
generating hydrogen from formic acid. A detailed analysis
of the catalyst solution via HRMS, NMR, and IR studies
revealed that from the in situ catalyst mixture and molecular-
defined [Fe(CO)3(PPh3)2] the same active species are formed
which gave comparable activities. IR measurements confirmed
two isomers of [Fe(CO)3(PPh3)2] which showed different
behaviors under light irradiation. Accompanying DFT calcula-
tions revealed that under light irradiation dissociation of PPh3
is favored compared to carbon monoxide. On the other hand
loss of CO should lead to deactivation of the catalytic system,
which is slowed down by the use of N-ligands.
(41) For recent applications of iron-catalyzed hydrogenations and transfer
hydrogenations, see: (a) Enthaler, S.; Erre, G.; Tse, M. K.; Junge, K.;
Beller, M. Tetrahedron Lett. 2006, 47, 8095. (b) Enthaler, S.;
Hagemann, B.; Erre, G.; Junge, K.; Beller, M. Chem. Asian J. 2006,
1, 598. (c) Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009,
131, 8772. (d) Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J.
Organometallics 2008, 27, 1470. (e) Casey, C. P.; Guan, H. J. Am.
Chem. Soc. 2009, 131, 2499. (f) Casey, C. P.; Guan, H. J. Am. Chem.
Soc. 2007, 129, 5816. (g) Lagaditis, P. O.; Mikhailine, A. A.; Lough,
A. J.; Morris, R. H. Inorg. Chem. 2010, 49, 1094. (h) Morris, R. H.
Chem. Soc. ReV. 2009, 38, 2282. (i) Mikhailine, A. A.; Lough, A. J.;
Morris, R. H. J. Am. Chem. Soc. 2009, 131, 1394. (j) Meyer, N.;
Lough, A. J.; Morris, R. H. Chem.sEur. J. 2009, 15, 5605. (k) Sui-
Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem., Int.
Ed. 2008, 47, 940.
(40) (a) Sweany, R. L. J. Am. Chem. Soc. 1981, 103, 2410. (b) Geoffroy,
G. L.; Bradley, M. G. Inorg. Chem. 1977, 16, 744.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 8933

A R T I C L E S
Boddien et al.
1075.1, 1022.6, 988.5, 803.3, 760.3, 693.8, 634.9 cm-1; HRMS
(ESI-TOF) m/z calcd for C29H24N3 + H 414.1965, found 414.1967.
Fe(CO)3(PPh3)2. General, experimental procedure: A double
walled reaction vessel equipped with a stirring bar was purged with
argon to remove all other gases. It was filled with benzylidenac-
etonetricarbonyliron (2.1 mmol, 0.6 g), triphenylphosphine (4.6
mmol, 1.21 g), and 20 mL of toluene. The vessel was closed and
connected to a thermostate and was maintained at 60 °C for 16 h.
The reaction mixture was cooled to room temperature, and the
solution was reduced to 3 mL. N-Hexane (10 mL) was added, and
the solution was kept at -30 °C for 16 h. The resulting yellow
precipitate was filtered and washed (3 × 2 mL n-hexane; 3 × 2
mL diethylether) while excluding air. The bright yellow crystals
were dried under reduced pressure. Resulting yield: 1.85 mmol,
(s), cm-1; HRMS (ESI-TOF) m/z calcd for C29H24N3 + H 664.1014,
found 664.1022.
Acknowledgment. This work has been supported by the state
of Mecklenburg-Vorpommern, BMBF, and DFG (Leibniz prize and
GRK1213). We thank D. Mellmann, A. Koch, S. Buchholz, C.
Fischer, A. Lehmann, A. Kammer, and K. Mevius for their excellent
analytical and technical support. F.G. thanks the Fonds der
Chemischen Industrie (FCI) for a Kekule´ grant.
Supporting Information Available: Complete refs 8b, 33;
1
experimental data; H NMR data for all new compounds; and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
1.23 g, 88%. H NMR (CDCl3): δ ) 7.3-7.6 (m, Ph) ppm; 31P
NMR (CDCl3): δ ) 82.9 (PPh3) ppm; FTIR (ATR) ν(CO) 1881
JA100925N
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8934 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010