Ortho-metalation of iron(0) tribenzylphosphine complexes: Homogeneous catalysts for the generation of hydrogen from formic acid

Article abstract of DOI:10.1002/anie.201004621

Without noble metals: A new catalyst derived from a non-noble metal has been developed for the selective generation of hydrogen from formic acid. DFT calculations and NMR spectroscopy support the formation of active ortho-metalated iron species as key intermediates during catalysis.

Full text of DOI:10.1002/anie.201004621

Angewandte
Chemie
DOI: 10.1002/anie.201004621
Hydrogen Generation
ortho-Metalation of Iron(0) Tribenzylphosphine Complexes:
Homogeneous Catalysts for the Generation of Hydrogen from Formic
Acid**
Albert Boddien, Felix Gꢀrtner, Ralf Jackstell, Henrik Junge, Anke Spannenberg,
Wolfgang Baumann, Ralf Ludwig,* and Matthias Beller*
As a result of the ever-increasing global demand for energy,
the establishment of alternative resources and more sustain-
able technologies constitutes a major challenge for science
and engineering in the coming decades. Given the limited
fossil fuels there is a move towards renewable resources like
biomass, wind, hydroelectric and geothermal energy, as well
as sunlight as an almost unlimited energy source. In addition,
new efficient routes for energy storage have to be established.
Among the currently discussed chemical energy carriers,
hydrogen in combination with fuel cell technology is generally
accepted as a clean technology and has gained remarkable
regarded as a sustainable hydrogen-storage material. Overall,
the ideal sustainable hydrogen generation, storage, and
utilization cycle is depicted in Scheme 1.
[
4,5]
Although several efficient homogeneous
and hetero-
[4a,6]
geneous
catalyst systems for the selective generation of
hydrogen from formic acid or formate have been developed,
[1]
attention in the last decades. Despite research in the field of
hydrogen storage, no general process exists that meets all the
industrial requirements. Hydrogen storage at ambient tem-
perature and pressure in systems having high power to weight
[
2]
and volume ratios needs to be improved.
In addition to methanol, formic acid is a potential liquid
hydrogen-storage media, and in recent years several research
groups have developed more and better catalysts for the
release of hydrogen from formic acid. The hydrogen content
À1
of HCO H (4.4 wt% of hydrogen; 5.22 MJkg ) is lower than
2
that of MeOH; however, it has an energy content that is at
least five times higher than that of commercially available
lithium ion batteries. Advantageously, formic acid is a liquid,
it is nontoxic, and most importantly, the reversible hydrogen
storage can be achieved under ambient conditions in the
Scheme 1. Cycle for sustainable hydrogen generation, storage, and
utilization.
very few efforts have been made in the field of non-noble-
metal-catalyzed formic acid decomposition. This is in contrast
to the increasing interest in bioinspired homogeneous metal
[3]
presence of suitable catalysts. If hydrogen is produced, for
example, by photocatalytic water splitting, and using carbon
[
7]
dioxide (CO ) as a feedstock, then formic acid can be
catalysis, e.g. with iron. In 2010, we reported the first
homogeneous iron catalyst system capable of generating
2
[8]
hydrogen from formic acid under near ambient conditions.
By applying an in situ catalyst system formed from
Fe (CO) ], 2,2’:6’2’’-terpyridine (tpy), and triphenylphos-
[
+]
[+]
[
*] A. Boddien, F. Gꢀrtner, Dr. R. Jackstell, Dr. H. Junge,
Dr. A. Spannenberg, Dr. W. Baumann, Prof. Dr. M. Beller
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢀt Rostock
Albert-Einstein-Strasse 29a, 18059, Rostock (Germany)
Fax: (+49)381-1281-5000
[
3
12
phine (PPh ), hydrogen generation is possible under irradi-
3
ation using visible light and ambient temperature. In addition
monomeric iron(0)/phosphine complexes were detected as
the key intermediates. Under optimized conditions turnover
numbers (TONs) of up to 126 were observed, which
constitutes the highest activity known to date for nonpre-
cious-metal-catalyzed hydrogen generation from formic acid.
Herein, we present a novel, improved iron phosphine
catalyst system, which showed a one order of magnitude
improvement in catalyst activity over the iron/triphenylphos-
phine system. The key to the success of the new system is the
use of benzylphosphine as a ligand, which undergoes a
remarkable ortho-metalation reaction upon irradiation with
visible light. Light has an impact on the generation of the
E-mail: matthias.beller@catalysis.de
Homepage: http:\\www.catalysis.de
Prof. Dr. R. Ludwig
Abteilung fꢁr Physikalische Chemie
Institut fꢁr Chemie, Universitꢀt Rostock
Dr.-Lorenz-Weg 1, 18059, Rostock (Germany)
E-mail: ralf.ludwig@uni-rostock.de
+
[
] These authors contributed equally to this work.
[
**] This work was supported by the State of Mecklenburg Vorpommern,
the BMBF, and the DFG (Leibniz prize). F.G. thanks the Fond der
Chemischen Industrie (FCI) for a Kekulꢂ grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004621.
Angew. Chem. Int. Ed. 2010, 49, 8993 –8996
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8993

Communications
active complex and hydrogen. This unusual activation process
and [Fe(CO) (PPh Bn) ] (1a) were synthesized and fully
3 2 2
involving ortho-metalation was studied in detail by using
NMR spectroscopy and DFT calculations.
characterized (see Figure 1 and the Supporting Information).
Both structures contain a pentacoordinated trigonal-bipyra-
midal iron(0) center surrounded by two axial phosphine
ligands and three equatorial CO molecules. The P-Fe-P
angles are 178.68 (1a) and 178.98 (1), and the average P-Fe-
CO angles are close to 908 (90.028 for 1a and 89.98 for 1). All
bond lengths are within the range of those for similar
In our ongoing search for active iron catalysts for the
generation of hydrogen from formic acid, we investigated
several phosphine ligands in the presence of iron carbonyl
complexes. Among the ligands studied, only those bearing
benzyl groups showed improved activity compared to our
previous triphenylphosphine-based catalyst. Selected results
from this screening of ligands for hydrogen generation are
shown in Table 1. In standard experiments the catalyst was
formed in situ from 10 mmol [Fe (CO) ], 1.0 equivalent
[
9]
complexes of the type [(CO) Fe(PR ) ].
3
3 2
By using both 1 and 1a, active systems were obtained in
the absence of terpyridine (tpy), but the resulting catalyst
system was not stable over a long time period (Table 1,
entries 5 and 6). However, in the presence of tpy, a stable
system is obtained which shows comparable activity to that of
the in situ formed catalyst (Table 1, entries 7 and 8; Figure 2).
As shown in entries 1 and 2 in Table 2, the catalyst activity
can be additionally improved by increasing the amine
concentration. This observation is in agreement with results
obtained for ruthenium complexes, wherein the nature of
3
12
2
,2’:6’2’’-terpyridine (tpy), and 1.0 equivalent phosphine in
N,N’-dimethylformamide (DMF) under xenon-light irradia-
tion at 608C. Both light and base are necessary for the
reaction to occur. The evolved gases were collected by
Table 1: Investigation of different benzyl phosphine ligands in the iron-
catalyzed generation of hydrogen from HCO
2
H.
[4c]
base influences the catalyst activity.
By applying 1,3-
[
a]
[b]
[b]
Entry
Iron source
[Fe (CO) ]
Ligand
PBn3
V [mL]
TON
1
2
3
4
203 (1033)
132 (344)
96 (153)
31 (272)
57
138 (702)
90 (234)
65 (104)
21 (185)
77
3
12
[Fe (CO) ]
PPh Bn
3
12
2
[Fe (CO) ]
PPh3
3
12
[Fe (CO) ]
P(CH CH Ph)
3
12
2
2
3
[
[
[
[
c,d]
c,d]
c]
5
6
7
8
1a
1
1
–
–
125
170
-
182 (573)
162 (486)
247 (778)
220 (660)
c]
[e]
[Fe (CO) ]
PBn3
3
12
[
a] Reaction conditions: 10 mmol [Fe (CO) ] (30 mmol Fe), 1.0 equiv
3 12
phosphine, 1.0 equiv tpy, 5 mL preformed NEt /HCO H (2:5) mixture,
3
2
1
.0 mL DMF, 608C, 3 or 15 h, 300 W Xe-light irradiation, no filter; H2/
CO gas mixture is 1:1; gas volumes detected using an automatic gas
2
burette; qualitative gas analysis using GC methods. [b] Values recorded
after 3 h; values in parentheses are those recorded after 15 h. [c] Used
Figure 1. Molecular structure of compound 1 (left) and 1a (right) in
1
5 mmol [Fe]. [d] Without tpy. [e] Used 2.0 equiv phosphine.
the crystal. The thermal ellipsoids are drawn at 30% probability, and
[
15]
the hydrogen atoms are omitted for clarity.
automatic gas burettes and then analyzed by gas chromatog-
raphy methods. In addition to traces of solvents and a 1:1
mixture of H to CO , CO was detected (0.2–2 vol%). To
2
2
calculate the TONs (TON = H /Fe or CO /Fe) the total
2
2
amount of evolved gas was corrected by taking the CO
content into account.
Compared to triphenylphosphine, tribenzylphosphine and
benzyldiphenylphosphine showed significantly improved
activity as well as stability (Table 1, entries 1–3). The system
using PPh3 is deactivated after 3 hours, and the turnover
number is 65 (Table 1, entry 3). However, the use tribenzyl-
phosphine gave a TON of 138 after 3 hours, and more
importantly remained stable for up to 15 hours (TON of 702;
Table 1, entry 1). Notably, this effect is limited to benzyl-
substituted phosphines; that is, the more benzyl moieties
present in the ligand, the higher the activity and stability of
the system (Table 1, entries 1 and 2).
The introduction of an ethyl bridge into the ligand
resulted in neither a stable system nor increased activity
Figure 2. Comparison of gas-evolution experiments with the catalyst 1
black line) with those of the in situ systems (1 with 1.0 equiv tpy:
blue line; [Fe (CO) ], 2 equiv PBn , 1.0 equiv tpy: red line). Reaction
(
relative to that of PPh (Table 1, entry 4); thus, it is unlikely
3
3
12
3
that steric effects of the ligands account for the increase in
activity. Notably, both light as well as base are essential for
catalysis. Next, the model compunds [Fe(CO) (PBn ) ] (1)
conditions: 5.0 mmol [Fe (CO) ] (15.0 mmol Fe), 5 mL preformed
3
12
NEt /HCOOH (2:5) mixture, 1.0 mL DMF, 608C, 3 h, 300 W Xe-light
3
irradiation, no filter. Gas mixture: H /CO =1:1.
3
3
2
2
2
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Chemie
Table 2: Influence of base upon the catalyst activity.
[
a]
[b]
[b]
Entry
Base
V [mL]
TON
1
NEt3
NEt3
DBU
HexNMe2
203 (1033)
430 (872)
390 (954)
58
138 (702)
292 (592)
265 (648)
39
[
c]
2
3
4
[
a] Reaction conditions: 10 mmol [Fe (CO) ] (30 mmol Fe), 1.0 equiv
3 12
PBn , 1.0 equiv tpy, 5 mL preformed base/HCO H (2:5) mixture, 1.0 mL
3
2
DMF, 608C, 3 or 15 h, 300 W Xe-light irradiation, no filter; H /CO gas
2
2
mixture is 1:1; gas volumes detected using an automatic gas burette;
qualitative gas analysis using GC methods. [b] Values recorded after 3 h;
values in parentheses are those recorded after 15 h. [c] Used NEt3/
HCO H (3:4) mixture.
2
diaza[5.4.0]bicycloundecene (DBU), an increased activity is
observed in the first 6 hours of reaction (Table 2, entries 1 and
3
); however, under these conditions the catalyst is fully
deactivated after 15 hours. N,N-Dimethyl-n-hexylamine
Figure 3. Relative energy for the conversion of [Fe(CO) (PBn ) ] (1)
3
3 2
(
HexNMe ) resulted in only low activity (Table 2, entry 4).
into the metalated species [HFe(C
6
H
4
CH
PBn )(CO) ] (4) and
2 2 3
2
[
HFe(C H CH PBn )(PBn )(CO) ] (7) via the intermediates [Fe-
Finally, a solution of HCO H/NEt3 (5:1; 10 mL) was
6
4
2
2
3
2
2
[
13,14]
(
PBn ) (CO) ] (5) or [Fe(PBn )(CO) ] (2).
3 2 2 3 3
subjected to 20 mmol [Fe (CO) ], 1 equivalent PBn , and
3
12
3
1
equivalent tpy, and a TON of 1266 (3728 mL gas) was
obtained after 51 hours. This represents the highest produc-
tivity for any non-noble-metal-catalyzed hydrogen produc-
tion from formic acid and is one order of magnitude higher
than all previously reported catalysts.
After testing the different ligands, there was no clear
trends resulting from either electronic or steric parameters.
Apparently, the significant difference in activity and stability
NMR measurements strongly support the formation of
metalated species during irradiation of 1 in [D ]DMF.
7
Whereas thermal treatment of 1 at 1008C did not induce
1
13
31
any change in the H, C, and P NMR spectra, significant
changes are obtained under light irradiation with Xe light,
thus supporting our hypothesis of metalation: a) Light
is not caused by these factors. Unlike PPh , PBn3 can
induces a dissociation of the PBn ligand (observed at
3
3
3
1
potentially undergo ortho-metalation to form a five-mem-
bered metallacycle, which could account for the increased
stability and activity. To the best of our knowledge ortho-
metalation of iron(0)/phosphine complexes is only known
with 1,2-bis(diphenylphosphine)ethane (dppe) as the
d( P) = À9.3 ppm in [D ]DMF) or CO leading formally to
7
the fragments [(CO) Fe(PBn )] (2) and [(CO) Fe(PBn ) ] (5).
3
3
2
3 2
For both fragments ortho-metalation of the ligands are
energetically favored (Figure 3); b) hydride formation takes
place exclusively during irradiation of 1 in [D ]DMF, showing
7
[10]
ligand. Four-membered metallacycles can be formed ther-
mally and photochemically when either [Fe(dppe) (C H )] or
clearly that hydrogen transfer from the ligand to the iron
1
31
center takes place; c) through H– P HMQC spectroscopy it
2
2
4
[11]
[
Fe(dppe) H ] are used as precursors. Such structures can
was shown that the hydride belongs to the coordinated
2
2
[
11a]
[12]
be effectively used for the activation of C(sp)ÀH bonds.
In
phosphine; d) the observed H–P coupling constant, JH,P =
the case of PBn ortho-metalation could lead to an even more
58 Hz, of the hydride is in acceptable agreement with the
calculated H–P coupling constants for 7 (JH,P = 45.9 Hz);
e) the same hydride signal is observed during the catalytic
reactions with formic acid, making it likely that this species
3
stable five-membered metallacycle, and to substantiate this
proposal, we carried out NMR experiments as well as
[
12]
theoretical calculations.
1
3
To undergo ortho-metalation, a free coordination site at
the iron center is needed. The dissociation of either CO or a
PBn ligand from the [Fe(CO) (PBn ) ] complex (1) under
plays a key role in the catalytic cycle; f) in the CNMR
spectrum new signals appear at d = 138 ppm (shifted down-
field by approximately 8 ppm), which is in good agreement
with the calculated downfield shifts for carbon atoms that are
3
3
3 2
irradiation leads to the unsaturated complexes 5 and 2,
[
12]
respectively. In Figure 3 it is shown that the dissociation of
a to the carbometalated center.
À1
CO costs about 10 kcalmol more in energy than that of
In conclusion, we have developed a new state-of-the-art
catalyst system for non-noble-metal-catalyzed decomposition
of formic acid into hydrogen and carbon dioxide. The
complexes [Fe(CO) (PBn ) ] (1) and [Fe(CO) (PPh Bn) ]
PBn . However, in both cases the ortho-metalated species 4
3
À1
and 7 are about 10 kcalmol lower in energy. There are also
intermediate states 3 and 6 wherein the hydrogen atom is
located between the Fe and the ligand. For species 7 we can
predict a coupling constant between the hydrogen atom and
both phosphorus nuclei of about 45.9 Hz, as well as a
3
3
2
3
2
2
(1a) were synthesized and fully characterized by single-
crystal X-ray diffraction. Mechanistic investigations by DFT
and NMR spectroscopy indicate the formation of ortho-
1
3
downfield shift of approximately 8 ppm in the C NMR
spectrum for the C atom beside the carbometalated center of
the phenyl ring for 4 and 7.
metalated iron species from Fe(PBn ) fragments, which can
3
account for the higher activity and stability in the presence of
PBn compared to PPh .
3
3
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Communications
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