Selective hydrogen production from methanol with a defined Iron pincer catalyst under mild conditions

Article abstract of DOI:10.1002/anie.201307224

Molecularly well-defined iron pincer complexes promote the aqueous-phase reforming of methanol to carbon dioxide and hydrogen, which is of interest in the context of a methanol and hydrogen economy. For the first time, the use of earth-abundant iron complexes under mild conditions for efficient hydrogen generation from alcohols is demonstrated. Ironing out the hydrogen: A molecularly defined iron pincer complex is able to catalyze the dehydrogenation of aqueous methanol at low temperatures. This represents a further step towards the implementation of a "methanol/hydrogen economy". Copyright

Full text of DOI:10.1002/anie.201307224

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Angewandte
Communications
DOI: 10.1002/anie.201307224
Renewable Energy
Selective Hydrogen Production from Methanol with a Defined Iron
Pincer Catalyst under Mild Conditions
Elisabetta Alberico, Peter Sponholz, Christoph Cordes, Martin Nielsen, Hans-Joachim Drexler,
Wolfgang Baumann, Henrik Junge, and Matthias Beller*
Abstract: Molecularly well-defined iron pincer complexes
promote the aqueous-phase reforming of methanol to carbon
dioxide and hydrogen, which is of interest in the context of
a methanol and hydrogen economy. For the first time, the use
of earth-abundant iron complexes under mild conditions for
efficient hydrogen generation from alcohols is demonstrated.
Recently, molecularly defined ruthenium catalysts have
been developed by our group[4] (Scheme 1, complex 1,
Equation (2)) and Grꢀtzmacher et al.[2c] (Scheme 1, com-
plex 2, Equation (1)) that make aqueous-phase dehydrogen-
ation of methanol feasible at low temperatures (< 1008C) and
ambient pressure. Both catalysts make use of “non-innocent”
pincer-type ligands,[5] which allow for improved activities and
selectivities in alcohol dehydrogenations.[6,7]
H
ydrogen is discussed as a possible benign energy carrier
within the frame of an H2-based economy.[1] In combination
with low-temperature proton-exchange membrane fuel cells
(PEM-FCs) it allows for the efficient conversion of chemical
energy into electricity. Due to its physical properties, hydro-
gen poses safety issues as to handling and transportation.
Most conveniently, it can be “chemically” stored in the form
of methanol, which is liquid at room temperature and has
a relatively high gravimetric hydrogen content (12.6%).[2] In
order to use methanol as hydrogen source, reformed-meth-
anol fuel cells (RMFCs) have been developed where reform-
ing takes place according to Equation (1) in Scheme 1. This
Because of the limited availability and high cost of
precious metals, the development of catalytic systems for
alcohol dehydrogenation that are based on non-noble metals
is highly desirable. For instance, the cationic cobalt(II) alkyl
complex [(HN(CH2CH2PCy2)Co(CH2SiMe3)]BArF is effec-
4
tive in the acceptorless dehydrogenation of secondary alco-
hols as well as in the hydrogenation of olefins and ketones.[8]
In addition, complexes of several transition metals, including
first-row ones,[9] have been prepared with aliphatic pincer
ligands of the type HN(CH2CH2PR2)2. However, to the best
of our knowledge, no report is known describing the
corresponding Fe complexes. Based on our experience in
iron catalysis and inspired by the work of Milstein et al. using
different Fe complexes with pyridine-based PNP pincer
ligands,[10] we set out to develop an iron-catalyzed dehydro-
genation of methanol.
Before attempting the synthesis of discrete iron pincer
complexes, we explored the reactivity of in situ generated
catalysts. Hence, we tested various iron precursors ([Fe-
(H2O)6](BF4)2, FeF2, FeCl2, FeBr2, FeI2, and [Fe3(CO)12]) in
combination with 1 equivalent of ligand 3 (see Scheme 2).[11]
Unfortunately, no hydrogen evolution was observed. On the
other hand, the preformed iron complexes 5 and 6 (Scheme 2)
Scheme 1. Catalysts for homogeneously catalyzed aqueous-phase
methanol dehydrogenation under mild conditions.
latter reaction, known as methanol steam-reforming, is
usually promoted by heterogeneous catalysts and requires
high temperatures.[3]
[*] Dr. E. Alberico,[+] P. Sponholz,[+] C. Cordes, Dr. M. Nielsen,
Dr. H.-J. Drexler, Dr. W. Baumann, Dr. H. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
Dr. E. Alberico[+]
Istituto di Chimica Biomolecolare, CNR, Sassari (Italy)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307224.
Scheme 2. Synthesis of FeII complexes 5 and 6.
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were indeed able to promote the dehydrogenation of aqueous
methanol at low temperature with a turnover frequency
(TOF) of 414 hÀ1 and 702 hÀ1, respectively, in the first hour
(Scheme 3).[12]
In the 1H NMR spectrum, the hydride ligand resonates at d =
À19.5 ppm (2JHP = 50.0 Hz) as a sharp triplet, while the h1-
BH4 ligand gives rise to a broad signal centered at d =
À2.8 ppm. In the IR spectrum broad bands at 2349 cmÀ1 and
2051 cmÀ1 are ascribed to the terminal and bridging B H
stretching modes, respectively, of the BH4 anion. Bands at
À
À
1892 cmÀ1 and 1833 cmÀ1 belong to the coordinated CO in the
two isomers. As shown in Figure 1, the X-ray crystal structure
Scheme 3. Dehydrogenation of aqueous methanol catalyzed by FeII
complexes 5 and 6.
The common precursor to both complexes 5 and 6 is the
blue complex 4 which was prepared in good yield from the
reaction of FeBr2·(THF)2 with amine 3 under an atmosphere
of CO (see Scheme 2). The 31P{1H} NMR spectrum shows
a singlet at d = 67.8 ppm, indicating that the two phosphorus
atoms of the ligand are equivalent, although the methyl
protons of the iPr groups give rise to four doublets and the
methinic protons to two almost superimposed multiplets in
the 1H{31P} NMR spectrum. The IR spectrum of 4 shows
a strong band at 1929 cmÀ1 indicative of a coordinated CO
molecule.
The hydride complex 5 was prepared by treating 4 with
1 equivalent of NaHBEt3 in THF and was isolated in 55%
yield as an orange-brownish solid (Scheme 2). The complex
was characterized by multinuclear NMR spectroscopy, high-
resolution mass spectrometry, and IR spectroscopy. The
31P{1H} NMR spectrum in benzene shows the presence of
two species: two singlets at d = 93.9 ppm and d = 95.5 ppm in
a ratio of 84:16 are observed. These signals correlate with the
two triplets having the same relative intensity at d =
Figure 1. Molecular structure of complex 6 with thermal ellipsoids set
at 30% probability (hydrogen atoms, except those on N, B, and Fe,
which could be refined from electron density, are omitted for clarity).
Selected bond lengths [ꢂ] and angles [8]: Fe1–N1 2.0669(12), Fe1–C17
1.7214(16), Fe1–P1 2.2188(4), Fe1–P2 2.2067(4), Fe1–H2 1.42(2), Fe1–
H11 1.691 (18), C17–O1 1.161 (2); N1-Fe1-C17 175.51(7), P1-Fe1-P2
165.74(2), H2-Fe1-H11 176.9(10), N1-Fe1-H2 86.4 (8).
analysis of 6 reveals a distorted octahedral coordination
geometry around the FeII center, with the CO ligand located
trans to the nitrogen atom and the hydride ligand located
trans to the h1-coordinated hydroborate ligand. Besides, the
hydrogen atoms at Fe and N are arranged anti to each other.
After demonstrating the reactivity of the molecularly
defined complexes 5 and 6 for methanol dehydrogenation,
the latter complex was chosen to assess the influence of the
reaction conditions (base, its concentration, temperature,
water content) on the performance of the catalyst (Table 1).
Notably, hydrogen evolution (TOF1h 1.5) was observed
even in the absence of base (Table 1, entry 1) from a 9:1
MeOH/H2O solution. In this case, the evolved gas contained
H2 and CO2 in a ratio of about 2.3 to 1.[14] This confirms that
the hydroborate complex 6 is able to directly generate the
active species. However, in the presence of KOH (0.5m;
Table 1, entry 2) the volume of hydrogen evolved increased
(TOF1h 10.3), showing that a base effectively promotes
catalysis. Under basic conditions, the evolved gas was
almost exclusively hydrogen as any produced CO2 was
trapped as carbonate. A boost in activity was observed at
higher base concentration (Table 1, entries 3 and 4), allowing
to achieve a good TOF1h of 702 hÀ1 with 8.0m KOH. Lower
activities were observed with 8.0m NaOH (Table 1, entry 5,
TOF1h 485 hÀ1) and 8.0m tBuOK (Table 1, entry 6,
TOF1h 646 hÀ1), and a more rapid catalyst deactivation was
observed with the latter.
À22.7 ppm (2JHP = 52.8 Hz) and d = À22.6 ppm (2JHP
=
54.8 Hz), respectively, in the 1H NMR spectrum which
account for the hydride ligand in the two species. The
chemical shifts of the hydride ligands suggest a trans position
with respect to the bromide ligand in both complexes which
are most likely isomers that differ in the relative orientation,
either syn or anti, of the hydrogen atoms at N and Fe. Two
bands at 1897 cmÀ1 and 1853 cmÀ1 in the IR spectrum indicate
that CO is retained in the coordination sphere of the metal,
a fact further supported by the detection of a triplet at d =
224.1 ppm (2JCP = 26.0 Hz) in the 13C NMR spectrum for the
major isomer.
To perform catalytic experiments in the absence of base,
the bright yellow hydrido hydroborato complex 6 was
prepared in 68% yield by treating 4 with excess NaBH4 in
ethanol (Scheme 2). As shown in the case of Ru[13] and Fe[10b]
hydrido tetrahydroborato complexes, the BH4À anion is labile
and can easily dissociate from the metal affording an active
hydride species ready to enter the catalytic cycle. The 31P{1H}
NMR spectrum of 6 shows a mixture of two isomers at d =
99.5 ppm (major isomer) and d = 100.9 ppm (minor isomer).
The addition of 10 equivalents of either potassium for-
mate (a plausible intermediate in the dehydrogenation of
methanol) or potassium carbonate only slightly decreased the
activity of the catalyst (Table 1, entries 7 and 8, TOF1h 626 hÀ1
and 558 hÀ1, respectively). Next, the effect of the water
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Table 1: Aqueous-phase dehydrogenation of methanol promoted by catalyst
precursor 6: variation of reaction conditions.[a]
(see the Supporting Information). Both formate and
carbonate/bicarbonate ions[15] were observed by
NMR spectroscopy, indicating that dehydrogenation
of methanol proceeds beyond the stage of formal-
dehyde to formate and further to CO2 which is
trapped as carbonate under the basic conditions
employed. Besides, free ligand was detected in
solution during the reaction, its relative amount
increasing over time. The positive effect of added
ligand on catalyst lifetime is therefore ascribed to
impeding catalyst decomposition.
Entry Additive
c(base)[b]
T
TOF1h TOF2h TOF3h TON3h TONmax
/
[molLÀ1
]
[8C][c] [hÀ1 [d] [hÀ1 [hÀ1
]
]
]
duration
1
2
3
4
–
–
–
–
–
65
72
79
91
1.5
10.3
59
4.0
9.5
64
5.3
8.3
59
15.9 69/20 h
0.5
4.0
8.0
24.9
177
–
–
702
594
510
1530
6270/
To assess the homogeneous nature of our cata-
lytic system, poisoning experiments[16a,b] were con-
ducted in the presence of PMe3 (Figures SI24,25 in
the Supporting Information). While a large excess of
PMe3 (20 equiv with respect to iron) almost stopped
the catalytic activity, 0.12 equivalents of PMe3 did
not hamper hydrogen evolution. The latter fact
argues against the presence of a previously reported
kind of nanoparticles.[16c] In addition, none of our
gas-evolution curves is sigmoidally shaped (Figur-
es SI21–23 in the Supporting Information), which
clearly indicates that there is no induction period,
which would be required to form nanoparticles from
the iron(II) complex.
43 h
5
6
7
–
–
8.0[e]
8.0[f]
8.0
91
91
91
485
646
626
426
428
528
380
333
465
1140
999
1395
–
–
–
HCOOK
10 equiv
K2CO3
10 equiv
–
8
8.0
8.0
91
89
558
411
486
382
433
362
1299
1086
–
9[g]
5303/
60 h
–
9834/
46 h
–
10[h]
11[j]
–
–
8.0
8.0
91[i] 734
91
620
644
528
617
1584
1851
635
12
13
3 1 equiv 8.0
3 5 equiv 8.0
91
91
613
644
555
570
506
520
1518
1560
9184/
111 h
Due to the “non-innocent” nature of the ligand,[5]
it is reasonable to postulate that this ligand is directly
involved in catalysis by means of an outer-sphere
mechanism. As the catalytically active species we
suggest the amide complex 6b which is generated
from 5 or 6 by the action of base (Scheme 4). Such
a complex should be able to abstract hydrogen from
the substrate—either methanol, formaldehyde (or its
hydrated form methandiol), or formate—to afford
[a] Reactions were performed under argon and at reflux using MeOH/H2O (10 mL,
ratio 9:1, unless otherwise given) and 6 (4.18 mmol), except for entry 11. H2/CO2
ratio >200 and CO<10 ppm were ascertained in all experiments except the one in
entry 1 by gas-phase GC. For entry 1 the relative amounts of gases were the
following: H2 69%, CO2 29.5%, CO 1.5%. Hydrogen evolution by decomposition of
BH4À would lead to a TON of 2. [b] KOH as base unless otherwise given. [c] Inner
temperature of the solution at reflux. [d] Volume of evolved hydrogen determined by
burette measurements. [e] NaOH as base. [f] KOtBu as base. [g] MeOH/H2O=4:1.
[h] Neat MeOH. [i] Set temperature. [j] The experiment was performed using 1 mmol
of catalyst.
content was investigated: the activity nearly doubled going
from a MeOH/H2O volume ratio of 4:1 (Table 1, entry 9,
TOF1h 411 hÀ1) to neat MeOH (Table 1, entry 10,
TOF1h 734 hÀ1). When the catalyst loading was lowered
from 4.16 mmol to 1 mmol under standard conditions, an
increase in activity was observed (Table 1, entry 11,
TOF1h 635 hÀ1) with a good total turnover number (TON)
of nearly 10000 after 46 h.
The evolved CO2, trapped as carbonate, could be released
as gas by treating the reaction mixture with HCl, proving the
complete decomposition of methanol to H2 and CO2 as
products (see Section SI4 in the Supporting Information).
Although the complexes were active for 43–66 h, we tried
to improve the stability further. Thus, experiments were
performed in the presence of an excess of ligand 3 (up to
5 equiv) under standard conditions (MeOH/H2O 9:1, 8.0m
KOH). While the activity was slightly reduced in the short
term (Table 1, entries 12 and 13, TOF1h 613 hÀ1 and 644 hÀ1,
respectively), the positive effect on catalyst stability became
apparent in the long term, and evolution of hydrogen
proceeded up to 5 days!
Scheme 4. Suggested structures of the catalytically active species and
resting states arising from FeII complexes 5 and 6 during aqueous
methanol reforming.
a dihydride species from which hydrogen is then released.
MeOH, H2O, and HCOOH may also add to the reactive
complex 6b giving rise to species 6c–e. The analogous Ru
complexes have been detected in solution by NMR spectros-
copy during methanol dehydrogenation promoted by
[Ru(H)(Cl)(CO){HN(CH2CH2PiPr2)2}].[4] The positive effect
To gain more insight into catalyst deactivation, additional
experiments were carried out under catalytic-like conditions
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2013, 52, 3981 – 3984; f) C. A. Huff, M. S. Sanford, J. Am. Chem.
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originate directly from the corresponding iron alkoxide 6d or
iron formate 6e through b-hydride elimination. This possi-
bility has been recently suggested by Milstein et al. for the
liberation of a molecule of CO2 from an iron formate
complex,[10d] and has before been shown to be a viable
reaction pathway by DFT calculations.[18]
In conclusion, we have shown for the first time that
molecularly defined iron pincer complexes are able to
promote dehydrogenation of methanol at low temperatures.
This opens the possibility to perform aqueous methanol
reforming with the waste heat from PEM-FCs. Clearly, the
observed catalyst turnover numbers (up to 10000) still have to
be improved for practical applications. Nevertheless, this
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Experimental Section
All reactions were performed under argon with exclusion of air. A
solution (10 mL) of MeOH and H2O in a given ratio, containing
a defined amount of base (and a defined amount of ligand, for
experiments with in situ generated catalyst or added ligand), was
heated to the desired temperature and let equilibrate for 30 min.
Then, the iron precursor (for in situ experiments) or either 5 or 6
(4.18 mmol) was added, which set the starting point for measuring the
evolved gas volume. Gas evolution was measured by manual or
automatic gas burettes.[20] The identity of the gas components and
their ratio were determined by gas-phase chromatography. Synthetic
procedures for the preparation of the new complexes 4, 5, and 6, and
their characterization, and plots of volume amount of gas evolved as
a function of time for all experiments reported in Table 1 can be found
in the Supporting Information.
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Received: August 16, 2013
Revised: October 10, 2013
Published online: December 6, 2013
Keywords: homogeneous catalysis · hydrogen · iron catalysts ·
.
methanol · renewable energy
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[12] TOF: moles of H2 produced per mole of catalyst per hour.
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through three consecutive steps, each catalyzed by the same
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take in hydrogenation reactions. A detailed description of the
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[14] The system under investigation is very complex and made up of
three discrete reactions which run parallel and possibly at
different rates that vary over time depending on the actual
concentrations of the species involved. In order to observe the
exact ratio of H2 to CO2 expected for aqueous MeOH reforming
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