Base-free production of H2 by dehydrogenation of formic acid using an iridium-bisMETAMORPhos complex

Article abstract of DOI:10.1002/chem.201302230

Erase the base: An iridium complex based on a cooperative ligand that functions as an internal base is reported. This complex can rapidly and cleanly dehydrogenate formic acid in absence of external base, a reaction that is required if formic acid is to be exploited as an energy carrier (see scheme). Copyright

Full text of DOI:10.1002/chem.201302230

COMMUNICATION
Erase the base: An iridium complex
based on a cooperative ligand that
functions as an internal base is
reported. This complex can rapidly
and cleanly dehydrogenate formic acid
in absence of external base, a reaction
that is required if formic acid is
exploited as energy carrier (see
scheme).
Cooperative Effects
S. Oldenhof, B. de Bruin, M. Lutz,
M. A. Siegler, F. W. Patureau,
J. I. van der Vlugt,
J. N. H. Reek* ................. &&&& — &&&&
Base-Free Production of H2 by Dehy-
drogenation of Formic Acid Using An
Iridium–bisMETAMORPhos Complex
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DOI: 10.1002/chem.201302230
Base-Free Production of H2 by Dehydrogenation of Formic Acid Using An
Iridium–bisMETAMORPhos Complex
Sander Oldenhof,[a] Bas de Bruin,[a] Martin Lutz,[c] Maxime A. Siegler,[c]
Frederic W. Patureau,[a, b] Jarl Ivar van der Vlugt,[a] and Joost N. H. Reek*[a]
Hydrogen holds the potential to be one of the major
energy carriers for the future. However, a hydrogen-based
economy requires technology that allows efficient and safe
storage and release of H2. In this light, the reversible storage
of dihydrogen in the form of formic acid (HCOOH) pro-
vides an interesting H2 storage-release system. Formic acid
quires purification to remove traces of volatile amines prior
to application in fuel cells.[2f] To date, only a few catalysts
that are active in the absence of base have been report-
AHCTUNGTRENNUNG
ed.[2a,g,h] In the work of Puddephatt and co-workers[2a] and
Beller and co-workers,[2h] Ru H and Fe H are respectively
suggested to act as a base via protonation of the hydride
and release of H2, and also Himeda[2g] suggested a mecha-
nism, in which an IrIII catalyst functions as a base. Ligand
cooperativity could provide an interesting approach in this
area, because metal complexes with an internal base as part
of the ligand could be developed.[4] For formic acid dehydro-
genation, only one recent report employs this strategy.[1g]
Herein, we present a new iridium–bisMETAMORPhos com-
plex, which is active in the dehydrogenation of HCOOH in
the absence of external base. METAMORPhos ligands are
based on a phosphine-functionalized sulfonamide scaffold, a
functionality that exists as an equilibrium mixture of PIII and
PV tautomers.[5] These ligands can coordinate as either neu-
tral or anionic donors, and this ambivalent behavior plays a
key role in the mechanism of Rh-catalyzed asymmetric hy-
drogenation, in which H2 is heterolytically split in a intramo-
lecular fashion.[5a] The reversible protonation stimulated us
to utilize such ligands in metal–ligand cooperative dehydro-
genation of HCOOH, because they can potentially function
as internal Brønsted base alongside their ligating properties.
A new bisMETAMORPhos ligand (3) was synthesized in
a three-step route from commercially available precursors
(see Scheme 1 and the Supporting Information for full ex-
perimental details). Compound 2 exists as a racemic mixture
of diastereoisomers and after condensation with 4-butyl-
phenyl-sulfonamide, the mesomeric form PR/SPS/R of ligand
(3) was obtained selectively, as was also evidenced by its
molecular structure (see the Supporting Information for
crystal structure). Characterization by 31P NMR spectrosco-
py showed that 3 exists in two tautomeric forms 3a (PIII/PIII)
and 3b (PV/PIII) with a 3a/3b ratio of 2.5 in CH2Cl2.[6] The
PV/PV tautomer was not observed. Upon reaction with [Ir-
À
À
can be obtained from the catalytic hydrogenation of
[1a–c]
CO2
or by direct hydrothermal oxidation of biomass.[1d–f]
Since the early nineties, substantial developments in the pro-
duction of HCOOH have stimulated interest in the reverse
reaction for hydrogen release on demand.[2] Important
recent breakthroughs are the highly active Fe-based dehy-
drogenation catalyst reported by Laurenczy, Beller, and co-
workers[2h] and work by Fujita and co-workers on an iridium
catalyst that is highly active for CO2 hydrogenation or
HCOOH dehydrogenation, depending on pH.[2k] The dehy-
drogenation of HCOOH to H2 and CO2 is typically per-
formed in the presence of substoichiometric amounts of
base, which drastically decreases the hydrogen content
(from the theoretical maximum of 4.4 wt% in pure
HCOOH to 2.3 wt% based on the typical 5:2 HCOOH/
NEt3 mixture). In the context of developing efficient hydro-
gen-based fuel cells, HCOOH/base mixtures were recently
optimized for H2 release using a ruthenium-based catalysts.[3]
Ideally, catalytic dehydrogenation should be performed in
the absence of base and other additives, thereby maximizing
the overall hydrogen storage capacity (4.4 wt%). Besides
this, hydrogen produced from HCOOH/NEt3 mixtures re-
[a] S. Oldenhof, Prof. Dr. B. de Bruin, Prof. Dr. F. W. Patureau,
Dr. Ir. J. I. van der Vlugt, Prof. Dr. J. N. H. Reek
vanꢁt Hoff Institute for Molecular Sciences
University of Amsterdam, Science Park 904
1098 XH, Amsterdam (The Netherlands)
Fax: (+31)20-525-5604
E-mail: j.n.h.reek@uva.nl
Homepage: www.science.uva.nl/research/imc/HomKat/
[b] Prof. Dr. F. W. Patureau
ACHUTNGREN(NUG acac)CAHTUNGTRNE(NUGN cod)] (acac=acetylacetonate; cod=cycloocta-1,5-
Current address: Fachbereich Chemie
Technische Universitꢂt Kaiserslautern
Erwin-Schrçdinger-Strasse, Geb. 52
67663 Kaiserslautern (Germany)
diene), ligand 3 coordinates as a monoanionic tetradentate
P2O2 ligand, giving IrI complex (4; Figure 1). Complex 4 is
formed through deprotonation of 3 by acacÀ, leading to dis-
placement of acacH and cod.[7] The room-temperature
31P NMR spectrum of complex 4 is deceptively simple, dis-
playing only a singlet at d=31.4 ppm due to fast proton ex-
change between the neutral and anionic arms of the ligand.
[c] Dr. M. Lutz, Dr. M. A. Siegler
Bijvoet Center for Biomolecular Research
Utrecht University (The Netherlands)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302230.
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Base-Free Production of H2
COMMUNICATION
fusion of pentane into a solu-
tion of 5 in CH2Cl2. It was
found that 5 crystallizes as the
dinuclear
complex
52
(Figure 2) with the hydride anti
to the xanthene backbone. The
structure exhibits centro-in-
verse symmetry, and the ligand
is coordinated as a dianionic
P2O2 entity. Ligand deprotona-
tion results in substantial short-
À
ening of the ligand N S bond
lengths.[8] In solution, complex
52 dissociates into its mononu-
clear form 5, as was observed
by detailed NMR studies.[9] It
should be noted that upon co-
ordination by one of the oxy-
gens, the sulfonamide becomes
chiral, and a mixture of diaste-
Scheme 1. Synthesis of bisMETAMORPhos ligand 3.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(PRPSSSSS and its enantiomer
PSPRSRSR) was observed in the
solid-state structure. Dissolving
5 in CD2Cl2 initially resulted in
a solution containing only one
À
Figure 1. Complex 4, formed by reaction of [Ir
addition of the ligand, generating complex 5.
ACHUTGTNREN(NUG acac)ACHTUNGTNER(NUGN cod)] with 3 (1 equiv), undergoes formal N H oxidative
However, at À858C, this equilibrium is sufficiently slow on
the NMR timescale to observe separate peaks for both the
neutral and anionic phosphorus atoms as two broad sig-
nals.[6] Complex 4 is the kinetic product, and slow proton
transfer from the neutral (proton-containing) ligand arm to
the metal (formal oxidative addition) resulted in formation
of the IrIII–hydride complex 5 (Figure 1). A similar oxidative
addition process of METAMORPhos was previously sug-
gested to occur in cooperative Rh-catalyzed asymmetric hy-
drogenation, but the observed formation of 5 provides the
first unambiguous experimental evidence for this intramo-
lecular process with this ligand type.[5a] Single crystals suita-
ble for X-ray diffraction analysis were obtained by slow dif-
isomer, as was evidenced by the simplicity of the 31P NMR
spectrum. However, in the presence of acacH, that is, when
it is not removed during the synthesis of 5, isomerization re-
sults in a mixture of several isomers (5’).[6] Also, the addition
of one equivalent of acacH to pure 5 resulted in complete
isomerization within 72 h. Analysis by diffusion-ordered
spectroscopy (DOSY) NMR showed that all isomers in the
mixture 5’ have the same diffusion coefficient as pure 5.[6]
Also, analysis by MS (FAB) showed identical spectra of 5’
and pure 5. Most likely, reversible protonation of the anion-
ic ligand at the nitrogen by acacH labilizes the coordinated
sulfone group, facilitating its dissociation, rotation and re-
coordination to the metal. Interestingly, crystals isolated
from 5’ were analyzed by X-ray diffraction crystallography,
and the molecular structure was confirmed to be that of
dimer 52.
We anticipated that 5 could function as a bifunctional cat-
alyst, in which the dianionic (i.e., doubly deprotonated)
P2O2 ligand functions as an internal Brønsted base. To inves-
tigate whether
5 is active in the dehydrogenation of
HCOOH, experiments were performed under standard con-
ditions (HCOOH/NEt3 5:2), and the reaction progress was
monitored volumetrically. Under these conditions, 5 effi-
ciently decomposed formic acid with a TOF of 929 hÀ1. Im-
portantly, the catalyst is also active in the absence of exter-
nal base, albeit with a slightly lower activity (TOF of
652 hÀ1). Remarkably, using the mixture of isomers (5’; see
below) turned out to lead to substantially faster dehydro-
genation of HCOOH than with pure isomer 5, giving rise to
Figure 2. Schematic representation and molecular structure of complex
52, 4-butylphenyl groups, hydrogen atoms (except for calculated hy-
drides), and solvent are omitted for clarity.
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J. N. H. Reek et al.
a threefold higher TOF (2938 hÀ1) in the presence of base
(HCOOH/NEt3 5:2; Table 1, entry 1). Similar activities were
found in dioxane and THF (TOF of 2625 and 3149 hÀ1, re-
spectively), indicating only a minor solvent effect. Notably,
genation of formic acid is to be used in fuels cells, because
CO is known to inhibit the functioning of such devices.[10]
To investigate the stability of the catalyst, 5’ was exposed
to air for a week, during which no deactivation occurred
(Table 1, entry 6). Utilizing the same sample after two more
batch experiments going to full conversion (overall TON of
3000), and exposing the sample to air for two more weeks
only led to a slight drop in catalytic activity (TOF 939 hÀ1).
The stability of the catalyst in pure HCOOH was also inves-
tigated. If the catalyst is stirred in pure HCOOH for 24 h
and subsequently used in a dehydrogenation experiment,
the activity of the catalyst was as observed for normal ex-
periments, indicating high catalyst stability in HCOOH.[6]
These experiments demonstrate the robustness of the cata-
lyst, which is an essential criterion for long-term application
in hydrogen fuel-cell devices.
Table 1. Formic acid (FA) dehydrogenation activity of 5’.[a]
[c]
Entry[b]
Solvent
Additive
TOF [hÀ1
]
1
2
3
4
toluene
THF
dioxane
toluene
toluene
toluene
THF
dioxane
NEt3
NEt3
NEt3
–
–
–
–
–
–
–
2938
2625
3149
1050Æ14
3092
1074
632
605
951
3271
5[d]
6[e]
7
8
9
dioxane/FA (1:1)
dioxane/FA (1:1)
10[d]
[a] Catalyst 5’ (5ꢃ10À3 mmol), HCOOH (5 mmol), 658C, solvent (1 mL).
[b] For all dehydrogenation curves, see the Supporting Information.
[c] TOFs were determined between 12–35% conversion, by volumetric
gas measurements. [d] Dehydrogenation was performed at 858C. [e] Ac-
tivity of 5’ after exposure to air for one week.
To verify that the bisMETAMORPhos ligand is indeed in-
volved in the reactivity, protonation experiments were per-
formed by using various acids. If the catalyst was treated
with two to ten equivalents of strong acid (CF3COOH, HCl,
or HBF4·Et2O) prior to a dehydrogenation experiment, cata-
lyst activity (under base-free conditions) either dropped sig-
nificantly, or complete deactivation was observed. Subse-
quent addition of an equimolar amount of base (NEt3) re-
stored the catalytic activity. The observed deactivation likely
originates from (double) protonation of the ligand, thereby
blocking its functionality as an internal base (Scheme 2).
The hydride is not affected by the presence of acid, accord-
ing to 1H NMR spectroscopy. Addition of acids with a
higher pKa compared to formic acid (e.g., CH3COOH) had
no effect on catalyst activity. These experiments confirm the
bifunctional mode of action (ligand and metal) of Ir—bis-
METAMORPhos complex. This was also supported by DFT
calculations, which showed that protonation of the nitrogen
in the ligand is energetically more favorable than protona-
tion of the oxygens of the sulfon groups (by 18.7 kcalmolÀ1).
also this isomeric mixture 5’ is very active in the absence of
external base. Turnover frequencies of 1050 and 3092 hÀ1
were achieved in toluene at 65 and 858C, respectively,
(Table 1, entries 4 and 5). Interestingly, although the solvent
hardly affected the catalytic activity of 5’ when using
HCOOH/NEt3 5:2 mixture, strong solvent effects were ob-
served in the absence of NEt3. In polar solvents (THF and
dioxane, entries 7 and 8) the dehydrogenation activity is
substantially lower than in toluene. Polar solvents likely
compete with HCOOH for the vacant axial coordination
site at Ir, which is not the case in the presence of NEt3, be-
cause formate (HCOOÀ) coordination to Ir prevails, even in
polar solvents. Interestingly, high activities were also ob-
tained in polar solvents by using a 1:1 dioxane/HCOOH
mixture. Under these conditions, TOFs of 951 and 3271 hÀ1
were obtained at 65 and 858C, respectively (Table 1, en-
tries 9 and 10). Even when the dehydrogenation was per-
formed in pure HCOOH, the catalyst was still working, al-
though it was significantly less active (TOF 17 hÀ1 at 858C).
This lower activity is most likely caused by the low solubility
of the catalyst in pure HCOOH; therefore the obtained
TOF is not very informative. Performing the catalytic runs
in presence of acacH (0–2 equiv) did not influence the cata-
lytic activity, showing that acacH only affects the isomeriza-
tion of 5 to 5’. The H2 pro-
À
Also, the protonation of the Ir H was found to be unfavor-
A
at the nitrogen.[6]
Kinetic studies by using 5’ showed a first-order rate de-
pendence (1.03) in catalyst (in the concentration range of
2.5–20 mm) confirming that 5’ is present only as a monomer
and that in solution no relevant pre-equilibrium with 52
exists.[6] From deuterium-labeling studies with DCOOD, we
observed a relatively small kinetic isotope effect (KIE) of
2.06Æ0.04.[2h,11] A KIE of 2.08Æ0.06 was observed by using
duced in all dehydrogenation
reactions was analyzed and
proven to be of high purity;
the levels of CO, which could
potentially be formed by dehy-
dration of HCOOH, were
below the detection limit of
the GC (d=10 ppm). These
low CO levels are essential, if
the H2 released by dehydro-
Scheme 2. Ligand deactivation of and activation of 5 by addition of strong acid and subsequent base.
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Base-Free Production of H2
COMMUNICATION
DCOOH, and a KIE of 1.00Æ0.02 was observed by using
tive catalysis concept, employing a bifunctional ligand, may
hold promise for the efficient and effective (storage and) re-
lease of H2 ideally generated from a renewable source, such
as solar energy. We are currently investigating the reversible
nature of this process by using bisMETAMORPHOS-type
ligand systems.
HCOOD.[6] These results suggest that C H bond breaking is
À
the rate-limiting step, which is in agreement with previous
reports.[2e,h] Based on these data and results from DFT calcu-
lations, we propose the following mechanism for HCOOH
dehydrogenation with this system (Scheme 3). Initial coordi-
Acknowledgements
We kindly acknowledge NWO for financial support.
Keywords: bifunctional activation · cooperative effects ·
dehydrogenation · H2 production · iridium
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Scheme 3. Proposed mechanism for the cooperative dehydrogenation of
formic acid with 5’.
nation of formic acid to iridium complex 5a generates spe-
cies 5b (step a). Subsequently, deprotonation of HCOOH
by the ligand and direct hydride transfer to iridium takes
place concomitant with the release of CO2 (step b), leading
to species 5c. A similar mechanism was proposed by Norrby
and co-workers for a Pd0–alkyne hydroarylation catalyst
with formate as additional ligand.[12a] Herein, a metal hy-
dride is formed by direct “second-coordination sphere” hy-
dride transfer from HCOOÀ to the metal. After protonation
of the ligand, H2 is eliminated by coupling of the metal hy-
dride with the generated ligand proton.[12] However, at this
point, we cannot exclude an alternative mechanism, which
involves Ir-formate formation and subsequent b-hydrogen
elimination. In the final step, H2 is evolved with regenera-
tion of species 5 (step c). This last step possibly involves
direct protonation of a hydride moiety by formic acid or by
(solvent- or HCOOH-mediated) protonation by NH of
hemilabile ligand.
In conclusion, a new bisMETAMORPhos ligand 3 and its
iridium complexes 5’, in which the ligand is in the dianionic
state, are reported herein. The anionic form of the ligand
can function as an internal base, and we have utilized this
feature to develop a catalytic system for the “base-free” de-
hydrogenation of HCOOH. In the context of using formic
acid as a convenient carrier for hydrogen storage, base-free
dehydrogenation is important, because it increases the hy-
drogen content from 2.3 wt% in typical HCOOH/NEt3 5:2
mixtures to 4.4 wt% in pure HCOOH. Catalyst 5’ not only
operates under such base-free conditions, but also produces
clean, CO-free dihydrogen and is very robust and active
(base free, TOF 3092 hÀ1 in toluene). As such, the coopera-
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[6] See the Supporting Information.
[7] The formation of acacH and cod was observed by 1H NMR spec-
ACHTUNGTRENNUNGtroscopy.
À
[8] N S bond lengths of ligand 3 (free or coordinated form): 1.621–
À
1.643 ꢈ. N S bond lengths in complex 52: 1.537–1.550 ꢈ.
[9] DOSY NMR spectroscopy was performed by using a mixture of 5
and a dinuclear Rh complex obtained from ligand 3 (this complex is
similar to previously described Rh–METAMORPhos complexes
(Ref. [5]), and its dimeric nature was confirmed by X-ray diffraction
analysis). These complexes were found to have different diffusion
constants. When 5 was mixed with a mononuclear complex based on
Received: June 12, 2013
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