Base-Free Dehydrogenation of Aqueous and Neat Formic Acid with Iridium(III) Cp*(dipyridylamine) Catalysts

Article abstract of DOI:10.1002/cssc.201802275

The selective dehydrogenation of formic acid by iridium(III) Cp*(dipyridylamine) catalysts is reported. The electron-enriched catalyst [Ir^IIICp*{(4-dimethylaminopyridin-2-yl-κΝ)(pyridin-2′-yl-κΝ)amine}(OSO₃)] gave the best performanc

Full text of DOI:10.1002/cssc.201802275

Accepted Article
Title: Base-free dehydrogenation of aqueous and neat formic acid with
Iridium(III)Cp*(dipyridylamine) catalysts
Authors: Shengdong Wang, Haiyun Huang, Thierry Roisnel, Christian
Bruneau, and Cedric Fischmeister
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10.1002/cssc.201802275
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Base-free dehydrogenation of aqueous and neat formic acid with
Iridium(III)Cp*(dipyridylamine) catalysts
Shengdong Wang,[a]≠ Haiyun Huang,[a] ≠ Thierry Roisnel,[b] Christian Bruneau,[a] Cédric Fischmeister*[a]
Dedication ((optional))
Abstract: The selective dehydrogenation of formic acid by
Iridium(III)Cp*(dipyridylamine) catalysts is reported. The electron
enriched catalyst Ir6 led to the best performances enabling the base
free dehydrogenation of aqueous and neat formic acid. In both cases
the reaction was selective with no carbon monoxide detectable. The
latent behavior of Ir6 was demonstrated which may be of practical
utility. Experimental results suggest an outer-sphere interaction with
the ligand.
particular basic additives such as alkylamines. Such additives
would complicate an overall fuel cell process since an additional
hydrogen post-purification step would be necessary for their
removal. To date, only a few catalysts able to promote formic
acid dehydrogenation under base-free conditions have been
reported.[6] It must be noted that most of these results were
obtained using a solvent, in general water. Only few examples of
pure formic acid dehydrogenation have been reported. In 2013,
Xiao described a series of cyclometallated iridium complexes
operating without solvent with an azeotropic mixture of formic
acid and triethylamine. The highest initial TOF obtained was
2570 h-1 but when studying the continuous formic acid
dehydrogenation they observed extremely high initial activity
(TOF 147 000 h-1) upon reloading formic acid.[7] In 2016,
Williams reported the efficient formic acid dehydrogenation
under neat conditions using an iridium catalyst bearing a P,N-
chelate. A TOF of 3000 h-1 was obtained in the presence of a
base.[8] Some reports briefly mention attempts to perform the
base-free dehydrogenation of pure formic acids but very low
activities were obtained.[6a, 9]
Hydrogen will undoubtedly become an essential source of
energy for future generations provided some issues related to
large scale utilization of this energy vector are solved. In
particular hydrogen storage and availability on demand are of
major importance. Many options have received attention and are
still attracting interest of researchers. Among them, the chemical
storage of hydrogen in the form of formic acid is gaining
increasing importance. Indeed, formic acid is a stable liquid
without important safety and hazard issue, it is therefore easy to
handle and transport. Considering the reversibility of formic acid
dehydrogenation, i.e. CO2 hydrogenation, a carbon neutral
Herein, we report a new iridium catalyst operating under base
free conditions. The catalyst is active in water and under neat
conditions and owing to a very good stability it can be used in a
semi-continuous mode leading to high cumulative TONs.
energy cycle is in theory accessible.[1]
A
number of
heterogeneous[2] and homogeneous catalysts[3] have been
reported for the dehydrogenation of formic acid. Heterogeneous
catalysts are essentially based on palladium. They are in
general reusable but they display low activity. Although the
dehydrogenation of formic acid with homogeneous catalysts has
been known for more than fifty years,[4] it is only recently after
the reports by Beller and Laurenczy that formic acid has been
intensively investigated as a hydrogen reservoir.[5] Iridium-based
complexes are the most studied and efficient homogeneous
catalysts reported to date. In 2015, Li reported a TOF value of
487 500 h-1 obtained with a cationic iridium complex, which to
the best of our knowledge is still the highest reported TOF. This
catalyst, as most iridium catalysts is a 5-membered iridacycle
operating in water, which is one of the prerequisite for this
transformation considering potential application in fuel cells.
Another important issue to consider is the use of additives, in
Recently, our group has been investigating the reduction of
levulinic acid into γ-valerolactone using new ruthenium and
iridium complexes bearing dipyridylamine ligands.[10] This type of
catalyst was earlier identified as efficient transfer hydrogenation
catalysts of ketones in water.[11] In particular, we have
demonstrated that the transfer hydrogenation of levulinic acid
with formic acid was in fact proceeding essentially by
hydrogenation resulting from a very fast initial formic acid
dehydrogenation under base-free conditions. We have also
reported that iridium catalysts were very efficient for this
transformation contrary to a related dipyridylamine-ruthenium
catalyst.[12] These results prompted us to investigate in more
details the dehydrogenation of formic acid with these 6-
membered iridacycles bearing dipyridylamine ligands. We have
thus studied some previously reported complexes and also
prepared new complexes featuring dimethylamino electron
donating substituents (Figure 1) considering previous reports[13]
and the studies on aqueous hydricity of late metal catalysts by
Miller.[14] Ir1-4 and the new complexes Ir5-7 were prepared
according to a previously reported procedure (see supporting
information).
[a]
S. Wang, H. Huang, Dr. C. Bruneau, Dr. C. Fischmeister
≠ Contributed equally to the work
Univ Rennes. UMR CNRS 6226 (Institut des Sciences Chimiques
de Rennes.
Université de Rennes 1
263, avenue du général Leclerc
E-mail: cedric.fischmeister@univ-rennes1.fr
V. Dorcet, Dr. T. Roisnel,
[b]
Univ Rennes. UMR CNRS 6226 (Institut des Sciences Chimiques
de Rennes
Centre de diffractométrie X
Université de Rennes 1
263, avenue du général Leclerc
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cssc.201802275
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Table 1 Cyclic voltammetry analysesa
Entry
Complex
Epc (V)b
-1.67
-2.03
-2.14
-1.95
-2.17
-1.63
1
2
3
4
5
6
Ir1
Ir2
Ir4
Ir5
Ir6
Ir8
b
aComplex 0.0025 M in acetonitrile, Bu4NPF6 (0.25 M),, v= 100 mV.s-1.
+
Cathodic (reduction) potentials are reported in V vs FeCp2/FeCp2 as an
internal standard.
First of all, comparing entries 2 and 6, it clearly appears that the
dipyridylamine ligand makes Ir2 more electron rich than its
bipyridine analogue Ir8. Comparing data within the
dipyridylamine-iridium complexes also brings key information. As
observed upon comparing the reduction potentials of Ir1 (NH)
and Ir4 (NBz) (Table 1, entries 1,3), the functional modification
of the bridging nitrogen in the dipyridylamine ligand also
translates in a lower reduction potential. As expected, the
introduction of a dimethylamino- group in the 4-position of one
pyridine led to more electron rich complexes; Ir1 vs Ir5 (Table 1,
entries 1 and 4) and Ir2 vs Ir6 (Table 1, entries 2 and 5).
Figure 1. Iridium complexes used in this study
Crystals suitable for X-Ray diffraction for molecular structure
determination were obtained for Ir5 (Figure 2). Analysis of bond
lengths and bond angles in Ir5 revealed no significant
differences with those of the parent dimethylamino-free complex
Ir1 reported earlier (see supporting information). Hence, without
important alteration of the steric properties, the catalytic
properties of the new complexes Ir5-7 should find their origin in
the variation of the electronic properties of the ligand in
particular in their stronger electron donation.
Complexes Ir1-7 were evaluated in the base-free
dehydrogenation of formic acid in water at 80 °C with a low
catalyst loading (Scheme 1). All the complexes were fully
soluble under the experimental conditions. The gas evolution
was measured with a digital flowmeter from which the catalyst
activity was determined after 10 minutes.
Scheme 1 Base-free formic acid dehydrogenation
Figure 2. Molecular structure of Ir5 represented at 70% ellipsoid probability. H
atoms, Cl-, and solvent are omitted for clarity.
As depicted in Table 2, the best result was obtained with the
most electron rich Ir6 catalyst (Table 2, entry 6). These results
also demonstrate the inhibition or low activity of chloro- complex
Ir1 as compared to the more electron rich and zwitterionic
sulfato-complex Ir2. This is further evidenced by comparing the
results obtained with catalysts Ir5 and Ir6 (Table 2, entries 5 and
6). The low lability of the chloride ligand is a reasonable
hypothesis for explaining the poor activity of chloride complexes.
The substitution pattern of the bridging nitrogen atom is also a
key factor as both Ir3 and Ir4 displayed very low activity (Table 2,
entries 3 and 4). In particular, Ir3 displayed a much lower activity
than the unsubstituted Ir2 catalyst. All together, these results
indicate the three main trends for an efficient catalyst: i) Ir-
chloride complexes are the least active catalysts. ii) The highest
activity is obtained with an electron rich complex. iii) the
substitution pattern of the remote bridging amine strongly
influences the catalyst efficiency hence suggesting the potential
involvement of this bridging N-H group in the catalytic
mechanism.
In order to gauge the electronic properties of the ligands,
complexes Ir1, Ir2, Ir4, Ir5 and Ir6 were analyzed by cyclic
voltammetry. In addition, the iridium-bipyridine complex
[Cp*Ir(bpy)OSO3][12] Ir8 closely related to Ir2 was also studied as
a reference. All these complexes display a non-reversible
reduction process (see SI). Their Epc was measured in
acetonitrile vs ferrocene (Table 1).
Several important information can be obtained from these
results. Due to the possible exchange of the sulphato-ligand for
acetonitrile in the zwitterionic complexes Ir2 and Ir6, information
should only be extracted by comparing complexes with the same
X-ligand (X= -Cl, -OSO3).
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10.1002/cssc.201802275
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COMMUNICATION
lower than those obtained in water, it demonstrates the
feasibility of a base-free dehydrogenation of neat formic acid, a
process of high interest for hydrogen production. To the best of
our knowledge, this is the highest TOF reported for the base free
dehydrogenation of neat formic acid.
Table 2 Dehydrogenation of FA with Ir1-7a
Entry
Catalyst
Ir1
TOF (h-1)b
3047
1
2
3
4
5
6
7
Ideally, for practical utility, formic acid should be stored with
a catalyst and produce hydrogen on demand by application of a
stimuli (T, light. Catalyst exhibiting this property are quoted
as latent or dormant catalysts and should also display another
important property, which is a long term stability in the presence
of the reactant. To address this question, a stock solution of Ir6
(0.08 mol%) in 2 mL of FA was prepared and stored in a fridge
(T ± 4 °C). After 2 days, an aliquot was taken and subjected to
dehydrogenation by heating at 80 °C. This operation was
repeated after 10 days. In both cases, TOFs of 4687 and 4583
h-1 were obtained, respectively, hence corresponding to a slight
decrease when compared to 4800 h-1 obtained under the
standard conditions. These results demonstrate that the catalyst
Ir6 is stable in pure formic acid.
Ir2
8705
Ir3
3382
Ir4
2980
Ir5
4319
Ir6
12321
10781
Ir7
Conditions: aFA (4 mmol), catalyst (0.01 mol%), H2O (2 mL), 80 °C, 10 min.
bbased on gas volume released (see SI)
Ir6 was further investigated in order to determine the optimum
conditions of use. We first inspected the influence of the reaction
pH. Reactions were thus conducted with varying ratio of formic
acid and sodium formate. Interestingly, Ir6 delivered the best
performance at pH 1.8 (i. e. no added sodium formate) (Table 3).
This is in sharp contrast with other iridium complexes bearing
15]
bipyridine or bis-N-heterocyclic ligands.[6d,g,13a
for which the
best TOFs were obtained at pH 3.5 - 4 (i.e. ~ pKa of formic acid).
This characteristic makes catalyst Ir6 very interesting since, as
explained earlier, it is highly desirable to run the
dehydrogenation of FA under additive-free conditions in order to
eliminate a post-purification of H2 when volatile bases such as
trialkylamine are used. Furthermore, it opens the way towards
the base-free dehydrogenation of neat formic acid which, to the
best of our knowledge, has only been scarcely reported with low
efficiency (TOF < 800 h-1).[6a,9] Henceforth, we have investigated
the influence of formic acid concentration on the reaction
outcome (Figure 3).
Figure 3 Influence of the concentration on the efficiency of the base-free
dehydrogenation of FA. FA (4 mmol), Ir6 (0.01 mol%), H2O, 80 °C, 10 min.
Table 3. Influence of the pH on the catalyst performances.a
The temperature dependence of the catalyst activity was
investigated over a range of temperature going from 60 °C to
100 °C in aqueous solution of FA and neat FA (Table 4). It was
demonstrated that in both cases, the catalyst remained active
even at 60 °C albeit TOF values were low. Highest TOF were
obtained at 100 °C and noteworthy, a TOF of 13292 h-1 was
obtained for the base-free dehydrogenation of neat FA.
Entry
pH
1.8
3.2
3.7
4.2
7.6
HCOOH/HCOONa[b]
TOF[c]
12321
11584
8940
1
2
3
4
5
4/0
3/1
2/2
1/3
0/4
4821
Table 4. Catalyst activity vs T
TOF (h-1)
FA (3 M in H2O)a
TOF (h-1)
Pure FAb
2276
Entry
T (°C)
aIr6: 0.01 mol%; H2O: 2 mL, 80 °C, 10 min. bmol ratio maintaining total
mole number = 4 mmol. c TOF calculated from the released gas volume at
10 min.
1
2
3
4
5
60
70
3214
5825
1774
2678
4821
9174
13292
80
13058
23805
38236
The dehydrogenation of FA was conducted with Ir6 at
different concentrations ranging from neat formic acid (26.5 M)
to a 1 M aqueous solution. TOF higher than 11384 h-1 were
obtained with concentrations ranging from 2 M to 8 M with a
maximum TOF of 13058 h-1 obtained for a concentration of 3
mol.L-1. Lower concentration (1 M) led to a decrease of the
activity to 5800 h-1. Interestingly, the reaction performed in neat
formic acid displayed a TOF of 4800 h-1. Although this result is
90
100
aFA: 0.184 g (4 mmol); Ir6: 0.01 mol%; H2O: 1.3 mL, 80 °C, 10 min. bFA:
0.184 g (4 mmol); Ir6: 0.01 mol%, 80 °C, 10 min
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1
With these results in hands, we turned our attention to the
continuous use of the catalyst in water in order to investigate the
productivity of the catalyst. This process was investigated by
adding 40 mmol of FA to a solution of Ir6 (0.001 mol%) in 13.3
mL of water until gas evolution ceased. A new load a FA was
then added until no more catalytic activity was observed. After
10 days a cumulative TON of 710 000 could be obtained.
the structure of c was observed by H NMR in d6-DMSO (see
supporting information). This NMR did not reveal any
protonation of the bridging NH or dimethylamino-group. An
outer-sphere interaction with H2O acting as a proton shuttle then
releases H2 with regeneration of a. In the case of the
dehydrogenation of neat formic acid, the situation is more
intricate since the formation of a is not certain, albeit the formic
acid used is not anhydrous. Hence the activation of formic acid
may follow a different pathway under neat conditions. As
explained earlier, the absence of water may also influence the
hydrogen release. Further experimental and theoretical
calculations will be necessary to clarify this mechanism both in
water and neat conditions.
As mentioned earlier, the H2 purity is of prime importance for
application in fuel cells. In particular carbon monoxide resulting
from formic acid dehydration is strongly undesired as it is not
compatible with platinum electrodes.[16] The purity of the
released gas for the reactions performed with Ir6 catalyst in H2O
(FA 3M) and in neat FA was measured by gas chromatography
using a catharometer detector. In both cases, CO content was
below the detection limit of the measuring equipment (< 1 ppm,
see supporting information).
Different mechanisms have been proposed for the
dehydrogenation of formic acid.[17] The formation of a metal
hydride M-H may occur via the formation of a metal-formate
followed by a -hydride elimination. However, this mechanism
should require the presence of an external base. Another
pathway involves a concerted mechanism where the ligand
plays the role of the base through outer-sphere interactions. H-
bonding interactions with remote ligands have been proposed in
many examples as key features for the formation of metal-
hydrides species as well as for the elimination of H2. [17,18]
In order to get more insight into the key steps of the mechanism,
Kinetic Isotope Effect measurements were performed. As
depicted in Table 5, the rate of the reaction performed with
formic acid in D2O was not strongly impacted contrary to
reaction performed with deuterated formic acid in H2O or D2O.
These results clearly demonstrate that cleavage of the H-CO2H
bond to form an Ir-H species constitutes the rate determining
step of the reaction. Of note the same study performed in neat
FA did not reveal any significant KIE. In this case the rate
determining step is not the Ir-H formation and could rather be
the elimination of H2 since water-assisted H2 elimination has
been proposed for reactions conducted in water.[13a, 19]
Table 5 KIE in the dehydrogenation of FAa
Entry
Source
Solvent
H2O
TOF (h-1)
13058
8337
KIE
-
1
2
3
4
5
6
HCOOH
HCOOH
DCOOD
DCOOD
HCOOH
DCOOD
Figure 4 Mechanism proposal
D2O
1.6
3.9
4.4
-
H2O
3314
In conclusion, we have prepared a new catalyst bearing an
electron enriched dipyridylamine ligand. This catalyst performs
the base-free dehydrogenation of aqueous formic acid with high
efficiency at temperatures equal or below 100 °C. The base-free
dehydrogenation of neat formic acid was also achieved with a
TOF of 13292 h-1 obtained at 100 °C. In both cases carbon
monoxide was not detected in the released gas. The versatile
and tunable dipyridylamine ligands appear as promising ligands
D2O
2980
neat
neat
5122
4952
1.0(3)
a Source (4 mmol), Ir6 (0.01 mol%), H2O or D2O (1.3 mL), 80 °C, 10 min. b
based on gas volume released
possibly operating via outer-sphere interactions.
Further
investigations will be necessary to elucidate the catalytic
mechanism. The latent behavior of the catalyst highlighted in
this article will also require deeper investigations to fully assess
its potential and utility.
Based on these results and the involvement of the remote
bridging NH in the dipyridylamine ligand, a tentative catalytic
mechanism in water is proposed in Figure 4. An initial
displacement of the OSO3 ligand by water in Ir6 generates the
aquo-complex a. A concerted mechanism involving the bridging
2
NH (b) and SO4 - acting as a base then releases CO2 and the
hydride species c. An Iridium-hydride species compatible with
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The authors acknowledge the China Scolarship Council for a
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measurements and to Dr. Thierry Labasque (Observatoire des
Sciences de l’Univers, UMR 6118, Géosciences Rennes, CNRS,
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10.1002/cssc.201802275
ChemSusChem
COMMUNICATION
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COMMUNICATION
A new catalyst promotes the efficient
dehydrogenation of aqueous and neat
formic acid without any additive.
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COMMUNICATION
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