Selective hydrogen generation from formic acid with well-defined complexes of ruthenium and phosphorus-nitrogen PN3-pincer ligand

Article abstract of DOI:10.1002/asia.201600169

An unsymmetrically protonated PN³-pincer complex in which ruthenium is coordinated by one nitrogen and two phosphorus atoms was employed for the selective generation of hydrogen from formic acid. Mechanistic studies suggest that the imine arm participates in the formic acid activation/deprotonation step. A long life time of 150 h with a turnover number over 1 million was achieved. Grabbing hold: A PN³-pincer complex was employed for the selective hydrogen generation from formic acid. Mechanistic studies suggest the imine arm participates in the formic acid activation/deprotonation step. A long life time of 150 h with a turnover number over 1 million was achieved.

Full text of DOI:10.1002/asia.201600169

DOI: 10.1002/asia.201600169
Communication
Hydrogen Generation
Selective Hydrogen Generation from Formic Acid with Well-
Defined Complexes of Ruthenium and Phosphorus–Nitrogen PN -
Pincer Ligand
3
[
a]
[a]
[b]
[a]
[a]
Yupeng Pan, Cheng-Ling Pan, Yufan Zhang, Huaifeng Li, Shixiong Min,
[
b]
[a]
[b]
[b]
[a]
Xunmun Guo, Bin Zheng, Hailong Chen, Addison Anders, Zhiping Lai,
Junrong Zheng,* and Kuo-Wei Huang*
[b]
[a]
(
turnover number (TON)) of the catalysts are in general not suf-
3
Abstract: An unsymmetrically protonated PN -pincer com-
plex in which ruthenium is coordinated by one nitrogen
and two phosphorus atoms was employed for the selec-
tive generation of hydrogen from formic acid. Mechanistic
studies suggest that the imine arm participates in the
formic acid activation/deprotonation step. A long life time
of 150 h with a turnover number over 1 million was ach-
ieved.
ficiently long, because of various reasons, for example, instabil-
ity to moisture, air or acid or other trace amount of impurities
which exists in most formic acid supplies.
The decomposition of FA to H
ly favored, but the energy barrier is high and the selectivity is
poor due to the formation of H O and CO in the absence of
a suitable catalyst. H and CO generation from FA was studied
and CO is thermodynamical-
2
2
2
2
2
[
4]
several decades ago, but the potential of utilizing CO
storage material was not fully recognized until recently.
as a H
2
2
[
5]
A
number of homogeneous and heterogeneous catalyst systems
[
6]
have been developed for the generation of H from FA. Reac-
2
Hydrogen is a clean energy carrier with water as the only ex-
haust when combusted in engines or fuel cells. It is considered
to play a critical role in future renewable energy technolo-
tions that give significantly enhanced turnover frequencies
(TOFs) and turnover numbers (TONs) were achieved by using
the FA/NEt azeotrope or FA/formate mixtures. A few molecular
3
[1]
gies. Formic acid (FA), an adduct of CO and hydrogen and
catalysts also show good activities in the absence of base addi-
2
[
6f,m,x,z,ab,ac]
a major product of biomass, has been considered as a potential
tives.
Examples of very high TONs were reported re-
[
6y]
liquid source and storage material for hydrogen. In particular,
cently by the groups of Beller (TON>1000000) and Li (TON
À
[6ac]
its volumetric hydrogen capacity reaches 53 g H L , equivalent
of 2400000),
although the detailed mechanisms for the
2
À1
to an energy density of 1.77 kWhL , making it attractive for
high activity and selectivity are not clear. It has been suggest-
ed that metal hydride groups can serve as a base in some of
these external-base-free systems, while in other cases coopera-
tive ligands play the role as a base.
[2]
automotive and mobile applications. A sustainable and rever-
sible energy storage and utilization cycle can be envisioned by
combining CO and H into formic acid to store hydrogen and
2
2
remove CO , and decomposing formic acid to release hydro-
Two general strategies can be considered for selective hy-
drogen generation from formic acid (Scheme 1). One starts
with a metal hydride species which can react with the proton
2
[
3]
gen.
To release hydrogen under mild conditions, homogeneous
or heterogeneous catalysts are used to decompose formic
acid. Towards the goal of industrially viable applications, this
approach faces a couple of challenges. In many catalytic sys-
tems, trace amount of CO, which can poison the catalyst of hy-
drogen fuel cells, is generated as a by-product. The lifetimes
of a formic acid molecule to produce H and a metal formate
2
intermediate, a typical acid–base reaction between metal hy-
drides and a proton source. The catalytic cycle can be complet-
ed with b-hydride elimination (b-H) of the formate ligand to re-
generate the metal hydride species. One potential risk of this
[
a] Dr. Y. Pan, Dr. C.-L. Pan, Dr. H. Li, Dr. S. Min, Dr. B. Zheng, Prof. Z. Lai,
Prof. K.-W. Huang
Division of Physical Sciences and Engineering
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900 (Saudi Arabia)
E-mail: hkw@kaust.edu.sa
[
b] Y. Zhang, Dr. X. Guo, Dr. H. Chen, A. Anders, Prof. J. Zheng
Department of Chemistry
Rice University
Houston, TX 77005 (USA)
E-mail: junrong@rice.edu
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201600169.
Scheme 1. General strategies for selective hydrogen generation from formic
acid.
Chem. Asian J. 2016, 11, 1357 – 1360
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Communication
model is the sensitivity of the catalyst toward water. The other
type starts with the oxidative addition (OA) of formic acid to
a reduced metal complex to form a hydridometal formate
complex which can then undergo the b-hydride elimination to
give a dihydride intermediate. It can be followed by the reduc-
tive elimination (RE) of the two hydride ligands to give hydro-
gen and regenerate the reduced metal complex. This type of
compound may be reactive toward oxygen, and the reduced
metal center may facilitate the oxidative addition of the formyl
CÀH bond of formic acid to the metal, leading to a decarbony-
lation reaction. Thus, to achieve a high TON in a practical fash-
ion, the stability toward water is essential, as common formic
acid typically contains certain amounts of water; and in order
to avoid the undesired CO formation, reduced metal species
are better excluded from the catalytic cycle.
arm changes not only the thermodynamic stability of these
complexes toward water but also their kinetic behavior.
Indeed, our recent studies indicated that our catalysts go
through a different mechanism for hydrogen activation where
two water molecules (or protic molecules) are needed as the
[
9,10]
proton shuttle(s).
These exciting and unexpected results of
3
the catalysts based on our PN -pincer ligands imply that they
could be suitable for catalyzing the dehydrogenation reaction
of FA. To our delight, promising catalytic performances were
achieved under base-free conditions in dimethylsulfoxide
3
(DMSO) for 3 among three PN -pincer Ru complexes (Table 1),
II
[a]
Table 1. Ru -catalyzed hydrogen production from formic acid.
Cat:
HCOOHƒƒ!H
2 2
+CO
These previous encouraging observations and the mechanis-
tic considerations for a highly selective and active catalyst
toward hydrogenation generation from FA prompted us to
evaluate the catalytic activity of Ru complexes bearing a dear-
omatized pyridine moiety and an imine arm. Herein, we report
a rationally designed catalyst with high activity and selectivity
for hydrogen generation from FA under mild conditions
Entry
Solvent
Cat.
T [8C]
TON
9800
–
2400
–
95000
2000
4800
3500
3600
95000
1
2
3
4
5
6
7
8
9
DMSO
Toluene
DMSO
Toluene
DMSO
Toluene
MeCN
DMF
1
1
2
2
3
3
3
3
3
3
50
50
50
50
50
50
50
50
50
50
(
Figure 1). These catalysts are not air or water sensitive and no
THF
DMSO
[b]
1
0
[
a] Reaction conditions: Ru complex (1.0 mmol in 5.0 mL of the respective
solvent), 95% commercial formic acid was injected by the microinjector
see the Supporting Information). TON was estimated based on the H
(
2
collected by water displacement, with the solvent and water vapors ne-
glected. Carbon dioxide was absorbed by a saturated solution of potassi-
um hydroxide. [b] Catalyst 3 was exposed to the air for one month.
DMF=N,N-dimethylformamide.
while none to low activities were observed in toluene, suggest-
ing the reaction is sensitive to the solvent medium. Further
solvent screening revealed that the highest activity was ach-
ieved in DMSO (entries 5–9, Table 1). Complex 3 aged in air for
one month showed no apparent difference in the catalytic ac-
tivity (entry 10, Table 1). It is also very important to note that
no formation of CO was detected, indicating that the purity of
the regenerated gas is suitable for the use in hydrogen fuel
cells (see the Supporting Information).
3
Figure 1. A new class of PN -Ru complexes.
detectible CO is generated from the hydrogen generation reac-
tion. The best catalyst is active (turnover frequency (TOF)>
À1
7
000 h ) over a very long lifetime of 9000 min with a turnover
number more than 1 million at 908C. The reaction setup is
simple and no sophisticated devices are required. These are fa-
vored properties for various applications. Mechanistic insights
from NMR spectroscopy studies are also presented.
To gain insight into the high reactivity and selectivity,
We have recently developed a new class of dearomatized
pyridine-based pincer complexes through the deprotonation
a series of NMR spectroscopy studies and model reactions
1
were conducted. The H NMR spectrum of 3 in [D ]DMSO
6
2
of one NH-PtBu arm (Figure 1). This change has resulted in
shows three sets of sp CÀH signals at d=5.57 (d), 5.94 (d) and
2
a huge impact on the catalytic functions and stability of the
complexes compared to their methylene counterpart devel-
6.85 (t) ppm, indicative of the dearomatization of the central
pyridine ring. These results are consistent with two phospho-
[7]
31
oped by Milstein and co-workers. It was found that these
new complexes are air- and moisture-stable, and they catalyze
rus signals observed in the P NMR spectrum with an apparent
1
triplet for a hydride at d=À26.43 ppm in the H NMR spec-
reactions where the analogs with a CH spacer show limited
trum. The presence of water as indicated by a peak at d=
2
1
activities. For example, Milstein’s catalyst catalyzes the dehy-
drogenative acylation of amines, but our catalysts catalyze the
selective dehydrogenation (oxidation) of benzylamines into
3.33 ppm in the H NMR spectrum showed that this dearomat-
II
ized Ru complex did not react with H O in DMSO even during
2
heating in an oil bath at 1008C for 12 h. This observation sup-
ports the stability of 3 towards water. Upon treatment with 1.5
[
8]
imines. Our catalysts efficiently catalyze the hydrogenation of
[
9]
2
esters under mild conditions even in the presence of water.
These surprising discoveries strongly suggest that the NÀH
equivalents of FA, the sp CÀH signals were shifted to down-
field at d=6.33 and 7.27 ppm, indicating that the imine arm
Chem. Asian J. 2016, 11, 1357 – 1360
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
was re-protonated, leading to the rearomatization of the pyri-
dine ring. A new and more downfield hydride signal was
1
formed at d=À16.94 ppm in the H NMR spectrum, consistent
with the presence of a ligand trans to H.
This ligand was confirmed to be a formate group by the ob-
13
servation of a formate signal at d=165.8 ppm in the C NMR
spectrum and a formyl hydrogen at d=8.39 ppm in the
Scheme 3. Proposed mechanism for the exchange of RuÀH and RuÀOOCH
signals.
1
H NMR spectrum. The formation of hydridoruthenium formate
4
was thus assigned accordingly (Scheme 2). The formation of
creased by about five times, but formation of H and HD were
2
1
also observed in the H NMR spectrum, indicating that the b-
hydride elimination of the formate group may be rate-deter-
mining, and the deuterium scrambling is likely a result of fast
equilibrium between species 3–5, H (HD and D ), CO , and
2
2
2
HCOOH (DCOOH and DCOOD).
It has been shown that the presence of a base, for example,
Na CO or NEt , could enhance the activities and lifetime of the
2
3
3
[
5b,6r,y,11]
catalysts for formic acid dehydrogenation (Table 2).
High
Table 2. Effect of base on the efficiency of formic acid dehydrogenatio-
[a]
n.
À1
Entry
Solvent
Additive
Et
T [8C]
TOF [h
]
TON
1
2
3
DMSO
DMSO
DMF
3
N
90
90
90
7333
13123
31000
1100000
350000
93000
Scheme 2. Plausible mechanism for the hydrogenation of CO
tion of H from formic acid catalyzed by complex 3.
2
and produc-
Na
Et
2
CO
3
N
2
3
[
a] Reaction conditions: Ru complex 3 (1.0 mmol), TON was estimated
4
was also accompanied by the disappearance of the reddish
based on the gas produced. Base loading: NEt (1.50 mL) or Na CO
3
2
3
orange color of 3 in the solution (Figure S1). It was noted that
(0.624 g), FA was injected at the specified rate (see the Supporting Infor-
mation).
intermediate 4 slowly decomposed with production of H and
2
CO , and the color of the solution changed from very pale
2
yellow back to reddish orange, indicating the regeneration of
À1
3
, which was also confirmed by NMR spectroscopy.
activities were identified with an average TOF of 7300 h and
The plausible mechanism for the catalytic transformation in-
a TON of 1100000 at 908C. Interestingly, in the presence of
a more general base, Na CO , the performance of catalyst 3
cludes three steps: the protonation of the imine arm of the
2
3
À1
dearomatized pincer complex (3) to intermediate 4, the de-
showed a TOF of approximately 13100 h and a TON of
350000. Unfortunately, attempts in pushing the faster reaction
rates, for example, using DMF as the solvent in the presence of
NEt3 with an increased FA injection rate, only resulted in
composition of formate 4 with liberation of CO , and the elimi-
2
nation of hydrogen to regenerate the catalyst. During the
whole process, the oxidation state of Ru remains II. While the
proposed dihydride species (5) was not seen during the formic
À1
a slightly enhanced TOF of 31000 h with a significantly short-
1
acid dehydrogenation reaction monitored by H NMR spectros-
er life time. These results suggest that the stability of the cata-
lyst greatly depends on the balance between the base and
acid. It was found that catalyst 3 indeed decomposed in high
concentrations of formic acid to release free phosphines, evi-
copy, complex 5 can be formed in situ when hydrogen is intro-
duced to the DMSO solution of 3. It is also very important to
note that, in dry C D solution, no reaction between complex 3
6
6
31
and H was observed. However, when wet C D was used, in-
denced in the P NMR spectrum (d=56.9 ppm), presumably
2
6
6
stant conversion of 3 and H into dihydride 5 was achieved.
due to the over-protonation of the amine arm, leading to hy-
2
[
12]
These results are consistent with our earlier studies, which sug-
gest that water (protic) molecules are needed for hydrogena-
drolysis of the NÀP bonds. The NÀH arms may play a poten-
tial role in modulating the acidity of the reaction environment.
In conclusion, we have developed an active and selective
catalyst for hydrogen production from FA. The reaction starts
with the protonation of the dearomatized ligand, and during
the process the oxidation state of the Ru center does not
change. This avoids the decarbonylation process and thus
makes the reaction perfectly selective without the formation of
CO. The enhanced water and air stabilities are likely realized by
the tridentate binding of the ligand, the +2 oxidation state of
3
[7a]
tion of the dearomatized PN -Ru complexes. Upon treatment
with CO , 5 was converted into formate 4. Moreover, exchange
2
signals between Ru-H and Ru-OOCH were observed under 2D
NOESY experiments (see the Supporting Information), suggest-
ing a possible equilibrium between intermediate 4 and dihy-
dride 5 and CO (Scheme 3), and strongly supporting the role
2
of 5 in the proposed catalytic process and implying that the
hydrogen elimination is unlikely the rate determining step.
When DCOOH was used, not only was the initial rate de-
II
Ru , and the bulky steric effect of the tert-butyl groups. The
Chem. Asian J. 2016, 11, 1357 – 1360
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1359
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
ability of the local environment to be tuned by the two NÀH
arms may also contribute to the long lifetime of the catalyst.
This mechanism significantly reduces the chance for protons
to attack the coordination atoms and the metal center under
a well-controlled FA injection rate, and therefore diminishes
the decomposition of the catalyst induced by the attack of
protons. The tolerance to water of 3 may enable practically
feasible hydrogen generation from FA.
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Keywords: formic acid · homogeneous catalysis · hydrogen ·
pincer ligands · ruthenium
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