Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation Catalyzed by Iridium Complexes Bearing Pyridyl-pyrazole Ligands: Effect of an Electron-donating Substituent on the Pyrazole Ring on the Catalytic Activity and Durability

Article abstract of DOI:10.1002/adsc.201801323

Cp*Ir (Cp=pentamethylcyclopentadienyl) complexes with an N,N-bidentate ligand such as 2,2′-bipyridine serve as catalysts for both carbon dioxide (CO₂) hydrogenation to formate and formic acid dehydrogenation in water. Previously, it was shown that the introduction of an electron-donating substituent on 2,2′-bipyridine is an effective method to improve the catalytic activity. Especially, the highly electron-donating hydroxyl (OH) substituent performs much better than other substituents such as methyl or methoxy under basic conditions. However, the introduction of an OH substituent on the ligand has been limited to six-membered rings such as pyridine or pyrimidine. These results prompted us to develop a new ligand comprising a pyridyl-pyrazole with an OH group on the pyrazole moiety for Cp*Ir-catalyzed CO₂ hydrogenation and formic acid dehydrogenation. The resultant catalyst showed high catalytic activity in CO₂ hydrogenation and excellent robustness in formic acid dehydrogenation with a turnover number of 10 million. (Figure presented.).

Full text of DOI:10.1002/adsc.201801323

Title: Carbon Dioxide Hydrogenation and Formic Acid
Dehydrogenation Catalyzed by Iridium Complexes Bearing
Pyridyl-pyrazole Ligands: Effect of an Electron-donating
Substituent on the Pyrazole Ring on the Catalytic Activity and
Durability
Authors: Naoya Onishi, Ryoichi Kanega, Etsuko Fujita, and Yuichiro
Himeda
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801323
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Advanced Synthesis & Catalysis
10.1002/adsc.201801323
VERY IMPORTANT PUBLICATION
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Carbon Dioxide Hydrogenation and Formic Acid
Dehydrogenation Catalyzed by Iridium Complexes Bearing
Pyridyl-pyrazole Ligands: Effect of an Electron-donating
Substituent on the Pyrazole Ring on the Catalytic Activity and
Durability
a
a
b
a
Naoya Onishi, Ryoichi Kanega, Etsuko Fujita, Yuichiro Himeda *
a
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki,
3
05-8565, Japan
Phone: +81-29-861-4262, Fax: +81-29-861-4688, E-mail: himeda.y@aist.go.jp
Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973-5000, United States
b
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. Cp*Ir (Cp* = pentamethylcyclopentadienyl)
complexes with an N^N-bidentate ligand such as 2,2-
bipyridine serve as catalysts for both carbon dioxide
studied. Because of its relatively high gravimetric
hydrogen content (4.3 wt%), its low toxicity toward
the human body and the environment, and its easy
[1]
(
CO
2
)
hydrogenation to formate and formic acid
accessibility by biomass processing, formic acid
(FA) has been widely considered as a liquid storage
dehydrogenation in water. Previously, it was shown that
the introduction of an electron-donating substituent on
medium capable of releasing H via its catalytic
2
2
,2-bipyridine is an effective method to improve the
dehydrogenation. In addition, when FA is stored as
an aqueous solution (< 85 wt%), it is not combustible.
Toward the efficient production and storage of
hydrogen, it is of considerable interest to develop
highly active and durable catalysts that exhibit
catalytic activity. Especially, the highly electron-donating
hydroxyl (OH) substituent performs much better than
other substituents such as methyl or methoxy under basic
conditions. However, the introduction of an OH
substituent on the ligand has been limited to six-
membered rings such as pyridine or pyrimidine. These
results prompted us to develop a new ligand comprising a
pyridyl-pyrazole with an OH group on the pyrazole
selective FA dehydrogenation to H
convert thermodynamically stable CO
formate.
To date, much effort has been devoted to
discover new and effective homogeneous catalysts
2
and CO
2
2
, and
to
and H
2
moiety for Cp*Ir-catalyzed CO
formic acid dehydrogenation. The resultant catalyst
showed high catalytic activity in CO hydrogenation and
2
hydrogenation and
[2]
[3]
for both CO
dehydrogenation.
catalysts for CO
2
hydrogenation to formate and FA
2
[3j, 3ae, 4]
Remarkable performance of
hydrogenation has been achieved by
excellent robustness in formic acid dehydrogenation with
a turnover number of 10 million.
2
many researchers. Nozaki and coworkers showed that
an Ir complex coordinated by a PNP pincer ligand
exhibited the high TOF (turnover frequency) of
2
Keywords: CO hydrogenation; Formic acid
dehydrogenation; Iridium catalyst, Hydrogen storage
-
1
1
3
50,000 h and high TON (turnover number) of

,500,000 at 200 C and 5.0 MPa in an organic
[5]
solvent. Pidko and coworkers also found a pincer
Ru complex with the highest observed initial TOF
New fuels without emission of air pollutants or
greenhouse gases are being sought, and among them
the use of hydrogen gas has drawn much attention.
Safe and efficient utilization of hydrogen provides a
convenient way to help combat the pressing challenge
of global greenhouse gas emissions. However,
significant technical and safety concerns regarding
cryogenic liquid and compressed gaseous hydrogen
together with its high flammability result in
considerable barriers to widespread use. Therefore,
rapid and controlled storage/production of hydrogen
in a safe and efficient manner is being extensively
-1

[6]
(
1,100,000 h ) at 120 C and 4.0 MPa of H
2
/CO ).
2
However, many of these catalysts require harsh
conditions and the presence of an amine and/or
organic solvents. Of particular importance is the
design of an efficient catalyst exhibiting high activity
and durability without requiring any additives. In
addition, with the consideration of the reduction of
the environmental load, promising catalysts should
perform in aqueous solution. Examples of such
efforts include the achievement of CO
2
hydrogenation
[3f, 3p, 3ad, 7]
under ambient conditions in water
, and
1
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Advanced Synthesis & Catalysis
10.1002/adsc.201801323
high-pressure hydrogen generation technology by FA
substituent[11] and positional effect; in the latter case,
[8]
dehydrogenation without a mechanical compressor.
it was found that complexes B and C have different
rate determining steps (i.e., H formation from IrH
2
vs -hydride elimination from the bound formate to
[12]
evolve CO , respectively) , and therefore, the
2
catalytic activity of C was improved compared to the
low performance of B at more basic conditions tuned
[13]
by the addition of sodium formate (Figure 1).
These results indicated that the presence and position
of OH on the ligand would be important for
improving catalytic activity in both CO
hydrogenation and FA dehydrogenation.
2
2
Scheme 1. Cp*Ir catalysts for CO hydrogenation and FA
dehydrogenation.
Scheme 2. Acid-base equilibrium between hydroxyl-form
and oxyanion-form.
In our previous work on half-sandwich Cp*Ir
complexes, we have shown that catalytic activity in
Furthermore, we have previously found that
five-membered azole-type rings such as imidazole
and pyrazole play a significant role in the ligand
framework for CO2 hydrogenation and FA
both CO hydrogenation and FA dehydrogenation
2
depended strongly on the electron-donating ability of
the 2,2-bipyridine ligands. Good correlation between
TOF values and Hammett substituent constants was
observed, i.e., the stronger the electron donating
property of the bipyridine ligand, the higher the
catalytic activity. Scheme 1 shows our designed
[14]
dehydrogenation catalysts. In addition, catalysts D
and E possessing a pyrazole moiety in the ligands
were also found to be highly active catalyst in CO
2
[16]
[15]
hydrogenation
and FA dehydrogenation.
catalysts that were active for CO hydrogenation. For
2
Furthermore, a pyrazole ring was also shown to be
[17]
example, while the Cp*Ir complex bearing 2,2-
substituted by an OH group and this fact prompted
us to develop a new ligand consisting of a hydroxyl-
pyrazole moiety. Herein, we describe a new type of
bipyridine (A) as the N^N-bidentate ligand exhibited
-
1
the low TOF of 1 h in CO
with 1.0 MPa H /CO , the introduction of a strong
electron-donating OH substituent at the 4-position of
,2-bipyridine improved the catalytic activity (B,
2
hydrogenation at 50 C
2
2
Cp*Ir catalysts for both CO hydrogenation and FA
2
dehydrogenation in order to study the effect of an OH
group on a pyrazole moiety. The complex possessing
an OH group was highly efficient toward CO2
hydrogenation even under ambient conditions. Also,
it was discovered to be long-lived, consuming a very
large amount of substrate in FA dehydrogenation.
2
-1
[9]
TOF: 650 h ) under same conditions.
This
enhancement of catalytic performance was attributed
to the increase in the electron-donating ability derived

from the oxyanion (O ) generated by the
deprotonation of the OH group under basic conditions.
It has already been found that the OH group can be
reversibly protonated and deprotonated by a change
of pH conditions. In addition, under basic conditions,

the resonance through the O and pyridine ring was
expanded, leading to the increase in the electron
donation ability from ligand to the metal center
10]
(
Scheme 2).[ The position of the OH group on the
pyridine ring was also important in improving the
catalytic activity. In CO hydrogenation, catalyst C
2
possessing an OH group near the Ir center showed
much higher catalytic activity than A and B due to
the support of the cleavage of the H bond to generate
2
an Ir-H intermediate having a solvent water bridge
with the OH group.[
3f]
In addition to CO
2
hydrogenation, it was also observed in FA
dehydrogenation that the catalytic activity was
improved by a highly electron donating OH
2
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Advanced Synthesis & Catalysis
10.1002/adsc.201801323
Figure 1. pH dependences of FA dehydrogenation using
catalyst B and C. pH in the reaction solution was tuned by
the addition of sodium formate.
Table 1 summarizes the results along with those of
previously reported catalysts, namely B-E, for
comparison. The complex 1 bearing a non-modified
-1
pyrazole moiety exhibited a TOF of 20 h (Entry 1).
Although three methyl groups, which are known to be
slightly electron-donating compared to hydrogen
substituents, were introduced on pyrazole, the
catalytic performance of 2 was not promoted (Entry 2,
-1
TOF: 11 h ). In contrast, catalyst 3 in which a methyl
group on the pyrazole ring was replaced by an
electron-donating methoxy group led to a modest
improvement in CO
2
hydrogenation compared
-1
pyrazole complexes 1 and 2 (Entry 3, TOF: 130 h ).
Furthermore, the catalytic activity of 4 possessing an
OH substituent was greatly enhanced compared to
-1
other pyrazole complexes 1-3 (Entry 4, TOF: 650 h ).
This recognizable change revealed that there was a
good correlation between the catalytic activity and
the electronic nature of the substituents. As shown in
Figure 3, UV−vis spectra during a pH titration of 4
showed a bathochromic effect in the region from pH
Figure 2. Cp*Ir catalysts coordinated by pyridyl-pyrazole
derivatives.
3
to 6 that is attributed to a change in the electronic
properties of the ligand by deprotonation of the OH
group on the pyrazole ring. In other words, since the
calculated pK of 4 was 4.0, it can be assumed that
hydrogenation.a)
a
Table 1. The results of CO
2

the O generated by the deprotonation
greatly improved the catalytic activity
Cat./µM Time, TOF, TON Final formate conc.,
Entry
-
1
h
1
1
1
48
1
h
M
under the reaction conditions for CO
2
13
1
2
3
4
5
6
1 / 200
2 / 200
3 / 20
4 / 20
4 / 20
4 / 20
4 / 50
B / 20
B / 50
C / 20
C / 50
D / 20
E / 20
E / 50
20
11
20
11
130
7850
1350
1850
1250
7700
92
5150
330
388
638
327
0.004
0.002
0.003
0.156
0.027
0.037
0.063
0.150
0.005
0.103
0.016
0.008
0.013
0.016
hydrogenation (pH 7.9). In addition, C
NMR analyses of 4 under acidic and
basic conditions supported the results
130
650
1350
1850
34
790
7
1650
27
[18]
according to our previous work. The

b)
conversion of OH to O by changing the
solution pH from acidic to basic
resulted in a downfield shift of the
signal of a carbon atom binding to OH
c)
1
d)
7
140
33
24
9
33
1
[10a]
8
9
(
163 ppm → 169 ppm). However, the
d),[7a]
3f]
catalytic performance of 4 was lower
than B and C under pressurized
conditions (Entries 8 and 10). On the
other hand, catalyst 4 had almost the
same catalytic activity as D and E
(Entries 12 and 13).
1
0[
d),[3f]
1
1
1
1
1
2
3
4
[
15]
15]
388
638
11
[
1
72
d)
a)
Conditions: PH2/CO2 = 1.0 MPa (1/1), [NaHCO
3
] =

1
.0 M in aqueous solution at 50 C. All numbers are
b)

c)

d)
the average of two runs. At 80 C. At 100 C. At

2
5 C, PH2/CO2 = 0.10 MPa (1/1).
CO
2
Hydrogenation. To investigate the effect
of substituent on catalytic activity for CO
2
hydrogenation to formate, the series of the Cp*Ir
complexes 1-4 bearing pyridyl-pyrazole derivatives
were prepared as shown in Figure 2. Since, in the
case of the pyridine ligand, an OH group near the Ir
center performed effectively in CO hydrogenation as
2
mentioned above, we explored what the modification
of substituents on a pyrazole moiety would have on
Figure 3. UV−vis absorption spectra of complex 4 in a pH
titration (left) and absorbance changes at single
wavelengths (270 and 295 nm) as a function of the pH
the catalytic activity. Firstly, CO
2
hydrogenation was
carried out with catalysts 1-4 ([NaHCO
3
]
aq = 1.0 M,

[
cat] = 50-200 µM, 50 C, PH2/CO2 = 1.0 MPa (1/1)).
(
right). Boltzmann fits are depicted.
3
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Advanced Synthesis & Catalysis
10.1002/adsc.201801323
Next, we investigated CO
2
hydrogenation under
aq = 1.0 M, [cat] = 50
µM, , 25 C, PH2/CO2 = 0.10 MPa(1/1)). Complex 4
which meant that the catalytic activity was
remarkably improved only by the strong electron
donation ability of OH. Under similar conditions,
catalyst 4 showed much higher catalytic activity than
catalysts B-D (Entries 5-7) possessing an OH group
on the pyridine moiety of the ligand, which suggested
that the OH on a pyrazole moiety worked much better
than that on a pyridine ring. On the other hand, the
result of 4 was quite similar to E (Entry 8) which
possess two OH groups on the pyrimidine moiety.
We also evaluated the pH dependence (Table S1)
because the catalysis of some of the catalysts
possessing an OH group on the ligand near the Ir
center (such as C) is known to be strongly affected by
pH. When catalysts 1-3 were evaluated at pH 3.5 with
FA/sodium formate (1/1), the reaction rates decreased
compared to those at pH 1.7. On the other hand, 4
showed the highest activity at pH 3.5. The catalysis
ambient conditions ([NaHCO
3
]

-1
achieved an initial TOF of 34 h in 1 hr and total
TON of 1250 after 140 hrs (Entry 7) with formate
concentration of 62.5 mM. In the cases of B and C
under the same conditions (Entries 9 and 11), the
concentrations of produced formate increased at the
initial stage but stopped after 24 hrs as shown in
Figure 4. These results suggest that the OH group on
a pyrazole moiety would improve the catalytic
durability. In summary, the OH group on the pyrazole
moiety served as an efficient substituent on the ligand
for the improvement of the catalytic activity for CO
2
hydrogenation. However, the catalytic performance
of 4 is lower than other sophisticated catalysts such as
the picolinamide catalyst[
4t]
and pyrimidine
catalyst.[ In the case of pyrazole catalysts D and E,
the catalytic activities were much higher than that of
. Therefore, the modification of the pyridine ring of
7a]
of 4 exhibited the pseudo bell-shaped pH-TOF profile
1
b)
Cat./µM
TON
TOF,
Entry
-
1
h
1
2
3
4
5
6
7
8
1 / 500
2 / 100
3 / 100
4 / 100
B / 100
C / 100
D / 100
E / 100
2000
240
630
10000
10000
10000
10000
10000
10000
10000
2700
6720
2400
2450
2700
7050
[11]
[13]
[16]
[16]
which is similar to that with C (Figure S1).
a)
Table 2. The results of FA dehydrogenation.
a)
The reaction was carried out in deaerated 1 M aqueous
FA solution at 60 C. By HPLC analysis after the reaction,

it was confirmed that FA was completely converted in all
reactions. The initial TOF was measured after 10 min.
b)
4
catalyst for CO
detailed reaction mechanism will be studied
will be necessary to develop a more promising
2
hydrogenation. Additionally, the
[19]
according to previous reports.
The performance of 4 at various temperatures
was also studied (Table S2). The Arrhenius plot is
Figure 4. Comparison of TON time courses for CO
hydrogenation under ambient conditions with catalyst 4
red circle), B (blue square), and C (black triangle).
2
shown in Figure S2, and the activation energy (E )
a
-
1
was calculated to be 74.1 kJ mol which is almost the
same as those obtained with other Cp*Ir catalysts
(

-1 [11]
-1 [13]
Conditions: [cat] = 50 μM, [NaHCO
3
]
aq = 1.0 M, 25 C,
such as B (76.4 kJ mol ) and C (75.4 kJ mol ).
P
H2/CO2 = 0.10 MPa (1/1).
The effect of the FA concentration on catalytic
activity was investigated at 60 C, and detailed data

are shown in Table S3. Complex 4 showed good
catalytic activities under various FA concentration
conditions, and in particular, FA was completely
consumed over 72 hours without deactivation even at
the high concentration of 20 M (Figure S3(a)).
Furthermore, the FA dehydrogenation reaction in a
20 M FA solution with 4 proceeded completely under
reflux conditions (Figure S3(b)) based on HPLC
analysis, suggesting the high durability of 4.
Additionally, the evolved gases did not show CO
contamination (< 10 ppm) as confirmed by GC
analysis (Figure S4).
FA Dehydrogenation. Following the study of
the catalytic reaction for CO hydrogenation, FA
dehydrogenation with catalysts 1-4 was also
evaluated (Table 2). Similar to CO hydrogenation,
the catalytic performance was found to depend on the
electron-donating ability of substituent on the
pyrazole moiety with the rate increasing in the order
2
2
1
< 2 < 3 < 4 (Entries 1-4). From the results of UV-
vis spectra in a pH titration as shown in CO
hydrogenation, it was assumed that the OH group of
was not deprotonated under acidic conditions,
2
4
4
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Advanced Synthesis & Catalysis
10.1002/adsc.201801323
Figure 5. The structures of durable Cp*Ir catalysts for FA
dehydrogenation.
total TON was 10 million, and the evolved gas
For practical applications, we have investigated
the catalytic durability of FA dehydrogenation with
this class of Cp*Ir catalysts. For FA as a hydrogen
storage medium, the durability of the catalyst is of
key importance because the catalyst reacts with
enormous amounts of FA in long-term reactions.
Therefore, the catalyst must keep showing the
3
volume reached 2.5 m . To the best of our knowledge,
these values are the best so far reported in the
literature. Although a high TON (5 million) and
-1
average TOF (28,700 h ) were also achieved by Li et
al., their catalytic system required the addition of 6
equiv of ligand to each equiv of Ir which might
[21]
enhance the catalyst lifetime.
In contrast, our
activity under harsh conditions, and generate H on a
2
system achieved high TON without any additives
such as a corresponding ligand and thus it was
confirmed that the active species was quite durable.
Figure 7. Time courses of volume of released gasses (blue
line) and rate of released gases (orange line) in FA
dehydrogenation with the continuous addition of FA by a
pump. Conditions: [FA] = 4.0 M, 500 mL, [4] = 10 M,
large scale. We have already reported highly durable
catalysts including Py-IMN and DHPT (Figure 5).
Py-IMN could maintain the catalytic activity for 15
3
days with the evolution of 1 m of gases and a TON

[20]
of 2 million from FA at 50 C. Similarly, DHPT
could also give the high TON of 5 million for 83 days

[8a]
0
0
at 60 C from FA. To compare the catalytic activity
and durability in brief, FA dehydrogenation was
carried out under reflux conditions with complexes 4,


7
0 C, [FA]add = 80 wt% (= 20 M), at 70 C. The rate of
Py-IMN, and DHPT (Figure 6, Conditions: [FA] =

8
.0 M, 100 mL, [cat] = 10 μM, at 110 C). Figure 6
shows the time course plots of TOFs up to 300 min
after the start of the reaction. For complexes 4 and
DHPT, the reaction smoothly proceeded without any
appreciable decrease of reaction rate. On the other
hand, in the case of Py-IMN whose initial TOF was
much higher than those of complexes 4 and DHPT
-
1
-1
(
Py-IMN; 195,000 h , 4; 110,000 h , DHPT; 65,000
-1
h ), the reaction rate markedly decreased from its
initiation. From these results, the durability of
complex 4 is suggested to be higher than that of Py-
IMN, and be similar to DHPT. Complex 4 performed
more actively than DHPT.
FA addition; (a) = 0.07 mL/min, 420 hr, 35.3 mol, (b) =
stop, (c) = 0.05 mL/min, 250 hr, 15 mol, (d) = stop, total
FA amount = 52.3 mol.
Figure 6. Time courses of TOFs (▲; catalyst 4, ●; Py-
IMN, ■; DHPT) in FA dehydrogenation under harsh
conditions. (Conditions: [FA] = 8 M, 100 mL, [cat] = 10


M, at 110 C.
To study the reaction mechanism with 4, the
plausible reaction intermediate was traced by NMR
analysis (Figure S5). By reacting 1 equiv of a sodium
formate with 4, the peak derived from Ir-H was
observed around -12 ppm, which disappeared upon
Therefore, we evaluated the robustness of 4 for
FA dehydrogenation in long-term experiments. In the
course of the reaction, a 20 M FA (80wt%) solution
was continuously fed to 4 M of FA solution by a
the addition of 1 drop of D
2
SO . In other words, the
4
pump (Figure 7, conditions: [FA]
0
= 4.0 M, 500 mL,
present catalyst system is presumed to be the
following the same reaction mechanism as with other
Cp*Ir catalysts with N^N-bidentate ligands; (i) a
formate anion coordinated to the Ir center, (ii) a Ir-H
intermediate generated by the -hydride elimination

[
4]
0
= 10 M, 70 C). The rate of FA addition by the
pump was set at 0.07 mL/min at the beginning of the
reaction, and 0.05 mL/min after interruption for FA
addition. Although a slight decrease in the TOF was
observed (possibly from deactivation, the difficulty of
steady heating of the solution, and/or a change in FA
concentration), the reaction ran to completion. The
total amount of decomposed FA was 52 mol. The
of CO
2
from the formato complex, (iii) a regenerated
starting complex by protonation of the Ir-H complex,
releasing H
preliminary
2
and coordinating a formate anion. Our
kinetic
isotope
effect
(KIE)
5
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Advanced Synthesis & Catalysis
10.1002/adsc.201801323
measurements have not clearly identified the rate References
determining step (RDS), but the bell-shape pH-
dependent TOF implies the RDS is step (ii).
[1] a) F. Joó, ChemSusChem 2008, 1, 805-808; b) T. C.
Johnson, D. J. Morris and M. Wills, Chem. Soc. Rev.
2010, 39, 81-88; c) B. Loges, A. Boddien, F. Gärtner,
H. Junge and M. Beller, Top. Catal. 2010, 53, 902-914;
d) W.-H. Wang, Y. Himeda, J. T. Muckerman, G. F.
Manbeck and E. Fujita, Chem. Rev. 2015, 115, 12936-
12973.
[2] a) N. Onishi, G. Laurenczy, M. Beller and Y. Himeda,
Coord. Chem. Rev. 2018, 373, 317-332; b) K. Sordakis,
C. Tang, L. K. Vogt, H. Junge, P. J. Dyson, M. Beller
and G. Laurenczy, Chem. Rev. 2018, 118, 372-433; c)
D. Mellmann, P. Sponholz, H. Junge and M. Beller,
Chem. Soc. Rev. 2016, 45, 3954-3988.
[3] a) J. C. Tsai and K. M. Nicholas, J. Am. Chem. Soc.
1992, 114, 5117-5124; b) C.-C. Tai, T. Chang, B. Roller
and P. G. Jessop, Inorg. Chem. 2003, 42, 7340-7341; c)
H. Hayashi, S. Ogo and S. Fukuzumi, Chem. Commun.
2004, 2714-2715; d) Y. Himeda, N. Onozawa-
Komatsuzaki, H. Sugihara, H. Arakawa and K. Kasuga,
Organometallics 2004, 23, 1480-1483; e) C. Federsel,
A. Boddien, R. Jackstell, R. Jennerjahn, P. J. Dyson, R.
Scopelliti, G. Laurenczy and M. Beller, Angew. Chem.
Int. Ed. 2010, 49, 9777-9780; f) W.-H. Wang, J. F. Hull,
J. T. Muckerman, E. Fujita and Y. Himeda, Energy
Environ. Sci. 2012, 5, 7923-7926; g) C. Ziebart, C.
Federsel, P. Anbarasan, R. Jackstell, W. Baumann, A.
Spannenberg and M. Beller, J. Am. Chem. Soc. 2012,
134, 20701-20704; h) E. A. Bielinski, P. O. Lagaditis,
Y. Zhang, B. Q. Mercado, C. Würtele, W. H.
Bernskoetter, N. Hazari and S. Schneider, J. Am. Chem.
Soc. 2014, 136, 10234-10237; i) S. Moret, P. J. Dyson
and G. Laurenczy, Nat. Commun. 2014, 5, 4017; j) S.
Enthaler, A. Brück, A. Kammer, H. Junge, E. Irran and
S. Gülak, ChemCatChem 2015, 7, 65-69; k) G. A.
Filonenko, D. Smykowski, B. M. Szyja, G. Li, J.
Szczygieł, E. J. M. Hensen and E. A. Pidko, ACS Catal.
2015, 5, 1145-1154; l) C. Liu, J.-H. Xie, G.-L. Tian, W.
Li and Q.-L. Zhou, Chem. Sci. 2015, 6, 2928-2931; m)
O. Rivada-Wheelaghan, A. Dauth, G. Leitus, Y. Diskin-
Posner and D. Milstein, Inorg. Chem. 2015, 54, 4526-
In summary, we have achieved the development
of new Cp*Ir complexes bearing various pyrazole
moieties in N^N-bidentate ligands for the CO
2
hydrogenation and FA dehydrogenation. The
introduction of an OH group on the pyrazole ring
improves the catalytic activity, and this tendency was
similar to pyridine-type ligand moieties. Under basic
conditions for CO hydrogenation, the OH group is
2
converted to an oxyanion by deprotonation, and the
oxyanion substituent which was known to be more
strongly electron donating than an OH group
contributes to the enhancement of the electron density
on the Ir center. Furthermore, the catalyst 4 remained
catalytically active for about 35 days in FA
dehydrogenation to produce H gas with a TON of 10
2
million with 4 M FA. The fact that the OH group
works efficiently on a pyrazole ring in addition to a
pyridine ring will lead to the development of new
ligands for CO
2
hydrogenation and FA
dehydrogenation catalysts. As far as the initial
reaction rate is concerned, catalyst 4 is lower than
other active catalysts.[
3ad, 4j]
However, since the
robustness of the catalyst overtakes other catalysts,
converting the pyridine ring of 4 into a more efficient
ligand backbone may lead to the creation of highly
active and highly durable catalysts. The authors are
working on further improvement of the catalyst.
Experimental Section
General Procedure for CO
2
Hydrogenation: A NaHCO
3
aqueous solution (1.0 M, pH 7.9, 10 mL) with complexes
(
0.20 μmol) was degassed by three cycles of freeze-pump-
thaw in a flask and was transferred into a stainless
autoclave. After 10 min. stabilization at 50 °C under 1.0
2 2
MPa of H /CO (1/1) gas, the reaction was started by
stirring (1500 rpm). The stirring was stopped after 1 h and
the pressurized gas was released to measure the
concentration of formate in the resulting solution by HPLC.
4
538; n) K. Sordakis, A. Guerriero, H. Bricout, M.
General Procedure for FA Dehydrogenation: A freshly
prepared 10 mM solution of catalyst (100 L) was added
Peruzzini, P. J. Dyson, E. Monflier, F. Hapiot, L.
Gonsalvi and G. Laurenczy, Inorg. Chim. Acta 2015,
2
to a deaerated aqueous FA/HCO Na solution, and the
4
31, 132-138; o) R. Watari, Y. Kayaki, S.-i. Hirano, N.
mixture was stirred at the desired temperature. The volume
of released gases was determined by a wet gas meter. The
TOF was determined according to the released gases. The
average TOF of the initial 10 min was adopted for the
initial TOF. After the reaction completed, the residual FA
in the solution was quantified with HPLC. The TON was
calculated based on the catalyst loading and concentration
of residual FA or formate.
Matsumoto and T. Ikariya, Adv. Synth. Catal. 2015,
357, 1369-1373; p) S. Xu, N. Onishi, A. Tsurusaki, Y.
Manaka, W.-H. Wang, J. T. Muckerman, E. Fujita and
Y. Himeda, Eur. J. Inorg. Chem. 2015, 5591-5594; q)
C. M. Zall, J. C. Linehan and A. M. Appel, ACS Catal.
2
015, 5, 5301-5305; r) Y. Zhang, A. D. MacIntosh, J. L.
Wong, E. A. Bielinski, P. G. Williard, B. Q. Mercado,
N. Hazari and W. H. Bernskoetter, Chem. Sci. 2015, 6,
Acknowledgements
4
291-4299; s) F. Zhu, L. Zhu-Ge, G. Yang and S. Zhou,
ChemSusChem 2015, 8, 609-612; t) F. Bertini, N.
Gorgas, B. Stöger, M. Peruzzini, L. F. Veiros, K.
Kirchner and L. Gonsalvi, ACS Catal. 2016, 6, 2889-
We thank Dr. James T. Muckerman for a careful reading of the
manuscript. This work was financially supported by JST CREST
(
Grant Number JPMJCR1342, Japan). The work at BNL was
carried out under contract DE-SC00112704 with the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences.
2
893; u) S.-M. Lu, Z. Wang, J. Li, J. Xiao and C. Li,
Green Chem. 2016, 18, 4553-4558; v) K. Rohmann, J.
Kothe, M. W. Haenel, U. Englert, M. Hölscher and W.
Leitner, Angew. Chem. Int. Ed. 2016, 55, 8966-8969;
w) A. Z. Spentzos, C. L. Barnes and W. H.
6
This article is protected by copyright. All rights reserved.

Advanced Synthesis & Catalysis
10.1002/adsc.201801323
Bernskoetter, Inorg. Chem. 2016, 55, 8225-8233; x) C.
and
Y.
Himeda,
Chem.
Eur.
J.
2018,
M. Zall, J. C. Linehan and A. M. Appel, J. Am. Chem.
Soc. 2016, 138, 9968-9977; y) Y. Zhang, P. G. Williard
and W. H. Bernskoetter, Organometallics 2016, 35,
https://doi.org/10.1002/chem.201800428.
[5] R. Tanaka, M. Yamashita and K. Nozaki, J. Am. Chem.
Soc. 2009, 131, 14168-14169.
8
60-865; z) F. Bertini, M. Glatz, N. Gorgas, B. Stoger,
[6] G. A. Filonenko, R. van Putten, E. N. Schulpen, E. J.
M. Hensen and E. A. Pidko, ChemCatChem 2014, 6,
1526-1530.
[7] a) J. F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi,
R. Periana, D. J. Szalda, J. T. Muckerman and E. Fujita,
Nat. Chem. 2012, 4, 383-388; b) Y. Maenaka, T.
Suenobu and S. Fukuzumi, Energy Environ. Sci. 2012,
5, 7360-7367.
[8] a) M. Iguchi, Y. Himeda, Y. Manaka and H. Kawanami,
ChemSusChem 2016, 9, 2749-2753; b) M. Iguchi, Y.
Himeda, Y. Manaka, K. Matsuoka and H. Kawanami,
ChemCatChem 2016, 8, 886-890; c) H. Zhong, M.
Iguchi, F.-Z. Song, M. Chatterjee, T. Ishizaka, I. Nagao,
Q. Xu and H. Kawanami, Sustainable Energy & Fuels
2017, 1, 1049-1055.
[9] Y. Himeda, N. Onozawa-Komatsuzaki, H. Sugihara and
K. Kasuga, J. Photochem. Photobiol. A 2006, 182, 306-
309.
[10] a) Y. Suna, M. Z. Ertem, W.-H. Wang, H.
Kambayashi, Y. Manaka, J. T. Muckerman, E. Fujita
and Y. Himeda, Organometallics 2014, 33, 6519-6530;
b) Y. Himeda, N. Onozawa-Komatsuzaki, H. Sugihara
and K. Kasuga, Organometallics 2007, 26, 702-712.
[11] Y. Himeda, Green Chem. 2009, 11, 2018-2022.
[12] M. Z. Ertem, Y. Himeda, E. Fujita and J. T.
Muckerman, ACS Catal. 2016, 6, 600-609.
[13] W.-H. Wang, S. Xu, Y. Manaka, Y. Suna, H.
Kambayashi, J. T. Muckerman, E. Fujita and Y.
Himeda, ChemSusChem 2014, 7, 1976-1983.
[14] Y. Manaka, W.-H. Wang, Y. Suna, H. Kambayashi, J.
T. Muckerman, E. Fujita and Y. Himeda, Catal. Sci.
Technol. 2014, 4, 34-37.
[15] N. Onishi, S. Xu, Y. Manaka, Y. Suna, W.-H. Wang, J.
T. Muckerman, E. Fujita and Y. Himeda, Inorg. Chem.
2015, 54, 5114-5123.
[16] W.-H. Wang, M. Z. Ertem, S. Xu, N. Onishi, Y.
Manaka, Y. Suna, H. Kambayashi, J. T. Muckerman, E.
Fujita and Y. Himeda, ACS Catal. 2015, 5, 5496-5504.
[17] Y. Liu, G. He, K. Chen, Y. Jin, Y. Li and H. Zhu, Eur.
J. Org. Chem. 2011, 5323-5330.
M. Peruzzini, L. F. Veiros, K. Kirchner and L.
Gonsalvi, Chem. Sci. 2017, 8, 5024-5029; aa) S. A.
Burgess, A. J. Kendall, D. R. Tyler, J. C. Linehan and
A. M. Appel, ACS Catal. 2017, 7, 3089-3096; ab) Z.
Dai, Q. Luo, H. Cong, J. Zhang and T. Peng, New J.
Chem. 2017, 41, 3055-3060; ac) A. Dubey, L. Nencini,
R. R. Fayzullin, C. Nervi and J. R. Khusnutdinova,
ACS Catal. 2017, 7, 3864-3868; ad) R. Kanega, N.
Onishi, D. J. Szalda, M. Z. Ertem, J. T. Muckerman, E.
Fujita and Y. Himeda, ACS Catal. 2017, 7, 6426-6429;
ae) S. Siek, D. B. Burks, D. L. Gerlach, G. Liang, J. M.
Tesh, C. R. Thompson, F. Qu, J. E. Shankwitz, R. M.
Vasquez, N. Chambers, G. J. Szulczewski, D. B.
Grotjahn, C. E. Webster and E. T. Papish,
Organometallics 2017, 36, 1091-1106.
[
4] a) C. Fellay, P. J. Dyson and G. Laurenczy, Angew.
Chem. Int. Ed. 2008, 47, 3966-3968; b) B. Loges, A.
Boddien, H. Junge and M. Beller, Angew. Chem. Int.
Ed. 2008, 47, 3962-3965; c) A. Boddien, D. Mellmann,
F. Gärtner, R. Jackstell, H. Junge, P. J. Dyson, G.
Laurenczy, R. Ludwig and M. Beller, Science 2011,
3
33, 1733-1736; d) J. H. Barnard, C. Wang, N. G.
Berry and J. Xiao, Chem. Sci. 2013, 4, 1234-1244; e) T.
Zell, B. Butschke, Y. Ben-David and D. Milstein,
Chem. Eur. J. 2013, 19, 8068-8072; f) T. W. Myers and
L. A. Berben, Chem. Sci. 2014, 5, 2771-2777; g) F.
Bertini, I. Mellone, A. Ienco, M. Peruzzini and L.
Gonsalvi, ACS Catal. 2015, 5, 1254-1265; h) A.
Matsunami, Y. Kayaki and T. Ikariya, Chem. Eur. J.
2
015, 21, 13513-13517; i) S. Oldenhof, M. Lutz, B. de
Bruin, J. Ivar van der Vlugt and J. N. H. Reek, Chem.
Sci. 2015, 6, 1027-1034; j) Z. Wang, S.-M. Lu, J. Li, J.
Wang and C. Li, Chem. Eur. J. 2015, 21, 12592-12595;
k) J. J. A. Celaje, Z. Lu, E. A. Kedzie, N. J. Terrile, J.
N. Lo and T. J. Williams, Nat. Commun. 2016, 7,
11308; l) C. Fink, S. Katsyuba and G. Laurenczy, Phys.
Chem. Chem. Phys. 2016, 18, 10764-10773; m) I.
Mellone, F. Bertini, M. Peruzzini and L. Gonsalvi,
Catal. Sci. Technol. 2016, 6, 6504-6512; n) M. C.
Neary and G. Parkin, Dalton Trans. 2016, 45, 14645-
[18] Y. Himeda, S. Miyazawa and T. Hirose,
ChemSusChem 2011, 4, 487-493.
1
4650; o) Y. Pan, C.-L. Pan, Y. Zhang, H. Li, S. Min,
X. Guo, B. Zheng, H. Chen, A. Anders, Z. Lai, J. Zheng
and K.-W. Huang, Chem. Asian J. 2016, 11, 1357-1360;
p) G. Papp, G. Olveti, H. Horvath, A. Katho and F. Joo,
Dalton Trans. 2016, 45, 14516-14519; q) L. Wang, H.
Sun, Z. Zuo, X. Li, W. Xu, R. Langer, O. Fuhr and D.
Fenske, Eur. J. Inorg. Chem. 2016, 5205-5214; r) C.
Guan, D.-D. Zhang, Y. Pan, M. Iguchi, M. J. Ajitha, J.
Hu, H. Li, C. Yao, M.-H. Huang, S. Min, J. Zheng, Y.
Himeda, H. Kawanami and K.-W. Huang, Inorg. Chem.
[19] a) I. Jószai and F. Joó, J. Mol. Catal. A: Chem. 2004,
224, 87-91; b) G. Zhao and F. Joó, Catal. Commun.
2011, 14, 74-76; c) J. L. Drake, C. M. Manna and J. A.
Byers, Organometallics 2013, 32, 6891-6894.
[20] N. Onishi, M. Z. Ertem, S. Xu, A. Tsurusaki, Y.
Manaka, J. T. Muckerman, E. Fujita and Y. Himeda,
Catal. Sci. Technol. 2016, 6, 988-992.
[21] S.-M. Lu, Z. Wang, J. Wang, J. Li and C. Li, Green
Chem. 2018, 20, 1835-1840.
2
017, 56, 438-445; s) L. Wang, N. Onishi, K. Murata,
T. Hirose, J. T. Muckerman, E. Fujita and Y. Himeda,
ChemSusChem 2017, 10, 1071-1075; t) R. Kanega, N.
Onishi, L. Wang, K. Murata, J. T. Muckerman, E. Fujita
7
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10.1002/adsc.201801323
COMMUNICATION
Carbon Dioxide Hydrogenation and Formic Acid
Dehydrogenation Catalyzed by Iridium Complexes
Bearing Pyridyl-pyrazole Ligands: Effect of an
Electron-donating Substituent on the Pyrazole Ring
on the Catalytic Activity and Durability
Adv. Synth. Catal. Year, Volume, Page – Page
Naoya Onishi, Ryoichi Kanega, Etsuko Fujita,
Yuichiro Himeda*
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