Ligand Design for Catalytic Dehydrogenation of Formic Acid to Produce High-pressure Hydrogen Gas under Base-free Conditions

Article abstract of DOI:10.1021/acs.inorgchem.9b01624

A series of Cp*Ir (Cp? = pentamethylcyclopentadienyl anion) complexes with amino-functionalized ligands were developed for the production of high-pressure H₂ via catalytic dehydrogenation of formic acid (DFA) in water under base-free conditions. The Ir complexes with 2,2′-bipyridine (bpy) ligands bearing amino or alkylamino groups at the para positions exhibited high activity and stability for DFA compared with complexes containing bpy ligands bearing para-hydroxyl groups. In addition, para-amino groups afforded superior catalytic stability under high-pressure conditions compared with ortho-amino groups. By exploiting these amino-functionalized Cp*Ir complexes, it was possible to continuously produce high-pressure CO-free H₂ via selective DFA in water upon the addition of concentrated FA (>99.5 wt %) to the base-free solution. Systematic investigation of the ligand effects on DFA revealed that the presence of alkylamino groups on the bpy ligand enhanced the catalytic activity (initial turnover frequency, TOF), although the stability decreased with increasing alkyl chain length on the amino groups. According to a Hammett plot, the increased catalytic activity of the Ir complexes after the introduction of amino-functionalized ligands may be attributable to the electron-donating effect of para-amino groups on the bpy ligand. Based on the experimental results, a reaction mechanism is proposed that involves a hydride intermediate whose stability is affected by the position of the amino groups on the bpy ligand, as confirmed through NMR studies.

Full text of DOI:10.1021/acs.inorgchem.9b01624

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Article
Ligand Design for Catalytic Dehydrogenation of Formic Acid to
Produce High-pressure Hydrogen Gas under Base-free Conditions
Hajime Kawanami,* Masayuki Iguchi, and Yuichiro Himeda
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ABSTRACT: A series of Cp*Ir (Cp* = pentamethylcyclopenta-
dienyl anion) complexes with amino-functionalized ligands were
developed for the production of high-pressure H2 via catalytic
dehydrogenation of formic acid (DFA) in water under base-free
conditions. The Ir complexes with 2,2′-bipyridine (bpy) ligands
bearing amino or alkylamino groups at the para positions exhibited
high activity and stability for DFA compared with complexes
containing bpy ligands bearing para-hydroxyl groups. In addition,
para-amino groups afforded superior catalytic stability under high-
pressure conditions compared with ortho-amino groups. By
exploiting these amino-functionalized Cp*Ir complexes, it was possible to continuously produce high-pressure CO-free H2 via
selective DFA in water upon the addition of concentrated FA (>99.5 wt %) to the base-free solution. Systematic investigation of the
ligand effects on DFA revealed that the presence of alkylamino groups on the bpy ligand enhanced the catalytic activity (initial
turnover frequency, TOF), although the stability decreased with increasing alkyl chain length on the amino groups. According to a
Hammett plot, the increased catalytic activity of the Ir complexes after the introduction of amino-functionalized ligands may be
attributable to the electron-donating effect of para-amino groups on the bpy ligand. Based on the experimental results, a reaction
mechanism is proposed that involves a hydride intermediate whose stability is affected by the position of the amino groups on the
bpy ligand, as confirmed through NMR studies.
carbon-neutral and environmentally benign system for H2
production.10,12−18
INTRODUCTION
■
Molecular hydrogen (H2) has attracted considerable attention
as an alternative energy source for both direct combustion and
use in fuel cells owing to its unique advantages, such as a high
energy density by weight (highest among fuels) and the
emission of only water vapor upon combustion or electro-
chemical oxidation, which are beneficial for environmental and
energy security.1−3 However, the gaseous nature of H2 limits
its volumetric energy density and leads to huge challenges with
storage and transportation. Thus, the use of high-pressure H2 is
one solution for increasing the energy density, although the
compression process requires a substantial amount of energy,
which can be as much as 10−20% of the energy content of the
H2.4,5 To circumvent these difficulties, the current state of the
art method is to use liquid organic hydrogen carriers
(LOHCs), which offer high volumetric hydrogen storage
capacities and permit safe and efficient long-term storage and
transportation predominantly using the existing infrastructure.3
Among LOHCs, formic acid (FA; HCO2H), a common bulk
chemical that exists as a stable liquid under ambient conditions
and can be produced by various industrial processes (reaction
between methanol and CO, degradation of biomass, etc.),6−11
has been extensively studied for the generation of H2. In
addition, hydrogenation of CO2 also produces FA, and this
interconversion between CO2 and HCO2H is considered a
Hydrogen production via the dehydrogenation of FA (DFA;
HCO2H → H2 + CO2) is hindered by a competitive
dehydration reaction (HCO2H → H2O + CO) that hampers
the catalytic activity owing to the formation of CO.19 For
instance, the application of FA as a H2 generator in proton-
exchange membrane (PEM) fuel cells requires highly selective
DFA to minimize the production of CO, which rapidly poisons
electrocatalysts. Furthermore, CO2 is also formed from FA
alongside H2, necessitating a separation step for subsequent
application of the produced H2 in fuel cells as well as the
recovery of CO2 as a starting material to complete the CO2−
HCO2H cycle.20 After obtaining high-pressure gas via DFA,
effective H2/CO2 separation has been demonstrated using the
phase change phenomenon from the supercritical fluid phase
to the gas−liquid or gas−solid phase.21,22 The pressurization of
FA and subsequent reaction to generate high-pressure H2
avoids the consumption of the large amount of energy
required to compress H2 and reduces the equipment size.
Received: June 1, 2019
https://dx.doi.org/10.1021/acs.inorgchem.9b01624
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Figure 1. Available literature for high-pressure H2 production from FA using homogeneous catalysts at mild temperatures (p ≥ 10 MPa, T < 100
°C).
Despite the several distinct advantages of high-pressure DFA
over atmospheric-pressure processes, few studies involving
mild temperatures have been reported (Figure 1)21−26 owing
to the comparatively low activity and rapid deactivation of
catalysts under high-pressure conditions.27 In addition, the
DFA rate was also reported to decrease with increasing
generated gas pressures, especially above 10 MPa.28 Addition
of a base improved the DFA rate but reduced the FA
conversion as well as the generated gas pressure.27,29
Therefore, for the high-pressure H2 production from FA in
the absence of a base, the development of a highly active and
selective catalyst that retains its stability and durability under
acidic conditions at high FA concentrations and high H2 and
CO2 pressures remains a major challenge.
containing an aromatic N,N′-bidentate ligand as the active
catalyst, which maintained high activity (= TOF) and
selectivity even under the high-pressure conditions.21,22,38
Most of the developed strategies for the conversion of FA to
H2 involve amino-functionalized homogeneous or heteroge-
neous catalysts under atmospheric pressure.6,11,39 Heteroge-
neous catalysts functionalized with amino groups have been
reported to exhibit the highest activity (= TOF), whereas the
replacement of the amino groups with alkylamino groups or an
increased alkyl chain length reduced the catalytic efficiency (=
TOF), though the origin of this behavior remain unclear.40−42
For homogeneous catalysts, an amino group near the metal
center may play an important role in the DFA pathway via
protonation of the NH moiety or hydrogen-bonding
interactions between the amino groups and water.43−45 It has
been reported that the introduction of dimethylamino groups
to the para positions of 2,2′-dipyridylamine afforded a Cp*Ir
complex with enhanced catalytic activity (= TOF).46 Thus,
systematic studies are urgently needed to elucidate the role of
amino groups in DFA and develop efficient and robust
catalysts.
We have developed a series of Cp*IrIII (Cp* =
pentamethylcyclopentadienyl anion) complexes with N,N′-
bidentate ligands for performing DFA in water at mild
temperatures.12,13,30−36 The ligand was found to exert a
distinct influence on the catalytic activity in terms of turnover
frequency (TOF), which increased with increasing electron-
donating ability from the ligand to the Ir center of the Cp*Ir
complexes.12,31,33,35 Interestingly, the presence of a hydroxyl
group in the ligand present near the metal center (e.g., the
ortho positions of 2,2′-bipyridine (bpy)) increased the catalytic
activity (= TOF) owing to electronic effects and a proton relay
with water (pendent base effect).31,37 In our previous work, we
observed that an Ir complex containing para-hydroxyl groups
on the bpy ligand mediated DFA at high pressures with high
activity (= TOF) and durability (= TON), whereas rapid
deactivation occurred in the case of ortho-hydroxyl groups
owing to the formation of an Ir trihydride complex.27 Recently,
we also reported that it is possible to produce high-pressure H2
and CO2 gases above 100 MPa at 80 °C using an Ir complex
Herein, we focus on homogeneous Cp*Ir-based catalysts
containing amino-functionalized bpy ligands for DFA. We
investigate the influence of substituents and their positions on
the catalytic performance for producing high-pressure H2
without any base. Furthermore, a reaction mechanism is
proposed based on NMR analyses and kinetic isotope effect
(KIE) measurements.
RESULTS AND DISCUSSION
■
The Ir complexes containing amino-functionalized ligands (1b,
1c, 1d, and 2b in Figure 2) were prepared via the reaction of
Cp*Ir aqua complex ([Cp*Ir(H2O)3]SO4) with the corre-
B
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upfield shifts of only 0.01−0.04 ppm, suggesting the lack of
protonation or deprotonation of the amino groups. We further
measured the UV−vis absorption spectra of complexes 1a−1d
over the pH range from 2 to 12. No obvious red-shift of the
maximum absorption band was observed for complexes 1b−1d
(Figures S11−S14), which was in contrast to the case of
complex 1a with hydroxyl groups on the bpy ligand (Figure
S11).30 These results of H NMR spectroscopy and UV−vis
1
absorption results revealed no change in the electronic
properties of the amino-functionalized complexes (1b−1d)
over the examined pH range under the present reaction
conditions.30 The pKa values of the aqua ligands of 1a−1d
were determined via UV−vis absorbance spectroscopy (Table
1 and Experimental Section in the SI). Deprotonation of the
aqua ligands of the amino-functionalized complexes (1b−1d)
was observed at pH values ranging from 7.63 to 8.01, which are
lower than the pH value of 9.2 observed for the hydroxyl-
functionalized complex (1a). Among the complexes 1b−1d,
pKa increased upon replacement of the amino groups (1b) on
the bpy ligand with the dimethylamino groups (1c) and
remained similar for diethylamino groups (1d). Deprotonation
of the hydroxyl groups of complex 1a was observed at pH =
4.87, whereas no protonation or deprotonation of the amino
groups occurred for complexes 1b−1d, as no spectral change
was detected over the pH range from 2 to 8.
Figure 2. Ir complexes for DFA.
sponding bpy derivatives in water according to a previously
reported method.47 The resulting complexes were charac-
1
terized using H NMR and 13C NMR spectroscopy (Figures
1
S2−S6). For 1b, H NMR and 13C NMR analysis in D2O
revealed a broad peak corresponding to bpy derived from the
classical bifurcated hydrogen bonds between the NH and 5-
CH of pyridine48 and water (Figure S2), which was in contrast
to the other complexes (1a, 1c, 1d and 2a), (Figures S3−S6).
This peak became sharp and more intense as the temperature
was increased from ambient conditions to 50 °C (Figures S7
and S8), owing to the cleavage of the bifurcated hydrogen
bonds. In addition, the electronic properties of 1a changed
significantly with pH owing to the presence of a proton-
responsive ligand with hydroxyl groups.30 To investigate the
influence of pH on the electronic properties of the catalyst, 1H
NMR spectra and UV−vis absorption spectra of complexes
1b−1d were measured at various pH values to investigate the
influence of pH values (Figures S9 and S10−S15 and Table
Initially, we conducted the DFA in water under atmospheric
pressure using the Ir-complexes (Table 2). The complex
a
Table 2. DFA with the Ir Complexes
complex
conc.
/mM
initial
b
residual
initial FA
conc. /M
TOF
reaction FA conc.
entry complex
/h−1
time /h
/M
c
c
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
2a
2b
2b
1c
1c
1c
0.02
0.02
0.02
0.02
0.02
0.02
1.1
0.02
0.02
0.01
1
1
1
1
1
1
1
3
3
3
1900
156
6.8
5.9
5.9
123
10
72
9.7
8.3
13
0
0
0
0
0
0.9
0
0
0.52
0.53
10100
12700
12700
1
S1). In terms of the H NMR spectra in D2O, the chemical
shifts of each complex varied from acidic (pD < 2) to basic
conditions (pD > 12) as summarized in Table 1. For complex
1a, the signals corresponding to the 3, 3′, 5, 5′, 6, and 6′
positions of the bpy ligand displayed large upfield shifts in the
range of 0.11−0.22 ppm, indicating the deprotonation of the
OH group at pH = 4.87 (Table 1, entry 1). In contrast, the
corresponding signals of complexes 1b−1d showed small
c
e
c
2200
d
d
17
24
16200
25000
115500
e f
,
10
a
Reaction conditions: 60 °C, pressure = 0.1 MPa, catalyst amount =
1
b
Table 1. pKa Values and H NMR Chemical Shifts for the
2−4 μmol. Average value of two measurements conducted during
the initial 10−20 min. The errors for all initial TOFs were within
a
bpy Ligands in the Ir Complexes
c
5%. The TOF values obtained in this work; for reference, TOF
b
Δppm
values of 2400 and 2200 were previously reported for 0.2 mM 1a and
d
2a, respectively.12,31 The gas generation rate analysis was stopped
3,3′-
4,4′-
5,5′-
6,6′-
e
f
entry complex
pKa
position position position position
after 10 h. Addition of 15 mol % of NaOH. Temperature = 80 °C.
c
1
1a
4.87 0.14
9.14 0.18
7.63 0.11
7.91 0.09
8.01 0.11
4.7
0.22
−
0.17
0.11
containing amino groups on the bpy ligand at the para
positions (1b, entry 2 in Table 2) effectively catalyzed DFA
compared with the complex containing hydroxyl groups at the
same positions (1a, entry 1 in Table 2). The incorporation of
dimethylamino (1c) or diethylamino (1d) groups into the bpy
ligand slightly enhanced the catalytic activity (= TOF). Based
on Hammett plots, recent studies have reported positive effects
on DFA after the introduction of electron-donating sub-
stituents into the bpy ligand.12,31,49 Thus, we correlated the
2
3
4
5
1b
1c
1d
0.04
0.02
0.01
0.86
−
−
−
0.02
0.01
0.01
0.80
0.04
0.04
0.04
d
2a
0.59
−
10.6
6
2b
7.39 0.16
9.65 0.10
1.03
1.00
1.02
−
a
b
Determined from UV−vis absorption spectra. Determined from 1H
50
+
Hammett constants (σp ) of the studied electron-donating
substituents at the para positions of bpy and the catalytic
performance (TOF) of the corresponding complexes (Figure
3). A slope of −2.01 was calculated from the Hammett plot of
NMR spectra, where Δppm denotes the change in the chemical shift
value from acidic (pH < 2) to basic conditions (pH > 12). pH of
c
d
deprotonation of the hydroxyl groups on the bpy ligand. From ref
30.
C
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the addition of water promoted the reaction (Figure S17).
During the reaction in the presence of water, FA was
completely decomposed into gaseous H2 and CO2 and a
final TON of 909000 was reached. To confirm the effect of
water, DFA was conducted in both water and in a water/1,4-
dioxane mixture, where 1,4-dioxane is an aprotic solvent that is
miscible with water in any ratio (Table S2). The water
concentration appeared to exert a positive influence on the
reaction rate, which indicates that the complexes are stable
even at high concentrations of FA in water. For complexes 1b−
1d, the catalytic activity (= TOF) reached a maximum value at
pH = 3.0 0.1 and pH = 2.5 in 0.1 and 3 M FA solution,
respectively (Figure 4 and Figure S18). The shift of the pH
Figure 3. Hammett plot for the rate of DFA (= TOF) catalyzed by
the Ir complexes (Cp*Ir) containing various substituents at the para
position of the bpy ligand for various catalyst concentrations in 1 M
FA solution. The TOFR denotes the TOFs for the Cp*Ir catalysts
with bpy ligands bearing OH, OMe, Me, and NR2 (R = H, Me, Et;
cat. conc. = 0.02 mM) groups at the para porisions. TOFH denotes
the TOF for the Cp*Ir catalyst with bpy ligand (R = H). The TOF
values for the Cp*Ir catalysts with OH (cat. conc. = 0.2 mM), OMe
(cat. conc. = 0.2 mM), Me (cat. conc. = 2 mM), and H (cat. conc. = 2
mM) groups were taken from the literature.12 The Hammett
constants were also taken from the literature.50 The line was
calculated using the Hammett equation.
the −NH2, −OH, −OMe, Me, and H groups. An obvious
relationship was observed between the catalytic activity (=
TOF) and the electron-donating ability of the substituents,
although no such effect was observed for the dimethylamino
(1c) and the diethylamino (1d) groups where the electron-
donating ability increased with increasing alkyl chain length on
the amino groups. Note that the deviation of Hammett
Figure 4. pH dependence of the rate of DFA using Ir complexes (0.02
□
○
mM) and various FA concentrations at 60 °C: , 1b in 0.1 M FA;
,
●
1c in 0.1 M FA; , 1c in 3 M FA. The pH of the solutions were
adjusted by the addition of either NaOH or H2SO4.
+
constants (σp ) from the reference data may be attributable to
corresponding to the highest catalytic activity (= TOF) with
increasing FA concentration suggests that the reaction rate can
be expressed as a function of the formate and proton
concentrations.28 A nominal effect on the rate of FA
dehydrogenation was observed in the presence of various
the changes in the resonance effects between the pyridine
moieties and alkylamino chains.50 Notably, the Ir complex with
the bpy ligand containing para-amino groups (1b) exhibited a
higher TOF compared with that containing ortho-amino
groups (2b).51 During the H NMR analysis of 2b in D2O,
bases (OH−, HCO3 , CO32−, NEt3) and Lewis acids (Li+, Na+,
1
−
the peaks corresponding to the bpy ligand with ortho-amino
groups remained unchanged even after the addition of excess
amount of NaOD (Figure S10(c)), whereas new peaks
attributed to the complex containing protonated ortho-amino
groups appeared after the addition of excess D2SO4 (Figure
S10(b)). This result implies that under acidic conditions,
protonation of the amino groups of 2b occurred to form
K+, Ca2+) (Table S3). Furthermore, a log−log plot of the DFA
rate versus the initial complex concentration indicated a first-
order reaction with respect to the catalyst concentration
(Figure S19). The calculated activation energies (Ea) obtained
from the Arrhenius plots were similar for complexes 1b and 1c
(81.4−82.3 kJ/mol, Figure S20) and were in good agreement
with those in previous reports.30,31,51
+
−NH3 , which then acted as an electron-withdrawing group,
NMR analyses and KIE measurements were performed to
elucidate the mechanism of DFA under the present reaction
conditions. On the basis of the experimental results, we
proposed the reaction pathway shown in Scheme 1. DFA is
considered to proceed via (i) generation of the formate
intermediate, (ii) subsequent release of CO2 via β-hydride
elimination to produce a hydride intermediate, and, finally,
(iii) generation of H2 from the hydride intermediate.28,31,37
Although GC analysis confirmed the generation of H2 from the
stoichiometric reaction between complex 1b and sodium
formate after the addition of sulfuric acid (Scheme 2 and
Figure S21), formation of the hydride intermediate (3)
thereby accounting for the reduced TOF.
The conversion of FA to H2 was also accelerated by the
addition of base as well as by higher temperatures (entries 9
and 10 in Table 2). For instance, the TOF increased by 1.5-
fold upon addition of 15 mol % NaOH into the FA solution
and finally reached 115500 h−1 on half-concentration of
catalyst. Despite this enhanced reaction rate, the remaining FA
solution (entries 9 and 10 in Table 2) was converted to
formate (HCO2Na), which is inert as a substrate for H2
production. Kinetic studies of DFA were conducted at
atmospheric pressure. For complexes 1b−1d, the initial rate
of DFA increased with increasing FA concentration up to 3 M
and thereafter decreased (Figure S16). The DFA reaction
barely proceeded at a high concentration of FA (20 M), and
1
remained undetected by H NMR owing to its short lifetime
and the rapid H/D exchange between the hydride complex and
D2O.31 Hence, to confirm the formation of the hydride
D
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Scheme 1. Proposed Reaction Mechanism for the Dehydrogenation of Formic Acid Using the Ir Complexes
Scheme 2. Stoichiometric Reaction between 1b and Sodium Formate Followed by the H2 Release upon Addition of Sulfuric
Acid
a
intermediate of 1b, the reaction between 1b and sodium
formate was conducted followed by anion metathesis with
NaPF6, which precipitated the hydride complex out of solution
and allowed its NMR spectrum to be recorded in the aprotic
solvent, DMSO-d6 (3′, Experimental Section and Figure S22).
In contrast, the reaction of complex 2b with sodium formate
generated the hydride intermediate, as demonstrated by the
NMR spectroscopy (Figure S23 (b) and (c)), and the GC
analysis confirmed the release of H2 upon the addition of
sulfuric acid to the hydride intermediate (Figure S23 (c) and
(d)). These NMR and GC results support our proposed
mechanism. The NMR analysis also confirmed that the lifetime
of the hydride intermediate from 2b was longer than that from
1b, which could be correlated with the TOFs of the DFA
reactions in the presence of the corresponding catalysts
(entries 2 and 6 in Table 2). Furthermore, KIE measurements
were conducted to study the rate-determining step of DFA
using 1b through the isotopic substitution of the C−H and O−
H groups of FA. Table 3 summarizes the observed reaction
rates for the four labeled FA isotopologues (HCO2H, HCO2D
(HCO2H + D2O), DCO2H, and DCO2D). Comparison of the
KIE values (ratio of the reaction rate constant for the labeled
molecule to that for the unlabeled molecule) revealed that
deuteration at the carbon atom (k(DCO2H)/k(HCO2H) = 2.9) had
a greater effect on DFA than deuteration at the oxygen atom
Table 3. Kinetic Isotope Effect in DFA using 1b
b
c
entry
substrate
solvent
TOF /h−1
KIE
1
2
3
4
HCO2H
HCO2H
DCO2D
DCO2D
H2O
D2O
H2O
D2O
7700 200
5400 300
2700 100
2900 100
−
1.42 0.04
2.85 0.03
2.65 0.02
a
Reaction conditions: temperature = 60 °C, pressure = 0.1 MPa,
catalyst amount = 2 μmol, initial FA solution = 1 mol/L, 20 mL.
Average value over the first 30 min. KIE is defined as the TOF ratio
b
c
of entry 1 to entry n, where n is set to 2, 3, or 4.
(k(HCO2D)/k(HCO2H) = 1.4). Thus, the β-elimination step is the
rate-determining step for this reaction, whereas the KIE value
of k(DCO2D/k(HCO2H) = 2.7 was almost the same as the value of
k(DCO2H)/k(HCO2H) = 2.9 owing to the nominal influence of the
O−H group of FA on the reaction. This result is consistent
with the short lifetime of the iridium hydride intermediate,
which produces H2 more rapidly than CO2.
The influence of the temperature on the generation of high-
pressure H2 using complexes (1b−1d) was also examined. At
50 °C, >20 MPa of high-pressure gas was generated from 7 M
FA solution in the presence of 1b. This experiment could be
repeated at least three times without showing in the generation
rate (Figure S24). Furthermore, at 80 °C complex 1c produced
a very high gas pressure of 157 MPa within 4 h from the 21 M
E
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a
of FA solution (Figure 5). The resulting high-pressure gas was
composed of H2 and CO2 in the same molar ratio, whereas no
Table 5. Continuous DFA using the Ir Complexes
b
c
entry complex pressure /MPa stability /h CO /vol. ppm TON80%
d
1
2
3
4
5
6
7
1a
1a
1b
1b
1c
1d
PYIM
0.1
20
0.1
20
20
20
20
60
10
120
60
40
30
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
66500
5829
402000
72400
50300
39100
45800
e
30
a
Reaction conditions: temperature = 50 °C, catalyst amount = 16
μmol, initial FA solution = 5 M, 40 mL. FA was added at a rate of 0.6
mL/h over 10 h using a plunger pump and then stopped for several
b
hours. This process was repeated several times. Time elapsed from
c
the beginning until a decrease in the gas generation rate. Calculated
value when the gas flow rate had decreased by 80% of the initial rate.
d
e
Below the detection limit (<6 vol. ppm). Data taken from the
literature.22
< 6 vol ppm, Figure S28). From 20 to 30 h of FA addition, the
gas flow rate was measured as 0.62 L/h (13 mmol-H2/h), and
the corresponding FA flow rate was 0.4 mL/h (11 mmol/h).
However, the gas flow rate gradually decreased over time
during FA addition. When the gas flow rate had decreased by
80% from the initial rate, a TON of 66500 (TON80%, Table 5,
entry 1) was calculated from the volume of H2. In the case of
1b, the average gas flow rate from 10 to 20 h of FA addition
was 0.80 L/h (16 mmol-H2/h). When the gas flow rate had
decreased by 80% and 50% from the initial rate, TONs were
402000 (TON80%, Table 5, entry 2) and 515000 (TON50%),
respectively. The TON80% for complex 1b (402000) was
therefore 6-fold larger than that for 1a (TON80% = 66500,
Figures S29 and S30). Owing to the residual FA in the
solution, the gas generation continued even after the addition
of FA had been stopped. Finally, when the FA decomposition
had reached completion, a TON of 974000 was calculated
from the total gas volume of H2.
Next, the catalytic stability of the complexes was studied
during continuous DFA at 20 MPa and 50 °C, and the
generated gas flow rate was measured. Complex 1a became
deactivated after the system reached 20 MPa (entry 2 in Table
5) owing to the formation of polymeric compounds with the
generated H2 which indicated limited stability under high
pressure.21 As determined using the stop and flow method,
complex 1b was able to generate the high-pressure gas with a
constant pressure of 20 MPa over six cycles of FA addition,
although the pressure gradually decreased thereafter (Figure
6). Complex 1b retained its stability for 60 h under high
pressure and afforded a TON80% value of 72400 (entry 4 in
Table 5). Complexes 1c and 1d also allowed a constant flow
rate of the generated gas to be maintained for six and three
cycles of FA addition, respectively, and the TON80% for 1c
(50300, Figure S30) was greater than that for 1d (39100,
Figure S31). In addition, we also observed that the resonance
associated with the amine and pyridine groups, which increases
with increasing electron density of the pyridine nitrogen atom,
had less effect on the stability than on the activity of the
catalyst. As mentioned above, we observed the highest pressure
of 153 MPa during DFA using the Ir-PYIM catalyst, although
the PYIM ligand lost its stability after 30 h at 20 MPa with a
TON80% of 45800. Thus, the incorporation of amino groups at
the para positions of pyridine improves the catalytic activity (=
TOF) and stability (= TON80%) during high-pressure DFA. It
should be noted that during the continuous-flow measure-
Figure 5. Time course of the generated pressure by the DFA at 80 °C
in highly concentrated FA solution using the Ir complexes (2 mM): ×
○
, 1a in 20 M FA; , 1c in 21 M FA.
CO (<6 vol ppm) was detected by GC analysis (Figure S25).
Under similar conditions, a maximum gas pressure of 123 MPa
was obtained using 1a owing to catalyst deactivation.22
Furthermore, we compared the reaction rates for complexes
1a, 1b, 1c, and 1d and an Ir complex bearing 2-(2′-
pyridyl)imidazoline (PYIM) to reach a pressure of 40 MPa
(Table 4), as PYIM was previously reported to generated the
a
Table 4. High-Pressure DFA using the Ir Complexes
complex
initial
reaction
time /h
residual FA
conc. /M
b
entry complex conc. /mM TOF /h−1
1
2
3
4
5
6
1a
1b
1c
1d
PYIM
PYIM
0.4
0.2
0.2
0.2
0.2
0.4
1900
8500
14600
16900
6500
14.0
5.2
3.7
2.2
18.2
2.4
0.55
0.42
0.40
0.37
9.99
0.40
9000
a
Reaction conditions: temperature = 80 °C, pressure = 40 MPa,
catalyst amount = 8 or 16 μmol, initial FA solution = 16 M, 40 mL.
b
Average value over the initial 10−30 min.
highest pressure of 153 MPa from DFA.22 The highest TOF
(16900 h−1) was obtained in 16 M FA solution using 1d at 80
°C. The flow rate of the high-pressure gas (H2 and CO2)
generated by DFA increased gradually with time during the
reaction (Figure S26) and might be correlated with the FA
concentration. Under these conditions, the equilibrium
concentration of FA was estimated as 0.37
0.01 M. This
high-pressure reaction was performed using the low concen-
trations of 1b, 1c, and 1d, and a high concentration of PYIM.
Based on the residual FA concentration after the reactions, the
complexes were arranged in descending order of catalytic
stability (1d ≅ 1c ≅ 1b > PYIM > 1a).
To assess the long-term reactivity, the Ir complexes were
subjected to continuous-flow measurements at 50 °C (Table
5). We first studied the decomposition of FA using 1a and 1b
at atmospheric pressure by adding neat FA to the solution at a
constant rate via a liquid pump. In the case of 1a, FA was
continuously and selectively decomposed into H2 and CO2
gases over 200 h without any detection of CO (detection limit
F
https://dx.doi.org/10.1021/acs.inorgchem.9b01624
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
reaction mechanism; high-pressure gas generation by
DFA; GC chromatogram of the gaseous products; and
continuous DFA at atmospheric and high pressures
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Hajime Kawanami − Research Institute for Chemical Process
Technology, Department of Material and Chemistry, National
Institute of Advanced Industrial Science and Technology, Sendai,
Miyagi 983-8551, Japan; orcid.org/0000-0002-2679-
2662; Email: h-kawanami@aist.go.jp; Fax: +81-22-237-5388
Authors
Masayuki Iguchi − Research Institute for Chemical Process
Technology, Department of Material and Chemistry, National
Institute of Advanced Industrial Science and Technology, Sendai,
Miyagi 983-8551, Japan
Yuichiro Himeda − Research Institute of Energy Frontier,
Department of Energy and Environment, National Institute of
Advanced Industrial Science and Technology, Tsukuba, Ibaraki
305-8565, Japan; orcid.org/0000-0002-9869-5554
Figure 6. Continuous DFA using 1b (16 μmol) at 50 °C and 20 MPa.
Blue line, generated gas volume; green line, added FA volume; × ,
flow rate of generated gas. Time elapsed after reaching the pressure of
20 MPa. FA was added at high-pressure at a rate of 0.6 mL/h over 10
h, and the FA addition was repeated seven times.
ments, a decrease in the catalytic activity resulted in an increase
in the residual concentration of FA, which led to a change in
the reaction rate as a function of FA concentration. To
eliminate the influence of residual reactant, neat FA was
continuously added to the solution for 10 h after the gas
generation had stopped, and each experiment was repeated
several times.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b01624
Notes
CAUTION! Although hydrogen gas has no reactivity or toxicity
hazards, it can ignite upon exposure to air. Hence, the generated
gas from DFA (50% of H2 and 50% of CO2) should be diluted
with a large quantity of N2 gas to reduce its H2 content to less than
4% before releasing it to the air. Therefore, all experiments must be
conducted inside a fume-hood with appropriate safety precautions.
The authors declare no competing financial interest.
CONCLUSIONS
■
We have synthesized a series of Cp*Ir complexes containing
bpy ligands bearing amino or alkylamino groups to investigate
the influence of the ligand on DFA in water under base-free
conditions. The Ir complexes containing amino groups at the
para positions of the bpy ligand exhibited high activity even at
high pressures compared with those containing hydroxyl
groups or ortho-amino groups on the bpy ligand. Furthermore,
despite the enhanced catalytic activity obtained after replacing
the amino groups by alkylamino groups, the stability of the Ir
complexes decreased. A Hammett plot demonstrated that
electron donation from the substituents at the para positions of
the bpy ligand increased the rate of DFA. In addition, NMR
analyses revealed faster dehydrogenation from the hydride
intermediate in the case of the IrCp* catalyst having a para-
amino-substituted bpy ligand. The results of this work illustrate
the electronic effects of these functionalized ligands on the
activity and stability during the generation of high-pressure H2
from FA, and we anticipate that the strategy outlined herein
will help guide future catalyst development involving the
introduction of functional groups into these ligands.
ACKNOWLEDGMENTS
■
M.I. and H.K. thank Dr. Maya Chatterjee (National Institute
of Advanced Industrial Science and Technology in Sendai) for
help in preparing and correcting the manuscript through
discussions. The authors thank Dr. Alexander Pyatenko
(National Institute of Advanced Industrial Science and
Technology in Tsukuba) for performing the elemental analysis.
The authors would like to acknowledge the financial support of
the Japan Science and Technology Agency (JST), CREST
(JPMJCR1342), and the International Joint Research Program
for Innovative Energy Technology of the Ministry of Economy,
Trade, and Industry (METI) of Japan.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b01624.
■
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