Highly Robust Hydrogen Generation by Bioinspired Ir Complexes for Dehydrogenation of Formic Acid in Water: Experimental and Theoretical Mechanistic Investigations at Different pH

Article abstract of DOI:10.1021/acscatal.5b01090

Hydrogen generation from formic acid (FA), one of the most promising hydrogen storage materials, has attracted much attention due to the demand for the development of renewable energy carriers. Catalytic dehydrogenation of FA in an efficient and green manner remains challenging. Here, we report a series of bioinspired Ir complexes for highly robust and selective hydrogen production from FA in aqueous solutions without organic solvents or additives. One of these complexes bearing an imidazoline moiety (complex 6) achieved a turnover frequency (TOF) of 322 000 h^-1 at 100 °C, which is higher than ever reported. The novel catalysts are very stable and applicable in highly concentrated FA. For instance, complex 3 (1 μmol) affords an unprecedented turnover number (TON) of 2 050 000 at 60 °C. Deuterium kinetic isotope effect experiments and density functional theory (DFT) calculations employing a speciation approach demonstrated a change in the rate-determining step with increasing solution pH. This study provides not only more insight into the mechanism of dehydrogenation of FA but also offers a new principle for the design of effective homogeneous organometallic catalysts for H₂ generation from FA.

Full text of DOI:10.1021/acscatal.5b01090

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Article
Highly Robust Hydrogen Generation by Bio-Inspired Ir Complexes
for Dehydrogenation of Formic Acid in Water: Experimental
and Theoretical Mechanistic Investigations at Different pH
Wan-Hui Wang, Mehmed Z. Ertem, Shaoan Xu, Naoya Onishi, Yuichi Manaka, Yuki
Suna, Hide Kambayashi, James T. Muckerman, Etsuko Fujita, and Yuichiro Himeda
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01090 • Publication Date (Web): 30 Jul 2015
Downloaded from http://pubs.acs.org on August 17, 2015
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Highly Robust Hydrogen Generation by Bioꢀ
Inspired Ir Complexes for Dehydrogenation of
Formic Acid in Water: Experimental and Theoretical
Mechanistic Investigations at Different pH
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a
b
c
c
c,d
c
Wan-Hui Wang, Mehmed Z. Ertem, Shaoan Xu, Naoya Onishi, Yuichi Manaka, Yuki Suna,
c
b,*
b,*
c,d,*
Hide Kambayashi, James T. Muckerman, Etsuko Fujita, and Yuichiro Himeda
a
School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin
24221, China
1
b
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA
c
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5ꢀ1, 1ꢀ1ꢀ1
Higashi, Tsukuba, Ibaraki, 305ꢀ8565 Japan
d
Japan Science and Technology Agency, CREST, 4ꢀ1ꢀ8 Honcho, Kawaguchi, Saitama, 332ꢀ
0012 Japan.
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ABSTRACT: Hydrogen generation from formic acid (FA), one of the most promising hydrogen
storage materials, has attracted much attention due to the demand for the development of
renewable energy carriers. Catalytic dehydrogenation of FA in an efficient and green manner
remains challenging. Here, we report a series of bioꢀinspired Ir complexes for highly robust and
selective hydrogen production from FA in aqueous solutions without organic solvents or
additives. One of these complexes bearing an imidazoline moiety (complex 6) achieved a
turnover frequency (TOF) of 322,000 h−1 at 100 ºC, which is higher than ever reported. The
novel catalysts are very stable and applicable in highly concentrated FA. For instance, complex 3
0
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(
1 ꢀmol) affords an unprecedented turnover number (TON) of 2,050,000 at 60 ºC. Deuterium
kinetic isotope effect experiments and density functional theory (DFT) calculations employing a
speciation” approach demonstrated a change in the rate determining step with increasing
“
solution pH. This study provides not only more insight into the mechanism of dehydrogenation
of FA, but also offers a new principle for the design of effective homogeneous organometallic
catalysts for H generation from FA.
2
KEYWORDS: Formic acid dehydrogenation, Ir complexes, Mechanism, Kinetic isotope effect,
pHꢀdependence
Introduction
A hydrogen economy is a promising alternative to the hydrocarbon economy for solving
1
,2
increasingly severe energy and environmental problems. Efficient hydrogen storage is one of
3ꢀ5
the bottlenecks for developing a hydrogen economy. Recently, hydrogen storage using formic
6
ꢀ9
acid (FA) as a carrier has attracted much attention because of its favorable properties. While
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FA has the relatively low hydrogen content of 4.4 wt%, it is nonꢀtoxic, bioꢀdegradable, and
environmental friendly. More importantly, it is a liquid under ambient conditions, which makes it
easy to store, transport and handle. FA can be selectively generated by catalytic hydrogenation of
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CO with a suitable catalyst under mild conditions and can also be used in fuel cells directly.
2
Moreover, FA dehydrogenation can selectively release pressurized H /CO with an appropriate
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catalyst. In some cases, the hydrogen storage capability (i.e., reversible interconversion between
CO and FA) can be achieved using the same catalyst by changing reaction conditions such as
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1
0ꢀ17
pressure, temperature, and pH.
Various heterogeneous and homogeneous catalysts for FA dehydrogenation have been
6
ꢀ8,18
19ꢀ22
reported.
Despite recent important progress,
heterogeneous catalysts often suffer from
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,19
low activity and selectivity (including the generation of CO that poisons fuel cell catalysts).
10,23ꢀ29
Homogeneous catalysts are generally more effective and attractive,
but are far from
practical use owing to their limited catalytic efficiency and robustness. The design of highly
efficient and robust homogenous catalysts remains a significant challenge. Nature uses
hydrogenase enzymes to accomplish reversible H oxidation (i.e., H splitting and generation,
2
2
Scheme 1a). An important synergistic effect between the metal center and cooperating ligands in
hydrogenases has inspired many chemists to explore such a catalyst design principle (Scheme
3
0ꢀ33
1
b).
mimicking the structure of [FeFe]ꢀhydrogenase to catalyze various transformations.
complexes with nonꢀinnocent ligands have also shown outstanding catalytic activity in FA
DuBois and Bullock et al. have developed a series of complexes with a pendent amine
3
4ꢀ36
Pincer
1
0,28,37
dehydrogenation in organic solvents.
Inspired by the active center of [Fe]ꢀhydrogenase
we have designed and synthesized a series of protonꢀresponsive complexes
Scheme 1d) for efficient catalytic interconversion of formic acid/formate and H /CO in
3
8,39
(Scheme 1c),
(
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water.
The bipyridine or bipyrimidine ligands bearing pendent OH groups are redox nonꢀ
innocent, protonꢀresponsive, and act as cooperating ligands which have been demonstrated to be
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largely responsible for the high activity of the complexes. Moreover, the protonꢀresponsive
complexes bearing functional OH groups showed tunable activity and can control the
equilibrium between formic acid/formate and H /CO under different pH conditions. Under
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acidic conditions, they decompose FA to release H /CO , while under basic conditions they
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catalyze CO hydrogenation to provide formate. This unique property is mainly attributed to the
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pH switchable OH substituent which deprotonates to give the strong electronꢀdonating oxyanion
and improves the activity. Therefore, determination of the exact species in aqueous solution
under different pH is highly desirable.
Scheme 1. Bioꢀinspired catalyst design for H splitting and generation: (a) The H splitting and
2
2
generation processes catalyzed by [FeFe]ꢀhydrogenase; (b) Catalyst design with cooperating
ligand bearing functional groups for H activation and generation by mimicking the enzyme; (c)
2
Structure of the guanylyl pyridinol (FeGP) cofactor in [Fe]ꢀhydrogenase (GMP: guanidine
monophospate); (d) H splitting and generation with our complexes via a synergistic effect
2
between the metal center and cooperating ligands.
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Although various homogeneous catalysts with precious metals or earthꢀabundant metals have
been developed for FA dehydrogenation, high performance (i.e., high TON and TOF) catalysts
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usually require the presence of organic solvents or additives.
Catalysts based on earthꢀ
abundant metals typically suffer from low selectivity, and CO contamination is usually
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observed.
The presence of the amine in a FA/amine system considerably decreases the
hydrogen capacity and results in loss of the volatile amine. In contrast, FA dehydrogenation in
water without organic additives is ecoꢀfriendly, and a highly concentrated FA solution can also
provide comparable hydrogen capacity. However, reports of highly active and stable catalysts in
1
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aqueous solution are scarce.
We recently reported that a fiveꢀmembered azole moiety is an
efficient electron donor, and its incorporation into a catalyst complex is effective for FA
49
dehydrogenation in water. This is supposedly due to the high electron donating ability of the
azole moieties. Taking into account that pyridine and pyrimidine ligands bearing pendent OH
groups are effective for H activation, we focused on designing new catalysts that combine
2
pyridine or pyrimidine moieties bearing pendent OH groups with electronꢀdonating azole and
azoline moieties including pyrazole, imidazole and imidazoline. Herein, we report
unprecedentedly durable bioꢀinspired complexes for catalyzing FA dehydrogenation in water
(
TON of 2,050,000 at 60 ºC). Furthermore, an initial TOF as high as 332,000 h−1 could be
achieved in 4 M HCO H/HCO Na (3/2) using a complex based on an imidazoline ligand.
2
2
The deuterium kinetic isotope effect (KIE) is an effective method for exploring reaction
1
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mechanisms.
We have previously investigated the mechanism of formic acid
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0
dehydrogenation with KIE studies, and demonstrated a significant effect depending on the
specific ligands with or without pendent OHs in the complexes. Complexes with 6,6′ꢀdihydroxyꢀ
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,2′ꢀbipyridine (OH substituents in ortho positions) exhibit a different pH dependence than those
with 4,4′ꢀdihydroxyꢀ2,2′ꢀbipyridine (OH substituents in para positions) owing to the two classes
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of complexes having different rate determining steps (RDS) in the catalytic cycle.
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Dehydrogenation of formic acid is known to exhibit a pH dependence, which suggests that the
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solution pH has an important influence on the reaction rate,
however, to the best of our
knowledge, the effect of solution pH on the mechanism has not yet been comprehensively
studied. Here we demonstrate for the first time that the rate determining step for a catalyst that
has a 6,6′ꢀdihydroxyꢀ2,2′ꢀbipyridineꢀlike ligand (OH substituent in an ortho position) changes
upon altering the solution pH, and rationalize the bellꢀshaped pH dependence of the activity of
such catalysts through a combined experimental and computational effort. In the calculations, we
employed a “speciation” approach which helps to determine the actual species (i.e., fully
deprotonated or partially deprotonated) in the reaction solution.
Results and discussion
Catalyst screening
The waterꢀsoluble complexes 1ꢀ6 (Chart 1) were synthesized using methods similar to those
previously reported (see SI). With these complexes in hand, we carried out the catalytic
dehydrogenation of formic acid in aqueous solutions at low FA concentration (1 M) and
temperature (60 °C, Table 1). Complex 1 bearing a MeO group showed moderate activity (TOF:
−1
1010 h , entry 1) in 1 M HCO H solution (pH 1.7) and apparent decreased activity (TOF: 160
2
−
1
h , entry 2) in 1 M HCO H/HCO Na (1/1, pH 3.5). In contrast, complex 2 with a pendent OH
2
2
group showed significant improvement in activity and stability (entries 3 and 4). The TOF
−
1
−1
increased to 2740 h in 1 M HCO H, and further increased to 3620 h in 1 M HCO H/HCO Na
2
2
2
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(
1/1, pH 3.5). These pH dependences are consistent with our previous report that complexes with
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1
pendent OH groups in the second coordination sphere exhibit a bellꢀshaped pH vs. rate profile.
−
1
In 1 M FA solution, complex 3 gave an initial TOF of 7050 h at 60 °C (entry 5). The initial
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TOF in 1 M HCO H/HCO Na (1/1, pH 3.5) reached 11,400 h , which is triple that of complex 2
2
2
(entry 6). Similar to all the complexes with pendent OH groups, its pH vs. rate profile also was
−
1
bellꢀshaped (Figure 1), and the TOF peaked at pH 2.8 (TOF: 18,000 h , entry 7). Changing the
position of the N in the pyrimidine ring led to a considerable decrease in the activity of the
complex as shown in the pH vs. rate profile (Figure 1, 3 vs. 4). Complex 4 gave a TOF of 5150
−1
−1
h
at pH 1.7 and 60 °C (entry 8). The TOF peaked at pH 3.0 (TOF: 10,800 h , entry 9).
Replacing the pyrazole ring with an imidazole ring led to both low stability and activity at low
solution pH. Complex 5 was found to have partially precipitated from the solution at some point
during the reaction at pH below 2.6, leading to decreased activity. The TOF at pH 1.7 was 4290
−
1
h , which is even lower than that of complex 4 (entry 10). Interestingly, the reaction rate
−
1
increased significantly with increasing solution pH. The maximum TOF value (19,900 h , entry
1
1) at pH 3.0 is even higher than that of complex 3 (Figure 1, 3 vs. 5). When complex 6 bearing
an imidazoline ring was used, we observed no apparent catalyst precipitation, but similar to
complex 5 the reaction rate increased significantly with increasing solution pH (Figure 1, 6). It
−
1
exhibited the highest initial TOF of 32,500 h at pH 3.0 (entry 13). The catalytic efficiency of
complex 6 is even higher than the most efficient Ir dinuclear THBPM complex under similar
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HO
N
N
OH
Ir
OMe
OH
Ir
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N
N
N
N
N
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Cp*
Cp*
Cp*
SO
SO42
Cl
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Ir
N
N
OH
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Cl
OH
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HO
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OH
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OH
HO
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OH
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Cp*
Cp*
Cp*
SO42
SO42
SO42
Ir
Ir
N
OH
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OH
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OH
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HN
N
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Chart 1. Novel biomimetic complexes for FA dehydrogenation in aqueous solution.
a
Table 1. Dehydrogenation of FA with Ir complexes 1ꢀ6.
b
−1
Entry
Catalyst / ꢀM
1 / 100
1 / 100
2 / 100
2 / 100
3 / 100
3 / 100
3 / 100
4 / 100
4 / 100
5 / 100
5 / 100
6 / 100
6 / 100
Solution pH
Time / h
TON
TOF / h
1
2
3
4
5
6
7
8
9
1.7
3.5
1.7
3.5
1.7
3.5
2.8
1.7
3.0
1.7
3.0
1.7
3.0
5
2570
1010
5
430
160
7
10,000
5000
2740
7
3620
7
10,000
5500
7050
1
11,400
18,000
5150
1.5
3.5
1.5
5
8700
10,000
7640
10,800
4290
1
1
1
0
1
2
10,000
7850
1
19,900
5520
3
10,000
7850
13
0.5
32,500
a
Reaction conditions: 1 M HCO H or HCO H/HCO Na solution (10 mL) with Ir complexes
2
2
2
1
ꢀmol at 60 °C. All numbers are an average of two runs. FA dehydrogenation reactions with
complexes 2ꢀ6 yielded complete FA decomposition. Average TOF over initial 10 min.
b
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Figure 1. pH dependence of FA dehydrogenation using complexes 3ꢀ6 (100 ꢀM) in 1 M
HCO H/HCO Na (10 mL) solution at 60 °C.
2
2
Durability tests
For practical use in FA dehydrogenation, a high concentration of FA and a high temperature
are required. Therefore, we carried out FA dehydrogenation under harsher conditions with
complexes 3ꢀ6 that showed remarkable activity in the preliminary experiments. The experiments
examining the temperature dependence with complex 3 suggested that the reaction rate increased
significantly upon increasing the reaction temperature from 45 to 70 °C. (SI, Figure S1). The FA
concentration was also found to be an important factor for the increase in the reaction rate. The
reaction rate reached a maximum in 4 M FA solution using either complex 3 or 4 (SI, Figures S2
and S4). In addition, the capability of generating high pressure gases is desirable for the
convenience of gas delivery. We carried out the FA dehydrogenation with complex 3 in a closed
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system. The inner pressure of the autoclave reached a final pressure of 4 MPa after the reaction
completed (SI, Figure S5). According to the HPLC analysis, 99.4 % of the FA was converted.
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This is consistent with our previous reports
and suggests that high pressure does not inhibit
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the catalytic activity of complex 3. Therefore, under acidic conditions, the equilibrium between
CO /H and HCO H of this catalytic system is strongly biased toward gas generation, and
2
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2
pressure only weakly affects the equilibrium. Moreover, no CO was detected by GC (detection
limit 1 ppm) from the released gas. This indicates that the catalytic reaction is highly selective
and goes through no dehydration, which generates CO and H O. These properties are important
2
for providing high pressure H which facilitates gas delivery and utilization in fuel cells because
2
CO is a poison for the catalyst in fuel cell electrodes.
Complexes 3 and 4 were found to be stable in highly concentrated FA solutions at both low
(60 °C, SI, Figure S6) and high (100 °C, SI, Figures S9ꢀS11) temperature. The reaction results in
highly concentrated FA solution and high temperature are listed in Table 2. All the FA was
completely decomposed at 100 °C (110 °C bath temperature; the actual reaction temperatures
measured were close to 100 °C) with only 20 ꢀM catalyst concentration (Table 2, entries 1ꢀ4). In
contrast, complexes 5 and 6 were relatively less stable under the same conditions. Higher catalyst
concentration (100 ꢀM) was required for complete conversion of the FA (entries 5 and 6).
Complex 3 showed excellent stability. This complex is quite stable in highly concentrated FA
solutions (even 80 wt% FA), and all the FA could be dehydrogenated (TON: 200,000) despite
the fact that the TOF decreased considerably (SI, Figure S7). Interestingly, we observed an
apparent increase of reaction rate with consumption of FA and decrease of FA concentration.
The rate around 9000 min is higher than that at the initial stage or around 1500 min. This is in
agreement with our study of the effect of FA concentration as shown in Figures S2 and S4.
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Complex 3 gave the high TOF of 178,000 h at 100 °C in 4 M FA (entry 1). The initial
reaction rate decreased slightly upon increasing the FA concentration (entries 1ꢀ3). The reaction
in 8 M FA was finished in 4 h and exhibited the high TON of 400,000 (entry 3). Using complex
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60
−1
4
in 4 M FA at 100 °C, the TOF of 120,000 h was slightly lower than that of complex 3 (entry
4 vs. entry 1). We obtained relatively lower TOF values with complexes 5 and 6 in 4 M FA
entries 5 and 6). As the pH vs. rate profile suggested, addition of formate can significantly
improve the reaction rate. Thus we tried a 4 M HCO H/HCO Na (98/2) solution at 100 °C (entry
(
2
2
−1
7). Accordingly, complex 3 achieved a significantly elevated initial TOF of 269,000 h . With
the same strategy, we optimized the reaction conditions and obtained the unprecedented initial
−
1
TOF of 322,000 h with complex 6 (entry 9), which is higher than for any catalyst previously
1
0,25,28,55
reported.
These results indicate that both complexes 3 and 6 show much higher activity
14
than the most effective dinuclear Ir complex previously reported. These water soluble
complexes in aqueous media are even more efficient than the very recently reported PNPꢀRu in
1
0
DMF/Et N for FA dehydrogenation.
3
Based on the results obtained in these preliminary experiments with highly stable and efficient
complex 3, we sought to maximize its TON for FA dehydrogenation. Unprecedentedly high
TONs (more than 2 million) were reproducibly obtained in highly concentrated aqueous FA
solutions for long reaction times (Figure 2). Sodium formate was not added to increase reaction
rate because sodium formate can't be decomposed and decreases the total TON. Considering that
the practical use would probably be carried out under milder conditions to achieve a lower
energy cost, we carried out the reaction at the relatively lower temperature of 60 °C. The
reaction proceeded smoothly using only 1 ꢀmol of catalyst (catalyst concentration: 5 ꢀM) in 200
mL 6 M FA aqueous solution. After 150 h, the rate of dehydrogenation became slower due to the
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2
2
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3
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5
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decrease of FA concentration. After 55 L of gas was released (167 h), a degassed 50 wt%
solution of FA (74 g, approximately 0.8 mol FA) was added (see SI). The addition of FA
restarted the gas generation and indicated that the catalyst was still active after the first run.
Finally, 100.2 L of gas was evolved and afforded the extraordinary TON of 2,050,000 which, to
the best of our knowledge, is higher than ever reported for either an aqueous or organic solvent
system.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 2. H generation from dehydrogenation of FA at high concentration and high
2
a
temperature with Ir complexes 3ꢀ6.
b
−1
Entry
Catalyst /ꢀM
Conc.
& Time / h
TON
TOF / h
Substrate
4 M FA
6 M FA
8 M FA
4 M FA
4 M FA
4 M FA
4 M 98/2
1
2
3
4
5
6
7
3 / 20
3 / 20
3 / 20
4 / 20
5 / 100
6 / 100
3 / 20
6 / 40
6 / 40
2
200,000
300,000
400,000
200,000
40,000
178,000
176,000
173,000
120,000
89,400
3.5
4
3.5
1
1.5
6
40,000
68,600
d
196,000
80,000
269,000
258,000
322,000
c
d
d
8
4 M 80/20
4 M 68/32
5
c
9
0.5
68,000
a
Reaction conditions: 0.2ꢀ1 ꢀmol Ir complexes in 10 mL HCO H or HCO H/HCO Na solution
2
2
2
at 100 °C (110° C bath temperature; the actual reaction temperatures measured were close to 100
b
c
°
C). All numbers are an average of two runs. Average TOF over initial 10 min. 50 mL
d
HCO H/HCO Na solution. The ratio of HCO H to HCO Na.
2
2
2
2
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41
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Time course of formic acid dehydrogenation using complex 3 (1 ꢁmol) in a 6 M
aqueous FA solution (200 mL, 1.2 mol). Additional FA (50 wt% solution 74 g, ~0.8 mol) at 60
°
C was added after 167 h. A total of 100.2 L gas was generated (see SI).
KIE studies
The proposed mechanism for FA dehydrogenation (Scheme 2) starts with the binding of
HCO H or formate resulting in a formateꢀbound intermediate B (Step I). Then, βꢀhydride
2
elimination with concomitant CO evolution leads to the formation of an Irꢀhydride intermediate
2
C (Step II). Following this step, H is formed via proton transfer from a hydroxy group of the
2
43
pyrimidine ring of 3 to an Irꢀhydride moiety (Step III) assisted by a water molecule, completing
the catalytic cycle (Schemes 3 and 4).
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1
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2
2
2
3
3
3
3
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3
3
3
3
4
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0
1
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6
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9
0
1
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 2. Change in rate determining step with altered solution pH for complex 3 in FA
dehydrogenation.
We have reported different rate determining steps for complexes with and without pendent OH
41
groups based on deuterium kinetic isotope effect studies. However, the KIE studies were
limited to the regime in which the reaction rates were increasing (see Figure 1). The KIE results
can explain the rate increase with increasing solution pH, but cannot explain the reaction rate
decrease with further increase of solution pH. The high catalytic activity of these complexes at
low pH and high pH above and below the peak motivated further studies of the KIE values at pH
values on both sides of the peak of the bellꢀshaped activity curve. Accordingly, we performed a
KIE study with complex 3, which peaked at pH 2.8 as shown in the pH vs. rate profile of Figure
1. The reactions were carried out in 1 M formic acid solution (pH 1.7) and 1 M HCO HꢀHCO Na
2
2
(
1/1) solution (pH 3.5), and the results are shown in Table 3. The KIE study at pH 1.7 suggests
that DCO D (KIE: 2.0, entry 3) is more influential than D O (KIE: 1.5, entry 2) on the reaction
2
2
rate. Therefore, the βꢀhydride elimination (Scheme 2, Step II), which involves Ir–D bond
formation when DCO D is used, was designated as the rateꢀdetermining step. This result is
2
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26,51,52
consistent with our previous report and other reports.
The examination of FA
concentration as shown in Figure S2 and S4 also supports step II as being rateꢀlimiting around
pH 1.7. The rate improvement from 1 M to 4 M FA could be explained by considering a preꢀ
equilibrium associated with the formation of a formate bound Ir complex (B in Scheme 2) prior
to the rate determining βꢀhydride elimination step (see Figure S3 and SI for details). Since
formation of B requires deprotonation, decreasing the pH with increased FA concentration will
slow down the reaction. On the other hand, increasing concentration of one of the reactants (e.g.,
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59
60
HCO H) will favor formation of B, hence the net outcome of increasing FA concentration will be
2
determined by these two counterꢀeffects. Using a simple rate expression for the βꢀhydride
elimination step including the preꢀequilibrium for the formation of B indicates that the rate will
be enhanced by 1.6 times by changing the FA concentration from 1 M to 4M which is in good
agreement with the experimental observation (Figure S2). Further increase in FA concentration
will slow down the deprotonation step for the formation of B to a greater extent and will result in
a decrease in the overall reaction rate.
When a KIE study was carried out at pH 3.5 using HCO HꢀHCO Na in the ratio of 1/1, the
2
2
results were surprising. The KIE value (2.7) using D O is higher than that (1.5) of using DCO Dꢀ
2
2
+
DCO Na (entry 2 vs. entry 3), suggesting that H O or H O is involved in the rateꢀdetermining
2
2
3
step. Therefore, as shown in Scheme 2, the RDS should be H formation (Step III) when pH >
2
2
.8. Based on these results, we propose that the rate determining step is βꢀhydride elimination
(Scheme 2, Step II) at pH 1.7ꢀ2.8 and changes to H formation (Scheme 2, Step III) when pH >
2
2
.8 (see Computational Studies below). We carried out a similar KIE study with complex 6
Table 4). Similarly, deuterated solvent showed much more influence than the deuterated
substrate upon changing solution pH from 1.7 to 3.5. These results are consistent with those of
(
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5
5
5
5
6
complex 3, and indicate the altering of the RDS. The interesting RDS change is attributed to the
5
6
loss or gain of one or more protons by the intermediate along the reaction pathway, therefore
detailed computational investigation of the actual forms of the intermediates of the proton
responsive catalysts was performed using DFT calculations.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
Table 3. Kinetic isotope effect in the dehydrogenation of FA using Ir complex 3.
b
c
b
c
Entry Substrate/
Solvent (pH 1.7)
HCO H / H O
TOF
/ KIE
Substrate/
TOF / KIE
−1
−1
h
Solvent (pH 3.5)
HCO HꢀHCO Na / H O
h
1
2
3
4
14,000
9570
6870
4430
ꢀ
16,600
6130
ꢀ
2
2
2
2
2
HCO H / D O
1.5
2.0
3.2
HCO HꢀHCO Na / D O
2.7
2
2
2
2
2
DCO D / H O
DCO DꢀDCO Na / H O
11,200 1.5
5090 3.3
2
2
2
2
2
DCO D / D O
DCO DꢀDCO Na / D O
2 2 2
2
2
a
b
c
Reaction conditions: 10 mL 1 M FA solution or FA/formate (1/1), 1 ꢀmol catalyst, 70 °C.
Average TOF over the initial 10 min. The errors are less than 2 %.
KIE = TOF(entry 1)/TOF(entry n), (n = 2, 3, and 4). The exchange of all labile protons was
taken into account.
a
Table 4. Kinetic isotope effect in the dehydrogenation of FA using Ir complex 6.
b
c
b
c
Entry Substrate/
Solvent (pH 1.7)
HCO H / H O
TOF
/ KIE
Substrate/
TOF / KIE
−
1
−1
h
Solvent (pH 3.5)
HCO HꢀHCO Na / H O
h
1
2
3
13,300
9080
5770
3440
ꢀ
28,800
ꢀ
2
2
2
2
2
HCO H / D O
1.5
2.3
3.9
HCO HꢀHCO Na / D O
23,200 1.2
25,300 1.1
18,200 1.6
2
2
2
2
2
DCO D / H O
DCO DꢀDCO Na / H O
2 2 2
2
2
4
a
DCO D / D O
DCO DꢀDCO Na / D O
2 2 2
2
2
Reaction conditions: 10 mL 1 M FA solution or FA/formate (1/1), 1 ꢀmol catalyst, 70 °C.
Average TOF over the initial 10 min. The errors are less than 2 %.
KIE = TOF(entry 1)/TOF(entry n), (n = 2, 3, and 4). The exchange of all labile protons was
b
c
taken into account.
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Computational Studies
5
7
We performed density functional theory (DFT) calculations at the M06 level of theory with
58
the SMD aqueous continuum solvation model (see Computational Methods in the SI for
details) to investigate the mechanistic details associated with FA dehydrogenation by complexes
2, 3, 5, and 6. We have demonstrated that the protonꢀresponsive complexes containing a
10
11
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15
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17
18
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48
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50
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60
−
proximal ꢀO group can facilitate the hydrogen splitting through hydrogen bonding (Scheme
4
3
−
1
d). Thus, in the reverse reaction, the proximal ꢀO can also assist the hydrogen formation. A
5
9
sulfonamide ligandꢀassisted FA dehydrogenation via hydrogen bonding has been reported.
Further considering the different hydrogen bonding model and different electron donating ability
−
between ꢀOH and ꢀO , it is highly desirable to determine the real forms of the protonꢀresponsive
complexes in the solution at different pH, i.e., the protonated form or a fully or partially
deprotonated form. Therefore we applied a speciation approach in which we calculated the
relative free energies of the intermediates and transitionꢀstate structures for all possible
protonation states of the complexes and followed the changes in relative free energies with
changes in pH. For instance, for FA dehydrogenation by 3, we generated freeꢀenergy profiles at
pH 1.7 and 3.5 following the relative free energies of intermediates and transition state structures
differing in protonation states as depicted in Schemes 3 and 4. Similar freeꢀenergy plots for
complexes 2, 5, and 6 are presented in the SI (Schemes S1ꢀS19).
For the following energetic analysis of the proposed catalytic cycle, we have used the free
energies associated with the most stable species for each reaction intermediate as shown in
Schemes 3 and 4 connected by a black solid line. The reasoning behind this convention (or
“
speciation” approach) is that deprotonation of an OH group will be more facile than traversing a
transition state for βꢀhydride elimination or H formation so that the most stable protonation state
2
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1
2
2
2
2
2
2
2
2
2
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3
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5
6
of each species will be predominant in the solution. At pH 1.7, the binding of formic acid is
uphill by 1.9 kcal/mol and the following βꢀhydride elimination step features a free energy of
‡
activation (ꢂG ) of 16.0 kcal/mol (Scheme 3). H formation from the generated Irꢀhydride
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
‡
species occurs with ꢂG = 13.0 kcal/mol, releasing H2(g) and regenerating the catalyst (Scheme
3). At pH 3.5, the formation of the formate complex requires no energy input (ꢂG = 0.0
‡
kcal/mol) and the ꢂG for the following βꢀhydride elimination step is 15.4 kcal/mol (Scheme 4).
‡
The following H formation step involves a ꢂG of 15.7 kcal/mol at pH 3.5 (Scheme 4). The
2
‡
calculated ꢂG s are 16.0 kcal/mol and 13.0 kcal/mol at pH 1.7 corresponding to the βꢀhydride
elimination and H generation steps, respectively, indicating that the former is the rateꢀ
2
determining step (Scheme 3). The calculated overall energy barrier for the βꢀhydride elimination
step (17.9 kcal/mol) including the freeꢀenergy cost for HCO H binding is consistent with the
2
experimental observation of an activation energy of 18.7 kcal/mol (SI, Figure S1).
(
26.0)
HO
N
N
OH
O
O
N
N
OH
O
N
N
N
N
(
20.4)
18.2)
(
HO
N
N
O
N
N
(
17.9)
(
17.6)
N
N
N
N
(
11.1)
(11.2)
O
O C
CO2, -H+
-
OH
HO
N
N
N
H
IrIII
(7.8)
H
O
N
(5.2)
(
2.8)
2.8)
(1.9)
+
H
(5.2)
H O, H
H
2
O
N
N
OH
(
H
N
IrIII
(
1.7)
HCO H
2
N
(
0.1)
(
0.0)
O
HO
-H O, -H
2 2
HC
(-0.8)
(-1.8)
N
N
OH
O
O
N
OH
N
N
N
IrIII
IrIII
N
O
N
N
OH
N
H
IrIII
N
N
(-11.2)
O
N
N
N
OH
IrIII
N
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Scheme 3. Proposed mechanism for FA dehydrogenation by complex 3 at pH 1.7 and 333.15
K. The relative free energies are reported in units of kcal/mol. The speciation of the protonation
state of each intermediate and transition state is indicated by the color code of the structures in
the upper right of the scheme.
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13
14
15
16
17
18
19
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22
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25
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56
57
58
59
60
(20.5)
HO
N
N
OH
O
O
N
N
OH
O
N
N
N
N
(
(
17.9)
17.6)
15.4)
(
(
14.9)
HO
N
N
O
N
N
N
N
N
N
(
11.2)
O
(
7.9)
O C
O
N
N
N
OH
H
(
5.6)
H
O
IrIII
-CO2
N
H O, H+
2
H
H
(2.5)
1.7)
0.0) HCO
(1.9)
(0.0)
(2.3)
O
N
N
N
OH
H
(
(
IrIII
H,-H+ (0.0)
2
N
(-0.8)
(-2.6)
-H
2
O, -H
2
O
HO
HC
O
O
N
N
OH
N
N
O
N
N
IrIII
IrIII
(-4.5)
N
N
O
N
OH
H
N
IrIII
N
N
(
-11.2)
O
N
OH
N
IrIII
N
N
Scheme 4. Proposed mechanism for FA dehydrogenation by complex 3 at pH 3.5 and 333.15
K. The relative free energies are reported in units of kcal/mol. The speciation of the protonation
state of each intermediate and transition state is indicated by the color code of the structures in
the upper right of the scheme.
‡
‡
On the other hand, at pH 3.5, ꢂG = 15.4 kcal/mol for βꢀhydride elimination and ꢂG = 15.7
kcal/mol for H generation, indicating that the latter is becoming the rateꢀdetermining step as the
2
pH is increased (Scheme 4). Based on these data, the βꢀhydride elimination is the rateꢀ
determining step at low pH, but the freeꢀenergy cost for H generation increases with increasing
2
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3
3
3
3
3
3
3
3
3
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5
5
5
6
‡
pH. The catalyst displays the highest rate at moderate pH when the ꢂG s of the two steps are
very close, consistent with the experimental observation of the bell shaped pH vs. rate profile
(
Figure 1) and the H generation step becoming rateꢀdetermining with further increase in pH as
2
0
1
2
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5
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9
0
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9
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‡
suggested by the KIE experiments. It should be kept in mind that the difference in the ꢂG s at
pH 3.5 is probably within the error in both the calculated and experimental values, but the trend
from pH 1.7 should be more reliable.
Table 5. Calculated kinetic isotope effects for βꢀhydride elimination and H generation steps for
2
complex 3 at T = 343.15 K.
Substrate/Solvent
HCO H/D O
βꢀhydride elimination
H generation*
2
1.0
2.4
2.4
4.7
2
2
DCO D/H O
1.0
4.5
2
2
DCO D/D O
2
2
* The reactant complex, similar to the transition state, involves additional explicit water for KIE
calculations. The exchange of all labile protons was taken into account.
We also calculated KIEs for the βꢀhydride elimination and H generation steps for complex 3
2
associated with deuterium substitution either on the substrate or solvent (Table 5). It is clear from
the calculated KIEs that βꢀhydride elimination is mostly affected by deuteration of formic acid
and the H generation step is more affected by the deuteration of water. The experimentally
2
observed dependence of KIEs on pH, taken together with the calculated KIEs, further support the
notion of a change in rate determining step from βꢀhydride elimination to H generation with
2
increasing pH, and at pH 3.5 is approaching the value at which that change will occur.
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The calculated ꢂG s for the βꢀhydride elimination and H generation steps at different pH for
2
complexes 2, 3, 5 and 6 provide some insight for the design of improved catalysts for FA
‡
dehydrogenation (Table 6). Comparison of ꢂG s of the rate determining steps at pH 1.7 and 3.5
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for complexes 2 and 3 indicates that the substitution of the hydroxy pyridine ligand in 2 with the
dihydroxy pyrimidine ring in 3 decreases the free energies of activation, consistent with the
‡
experimentally observed enhanced activity of 3. For complexes 2 and 3, the ꢂG s clearly
indicate the change of RDS from βꢀhydride elimination to H generation with increasing pH from
2
‡
1.7 to 3.5. Although the ꢂG s of complexes 5 and 6 showed no switching of RDS at 333.15 K,
‡
the ꢂG s for the H generation step are getting larger and the difference between the two steps is
2
becoming smaller. From this viewpoint, this tendency also supports the change of RDS with
increasing solution pH. From the results of our calculations, we expect the switchover point for
complexes 5 and 6 to be larger than the experimental observation (pH 3.1), which could be
attributed to the associated errors in the DFT calculation. We also calculated the free energies of
activation at 373.15 K for complexes 3 and 6. At this temperature, the H formation step
2
‡
becomes the rateꢀdetermining step for both complex 3 (ꢂG = 17.6 kcal/mol) and complex 6
‡
(ꢂG = 20.0 kcal/mol) with the former showing the highest activity at pH 2.8 whereas the latter
showing enhanced activity at the slightly elevated pH of 3.1 (SI, Schemes S1ꢀS19).
‡
Table 6. Calculated free energies of activation (ꢂG ) associated with relevant TSs for βꢀhydride
elimination and H generation steps for complexes 2, 3, 5, and 6 at T = 333.15 K for different pH
2
values.
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Free Energies of Activation (kcal/mol) Associated with Relevant TSs
Complex 2
βꢀhydride elimination 17.2
H generation 15.7
βꢀhydride elimination 15.9
H generation 16.1
Complex 3
16.0
Complex 5
18.1
Complex 6
18.4
pH 1.7
pH 3.5
0
1
2
3
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9
0
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13.0
12.9
13.6
2
15.4
16.7
17.6
15.7
14.8
16.4
2
Conclusions
We have designed and synthesized a series of biomimetic Ir complexes with hydroxyl
substituted pyridineꢀazole and pyrimidineꢀazole/azoline ligands. These waterꢀsoluble complexes
are demonstrated to be highly efficient and stable in catalytic FA dehydrogenation at high
temperature in highly concentrated FA aqueous solutions. We have achieved the unprecedented
−
1
TOF of 322,000 h and TON of 2,050,000 with the most effective complexes 6 and 3,
respectively. These complexes are capable of providing high pressure COꢀfree H . The
2
extraordinary catalytic performance indicates that these complexes should be highly useful in the
practical supply of H from FA and contributes significantly to the development of a hydrogen
2
economy. Investigation of the mechanism using deuterium KIE studies and DFT calculations
suggests that the rateꢀdetermining steps are altered upon changing the solution pH. The DFT
calculations employing a “speciation” approach provide insight into the mechanism and support
the RDS change. This mechanistic insight rationalizes the bellꢀshaped pH vs. rate profile, and
helps to determine the optimal conditions of FA dehydrogenation by addition of formate and
adjustment of the solution pH.
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ACKNOWLEDGMENT
Y. H. and Y. M. thank the Japan Science and Technology Agency (JST), CREST for financial
support. W.ꢀH. W. thanks the financial support from Dalian University of Technology (the
Fundamental Research Funds for the Central Universities, Grant No. DUT14RC(3)082; Grant
No. 844401) and National Natural Science Foundation of China (Grant No. 21402019). The
work at BNL was carried out under contract DEꢀSC00112704 with the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, and utilized resources at the BNL
Center for Functional Nanomaterials.
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Supporting Information. Experimental section containing general procedures and synthesis
of ligands and Ir complexes, a plot of TOF vs. temperature for FA dehydrogenation, plots of FA
dehydrogenation, TOF vs. [HCO H], plots of time courses of FA decomposition, computational
2
methods, freeꢀenergy profiles derived from DFT calculations for FA dehydrogenation for
1
complexes 2ꢀ6 at pH 1.7 and 3.5, tables of optimized coordinates and energies, and H NMR
spectra of ligands. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Yuichiro Himeda, himeda.y@aist.go.jp
Etsuko Fujita, fujita@bnl.gov
James T. Muckerman, muckerma@bnl.gov
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Newly designed water soluble complexes exhibited unprecedented activity and durability for
hydrogen generation from catalytic dehydrogenation of formic acid in aqueous solutions.
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