Formic acid dehydrogenation with bioinspired iridium complexes: A kinetic isotope effect study and mechanistic insight

Article abstract of DOI:10.1002/cssc.201301414

Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H₂O)]SO₄ (TH4BPM: 2,2′,6,6′-tetrahydroxyl-4,4′-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39 500 h^-1 at 80 °C in a 1 M aqueous solution of HCO₂H/HCO₂Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is β-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions. The key controls the mechanism: A deuterium kinetic isotope effect (KIE) study clearly indicates a different mechanism for complexes with OH at different positions of the ligands. The rate-limiting step is β-hydrogen elimination from the iridium-formate intermediate for complexes with OH at ortho positions owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of generation of H₂.

Full text of DOI:10.1002/cssc.201301414

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FULL PAPERS
DOI: 10.1002/cssc.201301414
Formic Acid Dehydrogenation with Bioinspired Iridium
Complexes: A Kinetic Isotope Effect Study and Mechanistic
Insight
[
a, b]
[a]
[a]
[a]
[a]
Wan-Hui Wang,
Shaoan Xu, Yuichi Manaka, Yuki Suna, Hide Kambayashi,
[c]
[c]
[a, b]
James T. Muckerman, Etsuko Fujita, and Yuichiro Himeda*
Highly efficient hydrogen generation from dehydrogenation of
formic acid is achieved by using bioinspired iridium complexes
that have hydroxyl groups at the ortho positions of the bipyri-
dine or bipyrimidine ligand (i.e., OH in the second coordination
sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)-
ic isotope effect study clearly indicates a different rate-deter-
mining step for complexes with hydroxyl groups at different
positions of the ligands. The rate-limiting step is b-hydrogen
elimination from the iridium–formate intermediate for com-
plexes with hydroxyl groups at ortho positions, owing to
a proton relay (i.e., pendent-base effect), which lowers the
energy barrier of hydrogen generation. In contrast, the reac-
tion of iridium hydride with a proton to liberate hydrogen is
demonstrated to be the rate-determining step for complexes
that do not have hydroxyl groups at the ortho positions.
(
H O)]SO4 (TH4BPM: 2,2’,6,6’-tetrahydroxyl-4,4’-bipyrimidine;
2
Cp*: pentamethylcyclopentadienyl) has a high turnover fre-
À1
quency of 39500 h at 808C in a 1m aqueous solution of
HCO H/HCO Na and produces hydrogen and carbon dioxide
2
2
without carbon monoxide contamination. The deuterium kinet-
Introduction
It is imperative to find sustainable energy carriers because of
diminishing fossil-fuel reserves and serious environmental
liquid-phase hydrogen storage materials, formic acid (FA) is of
particular interest and has attracted increasing attention. FA is
a nontoxic, biodegradable liquid under ambient conditions;
thus it can be easily handled and transported. Dehydrogena-
[
1]
problems caused by greenhouse gases. Hydrogen is consid-
ered as a promising alternative because of its high gravimetric
À1
energy density (33.3 kWhkg ) and its generation of only water
tion of FA is thermodynamically favorable (DG8=
À1 [4]
upon combustion or utilization in fuel cells. However, its gas-
À32.8 kJmol ); therefore, H can be released under relatively
2
À1
eous property and low volumetric energy density (2.5 WhL )
mild conditions. More importantly, FA is a renewable hydrogen
at atmospheric pressure are principal obstacles to its use on
a large scale. Physical storage methods, such as compressed or
liquefied hydrogen and physisorption by porous materials,
storage material because H can be stored or released by the
2
interconversion between FA and H /CO with a suitable cata-
2
2
lyst.
Considerable effort has been concentrated on the hydroge-
[
1,2]
suffer from cost or safety issues.
Recently, chemical hydro-
[4,5]
gen storage by using liquid-phase chemicals as hydrogen vec-
tors has emerged as a promising method owing to such attrac-
nation of CO to FA, and much progress has been achieved.
2
In contrast, the reverse reaction has been less studied.
Although the first FA dehydrogenation with transition-metal
[
3]
tive features as favorable stability and storability. Among
[6]
catalysts was reported early in 1967, this reaction has re-
ceived renewed attention in the last five years because FA as
+
[
a] Dr. W.-H. Wang, Dr. S. Xu, Dr. Y. Manaka, Dr. Y. Suna, Dr. H. Kambayashi,
Dr. Y. Himeda
[7]
a hydrogen storage medium has attracted interest. Beller
National Institute of Advanced Industrial Science and Technology
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba
Ibaraki, 305-8565 (Japan)
et al. reported H generation from HCO H/NEt by using Ru/
2
2
3
PPh3 catalysts with the high turnover frequency (TOF) of
À1
[8]
E-mail: himeda.y@aist.go.jp
3630 h (initial 20 min) at 408C. Independently, Laurenczy
et al. studied FA dehydrogenation in aqueous solution by
using an in situ generated ruthenium catalyst, and obtained
+
[
b] Dr. W.-H. Wang, Dr. Y. Himeda
Japan Science and Technology Agency
ACT-C, 4-1-8 Honcho, Kawaguchi
Saitama, 332-0012 (Japan)
À1
[9]
a TOF of 460 h at 1208C. Fukuzumi and co-workers report-
ed decomposition of FA with {RhCp*} complexes at room tem-
perature and obtained optimal efficiency by adjusting the pH
[c] Dr. J. T. Muckerman, Dr. E. Fujita
Chemistry Department, Brookhaven National Laboratory
Upton, NY 11973-5000 (USA)
[10]
À1
of the solution. An improved initial TOF of 426 h was ach-
+
[
] Current address:
ieved with a heterodinuclear complex [Ir(bpm)(Cp*)(H O)Ru-
2
School of Petroleum and Chemical Engineering
Dalian University of Technology
Panjin 124221 (PR China)
(
bpy) ](SO ) (bpm: 2,2’-bipyrimidine; Cp*: pentamethylcyclo-
2 4 2
[11]
pentadienyl; bpy: 2,2’-bipyridine). Wills et al. reported the
very high TOF of 18000 h and turnover number (TON) of
À1
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201301414.
25000 with commercially available complex [RuCl (dmso) ] in
2
4
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[
12]
HCO H/NEt at 1208C. We investigated FA dehydrogenation
Herein, we report highly efficient FA dehydrogenation with
bioinspired iridium complexes 2a, 3, and 4. For complexes 1–
4, we performed a deuterium KIE study and provide mechanis-
tic insight that rationalizes their different performance under
different pH conditions.
2
3
with [Ir(Cp*)(4DHBP)(H O)]SO (1a; 4DHBP: 4,4’-dihydroxy-2,2’-
bipyridine) under various conditions and obtained a TOF of
2
4
À1
[13]
1
4000 h at 908C. Recently, we achieved the highest TON
308000) and TOF (228000 h , at 908C) yet reported by using
À1
(
the bioinspired dinuclear complex [(Cp*IrCl) (THBPM)]Cl2
2
(
THBPM: 4,4’,6,6’-tetrahydroxy-2,2’-bipyrimidine) for FA dehy-
[
14]
drogenation in water. Very recently, we demonstrated that Results and Discussion
Ir(Cp*)}–aqua complexes with biazole ligands were highly effi-
{
[
15]
HCO H dehydrogenation
2
cient. [Ir(Cp*)(TMBIM)(H O)]SO (TMBIM: 4,4’,5,5’-tetramethyl-
2
2
4
À1
,2’-biimidazole) gave the high initial TOF of 34000 h at
08C.
All of the complexes in Scheme 1, except complex 3, were syn-
8
thesized according to our reported procedures (see the Experi-
[18,19]
Our collaboration has been focused on pursuing FA as a hy-
mental Section).
We tested the catalytic activity of these
drogen storage medium by using transition-metal complexes
with proton-responsive ligands as catalysts for the storage and
complexes for dehydrogenation of FA in an aqueous solution
of HCO H or HCO H/HCO Na (1:1). The results are listed in
2
2
2
[
16]
release of hydrogen.
We developed the first OH-bearing
and more recently hydrogenase-mimicking
complexes, including [Ir(Cp*)(6DHBP)(H O)]SO (2a; 6DHBP:
Table 1. Complex 2a showed activity similar to that of complex
[
13a,17]
complex 1a
1a in a 1m solution of HCO H (Table 1, entries 1 and 3). When
2
2
4
[
18]
6
,6’-dihydroxy-2,2’-bipyridine), [Ir(Cp*)(TH4BPM)(H O)]SO (4;
2
[
4
19]
TH4BPM:
2,2’,6,6’-tetrahydroxy-4,4’-bipyrimidine),
(Cp*IrCl) (THBPM)]Cl , that had OH groups in the second co-
and
[
14]
[
2 2
ordination sphere (i.e., pendent bases) for catalytic transforma-
[
16b,c]
tion between CO /H and HCO H.
Similar to hydrogenase,
2
2
2
which can promote reversible oxidation of dihydrogen, our
complexes can efficiently catalyze the reversible interconver-
sion between CO /H and HCO H by adjusting the pH of the
2
2
2
[
13b]
solution; this was demonstrated with complexes 1a
and
[
14]
[
(Cp*IrCl) (THBPM)]Cl . In addition, bioinspired complexes 2a
and 4 have shown high activity for CO hydrogenation.
2
2
[
18,19]
2
With the hypothesis that these complexes are also capable of
catalyzing FA dehydrogenation efficiently, we have investigat-
ed FA dehydrogenation by using bioinspired complexes 2a
and 4 and a newly designed complex 3 with 4,4’,6,6’-tetrahy-
droxy-2,2’-bipyridine in this study. Their catalytic capabilities
have been examined under a variety of conditions.
Scheme 1. Iridium complexes for FA dehydrogenation.
Although the dehydrogenation of FA has attracted increas-
ing attention, the mechanism is not yet unambiguously under-
stood. The groups of Laurenczy and Beller performed mecha-
nistic studies of their phosphine complexes with FTIR and NMR
[
a]
Table 1. Dehydrogenation of HCO
Entry Cat./conc. [mm]
2 2
H/HCO Na with Ir complexes.
[
9b,20]
spectroscopy and DFT calculation methods.
They provided
[
g]
t
T
Initial TOF TON
À1 [d]
Residual [formate]
significant information for understanding the catalytic mecha-
nism with phosphine complexes. Because kinetic isotope effect
À1
[
h] [8C] [h
]
[molL
]
[
[
[
b,c]
c]
1
2
3
4
5
6
7
8
9
0
1a/200
1a/200
2a/100
2a/100
2a/33.3
2b/200
2c/4000
3/50
4
27
8
60
60
60
2400
550
2450
5440
5000
0
(KIE) studies are useful for exploring reaction mechanisms, Fu-
2300 0.54
10000
5300 0.46
30000
2500 0.48
25
7650
4800 0.52
6340 0.37
11000 0.45
kuzumi et al. investigated the FA dehydrogenation mechanism
0
2
+
with [Rh(bpy)(Cp*)(H O)] and a heterodinuclear IrÀRu com-
4.5 60
4.5 90 19800
8
9
2
c]
[
10,11]
0
plex in a KIE study.
We studied the mechanism for complex
60
60
910
6
3890
3300
1
a with KIE methods and determined that the rate-limiting
–
–
[
c]
step of the dehydrogenation reaction was the protonation of
2.5 60
[
21]
[e]
the IrÀH intermediate. We also investigated CO hydrogena-
3/100
4/100
4/50
6
6
8
60
2
1
60 12200
80 39500
tion in KIE studies by using bioinspired complexes 2a and 4,
with pendent bases, and obtained clear evidence of the in-
volvement of a water molecule in the rate-determining heter-
[f]
11
[
a] The reaction was performed in a 1m aqueous solution of HCO
2
H/
HCO Na (10 mL; 1:1) unless otherwise noted. The errors of TON and TOF
2
olysis of H , and accelerated proton transfer by formation of
2
were less than 5%. [b] Data from Ref. [14]. [c] The reaction was performed
a water bridge in CO hydrogenation catalyzed by these com-
2
in 1m HCO H. [d] Average TOF in the initial 10 min. [e] This value was cal-
2
[
18,19]
plexes.
This prompted us to study the potential influence
culated based on no precipitation of the catalyst. [f] Average TOF in the
initial 5 min. [g] Minimum TON based on catalytic concentrations and FA
decomposed.
of the pendent base on the mechanism of FA dehydrogenation
with bioinspired complexes.
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the reaction was performed in 1m HCO H/HCO Na (1:1), the in-
2
2
itial TOF doubled for 2a (Table 1, entry 4), whereas it decreased
À1
for 1a (TOF: 552 h ; Table 1, entry 2). This result indicates that
complex 2a is more active than complex 1a, which is consis-
[
18]
tent with their performance in CO hydrogenation. It also
2
suggests that complexes 1a and 2a exhibit different pH de-
pendence for FA dehydrogenation. The difference is most
likely to be caused by different mechanisms being operative
(
see below). When the reaction was performed at 908C in 1m
À1
HCO H, the high TOF of 19800 h and a TON of 30000 were
2
obtained (Table 1, entry 5). Complexes 2b and 2c, with less-
electron-donating MeO and Me substituents on the bipyridine
À1
ligand, showed much lower reaction rates (TOF: 910 and 6 h ,
respectively; Table 1, entries 6 and 7) than the strong-donor
OH-substituted analogue 2a. This result suggests that the elec-
tron-donating ability of the substituents plays an important
role in activating the complex; this is consistent with our previ-
Figure 1. Plot of the rate of gas generation versus concentration of catalyst
with complex 2a in 1m HCO H/HCO Na (1:1, 10 mL) at 608C.
2 2
[
13a]
ous report for the 4,4’-substituted analogues.
Bipyridine-
based complex 3 with four hydroxyl donors showed higher ac-
À1
tivity (TOF: 3890 h ) than complexes 1a and 2a; this also
demonstrates the importance of the electron-donating ability
of the ligand (Table 1, entry 8 vs. entries 1 and 3). Unfortunate-
ly, complex 3 exhibited low stability in 1m FA (Table 1, entry 8)
and low water solubility in a 1m solution of HCO H/HCO Na
2
2
(1:1; Table 1, entry 9). The TOF is smaller than that of 2a
(Table 1, entry 4) owing to the low solubility. Bipyrimidine-
based complex 4, which also has four OH groups, decomposed
quickly in 1m FA (pH 1.7). Surprisingly, it was stable in 1m
HCO H/HCO Na (1:1) and dissolved in water more readily and
2
2
exhibited extraordinary activity. In 1m HCO H/HCO Na (1:1) at
2
2
À1
6
08C, the high initial TOF of 12200 h was obtained (Table 1,
entry 10). In addition, with complex 4, the apparent gas release
was accomplished in 1 h (Figure S1 in the Supporting Informa-
tion), but decomposition of formate (albeit very slow) was ob-
served by prolonging the reaction time. Finally, only 0.37m for-
mate was detected after 6 h (Table 1, entry 10). A TOF as high
Figure 2. Plot of the initial TOF (average rate over initial 10 min) versus con-
centration of the reaction solution (10 mL, HCO H/HCO Na=1:1) with
0.1 mm of 2a (0.3 mm for 8m formate solution and 0.1 mm for others) at
08C.
2
2
À1
as 39500 h was achieved at 808C with 50 mm of 4 (Table 1,
6
entry 11). This high activity is comparable to the most efficient
dinuclear complex [(Cp*IrCl) (THBPM)]Cl we have previously
2
2
[
14]
reported, and suggests that complex 4 is the most efficient
mononuclear complex for dehydrogenation of FA.
The reaction conditions influence the reaction rate consider-
ably and were therefore studied in detail. Increasing the con-
centration of the catalyst led to a higher gas generation rate.
For complex 2a, the gas generation rate varied linearly with
the concentration of 2a (Figure 1). This indicates that FA dehy-
drogenation is first order in iridium complex concentration.
The effect of the concentration of FA/formate was also studied
À1
with 2a. As shown in Figure 2, the rate (7600 h ) peaked in
a 4m solution of HCO H/HCO Na (1:1) and decreased when the
2
2
concentration further increased. The temperature dependence
of the initial TOF showed distinct Arrhenius behavior for all of
the complexes examined (Figure 3). By analyzing the Arrhenius
plots, the activation energies (E ) were calculated to be 75.7
a
À1
and 76.4 kJmol , respectively, for 2a and 4. The E values of
a
the two complexes are markedly lower than the Ea
Figure 3. Arrhenius plots for HCO H dehydrogenation with 2a and 4. Reac-
tion conditions: 0.1 mm catalyst in 1m HCO H/HCO Na (1:1, 10 mL).
2 2
2
À1
[22]
(203.9 kJmol ) of dehydrogenation without a catalyst.
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2
+
Decomposition of FA can give CO and H O by dehydration
[Ir(Cp*)(6,6’-R -bpy)(OH )] complexes (except for complex 2c,
2 2
2
or release H and CO by dehydrogenation. Carbon monoxide
R=Me) were used, either in 1m HCO H (Figure 4) or 1m
2
2
2
is a poison for some catalysts in fuel cells and is usually re-
quired to have a concentration of less than 10 ppm in practical
use. We tested the released gases from the decomposition of
HCO H/HCO Na (1:1; Figure S4 in the Supporting Information),
2
2
the Hammett plots showed a similar tendency; this demon-
strates the same electronic substituent effect as that in the
4,4’-substituted complexes.
1
m HCO H by using gas chromatography, and found no de-
2
tectable CO (Figure S2 in the Supporting Information). It was
also confirmed that the gas mixture contained H and CO in
The reaction with complex 2c (R=Me) proceeded very
slowly (initial TOF: 6 h ), even slower than that for the unsub-
À1
2
2
a ratio of 1:1; this demonstrates that FA was selectively decom-
posed into H and CO . For easy gas delivery and practical use,
stituted analogue [Ir(bpy)(Cp*)(H O)]SO (1d). This result indi-
cates that the bulky methyl group (Taft steric constant (Es):
2
4
[23]
2
2
high pressure is usually required. Therefore, we examined de-
hydrogenation at high pressure with 2a in an autoclave
equipped with a backpressure regulator (see the Experimental
Section). Under a pressure of 1 MPa, FA (1m, 50 mL) was com-
pletely decomposed at 608C in 10 h and no significant inhibi-
À1.24) at the 6,6’-positions has a pronounced steric effect that
prohibits access of formate to the iridium metal center for the
formation of the formato complex. In contrast, the less-hin-
[23]
dered MeO substituent (E : À0.55)
showed no inhibiting
s
effect on the reaction rate.
À1
tion of the reaction rate (initial TOF: 2050 h ) was observed
(
Figure S3 in the Supporting Information). These catalytic per-
pH dependence of TON and TOF
formances suggest that these complexes are potentially useful
for providing gases for fuel cells from the dehydrogenation of
FA.
As mentioned above, we have demonstrated the important
effect of electron donors on the catalytic performance of the
complexes. In CO hydrogenation and transfer hydrogenation,
2
we have also observed that OH deprotonates to give the
Electronic and steric effects of the substituents
+
stronger donor oxyanion (sp =À2.31) under basic conditions
[16a,17,24]
(
Scheme 2) and significantly enhances the reaction rate.
We have reported the electronic effect of the substituent (R) in
2
+ [13a]
HCO H dehydrogenation with [Ir(Cp*)(4,4’-R -bpy)(OH )]
.
2
2
2
We investigated the substituent effect with a series of 6,6’-sub-
2
+
stituted complexes [Ir(Cp*)(6,6’-R -bpy)(OH )] (R=OH, OMe,
2
2
Me, H) in a 1m solution of HCO H at 608C for comparison,. Ac-
2
+
cording to the Hammett constants (sp ), the electron-donating
+
ability of the substituents is in the order of OH (s =À0.91)>
p
+
+
+
OMe (sp =À0.78)>Me (sp =À0.31)>H (sp =0), which is in
agreement with the catalytic activity of the corresponding 4,4’-
2
+
substituted complexes. For the [Ir(Cp*)(4,4’-R -bpy)(OH )]
Scheme 2. The acid–base equilibrium of proton-responsive complex 2a.
2
2
complexes, the logarithm of the initial TOF showed a good
linear correlation with the Hammett constants of the substitu-
ents (Figure 4); this confirms that strong electron-donating
substituents lead to high activity of the catalyst. When
Hence, we studied the pH dependence of the proton-respon-
sive complexes 1a, 2a, and 4 (R=OH) in FA dehydrogenation
to test potential activation by deprotonation of the OH. For
comparison, the proton nonresponsive analogues 1b and 2b
(
R=OMe) were also studied. We tested the reaction rate
(
Figure 6) and turnovers (Figure 5) with the iridium complexes
in a mixed solution of FA and sodium formate (CHCO2H
+
CHCO2Na =1m) at 608C. The pH values of 1m HCO H and 1m
2
HCO Na are 1.8 and 7.6, respectively. The pH of the reaction
2
solution was adjusted by adding sodium formate, while keep-
ing the total concentration of HCO H plus HCO Na constant
2
2
(
CHCO2H +CHCO2Na =1m). In all experiments, the FA fraction was
completely decomposed, but sodium formate added generally
remained, even after prolonging the reaction time (Figure 5,
right y axis). Thus, addition of formate to the solution of FA led
to a decrease in the total TON (Figure 5, left y axis). Interesting-
ly, the reaction rate can be improved by increasing the solu-
tion pH (i.e., adding formate) in some cases (Figure 6).
The pH dependence of total turnovers (Figure 5) was the
same for all of the complexes because the FA fraction in the
mixture completely decomposed. In contrast, the pH depend-
ence of the reaction rate was clearly different for different
2 2
Figure 4. Hammett plots for HCO H dehydrogenation with [Ir(Cp*)(4,4’-R -
2+
2
+
bpy)(OH
2
)] (open squares) and [Ir(Cp*)(6,6’-R
2
-bpy)(OH
2
)] (triangles) for
R=OH, OMe, Me, and H in a 1m solution of HCO
2
H (pH 1.8).
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(
Scheme 2), and thereby improves the activity of the complex.
The reaction rate is considerably affected by the pH of the so-
lution through controlling the acid–base equilibrium (i.e., elec-
tronic effect) of the ligand. On the other hand, an increase in
formate concentration also contributes to the improvement of
the reaction rate by affecting the catalytic process (Step I,
Figure 7). The TOF of 4 also peaked at pH 4, which was similar
to the case of 2a. We believe the same pH dependence results
from the same cause as that illustrated for 2a. Interestingly,
complex 1a, which is proton responsive, showed a pH de-
pendence similar to those of proton nonresponsive complexes
1
b and 2b (Figure 6). The TOF dependence of complex 1a,
with OH groups at the para positions, decreased with increas-
ing pH of the solution and was apparently different from that
of complex 2a, which had OH groups at the ortho positions.
This tendency suggests that the proton concentration exhibits
a much more important effect on the reaction (through Ste-
p III, Figure 7) for 1a. These results indicate that the different
pH dependence for complexes 1a, 2a, and 4 may be associat-
ed with their different catalytic mechanisms. This prompted us
to seek insights into the reaction mechanism.
Figure 5. Plot of TON (closed circles) and concentration of residual formate
(closed squares) versus the pH of the solution. Reaction conditions: complex
2
a (1 mmol), solution of HCO H/HCO Na (10 mL; CHCO2H +CHCO Na =1m), 608C.
2
2
2
KIE study and mechanistic insight
FA dehydrogenation generally proceeds through three steps
(
Figure 7): formation of formato complex B (Step I), release of
CO2 by b-hydrogen elimination to generate iridium hydride
complex C (Step II), and production of H from the reaction of
2
[
Ir]ÀH and a proton (Step III). Under acidic conditions, [Ir]ÀH
[21]
complexes can be observed by ESI-MS, but are undetectable
1
by H NMR spectroscopy owing to fast H/D exchange (see
1
below). However, they can be detected by H NMR spectrosco-
Figure 6. The pH dependence of HCO
2
H dehydrogenation rate with 1, 2,
py under basic conditions in the reaction between the iridium
and 4 in a solution of HCO H/HCO Na (10 mL; 1m) at 608C. The pH of the
2
2
[18,19]
complex and H2.
We found that the chemical shift of the
2 2
solution is adjusted by changing the ratio of HCO H and HCO Na while
hydride of 2a (d=À10.56 ppm) was similar to that of 4 (d=
À10.32 ppm), and downshifted relative to the hydride of 1a
keeping their total concentration constant (1m).
(
d=À11.11 ppm; Figure S5 in the Supporting Information).
complexes. For complexes 2a and 4, with OH groups at the
ortho position, the initial TOF increased with increasing pH of
With this understanding of the overall reaction process, we
performed a deuterium KIE study with complexes 1a, 2a, 2b,
and 4 to investigate how the proton concentration, formate
concentration, and pendent base affected the reaction rate.
the solution by increasing the HCO Na fraction, and reached
2
a maximum and then decreased (Figure 6). The rate of com-
plex 2a reached a maximum at
pH 4.0, which was close to the
[
18]
pK values of complex 2a and
a
FA consistent with our observa-
tion for the dinuclear complex
[
14]
[
(Cp*IrCl) (THBPM)]Cl . We pro-
2 2
pose that these results are de-
rived from the combined effect
of deprotonation of the OH
groups on the ligands (i.e., elec-
tronic effect) and the deprotona-
tion of FA with increasing pH of
the solution. The deprotonation
of the OH group generates
much stronger electron-donat-
Figure 7. Proposed mechanism and different rate-determining steps for complexes with and without OH groups
À
ing O at a relatively higher pH in the ortho positions in FA dehydrogenation.
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[
a]
Table 2. KIE in the dehydrogenation of FA using Ir complexes.
[
b]
[c]
[c]
Entry
Substrate/
Solvent
1a
]
2a
]
2a
]
2b
]
4
[
d]
À1
[e]
[d]
À1
[e]
[d]
À1
[e]
[d]
À1
[e]
[d]
À1
[e]
TOF [h
KIE
TOF [h
KIE
–
TOF [h
KIE
TOF [h
KIE
–
TOF [h
]
KIE
–
1
2
3
4
HCO
HCO
DCO
DCO
2
2
2
2
H/H
H/D
D/H
D/D
2
O
O
2400
1140
1660
940
–
2160
1610
1100
905
5400
4300
2970
2560
–
2030
1130
1340
818
12200
7840
5350
4230
2
2.1
1.4
2.6
1.3
2.0
2.4
1.2
1.8
2.1
1.8
1.5
2.5
1.6
2.3
2.9
2
O
O
2
[
a] Reaction conditions: solution of FA (1m, 10 mL), catalyst (2 mmol), 608C. [b] Data from Ref. [21]. [c] The substrates were HCO
DCO D/DCO Na (1:1); catalyst loading: 1 mmol. [d] Average TOF over the initial 10 min. The errors were less than 5%. [e] KIE=TOF(entry 1)/TOF(entry n)
n=2, 3, and 4).
2 2
H/HCO Na (1:1) and
2
2
(
The results are summarized in Table 2. When the substrates or
solvents were replaced with deuterated reagents, the reaction
rate decreased considerably. For complex 2a, the KIE values
are similar for reactions at pH 1.8 (1m HCO H) and pH 3.5 (1m
2
HCO H/HCO Na (1:1)); this indicates that the rate-determining
2
2
step is not altered upon changing the pH of the solution from
.8 to 3.5. The KIE values are about two when DCO D replaces
1
2
Scheme 3. Hydrogen generation assisted by the pendent base and hydroni-
um through a proton relay in Step III of FA dehydrogenation (pH 3.5) with
complex 2a.
HCO H or NaCO D/DCO D (1:1) is used instead of NaCO H/
2
2
2
2
HCO H (1:1), whereas the KIE values are about one when D O
2
2
replaces H O (2a; Table 2, entries 2 and 3). The KIE experiments
2
with 2a and 4 suggest that the deuterated formate substrate
influences the reaction rate to a greater extent than the deu-
terated solvent; this indicates that deuterated formate is in-
action, under acidic conditions, a water molecule (in the form
of hydronium) and a hydroxyl group at the ortho position (i.e.,
complexes 2a and 4) can also form a proton relay and assist
the reaction of [Ir]ÀH with a proton (Scheme 3). The proton
volved in the rate-limiting step rather than deuterated solvent
+
(
D O) or D . Accordingly, we propose that, for complexes 2a
2
and 4, the generation of [Ir]ÀH from the formato complex
relay stabilizes the [Ir]ÀH transition state and lowers the
2
(
Step II, Figure 7) should be rate determining rather than H re-
energy barrier for generating H . Consequently, b-hydrogen
2
2
+
lease from the reaction of [Ir]ÀH with H . This is consistent
elimination of the formato complex (Step II, Figure 7) becomes
the rate-limiting step for the complexes with OH groups at the
ortho positions. In contrast, proton nonresponsive complexes
and complex 1a, without a pendent base, have a relatively
higher energy barrier for generating H2 from [Ir]ÀH and
a proton (Step III, Figure 7), which remains rate limiting.
[
18]
with the DFT calculations we previously reported. Therefore,
increasing the pH of the solution from two to four could in-
crease the reaction rate (complexes 2a and 4; Figure 6) be-
cause it could facilitate the formation of the formato inter-
mediate (B) and enhance the rate in a manner that was consis-
[
10,11]
tent with that reported by Fukuzumi et al.
The increased
In addition, we have analyzed the released gases in the KIE
study by GC (Table S1 in the Supporting Information). We
reaction rate may be partially due to deprotonation of the OH
groups on the ligand and consequent increased electron-do-
nating ability with increasing solution pH, as discussed above.
found that, if deuterated solvent (i.e., D O) was used instead of
2
H O, the main fraction of hydrogen generated was D (>86%;
2
2
[
21]
In contrast, KIE experiments of 1a and 2b with D O in
Table S1, entry 2, and Figure S6a in the Supporting Informa-
tion). If H O was used, the main fraction changed to H
2
place of H O lead to higher KIE values than those with DCO D
2
2
2
2
instead of HCO H (Table 2, entry 2 vs. entry 3). Therefore, D O
(>83%; Table S1, entry 3, in the Supporting Information), even
when using the deuterated substrate. When a mixture of FA
and formate were used as substrates, similar results were ob-
tained (Table S1 in the Supporting Information). This indicates
that complex 2a undergoes fast H/D exchange with solvent in
2
2
+
or D is most likely to be involved in the rate-determining
step. The reaction of [Ir]ÀH (C) with a proton to release dihy-
drogen (Step III; Figure 7) is proposed to be rate limiting for
complexes 1a and 2b. Thus, a high proton concentration (low
pH) will lead to high reaction rates; the reaction rate will de-
crease with increasing pH of the reaction solution, which is
consistent with the pH dependence observed (complexes 1a
and 2b; Figure 6).
[25]
the reaction. When deuterated solvent and substrate were
used, a high purity of D was obtained (up to 99%; Table S1,
2
entry 4, and Figure S6b in the Supporting Information). Com-
paring these data with our previously reported results for com-
[21]
The principal difference between complexes with 4DHBP
plex 1a, we find that the compositions of the generated
gases are almost identical (1a vs. 2a; Table S1, entries 2 and 3,
in the Supporting Information). This result suggests that the H/
D exchange process is faster than any other steps in this reac-
(
1a) and 6DHBP (2a) is the position of the OH groups. We
have demonstrated by experimental and computational meth-
ods that hydroxyl groups at ortho positions lead to a proton
relay incorporating a molecule of H O that facilitates the heter-
tion, such as H heterolysis or b-hydrogen elimination, and is
2
2
[
18,19]
olysis of H in CO hydrogenation.
Thus, in the reverse re-
not affected by the pendent base or proton relay.
2
2
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Synthesis of 4,4’,6,6’-tetramethoxy-2,2’-bipyridine: 4,4’,6,6’-Tetrame-
thoxy-2,2’-bipyridine was synthesized from 2-chloro-4,6-dimethoxy-
Conclusions
[27]
pyridine according to the reported procedure. The residue was
recrystallized from isopropanol to give the product as a white
Highly efficient hydrogen generation from dehydrogenation of
FA has been achieved by using bioinspired iridium complexes
1
solid. It was dried and used directly for next reaction. H NMR
2
a and 4. In particular, complex 4 gave the high TOF of
(
4
CDCl , 400 MHz): d=7.63 (d, J=2 Hz, 2H), 6.22 (d, J=2 Hz, 2H),
3
À1
39500 h at 808C. This indicates that bioinspired complexes
13
.01 (s, 6H), 3.88 ppm (s, 6H); C NMR (CDCl , 125 MHz): d=168.7,
3
2
a and 4 are among the best catalysts for FA dehydrogenation
1
65.2, 154.0, 103.2, 94.0, 55.4, 53.5 ppm.
in water. More importantly, these catalysts can produce high-
pressure hydrogen without CO contamination, which indicates
that these bioinspired iridium complexes are potentially useful
Synthesis of 4,4’,6,6’-tetrahydroxy-2,2’-bipyridine (THBP): HBr
9.0 mL, 27 mmol; 48 wt%) was added to a solution of 4,4’,6,6’-tet-
ramethoxy-2,2’-bipyridine (0.25 g, 0.92 mmol) in acetic acid
(15 mL). The mixture was heated at reflux for 72 h. The mixture
was cooled to room temperature and the solvent was removed
under vacuum. The residue was dissolved in 1m NaOH and then
(
in providing H for fuel cells. Our deuterium KIE study clearly
2
indicated a different mechanism for complexes with OH at the
ortho and para positions. The reaction of iridium hydride with
a proton to liberate H2 was the rate-determining step for
neutralized with 1m HCl. The precipitated solid was collected by
1
filtration and dried (0.18 g, 88.8%). H NMR (D O/KOD, 400 MHz):
“
normal” complexes, such as the proton nonresponsive com-
2
1
3
d=6.40 (d, J=1.6 Hz, 2H), 5.56 ppm (d, J=1.6 Hz, 2H); C NMR
D O/KOD, 125 MHz): d=179.1, 169.0, 144.0, 106.1, 99.9 ppm.
plexes (i.e., 2b) and 4DHBP complex 1a. In contrast, this step
is not rate limiting for complexes with OH at ortho positions,
such as 2a, 3, and 4, owing to the proton relay (i.e., pendent-
base effect), which lowers the energy barrier for generation of
(
2
Synthesis of 3: THBP (88.1 mg, 0.4 mmol) was added to an aque-
ous solution of [Ir(Cp*)(H O) ]SO (20 mL, 198.8 mg, 0.4 mmol)
2
3
4
under an argon atmosphere. After being stirred at room tempera-
ture for 12 h, the precipitated solid was collected by filtration,
H . Instead, b-hydrogen elimination was found to be rate limit-
2
ing for complexes with OH groups at ortho positions. The ap-
parently different pH dependences were also rationalized by
considering the different rate-determining steps. This mecha-
nistic insight provides important information that can help to
develop an effective hydrogen release system and more effi-
cient biomimetic catalysts for the dehydrogenation of FA.
washed with cold water, and dried under vacuum at 508C for 12 h
1
to give 3 as a yellow solid (158 mg, 59.7%). H NMR (D O/NaOD,
2
4
1
00 MHz): d=6.36 (d, J=2.4 Hz, 2H), 5.59 (d, J=2.4 Hz, 2H),
13
.47 ppm (s, 15H); C NMR (D O/KOD, 125 MHz): d=175.9, 172.2,
2
157.0, 103.4, 101.2, 84.4, 9.1 ppm.
Procedure for catalytic dehydrogenation of FA/formate: A freshly
prepared 5 mm solution of catalyst (67–400 mL), except for catalyst
2
c, was added to a deaerated 1m aqueous solution of HCO H/
2
HCO Na (10 mL), and the mixture was stirred at the desired tem-
2
perature. The volume of evolved gas was determined by means of
a gas meter and the generation rate was detected with a film flow
meter. After the reaction was complete, residual FA in the solution
was quantified by HPLC. The TON was calculated based on the cat-
alyst loading and concentration of residual formate. The reactions
were generally performed more than twice and average values of
TOF are presented. The errors are less than 5% under the same
conditions of temperature and atmospheric pressure.
Experimental Section
Materials and methods: All manipulations were performed under
an argon atmosphere by using standard Schlenk techniques or in
a glove box, and all aqueous solutions were degassed prior to use.
1
13
H and C NMR spectra were recorded on Bruker Avance 400 and
00 spectrometers by using tetramethylsilane or sodium 3-(trime-
5
thylsilyl)-1-propanesulfonate as an internal standard. The pH values
were measured on an Orion 3-Star pH meter with a glass electrode
after calibration with standard buffer solutions. The amount of
evolved gas from the dehydrogenation was measured by means of
a wet gas meter (Shinagawa Corp., W-NK-05) and the gas flow rate
was simultaneously detected by a film flow meter (Horiba STEC
Corp., SF-1U). The content of the generated gas was analyzed by
Procedure for catalytic dehydrogenation of FA/formate under pres-
surized conditions: A high-pressure glass cylinder (Hiper Glass Cyl-
inder, Taiatsu Techno Co., 90 mL, max. 3 MPa) equipped with
a backpressure regulator (adjusted to 1 MPa) was charged with 2a
(
(
2.54 mg, 2.5 mmol) and a freshly degassed 1m solution of FA
50 mL). The apparatus was flushed with high-pressure CO , and
2
means of a GL Science GC-390 gas chromatograph. H was detect-
ed by a thermal conductivity detector (TCD) using an activated 60/
2
the mixture was vigorously stirred at 608C. The volume of evolved
gas was measured with a gas meter (Shinagawa Corp., W-NK-05).
After the reaction was complete, residual FA in the solution was
quantified by HPLC.
8
0 carbon column and CO2 and CO were detected by using
a flame ionization detector (FID) equipped with a methanizer using
a Porapak Q 80/100 column at 508C. H , HD, and D gases were an-
2
2
alyzed with a TCD (1508C, 60 mA) on a GL Science GC-390 gas
chromatograph (Ne carrier, Hydro Isopack (2.0 m, 4.0 mm i.d., GTR
TEC Co., Ltd.) at 77 K (liquid N )). Formate concentrations were
2
monitored by means of HPLC on an anion-exclusion column
+
Acknowledgements
(
Tosoh TSKgel SCX(H )) with an aqueous solution of H PO (20 mm)
3
4
as the eluent and a UV detector (l=210 nm). Water used in the re-
actions was obtained from a Simplicity water purification system
Y.H. and W.-H.W. thank the Japan Science and Technology
Agency (JST), ACT-C for financial support. The work at BNL was
performed under contract DE-AC02–98CH10886 with the U.S. De-
partment of Energy and supported by its Division of Chemical
Sciences, Geosciences & Biosciences, Office of Basic Energy
Sciences.
(
Millipore).
Synthesis of 2-chloro-4,6-dimethoxypyridine: 2-Chloro-4,6-dime-
thoxypyridine was prepared from 2,4,6-trichloropyridine according
[26] 1
to the reported procedure.
H NMR (CDCl , 400 MHz): d=6.51 (d,
3
J=2 Hz, 1H), 6.12 (d, J=2 Hz, 1H), 3.91 (s, 3H), 3.81 ppm (s, 3H).
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[
14] J. F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda,
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[
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Published online on && &&, 0000
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The key controls the mechanism:
A deuterium kinetic isotope effect (KIE)
study clearly indicates a different mech-
anism for complexes with OH at differ-
ent positions of the ligands. The rate-
limiting step is b-hydrogen elimination
from the iridium–formate intermediate
for complexes with OH at ortho posi-
tions owing to a proton relay (i.e.,
pendent-base effect), which lowers the
energy barrier of generation of H2.
W.-H. Wang, S. Xu, Y. Manaka, Y. Suna,
H. Kambayashi, J. T. Muckerman, E. Fujita,
Y. Himeda*
&& – &&
Formic Acid Dehydrogenation with
Bioinspired Iridium Complexes: A
Kinetic Isotope Effect Study and
Mechanistic Insight
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