Unusually large tunneling effect on highly efficient generation of hydrogen and hydrogen isotopes in pH-selective decomposition of formic acid catalyzed by a heterodinuclear iridium-ruthenium complex in water

Article abstract of DOI:10.1021/ja910349w

(Chemical Equation Presented) A heterodinuclear iridium-ruthenium complex [Ir^III(Cp*)(H₂O)(bpm)Ru^II(bpy) ₂](SO₄)₂ {1(SO₄)₂, Cp* = η⁵-pentamethylcyclopentadienyl, bpm = 2,2′-bipyrimidine, bpy = 2,2′-bipyridine} acts as the most effective catalyst for selective production of hydrogen from formic acid in an aqueous solution at ambient temperature among catalysts reported so far. An unusually large tunneling effect was observed for the first time for the catalytic hydrogen production in H₂O vs D₂O.

Full text of DOI:10.1021/ja910349w

Published on Web 01/19/2010
Unusually Large Tunneling Effect on Highly Efficient Generation of Hydrogen
and Hydrogen Isotopes in pH-Selective Decomposition of Formic Acid
Catalyzed by a Heterodinuclear Iridium-Ruthenium Complex in Water
Shunichi Fukuzumi,* Takeshi Kobayashi, and Tomoyoshi Suenobu
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan
Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan
Received December 8, 2009; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Hydrogen has attracted much attention as an alternative clean
energy, because the reaction of hydrogen with oxygen produces
the requested energy and only water as an environmentally friendly
1
,2
side product. However, the storage and transfer of hydrogen are
difficult with current technology, because hydrogen gas is explosive
3
with a poor volumetric energy density. In this context, the
2 2
interconversion of H with CO to formic acid (HCOOH) (eq 1) in
4
the presence of an appropriate catalyst has merited special attention.
Although there have been a number of reports on the formation of
H +CO a HCOOH
(1)
2
2
Figure 1. pH Dependence of the H
decomposition of HCOOH/HCOONa (0.83 M) (black circles) and DCOOH/
DCOONa (0.83 M) (red circles) catalyzed by 1(SO (0.5 mM) in deaerated
O at 298 K.
2
and HD evolution rates (TOFs) in the
5
,6
H
2
by the catalytic decomposition of formic acid, improvement
in the catalytic activity is highly desired.
On the other hand, an H/D exchange between HCOOH and D O
2
4 2
)
H
2
would lead to production of D
as research tools in the various fields of science.
large deuterium kinetic isotope effects (KIEs) were recently reported
2
and HD, which have been utilized
7
–10
Abnormally
1
1
in protonolysis/deuterolysis of metal-alkyl complexes. Such
hydrogen tunneling has frequently been observed for C-H bond
1
2,13
activation by transition metal complexes.
However, there has
been no report on hydrogen tunneling for the catalytic production
of hydrogen.
We report herein a large tunneling effect on the highly efficient
production of hydrogen for the first time in the pH-selective
decomposition of formic acid (HCOOH, DCOOH, or DCOOD),
which is catalyzed by a heterodinuclear iridium-ruthenium complex
Figure 2. Arrhenius plots of TOF for the decomposition of HCOOH/
HCOONa (3.1 M) catalyzed by 1(SO ) (0.3 mM) in deaerated H O (black
circles) or D O (red circles) at pH 3.8 or pD 3.8, respectively.
2
III
II
5
4
2
2
[
Ir (Cp*)(H
tamethylcyclopentadienyl, bpm ) 2,2′-bipyrimidine, bpy ) 2,2′-
bipyridine} in H O and D O.
The catalyst 1(SO was synthesized quantitatively by a reaction
2 2 4 2 4 2
O)(bpm)Ru (bpy) ](SO ) {1(SO ) , Cp* ) η -pen-
2
2
knowledge, the value is the highest in the catalytic decomposition
of formic acid in H O at ambient temperature. The decrease in TOF
at pH > 3.8 indicates that the rate-determining step is the reaction
)
4 2
2
II
III
of [Ru (bpy)
98 K (see Experimental Section and S1 in the Supporting
Information (SI)).
Hydrogen was produced efficiently when 1(SO
2 4 2 3 4 2
(bpm)](SO ) with [Ir (Cp*)(H O) ](SO ) in H O at
III
II
3+
2
of the hydride complex [Ir (Cp*)(H)(bpm)Ru (bpy)
2
+
]
(3), pro-
duced by ꢀ-hydrogen elimination from 2, with H
When HCOOH/HCOONa was replaced by DCOOH/DCOONa
in H O, deuterium kinetic isotope effects (KIE ) 2.0 at pH 3.8
3
O .
4
)
2
was added to
was formed
1
4
a deaerated aqueous solution of HCOOH. CO
together with H
that HCOOH decomposes selectively to H
formation of CO and H O. The rate of hydrogen production
increases linearly with increasing [1(SO ] (S2). The turnover
2
2
2
, but no CO was detected by GC. This suggests
and CO without
and 4.9) were observed as shown in Figure 1 (red circles). The
KIE value increases with decreasing pH, KIE ) 9.7 at pH 2.9 (S4).
2
2
2
This indicates that a rate-determining step may be changed from
+
4
)
2
the reaction of 3 with H
3
O
at higher pH (e.g., pH 4.9) to the
frequency (TOF) gradually increases with increasing [HCOOH/
HCOONa] (total concentration of formate in the mixed solution of
ꢀ-hydrogen elimination step to afford 3 accompanied by CO
evolution at lower pH (e.g., pH 2.9).
2
HCOOH with HCOONa) to approach a limiting value at pH 3.8
The temperature dependence of the TOF value was examined,
and the Arrhenius plot in Figure 2 (black circles) afforded the
-
(
S3). Such a saturation dependence of TOF on [HCOO ] indicates
-1
that hydrogen is produced via the formate complex
activation energy of 17 kcal mol . This value is much smaller
than the activation energy of the decomposition of HCOOH without
III
II
3+
[
Ir (Cp*)(O(CO)H)(bpm)Ru (bpy)
2
]
(2). The pH dependence of
-1
17
the limiting value of TOF is shown in Figure 1 (black circles),
catalyst (78 kcal mol ).
When the decomposition of HCOOH/HCOONa was performed
with 1(SO in D O, HD was mainly formed together with D at
-1
15
where the maximum TOF value (426 h ) is obtained at pH 3.8
1
6
which agrees with the pK
a
value of HCOOH. To the best of our
4
)
2
2
2
1
496 9 J. AM. CHEM. SOC. 2010, 132, 1496–1497
10.1021/ja910349w  2010 American Chemical Society

C O M M U N I C A T I O N S
) was virtually the same as the rate of D
formation in the decomposition of DCOOD/DCOONa catalyzed
by 1(SO in D O (Table 1, entries 2 and 4). On the other hand,
a KIE value in the decomposition of DCOOH/DCOONa catalyzed
by 1(SO in H O is significantly smaller than the KIE values in
the decomposition of HCOOD/HCOONa and DCOOD/DCOONa
in D O (Table 1, entries 2-4). These indicate that the rate-
Scheme 1
production (HD and D
2
2
4
)
2
2
)
4 2
2
2
determining step in the overall hydrogen evolution reaction is not
+
the ꢀ-hydrogen elimination step but the reaction of 3 with H
to evolve H at pH 3.8.
3
O
2
In summary, a heterodinuclear iridium-ruthenium complex 1
acts as the most effective catalyst for selective production of
hydrogen from formic acid in an aqueous solution among catalysts
reported so far. The unusually large tunneling effect was observed
for the first time for the catalytic hydrogen production in H
2
O vs
Table 1. Rates of Hydrogen Evolution (v), KIEs, and Isotope
D
2
O. The selective formation of HD was also achieved by adjusting
Distribution for Evolved Hydrogen Gas in Decomposition of Formic
pH, providing a convenient way to produce HD, which would
otherwise be quite difficult or expensive to obtain.
a
4 2 2 2
Acid Catalyzed by 1(SO ) in H O or D O
c
fraction, mol %
v, µmol
min
Acknowledgment. This work was supported by a Grant-in-Aid
from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan, and KOSEF/MEST through WCU Project R31-
-
1
b
KIE
entry
substrate
solvent
H
2
HD D
2
1
2
3
4
HCOOH
HCOOD
DCOOH
DCOOD
H
D
H
D
2
O
2
O
2
O
2
O
4.8
0.12
3.2
-
100
3
29
0
0
57
71
5
0
40
0
d
40
2
008-000-10010-0.
1.5
40
e
0.12
95
Supporting Information Available: Experimental procedures,
a
figures, and tables (S1-S9). This material is available free of charge
via the Internet at http://pubs.acs.org.
4 2
Reaction conditions: 1(SO ) (0.3 mM) and formic acid (HCOOH/
HCOONa, HCOOD/HCOONa, DCOOH/DCOONa, or DCOOD/
DCOONa) (3.1 M) in deaerated H O or D O at 298 K at pH or pD 3.8.
2
2
b
c
KIE ) V (entry 1)/V (entry n) (n ) 2, 3, and 4). Analyzed by GC.
Commercially available HCOOD contains HCOOH in ca. 2%.
Commercially available DCOOD contains HCOOH, HCOOD, and/or
d
e
References
(
1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729.
DCOOH in ca. 2%.
(2) Bockris, J. O’M. Int. J. Hydrogen Energy 2008, 33, 2129.
(
(
(
3) (a) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670.
2
various pD (S5). The ratio of generated D and HD was constant
(b) Schlapbach, L.; Z u¨ ttel, A. Nature 2001, 414, 353.
4) (a) Fukuzumi, S. Eur. J. Inorg. Chem. 2008, 1351. (b) Enthaler, S.
ChemSusChem 2008, 1, 801. (c) Jo o´ , F. ChemSusChem 2008, 1, 805.
5) (a) Fellay, C.; Dyson, P. J.; Laurenczy, G. Angew. Chem., Int. Ed. 2008,
47, 3966. (b) Loges, B.; Boddien, A.; Junge, H.; Beller, M. Angew. Chem.,
Int. Ed. 2008, 47, 3962. (c) Boddien, A.; Loges, B.; Junge, H.; Beller, M.
ChemSusChem 2008, 1, 751. (d) Fellay, C.; Yan, N.; Dyson, P. J.;
Laurenczy, G. Chem.sEur. J. 2009, 15, 3752. (e) Wiener, H.; Sasson, Y.
J. Mol. Catal. 1986, 35, 277. (f) Gao, Y.; Kuncheria, J. K.; Jenkins, H. A.;
Puddephatt, R. J.; Yap, G. P. A. J. Chem. Soc., Dalton Trans. 2000, 3212.
during the catalytic reaction (S6). The formation of D
2
results from
+
H/D exchange of 3 with D
3
O . The proposed mechanism of the
2
catalytic formation of HD and D in the decomposition of HCOOH
with 1-d
Ir-D complex (3-d
2
is summarized in Scheme 1. The H/D exchange of the
+
1
) with H
3
O occurs via the dissociation of the
I
Ir-D complex with
H
2
O
to produce the Ir complex
I
II
]2+ (4) and H
+
[
Ir (Cp*)(bpm)Ru (bpy)
2
2
DO . At high pH, 3 is indeed
(6) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. ChemSusChem 2008, 1, 827.
(7) Loery, T.; Richardson, K. S. Mechanism and Theory in Organic Chemistry;
Harper & Row: New York, 1987.
I
converted to 4 (S7). The hydride complex 3 and Ir complex 4 are
well characterized by the 1H NMR spectra (see Experimental
(8) (a) Ogo, S.; Kabe, R.; Uehara, K.; Kure, B.; Nishimura, T.; Menon, S. C.;
Harada, R.; Fukuzumi, S.; Higuchi, Y.; Ohhara, T.; Tamada, T.; Kuroki,
R. Science 2007, 316, 585. (b) Kure, B.; Matsumoto, T.; Ichikawa, K.;
Fukuzumi, S.; Higuchi, Y.; Yagi, T.; Ogo, S. Dalton Trans. 2008, 4747.
a
Section in SI and S8). The pK value of 3 was determined to be
3
.9 by the pH titration (S9).
The presence or absence of tunneling can be determined from
(9) Inorganic Isotopic Synthesis: Herber, R. H., Ed.; Benjamin: New York,
1
962.
1
1,12,18
the temperature dependence of the KIE.
Arrhenius parameters A
mol together with a large KIE value at 298 K (KIE > 9) are
generally taken to unambiguously demonstrate the involvement of
Values for the
(H) > 1.2 kcal
(10) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155. (b)
Kubas, G. J. Chem. ReV. 2007, 107, 4152. (c) Yarnell, A. T. Chem. Eng.
News 2009, 87, 36.
H
/A
D
, 1 and E
a
(D) - E
a
-1
(
11) Bercaw, J. E.; Chen, G. S.; Labinger, J. A.; Lin, B.-L. J. Am. Chem. Soc.
008, 130, 17654.
(12) Pan, Z.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2008, 130, 7776.
2
11–13,18
tunneling.
The Arrhenius plots for TOF in D
2
O (red circles)
(
13) Kohen, A.; Klinman, J. P. Acc. Chem. Res. 1998, 31, 397.
vs H
2
O (black circles) in Figure 2 afford A(H
2
O)/A(D
2
O) ) 3.1 ×
(14) The presence of oxygen retarded hydrogen production, because the catalyst
is used for the reduction of oxygen until oxygen is consumed.
-
5
-1
10 , E
a
(D) - E (H) ) 8.2 kcal mol , and an unusually large KIE
a
(
15) The value was derived from TON () 142) for 20 min. A mononuclear
2+
value at 298 K (KIE ) 40), which clearly indicate a tunneling
pathway.
2
iridium complex [Ir(Cp*)(bpm)(H O)] exhibits negligibly lower TOF
1
value (0.19 h- ) than 1 in the decomposition of HCOOH/HCOONa under
otherwise the same experimental conditions at pH 3.8.
The production of hydrogen from the reaction of 3 with H
3
O+
(16) Handbook of Chemistry and Physics, 84th ed.; Lide, D. R., Ed.; CRC Press:
+
Boca Raton, FL, 2004.
involves the H-O bond cleavage in H
3
O , accompanied by the
(17) Ruelle, P.; Kesselring, U. W.; Nam-Tran, H. J. Am. Chem. Soc. 1986, 108,
formation of H-H bond with the hydride of 3.
When the decomposition of HCOOH/HCOONa was performed
with 1(SO in D O at pD 3.8 at 298 K, the rate of hydrogen
371.
(18) Kwart, H. Acc. Chem. Res. 1982, 15, 401.
4
)
2
2
JA910349W
J. AM. CHEM. SOC. 9 VOL. 132, NO. 5, 2010 1497