Formic acid acting as an efficient oxygen scavenger in four-electron reduction of oxygen catalyzed by a heterodinuclear iridium-ruthenium complex in water

Article abstract of DOI:10.1021/ja104486h

A heterodinuclear iridium-ruthenium complex [Ir^III(Cp*) (H₂O)(bpm)Ru^II(bpy)₂](SO₄) ₂ (Cp* = φ⁵-pentamethyl-cyclopentadienyl, bpm = 2,2′-bipyrimidine, bpy = 2,2′-bipyridine) acts as an effective catalyst for removal of dissolved O₂ by the four-electron reduction of O₂ with formic acid in water at an ambient temperature.

Full text of DOI:10.1021/ja104486h

Published on Web 08/05/2010
Formic Acid Acting as an Efficient Oxygen Scavenger in Four-Electron
Reduction of Oxygen 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, Suita,
Osaka 565-0871, Japan and Department of Bioinspired Science, Ewha Womans UniVersity, Seoul 120-750, Korea
Received May 24, 2010; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Abstract: A heterodinuclear iridium-ruthenium complex
III
II
5
[
Ir (Cp*)(H
cyclopentadienyl, bpm ) 2,2′-bipyrimidine, bpy ) 2,2′-bipyridine)
acts as an effective catalyst for removal of dissolved O by the
with formic acid in water at an
2
O)(bpm)Ru (bpy)
2
](SO
4
)
2
(Cp* ) η -pentamethyl-
2
four-electron reduction of O
ambient temperature.
2
2
Oxygen (O ) dissolved in water is known to cause the
1
corrosion of components in a large variety of systems. Thus,
extensive efforts have been devoted to develop methods to
2
-5
the four-electron reduction of O
(eq 3).
2
by 2 occurs efficiently to produce
substantially reduce the oxygen content in water.
Although
1
the principles of vacuum degasification and thermal deaeration
are simple, these methods require a lot of energy for operation.
+
3
+ 1/2 O + 2H f 1
(3)
Alternatively, dissolved O
hydrazine and hydrogen.
2
-5
can be removed by reduction with
However, the reductive removal of
2
2
The time profile of the reaction of 3 with O
2
was monitored by
oxygen requires a highly active catalyst to obtain a sufficient
reduction rate for the removal of dissolved O in water at an
a stopped flow technique. During the conversion from 3 to 1, two
reaction intermediates were observed as transient absorption spectra
as shown in Figure 2. First, 3 (red: λmax ) 512 nm) is converted to
an intermediate A (blue: λmax ) 420 and 540 nm) at 0.05 s, and
then A is changed to another intermediate B (green: λmin ) 410
and 590 nm) at 2.3 s. B is slowly changed to 1, which corresponds
to the black line that is the difference spectrum obtained by
subtracting the final spectrum (red line in Figure 1) from the
spectrum observed at 10 s. In such a case, B is observed as a
negative absorption spectrum (green) in Figure 2.
The rate of the first step to produce A is too fast to determine
2
the accurate rate of the reaction of 3 with O even at low
temperature (e.g., 275 K). The rate of the second step to produce
B remains unchanged irrespective of pH (> 4.2) (see Supporting
Information Figure S1). The rate of the final step to produce 1
increases with increasing concentration of HCOO- (Figure S2).
2
ambient temperature, because it is practically impossible to
enhance the reduction rate by heating the vast amount of
2
-5
water.
soluble catalyst to remove dissolved O
There has so far been no report on an efficient water-
2
in water by an
appropriate reductant at an ambient temperature.
We report herein a heterodinuclear iridium-ruthenium complex
III
II
5
[
Ir (Cp*)(H
cyclopentadienyl, bpm ) 2,2′-bipyrimidine, bpy ) 2,2′-bipyridine)
acting as an effective catalyst for removal of dissolved O by the
four-electron reduction of O with formic acid in water at an ambient
temperature. This is followed by the catalytic decomposition of
2 2 4 2
O)(bpm)Ru (bpy) ](SO ) (1, Cp* ) η -pentamethyl-
2
2
6
2
formic acid to hydrogen and CO . The catalytic mechanism of
the four-electron reduction of oxygen with formic acid is examined
by monitoring the reaction intermediates. The present method
provides a convenient way to remove dissolved O
completely at an ambient temperature.
2
in water
The catalyst
1
was synthesized by
a
reaction of
) in H O at 298
II
III
[
2
Ru (bpy) (bpm)](SO
4
2 3
) with [Ir (Cp*)(H O) ](SO
4
2
6
K as reported previously. When 1 equiv of 1 was added to a
deaerated aqueous solution of HCOOH at pH 5.2, where HCOOH
-
-
is deprotonated to form HCOO (pK
a
) 3.8), 1 reacts with HCOO
III
II
3+
to afford the hydride complex [Ir (Cp*)(H)(bpm)Ru (bpy)
2
]
(2)
6
and CO
to afford the Ir complex [Ir (Cp*)(bpm)Ru (bpy)
because the pK value (3.9) is much lower than pH 5.2. Both 2
and 3 were well characterized by the H NMR spectra.
2
(eq 1). The hydride complex 2 is deprotonated at pH 5.2
I
I
II
2+
2
]
(3) (eq 2),
6
a
1
6
I
The Ir complex (3) reacts efficiently with O
2
as shown by the
Figure 1. UV-vis-NIR absorption spectral changes in the reaction of 3
spectral titration in Figure 1, where the absorption spectra due to
2 2
with various concentrations of O in H O (pH 5.2) at 298 K. 3 is produced
3
are changed to those of 1. The titration curve in the inset of Figure
indicates that 3 reacts with 0.5 equiv of O to produce 1. Thus,
by the reaction of 1 (1.0 mM) with HCOOH/HCOONa (1.0 mM). Inset:
Plot of absorbance at λ ) 512 nm due to 3 vs the ratio of [O ]/[3].
1
2
2
1
1866 9 J. AM. CHEM. SOC. 2010, 132, 11866–11867
10.1021/ja104486h  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. UV-vis difference absorption spectral changes observed in the
reduction of O (0.13 mM) by 3 (red line, 0.10 mM) in the presence of
HCOOH/HCOONa (0.10 M) in H O at pH 5.2 at 298 K.
2
2
Figure 3. Time course of the turnover number (TON) of the H
in the decomposition of HCOOH/HCOONa (0.83 M) catalyzed by 1 (0.5
mM) in deaerated (freeze-pump-thaw cycles, 5 times) H O (O) and in
aerated H O (b) at pH 3.6 at 298 K.
2
evolution
Scheme 1
2
2
by freeze-pump-thaw cycles (5 times), no induction period was
observed as shown in Figure 3 (O). O dissolved in the reaction solution
and in the head space (total 6.0 µmol) was completely eliminated (TON
5.0, [O ] < 1 ppb in solution below the detection limit of an oxygen
electrode: YSI Model 5300A) after 150 min in the decomposition of
formic acid in aerated H O. Essentially the same slope of the plots in
Figure 3 after O removal implies the robustness of the complex 1 in
the catalytic O reduction.
In summary, we have achieved the efficient four-electron
reduction of O with formic acid in water at ambient temperature
2
)
2
2
2
2
2
by using complex 1, which is the most effective catalyst for
hydrogen generation in the decomposition of formic acid in water
6
Judging from such kinetic behaviors of the intermediates, three well-
resolved steps for the reaction of 3 with O are proposed as given
by eqs 4-6. First, 3 reacts with O to give the peroxo complex
A) (eq 4). The O-O bond of the peroxo complex (A) is
at room temperature. The complete deoxygenation of water ([O
2
]
< 1 ppb) has been achieved successfully. The four-electron reduction
of O is suggested to proceed via an iridium(III)-peroxo complex
and an iridium(V)-oxo complex, both of which are regarded as the
2
2
2
(
9
heterolytically cleaved to produce the Ir(V)-oxo complex (B)
key intermediates in Ir catalyzed water oxidation.
+
irrespective of the concentration of H at a pH higher than 4.2 (eq
Acknowledgment. This work was supported by a Grant-in-Aid
Nos. 20108010 and 21550061) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan, and by KOSEF/
MEST through WCU Project R31-2008-000-10010-0.
-
5
). B reacts with HCOO to yield 1 and CO
2
(eq 6). The formation
(
of an iridium-peroxo complex by the reaction of a low-valent
7
2
iridium complex with O has previously been reported. The
formation of an iridium(V)-oxo complex with cleavage of the O-O
8
bond of an iridium(III)-peroxo complex has also been reported.
Supporting Information Available: Experimental procedures,
figures (S1-S3). This material is available free of charge via the Internet
at http://pubs.acs.org.
In addition, the observed spectrum (green line in Figure 2) agrees
with the authentic spectrum of an iridium(V)-oxo complex (B) (λmin
)
410 nm, 590 nm) generated by the addition of peracetic acid
(
CH C(O)OOH) to a water solution of complex 1 (Figure S3).
3
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HCOO (eq 1), deprotonates to generate the low-valent complex 3.
-
(6) The catalytic decomposition of formic acid to hydrogen and CO with 1 has
by HCOO with 1 is shown in Scheme 1.
2
2
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2
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-
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I
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2
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during an induction period observed for the hydrogen evolution reaction
in the decomposition of formic acid catalyzed by 1 in aerated water
(
(
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2
(9) The more definitive assignment of the intermediates is now in progress.
2
as shown in Figure 3 (b). When O in water was completely removed
JA104486H
J. AM. CHEM. SOC. 9 VOL. 132, NO. 34, 2010 11867