Catalytic O2 evolution from water induced by adsorption of [(OH2)(Terpy)Mn(μ-O)2Mn(Terpy)(OH2)] 3+ complex onto clay compounds

Article abstract of DOI:10.1021/ja039780c

Water oxidation to evolve O₂ in photosynthesis is catalyzed by an enzyme whose active site contains a μ-oxo-bridged manganese core. Catalytic O₂ evolution has been difficult to establish by manganese-oxo complexes in homogeneous aqueous solutions. The reaction of [(OH₂)(terpy)Mn^III(μ-O)₂Mn^IV(terpy)(OH₂)]³⁺ (terpy = 2,2′:6′,2″-terpyridine) (1) with a Ce^IV oxidant leads to the decomposition of 1 to the permanganate ion without O₂ evolution in an aqueous solution but catalytically produces O₂ from water when 1 is adsorbed on clay compounds. ¹⁸O-labeling experiments showed that the oxygen atoms in O₂ originate exclusively from water. Catalysis of O₂ evolution requires cooperation of 2 equiv of 1 adsorbed on clay compounds. Copyright

Full text of DOI:10.1021/ja039780c

Published on Web 06/11/2004
Catalytic O2 Evolution from Water Induced by Adsorption of
[(OH2)(Terpy)Mn(µ-O)2Mn(Terpy)(OH2)]3+ Complex onto Clay Compounds
Masayuki Yagi* and Komei Narita
Faculty of Education and Human Sciences, and Center for Transdisciplinary Research, Niigata UniVersity,
8050 Ikarashi-2, Niigata 950-2181, Japan
Received November 25, 2003; E-mail: yagi@ed.niigata-u.ac.jp
Water oxidation to evolve O2 (eq 1) is an important and funda-
mental chemical reaction in photosynthesis. This reaction is cat-
alyzed by a unique manganese enzyme referred to as oxygen-
evolving complex (OEC), whose active site is comprised of an oxo-
bridged tetramanganese cluster.1-4 Though synthetic manganese-
oxo complexes have guided thoughts on the chemical and electronic
structures of the OEC,1,2 most of the structural models have not
catalyzed water oxidation to evolve O2 in a homogeneous aqueous
solution so far.5,6
2H2O f O2 + 4H+ + 4e-
(1)
Limburg et al. reported O2 evolution from water by the reac-
tion of [(OH2)(terpy)MnIII(µ-O)2MnIV(terpy)(OH2)]3+ (terpy )
2,2′:6′,2′′-terpyridine) (1) with NaClO or KHSO5.7,8 However, the
mechanism of the O2 formation is completely unclear, including
even disproportionation of 2ClO- f O2 + 2Cl-, which is known
to be catalyzed by MnII and other Lewis acids.5 To exclude this
possibility, O2 evolution experiments should be conducted using
oxidizing agents that do not contain any oxygen atoms.
We reported that in water oxidation by [Ru(NH3)5Cl]2+, a
favorable intermolecular interaction is provided for the cooperative
catalysis in a solid polymer matrix relative to a homogeneous solu-
tion to increase the catalytic activity.9 This result encouraged us to
design a heterogeneous catalysis system as a strategy for realizing
catalytic O2 evolution from water by manganese-oxo complexes.
The adsorption of 1 onto clay compounds yielded a successful
functional mimic for OEC. Herein we report that the reaction of 1
with a CeIV oxidant leads to decomposition of 1 to MnO4- without
O2 evolution in an aqueous solution but catalytically produces O2
from water when 1 is adsorbed on clay compounds.
Water oxidation by 1 was investigated in a solution using a CeIV
oxidant. Figure 1a shows the time course of the amount of O2
involved in the aqueous solution containing 0.42 mM 1 and a large
excess of CeIV ion (50 mM) measured by a Clark-type oxygen
electrode. O2 evolution was not observed compared with a blank
experiment without 1 (Figure 1b), nor was O2 evolved using a 10-
fold larger concentration of 1 (5 mM 1, 100mM CeIV). It was
concluded that the reaction of 1 with a CeIV oxidant evolves no
O2,10 which contrasts markedly with the catalytic O2 evolution
reported previously using 1 and either NaClO or KHSO5 as
oxidants.7,8 A detailed re-examination of the proposed O2 evolution
mechanism should be required in the previous system.
Surprisingly, when a comparable amount (0.72 µmol) of 1 as
used in solution was adsorbed onto Kaolin clay, the addition of a
large excess of CeIV ion to its aqueous suspension produced a sig-
nificant amount of O2, as shown in Figure 1c. The control exper-
iment using the same amount of Kaolin clay without 1 did not
exhibit any O2 evolution (Figure 1d). A prolonged experiment over
7 days using a gas chromatograph for O2 detection gave a turnover
number (TN) of 13.5 ( 1.1 (average for three trials) under the
Figure 1. Time courses of the amount of O2 evolved in reaction of 1 and
a 50 mM CeIV oxidant. (a) Aqueous solution of 1 (0.84 µmol; 0.42 mM).
(b) Aqueous solution without 1 for a blank experiment. (c) Aqueous
suspension of Kaolin clay (75 mg) adsorbing 1 (0.72 µmol (0.46 mg); 9.6
µmol (6.2 mg)/clay g). (d) Aqueous suspension of Kaolin clay (75 mg)
without 1 for a control experiment; liquid volume, 2.0 mL; pH ) 1.0.
following conditions: 0.46 µmol of 1/50 mg of clay, 100 mM CeIV,
10 mL. These results evidently reveal that adsorbed 1 catalyzes O2
evolution.
To identify an oxygen atom source for O2 evolution, 18O-labeling
experiments were conducted using H218O. The evolved gas was
analyzed on an electron-impact-ionization mass (EIMS) spectrom-
eter. The experiments in H218O media gave peaks at m/z ) 34 and
36, corresponding to 16O18O and (18O)2, in addition to the peak for
(16O)2 at m/z ) 32, in contrast to no peaks at m/z ) 34 and 36 for
natural abundance water (Figure S1). The content fraction of 18O
atom in O2 evolved is consistent with the 18O content in the water.
(The content fractions are 23 and 42 for 23.8 and 47.5 v/v % H218O,
respectively; see Table S1.) The O2 evolution was thus confirmed
to come exclusively from water.
To define the catalysis by 1 adsorbed on the clay, similar O2
evolution experiments were extended to various manganese species,
including manganese oxides (MnO2 and Mn2O3), Mn2+, Mn3+, and
MnO4 ions in solutions, as well as adsorbed Mn2+, Mn3+, and
-
terpyHnn+ on Kaolin clay. In none of the trials using these species
was O2 evolution detected. These results show the uniqueness of
the catalysis by 1 adsorbed on the clay.
UV-visible diffuse reflectance and X-ray absorption spectro-
scopic measurements were carried out to characterize 1 adsorbed
on clay (Figures S2-S6). The diffuse reflectance spectrum of the
1/clay adsorbate was similar to the absorption spectrum of the
MnIV-MnIV state in water rather than the MnIII-MnIV state. The
oxidation to MnIV-MnIV was supported by Mn K-edge X-ray
absorption near-edge structure (XANES) spectroscopic data in
which the Mn K-edge shifted to a higher energy region by 3.2 eV
at the peak versus that for 1 as a powder. Mn K-edge extended
X-ray absorption fine structure (EXAFS) spectrum of the 1/clay
adsorbate did not show an appreciable change compared with that
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J. AM. CHEM. SOC. 2004, 126, 8084-8085
10.1021/ja039780c CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
for 2/clay, but not at all for the solution. However, the saturated
amount of O2 was 0.51 µmol at 99 h for 2/clay, and the TN (0.63)
of 2 is less than unity. The VO2 for 2 (Figure 2c) is much lower
than that for 1 comparing the same amount of the adsorbed complex
under the conditions employed.13 These results suggest that the
terminal water ligands are involved in the catalysis. It may take
place by intermolecular coupling of MnV ) O that could be formed
by successive oxidation of a terminal water ligand on 1. However,
no evidence for involvement of MnV ) O in the catalysis is
obtained. The adsorption of 1 onto the clay suppressed the
decomposition to form MnO4- and results in a highly concentrated
condition compared with solutions, thus facilitating the cooperative
catalysis to form O2. This could account for the adsorption-induced
catalytic activity.
Figure 2. Plots of initial rate (VO (mol s-1)) of O2 evolution and yield
2
The adsorption of 1 on Montmorillonite MK10 clay also
produced catalytic O2 evolution from water, extending the generality
(ΦMnO ) of MnO4- formation versus the amount of the adsorbed complex
-
4
-
on clay; (a) VO for 1 (closed red circle), (b) ΦMnO for 1 (open red circle),
and (c) VO for22 (closed blue square). Kaolin clay4amount was 75 mg. The
of the adsorption-induced catalytic activity of 1. The plots of vO
2
2
conditions are indicated in Figure 1.
vs the amount of adsorbed 1 on Montmorillonite also provided an
upward curvature (Figure S9). The observed catalytic activity
depends on the degree of the concentration onto clays, and the
intrinsic catalytic activity of 1 is basically the same between both
clays. These results support the cooperative interaction proposed
in the adsorption-induced catalytic activity of 1. The present paper
illustrates that the adsorption of 1 onto a heterogeneous matrix is
required for catalytic O2 evolution from water.
for 1 as a powder. These data could suggest that 1 is autoxidized
to the MnIV-MnIV state on the clay possibly by a silicate layer
without a significant change in its original coordination structure.
Preliminary X-ray absorption spectroscopic data of the 1/clay
adsorbate treated with an CeIV oxidant indicated that the MnIV-
MnIV species exists before and after the catalysis for 30 min, in
contrast to the further oxidized species suggested by the corre-
sponding data for 1 with a CeIV solution.
Acknowledgment. We thank Dr. T. Sato for help with EIMS
experiments and Dr. K. Shimizu for help with X-ray absorption
spectroscopic measurements. Research was supported by Toray
Science and Technology Grant, Nissan Science Foundation, and
Grant for Promotion of Niigata University Research Projects.
The visible absorption spectral change of the aqueous solution
containing 1 and a large excess of CeIV ion was followed to see
what reactions are happening in solution. The spectral change
-
indicated the formation of MnO4 from 1. The kinetic analysis
showed that a bimolecular reaction of 1 is involved in a rate-
determining step of the MnO4 formation (Figures S7 and S8). It
is possibly formed by disproportionation of the high oxidation state.
Supporting Information Available: Detailed experimental pro-
cedures for catalysis, spectroscopic data (UV-vis, XANES, and
EXAFS) of 1/clay adsorbate, kinetic analysis for MnO4 formation,
and VO2 data for 1 or 2/Montmorillonite (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
-
-
-
-
The yield (ΦMnO ) of MnO4 formed during a 30 min reaction
4
was 30-86% (0.05-2 mM 1, 50 mM CeIV, 2.0 mL) in the solution.
For the 1/clay adsorbate, MnO4 formation was also observed in
-
References
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centrifugation. However, ΦMnO (18-32%; 0.15-0.81 µmol 1/75
4
2950.
mg clay, 2.0 mL) is much less than in the homogeneous solution,11
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showing that adsorption of 1 onto the clay significantly suppresses
-
their disproportionation to form MnO4
.
The plots of the initial O2 evolution rates (VO (mol s-1)) vs the
2
amount of 1 adsorbed on the clay gave upward curvature in Figure
2a, showing that the specific O2 evolution rate increases as the
amount of 1 on the clay increases. The kinetic analysis of VO
2
suggests that the predominant O2 evolution is produced by a
bimolecular reaction of adsorbed 1 (see Supporting Information).
Most likely, two molecules of 1 cooperatively catalyze O2 evolution
between complexes adsorbed in close proximity to each other on
(10) A similar experiment using the aqueous solution containing 0.42 mM 1
and 50 mM CeIV at pH ) 3.0 did not yield O2 evolution at all, though 1
is intact at pH ) 3-6 in the solution.
-
the clay. In contrast, ΦMnO decreased with the amount of 1, as
4
-
(11) MnO4 formed in the clay should come out of the anionic clay to the
illustrated in Figure 2b. There could be local adsorption equilibria
-
bulk solution. We confirmed that MnO4 cannot be adsorbed on clay at
-
of 1 at the interface between the clay and liquid phase. MnO4
all.
-
(12) Without the catalytic activity of 1 adsorbed on clay, the ΦMnO could
4
prefers to be formed in the liquid-phase rather than on the clay
increase linearly with the adsorption amount of 1 by the increased
-
since the MnO4 formation is suppressed by the adsorption of 1
-
bimolecular decomposition of 1. The ΦMnO decrease suggests that the
4
-
O2 evolution is correlated to MnO4 formation. This could be explained
on the clay (vide supra). As the amount of adsorbed 1 increases,
the fraction of 1 subject to MnO4- formation decreases in compe-
by the O2 evolution by adsorbed 1 that competes with the bimolecular
-
decomposition to form MnO4
.
-
tition with facilitated bimolecular O2 evolution.12 The MnO4
(13) VO increased linearly with the amount of 2 on Kaolin (Figure 2c),
2
indicating unimolecular O2 evolution in contrast with bimolecular catalysis
of 1. The unimolecular O2 evolution might be explained by either O-O
coupling of di-µ-O bridges or attack of outer-sphere water onto a µ-O
bridge in high oxidation species, probably including µ-O- radical bridges.
A coupling of di-µ-O- radical bridges in a Mn(µ-O)2Mn unit was proposed
by Yachandara et al. as a possible mechanism of O2 production in OEC
based on EXAFS results.
formation might still occur on the clay, but it could be much slower
relative to the O2 evolution.
To evaluate the mechanism of the O2 evolution by 1, [(bpy)2-
MnIII(µ-O)2MnIV(bpy)2]3+ (2), which has a structure comparable
with 1 but no terminal water ligands, was used for similar
experiments in both solution and on Kaolin clay. O2 was evolved
JA039780C
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J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004 8085