Characterization and activity analysis of catalytic water oxidation induced by hybridization of [(OH2)(terpy)Mn(μ-O)2Mn(terpy) (OH2)]3+ and clay compounds

Article abstract of DOI:10.1021/jp063679n

Hybridization of [(OH₂)(terpy)Mn(μ-O)₂Mn(terpy) (OH₂)]³⁺ (terpy = 2,2′:6′,2″- terpyridine) (1) and mica clay yielded catalytic dioxygen (O₂) evolution from water using a Ce^IV oxidant. The reaction was characterized by various spectroscopic measurements and a kinetic analysis of O₂ evolution. X-ray diffraction (XRD) data indicates the interlayer separation of mica changes upon intercalation of 1. The UV-vis diffuse reflectance (RD) and Mn K-edge X-ray absorption near-edge structure (XANES) data suggest that the oxidation state of the di-μ-oxo Mn₂ core is Mn^III-Mn^IV, but it is not intact. In aqueous solution, the reaction of 1 with a large excess Ce^IV oxidant led to decomposition of 1 to form MnO₄^- ion without O₂ evolution, most possibly by its disproportionation. However, MnO₄^- formation is suppressed by adsorption of 1 on clay. The maximum turnover number for O₂ evolution catalyzed by 1 adsorbed on mica and kaolin was 15 and 17, respectively, under the optimum conditions. The catalysis occurs in the interlayer space of mica or on the surface of kaolin, whereas MnO ₄^- formation occurs in the liquid phase, involving local adsorption equilibria of adsorbed 1 at the interface between the clay surface and the liquid phase. The analysis of O₂ evolution activity showed that the catalysis requires cooperation of two equivalents of 1 adsorbed on clay. The second-order rate constant based on the concentration (mol g ^-1) of 1 per unit weight of clay was 2.7 ± 0.1 mol ^-1 s^-1 g for mica, which is appreciably lower than that for kaolin (23.9 ± 0.4 mol^-1 s^-1 g). This difference can be explained by the localized adsorption of 1 on the surface for kaolin. However, the apparent turnover frequency ((k_O2) _app/s^-1) of 1 on mica was 2.2 times greater than on kaolin when the same fractional loading is compared. The higher cation exchange capacity (CEC) of mica statistically affords a shorter distance between the anionic sites to which 1 is attracted electrostatically, making the cooperative interaction between adsorbed molecules of 1 easier than that on kaolin. The higher CEC is important not only for attaining a higher loading but also for the higher catalytic activity of adsorbed 1.

Full text of DOI:10.1021/jp063679n

J. Phys. Chem. B 2006, 110, 23107-23114
23107
Characterization and Activity Analysis of Catalytic Water Oxidation Induced by
3
+
Hybridization of [(OH2)(terpy)Mn(µ-O)2Mn(terpy)(OH2)] and Clay Compounds
Komei Narita, Takayuki Kuwabara, Koji Sone,†,§ Ken-ichi Shimizu,‡,| and
†
†,§
Masayuki Yagi*,
†,§
Faculty of Education and Human Sciences, Niigata UniVersity, 8050 Ikarashi-2, Niigata 950-2181, Japan,
Graduate School of Science and Technology, Niigata UniVersity, 8050 Ikarashi-2, Niigata 950-2181, Japan,
and Center for Transdisciplinary Research, Niigata UniVersity, 8050 Ikarashi-2, Niigata 950-2181, Japan
ReceiVed: June 13, 2006; In Final Form: September 1, 2006
)]3 (terpy ) 2,2′:6′,2′′-terpyridine) (1) and mica clay
+
Hybridization of [(OH
yielded catalytic dioxygen (O
2
)(terpy)Mn(µ-O)
2
Mn(terpy)(OH
2
IV
2
) evolution from water using a Ce oxidant. The reaction was characterized by
2
various spectroscopic measurements and a kinetic analysis of O evolution. X-ray diffraction (XRD) data
indicates the interlayer separation of mica changes upon intercalation of 1. The UV-vis diffuse reflectance
(
RD) and Mn K-edge X-ray absorption near-edge structure (XANES) data suggest that the oxidation state of
III
IV
the di-µ-oxo Mn core is Mn -Mn , but it is not intact. In aqueous solution, the reaction of 1 with a large
2
IV
-
excess Ce oxidant led to decomposition of 1 to form MnO
4
ion without O
formation is suppressed by adsorption of 1 on clay. The maximum
evolution catalyzed by 1 adsorbed on mica and kaolin was 15 and 17, respectively,
under the optimum conditions. The catalysis occurs in the interlayer space of mica or on the surface of
2
evolution, most possibly by its
-
disproportionation. However, MnO
turnover number for O
4
2
-
kaolin, whereas MnO
at the interface between the clay surface and the liquid phase. The analysis of O
that the catalysis requires cooperation of two equivalents of 1 adsorbed on clay. The second-order rate constant
4
formation occurs in the liquid phase, involving local adsorption equilibria of adsorbed
1
2
evolution activity showed
-
1
-1 -1
based on the concentration (mol g ) of 1 per unit weight of clay was 2.7 ( 0.1 mol
s
g for mica, which
-
1
-1
is appreciably lower than that for kaolin (23.9 ( 0.4 mol
s
g). This difference can be explained by the
-
1
localized adsorption of 1 on the surface for kaolin. However, the apparent turnover frequency ((kO2
)
app/s )
of 1 on mica was 2.2 times greater than on kaolin when the same fractional loading is compared. The higher
cation exchange capacity (CEC) of mica statistically affords a shorter distance between the anionic sites to
which 1 is attracted electrostatically, making the cooperative interaction between adsorbed molecules of 1
easier than that on kaolin. The higher CEC is important not only for attaining a higher loading but also for
the higher catalytic activity of adsorbed 1.
Introduction
devices for providing a socially acceptable renewable energy
3
,4
source. A development of a catalyst for water oxidation to
evolve O2 (eq 1) is a key task to yield a breakthrough for a
technology of water splitting.
Natural and synthetic clay compounds are favorable materials
for “green” environmental sustainability to which much attention
has been paid recently. They consist of hexahedral silicate sheets
+
+
+
or octahedral aluminate sheets and cations such as H , Li , Na ,
water oxidation catalyst
+
+
-
K , and so forth sandwiched in an interlayer of a nanometer
2H O
8 O + 4H + 4e
(1)
2
2
scale between these sheets.1 This unique structural feature is
capable of incorporating functional molecules or ions into the
interlayer by cation exchange. The interlayer space could be
expected as a potential site for a specific chemical reaction that
cannot be seen in homogeneous solutions. The expanding and
deepening of research related to hybridization of clay com-
pounds with functional molecules could give a guided thought
to produce promising novel nanofunctional materials.
,2
Water oxidation is catalyzed in photosynthetic oxygen
5-7
evolving complex (OEC)
whose active site is known to
consist of manganese-oxo cluster. However, the mechanism
5-7
of water oxidation at OEC is still a question under debate.
OEC model studies using synthetic manganese complexes are
important for providing experimentally proven key reactions
for proposals of the water oxidation mechanism by OEC, and
Many scientists and engineers have been seeking practical
catalysts to activate water for both O2 and H2 production, which
are indispensable to construction of artificial photosynthetic
the several trailblazing works have been reported to give
3,4,8-15
important key reactions.
However, clear demonstrations
of the catalytic O2 evolution from water by synthetic manganese-
oxo complexes had not been reported.
Several years ago, Limburg and co-workers reported that O2
*
Author to whom correspondence should be addressed. Fax: +81-25-
2
62-7151; e-mail: yagi@ed.niigata-u.ac.jp.
†
Faculty of Education and Human Sciences.
Graduate School of Science and Technology.
Center for Transdisciplinary Research.
is evolved by the reaction of [(OH2)(terpy)Mn(µ-O)2Mn(terpy)-
‡
3+
(
OH2)] (terpy ) 2,2′:6′,2′′-terpyridine) (1) with the oxygen
§
1
2,13
|
donor agent of NaClO or KHSO5.
Their works have
Present address: Department of Applied Chemistry, Graduate School
of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.
triggered active research fields on syntheses, redox reactions,
1
0.1021/jp063679n CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/11/2006

23108 J. Phys. Chem. B, Vol. 110, No. 46, 2006
Narita et al.
catalyses related to 1, and its derivatives.16-20 Recently, Baffert
et al. reported that electrochemical oxidation of 1 cannot produce
O2 without the oxygen donor agent in solution.18 We prelimi-
IV
narily reported that the reaction of 1 with a Ce oxidant
catalytically produces O2 from water when 1 is adsorbed on
2
0
kaolin clay. This illustrated a successful design of a hetero-
geneous catalyst by hybridization of 1 and a clay compound.
However, the catalysis by 1 on solid clay matrixes has not yet
been sufficiently understood because of only an example for
investigation for 1/kaolin, in which the experimental conditions
are unfortunately restricted by a low cation exchange capacity
(CEC) of kaolin. Herein, water oxidation catalysis is tested by
1
adsorbed on mica clay which is known to possess an 8∼60
times higher CEC than that of kaolin. This allows not only
reliably analyzable spectroscopic data to be gained for charac-
terization of the 1/clay hybrid but also a wide concentration
range to be applied for a kinetic analysis of the catalysis. The
catalytic O2 evolution induced by hybridization of 1 and mica
is newly presented and characterized. The kinetic data for
Figure 1. Adsorption isotherms for adsorption of 1 onto (a) mica and
(b) kaolin.
1
/kaolin in a preliminary report20 is partially included to
photometer (Shimadzu, Multispec-1500). UV-vis DR spectra
of the 1/mica hybrid were measured using a UV-vis spectro-
photometer (JASCO V-550) in a diffuse reflectance mode. XRD
patterns of the 1/clay hybrids were taken in an X-ray diffrac-
tometer (MAC Science, MX-Labo). Mn K-edge X-ray absorp-
tion near-edge structure (XANES) measurements were carried
out in a transmission mode at BL-9A (Photon Factory in High
Energy Accelerator Research Organization, Tsukuba, Japan)
with a Si(111) double-crystal monochromator. Ionization cham-
bers filled with N2 for I0 (17 cm) and N2 (85%)-Ar (15%) for
I (31 cm) were used. High-energy X-rays from high-order
reflections were removed by a pair of flat quartz mirrors coated
with Rh/Ni that were aligned in parallel. The energy was defined
by assigning the first inflection point of the Cu foil spectrum
to 8980.3 eV. Normalization of XANES analysis was carried
out using REX2000 (Rigaku).
understand comprehensively the catalysis by 1 on solid clay
matrixes. Evidence for catalytic O2 evolution from water and
insights into the unique catalysis, including important influenc-
ing factors for the catalytic activity, will be provided.
Experimental Section
Materials. Mn(NO3)2, KMnO4, Ce(NH4)2(NO3)6 (Wako Pure
Chemical Industries, Ltd.), terpy, 2,2′-bipyridine (bpy), and 1,-
1
0-phenanthroline (phen) (Aldrich Chemical Co. Inc.) were
purchased and used as received. Kaolin (Aldrich Chemical Co.
Inc.) and somasif (ME-100) of mica clay (Co-op Chemical
Co., Inc., Japan) were given and used without further purifica-
tion.
III
IV
Syntheses of [(OH2)(terpy)Mn (µ-O)2Mn (terpy)(OH2)]-
IV
IV
(
(
[
NO3)3 (1(NO3)3), [(SO4)(terpy)Mn (µ-O)2Mn (terpy)(SO4)]
O2 Evolution Experiments. An aqueous solution of 1, 2, or
III
IV
1a), [(bpy)2Mn (µ-O)2Mn (bpy)2](ClO4)3 (2(ClO4)3), and
(phen)2Mn (µ-O)2Mn (phen)2](ClO4)3 (3(ClO4)3). 1(NO3)3
3
was adjusted to pH ) 4.0 with HNO3, and then the adequate
small portion of an aqueous solution (1.0 M) of a Ce(NH4)2-
NO3)6 oxidant was added to the solution. Otherwise, to the
III
IV
was prepared by a similar procedure to literature by Collomb
(
et al.21 An aqueous solution (3 mL, pH ) 3.0) of Mn(NO3)2
aqueous suspension containing 10∼75 mg of the 1, 2, or 3/clay
(1.7 mmol) was mixed with an aqueous suspension (40 mL,
hybrid was added the adequate small portion of the aqueous
pH ) 3.0) of terpy (1.7 mmol) to result in the yellow solution
IV
solution of Ce . The reaction was typically followed in 2.0
2+
(
because of formation of [Mn(terpy)2] ), and then KMnO4 (0.62
mL of a liquid-phase volume at pH ) 1.0 and 25 °C under the
mmol in 5.8 mL water) was dropped into the solution with
stirring to give a dark green solution. A saturated NaNO3 (5
mL, pH ) 3.0) was added to the solution after filtration.
Microcrystalline 1(NO3)3 was given by cooling in a refrigerator
and was washed with ice water and diethyl ether. (0.44 g, 62%
yield) 1a, 2(ClO4)3, and 3(ClO4)3 were synthesized according
IV
conditions of a large excess (50∼100 mM) of Ce versus 1
IV
(
pH of the 1 solution changes to 1.0 by adding a Ce ). The
amount of O2 evolved was analyzed using a Clark type oxygen
electrode (Hansatech Instruments, Oxygraph OXYG1 and DW1/
AD unit) with 1 µM of the detection level of O2 dissolved in
water. Gas chromatograph (Shimadzu, GC-8A) equipped with
a 5 Å molecular sieve column (argon carrier gas) was used to
analyze more than the O2 amount restricted for our oxygen
electrode instrument because of release of O2 gas from the
13
22
to the literature by Limburg et al., Cooper and Calvin, and
Manchanda et al.,23 respectively. These complexes were char-
acterized by IR and UV-vis absorption spectra.
Preparations of 1, 2, or 3/Clay Hybrids. To prepare 1, 2,
or 3/clay hybrids with the wide-ranging amount of the com-
plexes adsorbed, an aqueous solution (0∼1.2 mM, pH ) 4.0)
of 1, 2, or 3 was added to an aqueous suspension (5∼15 mL,
pH ) 4.0) of mica or kaolin (50∼400 mg). The resulting
suspension was filtrated after stirring for 30 min and then was
dried under vacuum to yield a 1, 2, or 3/clay hybrid. The amount
of 1, 2, or 3 adsorbed was measured by the visible absorption
spectral change of the aqueous solution before and after adding
clay.
18
18
electrode. For O-labeling experiments using H2 O, the evolved
gas was analyzed on an electron-impact-ionization mass spec-
trometer (JEOL, JMS-SX102).
Results and Discussion
Adsorption of 1 onto Clay Compounds. When an aqueous
solution of 1 was added to an aqueous suspension of mica or
+
kaolin, 1 was adsorbed onto clay by cation exchange with Na .
The adsorption isotherms of 1 onto mica and kaolin are shown
Spectroscopic Measurements. Reactions of 1 with a CeIV
oxidant in an aqueous solution were followed by UV-vis
spectral change as measured using a photodiode array spectro-
in Figure 1. The concentration (wads/mol g ) of 1 adsorbed on
-1
clay rose steeply at very low (almost zero) equilibrium
concentration (ceq/M) of 1 in the solution, indicating the high

Activity Analysis of Catalytic Water Oxidation
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23109
Figure 3. UV-vis DR spectra of 1/mica hybrids with various wads (fads
)
as indicated on figure. Inset shows the plots of Kubelka-Munk at 400
nm vs fads
.
Figure 2. XRD patterns of mica and 1/mica hybrids. (a) Dry mica
and (b) mica kept in aerobic conditions. The 1 concentration, wads (fads),
for 1/mica hybrids are (c) 0.012 (3%), (d) 0.041 (10%), (e) 0.082 (20%),
is different from the DR spectrum of 1 (as a powder) with a
shoulder at 414 nm and peaks at 556 and 662 nm, suggesting
that adsorbed 1 is not intact on mica. Figure 3 shows the UV-
vis DR spectra of the 1/mica hybrid with the various concentra-
tions of 1 adsorbed. The Kubelka-Munk (KM) values increased
almost linearly with fads below fads ) 20% (inset of Figure 3),
with the unchanged peak wavelength (λmax ) 406 nm) and
spectral shape in the whole concentrations employed. This
indicates that the different spectrum between 1 and 1/mica
hybrid is independent of its concentration on mica. The KM
values plateaued at more than fads ) 20%. This can be explained
by a sort of a filter effect; the species close to the mica surface
could absorb light sufficiently under the high fads conditions,
so that light absorption of the deeply intercalated species could
not be reflected in the KM values.
(
f) 0.12 (30%), (g) 0.15 (36.5%), (h) 0.16 (40%), (i) 0.25 (60%), and
-
1
(j) 0.33 mmol g (80%).
adsorption equilibrium constant of 1 onto clays. The maximum
concentration ((wads)max ) 0.411 mmol g ) of 1 adsorbed onto
mica was 20 times higher than that (0.022 mmol g ) for kaolin.
-
1
-1
-
1
These values are consistent with CEC of mica (1.2 meq g )
-
1
and kaolin (0.02∼0.15 meq g ) when considering trivalence
of 1 cation. The fractional loading (fads%) of 1 onto clay was
defined as fads% ) wads/(wads)max and was used in some cases
for convenience.
Characterization of a 1/Mica Hybrid. XRD Spectroscopic
Measurements. An XRD spectroscopic technique was used to
evaluate the layer structure of mica and kaolin adsorbing 1. The
XRD pattern of kaolin with an intense sharp peak at 2θ ) 12.36
degrees did not change by adsorption of 1, showing that 1 does
not change the layer structure of kaolin. 1 is considered to be
adsorbed onto the surface of kaolin. By contrast, the XRD
pattern of mica changed drastically by adsorption of 1, as shown
in Figure 2. The XRD pattern (Figure 2a) of dry mica gave an
intense peak at 2θ ) 9.2 degrees, corresponding to an interlayer
distance of 9.56 Å. However, when mica was kept in air, the
peak shifted to 2θ ) 7.18 degrees (interlayer distance of 12.3
The Mn K-Edge XANES Measurements. The Mn K-edge
XANES spectra of the 1/mica hybrid are shown in Figure 4,
III
IV
IV
including those of 1 (Mn -Mn ) and [(SO4)(terpy)Mn (µ-
IV
IV
IV
O)2Mn (terpy)(SO4)] (1a) (Mn -Mn ) as powders. The
XANES spectrum of the 1/mica hybrid (wads ) 0.082 mmol
-
1
g , fads ) 20%) exhibited a shoulder at 6551.1 eV and a peak
at 6561.6 eV, which are almost identical to that for fads ) 60%
(a shoulder at 6550.9 eV and a peak at 6561.6 eV). This is
consistent with the UV-vis DR spectroscopic data independent
of the concentration of 1 (vide supra). It is well-known that the
Mn K-edge shifts to a high-energy region by an increase in an
oxidation state of Mn centers in a di-µ-oxo Mn2 complex.24 The
comparison in the Mn K-edge among the 1/mica hybrid, 1, and
1a could provide an insight into the oxidation state of Mn centers
in the adsorbed species on mica. K-edge energy was evaluated
by energy at the local minimum around 6551 eV for the second-
order derivative Mn K-edge XANES spectra (a lower item in
Figure 4). The Mn K-edge energy (6551.1 eV) of 1 was lower
by 2.0 eV than that (6553.1 eV) of 1a. This is consistent with
the result reported by Visser et al. on Mn K-edge XANES
+
Å) by hydration of Na ions (Figure 2b). The XRD pattern of
the 1/mica hybrid exhibited a peak at 2θ ) 7.12 degrees and a
shoulder at 2θ ) 7.92 degrees in the low concentration (wads )
0
increased, the peak intensity increased, indicating that 1 is
intercalated into the interlayer of mica. At wads ) 0.12 mmol
g
-1
.012 mmol g , fads ) 3%) of adsorbed 1. As the concentration
-
1
(fads ) 30%), the shoulder almost disappeared, resulting in
an intense and sharp peak at 2θ ) 6.96 degrees (interlayer
distance of 12.7 Å). The peak was lowered at fads ) 60% and
ultimately became very broad at 2θ ) 6.9∼5.4 degrees at fads
)
3
80%. The more ordered layer structure was given at fads )
0% relative to the other concentrations (except fads ) 0%).
3+/4+
measurements of [(phen)Mn(µ-O)2Mn(phen)]
in an aceto-
The interspace distance between the layers at fads ) 30% can
be given as 6.1 Å from the interlayer distance (12.7 Å) and the
thickness (6.6 Å) of the layer. This is consistent with the
molecular size of 1 (13.2 × 11.1 × 4.8 Å) on the basis of crystal
graphic data and van der Waals radius (1.2 Å) of a hydrogen
atom.
nitrile solution, in which the K-edge shifted to a high-energy
III
IV
region by 2.2 eV in oxidation from the Mn -Mn to
IV-
IV
24
Mn Mn states of the di-µ-oxo Mn2 core. The K-edges
(6551.1 eV for fads ) 20%, 6550.9 eV for fads ) 60%) for the
1/mica hybrid were almost identical to that (6551.1 eV) of 1
rather than that of 1a. This suggests that the oxidation state of
the di-µ-oxo Mn2 core in the adsorbed species on mica could
UV-Vis DR Spectroscopic Measurements. The UV-vis DR
spectrum of the 1/mica hybrid exhibited a peak at 406 nm. This
III
IV 25
be Mn -Mn .

23110 J. Phys. Chem. B, Vol. 110, No. 46, 2006
Narita et al.
Figure 5. Visible absorption spectral change of an aqueous solution
IV
containing 0.1 mM 1 and 2.0 mM Ce ion at pH ) 1.0 and 25 °C.
Reaction time was indicated on figure.
Figure 4. Mn K-edge XANES spectra (upper) and the second-order
-
1
derivative spectra (lower). (a) 1/mica with wads ) 0.082 mmol g (fads
-
1
)
20%), (b) 1/mica with wads ) 0.25 mmol g (fads ) 60%), (c) 1
powder, and (d) 1a powder. The inset is the magnified spectra for the
pre-edge peak around 6540 eV.
The magnified spectra in the range of 6535∼6547 eV are
shown in the inset of Figure 4 for the pre-edge peak around
6
540 eV, assigned to a dipole-forbidden 1s f 3d transition of
the Mn center. The spectrum of 1 exhibited a pre-edge peak at
539.3 eV, while the spectra of the 1/mica hybrid exhibit two
6
separate pre-edge peaks at 6539.8 and 6542.1 eV. Pre-edge peak
energy corresponds to the energy difference between the 1s level
and the unoccupied 3d levels of the center ion, the latter of
which are influenced by the ligand field.2
6-28
The higher pre-
4
-
Figure 6. Plots of initial rate (VMnO4-) of MnO
concentration (c
Concentration of Ce , 10 mM; pH ) 1.0; 25 °C.
formation vs
0
0
) of 1. Inset shows plots of log VMnO4- vs log c .
edge energy (at 6542.1 eV) for the 1/mica hybrid could be
explained by an increase of the unoccupied 3d levels, possibly
because of a distorted local coordination structure of Mn ions.
However, it is also established that pre-edge peak energy
depends on the valence state of the center ions; pre-edge peak
energy shifts to higher energy with an increase in the oxidation
IV
spectral change in a reaction with 8 equiv of CeIV exhibited the
appearance of the absorption band at λmax ) 545 nm with
vibration structures that are a characteristic feature of MnO4
ion, concomitantly with the formation of the Mn -Mn
species. In a reaction with 20 equiv of Ce , stoichiometric
formation of MnO4 from 1 was indicated by appearance of
the absorption band at λmax ) 545 nm with an isosbestic point
-
IV
IV
number of iron.2
9-31
The possible explanation by the higher
IV
valence state could not be excluded for the higher pre-edge
energy in Mn K-edge XANES spectra of the 1/mica hybrid.
We are considering a significant distortion of 1 adsorbed on
mica. However, it is still unproven whether it can rationalisti-
cally explain the UV-vis DR spectroscopic features (Figure
-
at 489 nm, as shown in Figure 5.
To reveal the mechanism of MnO4- formation, its kinetics
was studied under the conditions of a large excess of Ce . The
IV
3
).
Reactions of 1 with a Ce Oxidant in Water. Reactions
-
IV
initial rate (VMnO4-) of MnO4 formation was given from the
IV
initial slope of the absorbance change at λ
2
) 545 nm (ꢀ )
of 1 with a Ce oxidant in water were investigated using a
UV-vis absorption spectroscopic technique. The absorption
spectral change in a reaction of 1 with 1 equiv of Ce shows
max
-
1
-1
344 M cm ). It increased with the concentration (c0) of 1
IV
as shown in Figure 6, and the plots of log VMnO4- versus log c0
gave a straight line with the slope of 2.07 ( 0.17 and intercept
the absorption increase at 440 nm and the absorption decrease
at 700 nm with an isosbestic point of 580 nm. This is close to
the absorption spectral change in formation of [(OH2)(terpy)-
-
)
1.32 ( 0.66 (inset of Figure 6). The slope shows that MnO4
formation can be analyzed as a second-order reaction with
IV
IV
4+
IV
IV
respect to 1. It is most possibly formed by disproportionation
of the high oxidation state of 1 in a rate-determining step. The
second-order rate constant for MnO formation was calculated
Mn (µ-O)2Mn (terpy)(OH2)] (Mn -Mn ) by dispropor-
tionation of 1 promoted by a pH decrease, as reported by Chen
-
et al.17 (The absorbance at 450 nm increased and the absorption
4
1
-1 -1
to be 2.1 × 10 M
s
from the intercept.
at 660 nm decreased with an isosbestic point at 565 nm.) This
IV
result suggests that the reaction of 1 with 1 equiv of Ce
O2 evolution by 1 from water was tested in an aqueous
solution containing 0.1∼0.5 mM 1 and a large excess (100 mM)
IV
IV
32
stoichiometrically produces Mn -Mn in solution. The

Activity Analysis of Catalytic Water Oxidation
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23111
TABLE 1: Summary of Amount (nO2) of O
2
Evolved and Turnover Number (TN) of Complexes for O
2
Evolution Experimentsa
compounds
n
ads/µmol
clay
amount of clay/mg
n
O2/µmol
TN
+
]3 (1)
nob
no
no
kaolin
kaolin
kaolin
kaolin
mica
[
Mn
2
(µ-O)
2
(terpy)
2
(OH
2
)
2
1.0
0.0
0.0
0.0
0.0
2.2 ((0.21)
3.4 ((0.02)
6.6 ((0.53)
0.0
0.0
0.0
0.0
b
b
5.0
50.0
0
0
0
0
0
0
0
1
1
.0
50
50
50
50
10
10
10
10
10
.13
.26
.46
.0
.25
.50
.0
17 ((1.6)
13 ((0.1)
14 ((1.2)
mica
mica
mica
0.95
3.8
3.7 ((0.15)
15 ((0.6)
23 ((1.5)
0.0
7.3 ((0.3)
15 ((0.6)
15 ((1.0)
0.0
.5
mica
]3 (2)
+
1.0
no
b
[Mn
2
(µ-O)
(µ-O)
2
(bpy)
4
0
1
.46
.5
kaolin
mica
50
10
0.26 ((0.01)
0.52 ((0.02)
1.0 ((0.02)
0.68 ((0.01)
+
]3 (3)
b
[Mn
2
2
(phen)
4
1.0
no
0.0
0.0
0
1
.46
.5
kaolin
mica
50
10
0.26 ((0.01)
0.71 ((0.12)
0.0
0.0
0.0
0.0
0.0
0.57 ((0.02)
0.47 ((0.08)
0.0
0.0
0.0
0.0
0.0
-
b
MnO
MnO
Mn
4
10.0
10.0
no
no
no
c
2
2
+
b
0.92
.92
0.92
0
kaolin
kaolin
50
50
n+
terpyH
n
a
Reaction conditions are as follows: the amount of CeIV oxidant, 1.0 mmol; reaction time, 4 days; liquid-phase volume, 10 mL; gas-phase
volume, 3 mL. nO2 was measured using a gas chromatograph. b Homogeneous solution. c Suspension.
of CeIV oxidant, but it was not observed at all, nor was O2
TABLE 2: Summary of Amount (nMnO4-) and Yield
-
IV
(Φ
MnO4-a) of MnO
4
Formed in Reactions of 1 and a Ce
evolved using a 10∼50-fold larger concentration of 1 (5 mM
Oxidant
IV
1
, 100 mM Ce ) (see Table 1).
amount of
clay/mg 10 mol 10 mol
n
ads
-7
/
n
MnO4-b/
-7
Reactions of 1/Clay Hybrids with a CeIV Oxidant in
c
system
Φ
MnO4-
%
IV
Water. Reactions of the 1/clay hybrids with a large excess Ce
oxidant in water produced a significant amount of O2 in contrast
to no O2 evolution by 1 in solution. Data on O2 evolution are
summarized in Table 1. O2 evolution was not observed when
using mica without 1 as a control. The amount (nO2/mol) of O2
evolved increased with the amount (nads/mol) of 1 adsorbed on
either kaolin or mica, showing that O2 is evolved by 1 adsorbed
on clay. The maximum turnover number (TN) of 1 was almost
the same for mica (TN ) 15) and kaolin (TN ) 17). This result
corroborates that 1 adsorbed on clay catalyzes O2 evolution.
To define the catalysis, similar O2 evolution experiments were
homogeneous solution
1.2
.4
0.90
2.1
2.7
0.47
0.77
1.3
0.95
1.2
75.0
85.8
56.3
31.3
25.7
18.3
19.8
24.6
19.0
2
4.8
1.5
3.0
7.0
4.8
4.8
5.1
1/kaolin
75
7
7
5
5
1
/mica
10
4
7
0
5
0.97
a
Reaction conditions are as follows: the concentration of CeIV
oxidant, 50 mM; reaction time, 30 min; liquid-phase volume, 2 mL.
b
n
MnO4- was measured from absorbance at λmax ) 545 nm (ꢀ ) 2344
2+
-1
cm-1). ΦMnO4- was defined as nMnO4- normalized by nads
.
c
extended to various manganese species including MnO2, Mn ,
M
-
2+
n+
and MnO4 in water, as well as adsorbed Mn and terpyHn
much less than in solutions under the comparable conditions.
This shows that adsorption of 1 onto the clay significantly
on mica or kaolin. In none of the trials using these species was
O2 evolution detected. These results show the unique catalysis
by 1 adsorbed on the clay.
-
suppresses the decomposition of 1 to form MnO4 .
1
8
18
O-Labeling O2 Evolution Experiments using H2 O. To
III
IV
3+
III
[
(bpy)2Mn (µ-O)2Mn (bpy)2] (2) and [(phen)2Mn (µ-O)2-
18
identify an oxygen atom source for O2 evolution, O-labeling
experiments were carried out using H2 O, and the evolved gas
was analyzed on an electron-impact-ionization mass (EIM)
spectrometer. In a H2 O media (47.5 and 23.8 v/v % H2 O),
IV
3+
Mn (phen)2] (3) have comparable structures with 1 but no
terminal water ligands. The similar experiments were carried
out using 2 or 3 in solution and on clays to compare with the
18
18
18
IV
reactions of 1 with a Ce oxidant. For neither 2 nor 3 was O2
the EIM spectra gave the peaks at m/z ) 34 and 36, corre-
evolved in the solution, but it was for both 2/clay and 3/clay.
However, TN values are less than unity for 2/clay (TN )
16 18
18
16
sponding to O O and O2, additionally to the peak for O2
at m/z ) 32, in contrast to no peaks at m/z ) 34 and 36 for
3
3
0
2
.5∼0.7) and for 3/clay (TN ) 0.5∼0.6), showing that neither
16
natural abundance water. (Figure 7) The amounts of O2,
nor 3 adsorbed on clay works as a catalyst for O2 evolution.
16 18
18
O O, and O2 are analyzed from the intensities at m/z ) 32,
This result suggests that terminal water coordination sites could
be important for the catalysis by adsorbed 1.
For clay hybrid systems, the MnO4- ion formed by decom-
34, and 36, respectively. The O2 evolution data are summarized
18
in Table 3. The content fraction (Φ18O) of an O atom in O2
evolved is consistent with the O content in the water media.
18
position of 1 was also observed in a liquid phase after removal
It was thus corroborated that the oxygen atoms in O evolved
are exclusively originated from water.
Kinetic Analysis and Influencing Factors of the O2
2
34
of the clay hybrids by centrifugation. The amount (nMnO4-/
-
mol) and yield (ΦMnO4-) of MnO4 formed for a 30-min reaction
are summarized in Table 2 including the data for the solution.
ΦMnO4- was 56.3∼85.8% in solutions under the conditions.
However, ΦMnO4- (18.3∼31.3%) for the 1/clay hybrids was
Evolution for the 1/Clay Hybrids. The time course of O2
evolved in the reaction of 1/mica with a large excess Ce ion
is shown in Figure 8, including data for a homogeneous solution
IV

23112 J. Phys. Chem. B, Vol. 110, No. 46, 2006
Narita et al.
Figure 7. Electron-impact-ionization mass (EIM) spectra for 18O-
labeling O evolution experiments using 1/kaolin (75 mg) with wads
2
-
)
Figure 9. Plots of initial rate (VO2/mol s-1) of O
2
evolution vs the
amount (nads) of 1 for the various amounts of mica. The amounts of
1
9
.3 µmol g ; reaction time, 7 days; liquid volume, 1.0 mL; gas-phase
volume, 6.3 mL.
mica are (a) 10 mg (closed circles), (b) 40 mg (half-closed circles),
and (c) 75 mg (open circles) for 1/mica. 50 mM Ce ; liquid volume,
IV
TABLE 3: Summary of O
2
Evolution Data in 18O-Labeling
2
.0 mL; pH ) 1.0. The inset shows the magnified figure for comparison
a
Experiments Using 1/Kaolin
between 1/mica and 1/kaolin. Open circles in the inset are the same
data as plots (c) for 1/mica (75 mg). Open squares are (d) VO2 data for
1
n
O2/µmolb
/kaolin (75 mg). The same VO2 data in a low nads region for 1/kaolin
H
2
18O v/v %
0
16O
2
16
O O
18
18O
0
0.24
1.0
2
Φ
18O
%
c
(open squres) were cited from ref 20.
5.6
2.7
1.9
0
0
23
41
2
4
3.8
7.5
1.6
2.4
a
Kaolin, 75 mg; nads ) 0.7 µmol; reaction time, 7 days; liquid
b
volume, 1.0 mL; gas-phase volume, 6.3 mL. Obtained from intensity
2
at m/z ) 32, 34, and 36 using calibration on O Φ18O is
abundance. c
1
8
2
fraction of O atom in O evolution.
-
4
formed vs the amount
Figure 10. Plots of amount (nMnO4-) of MnO
ads) of 1 for various amounts of clay. The amounts of mica are (a) 10
(n
mg (closed circles), (b) 40 mg (half-closed circles), and (c) 75 mg (open
IV
circles) for 1/mica. (d) 75 mg (open squares) for 1/kaolin, 50 mM Ce ;
liquid volume, 2.0 mL; pH ) 1.0. nMnO4- was measured after a 30-
min reaction.
The plots of VO2 versus nads are shown in Figure 9 for 1/mica
with various amounts of mica (10∼75 mg). VO2 increased
upward with nads under the conditions employed for 40 and 75
mg mica. However, VO2 for 10 mg mica thereafter departed from
the upward curvature above nads ) 2.0 µmol, which is ascribed
to elution of 1 from mica into a liquid phase to promote
decomposition of 1 to MnO4 ion. VO2 was higher as the amount
of mica is lower comparing the same nads below nads ) 2.0 µmol.
(VO2 order: 10 > 40 > 75 mg).
Figure 8. Time courses of the amount of O
1
0
2
evolved in reactions of
and a 50 mM CeIV oxidant. (a) Aqueous solution of 1 (1.5 µmol;
.75 mM), (b) aqueous solution without 1, (c) aqueous suspension of
-
1
mica clay (10 mg) adsorbing 1.5 µmol 1 (wads ) 0.15 mmol g ; fads
6.5%), and (d) aqueous suspension of mica clay (10 mg) without 1.
Liquid volume, 2.0 mL; pH ) 1.0.
)
3
-
of 1 containing the same amount (1.5 µmol) of 1 for comparison.
-
1
The initial O2 evolution rate (VO2/mol s ) was calculated from
the initial slope of the time course. The inset in Figure 9 shows
the comparison of VO2 between the 1/mica and the 1/kaolin
hybrids (both clay amounts are 75 mg). The nads range for 1/mica
is much wider than that for 1/kaolin, and the maximum VO2
value for the former is 7.6 times higher than that for the latter
under the conditions employed (nads < 8 µmol). This result is
ascribed to the higher CEC of mica.
On the other hand, nMnO4- measured in parallel with the V
O2
3
5
increased with nads, as shown in Figure 10. In contrast to the
VO2 data (Figure 9), the profiles of nMnO4- versus nads were the
same between 1/kaolin and 1/mica and were independent of
-
the amount of mica. These results suggest that MnO4 formation
from adsorbed 1 takes place in the liquid phase, whereas the
catalytic O2 evolution takes place in the interlayer space of the

Activity Analysis of Catalytic Water Oxidation
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23113
Figure 11. Plots of (kO2)app vs the amount (nads) of 1. The amounts of
mica are (a) 10 mg (closed circles), (b) 40 mg (half-closed circles),
and (c) 75 mg (open circles) for 1/mica and (d) 75 mg (open squares)
for 1/kaolin. The conditions are indicated in Figure 9.
Figure 12. Plots of (kO2
0 mg (closed circles), 40 mg (half-closed circles), and 75 mg (open
circles). (b) 1/kaolin; 75 mg (open squares).
)app vs the concentration (wads) of 1. (a) 1/mica;
1
mica. There could be local adsorption equilibria of adsorbed 1
at the interface between the clay surface and the liquid phase.
-
MnO4 prefers to be formed in the liquid phase rather than on
-
the clay. This is supported by MnO4 formation suppressed by
-
the adsorption of 1 on the mica. (vide supra) MnO4 formation
might still occur on clay, but it could be much slower relative
to the O2 evolution.
The upward curvature in VO2 versus nads plots (Figure 9) shows
that the specific O2 evolution increases with increasing nads. The
O2 evolution was analyzed by the kinetic model (eq 2) assuming
on combination between first- and second-order O2 evolutions
with respect to the adsorbed 1:
2
VO2 ) k1nads + k2nads
(2)
where k1/s- and k2/mol-1 s-1 are first-order and second-order
rate constants for O2 evolution, respectively. For comparison
of the turnover frequency of 1 adsorbed on clays, VO2 was
normalized by nads to define the apparent turnover frequency,
1
Figure 13. Plots of (kO2
)app vs the loading amount (fads) of 1. (a) 1/mica;
1
0 mg (closed circles), 40 mg (half-closed circles, and 75 mg (open
circles). (b) 1/kaolin; 75 mg (open squares).
-
1
(kO2)app/s , as eq 3.
of the linear plots of (kO2)app versus wads yields k2′ ) 2.7 ( 0.1
-
1 -1
mol
s
g of a second-order rate constant on the basis of wads.
(
k ) ) V /n ) k + k n
(3)
For comparison, (kO2)app for 1/kaolin was plotted versus wads in
O2 app
O2 ads
1
2 ads
-
1
Figure 12 together. The k2′ for 1/kaolin was 23.9 ( 0.4 mol
-
1
The plots of (kO2)app versus nads gave the straight lines with
significant slopes passing through the origin for each 1/clay (at
nads < 2.0 µmol for 10 mg mica and at nads < 0.9 µmol for
s
g, which is 8.9 times higher than that for 1/mica. This
difference could be explained by localized adsorption of 1 on
the kaolin surface. For 1/mica, 1 is intercalated into an interlayer
as well as adsorbed on a surface of mica (see Figure 2). On the
other hand, 1 is adsorbed only on a surface for 1/kaolin (vide
supra), resulting in a localized adsorption of 1 on the kaolin
surface. Nevertheless, the maximum (kO2)app for 1/mica was
higher than that for 1/kaolin under respective optimum condi-
tions. This is attributed to the higher loading of 1 because of
the higher CEC of mica.
To reveal the influence of CEC on (kO2)app, it was replotted
versus fads% in Figure 13 for both the 1/mica and the 1/kaolin
hybrids. The (kO2)app increased linearly at a low fads region, and
the maximum (kO2)app was observed around fads ) 50% for both
the hybrids. The (kO2)app for 1/mica was 2.2 times higher in
average (in the slope of the line) than that for 1/kaolin comparing
the same fads. This can be explained by considering the statistical
distance between anionic sites on clay to which 1 is attracted
electrostatically. The higher CEC of mica statistically affords a
shorter distance between the anionic sites, making the coopera-
1
/kaolin), as shown in Figure 11. The significant slope (corre-
sponding k2 in eq 3) suggests that O2 is predominantly evolved
by a bimolecular reaction of 1 adsorbed on clay. It is most likely
that adsorbed 1 works cooperatively for the catalysis. It may
take place by intermolecular coupling of Mn )O that could be
formed by successive oxidation of a terminal water ligand on
V
1
, on the basis of noncatalytic activity of 2 and 3 for O2
V
evolution. However, no evidence for involvement of Mn )O
in the catalysis is obtained. The fitting of eq 3 to the (kO2)app
data yielded almost zero for the k1 and the k2 values depending
2
on the amount of mica ((2.7 ( 0.3) × 10 , (0.69 ( 0.01) ×
2
2
-1 -1
1
0 , and (0.34 ( 0.03) × 10 mol
s
for 10, 40, and 75 mg
mica, respectively).
All the (kO2)app data for 10∼40 mg mica were replotted versus
-
1
concentration wads (mol g ) instead of nads (mol) in Figure 12.
The plots exhibited the significant linearity below 0.2 mmol
g , showing that (kO2)app is principally related to wads. The slope
-
1

23114 J. Phys. Chem. B, Vol. 110, No. 46, 2006
Narita et al.
tive interaction between adsorbed molecules of 1 easier than
that on kaolin. The higher CEC is important not only for
attaining a higher loading but also for the higher catalytic activity
of adsorbed 1.
(13) Limburg, J.; Vrettos, J. S.; Chen, H. Y.; de Paula, J. C.; Crabtree,
R. H.; Brudvig, G. W. J. Am. Chem. Soc. 2001, 123, 423.
(
14) Naruta, Y.; Sasayama, M.; Sasaki, T. Angew. Chem., Int. Ed. Engl.
994, 33, 1839.
15) Shimazaki, Y.; Nagano, T.; Takesue, H.; Ye, B.-H.; Tani, F.; Naruta,
1
(
Y. Angew. Chem., Int. Ed. 2004, 43, 98.
Conclusion
(16) Chen, H.; Tagore, R.; Das, S.; Incarvito, C.; Faller, J. W.; Crabtree,
R. H.; Brudvig, G. W. Inorg. Chem. 2005, 44, 7661.
(17) Chen, H.; Faller, J. W.; Crabtree, R. H.; Brudvig, G. W. J. Am.
Chem. Soc. 2004, 126, 7345.
The present paper illustrates evidence of catalytic O2 evolution
from water induced by adsorption of 1 onto solid clay. It requires
cooperative interaction of two equivalents of 1 adsorbed. The
catalysis occurs in the interlayer space of mica or on the surface
(
18) Baffert, C.; Romain, S.; Richardot, A.; Lepretre, J.-C.; Lefebvre,
B.; Deronzier, A.; Collomb, M.-N. J. Am. Chem. Soc. 2005, 127, 13694.
19) Wolpher, H.; Huang, P.; Borgstroem, M.; Bergquist, J.; Styring,
(
-
of kaolin, whereas the decomposition of 1 to form MnO4 ions
S.; Sun, L.; Akermark, B. Catal. Today 2004, 98, 529.
(20) Yagi, M.; Narita, K. J. Am. Chem. Soc. 2004, 126, 8084.
occurs in the liquid phase, involving local adsorption equilibria
of adsorbed 1 at the interface between the clay surface and the
liquid phase. Nevertheless, MnO4- formation is suppressed by
adsorption of 1 on clay. The concentration and localization of
(21) Collomb, M. N.; Deronzier, A.; Richardot, A.; Pecaut, J. New J.
Chem. 1999, 23, 351.
(
(
22) Cooper, S. R.; Calvin, M. J. Am. Chem. Soc. 1977, 99, 6623.
23) Manchanda, R.; Brudvig, G. W.; Degala, S.; Crabtree, R. H. Inorg.
1
adsorbed on clay, as well as the distance between anionic
Chem. 1994, 33, 5157.
(24) Visser, H.; Anxolabe, E.; Lhe‘re-Mallart; Bergmann, U.; Glatzel,
P.; Robblee, J. H.; Cramer, S. P.; Girerd, J.-J.; Sauer, K.; Klein, M. P.;
sites on clay statistically related to its cation exchange capacity,
are the important factors for the cooperative catalysis. These
results undoubtedly provide a first step for development of
nanohybrid catalysts for water oxidation to evolve O2 using clay
compounds.
Yachandra, V. K. J. Am. Chem. Soc. 2001, 123, 7031.
20
(
25) In the earlier communication, we considered that 1 is adsorbed
IV
IV
on kaolin as the Mn -Mn state on the basis of the UV-vis diffuse
reflectance spectroscopic data. However, we reconsider that the species
III
IV
adsorbed on kaolin is the Mn -Mn state from the present Mn K-edge
XANES data for the 1/mica hybrid.
Acknowledgment. Research was supported by Grant-in-Aid
for Young Scientists (B) from the Ministry of Education,
Culture, Sports, Science, and Technology (No. 16750113) and
grant from the Nissan Science Foundation. We thank Dr. T.
Sato for help with EIMS spectroscopic measurements. The X-ray
absorption experiments were performed under the approval of
the Photon Factory Program Advisory Committee (Proposal No.
(
26) Shimizu, K.; Maeshima, H.; Yoshida, H.; Satsuma, A.; Hattori, T.
Phys. Chem. Chem. Phys. 2001, 3, 862.
(27) George, S. J.; Lowery, M. D.; Solomon, E. I.; Cramer, S. P. J. Am.
Chem. Soc. 1993, 115, 2968.
(
28) Farges, F.; Brown, G. E., Jr.; Rehr, J. J. Phys. ReV. B 1997, 56,
809.
29) Linkous, C. A.; O’Grady, W. E.; Sayers, D.; Yang, C. Y. Inorg.
Chem. 1986, 25, 3761.
30) Westre, T. E.; Kennepohl, P.; DeWitt, J. G.; Hedman, B.; Hodgson,
K. O.; Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 6297.
31) Roe, A. L.; Schneider, D. J.; Mayer, R. J.; Pyrz, J. W.; Widom, J.;
Que, L. J. Am. Chem. Soc. 1984, 106, 1676.
32) Chen reported that a linear tetranuclear Mn complex is slowly
1
(
(
2
005G-189).
(
References and Notes
IV
(
(1) Clay Surfaces - Fundamentals and Applications; Wypych, F.,
IV
IV
formed from Mn -Mn in an acidic aqueous solution, which is shown
Satyanarayana, K. G., Eds.; Academic Press Inc: San Diego, CA, 2004.
by the absorption spectral change with the absorption decrease at 400-
(
(
(
(
2) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399.
3) Ruettinger, W.; Dismukes, G. C. Chem. ReV. 1997, 97, 1.
4) Yagi, M.; Kaneko, M. Chem. ReV. 2001, 101, 21.
17
4
20 nm and the increase at 470 nm. The formation of the linear tetranuclear
IV
Mn complex was not observed up to 100 min under the conditions of 1.0
mM 1 and pH ) 1.0 in the present work.
5) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata,
IV
(33) Kaneko reported that O2 is evolved when a Ce oxidant is added
S. Science 2004, 303, 1831.
6) Photosynthetic Water Oxidation special issue. In Biochim. Biophys.
Acta-Bioenerg.; Nugent, J., Ed.; 2001; Vol. 1503.
7) Loll, B.; Kern, J.; Saenger, W.; Zouni, A.; Biesiadka, J. Nature
London) 2005, 438, 1040.
8) Ramaraj, R.; Kira, A.; Kaneko, M. Angew. Chem., Int. Ed. Engl.
986, 25, 825.
9
to an aqueous suspension of 2/kaolin. However, the turnover numbers
(
(
0.38∼0.76) of 2 was less than unity, which is consistent with the present
-
data. In the present work, the decomposition of 2 or 3 to form MnO4 ions
was observed after the reaction for a 2 or 3 solution and 2 or 3/clay.
(
(
-
(
(34) We confirmed that MnO4 cannot be adsorbed on mica and kaolin
at all. Other cationic decomposed species might be adsorbed on clay, but
they are not analyzed sufficiently.
1
(9) Ramaraj, R.; Kira, A.; Kaneko, M. Chem. Lett. 1987, 261.
(
10) Ruettinger, W.; Yagi, M.; K. Wolf; Bernasek, S.; Dismukes, G. C.
(35) The plots of nMnO4- versus nads should give an upward curvature
as a MnO4 ion is formed by a bimolecular reaction of 1 for 1/mica.
-
J. Am. Chem. Soc. 2000, 122, 10353.
11) Yagi, M.; Wolf, K. V.; Baesjou, P. J.; Bernasek, S. L.; Dismukes,
G. C. Angew. Chem., Int. Ed. 2001, 40, 2925.
12) Limburg, J.; Vrettos, J. S.; Liable-Sands, L. M.; Rheingold, A. L.;
Crabtree, R. H.; Brudvig, G. W. Science 1999, 283, 1524.
(
However, these plots declined at high nads. This can be explained by side
-
reactions with a consumption of the formed MnO4 ion at high nads. The
-
(
consumption of the MnO4 ion formed was observed in a solution under
IV
the conditions of 0.1 mM 1 and 10 equiv Ce after 30 min.