Pentanuclear iron catalysts for water oxidation: Substituents provide two routes to control onset potentials

Article abstract of DOI:10.1039/c9sc00678h

The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, Nature, 2016, 530, 465-468). However, the development of strategies to decrease onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex. These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the faradaic efficiencies of the reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox potentials of the pentairon complex and control of the reaction mechanism.

Full text of DOI:10.1039/c9sc00678h

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DOI: 10.1039/C9SC00678H
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ARTICLE
Pentanuclear Iron Catalysts for Water Oxidation: Substituents
Provide Two Routes to Control Onset Potentials
Vijayendran K. K. Praneeth, Mio Kondo,a,b,c,d,† Masaya Okamura, Takuya Akai, Hitoshi Izu,a,b and
Shigeyuki Masaoka*
a,†
a
a,b
Received 00th January 20xx,
Accepted 00th January 20xx
,
a,b,d
DOI: 10.1039/x0xx00000x
The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation
reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-
based water oxidation electrocatalyst (Nature, 2016, 530, 465-468.). However, the development of strategies to decrease
onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron
complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two
newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex.
These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the Faradaic efficiencies of the
reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent
complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox
potentials of the pentairon complex and control of the reaction mechanism.
www.rsc.org/
Introduction
Water oxidation (2H O → O + 4H + 4e ) is considered the main
2 2
drive the reaction under mild condition.61 The active site of the
+
−
OEC contains a multinuclear metal complex, a Mn
which has several water coordination sites. Due to the
multinuclear structure of the Mn CaO cluster, the OEC can
4 5
CaO cluster,
bottleneck in the production of chemical fuels from sunlight
and/or electricity;1-7 this is because the reaction requires the
transfer of four electrons and the generation of O-O an bond,
and is both thermodynamically and kinetically demanding.
Therefore, the development of a highly active artificial catalyst
for the oxidation of water is of great importance. In this context,
since the discovery of the first molecular water oxidation 10 mV s ) and (c) sequential oxidation of iron ions in [Fe
catalyst, “Blue dimer”, a significant number of molecular water
oxidation catalysts have been reported.
4
5
smoothly accumulate the oxidative equivalents required for the
reaction via the formation of five distinct redox intermediates,
3
+
Scheme 1 (a) Chemical structure of [Fe
5
-H] , (b) a cyclic voltammogram of [Fe
5
-
3+
H] (0.2 mM in MeCN solutions containing 0.1 M TBAP under Ar at a scan rate of
–1
3+
5
-H] and a reaction of
4
the S state to generate dioxygen.
8
9
-24
More recently,
metal complexes containing earth-abundant transition metal
ions such as Mn2
5-28
Fe
29-36
, Co
37-46
, and Cu
47-55
ions have also
,
been intensely studied. Nevertheless, the development of
efficient metal-complex-based catalysts that consist of earth-
5
6-57
abundant transition metals is still very challenging.
In nature, water oxidation is catalysed by the oxygen
5
8-60
evolving complex (OEC) in photosystem II
. The OEC is a
highly active and robust catalyst for water oxidation that can
a. Department of Life and Coordination-Complex Molecular Science, Institute for
Molecular Science (IMS), 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787,
Japan.
b.
SOKENDAI [The Graduate University for Advanced Studies], Shonan Village,
Hayama, Kanagawa 240-0193, Japan.
ACT-C, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
c.
Saitama 332-0012, Japan.
Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular
d.
Science (IMS), 38 Nishigo-naka, Myodaiji, Okazaki, Aichi 444-8585, Japan
†
These authors contributed equally.
Electronic Supplementary Information (ESI) available: Synthetic details,
characterization, crystal structures and experimental details.
See DOI: 10.1039/x0xx00000x
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to yield 1,3-di(2-pyridyl)-1,3-propanedione. The methylation of
View Article Online
DOI: 10.1039/C9SC00678H
the product and further treatment with hydrazine monohydrate
afforded Me-Hbpp. The total synthetic yield of Me-Hbpp was
2
6% (see the Experimental Section for details). Br-Hbpp was
synthesized in moderate yield (65%) by the bromination of
6
3
1
13
Hbpp. Both ligands were characterized by H and C NMR
spectroscopy and elemental analyses. The syntheses of two
pentairon complexes with the obtained ligands were performed
by reacting the corresponding ligand (6 eq.) with FeSO
4 2
•7H O
(5 eq.) in the presence of a base (NaOH, 6 eq.) in methanol at
Scheme 2. Schematic illustrations of the series of pentanuclear iron complexes 80 °C (Scheme S2). The reaction mixture was further treated
bearing 4-substituted-3,5-bis(pyridyl)pyrazole (R-Hbpp. (R = H, Me, and Br))
ligands investigated in this study
with a saturated solution of aqueous NaBF
obtained precipitate was collected by filtration. The precipitate
n
states, where the subscript indicates the number of was recrystallized from MeCN/Et O to afford crystalline
4 6
or NaPF , and the
the S
stored oxidative equivalents (n = 0−4). After the formation of products. The electrospray ionization mass spectrometry (ESI-
2
the S
4
state, H
2
O reacts with the S
4
state to generate O
2
and MS) and elemental analysis data of the obtained crystalline
samples confirmed the formation of the desired pentairon
protons.62
II
III
3+
3+
Recently, we demonstrated that a pentanuclear iron complexes, [Fe Fe (μ -O)(Me-bpp) ]
complex [FeII
bis(pyridyl)pyrazole), can serve as a highly active catalyst for the complexes were 62 and 43% for [Fe -Me] and [Fe -Br] ,
electrocatalytic water oxidation.
accumulate four oxidative equivalents via the successive the reported procedure and characterized by ESI-MS and
oxidation of each of the iron centres in the complex (Scheme elemental analysis.
5
([Fe -Me] ) and
4
3
3+
6
III
3+
3+
II
III
3+
4
Fe (μ
3
-O)(bpp)
6
] , [Fe
5
-H] (Scheme 1a, Hbpp = [Fe Fe (μ -O)(Br-bpp) ] ([Fe -Br] ). The synthetic yields of
4
3
6
5
3
+
3+
5
5
3
3
3+
3+
5
[Fe -H]
can also respectively. The parent complex, [Fe -H] , was synthesized by
5
3
3
3
+
1
b). In the catalysis mediated by [Fe
5
-H] , the four-electron-
III
7+
7+
Crystal Structures of the Pentairon Complexes
oxidized species, [Fe
with H
durability of [Fe
5
(μ
3
-O)(bpp)
6
]
([Fe
5
-H] , S
4
state), reacts
3
+
3+
2 2
5 5
O to generate O (Scheme 1c). The reaction rate and Single crystals of [Fe -Me] and [Fe -Br] suitable for single-
3
+
5
-H] are the highest among iron-based water crystal X-ray diffraction (SCXRD) were obtained by vapor
oxidation catalysts (Fe-WOCs) reported thus far. However, a diffusion of diethyl ether (Et O) into saturated solutions of the
2
relatively large onset potential is required for the catalysis respective complexes in acetonitrile (MeCN). Note that a few
3
+
4
5
because the S state is only generated at high potentials. drops of MeOH were added to the MeCN solution of [Fe -Me]
Therefore, the development of a novel strategy for designing to prevent the oxidation of the complex. As we previously
catalysts that can drive the reaction at low onset potentials is
essential.
Here, we report two approaches for decreasing the onset
potential of pentairon water oxidation systems. Two
approaches involving the installation of substituents onto the
Hbpp ligand have been demonstrated. Two kinds of ligands, one
with electron-donating and the other with electron-
withdrawing groups at the 4-position of the Hbpp (Me-Hbpp
and Br-Hbpp in Scheme 2) have been employed, and the new
pentairon complexes were constructed utilizing these ligands.
The newly synthesized complexes catalysed the oxidation of
water with high Faradaic efficiencies, and the onset potentials
of these complexes were lower than that of the parent complex.
The mechanistic studies also revealed that two distinct routes
exist to decrease the onset potentials for water oxidation in
pentanuclear iron systems.
Results
Syntheses and Characterization of Ligands and Pentairon
Complexes
Me-Hbpp was prepared by the three-step synthetic route
shown in Scheme S1 in Electronic Supplementary Information Figure 1. ORTEP drawings of the cationic moieties (left) and core structures (right)
of (a) [Fe -Me](PF and (b) [Fe -Br](BF . The atoms are represented by the
5
6
)
3
5
4 3
)
(
ESI). Initially, 2-acetylpyridine and pyridine-2-carboxylic acid following colours: Fe, orange; O, red; N, blue; C, grey; and Br, purple. Hydrogen
atoms and crystal solvent molecules are omitted for clarity. Thermal ellipsoids are
methyl ester were reacted in the presence of sodium ethoxide shown at 30% probability.
2
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reported,33 single crystals of [Fe
-H]3+ were obtained by slow
O (v/v) solution
-Me] was obtained as the PF salt, and
-H]3+ were obtained as the BF
salts. The
5
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DOI: 10.1039/C9SC00678H
evaporation of acetonitrile from a 1:1 MeCN-H
2
3
+
of the complex. [Fe
Fe
-Br]3+ and [Fe
5
6
[
5
5
4
ORTEP diagrams of the cationic moieties of the three complexes
are shown in Figures 1 and S1, and the crystallographic data for
the newly synthesized complexes is summarized in Table S1. All
complexes displayed the same structural motif consisting of a
central [Fe
3
(μ
3
-O)] core connected by two apical Fe ions through
six R-bpp‒ units. [Fe
3+
-Me] crystallized in P1, and the
5
asymmetric unit contains one cationic pentairon complex and
three PF
anions. The asymmetric unit of the R3 crystal of [Fe
Br] is composed of one-third of the cationic pentairon complex
6
5
-
3
+
3
+
4 5
and one BF anion. The crystal structure of [Fe -H] belongs to
3
+
Figure 2. Cyclic voltammograms of 0.2 mM solutions of [Fe
5 5
-Me] (red line), [Fe -
the I4 space group, and the asymmetric unit contains half of the Br]3+ (blue line) and [Fe -H]3+ (black line). Measurements were performed in MeCN
cationic pentairon complex and one and half of the BF
The bond distances between the iron atoms and the N atoms
on R-bpp ligands are not significantly changed by either bromo Table 1. Redox potentials (E , V vs. Fc/ Fc
or methyl substitution (Table S2). These results clearly (0.1 M). E (−1), E (1), E (2), E (3), and E (4) correspond to the E values of the
demonstrate that the substituents do not affect the redox couples, respectively.
pentanuclear core structure of the complexes (Figure S2).
5
–1
solutions containing 0.1 M TBAP under Ar at a scan rate of 10 mV s . Working
anions. electrode, glassy carbon; counter electrode, Pt wire; reference electrode, Ag/Ag
CV scans were initiated from the respective open-circuit potentials.
+
4
.
−
+
1
/2
) and the onset potentials for water oxidation
+
(
E
onset, V vs. Fc/ Fc ) of a series of pentairon complexes in acetonitrile solutions with TBAP
1
/2
II
1/2
III
1/2
II
1/2
II
1/2
3
1/2
II
III
II
III
III
II
III
II III
II
III
III
II III
Fe
4
Fe /Fe
5
, Fe
3
Fe
2
/Fe
4
Fe , Fe
2
Fe
3
/Fe
Fe
2
, Fe Fe
4
/Fe
2
Fe
3
, and Fe
5
/Fe Fe
4
E
1/2
1
)
Complex
E
onset
UV-Vis Absorption Spectra
E1/2(−1)
E1/2(1)
E1/2(2)
E
1/2(3)
E1/2(4)
[
[
Fe
Fe
Fe
5
-Me]3+
-Br]3+
-H]3+
−0.60
−0.32
−0.55
0.05
0.32
0.13
0.24
0.48
0.30
0.62
0.87
0.68
1.01
1.27
1.08
1.09
1.15
1.18
5 5
The UV-Vis absorption spectra of [Fe -Me] , [Fe
-Br]3+ and
Fe -H] in MeCN are shown in Figure S3. The complexes
5
exhibited intense bands at approximately 400-410 nm and
shoulder peaks at 480-490 nm. These bands are attributed to
the charge transfer from the low-spin Fe centre to the ligand
In our previous study, Mössbauer spectroscopic
-H] showed that the two iron centres at
the apical positions are low-spin Fe ions and that the [Fe
3
+
5
3
+
[
[
5
1
2
) Condition: [cat] = 0.2 mM, [H O] = 5 M, working electrode: glassy carbon, scan
–1
II
rate: 10 mV s
.
3,64
(
MLCT).3
positively shifted (Figure 2 and Table 1). These trends are
consistent with the electron-donating and electron-
withdrawing properties of the methyl and bromo substituents,
respectively. This result clearly demonstrates that the redox
potentials of the pentairon complexes can be tuned by the
introduction of substituents on the ligands. The open-circuit
3
+
measurements of [Fe
5
II
3
(μ
3
-
II
O)] core is composed of two high-spin Fe ions and one high-
III
33
spin Fe ion. The UV-Vis absorption spectra strongly indicate
that the oxidation and spin states of the iron centres of [Fe
5
-
3
+
3+
3+
Me] and [Fe
the MLCT bands of [Fe
redshifted and blueshifted, respectively, compared to those of
5
-Br] are identical to those of [Fe
5
-H] . Note that
3
+
potentials of the complexes, located at –0.10 ([Fe
5
-Me] ), –
3
+
-Br]3+ are slightly
5
-Me] and [Fe
5
3
+
3+
0
.18 ([Fe
5
-Br] ), and –0.26 V ([Fe
5
-H] ), indicate an initial state
FeIII in solution. These initial oxidation states of the
complexes in solution states determined by the electrochemical
measurements are fully consistent with those estimated from
the UV-vis absorption spectra (vide supra).
II
of Fe
4
3
+
[
5
Fe -H] , reflecting the electron-donating or electron-
withdrawing nature of the substituents.
Electrochemical Properties
The influence of the electron-donating and electron-
withdrawing groups on the redox properties of the pentairon
complexes was investigated by cyclic voltammetry. The cyclic
Catalytic Activity for Water Oxidation
The electrocatalytic activities of [Fe
5 5
-Me]3+ and [Fe -Br]3+ for
water oxidation were examined by electrochemical
measurements of the solution containing 0.2 mM of the
complexes in the presence of 5 M H
both complexes exhibited a large irreversible current in the >
3
+
3+
3+
5 5 5
voltammograms (CVs) of [Fe -Me] , [Fe -Br] and [Fe -H]
(
0.2 mM) in dry MeCN containing 0.1 M Bu
4
NClO (TBAP) under
4
2
O. Under these conditions,
an Ar atmosphere are shown in Figure 2, and the
electrochemical data are summarized in Table 1. All the
complexes displayed one reversible and four successive
reversible one-electron oxidation waves assigned to the
3
+ 33
1
5
.0 V region (Figure 3). As previously reported for [Fe -H] ,
such an increase in the current is indicated of promotion of the
electrocatalytic water oxidation reaction. To further verify
whether the electrocatalytic current actually corresponds to the
activity in the catalytic oxidation of H
electrolysis (CPE) was carried out at an indium tin oxide (ITO)
II
III
II
II
III
II
III
II
III
II
III
Fe
4
Fe /Fe
5
,
Fe
Fe Fe
3
Fe
2
/Fe
4
Fe ,
2 3 3 2
Fe /Fe Fe ,
II III
/FeII
III
III
II III
Fe Fe
4
2
3
, and Fe /Fe Fe redox couples, indicating
5 4
2
O, controlled potential
that the electron transfer ability arising from the pentairon
structure is preserved even after the introduction of
substituents on the ligands. Importantly, all the redox waves of
3
3
electrode using a customized two-compartment cell system.
3
+
+
5
After 2 h of electrolysis of [Fe -Me] at 1.42 V (vs. Fc/Fc ), 17.2
[
Fe
5
-Me]3+ were shifted to a more negative potential relative to
C of charge was passed, and gas chromatography (GC) had
3
+
3+
those of [Fe
5 5
-H] , whereas the redox waves of [Fe -Br] were
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3
+
3+
Figure 4. Controlled potential electrolysis data of (a) [Fe
in a MeCN/H O mixed solution system containing 0.1 M TBAP using an ITO
electrode under an argon atmosphere. Conditions: [cat] = 0.2 mM, [H O] = 5 M
5 5
-Me] and (b) [Fe -Br]
2
2
+
and applied potential: 1.42 V vs. Fc/Fc , pH = 5
.
almost identical (Figures S7 and S8), suggesting no formation of
heterogeneous deposits during the electrolysis. We also
analysed the electrolyte solutions after the electrolysis by
dynamic light scattering (DLS) measurements and no formation
of heterogeneous nano particles were detected (Figure S9).
3
+
3+
These experimental results show that [Fe
5 5
-Me] and [Fe -Br]
can serve as homogeneous electrocatalysts for water oxidation.
3
+
3+
Figure 3. CVs of 0.2 mM solutions of (a) [Fe
5
-Me] and (b) [Fe
5
-Br] in MeCN Onset Potentials for Water Oxidation
containing 0.1 M TBAP in the absence of H
2
O (black lines) and the presence of 5
M H
2
O (pH = 5, red lines). The CVs were measured using a GC electrode under an
3
+
–1
5
Although both of the novel pentanuclear complexes, [Fe -Me]
Ar atmosphere at a scan rate of 10 mV s .
3
+
5
and [Fe -Br] , exhibited catalytic activity for water oxidation,
detected 39.5 μmol of O
2
as product (Figure 4a). In the case of the electrochemical responses of these complexes in the
3
+
3+
[
Fe
5
2 5
-Br] , the electrolysis under identical conditions afforded a presence of H O were different. In the case of [Fe -Me] , a
charge of 4.7 C and generated 9.3 μmol of O
2
(Figure 4b). The large irreversible current attributed to the catalytic water
Faradaic efficiencies of the reaction based on the 4e process oxidation was observed at a potential close to the fourth redox
−
3
+
3+
III
II III
were 92 and 86% for [Fe
5
-Me] and [Fe
5
-Br] , respectively. couple (Fe
O (Figure 3a). The onset potential (Eonset, Table 1) for the
TOFs) and turnover numbers (TONs) for water oxidation were reaction was estimated from the cross point of two lines that
5 4
/Fe Fe , E1/2(4) in Table 1) in the presence of 5 M
Based on the results of CPE experiments, turnover frequencies
H
2
(
3
+
2
roughly estimated. For [Fe
s
5
-Me] , TOF and TON were 3 × 10
and 2 × 10 , respectively, and TOF and TON values of [Fe
were obtained by extrapolating the slopes of the catalytic
current and non-Faradaic current. The Eonset of [Fe
−
1
6
3+
5
-
5
-Me] was
3
+
−1
5
Br] were estimated to be 20 s and 1 × 10 , respectively (for determined to be 1.09 V, which corresponds to the
the details of calculation see the ESI (P.S24)). Although these overpotential (η) of 0.65 V at pH = 5.0, and was slightly larger
3
+
3 -1
values were lower than those of [Fe
7
5
-H] (1 × 10 s (TOF) and than E1/2 of the fourth redox couple (1.01 V). In contrast, the
6
3+
.5 × 10 (TON)) estimated by using the same method, they catalytic current for water oxidation for [Fe
5
-Br] was observed
were substantially higher compared to the reported iron- at a more negative potential than the fourth redox couple (1.27
2
9-32,35
3+
complex-based catalyst for water oxidation.
the electrolyzed solutions were treated with oxo[5,10,15,20- = 0.71 V at pH = 5.0). These results clearly demonstrate that
5
In both cases, V, Figure3b); Eonset of [Fe -Br] was determined to be 1.15 V (η
6
5
3+
3+
tetra(4-pyridyl)porphyrinato]titanium (IV) as a chemical probe
5 5
[Fe -Me] and [Fe -Br] catalyse the water oxidation reaction
) was not detected by a distinct reaction mechanism. Notably, the Eonset of these
for details of experimental procedure, see the ESI (P.S22-23)). complexes is lower than that of [Fe -H] (1.18 V, η = 0.74 V at
5
−
and the 2e oxidized product of H
2
O (H
2
O
2
3
+
(
After the CPE experiment, the ITO working electrodes used in pH = 5.0) under identical experimental conditions.
the electrolysis were gently rinsed with small amounts of water
and MeCN, and then, a second round of electrolysis was
performed using the solution without the catalyst. Significantly
Discussion
small currents were observed in the second electrolysis
Reaction Mechanism of [Fe
-Me]3+
5
compared to the first electrolysis in both cases (Figures S4 and
S5), which indicates that the species homogeneously dissolved
in the solution are catalytically active. CV measurements of the
solution after the CPE experiments also clarified the presence
of pentanuclear complexes in the solution phase (Figures S6).
Additionally, the UV-vis absorption spectra of the ITO
electrodes before and after the CPE experiments remained
As described above, the Eonset of [Fe
slightly more positive potential than the E1/2 of the fourth redox
couple (Table 1). In other words, the formation of the four-
electron oxidized species (Fe
reaction with a water molecule and the subsequent oxidation
of water in this case. A similar trend was also observed in the
5
previously reported electrocatalysis by [Fe -H] ; the onset of
5
-Me]3+ is located at a
III
5
, the S
4
state) triggers the
3
+
4
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the catalytic wave is coupled with the formation of the four-
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5
electron oxidized species. Therefore, it is suggested that [Fe -
3
+
Me] probably promotes electrocatalytic water oxidation
3
+
through a catalytic cycle similar to that of [Fe
5
-H] , which we
previously proposed based on experimental and computational
3
3
studies (Figure 5, see also the ESI (P.S32)). In the catalytic
cycle, the successive four-step, one-electron oxidation of the
II
III
resting Fe
4
Fe (the S
0
state) initially generates the four-
III
electron oxidized species Fe
5
(the S
4
state) via the S
1
, S
-O)] core
O to this
state generates the
) species (intermediate A). Intermediate
O molecule to generate O
state. Therefore, the shift of the
-Me] (approximately 90 mV lower than that of
-H] ) is attributed to the electron-donating nature of the
2 3
and S
states. In the S
4
state, all the iron atoms in the [Fe
3
(μ
3
III
are in the Fe state. Subsequently, the addition of H
2
fully oxidized [Fe
3
(μ
(OH
3
-O)] core of the S
4
III
water-bound Fe
5
2
A then reacts with an additional H
and regenerate the initial S
2
2
0
3
+
E
onset of [Fe
5
3
+
[
Fe
5
Figure 5. Proposed reaction mechanism for the water oxidation catalyzed by [Fe
5
-
methyl groups, which reduces the potential to generate the S
4
state.
Me]3 and [Fe
+
-H] . Fe , blue; Fe , red; and Fe , purple.
3+ 33
II
III
IV
5
3
+
3
Figure 6. Cyclic voltammograms of [Fe
5
-Br] (0.2 mM) in the presence of (a) 1.5, (b) 2.0, (c) 3.0, (d) 4.0, and (e) 5.0 M H
2
O at pH = 5.0 (red lines) and in the absence of
O. CVs were measured in acetonitrile solutions with TBAP (0.1 M) on a GC electrode at a
scan rate of 10 mV s . The onset potentials were estimated from the cross points of two lines, which are obtained by extrapolating the slopes of the catalytic current
and non-Faradaic current.
+
H
2
O (black lines). (f) Overlaid CVs of [Fe -Br] at various concentrations of H
5 2
–1
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Page 6 of 12
ARTICLE
Journal Name
View Article Online
DOI: 10.1039/C9SC00678H
3
+
3
+
5
Figure 8 UV-Vis absorption spectra of [Fe -Br] (0.05 mM) at various applied
+
5
Figure 7 UV-Vis absorption spectra of [Fe -Me] (0.05 mM) at various applied
+
potentials ((a) 0.42, (b) 0.64 and (c) 1.00 V (vs. Fc/Fc )) in 0.1 M TBAP/MeCN.
Solutions were purged with Ar for 15 min prior to measurements. Weak Ar flow
was maintained throughout the measurements.
potentials ((a) 0.17, (b) 0.50 and (c) 0.86 V (vs. Fc/Fc )) in 0.1 M TBAP/MeCN.
Solutions were purged with Ar for 15 min prior to measurements. Weak Ar flow
was maintained throughout the measurements.
-Br]3+
3 5
state of [Fe
measurements were performed at various concentrations of
Reactivity of the S
5 5
Unlike [Fe -H] , the catalytic current for [Fe -
-Me]3+ and [Fe
3
+
[
5 3
Fe -Br] to confirm if the S state undergoes unimolecular or
3+
5
bimolecular reactions. As shown in Figure S12, the intensity of
the catalytic peak current was linearly dependent on the
3
+
Br] was not coupled with the fourth redox wave. Instead, the
catalytic current (Eonset = 1.15 V) arises in the potential region
between third (0.87 V) and fourth (1.27 V) redox waves (vide
3
+
5
concentration of [Fe -Br] , suggesting that the rate is first order
to the catalyst concentration, and therefore bimolecular path
requiring the association of catalysts is ruled out. Second, we
II III
supra). This result indicates that the Fe Fe
4 3
state (the S state)
generated by the third redox process should undergo a
chemical reaction (EC process), and the formed species should
be further oxidized at 1.15 V. Alternatively, a concerted
chemical and electrochemical process should occur at 1.15 V. In
hypothesized that a reaction of the S
the H O bound species and subsequently oxidise the formed
species is a possible pathway. To validate this hypothesis, CVs
were collected at various scan rates in the presence of 5 M H
3 2
state with H O to form
2
2
O
any case, the S
water oxidation in the case of [Fe
Intrigued by this finding, we set out to investigate the
3
state is considered the key intermediate for
by reversing the scan of potentials at 1.04 V. As shown in Figure
S13a, the redox potentials (E1/2 values) and the wave shapes of
the first three redox couples remained unchanged. Additionally,
3
+
5
-Br] .
3
+
3 5
reactivity of the S state of [Fe -Br] . Initially, electrochemical
6
| J. Name., 2012, 00, 1-3
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PleaseC dh oe mn oi ct a al dS cj ui es nt cme argins
Journal Name
the reversibility of the third redox couple (Fe FeIII
ARTICLE
II
/FeII
FeIII
II III
-O)]4+ (the S
4
2
3
)
bpp)
3 2
}
Fe Fe
2
(μ
3
1
state). In the second step, the
View Article Online
was investigated by plotting the intensity of the anodic and intensity of the MLCT band at 406 nmDdOraI:s1t0i.c1a0l3l9y/Cd9eScCr0e0a6s7e8dH,
cathodic peak currents against the square root of the scan rates suggesting that both iron centres at the apical positions are
(Figure S13b). The linearity of the obtained plot confirms that oxidised during the second step. Therefore, in the second step,
the third redox process is fully reversible and that no EC process the oxidation of the complex induces an intramolecular electron
III
occurs in this potential range. Therefore, the reaction of the S
state with H
this pathway can be excluded. Third, the possibility of the S
3
transfer process, and the species with two Fe ions at apical
III
II
2
O does not proceed in this potential region, and positions and one Fe and two Fe ions in the central core,
III
II
III
5+
3
3 2 2 3 2
[{Fe (μ-bpp) } Fe Fe (μ -O)] (the S state), forms. Further
state undergoing a proton-coupled electron transfer (PCET) oxidation of the complex increased the intensity of the band at
reaction was considered because catalytic water oxidation approximately 550 nm, which was attributed to the oxidation of
1
5
III
II
III
6+
reactions often involve such a process. However, this process the central core affording, [{Fe (μ-bpp)
hardly occurs because no dissociative proton exists in the S state). Notably, these redox transformations of the
-Br] recorded under various complexes, which involve an intramolecular electron transfer
pH conditions showed no change in the onset potential for process in the second step, are fully consistent with the
3 2 2 3
} Fe Fe (μ -O)] (the
3
S
3
3
+
state. Moreover, the CVs of [Fe
5
3
3,64
water oxidation (Figure S14). Therefore, the S
a PCET process is also unlikely. Finally, an electron transfer similar trend was also observed for [Fe
3
state undergoing previously reported Mössbauer study in the solid-state.
A
3
+
5
-Me] , as shown in
-Me] undergo an
3
+
3+
reaction coupled with water binding to the S
3
state was Figure 7. Therefore, [Fe
-Br] at various concentrations of H
were acquired. As shown in Figures 6 and S15, the onset conversions from Fe
potentials of the electrocatalytic current gradually shifted to states serve as key intermediates in the catalytic reaction.
5
-H] and [Fe
5
3
+
investigated. CVs of [Fe
5
2
O
electron transfer reaction in an identical manner during their
II
III
III
4
5 4
Fe to Fe (Scheme 3, Path A), and the S
3
+
lower potentials as the content of H
2
O increased. Note that the
5
In the case of [Fe -Br] , the spectral changes that occurred
onset potential of water oxidation was not affected by the upon the first and second oxidations were quite similar to those
3
+
3+
3+
concentrations of H
2 5 5 5
O in the case of [Fe -H] (Figure S16). This of [Fe -H] and [Fe -Me] ; a slight decrease in the MLCT band
result clearly demonstrates that the electron transfer reaction at 402 nm and the emergence of the new band at approximately
coupled with the binding of H
2
O to the S
3
state is the key step 660 nm in the first step and a drastic decrease in the MLCT band
in the second step (Figures 8 a and b). Therefore, the iron ion at
3
+
5
in the [Fe -Br] -catalysed reaction.
the central core is oxidised in the first step, and an oxidation-
Electronic Structures of S
3
states
III
induced intramolecular electron transfer affords [{Fe (μ-Br-
II
III
5+
To clarify the origin of the unique reactivity of the S
3
state of bpp)
3 2 2 3 2
} Fe Fe (μ -O)] (the S state) in the second step.
3
+
[
Fe
5
-Br] , the electronic structures of the S states of a series of However, in the third step, the complex exhibited spectral
3
3
+
3+
pentanuclear iron complexes were investigated. As we changes that were unlike those of [Fe
5
-H] and [Fe
-H]3+ this step, the intensity of the band at approximately 580 nm
state as evidenced by UV-vis absorption decreased (Figure 8c), whereas the corresponding bands for
5
-Me] . In
3
3
previously reported, the three-step oxidation of [Fe
affords the S
spectroscopy (Figures S17 and S18). In the first step, a slight [Fe
decrease in the MLCT band at 406 nm and a growth of the new that all iron atoms in the [Fe
5
3
3
+
3+
5
-H] and [Fe
5
-Me] grew (vide supra). This result indicated
(μ -O)] core are oxidized in [Fe
broad peak at approximately 640 nm were observed. This newly Br] . In other words, an additional intramolecular electron
3
3
5
-
6
+
II III
observed peak is attributed to the formation of the [Fe Fe
2
(μ
3
-
transfer reaction generates the three-electron oxidized species,
O)] central core.33 In other words, the first step corresponds to and the oxidation states of the iron ions can be described as
II
II
III
III
6+
3 3 3 3 3
the oxidation of the central core, which yields [{Fe (μ- [{Fe (μ-Br-bpp) }{Fe (μ-Br-bpp) }Fe (μ -O)] (the S state).
Scheme 3 Oxidation processes of (Path A) [Fe
5 5 5
-H]n+ and [Fe -Me]n+ and (Path B) [Fe -Br]n+
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Page 8 of 12
ARTICLE
Journal Name
3
+
5 3
View Article Online
In the [Fe -Br] -catalysed reaction, the formed S state reacts groups at the 4-position of the Hbpp ligand were successfully
DOI: 10.1039/C9SC00678H
with water via an electron transfer reaction coupled with the synthesized, and the complexation of the ligands (Me-Hbpp and
3
+
biding of H
afford the key intermediate can be summarized as shown in complexes, [Fe
Scheme 3, Path B. These results clearly demonstrate that the similar to that of the parent complex, [Fe
2 5
O (vide supra). Thus, the oxidation of [Fe -Br] to Br-Hbpp) with iron resulted in the formation of pentanuclear
3
+
3+
5
5
-Me] and [Fe -Br] , which have structures
3
+
5
-H] .
All of the three complexes exhibited five reversible redox
3
+
electronic structure of the S
different from those of [Fe
-H]3+ and [Fe
the origin of the unique reactivity of [Fe
3
state of [Fe
5
-Br] is completely
3+
5
5
-Me] , which may be waves, which can be attributed to the sequential redox
3
+
5
2
-Br] during catalysis. processes of iron ions in the absence of H O. However, the
redox potentials of the complexes were significantly changed by
the electronic effect of the substituents installed on the ligands.
Reaction Mechanism of [Fe
-Br]3+
5
3
+
Based on the aforementioned experimental evidence, a The redox potentials of [Fe
5
-Me] were shifted to more
3
+
plausible reaction mechanism for the water oxidation reaction negative potentials compared to those of [Fe
5
-H] , whereas the
-Br]3 was proposed. As depicted in Scheme 3, redox waves of [Fe
-Br] were observed at more positive
5 5
+
3+
catalysed by [Fe
the first step involves a sequential, stepwise three-electron potentials than those of [Fe
3
+
5
-H] . In the presence of H
2
O, both
3
+
3+
oxidation of the initial S
and S states), which includes a two-step intramolecular to water oxidation reactions, similar to [Fe
electron transfer process. In the S state of the complex, all iron experiments of the complexes revealed that the Faradaic
atoms in the [Fe (μ -O)] core are in the Fe state. Subsequently, efficiencies for the catalysis were 92 and 86% for [Fe
a concerted process involving water binding to the fully oxidized [Fe
Fe (μ -O)] core coupled with a one-electron oxidation process by [Fe
gives the water-bound Fe
Scheme 3, Path B). Intermediate
intermediates B and C, and the release of O
C regenerates the initial S state and produces O
Figure S19). Thus, the formation of the S state is favorably parent [Fe
0 3 5 5
state to produce the S state (via the [Fe -Me] and [Fe -Br] exhibited catalytic currents attributed
3
+
S
1
2
5
-H] . CPE
3
III
3+
3
3
5
-Me] and
-Br] , respectively. The onset potentials for water oxidation
-Me] and [Fe -Br] were lower than that of [Fe -H] .
5 5 5
3
+
5
3
+
3+
3+
[
3
3
III
5
(OH
2
) species, intermediate A
To clarify the origin of the lower onset potentials of the
(
A
then generates complexes, their catalytic mechanisms were investigated. In the
3
+
2
from intermediate case of [Fe
5 4
-Me] , the formation of the S state triggers the
0
2
as a product catalytic reaction, which is similar to the pathway seen with the
3
+
(
4
5
-H] complex. Therefore, the decrease in the onset
3
+
3+
bypassed in the catalytic cycle of [Fe
potentials to form the S
for the catalysis in the case of [Fe
5
5
-Br] , whereas the redox overpotential for [Fe -Me] is attributed to the electron-
4
states determine the onset potentials donating nature of the methyl substituents, which allows the
3
+
3+
5
5 4
-Me] and [Fe -H] . As a generation of the S state in the more negative potential region.
3
+
result, the onset potential for water oxidation was lower for In contrast, in the catalysis mediated by [Fe
5
-Br] , the three-
-Br]3+ compared to [Fe
-H] if a sufficient amount of electron oxidized species (the S
3+
state) served as a key
[
Fe
5
5
3
substrate was added to the reaction mixture (Figure 9). The intermediate due to its unique electronic structure, and the
result also implies that the generation of the fully oxidized state undergoes a water binding reaction coupled with an
[
Fe
3
(μ
3
-O)] core is essential for initiating catalysis.
electron transfer to initiate the catalytic reaction. In other
words, the generation of the S state was bypassed in this
4
system, which enables the catalytic reaction to occur at a lower
onset potential. Our results reveal that not only the simple
tuning of the redox potentials by the introduction of an
electron-donating group but also the control over the reaction
mechanism by the introduction of an electron-withdrawing
group can be valuable strategies for controlling onset
potentials.
Experimental Section
Materials
Pyridine-2-carboxylic acid methyl ester, 2-acetyl pyridine, 3,5-
bis(2-pyridyl)pyrazole, methyl iodide, oxo[5,10,15,20-tetra(4-
2
Figure 9. A plot of onset potentials as a function of the concentration of H O for
3
+
3+
pyridyl)porphyrinato]titanium
(IV),
1,8-
[
5 5
Fe -Br] (red line) and [Fe -H] (black line)
diazabicyclo[5.4.0]undec-7-ene and tetraethylammonium
perchlorate were purchased from Tokyo Chemical Industry Co.,
Ltd. NaBF
Chemical Industries, Ltd. FeSO
4
and NaOH were purchased from Wako Pure
·7H O was purchased from
Conclusions
4
2
In conclusion, we have demonstrated the syntheses,
electrochemical behaviours, and catalytic activity of a series of
pentanuclear iron complexes. Two types of ligands bearing
electron-donating (methyl) or electron-withdrawing (bromo)
Kanto Chemical Co., Inc. Potassium bis(trimethylsilyl)amide and
tris(4-bromophenyl)ammoniumyl hexachloroantimonate were
purchased from Aldrich and were used as received. An ITO-
coated glass working electrode (0.7 mm thick, 10 Ω/sq) was
8
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
purchased from Furuuchi Chemical Co. Ltd. [FeII
O)(bpp) ](BF 7H ([Fe -H](BF 7H
using the reported method. All solvents and reagents were of washed with purified H
III
electrodes, respectively. Between scans, the working electrode
4
Fe (μ
3
-
View Article Online
DOI: 10.1039/C9SC00678H
6
)
4 3
2
O
5
3
)
4 3
2
O) was synthesized was polished with 0.05 μm alumina paste (from BAS Inc.) and
3
2
O. All the redox potentials of the
the highest quality available and were used as received except samples presented in this paper were calibrated against the
+
for
tetrabutylammonium
perchlorate
(TBAP). redox potential of the ferrocene/ferrocenium couple (Fc/Fc ).
Tetrabutylammonium perchlorate was recrystallized from
absolute ethanol.
Controlled Potential Electrolysis
Controlled potential electrolysis experiments were performed
General Methods
in
JEOL JNM-LA 400 separated by an anion-exchange membrane. In the first
spectrometer. UV-Vis spectra were recorded on a SHIMADZU compartment, the ITO working electrode (1.0 cm × 1.5 cm) and
UV-2550UV-Vis spectrophotometer or UV-Vis Agilent Ag/Ag+ reference electrode were immersed in an electrolyte
Cary8454 spectrophotometer with a conventional quartz solution (0.1 M Bu NClO in acetonitrile/water (10:1) mixed
a
custom-designed gas-tight two-compartment cell
3
3
NMR spectra were recorded on
a
a
4
4
cuvette (path length, l = 1 cm). Spectroelectrochemical studies solvent) containing the catalyst (0.2 mM). In the second
were performed using a BAS Inc. spectroelectrochemical quartz compartment, the platinum auxiliary electrode was immersed
cell (l = 1 mm) containing Pt gauze (working electrode), a Pt wire in the electrolyte solution. The amount of evolved oxygen in the
(
auxiliary electrode) and a Ag/Ag+ (reference electrode) in headspace of the reaction cell was quantified by gas
conjunction with the CH Instruments potentiostat. Elemental chromatography. Subsequently, the potential production of
analyses were performed on a J-SCIENCE LAB MICRO CORDER liquid products (e.g., H ) in the reaction was analysed by
JM10 elemental analyser. ESI-TOF mass spectra were recorded treating the electrolyzed solution with oxo[5,10,15,20-tetra(4-
2 2
O
6
5
on a JEOL JMS-T100LP mass spectrometer. Gas analysis of O
2
pyridyl)porphyrinato]titanium (IV) as a chemical probe.
was performed using a Shimadzu GC-2014 gas chromatograph
equipped with a thermal conductivity detector and fitted with a
Syntheses
molecular sieve (5 Å) column, and the system was calibrated 1,3-Bis(2-pyridyl)-propane-1,3-dione (I)
with air. Dynamic light scattering (DLS) data were measured To a solution of pyridine-2-carboxylic acid methyl ester (2 g,
using Photal OTSUKA ELECTRONICS ELSZ-1000 zeta-potential 16.5 mmol) in anhydrous toluene (40 mL) under an argon
and particle size analyser, equipped with a 785 nm red laser atmosphere was added freshly prepared sodium ethoxide
source (detection limit: 0.6 nm in particle diameter).
solution (9.9 mL, 2 M, 19.8 mmol). After heating the reaction
mixture to 55 °C, a solution of 2-acetyl pyridine (2.26 g, 16.5
mmol) in anhydrous toluene (10 mL) was added. After stirring
X-ray Crystallography
Data collection for [Fe
-Me]3+ and [Fe -Br]3+ was performed at the resulting mixture for 2 h at 55 °C, a dark yellow precipitate
5 5
1
23 K on a ROD, Synergy Custom system (Rigaku Oxford appeared, and the reaction was stirred overnight at room
Diffraction) equipped with confocal monochromated Mo-Kα temperature. The solvent was then evaporated, and the crude
radiation, and data were processed using CrysAlisPro product was poured into ice and neutralized to pH 7 with acetic
1
.171.39.43c (Rigaku Oxford Diffraction). The structures were acid (50%). The resulting solid was collected by filtration and
6
6
solved by direct methods using SIR-92 and refined by the full- dried under vacuum to give compound I (1.92 g, 52%). The
matrix least squares techniques on F (SHELXL-97). All product was used for the next step without further purification.
nonhydrogen atoms were refined anisotropically and refined 1,3-Bis(2-pyridyl)-propane-2-methyl-1,3-dione (II)
2
67
with a riding model with Uiso constrained to be 1.2 times Ueq of Under an argon atmosphere, compound I (0.5 g, 2.21 mmol)
the carrier atom. The diffused electron densities resulting from was dissolved in 30 mL of anhydrous toluene. Potassium
the residual solvent molecules were removed from the data set bis(trimethylsilyl)amide (6.63 mL, 0.5 M, 3.32 mmol) was then
6
8
using the SQUEEZE routine of PLATON and refined further added to the solution. The resulting suspension was heated to
using the generated data. Crystallographic data have been 80 °C, and the colour of the reaction mixture changed from
deposited with the Cambridge Crystallographic Data Centre: orange to green. Then, methyl iodide (1.57 g, 11.05 mmol) was
Deposition numbers CCDC 1872481 and 1872482 for [Fe
5
-
added. After heating the resulting mixture at 80 °C for 5 h, the
Me](PF
can
6
)
3
and [Fe
be obtained
5 4 3
-Br](BF ) , respectively. Copies of the data reaction was stirred overnight at 50 °C. After cooling to room
free
of
charge
via temperature, the reaction was quenched by the addition of 10%
www.ccdc.cam.ac.uk/data_request/cif.
NaHCO (10 mL) followed by brine (10 mL) and extracted with
3
CH
2 2 2 4
Cl . The organic phases were dried over anhydrous Na SO
Electrochemical Studies
and filtered. The filtrate was concentrated under reduced
Electrochemical experiments were performed on a BAS ALS pressure. The crude oily product was purified by silica gel
Model 650 DKMP electrochemical analyser at room column chromatography using 30% EtOAc/n-hexane as the
temperature under Ar. Cyclic voltammetry experiments were eluent to give compound II as a light yellow solid (0.43 g, 82%).
performed using a one-compartment cell with a standard three-
1
3
H NMR (400 MHz, CDCl ): δ = 8.48 (m, 2 H), 8.07 (dt, J = 7.7 Hz,
electrode configuration, which consisted of a glassy carbon disk J = 1.2 Hz, 2 H), 7.81 (td, J = 7.7 Hz, J = 1.7 Hz, 2 H), 7.40 (m, 2H),
(
diameter 3 mm, from BAS Inc.), a Ag/Ag+ couple, and a 5.72 ppm (q, J = 7.06 Hz, 1 H), 1.54, J = 7.02 Hz, 3 H); C NMR
13
platinum wire as the working, reference and auxiliary (101 MHz, CDCl ): δ = 199.3, 152.4, 148.9, 137.2, 127.1, 122.5,
3
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Page 10 of 12
ARTICLE
Journal Name
: C [FeII
Fe (μ
III
-O)(Br-bpp)
](BF
([Fe
-Br](BF
)
)
5
6
4
0.2, 13.2 ppm; Elemental analyses calcd (%) for C14
9.99, H 5.03, N 11.66; found C 69.98, H 4.92, N 11.60.
-Methyl-3,5-bis(2-pyridyl)pyrazole (Me-Hbpp)
H
N O
12 2 2
4
3
6
4
)
3
5
4
3
View Article Online
DOI: 10.1039/C9SC00678H
A solution of FeSO 7H O (0.08 g, 0.28 mmol) in methanol (3 mL)
was added to a stirred solution of Br-Hbpp (0.10 g, 0.33 mmol)
4
2
Compound II (0.33 g, 1.4 mmol) was dissolved in anhydrous and NaOHaq (1 M, 0.33 mL, 0.33 mmol) in MeOH (10 mL). The
ethanol (25 mL), and the solution was degassed with Ar for 30 resulting dark red solution was refluxed under air at 80 °C for 12
min. To this solution was added hydrazine monohydrate (0.28 h. The reaction mixture was then filtered to remove the
g, 5.6 mmol), and the reaction mixture was refluxed at 95 °C undissolved residue. The obtained filtrate was precipitated with
4
under Ar for 3 h. After concentrating the resulting solution by a saturated aqueous solution of NaBF to give a brown
rotary evaporation, the solution was kept in a refrigerator precipitate, which was collected by filtration, washed with
overnight. A precipitate formed, and it was collected by water, and dried under vacuum. The obtained precipitate was
filtration and washed with a small amount of cold ethanol to dissolved in acetonitrile and subjected to vapour diffusion in
1
II
III
give Me-Hbpp as a white solid (0.19 g, 61%). H NMR (400 MHz, diethyl ether to afford dark red crystals of [Fe
CDCl ): δ = 12.04 (s, 1 H), 8.69 (m, 2 H), 7.80 (d, J = 7.8 Hz, 2 H), bpp) ](BF 4H O. The crystals were collected by filtration and
.78 (m, 2H), 7.25 (m, 2H), 2.74 ppm (s, 3 H); C NMR (101 MHz, dried under vacuum. Yield 0.06 g (43%). Elemental analysis
CDCl ): δ = 151.5, 149.5, 136.9, 122.5, 121.8, 113.8, 11.1 ppm; calcd. (%) for Fe : C 38.58, H 2.32, N 13.84;
Elemental analysis calcd (%) for C14 : C 71.17, H 5.12, N found C 38.79, H 2.45, N 13.77. ESI-TOF MS (positive ion,
3.71; found C 70.74, H 5.13, N 23.43.
-Bromo-3,5-bis(2-pyridyl)-pyrazole (Br-Hbpp)
4 3
Fe (μ -O)(Br-
3
6
4
)
3
2
1
3
7
3
5
C
78
H
56Br
6
N
24
B
3
F
12
O
5
12 4
H N
II
III
3+
2
4
acetonitrile): m/z: 698.54 [Fe
4
Fe (μ
3
-O)(Br-bpp)
6
] .
This compound was synthesized using the reported
procedure.63 3,5-Bis(2-pyridy)pyrazole (0.4 g, 1.8 mmol) was
Conflicts of interest
dissolved in CH
mL) in aqueous Na
2
Cl
2
(60 mL) at 0 °C. A solution of bromine (0.4 The authors declare no competing financial interests.
CO (1 N, 25 mL) was then added dropwise,
2
3
and the reaction was allowed to stir for 30 min. The reaction
mixture was neutralized to pH 7 with aqueous 1 M NaOH. The Acknowledgements
aqueous phase was extracted with CH
phase was dried over anhydrous Na
filtrate was concentrated under reduced pressure to yield the
crude product, which was purified by column chromatography
2
Cl
2
(80 mL). The organic
and filtered. The
This work was supported by a Grant-in-Aid for Scientific
Research (B) (No. 16H04125, to S.M.), a Grant-in-Aid for Young
Scientists (A) (No. 15H05480, to M.K.) and a Grant-in-Aid for
Challenging Research (Exploratory) (No. 17K19185, to M.K.)
from the Japan Society for the Promotion of Science. This work
was also supported by a Grant-in-Aid for Scientific Research on
Innovative Areas "Hybrid Catalysis" (No. 17H06444, to S.M.), a
Grant-in-Aid for Scientific Research on Innovative Areas
2
SO
4
on silica gel using 5% MeOH/CH
yellow solid (0.35 g, 65%). H NMR (400 MHz, CD
2
Cl
2
to afford Br-Hbpp as a pale-
Cl ): δ = 12.01
1
2
2
(
br, 1 H), 8.70 (m, 2 H), 8.21 (d, J = 7.9 Hz, 2 H), 7.84 (td, J = 7.9
1
3
Hz, J = 1.8 Hz), 7.34 (m, 2H); C NMR (101 MHz, CD
49.4, 149.3, 144.9, 136.8, 123.4, 121.8, 91.1 ppm; elemental
analysis calcd (%) for C13 Br: C 51.59, H 3.01, N 18.60; found
C 51.59, H 3.13, N 18.28.
2 2
Cl ): δ =
1
"
Coordination Asymmetry" (No. 17H05391, to M.K.), and JST
9 4
H N
ACT-C Grant Number JPMJCR12YH, Japan. M. K. is grateful to
the Tokuyama Science Foundation.
II
III
[
4 3 6 6 3 5 6 3
Fe Fe (μ -O)(Me-bpp) ](PF ) ([Fe -Me](PF ) )
Me-Hbpp (0.040 g, 0.17 mmol) was dissolved in degassed
methanol (10 mL), NaOHaq (1 M, 0.17 mL, 0.17 mmol) was
added, and the mixture was stirred to dissolve the Me-Hbpp.
Notes and references
1
2
3
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4 2
Subsequently, FeSO 7H O (0.038 g, 0.14 mmol) was added to
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4
5
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6
(
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precipitate was collected by filtration, washed with water and
dried under vacuum. The obtained precipitate was dissolved in
a mixture of acetonitrile and MeOH and subjected to vapour
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8
2
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II
III
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Fe
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1
(62%). Elemental analysis calcd. (%) for Fe
5 84 76 24 3 18 6
C H N P F O : C
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MS (positive ion, acetonitrile): m/z: 569.08 [Fe
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6
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Fe (μ
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+
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0 | J. Name., 2012, 00, 1-3
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