Thiolate-bridged dinuclear ruthenium and iron complexes as robust and efficient catalysts toward oxidation of molecular dihydrogen in protic solvents

Article abstract of DOI:10.1021/jacs.5b00584

Thiolate-bridged dinuclear ruthenium and iron complexes are found to work as efficient catalysts toward oxidation of molecular dihydrogen in protic solvents such as water and methanol under ambient reaction conditions. Heterolytic cleavage of the coordinated molecular dihydrogen at the dinuclear complexes and the sequential oxidation of the produced hydride complexes are involved as key steps to promote the present catalytic reaction. The catalytic activity of the dinuclear complexes toward the chemical oxidation of molecular dihydrogen achieves up to 10000 TON (turnover number), and electrooxidation of molecular dihydrogen proceeds quite rapidly. The result of the density functional theory (DFT) calculation on the reaction pathway indicates that a synergistic effect between the two ruthenium atoms plays an important role to realize the catalytic oxidation of molecular dihydrogen efficiently. The present dinuclear ruthenium complex is found to work as an efficient organometallic anode catalyst for the fuel cell. It is noteworthy that the present dinuclear complex worked not only as an effective catalyst toward chemical and electrochemical oxidation of molecular dihydrogen but also as a good anode catalyst for the fuel cell. We consider that the result described in this paper provides useful and valuable information to develop highly efficient and low-cost transition metal complexes as anode catalysts in the fuel cell.

Full text of DOI:10.1021/jacs.5b00584

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Article
Thiolate-Bridged Dinuclear Ruthenium- and Iron-
Complexes as Robust and Efficient Catalysts toward
Oxidation of Molecular Dihydrogen in Protic Solvents
Masahiro Yuki, Ken Sakata, Yoshifumi Hirao, Nobuaki
Nonoyama, Kazunari Nakajima, and Yoshiaki Nishibayashi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 10 Mar 2015
Downloaded from http://pubs.acs.org on March 18, 2015
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Thiolate-Bridged Dinuclear Ruthenium- and Iron-Complexes as
Robust and Efficient Catalysts toward Oxidation of Molecular
Dihydrogen in Protic Solvents
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Masahiro Yuki,† Ken Sakata,*‡ Yoshifumi Hirao,§ Nobuaki Nonoyama,§ Kazunari Nakajima,† and
Yoshiaki Nishibayashi*†
Abstract: Thiolate-bridged dinuclear ruthenium- and iron-complexes are found to work as efficient
catalysts toward oxidation of molecular dihydrogen in protic solvents such as water and methanol
under ambient reaction conditions. Heterolytic cleavage of the coordinated molecular dihydrogen at
the dinuclear complexes and the sequential oxidation of the produced hydride complexes are
involved as key steps to promote the present catalytic reaction. The catalytic activity of the dinuclear
complexes toward the chemical oxidation of molecular dihydrogen achieves up to ca. 10000 TON
(turnover number) and electrooxidation of molecular dihydrogen proceeds quite rapidly. The result
of the density functional theory (DFT) calculation on the reaction pathway indicates that synergistic
effect between the two ruthenium atoms plays an important role to realize the catalytic oxidation of
molecular dihydrogen efficiently. The present dinuclear ruthenium-complex is found to work as an
efficient organometallic anode catalyst for the fuel cell. It is noteworthy that the present dinuclear
complex worked not only as an effective catalyst toward chemical and electrochemical oxidation of
molecular dihydrogen but also as a good anode catalyst for the fuel cell. We consider that the result
described in this paper provides useful and valuable information to develop highly efficient and
low-cost transition metal complexes as anode catalysts in the fuel cell.
cat.
2 H+
2 eĞ
H (1 atm)
2
+
H O, rt
2
(OTf)2
S
M
Ru
S
P
Ph
cat.
M = Ru, Fe
_
______________________________________________________________________________
†Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi,
Bunkyo-ku, Tokyo 113-8656, Japan
‡
Faculty of Pharmaceutical Sciences, Hoshi University, Ebara, Shinagawa-ku, Tokyo 142-8501,
Japan
§
4
Fuel Cell System Development Center, Toyota Motor Corporation, Mishuku, Susono, Shizuoka
10-1193, Japan
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Molecular dihydrogen is one of the most leading candidates as a fuel source for the future of green
chemistry.
The best catalyst for the electrochemical production in oxidation of molecular
dihydrogen is platinum metal. In fact, platinum is currently used as the catalyst in fuel cells for
automotive transportation. However, the high cost of platinum provides a strong motivation for
development of less expensive alternatives. Toward the goal of the achievement of efficient catalysts
for the production and oxidation of molecular dihydrogen, many trials have already been investigated
by using a variety of transition metal complexes as catalysts. For the production of molecular
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dihydrogen, there are a number of reports on the catalytic reduction of proton into molecular
1,2
dihydrogen, including photoelectrochemical systems.
On the other hand, successful examples of
oxidation of molecular dihydrogen under ambient reaction conditions are still limited to selected
1
,3-9
reaction systems.
In biochemical sciences, hydrogenases reversibly catalyze both the oxidation of molecular
dihydrogen into proton and electron and the formation of molecular dihydrogen from proton and
10-12
electron under ambient reaction conditions.
Crystal structural studies of hydrogenases have
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0-12
provided useful information on the active sites of the enzyme.
Based on information of
structures of hydrogenases, the construction of model complexes mimicking the enzymes structurally
and functionally has been one of the challenging topics for chemists. Ogo and co-workers designed
and prepared model complexes of [NiFe] hydrogenases such as [NiRu] and [NiFe] dinuclear
complexes, where the [NiRu] dinuclear complexes worked as catalysts toward the oxidation of
5
molecular dihydrogen (up to 17 TON at 50 °C under 8 atm of H ). Thiolate-bridged dinuclear iron
2
complexes bearing either a ferrocenylphosphine as a redox active unit or the second coordination
sphere pendent amine moiety were reported as the mimicking core of [FeFe] hydrogenases, where
8
only a low catalytic activity was observed under ambient reaction conditions (up to 6 TONs).
A series of mononuclear nickel-diphosphine complexes bearing the second coordination sphere
pendent amine moiety were designed and prepared as mononuclear mimicking compounds, where
4
the presence of the amine moiety significantly improved the reaction rate of electrocatalysis.
4e
4h
Interestingly, the incorporation of glycine or arginine as an amino group to the diphosphine
ligand realized catalytic oxidation of molecular dihydrogen in water. More recently, Bullock and
co-workers reported mononuclear iron- and ruthenium-diphosphine complexes bearing the pendent
amine moiety in electrocatalysis, where the presence of the amine moiety also plays a critical role to
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promote the catalytic oxidation of molecular dihydrogen under ambient conditions (up to 25 TON).
In these reaction systems by using mononuclear metal complexes in electrocatalysis, the amine
moiety proximal to the metal center was considered to accelerate the heterolytic cleavage of the
coordinated molecular dihydrogen and also plays an important role as a proton relay.
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As an extension of our study on the development of novel catalytic reactivity of thiolate-bridged
dinuclear ruthenium- and iron-complexes [Cp*Ru{PhP(C H -o-S) }RuCp*](OTf) ([1a](OTf) ;
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4
2
2
2
5
Cp* =  -C Me ; OTf = OSO CF ), [Cp*Fe{PhP(C H -o-S) }RuCp*](OTf) ([1b](OTf) ), and
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2
3
6 4
2
2
2
5
13
[
CpRu{PhP(C H -o-S) }RuCp*(H O)](OTf) ([1c](OTf) ; Cp =  -C H ), we have now found
6 4 2 2 2 2 5 5
that these dinuclear complexes work as robust and efficient catalysts toward oxidation of molecular
dihydrogen under ambient reaction conditions (Scheme 1). In the chemical oxidation, an outstanding
catalytic performance (up to ca. 10000 TON) has been achieved especially in protic solvents such as
water and methanol. We also performed the DFT (density functional theory) study on the reaction
pathway based on the stoichiometric and catalytic reactions. The combined experimental and
theoretical studies revealed that the thiolate-bridged dinuclear core structure plays a critical role to
promote the catalytic oxidation. The present dinuclear ruthenium-complex such as 1a is found to
work as an efficient organometallic anode catalyst for the fuel cell. It is noteworthy that the present
dinuclear complex worked not only as an effective catalyst toward chemical and electrochemical
oxidation of molecular dihydrogen but also as a good anode catalyst for the fuel cell. Herein, we
describe the results of oxidation of molecular dihydrogen catalyzed by the thiolate-bridged dinuclear
ruthenium- and iron-complexes together with a theoretical study on the proposed reaction pathway by
DFT calculations and the electrochemical property of these dinuclear complexes including the ability
of the dinuclear ruthenium-complex as anode catalysts in the fuel cell.
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Results and discussion
Stoichiometric reactions of dinuclear complexes with molecular dihydrogen. Treatment of
[1a](OTf) in water at room temperature for 10 min under 1 atm of molecular dihydrogen gave the
2
corresponding diruthenium-hydride complex [Cp*Ru{PhP(C H -o-S) }RuHCp*](OTf) ([2a]OTf)
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in 96% isolated yield (Scheme 2).
The formation of a stoichiometric amount of
trifluoromethanesulfonic acid (HOTf) was observed in an aqueous phase after the reaction. The
amount of acid was determined by titration with KOH aqueous solution. This result indicated that
the heterolytic cleavage of molecular dihydrogen at the diruthenium core should occur to form a
hydride complex ([2a]OTf) and proton, respectively, where water worked as a base to deprotonate the
1
4
1
molecular dihydrogen coordinated to Ru (Scheme 3).
H NMR spectrum of [2a]OTf in
dichloromethane-d exhibited a doublet signal at -16.44 ppm (J = 3 Hz) assignable to the hydride
2
PH
on the ruthenium center, together with other signals involving two Cp* ligands and the
PhP(C H -o-S) } moiety.
{
6
4
2
When [1a](OTf) in dichloromethane-d was exposed to an atmosphere of molecular dihydrogen
2
2
at room temperature in the presence of a stoichiometric amount of base such as 2,6-lutidine and
P(o-MeC H ) , the solution color of the reaction mixture immediately turned from green into
6 4 3
1
orange-brown. H NMR spectrum of the reaction mixture indicated the formation of [2a]OTf. On
the other hand, the reaction of [2a]OTf with a stoichiometric amount of HOTf in dichloromethane-d
2
at room temperature under 1 atm of argon (Ar) gave [1a](OTf) in 99% NMR yield together with the
2
formation of molecular dihydrogen (97% GC yield). These results clearly indicated the reversibility
2
+
+
between [1a] and [2a] as shown in Scheme 3.
Next, we have investigated the reactivity of [2a]OTf toward the oxidant. The chemical oxidation
of [2a]OTf with 1 equiv of [Cp Fe]OTf in dichloromethane at room temperature for 5 min under 1
2
atm of N gave [1a](OTf) and ferrocene (Cp Fe) in 91% and 61% NMR yields, respectively,
2
2
2
together with molecular dihydrogen in 94% GC yield (Scheme 4). The slightly lower yields of
Cp Fe and molecular dihydrogen are considered to be due to unidentified side reactions. In order to
2
obtain more information on the reaction pathway for the formation of molecular dihydrogen, we
investigated the electrochemistry of both [1a](OTf) and [2a]OTf. The cyclic voltammogram of
2
[1a](OTf) in dichloromethane displayed two reversible one-electron reduction waves at -0.62 and
2
+
/0
2+/1+
1+/0
and [1a]
-
1.31 V vs Cp Fe corresponding to [1a]
redox couples. On the other hand, the
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voltammogram of [2a]OTf showed a reversible one-electron reduction wave at -1.68 V of [2a]
and an irreversible oxidation wave at +0.22 V, respectively. The irreversible wave of [2a]OTf was
assigned to ECE (Electrochemical-Chemical-Electrochemical) reaction shown in Figure 1. The ECE
+
reaction was initiated by one-electron oxidation of diruthenium-hydride complex 2a followed by the
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formation of monocationic complex together with proton. Subsequent one-electron oxidation of the
2+
monocationic complex afforded dicationic complex 1a .
The liberated proton reacted with
+
2+
diruthenium-hydride complex 2a to afford another 1a together with a stoichiometric amount of
+
2+
molecular dihydrogen (Scheme 3). As a whole, 2a was oxidatively converted into 1a together
with 0.5 equiv of molecular dihydrogen (total reaction in Scheme 3).
The result of the ECE reaction shown in Figure 1 prompted us to investigate the chemical
oxidation of [2a]OTf in the presence of a stoichiometric amount of base. In this reaction system, the
base may trap the liberated proton and the generation of molecular dihydrogen may be inhibited.
Actually, the reaction of [2a]OTf with 2 equiv of [Cp Fe]OTf and 1 equiv of P(o-MeC H ) at room
2
6 4 3
temperature for 5 min under 1 atm of N gave [1a](OTf) and Cp Fe in 91% and 64% NMR yields,
2
2
2
respectively, together with [HP(o-MeC H ) ]OTf in 66% NMR yield (Scheme 5) and no formation
6
4 3
of molecular dihydrogen was observed. This result indicated that the conversion of hydride in
2a]OTf into proton occurred during the oxidation of [2a]OTf. Based on the result of stoichiometric
reaction of [1a](OTf) with molecular dihydrogen (Scheme 2) and that of [2a]OTf with oxidant
[
2
(Scheme 4), we have envisaged that [1a](OTf) may promote the catalytic conversion of molecular
2
dihydrogen into 2 equiv of proton and 2 equiv of electron under ambient reaction conditions (Scheme
1). The detailed results are shown in the next section.
Catalytic behavior of dinuclear complexes toward molecular dihydrogen.
The catalytic
oxidation of molecular dihydrogen using [1a](OTf) as a catalyst was carried out according to the
2
following procedure. A dichloromethane solution (5 mL) of [Cp Fe]OTf (0.40 mmol) as an oxidant
2
and P(o-MeC H ) (0.40 mmol) as a base in the presence of a catalytic amount of [1a](OTf) (0.002
6
4 3
2
mmol) was stirred at room temperature for 9 h under 1 atm of molecular dihydrogen. After the
reaction, the formation of [HP(o-MeC H ) ]OTf and Cp Fe in 95% and 98% yields, respectively,
6
4 3
2
was observed by NMR (Scheme 6). The consumption of molecular dihydrogen was monitored to
check the state of progress. In the absence of [1a](OTf) , no reaction occurred under the same
2
reaction conditions. It was also confirmed that no reaction took place under 1 atm of N , in place of
2
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H . These results clearly indicated that molecular dihydrogen was catalytically converted into 2
2
equiv of proton and 2 equiv of electron under ambient reaction conditions.
When the reaction was carried out in water as solvent, it proceeded quite smoothly even in the
absence of P(o-MeC H ) as a base. Typical results are shown in Table 1. In this reaction system,
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the reaction was complete within 1.5 h, where turnover frequency (TOF) was 62 h (Table 1, entry 1).
Under the aqueous conditions, water worked as a base to form HOTf and actually the quantitative
formation of HOTf was confirmed after the reaction. A similar catalytic activity of [2a]OTf was also
achieved under the same reaction conditions (Table 1, entry 2), showing that [2a](OTf) was involved
as a key reactive intermediate toward the catalytic reaction. When a smaller amount of [1a](OTf)
2
(
0.0002 mmol and 0.00004 mmol) was used as a catalyst, the complete conversion of molecular
dihydrogen was achieved after 15 h and 72 h, respectively (Table 1, entries 3 and 4). In these cases,
turnover numbers (TONs) were 975 and 4700, respectively, where the catalytic activity of [1a](OTf)
2
-
1
(
ca. 65 h ) was maintained for over 72 h. At higher reaction temperature (70 °C), the reaction by
using [1a](OTf) (0.00004 mmol) was dramatically accelerated and completed within 6 h, where the
2
-
1
catalytic activity was increased (ca. 800 h ) (Table 1, entry 5). Even in the presence of 0.00002
mmol of [1a](OTf) as a catalyst, the reaction at 70 °C proceeded smoothly without the loss of the
2
catalytic activity (Table 1, entry 6). Separately, no consumption of molecular dihydrogen was
observed at all in the absence of [1a](OTf) as a catalyst under the same reaction conditions. This
2
result indicates that the catalytic use of the dinuclear complexes promoted the oxidation of molecular
dihydrogen. The utility of [Cp Fe]OTf as an oxidant in water is discussed in the Supporting
2
Information.
In order to investigate the catalytic performance under an aqueous solution, the use of stronger
3+/2+
15
oxidant [Fe(NO ) ] (E°(Fe
(aq)) = +0.77 V vs SHE) in place of [Cp Fe]OTf (E°(aq) = +0.40 V
2
3
3
15
vs SHE) gave a similar result under the same reaction conditions (Table 1, entry 7). In contrast, no
2+/+
reaction occurred at all when weaker oxidant [CuSO ] (E°(Cu
(aq)) = +0.15 V vs SHE,
4
2
+/0
15
E°(Cu
(aq)) = +0.34 V vs SHE) was used in place of [Cp Fe]OTf (Table 1, entry 8). Namely,
2
the nature of oxidants is considered to be one of the important factors to promote the catalytic
transformation. On the other hand, oxidation of molecular dihydrogen proceeded quite rapidly when
methanol was used as solvent. In the presence of a catalytic amount of [1a](OTf) (0.002 mmol), the
2
-
1
reaction in methanol was complete within 0.05 h, where the highest TOF (ca. 2000 h ) was observed
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(Table 1, entry 9). Although the exact role of methanol is not yet clear in this catalytic oxidation of
molecular dihydrogen, methanol may work as a stronger nucleophile than water and it may
16
dramatically accelerate the deprotonation step of the coordinated dihydrogen.
Other thiolate-bridged dinuclear ruthenium- and iron-complexes [1b](OTf) and [1c](OTf) have
2
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0
been found to work as effective catalysts as well toward oxidation of molecular dihydrogen in water
Table 1, entries 10 and 11). Here, the reaction using [1c](OTf) as a catalyst proceeded slightly
(
2
slower, probably because of a less bulky Cp moiety of [1c](OTf) allowing water solvent to be
2
coordinated to the active ruthenium center.
To obtain more information on the reaction pathway, we monitored the consumption of molecular
dihydrogen in the reaction with [Cp Fe]OTf in the presence of a catalytic amount of [1a](OTf) . The
2
2
time profile is shown in Figure 2 and typical results are shown in Table 2. The consumption of
-
1
molecular dihydrogen was observed with the constant rate of 0.13 mmol h until the end of the
reaction when the reaction with [Cp Fe]OTf (0.40 mmol) was carried out in the presence of
2
[
1a](OTf) (0.002 mmol) under 1 atm of molecular dihydrogen. The linearity of the consumption of
2
molecular dihydrogen indicated that the reaction rate was independent on both the concentration of
the oxidant and the pH value of the reaction mixture. The decrease in the rate of the consumption of
molecular dihydrogen was observed under a lower partial pressure of molecular dihydrogen (Table 2,
entries 2 and 3). Furthermore, a substantial isotope effect (k /k = 1.4) was observed when D
H2 D2
2
was used in place of H (Table 2, entry 4). These results indicated that both the heterolytic cleavage
2
of molecular dihydrogen on the ruthenium complex and the deprotonation from the produced hydride
complex may be involved as rate-determining steps of the catalytic reaction.
Reaction pathway for catalytic oxidation of molecular dihydrogen. Based on the stoichiometric
and catalytic reactions, we have proposed a reaction pathway shown in Scheme 7. At first, the
formation of dicationic ruthenium-dihydrogen complex B by the coordination of molecular
dihydrogen to dicationic ruthenium complex A occurs and then the nucleophilic attack of water to the
coordinated dihydrogen gives the corresponding monocationic ruthenium-hydride complex C
together with an acid. One-electron oxidation of the complex C gives the corresponding dicationic
ruthenium-hydride complex D and further deprotonation from D with water leads to the formation of
monocationic ruthenium complex E. Further one-electron oxidation of the complex E affords the
starting complex A.
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The proposed reaction pathway is supported by DFT calculations using the B3LYP functional
1
7-20
(LANL2DZ for Ru atom and 6-31G* for other atoms).
The polarizable continuum model
21
(PCM) was utilized for the solvent effects of water.
We investigated the model oxidation reaction
of hydrogen molecule catalyzed by the dicationic thiolate-bridged dinuclear ruthenium complex
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[
1a] (I) (Figure 3), where the deprotonation from the hydrogen-coordinated complex and the
one-electron oxidation of the produced hydride complex proceed alternately (eq(1)):
+
cat. [1a]2+
2 H O2+ + 2 Fe
H2 + 2 (H O)2 + 2 Fe
2
5
(eq 1)
A molecular dihydrogen coordinates to the dicationic complex I in a side-on fashion via the
transition state TS(I-II) to afford dicationic ruthenium-dihydrogen complex II. The barrier height of
TS(I-II) from the initial state is 7.8 kcal/mol, and the relative energy of II, E, is +5.1 kcal/mol. The
coordinated hydrogen atoms in II have the same positive atomic charge (+0.16 in the natural
22
population analysis (NPA) ), which is likely to be attacked by nucleophiles such as solvent water

molecules and OTf ion. We examined the proton abstraction from II by water dimer (H O) (II →
2
2
V). The nucleophilic attack of the oxygen atom in the water dimer gives the complex IV (E = 5.7
+
kcal/mol), which consists of the mono-cationic ruthenium-hydride complex and zundel cation H O ,
5
2
through the reactant complex III (E = 2.1 kcal/mol) and the transition state TS(III-IV) (E = 0.0
kcal/mol). The barrier height of TS(III-IV) from III is quite small (2.1 kcal/mol), so this
deprotonation step can occur smoothly. Then, one-electron oxidation of the isolated mono-cationic
ruthenium-hydride complex V leads to the corresponding dicationic hydride complex VI. By
+
considering that an electron is trapped by Cp Fe ion, the complex VI (E = 10.2 kcal/mol) is
2
calculated to be more stable than mono-cationic complex V (E = +0.5 kcal/mol) by 10.7 kcal/mol in
energy. The subsequent deprotonation from VI caused by (H O) was examined. The barrier height
2
2
of TS(VII-VIII) from the reactant complex VII is 6.6 kcal/mol, and the deprotonation process from
VII to VIII is slightly exothermic by 0.7 kcal/mol. Although the reaction barrier is slightly higher
than that of the first deprotonation step, this deprotonation step can also occur smoothly. Finally, the
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oxidation of the isolated monocationic complex [1a] (IX) reproduces the starting dicationic
complex I, and this step gives large stability in energy. As a whole, the present results achieved by
the DFT calculations support the proposed reaction pathway shown in Scheme 7.
In the propargylic substitution reaction catalyzed by thiolate-bridged diruthenium complex, it has
been proposed that the flexible RuRu structure plays a crucial role for the catalytic cycle.23 As
shown in Figure 3, the bond distance between two ruthenium atoms alters along the reaction pathway.
This result indicates the importance of similar dimetallic effect in the present oxidation reaction
system. In fact, we have examined the changes in electron population through two deprotonation
steps (II → V and VI → IX) as shown in Figure 4. In the first deprotonation step (II → V), the
atomic charge of the coordinated hydrogen atom in II is +0.16, which means that the hydrogen, which
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
is to be lost in the deprotonation, possesses the electron population of 0.84e . By eliminating the
proton nuclei from II, the electron population of the hydrogen atom in II is distributed not only to the


coordinated Ru site (0.38e ) but also to the thiolate-bridge site (0.22e ) and to the other Ru site

(
0.24e ) in V. This result shows that both the other Ru site and the thiolate-bridge site play as
electron storages. In the second deprotonation step (VI → IX), the electron population of the


hydrogen atom in VI (0.78e ) is distributed to the thiolate-bridge site (0.53e ) and to the other Ru site

(
0.28e ) in IX, and the electron population at the coordinated Ru site decreases slightly in an opposite
manner. Thus, the synergistic effect between two ruthenium atoms supported by the thiolate ligands
plays an important role also in the present reaction system.
Electrocatalysis of dinuclear complex toward molecular dihydrogen. We investigated the
electrocatalysis of [1a](OTf) toward the oxidation of molecular dihydrogen.
The cyclic
2
voltammogram of [1a](OTf) in methanol under 1 atm of N showed an irreversible oxidation wave
2
2
+/0
at +0.08 V vs Cp Fe (Figure 5, black trace). In contrast, the cyclic voltammogram of [1a](OTf)
2
2
in methanol under 1 atm of H showed a plateau shaped oxidation wave with the increased current,
2
corresponding to the catalytic current of oxidation of molecular dihydrogen (Figure 5, red trace).
The rate constant k of the electrooxidation of molecular dihydrogen catalyzed by [1a](OTf) was
obs
2
-
1
24
roughly estimated to be 1.6 s by using equation 2 (see the Supporting Information in details). For
the electrochemical oxidation, the value of the rate constant k of [1a](OTf) is lower than that of
obs
2
-
1
4
the most effective homogeneous nickel catalyst (210 s ; under 1 atm of H ) as well as
2
-
1 10
heterogeneous platinum catalyst (15 s ).
However, the use of [1a](OTf) as a catalyst led to ca.
2
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10000 TON in the chemical oxidation of molecular dihydrogen with oxidant and the catalytic activity
was maintained for over a few days (vide supra). This means that [1a](OTf) works as the one of the
2
most robust and efficient catalyst toward the chemical oxidation of molecular dihydrogen under
ambient reaction conditions.
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icat  nFAC Dkobs (eq 2)
Ability of dinuclear complex as an anode catalyst in the fuel cell. Finally, we evaluated the
ability of [1a](OTf) as an anode catalyst under carbon supported conditions for the fuel cell system.
2
A preliminary result of the polarization curve showed the open circuit voltage (OCV) of 1.04 V and
2
the maximum power density of 8.3 mW/cm at 0.38 V at 40 °C (Figure 6) (See the Supporting
Information in details). This result indicates that [1a](OTf) can work as an efficient organometallic
2
anode catalyst of the fuel cell although the present ability of [1a](OTf) as an anode catalyst is lower
2
than that of Pt catalyst in the fuel cell. Previously, Ogo and co-workers reported the use of the [NiRu]
5
c,5e
dinuclear complexes as organometallic anode catalysts in the fuel cell.
For comparison, the use
of the [1a](OTf) as an organometallic anode catalyst has significantly improved the ability as the
2
fuel cell. To the best of our knowledge, the thiolate-bridged dinuclear ruthenium-complex has the
2
5
highest performance as organometallic fuel cell.
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Conclusion
We have found that thiolate-bridged dinuclear ruthenium- and iron-complexes work as robust and
efficient catalysts toward oxidation of molecular dihydrogen in protic solvent such as water and
methanol under ambient reaction conditions. The catalytic activity of the complexes toward the
chemical oxidation of molecular dihydrogen achieves up to ca. 10000 TON and electrooxidation of
molecular dihydrogen proceeds quite rapidly. The high proton conductivity by the Grotthus
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mechanism in protic solvents seems to play a crucial role for the high performance of the catalysis.
The experimental result and the DFT calculation indicate that heterolytic cleavage of the coordinated
molecular dihydrogen at the dinuclear complexes and the sequential oxidation of the hydride
2
7
complexes are key steps to promote the present catalytic reaction.
Synergistic effect between the
two ruthenium atoms is observed during the catalytic transformation of molecular dihydrogen. We
have confirmed the ability of the thiolate-bridged dinuclear ruthenium-complex as a good anode
catalyst for the fuel cell although the present ability of the dinuclear ruthenium-complex as an anode
catalyst is lower than that of Pt catalyst in the fuel cell. We believe that the thiolate-bridged dinuclear
ruthenium-complex has the highest performance as organometallic fuel cell. It is noteworthy that the
present dinuclear complex worked not only as an effective catalyst toward chemical and
electrochemical oxidation of molecular dihydrogen but also as a good anode catalyst for the fuel cell.
We consider that the result described in this paper provides useful and valuable information to
develop highly efficient and low-cost transition metal complexes, instead of Pt, as anode catalysts in
2
8,29
the fuel cell.
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ASSOCIATED CONTENT
Supporting Information.
Full preparative, analytical, and computational details in pdf format. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
sakata@hoshi.ac.jp
ynishiba@sogo.t.u-tokyo.ac.jp
ACKNOWLEDGMENT
We would like to express the deepest appreciation to Dr. Haruyuki Nakanishi and Dr. Yukihisa
Katayama of Toyota Motor Corporation for their great contribution and their generous support to the
present project. Y.N. thanks Grants-in-Aid for Scientific Research (Nos. 26288044 and 26620075)
from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture,
Sports, Science and Technology of Japan (MEXT) and ENEOS Hydrogen Trust Fund. We thank
Grants-in-Aid for Scientific Research on Innovative Areas “Advanced Molecular Transformations by
Organocatalyst” from the Ministry of Education, Culture, Sports, Science and Technology, Japan
(Nos. 26105708 to Y.N. and 26105756 to K.S.).
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References and Notes
1. For recent reviews, see: (a) Rakowski DuBois, M.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62. (b)
Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100. (c) Ogo, S. Chem. Commun. 2009,
3
317. (d) Barton, B. E.; Olsen, M. T.; Rauchfuss, T. B. Curr. Opin. Biotech. 2010, 21, 292. (e)
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Rakowski DuBois, M.; DuBois, D. L. In Catalysis Without Precious Metals; Bullock, R. M., Ed.;
Wiley: Weinheim, 2010, Chapter 7, p165. (f) Yang, J. Y.; Bullock, R. M.; Rakowski DuBois, M.;
DuBois, D. L. MRS Bull. 2011, 36, 39. (g) DuBois, D. L.; Bullock, R. M. Eur. J. Inorg. Chem. 2011,
1
017. (h) Shaw, W. J.; Helm, M. L.; DuBois, D. L. Biochim. Biophys. Acta 2013, 1827, 1123. (i) Thoi,
V. S.; Sun, Y.; Long, J. R.; Chang, C. J. Chem. Soc. Rev. 2013, 42, 2388. (j) Wang, N.; Wang, M.;
Chen, L.; Sun, L. Dalton Trans. 2013, 42, 12059. (k) Ogo, S. Chem. Rec. 2014, 14, 397. (l) McKone,
J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Chem. Sci. 2014, 5, 865. (m)
Bullock, R. M.; Appel, A. M.; Helm, M. L. Chem. Commun. 2014, 50, 3125. (n) DuBois, D. L. Inorg.
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Chem. Soc. 2005, 127, 12717. (b) Le Goff, A.; Artero, V.; Jousselme, B.; Tran, P. D.; Guillet, N.;
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M. P.; Bullock, R. M.; Rakowski DuBois, M.; DuBois, D. L. Science 2011, 333, 863. (d) Karunadasa,
H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. Science 2012, 335, 698. (e)
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T. B.; Carroll, P. J. J. Am. Chem. Soc. 2012, 134, 18843. (i) Khnayzer, R. S.; McCusker, C. E.; Olaiya,
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M.-N.; Deronzier, A,; Palomares, E.; Llobet, A. J. Am. Chem. Soc. 2014, 136, 7655.
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3. Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E.; Lewis, N. S.; Lopez, M. A.; Guilard, R.;
L’Her, M.; Bother-By, A. A.; Mishra, P. K. J. Am. Chem. Soc. 1992, 114, 5654.
4. (a) Curtis, C. J.; Miedaner, A.; Ciancanelli, R.; Ellis, W. W.; Noll, B. C.; Rakowski DuBois, M.;
DuBois, D. L. Inorg. Chem. 2003, 42, 216. (b) Wilson, A. D.; Newell, R. W.; McNevin, M. J.;
Muckerman, J. T.; Rakowski DuBois, M.; DuBois, D. L. J. Am. Chem. Soc. 2006, 128, 358. (c) Yang,
J. Y.; Chen, S.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M.; DuBois, D. L.; Raugei, S.;
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. (a) Ogo, S.; Kabe, R.; Uehara, K.; Kure, B.; Nishimura, T.; Menon, S. C.; Harada, R.; Fukuzumi,
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Ohhara, T. Science 2013, 339, 682. (e) Matsumoto, T.; Kim, K.; Nakai, H.; Hibino, T.; Ogo, S.
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. (a) Camara, J. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2011, 133, 8098. (b) Camara, J. M.;
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4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3630.
10. Cracknell, J. A.; Vincent, K. A.; Armstrong, F. A. Chem. Rev. 2008, 108, 2439.
11. (a) Artero, V.; Fontecave, M. Coord. Chem. Rev. 2005, 249, 1518. (b) Canaguier, S.; Artero, V.;
Fontecave, M. Dalton Trans. 2008, 315. (c) Simmons, T. R.; Atrero, V. Angew. Chem. Int. Ed. 2013,
52, 6143. (d) Simmons, T. R.; Berggren, G.; Bacchi, M.; Fontecave, M.; Artero, V. Coord. Chem. Rev.
0
1
2
3
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9
0
1
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8
9
0
1
2
3
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5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
1
1
1
014, 270-271, 127.
2. Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245.
3. Yuki, M.; Miyake, Y.; Nishibayashi, Y. Organometallics 2010, 29, 5994.
4. Such Ru-dihydrogen complexes have been well known: (a) Jessop, P. G.; Morris, R. H. Coord.
Chem. Rev. 1992, 121, 155. (b) Kubas, G. J. Chem. Rev. 2007, 107, 4152. (c) Morris, R. H. Coord.
Chem. Rev. 2008, 252, 2381.
15. (a) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry,
Sixth Edition, Chapt. 17; Wiley: New York, 1999. (b) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996,
9
6, 877.
6. We observed the H/D exchange between H and CD OD under this catalytic condition.
1
2
3
Treatment of molecular dihydrogen with methanol-d in the presence of a catalytic amount of
4
[
1a](OTf) led to the formation of HD and D and the consumption of H was observed by NMR after
2 2 2
the reaction.
This result indicates that the reversibility between the deprotonation from the
ruthenium-dihydrogen complex and the protonation of the ruthenium-hydride complex is involved in
the present reaction system.
1
1
9
1
7. (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B
988, 37, 785. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994,
8, 11623.
8. Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
19. Hehre, W. J.; Radom, L.; Schleyer, P, v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory;
Wiley: New York, 1986.
20. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.;
1
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2
2
2
2
2
2
2
2
2
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3
3
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3
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5
6
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant,
J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J.
V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009.
0
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0
2
1. (a) Miertuš, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117129. (b) Scalmani, G.; Frisch,
M. J. J. Chem. Phys. 2010, 132, 114110.
2. Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735.
2
23. Ammal, S. C.; Yoshikai, N.; Inada, Y.; Nishibayashi, Y.; Nakamura, E. J. Am. Chem. Soc. 2005,
127, 9428.
24. Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706.
2
5. (a) As a half cell system using an immobilized Ni anode catalyst, a current density of ca. 2
2
2b
mA/cm at 500 mV overvoltage was reported by Artero, Palacin, and co-workers.
(b) The use of rhodium complexes was reported as organometallic fuel cell via oxidation of alcohol;
Annen, S. P.; Bambagioni, V.; Bevilacqua, M.; Filippi, J.; Marchionni, A.; Oberhauser, W.;
Schönberg, H.; Vizza, F.; Bianchini, C.; Grützmacher, H. Angew. Chem. Int. Ed. 2010, 49, 7229.
2
6. The following is a translation of the original French paper ”de Grotthuss, C. J. T. Ann. Chim.
Paris) 1806, 58, 54”; de Grotthuss, C. J. T. Biochim. Biophys. Acta 2006, 1757, 871.
7. (a) We consider that no heterolytic cleavage of the coordinated molecular dihydrogen on the Ru-S
(
2
bond occurred due to the unique structure of the thiolate-bridged diruthenium complexes, where the
unshared electron pair of the S atom as the bridging thiolate moiety does not have a suitable position
to promote the heterolytic cleavage of the coordinated molecular dihydrogen. At present, we believe
that no proton replay took place in the present reaction system. (b) When the catalytic reaction was
carried out in the presence of a catalytic amount of ruthenium nano particles under the same reaction
conditions, the reaction did not proceed substantially. This experimental result supports the proposal
shown in the present manuscript. However, we can not exclude the possibility that ruthenium nano
particles worked as real reactive species.
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28. Ogo and co-workers have recently reported that [NiFe]hydrogenase from Citobacter sp. S-77
worked as an effective electrode for the oxidation of molecular dihydrogen; Matsumoto, T.; Eguchi,
S.; Nakai, H.; Hibino, T.; Yoon, K.-S.; Ogo, S. Angew. Chem. Int. Ed. 2014, 53, 8895.
2
9. Reversible iridium-catalyzed dehydrogenation of alcohols has recently been reported; Bonitatibus
0
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0
Jr., P. J.; Chakraborty, S.; Doherty, M. D.; Siclovana, O.; Jones, W. D.; Soloveichik, G. L. Proc. Natl.
Acad. Sci. U.S.A. 2015, 112, 1687–1692.
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cat.
H (1 atm)
2 H+
+
2 eĞ
2
H O
2
rt
(
OTf)2
(OTf)2
(OTf)2
0
1
2
3
4
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9
0
1
2
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0
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0
1
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0
1
2
3
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8
9
0
S
S
S
Ru
Ru
Fe
Ru
Ru
Ru
S
S
S
P
P
P
OH2
Ph
Ph
Ph
[
1a](OTf)2
[1b](OTf)2
[1c](OTf)2
Scheme 1. Catalytic oxidation of molecular dihydrogen.
(OTf)2
OTf
+
S
S
Ru
Ru
+
H2
Ru
Ru
HOTf
H O
(
1 atm)
2
S
S
H
rt, 10 min
P
P
Ph
Ph
[
1a](OTf)2
[2a]OTf
Scheme 2. Stoichiometric reaction of dinuclear ruthenium complex with
molecular dihydrogen.
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H+
H2
2+
2+
+
S
S
S
Ru
Ru
Ru
Ru
Ru
Ru
H
S
S
S
H
9
H
P
P
P
Ph
Ph
Ph
10
11
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59
60
H+
H2
[1a]2+
[2a]+
Scheme 3. Reversibility of dinuclear ruthenium complex via ruthenium-dihydrogen complex.
(OTf)2
+
OTf
OTf
N2
S
S
(
1 atm)
Ru
Ru
Ru
Ru
+
Fe
+
1/2 H2
Fe
CH Cl
2
S
S
2
H
P
P
Ph
Ph
rt, 5 min
[
2a]OTf
[1a](OTf)2
Scheme 4. Chemical oxidation of [2a]OTf.
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OTf
OTf
S
Ru
Ru
+
2
Fe
S
P(o-MeC H )
+
6
4 3
S
H
P
Ph
0
1
2
3
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9
0
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0
(OTf)2
[HP(o-MeC H ) ]OTf +
[
2a]OTf
N2
(1 atm)
Ru
Ru
+
2
6
4 3
Fe
CH Cl
S
2
2
P
Ph
rt, 5 min
[1a](OTf)2
Scheme 5. Chemical oxidation of [2a](OTf) in the presence of base.
OTf
cat. [1a](OTf)2
(
0.002 mmol)
H2 + 2 P(o-MeC H ) + 2 Fe
2 [HP(o-MeC6H4)3]OTf
+
2
Fe
6
4 3
CH Cl
2
2
(1 atm)
(
5 mL)
(
0.40 mmol)
(0.40 mmol)
rt, 9 h
(0.38 mmol;
95% yield)
(0.39 mmol;
98% yield)
TON 98, TOF 11 h-1
Scheme 6. Catalytic oxidation of molecular dihydrogen in the presence of [1a](OTf)2.
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2+
S
eĞ
Ru
Ru
H (1 atm)
2
S
P
Ph
10
11
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60
A
+
2+
S
S
Ru
Ru
Ru
Ru
S
H
S
P
H
P
Ph
Ph
E
B
H O
2
+
H O
3
+
H O
3
H O
2
2+
+
S
S
Ru
Ru
Ru
Ru
S
S
H
H
P
P
Ph
Ph
eĞ
C
D
Scheme 7. Proposed reaction pathway of oxidation of molecular dihydrogen catalyzed
by [1a](OTf)2.
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electrochemical
reaction
chemical
reaction
electrochemical
reaction
- eĞ
- eĞ
+
2+
+
2+
[
Ru]
[Ru]
[Ru]
[Ru]
H
H
[2a]+
[2a]2+
[1a]+
[1a]2+
H+
0
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8
9
0
total reaction
+
2+
- eĞ
S
S
+
1/2 H2
Ru
Ru
Ru
Ru
S
S
H
P
P
Ph
Ph
2a]+
[1a]2+
[
Figure 1. Electrochemical-Chemical-Electrochemical reaction.
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ǙDZÇÃÉsÉNÉ`ÉÉ
ê
QÇL u¾í £i cåÉ
k
©v
T
ÉÇi mçÈ ÉeÇ Oýž ÇÉÇ
½
â²
É
Ç
Ä
…ÇÕïKóvÇ-ÇÅB
Figure 2. Time course of the consumption of molecular dihydrogen in the oxidation of molecular
dihydrogen with [Cp Fe]OTf (0.40 mmol) in the presence of [1a](OTf) (0.002 mmol) in water (5
2
2
mL) at room temperature.
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6

= 0.24e
2+
= 0.38e
+

(
0.32e)
(0.06e)
Ru
S
H +
S

Ru
Ru
Ru
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
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9
0
1
2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
S
S
H
H
P
H
0.84e
P
Ph
Ph

= 0.22e
II
V
 = 0.28e
2
+
 = -0.04e
+

(0.23e )
S
H +
S

Ru
Ru
Ru
Ru

(
-0.27e )
S
S
H
0.78e
P
P
Ph
Ph

= 0.53e
VI
IX

Figure 4. Changes in electron population (e ) estimated by NPA through two deprotonation steps (II
→
V and VI → IX).
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ǙDZÇÃÉsÉNÉ`ÉÉ
ê
QÇL u¾í £i cåÉ
k
©v
T
ÉÇi mçÈ ÉeÇ Oýž ÇÉÇ
½
â²
É
Ç
Ä
…ÇÕïKóvÇ-ÇÅB
Figure 5. Cyclic voltammogram of a solution of [1a](OTf) in methanol (1.0 mM) under 1 atm of
2
molecular dinitrogen (black) and molecular dihydrogen (red) (v = 10 mV/s, 20 °C, glassy carbon
n
working electrode, 0.1 M Bu NClO as supporting electlyte).
4
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6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
0
.8
.6
6
0.4
4
0
.2
0
2
0
0
5
10
15
20
25
30
‐
Current density / mAcm
Figure 6. I-V curve and power density curve for the fuel cell using [1a](OTf) as an anode catalyst
2
and Pt as a cathode catalyst. The cell was operated at 40 °C with the constant flow (500 mL/min
each) of hydrogen and oxygen gases into anode and cathode, respectively. The polarization curve
was recorded at the current sweep rate of 0.5 mA/s.
2
8
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Journal of the American Chemical Society
1
2
3
4
Table 1. Catalytic oxidation of molecular dihydrogen in the presence of catalyst in water.a
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
OTf
cat.
catalyst
H2
+
2
Fe
2 HOTf
+
2
Fe
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H O (5 mL)
2
(
1 atm)
rt
(
0.40 mmol)
TONd
TOFe
entry catalyst
mmol)
reaction
time (h)
amounts
amount of
(
consumed H2
(mmol)
-1
(
h )
HOTfb
ferrocenec
(mmol)
(mmol)
1
2
3
[1a](OTf)2 (0.002)
[2a](OTf) (0.002)
[1a](OTf)2 (0.0002)
1.5
1.5
15
>0.39
0.37
0.39
0.39
>0.19
>0.19
Ğ
93
98
62
65
65
>0.39
>0.39
975
4
[1a](OTf)2 (0.00004)
[1a](OTf)2 (0.00004)
72
6
>0.39
>0.39
0.38
0.38
Ğ
Ğ
4700
4775
65
5f
796
825
6f
12
>0.39
0.39
Ğ
[1a](OTf)2 (0.00002)
9900
7g [1a](OTf)2 (0.002)
100h
0
1.5
5
Ğ
Ğ
Ğ
Ğ
>0.19
0
67
8i
9j
[1a](OTf)2 (0.002)
[1a](OTf)2 (0.002)
Ğ
0.05
>0.39
0.38
>0.19
95
1900
1
1
a
0
[1b](OTf)2 (0.002)
[1c](OTf)2 (0.002)
1.5
3
>0.39
>0.39
0.40
0.38
>0.19
>0.19
100
95
67
32
1
Reactions of [Cp Fe]OTf (0.40 mmol) under 1 atm of H in the presence of catalyst in water (5
2
2
mL) at room temperature.
b
The amount of acid was estimated by titration with KOH aqueous solution.
c
d
e
f
1
The amount of ferrocene was determined by H NMR with an internal standard.
TON = (amount of ferrocene (mmol))/(amount of catalyst (mmol))/2.
TOF = TON/(reaction time (h)).
At 70 ¡C.
g
h
i
[
Fe(NO ) ] (0.40 mmol) was used in place of [Cp Fe]OTf.
3 3 2
The value of TON was estimated on the amount of consumed H2.
CuSO á5 H O] (0.20 mmol) was used in place of [Cp Fe]OTf.
[
4
2
2
j
MeOH (5 mL) was used in place of H O as solvent.
2
2
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