Electrocatalytic Water Oxidation with α-[Fe(mcp)(OTf)2] and Analogues

Article abstract of DOI:10.1021/acscatal.0c05439

The complex α-[Fe(mcp)(OTf)2] (mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst in the presence of sacrificial oxidants. However, because chemical oxidants are likely to take part in the reaction mechanism, mechanistic electrochemical studies are critical in establishing to what extent previous studies with sacrificial reagents have actually been meaningful. In this study, the complex α-[Fe(mcp)(OTf)2] and its analogues were investigated electrochemically under both acidic and neutral conditions. All the systems under investigation proved to be electrochemically active toward the WO reaction, with no major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp-iron complexes proceeds via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface. Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7 (KIE ～10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an electron transfer (ET).

Full text of DOI:10.1021/acscatal.0c05439

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Research Article
Electrocatalytic Water Oxidation with α‑[Fe(mcp)(OTf) ] and
2
Analogues
Silvia D’Agostini, Konstantin G. Kottrup, Carla Casadevall, Ilaria Gamba, Valeria Dantignana,
Alberto Bucci, Miquel Costas,* Julio Lloret-Fillol,* and Dennis G.H. Hetterscheid*
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sı Supporting Information
ABSTRACT: The complex α-[Fe(mcp)(OTf) ] (mcp = N,N′-dimethyl-N,N′-bis-
2
(
pyridin-2-ylmethyl)-cyclohexane-1,2-diamine and OTf = trifluoromethanesulfonate
anion) was reported in 2011 by some of us as an active water oxidation (WO) catalyst
in the presence of sacrificial oxidants. However, because chemical oxidants are likely to
take part in the reaction mechanism, mechanistic electrochemical studies are critical in
establishing to what extent previous studies with sacrificial reagents have actually been
meaningful. In this study, the complex α-[Fe(mcp)(OTf) ] and its analogues were
2
investigated electrochemically under both acidic and neutral conditions. All the systems
under investigation proved to be electrochemically active toward the WO reaction, with no
major differences in activity despite the structural changes. Our findings show that WO-catalyzed by mcp−iron complexes proceeds
via homogeneous species, whereas the analogous manganese complex forms a heterogeneous deposit on the electrode surface.
Mechanistic studies show that the reaction proceeds with a different rate-determining step (rds) than what was previously proposed
in the presence of chemical oxidants. Moreover, the different kinetic isotope effect (KIE) values obtained electrochemically at pH 7
(
KIE ∼ 10) and at pH 1 (KIE = 1) show that the reaction conditions have a remarkable effect on the rds and on the mechanism. We
suggest a proton-coupled electron transfer (PCET) as the rds under neutral conditions, whereas at pH 1 the rds is most likely an
electron transfer (ET).
KEYWORDS: water oxidation, iron complexes, manganese complexes, electrocatalysis, chemical oxidants, isotope effects
INTRODUCTION
developed the Fe−TAML (TAML = tetra-amido macrocyclic
−1
24
■
ligand) systems, which exhibited a TOF value of 1.3 s .
Masaoka et al. reported a pentanuclear iron catalyst showing a
The goal of reducing the world energy problems by moving
away from fossil fuels has raised ever-growing interest in the
search for renewable energy sources. Solar energy is a well-
known and promising source, and can be harvested in sufficient
−1
32
remarkable turnover frequency of 1900 s . Some of us
developed iron-based homogeneous water oxidation catalysts,
−1
25,37,38
1
which yielded TONs of about 3400 and TOFs of 0.4 s .
amounts to power the planet. Because of intermittency
However, most of the catalysts mentioned have been studied in
reasons, it is important to store excess solar energy in the form
of a chemical fuel. This can be achieved by splitting water into
the presence of chemical oxidants, such as cerium ammonium
25,37,39−43
1
nitrate (CAN) and sodium periodate (NaIO ).
4
O and H . In order to make water splitting more efficient, a
2
2
When using chemical oxidants, the choice of the reaction
conditions is limited to the stability of the oxidant, which lies
in a limited pH region, and no redox-potential control can be
better understanding of the catalytic water oxidation (WO)
process is required. Thus far, many research groups have
directed their attention to the development of catalysts based
on transition metals that are able to perform efficient WO. The
complexes that are proved to be the most active are based on
achieved. Furthermore, it has been shown that these oxidants
25,37,41
can take active part in the reaction mechanism.
In this
2
−11
12−18
regard, electrochemical methods are crucial to elucidate
whether the observed catalytic properties are actually
independent of sacrificial oxidants. Recently, some of us used
electrochemical methods coupled with mass spectrometry to
ruthenium
and iridium.
Ruthenium-based catalysts
6
19
exhibited turnover numbers (TONs) exceeding 10 , turnover
4
−1 5,19
frequencies (TOFs) in the range of 5 × 10 s ,
and an
2
0
overpotential of 180 mV. Iridium-based catalysts showed
6
21
TONs exceeding 10 with an overpotential of 250 mV.
However, an important requirement toward sustainability is
Received: December 11, 2020
Revised: January 26, 2021
Published: February 11, 2021
22,23
the use of earth-abundant elements.
In this regard, iron is
biocompatible, abundant, has a rich redox chemistry, and plays
a prominent role in oxidation chemistry. Several iron
complexes have been developed that can act as molecular
2
4−36
water oxidation catalysts.
Collins and co-workers
©
2021 The Authors. Published by
American Chemical Society
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study metal-catalyzed WO reactions. Fe(cyclam) (cyclam =
1
,4,8,11-tetraazacyclotetradecane) complexes with two cis
vacant sites or having a fifth donor axial ligand and the
dinuclear iron complex [{(MeOH)Fe(Hbbpya)} (μ−O)]-
2
(
OTf) (Hbbpya = N,N-bis(2,2′-bipyrid-6-yl)amine) were
4
31,33
found to be electrocatalytically active toward WO.
In the
latter case, further studies benchmarked an overpotential of
−
1
3
00−400 mV and a TOF of 0.12 s . On the other hand,
electrochemical studies with [{(MeOH)Fe(Hbbpya)} (μ−
O)](OTf) have also shown that the choice of the electrode
2
4
material is important as the complex exhibited enhanced water
oxidation activity in combination with graphitic working
electrodes.
Figure 1. CV in the presence (black) and the absence (gray) of α-
3
3
[Fe(mcp)(OTf)
] (1 mM) in 0.1 M Na SO , scanning at 20 mV/s,
2 2 4
using a GC working electrode.
In this study, we have investigated α-[Fe(mcp)(OTf)2]
mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-cyclohex-
(
ane-1,2-diamine; OTf = trifluoromethanesulfonate anion), the
using a gold working electrode is more complex because of the
oxidation of the gold surface (Figure S1). The advantage of
BDD is that very small background currents are recorded, yet
not useable for some applications because of very slow electron
transfer processes. In contract, the PG, fluorine-doped tin
oxide (FTO), and gold electrodes used are more appropriate
for bulk electrolysis applications, which will be discussed later.
Heterogeneous iron-based materials such as Fe O are
III
Fe analogue α-[Fe(mcp)(Cl) ]Cl, the deuterated analogue
2
α-[Fe(D4-mcp)(OTf) ], β-[Fe(mcp)(OTf) ], and the man-
2
2
ganese coordination complex α-[Mn(mcp)(OTf) ] as WO
2
electrocatalysts (Chart 1). Our findings show that water
Chart 1. Iron and manganese complexes evaluated in this
study.
2
3
44−47
known to be active water oxidation catalysts
and may
potentially form through the decomposition of homogeneous
complexes under the harsh oxidative conditions applied. The
potential formation of catalytically active heterogeneous
materials is a constant concern in the field of homogeneous
48
WO catalysis. In this regard, electrochemical quartz crystal
microbalance (EQCM) methods have been found useful to
rule out the formation of catalytically active heterogeneous
species that may become deposited at the surface of working
49,50
electrodes.
The working electrode in EQCM experiments
consists of a thin layer of gold, deposited on a quartz crystal
oscillator. Mass changes at the working electrode can be
detected by measuring the changes in the resonance frequency
51
of the quartz crystal. To avoid damaging the thin gold layer
of the EQCM electrode and because of the mass change
associated with gold oxide formation and reduction, the
potential was kept above 1.3 V during EQCM experiments to
avoid gold oxide reduction. To understand the nature of the
catalytically active species, we have performed an EQCM
experiment between 1.3 and 2.0 V at 1 mV/s, recorded in the
oxidation catalyzed by mcp iron complexes proceeds via
homogeneous species, also under electrocatalytic conditions,
and that a proton transfer is involved in the rate-determining
step (rds). In contrast, the analogue Mn complex α-
presence of α-[Fe(mcp)(OTf) ] (pH = 7; Figure 2). The first
2
[
Mn(mcp)(OTf) ] forms a heterogeneous deposit on the
scan of the experiment (Figure 2a) shows an increase in the
oscillation frequency, which translates into an apparent
decrease in mass of the electrode. This observation has been
previously attributed to bubble-formation at the working
electrode, which causes changes in hydrophobicity of the
2
electrode surface instead.
RESULTS
■
Voltammetry under Neutral and Acidic Conditions.
First, we examined the behavior of complex α-[Fe(mcp)-
52
solute. After the initial increase in oscillation frequency
during the first scan, no further changes in frequency take place
in subsequent scans, which indicates that catalytically active
species stay homogeneous and suggests that they are molecular
in nature. In contrast to the results obtained with α-
(
OTf) ] by cyclic voltammetry (CV) in aqueous NaClO (0.1
2
4
M) and Na SO4 (0.1 M) electrolyte solutions. In both
2
electrolyte solutions, identical results were obtained showing a
reversible redox couple at 0.8 V versus RHE. The cyclic
voltammogram (CV) recorded with a glassy carbon electrode
[Fe(mcp)(OTf) ], an EQCM experiment conducted in the
2
(
GC) from 0 to 2 V (all potentials are given vs RHE unless
presence of Fe(OTf) shows a clear decrease in oscillation
frequency, which is consistent in both the first and second
2
otherwise specified) shows a reversible redox wave at 0.8 V
II
III
assigned to the Fe /Fe redox couple (Figure 1), and an
irreversible oxidation wave starting at about 1.7 V, which could
be attributed to WO electrocatalytic activity. Very similar
results were obtained using basal plane pyrolytic graphite (PG)
and boron-doped diamond (BDD) as working electrodes
scans and indicates the formation of a deposit on the surface of
33
the gold electrode (Figure 2b).
To assess the WO capabilities of α-[Fe(mcp)(OTf)2],
online electrochemical mass spectrometry (OLEMS) experi-
ments were carried out in combination with CV. In OLEMS
measurements, the gaseous products are sampled close to the
(Figure S1), while the electrochemistry of α-[Fe(mcp)(OTf)2]
2
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Figure 2. CV in combination with quartz crystal microbalance (EQCM) experiment at a gold working electrode, cycling the potential between 1.3
and 2.0 V vs RHE at 1 mV/s. (a) CVs and frequency changes of α-[Fe(mcp)(OTf) ] (1.1 mM) in aqueous NaClO (0.1 M). Shown are the first
2
4
scan (top) and third scan (bottom) of the EQCM experiment. (b) CVs and frequency changes of Fe(OTf) (1.0 mM) in aqueous Na SO (0.1 M).
2
2
4
Shown are the first scan (top) and second scan (bottom) of the EQCM experiment. Solid lines represent the frequency trace and the current is
given in dashed lines. Arrows indicate the direction of the scan. Figure 2b top was reprinted with permission from ref 33.
Figure 3. CV in combination with OLEMS of α-[Fe(mcp)(OTf) ] (1.1 mM) in 0.1 M NaClO using a PG working electrode at a scan rate of 1
2
4
mV/s. Depicted are the m/z traces of O (top), CO (middle), and the corresponding current (bottom). The forward sweep of both the first scan
2
2
(left) and the second scan (right) is shown.
electrode surface in solution and selected m/z values are
and CO measured during the OLEMS experiment show
onsets at about 1.8 V.
2
53
recorded as a function of the applied potential. During all
OLEMS experiments, m/z = 32 and 44 were recorded to
monitor the oxygen evolution reaction and possible oxidative
decomposition of the ligand (CO2 formation) under the
The formation of CO has been routinely detected in
2
OLEMS experiments with a PG working electrode for all metal
complexes studied in our group and even in the absence of any
48,54−56
31,33
strongly oxidizing conditions applied.
experiments, the potential was cycled between 1.3 and 2.0 V at
mV/s for a total of three cycles. Herein, rough PG electrodes
In all OLEMS
metal complex in solution (see Figure S2).
To clarify the
origin of the observed CO formation during the experiment
2
1
with α-[Fe(mcp)(OTf) ], an additional OLEMS experiment
2
were used, which allow for a sufficiently large production of O
per cm geometric surface area for detection by OLEMS.
In OLEMS experiments with a PG working electrode, α-
was performed with a gold working electrode. Although the
first CV scan shows additional features because of the
formation of gold oxide, the experiment excludes the mcp
2
2
[
Fe(mcp)(OTf) ] demonstrates WO activity (Figure 3). In the
ligand as the source of CO . In a second scan and subsequent
2
2
current trace, an oxidative current can be seen starting at 1.7 V
with a sharp increase at about 1.8 V. The mass traces for O2
scans of the OLEMS experiment on gold, in the presence of α-
[Fe(mcp)(OTf) ], a sharp increase in the recorded current is
2
2
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Figure 4. CV in combination with OLEMS of α-[Fe(mcp)(OTf) ] (1.1 mM) in 0.1 M NaClO using a gold working electrode at a scan rate of 1
2
4
mV/s. Depicted are the m/z traces of O (top) CO (middle) and the corresponding current (bottom) as a function of applied potential. The
2
2
forward sweep of the first scan showing a gold oxidation wave at 1.8 V vs RHE (left) and the second scan (right) is shown.
visible, starting between 1.8 and 1.9 V. The mass trace for m/z
=
32 shows an onset of dioxygen evolution, which correlates
with the oxidative current while the mass trace for m/z = 44
shows no signs of CO formation (Figure 4). The onset of
2
dioxygen formation appears to be slightly delayed in the
OLEMS experiments with a gold working electrode compared
to the experiments with a PG working electrode. However, the
contribution of CO2 formation to the current in the
experiment with a PG electrode makes it more difficult to
precisely determine the onset of dioxygen evolution. The lack
of CO formation in the experiment with a gold working
2
electrode on the other hand indicates that the CO formation
2
observed for the case of a PG working electrode is mainly
because of the oxidation of the electrode material itself.
In addition to qualitative measurements to determine the
oxygen production as a function of applied potential, also
quantitative bulk experiments were performed to determine
the Faradaic efficiency of the water oxidation reaction. A
Faradaic Yield (FY) of 87% was obtained when an FTO
electrode was used (Figure 5), while the FYs dropped
significantly when carbon-based electrodes were used instead.
Because previous studies showed that α-[Fe(mcp)(OTf)2]
shows good catalytic rates under acidic conditions in the
presence of CAN, we decided to also investigate the complex
by means of electrochemistry under acidic conditions.
However, the Fe(II) complex is not stable for extended
periods at low pH because of demetallation. Demetallation at
low pH is not an issue for the catalytic water oxidation
experiments performed with CAN because upon addition of
CAN, the Fe(II) complex is quickly oxidized to higher
oxidation states, which are significantly more stable against
demetallation (vide infra). However, in electrochemical
experiments, the bulk of the complex present in solution
remains in the Fe(II) resting-state throughout the experiment.
As a result, demetallation occurs, causing noticeable degrada-
tion of the complex on the time scale of the electrochemical
experiments. Previous studies showed that α-[Fe(mcp)-
Figure 5. Determination of O produced (red line) and FY (blue
2
dots) for α-[Fe(mcp)(OTf) ] during chronoamperometry experiment
2
(
E = 1.8 V vs RHE, 30 min) monitored using an oxygen sensor probe.
The blue-dashed line represents the FY measured. In this experiment,
α-[Fe(mcp)(OTf) ] was used at a concentration of 1.1 mM, in 10
2
mM phosphate buffer at pH 6.8.
having the iron center in the +III oxidation state beforehand
prevents the ligand dissociation. According to UV−vis
experiments, the iron(III) complex α-[Fe(mcp)(Cl) ]Cl is
2
stable in the acidic electrolyte (pH 1, 0.1 M H SO , Figure 6).
2
4
Concordantly, we have investigated the electrochemistry of
α-[Fe(mcp)(Cl) ]Cl under both neutral and acidic conditions
2
(Figures S6 and 7). In contrast to the CV studies in the
presence of α-[Fe(mcp)(OTf) ], solutions of α-[Fe(mcp)-
2
(Cl) ]Cl produce multiple redox couples between 0.5 and 1.2
2
V versus RHE. This suggests the presence of multiple species
in solution, probably as a result of dimerization or partly
(de)coordination of chloride. The oxidation of chloride can be
ruled out, given that this process takes place at a significantly
higher redox potential.
OLEMS experiments of α-[Fe(mcp)(Cl) ]Cl were carried
2
(
OTf) ] is hydrolyzed at low pH such as in solutions of 0.1
out in both 0.1 M Na SO and 0.1 M H SO electrolyte
solutions. The presence of chloride anions in α-[Fe(mcp)-
2
2
4
2
4
4
2,57
M triflic acid.
The ligand demetallation in the iron(II)
precursor is too fast to exclude free iron in solution, which
potentially can affect the CV (Figures S3 and S4a). However,
(Cl) ]Cl is not compatible with a gold electrode at potentials
exceeding 1.2 V versus RHE because of the facile formation of
2
2
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Figure 6. Evolution of the UV−vis absorption of α-[Fe(mcp)(Cl) ]Cl
2
(
0.05 mM) in a 0.1 M H SO solution over 2 h, recorded in 15 min
2 4
intervals. Depicted are the UV−vis spectra recorded at t = 0 min
solid line) and at t = 120 min (dotted line) and the intermediate
UV−vis spectra recorded every 15 min (gray lines).
(
Figure 8. CV in combination with OLEMS of 1.1 mM α-
[
Fe(mcp)(Cl) ]Cl in 0.1 M H SO solution using a PG working
2
−
31
2 2 4
[
AuCl4]
under oxidative conditions.
Consequently,
electrode at a scan rate of 1 mV/s. Depicted are the m/z traces of O2
(top), CO (middle), and the corresponding current (bottom). For
the sake of clarity, only the forward sweep of the second scan is
OLEMS experiments in the presence of α-[Fe(mcp)(Cl) ]Cl
were exclusively performed with a PG working electrode. The
2
2
results of the OLEMS experiments of α-[Fe(mcp)(Cl) ]Cl in
shown.
2
0
.1 M Na SO solution are qualitatively similar to those
2 4
IV
obtained from OLEMS experiments of α-[Fe(mcp)(OTf) ] in
produces a 10-fold increase in the stability of the Fe (O)
2
0
.1 M NaClO solution (Figure S8). In both cases, the current
resting state intermediate and a 5-fold increase in the water
oxidation activity of the iron coordination complex. β-
4
38
trace shows an oxidation wave with a sharp increase in current
starting near 1.8 V. The m/z traces for both experiments show
an onset of dioxygen evolution around 1.8 V, which correlates
[Fe(mcp)(OTf) ] on the other hand is less stable with only 1/
2
37
2 the half-life of α-[Fe(mcp)(OTf)2].
with the sharp increase in current. The onset of CO formation
lies above 1.8 V, correlating with the onset of oxidative current
in the current trace. The OLEMS experiment of α-[Fe(mcp)-
Considering these pronounced differences in stability of the
different Fe(mcp)-analogues under catalytic conditions in the
presence of chemical oxidants, we decided to investigate
whether these differences translate into the realm of electro-
catalysis as well. Given that the methylene backbone of the
mcp ligand was shown to be unstable under strongly oxidizing
conditions at reaction times of minutes to hours, we have
compared the CV of the deuterated α-[Fe(D4-mcp)(OTf)2]
complex with that of the nondeuterated one, α-[Fe(mcp)-
2
(
Cl) ]Cl in 0.1 M H SO (Figure 8) shows a significantly
2 2 4
higher current compared to that observed in 0.1 M Na SO
2
4
(Figure S8a). From 1.8 V onward, oxidative current can be
seen in the current trace (Figure 8, bottom) which correlates
with the onset of CO formation (Figure 8, middle). The onset
2
of dioxygen evolution lies near 1.8 V (Figure 8, top),
correlating with the sharp increase in oxidative current visible
in the current trace.
Structural Modifications. To further assess the stability of
the WO catalysts under electrocatalytic conditions, we have
(OTf) ]. We did not observe any significant differences in the
2
activity and stability of both complexes under the conditions
explored in this contribution (Figures S9 and S10), which
confirms that under the studied catalytic conditions, the α-
also evaluated the activity of α-[Fe(D4-mcp)(OTf) ] and β-
[Fe(mcp)(OTf) ] complex is stable during the course of the
experiments.
2
2
[
Fe(mcp)(OTf) ] under electrocatalytic conditions. Previous
2
work showed that, when driven by chemical oxidants CAN and
To better compare the WO catalytic activities of α-
[Fe(mcp)(OTf) ] and β-[Fe(mcp)(OTf) ], BDD disks were
NaIO , the deuterated analogue α-[Fe(D4-mcp)(OTf) ] is
4
2
2
2
considerably more robust and resistant toward ligand
degradation under oxidative conditions compared to the
nondeuterated analogue. We previously reported that the
deuteration of the methylene positions of the mcp ligand
used (Figure 9). The background current of the BDD
electrode is relatively marginal and does not change
significantly under oxidative conditions in contrast to GC
II
III
and PG. The Fe /Fe redox couple of β-[Fe(mcp)(OTf) ] is
2
Figure 7. Voltammograms of 1.1 mM α-[Fe(mcp)(Cl) ]Cl in 0.1 M H SO solution. Shown are (a) range between 0.4 and 1.0 V vs RHE, starting
2
2
4
at 0.7 V vs RHE, recorded with a gold working electrode and (b) range between 0.0 and 2.0 V vs RHE, starting at 0.7 V vs RHE recorded with a PG
working electrode. Both voltammograms were recorded at a scan rate of 100 mV/s.
2
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Manganese Catalysts. Manganese complexes bearing
tetradentate aminopyridine ligands chemically closely related
58
to mcp have recently been described as WO electrocatalysts.
In addition, α-[Mn(mcp)(OTf) ] and α-[Fe(mcp)(OTf) ] are
2
2
known to exhibit similar catalytic activity and selectivity in the
oxidation of organic substrates, and their mechanisms are
59−66
proposed to be virtually identical.
to explore the electrocatalytic water oxidation activity of α-
Mn(mcp)(OTf) ]. The manganese complex was investigated
These precedents let us
[
2
electrochemically under the same conditions as α-[Fe(mcp)-
(
OTf) ] and with the same working electrodes (Figure S12).
2
Figure 9. Voltammograms of 1 mM α-[Fe(mcp)(OTf) ] (black line)
2
Experiments showed a common trend: in the forward scan of
the CV experiments with α-[Mn(mcp)(OTf) ], two oxidation
events were observed between 1.1 V and 1.5 V, whereas in the
backward scan, a reduction event was observed around 1.2 V
(
and β-[Fe(mcp)(OTf) ] (gray line) in 0.1 M Na SO at a scan rate of
2
2
4
2
2
0 mV/s, using a BDD working electrode. For the sake of
comparison, the catalytic current (i) was normalized with the peak
current (i ) of the oxidation event of the redox couple (i/i ).
p
p
Figure 11a). This behavior can be related to the formation of
a catalytically active heterogeneous species on the surface of
the working electrode (Figure 11a). In order to detect any
possible deposition of material on the electrode surface, after a
regular CV the electrode was taken out of the solution
containing the manganese catalyst, rinsed with Milli-Q water,
and placed in a cell containing the electrolyte solution without
any complex present. A CV was then measured and compared
to a blank measurement of the same electrode, previously
measured under the same conditions, in the absence of the
complex. Figure 11b shows clear signs of a deposition of a
catalytically active material present on the surface of the
working electrode. In contrast, no signs of deposition are
observed if the same experiment is carried out in the case of α-
shifted 40 mV to higher potentials in comparison with α-
Fe(mcp)(OTf) ]. Besides this small shift in the position of
the redox couple, the two complexes show a similar current
profile and onset for the water oxidation reaction.
Mechanistic Studies. Mechanistic analyses were per-
formed to shed light on the reaction mechanism of
[
2
electrocatalytic water oxidation with α-[Fe(mcp)(OTf) ]. In
2
order to determine the rate order in the catalyst, CV of α-
[
Fe(mcp)(OTf) ] was performed at different concentrations,
2
between 2.0 and 0.3 mM. For the calculation of the rate order,
the values of the catalytic currents at different concentrations
were normalized with the corresponding peak current values of
the oxidation event of the redox couple. The ratio between
these values, namely i /i , was plotted versus the concen-
[
Fe(mcp)(OTf) ] (Figure S13).
In order to further investigate the formation of any surface
2
cat
p
deposit, electrochemical quartz crystal microbalance (EQCM)
tration. For this system, a first order rate in [Fe] was found
4
9,50
experiments were carried out.
Figure 12a shows the results
]. A
(
see Figure S11).
Next, the electrocatalytic WO activity of α-[Fe(mcp)-
of an EQCM experiment with α-[Mn(mcp)(OTf)
2
(
OTf) ] was investigated in water and in deuterium oxide
decrease of the catalytic current over the first 25 scans was
observed simultaneously with a decrease in the resonance
frequency. The decrease in frequency was already present in
the first scan and became consistent in the further scans. This
behavior indicates a mass deposition, probably manganese
2
(
Figure 10). The CV experiments were performed in 10 mM
phosphate buffer at pH 6.8. Under the experimental conditions
used, a KIE ∼ 10 was found, which suggests that a proton-
coupled event is involved in the rds (Figure 10a). Using α-
67
[Fe(mcp)(Cl) ]Cl in a 0.1 M H SO solution of pH 1, a KIE
oxide, on the electrode surface.
2
2
4
of 1 was found, which points to a different rds under acidic
conditions (Figure 10b). When chemical oxidants are used to
Microbalance studies in chronoamperometry mode were
performed, by applying a potential of 2.0 V for 10 min while
monitoring the change in the resonance frequency. The value
of the resonance frequency of the quartz crystal shows a linear
decay over time, indicating a constant mass deposition on the
electrode surface (Figure 12b). Mass deposition was observed
drive WO, both in neutral media (NaIO , see Table S1) and in
4
2
5
acidic media (CAN), a KIE of 1 was found. Additional
details about the calculations of the KIE values are available in
the Supporting Information.
Figure 10. (a) Results of CV experiments of α-[Fe(mcp)(OTf) ] (1 mM) using a GC working electrode, at a scan rate of 20 mV/s. The
2
experiments were performed in 10 mM phosphate buffer solution at pH 6.8 with 0.1 M Na SO to maintain the ionic strength. The figure displays
2
4
the first scans of the experiments performed in water (black) and in deuterium oxide (gray). (b) Results of CV experiments of α-[Fe(mcp)(Cl) ]Cl
2
(
1 mM) using a GC working electrode, at a scan rate of 20 mV/s. The experiments were performed in 0.1 M H SO at pH 1. The figure shows the
2
4
first scans of the experiments performed in water (black) and in deuterium oxide (gray).
2
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Figure 11. CV of 1 mM α-[Mn(mcp)(OTf) ] (a) in 0.1 M Na SO , at a scan rate of 20 mV/s, using a BDD working electrode. Shown are the first
2
2
4
(
dotted line), second (dashed line), and third scans (solid line). (b) CV of a BDD working electrode in 0.1 M Na SO blank solution at a scan rate
2 4
of 20 mV/s, recorded before (gray) and after (black) the CV shown in (a).
Figure 12. (a) CV in combination with EQCM of α-[Mn(mcp)(OTf) ] (1 mM) in 0.1 M Na SO at a scan rate of 100 mV/s, using a gold working
2
2
4
electrode. Shown are scan 1 (black) and scans 2 to 25 (gray) of the CV experiment (bottom) and the corresponding change in resonance frequency
(
top). (b) Chronoamperometry in combination with EQCM of α-[Mn(mcp)(OTf) ] (1 mM) in 0.1 M Na SO , using a gold working electrode,
2
2
4
keeping the potential at 2.0 V vs RHE for 10 min (solid line), at 1.5 V vs RHE for 10 min (dashed line) and at 1.3 V vs RHE for 10 min (dotted
line).
also at lower potential values. Therefore, a chronoamperom-
etry experiment was performed at 1.5 V versus RHE for 2 h in
order to first produce a thick manganese oxide layer on top of
the gold electrode. After this experiment, the electrode was
placed in a blank electrolyte solution to perform an OLEMS
experiment, which confirmed that the deposit indeed produces
large amounts of dioxygen (Figure 13). No traces of carbon
dioxide were detected in the process, which suggests that active
manganese sites are mainly produced at the top of a relative
thick oxide layer. All evidence points to the conclusion that in
the case of α-[Mn(mcp)(OTf) ], the catalytic process takes
2
place on the electrode surface because of the formation of a
67−71
manganese oxide layer, which is active toward WO.
There
is no sign of a catalytic reaction taking place in solution. We
conclude that even though the Fe(mcp) and Mn(mcp)
complexes are very similar in structure, their behavior under
catalytic water oxidation conditions is largely different, which is
most likely because of manganese(II) ions being notoriously
labile.
DISCUSSION
■
CV of α-[Fe(mcp)(OTf) ] in aqueous NaClO (0.1 M) and
2
4
Na SO (0.1 M) electrolyte solutions shows an irreversible
2
4
oxidation wave starting at about 1.7 V associated with water
oxidation. The same wave is observed when using a glassy
carbon (GC) electrode, a basal plane pyrolytic graphite (PG)
electrode, a boron-doped diamond (BDD) electrode, and a
gold electrode. Online electrochemical mass spectrometry
(
OLEMS) experiments show that the electrochemical wave
correlates with the generation of O , demonstrating that it
2
corresponds to an electrocatalytic water oxidation wave. In the
case of PG electrodes, water oxidation occurs in combination
with oxidation of the electrode, producing CO . However, with
2
noncarbonaceous electrodes, CO formation is not observed,
2
Figure 13. CV in combination with OLEMS of a gold working
suggesting that it is not produced by oxidation of the ligand of
the catalyst but rather by oxidation of the carbon-based
electrode material. Thus, the data strongly suggest that
catalytic water oxidation is performed by the intact catalyst.
In addition, electrochemical quartz crystal microbalance
(EQCM) experiments show that there is no deposition of
electrode covered by a MnO layer, recorded in 0.1 M Na SO at a
2
4
scan rate of 1 mV/s. Depicted are the m/z traces of O (top), CO
2
2
(
middle), and the corresponding current (bottom). For the sake of
clarity, only the forward sweep of the second scan is shown. The MnO
layer was produced during a chronoamperometry experiment while
keeping the potential at 1.5 V for 2 h.
2
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(
catalytically active) material on the electrode, strongly
(OTf) ], the onset potential was found at about 1.7 V versus
2
suggesting that the catalytic water oxidation takes place in
the homogeneous phase.
In terms of molecular nature and catalytic current, the
performance of the Fe(mcp) systems is similar to that of the
dinuclear iron catalyst [(MeOH)Fe(Hbbpya)−μ−O−
RHE under electrochemical conditions (pH 5). This value
correlates well with the previously calculated redox potentials
V
IV
of the Fe /Fe redox couple for the α-[Fe(mep)(OTf)2]
complex (mep = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-
ethylenediamine). A redox potential value of 1.73 V versus
33
IV
V
(
Hbbpya)Fe(MeOH)](OTf) . Moreover, both complexes
NHE was found for the Fe (O)(OH ) to Fe (O)(OH)
2
73
4
showed an overpotential in the range of 300−500 mV with
respect to the thermodynamic water oxidation potential of 1.23
V. In terms of TOF the Fe(mcp) systems showed a value of
transition (computed at pH 1). In addition, previous studies
IV
determined that Fe (O)(OH ) species in related nonheme
2
complexes were not able to form the O−O bond required for
−
1
25,37,38
74
0
.41 s in the presence of chemical oxidants
and the
oxygen evolution. Therefore, it is reasonable to suggest that
−
1
Fe(Hbbpya) system showed a value of 0.12 s under
the onset of the electrocatalytic wave may correspond to the
33
V
IV
electrochemical conditions. . The advantage of the Fe(mcp)
system is that it was proved to be active under both neutral and
acidic pH conditions, providing that one starts with a +III
precursor, whereas [(MeOH)Fe(Hbbpya)−μ−O−(Hbbpya)-
Fe(MeOH)](OTf) did not show stability at pH 1. It should
be mentioned though that α-[Fe(mcp)(OTf) ] showed good
catalytic rates at low pH in the presence of cerium(IV)
ammonium nitrate (CAN), but the electrochemical studies of
α-[Fe(mcp)(OTf) ] under such low pH conditions were
formation of Fe (O)(OH) from Fe (O)(OH ) species, and is
2
the rds.
When comparing the catalytic mechanism between electro-
chemical conditions and chemical oxidant-driven conditions
for Fe(mcp) and related complexes, clear differences between
the two regimes become apparent. These differences are most
notable when comparing the KIE values obtained under
electrocatalytic conditions to KIE values obtained when water
oxidation is driven by chemical oxidants. The [Fe-
4
2
2
excluded because of the demetallation of the iron(II)
precursor, which occurs on the time scale required for the
electrochemical experiment. In the presence of CAN, the
iron(II) precursor is instantaneously oxidized to the iron(III)
species, which is stable against hydrolysis. Following a rational
(OTf) (pytacn)] (pytacn = 1-(2-pyridylmethyl)-4,7-dimethyl-
1,4,7-triazacyclononane) catalyst has been shown to perform
2
WO with a KIE of approximately 1 when using NaIO and
4
CAN, compared to a KIE of 10 under electrocatalytic
25
conditions. In order to determine whether these differences
in KIE arise from the different nature of the catalyst or by the
type of terminal oxidant (electrocatalysis vs chemical
oxidants), we have evaluated the KIE for the α-[Fe(mcp)-
design, the iron(III) complex α-[Fe(mcp)(Cl) ]Cl was proved
2
to be stable and active in the acidic electrolyte (pH 1, 0.1 M
H SO ). The TOF values found with our catalysts are still low
2
4
when compared to the pentanuclear iron catalyst reported by
Masaoka et al., with a TOF = 1900 s . However, the
pentanuclear system operated at an overpotential higher than
(OTf) ] complex when using chemical oxidants. Turnover
2
−1
32
frequencies were determined by manometry and are collected
in Table S1 in the Supporting Information. When using CAN,
which operates at low pH (pH = 0.8), we have found that the
KIE is ca. 1.0, which matches well with the electrochemistry
experiments under acidic conditions. Considering that a KIE of
10 was obtained electrochemically at higher pH values, we
have also evaluated the KIE at different pH values ranging from
5
00 mV and in an acetonitrile/water mixture. Therefore, our
Fe(mcp) water oxidation catalysts offer the advantages of
stability and activity at neutral and acidic pH values, a lower
overpotential compared to some of the benchmark systems,
2
6
31
33
including Fe(dpaq), Fe(cyclam), Fe(bbpya), and other
29
tetradentate polypyridyl type ligands, and the ability to
operate in aqueous solutions (dpaq = 2-[bis(pyridine-2-
ylmethyl)]amino-N-quinolin-8-yl-acetamido and bbpya =
N,N-bis(2,2′-bipyrid-6-yl)amine).
4.6 to 10, using NaIO as the oxidant (see Table S1). Again,
4
under these conditions, a KIE of approximately 1 is observed.
Therefore, chemical and electrochemical water oxidation
conducted at neutral pH values exhibits different KIE values
indicating that they have different rds.
However, in contrast to the results obtained for the iron
catalysts reported in this study, the manganese catalyst α-
Previous studies have addressed the mechanism of water
oxidation performed with the catalysts studied in this work in
[
Mn(mcp)(OTf) ] exhibits signs of complex degradation and
2
37,41,73
deposition of manganese oxide on the electrode, which is
responsible for the observed catalytic activity.
the presence of chemical oxidants.
In the particular case
of α-[Fe(mcp)(OTf) ], mechanistic studies under acidic
2
IV
Overall, the reaction rates for electrocatalytic water
oxidation in the presence of the Fe(mcp) systems are modest,
with an overpotential on the scale of approximately 500 mV
with respect to the thermodynamic potential of the water
oxidation reaction of 1.23 V, suggesting the existence of kinetic
barriers for the reaction. In this regard, previous work has
conditions using Ce as a sacrificial oxidant, aided by
spectroscopic analysis, have identified the high-valent inter-
IV
mediate Fe (O)(OH ) as the resting state at a low
2
IV
concentration of Ce . This species undergoes a one electron
oxidation to form Fe (O)(OH) via an inner sphere electron
V
IV
transfer process that proceeds through a heterometallic Fe −
III
IV
IV
shown that Fe /Fe oxidation on related complexes entails a
O−Ce species. The latter accumulates when large concen-
III
IV
IV
slow Fe (OH)/Fe (O) proton-coupled electron transfer
associated with relatively large reorganization energy values,
trations of Ce are used, enabling mass spectrometric and
37
IV
spectroscopic characterization. The heterometallic Fe −O−
Ce species constitutes the last detectable intermediate of the
72
IV
which presumably arise from a spin-state barrier.
Most notably, our mechanistic analyses indicate that the
electrocatalytic reaction is first order in the iron catalyst and
that the rds has a large KIE ∼ 10 at neutral pH.
In order to understand the origin of this large KIE, it should
be considered that under electrocatalytic conditions, the water
oxidation onset potential can be ascribed to the formation of
the catalytically active species. In the case of α-[Fe(mcp)-
catalytic cycle prior to dioxygen formation and its evolution
toward the reactive Fe (O)(OH) species via internal electron
V
transfer presumably constitutes the rds of the reaction.
Computational analysis indicates that attack of the water
V
molecule on Fe (O)(OH) is initially assisted by an interaction
73
with the hydroxide ligand at the same iron center. Proton-
coupled electron transfer from the incoming water molecule to
2
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V
IV
Fe (O)(OH) forms an {Fe (O)(OH )·OH} intermediate
highlight the importance of the electrochemical studies to
evaluate water oxidation catalysts.
2
from which the O−O bond is rapidly formed. The reaction of
V
Fe (O)(OH) with the water molecule must be fast and takes
place after the rds, accounting for the lack of an isotope effect.
The large KIE values obtained with the electrocatalytic WO
at pH 7 require an alternative explanation. Large KIE values in
water oxidation reactions are uncommon, but several notable
precedents have been reported. Electrochemical studies of
Meyer et al. on [Ru(Mebimpy)(bpy)(OH )] (Mebimpy =
,6-bis(1-methylbenzylimidazol-2-yl)pyridine complexes) re-
vealed a KIE value ranging from 6.6 in pure water to 2.3 in
the presence of HPO4 as the base, suggesting that added
bases accelerate the O−O bond formation by concerted atom-
proton transfer (ATP). Higher reaction rates were observed
EXPERIMENTAL SECTION
■
The reagents and solvents used were commercially available
and purchased from Panreac, Scharlau, and Aldrich. Prepara-
tion and handling of air-sensitive materials were carried out in
a N glovebox (MBraun ULK 1000) with O and H O
2
2
2
2+
2
concentrations < 5 ppm. Cerium(IV) ammonium nitrate
2
(
CAN) ≥99.5% trace metals basis were purchased from Sigma-
Aldrich. Water (18.2 MΩ cm) was purified with a Milli-Q
Millipore Gradient AIS system. D O was purchased and
2−
2
directly used from Sigma Aldrich. The complexes α-[Fe(mcp)-
(
OTf) ], α-[Fe(D4-mcp)(OTf) ], α-[Fe(mcp)(Cl) ]Cl, β-
2 2 2
7
with conjugated bases with a higher pK value. Another
example of a large KIE (20) under electrocatalytic conditions
[Fe(mcp)(OTf) ], and α-[Mn(mcp)(OTf) ] were synthesized
2 2
38,59,60
a
and characterized according to previous procedures
and
was found for the water oxidation catalyst {[(Me TMPA)-
except for the iron(III) complex stored in an argon-filled
glovebox to prevent oxidation in air.
2
II
Cu ] -(μ-OH) }(OTf) (Me TMPA = bis((6-methyl-2-
2
2
2
2
pyridyl)methyl)(2-pyridylmethyl)-amine). In this case, a
Electrochemical Experiments. All electrochemical meas-
urements with the exception of the EQCM experiments
(details below) were performed in a custom-made single-
compartment glass cell using a three-electrode setup. In all
cases, the data were recorded either on an Ivium potentiostat,
operated by IviumSoft software, or on an Autolab PGstat10
potentiostat operated by NOVA 2.1.2 software. The working
electrodes used in the experiments were a pyrolytic graphite
II
II
II
III
PCET step ({Cu (μ-OH) Cu } → {Cu (μ-OH)(μ-O)Cu }
2
−
+
+
e + H ) is considered to be responsible for the large kinetic
75
barrier.
Therefore, the large KIE value of 10 that we observe under
electrocatalytic conditions at neutral pH is reminiscent of the
PCET process observed in the high-valent copper active
species, and may indicate that PCET from the Fe (O)(OH )
to Fe (O)(OH) takes place during the rds.
IV
2
V
2
(PG) disk with a (geometric) surface area of 0.20 cm , two
2
gold disks with a (geometric) surface area of 0.13 cm and 0.18
2
2
cm , a glassy-carbon (GC) rod with a surface area of 0.07 cm ,
CONCLUSIONS
■
a boron-doped-diamond (BDD) disk with a surface area of
The electrochemical experiments performed on the Fe(mcp)
systems under acidic conditions showed that the results
obtained with chemical oxidants can largely be reproduced
electrochemically. Furthermore, it was proven that these
systems are electrocatalytically active toward water oxidation
under pH-neutral conditions, which are more suitable for
applications compared to strongly acidic conditions. The
evolution of dioxygen was proven with OLEMS experiments
2
0
.07 cm , and indium tin oxide (ITO)-coated glass plates. A
large surface area gold plate was used as a counter electrode.
The reference electrode was a reversible hydrogen electrode
(
RHE) made up of a platinum mesh in the H -saturated
2
electrolyte at the same pH as the working solution. The cell
and the reference electrode were connected via a Luggin
capillary.
The gold electrode was prepared before each experiment by
oxidizing the surface at 10 V for 30 s in a 10% H SO solution,
and no CO formation originating from the organic ligands was
2
2
4
detected, confirming the stability of the catalysts. Moreover,
these catalysts were proved to be molecular systems, as
confirmed by EQCM experiments where no formation of
deposits on the working electrodes was observed. A counter-
check of the molecularity of the systems was obtained by
followed by stripping of the gold oxide layer in 6 M HCl
solution and subsequent electropolishing of the electrode by
scanning for 200 cycles between 0.0 and 1.75 V versus RHE at
1
V/s in 0.1 M HClO electrolyte solution.
A fresh PG surface was prepared before each experiment by
4
studying the manganese catalyst α-[Mn(mcp)(OTf) ], for
2
polishing the working electrode with sandpaper and sub-
sequently removing excess debris by sonication in Milli-Q
water for at least 5 min.
For the experiments with an ITO working electrode, a small
slice of ITO-covered glass (ca. 0.5·1.5 cm) was used. While the
gold and PG working electrodes were used in the hanging
meniscus configuration, the ITO electrode was partially
submerged in the electrolyte solution.
The GC electrode was pretreated before each experiment by
polishing the electrode surface with alumina suspensions (1.0
μm followed by 0.3 μm and 0.05 μm). The polishing was
followed by removing the excess debris by sonicating the
electrode in Milli-Q water for 10 min.
which a heterogeneous species formed at the surface of the
working electrode is instead responsible for the catalytic
activity. Apart from the manganese system, all the complexes
studied showed comparable activity toward the water oxidation
reaction. By studying various structural analogues of the
Fe(mcp)-complex, the stability of the catalytic system under
electrochemical conditions was confirmed. The mechanistic
studies provided more insights into the catalytic mechanism,
showing that the reaction proceeds with a different rds with
respect to what was previously proposed when using chemical
2
5,38
oxidants.
This reflects the significance of detailed electro-
catalytic studies in addition to the use of chemical oxidants,
which take active part in the reaction mechanism. As shown in
previous studies, the reaction of the iron complex with Ce
generates a Fe −O−Ce complex, which acts as an inner-
sphere electron transfer intermediate, leading to the 1 e
The BDD electrode was prepared by sonication for 5 min in
Milli-Q water. Subsequently, the electrode was electropolished
by scanning 200 cycles between −1.0 and 2.25 V versus RHE
at 1 V/s in 0.1 M H SO solution.
IV
IV
IV
−
2
4
oxidation of the iron center, which later undergoes a water
nucleophilic attack. Overall, the findings of this study
All glassware used in the electrochemical measurements was
routinely cleaned of any organic contamination overnight with
38
2
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0
.5 M H SO solution containing 1 g/L KMnO . The metal
VO2
2
4
4
FY =
*100
particles were afterward removed by cleaning the glassware for
0 min with Milli-Q grade water (>18.2 MΩ cm resistivity)
containing a few droplets of concentrated H SO and 35%
V (th)
O
2
3
2
4
where
H O . The glassware was then cleaned by threefold rinsing
2
2
i*t RT
4
6
with Milli-Q grade water and boiling it in Milli-Q grade water.
The electrolyte solutions were prepared from p.a. grade
chemicals obtained from Merck (Suprapur) and Milli-Q water
V (th) =
·
·10
O
2
F
V (th) is the theoretical amount of O in μL, i is the current
passed, t is the time, factor 4 accounts for the stoichiometric in
WO, R is the gas constant (0.0821 L atm mol K ), T is the
temperature (298 K), and F is the Faraday constant (96,485 C
mol ).
Sample Preparation. Samples of complexes α-[Fe(mcp)-
OTf) ], α-[Fe(D4-mcp)(OTf) ], β-[Fe(mcp)(OTf) ], and α-
O
2
2
(
resistivity >18.2 MΩ cm). The phosphate buffer solutions (10
mM) were prepared by using NaH PO and Na HPO with
2
4
2
4
−1 −1
the addition of Na SO (0.1 M) to maintain the ionic strength.
2
4
Prior to measurements, the electrolyte solution was purged
of air by bubbling with argon (Linde, Ar 5.0) for at least 20
min. During the measurements, the cell was constantly kept
under argon flow to prevent air from entering.
For the OLEMS measurements, the gases formed at the
working electrode were collected via a hydrophobic tip (KEL-F
with a porous Teflon plug) in close proximity to the surface of
the working electrode and analyzed using a QMS 200 mass
spectrometer. A detailed description of the OLEMS setup is
available elsewhere. All electrochemical potential cycling in
combination with OLEMS was done at a scan rate of 1 mV/s.
EQCM experiments were performed in a 3 mL Teflon cell
purchased from Autolab. The top part of the cell was modified
to allow for electrochemical measurements under an inert
atmosphere. The EQCM was controlled using an Autolab
potentiostat operated by NOVA 2.0 software. Autolab EQCM
−
1
(
[
2
2
2
Mn(mcp)(OTf) ] were weighed in an argon atmosphere
2
inside the glovebox and stored in a closed vessel. Prior to the
experiment, the complexes were dissolved in a small amount of
electrolyte solution (typically 1−2 mL) taken from the cell,
which had previously been purged with argon and
subsequently added to the electrochemical cell. The electrolyte
solution was then purged again by bubbling with argon for
several minutes.
Samples of complex α-[Fe(mcp)(Cl) ]Cl were weighed in
air and subsequently added to the cell in a manner analogous
to that described for the iron(II) complexes.
53
2
For the electrochemical experiments, 1.0 mM and 1.1 mM
concentrations of the catalyst were used.
2
electrodes with a surface area of 1.5 cm consisting of a 200 nm
gold layer deposited on a quartz crystal were used as working
electrodes. A custom-made RHE reference electrode was used,
General Procedure for the Chemically Driven WO
Reactions. In a crimped 20 mL vial, CAN (685.29 mg, 1000
equiv, final concentration = 125 mM) was dissolved in Milli-Q
water (9.5 mL) at room temperature leading to an orange
solution. The headspace of the vial containing the resulting
solution was monitored with an atmospheric pressure sensor
transducer. After equilibration of the pressure signal, iron
54
which is described elsewhere.
Online analysis of the gas mixture during long-term
chronoamperometry to determine the FY was performed
using a NeoFox oxygen-sensing system equipped with a
FOSFOR-R probe. A tight three-electrode electrochemical cell
was filled with 15 mL of the electrolyte solution (10 mM
phosphate buffer at pH 6.8) and, after sealing, a constant flow
of Ar gas (40 mL/min) was applied, with the O sensor probe
placed in the headspace of the cell. Meanwhile, ca. 11 mg of
the α-[Fe(mcp)(OTf) ] complex was weighed and kept under
an inert atmosphere until the electrolyte solution became O2-
free. At this point, about 2 mL of this solution was withdrawn
from the cell and used to dissolve the catalyst powder. Once
the catalyst stock solution was prepared, it was rapidly injected
in the electrochemical cell. The final concentration was 1.1
mM in 15 mL of the electrolyte. To perform the experiment,
complex α-[Fe(mcp)(OTf) ] (0.5 mL of a stock solution of 2
2
mg in 11.8 mL Milli-Q water, final concentration = 12.5 μM)
was added and the evolved gas was monitored along the
reaction time. Then, an aliquot of the headspace (150 μL) was
analyzed in the GC-TCD to quantify the O and CO present
2
2
2
2
in the headspace.
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05439.
■
*
the cell was equipped with the FTO-covered glass slide (1 ×
The CV of α-[Fe(mcp)(OTf)
electrodes; UV−vis stability tests on [α-Fe(mcp)-
(OTf) ] and on α-[Fe(mcp)(Cl) ]Cl; CV of α-[Fe-
(mcp)(Cl) ]Cl, OLEMS of α-[Fe(mcp)(Cl) ]Cl, and
OLEMS of α-[Fe(D4-mcp)OTf) ]; mechanistic studies;
] with different working
2
] with different working
2
2
.5 cm ) as the working electrode, a SCE reference electrode,
and a Pt wire as the counter electrode. Chronoamperometry
was run at a constant potential of 1.65 V versus RHE for 30
min.
2
2
2
2
2
The volume (in μL) of O evolved was determined using the
and CV of α-[Mn(mcp)(OTf)
2
2
following equation
electrodes (PDF)
pM
6
AUTHOR INFORMATION
Corresponding Authors
VO2
=
V ·10
■
HS
p
Miquel Costas − Institut de Química Computacional i Catàlisi
(IQCC) and Departament de Química, Universitat de
Girona, 17003 Girona, Spain; orcid.org/0000-0001-
where pM is the measured O pressure (in torr), p is the
ambient pressure (760 torr), V is the volume of the
headspace, and the factor 10 is used to convert the volume in
μL. To obtain the FY, the VO2 value is divided by theoretical
2
HS
6
6326-8299; Email: miquel.costas@udg.edu
Julio Lloret-Fillol − Institute of Chemical Research of
Catalonia, Spain (ICIQ), The Barcelona Institute of Science
and Technology, 43007 Tarragona, Spain; Catalan
amount of O produced, based on the chronoamperometry, as
2
follows
2
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Institution for Research and Advanced Studies (ICREA),
oxidation activity comparable to that of photosystem II. Nat. Chem.
012, 4, 418−423.
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(
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8010 Barcelona, Spain; orcid.org/0000-0002-4240-
9
512; Email: jlloret@iciq.es
Bioinspired Molecular Ru Water Oxidation Catalysts with Negatively
Dennis G.H. Hetterscheid − Leiden Institute of Chemistry,
Leiden University, 2300 RA Leiden, The Netherlands;
orcid.org/0000-0001-5640-4416;
Charged Backbone Ligands. Acc. Chem. Res. 2015, 48, 2084−2096.
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Email: d.g.h.hetterscheid@chem.leidenuniv.nl
Authors
(6) Wang, L.; Duan, L.; Wang, Y.; Ahlquist, M. S. G.; Sun, L. Highly
Silvia D’Agostini − Leiden Institute of Chemistry, Leiden
University, 2300 RA Leiden, The Netherlands
Konstantin G. Kottrup − Leiden Institute of Chemistry,
Leiden University, 2300 RA Leiden, The Netherlands
Carla Casadevall − Institute of Chemical Research of
Catalonia, Spain (ICIQ), The Barcelona Institute of Science
and Technology, 43007 Tarragona, Spain; orcid.org/
efficient and robust molecular water oxidation catalysts based on
ruthenium complexes. Chem. Commun. 2014, 50, 12947−12950.
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Catalytic and Surface-Electrocatalytic Water Oxidation by Redox
Mediator−Catalyst Assemblies. Angew. Chem., Int. Ed. 2009, 48,
0
000-0002-3090-4938
Ilaria Gamba − Institut de Química Computacional i Catàlisi
IQCC) and Departament de Química, Universitat de
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9) Meyer, T. J.; Sheridan, M. V.; Sherman, B. D. Mechanisms of
Girona, 17003 Girona, Spain
molecular water oxidation in solution and on oxide surfaces. Chem.
Soc. Rev. 2017, 46, 6148−6169.
Valeria Dantignana − Institut de Química Computacional i
Catàlisi (IQCC) and Departament de Química, Universitat
de Girona, 17003 Girona, Spain
Alberto Bucci − Institute of Chemical Research of Catalonia,
Spain (ICIQ), The Barcelona Institute of Science and
Technology, 43007 Tarragona, Spain
(10) Vannucci, A. K.; Alibabaei, L.; Losego, M. D.; Concepcion, J. J.;
Kalanyan, B.; Parsons, G. N.; Meyer, T. J. Crossing the divide
between homogeneous and heterogeneous catalysis in water
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Complete contact information is available at:
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1
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04, 4029−4030.
12) Bucci, A.; Savini, A.; Rocchigiani, L.; Zuccaccia, C.; Rizzato, S.;
Albinati, A.; Llobet, A.; Macchioni, A. Organometallic Iridium
Catalysts Based on Pyridinecarboxylate Ligands for the Oxidative
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Author Contributions
S.D. and K.G.K. contributed equally. This article was written
through contributions of all authors. All authors have given
approval to the final version of the manuscript.
(
13) Diaz-Morales, O.; Hersbach, T. J. P.; Hetterscheid, D. G. H.;
Reek, J. N. H.; Koper, M. T. M. Electrochemical and Spectroelec-
trochemical Characterization of an Iridium-Based Molecular Catalyst
for Water Splitting: Turnover Frequencies, Stability, and Electrolyte
Effects. J. Am. Chem. Soc. 2014, 136, 10432−10439.
Funding
NWO Echo grant 717.014.008. PGC2018-101737-B-I00 and
2017 SGR 00264 grants.
(
14) Menendez Rodriguez, G.; Bucci, A.; Hutchinson, R.;
Notes
Bellachioma, G.; Zuccaccia, C.; Giovagnoli, S.; Idriss, H.;
Macchioni, A. Extremely Active, Tunable, and pH-Responsive Iridium
Water Oxidation Catalysts. ACS Energy Lett. 2017, 2, 105−110.
The authors declare no competing financial interest.
(
15) Blakemore, J. D.; Schley, N. D.; Olack, G. W.; Incarvito, C. D.;
ACKNOWLEDGMENTS
■
Brudvig, G. W.; Crabtree, R. H. Anodic deposition of a robust
iridium-based water-oxidation catalyst from organometallic precur-
sors. Chem. Sci. 2011, 2, 94−98.
The Netherlands’ Organization for Scientific Research (NWO)
is acknowledged for financial support (Echo grant
7
17.014.008). We acknowledge the support of the Spanish
Ministry of Science, Innovation, and Universities (PGC2018-
01737-B-I00) to M.C. and the Generalitat de Catalunya
2017 SGR 00264 and ICREA Academia). Thanks to
(16) Blakemore, J. D.; Mara, M. W.; Kushner-Lenhoff, M. N.;
Schley, N. D.; Konezny, S. J.; Rivalta, I.; Negre, C. F. A.; Snoeberger,
R. C.; Kokhan, O.; Huang, J.; Stickrath, A.; Tran, L. A.; Parr, M. L.;
Chen, L. X.; Tiede, D. M.; Batista, V. S.; Crabtree, R. H.; Brudvig, G.
W. Characterization of an Amorphous Iridium Water-Oxidation
Catalyst Electrodeposited from Organometallic Precursors. Inorg.
Chem. 2013, 52, 1860−1871.
1
(
MINECO FPU14/02550 (C.C.), ICIQ Foundation, ERC for
project FP7-PEOPLE-2010-ERG-268445 (J.L.F.), and CEL-
LEX Foundation (CELLEX-ICIQ high-throughput platform).
(17) Hintermair, U.; Sheehan, S. W.; Parent, A. R.; Ess, D. H.;
Richens, D. T.; Vaccaro, P. H.; Brudvig, G. W.; Crabtree, R. H.
Precursor Transformation during Molecular Oxidation Catalysis with
Organometallic Iridium Complexes. J. Am. Chem. Soc. 2013, 135,
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