Systematic ligand variation to modulate the electrochemical properties of iron and manganese complexes

Article abstract of DOI:10.1039/c9dt01343a

A series of iron(+iii) and manganese(+ii) complexes based on the dpaq^R-ligand system (dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamide) were investigated using cyclic voltammetry (CV) to elucidate how the electronic properties of the ligands influence the overpotential and catalytic current in the context of water oxidation catalysis. For the Fe-complexes an electron withdrawing NO₂ or CF₃ group attached to the 5-position of the quinoline unit increased the catalytic current, but only with a simultaneous increase of the overpotential. However, when a pyrene moiety was attached to the dipicolylamine unit of the ligand, the overpotential decreased with concomitant increase of the catalytic current. Although the manganese complexes showed no reversible formation of a molecular catalytically active species for water oxidation, the variations of the ligand scaffold affected largely the same trends in their electrochemical behavior.

Full text of DOI:10.1039/c9dt01343a

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Systematic ligand variation to modulate the
electrochemical properties of iron and
Cite this: DOI: 10.1039/c9dt01343a
manganese complexes†
Stefan S. Rohner, a Niklas W. Kinzel, a,b Christophe Werlé *b and
a,b
Walter Leitner
*
A series of iron(+III) and manganese(+II) complexes based on the dpaqR-ligand system (dpaq = 2-[bis(pyri-
dine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamide) were investigated using cyclic voltammetry (CV) to
elucidate how the electronic properties of the ligands influence the overpotential and catalytic current in
the context of water oxidation catalysis. For the Fe-complexes an electron withdrawing NO2 or CF3 group
attached to the 5-position of the quinoline unit increased the catalytic current, but only with a simul-
taneous increase of the overpotential. However, when a pyrene moiety was attached to the dipicolyl-
amine unit of the ligand, the overpotential decreased with concomitant increase of the catalytic current.
Although the manganese complexes showed no reversible formation of a molecular catalytically active
species for water oxidation, the variations of the ligand scaffold affected largely the same trends in their
electrochemical behavior.
Received 29th March 2019,
Accepted 5th June 2019
DOI: 10.1039/c9dt01343a
rsc.li/dalton
water nucleophilic attack (WNA) or radical coupling, but con-
trolling the detailed mechanistic cycles remains a challenge.6
Introduction
Energy generation from renewable energy sources such as The use of the [Fe(dpaqH)(H2O)](ClO4)2 complex (1a) as a WOC
wind and solar energy is becoming increasingly important on in propylene carbonate (PC) with water as a substrate was
a global level. Due to their intermittent availability, there is a reported in 2014 by the group of Meyer.7 The catalysis was
great need for efficient energy storage, for example in the form reported to take place at 1.58 V vs. NHE, resulting in an over-
of chemical bond energy.1 In this context, the electrochemical potential of ca. 760 mV for the water oxidation at pH 7.8 In
splitting of H2O into O2 and H2 plays a crucial role.2 However, electrolysis experiments a TON of 29 with a faradaic efficiency
the water oxidation half-reaction (eqn (1)) is thermo- of 45% was measured. Furthermore, the stability of the
dynamically (E = 1.23 V vs. NHE at pH = 0) as well as kinetically complex under reaction conditions was concluded from UV-Vis
demanding. The development of water oxidation catalysts spectroscopy and CV measurements before and after the
(WOCs) has become a crucial subject of current research to electrolysis. For the oxygen–oxygen bond formation, a WNA
reduce the overpotential and hence facilitate water splitting.3
mechanism was proposed. In this case, the active high-valent
FeV-oxo species acts as an electrophile. This reactivity might be
influenced by electronic variation of the ligand framework,
provided that the active iron center is within the same coordi-
nation sphere at this stage.9
Systematic modification of the ligand backbone to influ-
ence the electronic structure and steric environment of metal
centers is a typical approach to improve performance of lead
structures in organometallic catalysis. Nevertheless, there are
only few studies concerning the topic of systematic ligand
modifications and their intrinsic effects on activity and overpo-
tential for earth-abundant transition metal complexes.10 In
this article, we report the influence that various modifications
of the electronic properties of the H-dpaqR ligand system have
on the catalytic current of the respective iron(+III) complexes
(see Fig. 1a).11 In a second step, we investigate the influences
2H2O ! O2 þ 4Hþ þ 4eꢀ ð1Þ
While ruthenium and iridium catalysts have so far shown
the highest activity,4 first-row transition metals are increas-
ingly becoming the focus of current research due to their
greater abundances and lower costs.5 Formation of the
oxygen–oxygen bond is generally assumed to occur via either
aInstitut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Worringerweg 2, 52074 Aachen, Germany. E-mail: walter.leitner@cec.mpg.de
bMax-Planck-Institute for Chemical Energy Conversion, Stiftstraße 34-36,
45470 Mülheim an der Ruhr, Germany. E-mail: christophe.werle@cec.mpg.de
†Electronic supplementary information (ESI) available: Synthesis and character-
ization of all used ligands and complexes as well as supporting figures (PDF).
See DOI: 10.1039/c9dt01343a
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Fig. 1 Relevant iron (left) and manganese (right) dpaq complexes.
of the ligand modifications on the analogous manganese(+II) tron withdrawing nitro group,11a to evaluate the effect of
complexes (see Fig. 1b).12
increased electrophilicity at the iron center. As shown by cyclic
voltammetry (CV) in Fig. 3, complex 1e leads to a catalytic
current icat almost twice as high as that of 1a. Further CV
investigations support the assumption that the ligand modifi-
cation did not affect the principle reactivity of the iron core and
that the observed catalytic current reflects water oxidation. The
Results and discussion
As starting point for the ligand variation, we chose complex 1a
as it has been proven to be an active water oxidation catalyst
with a TON of 29.7 We reproduced and confirmed the presence
of a catalytic current for 1a at 1.58 V vs. NHE in a mixture of
propylene carbonate and water (3.2 vol%, Fig. 3). Using this
benchmark, we studied complex (1e) in which the proton at
the 5-position of the quinoline ring was replaced by an elec-
2
square of the catalytic current icat of 1e increased linearly with
the water concentration (Fig. 2) and a linear dependence of icat
on the concentration of 1e was confirmed (Fig. S18†). These
data are fully consistent with a WNA mechanism as proposed
by the group of Meyer for the unsubstituted catalyst 1a.7 In
Fig. 3 CV of [Fe(dpaq5-R)(H2O)](ClO4)2 complexes (1.0 mM) in PC (3.2
vol% H2O (1.85 M), 200 mM LiClO4) at a scan rate of 75 mV s−1 with R =
H (1a, red), OMe (1b, orange), NO2 (1e, blue) or CF3 (1f, green) and in the
Fig. 2 CV of [Fe(dpaq5-NO )(H2O)](ClO4)2 complex (1e, 1.0 mM) in PC
2
(3.2 vol%, 200 mM LiClO4) with an increasing water concentration
(0.2 M–1.8 M) at a scan rate of 75 mV s−1. Inset: Plot of icat vs. the
2
absence of a complex (grey). Inset: Plot of Ei vs. the Hammett substi-
cat
concentration of water.
tution constants σp of R.13
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addition, the presence of the reduction wave FeIII(OH2)2+/ the onset potential of 70 mV is observed for the pyrene-modi-
FeII(OH2)+ of 1e in the backward scan underlines that the for- fied complex 1d compared to 1a as well as a higher catalytic
mation of the catalytically active species is reversible (Fig. S18†). peak current (see Fig. 4). The catalytic current is linearly depen-
Although the catalytic current could be increased by the dent on the concentration of 1d (see Fig. S20†). Even after
ligand modification with an electron withdrawing NO2 group, running 50 cycles the linear relation remains valid, supporting
the oxidation of the FeIII(OH2)2+ to the FeV(O)2+ species the conclusion that a species in solution rather than adsorbed
becomes less favorable due to the lower electron density at the electrode is responsible for the activity (see Fig. S21,†
located at the iron center. This results in a potential of Ei
=
left). Furthermore, the catalytic current of 1d is slowly decreas-
cat
1.89 V vs. NHE, corresponding to a high overpotential of ing with an increasing number of scans (see Fig. S21,† right),
1.07 V and an increase of more than 300 mV compared to 1a. confirming that the higher current is not caused by a partial
The influence of the NO2 group can also be observed for the attachment of the pyrene moiety onto the electrode over time
reduction wave FeIII(OH2)2+/FeII(OH2)+ in the backward scan. that would result in a concomitant better conductivity.
Compared to 1a the respective potential for 1e is shifted by
To expand the investigations of the influence of the ligand
130 mV to 0.43 V vs. NHE. To further probe whether the modifications on the catalytic current to other metals with
observed effects are based on the electron withdrawing prop- potential for WOCs, we turned our attention from iron to the
erty of the NO2 group, we synthesized a new H-dpaqCF ligand respective manganese analogues. In a first step, we focused on
3
(5f, see ESI† for a detailed synthetic procedure) and obtained the unsubstituted [Mn(dpaqH)](ClO4) complex (2a).12 When
the corresponding [Fe(dpaqCF )(H2O)](ClO4)2 complex (1f). A dissolving the pale yellowish complex in acetonitrile and stir-
3
catalytic current at Ei = 1.82 V vs. NHE further confirms the ring the solution under air, the color changes to dark brown
cat
electronic effect of the para-substituent (see Fig. 3). within a few minutes suggesting an oxidation to a MnIII
Conversely, the overpotential should decrease with an increas- species similar to what was recently reported for the [Mn
ing electron density at the metal. Hence, in the next step, we (dpaqH)](OTf) complex by the group of Jackson.15 By detailed
used the H-dpaqOMe ligand (5b) with an electron donating NMR investigations they showed that an equilibrium between
methoxy group at the 5-position of the quinoline moiety and a μ-O bridged dimer and two mononuclear [MnIII(OH)(dpaq)]+
investigated the electrochemical properties of the corres- species is formed. In the presence of water, the equilibrium is
ponding [Fe(dpaqOMe)(H2O)](ClO4)2 complex (1b). As expected, shifted to the mononuclear species.15
the oxidation of FeIII(OH2)2+ to FeV(O)2+ is shifted to a more
negative potential compared to 1a, but in contrast almost no three quasi-reversible oxidation waves at E1/2 = 0.62 V, 1.33 V
catalytic current can be observed (see Fig. 3).
and 1.58 V vs. NHE at a scan rate of 300 mV s−1 (see Fig. S22†).
The cyclic voltammogram of 2a dissolved in MeCN shows
When comparing the different values for Ei in more At slower scan rates, the third oxidation wave becomes irrevers-
cat
detail, the potential and the associated Hammett substitution ible, showing that the oxidized species is stable only for a
constants (σpara) exhibit a strong correlation, suggesting that short time. Upon addition of water to the solution, a large
the change in onset potential is due to the electronic modifi- increase in current can be observed following the third oxi-
cation by the different ligand substituents (see inset of Fig. 3). dation wave, suggesting water oxidation taking place (see
This inductive electronic substitution effect can only be Fig. S23,† left). The square of the peak current at E = 1.58 V vs.
observed at the 5-position of the quinoline moiety which is
para to the anionically charged nitrogen of the amide function-
ality in the dpaq-ligand system. If the methoxy group is incor-
porated at the neighboring 6-position of the H-dpaq6-OMe
-
ligand (5c) in [Fe(dpaq6–OMe)(H2O)](ClO4)2 (1c), almost no
difference in the onset potential and only a small decrease of
the catalytic current is observed compared to the unsubsti-
tuted complex 1a (see Fig. S19†).
Since higher catalytic currents achieved by electron with-
drawing substituents at the quinoline moiety also led to
higher onset potentials, hence, the overall electrochemical pro-
perties could not be improved substantially relative to the
parent ligand toward the aim of water oxidation catalysis.
Therefore, as a next step, we modified the dipicolylamine unit
of the ligand. We successfully synthesized a new H-dpaq-based
ligand with an additional pyrene moiety covalently bound to
one of the pyridine rings (5d, see ESI† for a detailed synthetic
procedure) and obtained the corresponding [Fe(dpaqPyr)(H2O)]
(ClO4)2 complex (1d). The pyrene moiety increases the elec-
tronic π-conjugation of the ligand and hence may have a posi-
Fig. 4 CV of 1a (red) and 1d (black) (both 1.0 mM) in PC (3.2 vol% H2O
tive effect on the overpotential.14 Indeed, a cathodic shift of (1.85 M), 200 mM LiClO4) at a scan rate of 75 mV s−1
.
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NHE increases linearly with [H2O] (see Fig. S23,† right). buffer solution.17 The oxidation wave at 1.04 V vs. NHE shifts to
Furthermore, the current increases linearly with [Mn] (see a more negative potential with increasing pH (see Fig. 6). If the
Fig. S24†) which is in accordance with a reaction with first- potential is plotted vs. the pH value in a Pourbaix diagram, a
order kinetics.16
slope of −67 mV pH−1 is achieved, which agrees with a proton-
At low water concentrations the oxidation wave at E1/2 = 1.33 V coupled electron transfer (PCET) reaction (see Fig. S26†). We
vs. NHE remains reversible. However, at higher water concen- thus attribute the wave to the oxidation of [MnIII–OH]+ to a
trations the reversibility decreases, as shown by an increasing [MnIVvO]+ species. In contrast, the potential of the current at
ΔEp. The oxidation wave at 0.62 V vs. NHE, which is visible in 1.56 V vs. NHE is independent of the pH value. However, the
pure acetonitrile (see Fig. S22†), disappears almost completely current significantly increases with the pH value (see Fig. 6).
upon addition of water. In acetonitrile this oxidation wave pre-
Thereafter, we probed the effects of modifying the elec-
sumably corresponds to the MnII/III couple. Like mentioned tronic properties of the dpaq-ligand system on the behavior of
above, the dissolved MnII species can be oxidized under air to a the [Mn(dpaqR)](ClO4) complexes in analogy to the iron com-
μ-O bridged MnIII-dimer, which then forms a [MnIII(OH)(dpaq)]+ plexes (see Fig. 7).
species in the presence of water. If this species was not reducible
to a MnII species, it would explain why the MnII/III oxidation wave the dpaq-ligand showed the same trend in the electrochemical
disappears after addition of water. response of the Mn-complexes as in the Fe-complexes. The
Interestingly, the modifications at the quinoline moiety of
A large current increase can be observed not only in aceto- electron withdrawing NO2 group of 2e elevates the potential of
nitrile but also in aqueous buffer systems at a potential of the current increase by about 210 mV compared to 2a. In con-
1.56 V vs. NHE when using the [Mn(dpaqH)](ClO4) complex trast, the electron donating OMe group at the 5-position of 2b
(2a) pre-dissolved in MeCN. Scan rate dependent CVs show reduces the oxidation potential of the second oxidation wave
that the normalized current (i/ν0.5) at 1.56 V vs. NHE decreases by approximatively 270 mV. However, all complexes show an
with increasing scan rate, consistent with a rate-determining irreversible degradation under the test conditions in aqueous
step (see Fig. S25†). Just as in acetonitrile, the current at 1.56 V solution, irrespective of the ligand modification.
vs. NHE in a borate buffer linearly increases with the complex
concentration, indicating a reaction with first-order kinetics [Mn(dpaqPyr)]ClO4 complex (2d). With 2d the potential of the
with respect to the complex (see Fig. 5). peak current is reduced by 25 mV in a PC solution with water
However, in contrast to the [Fe(dpaqR)(H2O)](ClO4)2 com- (3.2 vol%) compared to 2a, while the current itself is slightly
plexes (1a–f) in propylene carbonate and the [Mn(dpaqH)](ClO4) increased (see Fig. S27†). Hence,
higher electronic
We then used the H-dpaqPyr ligand (5d) to synthesize the
a
complex (2a) in acetonitrile, 2a in borate buffer does not show a π-delocalization of the pyrene moiety shows the same effect on
reversible reduction wave in the reverse scan. Therefore, it must the electrochemical behavior as for the analogous iron
be assumed that the structure of the complex changes irreversi- complex 1d, albeit the influence is significantly lower.
bly under the given reaction conditions.
In summary, we studied the electrocatalytic properties of a
In a next step, the pH-dependence of the oxidation waves of series of iron and manganese complexes based on the
complex 2a was investigated by CV measurements in a borate H-dpaqR ligand system in the context of electrochemical water
oxidation. Modification of electronic properties by variation of
Fig. 5 CV of 2a with increasing concentration from 0.1 mM up to
1.0 mM in borate buffer (0.2 M, pH 8) at a scan rate of 50 mV s−1. Inset:
Plot of the current at 1.56 V vs. the concentration.
Fig. 6 CV of 2a (1.0 mM) with increasing pH from 7.0 up to 9.4 in
borate buffer (0.2 M) at a scan rate of 50 mV s−1
.
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Sigma-Aldrich, Alfa-Aesar, abcr, Acros Organics, TCI chemicals
and used without further purification. The H-dpaqR ligands
(5a–f) were synthesized based on modified literature pro-
cedures (for further details see ESI†). The different solvents
were purified using solvent purification-systems and were
stored and handled appropriately under argon. NMR-spectra
were recorded on Bruker AV400, AS400 or AS600 at the indi-
cated temperature. Chemical shifts (δ) are given in ppm and
were referenced against the solvent signal (acetonitrile-d3: δH
1.94 ppm and δC = 118.3 ppm; CDCl3: δH = 7.26 ppm and δC
77.1 ppm; CD2Cl2: δH = 5.32 ppm and δC = 53.8; CD3OD: δH
=
=
=
3.31 ppm and δC = 49.0).19 The coupling constants (J) are given
in Hz. Infrared (IR) spectra were recorded using a Bruker
Alpha–P FT-IR spectrometer. ESI-HRMS spectra were recorded
on a ThermoFisher Scientific LTQ-Orbitrap XL spectrometer.
Elemental analyses (C, H, N) were recorded at the
Mikroanalytisches Laboratorium Kolbe in Oberhausen,
Germany. Cyclic voltammetry (CV) was performed with a
Metrohm Autolab PGSTAT101 potentiostat and processed
using Metrohm Autolab Nova software 2.1. A three-electrode
setup constituted of a glassy-carbon working electrode (3 mm
diameter), a platinum-wire counter electrode and an Ag/AgCl
reference electrode (3 M NaCl) was used. All voltammograms
were referenced against the normal hydrogen electrode (NHE)
by adding 0.208 V to the measured potential against the Ag/
AgCl reference electrode. If the catalytic current was not pro-
nounced as a clear peak but only as a shoulder, the current
was measured at the inflection point of the curve. The glassy-
carbon working electrode was conditioned prior to each experi-
ment using a 0.05 μm alumina paste on a polishing pad and
washed with ultra-pure water.
Fig. 7 CV of [Mn(dpaqR)](ClO4) complexes (1.0 mM) in phosphate
buffer (0.1 M, pH 7) at a scan rate of 50 mV s−1 with R = H (2a, red),
5-OMe (2b, orange), 6-OMe (2c, green) or NO2 (2e, blue) and in the
absence of a complex (grey).
substituents in the ligand framework of the iron complexes
was found to exhibit a big influence on the overpotential as
well as the catalytic current. Electron withdrawing groups at
the 5-position of the quinoline moiety, such as NO2 and CF3
increase the catalytic current due to the higher electrophilicity
at the active species, albeit on the expense to increase the over-
potential simultaneously. In contrast, electron donating
groups, such as OMe, increase the electron density at the
metal center to a point where nucleophilic attack by water can
no longer take place. However, by incorporating a pyrene
moiety into one of the pyridine groups, the overpotential for
the [Fe(dpaqPyr)(H2O)](ClO4)2 complex (1d) was reduced and
the catalytic current was increased at the same time. This may
be at least partly due to the greater electronic π-conjugation of
pyrene. Although, the investigated manganese complexes
showed no reversible catalytic water oxidation behavior, the
ligand modifications affected the electrochemical properties in
the same way as it was the case with the iron complexes. While
the stability of the ligand framework under oxidative con-
ditions poses severe limitations for the systems discussed
herein, these results substantiate the possibility to control and
improve the catalytic performance of first-row metal-based
WOCs by systematic variation of their coordinative environ-
ment. This may eventually provide design criteria that can be
translated into catalytic materials.18
General procedure for the synthesis of [Fe(dpaqR)(H2O)](ClO4)2
complexes (1a–f)
Caution! Perchlorate salts of metal complexes are potentially
explosive. The complexes should be prepared only in small
quantities, and should be handled with great care. The synth-
eses were performed according to a modified reported proce-
dure.11b A solution of Fe(ClO4)3·6H2O in methanol was added
to a solution of the H-dpaqR ligand and triethylamine in
methanol or dichloromethane. The mixture was allowed to
stand overnight at room temperature before the precipitate
was recovered by filtration over fritted glass. The residue was
washed with diethyl ether (10 mL) and dried under high
vacuum.
1a. Fe(ClO4)3·6H2O (110 mg, 0.24 mmol) in methanol
(1 mL), H-dpaqH ligand (5a, 99.7 mg, 0.26 mmol) and triethyl-
amine (0.036 mL, 0.13 mmol) in methanol (1 mL). Yield =
58.1 mg, 0.09 mmol, 34%. Dark green solid. Anal. calcd for
[Fe(dpaqH)(H2O)](ClO4)2(H2O)(MeOH)0.5: C: 40.95; H, 3.80; N,
10.16; found: C: 40.84; H, 3.48; N, 9.82. HRMS-ESI (m/z): calcd
for C23H20FeN5O+: 438.10173; found: 438.09970. FT-IR:
1616 cm−1 (amide CvO).
Experimental section
General considerations
All syntheses involving air sensitive compounds were carried
1b. Fe(ClO4)3·6H2O (110 mg, 0.24 mmol) in methanol
out under argon using flame-dried glassware, ensuring rigor- (1 mL), H-dpaq5-OMe ligand (5b, 107.5 mg, 0.26 mmol) and tri-
ously inert conditions. The chemicals were purchased from ethylamine (0.036 mL, 0.13 mmol) in methanol (1 mL). Yield =
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58.2 mg, 0.09 mmol, 32%. Dark green solid. Anal. calcd for C23H20MnN5O+: 437.10483; found: 437.10275. FT-IR:
[Fe(dpaq5–OMe)(H2O)](ClO4)2(H2O)0.4(MeOH)0.3 C: 41.57; H, 1542 cm−1 (amide CvO).
3.73; N, 9.98; found: C: 41.61; H, 3.76; N, 10.04. HRMS-ESI 2b. Mn(ClO4)2·6H2O (110 mg, 0.31 mmol) in acetonitrile
(m/z): calcd for C24H22FeN5O2+: 468.11174; found: 468.11139. (0.5 mL), H–dpaq5–OMe ligand (5b, 107.5 mg, 0.26 mmol) and
FT-IR: 1609 cm−1 (amide CvO).
triethylamine (0.050 mL, 0.36 mmol) in acetonitrile (2 mL).
:
1c. Fe(ClO4)3·6H2O (110 mg, 0.24 mmol) in methanol The reaction mixture had to be stored at −20 °C for several
(1 mL), H-dpaq6-OMe ligand (5c, 107.5 mg, 0.26 mmol) and tri- days before the precipitate could be recovered. Yield = 85.5 mg,
ethylamine (0.036 mL, 0.13 mmol) in methanol (1 mL). Yield = 0.15 mmol, 58%. Pale beige-yellowish solid. HRMS-ESI (m/z):
+
46.9 mg, 0.06 mmol, 25%. Dark green solid. Anal. calcd for calcd for C24H22MnN5O2 : 467.11485; found: 467.11456. FT-IR:
[Fe(dpaq6–OMe)(H2O)](ClO4)2(MeOH)0.7: C: 41.92; H, 3.82; N, 1549 cm−1 (amide CvO).
9.90; found: C: 41.91; H, 3.87; N, 9.95. HRMS-ESI (m/z): calcd
2c. Mn(ClO4)2·6H2O (110 mg, 0.31 mmol) in acetonitrile
for C24H22FeN5O2 : 468.11174; found: 468.11209. FT-IR: (0.5 mL), H–dpaq6–OMe ligand (5c, 107.5 mg, 0.26 mmol) and
1611 cm−1 (amide CvO).
triethylamine (0.050 mL, 0.36 mmol) in acetonitrile (2 mL).
+
1d. Fe(ClO4)3·6H2O (55 mg, 0.12 mmol) in methanol Yield = 82.5 mg, 0.15 mmol, 56%. Pale beige-yellowish solid.
(0.3 mL), H-dpaqPyr (5d, 76.0 mg, 0.13 mmol) and triethyl- Anal. calcd for [Mn(dpaq6–OMe)](ClO4)(H2O)3.4: C: 45.89; H,
amine (0.018 mL, 0.13 mmol) in dichloromethane (0.5 mL). 4.62; N,11.15; found: C: 45.52; H, 4.12; N,11.05. HRMS-ESI
+
The mixture was allowed to stand at −20 °C overnight. Yield = (m/z): calcd for C24H22MnN5O2 : 467.11485; found: 467.11429.
47.2 mg, 0.06 mmol, (46%). Dark green solid. Anal. calcd for FT-IR: 1542 cm−1 (amide CvO).
[Fe(dpaqPyr)(H2O)](ClO4)2(H2O)2: C: 52.55; H, 3.84; N, 7.86;
2d. Mn(ClO4)2·6H2O (110 mg, 0.31 mmol) in acetonitrile
found: C: 52.54; H, 3.68; N, 7.70. HRMS-ESI (m/z): [M + OMe]+ (0.5 mL), H–dpaqPyr (5d, 76.0 mg, 0.13 mmol) and triethyl-
calcd for C39H28FeN5O2+: 669.18327; found: 669.18250. FT-IR: amine (0.025 mL, 0.18 mmol) in a mixture of dichloromethane
1606 cm−1 (amide CvO).
(0.5 mL) and acetonitrile (2 mL). Yield = 62.7 mg, 0.09 mmol,
1e. Fe(ClO4)3·6H2O (110 mg, 0.24 mmol) in methanol 65%. Dark brownish solid. Anal. calcd for [Mn(dpaqPyr)](ClO4)
(1 mL), H-dpaqNO ligand (5e, 111.4 mg, 0.26 mmol) and tri- (H2O)2.8: C: 59.48; H, 4.30; N, 8.89; found: C: 59.43; H, 4.17; N,
2
ethylamine (0.036 mL, 0.13 mmol) in methanol (1 mL). Yield = 8.87. HRMS-ESI (m/z): calcd for C39H28MnN5O+: 637.16744;
130.1 mg, 0.19 mmol, 72%. Dark green-brownish solid. Anal. found: 637.16681. FT-IR: 1539 cm−1 (amide CvO).
calcd for [Fe(dpaqNO )(H2O)](ClO4)2: C: 39.45; H, 3.02; N, 12.00;
2e. Mn(ClO4)2·6H2O (110 mg, 0.31 mmol) in acetonitrile
2
found: C: 40.26; H, 3.99; N, 12.87. HRMS-ESI (m/z): calcd for (0.5 mL), H-dpaqNO ligand (5e, 111.4 mg, 0.26 mmol) and tri-
2
+
C23H19FeN6O3 : 483.08681; found: 483.08484. FT-IR: ethylamine (0.050 mL, 0.36 mmol) in acetonitrile (2 mL).
1611 cm−1 (amide CvO).
Yield = 105.9 mg, 0.18 mmol, 70%. Orange solid. Anal. calcd
1f. Fe(ClO4)3·6H2O (110 mg, 0.24 mmol) in methanol for [Mn(dpaqNO )](ClO4)(H2O)3.6: C: 42.72; H, 4.08; N, 13.00;
2
(1 mL), H-dpaqCF ligand (5f, 117.4 mg, 0.26 mmol) and tri- found: C: 42.15; H, 3.41; N, 12.87. HRMS-ESI (m/z): calcd for
3
+
ethylamine (0.036 mL, 0.13 mmol) in methanol (1 mL). Yield = C23H20MnN6O3 : 482.08991; found: 482.08755. FT-IR:
46.9 mg, 0.065 mmol, 39%. Dark green-purple solid. Anal. 1531 cm−1 (amide CvO).
calcd for [Fe(dpaqCF )(H2O)](ClO4)2: C: 39.86; H, 2.93; N, 9.68;
3
found: C: 40.04; H, 3.11; N, 9.75. HRMS-ESI (m/z): calcd for
C24H19F3FeN5O+: 506.08911; found: 506.08868. FT-IR: Conflicts of interest
1609 cm−1 (amide CvO).
The authors declare no competing financial interest.
General procedure for the synthesis of [Mn(dpaqR)](ClO4) com-
plexes (2a–e)
Acknowledgements
The syntheses were performed according to a modified
reported procedure.12 A solution of Mn(ClO4)2·6H2O in aceto-
nitrile was added to a solution of the H-dpaqR ligand and tri-
ethylamine in dichloromethane and/or acetonitrile. The reac-
tion mixture was stirred at room temperature for 2 h before the
precipitate was recovered by filtration over fritted glass. The
residue was washed with acetonitrile (5 mL) and diethyl ether
(5 mL) and dried under high vacuum.
We gratefully acknowledge generous financial supports by the
MPG and the RWTH Aachen University. This work was sup-
ported by the Bundesministerium für Bildung und Forschung
as part of the project “MANGAN” (03SF0508). The authors
acknowledge Yannick Meiners and Maurice W. Vennewald for
their experimental support and Dr Markus Hölscher for fruit-
ful discussion. Open Access funding provided by the Max
Planck Society.
2a. Mn(ClO4)2·6H2O (110 mg, 0.31 mmol) in acetonitrile
(0.5 mL), H-dpaqH ligand (5a, 99.7 mg, 0.26 mmol) and tri-
ethylamine (0.050 mL, 0.36 mmol) in acetonitrile (2 mL).
Yield = 89.3 mg, 0.17 mmol, 64%. Pale beige solid. Anal. calcd
for [Mn(dpaqH)](ClO4)(H2O)1.3: C: 49.31; H, 4.07; N, 12.50;
found: C: 49.46; H, 4.27; N, 12.50. HRMS-ESI (m/z): calcd for
References
1 (a) R. Schlögl, Angew. Chem., Int. Ed., 2015, 54, 4436–4439;
(b) R. Schlögl, Top. Catal., 2016, 59, 772–786.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
2 (a) H. Dau, E. Fujita and L. Sun, ChemSusChem, 2017, 10,
4228–4235; (b) D. G. Nocera, Acc. Chem. Res., 2017, 50, 616–
619.
8 P. M. Wood, Biochem. J., 1988, 253, 287–289.
9 We note that secondary ligand effects resulting from the
formation of active catalytic clusters might equally apply.
3 (a) X. Du, Q. Shao and X. Zhang, Dalton Trans., 2019, 48, 10 (a) P. Xu, S. Hu, H. D. Zhang and X. Zheng, Phys. Chem.
1322–1331; (b) X. Q. Du, X. S. Zhang, Y. Li and M. Zhao,
Int. J. Hydrogen Energy, 2018, 43, 19955–19964; (c) X. Q. Du,
Z. Yang, Y. Li, Y. Q. Gong and M. Zhao, J. Mater. Chem. A,
2018, 6, 6938–6946; (d) J. Li, R. Güttinger, R. Moré, F. Song,
W. Wan and G. R. Patzke, Chem. Soc. Rev., 2017, 46, 6124–
6147; (e) T. J. Meyer, M. V. Sheridan and B. D. Sherman,
Chem. Soc. Rev., 2017, 46, 6148–6169; (f) J. D. Blakemore,
R. H. Crabtree and G. W. Brudvig, Chem. Rev., 2015, 115,
12974–13005; (g) M. D. Kärkäs, O. Verho, E. V. Johnston
Chem. Phys., 2018, 20, 14919–14926; (b) J. Shen, M. Wang,
J. Gao, H. Han, H. Liu and L. Sun, ChemSusChem, 2017, 10,
4581–4588; (c) T. Chantarojsiri, Y. Sun, J. R. Long and
C. J. Chang, Inorg. Chem., 2015, 54, 5879–5887;
(d) Z. Codola, I. Garcia-Bosch, F. Acuna-Pares, I. Prat,
J. M. Luis, M. Costas and J. Lloret-Fillol, Chem. – Eur. J.,
2013, 19, 8042–8047; (e) W. A. Hoffert, M. T. Mock,
A. M. Appel and J. Y. Yang, Eur. J. Inorg. Chem., 2013, 2013,
3846–3857.
and B. Åkermark, Chem. Rev., 2014, 114, 11863–12001; 11 (a) Y. Hitomi, K. Arakawa and M. Kodera, Chem. – Eur. J.,
(h) D. J. Wasylenko, R. D. Palmer and C. P. Berlinguette,
Chem. Commun., 2013, 49, 218–227.
4 (a) J. Creus, R. Matheu, I. Penafiel, D. Moonshiram,
2013, 19, 14697–14701; (b) Y. Hitomi, K. Arakawa,
T. Funabiki and M. Kodera, Angew. Chem., Int. Ed., 2012,
51, 3448–3452.
P. Blondeau, J. Benet-Buchholz, J. Garcia-Anton, X. Sala, 12 Y. Hitomi, Y. Iwamoto and M. Kodera, Dalton Trans., 2014,
C. Godard and A. Llobet, Angew. Chem., Int. Ed., 2016, 55, 43, 2161–2167.
15382–15386; (b) J. M. Thomsen, D. L. Huang, 13 D. H. Mcdaniel and H. C. Brown, J. Org. Chem., 1958, 23,
R. H. Crabtree and G. W. Brudvig, Dalton Trans., 2015, 44, 420–427.
12452–12472; (c) S. W. Sheehan, J. M. Thomsen, 14 P. Garrido-Barros, C. Gimbert-Surinach, D. Moonshiram,
U. Hintermair, R. H. Crabtree, G. W. Brudvig and
C. A. Schmuttenmaer, Nat. Commun., 2015, 6, 6469.
A. Picon, P. Monge, V. S. Batista and A. Llobet, J. Am. Chem.
Soc., 2017, 139, 12907–12910.
5 (a) N. Wang, H. Q. Zheng, W. Zhang and R. Cao, 15 (a) D. B. Rice, S. D. Jones, J. T. Douglas and T. A. Jackson,
Chin. J. Catal., 2018, 39, 228–244; (b) M. D. Karkas and
B. Akermark, Dalton Trans., 2016, 45, 14421–14461;
(c) M. Kondo and S. Masaoka, Chem. Lett., 2016, 45, 1220–
Inorg. Chem., 2018, 57, 7825–7837; (b) G. B. Wijeratne,
B. Corzine, V. W. Day and T. A. Jackson, Inorg. Chem., 2014,
53, 7622–7634.
1231; (d) M. A. Asraf, H. A. Younus, M. Yusubov and 16 (a) E. S. Rountree, B. D. McCarthy, T. T. Eisenhart and
F. Verpoort, Catal. Sci. Technol., 2015, 5, 4901–4925;
(e) A. R. Parent and K. Sakai, ChemSusChem, 2014, 7, 2070–
2080; (f) W. Schöfberger, F. Faschinger, S. Chattopadhyay,
S. Bhakta, B. Mondal, J. A. Elemans, S. Müllegger, S. Tebi,
J. L. Dempsey, Inorg. Chem., 2014, 53, 9983–10002;
(b) C. P. Andrieux, C. Blocman, J. M. Dumas-Bouchiat,
F. M’Halla and J. M. Savéant, J. Electroanal. Chem., 1980,
113, 19–40.
R. Koch, F. Klappenberger, M. Paszkiewicz, J. V. Barth, 17 Starting at pH 7, the pH value was adjusted by adding
E. Rauls, H. Aldahhak, W. G. Schmidt and A. Dey, Angew.
Chem., Int. Ed., 2016, 55, 2350–2305; (g) W. T. Lee,
small amounts of NaOH (0.2 M) to keep the overall ion con-
centration in solution at a constant level.
S. B. Muñoz 3rd, D. A. Dickie and J. M. Smith, Angew. 18 J. Q. Guan, Z. Y. Duan, F. X. Zhang, S. D. Kelly, R. Si,
Chem., Int. Ed., 2014, 53, 9856–9859.
6 I. Funes-Ardoiz, P. Garrido-Barros, A. Llobet and
F. Maseras, ACS Catal., 2017, 7, 1712–1719.
7 M. K. Coggins, M. T. Zhang, A. K. Vannucci, C. J. Dares and
T. J. Meyer, J. Am. Chem. Soc., 2014, 136, 5531–5534.
M. Dupuis, Q. G. Huang, J. Q. Chen, C. H. Tang and C. Li,
Nat. Catal., 2018, 1, 870–877.
19 G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb,
A. Nudelman, B. M. Stoltz, J. E. Bercaw and K. I. Goldberg,
Organometallics, 2010, 29, 2176–2179.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.