Water oxidation catalyzed by molecular di- and nonanuclear Fe complexes: Importance of a proper ligand framework

Article abstract of DOI:10.1039/c6dt01554a

The synthesis of two molecular iron complexes, a dinuclear iron(iii,iii) complex and a nonanuclear iron complex, based on the dinucleating ligand 2,2′-(2-hydroxy-5-methyl-1,3-phenylene)bis(1H-benzo[d]imidazole-4-carboxylic acid) is described. The two iron complexes were found to drive the oxidation of water by the one-electron oxidant [Ru(bpy)₃]³⁺.

Full text of DOI:10.1039/c6dt01554a

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A. Karlsson, T. Åkermark, A. Shatskiy, S. Demeshko, R. Liao, T. M. Laine, M. Haukka, E. Zeglio, A. Abdel
Magied, P. Siegbahn, F. Meyer, M. Kärkäs, E. Johnston, E. Nordlander and B. Åkermark, Dalton Trans.,
2016, DOI: 10.1039/C6DT01554A.
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DOI: 10.1039/C6DT01554A
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Water oxidation catalyzed by molecular di‐ and nonanuclear Fe
complexes: Importance of a proper ligand framework
Received 00th January 20xx,
Accepted 00th January 20xx
a†
b†
b†
b
b
Biswanath Das, Bao‐Lin Lee, Erik A. Karlsson, Torbjörn Åkermark, Andrey Shatskiy, Serhiy
c
d
b
e
f
Demeshko, Rong‐Zhen Liao, Tanja M. Laine, Matti Haukka, Erica Zeglio, Ahmed F. Abdel‐
DOI: 10.1039/x0xx00000x
b
b
c
b
b
Magied, Per E. M. Siegbahn, Franc Meyer, Markus D. Kärkäs,* Eric V. Johnston,* Ebbe
Nordlander, and Björn Åkermark*
a
b
www.rsc.org/
The synthesis of two molecular iron complexes, a dinuclear straightforward mechanistic studies compared to their
iron(III,III) complex and a nonanuclear iron complex, based on the heterogeneous counterpart, the insight gained from such
dinucleating
phenylene)bis(1H‐benzo[d]imidazole‐4‐carboxylic
described. The two iron complexes were found to drive the surfaces in water splitting devices.
ligand
2,2'‐(2‐hydroxy‐5‐methyl‐1,3‐ studies will certainly be of value for creating a better
acid) is understanding of the high‐valent metal species generated at
3
3
+
oxidation of water by the one‐electron oxidant [Ru(bpy) ] .
The efforts to develop synthetic WOCs have focused on
3
4
catalysts based on noble metals, such as ruthenium, due to its
The development of technologies to provide renewable and
clean energy is one of the most important challenges of the
robustness. However, the scarcity of such metals might limit
their use in large‐scale applications and the development of
WOCs comprised of abundant, inexpensive first‐row transition
2
1st century. In fact, access to sustainable and inexpensive
5,6,7,8,9,10,11,12
energy will probably become a major global problem within
the next few decades. Solar energy is perhaps the most
promising energy source that has the potential to satisfy the
demand of present and future generations. Creating an
artificial photosynthetic device would permit the conversion of
solar light and water into oxygen and a storable fuel, e.g.
hydrogen gas, an important complement to the generation of
metals is therefore of particular importance.
The dinuclear manganese(II,III) complex
13,14
1
(Fig. 1),
and
15
related manganese complexes, together with a similar
16
dinuclear ruthenium complex have recently been studied as
potential WOCs. However, none of these complexes catalyze
IV
water oxidation using Ce as the chemical oxidant. It is
believed that the reason is two‐fold—hydrolysis of the
complexes under acidic conditions and oxidation of the labile
benzylic amine. It was therefore surprising that a series of iron
complexes, containing ligands with benzylic amine functions,
1
electricity.
The assembly of such a device requires the combination of
several complex processes: light harvesting, charge separation,
electron transfer, water oxidation and reduction of protons to
molecular hydrogen. Currently, water oxidation is the limiting
factor and the pursuit of stable and efficient water oxidation
catalysts (WOCs) remains a challenge despite the considerable
IV
were quite efficient water oxidation catalysts, using Ce as
7
oxidant, without affecting the ligand system.
Recently, our group designed the bio‐inspired ligand 2,2'‐(2‐
hydroxy‐5‐methyl‐1,3‐phenylene)bis(1H‐benzo[d]imidazole‐4‐
2
progress during the last years. Although molecular WOCs
carboxylic acid) (H L) bearing imidazole groups in place of the
5
offer flexibility in catalyst design and are amenable to
benzylic amines, and negatively charged carboxylate
functionalities, and reported on the synthesis of the
a.
corresponding manganese(II,III) complex 2 (Fig. 1). Complex 2
Inorganic Chemistry Research Group, Chemical Physics, Center for Chemistry and
Chemical Engineering, Lund University, PO Box 124, SE‐22100 Lund, Sweden.
Department of Organc Chemistry, Arrhenius Laboratory, Stockholm University, SE‐
was able to catalyze the oxidation of water to molecular
oxygen, thus circumventing the problematic issue with
oxidative decomposition. The negatively charged groups
b.
10691 Stockholm, Sweden.
1
0
c.
Institut für Anorganische Chemie, Georg‐August‐Universität Göttingen,
Tammannstrasse 4, D‐37077 Göttingen, Germany.
have been shown to dramatically reduce the redox potentials
d.
Key Laboratory for Large‐Format Battery Materials and System, Ministry of
Education, School of Chemistry and Chemical Engineering, Huazhong University of
Science and Technology Wuhan 430074, China.
Department of Chemistry, University of Jyväskylä, PO Box 35, FI‐40014 University
of Jyväskylä, Finland.
Biomolecular and Organic Electronics, Department of Physics Chemistry and
Biology, Linköping University, SE‐58183 Linköping, Sweden
These authors contributed equally to this work.
16,17
of the metal center(s),
photocatalytic system with [Ru(bpy)
which is required for use in a
2+
3
] ‐type photosensitizers
e.
f.
(
bpy = 2,2’‐bipyridine). Encouraged by these results it was
decided to synthesize and study the corresponding dinuclear
iron complex (Fig. 1).
Herein we show that the molecular iron complex is indeed
3
†
Electronic Supplementary Information (ESI) available: Experimental details,
including procedures, syntheses and characterization of new compounds. See capable of catalyzing oxidation of water, when driven by mild
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C6DT01554A
n
O
M
O
X
O O
X
O
M
O
O
O
O
O
OH
N
HO
N
O
N
O
O
M
M
N
N
O
N
N
OH
O
N
N
N
N
N
H
H
H
H
1
: M = Mn, X = N, n = 2
2: M = Mn
3: M = Fe
5: M = Fe, X = C−O, n = 1
H5L
5
L, and dinuclear iron complexes 3 and 5.
Fig. 1. Structures of dinuclear manganese complexes 1 and 2, ligand H
3+
3+
one‐electron [Ru(bpy)
crystals of complex
gave to our surprise the nonanuclear iron cluster
3
] ‐type oxidants. Attempts to grow added to the oxidant ([Ru(bpy)
from DMSO for X‐ray crystallography observed (Fig. 2a). At a concentration of 26 μM, a turnover
, which was number (TON) of 4 was reached. Subsequent kinetic
3
] ), oxygen evolution was
3
4
~
also found to be catalytically active. The bio‐inspired ligand H5L experiments demonstrated that the initial rates of oxygen
was prepared in a two‐step synthetic route (Scheme S1). The evolution exhibited a linear dependence on the catalyst
III,III
+
[
Fe2 (H L)(µ‐OMe)(OAc)] complex 3 was prepared by concentration, with an apparent first‐order rate constant of
2
–
1
addition of a methanolic solution of Fe(BF ) ∙ 6H O (2 equiv.) 0.71 min (Fig. 2b). These findings suggest that the rate‐
4
2
2
to a methanol/Et N (9:1) solution containing ligand H L (1 limiting and preceding steps of the catalytic cycle include only
3
5
equiv.) and sodium acetate (2 equiv.). Stirring at room one molecule of the catalyst. As O–O bond formation is
temperature overnight afforded the title complex as a dark typically the rate‐limiting step of catalytic water oxidation, the
solid (see Scheme S1 and Supporting Information for details).
data suggests that O–O bond formation takes place
Unfortunately, attempts to obtain crystals suitable for X‐ray intramolecularly through coupling of two iron‐oxo units or
crystallography have so far failed. However, high‐resolution through nucleophilic attack by water on a high‐valent iron‐oxo
mass spectrometry (HRMS), elemental analysis, Mössbauer center. Photochemical experiments were subsequently carried
spectroscopy and UV‐vis spectroscopy of complex
3 are all in
a)
agreement with the suggested molecular structure (see Figs.
S1–S3). The HRMS spectrum of the dinuclear iron(III,III)
complex in negative mode displayed a peak at m/z 624.9751
with a characteristic isotope pattern that was assigned to the
III,III
–
molecular ion [Fe2 (L)(OAc)(OMe)] (see Fig. S1). Mössbauer
spectroscopy was conducted on the dinuclear complex in
order to support the oxidation state and the spin state of the
iron centers in complex
3 (Fig. S3). The spectrum revealed a
single doublet with an isomer shift and quadrupole splitting of
–
1
–1
δ = 0.48 mm s and ΔE = 0.89 mm s , respectively, which are
Q
characteristic for high‐spin iron(III) species. Similar values have
b)
been found for other dinuclear and tetranuclear complexes
III III
18b,19,20
with an [Fe Fe ] core.
The electrochemistry of iron complex
3
was studied in
buffered aqueous solutions (phosphate buffer, 0.1 M, pH 7.2),
resembling the conditions employed in the catalytic
experiments (vide infra), and displayed an onset potential for
electrocatalytic water oxidation at 1.25 V vs. the normal
~
hydrogen electrode (NHE) (Fig. S4). The differential pulse
voltammogram (DPV) of complex
peaks in the region 0.3 < E < 1.3 V (Fig. S5). The first peak at
3 displayed two oxidation
III,III
0
→
.74 V vs. NHE was assigned to the formal oxidation of Fe2
III,IV
Fe
2
and the second one, occurring at 1.14 V, was
III,IV
attributed to Fe2
IV,IV
→
Fe2
. Importantly, both oxidations
Fig. 2. (a) O evolution kinetics by iron complex 3 as a function of time. (b) Initial rate of
2
appear at a low enough potential to be thermodynamically oxygen evolution as a function of the concentration of catalyst 3. The initial rates of
3
+
–1
oxygen evolution (nmol s ) were calculated from the plot of oxygen evolution as a
accessible by the [Ru(bpy)3] oxidant (E1/2 = 1.26 V vs. NHE).
The catalytic activity of iron complex in water oxidation
function of time (60–180 s). Conditions: Experiments were performed by adding an
aqueous phosphate buffer solution (0.1 M, pH 7.2, 0.5 mL) at 20 °C containing iron
complex 3 to the chemical oxidant [Ru(bpy)
3
was evaluated using the one‐electron oxidant [Ru(bpy) ](PF ) .
When a buffered aqueous solution containing complex
3
6 3
3 6 3
](PF ) (3.0 mg, 3.0 μmol). Concentration of
3
was
complex 3: 80 μM ( ), 52 μM ( ), 26 μM ( ).
2
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out in a three‐component system using [Ru(bpy)
2 6 2
(deeb)](PF )
View Article Online
DOI: 10.1039/C6DT01554A
(
deeb diethyl (2,2’‐bipyridine)‐4,4’‐dicarboxylate) as
=
III
II
photosensitizer (E1/2 Ru /Ru = 1.40 V vs. NHE) and Na S O as
2
2
8
sacrificial electron acceptor, and showed that complex
3
is
able to promote photoinduced water oxidation (Fig. S7). The
low TONs obtained in the O2 evolution experiments are
probably due to a combination of the slow kinetics, low
3+
thermodynamic driving force when [Ru(bpy)3] is used as the
oxidant (the redox potential of this oxidant, E1/2 = 1.26 V vs.
NHE, is essentially equal to the onset potential for
electrocatalytic water oxidation mediated by complex 3, E
onset
3
+
=
1.25 V vs. NHE), and the limited stability of the [Ru(bpy) ] ‐ Fig. 3. (Left) Structure of the nonanuclear iron complex 4 generated from a DMSO
~
3
solution of the dinuclear iron complex 3. (Right) Enlargement of the nonanuclear iron
type oxidants in neutral aqueous solutions, which contribute
to unproductive reaction pathways.
core. Hydrogen atoms have been omitted for clarity.
Quantum chemical calculations were also performed on the
Attempts to crystallize the complex by dissolution in warm
DMSO, followed by cooling and standing for two weeks,
generated a microcrystalline precipitate. Despite the small size
of the crystals, an X‐ray structure could be obtained. The
dinuclear iron(III,III) complex
3. For the related dinuclear
manganese(II,III) complex , it has previously been established
2
that in aqueous phosphate buffer solutions, the bridging
methoxide and acetate ligands are replaced by hydroxide and
phosphate ligands, respectively, which generates the
precipitate turned out to be the iron‐oxo cluster [Fe (H L) (O) ]
9
2
6
9
21
(
4), composed of nine iron atoms and six ligand molecules (Fig.
catalytically active species. We therefore propose that under
catalytic conditions iron complex undergoes a similar
3
). Each of the six peripheral iron centers are coordinated by
3
one nitrogen, one phenoxy group and one carboxylate moiety
from the ligand, in addition to the two carboxylates of two
neighbouring ligands and one oxo‐bridge to one of the three
central irons. Each of the central irons in turn is linked to two
peripheral and one of the central irons by oxo‐bridges in
addition to three ligand carboxylates. The catalytic effect of
transformation and acquires a bridging hydroxide and a
phosphate ligand. Quantum chemical calculations suggest that
iron complex
3 exists as three different isomers and are shown
in Fig. S9 (for a detailed discussion, see Supporting
Information).
To verify that the ligand frameworks must be resistant to
oxidation if they are to promote water oxidation, the related
iron cluster
and it was found that it was capable of evolving oxygen (Fig.
S8). When using a concentration of 4.4 M of the cluster, a
4 in water oxidation was subsequently investigated
1
8c
dinuclear iron(III,III) complex
Compared to complex , complex
previously reported iron‐based WOCs,
5
was synthesized (Fig. 1).

3
5
is more similar to
since it is based on a
22,23
7
a–c
TON of 27 was obtained.
~
An important question is whether complexes
3 and 4 act as
benzylic amine ligand scaffold. However, neither with
3
+
IV
molecular catalysts or if they are merely pre‐catalysts. Lau and
co‐workers previously reported that nonheme iron complexes
decompose at neutral pH, generating iron oxide nanoparticles,
[
(
Ru(bpy)3] (in a buffered neutral aqueous solution) nor Ce
in an acidic aqueous solution) as oxidants, could oxygen
. The most probable
evolution be observed with complex
5
24
which catalyze oxygen evolution in aqueous borate buffer.
explanation is that degradation of the ligand occurs, as we
have previously observed with related ligands. The fact that
They noted that even with a simple iron precursor, Fe(ClO4)3,
oxygen was evolved, with a TON of 150. However, when a
~
complex
5
does not catalyze water oxidation highlights that
is not
phosphate buffer was used instead, no oxygen evolution was
observed. Similar results were also obtained by Fukuzumi and
the observed activity for the dinuclear iron complex
3
general and that special ligand frameworks are required to
promote oxidation of water.
Further, control experiments using simple iron(II) salts, such
7c
co‐workers. This dramatic difference was attributed to the
–
29
formation of the insoluble FePO4 (Ksp = 9.92∙10 ) in
phosphate buffer solutions. Since iron complexes and were
3
4
as iron(II) chloride, in the absence of ligand H L gave no
5
established to be active in phosphate buffer, both complexes
most likely function as molecular catalysts. This is further
detectable amounts of oxygen. Omitting complex
decomposition of the oxidant, [Ru(bpy) ](PF ) , without any
3 resulted in
3
6 3
supported by the fact that the activity of complex
3 is about
detectable oxygen production. Finally, to demonstrate that the
produced oxygen originates from water, the experiments were
the same in borate buffer at pH 8.5 as in phosphate buffer.
Additional dynamic light scattering (DLS) experiments on iron
18
repeated with O‐labeled water under otherwise unchanged
18
complex
3 (Figure S10a) and iron complex 4 (Figure S10b)
conditions (Fig. S6). With a relative O concentration of 6.3%,
3+
16,18
16,16
under similar conditions to [Ru(bpy) ] ‐driven water oxidation
the ratio of the two isotopomers
O2/
2
O of the evolved
3
did not reveal any nanoparticle formation (1–1000 nm) after
catalysis. The low TONs are probably due to slow kinetics and
oxygen was found to be close to the theoretical value for both
oxygen atoms being derived from water.
We also tried to obtain single crystals of the dinuclear
3+
the low thermodynamic driving force when [Ru(bpy)3] is used
as oxidant (vide supra). In addition, ligand degradation and
leaching of iron from the complex leads to the formation of
insoluble iron phosphate, which is catalytically inactive.
complex
solubility of complex
using common solvents such as acetonitrile, MeOH and CH Cl .
3
for X‐ray crystallography. However, the limited
3
in organic solvents prevented us from
2
2
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In conclusion, we have successfully prepared a dinuclear iron
View Article Online
DOI: 10.1039/C6DT01554A
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5
6
7
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driven by [Ru(bpy) ] ‐type oxidants. Kinetic experiments
3
revealed
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the two iron centers in complex . DLS experiments together
a
first‐order dependence on the catalyst
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7
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4 was
3
3
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,
Financial support from the Knut and Alice Wallenberg
Foundation, the Swedish Research Council (621‐2013‐4872),
the Carl Trygger Foundation, DFG (joint IRTG Metal Sites in
Biomolecules: Structures, Regulation and Mechanisms; see
www.biometals.eu) and the Swedish Energy Agency, is
gratefully acknowledged.
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