Photosensitized water oxidation by use of a bioinspired manganese catalyst

Article abstract of DOI:10.1002/anie.201104355

Manganificent potential: A dinuclear manganese catalyst contains metal centers that are coordinated by a central phenolate, with adjoining imidazole and carboxylate groups, which are all important functionalities in the natural oxygen-evolving center. The complex catalyzes the conversion of water to oxygen in the presence of a single-electron oxidant [Ru(bpy)₃]³⁺ (see picture, bpy=2,2′-bipyridine, A=acceptor). Copyright

Full text of DOI:10.1002/anie.201104355

DOI: 10.1002/anie.201104355
Photocatalysis
Photosensitized Water Oxidation by Use of a Bioinspired Manganese
Catalyst**
Erik A. Karlsson, Bao-Lin Lee, Torbjçrn ꢀkermark, Eric V. Johnston, Markus D. Kꢁrkꢁs,
Junliang Sun, ꢂrjan Hansson, Jan-E. Bꢁckvall, and Bjçrn ꢀkermark*
The search for renewable and clean energy sources is one of
the most important challenges of the 21st century.[1–3] The
utilization of sunlight is an environmentally friendly alter-
native, and together with nuclear fusion, it is the only
sustainable and carbon-free energy source with sufficient
potential to replace fossil fuels and satisfy future energy
demands. A promising approach is to mimic nature and
develop a system for artificial photosynthesis that converts
water into oxygen and hydrogen gas. The key determinant for
a breakthrough is the development of an efficient catalyst for
water oxidation. Although a series of catalysts based on
ruthenium have been developed,[4–15] catalysts based on earth-
abundant metals are rare. However, a few catalysts based on
cobalt[16,17] and iron[18] have been reported.
Homogeneous manganese-containing complexes have
been widely studied as models of the oxygen-evolving
center (OEC) in photosystem II, but only a few of these
Scheme 1. Synthesis of ligand 3 and complex 4.
complexes have been reported to evolve oxygen gas, mainly
by using oxygen atom donors such as HSO5À, NaOCl, and
tBuOOH.[19–26] However, none of these complexes has been
shown to oxidize water catalytically when using single-
electron oxidants under homogeneous conditions. Herein
we describe complex 4 (Scheme 1), which is the first
manganese complex capable of oxidizing water to oxygen in
homogeneous solution when using a single-electron oxidant,
[Ru(bpy)3]3+ (bpy = 2,2’-bipyridine). Complex 4 also cata-
lyzes the photochemical oxidation of water when using a
[Ru(bpy)3]2+-type photosensitizer and Na2S2O8 as electron
acceptor.
We have previously prepared a dinuclear manganese
complex that is capable of delivering three electrons to a
photochemically generated electron acceptor.[27] In this
II,II
III,IV
process, the complex was oxidized from Mn2
to Mn2
.
A fourth electron transfer probably takes place, just as in the
OEC, but leads to oxidative decomposition of the catalyst
rather than water oxidation. A probable decomposition
pathway could start with oxidation of the benzylic amines in
the ligand.
We have therefore prepared complex 4, which is based on
a new ligand, 3, that contains imidazole groups in place of the
benzylic amines present in many previously reported man-
ganese complexes (Scheme 1).[21,27,29] The ligand also contains
negatively charged carboxylate groups, shown to dramatically
reduce the redox potentials of the metal center,[14,15,28,29]
which is required for use in a catalytic system with [Ru-
(bpy)3]2+-type photosensitizers. It is interesting to note that
both imidazole and carboxylate groups act as ligands in the
OEC.[30,31] Ligand 3 was prepared from commercially avail-
able starting materials (dialdehyde 1 and amine 2) by
combined condensation and reductive cyclization. Complex-
ation with manganese(II) acetate afforded complex 4 in an
overall yield of 56% (Scheme 1). Complex 4 was character-
ized by ESI-MS (Figure S12 in the Supporting Information)
and elemental analysis.
[*] Dr. E. A. Karlsson, B.-L. Lee, E. V. Johnston, M. D. Kꢀrkꢀs,
Prof. J.-E. Bꢀckvall, Prof. B. ꢁkermark
Department of Organic Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
E-mail: bjorn.akermark@organ.su.se
Dr. T. ꢁkermark, Dr. J. Sun
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
Dr. ꢂ. Hansson
Biophysics Group, Department of Chemistry
University of Gothenburg
Crystals suitable for X-ray structure determination were
obtained by heating complex 4 to reflux in methanol followed
by crystallization at room temperature, which resulted in loss
of the bridging acetate ion and formation of an S2-symmetric
dimer, 5 (Figure 1). Interestingly, this structure has four
proximate Mn atoms that are bridged by oxygen atoms,
reminiscent of the Mn4Ca cluster in the OEC. Based on bond
P.O. Box 462, 40530 Gothenburg (Sweden)
[**] We thank T. Astlind for help with EPR spectroscopy and the Knut and
Alice Wallenberg Foundation, the Swedish Energy Authority, and the
European Research Council (ERC AdG 247014) for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104355.
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Communications
Figure 2. Oxygen evolution using [Ru(bpy)3]3+ as stoichiometric oxi-
dant. A solution of complex 4 (0.5 mL, 84 mm) in phosphate buffer
(0.1m, pH 7.2) was added to [Ru(bpy)3](PF6)3 (20 mg, 20 mmol) at
t=0. The jump at t=45 min is due to bursting of gas bubbles.
of an induction period when catalyst 4 was used could be an
indication that the active catalyst is present from the
beginning of the reaction, and is thus not produced by
decomposition. Finally, to demonstrate that the produced
dioxygen indeed originates from water, the experiment was
repeated with 18O-labeled water (Figure S10). With a relative
18O concentration of 5.8%, the ratio 16,18O2/16,16O2 was found
to be 0.10, which is close to the theoretical value 0.12
(= 2(1À0.058)0.058/(1À0.058)2), when both oxygen atoms
are derived from water.
Light-induced oxygen formation was also studied. In these
experiments, [Ru(bpy)3](PF6)2 was initially used as photo-
sensitizer, and give a TON of approximately 1. However,
when using [Ru(bpy)2(deeb)](PF6)2 (deeb = 4,4’-bis(ethoxy-
carbonyl)-2,2’-bipyridine) as photosensitizer, and thereby
increasing the RuIII/II redox potential from + 1.26[14] to
+ 1.40 V[32] versus the normal hydrogen electrode (NHE),
oxygen evolution occurred with a TON of approximately 4
(Figure 3). Several control experiments were performed.
Figure 1. X-ray crystal structure of dimer 5 at 50% probability level
(upper) and structural formula of 5 (lower).
valence calculations (Table S1), the oxidation states of the
two unique Mn atoms were determined to be MnII and MnIII.
In the same way, O3 and O6, which are coordinated to both
Mn atoms, were determined to be deprotonated, while O7
and O8 are protonated. Considering hydrogen bonds in the
crystal structure (Table S2), one proton was added to N3 to
form a hydrogen bond with O2 while there is no proton on N2
since it acts as acceptor for the hydrogen bonds with O7 and
O8. Since no additional ions were found in the crystal
structure, dimer 5 was considered to be neutral, which also is
in accordance with the above-mentioned oxidation and
protonation states.
The catalytic activity of complex 4 was investigated by
addition of a 480-fold excess of the single-electron oxidant
[Ru(bpy)3](PF6)3 in phosphate buffer (0.1m, pH 7.2). The
reaction was performed in an evacuated reaction vessel
connected to a mass spectrometer (see the Supporting
information). Oxygen evolution was detected with an initial
turnover frequency (TOF = amount of O2/(amount of 4 ꢀ
time)) of approximately 0.027 sÀ1, which lasted for about
1 hour, thus giving a turnover number (TON = amount of O2/
amount of 4) of approximately 25 (Figure 2). A control
experiment using manganese(II) acetate in the absence of
ligand 3 gave only substoichiometric amounts of oxygen
(Figure S9). In another control experiment, buffer solution
was added to [Ru(bpy)3](PF6)3, which resulted in decompo-
sition of the oxidant without any detectable oxygen produc-
tion. However, a small amount of carbon dioxide was
produced.
In principle, decomposition of the oxidant could generate
a ruthenium species that could react with ligand 3 to give an
active catalyst. When a solution of ligand 3 in phosphate
buffer was added to [Ru(bpy)3](PF6)3, no oxygen evolution
was observed, thus confirming that manganese complex 4 is
necessary for the catalytic activity. Furthermore, the absence
Figure 3. Light-driven oxygen evolution catalyzed by complex 4. A
solution of complex 4 (1.0 mL, 84 mm) in phosphate buffer (0.1m,
pH 7.2) was added to Na2S2O8 (4.7 mg, 20 mmol) and [Ru(bpy)2-
(deeb)](PF6)2 in acetonitrile (50 mL, 10 mm). The light was switched on
at t=0.
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Under identical conditions, no oxygen was produced in the
absence of catalyst 4. Omitting the photosensitizer resulted in
no oxygen evolution, and switching off the light terminated
oxygen evolution, thus confirming that complex 4 and the
photosensitizer are essential for the light-induced water
oxidation.
The properties of complex 4 in solution were studied in
the presence of K3PO4 by 1H NMR spectroscopy and X-band
EPR spectroscopy in D2O and H2O, respectively (Figures S1–
S8). The relatively narrow chemical shift range in combina-
In conclusion, we have reported a homogeneous manga-
nese-based catalyst 4, which is capable of catalyzing oxidation
of water to molecular oxygen in the presence of a single-
electron oxidant [Ru(bpy)3]3+. More importantly, 4 also
catalyzes photochemical water oxidation when using either
[Ru(bpy)3]2+ or the related [Ru(bpy)2(deeb)]2+ complex as
photosensitizer. A major difference between catalyst 4 and a
series of related catalysts developed by us and other research
groups is that the benzylic amine function has been replaced
by imidazole, thus making the ligand more resistant towards
oxidation. Furthermore, it may be even more important that
imidazole, as in many natural systems, can promote proton-
transfer reactions as the oxidation states of the manganese
atoms change.
tion with the absence of any EPR signal at 77 K indicate the
III,III
presence of an antiferromagnetically coupled Mn2
com-
plex with a singlet ground state, that is, the Mn2II,III complex 4
III,III
was oxidized to Mn2
by atmospheric oxygen because the
redox potential is lowered as the pH value is increased. The
broadening of the NMR signals could be attributed to the
population of excited spin states at ambient temperature.
Received: June 23, 2011
Published online: October 7, 2011
During the course of the water oxidation, the complex is
III,III
Keywords: homogeneous catalysis · manganese · N,O ligands ·
photocatalysis · water oxidation
expected to cycle between the initial Mn2
oxidation state
.
and Mn2V,V. In contrast with the EPR experiments in aqueous
III,III
solution, where the complex had been oxidized to Mn2
,
the spectrum recorded in dimethyl sulfoxide (DMSO) at 77 K
shows a broad (100 mT) signal at g = 2.0 (Figure S11), which
is indicative of a Mn2II,III complex.[33] This signal is overlapped
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