Water oxidation by single-site ruthenium complexes: Using ligands as redox and proton transfer mediators

Article abstract of DOI:10.1002/anie.201205018

Light me up: Through the use of an imidazole motif it is possible to introduce a combined redox and proton-transfer mediator into single-site ruthenium water-oxidation catalysts. With the complex (see picture), high turnover numbers and high initial turnover frequencies were attained with the mild oxidant [Ru(bpy)₃]³⁺ (bpy=2,2′-bipyridine). Copyright

Full text of DOI:10.1002/anie.201205018

Angewandte
Chemie
DOI: 10.1002/anie.201205018
Catalytic Oxidation
Water Oxidation by Single-Site Ruthenium Complexes: Using Ligands
as Redox and Proton Transfer Mediators**
Markus D. Kꢀrkꢀs, Torbjçrn ꢁkermark, Eric V. Johnston, Shams R. Karim, Tanja M. Laine,
Bao-Lin Lee, Tobias ꢁkermark, Timofei Privalov, and Bjçrn ꢁkermark*
The splitting of water into molecular oxygen and hydrogen
gas is an attractive option for the production of sustainable
energy. The process of oxidizing water is the key step for
realizing this goal. The possibilities are enormous, as the
hydrogen can be used as a fuel, stored for future demands, or
even used for the production of more complex fuels and
chemicals.[1] The challenge is that the splitting of water
requires both a relatively large input of energy, E8 = 1.23 V
versus normal hydrogen electrode (NHE; at pH 0), and
a catalyst that allows the reaction to proceed at a reasonable
rate. In Nature, light-driven water oxidation is carried out by
a tetranuclear manganese cluster,[2] located in the oxygen
evolving complex (OEC), at a rate which we are only now
starting to approach with synthetic mimics. To facilitate
mechanistic studies, these mimics generally consist of well-
defined molecular water oxidation catalysts (WOCs) con-
taining one or several high-valent redox-active metals in the
center. Intensive efforts during recent years have resulted in
the development of a number of mono- and dinuclear Ru-,[3]
Ir-,[4] Mn-,[5] Co-[6] and Fe-based[7] complexes.
tions. Therefore, PCET can allow for the accumulation of
multiple redox equivalents, a prerequisite for carrying out
multi-electron reactions. Another way to affect the redox
potential of the system is to attach electron-donating and
redox-active ligands to the metal centers. This would also
influence the balance between efficiency and stability of the
WOCs.
It seemed plausible that the introduction of imidazole and
phenol motifs, in combination with carboxylate groups, could
facilitate proton coupled reactions and result in WOCs with
the ability to form high-valent metal–oxo species at low
potentials. Therefore, the meridionally coordinating benz-
imidazole based ligands 1 and 2, and the related single-site
ruthenium complexes 3 and 4 were synthesized (Scheme 1;
A major drawback is that most of the catalysts developed
thus far require a powerful sacrificial oxidant (CeIV), to
oxidize water. However, for the development of a sustainable
water splitting system, the oxidant needs to be a light-
absorbing component (a photosensitizer), which can be
regenerated. A major obstacle frequently encountered in
light-driven water oxidation is the mismatch between the
relatively high redox potential at which a catalyst assumes its
active state and the lower potential attainable with a photo-
sensitizer. One way to decrease the redox potential is to
involve proton-coupled electron transfer (PCET), which is
a fundamental process in Nature.[8] It involves simultaneous
transfer of an electron and a proton and has a profound
influence on mechanisms and energetics of chemical reac-
Scheme 1. Molecular structures of ligands 1 and 2 and the single-site
ruthenium complexes 3 and 4. L=4-picoline.
see the Supporting Information for further details). Ligands
1 and 2 both contain imidazole and phenol motifs, which are
important components in the natural system. In fact, by using
the imidazole motif, it was possible to introduce a combined
redox and proton transfer mediator, a highly active and
essential element, into these single-site ruthenium WOCs.
With complex 3 in particular, we were able to reliably reach
turnover numbers (TONs) and initial turnover frequencies
(TOFs), of up to 4000 and > 7 sꢀ1, respectively, with [Ru-
(bpy)3]3+ as the oxidant; to the best of our knowledge, these
[*] M. D. Kꢀrkꢀs, T. ꢁkermark, Dr. E. V. Johnston, S. R. Karim,
T. M. Laine, B.-L. Lee, Prof. T. Privalov, Prof. B. ꢁkermark
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 10691 Stockholm (Sweden)
E-mail: bjorn.akermark@organ.su.se
Dr. T. ꢁkermark
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
are the highest values reported for any metal-based water
V
oxidation catalyst. Also, [Ru O]n+, the postulated active key
=
intermediate in water oxidation was successfully character-
ized by high-resolution mass spectrometry (HRMS).
[**] Financial support from the Swedish Energy Agency and the Knut
and Alice Wallenberg Foundation is gratefully acknowledged.
The single-site ruthenium complexes 3 and 4 were
synthesized according to Figure S1 (see the Supporting
Information). To study the oxidation of water with a chemical
oxidant, water oxidation catalyzed by complexes 3 and 4 was
Supporting information for this article, including detailed synthetic
methods and procedures, characterization, and quantum chemical
calculations, is available on the WWW under http://dx.doi.org/10.
1002/anie.201205018.
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Table 1: Comparison of TONs for different WOCs utilizing [Ru(bpy)3]3+
as an oxidant.
first evaluated with [Ru(bpy)3]3+. Upon addition of an
aqueous phosphate buffer solution of catalyst 3 or 4 to
[Ru(bpy)3]3+, immediate O2 evolution was observed (Fig-
ure 1a,b, respectively). In the absence of catalyst 3 or 4 only
Complex
TON[a]
Reference
3
4
4000 (4000)
180 (180)
18 (4.5)
30 (30)
1000 (250)
this work
this work
Ref. [9]
Ref. [10]
Ref. [6b]
[{Ru4O4(OH)2(H2O)4}(g-SiW10O36)2]10ꢀ
[Ru(bda)(pic)2]
[Co4(H2O)2(PW9O34)2]10ꢀ
[a] TON=nO =ncat; numbers in parentheses are [TON/unit metal].
2
bda=2,2’-bipyridine-6,6’-dicarboxylate, pic=4-picoline.
oxidation at the potential generated by [Ru(bpy)3]2+, 1.26 V
vs. NHE (see Table 1). This table clearly demonstrates that
complex 3 stands out from the rest.
To evaluate the possibility of carrying out light-driven
water oxidation under homogeneous, neutral conditions
(pH 7.2), a system consisting of catalyst 3 or 4, a photo-
sensitizer ([Ru(bpy)3]2+ or [Ru(bpy)2(deeb)]2+; deeb = 4,4’-
di(ethoxycarbonyl)-2,2’-bipyridine) and a sacrificial electron
acceptor (Na2S2O8) was employed. This is a well-studied
reaction, which proceeds by quenching of the RuII sensitizer
to give the RuIII oxidant and a SO4ꢀ radical, which generates
a second molecule of RuIII. The RuIII complexes then oxidize
the WOC, which in turn oxidizes water to oxygen.[11]
A
curious fact is that although the SO4ꢀ radical has a very high
oxidation potential (E8 = 2.4 V vs. NHE), there is at least
another clear example showing that negligible amounts of
oxygen are formed by direct oxidation of the WOC by the
sulfate radical in the presence of an excess of sensitizer (see
the Supporting Information).[3j] Evolution of molecular
oxygen was detected upon illumination of this three compo-
nent system. Figure S4 displays the kinetics of oxygen
evolution measured by mass spectrometry (MS) of a phos-
phate buffer solution (0.1m, pH 7.2) containing catalyst 3 or 4,
a photosensitizer, and the acceptor. In the presence of the
[Ru(bpy)3]2+ photosensitizer low TONs of ca. 20 were
obtained for catalysts 3 and 4. The photocatalytic system
was therefore evaluated with [Ru(bpy)2(deeb)]2+ as the
photosensitizer. Replacing [Ru(bpy)3]2+ (E [RuIII/RuII] =
1.26 V vs. NHE) by the more strongly oxidizing photosensi-
tizer [Ru(bpy)2(deeb)]2+ (E [RuIII/RuII] = 1.4 V vs. NHE)
a significantly higher TON of close to 200 could be reached
for complexes 3 and 4 (Figure S4). When diluting the
concentration to 1 mm for complex 3, a TON of 300 was
obtained under otherwise unchanged conditions (not shown).
A series of control experiments confirmed that catalyst,
photosensitizer, persulfate, and photons are all necessary to
achieve O2 evolution. These findings confirm that catalyst 3 or
4 is required to achieve visible-light-induced water oxidation.
The origin of the O atoms in the evolved O2 was
investigated by employing isotopically labeled water
(H218O) and then measuring the enriched O2 by MS (Fig-
ures S5 and S6). Using a relative concentration of 18O of 7%,
the relative abundance of the different oxygen isotopes 18,18O2,
16,18O2, and 16,16O2 (m/z = 6, 34 and 32) were recorded and
analyzed. The theoretical values, assuming that both oxygen
atoms are derived from H2O, were calculated for the different
isotopes. The relative abundance of the different oxygen
Figure 1. Chemical water oxidation catalyzed by a) complex 3 and
b) complex 4 using [Ru(bpy)3]3+ as an oxidant. Conditions: A deoxy-
genated aqueous phosphate buffer solution (0.1m, pH 7.2, 0.5 mL) of
catalyst 3 or 4 was added to [Ru(bpy)3](PF6)3 (5.1 mg, 5.1 mmol).
~
&
*
^
33 mm ( ), 3.3 mm ( ), 0.33 mm ( ), 0.033 mm ( ).
negligible amounts of O2 were detected, if any. A series of
experiments were conducted and it was revealed that catalyst
3 was a highly efficient catalyst for water oxidation. With
a concentration of 33 nm of catalyst 3 and 10 mm [Ru(bpy)3]3+,
a turnover number (TON, nO =ncatalyst) of 4000 was obtained
2
within 15 min. The initial turnover frequency was calculated
to be > 7 sꢀ1 (Supporting Information, Table S1), which is
among the highest reported for any catalytic system employ-
ing a chemical oxidant (regardless of whether CeIV or
[Ru(bpy)3]3+ was employed as the oxidant). Experiments
conducted with catalyst 4 resulted in a considerably lower
TON, perhaps because of the fact that it is more sensitive to
oxidation. The final yields of evolved O2 based upon [Ru-
À
Á
(bpy)3]3+ 4nO =n
were calculated to be 53, 27, 14,
3þ
½RuðbpyÞ3ꢁ
2
and 5.2% for catalyst 3, at 33, 3.3, 0.33, and 0.033 mm,
respectively. For catalyst 4, the yields were determined to be
52, 6.5, and 2.2% at 33, 3.3, and 0.33 mm, respectively.
However, at lower pH, the use of [Ru(bpy)3]3+ as an oxidant
results in inefficient catalytic H2O oxidation.
If photochemically generated [Ru(bpy)3]3+ is to replace
CeIV, the redox potentials of the catalysts have to be decreased
from ca. 1.6–1.7 V to < 1.4 V vs. NHE. At present, only a few
reported complexes have been reported which catalyze water
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isotopes is in agreement with the calculated values for both
complexes. These results clearly demonstrate that H2O is the
source of oxygen in the evolved O2, a question which has been
of major concern with previously reported water oxidation
catalysts.[12]
3 and 4, respectively, which indicate that the picoline–water
exchange should be facile.
Solid samples of complexes 3 and 4 were analyzed by
electron paramagnetic resonance (EPR) at 298 K (Fig-
ure S27a,b). Both are ruthenium(III) complexes with low-
spin d5, S = 1/2, and have g values of 2.18 and 2.14, for
complexes 3 and 4, respectively. These values are relatively
low for ruthenium(III) complexes and suggest that the
electron in the singly occupied molecular orbital (SOMO)
of the complexes is partly ligand centered and there is
a considerable ligand spin delocalization existing in both
complexes, which is further supported by DFT calculations
(Figure S27c,d). In the complexes ruthenium has a d5 config-
uration, although the Mulliken spin population of the
ruthenium is about 0.7, not exactly 1, because of delocaliza-
tion of the SOMO to the ligand. The spin density distribution,
shown in Figure S27c,d, is thus metal-centered, with strong
contributions from the ligand (see also, Figures S20 and
S21).[14]
The electronic absorption spectra of the two mononuclear
ruthenium(III) complexes in an aqueous solution display
intense absorption bands in the near-UV region of 300–
400 nm (Figure S8), which are assigned to p–p* transitions in
the ligands. Indeed, the p system appears to be
involved in the HOMOs and LUMOs, as well as the
total spin densities (Figures S18, S20, and S21). The
absorption at about 390 nm and 370 nm corresponds
to electronic transitions with energies close to the
HOMO–LUMO gap.
The 1H NMR spectra in [D4]MeOH of complexes 3 and 4
showed the characteristics of paramagnetic compounds and
displayed broad signals. Upon the addition of ascorbic acid,
which reduces the ruthenium(III) complexes to ruthen-
1
ium(II), the complexes could be characterized by H NMR,
thus confirming the structures of complexes 3 and 4 (Fig-
ures S2 and S3, respectively). Analysis of the ruthenium(III)
complex 3 by HRMS, in positive mode, displayed three signals
at m/z = 633.1320, 540.0727, and 447.0160, which correspond
to the monocationic species [3 + H+]+, [3ꢀL + H+]+, and
[3ꢀ2L+H+]+ (L = 4-picoline; Figure S7a–c), respectively. For
complex 4, three signals were similarly observed at m/z =
605.1372, 512.0781, and 419.0210, which were assigned to
[4 + H+]+, [4ꢀL + H+]+ and [4ꢀ2L + H+]+, respectively (Fig-
ure S7d–f).
The calculated structures of the mononuclear ruthenium
complexes 3 and 4 with either the 4-picoline or water
coordinated at the equatorial position are shown in Figure 2
The electrochemistry of complexes 3 and 4 was
studied by cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) in an aqueous solution of
phosphoric acid at pH 1.0 (Figures S13 and S14) and
in an aqueous phosphate buffer (0.1m, pH 7)
solution (Figure S15). Both complexes exhibited
an electrocatalytic wave for water oxidation with an
onset potential of ca. 1.4 V vs. NHE. At pH 7 the
onset potential was significantly decreased to ca.
Figure 2. DFT-calculated structures of the mononuclear ruthenium complexes a) 3
and b) 4 with 4-picoline ligands. Distances shown are in ꢁ.
1.24 V vs. NHE for the two complexes. By introduc-
ing the negatively charged ligands 1 and 2, the redox
potentials for complexes 3 and 4 were dramatically
and in Figure S21, respectively.[13] Since ligands 1 and 2 both
have a charge of ꢀ3, the RuIII complexes are neutral overall.
If reduced to the RuII state, complexes with a protonated
imidazole fragment are neutral overall (shown in Figure S18).
It has been emphasized in the literature that ease of access of
a water molecule to the ruthenium center is a prerequisite for
an efficient WOC. In the aqueous media described within the
self-consistent solvent model, complexes 3 and 4 with
a coordinating picoline were calculated to be more stable
than the water-coordinating analogues. According to our
calculations, the equatorial picoline-water ligand exchange
was calculated to be endothermic by about 5 kcalmolꢀ1 in the
ruthenium(III) state. To verify this, spectrophotometric titra-
tion of the two ruthenium(III) complexes was conducted. The
spectral changes upon the gradual addition of water to
a solution of complex 3 or 4 were monitored by UV/Vis
spectroscopy (at l = 400 nm; Figures S9–S12). The solvolysis
equilibrium constants were calculated to be 0.29 and 0.43 for
decreased compared to complexes containing neutral
ligands.[3b] The electrochemical data for the two ruthenium
complexes in aqueous media is summarized in Table S4 and
the calculated structures of plausible complexes are shown in
Figures S19, S21s, and S23–S25. To obtain further insight into
the electrochemical pH dependence of complex 3, a Pourbaix
diagram was constructed. All three oxidation steps, from
ruthenium(II) to ruthenium(V) displayed a pH dependence
with a slope of 59 mV/pH, indicating a dependence on the
removal of a proton in the one-electron redox processes
(Figure S16 and Scheme 2).
The observation and characterization of active intermedi-
ates during water oxidation catalysis is of fundamental
importance in the pursuit of more active and robust
WOCs.[15] Evidence of a high-valent ruthenium(V)–oxo
intermediate, derived from complex 4, was found by high-
resolution mass spectrometry (Figure 3). This species is
regarded as the active key intermediate in the oxidation of
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valent, redox-active metal centers, a key element for oxidizing
water. Thus far, the majority of the catalysts developed suffer
from the drawback of requiring high redox potentials to
oxidize water. Only a few of these complexes are able of
carrying out photocatalytic water oxidation. Combining both
redox and proton transfer mediator motifs into the WOC
would facilitate the simultaneous transfer of electrons and
protons, thus avoiding high-energy intermediates and giving
access to new reaction pathways. To both decrease the redox
potentials and permit coupled proton–electron transfer,
imidazole and carboxylate were introduced as mediators
into the ligands. Indeed, by introducing the redox and proton
transfer mediator motif (imidazole) the WOCs were able to
catalyze water oxidation, under neutral conditions, both by
pre-generated and photogenerated [Ru(bpy)3]3+. Electro-
chemical and quantum chemical studies confirmed the
interesting redox properties of these complexes. The details
of how electrons and protons are transferred in the splitting of
water are the fundamental core of understanding and realiz-
ing sustainable artificial energy conversion. These catalysts
bring us one step closer to achieving artificial photosynthesis
by virtue of the potential they have in terms of synthetic ease,
cost, high catalytic activity, and the possibility of designing
molecular assemblies where they can be coupled to co-
catalysts or photosensitizers in light-harvesting water splitting
devices.
Scheme 2. Possible ligand involvement during water oxidation catalysis
mediated by ligand 1 and 2.
Received: March 7, 2012
Published online: September 28, 2012
Keywords: electrochemistry · homogeneous catalysis ·
.
photocatalysis · ruthenium · water oxidation
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