A tailor-made molecular ruthenium catalyst for the oxidation of water and its deactivation through poisoning by carbon monoxide

Article abstract of DOI:10.1002/anie.201210226

No CO-operation: Two single-site ruthenium complexes, based on a tetradentate ligand, despite structural similarities, display a remarkable difference in their catalytic and chemical behavior (see picture). The fact that the CO-containing complex is forme

Full text of DOI:10.1002/anie.201210226

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DOI: 10.1002/anie.201210226
Oxidation of Water
A Tailor-Made Molecular Ruthenium Catalyst for the Oxidation of
Water and Its Deactivation through Poisoning by Carbon Monoxide**
Markus D. Kꢀrkꢀs, Torbjçrn ꢁkermark, Hong Chen, Junliang Sun, and Bjçrn ꢁkermark*
The transition to new sustainable energy sources is one of the
greatest challenges facing the human society today.[1] Overall
H2O splitting, employing distinct catalysts and solar energy,
has been a central issue for the scientific community during
the past decades because of its potential of producing green
and renewable energy, for example, molecular hydrogen. The
splitting of H2O can thus be divided into two half-reactions;
the oxidation of water and the reduction of protons. The fact
that the former half-reaction requires highly oxidizing con-
ditions, makes H2O oxidation more troublesome and discov-
ering catalysts for this transformation is highly challenging
and is thus far the limiting step and the linchpin for durable
artificial photosynthetic systems.
The oxidation of H2O to O2 is complex, in that it invokes
the consecutive removal of four electrons and reshuffling of
several bonds to finally liberate O2. During three billion years,
nature has evolved and fine-tuned a sophisticated system to
achieve photocatalytic H2O oxidation. In this multi-step
process, solar energy is harnessed to convert simple starting
materials into valuable feed stocks for biological organisms.
At the heart of PS II is the oxygen evolving complex (OEC),
containing the catalytically active Mn4Ca-core[2] which man-
ages to catalyze the oxidization of H2O to O2 at a tremendous
rate and with a high turnover, thus reflecting considerable
stability. Nonetheless, even this system is degraded and
continuously has to be reassembled.
Thus far, the artificial water oxidation catalysts (WOCs)
are decidedly less stable, although there have been significant
advances during the last decade.[3] The intensive quest for
a viable WOC has contributed to a range of heterogeneous,[4]
homogeneous,[5–9] and also some examples of immobilized
homogeneous catalysts.[10] The prevailing homogeneous
WOCs consist of complexes of Ru[5] and Ir[6] from the
second and third row, respectively, and recently comple-
mented by complexes of the first-row transition metals Mn,[7]
Fe,[8] and Co.[9]
Under the highly oxidizing conditions required to oxidize
H2O, the WOCs are decomposed and/or deactivated after
a certain time period. This is a serious and general issue
encountered with WOCs and insight into the deactivation
pathways is necessary if more stable WOCs are to be realized.
The current information regarding the deactivation of molec-
ular WOCs is quite dispersed and ligand dissociation[5g] and
oxidative decomposition[11] have been pointed out as the main
deactivation pathways.
Ligand architecture and catalyst optimization are of
primary importance for developing WOCs with a longer
lifetime. The main obstacle encountered with the majority of
WOCs is the high oxidation potentials required to achieve
H2O oxidation. One way of overcoming this problem is to
introduce anionic ligands to stabilize and lower the high
oxidation potentials.[5d,f,12] Recently, we employed a bioin-
spired strategy to synthesize ligands of this type, for the
development of single-site ruthenium complexes which
showed high activity in the oxidation of H2O with the mild
[*] Dr. M. D. Kꢀrkꢀs, Prof. B. ꢁkermark
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
10691 Stockholm (Sweden)
E-mail: bjorn.akermark@organ.su.se
oxidant [Ru(bpy)3]3+ [5f]
Inspired by this work it was thus
.
Dr. T. ꢁkermark, Prof. J. Sun
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
envisioned that a tailored catalyst design based upon the
tetradentate ligand scaffold rendered by ligand 3 [H2bpb =
N,N’-1,2-phenylene-bis(2-pyridine-carboxamide)]
would
H. Chen
afford highly versatile ruthenium-based WOCs. The readily
accessible tetradentate ligand scaffold 3 offers an easily
modified ligand environment, an important factor for tuning
WOCs and for the construction of supramolecular assemblies.
Herein we report two single-site ruthenium complexes,
based upon the tetradentate ligand 3, 4 ([Ru(bpb)(pic)2]Cl)
and 5 ([Ru(bpb)(CO)(OH2)]) (Figure 1), which despite
structural similarities display a remarkable difference in
their catalytic and chemical behavior. During catalytic H2O
oxidation by 4, complex 5 is generated. This contains a coor-
dinated carbon monoxide (CO) molecule and proved to be
inactive in H2O oxidation when using [Ru(bpy)3]3+ as oxidant.
CO is one of the gaseous byproducts generated by oxidative
decomposition (either from the catalyst itself or from [Ru-
(bpy)3]3+-type complexes, both pre-generated and photo-
Berzelii Center EXSELENT on Porous Materials and
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
and
Faculty of Material Science and Chemistry
China University of Geosciences
Wuhan 430074 (P.R. China)
[**] This project was supported by the Swedish Energy Agency, the Knut
and Alice Wallenberg Foundation, the Swedish Research Council
(VR), and the Swedish Governmental Agency for Innovation
Systems (VINNOVA) through the Berzelii Center EXSELENT. H.C.
would also like to thank the China Scholarship Council for his
scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210226.
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molecular ruthenium-based catalysts and inorganic polyoxo-
metallates being the notable exceptions.[5d,f,14]
Upon the addition of an aqueous solution of complex 4 to
the oxidant, gas bubbles were immediately observed. Real-
time mass spectrometry measurements enabled us to confirm
the evolution of O2 and the high activity expressed by
complex 4 (turnover number, TON = 200; turnover fre-
quency, TOF ꢀ 0.12 sꢁ1). Control experiments were also
carried out to establish the catalytic importance of complex 4,
where an equimolar amount of RuO2 was used instead of
complex 4. This resulted in negligible amounts of produced
O2, thus confirming the importance of complex 4.
Detailed studies were carried out to probe the stability of
complex 4 towards ligand dissociation and oxidative decom-
position. It is bench-stable and can be stored under an air
atmosphere without any detectable decomposition. Com-
plex 4 is also hydrolytically stable in the range 1 < pH < 7 for
at least seven days at ambient temperature. This was
confirmed by examination of the UV/Vis absorption spectra
as well as by NMR spectroscopy (Figures S24–S27).
To provide further support that complex 4 is the active
catalyst during the oxidation of H2O and that it retains its
identity after catalytic cycling, a solution of the reaction
mixture after about 20 turnovers was analyzed by HRMS (see
the Supporting Information for details). This resulted in the
appearance of a peak, in positive mode, at m/z = 604.1149
which is ascribed to [Ru(bpb)(pic)2]+ (Figure S13). This shows
that the structure of the catalyst is retained.
The reaction mixture was also examined by HRMS after
the O2 evolution had ceased. A major peak at m/z = 462.9985
was observed, in negative mode, corresponding to [Ru-
(bpb)(CO)(OH2) (5)ꢁH+]ꢁ (Figure S14). The fact that in
addition to the evolution of O2, CO is generated as one of the
gaseous reaction byproducts (by decomposition from the
catalyst itself or from the oxidant [Ru(bpy)3]3+) during the
H2O oxidation experiments and explains the formation of
complex 5 from 4. A probable intermediate in the conversion
of complex 4 into 5 could also be found (in positive mode) at
m/z = 539.0548 corresponding to [Ru(bpb)(pic)(CO)]+ (Fig-
ure S15).
Figure 1. Molecular structures of the single-site ruthenium com-
plexes 4 and 5.
chemically generated) during the catalytic oxidation of H2O.
This catalytic difference suggests a novel, and maybe general,
pathway for deactivation of ruthenium-based WOCs. This is
a key observation because thus far CO has been considered as
an innocent spectator molecule but our results imply that it
can contribute to the deactivation of ruthenium-based WOCs
during catalytic H2O oxidation.
Ligand 3[13] was synthesized from 1,2-phenylenediamine
(1) and 2-pyridinecarboxylic acid (2) by use of triphenyl
phosphite as coupling reagent. The corresponding single-site
ruthenium complex 4 was obtained by refluxing with [Ru-
(DMSO)4Cl2], in the presence of Et3N and 4-picoline (see the
Supporting Information for further details). The structure of
complex 4 was confirmed by high-resolution mass spectrom-
etry (HRMS), NMR spectroscopy, and UV/Vis spectroscopy.
Evaluation of the catalytic activity toward the chemically
driven oxidation of water for complex 4 was carried out with
[Ru(bpy)3]3+ as chemical oxidant in a buffered aqueous
solution (0.1m phosphate buffer, pH 7.2) and is depicted in
Figure 2 and Figure S5. The primary advantage of employing
this oxidant is its activity at near-neutral conditions, compared
to CeIV which requires acidic pH. However, only a few WOCs
have been characterized, employing [Ru(bpy)3]3+ as oxidant,
probably because of the high oxidation potential required to
activate a majority of the reported WOCs, with a few
These findings indicate that complex 5 is not an active
catalyst for H2O oxidation. To verify this, complex 5 was
separately synthesized by refluxing ligand 3 in N,N-dimethyl-
formamide in the presence of RuCl3, and KH as base, with
in situ generation of CO by thermal decomposition of N,N-
dimethylformamide (see the Supporting Information for
further details). X-ray quality single-crystals were grown by
slow diffusion of a toluene solution into a methanol solution
containing 5. The crystal structure of complex 5 is depicted in
Figure 3. In complex 5, the ruthenium atom is in a six-
coordinate configuration, with the tetradentate ligand 3
bound to the ruthenium center together with a H2O molecule
and carbon monoxide occupying the axial positions. The angle
of N1-Ru1-N4 is 115.08, and larger than the ideal 908 of an
octahedral configuration and may supply a seventh coordi-
nation site for complexes containing ligand 3. The details of
selected angles and bond lengths can be found in the
Supporting Information in Table S1, together with the
structure of the hydrogen-bonding network (Figure S16).
*
Figure 2. Kinetic curves for the evolution of O2 by complex 4 ( ) and 5
&
( ) versus time. Conditions: An aqueous phosphate buffer solution
(0.1m, pH 7.2, 0.5 mL) containing the complex (30 mm) was added to
the oxidant [Ru(bpy3)](PF6)3 (3.0 mg, 3.0 mmol).
2
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stability of the ruthenium(III) redox levels and also the
reactivity difference towards the catalytic oxidation of H2O.
The cyclic voltammogram for complex 4, obtained in an
aqueous solution at neutral pH (Figure S19) displays a cata-
lytic current for the oxidation of H2O starting at about 1.23 V
vs. NHE. This onset potential is shifted to higher anodic
potentials for complex 5 (E > 1.28 V vs. NHE). The oxidant
[Ru(bpy)3]3+ delivers a potential of 1.26 V vs. NHE, thus
making it thermodynamically unfavorable to drive H2O
oxidation by complex 5.
DPV measurements of complex 4 in acidic medium
(pH 1) also resulted in three well-separated peaks (Fig-
ure S18), where two of them are shifted to a more positive
potential compared to the potential at pH 7. The first
oxidation process at 0.17 V, ascribed to the RuIII/RuII redox
couple, is slightly affected. The second and third oxidation
processes at 0.98 and 1.21 V, corresponding to the formal
oxidations of RuIII!RuIV!RuVI, are shifted to higher
potentials. This means that these oxidation reactions are
associated with the loss of a proton, that is, these processes are
proton-coupled electron transfer (PCET) processes. For
complex 5, the previously observed, single wave at neutral
pH, splits into two waves occurring at 0.92 and 1.08 V in acidic
medium (pH 1).
Figure 3. Crystal structure of complex 5 with thermal ellipsoids at 30%
probability. Hydrogen atoms and solvent are omitted for clarity.
For a detailed discussion regarding the coordination proper-
ties of the two ruthenium complexes see the Supporting
Information.
The attempts to use the CO containing complex 5 for H2O
oxidation, with [Ru(bpy)3]3+ as oxidant, failed, with no
detectable amounts of generated O2 (Figure 2 and Figure S6).
Also when CeIV was used as oxidant no O2 was produced
(compared to complex 4 which was able to produce O2). This
was clearly surprising since oxidation of coordinated CO to
CO2 has frequently been used in iron CO chemistry to form
an empty coordination site.[15] In addition, efficient iridium-
based WOCs which contain CO have been reported.[6a] The
iridium WOCs have been reported to lose the CO ligand by
irreversible oxidation (to CO2), leaving an empty coordina-
tion site for an aqua ligand in the oxidation of H2O. This
different behavior of ruthenium and iridium is enticing and
highlights a key catalytic divergence among metal-based
WOCs. These observations and insights might thus shed light
on the deactivation of ruthenium-based WOCs.
To get a more comprehensive insight into the electro-
chemical properties of complexes 4 and 5, the dependence of
the potentials on the pH (Pourbaix diagrams) for the two
complexes were examined in the region 1.5 < pH < 8.0
(Figure 4). The Pourbaix diagram for complex 4 (Figure 4a)
Electrochemical measurements were performed on the
two single-site ruthenium complexes under relevant catalytic
turnover conditions (neutral pH) to understand the striking
difference in the reactivity exhibited by complexes 4 and 5.
This would also give further insight into the catalytically
active species and the mechanism of the oxidation of water.
The differential pulse voltammograms (DPVs) of the com-
plexes at pH 7.2 (0.1m phosphate buffer/acetonitrile 95:5)
provide further insight into their redox chemistry (Fig-
ure S17). Complex 4 displayed peaks at + 0.15, 0.70, and
0.98 V versus the normal hydrogen electrode (NHE) corre-
sponding to the formal oxidation reactions of RuII!RuIII
!
RuIV!RuVI. The catalytic oxidation of water occurs after the
RuIV!RuVI process, thus affirming that the RuVI oxidation
state initiates the evolution of oxygen. By contrast, complex 5
only shows a single peak at 0.89 V corresponding to the
formation of a high-valent ruthenium species. Obviously, the
axial ligands have a great influence on the level of p-
backbonding in the two complexes, thus reflecting the
Figure 4. Pourbaix diagrams of a) complex 4 and b) complex 5, in the
range 1.5<pH<8.0 (pKa values are denoted by the vertical dashed
lines). Experimental conditions: Pourbaix diagrams of complexes 4 and
5 (30 mm) were obtained from DPV measurements, in a 0.1m Britton-
Robinson buffer solutions in the range of 1.5<pH<8.0. The pH of
the solution was changed by using a 0.2m NaOH aqueous solution.
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reveals that the potential for the RuIII/RuII couple (0.17 V vs.
NHE) is pH independent from pH 1.5 up to 6.8. At pH > 6.8
the potential for the RuIII/RuII couple follows a Nernstian
behavior, where the slope is equivalent to ꢁ(m/n)·59 mV per
pH unit (where m and n are equivalent to the number of
protons and electrons transferred, respectively). The slope
was found to be ꢁ59 mV/pH and reveals that the one-electron
oxidation is accompanied by the transfer of a proton (PCET),
ascribed to the [RuIII-OH]/[RuII-OH2] redox couple.
From the Pourbaix diagram it could thus be determined
that the pKa value for the ruthenium–aqua complex, [RuIII-
OH2]+, is 6.8. This pH dependent oxidation of complex 4
signals that coordination of H2O occurs to the low valent
ruthenium center (RuII) as a seventh ligand, perhaps because
of the large bite angle (1158) offered by the tetradentate bpb2ꢁ
ligand. This coordination provides the ruthenium center with
easy access to high-valent ruthenium species through PCET
events, without ligand exchange. By contrast, the RuIV/RuIII
redox couple is pH dependent in the whole region from
pH 1.5 to 6.8, also with a slope of ꢁ59 mV per pH unit,
corresponding to the formation of [RuIV-OH]+. In fact, this
species could be detected by HRMS (Figure S12). The next
process, the oxidation of [RuIV-OH]+ is unique, in the sense
that this process has a slope of ꢁ29 mV per pH unit over the
whole pH range (1.5 < pH < 8.0) and is thus a one-proton-
For complex 4, a kinetic study was performed, where the
initial rate of O2 formation was found to be first-order in the
catalyst concentration (Figures S8 and S9), suggesting that
bimolecular reactions do not contribute to the catalytic H2O
oxidation. Based on the kinetics of the catalysis with
complex 4, together with the electrochemical results, we
suggest a mononuclear mechanism with the involvement of
a [RuVI O] species. The key structural feature of complex 4 is
=
the large bite angle of 1158 provided by the tetradentate
ligand 3, allowing for easy access of an aqua ligand to the
metal center. For complex 4, the first oxidation process
furnishes a seven-coordinated [RuIV-OH]+ species through
a proton-coupled electron event (at neutral pH). This one-
electron species was also detected by HRMS, after the
addition of 4 equivalents of the oxidant [Ru(bpy)3]3+ to
a solution containing 4 (Figure S12). The subsequent oxida-
tion process is also proton-coupled and is assumed to generate
2+
a [RuVI O] intermediate, the crucial species for H2O
=
oxidation catalysis. This highly electrophilic [RuVI O]
2+
=
species presumably undergoes a nucleophilic attack by
a water molecule, accompanied by loss of a proton, to
generate a hydroperoxo species [RuIV-OOH]+.[16] A further
proton-coupled oxidation step then furnishes the fragment
[RuV-OO]+ which liberates molecular O2 and regenerates the
starting RuIII species.
two-electron redox process. This actually suggests that
To conclude, the examination of the two related single-site
ruthenium complexes 4 and 5 containing the easily accessible
and modifiable tetradentate ligand scaffold H2bpb (3), reveals
striking differences in the reactivity of the two complexes.
While complex 4 is an efficient catalyst for the oxidation of
water, 5 turns out to be inactive. The fact that complex 5 is
formed from 4 under catalytic conditions is a key observation
and an important feature in the search for more stable and
efficient WOCs. The difference in the electrochemical
properties of the two structurally similar single-site ruthenium
complexes is intriguing and highlights the strong impact
exerted by the axial ligands on the catalytic activity of the
ruthenium centers. Ease of access of an aqua ligand to the
metal center is important for WOCs but although complex 5
has relatively low oxidation potentials and contains an axially
coordinated aqua ligand, this does not result in an active
WOC. Recognition of these features constitutes a basic design
principle for the development of future WOCs.
2+
a ruthenium(VI)-oxo ([RuVI O] ) or ruthenium(V)-oxyl
=
species ([RuV-oxyl]2+), is generated and involved in the
catalytic cycle of H2O oxidation. This differs from a majority
of the reported catalysts for H2O oxidation, in which the
catalysts usually are oxidized to the ruthenium(V) state,
which is the catalytically important intermediate.
The Pourbaix diagram of complex 5 is different from that
of complex 4, and contains peculiar features (Figure 4b). The
first redox process, assigned to the [RuIII-OH2]+/[RuII-OH2]
couple, is pH independent over a wide range of proton
concentrations (from pH 1.5 up to pH 6.5). The next oxida-
tion process displays a complex behavior over the studied pH
range. In the region of pH 1.5–2.6 the potential of this process
is pH independent and is assumed to involve a two-electron
oxidation process, corresponding to [RuV-OH2]3+/[RuIII-
OH2]+. From pH 2.6 to 4.8, the oxidation changes behavior
and has a slope of ꢁ29 mV per pH unit, implying that this
process is now a one-proton-two-electron oxidation to form
the species [RuV-OH]2+. Between pH 4.8–6.5 the dependence
of the potential has a slope of ꢁ59 mV per pH unit and
corresponds to a two-proton-two-electron-oxidation involv-
Received: December 21, 2012
Published online: && &&, &&&&
+
III
+
ing the redox couple [RuV O] /[Ru -OH2] . At pH > 6.5 the
Pourbaix diagram has a slope of ꢁ40 mV per pH unit and
astonishingly hints that this process is a two-proton-three-
Keywords: energy conversion · homogeneous catalysis ·
=
.
oxidation of water · oxygen evolution · ruthenium
electron oxidation and that it is possible to directly go from
+
[RuII-OH2] to [RuV O] . The difference in reactivity towards
=
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4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Oxidation of Water
No CO-operation: Two single-site ruthe-
nium complexes, based on a tetradentate
ligand, despite structural similarities,
display a remarkable difference in their
catalytic and chemical behavior (see
picture). The fact that the CO-containing
complex is formed in the catalytic oxida-
tion of water suggests a novel pathway for
the deactivation of ruthenium-based cat-
alysts.
M. D. Kꢁrkꢁs, T. ꢂkermark, H. Chen,
J. Sun, B. ꢂkermark*
&&&& — &&&&
A Tailor-Made Molecular Ruthenium
Catalyst for the Oxidation of Water and Its
Deactivation through Poisoning by
Carbon Monoxide
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!