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Synthesis, structure, spectroscopy and reactivity of
new heterotrinuclear water oxidation catalysts†
Cite this: DOI: 10.1039/c5sc04672f

Lorenzo Mognon,a Sukanta Mandal,b Carmen E. Castillo,cd Jerome Fortage,cd
´ˆ
Florian Molton,cd Guillem Arom´ e Jordi Benet-Buchhlolz,a Marie-Noelle Collombcd
ı,
¨
and Antoni Llobet*af
Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bppÀ) and 2,20 :60 ,200 terpyridine (trpy) of the general formula {[RuII(trpy)]2(m-[M(X)2(bpp)2])}(PF6)2, where M ¼ CoII, MnII and X ¼
ClÀ, AcOÀ (M ¼ CoII, X ¼ ClÀ: Ru2Co–Cl2; M ¼ MnII, X ¼ ClÀ: Ru2Mn–Cl2; M ¼ CoII, X ¼ AcOÀ: Ru2Co–
OAc2; M ¼ MnII, X ¼ AcOÀ: Ru2Mn–OAc2), have been prepared for the first time. The complexes have
been characterized using different spectroscopic techniques such as UV-vis, IR, and mass spectrometry.
X-Ray diffraction analyses have been used to characterize the Ru2Mn–Cl2 and Ru2Mn–OAc2 complexes.
The cyclic voltammograms (CV) for all four complexes in organic solvent (CH3CN or CH2Cl2) display
three successive reversible oxidative waves corresponding to one-electron oxidations of each of the
three metal centers. The oxidized forms of the complexes Ru2Co–OAc2 and Ru2Mn–OAc2 are further
characterized by EPR and UV-vis spectroscopy. The magnetic susceptibility measurements of all
complexes in the temperature range of 2–300 K reveal paramagnetic properties due to the presence of
high spin Co(II) and Mn(II) centers. The complexes Ru2Co–OAc2 and Ru2Mn–OAc2 act as precatalysts for
the water oxidation reaction, since the acetato groups are easily replaced by water at pH ¼ 7 generating
the active catalysts, {[Ru(H2O)(trpy)]2(m-[M(H2O)2(bpp)2])}4+ (M ¼ CoII: Ru2Co–(H2O)4; M ¼ MnII: Ru2Mn–
(H2O)4). The photochemical water oxidation reaction is studied using [Ru(bpy)3]2+ as the photosensitizer
and Na2S2O8 as a sacrificial electron acceptor at pH ¼ 7. The Co containing complex generates a TON of
Received 4th December 2015
Accepted 1st February 2016

50 in about 10 minutes (TOFi ¼ 0.21 sÀ1), whereas the Mn containing complex only generates a TON of
8. The water oxidation reaction of Ru2Co–(H2O)4 is further investigated using oxone as a sacrificial
chemical oxidant at pH ¼ 7. Labelled water oxidation experiments suggest that a nucleophilic attack

DOI: 10.1039/c5sc04672f

mechanism is occurring at the Co site of the trinuclear complex with cooperative involvement of the two

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Ru sites, via electronic coupling through the bppÀ bridging ligand and via neighboring hydrogen bonding.

Introduction
Water oxidation catalysis is a eld that has expanded enormously over the last few years.1–10 This development has been
fuelled by interest in the topic for the achievement of new
energy conversion schemes based on water and sunlight to
a

Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and
Technology, Avinguda Pa¨ Catalans 16, 43007 Tarragona, Spain. E-mail: allobet@iciq.cat
ısos

b

Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur721302, West Bengal, India

c

Univ. Grenoble Alpes, DCM, F-38000 Grenoble, France

d

CNRS, DCM, F-38000 Grenoble, France

Departament de Qu´mica Inorg`nica, Universitat de Barcelona, Diagonal 645, 08028
ı
a
Barcelona, Spain

e

Departament de Qu´
ımica, Universitat Aut`noma de Barcelona, Cerdanyola del Vall`s,
o
e
08193 Barcelona, Spain

f

† Electronic supplementary information (ESI) available: Complete experimental
procedures, compound characterization data and experimental data. CCDC
1440794 and 1440795. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5sc04672f

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obtain so-called solar fuel (such as H2), mimicking natural
photosynthesis. Water oxidation is the main reaction that
occurs at the OEC-PSII,11 yielding molecular oxygen, four
protons and four electrons, which can later react further. The
development of new water oxidation catalysts (WOCs) has been
based mainly on mononuclear12 and dinuclear13,14 Ru
complexes containing polypyridilic ligands. However, recently,
a number of Ir and rst row transition metal complexes have
also been reported to be able to oxidize water.3,15,16
While mononuclear complexes are in general easier to
prepare, targeted polynuclear complexes can present signicant
synthetic challenges. Nevertheless, polynuclear complexes
containing bridging ligands that can electronically couple the
metal centers can attain important benets from the perspective of a WOC. For example, multiple electronically coupled
redox active metal centers can cooperate during the four electron transfers needed for the water oxidation reaction. On the
other hand, non-redox active centers can be of interest to exert
electronic perturbation over the redox active center.17,18 They

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can also provide aqua/hydroxo ligands which, when strategically situated, can help to lower activation energies by hydrogen
bonding with key active species bonded to the active metal, or
even participate in O–O bond formation, as has been proposed
for the Ca–OH2 moiety of the OEC-PSII on a number of
occasions.19,20
With these considerations in mind, we undertook the preparation of heterotrinuclear complexes where all of the metal
centers are redox active and possess aqua ligands, to favor the
achievement of high oxidation states via Proton Coupled Electron Transfer (PCET). Herein, we report the synthesis, structural, spectroscopic and electrochemical characterization of
a new family of heterotrinuclear complexes containing Ru and
Co or Mn as metal centers, together with their capacity to
oxidize water to dioxygen.

Results and discussion
Synthesis and solid state structure
The well-known Hbpp ligand is an asymmetric molecule which,
once deprotonated, presents two equivalent coordination environments. The protonated ligand thus allows the preparation of
mononuclear complexes that can be used as starting materials
for the preparation of homo-21 or heterodinuclear, or heterotrinuclear22 complexes. In the present report we use out[Ru(Cl)(Hbpp)(trpy)]+, out-0, as the starting material for the
preparation of heterotrinuclear complexes containing redox
active metals, as displayed in Scheme 1.
The pyrazolato proton of out-0 is removed using NaOMe as
a base, and then MnCl2 or CoCl2 salts are used to react with the
vacant coordination sites of the bppÀ ligand, as shown in
eqn (1) for the case of cobalt.
2out-½RuðClÞðHbppÞðtrpyފþ þ 2MeOÀ þ CoCl2
/
out-0
È
À Â
ÃÂ
ÃÁÉ2þ
½Ruðtrpyފ2 m- CoðbppÞ2 ðm-ClÞ2
þ 2ClÀ þ 2MeOH
(1)
Ru2 Co-Cl2

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Complex Ru2Co–Cl2 is obtained in 58% yield, whereas
a 70% yield is obtained for Ru2Mn–Cl2 following a similar
synthetic procedure.22 Treatment of these trinuclear
complexes with sodium acetate at 75  C in an acetone : water
(5 : 1) solution replaces the chlorido bridging ligands by the
more labile acetato ligands, as indicated in the following
equation.
È
À Â
ÃÂ
ÃÁÉ2þ
½Ruðtrpyފ2 m- CoðbppÞ2 ðm-ClÞ2
þ 2OAcÀ /
Ru2 Co À Cl2
À Â
ÃÂ
ÃÁÉ2þ
È
þ 2ClÀ
½Ruðtrpyފ2 m- CoðbppÞ2 ðm-OAcÞ2
Ru2 Co À OAc2
(2)
Finally, substitution of the acetato bridges by monodentate
aqua ligands leads to the formation of the corresponding tetraaqua complex in neutral pH (eqn (3)).
À Â
ÃÂ
ÃÁÉ2þ
È
½Ruðtrpyފ2 m- CoðbppÞ2 ðm-OAcÞ2
þ 4H2 O /
Ru2 Co À OAc2
À Â
ÃÁÉ4þ
È
þ 2OAcÀ
½RuðH2 OÞðtrpyފ2 m- CoðH2 OÞ2 ðbppÞ2
Ru2 Co À ðH2 OÞ4
(3)
In contrast, under acidic conditions, the trinuclear
complexes decompose to the corresponding mononuclear in[Ru(Hbpp)(trpy)(H2O)]2+, in-1, and free Co(II) (Fig. S27†).
The synthetic strategies used for the preparation of Ru2Mn–
OAc2and Ru2Mn–(H2O)4 are analogous to those for the cobalt
counterparts.
All of these new complexes have been characterized by
analytic, spectroscopic or electrochemical techniques. Further,
X-ray diffraction analysis has been carried out for the Mn
complexes, Ru2Mn–Cl2 and Ru2Mn–OAc2, and ORTEP views of
their cationic moiety are presented in Fig. 1.
All the metal centers present pseudo-octahedral symmetry
around the rst coordination sphere. In the Ru2Mn–Cl2 case,
the ruthenium atoms are coordinated by ve N-atoms, three
from a meridional trpy ligand and two from bppÀ, while the
sixth coordination position is occupied by a chlorido ligand.
The manganese center is coordinated by the same two bridging
chlorido moieties, in a cis fashion, and by the two chelating
bppÀ ligands. The “Mn(bpp)2” moiety can thus be considered as
a bridge between the two Ru centers (see Fig. 1 and Scheme 1). A
very similar structure is obtained for Ru2Mn–OAc2 where the
bridging chlorido ligands have been substituted by bridging
acetato ligands. For both structures, the bonding distances and
angles presented by the Ru(II) and Mn(II) centers are
unremarkable.23,24
Redox properties and UV-vis spectroscopy in organic solvents
for Ru2M–OAc2

Scheme 1 Synthetic strategy followed for the preparation of trinuclear
complexes and their labelling.

Chem. Sci.

The cyclic voltammogram of Ru2Co–OAc2 in CH3CN (Fig. 2)
displays three successive reversible oxidation waves at E1/2 ¼
0.70 (DE ¼ 75 mV), 1.08 (DE ¼ 60 mV), and 1.22 V (DE ¼ 60 mV),
and one reversible reduction wave at E1/2 ¼ À1.23 V (DE ¼ 80
mV). All the redox potentials in this work are reported vs. NHE.
Each of the three oxidation processes corresponds to the

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ORTEP plots (50% probability) for the cationic structures of
complexes Ru2Mn–Cl2 (top) and Ru2Mn–(OAc)2 (bottom). Hydrogen
atoms have been omitted.
Fig. 1

Fig. 2 (Left) Cyclic voltammogram of a 0.25 mM solution of Ru2Co–
OAc2 in 0.1 M [(nBu4N)ClO4] in CH3CN, at a scan rate of 100 mV sÀ1.
(Right) UV-visible absorption spectra of a 0.25 mM solution of Ru2Co–
OAc2 in 0.1 M [(nBu4N)ClO4] in CH3CN, in oxidation states (black)
Ru2(II)Co(II), (red) Ru2(II)Co(III) and (blue) Ru2(III)Co(III).

exchange of one electron per molecule of complex, as evidenced
by rotating disk electrode experiments (Fig. S19†). The rst
process is assigned to the oxidation of the cobalt Co(III)/Co(II)
and the last two to the oxidation of the two Ru sites, Ru2(II,III)/
Ru2(II,II) and Ru2(III,III)/Ru2(II,III). The presence of two distinct
one-electron redox processes, close in potential (DE1/2 ¼ 140
mV), instead of a two-electron single wave, is in agreement with
there being two identical electroactive centers in the molecule
that can electronically communicate.24 This is the case of the
two Ru sites interacting through the conjugation of the bridging

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bppÀ ligands and the acetato bridge via the central Co3+ core.
The two electron reduction wave at À1.23 V is assigned to the
reduction of the terpyridine units of the Ru(II) centers. The
shoulder observed at À1.18 V indicates that the two terpyridine
ligands are not fully equivalent, which is in accordance with
there being weak electronic coupling between the two Ru
subunits.
The two oxidized forms of the complex, Ru2(II)Co(III) and
Ru2(III)Co(III) are stable in CH3CN, as tested by successive electrolysis at E ¼ 0.85 V and E ¼ 1.40 V, which consume, respectively, one and two electrons (Fig. S18†). Rotating disk electrode
experiments conrm the quantitative formation of these
species (Fig. S19†). The electrogenerated solutions have also
been analyzed by UV-visible (Fig. 2) and EPR spectroscopy
(Fig. 4, see below).
The three stable oxidation states, Ru2(II)Co(II), Ru2(II)Co(III)
and Ru2(III)Co(III), have distinct UV-visible signatures. The
initial orange Ru2(II)Co(II) solution exhibits two intense visible
bands at 366 and 500 nm (with a shoulder at 530 nm) and two
less intense shoulders at 620 and 710 nm. The oxidation of the
central Co(II) unit into Co(III) leads to a shi of the intense
visible bands to 396 and 473 nm (shoulder at 500 nm) and of the
two shoulders to 580 and 655 nm. A more pronounced color
change of the solution is observed when the two Ru(II) species
are oxidized into Ru(III), as indicated by the replacement of all
the previous visible bands by new ones at 390 (shoulder) and
545 nm. An increase in absorption is also observed between 600
and 1000 nm. As the more evident changes occur aer the
oxidation from Ru2(II)Co(III) to Ru2(III)Co(III), the intense visible
absorption bands originate from the ruthenium units.
Back electrolysis of the nal solution conducted at 0.35 V
(three electrons exchanged) restores the initial complex, Ru2(II)
Co(II), quantitatively (Fig. S20†). This demonstrates the perfect
stability of the different oxidation states of the trinuclear
compound, and the reversibility of the processes.
The cyclic voltammogram of Ru2Mn–OAc2 in CH3CN (Fig. 3
and S21†) also displays three successive one-electron reversible
oxidation waves at E1/2 ¼ 0.85 V (DE ¼ 60 mV), 0.96 V (DE ¼ 70
mV) and 1.47 V (DE ¼ 100 mV) at a scan rate of 50 mV sÀ1 and
a reversible two-electron terpyridine-centered reduction wave at
E1/2 ¼ À1.23 V (DE ¼ 80 mV). The two rst oxidation waves, very
close in potential (DE1/2 ¼ 110 mV), are thus assigned to

Fig. 3 (Left) Cyclic voltammogram of a 0.41 mM solution of Ru2Mn–
OAc2 in 0.1 M [(nBu4N)ClO4] in CH3CN, at a scan rate of 50 mV sÀ1.
(Right) UV-visible absorption spectra of a 0.41 mM solution of Ru2Mn–
OAc2 in 0.1 M [(nBu4N)ClO4] in CH3CN, in oxidation states, (black)
Ru2(II)Mn(II), (red) Ru2(III)Mn(II) and (blue) Ru2(III)Mn(III).

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the oxidation of the Ru sites, Ru2(II,III)/Ru2(II,II) and Ru2(III,III)/
Ru2(II,III) and the last one to the Mn central unit, Mn(III)/Mn(II).
The stability of the two oxidized states, Ru2(III)Mn(II) and
Ru2(III)Mn(III), has been evaluated by two successive electrolyses
at E ¼ 1.11 V and 1.69 V. The rst electrolysis at 1.11 V
consumed two electrons per molecule of initial complex and
lead to the quantitative formation of the Ru2(III)Mn(II) species.
An additional one electron oxidation carried out at 1.69 V leads
to the formation of the fully oxidized form, Ru2(III)Mn(III). At this
stage, the presence of an additional reversible process at E1/2 ¼
1.23 V with very small intensity should be pointed out, which is
probably related to minor decomposition of the complex by
decoordination of the Mn ion (less than 5%, Fig. S21†). Indeed,
the potential of this new process is similar to that of a RuN6
mononuclear complex such as in-[Ru(Hbpp)(trpy)(CH3CN)]2+.
Both electrolysis processes have been monitored by UV-vis and
X-band EPR spectroscopy (Fig. 3 and 4, respectively).
The UV-vis absorption spectrum of Ru2Mn–OAc2, with two
intense visible bands at 365 and 497 nm (with a shoulder at 530
nm) and two less intense shoulders at 620 and 710 nm, is nearly
identical to that of Ru2Co–OAc2 (see Fig. S9† for a comparison of
the spectra). These observations conrm that the visible
absorption bands originate mainly from the Ru units. Once the
two Ru(II) units have been oxidized, formation of the Ru2(III)
Mn(II) species leads to signicant changes with the replacement
of the initial visible bands by new ones at 400 (shoulder) and 553
nm and two shoulders at 710 and 764 nm. For the fully oxidized
solution, the oxidation of the central Mn(II) unit into Mn(III) leads
to minor changes, with a shi of the band at 553 nm to 538 nm
and a small increase of the absorption around 450 nm (Fig. 3).
The initial reduced state of the complex Ru2Mn–OAc2 is
restored almost quantitatively by a back electrolysis of the nal
solution at 0.35 V (Fig. S22†).
EPR properties
The X-band EPR spectra of the initial and electrochemically
oxidized solutions of Ru2Co–OAc2 have been recorded at low
temperature (13 K) (Fig. 4). The initial solution of Ru2Co–OAc2
shows an EPR signal characteristic of a Co(II) ion (d7) in the high
spin state (S ¼ 3/2),25 arising from the central Co(II) unit, as both
Ru units (Ru(II), d6) are diamagnetic and thus EPR silent. The

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analysis of high spin Co(II) centers is difficult because the zero
eld splitting (ZFS) energy is usually greater than the Zeeman
interaction, leading to spectra insensitive to the magnitude of
the axial term (D) of the ZFS. Consequently, only the real
g-values (greal) and the rhombicity (E/D ratio) of the system can
be extracted from the simulation of the EPR spectra. Simulation
of the experimental data provides the following spin-Hamiltonian parameters for the Ms ¼ |Æ1/2i: greal(x, y) ¼ 2.42, greal(z) ¼
2.31, and E/D ¼ 0.23.
Aer the one-electron oxidation into Ru2(II)Co(III), the solution becomes EPR silent, in accordance with the formation of
the low-spin Co(III) species (d6) (S ¼ 0).26
The EPR spectrum of the fully oxidized solution of Ru2(III)
Co(III) displays two features ascribable to an effective S ¼ 1 spin
state which results from a magnetic interaction between two S ¼
1/2 spin states located on the Ru(III) metals. The characteristic
rhombic signature of the low spin Ru(III) S ¼ 1/2 spin state (d5)
between 210 and 360 mT is retained in the spectrum.27,28 A weak
half-eld EPR line is also observed at 139 mT, which is consistent with the S ¼ 1 spin state induced by the magnetic interaction of two Ru(III) ions. EPR spectra with similar features have
been previously reported for magnetically coupled dinuclear
Ru(III)–Ru(III) complexes.28
The EPR signal of the initial Ru2Mn–OAc2 solution at 100 K
exhibits a 6-line signal characteristic of a high-spin Mn(II) ion
(3d5, S ¼ 5/2), related to the central Mn(II) unit of the complex
(Fig. 4).27 This signal fully disappears aer the oxidation of the
two diamagnetic Ru(II) units into Ru(III). Although an EPR signal
is expected for a magnetically coupled system involving two
Ru(III) (S ¼ 1/2) ions and one Mn(II) (S ¼ 5/2), no EPR signal was
detected regardless of temperature (from 13 K to 200 K),
presumably due to fast relaxation. The fully oxidized solution of
Ru2(III)Mn(III) is also EPR silent, which is in agreement with
there being magnetic coupling between the two Ru(III) (S ¼ 1/2)
and the Mn(III) (d4, S ¼ 2) ions, which must produce a integer
spin ground state. The low-intensity signals observed around
g ¼ 2 are attributed to some decomposition of the trinuclear
Ru2(III)Mn(III) (OAc)2 complex (less than 5%, as shown by electrochemistry) into “free” Mn(II) (six line feature centered at g ¼ 2.0,
339 mT) and the mononuclear [RuIII(bpp)(trpy)(CH3CN)]3+
complex (S ¼ 1/2) (the two features at a low magnetic eld
compared to the six line feature).

Magnetic properties

(Left) X-band EPR spectra recorded at 13 K of a 0.25 mM
solution of Ru2Co–OAc2 in 0.1 M [(nBu4N)ClO4] in CH3CN, in oxidation
states (a) Ru2(II)Co(II), (b) Ru2(II)Co(III) and (c) Ru2(III)Co(III) (black), and
corresponding simulated spectra (red). (Right) X-band EPR at 100 K of
a 0.41 mM solution of Ru2Mn–OAc2 in 0.1 M [(nBu4N)ClO4] in CH3CN,
in oxidation states (a) Ru2(II)Mn(II), (b) Ru2(III)Mn(II) and (c) Ru2(III)Mn(III).
Fig. 4

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Magnetic susceptibility measurements were performed under
a constant magnetic eld of 5000 Oe, in the temperature range
of 2–300 K. Variable eld reduced magnetization at 2 K was
performed for all complexes. Isoeld variable temperature
reduced magnetization measurements were also performed
under various elds. These experiments served as criteria of
purity, since this chemistry oen leads to the formation of small
impurities of Ru(0) particles that become very visible when the
magnetic properties are examined.
The constant eld and variable temperature magnetic
susceptibility results are represented in the form of cT vs. T
curves (where c is the molar paramagnetic susceptibility) (Fig. 5).
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The cT products for the complexes Ru2Co–Cl2 and Ru2Co–
OAc2 are, at 300 K, 3.44 and 2.96 cm3 K molÀ1, respectively,
which are signicantly higher than the expected values for spinonly Co(II) ions (calculated as 1.875 cm3 K molÀ1 for g ¼ 2.0 and
S ¼ 3/2). This product decreases with an increasing rate as the
temperature declines. Both observations are accounted for by
the fact that octahedral high spin Co(II) exhibits an orbital
angular momentum (L ¼ 1) in addition to the spin state (S ¼ 3/2).
This causes the observation of an effective g value that is much
higher than expected for a spin only system (here calculated as
2.51 and 2.71, respectively). The differences between both ions
are caused by their slightly differing coordination geometries,
as well as the varying ligand eld strength of some of the
donors.29 The depopulation of the J multiplets arising from the
spin–orbit coupling explains the decline of cT upon cooling.
The reduced magnetization curves measured at 2 K (Fig. S25†)
saturate for both compounds at values lower than expected for
an S ¼ 3/2 system. This is because at such a low temperature,
only the lowest J multiplet is populated. Isoeld magnetization
curves (at temperatures below 7 K and under several elds)
produce the same saturation values and show quasi-superposition of the various curves (Fig. S26†). This shows that the
anisotropy of these low energy states is very small. These results
are consistent with the above characterization and with the
interpretation of the EPR results.
At 300 K, the cT products for the complexes Ru2Mn–Cl2 and
Ru2Mn–OAc2 are almost identical; 4.43 and 4.45 cm3 K molÀ1,
respectively, which correspond to the expected spin-only value
for an isolated Mn(II) center with S ¼ 5/2 spin state and g ¼ 2.01
and 2.02, respectively, consistent with the EPR and electrochemical results. This value stays nearly constant until around 5
K, where a decline down to 3.98 and 4.04 cm3 K molÀ1 at 2 K,
respectively, is observed. The low temperature decline can be
due to a small value of ZFS, or weak antiferromagnetic intermolecular interactions. The variable eld reduced magnetization curves, measured at 2 K (Fig. S23†), and the isoeld
magnetization curves (Fig. S24†) are the result of Mn(II) saturating near 5 Bohr magneton, and this is consistent with the
above results. The fact that the isoeld lines superimpose with
each other for the various elds employed conrms that the
amount of ZFS is very small for both complexes.

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Fig. 6 DPV of (black) Ru2Co–(H2O)4, (red) Ru2Mn–(H2O)4, (blue) in-1
and (green) blank in a pH ¼ 7.0 (50 mM) phosphate buffer solution and
CF3CH2OH mixture (19 : 1).

Redox properties in aqueous solutions
The redox properties of the trinuclear complexes Ru2Co–(H2O)4
and Ru2Mn–(H2O)4 were investigated by means of CV and DPV
in a mixture of a pH ¼ 7.0 phosphate buffer (50 mM) and
CF3CH2OH (19 : 1), the use of the latter forced by the poor
solubility of the complexes in water. While very nicely dened
waves were observed in organic solvents for their acetato
counterparts as shown previously, in aqueous solution, the
waves are very wide (Fig. S28†). Nevertheless, the DPVs in Fig. 6
allow the observation of three faradaic processes situated at
approximately 0.6, 0.7 and 1.0 V for both Ru2Co–(H2O)4 and
Ru2Mn–(H2O)4, which are plotted together with the mononuclear aqua complex in-1 for comparative purposes. However,
the most important feature is the large electrocatalytic wave
displayed by these complexes, starting at 1.3 V for Ru2Co–
(H2O)4 and at around 1.5 V for Ru2Mn–(H2O)4 and in-1, which is
associated to water oxidation catalysis.
Water oxidation catalysis
Photochemically driven water oxidation catalysis has been
carried out through the photogeneration of [Ru(bpy)3]3+ as
a chemical oxidant using persulfate as a sacricial electron
acceptor (eqn (4)–(6)).
[RuII(bpy)3]2+ + hn / [RuII(bpy)3]2+*

(4)

[RuII(bpy)3]2+* + S2O82À / [RuIII(bpy)3]3+ + SO42À + SO4Àc
(5)
[RuII(bpy)3]2+ + SO4Àc / [RuIII(bpy)3]3+ + SO42À

Plots of cT vs. T (c is the molar paramagnetic susceptibility) for
complexes (blue) Ru2Co–Cl2, (red) Ru2Mn–Cl2, (green) Ru2Co–OAc2,
and (black) Ru2Mn–OAc2, under a constant magnetic field of 5000 Oe.
Fig. 5

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(6)

The oxygen generation prole as a function of time is presented in Fig. 7 for the systems Cat 50 mM/[Ru(bpy)3]2+ 0.5 mM/
Na2S2O8 20 mM/pH ¼ 7.0 (50 mM phosphate buffer), with
a total volume of 2.0 mL (H2O : CF3CH2OH ¼ 19 : 1) using a 300
W xenon lamp with a band pass lter of 440 nm, thermostated
at 298 K. Under these conditions, Ru2Co–(H2O)4 generates
a TON of 50 in about 10 minutes (TOFi ¼ 0.21 sÀ1) whereas
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Fig. 7 Photochemically induced oxidation of Ru2Co–(H2O)4 (black),
Ru2Mn–(H2O)4 (red) and in-1 (blue). Reaction conditions: [catalyst] ¼
50 mM, [[Ru(bpy)3](ClO4)2] ¼ 0.5 mM; [Na2S2O8] ¼ 20 mM; total volume
¼ 2 mL in a pH ¼ 7.0 (50 mM) phosphate buffer solution and CF3CH2OH mixture (19 : 1). A 300 W xenon lamp was used to illuminate
the sample through a band pass filter of 440 nm at 298 K. The O2 yields
based on persulfate are: 25%, 4% and <1%, respectively, for Ru2Co–
(H2O)4, Ru2Mn–(H2O)4 and in-1.

Ru2Mn–(H2O)4 only generates a TON of 8 and the mononuclear
complex in-1 does not generate any molecular oxygen within
this time frame. These results are consistent with the electrochemical analysis, as the potentials for catalysis for Ru2Mn–
(H2O)4 and in-1 are too high for efficient catalysis, given the fact
that the Ru(III)/Ru(II) potential for [Ru(bpy)3]2+/3+ is E1/2 ¼ 1.26 V.
In light of the good results obtained in the photoactivated
experiment, water oxidation catalysis with Ru2Co–(H2O)4, and
with the mononuclear in-1 for comparison, was also investigated using oxone (KHSO5) as a chemical oxidant. The oxygen
evolution proles as a function of time are presented in Fig. 8.
The Co containing complex generates 1.0 mmol of oxygen (0.41
mmol are subtracted due to the activity of the blank under the
same conditions) that correspond to a TON of about 13.
Labelling experiments using H218O were also carried out in
order to extract mechanistic information regarding the O–O
bond formation event when using oxone as a chemical oxidant.
The O2 36/34/32 isotope ratio was followed by on-line Mass
spectrometry. Two different degrees of H2O labelling were
employed, and the results obtained are shown in Fig. 8.
For discussion of the mechanism, we consider rst the
experiment with 97% H218O, and then test our hypothesis on the
data obtained from the experiment with 15% H218O. However,
before making a hypothesis on the mechanism of oxygen
evolution, it is paramount to obtain information on the labeling
state of the system at the moment of the oxygen evolving event.
(a). Raman experiments carried out under the same conditions as the water oxidation labelling experiments showed that
no 18O exchange occurs at all between water and oxone for at
least 10 h (Fig. S30†).30
(b). Substitution reactions of aqua ligands at low oxidation
states (II) for Ru and Co complexes occur rapidly. As the

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Fig. 8 (Top) Oxygen evolution profile using oxone (4.7 mM) as
a chemical oxidant for 40 mM Ru2Co–(H2O)4 (black), 80 mM in-1 (blue)
and a blank without catalyst (red), using a pH ¼ 7.0 (50 mM) phosphate
buffer up to a total volume of 2.0 mL (H2O : CF3CH2OH ¼ 19 : 1) in
a 298 K thermostated cell. The O2 yields based on oxone for Ru2Co–
(H2O)4 and in-1 are: 21% and <1%, respectively. (Bottom) Isotopic
oxygen generation profile monitored by on-line MS for Ru2Co–(H2O)4
using oxone in the same conditions. Bottom left using 97% H218O, and
bottom right using 15% H218O. Color code: black 32O2, red 34O2,
blue36O2.

precatalyst presents all acetate bridge ligands, we can assume
that all the aqua ligands are initially present as H218O.
(c). Peroxide oxidations of Ru-aqua polypyridyl complexes
occur through dehydrogenation pathways.31 Thus, oxone will
react with low oxidation states of Ru-aqua as follows (auxiliary
ligands not shown):
RuII–18OH2 + [HO–S(O)2(OO)]À / RuIV]18O + H2O
+ HSO4À

(7)

(d). Oxidation of rst row transition metals, particularly iron
and cobalt polyridylic complexes, with peroxides occurs
primarily via nucleophilic substitution.32–34
CoII–18OH2 + [HO–S(O)2(OO)]À / CoII–OO–S(O)2(OH)
+ H2O18
CoII–OO–S(O)2(OH) / CoIV]O + HSO4À

(8)
(9)

(e). Tautomeric equilibrium between the oxo-hydroxo
complexes, as described in eqn (10), would produce labelling
scrambling, as has been earlier proposed for related
complexes.35
CoIV(18O)(OH) # CoIV(O)(18OH)

(10)

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Scheme 2 Fast tautomeric equilibrium proposed for the active
species of the trinuclear Ru2Co–(H2O)4 complex just before the O–O
bond formation step. The bpp-ligand is represented by an arc and the
trpy ligands are omitted for clarity purposes. XO represents the oxidant
oxone (KHSO5). The blue oxygens are 16O and the red oxygens are 18O.

For the Ru2Co–(H2O)4 complex, this situation at the oxygen
evolving event is described in the top section of Scheme 2. This
assumes that for 100% H218O labeling at oxidation state II, all the
aqua groups are exchanged. For the Ru center reaching high
oxidation states with non-labeled oxone, [Ru(IV)]18O] will be
produced with 100% 18O labeling. However, for the cobalt center,
a mixed labeled [Co(IV)(16O)(18OH)] will be generated because of
the nucleophilic attack mechanism mentioned above.
At this point, the following conclusions are extrapolated
from the oxygen data shown in Fig. 8.
(1). The practically negligible amount of 36O2 in the 97%
18
H2 O experiment indicates the inexistence of intramolecular
pathways between the Ru]O and Co]O moieties. In addition,
it also precludes the existence of bimolecular pathways
involving only the Ru]O groups. On the other hand, potential
bimolecular pathways involving only Co centers are sterically
highly disfavorable.
(2). The mononuclear complex in-1 does not generate sufficient oxygen to be signicant under the applied conditions and
thus the inexistence of O2 coming independently from Ru-aqua/
Ru-oxo moieties is ruled out.
(3). Cis O–O coupling within the same Co metal center would
be compatible with the ratios of isotopic labelling obtained,
although this mechanism has been discarded based on DFT in
a number of examples.36,37
(4). The 97% labelling experiment (Fig. 8 and S31†) is
consistent with the existence of a very fast tautomeric equilibrium where species A and B exist in a 2 : 1 ratio respectively (see
Scheme 2). The origin of the higher stabilization of species A is
proposed to come from the higher hydrogen bond capacity of
isomer A with regard to that of B. Oxidation states of Ru and Co
are tentatively assigned from the apparent removal of 4/5 electrons from the oxidation state II,II,II, as judged from the electrocatalytic wave displayed by the complex. Assuming this
equilibrium, the reaction of oxone (XO) with A and B would
generate molecular oxygen with a ratio of isotopes of 34O2/32O2
¼ 2/1, which is what is found experimentally.
(5). Finally, changing the ratio of labelled water to 15% 18O
(Fig. 8 and S31†), will generate molecular oxygen with a ratio of
34
O2/32O2 ¼ 1/9. Experimentally, we obtained a ratio of 1/10,
which is in very good agreement with the proposed mechanism.

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Overall, the labelling experiments carried out together with
the rest of the electrochemical and spectroscopic properties are
in agreement with the presence of a water nucleophilic attack
(WNA) mechanism occurring at the Co site of the trinuclear
complex, with cooperative interaction of the two Ru sites via
electronic coupling through the bppÀ bridging ligand and via
neighboring hydrogen bonding.
It is worth mentioning that in our previous work with related
dinuclear Ru21 and Co3 complexes containing the Hbpp ligand,
namely {[M(H2O)(trpy)]2m-[(bpp)}3+ (M ¼ Ru or Co), the O–O
bond formation occurred via an intramolecular mechanism
(I2M).38 However, with the new trinuclear complexes described
here, given the new substantially different geometries and
electronic interactions between the metal centers, the mechanism changes to a WNA.

Conclusions
We have prepared and isolated a family of trinuclear complexes,
Ru2M–X2 (M]Co or Mn; X ¼ ClÀ or OAcÀ), where the central Co
or Mn atom, together with two bppÀ ligands, acts as a bridge
between the two external Ru moieties. These complexes have
been characterized in the solid state by X-ray diffraction analysis
and by magnetic measurements. In solution they have been
characterized by spectroscopic (EPR, UV-vis) and by electrochemical (CV, DPV) techniques. Overall, all these experiments
for the chlorido or acetato bridge complexes show the presence
of a relatively weak electronic coupling between the metal
centers, transmitted through the bridging ligands. In addition,
it is very interesting to see how the redox pattern radically
changes from the Mn complexes with regards to the Co
complexes manifesting the intrinsically different electronic
properties of the two transition metals. In aqueous acidic media
the trinuclear complexes revert to their Ru mononuclear counterparts and free Co(II) or Mn(II). However at pH ¼ 7.0 the
integrity of the complex is fully retained. Of particularly interest
is the heterotrinuclear Ru2Co–(H2O)4 complex, that is capable at
this pH of catalytically oxidizing water to molecular dioxygen
chemically, electrochemically and photochemically. Finally,
while a few detailed H218O labelling experiments have been
described for Ru complexes13,39–51 that allow the tracing of the
O–O bond formation step, this type of information is lacking for
rst row transition water oxidation catalysts. Our H218O labelling experiments constitute the rst example where the O–O
bond formation step has been elucidated based on labeling
experiments.

Acknowledgements
We thank MINECO (CTQ-2013–49075-R, SEV-2013-0319,
CTQ2014-52974-REDC,
ENE2014-52280-REDT,
CTQ201232247), the Feder funds and the EU COST actions CM1202 and
CM1205. L. M. thanks “ICIQ-Foundation” for a Ph.D. grant. S.
M. gratefully acknowledges “INSPIRE Faculty Award” by
Government of India (No. DST/INSPIRE Faculty Award/2012/
CH-72), and ISIRD research grant from IIT Kharagpur (project
code: SMR). C. E. C.'s acknowledges the MICINN for

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a postdoctoral grant. The authors wish to thank (for nancial
support) the LABEX ARCANE (ANR-11-LABX-0003-01) for the
project H2Photocat including the C. E. C.'s post-doctoral
fellowship. This work was also supported by ICMG FR 2067 and
COST CM1202 program (PERSPECT H2O). The Reseau National
de RPE interdisciplinaire TGE CNRS is also thanked for EPR
measurements and facilities.

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