“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic
Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To
access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/ic502603e

The Oxo-bridge Scenario Behind Single Site Water
Oxidation Catalysts
Isidoro López,a Somnath Maji,a J. Benet-Buchholza and Antoni Llobeta,b,*
a

Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans, 16, 43007 Tarragona,
Spain.
b

Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193
Barcelona, Spain.

KEYWORDS. Water oxidation, redox catalysis, Ru-oxo bridged complexes, transition metal
complexes.

1

ABSTRACT. High oxidation state decay of mononuclear complexes [RuTB(H2O)]2+, X2+,
(where B = 2,2’-bpy or bpy for X = 1; B = 5,5'-F2-bpy for X = 2; B = 6,6'-F2-bpy for X = 3; T =
2,2’:6’,2’’-terpyridine) oxidized with a large excess of Ce(IV) generates a manifold of
polynuclear oxo-bridged complexes. This include: a) dinuclear, [TB-RuIV-O-RuIV-(T)(O)OH2]2+
(1-dn4+), [TB-RuIII-O-RuIII-T(MeCN)2]4+ (1-dn-N4+), {[RuIII(trpy)(bpy)]2(µ-O)}4+, (1-dm4+); b)
trinuclear {[RuIII(trpy)(bpy)(µ-O)]2RuIV(trpy)(H2O)}(ClO4)56+ (1-tr6+), {[RuIII(trpy)(bpy)(µO)]2RuIV(pic)2}(ClO4)4 (1-tr-P4+; where P is the 2-pyridine carboxylate anion); and c)
tetranuclear,

[TB-RuIII-O-TRuIV(H2O)-O-TRuIV(H2O)-O-RuIII-TB]8+

TRuIV(AcO)-O-TRuIV(AcO)-O-RuIII-TB]6+

(1-tn-Ac6+)

and

(1-tn8+),

[TB-RuIII-O-

[TB-RuII-O-TRuIV(MeCN)-O-

TRuIV(MeCN)-O-RuII-TB]6+ (1-tn-N6+) complexes. These complexes have been structurally
characterized by single crystal X-ray diffraction analysis and their structural properties correlated
with their electronic structure. The dinuclear 1-dm4+ complex has been further characterized by
spectroscopic and electrochemical techniques. Addition of excess Ce(IV) to 1-dm4+ generates
dioxygen in a catalytic manner. However resonance Raman spectroscopy points out to the in situ
formation of 1-dn4+ as the active species.

2

1. Introduction.
The oxo-bridge metal unit is a ubiquitous motif in bioinorganic chemistry. There is a large
number of metalloproteins1-3 that contain the Fe-O-Fe core whereas the oxygen evolving
complex (OEC) of photosystem II (PSII) contains Mn-O-Mn units, within its CaMn4 cluster.
The latter has gained an increasing attention because it performs the key reaction that needs to be
fully understood and mastered to be able to come up with the development of commercial
devices, for the efficient conversion of sunlight into chemical fuels such as H2 or CH3OH.
To mimic the functional properties of OEC, many polynuclear µ-oxo metal complexes with
high oxidation states have been synthesized. The blue dimer was the first reported molecularly
well characterized water oxidation catalyst (WOC) and contains a RuIII-O-RuIII unit. Other
polynuclear WO catalysts recently reported include complexes with rigid aromatic organic
bridging ligands instead of the oxo-bridge4-8 as well as a family of polyoxometalate complexes.912

On the other hand, the recent discovery of single site WOCs5, 13, 14 has generated a large body
of complexes that are proposed to remain as mononuclear species during the catalytic cycle. The
successive pathways that lead to the evolution of dioxygen in complexes related to
[RuTB(H2O)]2+, X2+, (where B = 2,2’-bpy or bpy for X = 1; B = 5,5'-F2-bpy for X = 2; B = 6,6'F2-bpy for X = 3; T = 2,2’:6’,2’’-terpyridine) have been extensively studied13, 15 and the accepted
mechanism considers the key O-O bond forming step to be a water nucleophilic attack on a RuVO moiety. Although this description is now widely accepted16, 17 the corresponding deactivation
pathways associated with this type of catalysts have not been reported. In this regard only the
potential dissociation of bpy and subsequent oxidation to 2,2’-bipyridine N, N-dioxide has been
pointed18 out, as a hypothetical decomposition path.

3

Recently our group has found19 that mononuclear complexes partially transform to dinuclear
oxo bridge structures of general formula [TB-RuIV(µ-O)RuIVT(O)(H2O)]4+ (X-dn4+; X = 1, 2 or
3), when high oxidation states are reached. The generation of these dinuclear complexes,
involves necessarily the release of bpy from the first coordination sphere of the initial complex.
The formed dinuclear complexes are active and rugged water oxidation catalysts and its presence
complements the accepted mechanism, showing that the system is much more complex than
originally described. Furthermore, the formation of these µ-O structures uncovers a tendency for
the production of oxo-bridge oligomers by mononuclear Ru complexes in high oxidation states
as it happens with Fe2, 20 and Cu.20-22
In the present report we examine the interconnection between µ-oxo species generated after the
chemical and electrochemical oxidation of mononuclear complexes related to [TB-RuII(H2O)]2+,
12+, and discuss structural and electronic parameters.

4

2. Experimental section.
Materials. All reagents used in the present work were obtained from Aldrich Chemical Co.
and were used without further purification. RuCl3·3H2O was supplied by Alfa Aesar and was
used as received. Trifluoromethanesulfonic acid (HOTf) was pursued from CYMIT. Reagentgrade organic solvents were obtained from SDS and high-purity deionized water was obtained by
passing distilled water through a nanopure Milli-Q water purification system.
Preparations.
[Ru(trpy)(bpy)(H2O)](PF6)2 (12+),23 [Ru(trpy)(5,5’-F2-bpy)(H2O)](PF6)2 (22+),24 [Ru(trpy)(6,6’F2-bpy)(H2O)](PF6)2 (32+),24 [(trpy)(bpy)RuIV(µ-O)RuIV(trpy)(O)(H2O)](ClO4)4 (1-dn4+)19 and
[(trpy)(5,5’-F2-bpy)RuIV(µ-O)RuIV(trpy)(O)(H2O)] (ClO4)4 (2-dn4+)19 were prepared as described
in the literature.
{[Ru(trpy)(bpy)] 2(µ-O)}(ClO4)4·7H2O (1-dm4+). [Ru(trpy)(bpy)(H2O)](PF6)2 (101.4 mg, 0.127
mmols) was dissolved in 0.1 M HOTf (127 mL). A amount of 100 equivalents of
(NH4)2Ce(NO3)6 (7.10 g, 12.7 mmols) dissolved in the minimum amount of 0.1 M HOTf was
added to the previous stirred solution. The solution is left stirred for approximately 1 week and
UV-vis monitored periodically. When the band at 688 nm seemed to achieve a maximum value,
a saturated aqueous solution of NaClO4 (7 mL) was added. A dark green solid precipitated which
was filtered and washed with some drops of cold water. The solid was collected and dried under
vacuum overnight. Yield: 39.9 mg (45%). Anal. Calcd. for C50H50Cl4N10O23Ru2: C, 39.48; H,
3.45; N, 9.21. Found: C, 39.24; H, 3.24; N, 9.07.

5

Single crystal preparations.
[(trpy)(6,6’-F2-bpy)RuIV(µ-O)RuIV(trpy)(O)(H2O)](ClO4)4·3H2O (3-dn4+). Single crystals of 3dn4+ could be grown at RT one week after the addition of some drops of a sat. aq. NaClO4
solution to a catalytic solution of 32+ in 0.1 M HOTf when oxygen evolution had finished.
[(trpy)(bpy)RuIII(µ-O)RuIII(trpy)(CH3CN)2](PF6)4·H2O·CH3CN

(1-dn-N4+),

[(trpy)(6,6’-F2-

bpy)RuIII(µ-O)RuIII(trpy)(CH3CN)2](PF6)4·CH3CN (3-dn-N4+). 1-dn4+ or 3-dn4+was dissolved in
CH3CN and approximately 10 µL of a sat. aq. solution of NH4PF6 were added. Crystals were
grown by slow vapor diffusion of Et2O into the solutions.
{[RuIII(trpy)(bpy)] 2(µ-O)}(ClO4)4·4H2O (1-dm4+). Single crystals could be obtained at RT 2
days after the addition of some drops of a sat. aq. NaClO4 solution to concentrated solutions of 1dm4+ in 0.1 M HOTf.
{[RuIII.5(trpy)(bpy)] 2(µ-O)}(ClO4)4·3/2H2O (1-dm5+). Single crystals of 1-dm5+ could be grown
at RT 5 days after the addition of some drops of a sat. aq. NaClO4 solution to a catalytic solution
of 1-dm4+ in 0.1 M HOTf when oxygen evolution had finished.
{[RuIII(trpy)(bpy)(µ-O)]2RuIV(trpy)(H2O)}(ClO4)5(PF6)·2H2O (1-tr6+). Single crystals for this
complex were grown at RT 3 days after the addition of acetone to a solution of 1-dn4+ in 0.1 M
HOTf containing some drops of a sat. aq. solution of NH4PF6. The ratio acetone/water was
approximately 1:1.
{[RuIII(trpy)(bpy)(µ-O)]2RuIV(pic)2}(ClO4)4 (1-tr-P4+). A amount of 10 equivalents of picolinic
acid was added to a catalytic solution of 12+ in 0.1 M HOTf when oxygen evolution had finished.
The formation of 1-tr-P4+ was tracked by the increase of a band at 688 nm in the UV-vis
spectrum. Single crystals could be precipitated at RT 5 days after the addition of some drops of a
sat. aq. NaClO4 solution to the above solution when the increase of the band was negligible.

6

{[(trpy)(5,5’-F2-bpy)RuIII(µ-O)RuIV(trpy)(CH3COO)]2(µ-O)}(ClO4)6·2H2O

(2-tn-Ac6+),

{[(trpy)(bpy)RuII(µ-O)RuIV(trpy)(CH3CN)] 2(µ-O)}(PF6)6·xCH3CN (1-tn-N6+), and {[(trpy)(6,6’F2bpy)RuII(µ-O)RuIV(trpy)(CH3CN)] 2(µ-O)}(PF6)4(ClO4)2·3/2CH3CN (3-tn-N6+). Single crystals
for all these complexes could be obtained by the same procedure. The corresponding dinuclear
compounds (1-dn4+, 2-dn4+ or 3-dn4+) were dissolved in CH3CN and approximately 10 µL of a
sat. aq. solution of NH4PF6 were added. Crystals were grown by slow vapor diffusion of Et2O
into the solutions.
Equipment and measurements. UV/Vis spectroscopy was performed on a Cary 50 (Varian)
UV/Vis spectrophotometer in 1 cm or 0.2 cm when indicated quartz cuvettes. Cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performed on an
IJ-Cambria CHI-660 or a Bio-Logic SP-150 potentiostat using a three-electrode cell. Typical CV
experiments were carried out at a scan rate of 100 mV s-1. DPV experiments were carried out
with the parameters: Pulses Height = 50 mV, Pulses Width = 50 ms, Step Height = 4 mV and
Step Time = 200 ms. A glassy carbon electrode (3 mm diameter) was used as working electrode,
platinum wire as auxiliary electrode, and SSCE as a reference electrode. Working electrodes
were polished with 0.05 micron alumina paste, and rinsed with distilled water and acetone
followed by blow-drying before each measurement. All cyclic voltammograms presented in this
work were recorded in the absence of light and inside a Faradaic cage. The electrochemical
experiments were carried out in 0.1 M CF3SO3H (pH 1.0). E1/2 values reported in this work were
estimated from CV experiments as the average of the oxidative and reductive peak potentials
(Ep,a + Ep,c)/2 or taken as E(Imax) from DPV measurements.
A 400 MHz Bruker Avance II spectrometer and a Bruker Avance 500 MHz were used to carry
out NMR spectroscopy at room temperature. Samples were run in 0.1 M DOTf or CD3CN with

7

internal references (residual protons). Elemental analysis was performed using an EA-1108,
CHNS-O elemental analyzer from Fisons Instruments.
Samples for resonance Raman spectroscopy were prepared typically by mixing a 1 mM
solution of the starting complex with the desired amount of (NH4)2Ce(NO3)6 and transferring 100
µL of the reaction solution to a aluminium crucible and subsequently frozen at appropriate times
in liquid N2. Then, the crucible was placed into a Linkam THMS 600 temperature controlled
cryo stage to keep the temperature at -12°C. The rR spectrum was acquired using a Renishaw
inVia Reflex RAMAN confocal microscope (Gloucestershire, UK), equipped with an Ar-ion
laser at 514 nm and a Peltier-cooled CCD detector (-70°C) coupled to a Leica DM-2500
microscope. Calibration was carried out daily by recording the Raman spectrum of an internal Si
standard. Rayleigh scattered light was appropriately rejected by using edge-type filters.
Spectrums were recorded with the accumulation of 5 scans with a 20 s scan time each one. A 10x
working distance microscope objective was used to focus 50% of the laser power (25 mW) onto
the sample.
On-line manometric O2 measurements were carried out on a Testo 521 differential pressure
manometer with an operating range of 1-100 hPa and accuracy within 0.5 % of the measurement.
The manometer was coupled to thermostated reaction vessels for dynamic monitoring of the
headspace pressure above each reaction. The manometer’s secondary ports were connected to
thermostated reaction vessels containing the same solvents and headspace volumes as the sample
vials. A typical experiment consists in the addition of 100 equivalents of (NH4)2Ce(NO3)6
previously dissolved in 100 µL of 0.1 M HOTf upon a solution of the catalyst in 1.850 mL of the
same solvent containing the suitable amount of complex to yield 1 mM final concentration. This
combination is termed catalytic solution.

8

Single-Crystal X-Ray Structure Determination. All measured crystals were prepared under
inert conditions immersed in perfluoropolyether as the protecting oil for manipulation.
Data collection. Crystal structure determination was carried out using a Apex DUO Kappa 4axis goniometer equipped with an APPEX 2 4K CCD area detector, a Microfocus Source E025
IuS using MoKα radiation, Quazar MX multilayer Optics as monochromator and an Oxford
Cryosystems low temperature device Cryostream 700 plus (T = -173 °C). Full-sphere data
collection was used with ω and ϕ scans. Programs used: Data collection APEX-2,25 data
reduction Bruker Saint V/.60A.26
Structure solution and refinement. Crystal structure solution was achieved using direct
methods as implemented in SHELXTL27 and visualized using the program XP. Missing atoms
were subsequently located from difference Fourier synthesis and added to the atom list. Leastsquares refinement on F2 using all measured intensities was carried out using the program
SHELXTL. All non hydrogen atoms were refined including anisotropic displacement
parameters. In the case of 1-tn-N6+ the program SQUEEZE28 was applied in order to avoid the
highly disordered solvent molecules leading to a refined model with a R1 value of 6.34 % in
which all the solvent molecules were removed. The crystals obtained for 1-tn-N6+ were
diffracting extremely weak so that only a completeness of 93.6 % could be reached at a
resolution of sin(theta/lambda) = 0.6. Compound 1-dm4+ crystallized as a two domain crystal
(ratio 73:27). The collected data were processed with TWINABS taking in account overlapping
reflections.

9

3. Results and discussion.
3.1. Synthesis.
In a previous report19 we have demonstrate that addition of an excess of Ce(IV) to the water
oxidation catalyst [TB-RuII(H2O)]2+, 12+, generates oxygen and that during turnover the bpy
ligand slowly falls off the first coordination sphere and generates [T-RuVI(O)2(H2O)]2+. The latter
in turn reacts with 12+, to form the dinuclear complex, 1-dn4+, as indicated below and in Scheme
1,
[TB-RuII-OH2]2+ (12+) + [T-RuVI-(O)2OH2]2+
[TB-RuIV-O-RuIV-(T)(O)OH2]2+ (1-dn4+) + H2O

(1)

One electron oxidation of 1-dn4+ produces a dinuclear complex in oxidation state IV,V that is
associated with a large electrocatalytic wave responsible for water oxidation. On the other hand
the 1-dn4+ complex is also a very strong oxidative reagent specially towards organic substrates.
For instance, addition of MeCN produces the immediate reduction of the dimer to lower
oxidation states III,III followed by solvent coordination. The latter is indicated in the equation
below,
[TB-RuIII-O-RuIII-T(OH2)2]4+ + 2 MeCN ->
[TB-RuIII-O-RuIII-T(MeCN)2]4+ (1-dn-N4+) + 2H2O

(2)

and maintaining the original Ru-O-Ru backbone. In an analogous manner, addition of
ether/MeCN produces tetranuclear complexes in different oxidation states. As in the previous
case, the addition of organic compounds give rise to the immediate reduction of the initial
dinuclear complex, X-dn4+, followed by an oligomerization process generating,

10

We have also found qualitative evidence that the dinuclear complex 1-dn4+, could be obtained
electrochemically after an applied potential of 1.2 V overnight from a solution of 12+. This
indicates the existence of multiple routes to prepare the dinuclear complexes from their
respective mononuclear ones and is in agreement with Ce(IV) acting only as an outer sphere
electron transfer agent.

[TB-RuIII-O-TRuIV(H2O)-O-TRuIV(H2O)-O-RuIII-TB]8+ (1-tn8+) + 2 AcO-

->

[TB-RuIII-O-TRuIV(AcO)-O-TRuIV(AcO)-O-RuIII-TB]6+ (1-tn-Ac6+) + 2H2O

(3)

where the acetate ligand coordinates in a monodentate terminal fashion. The origin of the
acetato ligand is not clear but might come from the hydrolysis of MeCN catalyzed by Ru
complexes as we and others have shown previously.29,30
Further reduction of 1-tn8+ and solvent coordination yields the tetranuclear complexes, X-tnN6+, as indicated below,
[TB-RuIII-O-TRuIV(H2O)-O-TRuIV(H2O)-O-RuIII-TB]8+ (1-tn8+) + 2 MeCN + 2e- ->
[TB-RuII-O-TRuIV(MeCN)-O-TRuIV(MeCN)-O-RuII-TB]6+ (1-tn-N6+) + 2H2O

(4)

Addition of acetone to the initial dinuclear complex, 1-dn4+, produces an immediate reduction
followed by the oligomerization process that in this case generates the trinuclear complex , [TBRuIII-O-TRuIV(H2O)-O-RuIII-TB]6+, 1-tr6+, where the Ru-aqua ligands is maintained thanks to
the weak coordination capacity of acetone. In an analogous manner, addition of picolinic acid to
a solution of 1-dn4+ generates the formation of a trinuclear complex 1-tr-P6+, with similar
structure as 1-tr6+, but where the [T-Ru-(H2O)-O] moiety has been substituted by the [P2-Ru-O]
group.

11

Finally, in the presence of bpy but in the absence of organic substrates the IV,IV oxidation
state slowly decays (about 1 week) to lower oxidation states giving complexes such as the dimer,
[TB-RuIII-O-RuIII-TB]4+, 1-dm4+, in 85 % yield, whose formation can be monitored by UV-vis
spectroscopy (see sup. inf.) thanks to the presence of a highly characteristic band at 688 nm as
depicted in Figure 3.
All the above synthetic results combined point out the existence of an oligomerization process
where the [T-Ru-(H2O)-O] moiety acts as a repeating unit whereas the [TB-Ru] moiety acts as a
stopper to the polymerization process, that is,
n [T-RuIV-(O)(OH2)2]2+

->

[T-RuIV-(O)OH2]n2n+

+ n H2O

(5)

2 [TB-RuIII-OH2]3+ + [T-RuIV-(O)OH2]n2n+ ->
{TB-Ru-[O-TRu(H2O)]n-O-Ru-TB}(2n+4)+ + 2H+ + H2O (6)
Depending on time and solubility oligomers with a different number of repeating units are
obtained.

12

Color codes for formal Ru oxidation states: VI, fuchsia; IV, blue; III, green; II, orange.

13

Scheme 1. Synthetic strategy and complex nomenclature. X=1 for unsubstituted bpy, X = 2 for 5,5’-F2-bpy, and X = 3 for 6,6’-F2-bpy.

3.2. RuORu Structure and electronic properties.
The crystal structure of all the Ru oxo bridged complexes discussed in the present work are
displayed in Figure 1. In all cases the metal center exhibits a distorted octahedral coordination
and the most interesting features of these complexes are their Ru-O-Ru bond distances and
angles that are summarized in Table 1 together with related oxo-bridged Ru complexes
previously reported in the literature.
The RuORu bonding parameters depend on the oxidation state as well as on the degree of
electronic coupling between the metal centers through the oxygen bridging atom. A qualitative
MO diagram for π -system of the RuORu type of complexes31, 32 is shown in Figure 2 and sheds
some light into the orbitals involved in this type of bonding for the RuORu moiety

14

1-tr-P6+

1-tr6+

3-tn-N6+

3-dn-N4+

F, green. H atoms are not shown except for monodentate ligands.

15

Figure 1. Ortep plot (ellipsoid drawn at 50 % probability) of the X-ray structures. Color codes: Ru, cyan; O, Red; N, Blue; C, black;

3-dn4+

1-dm4+

Table 1. Comparison of important bond distances (Å) and angles (deg) for oxo-bridge
polynuclear metal complexes.
Complexa

Ru(2)O(2)

Ru(2)O(1)

Ru(1)O(1)

Ru(1)-ORu(2)

Ref.

1-dn4+

1.747(3)b

1.832(3)

1.848(3)

175.43(18)

Tw.d

2-dn4+

1.738(4)b

1.830(3)

1.855(3)

171.5(2)

Tw.

3-dn4+

1.731(3)b

1.833(3)

1.850(3)

172.63(16)

Tw.

{[(OEP)ClRuIV]2O}c

1.793(2)

1.793(2)

180

36

{[(OEP)(OH)RuIV]2O}c

1.847(13)

1.847(13)

180

37

{[Cl5RuIV]2O}4-c

1.80

1.80

180

38

1-dm5+

1.846(2)

1.848(2)

169.90(13)

Tw.

[(bpy)2(H2O)RuIIIORuIV(OH)(bpy)2]4+

1.847(12)

1.823(12)

170.0(7)

35

[(bpy)2(Cl)RuIIIORuIV(Cl)(bpy)2]4+

1.845(9)

1.805(9)

170.7(5)

35

[(TACN)RuIII(O)(CH3CO2)2RuIV(TACN)]3+

1.849(5)

1.837(5)

130.1(3)

33

1-dn-N4+e

1.864(4)f

1.865(4)

169.9(2)

Tw.

3-dn-N4+

1.912(5)f

1.891(5)

166.1(3)

Tw.

1-dm4+

1.8810(12)

1.8819(12)

164.31(7)

Tw.

{[RuIII(trpy)(C2O4)]2(µ-O)}

1.846(8)

1.841(8)

148.5(4)

51

{[RuIII(TACN)(acac)]2(µ-O)}2+

1.913(1)

1.913(1)

180.0(1)

47

{[RuIII(bpy)2(NH3)]2(µ-O)}4+

1.894(2)

1.894(2)

158.2(4)

52

{[RuIII(bpy)2(H2O)]2(µ-O)}4+

1.869(1)

1.869(1)

165.4(3)

53

{[RuIII(bpy)2(NO2)]2(µ-O)}2+

1.876(6)

1.890(7)

157.2(3)

54

{[RuIII(TACN)]2(µ-O)(µ-CH3CO2)2}2+

1.884(2)

1.884(2)

119.7(2)

33

{[RuIII(tpm)]2(µ-O)(µ-O2P(O)(OH))2}

1.868(2)

1.868(2)

124.7(4)

32

RuIV-O1-RuIV

RuIII-O1-RuIV

RuIII-O1-RuIII

16

MIII-O1-RuIV-O-MIII, g,h
1-tr-P4+, l

1.805(5),
1.799(5)i

1.872(5),
1.875(5)j

173.5(3),
176.6(3)k

Tw.

1-tr6+

1.821(3)i

1.881(3)j

166.0(2)k

Tw.

{[(NH3)5RuIII(µ-O)]2RuIV(en)2}6+

1.850(4)i

1.891(4)j

177.2(4)k

55

{[(BuNH2)((DPG)BF2)2FeIII(µO)]2RuIV(TPP’)}l

1.796(8),
1.801(8)i

1.786(8),
1.788(8)j

174.8(5),
175.1(5)k

41

{[(Salmah)FeIII(µ-O)]2RuIV(TPP)}

1.866(6)i

1.848(6)j

155.2(5)k

56

Ru1II-O1-Ru2IV-O2-RuIV-O-RuII, g
1-tn-N6+

1.8496(3)

1.839(2)

1.883(2)

169.67(15)

Tw.

2-tn-Ac6+

1.8364(3)

1.823(2)

1.865(3)

169.44(16)

Tw.

3-tn-N6+

1.8378(8)

1.829(6)

1.892(6)

168.10(4)

Tw.

a

Ligand abbreviation: OEP = octaethylporphinate, TACN = 1, 4, 7-trimethyl-1, 4, 7-triazacyclononane, acac =
pentane-2,4-dionate, en = ethylenediamine, BuNH2 = n-butylamine, (DPG)BF2 = (difluoroboryl)diphenylglyoximate, TPP’ = tetrakis(4-methoxyphenylporphyrinate), Salmah = N,N’-(4-methyl-4-azaheptane-1,7diyl)bis(salicylaldiminaate), TPP = 5,10,15,20-tetraphenylpophyrinate. b Ru(2) corresponds to Ru bound to the oxo
terminal ligand which is labeled as O(2). c High symmetry complexes with Ru(1)-Obridge = Ru(2)-Obridge. dThis work.
e
The bipyridine and the acetonitrile ligands are disordered interchanging its positions with a ratio of 86:14 which
means that Ru(1) and Ru(2) also do it. fRu(2) corresponds to Ru bound to two CH3CN ligands. gUsually
centrosymmetric complexes hM(III) = Ru(III) in 1-tr-P4+ and 1-tr6+ and M(III)=Fe(III) in the remaining complexes.
i
Ru(2) corresponds to the central Ru(IV) atom. jRu(1) corresponds to the outer M(III) atoms. kRu(IV)-O-M(III)
angle. lThe complex is non-centrosymmetric, so that a couple of distances or angles are showed in each field.

17

Here the Ru-O bond is taken as the z axis and thus the dxz and dyz orbitals from the metal atoms
mix with two π-type p orbitals from the oxo brigding atom producing three sets of bridge-based
orbitals. The first set consists of bonding orbitals (π1b, π2b) which have a large pO character. The
next set contains two non-bonding or slightly antibonding orbitals (π1nb, π2nb) which have a large
dπRu character. Finally, the third set comprises two antibonding orbitals (π1*, π2*) which have
also a large dπRu character. For complexes with a linear geometry, that is with a RuORu angle of
180° π1* and π2* are degenerate. However when the RuORu angles are smaller than 180°, the
degeneracy is lost and their energy difference is a function of the RuORu angle.
As a consequence of this, the electronic configuration of the dinuclear oxo bridge d5 Ru(III)
complexes will be a function of the RuORu angle since the π* orbitals are now populated. If the
energy gap between π1* and π2* is sufficiently large then the electronic configuration is (π1b)2
(π2b)2 (π1nb)2 (π2nb)2 (π1*)2, and thus the complexes will be diamagnetic. This is actually the case
for {[RuIII(TACN)]2(µ-O)(µ-CH3CO2)2}2+33 and {[RuIII(tpm)]2(µ-O)(µ-O2P(O)(OH))2}32 with
RuORu angles of 120° and 125° respectively. In contrast, dinuclear oxo bridge d5 Ru(III)
complexes with RuORu angles close to 180° display a paramagnetic behavior due either the
degeneracy of π1* and π2* or because they are very close in energy and thus both will be
populated at RT.

18

Figure 2. Qualitative MO diagram for the π system of Ru-O-Ru complexes. ∆E depends on the
angle of the moiety, the smaller the angle the larger the energetic separation. The electronic
configuration illustrates the case of a d5-d5 Ru(III)-O-Ru(III) complex with a linear angle (180°).

For the case of 1-dm4+, the Ru-O-Ru angle is 164° and thus has a paramagnetic behavior at RT
that is manifested by the broad range of the chemical shift (-30 to +30 ppm) where the
resonances are observed in the 1H-NMR spectrum (see sup inf). A similar shift has been
observed for related RuIII-O-RuIII complexes.34 The number of signals agrees with two
symmetric [Ru(trpy)(bpy)] halves manifesting the existence of symmetry in solution.
For the one electron oxidized complex, 1-dm5+, the Ru-O distance is shortened by
approximately 0.035 Å, in agreement with a higher bond order for Ru-O because of the removal
of an electron from the antibonding orbital π2* if compared to 1-dm4+. In addition the two Ru-O

19

bonds in 1-dm5+ are almost identical suggesting a high degree of electronic coupling between the
two Ru metal centers through the oxo bridge.
The dinuclear complexes 1-, 2- and 3-dn4+ have two radically different sites; while one of
them contains a [TB-RuIV] moiety the other one is a [TRuIV(O)(H2O)]. This is manifested in the
non-symmetric nature of the Ru-O-Ru bonds as can be observed in Table 1. The same effect has
been also found in the III,IV form of the blue dimer35.
This is in sharp contrast with previous reported36-38 symmetric complexes where the RuIV-ORuIV bonds are practically identical (Table 1). Finally in the crystal structure of 3-dn4+ we find
the shortest Ru-O terminal bond ever reported.39
The RuIV-O-RuIV angles are slightly bent with respect to the ideal 180˚ found in previously
reported complexes, because the steric constraint due the proximity of the bpy ligand to the aquo
group. In this way the less sterically demanding bpy in 1-dn4+ generates a RuORu angle of
175.5˚, whereas in the more sterically demanding Fluoro-bpys are 171.5o in 2-dn4+ and 172.6o in
3-dn4+.
A related structural motif is found for complexes 1-dn-N4+ and 3-dn-N4+ (Figure 1 and Supp
Inf. and Table 1) where the aqua group have been replaced by MeCN and the Ru metal centers
are at oxidation state III. As a consequence of this, the Ru-O bond distances are slightly
elongated. (See Table 1).35
Two trinuclear complexes with a RuIII-O-RuIV-O-RuIII backbone have been obtained, 1-tr6+
and 1-tr-P4+, (see Figure 1) and whose structures are reminiscent of Ruthenium Red,40
{[(NH3)5RuIII(µ-O)]2[µ-RuIV(NH3)4]}6+. There are only a few crystallographically characterized
trinuclear Ru-oxo-bridged complexes including some heterotrinuclear ones,41 where the Ru is
situated at the central position in oxidation state IV.(See Table 1)

20

In 1-tr6+ the central Ru atom adopts a trans dioxo arrangement and bears an aquo ligand. The
RuIV-O and the RuIII-O distances are significantly different suggesting a weak coupling among
metallic atoms. (Table 1) The trans-dioxo nature of the central Ru together with the presence of
the aqua ligand attached to the same metal center converts it into a potentially powerful WOC.19
The main structural parameters for 1-tr-P4+ are also collected in Table 1. As in the previous
case the RuIV-O distance is considerably shorter than that of RuIII-O (∆d = 0.078 Å) suggesting
again a weak electronic coupling. It is worth mentioning that the RuIV-OCarboxylate bond (2.034 Å)
is shorter than that reported42-44 for mononuclear picolinate Ru(II) complexes (2.085-2.102 Å)
and similar to that of a described45 in a Ru(IV) piridinecarboxylate compound (2.068(3) Å),
supporting the oxidation state assignment.
Tetranuclear complexes 1-tn-N6+ and 3-tn-N6+ with the Ru1II-O1-Ru2IV-O2-Ru2’IV-O1’-Ru1’II
backbone have also been obtained and their Orteps are presented in Figure 1 and the sup inf,
whereas selected bond distances and angles are displayed in Table 1.
These complexes contain an inversion center, “i”, situated at the central O2 atom. For complex
1-tn-N6+ the Ru1-O distance is 1.88 Å that is significantly longer than the Ru2-O distances that
are 1.85 and 1.84 Å. This Ru-O bonding distances advocate for a [(RuII-O-RuIV)2-(µ-O)]
assignment rather than a [(RuIII-O-RuIII)2-(µ-O)] which would also be in agreement with the
overall charge of the cationic part of the molecule. A similar analysis can be carried out for 3-tnN6+. However the two electron oxidized complex 3-tn-Ac6+, where in addition the MeCN ligands
have been substituted by acetate acting as a monodentate ligand, have similar Ru2-O distances
but slightly shorter Ru1-O, and thus support a [(RuIII-O-RuIV)2-(µ-O)] assignment. In turn this
also support the [(RuII-O-RuIV)2-(µ-O)] assignment for 1-tn6+ and 3-tn6+ as opposed to

21

intermediate oxidation state options such as [(RuII.5-O-RuIII.5)2-(µ-O)], although further
spectroscopic analysis is needed for complete oxidation state characterization.
3.3. The chemistry related to 1-dm4+.
As mentioned in the synthetic section 1-dm4+ can be obtained in high yield by adding 100
equivalents of Ce(IV) to a 0.1M triflic acid aqueous solution containing 1 equivalent of 12+. As
reported earlier19 this generates a system containing two parallel catalytic systems based on the
oxidation of 12+ and the formation of 1-dn4+ arising from bpy loss from the former, as indicated
in equation 1. Once Ce(IV) is depleted the two catalytic cycles stop generating species at high
oxidation states such as [trpy-bpy-RuIV=O]2+ and 1-dn4+. These species are not stable in acidic
solution and decay slowly to lower oxidation state species in the presence of free bpy. Over a
period of one week this mix decays mainly to the formation of 1-dm4+ in 85 % yield judging
from UV-vis.
Complex 1-dm4+, has been characterized in solution by UV-vis and NMR spectroscopy and in
the solid state by X-ray diffraction analysis as shown in Section 3.2. In addition the complex has
been characterized electrochemically by means of CV and DPV and its reactivity tested upon
addition of a large excess of Ce(IV).
The UV-vis spectrum of 1-dm4+ in a 0.1 M trilfic acid solution is displayed in Figure 3. The
most interesting feature of this spectrum is the band at 688 nm typical of RuIII-O-RuIII type of
complexes46,

47

which is associated with the overlapping of MLCT and bridge-based

transitions.48, 49 Interestingly the oxidation of the mononuclear complexes 12+ and 22+ with only 2
or 3 equivalent of Ce(IV) also produces this band manifesting the tendency toward oxo-bridge
formation of mononuclear systems. The 1H-NMR at RT for the paramagnetic complex 1-dm4+
was also registered in 0.1 M CF3SO3D in D2O. The spectrum shows resonances in a very wide

22

range due to the paramagnetic shift exerted by the Ru(III) d5 ions. A DOSY experiment was also
carried out that nicely reflects that the hydrodynamic radius, according to the Stokes-Einstein
equation is 1.9 times larger than for [Ru(trpy)(bpy)(CH3CN)]2+, used as a mononuclear standard.

Figure 3. UV-vis spectrum of 1-dm4+ in 0.1 M HOTf.
The CV of 1-dm4+ in 0.1 M trilfic acid is depicted in Figure 4. The initial anodic scan from the
open circuit potential up to 1.40 V shows a reversible wave at E1/2 = 1.08 V (∆E = 55 mV). This
electrochemical process is attributed to the one electron oxidation in eq. 1, according to
previously reported46, 47 related complexes.
RuIII-O-RuIII - 1e- → RuIII-O-RuIV

E1/2 = 1.08 V

(7)

After a subsequent cathodic scan until 0 V a chemically irreversible wave appears at Ep,c =
0.22 V which is consistent with the 2 e-/1H+ transfer as indicated in equation 8, and similarly
reported in the literature for related complexes, followed by Ru-O bond breaking and formation
of the mononuclear complex 12+,
RuIII-O-RuIII + 2e- + H+ → RuII-(OH)-RuII

Ep,c = 0.22 V

(8)

23

RuII-(OH)-RuII + H2O + H+

→ 2 RuII-OH2

(9)

This is nicely seen in the second CV cycle where a new chemically reversible and an
electrochemically quasireversible waves appears at E0 = 0.82 V and E0 = 0.98 V respectively,
which are associated with 12+.

Figure 4. CV of 1-dm4+ in 0.1 M HOTf, 1st cycle (solid line), 2nd cycle (dashed line). The inset
shows a comparison of the second cycle (black) and a CV from a solution of 12+ (red) in the
same solvent. Polished glassy carbon was used as working electrode, a Pt wire as counter
electrode and SSCE as reference electrode. Scan rate = 100 mV s-1.

Addition of 100 equivalents of Ce(IV) to 1-dm4+ generates oxygen with a TN of 7.6 and a
TOFi of 6.19 h-1 as can be extracted from the oxygen generation profile as a function of time that
is presented in the Sup Inf. This was a surprising result since given the absence of an aqua group
bonded to the Ru metal it was not expected to be an active WOC. In order to decipher what were
the active species we followed the reaction by rR spectroscopy.

24

Figure 5 shows the rR spectrum of 1-dm4+ in 0.1 M triflic acid before the addition of Ce(IV) in
a black line. After addition of Ce(IV) two spectra were recorded one after 30 s (red line) and the
second one after 2.73 h (blue line). First, a vibration with high intensity at 392 cm-1 observed in
1-dm4+ before oxidant addition, due to the νS Ru-O-Ru mode,35, 49, 50 shifts to 404 cm-1. This blue
shift is in agreement with a higher bond order of the Ru-O-Ru moiety and thus can be associated
with the one electron oxidation of the initial dimer to form 1-dm5+. Incidentally addition of a
large amount of ClO4- to this solution generates nice brown crystals of 1-dm5+, that confirm this
assignment. Secondly and surprisingly both spectra at 30 s and at 2.73 h present a vibration at
801 cm-1 which is the typical signature of 1-dn4+, as we have reported earlier.19 This clearly
suggest that upon addition of Ce(IV) the 1-dm4+ dimer partially reorganizes to form a small
amount of 1-dn4+, which in turn is responsible for the observed catalytic oxygen generation, as
we have demonstrated earlier. No RuO2 was detected in our experiments which is in agreement
with the oxidative robustness of the ligands used and the stability of the Ru-oxo-bridged moiety.
The formation of RuO2 as a black solid is very obvious when using Ru complexes with ligands
that are easy to oxidize, typically containing benzyllic, pyridylbenzyllic or amine type of
groups.57,58 We also have a nice correlation of dimer formation and oxygen generation19 that
again is in agreement with the absence of RuO2 in our systems.

25

Figure 5. rRaman spectrums 30 s (red) and 2.73 h (blue) after the addition of 100 equivalents of
CAN to a 1 mM solution of 1-dm4+ in 0.1 M HOTf at 25 °C. The spectrum before the reaction is
also included (black). The inset shows en enlargement in the 200-600 cm-1 region.

4. Conclusions.
Mononuclear [TBRuII-OH2] complexes behave as water oxidation catalysts in the presence of a
large excess of Ce(IV). In parallel this mononuclear complexes also suffer from ligand loss and
formation of a dinuclear oxo-bridged complexes that are also active WOCs. As expected their
high oxidation states are also highly active agents for the oxidation of organic substrates,
producing a series of polynuclear complexes at reduced oxidation states and manifesting the
existence of an oligomerization process.

26

We have isolated dinuclear, trinuclear and tetranuclear oxo-bridged complexes that have been
structurally characterized by single crystal X-ray diffraction analysis. Their structural properties
have been correlated to their electronic structure.
Finally the structural, electrochemical and reactivity properties of the oxo-bridged dimer 1dm4+ has been thoroughly studied. In particular rR spectroscopy clearly shows that the catalytic
oxidation of water to dioxygen observed is due to a reorganization of 1-dm4+ into 1-dn4+, after
the addition after the addition of Ce(IV).

ASSOCIATED CONTENT
Supporting Information. Electronic supplementary information (ESI) available: Additional
spectroscopic, electrochemical and reactivity experiments together with CIF files (CCDC #
1016623-1016632) and crystallographic information. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: allobet@iciq.cat.
Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENT

27

Support from MINECO (CTQ-2013-49075-R, SEV-2013-0319 and PRI-PIBIN-2011-1278) is
gratefully acknowledged. SM thanks MINECO for a Torres Quevedo contract and IL for a FPU
grant.

28

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TOC

Synopsis: A variety of linear polynuclear oxo-bridged Ru complexes including dinuclear,
trinuclear and tetranuclear Ru complexes have been obtained within the framework of the side
reactions involved in the so-called single site water oxidation catalyst. These complexes have
been structurally characterized by single crystal X-Ray diffraction analysis and the water
oxidation activity of some of them is discussed.

34