"This is the peer reviewed version of the following article:,Catalytic Oxidation of Water to
Dioxygen by Mononuclear Ru Complexes Bearing a 2,6‐Pyridinedicarboxylato Ligand, which
has been published in final form at:
https://onlinelibrary.wiley.com/doi/10.1002/cssc.201802996

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Catalytic Oxidation of Water to Dioxygen by Mononuclear Ru Complexes
Bearing a 2,6‐Pyridinedicarboxylato Ligand

Md Asmaul Hoque,1 Jordi Benet‐Bucholz,1 Antoni Llobet1,2, Carolina Gimbert‐Suriñach,1,*
1

Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and
Technology, Av. Països Catalans 16, 43007 Tarragona, Spain.

2

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

Abstract

The synthesis, purification and isolation of two new Ru complexes containing the tridentate
dianionic meridional ligand pyridyl‐2,6‐dicarboxylato (pdc2‐) of general formula [RuIII(pdc‐ҡ3‐
N1O2)(bpy)Cl], 1III, and [RuII(pdc‐ҡ2‐N1O1)(bpy)2], 2II, (bpy is 2,2’‐bipyridne) is reported. These two
complexes and their derivatives have been thoroughly characterized based on spectroscopic
(UV‐vis, NMR) and electrochemical techniques (CV, DPV and Coulometry). Under a high anodic
applied potential both complexes evolve towards the formation of new Ru‐aquo species namely,
[RuIII(pdc‐ҡ3‐N1O2)(bpy)(OH2)]+, 1-O, and [RuIV(O)(pdc‐ҡ2‐N1O1)(bpy)2], 2-O. These two new
complexes are active catalysts for the oxidation of water to dioxygen and their catalytic activity
is analyzed based on electrochemical techniques. A TOFmax = 2.4‐3.4 x103 s‐1, has been calculated
for 2-O.

Graphical Abstract

Keywords

Water oxidation, redox properties transition metal complexes, Ru complexes, redox catalysis

1-Introduction
Water oxidation (WO) catalysis is one of the key process involved in the light induced water
splitting (WS) reaction. This hydrogen as a clean fuel from an inexhaustible source of energy,
sunlight1. The overall reaction is depicted in Eq. 1, that can be split in the respective two half
reactions in Eq. 2 (WO) and Eq. 3 for proton reduction (PR).2 Given the beneficial impact of such
a sustainable process on our society, the development of efficient catalysts to perform the WO
reaction has experienced increasing interest.3‐5 Among the most efficient catalysts described to
date, ruthenium coordination complexes containing flexible, adaptative, multidentate and
equatorial (FAME) ligands have shown to perform remarkably well.3, 6 Another key feature of
the best performing WO catalysts is the presence of carboxylate groups in the coordination
sphere of the metal center, providing stability to the metal high oxidation states and lowering
the overpotential of the reaction.7‐9 In addition, the presence of dangling carboxylate group,
strategically situated so that it can intramolecularly accept a proton at the water nucleophilic
attack stage, significantly reduces the energy of activation at this step and thus greatly increases
reaction rate.3, 7, 10

H2O + 4h  H2 + O2

(1)

2H2O +  O2 + 4H+ + 4e‐
4H+ + 4e‐  2H2

(2)

(3)

Ruthenium complexes have also been crucial to understand the mechanistic pathways
responsible for the O‐O11‐13 bond formation and how the coordination sphere around the metal
center influence these pathways. Other factors such as the pKa of ruthenium aquo (Ru‐OH2)
intermediate species have also been shown to strongly impact the performance of the water
oxidation catalysts.14, 15 All these insights are of paramount importance because they have
allowed the rational design of catalysts that nowadays can perform as fast as 7,900 s‐1 at neutral
pH.7
In this work, we explore the water oxidation catalytic activity of Ru complexes containing the
meridional tridentate dianionic ligand 2,6‐pyridinedicarboxylato (pdc2‐), which can show
different coordination modes.16 For instance, it can coordinate in a tridentate ҡ‐N1O2 meridional

fashion and provide a strong sigma donation to the metal center16‐18 but also it can bind in a
bidentate ҡ‐N1O1 mode leaving a pendant carboxylate.16
Here on we report the preparation, purification and isolation of two new Ru complexes
[RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III, and [RuII(pdc‐ҡ2‐N1O1)(bpy)2], 2II, shown in Scheme 1, that in
addition to the pdc2‐ ligand also contain one or two neutral didentate 2,2’‐bipyridne (bpy) ligands
respectively. We have studied the spectroscopic, redox and structural properties of these two
complexes and we have also shown that are precursors to Ru complexes capable of catalytically
oxidize water to dioxygen. The activity of the catalysts is evaluated based on electrochemical
techniques.

2-Experimental Section
Materials. RuCl3·×H2O was purchased from Alfa‐Aesar. The precursor complex [RuCl2(DMSO)4]
was prepared according to a reported procedure.19 2,6‐pyridindicarboxylic acid (H2pdc) and
other chemicals were obtained from Aldrich and used as received. Solvents were dried with a
SPS® system and degassed by bubbling nitrogen before starting the reactions. High purity de‐
ionized water used for the electrochemistry experiments was obtained by passing distilled water
through a nanopure Mili‐Q water purification system. For other spectroscopic and
electrochemical studies, HPLC‐grade solvents were used.
Instrumentation and methods. A Bruker Avance 500 MHz were used to carry out NMR
spectroscopy. ESI‐Mass spectra were recorded using micromass Q‐TOF mass spectrometer.
Elemental analyses were carried out on Perkin‐Elmer 240C elemental analyzer. The EPR
experiments were carried out at 4 K on frozen solutions by using a X‐band spectrometer (Bruker
ELEXYS E580). The pH of the solutions was determined by a pHmeter (CRISON, Basic 20+)
calibrated before measurements through standard solutions at pH 4.01, 7.00 and 9.21. Oxygen
evolution was analyzed with a gas phase Clark type oxygen electrode (Unisense Ox‐N needle
microsensor) and calibrated by the addition of small quantities of oxygen (99%) at the end of
the experiment. All electrochemical experiments were performed in an IJ‐Cambria CHI‐660
potentiostat using a three‐electrode one compartment cell for cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) or two compartment cell for bulk electrolysis. 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 from DPV. The Reference Electrode (RE) was Hg/Hg2SO4
(K2SO4 saturated) and potentials were converted to NHE by adding 0.65 V. Glassy carbon disk (ф
= 0.3 cm, S = 0.07 cm2), Pt disk and Hg/Hg2SO4 (K2SO4 saturated) were used as Working Electrode
(WE), Counter Electrode (CE) and Reference Electrode (RE) respectively, unless explicitly
mentioned. Glassy carbon electrodes were polished with 0.05 μm alumina (Al2O3) and rinsed
with water. CVs and DPVs were iR compensated by the potentiostat in all the measurements.
CVs were recorded at 100 mV·s−1 scan rate. DPV parameters were ΔE = 4 mV, Amplitude = 0 mV,
Pulse width = 5 s, Sampling width = 0.0167 s, Pulse period = 5 s. The complexes were dissolved
in dichloromethane or acetone containing [(n‐Bu)4N][PF6] (0.1 M) as supporting electrolyte. In
aqueous solution the electrochemical experiments were carried out in I = 0.1 M phosphate
buffer solutions with desired pH. The Pourbaix diagrams were built using the following buffers:
sodium dihydrogen phosphate/phosphoric acid up to pH = 4 (pKa = 2.12), sodium hydrogen
phosphate/ sodium dihydrogen phosphate up to pH = 9 (pKa = 7.67), sodium hydrogen
phosphate/sodium phosphate up to pH = 13 (pKa = 12.12) and also 0.1 M CF3SO3H for pH=1.0.

For routine bulk electrolysis experiments in figure 4b bottom left, a Pt grid was used as a WE,
another Pt grid as a CE and a Hg/Hg2SO4 (K2SO4 saturated) as a RE. For the bulk electrolysis
experiment for oxygen detection, a glassy carbon rod (S = 8.2 cm2) was used as a working
electrode and Ag/AgCl (sat. KCl) as a RE.

For Figure S11, to generate [RuIII(pdc‐ҡ3‐N1O2)(bpy)(OH2)]+, 1-O, complex from 1 mM of
[RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III, bulk electrolysis experiment was carried out in three‐electrode
one compartment cell for 5 min at Eapp = 1.6 V without stirring. A glassy carbon disk was used
as a WE, Pt disk as a CE and a Hg/Hg2SO4 (K2SO4 saturated) as a RE. For figure S15, to see the
coordination of DMSO, [RuII(pdc‐ҡ2‐N1O1)(bpy)(dmso)Cl], to the complex [RuIII(pdc‐ҡ3‐
N1O2)(bpy)Cl], 1III, bulk electrolysis experiment was carried out in three‐electrode one
compartment cell for 2 min at Eapp = 0 V without stirring. A glassy carbon disk was used as a WE,
Pt disk as a CE and a Hg/Hg2SO4 (K2SO4 saturated) as a RE. iR compensation by the potentiostat
was not applied in this technique.

Single Crystal X-Ray Structure Determinations
Crystal preparation: Crystals of [RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III, was obtained from reaction in
methanol solvent. [RuII(pdc‐ҡ2‐N1O1)(bpy)2], 2II, and [RuIII(Hpdc‐ҡ2‐N1O1)(bpy)2]2+, 2III, were
grown by slow evaporation of methanol:hexane and water:acetonitrile respectively. The
measured crystals were prepared under inert conditions immersed in perfluoropolyether as
protecting oil for manipulation.
Data Collection: Crystal structure determination for compounds 1III, 2II and 2III were carried out
using a Rigaku diffractometer equipped with a Pilatus 200K area detector, a Rigaku MicroMax‐
007HF microfocus rotating anode with MoK radiation, Confocal Max Flux optics and an Oxford
Cryosystems low temperature device Cryostream 700 plus (T = ‐173 °C). Full‐sphere data
collection was used with  and  scans. Programs used: Data collection and reduction with
CrysAlisPro20 V/.60A and absorption correction with Scale3 Abspack scaling algorithm. 21
Structure Solution and Refinement: Crystal structure solution was achieved using the computer
program SHELXT.22 Visualization was performed with the program SHELXle.23 Missing atoms
were subsequently located from difference Fourier synthesis and added to the atom list. Least‐
squares refinement on F2 using all measured intensities was carried out using the program
SHELXL 2015. All non‐hydrogen atoms were refined including anisotropic displacement
parameters.

Comments to the structures: [RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III : The asymmetric unit contains one
molecule of the metal complex and one methanol molecule. [RuII(pdc‐ҡ2‐N1O1)(bpy)2], 2II : The
asymmetric unit contains one molecule of the metal complex and two molecules of water.
[RuIII(Hpdc‐ҡ2‐N1O1)(bpy)2]2+, 2III: The asymmetric unit contains one molecule of the metal
complex, 1 ½ PF6‐anions, 1.75 molecules of acetonitrile and 0.25 molecules of dichloromethane.
In this metal complex one of the carboxylates is protonated with 0.5 occupancy (although
hydrogen atoms can be only be localized with difficulties, the distances indicate unambiguously
that one of the oxygen atoms is protonated). In one of the solvent position, an acetonitrile and
a dichloromethane molecule are sharing its position by disorder with a ratio of respectively
75:25.
Synthesis of [RuIII(pdc-ҡ3-N1O2)(bpy)Cl], 1III. In a 100 mL two neck round‐bottom flask,
RuCl3.xH2O (262 mg, ca. 1 mmol) and LiCl (42 mg, 1mmol) were dissolved in 20 mL of degassed
methanol. Then, a 10 mL degassed aqueous solution of 2,6‐pyridine dicarboxylic acid (167 mg,
1 mmol) and sodium carbonate (106 mg, 1mmol) were added slowly to the reaction mixture.
After 20 minutes of stirring at room temperature, 10 mL of a degassed methanol solution of
2,2´‐bipyridine (156 mg, 1 mmol) was added slowly and refluxed for 4 hours under N2
atmosphere. The resulting orange‐red crystalline solid was filtered and washed with methanol
and diethyl ether (320 mg, 0.70 mmol, Yield: 70 %). Single crystals were selected from this batch
to perform single crystal x‐ray diffraction analysis. Anal. Calc. for (C17H11ClN3O4Ru·CH3OH): C,
44.13%; H, 3.09 %; N, 8.58 %; S. Found: C, 43.95 %; H, 2.76 %; N, 8.58 %. ESI+‐HRMS (MeOH) m/z
calc. for [M+Na]+ : 480.9406, found m/z: 480.9376.
Synthesis of [RuII(pdc-ҡ2-N1O1)(bpy)(DMSO)Cl] in situ. In a NMR tube or in a UV‐Vis
spectroscopy cell, 20 L of trimethylamine were added to a solution of 0.5 mL [RuIII(ҡ3‐
pdc)(bpy)Cl] in d6‐dmso for the NMR and dmso for the UV‐vis. 1H‐NMR (500 MHz, [d6]‐dimethyl
sulfoxide + triethylamine) δ: 9.31 (d, J = 4.9 Hz, 1H), 8.48 (d, J = 5.05 Hz, 1H), 8.29 (t, J = 8.35 Hz,
2H), 7.88 (t, J = 7.55 Hz, 1H), 7.81 (m, 3H), 7.38 (dd, J = 12.2 Hz and 5.05 Hz, 2H), 7.05 (dd, J = 7.1
Hz and 2.1 Hz, 1H). 13C‐NMR (125 MHz, [d6]‐dimethyl sulfoxide + triethylamine) δ: 172.3, 167.8,
164.3, 161.4, 161.2, 155.4, 152.9, 150.3, 137.2, 134.9, 134.7, 124.5, 123.8, 123.7, 122.9, 121.9,
121.4.
Synthesis of [RuIII(pdc-ҡ3-N1O2)(bpy)(OH2)] in situ, 1-O. In a 100 mL two neck round‐bottom
flask, [RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl] (100 mg, 0.21 mmol) and AgClO4 (50 mg, 0.24 mmol) in 40 mL
of a mixture of acetone: water (75:25) were heated at reflux under N2 atmosphere for 3h. The
color of the solution changed from orange red to green. A CV analysis of the reaction crude

mixture shows almost complete conversion of the starting material to a new species with a
Ru(III/II) couple consistent with the corresponding Ru‐OH2 complex (Figure S14 in the supporting
information). After several attempts to purify this compound, it was not possible to isolate it in
a pure form due to the formation of higher nuclearity oxo‐bridged species as suggested by UV‐
Vis spectroscopy, which showed typical absorptions in the range of 650‐800 nm.
Synthesis of [RuII((pdc-ҡ2-N1O1)(bpy)2]·3H2O, 2II. In a 100 mL two neck round bottom flask
[RuII(pdc‐ҡ3‐N1O2)(Cl)(DMSO)2]16 (560 mg, 1 mmol) and 2,2´‐bipyridine (312 mg, 2 mmol) were
dissolved in degassed methanol (40mL) and refluxed for 4 hours. The mixture was then
evaporated to dryness and the resulting solid dissolved in CH2Cl2 and purified by column
chromatography with neutral alumina using a mixture of CH2Cl2/MeOH (100:5, v/v) as eluent. A
red color fraction was collected giving a solid identified as the product (350 mg, 0.60 mmol,
Yield: 60 %). Single crystals were grown by slow evaporation of the complex in a 1:1 mixture of
methanol:hexane. Anal. Calc. for (C27H19N5O4Ru ·3H2O): C, 51.31 %; H, 3.63 %; N, 11.02 %. Found:
C, 51.26 %; H, 3.98 %; N, 11.07 %. 1H‐NMR (500 Hz, [d4]‐Methanol) δ: 8.67 (d, J = 5.7 Hz, 1H),
8.60 (d, J = 5.6 Hz, 1H), 8.58 (d, J = 8.1 Hz, 1H), 8.56 (d, J = 8.2 Hz, 1H), 8.48 (d, J = 8.1 Hz, 1H),
8.33 (d, J = 8 Hz, 1H), 8.09‐8.06 (m, 3H), 7.97 (t, J = 7.7 Hz, 1H). 7.85 (td, J = 1.4 and 7.9 Hz, 1H),
7.75‐7.70 (m, 2H ), 7.62 (t, J = 6.5 Hz, 1H ), 7.56 (t, J = 6.4 Hz, 1H), 7.39 (d, J = 5.1 Hz, 1H), 7.34
(dd, J = 1.5 and 7.7 Hz, 1H), 7.21 (t, J = 6.6 Hz, 1H), 7.06 (t, J = 6.7 Hz, 1H). 13C‐NMR (500 MHz,
[d4]‐Methanol) δ: 175.6, 170.7, 165.8, 160.9, 160.4, 160.0, 159.4, 155.5, 154.8, 152.9, 152.5,
150.6, 139.2, 137.8, 137.7, 137.5, 136.3, 127.9, 127.4, 127.4, 126.6, 126.4, 126.4, 124.5, 124.3,
124.3, 123.8. (ESI+‐HRMS; MeOH) m/z calc. for [M]+: 580.0585, found m/z: 580.0563.
Synthesis of [RuIII(Hpdc-ҡ2-N1O1)(bpy)2](PF6)2, 2III. In a 25 mL round bottom flask, a solution of
cerium(IV) ammonium nitrate (21 mM, 1.05 eq, 1 mL in pH1) was added dropwise to a solution
of [RuII(pdc‐ҡ2‐N1O1)(bpy)2] in water (2.0 mM, 10 mL) and the mixture was stirred at room
temperature for 15 minutes. A green color precipitate was obtained when a saturated aqueous
solution of KPF6 was added. The solid was filtered and washed with water, methanol and diethyl
ether (10 mg, 0.0135 mmol, Yield: 65%). Single crystals were obtained from slow evaporation of
a solution in a mixture of water and acetonitrile.
Synthesis of [RuIV(pdc-ҡ3-N1O2)(bpy)2]2+, 2IV. Inside a NMR tube, a solution of cerium(IV)
ammonium nitrate (42 mM, 2.1 eq, 0.1 mL in pD1) was added dropwise to a solution of [RuII(pdc‐
ҡ2‐N1O1)(bpy)2] in deuterated water (2 mM, 1.0 mL) and the mixture was stirred for 5 minutes.
1

H‐NMR (500 MHz, [d2]‐water) δ: 8.85 (t, J = 7.7 Hz, 1H), 8.77 (d, J = 8.1 Hz, 2H), 8.72 (d, J = 8.2

Hz, 2H), 8.59 (d, J = 7.7 Hz, 2H), 8.52 (t, J = 7.9 Hz, 2H), 8.31 (t, J = 7.9 Hz, 2H), 7.87‐7.83 (m, 4H),

7.64 (t, J = 6.8 Hz, 2H). 7.58 (t, J = 6.7 Hz, 2H). 13C‐NMR (500 MHz, [d2]‐water) δ: 168.1, 155.1,
151.5, 149.9, 148.9, 147.7, 147.1, 144.8, 142.9, 133.9, 129.9, 128.5, 127.2, 126.6.

3-Results and discussion
3.1 Synthesis, spectroscopic and structural characterization
The complex 1III was synthesized by slow addition of an aqueous solution of sodium 2,6‐
pyridindicarboxylate to a MeOH solution of the ruthenium precursor RuCl3.×H2O followed by
addition of 1 equivalent of bpy also in MeOH as indicated in Scheme 1. The final solution was
refluxed for 4h and on cooling an orange‐red crystalline solid of the desired complex precipitates
with 70% yield. Crystals suitable for single crystal x‐ray diffraction studies were obtained and its
molecular structure is shown in Figure 1A. It has a highly distorted octahedral geometry due to
the strain imposed by the pdc2‐ meridional ligand with O‐Ru‐O angle of 157o as opposed to the
180o expected for an ideal octahedron. It shows similar bond distances and angles to those
reported for related complexes.16‐18
Reduction of 1III in DMSO with NEt3, generates a new complex, [RuII(pdc‐ҡ2‐
N1O1)(bpy)(DMSO)Cl]‐, where the pdc2‐ changes its coordination mode from tridentate ҡ‐N1O2
to bidentate ҡ‐N1O1 as evidenced by NMR spectroscopy and Cyclic Voltammetry (CV)
experiments (Figures S1 and S15 in the SI). As a consequence of the DMSO coordination the
complex loses its Cs symmetry and thus all pdc2‐ proton resonances are different.
On the other hand, complex 2II was prepared in 60 % yield by reacting the ruthenium precursor
[RuII(ҡ3‐N1O2)(Cl)(DMSO)2]‐16 dissolved in MeOH with 2 equivalents of bpy ligand under reflux.
The 1H NMR spectrum of the product shows the non symmetric nature of the complex with two
sets of resonances for the bpy ligands as well as the corresponding non symmetric resonances
for the pdc2‐ protons, in agreement with the bidentate ҡ‐N1O1 coordination mode of the latter
(Figure 2 and Figures S3‐S4 in the supporting information). Upon oxidation with 1 equivalent of
cerium(IV) ammonium nitrate (Ce(IV)), the corresponding Ru(III) derivative, [RuIII(Hpdc‐ҡ2‐
N1O1)(bpy)2]2+, 2III, was isolated.
Single crystals of both the Ru(II) and Ru(III) species were obtained and their ORTEP structures
are shown in Figures 1B and 1C, respectively. Complex 2II displays the typical slightly distorted
octahedral geometry around the ruthenium, as expected for low‐spin d6 Ru(II) ion.24‐26 The bpy
ligands occupy both axial and equatorial positions assuming the ҡ‐N1O1‐pdc2‐ ligands binds in
the equatorial plane, with a dangling carboxylate not bonded to Ru.
The one electron oxidized Ru(III), 2III, shows a very similar structure with the Hpdc‐ ligand also
coordinating in a bidentate ‐N1O1 mode but with the nonbonding carboxylate protonated. The

Ru‐O bond distance of the Ru(III) compound is slightly shorter than that of its parent Ru(II)
complex, average 2.00 (1) Å vs. 2.08 (1) Å, respectively, as expected.
The addition of two equivalents of Ce(IV) to a solution of 2II generates the Ru(IV) derivative,
[RuIV(pdc‐ҡ3‐N1O2)(bpy)2]2+, 2IV, which slowly converts to the Ru(III) compound over time as
monitored by NMR spectroscopy (see Figures S7), hindering the formation of high quality
crystals suitable for single crystal x‐ray diffraction. However, it was possible to fully characterize
the Ru(IV) species by NMR and UV‐Vis spectroscopy (Figure 2, Figures S6 and S9). All the analysis
are consistent with a diamagnetic compound, corresponding to a low‐spin d4 Ru(IV) center with
a (dxz ,dyz)4 electronic configuration and pentagonal bipyramidal geometry.15, 27 [RuIV(pdc‐ҡ3‐
N1O2)(bpy)2]2+ shows less number of resonances in the 1H NMR spectra compared to its Ru(II)
derivative [RuII(pdc‐ҡ2‐N1O1)(bpy)2] in agreement to the symmetry increase. In addition, they are
shifted to lower field in accordance with the higher oxidation state of the Ru center.
Complex 2III is low spin d5 with an unpaired electron. As a consequence, all resonances in the 1H
NMR spectrum are broadened and highly shifted with regard to the Ru(II) analogue (Figure S5a
in the supporting information). On the other hand, it exhibits typical EPR features of
unsymmetrical Ru(III) complex with gx = 2.69, gy = 2.42, gz = 2.04 (Figure S5c in the supporting
information). The large g anisotropy and the deviation of the average g factor from the free
electron value of 2.0023 point to significant contributions from the heavy metal with its high
spin−orbit coupling constant to the spin distribution.28, 29 The Ru(III) chlorido complex 1III, also
shows a characteristic signal of the corresponding unpaired electron, but with a broader
signature (Figure S5e in the supporting information).24 Both Ru(II) and Ru(IV) derivatives, 2II and
2IV, are EPR‐silent as expected for complexes with no unpaired electrons.
The UV−vis spectra of complexes 2II, 2III and 2IV were recorded in 0.1 M triflic acid aqueous
solutions (pH 1.0) (Figure S9 in the supporting information). Typical Ru−bpy metal to ligand
charge transfer (MLCT) bands are observed in the 380−550 nm range for the Ru(II) compound,
where as a single transition at 360 nm is observed in that range of the spectrum for Ru(III), which
is essentially featureless for Ru(IV). Analogous spectra could be obtained by spectrophotometric
redox titration of 2II with Ce(IV), exhibiting isosbestic points as displayed in the Figure S10 in the
supporting information.

3.2 Electrochemical characterization
The electrochemical behavior of complexes 1III and 2II were analyzed by cyclic voltammetry (CV),
differential pulse voltammetry (DPV) and bulk electrolysis experiments in dichloromethane
(DCM) containing 0.1M of [(n‐Bu)4N][PF6] (TBAH) and 0.1 M ionic strength buffered aqueous
solutions at different pHs. All redox potentials reported in this work are referred to the NHE
electrode.
In DCM complex 1III shows a reversible redox wave at E1/2 = 0.31 V (ΔE = 75 mV) attributed to the
Ru(III/II) couple (Figure 3, left). On the other hand complex 2II shows two chemically reversible
and electrochemically quasi‐reversible waves at E1/2 = 1.05 V (ΔE = 62 mV) and E1/2 =1.32 V (ΔE =
120 mV) attributed to the Ru(III/II) and Ru(IV/III) couples respectively (Figure 3, right).30 The
relatively easy access to the IV/III redox potential at only 270 mV above the III/II is a clear
indication of the 7 coordinated nature of the oxidized compound, as already been proven by
NMR spectroscopy (Figure 2).
The chlorido complex 1III in aqueous solution at a pH 7 phosphate buffer (phbf), shows a Ru(III/II)
redox wave at E1/2 = 0.44 V (ΔE = 65 mV) (Figure S11 in the supporting information) and a second
wave at 1.45 V which is chemically irreversible. It thus indicates that Ru(IV)‐Cl complex is not
stable and undergoes oxidative Ru‐Cl degradation to form most likely Cl2 (g) as has been
proposed for related complexes. This produces the in situ generation of the Ru‐OH2 complex,
[RuII(pdc‐ҡ3‐N1O2)(bpy)(H2O)]‐ that can act as a water oxidation catalyst. Indeed, a bulk
electrolysis experiments of 1III at 1.6 V for 5 minutes at pH 7 involved a charge of 7.2 mC, which
implies 0.05 mols of electrons per mol of 1III. The shape of the current vs. time is in agreement
with the in situ generation of a water oxidation catalyst. A CV and DPV analysis of the solution
after the bulk electrolysis experiments reveals the generation of three new waves at E = 0.27 V,
E = 0.80 V and E = 1.41 V that can be tentatively assigned to the III/II, IV/III and V/IV couples of
[RuII(pdc‐ҡ3‐N1O2)(bpy)(H2O)]‐ respectively. The latter one being responsible for the catalytic
phenomenon.

On the other hand, the CV of complex 2II at pH 7 shows two pH independent redox waves at E1/2
= 0.89 V (ΔE = 90 mV) and 1.23 V (ΔE = 70 mV) associated with the III/II and IV/III couples as can
be observed in Figure 4A. Figure 4B shows the effect of carrying out 100 repetitive CV within the
potential range 0‐1.6 V at the same pH. As can be observed in the inset as the number of cycles
increases new small waves appear that are indicated with blue arrows together with the
presence of a large electrocatalytic wave at 1.4‐1.6 V.

In order to get more insights into the new species formed upon cycling, bulk electrolysis
experiments at Eapp = 1.45 V for 2h were conducted (Figure 4C) which involved a charge of 7.93
C and 4.8 mols of electrons per mol of initial Ru(II) clearly indicating the presence of an
electrocalytic process. The solution generated under these conditions was analyzed by DPV
experiments that are shown in Figure 4D. The DPV shows that the waves associated with the
initial complex have drastically decreased and a set of new waves appear at E = 0.25 V, E = 0.46
V, E = 0.73 V and E = 1.03 V. Further as shown by DPV at different pH all these waves are pH
dependent and therefore involving proton coupled electron transfer (PCET) processes that in
turn indicate the presence of Ru‐OH2 groups in the new species generated (Figure S13 in the SI).
Finally, a very large an intense wave can be observed at approximately 1.35 V attributed to the
catalytic oxidation of water to dioxygen.
It is important to note that waves at 0.25 and 0.73 V assigned to [RuII(pdc‐ҡ3‐N1O2)(bpy)(H2O)],
1-O, species generated from the Coulometry of 1III coincide with those generated by the
Coulometry of 2II, meaning that one of the transformation processes involves bpy ligand loss as
indicated in Scheme 2. Further by checking the potential as a function of pH we were able to
generate a Pourbaix diagram that is presented in the left hand side of Figure 5. The pka
calculated from the slope changes are 4 for Ru(II) and 11 for Ru(III) and are gathered in Table 1
together with similar data for related complexes previously described in the literature. The
strong sigma donating effect of the pyridil dicaroxylato ligand can be clearly observed on the
increase of pka’s (10‐>11 (II); 2‐>4 (III)) and reduction of the V/IV redox couples when comparing
for instance with trpy‐bpy‐Ru‐OH2. 31(REF)
On the other hand, the waves at 0.46 and 1.03 V are assigned to a new species where a one of
the carboxylate arms of pdc2‐ is substituted by an oxo group generating the seven coordinated
species [RuIV(O)(pdc‐ҡ2‐N1O1)(bpy)2], 2-O. The Pourbaix diagram obtained for this complex is
presented on the right hand side of Figure 5. Here the III/II redox potential is significantly hgiher

than for [RuII(pdc‐ҡ3‐N1O2)(bpy)(H2O)]‐, 1-O, since in the present complex the pdc2‐ ligand acts in
a k‐N1O1 mode and thus only one of the two anionic charges is directly felt by the Ru center. In
sharp contrast the V/IV redox potentials are similar which is due to the cancelling effect of CN7
vs CN6 1 vs. 2 anionic charges, a phenomenon that has been previously described for related
Ru‐aquo complexes. 14(REF, .31‐34)
The water oxidation catalytic cycles proposed for 1-O and 2-O, are presented in Scheme 2. The
main differentiating feature for the two cat cycles is that for 2-O the high oxidation state species
are CN7 and a dangling carboxylate is ready for an intramolecular proton transfer at the O‐O
bond formation step that is generally the rds, and thus radically decreases the energy of
activation at this step as has been previously shown for related complexes.
Indeed, a FOWA analysis35‐37 of the catalytic current (see Figure 6) for the mixture of 1-O and 2O, gives a TOFmax value for the catalytic process of 2.4‐3.4 x103 s‐1, for 2-O, assuming that the
initial current at the foot is solely due to the fastest WOC.
Finally, an analysis of the gas phase, of a bulk electrolysis experiments of 1 mM 2II at an applied
potential of 1.45 V for 1.2 h (4.25 C; 15 mols of electrons/mols of 2II; 3.5 TN) confirms the
evolution of O2 gas with a Faradaic efficiency of 90%. (see Figure S16)

In conclusion, two new Ru complexes RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III, and [RuII(pdc‐ҡ2‐
N1O1)(bpy)2], 2II, are reported that under high anodic potentials evolve towards the formation
of Ru‐aquo complexes [RuIII(pdc‐ҡ3‐N1O2)(bpy)(OH2)]+, 1-O, and [RuIV(O)(pdc‐ҡ2‐N1O1)(bpy)2], 2O, that are powerful and rugged water oxidation catalysts. These two complexes operate water
oxidation catalysis with active species that involve six coordination for the Ru center in 1-O and
seven coordination in 2-O. The present work uncovers and highlights the complexity involved in
water oxidation catalytic processes when transition metal complexes are exposed to high
oxidation potentials needed for water oxidation catalysis.

5-Acknowledgements
A.L. thanks MINECO, FEDER and AGAUR for grants CTQ2016‐80058‐R, CTQ2015‐73028‐EXP, SEV
2013‐0319, ENE2016‐82025‐REDT, CTQ2016‐81923‐REDC, and 2017‐SGR‐1631.

6- Supporting Information. Additional experimental details, electrochemical and spectroscopic
data. CIF files for complexes 1III, 2II and 2III with CCDC numbers 1881914, 1881913 and 1881915
respectively are available at https://www.ccdc.cam.ac.uk/.

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Table1. Thermodynamic and Catalytic Data for Ru‐pdc and for Related Ru Complexes Described in the Literature at pH = 7.0.

Entry
131
232
332
4 38
539
6e
740
810
9e
a

Complexesa

V/IV
2+

[Ru(trpy)(bpy)(H2O)]
Cis‐[Ru(trpy)(pic)(H2O)]1+
Trans‐[Ru(trpy)(pic)(H2O)]1+
out‐[Ru(Hbpp)(trpy)(H2O)]2+
[Ru(bpc)(bpy)(H2O)]1+
[Ru(pdc)(bpy)(H2O)]1+
Ru(bda)(isq)2(H2O)]
[Ru(tda)(py)2OH]
[Ru(pdc)(bpy)2OH]

1.86

‐
‐
‐
‐
1.41
1.11
1.43
1.47

E1/2 (V) vs NHE
IV/III
0.83
0.80
0.69
0.85

‐
0.73
0.88
0.87
0.93

III/II

b

E

RuII‐OH2

pKa
RuIII‐OH2

0.72
0.62
0.45
0.52
0.56
0.25
0.55
0.70

110
180
240
370

9.8
10.0
10
11.1
10.6
11
5.5

1.7
3.7
2.0
2.8
2.6
4
12.9

‐
480
330
170

‐
‐

‐
‐

RuIV‐OH
‐
‐
‐
‐
‐
‐
‐
IV

Ru (5.5)
RuIV (5.0)‐

TOFc,d
1.5× 101

‐
‐
‐
1.6× 102
3.0× 102
8.0× 103
3.4× 103

Ligand abbreviations: trpy = 2,2’:6’,2”‐terpyridine, bpy = 2,2’‐bipyridine, pic = 2‐picolinate, Hbpp = 3,5‐bis(2‐pyridyl)pyrazole, bpc = 2,2′‐bipyridine‐6‐carboxylate, pdc =

2,6‐pyridinedicarboxylate, bda = 2,2’‐bipyridine‐6,6’‐dicarboxylate, tda = 2,2′:6′,2″‐terpyridine‐6,6″‐dicarboxylate, py = pyridine. bE= E(IV/III)‐E(III/II). cTOF stands for
initial Turn Over Frequencies in cycles per second. These values are extracted for the catalytic reactions involving 1.0 mM Cat/100 mM Ce(IV)in a 0.1 M triflic acid solution
with a total volume of 2 mL (entry 1, 5 and 7). dTOF stands for Maximum Turn Over Frequencies per second. These value has been extracted from Foot of the Wave
Analysis of CV and DPV experiment in pH7 (entry 8 and 9). e this work.

Scheme 1. Synthetic scheme and labelling.

Scheme 2. Generation of water oxidation catalytically active species from 2II.

A)

B)

C)

Figure 1. ORTEP plots at 50% probability for [RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III (A), [RuII(pdc‐ҡ2‐
N1O1)(bpy)2], 2II (B) and [RuII(Hpdc‐ҡ2‐N1O1)(bpy)2]2+ , 2III (C).

Figure 2. 1H NMR (500 MHz, 298 K, [d2]‐water) of [RuII(pdc‐ҡ2‐N1O1)(bpy)2], 2II (black, bottom)
and [RuIV(pdc‐ҡ3‐N1O2)(bpy)2]2+, 2IV (blue, top). The Crystal Field Splitting of d‐orbitals and
electronic configuration under 6‐coordination‐octahedral (Oh) or 7‐coordinated pentagonal
bipyramidal (D5h) geometries are indicated next to each spectrum.

12

10

RuIII/RuII
RuIV/RuIII

8
III

Ru /Ru
i (A)

i (A)

5
0

II

4
0

-5
-4

-10
-8

0.0

0.2

0.4
0.6
0.8
E (V) vs NHE

1.0

0.6

0.8

1.0

1.2

1.4

1.6

1.8

E (V) vs NHE

Figure 3. Cyclic voltammetry experiments in dichloromethane‐0.1M [(n‐Bu)4N][PF6] of a 1mM
solution of for [RuIII(pdc‐ҡ3‐N1O2)(bpy)Cl], 1III (left) and [RuII(pdc‐ҡ2‐N1O1)(bpy)2], 2II (right).
WE: glassy carbon disk; CE: platinum disk; RE: Hg/Hg2SO4. Scan rate = 100 mV/s.

30
III/II

i (A)

20

IV/III

10
0
-10

10

0.4

0.8
1.2
E (V) vs NHE

1.6

3

2
4
1

2

100
i (mA)

6

80
60
40
20

0

0
0.0

0.5

CAT 2
CAT 1

120

8

Q (C)

140

4

Charge
Current

i (A)

-20
0.0

1.0
t (h)

1.5

2.0

0
0.0

III/II III/II
A
B

0.4

III/II
non-aqua
IV/III IV/III
IV/III
B non-aqua
A

0.8
1.2
E (V) vs NHE

1.6

Figure 4. A, CV of a pH 7 phbf solution of 1 mM 2II. B, 100 consecutive CV cycles. Inset,
enlargement of the 0‐0.8 V range. The blue arrows indicate small new waves growing. C, bulk
electrolysis of a pH 8 phbf solution of 2 mM 2II at Eapp = 1.45 V for 2 h. D, DPV of a pH 7 phbf
solution of 2mM 2II (black) and of the solution obtained after the bulk electrolysis in C adjusted
to pH 7 (red).

Figure 5. Pourbaix diagrams of [Ru‐OH2] species derived from bulk electrolysis of 2II. Left, 1O; Right, 2-O. The black solid lines indicate the redox potentials for the different redox
couples, whereas the dashed vertical lines indicate the pKa. The zone of stability of the
different species is indicated only with the Ru symbol, its oxidation state, and its degree of
protonation of the aqua ligand. For instance, “Ru(V)=O” is used to indicate the zone of stability
of [RuV(O)(pdc‐κ3‐N1O2)(bpy)]+ for the 1-O derived species (left) and [RuV(O)(pdc‐ҡ3‐
N1O1)(bpy)2]+ for the 2-O derived species (right).

Figure 6. CV of a mixture of 0.92 mM [RuII(pdc‐ҡ3‐ N1O2)(bpy)(H2O)], 1-O and 1.08 mM
[RuIV(O)(pdc‐ҡ2‐N1O1)(bpy)2], 2-O at pH 7.0 phbf. Inset: FOWA plot of the catalytic current.
The gray line represents the experimental data used for the FOWA analysis, and the black
solid line shows the experimental data used for the extraction of TOFmax