“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic
Chemistry, copyright ©2020 American Chemical Society after peer review and technical editing by the publisher. To
access the final edited and published work see:
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03595

A Ru-bda complex with a dangling carboxylate group: synthesis and
electrochemical properties
Abolfazl Ghaderian,1,2 Jan Holub,1 Jordi Benet-Buchholz,1 Antoni Llobet,1,3,* 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

Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Campus Sescelades, C/Marcel·lí
Domingo, s/n, 43007 Tarragona, Spain

3

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

*allobet@iciq.cat
*cgimbert@iciq.cat
Abstract
Ruthenium complexes containing the tetradentate 2,2'-bipyridine-6,6'-dicarboxylato (bda 2-) equatorial ligand
and ortho-subsituted pyridines in the axial position have been prepared and characterized using spectroscopic,
crystallographic

and

electrochemical

techniques.

Complexes

[Ru(Hbda)(DMSO)(pyC)]

(1)

and

[Ru(bda)(DMSO)(pyA)] (2) (where pyC is 2-pyridinecarboxylate, pyA is pyridine-2-ylmethanol and DMSO is
dimethylsulfoxide) have been isolated in moderate to high yields. The solid state structures of (1-H)- and 2 reveal
the strong chelate effect of the axial pyridine ligand that coordinates in a bidentate fashion leaving the bda2equatorial ligand coordinating in a tridentate mode. In solution, compound 2 shows a dynamic equilibrium
between different coordination modes of the bda2- and pyA ligands. This phenomenon does not occur for 1
because the carboxylate binds stronger than the labile alcohol in 2. Cyclic voltammetry analysis of 1 reveal a
complex behavior with a pH independent wave at E1/2 = 1.12 V that is tentatively associated with the twoelectron RuIV/II. In sharp contrast, complex 2 shows a pH dependent one-electron wave at E1/2 = 0.83 V (pH 1),
assigned to the proton couple electron transfer process of the Ru III/II couple and a pH independent wave at E1/2
= 1.06 V assigned to the RuIV/III couple. Compound 2 was used to prepare complex [Ru(bda)(pic)(pyA)] (4). This
complex is air sensitive and converts to complex [Ru(bda)(pic)(pyE)] (5) (where pyE is methyl 2-pyridine
carboxylate) in the presence of methanol. This oxidation also occurs by applying a positive potential to an
aqueous solution of 4, producing the derivative [Ru(bda)(pic)(pyC)] (3). Cyclic voltammetry of 3 shows two pH
independent one-electron oxidation waves at E1/2 = 0.64 V and E1/2 = 1.0 V, corresponding to the RuIII/II and RuIV/III
couples, respectively. In addition, a water oxidation catalytic wave appears at Eonset ≈ 1.4 V. Foot of the wave
analysis of this catalytic wave based on a water nucleophilic attack accounts for a TOFmax = 0.63-0.74 s-1.

1

Key words
Ruthenium, water oxidation catalysis, dynamic behavior, benzylic alcohol oxidation, ligand design

2

1-Introduction
In the field of water oxidation to dioxygen catalysis, ruthenium complexes have played an important role not
only allowing the elucidation of catalytic mechanisms but also generating the fastest and most rugged
molecular catalysts reported to date.1 The recent advances have allowed the scientific community to identify
a series of features that are crucial to reach turnover frequencies (TOFs) above 10 4 s-1 and stabilities beyond
millions of turnover numbers (TONs).2 Among them it is important to highlight the ligand robustness, the
possibility to reach seven coordination number at high metal oxidation states and second coordination sphere
effects. There are two families of complexes that comply with these requirements and are characterized by
containing an equatorial polypyridine ligand with one or two carboxylate groups bound to the metal center.
In particular, ruthenium complexes containing the tetradentate bda2- (2,2'-bipyridine-6,6'-dicarboxylato)
ligand and pentadentate tda2- ([2,2':6',2''-terpyridine]-6,6''-dicarboxylato) ligand are those showing record
rates and stabilities (see Figure 1 for catalyst precursors).3 The former bda family of complexes are known to
follow a bimolecular mechanism in respect to Ru where two Ru-oxo groups react in the key O-O bond
formation step (I2M) while the latter tda family of catalysts follows a water nucleophilic attack mechanism
(WNA) assisted by the dangling carboxylate group in the equatorial ligand. The κ-N 2O2 coordination fashion of
the bda2- ligands provides high electron density to the metal center mainly due to the two carboxylate groups
that results in the lowest overpotential for water oxidation reaction based on ruthenium molecular complexes
reported until now. On the other hand, the κ-N 3O1 coordination mode of the tda2- ligand, with only one
carboxylate group bound to the metal center results in higher overpotential. However, the presence of a
dangling carboxylate group in the tda2- is key to provide fast catalysis through a WNA mechanism, and to
incorporate such catalytic centers in conductive and semiconductive solid supports. 2a,

4

This is not

straightforward with the bda family of complexes that follow an I2M mechanism, for which the incorporation
into electrodes results in decomposition to ruthenium oxide due to the restricted mobility of the ruthenium
centers once attached on a surface.5
In this work, we take inspiration from the best features of the bda and tda family of complexes and explore
the synthesis of the complex [RuIII(bda)(pic)(pyC)] where pic is 4-methylpyridine and pyC is 2pyridinecarboxylate (Figure 1, right). The complex has a similar coordination sphere around the metal center
to that of the bda family (Figure 1, left), this means, 4 neutral pyridine ligands and 2 anionic carboxylate
ligands. The main difference is that the second coordinated carboxylate group is provided by an axial pyridine
ligand instead of the equatorial bda2- ligand. At the same time, the target complex has a dangling carboxylate
that may assist a water nucleophilic attack to the high valent Ru-oxo species, in a similar fashion to that of the
tda complex (Figure 1, middle).

3

Figure 1. Left) Ruthenium complexes that are precursors of water oxidation to
dioxygen catalysts. Right) Catalyst designed in this work. Abbreviations: bda2- is
2,2'-bipyridine-6,6'-dicarboxylato,
tda2is
[2,2':6',2''-terpyridine]-6,6''dicarboxylato, pic is 4-methylpyridine, py is pyridine and pyC is 2pyridinecarboxylato.

4

2-Experimental Section
Synthetic materials: The bda2- ligand 2,2'-bipyridine-6,6'-dicarboxylato3a and the ruthenium precursor
[Ru(DMSO)4Cl2]6 and intermediate [Ru(bda)(DMSO)(X)] (where X = MeOH or DMSO)7 were prepared according
to the literature. The ortho-functionalized pyridines 2-pyridinecarboxylic acid (HpyC), pyridine-2-ylmethanol
(pyA) and other reagents including 4-picoline (pic) were obtained from Sigma-Aldrich and used as received.
When required, solvents were dried by following the standard procedures, distilled under nitrogen and used
immediately. High purity de-ionized water used for the electrochemistry experiments was obtained by passing
distilled water through a nanopure Milli-Q water purification system. For other spectroscopic and
electrochemical studies, HPLC-grade solvents were used. Instrumentation and methods: A 500 MHz Bruker
spectrometer was used to carry out NMR spectroscopy. ESI-Mass experiments were performed by using
micromass Q-TOF mass spectrometer. UV/Vis spectroscopy was performed on a Cary 50 (Varian) UV/Vis
spectrophotometer in 0.1 cm quartz cuvettes. Elemental analyses were carried out on Perkin-Elmer 240C
elemental analyzer. The pH of the electrolytic solutions was determined by a pH meter (CRISON, Basic 20+)
calibrated before measurements through a standard solutions at pH 4.01, 7.00 and 9.21. Manometric
measurements were performed on a Testo 521 differential pressure manometer with an operating range of
0.1-10 kPa and accuracy within 0.5% of the measurements. The manometer was coupled to thermostatic
reaction vessels for dynamic monitoring of the headspace pressure above each reaction solution. The
manometer’s secondary ports were connected to thermostatic reaction vessels containing the same solvents
and headspace volumes as the sample vials. Each measurement for a reaction solution (2.0 mL) was performed
at 298 K. Electrochemical techniques and materials: all electrochemical experiments were performed with an
IJ-Cambria CHI-660 potentiostat using a one-compartment three-electrode 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. All the potentials reported in this work were measured vs Hg/Hg2SO4 (K2SO4
saturated) as reference electrode (unless indicated), and converted to NHE by adding 0.648 V. Glassy carbon
disk (ф = 0.3 cm, S = 0.07 cm2) and Pt disk were used as Working Electrode (WE) and Counter Electrode (CE),
respectively, unless explicitly mentioned. Glassy carbon electrodes were polished with 0.05 μm alumina (Al2O3)
and rinsed with water. CVs and DPs were iR compensated at 85%. Cyclic Voltammograms (CV) were recorded
at 100 mV·s−1 scan rate, unless explicitly metnioned. The DPV parameters were ΔE= 4 mV, Amplitude = 50 mV,
Pulse width = 0.05 s, Sampling width = 0.0167 s, Pulse period = 0.5 s. The electrolytes used in the
electrochemical experiments were 0.1 M triflic acid solution for pH 1 and I = 0.1 M phosphate buffer solutions
with desired the pH. For routine bulk electrolysis experiments, a Pt mesh (cylindrical Pt mesh electrode, ø: 10
mm) was used as a WE, another Pt grid (square Pt mesh 20*20 mm) as a CE and a Hg/Hg 2SO4 (K2SO4 saturated)
as a RE.

5

Synthesis of the [Ru(Hbda)(DMSO)(pyC)] (1). [Ru(bda)(DMSO)(X)] (X = MeOH or DMSO) (150 mg, c.a. 0.3
mmol) and pyridine-2-carboxylic acid (HpyC) (40 mg, 0.32 mmol) were dissolved in anhydrous MeOH (200 mL)
and stirred (600 rpm) at room temperature overnight under Ar. The product precipitated as a red powder,
which was filtered off and washed with acetone (1 x 5 mL), Et2O (3 x 5 mL) and dried under vacuum (0.165
mmol, 55% yield). 1H NMR (500 MHz, D2O): δ (ppm) = 2.47 (s, 3 H), 2.67 (s, 3 H), 7.49 (d, J=7.72 Hz, 1 H), 7.76
(dd, J=4.93, 9.94 Hz, 1 H), 8.04 (d, J= 4.24 Hz, 2 H), 8.09 (t, J=7.96 Hz, 1 H), 8.14 (d, J=7.72, 1 H), 8.22 (t, J=7.98
Hz, 1 H), 8.40 (d, J=7.36, Hz 1 H), 8.56 (d, J=8.08 Hz, 1 H), 9.76 (d, J=5.52 Hz, 1 H). 13C NMR (125 MHz, MeOD):
δ= 42.27, 43.37, 124.20, 125.46, 126.27, 126.59, 127.31, 128.23, 138.26, 138.32, 139.94, 151.94, 155.22 ppm.
UV/Vis [λmax, nm (ε, M-1 cm-1)]: 256 (14698), 296 (20017), 396 (6083). Dark red single crystals of
Na[Ru(bda)(DMSO)(pyC)], Na[1-H], suitable for X-ray diffraction studies formed after dissolving 1 in the
minimum amount of MeOH at room temperature and left it in the fridge overnight, as a result of
deprotonation of the carboxylic acid in 1 and precipitation with sodium countercation from traces of inorganic
salts. Elemental analysis calcd (%) for C20H16N3O7RuS-Na+2H2OMeOH (C21H24N3NaO10RuS): C 39.75, H 3.81, N
6.62. Found C 39.02, H 3.93, N 6.83. MS (ESI, positive mode, MeOH): m/z+ = 589.95 (M + 2Na+).
Synthesis of the [Ru(bda)(DMSO)(pyA)] (2). [Ru(bda)(DMSO)(X)] (X = MeOH or DMSO) (200 mg, c.a. 0.4 mmol)
and pyridine-2-ylmethanol (pyA) (60.6 µl, 0.8 mmol) were dissolved in anhydrous MeOH (20 mL) and heated
to reflux for 3 h under Ar. A red solid formed, which was filtered to remove unreacted solid ruthenium
precursor. The filtered solution was concentrated by rotary evaporator and diethyl ether added inducing
precipitation of compound 2, which was filtered and washed with additional diethyl ether (0.38 mmol, 95%
yield). Single crystals of 2 were obtained by dissolving it in the minimum amount of methanol at room
temperature and left in the fridge overnight. 1H NMR (500 MHz, MeOD): δ (ppm) = 3.09 (s, 6 H), 5.43 (s, 2 H),
7.23 (t, J=6.32 Hz, 1 H), 7.29 (d, J=5.08 Hz, 1 H), 7.67 (d, J= 7.84 Hz, 1 H), 7.92 (t, J=3.35 Hz, 1 H), 8.06 (d, J=7.64,
2 H), 8.23 (t, J=7.92 Hz, 2 H), 8.71 (d, J=8.00 Hz, 2 H). 13C NMR (125 MHz, MeOD): δ= 42.65, 69.56, 120.70,
124.78, 126.83, 137.05, 138.53, 139.06, 147.96, 158.80, 161.92, 172.09 ppm. UV/Vis [λmax, nm (ε, M-1 cm-1)]:
242 (12018), 296 (26221), 407 (3691). Elemental analysis calcd (%) for C20H19N3O6RuS·2H2O (C20H23N3O8RuS):
C 42.40, H 4.09, N 7.42. Found C 42.84, H 3.84, N 7.39. MS (ESI, positive mode, MeOH): m/z+ = 553.9 (M + Na+).
Synthesis of the [Ru(bda)(pic)(pyA)] (4). [Ru(bda)(DMSO)(pyA)] (2) (151 mg, 0.27 mmol) and 4-methylpyridine
(pic) (0.3 ml, 3 mmol) were dissolved in anhydrous MeOH (20 mL) and heated to reflux overnight under Ar. A
dark-red solution appeared which was evaporated to dryness. The residual solid was purified by column
chromatography on silica gel using DCM/methanol (100:17) as eluents. Compound 4 was isolated as a red solid
(0.042 mmol, 15% yield). 1H NMR (500 MHz, MeOD): δ (ppm) = 2.40 (s, 3 H), 4.72 (s, 2 H), 7.21 (d, J= 6.10 Hz,
2 H), 7.47 (m, 2 H), 7.83 (d, J= 7.60 Hz, 2 H), 8.00 (t, J=3.15 Hz, 1 H), 8.06 (t, J=7.90 Hz, 1 H), 8.16 (d, J=6.60, 2
H), 8.37 (t, J=7.85 Hz, 1 H), 8.60 (d, J=4.02 Hz, 2 H). 13C NMR (125 MHz, MeOD): δ=19.36, 55.96, 123.34, 124.93,
125.86, 127.44, 129.63, 134.81, 136.70, 149.71, 150.30, 151.44, 151.58, 159.10, 171.31, 173.31. UV/Vis [λmax,

6

nm (ε, M-1 cm-1)]: 255 (15100), 301 (22998), 346 (6430), 419 (5325), 422 (5591). Elemental analysis calcd (%)
for C24H20N4O5Ru·H2OMeOHDCM (C26H28Cl2N4O7Ru): C 45.89, H 4.15, N 8.23. Found C 45.79, H 3.65, N 8.56.
MS (ESI, positive mode, MeOH): m/z+ = 575.9 [M-H++MeO]. Single crystals of derivative 5 were grown by
dissolving 4 in DCM/methanol and by diffusion of diethyl ether under air. Compound 5 is the result of the
oxidation of the benzylic alcohol in compound 4 to give the corresponding methyl ester group, see main text
for discussion.
X-ray Crystallography. Data collection: Crystal structure determination for compounds 1 and Na2[Ru(bda)2]
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 CrysAlisPro8 V/.60A and absorption
correction with Scale3 Abspack scaling algorithm9. Crystal structure determination for compounds 2 and 5
were carried out using a Apex DUO Kappa 4-axis goniometer equipped with an APPEX 2 4K CCD area detector,
a Microfocus Source E025 IuS using MoK radiation, Quazar MX multilayer Optics as monochromator and an
Oxford Cryosystems low temperature device Cryostream 700 plus (T = -173 °C).

Crystal structure

determination for samples Full-sphere data collection was used with  and  scans. Programs used: Data
collection APEX-210, data reduction Bruker Saint11 V/.60A and absorption correction SADABS.12 Structure
Solution and Refinement: Crystal structure solution was achieved using the computer program SHELXT13.
Visualization was performed with the program SHELXle.14 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 2015.15 All non-hydrogen atoms were refined including
anisotropic displacement parameters. Comments to the structures: Na2[Ru(bda)2]: The asymmetric unit
contains one molecule of the metal complex, two sodium cations, 1.65 molecules of water (partially
disordered) and 4.60 molecules of methanol (partially disordered). The distance between the two
independent Sodium cations is of 3.42 Å and they are additionally connected through oxygen bridges coming
from the dimethyl sulfoxide groups. 1: The asymmetric unit contains one molecule of the metal complex, a
sodium cation, two water molecules and a methanol molecule. The sodium cation is coordinated to the metal
complex through the dimethylsulfoxide group and two carboxylates. Additionally, the sodium cation is
coordinated to the two water molecules and forms a dimer with another sodium cation. The distance between
sodium atoms is of 3.7 Å and they are connected through oxygen bridges coming from the dimethyl sulfoxide
groups. The dimer formed by the sodium cations has Ci symmetry. 2: The asymmetric unit contains two
independent molecules of the metal complex and highly disordered solvent molecules. A diethyl ether
molecule with an occupancy of 50 % is disordered around an inversion center shared with the neighboring
unit cell and could be refined properly. A second highly disordered solvent molecule, which could contain
7

diethyl ether or ethanol, could not be refined properly. In order to avoid these highly disordered solvent
molecule the program SQUEEZE16 was applied to remove it from the electron density. 5: The asymmetric unit
contains one independent molecule of the metal complex, one methanol molecule and half water molecule
disordered in two positions (occupancy 0.3:0.2).

8

3-Results
3·1-Preparation and characterization of compounds 1-4
The synthesis of complexes with general formula [Ru(bda)(py’)(py’’)] where py’ and py’’ are two different
monodentate pyridine ligands can be achieved by two consecutive substitution reactions from
[Ru(bda)(DMSO)(X)] where X is MeOH or DMSO (Scheme 1).17,7 The intermediate [Ru(bda)(DMSO)(X)] was
obtained as a brown solid with a yield of ca. 67 %, along with a side product identified as the bis-substituted
bda complex [Ru(bda)2]2- (ca. 5-7 %). The latter compound has never been reported before, it has been fully
characterized by NMR spectroscopy, single crystal x-ray diffraction (XRD) and elemental analysis as described
in detail in the experimental section and the supporting information.
The reaction of [Ru(bda)(DMSO)(X)] with 2-pyridinecarboxylic acid (HpyC) affords complex 1 in 55 % yield,
which has been fully characterized by spectroscopic, analytic and electrochemical techniques. Single crystals
of the anionic complex [Ru(bda)(DMSO)(pyC)], (1-H)-, suitable for XRD studies were obtained and its ORTEP
plot is shown in Figure 2. Compound (1-H)- is a RuII complex with pseudo-octahedral geometry with Ru-N, RuO and Ru-S bond distances similar to those of other Ru complexes of the bda family. 18 Interestingly, the bda2ligand coordinates in a κ-N2O1 fashion while the pyC ligand acts as a bidentate ligand with the carboxylate
occupying the fourth equatorial position. As a consequence, the angle O-Ru-O formed by the equatorial ligands
is 93.72˚, close to the 90˚ expected for a perfect octahedral geometry. These angle values are in sharp contrast
to those observed for Ru complexes, where the bda2- coordinates in a κ-N2O2 mode resulting in broad O-Ru-O
angles in the range of 121° to 124°.18a, 19 The axial positions in 1 are occupied by DMSO and the pyridine of the
pyC ligand. The 1H NMR spectrum of 1 in deuterated water shows a non-symmetric nature of the two pyridine
rings of the bda2- ligand, even at high temperature, and indicates that the structure just described in the solid
state is maintained in solution (Figure S7 and Figure S12).
The reaction of 1 with 4-picoline (pic) to afford the targeted compound [Ru(bda)(pic)(pyC)]-, 3, was not
possible after several attempts at high temperature, long reaction times and different combination of solvents.
The main products observed after reaction were the starting materials, [Ru(bda)(pic)(DMSO)] and
[Ru(bda)(pic)2].
In view of the unsuccessful reaction to prepare 3 using 1 as precursor, an alternative route was designed,
based on the intermediate [Ru(bda)(DMSO)(pyA)], 2 in Scheme 1, that contains an hydroxymethyl group in
the orto position of the axial pyridine. Compound 2 was prepared in 95 % yield starting from precursor
[Ru(bda)(DMSO)(X)] (where X = MeOH or DMSO). The solid state structure of the neutral complex 2 was
determined by single crystal XRD and its ORTEP plot is shown in Figure 2. The alcohol in the axial ligand is
coordinating the metal center leaving a dangling carboxylate of the bda2- ligand, that binds in a κ-N 2O1 mode.
It shows an analogous geometry to that of compound 1 with similar bond distances and angles.

9

Scheme 1. Synthetic scheme for the preparation of [Ru(Hbda)(DMSO)(pyC)] (1), [Ru(bda)(DMSO)(pyA)] (2), [Ru(bda)(pic)(pyC)]- (3),
and [Ru(bda)(pic)(pyA)] (4).

Figure 2. ORTEP plots at 50% probability for (1-H)-, 2 and 5. H atoms are omitted for clarity except for the H atom coordinated to
oxygen atom in 2.

On the other hand, NMR characterization of 2 in methanol at room temperature shows only one set of
resonances for the two pyridines of the bda2- ligand, indicative of a symmetric geometry as opposed to what
we observed in the solid state (Figure 3, blue spectrum). By lowering the temperature to 223 K, the bda 2- ligand
resonances become broad suggesting a fast equilibrium between species with different connectivity as
indicated in the sequence of reactions in the top of Figure 3. These types of equilibria have been observed
before for ruthenium complexes containing the bda2- and tda2- equatorial ligands.3b,

10

20

In addition, the

characteristic benzylic resonance at  = 5.43 ppm split upon lowering the temperature, supporting the
involvement of this group in the fast equilibria (Figures S18-S19).
These results highlight a crucial difference between 1 and 2. While the carboxylate group in the axial pyridine
of 1 binds very strongly to the ruthenium, the alcohol in 2 is more labile and can coordinate and decoordinate
easily at room temperature.

Figure 3. Zoom of the aromatic region of a 1H-NMR of 2 in [d4]-Methanol at 223 K (Red) and 298 K (blue).

Compound 2 is also more reactive than 1, as it was possible to substitute the DMSO axial ligand for a pic ligand
to give compound [Ru(bda)(pic)(pyA)], 4, as indicated in Scheme 1. Compound 4 is fairly air sensitive and gets
oxidized to its RuIII derivative leading to further decomposition when exposed to open atmosphere for
prolonged times. It was fully characterized by spectroscopic and analytical techniques as described in detail in
the experimental section. The 1H NMR spectrum of 4 shows a symmetric bda2- backbone, indicating that in
solution this compound is involved in a fast dynamic behavior analogous to that of its parent compound 2
(Figures S20-S25). In an attempt to crystallize compound 4, we obtained the derivative product 5 in Figure 2
which is the result of the oxidation of the benzylic alcohol to a methyl ester group. Indeed, benzylic groups are
prone to oxidation, which could be a consequence of the nucleophilic attack of a methanol solvent molecule
used in the crystallization mixture, to the electrophilic carbon of the coordinated alcohol group. Alternatively,
the close proximity of the methylene group in 4 to a putative high valent and strongly oxidizing Ru-OH or Ru=O
group could also be responsible of the oxidation to a carboxylic acid that esterifies in the presence of

11

methanol.21 The formation of the Ru-OH or Ru=O reactive species implies i) oxidation of the initial RuII by air
and ii) the coordination of an aquo ligand coming from moisture in the crystallization solvent.

12

3·2-Electrochemistry of 1 and 2
The electrochemical properties of complexes 1 and 2 were investigated in aqueous solution at pH 1 and pH 7
by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In this manuscript all potentials
are reported versus the normal hydrogen electrode (NHE).
The cyclic voltammetry of compound 1 in pH 1 ( Figure 4) shows a quasi-reversible wave at E1/2 = 1.12 V with
a rather low peak to peak separation (∆E = 48 mV) suggesting that either an adsorption phenomenon or a twoelectron process is taking place at this potential. Analysis of the ip of the anodic peak versus the square root
of the scan rate (υ) gives a good linear fit, pointing to a homogeneous phase electron transfer process (Figure
S27). In addition, a plot of the logip versus logυ shows a linear trend with a slope of 0.54, supporting a high
homogeneous contribution to the process.22 From these experiments, we tentatively assign the wave at E1/2 =
1.12 V to a two-electron process, where E2 < E1 in Scheme 2, which could be a consequence of the formation
of a putative seven coordinated species at oxidation state RuIV. In the second CV scan, a new wave appears at
E31/2 = 0.54 V (∆E = 59 mV), that we attribute to a Ru-OH2/Ru-OH complex, that is formed upon substitution of
one of the carboxylate groups by a water solvent molecule at high oxidation states (Scheme 2, bottom). This
assignment is supported by the pH dependence behavior of this wave and because it does not appear when
compound 1 is analyzed in organic solvents (Figure S28). This type of carboxylato/aquo substitution is common
in Ru-bda type of complexes.20 Three additional weak redox processes at E ≈ 0.26 V, E ≈ 0.98 V and E ≈ 1.40 V
also appear after the first scan (see also Figure S26). We attribute them to side-products derived from DMSO
ligand oxidation or to isomers of complex 1 that differ in the coordination modes of the bda2-, pyC or DMSO
ligands. The intensities of these waves are higher at pH 7 (Figure S26). At this pH, the redox wave at E11/2 =
1.12 V is not fully reversible, consistent with the presence of higher amount of side-products. In addition, a
redox wave appears at E1/2 = 0.30 V (∆E = 80mV) at pH 7 after sweeping the cycle to the second wave, which
corresponds to the Ru-OH2/Ru-OH complex that appears at E31/2 = 0.54 V at pH1.

Figure 4. Two consecutive CV cycles of 1 at pH 1 (TfOH = 0.1 M), ν = 100 mV/s. [Ru]: 1mM, WE: glassy carbon electrode;
CE: platinum electrode; RE: Hg/Hg2SO4. Asterisks (*) show unidentified redox waves.

13

Scheme 2. Reactions involved in the redox chemistry of 1.

Despite the similarities in the coordination environment of complexes 1 and 2 in the solid state structures
depicted in Figure 2, the cyclic voltammetry of 2 shows a very different redox profile when compared to that
of 1. In the first scan of a cyclic voltammetry at pH 1 compound 2 shows two redox processes at E41/2 = 0.83 V
(∆E ≈ 127 mV) and E51/2 = 1.06 V (∆E ≈ 262 mV) associated with the RuIII/II and RuIV/III couples respectively (red
trace in Figure 5). The first wave is pH dependent and appears at E41/2 = 0.51 V (∆E = 95 mV) at pH 7 (blue trace
in Figure 5). The Nernstian behavior of the first wave E4 is a clear indication that a proton coupled electron
transfer (PCET) process is taking place.23 As shown in Scheme 3, this PCET is attributed to the RuIII/II couple with
concomitant proton loss from the coordinated alcohol. The RuIV/III oxidation wave of 2 is pH independent and
appears at E51/2 = 1.06 V (∆E = 141 mV) at pH 7. In the cathodic scan of the cyclic voltammetry of 2 in pH 7
solution, a new redox wave appears at Ec ≈ 0.55 V only when the anodic scan is swept beyond the second
oxidation while the first wave at E41/2 = 0.51 V decreases in intensity (see also Figure S29 and S30). This
phenomenon also occurs at pH 1 and is progressive upon cycling (Figure S31). These results suggest that 2
converts to a new species that we attribute to DMSO ligand oxidation, to the oxidation of the pyA ligand to
give the corresponding carboxylic acid pyC ligand or both. Indeed, the new wave appearing upon scanning
appears at E ≈ 1.1 V, in a very similar potential to that observed for compound 1 containing the carboxylate
group.

14

Figure 5. CV of 2 at pH 1 (blue) and pH 7 (red). Conditions: pH 1 (TfOH= 0.1 M), pH 7 (phosphate buffer aqueous
solution), ν = 100 mV/s. [Cat]: 1mM, WE: glassy carbon electrode; CE: platinum electrode; RE: Hg/Hg2SO4. Black arrow
shows the scan direction. Asterisk (*) shows unidentified redox wave that appears only after scanning the second
oxidation wave.

Scheme 3. Reactions involved in the redox chemistry of 2.

15

3·3-Electrochemistry of 4 and its transformation to a water oxidation catalyst
The electrochemical properties of complex 4 were investigated in aqueous solution at pH 7 by means of CV,
DPV and bulk electrolysis experiments.
The first CV scan of a solution of 4 in pH 7 shows a complex behavior with five waves appearing at E61/2 = 0.64
V (∆E ≈ 66 mV), E71/2 = 0.86 V (∆E ≈ 89 mV), E81/2 = 1.0 V (∆E ≈ 74 mV), E9p,a = 1.33 V and E10p,a = 1.42 V (Figure
6, left). Upon cycling in the potential range from 0.1 V to 1.6 V, the CV profile simplifies leaving only the
reversible waves E6 and E8 (Figure 6, right and Figure S31). In addition, the appearance of a very intense,
irreversible wave at Eonset ≈ 1.4 V appears upon cycling, that we tentatively attribute to water oxidation
catalysis. Indeed, a bulk electrolysis experiment of a solution of 4, shows a steady current versus time profile
upon applying a potential of Eapp= 1.5 V, indicative of a catalytic process (Figure 7 and Figure S35). Importantly,
the waves at E6 and E8 are fully recovered after the catalytic wave, a proof of the integrity of the complex upon
turnover. These two waves are pH independent and are consistent with two successive one-electron
oxidations from RuII to RuIV of a complex that do not contain a Ru-OH2 group (Figure S33). Thus, the aquo ligand
must coordinate at the RuIV or RuV oxidation states to form a Ru-oxo/hydroxo species responsible of triggering
the catalysis. A foot of the wave analysis (FOWA) of a cyclic voltammetry containing the catalytic species gives
a maximum turnover frequency (TOFmax) of 0.63-0.74 s-1 (Figure S34).

Figure 6.Left) First CV scan of 4 in a phosphate buffer aqueous solution at pH 7. Right) Comparison of CV of 4 before (red)
and after 200 cycles (black). [Ru]: 1mM, ν = 100 mV/s, WE: glassy carbon electrode; CE: platinum electrode; RE:
Hg/Hg2SO4.

16

Figure 7. Bulk electrolysis of 4 in a phosphate buffer solution at pH 7 solution at Eapp= 1.5 V. During the first 55 minutes,

complex 4 converts to an active complex (green area). From this time, the current passed is attributed to the water
oxidation catalysis process by the active species, giving a TON = 587, based on the active catalyst on the surface, see
supporting information.24 Conditions: [Ru] = 1 mM, Volume = 3mL, Eapp= 1.5 V, WE: glassy carbon rod, CE: Pt mesh
electrode (simple square mesh net 20*20 mm), RE: Hg/Hg2SO4.

4-Discussion
The first attempt towards the synthesis of the target compound [Ru(bda)(pic)(pyC)], 3, from the intermediate
[Ru(Hbda)(DMSO)(pyC)], 1, by substitution of the DMSO ligand by pic was unsuccessful due to the low
reactivity of 1. Thus, an alternative route based on the intermediate [Ru(bda)(pic)(pyA)], 4, successfully
prepared from [Ru(bda)(DMSO)(pyA)], 2, was pursued. The distinct reactivity towards DMSO substitution of
related compounds 1 and 2 can be understood from their respective unique structural and electronic features.
While the carboxylate group of the axial pyC ligand is strongly bound to the ruthenium, the alcohol group in
the pyA ligand is labile and is involved in a dynamic behaviour in solution. This is demonstrated by low
temperature 1H NMR experiments that show a non-symmetric nature of the bda2- ligand while analogous
experiments done at room temperature show a perfectly symmetric compound, a result of an averaged of all
the species involved in the equilibrium (Figure 3). In addition, the electrochemical behaviour of 1 and 2 are
radically different. Compound 2 shows two consecutive one-electron oxidation, as expected for this type of
complexes. In sharp contrast, complex 1 shows a complex redox behavior with a main wave that has been
assigned to a single two-electron oxidation event, likely a consequence of the favored seven coordination
(CN7) environment around the metal center upon oxidation (Scheme 2).1a, 25
The benzylic alcohol in 4 is susceptible to oxidation, and upon exposure to the air in a methanolic solution it
produces the ester derivative 5 in Figure 2. This is the first indication of the feasibility of preparing the desired
complex 3 by oxidation of the alcohol precursor 4 in the absence of an esterification agent. Indeed, the
methylene group in the ortho-position of the pyridine ligand pyA is situated in a very close proximity to the
17

ruthenium center, which can catalyze the reaction. This oxidation can be accelerated electrochemically, by
cycling towards oxidative potentials to produce a high valent ruthenium complex, which can coordinate a
solvent water molecule generating compound 4IV(O) in Scheme 4. This RuIV=O species is very reactive and can
undergo the oxidation of the alcohol to carboxylic acid through an oxygen transfer reaction that would
produce the target compound 3.21 This hypothesis is supported by the electrochemical behaviour of 4 in water,
summarized in Figure 6. While the first scan of a cyclic voltammetry experiment shows a complex profile with
five redox waves, indicative of a mixture of species present in solution, it progressively evolves towards a much
simpler profile. As shown in the black trace of Figure 6 (right), only two consecutive one-electron oxidation
waves are observed, attributed to the RuIII/II and RuIV/III redox couples, followed by a catalytic process with Eonset
= 1.4 V. This behavior is fully consistent with complex 3, which at low oxidation state, does not coordinate any
aqua ligand due to the strong binding of the carboxylate of the axial pyC ligand leading to two pH independent
redox events (Scheme 4, bottom, and Table 1). However, at high oxidation states, the possibility to reach CN7
facilitates the coordination of a water molecule producing the Ru-oxo species 3(O)IV, which is further oxidized
to 3(O)V before catalysis starts. At pH 7, the dangling carboxylate group in 3(O)V is deprotonated and

can better assist the water nucleophilic attack of a water molecule to the Ru=O group in the O-O
bond formation step by accepting a proton of the incoming water.

Scheme 4. Putative mechanism to form the active water oxidation catalyst, 3(O), after bulk electrolysis of 4 in a
phosphate buffer solution at pH 7 and at Eapp= 1.5 V.

18

Foot of the wave analysis of the cyclic voltammetry of a solution containing electrogenerated 3 accounts for a
TOFmax= 0.63-0.74 s-1 assuming a water nucleophilic attack mechanism. This value is one of the highest
reported in the literature but it is significantly lower than that of the widely used and related [Ru(bda)(pic) 2]
complex with TOFmax= 11 s-1 based on a mechanism involving the interaction of two M=O groups (Table 1). 3a,
25

Complexes 3 and [Ru(bda)(pic)2] have a similar coordination sphere with four pyridine type of ligands and

two carboxylate groups. However, they have a significantly different geometry; while compound
[Ru(bda)(pic)2] shows a highly distorted octahedral geometry due to the wide O-Ru-O angle around 124°
provided by the bda2- ligand, complex 3 is expected to be close to the ideal octahedral coordination with a ORu-O around 90°, as suggested by the XRD structures of (1-H)- and 5 in Figure 2. This is a consequence of the
chelate effect of the axial pyC ligand, that strongly binds to the ruthenium leaving a carboxylate of the bda 2ligand dangling. This key feature can determine the final geometry of the reactive 3(O)V, which can be either
six or seven coordinated, influencing the overpotential of the reaction.26
Another key feature of 3 that differs from compound [Ru(bda)(pic)2] is the presence of a dangling carboxylate
group if the active 3(O)V is CN7 or even two dangling carboxylate groups if it has CN6 (Scheme 4). Interestingly,
this is not enough to outperform the high catalytic activity of the fastest molecular water oxidation catalyst
reported to date [Ru(tda)(py)2] with TOFmax = 7700 s-1 (Table 1).3b The latter is characterized by a unique
mechanism based on a water nucleophilic attack assisted by the well positioned dangling carboxylate group
of the equatorial ligand (Table 1).
Table 1. Electrochemical and water oxidation data at pH 7 for complex 3 and structurally related compounds
[Ru(bda)(pic)2] and [Ru(tda)(py)2] reported in the literature.
E in volts vs NHE at pH 7

TOFmax[a]

RuIII/II

RuIV/III

Ecat,onset

(s-1)

[RuIII(κ-N2O1-bda)(pic)(κ-N1O1-pyC)], 3

0.65[b]

1.01[b]

1.4[c]

[RuII(κ-N2O2-bda)(pic)2]

0.55

0.82

1.1

11

3a, 27

[RuII(κ-N3O1-tda)(py)2]

0.55[b]

1.10[b]

1.3[c]

7700

3b, 27

Ru complex

Ref.

0.63-0.74 This work

[a] Calculated using the FOWA method27 and assuming a mechanism based on the interaction between two M=O groups
(I2M) for [RuII(κ-N2O2-bda)(pic)2] and a water nucleophilic attack mechanism (WNA) for 3 and [RuII(κ-N3O1-tda)(py)2]. [b]
Data for the complex with no hydroxo/oxo coordinated to the ruthenium. [c] Data for the ruthenium oxo species that
forms only at high oxidation states.

19

5-Conclusions
An intramolecular oxidation of the benzylic alcohol of complex [Ru(bda)(pic)(pyA)], 4, allowed us to prepare
the corresponding carboxylic acid complex [Ru(bda)(pic)(pyC)], 3, in situ by electrochemical techniques.
Compound 3 is a precursor of a water oxidation catalyst that has similar coordination sphere to those of wellknown [Ru(bda)(pic)2] and [Ru(tda)(py)2] complexes, which are very active catalysts.
While the catalytic results obtained for 3 are amongst the best reported to date, with TOFmax = 0.63-0.74 s-1 at
an onset potential of 1.4 V, it doesn’t overcome the results obtained for [Ru(bda)(pic)2] and [Ru(tda)(py)2]
under the same experimental conditions (TOFmax = 11 s-1 at 1.1 V and TOFmax = 7700 s-1 at 1.3 V, respectively).
We attribute the lower performance of 3 to the different geometrical requirements of the equatorial and axial
ligands. In particular, to the stabilization effect of the pyridine-2-carboxylate ligand (pyC) in 3 that binds in a
bidentate fashion and leaves a dangling carboxylate from the bda2- axial ligand. As a consequence, 3 adopts a
coordination sphere that is close to a perfect octahedral geometry with angles around 90°, as observed for
the related compounds [Ru(bda)(DMSO)(pyC)], (1-H)-, and 5 in Figure 2, that were studied in the solid state.
In sharp contrast, the axial ligands in [Ru(bda)(pic)2] and [Ru(tda)(py)2] are monodentate, thus the equatorial
ligands bda2- and tda2- act as tetradentate ligands, forcing a distorted octahedral geometry with O-Ru-O and
Neq-Ru-O angles of 124° and 141° at oxidation state RuII, respectively. In addition, the tda2- ligand can act as a
pentadentate ligand at RuIV forming a CN7 complex. These characteristics together with additional second
coordination sphere effects, provide the optimum electronic and geometric features to undergo fast water
oxidation catalysis.
This work highlights the importance of geometry around the ruthenium metal center in complexes that are
active for the water oxidation catalysis. It also provides evidence of the reactivity of Ru towards intramolecular
oxidation of neighboring benzylic groups.

Acknowledgements
Support from MINECO, FEDER and AGAUR are gratefully acknowledged through grants CTQ2016-80058-R,
CTQ2015-73028-EXP, SEV 2013-0319, ENE2016-82025-REDT, CTQ2016-81923-REDC, and 2017-SGR-1631. A.
G. thanks MINECO for a FPI grant (BES-2015-073069).

Supporting Information
Additional crystallographic, spectroscopic, mass spectrometry and electrochemistry data.

20

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22

Graphical Abstract

Synopsis
Intramolecular oxidation of a benzylic alcohol by a high valent ruthenium-oxo group of a non-symmetric Rubda complex leads to an active catalyst for the water oxidation catalysis, which is characterized by leaving a
dangling carboxylate of the bda equatorial ligand that coordinates in a tridentate mode. The geometrical and
electronic characteristics of this catalyst provide distinct catalytic performance compared to the well-known
symmetric Ru-bda complexes, in which the bda ligand binds in a tetradentate fashion.

23