Polymeric binuclear ruthenium complex as efficient electrocatalyst for oxygen evolution reaction

Article abstract of DOI:10.1016/j.electacta.2018.06.138

A robust polymeric material constituted by chains of binuclear aqua ruthenium terpyridylphenanthroline complexes connected through benzene ring meta-positions, exhibiting an unusual two electrons and two protons PCET (proton-coupled electron transfer) process and enhanced oxygen evolution reaction (OER) activity, is described. The catalytic active [Ru^V[dbnd]O] species is generated at potentials more positive than +1.5 V (NHE) in the poly-[{Ru(H₂O)(phen)}₂(tpy₂ph)] film, as confirmed by the rise of the ruthenyl (Ru^V[dbnd]O) stretching band at 810 cm^?1 in the Raman spectrum, and the clear change to a higher activity regime as confirmed by EIS. Faradaic efficiency for OER as high as 39% was achieved at +1.8 V, indicating that poly-[{Ru(H₂O)(phen)}₂(tpy₂ph)] is one of the most active ruthenium terpyridine complexes ever prepared. The synthesis and characterization of the monomeric binuclear [{RuCl(Clphen)}₂(tpy₂ph)]²⁺ and its respective [{Ru(H₂O)(Clphen)}₂(tpy₂ph)]⁴⁺ aqua complex are also fully described.

Full text of DOI:10.1016/j.electacta.2018.06.138

Accepted Manuscript
Polymeric binuclear ruthenium complex as efficient electrocatalyst for oxygen
evolution reaction
Tiago A. Matias, André L.A. Parussulo, Paola A. Benavides, Robson R. Guimarães,
André H.B. Dourado, Marcelo Nakamura, Susana I.Córdoba de Torresi, Mauro
Bertotti, Koiti Araki
PII:
S0013-4686(18)31426-9
10.1016/j.electacta.2018.06.138
EA 32131
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 27 February 2018
Revised Date: 20 June 2018
Accepted Date: 21 June 2018
Please cite this article as: T.A. Matias, André.L.A. Parussulo, P.A. Benavides, R.R. Guimarães,
André.H.B. Dourado, M. Nakamura, Susana.I.Có. de Torresi, M. Bertotti, K. Araki, Polymeric binuclear
ruthenium complex as efficient electrocatalyst for oxygen evolution reaction, Electrochimica Acta (2018),
doi: 10.1016/j.electacta.2018.06.138.
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ACCEPTED MANUSCRIPT
Polymeric Binuclear Ruthenium Complex as Efficient Electrocatalyst for Oxygen
Evolution Reaction
Tiago A. Matias1*, André L. A. Parussulo1, Paola A. Benavides1, Robson R. Guimarães1,
André H. B. Dourado1, Marcelo Nakamura1, Susana I. Córdoba de Torresi1, Mauro
Bertotti1, Koiti Araki1*
1Department of Fundamental Chemistry, Institute of Chemistry, University of Sao Paulo,
Av. Prof. Lineu Prestes 748, Butanta, Sao Paulo, SP, 05508-000, Brazil.
Abstract:
A robust polymeric material constituted by chains of binuclear aqua ruthenium
terpyridylphenanthroline complexes connected through benzene ring meta-positions,
exhibiting an unusual two electrons and two protons PCET (proton-coupled electron
transfer) process and enhanced oxygen evolution reaction (OER) activity, is described. The
catalytic active [RuV=O] species is generated at potentials more positive than +1.5 V
(NHE) in the poly-[{Ru(H2O)(phen)}2(tpy2ph)] film, as confirmed by the rise of the
ruthenyl (RuV=O) stretching band at 810 cm-1 in the Raman spectrum, and the clear change
to a higher activity regime as confirmed by EIS. Faradaic efficiency for OER as high as
39% was achieved at +1.8 V, indicating that poly-[{Ru(H2O)(phen)}2(tpy2ph)] is one of the
most active ruthenium terpyridine complexes ever prepared. The synthesis and
characterization of the monomeric binuclear [{RuCl(Clphen)}2(tpy2ph)]2+ and its respective
[{Ru(H2O)(Clphen)}2(tpy2ph)]4+ aqua complex are also fully described.
Keywords: Oxygen evolution reaction, ruthenium complex, binuclear catalyst, high
valence complex, electrocatalysis, modified electrodes
Corresponding author:
E-mail address: koiaraki@iq.usp.br (K. Araki), Phone: +55 11 3091 8513.
E-mail address: tiagomatias@usp.br (T. A. Matias), Phone: +55 11 3091 3819.

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Highlights:
-
-
-
Polymeric binuclear aqua ruthenium terpyridylphenanthroline complex as OER catalyst
Robust and efficient RuV=O based electrocatalyst for tetraelectronic water oxidation
Local microenvironment induced a 2e–+2H+ PCET process that is absent in solution
1. Introduction:
The production of solar fuels by artificial photosynthesis, that is the conversion of
sun light into high energy chemicals such as H2, CO, CH4, etc., is a promising approach to
renewable and clean energy sources.[1-11] For this, it is necessary to develop strategies for
efficient activation of small molecules like CO2, H2O, and N2 [12-15] allowing their fast
conversion to solar fuels. Also great deal of efforts have been spent on the development of
devices for efficient artificial photosynthesis and photoelectrochemical devices for
conversion of light into electricity.[4, 5, 7-9, 16-18] Both can be obtained by assembling
together two electrodes connected by an external circuit. However, in the first one, the
anode made of a metal oxide semiconductor generally is sensitized with a light absorbing
species able to inject an electron in the conduction band when in the excited state,
generating a catalytic active species able to promote the oxidation of the substrate present
in the electrolyte solution. On the other hand, the cathode that receive the electron is
covered up with a thin film of a material that catalyses the reduction reaction storing the
photons energy as chemicals.[4, 5, 7-9, 16-18] One of the most interesting molecules is
water since it dissociates generating electrochemically active H+ and OH- that can be
reduced and oxidized, respectively to hydrogen and oxygen gas, that when combined in
internal combustion engines or in fuel cells just regenerates pure water.[19-22]
Accordingly, this molecule is pointed out as a key compound in the development of
sustainable and environmentally friendly artificial photosynthesis devices for production of
solar fuels.[8-10] However, to realize such a water-splitting reaction it is necessary to find
durable and efficient catalysts to accelerate the formation of hydrogen, but especially to

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accelerate the complex oxidation half-reaction described by the equation 2H2O → O2 + 4H+
+ 4e–. Despite all efforts, this multielectron/multiproton reaction involving the formation of
an O=O bond is imposing a major challenge that has not being overcame yet.[8, 9]
Among the molecular species, ruthenium polypyridyl complexes play a fundamental
role in coordination chemistry, and are well known as potential catalysts for the water
oxidation reaction,[8, 9, 16-18, 23-25] being explored as key versatile components in
artificial photosynthesis devices.[16-18] Many mononuclear ruthenium catalysts for water
oxidation have been developed[9] since the seminal report by Thummel et al,[26]
challenging the previous model proposed by Meyer et al, in which the activated species
involved the simultaneous interaction of a water molecule with two high valence metal
centres. Some of them were proven to be very efficient exhibiting catalytic activities similar
to that of Photosystem II (PSII), the biological system specialized in the oxidation of water
to oxygen in green plants.[27] Nevertheless, the activated complex of the mononuclear
OER catalysts seems to be a binuclear intermediate eventually formed by hydrogen bond,
π-π and other intermolecular and/or intramolecular interactions, showing the great
relevance of understanding the fundamentals of catalysed water oxidation reactions.
On the other hand, it is fundamental integrating the water oxidation catalyst to
artificial photosynthesis devices. In this context, molecules with carboxylate and
phosphonate groups can be easily bonded to the surface of metal oxides, thus transferring
their electrocatalytic properties to electrodes surface, a fundamental step in that direction.
The stability of such monolayer assembly is pH dependent however, and can result in
desorption of catalyst and loss of efficiency or, in more drastic cases, to its complete
deactivation.[28, 29] A very interesting approach to increase the surface concentration and
enhance the interfacial stability of ruthenium polypyridyl complexes is by
electropolymerization and formation of a thicker polymeric network.[29-32]
Recently we reported on a weakly coupled binuclear ruthenium polypyridyl
catalyst, [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4, where bpy = 2,2′-bipyridine and tpy2ph = 1,3-
bis(4′-2,2′:6′,2′′-terpyridin-4-yl)benzene, exhibiting a rate constant for water oxidation
kcat(O2) about 20 times larger than that of the analogous mononuclear complex.[33] In this
way, we were encouraged to immobilize that complex to a surface and investigate the

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catalytic activity of the resulting modified electrodes. The 5-Chloro-1,10-phenanthroline
(Clphen) substituted derivative of that binuclear complex was chosen for that purpose since
this ligand is known to form a C-C bond directly connecting two phenanthroline ligands
upon reductive elimination of chloride. However, the presence of not one but two
[Ru(Clphen)(tpy)] groups in the binuclear complex [{Ru(H2O)(Clphen)}2(tpy2ph)] allows
the formation of polymer chains (Fig. 1), in a similar way as in mononuclear complexes
having two of that polypyridyl ligand.[30-32] Accordingly, the new binuclear complex
enabled the preparation of electrodes modified with robust electrochemically active
polymeric films, whose repeating unit is the precursor of the catalytic active species. On the
other hand, the close proximity of the ruthenium complexes allows many statistically
different local arrangements and configurations with enhanced activity towards the oxygen
evolution reaction.
Figure 1: Schematic wireframe representation of the structure of the repeating unit of poly-
[{Ru(H2O)(phen)}2(tpy2ph)] material. The trifluoromethanesulphonate, TfO, counter ion
was omitted for clarity.
2. Experimental Section
1
Analytical-grade reagents were used without purification in all experiments. H
NMR spectra were recorded in a Bruker DRX 800 or 500 MHz instrument by using
suitable deuterated solvents and tetramethylsilane (or the solvent resonance) as reference.
Mass spectra (ESI-MS) were collected using a Bruker Daltonics Esquire 3000 Plus
spectrometer setting the capillary potential to 4 kV and the sample injection rate to 180 ꢀL
h–1. Elemental analyses were performed using a Perkin Elmer 2400 series II analyser.
UV/Vis absorption spectra were obtained in a Hewlett Packard 8453A diode array

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spectrophotometer. Cyclic voltammograms and electrochemical impedance spectra were
recorded using an Autolab PGSTAT30n potentiostat/galvanostat, and a conventional three
electrodes cell consisting of a platinum (organic solvent) or glassy carbon disc (aqueous
medium) working electrode, a platinum wire auxiliary electrode, and a Ag/AgNO3 (10 mM
in acetonitrile; +0.503 V vs. NHE) or Ag/AgCl (1.0 M KCl; +0.222 V vs. NHE) reference
electrode for organic and aqueous media, respectively. The electrochemical impedance
spectra were recorded in the 1 Hz to 10 kHz range, superimposing a 20 mV amplitude
sinusoidal alternating voltage perturbation onto the 1.3, 1.4, 1.5, 1.6, 1.7 and 1.8 V base
potential. Tetrabutylammonium perchlorate (Bu4NClO4; 0.10 M) and KNO3 (0.10 M) were
used as electrolyte respectively in organic and aqueous media. A solution of Na4[Fe(CN)6]
(E1/2 = +0.360 V vs. NHE) was used as an external reference for potential calibration.
UV/Vis absorption spectroelectrochemistry was performed by using a custom-built
electrochemical cell consisting of a gold minigrid working electrode, a Ag/AgNO3 or
Ag/AgCl reference electrode, and a platinum wire auxiliary electrode mounted inside a
quartz cuvette with a path length of 25 ꢀm. The potentials were applied using an EG&G
PAR 173 potentiostat/galvanostat, while spectra were recorded using the HP 8453A
spectrophotometer. Confocal Raman measurements were performed using a WITEC Alpha
300R microscope equipped with Ar ion laser (λ = 488 nm). The faradaic efficiency
measurements were carried out by rotating ring-disc electrode (RRDE) voltammetry using
the Autolab PGSTAT30n bipotentiostat in a hanging meniscus rotating disc electrode
(HMRDE)[34, 35] configuration cell. The working electrode was a glassy carbon disk, with
a Pt ring (collection efficiency N = 0.38, nominally). A Pt wire and a normal hydrogen
electrode (NHE) were used as counter and reference electrodes, respectively.
The tpy2ph ligand and the [{RuCl3}2(tpy2ph)] intermediate complex were prepared
according to previously described methods.[33]
[{RuCl(Clphen)}2(tpy2ph)](TfO)2: The [{RuCl3}2(tpy2ph)] intermediate complex (100.0
mg, 0.10 mmol) was reacted under reflux with the Clphen ligand (43.0 mg, 0.20 mmol),
LiCl (42.0 mg, 10 mmol), and 4-ethylmorpholine (1 mL) in ethanol/water mixture (3:1 v/v,
50 mL) for 8 h, under a N2 atmosphere. The volume of the mixture was reduced to about 10
mL in a flash evaporator, and the complex precipitated out with an aqueous Li(TfO)

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solution. The solid product was filtered off, washed with cold deionized water, dried in a
desiccator under vacuum, and purified by column chromatography (neutral alumina,
1
CHCl3/CH3CN as eluent). Yield: 72.0 mg (45 %). H NMR (DMSO-d6, 800 MHz) δ (ppm)
(m, J, H): 10.44 (d, J = 4.9, 1H), 10.36 (d, J = 5.1, 1H), 9.44 (s, 2H), 9.43 (s, 2H), 9.12 (s,
1H), 9.10 (d, J = 8.6, 1H) 9.03 (dd, J= 8.1, 4H), 8.95 (d, J = 8.31, 1H), 8.80 (s, 1H), 8.56
(m, 4H), 8.52 (d, J = 8.3, 1H), 8.49 (dd, J = 8.2, 1H), 8.38 (d, J = 8.0, 1H), 8.10 (t, J = 7.7,
1H), 8.03 – 7.99 ( m, 5H), 7.92 (d, J = 5.6, 1H), 7.59 (d, J = 5.9, 1H), 7.58 (d, J = 5.5, 2H),
7.54 (d, J = 5.3, 2H), 7.50 (dd, J = 7.9, 1H), 7.30 – 7.26 (m, 4H). ESI-MS: m/z = 621.4,
C62H38Cl4F6N10O6S2Ru2·5H2O (1631.17) calcd. C 45.65, H 2.97, N 8.59; found C 45.87, H
2.93, N 8.85.
[{Ru(H2O)(Clphen)}2(tpy2ph)](TfO)4: The binuclear ruthenium aqua complex was
prepared by reacting [{RuCl(Clphen)}2(tpy2ph)](TfO)2 (50.0 mg, 0.03 mmol) in 2 mL of
CF3SO3H (Trifluoromethanesulfonic acid) at room temperature, under stirring and inert
atmosphere. After 3 hours, the product was precipitated out with an aqueous LiTfO
solution, filtered using a sintered glass filter and washed with cold diethyl ether, and finally
dried in a desiccator under vacuum. Yield: 38.0 mg (68 %). 1H NMR (D2O:acetone-d6 (7:3)
500 MHz) δ (ppm) (m, J, H): 10.04 (d, J = 5.1 Hz, 1H), 9.97 (d, J = 5.2 Hz, 1H), 9.21 (s,
1H), 9.20 (s, 4H), 8.90 (d, J = 8.3 Hz, 2H), 8.77 (d, J = 8.0 Hz, 4H), 8.56 (d, J = 5.8 Hz,
2H), 8.54 (d, J = 3.1 Hz, 1H), 8.46 (dd, J = 8.3, 5.2 Hz, 1H), 8.41 (d, J = 8.2 Hz, 2H), 8.33
(s, 1H), 8.26 (d, J = 8.4 Hz, 1H), 8.08 (t, J = 7.8 Hz, 1H), 8.03 (t, J = 7.9 Hz, 4H), 7.95 (d, J
= 5.5 Hz, 1H), 7.87 (d, J = 5.5 Hz, 1H), 7.75 (d, J = 5.6 Hz, 4H), 7.50 (dd, J = 8.4, 5.5 Hz,
1H), 7.40 (dd, J = 8.2, 5.5 Hz, 1H), 7.32 – 7.28 (m, 4H). C62H42Cl2F12N10O14S4Ru2·5H2O
(1894.43) calcd.%: C 40.58, H 2.77, N 7.39; found: C 40.70, H 2.61, N 7.81.
Poly-[{Ru(H2O)(phen)}2(tpy2ph)]:
30
ꢀL of
a
1.0
mM
solution
of
[{Ru(H2O)(Clphen)}2(tpy2ph)](TfO)4 in acetone was drop-casted on by transferring it onto
a recently polished glassy carbon or cleaned FTO electrode surface, generating a thin film.
The electropolymerization reaction was carried out by scanning the modified electrode in
pH 7.0, aqueous HNO3 solution, in the 0.1 to -1.45 V potential range.
3. Results and Discussion

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3.1. Spectroscopic and electrochemical behaviour of compounds in solution
The electronic spectra of the compounds are shown in Figure 2. The dinuclear
[{RuCl(Clphen)}2(tpy2ph)](TfO)2 complex in DMF solution is magenta coloured and
presents bands in the visible region. The strong band at 524 nm is assigned to RuII(dπ) →
pπ*(tpy ph + Clphen) metal-to-ligand charge-transfer (MLCT) transition, whereas the
2
shoulder at 454 nm is attributed to RuII(dπ) → pπ*(Clphen) MLCT transition.[36] The
bands in the UV region at 321 and 289 nm are π → π* transitions characteristic of the
tpy2ph ligand,[33] whereas the band at 273 nm is typically a π → π* transition of the
Clphen
ligand.
The
respective
aqua
complex,
the
dinuclear
[{Ru(H2O)(Clphen)}2(tpy2ph)](TfO)4 complex, exhibits a very similar electronic spectrum
profile but the exchange of the chloride ligand for water causes a hypsochromic shift in all
MLCT bands, indicating an increase of the dπ → π*(tpy ph + Clphen) energy gap as
2
consequence of the replacement of the π-donor ligand Cl- by H2O.[33]
Figure 2: (A) Wire frame structure and (B) UV/Vis absorption spectrum of the dinuclear
[{RuCl(Clphen)}2(tpy2ph)]2+ complex (in DMF, black line) and of the respective aqua
complex [{Ru(H2O)(Clphen)}2(tpy2ph)]4+ (in water, blue line).

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The electrochemical behaviour of [{RuCl(Clphen)}2(tpy2ph)](TfO)2 in DMF
solution is shown in Figure 3A. The reversible redox process at E1/2= +0.96 V was assigned
to the oxidation of both Ru ions, one electron for each metal centre. This means that the
electronic communication between the metallic centres is weak.[33] No other redox process
was observed up to 2.0 V (see Figure S1 in Supp. Info.). That oxidation process led to the
complete disappearance of the MLCT bands at 524 and 454 nm (Fig. 3B), confirming the
attribution of the respective CV process. The redox process at E1/2= -1.16 V is also
reversible and has similar intensity of the wave at +0.96 V, and can be assigned to
processes associated with the monoelectronic reduction of the two Clphen ligands.[33] The
reduction of the tpy2ph ligand was observed by differential pulse voltammetry at Epc= -1.52
V as shown in Fig. S2 (Supp. Info.).

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Figure 3: (A) Cyclic voltammograms of [{RuCl(Clphen)}2(tpy2ph)]2+ complex in DMF,
0.10 M (Bu4N)ClO4. (B) Spectroelectrochemical changes associated with the oxidation of
[{RuIICl(Clphen)}2(tpy2ph)]2+ to [{RuIIICl(Clphen)}2(tpy2ph)]4+ in DMF solution. (C)
Cyclic voltammogram of [{Ru(H2O)(Clphen)}2(tpy2ph)]2+ complex in DMF solution.
Electrolyte 0.10 M (Bu4N)ClO4, v = 100 mV s-1, platinum disc working electrode.
The CV of the corresponding aqua complex [{Ru(H2O)(Clphen)}2(tpy2ph)](TfO)4
(Fig. 3C) exhibited two successive oxidations waves corresponding to two electrons each at
E1/2= +0.96 V and +1.21 V assigned to two proton-coupled electron-transfer (PCET)
processes
involving
respectively
the

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[{RuII(H2O)(Clphen)}2(tpy2ph)]2+/[{RuIII(OH)(Clphen)}2(tpy2ph)]4+
and
[{RuIII(OH)(Clphen)}2(tpy2ph)]4+/[{RuIV(O)(Clphen)}2(tpy2ph)]4+ processes.[33] The wave
at E1/2= -1.16 V was assigned to the monoelectronic reduction of two Clphen ligands. The
electrochemical behaviour of the chloro and the aqua complexes is consistent with the
behaviour
observed
in
previously
reported
2,2’-bipyridine
derivatives
[{RuCl(bpy)}2(tpy2ph)]2+ and [{Ru(H2O)(bpy)}2(tpy2ph)]4+.[33]
3.2. Poly-[Ru2(H2O)2(phen)2(tpy2ph)] film assembly.
The Clphen derivatives can be reductively polymerized leading to the elimination of
Cl- and formation of C-C bonds generating bis-phen ligands connected through the 5-
position. [26, 37, 38] The electropolymerization process was carried out by scanning
successively the working electrode potential from 0 to -1.3 V for 50 times at a scan rate of
100 mV s-1 in a 1.0 mM solution in DMF. The current at -0.6 and -0.8 V was increased
after each scan (Fig. 4A), as expected for the deposition of the complex on the electrode
surface. These two waves, tentatively associated with the first e second monoelectronic
reduction of phen-phen bridges, shows that the polymer is reduced at slightly less negative
potentials than the corresponding monomers, as expected for the increased conjugation
length in the polymeric material (see Fig. 1).[39] A thin electropolymerized film is
generated since the binuclear complex possesses two Clphen ligands in opposite positions,
•
that generate phen radicals that couple together forming a polymeric chain (Fig. 1) upon
reduction. When long enough, its solubility decreases and an electrochemically active
polymeric film is deposited on the electrode surface.[37, 38]
The growth of films by electropolymerization directly from solution was shown to
be sluggish and generated poor quality films. A very interesting alternative approach is first
depositing a film of the monomer by drop-casting and then carrying out the
electropolymerization process in a solvent in which the complex is poorly soluble. In this
way, a more compact and reticulated polymeric structure is generated as a consequence of
the proximity and pre-organization of the monomer molecules during the solvent
evaporation process, thus increasing the efficiency of the C-C coupling reaction as
demonstrated previously.[37] Accordingly, a film of [{Ru(H2O)(Clphen)}2(tpy2ph)](TfO)4

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complex was drop-casted on a GC electrode surface using a methanol solution, and the
modified electrode cycled in the 0.1 to -1.45 V range in pH 7, 0.1 M phosphate buffer
solution. In this case, one cycle was enough to complete the process, and the modified
electrode was washed with water, then with acetone, and dried. The ruthenium complex
film, previously very soluble in acetone, became completely insoluble in this solvent
confirming the success of the electropolymerization process.
Figure 4: (A) Successive CVs showing the growth of poly-[{Ru(H2O)(phen)}2(tpy2ph)]
from a 1 mM solution of the [{Ru(H2O)(Clphen)}2(tpy2ph)]4+ complex in DMF, 0.10 M
(Bu4N)ClO4, v = 100 mV s-1; and (B) CVs of a GC disc electrode modified with poly-
[{Ru(H2O)(phen)}2(tpy2ph)] in 1 mM, pH 1 (trifluoromethanesulfonic acid), 0.1 M KNO3
solution, v = 10 (a), 30 (b), 50 (c), 80 (d), 100 (e), 120 (f), 150 (g), 180 (h), 200 (i) mV s-1.
The voltammogram of a GC electrode modified with electropolymerized film
showed a single wave at E1/2= +0.97 V (Figure 4B), instead of a couple of PCET waves
characteristic of the starting [{Ru(H2O)(Clphen)}2(tpy2ph)](TfO)4 complex in solution.
Accordingly, that wave was tentatively attributed to a RuII-H2O/RuIV=O PCET process, i.e.

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an unusual two protons two electrons process per ruthenium centre.
Spectroelectrochemistry is a powerful technique to determine the redox state and
consequently the redox reaction taking place with ruthenium complexes,[33] and the
behaviour of a poly-[{Ru(H2O)(phen)}2(tpy2ph)] film on FTO electrode is shown in Figure
5A to confirm the above hypothesis. Its initial spectrum is analogous to that of the
[{Ru(H2O)(Clphen)}2(tpy2ph)]4+ monomer in solution, exhibiting the typical RuII(dπ) →
pπ*(tpy2ph and phen) MLCT band at 510 nm,[36] that disappears when a potential of +1.37
V is applied to the electrode. Concomitantly, a band assigned to a O2–(pπ)→RuIV(dπ*)
LMCT (ligand-to-metal charge-transfer) band appeared at 370 nm.[33] The initial UV-vis
spectrum clearly indicated the presence of the [{RuII(H2O)(phen)}2(tpy2ph)]4+ species that
is oxidized only at +1.37 V, a potential well above the RuIII/II process of
[{Ru(H2O)(Clphen)}2(tpy2ph)]2+ or [{Ru(H2O)(bpy)}2(tpy2ph)]4+ in DMF solution.[33] No
spectroscopic evidence of a RuIII-OH2 or RuIII-OH species could be found. The lack of a
couple of processes related to the RuIII/RuII and RuIV/RuIII processes but a single
bielectronic process in the voltammogram of the film can be explained based on a fast
disproportionation reaction of the RuIII-OH species.
The ligands play a fundamental role in ruthenium polypyridines complexes
influencing their spectroscopic and electrochemical properties. For example, ligands with
electron-donating substituents such as methyl, amino, methoxy, etc, tend to stabilize the
Ru(III) species shifting the RuIII/II process to less positive potentials, whereas electron-
withdrawing groups such as halogens, nitro, carboxylic acid, stabilize the Ru(II) species
increasing the redox potential.[40-42] In the oxidation state II, The predominant factor in
the Ru(II) complexes is the back-bonding involving the empty ligand π* and filled Ru(dπ)
orbitals. On the other hand, the behavior of the Ru(III) and Ru(IV) species is dominated by
the charge transfer interactions of the hydroxyl (OH-) and oxo (O2-) ligand pπ orbitals with
the Ru(dπ) orbitals.[41, 42] In this way, the Ru(II) and Ru(IV) species are possibly being
strongly stabilized while the Ru(III) species become thermodynamically unstable and
susceptible to disproportionation reaction.[42, 43] In our case, the proximity of the metal
centres connected by phen-phen bridges increases the electrostatic repulsions facilitating
the successive deprotonation of the RuIII-OH2 species, which undergoes a
disproportionation reaction generating the RuIV=O species. Finally, the anodic (iap) and

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cathodic (icp) peak currents of poly-[{Ru(H2O)(phen)}2(tpy2ph)] are linearly dependent on
the scan rate, consistent with a process independent of diffusion control as expected for an
electrochemically active material attached to the electrode surface (Figure S4).[31]
Figure 5: (A) Spectroelectrochemical changes associated with the oxidation of a poly-
[{Ru(H2O)(phen)}2(tpy2ph)] film deposited on FTO, in pH 1, in 1.0 mM KNO3 solution.
(B) AFM image of a typical poly-[{Ru(H2O)(phen)}2(tpy2ph)] film electropolymerized on
FTO surface.
The morphology of the poly-[{Ru(H2O)(phen)}2(tpy2ph)] thin film on FTO
(Fluorine doped Tin Oxide) electrode was characterized by atomic force microscopy, AFM
(Figure 5B). The topography of the electropolymerized film is dominated by granular
structures extending throughout the surface. Similar behaviour was observed previously
after electropolymerization of preformed molecular films, [37, 44] suggesting that they are
not completely homogeneous. In fact, they should have regions where the molecular

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orientation is more favourable for the electropolymerization reaction than others, thus
generating such a granular pattern.
3.3. Electrocatalytic oxidation of water
The CVs of poly-[{Ru(H2O)(phen)}2(tpy2ph)] modified GC electrode in pH 1.0
aqueous solution showed a well-defined wave at E1/2= +0.97 V assigned to RuII-
H2O/RuIV=O redox process (Fig. 6). In addition, one can notice a large increase in the
anodic current at more positive potentials than about +1.5 V, by comparing the
voltammograms recorded with the bare and the modified GC electrodes. This
electrocatalytic behaviour for oxidation of water is consistent with that observed for the
analogous bpy derivative, [{Ru(H2O)(bpy)}2(tpy2ph)], indicating the formation of the
[RuV=O]3+ species in the film. This more reactive species is able to oxidize water to
dioxygen, [33] whereas the [RuIV=O]2+ species is relatively stable and unreactive. This
result associated with the fact that there is no other electrochemical process unless the one
at E1/2= +0.97 V, is a good evidence reinforcing its assignment to a RuII-H2O/RuIV=O
PCET redox process.
The electrocatalytic oxidation of water using a poly-[{Ru(H2O)(phen)}2(tpy2ph)]
modified GC electrode was studied by EIS in order to evaluate its efficiency. The Nyquist
plots of the modified electrode recorded at different applied potentials are shown in Figure
7A. The diameter of the semicircle is directly related to the charge transfer resistance (Rct).
In other words, larger diameters indicate larger resistances and slower processes, whereas
smaller diameters are associated with faster and more efficient processes. It is clear that the
oxygen evolution reaction becomes faster and faster as the potential increased from 1.3 to
1.8 V, as evidenced by the large decrease in the semicircles diameter. In fact, the resistance
stepped down significantly at potential values more positive than 1.5-1.6 V, which are more
or less coincident with the onset of the OER observed in the voltammograms (Fig. 6).
Accordingly, this was associated with the beginning of the formation of the electrocatalytic
active [RuV=O] species, whose surface concentration increases exponentially as a function
of the applied potential, thus speeding up the oxygen evolution reaction, as confirmed by
the exponential decrease of Rct.

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Figure 6: Cyclic voltammogram of poly-[{Ru(H2O)(phen)}2(tpy2ph)] modified GC
electrode (blue line) as compared with a bare GC electrode (black line) in pH 1, 0.1 M
HNO3 aqueous solution, at v = 10 mV s-1.
The Rct parameter associated with the OER, at applied potentials in the 1.3-1.8 V
range, were determined by simulating the impedance data using a simple Randles circuit
(Figure S5). The good matching achieved with the experimental data can be seen in Figure
7A. Also, the log plot of Rct as a function of the applied potential (Fig. 7B) exhibited a clear
change in the slope, as expected for the presence of a more active electrocatalytic species at
potentials more positive than +1.5 V, as expected for the electrochemical conversion of the
[RuIV=O] to [RuV=O] species in the polymeric film. This hypothesis was confirmed by
monitoring in situ and in real-time the evolution of the Raman spectra of the film under
oxygen evolution reaction condition. In fact, it was possible to observe the rise of a new
band at 810 cm-1 in the Raman spectrum of poly-[{Ru(H2O)(phen)}2(tpy2ph)],
characteristic of the ruthenyl (RuV=O) stretching mode,[45-47] at an applied potential of
+1.80 V in pH 1.0 HNO3 solution, as shown in Figure 8.
The
results
discussed
above
confirmed
the
conversion
of
[{RuII(H2O)(phen)}2(tpy2ph)]4+ directly to the [{RuIV(O)(phen)}2(tpy2ph)]4+ species by a
(2e- + 2H+) PCET process, which is absent in the dinuclear ruthenium aqua complex in
solution. This is quite an unexpected result that can be assigned to electronic and structural

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effects, probably caused by the proximity of ruthenium centres in the polymer, as
consequence of the formation of the C-C bonds connecting directly two phen ligands (Fig.
4), as well as the close proximity of the ruthenium complexes generating different local
arrangements favouring the rapid disproportionation of the [{RuIII(OH)(phen)}2(tpy2ph)]4+
intermediate species into the respective [RuII–OH2] and [RuIV=O] species, in analogy to
other similar ruthenium polypyridyl complexes described in the literature.[48]
Figure 7: (A) Nyquist plots of EIS data obtained using GC electrodes modified with a poly-
[{Ru(H2O)(phen)}2(tpy2ph)] film in the 1.3 to 1.8 V range, in pH 1, 0.1 M aqueous HNO3
solution, superimposing an alternating sinusoidal perturbation (amplitude = 20 mV) with
frequency in the 1 Hz to 10 kHz range (symbols), where the respective simulated curves
using a simple Randles equivalent circuit are also presented (solid lines). (B) Log plot of

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the R1 parameter as a function of the applied potential clearly showing a break and change
to an higher activity regime at potentials more positive than +1.5 V.
Figure 8: Raman spectra of a poly-[{Ru(H2O)(phen)}2(tpy2ph)] film deposited on FTO
electrode at 0.0 and at +1.8 V (under oxygen evolution reaction condition), in 0.1 M
aqueous HNO3 solution.
The efficiency of poly-[{Ru(H2O)(phen)}2(tpy2ph)] as electrode material
(electrocatalyst) for oxygen evolution reaction was further investigated by Rotating Ring-
Disc-Electrode (RRDE) voltammetry, by applying a linear potential sweep to the disk and a
fixed potential to the ring. The GC disc of a RRDE was modified with the polymer and the
potential scanned in the 1.4 to 1.8 V range, while the amount of oxygen generated was
monitored at the Pt ring (potential set to -0.4 V to reduce O2 to H2O), [49] at a rotation rate
of 1800 rpm (Fig. S6). The tetraelectronic oxidation of water to oxygen started around +1.5
V in the modified GC disc and the current increased linearly as a function of the applied
potential. This behaviour was matched by the current in the ring confirming the formation
of increasing amounts of oxygen at the poly-[{Ru(H2O)(phen)}2(tpy2ph)] film as the
potential was scanned above 1.5 V. The faradaic efficiency of the material for oxidation of
water was determined by measuring the ratio of the ring and the disc current, the first one
corrected by the corresponding collection efficiency factor N=0.40 (determined by using
the well-behaved ferricyanide/ferrocyanide redox pair, as shown in Figure S7). These data

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were previously corrected by subtracting the voltammetric curve obtained with the bare GC
disc electrode, before modification with the polymer film. As shown in Figure 9, the
faradaic efficiency of the oxygen evolution reaction electrocatalyzed by poly-
[{Ru(H2O)(phen)}2(tpy2ph)] started to increase at +1.52 V and reached the maximum of
39% at +1.8 V (Fig. 9A), demonstrating to be one of the most active catalysts based on a
ruthenium terpyridyl complex reported to date. Similar experiments carried out by Meyer et
al [31, 49, 50] using ruthenium complexes as electrocatalyst showed comparable
efficiencies, but generally lower than that measured for the polymeric dinuclear complex
(see Table 1). This means that about 60% of the anodic current delivered to the poly-
[{Ru(H2O)(phen)}2(tpy2ph)] film is not being used to produce dioxygen, suggesting that
there are competing anodic processes that may lead to deactivation of the molecular
catalyst. The oxidation of the glassy carbon disk surface underneath [31, 49, 50] and partial
catalyst decomposition, probably by over oxidation and decomposition of the Ru-
polymer,[51] are some possibilities.
Accordingly, an electrolysis experiment (i−t) was carried out at a constant potential
of +1.80 V to evaluate the performance and robustness of the synthesized material as OER
catalyst. As shown in Figure 9B, a more or less stable current density was sustained during
up to 7000 s, confirming the robustness as electrode material for oxygen evolution reaction.
The faradaic efficiency for OER evaluated after 2000 s was found to be analogous (38%) to
that in the beginning of electrolysis (see Figure S8), except for the decrease in current
density. This indicates that poly-[{Ru(H2O)(phen)}2(tpy2ph)] still is the catalytic active
material present on the electrode surface but in lower amount, since the high initial
electrochemical activity at +1.80 V generating large amounts of O2 bubbles that removed
partially the material from electrode surface. Accordingly, one can suppose that poly-
[{Ru(H2O)(phen)}2(tpy2ph)] can be integrated to a semiconductor surface to produce
photoanodes for oxygen evolution reaction, and possibly electrodes suitable for electron
transfer, atom transfer, C-H bond insertion and multielectronic redox processes coupled to
proton transfer reactions,[52] useful for the assembly of dye sensitized
photoelectrosynthesis cells (DSPEC) using organic molecules as proton and electron
source.[18, 53]

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Figure 9: (A) Faradaic efficiency for oxygen evolution reaction electrocatalyzed by poly-
[{Ru(H2O)(phen)}2(tpy2ph)] as a function of potential; (B) Chronoamperometric curves
comparing the current as a function of time measured at +1.8 V with a bare GC electrode
and a GC electrode modified with a poly-[{Ru(H2O)(phen)}2(tpy2ph)] film, in pH 1, 0.1 M
aqueous HNO3 solution.
Table 1: Maximum faradaic efficiency of glassy carbon and FTO electrodes modified with
ruthenium complexes, at pH 1.
Compound
Faradaic efficiency (%)
39 (This work)
27 [49]
poly-[{Ru(H2O)(phen)}2(tpy2ph)]
[Ru(H2O)(tpy)(PO(OH)2)2-bpy]]2+
poly-[Ru(bda)(4-vinylpyridine)2]
[(bpy)2Ru(4-Mebpy-4’-bimpy)Ru(H2O) (tpy)]4+
[Ru(H2O)(Mebimpy)(PO3OH2)2-bpy]]2+
13 [31]
28 [50]
50 [49]

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tpy = 2,2’:6’,2’’-terpyridine, H2bda =2,2’-bipyridine-6,6’-dicarboxylic acid, PO(OH)2)2-bpy= 4,4′-bis-
methlylenephosphonato-2,2′-bipyridine, bpy=2,2’-bipyridine, 4-Mebpy-4’-bimpy=4-(methylbipyridin-4’-yl)-
N-(benzimidazole)-N’-pyridine), (PO3OH2)2-bpy) = 2,2′-bipyridine-4,4′-diyldiphosphonic acid.
4. Conclusions
In summary, described is a robust polymeric catalyst based on a binuclear
ruthenium polypyridyl complex prepared by an efficient electropolymerization process. The
polymeric material is constituted by chains of binuclear aqua ruthenium
terpyridylphenanthroline complexes connected at the meta-positions of benzene ring,
whose activity is enhanced by the local conformation and microenvironment generated by
their interaction, as confirmed by the change of the oxidation mechanism to an unusual two
electrons and two protons PCET process. The preparation and characterization of the
monomeric binuclear [{RuCl(Clphen)}2(tpy2ph)]2+ and its respective aqua complex
[{Ru(H2O)(Clphen)}2(tpy2ph)]4+, an easily electropolymerizable species through the
Clphen ligands, was fully described. The poly-[{Ru(H2O)(phen)}2(tpy2ph)] film was
characterized by atomic force microscopy, as well as by spectroscopic and electrochemical
techniques to demonstrate the formation of the catalytic active [RuV=O] species at
potentials starting from +1.5 V, as confirmed by the rise of the ruthenyl (Ru=O) band at
810 cm-1 in the Raman spectra and the EIS result showing a clear change to a higher
activity regime. A faradaic efficiency for oxygen evolution reaction as high as 39% was
achieved for poly-[{Ru(H2O)(phen)}2(tpy2ph)] indicating its potentiality for the
development of electrode materials for OER and artificial photosynthesis devices.
5. Acknowledgements
Authors gratefully acknowledge Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
(CAPES), and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the
financial support (CNPq 402281/2013-6 and 401581/2016-0; FAPESP 2013/24725-4 4 and
2015/26308-7)) and fellowships (PAB, CNPq 133876/2015-2; RRG, CNPq 141386/2011-
8; AHBD, FAPESP 2013/25592-8).

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