Catalytic Water-Oxidation Activity of a Weakly Coupled Binuclear Ruthenium Polypyridyl Complex

Article abstract of DOI:10.1002/ejic.201600889

The catalytic oxidation of water by the binuclear complex [Ru₂(H₂O)₂(bpy)₂(tpy₂ph)](PF₆)₄[bpy = 2,2′-bipyridine; tpy₂ph = 1,3-bis(4′-2,2′:6′,2′′-terpyridin-4-yl)benzene] was investigated comparatively to its mononuclear counterpart [Ru(H₂O)(bpy)(phtpy)](PF₆)₂(phtpy = 4′-phenyl-2,2′:6′,2′′-terpyridine). These catalysts were prepared from the synthesis of their precursor chloride complexes, which were also extensively characterized in this work. The H₂O–Ru^IIcomplexes were found to undergo proton-coupled electron-transfer processes to generate the redox species HO–Ru^III, O=Ru^IV, and O=Ru^V. The catalytically active species, [Ru^V₂(O)₂(bpy)₂(tpy₂ph)]⁶⁺and [Ru^V(O)(bpy)(phtpy)]³⁺, were generated electrochemically and by using cerium(IV) ammonium nitrate. In the presence of Ce^IV, the catalytic rates for O₂production by the binuclear and mononuclear species were 1.9 × 10^–3and 9.5 × 10^–5s^–1, respectively. This superior catalytic performance of the binuclear complex suggests that, despite weak electronic coupling between the Ru centers, the second site could play an important mechanistic role in the formation of the activated species [(bpy)(OO)Ru^IV(tpy₂ph)Ru^III(OH)(bpy)]⁴⁺.

Full text of DOI:10.1002/ejic.201600889

A Journal of
Accepted Article
Title: Catalytic Water-Oxidation Activity of a Weakly Coupled Binuclear Ruthenium
Polypyridyl Complex
´
Authors: Tiago Arau´jo Matias; Ana Paula Mangoni; Sergio Hiroshi Toma; Francisca
´
Natalina Rein; Reginaldo Cesar Rocha; Henrique Eisi Toma; Koiti Araki
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201600889
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European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
Catalytic Water-Oxidation Activity of a Weakly Coupled Binuclear
Ruthenium Polypyridyl Complex
Tiago A. Matias,[a] Ana P. Mangoni,[a] Sergio H. Toma, [a] Francisca N. Rein,[b] Reginaldo C. Rocha, [b]
Henrique E. Toma, [a] Koiti Araki*[a]
Abstract: The catalytic oxidation of water by the binuclear complex
[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 (tpy2ph 1,3-bis(4’-2,2’:6’,2’’-
centers are intimately connected by a µ-oxo bridge.[4b, 4c, 5] This
=
complex exhibits several oxidation states up to RuV=O, the
4c]
terpyridin-4-yl)benzene) was investigated comparatively to its
mononuclear counterpart [Ru(H2O)(bpy)(phtpy)](PF6)2 (phtpy = 4’-
phenyl-2,2’:6’,2’’-terpyridine). These catalysts were prepared from
the synthesis of their precursor chloride complexes, which were also
extensively characterized in this work. The H2O-Ru(II) complexes
undergo proton-coupled electron-transfer processes to generate the
redox species HO-Ru(III), O=Ru(IV) and O=Ru(V). The catalytically
active species [RuV2(O)2(bpy)2(tpy2ph)]6+ and [RuV(O)(bpy)(phtpy)]3+
were generated electrochemically as well as by using cerium(IV)
ammonium nitrate. In the presence of Ce(IV), the catalytic rates for
O2 production by the binuclear and mononuclear species were
1.9×10-3 s-1 and 9.5×10-5 s-1, respectively. This superior catalytic
performance by the binuclear complex suggests that, despite the
weak electronic coupling between Ru centers, the second site can
play an important mechanistic role in the formation of the activated
species [(bpy)(OO)RuIV(tpy2ph)RuIII(OH)(bpy)]4+.
catalytically active species for oxidation of water[4b,
upon
successive proton-coupled electron transfer (PCET) reactions.
The groups of Tanaka[6] and Llobet[7] extended this approach to
other binuclear ruthenium polypyridyl complexes in which the
metal centers are close enough to interact with each other and
promote O2 evolution from water.
This idea on the necessity of simultaneous interaction of at
least two metal centers to activate the oxidation of water
persisted until 2005, when Thummel et al. demonstrated high
catalytic activity in mononuclear ruthenium complexes.[8] Soon
afterwards, Meyer et al.[9] proposed a mechanism to explain
such activity by using ceric ammonium nitrate (CAN) as
oxidizing agent in highly acidic media. Since then, many groups
have contributed to this exciting area. For example, Berlinguette
et al. carried out a systematic study describing the effect of
electron donor and acceptor substituents on bpy and tpy ligands
of [Ru(L)(bpy)(tpy)]n+ (L = Cl- or H2O, n = 1 or 2) on the
electrocatalytic activity and stability of these complexes.[10] In
addition, they studied how the induced electronic effects
influence the interaction of those complexes with CAN, and the
influence of the mineral acid on the activity of this oxidizing
agent.[10-11] Sun et al.[12] studied the ligand substitution with
coordinating groups and demonstrated that the presence of
electron donors decreases the oxidation potentials while
maintaining relatively high electrocatalytic activity for oxidation of
water.
Introduction
The development of renewable and clean energy
sources,[1] independent of fossil fuels,[1e, 2] has been needed for
the continued development of our society. In this context, H2
production finds application in the direct use of H2 as a fuel due
to its high energy content[3] as well as in the production of
hydrocarbon fuels from CO2. Water splitting by the so-called
Several other water-oxidation mononuclear catalysts have
been reported, but the necessity or not of a binuclear active site
cannot be completely ruled out because the reaction mechanism
of such catalysts seems to involve the interaction of two
molecules of the mononuclear complex to generate the activated
state. Furthermore, the role of a strong electronic coupling[4b, 6, 13]
observed in the blue dimer and other dimeric catalysts is not
clear, despite some theoretical studies in this direction.[13]
Following our interest in this problem, here we report on the
synthesis, characterization, and catalytic properties of the
binuclear complex [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4, where the
two metal centers are almost electronically independent of each
other, as demonstrated here by comparison with the related
mononuclear complex [Ru(H2O)(bpy)(phtpy)](PF6)2 (phtpy = 4’-
phenyl-2,2’:6’,2’’-terpyridine). Surprisingly, this weakly coupled
binuclear complex presented an activity for oxidation of water
with a kcat(O2) about 20 times that of the mononuclear complex,
demonstrating a significant cooperation by the second Ru site.
artificial
photosynthesis
provides
a
sustainable and
environmentally friendly production of H2 by the catalyzed
reaction: 2 H2O → 2 H2 + O2.[4] The oxidation half-reaction
(2 H2O → O2 + 4H+ +4 e-) is responsible for the production of
protons and electrons to generate H2.[4] The complex multi-
electron/multi-proton mechanism associated with the O-O bond
formation has presented major challenges in the development of
efficient water-splitting catalysts.[4b]
Many attempts to develop electrocatalysts for water
oxidation using ruthenium polypyridyl complexes have been
carried out since the 1980's.[4b] The first well-characterized
system
was
the
blue
dimer,
[(bpy)2(OH2)RuIII-O-
RuIII(OH2)(bpy)2]4+ (bpy = 2,2’-bipyridine), where two ruthenium
[a]
[b]
T.A. Matias, A.P. Mangoni, S.H. Toma, H.E. Toma, K Araki*
Department of Chemistry, Institute of Chemistry, University of São
Paulo
Av. Lineu Prestes 748, Butantã, São Paulo, SP 05508-000, Brazil.
E-mail: koiaraki@iq.usp.br
F.N. Rein, R.C. Rocha
Results and Discussion
Los Alamos National Laboratory,
Los Alamos, New Mexico, NM 87545, USA.
Synthesis and Characterization
Supporting information for this article is given via a link at the end of
the document
This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
The phtpy and tpy2ph ligands were prepared by the
condensation reaction of 2-acetylpyridine with benzaldehyde or
isophthalaldehyde in the presence of an ammonia source,[14] and
characterization by ESI-MS and elemental analysis, the
presence of the Cl- ligand was confirmed by the signal of the H6”
in the NMR spectra (Fig. S3 and S4) at 10.11 and 10.13 ppm for
the mono- and binuclear complexes, respectively.[10a] Finally, the
1
characterized by H NMR spectroscopy (Fig. S1 and S2), mass
spectrometry (ESI-MS) and elemental analysis. The ruthenium
aquo complexes were prepared in three steps. First, the
tridentate ligands were reacted with RuCl3·H2O to obtain the
respective trichloro complexes. Then, the [RuCl3(phtpy)] and
[Ru2Cl6(tpy2ph)] precursors were reacted with bpy in the
presence of the reducing agent 4-ethylmorpholine to obtain the
respective chloro complexes [RuCl(bpy)(phtpy)](PF6) and
mono-
and
binuclear
aquo
complexes
[Ru(H2O)(bpy)(phtpy)](PF6)2 and [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4
were prepared by facilitated substitution of Cl- by H2O in the
presence of AgNO3 or CF3SO3H, which was confirmed by the
shift of the H6” signal to 9.65 and 9.54 ppm, respectively, in the
NMR spectra of the mono- and binuclear complexes (Fig. S5
and S6).[10a]
[Ru2Cl2(bpy)2(tpy2ph)](PF6)2.
In
addition
to
structural
Scheme 1. Synthetic routes for preparation of the mono- and binuclear aquo complexes [Ru(H2O)(bpy)(phtpy)](PF6)2 and [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4.
The mono- and binuclear chloro and aquo Ru(II)
complexes exhibit the typical pseudo-octahedral structure and
singlet ground state (low-spin dπ6), as supported by DFT
calculations. The fully optimized structures are shown in Fig. 1,
and bond distances and angles are provided in the Supporting
Information (Tables S1-S4). In the geometry optimization of the
aquo complexes, two solvent water molecules were explicitly
included as the first hydration shell around the water ligand. As
shown in a previous report on [Ru(H2O)(bpy)(tpy)]2+[15] this
approach significantly improves the characterization of electronic
structure as well as the spectral match between calculated (TD-
DFT) and experimental electronic absorptions, which is thus
relevant to the spectroscopic results below. For evaluation of the
DFT
structures,
the
optimized
geometries
of
[Ru(Cl)(bpy)(phtpy)]+ and [Ru(H2O)(bpy)(phtpy)]2+·(2H2O) were
compared with the X-ray crystallographic data for the related
complexes
[Ru(Cl)(bpy)(tpy)](PF6)[16]
and
[Ru(H2O)(bpy)(tpy)](ClO4)2,[17] respectively, and shown to be in
close agreement (Tables S1 and S3).
Figure 1. DFT structures of (A) [RuCl(bpy)(phtpy)]+, (B) [Ru2Cl2(bpy)2(tpy2ph)]2+, (C) [Ru(H2O)(bpy)(phtpy)]2+, and (D) [Ru2(H2O)2(bpy)2(tpy2ph)]4+. Full geometry
optimizations were performed at the B3LYP//6-31G*(C,H,N,O,Cl)/SDD(Ru) level. The explicit inclusion of two solvent H2O molecules (W1 and W2) per H2O ligand
(WL) to give the adducts B and D is discussed in the text. Hydrogen atoms of phtpy and tpy2ph are omitted for clarity (atom colors: Ru = purple, N = blue, O = red,
Cl = green, C = dark gray, H = light gray). Selected structural parameters are provided in Tables S1-S4, along with additional views of the molecular structures.
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European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
In the complexes shown in Fig. 1, the distortion from
octahedral geometry around the metal is due to the restricted
bite angle of the tridendate ligand. The calculated N1RuN3
angle for the tridendate phtpy and tpy2ph ligands (157.6158.3°)
and the N4RuN5 angle for the bidentate bpy ligand
(77.478.2°) are very similar to those of tpy and bpy in
{RuII(bpy)(tpy)} moieties.[18] For phtpy and tpy2ph, the calculated
RuN distance involving the N1 and N3 atoms trans to each
other is 2.102.11 Å, whereas that involving the central N2 is
much shorter (1.981.99 Å) as a result of the constraint by such
mer-arranged tridentate ligands.[19] For bpy, the RuN distance
is 2.112.12 Å for N5 and 2.072.09 Å for N4, reflecting a
stronger RuIIbpy π-backdonation at the N atom trans to the
donor Cl–/H2O ligand. The RuCl distance of 2.42 Å and RuOWL
distance of 2.17 Å are also typical of related chloro and aquo
complexes.[17, 20] The angle between the planes of the adjacent
rings of phenyl and middle pyridyl groups is 34.637.4° for the
mononuclear complexes (phtpy) and 42.843.1° for the
dinuclear complexes (tpy2ph). The two solvent water molecules
(W1 and W2) form H bonds with the Ru-bound water ligand
(WL), with OWL···OWsolv and (OWL)H···OWsolv distances of 2.74 Å
and 1.75 Å, respectively (Tables S3 and S4).
Electronic Spectroscopy
The electronic absorption spectra of all mono- and
binuclear species (Fig. 2) exhibit
a similar pattern, with
polypyridyl ligand localized pπ→pπ* and n→pπ* transitions
below 350 nm, and Ru(dπ)→pπ* metal-to-ligand charge-transfer
(MLCT) transitions above 350 nm. These electronic transitions
were assigned in detail with the support of TD-DFT calculations
(Fig. S7-S12 and Table S5-S12) by using a well-established
approach that has been successfully applied to structurally
related complexes of the type [Ru(L)(bpy)(tpy)]n+ (L = Cl- or H2O,
n = 0 or 1).[15, 21]
Figure 2. Experimental (blue line) and calculated (red line) UV-Vis absorption spectra. Vertical bars (in red) indicate discrete electronic transitions. The spectra of
(A) [RuCl(bpy)(phtpy)](PF6) and (B) [Ru2Cl2(bpy)2(tpy2ph)](PF6)2 were obtained in acetonitrile solution, and the spectra of (C) [Ru(H2O)(bpy)(phtpy)](PF6)2 and (D)
[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 were obtained in water. The TD-DFT electronic excitations were calculated at the B3LYP//6-31G*(C,H,N,O,Cl)/SDD(Ru) level.
As listed in Table S5, the three highest occupied molecular
orbitals (HOMOs) of the mononuclear [RuCl(bpy)(phtpy)]+
complex are mainly from Ru(II) t2g orbitals with minor
contributions from Cl-, phtpy and bpy pπ orbitals. The two lowest
unoccupied molecular orbitals (LUMOs) are nearly degenerate,
with one largely dominated by bpy pπ* and the other by phtpy p
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European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
π*. The most intense band at 518 nm was assigned to Ru(dπ)
→pπ*bpy+phtpy (transition #7), for which the natural transition
orbital (NTO) pairs is shown in Fig. S8. The weak, broad
absorption between 350 and 400 nm is associated with multiple
MLCT transitions involving low-lying bpy and phtpy π* orbitals.
In the UV region, the band at 321 nm is assigned to phtpy pπ→
pπ* transitions and the band at 288 nm to internal pπ→pπ*
transitions of both phtpy and bpy ligands.
mononuclear species. The MLCT and internal ligand transitions
associated with the bands below 400 nm also seem to be only
weakly perturbed. The assignment of these bands is analogous
to the mononuclear complex, as supported by the TD-DFT
calculations (Tables S7 and S8).
As expected, the substitution of Cl- by H2O perturbed the
electronic structure of these complexes, causing an increase in
the energy difference between the HOMO and LUMO orbitals
(see Fig. S10 and S12). In the mono- and binuclear aquo
The binuclear [Ru2Cl2(bpy)2(tpy2ph)]2+ species can be
viewed as two [RuCl(bpy)(phtpy)]+ complexes connected by a
phenyl bridge, and thus displays a similar electronic structure
with only small perturbation. Not surprisingly, the main Vis band
at 522 nm associated with the Ru(dπ)→pπ*bpy+phtpy transition
(NTO pair #13; Fig. 3) is only 4 nm apart from that of the
complexes
[Ru(H2O)(bpy)(phtpy)]2+
and
[Ru2(H2O)2(bpy)2(tpy2ph)]4+, the main MLCT band in the Vis
region is blue shifted to 485 nm and 495 nm, respectively, and
assigned in detail as shown in Fig. 3 and S11.
Figure 3. Natural transition orbital (NTO) pairs for the main transition in the visible region for [Ru2Cl2(bpy)2(tpy2ph)]2+ (left) and [Ru2(H2O)2(bpy)2(tpy2ph)]4+ (right).
Electrochemistry and Spectroelectrochemistry
phtpy/tpy2ph, is involved in each reduction process. Typical
cyclic voltammograms of the complexes in DMF (chloro species)
and aqueous solution at pH 1.0 (aquo species) are shown in Fig.
4, and redox potentials are listed in Table 1.
Ruthenium polypyridyl complexes generally exhibit a rich
electrochemistry due to accessible redox states associated with
metal center and ligands. The presence of two polypyridyl
ligands increases the complexity in identifying which one, bpy or
Table 1. Redox potentials (E1/2 vs. NHE) of mono- and binuclear complexes.
Compounds
RuII/III [c]
+0.92
RuIII/IV [d]
RuIV/V [e]
bpy0/-
-1.25
-1.24
-
phtpy0/-, tpy2ph0/-
-1.43
bpy-/2-
-
[RuCl(bpy)(phtpy)]+ [a]
[Ru2Cl2(bpy)2(tpy2ph)]2+ [a]
[Ru(H2O)(bpy)(phtpy)]2+ [b]
[Ru2(H2O)2(bpy)2(tpy2ph)]4+ [b]
-
+0.94 [f]
+0.96
-
-1.41
-1.64
+1.07
+1.09 [f]
+1.75
+0.98 [f]
+1.75 [f]
-
[a] in DMF; [b] in water at pH 1.0; [c] RuII-OH2/RuIII-OH2 couple; [d] RuIII-OH2/RuIV=O couple; [e] RuIV=O/RuV=O
couple; [f] same redox potential for both Ru centers.
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European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
Figure
[RuCl(bpy)(phtpy)](PF6) (black line,
[Ru2Cl2(bpy)2(tpy2ph)](PF6)2 (blue line, v = 50 mV s-1) complexes in DMF, 0.10
TBAClO4. (B) Cyclic voltammograms of the mononuclear
4.
(A)
Cyclic
voltammograms
of
the
mononuclear
v
=
100 mV s-1) and binuclear
M
[Ru(H2O)(bpy)(phtpy)](PF6)2 (red line,
v =
10 mV s-1) and binuclear
[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 (magenta line, v = 10 mV s-1) complexes in
aqueous solution at pH 1.0, 0.10 M KNO3.
The ligand reduction processes are accessible only in
DMF solution, as these processes are outside the working range
of water. In the aquo complexes, the coordinated water strongly
influences the results because of chemical reactions coupled to
the redox process. Thus, only the chloro complexes were
evaluated in the region of ligand reductions. The mononuclear
[RuCl(bpy)(phtpy)]+ species exhibited two redox processes at
1.25 and 1.43 V. The first was assigned to the bpy0/- couple,
since spectroelectrochemistry (Fig. S13) in this potential region
led to the decrease of the band at 288 nm, which is associated
with a pπ→pπ* transition of bpy. Similarly, the redox process at
1.43 V was assigned to the phtpy0/- couple due to the decrease
of the band at 321 nm after reduction of the complex under a
potential of 1.6 V (Fig. S13).
Three redox processes at 1.24, 1.41 and 1.64 V were
found for the [Ru2Cl2(bpy)2(tpy2ph)]2+ species. The first was
assigned to the bpy0/- couple on both moieties, as it led to the
decrease of the bpy pπ→pπ* transitions at 288 and 321 nm as
well as the MLCT band at 522 nm (Fig. S14 ). The processes at
[22]
1.41 and 1.64 V were assigned to the tpy2ph0/-
and both
bpy-/2- couples, as confirmed by the spectroelectrochemical
changes shown in Fig. S14, particularly the disappearance of
the bands at 321 and 295 nm, respectively.
Figure 5. Spectroelectrochemical changes associated with the oxidation of
[RuCl(bpy)(phtpy)](PF6) in DMF (A), [Ru(H2O)(bpy)(phtpy)](PF6)2 and
[Ru(OH)(bpy)(phtpy)](PF6)2
in
water
at
pH
7.0
(B,
C),
[Ru2Cl2(bpy)2(tpy2ph)](PF6)2 in DMF (D), and [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4
and [Ru2(OH)2(bpy)2(tpy2ph)](PF6)4 in water at pH 7.0 (E, F).
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European Journal of Inorganic Chemistry
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FULL PAPER
As for the oxidation processes, the chloro complexes exhibit
At pH 1, the two reversible processes separated by 110 mV
only one reversible redox process at 0.92-0.94 V assigned to the
(E1/2
=
+0.96
and
1.07
V)
for
the
mononuclear
13b, 22]
Ru(III/II) couple,[13a,
which is spectroelectrochemically
[Ru(H2O)(bpy)(phtpy)]2+ species (Fig. 4A) were assigned to the
1-electron [RuII-OH2]2+/[RuIII-OH2]3+ process and 1-electron/2-
proton [RuIII-OH2]3+/[RuIV=O]2+ process, respectively.[14c] At such
acidic condition (pH 0 to ~1.4), all but the high valence
[RuIV=O]2+ species are protonated (Fig. 6A). However, both
redox potentials decrease with a linear dependence of about 59
mV/pH as the pH increase from ~1.4 to 10, as described by the
supported by the disappearance of the MLCT band at 500-530
nm and the decrease of the ligand pπ→pπ* transition around
290 nm (Fig. 5A and 5D). The fact that oxidation occurs on both
Ru centers at the same potential (i.e. without separation) clearly
show the absence of significant electronic communication
between metals, confirming that the redox behavior of Ru
centers is independent of each other.
PCET
reactions
[RuII-OH2]2+/[RuIII-OH]2+
and
[RuIII-
Ligand substitution of Cl- by H2O shifted that redox process
to 0.96-0.98 V and promoted the appearance of a new reversible
oxidation at 1.07-1.09 V. Consistent with the above observations
for chloro complexes, the behavior of both mono- and binuclear
aquo species is very similar and indicates that the electronic
coupling between ruthenium sites in the binuclear complex is too
weak and insufficient to split their redox potentials. Since each
moiety behaves independently, the only difference is that the
current intensity of voltammetric waves corresponds to two
electrons in the binuclear complex and one electron in the
mononuclear species. As detailed below, the redox process at
1.07-1.09 V is assigned to the RuIII-H2O/RuIV=O process. Notice
that only minor spectroelectrochemical changes was observed
for this last process, since the Vis spectral features associated
with MLCT vanishes upon the first oxidation to Ru(III).
OH]2+/[RuIV=O]2+.[14c]
The UV-Vis absorption spectra of the [RuII-OH2]2+, [RuIII-
OH]2+ and [RuIV=O]2+ species as well as spectroelectrochemical
changes at pH 7.0 are shown in Fig. 5. When the [RuII-OH2]2+
complex is oxidized to [RuIII-OH]2+, the MLCT band decreases
concomitantly with the rise of a new absorption band at 387 nm,
characteristic of the OH-(pπ)→RuIII(dπ) ligand-to-metal charge
transfer (LMCT) transition.[23] Notice that, in the PCET oxidation
of [RuIII-OH]2+ to [RuIV=O]2+, this LMCT band disappears and two
new bands arise at 334 and 354 nm; the first one was tentatively
assigned to the O2-(pπ)→RuIV(dπ*) LMCT transition.
At pH < 1.4, the intermediate PCET reaction, whose E1/2
shifts with
a
slope of -118 mV/pH, involves the [RuIII-
OH2]3+/[RuIV=O]2+ redox process.[14c] Finally, the pH-independent
[RuIV=O]2+/[RuV=O]3+ process was observed at 1.7 V (Fig. 6A).
This process was revealed by square wave voltammetry and the
potentials refer to the anodic peak potentials obtained at each
pH (Fig. S15). An analogous behavior was observed for the
binuclear species [Ru2(H2O)2(bpy)2(tpy2ph)]4+, except for slight
shifts of pKa's and redox potentials in the Pourbaix diagram
shown in Fig. 6B.
Proton-Coupled Electron Transfer (PCET) Reactions
The mono and binuclear aquo complexes can undergo
PCET and thus are prone to pH-dependent redox reactions. A
careful electrochemical study was carried out as a function of pH
for the [Ru(H2O)(bpy)(phtpy)]2+ and [Ru2(H2O)2(bpy)2(tpy2ph)]4+
species in order to obtain the Pourbaix diagram shown in Fig. 6.
Catalytic Oxidation of Water
Initially, qualitative evidence for catalytic activity was
observed electrochemically. The voltammograms of the
mononuclear
[Ru(H2O)(bpy)(phtpy)](PF6)2
and
binuclear
[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 complexes in the expanded
0.01.9 V range exhibit the pair of waves typical of the [RuII-
OH2]2+/[RuIII-OH2]3+ and [RuIII-OH2]3+/[RuIV=O]2+ redox processes
between 0.8 and 1.2 V (Fig. S16). In addition, the anodic wave
assigned to the [RuIV=O]2+/[RuV=O]3+ process is observed
around 1.6-1.7 V, at the onset of the exponential increase of the
10b]
current due to the oxidation of water to dioxygen.[9b,
The
[RuIV=O]2+ species is relatively stable and can be easily
characterized by spectroscopic or electrochemical methods. In
contrast, the [RuV=O]3+ species is transient due to its catalytic
activity for oxidation of water to dioxygen.
In order to obtain quantitative confirmation of catalytic
water-oxidation activity by these complexes, direct measurement
of O2 production was performed in liquid phase using an oxygen
detection system equipped with a Clark-type electrode (see
experimental section for detailed method). In these experiments,
the chemical oxidant (NH4)2[Ce(NO3)6] (CAN; 10 mM in 0.1 M
HNO3) was used as oxidizing agent in the Ru-catalyzed reaction:
4 Ce(IV)(aq) + 2 H2O(l) → 4 Ce(III)(aq) + O2(g) + 4 H+(aq). Kinetic
curves based on O2 formation exhibiting the typical saturation
profiles for about 4 h of reaction are represented in Fig. 7.
Figure 6. Pourbaix diagrams for the (A) mononuclear complex
[Ru(H2O)(bpy)(phtpy)](PF6)2
and
(B)
binuclear
complex
[Ru2(OH)2(bpy)2(tpy2ph)](PF6)4.
This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
Figure 8. Plots of initial rates for O2 evolution (kobs) as a function of catalyst
concentration, from which the catalytic rates kcat(O2) were determined to be
1.9×10-3 s-1 for [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 (blue line) and 9.5×10-5 s-1 for
[Ru(H2O)(bpy)(phtpy)](PF6)2 (red line).
The oxidation of water by the mononuclear complex should
involve the oxidation of [RuII(H2O)(bpy)(phtpy)]2+ to the active
[RuV(O)(bpy)(phtpy)]3+ by a stepwise mechanism in which two
proton-coupled electron transfer processes give the high-valent
[RuIV(O)(bpy)(phtpy)]2+ that is further oxidized by one electron-
transfer process to the catalytically active species.[9, 10b, 13c] Then,
the O–O bond is formed by water nucleophilic attack (WNA) to
10b,
13c]
the oxo group,[9,
generating the intermediate
[RuIII(OOH)(bpy)(phtpy)]2+ species that is oxidized by a PCET
process to [RuIV(OO)(bpy)(phtpy)]2+, which releases O2 and
regenerates the starting species upon bonding of a water
molecule. This should be the rate-limiting step in the catalytic
Figure 7. O2 evolution as a function of reaction time for 1.0 mL aqueous
solutions (0.10 HNO3; pH 1.0) of the Ru complexes in the presence of 10 mM
CAN. In the representative curves above, the concentrations of
[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 (blue line) and [Ru(H2O)(bpy)(phtpy)](PF6)2
(red line) are 50 M and 100 M, respectively, to facilitate a direct comparison
of catalytic activity per Ru site (i.e. same amount of Ru in both runs).
cycle. Decomposition occurs by intramolecular electronic
13c]
redistribution and charge transfer with release of O2.[9,
A
similar mechanism leading to the formation of the (OH)RuIII–
RuIV(O-O) species after intramolecular PCET mediated by a
water molecule is proposed for binuclear catalysts with weakly
interacting sites.[13c]
By varying the concentration of Ru catalysts (Fig. S17), the
values of kobs for O2 evolution were determined by the initial rate
method as a linear function of the concentration; i.e. a first-order
dependence, with d[O2]/dt = kobs = kcat[Ru complex].[9, 10b] The
rate constants (kcat) evaluated from the slope of the kobs vs [Ru
complex] plots (Fig. 8) were found to be 1.9×10-3 s-1 for
According to our DFT calculations, the Ru-Ru distance in
the binuclear species is 13.9 Å, precluding the interaction of the
Ru=O sites and direct coupling to form the O=O bond.[13a, 13c]
Therefore, WNA should be the operating mechanism for this
binuclear catalyst, where each site operates independently from
13c]
one another.[13a,
In this case, WNA to the
[Ru2(H2O)2(bpy)2(tpy2ph)]4+
and
9.5×10-5
s-1
for
[(bpy)(O)RuV(tpy2ph)RuIV(O)(bpy)]5+ intermediate generates the
[(bpy)(HOO)RuIII(tpy2ph)RuIV(O)(bpy)]4+
species that is
[Ru(H2O)(bpy)(phtpy)]2+. These rates correspond to turnover
frequencies (TOF) of 0.11 min-1 and 5.7x10-3 min-1, indicating
that the catalytic activity of the binuclear complex is much higher
(by about 20 times) compared to the mononuclear species.
Another significant difference is that an induction period of 100-
200 s was typically observed for the mononuclear complex (Fig.
S17), while the binuclear complex exhibited water-oxidation
activity immediately upon mixing with Ce(IV). This observation
suggests that an intermediate species formed in situ (possibly
involving the association of another Ru site) is the actual active
species from the mononuclear complex. These results show that
the binuclear complex is a superior catalyst overall.
converted to the activated complex responsible for O2 release by
a PCET modulated by a water molecule. Thus, the second Ru
center plays a key role by effectively acting as an oxidant and
nucleophile responsible for converting [RuIII(OOH)]2+ into
[RuIV(OO)]2+, whereas [RuIV(O)]2+ is reduced to [RuIII(OH)]+ in the
formation of [(bpy)(OO)RuIV(tpy2ph)RuIII(OH)(bpy)]4+. The
superior performance of the binuclear catalyst compared to the
mononuclear confirms that the PCET process mediated by a
water molecule is a key and possibly rate-limiting step in the
catalytic cycle (as proposed by Van Voorhis) and that the
binuclear species has a more suitable structure to generate the
(OH)RuIII–RuIV(O-O) species responsible for the production of O2.
Conclusions
The
structurally
related
and
mononuclear
binuclear
[Ru(H2O)(bpy)(phtpy)](PF6)2
[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 complexes were synthesized,
characterized, and utilized to comparatively study the
performance of water-oxidation catalysts with one versus two
weakly coupled catalytic sites. Unlike structures designed for
close cooperation between active sites, the binuclear complex
reported here features Ru centers that are 13.9 Å apart and very
weakly coupled electronically. Interestingly, however, the ability
of this binuclear catalyst toward water oxidation with relatively
This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
(DFT) calculations were carried out using the Gaussian 09 revision D.01
software.[26] The hybrid B3LYP[27] functional with the SDD[28] relativistic
effective core potential and associated basis set for Ru and the 6-31G(d)
basis set[29] for all the other elements were used for geometry
optimization without any constraint. Following optimization, vibrational
frequencies were computed at the same level of theory to verify the
nature of all stationary points as true minima with no imaginary
frequencies. The electronic excitation spectra in the UV/Vis region were
obtained by time-dependent DFT (TD-DFT)[30] calculations, which
included the first (lowest-energy) 100 transitions using the same level of
theory as above for geometry optimization. Natural transition orbitals
(NTOs)[31] were generated to facilitate visualization and interpretation of
electronic transitions from TD-DFT calculations.
fast O2 evolution rates (kcat) was found to outperform its
mononuclear counterpart by over an order of magnitude. Adding
to the complexity in the design and uderstanding of water-
oxidation catalysts, these findings suggest that the presence of
such remote, weakly interacting Ru sites can be structurally
beneficial in some cases, presumably by facilitating the
intramolecular PCET leading to the formation of activated
species.
Experimental Section
Analytical grade reagents were used without purification in all
experiments. HNO3 stock solutions were prepared with bi-distilled acid.
Experiments were performed in 9:1 v/v water/CF3CH2OH solution to
increase the solubility of the ruthenium complexes. 1H NMR spectra were
recorded on a Bruker DRX 500 MHz instrument, using suitable solvents
and tetramethylsilane (or the solvent peak) as reference. Mass spectra
(ESI-MS) were collected using a Bruker Daltonics Esquire 3000 Plus
equipment with the capillary potential set to 4 kV and sample injection
rate to 180 µL h-1. Elemental analyses were performed using a Perkin
Elmer 2400 series II analyzer. UV-Vis absorption spectra were obtained
using a Hewlett Packard 8453A diode array spectrophotometer. Cyclic
voltammograms were measured using an Autolab PGSTAT30
potentiostat and a conventional three-electrode cell consisting of a
platinum (organic solvent) or glassy carbon (aqueous medium) working
electrode, a platinum wire as auxiliary electrode, and a reference
electrode made of Ag/AgNO3 (10 mM; +0.503 V vs. NHE) or Ag/AgCl
(1.0 M KCl; +0.222 V vs. NHE) for organic and aqueous media,
respectively. Tetrabutylammonium perchlorate (TBAClO4; 0.10 M) and
KNO3 (0.10 M) were used as electrolyte in organic and aqueous media. A
solution of Na4[Fe(CN)6] (E1/2 = +0.360 V vs NHE) was also used as
external reference for calibration of potentials.[24] Measurements as a
function of pH were performed using 5 mL of an aqueous solution of 1
mM complex in 0.10 M KNO3 buffered with 10 mM Britton-Robinson
buffer;[25] the pH was adjusted by controlled addition of microliter volumes
of 1.0 or 4.0 M KOH solution. UV-Vis absorption spectroelectrochemistry
was carried out using a custom-built electrochemical cell consisting of a
gold minigrid working electrode, an Ag/AgNO3 (10 mM in acetonitrile) or
Ag/AgCl (1.0 M KCl; +0.222 V vs. NHE) reference and a platinum wire
auxiliary electrode mounted inside a quartz cuvette with a pathlength of
25 m. The application of potentials was controlled by an EG&G PAR
173 potentiostat and the spectra were recorded on a HP 8453A
spectrophotometer. Oxygen evolution was measured in liquid phase
using a Hansatech Instruments, Oxygraph System (OXYG1 and DW1/AD
unit) equipped with a Clark-type oxygen electrode (S1 or S1/MOD). The
electrode surface was protected from the sample by a freshly installed
Teflon membrane. Detection of O2 was recorded at 1-second intervals
using the Oxygraph Plus software. The signal was calibrated using
oxygen-free and air-saturated aqueous solutions at a temperature of 20
ºC and local atmospheric pressure of 77 kPa. Freshly prepared stock
solutions of Ru complexes and CAN in 0.1 M HNO3 were used in the
experiments. Solutions of the complexes were diluted to the appropriate
concentrations and flushed in the reaction chamber with argon for 15-20
min to completely remove dissolved oxygen (steady baseline readout
around 1 nmol mL-1). The gas in the headspace above the solution was
expelled using a gas-tight threaded plunger. To start the oxygen
evolution reaction, 10 L of a fully deoxygenated 1.0 M CAN solution was
injected using a high-precision Hamilton syringe through the plunger into
1.0 mL of the Ru sample at various concentrations (20-200 M). Initial
rates of O2 production were determined for nearly linear regions of the
kinetic curves within the first minutes of reaction. During the experiments,
the temperature was kept constant at 20 °C. Density functional theory
4’-phenyl-2,2’:6’,2’’-terpyridine (phtpy): Benzaldehyde (1.10 g, 10
mmol) in 50 mL of ethanol) was reacted at room temperature with KOH
(1.50 g), 2-acetylpyridine (2.42 g, 20 mmol) and 30 mL of 28% NH4OH.
The precipitate formed after 4 h was filtered off, washed with water and
ethanol, and purified by recrystallization in a 7:3 v/v ethanol/water
mixture. Yield: 2.0 g (62%). 1H NMR (chloroform-d, 500 MHz) δ (ppm) (m,
J, H): 8.77 (s, 2H), 8.75 (dd, 4.8 Hz, 2H), 8.70 (d, 7.9 Hz, 2H), 7.93 (d,
7.0 Hz, 2H), 7.91 (t, 7.8 Hz, 2H), 7.53 (dd, 7.4 Hz, 2H), 7.47 (t, 7.3 Hz,
1H), 7.38 (t, 7.7 Hz, 2H). C21H15N3 (MW = 309.4), CHN exp(calc)%:
80.27(81.53); 4.72(4.89); 13.21(13.58); ESI-MS (m/z) exp(calc) [phtpy-
H]+: 310.1(310.3).
1,3-bis(4’-2,2’:6’,2’’-terpyridyl)benzene (tpy2ph): A solution of KOH
(1.67 g) in polyethylene glycol 300 (35 mL) was transferred to a beaker
and cooled in an ice/water bath. Then, 2-acetylpyridine (3.61 g, 29.84
mmol) and m-phthalaldehyde (1.0 g, 7.46 mmol) were added and the
homogenized mixture was maintained at 0 °C for 2 h. Finally, 25 mL of
28% NH4OH was added and the reaction mixture kept at 100 °C for
additional 2 h. The precipitate formed was filtered off, washed with water
and ethanol, and purified by recrystallization in a 7:3 v/v ethanol/water
mixture. Yield: 1.6 g (40%). 1H NMR (chloroform-d, 500 MHz) δ (ppm) (m,
J, H): 8.83 (s, 4H), 8.75 (d, 4.8 Hz, 4H), 8.71 (d, 7.9 Hz, 4H), 8.37 (s, 1H),
7.99 (d, 7.6 Hz, 2H), 7.91 (t, 7.8 Hz, 4H), 7.68 (t, 7.6 Hz, 1H), 7.37 (d, 7.4
Hz, 4H). C36H24N6·H2O (MW = 558.6) CHN exp(calc)%: 77.40(76.93),
4.69(4.67), 15.04(15.02); ESI-MS (m/z) [tpy2ph-H]+: 541.1(541.6).
[RuCl3(phtpy)]: This mononuclear precursor was prepared by refluxing
phtpy (0.10 g, 0.32 mmol) and RuCl3·H2O (0.080 g, 0.35 mmol) in 50 mL
of anhydrous ethanol for 4 h. After cooling to room temperature, the
brown solid was filtered off, washed with ethanol, and dried under
vacuum. Yield: 0.150 g (90%).
[Ru2Cl6(tpy2ph)]: This binuclear precursor was prepared by refluxing
tpy2ph (0.180 g, 0.33 mmol) and RuCl3·H2O (0.150 g, 0.66 mmol) in 50
mL of anhydrous ethanol for 12 h. A brown solid was filtered off, washed
with ethanol, and dried under vacuum. Yield: 0.255 g (80%).
[RuCl(bpy)(phtpy)](PF6): The crude [RuCl3(phtpy)] complex (0.10 g,
0.02 mmol) was refluxed with bpy (0.031 g, 0.02 mmol), LiCl (0.043 g,
1.0 mmol) and 4-ethylmorpholine (1.0 mL) in 30 mL of a 3:1 v/v
ethanol/water mixture. After
4 h, the solution was filtered and
concentrated in a flash evaporator under vacuum. Then, the complex
was precipitated using an aqueous NH4(PF6) solution. The
hexafluorophosphate product was filtered off, washed with water, dried in
a desiccator under vacuum, and purified by column chromatography
using neutral alumina as stationary phase and mixtures of CHCl3 and
1
CH3CN with increasing polarity as eluent. Yield: 0.115 g (80%). H NMR
(DMSO-d6, 500 MHz) δ (ppm) (m, J, H): 10.11 (d, 5.7 Hz, 1H), 9.19 (s,
2H), 8.94 (d, 7.9 Hz, 2H), 8.92 (d, 8.9 Hz, 1H), 8.64 (d, 8.2 Hz, 1H), 8.37
(t, 8.4 Hz, 1H), 8.32 (s, 1H), 8.08 (t, 8.0 Hz, 1H), 8,03 (t, 7.8 Hz, 2H),
This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600889
FULL PAPER
7.78 (t, 7.9 Hz, 1H), 7.72 (d, 7.6 Hz, 2H), 7,64 (d, 4.8 Hz, 2H), 7.62 (t, 7.6
Hz, 1H), 7.42 (d, 5.8 Hz, 1H), 7.40 (dd, 7.4 Hz, 2H), 7.08 (t, 7.8 Hz, 1H).
λmax nm (DMF) (ɛ (M-1 cm-1)): 518 (1.2×104), 368 (1.0×104), 321 (2.5×104),
288 (4.9×104). C31H23ClF6N5PRu (MW = 747.0) CHN exp(calc)%: 49.55
The authors thank the São Paulo Research Foundation
(FAPESP) and the National Council for Scientific and
Technological Development (CNPq) for funding, including a
CNPq fellowship (TAM and APM).
(49.84),
3.52(3.10),
9.06(9.37).
ESI-MS
(m/z)
exp(calc)
[Ru(Cl)(bpy)(phtpy)]+: 602.1 (602.1).
Keywords: catalysis • electrocatalysis • electronic coupling •
ruthenium • water oxidation
[Ru2Cl2(bpy)2(tpy2ph)](PF6)2: The crude [Ru2Cl6(tpy2ph)] complex (0.10
g; 0.10 mmol) was refluxed with bpy (0.032 g; 0.20 mmol), LiCl (0.042 g;
10 mmol) and 4-ethylmorpholine (1mL) in 50 mL of
a 3:1 v/v
References
ethanol/water mixture for 8 h, under N2 atmosphere. The volume of the
reaction mixture was reduced to about 10 mL in a flash evaporator and
the complex was precipitated with an aqueous NH4PF6 solution. The
solid product was filtered off, washed with cold deionized water, dried in
a desiccator under vacuum, and purified by column chromatography
using neutral alumina as stationary phase and mixtures of CHCl3 and
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CH3CN with increasing polarity as eluent. Yield: 0.060 g (40%). H NMR
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4H), 9.07 (s, 1H), 9.01 (d, 8.1 Hz, 4H), 8.95 (d, 8.1 Hz, 2H), 8.68 (d, 8.2
Hz, 2H), 8.52 (d, 8.1 Hz, 2H), 8.39 (t, 8.0 Hz, 2H), 8.10 (t, 6.5 Hz, 2H),
8.07 (t, 1H), 8.06 (t, 7.3 Hz, 4H), 7.82 (t, 7.7 Hz, 2H), 7.67 (d, 5.5 Hz, 2H),
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reacting [RuCl(bpy)(phtpy)](PF6) (0.10 g, 0.13 mmol) with AgNO3 (0.022
g, 0.13 mmol) in 30 mL of a 3:1 v/v acetone/water mixture at 60 ºC. The
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1
dried in a desiccator under vacuum. Yield: 0.105 g (90%). H NMR (D2O:
acetone-d6 (7:3), 500 MHz) δ (ppm) (m, J, H): 9.65 (d, 5.8, 1H), 8.97 (s,
2H), 8.80 (d, 8.5, 1H) 8.72 (d, 8.55 Hz, 2H), 8.47 (d, 8.8 Hz, 1H), 8.42 (t,
9.4 Hz, 1H), 8.17 (d, 7.63 Hz, 2H), 8.13 (t, J= 8.1 Hz, 1H), 8.05 (t, J=
9.50 Hz, 2H), 7.90 (d, J= 5.76 Hz, 2H), 7.78 (t, J= 8.4 Hz, 1H), 7.75 (t, J=
8.2 Hz, 2H), 7.67 (t, J= 8.7 Hz, 1H), 7.49 (d, J= 6.1 Hz, 1H), 7.45 (t, J=
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485
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315
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284
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292.5 (292.3)
:
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[Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4:
The
[Ru2Cl2(bpy)2(tpy2ph)](PF6)2
complex (0.10 g) was refluxed with 5 mL of CF3SO3H in 10 mL of o-
dichlorobenzene under N2 atmosphere for 2 h. The mixture was cooled in
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0.085 g (70%). 1H NMR (D2O: acetone-d6 (7:3), 500 MHz): 9.54 (d, 6.0
Hz, 2H), 9.03 (s, 4H), 8.77 (s, 1H), 8.69 (d, 8.8 Hz, 2H), 8.66 (d, 8,7 Hz,
4H), 8.37 (d, 8.8 Hz, 2H), 8.31 (t, 8.1 Hz, 2H), 8.29 (d, 2H), 8.03 (t, 7.2
Hz, 2H), 7.96 (t, 8.1 Hz, 4H), 7.94 (t, 1H), 7.80 (d, 5.9 Hz, 4H), 7.67 (t,
8.2 Hz, 2H), 7.34 (t, 7.1 Hz, 4H), 7.28 (d, 6.3 Hz, 2H), 6.87 (t, 7.2 Hz, 2H).
λmax nm (H2O, pH 3.0) (ɛ (M-1 cm-1)): 491 (2.7×104), 314 (7.4×104), 287
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=
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Acknowledgements
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European Journal of Inorganic Chemistry
10.1002/ejic.201600889
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This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600889
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:
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Water oxidation, Ruthenium
polypyridyl, Electronic coupling
Tiago A. Matias, Ana P. Mangoni, Sergio
H. Toma, Francisca N. Rein, Reginaldo
C. Rocha, Henrique E. Toma, Koiti
Araki*
Text for Table of Contents
Page 1. – Page 12.
Catalytic Water-Oxidation Activity of
a Weakly Coupled Binuclear
Ruthenium Polypyridyl Complex
Text for Table of Contents
Weakly coupled binuclear complex shows up to twenty times larger rate constant for oxygen evolution as
compared with the analogous mononuclear complex, indicating a major role of the second site on the
catalytic activity.
This article is protected by copyright. All rights reserved