Efficient water oxidation catalyzed by mononuclear ruthenium(II) complexes incorporating schiff base ligands

Article abstract of DOI:10.1002/chem.201305011

Four new charge-neutral ruthenium(II) complexes containing dianionic Schiff base and isoquinoline or 4-picoline ligands were synthesized and characterized by NMR and ESI-MS spectroscopies, elemental analysis, and X-ray diffraction. The complexes exhibited

Full text of DOI:10.1002/chem.201305011

DOI: 10.1002/chem.201305011
Full Paper
&
Water Oxidation
Efficient Water Oxidation Catalyzed by Mononuclear
Ruthenium(II) Complexes Incorporating Schiff Base Ligands
[a]
[a]
[b]
[b]
[a]
[a]
Ting-Ting Li, Yong Chen, Fu-Min Li, Wei-Liang Zhao, Chuan-Jun Wang, Xiao-Jun Lv,
[b]
[a, b]
Quan-Qing Xu, and Wen-Fu Fu*
Abstract: Four new charge-neutral ruthenium(II) complexes
containing dianionic Schiff base and isoquinoline or 4-pico-
line ligands were synthesized and characterized by NMR and
ESI-MS spectroscopies, elemental analysis, and X-ray diffrac-
tion. The complexes exhibited excellent chemical water oxi-
dation activity and high stability under acidic conditions
or. The high catalytic activities of these complexes for water
oxidation were sustained for more than 10 h at low concen-
trations. High turnover numbers of up to 3200 were ach-
ieved. A water nucleophilic attack mechanism was proposed.
V
A Ru =O intermediate was detected during the catalytic
cycle by high-resolution mass spectrometry.
(
pH 1.0) using (NH ) Ce(NO ) as a sacrificial electron accept-
4 2 3 6
Introduction
of strong oxidizing agents under acidic conditions, and under-
go successive and simultaneous loss of protons and electrons
through proton-coupled electron-transfer (PCET) processes.
Most of those studies have focused on Ru-based catalysts. The
first molecular WOC, blue dimer cis,cis-[(bpy) (H O)RuORu(H O)-
Photochemical water splitting is a promising strategy to con-
vert solar energy into chemical energy to meet rising global
[
1–3]
energy demands.
The photolysis of water, which involves
2
2
2
4
+
a multielectron process coupled with a multiproton transfer-
ence, consists of two half reactions whereby protons are re-
duced to dihydrogen and water is oxidized to dioxygen. Be-
(bpy)2] (bpy=2,2’-bipyridine), was pioneered by Meyer et al.
[15]
in the early 1980s. In 2005, Zong and Thummel reported
a well-defined mononuclear ruthenium complex that can cata-
lyze water oxidation processes in the presence of (NH ) Ce-
0
cause this transformation requires energy (E =1.23 V versus
4
2
[16]
the normal hydrogen electrode, NHE), water oxidation is con-
(NO ) (CAN). More recently, Sun et al. employed 2,2’-bipyri-
3 6
[4]
sidered to be the key step of the water-splitting process. In
nature, water oxidation occurs in the photosystem II (PSII) and
the tetranuclear manganese cluster (CaMn O ) serves as the
dine-6,6’-dicarboxylic acid (H bda) as an anionic ligand to pre-
2
pare mononuclear ruthenium complex [Ru(bda)(pic) ] (pic=4-
2
picoline), which displayed excellent catalytic activity towards
water oxidation processes under both thermal and visible-
4
x
[
5–8]
oxygen-evolving complex.
The metal centers in the complex
[17]
can be oxidized to higher oxidation states to promote the
transformation of metal–aquo to metal–oxo intermediates
light-driven conditions. Furthermore, by replacing the axial
ligand with isoquinoline (isoq), the prepared [Ru(bda)(isoq)2]
was capable of chemically catalyzing water oxidation processes
[
9]
prior to dioxygen production. Based on the PSII, chemists
have attempted to develop water oxidation catalysts (WOCs)
for artificial photosynthesis processes. Mono-, di-, and polynuc-
À1 [18]
with a high turnover frequency of 303 s . However, the cat-
II
alytic lifespan of most Ru -based WOCs is limited to the
[
10]
[11]
[12]
[13]
[14]
lear Ru, Ir, Mn, Fe, and Co complexes that exhibit
catalytic characteristics have been reported. These complexes
are easily oxidized to higher oxidation states in the presence
minute scale. The dissociation of axial ligands and oxidative
decomposition of equatorial ligands have been considered as
[15,19]
the major catalyst deactivation pathways.
Thus, the fabri-
cation of WOCs with well-defined structures that have both
a high turnover and a long catalytic lifespan remains a chal-
lenge.
[
a] T.-T. Li, Prof. Dr. Y. Chen, C.-J. Wang, Dr. X.-J. Lv, Prof. Dr. W.-F. Fu
Key Laboratory of Photochemical Conversion and Optoelectronic Materials
and HKU-CAS Joint Laboratory on New Materials, Technical Institute of
Physics and Chemistry and University of Chinese Academy of Sciences
Chinese Academy of Sciences
Beijing 100190 (P. R. China)
Fax: (+86)10-6255-4670
E-mail: fuwf@mail.ipc.ac.cn
To avoid decomposition of the organic ligands in molecular
WOCs, some research groups have attempted to improve the
durability of the catalytic system by immobilizing catalysts on
the electrode surface, which dramatically suppressed the oxi-
[20]
dative decomposition pathway. Compared with heterogene-
ous systems, the longevity of WOCs in homogeneous systems
can be extended by increasing the rigidity of the equatorial li-
[
b] F.-M. Li, W.-L. Zhao, Dr. Q.-Q. Xu, Prof. Dr. W.-F. Fu
College of Chemistry and Engineering
Yunnan Normal University
gands. Compared to [Ru(bda)(pic) ] catalyst, Sun and co-work-
Kunming 650092 (P. R. China)
2
ers discovered that [Ru(pda)(pic) ] complex bearing rigid ligand
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201305011.
2
1,10-phenanthroline-2,9-dicarboxylate (pda) exhibited im-
Chem. Eur. J. 2014, 20, 8054 – 8061
8054
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
proved catalytic stability and different catalytic water oxidation
to afford dark red solid 1. A similar procedure was adopted for
the preparation of complexes 2–4. H NMR analysis revealed
[
21]
1
pathways.
Based on coordination geometry, most WOCs feature nitro-
gen-heterocyclic ligands, such as polypyridyl and poly(pyrazol-
yl), but the ligands are generally more difficult to prepare and
modify when compared with some other simple ligands. Con-
siderable efforts have been devoted to developing new ligands
for water oxidation. These ligands should meet the following
requirements: 1) the synthetic process needs to be relatively
easy and afford easy modulation of the structural, electronic,
and electrochemical properties, and catalyst activity of the re-
sulting complex; 2) high chemical and photochemical stability
under strong oxidizing conditions; and 3) improved p-electron-
donating capabilities, that is, negatively charged ligands, af-
fording reduced metal complex oxidation potentials and cata-
lytic overpotential. Schiff bases are suitable candidates that are
easily synthesized from an aromatic amine and a carbonyl
compound through nucleophilic addition. A variety of metal
complexes featuring Schiff base ligands have been prepared
and used extensively in thermal catalysis and synthetic reac-
that the complexes featured a C2v symmetry (Figure S1 in the
Supporting Information). Single crystals of 2 and 4 were grown
by slow diffusion of diethyl ether into an acetonitrile solution;
the corresponding molecular structures are shown in Figure 1.
Figure 1. Molecular structures of 2 and 4 with thermal ellipsoids at 30%
probability. Hydrogen atoms and solvents are omitted for clarity.
[
22]
tions. Herein, we report the synthesis of four mononuclear
ruthenium complexes comprising dianionic Schiff base ligands
(Scheme 1). The obtained complexes exhibit remarkable cata-
Detailed crystal data, and selected bond lengths and angles
are listed in Table S1 in the Supporting Information and
II
Table 1, respectively. The C2v symmetry around the Ru center
was further confirmed by X-ray analysis; the bond angles at
[23]
the ruthenium atom vary from 82.3 to 95.18. The N atoms in
the isoquinoline or 4-picoline ligands are bound to the metal
in a nearly linear fashion for 2 and 4 with bond angles of 175.0
and 172.78, respectively. The RuÀO1/O2 distances of 2.055(5)/
2
.066(5) ꢁ in 2 are significantly shorter than those in [Ru(bda)-
[17a]
(
pic) ] and complex 4.
The RuÀN distances were within
2
Scheme 1. Syntheses of complexes 1–4.
a range of 1.971–2.081 ꢁ, which is similar to that of [Ru(bda)-
(
pic) ], and the axial RuÀN bond lengths of the complexes
2
were slightly longer than the equatorial bonds. Intermolecular
lytic activities towards chemical water oxidation in the pres-
ence of CAN as oxidant under acidic conditions. Despite their
similar ligand structure, the complexes displayed markedly dif-
ferent catalytic activities. Notably, the complexes exhibited
high stabilities, as indicated by the sustained catalytic activity
over a period of >10 h. To the best of our knowledge, this is
p–p interactions of adjacent molecules along the a axis (Fig-
Table 1. Selected bond lengths [ꢁ] and angles [8] for complexes 2 and 4.
2
4
II
the first report examining Ru complexes incorporating a Schiff
Ru1ÀN1
Ru1ÀN2
Ru1ÀN3
Ru1ÀN4
Ru1ÀO1
Ru1ÀO2
1.971(6)
1.993(6)
2.081(5)
2.071(5)
2.055(5)
2.066(5)
82.8(2)
91.4(2)
91.0(2)
94.5(2)
91.9(2)
91.0(2)
94.0(2)
87.8(2)
88.3(2)
87.8(2)
89.5(2)
88.7(2)
Ru1ÀN1
Ru1ÀN2
Ru1ÀN3
Ru1ÀN4
Ru1ÀO1
Ru1ÀO2
1.997(3)
1.990(3)
2.078(4)
2.081(4)
2.090(3)
2.091(3)
82.3(1)
90.9(2)
95.1(1)
93.5(1)
91.4(2)
93.6(1)
93.3(1)
89.0(1)
86.8(1)
86.5(1)
87.6(1)
91.0(1)
base ligand for catalytic water oxidation under acidic condi-
tions.
N1-Ru1-N2
N1-Ru1-N3
N1-Ru1-N4
N1-Ru1-O1
N2-Ru1-N3
N2-Ru1-N4
N2-Ru1-O2
N3-Ru1-O1
N3-Ru1-O2
N4-Ru1-O1
N4-Ru1-O2
O1-Ru1-O2
N1-Ru1-N2
N1-Ru1-N3
N1-Ru1-N4
N1-Ru1-O1
N2-Ru1-N3
N2-Ru1-N4
N2-Ru1-O2
N3-Ru1-O1
N3-Ru1-O2
N4-Ru1-O1
N4-Ru1-O2
O1-Ru1-O2
Results and Discussion
Synthesis and characterization
The Schiff base ligands were synthesized by the procedure
shown in the Supporting Information. The preparation of com-
plex 1 was performed by heating a mixture of H L1, [Ru-
2
(
dmso) Cl ], and potassium acetate in N,N-dimethylformamide
4 2
at 708C overnight under N atmosphere, followed by addition
2
of excess isoquinoline. The solvent was removed in vacuo and
the resulting residue was purified by column chromatography
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8055
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
ure S2 in the Supporting Information) are characterized by the
short distance of about 3.2 ꢁ between the planes of the two
isoquinoline units.
1 equivalent of CAN to complex 2 in aqueous solution of
pH 1.0 resulted in the bleaching MLCT band and increased ab-
sorbance around 430 nm. On adding a further 1 equivalent of
CAN, the change in the higher-energy wavelength region only
was observed (Figure S8 in the Supporting Information).
Absorption spectroscopy
The absorption spectra of 1–3 in acetonitrile are shown in
Figure 2. Complex 1 displays strong bands in the near-UV
region (200–300 nm), which correspond to intraligand p–p*
Electrochemistry
The redox properties of complexes 1–3 were examined by
cyclic voltammetry (CV) and differential pulse voltammetry
(
DPV) in both organic and mixed water solvents, and the elec-
trochemical data are summarized in Table 2. The electrochemi-
cal behaviors of complexes 1–3 were first evaluated in dry ace-
tonitrile, as shown in Figure 3. The complexes displayed mark-
III/II
edly lower E (Ru ) values than those of WOCs containing
1
=
2
[
26,27]
neutral ligands.
The low potentials are most likely because
of the improved p-electron-donating capabilities of the dia-
nionic Schiff base ligands. Complex 1 with the electron-with-
III/II
drawing (ÀF) group exhibits an E
1
=2(Ru ) value of 0.28 V versus
NHE, which is more positive than the corresponding potentials
of 2 and 3.
The DPV and CV curve of 3 in aqueous solution (pH 1.0) dis-
play three oxidation waves at 0.21, 0.61, and 1.23 V, corre-
III/II
IV/III
sponding to the oxidation potentials of E(Ru ), E(Ru ), and
À5
Figure 2. UV/Vis absorption spectra of complexes 1–3 (2.5ꢂ10 m) in aceto-
nitrile solution.
Table 2. Oxidation potentials Eox (V vs. NHE) of 1–3.
(
II/III)
(III/IV)
(IV/V)
Complex
Ru
Ru
–
Ru
–
[
a]
1
1
2
2
3
3
0.31
0.28
0.22
0.24
0.19
0.21
electron transitions. The characteristic long-wavelength ab-
sorptions in the visible region are assigned to metal-to-ligand
[
b]
1.04
0.65
0.61
–
1.27
1.25
1.23
–
[a]
[
24,25]
[
[
[
b]
a]
b]
charge-transfer (MLCT) transitions.
Although complexes 2
and 3 show similar absorption bands, complex 3 displays
slightly weaker absorbance in the visible region when com-
pared with those of 1 and 2. The three complexes could be
gradually oxidized by molecular oxygen under aerobic condi-
tions in aqueous CF SO H solution (pH 1.0), which was accom-
–
–
[a] Measured in acetonitrile containing 0.1m (Bu)
4
NPF
6
. [b] Measured in
pH 1.0 solution (0.1m CF
3
SO
2+
3
H) with 30% trifluoroethanol. The scan rate
À1
was 100 mVs . [Ru(bpy)
NHE.
3
]
1
=
2
was used as a reference with E =1.26 V vs.
3
3
panied by color changes from red to green. Spectral changes
as a function of time for 2 in aqueous CF SO H solution under
3
3
air atmosphere showed the reduced absorbance at around
60 nm that was assigned to the bleaching MLCT transitions,
5
and the isosbestic points at 620, 582, and 625 nm were clearly
observed in the time-resolved spectra of complexes 1, 2, and
3
, respectively, which indicated the existence of a new species
(Figures S3–S5 in the Supporting Information). Based on high-
resolution mass spectrometry (HRMS; found: 674.1214; calcu-
lated: 674.1261 for the complex 2), this new species was tenta-
III
tively ascribed to Ru complexes (Figure S6 in the Supporting
1
Information). The H NMR spectrum of this green sample in
CDCl3 featured broad signals, indicative of the formation of
III
[23]
paramagnetic Ru species. However, addition of an excess
amount of ascorbic acid to the solution restored the absorp-
tion spectrum and original solution color (Figure S7 in the Sup-
III
porting Information), thereby implying the reduction of Ru to
II
Ru . After the isolated green solid was treated with ascorbic
Figure 3. CV curves of complexes 1–3 (1.0 mm) in acetonitrile with (Bu)
4
NPF
6
1
acid, its H NMR spectrum (Figure S1 in the Supporting Infor-
(
0.1m) as the supporting electrolyte, a glassy carbon disk as the working
À1
mation) was identical with that of complex 2. Addition of
electrode, and Ag/AgCl as the reference electrode. Scan rate: 100 mVs .
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8056
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
V/IV
E(Ru ), respectively (Figure 4). Complexes 1 and 2 exhibit sim-
ilar DPV and CV curves, and they featured higher potentials at
the high oxidation state, as shown in Figures S9 and S10 in the
Supporting Information and Table 2. These differences were at-
Figure 5. Pourbaix diagram of 4 in Britton–Robinson buffer aqueous solu-
tion. Conditions: the potentials based on the pH value were obtained from
DPV measurement; the pH of the solution was adjusted by using 0.2m
NaOH aqueous solution.
and Figure S12 in the Supporting Information). Figure 5 shows
III/II
the pH independence of the Ru process (0.35 V vs. NHE)
III
from pH 1.0 up to 6.4 and the pK value of Ru ÀOH is 6.4. At
a
III/II
2
pH>6.4 the potential for the Ru process follows a well-de-
fined line with the slope close to À59 mV per pH unit, ascribed
À
+
III
II
to a 1 e /1 H PCET process of the [Ru ÀOH]/[Ru ÀOH ] redox
2
IV/III
couple. The Ru process is pH dependent over the whole pH
range from 1.0 to 8.0, with a slope of À59 mV per pH unit,
À1
Figure 4. DPV and CV curves (scan rate: 100 mVs ) of 3 in aqueous
which is a typical one-electron, one-proton PCET process. The
3 3
CF SO H solution (pH 1.0) containing trifluoroethanol (30%). A glassy carbon
disk and Ag/AgCl were used as the working and reference electrodes, re-
spectively.
V/IV
next process, Ru , also has a slope of À59 mV per pH unit
from 1.0 to 4.0. Accordingly, the Pourbaix diagram of 4 sup-
ports the coordination of H O to the low-valent Ru center
2
II
even at the Ru state and catalytic water oxidation of 4 in low
tributed to the two substituted groups on the Schiff base
ligand in the complexes. The results demonstrate that nega-
pH value aqueous solution follows the redox processes from
II
III
IV
V
Ru ÀOH to Ru ÀOH and to Ru ÀOH then to Ru =O.
2
2
II
tively charged ligands increase electron density on the Ru
center, consequently lowering the oxidation potentials to gain
Chemical water oxidation
[
28]
an advantage in catalytic water oxidation. The observed cat-
alytic onsets of 1–3 are comparable with their fast current in-
creases at around 1.30 V. The linear potential sweep voltammo-
grams of complexes 1–3 in the mixed trifluoroethanol and
CF SO H (1:2 v/v) solution were recorded and are shown in Fig-
Chemical water oxidation was performed using CAN as the oxi-
dant in an acidic solution. Typically, rapid oxygen evolution, as
monitored by a photosensitive Ocean Optics probe, was ob-
served upon addition of an acetonitrile solution of 1, 2, or 3
(4 mL, 1.0 mm) to CF SO H solution (4 mL, pH 1.0) containing
3
3
ure S11 in the Supporting Information. It was observed that
complex 3 with electron-donating methoxy groups displayed
the highest current response in comparison with those of com-
plexes 1 and 2, which is consistent with the high catalytic effi-
ciency of oxygen evolution. In the present system, the intro-
duction of the phenol group has the following advantages:
3
3
CAN (20 mm) under vigorous stirring. The addition of acetoni-
trile increased the solubility of the catalyst in aqueous solution.
The amount of evolved oxygen in the headspace of the reac-
tion tube was quantified by gas chromatography (GC). Figure 6
depicts oxygen evolution curves as a function of reaction time.
Oxygen production measurements were performed twice
under the same conditions for the purpose of excluding the
error. After 10 h, the oxygen evolution approached a plateau,
generating turnover numbers (TONs) of 2400 and 3200 for 2
and 3, respectively, whereas complex 1 produced a smaller
TON of 2200. Interestingly, these complexes were remarkably
stable, and oxygen evolution proceeded for over 10 h. To our
1) the phenol groups in WOCs possibly form a hydrogen bond
with the water molecules, which can facilitate water nucleo-
philic attack (WNA) reactions; and 2) the anionic group can sta-
II
bilize the higher oxidation states of the Ru -based WOCs by
promoting interactions between the filled p orbitals of the
oxygen atom and the empty d orbital of the metal center.
To get an insight into the redox properties of these com-
plexes, the dependence of the potentials on the pH (Pourbaix
diagram) for complex 4, which has good solubility in aqueous
solution, was recorded in the pH region of 1.0 to 8.0 (Figure 5
II
knowledge, mononuclear Ru -based WOCs exhibiting good du-
rability are scarce. The control experiments showed that in the
absence of catalysts and under the same other conditions, no
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8057
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 6. Oxygen evolution as a function of reaction time in the presence of
1
4
(1.0 mm), 2 (1.0 mm), or 3 (1.0 mm) in an aqueous CF
mL) containing CAN (20 mm).
3 3
SO H solution (pH 1.0,
oxygen evolution was detected. Previous studies performed
under strong oxidizing conditions reveal that RuO , which is
2
catalytically active towards water oxidation processes, is possi-
[
29,30]
bly involved in the catalytic reaction.
Thus, the possible in
situ formation of RuO nanoparticles (Figure S13 in the Sup-
2
porting Information) was assessed by dynamic light scattering
(DLS) experiment. The analysis confirmed that nanosized parti-
cles were not produced during the catalytic reaction. The sus-
tained activity of the herein prepared catalysts can be ex-
plained as such: 1) the rigid Schiff base ligands are beneficial
towards the robustness of the WOCs; and 2) phenolate is
a hard ligand that strongly chelates to the high-valent rutheni-
um center.
Figure 7. (top) Absorption spectral changes of (NH
60 nm at varying concentrations of 3 in an aqueous HNO
pH 1.0). Data were collected about 5 s after catalyst injection. (bottom) Initial
rates of (NH Ce(NO consumption as a function of concentration of 3.
4
)
2
Ce(NO
3
)
6
monitored at
3
3
solution (0.1m,
4
)
2
3 6
)
Mechanistic interpretation
and/or hydrogen-bond acceptors that are beneficial towards
[32]
The reaction and decay kinetic profiles were investigated by
measuring the absorption decay of CAN at 360 nm after
mixing the WOC with different concentrations of CAN in aque-
ous HNO3 solution (0.1m, pH 1.0) through real-time UV/Vis
spectroscopy. A linear dependency between the CAN depletion
and the concentration of 3 was observed; the initial rate of
efficient water oxidation processes.
V
Generally, high-valent Ru -oxo species are considered as the
key active intermediate in the water oxidation by Ru-based
[33]
WOCs. Meyer et al. confirmed the existence of intermediate
V
L Ru =O (in which L denotes a polypyridylic ligand) in cata-
5
5
lyzed water oxidation processes by spectroscopic study and
density functional theory calculation. Also, the nucleophilic
À1
CAN consumption was measured as 0.65 s (Figure 7). This
V
result indicates that complex 3 probably catalyzes water oxida-
attack of a water molecule on Ru =O, which generated three
[34]
tion through a water nucleophilic attack mechanism proposed
peroxidic intermediates, was reported. In addition, a seven-
[
31]
V
[35]
by Meyer et al. Figure 8 shows the initial rates of oxygen
evolution catalyzed by 3 as a function of catalyst concentra-
tion; the obtained profile is characteristic of a first-order reac-
coordinated Ru =O species is favored for Ru–bda WOCs. The
detection and characterization of stable intermediates during
the catalytic cycle are of fundamental importance to study cat-
alytic reaction mechanisms. To this effect, HRMS was used to
probe active intermediates under the present catalytic condi-
tions. In a typical run, CAN (10 equiv) was added to an aque-
ous solution of 2 (pH 1.0) under vigorous stirring and the solu-
tion was immediately injected into the mass spectrometer
without any treatment. The resulting mass spectra are shown
in Figure 9 and Figures S18 and S19 in the Supporting Informa-
tion. A signal at m/z=674.1213 was observed in the positive
À1
tion with a first-order rate constant of 0.185 s . Compared
with that of 3, complexes 1 and 2 produced lower turnover
frequencies of 0.097 and 0.128 s , respectively (Figures S14–
À1
S17 in the Supporting Information). Moreover, the kinetic
measurements analysis suggests that molecularly dissolved
catalysts rather than RuO participate in the catalytic reaction.
2
This indicates that the presence of the two methoxyl groups in
3
accelerates oxygenÀoxygen bond formation. A previous
III
+
study demonstrated that oxygenated groups in the vicinity of
the WOC active sites are promising electron or proton donors
mode that was assigned to [(L2)Ru (isoq) ] species. The weak
2
signal at m/z=690.1073 was attributed to intermediate [(L2)-
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8058
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
V
+
2
Figure 9. HRMS spectrum of [(L2(isoq) Ru )=O] : measured isotopic distribu-
tion.
Figure 8. (top) Oxygen evolution profiles over 5 min at various concentra-
tions of 3 in an aqueous CF SO H solution (pH 1.0, 4 mL) containing CAN
20 mm) at room temperature. (bottom) Initial rate of O production at vari-
ous concentrations of 3. The data were measured using an oxygen probe
3
3
(
2
(YSI 5331) immersed in the reaction solution.
Scheme 2. Proposed mechanism for water oxidation catalyzed by WOCs.
V
+
(
isoq) Ru =O] (calculated: 690.1210), but we did not observe
2
II
+
II
+
the signals of [(L2)Ru (isoq)(H O)+H] or [(L2)Ru (H O) +H] .
volved in the catalyzed water oxidation process was successful-
ly characterized by high-resolution mass spectrometry. The in-
troduction of electron-donating methoxyl groups of the dia-
nionic Schiff base ligands accelerated the WNA process, which
resulted in increased water oxidation rates. Studies to integrate
these catalysts in a photocatalytic water redox system to inves-
tigate the structure–activity relationship are currently under
way.
2
2
2
However, under acidic conditions (pH 1.0), one phenolate liga-
tion in the complex is possibly labile and substitution by water
would generate a six-coordinated aqua ruthenium complex
II
Ru ÀOH . The dissociated phenolate moiety would then coordi-
2
nate to the high-valent metal center to form a seven-coordi-
nate species during the catalytic reaction, as evidenced by
HRMS measurements. Relevant work is in progress in our labo-
ratory. A series of high-valent intermediates is sequentially gen-
V
erated through PCET processes to form Ru =O species, which
III
then undergo OÀO coupling with water to generate Ru ÀOÀ
Experimental Section
IV
OH. The latter is rapidly oxidized to Ru ÀOÀO, which produces
General
oxygen through a reductive elimination step to regenerate the
II
[36]
[37]
Ru ÀOH species (Scheme 2).
[Ru(dmso) Cl ] and [Ru(bpy) Cl ] were prepared according to
4 2 3 2
2
the respective reported procedures. All of the commercially avail-
able reagents and solvents used in this study were analytically
pure and used without further purification. The solvents used for
Conclusion
1
UV/Vis measurements were of HPLC grade. H NMR spectra were
Four mononuclear ruthenium(II) complexes containing differ-
ent Schiff base and axial ligands were successfully synthesized.
The complexes show remarkable catalytic activities and high
stabilities towards CAN-driven water oxidation, and a high TON
of 3200 was achieved. Kinetics studies revealed a first-order re-
recorded on a Bruker Avance 400 spectrometer with chemical
shifts (d, ppm) relative to tetramethylsilane. The mass spectra were
determined on an APEX II Model FT ion cyclotron resonance mass
spectrometer. Matrix-assisted laser desorption/ionization time-of-
flight (MALDI-TOF) mass spectra were recorded on a Bruker BIFLEX
III mass spectrometer. Elemental analyses were performed on
V
+
action process. The active intermediate [(L)(isoq) Ru =O] in-
2
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8059
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
a Vario EL III instrument. UV/Vis absorption spectra were obtained
on a Hitachi U-3010 spectrophotometer. Single crystals of 2 were
obtained by slow diffusion of diethyl ether vapors into an acetoni-
trile solution. The diffraction data were collected on a Rigaku R-
AXIS RAPID IP X-ray diffractometer using a graphite monochroma-
tor with MoKa radiation (l=0.071073 nm) at 113 K. The molecular
structure of the prepared complexes was resolved by direct meth-
Dynamic light scattering analysis
DLS measurements were carried out on a Dybapro Nano Star
(Wyatt Technology) instrument. The experiments were performed
in an acidic solution (pH 1.0) that contained the catalyst (1.0 mm)
and CAN (20 mm).
2
ods and refined by full-matrix least-squares methods on all F data
Detection of intermediates by HRMS
(
SHELXL-97). Non-hydrogen atoms were refined anisotropically. The
position of the hydrogen atoms was calculated and refined iso-
tropically.
A solution of catalyst 2 (0.05 mm, 5 mL) in acetonitrile/water (1:9)
was introduced into a 20 mL flask and stirred vigorously using
a magnetic bar. Then, CAN (2.5 mmol, 10 equiv based on 2) in
aqueous CF SO H solution (100 mL, pH 1.0) was slowly introduced.
After addition of CAN, the reaction solution was immediately in-
jected into the mass spectrometer without any treatment.
3
3
Electrochemistry
Electrochemical measurements were carried out on a CHI660C
electrochemical potentiostat. Cyclic voltammetry (CV) and differen-
tial pulse voltammetry (DPV) were performed using a three-elec-
trode cell in aqueous CF SO H solution (pH 1.0) containing trifluor-
Synthesis of complexes 1–3
3
3
oethanol (30%). Tetrabutylammonium hexafluorophosphate
(Bu) NPF , 0.1m) in acetonitrile was used as the supporting elec-
A mixture of ligand H
0.32 mmol), or H L3 (119 mg, 0.32 mmol) and potassium acetate
(63 mg, 0.64 mmol) was dissolved in N,N-dimethylformamide
(10 mL) under a N atmosphere. The resulting solution was stirred
vigorously and heated at 708C until a color change from orange to
red was observed. [Ru(dmso) Cl ] (153 mg, 0.32 mmol) was then
added to this solution over the course of 30 min and the mixture
was warmed for 24 h. Then, excess isoquinoline (2 mL) was added
to the reaction mixture, which was stirred overnight. The solvent
was removed in vacuo and the resulting residue was purified by
column chromatography (1:1 (v/v) hexane/dichloromethane
2
L1 (113 mg, 0.32 mmol), H L2 (100 mg,
2
(
2
4
6
trolyte. A glassy carbon disk (diameter: 3 mm), a platinum plate,
and an Ag/AgCl electrode (3m KCl aqueous solution) were used as
the working, counter, and reference electrodes, respectively. The
working electrode was successively polished with 3 and 1 mm dia-
mond pastes and sonicated in ion-free water before use. The elec-
trolyte was degassed with nitrogen for 30 min. All potentials are
2
4
2
3
+
reported versus NHE, and the redox couple [Ru(bpy)3]
2
+
/[Ru(bpy)3] , E
1
=1.26 V versus NHE, was used as a standard; the
=
2
À1
scan rate was 100 mVs .
eluent) to afford dark red solid 1–3.
1
Complex 1: Yield: 46.3%. H NMR (400 MHz, [D ]DMSO): d=9.42 (s,
6
Oxygen evolution analysis
2
H), 8.82 (s, 2H), 8.45 (dd, J=5.6, 3.2 Hz, 2H) 7.76 (t, J=8.0 Hz,
An oxygen probe (YSI 5331), monitor (YSI 5300), gas chromato-
graph, and an Ocean Optics optical probe were used to measure
and calibrate the evolved oxygen.
4H), 7.64 (d, J=6.7 Hz, 4H), 7.55 (t, J=5.6 Hz, 4H), 7.40 (dd, J=5.9,
3.0 Hz, 2H), 7.15 (d, J=8.0 Hz, 2H), 6.93 (dd, J=11.3, 7.1 Hz, 2H),
13
6.85 ppm (td, J=7.6, 4.5 Hz, 2H); C NMR (101 MHz, CDCl
): d=
59.2, 157.3, 150.1, 147.3, 140.8, 137.9, 133.2, 130.3, 128.2, 127.0,
26.4, 126.0, 124.5, 123.8, 123.0, 121.9, 121.0, 120.1, 116.5 ppm; MS
3
1
1
Turnover frequency and kinetics of oxygen evolution
+
(MALDI-TOF): m/z 711.1 [M+H] ; elemental analysis calcd (%) for
C H N O F Ru·isoquinoline: C 67.29, H 3.97, N 8.35; found: C
38
26
4
2 2
The catalytic reaction was performed in a 16 mL Schlenk bottle
containing degassed aqueous CF SO H solution (pH 1.0, 4 mL) and
6
7.45, H 4.07, N 8.48.
3
3
1
Complex 2: Yield: 46.4%. H NMR (400 MHz, [D ]DMSO): d=9.32 (s,
Ce(NH ) (NO ) (20 mm) in an argon atmosphere. The initial rate of
6
4
2
3 6
2
H), 8.43 (dd, J=6.2, 3.1 Hz, 2H), 8.13 (d, J=8.2 Hz, 2H), 7.83 (d,
J=5.7 Hz, 2H), 7.78 (t, J=7.1 Hz, 2H), 7.75 (m, 4H), 7.70 (t, J=
.0 Hz, 2H), 7.62 (t, J=6.9 Hz, 2H), 7.55 (t, J=6.6 Hz, 4H), 7.35 (dd,
J=6.2, 3.1 Hz, 2H), 7.32 (d, J=8.0 Hz, 2H), 6.19 ppm (br, 2H);
oxygen evolution over the course of 5 min was measured using an
oxygen probe immersed in the reaction solutions. The degassed
acetonitrile solution (10–100 mL) of 1 (1 mm) or 2 (1 mm) was intro-
duced at the bottom of the bottle by using a microsyringe. Turn-
over frequency=(noxygen/ncatalyst)/time.
8
13
C NMR (101 MHz, CDCl ): d=153.4, 146.5, 143.9, 136.6, 136.4,
3
1
1
;
31.7, 131.2, 129.5, 128.7, 128.4, 128.0, 127.3, 127.2, 127.1, 126.1,
25.9, 121.3, 119.4, 114.2 ppm; MS (MALDI-TOF): m/z 675.1 [M+H]
+
Turnover number
elemental analysis calcd (%) for C H N O Ru·isoquinoline: C
38
28
4
2
7
0.31, H 4.39, N 8.72; found: C 70.59, H 4.70, N 8.48.
In a typical measurement, an aqueous solution (4 mL, 20 mm,
1
Complex 3: Yield: 49.0%. H NMR (400 MHz, CD CN: d=9.20 (s,
2
pH 1.0 adjusted with CF SO H) of CAN was introduced into
3
3
3
H), 8.52 (s, 2H), 8.10 (dd, J=6.1, 3.3 Hz, 2H), 7.60 (m, 4H), 7.53 (t,
a 25 mL reaction flask. The reaction mixture was stirred magnetical-
ly, degassed with argon for more than 30 min, and the flask was
sealed with a rubber stopper. The catalyst solution (4 mL, 1 mm)
was then added to the above solution. The evolved oxygen in the
headspace of the flask was monitored with a photosensitive Ocean
Optics probe that was introduced into the neck of the flask. When
evolution of oxygen was complete, an aliquot of the evolved gas
was sampled from the headspace of the flask by using a microsyr-
inge, and quantified by GC on a Shimadzu GC-2014 instrument
equipped with a 5 ꢁ molecular sieve column (3 m ꢂ 2 mm), a ther-
mal conductivity detector, and argon as carrier gas. The TONs were
calculated based on the catalysts.
J=5.6, 3.8 Hz, 2H),7.43 (m, 4H), 7.25 (d, J=6.1 Hz, 2H), 7.20 (dd,
J=6.1, 3.3 Hz, 2H), 6.92 (d, J=7.6 Hz, 2H), 6.68 (d, J=7.1 Hz, 2H),
5
1
1
5
13
.87 (br, 2H), 3.79 ppm (s, 6H); C NMR (101 MHz, CDCl ): d=
3
57.1, 154.4, 146.1, 143.4, 135.8, 134.9, 131.7, 131.3, 128.7, 127.7,
27.3, 127.2, 126.5, 126.0, 125.2, 121.8, 121.2, 120.1, 113.5,
+
5.9 ppm; MS (MALDI-TOF): m/z 735.1 [M+H] ; elemental analysis
calcd (%) for C H N O Ru·dmso: C 62.13, H 4.72, N 6.90; found: C
40 32
4
4
6
1.86, H 4.78, N 6.98.
1
Complex 4: Yield: 45.7%. H NMR (400 MHz, [D ]DMSO): d=9.35 (s,
2H), 8.33 (d, J=5.6 Hz, 2H), 7.68 (d, J=6.3 Hz, 4H), 7.27 (dd, J=
6.2, 3.3 Hz, 2H), 7.17 (d, J=8.0 Hz, 2H), 6.96 (d, J=5.8 Hz, 4H), 6.93
6
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8060
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(
d, J=7.4 Hz, 2H), 6.20 (td, J=12.3, 4.6 Hz, 2H), 2.12 ppm (s, 6H);
[13] a) W. C. Ellis, N. D. McDaniel, S. Bernhard, T. J. Collins, J. Am. Chem. Soc.
+
2
010, 132, 10990–10991; b) J. L. Fillol, Z. Codolꢆ, I. Garcia-Bosch, L.
MS (MALDI-TOF): m/z 639.1 [M+H] ; elemental analysis calcd (%)
Gꢇmez, J. J. Pla, M. Costas, Nat. Chem. 2011, 3, 807–813; c) D. Hong, S.
Mandal, Y. Yamada, Y. M. Lee, W. Nam, A. Llobet, S. Fukuzumi, Inorg.
Chem. 2013, 52, 9522–9531.
for C H N O F Ru: C 60.18, H 4.26, N 8.77; found: C 60.25, H 4.33,
N 8.61.
3
2
27
4
2 2
CCDC-934249 and 986430 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[
[
14] a) D. J. Wasylenko, C. Ganesamoorthy, J. Borau-Garcia, C. P. Berlinguette,
Chem. Commun. 2011, 47, 4249–4251; b) S. Berardi, G. La Ganga, M.
Natali, I. Bazzan, F. Puntoriero, A. Sartorel, F. Scandola, S. Campagna, M.
Bonchio, J. Am. Chem. Soc. 2012, 134, 11104–11107; c) T. Nakazono,
A. R. Parent, K. Sakai, Chem. Commun. 2013, 49, 6325–6327.
15] S. W. Gersten, G. J. Samuels, T. J. Meyer, J. Am. Chem. Soc. 1982, 104,
4
029–4030.
Acknowledgements
[
[
16] R. F. Zong, R. P. Thummel, J. Am. Chem. Soc. 2005, 127, 12802–12803.
17] a) L. Duan, A. Fischer, Y. Xu, L. Sun, J. Am. Chem. Soc. 2009, 131, 10397–
10399; b) L. Duan, Y. Xu, P. Zhang, M. Wang, L. Sun, Inorg. Chem. 2009,
This work was supported by the National Key Basic Research
Program of China (973 Program 2013CB834804 and
4
8, 209–215.
[
[
[
18] L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet, L. Sun,
2013CB632403) and by the Ministry of Science and Technology
Nat. Chem. 2012, 4, 418–423.
19] D. J. Wasylenko, C. Ganesamoorthy, B. D. Koivisto, M. A. Henderson, C. P.
Berlinguette, Inorg. Chem. 2010, 49, 2202–2209.
of China (2012DFH40090). We thank the Natural Science Foun-
dation of China (NSFC Grant Nos. 21267025, 21273257, and
U1137606), CAS-Croucher Funding Scheme for Joint Laborato-
ries, and Yunnan Key Project (2010CC007) for financial support.
Y.C. acknowledges the Chinese Academy of Sciences (100 Tal-
ents Program) for funding support.
20] a) Z. Chen, J. J. Concepcion, J. W. Jurss, T. J. Meyer, J. Am. Chem. Soc.
2
009, 131, 15580–15581; b) J. Mola, E. Mas-Marza, X. Sala, I. Romero, M.
Rodriguez, C. Vinas, T. Parella, A. Llobet, Angew. Chem. 2008, 120, 5914–
916; Angew. Chem. Int. Ed. 2008, 47, 5830–5832; c) F. Li, B. Zhang, X.
5
Li, Y. Jiang, L. Chen, Y. Li, L. Sun, Angew. Chem. 2011, 123, 12484–12487;
Angew. Chem. 2011, 123, 12484–12487.
[
21] L. Tong, L. Duan, Y. Xu, T. Privalov, L. Sun, Angew. Chem. 2011, 123, 465–
469; Angew. Chem. Int. Ed. 2011, 50, 445–449.
Keywords: homogeneous catalysis · ruthenium · Schiff bases ·
water oxidation · water splitting
[22] a) P. G. Cozzi, Chem. Soc. Rev. 2004, 33, 410–421; b) C. Baleiz¼o, H.
Garcia, Chem. Rev. 2006, 106, 3987–4043; c) K. C. Gupta, A. K. Sutar,
Coord. Chem. Rev. 2008, 252, 1420–1450.
[
[
1] E. Amouyal, Sol. Energy Mater. Sol. Cells 1995, 38, 249–276.
2] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729–
[
23] M. D. Kꢄrkꢄs, T. ꢁkermark, E. V. Johnston, S. R. Karim, T. M. Laine, B.-L.
Lee, T. ꢁkermark, T. Privalov, B. ꢁkermark, Angew. Chem. 2012, 124,
15735.
11757–11761; Angew. Chem. Int. Ed. 2012, 51, 11589–11593.
[3] D. G. Nocera, ChemSusChem 2009, 2, 387–390.
[
[
24] C. F. Works, C. J. Jocher, G. D. Bart, Inorg. Chem. 2002, 41, 3728–3739.
25] K. Chichak, U. Jacquemard, N. R. Branda, Eur. J. Inorg. Chem. 2002, 2002,
[4] J. L. Dempsey, A. J. Esswein, D. R. Manke, J. Rosenthal, J. D. Soper, D. G.
Nocera, Inorg. Chem. 2005, 44, 6879–6892.
357–368.
[5] K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science
[
26] N. Kaveevivitchai, R. Zong, H.-W. Tseng, R. Chitta, R. P. Thummel, Inorg.
Chem. 2012, 51, 2930–2939.
2004, 303, 1831–1838.
[
[
6] N. Kamiya, J.-R. Shen, Proc. Natl. Acad. Sci. USA 2003, 100, 98–103.
7] B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka, Nature 2005, 438,
[
27] a) H. W. Tseng, R. Zong, J. T. Muckerman, R. Thummel, Inorg. Chem.
2
008, 47, 11763–11773; b) Y. Xu, L. Duan, T. ꢁkermark, L. Tong, B. L. Lee,
1040–1044.
R. Zhang, B. ꢁkermark, L. Sun, Chem. Eur. J. 2011, 17, 9520–9528.
[
[
8] A. Zouni, H.-T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saenger, P. Orth,
Nature 2001, 409, 739–743.
9] G. C. Dismukes, R. Brimblecombe, G. A. Felton, R. S. Pryadun, J. E.
Sheats, L. Spiccia, G. F. Swiegers, Acc. Chem. Res. 2009, 42, 1935–1943.
[
[
[
[
[
28] L. Duan, Y. Xu, M. Gorlov, L. Tong, S. Andersson, L. Sun, Chem. Eur. J.
2
010, 16, 4659–4668.
29] K. Nagoshi, S. Yamashita, M. Yagi, M. J. Kaneko, J. Mol. Catal. A 1999,
44, 71–76.
30] Y. Zhang, E. C. Judkins, D. R. McMillin, D. Mehta, T. Ren, ACS Catal. 2013,
, 2474–2478.
1
[10] a) J. J. Concepcion, J. W. Jurss, M. K. Brennaman, P. G. Hoertz, A. O. T. Pa-
trocinio, N. Y. M. Iha, J. L. Templeton, T. J. Meyer, Acc. Chem. Res. 2009,
3
42, 1954–1965; b) X. Liu, F. Wang, Coord. Chem. Rev. 2012, 256, 1115 –
31] J. J. Concepcion, J. W. Jurss, J. L. Templeton, T. J. Meyer, J. Am. Chem.
Soc. 2008, 130, 16462–16463.
32] D. C. Marelius, S. Bhagan, D. J. Charboneau, K. M. Schroeder, J. M.
Kamdar, A. R. McGettigan, B. J. Freeman, C. E. Moore, A. L. Rheingold,
A. L. Cooksy, D. K. Smith, J. J. Paul, E. T. Papish, D. B. Grotjahn, Eur. J.
Inorg. Chem. 2014, 4, 676–689.
1136; c) L. Bernet, R. Lalrempuia, W. Ghattas, H. Mꢃller-Bunz, L. Vigara,
A. Llobet, M. Albrecht, Chem. Commun. 2011, 47, 8058–8060; d) M. Ven-
nampalli, G. Liang, C. E. Webster, X. Zhao, Eur. J. Inorg. Chem. 2014, 4,
715–721.
[
11] a) N. D. McDaniel, F. J. Coughlin, L. L. Tinker, S. Bernhard, J. Am. Chem.
Soc. 2008, 130, 210–217; b) J. F. Hull, D. Balcells, J. D. Blakemore, C. D.
Incarvito, O. Eisenstein, G. W. Brudvig, R. H. Crabtree, J. Am. Chem. Soc.
[
[
33] S. Romain, L. Vigara, A. Llobet, Acc. Chem. Res. 2009, 42, 1944–1953.
34] J. J. Concepcion, M. K. Tsai, J. T. Muckerman, T. J. Meyer, J. Am. Chem.
Soc. 2010, 132, 1545–1557.
2009, 131, 8730–8731; c) R. Lalrempuia, N. D. McDaniel, H. Mꢃller-Bunz,
S. Bernhard, M. Albrecht, Angew. Chem. 2010, 122, 9959–9962; Angew.
Chem. Int. Ed. 2010, 49, 9765–9768.
[
[
[
35] L. Duan, L. Wang, A. K. Inge, A. Fischer, X. Zou, L. Sun, Inorg. Chem.
2
013, 52, 7844–7852.
36] E. Duliꢈre, M. Devillers, J. Marchand-Brynaert, Organometallics 2003, 22,
04–811.
37] J. A. Broomhead, C. G. Young, Inorg. Synth. 1982, 21, 127–128.
[
12] a) F. M. Ashmawy, C. A. McAuliffe, R. D. Parish, J. Tames, J. Chem. Soc.
Dalton Trans. 1985, 7, 1391–1397; b) J. Limburg, J. S. Vrettos, L. M.
Liable-Sands, A. L. Rheingold, R. H. Crabtree, G. W. Brudvig, Science
8
1999, 283, 1524–1527; c) E. A. Karlsson, B.-L. Lee, T. ꢁkermark, E. V.
Johnston, M. D. Kꢄrkꢄs, J. Sun, ꢅ. Hansson, J.-E. Bꢄckvall, B. ꢁkermark,
Angew. Chem. 2011, 123, 11919–11922; Angew. Chem. Int. Ed. 2011, 50,
Received: December 23, 2013
11715–11718.
Published online on June 2, 2014
Chem. Eur. J. 2014, 20, 8054 – 8061
www.chemeurj.org
8061
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim