A trinuclear ruthenium complex as a highly efficient molecular catalyst for water oxidation

Article abstract of DOI:10.1039/c5dt04233j

A trinuclear ruthenium complex, 3, was designed and synthesized with the ligand 2,2′-bipyridine-6,6′-dicarboxylic acid (bda) and we found that this complex could function as a highly efficient molecular catalyst for water oxidation in homogeneous systems.

Full text of DOI:10.1039/c5dt04233j

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A Trinuclear Ruthenium Complex as Highly Efficient Molecular
Catalyst for Water Oxidation
L. L. Zhang,a Y. Gao,*a Z. Liu,a X. Ding,a Z. Yua and L. C. Sun*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
A trinuclear ruthenium complex 3 was designed and synthesized with the ligand 2,2’-bipyridine-6,6’-dicarboxylic acid
(bda), and we found that this complex can work as highly efficient molecular catalyst for water oxidation in homogeneous
systems. This trinuclear molecular water oxidation catalyst 3 displayed much higher efficiencies in terms of turnover
numbers and initial oxygen evolution rate than its counterparts, a binuclear catalyst 2 and a mononuclear catalyst 1, in
both chemically driven and photochemically driven water oxidation based on either the whole catalytic molecules or the
active Ru centers. The reasons for the superior performance of the catalyst 3 were discussed and we believe that the
multinuclear Ru centers involved in a single molecule are indeed beneficial for enhancing the opportunity of the formation
of O-O bond through intramolecular radical coupling pathway.
DOI: 10.1039/x0xx00000x
www.rsc.org/
radical coupling reaction mechanism.14 Based on this reaction
mechanism, several binuclear Ru-bda catalysts have been
designed and synthesized for catalytic water oxidation and
Introduction
Natural photosynthesis (NPS), utilizing solar energy to convert
water and carbon dioxide into carbohydrates in photosynthetic
organisms, produces essential energy and resources to sustain
the natural creatures on earth.1-3 Inspired by the NPS, artificial
photosynthesis is consistently regarded as one of the best
ways to obtain abundant sustainable and clean energy to meet
the decreasing traditional energy resources and the arising
environmental concerns. At present, this hot research topic
has attracted great attention around the world and much
effort has been paid on functional mimics of the active site of
photosynthesis. The most prospective method among them is
the production of hydrogen or the reduction of carbon dioxide
through light driven water splitting. While in both processes,
water oxidation is the key challenge which provides the
important electrons and protons to ensure the solar-to-fuel
conversion. Therefore, many attempts have been carried out
on development of highly efficient water oxidation catalysts
based on transition metal complexes, such as manganese
(Mn),4-8 Ru,9-19 iridium (Ir)20-23 in which Ru-based catalysts have
displayed the best performance so far.
considerably high turnover numbers (TONs) were obtained
based on the generated oxygen.17 The results indicated that O-
O bond formation should occur in binuclear Ru-bda catalyst
units through intramolecular radical coupling pathway.
In this present work, we report a trinuclear Ru-bda catalyst
3
together with a reference binuclear Ru-bda catalyst 2, their
structures are shown in Chart 1. Experimental results display
that further increase of the catalyst unit in a single molecule
leads to better catalytic activity, very likely due to the
increased opportunities of intramolecular O-O bond
formations. The enhancing performances of catalyst
3 on
catalytic water oxidation were studied and discussed in this
paper.
Among the developed Ru-based catalysts, a mononuclear
Ru-bda (bda = 2,2’-bipyridine-6,6’-dicarboxylic acid) type of
catalysts has been developed by our group and displayed
significant activities on water oxidation with a bimolecular
a.State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research
Center on Molecular Devices, Dalian University of Technology (DUT), Dalian
116024, China.
Chart 1. Structures of catalysts 1–3.
b.Department of Chemistry, School of Chemical Science and Engineering, KTH Royal
Institute of Technology, Stockholm 10044, Sweden.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Results and discussion
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Synthesis and characterization of complexes 2 and 3.
Cyclic voltammograms (CVs) and differential pulse
Complexes 2 and 3 were synthesized by linking the Ru(bda)(pic)2
unites through flexible chain containing benzene similar to the
synthetic route of complex Ru(bda)(pic)2 1 (pic = picoline). Their
voltammograms (DPVs) of complexes
M) and
(3.3×10-4 M) were taken in CF3SO3H (pH = 1.0)
aqueous solution containing 20% CH3CN as shown in Figure 1.
The CV curve of complex (Figure 1a, red line) displays two
1 2
(1×10–3 M), (5 × 10–4
3
1
structures were fully characterized by HNMR spectroscopy, as well
as HR-MS and elemental analysis (Figure S1-S4). The 1HNMR
spectrum of complex 2 displays that the proton resonances of the
equatorial ligand are assigned to the doublet at δ = 8.65 and 7.88
and triplet at δ = 7.81, of the axial ligand are assigned to the triplet
7.53 and the multiplet (two doublet) at δ = 7.1. Protons on the
benzene are assigned to the triplet at δ = 7.05 covered partly by the
multiplet and double at δ = 6.87 and singlet at δ = 6.77. Comparing
to complex 2, the proton resonances of equatorial ligand of
complex 3 are found at positions of 2’ and 3’ are coincident leading
to the multiplet. While the proton resonances of axial ligand are
assigned to the doublet at δ = 7.07 and 6.96 belong to the protons
at positions of H5 and H5’, respectively. The protons at position of
H4’ display a broad singlet because of the highly symmetrical
structure. The protons affiliation indicates the correct structures of
complexes 2 and 3. The HR-MS signals at m/z = 1185.1423 and
1727.1951 are assigned to the species of [2+Na]+ and [3+Na]+
confirm again the correct structures of complexes 2 and 3 above .
2
reversible redox peaks at E1/2 = 0.92 and 1.14 V (vs. NHE),
which are assigned to the redox couples of RuII/RuIII-OH2 and
RuIII-OH2/RuIV-OH, respectively. The complex
3 (Figure 1a, blue
line) exhibits similar profiles of RuII/III and RuIII/IV redox
processes at E1/2 = 0.92 and 1.15 V (vs. NHE), respectively. The
profiles of RuII/III and RuIII/IV redox processes of complexes
and 3 are quite similar to the complex 1 (Figure 1a, black line),
2
at E1/2 = 0.89 and 1.12 V (vs. NHE). The potential differences
between the anodic (Epa) and cathodic (Epc) peaks of RuII/III
redox couples are calculated to be about 59 mV. This
electrochemical behavior demonstrates the single-electron
redox process of RuII/III redox couple and indicates that Ru
centers of the dinulcear complex
are independent at low oxidation stage.
In addition to the oxidation processes of RuII/RuIII-OH2 and
RuIII-OH2/RuIV-OH, the oxidation peak at Epa = 1.30 V (vs. NHE)
is also detected, respectively, for catalysts and in the
2 and the trinuclear complex
3
1
,
2
3
DPV measurements (Figure 1b), which are assigned to the
oxidation process of RuIV-OH/RuV=O. Beyond this oxidation
Electrochemistry of complexes 1–3.
peak, the catalytic current of catalyst
higher than that of catalysts and , which indicates that
catalyst possesses superior catalytic activity on water
oxidation.
3 is found obviously
1
2
3
Water oxidation by chemical oxidants.
The water oxidation abilities by chemical oxidants of the
complexes
aqueous solution using (NH4)2[Ce(NO3)6]. For comparison, the
concentrations of 1×10–7 M for catalyst , 5×10–8 M for catalyst
and 3.3×10–8 M for catalyst
were selected to control the
1, 2 and 3 were investigated in CF3SO3H (pH 1.0)
1
2
3
same amount of catalytic Ru centers involving in catalytic
water oxidation systems. The oxygen evolution was detected
with an oxygen sensor and calibrated by GC, as shown in
Figure 2. Based on the generated oxygen and catalyst
molecules, a high turnover number (TON) of 44412 for catalyst
2
(red line) and a much higher TON of 86498 for catalyst 3
(blue line) were obtained as shown in Figure 2a. The initial
turnover frequencies (TOFs) are calculated to be 68 and 126 s-
1, respectively. Based on the amount of catalyst molecules, the
TON of catalyst
Even based on the number of Ru catalytic centers, the TON of
28832 and TOF of 42 s–1 per Ru center of catalyst
are still
(TON of 22206 and TOF of
3 is almost twice as much as that of catalyst 2.
3
much higher than those of catalyst
2
34 s–1 per Ru center) and the reference reported ruthenium
dimer (TON of 21420 and TOF of 20 s–1 per Ru center)17, as
Figure 1. CVs (a) and DPVs (b) of complexes 1 (black line, 1×10–3 M), 2 (red line,
5×10–4 M) and 3 (blue line, 3.3×10–4 M) in CF3SO3H (pH 1.0) containing 20%
CH3CN with the glassy carbon as the working electrode, Pt wire as the counter
shown in Figure 2b. Both binuclear catalyst
catalyst display significantly higher efficiency than
mononuclear catalyst
(TON of 1460 and TOF of 3.8 s–1).
These results illustrate the O-O bond formations is easier in
2 and trinuclear
3
1
electrode, Ag/AgCl as the reference electrode, scan rate 50 mV/s, ENHE = EAg/AgCl
0.20.
+
catalyst
2
and
3
during the catalytic water oxidation process
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and implies the mechanism changes from intermolecular confirms that the initial oxygen evolution rate is zero order to
reaction of catalyst
and
1
to intramolecular reaction of catalysts
2
the concentrations of CeIV under our experimental condition.
All these results indicate that utilization of lower or higher
concentration of Ce(IV) seems do not benefit for catalytic
water oxidation. Higher concentration of Ce(IV) might
accelerate the decomposition of the catalysts. Nevertheless,
3
.
catalyst 3 consistently displays higher efficiency than catalyst 2
in the whole experiments. When using high concentrations of
Ce(IV) (more than 5 × 10–2 M), lower decreasing tendency of
TONs of catalyst
might be due to that when one of the metal-based active
centers is damaged by excessive oxidant, catalyst can still
3 is found than that of catalyst 2. The reason
3
carry O-O bond formation through intramolecular radical
coupling with the two remaining active centers. On the
contrary, after losing one active center, catalyst
2 has to
perform O-O bond formation through intermolecular radical
coupling or nucleophilic attack of water upon the survived
single active center, particularly in such a low catalyst
concentration.
Figure 2. Oxygen evolution of catalysts 1 (1×10–7 M) (black line) and 2 (5×10–8 M)
(red line) and 3 (3.3×10–8 M) (blue line) with (NH4)2[Ce(NO3)6] (5.0×10–2 M) as
oxidant in CF3SO3H (pH 1.0) solution. (a) TONs based on the number of catalyst
molecules. (b) TONs based on the number of ruthenium catalytic units.
Additionally, as reported,25 local enrichment of the catalytic
centers can increase the catalytic activity of mononulcear
catalyst
catalytic activity of catalyst
with the number of Ru centers rising from 1 to 7 in each
nanocage. 25 Therefore, the better performance of catalyst
1
. The reported reference results showed that the
1
was found increase obviously
3
indicates that more Ru centers involved in a single molecular
catalyst are beneficial to enhance the catalytic activities. The
insight is that multinuclear units in a single molecule indeed
increase the opportunity of O-O bond formation through
intramolecular radical coupling.
Additional experiments on studying the performance of these
catalysts have been carried out by using different amount of
oxidant in the systems. Keeping the same concentrations of
Figure 3. TONs (a) and initial TOFs (b) of catalysts 2 (5×10–8 M) (black line) and 3
(3.3×10–8 M) (red line) with different concentrations of (NH4)2[Ce(NO3)6] as
oxidant in CF3SO3H (pH 1.0) solution based on the ruthenium catalytic units.
catalysts
2 3
(5 × 10–8 M) and (3.3 × 10–8 M), alteration of the
concentration of Ce(IV) from 2.5 × 10–2 M to 7.5 × 10–2 M leads
to different TONs based on ruthenium catalytic centers, as
shown in Figure 3a. It is obvious that the highest TONs of
catalysts
2
and
3
were obtained with the Ce(IV) concentration
and
Kinetic studies.
of 5 × 10–2 M. Moreover, the initial TOFs of catalysts
2
3
To understand the water oxidation mechanisms in more
based on the Ruthenium catalytic unites under different
concentrations of CeIV were calculated according to the data
details, the kinetic studies on catalysts 2 and 3 were carried
out by monitoring the decay of CeIV at 360 nm with UV-Vis
spectra (Figure 4a and 4c). Different amounts of catalysts were
from Figure S5 (Figure 3b). TOFs of catalysts 2 and 3 are
calculated to around 30 s-1 and 40 s-1, respectively. The result
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injected into CeIV (1.5×10–3 M) aqueous solution and the data
were collected after 2 seconds. The initial consumption rates
of CeIV with catalyst
3 show a much steeper linear dependence
on the catalyst concentrations with a first-order reaction
constant of 36.31 s–1 (Figure 4d), which is higher than that of
catalyst
(Figure 4b). These results confirm again that catalyst
possesses much higher catalytic activity on water oxidation. In
addition, the plots of log(rate) vs log([ ]) and log([ ]) have
2
with also a first-order reaction constant of 25.00 s–1
3
2
3
been calculated and shown in Figure S6. The slopes of the
fitting linear are found to be 1.07 and 1.04, respectively. The
results confirm again that the water oxidation of catalyst 2 and
3 are the first-order reaction. Overall, the obtained first-order
kinetic constant indicates that no intermolecular reaction
occurs during water oxidation process and the rate-
determining step of O-O bond formation takes place most
likely through intramolecular interaction. Nevertheless, the
reaction mechanism of nucleophilic attack of water could not
be totally excluded during this process.
Figure 4. (a) Absorbance change at 360 nm according to catalyst 2 (0.25, 0.5,
0.75, 1.00 µM) addition into CF3SO3H (pH 1.0, 2.5 mL) solution containing CeIV
(1.5×10–3 M). (b) Initial rates of CeIV consumption depend on the concentrations
of catalyst 2. (c) Absorbance change at 360 nm according to catalyst 3 (0.1, 0.2,
0.3, 0.4 µM) addition into CF3SO3H (pH 1.0, 2.5 mL) solution containing CeIV
(1.5×10–3 M). (d) Initial rates of CeIV consumption depend on the concentrations
of catalyst 3.
Photocatalytic water oxidation.
The light driven water oxidation experiments were
implemented in three component systems in phosphate buffer
(pH 7.2) with complexes
1 2 3
(1×10–6 M), (5×10–7 M) or
(3.3×10–7 M) as catalyst, Ru(bpy)2[4,4’-(EtOOC)2-bpy)(PF6)2]
(1×10-3 M) as photosensitizer, Na2S2O8 (1×10–2 M) as sacrificial
electron acceptor. The experiments were performed under
300 mW/cm2 light intensity with 300 W Xe lamp passing
through 400 nm filter at room temperature.
Figure 5. The light driven water oxidation measurements in three component
systems in phosphate buffer (pH 7.2, 10 mL), with complexes 1 (1×10–6 M), 2
(5×10–7 M) or 3 (3.3×10–7 M) as catalysts, Ru(bpy)2[4,4’-(EtOOC)2-bpy)(PF6)2]
(1×10–3 M) as photosensitizer, Na2S2O8 (1×10–2 M) as sacrificial electron acceptor;
under 300 mW/cm2 light intensity with 300 W Xe lamp passing through 400 nm
filter at room temperature.
As shown in Figure 5, for the systems with catalysts 1, 2 and 3,
the generated oxygen are collected respectively to be 1.29,
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3.69 and 5.35 µmol, the relative TONs are calculated to be 129, stirred under 100 °C for 24 hours and cooled to room
369 and 535. The obtained oxygen and TON from the system temperature. Then Pd/C catalyst was filtered to recycle and
with catalyst
system with catalysts
the slope of oxygen generation curve for the system involving CDCl3) δ 8.37 –8.36 (m, 6H), 7.18 – 7.16 (m, 6H), 6.77 (s, 3H),
catalyst is higher than that of catalysts and
during the 2.85 (t, 12H). TOF-MS (ES+): m/z+ Found: 394.2278 [M+H]+;
initial period of light illumination (12 minutes). These results Calcd: 394.2283.
3 are substantially higher than those from the the solvent was eliminated by rotary evaporator to give 1.85 g
1
1
and 2. In addition, it is easy to find that (96%) of compound d as a brown powder. H NMR (400 MHz,
3
1
2
also confirm that catalyst
and on catalytic water oxidation, even in the photocatalytic
process, keeping the same O-O bond formation mechanism of
radical coupling within the trinuclear catalyst . In addition,
the better performance of catalyst than catalyst also
3 is indeed superior to catalysts 1
General procedure for synthesis of complex 2.
2
A mixture of Ru(bda)(DMSO)(4-picoline) (300 mg, 0.6 mmol)
and compound c (87 mg, 0.3 mmol) in dry CH3OH (50 mL) was
3
refluxed overnight under N2 atmosphere. Then the solvent was
removed under reduced pressure and the product was isolated
2
1
indicates O-O bond formation through intramolecular reaction
is better than intermolecular reaction.
by column chromatography on silica gel (CH2Cl2:CH3OH = 10:1)
1
as a dark red powder. H
to yield 80 mg (23%) of complex
2
NMR (400 MHz, d6-DMSO) δ 8.65 (d, J = 8.0 Hz, 4H), 7.88 (d, J
= 6.8 Hz, 4H), 7.81 (t, J = 7.8 Hz, 4H), 7.53 (t, J = 6.0 Hz, 8H),
7.10 – 7.05 (m, 8H), 6.87 (d, J = 7.6, 1.3 Hz, 2H), 6.77 (s, 1H),
2.65 (d, J = 10.4 Hz, 8H), 2.19 (s, 6H). TOF-MS (ES+): m/z+
Found: 1185.1406 [M+Na]+; Calcd: 1185.1423. Elemental
analysis, Calcd: For C56H46N8O8Ru2·CH3OH·2H2O: C 55.78%, H
4.45%, N 8.86%; Found: C 55.60%, H 4.42%, N 9.11%.
Experimental
Synthetic procedures
General procedure for synthesis of complex 3.
A mixture of Ru(bda)(DMSO)(4-picoline) (300 mg, 0.6 mmol)
and compound
d (79 mg, 0.2 mmol) in dry CH3OH (50 mL) was
refluxed overnight under N2 atmosphere. Then the solvent was
removed under reduced pressure and the product was isolated
by column chromatography on silica gel (CH2Cl2:CH3OH = 5:1)
1
as a dark red powder. H
to yield 40 mg (12%) of complex
3
NMR (400 MHz, d6-DMSO) δ 8.61 (d, J = 7.3 Hz, 6H), 7.79 (d, J
= 7.5 Hz, 12H), 7.53 (s, 12H), 7.07 (s, 6H), 6.96 (s, 6H), 6.47 (s,
3H), 2.57 – 2.51 (m, 12H), 2.19 (s, 9H). TOF-MS (ES+): m/z+
1727.1924 [M+Na]+; Calcd: 1727.1951. Elemental analysis,
Calcd: For C81H66N12O12Ru3·3H2O: C 55.42%, H 4.15%, N 9.39%;
Found: C 55.28%, H 4.13%, N 9.56%.
Scheme 1. Synthesis route of catalysts 2 and 3. (i) H2, Pd/C, EtOH, 2 days. (ii)
CH3OH, reflux, overnight.
General procedure for synthesis of compound a and b.
The synthesis of compounds
to the reported literature.24
a and b was carried out according
Materials and methods
All reagent grade solvents were purchased from Aladdin
chemical company, which were dried and distilled according to
the standard methods. All other materials were commercially
available. All reactions were carried out under N2
atomosphere.
1H NMR Spectra were collected at 298 K using a Bruker DRX-
400 instrument. Electrospray ionization mass spectra and high
resolution mass spectra were recorded on a Q-Tof Micromass
spectrometer (Manchester, England). The cyclic voltammetry
General procedure for synthesis of 1, 3-bis (4-pyridylethyl)-5-
phenyl- phosphate c.
Compound a (1.8 g, 6.3 mmol), Pd/C (20 mg) were dissolved in
EtOH (20 mL) in a 100 mL autoclave. The pressure of H2 was
controlled to be 10 normal atmospheres. The mixture was
stirred under 100 °C for 24 hours and cooled to room
temperature. Then Pd/C catalyst was filtered to recycle and
the solvent was eliminated by rotary evaporator to give 1.7 g
1
as a brown powder. H NMR (400 MHz,
measurements were performed on
a
BAS-100W
(94%) of compound
c
electrochemical potentiostat in a three-electrode system. The
working electrode was a glassy carbon disk (diameter, 3 mm)
polished with 3 and 1 mm diamond pastes and sonicated in
ion-free water before use. The counter electrode was platinum
wire. The reference electrode was Ag/AgCl. Oxygen evolution
analyses were performed on a Techcomp GC 7890T instrument
equipped with a 5 Å molecular sieve column and a thermal
conductivity detector with argon as carrier gas and an oxygen
sensor (OceanOptics NEOFOX-KIT-PATCH Non-Invasive oxygen
monitoring kit and RE-FOS-4-KIT RedEye patch). The photo-
CDCl3) δ 8.29 (d, 4H), 7.19 (d, 2H), 7.13 (d, 2H), 6.75 (d, 4H),
2.84 (t, 8H). TOF-MS (ES+): m/z+ 289.1687 [M+H]+. Calcd:
289.1704.
General procedure for synthesis of 1, 3, 5-tri-(4-
pyridyethyl)bezene d.
Compound
b (1.9 g, 4.9 mmol), Pd/C (20 mg) were dissolved in
EtOH (20 mL) in a 100 mL autoclave. The pressure of H2 was
controlled to be 10 normal atmospheres. The mixture was
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8
9
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induce water oxidation experiments were performed under
300 mW/cm2 light intensity with 300 W Xe lamp. The kinetic
experiments were carried out on
spectrophotometer.
a
HP 8450
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A highly efficient trinuclear catalyst
3 has been developed
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based on Ru-bda type of catalysts for water oxidation in
homogeneous systems. Based on number of catalystic
molecules, the TONs of catalyst
3
are found over two times
in both chemically and
higher than that of binuclear catalyst
2
15 L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A.
Llobet, L. Sun, Nat. Chem., 2012, 4, 418-523.
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8937.
photochemically driven catalytic water oxidation systems.
Even based on the same number of active centers, the
trinuclear catalyst
than the binuclear catalyst
the O-O bond formation catalyzed by the trinuclear complex
and the dinuclear complex is through intramolecular radical
3
still displayed much higher efficiencies
2
. Kinetic studies demonstrate that
3
17 Y. Jiang, F. Li, B. Zhang, X. Li, X. Wang, F. Huang, L. Sun,
Angew. Chem. Int. Ed., 2013, 52, 3398-3401.
2
18 R. A. Binstead, C. W. Chronister, J. Ni, C. M. Hartshorn, T. J.
Meyer, J. Am. Chem. Soc., 2000, 122, 8464-8473.
coupling reaction. These results show that increasing the
number of catalytic units within one catalyst molecule is
beneficial to enhance the opportunity on formation of O-O
bond. Further work on applying the highly efficient trinuclear
19 R. Zong, R. P. Thummel, J. Am. Chem. Soc., 2005, 127
12802-12803.
,
20 N. D. McDaniel, F. J. Coughlin, L. L. Tinker, S. Bernhard, J.
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catalyst
3 in dye-sensitized photoelectrochemical cell is
21 J. F. Hull, D. Balcells, J. D. Blakemore, C. D. Incarvito, O. G.
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Acknowledgements
This work was supported by the National Basic Research
Program of China (973 program) (2014CB239402), the National
Natural Science Foundation of China (20923006,
21120102036, 21106015, 91233201 and 21573033), the
Fundamental Research Funds for the Central Universities
(grant number DUT13RC(3)103, DUT15LK08), the Basic
Research Project of Key Laboratory of Liaoning (LZ2015015),
the Swedish Energy Agency and the K & A Wallenberg
Foundation.
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Dalton Transactions
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DOI: 10.1039/C5DT04233J
Trinuclear catalyst 3 was synthesized based on Ru(bda) (bda = 2, 2’-bipyridine-6,6’-dicarboxylic
acid) type of catalysts for higher catalytic abilities in homogeneous water oxidation systems.