Functional Polypyridine Co Complex as an Efficient Catalyst for Photo-Induced Water Oxidation

Article abstract of DOI:10.1002/cjoc.201600223

A novel mononuclear cobalt complex 1 was synthesized by treatment of CoCl₂·6H₂O with a COOMe functionalized TPA ligand (TPA=tris(2-pyridylmethyl)amine). In a basic borate buffer, 1 acts as an efficient catalyst for water oxidation, which is confirmed by an extinct catalytic oxidant wave in electrochemistry. Visible light-driven water oxidation has been achieved by 1 with a TON of 127.7 and a TOF of 3.8 s^?1respectively in a homogeneous system. In comparison to the reference RC with naked TPA, the higher efficiency of 1 evidences COOMe on ligand can improve the catalytic efficiency, leading to an effective pathway towards construction of a robust and stable artificial photosynthesis system.

Full text of DOI:10.1002/cjoc.201600223

COMMUNICATION
DOI: 10.1002/cjoc.201600223
Functional Polypyridine Co Complex as an Efficient Catalyst
for Photo-Induced Water Oxidation
,a
a
a
a
b
,c
Hongyan Wang,* Zhijuan Xin, Ruijuan Xiang, Si Liu, Xuewang Gao, Chengbo Li*
a
Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi'an, Shaanxi 710119, China
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and
b
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
College of Chemistry & Materials Science, Northwest University, Xi'an, Shaanxi 710127, China
c
2 2
A novel mononuclear cobalt complex 1 was synthesized by treatment of CoCl •6H O with a COOMe func-
tionalized TPA ligand (TPA＝tris(2-pyridylmethyl)amine). In a basic borate buffer, 1 acts as an efficient catalyst for
water oxidation, which is confirmed by an extinct catalytic oxidant wave in electrochemistry. Visible light-driven

1
water oxidation has been achieved by 1 with a TON of 127.7 and a TOF of 3.8 s respectively in a homogeneous
system. In comparison to the reference RC with naked TPA, the higher efficiency of 1 evidences COOMe on ligand
can improve the catalytic efficiency, leading to an effective pathway towards construction of a robust and stable ar-
tificial photosynthesis system.
Keywords cobalt complex, photo-induced, ligand-design, water oxidation
Introduction
to produce novel catalysts. In 2013, polar porphyrin
ligands were developed to coordinate with cobalt salts
to afford cobalt catalysts, which fascinated the active
Artificial photosynthesis is considered as the poten-
tially most controlled technology for the solar-to-
chemical energy conversion.
center to be attacked by water molecule during catalysis
[
1,2]
Frontline research in
[6d]
under a light-driven system.
In 2015, a molecular
natural photosynthesis has served as inspiration and
guidelines for the study in artificial photosynthesis for
many years. In a global perspective, efforts have been
made towards achieving stable and robust catalysts
based on cheaper and earth-abundant element due to
economical viability. Recently, molecular design con-
tributes to highly stable and efficient molecular catalysts
since their redox and kinetic properties are, in principle,
easily tunable by molecular modification. The already
demonstrated oxygen evolving activity can be further
developed by using fundamental coordination chemistry
to design a broader set of efficient catalysts. Cobalt be-
comes promising and strong candidate on account of
high efficiency, low light competition with chromophore.
In particular, paramagnetic cobalt species with typical
EPR pronunciation fascinates an intensive investigation
into intermediates generation, which further allows bet-
ter exploration and understanding the O－O bond for-
cobalt(II) catalyst with a 2,6-bis(methoxydi(pyridine-
-yl)methyl)pyridine) was reported to enable water ox-
2
[6e]
idation catalysis by means of visible light irradiation.
Then, di(pyridin-2-yl)methanediol was developed to
coordinate with cobalt salts to afford cobalt catalysts,
which achieved the largest TON among metal-organic
complexes for photocatalytic water oxidation up to
[
3]
[6f]
now. Overall, ligand design and catalyst optimization
are instrumental for stabilizing the catalytic system and
[
4]
2
sustained O productivity. In 2014, we presented a
mononuclear Co complex RC and dinuclear cobalt
complex with the soft TPA ligand [TPA＝tris(2-pyridyl-
methyl)amine], both serve as molecular catalysts to
[6g,6h]
promote photo-induced water oxidation.
Since TPA
provides a scaffold to introduce functional groups, tai-
loring of the catalytic activity is hopefully achieved by
appropriate modification of the substituent. It is essen-
tial to find a robust ligand system that can hold the met-
al centre firmly and does not undergo self-oxidation in
the oxidizing environment. Here we report a novel
monometallic cobalt catalyst 1 [dichloride tris(2-pyri-
dylmethyl)amine cobalt (II)] with electron-withdrawing
COOMe functionalized TPA ligand for efficient light-
[5]
mation during water oxidation.
Up to now, a few molecular cobalt complexes have
been reported to promote visible light-driven water oxi-
dation in homogeneous systems. Some multi-coordi-
nating ligands containing N-heterocycles are available
[
6]
*
E-mail: hongyan-wang@snnu.edu.cn, lcb123@nwu.edu.cn
Received April 12, 2016; accepted June 22, 2016; published online XXXX, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600223 or from the author.
Chin. J. Chem. 2016, XX, 1—6
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1

Wang et al.
COMMUNICATION
Scheme 1 Synthesis of 1 and the structure of RC
NaBH4
PBr3
MeO
OMe
MeO
OMe
OH
MeO
Br
N
MeOH/CH Cl
N
CHCl3
N
2
2
O
O
O
O
N
O
OMe
N
Co
Cl
N
N
.
(
NH ) CO
CoCl2 6H O
O
Cl
4
2
3
2
N
MeO
N
N
N
Co
N
CH CN
CH CN, reflux
3
3
O
OMe
N
N
O
N
Cl
O
O
OMe
RC
Cl
OMe
TPACOOMe
1
driven water oxidation.
(d, J＝8 Hz, 1H), 7.87 (t, J＝8 Hz, 1H), 8.05 (d, J＝8
Hz, 1H).
Methyl 6-(bromomethyl) picolinate PBr
mL, 1.04 mmol) was added to methyl 6-(hydroxymethyl)
picolinate (40 mg, 0.24 mmol) in 30 mL anhydrous
3
(0.1
Experimental
General
CHCl
at room temperature and then neutralized at 0 ℃ with
an aqueous saturated K CO solution. After extraction
with CH Cl (2 mL×3), the combined organic layers
were dried with Na SO and the solvent was removed
3
at 0 ℃. The reaction mixture was stirred for 4 h
The UV-Vis absorption spectra were measured on a
U-3900H spectrophotometer. The emission spectra were
measured on a F-7000 emission spectrometer from
HITACHI. HPLC-MS data were obtained using an ek-
spert ultraLC 100-XL. Solvents used for HPLC: 0.05%
formic acid in H O and 0.05% formic acid in CH OH.
H NMR spectra were recorded on a Bruker-400 MHz
spectrometer at 293 K. Chemical shifts are referenced
internally to the residual solvent signal.
2
3
2
2
2
4
under reduced pressure, leading to pure compound as a
2
3
1
1
pale yellow solid (55.2 mg, 100%). H NMR (400 MHz,
CDCl
H), 7.95 (t, J＝8 Hz, 1H), 8.11 (d, J＝8 Hz, 1H).
Tris[6-(methoxycarbonyl)-2-pyridylmethyl]amine
3
) δ: 4.03 (s, 3H), 4.73 (s, 2H), 7.75 (d, J＝8 Hz,
1
COOMe
(TPA
)
In a thick-walled Schlenk tube, 20 mL
Materials
acetonitrile was added to methyl 6-(bromomethyl)pico-
linate (41.9 mg, 0.18 mmol) and ammonium carbonate
All reactions and operations were carried out under a
dry argon atmosphere with standard Schlenk technique.
All solvents were dried and distilled prior to use. Dime-
(
85.94 mg, 0.89 mmol). Then the tube was sealed with a
Teflon stopper. The mixture was heated to 75 ℃ with
vigorous stirring for 18 h. The mixture was allowed to
cool to room temperature and carefully vented in a fume
hood to release ammonia pressure. The solids were fil-
4 2
thyl pyridine-2,6-dicarboxylate, NaBH , and CoCl •
6H
2
O were purchased from HEOWNS and used as re-
ceived.
2 2
tered, and washed with copious amounts of CH Cl . The
combined filtrates were concentrated to the crude prod-
uct as yellow oil, which was further purified by column
Synthesis
Methyl 6-(hydroxymethyl) picolinate
NaBH
4
(
104.4 mg, 2.76 mmol) was slowly added to a solution
chromatography on silica gel using CH
Et N (V∶V∶V＝10∶1∶0.2) as eluent. Evaporation
of the solvent yielded 23 mg (34.4%) light yellow solid.
H NMR (400 MHz, CDCl ) δ: 8.01 (t, J＝6 Hz, 3H),
3
7.8 (m, 5H), 7.65 (d, J＝8 Hz, 1H), 4.09 (s, 2H), 4.03 (s,
2 2 3
Cl /CH OH/
of dimethyl pyridine-2,6-dicarboxylate (500 mg, 2.56
mmol) in a 20 mL dry 7∶3 (V∶V) mixture of MeOH/
3
1
CH
at room temperature and then neutralized with an aque-
ous saturated NH Cl solution. After extraction with
CH Cl (2 mL×3), the combined organic layers were
dried with Na SO and the solvent was removed under
reduced pressure. The resulting crude residue was puri-
fied by column chromatography (petroleum ether/ethyl
acetate＝1∶6) giving the product (243.5 mg, 57.4%) as
2
Cl
2
at 0 ℃. The reaction mixture was stirred for 6 h
2H), 4.00 (s, 9H) (Figure S1). Elemental analysis of
4
COOMe
TPA
24 4 6
calcd (%) for C24H N O (464.4706): C
2
2
62.06, H 5.21, N 12.06; found C 61.91, H 5.12, N 11.94.
Mononuclear cobalt complex 1 An oven-dried
Schlenk flask equipped with a magnetic stir bar was
evacuated and filled with nitrogen. In this flask
2
4
1
a white solid. H NMR (400 MHz, CDCl
3
) δ: 3.06 (br s,
2 2
CoCl •6H O (16 mg, 0.06 mmol) was dissolved in
1
H; OH), 4.02 (s, 3H; OCH
3
), 4.89 (s, 2H; CH O), 7.56
2
CH CN (2 mL), degassed with Ar, and heated to reflux.
3
2
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Chin. J. Chem. 2016, XX, 1—6

Functional Polypyridine Co Complex as an Efficient Catalyst for Photo-Induced Water Oxidation
COOMe
COOMe
To the above refluxing mixture, TPA
mmol) in 2 mL CH
(23 mg, 0.01
CoCl
2
•6H
2
O with TPA
3
in refluxed CH CN over-
3
CN was added dropwise to afford a
night with a yield of 50.7%.
green solution, which was kept refluxing overnight.
When the solution was cooled down, the solvent was
removed and ether was used to recrystallize product
It is considered that UV-Vis spectroscopy can be
employed to investigate the structure of the molecule
catalyst. Particularly, upon dissolving the compound

three times to give 18 mg 1 as green solids (50.7%).
into CH
vent CH
trile species [(L)M(CH
by a long wavelength absorption band pattern between
3
CN, the Cl ligand can be replaced by the sol-
＋
ESI-MS (m/z): 568.0985, [M－2Cl＋HCOO] (calcd
3
CN molecule resulting into a mono-acetroni-
2
＋
2＋
5
68.4212); 261.5501, [M2Cl] /2 (calcd 261.7019).
Elemental analysis of cobalt complex 1 calcd (%) for
Cl Co•12H O (810.3098): C 35.54, H 5.92,
3
CN)] , which is characterised
[9]
C
24
H
24
N
4
O
6
2
2
600 and 700 nm in the UV-Vis spectra. Like its parent
compound RC, at room condition, the CH CN solution
of 1 exhibits two absorption bands centered at 590 and
80 nm respectively, accompanied by intense absorption
N 6.91; found C 35.19, H 6.01, N 7.16.
3
Electrochemistry
6
before 300 nm from lignad π→π* transition, implying
the mono-substituted Co center with Cl ligand (Figure
Electrochemical measurements were performed us-
ing a CHI660E B14514a. A three-electrode system with
a 0.07 cm glass carbon working electrode, platinum
wire as auxiliary, and an Ag/AgCl (with saturated KCl
aqueous solution) as reference electrode, was used to
measure the cyclic voltammograms in borate buffer. The
working electrode was polished with a 0.05 μm alumina

2
S2). Differently, in the aqueous solution, the electronic
absorbance spectrum of 1 affords one intense ligand
π→π* peak around 275 nm with a weak shoulder within
visible-light absorption centered around 500 nm as-
2
+
signed to the Co d-d transition, indicating the for-
COOMe
mation of [Co(TPA
2 2
)(H O) ] in an octahedral com-
paste and sonicated for 15 min before use. The sample
[
10]
solution (10 mL) with 100 mmol•L1 borate buffer as
plex. The concentration dependence of absorbance
satisfies Beer's law, which implies that 1 remains as a
monomer under the condition (Figure S3). Monitoring
the spectrum for 11 h with 1×10 mol•L 1 (Figure 1),
almost identical line on each measurement confirms
monomer as the single species to sustain in the presence
of water.
supporting electrolyte was degassed with argon for 30
min before the measurement started. All cyclic voltam-
mograms shown were recorded at a scan rate of 100
4
1

1
mV•s .
Oxygen evolution measurements
The oxygen evolution was measured using a stand-
ard Clark-type oxygraph electrode (Hansatech Instru-
ments). The cell was thermostated at 25 ℃ for all ex-
periments. The signal was recorded for the entire dura-
tion of the experiment at 0.1 s intervals using the Oxy-
graph+ software (Hansatech Instruments). The signal
was calibrated using air saturated aqueous solutions
1
(
2
[O ]＝276 μmol•L , T＝25 ℃). For the oxygen evo-
lution experiments, the each component was mixed in
pH 8 borate buffer in the dark, flushed with argon to
remove dissolved oxygen and then irradiated with visi-
2
ble light [LEDs, λ＝(470±10) nm, 820 μE/(m •s)]. The
maximum turnover frequency (TOF) was determined at
the steepest slope of the oxygen evolution curve.
Results and Discussion
Figure 1 UV-Vis spectrum of 1 × 10 mol•L1 1 in 75
4

1
As shown in Scheme 1, the synthesis of tri-α-
COOMe substituted TPA ligand (TPA
mmol•L pH 8.4 borate buffer for 0－11 h. The inset is the en-
larged absorption spectra of 1 ranging from 350－700 nm at 0 h.
COOMe
) was started
with the preparation of methyl 6-(hydroxymethyl)pico-
linate through reduction of dimethyl pyridine-2,6-di-
The catalytic ability of 1 for water oxidation was
firstly examined with electrochemistry in 0.1 mol•L
1
carboxylate in the mixture of dried MeOH and CH
2
Cl
Subsequently, methyl
-(hydroxymethyl) picolinate was bromized by PBr in
CHCl and resulted in the smooth conversion to the
2
[
7]
with the addition of NaBH
4
.
borate buffer at pH 8.4. One irreversible oxidative wave
was observed, followed by the appearance of catalytic
6
3
[6d,6g]
3
current for water oxidation (Figure S4).
The onset
[7]
methyl 6-(bromomethyl)picolinate. Then, the direct
potential is located at 1.10 V with an overpotential of
330 mV. This value is comparable to those recently re-
condensation of 6-(bromomethyl)picolinate with
COOMe
(
NH
4
)
2
CO
3
in CH
3
CN led to TPA
as light yellow
ported for mononuclear molecular cobalt catalysts (300
[8]
[3c,6f,6c]
oil in a yield of 34.4%. The mononuclear cobalt com-
－600 mV).
Since the electrochemistry indicated
plex 1 was obtained as green solid by treatment of
that the appearance of 1 could promote water oxidation,
Chin. J. Chem. 2016, XX, 1—6
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3

Wang et al.
COMMUNICATION
up to 7.50 µmol•L does not alter the initial rate of O
1
a water oxidation in a light-driven system was consid-
ered to explore its catalytic behavior assisted by photo-
2
formation before the first 15 s, indicating that the rate is
not dependent on the catalyst concentration under the
In comparison with naked
TPA chelated monomer compound RC, under the same
2
＋
sensitizer [Ru(bpy)
As shown in Figure S4, [Ru(bpy)
potential at 1.41 V, more positive than the onset poten-
3
]
and sacrificial electron acceptor.
3
＋
[6f,12]
3
]
exhibits oxidation
conditions (Figure S5).
[3i]
1
tial for water oxidation at 1.1 V in the presence of 1.
Therefore, a system where [Ru(bpy)
controlled conditions with 5.00 µmol•L catalyst, TON
3
＋
3
]
is formed
of 12.6 and TOF of 0.3 were obtained (Figure S6).
Clearly, the COOMe groups on ligand can significantly
enhance the catalytic ability, which is in consistence
with the electrochemical analysis.
through an electron transfer from a photo-excited
2
＋
[
Ru(bpy)
3
]
to an electron acceptor can be used in
combination with 1 to give a thermodynamically favor-
able driving force for water oxidation. In comparison,
the onset overpotential of RC is 490 mV (versus NHE),
2
＋
3
] ,
it is clear that in the combination of 1 and [Ru(bpy)
the driving force for water oxidation is much higher.
Visible-light induced water oxidation catalyzed by
compound 1 was initiated in a borate buffer with
[
3 4 2 2 2 8
Ru(bpy) ](ClO ) and Na S O as photosensitizer and
electron acceptor, respectively, and evolution-oxygen
measurement was monitored by a Clark electrode. It is
found that the pH of the reaction mixture strongly in-
fluences the process. A pH of 8.4 was found optimal
(
see Table S1) as a lower pH renders photocatalysis
much less efficient, while higher pH gives rise to in-
creased initial oxygen evolution rates, but also fasters
deactivation with overall lower TONs. Besides, it is
noteworthy that the efficiency of oxygen evolution
strongly depends on the concentration of buffer. Both
the rate and yield of oxygen formed were enhanced
when the concentration of buffer was increased by 75
Figure 2 Light-induced water oxidation in a mixture containing
1
1
[
Ru(bpy)
3
](ClO
4
)
2
(0.4 mmol•L ) and Na
2
S
2
O
8
(5 mmol•L )
1
mmol•L , implying O
2
formation proceeded a proton
1
with 1 (0, 1.25, 2.50, 3.75, 5.00 and 7.50 µmol•L ) in borate
buffer (75 mmol•L , pH 8.4). The Clark cell was kept constant at
transfer with buffer base serving as the proton accep-
1
[11]
tor. Besides, the ionic strength resuling from concen-
trated buffer could also influence the electron transfer
2
0 ℃, and the system was irradiated using LEDs [λ＝(470±10)
2
nm, 820 µE/(m •s)]. The arrow indicates the start of the irradia-
tion.
[11]
rate. Successive increase in buffer concentration led
to a deactivation of the whole system, possibly due to
the low solubility of the sensitizer, in accordance with
the observation of red precipitate in the system.
Some reports showed that molecular cobalt systems
serve as mere pre-catalysts yielding heterogeneous co-
[13]
balt oxide active for water oxidation. Therefore, care
should be paid to ascertain whether such heterogeneous
species are formed during the catalysis. Several evi-
dences were collected to prove the stability of 1. As
shown in Figure 1, there were no apparent changes in
the UV-Vis spectrum even after 11 h. In contrast, the
Table 1
tions of 1
2
O evolution, TON and TOF for different concentra-

1
c(1)/(µmol•L )
0
1.25 2.50 3.75 5.00 7.50
n(O
2
)/µmol
44.7 204.3 337.5 473.3 598.2 677.8
2+
UV-Vis spectrum of the solution containing free Co is
TON
—
—
127.7 117.1 114.3 110.7 84.4
3.8 1.8 1.3 1.1 0.9
[13]
totally different from its initial profile after 2 h. Be-
TOF/s
1
sides, in a sample that has been illuminated for ~200 s,
there is no evidence of any light scattering that would
arise from particle formation, suggesting a homogene-
ous system remained during the illumination. In contrast,
under the same condition, although Co(ClO ) displayed
Figure 2 and Table 1 shows the effect of varying the
concentration of 1 in 75 mmol•L1 borate buffer at the
optimal pH. The control experiment with a combination
4
2
of [Ru(bpy)
3
](ClO
4
)
2
and Na
2
S
2
O
8
in the absence of any
, and it is clear
much higher activity, white and floccus precipitate de-
posited from the solution, implying the creation of het-
erogeneity. This point is consistent with the analysis
1
produces only a small amount of O
2
that 1 is required for efficient water oxidation (Figure 2).
The maximum turnover frequency (TOF) and the turn-
over number (TON) for water oxidation depend on the
catalyst concentration. At the lowest catalyst concentra-
[
6a,6b,6h]
obtained from DLS (Figure S7),
and further sup-
ported by Tyndall scattering analysis (Figure S8). An-
other evidence is related to an attempted recycling ex-

1
2＋
tion, TOF of 3.8 s and TON of 127.7 respectively were
periment where fresh [Ru(bpy)3] and Na S O were
2
2
8
given by 1. Interestingly, varying the concentration of 1
added to the illumination experiment when oxygen
4
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Chin. J. Chem. 2016, XX, 1—6

Functional Polypyridine Co Complex as an Efficient Catalyst for Photo-Induced Water Oxidation
2
+
evolution had ceased (after ~200 s). Unlike that in the
presence of Co(ClO , where the O evolution was rei-
nitiated, only an activity close to the background one
in the system can be reduced to form Co when it acts
like an electron acceptor, no obvious promotion of oxy-
gen evolution was observed in the triad system, indicat-
4
)
2
2
was found in the case of 1, indicating the intact of 1 is
2
ing under limited conditions, the O evolution catalyzed
[6h]
required for O
2
evolution (Figure S9).
Attempt to
by 1 is dominant; (3) a light control experiment shows
that the catalytic water oxidation in the system is driven
by light. Evidently, water oxidation in the system is ini-
tiated by 1, which serves as the dominant catalyst as
well. However, in comparison, the initial rate and turn-
over numbers for oxygen evolution when
separate the residues after O
2
evolution ceased for MS
analysis was unsuccessful because there were not any
precipitates obtained even if the pH of the mixture had
been adjusted from 1－14. All together, the lack of par-
ticles formation implies 1 performs as a real catalyst for
water oxidation. Usually, it is considered that the ligand
plays a crucial role in determining the nature of the cat-
alytic system. Electron-withdrawing COOMe on TPA is
assumed to successfully resist self-oxidation, but also
inhibits the dimerization of two cobalt centers in aque-
ous solution, leading to the enhanced stability of 1 for
[Co(NH
cantly lower than that with Na , which might result
2
from the fact that [Co(NH Cl]Cl is usually active as
3 5 2
) Cl]Cl was chosen as an acceptor are signifi-
2
S
2
O
8
3
)
5
[14]
an electron acceptor in acidic or neutral conditions.
To acquire more insights into the photo-induced wa-
ter oxidation, a steady-state emission spectroscopy was
employed to shed light on electron transfer events be-
tween all the components. It has been found that the
[6h]
water oxidation.
[
Ru(bpy)
3
](ClO
with a rate constant k
L•s . However, a much smaller quenching rate con-
4 2
) in borate buffer solution is quenched
9
1
by Na
2
S
2
O
8
q
of 9.8×10 mol •

1 [3i]
8
1 1
stant (k
of compound 1 and [Ru(bpy)
Therefore, it is concluded that the photocatalytic process
q
) of 3.5×10 mol •s is shown in the presence
3
](ClO (Figure S10).
4 2
)
2
＋
is initiated by the reaction between [Ru(bpy)
Na . Then the generated Ru(III) species can oxidize
to produce highly active cobalt intermediate for the
3
]
and
2 2 8
S O
1
subsequent water oxidation. Research on the nature of
the intermediate is on-going. This proposal was further
evidenced by the water oxidation in a mixture of 1 and
[
3 4 3
Ru(bpy) ](ClO ) . Although the self-degradation of
Ru(III) led to water oxidation unavoidably, it is clear
1
that 44 µmol•L
2
O was caused by the Co catalyst in
comparison to the background (Figure S11).
Figure 3 Light-induced water oxidation in a mixture containing

1
1
[
Ru(bpy) ](ClO ) (0.4 mmol•L ), [Co(NH ) Cl]Cl (4 mmol•L )
3 4 2 3 5 2
Conclusions

1
1
and 1 (30 µmol•L ) in phosphate buffer (100 mmol•L , pH 7).
The Clark cell was kept constant at 20 ℃, and the system was
irradiated using LEDs [λ＝(470±10) nm, 820 µE/m •s]. The
In summary, a water-soluble cobalt complex was
synthesized and displayed water oxidation activity as a
molecular catalyst under a photo-irradiation condition.
It is found that the modification of ligand scaffold with
electron-withdrawing COOMe group fascinates to in-
crease catalytic response. Although it is unclear whether
the enhancement in catalytic efficiency results from the
steric substitution on ligand or the electron-withdrawing
effect, this study provides promising results for future
designing robust and stable catalyst, to be readily in-
corporated into an efficient artificial photosynthesis
system. Research on the nature of intermediate and
mechanism is on-going.
2
arrow indicates the start of the irradiation.
Unlike Na
2
S
2
O
8
, where the generated strong oxi-
is suspected of oxidizing water,
is regarded as a useful electron accep-
tor employed in the photo-induced water oxidation to
－�
dants
SO4
[
Co(NH ) Cl]Cl
3 5 2
[
14]
suppress confusion resulting from Na
ingly, the photo-induced water oxidation of 1 was car-
ried out assisted by [Co(NH Cl]Cl as an electron ac-
ceptor and [Ru(bpy) ](ClO as a photosensitizer in a
neutral phosphate buffer. Typically, 46 µmol•L
2
S
2
O
8
.
Accord-
3
)
5
2
3
4 2
)
1
O
2
were generated in the presence of 30 µmol•L 1 (Figure
). To prove that the oxygen evolution was indeed pro-
moted by 1, several control experiments were conducted:
1
3
Acknowledgement
(
[
1) removing any one of the three components:
Ru(bpy) ](ClO , [Co(NH Cl]Cl and 1, no oxygen
This work is supported by the National Natural Sci-
ence Foundation of China (21402113), the Fundamental
Research Funds for the Central Universities
(GK201503033). We are grateful to Prof. Jing Liu at
Shaanxi Normal University for providing DLS instru-
ment.
3
4
)
2
3
)
5
2
was generated in the presence of light, confirming that 1
is indeed involved in the catalytic processes; (2) alt-
hough the cobalt ion was reported to serve as a moder-
3 5 2
ate catalyst for water oxidation and the [Co(NH ) Cl]Cl
Chin. J. Chem. 2016, XX, 1—6
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
5

Wang et al.
COMMUNICATION
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6
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