A mononuclear cobalt complex with an organic ligand acting as a precatalyst for efficient visible light-driven water oxidation

Article abstract of DOI:10.1039/c3cc48059c

N,N′-Bis(salicylidene)ethylenediaminecobalt(ii) (1) has been investigated as a highly efficient water oxidation precatalyst with a TON of 854 at pH = 9.0, using [Ru(bpy)₃](ClO₄)₂ as a photosensitizer and Na₂S₂O₈ as a sacrificial electron acceptor. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc48059c

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A mononuclear cobalt complex with an organic
ligand acting as a precatalyst for efficient visible
light-driven water oxidation†
Cite this: Chem. Commun., 2014,
50, 2167
Received 20th October 2013,
Accepted 30th December 2013
Shao Fu,a Yongdong Liu,a Yong Ding,*ab Xiaoqiang Du,a Fangyuan Song,a Rui Xianga
and Baochun Ma*a
DOI: 10.1039/c3cc48059c
www.rsc.org/chemcomm
N,N0-Bis(salicylidene)ethylenediaminecobalt(II) (1) has been investigated the presence of N, N0-bis(salicylidene)ethylenediaminecobalt(II)
as a highly efficient water oxidation precatalyst with a TON of 854 at (salen Co(II)) (1).
pH = 9.0, using [Ru(bpy)3](ClO4)2 as a photosensitizer and Na2S2O8 as a
sacrificial electron acceptor.
Catalytic water splitting is of fundamental importance to natural
and artificial photosynthesis as well as photochemical energy
storage and fuel production. Photolysis of water is composed of
two half-reactions: the reduction of protons to H2 and the oxidation
of water to O2 (H2O - H2 + O2).1 In artificial photosynthesis, the
oxidation of water, viewed as the bottleneck of the whole process, is
recognized as the vital step for supplying electrons to the other half-
reaction.2 Therefore, developing an efficient water oxidation
catalyst (WOC) is much significant to obtain inexpensive and
renewable pollution-free energy sources. In the past few decades,
lots of efforts have been devoted to develop various metal com-
plexes3–7 as efficient WOCs. Recently, cobalt-based compounds
have attracted attention of many researchers, because cobalt is
much more abundant and cheaper than noble metals such as
ruthenium and iridium. The cobalt complexes with organic ligands
such as [CoII(Me6tren)(OH2)]2+ (2),8 [CoIII(Cp*)(bpy)(OH2)]2+ (3),8
[Co(qpy)(OH2)2]2+ (4),9 CoTPPS (5)10 and CoSIp (6)11 and without
organic ligands such as [Co4(H2O)2(a-PW9O34)2]10ꢀ (7)12 and
[CoIIICoII(H2O)W11O39]7ꢀ (8)13 have been reported as efficient water
oxidation catalysts or precatalysts.
Variables of the photocatalytic reaction including catalyst con-
centrations, buffer types, pHs and photosensitizers were systemi-
cally investigated to obtain the optimal conditions (Fig. 1 and
Fig. S8–S10, ESI†). Oxygen formed very quickly (Fig. 1) and O2
evolution achieved a plateau value in 6 min. The sacrificial electron
acceptor of Na2S2O8 was consumed up, so that the amount of O2
formed is limited to a certain value. A maximum O2 yield of 54.6%
and O2 evolution amount of 13.7 mmol were achieved when the
concentration of 1 was 1.6 mM in 80 mM borate buffer at a pH of
9.0. The activity of salen cobalt was compared with those of other
cobalt oxide/hydroxide (Table S6, ESI†) and Co (Table 1) complexes
under the same reaction conditions. The optimum TON of 1 of 854
is among the highest values reported for photocatalytic water
oxidation so far (Table 1 and Table S1, ESI†). The initial reaction
rate was very fast with an initial rate of O2 evolution of 6.10 mmol
The development of efficient WOCs remains a major scientific
challenge despite the above considerable progress, especially for
the high TON, TOF and high quantum yield. Herein, we present
an efficient photocatalytic water oxidation reaction system using
[Ru(bpy)3]2+ (bpy = 2,20-bipyridine) as a photosensitizer, Na2S2O8
as a sacrificial electron acceptor and borate as a buffer reagent in
a Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province, College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou 730000, China. E-mail: dingyong1@lzu.edu.cn, mabaochun@lzu.edu.cn
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
† Electronic supplementary information (ESI) available: Experimental details and
characterization of the spectrum for catalysis reaction. See DOI: 10.1039/c3cc48059c
Fig. 1 Kinetics of O2 formation in the photocatalytic system using
different concentrations of salen Co (0 mM, black; 1.2 mM, red; 1.6 mM,
blue; 2.0 mM, green; 2.4 mM, pink). Conditions: LED lamp (Z420 nm),
15.8 mW; 1.0 mM [Ru(bpy)3](ClO4)2, 5.0 mM Na2S2O8, 80 mM sodium
borate buffer (initial pH 9.0); total reaction volume 10 mL and overall
volume B23 mL; vigorous agitation using a magnetic stirrer.
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Table 1 Photocatalytic water oxidation catalyzed by different compounds
Because experiments were carried out at room temperature,
thermal stability is not critical in part for our experiments. In order
to know whether 1 is hydrolytically stable, three experiments were
carried out. Firstly, a UV-vis study of 1 shows that the absorbance
curve of catalyst 1 in borate buffer (80 mM, pH = 9.0) almost
containing cobalta
Complex
concentration
(mM)
O2 yieldc
TONb (%)
Ref.
Complex
Salen Co(II) (1)
[CoII(Me6tren)(OH2)]2+ (2)
CoTCPP (5)
1.6
1.6
1.6
1.6
854 54.6
394 25.2
191 12.2
531 34.0
487 31.1
441 28.0
762 48.8
35 70.9
29 29.0
This work completely overlaps between 0 minute and 60 minutes, indicating
This work
that the borate buffer does not affect the stability of 1 during the
catalytic reaction (Fig. S11, ESI†). Secondly, kinetics of oxygen
This work
This work
CoSIp (6)
[Co4(H2O)2(PW9O34)2]107ꢀꢀ (7) 1.6
This work formation contrast tests (Fig. S12, ESI†) were conducted. Complex
[CoIIICoII(H2O)W11O39
Co(NO3)2
]
(8) 1.6
1.6
This work
This work
This work
1 aged for 1 h gave about 13.6 mmol O2, which is the same as that
of the fresh 1 (13.7 mmol O2), also suggesting that complex 1 is
Salen Co(II) (1)
50.0
[CoIII(Cp*)(bpy)(OH2)]2+ (3) 50.0
8
stable in buffer solution. Thirdly, no nanoparticles were detected
after compound 1 was aged for 1 hour in the buffer by dynamic
light scattering (DLS) measurements, which meant that cobalt
hydroxide/oxide nanoparticles did not form during the aging
period. These three experiments proved that complex 1 is hydroly-
tically stable and does not decompose in borate buffer.
a
Conditions: LED lamp (Z420 nm), 1.0 mM [Ru(bpy)3](ClO4)2, 5.0 mM
Na2S2O8, 80 mM sodium borate buffer (initial pH 9.0), total reaction
volume 10 mL and overall volume B23 mL, vigorous agitation using a
magnetic stirrer. In calculating these reported values, we subtracted the
maximal contribution of O2 generated from noncatalytic pathways
b
(obtained from the control experiment without catalyst). TON is defined
as the total number of moles of O2 per mole of precatalyst. Yield is
defined as twice the number of moles of O2 per mole of Na2S2O8.
c
In order to make clear the oxidative stability of this cobalt
complex, DLS measurements were conducted to detect the reaction
solution after illumination. It was shown that different sizes of
particles were found, ranging from several nm to several thousand
nm (Fig. S15, ESI†). Thus, formation of particles during the reaction
was confirmed.
The complete oxidation of ligand bis(salicylidene)ethylene-
diamine should produce water, CO2 and NOx. Among these
products, CO2 can be detected and quantified by gas chromato-
graphy (see Table S7, ESI†).
minꢀ1 (TOFinitial = TON1min/60 s = 6.4 sꢀ1), which is well compar-
able to the fastest rates of water oxidation systems including
thermal and light reactions.14–16 The quantum yield of FQY is ca.
38.6%, which is the third highest value (Table S1, ESI†) reported for
photocatalytic water oxidation among all the documented com-
pounds containing cobalt so far.17
The thermal (dark) water oxidation can also be catalyzed by 1
when using [Ru(bpy)3]3+ as the oxidant. An 83% oxygen yield, a TON
of 194 and a TOFinitial of 2.0 sꢀ1 were obtained after 3 minute
reaction, respectively. The O2 yield and TON are the highest among
all dark reactions reported using [Ru(bpy)3]3+ as an oxidant so far
(Table S2, ESI†). This result reveals that [Ru(bpy)3]3+ can oxidize 1.
This conclusion was also supported by cyclic voltammetry (CV)
measurement of 1 and [Ru(bpy)3]2+. The onset of the catalytic wave
due to water oxidation is observed at ca. 0.70 V (Fig. 2), and the redox
potential of Ru(bpy)32+/3+ in our system is 1.16 V. So, [Ru(bpy)3]3+ is
thermodynamically capable of promoting water oxidation.
To investigate the nature of the catalytic species in photo-
chemical water oxidation by the salen cobalt complex, the follow-
ing experiments were performed. First, a solution containing
0.2 mM 1, 2.4 mM of [Ru(bpy)3](ClO4)2 and 5.0 mM of Na2S2O8
in a borate buffer (80 mM) at pH 9.0 was irradiated under visible
light for 6 min at room temperature. Some solid precipitates were
obtained after reaction and then were washed completely using
CH2Cl2. The solution of dichloromethane was then analyzed by
electrospray ionization mass spectrometry (ESI/MS). The species
observed in the mass spectrum were [Ru(bpy)3]2+ (m/z: 285.2),
[Ru(bpy)3] (ClO4)+ (m/z: 669.1) and [Ru(bpy)3](ClO4)(ClO2)+ (m/z:
737.2) (Fig. S18, ESI†). No 1, free salen ligand and other cobalt
species (such as simple Co(II) aqua ions) were found at m/z between
50 and 1000. Second, the above precipitates formed after illumina-
tion were separated from the reaction solution by centrifugation and
washed by pure water repeatedly. The obtained solid was then dried
up overnight at 50 1C. CDCl3 was used to extract the above
Recently, differentiating homogeneous and heterogeneous
water oxidation catalysis has become a very critical issue for
researchers using homogeneous WOCs initially. There are three
kinds of stability with respect to water oxidation catalysts, i.e.
oxidative, hydrolytic and thermal.18
1
precipitates and was then analysed by H NMR. No salen ligand
1
and other organic species were observed in the H NMR spectrum
except for the solvent signals (Fig. S19, ESI†). These experimental
results revealed that no 1 existed in the precipitates, revealing that 1
decomposed completely under the photocatalytic reaction conditions.
The precipitates derived from 1 using the above method were
then analyzed by scanning electron microscopy (SEM) and
different sizes of particles were observed (Fig. S16, ESI†). Aggre-
gation of particles with the porosity on the surface can be seen
clearly. The size of the particles is slightly different accompanied
by different degrees of aggregation, which form secondary parti-
cles of sizes from 100 to 200 nm as shown in the SEM image.
Fig. 2 Cyclic voltammograms (CV) of 80 mM sodium borate buffer
solution at pH 9.0 with 1 mM [Ru(bpy)3](ClO4)2 (red line) and 1 mM of 1
(blue line). The black line displays the CV of 80 mM sodium borate buffer
solution at 1.0 M KCl (pH 9.0), Ag/AgCl electrode as reference.
2168 | Chem. Commun., 2014, 50, 2167--2169
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Scheme 1
and the organic ligand of complex 1 is subjected to deep oxidization.
Some active intermediates generated from the decomposition of
salen Co(II) in situ should be the true catalytic active species, which
are responsible for the oxygen evolution. Further research is under-
way in order to make clear what species and the quantity content are.
In conclusion, an efficient water oxidation system was devel-
oped using a precatalyst N, N0-bis(salicylidene)ethylenediamine-
cobalt(II). A TON of 854 and a TOF of 6.4 sꢀ1 were achieved, which
are among the highest values reported for photocatalytic water
oxidation so far.
Fig. 3 X-Ray photoelectron spectra of Co3O4 and precipitates derived
from 1 in the energy regions of Co 2p and O 1s. The binding energy of each
element was corrected by the C 1s peak (284.8 eV).
The X-ray photoelectron spectra (XPS) measurement was carried
out for the above particles, being washed several times by purified
water. Fig. 3 shows the XPS spectrum of Co 2p of the particles, with
2p3/2 and 2p1/2 appearing at 781.0 eV and 796.0 eV, respectively. No
satellite peaks of Co 2p were observed in the XPS spectrum of the
precipitate, indicating that no Co(II) species existed on the surface
of the precipitate.
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 21173105 and 21172098)
and the Fundamental Research Funds for the Central Universities
(lzujbky-2013-50, lzujbky-2013-ct 02).
Compared to Fukuzumi’s XPS binding energy values8 of particles
derived from [CoII(Me6tren)(OH2)]2+ (2), our compounds decom-
posed from 1 are different to them. Similarly, authentic Co3O4
shows two intense peaks at 779.8 eV for Co 2p3/2 and at 794.8 eV for
Co 2p1/2 with two satellite peaks at 790.1 eV and 805.5 eV,
respectively. Contrasted with the O1s peak of Co3O4 (530.1 eV), a
higher binding energy of 531.5 eV for the precipitate was observed,
revealing that metal hydroxide species existed in the solid.8 Based
on the ESI/MS, 1H NMR and XPS analysis, the precipitates derived
from 1 should be Co(III) inorganic species, which would be a mixture
of Co(III) containing oxide and/or Co(III) hydroxide.
The recycling of the true catalytic active species was evaluated
(Table S8, ESI†). The catalytic activity (40.9% O2 yield) of the
isolated precipitate derived from salen Co towards visible-light-
driven water oxidation was found to be considerably lower than
that of fresh salen Co (54.6% O2 yield). The sizes of most particles
in situ formed during the photocatalytic process (Fig. S15, ESI†) are
less than 100 nm. However, the sizes of isolated precipitates
derived from salen Co (Fig. S16, ESI†) are from 100 to 200 nm.
The number of active cobalt oxo sites on the surface of the active
intermediate available for O2 formation is expected to be more for
these small particles in situ formed. So, the catalytic activity of the
fresh catalyst of salen Co was better than the recovered one.
A proposed mechanism is shown in Scheme 1. The reaction
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2ꢀ
were quenched by two S2O8
through visible-light-accessible
metal-to-ligand charge-transfer, resulting in the generation of two
2ꢀ
ꢀ
[RuIII(bpy)3]3+ complexes and two SO4 and two SO4 radical
anions. The latter which is also a strong oxidant oxidizes two
another [RuII(bpy)3]2+ to give two more [RuIII(bpy)3]3+ accompanying
ꢁ
ꢀ
SO4 radical anions converting into SO42ꢀ. The light induced
ꢁ
products ꢀof [Ru(bpy)3]3+ (E[Ru(bpy)32+/3+] in our system is 1.16 V)
and SO4 [E1(SO4 ꢀ/SO42ꢀ) E 2.4 V]18 are both strong oxidants,
ꢁ
ꢁ
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Chem. Commun., 2014, 50, 2167--2169 | 2169