A Water-Soluble Copper-Polypyridine Complex as a Homogeneous Catalyst for both Photo-Induced and Electrocatalytic O2 Evolution

Article abstract of DOI:10.1002/chem.201504066

The water-soluble polypyridine copper complex [Cu(F₃TPA)(ClO₄)₂] [1; F₃TPA=tris(2-fluoro-6-pyridylmethyl)amine] catalyzes water oxidation in a pH 8.5 borate buffer at a relatively low overpotential of 610 mV. Assisted by photosensitizer and an electron acceptor, 1 also exhibits activity as a homogeneous catalyst for photo-induced O₂ evolution with a maximum turnover frequency (TOF) of (1.58±0.03)×10^-1 s^-1 and a maximum turnover number (TON) of 11.61±0.23. In comparison, the reference [Cu(TPA)(ClO₄)₂] [TPA=tris(2-pyridylmethyl)amine] displayed almost no activity under either set of conditions, implying the crucial role of the ligand in determining the behavior of the catalyst. Experimental evidence indicate the molecular catalytic nature of 1, leading to a potentially practical strategy to apply the copper complex in a photoelectrochemical device for water oxidation.

Full text of DOI:10.1002/chem.201504066

DOI: 10.1002/chem.201504066
Communication
&
Water Oxidation |Hot Paper|
A Water-Soluble Copper–Polypyridine Complex as
a Homogeneous Catalyst for both Photo-Induced and
Electrocatalytic O2 Evolution
Rui-Juan Xiang,[a, b] Hong-Yan Wang,*[a, b] Zhi-Juan Xin,[a, b] Cheng-Bo Li,[c] Ya-Xing Lu,[b]
Xue-Wang Gao,[d] Hua-Ming Sun,[b] and Rui Cao*[b, e]
tably, with a wide choice of redox-active metal centers and set
of ligands, earth-abundant cobalt, nickel, and iron complexes
are interesting candidates, the oxygen evolving activity of
which has prompted an opportunity to broaden the range of
available catalysts through fundamental coordination chemis-
try.[2] Inspired by the extensive biological and biomimetic activ-
ities of copper enzymes and complexes in nature,[3] fundamen-
tal progress has been made in the development of copper-
based models, some of which undergo facile and reversible
OÀO bond formation and cleavage.[4] These efforts have led to
an increasing focus on the application of copper complexes in
water oxidation. To date, only a few copper species have been
reported as molecular catalysts to enable electrocatalytic water
oxidation,[5] therefore practical strategies are required for
a better design of robust, efficient and stable copper catalysts.
Artificial photosynthesis by catalytic oxidation of water is
among the most-investigated technologies for solar-to-chemi-
cal energy conversion.[6,7] Molecular catalysts increase the prac-
ticality of this process, because they facilitate the integration
of chromophores or chromophore antennae. Moreover, their
derivatives can be further functionalized assembly into photo-
electrochemical devices. Owing to their relatively high stability
and low light absorption, functionalization of copper com-
plexes by well-defined coordination chemistry is a very appeal-
ing and promising strategy to develop catalysts for photo-in-
duced water oxidation. Herein, we report a water-soluble
copper polypyridine complex 1 (Figure 1), which functions as
a molecular catalyst for water oxidation under both visible-
light irradiation and electrochemical conditions. To our knowl-
edge, it is the first such photochemical investigation of
a copper catalyst for O2 evolution and this result may allow
a derivative to be incorporated into molecular assemblies for
photoelectrochemical water oxidation.
Abstract: The water-soluble polypyridine copper complex
[Cu(F3TPA)(ClO4)2] [1; F3TPA=tris(2-fluoro-6-pyridylmethyl)-
amine] catalyzes water oxidation in a pH 8.5 borate buffer
at a relatively low overpotential of 610 mV. Assisted by
photosensitizer and an electron acceptor, 1 also exhibits
activity as a homogeneous catalyst for photo-induced O2
evolution with a maximum turnover frequency (TOF) of
(1.58Æ0.03)”10À1 sÀ1 and a maximum turnover number
(TON) of 11.61Æ0.23. In comparison, the reference [Cu(T-
PA)(ClO4)2] [TPA=tris(2-pyridylmethyl)amine] displayed
almost no activity under either set of conditions, implying
the crucial role of the ligand in determining the behavior
of the catalyst. Experimental evidence indicate the molec-
ular catalytic nature of 1, leading to a potentially practical
strategy to apply the copper complex in a photoelectro-
chemical device for water oxidation.
Water oxidation has become one of the most attractive targets
for the conversion of solar energy or electricity into chemical
fuels.[1] In comparison with heterogeneous systems, molecular
catalysts are advantageous, since their redox and kinetic prop-
erties are, in principle, easily tunable by molecular design. No-
[a] R.-J. Xiang, Prof. Dr. H.-Y. Wang, Z.-J. Xin
Key Laboratory of Macromolecular Science of Shaanxi Province
School of Chemistry and Chemical Engineering
Shaanxi Normal University, Xi’an, 710119 (P. R. China)
[b] R.-J. Xiang, Prof. Dr. H.-Y. Wang, Z.-J. Xin, Y.-X. Lu, Dr. H.-M. Sun,
Prof. Dr. R. Cao
School of Chemistry and Chemical Engineering
Shaanxi Normal University, Xi’an, 710119 (P. R. China)
E-mail: hongyan-wang@snnu.edu.cn
ruicao@ruc.edu.cn
The mononuclear copper polypyridine compound 1 was ob-
tained as a light blue powder through direct coordination of
[c] Dr. C.-B. Li
a
a-fluorinated tris(2-pyridylmethyl)amine[8] (F3TPA) with
College of Chemistry & Materials Science
Northwest University, Xi’an, 710127 (P. R. China)
Cu(ClO4)2·6H2O in acetonitrile. Complex 1 exhibits good solu-
bility in both acetonitrile and water. The UV/Vis absorption
spectrum in basic borate buffer (0.1m, pH 9) displays a band at
lꢀ260 nm and a shoulder at lꢀ340 nm. Both absorption in-
tensities are linearly dependent on the catalyst concentration
(see the Supporting Information, Figure S1). The UV/Vis spectra
of 1 in basic borate buffer when measured after 0, 4, 6, 8, and
18 h (see the Supporting Information, Figure S2) display identi-
cal absorptions for each measurement, supporting the notion
[d] Dr. X.-W. Gao
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences, Beijing, 100190 (P. R. China)
[e] Prof. Dr. R. Cao
Department of Chemistry, Renmin University of China
Beijing, 100872 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504066.
Chem. Eur. J. 2016, 22, 1602 – 1607
1602
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
that the monomeric structure of 1 is stable under these condi-
tion. Complex 1 displays absorptions almost above l=400 nm,
implying that 1 does not compete for visible-light absorption
in the system containing chromophore. Vapor diffusion of Et2O
into an acetonitrile solution of 1 yielded light blue crystals suit-
able for X-ray diffraction (Figure 1). The Cu center is coordinat-
Figure 2. Light-induced water oxidation in a 1 mL reaction containing
1 (0 mm, 15 mm, 30 mm, 45 mm, 60 mm and 90 mm), [Ru(bpy)3](ClO4)2 (0.4 mm),
and Na2S2O8 (5 mm) in borate buffer (75 mm, pH 8.5). The temperature of
the Clark cell was kept constant at 208C, and the system was irradiated by
using LEDs [l=(470Æ10) nm, 820 mEcmÀ2 sÀ1]. Inset: plot of oxygen genera-
tion initial velocities (v) vs. [1]. The rate v, is calculated in the linear part
during the first 30 s of illumination and corrected for v when [1]=0.
Figure 1. The crystal structure of 1 with thermal ellipsoids set at 50% proba-
bility level. Solvent molecules and hydrogen atoms are omitted for clarity.
Table 1. TON and TOF (corrected for background oxygen solution activi-
ty) for the concentrations of 1 (without catalyst, [O2]=82Æ2 mm).
Concentration of
[O2] generated by
TOF [sÀ1
]
TON
ed by a F3TPA ligand through four N atoms, an acetonitrile sol-
1 [mm]
1 [mm]
À
vent molecule, and a ClO4 ion, to give a distorted tetragonal
0
15
30
45
60
90
–
–
–
137Æ7
348Æ7
397Æ9
489Æ6
515Æ2
(8.01Æ0.80)”10À2 9.06Æ0.51
(1.58Æ0.03)”10À1 11.61Æ0.23
(1.27Æ0.00)”10À1 8.94Æ0.09
(1.18Æ0.02)”10À1 8.15Æ0.11
(8.34Æ0.11)”10À2 5.72Æ0.02
bipyramidal coordination geometry. The tertiary amine N atom
and the acetonitrile N atom are located at the axial positions,
whereas the three pyridine N atoms and the perchlorate O
atom are in the equatorial plane.
In the presence of photosensitizer [Ru(bpy)3](ClO4)2 and elec-
tron acceptor Na2S2O8 with excitation at l=470Æ10 nm,
photo-induced water oxidation of compound 1 was monitored
by a Clark electrode. A pH 8.5 borate buffer was found to be
optimal for the process (see the Supporting Information
Table S1). A lower pH rendered photocatalysis much less effi-
cient, whereas a higher pH gave rise to increased initial
oxygen evolution rates and overall higher TONs but faster de-
activation, demonstrating the decomposition of 1. The concen-
tration of buffer solution also influenced the activity, implying
that O2 formation preceded proton transfer with the basic
buffer serving as the proton acceptor (Table S1).[5d] Figure 2
shows photo-induced O2 evolution versus time at various con-
centrations of 1 under the optimized conditions, where the
arrow indicates the start of the irradiation. Due to accumula-
tion of a steady-state concentration of the photogenerated in-
termediates, an initial lag time (ca. 9 s) was observed in the ki-
netic traces. The total amount of oxygen and the maximum
turnover rate depended on the catalyst concentration. In the
absence of 1, a combination of [Ru(bpy)3](ClO4)2 and Na2S2O8
produced only a small amount of O2; it is thus clear that 1 is
required for efficient water oxidation. To make clear the contri-
butions of the Cu catalyst to O2 evolution, the TON and TOF
were calculated after correction for background in the absence
of 1 (Table 1). Typically, the best condition, with TOF of (1.58Æ
0.03)”10À1 sÀ1 and TON of 11.61Æ0.23, was given by 1 at
30 mm. The initial velocities (v) of O2 generation under the
chosen conditions were then plotted versus [1] (Figure 2,
inset), giving a linear fit with a rate constant k=(6.42Æ0.30)”
10À2 sÀ1 at 208C. The first-order behavior with regard to [1] in-
dicates a possible pathway for the OÀO bond formation involv-
ing a highly oxidized Cu intermediate, which undergoes a rate-
determining nucleophilic attack by a water molecular.[2f,9] To
test the influence of the fluoro substituents in the ligand, the
reference complex [Cu(TPA)(ClO4)2] [2; TPA =tris(2-pyridylme-
thyl)amine] was synthesized. However, complex 2 proved
almost inactive under the same conditions (see the Supporting
Information, Figure S3).
To test whether the catalytic system was truly homogeneous
in nature, the molecular integrity of 1 during photocatalysis
was scrutinized by dynamic light scattering (DLS) experi-
ments.[2d,10] In a sample that had been illuminated for approxi-
mately 150 seconds, there was no evidence of any light scat-
Chem. Eur. J. 2016, 22, 1602 – 1607
www.chemeurj.org
1603
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
tering that would have arisen from particle formation, suggest-
ing that the system remained homogeneous during the illumi-
nation (see the Supporting Information, Figure S4). This point
was further supported by Tyndall scattering analysis of the
system containing 1 after photolysis (see the Supporting Infor-
mation, Figure S5). Moreover, the solution was still transparent
even after 2 days. In contrast, under the same conditions, al-
though Cu(ClO4)2 also displayed activity, a white, cloudy precip-
itate was deposited from the solution, implying heterogeneity.
Quite a similar occurrence was observed with Co(ClO4)2 as an
alternative catalyst, consistent with the analysis obtained from
DLS.[11] A further difference of 1 from Cu(ClO4)2 is related to an
attempted recycling experiment where fresh [Ru(bpy)3]2+ and
Na2S2O8 were added to the illumination experiment when
oxygen evolution had ceased (after ca. 150 s). For Cu(ClO4)2,
due to the formation of copper oxide in the first run, the activ-
ity could be reinitiated with fresh [Ru(bpy)3](ClO4)2 and
Na2S2O8. However, only near-background activity was found in
the case of 1, indicating that intactness of 1 is required for O2
evolution (see the Supporting Information, Figure S6).[12] To-
gether with differences in particle formation and deactivation
behavior in the comparison of 1 with Cu(ClO4)2, these observa-
tions imply that 1 performs as a real catalyst for water oxida-
tion. Interestingly, no CO2 was detected in the system contain-
ing 1 after photolysis, and the decomposition products of
1 could be precipitated by adding concentrated NaOH to in-
crease the pH to 11. These products were then analyzed by
using ESI-MS (see the Supporting Information, Figure S7). A
peak was detected at m/z=263, possibly attributable to the
product with one pyridyl in F3TPA detached from the copper
center. It is assumed that axial open sites such as this facilitate
metal oxidation and proton transfer during catalysis. Further
signals appear at m/z=425 and 223, corresponding to
[Cu(F3TPA)(OH)]+. Remarkably, the products remain bound to
the Cu center, consistent with the lack of particle formation
observed in this system.
dation-state intermediates for subsequent water oxidation.
This process is further supported by the observation of water
oxidation in a mixture of 1 and [Ru(bpy)3](ClO4)3. As shown in
Figure 3, in comparison to the background without 1, the Cu
Figure 3. Water oxidation in a 1 mL reaction without or with 1 (15 mm) in
the presence of [Ru(bpy)3](ClO4)3 (0.4 mm) in borate buffer (75 mm, pH 8.5).
The temperature of the Clark cell was kept constant at 208C.
catalyst brought about an O2 concentration of 92Æ4 mm. Inter-
estingly, a shorter lag time (<2 s) and a faster O2 evolution
compared with the photolysis experiment occurred with an ini-
tial TOF of (3.74Æ0.15)”10À1 sÀ1 and a TON of 6.12Æ0.25 with
addition of 15 mm 1, indicating that direct oxidation of 1 by
[Ru(bpy)3](ClO4)3 led to an accelerated electron transfer (the
TON and TOF were calculated after correction for background).
Notably, this efficiency is also higher than that reported for
[(bpy)CuCl2] or [(bpy)Cu(OH)2] with [Ru(bpy)3]3+ as the
oxidant.[5a,13]
Photo-induced water oxidation is based on electron transfer
between all components. Therefore, steady-state emission
spectroscopy was employed to acquire more insights into this
process. On increasing the concentration of Na2S2O8, the
steady-state emission of excited [Ru(bpy)3](ClO4)2 in borate
buffer solution was quenched with rate constant kq =9.8”
109 mÀ1 sÀ1 (see the Supporting Information, Figure S8). Mean-
while, during irradiation of [Ru(bpy)3](ClO4)2 with the electron
acceptor Na2S2O8, the absorption spectra show a decrease in
the RuII MLCT band together with an increased absorption be-
tween 550 nm and 700 nm centered at 657 nm, reflecting the
build-up of RuIII species by electron transfer from excited
For a photoelectrochemical device, it is necessary to evalu-
ate the electrocatalytic water oxidation activity of the catalyst.
The electrochemical properties of 1 were therefore investigat-
ed and its capability for O2 production was determined by con-
trolled potential electrolysis (CPE) as follows. Using 1 cm2 ITO
(indium tin oxide) electrode as the working electrode, cyclic
voltammetry of 1 was carried out in a 0.1m pH 8.5 borate
buffer under argon (Figure 4). The presence of 1 induced
a large and irreversible oxidation wave at around 1.2 V vs.
normal hydrogen electrode (NHE) and the current density
reached up to 3 mAcmÀ2, a great enhancement relative to the
background. This pronounced electrochemical response can
be explained by catalysis of water oxidation, demonstrating
that 1 can perform as an electrocatalyst for O2 evolution. In
comparison, the much lower catalytic peak in the presence of
2 raised at 1.4 V. Besides, under the same condition, RuIII/RuII
oxidation couple occurred at 1.31 V, positive shift with the po-
tential for starting electrochemical water oxidation observed in
the appearance of 1 (see the Supporting Information, Fig-
ure S11). Therefore, the RuIII species generated by electron
2À
[Ru(bpy)3]2+ to S2O8 (see the Supporting Information, Fig-
ure S9).[2 g,11a] Similarly, on addition of 1, the steady-state emis-
sion of excited [Ru(bpy)3](ClO4)2 was quenched, with an in-
crease in the concentration of Cu (see the Supporting Informa-
tion, Figure S10). A smaller rate constant of kq =9.2”108 mÀ1 sÀ1
was calculated from a Stern–Volmer plot (Figure S8). Therefore,
we conclude that photocatalytic reaction is triggered by oxida-
tive quenching of [Ru(bpy)3]2+ on addition of Na2S2O8. The re-
sultant RuIII species then oxidize the CuII complex to higher oxi-
Chem. Eur. J. 2016, 22, 1602 – 1607
www.chemeurj.org
1604
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
n is the scan rate, R is the gas constant, and T is the absolute
temperature. Accordingly, Dcat can be determined from the
slope of the line to be 1.74”10À5 cm2 sÀ1
.
nFvDcat
RT
1=2
ð2Þ
iel ¼ 0:4633nFA½CŠð
Þ
The ratio of Equations (2) and (1), calculated with all of the
constants, gives the simplified Equation (3) as follows:[2c,5a,f,g]
rffiffiffiffiffiffi
icat
iel
kcat
v
ð3Þ
¼ 1:38
kcat established in this equation is usually considered as the
apparent rate constant for a first-order or pseudo-first order O2
evolution reaction. In electrochemistry, the TOF of a catalyst is
usually referred to as the catalytic water oxidation rate con-
stant kcat. The electrocatalytic activity of 1 was estimated by
plotting the catalytic current over the diffusive peak current
icat/id as a function of the reciprocal of the square root of scan
rate. With scan rates decreasing (see the Supporting Informa-
tion, Figure S15), icat/id matches the trend from a pure diffusion
behavior region to a pure kinetic behavior region.[5g,16a] Accord-
ing to Equation (3), a TOF value of 0.38 sÀ1 can be calculated
from the slope of the fitting line in Figure S13 (see the Sup-
porting Information). Since the 4-electron water oxidation reac-
tion is a complicated process, the calculated values can only
be regarded as an estimate of the catalytic rate.
Figure 4. CVs of 1 mm 1, 2 or Cu(ClO4)2 in a 0.1m borate buffer at pH 8.5
with ITO (1 cm2) as the working electrode at 50 mVsÀ1. Inset: CV of 1 mm
1 at À0.4 V to 0.6 V.
transfer from photo-excited [Ru(bpy)3]2+ to the electron ac-
ceptor is thermodynamically capable of promoting water oxi-
dation with addition of 1, but inhibited in the presence of 2.
This observation is consistent with the above experiments.
As shown in Figure 4, in pH 8.5 borate buffer, the onset of
the catalytic wave with addition of 1 is around 1.34 V, which
gives an overpotential of 610 mV,[14] lower than that of previ-
ously reported copper complexes.[5a,h] Although higher pH
prompts the stronger catalytic current, accompanied by the
decreasing overpotential based on the measurement at
pH 7.1–9.0 (see the Supporting Information, Figure S12), pH 8.5
was chosen in order to retain conditions close to those of the
photo-induced experiment.
CPE experiments to evaluate the capability for water oxida-
tion of 1 were conducted at 1.80 V on a 1.0 cm2 surface-area
ITO working electrode in a closed and deoxygenated electroly-
sis cell. In the presence of 1, the catalytic charge was dramati-
cally enhanced (Figure 5a). The background for oxygen forma-
tion at the applied potential in the absence of catalyst was
negligible. During CPE, bubble evolution was clearly observed
on the electrode surface (Figure 5b). After 2 h, the reaction
headspace was analyzed by gas chromatography, and the
result revealed that 41 mmol of O2 had been produced with 8.2
catalytic turnovers based on the initial amount of 1 with a Fara-
From cyclic voltammetry measurements with the concentra-
tion of 1 ranging from 0 to 1 mm, the catalytic peak current
(icat) varied linearly with [1] (see the Supporting Information,
Figure S13). This first-order dependence is in line with a single-
site mechanism for water oxidation catalysis according to
Equation (1), where ncat =4, representing the number of elec-
trons transferred during water oxidation, F is Faraday’s con-
stant, A is the area of the electrode (in cm2), [C] is the catalyst
concentration (in molLÀ1), kcat is the catalytic rate constant,
and Dcat is the diffusion coefficient of the catalyst (in
cm2 sÀ1).[2c,5]
icat ¼ ncatFA½CŠDc1a=t2kc1a=t2
ð1Þ
Besides of catalytic peak of 1 in CV, a weak anodic wave at
0.27 V was induced with a cathodic wave at À0.11 V in the re-
verse scan (DE=380 mV at 50 mVsÀ1 scan rate; Figure 4, inset).
This process was assigned to the CuII/I couple with
E
1/2 =0.08 V.[5d,h,15] The cathodic current density (id) varied line-
arly with the square root of the scan rate (see the Supporting
Information, Figure S14). This result is consistent with the Ran-
dles–Sevcik relation given by Equation (2),[5,16] where n=1, rep-
resenting the electron transferred in the noncatalytic reaction,
Figure 5. a) CPE with and without 1 mm 1 in a 0.1m borate buffer (pH 8.5)
at 1.8 V versus NHE; b) the bubbles on the electrode surface during CPE.
Chem. Eur. J. 2016, 22, 1602 – 1607
www.chemeurj.org
1605
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
daic efficiency of approximately 93% (see the Supporting Infor-
mation, Figure S16). The calculated Faradaic efficiency is some-
what lower, as some of the O2 produced was solubilized in the
electrolyte and not detected by headspace sampling.[5c,d,f] Fur-
thermore, the pH decreased by 0.3 after CPE, in accordance
with consumption of OHÀ by water oxidation.[5f]
water oxidation, research into which is currently ongoing in
our laboratory.
Acknowledgements
Several different techniques were used to establish whether
any heterogeneous materials were produced during the reac-
tion. Figure 5a shows the total charge passed during 2 h elec-
trolysis, and the absence of an induction period at early elec-
trolysis times implies homogeneous water oxidation cataly-
sis.[5d] In addition, UV/Vis spectra of the bulk solution contain-
ing 1 were measured before and after electrolysis, showing
nearly identical absorption (see the Supporting Information,
Figure S17). Moreover, SEM (scanning electron microscopy)
was used to examine the morphology of the working electrode
after electrolysis, giving very similar results to the bare ITO sur-
face (see the Supporting Information, Figure S18a,b). Energy
dispersive X-ray spectroscopy (EDS) also revealed that the elec-
trode exhibited the same elemental composition as the bare
ITO electrode after catalysis, implying that the decomposition
of 1 could not have led to deposition of heterogeneous parti-
cles on the electrode (Figure S18c,d). After catalysis, the elec-
trolyte solution was also examined by DLS, and no obvious sig-
nals were ascribed to particles, confirming no heterogeneity
within the electrolyte mixture (see the Supporting Information,
Figure S19). This was further supported by Tyndall scattering
analysis (see the Supporting Information, Figure S20). The over-
all lack of evidence for particle formation, either adsorbed on
the working electrode or present in the electrolyte solution, is
collectively consistent with homogeneous catalysis.
This work was supported by the National Natural Science
Foundation of China (21402113), the Fundamental Research
Funds for the Central Universities (6K201503033), and the
“Thousand Talents Program” in China. We are grateful to Prof.
Jing Liu at Shaanxi Normal University for providing access to
the DLS instrument.
Keywords: copper · electrocatalysis · homogeneous catalysis ·
photochemistry · water oxidation
[1] a) J. J. Concepcion, J. W. Jurss, M. K. Brennaman, P. G. Hoertz, A. O. T. Pa-
trocinio, N. Y. Murakami Iha, J. L. Templeton, T. J. Meyer, Acc. Chem. Res.
2009, 42, 1954–1965; b) H. Dau, I. Zaharieva, Acc. Chem. Res. 2009, 42,
1861–1870; c) R. Eisenberg, Science 2009, 324, 44–45; d) K. N. Ferreira,
T. M. Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science 2004, 303, 1831–
1838; e) S. W. Kohl, L. Weiner, L. Schwartsburd, L. Konstantinovski,
L. J. W. Shimon, Y. Ben-David, M. A. Iron, D. Milstein, Science 2009, 324,
74–77; f) B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka, Nature 2005,
438, 1040–1044; g) D. G. Nocera, Acc. Chem. Res. 2012, 45, 767–776;
h) R. D. L. Smith, M. S. Prevot, R. D. Fagan, Z. Zhang, P. A. Sedach, M. K. J.
Siu, S. Trudel, C. P. Berlinguette, Science 2013, 340, 60–63; i) Y. Wu, M.
Chen, Y. Han, H. Luo, X. Su, M.-T. Zhang, X. Lin, J. Sun, L. Wang, L. Deng,
W. Zhang, R. Cao, Angew. Chem. Int. Ed. 2015, 54, 4870–4875; Angew.
Chem. 2015, 127, 4952–4957.
[2] a) D. K. Dogutan, R. McGuire, Jr., D. G. Nocera, J. Am. Chem. Soc. 2011,
133, 9178–9180; b) J. L. Fillol, Z. Codola, I. Garcia-Bosch, L. Gomez, J. Jo-
se Pla, M. Costas, Nat. Chem. 2011, 3, 807–813; c) Y. Han, Y. Wu, W. Lai,
R. Cao, Inorg. Chem. 2015, 54, 5604–5613; d) C.-F. Leung, S.-M. Ng, C.-C.
Ko, W.-L. Man, J. Wu, L. Chen, T.-C. Lau, Energy Environ. Sci. 2012, 5,
7903–7907; e) T. Nakazono, A. R. Parent, K. Sakai, Chem. Commun. 2013,
49, 6325–6327; f) E. Pizzolato, M. Natali, B. Posocco, A. M. Lopez, I.
Bazzan, M. Di Valentin, P. Galloni, V. Conte, M. Bonchio, F. Scandola, A.
Sartorel, Chem. Commun. 2013, 49, 9941–9943; g) H. Wang, Y. Lu, E. Mi-
jangos, A. Thapper, Chin. J. Chem. 2014, 32, 467–473; h) D. J. Wasylen-
ko, R. D. Palmer, E. Schott, C. P. Berlinguette, Chem. Commun. 2012, 48,
2107–2109.
It is usually considered that upon sequential oxidation of
a metal catalyst in water by chemical, electrochemical or pho-
tochemical means, it is first converted into a molecular metal–
oxo species or decomposing into a metal oxide prior to acti-
vating water oxidation.[2d] The ligand plays a crucial role, partic-
ularly in determining the nature of the catalyst.[17] It has been
demonstrated that the TPA ligand is essential for binding
firmly to the metal center to access the true catalytically active
species.[2g,12] Importantly, self-oxidation of the ligand in the oxi-
dizing environment can be significantly diminished relaxed by
electron-withdrawing fluoro substituents. Accordingly, it can
reasonably be assumed that the stability of 1 as a molecularly
well-defined homogeneous catalyst profits from the strong
binding of the robust and oxidation-resistant F3TPA ligand to
the copper center.
[3] a) J. P. Klinman, Chem. Rev. 1996, 96, 2541–2562; b) E. I. Solomon, P.
Chen, M. Metz, S.-K. Lee, A. E. Palmer, Angew. Chem. Int. Ed. 2001, 40,
4570–4590; Angew. Chem. 2001, 113, 4702–4724; c) E. I. Solomon,
U. M. Sundaram, T. E. Machonkin, Chem. Rev. 1996, 96, 2563–2606.
[4] a) H. Hayashi, S. Fujinami, S. Nagatomo, S. Ogo, M. Suzuki, A. Uehara, Y.
Watanabe, T. Kitagawa, J. Am. Chem. Soc. 2000, 122, 2124–2125; b) E. A.
Lewis, W. B. Tolman, Chem. Rev. 2004, 104, 1047–1076; c) L. M. Mirica, X.
Ottenwaelder, T. D. P. Stack, Chem. Rev. 2004, 104, 1013–1045; d) X. Ot-
tenwaelder, D. J. Rudd, M. C. Corbett, K. O. Hodgson, B. Hedman, T. D. P.
Stack, J. Am. Chem. Soc. 2006, 128, 9268–9269.
[5] a) S. M. Barnett, K. I. Goldberg, J. M. Mayer, Nat. Chem. 2012, 4, 498–
502; b) Z. Chen, P. Kang, M.-T. Zhang, B. R. Stoner, T. J. Meyer, Energy En-
viron. Sci. 2013, 6, 813–817; c) Z. Chen, T. J. Meyer, Angew. Chem. Int.
Ed. 2013, 52, 700–703; Angew. Chem. 2013, 125, 728–731; d) M. K. Cog-
gins, M.-T. Zhang, Z. Chen, N. Song, T. J. Meyer, Angew. Chem. Int. Ed.
2014, 53, 12226–12230; Angew. Chem. 2014, 126, 12422–12426; e) W.-
B. Yu, Q.-Y. He, X.-F. Ma, H.-T. Shi, X. Wei, Dalton Trans. 2015, 44, 351–
358; f) M.-T. Zhang, Z. Chen, P. Kang, T. J. Meyer, J. Am. Chem. Soc. 2013,
135, 2048–2051; g) T. Zhang, C. Wang, S. Liu, J.-L. Wang, W. Lin, J. Am.
Chem. Soc. 2014, 136, 273–281; h) X.-J. Su, M. Gao, L. Jiao, R.-Z. Liao,
P. E. M. Siegbahn, J.-P. Cheng, M.-T. Zhang, Angew. Chem. Int. Ed. 2015,
54, 4909–4914; Angew. Chem. 2015, 127, 4991–4996.
In summary, a water-soluble copper complex bearing an
F3TPA ligand was synthesized and displayed water oxidation
activity as a molecular catalyst under both photo-irradiated
and electrochemical conditions. In spite of moderate catalytic
behavior, the verification of a low-overpotential single-site
copper molecular homogeneous catalyst, which is active under
both electrochemical and photochemical conditions, opens up
an opportunity for the development of stable molecular
copper-based catalysts for water oxidation. More importantly,
these developments can facilitate the ready incorporation of
homogeneous catalysts into photoelectrochemical devices for
[6] Y. Tachibana, L. Vayssieres, J. R. Durrant, Nat. Photonics 2012, 6, 511 –
518.
Chem. Eur. J. 2016, 22, 1602 – 1607
www.chemeurj.org
1606
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[7] a) A. Sartorel, M. Bonchio, S. Campagna, F. Scandola, Chem. Soc. Rev.
2013, 42, 2262–2280; b) A. Magnuson, M. Anderlund, O. Johansson, P.
Lindblad, R. Lomoth, T. Polivka, S. Ott, K. Stensjç, S. Styring, V. Sund-
strçm, L. Hammarstrçm, Acc. Chem. Res. 2009, 42, 1899–1909.
[8] A. Machkour, D. Mandon, M. Lachkar, R. Welter, Inorg. Chem. 2004, 43,
1545–1550.
[14] The potential for the onset of the catalytic wave is taken as the point
where the current is 10% of the biggest catalytic peak; see: F. Maillard,
A. Bonnefont, M. Chatenet, L. Guetaz, B. Doisneati-Cottignies, H. Rous-
sel, U. Stimming, Electrochim. Acta 2007, 53, 811 –822.
[15] P. Zhang, M. Wang, Y. Yang, T. Yao, L. Sun, Angew. Chem. Int. Ed. 2014,
53, 13803–13807; Angew. Chem. 2014, 126, 14023–14027.
[16] a) J. M. Saveant, E. Vianello, Electrochim. Acta 1965, 10, 905–920; b) A. J.
Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals and Applica-
tions, John Wiley & Sons, Inc., New York, 2001.
[9] S. Maji, L. Vigara, F. Cottone, F. Bozoglian, J. Benet-Buchholz, A. Llobet,
Angew. Chem. Int. Ed. 2012, 51, 5967–5970; Angew. Chem. 2012, 124,
6069–6072.
[17] a) D. J. Wasylenko, C. Ganesamoorthy, B. D. Koivisto, M. A. Henderson,
C. P. Berlinguette, Inorg. Chem. 2010, 49, 2202–2209; b) D. J. Wasylenko,
C. Ganesamoorthy, J. Borau-Garcia, C. P. Berlinguette, Chem. Commun.
2011, 47, 4249–4251; c) L. Bernet, R. Lalrempuia, W. Ghattas, H. Mueller-
Bunz, L. Vigara, A. Llobet, M. Albrecht, Chem. Commun. 2011, 47, 8058–
8060.
[10] a) B. Das, A. Orthaber, S. Ott, A. Thapper, Chem. Commun. 2015, 51,
13074–13077.
[11] a) D. Shevchenko, M. F. Anderlund, A. Thapper, S. Styring, Energy Envi-
ron. Sci. 2011, 4, 1284–1287; b) H.-Y. Wang, J. Liu, J. Zhu, S. Styring, S.
Ott, A. Thapper, Phys. Chem. Chem. Phys. 2014, 16, 3661–3669.
[12] H.-Y. Wang, E. Mijangos, S. Ott, A. Thapper, Angew. Chem. Int. Ed. 2014,
53, 14499–14502; Angew. Chem. 2014, 126, 14727–14730.
[13] G. L. Elizarova, L. G. Matvienko, N. V. Lozhkina, V. N. Parmon, K. I. Zamar-
aev, React. Kinet. Catal. Lett. 1981, 16, 191–194.
Received: October 10, 2015
Published online on January 7, 2016
Chem. Eur. J. 2016, 22, 1602 – 1607
www.chemeurj.org
1607
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim