A Bio-inspired Cu4O4 Cubane: Effective Molecular Catalysts for Electrocatalytic Water Oxidation in Aqueous Solution

Article abstract of DOI:10.1002/anie.201803944

Inspired by the cubic Mn₄CaO₅ cluster of natural oxygen-evolving complex in Photosystem II, tetrametallic molecular water oxidation catalysts, especially M₄O₄ cubane-like clusters (M=transition metals), have aroused great interest in developing highly active and robust catalysts for water oxidation. Among these M₄O₄ clusters, however, copper-based molecular catalysts are poorly understood. Now, bio-inspired Cu₄O₄ cubanes are presented as effective molecular catalysts for electrocatalytic water oxidation in aqueous solution (pH 12). The exceptional catalytic activity is manifested with a turnover frequency (TOF) of 267 s^?1 for [(L_Gly-Cu)₄] at 1.70 V and 105 s^?1 for [(L_Glu-Cu)₄] at 1.56 V. Electrochemical and spectroscopic study revealed a successive two-electron transfer process in the Cu₄O₄ cubanes to form high-valent Cu^III and Cu^IIIO^. intermediates during the catalysis.

Full text of DOI:10.1002/anie.201803944

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Accepted Article
Title: Bio-inspired Cu4O4 "Cubane": Effective Molecular Catalysts for
Electrocatalytic Water Oxidation in Aqueous Solution
Authors: Li-Zhu Wu, Xin Jiang, Jian Li, Bing Yang, Xiang-Zhu Wei, Bo-
Wei Dong, Yi Kao, Mao-Yong Huang, and Chen-Hu Tung
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803944
Angew. Chem. 10.1002/ange.201803944
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10.1002/anie.201803944
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Bio-inspired Cu4O4 “Cubane”: Effective Molecular Catalysts for
Electrocatalytic Water Oxidation in Aqueous Solution
Xin Jiang,a,b Jian Li,a,b Bing Yang,a,b Xiang-Zhu Wei,a,b Bo-Wei Dong,c Yi Kao,c Mao-Yong Huang,a,b
Chen-Ho Tung,a,b Li-Zhu Wu*a,b
Abstract: Water oxidation is a vital component of natural and
artificial photosynthetic systems, which convert sunlight into storable
chemical fuels. Inspired by the cubical Mn4CaO5 cluster of natural
oxygen-evolving complex in Photosystem II, tetrametallic molecular
water oxidation catalysts, especially M4O4 cubane-like clusters (M =
transition metals), have aroused great interests in developing highly
active and robust catalysts for water oxidation. Among these M4O4
clusters, however, copper-based molecular catalysts are poorly
understood. Here we present the first bio-inspired Cu4O4 cubanes as
effective molecular catalysts for electrocatalytic water oxidation in
aqueous solution (pH = 12). The exceptional catalytic activity is
manifested with a turnover frequency (TOF) of 267 s-1 for [(LGly-Cu)4]
at 1.70 V and 105 s-1 for [(LGlu-Cu)4] at 1.56 V. Electrochemical and
spectroscopic study revealed a successive two-electron transfer
process in the Cu4O4 cubanes to form high-valent CuIII and CuIIIO•
intermediates during the catalysis.
With this in mind, we initiated to design biomimetic molecular
cubane copper catalysts for oxygen evolution. Considering that
multidentate Schiff base ligands are capable of forming multi-
metal clusters by intimately bonding two or more metal centers
in a close proximity,[15] we selected the Schiff base (3-methoxy-
salicylidene)-amino acid (glycine, glutamic acid) chelates to
coordinate with copper(II) ion. To our delight, mixing Cu(OAc)2
with the chelate ligands (1:1) in aqueous solution immediately
produced the cubane-like Cu4O4 core surrounded by the amino
acids in 88-89 % isolated yield (Figure 1). Recrystallization of
[(LGly-Cu)4] from acetonitrile/water solution gave rise to dark
green block-shaped crystals. The X-ray crystal structure
revealed that [(LGly-Cu)4] (LGly: 3-methoxy-salicylidene-glycine)
featured a unique tetrameric cubane-like structure. As shown in
Figure 1, each Cu atom is coordinated with one N atom and five
O atoms. Taking Cu1 as a representative, the N1, O2 and O3
atoms from one horizontal ligand and the O2’ atom from another
vertical ligand are sited in the equatorial plane, while the O2’’
and O1’ atoms are arranged in an apical fashion. As a result,
Cu1 are connected with three neighboring Cu atoms via μ-O2,
O2‘ and O2‘‘ bridges leading to the cubane structure. Notably,
one of the three Cu-O bonds is elongated and distributed on two
opposite faces of the cubane, which may offer an open-site for
water attack. Indeed, the as-synthesized cubane complexes
([(LGly-Cu)4] and [(LGlu-Cu)4]) exhibit superior oxygen evolution
ability in aqueous solution (pH 12). The turnover frequency
(TOF) is determined to be 267 s-1 for [(LGly-Cu)4] at 1.70 V and
105 s-1 for [(LGlu-Cu)4] at 1.56 V. The exceptional catalytic
efficiency, higher than those of mono- or binuclear ones
(0.4~100 s-1)[11-13] enable us to believe that the Cu4O4 cubane
core is promising for the catalytic oxygen evolution.
Photosynthesis in nature utilizes the oxygen-evolving complex,
Mn4CaO5 cluster, to efficiently catalyze water oxidation in
photosystem II (PS II).[1] The Mn4CaO5 cluster shown in Figure 1
can be considered as a cuboidal Mn3CaO4 unit tethered to the
fourth manganese atom via a cornor-oxo attachment and an
additional μ-oxo bridge.[2] This beautiful scheme has stimulated
much effort in simulating the natural oxygen-evolving complex
(OEC) structure[3] and understanding the accumulated four
oxidizing equivalents on the active site for oxygen evolution. In
spite of not fully elucidating these factors, there have been
numerous advances in the molecular catalysts design for water
oxidation.[4-9] Polynulear metal-based clusters, especially those
with cubane cores mimicking natural Mn4CaO5 active site, have
recently appeared at a forefront for oxygen evolution.[10] Due to
inexpensive and easily accessible to different oxidation state,
copper-based molecular water oxidation catalysts have been
documented with the focus on mono- and bi-nuclear copper
complexes.[11-13] Their superior biomimetic reactivity[14] with
oxygen, and the unique covalent metal-oxo bonding in Cu2(μ-O)2
core are expected to be advantage for oxygen evolution.
However, the copper-based cubane-like molecular catalyst for
water oxidation is to date unknown.
[a]
Xin Jiang, Jian Li, Bing Yang,, Xiang-Zhu Wei, Mao-Yong Huang,
Prof. Chen-Ho Tung, Prof. Li-Zhu Wu
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry, the Chinese
Academy of Sciences, Beijing 100190, P.R. China.
E-mail: lzwu@mail.ipc.ac.cn
[b]
[c]
Xin Jiang, Jian Li, Xiang-Zhu Wei, Mao-Yong Huang, Prof. Chen-Ho
Tung, Prof. Li-Zhu Wu
School of Future Technologies, University of Chinese Academy of
Sciences, Beijing 100190, P. R. China.
Bo-Wei Dong, Yi Kao
College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, P. R. China.
Figure 1. (a) Structures of the OEC in PSII. (b) Synthesis of [(LGly-Cu)4] and
[(LGlu-Cu)4]. (N, blue; O, red; C, gray; Cu, green; Mn, purple; Ca, turquoise; H
atoms omitted for clarity).
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201803944
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The basically overlapped absorption spectra of [(LGly-Cu)4] and
[(LGlu-Cu)4] (Figure S1) indicated that these two complexes had
the same structural framework. A characteristic d-d absorption of
copper center around 650 nm[16] for [(LGly-Cu)4] and [(LGlu-Cu)4]
was clearly observed. The linear dependence of absorbance on
the concentration of cubanes demonstrated undissociation of
[(LGly-Cu)4] and [(LGlu-Cu)4] in aqueous solutions.[12f] More
strikingly, temperature dependence of molar magnetic
susceptibility (χ) for [(LGly-Cu)4] exhibited an distinct
antiferromagnetic transition (Figure S2).[16] Extracting part of the
susceptibility data above the Nꢀel temperature to fit the Curie–
Weiss equation verified the antiferromagnetic coupling behavior
(see details in Supporting Information). The fitted negative
Weiss constant θ (−66.477 K) indicated strong interaction
among the copper centers.[17]
result, the four copper centers of the cubane Cu4O4 can be
considered as two equivalent groups for a two-electron reaction,
i.e., the quasi-reversible process at around -0.14 V can be
II
II
assigned to a two-electron reaction of Cu4 /Cu2 Cu2I couple, and
the anodic peak at around 1.00 V was assigned to a two-
II
II
electron oxidation of Cu4 /Cu2 Cu2III couple (Scheme 1).
It was worth noting that the profile of the differential pulse
voltammetry (DPV) of [(LGly-Cu)4] was dramatically changed by
replacing the solvent from acetonitrile to PBS aqueous solution
(pH 12, Figure S7-8). The peak current at 0.94 V was
II
analogously assigned to the two-electron oxidation of Cu4 to
II
Cu2 Cu2III. In PBS, the largely strengthened anodic current after
0.94 V suggested that water oxidation had occurred.[11-13] The
linear scan voltammograms (LSVs) for [(LGly-Cu)4] and [(LGlu
-
Cu)4] were further obtained at tin-doped indium oxide (ITO)
electrodes. The greatly enhanced catalytic current suggested
water oxidation occurrence (Figure S9). The overpotential
required for [(LGly-Cu)4] to reach 1 mA/cm2 is 620 mV. The
The redox properties of [(LGly-Cu)4] and [(LGlu-Cu)4] were
investigated by cyclic voltammetry (CV) in 0.2 M aqueous
phosphate buffer solution (PBS) at pH 12 (Figure 2 and Figure
S3). As shown in Figure 2, the oxidative current at E1/2 (-0.14 V,
all potentials in this paper are vs NHE, normal hydrogen
electrode) was linearly dependent on the square root of the scan
rate (pH 12.0), indicating a diffusion-controlled process. The
relatively higher overpotential of 760 mV observed for [(LGlu
-
Cu)4] might be attributed to the different R groups of amino acids
between these two complexes (R = H, Gly; CH2COOH, Glu).[12b]
The apparent rate constant (kcat), known as the turnover
potential difference between the cathodic and anodic peaks (∆Ep, frequency (TOF), was calculated to be about 267 s-1 for [(LGly
-
see the labels in Figure 2) was 38 mV (scan rate v = 25 mV/s),
which is much smaller than that for one-electron quasi-reversible
process (∆Ep > 57 / n mV).[18] Moreover, the ∆Ep increased as
the scan rate increased, agreeing fairly well with a quasi-
reversible process. When ∆Ep > 200 / n mV, the plot of the
quasi-reversible peak potential Ep versus ln(v) was fitted by
Laviron equation.[19] The fitted slope is equal to - RT/αnF and
RT/(I - α)nF for the cathodic peak (Ep,c) and the anodic peak
(Ep,a), where α (transfer coefficient) and n can be deduced as 0.5
and 2 respectively (see details in Supporting Information). As a
Cu)4] at 1.7 V and 105 s-1 for [(LGlu-Cu)4] at 1.56 V (the potentail
for TOF was determined from the DPV, see details in Supporting
Information).[11-13]
To verify the oxygen evolution, controlled-potential electrolysis
(CPE) of 0.25 mM [(LGly-Cu)4] in 0.2 M PBS (pH 12.0) was
performed in a gas-tight custom-designed cell at 1.50 V with a
ITO electrode (1.00 cm2) (Figure S11). After 3 h electrolysis, the
production of oxygen was measured by gas chromatography
and about 30 μmol oxygen was detected with a Faraday
efficiency of 97±2 % (irrespective oxygen dissolved in aqueous
solution). Based on the initial concentration of Cu4O4 catalyst,
the turnover numbers (TON) of 4.74±0.1 was obtained. What’s
more, hydrogen was detected with a ratio (2:1) of hydrogen to
oxygen. Together with the high Faraday efficiency, we believed
the water oxidation took place to split water effectively under
homogeneous catalytic condition. To examine the stability of the
cubane complex, the ITO electrode after electrolysis was quickly
removed, rinsed with water, and immediately put into a fresh
solution without catalysts. Neither significant changes in the CV
spectra of ITO in 0.2 M PBS (Figure S13) nor heterogeneous
deposits on the ITO could be detected by energy-dispersive X-
ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS)
(Figure S14-15).[11a] The almost identical UV-Vis spectra of
[(LGly-Cu)4] after 3 h electrolysis (Figure S16) further indicated
the intact structure of Cu4O4 core.
Next, electron paramagnetic resonance(EPR) spectra of [(LGly
-
Cu)4] were recorded in-situ to track the generated copper
intermediates at different potentials. As shown in Figure 3, the
native state of [(LGly-Cu)4] displayed a typical spectrum (g = 2.17)
II
of Cu4 species. The absence of hyperfine structures at both 90
K and 5 K may be attributed to the spin-spin coupling among the
four CuII centers.[20] During electrolysis at 1.00 V (around Ep,a of
II
Cu2 Cu2III/Cu4III), the observed decrease of in-situ EPR signal
indicated the generation of some EPR-silent species. In this
catalytic process, there were only CuIII or CuIV high-valent
intermediates. Since the outermost electronic configuration of
CuIV species is 3d7, which definitely contains an unpaired
electron (i.e., EPR-active), this EPR-silent species could be
assigned to the CuIII species (3d8). Theoretically, the 3d
electrons in CuIII have two types of arrangements, a low-spin and
Figure 2. CVs of 0.25 mM [(LGly-Cu)4] in 0.2 M PBS using GC electrode with
different scan rate (25-125 mV/s) at -0.75 – 0.25 V (a) and 0.25 – 1.80 V (b).
Inset: Plot of the ratio of the catalytic current at 1.70 V, ic, to the oxidative peak
I
current for the Cu4II/Cu2IICu2 wave, id, vs ν-1/2
. The background without
complex were shown was the orange line in Figure 2b.
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10.1002/anie.201803944
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a
high-spin configuration.[21] The high-spin CuIII contains
10-12) the signal of the higher-valent redox couples was
overlapped with the water oxidation current, thus preventing
from accurate determination of the redox current of the possible
unpaired electrons (i.e., EPR-active), while the low-spin CuIII is
EPR-silent. Taken all together, we presumed the generated
EPR-silent species as the CuIII with a low-spin configuration. The
decrease of spin count (from 2.915  1015 to 1.579  1015 /mm3)
for in-situ EPR result at 1.00 V agreed fairly well with the two-
intermediates in the Pourbaix diagram. To gain
a better
understanding of the redox properties, CV study was performed
under acidic and weak basic condition (pH 4-9), where the
II
II
III
IV
electron oxidation from Cu4 to Cu2 Cu2
species (i.e.,
catalytic current was negligible. The potential of Cu4III/Cu2IIICu2
II
II
Cu4 /Cu2 Cu2III). The presence of a signal g = 2.0 in the in-situ
EPR spectra at 5 K might be attributed to the anisotropy of g
factor (g⊥),[22] which was overlapped by spin-spin coupling in the
EPR spectra before electrolysis. The completely EPR-silent
signal at 1.20 V is consistent with the formation of Cu4III species.
couples exhibited a slope of -49 mV/pH, in line with the 2e-/2H+
proton-coupled electron transfer (PCET) process (-59 mV/pH,
Nernstian ideal).[25] Accordingly, a probable catalytic mechanism
was proposed. As shown in Scheme 1, under the catalytic
conditions (pH 10-12), OH- ions instead of water molecules were
expected to coordinate with the copper (III) centers giving rise to
a [Cu4III(μ-OH)2O4] species (Int 1). Such a fragment could be
further oxidized to [Cu4III(μ-O)2O4] (Int 2) through the 2e-/2H+
PCET process. In the succeeding step, Int 2 were transformed
to the μ4-coordinated peroxide (Int3) and Int 3 released O2 and
simultaneously coordinated with the OH- ions regenerating Int 1.
III
Alternatively, Int 3 released O2 and formed Cu4 species, which
then coordinated with OH- ions to form Int 1 for the following
catalytic cycle.
Figure 3. X-Band EPR spectra at 90 K (a) and 5 K (b) of [(LGly-Cu)4] and the
generated species by in-situ electrolysis in 0.2 M PBS using glassy carbon
electrode at different potentials.
As the potential increased to 1.50 V (to initiate the OER), the
peak intensity recovered with the band shifting to lower g factor
(g = 2.15). Accordingly, the generation of CuIV species or CuIII
species with the μ-oxoradical character (CuIIIO•) could be
concluded.[23] To confirm the formation of μ4-coordinated
peroxide in situ, we added H2O2 to chemically generate μ4-
coordinated peroxide.[24] The same peak position at 875 cm-1 of
the in situ generated and chemically generated species in their
Raman spectra (Figure S18) manifested the ṽ(O-O) of the μ4-
coordinated peroxide.[24] Based on the above results, it is
reasonable to consider that under the catalytic condition, the
Scheme 1. Proposed mechanism for water oxidation of Cu4O4 core in alkaline
solution.
In conclusion, we have successfully achieved the first set of
cubane-like copper-based molecular catalysts ([(LGly-Cu)4] and
[(LGlu-Cu)4]) for water oxidation. The unique Cu4O4 cubane cores
are structurally similar to the oxygen-evolving center Mn4CaO5
clusters in PS II, and show efficient electrocatalytic water
II
Cu4 species was oxidized via two successive two-electron
III
oxidations to Cu4 intermediates and further to higher-valent
CuIIIO• species.
Given that [(LGly-Cu)4] was only soluble in alkaline solution, we
utilized [(LGlu-Cu)4] with good solubility in all pH range of
aqueous solutions to further evaluate the pH-dependence of the
redox couples. As shown in the Pourbaix diagram (Figure S19),
oxidation activity with a turnover frequency of 267 s-1 for [(LGly
-
Cu)4] at 1.70 V and 105 s-1 for [(LGlu-Cu)4] at 1.56 V. The high
performance may be attributed to the synergy and the
successive two-electron transfer processes involved in the
tetranuclear copper cubane centers. EDX, XPS and
electrochemical studies confirm that no heterogeneous deposit
is formed during the catalysis. In-situ EPR and Raman
spectroscopy suggest the presence of high-valent CuIII and
II
II
III
II
III
Cu4 /Cu2 Cu2 and Cu2 Cu2III/Cu4 couples at about 1.00 V and
1.20 V both displayed a pH-independent redox potential change,
indicating that the two successive two-electron oxidation
processes (from Cu4 O4 to Cu4IIIO4) is not relied on the pH value
II
of aqueous solution. However, under the catalytic conditions (pH
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COMMUNICATION
CuIIIO• intermediates. Multi-electron redox properties of the
cuboidal Cu4O4 might be favorable to activate catalytic cycles for
water oxidation. It is anticipated that this example would promote
the investigation of successive electron transfer and the design
of more active water oxidation catalysts.
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Acknowledgements
We are grateful for the financial support from the Ministry of
Science and Technology of China (2014CB239402,
2017YFA0206903), the National Natural Science Foundation of
China (91427303, 21390404), the Strategic Priority Research
Program of Chinese Academy of Science (XDB17000000) and
Key Research Program of Frontier Sciences of the Chinese
Academy of Sciences (QUZDY-SSW-JSC029).
Keywords: water oxidation • molecular catalyst • cubane-like
core • copper complex • oxygen-evolution reaction
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10.1002/anie.201803944
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Xin Jiang, Jian Li, Bing Yang, Xiang-Zhu
Wei, Bo-Wei Dong, Yi Kao, Mao-Yong
Huang, Chen-Ho Tung, Li-Zhu Wu*
The first set of Cu4O4 cubanes show
exceptional activity for electrocatalytic
water oxidation in aqueous solution,
with a turnover frequency of 267 s-1 for
[(LGly-Cu)4] at 1.70 V and 105 s-1 for
[(LGlu-Cu)4] at 1.56 V, respectively. The
unique cuboidal structure and high
catalytic performance is remeniscent of
the natural Mn4CaO5 clusters in PS II.
Page No. – Page No.
Bio-inspired
Cu4O4
“Cubane”:
Effective Molecular Catalysts for
Electrocatalytic Water Oxidation in
Aqueous Solution
This article is protected by copyright. All rights reserved.