Oxygen reduction catalyzed by a water-soluble binuclear copper(ii) complex from a neutral aqueous solution

Article abstract of DOI:10.1039/c6cc09206c

A water-soluble binuclear Cu^II complex, synthesized using a polypyridine-polyamide ligand, was able to catalyze oxygen reduction to water from neutral aqueous solutions. Electrocatalytic results suggested that one Cu^II center was first reduced by one electron to form a Cu^IICu^I species, which reacted with O₂ to give a superoxide radical intermediate.

Full text of DOI:10.1039/c6cc09206c

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DOI: 10.1039/C6CC09206C
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Oxygen reduction catalyzed by a water-soluble binuclear
copper(II) complex from a neutral aqueous solution
Chengyu Liu,a Haitao Lei,a Zongyao Zhang,a Fangfang Chen,b and Rui Cao*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A water-soluble binuclear CuII complex, synthesized using a H2O. Although a variety of binuclear Cu complexes have been
polypyridine-polyamide ligand, was able to catalyze oxygen designed and used to investigate the Cu/O2 chemistry,27,28 few
reduction to water from neutral aqueous solutions. examples have been reported as homogeneous ORR catalysts
Electrocatalytic results suggested that one CuII center was first in neutral water.10 Herein we report a water-soluble binuclear
reduced by one electron to form a CuIICuI species, which reacted Cu complex and its catalytic features for O2 reduction from a
with O2 to give a superoxide radical intermediate.
neutral aqueous solution. Mechanism studies suggested that
formation of a CuIICuI species triggered the reduction of O2 to
give a superoxide radical, which was then released from or
further reduced at the dicopper site depending on the reaction
conditions. This work is important because it represents a rare
example of homogeneous ORR catalyzed by molecular
complexes from neutral aqueous solutions.
The oxygen reduction reaction (ORR) is an important process
in both biological systems and fuel cells,1 the latter of which is
a key for realization of a hydrogen-based society.2-5 Compared
to oxidation of H2 at the anode, reduction of O2 at the cathode
is kinetically slow.2-5 Developing efficient ORR catalyst is thus
the heart of renewable-energy technologies. Platinum-based
materials are active for ORR, but utilization of this noble metal
is restricted due to its low natural abundance and high cost.6
Therefore, catalysts made of cheap earth-abundant transition
metals attract major attention.2,3 Recent efforts have
indentified metal complexes of Cr,7 Mn,8,9 Fe,10-13 Co,14-18 Ni,19-
Cu complexes of polypyridine29,30 and polyamide31,32
ligands have been shown recently to be highly active for water
oxidation. The initial goal of this research is to synthesize
mononuclear Cu complexes of new polypyridine-polyamide
ligands and to examine their potentials as water oxidation
catalysts.33 We designed a five-coordinated polypyridine-
polyamide ligand L (Fig. 1a), and rationalized that it could
coordinate a Cu ion to give a mononuclear Cu complex.
However, unlike we expected, reaction of L and one equivalent
of Cu(ClO4)2 gave a binuclear Cu complex 1 (Fig. 1b), which was
found to be unstable when functioning as a water oxidation
catalyst (data not shown). Crystallographic studies revealed
that 1 crystallized in the monoclinic space group P21/c (see
Table S1 for details). In the structure of 1, two Cu ions were
grabbed together by two L ligands, and were each coordinated
by three pyridine N atoms and two amide N atoms to give a
square pyramidal geometry (Fig. 1c). The d9 CuII electronic
structure was suggested based on Cu−N bond lengths and
charge balance consideration. Importantly, the vacant sites of
the two Cu ions had a face-to-face orientation with a Cu···Cu
distance of 3.458(3) Å, which falls in the range of the Cu···Cu
distances found in the binuclear type 3 (T3) copper sites of
multicopper oxidases (the enzymatic Cu···Cu distances are
depended on the type of enzymes and also on the oxidation
state of Cu ions).24,34 This result suggests that the dicopper site
in 1 has the right space for the activation of an O2 molecule.
21
and Cu22,23 as catalysts for O2 reduction. Despite these
achievements, metal complexes that are able to catalyze
homogeneous O2 reduction from a neutral aqueous solution
are very rare.10
Copper-containing proteins and enzymes are important in
many biological redox processes.24 Cu complexes have also
been extensively studied as synthetic models because of their
intriguing redox chemistry.25,26 Particularly, metalloproteins
with multiple Cu sites, such as binuclear Cu enzymes (i.e.,
tyrosinase and catechol oxidase) and multinuclear Cu oxidases
(i.e., laccase, ascorbate oxidase), play significant roles in O2
activation.24 It is considered that multiple Cu sites have the
benefits to bind O2 molecule strongly and provide sufficiently
the electrons required for the complete reduction of O2 to
a. Department of Chemistry, Renmin University of China, Beijing 100872, China.
ruicao@ruc.edu.cn.
b. School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710119, China.
Electronic Supplementary Information (ESI) available: Experimental details for
syntheses, crystallographic and electrochemical studies; Scheme S1; Table S1;
Figures S1-S24; CCDC 1516460. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Cyclic voltammogram (CV) of 1 on a glassy carbon (GC) electrode only with potentials more than 0.58 V (VvisewfeArrtircolecOennlinee,
•−
DOI: 10.1039/C6CC09206C
electrode in dimethylformamide (DMF) under N2 showed a Fig. S12). (2) The disproportionation of O2 is confirmed by
quasi-reversible wave at −1.18 V (vs ferrocene) (Fig. 2a), which RRDE, from which H2O2 is detected with a ring electrode
corresponded to the CuII/CuI redox process. Careful analysis of potential of 0.70 V (vs ferrocene, Fig. S13). Moreover, the
this cathodic wave showed a shoulder peak at −1.33 V (vs amount of H2O2 does not decrease with the increase of HOAc
•−
ferrocene), which became more pronounced under higher concentration, indicating that O2 is released from the
scan rates (Fig. S21). In addition, the peak separation ΔEp was dicopper site followed by disproportionation. (3) Comparison
106 mV, while the ΔEp of ferrocene was 64 mV. These results of CVs of 1 with acids under N2 and O2 confirmed that ORR
reveal that the two Cu ions are reduced in sequence (CuIICuII contributed dominantly to the cathodic current (Fig. S14).
→ CuIICuI → CuICuI) due to possible Cu···Cu interaction. An
irreversible reduction wave at −1.58 V (vs ferrocene) was also
observed, which could be assigned to the CuI/Cu0 redox
process. Interestingly, CV of 1 in DMF under O2 displayed a
large catalytic wave with the onset at −1.08 V (vs ferrocene,
Fig. 2b). Comparison of CVs of 1 under N2 and O2 suggests that
ORR is triggered upon the formation of a CuIICuI species of 1.
Fig. 2 (a) CV of 1 in DMF under N2. (b) CVs of 1 in DMF under
Fig. 1 (a) Structure of the new polypyridine-polyamide ligand L. O2 or N2. (c) CVs of 1 in DMF under O2 with the addition of 0-
(b) Schematic representation of the binuclear Cu complex 1. (c) 170 mM acetic acid and 100 mV s−1 scan rate. (d) RRDE data of
Thermal ellipsoid plot (50% probability) of the X-ray structure 1 in DMF under O2 with or without 170 mM acetic acid. (e) CVs
of 1. Hydrogen atoms are omitted for clarity.
of 1 in DMF under O2 at different scan rates. (f) Ratios of the
peak currents of anodic to cathodic at different scan rates.
Significantly, in contrast to typical catalytic waves, there is Conditions: 0.1 mM catalyst, GC disk-Pt ring electrode, −0.72 V
a strong anodic wave peaked at −1.16 V (vs ferrocene) on the (vs ferrocene) ring potential, 2500 rpm rotation rate, 20 °C.
return scan (Fig. 2b). We assigned this anodic wave to the
oxidation of superoxide radical O2•−, which was generated by
Third, the ratios of anodic to cathodic currents increased
1e− reduction of O2 on the cathodic scan. There are several with scan rates (Fig. 2e and 2f). It is expected that at higher
•−
lines of evidence. First, addition of acetic acid (HOAc) caused scan rates, more O2 will retain intact for oxidation on the
the increase of the cathodic wave but the decrease of the return scan. Therefore, we can conclude that in DMF the
•−
anodic wave (Fig. 2c). It is known that O2 disproportionates CuIICuI state of 1 is the active species to reduce O2 to generate
rapidly to O2 and H2O2 in the presence of acids.35 With 170 mM O2
,
•− 36,37 which is then released from the dicopper site.
of HOAc, the oxidation wave of O2•− became negligible.
More importantly, 1 is soluble in water. Therefore, we
examined the catalytic ORR features of 1 in neutral aqueous
•−
Second, by using rotating ring-disk electrode (RRDE), O2
could be oxidized at the ring electrode under an applied solutions. As shown in Fig. 3a, CV of 1 in 0.1 M pH 7 phosphate
potential of −0.72 V (vs ferrocene, Fig. 2d). With 170 mM of buffer under N2 shows an Ep,c at −0.28 V vs normal hydrogen
•−
HOAc, the amount of O2 became undetectable, a result electrode (NHE) and an Ep,a at 0.18 V vs NHE. The broad
consistent with the CV experiment. The following three points features of both cathodic and anodic peaks and the large ΔEp
are worth noting. (1) The oxidation of H2O2 occurs at the ring of 460 mV indicate that the CuII/CuI redox processes of these
2 | J. Name., 2012, 00, 1-3
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two Cu ions are also separated in aqueous solutions. Under O2, (vs NHE, Fig. 3d). Representative RRDE data forV2iewaAnrtdicle3Onalirnee
the cathodic wave became a strong catalytic wave, indicating depicted in Fig. S18 and S19, respectiveDlyO.I:T1h0e.1s0e39r/eCs6uCltCs09v2e0r6ifCy
the reduction of O2 by 1. Complex 1 was stable during ORR the cooperation effect between the two Cu ions of 1 toward
catalysis, showing a constant current in prolonged electrolysis the selective reduction of O2 to H2O.
under −0.40 V (vs NHE, Fig. 3b). The GC electrode and the
The pH dependence of ORR with 1 was also investigated.
solution after electrolysis were analyzed, showing no sign of RRDE measurements in phosphate buffer solutions of different
decomposition of 1 (Fig. S15 and S16). In addition, the solution pHs were conducted. Representative data at pH 5, 7, and 9 are
after bulk electrolysis was analyzed using the KI protocol. Our provided in Fig. S20, and the results are summarized in Fig. 4.
results showed that no H2O2 was generated during O2 Based on these studies, the following conclusions can be
reduction, indicating the conversion of O2 to H2O (Fig. S22).
made. (1) In principle, the observed current under O2 is mainly
The number of electrons (n) transferred per O2 reduction due to catalytic ORR and hydrogen evolution reaction (HER).
was then determined by RRDE. The disk electrode was scanned By subtracting the HER current determined under N2 from the
with linear sweep voltammetry (LSV) from 0.40 to −0.90 V (vs observed total current under O2, we can obtain the pH
NHE), while the ring potential was held at 0.70 V (vs NHE) to dependence of the ORR current (red line, Fig. 4b). The resulted
identify possibly formed H2O2. Compared to the blank control, volcano-like profile shows that 1 displays the highest ORR
the onset of ORR wave with 1 shifted to the positive direction activity at pH 7. (2) The ring current due to H2O2 oxidation
by about 300 mV, and a negligible amount of H2O2 was found increased with the pH (Fig. 4a). As shown in Fig. 4b (black line),
in ORR with 1 (Fig. 3c). The calculated n was 3.95 (Fig. 3d). In the n values of ORR are almost 4 in pH 5, 6, and 7 buffer
addition, 1 was shown to be much less efficient in catalyzing solutions but started to decrease in basic solutions. Similar
the reduction of H2O2 (Fig. S17). Thus, 1 is able to catalyze the observation is reported by Nocera and co-workers.38 They
direct 4e− reduction of O2 to H2O in neutral aqueous solutions.
investigated the ORR features of a family of binuclear Co
complexes and demonstrated that the H2O2 content increased
with inefficient proton delivery. Based on these data, we can
conclude that 1 has the highest activity and selectivity for the
reduction of O2 to H2O in a pH 7 buffer solution. Possible
reaction mechanisms for ORR with 1 in organic and aqueous
solutions are proposed (Fig. S23). Identifying the homolytic vs
heterolytic O−O bond cleavage during ORR with 1 is the focus
of our ongoing studies.
Fig. 3 (a) CVs of 1 under O2 or N2. (b) Controlled potential Fig. 4 (a) The observed total current (red), HER current (green),
electrolysis of 1 under air. (c) RRDE data under O2 with or and ring current due to H2O2 oxidation (blue) in pH-dependent
without 1. (d) The n value of ORR with 1 (blue), 2 (red), and 3 RRDE measurements of 1 under O2. (b) The ORR current and n
(black). Conditions: 0.05 mM catalyst, 0.1 M pH 7.0 phosphate value in different pH buffers. Conditions: 0.05 mM catalyst, 0.1
buffer, GC disk-Pt ring electrode, 2500 rpm rotation rate, 20 M pH 5-9 phosphate buffer, currents measured at −0.9 V (vs
°C.
NHE), GC disk-Pt ring electrode, 2500 rpm rotation rate, 20 °C.
Terminal metal oxo species are generally considered to be
In conclusion, a water-soluble binuclear CuII complex 1 was
key intermediates produced upon the heterolytic O−O bond synthesized and examined as a homogeneous electrocatalyst
cleavage during O2 reduction.1,11,14 For late transition metals for the reduction of O2 to H2O from neutral aqueous solutions.
(i.e., Cu), their terminal metal oxo species are not stable and Electrochemical studies revealed that the CuIICuII center was
are unlikely to be generated.7 As a consequence, mononuclear first reduced by one electron to give CuIICuI, which reacted
late transition metal complexes usually catalyze the 2e− with an O2 molecule to generate O2•−. In DMF, O2•− was largely
reduction of O2 to H2O2.7 However, late transition metal released from the dicopper site; while in aqueous solutions,
•−
complexes can still catalyze the 4e− reduction of O2 to H2O O2 could be further reduced to H2O at the dicopper site. In
through a bimetallic homolytic O−O bond cleavage.1,22,23 addition, the pH dependence studies revealed that proton
Control experiments using mononuclear Cu analogues 2 and 3 delivery played a decisive role in O2 reduction with 1 in
(Fig. S1) showed that n = 2.91 for 2 and n = 2.77 for 3 at −0.9 V aqueous solutions: 1 showed much better activity and
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Acknowledgements
We are grateful for the support from the “Thousand Talents
Program” of China, the National Natural Science Foundation of
China under Grants 21101170 and 21573139, the Fundamental
Research Funds for the Central Universities, and the Research
Funds of Renmin University of China.
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