Synthesis and electrocatalytic properties of a dinuclear copper(II) complex for generating hydrogen from acetic acid or water

Article abstract of DOI:10.1080/00958972.2014.998657

The reaction of CuCl₂s2H₂O and a tetradentate amine phenol ligand affords a dinuclear Cu(II) complex, 1, a new molecular electrocatalyst, whose structure has been determined by X-ray crystallography. Electrochemical studies show that

Full text of DOI:10.1080/00958972.2014.998657

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Synthesis and electrocatalytic
properties of a dinuclear copper(II)
complex for generating hydrogen from
acetic acid or water
Ting Fanga, Wei Lib, Shu-Zhong Zhana & Xiao-Lan Weia
a College of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou, China
b School of Chemistry and Environmental Science, Guizhou Minzu
University, Guiyang, China
Accepted author version posted online: 15 Dec 2014.Published
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online: 07 Jan 2015.
To cite this article: Ting Fang, Wei Li, Shu-Zhong Zhan & Xiao-Lan Wei (2015) Synthesis
and electrocatalytic properties of a dinuclear copper(II) complex for generating hydrogen
from acetic acid or water, Journal of Coordination Chemistry, 68:4, 573-585, DOI:
10.1080/00958972.2014.998657
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Journal of Coordination Chemistry, 2015
Vol. 68, No. 4, 573–585, http://dx.doi.org/10.1080/00958972.2014.998657
Synthesis and electrocatalytic properties of a dinuclear
copper(II) complex for generating hydrogen from acetic acid
or water
TING FANG†, WEI LI‡, SHU-ZHONG ZHAN*† and XIAO-LAN WEI†
†College of Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou, China
‡School of Chemistry and Environmental Science, Guizhou Minzu University, Guiyang, China
(Received 4 September 2014; accepted 20 November 2014)
The reaction of CuCl2⋅2H2O and a tetradentate amine phenol ligand affords a dinuclear Cu(II)
complex, 1, a new molecular electrocatalyst, whose structure has been determined by X-ray crystal-
lography. Electrochemical studies show that 1 can efficiently produce hydrogen from acetic acid
with a turnover frequency (TOF) of 50.1 (in DMF) or from water with a TOF of 104.3 (pH 7.0)
moles of hydrogen per mole of catalyst per hour.
Keywords: Copper(II)-complex; Molecular electro-catalyst; Water reduction; Hydrogen evolution
*Corresponding author. Email: shzhzhan@scut.edu.cn
© 2015 Taylor & Francis

574
T. Fang et al.
1. Introduction
The ability to store and recover secure, clean, and sustainable energy is arguably the most
important scientific and technical challenge facing humanity in the twenty-first century.
Chemical fuels are ideal in this context since chemical bonds have high energy density;
hydrogen has attracted considerable attention as an alternative chemical fuel. On a mid- to
long-term basis, it is urgent to provide an appealing approach toward generating and storing
hydrogen [1–3]. In nature, hydrogenase enzymes [4, 5] can efficiently catalyze both the pro-
duction and oxidation of hydrogen using earth-abundant metals (nickel and iron). However,
enzymes are difficult to obtain in sufficient amounts to adapt for commercial applications
and their stability is often limited outside of their native environment [4–7]. Electrolysis of
water is the simplest way to produce hydrogen. To increase the reaction rate and lower the
overpotential, it is necessary to use an efficient hydrogen evolution reaction (HER) electro-
catalyst. These considerations have led to development of molecular catalysts employing
more abundant metals, and several complexes that contain nickel [8–10], cobalt [11–13],
iron [14, 15], and molybdenum [16], have been developed as electrocatalysts for reduction
of water to form H2. Although there has been significant progress in designing molecular
catalysts for H2 evolution, the search for robust and highly active catalysts that can operate
in purely aqueous solution still remains a challenge [11]. In designing a model featuring
both water oxidation and reduction functionality, we sought a synthetic cofactor with the
following properties: (1) the metal center coordination geometry is planar; (2) mild redox
couple closer to the H2/H+ couple, in the range −0.70 to −1.45 V versus SHE; (3) chemical
inertness, so that reactions would be localized at the metal centers. In general, copper com-
plexes are employed as electrocatalysts for water oxidation [17, 18]. Here, we present the
synthesis, structure and properties of a new dinuclear copper(II) complex, 1, that can cata-
lyze hydrogen evolution from acetic acid or water.
2. Experimental
2.1. Materials and physical measurements
All commercially available reagents were used as received without purification. UV–vis
spectrum was measured on a Hitachi U-3010 spectrometer. Cyclic voltammograms were
obtained on a CHI-660E electrochemical analyzer under oxygen-free conditions using a
three-electrode cell in which a glassy carbon electrode gas chromatograph (GC), 1 mm in
diameter, was the working electrode, a saturated Ag/AgNO3 electrode was the reference
electrode, and platinum wire was the auxiliary electrode. A ferrocenium/ferrocene couple
was used as an internal standard and E1/2 of the ferrocenium/ferrocene (FeCP2+/0) couple
under the experimental condition was 0.47 V. 0.1 M [(n-Bu)4 N]ClO4 was used as the sup-
porting electrolyte. Acetic acid was added by syringe. Controlled-potential electrolysis
(CPE) in DMF was conducted using an air-tight glass double compartment cell separated
by a glass frit. The working compartment was fitted with a glassy carbon plate and an
Ag/AgNO3 reference electrode. The auxiliary compartment was fitted with a Pt gauze
electrode. The working compartment was filled with 50 mL of 33.3 mM acetic acid in a
0.1 M [(n-Bu4 N)]ClO4 DMF solution, while the auxiliary compartment was filled with
35 mL of 0.1 M [(n-Bu4 N)]ClO4 DMF solution, resulting in equal solution levels in both

Electrocatalytic copper(II) complex
575
compartments. Both compartments were sparged for 60 min with N2 and cyclic voltammo-
grams were recorded as controls. Complex 1 (4.41 μM) was then added and a cyclic vol-
tammogram was recorded. CPE in aqueous media was also conducted using an air-tight
glass double compartment cell separated by a glass frit. The working compartment was fit-
ted with a glassy carbon plate and a Ag/AgCl reference electrode. The auxiliary compart-
ment was fitted with a Pt gauze electrode. The working compartment was filled with 50 mL
of 0.25 M phosphate buffer solution (CH3CN : H2O = 2 : 3 as solvent) at different pH
values containing 0.1 M KCl of the supporting electrolyte, while the auxiliary compartment
was filled with 35 mL dihydrogen phosphate buffer solution. Both compartments were
sparged for 60 min with N2 and cyclic voltammograms were recorded as controls. Complex
1 (2.20 μM) was then added and a cyclic voltammogram was recorded. After electrolysis
under a nitrogen atmosphere, a 0.5 mL aliquot of the headspace was removed and replaced
with 0.5 mL of CH4. A sample of the headspace was injected into the GC. GC experiments
were carried out with an Agilent Technologies 7890A gas chromatography instrument.
2.2. Synthesis of 1
To a solution of 4 mM of 2-hydroxyacetophenone in 30 mL of methanol was added a
solution of 2 mM of ethylenediamine in 10 mL of methanol. The mixture was heated at
reflux for 3 h, giving a yellow powder. To the yellow mixture was added 4 mM
CuCl2⋅2H2O with stirring; the color of the solution changed from yellow to violet. Single
crystals were obtained from the filtrate which was allowed to stand at room temperature for
several days, collected by filtration, and dried in vacuo (0.506 g, 50.2%). The elemental
analysis results (found: C, 43.67; H, 3.67; N, 5.71. C18H18Cl2Cu2N2O2 requires C, 43.87;
H, 3.66; N, 5.69) were in agreement with the formula of the sample used for X-ray
analysis.
Table 1. Crystallographic data for 1.
Empirical formula
C18H18Cl2Cu2N2O2
Formula weight
492.32
λ (Å)
0.71073
Crystal system
Monoclinic
Space group
P2(1)/n
a (Å)
7.6557(19)
b (Å)
18.573(5)
c (Å)
13.117(3)
α (°)
90
β (°)
94.392(3)
γ (°)
90
1859.6(8)
V (Å3)
Z
4
Dc/Mgm−3
1.758
F (000)
992
θ range for data collection
Reflections collected/unique
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
2.69–25.50°
12,279/3406
0.6629 and 0.4467
3406/0/237
1.055
R1 = 0.0588, wR2 = 0.1608
R1 = 0.0702, wR2 = 0.1696

576
T. Fang et al.
2.3. Crystal structure determination
The X-ray analysis of 1 was carried out with a Bruker SMART CCD area detector using
graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All empiri-
cal absorption corrections were applied by using SADABS [19]. The structure was solved
using direct methods and the corresponding non-hydrogen atoms are refined anisotropically.
All hydrogens of the ligands were placed in calculated positions with fixed isotropic
thermal parameters and included in the structure factor calculations in the final stage of
full-matrix least-squares refinement. All calculations were performed using the SHELXTL
computer program [20]. Crystallographic data for 1 are given in table 1 and selected bond
lengths in table 2.
Table 2. Selected bond distances (Å) and angles (°) for 1.
Bond distances (Å)
Cu(1)–O(2)
Cu(1)–O(1)
Cu(1)–Cu(2)
Cu(2)–O(1)
Cu(2)–Cl(1)
1.886(4)
1.908(4)
3.0128(11)
2.097(4)
2.1925(17)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(2)–O(2)
Cu(2)–Cl(2)
1.903(4)
1.912(4)
1.951(4)
2.1903(19)
Bond angles (°)
O(2)–Cu(1)–N(1)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–N(2)
O(2)–Cu(1)–Cu(2)
O(1)–Cu(1)–Cu(2)
O(2)–Cu(2)–O(1)
O(1)–Cu(2)–Cl(2)
O(1)–Cu(2)–Cl(1)
O(2)–Cu(2)–Cu(1)
Cl(2)–Cu(2)–Cu(1)
176.38(18)
94.96(18)
90.05(19)
39.04(12)
43.63(11)
75.45(15)
132.43(13)
106.72(11)
37.51(11)
116.50(6)
O(2)–Cu(1)–O(1)
O(2)–Cu(1)–N(2)
O(1)–Cu(1)–N(2)
N(1)–Cu(1)–Cu(2)
N(2)–Cu(1)–Cu(2)
O(2)–Cu(2)–Cl(2)
O(2)–Cu(2)–Cl(1)
Cl(2)–Cu(2)–Cl(1)
O(1)–Cu(2)–Cu(1)
Cl(1)–Cu(2)–Cu(1)
81.60(16)
93.36(18)
174.76(17)
137.36(14)
131.15(14)
100.25(15)
139.81(16)
105.68(7)
38.90(10)
137.56(5)
Scheme 1. Schematic representation of the synthesis of 1.

Electrocatalytic copper(II) complex
577
3. Results and discussion
3.1. Structure analysis
The reaction of CuCl2⋅2H2O and the tetradentate amine phenol ligand at one molar ratio set
of 2 : 1 in the presence of triethylamine provides a dinuclear Cu(II) complex, 1 (scheme 1)
(yield = 50%), which is air stable in the solid and soluble in DMF, CH2Cl2, and CH3CN.
As shown in figure 1, two copper(II) ions are triply bridged by the phenolic oxygens, O1
and O2. The distance of Cu(1) and Cu(2) is 3.0128(11) Å, which is shorter than those of
Figure 1. Molecular structure of 1.
6
4
2
0
-2
-4
-6
-8
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (V vs Ag/AgNO )
3
Figure 2. Cyclic voltammogram of 1 in 0.1 M of [n-Bu4 N]ClO4 DMF solution at a glassy carbon electrode and a
scan rate of 100 mV s−1. Ferrocene is the internal standard.

578
T. Fang et al.
the reported carboxylate-bridged copper(II) complexes (from 3.1032(10) to 3.1716(12) Å)
[21, 22]. Two Cu(II) ions are put in different environments. Cu(1) is coordinated by two
amines and two phenolates from the amine-phenolate ligand in a slightly distorted square-
planar coordination geometry. The average Cu–O and Cu–N distances are 1.897(4) and
1.9075(4) Å, respectively. These distances are in the range of those of conventional Schiff
base copper (II) complexes of square-planar coordination [23–25].
Cu(2) has a distorted tetrahedral coordination geometry and is linked by two phenolate
oxygens from the imine-phenolate ligand and two Cl ions. The Cu–Cl bond lengths are
2.1903(19) – 2.1925(17) Å. The average distances between Cu(2) and O of the Schiff base
ligand is 2.024(4) Å, longer than that for Cu(1). The Cu(1)–O(1)–Cu(2) and Cu(1)–O(2)–
Cu(2) bridging angles are 97.47(15) and 103.45(18)°, respectively. The bridging Cu(1),
100 mV/s
200 mV/s
12
250 mV/S
8
4
300 mV/s
350 mV/s
400 mV/s
0
-4
-8
-12
-16
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (V vs Ag/AgNO )
3
-3
-4
-5
-6
-7
-8
-9
-10
-11
-12
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
1/2( 1/2 -1/2)
V
v
s
Figure 3. Scan rate dependence of precatalytic waves for a 1.69 mM solution of 1 (0.1 M [n-Bu4 N]ClO4) at scan
rates from 100 to 400 mV s−1
.

Electrocatalytic copper(II) complex
579
O(1), Cu(2), and O(2) is planar. The torsion angle N(1)–Cu(1)–O(2)–Cu(2) (−179.4(3)°) is
the same as that for N(2)–Cu(1)–O(1)–Cu(2).
3.2. Electronic spectrum
The spectrum was recorded in CH2Cl2, with main features at 361 nm (ε = 9800) and
557 nm (ε = 205) (figure S1, see online supplemental material at http://dx.doi.org/10.1080/
00958972.2014.998657). The shoulder observed at 361 nm corresponds to an LMCT
8
4
0
-4
C
HOAc
-8
9.6 mM
12 mM
18.7 mM
23 mM
43 mM
-12
-16
54.7 mM
-2.0
-1.5
-1.0
-0.5
0.0
0.5 1.0
Potential (V vs Ag/AgNO
3)
Figure 4. Cyclic voltammograms of a 1.69 mM solution of 1 with varying concentrations of acetic acid in DMF.
Conditions: rt, 0.1 M [n-Bu4 N]ClO4 as supporting electrolyte, scan rate: 100 mV s−1, GC working electrode
(1 mm diameter), Pt counter electrode, Ag/AgNO3 reference electrode, Fc internal standard.
140
Overpotential
120
492 mV
592 mV
100
80
60
40
20
0
692 mV
792 mV
892 mV
992 mV
0
20
40
60
80
100
120
Time (s)
Figure 5. Charge buildup vs. time from electrolysis of a 8.1 μM 1 in DMF (0.1 M [n-Bu4 N]ClO4) under various
overpotentials. All data have been deducted blank.

580
T. Fang et al.
transition between the bridging phenoxo and copper ions. The band observed at 557 nm is
characteristic of Cu(II) d–d transitions. Similar UV–visible features have been observed for
several related (μ-phenoxo) dicopper(II) complexes [26, 27].
3.3. Cyclic voltammetry studies
Cyclic voltammogram of 1 in DMF was measured at a glassy carbon electrode (1 mm
diameter) in the presence of 0.1 M [n-Bu4 N]ClO4 as supporting electrolyte. All potentials
+/0
in this study were given versus the FeCp2 couple internal standard. As shown in figure 2,
cyclic voltammogram of 1 exhibits one quasi-reversible wave at −0.74 V and two reversible
couples at −1.30 and −1.98 V versus Ag/AgNO3. We assign the reversible couples to the
metal-centered Cu2IV/III, Cu2III/II, and Cu2 processes, respectively, which are different
II/I
from those of the distorted square pyramidal copper(II) complex (E1/2 = 0.087 and
−0.125 V versus Ag/AgCl) [21]. As observed in figure 3, the current responses of the redox
events both at −0.74, −1.30, and −1.98 V show linear dependence on the square root of the
scan rate, which is indicative of a diffusion-controlled process, with the electrochemically
active species freely diffusing in the solution. Such distinctive potential prompts possible
usage of this complex as an electrocatalyst for HER.
3.4. Catalytic hydrogen evolution in DMF
To determine possible electrocatalytic activity of this complex, cyclic voltammograms of 1
were recorded in the presence of acetic acid. Figure 4 shows a systematic increase in icat
observed near −1.37 V with increasing acid concentration from 9.6 to 54.7 mM. This rise
in current can be attributed to catalytic generation of H2 from acetic acid, with catalytic
onset shift to more positive potentials (from −1.62 to −1.44 V). Based on equation (1), this
reduction occurred at an overpotential of 272 mV.
50
40
30
20
10
500
600
700
800
900
1000
Overpotential (mV vs SHE)
Figure 6. TOF (mol H2/mol catalysts/h) for electrocatalytic hydrogen production by
1 (8.1 μM) under
overpotentials vs. SHE.

Electrocatalytic copper(II) complex
581
(1)
EꢀHA ¼ EꢀHþ ꢁ ð2:303RT=FÞpKaHA
A number of control experiments were carried out to verify that 1 is responsible for the
catalysis. In particular, acetic acid, the free ligand, CuCl2, and the mixture of the free ligand
and CuCl2 were each measured under identical conditions. As can be seen in figures
S2–S5, the catalytic competency achieved with 1 is not matched by just ligand, CuCl2, or
the mixture of the free ligand and CuCl2, as might arise from dissociation of the ligand, nor
can it be accomplished with the ligand bound to a redox-inactive metal. Thus, a combina-
tion of the redox-active Cu ion and the ligand is essential for catalytic activity.
Further evidence for the electro-catalytic activity was obtained by bulk electrolysis of a
DMF solution of 1 (8.1 μM) with acetic acid (50 mM) at variable applied potentials using a
glassy carbon plate electrode in a double-compartment cell. A series of applied potentials
(a)
600
C
complex 1
500
0 mM
0.04 mM
0.08 mM
400
0.12 mM
0.20 mM
0.24 mM
300
200
100
0
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
Potential (V vs Ag/AgCl)
700
600
500
400
300
200
100
0
(b)
0.00
0.05
0.10
0.15
0.20
0.25
[Catalyst] (mM)
Figure 7. (a) Cyclic voltammograms of 1 at various concentrations (pH 7.0, scan rate: 100 mV s−1). (b) Plot of
catalytic peak current vs. concentrations of 1.

582
T. Fang et al.
were chosen, corresponding to the electrocatalytic potentials observed in cyclic voltammo-
grams. Figure 5 shows the total charge of bulk electrolysis of 1 in the presence of acid; the
charge significantly increased when the applied potential was added. Assuming every cata-
lyst molecule was distributed on the electrode surface and every electron was used for the
reduction of protons. According to equation (2) [28], we calculated turnover frequency
(TOF) for the catalyst reaching a maximum of 50.1 M of hydrogen per mole of catalyst per
hour (equation (3) in supplementary material and figure 6),
TOF ¼ DC=ðFꢂn1ꢂn2ꢂtÞ
(2)
where ΔC is the charge from the catalyst solution during CPE minus the charge from the
solution without catalyst during CPE, F is Faraday’s constant, n1 is the moles of electrons
(a)
pH 5.0
pH 5.7
pH 7.0
pH 8.0
500
400
300
200
100
0
0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0
Potential (V vs Ag/AgCl)
500
400
300
200
100
(b)
5.0
5.5
6.0
6.5
7.0
7.5
8.0
pH
Figure 8. (a) CVs of 1 showing the variation in catalytic current with pH (scan rate: 100 mV s−1). (b) Catalytic
current maximum (ic) as a function of pH.

Electrocatalytic copper(II) complex
583
required to generate 1 M of H2, n2 is the moles of catalyst in solution, and t is the duration
of electrolysis.
3.5. Catalytic hydrogen evolution in aqueous media
We also tried to explore the catalytic hydrogen evolution in aqueous media, a much more
attractive medium for the sustainable generation of hydrogen. CVs were conducted in
0.25 M phosphate buffers at different pH values. As shown in figure 7, in the absence of
1, a catalytic current was not apparent until a potential of −1.5 V versus Ag/AgCl was
attained. With addition of 1, the onset of catalytic current was observed at −1.36 V versus
Ag/AgCl, and the current strength increased significantly with increasing concentrations of
1 from 0.00 to 0.24 mM. When concentration of 1 reached 0.24 mM, the potential at
−1.05 V and current strength did not change obviously. The potential moved positive
about 450 mV compared to that in the absence of 1. Furthermore, it is found that the
catalytic current maximum of 1 was also dependent on pH values of buffers (figure 8),
indicating this catalyst is functioning in a diffusion-controlled regime and is molecular in
nature.
Catalytic hydrogen production can also be achieved with 1 in buffer. Figure 9 shows the
total charge of bulk electrolysis of the solution containing 1 at pH 7.0. H2 production can
be observed at pH 7.0. When the applied potential was 1.40 V versus Ag/AgCl at pH 7.0,
the maximum charge was only 23.8 mC during 2 min of electrolysis in absence of 1. Under
the same conditions, the charge reached 234 mC with addition of 1 (8.1 μM) (figure S6),
which was confirmed to be H2 by GC analysis. The evolved H2 was analyzed by gas chro-
matography, figure 10, which gave ~12.8 mL of H2 over an electrolysis period of 1 h with
a Faradaic efficiency of 96% for H2 (figure 11). TOF for electrocatalytic hydrogen produc-
tion by 1 is 104.3 (pH 7.0) (equation 4 in supplementary material) moles of hydrogen per
mole of catalyst per hour (figure S7).
300
Overpotential
250
749mV
779mV
809mV
839mV
869mV
200
150
100
50
0
0
20
40
60
80
100
120
Time (s)
Figure 9. Charge buildup of 1 (8.1 μM) vs. applied potentials at various pH values. All data have been deducted
blank.

584
T. Fang et al.
4000
3000
2000
1000
0
H
2
N
2
O
2
CH
4
0
2
4
6
8
Time (min)
Figure 10. GC traces after a 1-h CPE at 839 mV vs. SHE of 8.1 μM 1 in 0.25 M phosphate buffer, pH 7.0. A
standard of CH4 was added for calibration purposes.
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
0
50
100
150
200
250
Time (min)
Figure 11. Practical (red) and theoretical (black) pH changes assuming a 100% Faradaic efficiency of 1 during
P
It
electrolysis (The theoretical pH change over time can be calculated by the equation pH ¼ 14 þ 1 g
where
FV
I = current (A), t = time (s), F = Faraday constant (96,485 CM−1), V = solution volume (0.05 L)) (see http://dx.doi.
org/10.1080/00958972.2014.998657 for color version).
4. Conclusion
We have constructed a copper complex, 1. Electrochemical studies show that 1 can generate
dihydrogen from acetic acid and from water. This discovery has established a new chemical
paradigm for creating reduction catalysts that are highly active and robust in both organic
and aqueous media. Ongoing efforts are focused on modifying ligands and related com-
plexes to further facilitate new functional studies, especially emphasis on chemistry relevant
to sustainable energy cycles.

Electrocatalytic copper(II) complex
585
Supporting information
CCDC 661639 contains the supplementary crystallographic data for this paper. This data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK;
Fax: (+44) 1223 336 033; or E-mail: deposit@ccdc.cam.ac.uk.
Funding
This work was supported by the National Science Foundation of China [grant number 20971045], [grant number
21271073]; the Student Research Program (SRP) of South China University of Technology [grant number
B15-B7050170]; the Science Foundation of Guizhou Province [grant number J-2012-2190].
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