Double bridged μ2-Halide Cu (II) complexes for the electrocatalytic reduction of CO2

Article abstract of DOI:10.1016/j.electacta.2019.134675

Two dicopper (II) complexes of general formula [CuLX₂]₂ {L = 2, 5-diphenyl-3, 4(2-pyridyl) cyclopenta-2, 4-dien-1-one, X = Cl (1) or Br (2)} have been synthesized and characterized by elemental analysis, IR, UV–Vis spectroscopic meth

Full text of DOI:10.1016/j.electacta.2019.134675

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Double bridged μ -Halide Cu (II) complexes for the electrocatalytic reduction of CO
2
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Maryam Sadehvand, Ahmad Amiri, Farzaneh Fadaei Tirani, Jun Gu, Kurt Schenk-Joß
PII:
S0013-4686(19)31546-4
DOI:
https://doi.org/10.1016/j.electacta.2019.134675
EA 134675
Reference:
To appear in: Electrochimica Acta
Received Date: 5 June 2019
Revised Date: 9 August 2019
Accepted Date: 10 August 2019
Please cite this article as: M. Sadehvand, A. Amiri, F.F. Tirani, J. Gu, K. Schenk-Joß, Double bridged
μ -Halide Cu (II) complexes for the electrocatalytic reduction of CO , Electrochimica Acta (2019), doi:
2
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https://doi.org/10.1016/j.electacta.2019.134675.
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Double Bridged µ2-Halide Cu (II) Complexes for the Electrocatalytic
Reduction of CO2
Maryam Sadehvand a, Ahmad Amiri a, *, Farzaneh Fadaei Tirani b, Jun Gu b, Kurt Schenk-Joß c
aDepartment of Chemistry, College of Science, University of Tehran, Tehran 14155-6455, Iran.
bInstitute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015
Lausanne, Switzerland.
cInstitute of Physics, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.

Abstract:
Two dicopper (II) complexes of general formula [CuLX2]2 {L=2, 5-diphenyl-3, 4(2-pyridyl)
cyclopenta-2, 4-dien-1-one, X= Cl (1) or Br (2)} have been synthesized and characterized by
elemental analysis, IR, UV-Vis spectroscopic methods. The crystal structures of both complexes have
been determined by single-crystal X-ray diffraction; 1 and 2 have binuclear structures and Cu (II)
centers in both complexes adopt a distorted square pyramidal geometry. Two [CuLX] (X=Cl or Br)
units in both complexes are linked via µ-X coordination bridge modes with the Cu-Cu distances of
3.534 and 3.716 Å for 1 and 2 respectively. The electrochemical behavior of the free L ligand and the
corresponding Cu (II) complexes was studied in acetonitrile. The cyclic voltammetry of the
complexes 1 and 2 show three ligand based reduction processes and two metal-centred reductions that
are assigned to copper (II) to copper (I) and copper (I) to copper (0). The electrocatalytic activity for
the reduction of CO2 of the two complexes was investigated; both complexes are active for the CO2
reduction and result in the formation of CO.
Keywords: Binuclear complexes, Copper (II) complexes, Crystal Structure, Electrocatalyst, CO2
reduction
1- Introduction
The current global energy landscape heavily relies on the burning of fossil fuels including coal, oil
and natural gas. Indeed, these sources of energy represent more than 85 percent of the energy used
worldwide [1]. Since fossil fuels are non-renewable, concerns about a possible energy crisis caused by
a dearth of fossil fuels are growing. Energy extraction from the burning of fossil fuels also results in a
foreboding emission of carbon dioxide (CO2). As this increase in the CO2 concentration in the
atmosphere is linked to global warming and climate changes, there is an urgent need for reducing the
accumulation of CO2 in the atmosphere. Indeed, Solomon et al.’s climate modelling studies have
shown that the effect of the latest increase in the CO2 concentration of the past decades would take up
to 1000 years to subside, even if new emission could be prevented. Hence, not only the release of CO2

into the environment has to cease as soon as possible to minimise the adverse impact of climate
changes, but the captured CO2 should also be turned into valuable fuels [2-4].
Different methods, including radio-, thermo-, photo-, electro- and bio-chemical ones, have been
reported for the conversion of CO2 to other chemicals like CO, formate, methane and methanol [5].
Among them, the electrochemical reduction of CO2 has drawn much attention because it has several
advantages: (1) Electro-chemical conversion can be performed at room temperature and ambient
pressure. (2) The supporting electrolytes can be fully recycled so that the overall chemical
consumption can be minimized to simply water or wastewater. (3) The electro-chemical reaction
systems are compact, modular, on-demand, and easy for scale-up applications. (4) The necessary
electricity to drive the process can be obtained, without generating any new CO2, from solar, wind,
hydroelectric, geothermal, tidal, and thermoelectric processes [6]. The reduction of CO2 is, however, a
difficult task because the carbon atom is in its most oxidized state and is thermodynamically stable.
Thus, the electro-chemical CO2 reduction proceeds only at high over-potential. These large over-
.
-
potentials are related to the initial conversion of the CO2 molecule into the CO
radical anion.
2
Electro-catalysts are therefore primordial for decreasing the over-potentials required for an electro-
chemical reaction and for opening up routes for energetically less demanding reactions [7]. Amongst
the catalysts used for the reduction of CO2, metallic catalysts need large over-potentials for CO2
reduction and the study of the mechanisms of these systems are usually difficult. Moreover, the
electrode surface suffers from poisoning by the intermediates or products of catalysis [8]. Transition
metal complexes can serve as molecular catalysts in the reduction of CO2 at lower over-potentials
through undergoing multiple reductions and thus storing multiple redox equivalents in both the
ligand and metal ion. Many complexes with different ion centres and various macrocyclic,
polypyridyl and phosphine ligands have been used as an CO2 reduction electro-catalysts [9-11].
Among them, the polypyridyl class of ligands offer many advantages. Indeed, polypyridyl ligands
have been shown to be redox active and capable of assisting in mediating multi-electron
transformations. Additionally, polypyridyl ligands are ubiquitous archetypes within inorganic
complexes and have generally been shown to be stable and synthetically versatile. These ligands

afford the ability to precisely control coordination geometries and electronic structures around a
desired metal centre [10-12].
In recent years, copper (II) complexes containing pyridyl ligands and amine donors have been studied
as electro-catalysts for the reduction of CO2 [13-22]. Because in these complexes nitrogen donor
atoms in the backbones of pyridyl ligands can be bound to CO2 in the form of carbonate [23-24].
Examples of such compounds are [N, N'-bis(2-R-phenyl-salicylidenaminato)Cu(II)] (R= Me, Cl),
[N,N'-bis (salicylaldimine)propylenediimino Cu(II)], [Cu2(m-xpt)2(NO3)2](PF6)2
xpt)2(Cl)2](PF6)2 (xpt= m-xylylene pyridyltriazole group) [23-24].
,
[Cu2(m-
We report the synthesis and characterization of a series of new binuclear copper complexes containing
the ligand L with bromo and chloro bridging ligands (Scheme 1). We also report the crystal structures
as well as the spectroscopic and electrochemical properties of the two Cu (II) complexes. The electro-
catalytic activities of these two complexes in the reduction of CO2 has also been investigated.
2- Experimental
2.1. Materials
All solvents and reagents were obtained commercially and used as received from Aldrich and Merck.
Silica gel 60 (70- 230 mesh, Fluka) was used for dehydration. The purity of CO2 and argon (Ar) was
99.99%, and they were used directly.
2.2. Physical measurements
Elemental analyses were performed on a Perkin–Elmer 2400II CHNS-O elemental Analyzer. UV–Vis
spectra were recorded on a double beam Halo DB-20 spectrophotometer. Infrared spectra (KBr
pellets) were recorded between 4000 and 400 cm-1 on a Rayleigh WQF-510A FTIR spectrometer
spectrophotometer. The electro-chemical measurements were performed on a SAMA 500 Research
Analyzer in a three electrodes system consisting of a glassy-carbon-working-electrode, a platinum
disk auxiliary electrode and an Ag wire as pseudo-reference electrode. The glassy carbon working
electrode (Metrohm 6.1204.110) with 2.0 ± 0.1 mm diameter was manually cleaned with 1 µm

alumina polish prior to each scan. Tetrabutylammonium hexafluorophosphate (TBAH) was used as
the supporting electrolyte. Acetonitrile was dried over CaH2. These solutions were deoxygenated by
purging them with Ar for 5 min. All electro-chemical potentials were calibrated versus the
ferrocene/ferrocenium (Fc/Fc+) couple as an internal reference under the same conditions [25].
Chronopotentiometry curves were collected out on a Gamry Reference 3000 electrochemical
instrument using a homemade air-tight two-chamber cell. Working and reference electrodes were
fixed in one chamber and the counter electrode in the other; the two chambers were separated by a
frit. Belt-shaped carbon cloths (1071 HCB) with a 1 cm2 area exposed to the electrolyte were used as
the working electrode. A double-junction protected Ag/AgNO3 electrode (with acetonitrile containing
1 mM AgNO3 and 0.5 M TBHP as filling solution) was used as reference electrode, and a platinum-
wire counter-electrode. The electrolyte in the chamber of the working electrode was vigorously stirred
during the experiments. The working electrode side was bubbled with CO2 for 30 min to get the
electrolyte saturated. The inlet gas was pre-humidified by acetonitrile to minimize the evaporation of
the electrolyte. Online test of gas-phase products was done on a homemade gas chromatograph (GC)
equipped with a Carboxen®-1010 PLOT capillary column and a thermal conductivity detector (VICI)
and helium as carrier gas. The flow rate of CO2 was set to 10 sccm.
2.3. Synthesis
2.3.1. Synthesis of the red ligand (L)
The red ligand 2,5-diphenyl-3,4(2-pyridyl)cyclopenta-2,4-dien-1-one (L) was synthesized by thermal
dehydration, over silica gel, of its white diol precursor, LH2(OH)2 according to the method reported in
the literature [26].
2.3.2. Synthesis of Cu2L2Cl4 (1)
84.5 mg (0.2 mmol) of the red ligand L were added to a solution of CuCl2 (26 mg, 2mmol) in acetone
(15 ml) in an ice bath. The solution was stirred for 15 min at room temperature. Diethyl ether was
then allowed to diffuse, at room temperature, slowly into the deep brown solution. The brown
crystalline precipitate was filtered off and dried in vacuum. Crystals of (1) suitable for X-ray

crystallography were obtained by recrystallization from dichloromethane–acetone (10:1 v/v) at room
temperature after three days. Yield: 56%. Anal. Calc. for C54H36 Cl4Cu2N4O2: C, 62.26; H, 3.48; N,
5.38. Found: C, 61.64; H, 3.61; N, 6.14%. FT-IR (KBr, cm-1) υmax : 1715 (s, C=O), 1585 (m, C=N),
1433 (m, C=C), UV–Vis (acetonitrile): λmax (nm) (ε, L mol-1 cm-1): 514 (300), 297 (3270), 272
(34200).
2.3.3. Synthesis of Cu2L2Br4 (2)
The complex 2 was prepared by the same method used for 1, except that CuBr2 was used instead of
CuCl2. Recrystallization from dichloromethane gave brownish tabular crystals after one day. The
crystals were filtered off, washed with a small amount of cold methanol, and dried under vacuum.
Yield: 60%. Anal. Calc. for C54H36 Br4Cu2N4O2: C, 53.18; H, 2.98; N, 4.59. Found: C, 53.50; H, 3.82;
N, 5.04%. FT-IR (KBr, cm-1) υmax : 1715 (s, C=O), 1583 (m, C=N), 1433 (m, C=C), UV–Vis
(acetonitrile): λmax (nm) (ε, L mol-1 cm-1): 517 (465), 292 (54171), 270 (58343).
Scheme 1. Synthetic route to Cu2L2X4 (X= Cl, Br)
2.4. X-ray crystallography
Suitable tabular single crystals of 1 were recrystallised from dichloromethane/acetone and of 2 from
dichloromethane. BRAGG intensities were collected at T= 100 K, using Cu Kα radiation (λ = 1.54184
Å) on a Rigaku SuperNova dual system diffractometer equipped with an Atlas CCD detector. The
datasets were reduced and corrected for absorption, with the help of a set of faces enclosing the
crystals as snugly as possible, with CrysAlisPro [27].
The solutions and refinements of the structures were performed by the latest available version of

ShelXT [28] and ShelXL [29]. All non-hydrogen atoms were refined anisotropically using full-matrix
least-squares based on |F|2. The hydrogen atoms were placed at calculated positions by means of the
“riding” model where each H-atom was assigned a fixed isotropic displacement parameter with a
value equal to 1.2 Ueq of its carrier C-atom (1.5 Ueq for the methyl groups). Details of the data
collection, structure solution and refinement of 1 and 2 are compiled in Table 1 and some geometrical
items in Table S1.
The structure 2, was refined as a two-component inversion twin yielding the value of 0.30(4) for the
BASF parameter. In this structure, a solvent mask was calculated with the help of the solvent-masking
program in OLEX2 [30] and 172.0 electrons were found in a volume of 1024.0 Å3 in one void. This is
consistent with the presence of one solvent molecule of dichloromethane per formula unit, which
accounts for 168.0 electrons.
Fig.1. The structure of the binuclear complex 1. Displacements ellipsoids are drawn at the 50% level. The dimer
contains a centre of inversion midway between the copper and the µ2-chlorine atoms.
The complex 1 grows in a tabular (100) habit and develops the {100} and {001} pinacoids and the
{011} prism. The complex 2 grows in a surprisingly centrosymmetric tabular (001) habit and
develops the {110} prism and the {001} pinacoid unforeseen in this point group.
Table 1. Data collection, structure-solution and refinement for 1 and 2.

Compound
Cu2L2Cl4 (1)
Cu2L2Br4.CH2Cl2 (2)
Formula
C55H38Br4Cu2N4O2Cl2
1.525
4.844
C54H36Cl4Cu2N4O2
1.495
3.637
Dcalc./ g cm-3
µ
/mm-1
Formula Weight
Colour
1219.59
clear intense brown
DCM
Tabular (100)
0.33×0.12×0.01
100.0(1)
1041.75
clear light green
DCM/ Acetone
Tabular (100)
0.23×0.15×0.02
100.0(1)
Cryst. Solvents
Habitus
Size/mm3
T/K
Crystal System
Space Group
a/Å
b/Å
c/Å
orthorhombic
Pca21
10.8257(2)
25.2316(5)
19.4423(4)
90
monoclinic
C2/c
22.7229(17)
9.7235(7)
22.4497(13)
90
111.088(8)
90
4628.0(6)
4
°
α
β
γ
/
/
°
90
°
90
/
V/Å3
5310.66(18)
4
Z
Z'
1
0.5
Wavelength/Å
Radiation type
1.54184
1.54184
CuKα
4.221
76.448
11573
4776
3474
0.0406
298
CuKα
°
4.444
76.623
26911
8875
7892
0.0405
595
θ min
θ max
/
°
/
Measured Refl.
Independent Refl.
Reflections with I > 2(I)
Rint
Parameters
Restraints
0
0
Largest Peak/e Å-3
Deepest Hole/e Å-3
GooF
1.381
-0.526
1.037
0.1057
0.1006
0.0484
0.0409
1.419
-0.849
1.038
0.1771
0.1514
0.0862
0.0595
wR2 (all data)
wR2
R1 (all data)
R1

Fig.2. The structure of the binuclear complex 2. Displacements ellipsoids are drawn at the 50% level.
3- Result and Discussion:
3.1. Synthesis of complexes 1 and 2
Two binuclear complexes containing chloro (1), and bromo (2) halogens in basal (µ2) and apical
positions and the bidentate ligand L were prepared in acetone according to Scheme 1. This solvent
imposed itself because of its more soluble and stabler complexes compared to those in polar and
coordinating solvents such as alcohols, acetonitrile, DMF etc.; indeed in the latter strongly solvated
species may compete with the formation of the target-complexes. The N–Cu–N bite-angle (84.15(14)°
for 1, 86.7(2)° for 2) is a valuable parameter controlled by the linkage between the two pyridine-rings
by the CPD-one, but in these structure the free rotation of the pyridyl rings provides some slack (the
pyramid only displays symmetry C1). The kinetic chelate strain effect [31] arising from an increase in
the Cu–N bond distance (2.024(5)Å for 1, 2.033(6)Å for 2) and the interaction of the solvent with the
copper(II) ions are both involved in the process of formation of the complexes.
3.2. Spectral Studies
The FT-IR spectra of the free red ligand L and its complexes 1 and 2 are shown in ESI (Fig. S1) and
full data are presented in section 2.3. The IR spectrum of L exhibits bands between 2860 – 3100 cm-1

which can be attributed to C-H stretching modes of the pyridine rings. Two strong bands at 1715 and
1599 cm-1 correspond to C=O and C=N stretching vibrations and are red-shifted by 5-20 cm-1 [26].
Additional shifts are expected for the ring breathing-mode of L occurring at 990 cm-1 and between
1020 – 1025 cm-1 in the IR spectra of the complexes [32, 33].
The electronic absorption spectra of L and its metal complexes were recorded in acetonitrile and full
data are listed in section 2.3. The intra-ligand n→ π* and π→π* transitions of L appear at 492 (ε=
1167), 324 (ε = 5021) and 258 nm (ε = 200010). These transitions are slightly red-shifted in the
corresponding metal complexes. In the UV-Vis spectrum of 1 (Fig. 3), the peaks in the region of 270-
370 nm are assigned to ligand-centred transitions, while the broad absorption bands at 514 nm (ε=
300) and 517 nm (ε = 465) are attributed to d-d transitions in 1 and 2, respectively, and are in
agreement with corresponding ML5 complexes with C4v symmetry [32-38].
Fig.3. UV-Vis spectra of 1 (a) (700-265 nm, 1 × 10-5 M; 700-350 nm, 1 × 10-4 M) and 2 (b) (700-260
nm, 1 × 10-5 M; 800-350 nm, 1 × 10-4 M) in acetonitrile.
The d-d transition in 1 appears at shorter wavelength than in 2 by ~3 nm, presumably owing to a more
efficient overlap between the donor orbitals of the L and Cl¯ ligands with the d-orbitals of the copper
(II) centre. This observation is in accord with the shorter Cu–Npy (2.032(3) Å) and Cu–Clbridge
(2.4590(8) Å) bonds in 1 as compared to Cu–Npy (2.041(3) Å) and Cu–Brbridge (2.6364(6) Å) in 2.
(Table S1).

3.3. Crystal Structures
The crystallographic and refinement data are summarized in Table 1 and selected bond distances,
bond and torsion angles are given in Table S1. The molecular structures of 1 and 2 are illustrated in
Figs. 1, and 2, respectively; the copper atoms are coordinated by the bidentate red ligand L and three
halogen atoms.
1 and 2 resemble each other a lot in the solid state (Fig. S2), but are yet different enough so as not to
build-up isostructural structures. In particular, 1 crystallises in the monoclinic space group C2/c but 2
in the orthorhombic, non-centrosymmetric, space group Pca21. The binuclear complex in 1 displays
point symmetry ī but that in the 2 presents only point symmetry 1. But the two complexes are
topologically similar and present close ADDISON-parameters (1500918), too (1: 0.22, 2: 0.17),
meaning that they form slightly distorted square pyramids. Each copper of the dimeric unit of a
complex is connected to two nitrogen atoms of the pyridine rings from L, a chloride ion in the apical
position and two chloride ions bridging the Cu(II) centres in the basal plane to form a distorted square
pyramidal geometry.
The basal planes in 1 are defined by N1, N2, Cl1 and Cl2 for Cu1 (Fig. 1). The bridging Cu2Cl2
moiety is planar with a Cu-Cl1-Cu11 angle of 91.75 (4). Also the Cu-Cl bond distances in the bridged
Cu2Cl2 part are unequal, 2.3389 (12) and 2.579 Å, which are longer than the terminal Cu-Cl2 bond
distance 2.2260 (12) Å. the Cu-Cu distance is 3.543 Å longer than the sum of the two van der Waals
radii for copper atom, thus indicating that there is no metal-metal bonding. These values are in good
agreement with other dimeric copper chloride compounds [35, 37, 39-41].
The basal planes in 2 are by N1, N2, Br1, Br3 around Cu1 and by N3, N4, Br2 and Br4 for Cu2 (Fig.
2). The Cu-Br bond distances in the Cu2Br2 planar bridge are 2.4355 (14) and 2.8359 (12) Å, and the
°
Cu-Br-Cu and Br-Cu-Br bond angles are 89.54 (4) and 90.67 (4)°, respectively, close to 90°, as
expected for a square pyramidal geometry around Cu (II) ion. The Cu-Cu distance of 3.716(2) Å,
longer than the sum of the two van der Waals radii for copper atoms indicates that there is no metal-
metal bonding. These values are in good agreement with similar compounds reported in other papers
[35, 38, 41, 42].

The structure of 1 consists of (100) layers. Each half layer is built of dimers the centre of which lie at
-
(
, 0) and of which the long axes point to either [011] or [011]. Nearest dimers in a (100) chunk are
related by a { |00 } glide reflection and nearest half layers are related by {
|
} glide reflection.
The structure owes its cohesion to a plethora of weak C-H...Cl hydrogen bonds (Table S2), C-H…π
interaction and, of course, van der Waals forces. Note that the lengths of the hydrogen bonds nicely
correspond to Steiner’s values [43]. All of the weak interactions are intra-molecular (Fig. S3).
The structure of 2 consists of (010) layers in which the long axes of the dimers are all parallel and
-
point, in subsequent blocks, to [011] and [011]. The structure owes its cohesion to a plethora of weak
C-H…O and C-H...Br hydrogen bonds (Table S2), C-H…π interactions and, of course, van der Waals
forces. Note that the lengths of the hydrogen bonds nicely correspond to Steiner’s values [38]. All, but
three of the weak interactions are intra-molecular; the inter-molecular ones (parallel to [010]) connect
(010) chunks and induce chains along [100] (Fig. S4).
3.4. Electrochemical Studies
The electrochemical properties of the free ligand L and its metal complexes 1 and 2 were investigated
at 25˚C, under an argon atmosphere. Cyclic voltammograms of each compound in 1mM acetonitrile
solution were recorded (with 0.1M of TBAH as the supporting electrolyte). Our previous studies [44-
46] on the electrochemical properties of ligands containing pyridine residues showed that these
ligands were electro-active in the potential range from 0 to -2 V.
The cyclic voltammogram of L (Fig. S5) shows three redox processes at -0.83, -1.32 and -1.54 V that
can be assigned to the reduction of the pyridyl rings and lead to the formation of a radical anion [44-
46].
Fig. 4 shows a typical cyclic voltammogram of 1 from open circuit potential towards negative values.
Three well defined reduction waves at -0.63, -0.81 and -1.63 V and one low current wave at -1.09 V
were detected in primary scans. It is not straightforward to assign these cathodic peaks to either metal-
centred or ligand-centred processes, but the literature suggests that the two reductions at -0.63 V and -
1.09 V are related to CuII/I and CuI/0 respectively, while the two reduction peaks at -0.81 V and -1.63

V can be attributed to the reduction of the pyridyl rings of coordinated L in 1 [20, 47-49]. More
evidence comes from the cyclic voltammograms of 1, measured in different potential ranges. As seen
in Fig. 4 by limiting the potential window between 0 ‒ -0.9 V (red curve) the oxidation peak at -0.32
V corresponding to Cu0/I disappears and confirms the statement that the CuI/0 reduction process does
not accrues until -0.9 V. The existence of the oxidation peak around -0.06 V was related to the
reduction at -0.63V attributed to the CuII/I couple. By extending the potential window to -1.15 V
(green curve), the oxidation peak corresponding to Cu0/I process appears and confirms that the
reduction at -1.09 V is due to the CuI/0 couple. The occurrence of a second metal-centred reduction
processes (CuI/0) at negative potentials implies that the species reduced are not stable and that metal
ions escape from the complex. These results are supported by the appearance of a very intense
oxidative wave at -0.32 V, characteristic of the release of deposited metallic copper from the surface
of the electrode.
Fig. 4. Cyclic voltammogram of 1 (1 mM) in the potential ranges of (a) 1.25 ‒ -2.2 V (blue curve), (b) 0.02 ‒ -
0.95 V (red curve), (c) 0.02 ‒ -1.15 V (green curve) in acetonitrile at 298 K, under an Ar atmosphere and at a
scan rate of 0.1 V s-1.

The next oxidation peak at -0.06 V was attributed to CuI/II and the electrochemically quasi-reversible
oxidation process at +0.62 V to the CuII/III couple, which is consistent with those reported previously
[47, 50].
Fig. 5. CuII/III oxidation process of 1 under an Ar atmosphere at scan rates of 100 – 500 mV s-1.
Fig. 5 reveals a cathodic shift of the CuII/III oxidation potential with a scan rate increasing from 0.1 to
0.5 Vs-1, and thus confirms the quasi reversible behaviour of the electrochemical reaction. The peak
current between 0.10 and 0.50 V s-1 (Fig. 5 inset) depends linearly on the square root of the scan rate;
such dependence is typical of a diffusion-controlled current. The number of electrons transferred in a
diffusion-controlled process was estimated as two, by using of Bard’s equations (see Supplementary
Information) [51-52].
The cyclic voltammogram of 2 in acetonitrile (Fig. S6) shows features similar to those observed for 1.
In general, the redox couples of 2 are shifted to more positive values relative to those observed for 1,
demonstrating the weaker electron-donation character of Br¯ in 2 relative to Cl¯ in 1. This behaviour
confirms the structural data obtained from the X-ray diffraction analysis (Table S1). The average bond
lengths of Cu-Clbridge (2.4590(8) Å) and Cu-Npy (2.032(3) Å) in 1 are shorter than Cu-Brbridge
(2.6364(6) Å) and Cu-Npy (2.041(3) Å) in 2.

3.5. Study of the activity of 1 and 2 in the Electrocatalytic CO2 reduction
The electrochemical reduction of carbon dioxide by means of homogeneous electrocatalysts has
always been of interest because of its low over-potential compared to that required in the direct
reduction on metal electrodes [53]. Since the active redox sites of the polypyridyl ligands are capable
of storing multiple redox equivalents, L just might play an active role in catalysis [54-55]. The
presence of multiple equivalents of electrons in a catalyst can facilitate multi-electron reactivity with
.
-
CO2 and evade the energetically costly one-electron reduction of CO2 to CO . Therefore, the reduced
2
polypyridyl rings just might act as an electron-reservoir in the electrocatalytic reduction of CO2 in
which the metal centre would mediate the transfer of these reducing equivalents to CO2, leading to an
electrochemical reduction [14].
Once the electrochemical behaviour of 1 and 2 had been studied, the catalytic activity of the latter was
also investigated in CO2 reduction by bubbling CO2 through an acetonitrile solution of the
corresponding complexes and comparing with the results obtained under an argon atmosphere under
the same experimental condition. The cyclic voltammograms of 1 in acetonitrile solution in the
absence (blue) and in the presence of CO2 (red) are presented in Fig. 6. The reduction potentials
between -0.63 and -1.09 V show no significant changes neither in potential nor in current, while the
last ligand-based reduction at -1.63 V is slightly shifted and accompanied by a well pronounced
cathodic current peak, corresponding to the CO2 reduction process (more than 2.3-fold increase in
current density with respect to that observed in the absence of CO2). Moreover, return anodic waves
corresponding to the ligand disappeared, too; this indicates that the reduced form of the L is
consumed by the reaction with CO2 [56]. So, we may conclude that a homogeneous electron transfer
mechanism in the outer sphere between CO2 and one of the reduced forms of the complex 1 is
responsible for the reduction of CO2. Additional evidence for this speculation is the appearance of
metal-centred CuI/0 and CuI/II oxidations in the voltammogram of 1 in the presence of CO2.

Fig. 6. Cyclic voltammetry of 1 (1.1 mM) under Ar (blue) or CO2 (red) in acetonitrile with 0.1 M TBAH
supporting electrolyte at 100 mV s.
Complex 2 also shows a 2.8-fold increase in current density of the CO2 irreversible reduction wave
when is saturated with CO2 in comparison to the saturated complex with Ar (Fig. 7). The other quasi-
reversible reduction waves do not show any significant increase in current density. So, complex 2 can
also be used as a moderate electro-catalyst for the reduction of CO2 (Fig. 7).

Fig. 7. Cyclic voltammogram of 2 (1 mM) in acetonitrile at 298 K, under an Ar (blue) and CO2 (red)
atmosphere and at a scan rate of 0.1 V s-1.
A comparison between the electrocatalytic results of 1 with those obtained for 2 reveals that the
presence of the Br¯ ligand in 2 with a weaker electron-donation character versus Cl¯ in 1 leads to an
increase in the catalytic activity of 2 relative to 1. The complex 2 with bromo ligands therefore seems
to perform better in the molecular catalysis of CO2 [57].
To determine the products of the reduction of CO2, long-term chrono-potentiometric electrolysis was
performed in 0.5 M acetonitrile solutions of TBHP as electrolyte and 1 mg·mL-1 concentration of
catalysts (1 and 2) using a homemade air-tight two-chamber cell (Fig. S7 and Fig. S8). The
experimental procedure of measurements was described in section 2.2.
As shown in Fig. 8 and Table 2, CO was the only gaseous product of the electrocatalytic reduction of
CO2 in the presence of the complexes 1 and 2. Carbon monoxide was not formed, however, when the
electrolysis was carried out in the absence of CO2; CO is therefore shown to be produced by the
electrocatalytic reduction of CO2.

Table 2. Faraday efficiencies of different products of 1 and 2 during chronopotentiometry tests at -20 mA·cm-2 in CO2
saturated 0.5 M TBHP acetonitrile electrolyte with and without H2O.
Complex
Faradic Efficiency (%)
CO
HCOO¯
H2
Without water
With water
10
23
25
14
--
--
--
--
--
47
--
1
2
Without water
With water
52
The role of water in the electrochemical reduction of CO2 by 1 and 2 was investigated by adding a
known concentration of H2O to the electrolyte solution. The reduction of CO2 in the presence
Fig. 8. GC analysis of a headspace sample of complexes 1 and 2 after 30 min electrolysis experiments in
absence and presence of 0.5 M H2O.
1
of H2O yielded CO and H2 for both complexes (Fig. 8 and Table 2). H-NMR of the electrolyte
solution (containing 0.5 M of H2O and the electro-catalyst) after electrolysis did not reveal any
production of formate. As shown in Table 2, the Faradic-yield of CO upon addition of 0.1 M water

increased from 10% to 23% in the gaseous phase for 1, but decreased from 25% to 14% for 2. It can
be concluded that the presence of Br¯ groups, in particular their electron-withdrawing capacity makes
electron transfer easier in 2 with respect to 1.
4- Conclusion
Two binuclear copper (II) complexes of general formula [CuLX2]2 [X= Cl¯ (1), Br¯ (2)] binding the
ligand L were synthesized and characterized by different spectroscopic methods. X-ray diffraction
revealed the two Cu (II) centres to lie in distorted square coordination pyramids, subtended by the
monarch-like1 bidentate ligand L and three halide ligands, two of which share an edge in order to
form a dimer. The bromo-complex, 2, incorporates a DCM solvate molecule, hidden behind the wings
of the monarch, into its non-centrosymmetric structure. Both complexes owe the cohesion of their
layer structures to a plethora of weakest interactions in the solid state. The cyclic voltammetry of both
complexes in acetonitrile solution showed three L-centred reduction waves and three Cu-based redox
processes under Ar-atmosphere. Electrocatalytic studies of 1 and 2 by cyclic voltammetry showed that
these complexes can perform an electrochemical reduction of CO2 to CO with moderate FEs and good
stability. The electrocatalytic activity of the bromo bridging-ligands turned out to be superior to that
of the chloro ones; this difference is conjectured to be due to an easier electron transfer from the
catalyst to CO2. We shall pursue the flight of the monarch by implementing various other ligands in
order to glean more insight into the mechanisms of these redox processes.
Acknowledgement
The University of Tehran is gratefully acknowledged for the financial support (01/ 1/ 391832) of this
research. This work has been supported by Iran National Science Foundation (94001794).
1
Indeed, the flexible phenyl moieties rather perfectly mimic the wings and movements of a generic butterfly
and our favourite is the Monarch.

Supplementary information:
Crystallographic data have been deposited with the CCDC under the following codes: 1 (1862696)
and 2 (1862695). Copies of the data can be obtained, free of charge, from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, U.K. (fax, (internat.) +44-1223-
336033; E-mail, deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk.
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