A novel copper(ii) complex with an unsymmetrical tridentate-Schiff base: synthesis, crystal structure, electrochemical, morphological and electrocatalytic behaviors toward electroreduction of alkyl and aryl halides

Article abstract of DOI:10.1039/c5ra10819e

This work describes the synthesis of a new unsymmetrical tetradentate copper(ii) Schiff base complex Cu(L)(Py)(ClO₄) containing N₃O donor atoms. The tridentate ligand (HL) has been prepared by condensation of dehydroacetic acid on 1,2-diaminopropane in methanol. The reaction of the ligand with an appropriate amount of copper(ii) perchlorate hexahydrate (1 : 1 ratio) in the same solvent and in the presence of an excess of pyridine (Py) yields the title compound. The tridentate ligand (HL) with pyridine act as mixed ligands where three nitrogen and an enolic oxygen atoms were chelated to the copper centre. This complex has been fully characterized by FT-IR, UV-Vis spectrophotometry, and cyclic voltammetry. Single crystal X-ray diffraction of this complex showed that the copper ion was coordinated by one ligand, one pyridine molecule with one perchlorate anion in a square pyramidal geometry. The Cu(L)(Py)(ClO₄) complex crystallizes in an orthorhombic system, space group of Pcab with a = 11.051, b = 15.58, c = 21.736 ? and Z = 8. The electrochemical reduction of the copper(ii) complex, in N,N-dimethylformamide (DMF) solvent using cyclic voltammetry, produces conducting polymeric films on different electrode substrates, such as glassy carbon (GC), indium tin oxide (ITO) and fluorine tin oxide (FTO). The catalytic activity of this complex in homogeneous and heterogeneous electrocatalytic media seems to be efficient for the electroreduction of bromocyclopentane and iodobenzene.

Full text of DOI:10.1039/c5ra10819e

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A novel copper (II) complex with unsymmetrical tridentateꢀSchiff base:
Synthesis, Crystal structure, Electrochemical, Morphological and
Electrocatalytic behaviors toward electroreduction of alkyl and aryl halides
Ali OURARI*, Wassila DERAFA, Djouhra AGGOUN*
Laboratoire d'Electrochimie, d'Ingénierie Moléculaire et de Catalyse Redox (LEIMCR),
Faculté de Technologie, Université Ferhat ABBAS Setifꢀ1, Route de Béjaia, Setif 19000ꢀ
Algeria.
Abstract
This work describes the synthesis of a new unsymmetrical tetradentate copper (II) Schiff
base complex Cu(L)(Py)(ClO4)containing N3O donor atoms. The tridentate ligand (HL) has
been prepared by condensation of dehydroacetic acidon 1,2ꢀdiaminopropane in methanol. The
reaction of the ligand with an appropriate amount of copper (II) perchlorate hexahydrate (1:1
ratio) in the same solvent and in the presence of an excess of pyridine (Py) yields the title
compound. The tridentate ligand (HL) with pyridine act as mixed ligands where three
nitrogen and an enolic oxygen atoms were chelated to the copper centre.This complex has
been fully characterized by FTꢀIR, UVꢀVis spectrophotometry, and cyclic voltammetry.
Single crystal Xꢀray diffraction of this complex showed that the copper ion was coordinated
by one ligand, one pyridine molecule with one perchlorate anion in a square pyramidal
geometry. The Cu(L)(Py)(ClO4) complex crystallizes in an orthorhombic system, space
group of ꢀPcab with a = 11.051, b = 15.58, c = 21.736Å and Z = 8. The electrochemical
reduction of copper (II) complex, in N,Nꢀdimethylformamide(DMF) solvent using cyclic
voltammetry, produces conducting polymeric films on different electrode substrates, such as
glassy carbon (GC), indium tin oxide (ITO) and fluorine tin oxide (FTO). The catalytic
activity of this complex in homogeneous and heterogeneous electrocatalytic media seems to
be efficient for the electroreduction of bromocyclopentane and iodobenzene.
KEY WORDS: Copper (II) complex, Xꢀray diffraction, Modified electrodes, Electrocatalysis,
Alkyl and aryl halides.
*Corresponding authors. Tel.: +213 776 28 15 33; fax: +213 36 61 11 54 (A. Ourari). Tel.: +213.697.78.81.45.
Eꢀmail addresses: alourari@yahoo.fr (A. OURARI); agghafida@gmail.com (Dj. AGGOUN).
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1. Introduction
Among many organic reagents currently used in coordination chemistry, Schiff bases have
been increasingly developed in their different aspects. In general, they are prepared in high
yield using one step procedure via condensation of common aldehydes or ketones with amines
[1]. In coordination chemistry, these kinds of ligands belonging to the bidentate, tridentate
and tetradentate Schiff base ligands are considered highly interesting and desirable
compounds owing to their exceptional characteristics, notably easy preparation procedures,
versatile structures and diverse physical and chemical properties [1].This class of compounds
and their transition metal complexes has been studied extensively with the aim of shedding
light on various aspects ofelectrocatalysis[2], corrosion protection [3], biosensors [4] and in
many important biological activities, such as antitumoral, antifungal, and antibacterial
activities [5].
As an example of starting ketones, dehydroacetic acid is a powerful material which is
involved in the synthesis of several heterocyclic compounds [6] and chelating agents such as
those of Schiff bases. These ligands are also usually used in the synthesis of Schiff base
complexes of transition metals [7]. In addition, it was shown that the heterocyclic compounds
resulting from this starting material occupied a dominating positionin coordination chemistry
research that explores some therapeutic and pharmacological activities [8] and is useful in the
treatment of human diseases as well.
Thus, Schiff base complexes develop an important catalytic activity under homogeneous
and heterogeneous conditions, particularly in oxidation reactions, such as those mimicking
cytochromes Pꢀ450 enzymes [9]. Considering their low cost and large availability, other
transition metal complexes have also been used as catalysts for a wide variety of reactions,
including olefin polymerization[10], molecular oxygen and carbon dioxide reduction [11,12].
As for tetradentate transition metal complexes containing two oxygen and two nitrogen
(OONN) as donor atoms, they have attracted the attention of many researchers [13] due to
their structural versatility and electronic properties which can be fineꢀtuned by choosing an
appropriate amine and salicylaldehyde derivatives judiciously substituted [14]. In fact, the
reductive electroꢀpolymerization of diverse complexes has been investigated [15] and the
electrochemical design of modified electrodes by electrodeposition of polymer films has also
been extensively developed.In addition, copper (II) Schiff base complexes have also aroused a
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wide interest because they possess a diverse spectrum of biological and pharmaceutical
activities. Copper (II) is known to form complexes with a variety of molecular geometries
(e.g., tetrahedral, square planar, square pyramidal, and octahedral). Furthermore, copper
complexes have been copiously studied because of their structural, catalytical and good
magnetic properties.
As part of our ongoing research into the design and synthesis of a new copper Schiff base
complex, a family of heterocyclic ligand is being presented here which isderived from
dehydroacetic acid, 1,2ꢀdiaminopropane, pyridine and copper perchlorate hexahydrate in a
methanolic solution. This complex was formed in one pot with only one azomethine (–
C(CH3)=N–) group yielding an unreacted amino group of 1,2ꢀdiaminopropane leading to an
acceptable yieldunder mild conditions.
In this case, we notice that the ring of dehydroacetic acid does not appear to be open during
the reaction as it was reported in the literature [16], particularly in the presence of nucleophile
agents such as pyridinic derivatives. This behavior may be due to an inhibition of the
nucleophilic effect of pyridine since the reaction was conducted in a methanolic solution at
room temperature and without reflux. Finally, the resulting compound was confirmed by
crystallographic studies as discussed further and the structure of this copper (II) Schiff base
complex was illustrated by the following Scheme 1.
«Scheme 1»
2. Experimental
2.1.Physical Measurements
All solvents and chemical were of reagent grade and were purchased from Aldrich and Fluka.
They were used as received without any further purification. The solvents were dried before
use with the appropriate drying reagents. IR spectra were recorded on Perkin and Elmer 1000ꢀ
FTꢀIR Spectrometer using KBr disks, while the electronic spectra (UVꢀVis) were obtained on
a Unicam UVꢀ300 Spectrophotometer having 1 cm as a path length cell. Electrochemical
experiments were performed on a voltalab 40, Potentiostat galvanostat controlled by
microcomputer. Thus, the cyclic voltammograms were recorded after compilation of
electrochemical data obtained with an individual cell of 5 ml, using a conventional threeꢀ
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electrode system in DMF solutions containing 0.1M tetraꢀnꢀButylammonium perchlorate
(TBAP) and 0.001 M copper complex. The electrodes were polished by diamond paste and
copiously rinsed with acetone and then by DMF. The working electrode was either a disc of
glassy carbon (diam. 3 mm) or a semiꢀtransparent electrode of indium tin oxide (ITO,
Resistance=50 ꢁ, thickness= 100 nm) while the counter electrode was a platinum wire, and
all potentials were expressed versus the saturated solutions of calomel electrode (SCE). ITOꢀ
and FTOꢀelectrodes were typically prepared by cutting the glass to suitable size (active area =
1 cm2).
The Xꢀray photoelectron spectroscopy (XPS) measurements were carried out in a
MULTILAB 2000 (THERMO VG) spectrometer equipped with an Al K _Xꢀray source
(1486.6 eV). Survey and high resolution spectra were recorded in constant pass energy mode
(100 and 20 eV, respectively). The CASAXPS program with a Gaussian–Lorentzian mix
function and Shirley`s background subtraction was employed to deconvolute the XP spectra.
The C 1s peak at 284.6 eV was used to correct charging effects.
2.2.Xꢀray Crystallography
Colorless crystals of Cu(L)(Py)(ClO4) suitable for Xꢀray diffraction were obtained by slow
evaporation from methanol solution. A single crystal was mounted on a APEXII, BrukerꢀAXS
diffractometer, MoꢀKα radiation (λ = 0.71073 Å), T = 150(2) K; Centre de Diffractométrie
(CDFIX), Université de Rennes1, France. The structure was solved by direct methods using
the SIR97 program [17], and then refined with fullꢀmatrix leastꢀsquare methods based on F2
(SHELXLꢀ97) [18] with the aid of the WINGX [19] program. All nonꢀhydrogen atoms were
refined with anisotropic atomic displacement parameters. H atoms were finally included in
their calculated positions.Crystal data and refinement results are given in Table 1 and
complete crystallographic results are further given as supplementary materials.
2.3. Synthesis
Synthesis of complex Cu(L)(Py)(ClO4)
Synthesis of copper complex was carried out via known chemical reactions, as depicted
in Scheme 1. So, this complex was obtained by mixing stoechiometric quantities of
dehydroacetic acid 0.168 g (1 mmol) with 0.075 g (1 mmol) of 1,2ꢀdiaminopropane in
methanol solution (10 ml). To this mixture was added an excess of pyridine (2mmol, 158mg)
and then copper perchlorate hexahydrate 0.370 g (1 mmol) was similarly dissolved in the
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same medium. After two hours of reaction, a blue precipitate was observed and recovered by
filtration. It was purified by successive cleaning steeps using cooled methanol and diethyl
ether and then dried over CaCl2 giving 72% as yield. Its suitable single crystals were obtained
by slow evaporation.
3. Results and discussion
The synthesized copper complex is stable in air and moisture. Its solubility is usually good in
the coordinating solvents such as acetonitrile, DMF and DMSO.
3.1. Spectral characterization
ꢀ Infrared Spectra
The IR spectrum of the copper complex shows two absorption bands at 3345 and 3297 cm–1.
These absorption bands are attributed to the ν(NH2) vibration [20]. Also, another band with a
moderate intensity was observed at 1691cmꢀ1 and ascribed to the vibration ν(O=CꢀO) of the
lactone function. The strong and sharp absorption band due to the azomethine υC=N group of
the Schiff base appears at 1649 cm−1 [21]. The complex shows a band at 1089 cm−1 suggesting
the presence of perchlorate as counter anion [22]. In addition, bands observed at 1457 and
1352 cmꢀ1 are assigned respectively to the vibration of coordinated phenolic Cu–O–C
stretching mode and the Cu–N stretching ν(C―N). The formation of the M–O and M–N bonds
were further supported by the appearance of the ν(M–O) and ν(M–N) bands in the regions
439ꢀ520 and 624ꢀ670 cm−1, respectively [23]. This indicates that phenolic oxygen and
azomethine nitrogen atoms are involved in coordination.
ꢀ Electronic Spectra
The formation of the copper (II) complex was also confirmed by UVꢀVis spectrum. The
electronic spectrum of Cu(L)(Py)(ClO4), recorded in DMF solutions, displays its most
important features between 300 and 370 nm. The intense absorption band observed at 310 nm
arose from a transition involving electron migration along the entire conjugate system of the
ligand associated with the C=N linkages (n–π*) [24]. The weak band at near 368 nm can be
attributed to the L→M charge transfer transition [25]. The electronic transitions, observed in
the visible region around 620 nm, are ascribed to the weak d–d transitions [26].
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3.2.Crystallographic Study
The Cu(L)(Py)(ClO4) crystallizes in the orthorhombic space group Pcab with 8 formula units
in the cell. The asymetric unit of copper complex, and the atomic numbering used, is
illustrated in Fig. 1. Relevant Xꢀray diffraction data and selected bond lengths and angles are
listed respectively in tables 1 and 2. The CuII ion is fiveꢀcoordinated in a squareꢀpyramidal
geometry by three of the four donors of the Schiff base complex (a nitrogen atom from the
amino group, a second nitrogen atom from the imino group and an oxygen atom of the
pyranone molecular residue), the fourth donor is the pyridine nitrogen atom. The coordination
around the metal ion is completed by a longer axial bond with the perchlorate O atom of
perchlorate unit. The bond lengths for the coordination CuII sphere range from 1.970 (3)to
2.036 (3)Å and 2.538 (2) Å for CuꢀN and CuꢀO distances respectively. These results are in
good agreement with similar compounds described in the literature [27,28]. Cisoid and
transoid angles for Cu deviate from their ideal values of 90° and 180° and are found to be in
the range 85.43 (11)– 95.38 (11)° and 172.95 (11)– 174.51 (11)°, respectively (Table 2).The
bond lengths and angles around C24 are also in agreement with sp3 hybridization of this atom.
The azomethine relationship is evident from the N14–C20 bond length (1.475 (4)) and the
C10–N14–C20 bond angle (120.4 (3)). All H atoms attached to C or N atoms were positioned
geometrically and treated as riding on their parent atoms with CꢀH = 0.95 Å (Caromatic), 0.98 Å
(Cmethyl) and 0.92 Å (NH2) with Uiso(H) = 1.2 Ueq(Caromatic, NH2) or 1.5 Ueq(Cmethyl). The
trigonality index τ = (174.51 – 172.95)/60 = 0.026 indicating that the complex has a nearly
perfect planar tetragonal geometry [O12ꢀCuꢀN13 = 174.51° and N4ꢀCuꢀN14 = 172.95°] [29].
«Figure 1»
«Table 1»
«Table 2»
3.3.Electrochemical Study
3.3.1. Cyclic voltammetry
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The redox properties of the monomer Cu(L)(Py)(ClO4) 10ꢀ3 M were studied by cyclic
voltammetry in DMF solutions containing 10ꢀ1M of TBAP using 100 mVsꢀ1 as scan rate after
bubbling for more than 15 minutes under nitrogen atmosphere. The cyclovoltammograms,
recorded in the range between 0.0 and +1.6V (Fig. 2, curve A), showed a redox couple at E1/2
= 0.415V exhibiting respectively two waves Cu (III) at 0.55 V and Cu (II) species at
0.28V/SCE. The peakꢀtoꢀpeak separation (ꢂEp) was found to be equal to 270 mV. The
theoretical value of ꢂEp for oneꢀelectron process is equal or slightly superior to 60 mV [30].
The large difference between the theoretical value and the experimental one would be due to a
quasiꢀreversible behavior of the Cu (III)/Cu (II) redox couple. This indicates a relative slow
redox transition and/or electron exchange process at the interface between the electrode and
Cu (III)/Cu (II) metallic redox centers [31], which implies that these electrochemical
processes are mainly diffusionꢀcontrolled. In addition, in the range from ꢀ0.2 to ꢀ1.6V, it can
be observed that the copper (II) complex shows a quasiꢀreversible redox system of the Cu
(II)/Cu (I) at E1/2 equal to ꢀ0.87 V/SCE and the peakꢀtoꢀpeak separation (ꢂEp=140mV) (Fig.
2, curve B).
«Figure 2»
3.3.2. Modified electrode polyꢀ[Cu(L)(Py)(ClO4)]/GC
In general, a modified electrode (ME) is prepared by electropolymerization of a monomer in
two steps. Firstly, the monomer is electrodeposited as polymer either by repetitive cycling or
by electrolysis at controlled potential. Secondly, when the modified electrode is obtained, it
was copiously rinsed with biꢀdistilled water, with DMF and then transferred into another fresh
electrolytic DMF solution no containing monomer. This ME was identified by an
electrochemical method as cyclic voltammetry giving electrochemical characteristics of
polymer, attesting the presence of the complex in the polymeric matrices.
On the anodic side, Fig. 3 (Curve A) shows the evolution of cyclovoltammograms of
Cu(L)(Py)(ClO4) complex in DMF solutions at a scan rate of 100 mV sꢀ1. The cycling
between ꢀ0.2 and 1.2 V/SCE exhibits a continuous increasing in ipa and ipc peak currents
characterizing Cu (III)/Cu (II) redox couple where the electrodeposited films are more and
more conductive. Moreover, as the number of scans increases, the anodic and cathodic peak
currents shifted to more positive and more negative potentials respectively. This is consistent
with an increase of the polymer thickness causing an increase in its electrical resistance. This
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fact indicates that the copper complex is deposited onto the GC electrode surface and the
modified electrode polyꢀ[Cu(L)(Py)(ClO4)]/GC is so obtained.
On the cathodic side, Fig. 3 (Curve B) shows obviously that the electropolymerization
reaction of Cu(L)(Py)(ClO4) is easily performed by repetitive cycling from 0.0 to ꢀ1.2 V at
scan rate of 50 mV sꢀ1 with GCꢀelectrode. The buildup of the polyꢀ[Cu(L)(Py)(ClO4)] films
can be characterized by the increase in the ipa and ipc peak currents occurring in each repeated
cycling which induces the deposition of a conductive polymer on GCꢀelectrode surface [32ꢀ
34].
«Figure 3»
In this context, we have investigated the scan rate dependence for Cu (II)/Cu (I) redox
couples in homogeneous and heterogeneous phases with their solution and solid phase
respectively. Thus, a wide range of scan rates was explored starting from 25 to 500 mVs−1
(See Fig. 4). In addition, it can be seen that the redox currents increase with the increasing of
scan rates and the potential values of anodic waves shift to the more positive potentials and
the cathodic peak currents shift also towards the more negative potentials as the scan rates
increase [35]. This phenomenon can be explained by the presence of the electroactive species
in the bulk of the polymer requiring more energy.
«Figure 4»
3.3.3. Scanning electron micrographs
Transparent conducting oxide films are frequently used with substrates in developing
electronics and optoelectronic devices, due to their conductive and optical properties. Among
these substrates, fluorineꢀdoped tin oxide and indium tin oxide deposited as thin films on glass
stand out in the development of solar cells by their high electrical conductivity, optical
transparency to visible radiation and low cost. Actually, it is possible to find studies focused
on metal complexes films deposited on FTO or ITO substrates. Some authors show that the
surface of the ITO and FTO film undergoes chemical and morphological changes during
electrochemical treatment, affecting the electric and optic properties of the original substrate
[36]. In this paper, we examine the morphological characteristics of FTO and ITO films
during electropolymerization.
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The modification of these two electrodes, polyꢀ[Cu(L)(Py)(ClO4)]/ITO and polyꢀ
[Cu(L)(Py)(ClO4)]/FTO are elaborated in experimental conditions similar to those mentioned
above for GCꢀmodified electrodes. These modified electrodes are also copiously rinsed with
biꢀdistilled water, with DMF and then transferred into another fresh electrolytic DMF solution
containing no monomer. These MEs were identified by SEM microscopy (Fig.5).
This technique was used in order to evaluate the morphological and chemical changes
occurringon the ITOꢀ (Fig.5 ABCD) and FTOꢀelectrode surfaces (Fig.5 EFGH). Thus, it was
revealed that the presence of electrolyte salt gave rise to the formation of a number of
remarkably different sizes of microparticles, as can be observed from SEM micrographs of
polyꢀ[Cu(L)(Py)(ClO4)] films with ITO and FTO (See Fig.5 AꢀG). A typical fractalꢀlike or
dendritic structures consisting of a long central backbone and sharp side branches showing a
preferential growth along two definite directions was observed as reported in the literature
[37,38]. In our case, a difference between ITO and FTO SEM images was noticed suggesting
that the ITO image shows the formation of crystalline structure [36,37], while with FTO
image, it exhibits microstructures of cubeꢀshaped uniformꢀsized grains.
«Figure 5»
3.3.4. XPS Characterization
Xꢀray photoelectron spectroscopy (XPS) was used to examine the surface composition of
[Cu(L)(Py)(ClO4)] and polyꢀ[Cu(L)(Py)(ClO4)]/ITO. XPS spectra of Cu sample reveal the
presence of chlorine, oxygen, carbon, nitrogen and Cu either in the [Cu(L)(Py)(ClO4 powder
or in the polyꢀ[Cu(L)(Py)(ClO4)] film (Fig. 6). Based on these analyses, copper is present
approximately at the same level in both cases (Cu free and Cuꢀpoly). Some kinetic energy
shifts were observed for polyꢀ[Cu(L)(Py)(ClO4)]/ ITO (Fig. 6b) comparing to free copper
[Cu(L)(Py)(ClO4)] (Fig. 6a) (XPS). So, figures 7a and 7b shows wideꢀscan Xꢀray
photoelectron spectra of [Cu(L)(Py)(ClO4)] and polyꢀ[Cu(L)(Py)(ClO4)]/ITO electrode. The
Cu coverage is clearly higher in [Cu(L)(Py)(ClO4)] than in polyꢀ[Cu(L)(Py)(ClO4)]/ITO.
The spectrum of the [Cu(L)(Py)(ClO4)]/ITO sample shows large C 1s and O 1s contributions,
weak Cu 2p and Cu Auger signals being also apparent. This indicates that the electrode
surface is covered with a layer of [Cu(L)(Py)(ClO4)]. In agreement with this, there is
contribution from the ITO substrate which is supported by the present ITO composition (Si
2p, In 3p, Sn 3p). The narrowꢀscan Cu 2p spectra of the aboveꢀmentioned samples (Fig. 6a
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and 6b) show the main Cu 2p3/2 and Cu 2p1/2 signals (They appearing at 934, and at 940 eV
respectively) [39ꢀ41]. The 2p electron BEs of the metals and the values for the carbon 1s,
nitrogen 1s, oxygen 1s, and chlorine 2p electron BEs are given in survey spectrum A and B.
3.3.5. Reduction of bromocyclopentane and iodobenzene
Transition metals complexes have been extensively studied as catalysts for the reduction
and synthesis of organic compounds [42]. When alkyl halides are directly elctroreduced, the
resulting products via alkyl radical coupling and disproportionations as well as an abstraction
of a hydrogen atom from the solvent by an alkyl radical [43]. As example of the literature
[44], the electroreduction of 1ꢀbromooctane using nickel(I)ꢀsalen as catalyst yields nꢀoctyl
radical that can be involved in three reactions: (1) coupling to give nꢀhexadecane; (2)
abstraction of a hydrogen atom from DMF to produce nꢀoctane; (3) disproportionation to give
nꢀoctane and 1ꢀoctene [44].
3.3.5.1.Effect of concentration
The electropolymerized films of polyꢀ(Cu(L)(Py)(ClO4)) and its monomer form were found
to be efficient for the electrocatalytic reduction of alkyl and aryl halide compounds such as
bromocyclopentane and iodobenzene in homogeneous and heterogeneous media.
So, the cyclic voltammogram, obtained in homogeneous phase presented in Fig. 7 A, shows a
large enhancement in the reduction peak current ipc of the metal center when the copper
species are involved in the electroreduction of bromocyclopentane. This is consistent with an
obvious electrocatalytic process. Indeed, these increased peak currents cannot be due to the
direct reduction of the bromocyclopentane since they are formally occurring at a more
negative potential value under the same experimental conditions. So, without catalyst the
bromocyclopentane is electroreduced at ꢀ2.5V/SCE whereas in the presence of a catalyst its
electroreduction occurs at very low potential values (ꢀ0.95V/SCE). The difference between
the two potential values corresponds to a significant gain in potential (ꢂEp = 1.55 V) attesting
an important electrocatalytic effect as it can be observed with the peak currents of their Cu
(II) /Cu (I) redox system (Fig. 7 A) [45].
Also, the electropolymerized films of polyꢀ(Cu(L)(Py)(ClO4)) were found to be
effective for the electrocatalytic reduction of bromocyclopentane and iodobenzene. Cyclic
voltammograms obtained on glassy carbon electrode, coated with a polymerized film of polyꢀ
(Cu(L)(Py)(ClO4)) in DMF solution, are illustrated in figure 7 B and C. The same remarks
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are observed confirming the presence of a significant electrocatalytic effect in the case of the
modified electrodes with heterogeneous catalysis.
«Figure 7»
3.3.5.2.Scan rate effect
Fig. 8 shows the effect of scan rates for the electroreduction of iodobenzene. It can be clearly
seen that there is an increase in the peak currents ipa and ipc of the redox system Cu (II)/Cu (I)
accompanying the increase of the scan rates. So, these cyclovoltammograms were recorded in
the presence of 5.10ꢀ3 M of iodobenzene using the polyꢀ(Cu(L)(Py)(ClO4)) as a modified
electrode. This figure shows that the increase of the scan rate induces a shift of iodobenzene
reduction potentials. These potential values were shifted to more cathodic values suggesting
that this electrocatalytic behavior is in good accordance with a slow electrochemical process
for an heterogeneous reduction reaction of iodobenzene using a Cu(L)(Py)(ClO4) modified
electrode.
«Figure 8»
In our case, the catalytic reduction of the alkyl halide leads probably to the production of a
dimer (dicyclopentyl) with a good yield (Way 1), cycloalkane (Way 2) and cycloalkene
(Way 3) [46]. These processes take place in small yields either via disproportionation of alkyl
radicals or via abstraction of a hydrogen atom from the solvent by an alkyl radical (Scheme
2).
«Scheme2»
Scheme 3 presents a set of mechanism reactions, showing different steps in which are
involved the donor effects of protons governing the catalytic reduction of an alkyl
monohalide. These chemical processes take into consideration the possibility of the
electroreduction of one electron of copper (II) complex, that offer ways for alkylation of the
imino bonds (C=N) of copper (II) complex during these processes. These facts can be
summarized in the mechanism proposed by P.W. Raess et al. (Scheme 3) [46].
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«Scheme3»
Scheme 4 provides a sequence of pathways for the copper (II) complex catalyzing
reduction of iodobenzene [47].
«Scheme4»
3.3.6. Chronoamperometry
Chronoamperometry results obtained for the two concentrations of iodobenzene solutions
using polyꢀCu(L)(Py)(ClO4) are shown in Fig. 9. This technique can be employed to estimate
the catalytic rate constant, kh for the reaction between iodobenzene and polyꢀ
Cu(L)(Py)(ClO4) according to Galus method [48].
ꢈγ
ꢂꢆꢇ
ꢀꢁ
ꢀ
= γ1/2[
π
ꢂꢃꢄ
γ +
ꢅ
]
Eq. 1
√
√
γ
ꢀꢁ
ꢀ
= γ1/2π1/2 = π1/2( khCb t )1/2
Eq. 2
Where ICis the catalytic current of iodobenzene and polyꢀCu(L)(Py)(ClO4), IL is the limiting
current in the absence of iodobenzene, and γ = khCbt (Cb is the bulk concentration of
iodobenzene) is the argument of the error function. In the cases where γ exceeds two (2), the
error function is almost equal to unity (1) and therefore, the above equation Eq. 1 can be
reduced to the Eq. 2 where t is the time elapsed in seconds. This equation can be used to
½
calculate the rate constant of the catalytic process kh, based on the slope of IC / IL vs. t plot,
kh can be obtained for a given iodobenzene concentration from the values of the slopes. An
average value of kh was found to be kh= 0,48 103 cm3mol−1 s−1. This value also explains the
sharp feature of the catalytic peak observed for the catalytic reduction of iodobenzene at the
surface of Cu(L)(Py)(ClO4) modified electrode.
«Figure 9»
4. Conclusion
By way of conclusion, a new unsymmetrical copper (II) Schiff base complex was synthesized
by a simple and efficient method. The structure of this compound was confirmed by different
12

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techniques, such as infrared spectroscopy, UVꢀVis spectrophotometry analysis and also by
cyclic voltammetry. In addition, molecular structure of the copper (II) complex was
confirmed by XRD diffraction analysis. This complex was easily electropolymerized by
cathodic reduction to its polyꢀ[Cu(L)(Py)(ClO4)] films yielding modified electrodes on which
catalytic sites of copper as Cu(II) ions were identified in their coordinated forms. These new
modified electrodes were found to be as efficient homogeneous and heterogeneous
electrocatalysts for the reduction of bromocyclopentane and iodobenzene. These new
materials can be used as modified electrodes in many applications, such as catalysis,
electrocatalysis, electroanalysis and as well as sensors.
Aknowledgement
We gratefully aknowledge the “Direction Générale de Recherche (DGR)” for their financial
support. The authors would like to thank Professor Spyridon Zafeiratos for his helpful
discussions, ICPEES, UMR 7515 du CNRSꢀUniversité de Strasbourg, ECPM, 25 Rue
Becquerel, Fꢀ67087 Strasbourg Cedex 8, France and Professor ZERROUAL Larbi,
Département Génie des procédés, Faculté de Technologie, Université Ferhat ABBAS Setifꢀ1,
Sétif 19000 Algeria.
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List of captions
Scheme:
Scheme 1. Reaction way leading to the formation of the Schiff base copper (II) complex.
Scheme 2. Proposed yields for electroreduction of bromocyclopentane [46].
Scheme 3. Mechanism for electroreduction of bromocyclopentane proposed by P.W. Raess et
al. [46].
Scheme 4. Mechanism for electroreduction of iodobenzene proposed by P.C. Gach et al.
[47].
Figures:
Fig. 1 ORTEP drawing of the complex with atomic labelling, 50% ellipsoid probability.
Fig. 2 Cyclicvoltammograms recorded with 1mM of Cu(L)(Py)(ClO4) in DMF solutions
containing 0.1 M TBAP using 100 mVsꢀ1 as scan rate going from: (A) +0.0 to +1.6 and (B) ꢀ
0.2 to ꢀ1.6 V/SCE.
Fig. 3 Polyꢀ[Cu(L)(Py)(ClO4)] films obtained on GC electrode in DMF solutions, 0.1 M
TBAP from ꢀ0.2 to +1.2 using 100 mV s−1 as scan rate (A) and then from 0 to ꢀ1.2 V/SCE (B)
using 50 mV s−1 as scan rate.
Fig. 4 Cyclic voltammograms, recorded from DMF solutions 1 mM of copper (II) complex
using a disc of GC as working electrode and 0.1 M TBAP with various scan rates going from
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ꢀ0.2 to ꢀ1.6 V/SCE (A) and then, from 0.0 to ꢀ1.2 V/SCE (B) of Cu(L)(Py)(ClO4) and polyꢀ
[Cu(L)(Py)(ClO4)] respectively.
Fig. 5 Scanning electron micrographs of Cu(L)(Py)(ClO4) complex deposited on ITO (A, B,
C and D) and FTO (E, F, G and H) substrates.
Fig. 6 XPS spectra of Cu sample. (a) Survey spectra of [Cu(L)(Py)(ClO4)] complex powder,
(b) Survey spectra of polyꢀ[Cu(L)(Py)(ClO4)]/ITO films.
Fig. 7 Cyclic voltammograms, recorded at 100 mV s–1 for the electroreduction of
bromocyclopentane on GCꢀelectrode in DMF solutions containing 0.1 M TBAP in
homogeneous phase (A), in heterogeneous phase (B) and for the electroreduction of
iodobenzene (C).
Fig. 8 Cyclovoltammograms of polyꢀ[Cu(L)(Py)(ClO4)] recorded in presence of 5.10ꢀ3 M of
iodobenzene at different scan rates:10, 25, 50,75, 100 mVsꢀ1.
Fig. 9 Chronoamperograms obtained for Two concentrations of iodobenzene solutions using
½
Cu(L)(Py)(ClO4) modified electrode, See curve (A); slope of IC / IL vs. t plot (B).
Tables:
Table 1. Crystal data and structure refinement for Cu(L)(Py)(ClO4).
Table 2. Geometric parameters (Å, °): Selected bond lengths [Ǻ] and angles [°].
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Scheme 1:
CH3
NH2
H3C
H2N
NH2
CH3
+
Methanol
N
O
O
O
Cu
O
+
N
Copper perchlorate
H3C
O
HO
O
O
C
+ excess of pyridine
l
O
O
CH3
O
CH3
Scheme 2:
Way 1
Good yield
Electrocatalytic
reduction
Way 2
Way 3
Br
H
Small yields
Scheme 3:
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H3C
H2N
N
H3C
H2N
N
H3C
H2N
N
CH3
CH3
CH3
N
O
O
N
O
O
N
O
O
+ eꢀ
ꢀ eꢀ
CuII
CuI
CuII
O
O
O
OClO3
OClO3
OClO3
CH3
CH3
CH3
1
2
3
(1) 2 (or 3) +
1
+
+
Br
Br
(2)
2
2
H
H
+
(3)
Solvant
Solvant
H
(4)
H
(5)
H3C
H3C
H2N
CH3
CH3
H2N
N
N
O
N
O
CuII
CuII
Br
O
O
+
O
O
Br
+
N
OClO3
CH3
OClO3
(6)
(7)
CH3
CH3
H3C
H3C
H2N
N
H
CH3
H2N
N
N
O
N
O
CuII
CuII
Solvant
O
O
O
O
OClO3
OClO3
CH3
CH3
O
H3C
H2N
N
H3C
H2N
N
CH3
CH
3 O
N
N
CuII
CuII
O
O
+
O
O
(8)
OClO3
OClO3
CH3
CH3
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Scheme 4:
Cu(II)L + eꢀ
Cu(I)L
(1)
(2)
(3)
(4)
(5)
ꢀ Iꢀ
Cu(III)L
Cu(II)L
I
+ Cu(I)L
eꢀ
Cu(III)L
Cu(III)L
+ Cu(I)L
Cu(II)L + Cu(II)L
Cu(II)L
+ Cu(II)L
+ OH
(6)
I
+ H2O
Fig. 1.
Fig. 2.
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80
60
40
20
0
A
ꢀ20
ꢀ0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8
E(SCE)
2,5
B
0,0
ꢀ2,5
ꢀ5,0
ꢀ7,5
ꢀ10,0
ꢀ12,5
ꢀ15,0
ꢀ17,5
ꢀ1,8 ꢀ1,6 ꢀ1,4 ꢀ1,2 ꢀ1,0 ꢀ0,8 ꢀ0,6 ꢀ0,4 ꢀ0,2 0,0
E(SCE)
Fig. 3.
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20
15
10
5
A
0
ꢀ5
ꢀ10
ꢀ0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
E(SCE)
B
15
0
ꢀ15
ꢀ30
ꢀ45
ꢀ60
ꢀ1,2
ꢀ1,0
ꢀ0,8
ꢀ0,6
ꢀ0,4
ꢀ0,2
0,0
E(SCE)
Fig. 4.
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10
0
A
ꢀ10
ꢀ20
ꢀ30
ꢀ40
ꢀ50
25mV.sꢀ1
100mV.sꢀ1
200mV.sꢀ1
400mV.sꢀ1
500mV.sꢀ1
ꢀ1,8
ꢀ1,5
ꢀ1,2
ꢀ0,9
ꢀ0,6
ꢀ0,3
0,0
E(SCE)
10
5
B
0
10 mV.sꢀ1
25mV.sꢀ1
50 mV.sꢀ1
75 mV.sꢀ1
ꢀ5
ꢀ10
100 mV.sꢀ1
ꢀ15
ꢀ1,2
ꢀ1,0
ꢀ0,8
ꢀ0,6
ꢀ0,4
ꢀ0,2 0,0
E(V/SCE)
Fig. 5.
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A
B
C
D
F
E
G
H
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Fig. 6.
(a)
(b)
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Fig. 7.
0
- 50
0
- 100
- 150
- 200
- 250
- 300
ꢀ10
ꢀ20
ꢀ30
ꢀ40
ꢀ50
)
m
2
c
/
A
µ
(
i
Without BrCP
6mM BrCP
12mM BrCP
ꢀ1,8
ꢀ1,5
ꢀ1,2
ꢀ0,9
ꢀ0,6
ꢀ0,3
0,0
E(SCE)
- 30,
- 2,5
- 20,
- 1,5
- 10,
-0 ,5
0 0,
E(SCE)
A
B
C
28

Page 29 of 30
RSC Advances
View Article Online
DOI: 10.1039/C5RA10819E
Fig. 8.
4
0
-4
-8
With 510ꢀ3 IPh 10 mV/s
25 mV/s
-12
-16
-20
50 mV/s
75 mV/s
100 mV/s
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
E(V/ SCE)
Fig. 9.
(A)
(B)
Table 1.
29