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

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 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 P̄cab with a = 11.051, b = 15.58, c = 21.736 Å 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.

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1. Introduction

Among the 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 a 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 of electrocatalysis,² corrosion protection,³ biosensors⁴ 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 compounds⁶ 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 position in coordination chemistry research that explores some therapeutic and pharmacological activities⁸ 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 P-450 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 researchers¹³ due to their structural versatility and electronic properties which can be fine-tuned by choosing an appropriate amine and salicylaldehyde derivatives judiciously substituted.¹⁴ In fact, the reductive electro-polymerization of diverse complexes has been investigated¹⁵ 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 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 is derived from dehydroacetic acid, 1,2-diaminopropane, 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 1,2-diaminopropane leading to an acceptable yield under 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 Reaction way leading to the formation of the Schiff base copper(II) complex.

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 1000-FT-IR Spectrometer using KBr disks, while the electronic spectra (UV-Vis) were obtained on a Unicam UV-300 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 three-electrode system in DMF solutions containing 0.1 M tetra-n-butylammonium 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 semi-transparent electrode of indium tin oxide (ITO, resistance = 50 Ω, thickness = 100 nm) while the counter electrode was a platinum wire, and all potentials were expressed versus the saturated solutions of calomel electrode (SCE). ITO- and FTO-electrodes were typically prepared by cutting the glass to suitable size (active area = 1 cm²).

The X-ray photoelectron spectroscopy (XPS) measurements were carried out in a MULTILAB 2000 (THERMO VG) spectrometer equipped with an Al Kα X-ray 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. X-ray crystallography

Colorless crystals of Cu(L)(Py)(ClO₄) suitable for X-ray diffraction were obtained by slow evaporation from methanol solution. A single crystal was mounted on a APEXII, Bruker-AXS diffractometer, Mo-Kα 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 full-matrix least-square methods based on F² (SHELXL-97)¹⁸ with the aid of the WINGX¹⁹ program. All non-hydrogen 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 ESI.†

Table 1 Crystal data and structure refinement for Cu(L)(Py)(ClO₄)^a

a R = {∑[w(|F0| − |Fc|)]/∑w(|F0|)}, Rw = {∑[w(|F0| − |Fc|)^2]/∑w(|F0|^2)}^1/2, wR2 = {∑[w(F0^2 − Fc^2)^2]/∑w(F0^2)}^1/2 w = 1/[σ^2(F0^2) + (0.0794P)^2 + 0.6853P] where P = (Fo^2 + 2Fc^2)/3.
Molecular formula	C16H20CuN3O3·ClO4
Molecular weight	465.34
Temperature (K)	293
Radiation λ	Mo-Kα (0.71073 Å)
Crystal system	Orthorhombic
Space group	Pcab
a/Å	11.051 Å
b/Å	15.58 Å
c/Å	21.736 Å
V/Å^3	3742.3 Å^3
Z	8
Dcalc (g cm^−3)	1.652
Crystal size (mm^3)	0.4 × 0.3 × 0.1
Crystal description	Prism
Crystal colour	Colourless
Absorption coefficient (mm^−1)	1.36
F(0 0 0)	1912
Reflections collected	4288 [Rint = 0.064]
Range/indices (h, k, l)	−13, 14; −20, 20; −27, 28
θ limit	3.6–27.5
No. of observed data, I > 2σ(I)	3007
No. of variables	256
No. of restraints	0
Goodness of fit on F^2	0.125
Largest diff. peak and hole (e Å^−3)	0.57 and −0.42
R1 [I > 2sigma(I)]	0.0484
wR2	0.1245

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2.3. Synthesis

2.3.1. 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 stoichiometric quantities of dehydroacetic acid 0.168 g (1 mmol) with 0.075 g (1 mmol) of 1,2-diaminopropane in methanol solution (10 ml). To this mixture was added an excess of pyridine (2 mmol, 158 mg) and then copper perchlorate hexahydrate 0.370 g (1 mmol) was similarly dissolved in the 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

3.1.1. 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 1691 cm⁻¹ and ascribed to the vibration ν_(O=C–O) of the lactone function. The strong and sharp absorption band due to the azomethine ν_C=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 cm⁻¹ 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 439–520 and 624–670 cm⁻¹, respectively.²³ This indicates that phenolic oxygen and azomethine nitrogen atoms are involved in coordination.

3.1.2. Electronic spectra. The formation of the copper(II) complex was also confirmed by UV-Vis 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

3.2. Crystallographic study

The Cu(L)(Py)(ClO₄) crystallizes in the orthorhombic space group Pcab with 8 formula units in the cell. The asymmetric unit of copper complex, and the atomic numbering used, is illustrated in Fig. 1. Relevant X-ray diffraction data and selected bond lengths and angles are listed respectively in Tables 1 and 2. The Cu^II ion is five-coordinated in a square-pyramidal 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 Cu^II sphere range from 1.970 (3) to 2.036 (3) Å and 2.538 (2) Å for Cu–N and Cu–O 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.

Fig. 1 ORTEP drawing of the complex with atomic labelling, 50% ellipsoid probability.

Table 2 Geometric parameters (Å, °): selected bond lengths [Å] and angles [°]

Cu1–O12	1.901 (2)	N13–Cu1–N4	95.38 (11)
Cu1–N14	1.970 (3)	C6–O12–Cu1	128.6 (2)
Cu1–N13	1.991 (3)	C15–N4–Cu1	121.2 (2)
Cu1–N4	2.036 (3)	C19–N4–Cu1	122.1 (2)
O12–Cu1–N14	90.77 (10)	C10–N14–Cu1	128.8 (2)
O12–Cu1–N13	174.51 (11)	C20–N14–Cu1	110.1 (2)
N14–Cu1–N13	85.43 (11)	C13–N13–Cu1	108.7 (2)
O12–Cu1–N4	88.87 (10)	Cu1–N13–H13A	110
N14–Cu1–N4	172.95 (11)	Cu1–N13–H13B	110

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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 C–H = 0.95 Å (C_aromatic), 0.98 Å (C_methyl) and 0.92 Å (NH₂) with U_iso(H) = 1.2U_eq(C_aromatic,NH2) or 1.5U_eq(C_methyl). The trigonality index τ = (174.51 − 172.95)/60 = 0.026 indicating that the complex has a nearly perfect planar tetragonal geometry [O12–Cu–N13 = 174.51° and N4–Cu–N14 = 172.95°].²⁹

3.3. Electrochemical study

3.3.1. Cyclic voltammetry. The redox properties of the monomer Cu(L)(Py)(ClO₄) 10⁻³ M were studied by cyclic voltammetry in DMF solutions containing 10⁻¹ M of TBAP using 100 mV s⁻¹ as scan rate after bubbling for more than 15 minutes under nitrogen atmosphere. The cyclovoltammograms, recorded in the range between 0.0 and +1.6 V (Fig. 2, curve A), showed a redox couple at E_1/2 = 0.415 V exhibiting respectively two waves Cu(III) at 0.55 V and Cu(II) species at 0.28V/SCE. The peak-to-peak separation (ΔE_p) was found to be equal to 270 mV. The theoretical value of ΔE_p for one-electron 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 quasi-reversible 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 diffusion-controlled. In addition, in the range from −0.2 to −1.6 V, it can be observed that the copper(II) complex shows a quasi-reversible redox system of the Cu(II)/Cu(I) at E_1/2 equal to −0.87 V/SCE and the peak-to-peak separation (ΔE_p = 140 mV) (Fig. 2, curve B).

Fig. 2 Cyclicvoltammograms recorded with 1 mM of Cu(L)(Py)(ClO₄) in DMF solutions containing 0.1 M TBAP using 100 mV s⁻¹ as scan rate going from: (A) +0.0 to +1.6 and (B) −0.2 to −1.6 V/SCE.

3.3.2. Modified electrode poly-[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 bi-distilled 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 s⁻¹. The cycling between −0.2 and 1.2 V/SCE exhibits a continuous increasing in i_pa and i_pc 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 fact indicates that the copper complex is deposited onto the GC electrode surface and the modified electrode poly-[Cu(L)(Py)(ClO₄)]/GC is so obtained.

Fig. 3 Poly-[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.

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 s⁻¹ with GC-electrode. The buildup of the poly-[Cu(L)(Py)(ClO₄)] films can be characterized by the increase in the i_pa and i_pc peak currents occurring in each repeated cycling which induces the deposition of a conductive polymer on GC-electrode surface.^32–34

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 mV s⁻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.

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 −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 poly-[Cu(L)(Py)(ClO₄)] respectively.

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, fluorine-doped 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.

The modification of these two electrodes, poly-[Cu(L)(Py)(ClO₄)]/ITO and poly-[Cu(L)(Py)(ClO₄)]/FTO are elaborated in experimental conditions similar to those mentioned above for GC-modified electrodes. These modified electrodes are also copiously rinsed with bi-distilled water, with DMF and then transferred into another fresh electrolytic DMF solution containing no monomer. These MEs were identified by SEM microscopy (Fig. 5).

Fig. 5 Scanning electron micrographs of Cu(L)(Py)(ClO₄) complex deposited on ITO (A–D) and FTO (E–H) substrates.

This technique was used in order to evaluate the morphological and chemical changes occurring on the ITO- (Fig. 5A–D) and FTO-electrode surfaces (Fig. 5E–H). 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 poly-[Cu(L)(Py)(ClO₄)] films with ITO and FTO (see Fig. 5A–G). A typical fractal-like 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 cube-shaped uniform-sized grains.

3.3.4. XPS characterization. X-ray photoelectron spectroscopy (XPS) was used to examine the surface composition of [Cu(L)(Py)(ClO₄)] and poly-[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 poly-[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 Cu-poly). Some kinetic energy shifts were observed for poly-[Cu(L)(Py)(ClO₄)]/ITO (Fig. 6b) comparing to free copper [Cu(L)(Py)(ClO₄)] (Fig. 6a) (XPS). So, Fig. 7a and b shows wide-scan X-ray photoelectron spectra of [Cu(L)(Py)(ClO₄)] and poly-[Cu(L)(Py)(ClO₄)]/ITO electrode. The Cu coverage is clearly higher in [Cu(L)(Py)(ClO₄)] than in poly-[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 narrow-scan Cu 2p spectra of the above-mentioned samples (Fig. 6a and b) show the main Cu 2p_3/2 and Cu 2p_1/2 signals (They appearing at 934, and at 940 eV respectively).^39–41 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.

Fig. 6 XPS spectra of Cu sample. (a) Survey spectra of [Cu(L)(Py)(ClO₄)] complex powder, (b) survey spectra of poly-[Cu(L)(Py)(ClO₄)]/ITO films.

Fig. 7 Cyclic voltammograms, recorded at 100 mV s⁻¹ for the electroreduction of bromocyclopentane on GC-electrode in DMF solutions containing 0.1 M TBAP in homogeneous phase (A), in heterogeneous phase (B) and for the electroreduction of iodobenzene (C).

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 electroreduced, 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 1-bromooctane using nickel(I)-salen as catalyst yields n-octyl radical that can be involved in three reactions: (1) coupling to give n-hexadecane; (2) abstraction of a hydrogen atom from DMF to produce n-octane; (3) disproportionation to give n-octane and 1-octene.⁴⁴
3.3.5.1. Effect of concentration. The electropolymerized films of poly-(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. 7A, shows a large enhancement in the reduction peak current i_pc 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 (ΔE_p = 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. 7A).⁴⁵

Also, the electropolymerized films of poly-(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 poly-(Cu(L)(Py)(ClO₄)) in DMF solution, are illustrated in Fig. 7B and C. The same remarks are observed confirming the presence of a significant electrocatalytic effect in the case of the modified electrodes with heterogeneous catalysis.

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 i_pa and i_pc 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 × 10⁻³ M of iodobenzene using the poly-(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.

Fig. 8 Cyclovoltammograms of poly-[Cu(L)(Py)(ClO₄)] recorded in presence of 5 × 10⁻³ M of iodobenzene at different scan rates: 10, 25, 50, 75, 100 mV s⁻¹.

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).

Scheme 2 Proposed yields for electroreduction of bromocyclopentane.⁴⁶

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

Scheme 3 Mechanism for electroreduction of bromocyclopentane proposed by P. W. Raess et al.⁴⁶

Scheme 4 provides a sequence of pathways for the copper(II) complex catalyzing reduction of iodobenzene.⁴⁷

Scheme 4 Mechanism for electroreduction of iodobenzene proposed by P. C. Gach et al.⁴⁷

3.3.6. Chronoamperometry. Chronoamperometry results obtained for the two concentrations of iodobenzene solutions using poly-Cu(L)(Py)(ClO₄) are shown in Fig. 9. This technique can be employed to estimate the catalytic rate constant, k_h for the reaction between iodobenzene and poly-Cu(L)(Py)(ClO₄) according to Galus method.⁴⁸Where I_C is the catalytic current of iodobenzene and poly-Cu(L)(Py)(ClO₄), I_L is the limiting current in the absence of iodobenzene, and γ = k_hC_bt (C_b 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 eqn (1) can be reduced to the eqn (2) where t is the time elapsed in seconds. This equation can be used to calculate the rate constant of the catalytic process k_h, based on the slope of I_C/I_L vs. t^1/2 plot, k_h can be obtained for a given iodobenzene concentration from the values of the slopes. An average value of k_h was found to be k_h = 0.48 × 10³ cm³ mol⁻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.

Fig. 9 Chronoamperograms obtained for two concentrations of iodobenzene solutions using Cu(L)(Py)(ClO₄) modified electrode, see curve (A); slope of I_C/I_L vs. t^1/2 plot (B).

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 techniques, such as infrared spectroscopy, UV-Vis 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 poly-[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.

Acknowledgements

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 CNRS-Université de Strasbourg, ECPM, 25 Rue Becquerel, F-67087 Strasbourg Cedex 8, France and Professor ZERROUAL Larbi, Département Génie des procédés, Faculté de Technologie, Université Ferhat ABBAS Setif-1, Sétif 19000 Algeria.

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Footnote

† CCDC 1048607 formula: C₁₆ H₂₀ Cl₁ Cu₁ N₃ O₇. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra10819e

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