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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.53 no.4 Concepción Dec. 2008

http://dx.doi.org/10.4067/S0717-97072008000400011 

J. Chil. Chem. Soc, 53, N° 4 (2008) págs: 1689-1693

 

ELECTROCHEMICAL AND COMPUTATIONAL STUDY OF COPPER (II) ALKYLPYRAZOLONE BASED ENAMINE COMPLEX

 

Y. MORENO1*, J. BELMAR1, F. BROVELLI2, A. BULJAN3, O. PEÑA4 L. MORENO5

1 Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile.
2Depto. Cs. Básicas, Universidad de Concepción, Los Angeles, Chile.
3 Facultad de Ciencias Químicas (Grupo QTC), Universidad de Concepción, Concepción, Chile.
4Sciences Chimiques de Rennes. UMR-CNRS 6226. Université de Rennes 1, Av. du General Leclerc, 35042 Rennes cedex, France.
5Depto. Cs. Básicas, Facultad de Ciencias, Universidad del Bío Bío, Casilla 447, Chillán, Chile.


ABSTRACT

The cyclic voltammograms (CV) of the copper complex CuL2, L: C19H26N30 [1-(n-hexyl)-3-methyl-4-[1-phenylaminopropylidene]-2-pyrazolin-5-one] have been studied. The CV profiles of CuL2 show one or two reduction and oxidation wave. The energy level corresponding to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CuL2 have been determined experimentally from the first oxidation and reduction onset potential, respectively. These valúes were also estimated from Density Functional calculations. The electrochemical energy gap deduced from these measurements (Eg~1.03 eV) agrees quite well with the theoretical value.


INTRODUCTION

Pyrazolones are an important family of organic compounds.12 They have been widely studied as a consequence of their numerous applications36 and in particular as chelating agents for solvent extraction of various metal ions.78 Schiff bases9 and enamines derived from pyrazolones10 are known but they present solubility problems. To overeóme this lack of solubility, convenient syntheses of 1-n-alkyl derivatives of pyrazolones were reported a few years ago.11

The electrochemical methods are very important tools to characterize organic or inorganic systems. In this field, cyclic voltammetry (CV) is recognised as an important technique for measuring band gaps, electrón affinities (i.e. the energy of LUMO, εLUMO) and ionization potential (i.e. the energy of HOMO, εHOMO)12-14 In this case, the oxidation process corresponds to removal of an electrón from the HOMO energy level whereas the reduction eyele corresponds to electrón addition to the LUMO.

Electrochemical studies using metal complexes with ligand based on 1-phenylpyrazolone Schiff bases have been reported.15 However the electrochemical behaviour of 1-alkylpyrazolone was not totally established,16 since it was not achieved to assign the nature of the voltammetric peaks observed.

In the present work we have determined several electronic properties of a copper (II) alkylpyrazolone based enamine complex (scheme 1) by electrochemical measurements (εLUMO, εHOMO and Eg). Furthermore we have compared the experimental with theoretical valúes obtained from density-functional calculations. For this we have calculated the electrón density frontier orbitals and the natural charges.17

EXPERIMENTAL

Copper complex synthesis

The copper (II) complex shown in the scheme 1 was prepared from 0.1000 g (0.32 mmol) of protonated ligand HL (C19H27N30) and 0.032g (0.16 mmol) of Cu(CH3COO)2H20 dissolved in ethanol (lOmL) and heated under reflux for 1 h. The solution was then evaporated to a final volume of 5 mi and allowed to cool at room temperature. The solid was filtered off and crystallized by slow evaporation from a chloroform-hexane mixture, 1:1 v/v (yield 0.08g, 73%; m.p. 396(2) K.18'19

Electrochemical setup

The experimental method to carry out the cyclic voltammetry (CV) has been described in the literature.20,21 A glassy carbón disc of 0.07 cm2 geometric área was used as working electrode. The reference electrode was Ag/AgCl in solution of tetraethylammonium chloride (Et4NCl). The potential was adjusted to 0.197 V ver sus normal hydrogen electrode (NHE).22 Platinum gauze was used as counter-electrode.

Prior each experiment, the working electrode was polished with alumina slurry (particle size 0.3 um) and the rinsed with bidestilled water and acetonitrile. Acetonitrile anhydrous (Aldrich Chemical Co) was used as solvent and it was manipulated with syringes. Tetrabutylammonium perchlrorate (TBAC104) (Aldrich Chemical Co) was used as supporting electrolyte and it was dried under vacuum at 40°C for 24 h.

The electrochemical profiles were obtained from 0.001 M copper complex and ligand solutions containing 0.1 M supporting electrolyte at 100 mV/s scanning rate.

All solutions were kept under flowing argón during 30 min before each experiment. The gas flux was inverted to keep an inert atmosphere over the solution while the electrochemical perturbation was applied. The electrochemical data were recorded in a BAS CV-50W system.

Computational calculations

The calculations were performed by using effective core pseudopotentials (ECP) basis set on the copper atom, and the standard basis set 6-31G** for carbón, oxygen, nitrogen, and hydrogen atoms.23 The Ar core electrons of Cu were replaced by an ECP and DZ quality Hay and Wadt Los Alamos ECP basis set (LANL2DZ)24 was used for the valence electrons. The LANL2DZ basis set has been successfully used in calculation on metalloporphyrins and derivatives and transition metal complexes.25,26

Electrón correlation effeets were considered employing density functional theory (DFT) methods, which have been considered as a practical and effective computational tool, especially for organometallic compounds.27,28 Among these functional procedures, the most reliable approximation is often thought to be the hybrid HF/DFT method using a combination of the three nonlocal correlation functional; this method is called B3LYP.29 GAUSSIAN 03W package30 was used in the calculations. The geometry optimization was carried out with the unrestricted B3LYP functional29,31 and using the above basis sets. The molecular structures were fully optimized in gas phase, without symmetry restrictions, and compared with single crystal X-ray structure.19,32 To plot the electrón densities of HOMO and LUMO levéis, we used the MacSpartanPro software.33 The natural population analysis was performed using the NPA keyword implemented in the GAUSSIAN 03W.34

RESULTS AND DISCUSSION

Electrochemical characterization

The electrochemical behaviour of the CuL2 complex and the ligand L were studied by cyclic voltammetry (CV) (see figure 1). The CVs profiles were performed inthe potential range of-1.60 to +1.35 V. The CuL2 and L profiles in acetonitrile show one or two reduction and oxidation waves depending on the potential limits imposed.

For the ligand, two irreversible anodic peaks were observed at +0.775 and +1.250 V in the anodic sean (-1.025 and -1.540 V for the cathodic sean). Other authors have studied similar enamines ligands (e.g. 1-(n-hexyl)-3-methyl-5-pyrazolone16) and they found only one irreversible anodic peak. In our situation, if the anodic potential limit is increased, another peak can be observed. This feature is more representative of a molecular system with several redox sites because the ligand used in this study is a more reactive enamine. On the other hand, the copper (II) complex shows anodic peaks potentials at +0.590 and +0.690 V for the anodic sean and -0.850 and -1.100 V for the cathodic sean respectively. The onset potentials (E ) for the first oxidation and reduction waves are situatedat+0.416 V and -0.615 V, respectively. The onset potentials were obtained from the intersection of the two tangents drawn at the raising current and the background charging current of the CVs. The feature found is very similar to that found for the ligand; nevertheless anodic peaks appear at a less anodic potential and, in addition, both peaks correspond only to the ligand.

An important aspect to emphasize is that during the formation of the copper complex a change in the ligand structure takes place, which allows the formation of new bonds with the copper (II) ion. In order to make a correct allocation of the observed electrochemical phenomena it is necessary to calcúlate the energy of the frontiers orbital in the complex so as to determine the most reactive sites in this molecule.

As a first approach, it is possible to determine the energy of the frontier orbital from the electrochemical parameters. The HOMO and LUMO energies can be determined from the first oxidation and reduction onset potentials, respectively. The potential difference can be used to estimate the energy gap of the copper complex, that is, the Eg = εLUMO- εHOMO . The energy level of the normal hydrogen electrode (NHE) is situated 4.5 eV below the zero vacuum energy level.35 From this valué and the redox potential of the reference electrode used in the present work Ag/AgCI (0.197 V), a simple relation can be written which allows estimating both energy values:36,37

The experimental energies values of HOMO and LUMO leveis for CuL2 are -4.28 and -5.31 eV respectively; therefore the band gap estimated from the electrochemical measurements is 1.03eV(table 1). The electrochemical energy gap deduced from these measurements is mayor than calculated theoretical value. This result is not strange, because it is well-known that the calculated band gap in many cases does not agree with the experimental value.38,39

Computational calculation

As a first approach we have carried out the geometry optimization of the ligand (L) and the complex (CuL2). However, duringthe geometric optimization process, convergence problems took place, increasing the calculation time; furthermore, it was not found a mínimum energy conformation in both cases. This is due to the great number of conformers that can exist in ligand and complex respectively. Due to this, in both the ligand and the complex, we have eliminated the aliphatic flexible chains and they were replaced by methyl groups, this allowed to avoid the problem of convergence and to find a mínimum (see scheme 2).

At this stage, a new problem was observed: the geometry around the copper ion in the optimized model invariably evolved toward a distorted tetrahedral. However the experimental geometry showed a plañe square coordination around copper ion, without solvent molecules in the axial positions. To make modellization more realistic in terms of coordination geometry, two molecules of acetonitrile were included at the axial positions. From now on, we will refer to this new model as CuL2S2 (S = CH3CN). In this way, in the optimization process we considered the effect of two solvent molecules on the equihbrium geometric parameters. We consider that this modification should not affect significantly the validity of the electrochemical parameters. The equihbrium geometries of the ligand L and of the complex CuL2S2 obtained through DFT calculations were compared with those reported in literature.19,32 Table 2 shows valúes of bond length for the optimized ligand model employed in this study, which are in agreement with the experimental data obtained by X ray diffraction (XRD). In addition table 2 shows the valúes of the optimized geometric parameters for the CuL2S2 complex model. The results are quite cióse to the valúes reported in the literature32 since the errors found in bond distances and angles are less than 0.2%, e.g. 0.04 Á and 0.5 degrees.

Once the geometries for the ligand L and the CuL2S2 complex were optimized, we have calculated the electrón density maps of HOMO and LUMO states for these models (see figure 2). Also, we calculated the electrón density maps of the CuL2 complex, our aim being to observe the effect of solvent molecules (CH3CN) on the electronic properties of the complex.

The electrón density maps suggest that the oxidation of the ligand involves mainly the pirazolone moiety (see figure 2a). The presence of several reactive sites in this molecule must be due to the presence of conjugated double bonds, which influences the distribution of the electronic density (the HOMO level is delocalized over several atoms). In the copper complex (figure 2c), the situation is very similar to that of the ligand; however it exists a location of the electronic density between the Cl, C2, N2, Cl', C2' and N2' atoms. This location must be mainly the effect of the copper ion. The electrón density map for the complex which includes the two acetonitrile axial molecules is a little different, because the localization is now over C3, C5, C3' and C5' atoms.

For the LUMO level the electronic density maps indícate that these molecules may suffer a nucleophilic addition to a,p unsaturated double bonds. However the effect of the copper ion has modified the distribution of the MO and it participates in the LUMO. Under these features it is possible to identify two oxidation processes that involve only ligand double bonds carbon-carbon, carbon-nitrogen and carbon-oxygen. In the reduction process, besides the ligand double bonds, it also involves the metal ion; in this case three reduction processes will exist, two for the ligand and one for the metal ion.

Table 3 shows the atomic charges from the natural population analysis (NPA) 17 for ligand L and CuL2 and CuL2S2 model complexes. We have included natural charges associated to atoms that are directly bonded to copper ion. The calculated charge on the copper ion is lower than the formal charge +2. It results from charge donation from the nitrogen and oxygen atoms of the ligand.

 

The atomic charge presented by the atoms OÍ and N3 in the ligand decreases after complex formation. This is because oxygen and nitrogen have the electron-donor's behavior on the copper ion (an electrophilic site). For the copper complex the negative región involved by the bonds of the ligand and the positive región is located in the metal ion and over Cl and C5. These results are coherent with those obtained with HOMO and LUMO electrón densities.

The band gap energy values calculated for CuL2S2 and CuL2 from DFT UB3LYP/6-31G**/LANL2DZ level theory calculations are 0.66 and 0.61 eV respectively.

CONCLUSION

Cyclic voltammograms of copper complex CuL2, L: C19H2(¡N30 [l-(n-hexyl)-3-methyl-4-[l-phenylaminopropylidene]-2-pyrazolin-5-one] show one or two reduction and oxidation waves. The electrochemical oxidation process involves only ligand double bond but the reduction involves the ligand and also the copper ion. The energy levéis corresponding to HOMO and LUMO levéis have been determined experimentally from the first oxidation and reduction onset potential respectively, and their valúes were estimated using DFT UB3LYP/6-31G**/LANL2DZ level theory. The electrochemical energy gap deduced from these measurements (E ~ 1.03 eV) it is something different than the calculated theoretical valué, although it falls inside the expected range.

ACKNOWLEDGEMENTS

The authors thank to DIUC 208.021.026-1.0 and FONDECYT 1040461 for the financial support and Spanish Research Council (CSIC) for providing us with a free-of-charge license to the CSD system.

Supplementary data

Supplementary data for this paper are available from the IUCr electronic archives (reference: OB1229)

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(Received: December 26, 2007 - Accepted: May 23, 2008)

*e-mail: ymoreno@udec.cl

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