A Schiff Base Nickel Complex. A New Material as Modifier for Selective Electrodes

Perla Elizondo Martínez, Blanca Nájera Martínez and Cecilia Rodríguez de Barbarín

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AZojomo (ISSN 1833-122X) Volume 2 April 2006

Topics Covered

Abstract

Keywords

Introduction

Experimental

Synthesis of N,N´-Bis(2-nitrobenzyl)ethylendiimine (L1)

Complexation reaction

Measurement of the Equilibrium Constants

Modification of Electrodes

Results and Discussion

Acknowledgements

References

Contact Details

Abstract

Our interest in Environmental Chemistry has prompted us to design new materials containing polyfunctional units able to bind certain metallic ions.  These materials can be used as modifying agents to produce chemically modified electrodes. We expect to facilitate the detection of organic pollutants in water by binding these materials to an electrode surface.  We report a Ni(II) and Co(II) complexes prepared from the Schiff base macroligand N,N´-bis(2-nitrobenzyl)ethylendiimine (L₁) with Ni(II) salt and L₁ with Co(II) salt.  The nature of the complex was established by common spectroscopic techniques.  A thermal gravimetric analysis of the complex showed that it is thermodynamically stable. Its formation constant was obtained by conductimetric analysis (K_f = 1.25x10⁶).  The affinity of L towards Ni(II) and Co(II) was determined by atomic absorption.  The electrochemical study of the complex [Ni(II)-L₁]SO₄ was also performed by the cyclic voltammetry technique.  The results showed that, under certain conditions, this complex is capable of polymerizing on the electrode surfaces made of vitreous carbon and indium tin oxide.  The chemical modification on the surface was probed by exploring the cyclic voltammograms, which showed oxidation and reduction peaks that are characteristic of the Ni(II)/Ni(III) pair.  The sensing capability towards 2,4-dichlorophenol of these modified electrodes is currently under study.

Keywords

Schiff base Ni(II) complex, electrodes, modifiers

Introduction

There has been a considerable effort in recent years towards the preparation of new materials containing polyfunctional units (ligands) able to bind metallic ions.  The Schiff base macroligands [1-6] are synthesized from the reaction of dialdehydes and amino compounds.  Given that these materials form stable complexes, they provide the opportunity to design new systems selective to specific metallic ions.  These materials could be applied in different areas such as electrochemistry, bioinorganic, catalysis, metallic deactivators, separation processes and environmental chemistry among others [7-9].

New Schiff base [2, 4, 10-11] macroligands have been synthesized to study their selectivity towards complexation of different metallic ions.  These newly synthesized compounds are also used as precursors for the formation of other acyclic and cyclic ligands.

Complexes of Schiff base ligands with structural similarities to phtalocyanines (N4-macrocycles) [7, 12-14], and other related compounds [15-20], are currently used as modifiers of the active surface of electrodes to improve their catalytic activity in the selective detection of organic pollutants [15, 21-24] and the entrainment of metals.  Along the same line, our interest is to evaluate the capability of N,N’-bis(2-nitrobenzyl)ethylendiimine L₁ (Figure 1), to bind diverse metallic ions such as Ni(II) and Co(II), and carry out selectivity studies.  These complexes are later studied to test their potential in the development of chemically modified electrodes for use in pollutant detection.

The selectivity studies showed that the complex [NiL₁]SO₄ is very stable at Standard temperature and pressure.  It is currently being tested as a possible modifier of the active surface of an electrode.  Under certain specific conditions this compound has been electrochemically polymerized over a vitreous carbon (VC) surface and over a glass surface covered with indium tin oxide (ITO) respectively.  These chemically modified electrodes are being tested for the detection and degradation of organochlorinated pollutants.

Experimental

All the reactants used to obtain the ligands and their corresponding complexes were analytic grade from Sigma-Aldrich Chemical Company Inc, USA.

Synthesis of N,N´-Bis(2-nitrobenzyl)ethylendiimine (L₁)

L₁ was obtained by a condensation of a Schiff base according to a procedure previously reported, [3].  The nature of this compound was probed by infrared (IR) spectroscopy, and Nuclear Magnetic Resonance (NMR) and by melting point.

Complexation reaction

The ligand L₁ was mixed directly in a refluxing solution with Co(II) and Ni(II) sulfates in equimolar amounts in acetonitrile.  The corresponding complexes were characterized by IR spectroscopy and thermogravimetric analysis.

The IR spectrum of the ligand shows a band at 1637 cm^-1 corresponding to the imine group.  The spectra of the complexes Ni(II)-L₁ and Co(II)-L₁ are consistent with the presence of the imine group showing the same band.  It was also observed a shift of the bands corresponding to nitro groups along with a decrease in their intensity.  This displacement suggests the presence of a heteroatom-M bond.  The spectra also show an intense band around 1080 cm^-1 corresponding to the sulfate ion.

A solution of Ba(II) and Pb(II) was added to both complexes and an instantaneous precipitation occurred, suggesting that the sulfate ion is outside of the coordination sphere.

Measurement of the Equilibrium Constants

The formation constants for the complexes of Ni(II) and Co(II) with L₁ were determined by conductimetry.  Conductivities were measured for a series of solutions of each complex using an Orion conductimeter model 162.  The formation constants are expressed in molar concentration at 25^oC.

Modification of Electrodes

The electrochemical studies of the complex [Ni(II)- L₁]SO₄ were performed with a conventional three-electrodic potentiostatic system (CV-50W/ LG-50) (BAS)). The working electrodes were either a vitreous carbon electrode (VC) (BAS) of 3 mm diameter polished prior to all experiments, or an indium tin oxide (ITO) electrode.  The potentials were measured with reference to the Ag/AgCl_(s), KCl (BAS) electrode, placed in a compartment containing the supporting electrolyte.  All the experiments were performed at room temperature.

Table 1. Infrared data for N,N’-bis(2-nitrobenzyl)ethylendiimine (L₁), [NiL₁]SO₄ and [CoL₁]SO₄.

Results and Discussion

The ligand L₁ was obtained as yellow crystals with 83% yield.  This yield was higher than the results reported in the literature [3].

The nature of the ligand was established using IR and NMR spectroscopy as well as analysis of monocrystal X- ray diffraction.  The results of the IR analysis verify that the condensation reaction of the Schiff base was successful.  The previous assumption was based on the presence of bands corresponding to the imine group at 1637 and the nitro group at 1335 and 1516 cm^-1 as well as absence of the characteristic signals of the carbonyl and amine L₁ groups, as can be seen in Table 1.

The spectrum ¹H NMR of the diimine ligand (L₁) presents signals in the interval of 8.75-8.85 ppm corresponding to CH=N.  This signal proves the integrity of the product. (Figure 2).  The resonance data of the ligand are consistent with the proposed structure.

A single crystal of L₁ for XRD studies was obtained.  The Figure 3 shows the molecular structure of L₁, it belongs to the monoclinic system.  The Crystallographic data for L₁ is reported in Table 2.

The interaction of Ni(II) and Co(II) with the ligand L₁ was investigated.  The complexes were obtained from direct reaction between the metallic ions and the ligand using acetonitrile as solvent.  The compounds showed different colors and were obtained with a 45 - 48% yield.

Table 2. Crystal data for N,N’-bis(2-nitrobenzyl)ethylendiimine (L₁).

With respect to the IR data for the complexes, they were consistent with the presence of the imine group.  Each complex exhibited a band between 1630 and 1637 cm^-1.  A shift and an intensity decrease of the bands of the nitro groups could also be observed.  This suggests the presence of a heteroatom-M bond.  The spectra also showed a band between 1080 and 1130 cm^-1 which corresponds to the sulfate ion.  The instantaneous precipitation of the sulfate ions upon addition a solution of Ba(II) and Pb(II) suggests that the sulfate is outside of the coordination sphere.

The selectivity studies were carried out by atomic absorption.  Table 3 shows the results which illustrates the preferences of the ligand to Ni(II) and Co(II) ions.

Table 3. Selectivity Studies of the ligand L₁ toward the Co(II) and Ni(II) ions determined by Atomic Absorption.

The conductimetric analysis for the complexes (shown in Tables 4 and 5) provide K_f = 1.25 x 10⁶ for [NiL₁]SO₄ and 5.6 x 10⁵ for [CoL₁]SO₄, these values show that the Ni(II) complex is slightly more stable than Co(II) complex.  The results are in agreement with the stability reported in the literature for a series of complexes of Ni(II) and Co(II) based on their ionic radius [25]. A change of color at 87^oC was observed while measuring the melting point of Ni(II)-L₁ complex.  This temperature is lower than 100^oC, therefore one could think that such change in color is the result of the loss of non-essential water (hygroscopic or hydration type).  The compound remained stable until 300^oC where it begins to decompose.

Table 4. Determination of the Formation Constant of [NiL₁]SO₄ by Conductimetry.

Kf = 1.25x10⁶

Table 5. Determination of the Formation Constant of [CoL₁]SO₄  by Conductimetry.

Kf = 5.60 x10⁵

The thermograms from thermal studies performed on the complex [Ni(II)-L]SO₄ are shown in Figure 4.  These results show a loss of weight at approximately 90^oC and also an endothermal peak at the same temperature.  It can be thought that the complex loses hygroscopic and/or crystallization water at around 90^oC (i.e. not directly bonded to the central metal ion).  Latter can be demonstrated with the color changes of the compound, that it was due to the loss of water molecules.

The alternatives previously mentioned for the case of the complex [Ni(II)-L₁]SO₄ could be possible because the Ni can form stable complexes acting with a coordination number of 4 or 6.  Nevertheless, the chances of the water being coordinated at that temperature where the loss of weight occurs, are very small.

Electrochemical studies were carried out on the complex [NiL₁]SO₄ and it was possible to polymerize this compound on the surface of vitreous carbon (VC), and indium tin oxide (ITO) electrodes.  The studies were made using cyclic voltammetry in sweeps of 30 cycles.  The chemical modification on the surface of the electrodes was proved by the presence of oxidation and reduction peaks which are characteristic of the Ni²⁺/Ni³⁺ redox pair.  Different conditions were tested regarding sweep velocity and pH with the main aim of chemical modification for both electrodes.  According to the obtained voltammograms, the best conditions were found at pH 10 and a sweep velocity of 250 mV/s as shown in Figures 5 and 6.

Conclusions

According to the results obtained, the Schiff base condensation is adequate for the synthesis of the ligand L₁.

The thermal gravimetric analysis and the formation constants for Ni(II)-L₁ (K_f =1.25 x10⁶) and Co(II)-L₁ (K_f = 5.60 x10⁵) suggest that both complexes are thermodynamically stable.

The selectivity studies show that L₁ has greater affinity towards Ni(II) than Co(II).

The complex [NiL₁]SO₄ is electrochemically polymerized modifying the active surface of vitreous carbon and ITO electrodes.  The results from the analysis show that the modification is favored on the VC conductor more than ITO conductor under the following conditions: pH 10, sweep velocity 250 mV/s, 30 cycles.  Solution 1 mM [Ni(II)-L₁]SO₄, 0.1 M Na₂SO₄, sweep of potential from 0 to 1000 mV.

The voltammograms suggest that a film of Ni(II) complex is formed because of the  anodic oxidation of Ni(II)-L₁.

According to the preliminary results the VC modified electrode it can detect organo chlorinated compounds nevertheless, it is necessary to carry out more studies in order to obtain the best conditions for detecting the organic pollutants.

Acknowledgements

We thank to PAICyT and Facultad de Ciencias Químicas de la Universidad Autónoma de Nuevo León for financial support The authors also aknowledge Perla Martínez for carrying out some of the experiments and Leonor Blanco and Olga Espinosa for the Cyclic Voltammetry studies.

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Contact Details

Perla Elizondo Martínez,  Blanca Nájera Martínez and Cecilia Rodríguez de Barbarín

Facultad de Química, Universidad Autónoma de Nuevo León
Ave Pedro de Alba S/N Cd. Universitaria,
San Nicolás de los Garza, N. L,
México, 66450

E-mail: [email protected]

This paper was also published in print form in “Advances in Technology of Materials and Materials Processing”, 8[1] (2006) 41-48.