Electrochemical properties of Cu4[PhN3C6H4N3(H)Ph]4(µ-O)2, a tetranuclear Copper(II) complex with 1-phenyltriazenido-2-phenyltriazene-benzene as ligand

Propriedades eletroquímicas do Cu4[PhN3C6H4N3(H)Ph]4(µ-O)2, um complexo tetranuclear de Cobre (II) com o ligante 1-feniltriazenido-2-feniltriazeno-benzeno

Electrochemical properties of Cu₄[PhN₃C₆H₄N₃(H)Ph]₄(µ-O)₂, a tetranuclear Copper(II) complex with 1-phenyltriazenido-2-phenyltriazene-benzene as ligand

Propriedades eletroquímicas do Cu₄[PhN₃C₆H₄N₃(H)Ph]₄( µ -O)₂, um complexo tetranuclear de Cobre (II) com o ligante 1-feniltriazenido-2-feniltriazeno-benzeno

A. Hermann^I; L. S. Silva^I; C. R. M. Peixoto^{I, *}; A. B. de Oliveira^II; J. Bordinhão^III; M. Hörner^II

^IDepartamento de Biologia e Química, Universidade Regional do Noroeste do Estado do Rio Grande do Sul (UNIJUI), Rua do Comércio, 3000, Bairro Universitário, 98.700-000 Ijui, RS, Brazil

^IIDepartamento de Química, Universidade Federal de Santa Maria (UFSM), Campus Universitário, 97.105-900 Santa Maria, RS, Brazil

^IIIInstituto de Química, Universidade Federal do Rio de Janeiro (UFRJ), Cidade Universitária CT Bloco A-641, 21945-970 Rio de Janeiro, RJ, Brazil

ABSTRACT

Bis-(µ₂-oxo)-tetrakis{[1-feniltriazene-1,3-diil)-2-(phenyltriazenil)benzene copper(II) is a tetranuclear complex which shows four Cu(II) ions coordinated by four 1,2-bis(phenyltriazene)benzene bridged ligands, with one diazoaminic deprotonated chain, and two O^2- ligands. The complex reduces at E_1/2 = -0.95 V vs Fc⁺/Fc, a two electrons process. Cyclic voltammetric and spectroelectrochemical studies showed a reversible process. When immobilized on carbon paste electrode, the complex electrocatalyses the reduction of O₂ dissolved on aqueous solution at –0.3 V vs SCE potential. The obtained current shows linearity with O₂ concentration.

Keywords: copper(II) complex; 1,2-bis(phenyltriazene)benzene ligand; spectroelectrochemical characterization; electrochemical sensor, oxygen reduction.

RESUMO

Bis-(µ₂-oxo)-tetrakis{[1-feniltriazeno-1,3-diil)-2-(feniltriazenil)benzeno cobre(II) é um complexo tetranuclear que apresenta quatro íons Cu(II) coordenados por quatro ligantes 1,2-bis(feniltriazeno)benzeno em ponte, com uma cadeia diazoamínica desprotonada, e dois ligantes O^2-. O complexo apresenta um processo de redução por dois elétrons a E_1/2 = -0,95 V vs Fc⁺/Fc. Voltametria cíclica e espectroeletroquímica demonstraram um processo reversível. Quando imobilizado em eletrodo de pasta de carbono, o complexo eletrocatalisa a redução de O₂ dissolvido em solução aquosa a –0,3 V vs SCE. A corrente obtida apresenta linearidade com a concentração de O₂.

Palavras-chave: complexo de cobre (II); ligante 1,2-bis(feniltriazeno)benzeno; espectroeletroquímica; redução de oxigênio.

Introduction

Compounds containing a chain with three nitrogen atoms, the triazenes, are largely described on the literature [1,2]. Among other uses, they were studied as chemotherapeutic agents. Triazenes and anionic triazenide ligands can show different types of coordination in metal complexes [1-5]. They can be monodentate, (N1,N3)-chelating towards one metal atom or (N1,N3)-bridging over two metal atoms.

Compounds containing two chains of three nitrogen atoms, the bistriazenes, are also known [6-14]. The possibilities of coordination are enhanced when two nitrogen chains are present on the ligand. Complexes of ortho-, meta- and para-bis(phenyltriazene)benzenes ligands were synthesized and characterized. As expected, several different coordination types were observed. Bistriazenes show the coordination properties of triazenes and also the capacity to force close contacts between metal atoms.

Although the structure of complexes with bistriazene ligands have been described, no reports were found in the literature related to electrochemical properties of these compounds. These studies are important for characterization and to a better understanding of their reactivity, stability and possible applications.

The 1,2-bis(phenyltriazene)benzene ligand, C₆H₅NNN(H)C₆H₄NNN(H)C₆H₅, was used to synthesize the complex bis-(µ₂-oxo)-tetrakis{[1-phenyltriazene-1,3-diil)-2-(phenyltriazenil)benzene copper(II), Cu₄[PhN₃C₆H₄N₃(H)Ph]₄(µ₂-O)₂, (which will be represented by Cu₄L₄). This molecule is formed by a tetramer of four copper(II) ions and four 1-phenyltriazenido-2-phenyltriazene benzene anions, [C₆H₅NNNC₆H₄NNN(H)C₆H₅]^-. Each ligand chelates one metal ion by the deprotonated N₃ chain and coordinates monodentate to the neighboring one, using the protonated N₃ chain. The p-oxo ligands occupy the twofold axis and bridge two opposite edges of the Cu₄ tetrahedron. The crystal structure of this complex was determined by X-Ray crystallography and was described elsewhere [12].

The objective of this work was the characterization of the electrochemical, spectroelectrochemical and electrocatalytic properties of the Cu₄L₄ complex. For electrocatalytic studies the complex was incorporated on a carbon paste electrode, and used as electrocatalyst for the O₂ reduction reaction in aqueous solution.

Experimental

Synthesis

The ligand 1,2-bis(phenyltriazene)benzene and the complex Cu₄L₄ were prepared as described on the literature [12].

Electrochemical measurements

Cyclic voltammetry was carried out with a Microquímica instrument, consisting of a MQPG-01 potentiostat. A platinum disc and saturated calomel (SCE) were used as working and reference electrodes. A platinum sheet was used as the auxiliary electrode. For controlled potential electrolyses, cylindrical Pt grids were used as working and auxiliary electrodes and the potential was fixed at -0.4 V vs SCE. For spectroelectrochemical measurements the Microquímica potentiostat was used in parallel with a Pharmacya Biotech, Ultrospec 2000 spectrophotometer. A three-electrode system was designed for a rectangular quartz cell of 0.03 cm internal path-length. A gold minigrid was used as a transparent working electrode, in the presence of a small saturated calomel reference and a platinum auxiliary electrode. Tetrahydrofuran P.A. (Vetec) was used as solvent and 0.1 mol L^-1 tetrabutylammonium hexafluorophosphate (Sigma), as supporting electrolyte, in nitrogen atmosphere. The concentration of the Cu₄L₄ complex was 1 mmol L^-1. Ferrocene (Merck) 1 mmol L^-1 was added to the Cu₄L₄ solution in the final of the experiment of cyclic voltammetry in tetrahydrofuran medium to use its potential of oxidation as an internal potential standard.

For oxygen reduction reaction studies, a carbon paste electrode was constructed by mixing 20 mg of graphite powder (Merck), 5 mg of Cu₄L₄ complex and one drop of mineral oil (Nujol). For electrochemical measurements, aqueous solution, at pH 4.5, containing potassium chloride (Reagen) 1 mol L^-1 as supporting electrolyte, was used. Dissolved oxygen concentration was controlled by bubbling N₂ or atmospheric air. Oxygen concentrations were determined by Winkler method [15].

Results and discussion

Electrochemical characterization

The cyclic voltammogram of Cu₄L₄ complex is represented in Fig. 1a. It was observed a reversible reduction process at E_1/2 = –0.95 V vs Fc⁺/Fc. In the cyclic voltammogram of the free ligand 1,2-bis(phenyltriazene)benzene no redox process occurs at this potential, indicating that this is due to copper(II) ions (Figure 1b).

Two electrons transfer was determined by controlled potential electrolyses. As there are four copper(II) ions, it may occur the process Cu(II)₄ Cu(II)₂Cu(I)₂, with localization of the two electrons in two copper ions, or delocalization of them around the copper tetramer.

Spectroelectrochemistry

The reversibility of the Cu₄L₄ redox process was also observed by spectroelectrochemistry. The electronic spectrum of the ligand 1,2-bis(phenyltriazene)benzene shows an intense band at 378 nm (ε=32120 mol^-1 L cm^-1) (Fig. 2c) due to transitions in the triazene group [2,13,14]. In the complex this band appears at 388 nm (ε=33860 mol^-1 L cm^-1) (Fig. 2a). The spectrum of the complex also shows a broad and low intense band in the 600-800 nm region, due to d-d transitions in the copper(II) ion [16]. The intensity of this band decreases and the band at 388 nm is shifted to 395 nm when the complex is reduced at –1.0 V vs SCE (Fig. 2b). When the complex is oxidized again at 0.5 V vs SCE the initial spectrum is observed (Fig. 2a), indicating that there was no decomposition of the reduced form.

Oxygen reduction reaction

The Cu₄L₄ complex incorporated on carbon paste electrode shows the reduction process at E_1/2 = -0.17 V vs SCE (Fig. 3a). When oxygen was dissolved in the aqueous solution, it was observed an increase in the cathodic and decrease in the anodic peak current (Fig. 3b), indicating electron transfer from reduced immobilized Cu₄L₄ to dissolved O₂.

In the Fig. 4 it is shown the intensity of the current observed for O₂ reduction by Cu₄L₄ as a function of the applied potential, obtained by chronoamperometry. It is observed that the electrocatalytic process begins at -0.1 V, and the current increases until -0.3 V vs SCE. In the carbon paste electrode with no Cu₄L₄ complex incorporated the oxygen reduction begins only at -0.4 V.

The current, obtained by chronoamperometry, for oxygen reduction at a fixed potential (-0.30 V vs SCE), is shown in Fig. 5. It was obtained a linear relationship with the concentration of dissolved oxygen in aqueous solution, represented by the equation I = 1.056[O₂] + 1.365, with a linear correlation coefficient of 0.9967, for oxygen concentrations between 1.1 and 5.2 ppm. In accord with this result, it is possible the use of this electrode as a sensor for O₂ in aqueous solution. Due to the insolubility of Cu₄L₄ in aqueous medium the electrode achieved high stability. It was used continuously and maintained the response for at least 180 hours.

Conclusions

This is the first study about electrochemical properties of a bistriazene complex. It was demonstrated the high capacity of the ligand to coordinate and to bridge metallic ions. The stability of the tetramer formed by four copper(II) ions in the bis-(µ₂-oxo)-tetrakis{[1-phenyltriazene-1,3-diil)-2-(phenyltriazenil)benzene copper(II) complex was not affected even after the reduction process, and this permitted the use of the complex as an electrocatalyst.

Acknowledgements

C.R.M.P. is indebted to FAPERGS. A. H. and L. S. S. are indebted to CNPq and to FAPERGS, respectively, for a fellowship. We apologize to Prof. Dr Ademir Neves and Msc. Ademir dos Anjos of the UFSC for use of his laboratory and help in the coulometric measurements.

Received June 02 2008

Accepted August 14 2008

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Publication Dates

• Publication in this collection
  28 Oct 2008
• Date of issue
  2008

History

• Accepted
  14 Aug 2008
• Received
  02 June 2008