Cobalt ( II ) Phthalocyanine Bonded to 3n-propylimidazole Immobilized on Silica Gel Surface : Preparation and Electrochemical Properties

A ftalocianina de cobalto(II) imobilizada sobre a sílica gel (área superficial SBET = 500 m 2 g) modificada com 3-n-propilimidazol, foi incorporada a um eletrodo de pasta de carbono e mostrou-se eficiente na eletrocatálise da oxidação do ácido oxálico. As interações intermoleculares do complexo, as quais podem normalmente interferir no processo redox, praticamente não foram observadas no presente caso, devido a baixa densidade média do complexo na superfície do material sintetizado δ = 4,7 x 10 mol cm (δ= Nf/SBET, onde Nf é a quantidade de Co-Ftalocianina adsorvida por grama de silica gel modificada ). A resposta linear do eletrodo para concentrações do ácido oxálico entre 6,5 x 10 e 3,2 x 10 mol L, associado com sua alta estabilidade química tornam o material com a ftalocianina de cobalto(II) ligado covalentemente muito atrativo na preparação de uma nova classe de sensores químicos.


Introduction
Phthalocyanine metal complexes have been known for a long time as homogeneous and heterogeneous catalysts for a great variety of chemical reactions [1][2][3] where the metal of the macrocyclic complex acts as a redox center.Therefore, it is very interesting that more applications for metallated phthalocyanines could be found.It is possible to extend many applications in solution phase of these complexes to a solid phase by an immobilization process on an inert matrix surface and by an impregnation or chemical bonding procedures 4 .Complexes immobilized by an impregnation procedure on an inert matrix surface have been used to prepare chemically modified electrodes viewing numerous electrocatalytic reactions [5][6][7][8][9][10][11][12] .Another procedure has been to immobilize the phthalocyanine or porphyrine complexes by a chemical bonding of the central metal atoms to basic groups grafted on solid surfaces [13][14][15][16][17][18][19][20][21] .
This work reports the electrochemical properties of the Co-phthalocyanine complex bonded to imidazole groups grafted on a porous silica gel surface.The main objectives were: (a) to obtain a highly dispersed covalently bonded macrocyclic complex and (b) to study a model catalytic reaction in order to have a better understanding of the mechanism of electron transfer process on a surface of an insulating material.Electroxidation of oxalic acid was studied in order to probe the potentiality of the material.

Preparation of the chemically modified silica
Preparation of the material was carried out in two steps.In the first step, 3.4 g (53 mmol) of imidazole and 9.2 cm 3 (50 mmol) of 3-chloropropyltrimethoxysilane from Aldrich were added into a reaction flask containing 250 cm 3 of dry toluene, and the mixture was refluxed for 2 h under nitrogen atmosphere.In the second step, 20 g of silica gel with specific surface area of 500 m 2 g -1 and previously degassed under vacuum (10 -4 Torr) at 393 K was immersed in a flask containing this mixture.The mixture was refluxed for more 8 h under nitrogen atmosphere.The resulting material was filtered, washed with toluene and ethanol.Furthermore, the solid was washed with 10 -3 M aqueous HCl solution, 0.5 wt% HCO 3 -solution and finally with bidistilled water.The solid was dried under vacuum (10 -4 Torr) at 393 K.

Adsorption of Co(II) phthalocyanine on 3-n-propylimidazole silica gel
Cobalt(II) phthalocyanine, CoPc, (0.015 g; 26 µmol) was dissolved in dimethylformamide and 1 g of the 3-npropylimidazole silica gel, hereafter designated as SiIm, was immersed in this solution, stirred and allowed to stand for 24 h.The solid was filtered and washed exhaustively with ethanol until a clear filtrate was obtained.Figure 1d shows the immobilized CoPc on SiIm.

Chemical analyses
The solid was treated with concentrate HNO 3 solution and heated until evaporation of the acid to dryness.Then the metal ion was extracted with 0.1 M HNO 3 solution and analyzed by atomic absorption spectrometry.The amount of adsorbed Co(II) phthalocyanine was 2.2 µmol g -1 .
The amount of the immobilized organic functional groups was determined on basis of the N elemental analysis.The analytical results gave 0.73 wt% of nitrogen corresponding to 0.26 mmol g -1 of the attached functional groups.

Measurements
The electronic absorption spectra of the solid samples were obtained immersing the material in carbon tetrachloride using a quartz cell with 1 mm path length.The equipment used was a Beckman DU 640 spectrophotometer.
The electrochemical measurements were made with an electrode prepared by mixing graphite (Fluka), 50 wt%, and SiIm/CoPc, 50 wt%, and a mineral oil as a binder.This electrode was used as the working electrode, the reference was the saturated calomel electrode, SCE, and a platinum wire was used as the counter electrode.All measurements were made in a electrochemical cell under nitrogen atmosphere.The equipment used was the potentiostat/galvanostat PAR 273 A, from EG&G.

Electronic absorption spectra
Figure 2a shows the electronic absorption spectrum of the solid SiIm/CoPc, obtained by immersing the solid in carbon tetrachloride.Scattering of the radiation is not ob- served because the refractive index of silica gel matrix (η D = 1.45845) is nearly the same of that of carbon tetrachloride (η D = 1.4595) 22 .Figure 2a is compared with the spectrum of CoPc dissolved in dimethylsulphoxide (Fig. 2b) and dimethylformamide (Fig. 2c).The maximum of the absorption band peaks of the complex in solution phase, named B and Q bands, are observed at 326 and 657 nm, respectively.For SiIm/CoPc the maximum of these absorption band peaks are observed at 354 and 672 nm.It is known that for metal phthalocyanine axially coordinated with a ligand, the π interaction between the metal d xz and d yz orbitals with the ring e g (π*) orbitals, or LUMO, is enhanced 23 .The decrease of the energy gap between these two levels upon axial interaction in comparison with that in the free complex, decreases the energy transitions, a 1u (π) → e g (π*) (Q bands) and a 2u (π),b 2u (π) → e g (π*) (B bands).Therefore, in the present case the shift of the Q and B bands to lower wavelength regions is attributed to the imidazole molecule bonding to the metal atom in CoPc 24 .

Electrochemical studies
Pulse differential voltammetry of SiIm/CoPc  12 .The other anodic peak at 0.023 V with lower current intensity was not assigned and its origin is not explained.

Electroxidation of oxalic acid
In order to study the capability for the immobilized Co(II) phthalocyanine to mediate oxidative processes, the oxalic acid oxidation was chosen.The cyclic voltammetry technique was used to carry out such study.
Figure 4 shows the cyclic voltammograms obtained for SiIm/CoPc at pH 3.6 in presence (Fig. 4b) and absence of oxalic acid (Fig. 4a).No current peak is observed when the potential is swept between 0 and 1 V and in the presence of oxalic acid 2.5 x 10 -3 mol L -1 an anodic current peak is observed.
Changing the scan rates, v, and plotting I pa against v 1/2 , a linear correlation was obtained (Fig. 5).This is an indication that the oxidation process is a diffusion controlled process.
The electroxidation of the oxalic acid at the solid-solution interface can be described by the following reactions: The oxalic acid oxidation is strongly influenced by the solution pH since the dissociation constants are pK 1 = 1.46 and pK 2 = 4.4 25 , for the first and second dissociation constants, respectively.The response of the electrode to oxalic acid concentrations was studied at pHs 3.5, 6 and 7.
Figure 6 shows the plot of I pa against [H 2 C 2 O 4 ] in the concentration range from 6.5 x 10 -4 to 3.2 x 10 -3 mol L -1 , in 0.5 mol L -1 NaCl solution and v = 20 mV s -1 .To keep the pH constant, the buffers acetic acid/acetate (pH 3.5), biphthalate (pH 6.0) and TRIS (pH 7.0) were used.In every case a linear correlation between I pa and [H 2 C 2 O 4 ] was obtained and the angular coefficients were 59.3, 3.4 and 10.2 for pHs 3.5, 6.0 and 7.0, respectively.A higher sensitivity is achieved for pH 3.5 where the HC 2 O 4 species is the predominant one.

Chemical stability of the electrode
The stability of the electrode was studied cycling the potential in a 2.5 x 10 -3 mol L -1 HC 2 O 4 -solution, 0.5 mol L -1 NaCl supporting electrolyte solution and scan rate of 20 mV s -1 .The I pa current plotted against the number of cycles (Fig. 7) showed that it did not decrease after 65 cycles, indicating that the electroactive species was not leached from the electrode surface.

Conclusions
The CoPc complex on 3-n-propylimidazole groups grafted on SiO 2 surface efficiently electrocatalyzed the oxalic acid oxidation on a carbon paste electrode surface made of this material.Intermolecular interactions of the complex species which can normally interfere in the redox process practically were not observed in the present case because of the low average surface density of the complex species (4.7 x 10 -13 mol cm -2 ) prepared.A linear response of the electrode for oxalic acid concentration, between 6.5 x 10 -4 and 3.2 x 10 -3 mol L -1 , associated with its high chemical stability make the covalently immobilized Cophthalocyanine complex material very attractive in preparing new class of chemical sensors.

Figure 3
Figure 3 shows the pulse differential voltammetry curve for SiIm/CoPc obtained in 0.1 mol L -1 NaCl supporting electrolyte solution and pH 6.The anodic sweeping show two peaks corresponding to the following oxidation processes: Co(I) → Co(II) = -0.55V and Co(II) → Co(III) = 0.70 V These oxidation potentials are very similar to those observed for cobalt(II) tetrasulfophthalocyanine com-