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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.49 no.3 Concepción Sept. 2004

http://dx.doi.org/10.4067/S0717-97072004000300014 

 

J. Chil. Chem. Soc., 49, N 3 (2004): 261-266

"CHEMICAL METALLATION OF POLY-P-TETRAAMINOPHENYLPORPHYRIN FILMS GROWN BY CYCLIC VOLTAMMETRY ON CONDUCTING GLASS ELECTRODES".

 

Galo Ramírez, Gabriela Cornejo, Mauricio Lucero, Andrea Riquelme, Ignacio Azócar, Francisco Armijo, María J. Aguirre and Ejnar Trollund*.

Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile. Av Libertador Bernardo O'Higgins 3363, Santiago, Chile. E-mail: etrollun@lauca.usach.cl

(Received: May 13, 2004 - Accepted: July 12, 2004)


ABSTRACT

In this paper we present a simple and mild procedure to incorporate cations of transition metals to a polymer of the free-ligand para-tetraaminophenylporphyrin (H2TAPP) in order to obtain similar responses as in the case of the direct electropolymerization of a metallic complex. Sometimes, it is not possible to obtain an electropolymerized complex under a determined set of conditions. Changes in the conditions of synthesis drastically alter the properties of the polymers and, thus, the modification of the parameters of synthesis is not a feasible method to obtain a desired polymer. In those cases, a polymer of the free-ligand can be obtained and then the metallic ions can be incorporated so that an identical polymer to the one that could be obtained from the direct polymerization of the complex is obtained.

Keywords: metallation, electropolymerization, aminophenylporphyrins, free-ligand, modified electrodes.


 

INTRODUCTION

Electrodes coated with electroactive polymers have been extensively studied because they have diverse applications in many fields: electrocatalysis, photosentitization, electrochromics, etc.1-10. One way to perform an electrochemical polymerization is with a functional group in the "monomer" capable to be oxidized and to form active species, which reacts with other neutral molecule2. Metalloporphyrins are very attractive to be polymerized on electrodic surfaces because in addition to be catalyst agents, they have an extended -system anchoring the polymer to the electrodic surface. When electropolymerized, its activity and stability as electrocatalyst are enhanced. They can be used in media where a modified electrode with single layers of non-polymerized complex loss its activity11. However, depending on the conditions and the electrodic surface, there are cases were the polymerization does not occur12. The behavior of electropolymerized azamacrocyclic complexes is strongly dependent on the conditions of electropolymerization and is not comparable a polymer obtained from a determined set of conditions with another obtained from a different set of conditions. For these reasons, it has been published an electrochemical method to metallize some kinds of polymers13. In this work, we present our preliminary results about a simple chemical way to bind transition metal cations to an electropolymerized film of free-ligand para-tetraaminophenylporphyrin (H2TAPP) grown on a conducting glass electrode. The studied cations are Cu (II), Co (II) and Ni (II). For the cobalt case, we compare the voltammetric response and the UV-visible spectra of the metallated polymer with a polymer grown starting from the cobalt complex dissolved in solution, a native cobalt polymer. In the cases of Ni (II) and Cu (II), this kind of comparison is not possible because the complexes does not polymerize in the conditions used here. A comparison between the UV-visible spectra are reasonable because there is a change in symmetry when the porphyrin is metallated or not, and as a consequence, the number of Q bands diminish from 4 to 214. In the case of the cobalt complex, it also were compared the voltammetric response and the electrocatalytic activity toward the oxidation of 2-mercaptoethanol of the native and the metallated cobalt polymer. There are some works concerning the chemical metallation of porphyrins under mild conditions15-19. Also, there are a few studies concerning the kinetic of metallation and the dependence of the degree of metallation with the metal nature20-21. However, at our knowledge, this is the first chemical attempt to incorporate metal cations to a polymer of free-ligand porphyrins grown by cyclic voltammetry.

EXPERIMENTAL

Non-metallated-para-tetraaminophenylporphyrin (H2TAPP), and the corresponding complexes of Co, Ni and Cu (MTAPP) (MidCentury Co.) were used without further purification. The conducting transparent electrode was prepared by SnO2:F deposition on a glass sheet. The electrosynthesis of polymers, native poly-CoTAPP and poly-H2TAPP were performed by continuously cycling the electrode potential between -0.1 and 1.1 V versus Ag/AgCl at 100 mVs-1 for 100 cycles. The electrolyte used in the electropolymerization consisted in a 0.1M tetraethylammonium perchlorate (TEAP) in a mixture of dimethylformamide, (DMF) and acetonitrile (AN); DMF (20%v/v)-AN (80%v/v) containing the monomers: either the complex or the free-ligand in a 1mM concentration. The conducting glass electrode was also modified with a drop of a DMF/AN solution containing the monomer (monomer-electrode). After evaporation of the solvent, the electrode was rinsed with the same solvent, ethanol and water in order to eliminate the excess of the monomer. The electrolyte was purged with pure N2 and kept at room temperature. After the polymerization, the modified electrodes were rinsed with DMF/AN, ethanol and distilled water. The electrochemical experiments were performed in a Wenking POS 73 potentioscan and a Graftec X-Y recorder. The electrochemical cell was a three-compartment glass cell. The working electrode was a SnO2:F conducting glass (2 cm2 of geometrical area), connected to the Ag/AgCl reference electrode compartment, with a Luggin capillary with a Pt wire. The counter electrode was a Pt wire of 10 cm2 of geometrical area. The UV-visible experiments were performed in a one-compartment quartz cuvette. A Varian Cary 1E spectrophotometer, along with CaryWinUV 2.5 software was used to obtain the UV-visible spectra. The chemical metallation of the polymeric free-ligand was performed by dipping the modified electrode in a solution of ethanol (absolute, p.a.) containing the chloride salt of the metal MCl2·nH2O (5% w/v) (M = Ni, Co, Cu). The modified electrode was submerged in the boiling solution for ten minutes, based on the metal insertion porphyrin-method22. The electrooxidation of 2-mercaptoethanol (Fluka, p.a) was achieved by polarization curves at 5mVs-1, in a deaerated aqueous solution (KOH 0.1M) containing 4.5 mM of the thiol. The range of potential used was the open circuit potential of each system and +0.5V versus Ag/AgCl.

RESULTS AND DISCUSSION

Structures of the metallic complexes and the free-ligand are shown in Figure 1. The complexes belong to the D4h symmetry group and the free ligand, to the D2h group. The phenyl substituents are practically perpendicular to the macrocycle23. In spite of the planarity of the core of the metallic complexes, only the cobalt complex electropolymerizes on a conducting glass surface. Neither the nickel nor the copper complexes can be electropolymerized using the same conditions. However, the free-ligand easily polymerizes on the electrodic surface as a thin film.

Fig. 1. Structures of the free-ligand, para-tetraaminophenylporphyrin (H2TAPP) and the metallic complex M-para-tetraaminophenylporphyrin (MTAPP), where M = Co, Ni, or Cu.

Figure 2 shows the voltammetric responses corresponding to the electropolymerization of the free-ligand (Figure 2A) and the cobalt complex (Figure 2B) on the conducting glass surface. The voltammetric behavior is very different in both cases.

Fig. 2A. Electropolymerization of the free-ligand (H2TAPP) by continuously cycling the SnO2:F electrode potential between -0.1 and 1.1 V versus Ag/AgCl at 100 mVs-1 for 100 cycles. The electrolyte used in the electropolymerization consisted in a 0.1M tetraethylammonium perchlorate (TEAP) in a deaerated mixture of dimethylformamide, (DMF) and acetonitrile (AN); DMF (20%v/v)-AN(80%v/v) containing the monomer in a 1mM concentration. The curves presented correspond to the cycles 1, 5, 10, 15 and 20.

Fig. 2B. Electropolymerization of the cobalt complex (CoTAPP) by continuously cycling the SnO2:F electrode potential between -0.1 and 1.1 V versus Ag/AgCl at 100 mVs-1 for 100 cycles. The electrolyte used in the electropolymerization consisted in a 0.1M tetraethylammonium perchlorate (TEAP) in a deaerated mixture of dimethylformamide, (DMF) and acetonitrile (AN); DMF (20%v/v)-AN(80%v/v) containing the monomer in a 1mM concentration. The curves presented correspond to the cycles 1, 5, 10 and 15.

In the cobalt complex system, no redox peaks are visible except the oxidation of the amino group peak. In the case of the free ligand, an intense semi reversible peak is obtained at ca. 0.40V versus Ag/AgCl. Also, a cathodic peak is obtained at ca. 0.20V. The different profiles between both polymers are difficult to understand, but normally, only the non-metalled system shows strongly voltammetric peaks during the polymerization24, 25, corresponding to a ligand couple.

The change in the electronic density of the redox couples due to the presence of the metal probably modifies its voltammetric response. Co(II) attracts the electronic density of the ligand and it is not oxidizable at the potentials showed for the Figure 2A. Both systems show similarity in the amino group oxidation peak, observable at the same potential, close to 0.9V. On the other hand, for the non-metallated film, the voltammetric peaks decrease and move during the polymerization as shown by the arrows. During the polymerization, the electrodic surface changes and the conductivity diminishes due to the formation of a semiconductor film on its surface. This is the reason of the decreasing in the current as the electropolymerization takes place. In both cases, polymerization takes place after the irreversible oxidation of the amino groups, i.e. cationic radicals formed attack neutral molecules. The obtained films are conductive and very stable. After the electropolymerization of the non-metallated-tetraaminophenylporphyrin, poly-H2TAPP, on a glass electrode, it is possible to coordinate different transition metals. The modified-electrode is immersed in a boiling solution of ethanol containing the metal chloride salt, MCl2·nH2O 5% w/v (M = Cu (II), Ni (II), Co (II)) during ten minutes. After this treatment, the polymer is rinsed with ethanol and water to eliminate the non-coordinate metals. Figure 3 shows the voltammetric response of three-modified electrode. In Figure 3 curve A, the response of the poly-H2TAPP is shown. Figure 3 curve B depicts the cobalt-metallated electrode, poly-metallated-CoTAPP and Figure 3 curve C shows the response of the native cobalt polymer, poly-CoTAPP. Both, the curve B and C are very similar, indicating that the polymerized free-ligand has incorporated the cobalt (II) ions in the same manner as the cobalt ions are in the native poly-CoTAPP. Also, as shown in the case of Figure 2, the presence of Co(II) modifies the electronic density of the ligand avoiding this redox couple that appears at ca. 1.0V.

Fig. 3. Voltammograms of SnO2:F electrodes modified with poly-H2TAPP (A), poly-metallated-CoTAPP (B) and native poly-CoTAPP (C). Electrolyte : deaerated DMF/0.1M TEAP. Scan rate: 100 mVs-1.

On the other hand, the electronic spectra of the three modified electrodes and one electrode with adsorbed monomeric cobalt complex, monomeric-Co-electrode are shown in Figure 4. The non-metallated porphyrin H2TAPP belongs to a D2h symmetry group and the metal-complex, MTAPP, to a D4h group. Both systems, in the visible region present to kind of bands, Soret and Q bands26 that are corresponding to - transitions. However, due to a change in the symmetry, the ligand shows four Q bands and the metal-complex, only two Q bands26.

Fig. 4. UV-visible spectra of SnO2:F electrodes modified with poly-H2TAPP ( _____ ), poly-metallated-CoTAPP (--------------), native poly-CoTAPP ( - · - · - · ) and an electrode modified with the monomer; monomeric-CoTAPP ( - - - - -) in air.

In the present case, it is difficult to know the symmetry of the polymers. In fact, the non-metalled complex does not show the characteristic 4 Q bands as expected. Indeed, it presents wide absorptions without defined bands. However, it present two wide signals at 500-650 nm and at 650-900 nm. The second absorption is not present in the case of the metalled polymers. In the case of a D2h complex, the Q band (two Q band) splits because each one generates a vibrational satellite23. Then, it is possible the presence of two Q bands for each wide absorption. In spite of the difficulty in the interpretation of the absorption of the non-metalled complex it is possible to compare the UV-visible spectra of the monomeric-electrodes, native-polymeric electrode and metallated-polymeric electrodes. In spite of the different intensity of the Soret band, at ca. 440 nm, very similar features are obtained between the native poly-CoTAPP and the poly-metallated-CoTAPP. The Soret band for the native poly-CoTAPP and the poly-metallated-CoTAPP show maxima at 440 nm and at 444nm, respectively. The monomeric-electrode shows a maximum at 448 nm and the poly-H2TAPP at 431 nm. The Q bands have maxima at 554 and 599 nm for native poly-CoTAPP and poly-metallated-CoTAPP. The monomeric-electrode also depicts their maxima at the same wavelengths. The characteristics of the poly-H2TAPP are very different, as can be seen in the amplified Figure. Then, the similar voltammetric response (Figure 3) and the same features observed for the electronic spectra (Figure 4) are evidences of a true-metallation with Co. In the case of the metallation with Cu and Ni, the metallated polymers cannot be compared with the electropolymerized complex because they do not polymerize in the conditions used in this work. For this reason, we only compare the spectra of the poly-metallated-MTAPP, the poly-H2TAPP and the monomeric-electrodes. Figure 5 shows the corresponding electronic spectra for the Cu case.

Fig. 5. UV-visible spectra of SnO2:F electrodes modified with poly-H2TAPP (-------------), an electrode modified with monomeric-CuTAPP (························ ) and poly-metallated-CuTAPP (- - - - -) in air.

The similarities between the maxima of the Cu-layer-electrode with the metallated film are less evident than in the case of cobalt. However, the metallated polymer shows more similarity with the monomeric-Cu-electrode than with the non-metallated polymer. The same behavior is obtained for the Ni case (see Figure 6).

Fig. 6. UV-visible spectra of SnO2:F electrodes modified with poly-H2TAPP (--------------), an electrode modified with monomeric-NiTAPP (- - - - -) and poly-metallated-NiTAPP (························) in air.

The electrocatalytic activity of both the metallated and the native poly-CoTAPP toward the oxidation of 2-mercaptoethanol was studied. Cobalt complexes like porphyrins and phthalocyanines catalyze the oxidation of this thiol to the formation of the bisulfide26. In order to check if the metallated polymer has the expected electrocatalytic activity, we compare the UV-Vis spectra of both polymers in the presence and absence of the thiol and after, we compare the polarization curves of the two polymers in the presence of the thiol. Figure 7 A shows the spectra of the native poly-CoTAPP with and without the presence of mercaptoethanol. Figure 7B depicts the spectroscopic behavior of the metallated polymer in the presence and absence of the thiol. In both cases, the behavior is the same.

Fig. 7A. UV-visible spectra of SnO2:F electrodes modified with native poly-CoTAPP with (··········) and without (--------------) 2-mercaptoethanol, Electrolyte, 0.1M KOH and 4.5 mM of the thiol.


Fig. 7B. UV-visible spectra of SnO2:F electrodes modified with the poly-metallated-CoTAPP with (··········) and without (------------) 2-mercaptoethanol, Electrolyte, 0.1M KOH and 4.5 mM of the thiol.

When the thiol is in the solution, the poly-CoTAPP and the metallated polymers show a shift in the Soret band to high energy. The poly-CoTAPP shifts the maximum from 441 nm to 424 nm and the metallated polymer shifts its band from 451 nm to 436 nm. In spite of the difference in the maxima of the Soret band, this kind of shift is expected for poly-CoTAPP in the presence of 2-mercaptoethanol27. On the other hand there is not any shift and any change for the poly-H2TAPP when the thiol is or not in the solution (not shown). Figure 8 compares the polarization curves of the non-metallated, metallated and native poly-CoTAPP in the presence of the mercaptoethanol.

The electrochemical oxidation reaction is28, 29 :

Co(I)-TAPP______ →Co(II)-TAPP + e-
Co(II)-TAPP + RS_____ →[Co(II)-TAPP····RS-]
[Co(II)-TAPP····RS -]_____ →[Co(I)-TAPP····RS·]
[Co(I)-TAPP····RS·]_____ →Co(I)-TAPP + RS·
RS· + RS·___________ →RS-SR

In Figure 8 it is shown that the metallated and the native poly-CoTAPP show an oxidation wave that starts at the same potential. The free-ligand polymer shows an oxidation wave that begins at higher potentials. In absence of the thiol, there is not response (any current) for all the systems in the potential range of the Figure. A high current is obtained for the metallated system probably because the quantity of active sites is higher because the free-ligand polymerizes better than the cobalt complexes and a thicker polymer is obtained. Nevertheless, in the case of the metallated polymer, a shoulder is observed at ca. 0.15V. This shoulder is not observable for the native poly-CoTAPP, but the current in this case is too small to show any curvature. On the other hand it is not possible to discard an inclusion of the cobalt salt used in the metallation that enhances the current and oxidizes at this potential.

For the Ni and Cu metallated polymers, at our knowledge, there is not very specific, metal-centered reactions of electrocatalysis that could be used as test for the metallation. However, the promissory results obtained for the cobalt case can be considered as a preliminary method to incorporate ions to this kind of polymers, in order to obtain polymeric modified electrodic surfaces with desirable characteristics given by the conditions of electropolymerization. Finally, all the metallated systems were more stables that the non-metallated system (adsorbed on the conducting glass electrode) when immersed in a H2SO4 0.1M solution. The non-metallated polymer after ten minutes was completely destroyed or dissolved into the solution, because the spectrum of the electrode after this time corresponds to a "clean" electrode. The metallated-systems after an hour where only partially destroyed or dissolved, because the spectra remains with minor changes (not shown).

Fig. 8. Polarization curves of the SnO2:F electrodes modified with the poly-H2TAPP (--------------), the native poly-CoTAPP (- · - · - · ) and the poly-metallated-CoTAPP (················) in the presence of 2-mercaptoethanol. Electrolyte, 0.1M KOH and 4.5 mM of the thiol. Scan rate: 5mVs-1. Potential limits: from the open circuit potential of each system to 0.5 V versus Ag/AgCl.

CONCLUSIONS

The results shown in this work can be considered as a preliminary method to incorporate ions of transition metals to a polymer of the free-ligand para-tetraaminophenylporphyrin (H2TAPP) in order to obtain similar responses as in the case of the direct electropolymerization of a metallic complex. We proved the similarities between two modified electrodes: the metallated and the native poly-Co-para-tetraaminophenylporphyrin. In the cases of Ni (II) and Cu (II), the evidence of metallation was obtained by comparison with layers of monomers adsorbed on the electrode.

ACKNOWLEDGEMENTS

Fondecyt Project N 1010695 finances this work. G.R., M.L., A.R., F.A and I.A. acknowledge a Conicyt doctoral fellowship. F.A. acknowledges a "Beca de apoyo Término de Tesis 2002 Conicyt" Proyecto Dicyt 05-03-42TO.

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