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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.48 no.1 Concepción Mar. 2003

http://dx.doi.org/10.4067/S0717-97072003000100009 

 

"ELECTROPOLYMERIZATION AND ELECTROCATALYTIC BEHAVIOR OF
ELECTRODES MODIFIED WITH Fe AND NON-METALLED TETRAAMINOPHENYLPORPHYRINS: EFFECT OF THE POSITION OF THE AMINO
GROUP ON THE LIGAND".

GABRIELA CORNEJO, GALO RAMÍREZ, MANUEL VILLAGRÁN, JUAN COSTAMAGNA, EJNAR TROLLUND, and MARÍA J. AGUIRRE.

Departamento de Química de los Materiales, Facultad de Química y Biología,
Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile. E-
mail: maguirre@lauca.usach.cl
(Received: May 15, 2002 - Accepted: october 25, 2002)

ABSTRACT

Three monomeric iron and non-metalled tetraaminophenylporphyrins, with amino groups located in defferent positions of the phenyl substituents, were electropolymerized. These processes were carried out through continuous cycling of the potential on glassy carbon electrode surfaces.

The aim of the work was to determine the effect of the amino groups’ position on the properties of the modified-electrodes.

Phenyl substituents of the porphyrins were not conjugated with the macrocycle. Therefore, their substituents were only expected to modify the electropolymerization and not the potential of the redox metallic couples on the polymer. However, it was found that, when the complex is polymerized, the position of the amino groups greatly changes the potential of the iron couples. Thus, the electrocatalytic activity of the polymer-modified electrode toward the reduction of oxygen strongly depends on the position of amino substituents.

Key words: Tetraaminophenylporphyrins, electropolymerization, molecular oxygen reduction, substituent effect, electrocatalytic activity.

INTRODUCTION

Azamacrocyclic complexes of transition metals have been widely studied as catalysts, in homogeneus phase (1, 2) as well as in heterogeneous phase (3), due to the fact they possess interesting characteristics, which make them very active. The macrocyclic ligand allows the complex to be very stable and oxidation or reduction of metallic centers to be reversible proceses (4). Considering D4h symmetry, the metal presents two free axial positions, allowing an easy interaction with different substrates (5). The extended p -system and the planarity of the ligand ring let them anchor on electrodic surfaces (6), and, in some cases, ligands can stabilize "unusual" states of oxidation of the metal (1). In general, the metal interacts through its d orbitals (SOMO) along with the HOMO or LUMO orbitals of the molecule to be oxidized or reduced (7), so that they are electronic inner-sphere transferences. Many of these complexes, especially Fe and Co azamacrocyclic complexes, have been successfully used as electrocatalysts when reducing molecular oxygen (8). In general, Co complexes promote the reduction of O2 into peroxide, showing that when oxygen interacts with the metal, it is not strong enough to be able to break the O-O bond (9). On the other hand, most iron phthalocyanines (7) promote reduction to water, implying breakage of the O-O bond. In the case of the reduction of molecular oxygen, as it is a very complex reaction, the design of a suitable catalyst becomes difficult. Nonetheless, there have been works with face to face Co porphyrins, where the distance between both metallic centers allows the formation of m -oxo bridges (10) which attain their reduction into water. Taking into account Fe complexes, in which the reduction can be up to 4 electrons, theoretical studies indicate that there are two positions of minimum energy: one, in which the oxygen and Fe interaction is weak and would correspond to the generation of a physically-asorbed adduct, and another, in which the distance is smaller and the interaction is strong and would form a chemically-adsorbed adduct (11). Porphyrins are found among the Fe azamacrocyclic complexes studied to reduce oxygen. Porphyrin complexes of transition metals have many applications as photocatalysts and electrocatalyst systems (12-14). Iron porphyrins have been used as catalysts, for example in the oxidation of sulfur oxoanions (15), and as oxygen sensor (16). However, iron porphyrins can be unstable in aqueous solutions. Depending on the peripheric substituents, they can be electropolymerized, changing their catalytic activities (17-19) and enhancing their stability. In this sense, it has been published that a demetallation process that occurs in Fe(II) tetrakis(p-aminophenylporphyrin) does not take place when polymerized (20). In this work ortho, meta and para iron (III) tetraaminophenylporphyrin chlorides and their ligands were electropolymerized. These complexes present a predominantly D4h structure in which phenyl rings substituted with amino groups are found perpendicular to the azamarocyclic core (21). The electropolymerization mechanism implies the oxidation of amino groups, forming radical cationic species, which react with monomeric species (22). Even though the electropolymerizetion mechanism is unknown as the structure of the polymers obtained, it has been stated that this occurs due to the attack of amino radicals towars the carbon atom of the macrocyclic ligand (22). Taking into account the porphyrins studied in this work, polymerization would occur due to the attack of the oxidized amino towards a carbon atom located in an "ortho" position. In this work, the polymer obtained is conductive and differences are observed not only in the resulting charge, but also in the voltammetric response of the polymeric modified electrodes compared to the electrodes modified with physically-adsorbed layers of the complexes or ligands. The metallic couples of the polymer drastically vary when using iron porphyrins as monomers with the amino substituents in a different position. In all the cases, the films obtained are very thin and morphologic characteristics are not detectable. However, it has been demonstrated that great changes occur when the complexes are polymerized using the reduction of molecular oxygen as a probe.

EXPERIMENTAL

The compounds iron o-tetraaminophenylporphyrine, Fe(2-TAPP), iron m-tetraaminophenylporphyrine, Fe(3-TAPP), iron p-tetraaminophenylporphyrine, Fe(4-TAPP), o-tetraaminophenylporphyrine H2(2-TAPP), m-tetraaminophenylporphyrine, H2(3-TAPP) ,and p-tetraaminophenylporphyrine H2(4-TAPP) were purchased from Midcentury Co. and used without further purification. The glassy carbon-rotating electrode, from Radiometer (EDI 101), had a geometrical area of 0.049 cm2. It was polished with 0.25-m m alumina and washed in dimethylformamide (DMF) in an ultrasonic bath during five minutes before each experiment. The electrosynthesis of the polymers on the glassy carbon surface were carried out by continuous cycling of the electrode potential between –0.6 and +1.05 V versus Ag/AgCl, at 200 mVs-1, for 100 potentiodynamic cycles. The electrolyte used in these experiments consisted of a 0.1M-tetrabutylammonium perchlorate solution in DMF containing also the free ligand or the complex in a millimolar concentration. The electrolyte was deaerated with nitrogen and kept at room temperature. After polymerization, the modified electrodes were rinsed with DMF, ethanol and deionized twice-distilled water. Solvents and electrolytes were analytical grade. The electrochemical experiments were performed in a three-compartment glass cell, one for each of the electrodes: the working electrode connected to a speed control unit (Radiometer, CTV 101), the reference electrode, saturated Ag/AgCl, and the counter electrode, a Pt coil (0.14 cm2). The electrolyte used for the aqueous solutions was 0.1M NaOH (pH 13). For the oxygen reduction O2 (ultra pure, AGA) was bubbled before and during the measurement. An AFCBP1 Pine bipotentiostat, along with PineChem 2.5 software was used for the electropolymerization and to obtain data for the electroreduction of molecular oxygen.

RESULTS AND DISCUSSION

1. Characterization of the complexes and free-ligands
The structures of the complexes used in this study are shown in Figure 1. These structures correspond to the ortho (2-TAPP), meta (3-TAPP), or para-tetraaminophenylporphyrins (4-TAPP). The Figure shows that the phenyl groups are perpendicular to the plane of the molecule and hence, they are not conjugated with the macrocycle where the metal center is located (21). For this reason, the position of the amino groups in the phenyls should not affect the potential of the metal center and its catalytic activity.


Fig.1. Structures of the free-ligand, tetraaminophenylporphyrin, H2-TAPP, and the iron complex, Fe-TAPP, with the amino groups in "orto" (2), "meta" (3) or "para" (4) position.

The electronic spectra of the ligands were recorded in DMF with the same electrolyte of the electropolymerization, i.e., tetrabutylammonium perchlorate, TBAP. These spectra are shown in Figure 2A. In the case of the H2(2-TAPP) and H2(3-TAPP) the spectra correspond to typical "etio-type porphyrins" according to the Stern definition (21). They have an intense Soret band near 400 nm corresponding to the allowed transition 1A1g – 1E’u and four Q quasi-forbidden bands (Q bands labeled IV, III, II, and I in the sense of higher to lower energy). The intensity of the Q bands varies as IV > III > II > I, for an etio-type, i.e., when porphyrin is symmetrically substituted (21). The case of H2(4-TAPP) is apparently different because it only presents three Q bands, a pattern that does not correspond to an etio-type porphyrin. The spectra corresponding to the iron-porphyrins, Fe(X-TAPP), belonging to D4h symmetry are shown in Figure 2B. Only two Q bands of very low intensity are seen in addition to the Soret band in the spectra. Figure 3 shows schematic diagrams of the electronic transitions that account for these spectra. According to these diagrams, the non-metalled etio-type porphyrins belong to a D2h symmetry and the Q bands have two different origins. While bands I and III correspond to A1g-B3u and A1g-B2u transitions respectively, bands II and IV are vibrational satellites of I and III respectively (21). In the case of protonation of the non-metalled molecule, complete ionization or formation of a metalloporphyrin, the energy levels B2u and B3u are combined into a singly degenerate level. Bands I and III merge into one, as a consequence of higher symmetry (21). For a porphyrin belonging to D4h symmetry, only two bands Q: band I, and its satellite band II can be observed. In the case of H2(4-TAPP), the spectrum is non-typical for a phenylporphyrin. It seems that two bands Q were combined into one. No porphyrins with a profile of three Q bands whose intensity vary as in the spectrum of H2(4-TAPP) have been published. In order to elucidate if two Q bans are merged into one, a deconvolution of these bands will be necessary (work in progress). These results indicate that the main difference between the ligand and the iron porphyrin is due to the inner hydrogens in the central cavity of the molecule. Hydrogens cause the symmetry to be reduced from D4h to D2h in the free ligand. Independent of the position of the amino groups, the free ligands and the iron complexes, respectively show a very similar pattern, except in the case of H2(4-TAPP).



Fig.2. UV-visible spectra of the free-ligands (A) and the iron complexes (B) in dimethylformamide/0.1M tetrabutylammonium perchlorate.


Fig.3. Schematic diagrams of the electronic transitions responsible of the UV-visible spectra of the free-ligands (H2-TAPP) and the iron complexes (Fe-TAPP).

Figure 4A shows a voltammogram corresponding to the Fe(2-TAPP) obtained after ten potentiodynamic cycles between –2.0 and +0.9V versus Ag/AgCl in DMF/PTBA and 1mM of the complex. The voltammograms of the other iron complexes are very similar (not shown). The voltammetric response of the corresponding ligand is shown in the same

Figure, pointing out that the potential of the corresponding peaks are very similar, but not their intensities. The cathodic peak II for the ligand shows a splitting or a shoulder. There is no explanation for this behavior. The same experimental conditions were used for the voltammograms of H2(2-TAPP) and H2(3-TAPP), Figure 4B. The response of H2(4-TAPP) (not shown) is very similar to that of H2(2-TAPP. The voltammograms corresponding to the iron complexes show three cuasi-reversible peaks labeled I, II and III and those corresponding to the non-metalled complexes show only two clearly defined peaks, except the 3-tetraaminophenylphorphyrin with three clearly defined peaks. The potential (versus Ag/AgCl) of the cathodic peaks of all the complexes is shown in Table1.


Table 1. Cathodic potential of the peaks of the complexes or ligands versus Ag/AgCl


 

peak I

peak II

peak III


Fe(2-TAPP)

-1.686

-1.062

-0.237

Fe(3-TAPP)

-1.742

-1.057

-0.238

Fe(4-TAPP)

-1.692

-1.022

-0.172

H2(2-TAPP)

-1.447

-1.085

 

H2(3-TAPP)

-1.504

-1.044

-0.715

H2(4-TAPP)

-1.549

-1.123

 

It is noticeable, in Table 1, similarities between the potentials of peak II in the voltammograms of Fe(2-TAPP), H2(2-TAPP) and Fe(3-TAPP), H2(3-TAPP). The potential of peak II for Fe(4-TAPP) is nearly the potential for H2(4-TAPP). Complexes which have the amino group in the same position show similar or nearly similar potentials, but not identical for peak I. It lies at ca. –1.7 V vs Ag/AgCl for the iron complexes and at ca. –1.5V for the ligands. In the voltammograms of iron complexes or free ligands, peak II practically appears at the same potential. The main difference among the compounds is peak III, that does not appear in the voltammogram of the free ligands, except for H2(3-TAPP). In the latter the potential of peak III in the ligand is very different from the potentials of peak III in the iron complexes. It seems that in the ligand is due to a redox process not observed with iron complexes and with other free ligands. The assignment of the peaks in iron complexes is controversial (23, 24). When they are reduced, they can be described as a Fe(I)-porphyrin, as a Fe(II)-reduced porphyrin, or as resonant structures where the charge is delocalized between the metal and the ligand. Peak I can be attributed to a resonant structure between Fe(I)/Fe(0) and the ligand and peak II can be attributed to a resonant charge between the ligand and Fe(II)/Fe(I) (24). In the above mentioned assignments, the great similarity observed in the potential of peak II between the metal complexes and their respective ligands suggests a major contribution of the ligand in peak II rather than peak I. Peak III can be assigned to a "pure" redox couple, i.e., Fe(III)/Fe(II) (23, 24).


Fig.4. Voltammetric response of the dissolved (1mM) Fe (2-TAPP) and the free-ligand H2(2-TAPP) (A) and the free-ligands H2(2-TAPP) and H2(3-TAPP) (B) in dimethylformamide/0.1M tetrabutylammonium perchlorate. Working electrode: glassy carbon. Electrolyte: 0.1M tetrabutylammonium perchlorate/DMF. Scan rate: 100 mVs-1.

2. Characterization of the polymer-modified electrodes
The electropolymerization of the different iron complexes or ligands were performed by continuously cycling the potential of a glassy carbon electrode between –0.6 and +1.05V versus Ag/AgCl at 200 mVs-1. The electrolyte contained 1mM of a given complex in DMF/0.1M TBAP. The profiles of charge (plots of I versus t) do not show a significant growth as in the case of poly-phthalocyanines (25). The profile of electropolymerization of Fe(3-TAPP) and the plot of I versus t are shown in Figure 5A. From graph I versus t, it can be observed that even 1500 seconds later, the profile does not show signs of polymeric growth, the stability of the response indicate that the film obtained is conductive; otherwise, a diminution in the charge should be observed. Polymerizations of Fe(3-TAPP) and Fe(4-TAPP) exhibited the same behavior. Results from the polymerization of H2(4-TAPP) and the corresponding plot of I versus t are shown in Figure 5B. As in experiments with iron complexes, the profiles of the ligand show a similar pattern. An anodic peak grows at the positive limit of potential. It can be assigned to the oxidation of the phenyl-amino groups (26). However, electropolymerization is not as evident as in the case of iron complexes because, in this case, the voltammograms do not show growing signals. Nonetheless, a "true" polymerization or oligomerization must be in effect because of the differences observed between the voltammograms. Voltammograms recorded in aqueous solutions of electrodes modified with the polymers or adsorbed layers of monomers are very different as seen in Figure 6. In this Figure it can be seen that electropolymerization changes the voltammetric profile of the modified electrode, but this change does not follow a common pattern. The monomer and the polymer of Fe(2-TAPP) (Figure 6A) show only a redox couple and the redox couple for the polymer is shifted to more positive potentials. In Fe(3-TAPP) (Figure 6B), the polymer depicts two redox couples and the monomer only one. For the Fe(4-TAPP) (Figure 6C), the monomer shows two cathodic peaks but only one anodic wave, and the polymer depicts two reversible peaks. It is difficult to explain the differences in the electrochemical behavior of the complexes or ligands, whether electropolymerized, because the morphology and structures that form the polymers are unknown. The films obtained are too thin, thus a SEM analysis does not permit to distinguish the different features of any polymer. It is, however, very surprising that the position of the amino groups can drastically modify the voltammetric response of the complexes or ligands when they are electropolymerized compared to those observed when they are not polymerized. The reasons for those changes are probably a strong dependency on the morphology and structures obtained during the polymerization. Indeed, only in two cases, the first redox couple (couple I) assigned to the delocalized ligand-Fe(II)/Fe(I) in aqueous media, appears in the potential interval studied (1). Iron porphyrins are complexes where the metal center has an oxidation state (III) at open circuit potentials. Then, the possible presence of the redox couple Fe(II)/Fe(I) in that range of potentials will be strongly dependent on the ligand, as mentioned above. The polymer, acting as a ligand, will permit or not a delocalization of charge that makes this redox couple available (24). The different morphologies and chemical structures obtained when the complexes are electropolymerized are supported by the different voltammetric profiles of the monomer or polymer ligand, as shown in Figure 7. The redox couples are ill-defined but the voltammograms allow to distinguish that there are differences in the number and position of the redox couples if the ligand is electropolymerized, as in the case of the Fe complexes. A possible mechanism of polymerization (19, 26-28), involves cationic radicals that lead to the formation of a N-C bond between the amino group of a phenyl and the carbon of another phenyl substituent. The formation of such bonds may involve important changes in the angle between pendant phenyls and the macrocyclic complexes. In some cases, this may favor a higher planarity that permits to delocalize the charge of the iron centers trough various bonded complexes. An important modification of the redox couple must take place under such delocalization.



Fig.5. (A). Voltammetric response of the electropolymerization of Fe(3-TAPP) on a glassy carbon electrode by cycling the electrode potential between –0.6 and +1.05V versus Ag/AgCl in a solution of dimethylformamide -0.1M tetrabutylammonium perchlorate containing 1mM of the complex. Scan rate: 200 mVs-1. The profile corresponds to 100 cycles. B. A plot of I versus time corresponding to the electropolymerization shown in Figure 5A.


Fig.6. Comparison between the voltammetric response of the glassy carbon electrodes modified with a drop-solution deposition (mono) carried out by simple contact of a solution containing the monomer (1mM, DMF/0.1M tetrabutylammonium perchlorate) and after an hour of contact the electrode was rinsed with DMF, ethanol and water, or by electropolymerization (poly). The electrolyte is an aqueous solution containing 0.1M NaOH. Scan rate: 100 mVs-1. (A) mono and poly Fe(2-TAPP). (B) mono and poly Fe(3-TAPP9 and (C) mono and poly Fe (4-TAPP).



Fig.7. Comparison between the voltammetric response of the glassy carbon electrodes modified with a drop-solution deposition (mono) carried out by simple contact of a solution containing the monomer (1mM, DMF/0.1M tetrabutylammonium perchlorate) and after an hour of contact the electrode was rinsed with DMF, ethanol and water, or by electropolymerization (poly). The electrolyte is an aqueous solution containing 0.1M NaOH. Scan rate: 100 mVs-1. (A) mono and poly H2(2-TAPP). (B) mono and poly H2(3-TAPP9 and (C) mono and poly H2 (4-TAPP).

3. Comparison between monomers and polymers toward the reduction of molecular oxygen
The voltammetric response of some monomers and polymers (ligands) under O2 atmosphere in aqueous solution at pH 13 is shown in Figure 8. In the case of the free ligand-modified electrodes, only H2(3-TAPP) shows significant differences if polymerized. Indeed, polymers of free-ligands do not enhance the electrocatalytic response relative to two monomers. Polymerization only changes the response of the less electrocatalytic ligand. In fact, polymerized ligands show a very similar catalytic activity. The position of the amino group has no influence on the response of these polymers. However, the iron complexes are very active to the reduction of molecular oxygen. A comparison between the reductions of oxygen of the three iron-polymers with the best iron monomer is shown in Figure 9. In this Figure it can be appreciated that polymerization, in all the cases, increases the electroactivity. The main effect, i.e., the poly-Fe(4-TAPP), shifts the potential required for the reduction by 200 mV. The beginning of the reduction wave with Fe(4-TAPP) is very different for the monomer and the polymer, cathodic peaks appears at ca. –0.2V and –0.4V for the polymer and monomer respectively. In other cases, polymers are more active than monomers but the foot of the reduction wave starts at similar potentials. These are unexpected results, as figure 6 has shown. The main differences, when the iron complexes are polymerized, correspond to Fe(II)/Fe(I) redox couples according to the changes observed in the voltammogram. However, the redox couple responsible of the molecular oxygen reduction is Fe(III)/Fe(II) because reduction begins at potentials where Fe is in a state of oxidation (III) and when the cathodic sweep reaches the potential of the couple Fe(III)/Fe(II), the reduction begins. Results observed in Fig. 9 demonstrate that the redox couple Fe(III)/Fe(II) is also changed with the polymerization. If both metal couples are changed it implies, in each case, a strong variation in the nature of the "polymeric ligand". In each one the results can be explained in terms of the different nature of the polymeric ligand. Results confirm that the redox couple Fe(II)/Fe(I) has an important contribution from the ligand as shown in the comparison between the voltammograms of the monomeric or polymerized species. This result agrees with the published data (23, 24). In a similar way, Fe(III)/Fe(II) also receives a strong contribution from the ligand because of the changes observed in the electroreduction of molecular oxygen. Then, couple Fe(III)/Fe(II) is not a "pure" metal couple in the case of the polymers, contrarily to that informed in the literature (23, 24). In this way, changes in the nature of the electropolymerized ligand, due to the position of the amino groups may explain the changes obtained in the electroreduction of the molecular oxygen promoted by the redox couple Fe(III)/Fe(II) and the different features of the voltammograms of monomers and polymers.


Fig.8. Comparison between the voltammetric response of (A) mono and poly H2(3-TAPP) and (B) mono and poly Fe(4-TAPP) toward the electroreduction of molecular oxygen. The electrolyte is an aqueous solution containing 0.1M NaOH. Scan rate: 100 mVs-1. Electrode area: 0.017 cm2


Fig.9. Comparison among the three iron polymeric modified electrodes towaScan rate: 100 mVs-1.Electrode area: 0.23 cm2. rd the electroreduction of molecular oxygen. The electrolyte is an aqueous solution containing 0.1M NaOH.

ACKNOWLEDGEMENTS

Fondecyt, project 1010695, has supported this work. G.R. acknowledges a Conicyt Doctoral Fellowship. Authors also acknowledge Dr. Guillermo Ferraudi`s interesting discussion.

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