Voltammetry and Spectroelectrochemical Behavior of a Novel

Oct 10, 2007 - ... J-aggregations in solution; synthesis, characterization and electrochemistry. Melike Sevim , M. Nilüfer Yaraşir , Atıf Koca , Mehme...
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J. Phys. Chem. C 2007, 111, 16558-16563

Voltammetry and Spectroelectrochemical Behavior of a Novel Octapropylporphyrazinato Lead(II) Complex Meryem N. Yaras¸ ir,† Atıf Koca,*,‡ Mehmet Kandaz,*,† and Bekir Salih§ Department of Chemistry, Sakarya UniVersity, 54140 Esentepe, Sakarya, Turkey, Chemical Engineering Department, Engineering Faculty, Marmara UniVersity, 34722 Go¨ztepe, Istanbul, Turkey, and Department of Chemistry, Faculty of Science, Hacettepe UniVersity, Beytepe Campus, 06532 Ankara, Turkey ReceiVed: February 20, 2007; In Final Form: August 16, 2007

The synthesis, characterization, and voltammetric and spectroelectrochemical investigation of [2,3,7,8,12,13,17,18-octapropyl porphyrazinato] lead(II) are reported. The characterization of newly synthesized lead porphyrazine was made by elemental analysis, 1H NMR, FT-IR, UV-vis, and MALDI-TOF. The solution redox properties and spectroelectrochemical investigation of the novel lead(II) porphyrazine were studied using various electrochemical techniques in DCM on a platinum electrode. Cyclic voltammetry and differential pulse voltammetry studies show that the complex gives two one-electron ligand-based reductions and a oneelectron oxidation wave having diffusion-controlled mass transfer character. Assignments of the redox couples were confirmed by spectroelectrochemical measurements. Spectroelectrochemical studies reveal that the complex is demetalized during the controlled potential coulometry measurement after the first reduction couple of the complex.

1. Introduction The synthesis, characterization, and applications of tetrapyrolic macrocycles such as phthalocyanines, porphyrins, and their aza-analogues have been investigated intensively in many directions, as semiconductor,1 electrochromic device (ECD),2-5 solar cell,6 nonlinear optics,7 the treatment of cancerous tissues,8 and gas sensing.9,10 Although phthalocyanines and porphyrins have been well studied, porphyrazines have received considerably less attention. However, due to the simple synthetic route for the synthesis of porphyrazines, via the template cyclotetramerization of maleonitrile, these kinds of macrocyclic compounds are now the subject of enhanced interest.11 There is also a great interest in exploring novel structural modifications to the porphyrazine system, including the study of polynuclear derivatives and derivatives with specially designed peripheral substituents that coordinate with alkaline or transition metals.12-15 Barrett, Hoffman, and co-workers have published extensively on the synthesis of porphyrazines bearing thiols, amines, or alcohols as ring substituents, with the conversion of these polydentate ligands to a variety of coordination complexes.12,16-18 Our group has been heavily engaged with the preparation of novel phthalocyanine (Pc) and porphyrazine (Pz) derivatives during the last years. Among these, Pc and/or Pz derivatives bearing long chains or functional groups such as nitrobenzo,19 aminobenzo,19 2′-aminophenylsulfanyl,9 2-aminophenoxy,9 propane 1,2-diolsulfanyl,20 thioethoxy(ethoxy)ethanol,21 and 6-hydroxyhexylthio5 can be cited. In the present work, we synthesized a novel porphyrazine structure carrying a lead(II) metal center and octapropyl substituents on the periphery for the first time. It has been found * Corresponding authors. Tel.: +90 216 3480292 (A.K.), +90 264 2955946 (M.K.). Fax: +90 216 3450126 (A.K.), +90 264 2955950 (M.K.). E-mail: [email protected] (A.K.), [email protected] (M.K.). † Sakarya University. ‡ Marmara University. § Hacettepe University.

that metal ions could be integrated to the macrocyclic core of the porphyrazines more effectively than that of Pc6,11 due to the relatively large radius as compared to the phthalocyanine core. The molecule containing heavy metals such as Sn and Pb processes enhancing optical limiting capability with relative instability.22,23 As a p-type semiconducting material, lead(II) phthalocyanine (PbPc) was reported to be sensitive to many gases, such as NO2,24 NH3,25 Cl2,26 and H2.27 The position of the lead atom in the tetrapyrolic macrocycle skeleton differs from the position of the other transition metal atoms in the core of the complexes. For instance, the radius of divalent lead(II) ions is greater than the radius of the inner core of the phthalocyanine and porphyrazine ring, so the metal ion does not fit inside the central cavity but it lies above the plane.6,11,28 Therefore, these compounds show anomalous redox behavior.6 Because the metal sits outside of the ring, the complex shows a tendency to demetallization in redox reactions.6,11,30 As compared to more intensely studied transition metal-phthalocyanines, papers on the synthesis, especially electrochemistry and spectroelectrochemistry,30,31 of lead(II) phthalocyanine (PbPc) have been relatively few, and there have been none on lead porphyrazine. Therefore, in this study, we have synthesized, characterized, and examined the voltammetric and spectroelectrochemical properties of the new lead porphyrazine, [Pb(II)Pz(Pr)8], bearing propyl peripheral chains. In this study, we report also the influence of the integration of lead(II) to the inner core of Pz ring on its voltammetric properties, as compared to the other metallo-porphyrazines and lead(II) phthalocyanines. 2. Experimental Section n-Hexanol and tetrahydrofuran were distilled from anhydrous CaCl2. Pb(O2Me)2‚nH2O was purchased from Aldrich. 2,3,7,8,12,13,17,18-Octapropyl porphyrazine, [H2Pz(Pr)8], was prepared according to a literature procedure.32 The reaction was carried out under dry N2 atmosphere unless otherwise noted. 1H NMR spectra were recorded on a Bruker 300 MHz spectrometer

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Spectroelectrochemistry of Lead Porphyrazine instrument. Multiplicities are given as s (singlet), d (doublet), t (triplet). FTIR (thin solution film) spectra were recorded on a ATI UNICOM-Mattson 1000 spectrophotometer. Elemental analysis (C, H, and N) was performed at the instrumental Analysis Laboratory of Marmara University. Routine UVvisible spectra were obtained in a quartz cuvette on a Unicom UV-2 spectrometer. Chromatography was performed with silica gel (Merck grade 60 and sephadex) from Aldrich. The homogeneity of the products was tested in each step by TLC (SiO2, CHCl3, and MeOH). 2.1. MALDI Sample Preparation. 3-Indole acrylic acid (IAA) (20 mg/mL in acetonitrile) MALDI matrix for [Pb(II)Pz(Pr)8] was prepared. MALDI samples were prepared by mixing complex solution (2 mg/mL in acetonitrile-water with a 1:1 ratio) with the matrix solution (1:10 v/v) in a 0.5 mL eppendorf micro tube. Finally, 1 µL of this mixture was deposited on the sample plate, dried at room temperature, and then analyzed. 2.2. MALDI Mass Spectrometry. Mass spectra were acquired on a Voyager-DE PRO MALDI-TOF mass spectrometer (Applied Biosystems, USA) equipped with a nitrogen UV laser operating at 337 nm. Spectra were recorded in reflectron mode with an average of 50 shots. 2.3. Electrochemistry. The cyclic voltammetry (CV), differential pulse voltammetry (DPV), double potential step coulometry (DPSC), and controlled potential coulometry (CPC) measurements were carried out with a Princeton Applied Research model VersaStat II potentiostat/galvanostat controlled by an external PC and utilizing a three-electrode configuration at 25 °C. For CV, DPSC, and DPV measurements, the working electrode was a Pt plate with a surface area of 0.10 cm2. The surface of the working electrode was polished with a H2O suspension of Al2O3 before each run. The last polishing was done with a particle size of 50 nm. A Pt wire served as the counter electrode. Saturated calomel electrode (SCE) was employed as the reference electrode and separated from the bulk of the solution by a double bridge. Ferrocene was used as an internal reference. Electrochemical grade tetrabutylammonium perchlorate (TBAP) in extra pure dichloromethane (DCM) was employed as the supporting electrolyte at a concentration of 0.10 mol dm-3. High-purity N2 was used for deoxygenating at least 15 min prior to each run and to maintain a nitrogen blanket during the measurements. During voltammetric measurements, the reference electrode tip was moved as close as possible to the working electrode so that uncompensated resistance of the solution was a smaller fraction of the total resistance, and therefore the potential control error was low. Positive feedback IR compensation was applied to further minimize the potential control error. For controlled potential coulometry (CPC) studies, Pt gauze working electrode (10.5 cm2 surface area), Pt wire counter electrode separated by a glass bridge, and SCE as a reference electrode were used. In the technique, the potential of the working electrode is held constant for a long time, minutes to hours, and the resulting integrated charge is recorded. All of the electrochemically active species, which are being electrolyzed, will react, resulting in 100% efficiency. The total charge passed in this technique will obey Faraday’s law, Q ) nFNo, where Q is the total charge passed, n is the overall number of electrons consumed in the experiment, F is Faraday’s constant (9.64853 × 104 C/equiv), and No is the total moles of redox species present. Positive feedback IR compensation was applied to further minimize the potential control error.

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Figure 1. Structure of the [Pb(II)Pz(Pr)8].

The spectroelectrochemical measurements were carried out with an Agilent model 8453 diode array spectrophotometer equipped with the potentiostat/galvanostat utilizing a threeelectrode configuration of thin-layer quartz spectroelectrochemical cell at 25 °C. The working electrode was transparent Pt gauze. Pt wire counter electrode separated by a glass bridge and a SCE reference electrode separated from the bulk of the solution by a double bridge were used. 2.4. Synthesis. 2,3,7,8,12,13,17,18-Octapropylporphyrazinato Lead(II), [Pb(II)Pz(Pr)8]: H2[Pz(Pr)8] (0.1 g, 0.12 mmol) and Pb(O2CMe)2 (0.045 g, excess) in a mixture of n-hexanol and chloronaphthalene (10 mL) (5:1) were heated to 120 °C for 24 h, after which the solvent mixture were removed by vacuum distillation. Petroleum green color octapropropylporphyrazininato lead(II) was taken up in CHCl3 (150 mL) and filtered. The volume was reduced to 5 mL by rotary evaporation, and the product was purified by silica gel chromatography (eluent; CHCl3). Finally, the resulting solid was washed with petroleum ether and then with MeOH. Yield of Pb[Pz(Pr)8]: 0.035 g (34.08%). Mp > 200 °C. IR νmax (cm-1): 2975, 2933, 2855 (aliph-CH), 1625 (CdC), 1553 (CdN), 1514, 1325, 1210, 1054, 1020, 869, 795, 720, 697. 1H NMR ([d3]-CDCl3) δ: 0.98, (t, 24H, 3J ) 7.1 Hz, -CH2CH2CH3), 1.65 (m, 16H, 3J ) 7.5 Hz, -CH2CH2CH3), 2.67 (t, 16H, 3J ) 7.2 Hz, -CH2CH2CH3). UV/ vis (2.0 × 10-4 mol dm-3 in CHCl3) λmax/nm (log ): 625 (4.01), 576 (1.26), 355 (3.24). Anal. Calcd for C40H56N8Pb (%): C, 56.14; H, 6.50; N, 13.10. Found (%): C, 56.03; H, 6.35; N, 12.88. MS (MALDI-TOF) m/z (%): 856 [M + H]+ (100%). 3. Results and Discussion 3.1. Macrocycle Synthesis. Pb[Pz(Pr)8] employed here is shown in Figure 1. The reaction of H2[Pz(Pr)8] (0.1 g, 0.12 mmol) with Pb(O2CMe)2 (excess) in the presence of DBU base in a mixture of n-hezanol and chloronaphthalene (10 cm3) (5:1) for 8 h produced the corresponding Pb[Pz(Pr)8] complex in a moderate yield. The complex is instable in air and shows no appreciable sensitivity to light. The eight peripheral substituents and pyramidal structure of Pb[Pz(Pr)8] confer on the macromolecules good solubility in polar and nonpolar solvents such as chloroform (CHCl3), dichloromethane (CH2Cl2), tetrahydrofuran (THF), ethanol (EtOH), dimethyl formamide (DMF), dimethy sulfoxide (DMSO), and dimethyl acetamide (DMAA). The complex was purified by column chromatography using CHCl3. 3.2. Spectroscopic Characterization. The synthesized complex was characterized by FTIR, UV/vis, MALDI-TOF-MS, 1H NMR, as well as elemental analysis. IR spectra of [Pb(II)Pz(Pr)8] were recorded in the fundamental region of 600-4000 cm-1, using KBr film technique. In the IR spectra of the complex, the most intense bands of the spectrum are the diagnostic absorptions at ca. 2975, 2933, and 2855 cm-1 due to the antisymmetric C-H stretching vibrations of the CH2 and CH3 groups for the propyl moieties.19,21,33,34 The most instinctive indicator for [Pb(II)Pz(Pr)8] is disappearance of the deuterium exchangeable signal belonging to inner core protons as a result of the 18 π-electron system of

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Figure 2. Positive ion and reflection mode MALDI-MS spectrum of [Pb(II)Pz(Pr)8] was obtained in 3-indole acrylic acid MALDI matrix using nitrogen laser accumulating 50 laser shots with 9300 resolution. Inset (a) spectrum shows expanded theoretical isotopic mass distribution of protonated molecular ion mass region of the complex, and inset (b) spectrum shows expanded experimental isotopic mass distribution of protonated molecular ion mass region of the complex.

the Pz ring in the lower field region of the 1H NMR (δ ) -1.45 ppm) upon complexation.19,21,33,34 As compared to the spectra of the free-base porphyrazine, the shifts and changes in the intensity of certain peaks in the 1H NMR spectra of [Pb(II)Pz(Pr)8] are consistent with the metallization. Because the complex is soluble in various solvents, electronic spectra were recorded in chloroform solvent in the range 300800 nm. The metalloporphyrazine complex gives rise to apparently simpler Q and B band absorptions different from those of the metal-free derivative. Insertion of Pb(II) ions into metal-free derivative has been proven by the change of the split Q-band absorption, which is due to π-π* transitions of the completely conjugated 18-π electron system into a sharp intense Q-band at 625 nm after metallization with Pb(II) ion.35 The Q-band absorption is assigned to a1u f eg transitions leading to a doubly degenerate state, 1Eu, which is observed as a single intense absorption at ca. 650 nm, whereas the lower symmetry of metal-free porphyrazines leads to a more complex set of orbitals in which the two lowest-energy, unoccupied orbitals are no longer degenerate. Thus, the Q-band for metal-free porphyrazines is well-resolved split into two components with Qx and Qy absorbances.9,36 Q-band data for the complex show that the Q-band absorption is particularly sensitive to the presence of substituents and metal atom in porphyrazine core. The substitution of the electron acceptor Pb metal was found to affect bathochromic shift approximately 20-40 nm in comparison to the peak observed in the spectrum of metalloporphyzazines having Zn(II), Co(II), and Mn(II) metal centers.19,21 3.3. MALDI-MS Spectra. Isotopic mass distribution of the protonated molecular ion peak of Pb[Pz(Pr)8] was observed between 852 and 859 Da by 1 Da mass increment (Figure 2). Experimental isotopic mass distribution of the protonated molecular ion peak of lead(II) porphyrazine, {Pb[Pz(Pr)8]} (inset b in Figure 2), was exactly overlapped with the mass of the lead complex calculated theoretically from the elemental composition of the complex (inset a in Figure 2). Beside the protonated molecular ion peak of the complex, the protonated molecular ion peak of [H2Pz(Pr)8] was observed due to the less stability of this lead porphyrazine even if the MALDI-TOFMS spectrum was carried out in slightly acidic matrix, 3-indole acrylic acid, at pH ) 6.0. There were two low-intensity fragment ions more beside the protonated molecular ion peak of the [Pb(II)Pz(Pr)8]. Despite these, the MALDI-TOF mass spectrum of the complex was very clear.

Figure 3. (A) Cyclic voltammograms of [Pb(II)Pz(Pr)8] (2.5 × 10-4 mol dm-3) at various scan rates on Pt in DCM/TBAP. (B) Differential pulse voltammograms of [Pb(II)Pz(Pr)8] (2.5 × 10-4 mol dm-3). Pulse width, 50 ms; pulse height, 100 mV; step height, 5 mV; step time, 100 ms; scan rate, 50 mV s-1.

3.4. Voltammetric Studies. The solution redox properties of the [Pb(II)Pz(Pr)8] complex were studied using cyclic voltammetry (CV), differential pulse voltammetry (DPV), and controlled potential coulometry (CPC) in DCM on a platinum electrode. CV and DPV studies reveal that the complex has welldefined two porphyrazine ring-based reduction couples and one ring-based oxidation couple. The separation between the first ring reduction and the first ring oxidation processes related with the HOMO-LUMO gap (1.63 V) and the separation between the first and second ring reductions (0.41 V) are in agreement with the reported separation for redox processes in metalloporphyrazine complexes.6,11,16,19,21 Figure 3 shows the CV and DPV of the complex in DCM containing TBAP. The complex gives two one-electron reduction processes at -0.83 and -1.24 V and a one-electron oxidation process at 0.86 V versus SCE at 100 mV s-1 scan rate. Based on the well-known electrochemical behavior of phthalocyanine complexes, all couples in Figure 3 are assigned to porphyrazine ring. The assignments of the redox couples are also confirmed below using spectroelectrochemistry measurements. For couples Ic/Ia and IIc/IIa, the anodic to cathodic peak currents (Ipa/Ipc) were near unity, and the anodic to cathodic peak separation (∆Ep) ranged from 70 to 130 mV with increasing scan rates, thus suggesting reversible to quasireversible behavior (∆Ep ) 100 mV at 0.100 mV s-1 scan rate was obtained for ferrocene standard). Although the reverse peak for the oxidation wave recorded at 0.86 V is not recorded during

Spectroelectrochemistry of Lead Porphyrazine the CV measurements, a reverse peak is observed with a relatively small peak current during the DPV measurement; thus this process is not irreversible completely. The peak currents increased linearly with the square root of scan rates, for scan rates ranging from 10 to 500 mV s-1, indicating that the electrode reactions are purely diffusion-controlled for the couples Ic/Ia and IIc/IIa.37,38 The IIIa wave is recorded at the end of the solvent window, so we do not analyze its behaviors. There was no effect on the voltammograms when the potential window was narrow (-1.5 to +1.1 V), or when starting at 0 V and scanning to 1.10 V. CPC was used to determine the overall number of electrons in a faradic reaction. Unlike voltammetric techniques where the electrode area and diffusion coefficient of the redox species must be known, CPC can determine the overall number of electrons in the redox process without prior knowledge of the electrode area or diffusion coefficient. Applied potentials were selected approximately 50 mV higher than the reduction/oxidation peak to compensate the possible change of the peak potential due to the different cell and electrode configuration of the CPC measurement. The CPC studies indicated that the number of electrons transferred for the oxidation process of the complex electrolyzed at 0.90 V was found to be approximately one; it was found to be four for two reduction processes electrolyzed at -1.40 V applied potential. When the potential was applied to -0.95 V, the number of electrons transferred up to first reduction potential was found to be one. Because both reduction processes have very similar peak currents, the number of electrons transferred for the second reduction process should also be 1. Therefore, remained two electrons should be attributed to the two-electron reduction of the Pb(II) center of the complexes, despite the fact that the reduction couple for Pb(II) center was not observed during the CV and DPV time scale (Figure 3). It is well known that in PbPz, the reduction leads to the loss of the Pd central metal and gives a sharp peak, but this is not observed in Figure 3. This might result from the very slow rate of the demetallization processes giving a very low amount of lead species; therefore, the demetallization processes could not be recorded in Figure 3 within the CV and DPV time scale. Hence, it is clear that while the complex gives common ligand center reduction processes during the CV and DPV measurement, it is demetalated just under the applied negative potential during the CPC measurements. Reduction of metal center resulting from a demetallization process is also characterized by spectroelectrochemical measurements explained below. To obtain a more accurate measurement of the redox equivalency, reduction processes were studied by DPSC. Plots of Q (charge) versus t1/2 (time) show linear behavior for both processes. Slopes with a 1:1 ratio were consistent with the involvement of one electron for I and II redox processes. However, when DPSC was applied from zero to the end of the negative window at -1.40 V vs SCE, it gave a 4:1 ratio with I redox process due to the two-electron reduction of the Pb(II) metal center of the complex. The identities of these electrode processes were further revealed by performing spectroelectrochemical experiments. DPSC experiments also showed the reversibility of I and II processes with the approximately same slopes of the Cottrell lines representing the two potential steps of the processes. 3.5. Spectroelectrochemical Studies. Spectroelectrochemical studies were employed to confirm the assignments in the CVs of the complex. Figure 4 shows the UV-vis spectra of the neutral [Pb(II)Pz(Pr)8] and its electrochemically generated species. Spectroelectrochemical studies reveal that different

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Figure 4. UV-vis spectra for the neutral [Pb(II)Pz(Pr)8] complex and its electrochemically generated species at the end of the controlled potential application at reduction/oxidation potentials of the couples.

redox reactions are observed during the CPC time scale than during the CV. Although all redox couples are assigned to the porphyrazine ring in CV and DPV measurements, significant spectroscopic changes are recorded at the potential just after the first reduction couple of the complex because of the demetallization of the complex. This was also observed by Kadish and co-workers39 with the phthalocyanine complexes during the potential application at just negative of the first reduction over a period of a few minutes. In our previous study,30 we indicated also the demetallization of the tetra- and octa-substituted PbPc’s just before the first reduction processes. In this study, demetallization of [Pb(II)Pz(Pr)8] was observed at more negative potential than the demetallization process of PbPc studied in our previous study30 due to the stronger coordination of the metal to the core of porphyrazines, which is larger than that of phthalocyanines. Figure 5 shows the in situ UV-vis spectra of the complex. During a controlled potential reduction of the complex at potential -0.90 V (Figure 5A), while the absorption of the Q-band at 625 nm, its shoulder at 575 nm, and the B-band at 358 nm decrease, new bands are observed at 532, 750, and 830 nm assigned to the MLCT or LMCT. The decrease in the Q-band without shift is characteristic of ring-based processes. Observation of a new band (532 nm) in the 500-600 nm region is typical30,39-41 for ring-based reduction for metalloporphyrazines and the formation of a [M(II)Pz(-3)]1- species. During this process, green color of the complex is turned into blue. The reduction of the complex was reversible in that applying 0 V resulted in the regeneration of the starting spectrum and color. As shown in Figure 5A, the process at -0.90 V potential application results with clear isobestic points at 312, 400, 492, 556, and 657 nm in the spectra. During a controlled potential application just after the first reduction of the complex at potential -1.00 V, the original spectrum assigned to the [M(II)Pz(-3)]1- species is changed to a spectrum having a split Q-band. A split Q-band confirms the formation of the metal-free porphyrazine with the formation of a new band at 657 nm and a shoulder at 592 nm with decrease of the absorption of Q-band at 625 nm. Small absorption increments on the band at 532 nm and small decrease on the bands at 750 and 830 nm are also recorded. Absorption of the B-band decreases with blue shift from 358 to 343 nm during this process. These spectroelectrochemical measurements suggest the formation of metal-free porphyrazine because of the

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Figure 5. In-situ UV-vis spectral changes of [Pb(II)Pz(Pr)8]. (A) At the first reduction at Eapp ) -0.90 V. (B) After the first reduction at Eapp ) -1.00 V. (C) After the second reduction at Eapp ) -1.30 V. (D) After the first oxidation at Eapp ) 0.90 V.

demetallization of the complex at this potential, although the demetallization processes could not be recorded in CV and DPV measurement time scales. The final spectrum in Figure 5B essentially shows a split Q-band, which could be a result of decrease in symmetry (such as demetallization). Demetallization was confirmed because this process of the complex was irreversible in that applying 0 V did not result in the regeneration of the starting spectrum. During this process, the blue color of the complex is turned into violet. As shown in Figure 5B, the process at -1.00 V potential applications resulted with clear isobestic points at 313, 410, 512, 640, and 696 nm in the spectra. Upon controlled potential application at -1.30 V (second reduction potential) (Figure 5C), one of the split-Q bands (657 nm) and one of the Q-band shoulders (575 nm) decrease in intensity, while the shoulder at 592 nm and the Q-band at 625 increase significantly. A small absorption increment on B-band at 343 nm without shift and absorption decrements on the bands at 750 and 830 nm are recorded. The decrease in the absorption of one of the split Q-band and increase in the other is characteristic for the ring reduction of the metal-free porphyrazine.6,11,20,39-42 The demetallization process of the complex was irreversible in that applying 0 V did not result in the regeneration of the starting spectrum. Therefore, we study the oxidation process with a new complex solution. As shown in Figure 5D, two new bands are formed at 532 and 663 nm during the first oxidation process of the complex, while the Q-band and B-band decrease in absorption. Formation of a new band at 532 nm and decrease of Q-band without shift are characteristic of a ring-based oxidation process.6,30,39-44 Isobestic points at this process are 310, 505, 563, and 650 nm. Based on the controlled potential coulometry and spectroelectrochemistry discussed above, the following mechanism

(eq 1) may be proposed for the reduction and oxidation processes of the complexes:

4. Conclusion The spectral and electrochemical properties of newly synthesized Pb[Pz(Pr)8] have been presented in this work for the first time. Although the complex showed ligand-based redox processes during the CV and DPV time scales, demetallization process was observed during the controlled potential coulometric and spectroelectrochemical measurements. Observation of the demetallization process supports the differences of the position of the lead atom in the Pb[Pz(Pr)8] skeleton. Demetallization process occurred because the metal ion does not fit inside the central cavity but it lies above the Pz plane; therefore, the complex shows a tendency to demetallization. The more negative potential demetallization of Pb[Pz(Pr)8] with respect to demetallization process of PbPc may be due to the stronger coordination of the metal to the core of porphyrazines, which is larger than that of phthalocyanines.

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