Unveiling the Structure of Poly-Tetraruthenated Nickel Porphyrin by Raman Spectroelectrochemistry

Unveiling the Structure of Polytetraruthenated Nickel Porphyrin byRaman SpectroelectrochemistryLuís M. C. Ferreira,† Daniel Grasseschi,† Mauro S. F. Santos,† Paulo R. Martins,‡ Ivano G. R. Gutz,†

Ana Maria C. Ferreira,† Koiti Araki,*,† Henrique E. Toma,† and Lucio Angnes*,†

†Instituto de Química, Universidade de Sao Paulo, CEP 05508-000 Sao Paulo, SP, Brazil‡Instituto de Química, Universidade Federal de Goias, CEP 74001970 Goiania, GO, Brazil

*S Supporting Information

ABSTRACT: The structure of polytetraruthenated nickel porphyrin wasunveiled for the first time by electrochemistry, Raman spectroelectrochemistry,and a hydroxyl radical trapping assay. The electrocatalytic active material,precipitated on the electrode surface after successive cycling of [NiTPyP{Ru-(bipy)2Cl}4]

4+ species in strong aqueous alkaline solution (pH 13), was found tobe a peroxo-bridged coordination polymer. The electropolymerization processinvolves hydroxyl radicals (as confirmed by the characteristic set of DMPO/•OHadduct EPR peaks) as reaction intermediates, electrocatalytically generated in the0.80−1.10 V range, that induce the formation of Ni−O−O−Ni coordinationpolymers, as evidenced by Raman spectroelectrochemistry and molecularmodeling studies. The film growth is halted above 1.10 V due to the formationof oxygen gas bubbles.

1. INTRODUCTION

Electrocatalytically active modified electrodes can be easilyprepared by voltammetric cycling of nickel(II) macrocyclecomplexes such as nickel(II) tetraazamacrocycles,1−5 Ni-phtalocyanines,6−16 Ni-porphyrins,5,8,17−20 and Ni-Salen com-plexes9,11 in strongly alkaline solutions (pH ≥13). Interestingmaterials17,20−24 capable of promoting the oxidation of manyorganic compounds are generated, including alcohols, atrelatively low overpotentials. Malinski et al.25,26 studied theelectropolymerization of tetrakis(3-methoxy-4-hydroxiphenyl)-porphyrin and proposed the use of carbon-fiber ultra-microelectrodes modified with the resultant material asamperometric sensors for the determination of nitric oxide ina single cell. The monomers were thought to be connectedthrough the meso-methoxyphenol substituents in the nickelporphyrin meso positions.20,21 However, cationic [Ni-TMPyP]4+ and anionic [Ni-TSPP]4− porphyrin complexes,20,27

where TMPyP4+ = meso-tetra(pyridyl)porphyrin and TSPP4− =meso-tetra(4-sulfonatephenyl)porphyrin, were also shown toexhibit electropolymerization properties generating similarelectroactive materials. More recently, Bedioui et al.28 preparedhybrid materials based on carbon nanotubes and tetrasulfo-nated nickel(II) phthalocyanine with similar electrochemicalproperties.The characteristic electrochemical behavior of polynickel

porphyrins and polynickel macrocycles is usually assigned tothe formation and subsequent deposition of coordinationpolymers of the corresponding nickel complex on the electrodesurface, where polymerization takes place through axial μ-oxobridges connecting the nickel centers in a reaction that could be

mediated by electrogenerated hydroxyl radicals.3 However, thesimilarity in the electrochemical behavior of all of thosepolymeric materials among themselves and with that of nickelhydroxide29−31 is intriguing, and the elucidation of theirstructure may help to shed light on the mechanism ofstabilization of nickel hydroxide in the alpha phase.31,32 Thus,the formation of nickel hydroxide after a kind of “electro-chemically induced demetalation” of the starting Ni-TSPPcomplex generating a TSPP/Ni(OH)2 conducting salt, wherethe porphyrin is present as a radical cation, has also beenproposed.18 Nevertheless, the stability of such a reactiveporphyrin species in a strongly alkaline medium is questionable.In fact, there are few reports showing unequivocal experimentalevidence to help elucidate their actual chemical structure, eitheras hybrid materials containing Ni(OH)2 or as μ-oxo-bridgednickel macrocycle coordination polymers. Also, no evidence ofpossible intermediate species participating in the polymerformation mechanism has been reported yet.3,5,8,14,33

Raman spectroelectrochemistry is a powerful technique forthe investigation of electrochemically generated intermediatesand products that can provide invaluable structural informationon both the species in solution and those adsorbed on theelectrode surface. In fact, pioneering work by Fleischmann,Hendra, and MacQuilan34 on the anomalous intensification ofRaman scattering from pyridine molecules adsorbed on silverand gold electrode surfaces culminated in the discovery of

Received: January 22, 2015Revised: March 21, 2015Published: March 26, 2015

Article

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© 2015 American Chemical Society 4351 DOI: 10.1021/acs.langmuir.5b00250Langmuir 2015, 31, 4351−4360

surface-enhanced Raman spectroscopy, SERS. Since then,Raman spectroeletrochemistry has been used to characterizedifferent types of interfaces, such as those based on porphyrinthin films obtained by interfacial polymerization35 as well asgraphene and semiconductor thin films.36,37

Tetraruthenated metalloporphyrins were extensively inves-tigated as redox mediators for the oxidation of severalcompounds for sensor applications, exploring the [Ru-(bipy)2Cl]

+ peripheral complexes as electrocatalytic activesites in slightly acidic media.38−40 Interestingly, the tetrar-uthenated nickel porphyrin, [NiTPyP{Ru(bipy)2Cl}4]

4+, canalso be electropolymerized in pH 13 solution by redox cyclingin the 0.0 to 1.0 V vs Ag/AgCl(KCl sat.) range, despite thepresence of bulky [Ru(bipy)2Cl(pyP)]

+ complexes at the mesopositions, thus making unlikely the formation of axial μ-oxopolymers. As expected, the nickel center is electrochemicallyactive only in highly alkaline media, where the modifiedelectrode exhibits a very sharp couple of waves at around 0.4−0.5 V, which are assigned to a Ni3+/Ni2+ process. Furthermore,the peripheral ruthenium complexes seem to play an importantrole in film growth because the electropolymerization processwas shown to be remarkably dependent on the limiting anodicpotential used in the redox cycling,19 providing new strategiesto investigate further the nature of that polymeric material.Accordingly, the influence of hydroxide ions, the role played byRu and Ni sites in the electropolymerization process and filmgrowth, and the composition and structure of poly-[NiTPyP-{Ru(bipy)2Cl}4] were elucidated by a detailed electronic andRaman spectroelectrochemical study.

2. EXPERIMENTAL SECTION2.1. Chemicals and Reagents. Supermolecular complex μ-meso-

tetra(4-pyridyl)porphyrinatenickel(II)-tetrakis[bis(bipyridine)-(chloro)ruthenium(II)](TFMS)4 and sodium meso-tetra(4-sulfonatephenyl)porphyrinate zinc(II), hereafter denoted respectivelyas [NiTPyP{Ru(bipy)2Cl}4]

4+ and [ZnTSPP]4−, were synthesized aspreviously reported.31,38−41 Sodium hydroxide and methanol werepurchased from Merck (Darmstadt, Germany). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was acquired from Sigma-Aldrich Co.and purified by distillation under reduced pressure, as recommended,before its use as a spin scavenger.42 All aqueous solutions wereprepared using ultrapure water from a Millipore Milli-Q system(resistivity ≥18.2 MΩ·cm).

2.2. Electrode Modification and Electrochemistry. Theelectrochemical characterization was performed using a μ-Autolabtype III potentiostat/galvanostat (EcoChemie, The Netherlands). Thevoltammetric measurements were carried out in a conventional 10 mLcell using a (modified) glassy carbon disk working electrode(geometric area: 0.071 cm2), Ag/AgCl(KCl sat.) reference electrode,and a platinum wire as an auxiliary electrode. Electrodes were modifiedas previously reported19 by performing 5 or 50 successivevoltammetric cycles in a 1.0 × 10−4 mol·L−1 solution of [NiTPyP-{Ru(bipy)2Cl}4]

4+ species in 0.1 mol·L−1 NaOH supporting electro-lyte in the 0.00 to 0.90 V range (scan rate = 0.1 V·s−1). The modifiedelectrodes were washed with the electrolyte solution and reserved forfurther use. All potentials are referred to the Ag/AgCl(KCl sat.) referenceelectrode.

The experimental conditions for electrode modification andelectrochemical assays were chosen after careful study by varying thedeposition pH and limiting the positive potential as shown respectivelyin Figures S4 and S5. The polymeric material was not formed at pH 14probably because of the shift in the oxygen gas evolution reaction tolower potentials.

Figure 1. Electronic spectrum of a 1 × 10−4 mol·L−1 [NiTPyP{Ru(bipy)2Cl}4](TFMS) solution in (a) methanol and (b) a 0.1 mol·L−1 aqueousNaOH solution. The black and red lines correspond to the experimental and calculated spectra by deconvolution with Gaussian functions. Note thatno substitution of the chloro ligands is expected in the peripheral R = py-[Ru(bipy)2Cl]

+ groups.

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2.3. Electronic Spectra. Electronic spectra in solution wererecorded in an HP 8453 diode array spectrophotometer using a 10.0mm optical path quartz cuvette. The electronic reflectance spectra ofmodified platinum electrodes were obtained using an AnalyticalSpectral Devices (ASD) Field Spec fiber optics probe spectropho-tometer equipped with a tungsten lamp. Platinum electrodes weremodified following the procedure previously described for themodification of glassy carbon (GC) electrodes. The spectrum wasregistered in reflectance mode and converted to the absorbance scaleusing the ASD Field Spec equipment software.2.4. Raman Spectra and Spectroelectrochemistry. Raman

spectra were recorded using a WITec Alpha 300R confocal Ramanmicroscope equipped with an EM-CCD analyzer and Ar (488 nm),Nd:YAG (532 nm), and He−Ne (633 nm) lasers. The Ramanspectrum of solid [NiTPyP{Ru(bipy)2Cl}4](TFMS)4 was recorded at488 and 532 nm (power = 0.15 and 2.00 mW) using an integrationtime of 10 s. The Raman spectrum of a 1.0 × 10−3 mol·L−1 solution ofthis supramolecular complex in aqueous 0.1 mol·L−1 NaOH wasrecorded at 488 and 532 nm (power = 7.50 mW for both lasers) usingan integration time of 120 s.Raman spectroelectrochemistry experiments were carried out using

a homemade three-electrode spectroelectrochemical cell (Figure S1 inSupporting Information) and a PalmSens potentiostat/galvanostat.The laser was focused on the GC electrode surface and threesuccessive Raman spectra (λ = 532 nm, power = 0.50 mW, integrationtime = 10 s) recorded after surface modification with a poly-[NiTPyP{Ru(bipy)2Cl}4] film while applying a potential in the 0.05 to0.85 V range.2.5. Electron Paramagnetic Resonance Spectroscopy. The

hypothesis of a radical species being the key intermediate species forthe formation of the poly-[NiTPyP{Ru(bipy)2Cl}4] film wasinvestigated using a GC electrode modified with an electrostaticallyassembled [NiTPyP{Ru(bipy)2Cl}4]

4+/[ZnTSPP]4− film39 polarizedat 0.90 V. The spin-trapping reaction for hydroxyl radical detectionwas carried out in a specially designed electrochemical flow cell(Figure S2 in Supporting Information). A stream of a 0.1 mol·L−1

NaOH solution was continuously passed over the modified GC surfaceand merged with a 2.0 × 10−4 mol·L−1 DMPO solution at a confluencepoint placed immediately after the working electrode. A peristalticpump was used to propel both solutions at 0.6 mL·min−1. Theresultant effluent solution was collected in an appropriate flask andinjected into a 200 μL flat quartz cell placed in the cavity of a BrukerEMX EPR spectrometer operating at the X band (9.5 GHz). Spectrawere registered at room temperature using the following parameters:modulation frequency = 100 kHz, modulation amplitude = 1 G, andmicrowave power = 20.21 mW.2.6. Molecular Modeling. The ground-state geometry optimiza-

tion and the Raman spectra calculation for the target species werecarried out using the Gaussian 09W software employing densityfunctional theory (DFT)43,44 and the B3LYP hybrid functional45

(Becke’s gradient-corrected exchange correlation46 in conjunction withthe Lee−Yang−Parr correlation functional with three parameters47).The 6-311G++(d,p) basis set for C, N, and H atoms and theLANL2DZ basis set for the nickel atom, with their respectivepseudopotentials for the inner shell, were employed in the calculations.The assignment of the vibrational spectra was carried out bycomparison with literature data using GaussView 05 and Gabedit2.4.0 software.

3. RESULTS AND DISCUSSION

3.1. Electronic Spectroscopy. The electronic spectrum of[NiTPyP{Ru(bipy)2Cl}4](TFMS)4 in methanol solution (Fig-ure 1a) shows absorption bands at 209, 244, 291, 355, 408, 434,486, 526, and 558 nm, consistent with the presence of twochromophores, the nickel(II) porphyrin and the peripheralruthenium complexes.41 The bands at 291, 355, 434, and 486nm were assigned to the Ru complexes, where the 291 nm bandwas assigned to characteristic π → π* internal transitions of the

bipy ligands. The Ru(II) ligand field transitions were found at355 nm whereas the bands at 434 and 486 nm were assigned toRu(II) → bipy and dπ → pπ* metal-to-ligand charge-transfertransitions (MLCT). The changes observed in the MLCTbands in methanol and NaOH aqueous solution can beattributed to solvatochromic effects. The characteristicporphyrin ring bands at 408, 526, and 558 nm were assignedto the Soret, β, and α transitions, respectively.According to the four frontier orbitals model48 for the

porphyrin electronic states, the Soret band is marked by atransition from the HOMO (with a2u symmetry) to the LUMO(eg) orbital. The a2u orbital exhibits a lower electronic densityon the pyrrolic carbons, being more sensitive to the nature ofsubstituents at the meso positions and the coordinated metalion and respective axial ligands. Therefore, changes in theelectronic properties of the peripheral ruthenium complexesand the coordination of hydroxide anions to the nickelporphyrin axial positions should cause a blue shift and decreasein the intensity of the Soret band, as observed in Figure 1b.Accordingly, one can assume that [Ni(OH)2TPyP{Ru-(bipy)2Cl}4]

2+ is the predominant species in the 0.1 mol·L−1

NaOH aqueous solution.After 50 voltammetric cycles in the 0.00 to 0.90 V range in

aqueous NaOH solution (0.1 mol·L−1), a thin film of poly-[NiTPyP{Ru(bipy)2Cl}4] is deposited on the platinum diskelectrode surface. This exhibits red-shifted porphyrin transitions(Soret, β and α) consistent with formation of a π-stackedporphyrin material (Figure 2), whereas the MLCT bands of theperipheral ruthenium complexes exhibited only a slight red shiftand broadening indicating a much lower degree ofintermolecular interactions.

3.2. Electrochemical Behavior and Characterization.The cyclic voltammograms (CVs) corresponding to 50successive redox cycles of a platinum disk electrode in a pH13 aqueous 1.0 × 10−4 mol·L−1 [NiTPyP{Ru(bipy)2Cl}4]-(TFMS)4 solution, in the 0.0−0.9 V range, are presented inFigure 3a. The precipitation of poly-[NiTPyP{Ru(bipy)2Cl}4]on the electrode surface is confirmed by the rise of a couple ofsharp anodic and cathodic peaks respectively at 0.49 and 0.40

Figure 2. Electronic spectra of poly-[NiTPyP{Ru(bipy)2Cl}4]deposited on a platinum disk electrode surface after successive cycling(50 times) in a pH 13 aqueous 1.0 × 10−4 mol·L−1 [NiTPyP{Ru-(bipy)2Cl}4]

4+ solution in the 0.00 to 0.90 V range (black line).Calculated absorption spectrum (red line) and set of bands obtainedby deconvolution of the actual spectrum using Gaussian functions.

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V, assigned to the Ni3+/Ni2+ redox couple. In particular, thefirst three cycles (Figure 3b) were found to be important in theinvestigation of the mechanism of polymer formation.The rise of an irreversible anodic peak at around 0.80 V,

starting from the very first cycle in the presence of thetetraruthenated nickel porphyrin, is evident in all 50 voltammo-grams, suggesting the possible contribution of the peripheralruthenium complexes to the mechanism of polymer formation.Ru3+/Ru2+ is a reversible monoelectronic process38,39,49 suchthat the intense irreversible wave can be assigned to anelectrocatalytic process. In fact, the electrolyte discharge isanticipated in 0.4 V (Figure S3 in Supporting Information)when the tetraruthenated nickel porphyrin is present insolution, suggesting the key role of the peripheral rutheniumcomplexes in that electrochemical reaction. Considering theabsence of any other species except those present in thesupporting electrolyte solution, it should involve the electro-catalytic oxidation of hydroxide anions to hydrogen peroxideanion, molecular oxygen, or hydroxyl radical. The lasthypothesis is particularly interesting because it may be involvedin the electropolymerization mechanism18 leading to theformation of poly-[NiTPyP{Ru(bipy)2Cl}4] species.The electrochemical generation of dioxygen on the electrode

surface at sufficiently positive potentials is well documented inthe literature, but the formation of the other two species ismuch more scarce. Thus, the hypothesis of hydroxyl radicalformation mediated by peripheral Ru(III) complexes waspursued by EPR. For this purpose, an especially designed flowcell was constructed in order to trap the •OH species withDMPO soon after being generated on a GC electrode modifiedwith a thin film of electrostatically assembled38 [NiTPyP{Ru-(bipy)2Cl}4]

4+/[ZnTSPP]4− material. This is quite insoluble

and allows us to explore only the electrochemical properties ofthe peripheral ruthenium complexes because the tetraanionicporphyrin stacks on the top of the tetracationic porphyrinspecies acting as a spacer, thus sterically hindering thenickel(II) axial positions and inhibiting the formation of μ-oxo or μ-peroxo bridges that may consume that radical species.Interestingly, the EPR spectrum of the resulting solution(Figure 4) clearly exhibited the typical signals of the

DMPO/•OH adduct, with a hyperfine constant of 15.6 G,consistent with the reference value of 15.01 G.50−53 This resultindicates the formation of significant numbers of hydroxylradicals at the GC electrode modified with electrostaticallyassembled porphyrins and polarized at 0.9 V.The oxidation of water to molecular oxygen by the

[RuIII(bipy)3] complex in alkaline media was previouslyreported in the literature.54−58 A similar reaction probablycan be promoted by the [RuIII(bipy)2Cl(pyP)] species underappropriated electrochemical conditions. Nevertheless, becausethere was no gas evolution at the modified electrode polarizedat 0.90 V, that reaction should not be significant under ourexperimental conditions. Instead, in a 0.1 mol·L−1 NaOHsolution, hydroxyl radicals seem to be generated as the majoroxidation product, suggesting that it may be the keyintermediate species involved in the electropolymerizationmechanism leading to the formation of poly-[NiTPyP{Ru-(bipy)2Cl}4].Thus, the [RuIII(bipy)2Cl(pyP)] species generated in alkaline

media at potentials above 0.80 V should be the active speciesresponsible for the formation of hydroxyl radicals, either bydirect electron transfer or involving an intermediate species asproposed by Ledney et al.57 (Scheme 1). Direct electrontransfer is thermodynamically forbidden (OH• + e− = OH, E0 =2.02 V vs SHE), but a reaction mechanism involving thenucleophilic attack of OH− or H2O molecules on the paraposition of one of the bipy ligands produced a protonatedRu(III) adduct whose rapid deprotonation reaction leads to theredistribution of the charge density corresponding to the pair ofelectrons and the formation of a hydroxylated bipyridyl radical.The deprotonation of the bipy N−H intermediate is facilitatedby the high hydroxide concentration in the electrolyte such thatthe subsequent reduction of the peripheral complex to theRu(II) state leads to the homolysis of the C−OH bond,

Figure 3. (a) Successive 50 cyclic voltammograms recorded whilemonitoring the growth of poly-[NiTPyP{Ru(bipy)2Cl}4] on a glassycarbon electrode from an aqueous 1.0 × 10−4 mol·L−1 monomersolution in 0.1 mol·L−1 NaOH solution (pH 13) at 0.100 V·s−1 and(b) CVs corresponding to the first three cycles in the 0.0 to 0.9 Vrange where the 0.0 to 0.7 V region is shown in detail in the inset. CVin pure electrolyte solution (black line), first (red line), second (blueline), and third (green line) scan in [NiTPyP{Ru(bipy)2Cl}4]-(TFMS)4 solution.

Figure 4. EPR spectrum of the solution generated in the electro-chemical flow cell set with a GC electrode modified with a[NiTPyP{Ru(bipy)2Cl}4]

4+/[ZnTPPS]4− film and polarized at 0.90V, showing the characteristic signals of the DMPO/•OH spin adduct.

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releasing a •OH radical species and regenerating the bipyligand. Then, the complex is rapidly oxidized back to theRu(III) state by the electrode, starting a new electrocatalyticcycle.The formation of poly-[NiTPyP{Ru(bipy)2Cl}4] is strongly

dependent on that electrochemical process because thecharacteristic Ni3+/Ni2+ waves at 0.50 and 0.40 V (peaks IIIand IV in Figure 3b) are not observed when the potentialsweep is limited to values lower than 0.80 V, and there is nosignificant production of the [RuIII(bipy)2(pyP)Cl] complex.Also, potentials higher than 1.10 V promote the formation ofmolecular oxygen, and bubbles released at the modifiedelectrode surface inhibit the deposition of that polymericmaterial. Thus, the above-described mechanism seems toexplain why the voltammetric pattern in alkaline media isdifferent from that obtained in acidic media39 as well as thedependence of the film growth on pH and the limiting anodicpotential. The intensity of the reversible redox couple with Epa= 0.31 and Epc = 0.24 V (peaks I and II in Figure 3B) does notchange significantly as a function of the number of scans andcan be assigned to a soluble species present in the electrolytesolution.In summary, the formation of poly-[NiTPyP{Ru(bipy)2Cl}4]

proceeds via an electrochemical−chemical−electrochemical

(ECE) mechanism taking place in a narrow potential range(0.80 V ≤ E < 1.10 V) and involving electrocatalyticallygenerated hydroxyl radicals. These highly reactive speciesshould trigger some sort of polymerization process leading tothe deposition of a slightly soluble material as a thin film on theelectrode surface. However, no clear evidence of what isresponsible for the polymerization process has been reported.Accordingly, Raman spectroscopy and spectroelectrochemistrystudies were carried out to shed light on the structure of such apolymeric material.

3.3. Raman Spectroscopy and Spectroelectrochemis-try. Raman studies were carried out in order to characterize thepoly-[NiTPyP{Ru(bipy)2Cl}4] deposited on a glassy carbonelectrode surface by comparing the spectra of [NiTPyP{Ru-(bipy)2Cl}4](TFMS)4 in the solid state, in an aqueous 0.1 mol·L−1 NaOH solution (Figure 5), and as a polymeric film after 5and 50 voltammetric redox cycles in the 0.0 to 0.9 V range(Figure 6) at a scan rate of 0.10 V·s−1.The characteristic porphyrin and bipyridine CC and C

N stretching modes can be seen in the 1450−1650 cm−1 rangeusing the 532 and 488 nm laser lines. The 532 nm laser is inresonance with the nickel porphyrin β transition so that theintensification of the corresponding normal modes can beexplained by the Herzberg−Teller mechanism,59 where the

Scheme 1. Possible Mechanism for the Electrocatalytic Generation of Hydroxyl Radicals Mediated by Peripheral[RuIII(bipy)Cl(pyP)] Complexes in Strong Aqueous Alkaline Solutiona

aAdapted from Ledney et al.57

Figure 5. Normalized Raman spectra of [NiTPyP{Ru(bipy)2Cl}4](TFMS)4 powder (red line) and the [Ni(OH)2TPyP{Ru(bipy)2Cl}4]2+ complex

present in aqueous 0.1 mol·L−1 NaOH solution (black line), acquired using the (a) 488 and (b) 532 nm laser lines.

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asymmetric modes should be more enhanced than thesymmetric ones. Accordingly, the asymmetric CC and CN stretching modes at 1515 cm−1 and the asymmetricporphyrin ring deformation modes at 454 and 116 cm−1 arethe most enhanced ones.The spectral profile and peak intensities of [NiTPyP{Ru-

(bipy)2Cl}4]4+ species in aqueous 0.1 mol·L−1 NaOH solution

closely resemble those obtained for the pure powder asdescribed above, confirming that no degradation takes placewhen the tetraruthenated nickel porphyrin is dissolved instrongly alkaline solution (pH 13). The intensity of the ν(Ru−Cl) stretching mode at 253 cm−1 (Table 1) decreased in NaOHsolution when probed with both 488 and 532 nm laser lines.This suggests that hydroxide anions may be substituting for thechloride ligand in the Ru coordination sphere, but no evidenceof such a reaction could be found electrochemically. The bandsassigned to the Ni−O stretching and Ni−O−H angledeformation modes were found respectively at 405 and 1252cm−1, indicating that hydroxo ligands are coordinated to the Niporphyrin axial positions. Accordingly, one can assume that[Ni(OH)2TPyP{Ru(bipy)2Cl}4]

2+ is the major species presentin aqueous 0.1 mol·L−1 aqueous NaOH solution, but thepresence of minor amounts of supramolecular nickel porphyrincontaining [Ru(bipy)2(pyP)(OH)]

+ groups cannot be com-pletely ruled out. The conventional electrochemical behavior ofa [NiTPyP{Ru(bipy)2Cl}4]

4+ solution was recovered after thepH was adjusted to 5.The tentative assignment of the Raman spectra shown in

Table 1 was carried out in comparison to spectra of theporphyrin and its derivatives60−62 found in the literature, inaddition to the theoretical Raman spectra of [NiP], [NiP-(OH)]−1, and [NiP(O2)]

−1 species, where P represents theporphyrin ring. These species were considered in thetheoretical calculations assuming the possibility that thedeposited material could be a coordination polymer wherenickel porphyrin units are connected through hydroxide orperoxide bridges. The optimized structures and respectivetheoretical Raman spectra are shown in the SupportingInformation (Figures S6 and S7).

Poly-[NiTPyP{Ru(bipy)2Cl}4] was deposited as a thin filmfrom an aqueous NaOH solution such that hydroxide anionsare coordinated to the axial positions of the nickel(II)porphyrin. The electrochemical and Raman data suggest the

Figure 6. Theoretical Raman spectra of the [NiP(O2)]1− species

(top), where P represents the porphyrin ring. Raman spectra of thepoly-[NiTPyP{Ru(bipy)2Cl}4] species on a glassy carbon electrodeafter 50 (middle) and 5 (bottom) voltammetric cycles in the 0.00 to0.90 V range, in 0.1 mol·L1 aqueous NaOH solution.

Table 1. Tentative Assignment of the Raman Spectrum ofthe [NiTPyP{Ru(bipy)2Cl}4]

4+ Species in Aqueous 0.1 mol·L−1 NaOH Solution in Comparison to the Spectrum of thePure [NiTPyP{Ru(bipy)2Cl}4](TFMS)4 Powder RegisteredUsing the 488 nm Laser Line and Typical Vibrational Modesof the Peripheral [Ru(bipy)2Cl]

+ Complex60−62

theoret.[NiP] Ru(bipy)2Cl [NiTRP]

exp: 488 nm (inNaOH solution) tentative assignment

3133 νsiCβ−H3117 νassCβ−H30871632 1600 1609 1608 νassCmCα

νsi CC and νsi CN(bipy)

1555 1558 1557 1565 νass CC and νass CN(bipy)

νsiCmCα

νsiCβCβ

1493 1487 1491 1490 νassCβCβ

νassCC andνassCN(bipy)

1447 1455 1438 νsiCβCβ + νsiCmCα(out of phase)

1369 13791348 1352 1355 δsi,opNCC +

δsi,ipCαCmCα (outof phase)

νNiN1338 1321 1324 νassNC

13191274 1271 ν(bipy)

1259 1252 νCm−py + δipNi−OH

1217 1217 ν(py)1168 1170 1173 1177 δipCβH

1090 10941052 1064 1070 1066 δsi,ipNNiN974 1027 1024 1021 ring breathing (P +

bipy) + νsiNiN1004 989

889 901843

807 δsi,ipCαCβCβ +δsi,ipCαCmCα

719 744696650 662 669 668 δsi,opCαCβH +

δsi,opCH(py)601557

447 459409 405 νNiO + δsi,opP389 381

371 371 370 νRuN348 320 325 νsiNiN

253 νRuCl216 230 214 νassNiN + δassNiN

116 δsi,opP + δsi,opNiNa[NiTRP] = [NiTPyP{Ru(bipy)2Cl}4]

4+ complex; [NiP] = non-substituted Ni porphyrin.

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formation of μ-hydroxo, μ-peroxo, or μ-oxo bridges connectingadjacent nickel porphyrin units. And according to the radicaltrapping assay, hydroxyl radicals should be involved in theelectropolymerization mechanism.Thus, the formation of poly-[NiTPyP{Ru(bipy)2Cl}4] was

monitored by Raman spectroelectrochemistry in order to provethat hypothesis and identify the most probable bridging group.The spectrum of the material deposited on the GC electrodesurface after 5 and 50 voltammetric cycles is shown in Figure 6,in comparison to the spectrum of [NiTPyP{Ru(bipy)2Cl}4]

4+

species in NaOH solution, where it is apparent that the rise ofan intense band at 399 cm−1 is assigned to a Ni−O stretchingmode.The O−O stretching generally appears as high-intensity

bands in the 800−1300 cm−1 range depending on itsoxidization state and bond order.63,64 According to thetheoretical Raman spectra of the [NiP(O2)]

1− species (Figure6, top), the O−O stretching mode should be found around1370 cm−1 and the O−O bond length should be 1.31 Å, asexpected for the formation of a peroxo bridge with high chargedelocalization into the porphyrin ring. Comparing thetheoretical spectra with that of poly-[NiTPyP{Ru(bipy)2Cl}4]obtained after 5 and 50 voltammetric cycles, one can see therise of new bands at 1382 and 1236 cm−1, which can beassigned to a ν(O−O) mode with slightly higher bond orderthan that theoretically predicted for a peroxide. Furthermore,the bands at 249 and 209 cm−1 can be assigned to δ(NiOO)angular deformation modes, reinforcing the hypothesis of a μ-peroxo-bridged Ni porphyrin polymer. No additional bandsassignable to μ-hydroxo or μ-oxo bridged species could befound in the characteristic 800 to 1300 cm−1 range.A red shift and broadening of most of the Raman peaks were

observed in the polymeric material obtained after 50 cycles ascompared to 5 cycles, indicating that intermolecular inter-actions become stronger as the electropolymerization processproceeds in NaOH solution. This behavior can be rationalizedby a higher degree of π-stacking of nickel porphyrin rings dueto the formation of a more densely packed phase, as expectedfor the film growth as a function of the number of voltammetriccycles.The negative charge density on the O−O bridge is

dependent on the charge delocalization to the nickel porphyrinring, thus changing its apparent oxidation state. For example,the band at 810 cm−1 can be assigned to a ν(Ni3+−O)vibrational mode, which presents a higher frequency than theanalogous ν(Ni2+−O) mode at 405 cm−1. The Ru4+Ostretching mode also can be found around 800 cm−1; however,this was disregarded because the possibility of chloride ligandsubstitution by hydroxide was shown to be very low, accordingto our electrochemical studies as a function of the pH of theelectrolyte solution.Finally, poly-[NiTPyP{Ru(bipy)2Cl}4] was studied by

Raman spectroelectrochemistry (Figure 7) in the 0.00 to 0.70V range, making it possible to correlate the structural changesassociated with a given redox process and reinforce theevidence supporting the proposed structure. The first point tobe highlighted here is the absence of a typical Ni(OH)2 bandaround 475 cm−1 considering the whole potential range (Figure7a−c), thus ruling out the hypothesis of electrochemicallyinduced decomposition of the NiP complex65 and theformation of authentic Ni(OH)2. The spectrum of the materialformed after five voltammetric cycles (Figure 5a) showed thatthe oxidation of Ni2+ to Ni3+ around 0.50 V is coupled with the

formation of Ni−O−O−Ni peroxo bridges, involving hydroxylradicals as an intermediate. This is confirmed by the rise of the1234 and 209 cm−1 bands assigned to the ν(O−O) andδ(NiOO) modes, respectively. Additionally, the red shift of theporphyrin ν(CC) and ν(CN) modes at 1610 and 1560 cm−1

shows the π stacking of the nickel porphyrin rings as theelectropolymerization process proceeds. These vibrationalmodes were not significantly influenced when higher potentialswere applied, confirming the assignment of the anodic wave at0.52 V to the Ni2+/Ni3+ process. This conclusion is reinforcedby the saturation behavior of the intensity of the 810 cm−1

peak, assigned to the ν(Ni3+−O) mode, that reaches its

Figure 7. Raman spectroelectrochemistry of poly-[NiTPyP{Ru-(bipy)2Cl}4] on a glassy carbon electrode in 0.1 mol·L−1 aqueousNaOH solution in three complementary wavenumber ranges.

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maximum in the 0.45 to 0.55 V range. In summary, the Ramanspectroelectrochemistry studies confirmed the hypothesis thatpoly-[NiTPyP{Ru(bipy)2Cl}4] is constituted by stacked nickelmacrocycles connected by μ-peroxo bridges instead of hybridcomposites of authentic Ni(OH)2 formed upon electrochemi-cally induced decomposition of the nickel porphyrin complex.This conclusion is also supported by the fact that the CV andabsorption spectral profile of the starting [NiTPyP{Ru-(bipy)2Cl}4]

4+ complex is recovered upon neutralization ofthe polymeric material.

4. CONCLUSIONS

Spectroscopic and Raman spectroelectrochemical studies werecarried out, demonstrating for the first time that the polymericmaterial, electrochemically deposited from strongly alkalinesolutions of the [NiTPyP{Ru(bipy)2Cl}4]

4+ complex, is a μ-peroxo-bridged coordination polymer. The electropolymeriza-tion process involves the reaction of hydroxyl radicals, asconfirmed by EPR using DMPO as a spin-trapping agent, withthe hydroxide anions coordinated to the nickel porphyrin axialpositions. In fact, the oxidation of the peripheral rutheniumcomplexes to Ru(III) leads to the electrocatalytic formation ofhydroxyl radicals in the 0.80−1.10 V range, inducing theformation of Ni−O−O−Ni bridges. However, the film growthis halted above 1.10 V because of the formation of oxygen gasbubbles that disturb the electrode surface, precluding theadhesion of poly-[NiTPyP{Ru(bipy)2Cl}4]. The presence of μ-peroxo bridges was confirmed by the rise of Raman peaksassigned to ν(O−O) and δ(NiOO) at 1234 and 206 cm−1 andthe enhancement of the 810 cm−1 peak assigned to the ν(Ni3+−O) vibrational mode as Ni2+(O2

−2) is oxidized to the respectiveNi3+ species. Our findings probably can be extended to othernickel porphyrins.

■ ASSOCIATED CONTENT

*S Supporting InformationScheme of Raman spectroelectrochemical cell. CV of GCE inNaOH in the presence and absence of nickel porphyrin.Successive CVs as a function of pH. CVs as a function of anodiclimiting potential in NaOH. DFT-optimized structures andcalculated DFT Raman spectra of NiP, [NiP(OH)]−1, and[NiP(O2)]

−1. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: [email protected]. Phone: ++ 55 11 3091 8513.*E-mail: [email protected]. Phone: ++ 55 11 3091 3828. Fax:++ 55 11 3091 3781.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Fundacao de Amparo a Pesquisa do Estado de SaoPaulo (FAPESP), Conselho Nacional de DesenvolvimentoCientifico e Tecnolo gico (CNPq), and Coordenacao deAperfeicoamento de Pessoal de Ensino Superior (CAPES) forfinancial support and a fellowship for D.G. (FAPESP 2011/00037-6). We also thank Dr. Tiago Araujo Matias for helpfuldiscussions.

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