A Study of NO Adducts of Iron Protoporphyrin IX Adlayer on Au

A Study of NO Adducts of Iron Protoporphyrin IX Adlayer on Au Electrode with in Situ. ATR-FTIR Spectroscopy. Min Ma, Yan-Gang Yan, Jin-Yi Wang, Qiao-X...
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J. Phys. Chem. C 2007, 111, 8649-8654

8649

A Study of NO Adducts of Iron Protoporphyrin IX Adlayer on Au Electrode with in Situ ATR-FTIR Spectroscopy Min Ma, Yan-Gang Yan, Jin-Yi Wang, Qiao-Xia Li, and Wen-Bin Cai* Shanghai Key Laboratory for Molecular Catalysis and InnoVatiVe Materials and Department of Chemistry, Fudan UniVersity, Shanghai 200433, China ReceiVed: January 14, 2007; In Final Form: April 20, 2007

In situ attenuated total reflection (ATR) Fourier transform infrared (ATR-FTIR) spectroscopy has been applied to probe the coordination of nitric oxide to iron protoporphyrin IX (FePP) adlayer on Au (FePP/Au) electrodes in 0.1 M HClO4. On the basis of potential controlled ATR-FTIR spectra on independent FePP/Au electrodes and multistep ATR-FTIR measurement on one FePP/Au electrode, for the first time, up to three IR bands corresponding to three types of nitrosyl adducts of FePP have been identified with their intensities (concentrations) varied with the potential applied. The 1915-cm-1 band, which shows up at relatively positive potentials and stabilizes in a rather narrow potential range, can be reasonably assigned to the FeIII(NO)(OH2)PP species or to its isoelectronic format FeII(NO)+(OH2)PP. The other two bands with much lower frequencies, which can stabilize over a much wider potential range and which can exhibit nearly opposite potential-dependent intensities, are basically characteristic of nitrosyl adducts of ferrous FePP. One band at ca. 1670 cm-1 with insignificant Stark effect can be attributed to FeII(NO)PP. The other above 1705 cm-1 with significant Stark effect could be ascribed to FeII(NO)2PP. The multinitrosyl adductions may be caused by the largely inhomogeneous structure of the FePP adlayer on Au electrodes.

Introduction Nitric oxide is unique among diatomic molecules in that it can bind to many metals of different oxidation states and different electron configurations. The coordination chemistry of NO with iron porphyrins has been intensively investigated, inspired by both fundamental and technological interests ranging from relevant biological functions including vasodilatation and neuronal communication to environmental concerns about (bio)catalytic reduction and oxidation of nitrites.1-23 A detailed characterization of the electronic and structural properties of nitrosyl adducts of iron porhyrins promotes the further understanding of the roles that these species play in the abovementioned processes. Enemark and Feltham1,17-18 developed the {MNO}n formalism to describe the number of metal d plus NO π* electrons present in the MNO unit. They found that, for n ) 6 or less, the MNO unit is linear, while for n ) 7 or greater, the MNO unit is bent. In terms of model heme-NO complexes, both FeIII-NO ({FeNO}6) and FeII-NO ({FeNO}7) complexes have been characterized spectroscopically and structurally. Historically, studies on NO binding with iron porphyrins were mainly limited to bulk-phase reactions,1-15,17-19 with a variety of techniques including electron paramagnetic resonance (EPR),8 UV-vis absorption,5,14 X-ray absorption fine structure (XAFS),14 nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR),11-12,15 and resonance Raman spectroscopes.13,21,23 As for electrochemical systems, cyclic voltammetry in conjunction with optical methods has been applied to monitor the formation and conversion of dissolved iron nitrosyl adducts in both aqueous and nonaqueous media.4,9-12 Recently, efforts have been extended to study the coordination of NO to an iron porphyrin and its subsequent conversion with an expectation to create or modify versatile functional solid-liquid interfaces.16,20-22 * To whom correspondence should be addressed. Phone: +86-2155664050. Fax: +86-21-65641740. E-mail: [email protected].

Specifically, iron(III) protoporhyrin IX (FePP) (or hemin, a prothetic group of many hemoproteins; for its basic structure, please refer to Chart 1), irreversibly adsorbed on graphite and metal surfaces, exhibits excellent electrocatalytic effects on the reduction of dioxygen,24 hydrogen peroxide,25 nitrite,16,22,26 and carbon dioxide27 as well as potential sensing for trace amounts of CO and NO21,28 because of its extraordinary binding ability. However, the information derived from previous studies on bulk phases may not be directly applicable to the same or closely related species adsorbed on electrode surfaces because of microenvironment difference of materials.20 In situ spectroscopies including X-ray absorption spectroscopy (XAS) and surface-enhanced Raman spectroscopy (SERS) have been applied to characterize the nitrosyl adducts of iron porphyrins adsorbed on carbon and silver electrodes.20-22 However, the nature of bonding interactions between center Fe ions and NO, especially in ferric porphyrin complexes at an electrode, is not well understood.2,20-21 In situ SERS of the adsorbed nitrosyl FePP on an oxidation-reduction-cycling (ORC)-roughened Ag electrode failed to reveal the bands attributed to bound NO. Rather, the formation of nitrosyl adduct can be judged by the change in relative intensities of porphyrin ring bands.21 The SERS spectral features of the nitrosyl FeIIIPP complex on a Ag electrode seemed more similar to FeIIIPP than to FeIIPP.21 Statistical fittings for X-ray absorption near edge (XANE) and XAFS suggested a nitrosyl adduct [FeII(TMPP)(NO)(OH2)] stabilized on a carbon electrode over a wide potential range, regardless of originally ferrous and ferric states of iron (TMPP: abbreviation for meso-tetramethoxyphenylporphyrinato(2-)).20 Nevertheless, possible photodissociation of iron nitrosyl porphyrins with focused X-ray, UV, and visible irradiations may compromise the analysis as inferred in the literature.7,18 Surface infrared spectroscopy is more suitable for directly detecting internal stretching vibration of these small molecules

10.1021/jp070314+ CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007

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CHART 1: Structure of Iron Protoporphyrin IX (FePP)

that possess large dipole moments. In particular, attenuated total reflection (ATR)-FTIR including ATR-SEIRAS (surfaceenhanced infrared absorption spectroscopy) has higher surface sensitivity and far less interference from bulk solution, facilitating in situ monitoring of surface adsorption and reaction.29-39 One interesting application of the ATR-FTIR has been reported by Ye and co-workers to the study of CO/NO ligand exchange on a triruthenium cluster monolayer assembled on a gold electrode.37-38 Very recently, we have characterized with this technique two types of carbonyl adducts of FeIIPP adsorbed at a Au electrode in our previous study.36 In this report, we will present our preliminary results of surface FePP nitrosyl adducts on Au electrodes by taking advantage of in situ ATR-FTIR spectroscopy. Compared to the carbonyl adducts, the ATR-FTIR characterization of the nitrosyl faces greater challenges not only because NO binds to both ferrous and ferric porphyrins but also because the bands from water and carbonyl groups are close to the internal stretch band of NO ligand. These difficulties can be circumvented by combining the potential-controlled ATRFTIR measurement on independent FePP/Au electrodes and the multistep ATR-FTIR measurement on the same FePP/Au electrode. For the first time, potential-dependent structural information of nitrosyl adducts of FePP self-assembled on a Au electrode has been unwrapped, with up to three types of nitrosyl adducts discernible depending upon the potential applied. Experimental Section A Au nanoparticle film (ca. 60 nm thick) was deposited on the total-reflecting plane of hemicylindrical Si prism with electroless plating method.32,35-36 After being thoroughly rinsed with ultrapure Milli-Q water, the Au film electrode was immersed in a 50 µM chloride iron(III) protoporphyrin IX (FeIIIPP) (Aldrich Chemical Co.) in 0.1 M borax solution overnight for self-assembly to form a FePP modified Au (FePP/Au) electrode. Afterward, the FePP/Au electrode was thoroughly rinsed again with ultrapure water and was assembled into a spectroelectrochemical cell with a Krestchmann ATR configureration (a prism/Au film (electrode)/solution geometry). A Pt sheet and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Potential control was achieved with a ZF-3 potentiostat (Fangzheng Instruments, Shanghai) or a CHI660B electrochemistry workstation (CH Instruments, Texas). All potentials in the following were quoted against SCE. In situ ATR-FTIR spectra were obtained in 0.1 M HClO4 by using Magna-IR E.S.P. System 760 FT-IR (Nicolet) with unpolarized infrared radiation at an incidence angle of ca. 65°. The spectral resolution was 4 cm-1. All the spectra were shown in the absorbance unit defined as -log(I/ I0), where I and I0 represent the sample and reference singlebeam spectra, respectively. Spectral analysis was carried out with the Grams 32 software package (Galactic, Inc).

Figure 1. (a) Cyclic voltammograms for a bare Au electrode in neat 0.1 M HClO4 (trace 3) and NO containing 0.1 M HClO4 (inset trace 4) and for a FePP/Au electrode in neat 0.1 M HClO4 (trace 1) and NO containing 0.1 M HClO4 (trace 2). Scan rate: 50 mV s-1; Ei: 0.7 V. (b) Cyclic voltammograms for a bare Au electrode in neat 0.1 M HClO4 (trace 1) and a FePP/Au electrode in NO containing 0.1 M HClO4 (traces 2 and 3 correspond to first and second cycle, respectively. Ei: 0.3 V, initial scan direction: negative). Scan rate: 50 mV s-1, potential range: -0.25∼1.4 V.

Nitric oxide gas was generated by adding NaNO2 (A.R.) powder into diluted H2SO4 (A.R.) and FeSO4 (A.R.) mixture stored in a separate flask full of Ar atmosphere. Prior to entering the (spectro)-electrochemical cell for 1 min, NO was passed through two washing flasks filled with a 3 M KOH solution to ensure the removal of the trace amount of NO2.22 Results and Discussion Voltammetric Behaviors. Shown in Figure 1a are cyclic voltammograms between -0.3 and 0.7 V for the FePP-modified Au electrode in neat 0.1 M HClO4 (trace 1) and NO-containing 0.1 M HClO4 (trace 2) as well as the corresponding cyclic voltammograms for a bare Au electrode (traces 3 and 4). The displacement of trace 1 toward negative currents at potentials lower than 0.2 V is most likely due to the electrocatalytic reduction of traces of dioxygen present in the electrolyte. Similar to our previous report,21,36,41 no significant peaks due to the FeIII/FeII redox of the FePP/Au electrode (trace 2) can be observed in 0.1 M HClO4 in this potential range. At a constant solution pH, the reversibility and midpoint potential (namely, intermediate transition potential) for the FeIII/FeII redox of the FePP adlayer depend more or less upon microenvironment conditions such as types of electrodes and immobilization methods.22,26,40-41 Although the redox peaks for FeIII/FeII of the FePP adlayer on the Au electrode in 0.1 M HClO4 are not readily identified, a midpoint potential between -0.15 to -0.25

NO Adducts of Iron Protoporphyrin IX Adlayer V (SCE) can be assumed, on the basis of previous relevant reports.22,40-41 The presence of significant redox peaks for FePP on an HOPG (highly ordered pyrolytic graphite) electrode and the absence of these peaks for FePP on a Au electrode may reflect the difference in potential-induced structural changes. In the former, virtually no reorientation occurs as revealed by scanning tunneling microscopy (STM)42-44 (possibly because of the stronger π-π interaction between FePP ring and HOPG than the interaction between HOPG and FePP peripheral carboxylate group), whereas in the latter some kind of reorientation of FePP may occur because of the strong interaction of carboxylate group with Au electrode as the potential is increased. The potentialinduced reorientation of porphyrin ring plane at least partly causes the kinetic dispersion for the electron transfer between central metal ion and the Au electrode and makes the redox peaks extremely broadened and insignificant. Other contributions may come from more disordered structure of FePP adlayer on Au nanofilm electrode than that on HOPG electrode. Significantly increased cathodic current was found both for the bare Au (trace 4) and the FePP-modified Au (trace 2) electrodes upon introduction of NO into 0.1 M HClO4, attributable chiefly to the reduction of NO dissolved in solution. In fact, de Groot et al.22 found that solution NO can be reduced into selective NH2OH with N2O as the minor species on a FePPmodified graphite electrode In the present case, a cathodic current peak around -0.2 V shown on trace 3 (Figure 1a) became too weak to be identified after bubbling Ar for 60 min (not shown) or after the successive oxidation removal of FePP adlayer at potentials higher than 0.8 V before Ar bubbling (trace 3 in Figure 1b). This peak may probably correspond to the electrocatalytic reduction of solution NO by the FePP adlayer. A feeble anodic current increase at 0.7 V on traces 2 and 4 (Figure 1a) may be attributed to the oxidation of NO in solution and at surface. In the following in situ ATR-FTIR measurement, the positive potentials will be limited to less than 0.8 V. In situ ATR-FTIR Spectroscopy. It is known that the FeIIIPP adlayer exists in the monomeric form in an acidic solution on Au electrode surface, whereas the µ-oxo form of FeIIIPP layer may exist in a neutral solution.36,41 The formed FePP adlayer on Au is very stable in the potential range for in situ ATRFTIR spectra. The detailed structure and precise thickness of FePP film on the Au electrode is not known. STM investigations of FePP adlayer on HOPG electrodes yield a somewhat different structure of FePP adlayer.42-44 According to Snyder and White,43 FePP molecules lie nearly flat on the HOPG surface constituting ca. 1.5 monolayers with irregular-shaped aggregation and random distribution. However, Tao et al.44 observed with STM a monolayer of FePP on HOPG with a rather ordered structure at the initial self-assembled stage and aggregation at extended times. Hence, it may be reasonable to assume that the present FePP adlayer on Au is about 1-1.5 monolayer thick on the average without long-ranged ordered structure. Shown in Figure 2 are the potential-dependent ATR-FTIR spectra for a FePP/Au electrode in neat 0.1 M HClO4 solution with the reference spectrum taken at 0.7 V. The upward bands at ca. 1710 cm-1 and the downward bands at 1382 cm-1 can be assigned to ν(CO) of a peripheral propionic group and νs(OCO) of a peripheral propionate group.36 Increasing the potential, the ν(CO) band of the propionic group decreases while the νs(OCO) band of the propionate group increases (i.e., becomes less negative). In other words, relatively positive potentials favor the carboxylate group on Au electrode, while relatively negative

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Figure 2. Potential-dependent ATR-FTIR spectra for a FePP/Au electrode in 0.1 M HClO4. Reference spectrum was taken at 0.7 V.

Figure 3. Controlled-potential ATR-FTIR spectra for four independent Au electrodes and FePP/Au electrodes in NO containing 0.1 M HClO4 at -0.1 V (A) and 0.6 V (B). Reference spectra were taken in neat 0.1 M HClO4 at the same potentials as sample spectra.

potentials favor the carboxylic group. The same potentialdependent adsorption states have been found for benzoic acid and p-nitrobenzoic acid molecules on Au and Pt electrodes.45-47 This FePP adlayer may change its orientation from nearly flat to somewhat tilted state as the potential shifts positively, especially at potential higher than ca. 0.40 V (i.e., around the open-circuit potential), as a result of enhanced interaction of peripheral propionate with Au surface through the loss of a proton from originally propionic group.36 The ν(NO) band for FeII(NO)PP adlayer is approximated in frequency to the δ(HOH) band of the interfacial H2O and the νCO band of the propionic group. In addition, the NO species possibly adsorbed on exposed Au sites (if any) should also be heeded for sounding spectral characterization. To address these issues, first, controlledpotential ATR-FTIR measurement on independent bare Au and FePP/Au electrodes in 0.1 M HClO4 was taken before and after the introduction of NO to the electrolyte at specifically constant potentials, the results of which are shown in Figure 3. It can be seen that for bare Au electrodes, introduction of NO into the electrolyte does not result in appreciable bands in the frequency range of concern, indicating that NO adsorption on a bare Au electrode can be negligible, and thus, the spectral interference from NO adsorbed exposed Au sites of a FePP/Au electrode can be excluded. In addition, it can be seen that the controlled-potential measurement does not incur appreciable interfacial water band (ranging from 1610 to 1650 cm-1) either. For FePP/Au electrodes, one band located at 1672 cm-1 at -0.1 V and at least two bands located at 1915 and 1722 cm-1 (the

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Figure 4. Controlled-potential ATR-FTIR spectra for independent FePP/Au electrodes in NO containing 0.1 M HClO4. Reference spectra and sample spectra were taken before and after bubbling NO at constant potentials as specified.

latter consists of two bands, vide infra) at 0.6 V can be clearly identified. The νNO for nitrosyl adducts of various bulk iron porphyrins has been studied extensively2-4,7,11-12,14,17-18 arriving at a general consensus that the frequency depends strongly on the iron oxidation state and coordination number. In general, six-coordinate mononitrosyls of ferric porphyrins exhibit the νNO band frequency above 1900 cm-1, and for their five-coordinate counterparts, a red shift of ca. 50 cm-1 was noticed.11,17 In particular, two bands at 1860 and 1940 cm-1 are detected for [Fe(NO)2TPP]+ (TPP: abbreviation for tetraphenylporphyrinato(2-)).4 In the case of five- and six-coordinate mononitrosyls of ferrous porphyrins, the νNO bands are located at around 1670 and 1620 cm-1, respectively.3,6,11 Formation of a bulk-phase bisnitroysl ferrous porphyrin was also reported [e.g., FeII(NO)2TPP] only under very low temperatures and sufficient NO pressures with one strong band at 1695 cm-1 attributable to νNO.19 Hence, the approximate 1722-cm-1 broad band (consisting of actually two bands, vide infra) can be tentatively attributable to νNO of nitrosyl adducts of ferrous porphyrins with the formalism of {FeNO}7 (for ligating one NO) or {FeNO}8 (for ligating two NOs) and the band at 1915 cm-1 to νNO of sixcoordinate FeIII(NO)(H2O)PP or FeII(NO+)(H2O)PP with the formalism of {FeNO}6.6,17-18 Ex situ ATR-FTIR measurements on FePP/Au before and after reaction with NO at the solid/gas interface may lend a kind of support to these assignments. (Refer to Figure 1s in the Supporting Information.) The above controlled-potential ATR-FTIR measurements were extended to a potential range of -0.3 to 0.8 V, as shown in Figure 4 in which a freshly prepared FePP/Au electrode was used each time a controlled-potential measurement was run. At lower potentials such as -0.3 to -0.1 V, only one band located at 1665-1672 cm-1 can be seen, and increasing the potential caused the broadening of this band because of the growth of a shoulder band on the high wavenumber side. In all these controlled-potential measurements, the band at 1385 cm-1 was not detected, which was attributable to a possible intermediate species, that is, HNO coordinated to Fe(II)PP during the reduction of NO2-.22-23 The band at ca.1670 cm-1 shifted only slightly in the potential range from -0.2 to 0.7 V (from 1668 to 1672 cm-1), whereas the shoulder band above 1705 cm-1 appeared to gain its intensity and blue-shifted significantly with positively moving potentials. The shoulder band cannot be assigned to νCO of a peripheral propionic group since the controlled-potential measurement ensures this band to be effectively subtracted. Also, the 1915-cm-1 band shows up at 0.3 V and grows until 0.6 V. All bands related to NO ligands

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Figure 5. Potential-dependent ATR-FTIR spectra for a FePP/Au electrode in NO containing 0.1 M HClO4. Reference spectrum was taken at 0.7 V.

disappeared at 0.8 V, suggesting the breakage of NO coordination, and the new weak bands at 1531 and 1554 cm-1 at 0.8 V may reflect the formation of FePP π-cations.11 As we addressed already, the controlled-potential measurement is quite useful to remove possible interference from other species (if any) on the νNO band of FeIIPP(NO). Nevertheless, because of the fluctuated overall IR effects for different Au electrodes, it is less strict to compare the potential-dependent intensity in Figure 4. Therefore, we further our discussion on the basis of the potentialdependent ATR-FTIR spectra for the same FePP/Au electrode in NO-containing 0.1 M HClO4. Shown in Figure 5 are such spectra obtained with multistep mode stepwise from 0.7 to -0.2 V and with the single beam spectrum at 0.7 V as the reference for the calculation. The downward-directed 1382-cm-1 band of the carboxylate group of FePP was again detected irrespective of the nitrosyl adduct formation. It can be expected that the corresponding upwarddirected band around 1710 cm-1 for the carboxylic group should be accompanied at lower potentials, although it was buried in stronger νNO bands of interest. To remove the disturbance of the vibrational bands from the carboxylic group and interfacial water involved in the multistep acquisition mode, calibration was made by subtracting a weighted spectrum in Figure 2 (defined as A) from its corresponding spectrum in Figure 5 (defined as B) for a specific potential, that is, B - λA, where λ is a scale factor tunable in the subtraction with Grams v32 for the 1382-cm-1 band to be completely subtracted. The calibrated spectra were replotted in Figure 6. Qualitatively, the 1913-cm-1 band in Figure 6 varied with potential in the same trend as in Figure 4. The downward-directed band at 1913 cm-1 at potentials more negative than 0.3 V was caused by the reference spectrum at 0.7 V. Figure 7 shows the integrated intensities of three bands versus potential plots on the basis of curve fitting. Deconvolution was applied to the two overlapping bands (ca. 1670 and ca. 1710). The intensity of the 1913-cm-1 band was calculated from replotted spectra (not shown) with the reference spectrum taken at -0.2 V where no such band formed (refer to Figure 4). These three bands correspond to three states of nitrosyl adducts of FePP, with their stability or concentration varied depending upon potential. The minor species, FeIII(NO)(OH2)PP or FeII(NO)+(OH2)PP, showing the 1913-cm-1 band, stabilizes within a relatively narrow potential range of ca. 0.3-0.7 V and maximizes its coverage around 0.5 and 0.6 V. This is in agreement with the reports that nitrosyl adducts of ferric porphyrins are rather unstable in bulk phases.15,20 (Much less stability of FeIII(NO)(OH2)PP can be further proved by an independent in situ ATR-FTIR measurement on nitrosyl adducts of FePP/Au

NO Adducts of Iron Protoporphyrin IX Adlayer

J. Phys. Chem. C, Vol. 111, No. 24, 2007 8653 adlayer tilted up,36 facilitating NO to coordinate from both sides of the macrocycle ring plane of FePP. The less tilted portion of the FePP adlayer, which is favored at lower potentials,36 may accommodate only one NO ligand for each Fe(II) center. The mixture of mono- and bis-nitrosyl adducts as well as their opposite dependence on potential appears to agree with an inhomogeneous structure for the FePP adlayer. Similarly, a larger Stark effect was also found for biscarbonyl adduct than for monocarbonyl adduct of FePP adlayer on a Au electrode.36 Although difference in microenvironments for the bulk and surface species could be cited as a possible explanation, more work needs to be done to understand why the FeII(NO)2PP on Au electrode may survive at room temperature while a bulkphase FeII(NO)2TPP cannot.19

Figure 6. Potential-dependent ATR-FTIR spectra for a FePP/Au electrode in NO containing 0.1 M HClO4 after weighted deduction of specific spectra in Figure 2 from their corresponding spectra in Figure 5. See text for details.

Figure 7. The plot of integrated intensities of the bands at 1913 cm-1 (4), 1670 cm-1 (9), and 1705-1722 cm-1 (O) (from Figure 5) as a function of potential. See text for more details.

electrode after Ar was bubbled in to “purge” NO in 0.1 M HClO4 solution; refer to Figure 2s in the Supporting Information for details.) The two bands around 1670 cm-1 and above 1705 cm-1 exist over a much wider potential window, with their intensities nearly opposite dependence on potential (except at potentials lower than -0.1 V or higher than 0.6 V where nitrosyl species may start to be either reduced or oxidized). The first band, attributable to FeII(NO)PP, has a very small Stark effect with potential, ranging from 1668 cm-1 at the negative limit to 1672 cm-1 at the positive limit. With decreasing potential from 0.7 V, this band gains intensity monotonically until -0.1 V. The loss of intensity at -0.2 V and more negative potentials correspond to the reduction peak in Figure 1, suggestive of the partial reduction of nitrosyl adducts in company with the electrocatalytic reduction of dissolved NO. The second band has a rather large Stark effect, ranging from 1705 to 1722 cm-1. In general, this band increases its intensity as the potential shifts from 0.7 to 0.5 V and then monotonically decreases with negative-going potentials. Lorkovic and Ford19 reported that bulk-phase FeIITPP(NO)2 showed a band at 1695 cm-1, ca. 30 cm-1 higher than that observed for FeIITPP(NO). We assume that this band may be due to bisnitrosyl adducts of FeIIPP adlayer, FeII(NO)2PP on Au electrode. It has been reported that the FePP adlayer formed on Au electrode from an alkaline FePP solution does not show a homogeneously ordered structure.42-44 On the basis of our in situ ATR-FTIR measurement on FePP/Au in 0.1 M HClO4 at higher potentials (but more negative than its oxidation potential), a larger portion of FePP

Conclusion The coordination of nitric oxide to iron protoporphyrin IX adlayer on Au (denoted as FePP/Au) electrodes in 0.1 M HClO4 has been investigated by using in situ ATR-FTIR spectroscopy. On the basis of potential-controlled ATR-FTIR measurement on independent FePP/Au electrodes and multistep ATR-FTIR measurement on the same FePP/Au electrode, for the first time, up to three IR bands corresponding to three types of nitrosyl adducts of FePP can be identified with their intensities (concentrations) varied with the potential applied. The first band at ca. 1915 cm-1, showing up at relatively higher potentials, is ascribable to the less stable FeIII(NO)(OH2)PP species or to its isoelectronic format FeII(NO)+(OH2)PP. The second band at ca. 1670 cm-1 with insignificant Stark effect can be attributed to FeII(NO)PP, while the band above 1705 cm-1 with significant Stark effect could be ascribed to FeII(NO)2PP. The latter two bands stabilize over a much wider potential range and exhibit nearly opposite potential-dependent intensities. The presence of multiple nitrosyl adducts may be due to the largely inhomogeneous structure of the FePP adlayer on Au electrodes. Acknowledgment. The ATR Si prism was a gift from Professor M. Osawa in the Catalysis Research Center, Hokkaido University. The NSFC (No. 20473025), the SRFDP (No. 20040246008), the NCET (No. 04-0349), and the SNPC (No. 0452nm0642) are gratefully acknowledged for financial support. Y.Y.G. appreciates the support of the Innovation Fund from Fudan University (No. CQH1615018). We also thank the State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University (No. 200303), for providing partial financial support and technical assistance through coordinator Prof. Z. Q. Tian. Supporting Information Available: Ex situ ATR-FTIR spectrum for NO coordination to FePP/Au, and in situ ATRFTIR spectra for a pre-formed nitrosyl FePP/Au electrode in 0.1 M HClO4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339. (2) Wayland, B. B.; Olson, L. W. J. Am. Chem. Soc. 1974, 96, 6037. (3) Maxwell, J. C.; Caughey, W. S. Biochemistry 1976, 15, 388. (4) Olson, L. W.; Schaeper, D.; Lancon, D.; Kadish, K. M. J. Am. Chem. Soc. 1982, 104, 2042. (5) Fujita, E.; Fajer, J. J. Am. Chem. Soc. 1983, 105, 6743. (6) Scheidt, W. R.; Lee, Y. J.; Hatano, K. H. J. Am. Chem. Soc. 1984, 106, 3191. (7) Yu, N.-T.; Kerr, E. A. In Biological Applications of Raman Spectroscopy; Spiro, T. G., Ed.; John Wiley and Sons: New York, 1986; Vol. 3, p 81. (8) Yoshimura, T.; Suzuki, S.; Nakahara, A.; Iwasaki, H.; Masuko, M.; Matsubara, T. Biochemistry 1986, 25, 2436. (9) Barley, M. H.; Takeuchi, K. J.; Meyer, T. J. J. Am. Chem. Soc. 1986, 108, 5876.

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