. I Phys. . Chem. 1995, 99, 10359-10364
10359
In Situ Extended X-ray Absorption Fine Structure of an Iron Porphyrin Irreversibly Adsorbed on an Electrode Surface Sunghyun Kim,? Donald A. Tryk,? In Tae Bae,? Marnita Sandifer,? Roger Cam,$ Mark R. Antonio,* and Daniel A. Scherson*J Department of Chemistry, Case Westem Reserve University, Cleveland, Ohio 441 06, Argonne National Laboratory, Chemistry Division, Argonne, Illinois 60439, and Stanford Synchrotron Radiation Laboratory, Stanford, California 94305 Received: February 7, 1995@
Structural changes accompanying the reduction of @-oxo)bis[(meso-tetrakis(methoxyphenyl)porphyrinato)iron], [Fe(TMPP)]20, irreversibly adsorbed on Black Pearls 2000 (BP) high area carbon in aqueous electrolytes have been examined in situ by X-ray absorption fine structure (XAFS). In the pH range 5-10.8, the average iron-to-porphinato nitrogen distance, d(Fe-N,), for the ferric species, using theoretical phases and amplitudes calculated from FEFF 3.28 (2.09 f 0.01 A), was found to be, within experimental error, the same as that reported for the closely related [Fe(TPP)]20 in crystalline form (2.087 A). At extreme pH values d(Fe-N,) was smaller (2.04 f 0.02 8, for pH = 1.2 and 3.1 and 2.05 f 0.02 A for pH = 13) than those observed in the intermediate pH range. The values in strong acid are consistent with those reported for the diaquo axially coordinated complex [Fe(TPP)(OH2)2]+ (d(Fe-N,) = 2.045 f 0.005 A), whereas that at pH = 13, d(Fe-N,) = 2.05 f 0.02, strongly suggests an axially coordinated dihydroxy complex as the predominant species. The latter assignment is in agreement with information derived from other spectroscopic methods for closely related ferric porphyrins in solution phase. In contrast, the corresponding ferrous counterparts displayed values for d(Fe-N,) (2.02 f 0.02 A) consistent with the iron center placed in the plane of the ring throughout the whole range of pH values. Insight into the possible axial ligation of the ferrous derivatives was obtained from the dependence of the potential associated with the voltammetric peaks on the solution pH. Spin-state/ stereochemical relationships derived from X-ray crystallography and magnetic measurements of wellcharacterized iron porphyrins indicate that the adsorbed ferric species are all high spin, whereas the ferrous counterparts are either high or intermediate spin.
Introduction The catalytic' and electrocatalytic2 activities of transition metal macrocycles are most often associated with the axial coordination of the reactant to the metal center. A detailed structural and electronic characterization of the metal-ligandreactant microenvironment may be regarded as of utmost importance, as it may lead to a better understanding of the factors that control the catalytic properties of these materials and, in addition, shed light on possible molecular modifications that could result in gains in both activity and stability. Of special interest to ongoing efforts in this laboratory are oxidationreduction processes mediated by transition metal macrocycles irreversibly adsorbed on carbon support^.^ This surface confinement strategy provides a means of promoting redox reactions involving multiple sequential electron transfers, where energetically favorable, such as the four-electron reduction of dioxygen in aqueous electrolyte^.^ Iron K-edge extended X-ray absorption fine structure (EXAFS) has been used in this work to determine in situ the average iron-to-porphinato nitrogen distance, d(Fe-N,), of @-oxo)bis[(meso-tetra.kis(methoxyphenyl)po~phy~inato)iron], [Fe(TMPP)]20 (and its various axially coordinated monomeric forms), irreversibly adsorbed on Black Pearls 2000 (BP) high area carbon in aqueous electrolytes. Measurements were performed with the adsorbed macrocycle both in its native (femc) and reduced forms by polarizing the high-area electrode at the appropriate potenCase Western Reserve University. Argonne National Laboratory. 5 Stanford Synchrotron Radiation Laboratory. 'Abstract published in Advance ACS Absrracrs, June 1, 1995. t f
0022-3654/95/2099-10359$09.00/0
tials. The information derived from these experiments has provided considerable insight into modifications in the metal center-ligand microenvironment associated with changes in the redox state as a function of the solution pH. Experimental Section X-ray Absorption Fine Structure (XAFS). Data Acquisition and Analysis. Iron K-edge XAFS data were obtained at beam-lines X-18B and X-6B at the NSLS (National Synchrotron Light Source), Brookhaven National Laboratory, operating at 2.5 GeV with ca. 110-220 mA of stored current. Doublecrystal Si(220) monochromators were used for these measurements. The X-ray beam size on the electrode, which was mounted at 45" to the beam axis, was ca. 2 mm x 20 mm. The iron fluorescence XAFS signal, If, was recorded at ambient temperature through a Mn filter (three absorption lengths) with an ion chamber detecto? filled with argon as well as with a silicon photodiode detector.6 The incident X-ray intensity, IO, was measured with a sealed ion chamber filled with helium to provide an absorbance of ca. 0.05 at 7000 eV. In order to minimize the harmonic content of lo, the orientation of the crystals was adjusted to pass ca. 50% of the maximum IO at every point in the scan. The iron XANES was obtained with integration times of 4 s/point at uniform energy steps of 0.77 eV/point. The EXAFS was acquired with time averaging up to 10 s/point at energy intervals of constant photoelectron wave vector k, 0.05 kl. The primary XAFS data, I& were normalized by use of least-squares approximations to the preand postedge signals, Le., two-term linear and three-term quadratic functions, respectively.7 The normalized iron XAFS 0 1995 American Chemical Society
10360 J. Phys. Chem., Vol. 99,No. 25, 1995 is available as supplementary material. For the conversion from eV to k (wave vector)-space, k = [0.263(E - EO)]'", an EO of 7120 eV was used. Good signal-to-noise ratio was obtained to 7675 eV and was about 11.7 A-'. The k3x(k) EXAFS was extracted from the normalized XAFS by use of four sections (2.923 A-' each) of cubic B-spline functions. Fourier transforms (FTs) were obtained over the region 0- 11.7 A-1 with a square window function and without phase shift correction. To isolate the Fe-N, backscattering contribution from the total signal, the EXAFS was Fourier-filtered by use of a wide (filtering) Hanning window (Ar = 1.95 8, for the spectra for the oxidized state and 1.88 A for the spectra for the reduced state) with a 12.5% slope, centered about the Fe-N, peaks in the FT data (see supplementary material). The resulting Fourierfiltered k3x(k)EXAFS, from 2 to 12 A-I, was employed in the nonlinear least-squares curve-fitting with the conventional, single-scattering description of the EXAFS e f f e ~ t . Theoretical ~.~ phase and backscattering amplitude functions were obtained from FEW 3.28.1° The nonlinear least-squares best fits (with theoretical functions) to the filtered EXAFS are available as supplementary material. The accuracy of the resulting best fit values for d(Fe-N,) was evaluated through the fitting of the Fourier-filtered k3x(k) EXAFS for the neat, microcrystalline model compound [Fe(TMPP)]zO, which was measured and treated in the fashion described above. The value of d(FeN,) was determined to be 2.09 f 0.01 A when assuming either a single Fe-N shell (which yielded a goodness of fit F = 0.69) or a two-shell (Le., Fe-N and Fe-0) fitting of the filtered data, for which F = 0.21. In the latter case, all parameters (r, N , a, and A E o for both Fe-0 and Fe-N) except for r and A E o for Fe-N or all parameters except for A E o for Fe-N were allowed to vary. In both cases, d(Fe-0) was found to be 1.75 f 0.02 A. These distances are in line with d(Fe-N,) (2.087(3) A) and d(Fe-0) (1.763(1) A) extracted from a single-crystal X-ray diffraction (XRD) study of the closely related @-oxo)bis[(mesotetrakis(phenyl)porphyrinato)iron], [F~(TPP)]zO." The values of d(Fe-N) obtained using the wide Fourier-filtering window for neat [Fe(TMPP)]20 and, by analogy, for all of the ferric p-oxo dimers adsorbed on BP were not compromised by the low-frequency Fe-0 @-oxo) backscattering. Such behavior may not be surprising, since for all five-coordinate high-spin porphinatoiron(III) complexes, d(Fe-N,) is in the range 2.0602.087 and is thus significantly longer than the Fe-0 bond distances in p-oxo type compounds.I3 For the reduced porphyrins in the whole pH range, and the oxidized porphyrins in strongly acidic and strongly alkaline media, where the structures of the adsorbed species may be quite different from those of well-characterized model compounds, the accuracy of the Fe-N, interatomic distances may be slightly compromised by the presence of axial ligands. Studies by Eisenberger et a l . I 4 demonstrate that the complete neglect of the contributions of axial ligands to the EXAFS of picket fence porphyrins intrpduces an error of up to 2% (ca. f 0 . 0 2 A) in the values of d(Fe-N,). Although the main focus of this EXAFS analysis was the determination of d(Fe-N,), considerable effort was made to extract from the data values for d(Fe0) attributed to iron-axial ligand interactions. To this end, two-shell fits were performed using optimized d(Fe-N,) and AE, values determined from one-shell fits as fixed inputs to calculate best fits for d(Fe-0). The in situ experimental data, however, were apparently of insufficiently good quality to yield reliable d(Fe-0) values, even in cases in which d(Fe-Np) and the XANES were clearly consistent with a p-oxo type compound. Furthermore, based on the length of the experimental Ak file (10 A-'), shells separated by less than d2Ak (ca. 0.15-
Kim et al. 0.20 A) cannot be resolved. Since the Fe-axial ligand distance (most probably oxygen from hydroxyl or water) for a non-poxo complex may be expected to be within this range, e.g., d(Fe-N,) = 2.045(8) and d(Fe-0) = 2.095(2) 8, for [Fe(TPP)(OH2)2]+,I5 the search for axial ligands for adsorbed species other than p-oxo complexes was not deemed warranted and therefore was not pursued. In summary (i) the agreement between the EXAFS and XRD results for neat [Fe(TMPP)20)] lends credence to the analysis of the in situ EXAFS for irreversibly adsorbed [Fe(ThPP)]20 and related species generated by changes in the pH of the adjacent media or in oxidation state and (ii) the estimated absolute error for d(Fe-N,) measured in situ is not larger than zkO.01 A for the adsorbed p-oxo-type species and no larger than f 0 . 0 2 8, for all other species. Electrochemistry. The adsorption of the macrocycle was effected by adding the BP carbon (60 mg) to a solution of [FeTMPP]20 (25 mg) in acetone (ca. 7.5 x M). The mixture was then stirred (with a Teflon-coated magnetic stirrer) for 1 h, ultrasonically agitated for 30 min, and subsequently filtered through a Gelman filter (Acro LC 25 PVOF, 0.45 pm) with a syringe. The amount of adsorbed macrocycle was estimated by determining the concentration of the filtrate solution using conventional transmission UV-visible spectroscopic techniques. This method represents an improvement over that described in an earlier work16 in that the possibility of microcrystallite formation following adsorption is significantly decreased. In particular, for the amounts of macrocycle and carbon used, the concentration of macrocycle in the filtered solution (as measured spectroscopically) was negligible. All in situ Fe K-edge EXAFS spectra were recorded at potentials sufficiently positive (E,,) or negative (Ered) so as to convert the adsorbed species to a single oxidation state in the following aqueous buffered solutions: 0.05 M H2SO4 (pH = 1.2; E,, = 0.40; Ered = 0.45 V vs SCE), 0.1 M NaH2P04 (pH = 3.1, adjusted with H2SO4; E,, = 0.13; Erd = 0.72 V vs SCE), 0.1 M NaHzP04 (pH = 5, adjusted with Na2HP04; E,, = 0.10; Ered = -0.80 V vs SCE), 0.1 M NaH2P04 (pH = 7, adjusted with NaOH; E,, = 0.10; Ered = 0.80 V vs SCE)), 0.1 M NazHP04 (pH = 8.8, adjusted with 0.1 M NaHzP04; E,, = 0.00; Ered = -1.00 V vs SCE), 0.1 M NazHP04 (pH = 10.8, adjusted with NaOH; E,, = 0.00; Ered = - 1.OO V vs SCE), and 0.1 M NaOH (PH = 13, E,, = 0.00; Ered= -1.00 V vs SCE).
Results and Discussion Plots of the normalized iron K-edge EXAFS, k3x(k),for 40% [FeTMPP]zO/BP in the oxidized (top panel) and reduced (bottom panel) states as a function of solution pH are displayed in Figure 1. The corresponding FT data (Figure 2) exhibit the characteristic peak pattem of a metallop~rphyrin.'~-~~ In each case, the strong f i s t peak in the FT data at ca. 1.6 A (before phase shift correction) is due to backscattering from the four Np atoms. The second peak and shoulder combination at 2.422.67 and 2.98-3.12 8, (before phase shift correction) is due to backscattering from the eight carbon atoms of the pyrrole ring bonded to nitrogen, Ca, and four methine carbon atoms, C,,,,, respectively. The third peak at 3.58-3.91 A (before phase shift correction) in the FT data of Figure 2 is due to backscattering by the eight carbon atoms of the pyrrole ring in the beta position, Cg. One exception to this three-peak porphyrinic fingerprint is noted for the reduced data at pH 13, wherein the C, peak is anomalously short at 2.28 A (before phase shift correction). This apparent shift is an artifact of the FT data due to the lower quality of the EXAFS in this strongly alkaline solution. Otherwise, these FTs of the k3x(k) EXAFS suggest that the porphyrin environment is maintained at all solution pH values
In Situ EXAFS of Iron Porphyrin
J. Phys. Chem., Vol. 99, No. 25, 1995 10361
, 0
2
1-
A
6
4
at ca. 1.2 8, (before phase shift correction) and shoulders on the low-distance side of the Fe-N, peaks. Their presence is suggestive of backscattering by axial ligands, most likely oxygen atom(s), either from a p-oxo bridge or from water, hydroxy, or oxyanions. The absence of these features in the corresponding ferrous counterparts suggests that the axial ligand(s), ifpresent, is further away from the iron center than the oxygen p-oxo bridge. Although no XRD data appears to be available for ferrous porphyrins involving oxygen-bound axial ligands, the iron center-axial ligand distances for low-spin diaxially substituted ferrous porphyrin, for example, involving other (axial) neighboring atoms, e.g., N and C, can range from 2.10 8, for (CO)FeTPP(pyridi~~e)*~ to 2.463 A for (NO)FeTPP(4meth~lpiperidine).~~ Comparison of the k3x(k) EXAFS in Figure 1 reveals a small peak at ca. 5.5 for the reduced states (bottom panel) that is absent in the oxidized [Fe(TMPP)]20/BP states. This feature is characteristic of the in-plane location of iron for the reduced, ferrous forms of the [Fe(TMPP)]20/BP macrocycle. Similar features are also found in the k3x(k) EXAFS of a hydroxoiron(111) basket-handle porphyrin d e r i ~ a t i v e 'and ~ in the k3x(k) EXAFS of a number of other iron and cobalt porphyrins20,22,26-29 at 5.0-5.5 8,-' (the exact position in k-space is determined by the choise of EO). The curve-fitting results for the Fourier-filtered k3x(k)in situ Fe EXAFS of the oxidized and reduced forms of (FeTMPP)zO/ BP as a function of pH are given in Tables 1 and 2, respectively, which include best fit parameters for Fe-N,. The best fits to the experimental data are given in the Supplementary Material. Plots of d(Fe-N,) for the oxidized (circles) and reduced (squares) species are displayed in Figure 3. It will be assumed throughout the discussion that, at a given pH, the adsorbed macrocycle is present in a single (or largely predominant) form. This may not be the case for all pH values, in which case d(Fe-N,) represents an average over all the species present. Ferric Species. As indicated in Table 1 and Figure 3 (circles), the value of d(Fe-N,) for the oxidized macrocycles for the more acidic electrolytes (pH = 1.2 and 3.1) is 2.04 i 0.02 8,. This distance is the same as that found from the XRD analysis of
10
12
k (A-1)
I
I
I
I
I
I
1
0
2
4
6
8
10
I
12
k (A-1) Figure 1. Background-subtracted iron K-edge EXAFS, k3x(k),for 40% [FeTMPP]2O/BP in the oxidized (top panel) and reduced (bottom panel) states as a function of solution pH. A = 1.2, B = 3.1, C = 5.0, D = 7.0, E = 8.8, F = 10.8,G = 13. The vertical scale is offset for clarity.
and applied potentials. Furthermore, most of the FT data for the ferric macrocycles (left panel, Figure 2) display small peaks
0
1
2
3
r' (4
4
5
6
0
1
2
3
4
5
6
r' (A)
Figure 2. Fourier transforms of the iron K-edge EXAFS based on the data in Figure 1 for the oxidized (left panel) and reduced (right panel) species as a function of solution pH. A through G are the same as specified in Figure I. The vertical scale is offset for clarity.
Kim et al.
10362 J. Phys. Chem., Vol. 99, No. 25, 1995
TABLE 1: Single-Shell Curve-Fitting Results for the of Fourier-Filtered k3x(k) Fe K-Edge EXAFS (Ak = 10 kl) the Oxidized Iron P rphyrin Obtained with a Filtering Window Ar = 1.95 pH r,A C. N. Au2,A2(x103) AE0,eV F
la#
Agreement was found in the pH range 5.0-10.8 between the values of d(Fe-N,) ((2.07-2.08) f 0.01 A) and that of bulk [Fe(TPP)]20, Le., 2.087(3) A.I1 Further support for the presence of an adsorbed p-oxo type species was provided by the large integrated areas and peak heights of the 1s 3d electronic transitions in the XANESI9 (see supplementary material). At pH 13, d(Fe-Np) (2.05 f 0.02 A) is significantly smaller than the corresponding values observed in the intermediate pH range, although similar to those in strongly acidic media. This result is consistent with a movement of the iron center into the porphyrin plane. Insight into the possible nature of the species present in this strongly alkaline media can be gained from results obtained from proton NMR spectroscopy for certain watersoluble femc tetraarylporphyrins. In particular, the position of the metal atom with respect to the porphyrin plane can be inferred from the analysis of the NMR splitting patterns of the meta protons of the peripheral phenyl group. Using this approach, Ivanca et al?O have recently shown that, for Fe(TPPS)3and F~(TMPYP)~+, the macrocycles are present as axially coordinated diaquo monomers at very low pH which undergo partial deprotonation at higher pH values to yield a monomeric mixed aquo hydroxyl-coordinated species. As the solution becomes more alkaline, the latter converts into the dimeric p-oxo-type form of the macrocycle, and for very concentrated basic media, the dimer cleaves to form (at least in the case of F~(TMPYP)~+) a monomeric complex assigned to the axially coordinated dihydroxyl derivative. Further evidence for the existence of a high-spin femc dihydroxyl species has also been obtained in the case of F ~ ( T M P Y P ) ~ +for , ~which I the effective magnetic moment was found to increase from that of a typical antiferromagneticallycoupled p-oxo-type dimer (ca. 3.0 p ~to) a much higher value (4.7 p ~ as) the pH was further increased. Although the behavior observed for F~(TMPYP)~+ is unique among the very few iron macrocycles investigated to such degree of detail, it strongly suggests that the adsorbed species most likely formed at this high pH is axially coordinated dihydroxyl(FeTMPP). The area and peak heights of the 1s 3d electronic transition in the XANES for this strongly alkaline solution (see supplementary material) are somewhat larger than those observed in the intermediate pH range. Because the uncertainty in the magnitude of these parameters (due in part to the large step size used in these measurements) has not as yet been determined, it is premature to attribute to this single last point any specific significance. Additional spectroscopic and electrochemical information is required to determine whether a single or an admixture of species may be present in these very alkaline electrolytes (vide infra). Ferrous Species. With the exception of the results obtained for acid solutions, the values of d(Fe-N,) for the irreversibly adsorbed ferrous porphyrins extracted from the EXAFS analysis of the in situ data were significantly smaller than those of their femc counterparts (see Table 2 and Figure 3). The contraction of the Fe-N, distance from the oxidized to the reduced states is consistent with a decrease in the out-of-plane doming of iron on going from the FelI1to Fen porphyrin. Such steric influences on d(Fe-N,) are well known. In general, d(Fe-N,) is shorter in planar than in domed porphyrin configurations. In view of the fact that the changes in d(Fe-N,) induced by the nature of the axial ligand for ferrous porphyrins of the same spin are small and that the expected Fe-axial ligand distances are too similar from an EXAFS viewpoint to be reliably resolved, the variations in d(Fe-N,) extracted from the in situ data do not provide very useful information regarding axial ligation. +
1.2 3.1
5.0 7.0 8.8 10.8 13.0
2.04 2.04 2.08 2.08 2.07 2.08 2.05
5.5 5.6
5.4 5.2 3.2 4.2 4.0 3.9 4.7
-3.5 -4.1 -7.3 -6.2 -5.5 -6.7 -4.6
0.0 4.5 4.0 3.2 5.5
0.37 0.35 0.56 0.40 0.40 0.42 0.44
a The number of independent data points, N,dp = 2AkArtn = 12, exceeds the number of fitted parameters (four). The backscattering were obtained from FEFF amplitudes (F,(k)) and phase shifts (a,@)) Version 3.25. Ak, range of k values over which the curve-fitting was performed; Ar, window for inverse Fourier transform; C. N., coordination number; r, distance in A; u, Debye-Waller factor in A; A&, energy threshold difference in eV; F, goodness of fit.
TABLE 2: Single-Shell Curve-Fitting Results for the Fourier-Filtered k3x(&)Fe K-Edge EXAFS (Ak = 10 A-1) of the Reduced Iron Porphyrins Obtained with a Filtering Window Ar = 1.88 A"# pH
r,A
C.N.
Au2,A2(x103)
AEo,eV
F
1.2 3.1 5.0 7.0 8.8 10.8 13.0
2.03 2.03 2.03 2.04 2.02 2.02 2.01
3.4 3.7 3.4 3.4 3.5 3.4 3.9
3.8 3.8 2.8 3.2 2.9 3.4 2.6
1.1 -1.8 -2.0 -2.9 -1.1
0.27 0.36 0.27 0.35 0.39 0.30 0.47
-1.6 0.8
a The number of independent data points, Nidp = 2AkArh = 12, exceeds the number of fitted parameters (four). The backscattering were obtained from FEFF amplitudes (F,(k)) and phase shifts (a,@)) Version 3.25. Ak, range of k values over which the curve-fitting was performed; Ar, window for inverse Fourier transform; C. N., coordination number; r, distance in A; u, Debye-Waller factor in A; AEo, energy threshold difference in eV; F, goodness of fit.
2.09
z
n
2.03 2.04
2.02 2.01
s
j
t t 1
0
.
1
.
2
1
4
.
I
.
I
.
8
6
I
.
l
.
1 0 ' 1 2
I
14
PH Figure 3. Plots of d(Fe-N,) obtained from the analysis of the Fourierfiltered k3x(k)EXAFS for 40% [FeTMPP]20/BP for the ferric (circles) and ferrous (squares) forms of the absorbed macrocycle as a function of pH.
the six-coordinate high-spin femc porphyrin [F~(TPP)(OHZ)Z]+, d(Fe-N,) = 2.045(8) A, in which Fe3+ is centered in the heme plane.I5 These EXAFS results are consistent with the XANES data (supplementary material) that reveal integrated areas and peak heights of the 1s 3d electronic transitions that are small and typical of 6-fold coordination.l 9
-
-
In Situ EXAFS of Iron Porphyrin
J. Phys. Chem., Vol. 99, No. 25, 1995 10363
Insight into this issue can be gained based on the analysis of the pH dependence of the redox peaks obtained by conventional voltammetric techniques. In particular, a change in the average peak potential, (Epe*(ave)) of ca. 60 mV per pH unit is consistent with a redox process involving a net transfer of the same number of electrons and protons. Such dependence has been observed in the intermediate pH range for coatings of the closely related [FeTPP] adsorbed on graphite surface^.^ At more extreme pH values, however, Epe*(ave) for such films was found to become independent of pH. A sequence of redox processes consistent with these observations and the EXAFS data presented in this study, including the @-oxo)-dimedmonomer equilibrium, are shown schematically below: Strong Acid
+ 3H,O + 2H' -2[Fe1"TMPP(H20)2]+ [Fe"'TMPP(H,O),]+ + e- - [Fe"TMPP(H,O),]
[Fe"'TMPP],O
Intermediate pH [Fe"'TMPP(H,O),]+
+ OH- [Fe"'TMPP(H,O)(OH)]
[Fe"'TMPP],O
+ H20
+ 2e- + 3H,O + 2H+ 2 [Fe"TMPP(H,O),]
[Fe"TMPP],O
+ 2e- + 3 H 2 02[Fe"TMPP(OH)(H,O)]-
[Fe"'TMPP],O
+ 2e- + 20H- + H,O 2[Fe"TMPP( 0H),l2-
Strong Base [Fe"'TMPP],O
-
+ H,O + 20H-
[Fe"'TMPP(OH),]-
+ e-
2[Fe"'TMPP(OH),]-
[Fe"TMPP(OH),J2-
&-Oxo)-DimerMonomer Equilibrium [Fe"'TMPP],O
+ 3H,O
-
2[Fe"'TMPP(H20)(OH)]
It may be noted that this set of reactions is also in harmony with the unusual double step shape of the EF&(ave) vs pH curves reported for FeTPP coatings: for which the redox process appears to be independent of pH in the close to neutral pH region. Spin State Considerations. An indirect measure of the spin states of ferrous and ferric porphyrins can be provided by the magnitude of d(Fe-N,).I2 Based on the data compiled by Reed and Scheidt, the average values of d(Fe-N,) for the oxidized forms of [FeTMPP]20/BP in the whole pH range (2.05-2.08 A) are typical of those for high-spin species (2.05-2.09 A). This is consistent-as pertains here-with all precedent porphyrin chemistry with weak field ligands such as water and hydroxide. The assignment of the adsorbed, reduced iron porphyrins, however, is not as certain, as (with the exception of the data obtained at pH = 13, see below) the values of d(Fe-N,) extracted from the in situ data (2.01-2.04 A) are smaller than those reported for XRD-characterized high spin (2.05-2.07 A) and higher than those of intermediate (1.972 A) or low-spin
(1.98-2.01 A) ferrous porphyrins. Since the possibility of these species being in low spin can be safely excluded, it appears conceivable that the adsorbed system consists of an admixture of high- and intermediate-spin states. In fact, quasi in situ data obtained in strongly alkaline solutions for [Fe(TMPP)]20 adsorbed on high area carbon using Mossbauer effect spectroscopy at cryogenic temperatures3, indeed revealed the presence of two doublets with isomer shifts and quadrupole splittings consistent with high-spin and an intermediate-spin forms of the material. Conclusions The analysis of iron K-edge XAFS for hemoproteins is a subject of maturity in bioinorganic chemistry. The analysis of the in situ iron K-edge EXAFS reported here for 40% [Fe(TMPP)]zO adsorbed on BP as a function of solution pH and applied potential has benefit from this precedence. The results that clearly emerge are that d(Fe-N,) is longer for the oxidized than for the reduced states. The range of d(Fe-N,) for the oxidized macrocycles (2.05-2.08 A) is typical of highspin Fe"', whereas the corresponding range for the reduced macrocycle (2.01-2.04 A) is smaller and typical of either highandor intermediate-spin Fe". The p o x 0 dimer structure, [Fe(TMPP)]20, is maintained from pH 5- 11 for the oxidized material. At lower solution pH values (1.2-3.1), the smaller value of d(Fe-N,) suggests the presence of a diaquo complex, [FeTMPP(OH2)2]+, whereas at higher pH values, Le., 13, the dihydroxy species appears to predominate. Acknowledgment. Support for this work was provided by the Gas Research Institute. We thank A. S . Bommannavar (X18B) and K. Huang (X-6B) for assistance with the XAFS measurements at the NSLS, which is supported by the US. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. We thank J. Woicik (NIST) for the loan of the photodiode fluorescence detector. The assistance of S. Wang and Professor Robert Scott (University of Georgia) in the implementation of the interface between FEFF and XFPAKG is gratefully acknowledged. Supplementary Material Available: Figures of normalized iron XAFS for 40% [FeTMPP]20/BP in the oxidized and reduced states as a function of solution pH including XANES, plots of heights and integrated areas of pre-edge peaks of oxidized species, Fourier transform data displaying wide filtering windows, Fourier-filtered k3&) EXAFS Fe-N, backscattering, including best theoretical fits based on phases and amplitudes calculated from FEW 3.28 (11 pages). Ordering information is given on any current masthead page. References and Notes (1) (a) Traylor, T. G. Acc. Chem. Res. 1981, 14, 102-109. (b) Ji, L. N.; Liu, M.; Hsieh, A. K.; Andy-Hor, T. S . J. Mol. Catal. 1991, 70, 247257. (2) See, for example, the following. (a) Zagal, J.; Sen, R. K.; Yeager, E. B. J. Electroanal. Chem. 1977,83,207-213. (b) Zagal, J.; Bindra, P.; Yeager, E. B. J . Electrochem. SOC. 1980, 127, 1506-1517. (c) Collman, J. P.; Denisevich, P.; Konai, Y.; Manocco, M.; Koval, C.; Anson, F. C. J . Am. Chem. SOC.1980,102,6027-6036. (d) Durand, R. R., Jr.; Bencosme, C. S . ; Collman, J. P.; Anson, F. C. J . Am. Chem. SOC. 1983, 105, 27102718. (3) (a) Tanaka, A. A,; Fierro, C.; Yeager, E. B.; Scherson, D. J. Phys. Chem. 1987, 91, 3799. (b) Tanaka, A. A.; Fierro, C.; Scherson, D. A.; Yeager, E. B. Mater. Chem. Phys. 1989, 22, 431-456. (4) Shigehara, K.; Anson, F. C. J. Phys. Chem. 1982.86.2776-2783. ( 5 ) Lytle, F. W. In Applications of Synchrotron Radiation; Winick, H., Xian, D., Ye, M. H., Huang, T., Eds.; Gordon and Breach: New York, 1988; Vol. 4, pp 135-223. (6) Bouldin, C. E.; Forman, R. A,; Bell, M. I. Rev. Sci. Instrum. 1987, 52, 1891-1894.
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