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Equilibrium and Structural Study of Chloro Complexes of Iron(III) Ion in Acidic Aqueous Solution by Means of X-Ray Absorption SpectroscopyYasuhiro Inada3 and Shigenobu Funahashib

a Research Center for Materials Science, Nagoya University, Nagoya 464-8602, Japan b Laboratory of Analytical Chemistry, Graduate School of Science, Nagoya University,

Nagoya 464-8602, Japan

Reprint requests to Prof. S. Funahashi. E-mail: sfuna@chem4.chem.nagoya-u.ac.jp

Z. Naturforsch. 54 b, 1517-1523 (1999); received August 23, 1999 EXAFS, Complexation, Formation Constant, Iron(III) Ion, Chloride Ion

Using an extended X-ray absorption fine structure (EXAFS) technique, the formation constant of the pentaaquachloroiron(III) complex was determined to be 3.8 ± 0.4 mol-1 dm3 at 25.0 ± 0 .1 °C in a 1.00 mol dm-3 H C I O 4 aqueous solution. The structures of [Fe(OH2)6]3+ and [FeCl(OH2)5]2+ were determined on the basis of the same EXAFS data. The F e-0 bond length (2.05 A) in the latter is longer than that (2.01 A) in the former due to the electron donation and charge neutralization by the coordinating chloride ion with the Fe-Cl bond length of 2.26 A.

1. Introduction

The extended X-ray absorption fine structure (EXAFS) technique is generally accepted as a powerful tool for structural analysis around metal ions [1]. Wide applicability to any sample condi­tions, such as powder, amorphous, and solution, is an advantage of the EXAFS technique [2], Recently, in addition to hydration structures [3 - 10], solvation structures in nonaqueous solvents for many metal ions have been determined by means of EXAFS spectroscopy [11 -23]. We have investigated the solvation structures in nonaqueous solvents, such as1,1,3,3-tetramethylurea [11], ethylenediamine [12], propylamine [13], 1,3-propanediamine [13], ni- triles [14, 15], trimethylphosphate [16], and alco­hols [16].

The X-ray absorption brings about the excitation of an inner-shell electron of the absorbing atom to give an excited photoelectron. Because the transi­tion to an excited state and its decay are much faster than the ligand exchange reaction on the metal cen­ter, structural information of the metal complexes under equilibria is contained in an X-ray absorp­tion spectrum. In the case of a solution in which the equilibria between M, M Lj, ML2, • • • , MLX are achieved according to eq. (1), the normalized EXAFS oscillation, x (k ) , is expressed by the sum of the products of the EXAFS oscillation of each

component (XMLt(&X i = 0, 1, 2, • • • , x ) and its mole fraction (P ml, ) as eq. (2).

M ^ ML, ^ ML2 ^ ••• ^ ML, (1)

X

X(k) = ^2 ^ ML‘ ^ ML- (k ) (2)*=o

It is then suggested that the determination of the equilibrium constants of eq. (1) should be possi­ble on the basis of the EXAFS spectra of sample solutions with a variety of P ml, as well as by the potentiometric, spectrophotometric, and calorimet- ric titrations.

By potentiometric titration we can determine the formation constants of complexes in an aqueous so­lution with high precision, but this method is rarely used in nonaqueous solvents where electrodes do not work well. The spectrophotometric titration is useful to determine the formation constants in both aqueous and nonaqueous solvents only in the case of reactions with a change in the UV/Vis absorp­tion. The calorimetric titration is advantageous to evaluate the formation constants for complexation without such color change. However, these tech­niques are not useful to detect directly the change in the coordination environment around the metal ion. On the other hand, because EXAFS spectroscopy

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1518 Y. Inada and S. Funahashi • Complexation Equilibrium and Structure of Chloro Complex of Fe(III) Ion

Table I. Compositions of sample solutions for EXAFS measurements“.

Sample Cci Cci/CVe — Mole fraction11 —mol dm-3 [Fe(OH2)6]3+ [FeCl(OH2)5]2+

FC0 0 0 1.00 0.00FC1 0.0501 0.258 0.90 0.10FC2 0.100 0.515 0.81 0.19FC3 0.150 0.773 0.73 0.27FC4 0.200 1.03 0.66 0.34FC5 0.501 2.57 0.40 0.60

3 CVe = 0.194 mol dm-3 and [H+] = 1.00 mol dm-3( H C I O 4 ) ; b calculated using the formation constant A'i determined by EXAFS, see in text.

extracts information about short-range interactions and detects directly the coordinating atoms around the metal center, the EXAFS technique will enable the analysis of chemical equilibria with variations in the coordination of metal ions. Uncertainties in the estimation of the background and smooth base absorptions for the data analysis of EXAFS are the origin of the low sensitivity of EXAFS to the coor­dination number of the metal ion. On using a fixed observation cell in order to keep the background ab­sorption and light-path length unchanged, however, a relative change in X-ray absorbance will be ob­served in response to a change in the composition of the sample solution. Thus, on this basis we can determine the formation constant of the metal com­plex in solution. In this study, we have examined the additivity of the EXAFS oscillation (eq. (2)) and demonstrated the determination of the forma­tion constant for complexation of the iron(III) ion with the chloride ion in an acidic aqueous solution. This is the first report on the determination of the formation constant of a metal complex by means of the EXAFS technique.

2. Experimental

2.1. Sample solutions

The sample solutions for the EXAFS mea­surements were prepared by dissolving adequate amounts of Fe(H2 0 )n(C104)3 (Wako) and NaCl (Wako) in distilled water containing 1.00 mol dm -3 HCIO4. The concentration of the iron(III) ion was determined by EDTA titration. The concentrations and compositions of the sample solutions are sum­marized in Table I together with the mole fractions

of component species calculated using the forma­tion constant of the pentaaquachloroiron(III) com­plex determined by EXAFS described later. Accord­ing to the literature [24], the hydrolysis and poly­merization of the aqua iron(III) ion are negligible under the present experimental conditions.

2.2. EXAFS measurement and data analysis

X-ray absorption spectra around the Fe K edge were recorded using station BL-12C at the Photon Factory of the National Laboratory for High Energy Physics [25]. The white synchrotron radiation was monochromatized by a Si( 111) double crystal, and the parallelism of the crystals was detuned to 70% of the maximal intensity at 7.111 keV in order to re­move the higher-order reflection. The incident and transmitted X-ray intensities were simultaneously measured by the ionization chambers of a length of 17 and 31 cm, filled with N2 gas and with a 1:4 mix­ture of Ar and N2 gas, respectively. A flow sample cell (path length = 1 cm) with windows made of py- rolytically prepared boron nitride plates (150 pm , Shinetsu) was used for measurements of a series of EXAFS spectra. In such a series the flow sam­ple cell was never moved geometrically whenever a sample solution was substituted. The geometric fixation of the sample cell is important to analyze quantitatively the observed EXAFS oscillation for the determination of the formation constant. The temperature of the sample solutions was controlled to 25.0 ± 0 . 1 °C by the thermostated water circu­lated around the cell.

The background absorption (p^) and the base ab­sorption (po) were estimated by fitting the Victoreen equation and a cubic spline function to the observed absorbances in the pre-edge and post-edge regions, respectively. In order to obtain the normalized EX­AFS oscillation, \obs(k), the residuals obtained by the subtraction of p\, and po fr°m the observed ab­sorbances were divided by the smooth K shell ab­sorbances calculated by referring to the McMaster coefficients using the po value at 7.224 keV [26]. The values of threshold energy (Eo) were first de­termined at 7.126 keV corresponding to a peak po­sition of the derivative function of the observed X- ray absorption spectrum. The analysis of the Xobs(^) values was based upon the theoretical EXAFS ex­pression

Y. Inada and S. Funahashi • Complexation Equilibrium and Structure of Chloro Complex of Fe(III) Ion 1519

Xcal (k) = E { T ^ t } F^ S™{2kRJ + ^ (k ) }

exp( - 2^ 2fc2- ä ) (3)

where Fj(k), 6j(k), a 2, A(k), and So2 are the back scattering amplitude from each of N j scatterers j at distance R j from the absorbing atom, the photoelec­tron phase shift, the mean-square variation in R j , the mean free path of an ejected photoelectron, and the many-body amplitude reduction factor, respec­tively. The values of Fj(k), 8j(k), and A(k) for the F e -0 and Fe-Cl interactions were calculated using the FEFF7 program [27]. In order to refine the struc­ture parameters in eq. (3), the least-squares fitting in the R space was carried out using the FEFFIT program [28].

Since crystal structure data of the [Fe(OH2)6]3+ ion were not available, we used a virtual compound [Fe(OH2)6]3+ with the Fe-O bond distance of 2.01 A determined by the X-ray diffraction method [3] as a theoretical standard of FEFF7 to calculate the F j(k ), 6j(k), and A(k) values for the Fe-O interac­tion in the [Fe(OH2)6]3+ ion. Similarly, the model [FeCl(OH2)5]2+ with the Fe-O and Fe-Cl bond dis­tances of 2.02 and 2.30 A, respectively, was used for the theoretical standard of the Fe-O and Fe-Cl interactions in the [FeCl(OH2)s]2+ ion. These bond distances are the average values of those observed in the crystal structures of [FeCl(dmf)5]2+ (dmf = N,N- dimethylformamide) [29] and [FeCl(dmso)5]2+ (dmso = dimethyl sulfoxide) [30]. The \ 0bs(k) data were weighted by k 3 and windowed between 2.4 < k < 13.3 A-1 using the Hanning window with dk = 0.5 A-1 . The obtained structure parameters did not change even if the minimum k value was in­creased to 4.5 A“ 1. All the fittings were carried out on both the real and imaginary parts of the Fourier transformations in the R region from 1.0 to 2.1 A. The So2 value of 1.19, which was obtained by the fitting for the sample solution FC0 (see Table I) in which only [Fe(OH2)ö]3+ was contained, was kept constant for the analyses of the other sample so­lutions (FC1-FC5) containing various amounts of [Fe(OH2)6]3+ and [FeCl(OH2)5]2+ ions. The A E 0 value of the Fe-O interaction was set at —3.6 eV, which was determined for solution FC0. That of the Fe-Cl interaction was first determined as 4.0 eV

Fig. 1. The observed EXAFS oscillations weighted by k~\

for solution FC5, in which the largest amount of the [FeCl(OH2)5]2+ ion was contained among the present sample solutions, and was held constant for the other solutions (FC1-FC4). Because the quan­tities of [Fe(OH2)ö]3+ were known by the equilib­rium analysis, the N values of the Fe-O interaction in [Fe(OH2)6]3+ and [FeCl(OH2)5]2+ and the F e- Cl interaction in the latter were fixed in the course of the fitting procedures with the values calculated according to the compositions given in Table I. Fur­thermore, since the R and a 2 values of [Fe(OH2 )6]3+ could be determined by the EXAFS analysis of solu­tion FC0, those values were fixed for the analyses of the other solutions (FC1-FC5). In order to prevent the correlation in the least-squares fitting between the a 2 values of the Fe-O and the Fe-Cl interactions in [FeCl(OH2)5]2+, the a 2 value of the former was kept constant as 0.0078 A2, which was determined by the analysis of solution FC5, which contained the largest amount of the [FeCl(OH2)s]2+ ion.

3. Results and Discussion

3.1. Formation constant o f the pentaaquachloro- iron(III) complex

Fig. 1 shows the k 3\obs(k) curves with the isos- bestic points at k / A-1 = 2.62, 3.45, 4.00, 5.10, 6.40, 7.08, and 8.02. The existence of the isosbestic points means that only one equilibrium between the hexaaquairon(III) ion and the chloride ion (reaction(4)) exists in the present reaction system and that the additivity of the EXAFS oscillations is held.

[Fe(OH,)6]3+ + C P ^ [FeCl(OH,)5]2+ (4)

[FeCl(OH2)52+]J \ 1 — ---------------- ------------ ( j )

[Fe(OH2)63+][Cl_ ]

1520 Y. Inada and S. Funahashi • Complexation Equilibrium and Structure of Chloro Complex of Fe(III) Ion

C c\ 1 mol drrr3

Fig. 2. The k 3\ obs(k) values at k / A-1 = 3.00 (a), 4.25 (b), 5.75 (c), 6.50 (d), and 7.30 (e) as a function of Co.

Because relatively large changes in the k 3\obs(k) value were observed at k / A -1 = 3.00, 4.25, 5.75, 6.50, and 7.30, we simultaneously used the k 3Xob s(fc) values at the live k values to determine the formation constant A'i defined by eq. (5) using eqs. (6) - (8).

C fe = [Fe(OH2)63+] + [FeCl<OH2)52+j (6)

Cci = [Cr] + [FeCl(OH2)52+] (7)k \ o U k ) = (A-’x Fe*a')[Fe(O H2)63+] (8)

+ PxFeCi;*®[FeCl(OH2)52+] ) / c Fe.

The value of K \ was determined to be 3.8 ± 0.4 mol“ 1 dm3 at 25.0 ± 0 . 1 °C and [HC104] = 1.00 mol dm-3 . Fig. 2 shows the plots of the k 3\obs(k) values against C q. The calculated curves obtained using the determined parameters of A’3x Fe3+(fc), k 3XFeCi2+(k), and A'i excellently re­produce the k 3Xobs(k) values.

There were many investigations of the complexa­tion of the iron(III) ion with the chloride ion in acidic aqueous solution [31]. The values of the formation constant determined under conditions similar to our experiments are summarized in Table II [32 - 38]. Our value obtained by the EXAFS in the present

Table II. Formation constants of the monochloroiron(III) complex in an acidic aqueous solution at 25 °C.

K \[mol" 'dm 3]

Medium Method* Ref.

3.8 ± 0.4 1.00 mol dm “ 3 HCIO4 EXAFS ,*

3.0 1.00 mol dm “ 3 HCIO4+HCI kinetics3 [32]5.2 1.00 mol dm “ 3 HCIO4+HCI sp [33]4.3 1.00 mol dm “ 3 HCIO4+HCI extraction [34]4.03 1.0 mol dm ' 3 HC104+NaC104 kinetics3 [35]3.9 1 mol dm “ 3 HCIO4 sp [36]2.95 1.0 mol dm “ 3 HCIO4 sp [37]2.89 1.0 mol dm “ 3 HCIO4 em f [37]4.6 1.00 mol dm “ 3 HCIO4+HCI extraction [38]

* sp = spectrophotometry, em f = electromotive force measure­ment; ** this work; 3 Determined from the ratio o f the rate constants for the forward and reverse reactions.

Fig. 3. Fourier transform magnitudes for solutions FC0 (a), FC1 (b), FC2 (c), FC3 (d), FC4 (e), and FC5 (f).

study is consistent with those previously reported. This indicates the validity of the procedures of the formation constant determination by means of the EXAFS technique.

3.2. Structure o f pentaaquachloroiron(IIl) complex

The absolute values of the Fourier transformation carried out using the k 3 X o b s ( k ) values are given in Fig. 3 without the phase-shift correction. Only one peak is observed around 1.60 A which is attributable to two kinds of Fe-O (Fe-O(O) in [Fe(OH2)6]3+ and Fe-O (l) in [FeCl(OH2)5]2+) and one kind of Fe- C1 interactions. There is clearly a small increase in the Fourier transform magnitude at the longer side of the main peak with increasing the quantity of [FeCl(OH2)5]2+, because the distance of the Fe-Cl

Y. Inada and S. Funahashi • Complexation Equilibrium and Structure of Chloro Complex of Fe(III) Ion 1521

Table III. Structure parameters around the iron(III) iona.

Sample Interaction13 N R [A ] <r2 [A2]

FCO Fe-O(O) 6C 2.01 0.0073FC2 Fe-O(O) 4.83c 2.01c 0.0073c

Fe-O(l) 0.97° 2.05 0.0078cFe-Cl 0.19C 2.25 0.0032

FC3 Fe-O(O) 4.36c 2.01c 0.0073cFe-O(l) 1.37° 2.05 0.0078cFe-Cl 0.27c 2.26 0.0050

FC4 Fe-O(O) 3.95c 2 .01c 0.0073°Fe-O(l) 1.71° 2.04 0.0078cFe-Cl 0.34c 2.26 0.0065

FC5 Fe-O(O) 2.40c 2.0 lc 0.0073cFe-O(l) 3.00c 2.04 0.0078Fe-Cl 0.60c 2.28 0.0092

a The E q value for the Fe-O and Fe-Cl interactions are 7.122 and 7.130 keV, respectively, and So2 = 1.19;b 0(0) and 0(1) represent oxygen atoms in [Fe(OH2)6]3+ and [FeCl(OH2)s]2+, respectively;c fixed during the curve fit­ting procedure.

0 1 2 3R / Ä

Fig. 4. The observed (dots) and calculated (solid lines) values of the imaginary (a) and real (b) parts of the Fourier transformations for solutions FCO (A) and FC5 (B).

interaction is longer than that of Fe-O (l). In or­der to determine the structure parameters around the iron(III) center in a sample solution in which [Fe(OH2)ö]3+ and [FeCl(OH2)5]2+ ions coexist, a least-squares calculation according to a three-shell model with the Fe-O(O), Fe-O (l), and Fe-Cl inter­actions has been performed for solutions FC1-FC5. The results are given in Table III and the observed and calculated values of the real and imaginary parts of the Fourier transformations, respectively, for so­lutions FCO and FC5 are depicted in Fig. 4. In the case of solution FC1, the analysis using the three shell model gave rise to wrong R values, such as 2.07 and 1.69 A for the Fe-O (l) and Fe-Cl in­teractions, respectively, because of the small mole

Table IV. Averaged bond lengths in 6-coordinate octahe­dral monochloro and dichloro iron(III) complexes.

— R [A] —Complex3 Fe-O Fe-Cl Ref.

[FeCl(OH2)5]2+ 2.05 2.26 *FeCl(dmf)5-(FeCl3)20 2.013 2.303 [29]FeCl(dmso)5-(FeCl4)2 2.03 2.30 [30]FeCl(crn)(OH2)3 2.026 2.254 [39]FeCl(oda)(OH2)2 2.033 2.230 [40](FeCl2(dmso)4)2-FeCl4 2.019 2.350 [41]FeCh(tppo)4 FeCU 2.012 2.350 [42]FeCl2(xbp)2-FeCl4 2.011 2.345 [43]

a crn = chloranilate, oda = oxydiacetate, tppo = tri- phenylphosphine oxide, and xbp = /V,jV’- l ,2-xylene- a , a ’-diylbis(pyridin-2-one);b Fe-Cl bond length in the monochloroiron(III) or dichloroiron(III) complex; * this work.

fraction of [FeCl(OH2)5]2+. Thus, the results for so­lution FC 1 are not included in Table III.

For solutions FC2-FC5 with different concentra­tions of the chloride ion, the structure parameters of [FeCl(OH2)5]2+ were almost identical. The values of R were then concluded to be 2.05 A for the Fe- 0(1) interaction and 2.26 A for the Fe-Cl interac­tion, respectively, by averaging the values obtained for solutions FC2-FC5. The averaged Fe-O and F e- Cl bond lengths are summarized in Table IV for the monochloro and dichloro iron(III) complexes with 6 -coordinate octahedral structures in single crys­tals [29, 30, 39 - 43]. The Fe-Cl bond lengths of monochloro iron(III) complexes in the crystal are in the range of 2.23 - 2.30 A [29, 30, 39, 40], and thus almost in accordance with the corresponding value of [FeCl(OH2)5]2+ (2.26 A) in an acidic aqueous solution. The Fe-C l bond length in [FeCl(OH2)5]2+ is clearly shorter than that of ca. 2.35 A for the dichloro iron(III) complexes [41 - 43].

The Fe-O bond distance (2.05 A) in [FeCl(OH2)5]2+ is also in agreement with those (2.01 - 2.03 A) in 6 -coordinate octahedral monochloro iron(III) complexes (see Table IV) and is slightly longer than that in [Fe(OH2)ö]3+ (2.01 A). The a- and 7r-donations by a chloride ion enhance the electronic repulsion on the iron(III) center and reduce the formal charge on the iron(III) ion. These effects contribute to the elongation of the Fe-O bonds in [FeCl(OH2)5]2+. Interestingly, according to the crystal structures of [FeCl(dmf)5 ]2+ [29] and [FeCl(dmso)5 ]2+ [30], the five Fe-O distances in

1522 Y. Inada and S. Funahashi • Complexation Equilibrium and Structure of Chloro Complex of Fe(III) Ion

each crystal are not different within to ± 0.03 A, although the Fe-O bond at the trans position of the chloride ion is expected to be longer than the other four bonds at the cis positions. This seems to be due to the 7r-donation of the chloride ion, which can electronically influence the cis position, in ad­dition to the cr-donation which makes the greatest elongating contribution to a trans ligand. Five sim­ilarly elongated F e -0 bonds are then expected for the monochloro iron(III) complexes.

This paper has demonstrated that EXAFS spec­troscopy has a unique advantage for analysis of equilibria in solution. The formation constants of metal complexes and the structure parameters around the metal center can be simultaneously de­

termined using the same experimental data. Because organic materials have high transparency to X-rays, the chemical equilibria concerning metal ions in living samples will be also examined in situ by the present EXAFS technique.

Acknowledgement

This work was financially supported by the Grants-in-Aid for Scientific Research (Nos. 10440221, 10740305, 10874081, 11354009, and 11554030) from the Ministry of Education, Science, Sports and Culture of Japan. The EXAFS measure­ments were performed under the approval of the Photon Factory Program Advisory Committee (Pro­posal No. 96G175).

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