IV) oxidation in aqueous solution using X-ray absorption spectroscopy

Inorganica Chimica Acta 363 (2010) 802–806

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In situ spectroelectrochemical investigation of Pt(II/IV) oxidation in aqueous solution using X-ray absorption spectroscopy Koichiro Takao *, Shinobu Takao, Andreas C. Scheinost, Gert Bernhard, Christoph Hennig * Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf e.V., P.O. Box 51 01 19, 01314 Dresden, Germany

a r t i c l e

i n f o

Article history: Received 13 August 2009 Received in revised form 3 November 2009 Accepted 19 November 2009 Available online 27 November 2009 Keywords: XAFS Spectroelectrochemistry Platinum(II/IV) Redox Molecular structure

a b s t r a c t 2

2

The oxidation from PtII Cl4 to PtIV Cl6 in HCl aq. was studied in situ by combining electrochemistry with 2 XAFS spectroscopy. During the oxidation of PtII Cl4 , isosbestic points were observed in Pt LIII and LII XANES spectra as a function of time, indicating that the Pt(II/IV) redox equilibrium is the only reaction in the system. The Pt LIII and LII X-ray absorption edge energies of the initial PtIICl42 are 11562.9 and 13271.8 eV, respectively, while those of the electrolyzed species are 11564.6 and 13273.7 eV which are 2 identical with those of a PtIV Cl6 reference sample. The coordination of the electrolyzed species was char2 acterized by structural parameters derived from the EXAFS curve fit, and identified to PtIV Cl6 . Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Platinum(II/IV) redox chemistry plays an important role in various functionalities of Pt compounds. Oxidative addition of a substance to Pt and its reductive elimination are key steps in organic syntheses using Pt-based catalysts [1,2]. The use of Pt compounds as anti-cancer chemotherapy drugs (e.g., cis-diamminedichloroplatinum(II), cisplatin) is also attracting special interest [3,4]. In this application, the Pt ion interacts with a basic site of DNA, and finally triggers apoptosis of the cancer. However, side effects caused by damages to DNA are still problematic in the therapy with the Pt anti-cancers. In both examples, a molecular structure is one of the factors to determine reaction selectivity. Therefore, in situ structural chemistry of Pt(II/IV) species during the redox process should be explored. A combination of spectroscopy and electrochemistry, spectroelectrochemistry, may facilitate such an in situ study. X-ray absorption fine structure (XAFS) is appropriate to gain information on the complex coordination [5–8], because extended XAFS (EXAFS) can be transformed to one-dimensional radial distribution function providing coordination numbers (N) and interatomic distances (R) of atoms around Pt. This spectroscopic method also provides an additional advantage that the redox reaction can be monitored by a variation of X-ray absorption edge which is sensitive to the oxidation state. Furthermore, the X-ray absorption near edge struc-

* Corresponding authors. Tel.: +49 351 260 2076; fax: +49 351 260 3553. E-mail addresses: [email protected] (K. Takao), [email protected] (C. Hennig). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.11.032

ture (XANES) reflects the electronic structure of the species as well as UV–Vis absorption spectroscopy. As a first step, we selected the simplest Pt(II/IV) redox system, PtIICl42/PtIVCl62 in HCl aq. (Eq. (1)), and studied the XAFS spectroelectrochemical behavior of this redox couple.

Cl Cl

PtII

Cl

2-

+ 2 Cl-

Cl

Cl Cl

Cl PtIV Cl

2-

Cl

+ 2 e-

ð1Þ

Cl

The advantage of taking this system is the simplicity of the complex structures with Cl as a strong X-ray backscatterer [9–11]. Since both Pt oxidation states are enough stable in the ambient condition, availability of pure PtIICl42 and PtIVCl62 reference samples is also expedient for this study.

2. Experimental 2.1. Sample preparation An HCl (1.00 M) solution dissolving K2PtIICl4 (1.59  102 M, Aldrich) was prepared. This solution was subjected to both cyclic voltammetric and in situ XAFS spectroelectrochemical experi2 2 ments. The solutions of PtII Cl4 (3.33  102 M) and PtIV Cl6 2 II (3.04  10 M) in 1.00 M HCl aq. were also prepared from K2Pt Cl4 and H2PtIVCl6 (Aldrich), respectively. These solutions were sealed

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in polyethylene tubes, and used as references in XAFS to define the oxidation state of Pt.

2.5 2.0

2.2. Methods in HCl aq. was studied The redox behavior of Pt by cyclic voltammetry. The three-electrode system consisted of a Pt disc working electrode (surface area: 7.1 mm2), a Pt plate counter electrode, and an Ag/AgCl reference electrode. This experiment was operated by using Autolab PGSTAT302 potentiostat/galvanostat (Eco Chemie BV). The in situ XAFS spectroelectrochemical experiments for the oxidation of PtIICl42 were performed at the Rossendorf Beamline (BM20) at the European Synchrotron Radiation Facility (6 GeV, 50–90 mA) [12]. We used the spectroelectrochemical cell with double confinement [8] with modifications that the counter electrode was separated from the main compartment by a glass tube filled with 1 M HCl aq., and connected to the sample solution through a porous glass diaphragm. The potential on the working electrode was controlled by PGU 20 V–100 mA potentiostat, and kept at 1.00 V vs. Ag/AgCl. During the electrolysis, the sample solution was convected by N2 gas bubbling and magnetic stirrer. The gas outlet was introduced in 0.1 M EDTA aq. and a membrane filter. Each experimental run using this cell required 12 ml of the sample solution. An Si(1 1 1) double-crystal monochromator was employed in channel-cut mode to monochromatize white X-ray light from the synchrotron. XAFS spectra of Pt LIII and LII edges were recorded in a fluorescence mode by using 13-element Ge solid state fluorescence detector (Canberra) at ambient temperature (295 ± 1 K) and pressure. Prior to starting the XAFS measurement, the X-ray energy was calibrated by Pt foil (LIII: 11 564 eV, LII: 13 273 eV). The in situ XAFS spectroelectrochemical experiment run was repeated twice to record both Pt LIII and LII edges. Time dependence of the LIII edge was recorded in every 10 min, while that of the LII edge was measured in every 13 min. The Pt LIII and LII absorption edges in the recorded spectra were derived from first-inflection points. EXAFS data extraction and fits were performed for the Pt LIII edge XAFS spectra with IFEFFIT [13]. The threshold energy E0 was defined at 11 575 eV, regardless of Pt oxidation states. The curve fit was performed in R space using phases and amplitudes calculated by FEFF8.20 [14] and the crystal structures of K2PtIICl4 [15] and K2PtIVCl6 [16]. A single-scattering path (SS) from coordinating Cl and multiple-scattering paths (MS) from the linear Cl–Pt–Cl were included in the EXAFS curve fit. The amplitude reduction factor S02 was fixed to 0.9, and the shifts in the threshold energy DE0 were constrained to be the same value for all shells. 3. Results and discussion In accordance with thermodynamics [17,18], PtIICl42 and Pt Cl62 are predominant at [Pt] = 16 mM and [HCl] = 1 M in the respective oxidation states. Fig. 1 shows a cyclic voltammogram of PtIICl42 (1.59  102 M) in 1.00 M HCl aq. Oxidation and reduction peaks were observed at 0.95 and 0.05 V vs. Ag/AgCl, which correspond to the forward and backward reactions of Eq. (1), respectively [19,20]. The irreversible character of the voltammogram arises from large overpotential for association and dissociation of the additional Cl ions. On the basis of the electrochemical behavior of PtIICl42/PtIVCl62, the potential applied on the working electrode in the in situ XFAS spectroelectrochemical cell was decided to be 1.00 V vs. Ag/AgCl, which is enough to oxidize PtIICl42 to PtIVCl62. Note that this potential value is not enough positive to generate O2 gas on the working electrode. Therefore, the potentiostatic oxidation of PtIICl42 to IV

-4

Cl42/PtIVCl62

i /10 A

1.5 II

1.0 0.5 0.0 -0.5 -1.0 0.0

0.5 E / V vs. Ag/AgCl

1.0

Fig. 1. Cyclic voltammogram of PtIICl42 (1.59  102 M) in 1.00 M HCl aq. Scan rate: 50 mV s1, initial scan direction: anodic from 0.45 V vs. Ag/AgCl.

PtIVCl62 would not be disturbed under the present condition. It should be noteworthy that the backward reaction of Eq. (1), the reduction of PtIVCl62, was not studied here, because of its complexity. The redox potential of PtIICl42/PtIVCl62 in the present experimental condition (0.738 V vs. NHE at I = 1 M) is very close to those of Pt(0)/PtIICl42 and Pt(0)/PtIVCl62 (0.744 and 0.764 V vs. NHE, respectively) [21]. Furthermore, the potential applied on the working electrode has to be much more negative than that of PtIICl42/PtIVCl62 because of the significant overpotential as indicated by the electrochemical irreversibility of Eq. (1). Under this condition, significant deposition of Pt(0), platinum-black, cannot be avoided. Consequently, the sample will be inhomogeneous, and the total concentration of soluble Pt species in the test solution will decrease. This situation disturbs the following XAFS experiment under a constant condition. The oxidation of PtIICl42 at 1.00 V vs. Ag/AgCl was studied by the in situ XAFS spectroelectrochemical technique. Fig. 2 shows the Pt LIII and LII XANES spectra as a function of time (t). In both spectral series, isosbestic points were observed at 11 564, 11 572, 11 577, and 11 583 eV in LIII [part (a)] and 13 272, 13 279, 13 287, 13 291, and 13 305 eV in LII [part (b)], indicating that oxidation of PtIICl42 solely occurs in this system. The convergence of the spectral change means completion of the reaction. The XANES spectra at the maximum of time t (185 min for LIII, 160 min for LII) are identical to those of PtIVCl62 as the reference sample shown in Fig. 3. The Pt LIII and LII absorption edges after the oxidation of PtIICl42 and those of the reference samples (PtIICl42 and PtIVCl62) were listed in Table 1. Both edge energies after the oxidation are identical with those of PtIVCl62. All results in the in situ XAFS spectroelectrochemical experiment suggest that the final product of the oxidation of PtIICl42 is PtIVCl62. Using the absorbance at 11566.5 eV (LIII) and 13275.0 eV (LII), mole fractions of PtIICl42 and the oxidation product were calculated, and are plotted as functions of t in parts (a) and (b) of Fig. 4. In these plots, one can realize that the oxidation of PtIICl42 proceeds with elapse of time. The LIII and LII absorption edge energies at each t are also attached in the corresponding plots. The t dependence of both edge energies also demonstrates the progress of the oxidation, and is in agreement with the tendency of the mole fractions. A semi-logarithmic plot of the current (i) against t during the electrolysis in the LIII edge experiment is displayed in part (c) of Fig. 4. The log i value linearly decreases with elapse of t. Thus, the i–t curve can be regarded as a single exponential decay expressed by

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185 min

1.0

(a) Pt LIII

Table 1 Pt LIII and LII edge energies.

Fluorescence Intensity

Sample II

(initial) Pt Oxidation product IV 2 Pt Cl6 (reference)

Initial

0.8

LIII (eV)

LII (eV)

11562.9 11564.6 11564.5

13271.8 13273.7 13273.6

0.6 0.4

11564.5 11.58 E /keV

11.60

160 min

2.5

(b) Pt LII

Mole Fraction

11.56

0.8 11564.0

0.6 0.4

II

Initial

0.2

2.0

2-

11563.5

: Pt Cl4 : Ox. product : Pt LIII edge

Pt LIII Edge Energy /eV

11.54

(a)

1.0

0.2

Fluorescence Intensity

Cl42

11563.0

0.0 1.5

0

2

4

6 8 3 t /10 sec

10

12

1.0

1.0

(b) 13273.5

13.32 E /keV

0.8

13.36

Fig. 2. Pt LIII (a) and LII (b) XANES spectral changes during oxidation of PtIICl42 (1.59  102 M) in 1.00 M HCl aq. at 1.00 V vs. Ag/AgCl.

Mole Fraction

13.28

0.4

II

(a) Pt LIII

13272.5

2-

: Pt Cl4 : Ox. product Pt LII edge

0.2

8 Fluorescence Intensity

13273.0

0.6

Pt LII Edge Energy /eV

13.24

13272.0

0.0

6

0

2

4

4

6 8 3 t /10 sec

10

100

(c)

8 6

2

12

4

11.56

11.58

11.60 E /keV

Fluorescence Intensity

7

11.62

11.64

2

i /mA

0 11.54

(b) Pt LII

10

8 6 4

6 2

: Expt. : Fit

5

1

8

4

0

3

2

4

6

8

10

12

3

t /10 sec

2 13.24

13.26

13.28 E /keV

13.30

Fig. 3. Pt LIII (a) and LII (b) XANES spectra of reference PtIICl42 (3.33  102 M, black) and PtIVCl62 (3.04  102 M, red) samples in 1.00 M HCl aq.

Fig. 4. Mole fractions of PtIICl42 and the oxidation product together with X-ray absorption edge energies as functions of t (a: LIII, b: LII) from Fig. 2, and semilogarithmic plot of current decay (i–t) curve during the electrolysis at 1.00 V vs. Ag/ AgCl in the in situ Pt LIII edge XAFS spectroelectrochemical experiment (part c, black dots). The smooth red line in part c is the best fit of Eq. (2) in the least-squares regression.

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: Exp. : Fit

(a)

Sample II

FT(k χ(k))

PtIVCl62 (reference)

3

3

k χ(k)

(b)

(b)

6

8 k /Å

10 -1

12

2

Pt Cl4 (initial) oxidation product

(a)

4

Table 2 Structural parameters from Pt LIII EXAFS curve fits for initial PtIICl42, oxidation product and PtIVCl62 reference sample.

0

1

2

3 4 R + Δ /Å

5

6

Fig. 5. k3-Weighted Pt LIII EXAFS spectra (left) and their Fourier transforms (right) of (a) initial PtIICl42 solution ([PtIICl42] = 1.59  102 M, [HCl] = 1.00 M) and (b) electrolyzed solution.

Shell

DE0 (eV)

N

R (Å)

r2 (Å2)

Cl Cl(MS) Cl Cl(MS) Cl Cl(MS)

4.9

3.9

3.2

5.9

2.1

5.7

2.32 4.65 2.33 4.67 2.33 4.67

0.0032 0.0028 0.0015 0.0012 0.0030 0.0011

R + D = 3.9 Å in both FTs. The R values of these MS paths were evaluated as 4.65 Å for PtIICl42 and 4.67 Å for the oxidation product, which are twice of their Pt–Cl distances. Furthermore, the estimated N of PtIICl42 is 3.9, whereas that of the electrolysis product is 5.9. The structural parameters conclude that the oxidation product is PtIVCl62. It should be noted that the structural parameters of PtIVCl62 as an electrolyzed species are identical with those of the reference PtIVCl62 (Fig. S1) as shown in Table 2, also demonstrating the oxidation of PtIICl42 to PtIVCl62 (Eq. (1)). 4. Conclusion

i ¼ ibkg þ ðiini  ibkg Þ expðktÞ

ð2Þ

where ibkg and iini are the non-Faradic background and the initial currents, respectively. The first-order rate constant of the electrolysis is denoted by k. The best regression of Eq. (2) to the i–t curve resulted in ibkg = 0.18 mA, iini = 125.74 mA, and k = 4.17  104 s1. If the passed electric charge is exclusively consumed by the oxidation of PtIICl42, the rate of the PtIICl42 oxidation will be equal to that in the i–t curve. The t dependence of the mole fractions in Fig. 4a was analyzed by the single exponential function similar to Eq. (2), where i, ibkg, and iini were replaced with the mole fractions at t, infinite t and t = 0, respectively. The calculated k value from Fig. 4a is 4.21  104 s1, which is consistent with that derived from the i–t curve. Accordingly, it was clarified that the oxidation of PtIICl42 smoothly proceeds in the spectroelectrochemical cell. The same analysis was performed to the species distribution function from the LII edge experiment (Fig. 4b). As a result, the evaluated k value (3.97  104 s1) was found to be close to that in the LIII edge experiment. This exhibits reproducibility of the in situ XAFS spectroelectrochemical experiment for PtIICl42. Since the XAFS spectrum, especially EXAFS, is sensitive to the coordination structure around the excited atom, it should be feasible to characterize an electrolysis product from the structural point of view. At the one hand, the Pt–Cl distances in PtIICl42 and PtIVCl62 are almost same (2.32–2.33 Å) [15,16]. No remarkable differences in the EXAFS frequency and the peak position of its Fourier transform (FT) are therefore predicted. On the other hand, the additional two Cl of PtIVCl62 will result in the larger EXAFS amplitude and the greater FT peak intensity than PtIICl42. The k3-weighted Pt LIII EXAFS spectra and Fourier transforms (FTs) of initial PtIICl42 and the oxidation product are shown in Fig. 5. The EXAFS frequency of the oxidation product seems to be similar to that of PtIICl42, while the former amplitude is clearly greater than the latter. This observation illustrates the similarity in the Pt–Cl distance of these species and the presence of the additional Cl in the electrolysis product. The structural parameters (coordination number N, interatomic distance R, and Debye–Waller factor r2) obtained from the EXAFS curve fits are summarized in Table 2. In both FTs, an intense peak attributable to the Pt–Cl SS path was observed around R + D = 1.9 Å without scattering phase correction. The Pt–Cl distances in PtIICl42 and the electrolysis product were determined as 2.32 and 2.33 Å, respectively. The additional peak arising from the linear Cl–Pt–Cl MS paths was also detected around

In this study, we performed the XAFS spectroelectrochemical experiment to investigate the PtIICl42/PtIVCl62 redox couple in HCl aq. As a result, it was confirmed that the electrolytic oxidation from PtIICl42 to PtIVCl62 can be detected by using the Pt LIII and LII X-ray absorption edges and spectral changes in the XANES and EXAFS regions. Isosbestic points in the spectra indicated that the reaction solely occurs in the present system. The correlation between the i–t curve and the species distribution as a function of t exhibited the electrolysis efficiency. The structural data extracted from the EXAFS spectrum are helpful information for identifying the electrolyzed species and for concluding the reaction scheme. Even if the product of an electrochemical reaction of interest is unknown, its structural insight or hint to speculate or estimate what species occurs could be provided by one-dimensional radial distribution function derived from the EXAFS spectrum. These points are the additional advantages of this method over other spectroelectrochemical ones like UV–Vis absorption spectroscopy. Acknowledgment We thank Mr. J. Claussner for his support in technical development of the XAFS spectroelectrochemical cell. S. Takao is supported by the stipend from the Alexander von Humboldt foundation. This work was supported by the Deutsche Forschungsgemeinschaft under contract HE 2297/2-2. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.11.032. References [1] [2] [3] [4] [5] [6] [7] [8]

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[17] A.E. Martell, R.M. Smith, NIST Critically Selected Stability Constants of Metal Complexes: NIST Standard Reference Database 46 Version 8.0, Developed by R.J. Motekaitis; Distributed by NIST Standard Reference Data, 2004. [18] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, IUPAC, Marcel Dekker, New York, 1985. [19] A.T. Hubbard, F.C. Anson, Anal. Chem. 38 (1966) 1887. [20] A. Kowal, K. Doblhofer, S. Krause, G. Weinberg, J. Appl. Electrochem. 17 (1987) 1246. [21] The redox potential values described here were quoted from Ref. [18] and roughly recalculated by ionic strength correction at I = 1 M using the Debye– Hückel theory.