Journal of Nuclear and Radiochemical Sciences, Vol. 13, No.1, pp. N1-N3, 2013

Notes
Observation of Iron-valence Detrapping in Mixed-valence Trinuclear Heterometal
Pentafluorobenzoate Cluster Complex
Cr3+Fe3+Fe2+ O(C6F5COO) 6 (C5H5N) 3 ·0.5C5H5N·CH2Cl2 by 57Fe-Mössbauer Spectroscopy
Ryo Ogisoa, Yoichi Sakai*, a, Satoru Onakaa,b, Tadahiro Nakamotoc, Tsutomu Takayamaa, andMasashi TakahashidaDaido University, Takiharu-cho, Nagoya, Aichi 457-8530, Japan
Professor Emeritus, Nagoya Institute of Technology, Gokiso-cho, Nagoya, Aichi 466-8555, JapancToray Research Center, Sonoyama, Otsu, Shiga 520-8567, JapandToho University, Miyama, Funabashi, Chiba, 274-8510, Japanb

Received: December 25, 2012; In Final Form: April 9, 2013
A novel mixed-valence trinuclear heterometal pentafluorobenzoate cluster complex Cr3+ Fe3+ Fe2+ O(C6F5COO) 6(C5H5N)3 ·0.5C5H5N·CH2Cl2 was synthesized, and its 57Fe-Mössbauer spectra were measured with a temperature
range of 78 to 298 K. It has been revealed that just one apparent valence state of Fe2.5+ was observed at roomtemperature.

1. Introduction
A number of 57Fe-Mössbauer spectroscopic investigations of
mixed-valence trinuclear iron carboxylate cluster complexes
with a triangle array of iron cations (Fe3+, Fe3+, Fe2+) have been
intensively carried out by many researchers; the general chemical formula is Fe3O(R-COO) 6L3 · nS, where R-COO - is a carboxylate anion, L, a ligand, and S, a crystalline solvatedmolecule (n = 0, 1, 2, 3…). The cluster molecular structure isdepicted in Figure 1. In such cluster complexes, much intensive
attention has been paid to the valence-detrapping (valenceaveraging) through the electronic interactions among three ironcations. The first pioneering observation of valence-detrappingby 57 Fe-Mössbauer spectroscopy was reported in 1984 by
Hendrickson’s group,1 who revealed that one set of quadrupole
doublet ascribing to the detrapping valence of Fe2.7+ was measured in Fe3O(CH3COO) 6 (C5H5N) 3 ·C5H5N at room temperature, while two separate sets of doublet due to Fe3+ and Fe2+ ina valence-trapped state below 100 K.
After that, a lot of trinuclear iron carboxylate complexes
have been so far studied using Mössbauer techniques and there
have been many interesting articles, 2-6 most of which wereabout the valence-detrapping phenomena among three iron cations. As a rare case, Sato et al.2 reported that valence-detrapping between one Fe3+ and one Fe2+ was observed at higher
temperature for Fe3O(CH2ClCOO) 6 (H2O)3 ·3H2O. This partial
valence-detrapping, in which one of two Fe3+ ions participates,
was explained from the sites of two Fe3+ ions being not strictly
equivalent.
In the present research, we measured temperature-dependent
57Fe-Mössbauer spectra of a novel mixed-valence trinuclear
heterometal pentafluorobenzoate cluster complex, Cr3+Fe3+Fe2+O(C6F5COO) 6 (C5H5N)3 ·0.5C5H5N·CH2Cl2 (1) synthesized by amodified method of our previous preparation procedure for Fe3O(C6 F5 COO) 6 (C5 H 5 N) 3 ·CH 2 Cl 2 (2).4,5 The purpose of thiswork is to examine whether the valence detrapping occurs ornot, and what cations participate in the detrapping if it occurs.

*Corresponding author. E-mail: yocsakai@daido-it.ac.jp

Figure 1. General cluster molecular structure of mixed-valence trinuclear metal carboxylate complex, M3O(R-COO) 6L3.
In this figure, L is a ligand such as pyridine and water. The threelargest circles are M 3+ (gray-colored) or M 2+ (white) ions. Small
gray- and black-circles represent C and O atoms, respectively. R is a
substituted group in a carboxylate anion (R-COO -). The dotted lines
represent the coordination bonds.

2. Experimental
2.1. Material preparation. Chromium(III) nitrate nonahydrate (1.60 g, 4.0 mmol), iron(II) chloride tetrahydrate (0.20 g,1.0 mmol), and pyridine (3 mL) were dissolved in ethanol (15mL), with a molar ratio of Cr3+ to Fe2+ of 4.7 The mixture wasstirred for 30 min, to which sodium pentafluorobenzoate (2.34g, 10.0 mmol) was added. The resultant blackish-purple solid
product was filtered and extracted with dichloromethane. The
solution was stored in a refrigerator for a few weeks to yield
polycrystalline product.
The sample was characterized by elemental analysis, TGMS, IR spectroscopy and so on.
Anal. Calcd (%) for CrFe2 O(C6F5COO) 6 (C5H5N) 3 ·0.5C5H5N·CH2 Cl2 : C, 40.19; Cl, 3.92; F, 31.53; H, 1.09; N, 2.71. Found(%): C, 39.11; Cl, 3.72; F, 31.47; H, 1.19; N, 3.06.

© 2013 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 04/23/2013

OgisoJ. Nucl. Radiochem. Sci., Vol. 13, No. 1, 2013

The atomic ratio of Fe /Cr in the present complex waschecked with an energy-dispersive X-ray fluorescence spectrometer, by measuring Kα X-rays of Fe and Cr emitted fromthe sample loaded on a piece of filter paper. Numbers andkinds of crystalline solvate molecules were determined as theabove chemical formula by a TG-MS measurement.
2.2. Measurements. 57 Fe-Mössbauer spectra were measured in an ordinary transmission mode against a 57Co(Rh)source with a temperature range of 78 to 298 K using a
Mössbauer spectrometer, fabricated by Topologic Systems Inc.
Measurement temperature was regulated by a liquid-N2 cryostat of Oxford DN1726. The spectral curve fitting was carriedout by using a commercially available program, MossWinn
4.0Pre. The isomer shift (IS) and Doppler velocity scale werecalibrated with respect to metallic iron at 298 K.

Fe2.5+

298 K

Relative intensity / a.u.

N2240 K
180 K
120 K
78 K

3. Results and Discussion

Fe-2

03+

Fe22+

Doppler velocity /mm/s

4Figure 2. Temperature-dependent 57 Fe-Mössbauer spectra of
Cr3+Fe3+Fe2+ O(C6F5COO) 6 (C5H5N)3 ·0.5C5H5N·CH2Cl2.

TABLE 1: 57Fe-Mössbauer parameters at 298 and 78 K of
Cr3+Fe3+Fe2+ O(C6F5COO) 6 (C5H5N) 3 ·0.5C5H5N·CH2Cl2
T/K
29878

IS / mm/s
0.72 (1)0.57 (1)1.23 (1)

QS / mm/s LW / mm/s
0.43 (2)0.52 (3)1.30 (1)0.48 (3)2.38 (2)0.39 (1)A/%
10056 (2)
44 (2)

IS ; isomer shift, QS ; quadrupole splitting, LW ; line-width

approximation, we can use the IS values of Fe3+ and Fe2+ of theclosely-related complex Fe3+ Fe3+ Fe2+ O(C6 F5COO) 6 (C5H 5 N) 3(3), revealed to be in a fully valence-trapped state at room temperature in our recent work.4,5 Using the observed IS values of0.43 mm/s and 1.12 mm/s for Fe3+ and Fe2+, respectively, forthe complex (3) at room temperature, the IS values wereapproximately estimated to be 0.66 mm/s for +2.7 state of thecomplex (2) and 0.78 mm/s for +2.5 state of the complex (1)though the estimated values are somewhat large. This discussion
for the estimation is illustrated and summarized in Figure 3.
The discrepancy between the observed and estimated values is1.2
Isomer shift / mm/sFigure 2 shows the temperature-dependent Mössbauer spectra of Cr3+ Fe3+ Fe2+ O(C6F5COO) 6 (C5H5N)3 ·0.5C5H5N·CH 2Cl2(1); the fitting curves are illustrated only for the spectra at 78and 298 K. The spectral shapes at 120, 180, and 240 K are
greatly complicated because of extensive line-broadening due
to a relaxation effect. It is known that satisfactory fittings areimpossible for such spectra by an ordinary least-square calculation method.
As seen in the figure, the spectrum at 78 K was deconvoluted into two doublets, assigned to the high-spin ferric (Fe3+)
species and the high-spin ferrous (Fe2+) species from their
Mössbauer parameters summarized in Table 1. The relative
ratio of Mössbauer absorption intensity (i.e., area) of Fe3+ toFe2+ was evaluated to be 1.27 /1 (=56/44), deviating somewhat
from an expected ratio of unity in the chemical composition of
the sample material. Such deviation might be probably due toincompleteness of valence-trapping even at 78 K, or to difference of recoilless fraction between the Fe3+ and Fe2+ sites. With
increasing temperature, the two doublets were collapsed andthen merged into only one doublet, as shown in Figure 2.
The IS and quadrupole splitting (QS) values are 0.72 mm/s
and 0.43 mm/s, respectively, at 298 K (Table 1). Interestingly
the IS value (0.72 mm/s) of the present heterometal complex(1) at 298 K is obviously larger than that (0.64 mm/s) of thetri-iron complex Fe3+ Fe3+ Fe2+ O(C6F5COO) 6 (C5H5N)3 ·CH2Cl2(2) at 300 K, as reported in our previous articles.4,5 Since theaverage oxidation state of the complex (2) is +2.7 [= (3+3+2)/3]
assuming complete valence-detrapping among two Fe3+ cations
and one Fe2+ cation, the larger IS value for the complex (1) atroom temperature indicates that the average oxidation stateshould be lower than +2.7 and be presumed to be +2.5 , which
is probably ascribable to the fast electron hopping between only
two iron cations in the CrFeFeO-complex (1). In contrast such
fast hopping takes place among three iron cations in theFeFeFeO-complex (2). We tentatively presume that valencedetrapping among two iron and chromium ions should notoccur owing to a substantial ionizing-energy difference (M3+ toM2+, or M2+ to M3+) between iron and chromium.
Validity of our argument above is checked by comparing theobserved IS values in a valence-detrapped states of the complexes (1) and (2) with those estimated as a weighted-average
IS value at room temperature for Fe 3 + and Fe 2 + , that is,(IS Fe3+ +IS Fe2+)/2 or (2IS Fe3+ +IS Fe2+)/3. However, we cannot
obtain any experimental data on ISFe3+ and ISFe2+ at room temperature for the complexes (1) and (2), where valence-trappedstates never appear at room temperature. Since the IS value of57
Fe nucleus increases with lowering the temperature due to thesecond Doppler shift, the degree of which is affected by complicate factors, it is impossible to evaluate the room-temperature IS values of Fe3+ and Fe2+ using those in the trapped stateof the complex (1) or (2) at lower temperature. As a first2+Fe

1.0
0.8

2.7+

Fe

2.5+Fe0.6
3+0.4 Fecomplex (3)complex (2)complex (1)Figure 3. Observed and estimated values of the isomer shift at room
temperature for complexes (1), (2), and (3). Triangles stand for theobserved values, and circles for the values calculated as a weightedaverage from the observed IS values of complex (3).

Observation of Iron-valence Detrapping in Mixed-valence Trinuclear

in the same direction and degree, suggesting the same origin ofthe discrepancy.
In summary, it is concluded from the present investigationthat one d-electron transfers or hops between the Fe3+ and Fe2+ions much faster than the 57Fe-Mössbauer measurement timewindow of 100 ns, resulting in valence-detrapped state, i.e.,
Fe2.5+, at room temperature. Thus, chromium cation Cr3+ is left
alone from the valence-detrapping; two iron cations among trinuclear metals participate in the detrapping phenomena, andthe detrapping is complete, leading to Fe2.5+.
Our finding is the first report of such “partial” and “complete” iron valence-detrapping among trinuclear metal cations
to the best of our knowledge.
References and Note
(1) S. M. Oh, D. N. Hendrickson, K. L. Hassett, and R. E.
Davis, J. Amer. Chem. Soc., 106, 7984 (1984).

J. Nucl. Radiochem. Sci., Vol. 13, No. 1, 2013

N3(2) T. Sato, F. Ambe, K. Endo, M. Katada, H. Maeda, T.
Nakamoto, and H. Sano, J. Amer. Chem. Soc., 118, 3450
(1996).
(3) T. Nakamoto, M. Hanaya, M. Katada, K. Endo, S.
Kitagawa, and H. Sano, Inorg. Chem., 36, 4347 (1997).
(4) Y. Sakai, S. Onaka, M. Takahashi, R. Ogiso, T. Takayama,and T. Nakamoto, Hyperfine Int., 205, 1 (2012).
(5) S. Onaka, Y. Sakai, T. Ozeki, T. Nakamoto, Y. Kobayashi,
M. Takahashi, R. Ogiso, T. Takayama, M, Shiotsuka, M.
Sakamura, and M. Ito, to be submitted (2013).
(6) References cited in Ref. (2, 3, 4, 5).
(7) When the molar ratio (Cr3+/Fe2+) was less than 2, the complex Cr3+Fe3+Fe2+O(C6F5COO)6(C5H5N)3·0.5C5H5N·CH2Cl2,could not be synthesized, yielding Fe3+2Fe2+O(C6F5COO)6(C5H5N)3·CH2Cl2, checked by Mössbauer measurements.
With a reactant mixture with a molar ratio of Cr3+:Fe2+:Fe3+of 1:1:1, the CrFe2O-complex was not obtained, but theFe3O-complex was formed.