Journal of Nuclear and Radiochemical Sciences, Vol. 7, No. 2, pp. N5-N7, 2006

Notes
Mössbauer Study on the Electron Transfer Rate Depending on the IntermolecularInteraction in Iron(II, III) Mixed-Valence Complex
Natsuko Motokawa,a Shinya Hayami,*,a,b Masahiko Yamamoto,a and Yonezo Maedaa
aDepartment of Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka,812-8581, Japanb
Department of Chemistry, Faculty of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima,739-8526, Japan
Received: October 5, 2006; In Final Form: December 28, 2006
57

Fe Mössbauer spectroscopy has revealed that carboxylato bridged binuclear iron complex, [FeIIFeIII(bpmp)(ena)2](ClO4) 2, exhibits a mixed-valence state depending on temperature, where bpmp = 2,6-bis[bis-(2-pyridylmethyl)aminomethyl]-4-methylphenol and Hena = enanthic acid. Both of the polycrystalline sample and the film sample
prepared by dispersing the complex in poly(methyl methacrylate) (PMMA) have been investigated. The rate of theintramolecular electron transfer between iron(II) and iron(III) ions for the polycrystalline sample abruptlyincreases above ca. 200 K, and that for the film sample is slightly increase in the temperature range of 80 K to 295
K. The results show that the intermolecular interactions affect the intramolecular electron transfer of the binuclear
mixed-valence iron complex.

It is well known that mixed-valence iron clusters of theactive sites of metalloproteins play important roles in biologicalsystems.1 The model compounds of these compounds havebeen studied intensively and characterized by using various
methods. One of the model complexes is a family of carboxylato mixed-valence diiron(II,III) complexes. For example,mixed-valence diiron complexes with a heptadentate polypyridine ligand (bpmp), 2 the ligand having imidazol groups(bimp),3 and the ligand having phenol groups (bbpmp),4 havebeen reported previously (scheme 1).
In the investigation of mixed-valence complexes with suchligands, knowledge of the mechanism of the electron transfer
between the two iron centers is most important. Furthermore,in the field of nanomaterial science, understanding the electrontransfer phenomena at single molecule level is also important.
The mixed-valence diiron(II,III) bpmp complexes with various
CH3NOHNNNNNHbpmp
CH3MeNNNNOHNMe

carboxylic acid were reported by Maeda et al. previously, andcharacterized by using Mössbauer spectroscopy. 5 They
reported that the electron trapped/detrapped states are affectedby the kind of carboxylic acids, being bridging ligands. They
also suggested that the rate of the intramolecular electron transfer is influenced by the crystal packings. Therefore it isthought that the intermolecular interactions play an importantrole in the electron transfer of the mixed-valence state.
We focused on the intermolecular interactions in themixed-valence state, and prepared two types of samples ofthe mixed-valence diiron(II,III) compound [FeII FeIII (bpmp)(ena) 2 ](ClO 4) 2 , where Hbpmp represents 2,6-bis[bis-(2-pyridylmethyl)-aminomethyl]-4-methylphenol and Hena represents
enanthic acid (Figure 1). One is a polycrystalline sample asprepared and the other is a film sample.6,7 The film sample
was prepared by mixing an acetonitrile solution of poly(methylmethacrylate) (PMMA) and an acetonitrile solution of the complex, and obtained as a transparent film.8 In the film sample,
the complex molecules were dispersed completely at a single
molecular level and thus the intermolecular interaction would
be very weak.
Herein, we report that the rates of the electron transfer forthe polycrystalline and film samples are different, dependingon the strengths of the intermolecular interactions. Hereafter,
the polycrystalline sample of [FeIIFeIII (bpmp)(ena) 2](ClO4) 2 isrepresented as 1, and the film sample as 2.
CH3

CH3MeNMeNNNHbimpOHNNNOHONNNHOFeNN

H3bbpmp

Scheme 1. Structures of the ligands.

NFeO O O OCNNCC6H13 C6H13*Corresponding author. E-mail: hayami@hiroshima-u.ac.jp.
FAX: 81-82-424-7418.

Figure 1. Chemical structure of [Fe2 (bpmp)(ena) 2] 2+.

© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2006

N6Motokawa

J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

The temperature dependences of the Mössbauer spectra of 1and 2 were measured.9 The Mössbauer spectra of 1 and 2 areshown in Figures 2(a) and (b). The Mössbauer spectrum of 1
shows an asymmetric doublet at 295 K, Q.S. (quadrupole splitting) = 0.76 mm s -1 and I.S. (isomer shift) = 0.96 mm s -1,
assigned to delocalized valence states for iron(II) and iron(III)species. The delocalized valence states were observed above
220 K. This indicates that the rate of the electron transfer
between iron(II) and iron(III) atoms becomes comparable tothe Mössbauer hyperfine frequency. On the other hand, thespectrum at 80 K of 1 shows two doublets; one is of iron(II)high-spin species with Q.S. = 2.80 mm s-1 and I.S. = 1.17 mm s-1,
the other is of iron(III) high-spin species with Q.S. = 0.55 mm
s-1 and I.S. = 0.55 mm s-1. Two doublets were observed below
220 K, and the area ratio of iron(II) : iron(III) species is 1 : 1.
Temperature dependence of Mössbauer spectra of 2 was different from that of 1. In the spectra of 2, two doublets wereobserved at 295 K. The parameters are Q.S. = 1.61 mm s-1 andI.S. = 0.90 mm s-1 for the iron(II) high-spin species, and Q.S. =0.50 mm s-1 and I.S. = 0.41 mm s-1 for the iron(III) high-spin
species at 295 K. At 80 K, the spectrum of 2 also shows two
doublets; one is of iron(II) high-spin species with Q.S. = 2.52mm s-1 and I.S. = 1.19 mm s-1, the other is of iron(III) high-spin
species with Q.S. = 0.53 mm s-1 and I.S. = 0.53 mm s-1. The
values of the parameters are similar to those of 1 at 80 K. Thearea ratio of iron(II) : iron(III) species in 2 is 1 : 1, which indicates that no further oxidation of iron(II) ions occurred while(a) 1.00
295 K

Transmittance

0.95
1.00
220 K

0.85
1.00
80 K

0.80
-6-4-20246

preparing the film. For 2, two doublets of iron(II) and iron(III)species were observed in the temperature range of 80 – 295 K.
At 80 K, the line width of Mössbauer spectra of 1 is 0.41 mm s-1for iron(II) species and 0.47 mm s-1 for iron(III) species. On
the other hand, that of 2 is 0.63 mm s-1 for iron(II) and 0.58
mm s-1 for iron(III) species. The line widths of the Mössbauer
spectra of 2 are broader than those of 1. In the film sample, itis thought that the molecules exist in the similar structure to
the solution sample, and the distorted environment around central metals introduced in PMMA matrix may cause the broadline width.
Furthermore, the rate of electron transfer between iron(II)and iron(III) ions can be estimated by fitting the Mössbauer
spectra of 1 and 2 using the method of Wickmann.10 To estimate the relaxation time τ and the rate of electron transfer λ,the fitting analysis was carried out by the following equations,τ=τ1τ2 ,
τ1 + τ2(1)

λ=1,
τ(2)

where τ1 and τ2 are the electron relaxation time of each transition for iron(II) and iron(III) ions. The temperature dependence of λ for 1 and 2 are shown in Figure 3. The value of therate is almost constant (6.3 × 105 s-1) below ca. 140 K for 1. On
heating, the value exponentially increases from 8.8 × 105 s-1 at
200 K to 3.6 × 10 7 s-1 at 295 K. On the other hand, the rateslightly increases from 1.6 × 106 at 80 K to 3.0 × 106 s-1 at 295
K for 2. The rate of the intramolecular electron transfer for 1
is almost the same as that for 2 below 200 K. The rate for 1 ismuch faster than that for 2 at 295 K. The results for 2 showthat the rate is hardly dependent on the temperature in the single molecule. Since the intermolecular interaction is weakenough in the film sample, and the intermolecular interactions
would affect the intramolecular electron transfer in the binuclear mixed-valence iron complex.
In conclusion, we prepared the polycrystalline sample 1 andthe film sample 2 of the binuclear iron complex, [FeIIFeIII(bpmp)(ena) 2 ](ClO 4 ) 2 , and observed the different temperaturedependence of the intramolecular electron transfer rate between
1 and 2 by using Mössbauer spectroscopy. The polycrystalline
sample exhibited trapped-valence state below 140 K anddetrapped-valence state above 200 K in the Mössbauer spectra.
On the other hand, the film sample exhibited the trapped-Velocity / mm s -1(b)1.00
300.981.00

λ / × 106 s -1Transmittance

295 K

220 K

0.96
1.00

201080 K

0.94

0100-6-4-2024150200

2503006Velocity / mm s -1
Figure 2. Temperature dependence of the Mössbauer spectra of 1 (a)and 2 (b).

Temperature / K
Figure 3. λ vs. T plots for 1 and 2. Full plots (ò) for 1 and open plots(ô) for 2.

Mössbauer Study on the Electron Transfer Rate Depending

valence state in the temperature range of 80 – 295 K. The
results suggest that the intermolecular interaction greatly influences the electron transfer in the mixed-valence diiron(II,III)
complex. The investigation of the electron transfer in single
molecules will play important roles in the field of the nanomaterial science in the future.
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J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

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(7) The polycrystalline sample was prepared as reported previously.6 Anal. Calcd. For C51H51N6Fe2O13Cl2: C, 55.02; H,4.62; N, 7.38; Fe, 9.81. Found: C, 53.25; H, 4.54; N, 7.46;
Fe, 9.57.
(8) A solution of PMMA (0.10 g) in acetonitrile (20 mL) wasadded to a solution of the complex (0.022 g, 20 mmol) inacetonitrile (20 mL). The mixture was stirred for 15 minutes and spread into schale. Dark-green films were collected after leaving one night.
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