Transcript Mochida_Biferrocenium.ppt

Unconventional valence transition
in biferrocenium charge-transfer
complexes
T. Mochidaa,b
aDepartment
of Chemistry, Faculty of Science, Toho University,
Miyama Funabashi, Chiba 274-8510 Japan
bDepartment of Chemistry, Faculty of Science, Kobe University,
Rokkodai, Nada, Kobe, Hyogo 657-8501 Japan
(E-mail: [email protected])
Valence States of Biferrocene
Redox Stages of Biferrocene
0
1+
Ⅱ
Ⅱ
Fe
Fe
Fe
+e
Ⅱ
Ⅲ
-e
-e
Fe
2+
Fe
Neutral
+e
Ⅲ
Monocation
Fe
Ⅲ
Dication
Valence State of Monocation
+
Fe
Fe
Ⅱ
+
Ⅲ
+
Ⅱ
Fe
Fe
Ⅲ
Fe
Valence
localization
Fe
Ⅲ
Ⅱ
Biferrocene is an electron donor and exhibits two-step redox behavior. The monocation
is a mixed-valence species, containing Fe(II) and Fe(III). 57Fe Mössbauer spectroscopy
has been used as a powerful tool to investigate the valence states of biferrocenium salts.
By means of 57Fe Mössbauer spectroscopy, unusual valence transitions were found in
biferrocenium charge-transfer complexes with organic acceptors.
The ionic(I)–ionic(II) phase transition
-2
+2
-1
+1
+2
-2
+1
-1
Divalent solid
Monovalent solid
Mössbauer spectroscopy revealed an "ionic(I)-ionic(II)" charge-transfer
transition in [dineopentylbiferrocene][F1TCNQ]3. The material is a monovalent
solid (D+A3–) at room temperature and changes to a divalent solid (D2+A32–) at
low temperatures, in association with a first-order phase transition at around 120
K. In terms of the valence state, this is analogous to a conversion between
Na+Cl– and Mg2+O2–.
T. Mochida, K. Takazawa, M. Takahashi, M. Takeda, Y. Nishio, M. Sato, K. Kajita,
H. Mori, M. M. Matsushita, T. Sugawara, J. Phys. Soc. Jpn., 74, 2214 (2005).
Crystal Structure
Magnetic Susceptibility
a
o
b
Structure and magnetism. The acceptor forms a
trimer in the unit cell. At the transition temperature,
intermolecular electron transfer from the donor to the
acceptor unit occurs. The magnetic susceptibility
exhibits an enhancement at the phase transition. The
transition is governed by the balance between the
ionization energy and the Madelung energy.
NC
CN
Ｆ
Fe
Fe
NC
CN
3
57Fe
Mössbauer Spectra
Valence States
＋
HT phase
Monovalent
solid
Fe(II)+ Fe(III)
－
NC
NC
Fe Ⅲ
FeⅡ
CN
CN
NC
NC
e－
CN
CN
Electron
Transfer
LT phase
Divalent
solid
Fe(III)
CN
CN
NC
NC
2＋
FeⅢ
NC
NC
Ⅲ Fe
2－
NC
NC
NC
NC
CN
CN
CN
CN
CN
CN
High-temperature phase. The donor is in the monocation state and contains one
Fe(II) ion and one Fe(III) ion. The spectrum shows a quadrupole doublet, which is
typical for a biferrocenium monocation undergoing rapid valence exchange.
Low-temperature phase. The donor is in the dication state, containing two
Fe(III) ions. The spectrum shows a clear singlet, characteristic of the ferrocenium
cation.
Two-step valence transition
coupling with a high TC spin-Peierls transition
+
Fe
Fe
Ⅱ
+
Ⅲ
Ⅱ
Fe
Fe
Ⅲ
Two-step charge localization
Mössbauer spectroscopy revealed a two-step valence transition in
[diisopropylbiferrocene][Ni(mnt)2] (mnt = maleonitrilodithionate). The
transition steps occur at around room temperature and at low temperature.
The latter is coupled with a spin-Peierls transition at TC = 133.2 K.
T. Mochida, K. Takazawa, H. Matsui, M. Takahashi, M. Takeda, M. Sato, Y.
Nishio, K. Kajita, and H. Mori, Inorg. Chem., 44, 8628 (2005)
57Fe
Mössbauer Spectra
Valence State
+
Valence-detrapped state
Partial localization
at around 250K
Fe
Fe
+
Ⅲ
Ⅱ
Fe
Ⅱ
Fe
Partially localized state:
Coexistence of detrapped
and trapped sites (1:1)
Ⅲ
Valence
localization
+
Full localization
below 130 K
Fe
Fe
Ⅲ
Ⅱ
Valence-trapped state
57Fe
Mössbauer spectra. At 298 K, a doublet is seen, which is characteristic of
the valence-detrapped state of the cation. Between 230 K and 130 K, the spectra
indicate the coexistence of valence-detrapped and valence-trapped species. At
77 K, the spectrum shows two doublets, characteristic of the valence-trapped
state.
Crystal Structure at RT
+
Fe
NC
S
S
CN
S
CN
Ni
Fe
NC
S
Structure. The complex contains a mixed-valent biferrocenium monocation.
The anions, which carry ½ spins, are regularly stacked to form one-dimensional
chains. The presence of two crystallographically independent biferrocenium
cations, together with the one-dimensional Peierls instability of the anion spin,
is the origin of the two-step transition:
- Cation A undergoes valence localization at around room temperature, due to
the asymmetry of the crystal environment.
- Cation B undergoes valence localization below 120 K, coupling with a spinPeierls transition. This transition accompanies dimerization of the acceptor,
which induces symmetry-lowering of the crystal environment.