Synthesis, structure and valence-trapping vs. detrapping for new trinuclear iron pentafluoro benzoate complexes: Possible recognition of organic molecules by 57Fe Moessbauer spectroscopy

Article abstract of DOI:10.1039/c3dt53623h

New mixed-valence trinuclear iron pentafluorobenzoate complexes were synthesized. Their valence-detrapping and/or valence-trapping phenomena were studied by ⁵⁷Fe Moessbauer spectroscopy and X-ray crystallography. For [Fe₃O(C₆F₅CO ₂)₆(py)₃]·CH₂Cl₂ (1), a valence-trapped state was observed at low temperatures, while the valence-detrapped state was observed at room temperature. Removal of CH ₂Cl₂ from 1 gives the de-solvated [Fe₃O(C ₆F₅CO₂)₆(py)₃] (2) where the valence was trapped at room temperature. The CH₂Cl ₂-free 2 can reversibly absorb and desorb CH₃CN; the process was followed by ⁵⁷Fe Moessbauer spectroscopy by monitoring valence-trapping and valence-detrapping phenomena. Organic molecules such as benzene, toluene, ethylbenzene, cumene, and xylene are also trapped by 2 and affect the iron valence states. However, small molecules such as H ₂O and CO₂ do not affect the valence-trapped state of 2. Three xylene isomers trapped within the nano-void of 2 were distinguished by ⁵⁷Fe Moessbauer spectroscopy at room temperature. This journal is the Partner Organisations 2014.

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Synthesis, structure and valence-trapping vs.
detrapping for new trinuclear iron pentafluoro
benzoate complexes: possible recognition of
organic molecules by 57Fe Mössbauer
spectroscopy†
Cite this: Dalton Trans., 2014, 43,
6711
Satoru Onaka,*a,b Yoichi Sakai,*a Tomoji Ozeki,c Tadahiro Nakamoto,d
Yusuke Kobayashi,c Masashi Takahashi,e Ryo Ogiso,a Tsutomu Takayamaa and
Michito Shiotsukaf
New mixed-valence trinuclear iron pentafluorobenzoate complexes were synthesized. Their valence-
detrapping and/or valence-trapping phenomena were studied by 57Fe Mössbauer spectroscopy and
X-ray crystallography. For [Fe3O(C6F5CO2)6(py)3]·CH2Cl2 (1), a valence-trapped state was observed at low
temperatures, while the valence-detrapped state was observed at room temperature. Removal of CH2Cl2
from 1 gives the de-solvated [Fe3O(C6F5CO2)6(py)3] (2) where the valence was trapped at room tempera-
ture. The CH2Cl2-free 2 can reversibly absorb and desorb CH3CN; the process was followed by
57Fe Mössbauer spectroscopy by monitoring valence-trapping and valence-detrapping phenomena. Organic
molecules such as benzene, toluene, ethylbenzene, cumene, and xylene are also trapped by 2 and affect
the iron valence states. However, small molecules such as H2O and CO2 do not affect the valence-
trapped state of 2. Three xylene isomers trapped within the nano-void of 2 were distinguished by
57Fe Mössbauer spectroscopy at room temperature.
Received 25th December 2013,
Accepted 14th February 2014
DOI: 10.1039/c3dt53623h
www.rsc.org/dalton
electron transfer behaviours in the solid state.4–10 57Fe Möss-
bauer spectroscopy is a strong tool to study such valence-
Introduction
Since the structural elucidation of [M3O(CH3CO2)6(H2O)3]Cl· trapping vs. detrapping phenomena. Hendrickson et al. have
2H2O (M = Cr and Fe) in 1965,1 many carboxylate analogues demonstrated that the solvent molecule plays an important
of the form [M3O(RCO2)6L3]+/0 (M = transition metal, L = role in determining the intramolecular electron transfer with a
ligand) have been synthesized and their magnetic and/or elec- cooperative phase transition for a series of [Fe3O(CH3CO2)6L3]-
tronic behaviours have been studied extensively.2,3 Among (S)n (L represents pyridine and/or its derivatives and S rep-
them, mixed-valence trinuclear iron derivatives (M3 = FeIII2FeII) resents a solvent molecule).6–10 Sorai et al. have verified the
have attracted many chemists because of their interesting phase-transition in accord with a change from a valence-
trapped state to a valence detrapped state by precise heat
capacity measurements for [Fe3O(CH3CO2)6(py)3]·py;11,12 two
aDepartment of Chemistry, Daido University, Takiharu-cho, Minami-ku, Nagoya,
phase transition temperatures at 111 K and 191.5 K among
457-8530, Japan. E-mail: yocsakai@daido-it.ac.jp
bDepartment of Environmental Technology, Graduate School of Engineering,
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan.
E-mail: onakaau19@se.starcat.ne.jp
observed four phase transitions correspond to Mössbauer
spectral changes caused by the change of the intramolecular
electron transfer rate. They have also observed unexpected
large entropy jumps in these phase transition temperature
regions. Further explorations by the use of heat capacity
measurements and/or 2H-NMR have shown that the solvent
molecules should play an essential role in valence-detrapping
via increasing the intramolecular electron transfer rate,
accompanied by the cooperative phase transitions induced by
re-orientation movements of the solvent molecules for the
series of [FeIII2FeIIO(CH3CO2)6L3]·S complexes.12 In addition,
cDepartment of Chemistry and Materials Science, Tokyo Institute of Technology,
O-okayama, Megro-ku, Tokyo, 152-8551, Japan
dToray Research Center, Ohtsu 520-8567, Japan
eDepartment of Chemistry, Toho University, Miyama, Funahashi, Chiba, 274-8510,
Japan
fDepartment of Socio Science and Engineering, Graduate School of Engineering,
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
†Electronic supplementary information (ESI) available. CCDC 975326. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3dt53623h
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they have found that the valence-detrapping occurs at spectroscopic studies on valence-trapping vs. valence-detrap-
room temperature upon substitution of other solvent mole- ping. We will also suggest possible recognition of organic
cules for solvated pyridine in [FeIII2FeIIO(CH3CO2)6(py)3]·py.6,8 molecules by 57Fe Mössbauer spectroscopy.
Nakamoto and Sano et al. have successfully synthesized a
variety of new mixed-valence trinuclear iron carboxylate deriva-
tives [Fe3O(RCO2)6L3] (L represents pyridine and/or H2O)
including long chain fatty acid anions, halogenoacetates, cyano-
acetates, and/or pivalates instead of acetates with or without
solvent molecules. They have verified for the first time that
Results and discussion
Synthesis
A new FeIII2FeIIO(C6F5CO2)6L3·xS (x = 1; L = py; S = CH2Cl2) (1)
complex has been synthesized in ethanol at ambient tempera-
ture with moderate yield according to the following equation:
long alkyl chains can induce valence-detrapping in spite of the
−
lack of solvent molecules for [Fe3O(RCO2)6(py)3] (RCO2
=
myristic anion, palmitic anion, and stearic anion and L rep-
resents pyridine).13 Powder X-ray patterns for these clusters
exhibited layered structures and these long alkyl chains are
supposed to play a role similar to solvent molecules for intra-
molecular electron transfer (electron hopping) demonstrated
by Hendrickson et al.12 They have also obtained interesting
results that an electronic dipole moment is produced from the
electronically trapped Fe3O molecule and the intermolecular
dielectric interaction by this dipole moment plays a crucial
role in valence-trapping vs. valence-detrapping phenomena for
the cyanoacetate derivative, [Fe3O(CH2CNCO2)6(H2O)3].14 They
have found quite interesting results for halogenoacetate deriva-
tives in which the partial valence-detrapping between one FeIII
site (FeIIIA) of two FeIII ions and the FeII ion is observed with
increasing temperature for [Fe3O(CH2ClCO2)6(H2O)3]·3H2O
where intra- and inter-molecular hydrogen bonds are
formed.15 They have suggested that the hydrogen-bonding
interaction affects the local environment of iron atoms and
induces valence delocalization between the two iron ions.15
R. Wu et al. have reported soon after that similar partial
valence-detrapping between one of the FeIII sites and the FeII
site is observed for [Fe3O(Me3CCO2)6(py)3] at low tempera-
tures.16 C.-C. Wu et al. have shown a more complicated
valence-detrapping phenomenon for [Fe3O(CH3CO2)6(3-Cl-py)3]·
3-Cl-py, that is the FeII signal splits into two doublets with
rising temperature from 110 K.9 They have interpreted this
phenomenon in terms of different disordered solvent environ-
ments. In spite of elaborate studies by Hendrickson et al.
and Sano et al., there remain some issues to be explored
further for understanding the mechanism of valence-trapping
and valence-detrapping.
2FeCl3 þ FeCl2 þ 6C6F5CO2Na þ LðexcessÞ
! FeIII2FeIIOðC6F5CO2Þ6L3 ꢀ xS
ð1Þ
This is a typical carboxylate-metal cluster with bridging
benzoate ligands and three pyridine ligands.18,19 TG-MS
measurement of 1 exhibits a step of one CH2Cl2 molecule loss
upon heating in the range from 100 to 160 °C and the result-
ing “solvent-free cluster 2” is stable up to 200 °C (Fig. S1,
ESI†); the species lost during this step was confirmed to be
CH2Cl2 by MS. The powdered sample of 1 no longer absorbed
CH2Cl2. The preliminary paper has been reported elsewhere.19
We are tempted to examine whether 2 may behave as a porous
material like pyrazin-2-carboxylate Zn derivatives to trap
medium-sized molecules such as methanol and/or ethanol.20
Quite interesting chemical behaviours of 2 to organic mole-
cules are described in the Mössbauer section.
X-ray structure analysis
The molecular structure of 1 is shown in Fig. 1. Complex 1
crystallizes in a hexagonal space group P63/m. The central
oxygen atom O1 is located at a crystallographic C3h position in
1. Therefore, three Fe atoms are equivalent, which is consistent
with the Mössbauer data that two FeIII and one FeII atoms are
in
a valence-detrapped state at room temperature. The
We have supposed that the local valence-detrapping
between one FeIII site of two FeIII ions and one FeII site may
occur for molecules with lower symmetry than isosceles
triangle without solvent molecules and/or without hydrogen
bonds if such a mixed valence [Fe3O(RCO2)6L3] complex is syn-
thesized. In addition, mixed valence iron benzoate derivatives
have not yet been reported so far to the best of our knowledge,
although [FeIII3O(PhCO2)6(py)3]ClO4·py has been synthesized
in 2001.17 One of us (S.O.) has also reported the synthesis of
mixed valence manganese analogues [Mn3O(C6F5CO2)6(py)3]·
x(CH2Cl2) (x = 0 or 1) in 2003.18 In this paper, we will
report the successful synthesis of new pentafluorobenzoate
derivatives [FeIII2FeIIO(C6F5CO2)L3]·x(CH2Cl2) (x = 0 or 1 and L
represents py), X-ray structure analysis, and 57Fe Mössbauer Fig. 1 Molecular structure of 1.
6712 | Dalton Trans., 2014, 43, 6711–6719
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Fig. 2 Packing diagram of 1 viewed along the c-axis.
observed Fe–O1 distance, 1.9088(4) Å, is just between typical
FeIII–O distances (1.85 to 1.89 Å) and typical FeII–O distances
(1.93 to 2.01 Å).3,6–16 Slight differences are observed for Fe–car-
boxylate O distances (2.0569(14) and 2.0674(14) Å). Three pyri-
dine rings are perpendicular to the mirror plane which
contains three Fe atoms and the central O atom.
Fig. 2 shows a packing diagram of 1 viewed along the c-axis.
Guest CH2Cl2 molecules are located in nano-voids which are
formed by C6F5 rings and pyridine rings from neighboring Fe3 Fig. 3 57Fe Mössbauer spectra of [Fe3O(C6F5CO2)6(py)3]·CH2Cl2 (1).
clusters. The distances among H atoms in CH2Cl2 and F atoms
in C6F5 rings are longer than those of typical hydrogen bonds.
Debye model, temperature dependence of A can be expressed
The guest CH2Cl2 molecules are very likely to be kept in the
in the form
void by van der Waals force. The specimen used for the single
crystal X-ray diffraction seems to have lost some portion of its
dðln AÞ=dT ¼ ꢁ3Eγ2=Mc2kθ2
ð2Þ
solvent of crystallization and is best refined as a 0.735CH2Cl2
solvated complex. The crystal structure of 1 is quite similar to
the manganese analogue of [Mn3O(C6F5CO2)6(py)3], which
totally lacks solvent CH2Cl2 molecules and belongs to the hexa-
gonal space group P63/m, suggesting that this Mn3O complex
is in the valence detrapped state similar to 1.18,21
where Eγ is the Mössbauer transition energy, M the effective
mass imparted by the recoil energy by emission of the Möss-
bauer γ-ray, θ the Debye temperature, c the velocity of light,
and k the Boltzmann constant. On the right-hand side of
eqn (2), the unknown physical quantities characteristic of this
chemical matter are M and θ, which cannot be evaluated inde-
57Fe Mössbauer spectra
pendently in the present analysis, just giving the product Mθ2.
The temperature-dependent Mössbauer spectra of [Fe3O- The quantity of Mθ2, obtained as −3Eγ /c2k (the slope of the
(C6F5CO2)6(py)3]·CH2Cl2 (1) are shown in Fig. 3. Mössbauer plot), is called “intermolecular force constant” by Matsubara
2
parameters are summarized in Table 1.
et al.,22 both the M and θ quantities being correlated positively
Two sets of quadrupole-split doublets are observed at very to the rigidity of solids. The slope of the plots is inversely pro-
low temperatures with the area ratio of 1 : 2, which are portional to Mθ2 and is discontinuous at around 80 K (for
assigned to high-spin FeII and high-spin FeIII sites, respect- below and above 80 K, the Mθ2 values were estimated to be
ively; valence is trapped in the low temperature regions. The 2.51(14) × 10−21 and 1.92(3) × 10−21 kg K2, respectively), being
two quadrupole-split doublets become indistinguishable and consistent with the observation (Fig. 3) that the valence-
then coalesce into one quadrupole-split doublet at room temp- trapped state has begun to change to the valence-detrapped
erature, showing an intermediate isomer shift value of FeII and state around this temperature for 1. Assuming that the
FeIII ions, that is, Fe2.7+. It is difficult to determine the exact effective mass M is that of one molecule of the complex, the
transition temperature from the valence-trapped state to Debye temperatures θ were calculated to be 30 K (below 80 K)
valence-detrapped state on the basis of temperature-dependent and 26 K (above 80 K) with or without a solvated molecule.23
Mössbauer spectra. Fig. 4 shows plots of Mössbauer absorp- Upon the loss of solvent CH2Cl2 from 1, the Mössbauer spectra
tion total area intensities, A, of these two pairs of doublets vs. change drastically for 2 as shown in Fig. 5; the valence is
temperatures; in the high temperature approximation of the trapped even at room temperature. Hendrickson et al. have
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Table 1 57Fe Mössbauer parameters for 1 and 2
[Fe3O(C6F5CO2)6(py)3]·CH2Cl2 (1)
δ (mm s−1
)
ΔEQ (mm s−1
)
Γ (mm s−1
)
Area (%)
T/K
FeIII
Fe(av)
FeII
FeIII
Fe(av)
FeII
FeIII
Fe(av)
FeII
FeIII
Fe(av)
FeII
300
220
120
60
0.640(5)
0.685(5)
0.724(5)
0.401(8)
0.322(7)
0.331(10)
0.248(12)
0.329(12)
0.502(16)
100
100
100
0.549(2)
0.544(1)
1.247(5)
1.234(2)
1.139(4)
1.210(1)
2.140(10)
2.186(3)
0.308(6)
0.257(2)
0.382(17)
0.283(4)
67(1)
62(1)
33(1)
38(1)
5
[Fe3O(C6F5CO2)6(py)3] (2)
δ (mm s−1
)
ΔEQ (mm s−1
)
Γ (mm s−1
)
Area (%)
T/K
FeIII
FeII
FeIII
FeII
FeIII
FeII
FeIII
FeII
300
270
230
190
160
120
78
0.434(3)
0.452(3)
0.473(2)
0.492(3)
0.504(2)
0.521(3)
0.529(2)
1.115(11)
1.172(10)
1.179(7)
1.236(7)
1.235(4)
1.263(5)
1.273(5)
1.250(5)
1.272(5)
1.286(5)
1.297(6)
1.297(4)
1.278(5)
1.292(4)
1.243(22)
1.362(19)
1.401(15)
1.514(13)
1.562(9)
1.641(10)
1.816(9)
0.291(9)
0.322(9)
0.307(8)
0.342(9)
0.316(6)
0.353(8)
0.318(5)
0.428(28)
0.420(25)
0.374(21)
0.351(20)
0.345(13)
0.330(15)
0.374(13)
65(2)
66(2)
68(2)
68(2)
67(1)
67(1)
65(1)
35(2)
34(2)
32(2)
32(2)
33(1)
33(1)
35(1)
Isomer shifts (δ mm s−1) refer to α-iron; ΔEQ = quadrupole splitting (mm s−1), Γ = full width at half maximum of the Lorentzian lines (mm s−1).
Statistical standard deviations are given in parentheses, estimated from fitting procedure using the MossWinn 4.0 Pre software.
Fig. 4 A plot of Mössbauer absorption total area intensities, A vs. temp-
eratures for 1.
reported that the solvent molecule plays an important role
in valence-detrapping and valence-trapping phenomena for a
series of [Fe3O(CH3CCO2)6L3]·xS (x = 0 or 1).6–10
Fig. 5 57Fe Mössbauer spectra of [Fe3O(C6F5CO2)6(py)3] (2).
57Fe Mössbauer spectroscopic study on absorption of organic
molecule vapour by 2
In the previous paper, we have reported that the solvent manganese-analogue [Mn3O(C6F5CO2)6(py)3] are isomor-
CH2Cl2 is reversibly lost and absorbed for the manganese phous.18,21 A channel-like structure is found in 1 as shown in
analogue of [Mn3O(C6F5CO2)6(py)3]·CH2Cl2.18,21 Single crystal Fig. 2. CH2Cl2-free [Mn3O(C6F5CO2)6(py)3] has a similar channel
X-ray analyses have shown that 1 and CH2Cl2 desolvated structure. Therefore, we are tempted to explore CH2Cl2
6714 | Dalton Trans., 2014, 43, 6711–6719
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Fig. 6 A plot of weight increase of 2 by absorbing CH2Cl2 at room
temperature.
Fig. 7 57Fe Mössbauer spectra of [FeIII2FeIIO(C6F5CO2)6(py)3] (2)
exposed to organic vapour at room temperature: (a) C6H6 vapour for
15 h; (b) C6H5CH3 vapour for 19 h; (c) C6H5C2H5 vapour for 39 h;
(d) C6H5CH(CH3)2 vapour for 20 h.
de-solvation and solvation phenomena for 1 first; one mole-
cule of CH2Cl2 is easily lost from 1 as described in the Syn-
thesis section. Vapour-trap experiments for 2 were done for
CH2Cl2 in a double vial apparatus as described below. Approxi-
mately 100 mg sample of 2 was placed in a small-sized vial
and then this vial was placed in a large-sized vial. A little
amount of CH2Cl2 was poured into the large vial and the large
vial was capped. The weight increase of 2 after exposing to
CH2Cl2 vapour for several hours corresponds to one molecule
of CH2Cl2, indicating restoration of 1 (Fig. 6). The 2·CH2Cl2
complex shows valence-trapped Mössbauer spectra at low
temperatures, while a valence-detrapped spectrum is observed
at room temperature; the spectra for the restored sample are
indistinguishable from that of 1 over the measured tempera-
ture region (78–294 K). These Mössbauer spectral changes also
strongly support that the CH2Cl2 adduct of 2 is 1. We imagined
that organic molecules with similar size to CH2Cl2 would be
trapped in the void of 2. In order to explore the possibility of 2
as a nano-porous material, the similar procedure as described
above was employed for several organic vapours. A powder
sample of 2 (a few-ten mg to 100 mg) was used and a small
amount of organic liquid was poured into the large vial for
each exposure experiment. The total pressure inside the larger
vial is the sum of the atmospheric pressure of the air and the
saturated vapour pressure of organic molecules. The exposed
periods ranged from 12 minutes to several-tens of hours.
Acetonitrile was used first to examine our idea. As shown in
Fig. S2c (ESI),† the valence-trapped state of 2 is changed to
the valence-detrapped state at room temperature after exposure
to CH3CN. The weight increase plot against exposure time
is quite similar to 2·CH2Cl2. The product with a tentative
formula 2·CH3CN was heated under a reduced pressure to
ascertain that CH3CN could be removed from the complex.
The spectrum of 2 is regained after this treatment as
illustrated in Fig. S2d (ESI);† the spectral shape resembles that
of 2 (Fig. S2b, ESI†), suggesting that 2·CH3CN almost returns
to 2. This finding suggests that other organic molecules would
be incorporated in an intermolecular nano-void of 2. Möss-
bauer spectral measurements of 2 after exposure to the vapour
of several organic molecules were done at room temperature;
similar spectral changes to 2·CH3CN were observed for
benzene, toluene, ethylbenzene, and cumene, as illustrated in
Fig. 7. Carbon tetrachloride, chloroform, THF, and ethanol
exhibited similar spectral changes (spectra for these molecules
were not shown). Such a spectral change, however, was not
observed for H2O and CO2; examined samples are exposed sub-
sequently to CH3CN vapour, resulting in a detrapped-type
spectrum. It is concluded from this observation that H2O and
CO2 cannot be kept in the intermolecular nano-voids of 2 or
the interaction of these small molecules with host nano-
channels of 2 are too weak to be detected by Mössbauer
spectroscopy at room temperature. The organic molecules
mentioned above may be easily trapped in hydrophobic
environments and/or channels which are yielded by C6F5 rings
and pyridine rings in 2. Our finding that even C6H5CH3 and
C6H5CH(CH3)2 are trapped in the nano-voids leads to further
exploration into recognition of xylene isomers, the size of
which are close to those of toluene and cumene. As shown in
Fig. 8, three xylene isomers afford clearly different Mössbauer
spectra from one another at room temperature. Mössbauer
spectra at 78 K for these three adducts look like that of 1 in
the valence-trapped state (Fig. 9), although completely valence-
trapped states are expected below 78 K. The supposed complex
2·o-xylene shows a quite similar Mössbauer spectrum to that
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of 1 at room temperature, indicating the valence-detrapped
state. The Mössbauer spectrum of the supposed complex
2·m-xylene at room temperature resembles that of 1 at 100 K and
the Mössbauer spectrum of the supposed complex 2·p-xylene
at room temperature resembles that of 1 at 85 K. These two are
transient spectra from valence-trapped to valence-detrapped
states. Thus transition temperatures from the valence-trapped
state to the valence-detrapped state for xylene adducts of 2 are
estimated in the order o-xylene < m-xylene < p-xylene. This
order strongly suggests that the shape and/or the size of nano-
voids are influenced by guest xylene isomers, and xylene
isomers play an essential role in determining the electron
hopping rate in xylene adducts of 2; xylene molecules should
play a similar role in determining electron hopping among the
Fe3 triangles in the case of [Fe3O(CH3CO2)6L3]·py.6 This is the
first report that the xylene isomers are distinguished by Möss-
bauer spectroscopy to the best of our knowledge. For further
exploration of how xylene isomers are distinguished by
57Fe Mössbauer spectroscopy, weight increases of 2 upon exposure
to xylene vapour were examined. The observed weight increase
for each xylene isomer is almost consistent with that of 1 : 1
adduct formation as in the case of CH2Cl2. Fig. 10 shows
powder X-ray diffraction (XRD) patterns of 1, 2, and organic
molecule adducts of 2 mentioned above. The observed XRD
pattern of 1 is in agreement with that calculated by the
Mercury 2.3 program based on the single crystal diffraction
data, while that of 2 is different from that of 1, indicating that
2 belongs to a different space group from P63/m. XRD patterns
of 1 : 1 adducts of 2 with aromatic molecules such as benzene,
toluene, ethylbenzene, cumene, and xylene isomers are quite
similar to that of 1 except the p-xylene adduct, indicating that
1 and these adducts are isomorphous, although lattice con-
stants are slightly changed depending on the size of organic
molecules incorporated in nano-voids of 2; detailed analysis of
lattice changes by guest molecules will be discussed in a forth-
coming paper. Observed similarity in XRD patterns of adducts
of 2 leads to a conclusion that two organic molecules occupy
the same sites of CH2Cl2 in 1 in the unit cell. However, the
observed XRD pattern of the p-xylene adduct of 2 is neither
similar to that of 1 nor that of 2. The shape of the nano-
channel and/or the nano-structure of 1 and/or 2 is expected to
be more flexible than those of designed nano-architectures
described in the literature,20,24–27 because the nano-architec-
ture is composed of C6F5 and pyridine rings; the characteristic
of nano-architectures in 1 and/or 2 is its “softness or flexi-
bility”. This feature presumably enables accommodations of
various sizes of organic molecules. The longest p-xylene
among the examined organic molecules, however, forces sig-
nificant deformation of the nano-structures and/or the unit
cell dimensions of the p-xylene adduct of 2 from the original
nano-structure of 1 by trapping the p-xylene molecule in 2
leading to another space group. Temperature-dependent 57Fe
Mössbauer spectral behaviours for o- and m-xylene isomers are
interpreted in terms of similar contributions of the solvent
molecule motions to that of [Fe3O(CH3CO2)6(py)3]·py.11,12
However, interpretation of temperature-dependent 57Fe
Fig. 8 Room temperature Mössbauer spectra of [FeIII2FeIIO-
(C6F5CO2)6(py)3] (2) exposed to xylene isomer vapour: (a) 2;
(b) o-xylene; (c) m-xylene; (d) p-xylene.
Fig. 9 57Fe Mössbauer spectra of xylene adducts of
2 at 78 K:
(a) o-xylene adduct; (b) m-xylene adduct; (c) p-xylene adduct.
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Paper
Experimental
Materials and general procedures
C6F5CO2H (Aldrich) was dissolved into a small amount of hot
water and then neutralized by adding Na2CO3. A large portion
of water was evaporated and most of the remaining water was
taken off from the slurry by air-blowing while hot. The residue
was dried on filter paper by keeping at room temperature. IR
spectra were recorded with a JASCO FT-IR-4200 spectrometer
for Nujol mull samples in the range 4000–400 cm−1 at room
temperature. TG-MS spectra were measured using a thermo-
gravimetric analyzer (Shimadzu TG-40) coupled with a quadru-
pole mass spectrometer (Shimadzu GCMS QP2010) connected
by an in-house developed interface. Powder X-ray diffractions
were measured by a Rigaku Geigerflex RAD-2C diffractometer
(Cu Kα) at room temperature.
Synthesis
[Fe3O(C6F5CO2)6(py)3]·CH2Cl2 (1). FeCl3·6H2O (1.08 g,
4 mmol) and FeCl2·4H2O (0.40 g, 2 mmol) were dissolved in
30 mL of ethanol by stirring. Then 4.8 mL of pyridine was
added and the resulting orange-brown suspension was stirred
for 10 min. To this was added C6F5CO2Na (2.92 g, 12.5 mmol)
and the mixture was stirred for 2 h to afford an almost
black suspension. Black-brown precipitates were collected on
filter-paper and the residue was extracted with each 20 mL of
CH2Cl2 more than ten times until almost white solid was
left on filter paper. Combined CH2Cl2 extracts were reduced to
approximately 1/4 volume by evaporation of CH2Cl2 under
a reduced pressure. Ethanol (10 mL) was added and 5 mL
of hexane was layered over the solution. The mixture was
stored in a refrigerator for several days to afford dark-brown
needle-like crystals (2.13 g, yield 30%). Anal. Calc. for
C57H15F30Fe3N3O13·CH2Cl2: C, 39.31; H, 0.97; N, 2.37; F, 32.16;
Cl, 4.00. Found: C, 39.81; H, 1.04; N, 2.43; F, 32.41; Cl, 3.09%.
TG-MS analysis indicates that one CH2Cl2 molecule is con-
tained in 1.28 IR(Nujol): ν(CO)asym; 1654(vs), 1636(s), 1606(m)
cm−1; ν(CO)sy: 1401(s)
Fig. 10 Powder X-ray diffractions for 1 and 2, and organic molecule
adducts of 2 after exposure for 21 h at room temperature.
Mössbauer spectral changes for the p-xylene adduct requires
other factors than molecule motions of the trapped p-xylene.
The observed XRD pattern for the p-xylene adduct of 2
strongly suggests that the space group is different from
P63/m. Contribution of factors other than entropy changes
by trapped p-xylene molecule motion to electron hopping is
thus expected to be significantly different from other organic
molecules employed in the present organic vapour exposure.
The “soft nano-voids” in 2 thereby tempts us to explore
an important new aspect of “soft nano-materials” in
addition to well-known “flexible supra-molecular nano-
architectures”.20,24–27
[Fe3O(C6F5CO2)6(py)3] (2). A 178.5 mg ground sample of 1
was placed in a 10 mL volume flask and this flask was heated
at about 130 °C (oil-bath) for 1.5 h under a reduced pressure
(0.1 Torr). These experimental conditions were determined on
the basis of TG-MS data for 1. Observed weight loss was about
4.4%, which was in good agreement with the loss of one
CH2Cl2 molecule. Anal. Calcd for C57F30Fe3H15N3O13: C, 40.57;
H, 0.90; N, 2.49; F, 33.78. Found: C, 39.90; H, 0.88; N, 2.38; F,
34.30. IR(Nujol): ν(CO)asym; 1654(vs), 1636(s), 1606(m) cm−1
ν(CO)sym; 1404(s)
;
It is worthwhile to describe the results of comparisons of IR
spectra of 1 and 2 and 2 and the CH3CN adduct of 2 in the
4000–400 cm−1 region; no distinct changes were detected for
these pairs by an ordinary IR measurement. 57Fe Mössbauer
spectroscopy is thus concluded to be a quite sensitive and
indispensable technique to study organic molecule absorption
vs. desorption phenomena for trinuclear iron pentafluoro-
benzoate complexes.
57Fe Mössbauer spectroscopy
Mössbauer measurements were performed in an ordinary
mode with a Mössbauer spectrometer (Topologic Systems Inc.
for 77–300 K and WissEl 260A system for 5–70 K) with
57Co(Rh) sources. Crushed but not ground crystals were used
for measurements. Examined temperatures ranged from 5 K to
300 K and/or from 78 K to 300 K. The spectral curve fitting was
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Dalton Trans., 2014, 43, 6711–6719 | 6717

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carried out using Moss Winn 4.0 Pre (http://www.mosswinn.
com/). The isomer shift and Doppler velocity scale were
9 C.-C. Wu, S. A. Hunt, P. K. Gantzel, P. Gűtlich and
D. N. Hendrickson, Inorg. Chem., 1997, 36, 4717.
calibrated by the six lines of a body centered cubic iron foil 10 S. M. Oh, T. Kambara, D. N. Hendrickson, M. Sorai, K. Kaji,
(α-Fe), the center of which was taken as zero isomer shift.
S. E. Woehler and R. J. Wittebort, J. Am. Chem. Soc., 1985,
107, 5540.
X-ray structure analysis
11 M. Sorai, K. Kaji, D. N. Hendrickson and S. M. Oh, J. Am.
Chem. Soc., 1986, 108, 702.
Single crystal X-ray data for 1 were collected by a Rigaku
Mercury CCD diffractometer with a graphite monochromated
Mo Kα radiation (λ = 0.71073 Å) at 123 K. Structures were
solved by SIR9229 and refined by the full-matrix least-squares
method.30 All nonhydrogen atoms were refined with anisotro-
pic displacement parameters while the hydrogen atoms were
refined with a riding model. The molecular structure (Fig. 1)
and the crystal packing diagram for 1 (Fig. 2) are drawn
by ORTEP for Windows31 and Diamond 3.2i.32 Crystal data for
1: C57H15F30Fe3N3O13·0.735CH2Cl2, M = 1749.7, hexagonal,
P63/m, a = 13.458(1), c = 20.967(1) Å, V = 3288.7(5) Å3, T =
123(2) K, Z = 2, μ(Mo Kα) = 0.862 mm−1. 15 970 reflections were
measured, of which 2775 were independent (Rint = 0.0408).
The final R1 = 0.0439 for 2114 reflections with I > 2σ(I) and
wR(F2) = 0.1349 for all data.
12 H. G. Jang, S. J. Geib, Y. Kaneko, M. Nakano, M. Sorai,
A. L. Rheingold, B. Montez and D. N. Hendrickson, J. Am.
Chem. Soc., 1989, 111, 173; Y. Kaneko, M. Nakano,
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Inorg. Chem., 1992, 31, 2265.
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CCDC reference number 975326 for 1.
Conclusion
New mixed-valence tri-iron pentafluorobenzoate complexes
have been synthesized. Iron valences are detrapped for
CH2Cl2-solvated 1, while valences are trapped for CH2Cl2-free 2
at room temperature. Solvent-free 2 behaves as a nano-porous
material and absorbs many kinds of organic molecule
vapours. These behaviours including valence-trapping and/or
valence-detrapping have been studied by 57Fe Mössbauer
spectroscopy. Three xylene isomers have been recognized by
57Fe Mössbauer spectroscopy for the first time.
16 R. Wu, M. Poyraz, F. E. Sowrey, C. E. Anson, S. Wocadlo,
A. K. Powell, U. A. Jayasooriya, R. D. Cannon, T. Nakamoto,
M. Katada and H. Sano, Inorg. Chem., 1998, 37, 1913.
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19 Y. Sakai, S. Onaka, M. Takahashi, R. Ogiso, T. Takayama
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195.
been published.18 The crystal data are as follows: a = b =
13.696(5) Å, c = 20.981(8) Å, γ = 120°, Z = 2 at 298 K.
The Mn–O(center) distance is 1.932(5) Å, which corre-
sponds to detrapped Mn–O distances.
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23 The Debye temperatures estimated here from Mössbauer
data are those probed by 57Fe locally. It is interesting to
compare these “local” Debye temperatures with “total”
Debye temperatures obtained by other methods, giving
information on the effective mass M.
8 S. M. Oh, S. R. Wilson, D. N. Hendrickson, S. E. Woehler,
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6718 | Dalton Trans., 2014, 43, 6711–6719
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Paper
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This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 6711–6719 | 6719