Crystal Structures and Phase Sequences of Metallocenium Salts with Fluorinated Anions: Effects of Molecular Size and Symmetry on Phase Transitions to Ionic Plastic Crystals

Article abstract of DOI:10.1002/chem.201603170

Sandwich compounds often exhibit various phase transitions, including those to plastic phases. To elucidate the general features of the phase transitions in metallocenium salts, the thermal properties and crystal structures of [Fe(C₅Me₅)₂]X ([1]X), [Co(C₅Me₅)₂]X ([2]X), and [Fe(C₅Me₄H)₂]X ([3]X) have been investigated, where the counter anions (X) are Tf₂N (=(CF₃SO₂)₂N^?), OTf (=CF₃SO₃^?), PF₆, and BF₄. The Tf₂N salts commonly undergo phase transitions from an ordered phase at low temperatures to an anion-disordered phase, followed by a plastic phase and finally melt at high temperatures. All these salts exhibit a phase transition to a plastic phase, and the transition temperature generally decreases with decreasing cation size and increasing anion size. The crystal structures of these salts comprise an alternating arrangement of cations and anions. About half of these salts exhibit phase transitions at low temperatures, which are mostly correlated with the order–disorder of the anion.

Full text of DOI:10.1002/chem.201603170

DOI: 10.1002/chem.201603170
Full Paper
&
Crystal Engineering
Crystal Structures and Phase Sequences of Metallocenium Salts
with Fluorinated Anions: Effects of Molecular Size and Symmetry
on Phase Transitions to Ionic Plastic Crystals
Tomoyuki Mochida,*[a] Yusuke Funasako,[b] Mai Ishida,[a] Shingo Saruta,[c] Takashi Kosone,[c]
and Takafumi Kitazawa[c, d]
Abstract: Sandwich compounds often exhibit various phase
transitions, including those to plastic phases. To elucidate
the general features of the phase transitions in metalloceni-
um salts, the thermal properties and crystal structures of
[Fe(C5Me5)2]X ([1]X), [Co(C5Me5)2]X ([2]X), and [Fe(C5Me4H)2]X
([3]X) have been investigated, where the counter anions (X)
phase, followed by a plastic phase and finally melt at high
temperatures. All these salts exhibit a phase transition to
a plastic phase, and the transition temperature generally de-
creases with decreasing cation size and increasing anion
size. The crystal structures of these salts comprise an alter-
nating arrangement of cations and anions. About half of
these salts exhibit phase transitions at low temperatures,
which are mostly correlated with the order–disorder of the
anion.
ꢀ
are Tf2N (=(CF3SO2)2Nꢀ), OTf (=CF3SO3 ), PF6, and BF4. The
Tf2N salts commonly undergo phase transitions from an or-
dered phase at low temperatures to an anion-disordered
Introduction
fest phase transitions from an ordered crystalline phase to in-
creasingly disordered structures with increasing temperatures.
Furthermore, the phase transitions in organometallic com-
pounds have attracted attention for many years from the as-
pects of crystal engineering and molecular motion in the solid
state.[4] For example, [FeCp2]PF6 (Cp=C5H5) exhibits two phase
transitions at 213 and 347 K, and the highest-temperature
phase is a plastic phase.[5] [CoCp2]PF6 has a similar phase se-
quence.[6] [Fe(C5Me4H)2]BF4 exhibits a plastic phase above
409 K.[7] Metallocenium salts often exhibit plastic phases at
high temperatures owing to the globular shape of the cation.
Molecular motion and phase transitions in various metalloceni-
um salts have been investigated by means of calorimetry, solid
state NMR spectroscopy, X-ray crystallography, and Mossbauer
spectroscopy.[4–8]
Organic ionic plastic crystals containing alkylammonium, alkyl-
phosphonium, and other onium cations, have recently attract-
ed much attention because they are useful as solid state elec-
trolytes owing to their high ionic conductivity.[1] Many studies
have been conducted on molecular motion, phase transitions,
and electrochemical properties of ionic plastic crystals.[1,2] Plas-
tic phases are often exhibited by solids containing globular
molecules, in which the molecules exhibit orientational or rota-
tional disorder.[3] Plastic crystals generally display a small entro-
py of fusion as
a consequence of the disorder (DS<
20 JKꢀ1 molꢀ1), and they often have highly symmetric crystal
lattices such as cubic or hexagonal. Such materials often mani-
Based on this background, we considered that systematic
exploration of organometallic ionic plastic crystals would
expand the possibilities of ionic plastic crystals. To date, many
studies have been conducted on the phase transitions of met-
allocenium salts with various anions,[4–8] including our previous
studies on [CoCp2][(SO2CnF2n+1)2N][9] and [Fe(C5Me4R)2]X (X=
CnF2n+1CO2ꢀ, CnF2n+1SO3ꢀ).[10] However, no systematic studies
have been conducted on the size and shape of the constituent
ions. Therefore, in this study, we investigate the phase transi-
tions in [Fe(C5Me5)2]X ([1]X), [Co(C5Me5)2]X ([2]X), and
[Fe(C5Me4H)2]X ([3]X), where the counter anions (Xꢀ) are Tf2Nꢀ
[a] Prof. Dr. T. Mochida, M. Ishida
Department of Chemistry, Faculty of Science
Kobe University, Rokkodai, Nada, Hyogo 657-8501 (Japan)
E-mail: tmochida@platinum.kobe-u.ac.jp
[b] Dr. Y. Funasako
Department of Applied Chemistry, Faculty of Engineering
Tokyo University of Science, Yamaguchi
Sanyo-Onoda, Yamaguchi, 756-0884 (Japan)
[c] S. Saruta, Dr. T. Kosone, Prof. Dr. T. Kitazawa
Department of Chemistry, Faculty of Science
Toho University, Miyama, Funabashi, Chiba 274-8510 (Japan)
(=(CF3SO2)2Nꢀ), OTfꢀ (=CF3SO3 ), PF6ꢀ, and BF4 (Figure 1).
The phase transitions and crystal structures of [1]OTf and
[2]OTf were previously reported.[10] The crystal structures of
ꢀ
ꢀ
[d] Prof. Dr. T. Kitazawa
Research Center for Materials with Integrated Properties
Faculty of Science, Toho University
Miyama, Funabashi, Chiba 274-8510 (Japan)
[7]
[1]PF6–[3]PF6[11–13] and [3]BF4 are known.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603170.
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order of the anions, as discussed later. 4) Tf2N salts exhibit
melting, whereas other salts decompose without melting at
high temperatures. Detailed discussions on each of the salts
are given in the following sections.
Phase transitions in Tf2N salts
The Tf2N salts commonly exhibit two phase transitions in the
solid state and melt at high temperatures (Figure 2a). As an ex-
ample, the DSC trace of [3]Tf2N is shown in Figure 3. During
the heating process, this salt exhibits phase transitions at TC1
=
189.0 K
54.2 JKꢀ1 molꢀ1
21.2 JKꢀ1 molꢀ1).
(DS=1.7 JKꢀ1 molꢀ1)
and melts
and
at
T
C2 =318.6 K
(DS=
)
Tm =496.2 K
(DSm =
Figure 1. a) Metallocenium cations, and b) fluorinated anions used in this
study. The volume of each ion is also shown.
The phase transitions at low temperatures (TC1) in these
salts, which accompany small transition entropies (DS=1.0–
2.5 JKꢀ1 molꢀ1), are correlated with the order–disorder of the
anion (see below). The transition entropies are smaller than
those expected for the ordering of the two-fold disorder (DS=
Rln2=5.76 JKꢀ1 molꢀ1), which is probably compensated by the
difference in lattice entropies. The phase transitions at high
temperatures (TC2) accompanying large transition entropies
(DS=42–54 JKꢀ1 molꢀ1) are transitions to plastic phases. Loss
of birefringence was observed above TC2 by using polarized mi-
croscopy, which indicated that the salts have a cubic or other
high-symmetry crystal lattice. In [3]Tf2N the melting entropy is
smaller than the transition entropy to the plastic phase, as
often observed in plastic crystals.
In Figure 1, the volume of each ion is shown; they decrease
in the order [Fe(C5Me5)2]+ ([1]+)>[Co(C5Me5)2]+ ([2]+)>
ꢀ
[Fe(C5Me4H)2]+ ([3]+) for the cation and Tf2Nꢀ >OTfꢀ >PF6
>
ꢀ
BF4 for the anion. This variation allows a systematic investiga-
tion of the effects of molecular size and shape on the phase
transitions. Cation [2]+ is a diamagnetic 18-electron species;
hence, its volume is smaller than [1]+, which is a paramagnetic
17-electron species. The Tf2N anion has often been used as
a component of ionic liquids, the melting points of which are
below 1008C.[14] We previously found that alkylmetallocenium
salts with Tf2N gave ionic liquids.[15] Therefore, an investigation
of the phase transitions in these salts bridges the gap in the
understanding of organometallic ionic plastic crystals and ionic
liquids. Plastic phases have also been found in onium salts
with the Tf2N anion.[2a–c]
The melting points of [1]Tf2N–[3]Tf2N were 586, 558, and
496 K, respectively, which decreased with decreasing cation
size. This tendency is characteristic of molecular crystals, indi-
cating that intermolecular interactions, rather than electrostatic
interactions, determine the melting points. Consistently, the
melting points of [MCp2]Tf2N containing smaller cations are
even lower than those observed here (Tm =409 K for M=Fe,
439 K for M=Co).[9,15a] The comparatively lower melting point
of [3]Tf2N than [1]Tf2N and [2]Tf2N may be in part due to the
lower symmetry of the cation. The appearance of melting only
in Tf2N salts can probably be ascribed to the larger size and
flexibility of the anion compared with other anions.
In this study, we discuss the phase sequences of these salts,
mainly focusing on the Tf2N salts, and then discuss their crystal
structures and structural changes at low temperatures. It
should be noted that a room-temperature organometallic ionic
plastic crystal has been found using this approach.
Results and Discussion
These salts exhibit two-step phase transitions from an or-
dered phase at low temperatures to a partially disordered
phase and then to a plastic phase at high temperatures. It can
be noted that such successive phase sequences have also
been found in organic ionic plastic crystals.[1,2]
General features of phase sequences
Investigation by differential scanning calorimetry (DSC) re-
vealed that all the salts exhibit one or more phase transitions
in the solid state. The phase sequences for [1]X–[3]X are
shown in Figure 2. The phase transition temperatures and en-
tropies are shown in the this graphic; the data shown for
[1]OTf and [2]OTf were reported in our previous study.[10] The
presence of plastic phases in [3]PF6 and [3]BF4 has been report-
ed in previous studies.[7,8] General features of the phase se-
quence observed in these salts are summarized as follows.
1) Most salts exhibit two phase transitions in the solid state.
2) The highest-temperature solid phase is a plastic phase in all
the salts, and the phase transition temperatures to the plastic
phase are correlated with the cation/anion radius ratio.
3) About half of these salts exhibit phase transitions at low
temperatures, which are mostly correlated with the order–dis-
Phase transitions in OTf, PF6 and BF4 salts
Most salts with OTf, PF6 and BF4 anions exhibit phase transi-
tions in addition to those to plastic phases (Figure 2b–d).
Among the OTf salts, [1]OTf exhibits a phase transition at low
temperature (DS=4.9 JKꢀ1 molꢀ1), which accompanies intramo-
lecular conformational change of the cation, as previously re-
ported.[10] Among the PF6 salts, [1]PF6 and [2]PF6 exhibit phase
transitions above room temperature (DS=11–12 JKꢀ1 molꢀ1),
though the characters of the transitions are unknown. The salt
[3]PF6 exhibits a phase transition at low temperature (DS=
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Figure 2. Phase sequences of [Fe(C5Me5)2]X ([1]X), [Co(C5Me5)2]X ([2]X), and [Fe(C5Me4H)2]X ([3]X). a) Tf2N salts, b) OTf salts, c) PF6 salts, and d) BF4 salts. The
phase transition temperatures and entropies are shown above and below each bar chart, respectively. The space groups are also shown in the charts, of
which those indicated by an asterisk were taken from the literature.[7,10–13] The phase diagram shown for [3]BF4, which has two polymorphs, is that of the
more stable form.[7]
Phase transition to plastic phases
The phase transition temperatures to plastic phases in [1]X–
[3]X are plotted in Figure 4. As shown, the transition tempera-
tures tend to decrease with decreasing cation size ([1]+ >[2]+
>[3]+) and increasing anion size (BF4ꢀ <PF6ꢀ <OTfꢀ <Tf2Nꢀ).
This trend is understood in terms of the reduction of the
volume difference between the cation and anion, which likely
facilitates molecular rotation and decreases the transition tem-
perature. Compared with the overall trend, the higher transi-
tion temperatures for the Tf2N salts are probably related to the
unsymmetrical shape of the anion. Although the plastic phases
in metallocenium salts reported to date occur at high tempera-
tures, it can be noted that [3]OTf exhibits the plastic phase at
Figure 3. DSC trace of [Fe(C5Me4H)2]Tf2N ([3]Tf2N). An enlarged view is also
shown for the low-temperature region.
room temperature above 16.58C.
Regarding the sum of the phase transition entropies in the
solid state, the value for [2]X is slightly smaller than that for
3.4 JKꢀ1 molꢀ1), which is correlated with the order–disorder of
[1]X, whereas that for [3]X is larger than both of them owing
to the lower symmetry of the cation (e.g., [1]Tf2N:
the anion, as shown later. Among the BF4 salts, [1]BF4 exhibits
successive transitions at low temperature (DStotal
=
47.9 JKꢀ1 molꢀ1
,
[2]Tf2N:
44.3 JKꢀ1 molꢀ1,
[3]Tf2N:
3.2 JKꢀ1 molꢀ1), which is also correlated with the order–disorder
of the anion (see below). Despite being isomorphous with
[1]BF4, [2]BF4 does not exhibit a phase transition below room
temperature until 100 K. This salt exhibits phase transitions
only above room temperature.
55.9 JKꢀ1 molꢀ1), although the BF4 salts deviate from this ten-
dency. The transition entropies for the Tf2N salts are larger
than those for other salts, which seems consistent with the
lower symmetry and greater conformational freedom of the
anion (e.g., [1]Tf2N: 47.9 JKꢀ1 molꢀ1, [1]X (X=OTf, PF6, BF4):
35.4–37.5 JKꢀ1 molꢀ1).
The structure of the plastic phase in [3]OTf was investigated
by powder X-ray diffraction at room temperature (Figure 5).
This salt was found to have a NaCl-type cubic lattice with a lat-
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tures, [1]X and [2]X are isomorphous for each anion, whereas
[3]X exhibits a different structure.
The crystal structures of the salts commonly comprise an al-
ternating arrangement of cations and anions. In addition, the
C5 axes of the cations are aligned in the same direction in the
unit cell in each salt except [1]OTf.[10] It is interesting that such
a molecular arrangement, which accompanies the uniaxial
magnetic anisotropy of the crystal,[15c] is commonly found in
these salts. Because these salts contain nearly spherical cations,
the radius ratio rules for inorganic ionic crystals may hold, not
only for the plastic phase, but also for the crystal phase. The
rule predicts a coordination number of eight for crystals with
1>0.73 and six for those with 0.73>1>0.41, where 1 is the
anion/cation radius ratio.[17] In agreement with the rule, it was
found that the number of counter ions surrounding an ion is
eight for [1]Tf2N and [2]Tf2N (1=0.77) and six for all the OTf,
PF6, and BF4 salts (0.63>1>0.54). The only exception is [3]Tf2N
(1=0.79), the coordination number for which is six, despite
the large radius ratio. This is probably because of the unsym-
metrical shape of the constituent ions.
Figure 4. Phase transition temperatures to plastic phases in [Fe(C5Me5)2]X
&
*
([1]X, ), [Co(C5Me5)2]X ([2]X, &), and [Fe(C5Me4H)2]X ([3]X, ), where X=Tf2N,
OTf, PF6, and BF4, respectively.
tice constant of 13.00 ꢁ. The interionic distance is 6.50 ꢁ,
which is in agreement with the sum of the ionic radii estimat-
ed by DFT calculations (6.95 ꢁ). The radii of the ions were ten-
tatively calculated by assuming spheres having the same
volume as the ions. The radius ratio (ranion/rcation) for this salt
was 0.65. Therefore, the structure is in accord with the radius
ratio rule in ionic crystals, which predicts that the radius ratio
range of a NaCl-type structure is 0.41–0.73.[17] We also mea-
sured the magnetic susceptibility of [3]OTf; this salt was a para-
magnetic ionic crystal (cT=0.76 emuKꢀ1 molꢀ1, at 300 K). Cat-
ions [1]+ and [3]+ are paramagnetic, and hence their salts are
paramagnetic crystals. Recently, an example of a paramagnetic
ionic plastic crystal, [choline][FeCl4], has been reported.[16]
These materials may lead to novel electromagnetic phenom-
ena.
Furthermore, low-temperature structure determinations re-
vealed that the low-temperature phase transitions observed in
many of the salts accompany the order–disorder of the anion,
except for [1]OTf.[10] Crystal structures of each salt are dis-
cussed in the following sections.
Crystal structures of Tf2N salts
Crystal structure determination for Tf2N salts revealed that
[1]Tf2N and [2]Tf2N are isomorphous crystals but [3]Tf2N exhib-
its a different structure. Each salt exhibits a phase transition at
low temperature, as shown above.
The packing diagrams of [1]Tf2N (TC1 =167.4 K) at 180 and
100 K are shown in Figure 6a. The cell volume of the low-tem-
perature phase (Z=2) is twice that of the room-temperature
¯
phase (Z=1), whereas the space group is P1 in both phases.
The molecular arrangements particularly, are the same in both
phases, where the cations and anions are alternately arranged
in the crystal lattice. Each anion is surrounded by eight cations
and vice versa, hence forming a highly deformed CsCl-like lat-
tice. The molecular structures of the anion, exhibiting the trans
(C2) conformation, are also shown in the figure. In addition to
the phase-transition temperature, the central moiety is disor-
dered over two sites with a 0.5 occupancy, and the CF3 groups
exhibit rotational disorder. This type of disorder is often seen
in Tf2N salts.[2a] The anion is ordered in the low-temperature
phase, which results in ion pair formation of the anion with
one of the two adjacent cations and cell doubling. This type of
anion ordering has also been observed in [CoCp2][(SO2CnF2n+
1)2N].[9] In the room temperature phases of [1]Tf2N and [2]Tf2N,
both the cation and anion are located on inversion centers;
hence, the asymmetric unit contains half of the molecules. In
the low-temperature phase, however, the inversion centers are
removed and the asymmetric unit contains a cation and anion.
The crystal structures of [2]Tf2N determined above (297 K)
Figure 5. Powder XRD pattern of [3]OTf at room temperature.
General features of crystal structures
The space groups of the room-temperature phase and low-
temperature phase of each salt, as determined by single crystal
X-ray crystallography, are shown in Figure 2. Crystal structure
determination failed for [3]OTf, which has a plastic phase at
room temperature. Regarding the room temperature struc-
and below (90 K) the low-temperature phase transition (TC1
=
176.5 K) are isomorphous with [1]Tf2N in the corresponding
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¯
¯
Figure 6. a) Packing diagrams of [Fe(C5Me5)2]Tf2N ([1]Tf2N) at 180 K (P1, Z=1) and 100 K (P1, Z=2). ORTEP drawings (50% probability ellipsoids) of the anion
at each temperature are shown below, where one of the disordered moieties of the anion is displayed in gray. b) Packing diagrams of [Fe(C5Me4H)2]Tf2N
([3]Tf2N) at 200 K (Pnma, Z=4) and 90 K (P212121, Z=4). c) Packing diagrams of [Fe(C5Me5)2]BF4 ([1]BF4) at 195 and 90 K. d) Packing diagram of
[Fe(C5Me4H)2]PF6 ([3]PF6) at 100 K. Only half of the unit-cell content is shown along the a-axis. Hydrogen atoms have been omitted for clarity.
phases, showing the order–disorder of the anion. The unit-cell
volume of [2]Tf2N (1282 ꢁ3 at 90 K) was 2% smaller than that
of [1]Tf2N (1304 ꢁ3 at 100 K), a difference that is probably re-
sponsible for their different phase-transition temperatures.
The packing diagrams of [3]Tf2N (TC1 =189.0 K) at 200 and
90 K are shown in Figure 6b. Each anion is surrounded by six
cations and vice versa. The space group changed from ortho-
rhombic Pnma (200 K) to P212121 (90 K), whereas the molecular
arrangements were fundamentally the same in both phases.
The phase transition accompanies the order–disorder of the
anion, but no cell doubling occurred (Z=4) in contrast to
[1]Tf2N and [2]Tf2N. Both the cation and anion are located on
mirror planes in the high-temperature phase, with the asym-
metric unit containing half of the molecules, whereas the
mirror planes are removed in the low-temperature phase. In
both phases, there are short contacts between the ring hydro-
gen of the cation and the oxygen atom in the anion
(CCpH···O=2.46 ꢁ at 200 K, 2.54 ꢁ at 90 K), which are shorter
than the van der Waals distance by about 0.2 ꢁ. Similar inter-
molecular anion–cation contacts have been observed in the
salts of octamethylferrocenium derivatives.[15c] We previously
reported that a charge-transfer complex of octamethylferro-
cene with an organic acceptor exhibits a phase transition ac-
companying an order–disorder of the C5Me4H ring.[18] In the
present salt, however, the cation exhibits no disorder in the
room-temperature phase, which is probably responsible for its
larger transition entropy to the plastic phase, as compared
with [1]Tf2N and [2]Tf2N.
Crystal structures of PF6 and BF4 salts
[11,12]
The crystal structures of [1]X and [2]X (X=PF6
and BF4) in
the room-temperature phase were found to be almost the
same, and exhibited the same space group and similar cell pa-
rameters. The differences are that the cell volume of the BF4
salts are slightly smaller (e.g., V=1021 ꢁ3 for [2]BF4, V=
1065 ꢁ3 for [2]PF6 at room temperature) and the anions are
disordered in the BF4 salts. In contrast, the crystal structures of
[7]
[13]
[3]BF4 and [3]PF6 are different, having a different intramo-
lecular ring conformation of the cation, and the anions are dis-
ordered in both salts. In all the PF6 and BF4 salts, each cation is
surrounded by six anions and vice versa. Low-temperature
structure determination revealed that the phase transitions in
[1]BF4 and [3]PF6 accompany the order–disorder of the anion.
Compound [1]BF4 exhibits successive phase transitions at
low temperatures (TC =147, 166, and 183 K). The packing dia-
grams of this salt at above and below the transition tempera-
tures are shown in Figure 6c. The space group changes from
P21/c (195 K) to C2m (90 K). At 195 K, the asymmetric unit con-
tains half of each of the cation and anion. The cation is located
on the mirror plane, and the C5Me5 rings seem to undergo lib-
eration or rotation around the C5 axis, judging from the elon-
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gated thermal ellipsoids. The anion, which is also located on
the mirror plane, exhibits disorder. At 90 K, however, the mirror
planes are removed, and the asymmetric unit contains a pair
of cations. The cations in the unit cell, which exhibits a slightly
canted orientation to each other, have fairly small thermal el-
lipsoids and the anions are nearly ordered. The phase transi-
tion accompanies cell doubling; hence, the successive phase
transition might be due to the formation of a long-range peri-
odic structure related to the ordering of the anion or cation.
The room-temperature structure of [3]PF6 has been reported
(space group C2/c, Z=4, V=1967 ꢁ3), where the asymmetric
unit contains half of each of the cation and anion, exhibiting
disorder of the anion.[13] The packing diagram of this salt, de-
termined at 100 K, below the phase transition temperature
(TC =117 K), is shown in Figure 6d. The space group is un-
changed, but the cell volume is significantly reduced (V=
1888 ꢁ3) and the anions are fully ordered. At 100 K, there are
short cation–anion contacts between the ring hydrogen and
the F atom (CringH···F=2.46 ꢁ), which are shorter than the van
der Waals distance by 0.20 ꢁ. These contacts are absent in the
room temperature phase; hence, the formation of the short
contacts may be related to the anion ordering.
radius ratio. Only the Tf2N salts exhibit melting at high temper-
atures, and the melting points decrease with decreasing cation
size.
The crystal structures of these salts comprise an alternating
arrangement of cations and anions, where the coordination
numbers depend on the cation/anion radius ratio. Compounds
[1]X and [2]X are isomorphous crystals for the same anion. It is
intriguing to note that the C5 axes of the cations are generally
aligned in the same direction in the unit cell. Such molecular
arrangement accompanies uniaxial magnetic anisotropy of the
crystal, which is interesting in terms of magnetic functionality.
This study gives an account of the phase transitions and
crystal structures of a series of metallocenium salts. Together
with our previous study on related ionic liquids,[15c] this study
provides information about the boundary between ionic liq-
uids and plastic crystals. Metallocenium salts with a globular
molecular shape generally give plastic phases, but the intro-
duction of an alkyl substituent, which lowers the molecular
symmetry and increases the conformational freedom of the
cation, leads to the loss of the plastic phase and appearance
of a liquid phase.
This study, focusing on organometallic plastic salts, extends
the field of ionic plastic crystals and contributes to the under-
standing of the effects of shape and size of constituent mole-
cules in ionic plastic crystals. In addition, the systematic investi-
gation leads to the finding of a room temperature organome-
tallic ionic plastic crystal. It should be noted that the salts con-
taining iron atoms are regarded as magnetic plastic crystals.
Details of the electronic properties of these salts will be report-
ed in the near future.
Conclusions
The phase behavior of [1]X–[3]X (Tf2N, OTf, PF6, and BF4) has
been investigated, and all the salts are found to exhibit phase
transitions in the solid state. The phase sequences, generally
accompanying stepwise enhancement of disorder or molecular
motion, resemble those for typical organic ionic plastic crystals.
All the salts exhibit phase transitions to the plastic phase, and
the transition temperature is dependent on the cation/anion
Table 1. Crystallographic parameters.
[1]Tf2N
[2]Tf2N
[3]PF6
100 K
180 K
90 K
297 K
100 K
empirical formula
formula weight
C22H30F6FeNO4S2
606.44
C22H30CoF6NO4S2
609.52
C18H26F6FeP
443.21
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
triclinic
¯
P1
triclinic
¯
P1
triclinic
¯
P1
triclinic
¯
P1
monoclinic
C2/c
12.723(9)
11.748(8)
12.821(9)
90
99.937(11)
90
1888(2)
4
9.8444(16)
11.2361(19)
12.660(2)
85.391(3)
74.641(2)
74.912(2)
1303.7(4)
2
8.1370(17)
8.7985(18)
9.854(2)
69.048(3)
88.975(3)
83.714(3)
654.7(2)
1
9.8077(11)
11.1638(13)
12.5748(14)
84.961(2)
74.289(2)
75.375(2)
1282.2(3)
2
8.1408(13)
8.8562(13)
9.9243(15)
69.627(2)
89.267(3)
84.306(3)
667.26(18)
1
a [8]
b [8]
g [8]
V [ꢁ3]
Z
1calcd [gcmꢀ3
F(000)
]
1.545
626
1.538
313
1.579
628
1.517
314
1.560
916
reflns collected
independent reflns
parameters
8837
6091
335
4467
2970
219
8209
5198
335
4542
2832
219
4854
1991
123
R(int)
0.0421
0.0198
0.0188
0.0181
0.0801
[b]
R1,[a] Rw (I>2s)
0.0598, 0.0964
0.1496, 0.1192
1.033
0.817, ꢀ0.970
0.0470, 0.1107
0.0640, 0.1187
1.108
0.509, ꢀ0.297
0.0331, 0.0895
0.0351, 0.0916
1.072
0.790, ꢀ0.650
0.0396, 0.1087
0.0415, 0.1108
1.059
0.375, ꢀ0.411
0.0833, 0.2202
0.1191, 0.2357
1.265
0.995, ꢀ0.941
[b]
R1,[a] Rw (all data)
GOF
D1max, min [eꢁꢀ3
]
[a] R1 =Sj jFo jꢀjFc j j/SjFo j. [b] Rw =[Sw(Fo ꢀFc2)2/Sw(Fo2)2]1/2
2
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Table 2. Crystallographic parameters.
[3]Tf2N
[1]BF4
[2]BF4
90 K
200 K
90 K
195 K
297 K
empirical formula
formula weight
crystal system
space group
a [ꢁ]
C20H26F6FeNO4S2
578.39
C20H30BF4Fe
413.1
monoclinic
P21/c
8.7716(19)
17.065(4)
14.122(3)
109.806(11)
1988.8(8)
4
C20H30BCoF4
416.18
monoclinic
C2/m
14.088(12)
8.710(8)
9.009(8)
112.521(14)
1021.2(15)
2
orthorhombic
P212121
8.8073(9)
12.0045(12)
22.759(2)
90
2406.2(4)
4
1.597
orthorhombic
Pnma
23.2063(19)
12.1995(10)
8.8167(7)
90
2496.1(4)
4
1.539
monoclinic
C2/m
14.1675(10)
8.6843(6)
8.8556(6)
110.6460(10)
1019.57(12)
2
b [ꢁ]
c [ꢁ]
b [8]
V [ꢁ3]
Z
1calcd [gcmꢀ3
F(000)
]
1.380
868
1.346
434
1.354
436
1188
1188
reflns collected
independent reflns
parameters
17375
5547
316
15551
3014
215
11323
4559
255
3308
1252
83
2635
990
83
R(int)
0.0590
0.0809
0.0290
0.0208
0.0512
[b]
R1,[a] Rw (I>2s)
0.0415, 0.0837
0.0534, 0.0881
1.033
0.556, ꢀ0.338
0.0432, 0.0784
0.0989, 0.0966
1.008
0.315, ꢀ0.340
0.0407, 0.01020
0.0631, 0.1149
1.011
0.885, ꢀ0.632
0.0455, 0.1249
0.0460, 0.1253
1.110
1.053, ꢀ0.698
0.0665, 0.1742
0.0695, 0.1782
1.100
1.250, ꢀ0.547
R1,[a] Rw (all data)
GOF
D1max, min [eꢁꢀ3
[b]
]
[a] R1 =Sj jFo jꢀjFc j j/SjFo j. [b] Rw =[Sw(Fo ꢀFc2)2/Sw(Fo2)2]1/2
.
2
92%; elemental analysis calcd (%) for C20H30BF4Co: C 57.72, H 7.27;
found: C 57.41, H 7.20.
Experimental Section
General
X-ray structure analysis
DSC measurements were performed by using a Q100 differential
scanning calorimeter (TA instruments) in the temperature range of
100–550 K at a heating/cooling rate of 10 Kminꢀ1; other rates were
applied as required. Loss of birefringence of the crystals in the
plastic phase was observed using a polarized microscope equipped
with a Linkam LTS350 hot stage. The melting points of Tf2N salts
were visually determined under a microscope. Molecular volumes
were estimated based on DFT calculations (B3LYP/LanL2DZ) using
Spartan ‘14 software (Wavefunction Inc.).
X-ray diffraction data for single crystals of [1]Tf2N–[3]Tf2N, [1]BF4,
[2]BF4, and [3]PF6 were collected on a Bruker SMART APEX CCD dif-
fractometer by using MoKa radiation (l=0.71073 ꢁ). Temperature
variation was performed at 1 Kminꢀ1. Crystallographic parameters
are listed in Table 1 and Table 2. The structures were solved by the
direct methods (SHELXS-97[23]). ORTEP-3 for Windows[24] and Mercu-
ry[25] were used for the molecular graphics. The disorder model
could not be applied to the methyl groups in [1]BF4 at 195 K.
CCDC 1062857, 1062858, 1062859, 1062860, 1062861, 1062862,
1062865, 1062866, 1062867 and 1487788 contain the supplemen-
tary crystallographic data for this paper. These data are provided
free of charge by The Cambridge Crystallographic Data Centre.
Materials
[Co(C5Me5)2]PF6 ([2]PF6) was purchased from Sigma–Aldrich and
was used after recrystallization from acetonitrile. The preparations
of [1]X (X=PF6,[19] BF4[20]) and [3]X (X=PF6,[21] BF4[7]) can be found
in the literature. The general procedure for the preparation of
other salts is as follows. A slight excess of an acetonitrile solution
of AgX (Xꢀ =Tf2Nꢀ,[22] OTfꢀ, PF6ꢀ) was added dropwise to an aceto-
nitrile solution of ferrocenes while being stirred. After stirring for
1 h at room temperature, silver precipitates were removed by filtra-
tion. The filtrate was evaporated under reduced pressure and dried
under vacuum. The obtained powders were dissolved in acetoni-
trile and filtered. Further, the filtrate was evaporated and dried
under vacuum. Single crystals suitable for characterization were
obtained by vapor diffusion of diethyl ether into acetonitrile solu-
tions of the salts. [1]Tf2N: Yield 88%; elemental analysis calcd (%)
for C22H30F6FeNO4S2: C 43.57, H 4.99, N 2.31; found: C 43.69, H 5.07,
N 2.25. [2]Tf2N: Yield 88%; elemental analysis calcd (%) for
C22H30F6NO4S2Co: C 45.35, H 4.96, N 2.30; found: C 43.34, H 5.00, N
2.28. [3]Tf2N: Yield 85%; elemental analysis calcd (%) for
C20H26F6FeNO4S2: C 41.53, H 4.53, N 2.42; found: C 41.61, H 4.60, N
2.36. [3]OTf: Yield 98%; elemental analysis calcd (%) for
C19H26F3FeO3S: C 51.01, H 5.86; found: C 50.83, H 5.86. [1]BF4: Yield
Acknowledgements
This work was financially supported by JSPS KAKENHI
(16H04132). We thank T. Tominaga (Kobe University) for help
with X-ray crystallography and polarized microscope observa-
tion.
Keywords: crystal structures
·
metallocenes
·
phase
transitions · plastic crystals · structure elucidation
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Published online on && &&, 0000
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FULL PAPER
Organometallic ionic plastic crystals:
Metallocenium salts [M(C5Me5)2]X
(M=Fe, Co) and [Fe(C5Me4H)2]X with
fluorinated anions commonly exhibit
ionic plastic phases. With increasing
temperature, these salts mostly undergo
phase transitions from an ordered
phase to an anion-disordered phase
and then to a plastic phase. The crystal
structures and transition temperatures
are correlated with the cation/anion
radius ratio.
&
Crystal Engineering
T. Mochida,* Y. Funasako, M. Ishida,
S. Saruta, T. Kosone, T. Kitazawa
&& – &&
Crystal Structures and Phase
Sequences of Metallocenium Salts
with Fluorinated Anions: Effects of
Molecular Size and Symmetry on
Phase Transitions to Ionic Plastic
Crystals
Chem. Eur. J. 2016, 22, 1 – 9
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These are not the final page numbers! ÞÞ