Organometallic ionic liquids from alkyloctamethylferrocenium cations: Thermal properties, crystal structures, and magnetic properties

Article abstract of DOI:10.1039/c3dt00084b

Alkyloctamethylferrocenium salts with the Tf₂N anion ([Fe(C ₅Me₄C_nH_2n+1)(C₅Me ₄H)][Tf₂N]; Tf₂N = bis(trifluoromethanesulfonyl)amide) were prepared, and their ionic liquid properties, thermal properties, crystal structures, and magnetic properties were investigated. The melting points of the Tf₂N salts were near room temperature, and decreased with increasing alkyl chain length up to n = 8 and then increased. The salts with PF₆ and NO₃ anions were also prepared. The melting points of the PF₆ salts were higher than 100 °C. Most of these salts exhibited phase transitions in the solid state. The sum of the entropies of the melting and solid phase transitions was nearly independent of the alkyl chain length for salts with short alkyl chains, whereas those for salts with longer alkyl chains (n ≥ 10 for Tf₂N salts, n ≥ 6 for PF₆ salts) increased with increasing alkyl chain length. Crystal structure determinations revealed that the short chain salts form simple alternately packed structures of cations and anions in the solid state, and that the long chain salts form lamellar structures, in which the alkyl chains are aligned parallel between the layers. The effects of magnetic fields on the crystallization of the paramagnetic ionic liquids were investigated, and revealed that the Tf₂N salts with n = 4 exhibited magnetic orientation when solidified under magnetic fields. The magnetic orientation was shown to be a bulk phenomenon, and the importance of the magnetic anisotropy of the crystal structure was suggested in comparison with the response of other Tf₂N salts.

Full text of DOI:10.1039/c3dt00084b

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PAPER
Organometallic ionic liquids from
alkyloctamethylferrocenium cations: thermal
Cite this: Dalton Trans., 2013, 42, 8317
properties, crystal structures, and magnetic properties†
Yusuke Funasako,a Takashi Inagaki,a Tomoyuki Mochida,*a Toshihiro Sakurai,b
Hitoshi Ohta,c Ko Furukawa‡d and Toshikazu Nakamurad
Alkyloctamethylferrocenium salts with the Tf2N anion ([Fe(C5Me4CnH2n+1)(C5Me4H)][Tf2N]; Tf2N = bis-
(trifluoromethanesulfonyl)amide) were prepared, and their ionic liquid properties, thermal properties,
crystal structures, and magnetic properties were investigated. The melting points of the Tf2N salts were
near room temperature, and decreased with increasing alkyl chain length up to n = 8 and then increased.
The salts with PF6 and NO3 anions were also prepared. The melting points of the PF6 salts were higher
than 100 °C. Most of these salts exhibited phase transitions in the solid state. The sum of the entropies of
the melting and solid phase transitions was nearly independent of the alkyl chain length for salts with
short alkyl chains, whereas those for salts with longer alkyl chains (n ≥ 10 for Tf2N salts, n ≥ 6 for PF6
salts) increased with increasing alkyl chain length. Crystal structure determinations revealed that the
short chain salts form simple alternately packed structures of cations and anions in the solid state, and
that the long chain salts form lamellar structures, in which the alkyl chains are aligned parallel between
the layers. The effects of magnetic fields on the crystallization of the paramagnetic ionic liquids were
investigated, and revealed that the Tf2N salts with n = 4 exhibited magnetic orientation when solidified
under magnetic fields. The magnetic orientation was shown to be a bulk phenomenon, and the impor-
tance of the magnetic anisotropy of the crystal structure was suggested in comparison with the response
of other Tf2N salts.
Received 10th January 2013,
Accepted 4th April 2013
DOI: 10.1039/c3dt00084b
www.rsc.org/dalton
nyl)amide (Tf2N) are frequently used, and magnetic anions
such as MXn (M = metal, X = anionic ligands),3 organic radi-
1. Introduction
Ionic liquids are salts with melting points below 100 °C, and cals,4 and metal complexes5 are used to add magnetic func-
they are intriguing from the viewpoint of fundamental physical tions to the liquids.
chemistry and electrochemistry.1 Most ionic liquids are
Recently, several interesting examples of onium-free ionic
formed from organic onium cations such as imidazolium, liquids composed of cationic metal complexes have been
ammonium, and phosphonium cations, and their modifi- reported.6 We have recently developed ionic liquids containing
cations have led to various functionalities.2 As the counter metallocenium cations7 and cationic chelate complexes,8
anion, fluorinated anions such as bis(trifluoromethanesulfo- which exhibit interesting properties such as magnetic orienta-
tion,7b,d color changes,8a gas absorption,7e,8a and chemical
reactivities.7e Ferrocenium cations are interesting from the
viewpoint of magnetism,9 and they are suitable components of
ionic liquids because of their bulkiness and charge delocaliza-
tion. However, ferrocenium ionic liquids are highly sensitive to
air.7a,d In this study, alkyloctamethylferrocenium cations were
used to obtain air-stable ferrocenium ionic liquids. There are a
few examples of liquid-like peralkylated ferrocenium salts,
such as alkali metal podates with ether substituents10 and
liquid crystal salts with mesogenic substituents.11
aDepartment of Chemistry, Graduate School of Science, Kobe University, Rokkodai,
Nada, Hyogo 657-8501, Japan. E-mail: tmochida@platinum.kobe-u.ac.jp
bCenter for Supports to Research and Education Activities, Kobe University,
Rokkodai, Nada, Hyogo 657-8501, Japan
cMolecular Photoscience Research Center, Kobe University, Rokkodai, Nada,
Hyogo 657-8501, Japan
dInstitute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan
†Electronic supplementary information (ESI) available: Preparation of alkylocta-
methylferrocnes, results of the DSC, XRD, and magnetic measurements. CCDC
900446–900451. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt00084b
In this paper, we report the preparation and physical prop-
erties of alkyloctamethylferrocenium salts with the Tf2N anion
([Fe(C5Me4CnH2n+1)(C5Me4H)][Tf2N], Scheme 1), which are air-
‡Present address: Center for Instrumental Analysis, Institute for Research Pro-
motion, Niigata University, Nishi-ku, Niigata 950-2181, Japan.
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were obtained as dark-green oils or solids, which were stable
under air. The 1-pentenyl derivative [C5′Fc][Tf2N] was prepared
similarly. The melting points (Tm) of the Tf2N salts were in the
range 15–70 °C. The PF6 salts ([C4Fc][PF6], [C6Fc][PF6], and [C10Fc]-
[PF6]) and NO3 salts ([C6Fc][NO3] and [C10Fc][NO3]) were
obtained as dark-green solids. The melting points of these
salts were above 100 °C except for [C10Fc][NO3], which melted
at about 80 °C.
Melting points of Tf2N salts and their precursors
The melting points, melting entropies, and glass transition
temperatures of the Tf2N salts are listed in Table 1. The
melting points are plotted in Fig. 1 as a function of the alkyl
Scheme 1 Structural formulae of alkyloctamethylferrocenium salts prepared in
this study. Abbreviations for the salts are also shown.
chain length. Compared with [Fe(C5Me4H)2][Tf2N] (Tm
=
213 °C),13 introduction of alkyl chains significantly decreased
the melting points. The melting points decreased with increas-
ing chain length up to n = 8 and then increased, where n is the
number of carbon atoms in the chain. This tendency is similar
to that observed in ionic liquids of alkylimidazolium salts.14
The melting points of the present salts are higher by 10–30 °C
than those of [Rmim][Tf2N] (Rmim = 1-alkyl-3-methylimidazo-
lium)14b and [Fe(C5H4R)(C5H5)][Tf2N]7a,d with the same alkyl
chain lengths, which is consistent with the larger molecular
weights and volumes of the cations. The melting point of the
alkenyl derivative [C5′Fc][Tf2N] (Tm = 56.9 °C) was higher than
that of the alkyl derivative [C5Fc][Tf2N] (Tm = 30.0 °C and
20.2 °C, vide infra) as a result of the decreased flexibility of the
substituent.
stable paramagnetic ionic liquids. 1-Pentenyloctamethylferro-
cene was also used as the cation, and several nitrate (NO3) and
hexafluorophosphate (PF6) salts were prepared for comparison.
The discussion of the crystal structure is useful in elucidating
the nature of ionic liquids.12 Although structural data were not
available for the ferrocenium ionic liquids, structural determi-
nation of the present salts was possible. The thermal proper-
ties of [C6Fc][Tf2N]7c and the effects of magnetic fields on the
crystallization of [C4Fc][Tf2N]7b have been reported previously.
A detailed investigation of magnetic phenomena in the latter
salt is described here, and compared with those of other Tf2N
salts.
The thermal properties of alkyloctamethylferrocenes, which
are the precursors of the salts, were also examined (Table 2).
They melted at around 20 °C, and exhibited glass transitions
at around −85 °C when cooled from the liquid state. Crystalli-
zation was observed in the heating process from the glassy
2. Results and discussion
Preparation and properties
The alkyloctamethylferrocenium salts were prepared according state, except for the pentyl and 1-pentenyl derivatives. It is
to Scheme 2. Octamethylformylferrocene was reacted with interesting that the melting points of alkyloctamethylferro-
Grignard reagents, and the resultant alcohols were reduced to cenes (open circles) and their Tf2N salts (filled circles) are
give alkyloctamethylferrocenes as yellow-orange oils, which comparable (Fig. 1).
were further reacted with AgX (X = Tf2N, PF6, and NO3) to give
The ratios of the melting points and glass transition temp-
the desired salts. The Tf2N salts ([C3Fc][Tf2N]–[C17Fc][Tf2N]) eratures (Tg/Tm) for the alkyloctamethylferrocenes were
0.61–0.65. In the Tf2N salts, the glass transition was only
observed in the pentenyl derivative [C5′Fc][Tf2N] (Tg = −67 °C)
when the liquid was cooled at a rate faster than 10 °C min−1
.
The Tg/Tm ratio for this salt was 0.62. These ratios are consist-
ent with the empirical relationship (Tg/Tm ≈ 2/3) known for
general molecular compounds15 and also for alkylimidazolium
ionic liquids.16
Phase transitions in the Tf2N salts
The phase sequences of the Tf2N salts are shown in Fig. 2.
[C4Fc][Tf2N], [C5′Fc][Tf2N], and [C17Fc][Tf2N] exhibited no
phase transitions in the solid state (Fig. 2a). [C3Fc][Tf2N],
[C10Fc][Tf2N], and [C12Fc][Tf2N] exhibited one phase tran-
sition and [C8Fc][Tf2N] exhibited two phase transitions in the
solid state (Fig. 2b). The large values of ΔS indicate that exten-
sive molecular motions occur above the phase transition
Scheme 2 Scheme for the preparation of alkyloctamethylferrocenium salts.
8318 | Dalton Trans., 2013, 42, 8317–8327
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Table 1 Melting points (Tm), melting entropies (ΔSm), phase transition temperatures (Tc1, Tc2), phase transition entropies (ΔSc1, ΔSc2), and the sums of the phase
transition entropies (ΔStotal) of the Tf2N, PF6, and NO3 salts
Tm/°C
ΔSm/J K−1 mol−1
Tc1/°C
ΔSc1/J K−1 mol−1
Tc2/°C
ΔSc2/J K−1 mol−1
ΔStotal a/J K−1 mol−1
[C3Fc][Tf2N]
[C4Fc][Tf2N]
[C5Fc][Tf2N]
67.4
34.4
30.0
20.2b
56.9
27.7
16.6
86.3
98.5
20.4
80.7
27.4
45.4
90.5
107.1
86.3
98.5
—
80.7
84.6
0.5b
44.0b
[C5′Fc][Tf2N]
[C6Fc][Tf2N]
21.4
−1.6b
1.7
11.0
8.7
55.2
39.7b
19.9
14.7
36.5
[C8Fc][Tf2N]
[C10Fc][Tf2N]
[C12Fc][Tf2N]
[C17Fc][Tf2N]
[C4Fc][PF6]
[C6Fc][PF6]
[C10Fc][PF6]
16.9
17.5
26.2
12.5
12.5
18.0
97.3
26.1
26.7
39.8
11.5
2.5
34.9
27.2
54.5
93.6
59.1
35.1
64.3
56.8
166.8
142.1
201.4
−53.8
109.9
−44.5
54.2b
4.4
1.1
108.1
132.2
49.2
28.6
7.3
7.0
20.5
32.9b
[C6Fc][NO3]
109.4
26.6
26.6
a ΔStotal = ΔSm + ΔSc1 + ΔSc2 b Phase transitions in metastable polymorphs.
.
Fig. 2 Phase sequences of Tf2N salts. Different crystal phases are represented
as I–III. The asterisks indicate metastable phases.
●
(
○
)
Fig. 1 Melting points of Tf2N salts
)
and alkyloctamethylferrocenes
(
plotted as a function of the alkyl-chain length. The values for octamethylferro-
cene and its Tf2N salt (n = 0) are also shown.
motions of the alkyl chains are probably frozen to form a
glassy crystal. [C5Fc][Tf2N] and [C6Fc][Tf2N] exhibited three
crystal phases, including metastable phases (Fig. 2c). The
differential scanning calorimetry (DSC) traces for [C5Fc][Tf2N]
are shown in Fig. S1 in the ESI.† The Gibbs free energy
diagram for this salt derived from the data is shown in Fig. 3.
The melting points of the stable phase (phase III) and the
metastable phase (phase I) were 30.0 °C and 20.2 °C, respect-
ively, and their melting entropies (ΔSm) were 98.5 J K−1 mol−1
and 20.4 J K−1 mol−1, respectively. The latter small entropy
indicates that phase I is a highly disordered phase. [C6Fc]-
[Tf2N] showed a similar but slightly different behavior, which
Table 2 Melting points (Tm), melting entropies (ΔSm), and glass transition
temperatures (Tg) of alkyloctamethylferrocenes
Tm/°C
ΔSm/J K−1 mol−1
Tg/°C
C3Fc
C4Fc
C5Fc
C5’Fc
C6Fc
C8Fc
17.5
27.6
—
—
22.1
23.8
68.9
80.6
—
—
84.7
34.7
−97
−88
−86
−84
−83
−84
temperatures, not merely rotational and/or conformational dis- we have reported previously.7c
order of anions.
The sums of the entropies of the melting and solid phase
The high-temperature phases in these salts are orientation- transitions for the Tf2N salts (ΔStotal) are shown in Fig. 4. The
ally disordered phases with small melting entropies (ΔS < 20 J values for the salts with n = 3, 4, 5, and 6 were comparable
K−1 mol−1).17 Among them, [C10Fc][Tf2N] showed a glass tran- (80–110 J K−1 mol−1), whereas the salt with n = 8 exhibited a
sition in the crystal phase (phase II) at −40 °C, where the much smaller value and the salts with n ≥ 10 displayed an
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comparable (Fig. 4 and Table 2), as well as the melting points,
as noted above. A linear correlation between the melting
points of the salts and the precursors has also been observed
in alkylferrocenium Tf2N salts.7d These results demonstrate
that the thermodynamic properties of the Tf2N salts reflect
those of their precursors.
Thermal properties of PF6 and NO3 salts
The melting points of [C4Fc][PF6], [C6Fc][PF6], and [C10Fc]-
[PF6] were 166.8 °C, 142.1 °C, and 201.4 °C respectively, and
they first decreased and then increased with increasing chain
length. The melting points of the PF6 salts were higher by
100–150 °C than those of the corresponding Tf2N salts. The
PF6 salts exhibited two phase transitions in the solid state,
showing three stable crystalline phases (phase I–III). Their
phase transition data are shown in Table 1. [C10Fc][PF6]
was obtained as a metastable glassy crystal, which was trans-
formed to the stable phase (phase I) by heating (Fig. S2 in the
ESI†). The sums of the phase transition entropies for the salts
Fig. 3 Schematic Gibbs free energy diagram for [C5Fc][Tf2N]. The free energy
curves are approximated by straight lines for simplicity.
with n = 4, 6, and 10 were 59.1 J K−1 mol−1, 35.1 J K−1 mol−1
,
and 64.3 J K−1 mol−1, respectively. The dependence on n is
similar to that observed in the Tf2N salts shown above. The
increase in the value for the last salt is accounted for by the
increased alkyl chain length, and the different tendency for
the salt with n = 4 is ascribed to different packing structures,
as shown below.
The melting point of [C6Fc][NO3] was 109.4 °C (ΔS = 26.6 J
K−1 mol−1), which is much higher than that of the correspond-
ing Tf2N salt ([C6Fc][Tf2N]: Tm = 27.7 °C). Crystallization from
the melt occurred at 23.5 °C when cooled at 2 °C min−1, but a
glass transition (Tg = −35 °C) was observed when the cooling
rate was faster than 10 °C min−1. [C10Fc][NO3] melted at
around 80–82 °C, but it is not a precise melting point because
of the small amount of remaining solvate molecules.
Fig. 4 Sums of phase transition entropies of Tf2N salts. The dark-gray areas are
the entropies of solid phase transitions, and the others are the melting
entropies.
Viscosities
The viscosities of [C6Fc][Tf2N] and [C10Fc][Tf2N] at 25 °C were
752 mPa s and 746 mPa s, respectively, and they were com-
parable. Both were Newtonian liquids, and they were much
more viscous than the imidazolium ionic liquids (e.g., [bmim]-
[Tf2N]: 49 mPa s, bmim = 1-butyl-3-methylimidazolium)19 and
ferrocenium ionic liquids (e.g., [Fe(C5H4nBu)(C5H5)][Tf2N]:
112.3 mPa s).7a,d The temperature dependences of the visco-
sities were fitted using the Arrhenius plot from which activation
energies (Ea) of 43.1 kJ mol−1 and 44.9 kJ mol−1, respectively,
were derived. These values are comparable to those of [bmim]-
increase with elongation of the alkyl chain. For molecules con-
sisting of a rigid core and alkyl chains, such as mesogens and
alkylimidazolium ionic liquids, conformational changes of the
alkyl chains provide an entropy change of about 10 J K−1
mol−1 per methylene unit.18 The Tf2N salts with n ≥ 10 fol-
lowed this tendency well, which indicates that the motion of
the alkyl groups dominates the phase change entropies in
these salts. The absence of such a tendency in the salts with
n ≤ 6 is partly because the entropy contributions from the alkyl
moieties are small in the short chain salts and, moreover, is
probably the result of different packing structures (vide infra).
The total phase transition entropy in [C5′Fc][Tf2N] (80.7 J K−1
mol−1) was smaller by about 20 J K−1 mol−1 than that in [C5Fc]-
[Tf2N] (98.5 J K−1 mol−1), which is consistent with the loss of
rotational freedom of two methylene groups.
[Tf2N] (31.3 kJ mol−1 19 and [Fe(C5H4nBu)(C5H5)][Tf2N] (38.8 kJ
)
mol−1).7a,d The Vogel–Tammann–Fulcher (VTF) equation (η =
η0 exp[DT0/(T − T0)])20 was also used to fit the data, where T0 is
the ideal glass transition temperature and D is a parameter
that shows deviation from Arrhenius behavior. The values T0
and D obtained for [C6Fc][Tf2N] were −57.2 °C and 1.76, and
those for [C10Fc][Tf2N] were −53.6 °C and 1.63, respectively.
The small D values, which are even smaller than that of
[bmim][Tf2N] (D = 4.65),21 suggest that they are strongly fragile
liquids.22
It is interesting to note that the total phase transition entro-
pies of alkyloctamethylferrocenes and the Tf2N salts are
8320 | Dalton Trans., 2013, 42, 8317–8327
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der Waals distances by 0.15–0.36 Å. The Tf2N salts exhibited
no such contacts.
The packing arrangements of the salts depended on the
alkyl chain length in the cation. In the salts with short alkyl
chains ([C3Fc][Tf2N] and [C4Fc][PF6]), the anions and cations
are packed alternately in the unit cell (Fig. 5a and b). [C4Fc]-
[Tf2N] also exhibited a similar structure.7b In contrast, lamellar
structures are formed in the salts with longer alkyl chains
([C6Fc][X] and [C10Fc][X]; X = PF6, NO3) (Fig. 5c–f). In these
crystals, the anions and cations are arranged alternately to
form layers, which are stacked with their alkyl chains extend-
ing outside the layers. The alkyl chains are in contact with
each other between the layers and there are no direct π–π inter-
actions between the cations. The interlayer distances in
[C10Fc][X] are longer than those in [C6Fc][X] as a result of the
longer alkyl chains. In the salts, except for [C10Fc][NO3], the
C5 axes of the ferrocenium moieties are aligned parallel to
the stacking direction of the layers. The difference between the
packing patterns of the long chain and short chain salts,
giving different lattice entropies, accounts for the observed
different tendencies of the phase change entropies. A similar
phenomenon has been observed in cobaltocenium salts.24
Although the crystal structures of the Tf2N salts with long alkyl
chains were not determined, it is likely that the Tf2N salts with
n ≥ 8 form lamellar structures, considering the tendency of
the phase transition entropies in Fig. 4.
Magnetic properties of [C4Fc][Tf2N]
The Tf2N salts are paramagnetic ionic liquids containing mag-
netically anisotropic paramagnetic cations. We investigated
the effects of magnetic fields on their crystallization. A remark-
able effect was found in [C4Fc][Tf2N], which we reported pre-
viously in a communication7b and briefly describe here. As
shown in Fig. 6 (open circles), when the liquid was cooled
Fig. 5 Packing diagrams of (a) [C3Fc][Tf2N], (b) [C4Fc][PF6], (c) [C6Fc][PF6], (d)
[C6Fc][NO3], (e) [C10Fc][PF6], and (f) [C10Fc][NO3]. Hydrogen atoms have been
omitted for clarity.
Crystal structures
The crystal structures of [C3Fc][Tf2N], [C4Fc][PF6], [C6Fc][X],
and [C10Fc][X] (X = PF6, NO3) were determined at −100 °C.
Their packing diagrams are shown in Fig. 5. The structure of
[C4Fc][Tf2N] was reported previously.7b [C3Fc][Tf2N] belongs to
ˉ
space group C2/c, [C4Fc][PF6] and [C10Fc][NO3] belong to P1,
and the others belong to P21/c. In the alkyl chains of the salts,
the C–C bond adjacent to the Cp ring exhibited the gauche con-
formation, whereas the other bonds exhibited the trans confor-
mation. [C4Fc][PF6] exhibited disorder in the alkyl chains. The
intramolecular Fe–C(Cp) bond lengths, of which Fe–CH was
the shortest, were comparable to those of substituted octa-
methylferrocenium cations.23 In all the salts, the C5 axes of the
ferrocenium moieties were aligned in nearly the same direc-
tion in the crystal. Weak hydrogen bonds were formed in the
NO3 and PF6 salts between the ring hydrogen of the cation and
the oxygen or fluorine atom in the anion (Fig. S3 in the ESI†).
Fig. 6 Temperature dependence of the magnetic susceptibilities of [C4Fc]-
[Tf2N] in the form of a χT–T plot. The data measured under 0.5 T are shown by
open circles, and those for a sample crystallized under a perpendicular field of
The CH⋯X (X = O or F) distances were shorter than the van 0.59 T are shown by filled circles (heating run).
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under a magnetic field (0.5–2 T), the magnetic susceptibility
suddenly increased upon solidification at 26 °C, and, when
heated, the value decreased again to the liquid value at 36 °C
upon melting. The magnetic field affected the susceptibility
increase, which increased with increasing field strength, but
not the phase transition temperature.7b A similar magnetic
response was also found in a ferrocenium ionic liquid [Fe-
(C5H4Et)2][Tf2N],7d albeit with less reproducibility and controll-
ability. We ascribe this phenomenon to magnetic orientation,
and detailed investigations of the phenomenon are described
below. To confirm the magnetic orientation of the field-cooled
sample, we first measured the anisotropy of the magnetic sus-
ceptibility. A liquid sample of [C4Fc][Tf2N] was crystallized
under a magnetic field of 0.59 T, and the magnetic suscepti-
bility (χT value) of this sample measured under a perpendicu-
lar field was 0.63 emu K mol−1 (Fig. 6, filled circles), which
was smaller than that in the liquid state, whereas the value
measured under the parallel field increased. This result
directly demonstrates that the orientation of the crystal is con-
trolled by the direction of the magnetic field.
Fig. 7 Angular dependence of ESR spectra recorded at intervals of 10°. Inset
shows the angular dependences of g-value at −269.4 °C, measured on a sample
of [C4Fc][Tf2N] crystallized under a magnetic field of 0.8 T. θ is defined as the
angle between the magnetic fields used for crystallization and ESR
measurements.
The magnetic susceptibility of [C4Fc][Tf2N] solidified under
a field of 0.8 T was 1.36 emu K mol−1 when measured under a
parallel field,7b whereas the value for a fully oriented sample is
calculated to be 1.73 emu K mol−1, using χT = [g2S(S + 1)]/8,
and intensities of the g and g⊥ peaks were obtained by fitting
where g = 4.3 (vide infra) and S = 1/2. Assuming that there are
k
k
the spectra, and then the peak areas of the angle-dependent
and -independent components were calculated from the simu-
lated spectra for θ = 0° and θ = 90°, respectively. The ratio of
the former component, which corresponds to the ratio of the
oriented fraction, was 0.6. The value is again in good agree-
ment with that estimated from the magnetic susceptibility data
(0.61) as shown above.
only two components in the sample, a fully oriented fraction
and a non-oriented isotropic fraction, the ratio of the former
fraction was calculated to be 0.61 from the susceptibility
values.
The magnetic orientation was further investigated using
ESR spectroscopy. In the ESR spectra of an oriented polycrys-
talline sample crystallized under 0.8 T, a sharp, symmetrical
peak was observed below −233 °C for a parallel orientation of
the magnetic fields for crystallization and ESR measurements
Mechanism of magnetic orientation in [C4Fc][Tf2N]
(Fig. S4a in the ESI†). In contrast, asymmetric peaks typical of To investigate the mechanism of the magnetic-field effect, the
a non-oriented material were observed for the sample crystal- direction of the magnetic orientation was examined using
lized without magnetic fields (Fig. S4b in the ESI†). Further- powder X-ray diffraction (XRD) measurements. A liquid sample
more, a clear angular dependence was observed in the ESR of [C4Fc][Tf2N] was placed on a glass plate and crystallized
spectra of the oriented sample recorded at −269.4 °C, as under the magnetic field (0.36 T) of a neodymium magnet
shown in Fig. 7. An upfield shift and broadening of the peak placed below the plate. The XRD patterns showed a significant
were observed with increasing angle (θ) between the magnetic increase in the intensity of the peaks at 2θ = 13°, 14.5°, and
fields for the measurement and crystallization. The g com- 23° for the out-of-plane measurement compared with that of
k
ponent along the C5 axis of the cation is observed for θ = 0°, an unoriented sample (Fig. S6 in the ESI†), but the crystal
and the g⊥ component appears with sample rotation. The structure was unchanged by the magnetic field. The peak at
small residual peak of the g component observed even for 2θ = 14.5° was indexed as (200), which shows that the a-axis,
k
θ = 90° is a non-oriented fraction. Thus, the spectrum for each along which the C5 axis of the cation is oriented, is oriented
rotation angle contains the angular-dependent component of along the magnetic field, in agreement with the magnetic an-
the oriented fraction and the angular-independent component isotropy of the crystal.7b In repeated experiments on the
of the isotropic fraction (Fig. S5 in the ESI†). The g-values oriented samples, differences were observed in the relative
obtained by fitting the angular dependence (Fig. 7, inset) were peak intensities; this results from an in-plane distribution of
g = 4.3, g⊥ = 1.7, and gav. = 2.84, which are comparable to the the growth direction. The crystal growth pattern on the surface
k
typical values for ferrocenium salts such as [Fe(C5Me5)2][PF6] accompanying the radial distribution was visually observable
(g = 4.43, g⊥ = 1.35, gav. = 2.78) and [Fe(C5Me4H)2][BF4] under a polarized optical microscope.7b
k
(g = 4.11, g⊥ = 1.42, gav. = 2.64).25 The average g-value calculated
The effect of the sample shape on the magnetic response
k
from the χT value in the liquid state (0.78 emu K mol−1) was 2.87, was also investigated. For liquid samples in fine tubes (inner
which is in good agreement with the analysis. The linewidths diameter 0.2 mm) or in the form of thin layers (thickness
8322 | Dalton Trans., 2013, 42, 8317–8327
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0.5 μm), no magnetic orientations were observed, even under the direction of the change was random, indicating that no
7 T. Magnetic orientation was also inhibited by adding quartz orientation is caused by the magnetic fields. The χT value in
wool to the liquid.7b In these circumstances, the orientation of the liquid state was 0.76 emu K mol−1, which increased or
the crystal growth may be affected largely by the surface rather decreased by less than 0.05 emu K mol−1 upon crystallization,
than by the external fields. These results indicate that mag- and the value further increased or decreased by about 0.1 emu
netic orientation is a bulk property, and that the phenomenon K mol−1 at the solid phase transition temperature. The
is not attributable to the orientation of independent mole- absence of magnetic orientation in this salt is probably corre-
cules, but is more likely to be a result of orientation of the lated with the rotational motion of the ferrocenium moiety,
crystal nucleus or crystal growth direction during crystalliza- probably present in the disordered phase, which reduces the
tion. This seems to be consistent with the less controllable magnetic anisotropy of the crystal. The larger change in sus-
response in a ferrocenium ionic liquid, [Fe(C5H4Et)2][Tf2N], ceptibility observed at the solid phase transition is ascribed to
with a much lower viscosity than the present salt,7d because the increased magnetic anisotropy of the crystal caused by
the slower crystallization of high-viscosity liquids may lead to ordering of the cation. In the solid phase transition, the orien-
better magnetic-field orientation. There might be a critical size tations of the cations are fixed according to the crystal lattice;
of crystal nucleus for magnetic orientation, and further investi- hence they are not affected by magnetic fields.
gations may be needed to explore its microscopic mechanism.
These results show that not all ferrocenium ionic liquids
The above results unfortunately suggest that fabrication of show a magnetic orientation effect, despite the magnetic an-
thin-layer magnetic memory devices based on this mechanism isotropy of the molecule, and that the phenomenon is instead
is unfeasible.
related to the crystal properties. The orientation mechanism is
therefore significantly different from the well-known magnetic
orientation of diamagnetic liquid crystal molecules. The mag-
netic-field alignments of materials upon solidification have
attracted much interest,26 and magnetic susceptibility changes
coupled with liquid–solid transformations have also been
observed in several molecular materials such as paramagnetic
ionic liquids4,27 and a solid composed of radical dimers.28 The
magnetic orientation of metallomesogens coupled with a
phase transition to a liquid crystalline phase is an interesting
phenomenon.29 In [C4Fc][Tf2N], the magnetic orientation
occurs near room temperature with hysteresis and with a large
susceptibility change, the degree of which can be controlled by
the strength of the applied field. These are unique magnetic
switching properties not seen in other materials.
Magnetic properties of [C5′Fc][Tf2N] and [C12Fc][Tf2N]
In contrast to [C4Fc][Tf2N], no significant magnetic orientation
was observed in [C5′Fc][Tf2N] and [C12Fc][Tf2N]. The magnetic
susceptibility of [C5′Fc][Tf2N] increased by only 0.05 emu K
mol−1 (0.73 emu K mol−1) upon crystallization at −16 °C under
2 T (Fig. S7 in the ESI†) and, with heating, the value returned
to the liquid value at 57 °C. Although the crystal structure is
unknown, the negligible magnetic change can probably be
ascribed to the small magnetic anisotropy of the crystal, if any.
When [C12Fc][Tf2N] was cooled from the liquid phase, it
crystallized into a disordered phase at 25 °C, which further
exhibited a phase transition in the solid state at around 7 °C.
The temperature dependences of the magnetic susceptibility
upon cooling under 0.5 T, 1 T, and 7 T are shown in Fig. 8.
Repeated experiments showed that this salt exhibited small,
random changes of the χT values at the phase transitions, but
3. Conclusion
A series of alkyloctamethylferrocenium salts with Tf2N, PF6,
and NO3 anions ([Fe(C5Me4CnH2n+1)(C5Me4H)][X]) were pre-
pared. Investigation of their thermal properties, phase tran-
sition behaviors, and crystal structures revealed the following.
The Tf2N salts are ionic liquids that are stable under air. The
alkyl chain length dependences of the melting points are
similar to those observed for ionic liquids of alkylimidazolium
salts. Most of the salts exhibit phase transitions in the solid
state. In the salts with short alkyl chains, the sum of the phase
change entropies is nearly constant, whereas the salts with
longer alkyl chains display an entropy increase with elongation
of the alkyl chain. Crystal structure determinations revealed
that the short chain salts exhibit alternate stacking of cations
and anions, whereas the long chain salts form lamellar struc-
tures, where the alkyl chains are aligned between the layers.
The effect of magnetic fields on crystallization of the Tf2N salts
was investigated in detail. The magnetic orientation pheno-
menon in the salt with n = 4 upon crystallization originates
from the magnetic anisotropy of the crystal structure, which is
Fig. 8 Temperature dependences of magnetic susceptibilities of [C12Fc][Tf2N]
in the form of χT–T plots, measured under 0.1 T (dotted line), 1 T (solid line),
and 7 T (broken line) in the cooling processes.
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a bulk property. Further investigation of these functional concentrated under reduced pressure. The crude product was
liquids should lead to novel molecular electronic applications.
purified by column chromatography (alumina, pentane). The
product was obtained as a yellow oil (0.253 g, yield 82.8%). H
1
NMR (400 MHz, CDCl3, TMS): δ = 0.90 (t, 3H, J = 7.6 Hz), 1.29
(q, 4H, J = 7.5 Hz), 1.65–1.73 (br, 24H), 2.17 (t, 2H, J = 7.4 Hz),
3.21 (s, 1H).
The other alkyloctamethylferrocenes were prepared in a
similar manner, and trans-1-pentenyloctamethylferrocene was
obtained by dehydration of 1-hydroxypentyloctamethylferro-
cene (see ESI†).
Experimental
General
Octamethylformylferrocene30 and silver bis(trifluoromethylsul-
fonyl)amide (AgTf2N)31 were synthesized according to litera-
ture methods. The preparations of butyloctamethylferrocene
and its Tf2N salt ([C4Fc][Tf2N]) were reported previously.7b
Other reagents and solvents were commercially available. 1H
NMR spectra were recorded using a JEOL JNM-ECL-400
spectrometer. Elemental analyses were performed using a
Yanaco CHN MT5 analyzer. DSC measurements were per-
formed using a TA Instrument Q100 calorimeter in the temp-
erature range −180 to 210 °C at a rate of 10 °C min−1. The
viscosities of the liquids were measured under an N2 atmos-
phere using a Toki Sangyo TV-22 viscometer with a 3° × R7.7
cone rotor, in the temperature range 14–36 °C. XRD data for
the powder samples were recorded on a Rigaku SmartLab dif-
fractometer using Cu Kα radiation (λ = 1.54056 Å). ESR spectra
were recorded on an X-band JEOL TE-260 spectrometer in the
temperature range −269.4 to −233 °C. For the ESR measure-
ments, a powder sample of [C4Fc][Tf2N] was loaded into a
quartz tube, which was sealed under nitrogen, and the sample
was heated to melt it. The field-oriented samples were pre-
pared by cooling the sample to crystallize under the magnetic
field of the ESR magnet (0.8 T). The non-oriented samples
were obtained by the same procedure, without applying mag-
netic fields.
Preparation of Tf2N salts
[C3Fc][Tf2N]. In the dark, AgTf2N (0.232 g, 0.60 mmol) was
added to a solution of propyloctamethylferrocene (0.200 g,
0.59 mmol) in CH2Cl2 (10 mL). After stirring the solution for a
few minutes, the mixture was filtered via a syringe equipped
with
a membrane filter to remove silver deposits and
unreacted AgTf2N. Removal of the solvent from the filtrate
under reduced pressure gave the product as a dark-green solid
(0.336 g, 92% yield), which was recrystallized from ethanol/
hexane. Dark-green plate crystals. Anal. Calcd (%) for
C23H32F6FeNO4S2 (620.47): C, 44.52; H, 5.20; N, 2.26. Found: C,
44.58; H, 5.23; N, 2.32. The other Tf2N salts were prepared in a
similar manner. For the purification of [C10Fc][Tf2N] and
[C12Fc][Tf2N], they were dissolved in methanol and washed
repeatedly with hexane to remove impurities. The solvents
were evaporated under reduced pressure, and the residue was
dried under vacuum at 80 °C for one day. [C5Fc][Tf2N]. Dark-
green oil, 99% yield. Anal. Calcd (%) for C25H36F6FeNS2O4
(648.53): C, 46.30; H, 5.60; N, 2.16. Found: C, 46.51; H, 5.74; N,
2.13. [C5′Fc][Tf2N]. Dark-green powder, 79% yield. Anal. Calcd
(%) for C25H34F6FeNO4S2 (646.51): C, 46.44; H, 5.30; N, 2.17.
Found: C, 46.54; H, 5.39; N, 2.26. [C6Fc][Tf2N]. Dark-green oil,
Preparation of propyloctamethylferrocene
(a) 1-Hydroxypropyloctamethylferrocene. All the manipulations 95% yield. Anal. Calcd (%) for C26H38F6FeNO4S2 (662.55): C,
were carried out under a nitrogen atmosphere. Ethyl bromide 47.13; H, 5.78; N, 2.11. Found: C, 47.27; H, 5.87; N, 2.13.
(1.0 mL, 12.8 mmol) was added dropwise to a stirred mixture [C8Fc][Tf2N]. Dark-green oil, 98% yield. Anal. Calcd (%) for
of magnesium turnings (0.330 g, 13.6 mmol) and ether C28H42F6FeNO4S2 (690.60): C, 48.70; H, 6.13; N, 2.03. Found: C,
(15 mL), and the solution was stirred until most of the mag- 48.52; H, 6.18; N, 2.13. [C10Fc][Tf2N]. Dark-green oil, 68.3%
nesium dissolved. The Grignard reagent thus prepared was yield (total yield from octamethylferrocene). Anal. Calcd (%)
added slowly to a solution of octamethylformylferrocene for C30H46F6FeNO4S2 (718.66): C, 50.14; H, 6.45; N, 1.95.
(0.30 g, 1.0 mmol) in dry THF (20 mL) cooled at −78 °C, and Found: C, 49.91; H, 6.55; N, 2.02. [C12Fc][Tf2N]. Dark-green
the solution was stirred for 30 min. The reaction mixture was solid, 92% yield. Anal. Calcd (%) for C32H50F6FeNO4S2
quenched at −78 °C with saturated NH4Cl solution (20 mL) (746.71): C, 51.47; H, 6.75; N, 1.88. Found: C, 51.44; H, 6.58;
and extracted with ether. The organic layer was dried over mag- N, 1.98. [C17Fc][Tf2N]. Dark-green solid, 38.3% yield (total
nesium sulfate and concentrated under reduced pressure. The yield from octamethylferrocene). Anal. Calcd (%) for
crude product was purified by column chromatography (silica, C37H60F6FeNO4S2 (816.84): C, 54.40; H, 7.40; N, 1.71. Found: C,
CH2Cl2). The product was obtained as a yellow solid (0.324 g, 54.18; H, 7.46; N, 1.96.
1
99.9% yield). H NMR (400 MHz, CDCl3, TMS): δ = 0.89 (t, 3H,
J = 7.6 Hz), 1.69–1.89 (m, 26H), 2.33 (s, 2H), 3.55 (s, 1H), 4.28
Preparation of PF6 salts
(t, 1H, J = 7.0 Hz). (b) Propyloctamethylferrocene. A solution of [C4Fc][PF6]. In the dark,
a solution of AgPF6 (51 mg,
BH3·SMe2 (6.3 mmol) in THF (3.0 mL, 2 m solution) was 0.20 mmol) in acetone (5 mL) was added to a solution of butyl-
added to a stirred solution of 1-hydroxypropyloctamethylferro- octamethylferrocene (55 mg, 0.16 mmol) in acetone (10 mL).
cene (0.320 g, 0.90 mmol) in THF (15 mL). After refluxing the After stirring the solution for a few minutes, the mixture was
solution for 1 h, the reaction was quenched with aqueous filtered via a syringe equipped with a membrane filter to
NH4Cl (20 mL), and the solution was extracted with CH2Cl2. remove silver deposits and unreacted AgPF6. Removal of the
The organic layer was dried over magnesium sulfate and solvent from the filtrate under reduced pressure gave a solid,
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which was dissolved in CH2Cl2 (10 mL) and washed with pressure, and the residue was dried under vacuum at 80 °C for
water. After drying the organic phase over magnesium sulfate, one day. Anal. Calcd (%) for C28H46FeNO3 (500.51): C, 67.19;
removal of the solvent gave the product as a dark-green solid H, 9.26; N, 2.80. Found: C, 66.01; H, 9.07; N, 2.74. Recrystalli-
(57 mg, 74% yield), which was recrystallized from ethanol zation from acetone–ether–pentane (1 : 30 : 70) gave a small
(−50 °C). Green plate crystals. Anal. Calcd (%) for C22H34FeF6P amount of single crystals.
(499.31): C, 52.62; H, 6.72. Found: C, 52.92; H, 6.86. The other
PF6 salts were prepared in a similar manner to that described
above. [C6Fc][PF6]. Green plate crystals, 93% yield. Anal. Calcd
(%) for C24H38FeF6P (527.37): C, 54.66; H, 7.26. Found: C,
54.45; H, 7.26. [C10Fc][PF6]. Green plate crystals, 63.3% yield.
To remove impurities, the product was dissolved in methanol
and washed repeatedly with hexane. Anal. Calcd (%) for
C28H46FeF6P (583.47): C, 57.67; H, 7.95. Found: C, 57.35; H,
7.79.
Magnetic measurements
Magnetic measurements were carried out using a Quantum
Design MPMS-XL7 SQUID susceptometer; 30–60 mg of
samples were loaded in a quartz tube and used for the
measurements. Samples of [C4Fc][Tf2N] in confined spaces
were prepared as follows. A glass capillary (inner diameter
0.2 mm, outer diameter 0.3–0.4 mm) was filled with a liquid
sample of [C4Fc][Tf2N] (11 mg), and cut into lengths of 5 mm.
The short capillary-tubes were aligned and loaded into a
quartz tube for SQUID measurements. To prepare thin-layer
samples of thickness 25 μm, a liquid sample of [C4Fc][Tf2N]
was loaded into a quartz sample-tube, in which 200 sheets of
spacers and separators prepared from copper TEM grids (dia-
meter 3 mm, thickness 25 μm) were alternately carefully
immersed. The magnetic susceptibility data were corrected for
the diamagnetic contribution so that the χT values became
constant in the high-temperature region. The data for [C4Fc]-
[Tf2N] and [C5′Fc][Tf2N] were further corrected for a very small
amount of ferromagnetic impurities (3 × 10−5 emu) from the
magnetization curve.
Preparation of NO3 salts
[C6Fc][NO3]. In the dark, a solution of AgNO3 (32 mg,
0.18 mmol) in water (2 mL) was added to a solution of hexyl-
octamethylferrocene (50 mg, 0.13 mmol) in acetone (2 mL)
under stirring. After stirring for a few minutes, silver deposits
and unreacted AgNO3 were removed by filtration. The solvent
was evaporated under reduced pressure, and the residue was
dissolved in CH2Cl2 (50 mL) and washed with water (50 mL).
The organic layer was dried over magnesium sulfate, and eva-
poration of the solvent under reduced pressure gave the product
as a green powder in 85% yield; it was recrystallized from
acetone–ether–pentane (1 : 30 : 70). Green plate crystals. Anal.
Calcd (%) for C24H36FeNO3 (444.22): C, 64.86; H, 8.62; N, 3.15.
X-ray structure determination
Found: C, 64.52; H, 8.62; N, 3.24. The other nitrate salts were Single crystals for X-ray structure determination were obtained
prepared in a similar manner. [C10Fc][NO3]. Green powder, by recrystallization. [C3Fc][Tf2N] was recrystallized from
48% yield from octamethylferrocene. To remove impurities, ethanol at −14 °C, and [C4Fc][PF6], [C6Fc][PF6], and [C10Fc]-
the product was dissolved in methanol and washed repeatedly [PF6] from ethanol at −50 °C. [C6Fc][NO3] and [C10Fc][NO3]
with hexane. The solvent was evaporated under reduced were obtained by slow diffusion of ether/pentane into acetone
Table 3 Crystallographic parameters for [C3Fc][Tf2N], [C4Fc][PF6], [C6Fc][PF6], [C6Fc][NO3], [C10Fc][PF6], and [C10Fc][NO3]
[C3Fc][Tf2N]
[C4Fc][PF6]
[C6Fc][PF6]
[C6Fc][NO3]
[C10Fc][PF6]
[C10Fc][NO3]
Empirical formula
Formula weight
C23H32F6FeNO4S2
620.49
C22H34F6FeP
499.31
C24H38F6FeP
527.36
C24H38FeNO3
444.40
C28H46F6FeP
583.47
C28H46F6FeP
500.51
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
Monoclinic
C2/c (No. 15)
16.389(3)
18.944(3)
19.262(3)
Triclinic
P1 (No. 2)
Monoclinic
P21/c (No. 14)
16.0571(16)
8.9074(9)
Monoclinic
P21/c (No. 14)
17.982(5)
8.762(2)
17.319(5)
Monoclinic
P21/c (No. 14)
19.6648(18)
8.7837(8)
Triclinic
P1 (No. 2)
ˉ
ˉ
12.6489(14)
13.2409(15)
13.8307(16)
91.767(2)
91.233(2)
94.617(2)
2307.1(5)
4
8.0742(13)
9.8377(16)
17.596(3)
90.473(2)
92.713(3)
97.649(3)
1385.5(4)
2
17.6910(18)
17.5494(16)
113.192(2)
96.246(2)
115.567(4)
102.498(2)
γ/°
Volume/Å3
5497.1(16)
8
1.499
2515.3(4)
4
1.393
2461.6(11)
4
1.199
2959.5(5)
4
1.310
Z
dcalcd/g cm−3
1.438
0.777
1.201
0.572
μ/mm−1
0.771
0.717
0.635
0.616
Reflections collected
Independent reflections
F(000)
14 381
5195
2568
12 027
8005
1044
595
0.0400, 0.1055
0.0511, 0.1133
1.040
12 924
4741
1108
11 578
4324
956
15 520
5600
1236
6926
4763
542
307
0.0349, 0.1051
0.0403, 0.1083
1.091
Parameters
343
298
271
334
R1 a, wR2 b (I > 2σ(I))
R1 a, wR2 b (all data)
Goodness-of-fit on F2
0.0390, 0.1087
0.0447, 0.1135
1.068
0.0641, 0.1901
0.0772, 0.2038
1.050
0.0584, 0.1196
0.0808, 0.1296
0.972
0.0392, 0.1061
0.0546, 0.1175
1.032
a R1 = Σ||Fo| − |Fc||/Σ |Fo|. b wR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2
.
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solutions (acetone–ether–pentane
=
1 : 30 : 70). XRD data
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were collected with a Bruker Smart1000 CCD diffractometer
using Mo Kα radiation (λ = 0.71073 Å) at −100 °C (cooling
rate: 1 °C min−1). The structures were solved by direct
methods and refined using SHELXTL.32 The crystallographic
data are listed in Table 3. The crystallographic data for the
structures have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication nos. 900446
([C3Fc][Tf2N]), 900447 ([C4Fc][PF6]), 900448 ([C6Fc][PF6]),
900449 ([C6Fc][NO3]), 900450 ([C10Fc][PF6]), and 900451
([C10Fc][NO3]).
Acknowledgements
This work was supported by KAKENHI (No. 24350073) from
JSPS and the IMS (Institute for Molecular Science) Joint
Studies Program. We thank Dr Yoshito Furuie (Kobe Univer-
sity) for elemental analyses and Masaru Nakama (Crayonsoft
Inc.) for providing a Web-DB system.
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