Synthesis and properties of symmetrical N,N′-bis(alkyl)imidazolium bromotrichloroferrate(III) paramagnetic, room temperature ionic liquids with high short-term thermal stability

Article abstract of DOI:10.1016/j.molliq.2018.06.016

A series of paramagnetic ionic liquids (PILs) based on N,N′-bis(alkyl)imidazolium bromotrichloroferrate(III), [(C_n)₂Im][FeCl₃Br] (Im = imidazole; C_n = ethyl, butyl, hexyl, octyl, decyl, dodecyl), was prepared and compared against the more traditional [C_nC₁Im][FeCl₃Br] (C₁ = methyl) analogs. Both series were extensively characterized by physical, chemical, and computational methods including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), spectroscopy (FTIR and UV‐vis-NIR), magnetic susceptibility, surface tensiometry, density, and viscosity. The X-ray crystal structure of [(C₁₂)₂Im][FeCl₃Br] was also obtained. Notably, all ionic liquids prepared in this study show short-term thermal stabilities exceeding 300 °C in air and have melting points equal to or lower than 25 °C, which is surprisingly low for ionic liquids containing symmetrical cations. In addition, all presented ionic liquids have relatively low viscosities allowing for easier transitioning into applications. Physical property trends are discussed.

Full text of DOI:10.1016/j.molliq.2018.06.016

Accepted Manuscript
Synthesis
and
properties
of
symmetrical
N,N′-
bis(alkyl)imidazolium bromotrichloroferrate(III) paramagnetic,
room temperature ionic liquids with high short-term thermal
stability
Kevin T. Greeson, Nicolas G. Hall, Neil D. Redeker, Jacob C.
Marcischak, Laina V. Gilmore, Jerry A. Boatz, Tammy C. Le,
Jeffrey R. Alston, Andrew J. Guenthner, Kamran B. Ghiassi
PII:
DOI:
Reference:
S0167-7322(18)30957-7
doi:10.1016/j.molliq.2018.06.016
MOLLIQ 9214
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
23 February 2018
1 June 2018
5 June 2018
Please cite this article as: Kevin T. Greeson, Nicolas G. Hall, Neil D. Redeker, Jacob
C. Marcischak, Laina V. Gilmore, Jerry A. Boatz, Tammy C. Le, Jeffrey R. Alston,
Andrew J. Guenthner, Kamran B. Ghiassi , Synthesis and properties of symmetrical N,N′-
bis(alkyl)imidazolium bromotrichloroferrate(III) paramagnetic, room temperature ionic
liquids with high short-term thermal stability. Molliq (2017), doi:10.1016/
j.molliq.2018.06.016
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ACCEPTED MANUSCRIPT
Synthesis and properties of symmetrical N,N’-bis(alkyl)imidazolium bromotrichloroferrate(III)
paramagnetic, room temperature ionic liquids with high short-term thermal stability
Kevin T. Greeson,1 Nicolas G. Hall,1 Neil D. Redeker,1 Jacob C. Marcischak,2 Laina V. Gilmore,1 Jerry
A. Boatz,2 Tammy C. Le,1 Jeffrey R. Alston,1,3 Andrew J. Guenthner,2,4 Kamran B. Ghiassi2*
1 ERC, Incorporated, Air Force Research Laboratory, Edwards AFB, CA 93524, USA
2 Air Force Research Laboratory, Aerospace Systems Directorate, Edwards AFB, CA 93524, USA
3
Joint School of Nanoscience and Nanoengineering, North Carolina A&T State University,
Greensboro, NC 27401, USA
4
Nano Hydrophobics Inc., Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
ABSTRACT
A series of paramagnetic ionic liquids (PILs) based on N,N’-bis(alkyl)imidazolium
bromotrichloroferrate(III), [(Cn)2Im][FeCl3Br] (Im = imidazole; Cn = ethyl, butyl, hexyl, octyl, decyl,
dodecyl), was prepared and compared against the more traditional [CnC1Im][FeCl3Br] (C1 =
methyl) analogs. Both series were extensively characterized by physical, chemical, and
computational methods including differential scanning calorimetry (DSC), thermogravimetric
analysis (TGA), spectroscopy (FTIR and UV-vis-NIR), magnetic susceptibility, surface tensiometry,
density, and viscosity. The X-ray crystal structure of [(C12)2Im][FeCl3Br] was also obtained.
Notably, all ionic liquids prepared in this study show short-term thermal stabilities exceeding 300
°C in air and have melting points equal to or lower than 25 °C, which is surprisingly low for ionic
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liquids containing symmetrical cations. In addition, all presented ionic liquids have relatively low
viscosities allowing for easier transitioning into applications. Physical property trends are
discussed.
KEYWORDS
Ionic liquid, imidazolium, symmetry, thermal stability, magnetism, crystallography
INTRODUCTION
Ionic liquids (ILs) have recently come to represent an important class of compounds due
to their versatility in applications and materials chemistry. Conventionally, ionic liquids are
defined as salts with melting points below 100 °C, although room temperature ionic liquids
(RTILs) are often further specified when the substance is a liquid under standard laboratory
conditions.1-2 ILs have been utilized as reaction solvents, extraction media for environmental
toxins, and even as crystal engineering agents for pharmaceutical applications.1-9 The extremely
low vapor pressure of ILs also provides the potential for use at elevated temperatures without
the hazards and instabilities associated with evaporation, although this particular aspect of IL
applications has received less attention.
Among the many varieties of ILs, paramagnetic ionic liquids (PILs or MILs) are relatively
new and less explored.10 These species commonly derive their magnetic properties from anion
complexation with transition metals (Mn, Fe, Co, Cu) or rare-earth metals (Eu, Gd, Dy) with
unpaired electrons.11-16 Since many ionic liquids contain a halide as the anion, a facile addition of
metal halide will generate the corresponding PIL. The most common cations for PILs include
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quaternary ammonium, imidazolium, pyridinium, and pyrrolidinium while the most common
anion is tetrachloroferrate(III), [FeCl4]–.5, 10, 17-18 From an inorganic and magnetic perspective,
[FeCl4]– is a perfect tetrahedral (Td), high-spin d5 ion. Since halides are common ligands in
transition metal chemistry, the utilization of these transition metal halides is fairly intuitive.
However, there are a few reports of metal-free paramagnetic ionic liquids.18
PILs have been investigated as process liquids for catalysis, cellulose degradation, gas
adsorption, microfluidic processing, and extraction.6, 10 Of note, the Whitesides group invented a
method using PILs for density determination via magnetic levitation.19 In most of these cases, the
PIL serves mainly as a carrier fluid with interesting magnetic properties. The potential for
enhanced stability at high temperatures, however, affords the opportunity for PILs to perform as
multi-functional liquids in high temperature environments. These performance characteristics
could facilitate a wide variety of relatively unexplored applications for these materials in
electronics, aerospace, and power generation applications. For instance, PILs that can survive
high temperatures for short periods of time, possess low viscosities, and have relatively low
surface tension could serve as heat-transfer fluids with magnetically-controlled convection.
To facilitate the exploration of these possibilities, two series of N,N’-substituted
imidazolium-based ionic liquids were investigated. Herein we report the synthesis,
characterization, and properties for two series of alkylated PILs, [(Cn)2Im][FeCl3Br] and
[CnC1Im][FeCl3Br] (Im = imidazole; C1 = methyl; Cn = 2 (ethyl), 4 (butyl), 6 (hexyl), 8 (octyl), 10
(decyl), 12 (dodecyl). Of note, we observed that the [(Cn)2Im][FeCl3Br] compounds were all room
temperature ionic liquids. Nonetheless, the crystal structure of [(C12)2Im][FeCl3Br] was also
obtained. Comprehensive thermal and physical characterization including differential scanning
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calorimetry (DSC), thermogravimetric analysis (TGA), spectroscopy (FTIR and UV-vis-NIR),
magnetic susceptibility, surface tensiometry, density, and viscosity revealed remarkable short-
term thermal stability above 300 °C in air with relatively low viscosity and surface tension values
compared to other ionic liquids (it is important to note that the term “thermal stability” does not
have a single or consistent definition.48 With regard to the work presented here, “short-term
thermal stability” refers to the temperature at which the decomposition onset is measured as 5%
weight loss under ramped temperature experiments). Additionally, theoretical calculations of the
gas-phase enthalpies and free energies for the synthesis of both series of PILs from the addition
of FeCl3•6H2O to the bis(alkyl)imidazolium bromide precursor were performed. These materials
therefore represent promising candidates for new and important applications of ILs where these
special characteristics are required.
RESULTS AND DISCUSSION
Synthesis. The 1-alkyl-3-methylimidazolium bromide intermediates, [CnC1Im]Br (Cn = 2, 4, 6, 8,
10, 12) are prepared readily by reacting 1-methylimidazole with 1-bromoalkane either neat or in
20-21
acetonitrile under reflux conditions.9,
The preparation of symmetrical, bis-alkylated
imidazolium intermediates, [(Cn)2Im]X (Cn = 2, 4, 6, 8, 10, 12; X = Cl or Br), typically involves in situ
treatment of imidazole with sodium hydride followed by two equivalents of alkyl halide.22 An
alternative synthesis involves the in situ reaction of imidazole with a base, such as potassium
carbonate or potassium hydroxide.23-24 Halides such as chloride and bromide are commonly used,
although iodide salts are also reported.20 Alkyl bromides were chosen for this study due to cost,
incorporation of a better leaving group, and to allow further reaction with FeCl3•6H2O (vide
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infra). We explored a different synthesis path involving the treatment of imidazole with sodium
hydroxide to produce the sodium imidazolate salt, NaIm, prior to reacting with the alkyl bromide.
This eliminates the need for strict anaerobic reaction conditions, such as those required when
handling sodium hydride (Scheme 1). This precursor, NaIm, can be stored excluding light and
moisture for months.25 The reaction of NaIm proceeds in the same manner as the sodium hydride
reduction of imidazole, requiring two equivalents of alkyl bromide in tetrahydrofuran. Although
the yields are comparable, but not as high as other literature methods, this route does verify
NaIm as a viable starting material.
Both series [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br] (Cn = 2, 4, 6, 8, 10, 12) can be prepared
by mixing equimolar amounts of imidazolium bromide and FeCl3•6H2O under an atmosphere of
nitrogen.26 Anhydrous FeCl3 can be utilized as well, but the reaction conditions become more
cumbersome due to the anaerobic conditions needed to store FeCl3.27-28 These preparation
methods are analogous for the synthesis of asymmetric imidazolium tetrahaloferrate(III)
compounds (e.g. [C4C1Im][FeCl4] also referred to as [BMIm][FeCl4]).27 The entire [CnC1Im][FeCl4]
analogs have been synthesized previously, along with only one [(Cn)2Im][FeCl4] compound,
[(C4)2Im][FeCl4], leaving almost the entire symmetrical series unexplored.7, 29-33 In addition,
relatively little work has been reported using the [FeCl3Br]– anion.7, 33 The specific compounds
prepared for those studies were [C4C1Im][FeCl3Br] and [C6C1Im][FeCl3Br], focusing on sulfur
extraction of fuels and regioselective benzylation.7, 33 Consequently, no physical or thermal
property data were reported. This gap prompted us to explore both series with the [FeCl3Br]–
anion.
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For facile synthesis, we used the iron(III) chloride hexahydrate starting material. The full
reaction is shown in Scheme 1. The reactions can be performed neat under a blanket of nitrogen
for the entire [CnC1Im][FeCl3Br] series and for Cn = 2, 4, 6, 8, and 10 of the [(Cn)2Im][FeCl3Br] series,
owing to the fact that the products are liquids at room temperature (25 °C). For the remaining
compound, [(C12)2Im][FeCl3Br], a solvent (methanol) was utilized to allow for proper mixing since
the product is solid at room temperature. After a sufficient amount of stirring, water and solvents
can be removed by vacuum while heating.
Scheme 1. Synthetic pathway for the preparation of [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br].
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Physical Properties. Table 1 shows physical properties and thermal data tabulated from
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), density, viscosity,
surface tensiometry, and magnetic susceptibility experiments. For this study, a standard Schlenk
line and oil heating bath (10-2 torr and 80 °C, respectively) were utilized to remove residual water,
after which the products were stored under an atmosphere of nitrogen in a desiccator
(phosphorous pentoxide as desiccant) until measurement. The samples were verified to be free
of water via TGA, using weight loss after 1 hour at 110 °C as an estimate for volatiles content. All
samples were dried until the estimated water content was <0.5% (Supporting Information). As
expected, for both series, the density at 25 °C (plotted as a function of linear alkyl chain length,
n, in Figure 1) decreases as the alkyl group size increases. In addition, the [CnC1Im][FeCl3Br]
compounds have higher density values than their [(Cn)2Im][FeCl3Br] counterparts due to the
lower volume fraction of alkyl groups for an identical value of n. More specifically, the density
are described very well by a group contribution model of molar volume in which each methylene
segment contributes 16.8 ± 0.2 cm3 mol-1, with the remainder of each molecule contributing
198.7 ± 1.7 cm3 mol-1. These values were determined by a linear regression of molar volume on
the number of methylene groups, that is, n-1 for the [CnC1Im][FeCl3Br] compounds and 2(n-1) for
the [(Cn)2Im][FeCl3Br] compounds. This group contribution value for methylene segments may
include contributions from both the actual volume required to add the segment, as well as a small
positive contribution due to the decrease in ion density associated with the addition of each
segment.
The kinematic viscosity values (25 °C) for both series also behave as expected. Viscosity
increases as a function of alkyl chain length. While the [CnC1Im][FeCl3Br] series viscosity values
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gradually rise to 87 cSt, the [(Cn)2Im][FeCl3Br] compounds rise sharply in comparison to 212 cSt,
as illustrated in Figure 2. These increases are mainly driven by increasing molecular size
characteristics, with the [(Cn)2Im][FeCl3Br] compounds being considerably larger for the same
value of n due to their two-armed architecture.
Surface tension values (25 °C) for both series were determined using the pendant drop
method34-35 and are shown in Figure 3 as a function of alkyl chain length. The [CnC1Im][FeCl3Br]
compounds have higher surface tension values compared to the [(Cn)2Im][FeCl3Br] analogs,
ranging from 33.8 to 52.3 mN m-1. However, the [(Cn)2Im][FeCl3Br] are quite close ranging from
29.2 to 50.5 mN m-1. Both series decrease in surface tension as alkyl chain length increases. The
lowest value observed, 29.2 mN m-1 for [(C12)2Im][FeCl3Br], is characteristic of long-chain alkanes
that includes some polar moieties, such as 1-decanol (29.6 mN m-1 at 10 °C )
Solubilities were determined for both series using hexanes, toluene, dichloromethane,
chloroform, tetrahydrofuran, diethyl ether, acetone, acetonitrile, methanol, and water. The
results are presented in Table 2. It is interesting to note that both series are insoluble in hexanes
and display solubility in the remaining nine solvents. Other reports of similar compounds have
labeled this class of PILs as “magnetic surfactants.”36 However, to our knowledge, this is the first
report focusing on symmetrical [(Cn)2Im][FeCl3Br] imidazolium species, possibly owing to the
generally higher melting points found in symmetrical species.37 It is therefore intriguing that we
were able to produce RTILs with symmetrical alkylated species.
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Figure 1. Density plot as a function of alkyl chain length for the [(Cn)2Im][FeCl3Br] and
[CnC1Im][FeCl3Br] series.
Figure 2. Viscosity plot as a function of alkyl chain length for the [(Cn)2Im][FeCl3Br] and
[CnC1Im][FeCl3Br] series.
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Figure 3. Surface tension plot as a function of alkyl chain length for the [(Cn)2Im][FeCl3Br] and
[CnC1Im][FeCl3Br] series.
Table 1. Physical and thermal properties of the [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br] series.
Compound
density viscosity
surface
tension
melting
short-term
thermal
stability
(°C)e
χmol
(m3
μeff
(μB)
(g cm-3)a
(cSt)b
point (°C)d
(mN m-1)c
mol-1 x
10-7)
1.77
1.76
1.76
1.73
1.75
1.79
50.5
41.3
37.1
34.5
31.7
29.2
[(C2)2Im][FeCl3Br]
[(C4)2Im][FeCl3Br]
[(C6)2Im][FeCl3Br]
[(C8)2Im][FeCl3Br]
[(C10)2Im][FeCl3Br]
[(C12)2Im][FeCl3Br]
1.573
1.397
1.300
1.241
1.158
1.142
12.8
42.6
58.7
97.8
136.9
212.5
–0.1
--
--
--
4.6
25.1
331
310
307
300
305
309
5.73
5.71
5.69
5.65
5.69
5.75
[C2C1Im][FeCl3Br]
[C4C1Im][FeCl3Br]
[C6C1Im][FeCl3Br]
[C8C1Im][FeCl3Br]
[C10C1Im][FeCl3Br]
[C12C1Im][FeCl3Br]
1.627
1.527
1.447
1.385
1.334
1.286
15.2
21.1
32.4
43.0
69.2
86.8
52.3
45.8
42.6
38.6
34.8
33.8
19.4
--
--
--
-9.4
14.5
327
335
318
311
315
307
1.78
1.73
1.76
1.78
1.73
1.70
5.73
5.64
5.71
5.73
5.65
5.60
a. Density measured at 25 °C.
b. Kinematic viscosity at 25 °C.
c. Surface tension determined from pendant drop method at 25 °C.
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d. Melting point determined via DSC endotherm and verified via visual melting point
determination. In several cases, denoted by --, melting points could not be observed via
either measurement method.
e. TGA temperature (air, 5 °C/min) at which 5 wt% loss of compound is observed.
Table 2. Solubility profile for the [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br] series.
hexanes toluene CH2Cl2 CHCl3 THF ether acetone CH3CN CH3OH H2O
[(C2)2Im][FeCl3Br]
[(C4)2Im][FeCl3Br]
[(C6)2Im][FeCl3Br]
[(C8)2Im][FeCl3Br]
[(C10)2Im][FeCl3Br]
[(C12)2Im][FeCl3Br]
N
N
N
N
N
N
P
P
P
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
P
P
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
P
P
P
P
Y
Y*
[C2C1Im][FeCl3Br]
[C4C1Im][FeCl3Br]
[C6C1Im][FeCl3Br]
[C8C1Im][FeCl3Br]
[C10C1Im][FeCl3Br]
[C12C1Im][FeCl3Br]
N
N
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
N
N
P/N
Y = soluble; N = insoluble; P = partially soluble; * = emulsion
Magnetic susceptibility. Due to the low cost and abundance of iron, the paramagnetic nature of
d5 iron(III) has drawn considerable attention. Many reports have shown paramagnetism at room
temperature, with high susceptibility to external magnets such as Nd2Fe14B and even lower
strength fields.10, 17, 19, 26-27, 30, 36, 38 From an applications standpoint, this makes chemical
extraction quite appealing especially in biphasic systems. For this study, magnetic susceptibility
was measured using an Evans balance. The molar magnetic susceptibility (χmol) can be measured
using the standard equation:
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퐶
푏푎푙
퐿푀(푅−푅 )
0
휒푚표푙
=
(Eq. 1)
9
10 푚
where Cbal is a balance calibration constant, L is the length of the sample in the tube, M is the
molar mass, R is the instrument reading, R0 is the instrument reading of the unloaded tube, and
m is the sample mass. With χmol determined, the effective magnetic moment (μeff) can also be
readily calculated at the measured temperature, T:
휇푒푓푓 = 2.828 휒푚표푙T
(Eq. 2)
√
For the [(Cn)2Im][FeCl3Br] series, the μeff values range from 5.65 to 5.75 μB. The [CnC1Im][FeCl3Br]
series have similar values ranging from 5.60 to 5.73 μB. For reference, the reported μeff for the
representative compound [C4C1Im][FeCl4] is 5.90 μB.15, 30 Similar values have been reported for
[C8C1Im][FeCl4] and [C10C1Im][FeCl4], with corresponding μeff values of 5.78 and 5.66 μB,
respectively.15, 30 When added to hexanes, we have demonstrated that all of the prepared PILs in
this study can be manipulated by an external magnetic field, as shown in Figure 4.
Figure 4. Photographs of [C12C12Im][FeCl3Br] showing the immiscibility in hexanes (left) and
magnetic susceptibility (right).
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Thermal Properties. The prepared PILs were examined using differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). The second-heat DSC traces are shown in Figures 5
and 6 for [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br], respectively.
Figure 5. Compiled second-heat DSC traces of the [(Cn)2Im][FeCl3Br] series. The y axis has been
offset vertically for clarity. A dashed line is shown to represent conventional ambient
temperature (25 °C).
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Figure 6. Compiled second-heat DSC traces of the [CnC1Im][FeCl3Br] series. The y axis has been
offset vertically for clarity. A dashed line is shown to represent conventional ambient
temperature (25 °C).
The DSC heat curves indicate the presence of crystalline melting transitions for C2, C10,
and C12 species in both the [CnC1Im][FeCl3Br] and [(Cn)2Im][FeCl3Br] series. Under the given
experimental conditions, no crystalline melt was observed for the C4, C6 and C8 compounds in
either series. Further investigation via visual melting point determination confirmed these
results. Curiously, there is not a dramatic difference in melt temperatures between the
[CnC1Im][FeCl3Br] and [(Cn)2Im][FeCl3Br] series; we observed less than 15 °C difference in melting
point between the symmetric and asymmetric species. This may be due to the bulky nature of
the [FeCl3Br]– anion, which could disrupt alkyl chain crystallization.37
Figures 7 and 8 show combined TGA plots for the [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br]
series, respectively. All compounds show excellent short-term thermal stability with onset of
degradation exceeding 300 °C in air for all cases. Short-term thermal stability appears to be
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inversely correlated with alkyl chain length in both [(Cn)2Im][FeCl3Br] and [CnC1Im][FeCl3Br] series,
likely due to the overall decrease in ionic character of the species as alkyl chain length increases.
The degradation mechanisms for both series are quite complex, as indicated by the multiple steps
in each compound’s trace. This multistep degradation behavior is also observed with the
[CnC1Im][FeCl4] series reported by Zhou and coworkers.29
Figure 7. TGA plot for the [(Cn)2Im][FeCl3Br] series in air. A dashed line is shown to aid the
representation for 5% weight loss. Heating ramp rate was performed at 5 °C/min after an
isothermal temperature hold at 110 °C for one hour.
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Figure 8. TGA plot for the [CnC1Im][FeCl3Br] series in air. A dashed line is shown to aid the
representation for 5% weight loss. Heating ramp rate was performed at 5 °C/min after an
isothermal temperature hold at 110 °C for one hour.
Crystal Structure of [(C12)2Im][FeCl3Br]. Crystals of [(C12)2Im][FeCl3Br] were grown by allowing the
compound to melt during the day and recrystallize overnight. Crystal selection was executed in
the morning while the temperature was lower than 25 °C. It was observed that the focused light
from the microscope accelerated the melting of the material. To combat this issue, two
crystallization dishes were utilized in a “matryoshka doll” fashion. The inner dish held the
microscope slide with the compound while the outer dish held ice to maintain cool temperatures.
Melting was not observed using this method. A suitable crystal was found and immediately
mounted on the goniometer head below a nitrogen cold stream at –183 °C. We attempted to
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obtain crystals of the other materials with suitable melting points, [(C2)2Im][FeCl3Br],
[(C12)2Im][FeCl3Br], [C2C1Im][FeCl3Br], and [C12C1Im][FeCl3Br] using appropriate refrigerators or
freezers, but were unsuccessful at this time.
The structure is found in the monoclinic setting with space group P21/c, Z = 4. The
asymmetric unit is composed of the salt, [(C12)2Im][FeCl3Br], illustrated in Figure 9. The cation is
fully ordered. The anion shows disorder between the Br and Cl sites. Full refinement details are
provided in the CIF (Supporting Information). The structure shows channels of [FeCl3Br]– anions
between sheets of [(C12)2Im]+ as shown in Figures 10 and 11. These channels are similar to those
seen with other tetrahaloferrate(III) structures. We attempted to produce a different polymorph
of [(DD)2Im][FeCl3Br] by growing crystals in a refrigerator (4 °C), under the 1.5 T field of an
Nd2Fe14B magnet, and within the aligned field of two 1.5 T Nd2Fe14B magnets. Much to our
chagrin, the crystals obtained from these methods were the same form as the room temperature
structure. Interestingly, the crystals obtained under magnetic influence were considerably better
in quality. It is also noteworthy that the structure is devoid of solvent and water.
Figure 9. The asymmetric unit of [(C12)2Im][FeCl3Br] drawn with 50% thermal contours. Hydrogen
atoms and minor sites for Br and Cl are omitted for clarity. The color scheme is as follows: red =
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bromine, green = chlorine, orange = iron, blue = nitrogen, black = carbon. Hydrogen atoms are
omitted for clarity.
Figure 10. The packing diagram for the crystal structure [(C12)2Im][FeCl3Br] looking down the
crystallographic b axis. The color scheme is as follows: red = bromine, green = chlorine, orange =
iron, blue = nitrogen, black = carbon. Hydrogen atoms are omitted for clarity.
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Figure 11. Space-filling model for [(C12)2Im][FeCl3Br]. The color scheme is as follows: red =
bromine, green = chlorine, orange = iron, blue = nitrogen, grey = carbon. Hydrogen atoms are
omitted for clarity.
Reaction Thermodynamic Calculations
Theoretical calculations of the gas phase enthalpies and free energies of reaction were
performed for the PIL synthesis reactions:
[(Cn)2Im][Br] + FeCl3•6H2O → [(Cn)2Im][FeCl3Br] + 6H2O
(Eq. 3)
and:
[CnC1 Im][Br] + FeCl3•6H2O → [CnC1Im][FeCl3Br] + 6H2O
(Eq. 4)
with the results summarized in Table 3. All of the computed reaction enthalpies (free energies)
are endothermic (exergonic) and, with the exception of symmetric n-butyl [(C4)2Im]+, are largely
independent of alkyl chain length. The release of six product water molecules from the starting
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FeCl3•6H2O complex provides an increase in entropy, consistent with the predicted exergonicity
of these reactions. The predicted endothermicity and exergonicity is qualitatively consistent with
the corresponding experimental synthesis, for which the reactions proceed spontaneously but
absorb heat.
Table 3. M06/aPCseg-2//M06/PCseg-2 gas-phase enthalpies and free energies of reactions
shown in Equations 3 and 4 in kJ mol-1.
298
푟푥푛
298
[CnC1Im ][FeCl3Br]; n
∆퐻
∆퐺
푟푥푛
2
79.3
-106.0
-111.5
-104.0
-112.4
-109.9
-110.7
4
71.5
68.9
71.3
71.3
71.3
6
8
10
12
[(Cn)2Im][FeCl3Br]; n
2
58.7
92.6
67.0
67.3
52.1
68.2
-100.9
-96.6
4
6
-116.7
-116.3
-117.9
-122.1
8
10
12
CONCLUSION
Two series of alkyl-substituted imidazolium-based paramagnetic room temperature ionic liquids
show promise as candidates for new types of paramagnetic ionic liquid applications that require
short-term thermal stability above 300 °C in air while retaining low viscosity and surface tension
characteristics. Contrary to expectations, both series show comparable melting points, with both
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symmetrically and asymmetrically substituted compounds exhibiting melting points at a or below
room temperature. Analysis of the X-ray crystal structure of one of these compounds indicates
that disruption of crystalline order by bulky [FeCl3Br]– anions may contribute to the observed low
melting points. The ability to retain room temperature ionic liquid characteristics with both
asymmetric and symmetric substitution patterns substantially increases the range of physical
properties available by providing for a greater range of linear alkyl content. The greater linear
alkyl content only modestly impacts the thermochemical stability of these compounds, with 5%
weight loss temperatures in air becoming relatively independent of alkyl chain length and
exceeding 320 °C for the longest chain lengths studied. In addition, surface energy is significantly
reduced as alkyl chain length is increased. Density functional theory predictions of the reaction
enthalpies and free energies for the formation of the PILs show these reactions to be entropically-
driven and surprisingly independent of alkyl chain length. In combination with magnetic
properties that were confirmed to be typical for imidazolium-based paramagnetic ionic liquids,
these compounds afford performance with the potential to enable new applications in
demanding thermal environments.
EXPERIMENTAL
General Considerations.
All solvents and reagents were obtained commercially and used as received with the exception
of tetrahydrofuran (THF), diethyl ether, and toluene which were dried, deoxygenated, and stored
on a solvent purification system similar to that described by Pangborn and Grubbs.39 NMR
solvents were dried and stored using appropriate molecular sieves. All reactions were performed
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under an atmosphere of dry nitrogen, unless otherwise specified. After preparation, NaIm,
[(Cn)2Im]Br, [CnC1Im]Br, [(Cn)2Im][FeCl3Br], and [CnC1Im][FeCl3Br] compounds were stored in a
desiccator under a blanket of nitrogen. In addition, NaIm was stored in an amber bottle.
Elemental analysis was conducted by Atlantic Microlab, Inc.
Spectroscopic Measurements.
1H and 13C NMR data were collected on a Bruker Avance III HD 400 MHz spectrometer using either
CDCl3 or DMSO-d6 as solvent. Chemical shifts were referenced with respect to solvent. The
intermediate [(Cn)2Im]Br and [CnC1Im]Br compounds agree with literature NMR values. IR data
were collected on a Nicolet iS50 FTIR spectrometer equipped with an ATR accessory. Spectra are
shown in Supporting Information. UV-vis-NIR absorption data were recorded on a Shimadzu UV-
3101PC spectrophotometer from 190-1100 nm. The expected signals at 534, 619, and 688 nm for
the [FeCl3Br]– anion were observed for all corresponding compounds.26
High-resolution Mass Spectrometry (HRMS).
HRMS was performed using ESI-Q-TOF-MS. Data were acquired using a Waters Synapt High
Definition Mass Spectrometer equipped with a non-ESI source in positive and negative modes
employing acetonitrile or methanol as solvent. MS conditions are as follows: capillary voltage (3.0
kV), sample cone voltage (33.0 V), extraction cone voltage (4.0 V), flow rate (1.000 nL/min),
acquisition range (m/z 100-700), source temperature (80 °C).
Physical and Thermal Properties Measurements.
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DSC data were recorded on a TA Instruments Q200 differential scanning calorimeter using a
heat/cool/heat cycle with temperature limits of –85 and 175 °C, a heating rate of 10 °C/min, and
a cooling rate of 5 °C/min. TGA data were recorded in air on a TA Instruments Q5000 from 40 to
600 °C with a heating rate of 5 °C/min. Additional TGA experiments using a heating rate of 10
°C/min are located in Supporting Information. Melting points were determined via DSC and
verified via visual melting point determination when possible. Density was measured on a
Rudolph Research Analytical DDM 2911 density meter at 25 °C. Kinematic viscosity was measured
using a SCHOTT CT-52 capillary viscometer at 25 °C. Surface tension was measured using
acquisition of a silhouette of the axisymmetric pendant fluid droplet.34-35 Iterative fitting of the
Young–Laplace equation was used to balance the gravitational deformation of the drop with the
restorative interfacial tension. A Future Digital Scientific Corporation, Optical Contact Angle
System (OCA-20) was used to capture silhouette images of the ionic liquid pendant drops in air,
within a temperature-controlled chamber at 25 °C. Three separate images of each IL were
analyzed using the open source software package Open Drop v1.1.
Single-Crystal X-ray Diffraction Experiment.
Crystals of [(C12)2Im][FeCl3Br] were prepared and selected as outlined above (Results and
Discussion). A brown plate was immediately mounted in the 90 K cold stream provided by a Cryo
Industries low-temperature apparatus on the goniometer head of a Bruker ApexII CCD
instrument employing Cu Kα (λ = 1.54056 Å) radiation. Data were reduced using Bruker SAINT,
and a multi-scan absorption correction was applied using SADABS.40 Structure solution and
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refinement were executed on SHELXT-2015 and SHELXL-2017, respectively.41-42 Crystallographic
data are reported in the Supporting Information. Refinement details are provided in the CIF.
Computational Methods.
Density functional theory (DFT) methods were utilized to compute the gas phase enthalpies and
free energies of the reactions of the imidazolium bromide ion pairs, [(Cn)2Im]Br and [CnC1Im]Br,
with FeCl3•6H2O to produce [(Cn)2Im][FeCl3Br] and [CnC1 Im][FeCl3Br] plus 6H2O molecules. The
Lebedev quadrature scheme with 250 radial and 1454 angular grid points was used for DFT
numerical integration. The structures of all reactants and products were fully optimized and
confirmed as local minima via diagonalization of the mass-weighted matrix of energy second
derivatives with respect to nuclear displacements; i.e., the hessian matrix. Harmonic vibrational
frequencies were scaled by 0.98343-44 in obtaining zero point vibrational energy corrections. The
M0643-44 meta-generalized gradient approximation (meta-GGA) functional was used in
combination with the segmented polarization consistent basis set (PCseg-2).45 Relative energies
were refined via single point energy calculations using the augmented extension of the PCseg-2
basis set, denoted as M06/aPCseg-2//M06/PCseg-2. Spin unrestricted methods were used in
computing the sextet electronic spin states of iron-containing species FeCl3•6H2O and
[(Cn)2Im][FeCl3Br]. All computations were performed using the GAMESS46-47 quantum chemistry
code.
Synthesis.
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The [(Cn)2Im]Br compounds can prepared according to literature methods.22 However, we utilized
an alternative synthesis path using NaIm as shown in Scheme 1. NaIm was prepared using
imidazole and sodium hydroxide according to a literature procedure that was not modified.25 The
reaction of NaIm with two equivalents of alkyl bromide took place in THF under an atmosphere
of nitrogen, outlined in detail below, using a modified work-up procedure to what was previously
published. The [CnC1Im]Br compounds were prepared according to literature methods. NMR data
for the [(Cn)2Im]Br and [CnC1Im]Br series agree with the literature.22 The [(Cn)2Im][FeCl3Br] series
31
were prepared neat for Cn = 2, 4, 6, 8, 10 according to literature methods.19,
For
[(C12)2Im][FeCl3Br] methanol was used as a solvent since the product is a solid.26 The entire
[CnC1Im][FeCl3Br] series was prepared neat. The characteristic visible spectroscopy bands at 534,
619, and 688 nm were observed for [FeCl3Br]–.26 In addition, samples were submitted for EA
and/or HRMS as required.
[(C2)2Im]Br. To a round-bottom flask equipped with a reflux condenser and magnetic stir bar,
NaIm (5.032 g, 55.87 mmol) was suspended in 100 mL THF. 1-Bromoethane (5.5 eq., 32.967 g,
302.56 mmol) was added to this stirred, yellow suspension. This mixture was refluxed for 24
hours and then allowed to cool to room temperature. The reaction mixture, containing a light
yellow liquor with off-white solids (NaBr), was filtered and washed with ca. 150 mL THF. Solvent
was removed via rotary evaporation and the resulting yellow oil was dissolved in
dichloromethane. This solution was filtered, solvent removed via rotary evaporation, and then
stirred vigorously with ca. 150 mL diethyl ether. The diethyl ether was removed via cannulation
because the low viscosity of the product did not allow for decantation. The washing step with
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diethyl ether was repeated twice, and solvent was removed from the final yellow oil by rotary
evaporation followed by vacuum line (10-2 torr). Yield: yellow oil, 5.327 g, 46.5 %.
[(C4)2Im]Br. This was prepared similarly to [(C2)2Im]Br using NaIm (4.018 g, 44.62 mmol) and 1-
bromobutane (14.021 g, 102.3 mmol). Yield: yellow oil, 9.250 g, 79.4%.
[(C6)2Im]Br. This was prepared similarly to [(C2)2Im]Br using NaIm (4.004 g, 44.47 mmol) and 1-
bromohexane (15.441 g, 93.53 mmol). Yield: yellow oil, 12.241 g, 86.74%.
[(C8)2Im]Br. This was prepared similarly to [(C2)2Im]Br using NaIm (4.660 g, 51.74 mmol) and 1-
bromooctane (20.105 g, 104.11 mmol). Yield: yellow oil, 10.637 g, 55.1%.
[(C10)2Im]Br. This was prepared similarly to [(C2)2Im]Br using NaIm (4.050 g, 44.97 mmol) and 1-
bromodecane (20.020 g, 90.51 mmol). Yield: white foam-like solid, 11.170 g, 57.80%.
[(C12)2Im]Br. This was prepared similarly to [(C2)2Im]Br using NaIm (3.522 g, 39.11 mmol) and 1-
bromododecane (19.277 g, 77.35 mmol). Yield: white solid, 10.817 g, 57.6%.
[(C2)2Im][FeCl3Br]. A 100 mL round-bottom flask with a magnetic stir bar was charged with
[(C2)2Im]Br (5.327 g, 25.97 mmol) and FeCl3•6H2O (7.021 g, 25.97 mmol). The mixture
immediately turned into a viscous red-brown liquid and was flushed with nitrogen for five
minutes. The reaction was stirred overnight. Water was removed in vacuo, forming the product
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in near-quantitative yield. After back-filling with nitrogen, the product was stored in a desiccator.
Mp: –0.1 °C. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C7H13N2 125.1079; Found: 125.1072. [M–]
Calc. for FeCl3Br 239.7598; Found: 239.7681. EA Calcd (%) for C7H13N2FeCl3Br: C, 22.89; H, 3.57;
N, 7.63. Found: C, 22.60; H, 3.48; N, 7.41.
[(C4)2Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [(C4)2Im]Br (4.831 g,
18.50 mmol) and FeCl3•6H2O (5.026 g, 18.59 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C11H21N2 181.1705; Found: 181.1703. EA Calcd (%)
for C11H21N2FeCl3Br: C, 31.20; H, 5.00; N, 6.62. Found: C, 31.28; H, 5.12; N, 6.87.
[(C6)2Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [(C6)2Im]Br (3.301 g,
10.40 mmol) and FeCl3•6H2O (2.857 g, 10.57 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C15H29N2 237.2331; Found: 237.2332. EA Calcd (%)
for C15H29N2FeCl3Br: C, 37.57; H, 6.10; N, 5.84. Found: C, 38.17; H, 6.37; N, 5.16.
[(C8)2Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [(C8)2Im]Br (10.637 g,
28.47 mmol) and FeCl3•6H2O (7.986 g, 29.54 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C19H37N2 293.2957; Found: 293.2959. EA Calcd (%)
for C19H37N2FeCl3Br: C, 42.61; H, 6.96; N, 5.23. Found: C, 42.35; H, 7.15; N, 5.16.
[(C10)2Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [(C10)2Im]Br (11.170 g,
26.01 mmol) and FeCl3•6H2O (7.023 g, 26.01 mmol). Near-quantitative yield, red-brown viscous
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liquid. Mp: 4.6 °C. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C23H45N2 349.3583; Found: 349.3596.
EA Calcd (%) for C23H45N2FeCl3Br: C, 46.69; H, 7.67; N, 4.73. Found: C, 46.62; H, 7.67; N, 4.76.
[(C12)2Im][FeCl3Br]. A flask was charged with [(C12)2Im]Br (10.134 g, 20.87 mmol) and FeCl3•6H2O
(5.574 g, 20.62 mmol). The mixture immediately turned into a red-brown liquid. Methanol (25
mL) was added and the reaction was stirred while flushing with nitrogen for five minutes. The
mixture was stirred for an additional 8 hours. Solvent and water were then removed in vacuo.
Quantitative yield, red-brown viscous liquid. Note: Depending on laboratory temperature, the
product can solidify. Mp: 25.1 °C. HRMS (ESI/Q-TOF) m/z: [M+] Calc. for C27H53N2 405.4209;
Found: 405.4210. EA Calcd (%) for C27H53N2FeCl3Br: C, 50.06; H, 8.25; N, 4.32. Found: C, 50.16; H,
8.61; N, 4.65.
[C2C1Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [C2C1Im]Br (11.233 g,
58.79 mmol) and FeCl3•6H2O (15.851 g, 58.64 mmol). Near-quantitative yield, red-brown viscous
liquid. Mp: 19.4 °C. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C6H11N2 111.0922; Found: 111.0926.
EA Calcd (%) for C6H11N2FeCl3Br: C, 20.40; H, 3.14; N, 7.93. Found: C, 20.48; H, 3.18; N, 7.69.
[C4C1Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [C4C1Im]Br (3.473 g,
15.85 mmol) and FeCl3•6H2O (4.285 g, 15.85 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C8H15N2 139.1235; Found: 139.1229. EA Calcd (%)
for C8H15N2FeCl3Br: C, 25.20; H, 3.96; N, 7.35. Found: C, 24.73; H, 3.85; N, 7.10.
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[C6C1Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [C6C1Im]Br (15.323 g,
61.99 mmol) and FeCl3•6H2O (16.742 g, 61.94 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C10H19N2 167.1548; Found: 167.1540. EA Calcd (%)
for C10H19N2FeCl3Br: C, 29.34; H, 4.68; N, 6.84. Found: C, 28.83; H 4.67; N, 6.63.
[C8C1Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [C8C1Im]Br (17.836 g,
64.80 mmol) and FeCl3•6H2O (17.503 g, 64.75 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C12H23N2 195.1861; Found: 195.1856. EA Calcd (%)
for C12H23N2FeCl3Br: C, 32.95; H, 5.30; N, 6.40. Found: C, 32.35; H 5.37; N, 6.28.
[C10C1Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [C10C1Im]Br (17.628 g,
58.12 mmol) and FeCl3•6H2O (15.704 g, 58.10 mmol). Near-quantitative yield, red-brown viscous
liquid. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C14H27N2 223.2174; Found: 223.2165. EA Calcd (%)
for C14H27N2FeCl3Br: C, 36.12; H, 5.85; N, 6.02. Found: C, 36.20; H 5.84; N, 6.07.
[C12C1Im][FeCl3Br]. This was prepared similarly to [(C2)2Im][FeCl3Br] using [C12C1Im]Br (18.709 g,
56.47 mmol) and FeCl3•6H2O (15.220 g, 56.31 mmol). Near-quantitative yield, red-brown viscous
liquid. Mp: 14.5 °C. HRMS (ESI/Q-TOF) m/z: [M+] Calcd for C16H31N2 251.2487; Found: 251.2466.
EA Calcd (%) for C16H31N2FeCl3Br: C, 38.94; H, 6.33; N, 5.78. Found: C, 39.14; H 6.33; N, 5.68.
ASSOCIATED CONTENT
29