Low-melting salts based on a glycolated cobalt bis(dicarbollide) anion

Article abstract of DOI:10.1021/ic2023665

A new series of low-melting quaternary ammonium salts based on a glycolated cobalt bis(dicarbollide) anion structure have been synthesized and characterized, and their spectroscopic and physicochemical properties have been studied. The lowest melting point was obtained for 1-butyl-3-methylimidazolium (～50 °C) followed by 1-butyl-1-methylpiperidinium (～80 °C), 1-butyl-1-methylpyrrolidinium (～95 °C), and 1-butyl-4-methylpyridinium salts (～115 °C). The salts were thermally stable up to 180 °C [decomposition of an oligo(ethylene glycol) chain] and contained variable amounts of water. The flexible oligo(ethylene glycol) chains contributed to the waxy state of salts. The solubility of the salts was determined for 76 solvents that are commonly used in organic chemistry. Generally, the solubility increased with the dipole moment and relative polarity of the solvent. Salts exhibited good solubility in ketones and esters; moderate solubility was observed in alcohols, aromates, and chlorinated solvents, and poor solubility was obtained in ethers. The salts were practically insoluble in higher hydrocarbons and water. Salts are dissolved in the form of ion pairs or separated ions, depending on the nature of the solvent.

Full text of DOI:10.1021/ic2023665

Article
pubs.acs.org/IC
Low-Melting Salts Based on a Glycolated Cobalt Bis(dicarbollide)
Anion
Jakub Rak,*,†,‡ Milan Jakubek,† Robert Kaplanek,† and Vladimír Kral*,†,‡
́
́
†Department of Analytical Chemistry, Faculty of Chemical Engineering, Institute of Chemical Technology, Technicka
Prague 6, Czech Republic
‡Zentiva R&D, part of Sanofi-Aventis, U Kabelovny 130, 102 37 Prague 10, Czech Republic.
́
5, 166 28
S
* Supporting Information
ABSTRACT: A new series of low-melting quaternary
ammonium salts based on a glycolated cobalt bis(dicarbollide)
anion structure have been synthesized and characterized, and
their spectroscopic and physicochemical properties have been
studied. The lowest melting point was obtained for 1-butyl-3-
methylimidazolium (∼50 °C) followed by 1-butyl-1-methyl-
piperidinium (∼80 °C), 1-butyl-1-methylpyrrolidinium (∼95 °C),
and 1-butyl-4-methylpyridinium salts (∼115 °C). The salts were
thermally stable up to 180 °C [decomposition of an oligo(ethylene
glycol) chain] and contained variable amounts of water. The
flexible oligo(ethylene glycol) chains contributed to the waxy state
of salts. The solubility of the salts was determined for 76 solvents
that are commonly used in organic chemistry. Generally, the
solubility increased with the dipole moment and relative polarity of the solvent. Salts exhibited good solubility in ketones and esters;
moderate solubility was observed in alcohols, aromates, and chlorinated solvents, and poor solubility was obtained in ethers. The salts were
practically insoluble in higher hydrocarbons and water. Salts are dissolved in the form of ion pairs or separated ions, depending on the
nature of the solvent.
solubility of many compounds, high viscosity, and high polarity.
These attributes make them unique stationary phases in gas
chromatography.9 The miscibility of ILs with acetonitrile facil-
itates their use for adjustment of the analyte mobility and
separation in nonaqueous capillary electrophoresis (CE).10
With the same principle as that shown for CE, imidazolium
cations can interact with silanol groups on the alkylsilica surface
in a liquid chromatography (LC) column and compete with
the polar group in the analytes for the silanol groups.11 ILs were
used as the sensing materials in quartz crystal microbalance
sensors for the detection of organic vapors.12 ILs can produce
many more homogeneous sample solutions, and they possess
greater vacuum stability than most solid matrixes. For these
reasons, the use of ILs as matrixes for matrix-assisted laser
desorption/ionization coupled with mass spectrometry
(MALDI-MS) could enhance reproducibility and sensitivity.13
Compared to traditional electrolyte solutions, ILs offer a
broader range of electrochemical potentials and favorable
optical properties for in situ vis/near-IR and Raman spec-
troelectrochemistry of nanocarbon species (single-walled
carbon nanotubes and fullerene peapods).14 Moreover, they
can act as templates and precursors to inorganic materials as
well as solvents, but their utilization in inorganic synthesis
INTRODUCTION
■
Ionic liquids (ILs) are nonvolatile, nonflammable, and
thermally stable low-melting salts that are frequently composed
of organic cations and polyatomic inorganic or organic anions.
ILs are defined as salts with melting points below 100 °C,
commonly even below room temperature (RTILs), and whose
melted composition consists of discrete anions and cations.
Their low melting temperatures are due to delocalization of a
charge on voluminous ions. The most widely used cations are
imidazolium, pyridinium, pyrrolidinium, piperidinium, ammo-
nium, or phosphonium derivatives bearing at least one alkyl
chain. The range of anions found in ILs is wide, including
the important family of weakly coordinating hydrophobic
species, such as halide anions, nitrate, perchlorate, tetrafluor-
oborate, hexafluorophosphate, tosylate, acetate, triflate [(trifluo-
romethyl)sulfonate], and bis[(trifluoromethyl)sulfonyl]imide.
The large number of possible cation−anion combinations
allows the design of tailor-made ILs for a desired task. The
rapid emergence of ILs as alternative solvents has also pro-
moted a rapidly growing number of applications, such as those
for separations and catalysis, energetic materials, lubricants,
electrolytes, and additives in paints, plastics, and cleaning
agents.1−5 Thus, they are useful in liquid−liquid extraction
(LLE),6 liquid-phase microextraction (LPME),7 and solid-
phase microextraction (SPME).8 ILs possess many favor-
able properties, such as nonvolatility, nonflammability, good
Received: November 1, 2011
Published: March 20, 2012
© 2012 American Chemical Society
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processes has only recently been explored.15−17 Recently,
biologically active ions and building blocks of active
pharmaceutical ingredients have been used to make new
pharmaceutically active ILs.18
Anionic derivatives based on the placement of a cobalt
bis(dicarbollide) moiety at position 8: hydrogen (abbreviated
in this paper as [CB]−), hydroxyl group ([CB-OH]−), or
oligo(ethylene glycol) chain with 2, 4, 6, 8, or 10 ethylene
glycol units ([CB-2EG]−, [CB-4EG]−, [CB-6EG], [CB-8EG]−,
and [CB-10EG]−, respectively) are shown on Figure 1.
Boron clusters (boranes, carboranes, and metallacarboranes)
and their derivatives were discovered more than 5 decades ago.
These artificial compounds display very specific properties,
such as high chemical inertness, three-dimensional aromaticity,
rigid skeleton structure, a high degree of lipophilicity, and a
delocalized negative charge (if any).19−22 They are able to form
dihydrogen bonds between a partially negatively charged
hydrogen that is bound to an electropositive boron atom
(hydrogen atoms in the B−H group have hydride-like char-
acter) and a partially positively charged hydrogen located on
common organic molecules (OH, NH, and SH).22,23 Despite
their chemical stability, boron cluster anions can be func-
tionalized to obtain several derivatives with different types
and degrees of substitution.24−27 Currently, scientists have
been primarily interested in the use of boron cluster anions
for pharmaceutical applications.28−35 The most important
technical application of boron clusters is their utilization as
selective extraction agents for lanthanide and actinide cations
from nuclear waste solutions.35−41 Boron clusters can be
utilized in the preparation of light-emitting materials
(fluorophores),42−46 in catalysis35,47,48 or in the construction
of ion-selective-electrode membranes,49−51 as special poly-
mers,52,53 or in material chemistry.35,54−57
The first low-melting salts containing a boron cluster as the
anion were synthesized a few years ago. These studies yielded a
variety of new of boron-containing salts, including imidazolium
salts with [closo-CB11H12]− derivatives,58 ILs of stannaboranes
([1-(R)-closo-SnB11H11]−),59 1-pentylpyridinium salt of [closo-
CB11H12]−,60 salts of [nido-C2B9H12]− and [closo-CB11H12]−,61
salts of [closo-B12H11NR3]− anions (R = alkyl) with a wide
range of cations (even Li+ or H+),62 and salts of [closo-1,2-
C2B10H11]−.63 A recent study combined either imidazolium or
phosphonium cations with four boron cluster anions, ([commo-
3,3′-Co(1,2-C2B9H11)2]−, [nido-C2B9H12]−, and two perhalo-
genated dianions [closo-B12Cl12]2− and [closo-B10Cl10]2−).64 The
latest study combines [nido-C2B9H12]− with diether-functionalized
ammonium cations.65
In this paper, we describe the preparation of a set of low-
melting salts based on the combination of 1-butyl-3-methyl-
imidazolium, 1-butyl-4-methylpyridinium, 1-butyl-1-methyl-
pyrrolidinium, and 1-butyl-1-methylpiperidinium cations
(abbreviated as [bmim]+, [bmpy]+, [bmpyr]+, and [bmpip]+,
respectively) with various anionic derivatives of the cobalt
bis(dicarbollide) moiety. In contrast to classic ILs, these new
salts display unusual properties because of the presence of the
cobalt bis(dicarbollide) moiety, such as atypical H−H inter-
actions, and, thus, they have great potential especially in the
field of separation methods (CG, LC, CE, LLE, LPME, SPME,
and MALDI).
It should be noted that based on IUPAC nomenclature the
correct name should be 3,3′-commo-cobaltabis(undecahydro-
1,2-dicarba-closo-dodecaborane) (1−)ate.66 In the literature,
alternative names are also used, such as bis(1,2-dicarbollido)-
cobalt(III) (1−)ate, cobalta(III) bis(1,2-dicarbollide), cobalt-
(III) bis(1,2-dicarbollide), cobalt dicarbollide, cobalt bis-
(dicarbollide), and COSAN. The term cobalt bis(dicarbollide)
seems the most widely used in the recent literature and will be
used within this article.67
Figure 1. Schematic structures of quaternary ammonium cations and
cobalt bis(dicarbollide) based anions. The abbreviations used in this
work are also shown.
Compounds have been characterized by NMR, Fourier
transform infrared, elemental analysis, powder X-ray diffraction,
differential scanning calorimetry, and thermogravimetric
analysis. In addition to these characterizations, other important
properties such as the decomposition temperature have been
studied, and the solubility of all newly synthesized compounds
in 76 commonly used organic solvents has been explored for
these compounds.
EXPERIMENTAL SECTION
■
General Comments and Chemicals. NMR spectra were
1
recorded on a Varian Mercury Plus 300 MHz (FT, H at 300 MHz,
13C at 75 MHz, 19F at 282 MHz) instrument using tetramethylsilane
and C6F6 as the external standards (solution of standards in capillary).
Chemical shifts are quoted in ppm (δ-scale; s, singlet; bs, broad
singlet; d, doublet; t, triplet; m, multiplet). Solvents: CDCl3 and
deuterated dimethyl sulfoxide (DMSO-d6).
The IR spectra were collected using an Avatar 320 FT-IR
spectrometer (Thermo Nicolet), and samples were prepared using
the KBr pellet method. Spectra were measured in the range of 4000−
400 cm−1, and 64 scans were accumulated per spectrum at 1 cm−1 re-
solution. For data analysis, the spectral data were processed using
OMNIC 7.1 software (Thermo Nicolet).
Thermogravimetric analysis (TGA) was performed using thermo-
balance TG-750 (Stanton-Redcroft, Great Britain). Samples were
heated in an argon atmosphere from room temperature to 150 °C at a
heating rate of 10 °C/min. TGA was used for the determination of the
water content of prepared salts. Samples contain nonstoichiometric
amounts of water, if any. The thermal stability was measured using
TG-750 and Setsys Evolution (Setaram, France), where the samples
were heated from room temperature to 900 °C under various atmo-
spheres (air, nitrogen, and argon).
Differential scanning calorimetry (DSC) measurements were per-
formed using a DSC 131 calorimeter (Setaram, France) from −50 to
200 °C in an argon atmosphere. The DSC data were collected upon
the first melting of samples. Selected samples have been measured by
repetitive heating and cooling, but because of slow crystallization
(some samples need days or weeks to crystallize), a significant amount
of information was lost after the first cycle. Curve fitting of the
experimental data was performed using nonlinear optimization
(Marquardt method) implemented in the software Calisto (Setaram).
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[Na]+[8-(C8H17O5)-(1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]−
([Na]+[CB-4EG]−): 1.69 g (79%)
Elemental analysis (C, H, and N) was performed using an Ele-
mentar Vario El III and Perkin-Elmer 2400 by heating samples up to
1200 °C in an oxygen atmosphere.
Powder X-ray diffraction (PXRD) data were collected at room
temperature with an X’Pert Pro θ−θ powder diffractometer with
parafocusing Bragg−Brentano geometry using Cu Kα radiation (λ =
1.5418 Å; U = 40 kV; I = 30 mA).
The chemicals used were as follows: cesium cobalt bis(dicarbollide)
([Cs]+[CB]−) and 8-dioxane-3-cobalt bis(dicarbollide) (Katchem Ltd.,
Czech Republic); sodium hydride (60% suspension in oil), diethylene
glycol, tetraethylene glycol, hexaethylene glycol, octaethylene glycol,
1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]+[PF6]−),
1-butyl-4-methylpyridinium hexafluorophosphate ([bmpy]+[PF6]−),
1-butyl-1-methylpyrrolidinium hexafluorophosphate ([bmpyr]+[PF6]−),
and 1-butyl-1-methylpiperidinium ([bmpip]+[PF6]−) hexafluorophos-
phate (all Sigma-Aldrich); silica gel (60−100 mm, Merck). All solvents
were purchased from Penta (Czech Rep.) and Sigma-Aldrich and dried
according to standard procedures. All reactions requiring anhydrous
conditions were performed using anhydrous solvents and in an inert
atmosphere.
[Na]+[8-(C12H25O7)-(1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]−
([Na]+[CB-6EG]−): 2.22 g (88%)
[Na]+[8-(C16H33O9)-(1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]−
([Na]+[CB-8EG]−): 2.03 g (71%)
[Na]+[8-(C20H41O11)-(1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]−
([Na]+[CB-10EG]−): 1.85 g (58%)
Water Content of Cobalt Bis(dicarbollide) Derivatives. The
water content of all cobalt bis(dicarbollide) derivatives was analyzed
using TGA. The samples contained nonstoichiometric amounts of
water, if any. The water content of [Cs]+[CB]− was below 0.05%, but
the sodium analogue [Na]+[CB]− contained 13.6%. The water content
of sodium salts generally decreases with the increasing length of
oligo(ethylene glycol) chain, namely, [Na]+[CB]− ∼ 13.6%, [Na]+[CB-
OH]− ∼ 10.3%, [Na]+[CB-2EG]− ∼ 5.2%, [Na]+[CB-4EG]− ∼ 0.82%,
[Na]+[CB-6EG]− ∼ 0.96%, [Na]+[CB-8EG]− ∼ 0.67%, and [Na]+[CB-
10EG]− ∼ 0.39% w/w.
General Procedure for the Preparation of Glycolated Cobalt
Bis(Dicarbollide) Ammonium Salts. These salts were prepared by
mixing solutions of the appropriate precursors in dichloromethane
with approximately 2% excess of a cationic precursor ([bmim]+[PF6]−,
[bmpy]+[PF6]−, [bmpyr]+[PF6]−, and [bmpip]+[PF6]−). Inorganic salt
([Na]+[PF6]− or [Cs]+[PF6]−) and an excess of a cationic precursor
have been removed by repeated extraction of the mixture between
dichloromethane and water. Inorganic salts and cationic precursors
are much more soluble in water than new ILs; thus, this separation is
very effective. After each extraction step, the purity was evaluated
(the presence of [PF6]− anion in product) by 19F NMR, which is an
advantage of [PF6]−-based precursors compared with slightly cheaper
chlorides. Elongation of the chain led to glycol-like characteristics in
the products. For this reason, all of the salts that were prepared were
originally waxy, and only [CB]−- and [CB-OH]−-based salts solidified
with aging. The products were allowed to dry under reduced pressure
and further dried under vacuum for 6 h at 100 °C. If necessary, water
residues were removed using azeotropic evaporation with anhydrous
ethanol, and then the salts were dried under vacuum. The purity was
checked by 19F NMR and confirmed by elemental analysis.
Water Content of Prepared Salts. All prepared salts were
immediately completely anhydrous after drying (checked by TGA and
confirmed by elemental analysis). Because the hydration of salts is an
important property, salts were left in open air for 3 days. The water
content of all salts was again analyzed using TGA. The samples
contained nonstoichiometric amounts of water, if any. The products in
the solid state ([CB]−- and [CB-OH]−-based salts) often contained
negligible amounts of water in contrast to the salts in the waxy state,
which tended to absorb more water. Salts based on [CB-4EG]−
contained the most moisture, with approximately two stoichiometric
water molecules. The data from TGA were in agreement with the data
from elemental analyses.
Sodium Cobalt Bis(dicarbollide) ([Na]+[CB]−). The cesium salt of
cobalt bis(dicarbollide) ([Cs]+[CB]−) was converted to sodium salts
using the extraction procedure. The general procedure used has been
previously described.68 The aqueous solution of the cesium salt was
acidified by sulfuric acid (50%). It was then extracted three times with
diethyl ether. The organic layer was shaken with an aqueous solution
of sodium carbonate and re-extracted five times with water. The
solvent was evaporated under reduced pressure and high temperature
(60 °C), and the solid residue was dried under vacuum at 100 °C. The
sodium salts of cobalt bis(dicarbollide) ([Na]+[CB]−) were obtained
at a nearly quantitative yield.
[Na]+[8-(OH)-(1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]− ([Na]+[CB-
OH]−). The compound was prepared using a modified procedure
described in the literature.69 Cesium cobalt bis(1,2-dicarbollide) (4 g,
8.8 mmol) was suspended in 60% sulfuric acid and heated for 4.5 h at
120 °C. After cooling, the reaction mixture was diluted with water
(250 mL) and extracted with diethyl ether (3 × 200 mL). The organic
phase was carefully neutralized/extracted with a saturated solution of
NaHCO3 (200 mL), dried over anhydrous Na2SO4, and evaporated.
The crude product was purified by column chromatography on silica
(eluent: 2:1 dichloromethane/acetonitrile), subsequently dried under
vacuum for 6 h at 100 °C, and yielded 2.69 g (84%) of [Na]+[CB-
OH]−.
−
[Na]+[8-(C4H9O3)-(1,2-C2B9H10)(1′,2′-C2B9H11)-3,3′-Co]
([Na]+[CB-2EG]−). The compound was prepared using a modified
procedure described in the literature.70 A solution of sodium
hydroxide (1.2 g, 40 mmol) in water (50 mL) was added to a stirred
solution of 8-dioxane-3-cobalt bis(dicarbollide) (4 mmol) in 1,2-
dimethoxyethane (50 mL), and this reaction mixture was stirred
at 75 °C for 45 min [thin-layer chromatography (TLC) monitor-
ing; CH2Cl2]. The reaction mixture was cooled, diluted with brine
(250 mL), and extracted with ethyl acetate (4 × 100 mL). The mixed
organic phase was subsequently extracted with water (100 mL)
and brine (100 mL), dried over anhydrous Na2SO4, filtered, and
dried under vacuum for 6 h at 100 °C to give pure [Na]+[CB-2EG]−
(1.71 g, 95%).
Characterization. The yields of synthesis, elemental analysis of
1
both hydrated and anhydrous forms of salts, and H NMR, IR, and
PXRD data for salts in the solid state at room temperature are
provided in the Supporting Information.
Solubility of Salts in Various Solvents. The solubility of salts in
76 commonly used solvents in organic chemistry has been determined
by the simple dissolution of small amounts of salt in appropriate
amounts of solvent. This was performed for three concentrations of the
compound−solvent system: 0.1 mg of salt was dissolved (completely or
partially) in 1 mL of solvent (∼0.1 g/L), 1 mg in 1 mL (∼1 g/L), and
1 mg in 0.1 mL (∼10 g/L). On the basis of the solubility of the salts, they
were assigned to one of four groups: <0.1 g/L (insoluble), 0.1−1 g/L
(poorly soluble), 1−10 g/L (soluble), and >10 g/L (well soluble).
General Procedure for the Preparation of Glycolated Cobalt
Bis(Dicarbollide) Derivatives. The compound was prepared using a
modified procedure described in the literature.71−73 A stirred
suspension of sodium hydride (5 mmol) in 1,2-dimethoxyethane (40
mL) was added to a solution of the corresponding ethylene glycol (4.5
mmol) in 1,2-dimethoxyethane (40 mL) at room temperature, and the
mixture was subsequently stirred at 70 °C in an inert atmosphere for
10 min. Then, 8-dioxane-3-cobalt bis(dicarbollide) (4 mmol) in 1,2-
dimethoxyethane (40 mL) was added, and the reaction mixture was
stirred at 70 °C for an additional 30 min (TLC monitoring; CH2Cl2).
After evaporation of volatile compounds, the crude product was
purified by column chromatography on silica [gradient from CH2Cl2
to 1:1 (v/v) CH2Cl2−MeCN]. The product was dried under vacuum
for 6 h at 100 °C.
RESULTS AND DISCUSSION
■
A total of 28 salts with low melting temperatures that were
based on the cobalt bis(dicarbollide) anion in combination
with 1-butyl-3-methylimidazolium, 1-butyl-4-methylpyridinium,
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Figure 2. Stoichiometric amounts of water of precursors (gray) and prepared low-melting salts (black).
Figure 3. DSC curves of [CB-OH]−-based salts.
1-butyl-1-methylpyrrolidinium, and 1-butyl-1-methylpiperidi-
nium cations, have been prepared and characterized. All but
one were prepared for the first time in this study. The salt based
on [bmim]+[CB]− has been previously described by Nieuwen-
huyzen et al.,64 but a different preparation method was used
here. The structures and abbreviations for the salts are pro-
vided in Figure 1.
It has been demonstrated by solid-state NMR that sodium
cations in [Na]+[CB]− are somehow separated from cobalt
bis(dicarbollide) anions, forming channels that are filled with
highly mobile water molecules.79 Oligo(ethylene glycol) chains
can bind sodium cations through oxygen atoms (similar to
binding of cations by crown ethers), and thus relatively
hydrophobic ethylene groups are directed to the surface of the
ion pair. It is expected that the sodium cation is close to the
central cobalt atom,53 which causes an increase in the hydro-
phobicity, a rapid decrease in the aqueous solubility, and a
decrease in the hygroscopicity by an increase in the length of
the chain.
Conversely, the chain length influences the properties of salt
products in the opposite way. An increasing hygroscopicity of
products occurs with prolongation of the chain, which is most
likely caused by interactions between free water molecules and
chains in the product. They can also form channels filled with
mobile water molecules. The [CB-4EG]−-based salts stand out
from the trend and display substantially higher water content
than other salts (approximately two stoichiometric water
molecules).
The salts were prepared by conventional metathetic reactions
(described in the Experimental Section). Products based on
[CB]− and [CB-OH]−, after drying and leaving in the air, con-
tained negligible amounts of water (mostly up to 0.2% by weight;
see Figure 2). An increasing chain length of oligo(ethylene glycol)
caused an increase in the water content for all salts, but [CB-
4EG]−-based salts stand out of the trend and demonstrate
substantially higher water content than all other salts (determined
by TGA). Generally, precursor ILs themselves tend to be
hydrophobic (mainly because of the hydrophobic character of the
[PF6]− anion) and also slightly hygroscopic (depending mainly
on the nature of the anion and the length of the aliphatic chain of
the cation).1,74−78 Cobalt bis(dicarbollide) based precursors also
tended to contain water (see Figure 2; for exact values, see the
Experimental Section), but the trends in the water content for
these precursors [decreasing with prolongation of the oligo-
(ethylene glycol) chain] differed from the trends observed for the
prepared salts.
The thermal stability of the salts has been determined using
TGA and PXRD. Selected samples have been heated up to 900 °C
using various conditions (two apparatuses; air, nitrogen, and
argon atmospheres). It has been observed that (oligo)ethylene
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a
Table 1. DSC Analysis of 28 Low-Melting Salts
Tg [°C]
Tm [°C]
[bmim]+[CB]−
−4.2
−4.8
−10.3
55.5
53.4
[bmim]+[CB-OH]−
[bmim]+[CB-2EG]−
[bmim]+[CB-4EG]−
48.1 (47.5, 56.2)
49.3 (40.3, 49.7, 64.8)
52.1 (33.4, 53.0)
48.7 (48.4, 49.2)
49.5 (33.5, 51.2, 68.3)
90.2, 117.1
[bmim]+[CB-6EG]−
[bmim]+[CB-8EG]−
[bmim]+[CB-10EG]−
[bmpyr]+[CB]−
−29.6
−26.4
−31.0
−4.8
−9.1
−9.5
−8.7
−4.5
−9.5
−8.4
−4.0
−9.3
−9.0
−3.1
−7.8
−7.8
−3.4
−8.5
−8.3
−7.9
−4.1
−8.5
−8.0
−7.5
53.5
51.4
51.1
52.5
52.5
51.5
52.6
55.3
54.5
54.5
58.1
53.2
53.7
50.7
53.4
54.6
53.7
52.7
49.1
50.8
52.9
[bmpyr]+[CB-OH]−
[bmpyr]+[CB-2EG]−
[bmpyr]+[CB-4EG]−
[bmpyr]+[CB-6EG]−
[bmpyr]+[CB-8EG]−
[bmpyr]+[CB-10EG]−
[bmpip]+[CB]−
[bmpip]+[CB-OH]−
[bmpip]+[CB-2EG]−
[bmpip]+[CB-4EG]−
[bmpip]+[CB-6EG]−
[bmpip]+[CB-8EG]−
[bmpip]+[CB-10EG]−
[bmpy]+[CB]−
[bmpy]+[CB-OH]−
[bmpy]+[CB-2EG]−
[bmpy]+[CB-4EG]−
[bmpy]+[CB-6EG]−
[bmpy]+[CB-8EG]−
[bmpy]+[CB-10EG]−
99.7 (83.9, 101.3)
−37.5
−34.7
−37.3
99
87.3 (81.8, 88.6)
75.1
−38.9
−34.3
−36.3
57.0
118.5
112.3
−30.3
−25.9
−30.5
a
Glass transition (Tg) and melting point (Tm) temperatures evaluated by deconvolution of the merged peaks are in brackets.
glycol chains tend to decompose at approximately 180 °C
and that the cations tend to decompose from 300 to 420 °C,
depending on stabilization by the counterions. The cobalt
bis(dicarbollide) cages start to oxidize (if oxygen was pre-
sented) at approximately 700 °C to form B2O3 and cobalt-
based microparticles. Oxidation has been observed by heat flow
and an increase in the weight. The products look like black balls
of wire and have been analyzed by PXRD and elemental
analysis.
Melting points and glass transition temperatures have been
analyzed using DSC (Figure 3). The temperatures are shown in
Table 1. The first observed glass transition appeared at −38.9
to −25.9 °C. It was observed only for salts based on [CB-
6EG]−, [CB-8EG]−, and [CB-10EG]− anions. Generally,
bmpyr and bmpip salts have lower glass transition temperatures
(−38.9 to −34.3 °C) than [bmim]+ and [bmpy]+ salts (−31.0
to −25.9 °C). There is not any clear trend in temperatures as a
function of the chain length; however, [CB-8EG]−-based salts
seem to have slightly higher temperatures than [CB-6EG]−-
and [CB-10EG]−-based salts. The second glass transition tem-
perature region was between −10.3 and −3.1 °C without any
clear trend in temperatures among the salts. The third tem-
perature region between 49.1 and 58.1 °C was detected for all
salts except [bmim]+-based salts. For [bmim]+-based salts, the
melting point was observed in this region. The [bmim]+-based
salts tend to form various polymorphs and display multiple
melting points. These melting temperatures are relatively similar,
and thus the table shows an average temperature (onset) and
temperatures evaluated by deconvolution of the merged peaks
Figure 4. Dependence of the chemical shift of the aromatic signal of
[bmim]+ as a function of the counterion (1H NMR in CDCl3) with a
cut of spectra (inset).
(values in parentheses). It should be noted that the intensity of
the peak corresponding to the melting point increases with aging
of the samples due to slow crystallization. In contrast with data
from the literature,64 we have observed melting points of
[bmim]+[CB]−, but this may be due to the measurement of an
older sample. The lowest melting points obtained were for
[bmim]+-based salts (∼50 °C), followed by [bmpip]+-based salts
(∼80 °C), [bmpyr]+-based salts (∼95 °C), and [bmpy]+-based
salts (∼115 °C). The chain length influenced the melting
points only up to 10 °C; thus, its effect is significantly smaller
than the effect of the cation.
NMR spectra for all samples have been measured in
deuterated chloroform and DMSO. In agreement with
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Table 2. Solubility of Low-Melting Salts in 76 Solvents and the Physical Properties of the Solvents
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Table 2. continued
expectations, the samples in chloroform tended to form ion
pairs; thus, the spectra of the cationic moieties are significantly
different in the presence of various anionic moieties (clearly
visible in aromatic regions of [bmim]+ and [bmpy]+ spectra; for
exact chemical shifts, see the Supporting Information). In the
case of [bmpy]+ salts, these effects seem to be stronger with
increasing length of the oligo(ethylene glycol) chain (Figure 4).
On the other hand, the spectra for samples dissolved in DMSO
display only negligible differences in comparison to the
superposed spectra of the precursors. Therefore, it seems that
samples dissolved in DMSO are presented only in the form of
separated ions or mainly in the form of separated ions when
both forms are present (i.e., ion pairs and separated ions).
The IR spectra for all samples were measured using the KBr
pellet method; the values and figures are in the Supporting
Information. Generally, peaks belonging to cations tended to
become wider with increasing length of the oligo(ethylene
glycol) chain. This finding correlates with crystalline salts
becoming waxy with increasing length of the chain.
visualized by gray scale; the darker gray shade means that the
salt demonstrated the higher solubility in the specified solvent.
Table 2 summarizes the basic properties of the solvents: dipole
moment,80 relative polarity,81,82 log Pow,80,83 solubility of the
solvent in water and solubility of water in the solvent.80,82
Dipole moments of molecules are due to the nonuniform
distribution of charges on various atoms, and generally, polar
compounds (compounds with permanent dipole moment)
prefer to dissolve in polar solvents. Relative polarity is a
normalized empirical parameter of solvent polarity. It is derived
from the UV/vis charge-transfer absorption at long-wavelengths
for the negatively charged solvatochromic pyridinium
N-phenolate betaine dyes (Dimroth and Reichardt), where
Me4Si = 0 and water =100 at 25 °C. The values for relative
polarity are normalized by measurements of solvent shifts in
the absorption spectra and were obtained from a literature.81
The octanol−water partition coefficient (Pow) is defined as the
ratio of the equilibrium concentrations of a dissolved compound in
a two-phase system consisting of octanol and water (Pow = co/cw).
The Pow is widely used as a hydrophobicity descriptor.84
The solubility of all 28 salts was determined in 76 different
solvents. The properties of the solvents and solubility of salts
are summarized in Table 2. For simplicity, the solubility is
Generally, the solubility of cobalt bis(dicarbollide) salts
increases with the dipole moment and relative polarity of the
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solvents. The prepared salts were very soluble in ketones and
esters, moderately soluble in alcohols, aromates, and chlori-
nated solvents, and not very soluble in ethers. The salts were
practically insoluble in higher hydrocarbons and water.
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CONCLUSIONS
■
In this study, we synthesized low-melting salts based on
1-butyl-3-methylimidazolium ([bmim]+), 1-butyl-1-methylpiper-
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melting salts, PXRD patterns of salts and their precursors, DSC
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Corresponding Author
*E-mail: jakub.rak@gmail.com (J.R.), vladimir.kral@vscht.cz
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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ek, R.; Matejícek, P.; Kral, V. Eur.
Financial support was provided from Specific university
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