Characterization
of Ionic Liquids:
[DBU][Ac] and
PEG-DIL
Chapter - 2
Synthesis and
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
2.1. Brief history of Ionic Liquids
The history of ionic liquids began in 1914; Walden reported the first synthesis of
room temperature molten salt (i.e., ethylammonium nitrate salt) [1]. He reported
the physical properties of ethylammonium nitrate, [EtNH3]NO3, which has a
melting point of 8 oC. [EtNH3]NO3 was formed by the reaction of ethylamine with
concentrated nitric acid. This salt is liquid at room temperature; but usually it
contains a small amount of water (200-600 ppm).
Hurley and Weir [2] developed the first ionic liquid with chloroaluminate
ions such as ethyl pyridinium bromide/AlCl3 for their use in electroplating
aluminium in 1948 at the Rice Institute. It was prepared by mixing and warming
1-ethylpyridinium bromide with aluminum chloride (AlCl3). The use of
chloroaluminate ionic liquids as electrolyte attracted interest from both
fundamental and applied research. In 1967, Swain et al. described the use of
tetra-n-hexylammonium benzoate as a solvent for kinetic and electrochemical
investigation [3]. Room temperature ionic liquids (RTILs) became more popular
in chemistry fraternity with the reopening of development in this area by the
groups of Osteryoung et al. [4] and Hussey et al. [5] in 1970 and 1980
respectively. They carried out extensive research on organic chloride-aluminium
chloride room temperature ionic liquids. The first major review on room
temperature ionic liquids was written by Hussey [6]. The ionic liquids based on
AlCl3 can be regarded as the first generation of ionic liquids.
The first report in which ionic liquids were described as new reaction media
and catalyst for Friedel–Crafts reaction was published in 1986 [7]. In 1990, the
use of ionic liquids as solvents for homogeneous transition metal catalysts was
described for the first time by Chauvin et al. They reported the dimerisation of
propene by nickel complexes dissolved in acidic chloroaluminate ionic liquid [8].
Osteryoung et al. reported the polymerization of ethylene by Ziegler–Natta
catalysts in chloroaluminate molten salt [9].
The hygroscopic nature of AlCl3 based ionic liquids had delayed the progress
in their use in applications of organic reactions. They must be prepared and
handled under inert gas atmosphere. Thus, the synthesis of air and moisture
stable ionic liquids, which are considered as the second generation of ionic
liquids, attracted further interest in the use of ionic liquids in various fields. In
52
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
1992, Wilkes and Zaworotko [10] reported the first air and moisture stable ionic
liquids
based
on
1-ethyl-3-methylimidazolium
cation
with
either
tetrafluoroborate or hexafluorophosphate as anions. Unlike the chloroaluminate
ionic liquids, these ionic liquids could be prepared and safely stored outside of an
inert atmosphere. Generally, these ionic liquids are water insensitive; however,
the exposure to moisture for a long time can cause some changes in their
physical and chemical properties. This is due to the formation of HF as a result of
decomposition of the ionic liquid in presence of water. Therefore, ionic liquids
based on more hydrophobic anions such as trifluoromethanesulfonate (CF3SO3¯),
bis-(trifluoromethanesulfonyl)imide
[(CF3SO2)2N¯]
and
tris-
(trifluoromethanesulfonyl) methide [(CF3SO2)3C¯] were developed [11–13].
These ionic liquids have received extensive attention not only because of their
low reactivity with water but also because of their large electrochemical
windows. Usually, these ionic liquids can be well dried with the water contents
below 1 ppm under vacuum at temperatures between 100-150 oC.
Besides Osteryoung, Wilkes, Hussey and Seddon who are the pioneers in the
field of ionic liquids, there are several researchers, viz. Rogers, Welton,
Wasserscheid, MacFarlane, Ohno, Endres, Davis, Jr. Abbott, and others, who
entered in this field having a strong impact in introducing the ionic liquids in
many applications.
Rogers is one of the highly cited authors in the field of ionic liquids. He has
focused on the synthesis and characterization of environmentally friendly ionic
liquids as green solvents. He has measured and published physicochemical
properties of many ionic liquids with the aim of providing data to start
evaluating the use of ionic liquids in a variety of processes. He also worked on
the development of new materials from cellulose utilizing ionic liquids [14].
Welton published many papers dealing with the applications of ionic liquids
as solvents for synthesis and catalysis. He focused on how the ionic liquids
interact with solute species to affect their reactivity and he worked on replacing
environmentally damaging solvents with more benign alternatives. He is also the
author of one of the most cited papers [15].
Wasserscheid is an active member of the ionic liquid community who focused
on the preparation and characterization of ionic liquids for use in the biphasic
53
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
catalysis. He showed that the use of hexafluorophosphate ionic liquids allowed
selective to biphasic oligomerization of ethylene to 1-olefins. Together with
Welton, he edited a very important book entitled “Ionic Liquids in Synthesis”
which represents the synthesis and physicochemical properties of ionic liquids
as well as their use in catalysis, polymerization, and organic and inorganic
synthesis [16].
Macfarlane worked on the synthesis of new air and water stable ionic liquids
with the objactive of employing such ionic liquids as indicators for sensing and
displaying environmental parameters such as humidity. This process was
controlled by the colour change of the ionic liquids where they were synthesized
with either a coloured cation or anion, so that the ionic liquids themselves were
sensors. He has published many papers on the use of ionic liquids in
electropolymerization and in batteries [17].
Ohno concentrated his work on the synthesis of a series of polymerizable
ionic liquids and their polymerization to prepare a new class of ion conductive
polymers. He prepared polymer electrolytes with high ionic conductivity and
good elasticity by mixing nitrite rubber [poly(acrylonitrile-cobutadiene) rubber]
with the ionic liquid N-ethylimidazolium bis(trifluoromethanesulfonyl)imide.
Recently, he edited a book entitled “Electrochemical Aspects of Ionic Liquids”
which introduces some basic and advanced studies on ionic liquids in the field of
electrochemistry [18].
Davis introduced the concept of ‘‘task-specific ionic liquids’’ (TSILs) in the
field of ionic liquids. TSILs are ionic liquids in which a functional group is
incorporated to behave not only as a reaction medium but also as a reagent or
catalyst in some reactions or processes [19].
Abbott has recently developed a range of ionic compounds, which are fluid at
room temperature. These ionic liquids are based on simple precursors such as
choline chloride (vitamin B4); which is cheap and produced on a multitone scale
and hence these ionic liquids can have the potential to be applied on large scale
processes
for
the
first
time.
Recently,
he
edited
a
book
entitled
“Electrodeposition from Ionic Liquids” which introduces the applications of ionic
liquids on electrodeposition of metals [20].
54
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
2.1.1. Synthesis of Ionic Liquids
Generally, the synthesis of ionic liquids consists of two major steps:
 In the first step, desired cation has to be generated. This is usually done
by direct alkylation/quaternization of a nitrogen or phosphorus atom.
 Anion exchange to form the desired IL or reaction with Lewis acid is
performed to form the desired IL.
A general schematic diagram (Scheme 2.1) for the synthesis of ILs is shown
below using 1-methylimidazole as an example.
In many cases the desired cation is commercially available at reasonable cost,
most commonly as a halide salt (especially tetra alkyl ammonium salts). The
formation of IL thus requires only the anion exchange reaction or a Lewis acid
reaction. The preparation of ionic liquid is fairly simple. The anion exchange
reaction normally occurs by a metathesis exchange reaction. The major problem
with the metathesis anion exchange reaction is the stoichiometric amount of
waste (MX, HX) that is formed. The Lewis acid reaction is normally used to
prepare chloroaluminate based ILs.
ILs can also be prepared directly by alkylating 1-alkylimidazoles,
trialkylamines and trialkylphosphines with methyl triflate or methyl tosylate
[11]. The alkyl triflets and tosylates are sensitive to hydrolysis and should
therefore be worked with an inert atmosphere. This method has a major
advantage since the desired IL is produced with no by products, no halide
55
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
impurities which could hinder catalysis by strongly co-ordinating to transition
metal catalysts.
2.1.1.1. Preparation of the cation
Innovations in the field of ILs are being reported continuously in the form of
novel cation and anion combinations. The formation of the precursor cation may
be carried out either by direct alkylation or quaternization of a nitrogen or
phosphorus atom, most commonly with a haloalkane. These halide salts can then
be converted to the desired ILs by reacting them with salts or acid containing the
desired anion. The common amines used are 1-alkylimidazoles but others such
as pyridine, isoquinoline, 1-methylpyrrolidone and trialkylamines can also be
used for quaternization. Haloalkanes include chlroalkanes, bromoalkanes and
iodoalkanes with reaction conditions being milder in the order Cl<Br<I with
chloroalkanes being the least reactive and the iodoalkanes being most reactive.
For example, a quaternization reaction of 1-methylimidazole and an alkyl
chloride can take 2-3 days at 80 °C whereas the reaction with an alkyl bromide
takes about 1 day at 50-60 °C [21]. The reactions with alkyl bromides are
exothermic and lead to highly coloured ILs. In general, the reactivity of the
haloalkane decreases with increasing alkyl chain length. The cation is formed by
simply stirring the amine or phosphine with the haloalkane under nitrogen (to
exclude water and oxygen) by heating. The reaction is normally not carried out
with a solvent since the reagents are miscible with one another and the halide
salt is immiscible forming a lower phase. Overheating the reaction mixture to
temperatures greater than 80 °C could result in the reversal of the
quaternisation reaction. The halide salts should be kept free of moisture as they
are hygroscopic.
Purification of the halide salt is normally performed by recrystallization from a
mixture of dry acetonitrile and ethyl acetate in the case of solid halide salt or in
the case of oil, washing with an immiscible solvent such as dry acetonitrile or
ethyl acetate. The use of microwave irradiation as an alternative approach to
produce the halide salts with high yields and short reaction times have been also
reported [22].
56
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
Besides organic cations based ionic liquids, lithium salts are increasingly
being developed particularly for secondary batteries and for storage of energy.
They have often lower lattice energy and therefore, lower melting points than
their neighbouring elements in the periodic table. As an example the mixture of
LiCl and EtCl2 gives a liquid, on a large range of composition, at temperatures
lower than 0 oC.
2.1.1.2. Preparation of desired IL by anion metathesis reactions
The anion chemistry has a large influence on the properties of IL. The most
commonly employed IL anions are polyatomic inorganic species. The
introduction of different anions has become more popular as an increasing
number of alternatives are being discovered. In the future, list of cations and
anions will be extended to a nearly limitless number. Various combinations of
cations and anions have provided finely designed ionic liquids for different
applications. In this approach, the desired IL is obtained by performing an anion
metathesis reaction of the halide salt with a salt (normally silver, group 1 or
ammonium salts) or an acid containing the desired anion (acid-base
neutralization reactions). Use of the acid is normally preferred since it produces
HCl, HBr or HI as a by-product which can be easily removed by washing the IL
with water. The metathesis anion exchange reaction, when using a free acid is
normally performed by cooling the aqueous halide mixture since the reaction is
exothermic. Where the acid is not available, metal or ammonium salts can be
used without any difficulty. Water immiscible ILs are more straightforward to
prepare than the water miscible ILs. With water immiscible ILs, the free acid or
metal salt is washed out with water to give the final IL Product. Water miscible
ILs are normally extracted out of the aqueous phase with dichloromethane [23].
In both cases, ILs are then dried under vacuum.
2.1.2. Classification of ionic liquids according to acidity/ basicity
The design and choice of ionic liquids commonly focus on physical properties
such as water-miscibility, conductivity, viscosity and solubility properties,
although how the chemical structure of the ionic liquid affects these various
characteristics is still poorly understood. However, there is another chemical
57
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
property that imparts a variety of physical characteristics to the ionic liquids that
has been little investigated is the relative acidity or basicity of the component
ions. Lewis base anions can exhibit a base catalysis phenomenon, which can be
utilized [24]. The acidity or basicity of reactive ionic liquids is governed by the
strength of the cation or the anion, or by the combination of the cation and anion.
2.1.2.1 Neutral Cations and Anions
Typical ionic liquid anions are those that can be described as neutral in the
acid/base sense or very weakly basic; these exhibit only weak electrostatic
interactions with the cation and thus impart advantageously low melting points
and
viscosities.
hexafluorophosphate,
bis(trifluoromethanesulfonyl)amide,
tetrafluoroborate, methanesulfonate (mesylate), thiocyanate, tricyanomethide
and p-toluenesulfonate (tosylate) are included in this class (figure 2.1). Ionic
liquids formed from these anions typically exhibit good thermal and
electrochemical stability. They are often utilized as inert solvents in a wide range
of applications [11, 13, 25, 26].
2.1.2.2. Acidic Cations and Anions
The most popular acidic ILs include either protonated alkylimidazole salts or
sulfonylalkylimidazolium salts. The simplest examples of slightly acidic ionic
liquids are those based on the protic ammonium, pyrrolidinium and imidazolium
58
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
ions. The well-known AlCl3 based ionic liquids are Lewis acidic when they
contain an excess of AlCl3 (Figure 2.2) [27-37].
2.1.2.3. Basic Cations and Anions
There are a number of ionic liquid forming anions that can be classed as basic.
These include the lactate, formate, acetate (and carboxylates generally) and the
dicyanamide (dca) anion. Since the basicity of these anions imparts different
advantageous properties to the ionic liquids such as different solubilizing and
catalytic properties. This category of ionic liquids is likely to grow considerably
in the coming years. An alternative to the design of ionic liquids utilizing a basic
anion is to incorporate a basic site into the cation. This may afford more
thermally stable ionic liquids than those containing basic anions, which
frequently exhibit relatively low decomposition temperatures (Figure 2.3) [24].
59
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
2.1.2.4. Amphoteric Anions
There are a few numbers of ionic liquid anions that fall into the interesting class
of amphoteric anions, with the dual potential to accept and donate protons
depending on the other substances present. The hydrogen sulfate (HSO4) and
dihydrogen phosphate (H2PO4) anions are simple examples of such anions [24].
2.1.3. Purity of ionic liquids
The physico-chemical properties of ionic liquids can be changed by the presence
of impurities arising from their preparation [38]. Purity of ILs is a major factor,
when using ILs as reaction media, especially for transition metal catalysis.
Although, a very few reports are available on “distillable ionic liquids” [39]. In
most of the reported methods neither the ILs were distilled nor recrystallized.
Also, the purification of ILs via column chromatography is tricky. As a
consequence, once the ILs has formed in the course of the synthesis, purification
can become a nuisance. The main contaminations in ILs are halide anions,
organic bases that are generally produced from unreacted starting material and
water [40]. A colorimetric method has been recently developed to determine the
level of unreacted alkyl imidazole (<0.2 mol %) in the ionic liquid [40]. Halide
impurities can have a detrimental effect on reactions (especially transition metal
catalyzed reactions). Halide impurities can be removed by washing of ILs with
water. Halide removel by titration with AgBF4 is quite expensive and may lead to
silver impurities in ILs. Alternatively, methods of preparations have been
proposed to avoid the use of halide containing starting materials.
Gallo et al. [41] have systematically studied the influence of halide impurities
on catalytic Michael addition reactions. They have found that the system is
strongly sensitive to the amount of halides present in the ILs, inhibiting the
activity of the transition metal catalyst. The total amount of halide impurities in
different IL batches is variable even if the same synthetic protocol is followed.
Klingshirn et al. [42] came to the same conclusions for a palladium catalyzed
copolymerization of styrene and carbon monoxide. Daguenet and Dyson [43]
explained this fact as a significance of the extremely weak interactions between
the halide anion and the imidazolium cation. Accordingly the dissociation of the
60
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
halide from a transition metal complex can become thermodynamically
disfavoured in ILs.
The second major purity problem is “colour.” Most ILs are colourless in their
pure form, but in reality, they are more likely to be pale yellow to dark orange.
The origin of this is still somewhat unclear, since these (often trace) impurities
are not detectable via NMR or IR spectroscopy. Most likely, the colour is due to
degradation of the starting material. By taking some precautions, colourless ILs
can be obtained by (1) using freshly distilled starting material for the synthesis;
(2) performing the alkylation step at the most modest temperatures possible
(i.e., avoiding overheating) under a protective atmosphere, and (3) by cleaning
the final IL product through stirring with activated charcoal [44].
The third issue regarding the purity is the amount of water present in the ILs.
This is not only a problem for running reactions with water-sensitive
compounds, but the amount of water can change the physical properties of an IL
dramatically. Therefore, it is always advisable to dry ILs at elevated temperature
under high vacuum with vigorous stirring overnight before using them. Stirring
is crucial here because of high viscosities and because the water desorption takes
place only via the surface of the liquid phase. In critical cases, the amount of
water present can additionally be checked by IR spectroscopy [45] or, of course,
by standard Karl Fischer titration. In some cases traces of water can generate the
decomposition of the anion and the formation of HF (e.g. PF6 based salts).
Organic solvents are usually purified by distillation before use. This method is
not suitable to clean up ionic liquids, due to their non-volatile nature. Due to
these reasons, the highest purity possible must be attained during synthesis
itself.
2.1.4. Physicochemical properties of ILs
The physical properties such as melting point, boiling point, density, surface
tension and viscosity are related to the mechanics and engineering components
associated with a process. For example, density, viscosity and surface tension
will determine important parameters including rates of liquid–liquid phase
separation, mass transfer, power requirements of mixing and pumping. Other
physical properties, such as refractive index is related to certain chemical
61
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
properties despite providing a bulk property description. Chemical properties
such as the polarity, relative hydrogen bonding, donating and accepting ability
are more obviously related to the molecular chemistry of their application [46].
Due to intermolecular interactions, these parameters measure and the chemical
properties are believed to play a major role in determining solubilities, partition
constants, and reaction rates.
The physico-chemical properties of ILs can be varied by the selection of
suitable cations and anions. Their properties can be adjusted to suit the
requirements of a particular process. Because of this reason, the ILs has been
referred to as “designer” solvents.
Thus, it is necessary to understand how the physico-chemical properties of
ionic liquids are able to affect organic reactivity as well as how they depend upon
their structural features. This will be illustrated on the basis of a few selected
examples which are as follows:
2.1.4.1 Melting point
The most important property of the IL is the melting point. The melting point of
IL lies below 100 °C. With a given cation the choice of anion has a strong effect on
the melting point [47]. The coordinating and hydrophilic anions like halides lead
to high melting points, whereas weakly coordinating and hydrophobic anions
result in low melting points. Also increase in size of the anion with same charge
leads to a decrease in melting points. However, all ILs do not follow this rule of
decreasing melting point with increasing anion size (e.g. [emim][PF6]). The effect
of anion size on the melting point of [emim]X ILs is shown in Table 2.1.
Table 2.1 Effect of anion size on the melting point [emim]X ionic liquids
Anion (X)
Melting Point (°C)
Reference
[Cl]ˉ
87
[44]
[NO2]ˉ
55
[9]
[NO3]ˉ
38
[9]
[BF4]ˉ
15
[45]
[PF6]ˉ
62
[48]
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Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
[CF3SO3]ˉ
-9
[11]
[N(SO2CF3)2]ˉ
-3
[11]
Both cations and anions contribute to the low melting points of the ILs. Cation
size and symmetry make an important impact on the melting points of ILs.
Symmetrically substituted cations can crystallize easily and therefore often lead
to ionic solids with high melting point. Low symmetry in substitution can
prevent easy crystallization, resulting in low melting points.
By variation of the alkyl chain length in the cation, fine-tuning of the melting
point can be achieved. Longer the alkyl chain, lower is the melting point, but only
up to a certain extent (e.g. 8-10 carbons for 1-alkyl-3-methylimidazolium
cations). Beyond that, prolongation of the alkyl chain raises the melting point
again. In addition to this, a good distribution of charge in cation and weak
intermolecular interaction such as weak hydrogen bonding are also responsible
for the lowering of melting points of ILs. The effect of changes in the cation for
different chloride salts is shown in Table 2.2. Alkali metal salts are known to
have high melting points and these melting points are reduced to temperatures
at or below room temperature by replacing the simple inorganic cations with
unsymmetrical organic cations [45].
Table 2.2 The effect of cation size on melting point
Cation
Melting Point
Reference
NaCl
803
[49]
KCl
772
[49]
[mmim]Cl
125
[44]
[emim]Cl
87
[44]
[bmim]Cl
65
[44]
2.1.4.2. Viscosity
Viscosity is a key property of ionic liquids. Ionic liquids tend to have higher
viscosities than conventional solvents. The value of the viscosity varies
tremendously with chemical structure, composition, temperature and the
presence of solutes of impurities. Viscosities are ranging from 10 mPa·s to about
63
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
500 mPa·s at room temperature. A high viscosity may produce a reduction in the
rate of many organic reactions and a reduction in the diffusion rate of the redox
species. The viscosity of ILs is determined by van der Waals forces and hydrogen
bonding. An increase in formation of hydrogen bonds between ions results in
higher viscosity of the IL. A stronger Van der Waals interaction between ions
results in a higher viscosity. Electrostatic forces may also play an important role.
Alkyl chain lengthening in the cation and fluorination in the cation/anion leads
to an increase in viscosity [50]. This is due to stronger van der Waals forces
between cations leading to increase in the energy required for molecular motion.
Also, the ability of anions to form hydrogen bonding has a pronounced effect on
viscosity. The fluorinated anions such as NTf2¯ and PF6¯ form viscous ionic
liquids due to the formation of hydrogen bonding. The more asymmetrical cation
lowers the viscosity of IL. It has been shown that the viscosity of imidazolium
based ILs can be decreased by using highly branched and compact alkyl chains
[51]. Phosphonium ILs also tends to have higher viscosities than imidazolium
based ILs. ILs based on the dicyanamide anions have been found to have the
lowest viscosity of all the ILs. In general, all ionic liquids show a significant
decrease in viscosity as the temperature increases [52]. The purity of ILs is
important when determining accurate values for viscosity [40]. A small amount
of water tends to decrease the viscosity while high chloride impurities increase
the viscosity [40].
The viscosity of an IL is determined by both the cation and anion. For the
same cation the viscosity decreases as follows with change in anion:
[Cl]ˉ > [PF6]ˉ > [BF4]ˉ > [NO3]ˉ > [N(SO2CF3)2]ˉ
The addition of co-solvent to ILs can dramatically decrease the viscosity of IL
without changing the cation or anion of the IL [53].
2.1.4.3. Density
Density is one of the basic and important physical properties of ILs. In general, IL
is denser than water with values ranging from 1 to 1.6 g cm-3. The density of an
ionic liquid depends on the length and type of substituents in the cation and also
on the kind of the anion. The molar mass of the anion, alkyl chain length and
bulkiness in the cation significantly affect the overall density of ILs [54].
64
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
The general order of increasing density with respect to the anion is
[CH3SO3]ˉ ~ [BF4]ˉ < [CF3CO2]ˉ < [CF3SO3]ˉ < [C3F7CO2]ˉ < [N(SO2CF3)2]ˉ
In contrast, for ILs with the same anion, the density decreases with increasing
cation mass.
[beim]+ > [bmim]+ > [eeim]+ > [emim]+
Phosphonium based ILs tends to have lower densities than imidazolium based
ILs. Impurities present in the ILs have a less theatrical effect on the density of ILs
compared to their effect on viscosity [40]. The density of ionic liquid is also
temperature dependent. As temperature changes from 293 to 313 K, the density
of [bmim][BF4] decreases linearly with increase in the temperature [46].
2.1.4.4. Vapour pressure and thermal stability
These are unique properties of ionic liquids. They have negligible vapour
pressure. Thus minimizing solvent losses by evaporation into the environment.
They are therefore potentially recyclable. Due to their negligible vapour pressure
and non-flammable nature, ILs have been developed recently as so called green
solvents which are better alternatives for volatile organic solvents (VOS). VOS
are the common industrial solvents for synthesis in petrochemical and
pharmaceutical industries. The known problems with VOS are that they are quite
volatile, highly flammable, toxic and they tend to damage the earth’s atmosphere.
Non-volatile nature of ILs is a great advantage from an industrial process
viewpoint. Product separation by distillation of a reaction mixture becomes more
effective. The well-known problem of azeotrope formation between the solvent
and the products does not arise.
The thermal stability of ionic liquids is limited by the strength of their
heteroatom-carbon and their heteroatom-hydrogen bonds. In general, most of
ILs have high thermal stability, the decomposition temperature reported in the
literature are generally <400 oC, with minimal vapour pressure below their
decomposition temperature.
Recently, TGA of pyridinium salts have been described and noted that the
thermal decomposition was heavily dependent on the salt structure. It indicated
that the experiments performed under N2 or air produces the same results. The
beginning of thermal decomposition was nearly similar for the different cations
65
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
but appeared to decrease as the anion hydrophilicity was increased. It has been
suggested that the stability dependence on the anion is [PF6]¯ >[NTf2]¯ ~[BF4]¯
>halides [50]. Halide anions dramatically reduce the thermal stability with the
onset of decomposition occurring at least 100 °C below the corresponding ILs
with non-halide anions.
2.1.4.5. Polarity
It is well known that the solvent chosen could dramatically affect chemical
reactions. The solvent effects are considered to be mainly dependent on the
solvent polarity. Therefore, polarity is an important property of ionic liquids. The
simplest qualitative definition for a polar solvent is one that will dissolve and
stabilize dipolar or charged solutes, e.g., 'like dissolves like'. Under this
definition, ionic liquids due to their salt nature will be highly polar. ILs have a
polarity that lies between those of water and chlorinated organic solvents [55].
Due to the presence of the cation and the anion in the IL, there is most likely to
be a much wider range of solvent-solute interaction than with conventional
organic solvents. Different solvent-solute interactions in ILs have been reported
using solvatochromic dyes such as Nile Red and Reichardt’s dye [56].
2.1.4.6. Water miscibility
The hydrophilic/hydrophobic behaviour of IL is important for the solvation
properties. It is necessary to dissolve reactants, but it is also significant for the
recovery of products by solvent extraction. Furthermore, the water content of ILs
can affect the rates and selectivity of reactions. The miscibility of ILs with water
is an important factor for the industrial application of these solvents. Extensive
data are available on the miscibility of ILs with water. The miscibility of these ILs
with water depends on the nature of the anion, temperature and the length of the
alkyl chain on the cation. The ILs which is immiscible with water have a tendency
to absorb water from the atmosphere. On the basis of IR studies, water molecules
absorbed from the air are mostly present in the free state [44]. It has been
bonded with [PF6],¯ [BF4],¯ [SbF6],¯ [HSO4],¯ [ClO4],¯ [CF3SO3]¯ and [NTf2]¯ via Hbonding. Most of the water molecules should exist in symmetrical 1:2 type Hbonded complexes: anion…HOH…anion. The strength of H-bonding between
66
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
anion and water increases in the order [PF6]¯ < [SbF6]¯ < [BF4]¯ <[NTf2]¯ < [ClO4]¯
<[NO3]¯ < [CF3CO2]¯ .
2.1.4.7. Surface tension
Surface tension may be an important property in multiphase processes. ILs are
widely used in transition metal catalyzed reactions, carried out under multiphase
conditions. The rates of these processes depend on surface tension.
The surface tensions of ILs (e.g. [bmim][PF6] = 48.8 Nm-1 and [bmim][BF4] = 46.6
Nm-1) are lower than water (72.7 Nm-1 at 20 °C) but higher than n-alkanes (e.g.
hexane = 18 Nm-1) [54]. Surface tension values vary with temperature and
affected by the alkyl chain length [57].
2.1.4.8. Conductivity
The ionic conductivity is a key criterion for selecting electrochemical solvents.
ILs have good ionic conductivities compared with those of conventional
solvents/electrolyte systems (up to ~10 mS cm-1). At room temperature their
conductivities are usually lower than those of concentrated aqueous electrolytes.
Ionic liquids have high conductivities due to the fact that ionic liquids are
composed solely of ions. The conductivity of any solution depends not only on
the number of charge carriers but also on their mobility. The large constituent
ions of ionic liquids reduce the ion mobility which leads to lower conductivities.
Besides, ion pair formation or ion aggregation lead to reduced conductivity. The
conductivity of ionic liquids is inversely proportional to their viscosity. Hence,
ionic liquids of higher viscosity exhibit lower conductivity. Increasing the
temperature increases conductivity and lowers viscosity.
2.1.4.9. Electrochemical window
Two main advantages of using ionic liquids in electrochemical devices include
non-volatility and prevention of electrolytes from drying during the operation. A
recent review reports such developments in the area of electrochemistry [58].
Ionic liquids such as [emim]BF4 and [emim][CF3SO3] were reported as novel
moisture-stable
ionic
liquids
for
electrochemical
devices
[10].
The
electrochemical window for aqueous solutions is about 1.23 V at the standard
67
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
state. Organic solvents like propylene carbonate and acetonitrile possess
electrochemical window above 4 V. Several ionic liquids can have high
electrochemical window, which renders these materials suitable choice for
electrochemical applications. For example, [BMP][CF3SO3] is reported to have
electrochemical window as 6 V [13]. Ionic liquids possessing high
electrochemical window as a result they can act as electrolytes. Several
applications of ionic liquids in electrochemical devices can be considered. For
example, ionic liquids can be used in electric double layer capacitors [59], fuel
cells [60], lithium batteries [61], solar cells [62] and actuators [63], etc.
2.1.5. Applications of ionic liquids
Ionic liquid is a wide concept in modern synthetic organic chemistry. Recently,
ILs has aroused unprecedented interest in organic synthesis due to their unique
properties, in combination with their tunability. Applications of ILs are
concentrated in two directions:
 To replace organic solvents with ionic liquids; due to their unique solvent
properties, and
 To replace catalyst with ionic liquid due to their variable acidity and
basicity.
A number of reactions have been successfully tested in ILs. In most of the cases,
ILs enhance rate of reactions, yields, selectivities in comparison to conventional
organic solvents. Because of attractive properties, ILs are employed in a broad
area of applications listed below:
 Solvent extraction [64]
 Physico-chemical processes [65]
 Media for nucleophilic substitution reactions [65]
 Mobile phase modifier in HPLC [66]
 In electrodeposition of metals and semiconductors [67]
 Chemical analysis [68]
 Dye-sensitized solar cells [69, 70]
 The nuclear fuel cycle: electrodeposition and extraction [71]
 Nuclear-based separations [72]
68
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
 Oil shale processing [73]
 Separation of petrochemical relevance [74]
 Synthesis of functional nanoparticles and inorganic nanostructures
[75]
 Solvents for electrochemistry [76]
 Solvents for polymerization processes [77]
 Chemical and biochemical transformations [78]
 materials chemistry [79]
 Biocatalysts in ILs [80]
2.1.6. Task Specific Ionic liquids
Task-specific ionic liquids (TSILs) may be defined as ionic liquids in which a
functional group is covalently bonded to the cation or anion or both of the ILs.
These ILs can act as reagents or catalysts in organic reactions. Recently, many
attempts have been made to explore functional ionic liquids through
incorporation of additional functional groups as a part of the cation and/or
anion. Various types of ‘‘task specific ionic liquids’’ (TSILs) have been designed
and synthesized for specific purposes such as catalysis, organic synthesis,
separation of specific materials as well as for the construction of nanostructure
materials and ion conductive materials etc [19]. It is well known that polar
compounds or compounds containing strong proton donor functionality (such as
phenols, carboxylic acids, diols as well as ionised compounds) interact strongly
with ILs [16]. On other hand, compounds such as ketones, aldehydes and esters
with weak proton donor/accepter functionality interact with ILs through
induced ion dipole or weak van der Walls interactions [16].
2.1.7. Chiral Ionic Liquids
Chiral ionic liquids are emerging as a new type of asymmetric organocatalysts
and non-classical chiral ligands which combine the green credential of ionic
liquids and the catalytic principles of modern asymmetric catalysis. They can
provide not only an entire ion condition, but a chiral condition for the
asymmetric organic reactions. In the last few years, research on the development
69
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
of new chiral ionic liquids has led to an epoch-making change in asymmetric
organic reactions. Chiral ionic liquids include the normal chiral ionic liquids and
the supported chiral ionic liquids. Moreover based on the structure of the chiral
ionic liquids and the position of the chiral sites, they further fall into two general
categories: the normal chiral ionic liquids with a chiral cation, which is the
largest portion of the chiral ionic liquids and the chiral ionic liquids with a chiral
anion. In the first report related to chiral ionic liquid lactate was used as anion.
This still remains one of the few examples where chirality is induced in an anion
[81].
Chiral ionic liquids are attractive for their potential application to chiral
discrimination including asymmetric synthesis and optical resolution of
racemates. Novel chiral ionic liquids have been synthesized directly from the
‘chiral pool’. They are interesting solvents for enantioselective reactions and
useful in chiral separation techniques [82].
Due to their ease of synthesis and their peculiar properties, these new chiral
solvents should play a central role in enantioselective organic synthesis and
hopefully expand the scope of chiral solvents. Most reports deal with the
synthesis and properties of the new chiral ILs and only a few deals with their
application in organic reactions [83].
2.1.8. Supported ionic liquids (SILs)
Supported ionic liquid is a concept which combines the ionic liquids with
heterogeneous support materials. The prepared supported ionic liquid exhibit
similar or advanced chemical behaviours and have the advantage of being a solid.
This is an important feature as it facilitates the convenient separation of catalysts
from reaction mixtures. The preference for heterogeneous catalyst systems is
primarily motivated by the advantages of easy separation and the ability to use
fixed bed reactors. Another advantage of supported ionic liquid phases over
biphasic reaction systems is that biphasic systems always require larger
amounts of ionic liquid which is costly and may affect the economic viability of a
potential process.
In a very early example of supported ionic liquid catalysis a eutectic mixture
of palladium chloride/copper chloride was supported on a porous silica gel and
70
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
investigated for the partial oxidation of olefin (Wacker catalysis). Although the
melting point of this supported molten salt (423 K) was slightly higher than of
room temperature ionic liquids [84].
Subsequently the first supported Lewis acidic ionic liquid systems were
prepared and explored for catalysis applications [85]. A solid support material
was impregnated with a pre-formed ionic liquid which was also the catalytically
active species. Most commonly these ionic liquids consisted of aluminium
chloride derivatives. They were largely tested for Friedel–Crafts reactions [86].
Novel catalyst systems can be designed and prepared by using structured
supports like membranes or large pore-size zeolites. These combine the
advantages of homogeneous and heterogeneous catalysis. The recent advances in
supported ionic liquid catalysis have showed a tremendous potential and will
help to accelerate their introduction into commercial processes. Some of the
recently used supported ionic liquids are shown in Figure 2.4
2.1.9. Biodegradability of ionic liquids
The physicochemical properties of ionic liquids have been studied intensively
since their discovery. Diverse modifications of cations and anions have provided
ionic liquids with desired properties for many technical applications [87].
71
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
However, the design of an ionic liquid with suitable properties for a specific
application in view is not enough and more attention needs to satisfy the total
life-cycle. Studies on the biodegradation of ionic liquids and their potential
accumulation in the environment have begun in recent times [88]. Degradation
of organic compounds can be either aerobic or anaerobic. In both processes
micro-organisms require a source of nitrogen and other essential nutrients in
order to decompose an organic substrate into carbon dioxide and water. A
difference between both processes is that an aerobic treatment requires a source
of oxygen and an anaerobic treatment requires an electron acceptor such as Fe3+.
2.1.10. Recyclability of ionic liquids
Environmental considerations require the recovery of ionic liquids after their
use. Ionic liquids are quite expensive. Their recycling is also necessary due to
economic reasons. A review by Olivier-Bourbigou and Magna reports that ILs
have been successfully recycled in many reactions [89]. Several procedures for
recycling ionic liquids have been reported, and the efficiency of the recycling
varies from poor to very good recovery.
Recyclability requires rates and yields to be maintained at a reasonable level
after repeated reactions. Generally, recycling is based on the non-volatile nature
of ionic liquids and the solubility differences between ionic liquids, organic
compounds and water. Products can be extracted from ionic liquids with a nonpolar solvent or they can be separated by distillation. A water immiscible ionic
liquid can be washed with water to get a water soluble product or side products
out of the reaction mixture.
2.1.11. Salient features of ILs
Some of the salient features of ILs which makes them so attractive are as follows:
 They show negligible vapour pressure and are non-flammable nature.
 They have high thermal stability.
 They serve as a good medium to solubilize gases such as H2, CO, O2 and
CO2. Many reactions are now being performed using ionic liquids and
supercritical CO2.
72
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
 Their ionic character enhances the reaction rates to a great extent in
many reactions including microwave assisted and ultrasound promoted
organic synthesis.
 They are to dissolve a wide range of inorganic, organic, organometallic
compounds and even polymeric materials.
 They are highly polar.
 Most of the ionic liquids may be stored without decomposition for a long
period of time.
 They exhibit Brønsted, Lewis and Franklin acidity, superacidity as well as
basicity.
 They are immiscible with a number of organic solvents and provide a
non-aqueous, polar alternative for two-phase systems. Hydrophobic ionic
liquids can also be used as immiscible polar phases with water.
 Because of their non-volatile nature, products could be easily isolated by
vacuum distillation, leaving behind the IL pure enough for recycling after
the reaction.
 They are relatively cheap and easy to prepare.
 ILs may be termed as “designer” and ‘neoteric’ solvents since their
properties can be adjusted to suit for the particular process by changing
anion or cation or both.
These properties of ionic liquid provoked to carry out the synthesis of ionic
liquids and used in multicomponent reaction to push the reaction in to the
forward direction. Accordingly the work described in this chapter deals with
following aspects.
 Synthesis of Ionic Liquids [DBU]Ac and PEG-DIL
Modified synthesis of 1,8-diazabicyclo[5.4.0]-undec-7-en-8-ium acetate
[DBU][Ac] via ultrasound and synthesis of poly(ethylene glycol)
functionalized dicationic ionic liquid (PEG-DIL) by conventional method.
 Characterization of Ionic liquids.
Both the ionic liquids [DBU][Ac] and PEG-DIL were characterized by 1H
NMR, and 13C NMR spectroscopy. The obtained spectroscopic data are in
agreement with literature.
73
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
2.2. EXPERIMENTAL
2.2.1. Materials
All the chemicals were of research grade and used without further purification.
The reactions were performed in D-compact ultrasonic cleaner with a frequency
of 50 kHz and power 250 W. 1H NMR and
13C
NMR spectra were recorded on
Bruker Avance 400 MHz spectrometer by using D2O and DMSO-d6 as the
solvents.
2.2.2. Preparation of Ionic liquids
2.2.2.1. Preparation of [DBU][Ac]
Basic task-specified ionic liquid, 1,8-diazabicyclo[5.4.0]-undec-7-en-8-ium
acetate ([DBU][Ac]) synthesized through the simple neutralization reaction of
equal molar amounts of DBU and acetic acid. The general procedure for the
synthesis is given below.
2.2.2.2. General procedure for preparation of [DBU][Ac] under ultrasound
irradiation
A 100 ml, three-necked, round bottom flask was charged with 1,8Diazabicylco[5.4.0]-undec-7-ene (DBU). Aliquots of acetic acid (1 equiv.) were
added over a period of 15 min to DBU (1 equiv.) by maintaining the temperature
below 5 °C in an ice bath under ultrasound irradiation. After the addition was
over the ice bath was removed and the reaction mixture was exposed to
ultrasound irradiation for an additional period of 15 min at ambient
temperature. The round-bottomed flask was suspended at the centre of the
cleaning bath, 5 cm below the surface of the liquid to get maximum irradiations.
After the completion of reaction, obtained oily residue was dried in vacuum at 60
°C for 1 h to afford [DBU][Ac] as a light yellow, viscous liquid.
74
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
We successfully synthesized [DBU][Ac] under ultrasound irradiation within
90 min. [90]. The same transformation under conventional conditions as
reported by Chen et al. took 48 h to complete [91].
2.2.2.3. Preparation of PEG-DIL
The poly(ethylene glycol) functionalized dicationic ionic liquid (PEG-DIL) was
prepared by reacting PEG600 with 1-butylimidazole (Scheme 2.2). The general
synthesis split into two steps. The first step involves synthesis of PEG600OSO2CH3
via a reaction of PEG600 with methanesulfonyl chloride using pyridine as a base
in methylene dichloride. The second step, key step, in the synthesis of PEG-DIL
was a nucleophilic substitution between PEG600OSO2CH3 and 1-butylimidazole to
generate the desired PEG functionalized dicationic ionic liquid.
2.2.2.4. General procedure for preparation of PEG-DIL
A 100 ml, three-necked, round bottom flask was equipped with a magnetic
stirring needle and air condenser. The flask was charged with PEG600 (1 equiv.)
and pyridine (3 equiv.) in methylene dichloride and cooled in ice bath to 0 oC.
Methanesulfonyl chloride (3 equiv.) in methylene dichloride was added drop
wise to the stirred mixture over a period of 30 min by maintaining the
temperature below 5 °C. After complete addition, the ice bath was removed and
stirring was continued at room temperature for 12 h. The white precipitate
obtained was removed by filtration and the filtrate was washed twice with brine
and dried over anhydrous
sodium
sulphate.
The intermediate,
PEG
methanesulfonate was obtained after removal of the solvent under vacuum as
light yellow oil. A 250 ml round-bottom flask was charged with the obtained PEG
75
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
methanesulfonate and 1-butylimidazole in toluene. The reaction mixture was
stirred at 80 oC for 24 h. After the completion of reaction, the lower phase was
washed twice with 5 mL of toluene and once with 10.0 mL of ethyl ether.
Removal of the solvent under vacuum afforded PEG-DIL as light yellowish oil.
2.3. CHARACTERIZATION
2.3.1. Characterization of [DBU][Ac]
[DBU][Ac] was characterized by 1H NMR, and 13C NMR spectroscopy. The spectra
were run on Brucker Avance 400 MHz NMR spectrophotometer using D2O
assolvent. The shift values are obtained in δ ppm.
2.3.1.1. NMR Spectral Study
In order to confirm the structure of ionic liquid, 1H NMR and 13C NMR analysis of
[DBU][Ac] were performed. 1H NMR spectrum of 1,8-Diazabicyclo[5.4.0]undec-7en-8-ium acetate showed six different signals. All the values were in good
agreement with reported data in literature [91]. The signal for three protons of
CH3 group appeared as a singlet at 1.766 δ value. The signal appeared at 1.5171.675 δ value as a multiplet was interpreted for six protons at position 3, 4, 5.
The signal for two hydrogen at position 10 appeared at 1.842-1.901 δ value as a
multiplet. The signal appeared at 2.394-2.650 δ value as a multiplet was
interpreted for two protons at position 6. The two protons attached to ring
carbon having nitrogen atom on one side (at position 2) appeared as a multiplet
at slightly downfield region of the spectrum i.e. 3.165-3.193 δ. Similarly the four
protons at position 9 & 11 appeared as a multiplet at 3.278-3.437 δ.
13C
NMR
spectrum showed nine signals in aliphatic region at 19.5-54.1 δ. Two signals at
165.9 δ and 179.4 δ stands for the carbon at position 7 and carbonyl carbon
respectively present in the structure. All the spectroscopic data were in
agreement with the expected structure of the ionic liquid. 1H NMR and 13C NMR
spectrum of [DBU][Ac] are shown in figure 2.5 and 2.6 respectively.
76
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
Figure 2.5 1H NMR spectrum of [DBU][Ac]
Figure 2.6 13C NMR spectrum of [DBU][Ac]
77
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
2.3.2. Characterization of PEG-DIL
PEG methanesulfonate and PEG-DIL were characterized by 1H NMR, and APT
NMR spectroscopy.
2.3.2.1. NMR Spectral Study of PEG methanesulfonate
The PEG methanesulfonate (CH3O2SO-PEG-OSO2CH3) was characterized by 1H
NMR and APT NMR spectroscopy. 1H NMR spectrum of PEG methanesulfonate
showed three different signals. The signal for six protons of two CH3 group
appeared as a singlet at 3.100 δ ppm. The signal appeared at 3.655-3.790 δ ppm
as a multiplet was interpreted for 68 protons of PEG backbone. The signal for
four protons of two CH2 group appeared at 4.397 δ ppm as a triplet. APT NMR
spectrum showed three signals in aliphatic region. Signal at 37.0 δ ppm was
assigned the carbon of terminal CH3 groups. Two downward signals at 68.4 δ
ppm and 69.6 δ ppm stands for the carbon of CH2 groups present in the
structure. All the spectroscopic data were in agreement with the expected
structure of PEG methanesulfonate. 1H NMR and APT NMR spectrum of PEG
methanesulfonate are shown in figure 2.7 and 2.8 respectively.
Figure 2.7 1H NMR spectrum of PEG methanesulfonate
78
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
Figure 2.8 APT NMR spectrum of PEG methanesulfonate
2.3.2.2 NMR Spectral Study of PEG-DIL
In order to confirm the structure of ionic liquid, 1H NMR and APT NMR analysis
of PEG-DIL were performed. 1H NMR spectrum of PEG-DIL showed ten different
signals. The signal for six protons of two CH3 groups (-CH2CH2CH2CH3) appeared
as a triplet at 0.907 δ ppm. The signal appeared at 1.214-1.287 δ ppm as a
multiplet could be interpreted for four protons of two CH2 groups (CH2CH2CH2CH3). The signal for four protons of two CH2 groups (-CH2CH2CH2CH3)
appeared at 1.758-1.813 δ ppm as a multiplet. The signal for six protons of two
CH3 groups (CH3SO3-) appeared as a singlet at 2.354 δ ppm. The signal appeared
at 3.515-3.801 δ ppm as a multiplet could be interpreted for 68 protons of
((OCH2CH2)n). The four protons attached to carbon having nitrogen atom on one
side (-NCH2) appeared as a triplet at 4.203 δ ppm. Similarly the signal for four
protons of two CH2 groups (-NCH2) appeared as a triplet at 4.313 δ ppm. The
signal appeared at 7.700 δ ppm as a singlet was interpreted for two protons (–NCH=CH-N-) of the imidazole ring. Similarly the signal for two protons (–NCH=CH-N-) of the imidazole ring appeared as a singlet at 7.807 δ ppm. The
proton attached to ring carbon having nitrogen atom on both side appeared as a
79
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
singlet in the downfield region of the spectrum i.e. 9.157 δ ppm. APT NMR
spectrum shows two upward signals at 13.7 δ ppm and 37.31 δ ppm stands for
the carbon of CH3 groups and seven downward signals at 19.3, 31.9, 44.0, 48.7,
48.9, 68.6 and 70.2 δ ppm stands for the carbon of CH2 groups present in the
structure. Three upward signals at 120.4, 122.5 and 123.28 δ ppm stands for the
methane carbon of the imidazole ring present in the structure. All the
spectroscopic data were in agreement with the expected structure of the ionic
liquid. 1H NMR and APT NMR spectrum of PEG-DIL are shown in figure 2.9 and
2.10 respectively.
Figure 2.9 1H NMR spectrum of PEG-DIL
80
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
Figure 2.10 APT NMR spectrum of PEG-DIL
81
Chapter 2
Synthesis of [DBU][Ac] and PEG-DIL
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