Chloride Salt - [C 16 -mim] [Cl]
4.3. General Procedure for the Substitution of Anions with Imidazolium Cations Imidazolium Cations
Foreword:
The anion exchange reactions of low temperature ionic liquids can be divided into two distinct categories as laid down by Welton [15]: (I) direct treatment of halide salts with Lewis acids and (II) the formation of ionic liquids by anions metathesis. For the purposes of this work, the later approach was followed. There are wide ranges of methods that have been applied over the years and many authors have suggested refinements of the methods employed [18 - 19]. The preparative methods employed by many authors generally follow similar lines [20]. The main objective of all anion exchange reactions is the formation of the desired ionic liquid uncontaminated with unwanted cations of anions. Cammarata
[21] and Fuller [22] demonstrated metathesis reactions can be carried out entirely in organic solvents.
Purification of ionic liquids formed by anion metathesis can expose a number of problems as this author experienced. The most common impurities are halide anions such as chlorine ion, which during anion exchange synthesis, the chloride ion is ineffectively separated from the final product. The presence of such impurities is not only extremely detrimental for further electrochemistry investigations, but the presence of such undesired impurities can also alter the nature and behaviours the materials. Many authors [22 - 27] have reported that the physical properties such as densities and viscosities of affected materials can be radically altered by the presence of unwanted ions. In practical terms, it is suggested by this author and others that in any application where the presence of halide ions many cause problems, the concentrations of these undesired impurities should be monitored. Clearly, the impurity likely to be present in large concentrations in most ionic liquids is water. The removal of reaction solvents is generally easily achieved by heating the ionic liquid under vacuum. Water is generally one of the most problematic impurities to remove, and as described in this section during synthesis, the presence of water was kept to a minimum (35ppm) during all preparations. The ionic liquids should be heated to at least 70 °C for several hours with stirring to achieve an acceptable level of reduced water. According to Welton [16] 1-N-headecyl-3-methylimidazolium chloride can absorb 2 wt % water, when exposed to air for an extended period. Therefore all ionic liquids must be stored in inert environments i.e. a glove box. If an exact knowledge of the amount of water content water present in a sample is required, then the water content of the material in question can be determined by utilising the Karl-Fischer35, titration method, and the less precise method of IR spectroscopy.
35Karl Fischer (Model CA-02, AbimED Analysen- Technik)
Synthesis:
LiPF6 N N
H H
H
(CH2)15-CH3 N
N H H
H
(CH2)15-CH3 Cl + PF6 LiCl
1-N-hexadecyl-3-methylimidazolium hexafluorophosphate 1-N-hexadecyl-3-methylimidazolium chloride
+ + +
-Acetone
57 °C
Procedure:
The following description is the general procedure applied to all anion exchanges that where performed with the 1-N-hexadecyl-3-methylimidazolium chloride ionic liquids and the named lithium salts already outlined in table (4.1.1).
In argon filled glove box an exact amount of purified 1-n-hexadecyl-3-methylimidazolium chloride powder is weighted into a nitrogen flask. This is followed by the corresponding calculated ss of the respective lithium salt. Approximately five times the mass of the comb ts of dried ethyl acetate (99.98 % - dried 35 ppm H20) is added and the reactants or 24 hrs. The solution is permitted to cool down and recrystallise where d to form. The crystal suspension is filtered to remove the precip chloride salt. The organic phase is removed by drying under vacuum. The crystalline p oduct is redissolved in diethyl ether and washed three times with dried acetone
≈125 ppm) within the acetone assists in the further precipita ium chloride. The organic phase is cooled with ice to 4 °C encouraging drying of the product for 6 hrs; the recrystallised product as a fine crystalline powder. The recrystallisation process was repeated three times to ensure that all LiCl was removed from the product and that the product was a pure as possible. The pure recrystallised product was further vacuum dried in a desiccator for
s was checked by titration, 1H NMR, 13C NMR, DSC and MS.
mole ma ined reactan
are stirred f
fine white crystals are observe itated lithium
r
(98.5 % analysis). The water content ( tion of lith
recrystallisation. After low vacuum was observed to form
24 h. The purity of all salt
4.3.1.
Purification of Newly Synthesised Imidazolium Salts – Titration of LiCl:
At the beginning of this chapter, the author emphasised the current difficulties researchers are encountering in acquiring reliable published data of various statistical values of many ionic liquids. It is often the case that the data values are influenced by impurities left-over from their syntheses. From the perspective of characterisation of newly synthesised ionic liquids in many circumstances, certain quantities of impurities can be tolerated. But in the circumstances of electrochemical experiments very sample amounts of impurities can lead to serious erroneous if not invalid data results. Therefore, every possible precaution must be taken to ensure that the samples are as pure as possible.
In the previous section {4.3}, the author presented a method for an anion exchange between imidazolium and the lithium salts. The chief by-product of this procedure is LiCl.
The purification of the salts and the removal of the LiCl proved extremely difficult. In normal circumstances, LiCl is extracted by washing the salt with purified water, in which the LiCl salt in most circumstances falls out of the solution. In the case of our newly synthesised lithium salts that were presented earlier in sections {4.1.1}, {4.1.2}, and {4.1.3}, the application of water as a means of removal of LiCl was out of the question due to their extreme sensitivity to moisture. The above procedure of anion exchange and purification required both patience and a good sense of feel and common sense. During literature searches, the author found presently no reliable non-aqueous method of extraction for LiCl. The aforementioned method in section {4.3} of purification was extremely effective in this work.
In most salts after numerous washings with acetone and ethyl acetate, extremely low concentrations of LiCl were detected by means titration.
Titration Method:
The most common method for the measurement of the remaining amount of LiCl in the substituted imidazolium salts is accomplished by the titration of LiCl with silver nitrate solution.
Titration American Chemical Society Method[32]:
Weigh accurately between 0.15 to 0.20 g of salt sample and dissolve in 50 ml of ter (all samples are water-soluble) in a 250 ml glass-stoppered flask. Add 1 ml of ichlorofluorescein. While stirring, mix and titrate with 0.1 N silver nitrate solutions. The silv r n
Millipore wa d
e itrate flocculates and the mixture changes to a faint pink colour.
(g) wt Sample
239 . 4 x AgNO N
x
%LiCl = ml 3 Equ. [4.1.]
In most salts, the percentage of LiCl was found to have less than 0.001 % which l within tolerances. Other techniques were also utilised for the detection of the rem
is
wel aining
LiCl. The method of mass spectroscopy is also an extremely useful technique in determining the extent of the re
sim of
LiC a
Bun ing
flam sed
over a flam No
but
nec ilar
experim
con ily
and the
ove of
this he
aterials and re sources for evaluation. The characteristics and behaviours of the substituted anions can be seen in tables (4.3.1) to (4.3.4).
moval of the newly formed LiCl by-product after purification. Another pler and quicker test that can be carried out in the laboratory to check for the presence
l in any sample is to use a flame test. By passing a small amount of purified sample over sen flame one can determine if the substitution reaction was a success from the ensu
e colour. The compound lithium chloride exhibits a distinctive red colour on being pas e.
te:
The presence of water within the acetone organic solvent, played important essary role in assisting in the removal of the remaining dissolved LiCl. In sim
ents of other authors, all the substituted products were washed in water. Acetone was sidered by the author to be the appropriate solvent that dissolves all the reactants read retained enough water to promote the expulsion of LiCl without being detrimental to rall process. Later titration, IR, MS, and 1H-NMR investigations confirmed the wisdom procedure. For similar samples in the literature, good repeatability was observed. T
e characterisations of these m following tables (4.3.1) to (4.3.4) summarise th
where possible literatu
Table. (4.3.1)
Synthesis : 1-N-hexadecyl-3-methylimidazolium hexafluorophosphate [C16-mim] [PF6]
horthand Structure M:
g mol-1 K 7.71–7.78 doublet -N-CH-CH-N-
0.85 triplet -CH3
Anion
lithium
hexafluorophosphate LiPF 151.90 NA. N.
LiPF6
Synthesis : 1-N-hexadecyl-3-methylimidazolium tetrafluoroborate [C16-mim] [BF4]
Shorthand Structure M:
g mol K 7.71–7.78 doublet -N-CH-CH-N-
0.85 triplet -CH3
Tab. (4.3.3)
Synthesis : 1-N-hexadecyl-3-methylimidazolium bis-[1,2-benzenediolato(2-)-O,O’]borate [C16-mim] [H8]
Melting Point
DSC Mass Spectrometry MS
1H-NMR - [C16-mim Solvent - DMSO] [H8]
Name
Shorthand Structure M:
g mol-1 K
-7.71–7.79 doublet
-N-H
Synthesis : 1-N-hexadecyl-3-methylimidazolium bis-[3,4,5,6-tetrafluoro-1,2-benzenediolato(2-)-O,O’]borate [C -mim] [F8] 16
Melting Point
DSC Mass Spectrometry MS
1H-NMR - [C -mim] [F Solvent - DMSO 16 8]
Name
Shorthand Structure M:
g mol-1 K
F 7.71–7.78 doublet -N-CH-CH-N-
0.85 triplet -CH3