Synthesis of Novel Imidazolium Ionic Liquids and their Evaluation as
Selective Adsorbents.
Marcus P. Merrin and Robert D. Singer*
Department of Chemistry, St. Mary’s University, Halifax, Nova Scotia, B3H 3C3 CANADA
We have synthesized a selection of novel imidazolium tetrafluoroborate and hexafluorophosphate
ionic liquids 1, 2, 3, 4 containing an aldehyde or ketone substituent by direct substitution using
diethylacetal-protected chloroaldehydes and unprotected chloroketones under inert atmosphere.
The imidazolium chlorides thus generated were converted to the corresponding tetrafluoroborate
or hexafluorophosphate salts by metathesis reactions performed by several methodologies. We
also report the results of investigations into the formation of, and removal of dithiols from these
compounds and our preliminary evaluation of their potential usefulness as “getting” agents for
thiols and related contaminants in gas streams.
Green Context
Sulfur compounds in natural gas result in “sour gas” which upon combustion produce oxides which are highly
acidic, contributing to acid rain and resulting in corrosion of burner equipment. Distillation and flaring for liquid
fuels and washing for gas streams to afford “sweet gas” are strategies currently in use to address these problems.
Aqueous washing of gas streams results in wet gas which is corrosive to pipelines. Room temperature ionic
liquids designed specifically for desulfurization have the advantage over other solvents that, being effectively nonvolatile they are more readily cleaned and reactivated for reuse, avoiding release of volatile solvents to the
environment.
Introduction
The growing number of review articles1-4 attest to the recent growing interest in the use of ionic
liquids as novel, efficient and environmentally safe alternatives to conventional solvents in a
wide variety of chemical reactions and processes. The non-volatile nature of ionic liquids
combined with their polar but non-protic nature renders these compounds prime candidates for a
wide variety of industrial processes where the costs of replenishment or recycling are a
significant factor. In addition, the study of the synthesis of ionic liquids has matured to the point
where “designer” liquids can be produced with specific functionality appropriate for particular
processes. With this philosophy in mind, we set out to address the problem of the removal of
sulfur-containing contaminants from gaseous fuel streams such as natural gas to produce
essentially sulfur-free fuels known as “sweet gas”. Our approach involves adding aldehyde or
ketone functionality to the 1- and / or 3-substituents of 1,3-disubstituted imidazolium ionic
liquids to investigate the tendency of these substances to form thioketals / thioacetals and to
investigate methods of regenerating the original ionic liquid upon liberation of the captured sulfur
-1-
compounds. We also assess the degree to which the liquids degrade in functionality on repeated
cycling through the thioketal / thioacetal formation and removal.
S
S
N
N
R
S
N
5a R = CH3
N
R S
6a R = CH3
Results and Discussion
Preparation
The reaction of 2-chloroacetaldehyde diethylacetal with either of the imidazole reagents proceeds
quite slowly and the reactions of 3-chloropropionaldehyde diethyacetal are slower still. On the
other hand, the reactions of neat chloroacetone (2,4) proceed very rapidly with the evolution of
considerable heat, resulting in polymeric intractable products. It was necessary to conduct these
reactions in THF solution with initial cooling to 0oC
Attempts to metathesize the diethylacetal of compound 1a using aqueous tetrafluoroboric acid
with concomitant deprotection of the aldehyde function were unsuccessful. In order for this
procedure to work, it is neccessary to provide an organic phase into which the product
tetrafluoroborate can partition. To date we have not succeeded in finding a water-imiscible
solvent that will dissolve the tetrafluoroborate. It was, however possible to secure the
tetrafluoroborate of the diethylacetal by treating the chloride with an aqueous solution of silver
tetrafluoroborate, filtering off the silver chloride precipitate.
Deprotection of this compound proved to be problematic,. Reflux (100oC) with aqueous
trifluoroacetic acid for two days produced no detectable loss of protection. Presumably this is as
a result of the proximity of the positive imidazolium centre. Reflux with 5M HCl yielded the
deprotected aldehyde in virtually quantitative yield.
-2-
Table 1 1H NMR data for 1,3 in methanol-d5 δppm
C1 Me
C2
C3
C4
C5
C6
C7,8
C9,10
1a
4.13s
(diethyl- (NMe)
acetal)
9.12s
7.92s
7.93s
4.55 4.63d
4.98 5.15 t
3.53 3.90 q,d
1.25 1.02 t
1a
4.037s
9.04
7.69
7.62
4.33d
4.834.95
-
-
3a
In
Progress
3b
In
Progress
C6
C7
C8
Table 2 13C NMR Data for 1, 3 in methanol-d5 δppm
C1 Me
C2 arom C3 arom C4 arom C5
O-CH2- CH2-CH3
1a
(diethyl36.20
acetal)
138.63
123.73
123.51
63.14
100.77
51.28
14.97
1a
36.62
138.90
124.74
124.50
54.92
95.67
-
-
3a
In
Progress
3b
In
Progress
-3-
Table 3 1H NMR data for 2, 4 in methanol-d5 δppm
C1 Me
C2 arom
C3arom
C4 arom
C5
C6
C7,8
C9,10
2a
4.03s
9.06s
7.72s
7.66s
5.46s
2.33s
-
-
2b
4.11s
7.82s
7.18s
7.18s
3.80m
2.21m
1.72s
4a
-
8.68
7.49
7.49
5.37
2.28
-
-
4b
In
Progress
C7
C8
Table 4 13C NMR Data for 2, 4 in methanol-d5 δppm
C1 Me
C2arom
C3arom
C4arom
C5
C6
2a
36.68
138.82
124.80
124.28
68.61
27.17
2b
38.52
138.79
127.95
125.27
48.43
36.90
148.01
26.31
4a
-
135.46
124.36
124.36
68.73
27.07
-
-
4b
In
Progress
Table 6 1H NMR data for 5,6 in methanol-d5 δppm
5a
6a
C1
C2arom C3arom C4arom C5
In
Progress
In
progress
C6
S-CH2
Table 6 13C NMR data for 5,6 in methanol-d5 δppm
5a
6a
C1
C2
In
progress
In
progress
C3
C4
C5
C6
Table 7 Infrared Data
Compound
1a
2b
4a
Wavenumbers (cm-1)
623s, 1030vs,1175s, 1449m, 1580s, 1626s, 1745m, 2946s, 3143s
666s, 752m, 908s, 1080m, 1231s, 1518s, 1709s, 2954s, 3087m
758m, 832m, 1064s , 1584m, 1726s, 2829s, 2932s, 3113s
-4-
Table 8 Physical Properties
Compound
1a
mp/oC
<20
1b
2a
<20
2b
3a
3b
4a
<20
36-38
bp
M+
C6H9N2O2+ 125m/z
C4H7N2+ 83m/z [M - CHO]+
C5H7N2+ 95m/z [M – C2HO]+
M+ + CH3OH 157m/z
C7H11ON2+
139m/z
C9H15ON2+ 167m/z
C9H13N2O2+
181m/z
4b
Experimental Section
Unless otherwise stated, all reagents were purchased from the Aldrich Chemical Company Inc.
Milwaukee U.S.A. and were used without further purification. Inert atmospheres were highpurity (4.8 grade) Argon.
NMR Spectra were acquired using a Bruker AV500 400MHz Spectrometer (Atlantic Region
Magnetic resonance Centre), Deuterated solvents were obtained from CIL. All shifts are quoted
relative to TMS (1%).
Mass Spectral data were obtained on an Agilent ion-trap LCMS instrument using direct injection
of dilute methanol (ca. 20 μg.mL-1) solutions.
Preparation of 1-methyl-3-ethanal imidazolium chloride 1a
To a stirred flask fitted with a reflux condenser containing 2.5 mL (31 mmol) methylimidazole
under inert atmosphere, was added 40. mL (26 mmol) chloroacetaldehyde diethyl acetal. The
mixture was maintained at a temperature of 70 - 80 oC for 3 days.
The mixture was washed with dry distilled tetrahydrofuran (3 x 20 mL) to remove unreacted
starting material, to afford a highly viscous red-brown oil. This oil was subjected to high vacuum
at 80oC for 8 hours to remove residual methylimidazole.
The resulting oil was subjected to flash column chromatography on silica (Caledon 40 - 60
micron) with Chloroform / Methanol 85:15 eluent. The solvent was removed under vacuum, and
residual entrained solvent removed under high vacuum at 60oC to afford a brown liquid 0.42 g
(7.0 %) corresponding to the diethylacetal of structure 1a.
This was treated with 15M hydrochloric acid (10 mL) under reflux for 24 hours. On removal of
water, the crude solid material was recrystallized from methanol to give the free aldehyde as a
white crystalline solid.
-5-
Preparation of 1-methyl-3-propanal Imidazolium Chloride 1b
To a stirred flask fitted with a reflux condenser containing 1.14 mL (14 mmol) methylimidazole
under inert atmosphere, was added 2.0 mL (12 mmol) 3-chloropropionaldehyde diethyl acetal.
The mixture was maintained for 6 days at a temperature of 70 - 80 oC.
The mixture was washed with 3 x 20 mL dry distilled tetrahydrofuran to remove unreacted
starting material, to afford a highly viscous red-brown oil. This oil was subjected to high vacuum
at 80oC for 8 hours to remove residual methylimidazole.
Preparation of 1-methyl-3-propanone Imidazolium Chloride 2a
To a cooled (0oC) stirred flask fitted with a septum and inert atmosphere line containing 1.14 mL
(14 mmol) methylimidazole dissolved in 10 mL tetrahydrofuran, was added 2.0 mL (12 mmol) 3chloroacetone dissolved in 10 mL tetrahydrofuran over 1 hour. The solution was allowed to
attain room temperature and stirred for 2 days. The product precipitated from solution as a
cream-coloured solid. The mixture was allowed to settle and the THF removed with a cannula.
The mixture was washed with 3 x 20 mL dry distilled tetrahydrofuran to remove unreacted
starting material, and the resultant solid was subjected to high vacuum at 80oC for 8 hours to
remove residual solvent.
Preparation of 1,3-di(ethanal) imidazolium chloride 3a
Sodium hydride 0.37g (14.7 mmol) was weighed into a dry r.b. flask equipped with a teflon
stirrer flea and rubber septum under an argon atmosphere. Dry, distilled tetrahydrofuran (10 mL)
was introduced through the septum with a syringe and cannula. Imidazole 1.0 g (14.7 mmol) was
weighed into a second r.b. flask equipped with a rubber septum under argon atmosphere. Dry,
distilled tetrahydrofuran (10 mL) was introduced through the septum with a syringe and cannula.
Both the imidazole solution and the stirred suspension or sodium hydride in THF were cooled in
an ice bath for 1 hour. The imidazole solution was added dropwise to the NaH suspension over
20 minutes. This mixture was stirred for 2 hours whilst warming to room temperature.
Chloroacetaldehyde diethyacetal 4.40 mL (29.4 mmol) was added dropwise over 1 hour, and the
mixture raised to reflux temperature (70oC)and maintained at this temperature for 4 days.
Preparation of 1,3-di(3-propanal) imidazolium chloride 3b
Sodium hydride 0.37g (14.7 mmol) was weighed into a dry r.b. flask equipped with a teflon
stirrer flea and rubber septum under an argon atmosphere. Dry, distilled tetrahydrofuran (10 mL)
was introduced through the septum with a syringe and cannula. Imidazole 1.0 g (14.7 mmol) was
weighed into a second r.b. flask equipped with a rubber septum under argon atmosphere. Dry,
distilled tetrahydrofuran (10 mL) was introduced through the septum with a syringe and cannula.
Both the imidazole solution and the stirred suspension or sodium hydride in THF were cooled in
an ice bath for 1 hour. The imidazole solution was added dropwise to the NaH suspension over
-6-
20 minutes. This mixture was stirred for 2 hours whilst warming to room temperature. 3chloropropionaldehyde diethyacetal 4.40 mL (29.4 mmol) was added dropwise over 1 hour, and
the mixture raised to reflux temperature and maintained at this temperature for 4 days.
Preparation of 1-methyl-3-ethanal imidazolium tetrafluoroborate 3a
Attempts to metathesize compound 1a using aqueous tetrafluoroboric acid 8.1 g (48% solution)
with concomitant deprotection of the aldehyde function were unsuccessful.
However, dissolving the diethylacetal in distilled water and treating with a solution of silver
tetrafluoroborate (1 equiv. In 10 mL water) precipitated silver chloride. The soludtion was then
extracted with 3 x 20 mL chloroform. The chloroform layers were combined , dried over sodium
sulfate and evaporated under vacuum to yield the tetrafluoroborate of the diethylacetal ( 10%
yield). This was then dissolved in 20 mL water and subjected to treatment4 with 50%
trifluoroacetic acid in chloroform (10 mL) for 90 min. at 0oC.
Preparation of 1,3-di(3-ethanal) imidazolium chloride 3a
Sodium hydride 0.75g (28.4 mmol) was weighed into a dry r.b. flask equipped with a teflon
stirrer flea and rubber septum under an argon atmosphere. Dry, distilled tetrahydrofuran (10 mL)
was introduced through the septum with a syringe and cannula. Imidazole 2.0 g (29.4 mmol) was
weighed into a second r.b. flask equipped with a rubber septum under argon atmosphere. Dry,
distilled tetrahydrofuran (10 mL) was introduced through the septum with a syringe and cannula.
Both the imidazole solution and the stirred suspension or sodium hydride in THF were cooled in
an ice bath for 1 hour. The imidazole solution was added dropwise to the NaH suspension over
20 minutes. This mixture was stirred for 2 hours whilst warming to room temperature. 3chloroacetaldehyde diethyacetal 8.70 mL (58 mmol) was added dropwise over 1 hour, and the
mixture raised to reflux temperature and maintained at this temperature for 4 days. In Progress.
Preparation of 1,3-di(3-pentanone) imidazolium chloride 4b
Sodium hydride 0.75g (28.4 mmol) was weighed into a dry r.b. flask equipped with a teflon
stirrer flea and rubber septum under an argon atmosphere. Dry, distilled tetrahydrofuran (10 mL)
was introduced through the septum with a syringe and cannula. Imidazole 2.0 g (29.4 mmol) was
weighed into a second r.b. flask equipped with a rubber septum under argon atmosphere. Dry,
distilled tetrahydrofuran (10 mL) was introduced through the septum with a syringe and cannula.
Both the imidazole solution and the stirred suspension or sodium hydride in THF were cooled in
an ice bath for 1 hour. The imidazole solution was added dropwise to the NaH suspension over
20 minutes. This mixture was stirred for 2 hours whilst warming to room temperature. 5chloro2-pentanone 6.70 mL (58 mmol) was added dropwise over 1 hour, and the mixture raised to
reflux temperature and maintained at this temperature for 4 days. In Progress
-7-
Metathesis of Chloride 1a to Hexafluorophosphate form.
Compound 2a chloride form (0.5g) was dissolved in water (10 mL). This was treated with 60%
hexafluorophosphoric acid and stirred for 10. The resulting aqueous solution was washed with
chloroform (3 x 10 mL)and then extracted with ethyl acetate(3 x 10 mL). These extracts were
combined, washed with saturated sodium bicarbonate solution and evaporated. This solid was
filtered off, washed with water (3 x 20 mL) and dried under vacuum to yield 0.8 grams of brown
oil.
Metathesis of Chloride 2a to Hexafluorophosphate form.
Compound 2a chloride form (0.5g) was dissolved in water (10 mL). This was treated with 60%
hexafluorophosphoric acid and stirred for 10 minutes whereupon a distinct brown non-aqueous
layer formed. Attempts to extract this into chloroform (10 mL) caused the brown layer to
solidify. This solid was filtered off, washed with water (3 x 20 mL) and dried under vacuum to
yield 0.8 grams of tan solid.
Metathesis of Chloride 4a to Hexafluorophosphate form.
Compound 4a chloride form (0.5g) was dissolved in water (10 mL). This was treated with 60%
hexafluorophosphoric acid (10 mL) and stirred for 10 minutes whereupon a distinct change to a
light orange colour was noted. The resulting aqueous material was ectracted with three 10 mL
portions of chloroform, and then three 10 mL portions of ethyl acetate. The ethyl acetate was
combined and dried over anhydrous sodium sulphate overnight. The solution was filtered and the
solvent removed under vacuum
Formation of Thioketals from PF6- salts of 2a.
Preparation of 1,3-dithiolane ketal of 2a (PF6- salt) (5).
Molecular sieves 3Å 2.5 g was heated in excess of 160 oC for two days5. It was then held at
140oC under high vacuum for a further 6 hours before use. The desired imidazolium
hexafluorophosphate (2a, 0.05g, 0.17 mmol ) was added to the sieves in a round bottomed flask
flushed with dry argon and equipped with a rubber septum. Dry acetonitrile (10 mL) was added,
followed by dithioethane (0.15 mL 1.6 mmol). The mixture was allowed to stand for 8hours at
room temperature and then raised to reflux temperature for 24 hours.
-8-
1-methyl-3-(propanone dithiocresol ketal)imidazolium hexafluorophosphate 6
Molecular sieves 3Å 2.5 g was heated in excess of 160 oC for two days. It was then held at
140oC under high vacuum for a further 6 hours before use. The desired imidazolium
hexafluorophosphate (2a, 0.05g, 0.17 mmol ) was added to the sieves in a round bottomed flask
flushed with dry argon and equipped with a rubber septum. Dry acetonitrile (10 mL) was added,
followed by thiocresol (0.2g 1.4mmol). The mixture was allowed to stand for 8hours at room
temperature and then raised to reflux temperature for 24 hours.
The above method was thought at first to be promising, but on workup it was determined that the
product and / or starting imidazolium salt was irretrievably stuck to the molecular sieves. In
addition, the method is extremely limited in the choice of solvent for the reaction. Obviously
aldehydes, esters and ketones cannot be used and protic solvents are also excluded. Of the
remaining choices of common solvents, the salt 2a-PF6 was soluble only in acetonitrile. We
therefore embarked on a novel approach as follows:
Method II
1-butyl-3-methylimidazolium hexafluorophosphate (BMimPF6)(5 mL) was placed in a round
bottomed flask equipped with a stir-bar and flushed with dry argon. The flask was pumped under
high vacuum for 6 hours at 100oC to remove water. The salt 2a-PF6 (1.2g, 4.2 mmol) was
dissolved in this liquid. Thiocresol (2.3g, 18.5 mmol) was added, and the mixture stirred at
100oC under high vacuum for 8 hours. The mixture was allowed to cool, and the BMimPF6 ionic
liquid extracted with chloroform (3 x 10 mL). These extracts were combined and evaporated to
dryness.
Conclusions
We have successfully prepared samples of a selection of the proposed ionic liquids and we have
shown that at least one of them (1a) can be successfully converted from the diethylacetal to the
free aldehyde under conditions that do not degrade the functional species. This is especially
encouraging as this particular molecule has the aldehyde function in close proximity to the
imidazolium positive charge. The deprotection is an acid-catalyzed reaction and the positive
imidazolium centre is expected to discourage the approach of protons. We anticipate therefore
that adjusting the length of the side-chain (n in Fig. 1) may be used to fine-tune the reactivity of
the functional group towards the protection / deprotection cycle. The compounds produced are
all extremely hygroscopic, and this powerful affinity for water may also be of added benefit in
the proposed application, since when used as absorbents for sulfur, they may additionally remove
traces of water from the gas stream affording extremely dry gas.
Having successfully prepared ketone-type room temperature ionic liquids and diethylacetals of
the aldehyde-type room temperature ionic liquids, and having proved that the latter can be
deprotected under conditions that do not degrade the functionality, we are now in the process of
scaling up the syntheses and preparing bulk samples. These will be used to investigate the
optimum conditions for capture/ release of thiols, and suitable methodologies for recycling the
room-temperature ionic liquids.
-9-
References
1. Welton, T. Chem. Rev. 1999, 99, 2071
2. Holbrey, J.D.; Seddon, K.R.; Clean Products and Processes 1 1999, 233
3. Wasserscheid, P.; Welton, T. Angew. Chem. Int. Ed. 2000, 39, 3772
4. Zacharie, B.; Connolly, T.P.; Penney, C.L. J. Org. Chem. 60, 1995, 7072
5. P. Kumar, R.S. Reddy, A.P Singh, B. Pandey Tetrahedron Lett. 33,1992, 825
- 10 -