Strategies toward the design of energetic ionic liquids: Nitro- and nitrile-substituted N,N′-dialkylimidazolium salts

Article abstract of DOI:10.1039/b509260d

Twelve novel 1,3-dialkylimidazolium salts containing strongly electron-withdrawing nitro- and cyano-functionalities directly appended to the cationic heterocyclic rings have been synthesized; the influences of the substituents on both formation and thermal properties of the resultant ionic liquids have been determined by DSC, TGA, and single crystal X-ray diffraction, showing that an electron-withdrawing nitro-substituent can be successfully appended and has a similar influence on the melting behaviour as that of corresponding methyl group substitution. Synthesis of di-, or trinitro-substituted 1,3-dialkylimidazolium cations was unsuccessful due to the resistance of dinitro-substituted imidazoles to undergo either N-alkylation or protonation, while 1-alkyl-4,5-dicyanoimidazoles were successfully alkylated to obtain 1,3-dialkyl-4,5-dicyanoimidazolium salts. Five crystal structures (one of each cation type) show that, in the solid state, the NO₂-group has little significant effect, beyond the steric contribution, on the crystal packing. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b509260d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Strategies toward the design of energetic ionic liquids: nitro- and
nitrile-substituted N,N0-dialkylimidazolium saltsw
a
Alan R. Katritzky,*a Hongfang Yang,a Dazhi Zhang,z Kostyantyn Kirichenko,a
Marcin Smiglak,b John D. Holbrey,y*b W. Matthew Reichertb and
Robin D. Rogers*b
Received (in St Louis, MO, USA) 6th July 2005, Accepted 2nd December 2005
First published as an Advance Article on the web 13th January 2006
DOI: 10.1039/b509260d
Twelve novel 1,3-dialkylimidazolium salts containing strongly electron-withdrawing nitro- and
cyano-functionalities directly appended to the cationic heterocyclic rings have been synthesized;
the influences of the substituents on both formation and thermal properties of the resultant ionic
liquids have been determined by DSC, TGA, and single crystal X-ray diffraction, showing that an
electron-withdrawing nitro-substituent can be successfully appended and has a similar influence
on the melting behaviour as that of corresponding methyl group substitution. Synthesis of di-, or
trinitro-substituted 1,3-dialkylimidazolium cations was unsuccessful due to the resistance of
dinitro-substituted imidazoles to undergo either N-alkylation or protonation, while 1-alkyl-4,5-
dicyanoimidazoles were successfully alkylated to obtain 1,3-dialkyl-4,5-dicyanoimidazolium salts.
Five crystal structures (one of each cation type) show that, in the solid state, the NO2-group has
little significant effect, beyond the steric contribution, on the crystal packing.
develop ‘‘energetic liquids’’ (non-volatile liquid high energy
Introduction
density materials)29 for energy storage and propellant uses.
Drake and co-workers30 have prepared energetic ILs by
combining cations containing energetic functionality with
relatively small inorganic energetic anions ([NO3]ꢀ, [ClO4]ꢀ,
and [N(NO2)2]ꢀ). In this, and related work by Shreeve and co-
workers,31,32 energetic low melting organic salts contained
amine-functionalized triazoles and tetrazoles as cations. Im-
portantly, it was suggested that the ready formation of ionic
liquids with these cations appeared to be due to a similarity in
topographical shape and charge conventional IL systems.
Although ILs with imidazolium cations have been widely
studied, modification of the structure has been largely re-
stricted to variations in ring-alkyl group substitution33,34 or
to the addition of hydroxy,35 ether,36,37 epoxy,38 amine, or
cyano39 groups in the N-substituent sidechains, with the
notable exceptions of quite complex substituents in ‘‘Task
Specific Ionic Liquids’’.40 Direct modification of functionality
in the heterocyclic cation core has previously received only
limited attention,41–43 perhaps because of anticipation that the
combination of size/shape effects with changes in the cation
polarization and dipole moments might promote the forma-
tion of salts with relatively higher melting points.42
Ionic liquids (ILs; conventionally defined with a melting point
of below 100 1C) have come under intense scrutiny in recent
years. The intrinsic non-volatility, thermal stability, and large
liquid ranges achievable with many ILs have proven to be
important drivers1–7 supporting many advances beyond the
initial investigation of ILs as liquid electrolytes. Current topics
include electrochemistry,8–10 separation science,11–14 chemical
synthesis,2,4,5,15–19 and catalysis.4,5,17
This interest in ILs and their applications has led synergis-
tically to the development of an extensive, and diverse range of
organic salts that can support low melting ionic liquid phases
(with examples ranging from simple mononuclear20 to com-
plex21–25 anions) and to the exploration of materials applica-
tions utilizing the novel characteristics of these ionic liquid
phases, including thermal fluids, lubricants, photovoltaics, and
fuel cell electrolytes.26–28 One such example is in the effort to
a Center for Heterocyclic Compounds, University of Florida,
Department of Chemistry, Gainesville, Florida 32611, USA.
E-mail: Katritzky@chem.ufl.edu; Fax: þ1 352 392 9199;
Tel: þ1 352 392 0554
b Center for Green Manufacturing and Department of Chemistry, The
University of Alabama, Tuscaloosa, Alabama 35487, USA.
E-mail: RDRogers@bama.ua.edu; Fax: þ1 205 348 0823;
Tel: þ1 205 348 4323
In the present work, the preparation and physical properties
of imidazolium-based ILs containing strongly electron-with-
drawing nitro- and nitrile-functionalities directly attached to
the ring have been investigated. To this end, we have prepared
a series of imidazolium salts bearing NO2-functionalized
heterocyclic cations to evaluate how the physical and thermo-
dynamic properties (thermal stability, melting, and decompo-
sition temperatures) of the resulting salts are related to the
presence of electron-withdrawing substituents in the cation
ring. The goal is to develop synthetic and predictive strategies
w Electronic supplementary information (ESI) available: Ca-
tionꢁ ꢁ ꢁanion close contact geometries (Table S1), bond distances and
angles in the imidazolium rings (Table S2), and crystallographic data
for compounds 3a, 5a, 7b, 10a, 13b. See DOI: 10.1039/b509260d.
z Present address: Department of Organic Chemistry, College of
Pharmacy, Second Military Medical University, Shanghai, 200433,
P. R. China
y Present address: The QUILL Research Centre, Queen’s University
of Belfast, Northern Ireland, UK BT9 5AG. E-mail: J.Holbrey@
qub.ac.uk
ꢂc
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New J. Chem., 2006, 30, 349–358 | 349

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Scheme 2 Reaction conditions: (a) MeOTf, toluene, 20 1C, 72 h for
10a or 48 h for 10c; (b) LiNTf2, water–dichloromethane, 20 1C.
zole 6 with dimethyl sulfate in toluene afforded the corre-
sponding quaternary salt 7a (72% yield).
Attempts to prepare unsymmetrical 1-ethyl-3-methyl-4-
nitroimidazolium salt 5, similarly, by quaternization of 1-
methyl-5-nitroimidazole 2 with diethyl sulfate failed and re-
sulted in recovery of compound 2 (Scheme 1). However,
treatment of 1-ethyl-4-nitroimidazole 4, available from
4-nitroimidazole 1 by alkylation with diethyl sulfate in aqu-
eous media in the presence of sodium hydroxide at 45 1C,46
with methyl triflate in toluene at 20 1C for 72 h gave the
imidazolium salt 5a in 97% yield (Scheme 1).
Scheme 1 Reaction conditions: (a) Me2SO4, toluene, 20 1C, 48 h; (b)
MeOTf, toluene, 20 1C, 72 h; (c) LiNTf2, water–dichloromethane,
20 1C.
1-Methyl-2-nitroimidazole47,48 9a and 1-ethyl-2-nitroimida-
zole49 9b (Scheme 2) were prepared by alkylation of 2-nitro-
imidazole 8 with the appropriate dialkyl sulfate in aqueous
media in the presence of sodium bicarbonate at 60–65 1C in
39% and 36% yields, respectively. Treatment of 9a and 9b
with methyl triflate in toluene at 20 1C for 48 and 72 h gave
quaternary salts 10a,c in 77% and 90% yields, respectively.
1-Methyl-4,5-dicyanoimidazole 12a was prepared in 81%
yield according to a published procedure50 from 4,5-dicyano-
imidazole 11 and dimethyl sulfate in an aqueous medium in
the presence of sodium bicarbonate at 60–65 1C (Scheme 3).
Quaternization of 12a with dimethyl sulfate in toluene at
20–25 1C for 72 h produced the corresponding quaternary salt
13b in only 10% yield; reaction in chloroform under reflux for
24 h increased the yield to 25%. Treatment of compound 12a
with methyl fluorosulfonate in chloroform at room temperature
for 48 h, followed by methanolysis of the intermediate fluor-
osulfonate 13a (not isolated) from methanol afforded the methyl
sulfate salt 13b in 71% yield (Scheme 3). Attempts to convert
12a into the asymmetric 1-ethyl-3-methyl-4,5-dicyanoimidazo-
lium methyl sulfate (13, R1 = Et, X = MeSO4) by the reaction
with diethyl sulfate failed and resulted in recovery of 12a.
Unsymmetrical 1-ethyl-3-methyl-4,5-dicyanoimidazolium
triflate 13d was synthesized in 67% yield by quaternization
of 1-ethyl-4,5-dicyanoimidazole 12b (available from 4,5-dicya-
noimidazole and triethyl orthoformate in 72% yield)51 with
methyl triflate in toluene at 20–25 1C for 72 h.
for the introduction of energetic functions and components
into ionic liquid systems, as well as to understand how to
engineer low melting or specific activity ILs.
We report here the synthesis and physical properties of 12
imidazolium-based ILs containing strongly electron-with-
drawing nitro- and cyano-functionalities linked directly to
the ring. The solid states of five examples have additionally
been characterized by single crystal X-ray crystallography,
identifying the principle cation–anion interactions.
Results and discussion
We have prepared 12 new N,N0-dialkylimidazolium salts con-
taining –NO2 and –CN substituents in the imidazolium ring
and characterized these in terms of structural and thermal
properties. The synthetic approach to the preparation of N,N0-
dialkylimidazolium salts utilized key quaternization of weakly
nucleophilic ring-substituted N-alkyl imidazoles.
Synthesis
A systematic strategy to prepare mono-, di-, and trinitro-
substituted 1,3-dialkylimidazolium cations was limited by the
resistance of dinitro-substituted imidazoles to undergo either
N-alkylation or protonation. Thus, only mononitro-substi-
tuted, but no dinitro-substituted imidazolium salts could be
isolated. In contrast, 1-alkyl-4,5-dicyanoimidazole was suc-
cessfully alkylated to obtain 1,3-dialkyl-4,5-dicyanoimidazo-
lium salts.
1-Methyl-5-nitroimidazole44 2 was prepared in 80% yield by
the reaction of 4-nitroimidazole 1 with dimethyl sulfate in
dioxane under reflux following a published procedure44
(Scheme 1). The treatment of 2 with dimethyl sulfate in
toluene45 at room temperature for 48 h gave the corresponding
quaternary 1,3-dimethyl-4-nitroimidazolium methyl sulfate 3a
in 98% yield. Similar reaction of 1,2-dimethyl-5-nitroimida-
1-Methyl-4,5-dinitroimidazole52 15 and 1-methyl-2,4-dini-
troimidazole52 18 (Scheme 4) were prepared from correspond-
ing dinitroimidazoles 14 and 17, and dimethyl sulfate
according to previously published procedures.52
The presence of two nitro groups in the imidazole ring
deactivated 15 and 18, and attempted quaternizations of
dinitroimidazoles 15 and 18 with dimethyl sulfate, methyl
fluorosulfonate and methyl triflate were unsuccessful, resulting
in either recovery or decomposition of the starting imidazoles.
ꢂc
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Scheme 3 Reaction conditions: (a) for 12a: Me2SO4, aqueous NaH-
CO3, 65 1C, 2 h, 81%; for 12b: (EtO)3CH, 100 1C for 1 h, 150 1C for
1 h, 72%; (b) for 13a: 12a, MeOSO2F, chloroform, 20 1C, 48 h; for
13b: 12a, Me2SO4, toluene, 20 1C, 72 h, 10%; or Me2SO4, chloroform,
reflux, 24 h, 25%; for 13d: 12b, MeOTf, toluene, 20 1C, 72 h, 67%; (c)
recrystallized from methanol–diethyl ether, 71% yield from 12a; (d)
LiNTf2, water–dichloromethane, 20 1C, 71%.
Scheme 4 Reaction conditions: (a) Me2SO4, aqueous NaHCO3,
20 1C, 12 h; (b) Me2SO4, MeOSO2F, or MeOTf.
Metathesis reactions
The functionalized imidazolium cations were initially pre-
pared, isolated and characterized as either methyl sulfate or
triflate salts, as shown in Table 1, through the direct alkylation
procedures (Schemes 1–4). In order to obtain a homogenous
set of anions for comparison with conventional ILs, the
bis(triflyl)imide salts of each cation were prepared by meta-
thesis of the water soluble triflate 5a, 10a or methyl sulfate 3a,
7a, 13b salts with LiNTf2 in water, yielding the insoluble
bis(triflyl)imide salts (3b, 5b, 7b, 10b, 13c) which precipitated
initially as colorless solids or, for 3b and 5b, as liquids and
were then extracted with dichloromethane.
The structures of the salts 3a,b, 5a,b, 7a,b, 10a–c, and 13b–d
were supported by their 1H and 13C NMR spectra, and
elemental analysis. The NMR spectra of 3a,b, 5a,b, 7a,b,
10c, and 13d showed the appearance of new signals in the
range 3.92–4.12 ppm (1H-) and 36.0–39.8 ppm (13C-spectra)
corresponding to the N-methyl group. The NMR spectra of
the salts 10a and 13b with symmetric cations showed a single
signal of increased relative intensity corresponding to the N-
methyl group and slight downfield shifts 4.09 - 4.13 and 3.92
1
- 4.06 ppm (in the H-) and 37.4 - 39.7 and 33.9 - 36.9
ppm (in the 13C-spectra) as compared with 9a and 12a,
respectively. Interestingly, the two N,N0-methyl groups in the
unsymmetric cation of 3a showed equivalent shifts and a single
signal in the 13C NMR spectrum at 37.1 ppm, although the
shifts of the hydrogens were differentiated as two signals at
Thermal investigations
The bis(triflyl)imide salts, 3b and 5b with 1,3-dimethyl-4-
nitroimidazolium and 1-ethyl-3-methyl-4-nitroimidazolium
cations, formed room temperature ILs; all the other salts
prepared were isolated as crystalline solids in good yields
and high purity. The melting transition, measured visually
1
3.93 and 4.09 ppm in the H spectrum. The NMR spectra of
3b, 5b, 7b, 10b, and 13c showed patterns of signals similar to
those observed for the corresponding salts 3a, 5a, 7a, 10a, or
13b used in their preparation.
Table 1 Thermal data for the nitro- and nitrile-substituted 1,3-dialkylimidazolium saltsa
Cation
Anion
Tm(onset) (ꢃ1 1C)
95
Tm(peak) (ꢃ1 1C)
104
DHfus (J gꢀ1
)
Tdec
ꢀ
ꢀ
ꢀ
3a
3b
5a
1,3-Dimethyl-4-nitroimidazolium
MeSO4
84
166
258
NTf2
ꢀ41b, 35c
ꢀ
1-Ethyl-3-methyl-4-nitroimidazolium
OTfꢀ
120f
92
122f
94
88f
50
260
270
ꢀ
5b
7a
7b
10a
10b
10c
13b
13c
NTf2
ꢀ45b, 18c
1,2,3-Trimethyl-4-nitroimidazolium
1,3-Dimethyl-2-nitroimidazolium
MeSO4
150
110
162
113
116
77
107
143d
111f
97
70
97
20
40
75
54
99f
36
196, 285g
269, 340g
234, 298g
248, 318g
230
ꢀ
NTf2
OTfꢀ
NTf2
105
72
105
130
ꢀ
1-Ethyl-3-methyl-2-nitroimidazolium
1,3-Dimethyl-4,5-dicyanoimidazolium
OTfꢀ
MeSO4
218
NTf2
OTfꢀ
84e, 105f
84
ꢀ
264
229
13d
1-Ethyl-3-methyl-4,5-dicyanoimidazolium
75–80e
140
148
58
a
Melting points (onset and peak positions) and heats of fusion were determined by DSC from the second heating cycle after initially melting the
salts, then cooling to ꢀ110 1C. Decomposition temperatures (Tdec) were determined by TGA from onset to 5 wt% mass loss, heating at 5 1C minꢀ1
b
c
d
e
f
under helium. Glass transition temperature. Two second order transitions observed. Broad melting transition. Solid transition. Only
g
during first heating. Change in the slope of the decomposition.
ꢂc
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Table 2 Melting points (1C) of selected quaternary imidazolium salts
R1
R2
R3
R4
R5
OTfꢀ
NTf2
ꢀ
MeSO4
ꢀ
Entry
1
2
3
4
5
6
7
8
Et
Et
Et
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
ꢀ65 (Tg)45
ꢀ933
ꢀ333
Me
7345
10933
633
2033
Me
ꢀ333
NO2
NO2
105 (10a)
72 (10b)
ꢀ41 (Tg) (3b)
NO2
95 (3a)
105 (10c)
92 (5a)
Et
NO2
NO2
ꢀ45 (Tg) (5b)
110 (7b)
Me
Me
Me
Et
Me
Me
150 (7a)
11645
130 (13b)
9
10
11
CN
CN
CN
CN
84 (13c)
140 (13d)
and by DSC, and thermal decomposition data, determined by
TGA from the onset of decomposition to 5 wt% mass loss, are
shown in Table 1. Crystalline mononitro-substituted dimethyl-
3a, 10a, trimethyl- 7a, ethylmethyl- 5a, 10c nitroimidazolium,
and dimethyl- 13b and ethylmethyl- 13d dicyanoimidazolium
salts, were obtained either as methyl sulfates or as triflates. For
each salt, a sharp, first order melting transition in the tem-
perature range 95–154 1C was observed on heating of the
solids. By comparison, transition temperatures for the corre-
sponding salts with bis(triflyl)imide anions were reduced by up
to 40 1C for the crystalline salts compared to the respective
methyl sulfate or triflate analogs, and for the 1,3-dimethyl-4-
nitroimidazolium 3a,b and 1-ethyl-3-methyl-4-nitroimidazo-
lium 5a,b examples, by up to 90 1C to the highest temperature
transition observed which was a second order transition.
Neither 3b nor 5b could be induced to crystallize on cooling,
and in the DSC experiment were observed to undergo two first
order transitions, at B35 and ꢀ41 1C and at B18 and ꢀ45 1C,
respectively.
mal, non-accelerated conditions) is that of conventional E2
elimination of N-alkyl substituents from the cations.
The contribution that the single nitro-substituent on the
imidazolium cations has on the melting points of these salts
appears principally to be that of a single non-interacting
functional group, with the variation in melting point contribu-
tion being somewhat equivalent to that of a methyl-substituent
at the same position. Thus, for example, the change from
1-ethyl-3-methylimidazolium triflate (Table 2, Entry 1) to
1-ethyl-2,3-dimethylimidazolium triflate (Entry 2) results in
an increase in melting point of approximately 120 1C on C(2)-
methylation. Subsequent mutation of the C(2)–CH3 group to
C(2)–NO2 (Entry 6, 10c) results in a 4 1C drop in melting
point. Comparisons of the group contributions are not so clear
between other sets of examples, for example, between 1,3-
dimethyl-2-nitroimidazolium (Entry 4, 10b) and 1,3-dimethyl-
4-nitroimidazolium (Entry 5, 3b) bis(triflyl)imides, movement
of the nitro-substituent destabilizes the crystalline state by at
least 37 1C (perhaps due to asymmetry), whereas the equiva-
lent transformation between 1-ethyl-3-methyl-2-nitroimidazo-
lium (Entry 6, 10c) and 1-ethyl-3-methyl-4-nitroimidazolium
(Entry 7, 5a) triflate leads to a net stabilization of 9 1C.
However, despite these alternative transitional indications, it
is clear that the introduction of an electron-withdrawing
energetic function, in the shape of a nitro-group, onto the
aromatic cation ring of an organic salt can still lead to the
potential for IL formation.
Comparison of 3 and 7 with the corresponding iodide,
perchlorate, and nitrate salts recently reported by Shreeve
and co-workers during the course of this work also clearly
highlights the effects of anion replacement as a means of
controlling the melting points and propensities of the salts to
form ionic liquids.32 The melting points of the salts of the two
cations (3 and 7) with the five anions decrease in the order
Iꢀ > [ClO4]ꢀ > [NO3]ꢀ > [CH3SO4]ꢀ > [NTf2]ꢀ with values
of 180, 172, 163, 104, 35 1C and 191, 186, 161, 150, 110 1C,
respectively, for the 1,3-dimethyl-4-nitroimidazolium (3) and
1,2,3-trimethyl-4-nitroimidazolium (7) cations.
X-Ray crystallography
Crystals suitable for single crystal X-ray structure determina-
tion were obtained for five of the twelve imidazolium salts (3a,
5a, 7b, 10a, and 13b) by crystallization at room temperature
from methanol solution by trituration with diethyl ether
(Table 3).z In each case, all non hydrogen atoms were fully
refined anisotropically and all hydrogen atoms refined isotro-
pically. The five structurally characterized salts are depicted in
Fig. 1–5 with each asymmetric unit shown oriented
Upper thermal stabilities, measured by TGA for all the salts
were in the range B250–300 1C, measured from the initial
onset of decomposition. Characteristic thermograms show a
single mass-loss event. The decomposition temperatures re-
ported here were determined from the onset to initial 5 wt%
mass loss on isocratic heating, a measure which provides a
more realistic representation of thermal stability at elevated
temperatures, and based on this measure53 are comparable to
other ILs with equivalent anions. This indicates that the initial
thermal decomposition mechanism of these salts (under nor-
z CCDC reference numbers 272562–272566. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b509260d
ꢂc
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Table 3 Crystal and structure refinement data
[1,3-Dimethyl-4-
nitroimidazolium]
[MeSO4]
[1-Ethyl-3-methyl-4-
nitroimidazolium]
[OTf]
5a
[1,2,3-Trimethyl-4-
nitroimidazolium]
[NTf2]
[1,3-Dimethyl-2-
nitroimidazolium]
[OTf]
[1,3-Dimethyl-4,5-
dicyanoimidazolium]
[MeSO4]
13b
Compound
3a
7b
10a
Color/shape
Formula
Formula weight
Crystal system
Space group
T (K)
Colorless/needles
C6H11N3O6S
253.24
Monoclinic
P21/n
173
Colorless/needles
C7H10F3N3O5S
305.24
Monoclinic
P21/c
173
Colorless/needles
C8H10F6N4O6S2
436.32
Triclinic
ꢀ
P1
Colorless/needles
C6H8F3N3O5S
291.21
Monoclinic
P21/c
173
Colorless/plates
C8H10N4O4S
258.26
Monoclinic
P21
173
173
a (A)
b (A)
c (A)
a (1)
b (1)
g (1)
8.366(3)
11.036(4)
11.379(4)
90
100.580(6)
90
9.872(2)
12.839(3)
9.5949(19)
90
96.630 (4)
90
8.113(2)
12.596(4)
17.035(5)
70.065(5)
81.042(5)
77.570(5)
4
6.882(3)
11.924(5)
13.324(6)
90
96.911(6)
90
8.4703(14)
7.9000(13)
8.9145(15)
90
111.568(3)
90
Z
4
4
4
2
V (A3)
1032.8(6)
1.629
1490 (Rint = 0.0497)/ 1735 (Rint = 0.0152)/ 4389 (Rint = 0.0183)/ 1561 (Rint = 0.0265)/ 1552 (Rint = 0.0151)/
1207.9(4)
1.678
1591.7(8)
1.821
1085.4(8)
1.782
554.75(16)
1.546
Dcalc (g cmꢀ3
Independent/
)
Observed reflections
GooF
R1, wR2 >I > 2s(I)] 0.0385, 0.0928
R1, wR2 (all data) 0.0531, 0.0992
1210 ([I > 2s(I)])
1.029
1639 ([I > 2s(I)])
1.061
0.0240, 0.0623
3501 ([I > 2s(I)])
1.040
0.0355, 0.0839
0.0505, 0.0924
1466 ([I > 2s(I)])
1.141
0.0308, 0.0808
0.0328, 0.0822
1438 ([I > 2s(I)])
1.088
0.0340, 0.0965
0.0379, 0.0986
0.0253, 0.0631
1
R1 = S8Fo| ꢀ |Fc8/S|Fo|; R2 = {S[w(|Fo|2 ꢀ |Fc|2)2]/S(w(|Fo|2)2]}2
perpendicularly to the plane of the imidazolium ring with the
shortest cation–anion hydrogen bonding contact displayed.
In only one case (13b) was any disorder observed. In this
structure the methyl sulfate anion exhibited two conforma-
tions with the sulfur, carbon, and one oxygen atom [O(4)] fully
occupied and three oxygen atoms [O(1)–O(3)] in half occupied
positions. The disorder appears to be a result of the alter-
natives for weak hydrogen bonding interactions to the cation.
When the anion is in the O(1)–O(3) conformation, the closest
contact is from the C6 methyl hydrogen atom to O(2); while
the alternate conformation [O(1A)–O(3A)] allows the closest
contact to be between the ring C(2) hydrogen atom and O(2A).
Compound 7b is the only one of the five salts characterized
to have more than one cation–anion pair in the asymmetric
unit. Here too, this can be attributed to alternatives in the
weak intermolecular hydrogen bonding. Careful inspection of
the ions reveals little difference in the two cations or in the
conformations of the two anions. However, as shown in Fig. 3,
the shortest hydrogen bonding interactions between cation
and anion are subtly different, with the C(51) hydrogen atom
making its close contact to O(31), while the C(52) hydrogen
atom makes its closest contact to O(42). The resulting relative
orientation in the two anions and cations is sufficient to break
any higher symmetry. Interestingly, this is the only NTf2ꢀ salt
of those prepared that does not fit the strict definition of an
‘ionic liquid,’ that is, with melting point o100 1C.
As the two above examples imply, the overall packing in
each case is dominated by attractive Coulombic forces, aug-
mented by hydrogen-bonding from imidazolium ring donor
hydrogen or alkyl hydrogen positions to available anionic
hydrogen-bond acceptor sites. All of the salts crystallized into
relatively simple close-packed structures and the principal
Fig. 1 ORTEP diagram showing connectivity, conformation, and the atom numbering scheme of the individual cations and anions present in the
asymmetric unit and packing diagram of [1,3-dimethyl-4-nitroimidazolium][MeSO4] (3a). The intermolecular contact with the largest negative
difference from van der Waals separation is noted.
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Fig. 2 ORTEP diagram showing connectivity, conformation, and the atom numbering scheme of the individual cations and anions present in the
asymmetric unit and packing diagram of [1-ethyl-3-methyl-4-nitroimidazolium][OTf] (5a). The intermolecular contact with the largest negative
difference from van der Waals separation is noted.
close-contacts and hydrogen-bonding interactions between the
ions in each case are shown in Table S1.w
There is little effect of substitution on the bond lengths and
angles within the imidazolium cations (Table S2).w In 7b
however, the methyl group in the C(2) position elongates the
central N(1)–C(2) and C(2)–N(3) bonds.
The nitro-substituents in imidazolium salts 3a, 5a, 7b, and
10a are twisted slightly out of the plane of the heterocycle ring,
reducing conjugation. The observed dihedral twist is, however,
small ranging from 2.61 10a to 10.71 5a comparable to that in
other nitroimidazolium and imidazolium structures. The ni-
tro-groups do not have any significant hydrogen-bond donor
Fig. 3 ORTEP diagrams showing connectivity, conformation, and the atom numbering scheme of the individual cations and anions present in the
asymmetric unit and packing diagrams of [1,2,3-trimethyl-4-nitroimidazolium][NTf2] (7b). The intermolecular contact with the largest negative
difference from van der Waals separation is noted.
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Fig. 4 ORTEP diagram showing connectivity, conformation, and the atom numbering scheme of the individual cations and anions present in the
asymmetric unit and packing diagram of [1,3-dimethyl-2-nitroimidazolium][OTf] (10a). The intermolecular contact with the largest negative
difference from van der Waals separation is noted.
Fig. 5 ORTEP diagram showing connectivity, conformation, and the atom numbering scheme of the individual cations and anions present in the
asymmetric unit and packing diagram of [1,3-dimethyl-4,5-dicyanoimidazolium][MeSO4] (13b, disordered anion). The intermolecular contact with
the largest negative difference from van der Waals separation is noted.
or acceptor interactions beyond simple close-packing in the
crystal structures which could influence the lattice energies and
melting points of these salts.
Substitution of hydrogens in the ring-carbon position of
imidazolium cations tends, in general, to increase the melting
points of the respective salts. For example, compare the
transition temperatures for 1-ethyl-3-methylimidazolium,
1-ethyl-2,3-dimethylimidazolium and 1-ethyl-3,5-dimethylimi-
dazolium salts in Table 2 (Entries 1–3) where it is clearly
illustrated that relatively large variations in melting points
can be achieved through changes in the cation substitution
patterns.
The packing diagrams for the five structurally characterized
salts are also depicted in Fig. 1–5. The cations are all parallel
in salts 3a and 7b, but not in the remaining salts. Zones of
fluorine atoms are apparent in 5a, but this is not the case for
either 7b or for 10a.
The five crystal structures (one of each cation type) show
that in the solid state the NO2-group has little significant
effect, beyond a steric contribution, to the crystal packing:
no major hydrogen-bonding or directional close contacts are
observed. This lends support to our initial analysis of the
group contribution of the NO2 function to IL properties as
roughly equivalent to that of a methyl-function at the same
ring-position.
Conclusion
In this work, the focus has been on establishing the influence
of substituent type and position on the properties of nitro- and
cyano-substituted imidazolium salts. Functionalization of het-
erocycles with electron-withdrawing substituents, such as these
described here, results in a reduction in nucleophilicity of the
heterocycle and thus reduced ability to form quaternary salts
by SN2 alkylation reactions. Mono-substituted crystalline
quaternary salts (and some liquids with bis(triflyl)imide) could
be isolated, but higher-substitution with electron-withdrawing
groups resulted in the imidazoles failing to undergo quaterni-
zation even with strong alkylating agents (methyl triflate or
fluorosulfonate) due to the reduced nucleophilicity of the
azoles. Imidazolium salts with two nitrile substituents (13b–
d) could be obtained. A computational investigation of proton
and methylene-group affinities, and cation stability relative to
the the parent azoles has recently been conducted.61
Experimental section
Caution
Although no problems were encountered during this work, the
introduction of energetic functionalities into heterocyclic com-
pounds has inherent risks and appropriate procedures must be
taken and all materials handled with extreme care.
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General methods
General procedure for the preparation of imidazolium triflates
(5a, 10a, 10c, and 13d)
NMR spectra were obtained in CDCl3, DMSO-d6 or CD3OD
with TMS as the internal standard for 1H or the solvent as the
internal standard for 13C. J values are given in Hz. All of the
chemicals were employed as supplied.
To a solution of appropriate 1-alkylimidazole 4, 9a, 9b, or 12b
(5 mmol) in toluene (3 cm3) was added methyl triflate (5 mmol,
0.57 cm3) at 20–25 1C under nitrogen. The reaction mixture
was stirred at 20–25 1C for 72 hours (48 h for 10c). The
precipitate was filtered and recrystallized from methanol–
diethyl ether to give 1,3-dialkylimidazolium triflate 5a, 10a,
10c, 13d.
Lithium bis(triflyl)imide (LiNTf2) was a gift from 3M
(St Paul, MN).
Materials
1-Ethyl-3-methyl-4-nitroimidazolium triflate (5a). White nee-
dles (97%), mp 114–117 1C (from methanol–diethyl ether); dH
(300 MHz, DMSO-d6) 9.49 (1 H, s), 9.16 (1H, s), 4.31 (2 H, q,
J 7.3, CH2CH3), 4.11 (3 H, s, CH3), 1.50 (3 H, t, J 7.3,
CH2CH3); dC (75 MHz, DMSO-d6) 139.2, 138.1, 124.5, 120.7
(q, J 320.5, CF3), 46.0, 37.2, 14.6. Found: C, 27.87; H, 3.24; N,
13.46. Calc. for C7H10F3N3O5S: C, 27.55; H, 3.30; N, 13.77%.
2-Nitroimidazole 8 was prepared according to the published
procedure by diazotization of 2-aminoimidazolium sulfate
(available from aminoacetaldehyde diethyl acetal and S-
methylisothiourea)54 followed by substitution with nitrite
anion in the presence of copper sulfate.55 2,4-Dinitroimidazole
17 was prepared by N-nitration56,57 of 4-nitroimidazole 1
followed by isomerization of intermediate 1,4-dinitroimida-
zole in chlorobenzene at 115 1C.58
1,3-Dimethyl-2-nitroimidazolium triflate (10a). White nee-
dles (77%), mp 112–114 1C (from methanol–diethyl ether);
Procedure for the preparation of 4,5-dinitroimidazole (14)
d
H (300 MHz, DMSO-d6) 8.10 (2 H, s), 4.13 (6 H, s, 2 ꢄ CH3);
dC (75 MHz, DMSO-d6) 139.5, 125.3, 120.7 (q, J 319.9, CF3),
To 4-nitroimidazole (2.0 g, 18 mmol) dissolved in a minimum
quantity of concentrated sulfuric acid was added a mixture of
concentrated sulfuric acid (10 cm3) and fuming nitric acid
(90%, 8 cm3, 0.17 mol). The mixture was heated under reflux
for 5 h. After cooling, the mixture was poured into ice, and the
pH was adjusted to 2 with sodium bicarbonate. The product
was extracted with ethyl acetate. The extract was concentrated
to give 4,5-dinitroimidazole 14 as yellow crystals from water
(1.55 g, 55%), mp 173–176 1C (lit.57 188–189 1C); dH
(300 MHz, DMSO-d6) 12.37 (1 H, br s, NH), 8.06 (1 H, s);
dC (75 MHz, DMSO-d6) 135.5, 134.9.
39.7. Found: C, 24.63; H, 2.69; N, 14.00. Calc. for
C6H8F3N3O5S: C, 24.75; H, 2.77; N, 14.43%.
1-Ethyl-3-methyl-2-nitroimidazolium triflate (10c). White mi-
crocrystals (90%), mp 105–106 1C (from methanol–diethyl
ether); dH (300 MHz, DMSO-d6) 8.17 (1 H, d, J 1.9), 8.13 (1 H,
d, J 1.9), 4.54 (2 H, q, J 7.2, CH2CH3), 4.12 (3 H, s, CH3), 1.47
(3 H, t, J 7.2, CH2CH3); dC (75 MHz, DMSO-d6) 139.1, 125.7,
123.9, 120.7 (q, J 319.9, CF3), 48.2, 39.8, 14.6. Found: C,
27.75; H, 3.21; N, 13.54. Calc. for C7H10F3N3O5S: C, 27.55;
H, 3.30; N, 13.77%.
1-Ethyl-3-methyl-4,5-dicyanoimidazolium triflate (13d).
White microcrystals (67%), mp 147–148 1C (from methanol–
diethyl ether); dH (300 MHz, DMSO-d6) 9.79 (1 H, s), 4.44 (2
H, q, J 7.1, CH2CH3), 4.05 (3 H, s, CH3), 1.49 (3 H, t, J 7.1,
CH2CH3); dC (75 MHz, DMSO-d6) 142.0, 120.6 (q, J 319.9,
CF3), 115.9, 114.5, 106.1, 106.0, 46.5, 36.9, 13.9. Found: C,
34.84; H, 2.78; N, 17.55. Calc. for C8H10N4O4S: C, 34.84; H,
2.92; N, 18.06%.
General procedure for the preparation of imidazolium methyl
sulfates (3a and 7a)
Dimethyl sulfate (7 mmol, 0.88 g, 0.66 cm3) was added to a
solution of 1-alkylimidazole 2 or 6 (7 mmol) at 20–25 1C under
nitrogen. The reaction mixture was stirred at 20–25 1C for
48 h. The solvent was removed and the residue was recrystal-
lized from methanol–diethyl ether to give imidazolium methyl
sulfate 3a, 7a.
Procedure for the preparation of 1,3-dimethyl-4,5-
dicyanoimidazolium methyl sulfate (13b)
1,3-Dimethyl-4-nitroimidazolium methyl sulfate (3a). Yellow-
ish crystals (98%), mp 101–103 1C (from methanol–diethyl
ether); dH (300 MHz, DMSO-d6) 9.38 (1 H, s), 9.02 (1 H, s),
4.09 (3 H, s, CH3), 3.93 (3 H, s, CH3), 3.37 (3 H, s, CH3); dC
(75 MHz, DMSO-d6) 139.9, 137.8, 125.7, 52.8, 37.1. Found: C,
28.64; H, 4.27; N, 16.35. Calc. for C6H11N3O6S: C, 28.46; H,
4.38; N, 16.59%.
To a solution of 1-methyl-4,5-dicyanoimidazole 12a (1.8 g,
13.6 mmol) in chloroform (25 cm3) was added methyl fluor-
osulfonate (13.6 mmol, 1.07 cm3) at 20–25 1C under nitrogen.
The mixture was stirred at 20–25 1C for 48 h. The precipitate
of 1,3-dimethyl-4,5-dicyanoimidazolium fluorosulfonate 13a
was collected by filtration. Dissolution of 13a in methanol
resulted in methanolysis of the anion to methyl sulfate. 1,3-
Dimethyl-4,5-dicyanoimidazolium methyl sulfate 13b was iso-
lated by precipitation with diethyl ether and was recrystallized
from methanol–diethyl ether as white microcrystals (2.5 g,
71%), mp 134–139 1C (from methanol–diethyl ether); dH
(300 MHz, DMSO-d6) 9.69 (1 H, s), 4.06 (6 H, s, 2 ꢄ CH3),
3.37 (3 H, s, CH3SO4); dC (75 MHz, DMSO-d6) 142.6, 115.6,
106.1, 52.9, 36.9. Found: C, 37.13; H, 3.80; N, 21.64. Calc. for
C8H10N4O4S: C, 37.21; H, 3.90; N, 21.69%.
1,2,3-Trimethyl-4-nitroimidazolium methyl sulfate (7a). Yel-
lowish microcrystals (72%), mp 153–154 1C (from methanol–
diethyl ether); dH (300 MHz, CD3OD) 8.74 (1 H, br s), 4.09
(3 H, s), 3.92 (3 H, s), 3.65 (3 H, s), 2.74 (3 H, s); dC
(75 MHz, CD3OD) 150.4, 126.2, 55.1, 36.8, 36.0, 10.8. Found:
C, 31.48; H, 4.87; N, 15.32. Calc. for C7H13N3O6S: C, 31.46;
H, 4.90; N, 15.72%.
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General procedure for the preparation of imidazolium
bis(triflyl)imides (3b, 5b, 7b, 10b, and 13c)
was between 0 and ꢀ0.5 mW, and the baseline drift was less
than 0.1 mW over the temperature range 0–180 1C. An empty
sample pan was used as reference.
An aqueous solution of LiNTf2 (0.29 g, 1 mmol, in 15 cm3 of
water) was added to an aqueous solution of the appropriate
salt 3a, 5a, 7a, 10a, or 13b (1 mmol in 15 cm3 of water) at
20–25 1C. The reaction mixture was stirred either at 20–25 1C
for 1 h (3b or 7b) or at 30 1C for 3 h (5b, 10b and 13c). Then
dichloromethane (25 cm3) was added and the mixture was
stirred for an additional 20 min to extract the product and the
organic phase was separated. The product was twice extracted
with dichloromethane in the same manner. The combined
extracts were dried over sodium sulfate and the solvent was
removed under reduced pressure using a rotary evaporator
over 5 h to give the corresponding bis(triflyl)imides in 50–75%
isolated yields.
Thermal decomposition temperatures were measured in the
dynamic heating regime using a TGA 2950 TA Instrument
under a helium atmosphere. Samples between 2–10 mg were
heated from 40–500 1C under constant heating at 5 1C minꢀ1
.
X-Ray crystallographic studies
Samples were recrystallized from methanol by trituration with
diethyl ether at 25 1C. Single crystals were collected in air,
mounted on fibers and transferred to the goniometer. The
crystals were cooled to ꢀ100 1C with a stream of nitrogen gas
and data were collected on a Siemens SMART diffractometer
with a CCD area detector, using graphite monochromated
MoKa radiation. The SHELXTL software, version 5, was
used for solutions and refinements.59 Absorption corrections
were made with SADABS.60 Each structure was refined by
full-matrix least-squares on F2.
1,3-Dimethyl-4-nitroimidazolium bis(triflyl)imide (3b). Col-
orless liquid (59%), Tg ꢀ41 1C (from dichloromethane); dH
(360 MHz, DMSO-d6) 9.37 (1 H, s), 9.02 (1 H, d, J 1.8), 4.08
(3 H, s, CH3), 3.92 (3 H, s, CH3); dC (90 MHz, DMSO-d6)
139.8, 125.60, 119.4 (q, J 320.0, CF3), 37.0 (N–CH3), 36.99
(N–CH3).
In each structure, the atoms were readily located and the
positions of all non-hydrogen atoms were refined anisotropi-
cally. The hydrogen atoms (except for those associated with
the disordered anion in 13b) were added in approximated
positions and allowed to refine unconstrained in order to
obtain proper close contact interactions.
1-Ethyl-3-methyl-4-nitroimidazolium bis(triflyl)imide (5b).
Colorless liquid (74%), Tg ꢀ45 1C (from dichloromethane);
dH (360 MHz, DMSO-d6) 9.46 (1 H, d, J 1.6), 9.14 (1 H, d, J
1.6), 4.28 (2 H, q, J 7.3, CH2CH3), 4.08 (3 H, s, CH3), 1.46 (3
H, t, J 7.3, CH2CH3); dC (90 MHz, DMSO-d6) 139.0, 124.4,
119.4 (q, J 319.8, CF3), 45.9 (N–CH2), 37.0 (N–CH3), 14.5
(CH2CH3).
Disorder was observed for the methyl sulfate anion in 13b
with two positions each at 50% occupancy resolved for O1,
O2, and O3. These three atoms were refined in alternate least-
squares cycles from their disordered counterparts (O1A, O2A,
and O3A). The remaining atoms in the anion (S1, O4, and
C10) were refined at full occupancy. No attempt was made to
include the methyl hydrogen atoms associated with C10 in the
refinement.
1,2,3-Trimethyl-4-nitroimidazolium bis(triflyl)imide (7b).
Colorless needles (68%), mp 111–112 1C (from dichloro-
methane); dH (360 MHz, DMSO-d6) 9.00 (1H, s), 4.00 (3 H,
s), 3.85 (3 H, s), 2.72 (3 H, s); dC (90 MHz, DMSO-d6) 148.8,
125.2, 119.4 (q, J 319.6 CF3), 35.8 (N(3)–CH3), 35.0 (N(1)–
CH3), 10.2 (C(2)–CH3).
Acknowledgements
This research was supported by the Air Force Office of
Scientific Research (Grant F49620-03-1-0357).
1,3-Dimethyl-2-nitroimidazolium bis(triflyl)imide (10b).
White microcrystals (53%), mp 74–75 1C (from dichloro-
methane); dH (360 MHz, DMSO-d6) 8.10 (2 H, s), 4.13 (6 H,
s, 2 ꢄ CH3); dC (90 MHz, DMSO-d6) 139.4, 125.1, 119.7
(q, J 257.4, CF3), 39.6 (N–CH3).
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358 | New J. Chem., 2006, 30, 349–358