Synthesis, limitations, and thermal properties of energetically- substituted, protonated imidazolium picrate and nitrate salts and further comparison with their methylated analogs

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PAPER

Synthesis, limitations, and thermal properties of energetically-substituted,

protonated imidazolium picrate and nitrate salts and further comparison

with their methylated analogsw

a

Marcin Smiglak,z C. Corey Hines,^aW. Matthew Reichert,y^aAdam S. Vincek,^b

Alan R. Katritzky,^bcJoseph S. Thrasher,^aLuyi Sun,z^aParker D. McCrary,^a

Preston A. Beasley,^aSteven P. Kelley^aand Robin D. Rogers*^a

Received (in Gainesville, FL, USA) 4th August 2011, Accepted 6th November 2011

DOI: 10.1039/c1nj20677j

The possibility of forming simple energetic ionic liquids via the straightforward protonation of

heterocyclic amines with nitric or picric acid was explored with 1-alkylimidazoles,

1-alkyl-2-methylimidazoles, and nitro, dinitro, and dicyano-substituted derivatives. The melting

points of most of the prepared salts were lower than expected and of the 30 compounds prepared,

more than half were found to melt below 100 1C. Limitations in the approach were found as a

result of the use of energetic electron withdrawing substituents, such as nitro or cyano, which

results in a reduction in nucleophilicity of the heterocycle and an inability to form salts with the

acids studied. Interesting thermal behavior was observed with several of the new salts including

supercooling and crystallization on heating. Comparison of the simple protonated imidazolium

nitrate and picrate salts with their methylated analogs indicated that the protonated ionic liquids

do not diﬀer substantially in their melting points from the methylated analogs. However, the

thermal stabilities of protonated imidazolium salts are much lower than their alkylated

derivatives. Nitrate salts with alkylated cations tend to be more thermally stable than the

corresponding picrate salts, but with protonated cations, the picrate salts tend to be

approximately 70–80 1C more stable than the nitrate salts. Moreover, accelerating rate

calorimetry (ARC) revealed that alkylated salts decompose much less exothermically (in some

cases endothermically) than the protonated analogs, and that among all the analyzed salts,

the most energetic materials found were protonated 1-methylimidazolium nitrate and

1,2-dimethylimidazolium picrate.

Introduction

is protonated as a result of the proton transfer reaction

between a Brønsted acid and neutral amine, whereas in AILs,

the quaternary ammonium center is fully alkylated, thus a

permanent cation is created, usually as the result of the

alkylation reaction of neutral amine and alkylating agent.

Even though PILs are much easier to prepare in comparison

to their alkylated analogs, the latter have received far greater

attention over the last decade. This may be a result of the often

high melting points and low thermal stabilities exhibited by

PILs compared to their aprotic analogs.²

Ionic liquids (ILs) are often divided into two major families

based on the structure of the constituent cation: protic ionic

liquids (PILs) and aprotic ionic liquids (AILs).^1,2Using

amines as an example, in PILs, the quaternary amine center

^aDepartment of Chemistry and Center for Green Manufacturing,

The University of Alabama, Box 870336, Tuscaloosa, AL 35467-0338,

USA. E-mail: RDRogers@as.ua.edu

^bCenter for Heterocyclic Compounds, Department of Chemistry,

University of Florida, Gainesville, FL 32611-7200, USA

^cDepartment of Chemistry, King Abdulaziz University, Jeddah,

Saudi Arabia

The existence of low melting protonated amine salts, however,

is not new and can be traced back to at least an article from

1888 on the synthesis of hydroxyethylammonium nitrate, with

a melting point of 52–55 1C,³and from 1914, a report of

ethylammonium nitrate (mp 12 1C).⁴Currently, PILs are being

studied for their potential in biological applications,^5,6organic

synthesis (oleﬁn oligomerization,⁷etheriﬁcation,⁸esteriﬁcation,⁹

and Friedel–Crafts alkylation),¹⁰catalysts,^11–13chromatography,¹⁴

proton conducting electrolytes,^15–17and self-assembly media.^18–20

w CCDC reference numbers 837939–837946. For crystallographic data

in CIF or other electronic format see DOI: 10.1039/c1nj20677j

z Current Address: IoLiTec, Ionic Liquids Technologies GmbH,

Satzstrasse 184, D-74076 Heilbronn, Germany.

y Current address: Department of Chemistry, University of South

Alabama, Mobile, AL 36688, USA.

z Current address: Department of Chemistry and Biochemistry,

Texas State University-San Marcos, San Marcos, TX 78666, USA.

c

702 New J. Chem., 2012, 36, 702–722

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We have also recently proposed the use of PILs in the pharma-

ceutical, food, and agrichemical industries where numerous

examples of active ingredients formulated as protonated salts

exist.^21–23

Limited examples of using PILs in research on energetic

materials can also be found in the literature. The synthesis and

characterization of protonated 1,5-diaminotetrazolium nitrate

(mp 138 1C) and perchlorate (mp 97 1C) salts have been brieﬂy

described.²⁴Also, the synthesis and characterization of six

energetic ethylene 1,2-bis(oxyamino) nitrates, perchlorates,

and dinitramides were reported, among which three (ethylene

bis(oxyamine) mononitrate, ethylene bis(oxyamine) mono-

dinitramide, and ethylene bis(oxyamine) bisdinitramide) can

be classiﬁed as ILs with melting points below 100 1C.²⁵

Recently, Gao et al.²⁶reported the synthesis and characterization

of a group of protonated ammonium and azolium salts with

the 2,4,5-trinitroimidazolate anion. Utilizing simple acid–base

reaction chemistry, neutral amines and azoles were paired with

a very strong organic acid, 2,4,5-trinitroimidazole, resulting in

formation of 17 new salts. However, only one compound in

the group had a melting point below 100 1C.

Scheme 1 Investigated imidazoles with functional groups appended

to the heterocyclic core.

allowed for screening of a large range of products as potential

EILs and subsequent determination of their properties.

Drummond and Greaves have reviewed applications of

PILs in several ﬁelds,²⁷however, speciﬁc literature reports

focusing on systematic investigations of structure vs. property

relationships of PILs are very limited. Examples include

reports from Hirao et al. on heterocyclic amine-based PILs

with the tetraﬂuoroborate anion,²⁸Greaves et al., on primary

amine-based PILs with organic carboxylate anions,²⁹Luo

et al., on thermal properties of imidazolium, ammonium,

amide, and guanidine based PILs,^30,31and Anouti et al., on

the synthesis and physicochemical characterization of a family

of alkylammonium and pyrrolidinium-based PILs.^32,33

While the basic principles for developing and investigating

protic ionic liquids are simple and well established, there exists

a need to develop design criteria for the formation of ionic

liquids containing energetic functional groups.^34–36Such criteria

would help to understand and predict the relationships between

addition of speciﬁc energetic functional groups and the

characteristics of the resultant salts. We have therefore studied

protonation of a group of imidazoles containing various alkyl

groups and variable substitution with common energetic nitro

and cyano groups. The most straightforward, fast, and simple

synthetic approach was employed for the formation of ILs via

acid–base neutralization reactions with nitric and picric acid to

form protonated azolium-based salts. Although the exact

ionicity (degree of proton transfer) of the reported compounds

in the liquid state is outside the scope of this work, we would

refer the reader to the works of Angell,³⁷MacFarlane,³⁸

Watanabe,³⁹and even our own work⁴⁰for a discussion of this

concept and the issues it raises. Here we will focus on those

aspects likely to aﬀect practical application, the observed

melting and decomposition behavior.

The main research objective addressed during our study was

to develop a better understanding of relationships between

structural modiﬁcations and resulting thermal properties.

Speciﬁc objectives included (i) learning the inﬂuence of the

electron withdrawing groups, such as –NO₂or –CN, on the

properties of the alkylimidazole ring, especially on the nucleo-

philicity of the nitrogen in the heterocyclic core, (ii) compar-

ison of the thermal properties of protonated derivatives of the

imidazolium cation substituted with diﬀerent functional

groups on the carbon positions in the heterocyclic ring

(–CH₃, –NO₂, –CN), and a variety of alkyl chains on the

nitrogen position of the imidazolium ring, (iii) evaluation of

the inﬂuence of the chosen counterions (picrate or nitrate) on

the properties of the salts and their ability to form ILs,

(iv) analysis of the inﬂuence of structural changes from the

protonated to methylated imidazolium salts on the thermal

properties, and (v) analysis of the potential energetic character

of the salts (as evaluated by self heat rate maximum values)

utilizing accelerating rate calorimetry (ARC).

Results and discussion

Synthesis of N-alkylated-substituted imidazoles

Alkylimidazoles (Scheme 1) were obtained from commercial

sources (1, 2, 4), previously reported literature procedures

(8–11, 16–18),⁴¹or Method A or B below. Alkylimidazoles

(3, 7, and 13–15) were prepared by the alkylation of corres-

ponding imidazoles with alkyl bromides in acetonitrile in the

presence of potassium carbonate under reﬂux (Method A).

Method A provided low isolated yields (14–44%) for products

3 and 7 likely due to the low boiling point of propyl and

iso-propyl bromides. When the conditions were changed to

potassium tert-butoxide in DMF at room temperature

(Method B), the isolated yields of 3 and 7 improved to

72–82%. Alkylimidazoles (5, 6, and 12) were prepared by

Method B.

The targeted imidazolium-based cations were initially selected

based on the substitution patterns and homological diﬀerences

in the imidazole ring structures (Scheme 1). Systematic changes

were implemented in the substitution patterns of N-alkyl-

imidazole cores, ranging from non-energetic C-alkyl-substituted,

to energetically-substituted cations (–NO₂and –CN groups

directly appended to the imidazolium core). This approach

c

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Synthesis of protonated, substituted imidazolium nitrate or

picrate salts

The syntheses of a series of protonated 1-alkylimidazoles con-

taining combinations of nitro, cyano, or alkyl substituents were

attempted (Scheme 2). The salts were prepared by neutralization

reactions of the corresponding neutral azoles with nitric or picric

acids in aqueous ethanol solutions. The protonated salts obtained

successfully with nitrate and picrate anions include 15 cations:

1-methylimidazolium ([1-Me-3-H-IM]⁺), 1,2-dimethylimidazolium

([1,2-diMe-3-H-IM]⁺), 1-propylimidazolium ([1-Pr-3-H-IM]⁺),

1-butylimidazolium ([1-Bu-3-H-IM]⁺), 1-butyl-2-methylimida-

zolium ([1-Bu-2-Me-3-H-IM]⁺), 1-pentyl-2-methylimidazolium

([1-Pent-2-Me-3-H-IM]⁺), 1-hexylimidazolium ([1-Hex-3-H-

IM]⁺), 1-methyl-2-nitroimidazolium ([1-Me-2-NO₂-3-H-IM]⁺),

1,2-dimethyl-5-nitroimidazolium ([1,2-diMe-5-NO₂-3-H-IM]⁺),

1-ethyl-2-nitroimidazolium ([1-Et-2-NO₂-3-H-IM]⁺), 1-ethyl-4-

nitroimidazolium ([1-Et-4-NO₂-3-H-IM]⁺), 1-isopropyl-4-nitroimi-

dazolium ([1-i-Pr-4-NO₂-3-H-IM]⁺), 1-butyl-2-methyl-4-

nitroimidazolium ([1-Bu-2-Me-4-NO₂-3-H-IM]⁺), 1-pentyl-2-

methyl-4-nitroimidazolium ([1-Pent-2-Me-4-NO₂-3-H-IM]⁺),

and 1-hexyl-4-nitroimidazolium ([1-Hex-4-NO₂-3-H-IM]⁺).

Scheme 3 Unsuccessful protonation reactions of 1-Me-2,4-diNO₂-IM

(16), 1-Me-4,5-diNO₂-IM (17), and 1-Me-4,5-diCN-IM (18) with

hydrochloric, picric, and nitric acids.

Fig. 1 50% probability ellipsoid plot of co-crystal of 1-methyl-2,

4-dinitroimidazole (16) and picric acid.

Limitations in the synthesis of protonated, substituted

imidazolium nitrate and picrate salts

The protonation reactions utilizing 1-methyl-2,4-dinitroimidazole

(16, 1-Me-2,4-diNO₂-IM), 1-methyl-4,5-dinitroimidazole (17, 1-Me-

4,5-diNO₂-IM), and 1-methyl-4,5-dicyanoimidazole (18, 1-Me-4,5-

diCN-IM) with both picric and nitric acids failed (Scheme 3).

The functionalization of these heterocycles with electron-

withdrawing –NO₂or –CN substituents is thought to reduce

the basicity of the heterocycle, and thus reduce its ability to

form quaternary salts by protonation. These results are in

agreement with the computational results presented by

Gutowski et al.,⁴²where it was concluded that the higher the

electron withdrawing character of the substituents on carbon

2, 4, or 5 of the imidazole ring, the more resistant the molecule

is to react with electrophiles. Furthermore, when electron

withdrawing groups are added to the heterocyclic ring system,

the application of stronger alkylating/protonating agents

becomes necessary, and in the case of dinitro-imidazolium

salts, when calculated under ideal conditions, only the

triﬂates were reactive enough to alkylate the dinitro-imidazole

precursor. We were unable to protonate these molecules even

with HCl.

Fig. 2 50% probability ellipsoid plot of co-crystal of 1-methyl-4,

5-dicyanoimidazole (18) and two equivalents of picric acid.

the nitro/cyano groups attached to the imidazole core, the

imidazole becomes inactive toward protonation.

Thermal investigation of protonated nitrate and picrate

salts – diﬀerential scanning calorimetry analyses

Results from the DSC experiments are presented in Table 1.

The results are presented such that the ﬁrst data value

represents the initial glass transitions for the samples which

exhibit supercooled behavior after initial melting and cooling.

The second value represents any liquid–liquid transitions

obtained for the samples during the second and third con-

secutive run. Finally, the third data value represents actual

melting transitions of the samples in their thermal equilibrium

state. The synthesized salts varied in their thermal behavior

from high melting solids, commonly observed for the highly

substituted, short alkyl chain imidazolium picrate salts, to

room temperature ionic liquids with glass transitions below

0 1C and no crystallization or melting transitions, as commonly

observed for long alkyl chain imidazolium nitrate salts.⁴³

From the group of 30 compounds, more than half (23) melt

below 100 1C.

Further evidence of this eﬀect was obtained from the

isolation of co-crystals of 1-Me-4,5-diCN-IM and 1-Me-2,4-

diNO₂-IM with picric acid (Fig. 1 and 2). This observation

conﬁrms that, due to the high electron withdrawing eﬀect of

Most of the crystalline salts displayed a sharp melting

Scheme 2 General synthetic protocol for the formation of protonated

substituted imidazolium nitrate and picrate salts.

transition on heating, and a crystallization transition on

c

704 New J. Chem., 2012, 36, 702–722

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Table 1 Melting point transitions of the protonated substituted imidazolium salts^a

Thermal transitions (1C)

[Picrate]^ꢀ(a)

[Nitrate]^ꢀ(b)

—

Cation precursor

[Cation]⁺

1

[1-Me-3-H-IM]⁺

—

159^f

(lit 158)⁴⁵

182^f

(lit 132)⁴⁷

97^e

—

60^f

(lit 70)⁴⁶

79^f

2

[1,2-diMe-3-H-IM]⁺

—

(lit 84)⁴⁶

—

3

4

5

6

7

8

9

10

11

12

13

14

15

[1-Pr-3-H-IM]⁺

ꢀ24^b

ꢀ38^b

—

ꢀ81^b

ꢀ76^b

ꢀ68^b

ꢀ72^b

—

64^c

—

[1-Bu-3-H-IM]⁺

34^c

—

6^d

—

[1-Bu-2-Me-3-H-IM]⁺

[1-Pent-2-Me-3-H-IM]⁺

[1-Hex-3-H-IM]⁺

104^f

127^f

41^d

66^c

48^c

—

ꢀ44^b

ꢀ25^b

—

79^d

98^f

[1-Me-2-NO₂-3-H-IM]⁺

[1,2-diMe-5-NO₂-3-H-IM]⁺

[1-Et-2-NO₂-3-H-IM]⁺

[1-Et-4-NO₂-3-H-IM]⁺

[1-i-Pr-4-NO₂-3-H-IM]⁺

[1-Bu-2-Me-4-NO₂-3-H-IM]⁺

[1-Pent-2-Me-4-NO₂-3-H-IM]⁺

[1-Hex-4-NO₂-3-H-IM]⁺

38^c

—

42^e

—

165^f

—

88^f

ꢀ33^b

ꢀ26^b

ꢀ28^b

ꢀ30^b

ꢀ33^b

51^c

42^c

—

ꢀ60^b

ꢀ51^b

—

38^c

—

40^f

—

57^e

—

63^c

—

71^c

67^c

—

67^f

—

79^d

—

ꢀ63^b

—

ꢀ31^f

11^d

a

Salts meeting the deﬁnition of ILs, with melting point o100 1C, are shown in bold. All transitions are presented as the onset temperature for that

b

thermal transition. Glass transition (T_g) on heating, as result of initial melting and cooling of the sample, with no crystallization occurring on

c

cooling, and formation of the glass. Secondary liquid–liquid transitions (T_l–l). Melting point of the sample (mp) where the crystallization event

d

e

occurs on heating prior to the melting point. Melting event occurring only during the ﬁrst heating cycle (for the rest of the experiment the sample

f

remained as a supercooled liquid). Reproducible melting point.

cooling from the melt. However, several of the lower melting

salts only crystallized slowly on heating after their initial

melting, cooling, and glass transitions. The melting transitions

of many of the salts were poorly deﬁned in the DSC, com-

monly having a characteristic broad transition with a strong

leading edge.

Only a few of the analyzed salts exhibited simple thermal

behavior with a sharp melting point on heating and subsequent

crystallization on cooling that were reproducible throughout

all heating and cooling cycles, and, interestingly, none of those

salts exhibited glass transitions (Fig. 3, Example A). These samples

include: [1-Me-3-H-IM][Pic] (1a), [1,2-diMe-3-H-IM][Pic] (2a),

[1-Bu-2-Me-3-H-IM][Pic] (5a), [1-Pent-2-Me-3-H-IM][Pic] (6a),

[1,2-diMe-5-NO₂-3-H-IM][Pic] (9a), [1-Me-3-H-IM][NO₃] (1b),

[1,2-diMe-3-H-IM][NO₃] (2b), [1-Me-2-NO₂-3-H-IM][NO₃] (8b),

[1,2-diMe-5-NO₂-3-H-IM][NO₃] (9b), [1-Bu-2-Me-4-NO₂-3-H-

IM][NO₃] (13b), and [1-Pent-2-Me-4-NO₂-3-H-IM][NO₃] (14b).

All other salts exhibited more complicated thermal behavior

and can be divided into three groups. The ﬁrst group includes

the compounds that, during the ﬁrst heating cycle, exhibited a

Fig. 3 Characteristic thermal behavior of examples: Example A – typical

melting point transition, but no crystallization was observed

thermal behavior with a melting transition on heating and crystallization

during the cooling cycle, leading to the formation of a glass as

on cooling; Example B – supercooled behavior with initial melting of the

a result of supercooled behavior. Those supercooled salts,

sample (mp), and cooling with no crystallization follow by glass transition

upon heating and after passing the glass transition, undergo

(T_g), crystallization, and melting on heating.

(i) crystallization transition just prior to the expected melting

point and (ii) melting transition in the expected temperature

range. Compounds exhibiting such thermal characteristics

include: [1-Hex-3-H-IM][Pic] (7a), [1-Hex-4-NO₂-3-H-IM][Pic]

(15a), [1-Bu-3-H-IM][NO₃] (4b), [1-Hex-3-H-IM][NO₃] (7b),

and [1-Hex-4-NO₂-3-H-IM][NO₃] (15b) (Fig. 3, Example B).

The second group includes the compounds that exhibited a

melting point only during their ﬁrst heating cycle, after which

the salts exhibited supercooled characteristics with no crystal-

lization on cooling, a glass transition on cooling and on

heating, and neither crystallization nor melting during

consecutive cycles. Those salts include: [1-Pr-3-H-IM][Pic]

(3a) and [1-i-Pr-4-NO₂-3-H-IM][Pic] (12a).

The last group of compounds includes the salts that exhibited

rather unusual thermal behavior. They showed (i) diﬃcult to

deﬁne broad transitions on heating, (ii) no crystallization

transitions on cooling, (iii) glass transitions on cooling and

heating, (iv) no crystallization or melting on heating, but only

(v) liquid–liquid transitions above room temperature that

resemble secondary glass transitions. Samples exhibiting this

kind of thermal behavior include: [1-Bu-3-H-IM][Pic] (4a),

c

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[1-Me-2-NO₂-3-H-IM][Pic] (8a), [1-Et-2-NO₂-3-H-IM][Pic] (10a),

[1-Et-4-NO₂-3-H-IM][Pic] (11a), [1-Bu-2-Me-4-NO₂-3-H-IM][Pic]

(13a), [1-Pent-2-Me-4-NO₂-3-H-IM][Pic] (14a), [1-Pr-3-H-

IM][NO₃] (3b), [1-Bu-2-Me-3-H-IM][NO₃] (5b), [1-Pent-2-Me-3-

H-IM][NO₃] (6b), [1-Et-2-NO₂-3-H-IM][NO₃] (10b), [1-Et-4-

NO₂-3-H-IM][NO₃] (11b), and [1-i-Pr-4-NO₂-3-H-IM][NO₃] (12b).

The characteristic DSC traces of this group of salts showed

a real glass transition below 0 1C, then a second glass-like

transition above room temperature during the second and

third DSC cycles. It was noted that the secondary transitions

occurred closely, with much reduced intensity, to expected

crystallization and melting transitions. The thermal transitions

of the above mentioned salts, described as liquid–liquid transi-

tions, appear to be the temperatures of crystallization of the

particular salts, but because of their slow crystallization processes,

the actual crystallization does not occur and only something

appearing like a secondary glass transition is recorded. To

support the argument that the observed liquid–liquid transitions

are real attempts of the sample to crystallize and melt, the below

two examples are presented (Fig. 4 and 5).

Fig. 4 presents a comparison of the second cycles of DSC scans

for [1-Hex-4-NO₂-3-H-IM][Pic] (15a) (mp = 79 1C) (Example A),

[1-Hex-3-H-IM][Pic] (7a) (mp = 41 1C) (Example B), and [1-Bu-

2-Me-4-NO₂-3-H-IM][Pic] (13a) (T_l–l= 71 1C) (Example C). All

of the samples exhibit glass transitions below room temperature

as the result of their supercooled behavior. For [1-Hex-4-NO₂-3-

H-IM][Pic] (15a), after initial melting during the ﬁrst heating

cycle the sample does not crystallize on cooling but undergoes a

glass transition and subsequent crystallization and melting on

heating only. [1-Hex-3-H-IM][Pic] (7a) exhibits a similar shape in

the DSC trace to [1-Hex-4-NO₂-3-H-IM][Pic] (15a) in the melting

region, but the crystallization/melting trace is much reduced in

intensity. The same is observed for the [1-Bu-2-Me-4-NO₂-3-H-

IM][Pic] (13a), where in the region suspected of exhibiting

melting behavior the crystallization/melting does not occur and

only a T_l–lis observed. Based on the structural diﬀerence between

Fig. 5 DSC traces of [1-Me-2-NO₂-3-H-IM][Pic] (8a) showing a

sharp melting point on heating (during the ﬁrst heating cycle) and a

glass transition and liquid–liquid transition (during the second heating

cycle) as a result of supercooled behavior.

[1-Hex-3-H-IM][Pic] (7a) and [1-Hex-4-NO₂-3-H-IM][Pic] (15a),

it was expected that 7a would exhibit a lower melting point

than 15a, due to the absence of the additional substituent in

the imidazolium ring, which usually tends to cause an increase

in the melting point of those salts. The diﬀerence in the melting

points between those two compounds was not, however,

expected to be very large.

Another example, the thermogram of [1-Me-2-NO₂-3-H-

IM][Pic] (8a) (Fig. 5) clearly shows that the sample after an

initial melting transition at 42 1C, and consecutive cooling, did

not crystallize but instead exhibited supercooled characteristics,

with a glass transition at ꢀ25 1C. When heated for the second

time, neither crystallization nor melting transitions were observed;

however, a liquid–liquid transition was seen in the temperature

region previously recognized as a melting point transition.

Furthermore, after the sample was left for a one week period in

a closed vial and then DSC analyses performed again, the sample

exhibited a melting point during the ﬁrst heating cycle in the

region as observed previously and no crystallization or melting

transitions in any of the consecutive runs. As a result, it was

decided to consider the observed secondary T_l–ltransitions as

attempts of the salt to crystallize; thus we have decided to treat

these liquid–liquid transitions as indications of the melting points

of the analyzed samples. Such assumptions allow for better

comparisons of melting points of the salts and aids in drawing

conclusions regarding the observed trends in melting points.

As expected, the general trends in the melting points of the

salts show that melting transitions of picrate-based salts are

always higher than that of their nitrate equivalents. Picrate-

based salts exhibit melting points ranging from 182 1C for [1,2-

diMe-3-H-IM][Pic] (2a) to 34 1C for [1-Bu-3-H-IM][Pic] (4a)

(recognized as a liquid–liquid transition). On the other hand,

the melting points of nitrate-based salts range from 98 1C for

Fig. 4 Comparison of exemplary DSC traces of supercooled behavior.

The DSC traces are shown for the second heating cycle.

c

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[1-Me-2-NO₂-3-H-IM][NO₃] (8b) to 6 1C for [1-Bu-3-H-

IM][NO₃] (4b).

Table 2 Thermal stabilities^a

Thermal stabilities (1C)

Picrates (a) Nitrates (b)

Analyzing the melting point variation as a function of the

N-alkyl chain length, it was noted that the longer the alkyl

chain, the lower the melting point of the resulting salts, until

the alkyl chain extends over four carbons, at which point an

increase of the melting point is observed in most cases. This

behaviour is not uncommon among other ILs, and it is often

observed that the melting point minimum for alkylimidazolium-

based salts ranges between three and six carbons in the alkyl chain,

after which the melting point starts to increase.⁴⁴The melting

points of the 1-alkylimidazolium picrate salts follow the trend:

159 1C for [1-Me-3-H-IM]⁺(1a), 97 1C for [1-Pr-3-H-IM]⁺(3a),

34 1C for [1-Bu-3-H-IM]⁺(4a), and 41 1C for [1-Hex-3-H-IM]⁺

(7a). A similar trend is observed for 1-alkylimidazolium nitrate

salts: 60 1C for [1-Me-3-H-IM]⁺(1b), 64 1C for [1-Pr-3-H-IM]⁺(3b),

6 1C for [1-Bu-3-H-IM]⁺(4b), and 79 1C for [1-Hex-3-H-IM]⁺(7b).

Analysis of the structure vs. melting point relationships

between diﬀerent 1-alkyl-2-methyl-4 (or 5)-nitro-substituted

imidazolium salts reveals a similar trend to the 1-alkylimidazolium

salts, where the longer the N-alkyl chain-substituted on the

nitrogen position, the lower the melting point of the resulting

salt. The replacement of the C2-H position with a methyl

group did not inﬂuence this trend. For both picrate and nitrate

salts the expected trend is easily recognizable; for picrates:

165 1C for [1,2-diMe-5-NO₂-3-H-IM]⁺(9a), 71 1C for [1-Bu-2-

Me-4-NO₂-3-H-IM]⁺(13a), and 67 1C for [1-Pent-2-Me-4-

NO₂-3-H-IM]⁺(14a); and for nitrates: 88 1C for [1,2-diMe-5-

NO₂-3-H-IM]⁺(9b), 67 1C for [1-Bu-2-Me-4-NO₂-3-H-IM]⁺

(13b), and ꢀ31 1C for [1-Pent-2-Me-4-NO₂-3-H-IM]⁺(14b).

Such behavior has also been reported previously.⁴⁴

Cation

precursor Cation

1

2

3

4

5

6

7

[1-Me-3-H-IM]⁺

184

209

177

198

224

226

188

112

145

107

157

137

188

137

131

143

150

121

137

138

82

[1,2-diMe-3-H-IM]⁺

[1-Pr-3-H-IM]⁺

[1-Bu-3-H-IM]⁺

[1-Bu-2-Me-3-H-IM]⁺

[1-Pent-2-Me-3-H-IM]⁺

[1-Hex-3-H-IM]⁺

8

9

[1-Me-2-NO₂-3-H-IM]⁺

[1,2-diMe-5-NO₂-3-H-IM]⁺

[1-Et-2-NO₂-3-H-IM]⁺

[1-Et-4-NO₂-3-H-IM]⁺

[1-i-Pr-4-NO₂-3-H-IM]⁺

[1-Bu-2-Me-4-NO₂-3-H-IM]⁺

123

69

10

11

12

13

14

15

83, 141^b

87, 192^b

102, 213^b

104, 203^b

102, 231^b

[1-Pent-2-Me-4-NO₂-3-H-IM]⁺176

[1-Hex-4-NO₂-3-H-IM]⁺

175

a

All transitions are presented as the onset temperature for the 5%

b

decomposition (T_5%dec). Sample with two distinguishable decompo-

sition slopes; the given value is the onset temperature for the second

decomposition step.

their thermal stabilities were observed depending on the

structural characteristics of the cation, as well as on the anion

used, ranging in decomposition temperatures from 226 1C to

69 1C. Most of the investigated salts exhibited simple thermal

decomposition behavior with a single decomposition step. The

exceptions from this behavior were evident in three examples of

nitro-substituted imidazolium nitrate salts: [1-Bu-2-Me-4-NO₂-

3-H-IM][NO₃] (13b), [1-Pent-2-Me-4-NO₂-3-H-IM][NO₃] (14b),

and [1-Hex-4-NO₂-3-H-IM][NO₃] (15b).

Generally, the analyzed imidazolium picrate salts decomposed

in the thermal range between 226 1C ([1-Pent-2-Me-3-H-IM][Pic]

(6a)) and 107 1C ([1-Et-2-NO₂-3-H-IM][Pic] (10a)). The thermal

stabilities of the analogous nitrate salts varied from 150 1C

([1-Bu-3-H-IM][NO₃] (4b)) to 69 1C ([1-Et-2-NO₂-3-H-IM][NO₃]

(10b)). Examination of data indicates that the thermal stabilities

of all nitrate-based salts are lower than that of their picrate-based

analogs. Typically, the diﬀerence between the thermal stabilities

of nitrate and picrate salts was ca. 50 1C, but in some cases varied

between 20 and 100 1C. Moreover, it was noticed that all

unsubstituted imidazolium salts exhibited higher stabilities of

ca. 50 1C, compared to any mononitro-substituted salts.

Further analysis of the inﬂuence of the cation structure on the

melting points of the obtained salts revealed that 1,2-dialkyl-

substituted salts melt at higher temperatures than those not

possessing the methyl substituent on the C2 carbon position in

the ring; 182 1C vs. 159 1C for [1,2-diMe-3-H-IM][Pic] (2a) vs.

[1-Me-3-H-IM][Pic] (1a), 79 1C vs. 60 1C for [1,2-diMe-3-H-IM][NO₃]

(2b) vs. [1-Me-3-H-IM][NO₃] (1b), 104 1C vs. 34 1C for [1-Bu-2-Me-

3-H-IM][Pic] (5a) vs. [1-Bu-3-H-IM][Pic] (4a), and 66 1C vs. 6 1C

for [1-Bu-2-Me-3-H-IM][NO₃] (5b) vs. [1-Bu-3-H-IM][NO₃] (4b).

To compare the inﬂuence of the position of the –NO₂group

on the melting point of the salts, two isomeric cations were

prepared, where the only diﬀerence between them was the

position of the –NO₂group on the heterocyclic ring. In the case

of [1-Et-2-NO₂-3-H-IM][NO₃] (10b) (mp 38 1C) and [1-Et-4-

NO₂-3-H-IM][NO₃] (11b) (mp 40 1C), the melting point

diﬀerence is 2 1C, and in the case of [1-Et-2-NO₂-3-H-IM][Pic]

(10a) (mp 51 1C) and [1-Et-4-NO₂-3-H-IM][Pic] (11a) (mp

42 1C) this diﬀerence is 9 1C. Based on this ﬁnding it was

concluded that the position of the –NO₂substituent does not

have any substantial inﬂuence on the melting point of the

formed salt, but as it will be shown later, it does have

substantial eﬀect on the thermal stabilities of the analyzed salts.

Comparison of the thermal stabilities of protonated,

non-nitro-substituted imidazolium picrate and nitrate salts

All non-nitro-substituted imidazolium salts exhibit simple

decomposition characteristics with single-step decomposition.

The T_5%decvalues for the picrate salts vary between 177 1C for

[1-Pr-3-H-IM][Pic] (3a) and 226 1C for [1-Pent-2-Me-3-H-

IM][Pic] (6a). For the nitrate analogs, they do not vary

substantially, with onset temperatures between 121 1C for

[1-Bu-2-Me-3-H-IM][NO₃] (5b) and 150 1C for [1-Bu-3-H-

IM][NO₃] (4b). Comparing the homologous series of compounds

[1-Me; 1-Pr; 1-Bu; 1-Hex-3-H-IM][Pic], the decomposition

temperatures do not seem to have any correlation with alkyl

chain length, with T_5%decof 177 1C (1-Pr) o 184 1C (1-Me) o

188 1C (1-Hex) o 198 1C (1-Bu). Comparing the nitrate

equivalents of the same group of cations, it was noted that

Thermal investigation of protonated nitrate and picrate

salts–thermogravimetric analyses

Table 2 records the onset temperatures for 5% decomposition

(T_5%dec) as obtained by TGA analyses of the salts. Variations of

c

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all of those salts decomposed in a very narrow temperature

range, and in fact, the maximum diﬀerence between T_5%dec

is 13 1C: 137 1C (1-Me) o 138 1C (1-Hex) o 143 1C (1-Pr) o

150 1C (1-Bu).

When considering compounds with a methyl substituent on the

C2 position of the ring, a noticeable trend arises for the picrate

salts. The C2-Me-substituted salts exhibit higher decomposition

temperatures (ca. 20 1C) than those that are unsubstituted at the

C2 carbon. Comparing [1-Me-3-H-IM][Pic] (1a) to [1,2-diMe-3-

H-IM][Pic] (2a), the T_5%decincreased from 184 1C to 209 1C and

from [1-Bu-3-H-IM][Pic] (4a) to [1-Bu-2-Me-3-H-IM][Pic] (5a),

the T_5%decincreased from 198 1C to 224 1C. Additionally, it was

noted that the decomposition temperatures of C2-Me-substituted

picrate salts increased slightly as the alkyl chain length increased;

from 209 1C (1,2-diMe-substituted) to 224 1C (1-Bu-2-Me-

substituted) to 226 1C (1-Pent-2-Me-substituted). Interestingly,

the C2-Me-substituted nitrate salts exhibit lower decomposition

temperatures than the unsubstituted analogs; 137 1C [1-Me-3-

H-IM]⁺(1b) 4 131 1C [1,2-diMe-3-H-IM]⁺(2b) and 150 1C

[1-Bu-3-H-IM]⁺(4b) 4 121 1C [1-Bu-2-Me-3-H-IM]⁺(5b).

Moreover, changing the length of the N-alkyl chain in the

C2-Me-substituted 1-alkylimidazolium nitrates does not seem

to signiﬁcantly inﬂuence the decomposition temperature of the

resulting salt: 131 1C (1,2-diMe-substituted), 121 1C (1-Bu-2-

Me-substituted), and 137 1C (1-Pent-2-Me-substituted).

From these results it could be concluded that protonated, non-

nitro-substituted imidazolium picrate salts are more thermally

stable than their nitrate salt equivalents, and that the C2-Me-

substituted picrate salts tend to be more stable than those which

are unsubstituted, however, they also have higher melting points.

It was also concluded that the thermal stability range for each

group of salts is more dependent on the structure of the anion

than on the presence of substituents on the C2 carbon or the

alkyl chain length of the N-alkyl substituents.

Fig. 6 Characteristic two step decomposition processes observed for

4-NO₂-substituted alkylimidazolium nitrates ([1-Pent-2-Me-4-NO₂-3-

H-IM][NO₃] (14b) shown).

(11a) and [NO₃]^ꢀ(11b) they are 157 1C and 83 1C, respectively.

In contrast, the melting points for those pairs of compounds

remained similar (with a maximum diﬀerence of o10 1C). The

substitution of the C2 carbon position with a –NO₂group in

comparison to a –Me group resulted in a decrease of stability

in the picrate salts from 209 1C ([1,2-diMe-3-H-IM]⁺(2a)) to

112 1C ([1-Me-2-NO₂-3-H-IM]⁺(8a)), as well as in the case of

nitrate analogs, 131 1C ([1,2-diMe-3-H-IM]⁺(2b)) to 82 1C

([1-Me-2-NO₂-3-H-IM]⁺(8b)).

When considering all 4-NO₂-substituted alkylimidazolium

nitrates, two separate decomposition steps were observed. The

ﬁrst occurs at ca. 100 1C and ends at ca. 150 1C, while the

second starts between ca. 180 1C and 200 1C. The loss of

weight during the ﬁrst decomposition step (ca. 18–28 wt%)

seems to match the loss of an HNO₃molecule as the result of a

deprotonation reaction and evacuation of the acid from the

system (Fig. 6). This deprotonation and subsequent evapora-

tion of the acid would lead to the formation of the starting

amine and its evaporation at ca. 200 1C. Such two-step

decomposition was not seen for any other salt with an N-alkyl

chain shorter than ethyl, and it was not observed for any of the

non-nitro-substituted salts.

Thermal stabilities of protonated, nitro-substituted imidazolium

picrate and nitrate salts

The T_5%decdecomposition temperatures for these picrate salts

vary between 107 1C for [1-Et-2-NO₂-3-H-IM][Pic] (10a) and

188 1C for [1-Bu-2-Me-3-H-4-NO₂-IM][Pic] (13a). For the

nitrates T_5%decvary less from each other, with T_5%decbetween

69 1C for [1-Et-2-NO₂-3-H-IM][NO₃] (10b)) and 123 1C for

[1,2-diMe-5-NO₂-3-H-IM][NO₃] (9b). Generally, the decom-

position temperatures for the nitrate and picrate salts of the

–NO₂-substituted alkylimidazoles seems to be ca. 20–50 1C

lower than of those not substituted with nitro groups. Increasing

the length of the alkyl chain on the nitrogen seems to cause a

slight decrease in the thermal stability of the nitro-substituted

imidazolium salts. This can be seen in the example of both

picrates (112 1C vs. 107 1C), albeit less clear a diﬀerence than in

the nitrates, (82 1C vs. 69 1C) of [1-Me-2-NO₂-3-H-IM]⁺and

[1-Et-2-NO₂-3-H-IM]⁺cation-based salts.

Syntheses of selected alkylated imidazolium nitrate and picrate

salts and thermal analysis for comparison with protonated analogs

Methyl-alkylated analogs of a series of nitrate and picrate salts

were also prepared in order to compare the thermal behavior

of families of protonated vs. alkylated salts (Scheme 4). The

compounds included 1,3-dimethylimidazolium picrate ([1,3-

diMeIM][Pic]) (19a), 1,2,3-trimethylimidazolium picrate

([1,2,3-triMeIM][Pic]) (20a), 1-butyl-3-methylimdazolium

picrate ([1-Bu-3-MeIM][Pic]) (21a), 1,3-dimethylimidazolium

nitrate ([1,3-diMeIM][NO₃]) (19b), 1,2,3-trimethylimidazolium

nitrate ([1,2,3-triMeIM][NO₃]) (20b), and 1-butyl-3-methyl-

imidazolium nitrate ([1-Bu-3-MeIM][NO₃]) (21b).

Next, it was noticed that by changing the position of the

–NO₂substituent from the C2 to the C4 carbon in the

imidazolium ring, an increase in the stability of the resulting

salt was observed for both picrates and nitrates. The T_5%decof

[1-Et-2-NO₂-3-H-IM][Pic] (10a) and [NO₃]^ꢀ(10b) are 107 1C

and 69 1C, respectively while for [1-Et-4-NO₂-3-H-IM][Pic]

c

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Scheme 4 Protonated vs. alkylated salts.

Nitrate salts were obtained from the halide salts by metathesis

reactions with silver nitrate, and the picrate salts were obtained

from the halide precursors by ion exchange with sodium picrate

followed by solvent extraction into chloroform. In all cases, the

salts were obtained in high yields. The absence of halide

byproducts, from the anion exchange reactions, was conﬁrmed

with a silver nitrate test. The results conﬁrmed removal of the

halide byproducts during the product extraction step. The

obtained thermal data for these salts are presented in Table 1,

along with the thermal data for their protonated analogs.

Fig. 7 Comparison of the TGA behavior between protonated (open

symbols) and methylated (ﬁlled symbols) alkylimidazolium nitrate

salts; protonated: (–&–) [1-Me-3-H-IM][NO₃] (1b), (–J–) [1,2-

diMe-3-H-IM][NO₃] (2b), (–n–) [1-Bu-3-H-IM][NO₃] (4b); alkylated:

(–’–) [1,3-diMeIM][NO₃] (19b), (–K–) [1,2,3-triMeIM][NO₃] (20b),

(–m–) [1-Bu-3-MeIM][NO₃] (21b).

(Fig. 8, open symbols) decompose in a similar way to the nitrates,

which is rapid with a small quantity of solid residue left after the

experiment, the alkylated imidazolium picrate derivatives (Fig. 8,

ﬁlled symbols) seem to decompose at a much slower rate, with

few distinguishable steps. The ﬁrst step is rapid and stops after

ca. 25–30% of the sample is decomposed, while the second

Comparison of thermal data for protonated and alkylated

imidazolium picrate and nitrate salts–TGA and DSC analyses

The protonated ionic liquids do not diﬀer too much in their

melting points from their methylated derivatives (Table 1) with

diﬀerences ranging from 2 1C to 20 1C with one exception;

[1,2-diMe-3-H-IM][Pic] (2a) and [1,2,3-triMeIM][Pic] (20a)

diﬀer in melting point by 64 1C (the protonated salt is the

higher melting). This melting point diﬀerence is especially

surprising since it was expected that the more symmetrical

cation ([1,2,3-triMeIM]⁺) would form the higher melting salt.

Similarly, the protonated [1,2-diMe-3-H-IM][NO₃] (2b) also

melts at a higher temperature than the alkylated analog 20b,

but in this case the melting point diﬀerence was not as large.

As can be seen from the TGA data, the thermal stabilities of

the protonated imidazolium nitrate salts (Fig. 7, open symbols)

are much lower than their alkylated derivatives (Fig. 7, ﬁlled

symbols) ranging between ca. 131 and 150 1C for protonated

ILs, while the alkylated analogs of the salts appear to be stable

in a higher temperature range between 215 and 268 1C. Nitrate

salts with quaternized cations tended to be more thermally

stable than the corresponding picrate (Fig. 7 vs. Fig. 8, ﬁlled

symbols) salts, but with protonated cations, the picrate salts

(Fig. 7 vs. Fig. 8, open symbols) tended to be approximately

70–80 1C more stable than the nitrate salts.

The rates of decomposition are also interesting. All nitrate

salts demonstrated similar behavior as they decompose rapidly

and almost completely before they reach 500 1C. More variable

behavior is observed in the picrate derivatives which decompose

in diﬀerent ways depending on either being protonated or

methylated, although their decomposition temperatures do

not vary signiﬁcantly between both classes of salts (Fig. 8).

While the protonated imidazolium picrate derivatives

Fig. 8 Comparison of the TGA behavior between protonated (open

symbols) and methylated (ﬁlled symbols) alkylimidazolium picrate

salts; protonated: (–&–) [1-Me-3-H-IM][Pic] (1a), (–J–) [1,2-diMe-

3-H-IM][Pic] (2a), (–n–) [1-Bu-3-H-IM][Pic] (4a); alkylated: (–’–)

[1,3-diMeIM][Pic] (19a), (–K–) [1,2,3-triMeIM][Pic] (20a), (–m–)

[1-Bu-3-MeIM][Pic] (21a).

c

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step is slower and lasts until ca. 40–50% of the sample is

decomposed (around 300–350 1C), and the last step continues

the decomposition up to 800 1C at a constant rate. These

results suggest that decomposition of methylated imidazolium

picrate derivatives involves diﬀerent decomposition steps

which occur progressively one after another and are strongly

dependent on the previous step.

From the available results for nitrate salts it was observed

that alkylated imidazolium salts are much more stable than

their protonated analogs. Moreover, based on the rates of the

exothermic reactions among the four nitrate-based salts

(Fig. 9A, B), [1-Me-3-H-IM][NO₃] (1b) is the most energetic,

with a SHR_maxof 1523.58 1C g^ꢀ1min^ꢀ1, then [1-Bu-3-H-

IM][NO₃] (4b) c [1-Bu-3-MeIM][NO₃] (21b) 4 [1,2-diMe-3-

H-IM][NO₃] (2b), ordered from most to least energetic in

Accelerating rate calorimetry (ARC) analysis

terms of SHR_max

.

Comparing the ARC results for [1,2-diMe-3-H-IM][NO₃]

(2b) and [1,2-diMe-3-H-IM][Pic] (2a) (Table 3), it is interesting

to note that the exotherm observed for [1,2-diMe-3-H-IM][Pic]

(2a) starts at a much higher temperature but progresses much

Accelerating rate calorimetry (ARC) is a useful analytical

technique for identiﬁcation of self-heating runaway reactions

and determination of information on the thermal stability of

materials. Here, ARC was used to investigate the thermal

decomposition of some of the synthesized salts in more detail.

The decomposition temperature data for samples studied in an

oxygen atmosphere in a titanium bomb are shown in Table 3.

The analyzed salts show thermal stability on heating, indicated

by the small residual increase in internal pressure until the

thermal decomposition temperature is reached, at which point

self-heating of the sample starts, resulting in an exothermic

decomposition and evolution of gaseous products. The data in

Table 3 describes the temperature value at which the self-heating

exothermic reaction starts (T_s), the self-heating rate maximum

(SHR_max) which describes the peak temperature change per mass

unit during auto-decomposition of the sample, and pressure rate

maximum (PR_max), which describes the peak pressure change

per mass unit during auto-decomposition of the sample.

Seven ionic liquids were studied by ARC under an oxygen

atmosphere: [1-Me-3-H-IM][NO₃] (1b), [1,2-diMe-3-H-IM][Pic]

(2a), [1,2-diMe-3-H-MIM][NO₃] (2b), [1-Bu-3-H-IM][NO₃]

(4b), 1-butylimidazolium chloride ([1-Bu-3-H-IM][Cl]) (4c),

[1-Bu-3-MeIM][Pic] (21a), [1-Bu-3-MeIM][NO₃] (21b), and

1-butyl-3-methylimidazolium chloride ([1-Bu-3-MeIM][Cl]) (21c).

The T_sand the SHR_maxduring their exothermic decompositions

were determined and the results show that both the cation and

anion components in an ionic liquid play an important role

regarding its energetic properties.

more rapidly, as shown by the SHR_maxof 2650.48 1C g^ꢀ1min^ꢀ1

,

making this salt the most energetic among the analyzed com-

pounds. Also, the pressure curves show that [1,2-diMe-3-H-

IM][Pic] (2a) is explosive when reaching its self-decomposition

temperature, at which point the pressure rate maximum (PR_max

reaches 2514.28 psi g^ꢀ1min^ꢀ1

)

.

Among the series of protonated and methylated 1-butylimi-

dazolium salts (Fig. 10 and 11), [1-Bu-3-MeIM][Pic] (21a) proved

to be the most energetic with SHR_maxof 1502.27 1C g^ꢀ1min^ꢀ1

.

In comparison, both [1-Bu-3-MeIM][NO₃] (21b) and [1-Bu-3-

MeIM][Cl] (21c) were found to be much less energetic with

SHR_maxreaching only 1.31 and 1.39 1C g^ꢀ1min^ꢀ1, respectively.

However, the energetic character of the protonated 1-butylimi-

dazolium nitrate salt (4b) is much higher than that of the alkylated

analog (21b), reaching a SHR_maxvalue of 487.80 1C g^ꢀ1min^ꢀ1

.

X-ray crystallographic studies

Single crystals were obtained for 6 salts including [1-Me-3-H-

IM][Pic] (1a), [1,2-diMe-3-H-IM][Pic] (2a), [1-Me-3-H-IM][NO₃]

(1b), [1,2-diMe-3-H-IM][NO₃] (2b), [1,3-diMe-IM][Pic] (19a),

[1,2,3-triMe-IM][Pic] (20a), as well as two co-crystals, 1-Me-

2,4-diNO₂-IMꢁHPic (16a), and 1-Me-4,5-diCN-IMꢁ1.5 HPic

(co-crystal) (18a) by vapor diﬀusion of diethyl ether into methanol

at 25 1C. Data was collected on a Bruker SMART diﬀractometer

Table 3 Accelerating Rate Calorimetry (ARC) results for the selected protonated and alkylated imidazolium-based salts

Compound No

Ionic Liquid

T_s(1C)

SHR_max(D1C g^ꢀ1min^ꢀ1

)

PR_max(Dpsi g^ꢀ1min^ꢀ1

)

1b

[1-Me-3-H-IM][NO₃]

150–160

1523.58

at 254 1C

2650.48

at 271 1C

0.69

1289.74

at 264 1C

2514.28

at 262 1C

0.86

2a

2b

[1,2-diMe-3-H-IM][Pic]

[1,2-diMe-3-H-IM][NO₃]

205–215

130–140

at 173 1C

0.53

at 174 1C

2.22

at 272 1C

487.80

at 225 1C

0.90

at 269 1C

1502.27

at 278 1C

1.31

at 278 1C

1566.82

at 215 1C

0.85

at 268 1C

1253.18

at 266 1C

3.05

4b

[1-Bu-3-H-IM][NO₃]

[1-Bu-3-H-IM][Cl]

[1-Bu-3-MeIM][Pic]

[1-Bu-3-MeIM][NO₃]

145–155

215–225

195–205

190–200

4c

21a

21b

at 262 1C

0.54

at 265 1C

4.74

at 280 1C

1.39

at 283 1C

2.00

21c

[1-Bu-3-MeIM][Cl]

190–200

at 257 1C

4.05

at 258 1C

1.00

at 381 1C

at 320 1C

c

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Fig. 9 ARC data for nitrate salts (A) bomb pressure vs. time and

(B) bomb temperature vs. time: (–K–) [1-Me-3-H-IM][NO₃] (1b),

(–m–) [1,2-Me-3-H-IM][NO₃] (2b), (–’–) [1-Bu-3-H-IM][NO₃] (4b),

(–E–) [1-Bu-3-MeIM][NO₃] (21b).

Fig. 10 ARC data for protonated and methylated 1-butylimidazolium

salts (A) bomb pressure vs. time and (B) bomb temperature vs. time:

(–’–) [1-Bu-3-H-IM][NO₃] (1b), (–m–) [1-Bu-3-H-IM][Cl] (1c), (–K–)

[1-Bu-3-MeIM][Pic] (21a), (–E–) [1-Bu-3-MeIM][NO₃] (21b), (–.–)

[1-Bu-3-MeIM][Cl] (21c).

with a CCD area detector using graphite monochromated Mo-Ka

radiation. The crystals were cooled to ꢀ100 1C during collection.

Data integration, scaling, and absorption corrections were

performed using the SAINT package distributed by Bruker

AXS.⁴⁸Structure solution and reﬁnement was carried out using

the SHELX-97 software suite.⁴⁹Structures were solved using

direct methods and reﬁned by full-matrix least squares reﬁnement

against F². Full-occupancy non-hydrogen atoms were readily

locatated from the diﬀerence map and reﬁned anisotropically.

For the 4 protic salts (1a, 1b, 2a, and 2b), the hydrogen atoms

bonded to the imidazolium nitrogen atoms were located from the

diﬀerence map and the N–H bond lengths were in the range of

0.82–0.93 A. In the co-crystal 16a, the hydrogen atom was located

from the diﬀerence map with an O–H bond length of 0.98(3) A. In

the co-crystal 18a, there are two unique picric acid molecules; one

of which lies on a crystallographic 2-fold rotation axis. The

hydrogen atom on the full occupancy picric acid molecule was

found from the diﬀerence map and the O–H bond length is

1.081 A. However, the hydrogen atom on the other picric acid

molecule is disordered by the crystallographic rotation axis and

could not be located from the diﬀerence map; instead it was

reﬁned with 50 : 50 disorder over 2 calculated positions using a

riding rotating model. 20a showed disorder of the oxygen atoms

on one of the nitro groups and the methyl groups at the 1 and 3

positions on the imidazolium ring. The disordered oxygen atoms

were located from the diﬀerence map and reﬁned anisotropically

at 50 : 50 occupancy, while the methyl group hydrogen atoms were

placed in calculated positions oﬀset by 60 degrees at 50 : 50

occupancy and reﬁned using a riding rotating model. All other

c

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Fig. 11A

c

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712 New J. Chem., 2012, 36, 702–722

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Fig. 11B From left to right: 50% probability ellipsoid diagrams of formula unit, environment around cation/imidazole, environment around

anion/picric acid, and packing (axes: red = a, green = b, blue = c).

c

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New J. Chem., 2012, 36, 702–722 713

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hydrogen atoms were placed in calculated positions and allowed

to ride on their carrier atoms.

The picrate salts all show a combination of hydrogen

bonding, dipole–dipole interactions, and stacking. In 1a, the

cation is surrounded by 7 anions. Three anions are oriented

nearly coplanar with the imidazolium ring and make hydrogen

bonds with the ring hydrogen atoms. N3-H makes a bifurcated

hydrogen bond to the deprotonated oxygen atom and a

neighboring nitro group oxygen atom on one anion, C4-H

makes two non-directional contacts to nitro group oxygen

atoms on another anion, and C5-H makes non-directional

contacts to nitro group oxygen atoms on two separate anions.

C2-H and the methyl group also make short contacts to nitro

groups on anions that are not in the plane of the ring; these

contacts are longer than the in-plane hydrogen bonds. The

cation also stacks with two picrate anions. For one, the picrate

ring stacks edge-over-center with the cation (closest approach:

C2ꢁ ꢁ ꢁC13, 3.381(5) A; angle between planes = 0.891). The

para nitro group on another picrate also overlaps with the

cation (closest approach: C5ꢁ ꢁ ꢁN5, 3.503(5) A; angle between

rings = 0.551). There are no apparent cation–cation contacts.

The anion makes contacts to 7 cations and 3 anions. The

anion-anion contacts are all between hydrogen atoms on C10

and C13 and nitro groups.

Changes in the bonds of non-hydrogen atoms were also

observed which give indications of ionicity. For each of the 4

protic salts, the carbon-N1 bonds are close or statistically

identical to the carbon-N3 bonds, diﬀering by at most 0.017 A.

This is due to the delocalization of the positive charge across

both sides of the ring and is observed in the dialkylimidazolium

salts 19a and 20a. By contrast, the co-crystals show distinctly

diﬀerent bond lengths for the pyrrole-like and pyridine-like

nitrogen atoms (N1 and N3, respectively). In 16a the C2–N1

and C2–N3 bond lengths diﬀer by 0.055 A, and in 18a they

diﬀer by 0.034 A. The picrate anions in the 4 picrate salts

(1a, 2a, 19a, and 20a) are also structurally diﬀerent from those

in the co-crystals. The C–O bonds in all salts are signiﬁcantly

shorter than in the co-crystals while the carbon–carbon bonds

to the ortho carbon atoms are lengthened, suggesting a greater

degree of enolate character in the anion. For the nitrate salts

1b and 2b, the N–O bond lengths range from 1.238(2) to

1.265(2) A, an appropriate range for nitrate anion bonds.

The bond lengths and angles for the imidazolium, imidazole,

picrate, picric acid, and nitrate molecules appear to be within

typical ranges. The nitrate ions in both nitrate salts are

asymmetric, with two short bonds and one long one. The

aromatic rings and nitrate ions are all planar. For picric acid

and picrate molecules, the para nitro group is always coplanar

with the ring while one or both of the ortho nitro groups are

twisted out of plane or even disordered, as was modeled in 20a.

Each of the 6 salts shows a unique combination of cation and

anion interactions. This is to be expected as the protons on C2

and N3 are both hydrogen bond donors, and each cation has a

diﬀerent combination of protons on these sites. For the nitrate

salt 1b, the cation is surrounded by 4 cations and 5 anions. The

anions all make short contacts through the hydrogen atoms on

the ring: N3-H makes a directional hydrogen bond to an oxygen

atom on the anion, C4-H makes a bifurcated hydrogen bond to

oxygen atoms on two separate anions, and C2-H and C5-H

make non-directional contacts to oxygen atoms. Two of the

cation-cation contacts are between a methyl group and a ring

hydrogen atom and appear to be due to the linking of two

cations by short contacts to the same anion. The other cation–

cation short contacts are due to stacking interactions to cations

above and below the ring, which all stack C5-over-center (closest

approach: C2ꢁ ꢁꢁC4 3.329(3) A; angle between ring planes =

4.101). The anion is surrounded by 5 cations and no anions, and

participates only in the aforementioned cation-anion contacts.

In the other nitrate salt, 2b, the cation is surrounded by

6 anions. N3-H and C4-H make directional hydrogen bonds to

nitrate ions, and C5-H and the methyl groups make non-

directional contacts to nitrate ions. There is also a nitro group

above the imidazolium ring plane which makes a short contact

to C2. There is no stacking of cations and nearby cation rings

do not overlap. The anion is surrounded by six cations and

interacts via the aforementioned contacts. The nitro group

makes short (less than the sum of the van der Waals radii)

contacts to two imidazolium ions solely through contacts to

the methyl groups and to a third only via the electrostatic

interaction with C2 as opposed to 1b, where all contacts

involved acidic ring hydrogen atoms.

The cation in 2a is surrounded by 6 anions and no cations.

N3-H makes a bifurcated hydrogen bond to the deprotonated

oxygen atom and a nitro group oxygen atom on a neighboring

anion. There are no other directional hydrogen bonds involving

the cation. The hydrogen atom on C4 makes a non-directional

contact to a nitro group oxygen atom. Both methyl groups make

contacts with anions, one of which (C6-H6Bꢁꢁ ꢁO5) is rather

short and directional. Anions also stack above and below the

ring; one is positioned vertex-over-vertex (closest approach:

C12ꢁ ꢁꢁC5, 3.397(3) A; angle between planes = 0.401) and the

other is positioned edge-over-center (closest approach:

C14ꢁ ꢁꢁC13, 3.277(3) A; angle between planes = 6.711). The

anion is surrounded by 6 cations and 6 anions. Both hydrogen

atoms on the picrate make short contacts to nitro groups on

other anions, and the ortho nitro groups make short contacts to

the hydrogen atoms on other anions. The para nitro group stacks

over the C–O bond of another anion forming a dipole–dipole

contact between the nitro nitrogen atom and O at 3.015(2) A.

The cation in 19a is surrounded by 6 anions and no cations.

C2-H makes a bifurcated hydrogen bond to the deprotonated

oxygen atom and a neighboring nitro group oxygen atom,

C4-H forms a directional hydrogen bond to a nitro group

oxygen atom. The C4 and C5 hydrogen atoms also interact

with the nitro group on another cation to form a 7-membered

ring. One of the methyl groups also makes short contacts to

nitro groups on 3 separate anions. Two anions make contacts

from above and below the imidazolium ring plane. One is a

dipole–dipole interaction between the para nitro group and

C2, and the other is through vertex-over-vertex stacking of the

picrate and imidazolium rings (closest approach: C2ꢁ ꢁ ꢁC11,

3.344(3) A; angle between planes = 2.821). The anion is

surrounded by 6 cations and 3 anions. One of the anion-anion

contacts appears to be due to sharing of a hydrogen atom on

the cation rather than an anion-anion interaction. The anion

stacks edge over center with one anion (closest approach:

C8–10, 3.210(3) A, angle between planes = 01) and dimerizes

with another anion through contacts between O1 and C8.

c

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20a has the weakest hydrogen bond donors of the cations and,

incidentally, the most disorder, with the methyl groups on N1

and N3 being disordered over 2 positions and one of the ortho

nitro groups also being disordered over 2 positions. The cation is

surrounded by 7 anions and 1 cation. The hydrogen atoms on C4

makes a directional hydrogen bond to the deprotonated oxygen

atom, and the hydrogen atom on C5 makes a directional

hydrogen bond to a nitro group oxygen atom. The methyl groups

are involved in weaker short contacts to nitro groups. There is

also vertex-over-vertex (closest approach: C4ꢁꢁꢁC4, 3.188(3) A;

angle between planes = 01) and edge-over-center (closest

approach: C4ꢁ ꢁꢁN1, 3.460(3) A; angle between planes = 01)

stacking between cations. The anion is surrounded by 7 cations

and 3 anions. One of the anions is stacked edge-over-edge (closest

approach: C11ꢁꢁꢁC12, 3.318(2) A, angle between planes = 01)

which positions the para nitro group over C–O and N–O dipoles

of the other anion. The other two anions interact through

dipole–dipole interactions between antiparallel N–O bonds.

Interestingly, the co-crystal 16a only has one strong hydrogen

bonding group in the picric acid molecule which is involved in an

internal hydrogen bond, so hydrogen bonding between molecules

is not prevalent. Instead, interactions between the electron

withdrawn carbon and nitro group nitrogen atoms with

electron-rich oxygen and imidazole-nitrogen atoms are the main

source of short contacts. The 2,4-dinitro-3-methylimidazole

molecule is surrounded by 3 imidazole molecules and 7 picric

acid molecules. The ring nitrogen atom N3 makes short contacts

to carbon atoms on a picric acid molecule. The carbon atoms C4

and C5 make short contacts to nitro groups on two diﬀerent

picric acid molecules. The oxygen atom on the C2 nitro group

makes short contacts to carbon atoms on two separate picric

acid rings, and an oxygen atom on the C4 nitro group makes

short contacts to the nitro group nitrogen atoms on neighboring

picric acid and imidazole molecules. The other nitro group

oxygen atoms are the only atoms on the imidazole which

interact with hydrogen atoms; one of which interacts with the

–OH group on a picric acid molecule, and, while the other

interacts with a picric acid ring hydrogen atom. The methyl

group makes short contacts to nitro groups on a picric acid

molecule and 2 imidazole molecules via the hydrogen atoms.

The ring hydrogen atom on C4 does not make any short

contacts, as it does not have the acidity seen on an imidazolium

cation. The picric acid molecules in 16a are surrounded by 7

imidazole molecules and 4 picric acid molecules. In addition to

the imidazole-picric acid contacts discussed, the picric acid

molecules interact with each other through two kinds of

contacts. One of the ortho nitro groups makes short contacts

to ring hydrogen atoms in one direction, and in a perpendicular

direction the –OH group and the para nitro group stack over

each other through dipole–dipole interactions.

The nitrile nitrogen atom N10 makes a short contact to a nitro

group nitrogen atom, and the methyl group makes short con-

tacts to nitro group oxygen atoms in addition to the nitrile group

of the other imidazole. The picric acid molecule which occupies

a special position is surrounded by 6 imidazole molecules and 4

picric acid molecules. The –OH group makes a hydrogen bond

to the nitro group of a symmetry-equivalent picric acid molecule

next to it and receives the same hydrogen bond at its para nitro

group. Nitro groups on picric acid molecules above and below

the ring also make short contacts to carbon atoms on the ring.

The other picric acid molecule is surrounded by 3 imidazole

molecules and 6 picric acid molecules, all of which are its

symmetry equivalents. Two of the picric acid molecules form

N–O dipole–dipole interactions. Two interact with the hydrogen

atom on the ring –OH group which engages in an internal

hydrogen bond. One makes short contacts between a ring

carbon atom and a nitro oxygen atom, as well as the ring

hydrogen atom and a nitro oxygen atom. The last one appears

to be linked through interactions to an imidazole ring and forms

a contact which does not appear to due to an interaction.

The packing in 1b consists of alternating layers of cations

and anions stacking along the c axis. The layers are organized

along the ab plane. Anion-cation hydrogen bonding propa-

gates in 2-dimensional sheets roughly along the ab diagonal,

and cation–cation pi–pi stacking propagates in chains down the

c axis. The other nitrate salt, 2b, has a similar salt-like packing

with the notable exception that there is no cation–cation

stacking. Alternating layers of cations and anions form

perpendicular to the c axis. The cation–anion hydrogen

bonding propagates in zigzag chains along roughly along b.

In 1a hydrogen bonding organizes the cations and anions

into inﬁnite 1-d chains which are hydrogen bonded to each

other to form pairs of antiparallel chains. The pairs of chains

stack perpendicular to the hydrogen bonding direction via

cation–anion stacking interactions to form inﬁnite 2-d sheets,

each 2 molecules wide. These sheets run roughly along the ac

diagonals and alternating (non-hydrogen bonded) sheets are

rotated 1801 from each other about the a axis.

In 2a the anions are organized into sheets perpendicular to

c by anion-anion hydrogen bonding along b and anion-anion

stacking along a. The cations are organized into sheets

through their interactions with the anions. This results in a

salt-like packing consisting of alternating sheets of cations and

anions going along the c axis.

In 19a chains of alternating cations and anions form along the

ac diagonal plane. Each chain stacks with an antiparallel chain

through cation-cation and anion-anion stacking, and these chain

pairs go on to stack through cation–anion stacking to form

sheets. There are no strong interactions between these sheets.

In 20a the packing is salt-like, with the cations and anions

arranged in columns along c through cation–cation or anion–

anion stacking. The columns are surrounded by columns of

counter ions forming a 3-d lattice.

In 18a, the 1-methyl-4,5-dicyanoimidazole molecule is sur-

rounded by 2 imidazole molecules and 6 picric acid molecules.

The contacts between imidazole molecules occur between nitrile

groups and methyl groups at the extremity of the molecule and

are not very short. The C2 hydrogen atom makes non-directional

hydrogen bonds to nitro groups on two picric acid molecules.

N3 accepts a directional hydrogen bond from the hydrogen

atom on the picric acid ring, and the nitrile carbon atom C7

makes a short contact to a picric acid nitro oxygen atom.

The packing in 16a resembles an inclusion compound. The

picric acid molecules are self-organized into pleated sheets

perpendicular to c. These sheets are bridged to each other on

both sides by imidazole molecules. The imidazole molecules are

also arranged in pleated sheets but there are no interactions

which suggest this is self-organized.

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The packing in 18a is interesting and may represent a case of

arrested crystallization. The two unique picric acid molecules

never interact with each other and as a result there are 3

distinct zones in the packing: One of the picric acid molecules

is self-organized into sheets composed of zigzag chains, and

the other is organized into a sheet of stacked, head-to-tail

hydrogen bonded molecules. The imidazole molecules bridge

these two sheets together.

(98%) were purchased from Aldrich. 1-Methyl-2-nitroimidazole

(1-Me-2-NO₂-IM, 8), 1,2-dimethyl-5-nitroimidazole (1,2-diMe-

5-NO₂-IM, 9), 1-ethyl-2-nitroimidazole (1-Et-2-NO₂-IM, 10),

1-ethyl-4-nitroimidazole (1-Et-4-NO₂-IM, 11), 1-methyl-2,4-

dinitroimidazole (1-Me-2,4-diNO₂-IM, 16), 1-methyl-4,5-

dinitroimidazole (1-Me-4,5-diNO₂-IM, 17), and 1-methyl-4,5-

dicyanoimidazole (1-Me-4,5-diCN-IM, 18) were prepared

following our recently published procedure.⁴¹All imidazoles

were distilled prior to use. Nitric acid (69.5% w/w water

solution) was obtained from Fisher (Waltham, MA). Picric

acid (containing not less than 30% w/w water) was obtained

from MERCK (Darmstadt, Germany). Acids were used as

diluted solutions, 3 M HNO₃solution in water, and 0.1 M

picric acid solution in EtOH.

Of the 6 salts which formed crystals, the picrate salts

typically had more short contacts, especially anion–anion

contacts, than nitrate salts which is in agreement with the

higher melting points recorded. The melting point data also

shows a relationship between hydrogen bond donor availability

and melting point. The protic picrate salts both have higher

melting points than their aprotic analogs, The lowest melting

of all the picrate salts, 16a, has only the least acidic C4 and

C5 protons available for hydrogen bonding.

NMR spectra were obtained in CDCl₃(unless otherwise

1

stated) with TMS as the internal standard for H (300 MHz)

or the solvent as the internal standard for ¹³C (75 MHz). All of

the chemicals were employed as supplied.

Crystallographic data. 1a–C₁₀H₉N₅O₇; M = 311.12; Monoclinic,

space group = P2₁/c; a = 8.018(2) A, b = 5.885(2) A, c =

27.215(7) A; b = 94.798(5)1; V = 1279.7(6) A³; Z = 4; T =

173 K; l(Mo-Ka) = 0.71073 A; R₁4 2s = 0.0580, wR²for

all reﬂections = 0.1216. 1b–C₄H₇N₃O₃; M = 145.13; Monoclinic,

space group P2₁/c; a = 8.957(7) A, b = 9.917(7) A, c =

7.143(6) A; b = 96.05(1)1; V = 630.9(8) A³; Z = 4; T =

173 K; l(Mo-Ka) = 0.71073 A; R₁4 2s = 0.0347, wR²for all

reﬂections = 0.0929. 2a–C₁₁H₁₁N₅O₇; M = 325.25; Monoclinic,

space group = Cc; a = 6.076(2) A, b = 15.089(5) A,

c = 14.771(4) A; b = 90.949(5)1; V = 1354.0(7) A³; Z = 4;

T = 173 K; l(Mo-Ka) = 0.71073 A; Flack parameter =

0.7(8); R₁4 2s 0.0228, wR²for all reﬂections = 0.0593.

2b–C₅H₉N₃O₃; M = 159.15; Monoclinic, space group = P2₁/n;

a = 7.863(1) A, b = 13.001(2) A, c = 7.971(1) A; b =

111.871(3)1; V = 756.2(2) A³; Z = 4; T = 173 K; l(Mo-Ka) =

0.71073 A; R₁4 2s = 0.0343, wR²for all reﬂections = 0.0918.

16a–C₁₀H₇N₇O₁₁; M = 401.23; Monoclinic, space group = P2₁;

a = 6.287(1) A, b = 11.646(2) A, c = 10.307(2) A; b =

93.367(3)1; V = 753.3(2) A³; Z = 2; T = 173 K; l(Mo-Ka) =

0.71073 A; Flack parameter = ꢀ0.4(8); R₁4 2s = 0.0236;

wR²= 0.0558. 18a–C₁₅H_8.5N_8.5O_10.5; M = 475.80; Ortho-

rhombic, space group = Pbcn; a = 43.98(1) A; b = 8.850(2) A;

c = 9.627(2) A; a = b = g = 901; V = 3747(2) A³; Z = 8;

T = 173 K; l(Mo-Ka) = 0.71073 A; R₁4 2s 0.0678; wR²for

all reﬂections = 0.1367. 19a–C₁₁H₁₁N₅O₇; M = 325.25; Tri-

Thermogravimetric analyses (TGA) were performed using a

TGA 2950, TA Instruments, Inc. (New Castle, DE). These

experiments were conducted under air atmosphere and measured

in the dynamic heating regime. Samples between 5–15 mg

were heated from 30–500 1C under constant heating ramp of

5 1C min^ꢀ1with a 30 min isotherm at 75 1C. Temperatures

reported for the decomposition proﬁles for all materials were

established as the onset temperature for decomposition of the

ﬁrst 5% of the sample (T_5%onset).

Melting point/glass transition analyses were performed by

diﬀerential scanning calorimetry (DSC) using a DSC 2920

Modulated DSC, TA Instruments, Inc. (New Castle, DE)

cooled with a liquid nitrogen cryostat. The calorimeter was

calibrated for temperature and cell constants using indium

(T_m= 156.61 1C; C = 28.71 J g^ꢀ1). Data were collected at

atmospheric pressure, where samples were initially heated at a

rate of 5 1C min^ꢀ1to a temperature not to exceed 50 1C below

the measured T_5%onset(obtained from TGA). The sample was

then held for a 5 min isotherm prior to two cycles of cooling

and heating (back to upper temperature limit from ﬁrst

heating) at a rate of 5 1C min^ꢀ1spaced by 5 min isothermal

holding at lower (T = ꢀ100 1C, unless otherwise stated) and

upper (as indicated above) endpoint temperatures. Samples

between 5–15 mg were used in aluminum sample pans (sealed,

then perforated with a pin-hole to equilibrate pressure

resulting from potential expansion of evolved gases). The

DSC was adjusted so that zero heat ﬂow was between 0 and

ꢀ0.5 mW, and the baseline drift was less than 0.1 mW over

the temperature range of 0–180 1C. An empty sample pan

served as the reference. Temperatures reported for the glass

transition (T_g) and melting (T_m) were established as the onset

temperature for the endothermic change in heat ﬂow measured

through the material and as the onset temperature for the

exothermic change in heat ﬂow measured in the case of

observed crystallization (T_cryst).

%

clinic, space group = P1; a = 7.210(2) A; b = 9.615(2) A; c =

10.265(2) A; a = 78.308(3)1; b = 82.099(4)1, g = 85.773(4)1;

V = 689.5(3) A³; Z = 2; T = 173 K; l(Mo-Ka) = 0.71073 A;

R₁4 2s = 0.0592, wR²= 0.1305. 20a–C₁₂H₁₃N₅O₇;

%

M = 339.27; Triclinic, space group = P1; a = 7.250(2) A,

b = 9.107(3) A, 11.946(4) A; a = 83.270(5)1, b = 74.608(5)1,

g = 79.684(5)1; V = 746.1(4) A³; Z = 2; T = 173 K;

l(Mo-Ka) = 0.71073 A; R₁4 2s = 0.0352, wR²for all

reﬂections = 0.0913.

Accelerating rate calorimetric (ARC) measurements were

carried out with a modiﬁed Arthur D. Little ARC 2000t,

which allows samples to be run in a speciﬁc atmosphere.⁵⁰

Titanium bombs (1 inch o.d., from Tricor Metals, Inc.) were

used for all the ARC runs. The standard ARC operational

mode of Heat-Wait-Search (HWS) was adopted.

Experimental

Chemicals

1-Methylimidazole (1-MeIM, 1) (98%), 1,2-dimethylimidazole

(1,2-diMeIM, 2) (98%), and 1-butylimidazole (1-BuIM, 4)

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Procedure for alkylimidazoles (method A) (3, 7, 13–15)

147.9. Anal. Calcd for C₆H₉N₃O₂: C, 46.45; H, 5.85; N, 27.08.

Found: C, 46.85; H, 5.82; N, 27.11.

A mixture of appropriate imidazole (10 mmol), alkyl bromide

(12 mmol), potassium carbonate (3.32 g, 24 mmol), and

tetrabutylammonium bromide (0.032 g, 0.1 mmol) in aceto-

nitrile (50 mL) was stirred vigorously under reﬂux for 2 h.

After cooling to room temperature, the precipitate was ﬁltered

oﬀ and washed with acetonitrile. The ﬁltrate was evaporated,

and the crude products were puriﬁed via column chromato-

graphy using ethyl acetate/hexane.

1-Butyl-2-methyl-4-nitroimidazole (1-Bu-2-Me-4-NO₂-IM,

13). Microcrystals from ethyl acetate/hexane, 84%^ayield,

mp 58–60 1C, ¹H NMR (300 MHz, [D₁] CHCl₃) d 0.99 (t, J =

7.4 Hz, 3H), 1.46–1.33 (m, 2H), 1.84–1.74 (m, 2H), 2.44 (s, 3H),

3.94 (t, J = 7.3 Hz, 2H), 7.71 (s, 1H), ¹³C NMR (75 MHz, [D₁]

CHCl₃) d 13.0, 13.4, 19.5, 32.1, 46.9, 119.5, 144.5, 146.2. Anal.

Calcd for C₈H₁₃N₃O₂: C, 52.45; H, 7.15; N, 22.94. Found:

C, 52.80; H, 7.33; N, 22.87.

Procedure for alkylimidazoles (method B) (3, 5–7, 12)

1-Pentyl-2-methyl-4-nitroimidazole (1-Pent-2-Me-4-NO₂-IM,

14). Microcrystals from ethyl acetate/hexane, 65%^ayield, mp

35–37 1C, ¹H NMR (300 MHz, [D₁] CHCl₃) d 0.93 (t, J = 6.7 Hz,

3H), 1.40–1.32 (m, 4H), 1.85–1.75 (m, 2H), 2.44 (s, 3H), 3.92

(t, J = 7.4 Hz, 2H), 7.70 (s, 1H), ¹³C NMR (75 MHz, [D₁]

CHCl₃) d 13.1, 13.7, 22.1, 28.4, 29.9, 47.2, 119.5, 144.5, 146.3.

Anal. Calcd for C₉H₁₅N₃O₂: C, 54.81; H, 7.67; N, 21.30.

Found: C, 55.13; H, 7.90; N, 21.22.

The appropriate imidazole (50 mmol) was dissolved in DMF

(10 mL). Potassium tert-butoxide (6.7 g, 60 mmol) was added

at 0–5 1C followed by the addition of appropriate alkyl

bromide (72 mmol). The reaction mixture was stirred at room

temperature overnight. Water (20 mL) was added to the mixture.

The solution was extracted with ethyl acetate (3 ꢂ 40 mL). The

extract was washed with brine and dried over anhydrous

magnesium sulfate. The solvent was evaporated under reduced

pressure (bath 60–70 1C) to remove DMF and the residue was

puriﬁed via column chromatography using ethyl acetate/hexane

to give the desired N-alkylimidazoles.

1-Hexyl-4-nitroimidazole (1-Hex-4-NO₂-IM, 15). Micro-

crystals from ethyl acetate/hexane, 62%^ayield, mp 39–41 1C,

¹H NMR (300 MHz, [D₁] CHCl₃) d 0.89 (t, J = 6.9 Hz, 3H),

1.36–1.28 (m, 6H), 1.90–1.80 (m, 2H), 4.04 (t, J = 7.1 Hz, 2H),

7.46 (d, J = 1.3 Hz, 1H), 7.79 (d, J = 1.4 Hz, 1H), ¹³C NMR

(75 MHz, [D₁] CHCl₃) d 13.7, 22.2, 25.8, 30.5, 30.9, 48.3,

119.1, 135.9, 147.9. Anal. Calcd for C₉H₁₅N₃O₂: C, 54.81;

H, 7.67; N, 21.30. Found: C, 55.13; H, 7.93; N, 21.10.

1

1-Propylimidazole (1-PrIM, 3)⁵¹. Oil, 60% yield, H NMR

(300 MHz, [D₁] CHCl₃) d, 0.91 (t, J = 7.4 Hz, 3H), 1.85–1.73

(m, 2H), 3.89 (t, J = 7.1 Hz, 2H), 6.90 (s, 1H), 7.04 (s, 1H),

7.45 (s, 1H), ¹³C NMR (75 MHz, [D₁] CHCl₃) d 10.8, 24.1,

48.3, 118.5, 129.0, 136.8.

1-Butyl-2-methylimidazole (1-Bu-2-MeIM, 5)⁵². Oil (87%^b);

¹H NMR (300 MHz, [D₁] CHCl₃) d 0.94 (t, J = 7.3 Hz, 3H),

1.33 (m, 2H), 1.72–1.67 (m, 2H), 2.36 (s, 3H), 3.81 (t, J = 7.2 Hz,

2H), 6.80 (d, J = 1.0 Hz, 1H), 6.88 (d, J = 1.0 Hz, 1H);

¹³C NMR (75 MHz, [D₁] CHCl₃) d 12.8, 13.3, 19.5, 32.5, 45.5,

118.8, 126.7, 144.0. Anal. Calcd for C₈H₁₄N₂: C, 69.52;

H, 10.21; N, 20.27. Found: C, 68.91; H, 10.62; N, 20.02.

General procedure for preparation of protonated imidazolium

nitrate and picrate salts

A variety of acid–base paired, protonated imidazolium salts

were prepared using a neutralization reaction procedure.⁵³

The appropriate imidazole (0.6 mmol) was dissolved in EtOH

(3 mL) and a solution of the appropriate acid (0.6 mmol; 3%

excess of nitric acid was used for the preparation of nitrates)

was added dropwise to the imidazole solution with stirring at

40 1C. All of the neutralization reactions were exothermic. The

solution was stirred for up to 10 h at 40 1C, after which the

solvent was removed on a rotary evaporator, under reduced

pressure, at 50 1C. Reaction progress was monitored using

TLC chromatographic techniques, using chloroform and

methanol as eluent, comparing the signatures of the obtained

products with the TLC signature of the starting materials.

Solid products were washed with cold EtOH to remove any

unreacted substrates. Similarly, liquid samples were washed

with dry acetone. Pure products were obtained in 82–99%

yields after drying under vacuum at 40 1C for 10 h. The

reactions with 1-methylimidazolium, 1,2-dimethylimidazolium,

and 1-butylimidazolium salts were performed using the same

procedure but on larger scale (50 mmol).

1-Pentyl-2-methylimidazole (1-Pent-2-MeIM, 6)⁵². Oil,

1

92%^byield, H NMR (300 MHz, [D₁] CHCl₃) d 0.90 (t, J =

7.0 Hz, 3H), 1.37–1.26 (m, 4H), 1.75–1.65 (m, 2H), 2.35 (s, 3H),

3.79 (t, J = 7.1 Hz, 2H), 6.79 (d, J = 1.2 Hz, 1H), 6.88 (d, J =

1.2 Hz, 1H), ¹³C NMR (75 MHz, [D₁] CHCl₃) d, 12.6, 13.5,

21.8, 28.2, 30.0, 45.6, 118.6, 126.5, 143.8. Anal. Calcd for

C₉H₁₆N₂: C, 71.01; H, 10.59; N, 18.40. Found: C, 70.25;

H, 11.09; N, 17.96.

1-Hexylimidazole (1-HexIM, 7)⁵². Oil, 44%^aand 72%^b

1

yield, H NMR (300 MHz, [D₁] CHCl₃) d, 0.88 (t, J = 7.6 Hz,

3H) 1.33–1.26 (m, 6H), 1.75 (p, J = 14.4 Hz, J = 7.0 Hz, 2H),

3.91 (t, J = 7.1 Hz, 2H), 6.90 (t, J = 1.2 Hz, 1H), 7.04 (t, J =

1.0 Hz, 1H), 7.45 (s, 1H); ¹³C NMR (75 MHz, [D₁] CHCl₃)

d 13.6, 22.1, 25.8, 30.7, 30.8, 46.6, 118.4, 128.9, 136.7. Anal.

Calcd for C₉H₁₆N₂: C, 71.01; H, 10.59; N, 18.40. Found:

C, 69.96; H, 10.98; N, 18.39.

1-Methylimidazolium Picrate ([1-Me-3-H-IM][Pic]) (1a).

1

Yellow solid, 98% yield; H NMR (360 MHz, [D₆] DMSO)

1-Isopropyl-4-nitroimidazole (1-i-Pr-4-NO₂-IM, 12). Plates

from ethyl acetate/hexane 78%^byield, mp 50–53 1C; ¹H NMR

(300 MHz, [D₁] CHCl₃) d 1.59 (d, J = 6.7 Hz, 6H), 4.56–4.47

(m, 1H), 7.58 (d, J = 1.1 Hz, 1H), 7.91 (d, J = 1.3 Hz, 1 H),

¹³C NMR (75 MHz, [D₁] CHCl₃) d 23.3, 50.9, 117.3, 134.3,

d = 3.85 (s, 3H, N-CH₃), 7.61 (s, 1H, C5-H), 7.65 (s, 1H, C4-H),

8.58 (s, 2H, picrate), 8.94 (s, 1H, N-H); ¹³C NMR (90 MHz,

[D₆] DMSO) d = 35.24 (N-CH₃), 120.14, 122.97 (C4/5),

124.13 (picrate), 125.14 (picrate), 135.87 (C2), 141.83 (picrate),

160.77 (picrate).

c

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New J. Chem., 2012, 36, 702–722 717

View Article Online

1,2-Dimethylimidazolium Picrate ([1,2-diMe-3-H-IM][Pic])

(2a). Yellow solid, 98% yield; ¹H NMR (360 MHz, [D₆]

DMSO) d = 2.54 (s, 3H, C2-CH₃), 3.72 (s, 3H, N-CH₃),

7.51 (s, 1H, C5-H), 7.56 (s, 1H, C4-H), 8.58 (s, 2H, picrate);

¹³C NMR (90 MHz, [D₆] DMSO) d = 10.22 (C2-CH₃), 33.84

(N-CH₃), 117.59, 122.95 (C4/5), 124.11 (picrate), 125.15 (picrate),

141.84 (picrate), 144.41 (C2), 160.77 (picrate).

7.15 (s, 1H, C5-H), 7.64 (s, 1H, C4-H), 8.61 (s, 2H, picrate),

11.42 (b, N–H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 37.05

(N–CH₃), 124.46 (picrate), 125.17 (picrate), 127.47, 128.48

(C4/5), 141.76 (picrate), 151.58 (C2), 160.52 (picrate).

1,2-Dimethyl-5-nitroimidazolium Picrate ([1,2-diMe-5-NO₂-

3-H-IM][Pic]) (9a). Yellow thin crystals, 92% yield; ¹H NMR

(360 MHz, [D₆] DMSO) d = 2.49 (s, 3H, C2-CH₃), 3.85

(s, 3H, N-CH₃), 8.27 (s, 1H, C4-H), 8.59 (s, 2H, picrate);

¹³C NMR (90 MHz, [D₆] DMSO) d = 13.05(C2-CH₃), 33.50

(N–CH₃), 124.52 (picrate), 125.13 (picrate), 129.06 (C4),

141.76 (picrate), 143.62 (C2), 150.09 (C5), 160.85 (picrate).

1-Propylimidazolium Picrate ([1-Pr-3-H-IM][Pic]) (3a). Yellow

solid, 95% yield; ¹H NMR (360 MHz, [D₆] DMSO) d = 0.82

(t, 3H, –CH₃), 1.80 (m, 2H, –CH₂–CH₃), 4.12 (t, 3H, N-CH₂–),

ꢀ ꢀ_ꢀ

7.59 (s, 1H, C5-H), 7.70 (s, 1H, C4-H), 8.59 (s, 2H, picrate), 8.93

(s, 1H, C2-H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 10.20

(–CH₃), 22.73 (–CH₂–CH₃), 49.44 (N–CH₂–), 120.79, 121.36

1-Ethyl-2-nitroimidazolium Picrate ([1-Et-2-NO₂-3-H-IM][Pic])

1

(10a). Yellow viscous liquid, 86% yield; H NMR (360 MHz,

ꢀ ꢀ_ꢀ

(C4/C5), 123.85 (picrate), 124.91 (picrate), 135.22 (C2),

141.54(picrate), 160.57 (picrate).

[D₆] DMSO) d = 1.38 (t, 3H, –CH₃), 4.40 (q, 2H, N–CH₂–), 7.17

(d, 1H, C5-H), 7.70 (d, 1H, C4-H), 8.59 (s, 2H, picrate), ca. 9.2

(b, N–H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 15.56 (–CH₃),

44.72 (N–CH₂–), 124.81 (picrate), 125.14 (picrate), 127.06, 128.86

(C4/5), 141.76 (picrate), 150.22 (C2), 160.41 (picrate).

1-Butylimidazolium Picrate ([1-Bu-3-H-IM][Pic]) (4a). Yellow

1

viscous liquid, 98% yield; H NMR (360 MHz, [D₆] DMSO)

d = 0.90 (t, 3H, –CH₃), 1.23 (sextet, 2H, –CH₂–), 1.76 (p, 2H,

–CH₂–), 4.13 (t, 2H, N–CH₂–), 7.52 (s, 1H, C5-H), 7.65 (s, 1H,

C4-H), 8.60 (s, 2H, picrate), 9.19 (s, 1H, C2-H); ¹³C NMR

1-Ethyl-4-nitroimidazolium Picrate ([1-Et-4-NO₂-3-H-IM][Pic])

1

(11a). Yellow viscous liquid, 83% yield; H NMR (360 MHz,

(90 MHz, [D₆] DMSO) d = 13.34 (–CH₃), 18.93 (ꢀCH₂–CH₃),

ꢀ ꢀ_ꢀ

[D₆] DMSO) d = 1.41 (t, 3H, –CH₃), 4.12 (q, 2H, N–CH₃), 7.91

(s, 1H, C5-H), 8.48 (s, 1H, C2-H), 8.61 (s, 2H, picrate), 9.35

(b, N–H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 15.70 (CH₂–CH₃),

42.51 (N–CH₂), 121.20 (C5), 124.31 (picrate), 125.15 (picrate),

136.95 (C2), 141.78 (picrate), 146.89 (C4), 160.58 (picrate).

31.02 (–CH₂–), 46.52 (N–CH₂–), 118.20, 121.85 (C4/C5),

ꢀ ꢀ_ꢀ

124.15 (picrate), 125.11 (picrate), 141.55 (picrate), 144.52

(C2), 160.75 (picrate).

1-Butyl-2-methylimidazolium Picrate ([1-Bu-2-Me-3-H-IM][Pic])

(5a). Yellow solid, 98% yield; ¹H NMR (360 MHz, [D₆]

DMSO) d = 0.91 (t, 3H, –CH₃), 1.29 (m, 2H, –CH₂–CH₃),

1-Isopropyl-4-nitroimidazolium Picrate ([1-i-Pr-4-NO₂-3-H-

IM][Pic]) (12a). Yellow solid, 91% yield; ¹H NMR (360 MHz,

[D₆] DMSO) d = 1.44 (d, 6H, –CH₃), 4.53 (m, 1H, N–CH–),

ꢀ ꢀ_ꢀ

1.71 (p, 2H, –CH₂–CH₂–), 2.58 (s, 3H, C2–CH₃), 4.07 (t, 3H,

ꢀ ꢀ_ꢀ

ꢀ ꢀ

N–CH₂–), 7.56 (s, 1H, C5-H), 7.64 (s, 1H, C4-H), 8.59 (s, 2H,

picrate); ¹³C NMR (90 MHz, [D₆] DMSO) d = 10.23 (–CH₃),

13.38 (C2–CH₃), 18.96 (–CH₂–CH₃), 31.01 (–CH₂–), 46.52

ꢀ ꢀ_ꢀ

7.95 (s, 1H, C5-H), 8.52 (s, 1H, C2-H), 8.59 (s, 2H, picrate);

¹³C NMR (90 MHz, [D₆] DMSO) d = 22.91 (–CH₃), 50.59

(N–CH–), 119.84 (C5), 124.52 (picrate), 125.27 (picrate),

ꢀ ꢀ_ꢀ

ꢀ ꢀ

(N–CH₂–), 118.11, 121.81 (C4/C5), 124.09 (picrate), 125.07

ꢀ ꢀ_ꢀ

135.95 (C4), 141.92 (picrate), 147.14 (C2), 160.69 (picrate).

(picrate), 141.47 (picrate), 144.06 (C2), 160.83 (picrate).

1-Butyl-2-methyl-4-nitroimidazolium Picrate ([1-Bu-2-Me-4-

1

NO₂-3-H-IM][Pic]) (13a). Yellow solid, 94% yield; H NMR

1-Pentyl-2-methylimidazolium Picrate ([1-Pent-2-Me-3-H-

1

IM][Pic]) (6a). Yellow solid, 96% yield; H NMR (360 MHz,

(360 MHz, [D₆] DMSO) d = 0.89 (t, 3H, –CH₃), 1.26 (m, 2H,

–CH₂–CH₃), 1.69 (p, 2H, –CH₂–), 2.35 (s, 3H, C2-CH₃), 3.97

ꢀ ꢀ_ꢀ

[D₆] DMSO) d = 0.87 (t, 3H, –CH₃), 1.30 (m, 4H, –CH₂–CH₂–),

1.73 (p, 2H, –CH₂–CH₂–), 2.56 (s, 3H, C2–CH₃), 4.05 (t, 3H,

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀ

(t, 2H, N–CH₂–), 8.32 (s, 1H, C5-H), 8.59 (s, 2H, picrate);

¹³C NMR (90 MHz, [D₆] DMSO) d = 12.48 (–CH₃), 13.32

(C2–CH₃), 18.99 (–CH₂–CH₃), 31.44 (–CH₂–), 46.10

ꢀ ꢀ_ꢀ

N–CH₂–), 7.54 (s, 1H, C5-H), 7.63 (s, 1H, C4-H), 8.59 (s, 2H,

picrate); ¹³C NMR (90 MHz, [D₆] DMSO) d = 10.47 (–CH₃),

13.92 (C2–CH₃), 21.77 (–CH₂–CH₃), 27.95 (–CH₂–), 28.88

ꢀ ꢀ_ꢀ

(N–CH₂–), 121.97 (C5), 124.35 (picrate), 125.10 (picrate),

ꢀ ꢀ_ꢀ

(–CH₂–), 46.84 (N–CH₂–), 118.39, 121.92 (C4/C5), 124.39

141.75 (picrate), 144.77 (C4), 145.18 (C2), 160.52 (picrate).

ꢀ ꢀ_ꢀ

(picrate), 125.34 (picrate), 141.37 (picrate), 144.00 (C2),

160.80 (picrate).

1-Pentyl-2-methyl-4-nitroimidazolium Picrate ([1-Pent-2-

Me-4-NO₂-3-H-IM][Pic]) (14a). Yellow solid, 95% yield;

¹H NMR (360 MHz, [D₆] DMSO) d = 0.86 (t, 3H, –CH₃),

1.29 (m, 4H, –CH₂–CH₂–), 1.71 (p, 2H, –CH₂–), 2.35 (s, 3H,

1-Hexylimidazolium Picrate ([1-Hex-3-H-IM][Pic]) (7a).

1

Yellow liquid, 97% yield; H NMR (360 MHz, [D₆] DMSO)

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀ

d = 0.84 (t, 3H, –CH₃), 1.25 (m, 6H, –CH₂–CH₂–CH₂–), 1.77

(p, 2H, –CH₂–CH₂–), 4.13 (t, 3H, N–CH₂–), 7.55 (s, 1H, C5-H),

C2–CH₃), 3.96 (t, 2H, N–CH₂–), 8.33 (s, 1H, C5-H), 8.59

(s, 2H, picrate); ¹³C NMR (90 MHz, [D₆] DMSO) d = 12.51

(–CH₃), 13.73 (C2–CH₃), 21.57 (–CH₂–CH₃), 27.67 (–CH₂–),

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀ

7.68 (s, 1H, C4-H), 8.50 (s, 2H, picrate), 8.86 (s, 1H, C2-H);

¹³C NMR (90 MHz, [D₆] DMSO) d = 13.74 (–CH₃), 21.81

(–CH₂–CH₃), 25.17 (–CH₂–), 29.48 (–CH₂–), 30.45 (–CH₂–),

ꢀ ꢀ_ꢀ

29.12 (–CH₂–), 46.32 (N–CH₂–), 122.01 (C5), 124.32 (picrate),

ꢀ ꢀ_ꢀ

125.18 (picrate), 141.76 (picrate), 144.79 (C4), 145.19 (C2),

160.56 (picrate).

ꢀ ꢀ_ꢀ

47.98 (N–CH₂–), 120.30, 121.40 (C4/C5), 124.15 (picrate),

ꢀ ꢀ_ꢀ

125.10 (picrate), 135.45 (C2), 141.76 (picrate), 159.84 (picrate).

1-Hexyl-4-nitroimidazolium Picrate ([1-Hex-4-NO₂-3-H-

1

IM][Pic]) (15a). Yellow viscous liquid, 94% yield; H NMR

1-Methyl-2-nitroimidazolium Picrate ([1-Me-2-NO₂-3-H-

IM][Pic]) (8a). Yellow slowly crystallizing viscous liquid;

¹H NMR (360 MHz, [D₆] DMSO) d = 3.99 (s, 3H, N-CH₃),

(360 MHz, [D₆] DMSO) d = 0.84 (t, 3H, –CH₃), 1.25 (m, 6H,

–CH₂–CH₂–CH₂–), 1.76 (p, 2H, –CH₂–), 4.05 (t, 2H,

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

c

718 New J. Chem., 2012, 36, 702–722

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N–CH₂–), 7.88 (d, 1H, C5-H), 8.44 (s, 1H, C2-H), 8.59 (s, 2H,

picrate); ¹³C NMR (90 MHz, [D₆] DMSO) d = 13.50 (–CH₃),

21.51 (–CH₂–CH₃), 24.93, 29.24, 30.22 (–CH₂–), 47.75

N–CH₂–), 7.57 (s, 1H, C5-H), 7.66 (s, 1H, C4-H); ¹³C NMR

ꢀ ꢀ_ꢀ

(90 MHz, [D₆] DMSO) d = 10.16 (–CH₃), 13.71 (C2–CH₃),

21.55 (–CH₂–CH₃), 27.73 (–CH₂–), 28.63 (–CH₂–), 46.67

ꢀ ꢀ_ꢀ

(N–CH₃–), 119.14 (C5), 124.87 (picrate), 125.18 (picrate),

(N–CH₂–), 117.94, 121.78 (C4/C5), 143.79 (C2).

ꢀ ꢀ_ꢀ

135.18 (C2), 141.53 (picrate), 144.20 (C4), 160.49 (picrate).

1-Hexylimidazolium Nitrate ([1-Hex-3-H-IM][NO₃]) (7b).

White solid, 98% yield; ¹H NMR (360 MHz, [D₆] DMSO)

d = 0.84 (t, 3H, –CH₃), 1.25 (m, 6H, –CH₂–CH₂–CH₂–), 1.79

1-Methyl-2,4-dinitroimidazolium Picrate ([1-Me-2,4-diNO₂-

3-H-IM][Pic]) (16a). This reaction failed. Yellow co-crystals of

1-methyl-2,4-dinitroimidazole and picric acid were isolated,

1-Me-2,4-diNO₂-IMꢁHPic.

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

(p, 2H, –CH₂–CH₂–), 4.18 (t, 3H, N–CH₂–), 7.70 (s, 1H, C5-H),

7.81 (s, 1H, C4-H), 9.17 (s, 1H, C2-H); ¹³C NMR (90 MHz,

ꢀ ꢀ_ꢀ

[D₆] DMSO) d = 13.76 (–CH₃), 21.82 (–CH₂–CH₃), 25.13

(–CH₂–), 29.32 (–CH₂–), 30.46 (–CH₂–), 48.45 (N–CH₂–),

ꢀ ꢀ_ꢀ

1-Methyl-4,5-dinitroimidazolium Picrate ([1-Me-4,5-diNO₂-

3-H-IM][Pic]) (17a). This reaction failed. No product was

observed, only a mixture of the reactants.

ꢀ ꢀ_ꢀ

119.89, 121.93 (C4/C5), 135.22 (C2).

1-Methyl-2-nitroimidazolium Nitrate ([1-Me-2-NO₂-3-H-

1

IM][NO₃]) (8b). White solid, 82% yield; H NMR (360 MHz,

1-Methyl-4,5-dicyanoimidazolium Picrate ([1-Me-4,5-diCN-

3-H-IM][Pic]) (18a). This reaction failed. Yellow co-crystals of

1-methyl-4,5-dicyanoimidazole and picric acid were isolated,

1-Me-4,5-diCN-IMꢁ1.5 HPic.

[D₆] DMSO) d = 4.00 (s, 3H, N–CH₃), 7.17 (s, 1H, C5-H), 7.65

(s, 1H, C4-H), 11.49 (b, N-H); ¹³C NMR (90 MHz, [D₆] DMSO)

d = 37.07 (N–CH₃), 127.49, 128.51 (C4/5), 141.31 (C2).

1-Methylimidazolium Nitrate ([1-Me-3-H-IM][NO₃]) (1b).

White solid, 99% yield; ¹H NMR (360 MHz, [D₆] DMSO)

d = 3.86 (s, 3H, N-CH₃), 7.66 (s, 1H, C5-H), 7.69 (s, 1H, C4-H),

9.05 (s, 1H, C2-H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 35.40

(N–CH₃), 119.66, 123.09 (C4/5), 135.86 (C2).

1,2-Dimethyl-5-nitroimidazolium Nitrate ([1,2-diMe-5-NO₂-

3-H-IM][NO₃]) (9b). White solid, 84% yield; ¹H NMR (360 MHz,

[D₆] DMSO) d = 2.52 (s, 3H, C2-CH₃), 3.87 (s, 3H, N–CH₃), 8.36

(s, 1H, C4-H), 12.1 (b, N–H); ¹³C NMR (90 MHz, [D₆] DMSO)

d = 12.81 (C2-CH₃), 33.62 (N–CH₃), 122.87 (C4), 138.55 (C2),

149.82 (C5).

1,2-Dimethylimidazolium Nitrate ([1,2-diMe-3-H-IM][NO₃])

(2b). White crystals, 99% yield; ¹H NMR (360 MHz, [D₆]

DMSO) d = 2.55 (s, 3H, C2-CH₃), 3.73 (s, 3H, N–CH₃), 7.52

(s, 1H, C5-H), 7.57 (s, 1H, C4-H); ¹³C NMR (90 MHz, [D₆]

DMSO) d = 10.10 (C2-CH₃), 33.83 (N–CH₃), 117.52, 122.91

(C4/5), 144.41 (C2).

1-Ethyl-2-nitroimidazolium

Nitrate

([1-Et-2-NO₂-3-H-

IM][NO₃]) (10b). Colorless liquid, 85% yield; ¹H NMR

(360 MHz, [D₆] DMSO) d = 1.38 (t, 3H, –CH₃), 4.39

(q, 2H, N–CH₂–), 7.17 (d, 1H, C5-H), 7.70 (d, 1H, C4-H);

¹³C NMR (90 MHz, [D₆] DMSO) d = 15.57 (–CH₃), 44.72

(N–CH₂), 127.08, 127.88 (C4/5), 149.16 (C2).

1-Propylimidazolium Nitrate ([1-Pr-3-H-IM][NO₃]) (3b).

1

White crystals, 96% yield; H NMR (360 MHz, [D₆] DMSO)

1-Ethyl-4-nitroimidazolium

Nitrate

([1-Et-4-NO₂-3-H-

IM][NO₃]) (11b). Colorless liquid, 89% yield; ¹H NMR

(360 MHz, [D₆] DMSO) d = 1.39 (t, 3H, –CH₃), 4.08

(q, 2H, N–CH₂–), 7.89 (s, 1H, C5-H), 8.46 (s, 1H, C2-H);

¹³C NMR (90 MHz, [D₆] DMSO) d = 15.70 (–CH₃), 42.50

(N–CH₂), 121.20 (C5), 136.95 (C2), 145.86 (C4).

d = 0.84 (t, 3H, –CH₃), 1.81 (m, 2H, –CH₂–CH₃), 4.15 (t, 3H,

N–CH₂–), 7.70 (s, 1H, C5-H), 7.90 (s, 1H, C4-H), 9.15 (s, 1H,

ꢀ ꢀ_ꢀ

C2-H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 10.26 (–CH₃),

22.72 (–CH₂–CH₃), 49.86 (N–CH₂–), 119.80, 121.84 (C4/C5),

ꢀ ꢀ_ꢀ

135.13 (C2).

1-Butylimidazolium Nitrate ([1-Bu-3-H-IM][NO₃]) (4b). Color-

less liquid, 99% yield; ¹H NMR (360 MHz, [D₆] DMSO)

d = 0.90 (t, 3H, –CH₃), 1.25 (sextet, 2H, –CH₂–), 1.80 (p, 2H,

–CH₂–), 4.21 (t, 2H, N–CH₂–), 7.72 (s, 1H, C5-H), 7.82 (s, 1H,

C4-H), 9.19 (s, 1H, C2-H); ¹³C NMR (90 MHz, [D₆] DMSO)

d = 13.25, 18.84, 31.45 (alkyl), 48.23 (N–CH₂–), 120.08,

122.00 (C4/5), 135.35 (C2).

1-Isopropyl-4-nitroimidazolium Nitrate ([1-i-Pr-4-NO₂-3-H-

IM][NO₃]) (12b). White solid, 89% yield; ¹H NMR (360 MHz,

[D₆] DMSO) d = 1.44 (d, 6H, –CH₃), 4.52 (m, 1H, N–CH–),

ꢀ ꢀ_ꢀ

ꢀ ꢀ

7.95 (s, 1H, C5-H), 8.52 (s, 1H, C2-H); ¹³C NMR (90 MHz,

[D₆] DMSO) d = 22.71 (–CH₃), 50.33 (N–CH–), 119.64 (C5),

135.75 (C4), 146.93 (C2).

ꢀ ꢀ_ꢀ

ꢀ ꢀ

1-Butyl-2-methyl-4-nitroimidazolium Nitrate ([1-Bu-2-Me-4-

1

NO₂-3-H-IM][NO₃]) (13b). White solid, 93% yield; H NMR

1-Butyl-2-methylimidazolium Nitrate ([1-Bu-2-Me-3-H-

1

IM][NO₃]) (5b). White solid, 97% yield; H NMR (360 MHz,

(360 MHz, [D₆] DMSO) d = 0.90 (t, 3H, –CH₃), 1.28 (m, 2H,

–CH₂–CH₃), 1.69 (p, 2H, –CH₂–), 2.35 (s, 3H, C2-CH₃), 3.97

ꢀ ꢀ_ꢀ

[D₆] DMSO) d = 0.90 (t, 3H, –CH₃), 1.28 (m, 2H, –CH₂–CH₃),

1.71 (p, 2H, –CH₂–CH₂–), 2.58 (s, 3H, C2–CH₃), 4.07 (t, 3H,

ꢀ ꢀ_ꢀ

ꢀ

ꢀ ꢀ_ꢀ

ꢀ

(t, 2H, N–CH₂–), 8.33 (s, 1H, C5-H); ¹³C NMR (90 MHz, [D₆]

ꢀ ꢀ_ꢀ

N–CH₂–), 7.57 (s, 1H, C5-H), 7.66 (s, 1H, C4-H); ¹³C NMR

DMSO) d = 12.49 (–CH₃), 13.32 (C2–CH₃), 18.97

(–CH₂–CH₃), 31.43 (–CH₂–), 46.07 (N–CH₂–), 122.07 (C5),

ꢀ ꢀ_ꢀ

(90 MHz, [D₆] DMSO) d = 10.15 (–CH₃), 13.33 (C2–CH₃),

18.91 (–CH₂–CH₃), 30.95 (–CH₂–), 46.47 (N–CH₂–), 117.93,

ꢀ ꢀ_ꢀ

144.71 (C4), 145.11 (C2).

ꢀ ꢀ_ꢀ

121.99 (C4/C5), 143.79 (C2).

1-Pentyl-2-methyl-4-nitroimidazolium Nitrate ([1-Pent-2-

Me-4-NO₂-3-H-IM][NO₃]) (14b). Colorless viscous liquid, 97%

yield; ¹H NMR (360 MHz, [D₆] DMSO) d = 0.86 (t, 3H,

–CH₃), 1.30 (m, 4H, –CH₂–CH₂–), 1.71 (p, 2H, –CH₂–), 2.35

1-Pentyl-2-methylimidazolium Nitrate ([1-Pent-2-Me-3-H-

1

IM][NO₃]) (6b). White solid, 96% yield; H NMR (360 MHz,

[D₆] DMSO) d = 0.87 (t, 3H, –CH₃), 1.30 (m, 4H, –CH₂–CH₂–),

1.73 (p, 2H, –CH₂–CH₂–), 2.58 (s, 3H, C2–CH₃), 4.07 (t, 3H,

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀ

(s, 3H, C2–CH₃), 3.96 (t, 2H, N–CH₂–), 8.34 (s, 1H, C5-H);

ꢀ ꢀ_ꢀ

c

This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientiﬁque 2012

New J. Chem., 2012, 36, 702–722 719

View Article Online

¹³C NMR (90 MHz, [D₆] DMSO) d = 12.28 (–CH₃), 13.50

(C2–CH₃), 21.33 (–CH₂–CH₃), 27.63 (–CH₂–), 28.88 (–CH₂–),

picrate salts were extracted from the aqueous phase using

chloroform. Following this, the salts were dried under high

vacuum at 60 1C for an additional 5 h. The possible presence

of chloride anions was tested using aqueous solution of silver

nitrate.

ꢀ ꢀ_ꢀ

46.07 (N–CH₂–), 121.79 (C5), 144.55 (C4), 145.10 (C2).

ꢀ ꢀ_ꢀ

1-Hexyl-4-nitroimidazolium Nitrate ([1-Hex-4-NO₂-3-H-

IM][NO₃]) (15b). Colorless viscous liquid, 95% yield;

¹H NMR (360 MHz, [D₆] DMSO) d = 0.85 (t, 3H, –CH₃),

1.25 (m, 6H, –CH₂–CH₂–CH₂–), 1.75 (p, 2H, –CH₂–), 4.03

1,3-Dimethylimidazolium Nitrate ([1,3-diMeIM][NO₃]) (19b).

White solid, 65% yield; ¹H NMR (360 MHz, [D₆] DMSO)

d = 3.83 (s, 6H, N-CH₃), 7.69 (d, 2H, C4/C5-H), 9.03 (s, 1H,

C2-H); ¹³C NMR (90 MHz, [D₆] DMSO) d = 35.59 (N–CH₃),

123.33 (C4/C5), 136.99 (C2).

ꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀꢀ ꢀ_ꢀꢀ ꢀ_ꢀ

ꢀ ꢀ_ꢀ

(t, 2H, N–CH₂–), 7.85 (d, 1H, C5-H), 8.44 (s, 1H, C2-H);

¹³C NMR (90 MHz, [D₆] DMSO) d = 13.76 (–CH₃), 21.82

(–CH₂–CH₃), 25.26, 29.79, 30.47 (–CH₂–), 47.26 (N–CH₃–),

ꢀ ꢀ_ꢀ

121.44, (C5), 125.10 (C2), 137.29 (C4).

1,2,3-Trimethylimidazolium Nitrate ([1,2,3-triMeIM][NO₃])

(20b). White crystalline solid, 70% yield; ¹H NMR (500 MHz,

[D₄] MeOH) d = 2.58 (s, 3H, C2–CH₃), 3.85 (s, 6H, N–CH₃),

1-Methyl-2,4-dinitroimidazolium Nitrate ([1-Me-2,4-diNO₂-

3-H-IM][NO₃]) (16b). No product was observed, only a mixture

of reactants.

ꢀ

7.43 (s, 2H, C4/C5-H); ¹³C NMR (125 MHz [D₄] MeOH) d ^ꢀ=

10.02 (C2–CH₃), 35.33 (N–CH₃), 123.26 (C4/C5), 146.42 (C2).

ꢀ ꢀ_ꢀ

1-Methyl-4,5-dinitroimidazolium Nitrate ([1-Me-4,5-diNO₂-

3-H-IM][NO₃]) (17b). No product was observed, only a mixture

of reactants.

1-Butyl-3-methylimidazolium Nitrate ([1-Bu-3-MeIM][NO₃])

1

(21b). Colorless liquid, 69% yield; H NMR (500 MHz, [D₆]

DMSO) d = 0.91 (t, 3H, –CH₃), 1.33 (m, 2H, –CH₂–CH₃), 1.87

(m, 2H, –CH₂–), 4.05 (s, 3H, N–CH₃), 4.29 (t, 2H, N–CH₂–),

ꢀ ꢀ_ꢀ

1-Methyl-4,5-dicyanoimidazolium Nitrate ([1-Me-4,5-diCN-

3-H-IM][NO₃]) (18b). No product was observed, only a mixture

of reactants.

ꢀ ꢀ_ꢀ

7.59 (s, 1H, C5-H), 7.77 (s, 1H, C4-H), 9.06 (s, 1H, C2-H); ¹³

C

NMR (125 MHz [D₆] DMSO) d = 13.45 (–CH₃), 20.10

(–CH₂–CH₃), 32.59 (–CH₂–CH₂–), 37.00 (N–CH₃), 49.93

ꢀ ꢀ_ꢀ

General protocol for the synthesis of alkylated imidazolium salt

derivatives

(N–CH₂–), 122.23, 123.87 (C4/C5), 138.67 (C2).

1,3-Dimethylimidazolium Picrate ([1,3-diMeIM][Pic]) (19a).

1

Yellow solid, 78% yield; H NMR (360 MHz, [D₆] DMSO)

1,3-Dimethylimidazolium chloride (1,3-diMeIM][Cl]) (19c)

and 1,2,3-trimethylimidazolium chloride ([1,2,3-triMeIM][Cl])

(20c) were prepared by purging chloromethane gas through an

EtOH solution of either 1-methylimidazole or 1,2-dimethy-

limidazole at 0 1C similar to a protocol presented in the

literature.⁵⁴1-Butyl-3-methylimidazolium chloride (1-Bu-3-

MeIM][Cl]) (21c) was prepared using the standard literature

protocol by reacting 1-methylimidazole with a 30% excess

of chlorobutane at 70 1C and later removal of unreacted

substrates under vacuum.⁵⁵

d = 3.82 (s, 6H, N-CH₃), 7.68 (d, 2H, C4/C5-H), 8.60 (s, 2H,

picrate), 9.02 (s, 1H, C2-H); ¹³C NMR (90 MHz, [D₆] DMSO)

d = 35.58 (N-CH₃), 123.32 (C4/C5), 124.07 (picrate), 125.10

(picrate), 136.95 (C2), 141.74 (picrate), 160.73 (picrate).

1,2,3-Trimethylimidazolium Picrate ([1,2,3-triMeIM][Pic]) (20a).

Yellow crystalline solid, 85% yield; ¹H NMR (500 MHz, [D₄]

MeOH) d = 2.56 (s, 3H, C2–CH₃), 3.75 (s, 6H, N–CH₃), 7.59

ꢀ ꢀ_ꢀ

(s, 2H, C4/C5-H), 8.59 (s, 2H, picrate); ¹³C NMR (125 MHz

1,3-Dimethylimidazolium nitrate ([1,3-diMeIM][NO₃]) (19b),

1,2,3-trimethyl imidazolium nitrate ([1,2,3-triMeIM][NO₃]) (20b),

and 1-butyl-3-methylimidazolium nitrate ([1-Bu-3-MeIM][NO₃])

(21b) were prepared from their chloride precursors by the anion

exchange reaction of the chloride salt with silver nitrate

(1 : 1 molar ratio) in aqueous solution utilizing a previously

reported protocol.⁵⁶Equimolar amounts of silver nitrate,

dissolved in water, and imidazolium chloride salt, dissolved in

water, were mixed together by dropwise addition of silver nitrate

solution into a stirred solution of chloride salt. After the solution

was stirred in the dark for 24 h, the silver chloride was removed

by ﬁltration, and the ﬁltrate checked for the presence of silver

ions using HCl. The solvent was then evaporated at 80 1C under

vacuum. A ﬁnal puriﬁcation step involved dissolution of the

obtained nitrate salt in dry ethanol in order to separate it from

any solid, inorganic by-product. The salts were then dried under

high vacuum at 60 1C for an additional 5 h.

[D₄] MeOH) d = 9.47 (C2–CH₃), 34.54 (N–CH₃), 121.81

(C4/C5), 124.03 (picrate), 125.09 (picrate), 141.70 (picrate),

ꢀ ꢀ_ꢀ

144.57 (C2), 160.70 (picrate).

1-Butyl-3-methylimidazolium Picrate ([1-Bu-3-MeIM][Pic])

1

(21a). Yellow viscous liquid, 84% yield; H NMR (360 MHz,

[D₆] DMSO) d = 0.93 (t, 3H, –CH₃), 1.36 (m, 2H, –CH₂–CH₃),

1.81 (m, 2H, –CH₂–), 4.00 (s, 3H, N–CH₃), 4.33 (t, 2H,

ꢀ ꢀ_ꢀ

N–CH₂–), 7.50 (s, 1H, C5-H), 7.71 (s, 1H, C4-H), 8.57 (s, 2H,

picrate), 9.02 (s, 1H, C2-H); ¹³C NMR (125 MHz [D₆] DMSO)

ꢀ ꢀ_ꢀ

d = 13.22 (–CH₃), 20.90 (–CH₂–CH₃), 33.02 (–CH₂–CH₂–),

37.28 (N–CH₃), 49.15 (N–CH₂–), 122.21, 123.65 (C4/C5),

ꢀ ꢀ_ꢀ

124.15 (picrate), 125.11 (picrate), 138.55 (C2), 141.38 (picrate),

160.76 (picrate).

Conclusion

1,3-Dimethylimidazolium, 1,2,3-trimethylimidazolium, and

1-butyl-3-methylimidazolium picrate salts ware prepared from

their chloride precursors by an anion exchange reaction of the

chloride salt with sodium picrate (1 : 1 molar ratio) in a

mixture of EtOH/H₂O. Solvent was evaporated using high

vacuum, the samples were re-dissolved in water, and the

The focus of this work was on exploring the possibility of

forming simple energetic ionic liquids via the straightforward

protonation of neutral imidazole derivatives with nitric or picric

acids. Moreover, the inﬂuence of substituent type (nitro-,

cyano-, alkyl-substituents) and position on the properties of

the resulting protonated imidazolium salts was investigated.

c

720 New J. Chem., 2012, 36, 702–722

This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientiﬁque 2012

View Article Online

This easy acid–base chemistry approach resulted in the formation

of a large family of protonated imidazolium picrate and

nitrate salts but was not without limitations. It was found

that functionalization of imidazoles with electron-withdrawing

substituents such as nitro or cyano resulted in a reduction in

nucleophilicity of the heterocycle. Mononitro-substituted

protonated imidazolium salts could be isolated, but further

ring substitution with electron-withdrawing groups resulted in

the imidazoles failing to undergo protonation even with a

strong acid such as HCl. The only products that were obtained

were co-crystals of 1-Me-4,5-diCN-IM and 1-Me-2,4-diNO₂-IM

with picric acid.

revealed that the alkylated salts were much less explosive than

the protonated analogs, and that among all the analyzed salts,

the most energetic materials investigated were protonated

[1-Me-3-H-IM][NO₃] (1b) and [1,2-diMe-3-H-IM][Pic] (2a) salts.

Notes and references

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The melting points of the protonated salts were much lower

than expected, and from the group of 30 analyzed compounds,

more than half were found to melt below 100 1C. General

trends in the melting points of the analyzed salts show that

melting transitions of picrate-based salts were always higher

than that of their nitrate analogs. Additionally, analysis of the

melting point variations as a function of the N-alkyl chain

length shows that the longer the alkyl chain, the lower the

melting point of the resulting salt until the alkyl chain extends

over four carbons, at which point an increase of the melting

point was observed. It was also found that 1,2-dialkyl-substituted

salts generally melted at higher temperatures than those not

possessing a methyl substituent on the C2 carbon in the ring.

Comparing the inﬂuence of the position of the –NO₂group on

the melting point of the salts using two isomeric compounds, it

was found that the melting point does not change greatly. On

the other hand, this structural change signiﬁcantly inﬂuenced

the thermal stability of the two isomers, and by changing the

position of the –NO₂group from C2 to C4 resulted in an

increase in stability for both nitrates and picrates.

Analysis of the thermal stabilities of the salts revealed that

all [NO₃]^ꢀ-based salts are less stable than their [Pic]^ꢀanalogs.

Typically, the diﬀerence between the thermal stabilities of

nitrate and picrate salts was ca. 50 1C. Moreover, it was found

that C2-Me-substituted picrate salts tended to be more stable

than those which are unsubstituted, unfortunately at the same

time exhibited higher melting points. It was found that the

decomposition temperatures for the nitrate and picrate salts

substituted with –NO₂groups seem to be ca. 20–50 1C lower

than those that were not nitro-substituted. It was concluded

that the thermal stability range for each group of salts is more

dependent on the structure of the anion than on the presence

of the substituents on C2 carbon position or the alkyl chain

length of the N-alkyl substituents.

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Comparing the simple protonated imidazolium nitrate and

picrate salts with their methylated analogs, it was found that

the protonated ionic liquids did not diﬀer substantially in their

melting points from the methylated analogs. However, the

thermal stabilities of protonated imidazolium salts were much

lower than their alkylated derivatives. Nitrate salts with

alkylated cations tended to be more thermally stable than

the corresponding picrate salts, but for protonated cations, the

picrate salts tended to be approximately 70–80 1C more stable

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Article Doi

Synthesis, limitations, and thermal properties of energetically- substituted, protonated imidazolium picrate and nitrate salts and further comparison with their methylated analogs

DOI: 10.1039/c1nj20677j

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Article abstract of DOI:10.1039/c1nj20677j

Full text of DOI:10.1039/c1nj20677j

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