Nitrile-functionalized pyridinium, pyrrolidinium, and piperidinium ionic liquids

Article abstract of DOI:10.1021/jp2027675

Two series of 1-alkylpyridinium and N-alkyl-N-methylpiperidinium ionic liquids functionalized with a nitrile group at the end of the alkyl chain have been synthesized. Structural modifications include a change of the alkyl spacer length between the nitrile group and the heterocycle of the cationic core, as well as adding methyl or ethyl substituents on different positions of the pyridinium ring. The anions are the bromide and the bis(trifluoromethylsulfonyl) imide ion. All the bis(trifluoromethylsulfonyl)imide salts as well as the bromide salts with a long alkyl spacer were obtained as viscous liquids at room temperature, but some turned out to be supercooled liquids. In addition, pyrrolidinium and piperidinium ionic liquids with two nitrile functions attached to the heterocyclic core have been prepared. The crystal structures of seven pyridinium bis(trifluoromethylsulfonyl)imide salts are reported. Quantum chemical calculations have been performed on model cations and ion pairs with the bis(trifluoromethylsulfonyl)imide anion. A continuum model has been used to take solvation effects into account. These calculations show that the natural partial charge on the nitrogen atom of the nitrile group becomes more negative when the length of the alkyl spacer between the nitrile functional group and the heterocyclic core of the cation is increased. Methyl or methoxy substituents on the pyridinium ring slightly increase the negative charge on the nitrile nitrogen atom due to their electron-donating abilities. The position of the substituent (ortho, meta, or para) has only a very minor effect on the charge of the nitrogen atom. The ¹⁵N NMR spectra of the bis(trifluoromethylsulfonyl)imide ionic liquids were recorded with the nitrogen-15 nucleus at its natural abundance. The chemical shift of the ¹⁵N nucleus of the nitrile nitrogen atom could be correlated with the calculated negative partial charge on the nitrogen atom. ? 2011 American Chemical Society.

Full text of DOI:10.1021/jp2027675

ARTICLE
pubs.acs.org/JPCB
Nitrile-Functionalized Pyridinium, Pyrrolidinium, and Piperidinium
Ionic Liquids
Kallidanthiyil Chellappan Lethesh,† Kristof Van Hecke,† Luc Van Meervelt,† Peter Nockemann,‡
Barbara Kirchner,§ Stefan Zahn,§ Tatjana N. Parac-Vogt,† Wim Dehaen,† and Koen Binnemans*,†
†Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, P.O. Box 2404, B-3001 Heverlee, Belgium
‡The QUILL Research Centre, School of Chemistry and Chemical Engineering, David Keir Building, Stranmillis Road,
Queen’s University Belfast, Belfast BT9 5AG, United Kingdom
§Wilhelm-Ostwald-Institut f€ur Physikalische und Theoretische Chemie, Universit€at Leipzig, Linnꢀestrasse 2, D-04103 Leipzig, Germany
S Supporting Information
b
ABSTRACT: Two series of 1-alkylpyridinium and N-alkyl-N-
methylpiperidinium ionic liquids functionalized with a nitrile
group at the end of the alkyl chain have been synthesized.
Structural modifications include a change of the alkyl spacer
length between the nitrile group and the heterocycle of the
cationic core, as well as adding methyl or ethyl substituents on
different positions of the pyridinium ring. The anions are the
bromide and the bis(trifluoromethylsulfonyl)imide ion. All the
bis(trifluoromethylsulfonyl)imide salts as well as the bromide
salts with a long alkyl spacer were obtained as viscous liquids at
room temperature, but some turned out to be supercooled liquids. In addition, pyrrolidinium and piperidinium ionic liquids with
two nitrile functions attached to the heterocyclic core have been prepared. The crystal structures of seven pyridinium bis-
(trifluoromethylsulfonyl)imide salts are reported. Quantum chemical calculations have been performed on model cations and ion
pairs with the bis(trifluoromethylsulfonyl)imide anion. A continuum model has been used to take solvation effects into account.
These calculations show that the natural partial charge on the nitrogen atom of the nitrile group becomes more negative when the
length of the alkyl spacer between the nitrile functional group and the heterocyclic core of the cation is increased. Methyl or methoxy
substituents on the pyridinium ring slightly increase the negative charge on the nitrile nitrogen atom due to their electron-donating
abilities. The position of the substituent (ortho, meta, or para) has only a very minor effect on the charge of the nitrogen atom. The
15N NMR spectra of the bis(trifluoromethylsulfonyl)imide ionic liquids were recorded with the nitrogen-15 nucleus at its natural
abundance. The chemical shift of the 15N nucleus of the nitrile nitrogen atom could be correlated with the calculated negative partial
charge on the nitrogen atom.
’ INTRODUCTION
dicyanamide, tetrafluoroborate, hexafluorophosphate, and the very
important bis(trifluoromethylsulfonyl)imide (= bistriflimide) ion.
However, a disadvantage of ionic liquids with weakly coordinat-
ing anions such as the bis(trifluoromethylsulfonyl)imide is their
limited solubilizing ability for inorganic salts. This is a draw-
back for applications that require high concentrations of dis-
solved metal salts, e.g., the electrodeposition of reactive metals.
A solution to this problem is to use ionic liquids with strongly co-
ordinating anions, for instance chloride ions, but these ionic
liquids have typically high melting points or there are stability
issues as in the case of ionic liquids with β-diketonate anions.24
Another possibility is to use functionalized ionic liquids, i.e., ionic
liquids with functional, coordinating groups appended to the
ionic liquid cation. A commonly used name for this type of ionic
Ionic liquids are solvents that are composed entirely of ions.1ꢀ3
They can possess properties that make them of interest for poten-
tial technological applications.4 For instance, many ionic liquids
have an extremely low vapor pressure, a very large liquidus range, a
very wide electrochemical window and an intrinsic ionic conduc-
tivity. Depending on their composition, they can be miscible or
immiscible withorganic solvents or water. Ionic liquids can be used
as solvents for chemical reactions, including catalytic reactions.5ꢀ8
They can act as solvents for the synthesis of unusual inorganic
compounds.9ꢀ13 Several ionic liquid exhibit liquidꢀcrystalline
behavior.14 They find use in electrochemical applications,15 for
example as electrolytes in batteries,16,17 and in photovoltaic
devices,18ꢀ20 but also as a medium for electrodeposition21ꢀ23 of
metals. Many ionic liquids contain a 1-alkyl-3-methylimidazolium,
1-alkylpyridinium, N-alkyl-N-methylpyrrolidinium, quaternary ammo-
nium, or a quaternary phosphonium group as the cation. Typical
anions are chloride, bromide, acetate, diethylphosphate, triflate,
Received: March 24, 2011
Revised:
May 24, 2011
Published: May 24, 2011
r
2011 American Chemical Society
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ARTICLE
Figure 1. Overview of the nitrile-functionalized cations.
liquid is “task-specific ionic liquids”,25,26 but we prefer the term
“functionalized ionic liquids”. An example of such a functiona-
lized ionic liquid is an imidazolium salt incorporating a thiourea
moiety, which has been used for the extraction of mercury(II)
and cadmium(II) from an aqueous phase.27 Functionalized ionic
liquids with appended tertiary phosphine groups have been used
to immobilize rhodium(I) organometallic catalysts in [C4mim]-
[PF6].28,29 During the last four years, we have developed ionic
liquids functionalized with a carboxylic acid group.30ꢀ33 These
ionic liquids are remarkable in their ability to dissolve large
amounts of metal oxides or hydroxides, but they have unfortu-
nately a poor stability against electrochemical reduction, so that
they are of limited interest as solvents for electrodeposition of
metals. Functionalized ionic liquids with a higher electrochemical
stability are those containing a nitrile group. Nitrile-functionalized
ionic liquids were first introduced by Dyson and co-workers.34
The nitrile function is in general appended to the cationic core via
an alkyl spacer,35ꢀ40 although Hardacre et al. prepared ionic
liquids with the nitrile group directly bonded to the heterocyclic
ring of the cation.41ꢀ43 Nitrile-functionalized methimazole-
based ionic liquids have been used for the extraction of Agþ
ions.44 Ionic liquids with nitrile-functionalized anions have also
been described.45,46 The dicyanamide anion is a well-known
anion for obtaining low-viscous ionic liquids.47 Recently we have
reported on nitrile-functionalized pyrrolidinium ionic liquids,
and we have found that the coordination chemistry of cobalt(II)
in these ionic liquids strongly depends on the length of the alkyl
spacer between the nitrile group and the nitrogen atom of the
pyrrolidinium cation.48 An important parameter determining the
coordinating behavior of the nitrile function is the value of the
negative partial charge on the nitrile nitrogen atom.
In order to investigate the influence of the structure of the ionic
liquid cation on the charge distribution of the nitrile group and on the
thermal properties, we synthesized a series of 50 nitrile-functionalized
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Table 1. Melting Points (Tm) and Decomposition Tem-
peratures (Td) of Ionic Liquids
Table 2. Physicochemical Properties of Room-Temperature
Nitrile-Functionalized Ionic Liquids
compounda
Tm (°C)
Td (°C)
Td
(°C)b
viscosity
(cP)c
F
water
content (ppm)e
d
compounda
(g cmꢀ3
)
1-Br
160
145
83
210
230
255
205
220
215
220
260
250
260
220
205
260
200
220
280
205
320
390
400
380
415
415
2-Br
4-Br
245
240
270
265
255
250
250
360
415
410
415
390
405
410
420
430
420
440
430
420
425
430
425
420
410
425
---f
---f
---f
---f
---f
---f
---f
n.d.g
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
60
3-Br
5-Br
6-Br
193
183
194
160
124
139
142
120
173
152
182
192
137
142
199
44
11-Br
n.d.
7-Br
14-Br
n.d.
8-Br
15-Br
n.d.
9-Br
16-Br
n.d.
10-Br
12-Br
13-Br
17-Br
18-Br
19-Br
21-Br
22-Br
23-Br
24-Br
25-Br
1-Tf2N
8-Tf2N
17-Tf2N
24-Tf2N
25-Tf2N
20-Br
n.d.
2-Tf2N
3-Tf2N
4-Tf2N
5-Tf2N
6-Tf2N
7-Tf2N
9-Tf2N
10-Tf2N
11-Tf2N
12-Tf2N
13-Tf2N
14-Tf2N
15-Tf2N
16-Tf2N
19-Tf2N
20-Tf2N
21-Tf2N
22-Tf2N
23-Tf2N
326
363
150
320
280
164
578
325
168
308
165
180
167
197
297
157
567
481
408
1.50
1.54
1.45
1.33
1.56
1.61
1.53
1.49
1.43
1.50
1.43
1.37
1.40
1.45
1.45
1.48
1.55
1.53
1.46
55
73
45
30
40
40
50
29
70
90
51
40
48
80
58
61
62
30
a Compounds not listed in this table are room-temperature ionic liquids.
Their physicochemical properties are given in Table 2.
58
83
67
pyridinium, pyrrolidinium, and piperidinium ionic liquids with
bromide and bis(trifluoromethylsulfonyl)imide anions. Pyrroli-
dinium and piperidinium ionic liquids with two nitrile functions
attached to the heterocyclic core have been prepared as well. It
should be noted that the piperidinium ring can be considered as
the saturated analogue of the pyridinium ring. An overview of the
cations considered in this study is given in Figure 1. The naming
of the actual ionic liquids is a combination of the cation number
and the bromide (Br) or bis(trifluoromethylsulfonyl)imide (Tf2N)
anion, for instance 1-Br or 1-Tf2N. The crystal structure of sev-
eral of the pyridinium ionic liquids has been determined in order
to determine the cationꢀanion interactions in these functiona-
lized ionic liquids. The charge distribution in the cation has been
determined by density functional theory (DFT) calculations and
was experimentally verified via the chemical shift of the nitrile
group in the 15N NMR spectra.
50
a The compounds not listed in this table are solid at room temperature.
b Onset of thermal decomposition. Determined by DSC. c The viscosities
have been measured at 19 °C. d Mass density, measured at 19 °C. e The
water content was measured by coulometric Karl Fischer titration. f The
compounds were obtained as very viscous liquids. g n.d. = not determined.
Karl Fischer Titrator, model DL39). The viscosity of the ionic
liquids was measured by the falling ball method (Gilmont Instru-
ments). Differential scanning calorimetry (DSC) measurements
were made on a Mettler-Toledo DSC822e module (scan rate of
10 °C minꢀ1 under helium flow). DSC was used to determine the
melting point and the onset temperature for thermal decom-
position. The organic chemicals were purchased from Acros or
from Sigma-Aldrich. Lithium bis(trifluoromethylsulfonyl)imide
was obtained from IoLiTec (Denzlingen, Germany). All chemicals
were used as received, without any additional purification step.
X-ray Crystallography. X-ray intensity data were collected at
100 K on a SMART 6000 diffractometer equipped with CCD
detector using CuꢀKR radiation (λ = 1.54178 Å). The images
were interpreted and integrated with the program SAINT from
Bruker.49 All seven structures were solved by direct methods and
refined by full-matrix least-squares on F2 using the SHELXTL
program package.50,51 Non-hydrogen atoms were anisotropically
refined, and the hydrogen atoms in the riding mode and isotropic
temperature factors were fixed at 1.2 times U(eq) of the parent
atoms (1.5 times for methyl groups). For the structure of com-
pound 11-Tf2N, the X-H distances could be left free to refine.
CCDC-824765-824771 contains the supplementary crystallographic
’ EXPERIMENTAL SECTION
General Techniques. Elemental analyses (carbon, hydrogen,
nitrogen) were conducted on a CE Instruments EA-1110 ele-
mental analyzer. 1H and 13C NMR spectra were recorded on a
Bruker Avance 300 spectrometer (operating at 300 MHz for 1H
and at 75.5 MHz for 13C). 15N NMR spectra were recorded on a
Bruker 600 MHz Avance II spectrometer at 60.83 MHz fre-
quency on a BBO 10 mm probe. Neat CH3NO2 was used as ex-
ternal standard for 15N chemical shift. The pulse program zgig was
used with a pulse length of 28.3 μs and a power level of ꢀ1.5 dB.
The water content of the ionic liquids was determined by a
coulometric Karl Fischer titrator (Mettler Toledo Coulometric
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Table 3. Crystallographic Data of Ionic Liquids 1-Tf2N, 3-Tf2N, 8-Tf2N, 9-Tf2N, 11-Tf2N, 17-Tf2N and 18-Tf2N
1-Tf2N
3-Tf2N
8-Tf2N
9-Tf2N
11-Tf2N
17-Tf2N
18-Tf2N
molecular formula
formula weight (g molꢀ1
C9H7F6N3O4S2 C11H11F6N3O4S2 C10H9F6N3O4S2 C11H11F6N3O4S2 C12H13F6N3O4S2 C11H11F6N3O4S2 C11H11F6N3O4S2
399.32 427.37 413.34 427.37 441.39 427.37 427.37
)
crystal dimensions (mm3) 0.4 ꢁ 0.3 ꢁ 0.3 0.4 ꢁ 0.3 ꢁ 0.25 0.25 ꢁ 0.25 ꢁ 0.2 0.4 ꢁ 0.3 ꢁ 0.25 0.4 ꢁ 0.35 ꢁ 0.2 0.3 ꢁ 0.15 ꢁ 0.1 0.3 ꢁ 0.1 ꢁ 0.05
crystal system
space group
a (Å)
triclinic
P1 (No. 2)
8.1408(1)
8.7005(1)
10.5820(2)
80.863(1)
76.270(1)
81.141(1)
713.55(2)
2
monoclinic
monoclinic
triclinic
P1 (No. 2)
8.2118(8)
8.6844(7)
12.621(1)
98.692(5)
104.289(6)
104.167(5)
824.0(1)
2
monoclinic
P21/n (No. 14)
11.5159(6)
12.2778(6)
12.5068(6)
monoclinic
P21/c (No. 14)
8.255(3)
triclinic
P1 (No. 2)
8.1340(6)
12.4922(9)
17.291(1)
72.707(4)
80.807(5)
80.637(4)
1643.6(2)
4
P21/n (No. 14) P21/n (No. 14)
11.2891(2)
12.2491(2)
12.4087(2)
12.687(4)
8.186(3)
15.537(6)
b (Å)
13.880(3)
c (Å)
14.205(4)
R (°)
β (°)
γ (°)
V (Å3)
97.027(1)
106.36(2)
96.726(2)
92.16(2)
1703.00(5)
4
1548(1)
4
1756.2(2)
4
1626.5(8)
4
Z
F
calc (gcmꢀ3
)
1.859
1.667
133.2
864
1.773
1.723
1.669
1.745
1.727
2θmax (°)
F(000)
140.1
143.0
143.0
142.6
133.2
133.2
400
832
432
896
864
864
measured reflections
unique reflections
14976
14505
2957
25000
2981
10750
17312
3352
15061
2849
26387
2608
3042
5697
obs. reflections (I > 2σ(I)) 2588
2847
2655
2610
3231
2676
4549
parameters refined
R1
218
235
227
237
253
236
471
0.0283
0.0750
0.0285
0.0752
1.089
4.338
0.0282
0.0721
0.0293
0.0730
1.051
3.678
0.0366
0.0838
0.0422
0.0872
1.046
0.0382
0.0300
0.0755
0.0311
0.0764
1.092
0.0291
0.0741
0.0310
0.0763
1.066
0.0527
wR2
0.0941
0.1228
R1 (all data)
wR2 (all data)
goodness of fit
0.0451
0.0675
0.0993
0.1328
1.076
1.037
μ (mmꢀ1
)
4.022
3.801
3.587
3.851
3.811
CCDC-entry
CCDC-824765 CCDC-824766
CCDC-824767
CCDC-824769
CCDC-824768
CCDC-824771
CCDC-824770
Figure 2. Packing in the crystal structure of compound 1-Tf2N.
data for this paper and can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2
1EZ, UK; fax: þ44ꢀ1223ꢀ336033; or deposit@ccdc.cam.ac.uk).
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Figure 3. Close contacts in the crystal structure of compound 1-Tf2N.
Figure 4. Close contacts in the crystal structure of compound 3-Tf2N.
Computational Details. The programs provided by the
Turbomole-suite have been employed.52 Geometry optimizations
were carried out using the BLYP-D density functional,53ꢀ55 with
the resolution of identity (RI).56ꢀ58 The BLYP-D functional has
proven to perform very well compared to MP2 for ionic liquid ion
pairs.59,60 For these systems dispersion forces play an important
role for equilibrium structure and interaction energy.61,62 The
TZVP basis set was applied for structure optimization.63 Further-
more, the convergency criterion of the iteration cycle was in-
creased to 10ꢀ8 Hartree for all calculations. Solvation was ap-
proximated by the continuum solvation model (COSMO) with
the permittivity set to 15.64,65 Within continuum models such as
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Figure 5. Packing in the crystal structure of compound 8-Tf2N.
Figure 6. Close contacts in the crystal structure of compound 8-Tf2N.
COSMO, it is not possible to consider microheterogeneity which
may be important for ionic liquids with longer side chains.66
However, COSMO was successfully applied in previous calcula-
tions to determine melting points of ionic liquids.67,68
atom of the heterocycle with ω-bromoalkanenitrile. The bis-
(trifluoromethylsulfonyl)imide salts were obtained from the cor-
responding bromide salts by a metathesis reaction with lithium
bis(trifluoromethylsulfonyl)imide in water. Because the bromide
ionic liquids are totally miscible with water and the bis(trifluoro-
methylsulfonyl)imide ionic liquids are immiscible, phase separa-
tion occurs during the metathesis reaction. The bis(trifluoro-
methylsulfonyl)imide ionic liquids were washed with plenty of
water until the AgNO3 test was negative for the washing water.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Compounds. The
bromide salts were prepared by quaternization of the nitrogen
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Figure 7. Close contacts in the crystal structure of compound 11-Tf2N.
Figure 8. Packing in the crystal structure of compound 9-Tf2N.
By drying of the ionic liquids in high vacuum (10ꢀ2 to 10ꢀ3 mbar)
for several hours at 75 °C, the water content could be reduced to
less than about 75 ppm (determined by coulometric Karl Fischer
(Table 1), and for the compounds that were liquid at room tem-
perature, the viscosity, density, water content, and the onset tem-
perature for thermal decomposition were determined (Table 2).
The melting points of the bromide salts strongly depend on the
alkyl spacer length between the cationic core and the nitrile func-
tion. Whereas the salts with only one CH2 group in the spacer
have high melting points (above 150 °C), the melting point de-
creases with increasing spacer lengths, so that compounds with a
1
titration). The compounds were characterized by H and 13C
NMR spectroscopy, CHN analysis and electrospray ionization
mass spectrometry (ESI-MS). For the compounds that were
solid at room temperature, the melting point and the onset tem-
perature of the thermal decomposition process was measured
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Figure 9. Close contacts in the crystal structure of compound 9-Tf2N.
Figure 10. Packing in the crystal structure of compound 18-Tf2N.
(CH2)5 or longer spacer are highly viscous liquids at room tem-
perature. The presence of methyl or ethyl groups at different
positions of the pyridinium ring are less efficient for lowering
the melting point than a longer alkyl spacer. The bis(trifluoro-
methylsulfonyl)imide salts are liquid at room temperature, or
they are low-melting solids. The room-temperature ionic liquids
are quite viscous, with viscosities ranging between 150 cP and
580 cP at 19 °C. The pyrrolidinium and piperidinium bromide
salts are high-melting solids, with melting points between 135
and 200 °C. The corresponding bis(trifluoromethylsulfonyl)-
imide salts are low-melting solids or highly viscous liquids at
room temperature. The ionic liquids with bis(trifluoromethylsul-
fonyl)imide anions have a high thermal stability. The onset of
thermal decomposition in an inert atmosphere (helium) was
observed only at temperatures above 400 °C.
liquids were obtained as viscous liquids at room temperature,
some of the compounds showed a tendency to crystallize upon
standing in a freezer. For seven compounds (1-Tf2N, 3-Tf2N, 8-
Tf2N, 11-Tf2N, 9-Tf2N, 17-Tf2N, and 18-Tf2N), single crystals
suitable for X-ray diffraction studies could be obtained. The cry-
stal structures of these complexes give insight into the packing of
cations and anions in the crystal structure and give informa-
tion on the interactions between cation and anions. The crystal-
lographic data are summarized in Table 3.
In the crystal structure of compound 1-Tf2N one pyridinium
cation and one bis(trifluoromethylsulfonyl)imide anion are found
in the asymmetric unit. The nitrile-functionalized alkyl chain is
almost perfectly planar with the pyridinium ring. (3.1° between
the planes). Two symmetry equivalent (inversion center) pyr-
idinium cations are contacting each other; their nitrile moieties
are each pointing toward a CꢀH hydrogen atom of the sym-
Crystal Structures. Although most of the nitrile-functiona-
lized ionic liquids with bis(trifluoromethylsulfonyl)imide ionic
metry equivalent pyridine ring (CꢀH N distance of 2.67 Å),
3 3 3
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Figure 11. Close contacts between the pyridinium rings and the first bis(trifluoromethylsulfonyl)imide anion in the crystal structure of compound
18-Tf2N.
Figure 12. Close contacts between the pyridinium rings and the second bis(trifluormethylsulfonyl)imide anion in the crystal structure of compound
18-Tf2N.
hence forming pyridinium dimers (Figure 2). XꢀY π inter-
In these structures, interactions have also been observed between
the CꢀH pyridinium ring atoms and the respective anions,
together with additional interactions between the functionalizing
groups and the respective anions.
The asymmetric unit of compound 3-Tf2N contains one
pyridinium cation and one bis(trifluoromethylsulfonyl)imide
anion. The nitrile-functionalized alkyl chain is almost perpen-
dicular with respect to the pyridine ring (76.0° between planes
through the respective atoms), and the nitrile moiety lies in the
3 3 3
actions are found in the packing between the pyridine ring and
two sulfonyl oxygen atoms of the bis(trifluoromethylsulfonyl)-
imide anion (both distances of 3.12 Å betweenthe O-atomand the
pyridine ring centroid). The bis(trifluoromethylsulfonyl)imide
sulfonyl oxygen, CF3 fluorine, and nitrogen atoms are involved
in several CꢀH O, CꢀH N, and CꢀH F interactions
3 3 3
3 3 3
3 3 3
between the pyridinium cation and the bis(trifluoromethylsul-
fonyl)imide anion. In total, seven hydrogen contacts are observed
with distances ranging from 2.43 to 2.62 Å (Figure 3). Structures
containing the same pyridinium cation were not found in the
CSD.69 However, structures with similar pyridinium cations have
been reported, containing short alkyl chains of one carbon atom
and a COOH, CH3, COH, CONH2 or CSeH groups.70ꢀ79
same plane as the alkyl chain (12.5°). XꢀY
π interactions
3 3 3
are found in the packing between the nitrile and the pyridine
ring (distance of 3.37 Å between the nitrile N-atom and the ring
centroid) and for a sulfonyl O-atom of the anion and the
pyridine ring (distance of 3.35 Å). CꢀH
O, CꢀH
N, and
3 3 3
3 3 3
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ARTICLE
Figure 13. Packing in the crystal structure of compound 17-Tf2N.
Figure 14. Close contacts in the crystal structure of compound 17-Tf2N.
CꢀH
F interactions are observed between the pyridinium
The asymmetric unit of 8-Tf2N consists of one pyridinium
cation and one bis(trifluoromethylsulfonyl)imide anion. Consid-
ering the pyridinium cation, the nitrile-functionalized alkyl is
almost planar with the pyridine ring (2.3° between planes through
the respective atoms). The nitrile group is pointing toward a CꢀH
pyridine ring hydrogen atom (2.58 Å), linking the pyridinium
cationstogetherinchains, parallelwith (010), running in the[101]
direction (Figure 5). Three of the bis(trifluoromethylsulfonyl)-
imide sulfonyl oxygen atoms are contacting the CꢀH hydrogen
atoms of the methyl group, the nitrile-functionalized chain, as
well as the pyridine ring, of several pyridinium cations. In total,
3 3 3
cation and the bis(trifluoromethylsulfonyl)imide anion. In fact, eight
hydrogen contacts (<3 Å), which originate from four different cations
are observed and range between 2.47 and 2.73 Å (Figure 4). Such
interactions between the CꢀH hydrogen atoms of the pyridi-
nium cation and the anion have also been reported for structures
of the same pyridinium cation, but with Clꢀ and [PdCl4]2- anions.80
Furthermore, structures with similar pyridinium cores have been
reported, containing alkyl chains with the same number of carbon
atoms, but with an end-standing methyl group, and show similar
interactions between the CꢀH pyridine hydrogen atoms and the
chloride81 and bromide82 anions, respectively.
10 CꢀH O/F contacts (in the range of 2.39ꢀ2.69 Å) are
3 3 3
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Table 4. Natural Partial Charge (NPA) of the Nitrile Group
for an Isolated Cation (qcation), for a CationꢀAnion Pair with
a Bis(trifluoromethylsulfonyl)imide Cation (qpair) and for an
Isolated Cation with a Solvation Approximated by the COS-
a
MO Model (qCOSMO
)
cation
qcation (a.u.)
qpair (a.u)
qCOSMO (a.u.)
I1
ꢀ0.173
ꢀ0.216
ꢀ0.247
ꢀ0.263
ꢀ0.277
ꢀ0.285
ꢀ0.292
ꢀ0.296
ꢀ0.300
ꢀ0.182
ꢀ0.221
ꢀ0.249
ꢀ0.278
ꢀ0.292
ꢀ0.301
ꢀ0.177
ꢀ0.247
ꢀ0.185
ꢀ0.249
ꢀ0.179
ꢀ0.250
ꢀ0.181
ꢀ0.251
ꢀ0.187
ꢀ0.253
ꢀ0.243
ꢀ0.283
ꢀ0.293
ꢀ0.304
ꢀ0.303
ꢀ0.312
ꢀ0.311
ꢀ0.315
ꢀ0.314
---
ꢀ0.306
ꢀ0.359
ꢀ0.386
ꢀ0.398
ꢀ0.404
ꢀ0.407
ꢀ0.408
ꢀ0.410
ꢀ0.410
ꢀ0.299
ꢀ0.360
ꢀ0.386
ꢀ0.404
ꢀ0.409
ꢀ0.410
ꢀ0.281
ꢀ0.383
ꢀ0.307
ꢀ0.386
ꢀ0.309
ꢀ0.387
ꢀ0.311
ꢀ0.387
ꢀ0.315
ꢀ0.388
I2
I3
I4
I5
I6
I7
I8
I9
II1
II2
II3
II5
II7
II9
III1
III3
IV1
IV3
V1
V3
VI1
VI3
VII1
VII3
Figure 15. Overview of the cations considered for the calculation of the
charge distributions summarized in Table 4.
---
---
The asymmetric unit of 9-Tf2N contains one pyridinium
cation and one bis(trifluoromethylsulfonyl)imide anion. The
pyridinium cation differs from the one in 8-Tf2N by an extra
methyl group in the meta-position. The nitrile function makes
contact with a CꢀH pyridine ring and a methyl hydrogen atom
(2.66 Å and 2.71 Å, respectively), in this way building up chains
of pyridinium cations, parallel with (100), in the [010] direction
---
---
---
---
---
---
---
(Figure 8). XꢀY π interactions are observed in the crystal
3 3 3
---
packing, between one bis(trifluoromethylsulfonyl)imide CF3 fluo-
---
rine and one sulfonyl oxygen atom, and a pyridine ring (CꢀF
3 3 3
centroid SꢀO centroid distance of 3.42 Å and 3.53 Å, respec-
---
3 3 3
tively). The contacts between the pyridinium cation and the
bis(trifluoromethylsulfonyl)imide anion resembles the observed
contacts in 8-Tf2N: four contacts with the nitrile-functionalized,
one contact with the CꢀH pyridine ring and one contact with the
meta-methyl (Figure 9). No contacts are observed with the ortho-
methyl group.
---
---
---
a The charges are expressed in atomic units (a.u.).
The asymmetric unit of 18-Tf2N contains two pyridinium
cations, each with a bis(trifluoromethylsulfonyl)imide counter-
anion. For both pyridinium cations, both the nitrile-functiona-
lized alkyl chains (0.8°, 5.1°) and alkyl chains (0.8°, 3.4°) are
almost planar with the pyridine ring. The nitrile group of the first
pyridinium cation contacts a CꢀH hydrogen atom of the second
involved in interactions between the bis(trifluoromethylsulfonyl)-
imide anion and four pyridinium cations (Figure 6). A structure
with a similar pyridinium cation has been reported, containing a
short hydroxymethyl chain, a propionic acid chain and a bromide
as the anion.83 The molecules are linked by bromide ions to form
two intermolecular hydrogen bonds COOH Br HO yield-
3 3 3 3 3 3
pyridinium cation (CꢀH N distance of 2.64 Å). The nitrile
ing chains. An additional contact between the bromide and an
alkyl chain CꢀH hydrogen atom exists.
3 3 3
group of this second pyridinium cation contacts a CꢀH hydro-
gen atom of a symmetry equivalent molecule of the first pyridi-
The asymmetric unit of 11-Tf2N consists of one pyridinium
cation and one bis(trifluoromethylsulfonyl)imide anion. The mole-
cular arrangement and packing environment is very similar to
that of 3-Tf2N, as the ring structure of 11-Tf2N only differs in
carrying an extra methyl group. The nitrile-functionalized alkyl chain
is staggered and almost perpendicular with respect to the pyridine
ring (82.3° between planes through the respective atoms). Xꢀ
nium cation (CꢀH N distance of 2.67 Å), in this way building
3 3 3
up a tetrameric arrangement of pyridinium cations (Figure 10).
An XꢀY π interaction is found in the packing between a
3 3 3
bis(trifluoromethylsulfonyl)imide fluorine atom and a pyridine
ring (distance of 3.19 Å between the fluorine atom and the
pyridine ring centroid). Considering the first bis(trifluoromethyl-
sulfonyl)imide anion, one of the sulfonyl oxygen atoms is con-
tacting a CꢀH hydrogen atom of a pyridine ring. Two other
sulfonyl oxygen atoms and the nitrogen atom make hydrogen
contacts with CꢀH hydrogen atoms of the nitrile-functionalized
alkyl chains. One CF3 group is also involved in hydrogen contacts
between the pyridine ring and the nitrile-functionalized alkyl
Y
π interactions are found in the packing between the nitrile
3 3 3
group and the pyridine ring (distance of 3.39 Å between the nitrile
nitrogen atom and the ring centroid) and for a sulfonyl oxygen
atom of the anion and the pyridine ring (distance of 3.36 Å).
CꢀH O, CꢀH N, and CꢀH F interactions are observed
3 3 3
3 3 3
3 3 3
between the pyridinium cation and the bis(trifluoromethylsulfonyl)-
imide sulfonyl oxygen and nitrogen atoms. In fact, nine hydrogen
contacts, which originate from three different cations, are observed
and range between 2.52 and 2.70 Å, including an additional
contact with the extra methyl group on the cation (Figure 7).
chain. In total, nine CꢀH O/N/F contacts (in the range of
3 3 3
2.21ꢀ2.64 Å) are involved in interactions between the bistri-
flimide anion and five pyridinium rings (Figure 11). Consider-
ing the second bis(trifluoromethylsulfonyl)imide anion in the
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Figure 16. Electrostatic potential at the van der Waals surface (0.002 electrons/bohr3) for the 1-(3-cyanopropyl)pyridinium cation (left) and the
1-butylpyridinium cation (right). Solvation was taken into account via the COSMO model.
asymmetric unit, no contacts with pyridine ring CꢀH hydrogen
atoms are observed. In fact, the bis(trifluoromethylsulfonyl)imide
nitrogen atom of the nitrile function (NCN) increases with an
increase in length of the alkyl spacer between the nitrile function
and the cationic heterocyclic ring. Significant differences between
the negative partial charges on the NCN atoms for the pyridinium
series I and the piperidinium series II are only observed for the
compounds with a short alkyl spacer. For a CH2CH2CH2 spacer,
thecharges on thecations ofthe twoserieshave already converged.
It is worth mentioning that for alkyl nitriles, the negative partial
charge on the nitrile nitrogen atom is virtually independent of
the alkyl chain length.48 However, the partial charge of NCN for
the pyrrolidinium cations with a long alkyl spacer are close to
those observed for the alkyl nitriles.48
sulfonyl, nitrogen, and fluorine atoms make seven CꢀH O/N/F
3 3 3
contacts (in the range of 2.38ꢀ2.74 Å) with the hydrogen atoms
ofthe nitrile-functionalizedalkyl chainsofthreepyridiniumligands
(Figure 12). Structures containing the same pyridinium cation
were not found in the CSD.69 A structure with a similar pyridinium
cation has been reported, containing a short hydroxyethyl chain, a
second propionic acid chain and a bromide as the anion.83 The
molecules form centrosymmetric dimers connected by a pair of
hydrogen bonds between COOH and OH groups, and the OH
groupfurtherinteracts withthe bromide ion. Several pyridine rings
and one alkyl chain CꢀH hydrogen atom are contacting the
bromide anion.
For the pyridinium series I, calculations have been made for an
ion pair consisting of a 1-(ω-cyanoalkyl)pyridinium cation and a
bis(trifluoromethylsulfonyl)imide anion. It was observed that the
negative partial charge on NCN has been increased in comparison
to the value obtained for one isolated cation in the gas phase. This
shows that solvation affects the charge of NCN . Unfortunately, an
adequate quantum chemical description of solvation is unfeasible
due to computational costs. One feasible solvation model is the
continuum solvation model (COSMO).64 Application of this
model led to an additional increase of the negative partial charge
at NCN compared to the isolated ion pair calculations. Further-
more, the partial charges at NCN for cations with long alkyl
spacers converge toward ꢀ0.410 au. If the partial charges at
the NCN atoms of members of the pyridinium series I and the
piperidinium series II with the same alkyl spacer length are
compared, significant differences can be observed only for the
shortest alkyl spacer lengths. As can be seen in Table 4, NCN of
cation III1 possesses the smallest negative partial charge. A methyl
or methoxy substituent on the pyridinium ring has an electron
donation effect, but the increases of the negative partial charge of
NCN is negligible. The position of the methyl group (ortho, meta,
or para) has also only a marginal effect on the negative partial
charge of NCN. In Figure 16, the electrostatic potentials at the van
der Waals surface (0.002 electrons/bohr3) of the 1-(3-cyano-
propyl)pyridinium cation (left) and the 1-butylpyridinium cation
are compared.
The asymmetric unit of 17-Tf2N contains one pyridinium cation
and one bis(trifluoromethylsulfonyl)imide anion. The pyridinium
cation differs from the ring structure in 18-Tf2N only in the para-
position of the ethyl chain. The nitrile function is hydrogen bonded
to a CꢀH pyridine hydrogen atom of two symmetry equivalent
(inversion center) pyridinium cations (CꢀH N distance of
3 3 3
2.56 Å and 2.51 Å, respectively). Because of the inversion center,
the latter pyridinium cations contact the former one in exactly the
same way, building up strands of cations, parallel with (001),
running in the [100] direction (Figure 13). XꢀY π interac-
3 3 3
tions are observed in the crystal packing, especially for one
bis(trifluoromethylsulfonyl)imide CF3 group and a pyridine ring
(CꢀF centroid distances of 3.63, 3.46, and 3.86 Å, respec-
3 3 3
tively) and one bis(trifluoromethylsulfonyl)imide sulfonyl oxy-
gen atom and another pyridine ring (SꢀO centroid distance
3 3 3
of 3.18 Å). Contacts between the pyridinium cation and bis-
(trifluoromethylsulfonyl)imide anion are observed: all bis-
(trifluoromethylsulfonyl)imide sulfonyl oxygen atoms contact
either CꢀH pyridine ring hydrogen atoms (two contacts), nitrile-
alkyl hydrogen atoms (three contacts), or an ethyl hydrogen atom
(one contact) (Figure 14).
Quantum Chemical Calculations. Quantum chemical calcu-
lations have been performed on different nitrile-functionalized
pyridinium and piperidinium ionic liquids to investigate the
influence of the alkyl spacer on the charge located at the nitrogen
atom of the nitrile function (Table 4). An overview of the investi-
gated cations is given in Figure 15. Please note, that most investi-
gated cations resemble those of the synthesized ionic liquids.
Additional cations were added in the computational investiga-
tion to highlight trends and influences of substituents. For a given
series of cations, it is found that the negative partial charge on the
15N NMR Measurements. Natural abundance nuclear mag-
netic resonance studies on the nitrogen-15 isotope (15N NMR)
have been performed in order to gain additional insight into the
structural and electronic properties of the synthesized ionic liquids.
The electronic environment of the nitrogen atom is greatly influ-
enced by changes in the molecular topology and intermolecular
interactions. Those electronic changes are usuallydirectly reflected
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ARTICLE
Figure 18 show a systematic trend in the chemical shift of both
the pyridine and nitrile nitrogen resonances. The simultaneous
decrease of the nitrile and increase of the pyridine 15N chemical
shifts is observed as the number of aliphatic carbons between
them increases. This implies that the pyridine nitrogen is more
deshielded, while the nitrile nitrogen NCN is more shielded in
compounds containing long aliphatic chains between them. This
is in the agreement with the theoretical study that predicts that
the quantity of the negative charge on NCN becomes larger as the
length of the spacer increases.
’ CONCLUSIONS
Within the scope of this work, a total of 50 nitrile-functiona-
lized pyridinium and piperidinium ionic liquids with bromide or
bis(trifluoromethylsulfonyl)imide anions have been prepared
to investigate the influence of structural modifications on the
properties of these ionic liquids. The most striking changes were
observed upon an increase in the length of the alkyl spacer
between the cationic core and the nitrile group. A longer alkyl
spacer leads to lower melting points, but also to a more negative
partial charge on the nitrogen atom of the nitrile group. The
charge distribution in the pyridinium compounds was probed by
15N NMR spectroscopy, and the measurements were in agree-
ment with DFT calculations. These results on the nitrogen partial
charge are of importance for the use of these functionalized ionic
liquids to form complexes with metal ions, as it was illustrated
earlier by some of us for nitrile-functionalized pyrrolidinium ionic
liquids.48 The coordination chemistry of these ionic liquids is
presently under investigation. Crystal structures of pyridinium
compounds with bis(trifluoromethylsulfonyl)imide anions reveal
the interactions between the cations and anions in the solid state.
Figure 17. 15N NMR spectra of ionic liquids 2-Tf2N (a), 4-Tf2N (b),
and 5-Tf2N (c), recorded at room temperature and using CH3NO2 as
external standard.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF-files of the crystal struc-
b
tures. Synthesis and characterization of ionic liquids. This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 18. 15N NMR chemical shifts of ionic liquids 1-Tf2N - 5-Tf2N as
a function of the number of carbon atoms in the aliphatic chain between
nitrile and pyridine nitrogen atoms.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: Koen.Binnemans@chem.kuleuven.be. Fax: þ3216327992.
in the shielding and NMR chemical shift of the nitrogen nucleus.84
15N NMR spectra of compounds 2-Tf2N, 4-Tf2N, and 5-Tf2N are
shown inFigure17. All three ionicliquids show three characteristic
15N resonances, corresponding to the three nitrogen atoms pre-
sent in the structure. According to previously published 15N chem-
ical shift tables,85 the resonance at ꢀ242 ppm was assigned to
the nitrogen atom in the bis(trifluoromethylsulfonyl)imide anion,
while the resonances at around ꢀ175 ppm and ꢀ135 ppm were
assigned to the pyridine and nitrile nitrogen atoms of the cationic
part, respectively. Although at the first glance the spectra shown
in Figure 17 look nearly identical, a careful analysis revealed that
the chemical shifts of the pyridine and nitrile nitrogen atoms varied
depending on the chemical structure. Since the only difference
among these ionic liquids is the number of carbon atoms in the
aliphatic spacer between the pyridine nitrogen and the nitrile
function (2 carbon atoms in compound 2-Tf2N, 5 in 4-Tf2N, and
10 in 5-Tf2N), the 15N NMR spectra of ionic liquids 1-Tf2N and
3-Tf2N, that contain one and three aliphatic carbon atoms, respec-
tively, have been recorded in a next step. The data summarized in
’ ACKNOWLEDGMENT
This project was supported by the K.U. Leuven (Projects
IDO/05/005and GOA 08/05, and a DBOFPhD granttoK.C.L.),
by the IWT-Flanders (SBO-project “MAPIL”) and by the FWO-
Flanders (Research Community “Ionic Liquids”). The authors
gratefully acknowledge the financial support of the DFG priority
program SPP 1191 “Ionic Liquids” and the ERA Chemistry pro-
gram, which allows collaboration under the project “A Modular
Approach to Multi-responsive Surfactant/Peptide (SP) and
Surfactant/Peptide/Nanoparticle (SPN) Hybrid Materials”.
The authors thank Ms. Kathleen Lava for assistance during the
thermal analysis measurements and Mr. Dirk Henot for perform-
ing CHN analyses.
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