Pyrazolium salts as a new class of ionic liquid crystals

Article abstract of DOI:10.1039/c2jm31939j

New alkyloxyphenyl substituted pyrazolium salts [H₂pz ^R(n)][A] (R(n) = C₆H₄OC_nH _2n+1; n = 8, 10, 12, 14, 16, 18; [A] = Cl^-, BF ₄^-, ReO₄^-, SbF₆ ^-, CF₃SO₃^-, CH₃-p-C ₆H₄SO₃^-) have been synthesised and characterised. Those containing Cl^-, BF₄^-, ReO₄^- and SbF₆^- anions behave as liquid crystal compounds exhibiting smectic A (SmA) mesophases. Because the precursor neutral pyrazoles [Hpz^R(n)] were not liquid crystals, their protonation and strategic choice of the counter-anion was found useful to achieve mesomorphism and to modulate the transition temperatures, so giving rise to a new class of ionic liquid crystals. The X-ray crystal structures of selected derivatives as representative examples of each type of compound have been resolved. In most of the cases, for X-ray purposes, the salts were designed to contain one methoxyphenyl-substituted pyrazolium cation. Different structural features have been found depending on the nature of the anion. Dimeric units held through hydrogen bonds are arranged in chains which pack in a layer-like array for compounds [H₂pz^R(10)][BF ₄], [H₂pz^R(1)][OTf] and [H₂pz ^R(1)][PTS], but 2D structures were found for [H₂pz ^R(1)][Cl] and [H₂pz^R(1)][ReO₄]. Relationships between the solid and the mesophase structures are proposed. The absence of mesomorphism for CF₃SO₃^- and CH ₃-p-C₆H₄SO₃^- derivatives is also explained on the basis of the main structural features.

Full text of DOI:10.1039/c2jm31939j

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 13239
www.rsc.org/materials
PAPER
Pyrazolium salts as a new class of ionic liquid crystals†
a
a
a
b
Ignacio Sanchez, Jose Antonio Campo, Jose Vicente Heras, M. Rosario Torres and Mercedes Cano
a
*
ꢀ
ꢀ
ꢀ
Received 28th March 2012, Accepted 14th May 2012
DOI: 10.1039/c2jm31939j
New alkyloxyphenyl substituted pyrazolium salts [H2pzR(n)][A] (R(n) ¼ C6H4OCnH2n+1; n ¼ 8, 10, 12,
14, 16, 18; [A] ¼ Clꢀ, BF4ꢀ, ReO4ꢀ, SbF6ꢀ, CF3SO3ꢀ, CH3-p-C6H4SO3ꢀ) have been synthesised and
characterised. Those containing Clꢀ, BF4ꢀ, ReO4ꢀ and SbF6ꢀ anions behave as liquid crystal
compounds exhibiting smectic A (SmA) mesophases. Because the precursor neutral pyrazoles [HpzR(n)
]
were not liquid crystals, their protonation and strategic choice of the counter-anion was found useful to
achieve mesomorphism and to modulate the transition temperatures, so giving rise to a new class of
ionic liquid crystals. The X-ray crystal structures of selected derivatives as representative examples of
each type of compound have been resolved. In most of the cases, for X-ray purposes, the salts were
designed to contain one methoxyphenyl-substituted pyrazolium cation. Different structural features
have been found depending on the nature of the anion. Dimeric units held through hydrogen bonds are
arranged in chains which pack in a layer-like array for compounds [H2pzR(10)][BF4], [H2pzR(1)][OTf] and
[H2pzR(1)][PTS], but 2D structures were found for [H2pzR(1)][Cl] and [H2pzR(1)][ReO4]. Relationships
between the solid and the mesophase structures are proposed. The absence of mesomorphism for
CF3SO3ꢀ and CH3-p-C6H4SO3ꢀ derivatives is also explained on the basis of the main structural features.
the mesogenic core and their thermal properties have been
described and occasionally compared.
Introduction
Ionic liquid crystals (ILCs) can be considered as an extension of
the typical ionic liquid mateꢁrials (organic salts with melting
temperatures lower than 100 C).1 In addition to their charac-
teristic ionic liquid properties (low vapour pressure, thermal
stability, polarity, electric behaviour, etc.),1,2 they exhibit liquid
crystal properties, such as molecular orientational order.2,3
Alternatively, ILCs can be regarded as a class of liquid crystals
constituted by cations and anions.3,4 Due to their dual behaviour
as liquid crystals and ionic liquids, they offer new opportunities
as ion conductive materials5–7 in electronic devices, as organised
reaction media8 and as template agents in materials science9 or in
bio-related science.10,11
Differences in mesophase range, thermal stability and clearing
temperature have been observed depending on variables such as
the chain length or the nature of the cation and anion present in
the salts.2–4,11,13 For instance, the mesophage stability range of
imidazolium and pyridinium salts was increased by increasing
the chain length, however, the clearing points were dependant on
the cation or the anion.3,13–15 At the same time, the melting points
were strongly dependent on the choice of counterion, and imi-
dazolium salts with fluorinated anions exhibited the lowest
melting temperatures.3 As a consequence, it seems to be impor-
tant to highlight that the thermal behaviour of different ILCs
does not follow the same pattern when modifying the same
variables, therefore a systematic study is required for each new
family of ionic salts investigated.
Examples of ILC materials include ammonium, phosphonium,
pyridinium, pyrrolidinium and imidazolium salts,2–6,10–23 the
latter of these being the most abundantly studied because of the
profuse applications of related imidazolium-based ionic
liquids.2,3,12 In all the above examples the cationic group acts as
We are now involved in the study of a new class of ILC based
on pyrazolium salts, taking into account our experience on liquid
crystalline pyrazole complexes. In addition, from the best of our
knowledge, ILCs based on pyrazolium salts have not been
described yet. Previous work from our lab has been directed
towards the design, preparation and study of liquid crystal
properties of metallomesogens containing pyrazole ligands of the
types 3-(4-alkyloxyphenyl)pyrazole, [HpzR(n)] and 3,5-di(4-alky-
loxyphenyl)pyrazole, [Hpz2R(n)], (R(n) ¼ C6H4OCnH2n+1; n ¼
4–18) (Scheme 1),24–27 most of them being ILCs in which the
metallorganic moieties are the mesogenic groups. In particular,
the ionic complexes of the type [M(HpzR(n))2][A] and
aDepartamento de Quꢀımica Inorgaꢀnica I, Facultad de Ciencias Quꢀımicas,
Universidad Complutense, 28040 Madrid, Spain. E-mail: mmcano@quim.
ucm.es; Fax: +34 91 394 4352; Tel: +34 91 394 4340
bCAI de Difraccioꢀn de Rayos-X, Facultad de Ciencias Quꢀımicas,
Universidad Complutense, 28040 Madrid, Spain
† Electronic
supplementary
information
(ESI)
available:
Characterisation of compounds, Table S1–S6 and Fig. S1. CCDC
reference numbers 873375–873379 for compounds Cl-1, BF4-10,
ReO4-1, OTf-1 and PTS-1. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2jm31939j
ꢀ
ꢀ
[M(Hpz2R(n))2][A] (M ¼ Ag, Au; A ¼ NO3ꢀ, BF4 PF6
,
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13239

View Online
Experimental
Materials and physical measurements
All commercial reagents were used as supplied. The precursor
pyrazoles [HpzR(n)] (R(n) ¼ C6H4OCnH2n+1; n ¼ 1, 8, 10, 12, 14,
16, 18) were prepared following a general procedure previously
reported by us.24,25
Elemental analyses for carbon, hydrogen, nitrogen and sulfur
were carried out by the Microanalytical Service of Complutense
University (validated range: %C 0.5–94.7, %H 0.5–7.6, %N 0.5–
23.0 and %S 0.5–30.6). IR spectra were recorded on a FTIR
Thermo Nicolet 200 spectrophotometer with samples as KBr
1
pellets in the 4000–400 cmꢀ1 region. H-NMR spectra were per-
formed at room temperature on a Bruker DPX-300 spectro-
photometer (NMR Service of Complutense University) from
solutions in (CD3)2CO or CDCl3. Chemical shifts (d) are listed
relative to Me4Si using the signal of the deuterated solvent as the
reference (2.05 and 7.26 ppm, respectively), and coupling
constants (J) are in hertz. Multiplicities are indicated as s
(singlet), d (doublet), t (triplet), m (multiplet), br s (broad signal).
Scheme 1 Pyrazole-type ligands.
1
The H chemical shifts and coupling constants are accurate to
ꢀ
28,29
CF3SO3ꢀ, CH3-p-C6H4SO3
)
have been shown to exhibit
ꢂ0.01 ppm and ꢂ0.3 Hz, respectively. Scheme 3 shows the
nomenclature used for the NMR assignments.
monotropic or enantiotropic smectic behaviour and present an
extensive range of stability of the mesophases. In addition, it was
also proved that the free 3,5-alkyloxyphenyl-disubstituted pyr-
azoles are mesomorphic26,30 in contrast to the absence of meso-
morphism of the related 3(5)-monosubstituted ligands.24,25
We have also investigated new ILC salts containing b-dike-
Phase studies were carried out by optical microscopy using an
Olympus BX50 microscope equipped with a Linkam THMS 600
heating stage. The temperatures were assigned on the basis of
optic observations with polarised light. Measurements of the
transition temperatures were made using a Perkin Elmer Pyris 1
differential scanning calorimeter with the sample (1–4 mg) sealed
hermetically in aluminium pans and with a heating or cooling
rate of 5–10 K minꢀ1. The X-ray diffractograms at variable
temperature were recorded on a Panalytical X’Pert PRO MPD
diffractometer in a q–q configuration equipped with an Anton
Paar HTK1200 heating stage (X-Ray Diffraction Service of
Complutense University).
2
tonylpyridinium cations with [ZnCl4] ꢀ anions (Scheme 2), which
exhibit enantiotropic smectic behaviour with a wide mesophase
range.31
Encouraged by these results, we are now interested in the
design of new ILCs based on cations produced by protonation of
the non-mesomorphic monosubstituted pyrazoles [HpzR(n)].
These cations should be combined with different counterions in
order to study the potential for inducting and tuning the meso-
morphism in the promesogenic moiety as a function of the anion
characteristics and/or the alkyl lateral chain length.
Pyrazolium compounds of the type [H2pzR(n)][A] ([A] ¼ Clꢀ,
BF4ꢀ; R(n) ¼ C6H4OCnH2n+1, n ¼ 1, 8, 10, 12, 14, 16, 18)
In this work we present the synthesis and mesomorphic
To a solution of 3 mmol of the corresponding [HpzR(n)] in
dichloromethane (50 mL), a hydrochloric acid solution (spec.
grav. 1.19) or HBF4$Et2O was added until a white or pale yellow
precipitate was formed. The mixture was stirred for 3 hours and
properties of new ionic liquid crystals based on pyrazolium
cations of the type [H2pzR(n) +
bearing one alkyloxyphenyl
]
substituent of variable chain length (n ¼ 8–18 carbon atoms) at
the position 3(5) of the pyrazole ring. We have used six different
counterions, Clꢀ, BF4ꢀ, ReO4ꢀ, SbF6ꢀ, CF3SO3ꢀ (OTf) and CH3-
p-C6H4SO3ꢀ (PTS) for each cationic unit, so extending our study
to six families of compounds I–VI (Scheme 3), four of them
exhibiting mesomorphism.
Scheme 3 Pyrazolium salts studied in this work, and atom numbering
Scheme 2 b-Diketonylpyridinium-based ILC.
13240 | J. Mater. Chem., 2012, 22, 13239–13251
used in the NMR assignments.
This journal is ª The Royal Society of Chemistry 2012

View Online
then cooled to ꢀ18 ꢁC. The solid was filtered, washed with water
and dried in vacuo. All compounds were characterised by IR and
1H-NMR spectroscopies and CHN analyses (deposited as ESI†).
Specific examples of the characterisation are given below.
[H2pzR(8)][Cl] (Cl-8): colourless solid (80%). Elemental
analysis: found: C, 65.8; H, 7.8; N, 9.1%. C17H25N2OCl requires
C, 66.1; H, 8.2; N, 9.1%. nmax(KBr)/cmꢀ1: 3144, 3080 n(NH),
1615 n(C]C + C]N). dH (300 MHz; CDCl3; Me4Si): 0.89 (3 H,
t, J 6.7, CH3), 1.30 (10 H, m, CH2), 1.81 (2 H, m, CH2), 4.01 (2 H,
t, J 6.7, OCH2), 6.71 (1 H, d, J 2.8, H4), 7.02 (2 H, d, J 8.9, Hm),
7.90 (1 H, d, J 2.8, H5), 7.93 (2 H, d, J 8.9, Ho).
[H2pzR(8)][PTS] (PTS-8): colourless solid (85%). Elemental
analysis: found: C, 64.7; H, 7.1; N, 6.3; S, 7.1%. C24H32N2SO4
requires C, 64.8; H, 7.2; N, 6.3; S, 7.2%. nmax(KBr)/cmꢀ1: 3218,
3136 n(NH), 1617 n(C]C + C]N), 1186, 1021 n(SO). dH (300
MHz; CDCl3; Me4Si): 0.89 (3 H, t, J 6.7, CH3), 1.29 (10 H, m,
CH2), 1.80 (2 H, m, CH2), 2.36 (3 H, s, CH3(PTS)), 3.99 (2 H, t, J
6.7, OCH2), 6.70 (1 H, d, J 2.7, H4), 6.97 (2 H, d, J 8.7, Hm), 7.20
(2 H, d, J 8.1, Ho(PTS)), 7.73 (2 H, d, J 8.7, Ho), 7.84 (2 H, d, J
8.1, Hm(PTS)), 8.05 (1 H, d, J 2.7, H5).
Crystallographic structure determinations
[H2pzR(8)][BF4] (BF4-8): pale yellow solid (69%). Elemental
analysis: found: C, 57.0; H, 6.9; N, 7.8%. C17H25N2OBF4
requires C, 56.7; H, 7.0; N, 7.8%. nmax(KBr)/cmꢀ1: 3385, 3241
n(NH), 1615 n(C]C + C]N), 1083 n(B–F). dH (300 MHz;
CDCl3; Me4Si): 0.89 (3 H, t, J 6.7, CH3), 1.30 (10 H, m, CH2),
1.81 (2 H, m, CH2), 4.02 (2 H, t, J 6.7, OCH2), 6.81 (1 H, d, J 2.8,
H4), 7.02 (2 H, d, J 8.9, Hm), 7.67 (2 H, d, J 8.9, Ho), 8.14 (1 H, d,
J 2.8, H5), 12.83 (br s, NH), 13.27 (br s, NH).
Data collection for all compounds was carried out at room
temperature on a Bruker Smart CCD diffractometer using
ꢁ
graphite-monochromated Mo-Ka radiation (l ¼ 0.71073 A)
operating at 50 kV and 35 mA for Cl-1 and BF4-10 and at 50 kV
and 30 mA for ReO4-1, OTf-1 and PTS-1. In all cases, the data
were collected over a hemisphere of the reciprocal space by
combination of three exposure sets. Each exposure was 20 s long
for Cl-1, BF4-10, ReO4-1 and OTf-1 and 10 s for PTS-1, and
covered 0.3ꢁ in u. The first 100 frames were recollected at the end
of the data collection to monitor crystal decay, and no appre-
ciable decay was observed. A summary of the fundamental
crystal and refinement data is given in Table 1.
ꢀ
Pyrazolium compounds of the type [H2pzR(n)][A] ([A] ¼ ReO4
,
SbF6ꢀ, OTf, PTS; R(n) ¼ C6H4OCnH2n+1, n ¼ 1, 8, 10, 12, 14,
16, 18)
The structures were solved by direct methods and refined by
full-matrix least-square procedures on F2.32 All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were
included in calculated positions and refined riding on the
respective carbon atoms, with some exceptions. The hydrogens
H1 and H2 bonded to N1 and N2 atoms for Cl-1, BF4-10 and
OTf-1, and H1, H2, H3, H4, H5 and H6 bonded to N1, N2, N3,
N4, N5 and N6, respectively, for PTS-1 were located in a Fourier
synthesis and refined riding on the respective bonded atoms. For
compound ReO4-1, they were located in a Fourier synthesis and
fixed.
To a solution of the [H2pzR(n)][Cl] in 40 mL of CH2Cl2 the
corresponding salt AgA (A ¼ ReO4ꢀ, SbF6ꢀ, OTf, PTS) in 8 : 2
mL of CH2Cl2–CH3CN and in a 1 : 1 molar ratio was added,
under a nitrogen atmosphere. The mixture was stirred for 24
hours in the absence of light and then filtered through Celiteꢀ.
The clear filtrate was concentrated in vacuo until a solid
precipitated. The white solid was filtered and dried in vacuo. All
1
compounds were characterised by IR and H-NMR spectros-
copies and CHN or CHNS analyses (deposited as ESI†). A
specific example of the characterisation for a compound of each
family is given below.
[H2pzR(8)][ReO4] (ReO4-8): colourless solid (75%).
Elemental analysis: found: C, 38.7; H, 4.8; N, 5.4%.
C17H25N2O5Re requires C, 39.0; H, 4.8; N, 5.4%. nmax(KBr)/
cmꢀ1: 3144, 3124 n(NH), 1616 n(C]C + C]N), 908 n(Re–O). dH
(300 MHz; CDCl3; Me4Si): 0.89 (3 H, t, J 6.7, CH3), 1.29 (10 H,
m, CH2), 1.81 (2 H, m, CH2), 4.02 (2 H, t, J 6.7, OCH2), 6.79
(1 H, d, J 2.3, H4), 7.04 (2 H, d, J 8.7, Hm), 7.72 (2 H, d, J 8.7,
Ho), 7.93 (1 H, d, J 2.3, H5).
[H2pzR(8)][SbF6] (SbF6-8): colourless solid (49%). Elemental
analysis: found: C, 40.5; H, 4.9; N, 5.6%. C17H25N2OSbF6
requires C, 40.1; H, 5.0; N, 5.5%. nmax(KBr)/cmꢀ1: 3336, 3167
n(NH), 1616 n(C]C + C]N), 665 n(Sb–F). dH (300 MHz;
CDCl3; Me4Si): 0.90 (3 H, t, J 6.7, CH3), 1.30 (10 H, m, CH2),
1.81 (2 H, m, CH2), 4.01 (2 H, t, J 6.7, OCH2), 6.74 (1 H, d, J 2.4,
H4), 7.01 (2 H, d, J 8.3, Hm), 7.63 (2 H, d, J 8.3, Ho), 7.87 (1 H, d,
J 2.4, H5).
[H2pzR(8)][OTf] (OTf-8): colourless solid (75%). Elemental
analysis: found: C, 50.9; H, 5.8; N, 6.6; S, 7.6%. C18H25N2SO4F3
requires C, 51.2; H, 6.0; N, 6.6; S, 7.6%. nmax(KBr)/cmꢀ1: 3136
n(NH), 1618 n(C]C + C]N), 1257, 1032 n(SO). dH (300 MHz;
CDCl3; Me4Si): 0.90 (3 H, t, J 6.7, CH3), 1.30 (10 H, m, CH2),
1.82 (2 H, m, CH2), 4.03 (2 H, t, J 6.7, OCH2), 6.79 (1 H, d, J 2.8,
H4), 7.03 (2 H, d, J 8.8, Hm), 7.70 (2 H, d, J 8.8, Ho), 8.11 (1 H, d,
J 2.8, H5), 13.67 (br s, NH), 14.51 (br s, NH).
Results and discussion
1H-NMR and IR characterisation
Table 2 depicts all the new ionic salts prepared in this work
including the family and the nomenclature used according to the
counterion and the number of carbon atoms in the alkyl chain.
All compounds were characterised by elemental analysis and
1
spectroscopic techniques (IR and H-NMR; see Experimental
section and ESI†).
The [H2pzR(n)][A] salts were prepared in two different ways (a
and b) according to Scheme 4. Chloride and tetrafluoroborate
derivatives (I and II) were obtained by protonation of the pyr-
azole precursor with hydrochloric (a1) or tetrafluoroboric (a2)
acid, respectively. The metathesis reactions (b) of the corre-
sponding pyrazolium chlorides (I) with silver perrhenate, silver
hexafluoroantimoniate, silver trifluoromethanesulfonate (OTf)
and silver p-toluenesulfonate (PTS) in a 1 : 1 molar ratio in
CH2Cl2 gave rise to the new salts III, IV, V and VI, respectively.
1
The H-NMR spectra of the new salts (I–VI) in CDCl3 solu-
tion at room temperature display the expected signals from the
pyrazolium and alkyloxyphenyl groups. Compound VI exhibits,
in addition, the signals for the protons of the PTS counteranion.
In all cases the NH signals are not observed (except for
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13241

View Online
Table 1 Crystal and refinement data for Cl-1, BF4-10, ReO4-1, OTf-1 and PTS-1
Cl-1
BF4-10
ReO4-1
OTf-1
PTS-1
Empirical formula
Formula weight
Crystal system
C10H11ClN2O
210.66
Orthorhombic
Pbca
C19H29BF4N2O
388.25
Triclinic
P(1)
2
6.1620(6)
9.2230(9)
19.067(2)
93.895(2)
94.406(2)
104.332(2)
1042.5(2)
2
C10H11N2O5Re
425.41
Orthorhombic
P212121
19
4.9991(4)
9.5975(8)
24.980(2)
90
90
90
1198.5(2)
4
C11H11F3N2O4S
324.28
Triclinic
P(1)
2
6.344(4)
10.275(6)
11.844(7)
70.91(1)
83.27(2)
89.32(2)
724.3(7)
2
C17H18N2O4S
346.39
Triclinic
P(1)
2
9.993(2)
14.007(3)
18.935(3)
103.999(4)
91.508(4)
96.957(4)
2548.5(8)
6
ꢂ
ꢂ
ꢂ
Space group
Space group number
61
ꢁ
a/A
11.998(2)
7.316(2)
23.079(3)
90
90
90
2025.9(5)
8
293(2)
880
1.381
ꢁ
b/A
ꢁ
c/A
a (ꢁ)
b (ꢁ)
g (ꢁ)
3
ꢁ
V/A
Z
T/K
F(000)
rc/g cmꢀ3
m/mmꢀ1
Scan technique
Data collected
296(2)
412
1.237
0.100
u and 4
296(2)
800
2.358
10.154
u and 4
296(2)
332
1.487
0.273
u and 4
293(2)
1092
1.354
0.214
u and 4
0.344
u and 4
(ꢀ11, ꢀ9, ꢀ18) to (15, (ꢀ7, ꢀ8, ꢀ22) to (7, (ꢀ6, ꢀ11, ꢀ30) to (6, (ꢀ7, ꢀ12, ꢀ11) to (7, (ꢀ11, ꢀ12, ꢀ22) to (11,
9, 27)
1.76 to 27.00
11 540
10, 22)
1.08 to 25.00
7996
9, 30)
1.63 to 26.00
9617
10, 14)
1.83 to 25.00
5594
16, 22)
1.11 to 25.00
19 528
q range (ꢁ)
Reflections collected
Independent reflections
Completeness to maximum 96.6
q (%)
2138 (Rint ¼ 0.0912)
3571 (Rint ¼ 0.0391) 2343 (Rint ¼ 0.0313) 2489 (Rint ¼ 0.0959)
8734 (Rint ¼ 0.1040)
97.0
99.6
97.3
97.2
Data/restraints/parameters 2138/0/127
Observed reflections [I > 943
2s(I)]
3571/0/246
1749
2343/0/163
2244
2489/0/192
1029
8734/0/655
2671
Ra
0.0428
0.1024
0.0619
0.2181
0.0199
0.0475
0.0641
0.1681
0.0614
0.1153
b
RwF
a
b
2
2
S[|Fo| ꢀ |Fc|]/S[|Fo|. {S[w(Fo ꢀ Fc )2]/S[w(Fo2)2]}1/2
.
compounds BF4-8 and OTf-8), which suggests the presence of
strong H-bond interactions in solution.29
As an interesting feature, it was remarkable to observe the
influence of both factors, the counterion nature and the solution
concentration, on the chemical shifts of aromatic and pyrazolium
ring protons. Thus, we carried out two additional experiments.
1
In the first, a H-NMR study at a constant concentration (C ¼
6.7 ꢃ 10ꢀ3 M) was performed for all the salts containing 8 carbon
atoms in the alkyl chain. In general, the signals in the aromatic
region were down-field shifted with respect to those of the neutral
pyrazole compounds.24 This deshielding is counterion-depen-
dent, the shift of the closest proton to the 1-NH pyrazolic
nitrogen atom being the most modified one. As depicted in Table
3, the presence of BF4ꢀ, OTf and PTS anions produced larger
modifications in the pyrazolium signals than those observed for
Scheme 4 Synthesis of the pyrazolium compounds I–VI.
Table 2 Nomenclature and formulation of the compounds described in the worka
Family
I
II
III
IV
V
VI
Type
n ¼ 1
[H2pzR(n)][Cl]
Cl-1
Cl-8
Cl-10
Cl-12
Cl-14
Cl-16
Cl-18
[H2pzR(n)][BF4]
[H2pzR(n)][ReO4]
ReO4-1
ReO4-8
ReO4-10
ReO4-12
ReO4-14
ReO4-16
ReO4-18
[H2pzR(n)][SbF6]
[H2pzR(n)][OTf]
OTf-1
OTf-8
OTf-10
OTf-12
OTf-14
OTf-16
OTf-18
[H2pzR(n)][PTS]
PTS-1
PTS-8
PTS-10
PTS-12
PTS-14
PTS-16
PTS-18
n ¼ 8
BF4-8
SbF6-8
n ¼ 10
n ¼ 12
n ¼ 14
n ¼ 16
n ¼ 18
BF4-10
BF4-12
BF4-14
BF4-16
BF4-18
SbF6-10
SbF6-12
SbF6-14
SbF6-16
SbF6-18
a
R(n) ¼ C6H4OCnH2n+1; OTf ¼ CF3SO3ꢀ; PTS ¼ CH3-p-C6H4SO3
.
ꢀ
13242 | J. Mater. Chem., 2012, 22, 13239–13251
This journal is ª The Royal Society of Chemistry 2012

View Online
ꢀ
ꢀ
the Clꢀ, ReO4 and SbF6 salts. A similar fact has also been
found for related alkyloxybenzyl-methyl-imidazolium based
ionic liquid crystals,11 and has been explained in terms of the
different morphology and ability of the anion to interact with the
cationic moiety, highlighting that these protons are very envi-
ronment-sensitive.
Table 4 Chemical shifts in the aromatic region for representative
examples of the salts containing 8 carbon atoms in the substituent alkyl
chain at various solution concentrations
Compound
C/mol Lꢀ1
H5
H4
Hortho
Hmeta
Cl-8
6.7 ꢃ 10ꢀ2
6.7 ꢃ 10ꢀ3
6.7 ꢃ 10ꢀ4
3.4 ꢃ 10ꢀ2
6.7 ꢃ 10ꢀ3
8.4 ꢃ 10ꢀ5
3.3 ꢃ 10ꢀ2
6.7 ꢃ 10ꢀ3
1.7 ꢃ 10ꢀ3
3.3 ꢃ 10ꢀ4
7.90
7.91
7.91
8.14
8.08
7.92
8.12
8.10
8.08
7.96
6.71
6.72
6.72
6.81
6.82
6.83
6.78
6.79
6.79
6.80
7.93
7.94
7.94
7.68
7.68
7.65
7.70
7.70
7.69
7.67
7.01
7.02
7.02
7.02
7.03
7.05
7.02
7.03
7.03
7.03
In the second experiment, concentration-dependance studies
were carried out for representative compounds (Table 4). When
the concentration was increased, a down-field shift of the signals
was generally observed, which could be attributed to the presence
of aggregates in solution, as was observed in related cases.11,12,16
These variations are of a different magnitude depending on the
counterion present in each salt.
BF4-8
OTf-8
The IR spectra of all salts in the solid state show the charac-
teristic bands of the pyrazolium moiety as well as those of the
corresponding counterions, where appropriate. In particular, the
n(C]N) + n(C]C) and n(NH) absorption bands from the pyr-
azolium cation appear at ca. 1600 and in the 3100–3400 cmꢀ1
range, respectively.33 Additionally, characteristic bands of the
in Table S1–S5 (see ESI†). Table 5 lists the hydrogen bond
geometries of these compounds.
Compound BF4-10 crystallises into the triclinic system, space
ꢂ
group P(1). The crystal structure consists of dimers containing
counter-anions are also observed. In particular, for BF4ꢀ, ReO4
ꢀ
two cations and two anions held by N/F H-bonding interac-
tions (Table 5), with both cations in a head to tail arrangement
(Fig. 1). Columns of dimers are built along the b axis and consist
ꢀ
and SbF6 salts, bands at ca. 1080, 900 and 650 cmꢀ1 corre-
sponding to n(BF), n(ReO) and n(SbF) from the corresponding
anions,33 appear at values that agree with the ionic nature of the
salts (II–IV). Also, the presence of the counterions OTf and PTS
ꢀ
of superposed rows of pyrazolium rings and single rows of BF4
anions, held together through conventional N/F and weak C/
F H-bonding interactions (Table 5). Additional, very weak, C–
(V–VI) is clearly confirmed by the two bands at ca. 1250 cmꢀ1
nas(SO3), and 1030 cmꢀ1, ns(SO3), for the former, and 1180 cmꢀ1
nas(SO3), and 1020 cmꢀ1, nas(SO3), for the latter.33
,
,
ꢁ
H/F interactions of 3.45 A could be suggested as evidence
supporting a potential 2D structure. The bond distances in the
pyrazole and benzene rings are evidence of a delocalised p system
(Table S1†). Additionally, they show a high degree of planarity,
as deduced from the dihedral angles of 4.1(1)ꢁ between both
rings.
Crystal structures of [H2pzR(1)][Cl] (Cl-1), [H2pzR(10)][BF4]
(BF4-10), [H2pzR(1)][ReO4] (ReO4-1), [H2pzR(1)][OTf] (OTf-1)
and [H2pzR(1)][PTS] (PTS-1)
The crystal packing of BF4-10 is shown in Fig. 6. The supra-
molecular arrangement is typical of a layered structure with
alternation of polar groups and hydrophobic chains. In the layer
the chains exhibit a trans orientation giving rise to a highly
interdigitated structure along the c axis. The long molecular axis
of the cation is tilted with respect to the normal layer. The
The BF4-10 salt from family II is proven to produce crystals of
sufficient quality for single-crystal X-ray analysis. However, for
the remaining compounds, I and III–VI, all attempts to grow
crystals were unsuccessful. We then tried to prepare and crys-
tallise related salts of cationic species containing one methox-
yphenyl substituent at the 3(5) position of the pyrazole ring,
which, as expected, crystallise more easily than compounds
containing long-chained cations. Therefore, crystals of Cl-1,
ReO4-1, OTf-1 and PTS-1 were obtained by slow vapour diffu-
sion of n-hexane into a dichloromethane solution of the corre-
sponding compound, and were characterised by single crystal
X-ray diffraction.
ꢁ
interlayer separation is 19.1(1) A.
The straight-chain nature of the alkyl groups is maintained
along the structure with carbon chain torsion angles approaching
180ꢁ. The chain configuration and the lack of any disorder in the
structure appear to be a consequence of the interdigitated
molecular packing. The formation of separate hydrophilic and
hydrophobic structural parts has been proved to be useful for the
formation of liquid-crystalline phases.17
The molecular structure of Cl-1, BF4-10, ReO4-1, OTf-1 and
PTS-1 as representative examples of families I, II, III, V and VI
are shown in Fig. 1–5. The bond lengths and angles are recovered
Table 3 Chemical shifts in the aromatic region for the salts containing 8
carbon atoms in the substituent alkyl chain at a constant solution
concentration of 6.7 ꢃ 10ꢀ3
M
[A]
H5
H4
Hortho
Hmeta
Clꢀ
BF4
7.91
8.08
7.93
7.87
8.10
8.05
6.72
6.82
6.79
6.74
6.79
6.70
7.94
7.68
7.72
7.63
7.70
7.73
7.02
7.03
7.04
7.01
7.03
6.97
ꢀ
ꢀ
ReO4
ꢀ
SbF6
OTf
PTS
Fig. 1 Ortep plot of [H2pzR(10)][BF4] BF4-10 with 30% probability.
Hydrogen atoms, except H1 and H2, have been omitted for clarity. The
dimer structure is depicted in the inset.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13243

View Online
Fig. 2 Ortep plot of [H2pzR(1)][Cl] Cl-1 with 40% probability. Hydrogen
atoms, except H1 and H2, have been omitted for clarity. The dimer
structure is depicted in the inset.
Fig. 5 Ortep plot of [H2pzR(1)][ ReO4] ReO4-1 with 30% probability.
Hydrogen atoms, except H1 and H2, have been omitted for clarity.
within the expected range (Tables S2–S4†). The pyrazolium head
group and the phenyl substituent are almost in the same plane for
PTS-1 and OTf-1 as they were observed in the BF4-10 derivative.
However, both planes are tilted by 21.5(1)ꢁ for Cl-1. Following the
alternation of the polar and hydrophobic parts found for BF4-10,
the [H2pzR(1)]+ cation present in the three salts can be considered as
containing an hydrophilic part (the pyrazolium group) and
a hydrophobic methyl group from the p-methoxyphenyl substit-
uent, which are in close proximity. The cations are bonded to their
corresponding anions by N–H/X H-bonds (X ¼ O for PTS-1
and OTf-1, Cl for Cl-1) and, in the three cases, dimers held by N–
H/X H-bonds between neighbouring units are formed (Table 5;
Fig. 2–4). From this point, similar structural packing features
were found for PTS-1 and OTf-1, but were different from those of
the Cl-1 salt. In PTS-1 and OTf-1, columns of dimers are defined
from weak C–H/O interactions, but a 2D arrangement is
obtained for Cl-1 through additional C–H/O H-bonds.
The columns in PTS-1 and OTf-1 are arranged in layer-like
arrays. The layers in the ac and ab planes, respectively (Fig. 7 and
Fig. S1†), can be defined as holding together rows of the cationic
part of the pyrazolium and the SO3 rows of the triflate or
p-toluenesulfonate groups in OTf-1 and PTS-1, respectively. The
remaining fragments, CF3 from the triflate and C6H4OCH3 from
the p-toluenesulfonate, are located each side of the layer pointing
out in opposite directions and are tilted towards the main plane
of the layer.
Fig. 3 Ortep plot of [H2pzR(1)][OTf] OTf-1 with 20% probability.
Hydrogen atoms, except H1, have been omitted for clarity. The dimer
structure is depicted in the inset.
The molecular structure of Cl-1 is displayed in Fig. 2. As was
mentioned, the [H2pzR(1) +
cation exhibits similar structural
]
characteristics to those presented in the related salts with OTf or
PTS, except for the dihedral angle between the pyrazolium and
benzene rings. So, each cation is NH-bonded to its chlorine
anion, and is again NH-bonded to the neighbouring unit,
generating bridge-chlorine dimers in which the Cl/N are prac-
tically identical (Table 5; Fig. 2). However, in this case, in
contrast to that observed for OTf-1 and PTS-1, each dimer is
surrounded another four and is bonded to them by weak C–H/
O interactions between the central carbon atom of the pyr-
azolium ring and the oxygen of the methoxy group, generating
a corrugated layer which is extended in the ac plane (Fig. 8). The
Fig. 4 Ortep plot of [H2pzR(1)][PTS] PTS-1 with 30% probability.
Hydrogen atoms, except H1, H2, H3, H4, H5 and H6, have been omitted
for clarity. The dimer structure is depicted in the inset.
The single crystal X-ray structures of representative
compounds of families I, V and VI show that Cl-1 crystallises
into the orthorhombic system, group Pbca, while PTS-1 and
ꢁ
layers are separated from one another by a distance of 5.4 A, and
no short contacts were found between layers.
ꢂ
OTf-1 crystallises into the triclinic one, group P(1). The
The supramolecular 2D structure of the chlorine salts of the
[H2pzR(1) +
] cation was clearly different to those of the related
salts with OTf, PTS or BF4ꢀ, which by contrast exhibit a similar
compounds Cl-1 and OTf-1 present one cation and one anion in
the asymmetric unit, while three cations and three anions are
observed for PTS-1 (Fig. 2–4). Bond lengths and angles are well
layer-like array of columns of cations and anions. For the latter
13244 | J. Mater. Chem., 2012, 22, 13239–13251
This journal is ª The Royal Society of Chemistry 2012

View Online
ꢁ
Table 5 Hydrogen bond geometries (lengths in A and angles in degrees) for Cl-1, BF4-10, ReO4-1, OTf-1 and PTS-1
Compound
D–H/A
d(D–H)
d(H/A)
d(D/A)
<(D–H/A)
Cl-1
N1–H1/Cl
0.92
0.92
0.93
1.05
1.08
0.93
0.90
0.96
0.93
1.05
1.04
0.93
0.98
0.97
1.09
1.00
1.04
0.94
0.93
0.93
2.11
2.16
2.57
1.77
1.77
2.40
1.88
1.78
2.44
1.76
1.89
2.46
1.82
1.90
1.67
1.93
1.73
1.91
2.35
2.37
3.019(3)
3.055(2)
3.32(1)
2.805(4)
2.774(4)
3.28(1)
2.751(6)
2.703(6)
3.20(1)
2.791(5)
2.817(5)
3.38(1)
2.752(6)
2.790(6)
2.754(5)
2.767(6)
2.757(6)
2.741(6)
3.24(1)
172.4
163.6
138.4
166.8
151.7
158.0
163.2
160.2
138.1
166.6
147.2
171.0
156.7
151.0
170.3
140.1
165.3
146.4
160.4
163.3
N2–H2/Cla
C4–H4/O1b
N1–H1/F2
N2–H1/F1c
C3–H3/F4d
N2–H2/O4
N1–H1/O3e
C5–H5/O3d
N1–H1/O2
N2–H2/O3f
C4–H4/O4d
N1–H1/O4
N2–H2/O6
N3–H3/O8
N4–H4/O2
N5–H5/O12
N6–H6/O10g
C38–H38/O11h
C21–H21/O7i
BF4–10
ReO4–1
OTf-1
PTS-1
3.28(1)
a
ꢀx + 1, ꢀy + 2, ꢀz + 1. b x ꢀ ½, y, ꢀz + ½. c ꢀx ꢀ 1, ꢀy + 1, ꢀz + 1. d x, y + 1, z. e x + ½, ꢀy + ½, ꢀz. f ꢀx, ꢀy + 1, ꢀz + 2. g ꢀx, ꢀy + 1, ꢀz + 1. h x +
1, y, z. i x ꢀ 1, y, z.
the ones, in the long or short chain pyrazolium compounds,
previously mentioned. In the same way as in the previous salts,
ꢀ
each cation is strongly H-bonded to its own ReO4 counter-
anion, but this unit does not produce dimers as in the previous
salts. In contrast, the pyrazolium cations and ReO4ꢀ anions form
double chains by stacking along the a axis through N–H/O H-
bonds involving two NH groups per cation and two oxygen
atoms per anion (Table 5). In each double chain, the cations
stack in such a way that the methoxyphenyl substituents alter-
Fig. 6 Packing of BF4-10 in the bc plane.
nate in orientation, giving rise to a herringbone-type chain
structure (Fig. 9). These double chains form a layer-like array in
the ac plane, in which neighbouring chains have opposite
distributions showing a clear interdigitation (Fig. 9).
In addition, a 2D bilayer structure can be defined in the ab
plane by new weak C–H/O interactions between chains. The
cations are oriented in a such way that an angle of 44.6ꢁ is
adopted between the bilayer plane and the benzene plane. The
Fig. 7 Packing of PTS-1 in the bc plane.
groups of salts, the interlayer distance was determined by the
anion, and the longest distances were produced by the bulkiest
anions (OTf, PTS).
Compound ReO4-1 crystallises into the orthorhombic system,
space group P212121. The asymmetric unit contains a crystallo-
ꢀ
graphically independent cation [H2pzR(1)]+ and an ReO4 anion
(Fig. 5).
The bond lengths and angles in the organic cations are broadly
within the expected range (Table S5†) and can be compared to
Fig. 8 View of the 2D structure of Cl-1 (a) in the ac plane and (b) in the
bc plane.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13245

View Online
ꢁ
bilayer thickness is 15 A and the separation between neigh-
process from the isotropic liquid, the formation of ba^tonnets was
observed, which, upon further cooling, gave rise to a fan-shaped
texture together with homeotropic areas (Fig. 10a–c). In addi-
tion, during heating, the formation of oily streaks upon defor-
mation of the samples confirms the identification of the
mesophase (Fig. 10d).
ꢁ
bouring bilayers is 12.5 A, suggesting certain interpenetration.
The above structures can be extrapolated to those of related
long-chained ionic salts [H2pzR(n)][A] (n ¼ 8–18), so that the H-
bonds responsible for the layer-like structure for BF4ꢀ salts, and
even the 2D structures in Clꢀ and ReO4 derivatives, can be
ꢀ
considered as an important factor for achieving the supramo-
lecular ordering of the fluid phases observed in the mentioned
ionic salts (see below). However, the anion size and the
morphology also play an important role, in such a way that the
bulkiest PTS and OTf anions prevent the supramolecular
arrangement of the fluid phases.
For the salts, including those with OTf (V) and PTS (VI) as
counteranions, the phase transition temperatures from the solid
to the isotropic liquid are increased by lowering the alkyl chain
length, indicating that the higher mobility of the longer substit-
uents disrupts the van der Waals interactions.
On heating, the DSC thermograms of compounds I–IV
revealed large enthalpy values for the Cr–Cr0 and Cr–SmA phase
transitions, as well as small enthalpy values which were consis-
tently observed for the SmA–I phase transitions. The related
reverse transitions on cooling exhibited the same behaviour and
are assigned to the I–SmA, SmA–Cr and Cr–Cr0 phase transi-
tions. A typical DSC thermogram of a mesomorphic compound,
Cl-10, is shown in Fig. 11.
Thermal behaviour
The thermal behaviour of all the new ionic compounds was
examined by polarised optical microscopy (POM) and differen-
tial scanning calorimetry (DSC). A detailed temperature-
dependent, low-angle powder X-ray diffraction study was also
performed for representative compounds of each family.
All the pyrazolium salts I–IV with n ¼ 8–18 show enantio-
tropic SmA mesophases, except the compound Cl-8 which
exhibits a monotropic SmA mesophase (Table 6). In contrast,
neither of the salts V or VI display liquid crystal behaviour
(Table S6†), which is consistent with the absence of meso-
morphism observed in related imidazolium-based ionic liquid
compounds containing bulky counteranions.5,13,16,18
In general terms, all members of each mesomorphic family
(I–IV) exhibit a similar pattern upon heating and cooling with
the exception of ReO4-10, ReO4-12 and ReO4-14 compounds for
which the phase transition involving solidification from the
mesophase could not be observed neither by the DSC thermo-
grams nor by POM observations. Further POM, DSC and
powder X-Ray diffraction experiments were carried out in order
to achieve new results. A description of the particular study of
these compounds is now presented.
The mesomorphic behaviour observed in compounds I–IV
shows significant differences with that of some other related
salts. For instance, the melting and clearing temperatures were
higher than those of 1-alkyl-3-methylimidazolium and 1-(4-
alkyloxybenzyl)-3-methylimidazolium with the same counter-
ions.3,4,11,14–16 The presence of H-bonding interactions, supported
by the X-ray crystalline structures described above, appears to be
responsible for the stabilization of the ordered phases.
Additonally, the mesomorphism of pyrazolium salts contain-
ing Clꢀ as counterion (I) contrasts with that of related salts
containing a pyridinium head, for which protonation on the
heterocyclic nitrogen and the presence of the Clꢀ counterion
constituted an impediment to acquiring liquid crystal
properties.22,23
New DSC thermograms of ReO4-10, ReO4-12 and ReO4-14
were recorded after heating the samples just above the melting
point and then cooling down from this temperature. In this way,
we were able to register an endothermic peak related to the
mesophase–solid phase transition, in agreement with the solid
phase observed by POM upon cooling from the mesophase when
the same thermal route was used. We also carried out new POM
studies by modifying the experimental conditions, as are now
described for ReO4-12. A sample of this compound was slowly
cooled down at 1 ꢁC minꢀ1 from the liquid, in 20 ꢁC steps,
holding the temperature for several hours after each step. The
formation of a solid phase was observed at 90 ꢁC after 18 hours.
A sample of the same compound was heated to the isotropic
phase and then quenched to room temperature, giving rise to
a solid phase, considered as a supercooled phase, which main-
tained the mesophase texture. Analogous temperature programs
were followed for X-Ray diffraction measurements in order to
confirm the existence of different final phases depending on the
cooling rate, which will be described later.
All the compounds I–VI exhibit a rich polymorphism, showing
several crystalline phases deduced from the DSC thermograms
and POM observations.
In all cases, the mesophases were undoubtedly recognised as
SmA on the basis of their optical textures. During the cooling
At this point of the manuscript, for a better understanding of
the thermal behaviour each type of compound and for compar-
ative purposes, we are going to present the results for each family
(I–IV).
I (Cl-n): all the chloride salts displayed enantiotropic meso-
morphism with exception of compound Cl-8 which exhibited
a monotropic mesophase. The melting and clearing tempera-
tures showed only small variations, including a slight increase in
the liquid crystal range with increasing alkyl chain length from
n ¼ 10 until n ¼ 14 and a decrease for n > 14. Thus, the
compound of the family with the widest mesophase range is
Cl-14 (Fig. 12a).
Fig. 9 Packing of the ReO4-1 in the ac plane.
13246 | J. Mater. Chem., 2012, 22, 13239–13251
This journal is ª The Royal Society of Chemistry 2012

View Online
Table 6 Thermal data of compounds of the families I (Cl-n), II (BF4-n), III (ReO4-n) and IV (SbF6-n)a
DH/kJ
molꢀ1
Compound
Transition
T/ꢁC
DH/kJ molꢀ1
Compound
Transition
T/ꢁC
Cl-8
Cr / I
150
150
139
150
166
157
135
97
144
170
168
133
140
170
162
128
99
140
167
148
123
105
138
166
158
129
103
86
30.6
ꢀ2.4
ꢀ14.7
23.1
2.1
ꢀ1.6
ꢀ12.5
ꢀ3.0
27.1
1.9
ꢀ1.7
ꢀ21.4
32.8
1.5
ꢀ1.1
ꢀ16.5
ꢀ4.4
41.0
1.7
ꢀ0.8
ꢀ14.0
ꢀ8.7
43.7
1.3
ꢀ0.8
ꢀ25.3
ꢀ5.7
17.0
1.0
ReO4-8
Cr / Cr0
124
155
164
162
113
118
190
183
64c
118
208
206
78
27.2
23.2
2.0
ꢀ2.0
ꢀ22.8
44.0
1.7
ꢀ0.9
ꢀ28.3
53.6
1.1
I / SmA
SmA / Cr
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
Cl-10
ReO4-10
ReO4–12
Cl-12
Cl-14
ꢀ0.7
ꢀ40.3b
ReO4-14
ReO4-16
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
114
195
189
70
43.0
0.6
ꢀ0.5
ꢀ38.8
3.6
Cl-16
Cl-18
87
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
112
162
162
108
61
113
148
147
104
78
45.0
0.6
ꢀ0.5
ꢀ7.4
ꢀ23.9
67.4
0.3
ꢀ0.2
ꢀ10.5
ꢀ25.4
0.2
ReO4-18
SbF6-8
Cr / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
BF4-8
110
106
80
ꢀ1.0
Cr / Cr0
100
112
147
d
Cr0 / Cr00
Cr00 / SmA
SmA / I
I / SmA
SmA / Cr
Cr0 / Cr0
0.2
11.8b
BF4-10
80
12.7
25.2
1.4
ꢀ1.5
ꢀ31.4
10.8
23.8
1.0
ꢀ1.0
ꢀ33.8
13.5
28.6
1.0
ꢀ0.9
ꢀ39.2
62.3
0.4
123
165
162
100
91
122
182
181
87
144
80
91
104
151
ꢀ0.6
ꢀ3.1
2.8
20.0
17.6b
SbF6-10
BF4-12
BF4-14
BF4-16
BF4–18
Cr / Cr00
Cr00 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
144
ꢀ14.2b
98
91
114
153
ꢀ18.7
122
187
186
91
104
179
175
90
SbF6-12
SbF6-14
Cr / Cr0
25.1
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
26.8b
142
132
88
113
144
166
155
100
87
118
129
209
121
132
199
175
99
ꢀ1.3
ꢀ25.1
ꢀ19.1
31.3
27.9
1.0
ꢀ0.4
ꢀ16.1
ꢀ13.2
49.4b
ꢀ0.7
ꢀ53.5
ꢀ2.2
63.7
0.2
Cr / Cr0
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
Cr / Cr0
78
102
162
144
81
ꢀ0.2
ꢀ69.7
SbF6-16
SbF6-18
Cr / Cr0
Cr0 / SmA
SmA / I
Cr / Cr0
dec.
72b
Cr0 / SmA
SmA / I
I / SmA
SmA / Cr
0.5
ꢀ0.5
ꢀ37.9
a
d
Temperatures from the DSC peak. b Overlapped processes. c Solidification observed on the second heating process. Temperature given by POM.
II (BF4-n): in this family the melting temperatures are lower
than those observed for the related Clꢀ salts and again, they were
almost unmodified by increasing the alkyl chain length (except
BF4-8), but showed a slight decrease for long-tailed derivatives.
In contrast, the clearing temperatures increased significantly
until n ¼ 14 and then they decreased. As a consequence, the
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13247

View Online
Fig. 10 POM photomicrographs of: (a) BF4-8 at 102.0 ꢁC during cooling; (b) ReO4-12 at 195.0 ꢁC during cooling; (c) SbF6-18 at 138.3 ꢁC during
cooling; (d) Cl-18 at 147.0 ꢁC during heating.
Two effects were taken into account in order to discuss the
different thermal behaviour of the compounds I–IV: the effect of
the counterion and the effect of the alkyl chain length. Consid-
ering the effect of the counterion on the transition temperatures,
we found that the geometry and size seem to modulate the
melting temperatures of the new salts. The spherical or quasi-
spherical Clꢀ and SbF6ꢀ anions give rise to higher melting points
in their salts (I and IV), which is consistent with a more
favourable packing in the solid state. In contrast, the salts con-
ꢀ
ꢀ
taining the tetrahedral BF4 and ReO4 anions (II and III)
exhibit lower melting temperatures as a consequence of an
apparently less favoured packing (Fig. 12). The anion size was
also important, with the bulkiest ones giving rise to the lowest
melting temperatures. So, considering both effects, the melting
Fig. 11 DSC trace for Cl-10.
ꢀ
ꢀ
temperatures were found to be in the order ReO4 # BF4
SbF6ꢀ < Clꢀ for a given cation (Fig. 12).
<
mesophase range reaches the highest value for the compound
BF4-16 (Fig. 12b).
Regarding the clearing transition, the compounds I and IV,
containing spherical anions, exhibit, in general terms, lower
clearing temperatures than those of the related derivatives II and
III, with tetrahedral anions. This fact suggests an increase in the
cation–cation electrostatic repulsions for the first case. Among
the latter (II and III), the bulky ReO4ꢀ group should disrupt the
cationic proximity more easily, therefore decreasing the cationic
repulsion and so giving rise to higher clearing temperatures.
However, this feature was observed only in the compound family
III when n # 14 carbon atoms in the hydrophobic tail, while for
longer chains an opposite variation was exhibited.
III (ReO4-n): the melting points of the compounds of this
series are, in general terms, slightly lower than those observed for
ꢀ
the analogue BF4 salts and follow the same pattern by the
increasing the alkyl chain length. However, the clearing points
depend strongly on this variable, giving rise to a mesophase
stability range which reaches the highest value for the compound
ReO4-12 (Fig. 12c).
IV (SbF6-n): the clearing and melting temperatures are close
to those of the Clꢀ salts, although for the longest alkyl chain
substituted compounds the liquid crystalline range expands due
to a slight decrease of the melting temperatures and a significant
increase of the clearing points, leading to wider mesophase
ranges for compounds SbF6-16 and SbF6-18 (Fig. 12d).
When studying the effect of the alkyl chain length, some
features have been determined. As the first, compounds con-
taining alkyl chains with n ¼ 8 appear to deviate from the general
trend, as is shown by the monotropic behaviour of Cl-8 or the
13248 | J. Mater. Chem., 2012, 22, 13239–13251
This journal is ª The Royal Society of Chemistry 2012

View Online
Fig. 12 Bar diagram comparing the stability ranges of mesophases for each family.
mesophase ranges exhibited by BF4-8 and ReO4-8, which are
narrower than those of the remaining derivatives in their fami-
lies. This feature suggests that the driving forces for the forma-
tion of the liquid crystalline state require a chain length of n $ 8
to be optimised. As we commented, in the remaining compounds,
the melting temperatures seem to be modulated by the anion
shape and size while the clearing points are governed by both
counteranion nature and cation length. Thus, for cations con-
taining short chains (n ¼ 10, 12, 14), the lowest melting
temperatures and the highest clearing temperatures, and so the
widest mesophase ranges, correspond to the salts with tetrahe-
dral anions, BF4ꢀ and ReO4ꢀ (Fig. 13a–c). However, going to the
longest cation-containing compounds, with n ¼ 16 and 18, the
clearing temperatures remain almost constant for Clꢀ salts but
tend to decrease for tetrahedral anion containing salts (showing
In summary, taking into account all the above results we can
establish that the best results for the mesophase emerging and its
stability are found for compounds in the order ReO4-12 and
ReO4-14 > BF4-16 and BF4-18 $ SbF6-16 and SbF6-18.
In relation to ionicity of the studied systems, the evidence of
the liquid crystal behaviour established for the pyrazolium salts,
[H2pzR(n)][A], which, by contrast was absent in their corre-
sponding neutral pyrazoles, HpzR(n), exclude the dissociation of
the protonated cationic form, at least in certain extension. The
establishment of this premise should be a motive for new studies
taking into account results previously described.34,35
Temperature-dependent powder X-ray diffraction studies
Selected mesomorphic derivatives of families I–IV were subjected
to temperature-dependent powder X-ray diffraction (XRD)
measurements. In all cases, the XRD patterns at the mesophase
temperature show, in the low-angle region, two to four sharp
peaks in a 1 : 2 : 3 : 4 ratio indexed as the (001), (002), (003) and
(004) reflections, which are indicative of a lamellar structure. In
ꢀ
an opposite variation in the order BF4 > ReO4ꢀ) and, finally,
ꢀ
they are markedly increased in SbF6 derivatives (Fig. 13d–e).
For these longest compounds, as the melting temperatures are
almost unaffected by the alkyl length growing, the widest mes-
ꢀ
ophase ranges among long tailed salts are observed in SbF6
ꢁ
addition, a broad halo in the medium-angle region, at ca. 4.5 A
derivatives.
Therefore, for the longest compounds, the van der Waals
interactions between the hydrophobic tails over the electrostatic
forces between ions in the mesophase must be a predominant
effect. In contrast, the electrostatic repulsions appear to be
determinant in compounds with the shortest chain lengths,
corresponds to the liquid-like order of the molten alkyl chains.
The results are summarised in Table 7. Fig. 14 shows the
diffraction pattern of compound ReO4-16 as a representative
example.
An interesting feature was observed by comparing the layer
spacing of compounds with equal alkyl chain lengths and different
counterions. A longer layer spacing was observed for ReO4-8 than
that of its homologue, BF4-8, in agreement with the larger size of
the perrhenate anion. This result appears to confirm the proposal
concerned with the higher clearing points found for ReO4ꢀ salts
due to the lower cationic proximity in the mesophase.
yielding higher clearing points for salts containing tetrahedral
ꢀ
anions. The singular order of the clearing temperatures, SbF6
>
ꢀ
BF4 > ReO4ꢀ, found for long-tailed derivatives, requires an
additional explanation, which emerges by considering the influ-
ence of the hydrogen bonding as a support of the interactions
that should contribute to govern the mesophase.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13249

View Online
Fig. 13 Bar diagram comparing the stability ranges of mesophases for each carbon atom in the chain.
Because of the singular behaviour of perrhenate derivatives
upon cooling, we carried out a further XRD study for ReO4-12
as a representative example, following analogous temperature
programs to those used in the POM observations (see thermal
behaviour section). In an initial experiment, a sample was slowly
cooled down (1 ꢁC minꢀ1) to room temperature (25 ꢁC) after
reaching the isotropic liquid phase (215 ꢁC). The diffraction
patterns obtained each 10–20 ꢁC were characteristic of a smectic
mesophase until 90 ꢁC. However, upon further cooling from this
temperature, the intensity of the sharp peaks at the low angle
room temperature. In a second experiment, a sample was
quenched to room temperature from the isotropic phase. In that
ꢁ
case, the diffractogram at 25 C, showing 3 sharp peaks at the
low angle region, and a broad halo in the middle angle region,
was characteristic of a smectic mesophase, suggesting the pres-
ence of a supercooled mesophase. This feature agrees with the
POM observation of a fan-shape texture at room temperature
after following a similar cooling cycle.
Conclusions
ꢁ
region gradually decreased and finally disappeared at 25 C in
New mesomorphic compounds based on long alkyl chain-
substituted pyrazolium cations with different counterions have
been prepared and characterised as mesomorphic materials. We
have established that the melting temperatures are modulated by
the shape of the counterion while both the size and shape of the
anion and the length of the alkyl chain should be considered for
the clearing temperatures. The spherical or pseudo spherical
shape of Clꢀ and SbF6ꢀ anions determines a favourable packing
in the solid and a cation–cation proximity in the mesophase,
which leads to higher melting temperatures but lower clearing
agreement with the formation of an amorphous solid phase at
Table 7 X-Ray diffraction data of selected compounds
ꢁ
d-spacing/A
Comp.
T/ꢁCa
Position/2q
Miller indices (hkl)
Cl-18
135
2.3
4.5
18.5
3.0
6.0
19.5
2.8
5.3
7.9
10.6
20.7
2.6
4.7
7.0
9.4
19.5
2.3
4.4
20.1
2.4
4.8
18.9
38.7
19.5
4.8
29.4
14.7
4.5
31.6
16.7
11.1
8.3
(001)
(002)
b
BF4-8
90
(001)
(002)
b
ReO4-8
140
(001)
(002)
(003)
(004)
b
4.3
ReO4-12
130
34.5
18.9
12.6
9.4
(001)
(002)
(003)
(004)
b
4.5
ReO4-16
SbF6–10
140
150
38.7
19.9
4.4
36.5
18.4
4.7
(001)
(002)
b
(001)
(002)
b
a
Upon cooling. b Halo of the molten alkyloxy chains.
Fig. 14 X-Ray diffraction pattern of compound ReO4-16.
This journal is ª The Royal Society of Chemistry 2012
13250 | J. Mater. Chem., 2012, 22, 13239–13251

View Online
A. Taubert, C. Palivan, O. Casse, F. Gozzo and B. Schmitt, J.
Phys. Chem. C, 2007, 111, 4077; A. Taubert, I. Arbell, A. Mecke
and P. Graf, Gold Bull., 2006, 39, 205; Q. T. Wang, X. B. Wang,
W. J. Lou and J. C. Hao, ChemPhysChem, 2009, 10, 3201.
10 W. Dobbs, B. Heinrich, C. Bourgogne, B. Donnio, E. Terazzi,
M.-E. Bonnet, F. Stock, P. Erbacher, A.-L. Bolcato-Bellemin and
L. Douce, J. Am. Chem. Soc., 2009, 131, 13338.
temperatures, respectively. The opposite effect is observed in the
ꢀ
ꢀ
derivatives containing the tetrahedral BF4 and ReO4 anions.
In those compounds, the van der Waals interactions between
chains are in competition with the electrostatic effects depending
on the size of the counterions as well as the alkyl chain length. In
ꢀ
particular, the use of tetrahedral anions, ReO4 combined with
11 W. Dobbs, L. Douce and B. Heinrich, Beilstein J. Org. Chem., 2009, 5,
62.
cations containing alkyl chains with 10 to 14 carbon atoms or
ꢀ
ꢀ
BF4 bonded to long-chained substituted cations (n > 14),
12 M. Trilla, R. Pleixats, T. Parella, C. Blanc, P. Dieudonne, Y. Guari
and M. W. C. Man, Langmuir, 2008, 24, 259.
13 A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston,
S. E. J. McMath and M. Nieuwenhuyzen, Chem. Mater., 2002, 14,
629.
14 C. J. Bowlas, D. W. Bruce and K. R. Seddon, Chem. Commun., 1996,
1625.
15 C. M. Gordon, J. D. Holbrey, A. R. Kennedy and K. R. Seddon, J.
Mater. Chem., 1998, 8, 2627; J. D. Holbrey and K. R. Seddon, J.
Chem. Soc., Dalton Trans., 1999, 2133.
16 W. Dobbs, L. Douce, L. Allouche, A. Louati, F. Malbosc and
R. Welter, New J. Chem., 2006, 30, 528.
produce better results than the use of spherical or quasi spherical
ꢀ
Clꢀ and SbF6 anions.
Structural relationships between the solid phase and the mes-
ophase have been found. The pyrazolium ionic salts exhibited 2D
or layer-like crystalline structures defined by strong and weak
hydrogen bond interactions, which can be related to those of the
lamellar mesophases observed in the Clꢀ, BF4ꢀ and ReO4ꢀ salts.
The bulkiest anions, OTf and PTS, expanded the thickness of the
layers by their CF3 or C6H4CH3 groups, respectively, included
laterally side by side. As a consequence, there is difficulty in
achieving the H-bonding interactions responsible for the supra-
molecular ordering of the fluid phases, which results in the
absence of mesomorphism for these OTf and PTS derivatives. In
contrast, the 2D, layered, crystalline structures of the ReO4ꢀ salts
agree with the best liquid crystal properties. Nevertheless, the
corrugated 2D, layered structure of Clꢀ derivatives seems to be
responsible for less extensive liquid crystal ranges in these salts.
17 A. Getsis and A.-V. Mudring, Cryst. Res. Technol., 2008, 43, 1187.
18 K.-M. Lee, Y.-T. Lee and I. J. B. Lin, J. Mater. Chem., 2003, 13,
1079.
19 K. Goossens, K. Lava, P. Nockemann, K. Van Hecke, L. Van
€
Meervelt, K. Driesen, C. Gorller-Walrand, K. Binnemans and
T. Cardinaels, Chem.–Eur. J., 2009, 15, 656.
20 V. Causin and G. Saielli, J. Mater. Chem., 2009, 19, 9153.
21 V. Causin and G. Saielli, J. Mol. Liq., 2009, 145, 41.
€
22 E. J. R. Sudholter, J. B. F. N. Engberts and W. D. de Jeu, J. Phys.
Chem., 1982, 86, 1908.
23 J. J. H. Nusselder, J. B. F. N. Engberts and H. A. Van Doren, Liq.
Cryst., 1993, 13, 213.
24 M. C. Torralba, M. Cano, J. A. Campo, J. V. Heras, E. Pinilla and
M. R. Torres, J. Organomet. Chem., 2001, 633, 91.
Acknowledgements
25 M. C. Torralba, M. Cano, J. A. Campo, J. V. Heras, E. Pinilla and
M. R. Torres, Inorg. Chem. Commun., 2002, 5, 887.
26 M. C. Torralba, M. Cano, J. A. Campo, J. V. Heras, E. Pinilla and
M. R. Torres, J. Organomet. Chem., 2002, 654, 150;
ꢀ
We thank the Ministerio de Economıa y Competitividad (Spain),
projects CTQ2010-19470 and CTQ2011-25172, and Universidad
Complutense de Madrid (Spain), project GR35/10-A-910300, for
ꢀ
M. C. Torralba, M. Cano, S. Gomez, J. A. Campo, J. V. Heras,
ꢀ
financial support. I. S. thanks the Ministerio de Educacion
J. Perles and C. Ruiz-Valero, J. Organomet. Chem., 2003, 682, 26.
27 M. C. Torralba, P. Ovejero, M. J. Mayoral, M. Cano, J. A. Campo,
J. V. Heras, E. Pinilla and M. R. Torres, Helv. Chim. Acta, 2004, 87,
250; M. C. Torralba, M. Cano, J. A. Campo, J. V. Heras and
E. Pinilla, Z. Kristallogr.-New Cryst. Struct., 2005, 220, 615.
(Spain) for the predoctoral scholarship from the FPU program.
Notes and references
28 M. J. Mayoral, P. Ovejero, J. A. Campo, J. V. Heras, E. Pinilla,
M. R. Torres, C. Lodeiro and M. Cano, Dalton Trans., 2008, 6912.
29 M. J. Mayoral, P. Ovejero, J. A. Campo, J. V. Heras, M. R. Torres,
C. Lodeiro and M. Cano, New J. Chem., 2010, 34, 2766.
1 P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3773;
K. R. Seddon, J. Chem. Technol. Biotechnol., 1997, 68, 351;
T. Welton, Chem. Rev., 1999, 99, 2071.
2 I. J. B. Lin and Ch. S. Vasam, J. Organomet. Chem., 2005, 690, 3498.
3 K. Binnemans, Chem. Rev., 2005, 105, 4148.
4 K. V. Axenov and S. Laschat, Materials, 2011, 4, 206.
5 M. Yoshio, T. Ichikawa, H. Shimura, T. Kagata, A. Hamasaki,
T. Mukai, H. Ohno and T. Kato, Bull. Chem. Soc. Jpn., 2007, 80,
1836.
ꢀ
30 J. Barbera, C. Cativiela, J. L. Serrano and M. M. Zurbano, Liq.
Cryst., 1992, 11, 887.
31 M. J. Mayoral, P. Ovejero, J. A. Campo, J. V. Heras, E. Pinilla,
M. R. Torres and M. Cano, Inorg. Chem. Commun., 2009, 12, 214.
32 G. M. Sheldrick, SHELX97, Program for Refinement of Crystal
€
€
Structure, University of Gottingen, Gottingen, Germany, 1997.
33 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, John Wiley & Sons, New York, 5th edn,
1986.
6 M. Yoshio, T. Mukai, H. Ohno and T. Kato, J. Am. Chem. Soc.,
2004, 126, 994.
7 T. Kato, N. Mizoshita and K. Kishimoto, Angew. Chem., Int. Ed.,
2006, 45, 38.
34 D. R. MacFarlane and K. R. Seddon, Aust. J. Chem., 2007, 60, 3.
35 D. R. MacFarlane, J. M. Pringle, K. M. Johansson, S. A. Forsyth and
M. Forsyth, Chem. Commun., 2006, 1905.
8 T. Welton, Coord. Chem. Rev., 2004, 248, 2459.
9 W. Dobbs, J.-M. Suisse, L. Douce and R. Welter, Angew. Chem., Int.
Ed., 2006, 45, 4179; A. Taubert and Z. Li, Dalton Trans., 2007, 723;
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13239–13251 | 13251