Designer ionic liquid crystals based on congruently shaped guanidinium sulfonates

Article abstract of DOI:10.1002/chem.201101925

Ionic liquid crystals are mesogenic compounds that consist of cations and anions, usually rod-like cations and spherical anions. Herein we report a new method for the synthesis of ionic liquid crystals by using cations and anions of the same molecular shape with oppositely charged head groups. Thus, 4-alkoxyphenylpentamethylguanidinium 4-alkoxyphenylsulfonate ion pairs have been synthesised. 4-Alkoxyphenylpentamethylguanidinium iodides were also prepared to determine the influence of congruently shaped anions, in comparison with their spherical counterparts, on mesophase behaviour, which was investigated by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). All the liquid crystalline salts exhibit smectic A mesophases with strongly interdigitated bilayer structures. The guanidinium sulfonate ion pairs show mesomorphic properties from shorter alkyl chain lengths (≥C₉) and lower melting points (≈10 K), whereas the corresponding guanidinium iodides are liquid crystalline for longer alkyl chain lengths (≥C₁₄). For chains with ≥C₁₈, however, the mesophase range decreases for the sulfonate ion pairs, but not for the iodide salts. Tuning mesophase stability: Guanidinium sulfonate ion pairs with congruently shaped anions and cations with short alkyl chains R (see scheme) have favourable mesogenic properties with respect to mesophase stability in comparison with their guanidinium counterparts with spherical iodide anions. Copyright

Full text of DOI:10.1002/chem.201101925

DOI: 10.1002/chem.201101925
Designer Ionic Liquid Crystals Based on Congruently Shaped
Guanidinium Sulfonates
Martin Butschies, Wolfgang Frey, and Sabine Laschat*[a]
Dedicated to Professor Gerhard Erker on the occasion of his 65th birthday
Abstract: Ionic liquid crystals are
termine the influence of congruently
shaped anions, in comparison with
their spherical counterparts, on meso-
phase behaviour, which was investigat-
ed by differential scanning calorimetry
(DSC), polarising optical microscopy
(POM) and X-ray diffraction (XRD).
All the liquid crystalline salts exhibit
smectic A mesophases with strongly in-
terdigitated bilayer structures. The gua-
mesogenic compounds that consist of
cations and anions, usually rod-like cat-
ions and spherical anions. Herein we
report a new method for the synthesis
of ionic liquid crystals by using cations
and anions of the same molecular
shape with oppositely charged head
groups. Thus, 4-alkoxyphenylpentame-
thylguanidinium 4-alkoxyphenylsulfo-
nate ion pairs have been synthesised.
4-Alkoxyphenylpentamethylguanidini-
um iodides were also prepared to de-
nidinium sulfonate ion pairs show mes-
omorphic properties from shorter alkyl
chain lengths (ꢀC9) and lower melting
points (ꢁ10 K), whereas the corre-
sponding guanidinium iodides are
liquid crystalline for longer alkyl chain
lengths (ꢀC14). For chains with ꢀC18,
however, the mesophase range decreas-
es for the sulfonate ion pairs, but not
for the iodide salts.
Keywords: ion pairs · liquid crys-
tals · mesophases · X-ray diffraction
Introduction
In particular, bulky spherical anions led to a major disturb-
ance of the smectic bilayer structure, resulting in significant-
ly decreased phase stabilities. In contrast to the above-men-
tioned “inorganic” anions, alkyl sulfonate anions have been
used in ionic liquid crystals to a much lesser extent. Promi-
nent examples are the liquid crystals obtained by ionic self-
assembly that were developed by Faul,[6] Mꢀllen[7] and
Nicoud[8] and their co-workers. However, in these cases
mostly disk-shaped cations were combined with rod-shaped
sulfonate anions. In contrast, Zhang and co-workers devel-
oped potassium cholesterylsulfonate derivatives, which ex-
hibit smectic A mesophases.[9] Therefore, in this study, meso-
genic guanidinium cations were combined with mesogenic
sulfonate anions to study the influence of symmetry in the
anion and cation on the mesomorphic properties. We antici-
pated that the use of congruently rod-shaped anions and cat-
ions, rather than a combination of a rod-shaped cation and a
spherical anion, should lead to more efficient packing of the
smectic mesophase, resulting in increased mesophase widths
and easier tuning of the phase-transition temperatures by
adjusting the anion and cation chain lengths separately
(Figure 1).
Ionic liquid crystals are an emerging area in materials
chemistry, combining the properties of both ionic liquids, for
example, fluidity and viscosity, which can be adjusted by the
appropriate choice of anions and/or cations, low vapour
pressure, ionic conductivity, wide electrochemical range and
high thermal stability, with those of liquid crystals, such as
anisotropic physical properties, birefringence, order and mo-
bility. Thus, they can be considered as partially ordered
ionic liquids.[1] In particular, in the class of imidazolium
ionic liquids, the influence of substituents, spacer and termi-
nal groups on the mesomorphic properties of the cation has
been intensively investigated.[2] In contrast, studies of the
effect of the anion on the mesophase have mainly focused
on halides, pseudohalides or complex anions, such as BF4,
BPh4, PF6 and NTf2.[2a,3] One example of an unusual anion,
namely closo-carbaborate anions, was published by Kaszyn-
ski and co-workers.[4] We have recently shown that the
phase widths of the SmA phase for guanidinium salts are
much larger for halides than for BF4, BPh4, PF6 and SCN.[5]
To investigate this hypothesis, we have synthesised several
rigid core guanidinium cations and the corresponding con-
[a] Dipl.-Chem. M. Butschies, Dr. W. Frey, Prof. Dr. S. Laschat
Institut fꢀr Organische Chemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax : (+49)711-685-64285
gruent sulfonate anions 1ACTHNUTRGNE(UNG Cn/Cm) with various alkyl chain
lengths Cn/Cm (Scheme 1). As a reference material with
spherical anions, the corresponding guanidinium iodides
2(Cn) were synthesised and used as a basis of comparison
for the investigation of mesophase stabilities. The results are
discussed below.
E-mail: sabine.laschat@oc.uni-stuttgart.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101925.
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FULL PAPER
Figure 1. a) Model of a smectic phase with small spherical anions be-
tween the layers. b) Increasing the size of the anions leads to a destabili-
sation of the layer structure.c) Congruently mesogenic anions could stabi-
lise the mesophase.
Scheme 2. Synthesis of guanidinium sulfonate ion pairs 1ACTHNUTRGNE(UNG Cn/Cm). Re-
agents and conditions: i) R1Br, K2CO3, NaI, MeCN, reflux, 12 h; ii) KOH,
EtOH, reflux, 24 h; iii) tetramethylchloroformamidinium chloride,
NaHCO3, CH2Cl2, reflux, 1 h; iv) R2Br, KOH , DMSO, 608C, 24 h;
v) POCl3, DMF, DCM, 1 h; vi) NaOMe, MeOH, RT, 30 min; vii) KOH,
MeOH, MeCN, reflux, 24 h.
Scheme 1. Calamitic guanidinium sulfonate ion pairs 1
dinium salts 2(Cn) with spherical iodide anions.
ACHTUGNTRENN(UGN Cn/Cm) and guani-
Results and Discussion
tration. To obtain pure guanidinium chlorides 6 we used
flash chromatography with silica loaded with hydrogen chlo-
ride (for details, see the Supporting Information). All organ-
ic impurities could be removed by flushing with ethyl ace-
tate. Pure guanidinium chlorides 6 were obtained in excel-
lent yields by flushing with an ethyl acetate/methanol mix-
ture and a trace of aqueous hydrochloric acid.
Methyl sulfonates 10 (Scheme 2) were derived from the
commercially available sodium 4-hydroxybenzenesulfonate
(7), which was alkylated with the corresponding alkyl bro-
mide in dimethyl sulfoxide with potassium hydroxide at
moderate temperatures (608C) as a general synthetic route.
Due to their low solubility in nearly all solvents, the sodium
salts 8 were used in the next step without further purifica-
tion and characterisation; the salts 8 were chlorinated with
phosphoryl chloride in dimethylformamide and dichlorome-
thane. It was possible to purify the relatively stable sulfonyl
chlorides 9 by column chromatography (yields 54–99%).
Further treatment with sodium methoxide gave the corre-
sponding methyl 4-alkoxybenzenesulfonates (10) in excel-
lent yields (75–98%) and were used in the next step without
further purification.
Synthesis: The synthesis route to the guanidinium sulfonate
ion pairs 1(Cn/Cm) is shown in Scheme 2 (for details, see the
ACHTUNGTRENNUNG
Experimental Section). In contrast to our previous strategy
for the synthesis of guanidinium chlorides 6,[5b] we have de-
veloped a new strategy starting from N-(4-hydroxyphenyl)a-
cetamide (3). Alkylation of 3 with various alkyl bromides in
acetonitrile, potassium carbonate and a catalytic amount of
sodium iodide gave the corresponding N-(4-alkoxyphenyl)a-
cetamides (4; 68–90%). Amide cleavage was carried out in
ethanol with potassium hydroxide to give amines 5 in excel-
lent yields (91–97%). In contrast to our previously pub-
lished method, in which amines were formed by palladium-
catalysed reduction of nitro groups under 1 atm of hydro-
gen, often in low yields, we could now obtain quantitative
conversion. Amines 5 were subsequently treated with tetra-
methylchloroformamidinium chloride[10] in the presence of
sodium hydrogen carbonate in dichloromethane to form the
corresponding guanidinium chlorides 6 (84–99%). The use
of sodium hydrogen carbonate has two major advantages.
First, no additional organic salts were formed during the re-
action, as in the case of triethylamine, for which the separa-
tion of ionic liquid crystals from the Et3NH+Clꢂ turned out
to be rather difficult. Secondly, the guanidinium chlorides 6
were not deprotonated due to the weak basicity of the hy-
drogen carbonate. Therefore the crude guanidinium prod-
ucts and the inorganic salts could simply be separated by fil-
The final coupling reaction between the guanidinium
chlorides 6 and the methyl sulfonates 10 was carried out in
acetonitrile at reflux temperature (Scheme 2). The guanidi-
nium chlorides 6 were first deprotonated with potassium hy-
droxide in methanol to the corresponding guanidine base
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S. Laschat et al.
ꢂ
ꢂ
ꢂ
and in a second step heated at reflux with 10 for 24 h in ace-
tonitrile. Finally, recrystallisation from ethyl acetate gave
the N1 C1, N2 C1 and N3 C1 bond lengths in the guanidi-
ꢂ
nium cation is 1.34 ꢃ, significantly shorter than a C N
the pure guanidinium sulfonate ion pairs 1
N
single bond (1.47 ꢃ), which indicates a well-delocalised
system in the guanidinium head group. The anionic sulfo-
nate head group is located next to the positively charged
guanidinium head group with a clear linear electrostatic in-
teraction between H6C of the guanidinium group and O2 of
the sulfonate (d=2.58 ꢃ, a=1718). The packing diagram re-
veals a tilted bilayer structure with strongly interdigitated
alkyl chains. The aryl rings of the cation and anion are not
located parallel to each other and so no direct p–p stacking
could be observed. However, there are nearly linear weak
interactions between the protons of the guanidinium aryl
yields (75–98%). Thus, the methyl sulfonate 10 was used as
the methylation reagent and then as the anion at the same
time. Therefore, no further anion exchange was needed.
For comparison, the corresponding N-methylated guanidi-
nium iodides 2(Cn) with spherical anions were synthesised
(Scheme 3). Thus, guanidinium chlorides 6 were N-methylat-
ed by using methyl iodide with potassium carbonate in ace-
tonitrile to give the pure iodides 2(Cn) in excellent yields
(89–99%) after recrystallisation from ethyl acetate.
ꢂ
ring and the p system of the sulfonate aryl ring (e.g., C9 H9
of the cation and C27 of the anion with d=3.25 ꢃ and a=
1528). Therefore the aryl rings are almost perpendicular to
each other. Inside the layers, however, a parallel orientation
of all guanidinium aryl rings and of all sulfonate aryl rings,
respectively, could be observed. Due to the alternating ar-
rangement of cations and anions inside the layers, no direct
interaction between these parallel aryl systems was ob-
served.
Scheme 3. Synthesis of guanidinium iodides 2(Cn) with spherical anions.
Reagents and conditions: i) MeI, K2CO3, MeCN, 508C, 12 h.
In addition, it was possible to obtain single crystals of the
corresponding iodide 2
shown in Figure 3. The iodide 2ACTHNUTGRNE(UNG C12) also crystallises in mon-
(C12).[11] The crystal structure is
ACHTUNGTRENNUNG
Single-crystal investigation: It was possible to grow single
crystals of the guanidinium sulfonate ion pair 1
The corresponding crystal structure and packing diagram
are shown in Figure 2. Ion pair 1(C12/C14) crystallises in
monoclinic space group P21/n with a=8.6740(1), b=
49.350(7), c=10.9489(17) ꢃ, b=93.297(14)8, V=
4670.0(14) ꢃ3, Z=4 and Dcalcd =1.102 gcmꢂ3. The average of
G
oclinic space group P21/n with a=6.9372(5), b=49.334(4),
c=7.9828(6) ꢃ, b=98.659(3)8,
D
calcd =1.312 gcmꢂ3, Z=4
ꢂ
3
A
and V=2701.0(4) ꢃ . The average N C bond length of the
guanidinium head group is again 1.34 ꢃ. The smallest dis-
tance between the iodide and the head group is 3.13 ꢃ for
ꢂ
C3 H and so the anion is located close to the guanidinium
head group, as expected. The rod-like molecules are strongly
interdigitated with the aryl rings stacked in parallel. The
iodide anions are orientated in ion channels located between
the bilayers and surrounded by guanidinium cations.
The single-crystal data reveal that the packing organisa-
tion is similar in both 1ACTHUNGTRENNG(U C12/C14) and 2AHCTUNGTRNE(NUNG C12). Therefore the
crystal packing in the two compounds in the solid state ap-
pears not to be significantly affected by the use of con-
gruently shaped sulfonate anions instead of spherical iodide
anions.
Thermotropic properties: The ionic compounds 1ACHTUNGTRENNUNG(Cn/Cm)
and 2(Cn) were investigated by polarised optical microscopy
(POM) and differential scanning calorimetry (DSC). All the
mesophases of the guanidinium salts 1ACTHUNRGTNEUNG(Cn/Cm) and 2(Cn)
show homeotropic alignment over almost the entire meso-
phase range upon cooling from the isotropic melt. These
findings indicate a strong perpendicular orientation along
the glass plates and therefore a smectic A layer structure
was concluded.[13] In addition, a few focal conical domains
and maltese crosses, indicative of SmA phases, were ob-
served. Some representative textures are shown in Figure 4.
The results of the calorimetric investigations on 1
are summarised in Table 1 and visualised in Figure 5. Deriv-
ative 1(C8/C8) with the shortest alkyl chain lengths does not
show a mesophase and it melts into the isotropic phase at
ACHTUNGTRENNUNG(Cn/Cm)
Figure 2. a) X-ray structure of 1ACHTNUTRGNE(UNG C12/C14) in the solid state. b) View along
AHCTUNGTRENNUNG
the c axis of the molecular arrangement of 1ACHTNUTRGNE(UGN C12/C14) showing a bilayer
structure.[12]
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Designer Ionic Liquid Crystals
FULL PAPER
Figure 4. a) Typical texture of the guanidinium sulfonate ion pair 1ACHTUNGTRENNUNG(C16/
C16) at 1528C upon cooling from the isotropic melt with homeotropic
alignment and maltese crosses indicative of a SmA mesophase. b) Analo-
gous mesophase texture of the iodide 2ACTHUNTRGNE(UNG C16) with maltese crosses and de-
fects along air bubbles at 1678C. (Magnification: 200ꢄ; cooling rate:
10 Kminꢂ1.)
Figure 5. Mesophase stabilities of 1ACTHNUTRGNE(UNG Cn/Cm) upon second heating.
Figure 3. a) X-ray structure of 2ACTHNUTRGNE(NUG C12) in the solid state. b) View along the
crease in the clearing points from 106 to 1608C was ob-
served, the clearing transition seems to reach an upper limit
c axis of the molecular arrangement of 2ACTHNUTRGNE(UNG C12) with a bilayer structure and
iodide channels between the layers.[12]
at 1638C for the ion pair 1
the clearing temperature decreases and drops markedly for
compound 1(C20/C20) with the longest alkyl chain lengths in-
vestigated. This destabilisation of the mesophase is believed
to occur through less favourable interactions between the
long alkyl chain lengths.[14]
For comparison, the mesophase stabilities of the guanidi-
nium salts 2(Cn) with spherical iodide anions were
investigated by DSC. The results are summarised in
ACHTNUGTNERN(NUG C16/C16) and then for higher n,m,
988C. For all the other salts 1
was observed with a nearly linear increase of the melting
points starting at 918C for 1(C9/C9) and ending at 1078C for
(C20/C20). In contrast, the clearing points follow a rather
unusual trend. Up to compound 1(C14/C14), a significant in-
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
1ACHTUNGTRENNUNG
Table 1. DSC data of guanidinium sulfonate ion pairs 1ACHTUNGTRENNUNG
Table 2 and visualised in Figure 6. For derivatives
with shorter alkyl chain lengths, 2ACTHNUGTRENN(UG C10) and 2ACHTUNGTRENNUNG(C12),
Compound
Phase
T [8C]
(DH
[kJmolꢂ1])
Phase
Phase
G
U
A
ACHTUNGTRENNUNG
with crystal-to-isotropic transitions at 119 and
1178C, respectively, no mesophases were observed.
In contrast, the guanidinium sulfonate ion pair 1-
1
1
1
1
1
1
1
1
N
Cr[b]
Cr[b]
Cr[b]
Cr[b]
Cr[b]
Cr[b]
Cr[b]
Cr[b]
98 (60.4)
91 (1.2)[c]
94 (52.9)
99 (60.1)
101 (77.6)
103 (88.1)
105 (97.7)
–
–
I
I
I
I
I
I
I
I
SmA
SmA
SmA
SmA
SmA
SmA
SmA
106 (1.7)
126 (2.0)
148 (2.2)
160 (1.8)
163 (1.6)
161 (1.3)
138 (0.8)
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
mesophase range of 21 K with a melting point at
1118C and a clearing point at 1328C. Compared
107 (110.3)
with the ion pair 1
(C14) is smaller (D=38 K) and has a higher melt-
ing point (D=10 K). However, derivative 2(C16)
with longer alkyl chains shows little difference in
ACHTNUGTNERN(NUG C14/C14), the mesophase range of
[a] Second heating, heating/cooling rate=10 Kminꢂ1. [b] All compounds show addi-
tional soft crystal–crystal transitions upon heating. For details see the Experimental
Section and the Supporting Information. [c] First heating: 898C (50.5 kJmolꢂ1).
2ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
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S. Laschat et al.
Table 2. DSC data of guanidinium iodides 2(Cn).[a]
ing density leading to higher melting points, closely
followed by the derivatives 1ACHTNURTGENNUG(C12/C14) and 1ACHTUNGTRENNUNG(C10/C12)
with an increase of two carbon atoms in the alkyl
chain of the sulfonate group. The lowest overall
Compound
Phase
T [8C]
(DH
[kJmolꢂ1])
Phase
T [8C]
ACTHNUGTREN(NUNG DH ACHTUNGTRENNUNG
[kJmolꢂ1])
Phase
G
G
2
2
2
2
2
2
A
Cr[b]
Cr[b]
Cr[b]
Cr[b]
Cr[b]
Cr[b]
119 (18.8)
117 (25.8)
111 (24.6)
110 (16.0)
110 (47.5)
112 (51.2)
–
–
–
–
I
I
I
R
melting point was achieved for the ion pair 1ACHTUNGTRENNUNG(C8/
E
SmA
SmA
SmA
SmA
132 (0.4)
171 (0.7)
201 (1.1)
224
C12). Similar behaviour was observed by Causin and
Saielli for symmetric and asymmetric dialkylated vi-
ologen salts.[15]
N
I
R
I
A
I[c]
[a] Second heating, heating/cooling rate=10 Kminꢂ1. [b] All compounds show addi-
X-ray investigations: The previously assigned smec-
tic layer structure was proven by small-angle X-ray
scattering (SAXS) and wide-angle X-ray scattering
tional soft crystal–crystal transitions upon heating. For details, see the Experimental
Section and the Supporting Information. [c] Compound 2ACTHNUGTRNEUNG(C20) shows initial decompo-
sition. Therefore the first heating run was used to give an approximation of the clear-
ing point.
(WAXS). A typical diffraction pattern of 1ACHTUNGTRENNUNG(C12/C12)
with a strong fundamental diffraction peak (001)
and a diffuse halo of the molten-like alkyl chains at
around 500 pm is shown in Figure 8, and is representative of
all ion pairs 1(Cn/Cm). Together with the information from
AHCTUNGTRENNUNG
the POM investigations, the mesophase geometry of 1
Cm) can be confirmed as smectic.
ACHTUNGTRENNUNG(Cn/
The diffraction peak (001) was used to calculate the layer
spacings d001 at various temperatures by using a Gaussian
Figure 6. Mesophase stabilities of 2(Cn) upon second heating. For 2ACHTUNRTGNEUNG(C20),
initial decomposition was observed.
the mesophase range (D=1 K) compared with the ion pair
(C16/C16). The mesophase stabilities of the salts with spheri-
cal iodide anions 2(C18) and 2(C20) are greater than those of
1ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
the corresponding ion pairs 1. This is due to the rapid in-
crease in clearing point with increasing alkyl chain length
coinciding with melting points that remain almost un-
changed.
To illustrate the diversity of the guanidinium sulfonate ion
pairs 1ACHTUNGTRENNUNG(Cn/Cm), several ion pairs with mixed alkyl chain
lengths Cn and Cm were synthesised. Their mesophase stabil-
ities are shown in Figure 7 (for details of transition tempera-
tures and enthalpies, see the Supporting Information). For
this purpose, ion pairs with one constant and one variable
alkyl chain length 1
Surprisingly, the clearing points for all derivatives 1
and 1(Cn/C12) (with n=m) are almost equal (e.g., T
C10)=1388C and T(C10/C12)=1378C) and increase linearly
ACHTUNGTREN(NUG C12/Cm) and 1ACHTNUGTRENNNUG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
with increasing alkyl chain length. The melting points, how-
ever, do not show such a clear trend: the ion pairs with
equal chain lengths 1ACHTNUTRGNE(NUG C12/C12) show the maximum melting
points and by decreasing the length of one of the chains
lower melting points are observed. Thus, salts with equal
chain lengths Cn/Cm (n=m) seem to have the highest pack-
Figure 7. Mesophase stabilities of 1ACTHNUGTRNENU(G C12/Cm) (top) and 1ACHUTGNTREN(NUGN Cn/C12) (bottom)
upon second heating.
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Designer Ionic Liquid Crystals
FULL PAPER
Table 3. Layer spacings dred of the guanidinium sulfonate ion pairs 1
Cm) at a common reduced temperature (Tred =0.95Tiso).
ACHTUNGTRENNUNG(Cn/
Compound
Tiso [8C]
Tred [8C]
dred [pm]
1
1
1
1
1
1
1
N
107
126
148
160
163
161
131
101
120
141
152
155
153
124
2813
3030
3224
3451
3560
3712
4136
tails, see Figure S1 in the Supporting Information). The re-
duced temperatures Tred are almost identical, regardless of
the alkyl chain length combination C12/Cm and Cn/C12. This
is also reflected in the layer spacings dred. Again, dred increas-
es with increasing alkyl chain length C12/Cm and Cn/C12, but
is almost constant for n=m. This means that the layer spac-
Figure 8. WAXS image of 1ACHTNUTRGNE(UNG C12/C12) at 1308C (small picture) with the
corresponding diffraction pattern.
ings of the guanidinium sulfonate ion pairs 1ACTHNUGTRNEUNG(Cn/Cm) are do-
distribution. The layer spacings of the guanidinium sulfonate
minated by the longer alkyl chain in the ion pair. Because
the layer spacings are independent of the location of the
longer alkyl chain (whether attached to the cation or anion),
the congruently shaped cations and anions are mixed up
within the layer structure. This confirms the proposed model
ion pairs 1
ACHTUNGTRENNUNG(Cn/Cm) show a strong linear temperature depend-
ence (Figure 9) and thus a layer spacing dred at a common
of the arrangement of guanidinium sulfonate ion pairs 1
Cm) in the mesophase (see Figure 1).
ACHTUNGTRENNUNG(Cn/
Smectic bilayer structures with strong interdigitated alkyl
chains were also observed for guanidinium iodides 2(Cn).
The temperature dependence of the layer spacings d001 for
the liquid crystalline derivatives 2(Cn) is shown in Figure 10
with layer spacings decreasing with increasing temperature.
Salt 2ACHTUNTRGNE(NUG C20) with the longest alkyl chain length was not in-
cluded because of partial decomposition at higher tempera-
tures. The layer spacings dred of 2(Cn) increase nearly linear
with increasing alkyl chain length (Table 5).
Conclusion
We have reported herein the first examples of guanidinium
sulfonate ion pairs with congruent molecular shapes. The
synthetic route to the guanidinium chloride precursors 6 has
been optimised significantly by a couple of modifications.
First, nitrophenols, which require costly palladium-catalysed
reductions to the corresponding amines, can be replaced by
cheap and commercially available N-(4-hydroxyphenyl)ace-
tamides (Paracetamol). Secondly, the use of sodium hydro-
gen carbonate instead of triethylamine simplifies the purifi-
Figure 9. Temperature dependence of the d001 layer spacings for 1ACHTUNGRTNEUNG(Cn/Cm)
~
*
^
obtained from SAXS investigations: + C9/C9; C10/C10
;
C12/C12
;
C14/
!
C14; ꢀ C16/C16
;
C18/C18; 3 C20/C20.
reduced temperature (d001 at 0.95Tiso)[2a] was determined; the
data are summarised in Table 3. The layer spacings point to
a bilayer structure with strongly interdigitated alkyl chains
(d001 >Lcalcd), for example, the layer spacing dred of 1ACTHUNRTGNE(UNG C12/C12)
is 3224 pm, and comparison with the molecular
[16]
lengths (Lcalcd
)
calculated for the cation with
2300 pm and the anion with 2100 pm (all trans con-
figuration) indicates a bilayer structure. Overall, the
layer spacings d001 increase almost linearly with
alkyl chain lengths Cn/Cm.
For a more detailed understanding of the layer-
thickness dependence, the layer spacings dred of ion
pairs 1 with varying alkyl chain lengths C12/Cm and
Cn/C12 (with n=m) were compared (Table 4, for de-
Table 4. Comparison of the layer spacings dred of the guanidinium sulfonate ion pairs
(C12/Cm) and 1(Cn/C12).
1
G
ACHTUNGTRENNUNG
Compound
(C12/C8)
(C12/C10)
(C12/C12)
(C12/C14)
(C12/C16)
Tred [8C]
dred [pm]
dred [pm]
Tred [8C]
Compound
(C8/C12)
(C10/C12)
(C12/C12)
(C14/C12)
(C16/C12)
1
A
115
131
141
146
152
3037
3119
3224
3354
3454
3087
3140
3224
3310
3437
116
130
141
147
151
1ACHTUNGTRENNUNG
1
1
1
1
N
1
1
1
1
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Chem. Eur. J. 2012, 18, 3014 – 3022
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3019

S. Laschat et al.
tures above 2008C.[5] For guanidinium sulfonates 1
and 1
(Cn/C12) (n,m¼12), unequal alkyl chain lengths seem
to have a major influence on the melting points. The sym-
metrical ion pair 1(C12/C12) has the highest melting point
ACHTUNGTRENNUNG(C12/Cm)
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
due to the highest packing density. All other alkyl chain
length combinations result in lower melting points. Overall,
the transition temperatures of 1ACTHUNTRGNE(UNG Cn/Cm) can be easily adjust-
ed to achieve the desired mesophase range by combining
cationic and anionic compounds with the appropriate alkyl
chain lengths. Finally, the congruently shaped ion pairs 1ACHTUNGTRENNUNG(Cn/
Cm) with sulfonates as the anions have favourable mesomor-
phic properties with shorter alkyl chain lengths due to the
stabilisation of the smectic layer structure by ion-pair inter-
actions relative to the corresponding ion pairs 2(Cn) with io-
dides. These results reveal that the use of rod-shaped meso-
genic anions and cations gives access to tailor-made smectic
liquid crystals in which the mesomorphic properties can be
adjusted by an appropriate combination of chain lengths. It
remains to be proven whether this is also true for other
cation/anion combinations.
Figure 10. Temperature dependence of the d001 layer spacings for 2(Cn)
~
!
obtained from SAXS investigations: C14/C14; ꢀ C16/C16
;
C18/C18.
Table 5. Layer spacings dred of the guanidinium iodides 2(Cn) at
a
common reduced temperature.
Compound
Tiso [8C]
Tred [8C]
dred [pm]
Experimental Section
2
2
2
N
132
171
201
125
162
191
3142
3295
3431
General methods: Commercially obtained chemicals were used as re-
ceived. Guanidinium chloride precursors 6 with alkyl chain lengths R=
C8H17, C10H21, C12H25 and C16H33 have been described in a previous
work.[5b] Tetramethylchloroformamidinium chloride was prepared accord-
cation procedure. Sodium chloride is the only byproduct,
which can easily be removed, unlike triethylamine hydro-
chloride. In addition, we have developed a large-scale and
environmentally friendly chromatographic method for the
purification of guanidinium chlorides 6, involving the use of
silica gel modified with hydrogen chloride and a non-chlori-
nated eluent. The key step in the preparation of ion pairs 1-
ing to literature procedures.[10] The syntheses of compounds 6
C14) and 2
A
ACHTUNGTRENNUNG(C14/
AHCTUNGTRENNUNG
below, all other derivatives are summarised in the Supporting Informa-
tion. For better clarity and simplicity, non-IUPAC nomenclature has been
used. Column chromatography was carried out on silica 60Dm (40–
70 mm). Silica gel sheets (TLC silica gel 60 F 254) from Merck were used
for thin-layer chromatography. 1H and 13C NMR spectra were recorded
with Bruker Avance 300 and 500 spectrometers at 296 K. The numbering
system used for the assignment of the 1H and 13C NMR signals is shown
in Scheme 4. FTIR spectra were recorded with a Bruker Vektor22 spec-
trometer with an MKII Golden Gate Single Reflection Diamant ATR
system. Mass spectrometry was performed on a Varian MAT 711 mass
spectrometer with EI ionisation (70 eV), a Finnigan MAT 95 spectrome-
ter with CI ionisation and methane as the reactant gas, and a Bruker mi-
crOTOF Q spectrometer with electrospray ionisation. Elemental analy-
ses were performed on a Carlo Erba Strumentazione Elemental Analyzer
Model 1106. DSC was performed with a Mettler Toledo DSC822 instru-
ment, melting points (uncorrected) and mesophase textures were ob-
tained by polarising optical microscopy using an Olympus BX 50 polaris-
ing microscope combined with a Linkam LTS 350 hot stage instrument.
X-ray powder experiments were performed with a Bruker Nanostar in-
strument with the monochromatic CuKa1 beam (l=1.5405 ꢃ) obtained by
using a ceramic tube generator (1500 W) with cross-coupled Gçbel mir-
rors as the monochromator. The X-ray patterns were calibrated with the
powder pattern of silver Behenate. Samples were prepared in glass tubes
(0.7 mm outside diameter) from Fa. Hilgenberg GmbH and tempered on
a temperature-controlled hot stage (ꢃ1 K).
ACHTUNGTRENNUNG(Cn/Cm) was the N-alkylation of the guanidinium precursor 6
with methyl sulfonates 10. This procedure has two major ad-
vantages: first, the sulfonates 10 operate as a methyl transfer
agent and, secondly, the corresponding leaving group is the
desired counter-ion. In addition, expensive and possibly not
quantitative anion-exchange reactions of the corresponding
guanidinium iodides 2(Cn), for example, silver salts, can be
avoided.
The mesomorphic properties of the ionic liquid crystalline
guanidinium sulfonate ion pairs 1ACTHNUTRGNE(UNG Cn/Cm) are caused by an
intricate balance of anion and cation effects. Whereas melt-
ing points for guanidinium iodides 2(Cn) remain almost con-
stant, they increase slightly for congruently shaped guanidi-
nium sulfonates 1ACHTUNGTRENNUNG(Cn/Cm) with increasing chain lengths. In
contrast, whereas a linear increase in the clearing points
with increasing chain lengths was observed for the guanidi-
nium iodides 2(Cn), the clearing points of the congruently
shaped guanidinium sulfonates 1ACTHNUTRGNE(UNG Cn/Cm) displayed a bell-
shaped curve with a steady increase from C9/C9 up to C16/C16
and a subsequent decrease. Thus, the use of alkyl sulfonate
anions might be useful to avoid thermal decomposition
upon repetitive heating/cooling cycles, which was observed
previously for guanidinium halides with clearing tempera-
Scheme 4. Numbering system used for the assignment of individual NMR
signals.
3020
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3014 – 3022

Designer Ionic Liquid Crystals
FULL PAPER
General procedure for the preparation of 4-(alkoxy)phenyltetramethyl-
guanidinium chlorides 6(Cn): The corresponding amine 5(Cn) (3.44 mmol)
was dissolved in dry dichloromethane and sodium hydrogen carbonate
(2.89 g, 34,4 mmol) was added. Tetramethylchloroformamidinium chlo-
ride (3.78 mmol) was added dropwise and the reaction mixture was
heated at reflux for 1 h. After cooling to room temperature, the solid was
filtered off and the solvent removed under reduced pressure. A column
was stacked with HCl-modified silica gel (10ꢄmass fraction of the crude
product), suspended in ethyl acetate and the crude product was stacked
on the top. The column was flushed with ethyl acetate until all impurities
were removed. Then the eluent was changed to EtOAc/MeOH (3:1) with
a trace of HCl and the pure guanidinium chlorides 6 were obtained in up
to quantitative yields.
was removed under reduced pressure. The residue was dissolved in di-
chloromethane, filtered and the solvent was again removed under re-
duced pressure. The crude product was recrystallised from ethyl acetate
to obtain the pure guanidinium iodides 2(Cn).
4-(Tetradecyloxy)phenylpentamethylguanidinium iodide
2ACTHUNGETRN(UNG C12): White
1
solid (113 mg, 0.207 mmol, 96%); H NMR (500 MHz, CDCl3, TMS): d=
0.88 (t, J=6.9 Hz, 3H; CH3), 1.18–1.39 (m, 20H; CH2), 1.41–1.49 (m,
2H; CH2), 1.73–1.81 (m, 2H; OCH2CH2), 2.68–3.38 (2 brs, 12H; N-
AHCUTNGERT(GNNUN CH3)2), 3.49 (s, 3H; NCH3), 3.94 (t, J=6.6 Hz, 2H; OCH2), 6.90–6.97
(m, 2H; 3-H), 7.04–7.11 ppm (m, 2H; 2-H); 13C NMR (125 MHz, CDCl3,
TMS): d=14.1 (CH3), 22.7, 26.0, 29.2, 29.37, 29.4, 29.58, 29.61, 29.66,
29.67, 29.69, 29.70, 31.9 (CH2), 41.5 (NCH3), 41.2, 42.3 (2 brs, NACHTUNGTRENNUNG(CH3)2)
68.5 (OCH2), 116.0 (C3), 123.6 (C2), 134.6 (C1), 157.7 (C4) 162.3 ppm
(C1’); FTIR (ATR): n˜ =2919 (s), 2848 (m), 1611 (s), 1548 (s), 1510 (s),
1467 (m), 1404 (vs), 1393 (w), 1236 (vs), 1188 (s), 1179 (m), 1113 (m),
4-(Tetradecyloxy)phenyltetramethylguanidinium chloride
6ACTHUNGETRN(UNG C14): White
solid (1.79 g, 3.41 mmol, 99%); 1H NMR (500 MHz, CDCl3, TMS): d=
0.88 (t, J=6.9 Hz, 3H; CH3), 1.19–1.39 (m, 20H; CH2), 1.40–1.48 (m,
ACTHNUTRGNEUNG
1040(m), 895 (w), 832 (s), 720 cmꢂ1 (m); MS(ESI): m/z: 418 [M]+;
HRMS (ESI): m/z calcd for C26H48N3O+: 418.3792 [M]+; found:
418.3797; elemental analysis calcd (%) for C26H48IN3O: C 57.24, H 8.87,
N 7.60; found: C 57.22, H 8.79, N 7.60; DSC: Cr1 58 (1.2 kJmolꢂ1) Cr2
2H; CH2), 1.72–1.80 (m, 2H; OCH2CH2), 2.50–3.58 (brs, 12H; NACTHNUTRGNEUNG(CH3)2),
3.91 (t, J=6.6 Hz, 2H; OCH2), 6.81–6.88 (m, 2H; 3-H), 7.05–7.12 (m,
2H; 2-H), 11.91 ppm (s, 1H; NH); 13C NMR (125 MHz, CDCl3, TMS):
d=14.1 (CH3), 22.7, 26.0, 29.25, 29.36, 29.42, 29.59, 29.61, 29.66, 29.67,
678C (3.4 kJmolꢂ1) Cr3 1118C (24.6 kJmolꢂ1) SmA 1318C (0.4 kJmolꢂ1
)
I.
29.69, 29.70, 31.9 (CH2), 40.6 (brs, NACHTNUTRGNENUG(CH3)2), 68.4 (OCH2), 115.5 (C3),
122.3 (C2), 130.6 (C1), 156.8, 158.8 ppm (C4, C1’);;FTIR (ATR): n˜ =2917
(vs), 2849 (s), 1624 (s), 1560 (s), 1505 (s), 1471 (s), 1419 (s), 1402 (s),
1314 (m), 1232 (vs), 1172 (m), 1117 (w), 1035 (m), 840 (s), 757 (m),
716 cmꢂ1 (s); MS
ACHTUNGTRENNUNG
[MꢂC2H6NꢂC14H29]+, 147 [MꢂC2H6NꢂC14H29O]+; HRMS (ESI): m/z
calcd for C25H46N3O+: 404.3635 [M]+; found: 404.3626; elemental analy-
sis calcd (%) for C25H46ClN3O: C 68.23, H 10.54, N 9.55; found: C 68.33,
Acknowledgements
Financial support by the Bundesministerium fꢀr Bildung und Forschung
(BMBF-Verbundprojekt Nr. 01 RI 05177), the Studienstiftung des Deut-
schen Volkes (fellowship for M.B.), the Deutsche Forschungsgemein-
schaft, the Ministerium fꢀr Wissenschaft, Forschung und Kunst des
Landes Baden-Wꢀrttemberg and the Fonds der Chemischen Industrie is
gratefully acknowledged. We would like to thank the referees for valua-
ble suggestions.
H
10.42,
(1.0 kJmolꢂ1) I.
General procedure for the preparation of the 4-(alkoxy)phenylpentame-
thylguanidinium 4-alkoxybenzenesulfonates 1(Cn/Cm): The corresponding
N ) SmA 2018C
9.63; DSC: Cr 1108C (41.8 kJmolꢂ1
AHCTUNGTRENNUNG
N-protonated guanidinium chloride 6(Cn) (0.227 mmol) was dissolved in
methanol (5 mL). Potassium hydroxide (38 mg, 0.681 mmol) was dis-
solved in methanol (5 mL) and added dropwise to the guanidinium solu-
tion. After 5 min, the solvent was removed under reduced pressure; the
residue dissolved in diethyl ether and the mixture filtrated. After remov-
ing the solvent, the corresponding methyl sulfonate 10(Cm) (0.227 mmol)
was added and the reaction mixture was heated at reflux in acetonitrile
(10 mL) for 24 h. After cooling to room temperature, the solvent was re-
moved under reduced pressure and the crude product was recrystallised
from ethyl acetate to obtain the pure guanidinium sulfonate ion pair 1-
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ACHTUNGTRENNUNG
nesulfonates 1ACHTUNGTRENNUNG(C14/C14): Colourless solid (145 mg, 0.184 mmol, 81%);
1H NMR (500 MHz, CDCl3, TMS): d=0.82–0.91 (m, 6H; CH3), 1.19–1.38
(m, 40H; CH2), 1.39–1.48 (m, 4H; CH2), 1.72–1.80 (m, 4H; OCH2CH2),
2.70–3.23 (2 brs, 12H; NACTHNUTRGNEUNG(CH3)2), 3.43 (s, 3H; NCH3), 3.88–3.96 (m, 4H;
OCH2), 6.79–6.84 (m, 2H; 3’’-H), 6.88–6.92 (m, 2H; 3-H), 7.01–7.06 (m,
2H; 2-H), 7.80–7.85 ppm (m, 2H; 2’’-H); 13C NMR (125 MHz, CDCl3,
TMS): d=14.1 (CH3), 22.7, 26.0, 29.21, 29.26, 29.36, 29.41, 29.44, 29.59,
29.60, 29.62, 29.66, 29.67, 29.69, 29.7, 31.9 (CH2), 40.2 (NCH3), 40.3, 41.2
(2 brs, NACHTUNGTRENNUNG(CH3)2), 68.1, 68.5 (OCH2), 113.6 (C3’’), 115.9 (C3), 123.4 (C2),
127.6 (C2’’), 135.1 (C1), 139.7 (C1’’), 157.5 (C4), 159.6 (C4’’), 162.3 ppm
(C1’); FTIR (ATR): n˜ =2918 (vs), 2850 (s), 1601 (m), 1553 (m), 1511 (m),
1468 (m), 1406 (m), 1297 (w), 1244 (s), 1213 (s), 1195 (s), 1119 (vs), 1029
(s), 1003 (m), 898 (w), 831 (m), 700 (m), 605 (s), 564 cmꢂ1 (s); MS
ACTHNUTRGNE(NUG ESI):
m/z: 418 [M]+, 222 [MꢂC14H29+H]+, 206 [MꢂC14H29O+H]+, 369 [M]ꢂ,
172 [MꢂC14H29]ꢂ; HRMS (ESI): m/z calcd for C26H48N3O+: 418.3792
[M]+; found: 418.3797; calcd for C20H33O4Sꢂ: 369.2094 [M]ꢂ; found:
369.2094; elemental analysis calcd (%) for C46H82N3O5S: C 70.09, H
10.36,
N 5.33; found: C 69.84, H 10.24, N 5.22; DSC: Cr1 538C
(ꢂ13.6 kJmolꢂ1) Cr2 1018C (77.6 kJmolꢂ1) SmA 1608C (1.8 kJmolꢂ1) I.
General procedure for the preparation of 4-(alkoxy)phenylpentamethyl-
guanidinium iodides 2(Cn): The corresponding N-protonated guanidinium
chloride 6(Cn) (0.216 mmol) was stirred with methyl iodide (0.03 mL,
0.432 mmol) and potassium carbonate (60 mg, 0.432 mmol) in acetonitrile
(10 mL) at 508C for 12 h. After cooling to room temperature, the solvent
Chem. Eur. J. 2012, 18, 3014 – 3022
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3021

S. Laschat et al.
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supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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crystals, which were assigned to water oxygen atoms. There is one
disordered oxygen per ionic pair with a disorder ratio of about 5:1.
The water molecule is evident only in the single-crystal structure.
Nevertheless, elemental analysis gave no indication of significant
water contamination.
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Received: June 22, 2011
Revised: September 27, 2011
Published online: February 1, 2012
3022
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3014 – 3022