Metal-free C-H functionalization of pyrrolidine to pyrrolinium-based room temperature ionic liquid crystals

Article abstract of DOI:10.1039/d1nj00647a

The development of ionic liquid crystals (ILCs) using pyrrolidinium cation has received considerable interest due to their higher electrochemical stability. However, high charge density associated with low charge distribution in the fully saturated pyrrol

Full text of DOI:10.1039/d1nj00647a

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Metal-free C–H functionalization of pyrrolidine to
pyrrolinium-based room temperature ionic liquid
crystals†
Cite this: New J. Chem., 2021,
45, 8064
a
Sumana Mandal,a Ravindra Kumar Gupta,
Suraj Kumar Pathak,a
b
a
D. S. Shankar Rao,
S. Krishna Prasad,b Achalkumar Ammathnadu Sudhakar
*
a
and Chandan K. Jana
*
The development of ionic liquid crystals (ILCs) using pyrrolidinium cation has received considerable
interest due to their higher electrochemical stability. However, high charge density associated with low
charge distribution in the fully saturated pyrrolidine ring leads to the high melting temperature of the
materials due to strong ionic interactions. Herein, the synthesis and evaluation of the structure–property
relationship of a new class of pyrrolinium-based ionic liquid crystals has been reported. The metal-free
direct C–H functionalization of pyrrolidine, followed by the N-alkylation of resulting pyrroline produced
the pyrolinium moiety that serves as the unprecedented polar head of the mesogens. These ionic
mesogens having a partially unsaturated pyrrolidine ring were found to stabilize numerous mesophases
over a wide thermal range including room temperature.
Received 7th February 2021,
Accepted 26th March 2021
DOI: 10.1039/d1nj00647a
rsc.li/njc
However, ILCs built on aromatic cations such as imidazolium,
pyridinium, and quinolinium have dominated the field.
Introduction
Ionic liquid crystals (ILCs) form a class of liquid crystals (LCs) However, a few examples corresponding to pyrrolidinium-
that contain anions and cations, and due to their ionic character, based ILCs containing fully saturated N-heterocycle have been
they differ significantly from the conventional neutral LCs.1 They developed.5–8 The pyrrolidinium cation is a five-membered
usually tend to stabilize layered phases due to the strong ionic nonplanar N-heterocyclic nonaromatic ring system that lacks
character but are also known to stabilize several uncommon charge delocalization and displays higher electrochemical
mesophases. Clearly, due to the ionic nature, they have the stability.9 However, high charge density associated with low
capability to transport ions and thus have the potential to be charge distribution leads to a high melting temperature of
used as electrolytes for batteries, fuel cells, and dye-sensitized the materials due to strong ionic interactions (Scheme 1).
solar cells.2 They merge the properties of ionic liquids and LCs, We anticipated that the increase in the charge delocalization
thus opening up a wide variety of applications to be utilized as by placing a partial unsaturation in the pyrrolidine ring would
designer solvents for the reactions due to the additional liquid decrease the melting temperature. Herein, we report a new type
crystalline order they bring about in comparison to ionic liquids, of pyrrolinium-based ionic mesogens, which exhibited a broad
in addition to the tunability with respect to the change of cations range of LC phases at lower/room temperature that are
and anions.3 Over the years, numerous ILCs based on the common to calamitic (rod-shaped) and discotic (disc-shaped) LCs.
different types of ionic heads such as ammonium salts, phos-
phonium salts, imidazolium salts, pyridinium salts, viologens,
pyrilium and thiopyrilium salts, dithiolium salts, vinamidinium
salts, alkali metal alkanoates (soaps), phosphates, and phosphonates
as well as several metal complexes have been developed.1,4
a Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
781039, Assam, India. E-mail: achalkumar@iitg.ac.in, ckjana@iitg.ac.in
b Centre for Nano and Soft Matter Sciences (CeNS), P. B. No. 1329, Prof. U. R. Rao
Road, Jalahalli, Bengaluru, 560 013, Karnataka, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Known pyrrolidinium-based ILCs.
d1nj00647a
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Scheme 2 Synthesis and structures of pyrrolinium-based ILCs.
A metal-free direct b-(sp3) C–H functionalization of pyrrolidine (Schemes 2 and S2, ESI†) with good to excellent yields. A series
has been developed for the synthesis of a series of pyrrolinium- of ions comprising the cationic pyrroline-containing n-alkoxy
based mesogens (Scheme 2).
aryl group as a head and different anions such as iodide,
bromide, or tetrafluoroborate have also been synthesized. The
number and positions of the alkoxy chains as well as the type of
the anions were systematically varied to understand the effects
of the structure on the thermotropic mesophase behavior of
ions. Different alkyl groups such as methyl, ethyl, propyl,
benzyl, and dodecyl with different counter anions (iodide,
bromide, tetrafluoroborate, etc.) have been introduced. In the
case of compounds with a long chain at the N-terminal, a low
conversion (12/3/C12I, PhC12Br) was noticed.
Results and discussion
Synthesis and characterization
The synthesis of mesogens commenced with the preparation of
aryl aldehyde bearing n-alkoxy groups with varying chain
lengths. The desired pyrroline derivatives 2a-l were produced
from the reactions of pyrrolidine and arylaldehydes 1 through
the direct b-C–H functionalization of pyrrolidine under metal-
free simple conditions (Scheme S1, ESI†).10,11 Selected pyrro-
lines possessing a polar head group and apolar aliphatic tails,
which are often assumed as important structural features for a
Thermal behavior
compound to show a mesophasic behavior, were evaluated All the ionic compounds were studied via polarizing optical
under POM. Unfortunately, none of the tested pyrrolines microscopy (POM), differential scanning calorimetry (DSC),
showed LC properties. Then, we decided to prepare iminium and X-ray diffraction (XRD) studies to examine the thermal
ions from those imines by treating them with different alkyl behavior. Table 1, bargraph shown in Fig. 1, Table S1, and S2
halides. The treatment of different alkylating reagents such (ESI†) represent the thermal behavior of ILCs. The first four
as methyl iodide, triethyloxoniumtetrafluoroborate, trimethyl- wedge-shaped compounds, namely 12/3/MI, 12/3/EI, 12/3/ProI,
oxoniumtetrafluoroborate or benzyl bromide with the b-substituted and 12/3/C12I, vary from each other only with the length of the
secondary cyclic imines gave the corresponding iminium ions alkyl chain attached to the nitrogen. Out of these, 12/3/MI and
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Table 1 Phase transition temperaturesa (1C) and corresponding enthalpies (kJ molÀ1) of the ionic LCs
Phase sequence
Entry
2nd Heating
1st Cooling
12/3/MI
10/3/MI
8/3/MI
Cr 9.2 (7.3) Colr108.8 (2.7) Colh 173.6 (0.01) I
Cr 55.7 (9.7) Colh 167.9 (0.01) I
Colob2 133.0 (7.2) Colob1 164.6 (0.1) I
Cr 66.3 (1.4) Colh2 82.3 (3.8) Colh1 155.2 (0.01) I
Cr1 35.3 (3.0) Cr2 93.5 (19.6) I
I 165.2 (0.01) Colh 97.7 (2.6) Colr3.1 (8.6) Cr
c
I 163.7 Colh 39.1 (2.2) Cr2 23.5 (0.7) Cr1
b
I 161.5 (0.01) Colob1 124.1 (8.3) Colob2
12/3/EI
I 144.1 (0.02) Colh1 63.8 (12.7) Colh2 16.4 (0.3) Cr
I 80.9 (16.5) Cr2 29.4 (2.7) Cr1
I 91.6 (0.5) Colh 54.6 (32.2) Cr
I 150 Colh1 39.7 (11.5) Colr À4.3 (0.5) Cr
I 148.3(0.3) Colh49.2 (14.4) Colrb
I 119 Crc
12/3/ProI
12/3/C12I
12/3/MBF
12/3/EBF
12/3/BnBr
Cr 78.6 (5.8) Colh 98.5 (38.4) I
Cr 9.1 (1.2) Colr 65.8 (5.5) Colh 155 I
Cr1 48.5 (2.3) Colr 69.1 (14.5) Colh151.3 (0.1) I
Cr1 50 (11.1) Cr2 73.4 (6.3) Cr3 144.6 (46.4) I
a
b
Peak temperatures in the DSC thermograms obtained during the first cooling and second heating cycles at 5 1C minÀ1
.
optical texture remains
c
frozen. the transition was observed only under POM and no transition was observed in DSC; (Cr = crystal; Colh = Columnar hexagonnal phase;
Colr = Columnar rectangular phase; Colr = Columnar oblique phase; I = Isotropic liquid.)
Thus, the mesophase stabilization of these wedge-shaped ionic
liquid crystals is more sensitive to the chain length of the N-alkyl
group at the tip of the wedge, rather than the length of the
peripheral chains. The dialkoxy derivatives 12/(3,4)/MI, 12/(3,4)/
MBF, 12/(3,4)/EBF, and 12/(3,5)/MI were mesomorphic, whereas
other dialkoxy derivatives 12/(2,3)/MI and 12/(2,4)/MI turned out to
be crystalline. Surprisingly, among the monoalkoxy derivatives
12/p/MI, 14/p/MI, and 12/p/EBF, only the last compound was
found to be liquid crystalline. This emphasizes the requirement
of more peripheral tails as well as the type of counteranion in
stabilizing the mesomorphic behavior in this class of compounds.
The mesomorphic behavior of some of the representative
compounds of each series is discussed in detail in the following
section.
Among the trialkoxy derivatives, compound 12/3/MI, 12/3/
MBF, and 12/3/EBF were bimesomorphic exhibiting columnar
rectangular (Colr) phase and columnar hexagonal (Colh)
phases. Other trialkoxy derivatives 10/3/MI and 12/3/EI
exhibited the Colh phase, while 12/3/C12I and 8/3/MI exhibited
Colr and columnar oblique (Colob) phases, respectively.
Compound 12/3/MI, which is a sticky viscous sample at room
temperature, was birefringent under POM. The compound
showed a transition around 109 1C with an enthalpy change
of 2.7 kJ molÀ1 (Fig. 2a, Table 1). Further heating led to a phase
transition, where the birefringent mass converts to an isotropic
liquid at 174 1C. The isotropic liquid on cooling yields a mosaic
texture that is often obtained for Col phases (Fig. 2b). Further
cooling leads to a transition at E98 1C (DH = 2.6 kJ molÀ1) with
Fig. 1 Bargraph representation of the thermal behavior of ILCs (2nd
heating scan in DSC is considered).
12/3/EI exhibited room temperature columnar (Col) phase, a slight change in the optical texture (Fig. 2c), before showing
while compound 12/3/ProI turned to be crystalline. Surprisingly, the crystallization at 9 1C (DH = 8.6 kJ molÀ1), as noted in DSC.
compound 12/3/C12I with the n-C12-chain exhibited a Col phase Thus, the compound was liquid crystalline at room temperature,
along with a crystalline phase (Fig. S4 and S11, ESI†).
as noted by POM and DSC observations. To investigate the phase
Replacing the alkyl chain on N with a benzyl group (12/3/ structure, we have carried out the XRD studies well within the
BnBr) also led to a crystalline nature, although the counter ion thermal ranges of the mesophase.
was bromide. The variation of the counter ions, i.e., BF4À as in the
The XRD pattern obtained at 130 1C showed a single peak
case of 12/3/MBF or 12/3/EBF instead of IÀ exhibited the Col phase (d-spacing of 35.1 Å) with high intensity at a low angle along
with a reduction in the isotropic temperature (Fig. S5 and S6, with a diffused peak in the wide-angle (Fig. 2d). The diffused
ESI†). Reduction in the length of the peripheral chain (with three peak at a wide angle corresponds to the packing of flexible alkyl
n-decyloxy chains or n-octyloxy chains) as in the case of 10/3/MI chains. The mosaic pattern of the optical texture is often
and 8/3/MI led to the lowering of the clearing point, while observed for columnar (Col) phases (Fig. 2b). However, the
the compound remained liquid crystalline (Fig. S1 and S2, ESI†). single peak at the low angle is often witnessed in the case of
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Fig. 2 DSC thermograms obtained for 12/3/MI for the first cooling (blue trace) and second heating (red trace) scans (a); POM image obtained for
compound 12/3/MI at 130 1C in the Colh phase (b); at 30 1C in the Colr phase (c); XRD profile of compound 12/3/MI depicting the intensity vs 2y obtained
for the Colh phase (d) and Colr phase at 28 1C (e). Scale bar corresponds to 100 mm.
columnar phases with the hexagonal symmetry (Colh phase). 01, 10 planes of a rectangular lattice (Fig. 3). The lattice
Though the single peak in the low angle does not unambiguously parameters for the rectangular unit cell was calculated to be
prove the existence of the Colh phase, there are several reports in a = 41 Å and b = 20.4 Å. Herein, the wide-angle, two diffused
the literature, where the Colh phase with a single peak in the peaks were observed corresponding to the d-spacings of 4.4 Å
low angle are reported. Such a pattern has been ascribed to a and 3.73 Å, respectively. The first diffused peak corresponds to
minimum in the form factor.12
the packing of flexible alkyl chains, while the second one
The hexagonal lattice parameter ‘a’ was found to be 40.51 Å, corresponds to the core–core stacking. This suggests that at
which refers to the sides of a hexagonal unit cell. The molecular low temperature the core–core interactions between the disks
length (L) calculated for compound 12/3/MI was found to be within the column has enhanced. Thus, the low-temperature
E25 Å. From the calculations of the lattice area (S) and volume phase can be unambiguously assigned the Colr phase with the
(V), we could arrive at the value, ‘Z’, i.e., the number of p2mm symmetry (Fig. 3). The number of molecules forming a
molecules forming a unit hexagonal cell, which was found to disk in the rectangular unit cell was found to be around 2. The
be E4. Such discoid formation is possible if four such ionic XRD studies carried out on the lower chain homologs of 12/3/MI,
LCs arrange with their ionic heads oriented towards the center, i.e., 10/3/MI and 8/3/MI showed that these compounds are
which eventually leads to the diameter of such a disk to be monomesomorphic exhibiting Colh and Colob phases, respectively
E50 Å. Such disks can self-assemble to form long columns, (Fig. S1 and S2 ESI,† Table 1). Compound 8/3/MI exhibited a
which then self-organize to form the Colh phase (Fig. 3). transition between Colob1 and Colob2 phases, where the Colob1
However, the value of a was around 20% less than the diameter phase remains stable even at room temperature, as evidenced
of the disk, pointing to a possible interdigitation of the from POM, DSC, and XRD studies (Fig. S2 and Table S1, ESI†). It
peripheral alkyl tails from the neighboring columns. Further, was very hard to detect the change in these two phases as the POM
the low-temperature phase was investigated at 28 1C, showing texture only showed a subtle change in birefringence. However, a
the signatures of the Colr phase. The diffraction pattern significant enthalpy change corresponding to the phase transition
obtained at 28 1C showed an intense peak, followed by a peak was noted (DH = 8.3 kJ) in DSC (Fig. S2 ESI† and Table 1). Here
of moderate intensity at a low angle region, corresponding to again, almost four molecules form a disc, which self-assemble in a
the d-spacings of 41 Å and 20.4 Å, which can be indexed to the parallelogram lattice to give a Colob phase.
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Fig. 3 Schematic showing the self-assembly of 12/3/MI into Colh and Colr phases. The optical textures observed for the Colh phase (at 130 1C) and Colr
phase (at 28 1C) is shown in the inset.
The substitution of the ethyl group in place of methyl as in 120 1C showed a strong peak in the low angle with a d-spacing
the case of 12/3/EI stabilized the Colh phase over a wide range of 39.1 Å, followed by another reflection with a d-spacing of
(Fig. S3, ESI†). However, the same with the n-dodecyl group 19.5 Å (Fig. 5c and Table S2, ESI†). The d-spacing ratio of these
(compound 12/3/C12I) led to the stabilization of the Colr phase two peaks is 1 : 0.5. Although it gives an impression of a layered
over a short-range (Fig. S4, ESI†). The variation of the counter phase, the POM textural observation supports this to be a Col
anion from iodide to tetrafluoroborate as in the case of 12/3/ phase, particularly a Colh phase, where the d-spacing corres-
MBF or 12/3/EBF led to the stabilization of both Colh and Colr ponding to the 11 planes of the hexagonal phase is missing.
phases (Fig. 4, Fig. S5 and S6, ESI†). In comparison to 12/3/MI, A diffused broad peak at wide-angle corresponding to a d-
these compounds exhibited a reduction in the clearing points. spacing of 4.41 Å confirms the liquid crystalline nature of the
However, in the case of 12/3/EBF, an increase in the melting sample (Fig. 5c). The calculated lattice constant a = 45.2 Å,
point was noticed. These differences in the thermal behavior which is less than the double of the molecular length (52 Å).
show that how a minute structural variation affects the self- This means that the individual disc is formed by more than one
assembly of ionic LCs.
molecule, and from the calculation, it was found to be around
The dialkoxy derivatives 12/(3,4)/MI, 12/(3,4)/MBF, 12/(3,4)/ eight molecules. It should be noted that the number of molecules
EBF and 12/(3,5)/MI were mesomorphic. Compound 12/(3,4)/ that form a disc, i.e., Z is more than trialkoxy derivatives, which is
MI exhibited a Colh phase (Table 2). The optical texture showed understandable as more number of dialkoxy derivatives are
a pseudo focal conic pattern (Fig. 5b). The XRD plot obtained at required to complete the disc formation. These molecules point
Table 2 Phase transition temperaturesa (1C) and corresponding enthalpies (kJ molÀ1) of the ionic LCs
Phase sequence
Entry
2nd Heating
1st Cooling
12(3,4)/MI
12(3,4)/MBF
12(3,4)/EBF
12(3,5)/MI
12(2,3)/MI
12(2,4)/MI
12/p/MI
Cr1 69.7 (5.7) Cr2 80.8 (10.4) Cr3 141.1 (14.3) Colh 177b
Cr 139.3 (24.6) Colh 212.1 (0.5) I
Cr 120.1 (30.8) Colh 163.6 (0.2) I
Colh 131.9 (0.2) I
I
I 175b Colh 58.8 (5.1) Cr
I 207.3 (0.3) Colh 92.5 (24.7) Cr
I 160.1 (0.4) Colh 82.5 (30.0) Cr
I 130 Colh (0.3) M
I 107.1 (50.1) Cr
I 165.2 (0.01) M1 97.7 (2.6) M2 3.1 (8.6) Cr
Cr 122.6 (49.2) I
Cr1 9.2 (7.3) M2 108.8 (2.7) M1 173.6 (0.01) I
Cr1 31.7 (0.6) Cr2 136.5 (11) Ic
Cr1 69.81(48.4) Cr2 136 (60.1) Ic
14/p/MI
12/p/EBF
Cr1 26.7 (4.7) Cr2 43.5 (0.8) Cr3 103.2 (10.2) Sm2 162.1 (0.4) Sm1 177bI
I 170.9 Sm1 146.6 (0.3) Sm2 21.1 (5.1) Sm3
d
Peak temperatures in the DSC thermograms obtained during the first cooling and second heating cycles at 5 1C minÀ1
.
the transition was
a
b
c
d
observed only under POM. isotropic liquid freezes after a long time. crystalliazation was not observed in DSC, but the optical texture remains
frozen (Cr = crystal; Colh = Columnar hexagonnal phase; Colr = Columnar rectangular phase; Sm = Smectic phase; I = isotropic liquid.)
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Fig. 4 POM image obtained for the Colh phase (a) and Colr phase (b)
obtained for ILC 12/3/MBF. Scale bar corresponds to 100 mm.
their polar head towards the center and pack to form a disc, and
such discs self-assemble to form columns, and these columns
organize in a hexagonal symmetry to form a Colh phase. The
formation of such discs is expected due to the increased polarity
of the ionic head, which makes two molecules to preferentially
align with the polar parts pointing towards each other.
The presence of two flexible chains per molecule drives them to
self-assemble to form discs rather than layered structures.
The variation of the counterion to tetrafluoroborate (12/
(3,4)/MBF) led to the stabilization of the Colh phase, which
did not alter even with a change in the alkyl group from methyl
to ethyl group (12(3,4)/EBF) (Fig. S8 and S9, ESI†). In these
cases, the ratio of the first three d-spacings in the low angle was
found to be in the ratio of 1 : 0.57 : 0.5, which is typical of a Colh
phase (Table S1, ESI†). However, the core–core peak was not
found in these cases in comparison to corresponding trialkoxy
derivatives. Notably, compound 12(3,5)/MI, where the alkoxy
tails are substituted in 3 and 5 positions, stabilized the Colh
phase down to room temperature even with good core–core
interactions, as evidenced from DSC and XRD studies (Fig. S10,
ESI†). Other dialkoxy derivatives 12(2,3)/MI and 12(2,4)/MI were
found to be crystalline (Fig. S11, ESI†) due to the unfavorable
substitution pattern.
Fig. 6 DSC thermograms obtained for 12/p/EBF for the first cooling (blue
trace) and second heating (red trace) scans (a); POM image obtained for
the Sm1 phase at 163 1C (b); for the Sm2 phase at 104 1C (c) and for the Sm2
phase at 28 1C on cooling from the isotropic state (d). Scale bar
corresponds to 100 mm.
DSC at E146.6 1C (DH = 0.3 kJ molÀ1) (Fig. 6 and Fig. S12a–d,
ESI†). The XRD studies carried out at 120 1C showed a similar
diffraction pattern as in the Sm1 phase but with an additional
reflection at a low angle. The d-spacing corresponding to the first
reflection at a low angle is exactly the multiple of the second
reflection, suggesting this to be a lamellar or layered phase
(Fig. S12e–g, ESI†). The d/L ratio obtained was E1.5, suggesting
it to be an interdigitated bilayer smectic phase (Sm2 phase)
(Table S2, ESI†). Further cooling has shown a transition at 21 1C
(DH = 0.3 kJ molÀ1), where the striations on focal conic fans were
noted in the POM observation. The XRD studies just above this,
i.e., around 28 1C revealed additional reflections along with a sharp
peak at a low angle. The first reflection was an exact multiple of
other reflections, confirming the lamellar order. The d/L ratio was
1.8 confirming the interdigitated bilayer structure of the smectic
phase (Fig. 7). The diffused reflection was seen at 4.64 Å along with
another relatively sharp peak at 4.44 Å, which may be due to the
core–core interaction.
The monoalkoxy derivative 12/p/EBF exhibited an interdigitated
bilayer smectic phase (Sm1 phase), as evidenced by the XRD
studies carried out at 156 1C on cooling the isotropic liquid
(Fig. 6). The POM image was typical of the SmA phase (Fig. S6b,
ESI†). On further cooling, the POM texture did not change much
except the birefringence, but a clear transition was noticed in the
Fig. 5 DSC thermograms obtained for 12(3,4)/MI for the first cooling (blue trace) and second heating (red trace) scans (a); POM image (b); XRD pattern
obtained at 120 1C (c). Scale bar corresponds to 100 mm.
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Conclusions
In summary, we synthesized a series of pyrrolinium-based
mesogens via the metal-free C–H functionalization of pyrrolidine.
In contrast to the pyrrolidinum-based LC, these mesogens
containing a partially unsaturated pyrrolidine ring stabilizes LC
phases at room temperature. Numerous molecular structures
have been evaluated with reference to the variation in the number,
length of the alkyl tails as well as the type of the counterion to
understand the structure–property relationships. In particular, the
trialkoxy and dialkoxy derivatives stabilized Col phases, while
monoalkoxy derivatives stabilized the interdigitated bilayer
smectic phase, which is due to the polarity of the ionic head
and nanophase segregation of the incompatible molecular
subunits. It is important to note that the type of counterion also
plays a major role in the liquid crystalline self-assembly. The
studies on the synthesis of chiral mesogens by incorporating a
chiral center in the pyrroline ring are underway in our laboratory.
Author contributions
SM synthesized and characterized all the intermediates and
final compounds; SP and RKG carried out the POM and DSC
studies; DSSR and SKP conducted the XRD studies and
analyses; AAS and CKJ conceptualized the project, supervised
the execution and jointly wrote to the manuscript with the
contribution from all coauthors.
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Conflicts of interest
‘‘There are no conflicts to declare’’.
Acknowledgements
CKJ acknowledge Science and Engineering Board (SERB), New
Delhi (CRG/2019/000330) for financial support.
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