Crystal engineering approach toward selective formation of an asymmetric supramolecular synthon in primary ammonium monocarboxylate (PAM) salts and their gelation studies

Article abstract of DOI:10.1021/cg401863s

A crystal engineering approach assisted by Cambridge Structural Database (CSD) analyses was adopted in synthesizing a new series of primary ammonium monocarboxylate (PAM) salts derived from variously substituted phenylacetic acid (PA) and chiral amines (R)- and (S)-1-phenylethyl amines (PEA) with the aim of ascertaining an asymmetric one-dimensional (1D) PAM synthon (designated as synthon W) over the other symmetric possibility (synthon X). Characterization of 32 such PAM salts by single crystal X-ray diffraction (SXRD) revealed the exclusive formation of synthon W. The results suggested that the chiral ammonium components must have played a crucial role in generating exclusive formation of synthon W. Interestingly, about 28% of the PAM salts thus synthesized herein displayed gelation ability thereby emphasizing the role of 1D PAM synthon W in gelation.

Full text of DOI:10.1021/cg401863s

Article
pubs.acs.org/crystal
Crystal Engineering Approach toward Selective Formation of an
Asymmetric Supramolecular Synthon in Primary Ammonium
Monocarboxylate (PAM) Salts and Their Gelation Studies
Tapas Kumar Adalder and Parthasarathi Dastidar*
Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur,
Kolkata 700032, West Bengal, India
S
* Supporting Information
ABSTRACT: A crystal engineering approach assisted by Cambridge Structural Database
(CSD) analyses was adopted in synthesizing a new series of primary ammonium
monocarboxylate (PAM) salts derived from variously substituted phenylacetic acid (PA) and
chiral amines (R)- and (S)-1-phenylethyl amines (PEA) with the aim of ascertaining an
asymmetric one-dimensional (1D) PAM synthon (designated as synthon W) over the other
symmetric possibility (synthon X). Characterization of 32 such PAM salts by single crystal X-ray
diffraction (SXRD) revealed the exclusive formation of synthon W. The results suggested that
the chiral ammonium components must have played a crucial role in generating exclusive
formation of synthon W. Interestingly, about 28% of the PAM salts thus synthesized herein
displayed gelation ability thereby emphasizing the role of 1D PAM synthon W in gelation.
containing 10-membered (asymmetric synthon W) and
alternating 8- and 12-membered (symmetric synthon X)
hydrogen bonded rings (Scheme 1). Occasionally, different
kinds of two-dimensional (2D) HBNs are also observed in
PAM salts.
The crystal structures of 34 PAM salts reported by us thus far
revealed that the number of PAM synthons W and X, and other
PAM synthons were 16, 14 and 4, respectively. Interestingly 13
out of 16 PAM salts containing PAM synthon W were gelators,
whereas only 3 out of 14 PAM salts containing PAM synthon X
exhibited gelation, which clearly indicated the effective role of
the PAM synthon W in gelation. Understandably, gaining
control over the exclusive formation of the PAM synthon W
appears attractive in the context of crystal engineering and
designing new gelators.
Thus, 32 PAM salts derived from variously substituted
phenylacetic acid (PA) and both (R)- and (S)-1-phenylethyl
amines (PEA) were synthesized (Scheme 2). Single crystal
structures of all the PAM salts revealed the exclusive formation
of synthon W. This article reports the rationale behind
achieving such remarkable control over the formation of
synthon W and its role in gelation in this new series of PAM
salts.
INTRODUCTION
■
Supramolecular gel, a solid-like material containing mostly
solvent and a small amount of solute (gelatoralso known as
low molecular weight gelators or LMWGs1), has gained
widespread appeal ranging from academic interests to
technological applications.2 However, the mechanistic inter-
pretation3 of the gelation process is poorly understood. Studies
indicate that the gelator molecules self-assemble to form fibrils
driven by various noncovalent interactions such as hydrogen
bonding,4 halogen bonding,5 π−π stacking,6 hydrophobic
interactions,7 charge transfer,8 etc. The fibrils then self-assemble
to form a network structure known as self-assembled fibrillar
networks (SAFINs).9 Immobilization of gelling solvent within
SAFINs via capillary forces ultimately results in gel. Macro-
scopic visualization of such an event is evident from the ability
of a gel sample to withstand its own weight against gravity.
Unfortunately, molecular level understanding of the gel-
forming mechanism is in its infancy, and therefore designing
a gelator molecule is indeed a daunting challenge. Nevertheless,
efforts have been made by various groups to design gelators.10
We have been engaged in designing salt-based organic
gelators following a supramolecular synthon11 approach in the
context of crystal engineering12 and have identified various gel-
forming supramolecular synthons, namely, primary ammonium
monocarboboxylate (PAM),13 primary ammonium dicarbox-
ylate (PAD),14 secondary ammonium monocarboxylate
(SAM),15 and secondary ammonium dicarboxylate (SAD).16
Among these synthons, the PAM synthon is quite intriguing,
and there are mainly two kinds of PAM synthonsboth a one-
dimensional (1D) columnar hydrogen bonded network (HBN)
Received: December 14, 2013
Revised: April 1, 2014
Published: April 1, 2014
© 2014 American Chemical Society
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A close look at the PAM synthon W revealed that there were
three kinds of hydrogen bonds involving the ammonium cation
and the carboxylate anion leading to the formation of a 1D
columnar network propagating along a chiral 2-fold screw axis
(21-screw). On the other hand, synthon X was intrinsically
centrosymmetric having a center of inversion relating
alternating 8- and 12-membered hydrogen bonded rings. In
fact, further analyses suggested that chirality of the reacting
components indeed had an influence on the formation of PAM
synthon W; thus, ∼48% of the PAM salts in category II having
synthon W had chiral amines, whereas nearly 67% of the PAM
salts containing synthon W in category IV contained chiral
amines. Moreover, a detailed study of PAM salts derived from
chiral amines and achiral acids indicated that the chirality of the
amine indeed played a crucial role in shaping up the resultant
supramolecular synthon;17 it was observed that ∼72% of the
salts studied in these reports displayed PAM synthon W. In a
series of systematic studies pertaining to the supramolecular
chirality transfer,18 it was observed that ∼79% PAM salts having
chiral cationic component displayed synthon W. In a recent
report19 on a series of chiral PAM salts displaying intriguing
gelation and slow release of pheromones for certain pests, it
was observed that all the salts for which the single crystal
structures could be determined showed synthon W. Thus, it
was clear that molecular chirality of one of the components of
PAM salts played a crucial role in promoting asymmetric PAM
synthon W. Moreover, there were reports that suggested
chirality played vital role in gelation.20 Thus, we considered a
new series of PAM salts derived from variously substituted
phenylacetic acid and a chiral amine, namely, (R)- and (S)-1-
phenylethyl amine for the present study.
Scheme 1. Most Frequently Observed PAM Synthons “W”
and “X”
RESULTS AND DISCUSSION
CSD Analyses. To achieve such a goal, we first looked into
■
the Cambridge Structural Database (CSD version 1.15) for
PAM salts with −COO− and NH3 as a search fragment. The
+
number of hits obtained was 2685. Excluding solvate and other
functionality capable of forming hydrogen bond resulted in 224
hits, which were then divided into four categories wherein all
the possible combinations of aromatic and aliphatic moieties of
both the cation and anion of the salts were considered. PAM
synthon W was found to be dominating in categories II and IV
(∼66 and ∼56%, respectively); categories I and III were not
considered due to an insignificant number of hits (Scheme 3).
Syntheses. All the PAM salts depicted in Scheme 2 were
synthesized by reacting the acid and the amine in a 1:1 molar
ratio in MeOH at room temperature. After complete
evaporation of the solvent, the isolated solids were subjected
to Fourier transform infrared spectroscopy (FT-IR). The
absence of a band at 1697−1708 cm−1 and appearance of a
Scheme 2. 32 PAM Salts Studied in the Present Study
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Scheme 3. CSD Search on PAM Salts
new band at 1533−1587 cm−1 clearly indicated that complete
deprotonation of the carboxylic acid moiety took place
establishing the stoichiometry of these salts as 1:1.
electron-withdrawing substitution, for example, −F, −NO2 at
2-position led to the space group P212121 (entry 26−29, Table
1) instead of P21, whereas an electron-donating substituent, for
example, −Me at 4-position displayed P212121 (entry 30−31,
Table 1) instead of C2. In each category of the crystals, there
were several subgroups containing isomorphous crystal
structures; there were 3, 2, and 5 isomorphous subgroups in
P21, C2, and P212121, respectively.
Thus, salts having opposite chirality in the cationic species
with identical anionic component or identically positioned
substituents in the anionic component (e.g., Me/NO2 or
halogens) were isomorphous except for the salts (R)-PEA·
4NPA (P212121) and (S)-PEA·4NPA (C2). Detail SXRD
analyses of these salts revealed that PAM synthon W was
present exclusively in all the crystal structures (Figure 1 and
Figures S1−S11, Supporting Information); the carboxylate
anion and the ammonium cation were involved in three
different kinds of hydrogen bonding sustained by N−H···O
interactions [N···O = 2.6822(15)−3.355(3) Å; ∠N−H···O =
145.2−179.4°] that led to the formation of 1D HBN consisting
of 10-membered hydrogen bonded rings (synthon W).
It was remarkable that the exclusive formation of PAM
synthon W did not depend on the nature of chirality (R or S)
of the cationic species, nor did it depend on the position and
electronic nature of the substituents on the anionic moiety
indicating the robustness of PAM synthon W in this series of
salts. In all the cases, the 1D HBN displaying PAM synthon W
was packed in parallel fashion along the shortest cell axis a or b.
In most of the cases, the packing of the 1D networks was
reinforced by various noncovalent interactions such as C−H···π
Single Crystal X-ray Diffraction (SXRD). X-ray quality
single crystals were obtained from various solvent mixtures
(Supporting Information); it may be noted that in each and
every crystallization experiment, commercial petrol was needed
to get good quality single crystals suitable for X-ray diffraction
studies. Fortunately, we were successful in growing single
crystals of all 32 salts. SXRD revealed that the salts belonged to
three different categories depending on their crystal system and
space group (Tables S1−S6, Supporting Information). While
most of the salts (17 salts) belonged to the non-
centrosymmetric orthorhombic space group P212121, the rest
displayed the non-centrosymmetric monoclinic space groups
P21 (8 salts) and C2 (7 salts). Since chirality (R or S) does not
influence the resultant space group (except in the cases of 11
enantiomorphic pairs of space groups), most of the salts having
a common carboxylate anion crystallized in the identical space
group irrespective of the chirality (R or S) of the ammonium
cation; there seems to be, however, an exception for the cases
of (S)-PEA·4NPA (C2) and (R)-PEA·4NPA (P212121) (entry
13 and 32 respectively, Table 1). On the other hand, the
substituents on the phenyl ring of the anionic component
appeared to have a significant influence on the resultant space
group; thus, unsubstituted or 2-substituted phenyl acetate salts
belonged to the space group P21 (entry 1−8, Table 1), whereas
4-substitution led to the space group C2 (entry 9−15, Table 1).
All the 3-substituted derivatives crystallized in the space group
P212121 (entry 16−25, Table 1). Interestingly, strongly
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Table 1. Various Crystallographic Parameter and Gelation Property of the Salts
a
entry
salt
space group
cell parameters (a, b, c, β)
synthon
G/NG
2
(R)-PEA·PA
(S)-PEA·PA
P21
15.748(9), 6.067(3), 15.816(8), 104.771(16)
15.748(9), 6.067(3), 15.816(8), 104.771(16)
9.1696(3), 6.1000(2), 14.0886(6), 102.164(2)
9.0497(4), 6.0462(3), 13.8277(6), 100.174(2)
9.1044(4), 6.1014(3), 14.0785(6), 102.4640(10)
8.9899(4), 6.0200(3), 13.8303(6), 100.963(3)
11.6182(10), 6.0241(5), 11.6695(10), 106.132(2)
11.6215(10), 6.0357(5), 11.6740(10), 106.130(2)
18.814(4), 5.7401(12), 14.803(3), 91.957(5)
18.7783(9), 5.7284(3), 14.7747(6), 91.946(5)
18.7305(10), 5.7286(3), 14.6009(8), 91.488(3)
18.2329(6), 5.6931(2), 14.5435(4), 92.2190(10)
18.4839(6), 5.86480(10), 14.7362(4), 94.277(3)
20.1479(17), 6.3269(5), 12.3797(10), 92.249(5)
20.1528(9), 6.3263(3), 12.3730(5), 92.303(4)
6.4736 (16), 12.144(3), 20.562(5),
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
W
G
1
P21
G
3
(R)-PEA·2BPA
(S)-PEA·2BPA
(R)-PEA·2CPA
(S)-PEA·2CPA
(R)-PEA·2MPA
(S)-PEA·2MPA
(R)-PEA·4BPA
(S)-PEA·4BPA
(R)-PEA·4CPA
(S)-PEA·4CPA
(S)-PEA·4NPA
(R)-PEA·4FPA
(S)-PEA·4FPA
(R)-PEA·3BPA
(S)-PEA·3BPA
(R)-PEA·3CPA
(S)-PEA·3CPA
(R)-PEA·3FPA
(S)-PEA·3FPA
(R)-PEA·3MPA
(S)-PEA·3MPA
(R)-PEA·3NPA
(S)-PEA·3NPA
(R)-PEA·2FPA
(S)-PEA·2FPA
(R)-PEA·2NPA
(S)-PEA·2NPA
(R)-PEA·4MPA
(S)-PEA·4MPA
(R)-PEA·4NPA
P21
NG
NG
NG
NG
NG
NG
NG
NG
NG
NG
G
4
P21
5
P21
6
P21
7
P21
8
P21
9
C2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
C2
C2
C2
C2
C2
NG
G
C2
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
P212121
NG
NG
NG
NG
G
6.4741(3), 12.1369(5), 20.5318(8)
6.4880(3), 12.0928(6), 20.1117(10)
6.4865(7), 11.9271(12), 19.820(2)
6.4720(5), 12.2437(8), 19.4058(14)
6.5083(2), 12.0454(3), 18.8508(5)
G
5.7242(3), 12.1610(7), 22.5922(13)
NG
NG
G
5.7217(3), 12.1561(7), 22.5944(12)
5.7484(5), 12.1833(10), 22.8871(18)
5.7438(3), 12.1764(7), 22.8889(12)
G
5.9075(11), 12.565(2), 20.215(4)
NG
NG
NG
NG
NG
NG
G
5.9102(7), 12.5547(15), 20.197(2)
6.8857(5), 12.8536(9), 18.8943(14)
6.873(4), 12.792(6), 18.270(9)
6.7277(8), 8.5988(10), 27.314(3)
6.7196(11), 8.5932(14), 27.330(5)
5.7057(8), 15.291(2), 18.545(3)
a
G = Gelator and NG = Nongelator.
Figure 1. Illustration of single crystal structures; (A) propagation of the 1D HBN displaying PAM synthon W present in all the 32 salts, (B−D)
representative examples of PAM synthon W present in the salts (S)-PEA·2MPA, (S)-PEA·2MPA, and (R)-PEA·3NPA belonging to the space group
category of P21, C2, and P212121, respectively.
(3.72−3.76 Å) and C−H···O (3.28−3.78 Å) (Supporting
Information). Powder X-ray diffraction (PXRD) confirmed the
crystalline phase purity of all the salts except (R)- and (S)-PEA·
4FPA (vide infra and Figures S12−S14, Supporting Informa-
tion).
ability of these salts by scanning 14 solvents, both polar and
nonpolar (Figure 1). The fact that out of 32 salts, only 9 salts
(∼28%) were found to gel various solvents with good to
moderate efficiency displaying minimum gelator concentration
(MGC) of 1.8−4.0 wt % with gel−sol dissociation temperature
(Tgel) of 55−112 °C clearly indicated that the 1D HBN was an
important criterion for gelation but not a necessary and
Gelation Studies. Having established the existence of 1D
supramolecular PAM synthon W, we evaluated the gelation
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Figure 2. Representative photographs of the 4.0 wt % gel of various gelator salts in different solvents; Panels A−F represent the 4.0 wt % gel derived
from (R)-PEA·PA, (R)-PEA·3NPA, (R)-PEA·4NPA, (S)-PEA·PA, (S)-PEA·3NPA, and (S)-PEA·4NPA, respectively.
sufficient condition (Table S7, Supporting Information). Other
parameters such as surface compatibility of gel network and
target solvent should influence gelation. The position and
nature of the substituents on the carboxylate moiety of the
PAM salts that might have significant influence on gel network/
target solvent interactions seemed to have played a crucial role
in gelation. Thus, salts having substituents at 2-position were all
nongelators, whereas electron-withdrawing substituents at 3-
and 4-positions and also no substitution seemed to have
promoted gelation. Salts having an unsubstituted anionic
moiety, namely, (R)- and (S)-PEA·PA, were found be the
most versatile displaying gelation of 10 solvents out of 14
solvents tested. Tgel versus [gelator] plots (Figure 3) of all the 9
gelator salts in various solvents revealed that Tgel increased with
the increase in gelator concentration in most of the cases
indicating the supramolecular nature of the gel network.21
Because of the lack of common solvent for gelation and
differences in MGC, it was not possible to comment on the
influence of the substituents on the thermal stability of the gels.
However, close inspection of Figure 3 revealed that there were
two comparable groups of gelator salts, namely, (S)-PEA·
4NPA, (R)-PEA·3FPA, (S)-PEA·3NPA and (R)-PEA·4NPA,
(S)-PEA·PA, (R)-PEA·3NPAboth groups gelled a common
solvent (p-xylene) with different starting gelator concentration
(4.0 and 3.0 wt %, respectively); it was observed that 4-NO2
substituted gelator salts were found to be producing thermally
more stable gels than the gels derived from gelator salts having
3-NO2 substitution in each group displaying the role of the
position of the substituents on gelation. It may be mentioned
here that all the gels were stable at room temperature for
several months.
To assess the viscoelastic response of some of the selected
gels, we carried out dynamic rheology wherein the viscous
modulus G′ and loss modulus G″ were plotted against
frequency ω (rad·s−1) with a constant strain of 0.1%. For this
purpose, we have chosen an enantiomeric pair of gelators
keeping the gelling solvent and concentration of the gelator
fixed within the pair. In all the cases studied, G′ was
significantly more than G″, and they were found to be
frequency invariant for a considerable time period thereby
establishing their viscoelastic (gel) nature (Figure 4). Critical
evaluation of the G′ revealed that for the salt PEA·PA, the (S)-
enantiomer was much stronger than (R)-enantiomer, whereas
the observation was opposite for the salt PEA·3NPA. For PEA·
4NPA, both the enantiomers displayed similar mechanical
strength (Table S8, Supporting Information).
Field emission scanning microscopy (FE-SEM) of some
selected gels revealed the morphology of the gel network
(Figure 5); a bundle of fibers, porous architecture, and highly
entangled fibrous networks were observed in the gels. Close
inspection of Figure 5C and D revealed the existence of twisted
fibers,22 which was further confirmed by atomic force
microscopy (AFM) (Figure 6) on one of the gel (S)-PEA·
3NPA in m-xylene); it was clear that most of the fibers
observed in the AFM were of left-handed chirality.
Figure 3. Tgel vs [gelator] plots; p-xylene [(S)-PEA·PA, (S)-PEA·
3FPA, (R)-PEA·3NPA, (S)-PEA·3NPA, (R)-PEA·4NPA], mesitylene
[(S)-PEA·4NPA, (S)-PEA·PA], methyl salicylate [(R)-PEA·3FPA],
and 1,2-dichlorobenzene [(S)-PEA·4FPA] were employed as gelling
solvents.
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Figure 4. Frequency sweep experiment; (A) 4.0 wt % nitrobenzene gel of (R)-PEA·PA and (S)-PEA·PA, (B) 4.0 wt % toluene gel of (R)-PEA·
3NPA and (S)-PEA·3NPA, (C) 4.0 wt % p-xylene gel of (R)-PEA·4NPA and (S)-PEA·4NPA at 25 °C.
Figure 6. AFM image of the xerogel of (S)-PEA·3NPA in m-xylene.
(A) Left-handed helices; (B) zoomed in view of left-handed helices,
and (C) height profile of the left-handed helix as marked a−b.
98% enantiomerically pure; as a result, the observation of a few
fibers having right-handed chirality in AFM could be due to the
presence of a small amount of other enantiomers.
Figure 5. SEM micrographs of the xerogels; (A) (S)-PEA·PA and (B)
(R)-PEA·PA in mesitylene, (C) (S)-PEA·3NPA in m-xylene, (D) (S)-
PEA·3NPA in toluene, (E) (S)-PEA·3NPA in o-xylene.
Structure−Property Correlation. To probe the role of
the supramolecular PAM synthon W on gelation, we attempted
structure−property correlation originally proposed by Weiss et
al.24 The PXRD patterns of the xerogels were compared with
that of the bulk solid and simulated PXRD patterns obtained
from SXRD data. It was revealed that in all the cases, except
(S)-PEA·4FPA, the patterns were nearly superimposable
suggesting that the crystal structure determined from SXRD
However, a small number of fibers having opposite chirality
were also observed. Such an occurrence of a mixture of both
left- and right-handed helices from a pure enantiomer was
reported to be rare, and it was explained by invoking chiral
symmetry-breaking theories.23 It may be noted here that the
chiral amine used for synthesizing these PAM salts was only
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Figure 7. PXRD comparison plots for the gelator salts under various conditions.
truly represented that in the bulk solid as well as in the xerogel
(Figure 7). Thus, PAM synthon W was present in these gel
networks in the corresponding xerogels. In the case of (S)-
PEA·4FPA, it appeared that PXRD patterns of simulated and
xerogel were nearly superimposable, meaning that in this case
also synthon W prevailed in the gel network in the xerogel.
However, the PXRD pattern of the bulk solid of (S)-PEA·
4FPA showed significant differences from that of the simulated
and xerogel PXRDs indicating that there were probably other
crystalline phases present in the bulk solid. It may be
mentioned that one cannot rule out the possibility of crystalline
phase changes or a new event of nucleation resulting in a new
crystal phase during xerogel formation due to solvent
evaporation. Therefore, ideally PXRD patterns of the gel state
should be considered. Acquiring a good quality PXRD pattern
from the gel state is often difficult due to the scattering of
solvent molecules and less amount of gelators in the gel.
However, there is no guarantee that such a phase change should
always occur. Thus, comparing PXRD patterns of xerogel in
structure−property correlation has been a reasonable com-
promise.
Conclusions. Control over the exclusive formation of an 1D
asymmetric PAM synthon, namely, synthon W, over the other
symmetric possibility (synthon X) was achieved in 32 PAM
salts by careful consideration of CSD analyses. Introduction of
the chiral ammonium cation [(R)/(S)-1-phenylethylammo-
nium] as one of the components in these PAM salts ensured
the exclusive formation of asymmetric PAM synthon W as
revealed by SXRD analyses of all the 32 PAM salts. However,
only 28% of the PAM salts displayed moderate to good gelation
ability with reasonable thermal stability with various polar and
nonpolar solvents emphasizing the fact that 1D HBN was an
important criterion for gelation but not the necessary and
sufficient condition. Position and electronic effects of the
substituents seemed to have played important roles in gelation.
The position of substituents seemed to have influenced the
thermal stability of the gels. Structure−property correlation
based on PXRD and SXRD established that the PAM synthon
W was present in the xerogel networks as well. Existence of
helical fibers in the xerogel of (S)-PEA·3NPA in m-xylene was
noteworthy, and a study toward the understanding of
expression of molecular chirality into macroscopic chirality
(as manifested in twisted fibers) will be undertaken in the
future.
EXPERIMENTAL SECTION
■
Materials and Methods. All the chemicals were commercially
available and used as received without further purification. Melting
points of all 32 salts were determined by Veego programmable melting
point apparatus, India. FT-IR spectra were recorded on a FT-IR-8300,
Shimadzu. The elemental compositions of the purified salts were
established by elemental analysis (Perkin-Elmer Precisely, Series-II,
CHNO/S Analyzer-2400). 1H NMR spectra were recorded on a
Brukar AVANCE DPX 300 (for 300 MHz) and III 500 (for 500
MHz). Rheological experiments were performed with an AR 2000
advanced rheometer (TA Instruments). FE-SEM was obtained in a
JEOL, JMS-6700F. AFM images were recorded by an AUTOPROBE
CP base unit, di CP-II instrument, model no. AP0100. Powder X-ray
patterns at various conditions were recorded on a Bruker AXS D8
Advance powder (Cu Kα1 radiation, λ = 1.5406 Å) diffractometer.
General Synthetic Procedure. All 32 salts were synthesized by
taking the equimolar amount of the individual components in
methanol in a 25 mL beaker. The reaction mixture was sonicated
for a few minutes followed by gentle warming if necessary. The
resulting solution was kept undisturbed in open air, and a white solid
was left after the complete removal of the solvent. The solids were
then washed with petroleum ether and recrystallized from either
MeOH/petroleum ether or MeOH/commercial petrol. The recrystal-
lized salts were then subjected to various physicochemical analyses and
gelation study.
Gelation Study. Tgel Measurements. In a typical experiment, 20
mg of each salt was taken in a test tube (10 × 100 mm) and dissolved
in 500 μL of the targeted solvent by gentle heating. The solution was
then allowed to cool to room temperature. Gel formation was
confirmed by tube inversion. Tgel was measured by the dropping ball
method; a glass ball weighing 242.0 mg was placed on top a 0.5 mL gel
taken in a test tube. The tube was then immersed in an oil bath placed
on a magnetic stirrer to ensure uniform heating. The temperature was
noted when the ball touched the bottom of the test tube.
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FE-SEM. Dilute solution of a gelator salt was drop casted on the
glass plate fixed with the standard metallic SEM stub and dried under
ambient condition. The samples were coated with platinum prior to
the SEM image recording.
AFM. The sample for the AFM was prepared by depositing a drop
of a dilute m-xylene solution of a gelator salt (0.2 wt %) on a cover
slide. The sample was dried under a vacuum at room temperature for
overnight prior to the recording of AFM images.
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package of SMART APEX. All structures were solved by direct method
and refined in a routine manner. In most of the cases, non-hydrogen
atoms were treated anisotropically. The most of the hydrogen atoms
were fixed geometrically. CCDC (CCDC Nos. 953786−953817)
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ASSOCIATED CONTENT
* Supporting Information
Physicochemical data, gelation data, additional figures, crystallo-
■
S
graphic parameters, molecular plot with hydrogen bonding
(5) (a) Meazza, L.; Foster, J. A.; Fucke, K.; Metrangolo, P.; Resnati,
G.; Steed, J. W. Nat. Chem. 2013, 5, 42−47. (b) Priimagi, A.; Cavallo,
G.; Metrangolo, P.; Resnati, G. Acc. Chem. Res. 2013, 46, 2686−2695.
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1
parameters, H NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org
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D. J. Org. Chem. 2007, 72, 7270−7278. (c) Allix, F.; Curcio, P.; Pham,
Q. N.; Pickaert, G.; Jamart-Greg
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J.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ocpd@iacs.res.in; parthod123@rediffmail.com.
Notes
■
́
oire, B. Langmuir 2010, 26, 16818−
16827. (d) Ryan, D. M.; Doran, T. M.; Nilsson, B. L. Langmuir 2011,
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Ed. 2010, 49, 6576−6579. (b) Haketa, Y.; Sasaki, S.; Ohta, N.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.K.A. and P.D. thank CSIR, New Delhi, for a senior research
fellowship and financial support, respectively. SXRD data were
collected at the DBT-funded X-ray diffraction facility under the
CEIB program in the Department of Organic Chemistry, IACS,
Kolkata.
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