Supramolecular chirality in organo-, hydro-, and metallogels derived from bis-amides of L -(+)-tartaric acid: Formation of highly aligned 1D silica fibers and evidence of 5-c net SnS topology in a metallogel network

Article abstract of DOI:10.1002/chem.201200871

A series of bis-amides derived from L-(+)-tartaric acid was synthesized as potential low-molecular-weight gelators. Out of 14 bis-amides synthesized, 13 displayed organo-, hydro-, and ambidextrous gelation behavior. The gels were characterized by methods

Full text of DOI:10.1002/chem.201200871

FULL PAPER
DOI: 10.1002/chem.201200871
Supramolecular Chirality in Organo-, Hydro-, and Metallogels Derived from
Bis-amides of l-(+)-Tartaric Acid: Formation of Highly Aligned 1D Silica
Fibers and Evidence of 5-c Net SnS Topology in a Metallogel Network
Uttam Kumar Das and Parthasarathi Dastidar*[a]
Abstract: A series of bis-amides de-
rived from l-(+)-tartaric acid was syn-
thesized as potential low-molecular-
weight gelators. Out of 14 bis-amides
synthesized, 13 displayed organo-,
hydro-, and ambidextrous gelation be-
havior. The gels were characterized by
methods including circular dichroism,
differential scanning calorimetry, opti-
cal and electron microscopy, and rheol-
ogy. One of the gels derived from di-3-
pyridyltartaramide (D-3-PyTA) dis-
played intriguing nanotubular morphol-
ogy of the gel network, which was ex-
conditions. A structure–property corre-
lation on the basis of single-crystal and
powder X-ray diffraction data was at-
tempted to gain insight into the struc-
tures of the gel networks in both
organo- and metallogels. Such study
led to the determination of the gel-net-
work structure of the CuII coordina-
tion-polymer-based metallogel, which
displayed a 2D sheet architecture made
of a chloride-bridged double helix that
resembled a 5-c net SnS topology.
ploited as
a template to generate
highly aligned 1D silica fibers. The ge-
lator D-3-PyTA was also exploited to
generate metallogels by treatment with
various CuII/ZnII salts under suitable
Keywords: amides · gels · supramo-
lecular chemistry · tartaric acid ·
X-ray diffraction
Introduction
priori a gelator molecule. Nevertheless, there have been ef-
forts by various groups to design gelator molecules. Weiss
et al. have designed a series of aromatic linker steroid
(ALS)-based gelators by rationalizing the observations of
existing literature.[2] van Esch et al. reported C3-symmetric
hydrogelator derivatives wherein the critical balance of hy-
drophobicity and hydrophilicity were rationally introduced
to effect hydrogelation.[18] Our group has shown that the su-
pramolecular synthon approach is quite successful in design-
ing organic-salt-based gelators.[19]
Chiral gels that represent supramolecular chirality are im-
portant in chiro-optical switches,[20] chiral catalysis,[21] helical
crystallization of proteins[22] and inorganic replicas,[5] chiral
resolution,[23] and so on. We have been engaged in develop-
ing easily accessible chiral gels.[24] Amide functionality is
known to impart gelation in many gelators reported.[25] It is
believed that the self-complementary 1D hydrogen-bonding
interactions of secondary amide[26] lead to gel formation. l-
(+)-Tartaric acid (TA) is a commercially available and inex-
pensive chiral source. We have therefore decided to exploit
TA by installing amide functionality. Surprisingly, there is
no amide-based gelator that has been derived from TA re-
ported in the literature. Oda et al., however, reported gels
derived from gemini surfactants derived from TA-based
salts.[27]
Low-molecular-mass organic gelators (LMOG) are small or-
ganic molecules that can entrap a large volume of solvent,
thus resulting in solidlike viscoelastic materials.[1] The aniso-
tropic supramolecular (noncovalent) interactions (such as
hydrogen bonding, van der Walls forces, p–p interactions,
and so on) among the gelator molecules lead to the highly
directional metastable self-assembled fibrillar networks
(SAFINs).[2] The SAFINs are further entangled to form a
3D network within which the solvent molecules are immobi-
lized by means of surface tension. For the last two decades,
research activities on supramolecular gels have increased be-
cause of their potential applications in, for example, conser-
vation of art,[3] electro-optics/photonics,[4] structure-directing
agents,[5] cosmetics,[6] drug delivery,[7] biomedical applica-
tions,[8] sensors,[9] oil recovery,[10] catalysis,[11] and so on. For
various reasons, such as wide structural diversities in known
gelator molecules (cholesterol and anthracene derivative-
s,[9a,12] surfactants,[13,27] porphyrins and phthalocyanines,[14,42a]
carbohydrates[15] and peptide derivatives,[16] bis-urea deriva-
tives,[17] and so on) or a lack of molecular-level understand-
ing of the gelation mechanism, it is difficult to design a
[a] U. K. Das, Dr. P. Dastidar
In this paper we report a series of TA-based bis-amide ge-
lators (Scheme 1). The amides were characterized by FTIR,
1H and 13C NMR spectroscopy, and HRMS. Most of the
amides showed gelation properties. Compound D-3-PyTA
showed interesting metallogelation behavior. Single-crystal
structures of three gelator and one nongelator molecules
have also been reported. The gels were characterized by dif-
Department of Organic Chemistry
Indian Association for the Cultivation of Science (IACS)
2A and 2B, Raja S. C. Mullick Road
Jadavpur, Kolkata, 700032, West Bengal (India)
Fax : (+91)33-2473-2805
E-mail: ocpd@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200871.
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thermal stability of the gels. Figure 1 depicts the plot of Tgel
versus [Gelator] wherein the gels were prepared in a binary
mixture of DMF and water with varying concentrations of
Scheme 1. Bis-amides of l-(+)-tartaric acid.
ferential scanning calorimetry (DSC), rheology, scanning
and transmission electron microscopy (SEM, TEM). Struc-
ture–property correlation has been attempted in some se-
lected cases by using powder and single-crystal X-ray dif-
fraction (PXRD, SXRD).
Figure 1. Tgel versus [Gelator] plots for some selected gels.
DMF (total volume=0.5 mL). The Tgel gradually increased
with [Gelator], which means that the gel-forming networks
were governed by supramolecular interactions (presumably
hydrogen-bonding, alkyl–alkyl hydrophobic interactions).
The fact that the hydrophobic interactions that involve the
alkyl chains indeed played a crucial role in gelation was
clear from the highest Tgel of the gel derived from the gela-
tor with the longest alkyl chain (DTDTA).
Results and Discussion
Gelation studies: All the compounds were prepared follow-
ing a modified literature procedure[28] (see the Experimental
Section). The compounds were scanned for gelation proper-
ties with 18 prototype solvents, both polar and nonpolar
(Table S15 in the Supporting Information).
All the gels (confirmed by the test-tube inversion
method[29]) were thermoreversible and stable for months
under ambient conditions. Many of the gelators showed am-
bidextrous (i.e., capable of gelling both organic and aqueous
solvents) gelling behavior. When the terminal substituents
were n-alkyl chains, the gelation abilities of the correspond-
ing bis-amide derivatives were quite versatile and displayed
the ability to gel most of the solvents studied herein. Inter-
estingly, when the terminal substituents were replaced with
aromatic moieties, the gelation behavior of the correspond-
ing compounds changed drastically. Whereas 3-pyridyl (D-3-
pyTA) and 4-nitro (B-4-NPhTA) derivatives displayed only
hydrogelation, the corresponding phenyl derivative
(DPhTA) was found to be a nongelator, and the 4-methoxy
derivative (B-4-MPhTA) displayed ambidextrous behavior.
The role of the pyridyl N in gelation was probed by carrying
out gelation of D-3-pyTA in a solution of DMF/water with
pH of varying acidity. It was observed that D-3-pyTA failed
to produce gel below pH 7.0 (Figure S1 in the Supporting in-
formation), thus indicating possible protonation of the pyr-
idyl N atom and thereby disruption of the hydrogen bond-
ing, which is presumably responsible for the formation of
the desired SAFIN required for gelation. The minimum ge-
lator concentration (MGC) and the gel dissociation temper-
ature Tgel were within the range of 0.35–4 wt% and 56–
1208C, respectively, thus indicating significant efficiency and
Calorimetric characterization of two selected gels derived
from the gelators with the longest and shortest alkyl chains
(DTDTA and DBuTA, respectively) in the series showed in-
triguing results (Figure 2).
In both the cases, there were two peaks in each endother-
mic and exothermic run. The peaks in DTDTA gel were
much sharper than those in DBuTA gel, which could be due
to the stronger gel strength of the DMSO gel of DTDTA.
Since the existence of two peaks in each run might indicate
solvotropic liquid-crystalline (LC) behavior,[30] we examined
these gels under hot-stage optical microscopy. However, the
microscopic observations did not support the formation of
LC phases (Figure S2 in the Supporting Information). Thus,
the peaks at 91 and 1038C for DMSO and toluene gel of
DTDTA and DBuTA, respectively, could be due to the crys-
talline phase change of the SAFINs during gel–sol transi-
tion; the other two peaks at 106 and 868C in respective
cases represented the gel–sol transition (Figure 1).
Since it is difficult to assess the enthalpy change (DH) as-
sociated with the process of the gel–sol transition because of
the generally observed broad peaks in the endotherm of the
DSC traces, we made an attempt to extract the DH values
associated with a gel–sol transition from the Tgel versus [Ge-
lator] plot (Figure 1).[31] Interestingly, when the semilog of
the mole fraction of each gelator at each concentration ex-
tracted from Figure 1 was plotted against 1/Tgel Kꢀ1, nice
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Bis-Amides of l-(+)-Tartaric Acid
FULL PAPER
tion and a definite amount of gelator is involved in the
phase transition. From the plots, the enthalpy DHm was cal-
culated to be within the range of 14.4–97.7 kJmolꢀ1.
Microscopy: SEM micrographs of the DMF/water xerogels
of some selected gelators revealed interesting results. In the
cases of alkyl-substituted gelators, the corresponding xero-
gels displayed discrete plate (DBuTA, DPenTA, DNonTA,
DUnDTA, DTDTA), discrete tape (B-4-MPhTA), and
highly entangled tape (DHexTA, DDTA) morphologies.
The most intriguing morphology was observed in the case of
D-3-PyTA, wherein highly aligned nanotubes could be seen.
The presence of nanotubular morphology also was con-
firmed in highly reproducible TEM images (Figure 4). This
Figure 2. DSC traces of A) approximately 4.0 wt% DMSO gel of
DTDTA; B) approximately 4.0 wt% toluene gel of DBuTA.
linear plots were obtained that represented the Schroeder–
van Laar equation [Eq. (1), Figure 3].[32]
ln½gelatorꢁ ¼ ꢀðDHm=RTgelÞ þ constant
ð1Þ
in which DHm is the enthalpy of melting and Tgel is the tran-
sition temperature of the gel–sol transition, which means
that the gel–sol transition can be considered a first-order
transition if we consider that the gel melts into an ideal solu-
Figure 4. SEM of selected xerogels and TEM of the xerogel of D-3-
PyTA; all the gels were prepared in DMF/water.
Figure 3. Semilog plot of the mole fraction of the gelators against 1/T.
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U. K. Das and P. Dastidar
observation prompted us to exploit the nanotubular mor-
phology of the gel network as template[5] to generate highly
aligned 1D fibrous silica from Si
in the Supporting Information).
ACHTNUGTRENN(UGN OC2H5)4 (TEOS; Figure S6
Rheology: Dynamic rheology was carried out on some se-
lected gels to evaluate their gel-like response.[33] In typical
frequency sweep experiments, the elasticity modulus G’ and
the viscosity modulus G’’ were plotted as a function of the
angular frequency ꢁwꢂ at a constant strain of 0.01%. In these
experiments, 4 wt% DMF/water gels of the gelators with
the shortest and longest alkyl chains (DBuTA and DTDTA,
respectively) were selected to see the effect of the length of
the alkyl chain on the viscoelastic behavior of the corre-
sponding gels. Figure 5 shows that in both the cases the G’
Figure 6. Yield stress versus the number of carbon atoms present in the
aliphatic amine of bis-tartaramide derivatives.
These counterintuitive observations could be because of
the more crystalline nature of the gel fibers that are derived
from the gelators with the shorter alkyl chains (Figure S3 in
the Supporting Information). In rotational flow curve ex-
periments, the viscosity of the 4 wt% DMF/water gels of
DBuTA and DTDTA showed a steady decrease as the shear
rate increased, which indicates that they were shear-thinning
materials (Figure 7). In the cases of aromatic-substituted ge-
Figure 7. A typical flow curve measurement of gels made in a binary mix-
ture of DMF and water.
Figure 5. Dynamic rheology of (DMF/water) gels of DTDTA and
DBuTA at 258C; the elasticity modulus G’ and viscosity modulus G’’ are
shown as a function of the angular frequency w. The strain used was
0.01%.
lators, namely, D-3-PyTA, B-4-MPhTA, and B-4-NPhTA,
the yield stress of the pyridyl-substituted gelator D-3-PyTA
(4 wt% DMF/water gel) showed maximum yield stress, pre-
sumably due to the participation of pyridyl N atoms in hy-
drogen bonding. Rotational flow curve experiments also
showed the expected trend (Figure S4 in the Supporting In-
formation).
values (191.9 and 15.2 kPa for DBuTA and DTDTA, respec-
tively) remained frequency-invariant over a considerable
period on the timescale and were significantly larger in mag-
nitude than the corresponding G’’ values, which supports
their gel-like response. It is, however, intriguing to note that
the gelator with the shortest alkyl chain in the series dis-
played better viscoelastic properties than the gelator with
the longest alkyl chain. Yield stress experiments with 10 gels
(all 4 wt% DMF/water gels) derived from the n-alkyl-substi-
tuted bis-amide gelators also indicated a similar trend. In
this cases, too, DBuTA displayed highest yield stress
(586 Pa), which gradually decreased as the chain length in-
creased (Figure 6).
Circular dichroism: CD spectra were recorded on two se-
lected gels, namely, DTDTA (4 wt% in toluene) and D-3-
PyTA (4 wt% in DMF/water); the spectra of the corre-
sponding solutions were also obtained. Both the gels and
the corresponding solutions displayed a positive Cotton
effect. The DTDTA gel showed a peak at approximately
223 nm and a trough at approximately 200 nm. The corre-
sponding solution also showed a similar peak and trough
near 208 and 198 nm, respectively. On the other hand, D-3-
PyTA gel showed a peak at approximately 251 nm and a
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Bis-Amides of l-(+)-Tartaric Acid
FULL PAPER
trough at approximately 224 nm; in the corresponding solu-
tion, the peak and trough appeared at approximately 252
and 233 nm, respectively. These CD signals are due to the
p–p* transition of the amide carbonyl, and the patterns are
similar to that of the b sheet in polypeptides.[34] The sharp
peaks at approximately 272 and 302 nm in 3-D-PyTA gel
were due to p–p* and n–p* transitions of the aromatic (pyr-
idyl) moiety and indicated the higher anisotropy in the gel
state. This observation is further supported by the absence
of such peaks in the corresponding solution. However, a
very sharp peak at approximately 245 nm in the solution of
DTDTA in toluene was due to the p–p* transition of the
solvent (Figure 8).
was somewhat prevented.[36] This hypothesis was most ex-
plicitly demonstrated by our group in various organic-salt-
based gelators.[19,37] Since bis-amide can also participate in
the 1D b sheet, we thought it was worthwhile to attempt to
determine the crystal structures of the compounds reported
herein and correlated their structures with the gelation/non-
gelation properties. The presence of two hydroxyl groups in
the tartaric acid backbone was also expected to participate
in hydrogen bonding. With this background, we attempted
to crystallize all the compounds reported in this article. Our
best efforts resulted in crystallization of four compounds, of
which three compounds were gelators (Table S16 in the Sup-
porting Information). The following sections describe the
single-crystal structures of the compounds.
Single-crystal structure of DHepTA: This compound was
crystallized in the non-centrosymmetric monoclinic space
group C2, and its asymmetric unit contained one gelator
molecule. The chiral backbone of the tartaric acid adopted a
conformation wherein the hydroxyl groups were gauche and
the amide moieties were staggered with respect to each
other. The conformations of the alkyl groups were found be
quite different. Whereas the methylene groups in one of the
alkyl chains all showed staggered conformation, the methyl-
ene groups in the other alkyl chain displayed gauche confor-
mation. The dihedral angle that involves the two amide moi-
eties was found be 74.88, which indicates that they were
almost orthogonal to each other. Under such an arrange-
ment, a b sheet that involves the bis-amide moiety could not
ꢀ
be formed. Whereas one of the amide N H groups was in-
volved in complementary hydrogen bonding with the neigh-
boring amide moiety of the adjacent gelator molecule
ꢀ
ꢀ
(N···O=3.064(4) ꢃ; aN H···O=147.48), the N H of the
other amide did not participate in hydrogen-bonding inter-
actions; the >C=O moiety of this amide group was involved
in hydrogen bonding with one of the hydroxyl groups of the
ꢀ
neighboring molecule (O···O=2.617(3) ꢃ; aO H···O=
176(11)8). The other hydroxyl group participated in extend-
ed hydrogen-bonding interactions with the crystallographi-
cally equivalent hydroxyl groups of the neighboring mole-
cules. The overall hydrogen-bonded network may thus be
described as 2D sheet architecture that was further packed
in parallel fashion without displaying any alkyl–alkyl interdi-
gitation (Figure 9).
Figure 8. CD spectra of some selected gels and their corresponding sols.
Single-crystal X-ray diffraction: It is believed that the gelati-
on is
a frustrated crystallization process wherein the
SAFINs are formed from a suitable solution that contains
the gelator molecule.[1] In fact, there are many occasions in
which single crystals of the gelator molecules are obtained
from aging gel samples.[35] The implication of the crystal
structure of the gelator molecule is enormous in designing
gelators.[19] It was proposed a decade ago that 1D hydrogen-
bonded network-promoted gelation and such anisotropic in-
teractions helped the molecules to grow as fibers with a
high aspect ratio, which consequently formed SAFINs under
suitable conditions to impart gelation. However, 2D or 3D
hydrogen-bonded networks either produced weak gel or no
gelation at all, and in these cases, the 1D growth of fibers
Single-crystal structure of D-3-PyTA: This compound was
crystallized in the non-centrosymmetric orthorhombic space
group P212121, and its asymmetric unit contained one mole-
cule. The hydroxyl groups and the terminal amide moieties
displayed a gauche and staggered conformation with respect
to each other. The relative orientation of the terminal pyrid-
yl N atoms was anti, whereas the conformation that involved
the pyridyl N and amide O in each terminal was found to be
syn–syn. The amide pyridyl moiety in each terminal dis-
played a planar conformation with dihedral angles of 13.3
and 22.08. The terminal pyridyl rings were found to be co-
planar and displayed a dihedral angle of 6.148. Interestingly,
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U. K. Das and P. Dastidar
The hydroxyl groups were involved in extensive hydrogen
bonding with the same moieties of the neighboring mole-
ꢀ
cules (O···O=2.850(3) ꢃ; aO H···O=151(3)8 and O···O=
ꢀ
2.9098(9) ꢃ; aO H···O=129(3)8) as well as with one of the
ꢀ
>C=O of the amide moiety (O···O=2.650(3) ꢃ; aO
H···O=162(4)8), thereby resulting in an overall 2D hydro-
gen-bonded network. The 2D networks were packed further
in a parallel fashion (Figure 11).
Figure 9. A) 2D hydrogen-bonding network (alkyl chains not shown for
better clarity) and B) parallel packing of the 2D sheets in DHepTA.
in this structure, typical amide···amide-extended hydrogen
bonding was not observed. Instead, one of the pyridyl N
atoms participated in hydrogen bonding with the amide NH
of a neighboring molecule in an extended fashion (N···N=
2.926(4) ꢃ; aN-H···N=174.58); the other pyridyl N atom
did not participate in any hydrogen-bonding interactions.
The hydroxyl groups in the tartaric acid backbone were in-
volved in hydrogen bonding with the hydroxyl groups of the
ꢀ
neighboring molecules (O···O=3.021(4)–2.765(4) ꢃ; aO
H···O=143(4)–147(5)8), thereby resulting in an overall 3D
hydrogen-bonded network structure (Figure 10).
Figure 11. A) 2D hydrogen-bonding network (phenyl rings not shown for
better clarity) and B) parallel packing of the 2D sheets in DPhTA.
Single-crystal structure of B-4-MPhTA: This compound was
crystallized in the non-centrosymmetric monoclinic space
group P21, and its asymmetric unit contained two molecules.
The chiral backbone of both the molecules showed similar
conformation wherein the amide and the hydroxyl groups
were staggered and gauche, respectively, with each other. In
both the molecules the terminal 4-methoxyphenyl amide
moieties were nonplanar. In one molecule, the terminal 4-
methoxyphenyl amide moieties displayed dihedral angles of
78.13 and 27.388, and the terminal phenyl groups formed a
dihedral angle of 2.98. In the other molecule, the terminal 4-
methoxyphenyl amide moieties showed dihedral angles of
52.11 and 17.788, and the terminal phenyl groups displayed
a dihedral angle of 51.68. In one molecule, the amide NH
moieties did not participate in any hydrogen-bonding inter-
actions. In another molecule, only one of the amide NH
moieties was involved in hydrogen-bonding interactions
with one of the >C=O groups of the amide moiety of the
ꢀ
ꢀ
Figure 10. 3D hydrogen-bonding network illustrating N H···N and O
H···O interactions in D-3-PyTA.
Single-crystal structure of DPhTA: Compound DPhTA was
crystallized in the non-centrosymmetric orthorhombic space
group P212121, and its asymmetric unit contained one mole-
cule. The chrial backbone adopted a conformation wherein
the amide and the hydroxyl groups were staggered and
gauche, respectively, with each other. The terminal phenyl
amide moieties were nonplanar and displayed dihedral
angles of 32.7 and 27.08, and the terminal phenyl groups
formed a dihedral angle of 17.38. The amide NH moieties
did not participate in any hydrogen-bonding interactions.
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Bis-Amides of l-(+)-Tartaric Acid
FULL PAPER
ꢀ
neighboring molecule (N···O=3.056(2) ꢃ; aN H···O=
145.58). The hydroxyl groups were involved in extensive hy-
drogen bonding with the same moieties of the neighboring
of a new crystalline phase of the gelator molecule, thereby
resulting in a mismatch of the corresponding PXRD pat-
terns. However, there is no guarantee that such an event
will always take place. To obtain the structure of the
SAFINs in the gel state, a more powerful synchrotron beam-
line followed by ab initio structure determination from the
PXRD data thus obtained should be employed. Since a syn-
chrotron beamline is not so readily accessible and ab initio
determination of the structure from PXRD has not yet
become routine,[39] the comparison of PXRD data of xerogel
with that of bulk and simulated data is the available option.
Thus, we performed PXRD experiments on selected aque-
ous gels of DHepTA, D-3-PyTA, and B-4-MPhTA as de-
picted in Figure 13. It was clear from the plots that in all
these cases the PXRD patterns under various conditions
were nearly superimposable, which meant that the single-
crystal structure in respective cases actually represented the
bulk as well as the SAFINs found in the xerogels.
crystallographically
equivalent
molecule
(O···O=
ꢀ
2.8070(19)–2.9837(7) ꢃ; aO H···O=151(3)–128(2)8), the
same moieties of the neighboring crystallographically none-
ꢀ
quivalent molecule (O···O=2.6582(19) ꢃ; aO H···O=
169(3)8), as well as with one of >C=O groups of the amide
ꢀ
moiety (O···O=2.6521(18) ꢃ; aO H···O=165(2)8), which
resulted in an overall 2D hydrogen-bonded network. The
2D networks were packed further in parallel fashion
(Figure 12).
Figure 12. A) 2D hydrogen-bonding network (4-methoxylphenyl rings not
shown for better clarity) and B) parallel packing of the 2D sheets in B-4-
MPhTA.
Powder X-ray diffraction: The key to the design of any ma-
terials lies in the detailed understanding of their structures.
In gel, a structural understanding of the SAFINs is key to
designing gelator molecules. The detailed structural descrip-
tion of the SAFINs would expectedly lead to the insights
into the various intermolecular interactions that involve the
gelator molecules that are indispensable in the successful
pursuit of designing gelators. Understandably, SXRD experi-
ments cannot be performed with SAFINs because of their
sub-X-ray size, and therefore one has to determine the
structure of SAFINs by an indirect method. One such
method, originally developed by Weiss et al., is employed
here.[38] In this method, the simulated, bulk, and xerogel
powder X-ray diffraction (PXRD) patterns are compared; a
near perfect match would indirectly determine the structural
description of the SAFINs at the atomic level. It might be
noted that the structure of the SAFINs present in the xero-
gel need not necessarily be identical with that in the gel
Figure 13. PXRD patterns of some of the selected gelator molecules
under various conditions.
Metallogelation by D-3-PyTA: On account of its terminal 3-
pyridyl rings, the gelator D-3-PyTA is expected to form a
coordination bond with a suitable metal center and thus
could be a good candidate for a metallogelling agent. When
a metal takes part in the process of gelation either by coor-
dination or being incorporated within the gel matrix as a
noncoordinating metal or nanoparticle, the resuling gel is
known as a metallogel.[40] Cross-linked coordination poly-
mers,[41] discrete metal complexes,[42] and well-defined coor-
state. During xerogel formation by evaporating
a gel
sample, new nucleation events might lead to the formation
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U. K. Das and P. Dastidar
dination polymers[43] are known as metallogelators. Much
effort is being directed toward metallogelation research be-
cause of the various potential applications of metallogels,
such as in photophysics,[44] catalysis,[45] sensing,[46] magnetic
materials,[47] and so on. Thus, we treated D-3-PyTA with dif-
ferent CuII and ZnII salts under various conditions. The
ligand only produced gel in DMF when treated with CuII
salts in a 1:1 (metal/ligand) ratio and keeping the overall
concentration within the range of 4.5–11.0 wt% (considering
all the reactants) (Scheme 2). All the gels were thermorever-
Figure 14. Dynamic rheology at 258C of gel B; the elastic modulus G’
and viscous modulus G’’ are shown as function of the angular frequency
w. The strain used was 0.01%.
Scheme 2. Metallogels and crystals obtained from D-3-PyTA.
sible, except gel C (Scheme 2) obtained from CuSO4; in this
case, the gel gradually turned into deep blue microcrystals
over the period of one month. Similar reactions with ZnII
did not produce any gel. It may be noted that the ligand
alone, being highly soluble in DMF, could not form any gel.
However, it formed gel in a binary mixture of DMF/water
wherein the maximum concentration of DMF was not more
than 17% v/v. Dynamic rheology experiments (frequency
sweep) on a selected gel (gel B) clearly demonstrated the
typical gel-like response of gel B. The elastic modulus G’
Figure 15. SEM of the DMF xerogels of A) gel A, B) gel B, C) gel C, and
D) gel D.
sponding metal salts in 1:1 (metal/ligand) molar ratio in
mixed solvent system was allowed to evaporate slowly, X-
ray-quality single crystals were obtained (Scheme 2,
Table S17 in the Supporting Information).
Crystal structure of [{(H2O)2Cu(m-D-3-PyTA)2Cu(m-D-3-
was frequency-invariant over
a
considerable frequency
PyTA)2ACHTNURTGE(NGUNN NO3)2}·ACHTUNREGTGNN(UN NO3)2]a ACHTUNGTREN(NUGN CP1): The crystals of CP1 belong
range and was found to be more than one order of magni-
tude higher than the viscous modulus G’’, thus indicating the
viscoelastic behavior of gel B (Figure 14).
Scanning electron micrographs of these xerogels displayed
highly aggregated particles instead of entangled fibers as
usually observed in various xerogels (Figure 15). The solvent
molecules are understandably immobilized within the gel
network, thus resulting in gel formation.
As structure–property correlations on the basis of SXRD
and PXRD data are often useful in understanding and de-
signing gelators (vide supra), we attempted to crystallize
some of the coordination compounds that produced gels.
When a highly diluted solution of the ligand and the corre-
to the non-centrosymmetric tetragonal space group P42212.
In the asymmetric unit, there are two Cu atoms (both sitting
at special positions with site-occupancy factors of 0.5 and
0.25), two half ligands coordinated to each Cu atom, one
fully occupied nitrate coordinated to one of the Cu, and one
uncoordinated disordered nitrate and one water molecule
coordinated to one of the Cu atoms. Both of the Cu atoms
showed slightly distorted octahedral geometry wherein the
equatorial positions were coordinated by the pyridyl N
atoms and the apical positions were coordinated by water in
one of the Cu centers and nitrate in the other. Thus the ex-
tended coordination of the bidentate pyridyl ligand to the
Cu center produced an undulating 2D grid architecture,
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Chem. Eur. J. 2012, 18, 13079 – 13090

Bis-Amides of l-(+)-Tartaric Acid
FULL PAPER
which were further packed in offset fashion that was sus-
tained by hydrogen-bonding interactions that involved the
ꢀ
amide···amide (N···O=2.935(7) ꢃ; aN H···N=154.48) and
ꢀ
hydroxyl···hydroxyl (O···O=2.795 ꢃ; aO H···H=128.928)
interactions, which resulted in a bilayer structure. Such bi-
layers are further self-assembled in parallel fashion sus-
tained by amide···amide hydrogen bonding (N···O=
ꢀ
2.893(9) ꢃ; aN H···O=165.88) (Figure 16).
Crystal structure of [{(H2O)Cu2(m-D-3-PyTA)
2ACHTUNGTRNE(NUNG m-Cl)}·
(SO4)(Cl)]a (CP2): The crystals of CP2 belong to the non-
ACHTUNGTRENNUNG
centrosymmetric orthorhombic space group F222. The asym-
metric unit contains one Cu (half-occupied), one fully occu-
pied ligand coordinated to the metal center, one chloride
(one-quarter occupied) that acts as a bridge by coordinating
adjacent to the metal center, one uncoordinated sulfate
(one-quarter occupied), and one uncoordinated chloride
(one-quarter occupied). Although CuSO4 was taken as the
reacting metal salt, incorporation of chloride in the crystal
structure might be due to the chloride ion present in the
water used during reaction. However, we regenerated CP2
in a deliberate synthetic attempt wherein both CuCl2 and
CuSO4 were used as metal salt as revealed in PXRD experi-
ments (see below). A critical look into the crystal structure
revealed an interesting network topology. The bipyridyl
ligand coordinated to the adjacent metal center in such a
way that a 1D helical coordination polymer chain was
formed; such chains were further intertwined into a double
helix sustained by the chloride bridge that connected the Cu
centers of the adjacent helical chain. Such extended coordi-
nation ultimately led to the formation of a 2D grid wherein
each side of the grid was made up of double helical chains
(Figure 17). The topological analysis by TOPOS40[48] also re-
vealed a 5-c net; its uninodal net and topological type were
found to be an sp2-periodic net with an extended point
symbol [4.4.4.4.4.4.4.4.6(3).6(3)] that resembled the topology
of the core SnS.[49] We could not model some electron densi-
ty that arose from the solvent and it was removed by
SQUEEZE,[50] which indicated the presence of approxi-
mately twelve lattice-occluded water molecules (Figure S7
in the Supporting Information).
Crystal structure of [{Zn2(m-D-3-PyTA)2(Br)2}·2DMF]a
ACHTUNGTRENNUNG(CP3): The crystals of CP3 belong to the non-centrosym-
metric, orthorhombic space group P212121. The asymmetric
unit is comprised of two Zn atoms, two ligands, two bromide
ions, and two solvate DMF molecules. The extended coordi-
nation of the ligand with the adjacent metal center produced
a 1D helical chain. Such chains were further packed into 2D
sheets by means of hydrogen bonding that involved hydrox-
ꢀ
yl···hydroxyl (O···O=2.855(7) ꢃ; aO H···H=175.38) inter-
actions of the ligand; the solvate DMF was found to interact
ꢀ
with one of the amide N H groups of the ligand backbone
ꢀ
(N···O=2.955(8) ꢃ; aN H···O=155.88) (Figure 18).
Figure 16. Single-crystal structure illustration of CP1: A) two 2D grids
packed in offset fashion, B) TOPOS view of two 2D grids, and C) parallel
packing of the 2D grids sustained by various hydrogen-bonding interac-
tions marked as dotted lines.
Powder X-ray diffraction: Detailed PXRD experiments
were carried out on the xerogels to gain insight into the
Chem. Eur. J. 2012, 18, 13079 – 13090
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13087

U. K. Das and P. Dastidar
PXRD matched well with that of the bulk, thus indicating
the high crystalline phase purity of the sample. However,
the corresponding xerogel of gel B was found to be amor-
phous. In the case of CP2, all the PXRD patterns matched
reasonably well, thereby indicating that the crystal structure
of CP2 represented the gel network in the xerogel of gel C.
It might be mentioned here that the deliberate synthetic at-
tempt of CP2 by using both CuCl2 and CuSO4 as metal salts
was successful as can be seen from the near superimposable
PXRD patterns of simulated, as-synthesized bulk by using
only CuSO4 (Bulk-A), and bulk using both the Cu salts
(Bulk-B) (Figure 19). The presence of chloride in CP2 was
also confirmed by energy-dispersive X-ray spectroscopy
(Figure S5 in the Supporting Information). Thus, PXRD
data that pertain to CP2 clearly demonstrated that a 5-c 2D
net that displayed SnS topology was responsible for the gel-
forming network in gel C, which, to the best of our knowl-
edge, is hitherto unknown.
Figure 17. Single-crystal structure illustration of CP2: A) double-helical
strand bridged by chloride, B) 2D grid composed of all double-helical
sides (the horizontal sides are shown in caped stick (white)), C) another
representation of a 5-c 2D network (part of the aromatic rings and hy-
droxyl moieties and hydrogen not shown for better clarity), D) TOPOS
diagram of the 2D grid (uncoordinated chloride and sulfate not shown
for better clarity), and E) TOPOS diagram of SnS.
Conclusion
An easy method of access to the supramolecular chirality in
the form of organo-, hydro-, and metallogels has been ach-
ieved from a new series of bis-amide compounds derived
from an inexpensively resourced chiral source, namely, l-
(+)-tartaric acid. Fourteen such bis-amides have been syn-
thesized by using various n-alkyl amines and a few aromatic
amines, namely, 3-pyridylamine, aniline, 4-methoxyaniline,
and 4-nitroaniline. Ninety-three percent of these bis-amides
showed gelation with various solvents; many of them
showed ambidextrous behavior. The CD spectra of some se-
lected gels (toluene gel of DTDTA and DMF/water gel of
D-3-PyTA) indicated the presence of a b-sheet-like struc-
ture in the gel state. Most of the gels displayed typical gel-
like response in rheological experiments. Structure–property
correlations on the basis of SXRD and PXRD data on three
selected cases (DMF/water gels of DHepTA, D-3-PyTA,
and B-4-MPhTA) established the gel-network structures in
the corresponding xerogel state. One such gel (DMF/water
gel of D-3-PyTA) displayed nanotubular morphology in the
electron micrographs and has been exploited as a template
to generate highly aligned silica fibers. The pyridyl-function-
alized bis-amide, namely, D-3-PyTA, has been exploited to
obtain coordination-polymer-based CuII and ZnII metallo-
gels. SXRD data of three corresponding coordination poly-
mers (CP1, CP2, and CP3) have been determined and cor-
related with the PXRD of the corresponding xerogels. In
one such case (CP2), the SXRD data revealed the existence
of a 5-c 2D-net SnS topology in the metallogel-forming net-
work. Interestingly, the 2D network structure was built from
double helical chains sustained by a chloride bridge. To the
best of our knowledge, such a 5-c 2D-net SnS topology re-
sponsible for the metallogel-forming network found in the
present study is unprecedented.
Figure 18. A) 1D helical chain and B) parallel packing of the helical
chains sustained by various hydrogen-bonding interactions marked as
dotted lines (solvate DMF shown in green) in CP3.
Figure 19. PXRD patterns of CP1 and CP2 under various conditions.
structure of the gel network in the corresponding gels for
the reasons stated above (Figure 19). In CP1, the simulated
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Chem. Eur. J. 2012, 18, 13079 – 13090

Bis-Amides of l-(+)-Tartaric Acid
FULL PAPER
Acknowledgements
Experimental Section
U.K.D. and P.D. thank CSIR, New Delhi for a Senior Research Fellow-
ship (SRF) and financial support, respectively. All the single-crystal X-
ray diffraction data were collected at the DST-funded National Single
Crystal Diffractometer facility at the Department of Inorganic Chemistry,
IACS, Kolkata.
Materials and physical measurements: Commercially available starting
materials, and reagents were used as usual without further purification.
Solvents were of laboratory reagent grade and used without further distil-
lation. Both 1H and 13C NMR spectroscopy were performed using a
300 MHz spectrometer (Bruker Avance DPX-300) and a 500 MHz spec-
trometer (Bruker Ultrasheild Plus-500). IR spectra were obtained using
an FTIR instrument (FTIR-8300, Shimadzu). The elemental composi-
tions of the purified compounds were confirmed by elemental analysis
(Perkin–Elmer Precisely, Series-II, CHNO/S Analyser-2400). SEM was
recorded using a JEOL JMS-6700F field-emission scanning electron mi-
croscope. TEM was recorded using a JEOL instrument with a 300 mesh
copper TEM grid. Optical microscopic pictures were taken using a Leica
CLS 150X. DSC was recorded using a Perkin–Elmer Diamond DSC in-
strument. Rheology studies were carried out using an SDT Q series ad-
vanced rheometer AR 2000. CD were performed using a Jasco J-815 CD
spectrometer. X-ray powder diffraction patterns were recorded using a
Bruker AXS D8 Avance powder X-ray diffractometer (CuKa1 radiation,
l=1.5406 ꢃ).
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3-PyTA was crystallized from
a DMF/MeOH solution, crystals of
DHepTA were obtained from an MeOH solution. Crystals of DPhTA
were obtained from a DMSO/acetonitrile mixture, whereas B-4-MPhTA
was crystallized from p-xylene. A single crystal of CP1 was obtained
when a DMF/water solution of D-3-PyTA and CuACTHNUTGRNEG(UN NO3)2 were allowed to
evaporate. Both CP2 and CP3 were obtained when a solution of D-3-
PyTA in DMF/ethanol was layered atop the aqueous solution of CuSO4
and ZnBr2, respectively, and allowed to evaporate slowly.
Single-crystal X-ray data were collected with MoKa radiation (l=
0.7107 ꢃ) using a SMART APEX-II diffractometer equipped with a
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fined in a routine manner. In all cases, non-hydrogen atoms were treated
anisotropically. Whenever possible, the hydrogen atoms were located on
a difference Fourier map and refined. In other cases, the hydrogen atoms
were geometrically fixed at their idealized positions.
CCDC-812385, -812385, -812385 and -812385 contain the 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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