P-Tolyl glycerol ether: Is it possible to find more simple molecular organogelator with pronounced chirality driven properties?

Article abstract of DOI:10.1039/b924948f

p-Tolyl glycerol ether not belonging to any of the known gelator families forms stable transparent gels in hydrocarbon media showing very good quantitative characteristics of gelling abilities, which are, in turn, strongly dependent on the chiral characteristics of the gelator. The crystal packing differences between the rac- and scal-substances could be the reason for such behaviour.

Full text of DOI:10.1039/b924948f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
p-Tolyl glycerol ether: is it possible to find more simple molecular
organogelator with pronounced chirality driven properties?w
Alexander A. Bredikhin,* Zemfira A. Bredikhina, Flyura S. Akhatova and
Aidar T. Gubaidullin
Received 26th November 2009, Accepted 12th March 2010
First published as an Advance Article on the web 8th April 2010
DOI: 10.1039/b924948f
p-Tolyl glycerol ether not belonging to any of the known gelator
families forms stable transparent gels in hydrocarbon media
showing very good quantitative characteristics of gelling
abilities, which are, in turn, strongly dependent on the chiral
characteristics of the gelator. The crystal packing differences
between the rac- and scal-substances could be the reason for such
behaviour.
The chemistry of low molecular mass organic gelators
(LMOG) is currently a rapidly progressing area of research
at the intersection of organic and nanochemistry. Soon after
publication of the encyclopaedic monograph by Weiss and
Terech on the subject,1 two special issues and a number of
reviews have been published in the leading journals devoted to
properties and applications of the LMOG formed gels.2,3
Scheme 1 Some chiral organic molecular gelators.
‘‘A quest for the simplest LMOG structures’’ is of great
CMG families, however no data are available on the gelling
abilities of their racemic and/or opposite enantiomeric
compounds. In addition, these compounds are characterized
by high molecular weight and possess several chiral centres
and therefore are not suitable candidates for the simplest
chiral molecular gelators.
interest from the point of view of understanding the effect of
the gelator structure on the gel formation and properties.4
Accepting this challenge, George and Weiss have marked out
normal alkanes,5 in particular n-hexatriacontane,6 as molecular
gelators having the simplest chemical structure. The dialkyl
ureas (RNHCONHR, R = Me, Et, n-Pr; MW = 88, 116, and
144 correspondingly) have been mentioned as LMOG with the
lowest molecular weight.7
Apparently, the lowest molecular weight chiral gelators
among all CMG described in the literature reside in the family
of amino acids derivatives, such as bis-leucine oxalyl amide
(4), N-dodecanoylserine (5), and the product of the enzymatic
aldol reaction between glycine and 2,3-O-isopropylidene-
glyceraldehyde (6). Both enantiopure amides 4 are well
pronounced gelators of water and dioxane, whereas rac-4 is
a poor gelator for these solvents.13 In the case of 5, hot
solutions of L- or D-enantiomers in toluene or water form
(micellar) gels after cooling, whereas the racemate gives planar
platelets in both solvents.14 While compound 6 forms gels in a
number of organic solvents, its diastereomer, which differs
only by the absolute stereochemistry at C3, crystallizes.15
Here, we introduce a new candidate for the role of the
lowest molecular weight chiral organic gelator—p-tolyl glycerol
ether 7. Racemic and enantiopure diols 7 were synthesized
from p-cresol and racemic or scalemic 3-chloropropane-1,2-
diols (see ESIw for details). Having a very simple chemical
structure and lower molecular weight compared to any of the
above mentioned CMG, this compound demonstrates very
pronounced chirality driven properties. Thus, both (R)- and
(S)-7 form stable transparent gels with a number of saturated
hydrocarbons (Fig. 1a and Table S1w); the characteristics of
their gelation abilities, such as weight percent of the gelator or
the number of the solvent molecules immobilized by a single
The chiral organic gelators (Scheme 1) form a special family
of LMOG whose gel-forming abilities, as well as their gel
properties and nano-associate structures, essentially depend
on the chirality characteristics of the substance.8,9 The ‘‘quest
for the simplest LMOG’’ could be initiated for this specific
class of gelators as well.
Information about the chiral molecular gelators (CMG) is
scattered over numerous sources and is not ranked on the
basis of CMG simplicity. Although 12-hydroxyoctadecanoic
acid (1) may be the simplest CMG from the point of view of
the chemical structure, Terech et al. have clearly established
that its racemic gels are comparable in every respect to the
gels of the single enantiomer.10 Dihydrolanosterol (2)11
and 1,3:2,4-di-O-benzylidene sorbitol (3)12 are the simplest
representatives of the vast steroid and carbohydrate based
A.E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Scientific Center, Russian Academy of Sciences, Arbuzov St., 8,
Kazan 420088, Russian Federation. E-mail: baa@iopc.ru;
Fax: +7 843 2731872; Tel: +7 843 2734573
w Electronic supplementary information (ESI) available: Synthesis,
crystals preparation, and gelation properties of compound 7. CCDC
755204 and 755205. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b924948f
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3523–3525 | 3523

View Article Online
Fig. 1 (a) The gel formed by (S)-7 and n-nonane after four months of
ambient temperature storage in the sealed vial. (b) SEM micrograph of
the non-conditioned xerogel formed by (S)-7 on aluminium support.
Fig. 3 H-bonded 1D P-helixes (grey zones) in the compound
(S)-7 crystals; intermolecular hydrogen bonds are denoted by dashed
lines.
gelator molecule, compare favourably with similar indices for
most of the LMOGs described in the literature. At the same
time, rac-7 forms no gel in the series of the solvents tested.
The scalemic p-tolyl glycerol ether (R)- or (S)-7 belongs to
the family of gelators forming self-assembled fibrillar networks,
which is easily demonstrated by SEM inspection of their
xerogels obtained by fast in vacuum drying of the thin nonane
gel films on a polished aluminium surface (Fig. 1b). The chiral
nature of the self-associated ensembles arising during the
gelling process could be traced by CD spectra. Since the
aromatic ring, the main chromophore in the molecule of 7,
is separated from the chiral centre by three chemical bonds,
the intensity of the CD spectrum of the scal-7 concentrated
gelling solution in hot nonane is negligible (Fig. 2a). A sharp
change in the CD spectrum (Fig. 2b) is observed at the
beginning of aggregation in the temperature region of ca. 40 1C,
which is close to Tgel (39–40 1C according to the steel
ball test). Such an induced CD signal may emerge when the
overall medium has a pronounced (homo) chiral character.
The crystallization peculiarities of scal- and rac-LMOG
might be the part of the reason for their different gelling
behaviour. As it was mentioned in the Brizard et al. review,8d
crystals of the gelators suitable for X-ray diffraction are
usually very difficult to obtain, and the crystallographic studies
are often performed on the analogous molecules that possess
poor or no gelling abilities. To our delight, we were able to
obtain single crystals of both scal- and rac-7 suitable for X-ray
diffraction analysis by liquid diffusion crystallization.z
crystallizes in the ‘‘achiral’’ Pc space group with two symmetry
independent molecules, subsequently referred to as A and B,
in the unit cell. The conformations of these A and B molecules
(ap, ap, sc, ap, ꢁsc for (S)-A and (S)-B molecules
C
Ar–O–CH2–CH(OH)–CH2–OH bond succession) are very
similar to one another and differ significantly from the
conformation (ap, ap, ꢁsc, ꢁsc, ꢁac for the same bond
succession) of the scalemic sample forming (S)-molecules
(Fig. S1w)
The change of the conformation is accompanied by the
change of the supramolecular pattern of the homochiral
crystal packing as compared with the heterochiral one. The
well pronounced motifs in the (S)-7 crystal lattice are the 1D
columns formed by H-bonded molecules along the 21 axes
(Fig. 3).
The intermolecular hydrogen bond columnar system is
formed by the hydrogen bonding to the terminal primary
hydroxyls of the upper and lower molecules and the secondary
hydroxyls of the left and right molecules in Fig. 3. The
important feature of this hydrogen bond system lies in its
chirality: the succession ‘‘closest donor – closest acceptor’’
(ꢂ ꢂ ꢂO1–H1ꢂ ꢂ ꢂO20–H200ꢂ ꢂ ꢂO10 0–H100ꢂ ꢂ ꢂ) organized along the b
axis in the crystals of (S)-7 gives rise to a right-handed P-helix
(Fig. 3). The hydroxyl groups that are not involved in this
H-bond helix take part in the neighbouring ones resulting
in a 2D ‘‘palisade-like’’ supramolecular bilayer pattern of
H-bonded molecules arranged parallel to crystallographic
plane ab.
The scalemic samples of p-tolyl glycerol ether crystallize in
the ‘‘chiral’’ P212121 space group with only one symmetry
independent molecule in the unit cell. The racemic sample
The 2D bilayer structures parallel to crystallographic plane
bc are also present in the crystal packing of the racemic sample
(Fig. 4). One layer of the pattern consists of A, and another
one of B molecules only; the layers are linked by the developed
system of the intermolecular hydrogen bonds in between.
However, there are no marked 1D hydrogen bond motifs in
this system: both endless zigzag chains and the adjacent
13-membered rings spread along b as well as c directions. It
seems that the crystal habitus and the association properties
of the microcrystals of scal- and rac-7 are at least partly
attributable to the above mentioned packing peculiarities.
The tendency to the formation of the 1D structures and the
ability of the structures to combine into more complex units
could be the reasons for the elongated straw-like crystal
Fig. 2 CD spectra of 3 ꢃ 10ꢁ3 M hot nonane solution of (S)-7
(a, blue), and of the gel formed by the same solution on cooling (b, red).
ꢀc
This journal is The Royal Society of Chemistry 2010
3524 | Chem. Commun., 2010, 46, 3523–3525

View Article Online
This work was supported by Russian Fund of Basic
Research, grant number 09-03-00308. We also thank Drs
V. P. Gubskaya, D. V. Zakharychev, and A. E. Vandyukov
for valuable technical assistance.
Notes and references
z Crystal data for rac-7: C10H14O3, M = 182.21, Monoclinic (Pc),
a = 17.264(2) A, b = 4.7225(7) A, c = 11.9008(17) A, b = 90.091(2)1,
V = 970.3(2) A3, Z = 4, m(Mo-Ka) = 0.091 mmꢁ1, 12 167 reflections
measured, 2236 unique (Rint = 0.0389). Final R = 0.0768 for 1597
reflections with I > 2s(I) and wR = 0.2257 (all data), GOF (F2) =
1.017. Crystal data for (S)-7: C10H14O3, M = 182.21, Orthorhombic
(P212121), a = 4.8768(2) A, b = 7.2147(3) A, c = 28.5203(11) A, V =
1003.48(7) A3, Z = 4, m(Cu-Ka) = 0.726 mmꢁ1. 7867 reflections
measured, 1673 unique (Rint = 0.0295). Final R = 0.0288 for 1630
reflections with I > 2s(I) and wR = 0.0771 (all data), GOF (F2) = 1.047.
CCDC reference numbers 755204 for rac-7 and 755205 for (S)-7.
1 Molecular Gels: Materials with Self-assembled Fibrillar Networks, ed.
R. G. Weiss and P. Terech, Springer, Dordrecht, The Netherlands,
2006.
2 (a) Sol–Gel Chemistry and Materials issue, Acc. Chem. Res., 2007,
40, N.9; (b) Low Molecular Weight Organic Gelators issue,
Tetrahedron, 2007, 63, N.31.
Fig. 4 Bilayer 2D H-bonded supramolecular pattern in the rac-7
crystals. Symmetry independent A and B molecules are denoted by
different styles.
3 (a) D. K. Smith, Molecular Gels—Nanostructured Soft Materials,
in Organic nanostructures, ed. J. L. Atwood and J. W. Steed, 2008,
WILEY-VCH, Weinheim, ch. 5, pp. 111–154; (b) F. Zhao,
M. Lung Ma and B. Xu, Chem. Soc. Rev., 2009, 38, 883;
(c) P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699; (d) A. R. Hirst,
B. Escuder, J. F. Miravet and D. K. Smith, Angew. Chem., Int. Ed.,
2008, 47, 8002; (e) M. Llusar and C. Sanchez, Chem. Mater., 2008,
20, 782.
4 M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489.
5 D. J. Abdallah and R. G. Weiss, Langmuir, 2000, 16, 352.
6 D. J. Abdallah, S. A. Sirchio and R. G. Weiss, Langmuir, 2000, 16,
7558.
7 M. George, G. Tan, V. T. John and R. G. Weiss, Chem.–Eur. J.,
2005, 11, 3243.
Fig. 5 Microphotographs of the crystals obtained during slow
crystallization of dilute scal-7 (a) and rac-7 (b) solutions; scale interval
10 mm.
8 For reviews, see: (a) S. Malik, N. Fujita and S. Shinkai, Gels as a
Media for Functional Chiral Nanofibers, in: Chirality at the Nanoscale:
Nanoparticles, Surfaces, Materials and more, ed. D. B. Amabilino,
WILEY-VCH, Weinheim, 2009, ch. 4, pp. 93–114; (b) M. Suzuki and
K. Hanabusa, Chem. Soc. Rev., 2009, 38, 967; (c) D. K. Smith, Chem.
Soc. Rev., 2009, 38, 684; (d) A. Brizard, R. Oda and I. Huc, Top. Curr.
Chem., 2005, 256, 167.
constitution and the entangled character of their agglomerates
obtained during slow crystallization of dilute scal-7 solutions
(Fig. 5a). In turn, the propensity to form strongly connected
2D structures poorly bounded to each other could cause
precipitation of the isolated thin crystal plates during crystal-
lization of rac-7 (Fig. 5b).
9 For resent examples, see (a) L. Frkanec and M. Zinic, Chem.
´
Commun., 2010, 46, 522; (b) A. Dawn, N. Fujita, S. Haraguchi,
K. Sada and S. Shinkai, Chem. Commun., 2009, 2100;
(c) Y.-S. Zheng, S.-Y. Ran, Y.-J. Hu and X.-X. Liu, Chem.
Commun., 2009, 1121; (d) F. Rodrıguez-Llansola, J. F. Miravet
´
In summary, p-tolyl glycerol ether was shown to be an
effective molecular organic gelator. It forms stable transparent
gels in hydrocarbon media showing very good quantitative
characteristics of the gelling abilities, which are in turn
strongly dependent on the chiral properties of the gelator.
p-Tolyl glycerol ether can serve as a convenient model
compound for studying the properties of the chiral gelators
as it has a simple chemical structure and is readily available in
both enantiomeric and racemic forms.
and B. Escuder, Chem. Commun., 2009, 209.
10 P. Terech, V. Rodrigez, J. D. Barnes and G. B. McKenna,
Langmuir, 1994, 10, 3406.
11 P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133.
12 E. A. Wilder, C. K. Hall, S. A. Khan and R. J. Spontak, Recent
Res. Dev. Mater. Sci., 2002, 3(Pt. 1), 93.
´ ´ ´
13 M. Jokic, J. Makarevic and M. Zinic, J. Chem. Soc., Chem.
Commun., 1995, 1723.
14 C. Boettcher, B. Schade and J.-H. Fuhrtop, Langmuir, 2001, 17, 873.
15 V. P. Vassilev, E. E Simanek, M. R. Wood and C.-H. Wong,
Chem. Commun., 1998, 1865.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3523–3525 | 3525