An organogel formed from a cyclic β-aminoalcohol

Article abstract of DOI:10.1039/c1cc13179f

A new organogelator with unique structural feature of a cyclic β-aminoalcohol is presented as the first example of gelation by aminoalcohol through hydrogen-bonding between hydroxy and amine.

Full text of DOI:10.1039/c1cc13179f

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An organogel formed from a cyclic b-aminoalcoholww
Chuanqing Kang, Zheng Bian, Yabing He, Fushe Han, Xuepeng Qiu and Lianxun Gao*
Received 30th May 2011, Accepted 12th August 2011
DOI: 10.1039/c1cc13179f
A new organogelator with unique structural feature of a cyclic b-
aminoalcohol is presented as the first example of gelation by
aminoalcohol through hydrogen-bonding between hydroxy
and amine.
strength with that from other groups,6 to the best of our
knowledge, no report has yet focused on gelation driven by
H-bonding between hydroxy and amine. With our long-standing
interest in the development of new gelators as well as their
application in catalytic chemistry, we have recently reported on
gelators that gel solvents through p-stacking or metal–ligand
Gels from low-molecular-mass organogelators (LMOGs) as a
fascinating field has attracted much attention owing to the
physical properties and potential applications in areas such as
drug delivery, electronics, sensors, catalysis and templates for
metal nanomaterials.1 Stimulus-responsive properties of some
gels also enable the nanomaterial to be used as a core component
for smart devices.2 A wide range of synthetic LMOGs varying
from the simplest alkanes to dendritic molecules have been
reported during long-term studies.3 The advances in gelator
structures and gel capabilities not only provide an intensive
understanding to the self-assembly therein but also shed more
light on new tunable applications of the soft materials.4 One
current research emphasis has been on investigation of a wide
range of structurally diverse gelators.
coordination.7,8 Herein, we present
a new gelator 1
(Scheme 1), a derivative of 1,2,3,4-tetrahydroisoquinoline
not belonging to any known gelator type. The gelator gels
solvents through unique H-bonding interaction between
hydroxy and amine as a new type of driver for gelation.
The new diastereomeric compounds 1 and 2 were prepared from
a L-DOPA derivative through Pictet–Spengler reaction followed by
reduction with lithium aluminium hydride. The structures of 1 and
2 were confirmed with various spectrascopies.9,10 Compounds 3
and
4
were synthesized according to the literature
procedures.11
The gelation ability of 1–4 was tested in various common
solvents by the inverse tube method.12 Only compound 1, with
a cis-arrangement of phenyl and hydroxymethyl, is capable of
reversibly gelling a number of polar and apolar solvents by
cooling the hot solution to room temperature. Compound 2,
as an epimer of 1, and 3 display good solubility in most solvents
while 4 can be easily crystallized in apolar solvents. The results
suggest the vital importance of the steric effect and chemical factor
for the gelation ability.5,13 The minimal gelation concentration
(Cgel) of 1 and the sol–gel transition temperature (Tgel) at Cgel
varies depending on the solvent. The value of Cgel is not more
than 0.8% (g mlÀ1) in toluene, o-xylene and mesitylene, 3.0,
3.6, 4.1 and 4.7% in butanone, isopropanol, dioxane and
acetonitrile, repectively, and 45.0% in other solvents. The
value of Tgel is 65 1C for the toluene gel, 74 1C for the xylene
gel, and 76 1C for the mesitylene gel, respectively. In addition,
gels from aromatic solvents are transparent, but the others are
translucent or opaque. Methanol, ethyl ether and hexane can
A gel is usually formed by the hierarchical self-assembly of
gelators from a 1D-predominating arrangement to an entangled
nanofibrous network that immobilizes liquids. This gelation is
manipulated by one or a combination of various noncovalent
interactions such as hydrogen-bonding (H-bonding), p-stacking,
dipole–dipole interaction, van der Waals forces and solvophilic–
solvophobic balance.3 Generally, the structure of gelator
comprises of a polar segment such as amide, urea, hydroxy or
metal coordination centre and an apolar segment such as
p-system and long alkyl group. These distinct segments provide
specific noncovalent interactions and control the subtle balance
between solution and gelation.5
Among the noncovalent interactions, H-bonding is the most
utilized in both biological and synthetic gel systems, which is
usually generated from amide, urea, thiourea, carboxylic acid,
carbamate, hydroxy, or a combination of them.4,5 While the
H-bonding between hydroxy and amine is of comparable
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Graduate School of Chinese Academy of Sciences, 5625 Renmin
Street, Changchun 130022, China. E-mail: lxgao@ciac.jl.cn;
Fax: +86-431-85262926; Tel: +86-431-85262352
w Electronic supplementary information (ESI) available: Experimental
materials, synthetic procedures and characterization, HRMS, NMR,
FTIR, gel tests, CIF data of single-crystal 4. CCDC 823631. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc13179f
Scheme 1
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10746 Chem. Commun., 2011, 47, 10746–10748
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Fig. 1 SEM images of xerogels from toluene (a), THF (b), mesitylene
(c), dioxane (d), acetonitrile (e) and butanone (f). Scale bars (black):
1 mm (a, b), 2 mm (c), 20 mm (d, e), and 50 mm (f) (see Fig. S13, ESIw for
larger images).
Fig. 2 Single crystal of 4: (a) crystal cell and interactions; (b)
staggered packing of 1D aggregations viewing along the a axis; (c)
head-to-head bilayer structure (two bilayers displayed). Hydrogens
except for those on nitrogen, oxygen and chiral centre are omitted for
clarity.
not be gelled by 1 (Table S2 and Fig. S12, ESIw). All gels display
good stability with the morphologies remaining unchanged
upon standing for more than a year at room temperature.
As shown in Fig. 1, micromorphological investigations to the
xerogels of 1 were carried out by scanning electronic microscopy
(SEM). Well-developed fibrous network, spongiform-like
porous network, well-crinkled films or ball clusters with size
ranging from nanometres to micrometers are formed in
different solvents.
axis to form 1D linear aggregates. The driving forces for 1D
aggregation are the O–HÁ Á ÁN H-bonding (b1 and b2) and
relative weaker C–HÁ Á Áp interactions (b3–5) between the two
adjacent molecules, in which CH is the chiral centre. Other
H-bondings of N–HÁ Á ÁO (b6 and b7) hold the 1D aggregates
together in a staggered head-to-head style to a bilayer structure
in a 2D direction with spacing of 13.78 A. The 2D bilayers
pack layer-by-layer through electrostatic attraction giving the
3D crystal structure.
The effect of metal ions on the stability of the toluene gel
was evaluated by visible morphological change when the gel
was contacted with metal ion solution (Table S3, ESIw). The
gel selectively responds to Ag(I), Cu(II) and Fe(III) ions with
morphological change within 1 h followed by complete breakdown
on contacting to these ions for 6, 10 and 8 h, respectively.
However, the gels are stable to alkali-metal ions, Mg(II),
Ca(II), Mn(II), Co(II), Ni(II), Zn(II), Cd(II) and Yb(III) ions
for at least one week. The gel covered with aqueous HCl
(pH = 2.5) did not display any change for more than six
months. The active metal ions trigger the collapse of the fibrous
network because their coordination to the gelator molecule, a
good N,O-ligand, breaks the H-bonding interactions between
hydroxy and amine. Transition metal-responsive organogels
are a rarely reported aspect of stimulus-responsive gels.7,14 The
result illustrates the importance of H-bonding for the gelation
by 1, as well as the potential of the gel in responsive application
of probing environment elements.2
The simulated XRD pattern of crystal 4 exhibits a lamellar
structure with diffraction planes of (001) and (100) corresponding
to the d-spacing of bilayer stacking and the period distance in 1D
aggregations, respectively (Fig. S15, ESIw). The powder XRD
of xerogel 1 from toluene demonstrates clear crystallisation
peaks comparable to crystal 4. As shown in Fig. 3, the peaks at
d-spacings of 17.6, 8.73, 5.75 and 4.24 A are ascribable to
diffractions from the (001) plane corresponding to the distance
of a lamellar structure. Given that 1 packs in compact stacking
or in inclined position, the distance is approximately equivalent
to twice of the long-axis length of 1 (ca. 10 A, Fig. S16, ESIw)
Thus, the distance is probably the thickness of a bilayer like
the spacing of the head-to-head arrangement of 4 in the
crystal. The diffractions from the (100) plane at d-spacings
of 7.89 and 3.98 A is attributed to the distance of a periodic
structure in another dimension. Probably, it is in the direction
along 1D aggregations, which looks like the counterpart of the
(100) plane in the crystal of 4.
The formation of H-bonding between hydroxy and amine as
the driving force for gelation is also demonstrated by FTIR
(Fig. S15, ESIw). A dilute fresh solution of 1 in dichloromethane
(2% g mlÀ1) displays a strong band at 3620 cmÀ1 and a weak
band at 3258 cmÀ1 corresponding to stretching vibration of
free and hydrogen-bonded hydroxy and amine, respectively.
For the xerogels, the former band disappears, while a strong
band appears at 3253 cmÀ1 corresponding to H-bonding
between hydroxy and amine.
Clearer insights into the self-assembly of the organogelator
to gelling solvents are derived from experimental powder
X-ray diffraction (XRD) analyses and single-crystal analysis
of the homologue 4 (attempts to obtain single crystals of 1
failed).15 High-quality single crystals of 4 were obtained by
extended standing of a toluene solution at room temperature.16
As shown in Fig. 2, in crystalline 4, molecules stack one-by-
one in nearly parallel style in the same direction along the a
Fig. 3 Powder XRD of xerogel 1 from toluene.
Chem. Commun., 2011, 47, 10746–10748 10747
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Fig. 4 Hierarchical self-assembly of 1 to gel.
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spectra. The MS show an exceptional [M À H]+ plus usual [M +
H]+, the former resulting from dehydrogenation under laser
irradiation (Table S1 and Fig. S1–4, ESIw): X. Lou,
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Thus, the hierarchical self-assembly of 1 from liquid
gelation is postulated as shown in Fig. 4. The molecule 1
assembles through H-bonding between hydroxy and amine to
1D linear aggregates and subsequently to a 2D bilayer
structure. The stacking of bilayers affords a lamellar structure
that grows and expands as entangled fibrous or flaky networks
to immobilize fluids as a gel. The cis-phenyl on 1 is supposed
to be advantageous for 1D aggregation in long range but
disadvantageous for 2D or 3D assembly in wide area.
However, the trans-phenyl on 2 may prevent the self-assembly
in a pattern of one dimension predominating over the other
dimensions, while compound 3, without phenyl substitution
retains high solvophilicity. The difference in gelation capability
among 1, 2 and 3 suggests that the cis-configuration of
substituents on the 1,2,3,4-tetrahydroisoquinoline scaffold is
a key factor for gelation over solution or crystallization.
Hence, the H-bonding between hydroxy and amine, associated
with stereochemistry influence, controls the pattern of
self-assembly of 1 to solvent gelation.
In summary, we have discovered a new organogelator 1 with
a unique structural feature of cyclic b-aminoalcohol, which is
able to form reversible gels in usual polar and apolar solvents
through H-bonding between hydroxy and amine. Toluene gel
is selectively responsive to transition metal ions such as Ag(I),
Cu(II) and Fe(III). The hierarchical self-assembly of 1 to a gel is
proposed according to FTIR and XRD analyses as well as from
X-ray single crystal analysis of analogue compound 4. Compound
1 is the first example of aminoalcohol organogelator with
gelation driven by H-bonding between hydroxy and amine.17
The discovery expands the scope and diversity of structures
and interactions for the design of new gelators. Further work
will focus on the extension of the structural scope of the
gelator scaffold, and application of the gel system as soft
materials, are in progress.
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16 Crystal data for 4: C10H13NO, M = 163.21, monoclinic, space
group P21, a = 9.6435(14), b = 6.500(1), c = 14.390(2) A, , b =
106.698(2)1, V = 864.0(2) A3, T = 295(2) K, Z = 4, 4797
reflections measured, 1863 independent reflections (Rint
=
0.0295). The final R1 values were 0.0321 (I 4 2s(I)). The final
wR(F2) values were 0.0793 (I 4 2s(I)). The final R1 values were
0.0362 (all data). The final wR(F2) values were 0.0820 (all data).
The goodness of fit on F2 was 1.052.
Notes and references
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17 So-called aminoalcohol organogelators in other reports are, in fact,
the corresponding amide derivatives based upon interactions
between amido, but not aminoalcohol themselves. See review:
´
L. Frkanec and M. Zinic, Chem. Commun., 2010, 46, 522.
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10748 Chem. Commun., 2011, 47, 10746–10748
This journal is The Royal Society of Chemistry 2011