Low molecular weight organogelators derived from threefold symmetric tricarbamates

Article abstract of DOI:10.1016/j.tetlet.2016.11.104

A group of new low molecular weight organogelators based on threefold symmetric tricarbamate were synthesized and characterized. The tricarbamates with long alkyl chains were able to gelate a wide range of polar and nonpolar organic solvents such as aceto

Full text of DOI:10.1016/j.tetlet.2016.11.104

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Low molecular weight organogelators derived from threefold symmetric
tricarbamates
⇑
Xiaodong Hou, Jonathan Butz, Jiao Chen, Zijun D. Wang, Julia X. Zhao, Tiffany Shiu, Qianli Rick Chu
Department of Chemistry, University of North Dakota, Grand Forks, ND 58202, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A group of new low molecular weight organogelators based on threefold symmetric tricarbamate were
synthesized and characterized. The tricarbamates with long alkyl chains were able to gelate a wide range
of polar and nonpolar organic solvents such as acetonitrile and cyclohexane, generally at concentrations
lower than 20 g/L. The best organogel formation was obtained using a threefold symmetric tricarbamate
in n-dodecane, in which a sufficiently transparent gel was formed at the critical gelation concentration
1 g/L. Intermolecular hydrogen bonding by the tricarbamate in a nonpolar solvent benzene-d6 was indi-
cated by 1H NMR spectra. Its maximum UV absorption was 11 nm higher in chloroform than in n-dode-
cane, and this red shift indicated increased conjugation between the benzene core and the carbamate
substituents, which confirmed a change in its conformation from nonpolar to polar solvent. The self-
assembling behavior of the tricarbamate in dilute solutions was investigated by TEM. Fiber-like networks
were observed in a large range of solution concentrations.
Received 4 November 2016
Revised 23 November 2016
Accepted 25 November 2016
Available online xxxx
Keywords:
Organogel
Low molecular weight gelator
Threefold symmetry
Carbamate
Supramolecular atropisomer
Hydrogen bond
LMOG
Ó 2016 Elsevier Ltd. All rights reserved.
LMWG
Introduction
compound counterparts such as amide, amino, and urea, only a
limited number of reports have studied LMOGs based mainly on
Low Molecular Weight Organogelators (LMOGs) have been
attracting attention not only for their potential applications rang-
ing from oil spill remediation1 to art conservation,2 but also for
their capacity to form a variety of interesting self-assemblies as
fibers, sheets, tapes or strands that researchers do not fully under-
stand.3 The gelation process of LMOGs is thermo-reversible: small
molecules self-assemble into three-dimensional supramolecular
networks that entrap solvents by surface tension. Models for
supramolecular structures of the aggregates have been proposed,
but the true nature of the aggregation phenomenon is still under
investigation.4,5 Meanwhile, despite the fact that many efforts have
been made to fully understand the mechanism and establish
guidelines for the rational design of organogelators, many novel
organogelators have been discovered by serendipity rather than
design.3c,6
intermolecular H-bonding of carbamate.9 There are a number of
advantages of this new tricarbamate system: Their syntheses were
accomplished using readily available and inexpensive benzene-
1,3,5-tricarbonyl trichloride, sodium azide, and alcohols in good
yields. Threefold symmetry allows installation of a corresponding
side-chain group to each carbonyl group at the same time,10 with
long alkyl group-containing chains generally improve solubility
of the resulting compound. The tricarbamates showed good
thermal stability due to the presence of a large conjugated system.
Moreover, considering the multitude of alcohols that are
commercially available or easy to synthesize, there is an opportu-
nity to further tweak the properties of the LMOGs of threefold
symmetric tricarbamates according to the future academic study
or per industrial production requirements.
In this communication, we report a new group of LMOGs based
on threefold symmetric tricarbamates that were found during our
study of a series of supramolecular atropisomers5a,7 and symmetric
monomers with multiple reactive centers.8 In contrast to the
numerous reports on LMOGs based on other N- and O-containing
Result and discussion
Nine threefold symmetric tricarbamates were readily synthe-
sized in excellent yield as shown in Scheme 1 by reacting the
corresponding primary alcohol with 1,3,5-triisocyanatobenzene,11
derived from 1,3,5-benzenetricarbonyl chloride in a two-step,
one-pot reaction. After purification by column chromatography,
tricarbamates were characterized by 1H NMR, 13C NMR, FT-IR,
⇑
Corresponding author.
E-mail address: chu@chem.und.edu (Q.R. Chu).
http://dx.doi.org/10.1016/j.tetlet.2016.11.104
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
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2
X. Hou et al. / Tetrahedron Letters xxx (2016) xxx–xxx
H
N
H
N
O
O
NCO
COCl
R
R
NaN3, H2O
rt
ClCH2CH2Cl
2.5 h, reflux
R-OH
Et3N,Toluene, reflux
O
O
O
R
NH
OCN
NCO
ClOC
COCl
O
R = n-CmH2m+1
Yld: 68%
84-95%
(m = 1-8 and 12)
Trialkyl N',N'',N'''-benzene-
1,3,5-tricarbamate
C-1 ~ C-8
C-12
and
Scheme 1. Synthesis of the tricarbamates.
and mass spectroscopy. The nine symmetric tricarbamates are
labeled as C-n (C-1 ꢀ C-8 and C-12), where the n is the carbon
number of its alkyl chain. Details of the synthetic procedure and
characterization are included in the supporting information. The
tricarbamates were very stable under ambient conditions and did
not decompose after one-year storage.
consistent with the presence of intermolecular hydrogen bonding
interaction in solution. Because the formation of strong
intramolecular hydrogen bonding is impossible for compound C-
12 due to the location and distance between the three carbamate
groups around the aromatic core, the concentration dependence
of the N-H chemical shift can be explained by intermolecular
hydrogen bonding. With the change of the concentration, the
geometry and composition of the hydrogen bonded aggregates
change accordingly, so the chemical environment change of hydro-
gen bonded N-H within the aggregates causes the upfield-shifting
of its 1H NMR signal. If the C-12 molecules had presented as indi-
vidual molecules in benzene-d6, the N-H would have been able to
freely find its most comfortable conformation despite its concen-
tration and there would have not been significant change upon
dilution.
As presented in Fig. 4, the absorption maximum was at 234 nm
in n-dodecane. In a more polar solvent, chloroform, it was shifted
to 245 nm, which was 11 nm higher than in n-dodecane. Similar
shifts of absorbance peak have been observed in benzene-1,3,5-tri-
carboxamides systems.13 The potential of forming six or more
hydrogen bonds makes it unfavorable for the tricarbamate C-12
or similar compounds to present as individual molecules in chloro-
form. Thus, the red shift might indicate an increase in conjugation
between the benzene unit and the carbamate substituent due to
conformational change in the hydrogen bonded aggregates from
nonpolar n-dodecane to polar chloroform solvent.
To explore the supramolecular structure of the C-12 organogel,
powder X-ray Powder Diffraction (XRD) analysis was performed.
There was only one small peak around 7 degrees and a broad peak
around 20 degrees in the powder XRD pattern (Fig. S8 in the ESI),
so the tricarbamate C-12 did not form a well-defined crystalline
aggregate as its tricarbamate and triamide analogues.5a,7 To
explore the microscopic structure of the transparent organogel,
electron microscopic analysis was performed. Fig. 5a shows a
Transmission Electron Microscopy (TEM) image of the n-
dodecane gel of tricarbamate C-12 at a concentration of 1 g/L
(1.3 Â 10À3 M). From the TEM images, entangled fiber-like
structures were observed with diameters ranging from about
twenty to several hundred nanometers. The elongated self-
assemblies reach dozens of micrometers in length, which is
corresponding to thousands of tricarbamate molecules assembled
together. During the testing of the critical gelation concentration
of tricarbamate C-12 in n-dodecane, it was found n-dodecane
could still be partially congealed when the concentration was
diluted to 5 Â 10À4 M. Three concentrations of solution ranging
from 10À4 to 10À5 M were cooled down from near the boiling
point of n-dodecane, dried in the air, and photographed using
TEM. As shown in the Fig. 5 (b–d), fiber-like structures were also
In contrast to the poor solubility of linear long-chain carba-
mates in most solvents,9d threefold symmetric tricarbamates are
generally soluble in common organic solvents. The increased
solubility is perhaps because it is difficult for the three-armed
carbamates to quickly form repeatable close packing in the solid
state, especially for those with long alkyl chains.5 Their high solu-
bility makes it easy to test the gelation properties. The gelation
capability of this group of tricarbamates was tested with different
solvents and shown in Table 1. In a typical concentration of 20 g/L
(2.6 Â 10À2 M) in n-dodecane, the tricarbamates such as C-1 and C-
2 with short alkyl side chains had relatively low solubility and
formed crystalline precipitate, and crystals grew on the sidewall
of the vial upon slow evaporation of the solvent. With the increase
of the alkyl side chain length, the tricarbamates solubility in n-
dodecane increased accordingly and did not precipitate rapidly.
The partial organogels of C-5 and C-6 showed high viscosity, but
could not pass the vial inversion test. However, as the alkyl chain
length was further increased, the solid gels formed quickly and
held their position during vial inversion. As shown in Fig. 1, while
C-7 organogel lost about 5% of n-dodecane solvent during the vial
inversion test, C-8 and C-12 organogel were able to completely
immobilize the solvent.
Tricarbamate C-12 was chosen to further study its gelation
capability. It was found that C-12 was able to form organogels with
a wide range of solvents from polar solvents, like acetonitrile, to
nonpolar solvents, such as toluene, generally at a critical gel con-
centration (CGC) around 20 g/L in these solvents. The results are
summarized in Table 2. Decalin and methylcyclohexane could be
gelled at concentration of 10 g/L. The best gelation result of tricar-
bamate C-12 was observed in a nonpolar solvent, n-dodecane, in
which the critical gelation concentration of 1 g/L (1.3 Â 10À3 M)
was remarkably low. To the best of our knowledge, the above value
of 1 g/L is among the best gelation capabilities of all reported
organogelators. A further interesting feature is that the 1 g/L of tri-
carbamate C-12 gel in n-dodecane is nearly transparent (Fig. 2),
which expands its potential applications.
The formation of intermolecular hydrogen bonding was indi-
cated by 1H NMR spectra of tricarbamate C-12 in benzene-d6. As
shown in Fig. 3, as the concentration of the solution decreases from
100 mM to 1.5 mM, the chemical shift of the active proton of the
carbamate group is gradually moved upfield.12 The substantial
upfield-shifting of the carbamate proton signal upon dilution is
Table 1
Gelation capability of C-1 ꢀ C-8 and C-12 in n-dodecane (20 g/L).
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C-12
Crystalline precipitate
Sparingly soluble
Suspension
Suspension
Partial gel
Partial gel
Solid gel
Solid gel
Solid gel
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X. Hou et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Fig. 1. Sample 1–8 corresponds to tricarbamate C-1 ꢀ C-8 and sample 9 is C-12 at 20 g/L (2.6 Â 10À2 M) in n-dodecane.
Table 2
Critical gel concentration (CGC) of tricarbamate C-12 for a variety of solvents.
ƻ
1.5 mM
6.0 mM
Solvents
CGCa
Solvents
CGCa
ƻ
n-Dodecane
Decalin
Methyl cyclohexane
Toluene
1a
n-Hexane
20
20
20
50
10
10
20
20
1-Dodecanol
Acetonitrile
Ethanol
ƻ
12.5 mM
25.0 mM
50.0 mM
Cyclohexane
Chloroform
Soluble
ƻ
a
The critical gelation concentration is showed as g/L.
ƻ
ƻ
100.0 mM
8
7
6
5
4
2
1
0
ppm
Chemistry Shift
Fig. 3. 1H NMR spectra of tricarbamate C-12 at various concentrations of benzene-
d6. Chemical shift of the N-H gradually moves upfield with decreasing solution
concentrations. The NMR solutions must remain below the gelation point or else
high resolution spectra cannot be obtained.
1.5
234 nm
245 nm
n-dodecane
chloroform
n-dodecane
1.0
0.5
0.0
chloroform
Fig. 2. Transparent gel of the tricarbamate C-12 in n-dodecane (1 g/L, or
1.3 Â 10À3 M).
formed with samples from the all three concentrations of
solutions. The shape and size of the aggregates were similar to
those observed with the 1 g/L (1.3 Â 10À3 M, Fig. 5a) gel further
showing that the gelation process is general and reliable.
200
250
300
Wavelength(nm)
350
400
In summary, a new family of LMOGs based on the threefold
symmetric tricarbamate were synthesized, characterized, and
studied. The tricarbamates with long alkyl chains were able to
gelate a variety of organic solvents and showed remarkable gela-
tion ability in nonpolar solvents.6a Fiber-like networks were
observed under TEM in a wide range of solution concentrations.
The readily available starting materials, ease of synthesis, good
solubility and stability of the threefold symmetric tricarbamates
make them attractive members of LMOGs.10 The potential applica-
tions of the organogels and their derivatives include serving as
Fig. 4. UV–Vis spectra of tricarbamate C-12 (10À5 M) in polar and nonpolar solvents
at room temperature. Both curves have been normalized for convenience of
comparison.
responsive soft materials or being used as templates and stabilizers
in the synthesis of polymeric, inorganic, and hybrid
nanomaterials.14
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4
X. Hou et al. / Tetrahedron Letters xxx (2016) xxx–xxx
a
b
Fig. 5. TEM images of nanofibers formed by tricarbamate C-12 in n-dodecane: (a) gel at 1 g/L (1.3 Â 10À3 M), (b) solution at 10À4 M, (c) solution at 5 Â 10À4 M, and (d)
solution at 5 Â 10À5 M. The solutions were prepared by heating close to the boiling temperature of n-dodecane and then cooled down in air.
(f) Leenders CMA, Jansen G, Frissen MMM, et al. Chem Eur J.
2016;22:4608–4615.
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(NSF, Grant # EPS-0814442) and the Doctoral New Investigator
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(PRF 52705-DNI7). The authors acknowledge Dr. Angel Ugrinov at
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