General approach to low-molecular-weight metallogelators via the coordination-induced gelation of an l-glutamate-based lipid

Article abstract of DOI:10.1021/la203569y

A twin-tailed glutamate-based lipid with a pyridine headgroup was prepared in good yield using standard amide coupling and protection/deprotection chemistry. The resulting Lewis basic lipid gels a wide array of hydrocarbon solvents at a critical gelation

Full text of DOI:10.1021/la203569y

Letter
pubs.acs.org/Langmuir
General Approach to Low-Molecular-Weight Metallogelators via the
Coordination-Induced Gelation of an L-Glutamate-Based Lipid
Robert E. Bachman,* Anthony J. Zucchero, and Jamie L. Robinson
Department of Chemistry, The University of the South, 735 University Avenue, Sewanee, Tennessee 37383, United States
*
S Supporting Information
ABSTRACT: A twin-tailed glutamate-based lipid with a pyridine headgroup was prepared
in good yield using standard amide coupling and protection/deprotection chemistry. The
resulting Lewis basic lipid gels a wide array of hydrocarbon solvents at a critical gelation
concentration (C ) of 0.3 wt %. The gelation of more polar solvents, such as ethanol, THF,
g
dichloromethane, and chloroform, occurs with a C of between 2 and 5 wt %, demon-
g
strating the importance of hydrogen bonding interactions in gel formation. The importance
of hydrogen bonding in this system was also demonstrated by IR observation of the amide
bands, which show a substantial shift upon gelation. Solutions of this new organogelator with concentrations below C rapidly
g
form gels upon the introduction of a wide variety of metal salts or complexes, providing a convenient general method for the
preparation of metallogelators. Spectroscopic evidence suggests that the enhanced gelation seen in the metal-containing systems
can be explained by a cross-linking of gel fibril aggregates similar to those formed by the unmetalated gelator.
8
ver the last two decades, there has been a rapidly growing
exploration of molecular gels, which are composite
which includes a single example of a metal-based gelator. In
this letter, we report the preparation of a new LMWG that
incorporates a Lewis basic pyridine headgroup (L). This lipid is
capable of forming organogels in a variety of hydrocarbon
solvents at concentrations of as low as 0.3 wt % (∼4 mM).
Additionally, the gelation ability of L is greatly enhanced by its
in situ coordination to a wide variety of metal centers. As such,
the synthesis of L provides a general approach to the formation
of a wide array of metallogels via coordination-induced gelation.
O
viscoelastic materials that spontaneously self-assemble when a
small amount (typically <2 wt %) of an appropriate low-
molecular-weight gelator (LMWG) is introduced into either
1
water or an organic solvent. Interest in these materials is
driven at a fundamental level by the desire to understand the
self-assembly processes responsible for gel formation. More-
over, given the fine control that chemists have over molecular
structure, gelation represents an elegant bottom-up approach to
the fabrication of structures on the nanoscale as well as on
2
larger organizational scales. Given their unique ability to
organize structure across several length scales, LMWGs have
been suggested to have numerous, diverse, practical applica-
tions, including low-dimensional conductors, membrane filters,
1
,2
drug-delivery vehicles, and sensors.
Although countless organic LMWGs have been identified,
1
The Lewis basic lipid (L) was prepared in three steps from
CBZ-protected glutamic acid using standard amide coupling
metal-containing systems have only more recently begun to
7
,9
3
and deprotection chemistry (Scheme 1).
Both the
receive significant attention. The incorporation of metal
intermediate CBZ-protected glutamate lipid and L readily gel
a variety of solvents, allowing purification via washing of the
crude solid followed by a gelation/filtration step. The amine
intermediate was purified by recrystallization from ethanol.
Typical reaction yields for each of the three steps were between
centers into gelators permit the design of materials that possess
propertiesmagnetism, color, catalytic activity, and redox
behaviorthat are difficult to achieve in organic systems.
Typically the metallogels (metal-containing gels) prepared to
date have been created using preformed metal complexes of
3
,4
organic gelators. Alternatively, multitopic ligands have been
8
5 and 97%, with much of the variability due to handling losses
used to form gels via the in situ construction of coordination
(
Supporting Information).
3
,5
polymers. In contrast, only a few reports of coordination-
induced in situ gelation by discrete complexes have appeared to
Organogels of L can be formed with a critical gelation
concentration (C ) of approximately 0.3 wt % (∼4 mM) in
6
g
date, and these systems have typically been designed to
both aromatic and selected aliphatic hydrocarbon solvents
function with a single metal species. Inspired by this initial
work, we have undertaken an effort to develop ligand systems
capable of forming gels with a wide selection of metal fragments
via in situ coordination to form discrete metal complexes. Our
design strategy is based on the elegant modular approach to7
glutamate-based gelators developed by Ihara and co-workers,
(
Table 1). The resulting gels may be characterized as
Received: September 12, 2011
Revised: November 17, 2011
Published: December 5, 2011
©
2011 American Chemical Society
27
dx.doi.org/10.1021/la203569y | Langmuir 2012, 28, 27−30

Langmuir
Letter
Scheme 1. Synthesis of L
Table 1. Critical Gel Concentration (CGC) in the Weight
Percentage for L in a Variety of Solvents
measurable evaporation of the solvent. The isolation of these
gelatinous masses by decantation followed by drying returns a
composition close to the measured C value for that solvent.
g
solvent
benzene
CGC (%)
solvent
CGC (%)
(
e.g., 5 wt % vs a measured C = 3 wt % for ethanol). This
g
0.3
0.3
0.3
0.3
0.3
0.3
0.3
3
EtOH
CHCl3
CH Cl
3
5
5
5
finding is consistent with the tendency, noted above, for gels
toluene
formed at or near C in hydrocarbon solvents to exclude solvent
g
xylene
2
2
as they age to reach a higher, and presumably more stable, fiber
to immobilized solvent ratio.
mesitylene
tetrahydronapthalene
cyclohexane
silicone oil
MeOH
THF
a
acetone
ppt/pg
1
The solution H NMR spectra of L depend on both the
Et O
2
insol
insol
insol
concentration and the solvent used. At higher concentrations
CH CN
3
(
7
∼1 wt %, ∼25 mM) in CDCl , the amide signals occur at 8.55,
3
H O
2
.05, and 5.94 ppm. On the basis of the coupling patterns
a
ppt/pg: precipitation with partial gel formation. insol: the gelator
does not dissolve even under extended heating.
(
Supporting Information), we assign these peaks to isonicoti-
namide linkage and the two lipid amide linkages. The strong
upfield shift of one of the lipid amide signals is consistent with
the presence of a strong intramolecular hydrogen bonding
interaction, most likely involving the N−H group of one
transparent to translucent, with the most transparent gels
forming in aromatic solvents (Figure 1). Initial X-ray scattering
experiments on dried xerogel samples show only broad features
indicative of an essentially amorphous fiber structure.
10
dodecyl tail with the carbonyl linkage of the other tail. The
isonicotinamide signal migrates downfield with decreasing
concentration, reaching approximately 10 ppm at [L] ≤ 0.25
wt %, and the other signals show little change in chemical shift
but become steadily broader. The differing behavior of these
signals suggests that L aggregates in solution via an
intermolecular hydrogen bonding interaction involving the
isonicotinamide group at concentrations well below C . It also
g
suggests that deaggregation leads to a higher degree of
fluxionality in the hydrogen bonding between the lipid amide
linkages. This picture is further supported by spectra taken in
hydrogen bond donor and/or acceptor solvents, such as
ethanol and THF. These spectra show a significant downfield
shift of the lipid amide signals to 8.24 and 8.11 ppm as the
hydrogen bonding is disrupted and the environments of the
amide groups become more similar.
Figure 1. Typical gel formed from 0.5 wt % L in toluene with cooling
in a water bath. Lines drawn on the background paper help to show
the high degree of translucency in these gels. The lines behind the gel
are of the same length as the two below the gel but are magnified by
the curvature of the vial.
Taken as a whole, the NMR data suggests that the
aggregation of L begins to occur well below C and that the
g
Aromatic hydrocarbon gels with [L] between 0.5 and 1 wt %
are robust and stable at room temperature for longer than a
month. Gels with [L] < 0.5 wt % slowly exclude solvent to
produce a stable gel with [L] higher than that initially
formulated along with a small amount of solution. The gel−
self-assembly process depends on both intra- and intermolecular
hydrogen bonding interactions, which are likely assisted by the
van der Waals packing of the aliphatic chains. The significance
of such alkyl chain packing in gel formation has been elegantly
11
demonstrated by Weiss and co-workers. This model is also
supported by IR spectral data, which shows a strong red shift of
sol transition temperature (T ) of mesitylene gels was found to
g
−1
increase rapidly at low [L], increasing from 34 °C at 0.3 wt %
to 55 °C at 1 wt %. Doubling [L] to 2 wt % has much less of an
both the N−H (3436 to 3298 cm ) and amide (1664 and 1557
−1
12
to 1635 and 1522 cm ) bands upon gelation. The tendency
impact, increasing T by only 7 °C to 62 °C. At [L] > 2 wt %, L
of L to aggregate in solution well below C also explains the
g
g
partially precipitates upon cooling rather than forming a
homogeneous gel.
Gels of L can be formed in a variety of polar solvents as well,
spontaneous formation of gel masses noted above.
Although no gelation was observed for mixtures of 1 wt % L
in CH Cl , the addition of half a molar equivalent of a
2
2
but with a C that is at least an order of magnitude higher than
soluble Ag(I) salt, such as AgBF or AgSbF (2:1 L/Ag(I))
g
4 6
that required for hydrocarbons (Table 1). However, sealed
leads to instantaneous gelation, presumably induced
+
6b,13
solutions of L with [L] < C form optically transparent self-
by the coordination of two molecules of L to one Ag .
g
supporting gel masses upon standing for several days without
The room-temperature CGC for this L/Ag+ system was
2
8
dx.doi.org/10.1021/la203569y | Langmuir 2012, 28, 27−30

Langmuir
Letter
estimated to be 0.5 wt % using a dilution/thermal
isotropization/regelation/inversion protocol. This result in-
dicates that the metal complexation enhances the gelation
ability of L by 10-fold, as measured by CGC, relative to free L
in CH Cl .
of the identity of the counterion for all Ag(I) salts examined. In
contrast, a significant counterion dependence was observed for
the Cu(II) salts. Although the origin of this difference remains
somewhat unclear, two hypotheses seem viable. First, the
anions employed in the Ag(I) case are essentially nonco-
ordinating and so do not compete with the gelator ligand for
metal binding. Second, the more variable coordination geo-
metry and number of Cu(II) may require a different L/metal
ratio than 2:1 to achieve gelation. It is also possible that both of
these factors can be operative simultaneously. Studies to
explore more fully both the generality of this gelation process
and its dependence on variables such as the counterion identity
and L/metal ratio are ongoing.
2
2
1
The H NMR spectra of very dilute (≤0.3 wt %) solutions
+
with a nominal composition of L Ag , in either CDCl or
2
3
CD Cl , show broad signals in the aromatic region at 8.85 and
2
2
8
.50 ppm. This downfield shift of the pyridine resonances
(
versus 8.75 and 7.74 ppm for free L) is consistent with the
binding of a silver ion to the basic nitrogen atom of the pyridine
moiety. Similar shifts are seen for the aromatic signals of non-
gel-forming species LAuCl and [LCH ]OTf, which appear at
3
8
.95 and 8.71 ppm and at 8.92 and 8.51 ppm, respectively. The
Attempts to induce gelation in 1 wt % ethanolic solutions
failed; however, the resulting solutions spontaneously form gel
masses analogous to those seen for pure L, suggesting that
gelation in ethanol should be possible at higher concentrations.
Additionally, the removal of ethanol provided powders that
readily formed metallogels upon swelling with toluene or other
hydrocarbon solvents, providing an alternative method for
producing metallogels of metal salts with limited solubility in
solvents such as CH Cl .
+
proposed formula of [L Ag] is also supported by viscometry
measurements on sub-C samples. Although the addition of any
amount of Ag results in increased viscosity compared to that of
solutions of L, the viscosity reaches a maximum near a ligand/
metal ratio of 2:1. However, the relative broadness of this curve,
along with the broad aromatic signals observed in the H NMR,
allows for the possibility of a dynamic system consisting of a
mixture of species with differing L/M ratios.
2
g
+
1
2
2
The metal-induced gelation of solutions of L is a general
phenomenon. The addition of a variety of metal salts or
complexes (Table 2) to 1 wt % solutions of L in CH Cl ,
The IR spectra of the metallogels, regardless of the solvent
used for preparation, look essentially identical to gels and
xerogels prepared from pure L, with the N−H band at ∼3300
cm and the amide bands at 1635 and 1530 cm . This would
seem to indicate that the hydrogen bonding network
responsible for the formation of the gel fibers in the metallogels
is essentially identical to that of free L.
2
2
−1
−1
Table 2. Results of Coordination-Induced Gelation in
CH Cl for the Metal Salt or Complex Examined
2
2
a
a
metal salt
gelation?
metal salt
gelation?
In trying to understand the mechanism for coordination-
AgBF4
G
G
G
G
G
G
G
G
NiCl ·6H O
2
2
G
G
G
S
induced gelation, it is also worth noting that fragments capable
AgSbF6
CuBr
Cu(NO )·2.5H O
+
3
of binding to a single unit of L, such as AuCl or CH , do not
AgNO3
3
2
induce gelation. Furthermore, enhanced gelation is independ-
ZnCl2
CuBr2
ent of whether the fragment linking the L moieties carries a
b
CdCl2
Cu(OAc)2
FeCl2
S
+
charge (e.g., Ag vs CoCl ). Indeed, the overall pattern of
2
CoCl2
S
behavior summarized in Table 2 supports the idea that
coordination-induced gelation by metal−L complexes is the
result of physically linking two or more L units, independent of
the geometry of the linkage. This latter point is clearly
demonstrated by the deep blue color of the cobalt-containing
SnCl ·2H O
FeCl3
S
2
2
Pd(PhCN) Cl2
thtAuCl
sol
2
a
G: gel stable to inverstion. sol: solution only. S: semigela
significant increase in viscosity but not complete immobilization
upon inversion. Decreasing the solvent volume by approximately 66%
b
15
gel, consistent with a tetrahedral geometry at the metal center
led to the formation of a gel.
and a “bent” cross-link as opposed to the linear cross-link
+
present for the Ag system. This model is consistent with the
CHCl , or THF results in essentially instantaneous gelation
3
+
recent report by Lam and Yam that uses Ag to cross-link Re(I)-
(
Figure 2). Although it is reasonable to expect differing ideal
6a
acetylide moieties and induce gelation. This interpretation is
also supported by SEM images of xerogels of free L (1 wt % in
+
toluene) and both the L/Ag and L/CoCl metallogel systems
2
(
1 wt % in CH Cl ) (Figure 3). Although the change in the gela-
2 2
tion solvent and the resulting different drying regimes are
certainly confounding variables in these measurements, the SEM
images reveal subtle differences in the gel morphology. All three
samples appear to be composed of fiberlike structures, which
further aggregate into the 2D motifs common in LMWG sys-
1a,2a,4
tems.
However, whereas xerogels of free L typically collapse
to tangled mats of fibers upon drying, the metal-containing
systems generate robust, porous 3D structures. Additionally, the
fiber dimensions in the metal-containing gels are typically larger
than in the metal-containing sytems, indicative of higher-order
aggregation as would be expected in the case of cross-link
formation.
From a macroscopic perspective, our model for the enhanced
gelation seen in the presence of metal ions is equivalent to the
formation of cross-links between preassembled gel fibrils of L
Figure 2. Addition of 0.5 equiv of CoCl to a 1 wt % solution of L in
2
THF produces instantaneous gelation of the system. The C in this
g
system is 0.5 wt %.
stoichiometries for such a wide array of metal species (e.g., one
14
might expect a 4:1 ratio for Ni(II) ), a 2:1 L/metal ratio was
maintained in these experiments to facilitate comparison. As
can be seen in Table 2, the gelation behavior was independent
2
9
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Langmuir
Letter
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AUTHOR INFORMATION
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Corresponding Author
E-mail: rbachman@sewanee.edu.
(
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ACKNOWLEDGMENTS
■
We thank the Research Corporation Cottrell College Scholars
Fund (CC5549), the Jesse Ball duPont Foundation, and The
University of the South for their financial support of this work.
Gunnar Glasser of the Max Planck Institute for Polymer
Research is acknowledged for his assistance with the acquisition
and interpretation of the SEM images.
3
0
dx.doi.org/10.1021/la203569y | Langmuir 2012, 28, 27−30