Liposomes by polymerization of an imidazolium ionic liquid: Use as microreactors for gold-catalyzed alcohol oxidation

Article abstract of DOI:10.1002/chem.200902395

The bipodal ionic liquid N, N'-bis(10-undecenyl)-2-methylimidazolium underwent polymerization in aqueous media with potassium persulfate to form submicrometric liposomes. A. TEM study indicated that the liposomes are the result of several morphological transformations of the polymer particles. Photopolymerization at room temperature led to spherical particles with some internal voids and polymer chains. We took advantage of the positive charge on the imidazolium rings and used a photocuring agent to form in a single step starting from the imidazolium monomer and NaAuCl4 spherical polymer particles containing gold nanoparticles (10-15 nm) in their interior. This system was found to promote the selective aerobic oxidation of 2-hydroxybenzyl alcohol to salicylaldehyde.

Full text of DOI:10.1002/chem.200902395

DOI: 10.1002/chem.200902395
Liposomes by Polymerization of an Imidazolium Ionic Liquid:
Use as Microreactors for Gold-Catalyzed Alcohol Oxidation
Mireia Buaki, Carmela Aprile, Amarajothi Dhakshinamoorthy, Mercedes Alvaro, and
Hermenegildo Garcia*[a]
Abstract: The bipodal ionic liquid
N,N’-bis(10-undecenyl)-2-methylimida-
zolium underwent polymerization in
aqueous media with potassium persul-
fate to form submicrometric liposomes.
A TEM study indicated that the lipo-
somes are the result of several morpho-
logical transformations of the polymer
particles. Photopolymerization at room
temperature led to spherical particles
with some internal voids and polymer
chains. We took advantage of the posi-
tive charge on the imidazolium rings
and used a photocuring agent to form
in a single step starting from the imida-
zolium monomer and NaAuCl4 spheri-
cal polymer particles containing gold
nanoparticles (10–15 nm) in their inte-
rior. This system was found to promote
the selective aerobic oxidation of 2-hy-
droxybenzyl alcohol to salicylaldehyde.
Keywords: gold
· ionic liquids ·
liposomes · nanoparticles · polymer-
ization · self-assembly
Introduction
peared as a general and powerful tool for the creation of
spatial inhomogeneities and spatial organization. The forces
arising between the solvent and the solute have been ap-
plied, for instance, in the synthesis of micro-/mesoporous sil-
icates using a structure-directing agent.[8–11]
The spontaneous self-assembly and spatial structuring of
ionic liquids and liquid crystals form the basis of many fun-
damental phenomena that have been exploited in numerous
applications.[1–4] The special features and amphiphilic charac-
ter of molecules comprising polar and apolar parts tend to
be reflected at the supramolecular level by the appearance
of intermolecular aggregations that maximize the favorable
and minimize the unfavorable interactions.[5] This self-as-
sembly is generally better expressed in media that have a
strong bias towards one type of polarity, water being the
most studied solvent. The strong hydrogen bonds present in
pure water are disturbed when apolar chains and substitu-
ents are introduced into the medium and the phenomena
observed under these conditions, particularly aggregation,
are generally described as arising from the hydrophobic
nature of these groups in a hydrophilic medium.[6,7] The in-
terplay between hydrophilicity and hydrophobicity has ap-
As a particular example of the use of amphiphilic mole-
cules in producing spatially structured objects, in this report
we describe the synthesis and polymerization of an imidazo-
lium ionic liquid bearing two long alkenyl chains with termi-
nal C=C bonds. Ionic liquids with imidazolium heads and
long alkyl chains have been used as templates for the prepa-
ration of mesoporous silicas because of their ability to self-
assemble in aqueous media.[12,13] In this case, the terminal
C=C bond should allow cross-linking of the chains, freezing
the shape of self-assembled monomer particles. The length
and conformational freedom of the alkyl chains should still
permit after polymerization a considerable flexibility and
even reshaping of the polymeric particle. The presence of
two alkyl chains may favor preferentially the formation of
vesicles and liposomes with a defined membrane and an
empty space with respect to spherical particles.[14,15] It has
frequently been observed that surfactants with a single alkyl
chain form micelles, whereas those surfactants bearing two
long alkyl chains tend to self-assemble to form lipo-
somes.[16–19] Liposomes with porous walls can allow mass
transfer from the exterior to the interior of the membrane
for substrates smaller than the pore diameter and this spatial
organization allows the location and immobilization in the
[a] M. Buaki, Dr. C. Aprile, Dr. A. Dhakshinamoorthy, Dr. M. Alvaro,
Prof. Dr. H. Garcia
Instituto Universitario de Tecnologꢀa Quꢀmica CSIC-UPV
and Departamento de Quꢀmica, Universidad Politꢁcnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia (Spain)
Fax : (+34)96-3877809
E-mail: hgarcia@qim.upv.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902395.
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FULL PAPER
interior of the liposomes guests that can be responsible for
some of the properties of the particle. In the case presented
herein we will show that the positive charge on the imidazo-
yl derivatives, although sometimes the presence of co-surfac-
tants can play a role in the self-assembly.[27]
The rationale behind the synthesis of imidazolium 1+ was
to obtain a surfactant with w-alkenyl moieties that have the
possibility, after spontaneous self-organization in an aqueous
medium, of polymerizing through the terminal C=C double
bonds and thus form cross-linked, structurally defined mi-
crometric particles that could be separated from solution by
filtration or centrifugation.
À
lium head can be advantageously used to attract AuCl4
through ion pairing and this monoatomic complex can act as
a precursor to gold nanoparticles. As a consequence of the
À
preferential location of AuCl4 , the resulting nanoparticles
will be mostly located in the interior of the polymeric parti-
cle. Gold nanoparticles can exhibit catalytic properties for
aerobic oxidations and other organic reactions.[20–23] In this
way, the system comprising encapsulated gold nanoparticles
inside porous polymeric particle of micrometric length re-
semble in a certain way the reported microreactors formed
by nanoscopic zeolite cavities containing catalytic sites.[24–26]
To obtain the liposomes, the as-prepared imidazolium
ionic liquid 1+ was dissolved in water and stirred for 1 h to
allow self-assembly of the precursor before the addition of
potassium persulfate as the radical polymerization initiator.
Potassium persulfate is a typical radical initiator and is
widely used to promote the thermal polymerization of sty-
rene and other alkenes in aqueous media.[28] To study the in-
fluence of the polymerization temperature on the shape, di-
mension, and homogeneity of the liposomes, polymerization
was performed at 40, 80, and 1008C. Under these conditions,
in all cases the signals typical of C=C double-bond hydrogen
atoms were shifted from 5.8 to 5.0 ppm in the 1H NMR
spectra, which clearly indicates that polymerization has oc-
curred to a significant extent, even at the lowest tempera-
ture. For the cross-linking reaction performed at 808C, the
area of the residual vinylic signals was about 12% of the ini-
tial peaks.
Results and Discussion
Synthesis and characterization of the liposomes: In the first
stage of our work we synthesized N,N’-bis(10-undecenyl)-2-
methylimidazolium bromide (1+BrÀ), a surfactant molecule
with a polar head and two hydrophobic tails long enough to
favor self-organization to form vesicle-like structures. The
synthesis was carried out in a straightforward manner by
treating 2-methylimidazole with 11-bromo-1-undecene in
toluene at reflux (Scheme 1).
Transmission electron microscopy (TEM) images obtained
from the polymeric material after centrifugation of the sus-
pension showed that all the particles have the same cell-like
system with a morphology defined by a hollow liposome
with a polymeric wall of 20–30 nm thickness and an internal
void. This characteristic morphology arises from the sponta-
neous self-organization of our bipodal ionic monomers, the
structure of which is reminiscent of natural phospholipids
(Scheme 2).
Scheme 1. Synthetic procedure followed to obtain N,N’-bis(10-undecen-
yl)-2-methylimidazolium bromide (1+BrÀ).
The purity of monomer 1+BrÀ was checked by reversed-
phase HPLC, which showed that the peak corresponding to
1+BrÀ was 77% of the total area and that it was accompa-
nied by three other peaks (see Figure S1 of the Supporting
Information). A second peak (5%) close to 1+BrÀ was at-
tributed to the imidazolium 1+ associated with a counteran-
ion different to BrÀ, for example, OHÀ, on the basis of its
similar retention time and optical spectrum. On the basis of
the synthetic route followed to obtain 1+BrÀ, the other two
peaks (15 and 3%) were assigned to monopodal imidazoli-
um associated with BrÀ or the other counterion. Careful in-
Scheme 2. Structure of the vesicles obtained by the self-assembly and
polymerization of N,N-dialkenylimidazolium.
1
tegration of the H NMR signals showed a 10% deficiency
with respect to the imidazolium protons in the area of the
vinylic signal corresponding to the undecenyl chain (see Fig-
ure S2 of the Supporting Information), which supports the
assignment of the second largest HPLC peak to the mono-
substituted protonated imidazolium. We anticipate that the
ability of compounds 1+ to self-assemble will not be much
disturbed by the presence of a proportion of monoundecen-
After statistical analysis of the vesicle size distribution by
TEM it was concluded that the most size-homogeneous vesi-
cles are formed at 408C and at this temperature the diame-
ter ranges from 400 to 600 nm (see Figure 1, left). At higher
temperatures we obtained vesicles with dimensions ranging
from 400 nm to 1 mm with a significantly broader liposome
size distribution. The yield of liposomes obtained in terms
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H. Garcia et al.
quires a threshold temperature to promote a high degree of
double-bond polymerization.
To circumvent the problems associated with the use of
thermal activation to promote the polymerization using per-
sulfate we considered the possibility of synthesizing the
spheres at room temperature using a photocuring agent,
namely 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophe-
none (2), as the radical source; compound 2 decomposes
into radicals upon irradiation with UV light. Thus, by keep-
ing the same concentration of the organic precursor as used
in the thermal cross-linking, we performed the polymeri-
zation of the ionic liquid 1+BrÀ by adding the water-soluble
compound 2 to the aqueous solution and irradiating the
samples through quartz tubes (Scheme 3).
Figure 1. TEM images of the vesicles prepared by heating the reaction
mixture containing 1+BrÀ at 408C (left) and the particles present in the
aqueous phase after sedimentation of the larger vesicles upon polymeri-
zation at 1008C (right).
of the percentage of the initial monomer recovered as lipo-
somes when the reaction was carried out at 408C was 65%,
which indicates that the C=C double-bond cross-linking pro-
moted by potassium persulfate is not complete at this tem-
perature.
In the case of the polymerization performed at 1008C, we
also observed that the liquid phase contains small spheres
(Figure 1, right) with diameters of 20–40 nm and an internal
polymeric structure. However, owing to their small dimen-
sions, these nanoparticles do not sediment under our centri-
fugation procedure, remaining in solution and being a part
of the 28% of the monomer 1+BrÀ that is not recovered as
vesicles under these conditions.
The monomer 1+BrÀ polymerizes by the common radical
À
chain mechanism initiated by homolytic O O bond rupture
Scheme 3. Alternative procedures for the formation of polymeric parti-
of the persulfate to form oxyl radicals. These oxygen-cen-
tered radicals attack the C=C double bond to generate
carbon-centered radicals that will propagate the chain.[29]
Under similar conditions, latex spheres of fairly homogene-
ous particle size have been obtained.[30] The spherical shape
of the polystyrene latex is believed to arise from hydropho-
bic interactions that lead to a geometrical body that offers
the least contact interface between water and the hydropho-
bic styrene.[31] The principle feature arising from the use of
the monomer 1+BrÀ is the spatial self-assembly of this mole-
cule to form hollow spheres in water rather than thick
spheres.[32] There are precedents reported in the literature
using silanes that have clearly shown the formation of rigid
silica hollow spheres rather than thick spheres when appro-
priate precursors are used.[33] The origin of this internal
space is the imbrication of the alkyl chains with the forma-
tion of a bilayer arrangement, as indicated in Scheme 2.
Vesicles and liposomes are well known in biological systems,
the individual component phospholipids structurally resem-
bling compound 1+BrÀ.
cles from the imidazolium ionic liquid 1+.
As in the previous case, the course of the polymerization
was conveniently followed by 1H NMR spectroscopy, period-
ically taking small aliquots of the liquid phase. We observed
that the polymerization was almost complete in 12 h. To
remove traces of unreacted compound 2 as well as some
radical recombination byproducts, the sample was extracted
with dichloromethane until the complete disappearance of
organic compounds, as indicated by GC–MS.
TEM images of the extracted suspension revealed some
morphological differences in the resulting polymeric parti-
cles, which appear as thick spheres rather than vesicles (Fig-
ure 2a). We attribute this change in the polymer morphology
to the mildness of the room-temperature photochemical
polymerization procedure. It appears that the spheres are
metastable intermediates in the formation of vesicles.
The particle size distribution of the spherical polymer par-
ticles ranges from 100 to 900 nm with an average of 400 nm
(Figure 2b). It seems that photocuring gives a somewhat
broader size distribution than the polymerization with potas-
sium persulfate. A careful inspection of the TEM images
after various intervals of time reveals the mechanism
through which vesicles are formed and provides a rationale
for the different morphologies obtained by thermal and pho-
tochemical polymerization. Thus, the TEM images reveal
that at short reaction times the particles still contain small
polymeric chains in the interior of the particles. These inter-
Concerning the influence of the polymerization tempera-
ture on the morphology and structure of the particles, we in-
terpret the results by considering that at 1008C the forces
effecting the self-assembly of the monomer are largely dis-
turbed and that, on the other hand, the radical polymeri-
zation is considerably faster. These factors have a detrimen-
tal influence on the size homogeneity of the vesicles. On the
other hand, the use of potassium persulfate as initiator re-
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From this experiment it can be deduced that the electron
beam causes the migration of the internal polymeric chains
towards the peripheral wall, an effect that we suggest also
occurs spontaneously by capillary forces during the thermal
evolution and aging of the particles. This migration of the
internal polymer chains promoted either by focalized elec-
trons in a few minutes or spontaneously during the thermal
curing of the system leads to the formation of the thick ex-
ternal walls of the vesicles. This reorganization produces the
empty internal volume of the liposomes.
The different stages through which the vesicles are
formed also explains why, for sufficiently cured samples,
images such as those presented in Figure 1, left show vesi-
cles with a clearly defined internal void, whereas for insuffi-
ciently cured samples, such as those shown in Figure 3a and
b and even Figure 1, right, the polymer particles still contain
polymer chains inside their volume.
Figure 2. a) TEM (left) and SEM (right) images of the spherical particles
obtained by room-temperature polymerization of imidazolium 1+
through UV photocuring using water-soluble 2. b) Size distribution of the
spheres obtained by room-temperature polymerization of imidazolium 1+
promoted photochemically by compound 2.
Gold nanoparticles inside polymeric spheres: One of the
aims of our work was to show that particles obtained by the
self-assembly of imidazolium 1+ can be used to organize
spatially nano objects. With this in mind, and after studying
the polymerization of monomer 1+, the next step was to
show that it is possible to prepare gold nanoparticles inside
the polymeric particles derived from 1+ (P1) and even to
show the catalytic application of our systems in gold-cata-
lyzed oxidation reactions.
nal chains exhibit some structuring and are not completely
dense. At this stage the external membrane has already de-
veloped. A representative image is shown in Figure 3a. If at
We thought that the cationic head of the imidazolium
ionic liquid precursor would interact with anionic tetra-
ACHUTNGRENcNUG hloroCAHTUNGTRNEaNUGN urate through coulombic forces during all the stages
of polymer formation and that, later, the location of the
gold nanoparticles in the polymeric particles would be large-
ly determined by the position of the imidazolium heads.
Thus, due to ion pairing with the imidazolium units, tetra-
chloroaurate should occupy the same positions as the imid-
À
azolium heads. Subsequently, the reduction of AuCl4 and
metal nanoparticle formation would lead to gold nanoparti-
cles that should associate with the polymer particles and be
located inside and outside of the polymeric particles.
However, when persulfate polymerization of imidazolium
À
monomer 1+ was attempted in the presence of AuCl4 , it
was observed that the presence of this metal complex se-
verely interferes with the mechanism of the radical polymer-
ization and samples of polymeric vesicles like those de-
scribed in the absence of AuCl4 could not be obtained sat-
isfactorily under the same conditions.
We circumvented this problem by performing the poly-
merization of monomer 1+ and the formation of gold nano-
particles from tetrachloroaurate photochemically rather
than thermally. It has previously been reported that the for-
mation of gold nanoparticles up to a particle size 10 nm can
be promoted efficiently by UV light irradiation by employ-
ing the same photocuring agents as those used in the radical
polymerization of alkenes.[34–36] Thus, it could be that the
same photochemical process is able to effect simultaneously
the formation of gold nanoparticles and the polymerization
of the double bonds of monomer 1+. This was confirmed ex-
Figure 3. Selected TEM images showing the changes in the polymer par-
ticle morphology towards the formation of vesicles under the effect of a
focused electron beam recorded a) almost immediately and after b) 2,
c) 4, and d) 8 min exposure time (scale bars=200 nm). The instrument
operating voltage was 100 keV.
À
this moment, at which the presence of internal polymeric
chains in the spherical particles is clearly visible, we take
TEM images at different exposure times using a sufficiently
high-power electron beam, changes in the morphology of
the particles can be observed with the internal polymeric
material migrating towards the wall to create an empty in-
ternal space and thicker external walls. The results of this in
situ experiment are shown in Figure 3.
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H. Garcia et al.
perimentally and we were able to obtain an Au-P1 sample
in which the gold nanoparticles are embedded in the micro-
metric polymer particles. Scheme 4 summarizes the process-
submicrometric diameter containing in their interior a con-
siderable number of small-sized gold nanoparticles. The
image shows that the synthesis solution does not contain
any gold nanoparticles that are not associated with the poly-
meric particles.
Figure 5 shows some selected images that illustrate the
type of polymer-encapsulated gold nanoparticles that can be
obtained. As can be seen, the density of the gold nanoparti-
Scheme 4. Concurrent process leading to the formation of gold nanoparti-
cles inside the vesicles on irradiation of a mixture of 1/AuCl4/2.
es that occur simultaneously upon irradiation of 1/AuCl4/2
mixtures in water. To effect this polymerization, we adapted
the procedure used for the preparation of the polymeric
nanoparticles by adding, after stirring for 1 h, a solution of
NaAuCl4 and then the photocuring agent 2 to give a mixture
of 1/AuCl4/2 in a molar ratio of 0.16:0.064:0.32. The mixture
was then irradiated with UV light (l>300 nm) at room tem-
perature. Also in this case, the polymerization was conven-
Figure 5. Selected TEM images showing the Au-P1 structure before (top
and bottom left) and after (bottom right) their use as a catalyst in the
aerobic oxidation of 2-hydroxybenzyl alcohol (see below).
1
iently followed by H NMR spectroscopy by monitoring the
cles increases significantly as we approach the center of the
polymer particle. We even notice that the gold nanoparticles
in Au-P1 are almost exclusively formed inside the polymeric
particles and not on the external surface. The average size
of the gold particles is about 10Æ5 nm, which is in the
range of values previously reported for the photochemical
formation of gold nanoparticles using compound 2 as the
photocuring agent.[34–36]
disappearance of the signals of the vinylic protons. At the
end of the reaction we observed the characteristic color of
gold nanoparticles. Figure 4 shows photographs before and
The reasons for this preferential internal location of the
gold nanoparticles are most probably the previously com-
mented morphology of the polymeric spheres and the mech-
anism of aggregation and shaping of the monomer 1+. Thus,
the internal polymer chains observed in the TEM images of
the spheres indicate that most of the imidazolium rings
occupy internal positions. As a result of ion pairing, most of
À
Figure 4. Photographs showing the color of solutions of the imidazolium
ionic precursor 1+ in the presence of photocuring agent 2 before and
after irradiation in the presence and absence of NaAuCl4. Left- and
right-hand images correspond to solutions with and without NaAuCl4, re-
spectively.
the gold atoms in AuCl4 should therefore be located inside
the polymer particle and as the gold nanoparticles evolve,
their formation should occur inside the polymer particles, as
is observed.
Thus, the incorporation of gold nanoparticles in the poly-
À
mer particle by preassociation between AuCl4 and imidazo-
after irradiation of solutions of the imidazolium ionic pre-
cursor 1+ in the presence of photocuring agent 2 with and
without tetrachloroaurate.
lium is a simple and highly efficient methodology for obtain-
ing these entities in a single step without dedicated polymer
synthesis.
In accord with the formation of spheres in the polymeri-
zation of 1+BrÀ by photocuring, the Au-P1 samples also
consisted of submicrometric polymeric particles. TEM
images of Au-P1 samples obtained by photochemical curing
recorded after extraction with dichloromethane show the
formation of the expected spherical polymer particles of
To address the effects of monomer structure and the
cross-linking process on the morphology of the resulting
Au-P1 particles, we performed an analogous experiment
using cetyltrimethylammonium bromide (CTAB) instead of
1+BrÀ as the surfactant. CTAB is a widely used structure-di-
recting agent and its ability to form micelles is firmly estab-
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Gold Inside Liposomes
FULL PAPER
lished. Ion pairing should work similarly for both 1+BrÀ and
CTAB, and this phenomenon should control the location of
the gold nanoparticles. The differences between the particles
derived from 1+BrÀ or from CTAB should arise from the
differences in the self-assembly and the chain cross-linking
of these two complexes.
As anticipated, after 24 h salicylaldehyde was obtained with
a selectivity of 99% and a yield of 33%. The total mass bal-
ance was 64%, which indicates that some residual material
is adsorbed on the polymeric spheres, which were not ana-
lyzed. Extraction showed additional amounts of HBA to
close the mass balance. In addition, we did not observe the
formation of the corresponding hydroxybenzoic acid and,
therefore, the process is highly selective for the oxidation to
aldehyde without over oxidation to the carboxylic acid.
TEM images of the catalyst before and after the reaction
show that the structure and morphology of the Au-P1 parti-
cles remain unaltered during the reaction (Figure 5). Au-P1
can be easily separated from the reaction medium and recy-
cled for two consecutive runs without a decrease in its activ-
ity. Even though the present process gives a moderate yield,
the results obtained demonstrate that our system can be
used as a heterogeneous catalyst for the oxidation of pri-
mary alcohols.
À
When a micellar solution of CTAB containing AuCl4 was
submitted to photocuring using compound 2 to generate the
gold nanoparticles under identical conditions to those used
for Au-P1, TEM images revealed that gold nanoparticles
(10–15 nm particle size) were present throughout the solu-
tion and mostly unassociated with the CTAB micelles.
Figure 6 shows an image of the resulting Au–CTAB sample.
Conclusion
An imidazolium ionic liquid bearing two C11 alkenyl chains
with terminal C=C double bonds is a versatile monomer
that undergoes self-assembly in water and after thermal or
photochemical polymerization gives cell-like or spherical
particles. We have shown that the spherical particles trans-
form into cell-like polymeric liposomes. We have taken ad-
vantage of the internal location of the majority of the imid-
Figure 6. TEM image obtained after the photocuring of a micellar solu-
tion of CTAB containing AuCl4À. The image shows small gold nanoparti-
cles (10–15 nm) in high contrast and the larger CTAB micelles (5–
150 nm) in low contrast. The scale bar corresponds to 200 nm.
It is reasonable to assume that the rigid membrane of the
À
À
liposomes and the internal location of AuCl4 should trap
azolium units and their ion pairing with AuCl4 ions to form
the gold nanoparticles after their formation in the lipo-
somes. However, in the micelles derived from CTAB, the
gold nanoparticles are free to diffuse into the aqueous solu-
tion, where they are predominantly observed. This control
experiment illustrates well the advantages of using a strat-
egy similar to that used with the ionic liquid 1+BrÀ.
gold nanoparticles inside the polymeric structure. The re-
sulting material, which consists of gold nanoparticles (10Æ
5 nm average size) embedded within the polymer structure,
exhibits notable catalytic activity for the selective aerobic
oxidation of 2-hydroxybenzyl alcohol to salicylaldehyde.
Catalytic activity of Au-P1 samples in the aerobic oxidation
of 2-hydroxybenzyl alcohol: Numerous studies have shown
that metal nanoparticles can be dispersed in imidazolium
ionic liquids and that these solutions are efficient, reusable
homogeneous catalysts for hydrogenation reactions.[37–39] In
a certain way, Au-P1 is a polymeric analogue of these liquid
systems because the gold nanoparticles in Au-P1 will also
be surrounded by imidazolium rings. Thus, the catalytic
properties of Au-P1 are of general interest in the context of
extending the use of ionic liquids in solid systems.
To explore the activity of our Au-P1 system as a heteroge-
neous catalyst for selective oxidation reactions, we selected
the aerobic oxidation of the primary alcohol 2-hydroxyben-
zyl alcohol (HBA). The current demand for environmentally
friendly processes requires the development of green oxida-
tion methods that employ clean oxidants and reaction media
and do not generate waste.[40–42] HBA was oxidized at neu-
tral pH by using molecular oxygen in water at 908C. Under
these conditions, the Au-P1 nanoparticles obtained by pho-
tocuring were directly used without any additional process.
Experimental Section
General: All the materials used in the present investigation were com-
mercially available and used as received. Combustion chemical analyses
to determine the organic content was performed in a FISONS CHNOS
analyzer. FTIR spectra were recorded with a Nicolet 710 spectrometer
with the samples prepared in KBr disks. 1H and 13C NMR spectra were
recorded with a Varian Gemini 3000 spectrometer at 300 and 75 MHz, re-
spectively. The reactions were monitored by NMR spectroscopy by
taking an aliquot of 5 mL from the quartz tube at 3 h time intervals. The
water was evaporated and the remaining solid was redissolved in CDCl3
to record the 1H NMR spectra. The TEM experiments were carried out
using a Philips CM300 FEG system with an operating voltage of 100 kV.
SEM images were obtained with a JEOL JSM-5410 instrument operating
at 20 kV.
Synthesis of the monomer N,N’-bis(10-undecenyl)-2-methylimidazolium
bromide: Triethylamine (1.2 equiv, 7.3 mmol, 0.738 g, 1.017 mL) and 11-
bromoundecene (2 equiv, 12.2 mmol, 2.84 g, 2.66 mL) were added to a so-
lution of 2-methylimidazole (6.1 mmol, 0.5 g) in toluene (50 mL). The
mixture was stirred at 908C for 48 h and filtered to remove the ammoni-
um salt formed during the course of the reaction. Then the toluene was
evaporated and the resulting oil was washed several times with hexane
until a yellowish powder was formed (0.282 g, 56.5%). The purity of 1+
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H. Garcia et al.
BrÀ was checked by reversed-phase HPLC using a C-18 column and a
mixture of methanol/acetonitrile (3:1) as eluent and a diode array as de-
tector.
1H NMR (300 MHz, DMSO): d=7.733 (s, 2H), 5.877–5.716 (m, 2H),
5.024–4.878 (dd, J1 =5.017, J2 =4.952, 4H), 4.156–4.039 (t, J1 =4.138, J2 =
4.114, J3 =4.090 Hz, 4H), 2.624 (s, 3H), 2.069–1.952 (m, 4H), 1.775–1.658
(m, 4H), 1.363–1.128 ppm (m, 24H); 13C (75 MHz, D2O): d=138.284,
121.575, 113.82, 48.007, 33.498, 29.34, 28.968, 28.695, 25.758, 9.732 ppm;
elemental analysis calcd (%) for C 64.0, H 10.40, N 6.15; found: C 63.1,
H 10.35, N 6.29.
and 2 (10À3 m). The irradiation was carried out through quartz under ni-
trogen for 12 h.
Procedure for the catalytic oxidation of 2-hydroxybenzyl alcohol: 2-Hy-
droxybenzyl alcohol (50 mg) was dissolved in a solution of potassium
phosphate (170 mg) and the catalyst in water (40 mL) under oxygen in a
50 mL round-bottomed flask fitted with a silicone septum. The reaction
mixture was maintained at 908C on a hot plate for 24 h. After the re-
quired time, the solution was extracted with dichloromethane (2ꢃ10 mL)
and the combined extracts were dried over anhydrous sodium sulfate.
Then the percentage conversion and yield were determined by gas chro-
matography on a Hewlett-Packard 5890 series II instrument equipped
with a FID detector and using a known amount of nitrobenzene as the
external standard.
Synthesis of liposomes
Thermal polymerization: The as-prepared imidazolium ionic liquid
(0.2 mmol, 0.0775 g) was dissolved in water (50 mL) and stirred for 1 h to
allow the self-organization of the monomer. After this time, a catalytic
amount of potassium persulfate (30 mg) was added to the solution. The
solution was stirred for 24 h at three different temperatures: 40, 60, and
1008C. Under these conditions, the polymerization was confirmed by
1H NMR spectroscopy since the signals corresponding to the C=C double
bonds decrease. The solution was allowed to cool to room temperature
and centrifuged at 9000 rpm for 30 min. The supernatant liquid was re-
moved and after lyophilization of the remaining solid, a white powder
was obtained (yields: 65, 68, and 72% for polymerization at 40, 60, and
1008C, respectively).
Acknowledgements
Financial support from the Spanish Ministry of Science and Innovation
(Projects CTQ2009-11583 and CTQ2007-67805/PPQ) is gratefully ac-
knowledged. C.A. thanks the Spanish Ministry of Education for a Juan
de la Cierva research associate contract.
Photopolymerization: Polymerization by the photochemical pathway was
carried out by irradiating the samples with UV light through quartz tubes
under nitrogen. In this procedure the imidazolium ionic liquid
(0.16 mmol, 0.062 g) was dissolved in water (20 mL) and stirred at room
temperature for 1 h. The water-soluble 2-hydroxy-4’-(2-hydroxyethoxy)-
2-methylpropiophenone (2; 0.32 mmol, 0.066 g) was dissolved in water
(20 mL) and added to the monomer solution after completion of the
[1] M. Gꢄmez-Lꢄpez, J. A. Preece, J. F. Stoddart, Nanotechnology 1996,
7, 183–192.
[2] M. E. Davis, C.-Y. Chen, S. L. Burkett, R. F. Lobo, Mater. Res. Soc.
Symp. Proc. 1994, 346, 831–842.
[3] “Templating, Self-Assembly, and Self-Organization”: Comprehensive
Supramolecular Chemistry, Vol. 9 (Eds.: J.-P. Sauvage, M. W. Hossei-
ni), Pergamon, New York, 1996.
[4] L. F. Lindoy, I. M. Atkinson, Self-Assembly in Supramolecular Sys-
tems, RSC, London, 2000.
[5] P. Behrens, G. D. Stucky, Angew. Chem. 1993, 105, 729–732; Angew.
Chem. Int. Ed. Engl. 1993, 32, 696–699.
[6] B. Widom, P. Bhimalapuram, K. Koga, Phys. Chem. Chem. Phys.
2003, 5, 3085–3093.
ACHTUNGTRENNUNGmonoACHTUNGTRENNUNGmer self-organization. The solution was irradiated for 12 h and the
1
polymerization was monitored by H NMR spectroscopy until the signals
corresponding to the C=C double bond had almost disappeared. Once ir-
radiation was complete, the solution was washed with dichloromethane
(3ꢃ40 mL) and the aqueous layer was reduced to 10 mL. The remaining
solution was lyophilized to give a white powder (0.0332 g, 53.5%).
Synthesis of gold nanoparticles inside the polymeric spheres
[7] E. E. Meyer, K. J. Rosenberg, J. Israelachvili, Proc. Natl. Acad. Sci.
USA 2006, 103, 15739–15746.
Thermal method: The imidazolium ionic liquid (0.2 mmol, 0.0775 g) was
[8] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge,
K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B.
McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc. 1992,
114, 10834–10843.
[9] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck,
Nature 1992, 359, 710–712.
[10] P. Behrens, M. Haak, Angew. Chem. 1993, 105, 729–732; Angew.
Chem. Int. Ed. Engl. 1993, 32, 696–699.
[11] P. Behrens, Adv. Mater. 1993, 5, 127–132.
dissolved in water (40 mL) and stirred under nitrogen for 1 h. Tetra-
ACHTUNGTRENNUNGchloroACHTUNGTRENNUNGauric acid (0.04 mmol, 0.016 g) dissolved in water (10 mL) was
added to this solution, which was stirred for 30 min. Then potassium per-
sulfate (30 mg) and ascorbic acid (70 mg) were added to the above solu-
tion and the temperature was maintained at 408C for 24 h. The solution
was then centrifuged followed by lyophilization to give the corresponding
polymeric spheres as the final material (0.0362 g, 42%).
Photochemical method: The photochemical polymerization was carried
out by irradiating the samples with UV light through quartz tubes under
nitrogen. The imidazolium ionic liquid (0.16 mmol, 0.062 g) was dissolved
in water (15 mL) and stirred at room temperature for 1 h. Next, tetra-
chloroauric acid (0.016 mmol, 0.008 g) dissolved in water (5 mL) was
added to this solution, which was stirred for 30 min. Then the water-solu-
[12] T. W. Wang, H. Kaper, M. Antonietti, B. Smarsly, Langmuir 2007,
23, 1489–1495.
[13] Y. Zhou, J. H. Schattka, M. Antonietti, Nano Lett. 2004, 4, 477–481.
[14] D. E. Discher, A. Eisenberg, Science 2002, 297, 967–973.
[15] H. Noguchi, M. Takasu, Phys. Rev. E 2001, 64, 041913.
[16] Y. J. Jeon, P. K. Bharadwaj, S. W. Choi, J. W. Lee, K. Kim, Angew.
Chem. 2002, 114, 4654–4656; Angew. Chem. Int. Ed. 2002, 41, 4474–
4476.
[17] H. Kukula, H. Schlaad, M. Antonietti, S. Foerster, J. Am. Chem.
Soc. 2002, 124, 1658–1663.
[18] A. M. Carmona-Ribeiro, Chem. Soc. Rev. 1992, 21, 209–214.
[19] R. Nagarajan, E. Ruckenstein, J. Colloid Interface Sci. 1979, 71,
580–604.
[20] A. Corma, H. Garcia, Chem. Soc. Rev. 2008, 37, 2096–2126.
[21] A. Grirrane, A. Corma, H. Garcia, Science 2008, 322, 1661–1664.
[22] A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180–3211.
[23] A. S. K. Hashmi, G. J. Hutchings, Angew. Chem. 2006, 118, 8064–
8105; Angew. Chem. Int. Ed. 2006, 45, 7896–936.
[24] T. S. Koblenz, J. Wassenaar, J. N. H. Reek, Chem. Soc. Rev. 2008, 37,
247–262.
ble
2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone
(2;
0.32 mmol, 0.066 g) was dissolved in water (20 mL) and added to this so-
lution. The solution was irradiated for 12 h and the polymerization was
monitored by 1H NMR spectroscopy until the signals corresponding to
the C=C double bond had almost disappeared. Once irradiation was
complete, the solution was washed with dichloromethane (3ꢃ40 mL) and
the aqueous layer was reduced to 10 mL. The remaining solution was
lyophilized to give a powder with a color characteristic of gold nanoparti-
cles (0.035 g, 45.2%). Another two samples were prepared with molar
ratios of 1+/AuCl4 of 0.016:0.032 and 0.016:0.064, keeping the concentra-
tions of 1+BrÀ and 2 constant. The exact molar ratio of 1+BrÀ/HAuCl4/2
was 0.16:0.032:0.32 or 0.16:0.064:0.32. The catalyst employed for the oxi-
dation of 2-hydroxybenzyl alcohol was the one prepared with 0.064 mmol
of HAuCl4.
An analogous procedure was performed using an aqueous solution
(20 mL) of cetyltrimethylammonium bromide (10À3 m), HAuCl4 (10À4 m),
[25] M. Guisnet, Methods Reagents Green Chem. 2007, 231–249.
13088
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 13082 – 13089

Gold Inside Liposomes
FULL PAPER
[26] G. A. Ozin, S. Ozkar, H. O. Pastore, A. J. Poe, E. J. S. Vichi, ACS
Symp. Ser. 1992, 499, 314–332.
[35] M. L. Marin, K. L. McGilvray, J. C. Scaiano, J. Am. Chem. Soc.
2008, 130, 16572–16584.
[27] D. Batra, S. Seifert, M. A. Firestone, Macromol. Chem. Phys. 2007,
208, 1416–1427.
[36] K. L. McGilvray, M. R. Decan, D. Wang, J. C. Scaiano, J. Am.
Chem. Soc. 2006, 128, 15980–15981.
[28] Y. S. Choi, H. T. Ham, I. J. Chung, Polymer 2003, 44, 8147–8154.
[29] R. C. Mundargi, S. A. Agnihotri, S. A. Patil, T. M. Aminabhavi, J.
Appl. Polym. Sci. 2006, 101, 618–623.
[37] D. Bradley, P. Dyson, T. Welton, Chem. Rev. 2000, 100, 18–21.
[38] Ionic Liquids in Synthesis (Eds.: P. Wasserscheid, T. Welton), Wiley-
VCH, Weinheim, 2003.
[30] L. Song, X. Ge, M. Wang, Z. Zhang, S. Li, J. Polymer Sci. A: Polym.
Chem. 2006, 44, 2533–2541.
[39] J. Dupont, R. F. de Souza, P. A. Z. Suarez, Chem. Rev. 2002, 102,
3667–3691.
[31] R. Zhang, A. Seki, T. Ishizone, H. Yokoyama, Langmuir 2008, 24,
5527–5533.
[32] S. Wu, Y. Ding, X. Zhang, H. Tang, Mater. Lett. 2008, 62, 3301–
3304.
[33] C.-Y. Chang-Chien, C.-H. Hsu, T.-Y. Lee, C.-W. Liu, S.-H. Wu, H.-P.
Lin, C.-Y. Tang, C.-Y. Lin, Eur. J. Inorg. Chem. 2007, 3798–3804.
[34] J. C. Scaiano, P. Billone, C. M. Gonzalez, L. Maretti, M. L. Marin,
K. L. McGilvray, N. Yuan, Pure Appl. Chem. 2009, 81, 635–647.
[40] R. A. Sheldon, Catal. Oxid. 1995, 239–265.
[41] Catalytic Oxidation: Principles and Applications (Eds.: R. A. Shel-
don, R. A. van Santen), World Scientific, Singapore, 1995.
[42] R. A. Sheldon, I. W. C. E. Arends, G.-J. ten Brink, A. Dijksman,
Acc. Chem. Res. 2002, 35, 774–781.
Received: August 31, 2009
Published online: October 23, 2009
Chem. Eur. J. 2009, 15, 13082 – 13089
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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13089