Fullerene-ionic-liquid conjugates: A new class of hybrid materials with unprecedented properties

Article abstract of DOI:10.1002/chem.201406067

A modular approach has been followed for the synthesis of a series of fullerene-ionic-liquid (IL) hybrids in which the number of IL moieties (two or twelve), anion, and cation have been varied. The combination of C60 and IL give rise to new unique propert

Full text of DOI:10.1002/chem.201406067

DOI: 10.1002/chem.201406067
Full Paper
&
Graphitic Nanostructures
Fullerene–Ionic-Liquid Conjugates: A New Class of Hybrid
Materials with Unprecedented Properties
[a]
[b]
[b]
[a]
Vincenzo Campisciano, Valeria La Parola, Leonarda F. Liotta, Francesco Giacalone,*
[a]
and Michelangelo Gruttadauria*
Abstract: A modular approach has been followed for the
synthesis of a series of fullerene–ionic-liquid (IL) hybrids in
which the number of IL moieties (two or twelve), anion, and
cation have been varied. The combination of C60 and IL give
rise to new unique properties in the conjugates such as sol-
onions and nanocages with few-layer graphene sidewalls,
which have been characterized by means of thermogravi-
metric analysis (TGA), X-ray photoelectron spectroscopy
(XPS), X-ray diffraction (XRD), scanning electron microscopy/
energy-dispersive X-ray analysis (SEM-EDAX), and high-reso-
lution transmission electron microscopy (HRTEM). Finally, the
material thus obtained was successfully applied as catalyst
in Suzuki and Mizoroki–Heck reactions in a concentration of
just 0.2 mol%. In the former process it was recyclable for
five runs with no loss in activity.
À1
ubility in water, which was higher than 800 mgmL in sev-
eral cases. In addition, one of the C –IL hybrids has been
60
employed for the immobilization of palladium nanoparticles
through ion exchange followed by reduction with sodium
borohydride. Surprisingly, during the reduction several
carbon nanostructures were formed that comprised nano-
Introduction
forms (CNFs)—namely, single- and multiwalled nanotubes,
nanohorns, and graphene—has been covalently modified with
[11]
In recent decades a great deal of interest has been devoted to
ionic-liquid moieties have been reported. Despite the large
number of examples for CNF–IL hybrids, just one report deals
with the synthesis of a series of fulleropyrrolidine–imidazolium
[
1]
the study and application of ionic liquids (ILs). The huge
potential of these neoteric solvents relies on the fact that
their physical and chemical properties can be finely tuned by
the selection of anions and cations. In such a way, ILs can be
also tailored to exhibit a specific function, thereby leading to
[12]
salts and the study of their solubility profiles. It is likely that
fullerene–IL hybrids will constitute a new class of materials
that could find applications in several fields. In this regard, full-
erene–IL mixtures have been successfully employed as glucose
[
2]
task-specific ionic liquids (TSILs). Nowadays ILs find promising
[
1a,3]
[13]
applications in several fields such as synthesis and catalysis,
sensors or in the preparation of stationary phases for gas
[
4]
[5]
[6]
[14]
biotechnology, separation science, energy storage, electro-
chromatography.
Analogously, zwitterionic multicharged
[
7]
[15]
chemistry, and sensing, among others. Recently, ILs have
begun to find applications in combination with carbon nano-
fullerene derivatives have been used for gene delivery or in
[
16]
medicinal chemistry. However, multilayered cross-linked sup-
ported ionic-liquid phases (mlc-SILP) proved to be excellent
supports for the immobilization of palladium as catalyst in CÀC
[8]
forms not only as reaction media for their functionalization,
[
9]
but also for preparing so-called bucky gels. In the latter, the
specific interactions between ILs and nanocarbons afford to
the hybrids unique properties as well as improved dispersibility
in various media that can be employed in electrochemical and
[
17]
bond formation.
With this in mind, herein we report the synthesis and
characterization of a series of fullerene–ionic-liquid hybrids in
which the number of IL moieties, anions, and cations have
been varied. Finally, one of these hybrids has been used for
the immobilization of palladium nanoparticles and used as a
catalyst for CÀC coupling reactions.
[
10]
energy-storage devices or as supports for catalysis. In addi-
tion, several examples in which the surface of carbon nano-
[
a] V. Campisciano, Dr. F. Giacalone, Prof. M. Gruttadauria
Department of Biological, Chemical and
Pharmaceutical Sciences and Technologies
University of Palermo, Viale delle Scienze s/n
Ed. 17, 90128 Palermo (Italy)
Results and Discussion
[
18]
E-mail: francesco.giacalone@unipa.it
Firstly, malonates 1a–c
were prepared by starting from
michelangelo.gruttadauria@unipa.it
malonyl chloride and the properly substituted propanol.
Hence, the fullerene derivatives endowed with two moieties of
IL 3a and 3b were obtained through Bingel cyclopropan-
[
b] Dr. V. L. Parola, Dr. L. F. Liotta
Istituto per lo Studio dei Materiali Nanostrutturati
ISMN-CNR Via Ugo La Malfa 153, 90146 Palermo (Italy)
[19]
ation
reaction followed by nucleophilic substitution with
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406067.
1-butylimidazole in quantitative yields (Scheme 1). Next, the
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Scheme 1. Synthesis of C60–IL 3a and 3b (DBU=1,8-diazabicyclo[5.4.0]undec-7-ene).
Scheme 2. Synthesis of C60–IL conjugates 5a–d.
syntheses of C –IL conjugates that bear twelve ionic-liquid
yield over two steps (Scheme 3). All the fullerene–IL hybrids
were thoroughly characterized and their spectroscopic data
are in agreement with the proposed structures. Mono-adduct
60
moieties arranged in an octahedral addition pattern were car-
[
20]
ried out by following the protocol of Sun et al. In this way it
has been possible to access highly symmetric hexakis-adducts
[18b]
13
precursors 2a and 2b
sp signals typical of C2v fullerene derivatives along with the
showed in the C NMR spectra 15
2
[24]
4
a–c with satisfactory yields (Scheme 2). Then the octahedral
intermediate 4b was in turn treated with 1-methylimidazole,
,2-dimethylimidazole, and 1-butylimidazole, and 4a with 1-
3
signals at d=71 and 52 ppm that belong to the two sp -
1
carbon atoms of C60 and the methanofullerene bridgehead, re-
spectively (see Figure S1 in the Supporting Information). The
NMR spectra of hybrid derivatives 3a and 3b were more com-
plicated, probably owing to strong intermolecular interactions
(Figures S2 and S3 in the Supporting Information). However,
butylimidazole in chloroform heated to reflux to afford the
hexakis C –IL hybrids 5a–d in quantitative yields. Recently, the
60
synthesis of several fullerene hexakis-adducts through the
copper(I)-catalyzed alkyne–azide cycloaddition (CuAAC) “click”
[
21]
1
13
reaction has been reported that employed a C60 dodecakis–
the hexakis-adduct has simpler H and C NMR spectra (see
Figures S4 and S5 in the Supporting Information). In the
proton spectra of 4a and 4b the multiplicity of the three
methylenic groups is clearly visible even if the signals are
slightly broadened, whereas in the carbon spectra the octahe-
[
15c,22]
[18a,23]
alkyne
or the dodecakis–azido derivative 4c.
Hence,
a different strategy to obtain access to C –IL conjugate with
60
a different cation has been designed.
In fact, by taking inspiration from the seminal work of Nier-
engarten and co-workers, derivative 4c could be employed to
build, through the CuAAC approach, a known dodecakis–tria-
2
dral substitution pattern of the C60 gives rise to two sp signals
3
at d=146 and 141 ppm along with an sp signal at d=
[
18a]
1
zole hybrid.
This could then be easily transformed into the
69 ppm. Interestingly, in the H NMR spectra of 1-substituted
corresponding triazolium salt 6 with benzyl bromide in 80%
imidazolium C –ILs 5a, 5c, and 5d, the proton at the C2
60
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13
whereas in its C NMR spectra (see Figure S10 in the Support-
ing Information) the peaks of the benzyl moiety appear in the
d=127–132 ppm region along with those of the parent
[18a]
triazole.
Unfortunately, owing to their highly charged nature, we
were unable to get good high-resolution (HR) ESI spectra of
the imidazolium-based fullerene–IL hybrids. However, the HR-
ESI spectrum of 6 clearly showed the presence of multicharged
species with 3, 5, and 6 positive charges (see Figure S11 in the
Supporting Information).
The presence of the IL moieties on the fullerene cage affords
to these hybrids an excellent solubility in polar solvents such
as methanol and water. Although several examples of water-
[25]
[26]
soluble fullerene mono-adducts and polymers have been
described so far, only a few water-soluble octahedral hexakis-
[27]
adducts have been reported. This is particularly important
given that these easily accessible multicharged molecules
[15a,b,16c,28]
might find applications in gene delivery.
The UV/Vis spectra of 5a–d recorded in water are reported
in Figure 2 (those of all the C –IL systems in methanol are
60
Scheme 3. Synthesis of C60–IL conjugates 6.
position of the imidazolium ring disappears after exchange
with the deuterated solvent (compare Figure 1 and Figure S8
1
with Figure S9 in the Supporting Information). The H NMR
spectrum of 5c shows the presence of all the expected signals
with their corresponding integrals, thus indicating the good
outcome of the reaction (Figure 1).
Figure 2. UV/Vis spectra of C –IL conjugates 5a–d in water. The inset shows
a picture of 100 mgmL water solutions of 5c (left) and 5d (right).
6
0
À1
1
The H NMR spectrum of the triazolium adduct 6 presents
all the signals including ten aromatic protons in the
d=7.0–8.0 ppm region plus the triazole proton at d=9.6 ppm,
reported in Figure S12 of the Supporting Information). The
spectra show no aggregation in water with three transitions at
about 240, 275, and 335 nm. Most interestingly, 5a–d display
an outstanding solubility in water at room temperature and
À1
À1
pH 7 higher than 800 mgmL (0.19–0.21 molL ) to form solu-
tions that were stable for months; this is probably the highest
value of solubility for fullerene derivatives reported so
[25,26,29]
far.
The same concentration could be achieved by
dissolving hybrid 6 in methanol.
With the hybrid C –IL systems in hand, we next explored
60
their possible practical application as supports for catalysts. In
this regard, we chose to immobilize palladium nanoparticles to
prepare a new catalyst for CÀC coupling reactions by using
our systems as a kind of supported ionic-liquid-like phase
[30]
(
SILLP) on the fullerene sphere. The synthesis of the catalytic
material was accomplished in two steps, as reported in
Scheme 4. First, we immobilized tetrachloropalladate ions
(
10 wt%) onto C –IL 5a through anion metathesis, hence the
60
1
2À
Figure 1. H NMR spectrum of 5c.
supported PdCl4 species was reduced with NaBH in etha-
4
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Figure 3. TGA of 5a and 8 under oxygen flow.
Scheme 4. Synthesis of catalyst 8.
[
31]
nol. The purification of material 8 from inorganic salts was
achieved by means of dialysis using a membrane with a cutoff
of 1000 Da to afford a finely dispersed insoluble black powder.
Catalyst 8 was characterized by means of several analytical
and spectroscopic techniques. Thermogravimetric analysis
(
TGA) was performed under oxygen flow from 100 to 10008C
À1
with a heating rate of 108Cmin to determine the total
amount of Pd species present in the sample and the degrad-
ation temperature range (Figure 3).
Figure 4. HRTEM pictures of 7.
As expected, the TGA trace of 5a shows a continuous
weight loss in the 140–5908C range, the temperature at which
the combustion was completed and no residue was detected.
Conversely, TGA of 8 presents a weight loss with the same
slope as 5a up to approximately 5008C, after which no addi-
tional loss was observed up to 8408C. Hence, a new small
weight loss of 2.7% can be accounted for by the quantitative
transformation of PdO to Pd since the former species is not
ganized and amorphous nano-objects in which ionic-liquid-like
speckled domains are discernable (Figure 4 and Figure S13 in
[33]
the Supporting Information).
Small-sized palladium-rich
aggregates are clearly visible and uniformly distributed with
a mean diameter of 1.4Æ0.5 nm (see Figure S13 in the Sup-
porting Information). It is likely that the repulsive action of the
positive charge of imidazolium groups prevents their agglom-
[
32]
[34]
stable over 8008C. By this last weight loss the Pd content in
sample 8 was calculated to result in a loading of 18% as
metallic Pd.
eration into larger domains. Nevertheless, TEM analysis of 7
also displayed the presence of a few larger Pd aggregates
(Figure S14 in the Supporting Information). However, the TEM
analysis of 8 was very surprising owing to the presence of
a number of organized carbon nanostructures with large
graphitic domains (see Figure S15 in the Supporting
Information) as well as to several carbon nano-onions with
diameters in the 20–30 nm range (Figure 5 and Figure S15 in
the Supporting Information).
The morphologic characterization of hybrids 7 and 8 was
carried out by means of high-resolution transmission electron
microscopy (HRTEM). The analysis of TEM images helps to shed
light on the organization of the hybrids, and in particular it
can provide useful information about the presence and dimen-
sion of palladium nanoparticles. Interestingly, 7 shows both or-
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broad peak at approximately 408 2q owing to metallic Pd (PDF
no. 180870) is observed. Metallic Pd particles smaller than
2
nm were found by applying the Scherrer equation that eval-
uates the full width at half-maximum (FWHM) of the reflection
line at approximately 408 2q. Such a result is in good agree-
ment with the particle size estimated by TEM analysis. The
broad peak at approximately 208 2q is attributed to a poor or-
[45]
dering of graphene sheets. In fact, reduced graphite oxide
or reduced graphene oxide might exhibit a peak in the range
between 20 and 258 owing to restacking. In that case, no
graphitic stacking is
achieved owing to lattice mismatch, structural defects, or the
[46]
presence of residual functional groups. X-ray photoelectron
spectroscopy (XPS) analysis was used to analyze the outer part
of the materials. Figure 6 shows the survey spectra of samples
Figure 5. HRTEM pictures of 8.
In all cases, the interplane distance estimated after noise
reduction and calculating the intensity profile was 0.345 nm.
This value, slightly higher than that of d002 value of graphite
(
0.335 nm), clearly indicates
a local structure similar to
[
35]
turbostratic graphite (d002 =0.344 nm).
In accordance with the XRD data (see below), the palladium
nanoparticles are highly dispersed throughout the sample and
large aggregates were not detected by TEM. Nevertheless, the
presence of these kinds of carbon nanoforms in the sample
raise the question about how these nanostructures were
formed. Per the data, carbon nano-onions can be prepared
[36]
through different methods such as electron irradiation, ion
[
37]
implantation,
plasma-enhanced chemical vapor deposi-
[
38]
[39]
Figure 6. XPS survey spectra of 7 and 8.
tion,
nanodiamonds,
sition-metal catalyst,
high-temperature annealing
or plasma spraying of
[
40]
chemical vapor deposition using a tran-
[
41]
[42]
counter-flow diffusion flames,
and
7 and 8. The comparison of the two spectra proved that dialy-
sis purification of the final product was successful in eliminat-
ing the Na, Br, and Cl present in precursor 7, as confirmed by
semiquantitative scanning electron microscopy/energy-
dispersive X-ray analysis (SEM-EDAX) analysis (Figure S17 in the
Supporting Information). The final product 8 shows, along with
carbon, the presence of O, N, and Pd.
[
43]
underwater arc discharge, the latter being the preferred one
to produce bulk amounts. However, all the above techniques
usually require high doses of energy, whereas in our case the
II
0
reduction of Pd to Pd , which seems to be the step in which
the nanocarbons are formed, is carried out at room tempera-
ture. To shed light on this process, we carried out a blank ex-
periment by treating C –IL 5a with NaBH in ethanol, but this
The results obtained by the analysis of the region of C1s,
O1s N1s, and Pd3d are summarized in Table 1. The C1s peak
shows for both samples three components at 284.4, 286.3, and
60
4
reaction only resulted in the hydrolysis of the hybrid, thus
giving rise to the formation of 3-(3-hydroxypropyl)-1-methyli-
1
2
3
midazolium bromide (see the H NMR spectra in Figure S16 of
288.3 eV owing to sp graphitic carbon, to the sp CÀC bond
[47]
the Supporting Information), in accord with other reported
and CÀO, and to the C=O bond, respectively. The main dif-
ference in the carbon region between samples of 7 and 8 is
[19b,44]
Bingel adducts in the presence of hydride ions/alcohol.
This latter result justifies the higher Pd content (18 wt%) with
respect to the anticipated one (10 wt%). Unfortunately, SEM
analyses carried out on 8 (Figure S17 in the Supporting Infor-
mation) did not help to add conclusive data. On the basis of
these findings, an explanation for the formation of
that, after reducing treatment in NaBH , the increase in the
4
graphitic component with respect to the others occurs, as evi-
denced by Figure 7a and Table 1. Oxygen and nitrogen regions
show binding energy values of 532 and 401 eV, respectively
the experimentally observed carbon nanostructures
cannot be proposed, and more in-depth studies,
which are out of the scope of the present work, will
be carried out in due course to completely reveal the
mechanism of formation of such nanocarbons.
Table 1. XPS binding energies [eV] and atomic ratios of elements constituting
samples 7 and 8. The relative percentages are given in parentheses.
C1s
Pd3d5/2
Pd/C
O/C
N/C
7
8
284.6 (67), 286.3 (30), 288.3 (3)
284.6 (85), 286.2 (13), 288.5 (2)
337.1
335.6 (35), 337.3 (65)
0.018
0.016
0.13
0.14
0.07
0.04
The X-ray diffraction (XRD) data of 8 is shown in
Figure S18 in the Supporting Information. A very
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[
a]
Table 2. Suzuki–Miyaura reactions catalyzed by 8.
Entry Substrate
1
Product
Conv. [%] Yield [%]
>99
>99
88
99
99
87
99
2
3
4
>99
Figure 7. High-resolution XPS of a) C1s and b) Pd3d of 7 and 8.
5
6
87
68
(
see Figure 6), typical of CÀO and aromatic nitrogenated spe-
cies. However, the Pd3d spectra, shown in Figure 7b, are char-
acterized by the typical two spin–orbit components, Pd3d5/2
>99
–
99
–
and Pd3d , separated by approximately 5.4 eV. The spectrum
[b]
[c]
[c]
3/2
7
8
II
relative to 7 is typical of Pd with a binding energy centered at
37.1 eV. The spectrum of 8 exhibits two doublets attributed
3
[d]
>99
99
0
II
[48]
to two different chemical species: Pd and Pd , respectively.
The relative amount of the two components indicated
[a] Reaction conditions: Phenylboronic acid (1.1 mmol), aryl halide
(
(
1 mmol),
K
2
CO
3
2
(1.2 mmol), EtOH (1.2 mL), H O (1.2 mL), catalyst
a degree of reduction of 35% after treatment with NaBH . It is
4
0.2 mol%, 1 mg), reaction time 3 h. [b] Reaction time: 19 h. [c] No reac-
worth noting that XPS is a surface-sensitive technique; this
means that the surface signal weight is much more than the
bulk, and this phenomenon is particularly important in the
case of very small nanoparticles like the ones that have been
found in this sample. For that reason, the real degree of
reduction could probably be higher than 35%.
tion. [d] Reaction conditions: Phenylboronic acid (1.1 mmol), aryl halide
1 mmol), CO (1.2 mmol), EtOH (0.6 mL), (0.6 mL), catalyst
(0.02 mol%).
(
K
2
3
2
H O
quantitatively with a turnover number (TON) of 5000 (Table 2,
entry 8). All the above results make this catalyst a good candi-
Material 8, which contained highly dispersed Pd nano-
particles, thus constitutes a good candidate for a catalyst in
Pd-mediated cross-coupling reactions given that metal nano-
particles are known to be more active than their particulate
[17b]
date for further tests under continuous-flow conditions.
These promising results prompted us to explore the catalytic
activity of material 8 in another Pd-promoted CÀC coupling
process such as the Mizoroki–Heck reaction (Table 3). Reaction
between a set of four aryliodides and methyl acrylate in DMF/
water at 908C in the presence of triethylamine (TEA) as base
gave the corresponding alkenes in high yields and excellent
selectivity, with the expected trans isomer as the only product,
once again by employing just 0.2 mol% of catalyst 8.
[
49]
metal counterparts in catalytic reactions.
Moreover, since
metal nanoparticles are thermodynamically unstable, the syn-
ergistic effect of coordination, steric, and electrostatic interac-
tions can be exploited in the present approach, thereby pre-
[
34b]
venting nanoparticle coalescence and percolation.
In this
regard, 8 was used as catalyst in the Suzuki reaction between
phenylboronic acid and a set of aryl bromides in ethanol/water
at 508C in the presence of K CO as base. All the reactions
2
3
Conclusion
were carried out for three hours using the catalyst in
a 0.2 mol% loading (Table 2). Conversions were often quantita-
tive with isolate yields that ranged from 68 to 99%. The cata-
lyst was shown to be inactive toward aryl chlorides, given that
Two different strategies have been reported that allow easy
access to several C –IL conjugates in which the anion, cation,
60
and side chain have been varied. For this class of new hybrid
materials, the combination of C60 and IL gives rise to outstand-
ing solubility profiles in classically non-C -friendly solvents
4-chlorobenzaldehyde gave no reaction under the common
reaction conditions (Table 2, entry 7).
60
In addition, the recyclability of the catalyst was checked for
the reaction between 4-bromobenzaldehyde and phenyl-
boronic acid (0.2 mol%, 3 h). After easy recovery of 8 by cen-
trifugation, it was used for five consecutive runs to give rise
to complete conversion and yield, thus showing no loss in
catalytic activity. Moreover, by lowering the catalyst loading
to just 0.02 mol%, the biphenyl-4-carboxaldehyde was formed
such as water and methanol in which imidazolium-based hexa-
À1
kis-adducts show solubilities higher than 800 mgmL . This un-
precedented solubility can be exploited in several key fields
such as medicinal chemistry for gene delivery, photodynamic
cancer therapy, and DNA photocleavage, but also in materials
chemistry, catalysis, analytical, and coordination chemistry,
among others. With this in mind, one of the C –IL hybrids has
60
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Financial support from the University of Palermo and COST
Action CM0905 ORCA are gratefully acknowledged. Some NMR
spectroscopic experimental data, mass spectra, and TEM
studies were provided by the Centro Grandi Apparecchiature -
UniNetLab - Universitꢁ di Palermo funded by P.O.R. Sicilia
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Published online on && &&, 0000
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Full Paper
FULL PAPER
Fullerenes go hybrid: Novel C –ionic-
liquid hybrids show unprecedented
60
&
Graphitic Nanostructures
V. Campisciano, V. L. Parola, L. F. Liotta,
F. Giacalone,* M. Gruttadauria*
properties such as an outstanding solu-
À1
bility in water higher than 800 mgmL .
These hybrids can be used for the prep-
aration of highly active and recyclable
catalysts for CÀC coupling reactions
upon immobilization of Pd nano-
particles.
&
& – &&
Fullerene–Ionic-Liquid Conjugates: A
New Class of Hybrid Materials with
Unprecedented Properties
Chem. Eur. J. 2015, 21, 1 – 9
www.chemeurj.org
9
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ