Templating effect of carbon nanoforms on highly cross-linked imidazolium network: Catalytic activity of the resulting hybrids with Pd nanoparticles

Article abstract of DOI:10.1002/aoc.4848

Two different carbon nanoforms (CNFs), namely multi-walled carbon nanotubes (MWCNTs) and carbon nanohorns (CNHs), have been chosen as support for the direct polymerization of a bis-vinylimidazolium salt. Transmission electron microscopy analyses revealed a templating effect of the CNFs on the growth of the polymeric network, which perfectly covers their whole surfaces creating a cylindrical or spherical coating for MWCNTs and CNHs, respectively. Subsequently, the CNFs-polyimidazolium have been used as stabilizers for Pd nanoparticles (Pd NPs), and the obtained materials have been characterized by means of analytical and spectroscopic techniques and then employed as easily recoverable and recyclable catalysts for Suzuki and Heck reactions. Quantitative conversions have been obtained in almost all the explored reactions, even employing low loading of catalyst (down to 0.007 mol%). Suzuki reactions were carried out in pure water under aerobic conditions. Both materials showed excellent activity and recyclability for the investigated C-C coupling reactions, with the CNHs-based material resulting slightly more active than the MWCNTs-based one due to a higher superficial exposure of Pd NPs.

Full text of DOI:10.1002/aoc.4848

Received: 5 October 2018
DOI: 10.1002/aoc.4848
Revised: 10 January 2019
Accepted: 13 January 2019
F U L L P A P E R
Templating effect of carbon nanoforms on highly cross‐
linked imidazolium network: Catalytic activity of the
resulting hybrids with Pd nanoparticles
Vincenzo Campisciano1
| Carla Calabrese1,2
| Leonarda Francesca Liotta3
|
Valeria La Parola3
| Alberto Spinella4
| Carmela Aprile2
|
Michelangelo Gruttadauria1
| Francesco Giacalone1
1 Department of Biological, Chemical and
Pharmaceutical Sciences and
Two different carbon nanoforms (CNFs), namely multi‐walled carbon nano-
tubes (MWCNTs) and carbon nanohorns (CNHs), have been chosen as support
for the direct polymerization of a bis‐vinylimidazolium salt. Transmission elec-
tron microscopy analyses revealed a templating effect of the CNFs on the
growth of the polymeric network, which perfectly covers their whole surfaces
creating a cylindrical or spherical coating for MWCNTs and CNHs, respec-
tively. Subsequently, the CNFs‐polyimidazolium have been used as stabilizers
for Pd nanoparticles (Pd NPs), and the obtained materials have been character-
ized by means of analytical and spectroscopic techniques and then employed as
easily recoverable and recyclable catalysts for Suzuki and Heck reactions.
Quantitative conversions have been obtained in almost all the explored reac-
tions, even employing low loading of catalyst (down to 0.007 mol%). Suzuki
reactions were carried out in pure water under aerobic conditions. Both mate-
rials showed excellent activity and recyclability for the investigated C‐C cou-
pling reactions, with the CNHs‐based material resulting slightly more active
than the MWCNTs‐based one due to a higher superficial exposure of Pd NPs.
Technologies (STEBICEF), Università
degli Studi di Palermo, V.le delle Scienze
Ed. 17, 90128 Palermo, Italy
2 Laboratory of Applied Material
Chemistry (CMA), University of Namur,
61 rue de Bruxelles, 5000 Namur, Belgium
3 Istituto per lo Studio dei Materiali
Nanostrutturati ISMN‐CNR, Via Ugo La
Malfa 153, 90146 Palermo, Italy
4 Centro Grandi Apparecchiature‐ATeN
Center, Università degli Studi di Palermo,
Via F. Marini 14, 90128 Palermo, Italy
Correspondence
Vincenzo Campisciano, Michelangelo
Gruttadauria and Francesco Giacalone,
Department of Biological, Chemical and
Pharmaceutical Sciences and
Technologies (STEBICEF), Università
degli Studi di Palermo, V.le delle Scienze
Ed. 17, 90128, Palermo, Italy.
KEYWORDS
Email: vincenzo.campisciano@unipa.it;
michelangelo.gruttadauria@unipa.it;
francesco.giacalone@unipa.it
C‐C coupling, Heck reaction, heterogeneous catalysis, nanotubes, Suzuki–Miyaura reaction
Funding information
ATeN Center ‐ University of Palermo;
University of Palermo
1 | INTRODUCTION
fullerenes, carbon nanotubes (CNTs) and graphene are
surely the most popular and studied members. They have
Among the large number of supports employed in the
field of heterogeneous catalysis, carbon nanoforms
(CNFs) have played a key role due to their peculiar and
advantageous features, such as high surface area and
thermal stability, exceptional mechanical resistance and
unique electronic properties. Within the CNF family,
been exploited in a plethora of applications, such as
energy conversion and storage,[1] electronics and
optoelectronics,[1e,2] sensing,[3] biomedicine,[4] and also
the production of high‐performance materials such as
films and fibers.[5] Conversely to the case of the above‐
mentioned CNFs, the applications of another class of
Appl Organometal Chem. 2019;e4848.
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© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4848

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CAMPISCIANO ET AL.
carbon allotropes, namely carbon nanohorns (CNHs),
were less explored. CNHs are constituted by graphene
tubes closed with a horn‐shaped tip. By virtue of their
structure, CNHs are very intriguing materials that share
some features and chemistry resembling both that of ful-
lerenes, due to the presence of the closed tips, and that of
single‐walled CNTs, thanks to their elongated shape.
Among the possible applications of CNFs, catalysis is
one of the most interesting emerging fields.[6] In this con-
text, transition metals catalyzed C‐C coupling reactions
play a central role both in academia and for synthesis of
fine chemicals and pharmaceuticals. Palladium is proba-
bly the most versatile and used transition metal for cata-
lyzed C‐C couplings, such as Suzuki, Heck and
Sonogashira reactions.[7] The use of Pd nanoparticles
(NPs) as heterogeneous catalysts of C‐C coupling reac-
tions has attracted increasing attention due to their
intrinsically advantageous nature that combines high
reactivity with very large surface area thanks to the high
surface to volume ratio. Because the catalytic activity
strongly depends on particles size and their distribution
on support material,[8] an optimal interaction of Pd NPs
with the support is a key priority. In light of the weak
interaction of metal NPs with pristine CNFs, big efforts
in functionalizing them have been made with the aim
to obtain a better dispersion of catalytically active species
on support surface and simultaneously promote the
debundling of aggregates typical of CNTs and CNHs in
their pristine form. Therefore, chemical modification of
CNFs is an essential requirement to obtain catalytic mate-
rials with high activity and tailored for reducing the possi-
ble leaching of metal. Focusing attention on the covalent
functionalization of CNTs and CNHs, the oxidative pre‐
treatment and the subsequent post‐modification of such
materials is surely the most employed method that pro-
vides access to a wide range of functionalities covalently
grafted onto their surface. However, oxidative pre‐
treatment is not the only way to modify CNFs and other
techniques to functionalize covalently pristine CNTs and
CNHs were developed, which can involve addition of aryl
diazonium salts, microwave‐assisted functionalization,
coating with polymers, among others.[9] In order to obtain
highly active heterogeneous catalysts in which CNTs and
CNHs can act as support materials of Pd NPs, the choice
of a suitable stabilizer of metal NPs to avoid their aggrega-
tion during the preparation, deposition onto support or
catalytic cycle is fundamental.[10]
possible to find a good number of examples about the
use of CNTs‐Pd hybrid systems in which CNTs were
opportunely modified with different stabilizers of metal
nanoparticles (MNPs) such as biguanide or imino‐
pyridine derivatives,[11] dendrimers,[12] Pd (II) com-
plexes[13] and many others,[14] on the other hand, CNHs
were less used in this context.[15]
In continuation of our ongoing research on the use of
imidazolium‐based catalysts and on the functionalization
of different CNFs for catalytic purposes,[16] multi‐walled
carbon nanotubes (MWCNTs) and CNHs were chosen
as support materials for Pd NPs. Among MNPs stabilizers,
ionic liquids (ILs) play a central role. In particular,
imidazolium‐based ILs constitute an excellent medium
in which MNPs are stabilized by means of both electro-
static and steric effects.[17] The union of CNFs and ILs
constitutes a successful marriage that results in a syner-
gistic combination of the properties of both members to
give hybrid systems with huge potential and
applications.[14c,16a,d,18]
Herein, the radical polymerization of
a
bis‐
vinylimidazolium salt in the presence of pristine
MWCNTs and CNHs is reported.
The CNF structures acted as templates for the growth
of the highly cross‐linked imidazolium network polymer
despite the random nature of the polymerization process.
The obtained materials, which displayed
a good
dispersibility in water, were used as supports for the
immobilization of Pd NPs, and were tested as heteroge-
neous and recyclable catalysts for Suzuki reaction in
aqueous medium and Heck coupling. The prepared
hybrid materials were characterized by means of different
techniques, including thermogravimetric analysis (TGA),
solid‐state NMR spectroscopy, transmission electron
microscopy (TEM), X‐ray photoelectron spectroscopy
(XPS) and microwave plasma‐atomic emission spectrom-
etry (MP‐AES).
2 | EXPERIMENTAL
Chemicals and solvents were purchased from commercial
suppliers and used without further purification. For thin‐
layer chromatography, silica gel plates (Merck 60 F254)
were used, and compounds were visualized by irradiation
with UV light and/or by treatment with a KMnO4 solu-
1
tion. H spectra were recorded with a Bruker 300 MHz
Furthermore, the robustness of both CNTs and CNHs
could ensure the recoverability and reusability of the cat-
alyst. The interest in obtaining highly performing hetero-
geneous catalysts is witnessed by the many examples
concerning the use of CNFs‐Pd hybrid systems used for
C–C coupling reactions. However, if on one hand it is
spectrometer. The 13C CPMAS NMR spectra were
acquired by means of a spectrometer Bruker Avance II
1
400 operating at 400.15 and 100.63 MHz for H and 13C
nuclides, respectively, and equipped with a 4‐mm (H‐X)
double‐channel CPMAS probe. All spectra were acquired
with a MAS speed of 8 kHz, 1024 scans, a contact time of

CAMPISCIANO ET AL.
3 of 12
2 ms, a delay time of 3 s and an excitation pulse of 4.7 μs
on the H nucleus. The optimization of the Hartmann–
was stirred at 90°C. After 21 hr a white precipitate was
formed and toluene was removed by simple decantation.
Afterwards, methanol was added and gently warmed to
solubilize the solid. The volume was reduced by rotary
evaporator obtaining a yellowish viscous oil that after
addition of diethyl ether and sonication gave rise to a white
precipitate. Diethyl ether was then removed and the white
solid was further washed twice with diethyl ether and dried
under vacuum at 40°C. Bis‐vinylimidazolium salt 1 was
obtained as a white solid (4.516 g; 90%).
1
Hahn condition has been obtained using an adamantane
standard sample. This compound was also used as the
external chemical shift reference. All samples were placed
in 4‐mm zirconia rotors filled with Kel‐F caps. TGA was
performed under nitrogen flow from 100 to 1000°C with
a heating rate of 10°C/min in a Mettler Toledo
TGA/DSC STAR System. TEM micrographs were
recorded on a high‐resolution transmission electron
microscope (HRTEM) JEOL JEM‐2100 operating at 200
kV accelerating voltage. Samples were dispersed in water
and drop cast onto carbon‐coated copper TEM grids for
HRTEM analysis. The imaging conditions were carefully
tuned by lowering the accelerating voltage of the micro-
scope and reducing the beam current density to a mini-
mum in order to minimize the electron beam‐induced
damage of the sample. MP‐AES analyses were carried
out with a 4200 MP‐AES, Agilent. Analyses were con-
ducted using a calibration curve, obtained by dilution.
XPS analyses were performed with a VGMicrotech ESCA
3000Multilab, equipped with a dual Mg/Al anode. The
spectra were excited with the unmonochromatized Al
Ka source (1486.6 eV) run at 14 kV and 15 mA. The ana-
lyzer was operated in the constant analyzer energy mode.
For the individual peak energy regions, a pass energy of
20 eV set across the hemispheres was used. Survey spec-
tra were measured at 50 eV pass energy. The sample pow-
ders were mounted on a double‐sided adhesive tape. The
pressure in the analysis chamber was in the range of
10−8 Torr during data collection. The constant charging
of the samples was removed by referencing all the ener-
gies to the C1s set at 284.6 eV. The invariance of the peak
shapes and widths at the beginning and end of the anal-
yses ensured absence of differential charging. Analyses
of the peaks were carried out with the software provided
by VG, based on the non‐linear least‐squares fitting pro-
gram using a weighted sum of Lorentzian and Gaussian
component curves after background subtraction accord-
ing to Shirley and Sherwood.[19] Atomic concentrations
were calculated from peak intensity using the sensitivity
factors provided with the software. The binding energy
values are quoted with a precision of 0.15 eV and the
atomic percentage with a precision of 10%.
2.2 | Synthesis of MWCNT‐imi 2 and
CNH‐imi 4
In a two‐neck round‐bottom flask, a suspension of
MWCNTs or CNHs (40 mg) and bis‐vinylimidazolium
salt 1 (674 mg, 1.67 mmol) in absolute ethanol (15 ml)
was sonicated for 20 min. Afterwards, AIBN (28 mg,
0.17 mmol) was added and the mixture was degassed by
bubbling argon for 20 min. The mixture was then heated
at 78°C while stirring under argon. After 20 hr the mix-
ture was filtered, and the solid washed with methanol
and diethyl ether. After drying under vacuum at 40°C,
MWCNT‐imi 2 was obtained as a black/gray solid (617
mg) and CNH‐imi 4 as a black solid (546 mg).
2.3 | Synthesis of MWCNT‐imi‐Pd 3 and
CNH‐imi‐Pd 5
PdCl2 (37.57 mg, 0.21 mmol) and NaCl (246.68 mg, 4.20
mmol) were transferred in a round‐bottom flask, and
water (15 ml) was added. The mixture was heated at
80°C for 30 min until the formation of a brown solution.
The obtained solution was allowed to reach room temper-
ature, and was added dropwise to a stirring suspension of
MWCNT‐imi 2 or CNH‐imi 4 (200 mg) in water (12.5 ml).
The resulting mixture was stirred at room temperature
for 22 hr. The suspension was then filtered, and the solid
was washed with water, methanol and diethyl ether.
Once dried, the resulting solid was suspended in absolute
ethanol (15 ml) and sonicated for 15 min. After the soni-
cation, the suspension was stirred, and a freshly prepared
suspension of NaBH4 (56.74 mg, 1.47 mmol) in absolute
ethanol (10 ml) was added dropwise under stirring to pre-
vious suspension. The resulting mixture was stirred at
room temperature for 6 hr. The suspension was then fil-
tered, and the resulting solid was washed with water,
methanol and acetone. After drying under vacuum at
40°C, MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5 were
obtained as black solids (183 and 184 mg, respectively).
2.1 | Synthesis of bis‐vinylimidazolium
salt 1
Bis‐vinylimidazolium salt 1 was prepared following a
reported procedure with minor changes.[20] A solution
of 1,4‐dibromobutane (1.50 ml, 12.43 mmol) and 1‐
vinylimidazole (2.45 ml, 26.53 mmol) in toluene (11 ml)

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CAMPISCIANO ET AL.
short pad of silica using a mixture of hexane and ethyl
acetate as eluent. The conversions were estimated by
1H‐NMR spectroscopy.
2.4 | General procedure for the Suzuki
reaction
Aryl bromide (0.5 mmol), boronic acid or phenylboronic
acid pinacol ester (0.55 mmol), diisopropylamine (1
mmol), catalyst 3 (0.70 mg–0.1 mol%) or 5 (0.52 mg–
0.07 mol%) and water (1.2 ml) were transferred in a glass
vial with a screw cap and heated at 100°C under stirring
for the required time. Then, the reaction mixture was
allowed to cool down to room temperature, diluted with
water and extracted three times with dichloromethane.
The combined organic layers were dried with anhydrous
Na2SO4 and the solvent was evaporated under vacuum.
The residue was filtered by a short pad of silica using a
mixture of hexane and ethyl acetate as eluent. The con-
2.7 | General recycling procedure in the
Heck reaction
4‐Iodoanisole (1 mmol), methyl acrylate (1.5 mmol),
triethylamine (2 mmol), catalyst 3 (1.40 mg–0.1 mol%) or
5 (1.05 mg–0.07 mol%) and DMF (2 ml) were transferred
in a glass vial with a screw cap and heated at 120°C under
stirring. After 3 hr the reaction mixture was allowed to
cool down to room temperature, diluted with diethyl
ether and centrifuged. Organic layer was recovered and
catalyst was further washed with diethyl ether, and then
four times with methanol, water, methanol again and
finally with diethyl ether. Once dry, the catalyst was used
in the next cycle. The recovered organic layers were com-
bined and evaporated under reduced pressure, diluted
with water and extracted three times with diethyl ether.
The combined organic layers were then dried with anhy-
drous Na2SO4 and the solvent was evaporated under vac-
uum. The residue was purified by column
chromatography using a mixture of hexane and ethyl ace-
tate as eluent.
1
versions were estimated by H‐NMR spectroscopy.
2.5 | General recycling procedure in the
Suzuki reaction
4‐Bromoacetophenone (1 mmol), phenylboronic acid
(1.1 mmol), diisopropylamine (2 mmol), catalyst 3 (1.40
mg–0.1 mol%) or 5 (1.05 mg–0.07 mol%) and water (2.4
ml) were transferred in a glass vial with a screw cap
and heated at 100°C under stirring. After 3 hr the reac-
tion mixture was allowed to cool down to room tempera-
ture, diluted with ethyl acetate and centrifuged. Organic
layer was recovered and catalyst was further washed
twice with ethyl acetate, then with a mixture of
methanol/ethyl acetate 2:1 and finally with diethyl ether.
Once dry, the catalyst was used in the next cycle. The
recovered organic layers were combined and evaporated
under reduced pressure, diluted with water and extracted
three times with dichloromethane. The combined organic
layers were then dried with anhydrous Na2SO4 and the
solvent was evaporated under vacuum. The residue was
purified by column chromatography using a mixture of
hexane and ethyl acetate as eluent.
3 | RESULTS AND DISCUSSION
Hybrid materials 3 and 5 were easily prepared through
the straightforward synthetic strategy depicted in
Scheme
1.
Radical
polymerization
of
bis‐
vinylimidazolium salt 1[20] onto the MWCNTs and CNHs
surface was started by thermal decomposition of radical
initiator 2,2′‐azobis(2‐methylpropionitrile) (AIBN) in eth-
anol, at reflux. The obtained polyimidazolium salt‐coated
materials, namely MWCNT‐imi 2 and CNH‐imi 4, were
dispersed in water in the presence of Na2PdCl4 to proceed
with ion‐exchange of bromides with tetrachloropalladate
ions, followed by reduction with NaBH4 in ethanol lead-
ing to the final materials MWCNT‐imi‐Pd 3 and CNH‐
imi‐Pd 5, respectively (Scheme 1).
2.6 | General procedure for the Heck
reaction
It is worth noting that the above approach represents a
versatile strategy to accede to a number of heterogeneous
catalysts, given that different MNP precursors can be
adopted and stabilized or even polyoxometalate species
can be immobilized through ionic‐exchange. The
polyimidazolium salt loadings of materials MWCNT‐imi
2 and CNH‐imi 4 were estimated by means of TGA under
N2 by considering the weight losses at 700°C correspond-
ing to 92.5 and 88.6 wt%, respectively (Figure 1).
Aryl
iodide
(0.5 mmol),
alkene
(0.75 mmol),
triethylamine (1 mmol), catalyst 3 (0.70 mg–0.1 mol%) or
5 (0.52 mg–0.07 mol%) and dimethylformamide (DMF;
1 ml) were transferred in a glass vial with a screw cap
and heated at 120°C under stirring for the required time.
Then, the reaction mixture was allowed to cool down to
room temperature, diluted with water and extracted three
times with diethyl ether. The combined organic layers
were dried with anhydrous Na2SO4 and the solvent was
evaporated under vacuum. The residue was filtered by a
Thermogravimetric analysis profiles of pristine
MWCNTs and CNHs showed very low weight losses at

CAMPISCIANO ET AL.
5 of 12
SCHEME 1 Synthetic procedure for the preparation of materials MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5
FIGURE 1 Thermogravimetric analysis (TGA) profiles under N2
of pristine (dashed lines) and functionalized (solid lines) multi‐
walled carbon nanotubes (MWCNTs) and carbon nanohorns
(CNHs)
the same temperature corresponding to 2.8 and 3.3 wt%,
respectively. Therefore, the calculated polyimidazolium
salt loading of materials MWCNTs‐imi 2 and CNH‐imi 4
corresponded to 2.28 and 2.13 mmol/g, respectively.
13C cross‐polarization magic angle spinning NMR (13C
CPMAS NMR) spectra of all materials MWCNT‐imi 2,
MWCNT‐imi‐Pd 3, CNH‐imi 4 and CNH‐imi‐Pd 5 were
recorded (Figure 2).
FIGURE 2 13C cross‐polarization magic angle spinning (CPMAS)
NMR spectra of materials multi‐walled carbon nanotube
(MWCNT)‐imi 2, MWCNT‐imi‐Pd 3, carbon nanohorn (CNH)‐imi 4
and CNH‐imi‐Pd 5
13C CPMAS NMR spectra of materials 2 and 4 con-
firmed the good outcoming of the polymerization step
as evidenced by the absence of signals that can be
ascribed to residual vinyl groups at about 110 ppm

6 of 12
CAMPISCIANO ET AL.
(Figure 2, red lines). On the other hand, the signals at low
fields (120–140 ppm) attributed to the carbon atoms of
the imidazolium rings and those of aliphatic carbon
atoms resonating in the 20–60 ppm region are clearly vis-
ible. NMR spectra of the final hybrids decorated with Pd
NPs, namely MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5,
showed no relevant difference with the spectra of their
precursors (compare red lines with black lines in
Figure 2).
In Figure 3, HRTEM micrographs of pristine
MWCNTs, MWCNT‐imi 2 and MWCNT‐imi‐Pd 3 and
CNHs, CNH‐imi 4 and CNH‐imi‐Pd 5 are displayed. Pris-
tine MWCNTs are aggregated forming ropes and bundles
(Figure 3a). However, polymerization of bis‐
vinylimidazolium salt leads to a better dispersion of the
CNTs, which are almost all uniformly wrapped by
polyimidazolium salt network (Poly‐imi) that creates a
perfect cylindrical coating with a mean outer diameter
of ~100 nm (Figures 3b and S1a,b). Similar results were
obtained by Li, Chen et al. that functionalized MWCNTs
with a methylimidazolium salt derivative by free‐radical
polymerization.[18l] It is noteworthy to point out that
polymerization of bis‐vinylimidazolium salts onto differ-
ent supports, such as silica, could result in a physical mix-
(Figures 3f and S2a,b) showed also that functionalization
of CNHs with bis‐vinylimidazolium salt 1 led to the for-
mation of a homogeneous spherical‐like coating of Poly‐
imi uniformly distributed onto the surface of CNHs. On
the other hand, only at high magnifications it was possi-
ble to observe the presence of very small Pd NPs uni-
formly distributed onto the 3D Poly‐imi network of
MWCNT‐imi‐Pd 3, whose dimensions could not be deter-
mined (Figures 3d and S1d). Conversely, on CNH‐imi‐Pd
5, Pd NPs are clearly visible (Figures 3g,h and S2c–e),
showing a bimodal distribution with a mean particle size
of 28.8 12.0 nm (n = 66) and 2.6 0.6 nm (n = 282;
Figure S2f) for the big and small NPs, respectively. It is
interesting to note that in this case the NPs are mainly
located on the outer part of the hybrid material being
thus more available for the catalysis.
Palladium loading of materials MWCNT‐imi‐Pd 3 and
CNH‐imi‐Pd 5 was determined by means of MP‐AES, and
was found to be 7.6 and 7.1 wt%, respectively.
Furthermore, materials MWCNT‐imi‐Pd 3 and CNH‐
imi‐Pd 5 were subjected to XPS analysis (Figure 4) to esti-
mate the degree of reduction of palladium after the treat-
ment with NaBH4. The results, summarized in Table 1,
showed how the degree of reduction was quite similar for
both materials, with the content of Pd(0) being equal to
61% and 63% for MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5,
respectively.[22] Moreover, it is worth noting that although
the palladium loading of material 3 is slightly higher than
that of 5 (7.6 vs. 7.1 wt%), the Pd/C atomic ratio deter-
mined by XPS is the same, revealing a higher surface expo-
sure of Pd NPs in the latter hybrid (Table 1). This finding is
in agreement with HRTEM analysis.
ture
composed
of
supported
Poly‐imi
and
homopolymerized imidazolium salt, as previously
reported.[21] Herein, no traces of isolated domains of
homopolymer were found. This finding could be
explained considering that CNTs can act as a sort of
templating agent that leads the polymerization of the
bis‐vinylimidazolium salt monomer to take place around
the nanotubes skeleton. TEM images of CNH‐imi 4
FIGURE 3 Representative high‐resolution transmission electron microscopy (HRTEM) images of: (a) pristine multi‐walled carbon
nanotube (MWCNT); (b) MWCNT‐imi 2; (c,d) MWCNT‐imi‐Pd 3; (e) pristine carbon nanohorns (CNHs); (f) CNH‐imi 4; (g,h) CNH‐imi‐
Pd 5

CAMPISCIANO ET AL.
7 of 12
TABLE 2 Screening of reaction conditions for the Suzuki–
Miyaura reactiona
T
t
Conv.
Entry Catalyst R
Base (°C) (hr) (%)b Solvent
1
2
3
3
3
3
CHO K2CO3 100
CHO K2CO3 50
CHO K2CO3 50
6
6
3
70
42
99
H2O
EtOH
H2O/
EtOH
(1:1)
4
5
6
3
3
3
CHO K3PO4 100
CHO DIPA 100
OCH3 K2CO3 50
6
2
5
75
99
36
H2O
H2O
H2O/
EtOH
(1:1)
7
3
5
5
5
OCH3 DIPA 100
OCH3 DIPA 100
OCH3 DIPA 100
CHO DIPA 100
1
1
1
2
87
99
93
99
H2O
8
H2O
H2O
H2O
9c
10c
aReaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (0.55
mmol), base (2 equiv.), catalyst (0.1 mol%), solvent (1.2 ml).
bConversion determined by 1H‐NMR.
FIGURE 4 High‐resolution X‐ray photoelectron spectroscopy
(XPS) of the Pd3d region of materials multi‐walled carbon
nanotube (MWCNT)‐imi‐Pd 3 and carbon nanohorn (CNH)‐imi‐Pd
5
cCNH‐imi‐Pd 5 used at 0.07 mol%.DIPA, diisopropylamine.
were quite similar to those collected with K2CO3 (com-
TABLE 1 Pd3d5/2 binding energies (eV), relative percentages (%)
and Pd/C atomic ratios of MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5
pare entries
1
and 4). However, the use of
diisopropylamine (DIPA) in water at 100°C gave rise to
full conversion in only 2 hr (entry 5). Then, the optimized
reaction conditions were employed in the Suzuki reaction
using the less reactive 4‐bromoanisole. Data reported in
Table 2 prove that the best conditions arise from the com-
bination of DIPA as base and water as solvent (compare
entries 6 and 7). On the grounds of this finding, CNH‐
Imi‐Pd 5 was tested at 0.1 mol% using DIPA as base and
water as solvent, showing a higher catalytic activity than
MWCNT‐imi‐Pd 3 (entries 7 and 8). Therefore, in the
light of the obtained results, the catalytic loading of
CNH‐imi‐Pd 5 was further decreased to 0.07 mol%.
Despite the lower catalytic loading, catalyst 5 allowed to
obtain high conversions into the corresponding biphenyls
when both 4‐bromoanisole and 4‐bromobenzaldehyde
were used (entries 9 and 10).
MWCNT‐imi‐Pd 3
CNH‐imi‐Pd 5
Pd(0)
Pd (II)
Pd/C
335.3 (61)
337.6 (39)
0.016
334.7 (63)
337.3 (37)
0.016
CNH, carbon nanohorn; MWCNT, multi‐walled carbon nanotube.
Once the materials were fully characterized, MWCNT‐
imi‐Pd 3 and CNH‐imi‐Pd 5 were initially tested in the
catalysis of the Suzuki reaction. Firstly, MWCNT‐imi‐Pd
3 was tested in water, at 100°C, in the presence of
K2CO3 as base, affording 70% of conversion into the
biphenyl‐4‐carboxaldehyde in 6 hr (Table 2, entry 1).
After the same reaction time, a lower conversion (42%)
was recorded by using ethanol as solvent, at 50°C (entry
2). On the other hand, when the reaction was run in 3
hr in a mixture of H2O/EtOH (1:1 v/v) at 50°C, the con-
version into the desired product was complete (entry 3).
No relevant improvements were observed when K3PO4
was used as base in water at 100°C. The obtained results
The previously optimized reaction conditions were
selected to check the catalytic versatility of the solids 3
and 5 in the Suzuki couplings between a broad spectrum
of substrates, including several aryl bromides, one aryl
chloride, boronic acids and one pinacol ester (Table 3).
In this context, MWCNT‐Imi‐Pd 3 and CNH‐Imi‐Pd 5

8 of 12
CAMPISCIANO ET AL.
TABLE 3 Suzuki–Miyaura reactions catalyzed by materials
MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5a
allows it to reach a higher turnover number (TON,
defined as moles of aryl halide converted/moles of palla-
dium) value even when conversion into the desired prod-
uct was lower than when the MWCNTs‐based catalyst
was used (entry 9; TONs values: 470 and 514). The some-
what different catalytic activity may be rationalized as a
result of different contributions: although MWCNT‐imi‐
Pd 3 has both a higher Pd content and lower Pd NPs
dimensions, the palladium seems to be uniformly distrib-
uted in the 3D polymeric network, being the inner layers
less available to the catalysis. On the contrary, CNH‐imi‐
Pd 5 has lower Pd content and higher NPs size, but the Pd
NPs are mainly exposed on the surface and easily accessi-
ble to the reactants during the catalysis.
A further experiment was performed by using a
reduced catalytic loading. The reaction between 4‐
bromoacetophenone and phenylboronic acid was chosen
for this purpose. In particular, catalyst MWCNT‐imi‐Pd
3 was tested at 0.01 mol%, whereas CNH‐Imi‐Pd 5 was
employed with a palladium loading of 0.007 mol%
(Table 3, entry 11). This reaction confirmed the different
activity of the two catalysts. After 16 hr, MWCNT‐imi‐
Pd 3 gave 95% of conversion whereas, in the same time,
CNH‐imi‐Pd 5 allowed full conversion into the corre-
sponding product. Taking into account the different cata-
lytic loading, it is noteworthy to mention the reached
TONs values, namely 9500 and 14 286 for catalysts 3
and 5, respectively, which highlight the different catalytic
activities of the two catalysts. Furthermore, an aryl chlo-
ride, 1‐choro‐4‐nitrobenzene, was tested in the Suzuki
reaction in the presence of catalyst 5 (Table 3, entry 12).
It worked with a conversion of 39%, which is remarkable
as 1‐chloro‐4‐nitrobenzene has a significantly lower reac-
tivity than the aryl bromides. Still the result is not opti-
mized yet. To obtain higher conversion, the amount of
catalyst could be increased or harsher reaction conditions
could be applied.
MWCNT‐ CNH‐imi‐
imi‐Pd 3 Pd 5
t
Conv.
Conv.
(%)b
Entry R1
R2
Product (hr) (%)b
1
4‐Ac
H
H
H
H
H
6c
3
99
99
99
95
78
69
99
81
47
99
95
99
99
99
99
93
84
99
80
36
99
99
39
2
3‐Ac
6d
6e
3
3
4‐CN
4‐NO2
3‐Me
5
4
6f
4
5
6 g
6 h
3
6
4‐OMe 2‐Me
5
7
4‐CHO 4‐OMe 6i
3
c
8
4‐Ac
–
6c
6
9
4‐OMe 4‐CHO 6j
4‐CHO 4‐CHO 6 k
5
10
11d
12e
24
16
24
4‐Ac
H
H
6c
6f
4‐NO2
aReaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (0.55
mmol), DIPA (1 mmol), catalyst (0.07–0.1 mol%), H2O (1.2 ml).
bConversion determined by 1H‐NMR.
cPhenylboronic acid pinacol ester was used.
dReaction conditions: 4‐bromoacetophenone (3 mmol), phenylboronic acid
(3.3 mmol), DIPA (6 mmol), catalyst (MWCNT‐imi‐Pd
3 0.01 mol% or
CNH‐imi‐Pd 5 0.007 mol%), H2O (3.6 ml).
e1‐Chloro‐4‐nitro‐benzene was used.
CNH, carbon nanohorn; MWCNT, multi‐walled carbon nanotube.
were tested using a Pd loading of 0.1 and 0.07 mol%,
respectively (Table 3).
Conversions in the corresponding biphenyls were from
good to excellent in almost all the cases, with both cata-
lysts being CNH‐imi‐Pd 5 slightly more active than
MWCNT‐imi‐Pd 3. This difference in activity is more pro-
nounced when 3‐bromotoluene was reacted with
phenylboronic acid or the electron‐rich 4‐bromoanisole
was coupled with 2‐tolylboronic acid (Table 3, entries 5
and 6).
In Table 4, a comparison between the more active CNH‐
imi‐Pd 5 and other catalytic systems reported elsewhere in
the Suzuki reaction between 4‐bromoanisole and
phenylboronic acid is shown, although a strictly direct
comparison is not possible as the majority of the reactions
were not carried out in pure water as in our case. Examples
concerning both Pd (II) complexes and Pd NPs supported
onto single‐walled carbon nanotubes (SWCNTs),
MWCNTs and oxidized CNHs were taken into account.
Table 4 shows that CNH‐imi‐Pd 5 exhibits higher or com-
parable activity with the other examined examples. Fur-
thermore, it was possible to compare the catalytic activity
of the herein presented system CNH‐imi‐Pd 5 with com-
mercial Pd/C catalyst used in the same reaction conditions
(water, DIPA, 100°C). In the case of Pd/C, a 93% yield into
the corresponding biphenyl derivative was obtained in just
Conversely, it is noteworthy to note that using
phenylboronic
acid
pinacol
ester
or
4‐
formylphenylboronic acid in combination with 4‐
bromoacetophenone and 4‐bromoanisole, respectively
(entries 8 and 9), the catalytic performances of both mate-
rials resulted to be quite similar. However, CNH‐imi‐Pd 5
still showed a slightly higher activity than MWCNT‐imi‐
Pd 3 because of the lower catalytic loading employed that

CAMPISCIANO ET AL.
9 of 12
TABLE 4 Comparison of catalytic activity between CNH‐imi‐Pd 5 and other reported catalytic systems in the Suzuki–Miyaura reaction
between 4‐bromoanisole and phenylboronic acid
Entry Catalyst
Pd loading (mol%) Conditions
Time
Yield Ref.
[24]
1
SWCNT‐DETA/Pd (II)
1
K2CO3, H2O/EtOH (1:1), 60°C
1.3 hr
1 hr
96%
2
SWCNT‐Metformine/Pd (II)
1
K2CO3, H2O/EtOH (1:1), 50°C
K2CO3, H2O/DMF (1:1), 60°C
K2CO3, H2O/EtOH (1:1), r.t.
Cs2CO3, EtOH, 80°C
90%
99%
80%
91%
96%
99%
99%
92%
92%
91%
[11a]
3
MWCNT‐Schiff base/Pd (II)
MWCNT‐(S)‐methyl histidinate/Pd (II) 0.09a
0.1
3 hr
[13a]
[25]
4
1 hr
[26]
[27]
[28]
[29]
[30]
[31]
5
SWCNT@Fe3O4@SiO2/Pd
MWCNT/Pd
1.5
0.1
0.3
0.1
0.5
0.2
0.05
1.5
0.07
15 hr
1 hr
6
K2CO3, TBAB, H2O, 120°C
Na2CO3, EtOH, 90°C
7
MWCNT/Pd
8 hr
8
MWCNT/Pd/PdO
Graphene/MWCNTs/Pd
MWCNT‐Schiff base/Pd
ox‐SWCNH/Pd
DIPA, H2O, 90°C
4 hr
9
K2CO3, H2O/EtOH (1:1), 60°C
K2CO3, H2O/EtOH (1:1), r.t.
1.5 hr
2.5 hr
10
11
12
13
Na2CO3, NMP/H2O (2:1), 120°C 2 hr
[15a]
[23]
Pd/C
DIPA, H2O, 100°C
DIPA, H2O, 100°C
20 min 93%
1 hr 93%
CNH‐imi‐Pd
This work
CNH, carbon nanohorn; DIPA, diisopropylamine; DMF, dimethylformamide; r.t., room temperature; SWCNT, single‐walled carbon nanotube; MWCNT, multi‐
walled carbon nanotube.
aAccordingly to the Pd loading determined by ICP the actual catalytic loading is 0.09 mol%, not 0.0022 mol% as reported in the paper.
20 min, but an over 20‐fold higher Pd loading than that
used in the case of CNH‐imi‐Pd 5 was employed (compare
entries 12 and 13).
The recyclability of MWCNT‐imi‐Pd 3 and CNH‐imi‐
Pd 5, at 0.1 and 0.07 mol%, respectively, was investigated
in the Suzuki reaction between 4‐bromoacetophenone
and phenylboronic acid. Both catalysts retained their
activity over five cycles affording the desired product in
quantitative yield.
Furthermore, MWCNT‐imi‐Pd 3 and CNH‐imi‐Pd 5
were tested in the catalysis of the Heck reaction with
the same loading used for the Suzuki reaction (0.1 and
0.07 mol%, respectively). Both electron‐rich and
electron‐poor aryl iodides were reacted with different
alkenes, such as styrene, 4‐chlorostyrene and methyl
acrylate using DMF as solvent and triethylamine (TEA)
as base at 120°C. The conversions into the corresponding
products were from good to excellent (Table 5). Once
again, taking into account its lower catalytic loading,
CNH‐imi‐Pd 5 showed a higher activity than MWCNT‐
imi‐Pd 3, especially when styrene and 4‐chlorostyrene
Regioselectivity towards trans alkene isomer in the
Heck reaction between methyl acrylate and several aryl
iodides was excellent, and no traces of by‐products were
detected (entries 11–19). However, when styrene and 4‐
chlorostyrene were employed, the regioselectivity was
lower with the formation of the corresponding gem‐
alkenes in variable amounts between 2 and 15 mol%
(entries 1–10).
A further experiment with a 10‐fold lower catalytic
loading was carried out using MWCNT‐imi‐Pd 3 and
CNH‐imi‐Pd 5 (0.01 and 0.007 mol%, respectively) in the
reaction between 4‐iodoanisole and methyl acrylate
(entry 20). In the adopted conditions, these low loadings
allowed a quantitative conversion into the desired prod-
uct after 5 hr.
Furthermore, the same reaction between 4‐iodoanisole
and methyl acrylate was chosen to assess the recyclability
of MWCNT‐imi‐Pd 3 and CNH‐ imi‐Pd 5 with Pd loading
of 0.1 and 0.07 mol%, respectively. Both catalysts gave rise
to complete conversion into the corresponding product
over the six cycles investigated. At the end of each cata-
lytic run, the solids were simply recovered by centrifuga-
tion and successfully reused retaining their catalytic
activity. No catalytic activity was observed in the recov-
ered solution.
were
used
(entries
1–10).
Moreover,
4‐
bromobenzaldehyde was coupled with styrene affording
good results (entry 10), and with methyl acrylate
affording excellent results (entry 19).

10 of 12
CAMPISCIANO ET AL.

CAMPISCIANO ET AL.
11 of 12
ACKNOWLEDGEMENTS
4 | CONCLUSION
In conclusion, we have applied
Financial support from the University of Palermo is grate-
fully acknowledged. The authors are also very grateful to
Dr Giorgio Nasillo for his valuable work with TEM mea-
surements provided by ATeN Center ‐ University of
Palermo.
a
straightforward
approach based on the direct radical polymerization of
a bis‐vinylimidazolium dibromide salt in the presence
of different pristine CNFs, namely MWCNTs and CNHs,
which allowed obtaining two CNFs‐polyimidazolium
hybrids. Interestingly, in both cases, the CNF structures
acted as templates for the growth of the highly cross‐
linked imidazolium network polymer despite the ran-
dom nature of the polymerization process. The hybrids
were hence employed as new supports and stabilizers
for Pd NPs, and the resulting materials were character-
ized by means of TGA, 13C CPMAS NMR, MP‐AES,
XPS and HRTEM. This latter technique revealed two
different situations regarding both the dimensions and
the distribution of Pd NPs on the materials MWCNT‐
imi‐Pd 3 and CNH‐imi‐Pd 5. If on the one hand, only
at high magnifications, it was possible to observe the
presence of very small Pd NPs uniformly distributed
onto material 3, on the other hand, material 5 is charac-
terized by the presence of bigger Pd NPs mainly allo-
cated on the outer part of the hybrid material. Because
of the small size of Pd NPs dispersed onto material 3,
it was not possible to estimate their mean
dimensions. On the contrary, material 5 showed a
bimodal distribution with a mean particle size of 28.8
12.0 nm and 2.6 0.6 nm. Considering the similar
imidazolium loading on materials 3 and 5, such a differ-
ence could be ascribed to the different morphology and
symmetry of the two CNFs. The two catalysts, MWCNT‐
imi‐Pd 3 and CNH‐imi‐Pd 5, were then extensively used
with a large set of substrates in the Suzuki reactions in
pure water under air atmosphere and in the Heck reac-
tions in 0.1 and 0.07 mol% loading, respectively, giving
C‐C coupled products in high conversions. In the case
of Suzuki reactions in pure water, results were
comparable or even better than those obtained with
commercial Pd/C[23] or other supported Pd‐based
catalysts.[11a,13a,15a,24–32] Both catalysts were recovered
after centrifugation and reused in up to five cycles with-
out loss in catalytic activity. Moreover, the catalytic
loading was also scaled down to 0.01 and 0.007 mol%
ORCID
Vincenzo Campisciano
https://orcid.org/0000-0002-6990-
8290
Carla Calabrese https://orcid.org/0000-0001-8348-0764
Leonarda Francesca Liotta
https://orcid.org/0000-0001-
5442-2469
Valeria La Parola https://orcid.org/0000-0001-7695-6031
Alberto Spinella https://orcid.org/0000-0001-9822-8537
Carmela Aprile https://orcid.org/0000-0002-3193-3239
Michelangelo Gruttadauria
6952-0712
https://orcid.org/0000-0002-
Francesco Giacalone
4568
https://orcid.org/0000-0001-7696-
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How to cite this article: Campisciano V,
Calabrese C, Liotta LF, et al. Templating effect of
carbon nanoforms on highly cross‐linked
imidazolium network: Catalytic activity of the
resulting hybrids with Pd nanoparticles. Appl
Organometal Chem. 2019;e4848. https://doi.org/
10.1002/aoc.4848