SBA-15/POSS-Imidazolium Hybrid as Catalytic Nanoreactor: the role of the Support in the Stabilization of Palladium Species for C?C Cross Coupling Reactions.

Article abstract of DOI:10.1002/adsc.201900350

Supported imidazolium modified polyhedral oligomeric silsesquioxanes (POSS) on SBA-15 have been used as platform for Pd(II) species. The so-obtained material was firstly characterized by means of TGA, solid state NMR, TEM, XPS, SAXS, porosimetry and ICP-OES and it was successfully tested as pre-catalyst in C?C cross couplings, namely Suzuki-Miyaura and Heck reactions. In both cases, the solid proved to be highly efficient and easily recoverable from the reaction mixture. The recyclability was verified for up to seven cycles without showing any activity decrease. Interestingly, only Pd(II) was detected in the reused catalyst in the Heck reaction. Therefore, the versatility of the material was investigated by using various aryl halides. Our palladium nanocomposite was able to promote both Suzuki and Heck reactions down to 0.0007 mol% showing outstanding turnover frequency (TOF) values of 114,286 and 32,381 h^?1, respectively. Comparison with the palladium catalyst on SiO₂-POSS-imidazolium support showed interesting differences in terms of stabilization of Pd species and recyclability. The excellent outcome of the reactions could be ascribed to the textural properties of the SBA-15 support and the presence of the imidazolium-POSS nanocage within the pores of SBA-15, that worked as a sort of nanoreactor. (Figure presented.).

Full text of DOI:10.1002/adsc.201900350

Title: SBA-15/POSS-Imidazolium hybrids as Catalytic Nanoreactor: the
role of the support in the stabilization of Palladium species for C
−C Cross Coupling Reactions
Authors: Carla Calabrese, Vincenzo Campisciano, Fabiana Siragusa,
Leonarda Liotta, Carmela Aprile, Michelangelo Gruttadauria,
and Francesco Giacalone
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900350
Link to VoR: http://dx.doi.org/10.1002/adsc.201900350

Advanced Synthesis & Catalysis
10.1002/adsc.201900350
VERY IMPORTANT PUBLICATION - VIP FULL PAPER
DOI: 10.1002/adsc.201900350
SBA-15/POSS-Imidazolium hybrids as Catalytic Nanoreactor:
the role of the support in the stabilization of Palladium species
for C−C Cross Coupling Reactions.
a,b
a
a
c
Carla Calabrese, Vincenzo Campisciano, Fabiana Siragusa, Leonarda F. Liotta,
b
a
a
Carmela Aprile, * Michelangelo Gruttadauria, * Francesco Giacalone *
a
Department of Biological, Chemical and Pharmaceutical Sciences and Technologies, University of Palermo Viale delle
Scienze, Ed. 17 90128, Palermo (Italy)
email: michelangelo.gruttadauria@unipa.it or francesco.giacalone@unipa.it
Laboratory of Applied Materials Chemistry (CMA), University of Namur, 61 rue de Bruxelles, 5000 Namur (Belgium)
email: carmela.aprile@unamur.be
b
c
Istituto per lo Studio dei Materiali Nanostrutturati ISMN-CNR, Via Ugo La Malfa 153, 90146 Palermo (Italy)
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. Supported imidazolium modified polyhedral Our palladium nanocomposite was able to promote both
oligomeric silsesquioxanes (POSS) on SBA-15 have been Suzuki and Heck reactions down to 0.0007 mol% showing
used as platform for Pd(II) species. Such material was firstly outstanding turnover frequency (TOF) values of 114,286
-
1
characterized by means of TGA, solid state NMR, TEM, and 32,381 h , respectively. Comparison with the
XPS, SAXS, porosimetry and ICP-OES and it was palladium catalyst on SiO -POSS-imidazolium support
2
successfully tested as pre-catalyst in C−C cross couplings, showed interesting differences in terms of stabilization of
namely Suzuki-Miyaura and Heck reactions. In both cases, Pd species and recyclability. The excellent outcome of the
the solid proved to be highly efficient and easily recoverable reactions could be ascribed to the textural properties of the
from the reaction mixture. The recyclability was verified for SBA-15 support and the presence of the imidazolium-
up to seven cycles without showing any activity decrease. POSS nanocage within the pores of SBA-15, that worked
Interestingly only Pd(II) was detected in the reused catalyst as a sort of nanoreactor.
in the Heck reaction. Therefore, the versatility of the
material was investigated by using various aryl halides.
Keywords: C−C coupling; Heck reaction; heterogeneous
catalysis; palladium; Suzuki-Miyaura reaction
catalyst by comparison to the corresponding
homogeneous reaction conditions. On this last point, it
is worth to look at the strict regulations on the
maximum allowable amount of metal contaminants,
for example, for pharmaceutical compounds (< 5
Introduction
The increasing demand of highly active and recyclable
catalytic materials for C−C cross coupling reactions
pushed the scientific research toward the design of
performing heterogeneous hybrids. The importance of
C−C bond formation process is well known in organic
synthesis including the preparation of pharmaceuticals, zeolites,
[3]
ppm). Palladium catalysts have been immobilized
onto a plethora of pre-functionalized solid supports
[
4]
[5]
[8]
such as mesostructured and amorphous silica,
[6]
[7]
organic polymers,
magnetic nanocomposites, carbon nanoforms,
ionic liquids,
[9]
[10]
drugs, agrochemicals, natural products, and the design
of organic semiconductors. In particular, palladium
catalyzed C−C cross coupling reactions emerged as
one of the most versatile tools as testified not only by
the awarding of the Nobel Prize in Chemistry in 2010
to R. F. Heck, E.-C. Negishi and A. Suzuki, but also
[11]
[12]
halloysite nanotubes,
supported imidazolium salts,
emerged as promising platform for both Pd
and MOFs . Moreover,
[13] [14]
or imidazole,
[15]
[8, 16]
nanoparticles (NPs)
and Pd(II) species.
In
doing so, silica stands out from other solid supports
because of its useful features, such as high surface area,
good thermal and mechanical stability, widespread
availability, and easy covalent functionalization
strategies covering a broad range of organic or organo-
[1]
by the continuous growing publications on this field.
In order to improve the sustainability of such reactions,
a broad variety of palladium based heterogeneous
catalysts has been developed.[2] In this context, the
reduction of waste generation is indeed connected to
an easy recovery and reuse of the catalyst itself.
Furthermore, a lower metal contamination of the final
products is typically achieved with heterogeneous
[5a, 17]
metallic moieties.
Recently, we reported a series
of highly active catalytic materials based on palladium
NPs supported on imidazolium-C60 hybrids covalently
grafted onto several supports, namely amorphous
1
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
silica gel, mesostructured SBA-15 and silica coated
maghemite.[ The use of silica supports for the system
Pd NPs@imidazolium salt@C60, allowed to obtain
designed a novel heterogeneous hybrid following a
modular synthesis in which POSS units were
previously grafted onto the solid support in order to be
modified with imidazolium chloride moieties. The
18]
heterogeneous
nanocatalysts
with
excellent
performances for C−C cross couplings. Polyhedral
oligomeric silsesquioxanes (POSS), have been
modified with imidazolium functionalities and used as
support for Pd(II).[ Such hybrids were employed as
pre-catalysts for Suzuki-Miyaura and Heck couplings.
POSS nanostructures are a family of appealing
solid was treated with Na
2
PdCl to give the final
4
hybrid by ionic exchange. Next, our material was
successfully tested as pre-catalyst in Suzuki-Miyaura
and Heck reactions.
19]
organic-inorganic hybrid molecules composed of an Results and Discussion
inorganic siloxane nanocage surrounded by tailoring
organic peripheries.[ POSS display high thermal and
mechanical stability together with a wide tunability of
functional side groups. Such interesting properties
result in a broad range of applications involving POSS
hybrids for the design of sensors, electronic devices,
polymer nanocomposites, liquid crystals, biomaterials,
20]
According to a literature procedure,[22] octakis(3-
chloropropyl)-octasilsesquioxane (compound 1) was
reacted with triethoxy-3-(2-imidazolin-
1yl)propylsilane to give compound 2 that was directly
grafted onto mesostructured SBA-15 to afford the
solid 3. The functionalization of supported POSS
nanostructures with imidazolium chloride peripheries
(material 4) was obtained from the reaction between
material 3 and 1-methylimidazole. Such convergent
synthetic approach allows to diversify the peripheries
of the POSS nanocage. The imidazolium loading of
solid 4 was estimated by means of combustion
chemical analysis (C, H, N) and was found to be 0.81
mmol/g. This synthetic strategy allowed to obtain a
promising catalytic support endowed with a specific
[
21]
solar cells and catalytic supports.
In a recent
work,[
22]
we reported the design, synthesis and
application of a series of imidazolium functionalized
POSS nanostructures covalently immobilized onto
silica supports. Such materials were successfully used
as heterogeneous catalysts for the conversion of
carbon dioxide into cyclic carbonates. Herein, we
propose imidazolium modified POSS hybrids grafted
onto mesostructured SBA-15 as solid supports for the
immobilization of Pd(II) species. Furthermore,
inspired by the promising catalytic performance of
2
surface area of 272 m /g, total pore volume of 0.87
3
cm /g, and mean pore size distribution around 14 nm.
[19a]
POSS imidazolium tetrachloropalladate salts,
we
2
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
Scheme 1. Synthesis of Pd(II) imidazolium functionalized POSS hybrid.
Considering its high value of pore size distribution,
enabling internal diffusion paths, the hybrid 4 was
designed as a sort of nanoreactor based on
imidazolium modified POSS units as solid support for
Pd species.[ For doing so, the solid 4 was treated
distance d (100) was computed as d = 20.55 nm and
the unit cell parameter as α = 23.76 nm. Moreover, the
0
patterns of POSS modified SBA-15 3 and 4 proved
that the ordered structure of pristine SBA-15 is
23]
preserved. Indeed, the pore-to-pore distance α is the
0
with a solution of Na
2
PdCl
4
to give the final material 5
same for all the solids. By comparison with pristine
SBA-15, in the case of functionalized material it is
possible to observe a decrease in intensity of the
signals due to the presence of POSS nanostructures.
However, the structural order of the pores of the
pristine solid support was not significantly affected,
being almost constant the ratio between the relative
reflections of SBA-15, samples 3 and 4.
by anionic metathesis reaction (Scheme 1). An
analogous route has been followed in order to prepare
the corresponding material based on amorphous silica
(
see SI). The thermal behavior of the final hybrid 5 and
its precursors 3 and 4 was investigated by means of
thermogravimetric analysis (TGA, Figure 1).
Figure 1. TGA profiles of 3, 4 and 5.
Figure 2. SAXS patterns of SBA-15, 3, 4.
The decomposition of organic functionalities started in
the range 240-250 °C, being the solid 5 slightly more
stable than 3 and 4. The good thermal robustness of the
final hybrid 5 is promising for its repeated use under
heating regimes. Then, palladium loading of 5 was
quantified by using inductively coupled plasma optical
emission spectroscopy (ICP-OES) to be 1.5 wt%. X-
ray photoelectron spectroscopy (XPS) analysis was
performed to evaluate the oxidation state of supported
palladium species onto the surface of the solid 5
1
3
29
C and Si solid state cross polarization-magic angle
spinning (CP-MAS) NMR measurements were
performed on both the final material 5 and its precursor
1
3
4
(Figures S2, S3). All the solid state C CP-MAS
NMR experiments were recorded using the TOSS
total suppression side band) pulse sequence to remove
(
the spinning side bands while leaving the isotropic
signals. By comparison of 4 and 5 NMR spectra, it is
interesting to note an excellent overlapping of the
signals proving that the subsequent immobilization of
palladium species had no remarkable effect on the
chemical shift. In CP-MAS C NMR spectra the
peculiar signals of the carbon atoms of imidazolium
ring were detected in the range δ = 122-137 ppm,
whereas the aliphatic ones resonated in the range δ =
(
Figure S1).
Pd 3d region showed the presence of Pd(II) species as
testified by the two peaks centered at 337.5 and 342.9
eV corresponding to Pd 3d5/2 and Pd 3d 3/2, respectively.
Small angle X-ray scattering (SAXS) measurements
were carried out on both pristine and functionalized
SBA-15 (Figure 2). SAXS analysis revealed the
ordered mesostructure of SBA-15 showing a scattering
pattern composed of a peak centered at 0.43° 2θ and
two peaks, not clearly resolved, at 0.71° 2θ and 0.73°
1
3
1
0-52 ppm. The weak signal located at δ = 165 ppm
can be ascribed to the C2 imidazolidinium carbon
2
9
atom. In CP-MAS Si NMR spectra the presence of
4
3
Q [(SiO)
4
Si] (δ = -118 ppm) and Q [(SiO) SiOH] (δ =
3
2
θ. The diffraction patterns are in line with the typical
[24]
-110 ppm) units was observed. The signal at δ = -753
profile for hexagonal (p6mm)
mesoporous silica
ppm was assigned to the completely condensed T
SBA-15. The most intense peak was indexed as (100)
reflection, the other two features, almost merging in a
broad one, were ascribed to (110) and (200) secondary
peaks. According to the Bragg’s law, the plane
silicon units [R-Si(OSi) ] of both POSS nanocage and
3
the organosilane silicon acting as linker between the
solid support and the POSS itself, whereas the signal
2
at δ = -67 ppm was attributed to T [R- Si(OSi)
2
OR]
3
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
Table 2. Heck reactions catalyzed by 5a).
silicon atoms bridging bulky SBA-15 to POSS units.
Then, material 5 was tested as heterogeneous pre-
catalyst at 0.07 mol% Pd loading in Heck and Suzuki-
Miyaura reactions. In both cases, the catalytic
performances were extensively evaluated in terms of
selectivity, turnover number (TON, calculated as
moles of aryl halide converted/moles of Pd), turnover
Entry R
Product
Conv.
(% )
99
S
TOF
-
1
-1
(%) (h )
frequency (TOF = TON h ), and recyclability. The
1
1
2
H
91
71
70
conversion of starting aryl halides was estimated by H
NMR analysis of the reaction mixture. Heck reactions
were performed by coupling several aryl iodides with
methyl acrylate or styrene using DMF as solvent and
triethylamine as base, at 120 °C. The reactions
between aryl iodides and methyl acrylate afforded
from high to quantitative conversion in 3 h (Table 1,
Figures S4-S10). Excellent results were also obtained
in terms of regioselectivity by giving trans isomer as
the only reaction product. Interestingly, also an aryl
bromide, 4-bromobenzaldehyde, gave a very good
result (Table 1, entry 8).
4-CH
3
98
99
89
3
4-COCH
3
92
71
4
5
4-OCH
3-OCH
3
3
98
89
86
87
70
64
6
4-NO
2-C
2
>99
94
71
Table 1. Heck reactions catalyzed by 5a).
7
4
H
3
S
>99
>99
95
96
71
60
8b)
4-CHO
Entry
1
R
Product
Conv. TOF
a)
-
1
Reaction conditions: aryl iodide (0.5 mmol), styrene (0.75
(
%)
(h )
476
mmol), triethylamine (1 mmol), DMF (1 mL), catalyst (2.5
mg), 120 °C, 20 h. 4-bromobenzaldehyde, 20 h.
H
>99
b)
2
3
4
4-CH
3
>99
99
476
471
471
Further experiments were performed with a decreased
palladium loading in the coupling between 4-
iodoanisole and methyl acrylate (Table 3). Improved
4-COCH
3
4-OCH
3-OCH
3
99
-1
TOF values were achieved ranging from 476 h to
-1
3
2,381 h .
5
3
99
471
Table 3. Screening of catalyst loading in the reactions
a)
between 4-iodoanisole and methyl acrylate.
6
4-NO
2-C
2
>99
91
476
433
286
Entry
Catalyst
mol%)
Conv.
(%)
TOF
(h )
-
1
(
7
4
H
3
S
1
2
3
0.07
0.007
0.0007
>99
99
68
476
4,714
32,381
8b)
4-CHO
>99
a)
a)
For the reaction conditions see Table S1.
Reaction conditions: aryl halide (0.5 mmol), methyl
acrylate (0.75 mmol), triethylamine (1 mmol), DMF (1 mL),
b)
catalyst (2.5 mg), 120 °C, 3 h. 4-bromobenzaldehyde, 5 h.
In order to investigate the recyclability of the material,
the above mentioned coupling reaction was selected as
benchmark process (Figure 3). In doing so, the solid
was tested up to six cycles at 0.07 mol% without
showing any decrease in the catalytic activity. Then,
the catalyst recovered from the sixth cycle was
successfully employed at 0.007 mol% for a seventh
Lower levels of regioselectivity were observed in the
Heck reactions between aryl iodides and styrene
(
Table 2, Figures S11-17). Such couplings were
carried out for 20 h leading to both trans isomer and
gem-alkene (5-14%). This finding, already reported in
literature,[ was ascribed to the α-migration step of
the aryl group during the catalytic cycle when electron
rich olefins are used. An aryl bromide, 4-
bromobenzaldehyde, gave also a very good results
th
25]
cycle. Interestingly, in the 7 cycle conversion was
very close to that observed in the first cycle with the
same catalytic loading (99%, Table 3, entry 2). This
result confirms the excellent robustness of material 5.
(
Table 2, entry 8).
4
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
catalysts and their study by XPS after reuse. On
unmodified SiO
2
, most of Pd(II), from Pd(acac) , was
2
[26]
reduced to Pd(0) during the Heck reaction. On the
other hand, bis-carbene-pincer complexes of Pd(II) on
montmorillonite K-10 or a polystyrene-supported
palladacycle did not show Pd(0) in the XPS analysis
[27]
after reuse.
TEM analysis on reused SBA-15
metformin-supported Pd(II) catalyst showed the
presence of Pd(0) particles.
[28]
Figure 3. Recycling tests of 5 in the Heck reaction between
4
-iodoanisole and methyl acrylate.
Transmission
electron
microscopy
(TEM)
measurements were carried out on both fresh and
reused material (Figure 4).
a)
Figure 5. XPS spectrum of 5 after recycling tests in the
Heck reaction.
We wondered if the absence of Pd(0) after reuse could
be ascribed to the particular nature of the SBA-15
support. Then, we used as support for Pd species the
b)
corresponding POSS immobilized on amorphous SiO
2
2
that possesses a specific surface area of 250 m /g, and
3
a total pore volume of 0.60 cm /g, and pore size
[22]
distribution around 4.6 nm.
TGA of palladium-
-POSS-imidazolium support
based catalyst on SiO
2
showed a very similar thermal stability (Fig. S18).
Heck reactions were carried out between 4-iodoanisole
and methyl acrylate under the same conditions for four
cycles giving quantitative yields in the first three
cycles and a 96% yield in the fourth cycle.
Differently from the reused catalyst 5, XPS analysis of
the SiO -based reused catalyst showed the presence of
2
both Pd(0) (46%) and Pd(II) species (54%) (see Figure
S19 and S20).
Figure 4. TEM pictures of the as-synthesized (a) and the
reused catalyst 5 in the Heck reaction (b).
In order to investigate the possible leaching of
palladium into the product, microwave plasma-atomic
emission spectrometry (MP-AES) analysis was
performed on the product obtained from the reaction
between 4-iodoanisole and styrene in the presence of
0.07 mol% of 5. About 4.5% of the starting palladium
was found into the product (17 ppm). On the other
hand, when such analysis was performed on the
product obtained in the second cycle, no palladium
was detected being its amount lower than the limit of
detection (<1 ppm). In addition, no detectable amount
of palladium was found in the reaction carried out in
the presence of 0.007 mol% of 5.
TEM analysis of the as-synthesized and the reused
material revealed the morphology of the bidimensional
mesostructured functionalized SBA-15 without
showing the presence of reduced Pd nanoparticles
(
Figure 4). The reused catalyst 5 in the Heck reaction
was further characterized by using XPS analysis. As
evidenced by Figure 5, the XPS spectrum of the Pd 3d
region clearly showed the absence of reduced Pd. Only
the presence of Pd(II) was detected.
Although the literature is plenty of supported
palladium-catalyzed Heck reactions, only a few
examples concern with the use of Pd(II)-based
5
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
In the case of palladium on SiO
support, no detectable amount of palladium was found
into the product already after the first cycle.
2
-POSS-imidazolium
rise to a 64% conversion whereas the reaction with the
o-tolylboronic acid led to an 88% conversion. On the
ground of this finding, a screening of palladium
loading was carried out in the reaction between 4-
bromobenzaldehyde and phenylboronic acid (Table 5).
In particular, the catalyst amount was decreased down
to 0.007 and 0.0007 mol% affording turnover
-1
frequency of 14,143 and 114,286 h , respectively.
Moreover, a further catalytic test was run at higher
temperature, namely 120 °C in 5 minutes with a
palladium loading of 0.0007 mol%. In this case, the
aryl bromide conversion was complete and led to the
-1
remarkable TOF value of 1,721,170 h .
Table 4. Suzuki-Miyaura reactions catalyzed by 5a).
Figure 6. Graphical representation of hybrid material 5.
In the case of material 5, the partial leaching of the
catalyst from the support, after the first cycle, could be
ascribed to Pd(II) species located on the outer surface
of SBA-15 channels, not able to provide the same
stabilization as well as inside the pore. Indeed, the
presence of the imidazolium POSS nanostructure
mainly within the inorganic SBA-15 could increase the
stabilization of the Pd(II) species inside the pore
R1
R2
Conv.b)
Entry
1
TOF
(h )
-
1
(
%)
H
4-CHO
>99
1429
2
3
4
H
H
H
4-CH
3-CH
4-CN
3
3
93
1329
1414
1429
99
>99
(
Figure 6). This phenomenon is, probably, less
important when amorphous silica support is used.
Such stabilization could play a role in the sense that
catalyst 5 probably acts as a reservoir of active anionic,
dicoordinate palladium(0) species [ obtained by
5
H
4-COCH
3-COCH
3
3
>99
1429
6
7
8
H
H
H
>99
>99
95
1429
1429
1357
29]
reduction of a palladium(II) precursor especially if
4-NO
2
we consider the reaction carried out in the presence of
only 0.0007 mol%. The no detectable amount of Pd(0)
by XPS and TEM after recycling could be ascribed to
the stabilizing nature of the POSS-imidazolium
nanocages within the pores and to a fast re-oxidation
to Pd(II) species since the reactions are not carried out
under inert atmosphere.[ In addition, the absence of
Pd(0) could be explained with the no ability of the
support to re-capture the leached palladium.
4-OCH
3-OCH
4-OCH
4-OCH
3
3
3
3
9
H
75
64
88
1071
305
1
1
0c)
1c)
H
29f]
2-CH
3
419
a)
Reaction conditions: aryl halide (0.5 mmol),
phenylboronic acid (0.55 mmol), K CO (0.6 mmol),
Next, the catalytic activity of 5 was investigated in
Suzuki reaction between phenylboronic acid and a set
of aryl bromides bearing either electron-donating or
electron-withdrawing groups. The catalytic tests were
run at 50 °C in a mixture ethanol/water (1:1) using
2
3
2
EtOH/H O (1:1, 1.2 mL), catalyst (0.07 mol%), 50 °C, 1 h.
b)
Conversion estimated by 1 H NMR analysis. Reaction
c)
performed in 3 h.
K
2
CO as base (Table 4, Figures S21-S31).
3
As showed in Table 4, aryl bromides with electron-
withdrawing substituents such as 4-CHO, 4-CN, 3-
Table 5. Screening of catalyst loading in the reactions
between phenylboronic acid and 4-bromobenzaldehyde.
COCH
3
,
4-COCH
3
,
and 4-NO
2
reacted with
phenylboronic acid providing the corresponding cross-
coupling products in quantitative conversions in 1 h.
On the other hand, aryl bromides with electron-
Entry
Catalyst
(mol%)
Conv.
(%)
TOF
(h )
-
1
1
2
3
4
0.07
>99
99
80
1,429
14,143
114,286
1,721,170
donating groups such as 3-CH
3
, 4-CH
3
, 3-OCH , 4-
3
0.007
0.0007
0.0007
OCH , gave slightly lower conversions. Moreover, 4-
3
bromo anisole was employed for further catalytic tests
with two additional phenyl boronic acids (entries 10-
>99
a)
For the reaction conditions see Table S2.
11). Such reactions were performed in 3 h. The
coupling between the electron-donating 4-iodoanisole
and the less reactive 4-formylphenylboronic acid gave
6
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
Again, in order to investigate the possible leaching of
palladium into the product, MP-AES analysis was
carried out on the biphenyl product obtained from the
detected by the appearance of the signals centered at
335 (3d5/2) and 340.4 (3d3/2) eV and resulted in a
percentage of metallic Pd of 44% respect to Pd(II).
TEM measurements were carried out on the reused
material in the Suzuki reaction (Figure 9). TEM
reaction
between
4-bromobenzaldehyde
and
phenylboronic acid in the presence of 0.07 mol% of 5.
No palladium was detected being its amount less than
th
analysis of the catalyst recovered from the 6 cycle
1
ppm. Finally, the recyclability of 5 in the Suzuki
showed a fine dispersion of palladium nanoparticles.
TEM pictures were collected using bright field and
dark field diffraction contrast imaging. In both cases
the presence of Pd NPs was clearly discernable.
reaction was verified with the coupling between 4-
bromobenzaldehyde and phenylboronic acid (Figure
7). It is worth to observe that 0.07 mol% of catalyst
allowed to obtain a quantitative yield and conversion
of the selected aryl bromide within 1 h in six
For the sake of comparison, as for the Heck reaction,
recycling investigations were also carried out using the
th
consecutive runs. Next, catalyst recovered from the 6
corresponding palladium catalyst on amorphous SiO .
2
cycle was employed for two additional recycling tests
with two decreased amounts of catalyst, namely 0.007
and 0.0007 mol%. By comparison with the results
obtained with the as-synthesized solid 5 (Table 5,
entries 2 and 3), it was possible to assess that the
reused catalyst is robust enough to be employed up to
seven cycles even with a decreased palladium loading.
This excellent recyclability agrees with the absence of
palladium into the final product.
The Suzuki reaction between 4-bromobenzaldehyde
and phenylboronic acid was carried for four
consecutive runs giving decreasing yields from
quantitative, in the first cycle, to 88% in the fourth
cycle.
a)
b)
Figure 7. Recycling tests of 5 in the Suzuki reaction
between 4-bromobenzaldehyde and phenylboronic acid.
Figure 9. TEM pictures of the reused catalyst 5 in the
Suzuki reaction in bright field (a) and in dark field (b).
Conclusion
Imidazolium functionalized POSS nanostructures
were successfully grafted onto mesostructured SBA-
1
5 allowing to obtain a suitable stabilizing support for
Pd(II) pre-catalytic species. The hybrid material 5 was
fully characterized by means of several techniques and
its catalytic activity was investigated toward Suzuki
and Heck cross-coupling reactions. In both processes,
the catalytic performances were evaluated in terms of
turnover frequency, versatility and recyclability. The
versatility of 5 was investigated with several aryl
halides at 0.07 mol% of Pd providing high conversions
Figure 8. XPS spectrum of 5 after recycling tests in the
Suzuki reaction.
Moreover, XPS analysis of the Pd 3d core level on the
spent catalyst revealed the presence of both Pd(0) and
Pd(II) species (Figure 8). The in situ reduction of
Pd(II) into Pd(0) during the catalytic tests was clearly
7
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
size and a counting time of 1.3 s per step. Solid state CP-
into the corresponding products. Additional tests were
carried out by using the material with a palladium
loading decreased down to 0.0007 mol% leading to the
outstanding TOF value of 1,721,170 h in the Suzuki
reaction between 4-bromobenzaldehyde and
1
3
MAS-TOSS C-NMR spectra were recorded at room
temperature, on a Bruker Avance 500 Spectrometer
operating at 11.7 T, using a contact time of 2 ms, a spinning
-1
29
rate of 5 KHz and a Bruker probe of 4mm. CP-MAS Si-
NMR spectra were recorded at room temperature, on a
Bruker Avance 500 Spectrometer operating at 11.7 T, using
a contact time of 2 ms, a spinning rate of 8 KHz and a Bruker
probe of 4mm. Transmission electron microscopy (TEM)
pictures were recorded on a Philips TECNAI 10 microscope
at 80 kV. Microwave Plasma Atomic Emission
Spectroscopy (MP-AES) analyses were performed with a
phenylboronic acid. Moreover, the solid proved to be
highly recyclable up to seven cycles without showing
any decrease in the catalytic activity. The recyclability
of the material was verified at 0.07 mol% of Pd for six
consecutive runs. Then, the catalyst recovered from
the sixth cycle was employed for further recycling
experiments by using a decreased amount of Pd
namely 0.007 and 0.0007 mol% for the Suzuki
reaction, and 0.007 mol% in the Heck reaction. The
direct comparison of the abovementioned catalytic
tests with the analogous ones performed at 0.007 and
4
200 MP-AES, Agilent. Analyses Were carried out using a
calibration curve, obtained by dilution.
Synthesis of 4
Hybrid 4 [22w] as synthesized according to literature
procedures.
0.0007 mol% of Pd with the as-synthesized material
Synthesis of 5
allowed to assess the high recyclability of the material.
The presence of the POSS nanocage and the textural
properties of SBA, can affect on the uniformly
distribution of Pd species within the pore walls, acting
as a nanoreactor, that, in turn, can help to overcome
the problems of aggregation of palladium and strongly
limiting leaching phenomena.
In a round bottom flask, supported imidazolium modified
POSS 4 (300 mg) was dispersed and sonicated in distilled
water (3 mL). In the meanwhile,
a
sodium
tetrachloropalladate solution was prepared by using PdCl
2
(
0.07 mmol, 12.4 mg), 20 equiv. of NaCl (1.4 mmol, 81.8
mg) in distilled water (4 mL). The above mixture was heated
at 80°C for 20 min. Then, it was allowed to cool down to
2 4
room temperature. The obtained brown Na PdCl solution
was added dropwise to the suspension of 4. The reaction
mixture was stirred at room temperature for 16 h. Then, the
solid was recovered by centrifugation and washed with
water, methanol and diethyl ether. Before each
centrifugation, the solid was sonicated for 5 min in the
washing solvent. After the last washing, it was dried
overnight at 60 °C obtaining catalyst 5 as pale yellow
powder (299 mg).
Experimental Section
Spectroscopic and analytical methods
Thermogravimetric analysis (TGA) measurements were
carried out under oxygen flow from 100 to 1000 °C with a
-
1
heating rate of 10 °C min in a Mettler Toledo TGA STAR
Combustion chemical analysis was performed on a
PerkinElmer 2400 Series II Elemental Analyzer. System.
General procedure for the Heck reaction
The catalytic tests were carried out in a 3 mL screw cap glass
vial in which pre-catalyst 5 (0.07 mol%), aryl iodide (0.5
mmol), alkene (0.75 mmol), triethylamine (1 mmol) and
DMF (1 mL) were placed. The reaction mixture was
sonicated for a short time and heated at 120 °C under stirring
for a given time. Then, the reaction mixture was allowed to
cool down to room temperature, diluted with water in order
to be extracted with diethyl ether. The organic layer was
2
N adsorption–desorption analysis was carried out at 77 K
by using a volumetric adsorption analyzer (Micromeritics
Tristar 3000). Before the analysis, the sample was pretreated
at 150 °C for 16 h under reduced pressure (0.1 mbar). The
0
BET method was applied in the p/p =0.05–0.30 range to
calculate the specific surface area, whereas the pore size
distributions were determined from the adsorption isotherm
using the BJH method.
dried with Na SO , filtered and evaporated under vacuum.
2 4
1
The conversions of the products were estimated by H NMR
analysis.
Inductively coupled plasma optical emission spectroscopy
(
ICP-OES) analysis was carried out with an Optima 8000
Recycling procedure of 5 in the Heck reaction
ICP-OES Spectrometer. X-ray photoelectron spectroscopy
XPS) analyses were performed with a Thermo Fisher
ESCALAB 250Xi instrument equipped with
(
Recycling tests were performed in a 10 mL screw cap glass
vial in which pre-catalyst 5 (0.07 mol%, 10 mg), 4-
iodoanisole (2 mmol), methyl acrylate (3 mmol),
triethylamine (4 mmol) and DMF (4 mL) were placed. The
reaction mixture was sonicated for a short time and heated
at 120 °C under stirring for 3h. Then, it was allowed to cool
down to room temperature and centrifuged in order to
recover the catalyst. The solid 5 was washed, sonicated and
centrifuged with ethyl acetate, methanol and diethyl ether.
Once dried, it was used for the next catalytic cycle. The
recovered organic layers were evaporated under reduced
pressure then treated with water and extracted with diethyl
a
monochromatic Al Kα X-ray source (1486.6 eV) and a
hemispherical deflector analyser (SDA) working at constant
pass energy (CAE) allowing to obtain a constant energy
resolution on the whole spectrum. The experiments were
performed using a 200 μm diameter X-ray spot. The charge
neutralization of the sample was achieved with a flood gun
using low energy electrons and argon ions. The pressure in
-
8
the analysis chamber was in the range of 10 Torr during
data collection. Survey spectra were recorded with a 150 eV
pass energy, whereas high-resolution individual spectra
were collected with a 25 eV pass energy. Analyses of the
peaks were carried out with the software Thermo Avantage,
based on the non-linear squares fitting program using a
weighted sum of Lorentzian and Gaussian component
curves after background subtraction according to Shirley
2 4
ether. The combined organic layers were dried with Na SO ,
filtered and evaporated under vacuum. The conversions of
1
the products were estimated by H NMR analysis.
[
30]
and Sherwood.
Full width at half maximum (FWHM)
General procedure for the Suzuki reaction
values were fixed for all the signals. Small-angle X-ray
scattering (SAXS) experiments were performed on a Bruker
Nanostar SAXS System equipped with 2D detector using
Cu K radiation ( = 1.5418A˚). The SAXS measurements
were collected in the range 0.3–1.5◦ 2 by using a 0.02◦ step
The catalytic tests were carried out in a 3 mL screw cap glass
vial in which pre-catalyst 5 (0.07 mol%), aryl bromide (0.5
2 3
mmol), phenylboronic acid (0.55 mmol) and K CO (0.6
mmol) were placed. Then, distilled water (0.6 mL) and
8
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Advanced Synthesis & Catalysis
10.1002/adsc.201900350
ethanol (0.6 mL) were added. The reaction mixture was
sonicated for a short time and heated at 50 °C under stirring
for a given time. Then, the reaction mixture was allowed to
cool down to room temperature, diluted with water in order
to be extracted with dichloromethane. The organic layer was
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The conversions of the products were estimated by H NMR
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55, 2007-2018.
A. Kumbhar, Top. Curr. Chem. 2016,
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Recycling procedure of 5 in the Suzuki reaction
3
Recycling tests were performed in a 10 mL screw cap glass
vial in which pre-catalyst 5 (0.07 mol%, 10 mg), 4-
bromobenzaldehyde (2 mmol), phenylboronic acid (2.2
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reaction mixture was sonicated for a short time and heated
at 50 °C under stirring for 1h. Then, it was allowed to cool
down to room temperature and centrifuged in order to
recover the catalyst. The solid 5 was washed, sonicated and
centrifuged with ethyl acetate, methanol and diethyl ether.
Once dried, it was used for the next catalytic cycle. The
recovered organic layers were evaporated under reduced
pressure then treated with water and extracted with
dichloromethane. The combined organic layers were dried
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Acknowledgements
We gratefully acknowledge the University of Palermo and the
University of Namur. C.C. gratefully acknowledges the University
of Palermo and University of Namur for a co-funded PhD
fellowship. This research used resources of the nuclear magnetic
resonance service located at the University of Namur. Dr. Luca
Fusaro is acknowledged for his support to the NMR
measurements. This service is a member of the “Plateforme
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0
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FULL PAPER
SBA-15/POSS-Imidazolium hybrids as Catalytic
Nanoreactor: the role of the support in the
stabilization of Palladium species for C−C Cross
Coupling Reactions
Adv. Synth. Catal. Year, Volume, Page – Page
Carla Calabrese, Vincenzo Campisciano, Fabiana
Siragusa, Leonarda F. Liotta, Carmela Aprile,*
Michelangelo Gruttadauria,* Francesco Giacalone*
1
1
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