Proximity Effect using a Nanocage Structure: Polyhedral Oligomeric Silsesquioxane-Imidazolium Tetrachloro- palladate Salt as a Precatalyst for the Suzuki-Miyaura Reaction in Water

Article abstract of DOI:10.1002/cctc.201600155

A polyhedral oligomeric silsesquioxane-imidazolium tetrachloropalladate salt (POSS-Imi-PdCl₄) was prepared by the reaction of a polyhedral oligomeric silsesquioxane-imidazolium chloride salt (POSS-Imi-Cl) with PdCl₂ and used as a pre-catalyst for the Suzuki-Miyaura reaction in water at 100 °C at a low loading (0.08-0.16 mol %). Biphenyl compounds were isolated in high to excellent yields. A comparison of the POSS-based catalyst with the corresponding catalyst without the nanocage structure (i.e., 1-butyl-3-methylimidazolium tetrachloropalladate) highlighted the role of the POSS structure to reach higher yields in the Suzuki-Miyaura reaction. This result is ascribed to a proximity effect of the imidazolium moieties linked to the nanocage structure. Fresh and used catalytic materials were characterised by using X-ray photoelectron spectroscopy and TEM.

Full text of DOI:10.1002/cctc.201600155

DOI: 10.1002/cctc.201600155
Full Papers
Proximity Effect using a Nanocage Structure: Polyhedral
Oligomeric Silsesquioxane-Imidazolium Tetrachloro-
palladate Salt as a Precatalyst for the Suzuki–Miyaura
Reaction in Water
Lucia Anna Bivona,[a, b] Francesco Giacalone,[a] Esther Carbonell,[b]
Michelangelo Gruttadauria,*[a] and Carmela Aprile*[b]
A
polyhedral oligomeric silsesquioxane-imidazolium tetra-
structure (i.e., 1-butyl-3-methylimidazolium tetrachloropalla-
date) highlighted the role of the POSS structure to reach
higher yields in the Suzuki–Miyaura reaction. This result is as-
cribed to a proximity effect of the imidazolium moieties linked
to the nanocage structure. Fresh and used catalytic materials
were characterised by using X-ray photoelectron spectroscopy
and TEM.
chloropalladate salt (POSS-Imi-PdCl4) was prepared by the reac-
tion of a polyhedral oligomeric silsesquioxane-imidazolium
chloride salt (POSS-Imi-Cl) with PdCl2 and used as a pre-catalyst
for the Suzuki–Miyaura reaction in water at 1008C at a low
loading (0.08–0.16 mol%). Biphenyl compounds were isolated
in high to excellent yields. A comparison of the POSS-based
catalyst with the corresponding catalyst without the nanocage
Introduction
The palladium-catalysed Suzuki–Miyaura cross-coupling reac-
tion has emerged as an important and efficient tool in organic
synthesis because it is one of the most convenient modern
methods for CÀC bond formation.[1] The main advantages of
this process are represented by the mild reaction conditions
employed[2] and the tolerance toward a wide range of func-
tional groups. In particular, this reaction is an important strat-
egy for the synthesis of polymers but also in the field of phar-
maceutical and natural products.[3]
tention has been paid to the use of eco-friendly and cheap sol-
vents or co-solvents, such as water.[5] Tetrachloropalladate salts
are useful Pd pre-catalysts. Imidazolium tetrachloropalladate
salts were employed as pre-catalysts at 1 mol% loading in
Suzuki–Miyaura reactions in 1-ethyl-3-methylimidazolium bis-
(trifluoromethylsulfonyl)imide at 1008C, which resulted in high
conversions.[6] Other imidazolium tetrachloropalladate salts of
the type [IL]2[PdX4] (IL=imidazolium cation, X=Cl, Br) were
used as catalyst precursors at 1 mol% loading in the Suzuki–
Miyaura reaction in iPrOH or iPrOH/H2O 1:1 at 408C.[7] In situ
ESI-MS experiments showed evidence of [(IL)xPd3] cluster for-
mation if complexes of the type [IL]2[PdX4] were used as cata-
lyst precursors.[8] Other anionic Pd complexes of the type
[IL]2[PdX4] have been reported and used as pre-catalysts in the
Suzuki–Miyaura reaction,[9] in the oxidative Heck reaction[10]
and other catalytic applications.[11] Recently, inorganic–organic
hybrid materials have attracted great interest thanks to their
excellent performance in terms of thermal and mechanical sta-
bility. Nanostructured polyhedral oligomeric silsesquioxanes
(POSS) are a class of organic–inorganic hybrid compounds
with the general formula (RSiO3/2)n that consist of an inorganic
silica-like core surrounded by organic groups.[12] In general,
their structure can be expressed by the formula TnRm, in which
T indicates the number of Si atoms and R denotes the organic
substituents. Typical POSS derivatives have a cube-octameric
structure (T8R8). Recently, increased interest in POSS nanostruc-
tures has been attributed to their high performances, which
come from a combination of inorganic and organic characteris-
tics. As the organic peripheries can be functionalised easily,
POSS can be designed for multi-functional nanocomposites. To
date, they have been applied widely in many materials, such
Traditionally, the Suzuki–Miyaura reaction proceeds by using
Pd complexes with N- and P-containing ligands. It has been
well established that ionic liquids (ILs) are interesting solvents
and/or ligands in which to conduct the Suzuki–Miyaura reac-
tion with Pd salts under “ligand-free” conditions. This process
eliminates the need for toxic solvents and expensive P-contain-
ing ligands. Moreover, in ILs it is possible to use PdII as a pre-
catalyst for the CÀC coupling reaction, which avoids the use of
reducing reagents.[4] To obtain greener conditions, special at-
[a] Dr. L. A. Bivona, Dr. F. Giacalone, Prof. M. Gruttadauria
Dipartimento di Scienze e Tecnologie Biologiche
Chimiche e Farmaceutiche (STEBICEF)
Sezione di Chimica, Università di Palermo
Viale delle Scienze s/n, Ed. 17, I-90128 Palermo (Italy)
E-mail: michelangelo.gruttadauria@unipa.it
[b] Dr. L. A. Bivona, Dr. E. Carbonell, Prof. C. Aprile
Laboratory of Applied Material Chemistry (CMA)
University of Namur
61 rue de Bruxelles, 5000 Namur (Belgium)
E-mail: carmela.aprile@unamur.be
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201600155.
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as biomaterials,[13] hybrid electrochromic devices,[14] solar
cells,[15] models of silica-supported catalysts,[16] additives in the
synthesis of periodic mesoporous organosilicas,[17] polymer
nanocomposites[18] and in catalysis.[19] In particular, there are
very few examples of POSS-stabilised Pd nanoparticles used as
catalysts.[20]
the reaction was performed in water. The final product was
1
characterised by elemental analysis, H and 13C NMR spectros-
copy, thermogravimetric analysis (TGA), TEM and X-ray photo-
1
electron spectroscopy (XPS). H and 13C NMR spectra in D2O, al-
though not resolved as the precursor POSS-Imi-Cl (see Sup-
porting Information), showed all the expected signals. Howev-
1
In recent years, we have been involved in investigations in
the development of new Pd-based catalysts[21] and, in addition,
we have presented a silsesquioxane-based nanostructure func-
tionalised with imidazolium chloride (POSS-Imi-Cl) as an effi-
cient catalyst for chemical fixation of CO2.[22] In this case, the
catalyst displayed an improved catalytic performance with re-
spect to unsupported 1-butyl-3-methylimidazolium chloride
and such enhanced activity was ascribed to the proximity
effect generated by the increased local concentration of imida-
zolium species that surround the inorganic silsesquioxane core.
As imidazolium-based ILs have been employed as homoge-
neous supports for Pd-based catalysts and to take advantage
of our synthetic strategy for the synthesis of POSS-Imi-Cl, we
envisaged a new approach for Pd-based POSS imidazolium
salts as pre-catalysts for Suzuki–Miyaura reactions. In this
paper, we describe the synthesis of a new imidazolium tetra-
chloropalladate salt immobilised on a POSS nanocage and its
use as a pre-catalyst at a low catalytic loading in the Suzuki–
Miyaura reaction in water. We also describe an improved cata-
lytic performance with respect to the corresponding unsup-
ported pre-catalyst.
er, different from the H NMR spectrum of POSS-Imi-Cl, which
did not show the signal of the proton at the C-2 position of
the imidazolium ring because of its exchange with D2O, the
1H NMR spectrum of POSS-Imi-PdCl4 showed the signal of this
proton clearly. After heating the samples to 958C, NMR spec-
troscopy showed a partial exchange of the above-mentioned
proton but did not show any significant modification of the
POSS-Imi-Cl structure. TGA was performed under oxygen flow
to 9008C with a heating rate of 108CminÀ1 (Figure 1). Under
these conditions, the residue is constituted by SiO2 and Pd0,[23]
which is in excellent agreement with the structure of POSS-
Imi-PdCl4.
Results and Discussion
Figure 1. TGA of POSS-Imi-PdCl4 under oxygen flow.
Polyhedral oligomeric silsesquioxane functionalised with imida-
zolium moieties was synthesised in a two-step procedure.[22]
We started from the commercially available T8R8 octavinyl-sub-
stituted silsesquioxane (POSS-vinyl; Scheme 1). POSS-vinyl was
reacted with 3-chloro-1-propanethiol through a thiol-ene reac-
tion in presence of 2,2-azobisisobutyronitrile (AIBN) as a radical
source to give POSS-Cl (Scheme 1). Then, POSS-Cl was reacted
with 1-methylimidazole to produce the imidazolium-functional-
ised silsesquioxane (POSS-Imi-Cl; Scheme 1). POSS-Cl and
POSS-Imi-Cl were fully characterised (1H, 13C and 29Si magic-
angle spinning cross-polarisation (MAS-CP) NMR spectroscopy,
FTIR spectroscopy and elemental analysis). POSS-Imi-PdCl4 was
synthesised by the treatment of POSS-Imi-Cl with PdCl2 in
water. Tetrachloropalladate-based ILs can be synthesised by
the treatment of the appropriate imidazolium salt with PdCl2
or PdCl2(cod) (cod=cyclooctadiene) in acetonitrile.[11c] Howev-
er, because of the low solubility of POSS-Imi-Cl in acetonitrile,
POSS-Imi-PdCl4 was then used as a catalyst for the Suzuki–
Miyaura reaction between phenylboronic acid and a set of aryl
bromides. The reactions were performed in water at 1008C for
4 h using K2CO3 as the base in the presence of 0.16 mol% cata-
lyst (Table 1). All the biphenyls were obtained in high or excel-
lent yields. The reactions can even be performed at a lower
temperature (508C) as demonstrated by the quantitative yield
obtained in the case of 4-bromobenzaldehyde and the high
yield (85%) obtained with 4-bromoanisole (entries 11 and 12).
To investigate the role of the silsesquioxane nanocage, addi-
tional catalytic tests were performed using the corresponding
imidazolium-based catalyst without the silsesquioxane nano-
cage (i.e., 1-butyl-3-methylimidazolium tetrachloropalladate;
bmim2PdCl4; Table 2). These catalytic tests were performed
using different aryl bromides and two catalyst loadings (0.16
Scheme 1. Synthesis of the POSS-Imi-PdCl4 pre-catalyst.
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Table 1. Suzuki–Miyaura reaction between phenylboronic acid and aryl
bromides catalysed by POSS-Imi-PdCl4.[a]
Table 2. Suzuki–Miyaura reactions between phenylboronic acid and aryl
bromides.[a]
Entry
Catalyst
[mol%]
R
Conv.[b]
[%]
Yield[c]
[%]
TON[d]
[b]
Entry
Biphenyl
Conv.
[%]
Yield[c]
[%]
TON[d]
1
2
3
4
5
6
7
8
bmim2PdCl4
(0.16)
bmim2PdCl4
(0.16)
bmim2PdCl4
(0.16)
bmim2 PdCl4
(0.16)
POSS-Imi-PdCl4
(0.08)
bmim2 PdCl4
(0.08)
POSS-Imi-PdCl4
(0.08)
bmim2PdCl4
(0.08)
2-COCH3
4-OCH3
3-OCH3
4-CH3
24
57
75
64
90
45
72
26
21
57
66
58
88
40
64
15
131
356
413
363
1100
500
800
188
1
2
3
4
5
6
99
99
99
80
95
93
97
99
99
76
95
88
606
619
619
475
594
550
4-OCH3
4-OCH3
4-CH3
4-CH3
7
8
9
88
99
99
79
99
99
494
619
619
[a] Reaction conditions: aryl bromide (1.0 mmol), phenylboronic acid
(1.1 mmol), K2CO3 (2.1 mmol), H2O (1.0 mL), catalyst (0.16 or 0.08 mol%).
[b] Calculated from 1H NMR spectra (Figures S32–S39). [c] Isolated yield
obtained after passing the residue through a short pad of silica gel.
[d] TON calculated as mmol product/mmol catalytically active sites.
10
77
69
431
Table 3. Suzuki–Miyaura reaction between phenylboronic acid and 4-bro-
moanisole.[a]
11[e]
12[e]
99
85
99
85
619
531
[a] Reaction conditions: aryl bromide (1.0 mmol), phenylboronic acid
(1.1 mmol), K2CO3 (2.1 mmol), H2O (1.0 mL), catalyst (0.16 mol%, 1.2 mg).
[b] Calculated from the 1H NMR spectra (Figures S20–S31). [c] Isolated
yield obtained after passing the residue through a short pad of silica gel.
[d] Turnover number (TON) calculated as mmol product/mmol catalytical-
ly active sites. [e] Reaction performed at 508C.
Entry
Catalyst
Conversion [%][b]
1
2
3
Na2PdCl4
Na2PdCl4+2bmimCl
Na2PdCl4+POSS-Imi-Cl
26
30
90
[a] Reaction conditions: 4-bromoanisole (2.0 mmol), phenylboronic acid
(2.2 mmol), K2CO3 (4.2 mmol), H2O (2.0 mL), Na2PdCl4 (3.2 mmol) or
Na2PdCl4 (3.2 mmol)+bmimCl (6.4 mmol) or Na2PdCl4 (3.2 mmol)+POSS-Imi-
and 0.08 mol%). If bmim2PdCl4 was employed in a 0.16 mol%
loading, the conversions and yields were much lower than
those obtained using POSS-Imi-PdCl4 (Table 2, entries 1–4 vs.
Table 1, entries 4–7).
1
Cl (0.8 mmol). [b] Calculated from H NMR spectra (Figures S41–S43).
Finally, we checked the less reactive 4-chloronitrobenzene in
the presence of POSS-Imi-PdCl4 and bmim2PdCl4 (Scheme 2).
Under such mild conditions (0.16 mol%, 1008C, 4 h), a 15%
conversion was obtained in the presence of POSS-Imi-PdCl4
(Figure S40). However, no product was observed if the same
reaction was catalysed by bmim2PdCl4.
The difference between the POSS-based catalyst and
bmim2PdCl4 was even more evident if the catalysts were em-
ployed at a lower loading (0.08 mol%; Table 2, entries 5–8).
4-Bromoanisole and 4-bromotoluene gave drastically lower
conversions if the reactions were performed in the presence of
bmim2PdCl4 (Table 2, entries 5–8). Notably, the POSS-Imi-PdCl4
catalyst worked well even at 0.08 mol% loading, a catalyst
loading much lower than other [IL]2[PdCl4] catalytic systems re-
ported previously.[6,7]
The two catalysts (POSS-Imi-PdCl4 and bmim2PdCl4) were
characterised by XPS before and after the Suzuki reaction (Fig-
To collect more information about the role of the POSS-Imi-
Cl nanocage, we performed the Suzuki–Miyaura reaction be-
tween phenylboronic acid and 4-bromoanisole using Na2PdCl4
or Na2PdCl4 in the presence of bmimCl or POSS-Imi-Cl (Table 3).
Again, the effect of the imidazolium-POSS nanocage was evi-
dent.
Scheme 2. Suzuki–Miyaura reaction between phenylboronic acid and
4-chloronitrobenzene.
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Table 4. XPS data of POSS-Imi-PdCl4 and bmim2PdCl4 before and after
the Suzuki–Miyaura reaction.[a]
Entry
Catalyst
PdII
[eV]
Pd0
[eV]
PdII
[%]
Pd0
[%]
1
POSS-Imi-PdCl4
(fresh)
bmim2PdCl4
(fresh)
POSS-Imi-PdCl4
(after reaction)
bmim2PdCl4
(after reaction)
POSS-Imi-PdCl4
(after heating)
bmim2PdCl4
(after heating)
337.4
337.2
336.8
336.9
336.6
336.9
335.7
–
93
99
13
12
83
71
7
2
trace
87
3
334.6
334.9
335.0
334.6
4
88
5[b]
6[b]
17
29
[a] The reference peak (C1s) is set at BE=284.6 eV. FWHM fit parameters
are fixed between 0.5 and 3.5 eV for all of the signals. [b] After heating at
1008C in water for 4 h.
S-PdCl2 (Scheme 3 and Supporting Information), which lacks
imidazolium units, and XPS was performed (Figure 4).
The S2p core-level XPS spectrum of POSS-Imi-PdCl4 (Fig-
ure 4a) can be resolved into a doublet (from the S2p3/2 and
S2p1/2 components that originate from the spin–orbit splitting
effect contributions). However, the presence of different sulfur
moieties in POSS-S-PdCl2 is shown in Figure 4b. At least two
different doublets can be thus resolved. The peak at a binding
energy (BE) of 161.9 eV (16%), can be ascribed to the presence
of a S!Pd interaction, as reported previously.[25] This interac-
tion is absent in POSS-Imi-PdCl4, which indicates that, in this
case, the sulfur atoms play a minor role to stabilise Pd species.
A TEM investigation on the silsesquioxane-based catalyst
before and after the Suzuki–Miyaura reaction was performed
as well (Figure 5). TEM analysis after the Suzuki–Miyaura reac-
tion confirmed the presence of Pd0. The Pd nanoparticles are
well distributed and have an average dimension of 3 nm. TEM
analysis after the same Suzuki–Miyaura reaction performed in
the presence of bmim2PdCl4 also shows Pd nanoparticles with
an average dimension of 3 nm. The only difference between
the two catalysts could be ascribed to a slightly better disper-
sion of the Pd nanoparticles in the POSS-based compound.
Neither TEM nor XPS analyses provide a clear explanation
for the improved catalytic performance of POSS-Imi-PdCl4 com-
pared to bmim2PdCl4. This higher catalytic activity could be as-
cribed to a proximity effect[26] displayed by POSS-Imi-PdCl4
(Figure 6a), in which the reaction is highly favoured by the
local concentration of imidazolium moieties. Probably, a kind
Figure 2. Pd3d core-level XPS spectra of fresh a) POSS-Imi-PdCl4 and
b) bmim2PdCl4.
ures 2 and 3 and Table 4). As expected, the fresh catalysts
showed the presence of PdII species, although, in the case of
POSS-Imi-PdCl4, a small amount of Pd0 was observed (entries 1
and 2). However, after the reaction, both catalysts showed
practically the same amount of Pd0 (entries 3 and 4 and
Figure 3). To disclose if the presence of Pd0 is imputable only
to the CÀC coupling reaction mechanism, the POSS-Imi-PdCl4
and bmim2PdCl4 catalysts were heated at 1008C in water for
4 h. For POSS-Imi-PdCl4, approximately 17% of the Pd0 species
was observed, whereas approximately 29% of the Pd0 species
was observed for bmim2PdCl4 (entries 5 and 6, Figure S19).
Such reduced species could be ascribed to the thermal de-
composition of the tetrachloropalladate anion that takes place
under the reaction conditions independent of the Suzuki–
Miyaura catalytic cycle.[24]
We wondered if the presence of sulfur atoms in the alkyl
chains in the POSS nanocage may have a role in the stabilisa-
tion of the Pd species. To shed some light on this question,
the high-resolution S2p core-level XPS spectrum of POSS-Imi-
PdCl4 was analysed. To make a comparison, we prepared POSS-
Scheme 3. Structures of POSS-S and POSS-S-PdCl2.
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Figure 3. Pd3d core-level XPS spectra of a) POSS-Imi-PdCl4 and
b) bmim2PdCl4 after the Suzuki–Miyaura reaction.
Figure 4. S2p core-level XPS spectra of a) POSS-Imi-PdCl4 and b) POSS-S-
PdCl2.
of phase-transfer catalysis caused by the imidazolium units
linked to the silica nanocage is operative. The imidazolium
units could create a local environment in which the reactants
are better dissolved with respect to the aqueous phase.
Indeed, ILs that contain imidazolium, phosphonium, ammoni-
um and pyridinium cations have been shown to act as phase-
transfers catalysts for fluorination, nucleophilic substitutions,
etherification and benzoin condensation reactions.[27] Recently,
it was described that an imidazolium-based organic–inorganic
hybrid silica enhanced the asymmetric Michael addition of 1,3-
dicarbonyl compounds to nitroalkenes in brine at 0.5 mol%
loading by acting as a phase-transfer catalyst.[28] These effects
cannot be operative if the bmim2PdCl4 catalyst is used because
of the absence of the nanocage (Figure 6b).
onic acid in water in the presence of a polyhedral oligomeric
silsesquioxane-imidazolium tetrachloropalladate salt (POSS-Imi-
PdCl4) as a pre-catalyst. POSS-Imi-PdCl4 was prepared by reac-
tion of a polyhedral oligomeric silsesquioxane-imidazolium
chloride salt with PdCl2 and used in water at 1008C at a low
loading (0.08–0.16 mol%).
The catalytic activity of this pre-catalyst was compared with
that of the corresponding pre-catalyst without the nanocage
structure (1-butyl-3-methylimidazolium tetrachloropalladate).
X-ray photoelectron spectroscopy and TEM did not evidence
any particular stabilising role of the POSS structure. Therefore,
the enhanced catalytic activity can be ascribed to a phase-
transfer catalysis preformed by imidazolium units linked to the
silica nanocage. The proximity of these units creates a local en-
vironment in which the reactants are better dissolved with re-
spect to the aqueous phase.
Conclusions
For the first time, a proximity effect has been observed in the
Suzuki–Miyaura reaction between arylbromides and phenylbor-
Figure 5. TEM images of a) fresh POSS-Imi-PdCl4, and b) POSS-Imi-PdCl4 and c) bmim2PdCl4 after the Suzuki reaction between 4-bromotoluene and phenyl-
boronic acid.
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gel was solubilised in dichloromethane (0.5 mL) and precipitated
with hexane (3”25 mL) at 08C. Finally, the gel was dried under re-
duced pressure to give POSS-Cl as a transparent viscous gel. Yield:
1
99%; H NMR (400 MHz, CDCl3): d=3.66 (t, 16H, J=6.2 Hz), 2.68 (t,
16H, J=8.7 Hz), 2.63 (t, 16H, J=8.7 Hz), 2.04 (m, 16H), 1.04 ppm
(t, 16H, J=8.7 Hz); 13C NMR (100 MHz, CDCl3): d=43.8, 32.4, 29.1,
26.3, 13.2 ppm; 29Si MAS NMR (99 MHz): d=À68 ppm; elemental
analysis calcd (%) for C40H80Cl8O12S8Si8 (1517.88): C 31.65, H 5.31, S
16.90; found: C 32.09, H 5.28, S 16.27.
Synthesis of POSS-Imi-Cl
POSS-Cl (670 mg, 0.441 mmol) was dissolved in toluene (7.2 mL),
and 1-methylimidazole (0.422 mL, 5.294 mmol, 12.00 equiv.) was
added. The reaction was stirred for 4 days at 908C, which resulted
in the precipitation of a brown gel. After this time, the reaction
mixture was cooled, and the supernatant was removed. The solid
was washed with dichloromethane (6”20 mL) by stirring at RT and
then dried under reduced pressure. Finally, it was lyophilised over-
night in milliQ water (2 mL) to give POSS-Imi-Cl as a brown viscous
solid. Yield: 68%; 1H NMR (500 MHz, D2O): d=7.37 (d, 8H, J=
2.0 Hz), 7.31 (d, 8H, J=2.0 Hz), 4.18 (t, 16H, J=6.9 Hz), 3.75 (s,
24H); 2.54 (t, 16H, J=8.9 Hz), 2.44 (t, 16H, J=7.0 Hz), 2.03 (m,
16H), 0.86 ppm (t, 16H, J=7.0 Hz); 13C NMR (125 MHz, D2O): d=
135.7, 123.4, 121.8, 47.9, 35.4, 28.4, 26.5, 25.2, 12.6 ppm;
29Si MAS NMR (99 MHz): d=À60.2, À69.9 ppm; elemental analysis
calcd (%) for C72H128Cl8N16O12S8Si8 (2174.71): N 10.31, C 39.77, H
5.93, S 11.79; found: N 9.32, C 34.54, H 6.28, S 9.47.
Figure 6. Schematic representation of Suzuki–Miyaura reaction in water cata-
lysed by a) POSS-Imi-PdCl4 and b) bmim2PdCl4. Route a shows that the reac-
tion is highly favoured by local concentration of imidazolium moieties.
Experimental Section
General methods
All of the substrates and solvents were purchased from Sigma Al-
drich and Fischer and used without further purification. Liquid-
state 1H and 13C NMR spectroscopy were performed by using
1
a JEOL ECX-400 spectrometer operated at 9.4 T (399.9 MHz for H
and 100.5 MHz for 13C). Chemical shifts were referenced to the re-
sidual 1H and 13C signals of the deuterated solvents. Liquid-state
29Si NMR spectra were recorded by using a Bruker Avance-500
spectrometer operated at 11.7 T (99.3 MHz for 29Si). Combustion
chemical analysis (C, H, N) was performed by using a Thermo Finni-
gan-FlashEA 1112 apparatus. TEM images were taken by using
a PHILIPS TECNAI 10 instrument at 80 kV. Samples were dispersed
in ethanol and deposited on a carbon-coated copper grid. XPS was
performed by using a Thermo Scientific K-Alpha spectrometer
equipped with a monochromatised Al anode (1486.6 eV): X-ray
source: 12 kV, 1.8 mA; X-ray spot size: 200 mm. A flood gun (elec-
trons and Ar ions at very low energy) was used to avoid possible
charging effects. The analyser was operated at constant pass
energy (CAE) to ensure a constant energy resolution over the
whole spectrum. The analyser was operated at 200 eV pass energy
for survey spectra and at 40 eV for high-resolution individual spec-
tra. Pressure in the chamber was approximately 10À7 mbar. Analysis
of the peaks was performed by using the software Thermo Avant-
age, based on a non-linear least squares fitting program using
a weighted sum of Lorentzian and Gaussian component curves
after background subtraction according to Shirley and Sher-
wood.[29] Full width at half maximum (FWHM) values were fixed for
all the signals. POSS-Imi-Cl was synthesised using a slight modifica-
tion of a previous procedure.[22]
Synthesis of POSS-Imi-PdCl4
In a round-bottomed flask, POSS-Imi-Cl (70.5 mg, 0.0324 mmol)
was dissolved in water (1.0 mL). PdCl2 was added (23.06 mg,
0.1300 mmol, Imi/PdCl2 =2:1), and the reaction mixture was
heated at 808C for 1 h. The orange mixture was cooled and stirred
at RT for 16 h. After this time, the supernatant was removed under
reduced pressure to give POSS-Imi-PdCl4 as an orange powder in
1
quantitative yield. H NMR (500 MHz, D2O): d=8.86 (8H), 7.59 (8H),
7.53 (8H), 4.42 (16H) 3.96 (24H) 3.01 (16H), 2.42 (16H), 2.24 (16H)
1.65 ppm (16H); 13C NMR (100 MHz, D2O): d=136.3, 124.0, 122.3,
48.2, 35.6, 33.0, 32.1, 27.9, 13.2 ppm; elemental analysis calcd (%)
for C72H128Cl16N16O12Pd4S8Si8 (2884.02): N 7.77, C 29.98, H 4.47, S
8.89; found: N 6.48, C 25.63, H 4.88, S 7.16.
Synthesis of Bmim2PdCl4
An aqueous solution of 1-butyl-3-methylimidazolium chloride
(90.7 mg, 0.5209 mmol) was prepared in
a volumetric flask
(2.0 mL). Then, this solution (0.1302 mmol, 0.5 mL) and PdCl2
(11.45 mg, 0.065 mmol) were added into a round-bottomed flask
and heated at 808C for 1 h. The red mixture was cooled to RT and
stirred for 16 h (as the procedure reported for the previous cata-
lyst). Finally, the supernatant was removed under reduced pressure
to give Bmim2PdCl4 as a red solid in quantitative yield. 1H NMR
(400 MHz, CD3OD): d=9.08 (s, 2H), 7.72 (d, 2H, J=2.0 Hz), 7.64 (d,
2H, J=2.0 Hz), 4.32 (t, 4H, J=8 Hz), 4.02 (s, 6H), 1.95 (m, 4H, J=
8 Hz), 1.46 (m, 4H, J=8 Hz), 1.04 ppm (t, 6H, J=8 Hz); 13C NMR
(100 MHz, CD3OD): d=136.6, 123.7, 122.4, 49.4, 35.3, 31.8, 19.0,
12.3 ppm; elemental analysis calcd (%) for C16H30Cl4N4Pd (526.67): C
36.49, H 5.74, N 10.64; found: C 36.85, H 5.45, N 10.73.
Synthesis of POSS-Cl
POSS-vinyl (300 mg, 0.474 mmol) was added to anhydrous toluene
(1.6 mL) under a N2 atmosphere. AIBN (30 mg, 0.183 mmol) was
added to the POSS-vinyl solution, and the reaction mixture was
heated to 408C. Then, the linker 3-chloropropanethiol (0.400 mL,
4.108 mmol, 8.67 equiv.) was slowly added to the mixture, and the
reaction was stirred for 6 h at 608C. After cooling the reaction to
RT, the supernatant was removed under reduced pressure, and the
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Full Papers
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Acknowledgements
The authors acknowledge the Università di Palermo and the Uni-
versity of Namur. L.B. gratefully acknowledges the Università di
Palermo and University of Namur for a co-funded PhD fellowship.
E.C. acknowledges the FNRS for a postdoctoral researcher (CR)
grant. The authors acknowledge Dr. Luca Fusaro and Dr. Pierre
Louette for their support in the NMR and XPS measurements, re-
spectively.
Keywords: cage compounds · CÀC coupling · palladium ·
phase-transfer catalysis · supported catalysts
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