Functionalized Magnetic Mesoporous Silica Nanoparticle-Supported Palladium Catalysts for Carbonylative Sonogashira Coupling Reactions of Aryl Iodides

Article abstract of DOI:10.1002/cctc.201500356

Magnetic mesoporous silica nanoparticles (MMSN) were prepared and used as a support for palladium catalysts. MMSN with a surface area of 1909 m² g^-1 were synthesized by a nanoemulsification process involving the dispersion of hydroph

Full text of DOI:10.1002/cctc.201500356

DOI: 10.1002/cctc.201500356
Full Papers
Functionalized Magnetic Mesoporous Silica Nanoparticle-
Supported Palladium Catalysts for Carbonylative
Sonogashira Coupling Reactions of Aryl Iodides
Suzana Natour and Raed Abu-Reziq*[a]
Magnetic mesoporous silica nanoparticles (MMSN) were pre-
pared and used as a support for palladium catalysts. MMSN
with a surface area of 1909 m2 gÀ1 were synthesized by a nano-
emulsification process involving the dispersion of hydrophobic
magnetite nanoparticles in chloroform in the presence of cetyl-
trimethylammonium bromide, followed by the addition of tet-
raethoxysilane and its polycondensation by a sol–gel route.
The MMSN were modified with phosphine and N-heterocyclic
carbene-based ligands, which provided coordination sites for
conjugation with a palladium catalyst. These modified particles
were fully characterized and employed as catalyst nanosup-
ports. The palladium catalyst was immobilized on the surface
and within the pores of MMSN and applied in copper-free car-
bonylative Sonogashira coupling reactions of aryl iodides with
terminal alkynes.
Introduction
Of the transition metal catalysts, palladium is arguably the
most widely utilized in organic transformations, including oxi-
dation,[1] alkylation,[2] hydrogenation,[3] hydroformylation,[4] car-
bonylation,[5] and cross-coupling reactions.[6]
reaction of aryl iodides with terminal alkynes in the presence
of the simplest C1 unit and atom-efficient carbon monoxide.
The first palladium-catalyzed carbonylative Sonogashira reac-
tion was reported by Kobayashi and Tanaka in 1981.[11] Custom-
arily, this reaction requires copper co-catalyst and a high pres-
sure of carbon monoxide. Accordingly, many enhancements
and various modifications of this catalytic transformation have
been established. For example, Ahmed and Mori reported
a direct palladium-catalyzed carbonylative Sonogashira cou-
pling reaction of aryl iodides with terminal alkynes under at-
mospheric pressure of carbon monoxide without copper co-
catalyst.[12] In another study, Yang and co-workers showed a pal-
ladium-catalyzed copper-free carbonylative Sonogashira reac-
tion of aryl iodides.[13] Fukuyama et al., reported an additional
copper-free route for the synthesis of alkynones in ionic liq-
uids.[14]
Palladium-mediated catalytic transformations are subject to
avid attention from both industrial and academic communities.
They play a crucial part in constructing and introducing func-
tional groups such as esters, carbonyls, amides, amines, and al-
kynones and in CÀC bond formation. These groups are indis-
pensable and ubiquitous building blocks in organic chemistry,
pharmaceuticals, and agrichemicals.[7]
The carbonylation reaction is a robust and direct catalytic
transformation through which aromatic carbonyl compounds
are attained. Of these compounds, a,b-alkynyl ketones have
gained considerable attention as they appear in myriad biolog-
ically active molecules and may serve as synthetic intermedi-
ates in natural product syntheses.[8] In this regard, several
Even though palladium-catalyzed carbonylative Sonogashira
coupling reactions exhibit high activities in homogeneous sys-
tems, there are difficulties in the isolation and recovery of the
catalyst from the reaction mixture that may lead to contamina-
tion of the desired product. Consequently, great attention has
been paid to the development of more environmentally
benign systems such as heterogeneous catalysts, in which the
catalytic species are supported on a solid support.[15] In this
regard, supported palladium catalysts have been investigated
intensively due to their chemical robustness, economic as-
pects, reusability, and broad utilization in many essential cata-
lytic transformations.
routes have been adopted for the synthesis of alkynones.[5, 9]
A
prevalent path involves alkynyl organometallic reagents with
acid chloride.[10] However, an alternative and more atom-eco-
nomic protocol for the preparation of such compounds is the
palladium-catalyzed direct carbonylative Sonogashira coupling
[a] S. Natour, Dr. R. Abu-Reziq
Institute of Chemistry, Casali Center of Applied Chemistry
Center for Nanoscience and Nanotechnology
The Hebrew University of Jerusalem
Jerusalem 9190401 (Israel)
Fax: (+972)2-6585469
E-mail: Raed.Abu-Reziq@mail.huji.ac.il
To the best of our knowledge, to date only a few solid cata-
lytic systems based on immobilized palladium catalysts have
been applied in carbonylative Sonogashira coupling reactions.
Such solid supports are based on cross-linked co-polymer-sup-
ported ionic liquid,[16] mesoporous silica MCM-41,[17] and bare
magnetite particles.[18]
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500356.
This publication is part of a Special Issue on Palladium Catalysis. To view
the complete issue, visit:
http://onlinelibrary.wiley.com/doi/10.1002/cctc.v7.14/issuetoc
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Conventional heterogeneous catalysts are known to be less
active than their homogeneous counterparts. Hence, nanoca-
talysis, a discipline that merges the merits of both the homo-
geneous and heterogeneous worlds, has been developed.[19]
As a part of this field, which is directed towards developing
highly efficient and recyclable catalysts, we report herein
a nanocatalytic system based on the immobilization of palladi-
um on modified magnetic mesoporous silica nanoparticles
(MMSN) and its utilization in the carbonylative Sonogashira
coupling reaction.
addition, bases are used for the hydrolysis of the silane mono-
mers to afford the silica matrix. For the synthesis of MMSN, our
study began with the synthesis of hydrophobic oleic acid-
coated MNP (MNP-OA). Various techniques have been broadly
utilized for the synthesis of MNP.[24] MNP-OA were readily pre-
pared in a one-pot reaction by co-precipitation of iron salts
and ferric and ferrous chlorides at 85–908C in a basic aqueous
medium in an inert atmosphere according to Massart’s
method,[25] followed by the addition of oleic acid. The pure su-
perparamagnetic MNP were spherical in shape with an average
diameter of 15–20 nm, as substantiated by transmission elec-
tron microscopy (TEM, Figure S1). Such particles have a high
tendency to agglomerate to diminish the high energy derived
from the high surface area-to-volume ratio. Moreover, non-
modified MNP are poorly dispersible in any medium. Thus, to
avoid this restriction, numerous surface-coating agents and
capping methods were utilized. In this case, the MNP were sta-
bilized with oleate groups, which coordinated with iron ions
present on the surface of the MNP through the oxygen atoms
of the carboxylate. The oleate groups, in addition to sterically
stabilizing the MNP, enhanced the solubility of MNP in solvents
such as chloroform.
Mesoporous silica nanoparticles (MSN), containing pores
with diameters in the range of 2–50 nm, have burgeoned as
powerful and attractive functional materials in the field of
chemistry and nanoscience.[20] MSN with controlled size and
morphology display attractive properties such as high specific
surface area with ample SiÀOH bonds at the surface, high ther-
mal, chemical, and mechanical stability, low toxicity, and high
compatibility,[21] making them ideal candidates for a number of
applications.[22] Owing to their nanometric dimensions, the iso-
lation of MSN from reaction media is generally challenging. A
commonly used strategy is the incorporation of magnetic
nanoparticles such as magnetite nanoparticles (MNP) into the
silica network to impart superparamagnetic properties to MSN
that enable their isolation by simple application of an external
magnetic field. In addition to the facile and expeditious isola-
tion process, the magnetic nanoparticles have proven highly
efficient as a catalyst support owing to their low toxicity and
cost, high stability, and high surface area-to-volume ratio.[23]
However, MNP will agglomerate over time and even oxidize if
exposed to the atmosphere for a long period of time, leading
to the loss of the distinctive magnetization. Therefore, deposi-
tion of silica on the MNP may prevent such undesirable fea-
tures.
The dispersion of MNP-OA in chloroform was utilized in the
preparation of the MMSN. The synthetic approach for estab-
lishing the MMSN is depicted in Scheme 1.
In the present work, MSN with hydrophobic MNP incorporat-
ed into the core of the silica framework (MMSN) were fabricat-
ed by a sol–gel route under basic conditions by using tetrae-
thoxysilane (TEOS) as the silane monomer, MNP, and cetyltri-
methylammonium bromide (CTAB) as both stabilizer and mes-
ostructure-templating surfactant agent. The MMSN were then
modified with various functional groups such as phosphine li-
gands and N-heterocyclic carbenes (NHCs) and utilized as
nanosupports for the immobilization of palladium species. The
synthesis, characterization, and catalytic activity of the support-
ed palladium in the model reaction, the carbonylative Sonoga-
shira coupling of aryl iodides and terminal alkynes, are de-
scribed here. The catalyst was recovered from the reaction
media and recycled efficiently by applying an external magnet-
ic field.
Scheme 1. Synthesis of MMSN Fe3O4@mSiO2. TDW=triple distilled water.
Practically, the entrapment of the MNP-OA within the silica
network was achieved by emulsification of the MNP in aque-
ous solution with use of a templating surfactant CTAB that
transferred the MNP-OA from the oil phase to the aqueous
phase by micellar dispersion. Subsequently, the mesoporous
silica network was attained by the slow addition of the silica
precursor TEOS, which was hydrolyzed and condensed in basic
medium by the conventional sol–gel process.
The surface morphology of MMSN was probed by scanning
electron microscopy (SEM) and TEM analyses, which verified
the formation of spherical porous silica nanoparticles with
MNP in the core of the network (Figure 1a and b, respectively).
The particle size distribution and zeta potential of the un-
functionalized MMSN were analyzed by using dynamic light
scattering (DLS) and Nano-ZetaSizer intruments, respectively,
as depicted in Figure 2. These analyses revealed the formation
of poly-dispersed MMSN with an average particle size of
304 nm and zeta potential of À27.8 mV. The negative zeta po-
tential charge was due to deprotonation of the silanol groups
present on the surface. Furthermore, the absolute value of the
Results and Discussion
Synthesis of MMSN (Fe3O4@mSiO2)
MMSN are inorganic materials generally prepared by deposi-
tion of a mesoporous silica layer onto MNP. In this process, var-
ious structure agents and co-agents are utilized to transfer hy-
drophobic MNP from the oil phase to the aqueous phase. In
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To obtain the mesostructure and gain access to the pores,
the templating surfactant had to be removed. This could be
achieved by calcination, however, this method is associated
with pore size reduction, a low degree of condensation of the
silica network, and particle agglomeration. In this work, we
have produced MMSN with a higher surface area than reported
previously, 1000 m2 gÀ1, without adopting the calcination pro-
cess. The specific surface area of the MMSN evaluated by Bru-
nauer–Emmett–Teller (BET) analysis was approximately
1909 m2 gÀ1 and the pore volume and diameter according to
Barrett–Joyner–Halenda (BJH) calculations were 0.216 cm3 gÀ1
and 1.89 nm, respectively. The nitrogen adsorption–desorption
isotherm of the MMSN is shown in Figure S2.
The chemistry of the silica matrix was investigated further
by using solid-state 29Si cross polarization magic angle spin-
ning (29Si CP-MAS) NMR spectroscopy (Figure 3). Pure MSN,
synthesized in the same manner as MMSN, exhibited two dis-
tinct signals at d=À100.07 and À110.51 ppm. These signals
were attributed to isolated silanol (Q3) and siloxane groups
(Q4), respectively.
Figure 1. a) SEM and b,c) TEM micrographs of MMSN.
The composition of the MMSN was probed further by X-ray
powder diffraction (XRD). The XRD pattern of the MMSN (Fig-
ure 4b) displayed the characteristic peaks of the MNP at 2q=
18.1, 30.1, 35.5, 43.4, 53.8, 57.4, and 62.88, which were identical
to the observed peaks of the pure MNP (Figure 4a). The broad
zeta potential indicated that the MMSN were stable owing to
the moderate to high degree of the electrostatic repulsion be-
tween adjacent MMSN.
Figure 3. 29Si CP-MAS NMR spectrum of MSN.
Figure 2. a) Particle size distribution and b) zeta (z) potential graph of MMSN.
Figure 4. XRD pattern of a) pure magnetite and b) MMSN.
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peak in the range 2q=18–288 was assigned to the amorphous
silica network.
CTAB molecules concealed within the pores. Additionally, the
weight loss at temperatures above 5008C was due to thermal
dehydroxylation and condensation of the silanol groups on
the internal surface of the MSN to form siloxane groups. To ex-
clusively remove the templating surfactants and residues of
the unreacted silica precursors, thermal treatment such as cal-
cination would be required. However, this process could
change the morphology, collapse the pore network, and may
lead to dehydroxylation of the surface silanol groups resulting
in particle agglomeration.
Preparation and characterization of amine-functionalized
MMSN (MMSN-NH2)
Surface modification of MMSN can be achieved mostly through
a post-synthetic approach, such as grafting or co-condensa-
tion. The amine functionality was introduced by reacting (3-
aminopropyl)trimethoxysilane (3-APTMS) with MMSN in etha-
nol at 808C in an inert atmosphere. The functionalization oc-
curred through condensation of the methoxy groups with the
surface silanol groups, resulting in the loss of methanol
(Scheme 2). The functionalization step did not affect the mor-
phology or particle size distribution according to SEM, TEM,
and DLS analyses (Figures S3a,b, and S4 respectively).
After functionalizing the MMSN with 3-APTMS, 33.65%
weight loss was observed (Figure 5d), which indicated that
more organic content, in this case ꢀSiÀ(CH2)3NH2, was present.
The IR spectra of MMSN and MMSN-NH2 are shown in Fig-
ure S5. The MMSN exhibited absorption peaks at 1110 cmÀ1
and at intervals of 3100–3707 cmÀ1, which were attributed to
the asymmetrical stretching of SiÀOÀSi and SiÀOH vibrations,
respectively (Figure S5a). In addition, absorption bands at 1643
and 2851–2920 cmÀ1 were assigned to the C=C and CÀH
stretching vibrations, respectively, of the oleate groups stabiliz-
ing the MNP. The amine-functionalized MMSN exhibited similar
absorption peaks to MMSN, in addition to peaks at 1550 and
1090 cmÀ1, which were ascribed to the NÀH bending and CÀN
stretching vibrations, respectively (Figure S5b).
Scheme 2. Amine-functionalized MMSN.
Further analysis to verify the presence of the organic units
on the MMSN was conducted by using thermogravimetric
analysis (TGA) in the temperature range of 25–9508C in an
inert atmosphere. TGA results from pure MNP, MNP-OA, MMSN,
and MMSN-NH2 are shown in Figure 5. Oleic acid-stabilized
MNP exhibited an increase in the organic content with 19%
weight loss (Figure 5b), in comparison to the pure MNP, which
underwent 3.5% weight loss (Figure 5a); this was attributed to
solvent residues and absorbed moisture. For MMSN the weight
loss of 29.73% (Figure 5c) could be ascribed to the removal of
surfactant molecules (CTAB), which decomposed in the tem-
perature range 243–2738C. Notably, CTAB was also encapsulat-
ed within the pores, generating interactions with the domestic
surface of the mesoporous silica framework. Hence, higher
temperatures were required for the decomposition of the
Preparation and characterization of phosphonated MMSN-
NH2 (MMSN-N-(PPh2)2)
Palladium complexes with phosphines as ligands have been
utilized in various carbonylation reactions, providing high cata-
lytic efficiency and selectivity. In our study, we supported
a phosphine ligand on MMSN by reacting MMSN-NH2 with (di-
phenylphosphino)methanol, prepared from diphenylphosphine
and paraformaldehyde, in dry and degassed methanol. The
synthetic process of MMSN-N-(PPh2)2 is depicted in Scheme 3.
After phosphonation of MMSN-NH2, its morphology, shape,
and particle size distribution were retained in MMSN-N-(PPh2)2,
as observed in SEM, TEM (Figure S6a and b, respectively), and
DLS analyses (Figure S7). Moreover, TGA of MMSN-N-(PPh2)2
displayed a higher organic content with a weight loss of
45.8% (Figure 5e) compared to the non-phosphonated system
MMSN-NH2, which ascertained that the phosphonation reac-
tion indeed occurred. IR spectroscopy revealed similar absorp-
tion bands as in previous systems, in addition to absorption
peaks at 1650, 1480, and 1080 cmÀ1, which were assigned to
the stretching vibrations of the aromatic C=C and CÀN bonds
(Figure S5c).
Figure 5. TGA curves of a) pure MNP, b) MNP-OA, c) MMSN, d) MMSN-NH2,
and e) MMNs-N-(PPh2)2.
Scheme 3. Preparation of MMSN-N-(PPh2)2.
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Preparation and characterization of bis-imidazolidine-based
NHC supported on MMSN (MMSN-Si-NHC)
NHCs have been established as efficient ligands in transition
metal-catalyzed reactions due to their excellent electron-donat-
ing ability as strong s donors. NHC-metal complexes are ubiq-
uitous in organometallic chemistry and have been utilized in
a broad range of reactions such as carbonylation and other CÀ
C and CÀN coupling, polymerization, and hydrogenation reac-
tions.[26]
We have prepared an additional catalyst support based on
the immobilization of 1,1’-[1,3-phenylenebis(methylene)]bis{3-
[3-(triethoxysilyl)propyl]-4,5-dihydro-1H-imidazol-3-ium} chlo-
ride [bis-imidazolidine silane (Si-Im)] on the surface of MMSN.
The preparation of this system was initiated by the synthesis
of Si-Im with a,a-dichloro-m-xylene and triethoxy-3-(2-imidazo-
lin-1-yl)propylsilane in ethanol at 808C. Subsequently, the Si-Im
was grafted on the surface of MMSN through reaction of the
triethoxysilane groups of the Si-Im ligand with the surface hy-
droxyl groups of the MMSN in ethanol for 24 h at 808C
(Scheme 4). The resulting system MMSN-Si-Im showed no sig-
nificant change in particle size, as verified in TEM and DLS
analyses (Figure S8a and b, respectively).
Figure 6. a) Zeta potential graph and b) TGA curve of MMSN-Si-Im.
Scheme 4. Preparation of MMSN-Si-Im.
Zeta potential measurements of the MMSN-Si-Im revealed
a potential of +25.8 mV, confirming the immobilization and
the presence of positively charged imidazolidine groups on
the surface of the MMSN (Figure 6a). Additionally, TGA verified
the presence of organic groups of the Si-Im on the MMSN, by
revealing a weight loss of 38.3% (Figure 6b).
Figure 7. IR spectra of a) Si-Im and b) MMSN-Si-Im.
Preparation and characterization of NiXantphos-supported
MMSN (MMSN-NiXantphos)
IR analysis of the MMSN-Si-Im (Figure 7b) showed the char-
acteristic absorption bands of Si-Im functional groups (Fig-
ure 7a), CÀN at 1078 cmÀ1 and C=C at 1445 and 1654 cmÀ1. In
addition, the typical asymmetric stretching vibrations of the
SiÀOÀSi and SiÀOH bonds could be distinctly identified.
The MMSN-supported NiXantphos ligand 4,6-bis(diphenylphos-
phino)phenoxazine was prepared in a similar manner to the
previous systems. MMSN was initially modified with isocyanate
groups by a condensation reaction of ÀOH groups on the sur-
face of MMSN with (3-isocyanatopropyl)triethoxysilane. Then,
the phenoxazine-based ligand NiXantphos was reacted with
the isocyanate moiety on the MMSN. The synthetic route is de-
scribed in Scheme 5.
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spectroscopy (STEM-EDS), XRD
and X-ray photoelectron spec-
troscopy (XPS) analyses were
conducted.
TEM (Figure 8) and STEMEDS
(Figure 9) of the aforementioned
systems clearly showed the exis-
tence of palladium species and
the entrapped iron components
of MNP-OA within the core of
the silica network.
Scheme 5. Preparation of MMSN-NiXantphos.
TGA analysis of (3-isocyanatopropyl)triethoxysilane-modified
MMSN (Figure S9a) revealed a 41% weight loss, whereas
MMSN-NiXantphos displayed a higher weight loss of 50% (Fig-
ure S9b), indicating attachment of the NiXantphos ligand. The
morphology and size distribution of MMSN-NiXantphos re-
mained intact, as shown in TEM and DLS analyses (Figure S10a
and b).
Immobilization of palladium catalyst
To evaluate the efficiency of the modified MMSN in catalysis,
palladium-based catalysts were supported on the functional-
ized MMSN. Several supported palladium systems based on
palladium(II) and palladium(0) on MMSN-N-PPh2, MMSN-Si-
NHC, and MMSN-NiXantphos were accomplished. Palladium(II)
and palladium(0) grafted on the modified MMSN, through co-
ordination with the phosphine or carbene groups, were syn-
thesized by reacting palladium catalysts with functionalized
MMSN in toluene at room temperature for 24 h (Scheme 6).
Pd/MMSN-Si-NHC was prepared by initially reacting MMNs-Si-
Im with an excess of a strong base, potassium tert-butoxide, in
dry methanol and an inert atmosphere followed by the addi-
tion of palladium precursor catalyst. Several palladium precur-
sors such as Pd(OAc)2, Pd(cod)Cl2 (cod=cyclooctadiene), and
Pd(CH3CN)2Cl2 were immobilized onto MMSN-N-(PPh2)2, MMSN-
Si-NHC, and MMSN-NiXantphos. To ascertain the presence of
the palladium species on the functionalized MMSN, TEM, scan-
ning transmission electron microscopy/energy dispersive X-ray
Figure 8. TEM micrographs of a) Pd(OAc)2/MMSN-N-(PPh2)2, b) Pd(cod)Cl2/
MMSN-N-(PPh2)2, c) Pd0/MMSN-Si-NHC, d) Pd(cod)Cl2/MMSN-Si-NHC,
e) Pd(OAc)2/MMSN-NiXantphos, and f) Pd(CH3CN)2Cl2/MMSN-NiXantphos.
XPS analysis (Figure S11) was utilized to identify the oxida-
tion state of the palladium on the modified MMSN. Pd(OAc)2/
MMSN-N-(PPh2)2 had characteristic peaks corresponding to pal-
ladium(II) species (Figure S11a). The peaks centered at 337.0
and 342.1 eV were attributed to Pd3d5/2 and Pd3d3/2, respec-
tively. Pd0/MMSN-Si-NHC displayed peaks at 335.1 and
340.5 eV, which were attributed to palladium(0) (Figure S11b).
Presence of palladium(0) on MMSN-Si-NHC was also proved
by XRD analysis. Four peaks at 2q=39.71, 46.00, 66.36, and
74.088 were assigned to the face-centered cubic palladium
crystal structure and the (100), (200), (220), and (311) reflec-
tions, respectively (Figure S12).
Catalysis
To comprehend and evaluate the catalytic efficacy of the sys-
tems, we began by testing the as-prepared catalysts Pd/
MMSN-N-(PPh2)2 in the model carbonylative Sonogashira reac-
tion of 4-iodoanisole and phenylacetylene with triethylamine
as base.
Different reaction temperatures, times, and carbon monox-
ide pressures with 1 mol% of palladium catalyst were investi-
gated as displayed in Table 1. Among the conditions tested,
a reaction temperature of 808C with 40 psi (1 psi=6.89 kPa) of
carbon monoxide, and reaction time of 24 h (entry 5) proved
the most efficient. If the reaction was conducted with 40 or
400 psi of carbon monoxide at room temperature, no reaction
Scheme 6. Palladium supported on a) MMSN-N-(PPh2)2, b) MMSN-Si-NHC,
and c) MMSN-NiXantphos.
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occurred. Polymerization of phe-
nylacetylene was observed at
a reaction temperature of 1308C.
The optimized reaction condi-
tions were adopted for evaluat-
ing the influence of solvents on
the catalytic efficiency of the
supported palladium precursors
on MMSN-N-(PPh2)2, MMSN-Si-
NHC, and MMSN-NiXantphos
(Table 2). Solvents with different
polarities were employed and
the results summarized in
Table 2. The polarity of the reac-
tion medium played an impor-
tant role in the reaction. With
the non-polar solvent toluene,
low to moderate yields where
obtained in all catalytic systems
(entries 2, 8, 14, 20, 24, and 28).
On the other hand, a polar sol-
vent with coordinative ability, in
this case acetonitrile, gave
mostly low yields (entries 5, 11,
17, and 22). If water and
[BMIm]PF6
(BMIm=1-butyl-3-
methylimidazolium) were used
as reaction solvents with MMSN-
N-(PPh2)2 and MMSN-Si-NHC as
the supported ligands in the
presence of Pd(OAc)2 and
Pd(cod)Cl2, the yields of the de-
sired product ranged between
57 and 81% (entries 6, 7, 12, and
19).
Figure 9. EDS analysis of a) Pd(OAc)2/MMSN-N-(PPh2)2, b) Pd(cod)Cl2/MMSN-N-(PPh2)2, c) Pd0/MMSN-Si-NHC,
d) Pd(cod)Cl2/MMSN-Si-NHC, e) Pd(OAc)2/MMSN-NiXantphos, and f) Pd(CH3CN)2Cl2/MMSN-NiXantphos.
The highest conversions and
yields were obtained when di-
methylacetamide (DMA) was uti-
lized as the reaction solvent
Table 1. Carbonylative Sonogashira reactions of 4-iodoanisole and
(Table 2, entries 1, 13, 18, 23, 27, and 31). Hence, we chose
DMA as the reaction solvent for examining and evaluating the
effect of base on the carbonylative coupling reaction of 4-io-
doanisole and phenylacetylene. For this purpose, we chose
Pd(OAc)2 and Pd(cod)Cl2 supported on MMSN-N-(PPh2)2 as the
catalysts and tested the influence of various bases on their cat-
alytic efficiency.
phenylacetylene catalyzed by PdII/MMSN-N-(PPh2)2.[a]
Entry
T
t
CO
Yield[b]
[%]
[8C]
[h]
[psi]
From the data illustrated in Table 3, pyridine gave the lowest
yields, whereas potassium carbonate, cesium carbonate, and
sodium acetate afforded moderate yields. From these data,
triethylamine was found to be the most efficient base for the
model reaction in DMA with Pd(OAc)2/MMSN-N-(PPh2)2 and
Pd(cod)Cl2/MMSN-N-(PPh2)2 as the catalytic systems (entries 1
and 6).
1
2
3
4
5
6
7
8
130
80
80
80
80
25
25
25
24
4
4
12
24
4
400
400
40
40
40
40
400
40
47
32
28
51
94
–
4
24
–
–
[a] General reaction conditions: 1 g PdII/MMSN-N-(PPh2)2 suspension con-
taining 5.9”10À3 mmol [Pd], 0.59 mmol 4-iodoanisole, 0.708 mmol phe-
nylacetylene, 0.59 mmol Et3N, and 4 mL DMA as reaction solvent. [b] De-
After finding the optimal reaction conditions, the generality
and scope of the carbonylative Sonogashira coupling reaction
with Pd(OAc)2/MMSN-N-(PPh2)2 was explored further with di-
verse aryl iodides and terminal acetylenes in DMA and under
1
termined by H NMR spectroscopy and GC analyses.
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Table 2. Effect of solvent and palladium catalyst on the carbonylative
Sonogashira coupling reaction.
Table 3. Carbonylative Sonogashira reaction of 4-iodoanisole and phenyl-
acetylene catalyzed by MMSN-N-(PPh2)2-supported palladium in the pres-
ence of various bases.
Entry Supported ligand
Palladium precursor Solvent
Yield[f] [%]
Entry
Palladium precursor
Base
Yield[c] [%]
[a]
1
MMSN-N-(PPh2)2
Pd(OAc)2
DMA
94
35
86
84
[a]
1
2
3
4
5
6
7
8
9
10
Pd(OAc)2
Et3N
K2CO3
Cs2CO3
pyridine
NaOAc
Et3N
94
84
83
13
69
96
34
83
10
70
2
3
4
5
toluene
DMSO
DMF
acetonitrile 45
H2O 57
[BMIm]PF6 75
6
[b]
Pd(cod)Cl2
7
8
9
[b]
K2CO3
MMSN-N-(PPh2)2
MMSN-Si-NHC[c]
MMSN-Si-NHC[d]
Pd(cod)Cl2
Pd(cod)Cl2
Pd(OAc)2
toluene
DMSO
DMF
acetonitrile 29
[BMIm]PF6 81
72
56
43
Cs2CO3
pyridine
NaOAc
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
[a] 1 g Pd(OAc)2/MMSN-N-(PPh2)2 suspension containing 5.9”10À3 mmol
[Pd], 0.59 mmol 4-iodoanisole, 0.708 mmol phenylacetylene, 0.59 mmol
base, and 4 mL DMA. [b] 1 g Pd(cod)Cl2/MMSN-N-(PPh2)2 suspension con-
taining 5.25”10À3 mmol [Pd], 0.525 mmol 4-iodoanisole, 0.63 mmol phe-
nylacetylene, 0.525 mmol base, and 4 mL DMA. [c] Determined by
1H NMR spectroscopy and GC analyses.
DMA
96
48
86
78
toluene
DMSO
DMF
acetonitrile 51
DMA 82
[BMIm]PF6 61
toluene
DMSO
82
90
acetonitrile 28
DMA
toluene
DMSO
acetonitrile 88
DMA
toluene
DMSO
acetonitrile 86
DMA 88
Table 4. Carbonylative Sonogashira reaction of various aryl iodides and
terminal alkynes catalyzed by Pd(OAc)2 /MMSN-N-(PPh2)2.[a]
88
63
86
MMSN-NiXantphos[e] Pd(OAc)2
94
64
73
MMSN-NiXantphos[e] Pd(CH3CN)2Cl2
Entry Aryl iodide
Terminal alkyne
Yield[b] [%]
1
2
3
4
5
6
7
8
9
iodobenzene
4-iodoacetophenone
3-iodotoluene
4-iodotoluene
1-iodo-4-nitrobenzene
phenylacetylene
phenylacetylene
phenylacetylene
phenylacetylene
phenylacetylene
85
80
73
65
50
81
[a] 1 g PdII/MMSN-N-(PPh2)2 suspension containing 5.9”10À3 mmol [Pd],
0.59 mmol 4-iodoanisole, 0.708 mmol phenylacetylene, 0.59 mmol Et3N,
and 4 mL solvent. [b] 1 g PdII/MMSN-N-(PPh2)2 suspension containing
5.25”10À3 mmol [Pd], 0.525 mmol 4-iodoanisole, 0.63 mmol phenylacety-
lene, 0.525 mmol Et3N, and 4 mL solvent. [c] 1 g PdII/MMSN-Si-NHC sus-
pension containing 4.67”10À3 mmol [Pd], 0.46 mmol 4-iodoanisole,
0.56 mmol phenylacetylene, 0.46 mmol Et3N, and 4 mL solvent. [d] 1 g
Pd0/MMSN-Si-NHC suspension containing 5.9”10À3 mmol [Pd],
0.59 mmol 4-iodoanisole, 0.708 mmol phenylacetylene, 0.59 mmol Et3N,
and 4 mL solvent. [e] 1 g PdII/MMSN-NiXantphos suspension containing
5.14”10À3 mmol [Pd], 0.514 mmol 4-iodoanisole, 0.62 mmol phenylacety-
1-chloro-4-iodobenzene phenylacetylene
4-iodoanisole
4-iodoanisole
4-iodoanisole
1-ethynyl-4-fluorobenzene 78
4-ethynylanisole
4-ethynyltoluene
80
92
[a] 1 g Pd(OAc)2/MMSN-N-(PPh2)2 suspension containing 5.9”10À3 mmol
[Pd], 0.59 mmol aryl iodide, 0.708 mmol acetylene, 0.59 mmol Et3N, and
1
4 mL DMA. [b] Determined by H NMR spectroscopy and GC analyses.
1
lene, 0.514 mmol Et3N, and 4 mL solvent. [f] Determined by H NMR spec-
troscopy and GC analyses.
was attained (entry 7) whereas higher yields of 80 and 92%
were reached if 4-ethynylanisole and 4-ethynyltoluene were
used (entries 8 and 9).
reaction conditions of 40 psi carbon monoxide, triethylamine
as base, 808C, and a reaction time of 24 h. The results are sum-
marized in Table 4.
Ensuring recyclability of the heterogeneous catalysts while
maintaining the catalytic efficiency is a prominent concern in
the field of catalysis. By entrapment of magnetic nanoparticles
in the core of a mesoporous silica network, isolation and re-
covery of the catalyst is achieved easily by exposure to an ex-
ternal magnetic field. Thus, the recyclability of Pd(OAc)2/
MMSN-N-(PPh2)2 was explored and evaluated in the carbonyla-
tive Sonogashira coupling reaction of 4-iodoanisole and phe-
nylacetylene under the optimized reaction conditions. The re-
cyclability of the of Pd(OAc)2/MMSN-N-(PPh2)2 was verified in
four consecutive cycles, with some decrease in the reactivity
With iodobenzene as the substrate, 85% yield was obtained.
80 and 81% yield were acquired when the aryl iodide was sub-
stituted with moderately and weakly electron-withdrawing
groups, respectively (Table 4, entries 2 and 6), whereas a strong-
ly electron-withdrawing group afforded 50% yield (entry 5). On
the other hand, electron-donating groups afforded moderate
yields (entries 3 and 4). We then investigated a few terminal al-
kynes containing different groups in the carbonylative reaction
with 4-iodoanisole. With 1-ethynyl-4-fluorobenzene, 78% yield
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observed in the third and fourth cycles (Figure 10). After each
catalytic reaction, the catalyst was separated readily by apply-
ing an external magnetic field (Figure 11), washed with DMA,
and utilized directly in the next cycle.
After performing the catalytic reaction, leaching of the palla-
dium species from the catalytic system was examined by in-
ductively coupled plasma mass spectrometry measurements
(ICP-MS). There was no detectable leaching of palladium. Fur-
thermore, to prove that the catalytic system was stable and
unaffected by the reaction conditions, the recovered catalyst
was examined by SEM and TEM analyses. The SEM and TEM
micrographs of the catalyst in Figure 12 affirmed that the mor-
phology and size distribution after three cycles remained large-
ly intact.
Figure 12. Recycled Pd(OAc)2/MMSN-N-(PPh2)2 after three cycles: a) SEM and
b) TEM micrographs.
TEM (Figure 12b), EDS, and XPS analyses (Figure 13a and b)
unambiguously confirmed the presence of palladium nanopar-
ticles on the surface of the MMSN system after the carbonyla-
tion reaction, indicating that palladium(II) was reduced to pal-
ladium(0) during the catalytic reaction.
Figure 10. Recyclability of Pd(OAc)2/MMSN-N-(PPh2)2 in the carbonylative
Sonogashira coupling reaction of 4-iodoanisole and phenylacetylene.
Figure 13. a) EDS and b) XPS analysis Pd0/MMSN-N-(PPh2)2 after the carbony-
lative Sonogashira reaction.
Conclusions
Highly mesoporous magnetic silica nanoparticles (MMSN) with
a surface area of 1909 m2 gÀ1 were prepared without the adop-
tion of any calcination process and utilized as catalyst nano-
supports. Of the various functionalized MMSN that were pre-
pared, palladium(II) acetate supported on MMSN-N-(PPh2)2
showed high activity in the carbonylative Sonogashira cou-
pling reaction under the optimized reaction conditions of
40 psi carbon monoxide in the presence of triethylamine as
the base and dimethylacetamide as reaction solvent. Owing to
the presence of magnetic nanoparticles in the core of the mes-
Figure 11. Magnetic separation of Pd(OAc)2/MMSN-N-(PPh2)2 from the
reaction mixture.
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oporous silica nanoparticles, the catalytic system was readily
isolated from the reaction mixture by simple application of an
external magnetic field. The catalyst was recycled up to four
times, exhibiting some decrease in the catalytic efficiency after
the fourth cycle.
Preparation of MMSN
MMSN were prepared by the addition of MNP-OA (50 mgmLÀ1
,
1 mL) to a solution containing CTAB (0.48 g), absolute EtOH
(33 mL), triple-distilled H2O (88 mL), and concentrated NH3(aq) solu-
tion (25 wt%, 0.1 mL). The resulting mixture was stirred mechani-
cally for 60 min at RT, followed by the addition of TEOS (1.27 g).
The reaction mixture was stirred for a further 24 h to generate the
MMSN, which were isolated magnetically and washed several times
with distilled H2O and then with EtOH. The system was redispersed
in EtOH (100 mL) and sonicated for 60 min.
Experimental Section
Materials and methods
CTAB, FeCl3·6H2O, FeCl2·4H2O, NH4OH 25%, oleic acid, and
Pd(OAc)2 were purchased from Acros Fischer Scientific. TEOS was
contributed by Sol-Gel Technologies and 3-APTMS (95%), a,a-di-
chloro-m-xylene and triethoxy-3-(2-imidazolin-1-yl)propylsilane pur-
chased from Sigma Aldrich. All substrates for the carbonylative So-
nogashira reactions were purchased from either Sigma Aldrich or
Acros and used without further purification. SEM was used to de-
termine the morphology of the MMSN system. The experiments
were performed on a high-resolution scanning electron micro-
scope (Sirion, FEI Company) with a Shottky-type field emission
source and secondary electron detector. The images were scanned
at a voltage of 5 kV. TEM and EDS were performed on a Tecnai
(S)TEM F20 G2 instrument (FEI Company) operated at 200 kV. The
IR spectra were recorded at RT in transmission mode on a Perki-
nElmer 65 FTIR spectrometer. TGA was performed on a Mettler
Toledo TG 50 analyzer. Measurements were performed in a temper-
ature range of 25–9508C at a heating rate of 108CminÀ1 in N2. DLS
and zeta potential were measured by using a Nano-Series Nano-
ZetaSizer ZEN3600 instrument (Malvern Instruments). GC (7890 A,
Agilent Technologies) with a capillary column (HP-5, 30 m) was
used to determine the yields of the carbonylative Sonogashira re-
actions. 1H NMR and 13C NMR spectra were recorded on a Bruker
DRX-400 instrument in CDCl3. XRD measurements were performed
on the D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germa-
ny) with a goniometer radius of 217.5 mm, secondary graphite
monochromator, 28 Soller slits and 0.2 mm receiving slit. Low-back-
ground quartz sample holders were filled with the powder sam-
ples. XRD patterns within the range 2q=1 to 908 were recorded at
RT by using CuKa radiation (l=1.5418 Š) under the following mea-
surement conditions: tube voltage 40 kV, tube current 40 mA,
step-scan mode with a step size of 2q=0.028, and counting time
1 sstepÀ1. XPS measurements were performed by using a Kratos
Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical)
with an AlKa monochromatic radiation source (1486.7 eV), pass
energy of 20 eV, and step size of 0.1 eV. The binding energies were
calibrated to a C1s energy of 285.0 eV. The specific surface areas
were calculated by means of the BET equation by utilizing a high-
speed gas sorption analyzer, Quantachrome Nova 1200e
instrument.
Preparation of MMSN-NH2
To a suspension of MMSN dispersed in EtOH (150 mL), 3-APTMS
(5 mmol, 0.9 g) was added and stirred mechanically under reflux in
an inert atmosphere for 24 h. The amine-modified MMSN were iso-
lated magnetically, washed with EtOH and MeOH, and redispersed
in dry, degassed MeOH (100 mL).
Preparation of MMSN-N-(PPh2)2
Diphenylphosphine (8.05 mmol, 1.4 mL), prepared according to the
literature,[27] was refluxed with paraformaldehyde (8.1 mmol,
0.24 g) in dry and degassed MeOH for 60 min, forming (diphenyl-
phosphino)methanol. The reaction mixture was transferred to
a 250 mL three-necked flask containing a suspension of MMSN-NH2
in dry and degassed toluene (150 mL) and stirred mechanically in
an inert atmosphere under reflux for 24 h. The system was cooled
to RT, separated magnetically, washed, and redispersed in toluene
(100 mL).
Supporting Pd(II) on MMSN-N-(PPh2)2
The phosphonated system MMSN-N-(PPh2)2 in toluene was sonicat-
ed for 15 min and the Pd precursor (0.088 mmol) was added. The
reaction mixture was stirred at RT for 24 h in an inert atmosphere.
The catalytic system was separated magnetically, washed, and re-
dispersed in toluene to reach 15 g suspension total. The Pd load-
ing was determined by ICP-MS analysis.
Synthesis of Si-Im
N-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole (14.5 mmol, 4 g)
and a,a-dichloro-m-xylene (7.28 mmol, 1.27 g) in dry toluene
(60 mL) were refluxed in an inert atmosphere for 48 h. The reaction
mixture was cooled to RT, the solvent evaporated, and the result-
ing material dried in vacuo. A highly viscous yellow-orange product
was obtained in 92% yield.
1H NMR (400 MHz, CDCl3): d=0.61 (t, J=8.8 Hz, 4H), 1.22 (t, J=
8 Hz, 18H), 1.73–1.80 (m, 4H), 2.35 (m, 4H), 3.72 (t, J=7.2 Hz, 4H),
3.82 (q, J=7.2 Hz, 12H), 4.10 (t, J=8.4 Hz, 4H), 4.90 (s, 4H), 7.14–
7.40 (m, 4H), 10.14 ppm (s, 2H); 13C NMR (100 MHz, CDCl3): d=7.3,
15.2, 18.3, 21.4, 48.0, 48.6, 50.7, 51.2, 58.4, 65.8, 125.2, 128.3, 128.9,
137.8, 159.0 ppm; elemental analysis calcd. (%) for C32H60N4O6Si2: C,
53.09; H, 8.35; N, 7.74; Cl, 9.79; found: C, 49.79; H, 8.30; N, 7.19, Cl,
9.86.
Preparation of MNP-OA
FeCl3·6H2O (11.7 g) and FeCl2·4H2O (4.4 g) were dissolved and
stirred mechanically in deionized H2O (400 mL) in an inert atmos-
phere. The mixture was heated at 85–908C followed by rapid addi-
tion of NH4OH (aq) solution (25%, 18 mL), producing a black suspen-
sion of MNP. The mixture was heated for a further 20 min, then
oleic acid (18 mL) added slowly to the suspension, which was al-
lowed to stir for a further 60 min at 85–908C until a black precipi-
tate formed. The mixture was cooled to RT and the hydrophobic
MNP-OA isolated by applying an external magnetic field and wash-
ing with deionized H2O and 4 times with acetone. Finally, the MNP-
OA were suspended in CHCl3 (100 mL) and sonicated for 90 min.
Synthesis of MMSN-Si-Im
A 250 mL three-necked flask was charged with MMSN in EtOH
(150 mL) and Si-Im (0.42 mmol, 0.3 g). The mixture was stirred me-
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chanically in an inert atmosphere and refluxed for 24 h. The system
was cooled to RT and isolated magnetically, washed with EtOH and
MeOH, and redispersed in dry MeOH (100 mL).
Keywords: carbonylation
nanoparticles · palladium · sol–gel processes
·
heterogeneous catalysis
·
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Reuse experiment
After completion of the carbonylative Sonogashira reaction of 4-io-
doanisole with phenylacetylene catalyzed by MMSN-N-(PPh2)2 to
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Acknowledgements
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We acknowledge funding support from the Casali Foundation.
S.N. is also grateful to the Ministry of Science, Culture and Sport
of Israel for her fellowship.
Received: March 31, 2015
Published online on June 26, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim