Thioether–NHC-Ligated PdII Complex for Crafting a Filtration-Free Magnetically Retrievable Catalyst for Suzuki–Miyaura Coupling in Water

Article abstract of DOI:10.1002/ejic.201800209

A new benzimidazolium salt, 1-[benzylaminocarbonylmethyl]-3-(2-phenylsulfanylethyl)-1H-benzimidazolium chloride (L), which is a precursor of a novel thioether-functionalized NHC, has been synthesized by subjecting 1H-benzimidazole to a sequence of reactions with 1,2-dichloroethane, sodium thiophenolate, and N-benzyl-2-chloroacetamide, successively. The moisture/air-insensitive complex [Pd(L–HCl)Cl₂] (1) was prepared by the reaction of L with PdCl₂. The molecular structure of 1, established by X-ray crystallography, revealed a square-planar geometry around Pd. Complex 1 was screened for Suzuki–Miyaura coupling reactions of various aryl/heteroaryl bromides (yields of up to 94 % in 2 h) in water at room temperature. Furthermore, complex 1 was immobilized onto the surface of aminopropyl-functionalized silica-coated magnetite nanoparticles [NPs, Fe₃O₄@SiO₂-(CH₂)₃-NH₂] by a stepwise modification strategy to develop the heterogeneous magnetically retrievable catalyst Fe₃O₄@SiO₂-1′, in which the amide functionality present in the side-arm of the NHC within the NHC–Pd^II complex serves as linker. This magnetic nanosupport, bearing palladium complexes incorporating a novel thioether-based NHC ligand that functions in aqueous aerobic medium and can be easily separated, renders Fe₃O₄@SiO₂-1′ a most desirable catalyst for the Suzuki–Miyaura coupling reaction. It was also observed that the catalyst was effective for up to seven cycles and was easily separated from the reaction medium by the use of an external magnet, further increasing its appeal.

Full text of DOI:10.1002/ejic.201800209

A Journal of
Accepted Article
Title: Thioether-NHC Ligated Pd(II) Complex in Crafting of Filtration-
Free Magnetically Retrievable Catalyst for Suzuki-Miyaura
Coupling in Water
Authors: Raj Kumar Joshi, Kamal Nayan Sharma, and Naveen
Satrawala
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800209
Link to VoR: http://dx.doi.org/10.1002/ejic.201800209

European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
Thioether-NHC Ligated Pd(II) Complex in Crafting of Filtration-
Free Magnetically Retrievable Catalyst for Suzuki-Miyaura
Coupling in Water
[
a]
[a]
[a]
Kamal Nayan Sharma,* Naveen Satrawala, and Raj Kumar Joshi*
Abstract: A new benzimidazolium salt, 1-[N-benzylacetamido]-3-[1-
2-phenylsulfanylethyl)] benzimidazolium chloride (L) which is
catalytic systems for Suzuki-Miyaura cross-coupling. The
reported methods have certain limitations with their uses like
difficulty in product separation, recycling of the catalysts, and
contamination of the desired product with heavy metal traces
which reduce the applications potential of homogeneous
catalytic systems. In light of increasing environmental ethics and
sustainability, the development of recyclable catalysts is highly
desirable which can efficiently beat the challenges of
homogeneous catalysis. For reuse the catalyst, heterogenization
of the homogeneous version of the catalysts on to the solid
support material could be a logical possibility, it also shows new
ways for design and synthesis of efficient, stable and recyclable
(
a
precursor of novel thioether functionalized NHC, was synthesized by
the subjection of 1H-benzimidazole to a sequence of reactions with
1
,2-dichloroethane, sodium thiophenolate, and N-benzyl-2-
chloroacetamide, successively. Moisture/air insensitive complex
Pd(L−HCl)Cl ] (1), was prepared by a palladium promoted reaction
of L with PdCl . The molecular structure of 1 established with X-ray
[
2
2
crystallography reveals the square planar geometry around Pd.
Complex 1 was screened for Suzuki-Miyaura coupling for various
aryl/hetero-aryl bromides (Yield up to 94% in 2 h) at rt in water.
Furthermore, the complex 1 was immobilized onto the surface of the
[
8]
[9]
aminopropyl
Fe @SiO
present on sidearm of NHC within the NHC-Pd(II) complex and a
heterogeneous magnetically retrievable catalyst (Fe @SiO -1’)
functionalized
silica-coated
magnetite
NPs
catalysts. Several solid supports which includes the silica,
[
10]
[11]
[
3
O
4
2
-(CH -NH through linking the amide functionality
2
)
3
2
]
silica-coated magnetite,
graphene oxide nanohybrid,
[12]
[13]
[14]
[15]
maghemite,
zeolites,
hydrogel,
porous glass
are
3
O
4
2
reported with palladium based heterogeneous recyclable
catalyst. However, the heterogeneous catalyst based on the
solid support of magnetic core-shell NPs, particularly have
achieved new horizons in catalysis due to easily separable
magnetic properties. Moreover, the coating of the magnetite NPs
within a silica-shell enhances the stability of core-shell NPs and
achieved. The magnetic nano-support, novel thioether based NHCs,
easily separable catalyst and aqueous aerobic medium of reaction
3 4 2
level up the properties of Fe O @SiO -1’ and make the most
desirable catalyst for Suzuki-Miyaura coupling. It was noticed that
the catalyst activated up to 7 cycles and easily separated by use of
an external magnet. This put extra stars and highly raised the
efficiency of the catalyst.
[
16]
prevents the magneto-static agglomeration.
The coordination
capability of ligand supported on to the surface of the solid with
palladium has been employed in the strategy for the
development of a solid supported catalyst for Suzuki coupling.
[
17]
−
[18]
The ligand frameworks designed using pyridine,
O /N,
Introduction
[19]
[20]
[21]
NHC,
amine functionalized NHC,
and/or phosphine
derived donor functionalities, have been used frequently for this
purpose. Due to the unique two-electron σ-donor and weak ᴫ-
acceptor properties, NHCs are known to increases the electron
density on metal centre and consequently favors the oxidative
[
1]
Suzuki-Miyaura reaction is the premier C-C cross-coupling
[2]
process, widely applied in synthetic chemistry. Lenience for the
various functional groups, commercially availability and ease in
handling of boron-based compounds, are the key advantages of
Suzuki-Miyaura coupling. The industrial applications of Suzuki-
Miyaura coupling for construction of biaryl units of
pharmaceuticals and fine chemicals are also remarkable.
Large numbers of homogeneous catalytic systems such as
Pd(II) complexes with bulky and electron-rich phosphines,
addition step with providing longer lifetime to real catalytic
[22]
species.
Moisture/air insensitive Pd(II) complexes of
chalcogen donor containing ligands have been found very
[
3]
[4]
[23]
[24]
efficient catalyst for C-C coupling reactions. The NHCs, as
well as chalcogen donors with N, or O donor chelating functions,
[
5]
[23]
are well explored,
However, the chelating ligands contains
[
6]
[7]
carbenes, and palladacycles have been reported as efficient
[25]
NHC and chalcogen donors are rare and not much explored.
Therefore, in the quest of air/moisture insensitivity, efficiency
and recyclability of catalyst for Suzuki coupling, the incorporation
of chelating chalcogen donor based functions in NHC, and
palladium coordination of such ligand for build up a magnetically
retrievable heterogeneous catalytic system is worth exploring. In
the best of our mind, till date, neither the magnetite NPs-
supported nor the other solids supported palladium complex of
(S, CNHC) type bidentate ligand has been reported for catalysis of
any organic transformation.
[
[
a]
*]
Dr. Kamal Nayan Sharma,* Mr. Naveen Satrawala, Dr. Raj Kumar
Joshi*
Department of Chemistry
Malaviya National Institute of Technology Jaipur
J.L.N. Marg, Jaipur 302017, Rajasthan, India
Corresponding authors
E-mail: rkjoshi.chy@mnit.ac.in (R. K. Joshi)
http://www.mnit.ac.in/dept_chemistry/preprofile.php
kamalnayaniitd@gmail.com (K. N. Sharma)
In present paper, we report a novel, homogeneous Pd(II)
catalyst chelating with thioether-NHC ligand, heterogeneous
Supporting information for this article is given via a link at the end of
the document.
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
[
27]
1
13
1
version of the catalyst was also synthesised through
immobilizing the catalyst on the silica encapsulated magnetite
NPs and developed a filtration-free, magnetically retrievable and
efficient palladium catalyst which is effectively working (up to
seventh cycles) for Suzuki-Miyaura reaction.
and PdCl
2
without using any base.
The H and C{ H} NMR
spectra of L (SI, Figure S7 and S8) and the complex 1 (SI,
Figure S10 and S11) supported the proposed molecular
structures shown in Scheme 1. A singlet at 10.54 ppm in
1
H
1
NMR spectrum of L, related to the highly deshielded proton (H )
attached with the pre-carbene carbon (C
1
), is in the range
[
25]
reported previously for imidazolium salts (δ 9.5-12.0 ppm),
1
Results and Discussion
and suggests its high acidity. This H NMR signal was not
observed in the proton NMR of complex 1, which indicates the
deprotonation of C
Pd(II). Furthermore, a most intense peak at (m/z) 402.1635 and
42.0305 in mass spectra of L and 1, respectively, correspond
1
proton followed by the coordination with
The strategy used for the syntheses of the benzimidazolium salt
(
2
L) and Pd(II) complex [Pd(L−HCl)Cl ] (1), is depicted in Scheme
5
1.
+
to [M−Cl] .
Crystal Structure
The suitable quality single crystals of 1 were grown by slow
evaporation of concentrated solution of 1 in CHCl -CH OH (4:1)
3 3
at 0 °C. X-ray diffraction was recorded to establish the molecular
structure of 1 (details of crystal data and refinements, see Table
S1 of SI). The molecular structure of 1 with thermal ellipsoid is
depicted in Figure 1 (for more detail, see Table S2 of SI). The
geometry is nearly square planar and ligand is coordinated with
˄
metal centre in a bidentate (CNHC S) mode forming a six-
membered chelate ring. In complex 1, the Pd-CNHC and Pd-S
bond lengths, are found 1.970(3) and 2.2838(8) Å, respectively,
and also in close agreement with the reported 1.976(4) and
[25]c
2.2823(11) Å values, respectively.
Scheme 1. Strategy Used in Preparation of Benzimidazolium Chloride (L) and
Pd(II)-NHC Complex (1).
1
-(2-Chloroethyl)-1H-benzimidazole (A) was prepared by
a
[26]
modified synthetic procedure reported previously.
The
presented procedure involved the use of K
butyl ammonium bromide in a reaction of 1H-benzimidazole with
,2-dichloroethane and exclusively produced the desired product
A) rather than an impure mixture containing the
2 3
CO with tetra-n-
1
(
[26]
dehydrohalogenated product.
The thioether functionalized
benzimidazole (B) was formed when ethanolic solution of A was
added to the in situ generated sodium thiophenolate in refluxing
Figure 1. Molecular Structure of 1 with Thermal Ellipsoids Set at the 30%
Probability Level. H Atoms and a Molecule of CHCl3 are omitted for Clarity.
Bond Lengths (Å): Pd(1)-C(1) 1.970(3), Pd(1)-S(1) 2.284(8), Pd(1)-Cl(1)
1
3
ethanol. The proton and
C
NMR and mass spectra
authenticating the formation of A, B, L and 1, are provided in
Figures S1-S12 of Supporting Information (SI). The NMR
spectra of B (SI, Figure S4 and S5) indicate a typical resonance
for aliphatic substituted benzimidazoles in the range 7.16-7.76
2.357(8), Pd(1)-Cl(2) 2.313(8); Bond angles (°): C(1)-Pd(1)-S(1) 91.91(8),
S(1)-Pd(1)-Cl(1) 84.99(3), Cl(1)-Pd(1)-Cl(2) 92.25(3), Cl(2)-Pd(1)-C(1)
90.75(8).
ppm for H3−6 protons, a singlet at 7.80 ppm for H
1
proton and
Evaluation of Catalytic Potential of 1 for Suzuki-Miyaura
Coupling
1
42.6 ppm for C carbon of B. The intense peak in the mass
1
spectrum (See SI, Figure S6) at (m/z) 255.1145 corroborating
+
The Pd(II) complex (1) was explored for catalytic investigation of
Suzuki-Miyaura coupling and found efficient under aqueous and
aerobic reaction conditions in water. 4-Bromobenzaldehyde was
selected as a model substrate for preliminary investigations of
catalysis. The solvents used in reaction also influence the
course of reaction and yield of the product. After performing a
series of reactions (Entries 1-5, Table 1), it was observed that
water is the best solvent. It was found that 0.01 mol% of 1 is
ideal for the reaction while further lowering in mol% of 1,
the presence of [M+H] , also supports the structure. The FT-IR
spectrum of B indicates the disappearance of C-Cl stretching
−
1
band (664 cm ) of A and slightly shifted (red shifted) bands
observed due to the less electronegative S than Cl. An efficient
atom economy reaction of B with N-benzyl-2-chloroacetamide,
afforded air-stable thio-ether functionalized NHC ligand
precursor (L) in nearly quantitative yield (92%). Thereafter, the
palladium(II) complex (1), was prepared by palladium metal
promoted direct reaction between the NHC ligand precursor (L)
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
significantly reduced the yield of product (Entries 6-9, Table 1).
The initial formation of desired products was detected after 20-
while slight warming (up to 55 °C) was required to activate the
inactivated aryl bromides.
30 minutes of the reaction, however, the best yield was obtained
in 2 h.
Table 2. Scope of Suzuki-Miyaura Coupling Reactions Using 1 as Catalyst.
Table 1. Solvent, Base and Catalyst Optimization for Suzuki-Miyaura Coupling
Catalyzed by 1.
Entry
No.
Aryl/Heteroaryl
Bromides
Coupling
Products
[
a]
Yield (%)
[
b]
b]
1
2
3
4
5
6
7
8
94
92
94
86
82
93
88
83
[
a
Entry
Base
Solvent
1 (mol%)
Yield (%)
1
2
3
4
5
6
7
8
9
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
Water
0.01
0.01
0.01
0.01
0.01
0.05
0.005
0.001
0.0005
0.01
0.01
0.01
0.01
0.01
0.01
94
94
75
85
25
94
65
25
NC
NC
85
88
65
68
35
[b]
DMF-water
DMF
[
[
[
[
b]
b]
b]
c]
Ethanol-water
THF
Water
Water
Water
Water
10
11
12
13
14
15
No base
Water
[c]
Na
2
2
CO
3
3
Water
Cs
CO
Water
9
KOH
Water
NaOAc
TEA
Water
1
0
1
Water
1
Phenylboronic acid (1.3 mmol, 0.158 g), 4-bromobenzaldehyde (1.0 mmol,
.185 g), base (2.0 mmol), solvent (5 mL), Temp. 55 °C, Time 2 h, NC (no
0
a
conversion), Isolated yield.
1
2
3
The time profile of reaction for some representative
aryl/heteroaryl bromides such as 4-bromobenzaldehyde, 4-
bromoanisole, and 2-bromopyridine with optimum reaction
conditions was studied (SI, Figure S13). It reveals that maximum
conversion can be achieved in 2 h, moreover further increasing
the time of the reaction with maintaining the similar parameters,
does not bring any significant change in the yield of product.
1
[
c]
1
4
5
93
[b]
1
91
[
b]
The formation of cross-coupled product was not observed in the
absence of the base, it indicates that the presence of the base is
an essential requisite for the reaction (Entry 10, Table 1). The
16
8
2
[
[
c]
c]
17
85
75
77
reaction was optimized using various bases such as Na
Cs CO , KOH, and CH COONa, and triethylamine (TEA). Of
these, K CO was found most suitable (Yield 94%, Table 1,
2 3
CO ,
2
3
3
2
3
18
Entry 1). Other bases also work for the reaction but demand
comperatively long time to bring the admirable conversions
[b]
19
(
Entry 11-15, Table 1).
-
4
The scope of the present coupling reaction was further explored
for the several aryl and heteroaryl bromides with phenyl,
substituted phenyl and nephthyl boronic acids and a high
conversion of the coupling product was observed (Table 2). A
significant transformation was also observed with deactivated
substrates i.e. 4-bromoanisole (Entry 8, Table 2). For activated
aryl bromides, the catalyst worked well at room temperature
Catalyst loading (0.01 mol%, 1.0×10 mmol); phenylboronic acid (1.3
mmol); Aryl/heteroaryl bromide (1.0 mmol); base (2.0 mmol); water (5 ml);
time (2 h); [a] isolated yield, [b] at room temperature, [c] at 55 °C. (Entries
1
5-19;
p-tolylboronic
acid,
2-naphthylboronic
acid
and
4-
fluorophenylboronic acid were used)
During the catalytic investigations, the formation of black
particles was experienced after 20-30 min of the reaction. The
decomposition of Pd(II) complexes of sulfur-containing ligands to
3
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European Journal of Inorganic Chemistry
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FULL PAPER
Pd(0) species and an equilibrium stage between NPs of Pd and
the actual active species during the catalytic reaction, reported
1') was obtained. The amide functionality, present on the side
arm of NHC ligand of 1 served as a linker to connect the NHC-
Pd(II) complex with amino propyl functionalized silica coated
[
28-30]
previously.
Here, we assumed that complex 1 is dispensers
of real catalytic species, either directly or via some intermediate
formation of nanophase.
magnetite NPs
[Fe
3
O
4
@SiO
2
-(CH
2
)
3
-NH
2
].
A
stepwise
modification strategy onto the surface of the silica coated
magnetite NPs was affianced for developing a heterogeneous
Pd(II)-NHC catalyst (Scheme 2).
To explore furthermore about the active catalyst, a typical
reaction of 4-bromobenzaldehyde with phenylboronic acid
catalyzed with 1 under optimum conditions was studied in detail
and the black particles were isolated and subjected for detailed
analysis with HR-TEM and TEM-EDX. The HR-TEM images
(
Figure 2) of the black material indicates the uniformly dispersed
spherical NPs and the size of 70% particles are in the range of
1.5-2.0 nm (inset Figure 2b). These NPs can be formulated as
Pd S on the basis of quantitative % determined through TEM-
EDX analysis (SI, Figure S14).
∼
2
[25]a
Scheme 2. Schematic Illustration of Synthesis of Solid Supported Magnetically
Retrievable Pd(II)-NHC Catalyst (Fe O @SiO -1').
3 4 2
Fe
3
O
4
NPs were synthesized using
a
previously reported
[31]
procedure.
Because of the aggregation tendency of free
Figure 2. HR-TEM Images of Pd-S NPs Formed From 1 at Some Stage of
Present Suzuki-Miyaura Coupling (a) at 100 nm Scale Bar (b) at 10 nm Scale
Bar and Size Distribution (Inset) (c) at 5 nm Scale Bar (d) Zoom In Image of
Focused Area by a Red Square Shape (See SI, Figure S14 For TEM-EDX
Analysis).
magnetic NPs and corroding tendency with acids; their coating
with silica seems favourable. Moreover, the shell of silica around
Fe
further surface modification. Therefore, the silica-coating of
freshly synthesized Fe NPs was performed by a known
method (sol-gel approach).
amine functionalities onto the surface of the core-shell NPs
Fe @SiO ), was obtained using 3-aminopropyl
triethoxysilane. The aminopropyl functionalized silica-coated
magnetite NPs (Fe @SiO -amine) were further reacted with
chloroacetyl chloride that lead to the covalent immobilization of
chloromethyl-amide onto the surface of Fe @SiO -amine
resulted in Fe @SiO -amide-CH Cl NPs. Further, the
Fe @SiO -amide-CH Cl NPs on reaction with 1-(2-
3 4
O , also helps in providing active sites (Si-OH groups) for
3 4
O
The catalytic potential of isolated NPs was also investigated for
some of the typical Suzuki-Miyaura coupling substrates i.e., 4-
[
32]
Thereafter, the introduction of
bromobenzaldehyde and PhB(OH)
conditions, the average yield (55%) of the product was obtained
with 10 mg of Pd S NPs. Moreover, the NPs deactivated in one
2
under similar reaction
(
3
O
4
2
[33]
2
3
O
4
2
cycle and failed to couple 4-bromoanisole (deactivated
substrate), it might be due to the probable leaching of active
discrete Pd in the solution during catalysis.
3
O
4
2
3
O
4
2
2
3
O
4
2
2
Immobilization of
core/shell NPs
3 4 2
1 onto the surface of Fe O @SiO
phenylsulfanyl-ethyl)-1H-benzimidazole (B) resulted in covalent
immobilization of N-heterocyclic carbene ligand on the modified
To overcome the reusability issue of catalyst, 1 was immobilized
NPs. Finally, the supported NHC ligand precursor (Fe
3 4 2
O @SiO -
onto the surface of silica coated magnetite NPs, and
Lʹ) were treated with PdCl
2
(using the analogous procedure
3 4 2
heterogeneous magnetically retrievable catalyst (Fe O @SiO -
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
−
1
employed in the synthesis of 1) to produce the solid supported
The distinguished Fe-O stretching vibrations at 593 cm and a
−
1
palladium catalyst (Fe
In the powder X-ray pattern (Figure 3) of core/shell NPs
Fe @SiO ), six characteristic Bragg's peaks centered at
0.2°, 35.6°, 43.3°, 53.7°, 57.1° and 62.7° (2θ) were observed,
which corroborate the crystal planes (220), (311), (400), (422),
511) and (440) reported in standard JCPDS card No. 019-0629.
Additionally, the appearance of a broad peak centered at 21.2°
is attributed to the presence of amorphous SiO shell around the
3
O
4
@SiO
2
-1ʹ).
broad band at 3344 cm are attributable to OH groups present
Furthermore, the Si-O-Si
symmetric, Si-O symmetric and Si-O-Si asymmetric stretching
[34]
on the surface of magnetite NPs.
(
3
O
4
2
−
1
3
vibration of Fe
3
32]
O
4
@SiO
respectively. A considerably decreased less intense band of
Fe-O stretching was recorded for Fe @SiO . It establishes the
2
observed at 802, 959 and 1103 cm ,
[
(
3
O
4
2
structural differences between the pure and silica-coated
magnetite NPs and also supports the presence of silica-shell
around Fe O core. The subsequent modification on to the
3 4
2
[32]
3
Fe O
4
core.
surface of core-shell magnetic nanosupport by 3-
aminopropyltriethoxysilane (APTES) is identified by the
−
1
−1
appearance of two distinct bands 2925 cm and 1630 cm of
CH and NH groups, respectively. An intense band appearing at
640 cm indicates the C=O stretching vibrations of amido
functional group present in Fe @SiO -amide-CH Cl NPs. The
FT-IR spectrum of Fe @SiO
-Lʹ shows the significant blue
2
2
−
1
1
3
O
4
2
2
3
O
4
2
shift in C=O stretching vibration and supports its formation.
Additionally, the noticeable shift in all bands was found in order
to suggest a coordinate bond formation between the metal and
ligand. The size and shape of the final solid supported palladium
catalyst was analyzed by transmission electron microscopy
(
TEM). The TEM micrograph of freshly prepared Fe
shows the range of 20-25 nm sized NPs with a dark nano-Fe
core surrounded by a grey spongy silica shell [Figure 5(b)].
3 4 2
O @SiO -1ʹ
3 4
O
Figure 3. Powder X-ray of Silica Coated Magnetite NPs.
FT-IR spectra of Fe
Fe @SiO -amide-CH
3
O
4
,
Fe
3
O
4
@SiO
@SiO
2
,
Fe
3
O
4
@SiO
2
-amine,
-1ʹ
3
O
4
2
2
Cl, Fe
3
O
4
2
-Lʹ and Fe
3
O
4
@SiO
2
found in close aggreement with the qualitative identifications of
functional groups present in the various NPs (Figure 4).
3 4 2
Figure 5. (a) TEM Images of Fe O @SiO
-1ʹ NPs and Size Distribution Plot
(inset) and (b) Zoom In Image of the Red Square Focused Area Showing
Presence of Core-Shell Morphology (See SI, Figure S15 for TEM-EDX
analysis).
No separate silica aggregates were seen in the TEM image of
SiO @Fe O -1ʹ NPs, which confirmed the coating of silica NPs
2 3 4
on the surface of the magnetic core. The Debye Scherrer
formula was used for the determination of average crystallite
size of the Fe O NPs using the peak of highest intensity
3 4
observed in powder X-ray patterns (Figure 3) i.e., the (311) peak
in present case and estimated to be 24 nm which is also
consistent with the size observed in TEM of Fe
The TEM-EDX analysis confirmed the availability of all the
constituent elements in the Fe @SiO
-1ʹ NPs, and also
revealed a successful immobilization of NHC-Pd(II) complex 1
on to the surface of Fe @SiO NPs (SI, Figure S15).
Moreover, there was no occurrence of Pd in the NPs form was
found on the surface of Fe @SiO
-1ʹ NPs; this supports the
fact that the available Pd in the Fe @SiO
-1ʹ NPs is in the
molecular complex form. The ICP-AES analysis indicates the
3 4 2
O @SiO NPs.
3
O
4
2
3
O
4
2
Figure 4. FT-IR Spectra of (i) Fe
3
O
4
, (ii) Fe
@SiO
3
O
4
@SiO
2
, (iii) Fe
3
O
4
@SiO
2
-amine,
-1ʹ.
3
O
4
2
(
iv) Fe @SiO -amide-CH Cl, (v) Fe
3
O
4
2
2
3
O
4
2
-Lʹ and (vi) Fe
3
O
4
@SiO
2
3
O
4
2
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
0.128 mmol/g (1.369% Pd by weight) of Pd content in the
3 4 2
Table 3. Catalytic Investigation of Fe O @SiO -1ʹ for Suzuki-Miyaura
Fe
Fe
3
O
O
4
@SiO
@SiO
2
-1ʹ. The core-level XPS analysis of the
-1ʹ was also performed to ascertain the oxidation
Coupling of Aryl/hetero-aryl Bromides with Aryl Boronic Acids.
[a]
3
4
2
Entry
No
Aryl/Heteroaryl
Bromides
Coupling
Products
Yield
state of Pd (SI, Figure S16). The XPS spectrum of Pd shows the
two characteristic peaks at binding energy of 339.3 eV and
(%)
5/2
3/2
[b]
344.7 eV due to the 3d and 3d states, respectively, this
1
2
3
4
5
6
7
8
93
confirmed the +2 oxidation state of palladium in Fe @SiO -1ʹ
NPs.
3
O
4
2
[
35]
[b]
91
[
[
b]
b]
94
Evaluation of Supported Palladium Catalyst (Fe
for Suzuki-Miyaura Coupling
3 4 2
O @SiO -1ʹ)
88
3 4 2
The catalytic potential of the Fe O @SiO -1ʹ was explored for
Suzuki-Miyaura coupling of several aryl/heteroaryl bromides
and the results are summarized in Table 3. A systematic
[b]
80
3 4 2
O @SiO
-1ʹ
[
[
[
b]
c]
c]
evaluation of the results revealed that 20 mg of Fe
is an ideal amount to produce the best results in 2 h at 55 °C in
water. The magnetically active Fe @SiO -1ʹ NPs fully
recovered after completion of the reaction by an external
magnet; moreover, catalyst still maintained its efficiency. After
isolation of catalyst, the obtained liquid was cooled down to
room temperature and analytically pure coupling product was
extracted in diethyl ether.
9
2
3
O
4
2
90
78
[c]
9
85
[
b]
c]
1
0
83
82
3 4 2
Recyclability of Fe O @SiO -1ʹ
[
After completion of the reaction, the catalyst was separated by
an external magnet. The isolated catalyst was washed with
water followed by methanol and then dried under vacuum and
reused for the present reaction. We have continuously explored
the catalyst up to the seven cycles. The experimental details
and %yield of each run is separately provided in SI, Table S3.
The 85% transformation was recorded in the 7 cycle, hence, it
indicates the high potential and robustness of the catalyst
towards the Suzuki-Miyaura coupling.
11
[
b]
b]
1
2
3
91
[
1
92
94
92
th
[
[
c]
b]
14
1
5
6
Nature of Catalysis: Homogeneous or Heterogeneous
[
b]
1
85
3 4 2
The heterogeneity of the catalyst Fe O @SiO -1' was examined
using hot filtration test. The Suzuki reaction of 4-
a
bromobenzaldehyde with phenyl boronic acid under optimum
conditions was executed. When formation of cross coupled
product in the reaction reached up to ~42% (NMR yield after 50
min of reaction), the reaction was stopped and the catalyst was
magnetically separated. The clear reactions mixture was divided
in two halves; one half was transferred into another round
bottom flask for reaction to run while the isolated NPs were
added into the remaining half. Both reactions were allowed to
run under same reaction conditions for another 2 h and product
formation was estimated by proton NMR. The conversion
exceed up to 94%, in the flask containing the catalyst, while in
the catalyst-free flask yield of desired product reached only to
17
[c]
88
[
c]
b]
1
8
9
78
84
[
1
Catalyst (0.02 g, Wt% of Pd 1.369%, 0.128 mmol/g, 0.002 mmol Pd, 0.2
mol% Pd); aryl/heteroaryl bromide (1.0 mmol); phenylboronic acid (1.3
mmol); base (2.0 mmol), solvent (5 mL), time (2 h); [a] isolated yield, [b]
at room temperature, [c] at 55 °C. (Entries 15-19; p-tolylboronic acid, 2-
naphthylboronic acid and 4-fluorophenylboronic acid were used)
4
9%. Here, we assumed, the palladium complex immobilized
onto the surface of magnetic NPs generates palladium-sulfide
NPs followed by the deposition onto the surface of Fe @SiO
Moreover, the leaching of discrete Pd(0) was tested using ICP-
AES analysis of Fe O @SiO -1' before and after the three
3 4 2
catalytic cycles. Wt% of Pd in the catalyst was recorded before
the catalysis (1.369%) and after three cycles it was slightly
reduced to 1.110%. However, no further significant changes in
Pd content was noticed even after seventh catalytic cycle. This
3
O
4
2
NPs. There are reports available where the metal coordinated
onto the surface of the solid material becomes stabilized after
forming the metal NPs through the deposition-reduction
[
36]
process.
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
observation favors a largely heterogeneous nature of catalysis.
the silica encapsulated magnetic NPs supported Pd(II) complex
using thioether based NHC. The two-phase test indicates, the
Pd(0) leaching makes the catalytic process of 1, homogeneous
while in case of solid supported catalyst 2, leaching of Pd(0) is
negligible and catalytic process is largely heterogeneous. The
catalyst recovery by an external magnet and continuously reuse
for 7 cycles without losing catalytic strength are the novel and
highly desirable features in the development of sustainable
processes. At last, the formation of a novel thioether based
NHC-Pd(II) catalyst works in feasible reaction conditions at low
temperature in aqueous green solution which further make the
process more effective and economical benign.
[37]
A two phase test
(SI, Page No. S21, Scheme S1) was also
conducted to establish the heterogeneous or homogeneous
nature of the catalysis. A mixture of 4-bromoacetophenone and
4-bromobenzoic acid immobilized on silica (as amide) and
phenylboronic acid, were allowed to react under optimum
reaction conditions. The liquid part of the reaction was filtered,
desired product was extracted in diethylether and ∼95% yield of
1
4
-acetylbiphenyl was recored in H NMR. Furtheromore, the
remaining solid residue of reaction mixture was hydrolyzed, and
1
after workup the resulting products were analyzed with H NMR.
It indicates ∼15% of the immobilized 4-bromobenzoic acid (as
amide) was converted to the cross-coupled product. These
observations suggest the minor leaching of homogeneous Pd
species (molecular or colloidal, responsible for homogeneous
catalysis) from the nanocatalyst. Thus, it proved that coupling
Experimental Section
catalyzed with the solid supported Pd catalyst Fe
significantly in heterogeneous manner.
3
O
4
@SiO
2
-1ʹ, is
Synthesis of 1-(2-Chloroethyl)-1H-benzimidazole (A)
A mixture of benzimidazole (1.772 g, 15.0 mmol), potassium carbonate
(
4.146 g, 30.0 mmol) and tetra-n-butylammonium bromide (1.612 g, 5.0
Furthermore, based on various reaction parameters (optimum
catalyst loading, reaction temperature, time, or atmosphere),
comparison of catalytic performance of 1 with the previously
reported N or P donor functionalized NHC-Pd(II) catalysts has
been made and provided in supporting informations (SI, Table
S4). It appears that the present Pd(II) complex 1 can be labeled
mmol) in 50 mL of 1,2-dichloroethane was stirred under reflux for 24 h. A
light yellow suspension observed was extracted in dichloromethane (50
mL). The solvent was completely removed on rotary evaporator and
resulting residue was purified by column chromatography on silica gel,
which affords the off-white crystalline solid of the product. Off-white solid,
a
1
Yield: 2.298 g (85%). Mp: 78 °C. H NMR (400 MHz, CDCl3) δ (ppm):
7
4
(
.91 (s, 1H, H1), 7.84-7.80 (m, 1H, H3), 7.34-7.28 (m, 3H, H5, H4 and H6),
as efficent catalyst; since, 94% yield in
2
h
under
13
1
.40 (t, J = 6.0 Hz, 2H, H8), 3.77 (t, J = 6.0 Hz, 2H, H9). C{ H} NMR
aqueous/aerobic reaction conditions was recorded. The noted
superior catalytic activity of 1 is attributed to the unique σ-donor
and weak ᴫ-acceptor properties of NHCs along with the
air/moisture insensitivity and ease of handling and synthesis of
thioether functionalized ligands which have made them viable
alternatives to classical air/moisture sensitive phosphine based
101 MHz, CDCl3) δ (ppm): 143.7 (C7), 143.2 (C1), 133.2 (C2), 123.1 (C4),
22.3 (C5), 120.5 (C6), 109.0 (C3), 46.4 (C8), 42.0 (C9). Mass (CH3CN)
1
+
+
[
M+H] (m/z) Found: 181.0753; Calc. value for [C9H10ClN2] : 181.0527.
−1
FT-IR (KBr, νmax/cm ): 3057 (m, νC-H aromatic), 2961 (m, νC-H aliphatic), 1614
(s, νC=N aromatic), 1493 (s, νC=C aromatic), 1257 (m, νC=N aliphatic), 742 (s, νC-H
aromatic), 664 (s, νC-Cl aliphatic).
[
5]
Pd-catalysts. The catalytic activity of NHC-Pd(II) (Fe
3 4 2
O @SiO -
1') was also found either superior or comparable with the
Synthesis of 1-(2-Phenylsulfanyl-ethyl)-1H-benzimidazole (B)
previously reported solid supported NHC-Pd catalysts (See SI,
Table S5). However, the homogeneous version (complex 1) of
The ethanolic solution 30 ml of sodium hydroxide (0.200 g, 5.0 mmol)
and thiophenol (0.551 g, 5.0 mmol) was refluxed for 30 minutes. 20 mL
solution of 1-(2-Chloroethyl)-1H-benzimidazole (0.903 g, 5.0 mmol) in
ethanol was added to it and further allowed to stirred under reflux for 12 h.
Thereafter, extraction in chloroform (4 × 25 mL), the extract was washed
with water (3 × 40 mL) and dried over anhydrous sodium sulphate. The
solvent removed under reduced pressure and obtained product was
purified by column chromatography on silica gel. Yellow oil, Yield: 1.079
the catalyst Fe
of catalyst loading but unfortunately deactivated after one cycle.
The solid supported catalyst Fe @SiO -1' was found
3 4 2
O @SiO -1' has higher catalytic activity in term
3
O
4
2
excellent due to the intact feature of reusability and recyclability
th
upto 7 cycles. It has been assumed, the excessive binding
capabilities of free amines or amide present onto the surface of
1
g (85%). H NMR (300 MHz, CDCl3) δ (ppm): 7.80 (s, 1H, H1), 7.76−7.69
3 4 2 2 3 4 2 2
core-shells Fe O @SiO -NH / Fe O @SiO -amide-NH NPs with
(
6
m, 1H, H3), 7.34−7.16 (m, 8H, H5, H4, H13, H12, H11 and H6), 4.27 (t, J =
13
1
PdCl at some stage of immobilization are responsible to raised
2
.9 Hz, 2H, H8), 3.24 (t, J = 6.9 Hz, 2H, H9). C{ H}NMR (75 MHz,
the Pd content in TEM-EDX anaylsis.
3 7 1 10 2
CDCl ) δ (ppm): 142.9 (C ), 142.6 (C ), 133.4 (C ), 132.7 (C ), 129.5
(
(
C11), 128.6 (C12), 126.4 (C13), 122.4 (C5), 121.6 (C4), 119.6 (C6), 108.9
+
C3), 43.3 (C8), 32.9 (C9). Mass (CH3CN) [M+H] (m/z) Found: 255.1145;
+
−1
Calc. value for [C15H15N2S] : 255.0950. FT-IR (KBr, νmax/cm ): 3055 (m,
νC-H aromatic), 2929 (m, νC-H aliphatic), 1613 (s, νC=N aromatic), 1582 (s, νC=C
aromatic), 1248 (m, νC‒N aliphatic), 738 (s, νC-H aromatic (bending)).
Conclusions
Synthesis of general and silica encapsulated magnetic solid
supported Pd(II) complexes of thioether containing NHC and
their comparative study of catalytic activity for Suzuki-Miyaura
coupling was explored with the homo- and heterogeneous
version of the catalyst. The structure of homogeneous catalyst 1
was established with X-ray crystallography. Both homogeneous
and heterogeneous catalytic systems are easy to handle,
synthesize, and stable in air/moisture. We are the first to report
Synthesis of 1-[N-Benzylacetamido]-3-[1-(2-phenylsulfanylethyl)]-
benzimidazolium Chloride (L)
Mixture of 1-(2-phenylsulfanylethyl)-1H-benzimidazole (0.254 g, 1.0
mmol) and N-benzyl-2-chloroacetamide (0.183 g, 1.0 mmol) in 10 mL of
toluene was stirred under reflux in an inert atmosphere to give L as white
solid which was precipitated in reaction medium after 12 h. The reaction
mixture was filtered; the obtained residue was washed with toluene and
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
dried in vacuo yielding L as white solid. White solid, Yield: 0.402 g (92%).
Mp: 128 °C. H NMR (300 MHz, CDCl3) δ (ppm): 10.54 (s, 1H, H1), 9.92
an external magnet, washed with methanol (10 mL) and CH2Cl2 (10 mL)
and dried in vacuo.
1
(
t, J = 5.8 Hz, 1H, NH), 7.88 (dd, J = 6.0, 3.1 Hz, 1H, H5), 7.62-7.48 (m,
2
4
H, H6, H3), 7.40 (dd, J = 5.9, 3.1 Hz, 1H, H4), 7.29 (dd, J = 8.6, 4.9 Hz,
H, H12, H19), 7.24-7.14 (m, 5H, H18, H11, H20), 7.11 (d, J = 7.1 Hz, 1H,
General Procedure for Suzuki-Miyaura Coupling Reaction
A mixture of phenylboronic acid (0.159 g, 1.3 mmol), K2CO3 (0.276 g, 2.0
mmol), aryl/hetero aryl bromide (1.0 mmol) and palladium catalyst in
appropriate solvent (5 mL) was stirred under ambient conditions. The
reaction was monitored with TLC until maximum conversion of aryl
bromide to the desired product observed. On completion, the reaction
mixture was extracted with diethyl ether (2 × 20 mL). The solvent was
removed in vacuo, the residue was considered chromatographic work up
on silica gel column, and desired cross-coupled product was isolated.
H13), 5.59 (s, 2H, H14), 4.62 (t, J = 6.3 Hz, 2H, H8), 4.37 (d, J = 5.9 Hz, 2H,
1
3
H16), 3.50 (t, J = 6.3 Hz, 2H, H9). C NMR (101 MHz, CDCl3) δ (ppm):
64.41 (C15), 143.11 (C1), 138.02 (C17), 132.98 (C10), 131.77 (C7), 130.86
C2), 130.72 (C18), 129.33 (C19), 128.38 (C11), 127.82 (C12), 127.51 (C6),
1
(
1
4
4
27.37 (C3), 127.05 (C13, C20), 114.59 (C5), 112.16 (C4), 49.89 (C14),
+
6.97 (C8), 43.53 (C16), 33.45 (C9). Mass (CH3CN) [M−Cl] (m/z) Found:
+
02.1635; Calc. value for
[C24H24N3OS] : 402.1635. FT-IR (KBr,
−
1
νmax/cm ): 3199 (m, νN-H amide), 3045 (m, νC-H aromatic), 2817 (m, νC-H aliphatic),
678 (s, νC=O), 1562 (s, νC=C aromatic), 1256 (m, νC‒N aliphatic), 753 (s, νC-H
1
aromatic (bending)).
Isolation of in situ Generated Nanoparticles from Pd(II)-complex (1)
During Suzuki-Miyaura Coupling
Synthesis of [Pd(L−HCl)Cl2] (1)
A mixture of 4-bromoanisole (0.185 g, 1.0 mmol), phenylboronic acid
(
(
0.159 g, 1.3 mmol), K2CO3 (0.276 g, 2.0 mmol) and Pd(II)-complex 1
A mixture of L (0.219 g, 0.5 mmol), PdCl2 (0.089 g, 0.5 mmol) in 10 mL of
DMF was stirred under inert atmosphere at 90 °C for 12 h. Thereafter,
the volatiles were removed by a rotary evaporator. The residue was
dissolved in minimum amount of dichloromethane. After addition of
diethyl ether, a pale yellow solid precipitated which was isolated by
filtration and dried under vacuum. The product was dissolved in CH2Cl2-
CH3OH (4:1) and suitable quality single crystals were grown by slow
0.288 g, 0.5 mmol) in water (5.0 mL) was stirred under ambient
conditions for 2 h. Thereafter, the solvent was decanted and the black
residue was thoroughly washed with water-acetone (1:3) and subjected
to further analytical characterization.
1
evaporation. Pale yellow solid, Yield: 0.263 g, 91%. Mp: 195 °C. H NMR
Supporting Information
(
4
400 MHz, DMSO-d6) δ (ppm): 9.25-8.98 (br m, 1H, NH), 7.79-7.63 (br m,
H, H3−6), 7.36 (br m, 10H, H18−20, and H11−13), 5.71 (br s, 2H, H14), 5.00
br s, 1H, H8), 4.89 (br s, 1H, H8), 4.38 (br s, 2H, H16), 3.80-3.56 (br m,
(
1
Physical measurements, chemical/reagents used, NMR spectra;
mass spectra; crystal and refinement data; bond lengths and
angles; CIF of 1 (CCDC No. 1585446); TEM-EDX, XPS and the
two-phase test.
1
3
H, H9), 3.17-3.09 (br m, 1H, H9). C NMR (101 MHz, DMSO-d6) δ
(
1
ppm): 166.0 (C15), 138.8 (C17), 134.23 (C10), 132.75 (C7), 132.53 (C2),
29.51 (18), 128.36 (C19), 127.86 (C11), 127.36 (C12), 126.89 (C6), 125.73
C3), 124.16 (C20), 123.93 (C13), 112.01 (C5), 111.72 (C4), 50.34 (C14),
(
4
+
6.58 (C8), 42.51 (C16), 30.33 (C9). Mass (CH3CN) [M−Cl] (m/z) Found:
+
5
42.0305; Calc. value for [C24H23ClN3OPdS] : 542.0302. FT-IR (KBr,
−
1
νmax/cm ): 3324 (m, νN-H amide), 3059 (m, νC-H aromatic), 2944 (m, νC-H aliphatic), Acknowledgements
1687 (s, νC=O), 1622 (s, νC=C aromatic), 1248 (m, νC‒N aliphatic), 745 (s, νC-H
aromatic (bending)).
R.K.J. thanks to Council of Scientific and Industrial Research
CSIR) for financial support (01(2760)/13/EMR-II). K.N.S. thanks
(
Synthesis of Fe3O4@SiO2-amide-CH2Cl
to Science and Engineering Research Board (SERB), New Delhi
for start-up research grant (YSS/2015/000698). Authors
acknowledge the MRC, MNIT Jaipur for characterization
facilities. SIC, IIT Indore for single crystal X-ray and SAIF, IIT
Bombay for ICP-AES analyses.
After the dropwise addition of chloroacetyl chloride (excess) to a solution
of nanostructured Fe3O4@SiO2-amine (1.00 g) dispersed in 25 mL of 1,2-
dichloroethane and triethylamine (10 mmol) with maintaining the
temperature between 0-5 °C, the mixture was overnight stirred at room
temperature. A brownish material was observed which was isolated from
the reaction mixture by means of an external magnet. It was washed
several times with water, saturated solution of sodium chloride and again
with water. Finally, it was dried under vacuum and NPs of Fe3O4@SiO2-
amide-CH2Cl obtained.
Keywords: Thioether-NHC • core/shell NPs
•
magnetically
retrievable
Coupling
•
Heterogeneous Catalysis
•
Suzuki-Miyaura
Synthesis of Fe3O4@SiO2-Lʹ
[1]
a) A. Suzuki, Angew. Chem. Int. Ed. 2011, 50, 6722-6737; b) A. Suzuki,
J. Organomet. Chem. 1999, 576, 147-168.
Fe3O4@SiO2-amide-CH2Cl (1.00 g) dispersed in 10 mL of toluene
through 30 min. sonication, further it was stirred for next 12 h under reflux
with 1-(2-phenylsulfanylethyl)-1H-benzimidazole (0.254 g, 1.0 mmol). A
brownish material was formed which was isolated from the mixture by
use of an external magnet. It was washed with toluene (25 mL), and
dried under vacuo to obtain Fe3O4@SiO2-Lʹ NPs.
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2]
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[
[
3]
4]
Synthesis of Fe3O4@SiO2-1ʹ
A mixture of Fe3O4@SiO2-Lʹ (1.0 g) NPs dispersed in dry DMF (10 mL)
and PdCl2 (0.089 g, 0.5 mmol) was stirred at 90 °C for 12 h under
nitrogen atmosphere. The Fe3O4@SiO2-1ʹ formed and isolated by using
[
[
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European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
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This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201800209
FULL PAPER
Entry for the Table of Contents
Dr. Kamal Nayan Sharma,* Mr. Naveen
Satrawala, and Dr. Raj Kumar Joshi*
Page No. - Page No.
Thioether-NHC Ligated Pd(II)
Complex in Crafting of Filtration-Free
Magnetically Retrievable Catalyst for
Suzuki-Miyaura Coupling in Water
Text for Table of Contents
2
A new benzimidazolium chloride (L), precursor of novel thioether functionalized NHC, and complex [Pd(L−HCl)Cl ] (1) were
synthesized. Structure of 1 was established with X-ray crystallography. 1 was found an efficient pre-catalyst for Suzuki-Miyaura
coupling of aryl/hetero-aryl bromides (Yield up to 94% in 2 h) at rt in water. To avoid the deactivation of catalyst in reuse, 1 was
3 4 2 3 4 2
immobilized on Fe O @SiO core/shell NPs to develop heterogeneous magnetically retrievable catalyst (Fe O @SiO -1'). The
magnetic nano-support and novel thioether-NHC make the catalyst easily separable and reusable up to 7 cycles.
1
0
This article is protected by copyright. All rights reserved.