Magnetically retrievable silica-based nickel nanocatalyst for Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1039/c4cy01736f

A new magnetically recoverable silica-based nickel nanocatalyst was synthesized, characterized and applied for the first time as a catalyst in Suzuki-Miyaura cross-coupling reaction. Excellent catalytic activity, ease of recovery and reusability up to six cycles without appreciable loss of performance make the present protocol beneficial from industrial and environmental viewpoints.

Full text of DOI:10.1039/c4cy01736f

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Magnetically retrievable silica based Nickel nanocatalyst for Suzuki-
Miyaura Cross coupling reaction
a
a
a
a
b
Rakesh Kumar Sharma,* Manavi Yadav, Rashmi Gaur, Yukti Monga and Alok Adholeya
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A new magnetically recoverable silica based nickel nanocatalyst was synthesized, characterized and
applied for the first time as a catalyst in SuzukiꢀMiyaura cross coupling reaction. Excellent catalytic
activity, ease of recovery and reusability upto six cycles without appreciable loss of performance, makes
the present protocol beneficial from industrial and environmental viewpoint.
catalyst. To circumvent the aforementioned drawbacks,
45 heterogenization of the existing homogeneous catalysts has
contributed greatly as an attractive solution, as the heterogeneous
catalytic systems can be efficiently reꢀused whilst keeping the
inherent activity of the catalytic centre.
To date, various attempts have been made for the development of
1
0
1. Introduction
Over the last few decades the quest for the most economic
ways in the formation of CꢀC bond has become a matter of
increasing importance among the industrial and academic
10
1
research units. This paradigm construction of biaryl moiety
11
5
5
6
6
7
7
0
5
0
5
0
5
Niꢀ based heterogeneous catalyst for Suzuki reaction. In 2000,
1
2
2
3
3
4
5
0
5
0
5
0
enables facile preparation of numerous structurally diverse and
complex molecules essential for the development of modern
pharmaceuticals, agrochemicals and organic materials. Owing to
its significant applications, transition metalꢀcatalyzed crossꢀ
coupling reactions have witnessed tremendous advances for
selective CꢀC bond formation. Among them, palladium (Pd) has
been the first choice and is still one of the most frequently
investigated metals for crossꢀcoupling reactions. However, due
to its robust demand and limited resources, dispersion of Pd from
the environment has raised a key concern regarding its
availability in near future. Therefore, there is an urgent need to
Lipshutz et al. reported Ni immobilized on charcoal as a
heterogeneous catalyst for Suzuki reaction. Later, they found
enhanced efficiency for Suzuki, when Ni was anchored on
graphite. Further, Wang et al. reported Niꢀmetal colloid using
11a
2
11b
11c
TBAB as
a
stabilizer for crossꢀcoupling reactions.
3
Unfortunately, these methodologies suffered from several
drawbacks such as harsh reaction conditions, high temperature,
longer reaction time, use of toxic reducing agents, and had both
storage and handling difficulties. However, in recent years, there
are some reports on the use of Ni nanoparticles as heterogeneous
catalyst for the Suzuki coupling reaction, but these catalysts faced
numerous limitations including low catalytic efficiency, lack of
4
5
explore other available and costꢀeffective metal catalyst. In this
context, a recent surge of interest is growing up for more
abundant firstꢀrow transitionꢀmetal based catalysts. As
promising solution to this issue, nickel (Ni) catalysis has emerged
12
stability, metal leaching issues and little reusability.
6
a
Recently, magnetic nanoparticles (MNPs) have engrossed
immense attention as sustainable alternatives to conventional
heterogeneous catalytic support. Besides, preserving all the
7
as a viable alternative to Pd catalyzed crossꢀcoupling reactions.
13
Also, being comparatively smaller in size, it has the ability to
undergo the oxidative addition step with aryl halides much more
readily than Pd counterparts. Indeed, as compared to Pd, Niꢀbased
catalysts are more specific and versatile due to their unique
reactivity and catalytic behavior and have been continuously
employed as powerful catalysts for the construction of a wide
variety of C–C, C–N, and C–P bond forming reactions.
Despite the substantial progress made towards the development
of Niꢀbased catalytic systems for various crossꢀcoupling
desirable attributes of both homogeneous and heterogeneous
catalysis, they offer an added advantage of facile separation by
magnetic forces, and paves the way for tedious procedures of
catalyst filtration or centrifugation, after completion of the
reaction. However, they have a tendency to aggregate, which is
eliminated by silica coating. Apart from screening the dipolar
attraction between MNPs, silica shell also imparts various
desirable properties to it such as, excellent thermal stability,
8
14
chemical inertness and ease of succedent functionalization.
9
reactions, a common feature involves homogeneous catalysis.
As a part of our ongoing research work in the development of
Arguably the greatest barrier to the wider adoption of
homogeneous catalysis is the difficulty in separation of the metal
from the product stream and the ability to recover and reuse the
15
green and sustainable protocols. We herein describe a first
report wherein magnificent properties of silica are incorporated
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with the magnetic properties of nanoparticles and catalytic
properties of nickel for performing SuzukiꢀMiyaura crossꢀ
coupling reaction. This system is of interest as it possesses
excellent stability, good activity, and is more costꢀeffective than
other previously reported catalysts.
tape. Sample was then dried using pressurized air and sputterꢀ
coated with a 3 nm AuꢀPd layer using mini sputter coater SC7620
of Qourum technologies under vacuum and finally the sample
was subjected to scanning electron microscope.
40 Magnetization measurements M (T, H) were performed by using
EVꢀ9, Microsense, ADE vibrating sample magnetometer. The
Fourier transform infrared spectra (FTꢀIR) of nanoparticles were
collected using PerkinꢀElmer Spectrum 2000. Digestions were
performed in Anton Paar multiwave 3000 microwave reaction
5
2
. Experimental
2
.1 General
4
5
5
5
0
5
system equipped with temperature and pressure sensor. The
amount of nickel in the catalyst and in the supernatant was
estimated by Atomic absorption spectroscopy (AAS) on Analytik
Jenas ZEEnit700P Atomic Absorption Spectrometer using an
acetylene flame. The derived products were analyzed and
confirmed by using Agilent gas chromatography (6850 GC) with
a HPꢀ5MS 5% phenyl methyl siloxane capillary column (30.0 m
TEOS (tetraethoxyorthosilicate) was procured from Sigma
Aldrich and 3ꢀAminopropylꢀtriethoxysilane (APTES) was
obtained from Fluka. Ferric sulphate and ferrous sulphate were
purchased from Sisco Research Laboratory (SRL). Nickel (II)
chloride hexahydrate, Ethanol, 1, 4ꢀDioxane and Ethyl acetate
EtOAc) were purchased from Merck. Double deionized water
was used throughout the study. All the other reagents used were
of analytical grade and commercially available.
10
15
20
25
30
35
(
×
0.25 mm × 0.25 mm) and a quadrupole mass filter equipped
5
975 mass selective detector (MSD) using helium as a carrier
gas. XPS studies were also performed to determine the oxidation
state of Nickel present in the synthesized catalyst.
The catalyst was analyzed through different techniques, which
ensured a complete composition, morphology, functionalization,
magnetization and size characterization. Xꢀray diffraction (XRD)
was performed using a D8 Discover Bruker AXS, (Karlsruhe,
Bundesland, Germany) diffractometer equipped with Cu/Kα
radiation at a scanning rate of 4°/min in the 2θ range of 5–80° (λ
2
.2 Catalyst preparation
The first step towards the synthesis of desired catalyst is the
preparation of magnetic nanoparticles by coꢀprecipitation
=
0.15406 nm, 40 kV, 40 mA). The morphology and uniformity
16
6
6
7
0
5
0
technique (Scheme 1). Considering the aggregation tendency of
naked magnetic nanoparticles (MNPs), they were further coated
with silica (SMNPs), which not only solves the problem of
aggregation but also provides suitable sites for surface
of nanoparticles were investigated by FEI TECHNAI G2 T20,
Transmission Electron Microscope operated at 200 kV by
dispersing sample on a copper grid coated with an amorphous
carbon film and the “ImageJ” software was used for image
processing and analysis.
The chemical composition of the particles was determined by Xꢀ
ray energyꢀdisperse spectroscopy (EDS) by using an Ametek
EDAX system. SEM was performed using Carl Zeiss India,
Scanning Electron Microscope for the characterization of
structural properties of prepared nanocomposites. Sample
preparation was done by dispersing nanoparticles in ethanol and
drop casting the sample on metal stub covered with an aluminium
functionalization. Finally, NH linker (APTES) was covalently
2
coupled to the hydroxyl group present on the surface of the
magnetic silica nanoparticles for the grafting of other substrates.
APTES functionalized silica encapsulated magnetic nanoparticles
(ASMNPs) were further reacted with ligand, thiopheneꢀ2
carboxaldehyde (TC) and the resulting NPs were metallated with
nickel chloride hexahydrate (NiCl .6H O) to produce the final
2
2
catalyst (NiꢀTC@ASMNPs).
Scheme 1 A schematic illustration for the formation of NiꢀTC@ASMNP nanocatalyst
2
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2
.2.1 Synthesis of APTES functionalized silica coated
magnetic nanoparticles
For this purpose, 6.0 g of Fe (SO ) and 4.2 g of FeSO was
2
4 3
4
o
5
0
5
dissolved in 250 mL water and was stirred at 60 C to give a
yellowishꢀorange solution. To this, 15 mL of 25% NH OH
4
solution was added with vigorous stirring and colour of the bulk
solution turned black. Stirring was continued for another half an
hour and the precipitated MNPs were separated using external
magnet and washed several times with deionized water and
ethanol. Silica coating over these MNPs was achieved via solꢀgel
5
0
Scheme 2 NiꢀTC@ASMNP nanocatalyst catalyzed SuzukiꢀMiyaura
crossꢀcoupling reaction
1
3
. Results and Discussion
15b
approach. Dispersed solution of activated 5.0 g of MNPs with
.1 M HCl (2.2 mL) in 200 ml ethanol and 50 mL water was
0
3.1 Characterizations
obtained under sonication. Then, 5 mL of 25% NH OH solution
4
55
3.1.1 Fourier-transform Infrared Spectroscopy
1
was added to the suspension at room temperature followed by the
Due to the superparamagnetic nature of magnetic nanoparticle,
NMR technique could not be used for the confirmation of its
surface modification. Instead, Fourier transform Infrared
spectroscopy was employed to demonstrate the functionalization
addition of 1mL of TEOS and the solution was kept for stirring at
o
6
0 C for 6 h. The obtained SMNPs were magnetically separated,
washed with ethanol and dried under vacuum. The obtained
SMNPs were further functionalized using APTES to afford
ASMNPs. This was done by adding APTES to the dispersed
solution of 0.1 g SMNP in 100 mL ethanol, under sonication and
−1
6
6
7
7
8
8
0
5
0
5
0
5
of the synthesized composites in the range of 4000–400 cm .
2
0
−1
Fig. 1a exhibits the characteristic band at 586 cm due to the
vibration of the Fe–O bond in the tetrahedral site of bare
magnetic nanoparticles and splitting corresponds to the Fe–O
o
the resulting mixture was stirred at 50 C for 6 h.
17a
−1
bond of bulk magnetite.
The broad band at 3367 cm is
2
5
2.2.2 Synthesis of Ni-TC@ASMNP
assigned to the O–H stretching vibration arising from adsorbed
water. The coating of silane matrix onto the surface of MNPs was
For covalent grafting of the ligand on ASMNPs, 1g of ASMNP
was refluxed with TC in dried methanol along with molecular
ꢀ1
ꢀ1
confirmed by two new bands at 1088 cm and 802 cm which
are ascribed to the symmetrical and asymmetrical vibrations of
o
17b
sieves at 70 C for 3 h. The obtained product was washed with
the Si–O–Si bonds (Fig. 1b). Also, the stretching absorption of
3
0
methanol and dried under vacuum. Finally, 1 g of grafted
TC@ASMNPs was stirred with a solution of 4 mmol of
ꢀ1
ꢀ1
Fe–O at 586 cm shifts to high wave number of 614 cm after
coating with silica. Moreover, a significant reduction in the
spectral intensity was observed, while moving from MNPs to
SMNPs. Thus, it was clearly concluded that MNP was
successfully coated with a silica layer. In the spectra of ASMNPs
NiCl .6H 0 in methanol for 3 h. The resulted NiꢀTC@ASMNPs
2
2
were separated magnetically and thoroughly washed with
deionized water and dried under vacuum.
3
5
(Fig. 1c), the appearance of the two broad bands at 3422 and
2
.3 Reaction procedure for Suzuki-Miyaura cross coupling
ꢀ1
1
633 cm were due to the N–H stretching and NH bending
2
reaction
17c
vibrations of free NH2 group, respectively.
After Schiff’s
condensation (Fig. 1d), a characteristic band of the imine group
1
5 mg of NiꢀTC@ASMNP catalyst was taken in an oven dried
−1 17d
(
C=N) appears at 1651 cm , which confirms that thiopheneꢀ2ꢀ
4
0
round bottom flask, and to this, PPh (20 mol%), aryl halide (0.5
3
carboxaldehyde is chemically bonded to the surface of
functionalized silica. Furthermore, on metallation with Nickel
chloride, it is observed that the prominent absorption peak of
mmol) and phenyboronic acid (0.6 mmol) were added. After this
K PO (0.75 mmol) was added, followed by the addition of 1 mL
3
4
dioxane. The reaction mixture was kept under N atmosphere and
2
−1
−1
C=N at 1651 cm is shifted to a lower wave number 1643 cm
o
was stirred at 100 C till the completion of the reaction (Scheme
in Fig. 1e, indicating strong metalꢀligand interaction (See ESIꢀ
S1).
4
5
2). The catalyst was recovered with permanent magnet. Reaction
was monitored using thin layer chromatography and the products
were extracted using ethyl acetate, dried over sodium sulphate,
concentrated under reduced pressure, and analyzed by GCꢀMS.
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40
Fig. 2 XRD pattern of (a) MNPs and (b) SMNPs
.1.3 SEM analysis
3
4
5
Fig. 3a and b represents the SEM images of bare magnetic
nanoparticles and silica encapsulated magnetic nanoparticles,
respectively. The smooth surface of magnetic core particles gets
spongy due to uniform silica coating around the surface of MNPs.
No separate aggregates of silica were found, thereby indicating
that the silica coating has occurred only along the surface of
MNPs. Fig. 3c shows the SEM image of the final catalyst Niꢀ
TC@ASMNPs, which further confirms that the surface
modification procedures did not affect the morphology of the
magnetite cores. Fig 3d displays the SEM image of recovered
catalyst after the completion of reaction which indicates that the
shape and morphology of the catalyst remains unchanged.
Fig. 1 FTꢀIR spectra of (a) MNP, (b) SMNP, (c) ASMNP, (d)
TC@ASMNP and (e) NiꢀTC@ASMNP
3
.1.2 X-Ray Diffraction studies
5
0
5
0
5
0
5
50
In order to investigate phase, purity and crystalline nature of
synthesized nanoparticles (MNP, SMNP, NiꢀTC@ASMNP) and
confirm their structural modification, powder Xꢀray diffraction
(XRD) studies were conducted. XRD of the bare MNPs in Fig.
1
1
2
2
3
3
2a displayed patterns consistent with standard XRD data of the
Joint Committee on Powder Diffraction Standards (JCPDS) card
number (19ꢀ0629) for Fe O crystal. All the observed diffraction
55
3
4
peaks were indexed to the inverse cubic spinel structure of Fe O ,
3
4
revealing the high phase purity and high crystallinity of the Fe O4
3
nanocore. As depicted from the diffractogram, the patterns
o
o
o
(a)
indicate a crystallized structure at 2θ: 16.181 , 30.366 , 35.663 ,
4
o
o
o
3.024 , 57.299 , and 62.865 , which are assigned to the (111),
(
2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) crystallographic faces
18a
of magnetite. The Debye–Scherrer equation (Dhkl 1/4kl/β cos
θ) was used to estimate the mean crystallite size from the XRD
patterns, where D is the size of the axis parallel to the (hkl) plane,
k is a constant with a typical value of 0.89 for spherical particles,
l is the wavelength of radiation, β is the full width at half
maximum (FWHM) in radians, and θ is the position of the
diffraction peak maximum. Here, the average crystallite size of
Fe O nanoparticles was calculated to be 11.4 nm by measuring
3
4
the (3 1 1) peak widths of the XRD lines. Same peaks were
observed in the silicaꢀcoated nanoparticle XRD patterns in Fig.
2
b, indicating retention of the crystalline spinel ferrite core
structure during the silicaꢀcoating process. A weak broad hump
appeared in the spectra of SMNP from 2θ = 20◦ to 30◦ which was
consistent with the formation of amorphous silica phase around
the magnetic core, owing to very small crystallite size (See ESIꢀ
(
c)
1
8b
S2). Other than this, no any substantial variation was observed
in the rest of the XRD pattern of SMNP, revealing the
preservation of the original crystallographic structural
characteristic of magnetite (Fe O ) even after the extreme
3
4
chemical and physical conditions created during the silica coating
process.
4
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Fig. 3 SEM images of (a) MNPs, (b) SMNPs, (c) NiꢀTC@ASMNPs and
d) Recovered NiꢀTC@ASMNPs
(
3
.1.4 Metal content determination and Oxidation state of
5
0
5
Metal in catalyst
(
a)
Energy dispersive Xꢀray Spectroscopy (EDS) displayed the
elemental composition after the grafting reaction, verifying the
presence of Ni, Fe and Si in the nanocatalyst (Fig. 4).The
quantitative determination of the metal content was obtained by
AAS analysis and was found to be 0.208 mmolg .
In the XPS spectrum of the NiꢀTC@ASMNPs, peaks were
observed at higher binding energy (855.98 eV and 855.71 eV)
1
ꢀ
1
2+
19
confirming Ni ion before and after catalysis.
S9)
(See ESIꢀS8,
1
(
e)
(
c)
Fig. 4 EDS pattern of NiꢀTC@ASMNPs
3.1.5 TEM analysis
2
2
3
3
4
0
5
0
5
0
The size and shape of synthesized nanoparticles were deduced
from the TEM images. TEM micrographs provide clearer picture
of the particle size and morphology of nanocomposites. The
uncoated MNPs show slight agglomeration (Fig. 5a), which
exists due to the absence of surfactants and the lack of any
20
repulsive forces between the magnetic nanoparticles. Highly
crystalline nature of nanocomposites is represented by the white
spots and array of bright diffraction rings in the SAED pattern of
nanoparticles (Fig. 5b). From HRTEM image (Fig. 5c), the
average interplanar distance of MNPs was measured to be ∼0.25
nm, which corresponds to (3 1 1) plane of inverse spinel
structured Fe O . Moreover, Fig. 5d showed that the silica
4
5
3
4
Fig. 5 TEM micrographs obtained at various stages of nanoparticle
synthesis: (a) MNPs, (b) SAED pattern of MNPs, (c) HRꢀTEM image of
MNP, (d) SMNPs and (e) inset: histogram illustrating size distribution of
MNPs
coating was almost homogeneous and a closer examination of the
image reveals that the silicaꢀcoated magnetic nanoparticles
display dark MNP cores due to their high density, surrounded by
a lighter amorphous silica shell of about 4ꢀ5 nm thick. Fig. 5e
depicts the size distribution histogram of about 100 colloidal
aggregates of MNPs with maximum particle size in the range of
10ꢀ12 nm (See ESIꢀS3). The size estimated from TEM
micrographs agrees well with the crystallite size estimated from
XRD line profile fitting. TEM analysis of the nanoparticles
indicated that both pure MNPs and SMNPs were quasi spherical
in shape.
50
3.1.6 VSM
The magnetization measurements of bare MNPs, SMNPs,
ASMNPs and NiꢀTC@ASMNPs were investigated at room
temperature (298K) using vibration sample magnetometer (VSM)
with the field sweeping from −20, 000 to 20,000 Oe. Fig. 6 shows
the magnetization curves of MNP, SMNP, ASMNP and Niꢀ
TC@ASMNP. At the same field, the saturation magnetization
value (Ms) for MNPs was found to be 58 emu/g, which is less
55
21a
than its bulk counterpart (90 emu/g). This decrease can be
attributed to the nanosize of the obtained MNPs as the size of
magnetic particles is directly proportional to its magnetization
value. For SMNPs, ASMNPs and NiꢀTC@ASMNPs, the
60
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magnetization value gets further reduced to 42, 26 and 15 emu/g
(Fig. 8). Lower temperature resulted in prolonged reaction time
respectively, due to the contribution of the nonꢀmagnetic silica 45 for the complete conversion into the desired product and the yield
shell and functionalized groups on the surface of MNPs. Even
though the Ms values of MNPs have evidently decreased, they
could still be efficiently removed from solution media with a
was lower. Besides this, a control experiment without catalyst
was conducted at 100 C and no product was observed even after
24 h of reaction. Similarly, the use of Ni@ASMNPs and 20
o
5
21b
permanent magnet. The enlarged magnetization curve (Fig. 6e)
mol% PPh resulted in trace yield after 24 h. We also performed
3
for all three nanoparticles showed typical characteristic features 50 the reaction without phosphine as additive but obtained no
of superparamagnetic behaviour i.e., zero coercivity and
negligible remanence, by passing through the origin on MꢀH
hysteresis. This also implies that magnetic materials can be
aligned only under magnetic field and will not retain their
product. Therefore, the addition of phosphine was essential, and
PPh gave desirable results. Finally, we performed the reaction
3
10
with the synthesized catalyst NiꢀTC@ASMNP with 20 mol%
PPh and found higher yield. This clearly indicated that, though
3
residual magnetism once the field is removed, thereby making 55 the reaction is catalysed by metal, but, the role of ligand is
magnetic nanoparticles good candidate for catalytic support.
equally important to carry out the transformation. In this case, the
S, Nꢀbidentate ligand with the strong σ donation acted as a coꢀ
catalyst and was effectively bound to the metal. The effect of
catalyst loading was also investigated by employing different
quantities of the catalyst (Table 1, entries 3-7). A significant
increase in the percentage yield of the product was observed
when the amount of catalyst was increased from 5 to 10 mg. This
60
4
g
suggests that the reaction rate depends on the Ni concentration.
Since the role of transition metal in the coupling reaction is to
provide an active site for the reaction to take place, rate of the
reaction gets enhanced as number of active sites increases with
the amount of the catalyst. The best result was obtained with 15
mg of the catalyst.
65
10
15
Fig. 6 Magnetization curves obtained by VSM at room temperature for (a)
MNPs, (b) SMNPs, (c) ASMNPs, (d) NiꢀTC@ASMNPs and (e) inset:
enlarged image near the coercive field.
3
.2 Catalytic studies
20
25
30
35
40
To investigate the catalytic efficacy of NiꢀTC@ASMNPs and to
determine optimal experimental conditions for SuzukiꢀMiyaura
crossꢀcoupling
reaction,
4ꢀbromoacetophenone
and
phenylboronic acid were chosen as model substrates and were
treated under different temperature conditions, using various
solvents and bases. To gain a better insight into the nature of
prepared nickel nanocatalyst and its best source, various
experiments were conducted without ligand, without PPh and
3
70
Fig. 7 Effect of temperature on model reaction for SuzukiꢀMiyaura crossꢀ
with different amount of nanocatalyst.
coupling reaction (Reaction conditions: 4ꢀbromoacetophenone (0.5
mmol), phenylboronic acid (0.6 mmol), dioxane (1 mL), K
3 4
PO (0.75
mmol), catalyst (15 mg), PPh (20 mol%), time (10 h)
3
3.2.1 Catalytic activity of Ni-TC@ASMNP catalyst in Suzuki-
Miyaura cross coupling reaction
As a starting point for optimizing SuzukiꢀMiyaura cross coupling
reaction, the impact of various bases on the efficiency of this
procedure was studied since its nature is known to be crucial in
this type of coupling reaction. The highest yield was obtained
when the reaction was carried out using K PO as a base.
3
4
t
However, when stronger alkalies (KO Bu or NaOH) were used,
product was obtained in traces. Cs CO and K CO were also
2
3
2
3
employed, and in both the cases low yields were obtained. Under
o
various conditions, 100 C was optimum temperature (Fig. 7) and
1
, 4ꢀdioxane was found to be the most effective solvent while the
use of toluene, DMF and DMSO were inefficient for the reaction
6
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*
Fig. 8 Effect of bases and solvents on the catalytic efficiency of Niꢀ
Reaction performed without PPh
3
TC@ASMNP in SuzukiꢀMiyaura crossꢀcoupling reaction (Reaction
conditions: 4ꢀbromoacetophenone (0.5 mmol), phenylboronic acid (0.6
mmol), solvent (1 mL), base (0.75 mmol), NiꢀTC@ASMNP (15 mg), PPh
1
2
2
5
0
5
To investigate the generality of this crossꢀcoupling reaction, we
studied the reaction of phenylboronic acid with the range of aryl
halides (Cl, Br, I) under the present optimized conditions.
Regardless of their electronic characters, phenylboronic acid
coupled smoothly with aryl bromides and iodides bearing both
electronꢀdeficient and electronꢀrich substituents, to afford the
corresponding products in good to excellent yields with high
turnover numbers (TON). It was also observed that the yield was
lower in case of orthoꢀ substituted aryl halides than those
obtained with paraꢀsubstituted ones, which might be due to steric
factors. While aryl chlorides showed lower yields, which can be
correlated with their poor reactivity toward oxidative addition in
the catalytic cycle. In order to further explore the scope of the
synthesized Nickel catalyst, we performed reaction of phenyl
3
5
(20 mol%), time (10 h)
Table 1 Screening of Nickel catalyst for SuzukiꢀMiyaura crossꢀcoupling
a
reaction
b
Entry
Catalyst
Phosphine
Time
h)
Yield (%)
(
1
.
c
No
PPh
PPh
3
3
24
ꢀ
2
.
Ni@ASMNPs
24
Trace
(
20mg)
NiꢀTC@ASMNP
5 mg)
NiꢀTC@ASMNP
10 mg)
NiꢀTC@ASMNP
15 mg)
NiꢀTC@ASMNP
20 mg)
NiꢀTC@ASMNP
15 mg)
d
e
3
.
PPh
PPh
PPh
PPh
ꢀ
3
10
10
10
10
10
Trace
52
(
4
.
3
3
3
(
f
5
.
94
(
g
6
7
.
93
30 boronic acid with aryl tosylates, mesylate and heteroaryl halides.
To our delight, all the substrates gave good to excellent yields for
SuzukiꢀMiyaura cross coupling reaction. In case of tosylates and
mesylate, excellent yields were obtained. It is also noteworthy
that in case of heteroaryl halides, yields were found to be slightly
35 lower. All reactions were performed on the same scale (0.5
mmol) and isolated yields of the products have been summarized
(
*
.
ꢀ
(
a
Reaction conditions: 4ꢀbromoacetophenone (0.5 mmol), phenylboronic
acid (0.6 mmol), dioxane (1 mL), K PO (0.75 mmol), PPh (20 mol%),
3
4
3
1
0
time (10 h)
Yield was determined by GCꢀMS
b
in
Table
2.
Amount of NiꢀTC@ASMNP: cꢀ20 mg, dꢀ5 mg, eꢀ10 mg, fꢀ15 mg, gꢀ20
mg
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Catalysis
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DOI: 10.1039/C4CY01736F
Science & Technology
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
a
Table 2. Suzukiꢀ Miyaura crossꢀcoupling reaction of phenyl boronic acid (1) with aryl halides (2) as per scheme 2
b
c
Entry
Boronic Acid
Aryl
Product
Time (h)
Yield (%)
TON
298
Halides/Tosylate/Mesylate
1
10
93
94
2
3
4
10
10
10
301
301
272
94
85
5
6
10
10
95
91
304
291
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7
11
87
278
Me
3g
8
9
11
10
10
12
12
87
92
98
83
86
278
294
314
266
275
1
0
1
2
1
1
1
3
13
77
246
1
1
4
5
10
15
75
78
240
250
Br
N
2
n
N
3i
N
3j
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1
1
6
7
10
10
82
263
N
3j
97
310
1
1
8
9
10
10
89
81
285
259
a
Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid (0.6 mmol), dioxane (1mL), K
3
PO
4
(0.75 mmol), NiꢀTC@ASMNP (15 mg), PPh
3
(20
mol%),
b
Yield was determined by GCꢀMS
c
TON= Calculated using the 0.208 mmol/g Nickel (Obtained by AAS for NiꢀTC@ASMNP)
5
Also, the present catalytic system was compared with previously 10 reaction time, stability, selectivity, turn over number, recovery
reported catalysts (Table 3), and observed that our organic–
inorganic hybrid catalytic system (NiꢀTC@ASMNP) is much
better than the literature precedents in terms of yield, cost,
and reusability. The high catalytic activity could be attributed to
the high dispersion of nikel active sites onto the outer surface of
the
TC@ASMNP.
15
Table 3. Comparison of NiꢀTC@ASMNP with literature precedents using Ni based homogeneous and heterogeneous catalysts for SuzukiꢀMiyaura cross
coupling reaction
S.No
Boronic Acid
Aryl halide
Catalyst
Reaction conditions Yield (%) Ref.
1
Ni(II) mounted on
graphite (Ni/Cg)
K PO , PPh , LiBr,
87
10b
3
4
3
dioxane,
o
1
80ꢀ200 C, 9h
2
Magnetic Fe–Ni
Alloy
NaOH, PCy dioxane,
3,
95
11a
o
1
20 C, 12h
1
0
|
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DOI: 10.1039/C4CY01736F
o
3
4
Monodisperse Ni and
NiO nanoparticles
DMSO, 135 C
98
11c
Cl
Nickel in charcoal
Ni/C)
KF, PPh , LiOH, MW,
3
91
10e
(
o
1
80 C, 30 min
NC
5
6
Ni(II)/Cg
PPh , K PO , dioxane,
LiBr, 180 C, 10 h
92
10d
3
3
4
o
HPMC stabilized Ni
Nanoparticles
Ethylene Glycol,
99
95
11b
7g
o
Cs CO , 100 C, 20 h
2
3
o
7
*
*
BrCF COOEt
Ni(NO ) .6H O
dioxane, bpy, 60 C,
2
3 2
2
2
4 h
8
Ni(II) σꢀaryl complex PPh , K CO , toluene,
95
80
20a
9b
3
2
3
o
1
10 C, 18 h
9
BisꢀNHC pincer Ni
complex
K PO , PPh , dioxane,
3 4 3
o
1
00 C, 24 h
1
1
0
1
BisꢀNHC pincer Ni
complex
K PO , PPh , dioxane,
77
98
9b
3
4
3
o
1
00 C, 24 h
NiꢀTC@ASMNP
NiꢀTC@ASMNP
K PO , PPh , dioxane,
Present
work
3
4
3
o
1
00 C, 10 h
1
2
K PO , PPh , dioxane,
94
Present
work
3
4
3
o
1
00 C, 10 h
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*
Homogeneous catalyst
3
.2.2 Catalytic Stability & Reusability
A long life span and the ability to easily recover are highly
desirable features of a catalyst for its industrial applications. In
this regard, the recyclability of NiꢀTC@ASMNP was investigated
by using 4ꢀIodobenzene and phenylboronic acid as model
substrates. The catalyst was smoothly separated using external
magnet after the completion of the reaction and was washed with
ethanol, dried under vacuum and reused in subsequent reactions.
Nearly, quantitative catalyst could be well retrieved from each
run. In a test of six cycles, the catalyst could be recovered without
any substantial loss of catalytic activity (Fig. 9). The recovered
catalyst after six runs had no change in composition, attributing
to EDS in comparison to the fresh one. Moreover, no
morphological change was observed in TEM (See ESIꢀS4, S6)
and SEM micrographs of the recovered catalyst. Furthermore,
magnetism exhibited by the used nanocatalyst is sufficiently good
for its effective separation from the reaction mixture via an
external magnet (Fig. 10).
5
0
5
0
1
1
2
25
Fig.10 Magnetization curve obtained by VSM at room temperature for reꢀ
used NiꢀTC@ASMNPs
3
.2.3 Split test
3
0
While using a heterogeneous catalyst, there is a possibility of
migration of the active species from solid support to the liquid
phase, as the leached species become more liable for catalytic
activity. Thus, to explore the catalyst leaching, a hotꢀfiltration or
split test was performed for desired reaction. After completion of
the reaction, the solid catalyst was separated through an external
magnet and the filtrate was analyzed through an atomic
absorption spectrometer which showed negligible leaching.
Again the standard reaction was conducted in the presence of the
catalyst for 6 hours (70% conversion by GC), and then the
catalyst was removed using a magnet and the reaction was carried
further. No coupling product was detected even up to 10 hours in
the reaction mixture under the same condition, thereby indicating
truly heterogeneous nature of the magnetic nanocatalyst.
1
00
8
0
6
0
4
0
35
2
0
0
1
st
2
nd
3
rd
No. of runs
4
th
5
th
6th
4
4
5
0
5
0
Fig. 9 Catalyst recycling test for successive six runs of SuzukiꢀMiyaura
cross coupling reaction
3.3 Proposed catalytic mechanism
The probable mechanism for NiꢀTC@ASMNP catalyzed Suzukiꢀ
Miyaura cross coupling is illustrated in Fig. 11, which involves
three sequential steps: oxidative addition, transmetalation and
8a
reductive elimination. The reaction starts with the in situ
generation of active Ni(0) precatalyst prior to main catalytic cycle
22
(
Fig. 11a). We assume that the role of additional PPh is to
3
stabilize the active catalyst. Further, this Ni(0) species undergoes
oxidative addition with aryl halide to yield Ni(II) σꢀaryl complex
(Fig. 11b) which contains a weakly coordinated halide ion. The
transmetalation of the aryl group of the activated arylboronic acid
55
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2
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to Ni(II) σꢀaryl complex gives Ni(II) diaryl complex (Fig. 11c).
Finally, reductive elimination of complex facilitates the catalytic
cycle by regenerating the active Ni(0) species while the cross
coupled product is produced (Fig. 11d).
5
Fig. 11 Proposed catalytic mechanism for NiꢀTC@ASMNP catalyzed SuzukiꢀMiyaura crossꢀcoupling reaction
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Conclusions
35
1
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Angew. Chem., 1986, 98, 504; (c) J. K. Stille, Angew. Chem. Int. Ed.
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D. R. Stuart, K. Fagnou, Science., 2007, 316, 1172.
1
1
2
2
0
5
0
5
In conclusion, we have developed a highly selective, economical,
and efficient silica based organic–inorganic hybrid Nickel
nanocatalyst for Suzuki crossꢀcoupling reaction. The targeted
coupling reactions were achieved, under relatively mild
experimental conditions and in reasonably high yield. Moreover,
the catalyst is stable, shows negligible leaching and maintains
high catalytic activity over six cycles. Followꢀup studies directed
at expanding the scope of this catalyst are underway.
Additionally, facile synthetic procedure, broad substrate scope,
short reaction time, high yield, effortless magnetic recovery and
reusability for the reaction make it a favourable protocol from
environmental and industrial view point.
40
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Acknowledgements
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Green Chemistry Network Centre, Department of Chemistry, University
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of Delhi, Delhiꢀ 110007, India. Fax: +91ꢀ011ꢀ27666250; Tel: 011ꢀ
27666250, Eꢀmail: rksharmagreenchem@hotmail.com
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