Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient, Efficient and Recyclable Catalyst for Suzuki–Miyaura Cross-Coupling Reactions

Catal Lett

DOI 10.1007/s10562-017-1987-7

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-

Palladium(II): A Convenient, Eﬃcient and Recyclable Catalyst

for Suzuki–Miyaura Cross-Coupling Reactions

K. Vishal¹· Bradley D. Fahlman²· B. S. Sasidhar³· Shivaputra A. Patil⁴·

Siddappa A. Patil¹

Received: 27 November 2016 / Accepted: 21 January 2017

Abstract A new magnetic nanoparticle-supported N-het-

erocyclic carbene-palladium(II) nanomagnetic catalyst was

synthesized and appropriately characterized using attenu-

ated total reﬂectance infrared spectroscopy (ATR-IR),

ultraviolet–visible spectroscopy (UV–Visible), inductively

coupled plasma-atomic emission spectroscopy (ICP-AES),

energy-dispersive X-ray spectroscopy (EDS), ﬁeld-emis-

sion scanning electron microscopy (FESEM), transmis-

sion electron microscopy (TEM), X-ray powder diﬀraction

(XRD), thermogravimetric analysis (TGA) and Brunauer–

Emmett–Teller surface area analysis (BET). The nanomag-

netic catalyst was used as convenient and eﬃcient catalyst

for the Suzuki–Miyaura cross-coupling reaction of various

aryl bromides/chlorides/iodide with phenylboronic acid.

The eﬀects of varying solvents, bases, temperature, time

and catalytic ratios on the performance of the Suzuki–

Miyaura cross-coupling reaction were investigated. The

notable advantages of this heterogeneous nanomagnetic

catalyst are excellent yields, mild reaction conditions, short

reaction times and easy work-up. Moreover, the new nano-

magnetic catalyst could be easily recovered with an exter-

nal magnet and could be reused at least ﬁve times without

loss of its catalytic activity.

* Siddappa A. Patil

p.siddappa@jainuniversity.ac.in

1

Centre for Nano and Material Sciences, Jain Global Campus,

Jain University, Bangalore, Karnataka 562112, India

2

Department of Chemistry and Biochemistry & Science

of Advanced Materials, Central Michigan University,

Mount Pleasant, MI 48859, USA

3

Organic Chemistry Section, National Institute

for Interdisciplinary Science and Technology (CSIR),

Trivandrum 695019, India

4

Pharmaceutical Sciences Department, College of Pharmacy,

Rosalind Franklin University of Medicine and Science, 3333

Green Bay Road, North Chicago, IL 60064, USA

Vol.:(0123456789)

1 3

K. Vishal et al.

Graphical Abstract

present in same phase as that of reactants. As a result, the

eﬃciency will be less and reaction rate will not be as high

as that using homogenous catalysts [5]. However, a distinct

advantage of heterogeneous catalysts is their easy isolation

after reaction completion. Therefore, there is a need for a

catalyst which can increase the eﬃciency of the reaction

(homogenous-like) and can be isolated easily after reaction

completion (heterogeneous-like).

Keywords Magnetic nanoparticles · N-heterocyclic

carbene-palladium(II) · Nanomagnetic catalyst · Synthesis

and Characterization · Suzuki–Miyaura cross-coupling

reaction

1 Introduction

In 1991, Arduengo isolated N-heterocyclic carbene

(NHC) as the ﬁrst free, stable carbene through deprotona-

tion of 1,3-bisadamantyl imidazolium chloride [6]. Later,

in 1995, Herrmann and co-workers reported t he ﬁrst use

of NHC ligands in catalytic applications [7]. After that,

NHC ligands have attracted much attention as appropri-

ate transition metal ligands. Recently, NHCs have drawn

much attention as substitutes for phosphines because of

higher dissociation energies, less toxicity, stability in air

and moisture, analogous σ-donor and low σ-acceptor abili-

ties, and the for mation of stronger bonds with various tran-

sition metals [8–18]. On the other hand, NHC-metal com-

plexes show high t hermal stabilities, and the electronic and

steric properties of NHCs may be tuned, which is an added

A catalyst plays an important role in chemical transforma-

tions by decreasing the time of reaction, increasing the con-

version of starting materials to products thereby increasing

the yield and reducing waste generation [1]. Two types of

catalysts, homogeneous and heterogeneous, are classiﬁed

based on their physical state in a chemical reaction. Both

homogenous and heterogeneous cat alysts have their own

advantages and disadvantages [2, 3]. An advantage of

homogenous catalysts is their placement in the same phase

as that of reactants; hence, they are more eﬃcient and the

rate of reaction will be high [4]. But for this same reason,

it is diﬃcult to separate homogeneous catalysts from prod-

ucts, lowering their recovery after completion of the reac-

tion. In the case of heterogeneous catalysts, they will not be

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

advantage for the development of novel organometallic

materials [19]. Consequently, NHC-metal complexes have

been used extensively as convenient and eﬃcient homo-

geneous catalysts in numerous organic transformations,

such as carbon–carbon (C–C) and carbon–nitrogen (C–N)

cross-coupling reactions [20–35]. NHC-palladium(II)

complexes have shown very good catalytic activity in the

Suzuki–Miyaura cross-coupling reactions, which have

many applications in the preparation of natural products,

ﬁne chemicals and various intermediates for drugs [36, 37].

Recently, various types of NHC-palladium(II) com-

plexes have been developed as heterogeneous catalysts

for C–C bond formation reactions through immobilizing

palladium(II) on various solid supports [38–42]. But these

heterogeneous catalysts suﬀer from many diﬃculties such

as easy agglomeration, and leaching of palladium(II) from

the catalyst to the reaction mixture. Hence, the design and

synthesis of new catalysts that are not only active and sta-

ble, but also easy to separate and reuse, is extremely neces-

sary to facilitate the C–C bond formation reactions. When

nanoparticles are employed as a catalytic support, it has the

advantage of being inert in most chemical environments,

allowing the immobilized catalyst to behave as it would

in its free state. But the separation of these nanoparticles

requires techniques like nanoﬁltration, centrifugation, or

precipitation-ﬂocculation [43]. To overcome these diﬃ-

culties, magnetite and other superparamagnetic nanopar-

ticles are employed as a catalyst support, which possess

the advantage of high surface area and easy recovery and

recycling of catalyst. In this regard, magnetic separation

oﬀers a convenient and eﬃcient method for C–C cross-

coupling reactions [44]. Also, the synthesis and charac-

terization of NHC-palladium(II) complexes on the surface

of magnetic nanoparticles, with high catalytic activity in

Suzuki–Miyaura cross-coupling reactions, are r are and only

few examples have been reported thus far [45–47].

reused in the next reaction cycle without signiﬁcant loss of

activity.

2 Experimental

2.1 Materials

Unless otherwise stated, all reactions were performed under

aerobic conditions in oven-dried glassware with magnetic

.

stirring. FeCl₃6H₂O, FeCl₂4H₂O, sodium carbonate,

potassium hydroxide, ammonium hydroxide, benzyl chlo-

ride, benzimidazole, 3-chloropropyl triethoxysilane, pal-

ladium acetate, aryl halides and phenylboronic acid were

purchased from Sigma-Aldrich chemical company and

were used without further puriﬁcation. Solvents were dried

and degassed using a JC Meyer company solvent puriﬁca-

tion system. Heating was accomplished by either a heating

mantle or silicone oil bath. Reactions were monitored by

thin-layer chromatography (TLC) performed on 0.25 mm

Merck TLC silica gel plates, using UV light as a visual-

izing agent. Puriﬁcation of reaction products was carried

out by ﬂash column chromatography using silica gel 60

(230–400 mesh). Yields refer to chromatographically pure

material. Concentration in vacuo refers to the removal of

volatile solvent using a rotary evaporator attached to a dry

diaphragm pump (10–15 mm Hg), followed by pumping to

a constant weight with an oil pump (<300 mTorr).

2.2 Characterization

Attenuated total reﬂectance infrared spectra were recorded

with Bruker Alpha Eco-ATR spectrometer. UV–visible

spectrophotometry was carried out using a SHIMADZU

UV-1800 A11454907691 spectrophotometer. Brunauer-

Emmett-Teller surface areas were obtained by physisorption

of N₂using Microtrac BELSORP MAX instrument. The

elemental palladium content of the nanomagnetic catalyst

was determined using a Thermo Electron IRIS INTREPID

II XSP DUO inductively coupled plasma-atomic emis-

sion spectrophotometer. Transmission electron microscope

images were obtained using Jeol/JEM 2100 microscope.

FESEM images along with energy-dispersive X-ray spec-

troscopy to observe morphology and elemental distribu-

tions respectively were obtained using a JEOL Model-

JSM7100F microscope. Thermogravimetric analysis was

carried out using a Perkin-Elmer Diamond TG/DTA, with

a heating rate of 10.0°C/min. Powder X-ray diﬀraction

was carried out using a Bruker AXS D8 Advance diﬀrac-

Since NHCs are stabilized by bulky substituents on the

nitrogen atoms, we have selected benzyl bromide for sub-

stitution on one nitrogen and 3-chloropropyltriethoxysilane

on the other nitrogen. Also, the strong aﬃnity of magnetite

towards silica makes silanes very useful for the function-

alization of magnetite [43]. Herein, we present the synthe-

sis and characterization of a new magnetic nanoparticle-

supported NHC-palladium(II) nanomagnetic catalyst for

Suzuki–Miyaura cross-coupling reactions of various aryl

bromides/chlorides/iodide with phenylboronic acid. The

nanomagnetic catalyst is air- and water-stable, and can be

synthesized in high yield with high purity from inexpensive

commercially available starting materials. Furthermore,

this catalyst is shown to exhibit high catalytic activity in

Suzuki–Miyaura cross-coupling reactions. In addition, the

nanomagnetic catalyst may be easily isolated from the reac-

tion medium by using an external magnet, and can then be

1

tometer. H NMR spectra were recorded at 400 MHz, and

1

are reported relative to CDCl₃(δ7.27). H NMR coupling

constants (J) are reported in Hertz (Hz) and multiplicities

are indicated as follows: s (singlet), d (doublet), t (triplet),

1 3

K. Vishal et al.

m (multiplet). Liquid chromatography mass spectra (LC-

MS) were recorded on Agilent technologies quadrupole

LC-MS system. Elemental analyses of compounds were

done on Euro Vector S.P.A, Euro EA 3000 CHNS elemen-

tal analyzer.

[M+H]⁺. Anal. Calc’d for C₁₄H₁₂N₂: C, 80.74; H, 5.81; N,

13.45%. Found: C, 80.71; H, 5.83; N.13.47%.

2.6 Synthesis of Magnetic Nanoparticle Tethered

1-Benzyl-1H-benzimidazolium chloride (NHC@

MNPs)

2.3 Synthesis of Hydroxyl Substituted Magnetic

Nanoparticles (Fe₃O₄)

Silyl chloride functionalized magnetic nanoparticles (2.0 g)

were suspended in toluene (40 mL) by ultrasonicating for

30 min. 1-Benzyl-1H-benzimidazole (1.0 g, 4.8 mmol) was

added in one portion to the reaction mixture and stirred at

110°C for 48 h. Then, the reaction mixture was cooled to

room temperature. The product was isolated by magnetic

decantation, washed with toluene (5×20 mL) and dried at

80°C for 12 h to yield a brown-coloured solid.

The hydroxyl-substituted magnetic nanoparticles (MNPs)

were synthesized through a chemical co-precipitation

.

method. A mixture of FeCl₃6H₂O (2.35 g, 8.7 mmol) and

.

FeCl₂4H₂O (0.86 g, 4.3 mmol) were dissolved in deion-

ized water (40 mL). The resultant solution was stirred for

30 min at 85°C to completely dissolve both the reactants.

Then, an ammonium hydroxide (5 mL) solution was added

slowly with vigorous stirring at 85°C to produce a black

solid. The reaction was continued for an additional 30 min.

The black-colored magnetic nanoparticles were isolated

by magnetic decantation, washed repeatedly with deion-

ized water until neutrality, and further washed with ethanol

(2×20 mL) and dried at 80°C for 6 h.

2.7 Synthesis of Magnetic Nanoparticle-Supported

N-Heterocyclic Carbene-Palladium(II)

Nanomagnetic Catalyst (NHC-Pd@MNPs)

To a solution of Na₂CO₃(0.5 M, 20 mL) in dimethyl-

formamide (DMF) (20 mL), palladium acetate (0.22 g,

0.98 mmol) was added and the resulting mixture was

stirred at room temperature for 15 min. Magnetic nanopar-

ticles tethered with 1-benzyl-1H-benzimidazolium chloride

(1.0 g) were added and stirring was continued at 50°C for

16 h. Then, the reaction mixture was cooled to room tem-

perature. The resultant compound was isolated by magnetic

decantation, washed with water (2×20 mL) followed by

ethanol (2×20 mL) and dried at 80°C for 6 h to yield a

blackish brown-coloured solid.

2.4 Synthesisof Silyl Chloride Functionalized Magnetic

Nanoparticles (SMNPs)

Hydroxyl-substituted magnetic nanoparticles (2.0 g) were

suspended in an ethanol (EtOH):water (H₂O) (1:1) mixture

(20 mL) by ultrasonicating for 30 min. 3-chloropropyl tri-

ethoxysilane (2.0 g, 8.3 mmol) was added to the suspension

and stirred at 45°C for 24 h. Then, the solution was cooled

to room temperature. The resultant dark brown silyl chlo-

ride functionalized magnetic nanoparticles were isolated

by magnetic decantation, washed with deionized water

(2×20 mL) followed by ethanol (2×20 mL) and dried at

80°C for 6 h.

2.8 General Procedure for Suzuki–Miyaura

Cross-coupling Reaction

NHC-Pd@MNPs nanomagnetic catalyst (0.15 mol% Pd),

aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol) and

potassium carbonate (2.2 mmol) were placed in a round-

bottomed ﬂask. To the mixture, EtOH:H₂O (1:1, 5 mL) was

added and stirred at 70°C for various times. The progress

of the reaction was monitored by TLC. After reaction com-

pletion, the mixture was cooled to room temperature and

the NHC-Pd@MNPs nanomagnetic catalyst was separated

by using an external magnet. To the ﬁltrate, ethyl acetate

(10 mL) and water (10 mL) were added. Ethyl acetate layer

was separated from the water layer using a separatory fun-

nel and dried with sodium sulphate. The dried ethyl ace-

tate was concentrated in vacuo and the product was puri-

ﬁed by column chromatography using n-hexane and ethyl

acetate as eluents to aﬀord the corresponding products in

good to excellent yields. All the coupling products were

known molecules and were conﬁrmed by comparing the

2.5 Synthesis of 1-Benzyl-1H-benzimidazole

Benzimidazole (1.0 g, 8.46 mmol) and potassium hydrox-

ide (0.71 g, 12.69 mmol) were stirred for 2 h in a minimum

amount of dimethyl sulfoxide (DMSO) at 100°C. The tem-

perature of the reaction mixture was reduced to 40°C and

benzyl bromide (1.45 g, 8.46 mmol) was added in one por-

tion and stirring was continued for an additional 2 h. After-

wards, the reaction mixture was cooled to room temperature

and ice cold water (20 mL) was added. The oﬀ-white solid

was separated by ﬁltration, washed with water and dried

at 50°C under vacuum to get 1-benzyl-1H-benzimidazole

1

as an oﬀ-white solid. Yield: 97%. H NMR (400 MHz,

CDCl₃): δ (ppm)=7.95 (s, 1H, NCHN), 7.82 (d, J=6.8 Hz,

1H, NC₆H₄N), 7.34–7.18 (m, 8H, NC₆H₄N and CH₂C₆H₅),

5.36 (s, 2H, CH₂C₆H₅). Mass data (m/z, LC-MS): 209.04

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

1

melting point, H NMR and mass spectroscopic data with

the authentic samples.

10. 4-Acetylbiphenyl (Table 7, entries 14 and 15): White

powder. Melting poin t=119–123°C; ¹H NMR

(400 MHz, CDCl₃): δ (ppm)=8.02 (d, J=8.4 Hz,

2H), 7.68 (d, J=8.8 Hz, 2H), 7.62 (d, J=7.6 Hz, 2H),

7.44–7.46 (m, 2H), 7.38–7.39 (m, 1H), 2.63 (s, 3 H).

LC-MS for C₁₄H₁₂O: m/z=197.17 [M+H]⁺.

1. Biphenyl (Table 7, entries 1 and 2): Colorless crys-

1

tals. Melting poin t=68–70°C. H NMR (400 MHz,

CDCl₃): δ (ppm)=7.35(t, J=7.6 Hz,2H), 7.42 (t,

J=7.6 Hz, 4H), 7.6 (d, J=8 Hz, 4H). LC-MS for

C₁₂H₁₀: m/z=155.07 [M+H]⁺.

11. 3-Phenylbenzoic acid (Table 7, entries 16 and 17):

Oﬀ-white crystals. Melting poin t=164–168°C; ¹H

NMR (400 MHz, CDCl₃): δ (ppm)=7.39–7.36 (m,

2H), 7.49–7.41(m, 1H), 7.64 (d, J=7.2 Hz, 3H), 7.74

(d, J=7.6 Hz, 2H), 7.84 (d, J=7.2 Hz, 1H). LC-MS

for C₁₃H₁₀O₂: m/z=199.06 [M+H]⁺.

2. 4-Methylbiphenyl (Table 7, entry 3): White crystalline

1

solid. Melting point=45–4 7°C; H NMR (400 MHz,

CDCl₃): δ (ppm)=7.79 (d, J=7.6 Hz, 2H), 7.65 (s,

1H), 7.40–7.36 (m, 4H), 7.32–7.28 (m, 2H), 2.27 (s,

3H). LC-MS for C₁₃H₁₂: m/z=169.05 [M+H]⁺.

12. 4-Phenylbenzoic acid (Table 7, entry 18): White solid.

Melting point=219–224°C; ¹H NMR (400 MHz,

CDCl₃): δ (ppm)=7.39–7.37 (m, 4H), 7.44–7.40

(m, 1H), 7.52–7.49 (m, 3H), 7.65 (d, J=7.2 Hz, 1H).

LC-MS for C₁₃H₁₀O₂: m/z=199.11 [M+H]⁺.

3. 4-Cyanobiphenyl (Table 7, entry 4): Oﬀ-white crys-

1

talline powder. Melting poin t=85–87°C; H NMR

(400 MHz, CDCl₃): δ (ppm)=7.43–7.41 (m, 1H),

7.50–7.47 (m, 2H), 7.58 (d, J=8 Hz, 2H), 7.66–7.73

(m, 4H). LC-MS for C₁₃H₉N: m/z=180.01 [M+H]⁺.

4. 4-Aminobiphenyl (Table 7, entry 5): Purple crys-

13. 2,4-Diﬂuoro-1,1′-biphenyl (Table 7, entry 19): Pale

1

yellow crystals. Melting point=61–6 5°C; H NMR

1

tals. Melting point=52–5 5°C; H NMR (400 MHz,

(400 MHz, CDCl₃): δ (ppm)=7.42–7.35 (m, 4H),

7.50–7.49 (m, 2H), 7.59 (d, J=7.2 Hz, 2H). LC-MS

for C₁₂H₈F₂: m/z=191.02 [M+H]⁺.

CDCl₃): δ (ppm)=7.54 (d, J=8.4 Hz, 2H), 7.41–

7.38 (m, 4H), 7.29–7.24 (m, 1H), 6.76 (d, J=8 Hz,

2H), 3.75 (s, 2H). LC-MS for C₁₂H₁₁N: m/z=170.03

[M+H]⁺.

2.9 Procedure for Recovery of NHC-Pd@MNPs

Nanomagnetic Catalyst

5. 4-Phenylbenzaldehyde (Table 7, entry 6): Yel-

low crystals. Melting point=57–5 9°C; ¹H NMR

(400 MHz, CDCl₃): δ (ppm)=10.07 (s, 1H), 7.52–

7.46 (m, 3H), 7.62 (d, J=8.4 Hz, 2H), 7.75 (d,

J=6.8 Hz, 2H), 7.94 (d, J=6.8 Hz, 2H). LC-MS for

C₁₃H₁₀O: m/z=183.09 [M+H]⁺.

NHC-Pd@MNPs nanomagnetic catalyst (0.15 mol%), aryl

halide (1.0 mol%), phenylboronic acid (1.1 mol%) and

potassium carbonate (2.2 mol%) were placed in a round-

bottomed ﬂask. To the reaction mixture, EtOH:H₂O (1:1,

5 mL) was added and stirred at required temperature for

various times. After completion of reaction, the reaction

mixture was cooled to room temperature and the NHC-

Pd@MNPs nanomagnetic catalyst was separated by using

an external magnet. The separated NHC-Pd@MNPs nano-

magnetic catalyst was washed with water (2×10 mL) fol-

lowed by ethanol (2×10 mL) and dried at 80°C for 6 h.

Then, the dried NHC-Pd@MNPs nanomagnetic catalyst

was used for next round of Suzuki–Miyaura cross-coupling

reactions.

6. 2-Phenylbenzaldehyde (Table 7, entries 7 and 8): Yel-

1

low oil. H NMR (400 MHz, CDCl₃): δ (ppm)=9.78

(s, 1H),7.30–7.26 (m, 3H), 7.39–7.36 (m, 2H), 7.51

(d, J=7.6 Hz, 1H), 7.65–7.71 (m, 2H). LC-MS for

C₁₃H₁₀O: m/z=183.05 [M+H]⁺.

7. 4-Nitrobiphenyl (Table 7, entry 9): Pale yellow crys-

tals. Melting point=112–11 4°C; ¹H NMR (400 MHz,

CDCl₃): δ (ppm)=7.51–7.45 (m, 3H), 7.62 (d,

J=8.8 Hz, 2H), 7.73 (d, J=8.8 Hz, 2H), 8.29 (d,

J=8.8 Hz, 2H). LC-MS for C₁₂H₉NO₂: m/z=200.04

[M+H]⁺.

8. 4-Methoxybiphenyl (Table 7, entries 10 and 11):

1

White powder. Melting poin t=86–90°C; H NMR

3 Results and Discussion

3.1 Synthesis

(400 MHz, CDCl₃): δ (ppm)=7.56–7.52 (m, 4H),

7.41 (t, J=7.2 Hz, 2H), 7.31 (t, J=7.6 Hz, 1H), 6.98

(d, J=8.8 Hz, 2H), 3.85 (s, 3H). LC-MS for C₁₃H₁₂O:

m/z=185.10 [M+H]⁺.

In the present study, a NHC-Pd@MNPs nanomagnetic

catalyst was synthesized through a multistep reaction

as illustrated in Scheme 1. First, hydroxyl-substituted

magnetic nanoparticles (1) were prepared by a chemi-

cal co-precipitation method, followed by the function-

alization of hydroxyl-substituted magnetic nanoparticles

by 3-chloropropyl triethoxysilane (2) in a EtOH:H₂O

9. 4-Hydroxybiphenyl (Table 7, entries 12 and 13):

White crystals. Melting poin t=164–166°C; ¹H NMR

(400 MHz, CDCl₃): δ (ppm)=7.53 (d, J=7.2 Hz,

2H), 7.48 (d, J=7.2 Hz, 2H), 7.39 (d, J=8.8 Hz, 2H),

7.31–7.28 (m, 1H), 6.90 (d, J=8.4 Hz, 2H), 4.8 (s,

1H). LC-MS for C₁₂H₁₀O: m/z=171.15 [M+H]⁺.

1 3

K. Vishal et al.

OH

HO

O

EtO

EtO Si

EtO

EtOH-H₂O

45 ⁰C, 24 h

b)

+

MNP

Cl

HO MNP OH

Si

Cl

OH

HO

OH

1

2

3

Toluene

110 ⁰C

48 h

N

4

Pd(OAc)₂

Cl

Cl Pd Cl

O

Na₂CO₃

MNP

O

Si

N

DMF

O

50 ⁰C, 16 h

6

5

NHC-Pd@MNPs

12

Scheme 1 Synthetic schemes for a MNPs (1), and b the NHC-Pd@MNPs nanomagnetic catalyst (6)

mixture at 45 °C. In the next step, silyl-functionalized

magnetic nanoparticles (3) were reacted with1-benzyl-

1H-benzimidazole (4) in toluene at 110 °C for 48 h.

The magnetic nanoparticles tethered with 1-benzyl-

1H-benzimidazolium chloride (5) were then treated with

palladium acetate in DMF in the presence of aqueous

sodium carbonate to get the desired NHC-Pd@MNPs

nanomagnetic catalyst (6).

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

3.2 Spectroscopic and Microscopic Characterization

Chemical structures of all the synthesized compounds

were conﬁrmed through the ATR-IR spectroscopic tech-

nique. Figure 1 demonstrates ATR-IR spectra of MNPs,

SMNPs and NHC-Pd@MNPs nanomagnetic catalysts.

The ATR-IR spectrum of MNPs (Fig. 1a) displays a char-

acteristic peak of Fe–O at 585 cm⁻¹and stretching vibra-

tions of O-H bonds at 3393 cm⁻¹. The bands observed

around 991 and 2912 cm⁻¹are attributed to the Si–O

and C–H stretching vibrations of the propyl group from

3-chloropropyl triethoxysilane apart from a Fe-O stretch-

ing vibration at 584 cm⁻¹of SMNPs (Fig. 1 (b)), which

conﬁrms the functionalization of chloropropyltriethox-

ysilane on the surface of the MNPs. The ATR-IR spec-

trum of NHC-Pd@MNPs nanomagnetic catalyst exhibits

typical bands in the range 3250–2902 cm⁻¹attributed to

aromatic C–H stretching vibrations (Fig. 1c) [48, 49]. In

addition, various signals characteristic of Pd–C vibra-

tions are observed in the spectrum (1648, 1490 and

680 cm⁻¹) (Fig. 1c), which conﬁrms the structure of the

NHC-Pd@MNPs nanomagnetic catalyst [50]. Figure 2

shows the ATR-IR spectra of the NHC-Pd@MNPs nano -

magnetic catalyst and reused NHC-Pd@MNPs nanomag-

netic catalyst. Even after ﬁve recycles, the NHC-Pd@

MNPs nanomagnetic catalyst composition remained the

same, which is further conﬁrmed by comparison of ATR-

IR spectra of the NHC-Pd@MNPs nanomagnetic catalyst

(Fig. 2 c) and recycled NHC-Pd@MNPs nanomagnetic

catalyst (Fig. 2d).

Fig. 2 ATR-IR spectra of c the NHC-Pd@MNPs nanomagnetic cata-

lyst and d the recycled NHC-Pd@MNPs nanomagnetic catalyst

are shown in Fig. 3. The λ_maxobserved for MNPs is

370 nm, whereas in the case of NHC-Pd@MNPs nano-

magnetic catalyst, the λ_maxobserved is at 387 nm. The

shift in λ_maxfrom 370 nm to 387 nm can be attributed

to the successful surface functionalization of MNPs [51,

52].

The surface functionalization of MNPs was further con-

ﬁrmed by analyzing the surface area by BET. Nitrogen

adsorption–desorption curves for (a) MNPs, (b) SMNPs,

and (c) the NHC-Pd@MNPs nanomagnetic catalyst are

shown in Fig. 4. The NHC-Pd@MNPs nanomagnetic

catalyst exhibited a type-II isotherm. The amount of N₂

The UV–visible spectra of a suspension of MNPs and

the NHC-Pd@MNPs nanomagnetic catalyst in methanol

Fig. 1 ATR-IR spectra of a MNPs, b SMNPs, and c the NHC-Pd@

Fig. 3 UV–Visible spectra of a MNPs and b the NHC-Pd@MNPs

MNPs nanomagnetic catalyst

nanomagnetic catalyst

1 3

K. Vishal et al.

Fig. 4 Nitrogen adsorption–desorption curves for a MNPs, b SMNPs, and c the NHC-Pd@MNPs nanomagnetic catalyst

adsorbed on bare MNPs was high compared to SMNPs

and the NHC-Pd@MNPs nanomagnetic catalyst. The ini-

tial surface area of bare MNPs (77.47 m²g⁻¹) got reduced

by surface functionalization with 3-chloropropyl triethox-

ysilane (76.60 m²g⁻¹) and was further reduced when the

NHC-Pd@MNPs nanomagnetic catalyst (56.25 m²g⁻¹)

was prepared, which is evident from data obtained from the

BET [53].

seen from the TEM image. The morphology of the recycled

NHC-Pd@MNPs nanomagnetic catalyst does not show

muc h change after being reused up to ﬁve times (Fig. 5c)

[54–56].

FESEM images of the NHC-Pd@MNPs nanomagnetic

catalyst and recycled NHC-Pd@MNPs nanomagnetic

catalyst are shown in Fig. 6. From the FESEM image,

it is clear that the NHC-Pd@MNPs nanomagnetic cata-

lyst was made up of uniform nanometer-sized spherical

particles [57–59]. The prepared NHC-Pd@MNPs nano-

magnetic cat alysts are narrowly distributed and well dis-

persed. Also, after ﬁve recycles the morphology of the

nanomagnetic catalyst was not altered, which is in good

agreement with TEM images of the recycled NHC-Pd@

MNPs nanomagnetic catalyst. The presence of palladium

atoms in NHC-Pd@MNPs nanomagnetic catalyst was

conﬁrmed by EDS (Fig. 7). The EDS spectrum shows

The morphologies and sizes of MNPs, the NHC-Pd@

MNPs catalyst and reused NHC-Pd@MNPs nanomagnetic

catalyst were investigated using TEM (Fig. 5). As shown

in Fig. 5a, MNPs were spherical in nature with sizes rang-

ing from 7 to 10 nm. After anchoring of the NHC-Pd(II)

complex, the support particles are quasi-spherical in nature

and the sizes were in the range of 10–15 nm (Fig. 5b).

Furthermore, the magnetic core as a dark spot inside the

spherical NHC-Pd@MNPs nanomagnetic catalyst can be

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

Fig. 5 TEM images of a

MNPs, b the NHC-Pd@MNPs

nanomagnetic catalyst, and c

the recycled NHC-Pd@MNPs

nanomagnetic catalyst

Fig. 6 FESEM images of a the NHC-Pd@MNPs nanomagnetic catalyst and b the recycled NHC-Pd@MNPs nanomagnetic catalyst

characteristic signals of C, N, O, Fe, Si and Pd, which

further conﬁrms the presence of a N-heterocyclic car-

bene-palladium(II) complex on the surface of the MNPs

[57]. In order to determine the exact quantity of Pd in

theNHC-Pd@MNPs nanomagnetic catalyst, ICP-AES

was carried out. ICP-AES analysis shown that 6.04% w/w

of palladium is loaded on the NHC-Pd@MNPs nanomag-

netic catalyst.

1 3

K. Vishal et al.

Fig. 7 EDS spectrum of the NHC-Pd@MNPs nanomagnetic catalyst

Fig. 8 XRD patterns of a

MNPs and b the NHC-Pd@

MNPs nanomagnetic catalyst

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

The XRD pattern of the NHC-Pd@MNPs nanomag-

netic catalyst in comparison with MNPs was investigated

(Fig. 8). The XRD pattern of MNPs (Fig. 8a) indicates a

cubic spinel structure with peaks attributed to Fe₃O₄at 2θ

of 30.09°, 35.59°, 43.07°, 53.43°, 57.37° and 62.90° cor-

responding to the reﬂection planes of (220), (311), (400),

(422), (511), and (440). On the other hand, the NHC-Pd@

MNPs nanomagnetic catalyst showed a similar XRD pat-

tern (Fig. 8b) as MNPs. Therefore, it is clear that even

after functionalization of MNPs, the phase remained same,

which is in good agreement with TEM images. Also, no

characteristic peaks of Pd were observed, which shows

excellent dispersion of Pd sites on the MNPs [56].

3.3 Catalytic Properties of the NHC-Pd@MNPs

Nanomagnetic Catalyst

After structural characterization of the prepared NHC-

Pd@MNPs nanomagnetic catalyst through spectroscopic

and microscopic techniques, its catalytic activity was

studied as an active and stable magnetically separable

nanomagnetic catalyst in Suzuki–Miyaura cross-coupling

reactions. The reaction conditions were optimized on

model Suzuki–Miyaura cross-coupling reaction between

bromobenzene and phenylboronic acid, as shown in

Scheme 2.

The primary requirement for green synthesis is the use

of green solvents. Therefore, EtOH and H₂O were given

high priority as solvents during the process optimization.

The reaction conditions were optimized through a series

of reactions as shown in Table 1. The preliminary results

revealed that using K₂CO₃base, EtOH:H₂O (1:1) solvent,

0.15 mol% Pd of NHC-Pd@MNPs nanomagnetic cata-

lyst at 70 °C aﬀorded the highest yield (Table 1, entry 8).

The thermal stability of the MNPs and NHC-Pd@MNPs

nanomagnetic catalyst was evaluated using thermogravi-

metric analysis, carried out under a nitrogen atmosphere

between 40 and 730°C at a rate of 10°C min⁻¹(Fig. 9).

The thermogram of magnetic nanoparticles (Fig. 9 a)

showed weight loss of 6% in the range 80–200°C corre-

sponding to the loss of surface hydroxyl groups and mois-

ture. The NHC-Pd@MNPs nanomagnetic catalyst (Fig. 9b)

decomposes in two stages. The ﬁrst step, 80–110°C, results

in a mass loss of 3% corresponding to the loss of adsorbed

water molecules. In the second step, the complete loss of

covalently attached organic moiety was observed in the

range 110–500°C with mass loss of 9%. Thus, on the basis

of these r esults, grafting of the NHC onto the MNPs is con-

ﬁrmed [1, 57]. Furthermore, it is clear that the NHC-Pd@

MNPs nanomagnetic catalyst could be used in reactions to

a temperature of ca.110°C without decomposition, which

is an added advantage for its use in Suzuki–Miyaura cross-

coupling reactions that are usually performed at elevated

temperatures.

The performance of

a

palladium catalyzed

Suzuki–Miyaura cross-coupling reaction is known to be

governed by a number of factors such as kind of solvent,

base, temperature, time, catalyst ratio, etc. Hence, the

eﬀect of diﬀerent solvents, bases, temperatures, time and

NHC-Pd@MNPs, Base

Solvent, Temperature, Time

Br + (HO)₂B

Scheme 2 Suzuki–Miyaura cross-coupling reaction between bro-

mobenzene and phenylboronic acid in the presence of NHC-Pd@

MNPs nanomagnetic catalyst

Table 1 Optimization of conditions for Suzuki–Miyaura cross-cou-

pling reaction of bromobenzene with phenylboronic acid in the pres-

ence ofNHC-Pd@MNPs nanomagnetic catalyst

Entry Base

Solvent

Tem-

Time (h) Yield (%)^a

perature

(°C)

1

2

3

4

5

6

7

8

K₂CO₃DMF

K₂CO₃DMF:H₂O (1:1)

K₂CO₃H₂O

K₂CO₃EtOH

NaOH EtOH

K₂CO₃EtOH:H₂O (9:1)

K₂CO₃EtOH:H₂O (8:2)

K₂CO₃EtOH:H₂O (1:1) 70

100

80

70

6

3

2

1

55

70

85

88

40

90

92

95

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl hal-

ide), base (2.2 mmol) and solvent (5 mL) in air

^aIsolated yield after separation by column chromatography; average

of two runs

Fig. 9 TGA curves of a MNPs and b the NHC-Pd@MNPs nanomag-

netic catalyst

1 3

K. Vishal et al.

Table 2 Suzuki–Miyaura cross-coupling reactions of bromobenzene

with phenylboronic acid in the presence of NHC-Pd@MNPs nano-

magnetic catalyst with various solvents

Table 4 Suzuki–Miyaura cross-coupling reactions of bromobenzene

with phenylboronic acid using the NHC-Pd@MNPs nanomagnetic

catalyst at various temperatures

Entry Base

Solvent

Tem-

Time (h) Yield (%)^a

Entry Base

Solvent

Tem-

Time(h) Yield (%a

perature

(°C)

perature

(°C)

1

2

3

4

5

6

7

8

K₂CO₃DMF

K₂CO₃H₂O

K₂CO₃EtOH

K₂CO₃MeOH

K₂CO₃THF

K₂CO₃Toluene

K₂CO₃Acetone

K₂CO₃EtOH : H₂O (8:2) 70

70

56

1

50

85

80

78

55

70

50

92

1

2

3

4

5

6

7

8

K₂CO₃EtOH:H₂O (1:1)

25

40

50

60

70

80

90

1

55

70

80

85

95

85

80

K₂CO₃EtOH:H₂O (1:1) 100

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl hal-

ide), base (2.2 mmol) and solvent (5 mL) in air

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl hal-

ide), base (2.2 mmol) and solvent (5 mL) in air

^aIsolated yield after separation by column chromatography; average

of two runs

^aIsolated yield after separation by column chromatography; average

of two runs

Table 5 Suzuki–Miyaura cross-coupling reactions of bromobenzene

with phenylboronic acid using NHC-Pd@MNPs nanomagnetic cata-

lyst at various time intervals

Table 3 Suzuki–Miyaura cross-coupling reactions of bromobenzene

with phenylboronic acid using NHC-Pd@MNPs nanomagnetic cata-

lyst with various bases

Time (h) Yield (%)^a

Entry Base

Solvent

Tem-

Time(h) Yield (%)^a

Entry Base

Solvent

Tem-

perature

(°C)

perature

(°C)

1

2

3

4

5

6

K₂CO₃EtOH:H₂O (1:1) 70

0.33

0.5

0.66

0.83

1

60

75

80

85

95

1

2

3

4

5

6

7

K₂CO₃EtOH:H₂O (1:1) 70

1

95

85

80

40

55

70

75

Na₂CO₃EtOH:H₂O (1:1) 70

KF

NaOH

KOH

EtOH:H₂O (1:1) 70

1.5

Na₃PO₄EtOH:H₂O (1:1) 70

CsCO₃EtOH:H₂O (1:1) 70

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl hal-

ide), base (2.2 mmol) and solvent (5 mL) in air

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl hal-

ide), base (2.2 mmol) and solvent (5 mL) in air

^aIsolated yield after separation by column chromatography; average

of two runs

^aIsolated yield after separation by column chromatography; average

of two runs

NHC-Pd@MNPs nanomagnetic catalyst. Suzuki–Miyaura

cross-coupling reactions were carried out in diﬀerent

solvents like DMF, H₂O, EtOH, tetrahydrofuran (THF),

toluene, acetone, methanol (MeOH) and EtOH:H₂O (8:2)

solvent mixture, as shown in Table 2. The results revealed

that reaction went well with polar solvents except with

DMF and acetone (Table 2, entries 1 and 7). Surprisingly,

the NHC-Pd@MNPs nanomagnetic catalyst showed fairly

good catalytic activity with the nonpolar solvent toluene

(Table 2, entry 6).

catalyst ratio were studied for the model Suzuki–Miyaura

cross-coupling reaction be tw een bromobenzene and phe-

nylboronic acid (Tables 2, 3, 4, 5, 6).

3.3.1 Eﬀect of Solvent on Suzuki–Miyaura cross-Coupling

Reaction

We studied the eﬀect of solvents on Suzuki–Miyaura

cross-coupling reactions between bromobenzene and

phenylboronic acid to understand the performance of the

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

Table 6 Suzuki–Miyaura cross-coupling reaction of bromobenzene

with phenylboronic acid using various NHC-Pd@MNPs nanomag-

netic catalyst ratios

3.3.2 Eﬀect of Base on Suzuki–Miyaura cross-Coupling

Reactions

Entry Base

Solvent

Tem-

Pd (mol%) Yield (%)^a

The eﬀect of bases on the eﬃciency of the NHC-Pd@

MNPs nanomagnetic catalyst in the Suzuki–Miyaura cross-

coupling reaction of bromobenzene and phenylboronic acid

was investigated by employing various bases (Table 3).

Among the bases employed (K₂CO₃, Na₂CO₃, KF, NaOH,

KOH, Na₃PO₄and CsCO₃), K₂CO₃, Na₂CO₃, KF, Na₃PO₄

and CsCO₃were found to be rather eﬀective (Table 3,

entries 1–3, 6 and 7). However, using bases like NaOH and

KOH led to lower conversion (Table 3, entries 4 and 5).

perature

(°C)

1

2

3

4

5

6

7

K₂CO₃EtOH:H₂O (1:1) 70

0.01

0.02

0.05

0.07

0.10

0.15

0.20

20

45

60

70

85

95

3.3.3 Eﬀect of Temperature on Suzuki–

Miyaura cross-Coupling Reactions

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), base (2.2 mmol) and solvent (5 mL) in air

^aIsolated yieldafter separation by column chromatography; average of

two runs

It is also found that with the NHC-Pd@MNPs nanomag-

netic catalyst, the reaction temperature has a great inﬂuence

Fig. 10 Plots depicting yields of Suzuki–Miyaura cross-coupling products against varying a temperatures, b time, and c catalyst ratio

1 3

K. Vishal et al.

Table 7 Suzuki–Miyaura cross-coupling reactions between aryl halides with phenylboronic acid catalyzed by NHC-Pd@MNPs nanomagnetic

catalyst

on the Suzuki–Miyaura cross-coupling reaction (Fig. 10a).

As shown in Table 4, a high conversion w as observed

at temperature 70 and 8 0°C (Table 4, entries 5 and 6).

At lower reaction temperatures (25, 40, 50 and 6 0°C)

(Table 4, entries 1–4), the yields were less, and at a high

reaction temperature (100°C) (Table 4, entry 8), the yield

was decreased. Hence, all the reactions were carried out at

70°C (Table 4, entry 5).

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

Table 7 (continued)

^aReaction conditions: Aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl halide), base

(2.2 mmol) and solvent (5 mL) in air

^bIsolated yield after separation by column chromatography; average of two runs

3.3.4 Eﬀect of Time on Suzuki–Miyaura cross-Coupling

Reaction

Suzuki–Miyaura cross-coupling reaction of bromoben-

zene with phenylboronic acid was carried out at diﬀerent

time periods from 0.33 to 1.5 h (Fig. 10b). It is clear from

the data obtained (Table 5) that with an increase in time,

the conversion increased resulting in a higher yield; a

To understand the rate of reaction with the prepared

NHC-Pd@MNPs nanomagnetic catalyst, the model

1 3

K. Vishal et al.

in order to ﬁnd out the appropriate quantity of the NHC-

Pd@MNPs nanomagnetic catalyst, we performed model

Suzuki–Miyaura cross-coupling reactions of bromoben-

zene with phenylboronic acid with varying amounts of

the NHC-Pd@MNPs nanomagnetic catalyst, starting from

0.01 mol% Pd to 0.20 mol% Pd (Fig. 10c). A higher con-

version or maximum yield was obtained using 0.15 mol%

Pd of the NHC-Pd@MNPs nanomagnetic catalyst, as

shown in Table 6 (Table 6, entry 6). Furthermore, the use

of higher quantities of the NHC-Pd@MNPs nanomagnetic

catalyst (0.20 mol% Pd) does not increase the conversion

(Table 6, entry 7).

Using the optimized reaction conditions, the NHC-Pd@

MNPs nanomagnetic catalyst was applied to a phenylbo-

ronic acid with a range of aryl bromides, aryl chlorides and

aryl iodide. The results are tabulated in Table 7. All the

bromides gave good to excellent yields except 2-bromoben-

zaldehyde (Table 7, entry 7). In detail, various electron-

withdrawing and electron-donating groups such as –NH₂,

–OH, –NO₂, –CH₃, –OCH₃, –CHO, –COCH₃and –COOH

on aryl bromide are well-tolerated aﬀording the desired

unsymmetric biaryls in good to excellent yields. Therefore,

it can be concluded that the reaction yield was not inﬂu-

enced by the electronic properties of the substituent on the

aryl bromides. However, aryl chlorides gave lesser yields

compared with their bromide counter parts under the same

reaction conditions (Table 7, entries 2, 8, 13, 15 and 17).

On the other hand, iodoanisole gave 100% conversion

in 10 min, as expected (Table 7, entry 11). These results

conﬁrmed the good performance of our NHC-Pd@MNPs

nanomagnetic catalyst on Suzuki–Miyaura cross-coupling

reactions. Furthermore, the selectivity of the prepared

NHC-Pd@MNPs nanomagnetic catalyst was conﬁrmed by

conducting the Suzuki–Miyaura cross-coupling reaction

using the optimized reaction conditions in the absence of

aryl bromide. The homo-coupling biphenyl product was

less than 2% even after maintaining the reaction for longer

time. Thus, it is conﬁrmed that the prepared NHC-Pd@

MNPs nanomagnetic catalyst is highly selective.

Fig. 11 The recycling eﬃciency of NHC-Pd@MNPs nanomagnetic

catalyst in Suzuki–Miyaura cross-coupling reaction of bromobenzene

and phenylboronic acid

Table 8 Recyclability of the NHC-Pd@MNPs nanomagnetic catalyst

in Suzuki–Miyaura cross-coupling reactions of bromobenzene with

phenylboronic acid

Entry Base

Solvent

Tem-

Catalyst Run % Yield^a

perature

(°C)

1

2

3

4

5

6

7

8

9

K₂CO₃EtOH:H₂O (1:1) 70

Fresh

1st recycle

2nd recycle 95

3rd recycle

4th recycle

5th recycle

6th recycle

7th recycle

8th recycle

95

94

93

88

81

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid

(1.1 mmol), NHC-Pd@MNPs (0.15 mol% Pd with respect to aryl hal-

ide), base (2.2 mmol) and solvent (5 mL) in air

3.4 Catalyst Recyclability and Leaching Studies

^aIsolated yield after separation by column chromatography; average

of two runs

Recyclability is an important factor to judge the sustain-

ability of a catalyst. Therefore, the NHC-Pd@MNPs

nanomagnetic catalyst was examined through the model

Suzuki–Miyaura cross-coupling reaction between bro-

mobenzene and phenylboronic acid. After completion of

the reaction, the NHC-Pd@MNPs nanomagnetic catalyst

can be conveniently and eﬃciently recovered from the reac-

tion mixture using an external magnet, and can be used in

the next run after washing with water followed by etha-

nol and drying. The results revealed that the NHC-Pd@

MNPs nanomagnetic catalyst can be reused without any

maximum yield was obtained in one hour (Table 5, entry

5). Also, a further increase in time beyond 1 h (1.5 h)

does not improve the yield (Table 5, entry 6).

3.3.5 Eﬀect of the Catalyst Ratioon Suzuki–

Miyaura cross-Coupling Reactions

The quantity of catalyst plays an important role in

Suzuki–Miyaura cross-coupling reactions. Therefore,

1 3

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

Table 9 Comparison of results for the NHC-Pd@MNPs nanomagnetic catalyst with other catalysts for the Suzuki–Miyaura cross-coupling reac-

tion between bromobenzene and phenylboronic acid

Entry

Catalyst

Solvent

Temp (°C)

Time (h)

Yield (%)

Ref.

1^a

2

Iron oxide-Pd complex

MNPs-NHC-Pd(II)

Fe₃O₄/SiO₂/HPG-OPPh₂-PNP

Catalyst-1

Co/C-ROMP gel Pd

Pd-Fe₃O₄NCS

Pd-IL-NH₂/SiO₂/Fe₃O₄

Pd(II)NiFe₂O₄

G₁MNP

MUA-Pd

Pd-MTAT

LDH-Pd(0)

DMF

50

12

1

0.17

12

24

5

6.5

5

12

10

2

77

93

87

92

70

87

93

89

92

85

94

90

95

90

[46]

[57]

[58]

[60]

[61]

[62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

Present work

EtOH:H₂O (1:1)

DMF:H₂O (1:1)

EtOH:H₂O (3:1)

THF:H₂O (1:2)

Dimethoxyethane:H₂O (3:1)

EtOH:H₂O (1:1)

EtOH:H₂O (9:1)

MeOH

R.T

70

65

Reﬂux

80

60

90

85

80

70

3

4^a

5

6

7

8

9

10

11

12

13

14

15^a

DMF

DMF:H₂O (1:5)

1,4-Dioxane-H₂O (1:1)

Isopropyl alcohol

EtOH:H₂O (1:1)

Pd-MSN-IPr

NHC-Pd@MNPs

1

^aCoupling reaction between 4-bromoanisole and phenyl boronic acid

modiﬁcation for up to ﬁve cycles, and no signiﬁcant loss

of activity was observed. In sixth recycle, NHC-Pd@MNPs

nanomagnetic catalyst shown good activity but from sev-

enth recycle onwar ds, a decrease in catalytic activity was

observed (Fig. 11) (Table 8). The TEM, FESEM images

and ATR-IR spectr um of ﬁve-times recycled NHC-Pd@

MNPs nanomagnetic catalyst was recorded. The TEM

image (Fig. 5c) shows that there is no change in the mor-

phology of the NHC-Pd@MNPs nanomagnetic catalyst

after recycling up to ﬁve cycles which is further conﬁrmed

by the FESEM image (Fig. 6b). The ATR-IR spectrum

(Fig. 2b) shows that the NHC-Pd@MNPs nanomagnetic

catalyst is intact after recycling. Furthermore, the pre-

pared NHC-Pd@MNPs nanomagnetic catalyst is stable at

room temperature for more than 4 months. This was con-

cluded by analyzing the performance of the NHC-Pd@

MNPs nanomagnetic catalyst after keeping it for more than

4 months at room temperature which showed no observable

change in activity.

3.5 Comparison of Catalysts

Table 9 demonstrates a comparison of results for the NHC-

Pd@MNPs nanomagnetic catalyst with other catalysts for

the Suzuki–Miyaura cross-coupling reaction between bro-

mobenzene and phenylboronic acid. Comparison of the

results shows a better catalytic activity and shorter reac-

tion time for the NHC-Pd@MNPs nanomagnetic cat a-

lyst in Suzuki–Miyaura cross-coupling reactions [60–69].

However, some of the major advantages of the NHC-Pd@

MNPs nanomagnetic catalyst are: (1) magnetically recover-

able, (2) mild reaction conditions, (3) use of green solvents,

(4) short reaction time, and (5) excellent selectivity. Also,

the NHC-Pd@MNPs nanomagnetic catalyst can be syn-

thesized by an incisive method using easily available chief

chemicals under aerobic conditions, which do not require a

sophisticated setup.

The leaching study of the NHC-Pd@MNPs nanomag-

netic catalyst was also performed to understand the stability

of the NHC-Pd@MNPs catalyst in the optimized reaction

conditions. This study was done by magnetic separation

of the NHC-Pd@MNPs catalyst after 10 min of reaction

time, and the same reaction was further continued for more

than 1 h and was monitored using TLC. Further conver-

sion was not observed and the isolated yield was 60%. This

proved that leaching of the NHC-Pd@MNPs nanomagnetic

catalyst was not occurring and is stable in the chemical

environment.

4 Conclusion

In summary, we have developed a new, practical, and eco-

nomic NHC-Pd@MNPs nanomagnetic catalyst. The struc-

ture of the synthesized NHC-Pd@MNPs nanomagnetic

catalyst was fully conﬁrmed using AT-IR spectroscopy,

UV–Visible spectrophotometry, ICP-AES, FESEM, EDS,

TEM, XRD, TGA and BET analysis. The NHC-Pd@MNPs

nanomagnetic catalyst showed extraordinary performance

in Suzuki–Miyaura cross-coupling reactions. The NHC-

Pd@MNPs nanomagnetic catalyst could be simply recov-

ered from the reaction media using magnetic separation

1 3

K. Vishal et al.

29. Giudici RE, Hoveyda AH (2007) J Am Chem Soc 129:3824

30. Kantchev EAB, Brien CJO, Organ MG (2007) Angew Chem

Int Ed 46:2768

and could be reused at least ﬁve times without signiﬁcant

loss in activity. This approach could be used to prepare var-

ious new magneticnanoparticle supported N-heterocyclic

carbene-palladium(II) nanomagnetic catalysts which could

oﬀer more opportunities for developing powerful and reus-

able catalysts for green organic synthesis.

31. Polshettiwar V, Hesemann P, Moreau JJE (2007) Tetrahedron

Lett 48:5363

32. Lebel H, Janes MK, Charette AB, Nolan SP (2004) J Am

Chem Soc 126:5046

33. Titcomb LR, Caddick S, Cloke FGN, Wilson DJ, McKerrecher

D (2001) Chem Commun 15:1388

Acknowledgements The authors thank DST-Nanomission, India

(SR/NM/NS-20/2014), DST-SERB, India (SERB/F/7013/2015-16)

and Jain University, India for ﬁnancial support.

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Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient,…

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Article Doi

Magnetic Nanoparticle-Supported N-Heterocyclic Carbene-Palladium(II): A Convenient, Efficient and Recyclable Catalyst for Suzuki–Miyaura Cross-Coupling Reactions

DOI: 10.1007/s10562-017-1987-7

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Full text of DOI:10.1007/s10562-017-1987-7

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