A convenient, efficient and reusable N-heterocyclic carbene-palladium(II) based catalyst supported on magnetite for Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions

Article abstract of DOI:10.1039/c7nj01876b

In the present work, a new magnetic nanoparticle supported N-heterocyclic carbene-palladium(ii) (NO₂-NHC-Pd@Fe₃O₄) nanomagnetic catalyst was synthesized by a facile multistep synthesis under aerobic conditions using inexpensive chemicals. The NO₂-NHC-Pd@Fe₃O₄ nanomagnetic catalyst was characterized by various analytical techniques such as attenuated total reflectance infrared spectroscopy (ATR-IR), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), energy-dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller surface area analysis (BET). The synthesized NO₂-NHC-Pd@Fe₃O₄ nanomagnetic catalyst showed excellent catalytic activity in both Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions for various substrates under mild reaction conditions. Recovery of the NO₂-NHC-Pd@Fe₃O₄ nanomagnetic catalyst from the reaction mixture was easily accomplished by applying an external magnet. The recovered NO₂-NHC-Pd@Fe₃O₄ nanomagnetic catalyst exhibited very good catalytic activity up to seven recycles in Suzuki-Miyaura and five recycles in Mizoroki-Heck cross-coupling reactions without considerable loss of its catalytic activity.

Full text of DOI:10.1039/c7nj01876b

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Page 1 of 39
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A Convenient, Efficient and Reusable N-Heterocyclic Carbene–
Palladium(II) Based Catalyst Supported on Magnetite for Suzuki–
Miyaura and Mizoroki–Heck Cross-Coupling Reactions
Vishal Kandathil,a Bradley D. Fahlman,b Sasidhar B. S.,c Shivaputra A. Patil,d Siddappa A. Patila*
aCentre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura,
Ramanagaram, Bangalore 562112, India.
bDepartment of Chemistry and Biochemistry & Science of Advanced Materials, Central
Michigan University, Mount Pleasant, MI 48859, USA.
cOrganic Chemistry Section, Chemical Sciences and Technology Division, CSIRꢀNational
Institute for Interdisciplinary Science and Technology, Thiruvananthapuramꢀ695019, India.
dPharmaceutical Sciences Department, College of Pharmacy, Rosalind Franklin University of
Medicine and Science, North Chicago, IL 60064 USA.
Corresponding author: Dr. Siddappa A. Patil
Tel: +91 80 27577254
Fax: +91 80 27577211
Eꢀmail: p.siddappa@jainuniversity.ac.in
1

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Abstract
In the present work, a new magnetic nanoparticle supported Nꢀheterocyclic carbene–
palladium(II) (NO2ꢀNHCꢀPd@Fe3O4) nanomagnetic catalyst was synthesized by a facile
multistep synthesis under aerobic conditions using inexpensive chemicals. The NO2ꢀNHCꢀ
Pd@Fe3O4 nanomagnetic catalyst was characterized by various analytical techniques such as
attenuated total reflectance infrared spectroscopy (ATRꢀIR), inductively coupled plasmaꢀatomic
emission spectroscopy (ICPꢀAES), energyꢀdispersive Xꢀray spectroscopy (EDS), fieldꢀemission
scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Xꢀray
powder diffraction (XRD), thermogravimetric analysis (TGA) and BrunauerꢀEmmettꢀTeller
surface area analysis (BET). The synthesized NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst
showed excellent catalytic activity in both SuzukiꢀMiyaura and MizorokiꢀHeck crossꢀcoupling
reactions for various substrates under mild reaction conditions. Recovery of NO2ꢀNHCꢀ
Pd@Fe3O4 nanomagnetic catalyst from the reaction mixture was easily accomplished by
applying external magnet. The recovered NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst exhibited
very good catalytic activity up to seven recycles in SuzukiꢀMiyaura and five recycles in
MizorokiꢀHeck crossꢀcoupling reactions without considerable loss of its catalytic activity.
Keywords: Magnetic nanoparticle; Nꢀheterocyclic carbeneꢀpalladium(II); nanomagnetic
catalyst; Synthesis and characterization; SuzukiꢀMiyaura and MizorokiꢀHeck crossꢀcoupling
reactions
2

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1. Introduction
Carbonꢀcarbon bond formation reactions of aryl halides with organoboronic acids in the
presence of palladium and a base, the Suzuki–Miyaura crossꢀcoupling reaction, is one of the
most important and flexible reactions for the synthesis of various biaryls which constitute
important structural units in various natural products, pharmaceutical intermediates, polymers
and many more.1ꢀ4 The notable advantage of using organoboronic acids is their increased
stability in air and moisture, mild reaction conditions, and the feasibility of reactions with a wide
range of functional groups.5 In addition, arylation of olefins in the presence of palladium and a
base, the Mizoroki–Heck crossꢀcoupling reaction, is an enormously useful and common strategy
for synthesizing functionalized olefins6, which contributes a wide range of molecules of use in
pharmaceutical, phytochemical, agrochemical, and polymer industries7. Thus, palladium
catalyzed crossꢀcoupling reactions are a very significant and powerful tool in the synthesis of
numerous molecules with diverse physical, chemical and biological activities.8, 9
Even though palladium catalyzed crossꢀcoupling reactions are very important, the
commercial production of such crossꢀcoupled products are expensive because of the precious
noble metal palladium. As a result, recovery and reuse of the precious palladium based catalyst
will help to reduce the cost of production minimize palladium poisoning in the active
pharmaceutical intermediates, and reduce waste generation.10 To improve their recyclability,
homogeneous catalysts are typically grafted onto different types of supports, rendering them
heterogeneous in nature.11ꢀ13 By implementing a magnetic support to a homogeneous catalyst
makes it easily recyclable with the help of an external magnet, thereby avoiding filtration or
centrifugation techniques for recovery of the catalyst.13ꢀ16
3

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Phosphine ligands are widely employed in palladium catalyzed crossꢀcoupling
reactions.17, 18 However, these ligands have some shortcomings like air and moisture sensitivity,
and they are also toxic in nature.19 The isolation of stable Nꢀheterocyclic carbenes (NHCs) in
1991 by Arduengo20, and subsequent research on NHCs, has revealed that they are good
substitutes for phosphines as ligands in catalysis.21ꢀ23 The chief advantages of NHCs over
phosphines are their comparable or superior activity, higher dissociation energy, ease of largeꢀ
scale synthesis, less toxicity, and increased stability in air and moisture.24, 25 The use of NHC
ligands has many advantages over phosphine analogs in many palladium catalyzed crossꢀ
coupling reactions such as Suzuki–Miyaura, Mizoroki–Heck, BuchwaldꢀHartwig and
Sonogashira crossꢀcoupling reactions.24, 26, 27 The strong σꢀdonor capability of NHC ligand
facilitates the oxidative addition step in the catalytic cycle of crossꢀcoupling reactions.25, 28ꢀ32
Also, reductive elimination in the last step of catalytic cycle is promoted by the increased steric
bulk around the metal in the NHC.28 The metalꢀleaching issue is less pronounced in the case of
NHC–metal complexes, which is due to the strong carbeneꢀmetal bond they form; for these
reasons, these complexes also possess high thermal stability.22
Thus, in continuation of our previous work33, which is focused on green and sustainable
pathways in the field of catalysis and the search for new potential catalysts, we report herein the
synthesis, characterization and catalytic activity of a new NO2ꢀNHCꢀPd@Fe3O4, nanomagnetic
catalyst for Suzuki–Miyaura and Mizoroki–Heck crossꢀcoupling reactions. The prepared NO2ꢀ
NHCꢀPd@Fe3O4 nanomagnetic catalyst is stable under air and moisture and can be synthesized
easily from commercially available inexpensive chemicals. The NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalyst exhibited excellent catalytic activity in both Suzuki–Miyaura and
Mizoroki–Heck crossꢀcoupling reactions with good recyclability. The better catalytic activity of
4

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NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst in Suzuki–Miyaura and Mizoroki–Heck crossꢀ
coupling reactions may be due to the increased steric bulk caused by the addition of nitro group
on the phenyl ring in the present NHC ligand as compared to previously reported
NHC‑Pd@MNPs nanomagnetic catalyst from our laboratory.33
2. Experimental
2.1Materials
Unless otherwise stated, all reactions were performed under aerobic conditions in ovenꢀ
.
.
dried glassware with magnetic stirring. FeCl3 6H2O, FeCl2 4H2O, sodium carbonate, cesium
carbonate, potassium carbonate, ammonium hydroxide, 4ꢀnitrobenzyl bromide, benzimidazole,
(3ꢀchloropropyl)triethoxysilane, palladium(II) acetate, aryl halides, phenylboronic acid, styrene,
tertꢀbutyl acrylate, sodium phosphate tribasic dodecahydrate, were purchased from Sigmaꢀ
Aldrich chemical company and were used without further purification. Solvents were dried and
degassed using a JC Meyer company solvent purification system. Heating was accomplished by
either a heating mantle or silicone oil bath. Reactions were monitored by thinꢀlayer
chromatography (TLC) performed on 0.25 mm Merck TLC silica gel plates, using UV light as a
visualizing agent. Purification of reaction products was carried out by flash column
chromatography using silica gel 60 (230ꢀ400 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 (10ꢀ15 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 a Bruker Alpha Ecoꢀ
ATR spectrometer. BrunauerꢀEmmettꢀTeller surface areas were obtained by physisorption of N2
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using a Microtrac BELSORP MAX instrument. The elemental palladium content of the NO2ꢀ
NHCꢀPd@Fe3O4 nanomagnetic catalyst was determined by Thermo Electron IRIS INTREPID II
XSP DUO inductively coupled plasma atomic emission spectroscopy. Transmission electron
microscope images were obtained using Jeol/JEM 2100 microscope. A JEOL JSM 7100F fieldꢀ
emission scanning electron microscope (FESEM), fitted with energy dispersive Xꢀray
spectroscopy, was used to observe the morphology and elemental distributions of samples.
Thermogravimetric analysis was carried out using a Perkin Elmer, Diamond TG/DTA with a
heating rate of 10.0 °C/min. Xꢀray powder diffraction patterns were obtained using a Bruker
AXS D8 Advance diffractometer. 1H NMR spectra were recorded at 400 MHz, and are reported
1
relative to CDCl3 (δ 7.27). H NMR coupling constants (J) are reported in Hertz (Hz) and
multiplicities are indicated as follows: s (singlet), d (doublet), t (triplet), and m (multiplet).
Liquid chromatography mass spectra (LCꢀMS) were recorded on an Agilent technologies
quadrupole LCꢀMS system. The singleꢀcrystal Xꢀray diffraction data was collected on a Bruker
AXS Kappa Apex 2 CCD diffractometer at 293(2) K. Crystallographic data of the structure
reported in this article was deposited to the Cambridge Crystallographic Data Center with the
deposition number 1552582. A copy of the data can be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 1223 336–033; e–mail:
deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk/deposit]
2.3 Synthesis of magnetite (Fe3O4, magnetic nanoparticles)
Magnetic nanoparticles (MNPs) were synthesized through a coꢀprecipitation method, as
.
.
described in our previous work.33 Briefly, FeCl3 6H2O (2.35g, 8.7 mmol) and FeCl2 4H2O (0.86
g, 4.3 mmol) were dissolved in deionized water (40 mL). The reaction mixture was then stirred
at 85 °C for 30 minutes to completely dissolve both the reactants. Ammonium hydroxide (5 mL)
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solution was then added slowly to the reaction mixture with vigorous stirring at 85 °C to
generate a black colored precipitate. The reaction was continued for an additional 30 minutes.
The precipitated magnetic nanoparticles were then isolated by magnetic decantation, washed
with deionized water repeatedly until neutral pH, further washed with ethanol (2 x 20 mL), and
dried at 80 °C for 6 h.
2.4 Synthesis of silyl chloride functionalized magnetic nanoparticles (SMNPs)
In an ethanol:water (EtOH:H2O) (1:1) mixture (20 mL), hydroxylꢀsubstituted magnetic
nanoparticles (2.0 g) were dispersed by ultrasonication for 30 minutes, followed by the addition
of (3ꢀchloropropyl)triethoxysilane (2.0 g, 8.3 mmol). Then, the reaction mixture was stirred at 45
°C for 24 h. The precipitated dark brown colored silyl chlorideꢀfunctionalized magnetic
nanoparticles were isolated by magnetic decantation after cooling to room temperature, washed
with deionized water (2 x 20 mL) and ethanol (2 x 20 mL) and dried at 60 °C in vacuo.
2.5 Synthesis of 1-(4-nitrobenzyl)-1H-benzimidazole
The synthesis of 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀbenzimidazole was carried out according to a
published literature procedure with slight modification.34 Benzimidazole (1.0 g, 8.46 mmol) was
added to a roundꢀbottomed flask containing methanol (20 mL). The reaction mixture was stirred
at room temperature for 10 minutes. Cesium carbonate was then added into the reaction mixture
under stirring, followed by cooling the reaction mixture to 5ꢀ10 °C. After 10 minutes, 4ꢀ
nitrobenzyl bromide was added under stirring at the same temperature. After 15 minutes, the
temperature was raised to room temperature, with continued stirring of the reaction mixture. The
progress of the reaction was monitored by TLC using ethyl acetateꢀhexane as an eluent. After
reaction completion, the reaction mixture was added to water (60 mL) with stirring to precipitate
the product as a yellowꢀcolored solid. The solid was filtered and washed with water (2 x 40 mL),
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and dried at 50 °C in vacuo to yield 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀbenzimidazole as a yellowꢀcolored
1
solid. Yield: 90%; Melting point = 83ꢀ86 °C; H NMR (400 MHz, CDCl3): δ (ppm) = 8.17 (d, J
= 8.0 Hz, 2H), 7.96 (s, 1H, NCHN), 7.41ꢀ7.38 (m, 4H), 7.32ꢀ7.29 (m, 2H), 5.25 (s, 2H,
CH2C6H5).
2.6 Synthesis of magnetic nanoparticle-tethered 1-(4-nitrobenzyl)-1H-benzimidazolium chloride
(NO2-NHC@Fe3O4)
In toluene (40 mL), silyl chlorideꢀfunctionalized magnetic nanoparticles (2.0 g) were
suspended by ultrasonication for 30 minutes. 1ꢀ(4ꢀNitrobenzyl)ꢀ1Hꢀbenzimidazole (1.0 g, 4.8
mmol) was added in one portion to the reaction mixture and stirred at 110 °C for 48 h. The
reaction mixture was then cooled to room temperature. The resulted brown colored solid was
isolated by magnetic decantation, washed with toluene (5 x 20 mL) and dried at 50 °C under
reduced pressure for 12 h to yield a brownꢀcoloured solid, NO2ꢀNHC@Fe3O4.
2.7 Synthesis of magnetic nanoparticle supported N-heterocyclic carbene-palladium(II) complex
(NO2-NHC-Pd@Fe3O4)
To a solution of Na2CO3 (0.5 M, 20 mL) in dimethylformamide (DMF) (20 mL),
palladium(II) acetate (0.22 g, 0.98 mmol) was added and the reaction mixture was stirred at room
temperature for 15 minutes. Magnetic nanoparticleꢀtethered 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀ
benzimidazolium chloride (1.0 g) was then added to the reaction mixture and stirred at 50 °C for
16 h. Then, the reaction mixture was cooled to room temperature. The resultant product was
isolated by magnetic decantation, washed with water (2 x 20 mL) and methanol (2 x 20 mL), and
dried under reduced pressure for 6 h at 45 °C to yield a brownꢀcoloured solid, the NO2ꢀNHCꢀ
Pd@Fe3O4 nanomagnetic catalyst.
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2.8 General procedure for Suzuki-Miyaura cross-coupling reactions
The magnetic nanoparticle supported NHCꢀpalladium(II) complex (NO2ꢀNHCꢀ
Pd@Fe3O4, 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 roundꢀbottomed flask. To the
reaction mixture, EtOH:H2O (1:1, 5 mL) was added and stirred at room temperature for
designated hours. The progress of the reaction was monitored by TLC. After reaction
completion, the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was separated by using an external
magnet. To the reaction mixture, ethyl acetate (10 mL) and water (10 mL) were added. The
crossꢀcoupled product was extracted to ethyl acetate from water using a separatory funnel and
dried with magnesium sulphate. The dried ethyl acetate was concentrated in vacuo, and the
product was purified by column chromatography using nꢀhexane and ethyl acetate as eluents to
afford the corresponding products in good to excellent yields. All the crossꢀcoupling products
1
were known molecules and were confirmed by comparing the melting point, H NMR and mass
spectroscopic data with authentic samples.
1
1. Biphenyl (Table 2, entries1, 2 and 3): Colorless crystals. Melting point = 68ꢀ70 °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 = 8.0
Hz, 4H). LCꢀMS for C12H10: m/z = 155.07 [M+H]+.
2. 4-Methylbiphenyl (Table 2, entries 4 and 5): White crystalline solid. Melting point = 45ꢀ47 °C;
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.57 (d, J = 6.0 Hz, 2H), 7.49 (d, J = 6.0 Hz, 2H), 7.42
(d, J = 6.0 Hz, 2H), 7.30 (t, J = 6.0 Hz,1H), 7.24 (d, J = 6.0 Hz, 2H), 2.38 (s, 3H). LCꢀMS for
C13H12: m/z = 169.05 [M+H]+.
1
3. 4-Acetylbiphenyl (Table 2, entries 6 and 7): White powder. Melting point = 119ꢀ123 °C; H
NMR (400 MHz, CDCl3): δ (ppm) = 8.03 (d, J = 8.0 Hz, 2H), 7.69 (d, J = 8.0 Hz, 2H), 7.63 (d,
J = 7.6 Hz, 2H), 7.45ꢀ7.48 (m, 2H), 7.39ꢀ7.37 (m, 1H), 2.63 (s, 3H). LCꢀMS for C14H12O: m/z =
197.17 [M+H]+.
4. 4-Hydroxybiphenyl (Table 2, entries 8, 9 and 10): White crystals. Melting point = 164ꢀ166 °C;
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.51 (d, J = 7.2 Hz, 2H), 7.47 (d, J = 7.2 Hz, 2H), 7.36
(d, J = 8.0 Hz, 2H), 7.30ꢀ7.28 (m, 1H), 6.89 (d, J=8.0 Hz, 2H), 4.81 (s, 1H). LCꢀMS for
C12H10O: m/z = 171.15 [M+H]+.
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5. 4-Cyanobiphenyl (Table 2, entry 11): Offꢀwhite crystalline powder. Melting point = 85ꢀ87 °C;
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.73ꢀ7.65 (m, 4H), 7.59 (d, J = 8.0 Hz, 2H), 7.50ꢀ7.47
(t, J = 8.0 Hz, 2H), 7.43ꢀ7.41 (m, 1H). LCꢀMS for C13H9N: m/z = 180.01 [M+H]+.
1
6. 4-Nitrobiphenyl (Table 2, entry 12): Pale yellow crystals. Melting point = 112ꢀ114 °C; H
NMR (400 MHz, CDCl3): δ (ppm) = 8.28 (d, J = 8.0 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H), 7.61 (d,
J = 8.0 Hz, 2H),7.50ꢀ7.44 (m, 3H). LCꢀMS for C12H9NO2: m/z = 200.04 [M+H]+.
7. 4-Aminobiphenyl (Table 2, entry 13): Purple crystals. Melting point = 52ꢀ55 °C; 1H NMR (400
MHz, CDCl3): δ (ppm) = 7.54 (d, J = 8.4 Hz, 2H), 7.44ꢀ7.41 (m, 4H), 7.28ꢀ7.24 (m, 1H), 6.78
(d, J = 8.0 Hz, 2H), 3.72 (s, 2H). LCꢀMS for C12H11N: m/z = 170.03 [M+H]+.
1
8. 4-Phenylbenzaldehyde (Table 2, entry 14): Yellow crystals. Melting point = 57ꢀ59 °C; H
NMR (400 MHz, CDCl3): δ (ppm) = 9.98 (s, 1H), 7.88 (d, J = 6.4 Hz, 2H), 7.68 (d, J = 6.0 Hz,
2H), 7.57 (d, J = 6.8 Hz, 2H), 7.42 (m, 3H). LCꢀMS for C13H10O: m/z = 183.09 [M+H]+.
1
9. 2-Phenylbenzaldehyde (Table 2, entries 15 and 16): Yellow oil. H NMR (400 MHz, CDCl3):
δ (ppm) = 9.98 (s, 1H), 8.01 (d, J = 6.0 Hz, 1H), 7.63 (t, J = 6.0 Hz, 1H), 7.47 (m, 5H), 7.38 (d,
J = 6.0 Hz, 2H). LCꢀMS for C13H10O: m/z = 183.05 [M+H]+.
10. 4-Phenylbenzoic acid (Table 2, entry 17): White solid. Melting point = 219ꢀ224 °C; 1H NMR
(400 MHz, CDCl3): δ (ppm) = 8.03 (d, J = 6.0 Hz, 2H), 7.81 (d, J = 6.0 Hz, 2H), 7.74 (d, J = 6.0
Hz, 2H), 7.52ꢀ7.49(t, J = 6.0 Hz, 2H), 7.44ꢀ7.41(t, J = 6.4 Hz, 1H). LCꢀMS for C13H10O2: m/z =
199.11 [M+H]+.
11. 3-Phenylbenzoic acid (Table 2, entry 18): Offꢀwhite crystals. Melting point = 164ꢀ168 °C; 1H
NMR (400 MHz, CDCl3): δ (ppm) = 8.18 (s, 1H), 7.93 (t, J = 7.2 Hz, 2H), 7.70 (d, J = 6.0 Hz,
2H), 7.61 (t, J = 7.2 Hz, 1H), 7.52ꢀ7.40 (m, 3H). LCꢀMS for C13H10O2: m/z = 199.06 [M+H]+.
12. 4-Methoxybiphenyl (Table 2, entries 19 and 20): White powder. Melting point = 86ꢀ90 °C;
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.54ꢀ7.51 (m, 4H), 7.40 (t, J = 7.2 Hz, 2H), 7.30 (t, J =
7.6 Hz, 1H), 6.98 (d, J = 8.0 Hz, 2H), 3.85 (s, 3H). LCꢀMS for C13H12O: m/z = 185.10 [M+H]+.
13. 2,4-Difluoro-1,1’-biphenyl (Table 2, entry 21): Pale yellow crystals. Melting point = 61ꢀ65
1
°C; H NMR (400 MHz, CDCl3): δ (ppm) = 7.58 (d, J = 6.0 Hz, 1H), 7.48 (d, J = 6.0 Hz, 2H),
7.42ꢀ7.36 (m, 4H), 6.91ꢀ6.89 (m, 1H). LCꢀMS for C12H8F2: m/z = 191.02 [M+H]+.
2.9 General procedure for Mizoroki-Heck cross-coupling reactions
The magnetic nanoparticle supported NHCꢀpalladium(II) complex (NO2ꢀNHCꢀ
Pd@Fe3O4, nanomagnetic catalyst) (1.0 mol% Pd), aryl halide (1.0 mmol), alkene (1.2 mmol)
and sodium phosphate tribasic dodecahydrate (2.0 mmol) were placed in a roundꢀbottomed flask.
To the reaction mixture, acetonitrile (5 mL) was added and stirred at 80 °C for designated hours.
The progress of the reaction was monitored by TLC. After reaction completion, the reaction
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mixture to room temperature, and the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was
separated by using an external magnet. To the reaction mixture, ethyl acetate (10 mL) and water
(10 mL) were added. The ethyl acetate layer was separated from water layer through separatory
funnel and dried with magnesium sulphate. The dried ethyl acetate was concentrated in vacuo,
and the product was purified by column chromatography using nꢀhexane and ethyl acetate as
eluents to afford the corresponding products in moderate to excellent yields. All the coupling
1
products were known molecules and were confirmed by comparing the H NMR spectral data
with the authentic samples.
1. (E)-1,2-diphenylethene (Table 5, entries 1 and 2): White solid; 1H NMR (400 MHz, CDCl3): δ
(ppm) = 7.53ꢀ7.51 (m,4H), 7.36 (t, J = 8.0 Hz, 4H),7.28ꢀ7.24 (m, 4H).
2. (E)-tert-butyl 3-(4-hydroxyphenyl)acrylate (Table 5, entries 3, 4 and 5): Light brown liquid;
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.53 (d, J = 15.8 Hz, 1H), 7.40 (s, 2H), 6.83 (s, 2H),
6.24 (d, J =15.8 Hz, 1H), 1.53 (s, 9H).
3. (E)-tert-butyl 3-(p-tolyl)acrylate (Table 5, entries 6 and 7): Light yellow liquid; 1H NMR (400
MHz, CDCl3): δ (ppm) = 7.40 (d, J = 8.0Hz, 3H), 7.29 (s, 2H), 6.27 (d, 16.0 Hz, 1H), 2.29 (s,
3H), 1.53 (s, 9H).
1
4. (E)-1-methyl-4-styrylbenzene (Table 5, entries 8 and 9): White solid; H NMR (400 MHz,
CDCl3): δ (ppm) = 7.88 (s, 2H), 7.64ꢀ7.39 (m, 9H), 2.18 (s, 3H).
5. (E)-tert-butyl 3-(4-acetylphenyl)acrylate (Table 5, entries 10 and 11): Yellow liquid; 1H NMR
(400 MHz, CDCl3): δ (ppm) = 7.33ꢀ7.05 (m, 5H), 6.11 (d, J =16.0 Hz, 1H), 2.25 (s, 3H), 1.25 (s,
9H).
6. (E)-tert-butyl 3-(2,4-difluorophenyl)acrylate (Table 5, entry 12): Light brown liquid; 1H NMR
(400 MHz, CDCl3): δ (ppm) = 7.72 (d, J = 15.8 Hz, 1H), 7.54 (d, J = 8.0Hz, 3H), 7.37 (d, J =
15.8 Hz, 1H), 1.28 (s, 9H).
1
7. (E)-tert-butyl 3-(4-cyanophenyl)acrylate (Table 5, entries 13 and 14): Colorless crystal; H
NMR (400 MHz, CDCl3): δ (ppm) = 7.84 (d, J = 8.0Hz, 3H), 7.70 (d, J = 8.0 Hz, 2H), 6.64 (d, J
= 16.0 Hz, 1H), 1.53 (s, 9H).
8. tert-butyl cinnamate (Table 5, entry 15): Offꢀwhite solid; 1H NMR (400MHz, CDCl3): δ (ppm)
= 7.56 (d, J = 15.8 Hz, 1H), 7.51 (s, 2H), 7.36 (d, J = 8.0Hz, 3H), 6.37 (d, J = 16.0Hz, 1H), 1.53
(s, 9H).
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1
9. (E)-tert-butyl 3-(4-methoxyphenyl)acrylate (Table 5, entries 16, 17 and 18): Yellow liquid; H
NMR (400 MHz, CDCl3): δ (ppm) = 7.62 (d, J = 15.8 Hz, 1H), 7.45 (s, 2H), 7.13 (d, J = 8.0 Hz,
2H), 6.24 (d, J = 16.0Hz, 1H), 3.82 (s, 3H), 1.52 (s, 9H).
2.10 Procedure for recovery of the NO2-NHC-Pd@Fe3O4 nanomagnetic catalyst
After reaction completion, the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was
separated using an external magnet. The NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was then
subjected to washing with ethanol (2 x 10 mL), water (2 x 10mL), and methanol (2 x 10 mL).
The isolated NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was dried at 45 °C under vacuum for 6
h. The dried NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was used for the next round of reaction
without further purification.
3. Results and Discussion
3.1 Synthesis of NO2-NHC-Pd@Fe3O4 nanomagnetic catalyst
In an extension of our previous, the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was
synthesized through a multistep strategy as described in Scheme 1. Hydroxylꢀsubstituted
magnetic nanoparticles (1) were prepared by a coꢀprecipitation method, followed by the
functionalization of magnetic nanoparticles with (3ꢀchloropropyl)triethoxysilane (2) in
EtOH:H2O mixture at 45 °C. 4ꢀNitrobenzyl bromide was reacted with benzimidazole in the
presence of cesium carbonate in methanol to get the intermediate 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀ
benzimidazole (4). Additionally, the structure of 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀbenzimidazole was
confirmed by singleꢀcrystal X–ray diffraction analysis. Suitable single crystals for Xꢀray
diffraction analysis of 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀbenzimidazole were grown by a slow evaporation
method. The molecular structures of the two independent molecules (A and B) of the 1ꢀ(4ꢀ
nitrobenzyl)ꢀ1Hꢀbenzimidazole (4) compound are illustrated in Fig. 1. 1ꢀ(4ꢀNitrobenzyl)ꢀ1Hꢀ
benzimidazole was then treated with silyl functionalized magnetic nanoparticles (SMNPs) (3) in
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toluene at 110 °C for 48 h. The obtained magnetic nanoparticleꢀtethered 1ꢀ(4ꢀnitrobenzyl)ꢀ1Hꢀ
benzimidazolium chloride (5) was treated with palladium(II) acetate in dimethylformamide
(DMF) in the presence of aqueous sodium carbonate to yield the NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalyst (6).
OH
OH
HO
HO
HO
O
O
O
EtO
EthanolꢀWater
45 0 C, 24 h
b)
+
MNP
OH EtO Si
EtO
Cl
MNP
Si
Cl
OH
OH
1
2
3
O2N
Toluene
110 0C
48 h
N
N
4
Pd(OAc)2
Na2CO3
O
Cl Pd Cl
Cl
O
MNP O
O
Si
N
N
MNP O
O
Si
N
N
DMF
50 0C, 16 h
5
NO2
6
NO2
NO2ꢀNHCꢀPd@Fe3O4
Scheme 1. Synthetic schemes of (a) magnetic nanoparticles (1) and (b) NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalyst (6).
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N
N
NO2
4
Molecule B
Molecule A
Fig. 1. A view of the molecular structure of the two independent molecules (A and B) of the 1ꢀ
(4ꢀnitrobenzyl)ꢀ1Hꢀbenzimidazole (4), with atom labelling. Displacement ellipsoids are drawn at
the 50% probability level.
3.2 Spectroscopic and microscopic characterization of the NO2-NHC-Pd@Fe3O4 nanomagnetic
catalyst
ATRꢀIR was employed for the spectroscopic characterization of products formed in each
step for the synthesis of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst. Fig. 2 shows ATRꢀIR
spectra of Fe3O4, silylꢀfunctionalized Fe3O4, and the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic
catalyst. The peaks at 585 cmꢀ1 and 3392 cmꢀ1 in the IR spectra of Fe3O4 are attributed to FeꢀO
and OꢀH stretching vibrations (Fig. 2a), respectively. The peak at 1641 cmꢀ1 corresponds to the
OꢀH overtone band, which confirms the presence of adsorbed water layer on Fe3O4. Two extra
peaks in Fig. 2b at 991 cmꢀ1 and 2912 cmꢀ1, when compared with Fig. 2a, are due to SiꢀO and Cꢀ
H stretching respectively, which confirms the surface functionalization of Fe3O4 with (3ꢀ
chloropropyl)triethoxysilane. The characteristic peaks at 1521, 1656, and 985 cmꢀ1 in the IR
spectrum of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst (Fig. 2c) are due to NꢀO, C=C and
SiꢀO vibrations, respectively, apart from the peaks at 2930 cmꢀ1 and 3379 cmꢀ1, which are due to
CꢀH and OꢀH stretching; this confirms the structure of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic
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catalyst. The IR spectra of the sevenꢀtimes recycled NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst
shows the structure is intact even though there is some shift in the peaks, as shown in Fig. 2d.
Fig. 2. ATRꢀIR spectra of (a) MNPs, (b) SMNPs, (c) NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic
catalyst and (d) seven times recycled NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst from Suzukiꢀ
Miyaura crossꢀcoupling.
BET analysis was performed to confirm the surface functionalization of Fe3O4. Fig. 3
shows the nitrogen adsorptionꢀdesorption curve for (a) MNPs, (b) SMNPs, and (c) the NO2ꢀ
NHCꢀPd@Fe3O4 nanomagnetic catalyst. The NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst
exhibited a typeꢀII isotherm. The amount of N2 adsorbed on bare MNPs was high as compared to
SMNPs and the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst. Fe3O4 showed a surface area of
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77.47 m2gꢀ1 that was reduced to 76.60 m2gꢀ1 after surface functionalization with (3ꢀ
chloropropyl)triethoxysilane. The surface area further decreased for the NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalyst to 69.61 m2gꢀ1, which confirms the surface functionalization of Fe3O4.
(a)
(b)
(c)
Fig. 3. Nitrogen adsorptionꢀdesorption curves for (a) MNPs, (b) SMNPs and (c) NO2ꢀNHCꢀ
Pd@Fe3O4 nanomagnetic catalyst.
TEM analysis was performed to observe the morphology and size of Fe3O4 nanoparticles
and the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst (Fig. 4). Quasiꢀspherical morphology was
observed for Fe3O4, with sizes ranging from 7ꢀ11 nm. After the grafting of the NHCꢀ
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palladium(II) complex, the particle size increased to 11ꢀ15 nm with quasiꢀspherical morphology
(Fig. 4b). Due to their relatively high contrast, TEM images show the magnetic core as dark
spots inside the quasiꢀspherical NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalysts. The TEM image
of seven times recycled NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst from SuzukiꢀMiyaura
crossꢀcoupling (Fig. 4c) shows an almost identical morphology and size.
(a)
(b)
(c)
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Fig. 4. TEM images of (a) MNPs, (b) NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst and (c) seven
times recycled NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst from SuzukiꢀMiyaura crossꢀ
coupling.
The morphology and size of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst and the
recycled NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst were evaluated through FESEM (Fig. 5).
The FESEM image of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst shows the formation of
uniform nanometerꢀsized spherical particles (Fig. 5a). Very interestingly, seven times recycled
NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst from SuzukiꢀMiyaura crossꢀcoupling reaction
displayed the same morphology and size and which is in good agreement with TEM results.
Elemental dotꢀmapping of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was carried out using
EDS (Fig. 6). The characteristic peaks of all the elements present (C, N, O, Fe, Si, Pd and Cl) in
the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was confirmed by energyꢀdispersive Xꢀray
spectroscopy EDS (Fig. 7). Thus, EDS spectrum confirms the successful grafting of NHC–
palladium(II) complex on Fe3O4. All the elements present in the catalyst are uniformly scattered,
which is evident from the area mapping by EDS, as shown in Fig. 6. Quantitative assessment of
palladium(II) loading in the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was determined by
ICPꢀAES. The ICPꢀAES analysis of the catalyst revealed that 2.48% w/w of palladium was
loaded onto the Fe3O4 nanomagnetic support.
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(a)
(b)
Fig. 5. FESEM images of (a) NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst and (b) seven times
recycled NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst from SuzukiꢀMiyaura crossꢀcoupling.
N
O
Si
Cl
Pd
Fe
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C
Fig. 6. EDS elemental mapping of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst.
Fig. 7. EDS spectrum of NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst.
The Xꢀray powder diffraction (XRD) pattern of Fe3O4 (Fig. 8a) shows a cubic spinel
structure, with peaks attributed to Fe3O4 at 2θ = 30.09°, 35.59°, 43.07°, 53.43°, 57.37° and
62.90°, which corresponds to the reflection planes (220), (311), (400), (422), (511) and (440),
respectively. From the XRD pattern of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst (Fig.
8b), it is clear that the phase remained unaltered even after anchoring of the NHCꢀpalladium(II)
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complex onto Fe3O4. No characteristic peaks for palladium were detected in the XRD pattern of
the nanomagnetic catalyst, which indicates that the palladium sites are well dispersed on the
magnetite support.35 The crystal size of Fe3O4, obtained using the Debye Scherrer equation, was
12.85 nm; the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was 13.52 nm, which is in agreement
with TEM images.
(b)
(a)
Fig. 8. XRD patterns of (a) MNPs and (b) NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst.
Thermogravimetric analysis (TGA) was performed to study the thermal stability of Fe3O4
nanoparticles and the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst, using a heating rate of 10
°C/min under a nitrogen atmosphere between 40 °C and 730 °C (Fig. 9). The loss of surface
hydroxyl groups and moisture resulted in a weight loss of 6% between 80ꢀ200 °C for Fe3O4 (Fig.
9a), whereas the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst showed decomposition in two
stages (Fig. 9b). The loss of adsorbed water molecules at a temperature up to 110 °C forms the
first stage; loss of covalently bonded organic moieties at a temperature up to 500 °C resulted in a
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weight loss of 7.8%. Also, it is clear that the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst is
stable up to 220ꢀ250 °C, which is advantageous for its use in highꢀtemperature reactions.
Fig. 9. TGA curve of (a) MNPs and (b) NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst.
3.3 Application of the NO2-NHC-Pd@Fe3O4 nanomagnetic catalyst in Suzuki-Miyaura cross-
coupling reactions
Subsequent to the complete characterization of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic
catalyst by a suite of analytical techniques, the catalytic potential of the nanomagnetic catalyst
for Suzuki–Miyaura crossꢀcoupling reactions was assessed. The Suzuki–Miyaura crossꢀcoupling
reaction between bromobenzene and phenylboronic acid was chosen as the model reaction.
Varying sets of conditions were assessed for the Suzuki–Miyaura crossꢀcoupling reaction (Table
1), which resulted yields as high as 95%. We found that increases in time and temperature did
not improve the yield, and the optimal catalyst ratio was 0.15 mol% of palladium. Also, the use
of potassium carbonate and EtOH:H2O (1:1) was superior relative to the use of other bases and
solvents. However, unlike our previously reported nanomagnetic catalyst33, the reaction
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proceeded well at room temperature. The increased steric bulk caused by the nitro group in NHC
ligand may be the reason for better activity of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst at
lower temperatures. Thus, through a series of reactions, the Suzuki–Miyaura crossꢀcoupling
reaction conditions were optimized with potassium carbonate as base, EtOH:H2O (1:1) mixture
as solvent, room temperature and 0.15 mol% of palladium as the catalyst ratio for 2 h.
Table 1. Optimization of conditions for SuzukiꢀMiyaura crossꢀcoupling reaction of
bromobenzene with phenylboronic acid in the presence of the NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalysta.
Pd (mol %)
Entry
1
Base
Solvent
Temp. (°C)
70
Time (h)
Yield (%)b
95
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
K2CO3 EtOH:H2O (1:1)
Na2CO3 EtOH:H2O (1:1)
Na2CO3 EtOH:H2O (1:1)
Na3PO4 EtOH:H2O (1:1)
CsCO3 EtOH:H2O (1:1)
0.15
0.15
0.15
0.15
0.15
0.15
0.10
0.10
0.2
1
2
1
1
2
3
2
3
1
2
2
3
3
3
3
3
3
2
3
4
5
6
7
8
9
10
11
11
12
13
14
15
16
70
50
95
95
85
95
95
83
85
95
95
80
90
73
78
75
78
58
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
0.2
0.15
0.15
0.15
0.15
0.15
0.15
0.15
K2CO3
K2CO3
K2CO3
EtOH
H2O
DMF
aReaction conditions: Bromobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), base (2.2 mmol) and
solvent (5 mL) in air.
bIsolated yield after separation by column chromatography; average of two runs.
The catalytic activity of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was explored in
SuzukiꢀMiyaura crossꢀcoupling between various aryl halides and phenylboronic acid, as shown
in Table 2. Among the various aryl halides employed, with both electronꢀdonating groups as well
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as electronꢀwithdrawing groups, the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst gave
comparable yields under the optimized conditions. It is observed that the steric effect greatly
hindered the reaction, and it has been clearly demonstrated by the comparison of the crossꢀ
coupling reaction yields of 2ꢀbromobenzaldehyde as well as 4ꢀbromobenzaldehyde with
phenylboronic acid. In addition, aryl iodides gave excellent yields when compared to their
bromide and chloride counterparts due to the increased reactivity of aryl iodides. Aryl bromides
in turn gave good yields when compared with their chloride counterpart. The selectivity of the
NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst was confirmed by the presence of a negligible
amount of homocoupled product obtained. For further authentication on selectivity, Suzuki–
Miyaura crossꢀcoupling reactions were carried out without aryl halides under optimized
conditions. The biphenyl homocoupled product obtained was in a trace amount, which proved
that the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst is highly selective. 36, 37
Table 2. SuzukiꢀMiyaura crossꢀcoupling reactions of different aryl halides with phenylboronic
acid by using NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalysta.
Entry
1
Aryl halide
Product
Time (h)
1
Yield (%)b
98
H
H
H
2
3
4
5
2
4
1
2
95
89
90
84
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6
7
2
3
95
80
8
1
2
6
2
2
2
93
86
52
94
96
91
9
10
11
12
13
14
15
3
7
85
57
16
17
18
24
2
ꢀ
90
91
2
19
20
1
2
99
93
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21
2
92
aReaction conditions: Aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol), NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalyst (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.
3.4 Recyclability of the NO2-NHC-Pd@Fe3O4 nanomagnetic catalyst in Suzuki-Miyaura cross-
coupling reactions
Recovery and recycling of the catalyst after reaction completion is the key benefit
associated with heterogeneous catalysts. In order to study the recycling efficiency of the NO2ꢀ
NHCꢀPd@Fe3O4 nanomagnetic catalyst, the Suzuki–Miyaura crossꢀcoupling reaction between
bromobenzene and phenylboronic acid was chosen. From the recycling study as shown in Table
3, the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst retained its activity up to seven recycles
without a considerable drop (Fig. 10). From the eighthꢀcycle onwards, a drop in yield is observed
which reached up to 81% by the end of the eleventh recycle. The reaction conditions employed
were milder when compared to our previous work33, which may also account for the increase in
number of recycles.
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Fig. 10. Recycling efficiency of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst for the reaction
of bromobenzene and phenylboronic acid.
Table 3. Recyclability of NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst in SuzukiꢀMiyaura crossꢀ
coupling reactions of bromobenzene with phenylboronic acida.
Entry
1
2
3
4
5
6
7
8
Base
Solvent
Temp. (°C)
R.T
Catalyst run
Fresh
Yield (%)b
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
95
95
95
95
95
94
94
93
90
87
83
81
R.T
R.T
R.T
R.T
R.T
R.T
R.T
R.T
1st recycle
2nd recycle
3rd recycle
4th recycle
5th recycle
6th recycle
7th recycle
8th recycle
9th recycle
10th recycle
11th recycle
9
10
11
12
R.T
R.T
R.T
aReaction conditions: Bromobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), NO2ꢀNHCꢀ
Pd@Fe3O4 nanomagnetic catalyst (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.
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3.5 Application of the NO2-NHC-Pd@Fe3O4 nanomagnetic catalyst in Mizoroki-Heck cross-
coupling reactions
Encouraged by the excellent results obtained from Suzuki–Miyaura crossꢀcoupling
reactions, we proceeded to study the catalytic activity of NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic
catalyst in Mizoroki–Heck crossꢀcoupling reactions. The crossꢀcoupling reaction between
bromobenzene and styrene was taken as the model reaction. The reaction conditions were
optimized by following a series of reactions, as listed in Table 4. Reactions were carried out by
varying different parameters such as base, solvent, temperature, and catalyst ratio to determine
the optimum conditions. Among the bases used for the reaction, sodium phosphate
dodecahydrate was found superior. Regarding solvents, acetonitrile (MeCN) was found
promising at a temperature of 80 °C, followed by 6 h reaction time with 1 mol% of palladium
catalyst.
Table 4. Optimization of conditions for the MizorokiꢀHeck crossꢀcoupling reaction of
bromobenzene with styrene in the presence of NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalysta.
NO2ꢀNHCꢀPd@Fe3O4
Br
+
Entry
1
Base
Solvent
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
MeCN
Temp. (°C) Pd (mol %) Time (h) Yield (%)b
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
R.T
50°C
70°C
70°C
70°C
70°C
70°C
80°C
100°C
80°C
80°C
80°C
0.15
0.15
0.15
0.25
0.5
1
1
1
1
1
6
6
6
6
6
6
6
6
6
10
23
50
60
75
78
85
88
90
92
92
92
2
3
4
5
6
7
8
9
K2CO3
.
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4 12H2O
10
11
12
6
6
12
MeCN
MeCN
2
1
28

Page 29 of 39
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ01876B
.
13
14
15
16
17
18
19
20
21
22
23
Na3PO4.12H2O
MeCN
MeCN
1,4ꢀDioxane
Acetone
Toluene
DMF
80°C
R.T
1
1
1
1
1
1
1
1
1
1
1
3
6
6
6
6
6
6
6
6
6
6
81
50
55
25
70
83
48
39
28
68
72
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4.12H2O
Na3PO4 12H2O
NaOH
80°C
56°C
80°C
80°C
78°C
80°C
80°C
80°C
80°C
EtOH
H2O
MeCN
MeCN
MeCN
Na2CO3
CsCO3
aReaction conditions: bromobenzene (1.0 mmol), styrene (1.2 mmol), NO2ꢀNHCꢀPd@Fe3O4
nanomagnetic catalyst (1 mol% Pd with respect to aryl halide), base (2.0 mmol) and solvent (5 mL) in air.
bIsolated yield after separation by column chromatography; average of two runs.
In order to examine the scope of the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst in
arylation of olefins, various aryl halides were coupled with alkenes under the optimized
conditions, as shown in Table 5. Due to increased reactivity of aryl iodides, coupling was faster
and resulted in excellent yields. Aryl bromides also reacted efficiently with alkenes under
optimized conditions resulting in good yields. But aryl chlorides gave lower yields when
compared to their iodide and bromide counterparts. On the other hand, aryl halides with both
electronꢀdonating groups and electronꢀwithdrawing groups were well tolerated and gave desired
crossꢀcoupling products in good to excellent yields in different time duration.38 From these
results, it can be concluded that the NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalyst can catalyze
Mizoroki–Heck crossꢀcoupling reaction efficiently.
Table 5. MizorokiꢀHeck crossꢀcoupling of different aryl halides with alkenes in the presence of
NO2ꢀNHCꢀPd@Fe3O4 nanomagnetic catalysta.
Entry
1
R
Olefin
Alkene
Time (h) Yield (%)b
96
5
29