Efficient catalytic activation of Suzuki-Miyaura C-C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides

Article abstract of DOI:10.1039/c5ra00441a

n-Bromodocosane reacts with Na₂S, generated in situ by the reduction of elemental sulfur with NaBH₄, to give n-didocosyl sulfide (L1), which acts as a protector for palladium nanoparticles (2-7) that are prepared using different palladium precursors in the presence of L1 (Pd:L1 ratio 1:2 and 4:1). The NPs have been characterized with powder X-ray diffraction, SEM, EDX, UV-vis spectroscopy and HRTEM. The size (nm) ranges for the majority of spherical NPs 2-7 are ～18-19, 4-5, 5-7, 4-6, 7-9 and 4-6 respectively. The precursor of palladium affects the size, shape and dispersion of the NPs. When [Pd(CH₃CN)₂Cl₂]/Na₂PdCl₄ was used as a precursor, uniformly dispersed NPs of narrow size range were obtained. L1 and its complex [Pd(L1)₂Cl₂] (1) have also been synthesized by the reaction of Na₂PdCl₄ with L1 and characterized with ¹H and ¹³C{¹H} NMR spectroscopy. The NPs show good catalytic activity for the Suzuki-Miyaura coupling (SMC) of various aryl chlorides/bromides with phenylboronic acid at low catalyst loading (0.1-0.5 mol% of Pd). The conversion is good for some aryl halides in a short reaction time of the order 1-2 h. Among 2-7, the highest activity is observed for Pd NPs obtained from Na₂PdCl₄, which is probably due to uniformity in their size and dispersion. The distinct advantage of NPs 2-7 is that they can be separated and reused at least up to five times. The complex 1, equivalent to 0.001 mol% Pd, is efficient for the SMC of some aryl halides, as good conversion into coupled products has been observed. Two phase tests, conducted for 1 and 3, suggest the contribution of both homogeneous and heterogeneous catalytic pathways in overall catalysis. This journal is

Full text of DOI:10.1039/c5ra00441a

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PAPER
Efficient catalytic activation of Suzuki–Miyaura
C–C coupling reactions with recyclable palladium
nanoparticles tailored with sterically demanding
di-n-alkyl sulfides†
Cite this: RSC Adv., 2015, 5, 20081
Satyendra Kumar, G. K. Rao, Arun Kumar, Mahabir P. Singh, Fariha Saleem
and Ajai K. Singh*
2 4
n-Bromodocosane reacts with Na S, generated in situ by the reduction of elemental sulfur with NaBH , to
give n-didocosyl sulfide (L1), which acts as a protector for palladium nanoparticles (2–7) that are prepared
using different palladium precursors in the presence of L1 (Pd : L1 ratio 1 : 2 and 4 : 1). The NPs have been
characterized with powder X-ray diffraction, SEM, EDX, UV-vis spectroscopy and HRTEM. The size (nm)
ranges for the majority of spherical NPs 2–7 are ꢀ18–19, 4–5, 5–7, 4–6, 7–9 and 4–6 respectively. The
precursor of palladium affects the size, shape and dispersion of the NPs. When [Pd(CH
Na PdCl was used as a precursor, uniformly dispersed NPs of narrow size range were obtained. L1 and
its complex [Pd(L1) Cl ] (1) have also been synthesized by the reaction of Na PdCl with L1 and
3 2 2
CN) Cl ]/
2
4
2
2
2
4
1
13
1
characterized with H and C{ H} NMR spectroscopy. The NPs show good catalytic activity for the
Suzuki–Miyaura coupling (SMC) of various aryl chlorides/bromides with phenylboronic acid at low
catalyst loading (0.1–0.5 mol% of Pd). The conversion is good for some aryl halides in a short reaction
2 4
time of the order 1–2 h. Among 2–7, the highest activity is observed for Pd NPs obtained from Na PdCl ,
which is probably due to uniformity in their size and dispersion. The distinct advantage of NPs 2–7 is that
they can be separated and reused at least up to five times. The complex 1, equivalent to 0.001 mol% Pd,
is efficient for the SMC of some aryl halides, as good conversion into coupled products has been
observed. Two phase tests, conducted for 1 and 3, suggest the contribution of both homogeneous and
heterogeneous catalytic pathways in overall catalysis.
Received 9th January 2015
Accepted 2nd February 2015
DOI: 10.1039/c5ra00441a
www.rsc.org/advances
surface area and morphology are important considerations for
catalytic performance. The high surface-to-volume ratio and
Introduction
Palladium catalysts are widely used for the Suzuki–Miyaura increase in atomic distribution on the surface are expected to
coupling (SMC), which is a powerful synthetic tool in organic enhance the catalytic activity. The synthesis of well-dened and
1
chemistry for C–C bond formation. They include both molec- mono-dispersed (small size range) Pd NPs continues to be a
2,3
ular and nano sized species. Though the catalysis is carried challenge. The ligands used to protect Pd NPs may reduce the
out by palladium(0) in the case of both Pd nanoparticles (NPs) catalytic activity. Thus, the protection strategy for NPs should be
14
and molecular complexes, the role of the ligand present appears such that the activity loss is minimal. Note that the role of the
to be crucial as it affects the efficiency of the catalyst. Therefore, protecting ligand is very crucial because it determines the
phosphorus, carbene, oxime and several other ligands have dispersion of the NPs and affects their catalytic efficiency,
15
been used to design molecular complexes of palladium, which recyclability, composition, solubility and size. Various agents,
4
–12
are suitable as efficient catalysts for SMC.
complexes do not catalyze the coupling but act as dispensers of compounds, ionic liquids and surfactants, have been found to
Generally, these such as dendrimers, organic ligands, organouorous
2,4,16
real palladium(0) containing catalytic species (via nano-sized protect metal NPs.
palladium if formed in the course of catalysis). When palla- as thiol, amine, carbonyl or hydroxyl, in a protector limits the
dium NPs are used as catalysts for this coupling reaction, their activity of NPs due to strong chemisorptions. Recently, some
organochalcogen ligands, mainly alkyl thiolates, have been
The presence of a functional group, such
13
15
utilized as protectors for designing catalytically active transition
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016,
16–18
India. E-mail: aksingh@chemistry.iitd.ac.in; ajai57@hotmail.com; Fax: +91 11 metal (such as Pd, Pt and Au) NPs.
The thiolate protected Pd
2
6581102; Tel: +91 11 26591379
and Pt NPs that have been reported as catalysts for organic
reactions include palladium NPs protected with alkylthiolate for
†
Electronic supplementary information (ESI) available: Spectral data of L1 and 1;
SEM, EDX and powder XRD of NPs. See DOI: 10.1039/c5ra00441a
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1
4b,17e
the catalysis of Suzuki–Miyaura coupling
tion of allyl alcohol,
and isomeriza- record UV-vis spectra. Melting points of the ligand and complex
17f–h
as well as Pt NPs, which are protected were determined in an open capillary sealed at one end with an
with alkylthiol and alkyl amine and have good potential for apparatus equipped with electric heating and reported as such.
methanol oxidation and hydrogenation of allyl alcohol and The products of catalytic reactions were separated and puried
17a–c,h
maleic acid, respectively.
Hydrogenation of allylamine has (if required) by chromatography using silica gel (60–120 mesh)
been successfully catalyzed by cyclodextrin thiolate protected Pt column. n-Hexane and its mixtures with chloroform/ethyl
17d
and Pd nanoparticles. Surprisingly, thioethers, which are so acetate in variable proportions were used as eluents. The
donors, less toxic and easy to handle in air, have been slightly products were authenticated by matching their NMR data with
explored as protectors to design catalytically active metal NPs. those reported in the literature. Commercially available
Recently, Obare and coworkers have reported the hydrogena- reagents were used as received. All the reactions were carried
18
tion of styrene using Pd NPs protected with a thioether ligand.
out in an oven-dried round bottom ask under ambient
However, predictable control of particle size with ligands conditions. Before use, commercial nitrogen gas was succes-
18
continues to be a challenge. The precursor of metal and pro- sively passed through traps containing solutions of alkaline
tecting ligand together may be envisaged as a suitable combi- anthraquinone–sodium dithionite, alkaline pyrogallol, and
nation for controlling, to some extent, the properties of NPs, concentrated H SO and KOH pellets. Nitrogen atmosphere, if
2 4
including catalytic activity. The bulky thioether ligand used as a required, was created using Schlenk techniques. An atomic
protector may result in good dispersion and, in turn, an active absorption spectrophotometer (AAS), Lab India AAS 7000, was
nano-sized metal catalyst. Herein, we report the Pd NPs used to determine Pd content in NPs. 10, 20, and 30 ppm
prepared using different Pd precursors and didocosyl sulde solutions of Pd(II) prepared from Na PdCl in deionised water
2
4
[
CH (CH ) CH –S–CH –(CH ) CH ] (L1) as a protecting were used as standards.
3 2 20 2 2 2 20 3
ligand. Their catalytic performance for SMC has been studied
and found to be promising; moreover, they are found to be
recyclable. The synthesis of the palladium(II) complex
Starting materials
2 2 4
[PdCl (L1) ] and its activity to catalyze SMC are also reported; Sulfur powder, NaBH , 1-bromodocosane, sodium tetra-
moreover, the complex is also an efficient catalyst (couples chloropalladate, palladium acetate, palladium(II) chloride,
some ArBr at a Pd loading level of as low as 0.0001 mol%). It potassium carbonate, caesium carbonate, phenylboronic acid
activates coupling via formation of Pd based NPs. Note that the and all aryl halides were procured from Aldrich (USA). Bis(ace-
Pd precursors affect size, shape, and dispersion of Pd NPs, tonitrile)dichloropalladium(II) was prepared by reuxing palla-
10,15b
which in turn affect their catalytic activity.
are described in this paper.
All these results dium chloride in acetonitrile.
Syntheses of L1 (n-C H ) S. Sulfur powder (0.06 g,
2
2
45 2
ꢁ
2.0 mmol) was stirred in dry ethanol (30 mL) for 1 h at 70 C.
Aqueous saturated solution of NaBH containing NaOH (0.20 g)
was added dropwise to it under a nitrogen atmosphere. Aer the
reaction mixture turned greenish yellow, solid 1-bromodoco-
4
Experimental section
Physical measurement
1
13
1
H and C{ H} spectra were recorded on a Bruker Spectrospin sane (1.55 g, 4 mmol) was added with stirring. Thereaer, the
ꢁ
DPX 300 NMR spectrometer at 300.13 and 75.47 MHz, respec- reaction mixture was stirred further at 60 C for 8 h. On
tively. The chemical shis are reported in ppm relative to the completion of the reaction, the precipitate was ltered, washed
1
3
internal standard tetramethylsilane. C DEPT NMR was used to with ethanol (3 ꢂ 30 mL) and dried in vacuo. Yield: white solid;
ꢁ
routinely determine the number of hydrogen atoms linked to (1.11 g) 85%; m.p. 68 C. Anal. found: C, 79.74%; H, 13.26%.
1
various carbon atoms. Elemental analyses were carried out with Calc. for C44
H
90S: C, 79.85%; H, 13.33%. H NMR (300 MHz,
ꢁ
a Perkin-Elmer 2400 Series II C, H, N analyzer. HRTEM studies CDCl
3
, 25 C, TMS); d (ppm): 0.88 (t, J ¼ 6.6 Hz, 6H, H22), 1.25–
2
13
1
were carried out with a Tecnai G 20 electron microscope 1.56 (m, 80H, H2, H3–H21), 2.49 (t, J ¼ 76.9 Hz, 4H, H1). C{ H}
ꢁ
operated at 200 kV. The specimens for this purpose were NMR (75 MHz, CDCl
3
, 25 C, TMS); d (ppm): 14.1 (C22), 22.7
prepared by dispersing the powdered solid in ethanol by ultra- (C21), 28.5–29.7 (C20–C4), 31.9 (C3), 32.1 (C2), 39.3 (C1).
sonication, dropping slurry onto a porous carbon lm sup- Synthesis of complex 1. Na [PdCl ] (0.147 g, 0.5 mmol) was
2
4
ported on a copper grid and then drying it in air. Carl Zeiss dissolved in 10 mL of methanol. The solution of ligand L1
EVO5O scanning electron microscope (SEM) equipped with an (0.65 g, 1 mmol) in a mixture of chloroform and hexane
EDX system model QuanTax 200, based on the SDD technology (20 mL each) was added to it with vigorous stirring. The mixture
was used to observe elemental composition of the nano- was further stirred for 2 h. Thereaer, the volume of the reac-
particles. The samples for SEM were mounted on a circular tion mixture was reduced to 10 mL on a rotary evaporator. The
metallic sample holder with a sticky carbon tape and scanned in resulting yellow precipitate was ltered, washed with methanol
different regions in order to minimize the error in the analysis (3 ꢂ 20 mL) and dried in vacuo. Yield: light yellow solid (0.59 g)
ꢁ
made for evaluating the morphological parameters. Powder X- 80%; m.p. 78 C(d). Anal. found: C, 70.27%; H, 11.81%. Calc. for
1
ray diffraction (PXRD) studies were carried out on a Bruker D8 Pd[Cl S(C H ) ]: C, 70.42%; H, 11.76%. H NMR (300 MHz,
2
22 45 2
ꢁ
advance diffractometer with Ni-ltered CuKa radiation using a CDCl , 25 C, TMS); d (ppm): 0.88 (m, 12H, H22), 1.25–1.86
scan speed of 1 s and scan step of 0.02 . Shimadzu UV2450 (m, 152H, H21–H3), 1.40–1.86 (m, 8H, H2), 2.67–2.78 (m, 8H,
3
ꢁ
1
3
1
ꢁ
spectrophotometer with UV probe soware has been used to H1).
C{ H} NMR (75 MHz, CDCl
3
,
25 C, TMS);
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d (ppm):14.1 (C22), 22.6 (C21), 28.2 (20), 28.9–29.7 (C19–C4), reaction mixture was heated on an oil bath, and the tempera-
ꢁ
30.9 (C3), 31.9 (C2), 36.1 (C1).
ture was maintained at 100 C. The progress of reaction was
Synthesis of palladium NPs 2–7. Pd(OAc)2 (0.224 g, monitored with NMR. Aer maximum conversion, the reactants
.0 mmol) for 2 and 5/Na [PdCl ] (0.294 g, 1.0 mmol) for 3 and were added in the same proportion to the original reaction
1
2
4
6
3 2 2
/Pd[(CH CN) Cl ] (0.259 g, 1.0 mmol) for 4 and 7 was dissolved mixture to monitor the catalytic recycling. Conversions were
1
in 50 mL of methanol. The solution of ligand L1 [1.30 g, 2 mmol estimated with H NMR (Table 6). Aer the completion of the
(
(
Pd : L1 ¼ 1 : 2) for 2, 3 and 4/0.162 g, 0.25 mmol for 5, 6 and 7 h catalytic cycle, the reaction mixture was cooled to room
Pd : L1 ¼ 4 : 1)] prepared in 100 mL of a mixture of chloroform temperature. The coupled product was extracted with 20 mL
and hexane (equal volumes of each) was added to the above- diethylether or ethyl acetate (in case of 4). The extract was
mentioned solutions with vigorous stirring. The reaction washed with water (3 ꢂ 20 mL) and dried over anhydrous
mixture was further stirred for 12 h at room temperature. For Na SO . The solvent in the extract was evaporated off and
2
4
NPs 3, 4, 6 and 7, the solution of NaBH (0.111 g, 3.0 mmol) in residue was subjected to NMR to authenticate the product.
4
methanol was added additionally dropwise to the reaction
mixture with stirring over a period of 30 min. Methanol acts as a
reducing agent for palladium acetate in the synthesis of NPs 2/5.
On the completion of reaction, the solvent in the mixture was
reduced to 10 mL on a rotary evaporator. The resulting residue
was centrifuged, washed with methanol (3 ꢂ 30 mL) and dried
in vacuo.
Catalytic recyclability of NPs aer separation
The coupling reaction of 4-bromobenzoic acid was carried out
with NPs 4 (0.2 mol% of Pd). Aer the completion of the reac-
tion, ethyl acetate and water were added to the reaction mixture
and it was centrifuged. Aqueous-organic layer was decanted and
the black residue that was obtained was washed thoroughly
with ethyl acetate and water to remove organic content and
base, respectively. The resulting residue was dried in vacuo. The
organic layer from ethyl acetate–water system was separated. Its
Procedure for Suzuki–Miyaura C–C coupling reaction
catalyzed with 1
1
An oven-dried round bottom ask was charged with aryl halide
solvent was evaporated and residue was subjected to H NMR
(
1 mmol), phenylboronic acid (1.2 mmol), K
2 3
CO (2 mmol) and
for estimating the conversion. The NPs separated were reused
for catalysis using an optimized procedure. This procedure was
repeated ve times and results are given in Table 8.
DMF/H O (3 mL/1 mL). An appropriate amount of complex 1 as
a solution made in chloroform was added. The ask was placed
on an oil bath at 100 C. Progress of the reaction was monitored
by H NMR. Aer the completion of the reaction, the product
was extracted with 10 mL of diethyl ether. The organic phase
was washed with water (3 ꢂ 10 mL) and dried over anhydrous
Na SO . The solvent was removed on a rotary evaporator and the
residue was subjected to H and C{ H} NMR. Further puri-
cation, if required, was performed by column chromatography
on silica gel using hexane/ethyl acetate mixture (90 : 10) as an
eluent.
2
ꢁ
1
Isolation of Pd NPs generated from 1 in coupling reaction
An oven-dried round bottom ask was charged with 4-bromo-
benzaldehyde (1 mmol), phenylboronic acid (1.2 mmol), K
2 3
CO
2
4
1
13
1
(2 mmol), DMF/H O (3 mL/1 mL) and large amount of complex
2
1
(200 mg). The ask was placed on an oil bath and the
ꢁ
temperature was maintained at 100 C. On the completion of
the reaction, the black particles settled down in the reaction
mixture. The aqueous-organic layer was decanted, and the black
residue that was obtained was centrifuged and thoroughly
washed with ethyl acetate and water to remove the base and
organic content. Then, it was washed again with acetone–water
Procedure for the Suzuki–Miyaura C–C coupling reaction
catalyzed with nanoparticles (2–7)
An oven-dried round bottom ask was charged with aryl halide
(20 + 10 mL). The resulting residue was dried in vacuo, and
2 3
(5 mmol), phenylboronic acid (6 mmol), K CO (8 mmol) and
HRTEM study revealed that the residue was in the form of
spherical NPs (Fig. 1g).
DMF/H O (9 mL/3 mL). Nanoparticles (one from 2–7) were
added (amount equivalent to 0.2 mol% of Pd in case of 2–4 and
2
0
.5 mol% in case of 5–7). The reaction mixture was heated on an
Hg poisoning test
ꢁ
oil bath, and the temperature was maintained at 100 C. The
progress of the reaction was monitored with NMR. On the
completion of the reaction, the product was extracted with
Excess of Hg (Hg : Pd; 400 : 1) was taken in a reaction ask
before the addition of reactants. Thereaer, the coupling reac-
tion of 4-bromobenzaldehyde (1 mmol) with phenylboronic acid
10 mL of diethylether. The extract was washed with water
(1.2 mmol) using 1/3 (0.1/0.2 mol% of Pd) as a catalyst under
(
3 ꢂ 10 mL) and dried over anhydrous Na
2
SO . The solvent of
4
optimal conditions was carried out. Aer carrying out the
reaction for 12 h, the cross-coupled derivative for 1 and 3 was
obtained in 100% and 67% yield, respectively.
the extract was evaporated off and the residue was subjected to
NMR.
Catalytic recyclability for NPs
PPh
To the coupling reaction of 4-bromobenzaldehyde (1 mmol)
(5.2 mg, 2 mol%) was
–4) were added (amount equivalent to 0.2 mol% of Pd). The added under optimal conditions. Thereaer, 1/3 (equivalent to
3
poisoning test
An oven-dried round bottom ask was charged with aryl
bromide (5 mmol), phenylboronic acid (6 mmol), K CO
2
3
(
2
2 3
8 mmol) and DMF/H O (9 mL/3 mL). Nanoparticles (one from with phenylboronic acid (1.2 mmol), PPh
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organic phases were combined together and the solvent was
1
evaporated. Then, the resulting residue was analyzed with H
NMR.
Results and discussion
Synthesis and characterization
The syntheses of L1, its Pd(II)–complex 1 and Pd NPs 2–7 (pro-
tected with L1) are summarized in Scheme 1. Note that these
compounds are stable in air; moreover, the high protection of
NPs, which is provided by L1, is revealed by the fact that
centrifugation of their dispersions in chloroform at 6000 rpm
does not result in any aggregation (see ESI; Fig. S29†). The air
insensitive L1 and diamagnetic palladium complex 1 have been
1
13
1
characterized using their H and C{ H} NMR spectra. These
spectra (see ESI; Fig. S1–S4†) are consistent with their structures
depicted in Scheme 1. The Pd NPs 2–7 have been synthesized
(
Scheme 1) using different palladium precursors in varying
metal to ligand ratios. The Pd : L1 ratio is 1 : 2 for 2, 3 and 4 and
: 1 for 5, 6 and 7; moreover, the palladium precursors
Pd(OAc) , Na PdCl and [Pd(CH CN) Cl ] have been used in
case of 2/5, 3/6 and 4/7, respectively (Scheme 1). All these NPs
2–7) have been subjected to powder XRD (see ESI Fig. S20–
4
2
2
4
3
2
2
(
S25†), HRTEM (Fig. 1), SEM and SEM-EDX (see ESI Fig. S5–S10
and S12–S17†) for characterization. The peaks at d ꢀ 2.28 (hkl:
111) and 1.97 (hkl: 200) observed in the powder XRD of 6 and 7
(
(
see ESI Fig. S24 and S25†) support the presence of palladium(0)
JCPDS s 88-2335) with face-centered cubic structure. Because
of high content of protector ligand (see SEM-EDX results), these
peaks do not appear in case of 2, 3 and 4. The peaks present at
Fig. 1 HRTEM images of nanoparticles. (a) 2 (scale bar 100 nm), (b) 3
ꢁ ꢁ
2.318 and 4.131 in powder XRD of ligand were found slightly
(
scale bar 20 nm), (c) 4 (scale bar 20), (d) 5 (scale bar 50 nm), (e) 6 (scale
shied in powder XRD of 2, 3 and 4 (see ESI Fig. S20–S22†),
indicating the interaction of ligand with NPs. The SEM-EDX
studies have revealed that Pd : S ratios (in atomic%) in NPs
bar 50 nm), (f) 7 (scale bar 20 nm), (g) NPs obtained in SMC of complex
1
(scale bar 20 nm), (h) NPs 6 after a catalytic reaction (scale bar 50).
2–4 are 35 : 65, 39 : 61 and 33 : 67, respectively. This ratio in
0
1
.1/0.2 mol% of Pd) was added to the reaction mixture. Aer case of 5, 6 and 7 has been found to be 40 : 60, 61 : 39 and
8 h of reaction, the cross-coupled product 4-formylbiphenyl 74 : 26, respectively. The size (nm) ranges for NPs (majority) of
was obtained in ꢀ100% yield in both the cases.
Pd are ꢀ18–19, 4–5, 5–7, 4–6, 7–9 and 4–6 respectively for 2–7.
The size distribution graphs given in ESI (Fig. S26–S28†)
Two/three-phase test
A mixture of 4-bromobenzoic acid-immobilized silica (0.20 g),
19
prepared using a reported procedure, phenylboronic acid
(
(
0.36 g, 3 mmol), 4-bromoacetophenone or 4-bromoanisole
ꢁ
1 mmol) and K
2
CO
3
(0.56 g, 4 mmol) was heated at 100 C for
12 h in a mixture of DMF/water (6 mL/3 mL) in the presence of
1/3 (0.5/0.1 mol%). Thereaer, the reaction mixture was cooled
and ltered through a G-4 crucible. The residue was washed
with 30 mL of water followed by diethyl ether (40 mL). The

ltrate and washings were collected together, and the resulting
mixture was extracted with 30 mL of diethyl ether. The solvent
of the extract was evaporated on a rotary evaporator and the
1
residue was subjected to H NMR. The residue in G-4 crucible
was hydrolysed with KOH (1.68 g dissolved in 10 mL of EtOH
ꢁ
+
5 mL of H O) at 90 C for 3 days. The hydrolysed solution was
2
neutralized with 20% (v/v) aqueous HCl and extracted with
dichloromethane (30 mL) followed by ethyl acetate (40 mL). The Scheme 1 Synthesis of L1, Pd(II) complex 1 and Pd nanoparticles 2–7.
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a
Table 1 Optimization of reaction conditions
CHO–C
6
H
4
–Br + PhB(OH)
2
/ CHO–C
6
H
4
–Ar
b
Entry no.
Solvent
Base
Yield (%)
1
2
3
4
5
6
7
8
9
DMF + Water
DMF + Water
DMF + Water
DMF + Water
DMF + Water
DMF
Toluene
THF
DMSO
Cs
CH
KOH
NEt
2
CO
3
91
67
82
10
96
81
11
29
78
3
COONa
3
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
3
3
3
3
3
Fig. 2 UV-vis spectrum of NPs 2–4, 6 and L1.
a
Reaction conditions: 1 mmol of 4-bromobenzaldehyde, 1.2 mmol of
phenylboronic acid, 2 mmol of base, 5 mL of solvent, temperature of
ꢁ
b
bath 100 C, 0.0001 mol% of 1 and reaction time was 10 h. NMR yield.
suggest that the pattern of growth varies for nanoparticles 2 to
7
2
. In the case of 2, the reduction of Pd(OAc) is slow as it is
caused by MeOH, a mild reducing agent. Thus, the reduction
time is prolonged, causing multi-nucleation and uneven
growth rate and has resulted in big Pd NPs with wider size
(
Table 2: entries 1 and 2) but at higher catalyst loading (1.0 mol%
of Pd). For aryl bromides, good catalytic conversions were
obtained with 1 even at its very low loading (10 mol% of Pd).
15b
range. Comparison of the size of 2 and 5 reveals that it may
also be contributed by high ligand : Pd(OAc) ratio. Probably,
ꢃ4
2
Moreover, air insensitivity of 1 is a distinct advantage. The extent
of variation in catalytic activity of 1 for aryl chlorides and
bromides was revealed when the coupling reaction of 4-chloro-
benzaldehyde under optimized reaction conditions (Table 2:
entry 2) was compared with that of 4-bromobenzaldehyde (Table
the reduction is slowed down due to increase in number of
ligand molecules in the reaction mixture, which in turn may
lead to further aggregation during the growth process. The
large size in case of NPs 2 leads to their lower catalytic activity
in comparison to that of 3 and 4 (Table 2). The NPs 3, 4, 6 and 7
have been synthesized by reduction with sodium borohydride,
a strong reducing agent, over a period of 30 min. Their small
size and good dispersion result is due to the fast reduction
process. The bulky and so donor site containing ligands
around Pd NPs are known to prevent their aggregation during
2
2
: entries 6 and 7). 89% yield of the cross-coupled product (Table
: entry 2) was obtained in 40 h using 1 mol% of catalyst loading
in the case of 4-chlorobenzaldehyde, whereas 4-bromo-
benzaldehyde gave yield of same order (Table 2: entries 7) within
9
change in catalyst loading on cross-coupled product formation
was studied for 4-bromobenzaldehyde, 1-bromo-4-nitrobenzene
and 4-bromoanisole. The yield of the coupled product in the
case of 4-bromobenzaldehyde remained almost the same
ꢃ4
h at a very low catalyst loading (10 mol% of Pd). The effect of
15b
growth. Thus, the quality of NPs (size and dispersion) is
determined by a balance between rate of reduction and control
of ligand over growth. In the reduction process, Pd(II) is fully
converted into Pd(0) as revealed by the UV-vis spectrum. There
(Table 2: entries 6, 7) on varying the catalyst loading between
2
ꢃ
is no peak at 235 nm due to the presence of Pd(II)/[PdCl
the dispersion of NPs. The appearance of the peak at around
05 nm due to Pd(0) along with a characteristic peak of ligand
4
]
in
3
a
Table 2 Suzuki–Miyaura coupling reactions catalyzed by 1
L1 at ꢀ250 nm (Fig. 2) in the UV-vis spectrum of each NP
indicates a weak interaction between the ligand and the surface
Ar–X + PhB(OH) / Ar–Ph (Ar: aryl/heteroaryl; X: Cl/Br)
2
15c
of palladium of NPs.
b
Entry no.
Aryl/heteroaryl halide
mol%
t (h)
Yield (%)
1
2
3
4
5
6
7
8
9
1-Chloro-4-nitrobenzene
4-Chlorobenzaldehyde
4-Bromobenzonitrile
1-Bromo-4-nitrobenzene
1-Bromo-4-nitrobenzene
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromoacetophenone
4-Bromotoluene
1
1
0.1
0.1
10
10
10
10
0.1
1
23
40
4
1
16
7
9
9
14
3
6
94
89
93
96
94
95
94
92
91
92
90
90
Catalysis
The complex 1 and NPs 2–7 have been explored for SMC of
several aryl halides, including heteroaryl bromides and aryl
chlorides. To optimize the reaction conditions, the coupling
reaction of 4-bromobenzaldehyde with phenylboronic acid in
the presence of 1 was studied (Table 1) using various bases and
ꢃ3
ꢃ2
ꢃ4
ꢃ2
solvents. K
2
CO
3
has been found to be the most appropriate base
1
1
1
0
1
2
4-Bromoanisole
4-Bromoanisole
2-Bromopyridine
because the yield of cross-coupled product is lower compared to
alternatives such as caesium carbonate, sodium acetate,
potassium hydroxide, and triethylamine. Furthermore, the
reaction was inuenced by solvents. A mixture of DMF and
water was found to give the best results (Table 1: entry 5).
Complex 1 was found to show catalytic activity for aryl chlorides
0.5
0.1
10
a
Reaction conditions: 1 mmol of aryl/heteroaryl halide, 1.2 mmol of
CO , DMF + H O (3 + 1 mL),
temperature of bath 100 C. Isolated yield.
phenylboronic acid, 2 mmol of K
2
3
2
ꢁ
b
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Table
Paper
3
Activity of Pd NPs 2–7 for Suzuki coupling of well as sulfur, and the approximate Pd : S ratio (in atom%) is
a
4
-bromoanisole
28 : 72.
The results of investigations on catalytic activities of NPs 2–7
Catalyst
for SMC are given in Tables 3, 4 and 5. 2–7 have been explored
for the reaction of 4-bromoanisole with phenylboronic acid
under optimum reaction conditions (Table 3). The order of
efficiency has been found as 3 > 5 > 4 > 6 > 2 > 7 (Table 3). The
2
3
4
5
6
7
b
Yield (%)
71
ꢀ100
85
90
72
Trace
a
Reaction conditions: 4-bromoanisole (1 mmol), phenyl boronic acid cross-coupled product was obtained in ꢀ100% yield aer the
(
1.2 mmol), K
2
CO
3
(2 mmol), DMF + H
2
O (3 + 1 mL), temperature of
reaction of 1 h in case of 3. These results show that metal-
ligand ratio and the precursor used in the synthesis of NPs
ꢁ
bath 100 C, time 1 h, amount of NPs 2–7 equivalent of 0.1 mol% of
b
:
Pd. NMR yield.
affect their catalytic activity. The NPs 2 show low catalytic effi-
ciency, which may be due to their large size as evident by their
TEM image (Fig. 1a). The TEM image recorded aer the catalytic
reaction of 4-bromoanisole with phenylboronic acid in the
presence of 6 (Table 3) suggests that the quality of NPs 6 does
not degrade and that they remain well dispersed even aer the
reaction (see Fig. 1e and h). The activity of NPs 2–7 has been
studied further for various aryl halides. The NPs 2 and 3 show
almost similar reactivity towards the coupling reaction of
ꢃ2
ꢃ4
1
0
and 10 mol% if the reaction time was slightly increased
for the lower loading. The yield did not vary signicantly on
varying the catalyst loading from 0.1 to 10 and from 1.0 to
.5 mol% of Pd in the case of 1-bromo-4-nitrobenzene and
-bromoanisole, respectively, if the reaction time was adjusted.
The reaction time required for the completion of the reaction
varied (Table 2: entries 4, 5, 10 and 11) signicantly from 16 to
and 6 to 3 h on increasing the catalyst loading in the case of
-bromo-4-nitrobenzene and 4-bromoanisole, respectively. The
efficiency of 1 is also reected in high conversion (92%; Table 2:
entries 10 and 11) of 4-bromoanisole, which is an electronically
deactivated substrate. In the course of coupling reaction
between 4-bromobenzaldehyde and phenylboronic acid cata-
lyzed with 1, the formation of black particles was observed.
Thus, this reaction was carried out using a larger amount of 1
ꢃ3
0
4
4-chlorobenzaldehyde/1-chloro-4-nitrobenzene. However, NPs 4
1
1
do not show any catalytic activity towards aryl chloride
substrates. The results given in Tables 4 and 5 reect that 2–7
can be successfully used to couple several aryl bromides in 1 to
4
h with their low loading (0.2–0.5 mol% of Pd). Generally, the
product obtained, using these catalysts is pure; therefore,
column chromatography is not required for further purica-
tion. The recyclability of 2, 3 and 4 was studied for the coupling
reaction of 4-bromobenzaldehyde, 4-bromoanisole and 4-bro-
mobenzoic acid, respectively, with phenyl boronic acid. 2 and 4
do not show any decrease in their efficiencies for the reaction of
(
see Experimental) to obtain good quantity of black particles for
further investigations. These black particles were isolated,
washed thoroughly, dried and characterized with HRTEM, SEM
and SEM-EDX. HRTEM studies have revealed that these particles
are of nanosize (ꢀ4 nm). They are catalytically active (Table 7)
4-bromobenzaldehyde and 4-bromobenzoic, respectively,
(Table 6) even aer the h reaction cycle. However, the cata-
lytic activity of 3 was reasonably decreased for the coupling
reaction of 4-bromoanisole aer the third reaction cycle
20,21
due to their protection with sulfur ligands
but are required in
higher amount to provide sufficient Pd loading for good
conversion in comparison to that of precursor molecular
complex. The presence of protecting ligands with black particles
is supported by SEM-EDX, which reveals that NPs contain Pd as
(Table 6). To check whether these NPs are active aer their
isolation from the reaction mixture, coupling of 4-bromo-
benzoic acid with phenyl boronic acid under optimal reaction
conditions was carried out by reusing the separated NPs 4.
a
Table 4 Suzuki–Miyaura C–C coupling reactions catalyzed with Pd NPs 2–4
Ar–X + PhB(OH)
2
/ Ar–Ph (X: Cl/Br)
2
3
4
b
b
b
Entry no.
Aryl halide
t (h)
Yield
91
t (h)
Yield
t (h)
Yield
c
1
4-Chlorobenzaldehyde
1-Chloro-4-nitrobenzene
4-Bromobenzaldehyde
4-Bromobenzoic acid
1-Bromo-4-nitrobenzene
4-Bromobenzonitrile
4-Bromotoluene
20
20
2
4
2
3
4
4
8
20
20
2
4
2
3
4
1
11
95
20
24
2
4
2
3
4
4
8
16
c
2
ꢀ100
ꢀ100
ꢀ100
94
ꢀ100
Trace
ꢀ100
ꢀ100
ꢀ100
ꢀ100
85
3
4
5
6
7
8
9
97
94
96
89
91
87
—
93
95
4-Bromoanisole
2-Bromopyridine
ꢀ100
96
95
86
a
ꢁ
3 2
(8 mmol), DMF + H O (9 + 3 mL), temperature of bath 100 C, amount of
, 3 and 4 equivalent to 0.2 mol% of palladium. NMR % yield. Amount of catalyst equivalent to 1.0 mol% of palladium.
Reaction conditions: ArX (5 mmol), phenylboronic acid (6 mmol), K
2
CO
b
c
2
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Table 5 Suzuki–Miyaura C–C coupling reactions catalyzed with Pd NPs 5–7
RSC Advances
a
Ar–Br + PhB(OH)
2
/ Ar–Ph
5
6
7
b
b
b
Entry no.
Aryl bromide
t (min)
Yield
t (min)
Yield
t (min)
Yield
1
2
3
4-Bromobenzaldehyde
Bromobenzene
4-Bromoanisole
90
90
90
ꢀ100
91
87
90
90
90
ꢀ100
ꢀ100
ꢀ100
90
90
90
ꢀ100
ꢀ100
ꢀ100
a
ꢁ
Reaction conditions: ArBr (5 mmol), phenylboronic acid (6 mmol), K
2
CO
3
(8 mmol), DMF + H
2
O (9 + 3 mL), temperature of bath 100 C, amount of
b
5, 6 and 7 equivalent to 0.5 mol% of Pd. NMR % yield.
Results shown in Table 8 indicate that the separated NPs are yield aer 18 h of reaction. Thus, catalysis is not heterogeneous
active even aer the fourth catalytic cycle. HRTEM of these NPs with 1. Because Pd NPs are protected with ligand molecules, the
recorded aer the rst catalytic cycle (Fig. 3) does not show possibility of a heterogeneous pathway cannot be presumed. On
signicant change in their size and distribution. The NPs 2–7 the basis of mercury and triphenylphosphine poisoning tests
protected with L1 are required in an amount (in terms of mol% alone, one cannot convincingly conclude the nature of the
of Pd) higher than that of 1 for the same SMC reaction in order catalytic process. Thus, a two/three-phase test (Scheme 2) was
to get comparable conversion but they are recyclable. The good carried out to ascertain the contributions of heterogeneous and
19
efficiency of 2–7 may be attributed to the large surface-to- homogeneous catalytic processes. This test provides a better
volume ratio resulting from the absence of any direct covalent idea about catalytically active metal species.
22
interaction between protecting ligand and surface palladium
Note that this test was developed by Rebek and co-workers.
atoms. In earlier studies, the weak binding between metal In this test, a coupling partner (aryl halide) is heterogenized and
atoms on the surface of NPs and ligands has been found to the second coupling partner (phenylboronic acid) along with
result in well dispersed nanoparticles of high catalytic another aryl halide (homogeneous) have been reacted in the
15a
reactivity.
presence of catalyst under optimum reaction conditions. If the
To establish the pathway of catalysis with molecular complex catalyst behaves in a heterogeneous manner, the supported aryl
1
and NPs 2–7, mercury and triphenylphosphine poisoning and halide is not expected to be converted to a coupled product.
two phase tests were performed. The coupling of 4-bromo- When palladium is released into a solution, the supported
benzaldehyde was carried out in the presence of 1/3 substrate is likely to be converted into a product. Thus, as
(0.1/0.2 mol% of Pd) for the mercury poisoning test. Aer 12 h shown in Scheme 2, 4-bromobenzoic acid immobilized silica
of reaction time, the cross-coupled product for 1 and 3 was was reacted with phenylboronic acid in the presence of 4-bro-
obtained in ꢀ100 and 67% yield, respectively. Thus, catalysis by moacetophenone or 4-bromoanisole under optimum reaction
molecular complex 1 is not quenched by Hg. However, the conditions using 1 and 3 as catalysts. Aer 12 h of reaction time,
1
catalysis by NPs has been found to be diminished to some the soluble part was separated and analyzed with H NMR. The
extent. The coupling reaction of 4-bromobenzaldehyde acti- yield of the cross-coupled product was ꢀ100% for 4-acetylbi-
vated with 1/3 was also smooth in the presence of triphenyl- phenyl and 81–87% for 4-methoxybiphenyl. The solid phase
phosphine because the product was formed in almost in ꢀ100% separated through G-4 crucible was hydrolyzed, and the
a
Table 6 Recyclability of NPs in Suzuki–Miyaura C–C coupling reactions
Ar–Br + PhB(OH)
2
/ Ar–Ph
Reaction cycle
Entry no.
Aryl bromide
1
2
3
4
5
b
1
2
3
4-Bromobenzaldehyde
t (h)
Yield
t (h)
Yield
t (h)
Yield
4
4
4
4
4
c
c
c
ꢀ100
ꢀ100
ꢀ100
ꢀ100
ꢀ100
b
4-Bromoanisole
1
1
1
1
1
ꢀ100
5
ꢀ100
ꢀ100
5
ꢀ100
ꢀ100
5
ꢀ100
85
5
ꢀ100
81
5
ꢀ100
b
4-Bromobenzoic acid
a
ꢁ
b
Reaction conditions: ArBr (5 mmol), phenylboronic acid (6 mmol), K
of 2–4 equivalent to 0.2 mol% of Pd. NMR % yield.
2
CO
3
(8 mmol), DMF + H
2
O (9 + 3) mL, temperature of bath 100 C. Amount
c
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Table 7 Suzuki–Miyaura C–C coupling reactions catalyzed by Pd NPs
a
obtained from complex 1
2
Ar–Br + PhB(OH) / Ar–Ph
NPs obtained from 1
b
Entry no.
Aryl bromide
t (min)
Yield (%)
1
2
3
4-Bromobenzaldehyde
Bromobenzene
4-Bromobenzoic acid
90
90
90
ꢀ100
87
91
a
Reaction conditions: ArBr (1 mmol), phenyl boronic acid (1.2 mmol),
ꢁ
K
2
CO
3
(2 mmol), DMF + H
2
O (3 + 1 mL), temperature of bath 100 C.
Amount of NPs equivalent to 0.5 mol% of Pd (calculated by AAS).
b
NMR yield.
Table 8 Recyclability of NPs 4 in Suzuki–Miyaura C–C coupling
a
reaction
COOH–C
6
H
4
–Br + PhB(OH)
2
/ COOH–C
6
H
4
–Ar
Reaction cycle
Scheme 2 Two phase test in the presence of 1 and 3.
Entry no. Aryl bromide
1
2
3
4
5
1
4-Bromobenzoic acid t (h) b
5
5
5
5
5
present results indicate that in both the cases, the reaction is
Yield ꢀ100 ꢀ100 ꢀ100 95 92 homogeneously as well as heterogeneously catalyzed with Pd
a
species. However, the contribution of homogeneous catalysis is
Reaction conditions: 4-bromobenzoic acid (1 mmol), phenylboronic
acid (1.2 mmol), K
of bath 100 C. Amount of 4 equivalent to 0.2 mol% of Pd. NMR %
yield.
2
CO
3
(2 mmol), DMF + H
2
O (3 + 1 mL), temperature marginally higher. It is possible that 1 behaves as a precatalyst
ꢁ
b
and generates nano-sized particles of palladium, which
contributes to a heterogeneous pathway. In the case of palla-
dium NPs, it could be concluded that the catalytically active
palladium leached from NPs is responsible for homogeneous
catalysis. Thus, the catalysis is neither purely homogeneous nor
purely heterogeneous and can be described as “cocktail-
1
resulting residue aer workup was analyzed with H NMR. In
the presence of 4-bromoacetophenone, the conversion of
immobilized 4-bromobenzoic to biphenyl-4-carboxylic acid was
observed to be 52% when complex 1 was used as a catalyst. The
conversion was 59% when NPs 3 were used as a catalyst. In the
presence of 4-bromoanisole, the conversions of immobilized
substrate for catalysts 1 and 3 were 58% and 51%, respectively.
21,23–25
like”.
Conclusions
The relative rates of reaction of immobilized and free ArBr are Palladium NPs 2–7 have been prepared using n-didocosyl
not of much signicance in concluding the nature of catalysis. sulde (L1) as a protector and different precursors of palladium
The absolute conversion of immobilized substrate indicates in different ratios with respect to ligand L1. The precursor of
whether catalysis is homogeneous or heterogeneous. The palladium affects the size, shape and dispersion of NPs. The
size (nm) ranges for NPs (majority) of Pd are: 2: ꢀ18–19; 3: ꢀ4–
5
; 4: ꢀ5–7; 5: ꢀ4–6; 6: ꢀ7–9; 7: ꢀ4–6. The NPs have been
characterized using powder X-ray diffraction, SEM, EDX, UV-vis
and HRTEM. The complex [Pd(L1) Cl ] (1) has also been
2
2
1
13
1
synthesized and characterized with H and C{ H} NMR spec-
troscopy. The NPs show good catalytic efficiency for the Suzuki–
Miyaura coupling (SMC) of various aryl halides with phenyl-
boronic acid at a low catalyst loading (0.1–0.5 mol% of Pd). The
conversion is good for some aryl halides even in a short reaction
time of the order 1–2 h. The size and dispersion were most
2 4
favourable for catalytic activity when Na PdCl was used as a
precursor. The distinct advantage associated with the NPs 2–7 is
that they remain active in solution even aer the reaction and
also aer their separation; moreover, they can be reused up to
ve times. The complex 1 is also efficient for SMC as its loading
Fig. 3 HRTEM image of NPs 4 after first catalytic cycle.
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RSC Advances
equivalent to 0.001–0.0001 mol% Pd is optimum for good 11 (a) C. Fleckenstein, S. Roy, S. Leuthaeusser and H. Plenio,
conversion of some aryl bromides into coupled products. A two
phase test, conducted in the presence of 1 and 3, suggests the
contribution of homogeneous as well as heterogeneous Pd
species in the catalytic processes.
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