Layered double hydroxides supported nanoplatinum catalyst for Suzuki coupling of aryl halides

Article abstract of DOI:10.1016/j.molcata.2005.07.016

Layered double hydroxides (LDH) supported nanoplatinum catalyst was prepared and employed for Suzuki cross coupling of aryl halides (iodides and bromides) with a broad range of arylboronic to afford the corresponding biaryls in good to excellent yields without using any external ligand. The catalyst is reused for several cycles with consistent activity.

Full text of DOI:10.1016/j.molcata.2005.07.016

Journal of Molecular Catalysis A: Chemical 241 (2005) 215–218
Layered double hydroxides supported nanoplatinum catalyst for
Suzuki coupling of aryl halides
Boyapati M. Choudarya,∗∗, Moumita Royb, Sarabindu Royb, M. Lakshmi Kantamb,∗
a
Ogene Systems(I) Pvt. Ltd., Hyderabad 500037, India
b
Indian Institute of Chemical Technology, Inorganic Chemistry Divison, Tarnaka, Hyderabad 500007, India
Received 24 March 2005; accepted 3 July 2005
Available online 22 August 2005
Abstract
Layered double hydroxides (LDH) supported nanoplatinum catalyst was prepared and employed for Suzuki cross coupling of aryl halides
(iodides and bromides) with a broad range of arylboronic to afford the corresponding biaryls in good to excellent yields without using any
external ligand. The catalyst is reused for several cycles with consistent activity.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Layered double hydroxides; Nanoplatinum; Suzuki; Aryl halides; Reusable catalyst
1. Introduction
lyst, which can activate aryl chlorides also. Herein we report
Suzuki cross coupling reaction catalyzed by a heterogeneous
layered double hydroxides supported nanoplatinum catalyst
(Scheme 1).
Suzuki [1] reaction, coupling of aryl halides with aryl-
boronic acids, is one of the most powerful methods for the
synthesis of biaryl compounds, important intermediates for
pharmaceuticals, herbicides and natural products. Although
several reports using palladium and nickel complexes [2] are
available, a few examples using platinum, the next group
member is cited for Suzuki cross coupling [3]. It is most
probable that the slow rate of reductive elimination of biaryl
group from platinum complexes compared with the rate for
the analogous palladium complex is responsible for the less
reactivity of platinum. Bedford et al. [3a] and Oh et al. [3b]
reported Suzuki biaryl coupling reactions of aryl bromides
and aryl iodides, respectively, using Pt complexes. In view of
the non-reusability of the expensive platinum catalyst and
use of costly ligands, it is highly desirable to develop a
reusable and recoverable platinum catalytic system without
using any external ligand. Previously we have reported lay-
ered double hydroxides (LDH) supported nano-Pd0 [4] cata-
2. Experimental
2.1. Materials and methods
FTIR spectra were recorded on a Perkin-Elmer spec-
1
trophotometer. H NMR spectra were recorded on Bruker
(300 MHz) spectrophotometer using CDCl3 as solvent and
TMS as the internal standard. X-ray photoelectron spec-
troscopy (XPS) spectra were recorded on a KRATOS AXIS
165withadualanode(MgandAl)apparatususingtheMgK␣
anode. X-ray powder diffraction (XRD) data were collected
on a Simens/D-5000 diffractometer using Cu K␣ radiation.
The particle size and external morphology of the samples
were observed on a JEOL JEM-100CX transmission elec-
tron microscopy (TEM). SEM–EDX was performed on a
Hitachi SEM S-520, EDX-Oxford Link ISIS-300 instrument.
Allknowncompoundswerecharacterizedbycomparingtheir
physical data with those in the literature. Solvents used for
∗
∗∗
Corresponding author. Tel.: +91 40 2719 3510; fax: +91 40 2716 0921.
Corresponding author.
E-mail addresses: bmchoudary@yahoo.com (B.M. Choudary),
lkmannepalli@yahoo.com (M.L. Kantam).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.07.016

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B.M. Choudary et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 215–218
Scheme 2. Synthesis of LDH-Pt catalyst.
Scheme 1. LDH-Pt0 catalyzed Suzuki cross coupling of aryl halides and
arylboronic acids.
experiments were dried and distilled according to literature
procedures. All the reactants were commercially available
and used without purification.
2.2. Preparation of LDH-PtCl6
Scheme 3. The structures of catalysts (a) LDH-PtCl6 and (b) LDH-Pt0.
LDH-Cl (Mg:Al = 3:1) was prepared according to the lit-
erature [5]. LDH-Cl (2 g, Mg:Al = 3:1) was suspended in
150 ml of aqueous K2PtCl6 (0.388 g, 0.8 mmol) solution and
stirred at 25 ◦C for 24 h under N2 atmosphere. The solid cat-
alyst was filtered, washed thoroughly with 500 ml of water
and vacuum-dried to obtain LDH-PtCl6.
3.2. Characterization of the LDH-Pt0 catalyst
3.2.1. X-ray powder diffraction
Patterns of initial LDH and LDH-Pt0 hardly differ in the
range 2θ = 3◦–65◦.The observed d0 0 3 basal spacing of sup-
˚
port that appeared at 7.9 A remained unchanged after the
anion exchange and subsequent reduction, which indicates
that the PtCl62− and Pt0 nano particles are mainly located at
the edge-on surface in the respective samples (Fig. 1).
2.3. Preparation of LDH-Pt0
LDH-PtCl6 (1 g) was reduced with hydrazine hydrate (1 g,
20 mmol) in ethanol (10 ml) for 3 h under nitrogen atmo-
sphere, filtered and washed with ethanol to give an air stable
black powder (0.374 mmol of Pt per gram of the catalyst).
3.2.2. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopic investigation of LDH-
Pt0 (Fig. 2) at Pt 4f level shows a 4f7/2 line at 72.87 eV and
4f5/2 at 75.19 eV, which clearly indicates that the metal is in
the reduced form [6].
2.4. General experimental procedure
In an oven dried 10 ml round-bottom flask aryl halide
(1 mmol), arylboronic acid (1.5 mmol), K3PO4 (2 mmol),
LDH-Pt0 (1 mol%), and dioxane:water [5:1] (2 ml) were
taken and stirred at 100 ◦C under N2 atmosphere. After the
completion of the reaction, monitored by TLC, the catalyst
was filtered. The filtrate was diluted with ethyl acetate and
washed with 10% aqueous NaOH solution and finally with
saturated aqueous NaCl solution. The organic layer was dried
with Na2SO4 and concentrated to get the crude product. The
crude product was column chromatographed using hexane
and ethyl acetate mixture as an eluent. All products were
characterized by NMR and mass spectroscopy.
3.2.3. Transmission electron microscopy
TEM image of the LDH-Pt0 catalyst shows that the aver-
agesizesofnanoplatinumparticlesareintherangeof4–6 nm.
The TEM images of the used catalyst indicate that the size
3. Results and discussion
3.1. Preparation of LDH-Pt0
Hexachloroplatinate was exchanged onto chloride satu-
rated LDH to obtain a light yellow colored LDH-Pt(IV) and
then reduced with hydrazine hydrate to give air stable black
colored LDH-Pt0 catalyst (Scheme 1). The possible struc-
tures for the catalysts are depicted in Schemes 2 and 3.
Fig. 1. XRD of: (a) LDH-Cl; (b) LDH- Pt0; (c) LDH-PtCl6.

B.M. Choudary et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 215–218
217
and morphology of the nanoplatinum remains unchanged in
the used catalyst.
boronic acids as expected. The presence of electron donat-
ing groups in the aryl halide system retards the reaction,
whereas electron-withdrawing group enhance the reaction.
Here proper choice of solvent plays a crucial role for effective
reaction. Without water, no reaction took place under identi-
cal condition. In dry dioxane, acetonitrile, DMF, toluene the
yields were very poor (5–10%). It may be due to the poor
solubility of inorganic bases in the organic solvents. Among
the bases screened [7] K3PO4 was found to be the best but
NaOH, KF were also good alternatives. Instead of LDH-Pt0,
LDH-PtCl6 can also catalyze the cross coupling reaction of
aryl iodides with arylboronic acids in good yields but in case
3.3. Catalytic Suzuki coupling of aryl halides
A variety of aryl iodides and bromides reacted in the pres-
ence of 1.0 mol% LDH-Pt0 with different arylboronic acids
at 100 ◦C temperature affording the corresponding cross cou-
pling biphenyl product in very good to excellent yields. The
results are summarized in Table 1.
From Table 1, it is clear that aryl iodides are more reactive
than the aryl bromides in the Suzuki coupling with aryl-
Table 1
Catalytic Suzuki couplinga of aryl halides and arylboronic acids using LDH-Pt0
Entry
Aryl halide
Arylboronic acid
Biaryl
Isolated yield (%)
90, 88b, 85c, traced
1
2
3
95
92
4
5
6
98
98
88
7
8
90
78
9
10
11
12
13
14
82, 80b, 55c
82
90
85
95
95
60
15
a
Aryl halide (1.0 mmol); arylboronic acid (1.5 mmol); K3PO4 (2.0 mmol); LDH-Pt0 (1 mol%); dioxane:water = 5:1 (3 ml) 100 ◦C; reaction time = 12 h (for
iodides) and 24 h (bromides).
b
Yield after fifth cycle.
LDH-PtCl6 (1.0 mol%) used as a catalyst.
K2PtCl6 (1.0 mol%) used as a catalyst.
c
d

218
B.M. Choudary et al. / Journal of Molecular Catalysis A: Chemical 241 (2005) 215–218
4. Conclusion
In summary, layered double hydroxides supported
nanoplatinum was prepared and employed for Suzuki cross
coupling of aryl halides (iodides and bromides) with a broad
range of arylboronic acids to afford the corresponding biaryls
in good to excellent yields without using any external ligand.
The simple procedure, easy recovery, and reusable catalytic
systems are expected to contribute to the development of
benign chemical process and products.
Acknowledgements
MR wishes to thank UGC, New Delhi, and SR wishes to
thank CSIR, New Delhi for financial support in the form of
Junior Research Fellowship.
Fig. 2. XPS scan of LDH-Pt0, in inset narrow scan for Pt (a and b) and Al
(c).
References
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