Palladium nanoparticles stabilized by sterically hindered phosphonium salts as Suzuki cross-coupling catalysts

Article abstract of DOI:10.1007/s11172-013-0088-z

Palladium nanoparticles with an average size of 2.5±0.5 nm formed from palladium acetate in the presence of tri-tert-butyl(decyl)phosphonium tetrafluoroborate without an additional reducing agent exhibit high activity as a catalyst for the Suzuki cross-coupling reaction involving bromo- and chloroarenes under mild conditions.

Full text of DOI:10.1007/s11172-013-0088-z

Russian Chemical Bulletin, International Edition, Vol. 62, No. 3, pp. 657—660, March, 2013
657
Palladium nanoparticles stabilized by sterically hindered
phosphonium salts as Suzuki crossꢀcoupling catalysts*
V. V. Ermolaev, D. M. Arkhipova, L. Sh. Nigmatullina, I. Kh. Rizvanov, V. A. Milyukov, and O. G. Sinyashin
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center, Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Eꢀmail: miluykov@iopc.ru
Palladium nanoparticles with an average size of 2.5 0.5 nm formed from palladium acetate
in the presence of triꢀtertꢀbutyl(decyl)phosphonium tetrafluoroborate without an additional
reducing agent exhibit high activity as a catalyst for the Suzuki crossꢀcoupling reaction involvꢀ
ing bromoꢀ and сhloroarenes under mild conditions.
Key words: palladium nanoparticles, bromoarenes, catalysis, crossꢀcoupling, arylꢀ
boronic acids.
The crossꢀcoupling between aryl halides and arylboꢀ
ronic acids discovered by A. Suzuki1 in 1979 is one of the
most demanded and efficient methods for the formation
of C—C bond between aryl fragments.2—4 According to
the commonly accepted mechanism, the reaction proꢀ
ceeds via three stages: oxidative addition of aryl halide to
palladium(0), transmetallation, and reductive elimination
of the product. The palladium(0) complexes with phosꢀ
phine ligands are used most frequently as catalysts of the
process.5 The variation of the phosphine ligand structure
makes it possible to "tune" the catalyst activity and to
make even poorly reactive chloroarenes to react.6
A substantial limitation for wide use of the Suzuki
crossꢀcoupling in practice is complicated recovery of the
catalysts based on molecular palladium(0) complexes and
high cost of phosphine ligands and palladium(0) comꢀ
pounds.
dendrimers,15 and surfactants16 act as stabilizers of pallaꢀ
dium nanoparticles. Ionic liquids as stabilizers of palladiꢀ
um nanoparticles are of most interest, since they have low
vapor pressure and are immiscible with the most organic
solvents, which provides, on the one hand, easy catalyst
recovery and, on the other hand, the entire corresponꢀ
dence to principles of green chemistry.
Presently, imidazolium salts (in which the carbene
complex with palladium acts as a catalyst),17,18 tetraꢀ
butylammonium salts,19 and other nitrogenꢀcontaining
salts20,21 are most popular as stabilizers of palladium nanoꢀ
particles. These stabilizing agents are unstable under basic
conditions because of the acidic proton, which restricts
their application in the reactions involving strong nucleoꢀ
philes.22 Therefore, phosphonium salts that are resistant
towards strong nucleophiles are interesting as potential
stabilizers of palladium nanoparticles. However, only two
examples for the stabilization of palladium nanoparticles
by trihexyl(tetradecyl)phosphonium salts are described to
date.23,24 The influence of the phosphonium cation strucꢀ
ture on the size and morphology of palladium nanopartiꢀ
cles and their catalytic activity remains unclear.
One of the most promising processing of the Suzuki
crossꢀcoupling is the use of nanosized palladium particles
as catalysts. I. P. Beletskaya pioneered in demonstrating
the possibility to carry it out in the "ligandꢀfree" version7—9
;
however, in these works the authors did not mention the
possibility of formation of palladium nanoparticles. This
approach was successfully used for the synthesis of funcꢀ
tional biaryls,10 but the necessity to use a significant
amount of palladium acetate (up to 5 mol.%) and imposꢀ
sibility to recover the catalyst impede the use of this method
in daily practice.
In this report, we present the data on the possibility of
using the phosphonium salt with the sterically hindered
cation, viz., triꢀtertꢀbutyl(decyl)phosphonium tetrafluoroꢀ
borate25 (1), for the stabilization of palladium nanopartiꢀ
cles and applying the obtained system as a catalyst for the
Suzuki crossꢀcoupling involving bromoꢀ and chloroarenes.
The methods based on using colloidal solutions of palꢀ
ladium nanoparticles are very popular.11,12 Polymers,13,14
*Dedicated to the Academician of the Russian Academy of
Sciences I. P. Beletskaya on the occasion of her anniversary.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0656—0659, March, 2013.
1066ꢀ5285/13/6203ꢀ657 © 2013 Springer Science+Business Media, Inc.

658
Russ.Chem.Bull., Int.Ed., Vol. 62, No. 3, March, 2013
Ermolaev et al.
Scheme 1
Results and Discussion
Palladium nanoparticles were obtained by the reducꢀ
tion of palladium acetate in the presence of phosphonium
salt. There are two possible routes for the reduction of
palladium(II) to palladium(0): decomposition of palladiꢀ
um acetate at 70—80 С and reduction with phenylboronꢀ
ic acid affording biphenyl as the homocoupling product.
The obtained nanoassociates were characterized by transꢀ
mission electron microscopy (Fig. 1).
In this case, both individual nanoparticles with a comꢀ
plicated shape and small aggregates are observed. The avꢀ
erage size of the prepared palladium nanoparticles stabiꢀ
lized by salt 1 is 2.5 0.5 nm, which is comparable with the
sizes of the palladium nanoparticles obtained in the presꢀ
ence of trihexyl(tetradecyl)phosphonium salts (7 2 nm
and 3.5 1 nm).23,24
2a—f
Ar
3a,b
a
b
R
H
F
4a—f
R
H
F
H
F
Ar
a
b
c
d
e
f
4ꢀMeC6H4
3ꢀMeC6H4
2ꢀMeC6H4
2,6ꢀMe2C6H3
2,3,4ꢀF3C6H3
2,4ꢀ(F3C)2C6H3
a
b
c
d
e
f
4ꢀMeC6H4
4ꢀMeC6H4
3ꢀMeC6H4
3ꢀMeC6H4
2ꢀMeC6H4
2ꢀMeC6H4
H
F
4g—i
R
H
F
Ar
4j—l
j
k
l
R
F
H
F
Ar
g
h
i
2,6ꢀMe2C6H3
2,6ꢀMe2C6H3
2,3,4ꢀF3C6H2
2,3,4ꢀF3C6H2
2,4ꢀ(F3C)2C6H3
2,4ꢀ(F3C)2C6H3
H
The system based on the palladium nanoparticles staꢀ
bilized by phosphonium salt 1 exhibits good activity as the
catalyst for the Suzuki crossꢀcoupling of bromoarenes 2
with phenylboronic acid 3a (Scheme 1, Table 1). Note
that the process takes place under mild conditions (30 С)
in the presence of 1 mol.% catalyst, and the yields of the
isolated products are 57—92%. The activity of this cataꢀ
lytic system is comparable26 with the "reference" cataꢀ
lyst Pd(OAc)2—PBut3. The formation of a small amount
(2—3%) of the homocoupling product of phenylboronic
acid is observed.
1,3ꢀdisubstituted: 5a, 6a,b; 1,4ꢀdisubstituted: 5b, 6c,d.
R = H (6a,c), R = F (6b,d)
The high conversion of bromoarenes in the Suzuki
crossꢀcoupling in the presence of palladium nanopartiꢀ
cles stabilized by salt 1 is also observed when 2,4ꢀdiꢀ
fluorophenylboronic acid (3b) is used (see Scheme 1 and
Table 1).
Reagents and conditions: i. nanoꢀPd, 1, Cs2CO3, toluene, 30 C,
12 h.
At the same time, the conversion of chloroarenes 7a—f,
under the same conditions, depends considerably on the
50 nm
50 nm
Fig. 1. TEM images of palladium nanoparticles obtained by reduction in the presence of triꢀtertꢀbutyl(decyl)phosphonium tetrafluoꢀ
roborate (1).

Phosphoniumꢀstabilized nanopalladium
Russ.Chem.Bull., Int.Ed., Vol. 62, No. 3, March, 2013
659
Table 1. Conversions of the starting bromoarenes 2a—f and 5a,b
and the yields of products 4a—l and 6a—d in the Suzuki crossꢀ
coupling between bromoarenes and arylboronic acids catalyzed
by the Pd(OAc)2—1 (I) and Pd(OAc)2—PBut (II) systems
Conversion of 2 (%)
100
90
80
70
60
50
40
30
20
10
Bromoꢀ
arene
Conversion of bromoarene (yield of product) (%)
PhB(OH)2 (3a) 2,4ꢀF2C6H3B(OH)2 (3b)
2a
2b
2c
Product
I
II
Product
I
II
2a
2b
2c
2d
2e
2f
4a
4c
4e
4g
4i
4k
6a
6c
99 (92)
99 (88)
93 (85)
61 (57)
86 (77)
93 (90)
84 (77)
69 (57)
99
77
91
93
99
94
35
94
4b
4d
4f
4h
4j
4l
6b
6d
99 (91)
87 (82)
83 (74)
54 (42)
83 (69)
46 (39)
73 (65)
83 (72)
88
76
62
93
99
99
72
9
1
2
3
4
5
N
Fig. 2. Activity of the Pd(OAc)2—1 catalytic system in the crossꢀ
coupling of bromotoluenes 2a—c and phenylboronic acid 3a;
N is the number of catalytic cycles.
5a
5b
Note. Reaction conditions: bromoarene (2 mmoles), boronic acid
(2.3 mmoles, 15% excess), Cs2CO3 (2.3 mmoles), toluene
(5 mL), Pd(OAc)2 (0.02 mmole, 1 mol.%), phosphonium salt 1
or PdBut3 (1 mmole), 30 С, 12 h (the yields of isolated products
are given).
(organic phase) of this threeꢀlayer system contains the
reaction products and haloarenes (if any), the medium
layer contains phosphonium salt 1 and palladium nanoꢀ
particles, and the bottom layer (inorganic phase) is an
aqueous solution of inorganic salts. The medium layer was
separated, washed with a mixture of water and hexane
(twice), and used in the subsequent catalytic cycles. The
results obtained in five catalytic cycles are shown in Fig. 2.
Additional experiments were carried out to determine
the dependence of the conversion of bromoarenes on the
nature of the phosphonium cation. The results are preꢀ
sented in Table 2. The control experiment in the absence
of phosphonium salts revealed that the salt is important
for the stabilization of catalytic particles in the course of
the reaction. In addition, the conversion of bromotoluꢀ
enes in the Suzuki crossꢀcoupling depends substantially
on the cation structure of the phosphonium salt. For exꢀ
ample, 93% conversion is observed in the coupling beꢀ
tween 2ꢀbromotoluene (2c) and phenylboronic acid (3a)
in the presence of salt 1, while the conversion is only 23%
under the same conditions in the case of less sterically
hindered phosphonium salt, viz., tributyl(decyl)phosꢀ
phonium tetrafluoroborate.
substituent in the substrate (Scheme 2). So, the converꢀ
sion of activated chloroarenes with electronꢀwithdrawing
substituents 7a—c ranges from 60 to 70%, while it is only
11% in the case of deactivated 4ꢀchlorotoluene (7f).
Scheme 2
X
7a—f
Product
Conversion
of chloroarene 7 (%)
4ꢀMeC(O)
4ꢀMeOC(O)
2ꢀMeOC(O)
4ꢀO2N
4ꢀCN
4ꢀMe
7a
7b
7c
7d
7e
7f
8a
8b
8c
8d
8e
4a
63
68
54
40
75
11
To conclude, it was found in this work that the crossꢀ
coupling of bromoꢀ and chloroarenes with phenylboronic
Reagents and conditions: nanoꢀPd, 1, Cs2CO3, toluene, 30 C,
12 h.
Table 2. Conversion (%) of bromotoluenes 2a—c in the
reaction with phenylboronic acid 3a vs structure of the
cation of phosphonium salt
Thus, the catalytic system based on palladium nanoꢀ
particles stabilized by phosphonium salt 1 is highly active
in the crossꢀcoupling of bromoarenes and activated chloroꢀ
arenes, providing high conversion under mild conditions.
The palladium nanoparticles stabilized by phosphoniꢀ
um salt 1 can readily be isolated from the reaction mixture
and recycled without loss of activity. For this purpose, we
developed the following procedure: after the solvent was
removed, a mixture of hexane and water was added to the
dry residue to form a threeꢀlayer system. The upper layer
Phosphonium salt
Bromoarene
2a
2b
2c
–
[But3PC10H21]+BF4 (1)
100
185
134
100
127
110
93
23
10
[Bu3PC10H21]+BF4
–
Without phosphonium salt

660
Russ.Chem.Bull., Int.Ed., Vol. 62, No. 3, March, 2013
Ermolaev et al.
4. F. Alonso, I. P. Beletskaya, M. Yus, Tetrahedron, 2008,
64, 3047.
acid can occur under mild conditions using palladium
nanoparticles stabilized by the phosphonium salts conꢀ
taining sterically hindered cation, viz., triꢀtertꢀbutylꢀ
(decyl)phosphonium tetrafluoroborate (1). The prepared
catalytic system is characterized by high activity, which
makes it possible to carry out the Suzuki crossꢀcoupling
involving bromoarenes and activated chloroarenes under
mild conditions with high conversions.
5. N. Miyaura, A. Suzuki, Chem. Rev., 1995, 95, 2457.
6. R. Martin, S. L. Buchwald, Acc. Chem. Res., 2008, 41, 1461.
7. N. A. Bumagin, V. V. Bykov, I. P. Beletskaya, Bull. Acad.
Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1989, 38, 2206
[Izv. Akad. Nauk SSSR, Ser. Khim., 1989, 2394].
8. N. E. Bumagin, V. V. Bykov, I. P. Beletskaya, Dokl. Akad.
Nauk SSSR, 1990, 315, 1133 [Dokl. Chem. (Engl. Transl.),
1990, 315, No. 5].
9. N. A. Bumagin, V. V. Bykov, I. P. Beletskaya, Bull. Acad.
Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1990, 39, 2426
[Izv. Akad. Nauk SSSR, 1990, 2672].
10. E. M. Campi, W. R. Jackson, S. M. Marcuccio, C. G. M.
Naeslund, Chem. Commun., 1994, 2395.
11. V. P. Ananikov, I. P. Beletskaya, Organometallics, 2012,
31, 1595.
12. D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem., Int. Ed.,
2005, 44, 7852.
13. C. Ornelas, A. K. Diallo, J. Ruiz, D. Astruca, Adv. Synth.
Catal., 2009, 351, 2147.
14. Y. Li, X. M. Hong, D. M. Collard, M. A. ElꢀSayed, Org.
Lett., 2000, 15, 2385.
15. Y. Tsuji, T. Fujihara, Inorg. Chem., 2007, 46, 1895.
16. J. P. Hallett, T. Welton, Chem. Rev., 2011, 111, 3508.
17. L. D. Pachon, С. J. Elsevier, G. Rothenberg, Adv. Synth.
Catal., 2006, 348, 1705.
18. C. Ye, J.ꢀC. Xiao, B. Twamley, A. D. LaLonde, M. G.
Norton, J. M. Shreeve, Eur. J. Org. Chem., 2007, 5095.
19. V. Calo, A. Nacci, A. Monopoli, F. Montingelli, J. Org.
Chem., 2005, 70, 6040.
Experimental
All procedures associated with the preparation of the starting
reagents, syntheses, and isolation of products were carried out
under argon using a standard Schlenk technique. The converꢀ
sion of bromoꢀ and chloroarenes was evaluated by gas chromatoꢀ
graphy coupled with mass spectrometry on a DFS instrument
(ThermoꢀScientific, USA), injector temperature 280 C, MSꢀ5
column (30 m, 0.25 mm, 0.25 m). TEM images of the cataꢀ
lyst surface before and after the reaction were obtained using
a PHILIPS/FEI CM20 transmission electron microscope. The
particle size distribution was determined on a Veeco MultiMode V
scanning probing microscope using the intermittentꢀcontact
atomic force method.
Suzuki crossꢀcoupling reaction (general procedure). A mixꢀ
ture of arylboronic acid (2.3 mmol), bromoꢀ or chloroarene
(2 mmol), Cs2CO3 (2.3 mmol), Pd(OAc)2 (0.02 mmol), triꢀtertꢀ
butyl(decyl)phosphonium tetrafluoroborate (1) (0.46 mmol), and
toluene (5 mL) was placed in a Schlenk flask and vigorously
stirred for 12 h at 30 С. To isolate the products, the reaction
mixture was filtered through Al2O3, the solvent was completely
removed in vacuo, and the obtained solid powder was recrystalꢀ
lized from petroleum ether.
20. J.ꢀH. Cha, K.ꢀS. Kim, H. Lee Korean, J. Chem. Eng., 2009,
26, 760.
21. M. T. Reetz, M. Winter, R. Breinbauer, T. ThurnꢀAlbrecht,
W. Vogel, Chem. Eur. J., 2001, 7, 1084.
22. L. Ford, F. Atefi, R. D. Singer, P. J. Scammells, Eur. J. Org.
Chem., 2011, 942.
23. A. Banerjee, R. Theron, R. W. J. Scott, ChemSusChem, 2012,
5, 109.
This work was financially supported by the Ministry of
Education and Science of the Russian Federation (state
contract no. 8451).
24. H. A. Kalviri, F. M. Kerton, Green Chem., 2011, 13, 681.
25. V. Ermolaev, V. Miluykov, I. Rizvanov, D. Krivolapov,
E. Zvereva, S. Katsyuba, O. Sinyashin, R. Schmutzler, Dalꢀ
ton Trans., 2010, 39, 5564.
References
1. N. Miyaura, K. Yamada, A. Suzuki, Tetrahedron Lett., 1979,
20, 3437.
26. A. F. Littke, G. C. Fu, J. Am. Chem. Soc., 2001, 123, 6989.
2. A. Balanta, C. Godard, C. Claver, Chem. Soc. Rev., 2011,
40, 4973.
3. K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew. Chem., Int.
Ed., 2005, 44, 4442.
Received January 16, 2013;
in revised form February 26, 2013