The role of Pd nanoparticles in ionic liquid in the heck reaction

Article abstract of DOI:10.1021/ja0430043

Pd(0) nanoparticles with ～2 nm diameter, immobilized in 1-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid, are efficient catalyst precursors for coupling of aryl halides with n-butylacrylate. In situ TEM analysis of the ionic liquid catalytic

Full text of DOI:10.1021/ja0430043

Published on Web 02/18/2005
The Role of Pd Nanoparticles in Ionic Liquid in the Heck Reaction
Claudia C. Cassol, Alexandre P. Umpierre, Giovanna Machado, Silvana I. Wolke, and
Jairton Dupont*
Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, AV. Bento Gon c¸ alVes,
9
500 Porto Alegre 91501-970 RS Brazil
Received November 19, 2004; E-mail: dupont@iq.ufrgs.br
The arylation/vinylation of alkenes promoted by palladium-based
Scheme 1
catalyst precursors is one of the most investigated transition-metal-
1
catalyzed C-C coupling reactions. The arsenal of procedures
involving Pd catalyst precursors, such as those associated with
phosphines, carbenes, N-, S-, P-, and Se-containing palladacycles,
and even ligand-free approaches, developed over the past few years
is impressive.2 In fact, the Heck reaction involving aryl iodides
and bromides is promoted by almost any Pd(II) or Pd(0) catalyst
,3
4
precursor, usually at elevated temperatures. In this respect, there
are now several indications pointing to, at least in the case of
phosphine-free palladium catalyst precursors, the involvement of
soluble Pd nanoparticles as a reservoir of catalytically active
immediately a black solution. After being stirred for 15 min at room
temperature, the black solution was dissolved in BMI‚PF , and the
6
volatiles were removed under reduced pressure. A small amount
of this ionic liquid “solution” was transferred to a carbon-coated
copper grid and analyzed by TEM (Figure 1, left).
5
species. Not surprisingly, there is a profusion of reports on the
use of colloidal Pd solutions as catalyst precursors in Heck coupling
6
reactions, but the question of whether the reaction is promoted by
the nanoparticles themselves or whether they serve as a reservoir
of soluble Pd catalytically active species still remains open.6,7
Analysis of the metal content in the organic phase at different
substrate conversions can provide indications of Pd leaching during
the reaction. Transmission electron microscopy (TEM) analysis
before and after catalysis can provide information about the
modifications in size and shape of the nanoparticles during
8
catalysis. However, it is not possible to perform TEM experiments
in situ since the sample must be treated (elimination of the solvents,
for example, which may be involved in the stabilization of the
nanoparticles) prior to analysis.
Imidazolium ionic liquids provide an excellent medium for the
formation and stabilization of transition-metal nanoparticles, and
Figure 1. Part of the TEM micrographs showing the Pd nanoparticles
dispersed in BMI‚PF6 ionic liquid before (left) and after (right) catalysis,
observed at 200 kV.
9
since these ionic fluids have no measurable vapor pressure, the size
and shape of the metal nanoparticles can be investigated in situ by
TEM.10 Therefore, the system formed by Pd nanoparticles dispersed
in imidazolium ionic liquids might be an ideal candidate for the in
situ investigations of the size and shape modifications of Pd
nanoparticles during the Heck coupling reaction.11 We wish to
disclose herein our results concerning the in situ TEM analysis of
Pd nanoparticles dispersed in 1-n-butyl-3-methylimidazolium hexa-
The particles display an irregular shape with a monomodal
particle size distribution (1.7 ( 0.3 nm). Energy Dispersion
Spectrometry confirms the presence of Pd on the sample (Figure
S1 in the Supporting Information). Another part of the ionic liquid
sample was centrifuged, affording a black powder that was washed
with dichloromethane, dried under reduced pressure, and analyzed
by X-ray diffraction analysis (XRD). The diffraction line (111) of
Pd(0) can be clearly observed in the diffraction pattern (Figure S2
in the Supporting Information).
6
fluorophosphate (BMI‚PF ) ionic liquid, before and after the
arylation of n-butyl acrylate, and the Pd leaching from the ionic
liquid phase to the organic phase at different substrate conversions.
The first challenge was to prepare stable and ligand-free Pd
The catalytic properties of the Pd nanoparticles dispersed in BMI‚
PF were tested in the coupling of aryl halides with n-butylacrylate
6
nanoparticles dispersed in the BMI‚PF
6
ionic liquid.12 It has been
at different temperatures (Table 1). First, we tested different bases
i
recently reported that the reaction of N-containing palladacycles
with allenes affords cationic N-heterocycles and Pd(0) (Scheme
(NaOAc, NEt( Pr) , DABCO, and Na CO ) in the reaction of
2
2
3
iodobenzene with n-butylacrylate at 80 °C ([PhI]/[Pd] ) 1000). A
mixture of the aryl halide (1.0 mmol) and n-butyl acrylate (1.2
mmol) with the Pd nanoparticles dispersed in 0.5 mL of ionic liquid
13
1
). The soluble Pd(0) species thus formed are relatively stable
and were removed from the organic solution as Pd(PPh after
treatment with PPh . Therefore, this reaction can constitute a source
3 4
)
i
3
gave a light-yellow solution in the presence of NEt( Pr) and
2
of Pd(0) species, which in the presence of stabilizing agents such
as imidazolium ionic liquids, generates stable Pd(0) nanoparticles.
The reaction of palladacycle 114 with a large excess of dimeth-
ylallene in dichloromethane at room temperature affords almost
suspensions in the presence of the other bases. After 14 h, the
organic products form an upper phase layer that was extracted with
diethyl ether and analyzed by GC (using methyl benzoate as internal
standard) and GC-MS.
3298
9
J. AM. CHEM. SOC. 2005, 127, 3298-3299
10.1021/ja0430043 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Heck Coupling between n-Butylacrylate (1.2 mmol) and
cycle or fall back to the nanoparticles reservoir. Moreover, these
results are in line with the de Vries propositions for the ligand-
free Pd catalysts in Heck coupling reactions.3
In summary, we have demonstrated that imidazolium ionic liquids
are the media of choice for the in situ investigations of the changes
in shape and in size of colloidal Pd catalysts. Further in situ
investigations employing EXAFS in the Heck reaction promoted
by Pd nanoparticles dispersed in imidazolium ionic liquids are under
investigation in our laboratory.
Aryl Halides (1.0 mmol) by Pd Nanoparticles Dispersed in BMI‚PF6
i
(
0.5 mL) for 14 h using NEt( Pr)2 (1.2 mmol)
T
conversiona
(%)
yield
(%)
entry
ArX
[ArX]/[Pd]
(°C)
1
2
3
4
5
6
PhI
PhI
PhI
PhI
1000
1000
1000
1000
1000
200
30
80
80
100
130
130
0
98
92
100
98
30
0
75
87
86
77
30
b
4-MeCOC6H4Br
4-MeOC6H4Br
Acknowledgment. Thanks are due to FAPERGS, CNPq, and
CTPETRO for financial support, and CAPES for scholarships.
a
Based on PhI consumption. b Using 0.2 mL of the ionic liquid.
Supporting Information Available: Experimental procedure, XRD,
and EDS spectra of the Pd nanoparticles. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(
1) (a) Handbook of Organopalladium Chemistry for Organic Synthesis;
Negishi, E., Ed.; John Wiley & Sons: New York, 2002. (b) Beletskaya,
I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009-3066.
(
2) (a) Bedford, R. B. Chem. Commun. 2003, 1787-1796. (b) Dupont, J.;
Pfeffer, M.; Spencer, J. Eur. J. Inorg. Chem. 2001, 1917-1927.
3) (a) Reetz, M. T.; de Vries, J. G. Chem. Commun. 2004, 1559-1563. (b)
de Vries, A. H. M.; Parlevliet, F. J.; Schmieder-van de Vondervoort, L.;
Mommers, J. H. M.; Henderickx, H. J. W.; Walet, M. A. M.; de Vries, J.
G. AdV. Synth. Catal. 2002, 344, 996-1002. (c) de Vries, A. H. M.;
Mulders, J. M. C. A.; Mommers, J. H. M.; Henderickx, H. J. W.; de Vries,
J. G. Org. Lett. 2003, 5, 3285-3288. (d) Reetz, M. T.; Westermann, E.;
Lohmer, R.; Lohmer, G. Tetrahedron Lett. 1998, 39, 8449-8452.
4) Gruber, A. S.; Pozebon, D.; Monteiro, A. L.; Dupont, J. Tetrahedron Lett.
2001, 42, 7345-7348.
(
Figure 2. Possible pathways involved in the Heck reaction promoted by
Pd nanoparticles dispersed in imidazolium ionic liquids.
(
(
5) (a) Rocaboy, C.; Gladysz, J. A. New J. Chem. 2003, 27, 39-49. (b) Reetz,
M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165-168. (c)
Tromp, M.; Sietsma, J. R. A.; van Bokhoven, J. A.; van Strijdonck, G. P.
F.; van Haaren, R. J.; van der Eerden, A. M. J.; van Leeuwen, P. W. N.
M.; Koningsberger, D. C. Chem. Commun. 2003, 128-129.
Almost complete iodobenzene conversion was observed in the
i
reaction experiment performed with NEt( Pr)
2
, whereas only <30%
conversion was attained with the other bases. A small portion of
the ionic liquid phase after the coupling reaction described in Table
(
6) See, for example: (a) Mandal, S.; Roy, D.; Chaudhari, R. V.; Sastry, M.
Chem. Mater. 2004, 16, 3714-3724. (b) Moreno-Manas, M.; Pleixats,
R. Acc. Chem. Res. 2003, 36, 638-643. (c) Biffis, A.; Zecca, M.; Basato,
M. J. Mol. Catal. A 2001, 173, 249-274. (d) Walter, J.; Heiermann, J.;
Dyker, G.; Hara, S.; Shioyama, H. J. Catal. 2000, 189, 449-455.
1, entry 2, was placed in a carbon-coated copper grid and analyzed
by TEM (Figure 1, right).
The Pd nanoparticles dispersed in the ionic liquid after the
reaction display an irregular shape with a monomodal size distribu-
tion of 6.1 ((0.7) nm.
The palladium content in the organic phase during the arylation
reaction was checked by ICP-AS and shows significant metal
leaching from the ionic phase to the organic phase at low substrate
conversions (34 and 12% at 3 and 11% iodobenzene conversion,
respectively) and drops to 5-8% leaching at higher conversions
(7) (a) Choudary, B. M.; Madhi, S.; Kantam, M. L.; Sreedhar, B.; Iwasawa,
Y. J. Am. Chem. Soc. 2004, 126, 2292-2293. (b) Kogan, V.; Aizenshtat,
Z.; Popovitz-Biro, R.; Neumann, R. Org. Lett. 2002, 4, 3529-3532. (c)
Le Bars, J.; Specht, U.; Bradley, J. S.; Blackmond, D. G. Langmuir 1999,
1
5, 7621-7625. (d) Zhao, F. Y.; Bhanage, B. M.; Shirai, M.; Arai, M.
Chem.sEur. J. 2000, 6, 843-848.
(
8) See, for example: Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004,
108, 8572-8580.
(9) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira,
S. R. J. Am. Chem. Soc. 2002, 124, 4228-4229.
(10) Scheeren, C. W.; Machado, G.; Dupont, J.; Fichtner, P. F. P.; Texeira, S.
R. Inorg. Chem. 2003, 42, 4738-4742.
(>60%). A similar trend was recently observed in Heck reactions
(
11) For Heck reactions in ionic liquids, see: (a) Carmichael, A. J.; Earle, M.
promoted by a heterogeneous catalyst precursor where it was
proposed that Pd dissolution and reprecipitation are inherent parts
of the catalytic cycle.15
J.; Holbrey, J. D.; McCormac, P. B.; Seddon, K. R. Org. Lett. 1999, 1,
9
97-1000. (b) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV.
2
002, 102, 3667-3691.
(12) For Pd nanoparticles in ionic liquids, see: (a) Anderson, K.; Fernandez,
S. C.; Hardacre, C.; Marr, P. C. Inorg. Chem. Commun. 2004, 7, 73-76.
The TEM and ICP-AS results are strong indications that Pd
nanoparticles dispersed in the ionic liquid act as a reservoir of
catalytically active Pd species (Figure 2). The recovered organic
phase extracted with diethyl ether after 4 h (containing around 25%
(b) Cal o´ , V.; Nacci, A.; Monopoli, A.; Laera, S.; Cioffi, N. J. Org. Chem.
2003, 68, 2929-2933. (c) Deshmukh, R. R.; Rajagopal, R.; Srinivasan,
K. V. Chem. Commun. 2001, 1544-1545. (d) Huang, J.; Jiang, T.; Han,
B. X.; Gao, H. X.; Chang, Y. H.; Zhao, G. Y.; Wu, W. Z. Chem. Commun.
2
003, 1654-1655. (e) Hamill, N. A.; Hardacre, C.; McMath, S. E. J. Green
Chem. 2002, 4, 139-142. (f) Cal o´ , V.; Nacci, A.; Monopoli, A.; Detomaso,
A.; Iliade, P. Organometallics 2003, 22, 4193-4197. (g) Zhao, D.; Fei,
Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. J. Am. Chem. Soc. 2004,
126, 15876-15882.
Pd) is inactive for the coupling of PhI and n-butylacrylate in the
i
presence of NEt( Pr)
2
at 80 °C after 20 h. Moreover, attempts to
identify Pd nanoparticles in the organic phase by TEM analysis
have been unsuccessful so far. Thus, it is highly probable that the
reaction proceeds through the oxidative addition of PhI on the
nanoparticles surface, and the oxidized Pd species thus formed are
detached from the surface and are entering in the main catalytic
cycle. The Pd(0) formed in the main catalytic cycle, after â-hydride
and reductive elimination steps, can either continue in the catalytic
(
13) Sirlin, C.; Chengebroyen, J.; Konrath, R.; Ebeling, G.; Raad, I.; Dupont,
J.; Paschaki, M.; Kotzyba-Hibert, F.; Harf-Monteil, C.; Pfeffer, M. Eur.
J. Org. Chem. 2004, 1724-1731.
(
14) Consorti, C. S.; Zanini, M. L.; Leal, S.; Ebeling, G.; Dupont, J. Org. Lett.
2003, 5, 983-986.
(15) Prockl, S. S.; Kleist, W.; Gruber, M. A.; Kohler, K. Angew. Chem., Int.
Ed. 2004, 43, 1881-1882.
JA0430043
J. AM. CHEM. SOC.
9
VOL. 127, NO. 10, 2005 3299