nanoparticles were formed via thermal decomposition of Pd-
(OAc)2 without any additional reducing agents.7 We therefore
expected that use of copolymer 2 might enable immobiliza-
tion of Pd(0) nanoparticles. Thus, a solution of Pd(OAc)2
and copolymer 2 in THF was heated at 66 °C for 3 h (entry
2), gradual color change (orange to dark brown) was
observed, and palladium was successfully immobilized,
although the loading level was low (entry 2).8-10 Encouraged
by this result, we investigated immobilization of Pd(NO3)2
by adding 1 or 2 equiv of sodium acetate (entries 3 and 4).
Interestingly, not only did immobilization occur, but also
significant increases of the loading levels were observed (vs
entry 2). It was speculated that sodium nitrate produced
during the formation of Pd(OAc)2 might assist the im-
mobilization.11,12
Indeed, when sodium nitrate was added to Pd(OAc)2, the
loading level was increased (entry 5). Addition of sodium
chloride or sodium acetate to Pd(OAc)2 was also found to
enhance the loading level (entries 6 and 7). Then, we
investigated the effect of metal ions on loading level (entries
3, 8, and 9). It turned out that the loading level was increased
dramatically when potassium acetate was used, whereas
lithium acetate showed almost the same loading level as that
of sodium acetate.13 The effect of those salts is likely an
electrostatic stabilization;11 the precise mechanism is as yet
unclear. This is the first example of the preparation of
polymer-incarcerated (PI) palladium using a reduction strat-
egy.
Scheme 1. Two Strategies for the Polymer-Incarcerated
Method
immobilization (Scheme 1, previous strategy), four triph-
enylphosphine moieties were lost during the preparation. We
envisaged that PI Pd could be altenatively prepared from
inexpensive Pd(II) salts under suitable reduction conditions
(Scheme 1, this strategy). As a consequence, a new practical
immobilization method of Pd was achieved. Remarkable
effects of alkali metal salts on palladium immobilization and
high catalytic activity were observed.
Initially, immobilization of palladium nitrate (Pd(NO3)2)
using copolymer 2 was investigated according to the general
PI procedure (Scheme 2, Table 1, entry 1). However, serious
Scheme 2. Preparation of Polymer-Incarcerated Palladium
Next, we focused on the application of these catalysts, PI
Pd A-H, to the Mizoroki-Heck reaction7a,14 of iodobenzene
(1a) with ethyl acrylate (2a) as a model reaction (Table 2).
(7) (a) Reetz, M. T.; de Vries, J. G. Chem. Commun. 2004, 1559. (b)
Reetz, M. T.; Maase, M. AdV. Mater. 1999, 11, 773. (c) Reetz, M. T.; Helbig,
W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995,
267, 367.
(8) It was observed by transmission electron microscope (TEM) analysis
that the size of immobilized palladium particles was less than 1.5 nm and
Pd dispersed uniformly on the polymer.
leaching of Pd occurred, and immobilization was unsuccess-
ful. Therefore, immobilization of Pd(II) without reduction
seemed to be difficult. Then, immobilization of Pd via
(9) Immobilization of Pd nanoparticles in the presence of tetrabutylam-
monium bromide (1 equiv) was also effective [yield 206 mg, 0.45 mmol/g
(70%)]. However, reactivity of this catalyst in Heck reaction of iodobenzene
and ethyl acrylate was low (49% yield).
(10) This observation suggested that formation of Pd(0) from Pd(II) and
subsequent stabilization by the polymer might be the key to the success of
the current immobilization method.
Table 1. Effect of Pd Sources and Salts
yield
loading
entry
“Pd”
additive
(mg) (mmol/g)b Pl Pd
1a
2a
3
4
5
Pd(NO3)2
Pd(OAc)2
112
0
(11) It has been reported that ionic compounds such as those containing
halides, carboxylates, or polyoxoanions might form an electrical double
layer around the metal particles to suppress aggregation of metals. (a) Finke
R. G. In Metal Nanoparticles: Synthesis, Characterization and Application;
Feldheim, D. L., Foss, C. A., Jr., Eds.; Marcel Dekker: New York, 2002;
Chapter 2, pp 17-54. (b) Labib, M. E. Colloids Surf. 1988, 29, 293. For
reviews, see: (a) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102,
3757. (b) Aiken, J. D., III; Finke, R. G. J. Mol. Catal. A 1999, 145, 1.
(12) Immobilization of Pd2(dba)3 using the previous PI method (ligand
exchange from dba to the polymer) in the absence of tetrabutylammonium
salts (1 equiv) was unsuccessful, while addition of the ammonium salts
enabled immobilization of Pd2(dba)3. Considering the role of the ammonium
salts as stabilizers for Pd(0), our initial success in the immobilization of
Pd(PPh3)4 suggested the effect of phosphine ligands as stabilizers as well.
Synthesis of Pd nonoparticles stabilized by phosphine ligands has recently
been reported; see: (a) Son, S. U.; Jang, Y.; Yoon, K. Y.; Kang, E.; Hyeon,
T. Nano Lett. 2004, 4, 1147. (b) Jansat, S.; Gomez, M.; Philippot, K.; Muller,
G.; Guiu, E.; Claver, C.; Castillon, S.; Chaudret, B. J. Am. Chem. Soc.
2004, 126, 1592. (c) Tamura, M.; Fujihara, H. J. Am. Chem. Soc. 2003,
125, 15742. (d) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003,
125, 8340.
189 0.19 (28%)
A
B
C
D
E
F
Pd(NO3)2 NaOAc (2 equiv) 169 0.44 (57%)
Pd(NO3)2 NaOAc (1 equiv) 166 0.63 (80%)
Pd(OAc)2 NaNO3 (2 equiv) 184 0.34 (48%)
6
Pd(OAc)2 NaCl (2 equiv)
187 0.31 (45%)
7
Pd(OAc)2 NaOAc (2 equiv) 165 0.50 (63%)
8
9
Pd(NO3)2 KOAc (2 equiv)
Pd(NO3)2 LiOAc (2 equiv)
177 0.71 (96%)
209 0.41 (66%)
G
H
a Immobilization of Pd using reduction of Pd(II) in THF under H2 (1
atm) at room temperature was also unsuccessful. b Determined by XRF
analysis. Percentages of Pd loaded are shown in parentheses.
reduction of Pd(NO3)2 or palladium acetate (Pd(OAc)2) in
THF under atmospheric hydrogen was tested. However,
immobilization failed, and precipitation of Pd-black was
observed. Meanwhile, it was recently reported that Pd(0)
(13) Transmission electron microscope (TEM) analysis showed that the
size of Pd particles of PI Pd G and H are 3-5 nm.
376
Org. Lett., Vol. 8, No. 3, 2006