Dendrimer-encapsulated Pd nanoparticles as fluorous phase-soluble catalysts [15]

Full text of DOI:10.1021/ja9936870

J. Am. Chem. Soc. 2000, 122, 1243-1244
1243
Scheme 1
Dendrimer-Encapsulated Pd Nanoparticles as
Fluorous Phase-Soluble Catalysts
Victor Chechik† and Richard M.Crooks*
Department of Chemistry, Texas A&M UniVersity
P.O. Box 30012, College Station, Texas 77842-3012
ReceiVed October 14, 1999
Here we describe the application of dendrimer-encapsulated
Pd nanoparticles to fluorous biphasic catalysis.1 Complexation
of Pd/dendrimer composites with perfluorinated carboxylic acids
renders the resulting nanocomposites preferentially soluble in
fluorinated hydrocarbons. These new catalysts show high activity
and selectivity for biphasic hydrogenation of alkenes and conju-
gated dienes. Moreover, the catalysts can easily be recovered and
used for multiple reactions.
they were noncovalently modified with perfluoropolyethers.,10,11
This was achieved by complexing the terminal dendrimer amine
groups with the carboxylic end groups of the perfluoropolyethers.
Figure 1 shows a biphasic toluene/perfluoro-2-butyltetrahy-
drofuran (Fluoroinert FC-75) mixture containing dendrimer-
encapsulated Pd nanoparticles complexed with poly(hexafluoro-
propylene oxide-co-difluoromethylene oxide) monocarboxylic
acid, MW ∼550. The dark brown Pd/dendrimer catalyst resides
exclusively in the fluorous phase. To the best of our knowledge,
this is the first example of a stable solution of metal nanoparticles
in fluorous solvents.
To probe the catalytic properties of this biphasic system, we
performed hydrogenation of alkenes. Tetrahydrofuran and FC-
75 were used as organic and fluorous solvents, respectively. The
structures of the substrates used in these experiments and the
corresponding turnover frequencies (TOFs) are shown in Table
1. The identity of the reaction products was confirmed by NMR
after isolation,12 and the TOFs were calculated from the rate of
hydrogen uptake or the accumulation of product assessed by
NMR. In some cases we calculated the TOF using both methods
and found the results to be nearly identical. Interestingly, we found
that addition of polar organic substrates to the reaction mixture
often caused precipitation of the catalyst. This is likely due to
competition between the substrate and the perfluorinated acid for
the amine groups of the dendrimer. This problem was circum-
vented by using a large excess of the perfluorinated acid. We
also note in passing that formation of a fine emulsion and vigorous
stirring are critical for obtaining reproducible TOF values. The
settling time for such emulsions depends on the substrate but
ranges from a few seconds to 5 min.
Reactions in biphasic fluorous/organic systems were suggested
by Horvath and Rabai in 19942 to facilitate recovery and recycling
of soluble catalysts. The general approach to biphasic catalysis
is illustrated in Scheme 1.3 The system consists of organic and
fluorous layers. The catalyst is selectively soluble in the fluorous
phase, while the reactants are preferentially soluble in the organic
solvent. Stirring, sonicating, and/or heating of the mixture leads
to formation of a fine emulsion and partial homogenization (with
some solvents, complete homogenization is obtained at elevated
temperatures), and the catalytic reaction proceeds at the interface
between the two liquids. When the reaction is over, the liquid
phases are separated, the product is isolated from the organic
phase, and the catalyst-containing fluorous layer is recycled. Such
easy separation and recycling are particularly attractive in terms
of “green chemistry”, and a number of fluorous phase-soluble
catalysts have been reported in the literature, including some based
on metal complexes.4,5 Preparation of fluorous phase-soluble metal
nanoparticles, however, has not previously been reported.
Catalytic properties of metal nanoparticles have been explored
since the pioneering studies of Rampino and Nord in the early
1940s.6 Over the past decade research in this area intensified,7
because catalysis by nanoparticles is the most efficient type of
heterogeneous catalysis. This is, of course, a consequence of the
increase in total surface area with decreasing particle size. For
example, 63% of Pd atoms in a 1.4-nm Pd particle are on the
surface of the particle and thus available to perform a catalytic
function. Apart from high efficiency, metal nanoparticles often
show unique selectivity properties, which in some cases are
superior to those of the bulk materials.8
The key result from this study is that the Pd/dendrimer
nanocomposites are catalytically active in fluorous biphasic
systems. Indeed, one catalyst was recycled 12 times, as shown
in Scheme 1, without appreciable loss of catalytic activity. Despite
significant mutual solubility of THF and FC-75, leaking of the
catalyst into the organic phase was not observed within experi-
mental error (∼1% of the total amount of catalyst).
An important drawback of colloidal catalysts is that they are
difficult to separate from reaction mixtures and recycle. However,
the fluorous-phase, biphasic strategy described here combines the
high efficiency and selectivity of homogeneous catalysis with the
ease of separation and recyclability of heterogeneous catalysis.
Our approach is based on the synthesis of dendrimer-encapsulated
metal nanoparticles.9 To enhance solubility of the catalyst-carrying
poly(amidoamine) (PAMAM) dendrimers in fluorous solvents,
(9) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364.
(10) Chechik, V.; Zhao, M.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121,
4910.
(11) Preparation of Pd nanoparticles encapsulated within the interior of
amine-terminated, fourth-generation PAMAM dendrimer was described in ref
10. An aqueous solution of dendrimer-encapsulated nanocomposites, which
contained 10 µmol of Pd and 1 µmol of dendrimer, was purified by dialysis,
concentrated to 1 mL by rotorary evaporation, and mixed with 20 mL of an
ethanolic solution of poly(hexafluoropropylene oxide-co-difluoromethylene
oxide) monocarboxylic acid (300 mg, MW 550, Aldrich Chemical Co.,
Milwaukee, WI). This mixture was evaporated and the product found to be
selectively soluble in fluorous solvents. That the brown color resulting from
the presence of the nanoparticles was found to reside exclusively within the
fluorous phase and that no agglomeration of Pd was observed serve to confirm
the presence of the Pd particles within the dendritic hosts. Interestingly,
perfluoroalkanoic acids CF3(CF2)nCOOH (n ) 11, 17) failed to solubilize Pd/
dendrimer composites in fluorinated solvents.
* To whom correspondence should be addressed. E-mail: crooks@tamu.edu.
Phone: (409) 845-5629. Fax: (409) 845-1399.
† Present address: Department of Chemistry, University of York, Hesling-
ton, York YO10 5DD, U.K.
(1) The term “fluorous phase” refers to solutions prepared using perflu-
orinated organic solvents.
(2) Horvath, I. T.; Rabai, J. Science 1994, 266, 72.
(3) de Wolf, E.; van Koten, G.; Deelman, B.-J. Chem. Soc. ReV. 1999, 28,
37.
(4) Cornils, B. Angew. Chem., Int. Ed. Engl. 1997, 36, 2057.
(5) Curran, D. P. Angew. Chem., Int. Ed. 1998, 37, 1174.
(6) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, 2745.
(7) Lewis, L. N. Chem. ReV. 1993, 93, 2693.
(8) Schmid, G.; Ba¨umle, M.; Geerkens, M.; Heim, I.; Osemann, C.;
Sawitowski, T. Chem. Soc. ReV. 1999, 28, 179.
(12) In the biphasic system, ∼95% of polar (e.g., vinyl acetate) and ∼85%
of nonpolar (e.g., hexene) substrates/products are contained in the organic
phase. Product assays were performed on combined organic fractions following
extraction of the fluorous phase with pure THF.
10.1021/ja9936870 CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/01/2000

1244 J. Am. Chem. Soc., Vol. 122, No. 6, 2000
Communications to the Editor
colloidal, rather than bulk, Pd catalysts. For instance, Hirai and
co-workers found that colloidal Pd can selectively hydrogenate
1,3-cyclooctadiene to cyclooctene (yield 99.9%), while conven-
tional Pd/C catalysts are only moderately selective (ca. 83%).13
The unoptimized dendrimer-encapsulated catalysts yield a 99%
conversion of 1,3-cyclooctadiene to cyclooctene (Table 1).
Some conclusions can be drawn by comparing the TOF values
obtained using biphasic catalysis with literature data for the same
reactions catalyzed by a polymer-supported Pd(0) catalyst (Table
1).14 High values of TOFs for both catalysts are consistent with
the presence of nanoparticles. The selectivity pattern exhibited
by the two types of catalysts is somewhat different. The range of
TOF numbers for biphasic catalysis is far greater than that for
conventional polymer catalysis, which suggests the possibility of
selectively hydrogenating one substrate in the presence of another.
We believe that this added selectivity arises mainly from the polar
nanoenvironment within the dendrimer interior in which the
catalytic reactions occur. For example, the data in Table 1 indicate
that more polar substrates undergo catalytic conversion faster in
the biphasic system. Taking into account that the dendrimer
interior is hydrophilic, one could reason that polar substrates
would partition in the dendrimer interior more easily, which would
promote encounters between the substrate and the surface of the
catalyst. As we have previously shown, selectivity can also be
influenced by the size of the substrate: for a given polarity,
smaller substrates more easily penetrate the sterically crowded
dendrimer surface.9 Finally, solubility of the substrate in the
fluorous phase can also affect the catalytic selectivity.
Figure 1. Two-phase system consisting of (top phase) toluene and
(bottom phase) perfluoro-2-butyltetrahydrofuran (Fluoroinert FC-75). The
dark color (brown) in the bottom phase indicates that the dendrimer-
encapsulated Pd nanoparticles, complexed with poly(hexafluoropropylene
oxide-co-difluoromethylene oxide) monocarboxylic acid, are selectively
extracted into the fluorous phase. No detectable color was observed in
the organic phase.
Table 1. Substrate Structures and Turnover Frequencies Obtained
for Hydrogenation in the Fluorous Biphasic System Described in the
Text
To summarize, we have described preparation of fluorous
phase-soluble dendrimer-encapsulated Pd nanoparticles. The
method is very general and could be applied to virtually any type
of inorganic material that can be sequestered within a dendrimer
interior. Moreover, the entire reaction sequence is carried out using
commercially available materials, and no covalent dendrimer
modification is required. The Pd/dendrimer catalysts are quite
active for alkene hydrogenation. We also showed that the
selectivity pattern exhibited by these new catalysts is enhanced
by the local dielectric environment within the dendrimer.15 Perhaps
most importantly, however, the methodology is very simple to
implement and the catalysts are easy to separate from the product
and recycle. Taken together, these factors suggest that these
materials are promising candidates for selective, environmentally
friendly catalysis applications.
TOF [mol H2 (mol Pd)-1 h-1
]
in biphasic with polymer-bound
substrate
mixture,a
400
Pd(0) particlese
565
50
10
820b
3
122
165
c
20
40d
884
Acknowledgment. We gratefully acknowledge the Office of Naval
Research for support of this work. We also thank Mr. Mark Kaiser
(Dendritech, Inc., Midland, MI) for providing technical information and
some of the Starburst PAMAM dendrimers used in this study. We
acknowledge Prof. David Bergbreiter (Texas A&M University) for use
of the hydrogenation apparatus and Dr. Lee Yeung for technical assistance
and valuable discussions.
a The substrate (10 mmol) was dissolved in THF (10 mL) and mixed
with FC-75 (5 mL) containing 1.5 g of acid-terminated perfluoropoly-
ether and the catalyst (10 µmol of Pd). The mixture was connected to
a hydrogen buret and stirred vigorously. Hydrogenation was performed
at room temperature and atmospheric pressure. Results are the average
of at least two runs with independently prepared batches of catalyst;
results obtained with different batches of catalysts were reproducible
to within ∼30%. b Butyl acrylate. c 1-Hexene was used as the starting
material. After 15 h the reaction mixture contained n-hexane and
1-hexene:2-hexenes:3-hexenes in a 1:10:1 ratio. d Product is a mixture
of cyclooctene (ca. 99%) and cyclooctane. e From ref 14.
JA9936870
(13) Hirai, H.; Chawanya, H.; Toshima, N. Makromol. Chem., Rapid
Commun. 1981, 2, 99.
(14) Selvaraj, P. C.; Mahadevan, V. J. Polym. Sci. A 1997, 35, 105.
(15) Toshima, N. Supramol. Sci. 1998, 5, 395.
These new catalysts can be used for hydrogenation of a range
of unsaturated compounds, and also for isomerization of terminal
alkenes (Table 1). The reaction selectivity resembles that of