Interfacially cross-linked reverse micelles as soluble support for palladium nanoparticle catalysts

Article abstract of DOI:10.1002/hlca.201100451

Reverse micelles (RM) were formed in heptane/CHCl₃ with a surfactant carrying the triallylammonium (=triprop-2-en-1-ylammonium) head group (Scheme). Photo-cross-linking with dithiothreitol (=rel-(2R,3R)-1,4- dimercaptobutane-2,3-diol; DTT) captured the RMs and afforded organic, soluble nanoparticles in a one-step reaction. Similar to dendrimers, the cross-linked reverse micelles could encapsulate palladium nanoparticles within their hydrophilic cores and protect them in catalytic reactions. Good to excellent yields were obtained in the Heck coupling of a range of alkyl acrylates (=alkyl prop-2-enoates) and iodobenzenes (Tables 1 and 2). The catalytic activity of the palladium nanoparticles was maintained in several repeated runs.

Full text of DOI:10.1002/hlca.201100451

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Interfacially Cross-Linked Reverse Micelles as Soluble Support for Palladium
Nanoparticle Catalysts
by Li-Chen Lee and Yan Zhao*
Department of Chemistry, Iowa State University, Ames, IA 50011-3111, USA
(phone: þ 1-515-2945845; fax: þ 1-515-2940105; e-mail: zhaoy@iastate.edu)
Reverse micelles (RM) were formed in heptane/CHCl3 with a surfactant carrying the triallylam-
monium (¼triprop-2-en-1-ylammonium) head group (Scheme). Photo-cross-linking with dithiothreitol
(¼ rel-(2R,3R)-1,4-dimercaptobutane-2,3-diol; DTT) captured the RMs and afforded organic, soluble
nanoparticles in a one-step reaction. Similar to dendrimers, the cross-linked reverse micelles could
encapsulate palladium nanoparticles within their hydrophilic cores and protect them in catalytic
reactions. Good to excellent yields were obtained in the Heck coupling of a range of alkyl acrylates
(¼alkyl prop-2-enoates) and iodobenzenes (Tables 1 and 2). The catalytic activity of the palladium
nanoparticles was maintained in several repeated runs.
Introduction. – By staying in the same phase and being readily accessible to
reactants, homogeneous catalysts may achieve very high activity. Their mechanistic
understanding and optimization in activity/selectivity are facilitated by the systematic
tuning of the ligand environments and active sites. Heterogeneous catalysts, on the
other hand, often distinguish themselves by the ease of product separation, long-term
stability, and recyclability. Chemists have long sought to bridge the gap between the two
catalyst types and create either heterogenized homogeneous catalysts or soluble
versions of heterogeneous catalysts [1]. Metal nanoparticles find many applications in
both homogeneous and heterogeneous catalysis because their large surface area
translates to a high percentage of surface atoms useful for catalysis [1 – 4]. When
deposited on solid supports, however, metal nanoparticles tend to vary greatly in size,
shape, and aggregational state, making it challenging to control the active sites and
selectivity in the reactions [5]. For these and other reasons, there have been intensive
efforts to identify suitable organic supports to stabilize metal nanoparticles, in the hope
that the organic supports may not only create more uniform metal nanoparticles but
also provide additional features not available to their inorganic counterparts [1 – 4].
Organic dendrimers [6], for example, have been shown to offer a range of useful
features including size selectivity [7], tailored solubility [8][9], and improved surface
deposition [5].
We recently reported a simple method to capture reverse micelles covalently [10].
Reverse micelles (RMs) are widely used as templates to create inorganic nanoparticles,
but the dimension of the final materials templated by RMs rarely correlates directly
with the size of the templates [11][12]. The discrepancy derives from fast collision of
RMs, accompanied by coalescence and rapid exchange of the internal contents
[13][14]. Although covalent fixation of RMs should in principle solve these problems,
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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the same problems in the RM-templated synthesis (i.e., fast collision and coalescence)
make it difficult to capture the dynamic noncovalent assemblies in the original states
[15][16]. Our covalent capture of RM relies on the three allyl (¼ prop-2-en-1-yl)
groups in the headgroup of 4-(dodecyloxy)-N,N,N-triprop-2-en-1-ylbenzenemethan-
aminium bromide (1), which create a high density of alkene functionality at the
surfactantꢀH2O interface (Scheme). When a H2O-soluble cross-linker, dithiothreitol
(rel-(2R,3R)-1,4-dimercaptobutane-2,3-diol; DTT), is used, the efficient thiolꢀene
radical addition [17 – 19] is facilitated by the proximity and high local concentration of
the thiol and the alkene moieties. The high cross-linking density near the head group is
also important to the successful capture of the RMs in the original size [20][21].
Scheme. Preparation of Interfacially Cross-Linked Reverse Micelles (ICRMs) by the ThiolꢀEne
Addition
In this work, we report our first step in using interfacially cross-linked RMs
(ICRMs) as soluble support for Pd nanoparticles and demonstrate their applications in
the Heck coupling. The Heck reaction belongs to an extremely important class of C,C
bond-formation reactions [22][23]. Although phosphine ligands are well-known to
stabilize the catalytically active Pd0 species for Heck reactions, they could be quite
expensive. Similar to most homogeneous catalysts, recovery and reuse of catalysts are
often impossible [24]. In addition to various inorganic supports [24][25], organic
polymers [26][27], dendrimers [28 – 31], lyotropic liquid crystal polymers [32], block
copolymer micelles [33][34], and polyhedral oligomeric silsesquioxane [35] have been
reported to afford catalytically active Pd nanoparticles for the Heck coupling. With Pd-
ICRM composites, we were able to perform efficient Heck reactions between
iodoarenes and various acrylates (¼ prop-2-enoates). No palladium black was
observed even after several reaction cycles. An important benefit of the ICRMs is
their good miscibility with common organic reactants, allowing us to perform the Heck
reaction without any solvents.
Results and Discussion. – ICRMs are unique templates for metal nanoparticles. Our
previous work shows that single or bimetallic nanoparticles could be prepared simply
2ꢀ
by extraction of anionic metal salts such as AuCl4ꢀ and PtCl6 followed by reduction
[10]. Because the interior of the ICRMs mainly contains weak ligands such as bromide,
thioether, and OH groups, we reasoned that they should serve as useful support for
catalytic nanoparticles. Our hypothesis was that the metal particle inside an ICRM
would be stabilized mainly by physical protection and thus remains catalytically active.
The rationale for the work was that the ICRMs resemble dendrimers in the multivalent

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surface functionality and a well-defined coreꢀshell structure but are prepared in a one-
step synthesis. If they can duplicate the key benefits of dendrimer templates [6], the
ease of synthesis would make them attractive for large-scale applications.
The details of ICRM synthesis were reported previously [10]. Briefly, optically clear
RM solutions were prepared in heptane/CHCl3 2 :1 with [H2O]/[1] ¼ 5. CHCl3 was
included mainly to help dissolve DTT, the cross-linker, in the nonpolar mixture. Upon
photolysis in the presence of 0.5 mol-% of the photoinitiator 2,2’-dimethoxy-2-phenyl-
acetophenone (¼2-methoxy-1-(2-methoxyphenyl)-1-phenylethanone), an efficient
thiolꢀene radical reaction [17 – 19] took place. As reported previously, the ICRMs
were characterized by a range of techniques including 1H-NMR spectroscopy, dynamic
light scattering (DLS), transmission electron microscopy (TEM), and IR spectroscopy
[10]. The ICRMs can interact with one another through van der Waals interactions of
the alkyl chains on the surface. Their cores consist of charged ammonium head groups,
and aggregation thus can benefit as well from electrostatic interactions, which could
operate over a long distance. A double-tailed surfactant afforded ICRMs free of
aggregation. We chose to use the single-tailed surfacant because the lower density of
alkyl coronas was possibly beneficial to mass transfer. Although the ICRMs of the
single-tailed surfactant 1 aggregated in a solvent-dependent fashion, the aggregates
(13 nm in diameter in THF and ca. 200 nm in CHCl3) were completely soluble in
solvents with intermediate polarity but insoluble in highly polar (e.g., MeOH or H2O)
or highly nonpolar solvents such as hexane.
Because an ICRM contains numerous ammonium head groups in the interior, it is
most suited to extract anionic metal precursors. When a solution of H2PdCl4 (10 mm) in
H2O (2 ml) was stirred with a solution of ICRM ([1] ¼ 10 mm) in CHCl3 (2 ml), the
orange phase turned colorless, and the organic layer became orange, indicating the
successful extraction of the metal salts into the ICRMs.
Many reducing agents including sodium borohydride [28][31][34], superhydride
[33], and EtOH [26] were reported to convert PdII to Pd0. When the above described
Pd-containing ICRM solution was heated to reflux in the presence of EtOH (2 ml) for
3 h, a brownish orange solution was obtained. No palladium black or precipitate was
formed during the process, indicating that the reduced palladium was protected by the
ICRMs. After evaporation of the solvents, the materials obtained could be redissolved
in common organic solvents such as CHCl3, CH2Cl2, and THF. The solution was stable
over at least a period of several months.
In our catalytic study (vide infra), we attempted Heck reactions with the Pd-ICRMs
without EtOH treatment because Et3N, the base used in our Heck coupling, is known to
reduce PdII during the reaction [24]. Since the materials with and without EtOH
reduction essentially afforded the same yields (vide infra), we adopted the simpler
preparative procedure without the reduction step.
The UV/VIS spectra of the ICRMs and the Pd-ICRMs with and without EtOH
treatment are shown in Fig. 1. The ICRMs had a major peak at 280 nm from the
aromatic groups of the ICRMs (see structure of 1). The major absorption of aqueous
H2PdCl4 solution had a weak broad absorption at 420 nm. After loading with the
palladium salt, the Pd-ICRMs had a very broad absorption band that started from
300 nm and tailed off into the VIS region. Overall, the materials obtained with and
without EtOH reduction looked very similar. Possibly, some or all of the palladium salts

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Fig. 1. UV/VIS Spectra of CHCl3 solutions of ICRM, Pd-ICRM, and Pd-ICRM after EtOH treatment.
[1] ¼ 0.1 mm; [H2PdCl4] ¼ 0.1 mm. The spectra were adjusted in intensity for easier comparison.
were already reduced after extraction into the ICRMs. According to our previous study,
AuCl4ꢀ extracted into the cores of the ICRMs could reduce spontaneously [10].
We also characterized the Pd-ICRM without EtOH treatment by TEM. Fig. 2
shows the representative micrograph of an unstained Pd-ICRM sample. The Pd
nanoparticles showed up as dark spots and averaged ca. 2 nm in diameter, in line with
the core size of the ICRMs. The ICRMs also seemed to cluster together, possibly due to
the aggregation of the alkyl shell.
Fig. 2. Micrograph of Pd-ICRM without EtOH treatment, obtained by TEM
Initially, we were not sure whether the Pd-ICRMs would be catalytically active and
stable during reactions. Soluble palladium clusters are known to form from Pd-
nanoparticles and are responsible for catalyzing cross-coupling reactions [36][37].
Without ligands, Pd0 is quite unstable and tends to quickly form palladium black and
lose activity [22 – 24]. Therefore, after Pd-atoms depart from the ICRM cores and move

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into the solution, even if these atoms are initially active, they would aggregate easily
and precipitate. To our delight, when a 1:2 mixture of iodobenzene and butyl acrylate
(¼ butyl prop-2-enoate) were heated with 1 mol-% of Pd-ICRMs and 1.2 equiv. of Et3N
at 808, reaction ran cleanly (Fig. 3). After 18 h, the limiting reagent, iodobenzene,
disappeared completely, and cinnamate was obtained as the only product. No solvent
was needed in the reaction, suggesting that the Pd-ICRMs were completely miscible
with the reactants, and the Pd metal inside the cores was readily accessible to the
reactants. Importantly, the reaction mixture stayed completely clear at the end of the
reaction, showing no formation of palladium black.
Fig. 3. 1H-NMR Spectra of a) the reaction mixture and b) a 1:2 mixture of iodobenzene and butyl acrylate
Table 1 shows the yields of the Heck reaction between various alkyl acrylates and
iodobenzene catalyzed by the Pd-ICRMs. As mentioned earlier, EtOH reduction made
no difference in the catalysis (Entries 1 and 2), possibly because either PdII was already
reduced during the Pd-ICRM preparation or reduced by the Et3N added [24]. t-Butyl
acrylate and the hydrophilic 2-hydroxyethyl acrylate both gave good yields, suggesting
that both hydrophobic and hydrophilic substrates are able to penetrate the core of the
cross-linked reverse micelles. The 2-ethylhexyl acrylate was less reactive, giving a
modest yield of 68% (Entry 5). Initially, we thought the lower yield was caused by the
alkyl corona of the ICRM having ꢂfilteringꢃ effects, similar to those found in high-
generation dendrimers [7]. To probe the reason for lower yield, we allowed a 1:1
mixture of butyl acrylate and 2-ethylhexyl acrylate to react with 1 equivalent of
iodobenzene and found that both acrylates were converted by similar rates. The lower
conversion in 2-ethylhexyl acrylate, therefore, could not come from the filtering effect
mentioned above. We also performed some control experiments. Without protection by

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Table 1. Pd-ICRM-Catalyzed Heck Reactions Between Alkyl Acrylate and Iodobenzene
Entry
R
Yield [%]a)
1
2
3
4
5
6
7
Bu
Bu
> 95
> 95b)
94
93
68
> 95c)
0d)
t-Bu
2-hydroxyethyl
2-ethylhexyl
Bu
Bu
a) Yields of the products were determined by 1H-NMR analysis. A yield of > 95% means that the
reaction went to completion with no detectable amount of iodobenzene by 1H-NMR spectroscopy.
Typical reaction conditions: alkyl acrylate (1.0 mmol), iodobenzene (0.5 mmol), Et3N (0.6 mmol), and
Pd-ICRM (1 mol-% of Pd with respect to iodobenzene); at 808 for 18 h. In Entry 4, 1 equiv. of acrylate
was used. b) Pd-ICRM with EtOH reduction was used. c) H2PdCl4 was used. Although the reaction
proceeded well, a large amount of palladium black formed during the coupling. d) ICRMs in the absence
of Pd were used.
the ICRMs, H2PdCl4 was insoluble in the reaction mixture and gave palladium black
immediately. Although complete conversion of the butyl acrylate was also observed,
the reaction, after 18 h, produced a large amount of black precipitate (Entry 6). In the
absence of Pd, the ICRMs, as expected, were unable to promote the reaction (Entry 7).
The catalysis could happen in two possible ways. First, Pd0-atoms departed from the
stock of palladium in the ICRM cores and moved into the solution to catalyze the
reaction. Second, the reactions mostly took place on the surface of the Pd nano-
particles, and catalytically active species never moved too far away from the ICRM
cores. Our data could not rule out either mechanism. Because H2PdCl4 without the
ICRMs also catalyzed the Heck coupling (Table 1, Entry 6), the ICRMs were not
necessary for the reaction to occur. On the other hand, since H2PdCl4 without the
ICRMs produced a large amount of palladium black, the ICRMs were clearly helpful
by stabilizing Pd0. Most likely, even if Pd0 migrated from the ICRM cores and went into
the solution to catalyze the reaction, the amount of Pd0 departed must be so small that
visible palladium, i.e., large aggregates of Pd0 could not form. Alternatively, because
the high tendency for Pd0 to agglomerate, the Pd0 atoms or small clusters that went into
the solution may return and merge with the Pd nanoparticles within the ICRM cores. It
should be noted that palladium black was never observed in Pd-ICRM-catalyzed
reactions even in repeated runs (vide infra).
The results so far are quite promising. Despite the simpler synthesis and structure,
the Pd-ICRMs gave comparable results as dendrimer-protected Pd nanoparticles in
Heck coupling [28 – 31]. The good miscibility of the ICRMs with organic reactants is
noteworthy, as solventless conditions are attractive for both economical and environ-
mental points of view. To study the scope of the reaction, we examined the coupling

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between butyl acrylate and a range of substituted iodobenzenes, with electron-donating
and -withdrawing groups at different positions. Most reactions proceeded well and gave
good to excellent yields. On average, electron-deficient iodobenzenes gave somewhat
higher yields than the electron-rich ones (Table 2).
Table 2. Pd-ICRM-Catalyzed Heck Reactions between Butyl Acrylate and Substituted Iodobenzenes
R’
Yield [%]a)
H
2-Me
> 95
82
2-MeO
2-Br
78
95
3-Br
> 95
> 95
88
3-NO2
2-NO2 and 5-MeO
4-Br
89
4-NO2
4-CN
78
82
4-MeO
84
a) Yields of the products were determined by 1H-NMR analysis. A yield of > 95% means that the
reaction went to completion with no detectable amount of iodobenzene by 1H-NMR spectroscopy.
Typical reaction conditions: alkyl acrylate (1.0 mmol), iodobenzene (0.5 mmol), Et3N (0.6 mmol), and
Pd-ICRM (1 mol-% of Pd with respect to iodobenzenes; at 808 for 18 h.
To test the reusability of the Pd-ICRMs, we took a mixture of butyl acrylate,
iodobenzene, Et3N, and 1 mol-% Pd-ICRMs and heated it to 808. After 18 h, another
batch of reactants without the extra equivalent of butyl acrylate was added. 1H-NMR
spectroscopy indicated complete conversion of the iodobenzene. The procedure was
repeated and, after the third cycle, the yield still remained quite good (85%). It is worth
mentioning that the reaction mixture stayed clear throughout the three cycles and
showed no formation of palladium black at the end of the third reaction cycle (Fig. 4).
The lower yield in the third cycle could result from surface-poisoning of the
nanoparticles. Similar results were reported in dendrimer-encapsulated Pd nano-
particles [28]. We also attempted to use the Pd-ICRMs for the Heck reactions of
bromo- and chloroarenes but found them to be much less reactive. The results most
likely derived from limitation of the Pd nanoparticles because similar observations
were made in dendrimer-encapsulated Pd nanoparticles [28 – 31].
Conclusions. – In summary, ICRMs are an excellent soluble support for catalytically
active Pd nanoparticles. Good to excellent yields were obtained for the Heck coupling
of a wide range of alkyl acrylates and iodobenzenes. The catalytic cores were readily
accessible to most reagents. Compared to dendrimer templates, the ICRMs are much
easier to prepare, via a one-step photo-cross-linking reaction. Extraction of metal salts

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Fig. 4. Photograph of the reaction mixture of butyl acrylate and iodobenzene after the third catalytic cycle
is straightforward. Although the current systems only extract anionic salts, similar
cross-linking concepts can be applied to anionic and nonionic surfactants, as long as
they carry multiple C¼C bonds near the head group. The ICRMs are insoluble in polar
organic solvents (MeOH or EtOH) and, if needed, can be easily recovered by
precipitation. We demonstrated some success in reusing the catalysts for multiple runs.
Given the simplicity of the synthesis and easy modification of the surfactants, ICRMs
with a wide range of properties should be readily available, making these materials
potentially useful in many catalytic applications.
Experimental Part
General. All reagents and solvents were of A.C.S. certified grade or higher, and were used as
received from commercial suppliers. The synthesis and characterization of ICRMs were reported
previously [10]. UV/VIS Spectra: Cary-50-Bio UV/VIS spectrophotometer; at r.t.. TEM: Philips-CM-30
instrument, operating at 200 kV. 1H-NMR Spectra: Varian-VXR-400 and Bruker DRX-400 spectrometer;
d in ppm rel. to Me4Si as internal standard, J in Hz.
Typical Preparation of Pd-ICRMs. An aq. soln. of H2PdCl4 (10 mm) was prepared by adding 0.4m
HCl (6 ml) to PdCl2 (1.2 mmol) in H2O (114 ml). A portion (2 ml) of this H2PdCl4 soln. was added to an
ICRM soln. in CHCl3 (2 ml, [1] ¼ 10 mm). After stirring overnight, the mixture was allowed to sit at r.t.
The orange org. phase was separated, washed with H2O (3ꢁ), and concentrated to give an orange
powder. For the UV/VIS measurement, 0.1 ml of the CHCl3 soln. was diluted by 9.9 ml of CHCl3 so that
the concentration of the cross-linked [1] was 0.1 mm in the final sample.
Typical Preparation of Pd-ICRMs with EtOH as the Reducing Agents. The Pd-ICRMs were prepared
as above. The CHCl3 layer was separated and combined with 2 ml of EtOH. The resulting mixture was
heated to reflux under N2 for 3 h. After the soln. was cooled to r.t., the org. layer was washed with H2O
and concentrated to give a brownish orange powder. For the UV/VIS measurement, 2 ml of CHCl3 was
added back to the above-described brownish orange powder. A portion (0.1 ml) of the CHCl3 soln. was
diluted by 9.9 ml of CHCl3 so that the concentration of cross-linked 1 was 0.1 mm in the final sample.
TEM Imaging. A soln. of Pd-ICRM in THF ([1] ¼ 1.0 · 10ꢀ4 m) was gently placed on a C-atom-coated
copper grid (C-atom type A, 300 mesh). The TEM grid was dried overnight at r.t., and subjected to TEM
observation.

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Typical Procedure for the Heck Reaction. A small J. Young tube was charged with butyl acrylate
(143.4 ml, 1 mmol), iodobenzene (56 ml, 0.5 mmol), Et3N (83.6 ml, 0.6 mmol), and the Pd-ICRM catalyst
([Pd] ¼ [1] ¼ 0.005 mmol). The mixture was stirred at 808 for 18 h with the tube closed. After the mixture
was cooled to r.t., CDCl3 was added, and yield of the reaction was determined by 1H-NMR spectroscopy.
We thank the U.S. Department of Energy, Office of Basic Energy Sciences (grant DE-SC0002142),
for supporting the research.
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Received November 16, 2011