Highly efficient and recyclable Au nanoparticle-supported palladium(II) interphase catalysts and microwave-assisted alkyne cyclotrimerization reactions in ionic liquids

Article abstract of DOI:10.1021/jo800524h

(Graph Presented) The gold nanoparticles with core diameter of 3.9-4.7 nm were stabilized with octanethiolate and dipyridylphosphinicamido undecanethiolate. Without varying the size of central Au cores, palladium complexes were immobilized onto these Au nanoparticles through chelation to the surface-bound dipyridyls. Hybrid catalysts of this type were dissolvable and precipitable, and their structures and reactions were investigated by solution nuclear magnetic resonance (NMR) spectroscopy with a resolution typically attained for soluble systems. These surface-bound Pd(II) complexes were highly effective catalysts for [2+2+2] alkyne cyclotrimerization reactions to give highly congested benzene rings with fairly good selectivity. The catalytic reactivity of these interphase catalysts was even higher than that of their unbound counterparts. In addition, they can be easily separated and quantitatively recovered by simple filtration. The recovered catalysts can be effectively recycled many times and their electron microscopy images and NMR spectra showed negligible difference from those of freshly prepared. The complete transformation by Au-bound Pd(II) catalyst with a loading of 4 mol % can be achieved within 1 h for most alkynes. The same catalysis can be further accelerated in ionic liquid under microwave conditions to give nearly 100% of cyclotrimerized products in minutes.

Full text of DOI:10.1021/jo800524h

Highly Efficient and Recyclable Au Nanoparticle-Supported
Palladium(II) Interphase Catalysts and Microwave-Assisted Alkyne
Cyclotrimerization Reactions in Ionic Liquids
Yu-Yun Lin, Shih-Chung Tsai, and Shuchun Joyce Yu*
Department of Chemistry and Biochemistry, National Chung Cheng UniVersity, Min-Hsiung,
Chia-Yi, Taiwan 621
chejyy@ccu.edu.tw
ReceiVed March 7, 2008
The gold nanoparticles with core diameter of 3.9-4.7 nm were stabilized with octanethiolate and
dipyridylphosphinicamido undecanethiolate. Without varying the size of central Au cores, palladium
complexes were immobilized onto these Au nanoparticles through chelation to the surface-bound dipyridyls.
Hybrid catalysts of this type were dissolvable and precipitable, and their structures and reactions were
investigated by solution nuclear magnetic resonance (NMR) spectroscopy with a resolution typically
attained for soluble systems. These surface-bound Pd(II) complexes were highly effective catalysts for
[2+2+2] alkyne cyclotrimerization reactions to give highly congested benzene rings with fairly good
selectivity. The catalytic reactivity of these interphase catalysts was even higher than that of their unbound
counterparts. In addition, they can be easily separated and quantitatively recovered by simple filtration.
The recovered catalysts can be effectively recycled many times and their electron microscopy images
and NMR spectra showed negligible difference from those of freshly prepared. The complete transformation
by Au-bound Pd(II) catalyst with a loading of 4 mol % can be achieved within 1 h for most alkynes. The
same catalysis can be further accelerated in ionic liquid under microwave conditions to give nearly 100%
of cyclotrimerized products in minutes.
Introduction
of hybrid catalyst should be in principle congregating the
advantages from both heterogeneous and homogeneous systems.
However, in many cases, supported catalysts suffer a series of
problems due to heterogenization. This has led to alternative
liquid phase methodologies to restore homogeneous reaction
conditions. Hence soluble supports1,4,5 have been used to obtain
detailed structures of the active catalysts, and much more
fundamental knowledge about the catalyst modification, deg-
radation, as well as interactions at the interface would then
become comprehensible. Since the alkanethiolate-protected gold
The strategy of immobilizing homogeneous transition metal
complexes to solid supports such as organic polymers,1 inorganic
metal,2 and metal oxides3 has been extensively applied in
sustainable chemistry to prepare recoverable catalysts. This type
(1) Recent reviews on polymer-supported catalysts: (a) Leadbeater, N. E.;
Marco, M. Chem. ReV. 2002, 102, 3217–3274. (b) McNamara, C. A.; Dixon,
M. J.; Bradley, M. Chem. ReV. 2002, 102, 3275–3299. (c) Dickerson, T. J.; Reed,
N. N.; Janda, K. D. Chem. ReV. 2002, 102, 3325–3344. (d) Cole-Hamilton, D. J.
Science 2003, 299, 1702–1706.
4920 J. Org. Chem. 2008, 73, 4920–4928
10.1021/jo800524h CCC: $40.75 2008 American Chemical Society
Published on Web 06/04/2008

Au Nanoparticle-Supported Pd(II) Interphase Catalysts
SCHEME 1. Synthesis of Spacer Ligand
HS(CH2)11NHP(O)(2-py)2 (4)
nanoparticles (Au NPs) have been known not only to possess
solid surfaces resembling the (111) surface of bulk gold6 but
also to behave like soluble molecules for their dissolvability,
precipitability, and redissolvability.7 Au NPs would suitably
become good candidates for metal complex catalyst-supports.
Some recent articles have reported the use of Au colloids as
catalyst-supports in various organic transformations.8–11 Herein
we report the synthesis of a recyclable hybrid catalyst system
consisting of Pd(II) complexes immobilized onto Au NPs
through coordination to the surface alkanethiolates. Hybrid
catalysts of this type are dissolvable and precipitable, so their
structures and reaction chemistry can be easily investigated by
solution phase nuclear magnetic resonance (NMR) spectroscopy
with a resolution typically obtained on soluble systems. We have
also demonstrated the excellent reactivity and sustainable
recyclability of this hybrid catalyst system in a series of
[2+2+2] alkyne cyclotrimerization reactions. By using trans-
mission electron microscopy (TEM) and solution NMR it will
be shown that the structures of the catalytic system remain
unchanged even after 5 cycles of catalysis. In addition, the rates
of these catalytic cyclotrimerization reactions can be further
accelerated in ionic liquid under microwave (MW) irradiation
conditions.
it is interesting to note that the Au NPs-bound Pd(II) complexes
in the present study are more effective than their SiO2-bound
analogues and their unbound counterparts.
Results and Discussion
Immobilization of complexes with use of covalent tethering
techniques is, at present, the most favorable approach to design
stable hybrid catalysts.4a The compound dipyridylphosphinic
amido undecanethiol, HS(CH2)11NHP(O)(2-py)2 (4) (Scheme
1), was synthesized to be used as the tethering linker since the
thiol end and the amido phosphinic dipyridyl end have been
demonstrated to readily bind surface Au and molecular Pd(II),
respectively.5a The Au NPs surfaces were functionalized with
use of the place-exchange method12,13 by treating 75 mg of
octanethiolate-covered Au NPs (Au-SR, 2.7 ( 0.5 nm)6a,13b,14
with various quantities of 4 (60-200 mg) in CHCl3 at 70 °C
for 16 h (Scheme 2). The resulting mixed thiolates-covered Au
NPs RS-Au-L (7) have diameters of 3.9-4.7 nm and L:RS mole
ratios ranging from 1:0.7 to 1:1.2, where RS ) octanethiolate
and L ) S(CH2)11NHP(O)(2-py)2 (see the Supporting Informa-
tion). Further treatment of 7 with Pd(CH3CN)2Cl2 would make
the molecular Pd(II) tethered onto Au NPs via direct binding
with surface dipyridyl to form a stable palladacycle. The Pd(II)-
functionalized Au NPs, RS-Au-L-PdCl2 (8), thus obtained has
average core sizes of 3.1-4.8 nm and L-PdCl2/RS ratios of
1/0.61 to 1/1.39 (see the Supporting Information).
Most surface-bound transition metal complex catalysts are
generally less reactive than their unbound analogues.3 However,
(2) (a) Chen, M.; Kumar, D.; Yi, C.-W.; Goodman, D. W. Science 2005,
310, 291–293. (b) Zhou, X.-S.; Dong, Z.-R.; Zhang, H.-M.; Yan, J.-W.; Gao,
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Ramesh, K.; Chen, F.; Chen, L.; White, T.; Tay, Q.; Yaakub, S. N.; Wang, Z.
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Mayer, H. A. Chem. ReV. 2002, 102, 3543–3578. (e) McMorn, P.; Hutchins,
G. J. Chem. Soc. ReV. 2004, 33, 108–122.
(4) For some examples of organic polymers or low molecular weight ionic
liquids as soluble supports for catalysts see: (a) Miao, W.; Chan, T. H. Acc.
Chem. Res. 2006, 39, 897–908. (b) Sommer, K.; Yu, W.; Richardson, J. M.;
Weck, M.; Jones, C. W. AdV. Synth. Catal. 2005, 347, 161–171. (c) Bergbreiter,
D. E.; Li, J. Top. Curr. Chem. 2004, 242, 113–176. (d) Bergbreiter, D. E. Chem.
ReV. 2002, 102, 3345–3384.
The TEM images of Au NPs 7 and 8 have shown that
sequential immobilization processes would not cause significant
size change on the central Au cores (see Supporting Informa-
tion). The UV-vis spectra of RS-Au-L (7) and RS-Au-L-PdCl2
(8) exhibit the characteristic surface plasmon resonance peak
at ∼520 nm (see Supporting Information).
(5) For some examples of soluble inorganic polymers like polysiloxanes or
polyphosphazenes as supports for catalysts see: (a) Fu, Y.-S.; Yu, S. J. Angew.
Chem., Int. Ed. 2001, 40, 437–440. (b) Grunlan, M. A.; Regan, K. R.; Bergbreiter,
D. E. Chem. Commun. 2006, 1715–1717. (c) DeClue, M. S.; Siegel, J. S. Org.
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Since the Pd(II)-functionalized Au NPs typically obtained
under our reaction conditions are soluble in many organic
solvents such as methanol, DMSO, and DMF, we could
therefore use simple solution 1H NMR other than the traditional
atomic absorption (AA) spectroscopy to determine the average
thiolate coverage and the amount of Pd(II) loading. It can be
clearly seen in the 1H NMR spectra (Figures 1 and 2) of thiolate-
protected Au NPs, Au-SR, RS-Au-L (7) and RS-Au-L-PdCl2
NPs (8), that the R-methylene proton (-CH2S-Au) resonances
of the surface thiolates were broadened to an extent to become
barely visible. All other proton resonances were also signifi-
cantly broadened and their multiplicity could no longer be
determined. The cause for peak broadening is still a subject
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Lin et al.
SCHEME 2. Synthesis of Pd(II)-Functionalized Au NPs, RS-Au-L-PdCl2 (8)
under discussion.15,16 The Au NPs surface-bound species and
their free forms can easily be distinguished by the sharpness of
the multiplets and their chemical shifts (referenced to chloroform
or DMSO). In addition, the IR spectra of the Au NPs-bound
thiolates also showed that the νasym(SH) bands of free octanethiol
(2569 cm-1) and the unbound 4 (2576 cm-1) disappeared17 upon
immobilization onto Au NPs (see Supporting Information).
Given that RS-Au-L-PdCl2 NPs 8 are soluble in polar solvents
such as DMSO, purification of these Au NPs for the use in
quantitative analyses can be easily performed by washing the
crude with less polar solvents such as CHCl3 followed by
centrifugation and filtration. In addition to TEM and AA,
solution NMR spectroscopy was heavily used in the current
study for quantitative analyses. The analytical data of Au NPs
7 and 8 were summarized in the Supporting Information. The
mole contents of surface thiolates in 1 g of RS-Au-L (7) were
determined to be 4.8 × 10-4-6.1 × 10-4 mol/g of RS and 5.0
× 10-4-7.2 × 10-4 mol/g of spacer thiolate L, whereas the
surface contents of Pd(II) and RS in 1 g of RS-Au-L-PdCl2
NPs (8) were found to be 2.4 × 10-4-6.0 × 10-4 and 3.3 ×
10-4-4.2 × 10-4 mol/g, respectively. The weight percentage
of immobilized Pd(II) (Pd wt %) in Au NPs 8 can then be
calculated accordingly.
benzenes, only a few synthetically useful catalyst systems have
been established.19,20 Recent progress in this area has revealed
that the Pd(II) metal center seems to be a potential candidate
for its air/moisture -stability and its abounding chemistry with
alkynes.21,22 In the current study, molecular Pd(II) immobilized
at the interphase, in a form of RS-Au-L-PdCl2 NPs (8), was
used as the active catalyst for [2+2+2] cyclotrimerization of
various alkynes (R1Ct CR2, R1 ) R2 ) Me, Et, nPr, Ph; R1 )
n
Me, R2 ) Et, Pr; and R1 ) Ph, R2 ) H,). Among several
solvents tested, all the catalyses were operated heterogeneously
in molecular solvents (CHCl3 or THF) or in ionic liquid
bmimPF6 for the idea of easy separation, catalyst recycling, and
developing greener synthesis protocols. Under very mild reaction
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Bonn˜aga, L. V. R.; Zhang, H.-C.; Maryanoff, B. E. Chem. Commun. 2004, 2394–
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It can be seen in the Supporting Information that the Pd wt
% of 2.5-6.4% obtained from 1H NMR spectroscopy was found
to be systematically smaller (<5%) than those obtained by using
AA spectroscopy (Pd wt % ) 2.8-6.7%). Since it would only
take 10 min or less to acquire data on a 1H NMR spectrometer,
we used NMR instead of AA to obtain analytical data for
catalyses thereafter.
The construction of highly congested benzene rings from
simple alkynes is a synthetically important transformation for
its extensive use in industry as well as in the laboratory.18
Though many transition metals have been employed for the
[2+2+2] cyclotrimerization of alkynes to polysubstituted
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4922 J. Org. Chem. Vol. 73, No. 13, 2008

Au Nanoparticle-Supported Pd(II) Interphase Catalysts
FIGURE 1. The 400 MHz 1H NMR (27 °C, CDCl3) spectra of (a) octanethiol (HSR), (b) octanethiolate-covered Au NPs (Au-SR), (c) phosphinic
amide-functionalized alkanethiol 4 (HL), and (d) RS-Au-L NPs 7 (* is CDCl3 and # is water in the system).
conditions, complete conversions could be found for both
terminal and internal alkynes to give desired products in a short
reaction time without any activator such as Zn/ZnI2 required
for the CoBr2 system23 or CuCl2 for the PdCl2 system.22a,27b
For example, in experiments conducted by using 11.6 mg of
RS-Au-L-PdCl2 NPs 8, which was equivalent to 4 mol % of
Pd(II) loading, 0.075 mmol of 3-hexyne was fully converted
into hexaethylbenzene in 30 min at 27 °C or in 10 min at 62
°C (entry 2, Table 3). As elucidated in Table 1, the 8-catalyzed
reactions of alkynes with smaller substituents such as Me, Et,
of the free end of the long alkyl chain. Also the molecular
catalysts Pd(CH3CN)2Cl2 and Pd(PhCN)2Cl2 bearing a short and
more rigid chain were found to be effective catalysts for the
same catalytic reactions. However, the catalytic efficiency of
Pd(CH3CN)2Cl2 under homogeneous conditions was only
40-75% of that found for 8 under heterogeneous conditions
for cyclotrimerization of 4-octyne, 2-pentyne, 2-hexyne, and
phenylacetylene (entries 3, 5-7, and 14-17, Table 1). In cases
of the most reactive 3-hexyne and the least reactive dipheny-
lacetylene, similar catalytic reactivity was obtained for both the
Au-bound and the free-form Pd(II) systems (entries 2, 4, and
11-13, Table 1).
n
and Pr underwent cycloaddition much more efficiently than
those with a larger phenyl substituent.
To study the cause of catalytic reactivity enhancement in the
Au nanosurface tethered Pd(II) hybrid system, we have also
investigated the catalysis on bulk SiO2 surfaces. It can be clearly
seen in entry 8 of Table 1 that the SiO2-bound Pd(II) catalyst
(Scheme 4)25 only had 6-23% of reactivity of those found for
8 under exactly the same reaction conditions. However, higher
conversions can be obtained when catalysis proceeded for a
much longer reaction time.5a As expected for most bulk-surface
supported metal complex catalysts systems, a decrease in
reactivity upon progressive heterogenization was also observed
for the SiO2-bound Pd(II) system. Additionally, the octanethi-
olate-covered Au NPs (Au-SR) were also tested for catalytic
activity. As expected, the Au cores were proven to have no
reactivity toward cyclotrimerization reactions.
It has been reported that transition metal complexes attached
to self-assembled monolayers of alkanethiolates on gold colloids
exhibited catalytic properties similar to those of their corre-
sponding homogeneous catalysts.8–11 It is interesting to note
that in our system the hybrid catalyst RS-Au-L-PdCl2 NPs (8)
is more reactive than the unbound free forms. For example, the
homogeneous analogues HO(CH2)11N(H)(O)P(2-py)2PdCl2 (9)
and Br(CH2)11N(H)(O)P(2-py)2PdCl2 (10) (Scheme 3) were used
as controls and they were proven to give no reactivity for
cyclotrimerization of alkynes even with a Pd(II) loading of up
to 20 mol % at 80 °C for over 24 h (entries 9 and 10, Table
1).24 The shutoff in catalytic activity of the Pd(II) center in both
9 and 10 was most probably due to the entropically unfavorable
formation of the activated complex caused by the free dangling
of the long alkyl chain. This is consistent with our findings that
the Au-bound Pd(II) is much more active due to immobilization
(25) The molecular Pd(II) complex catalysts can be tethered to a range of
polysiloxanes, which were specially designed to mimic the surface of silica gel,
but with properties that are more readily controlled than those of silica gel. The
molecular weight and degree of cross-linking of these polysiloxane materials
can be systematically varied to provide catalyst systems ranging from soluble,
homogeneous model compounds to heterogeneous three-dimensional networks.
For the synthetic details, structural information, and reaction chemistry of the
soluble form of SiO2-bound Pd(II) complex catalyst, see ref 5a.
(23) Hilt, G.; Vogler, T.; Hess, W.; Galbiati, F. Chem. Commun. 2005, 1474–
1475.
(24) For the case of 3-hexyne and 20 mol % of Br(CH2)11N(H)(O)P(2-
py)2PdCl2 (10), oxidative addition of ligand C-Br was the only observable reaction
in CDCl3 at 62 °C after 24 h.
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Lin et al.
1
FIGURE 2. The 400 MHz H NMR (27 °C, d6-DMSO) spectra of (a) molecular catalyst 9, (b) molecular catalyst 10, (c) RS-Au-L NPs 7, (d)
RS-Au-L-PdCl2 NPs 8, and (d) catalyst 8 recovered from the 5th cycle of the catalytic cyclotrimerization of 3-hexyne (* is DMSO, # is water, and
9 is the cyclotrimerized product hexaethylbenzene).
Similar observation of a significant increase in catalytic
activity has also been reported for ring-opening metathesis
polymerization (ROMP) of norbornene by Ru(III) catalysts
supported on Au colloids or on a flat Au surface9 and
hydrogenation of ketones by Rh(I) catalysts organized in
Langmuir-Blodgett (LB) films.26 A mechanistic model con-
nected with the orientation of the Rh(I) in a manner that favors
its interaction with the substrates and the growing products was
proposed by Milstein26 to explain a much higher reaction rate
obtained for the Rh(I) catalyst fixed on LB films. The same
model has also been used by Tremel et al. in their system to
explain a significant increase in catalytic activity of ROMP by
the Ru(III) catalyst supported on Au colloids or on flat Au
surface.9 However, the real cause of reactivity enhancement in
our system is not yet clear. More mechanistic insights may be
obtained from further investigation on the catalytic activity
versus the length of tethering ligands.
A number of plausible mechanistic pathways for the metal-
catalyzed alkyne cyclotrimerization reactions have been pro-
posed (Scheme 5).19b,22a,b,f,g,i,27b The 8-catalyzed cyclotrimer-
ization of unsymmetrically substituted alkyne such as MeCt CEt
and HCt CPh was found to give two kinds of hexasubstituted
benzenes, 1,3,5- and 1,2,4-C6R1 R2 , in a ratio of about 1:2 to
3
3
1:4 (entries 5 and 7, Table 1). These product distributions were
in agreement with the selectivity obtained for the reported
mechanisminvolvingametallacyclopentadieneintermediate.22f,28
However, when MeCt CnPr was used as the alkyne source,
three different products were obtained in a 1,3,5-/1,2,4-/1,2,3-
C6R1 R2 ratio of about 1:3:1 (entry 6, Table 1), which was
3
3
also consistent with another different mechanism having a
cyclobutadienyl complex as reactive intermediate.22e,i,27 It
has been demonstrated in the literature that the product
(26) To¨llner, K.; Popovitz-Biro, R.; Lahav, M.; Milstein, D. Science 1997,
278, 2100–2102.
4924 J. Org. Chem. Vol. 73, No. 13, 2008

Au Nanoparticle-Supported Pd(II) Interphase Catalysts
TABLE 1. RS-Au-PdCl2 NPs (8)-Catalyzed [2+2+2] Alkyne
liquids have proven to be better and greener solvents to couple
effectively with MW in either a continuous wave mode or in a
pulse mode.30 In the current study, we have performed the MW-
assisted 8-catalyzed alkyne cyclotrimerization reactions in
bmimPF6 ionic liquid. As shown in Table 2, the MW heating
method provided significant rate acceleration as compared to
the conventional thermal heating method. Most of the MW-
assisted, 8-catalyzed reactions were completed within 1-5 min
(entries 1-6, Table 2), whereas the same catalyses would take
10 min to 5.5 h under thermal conditions. Even with the less
reactive diphenylacetylene the reaction under MW for 10 min
gave 32% yield, while the same mixture at 62 °C for 10 min
provided 0% conversion but would give 60% yield after 24 h.
In addition to ionic bmimPF6, molecular THF was also used as
the reaction media for the same microwaved catalyses. It can
be clearly seen in Table 2 that the catalyses proceeded much
more efficiently in bmimPF6 than in THF (entries 12-15). The
Pd(CH3CN)2Cl2-catalyzed cyclotrimerizations can also be sub-
stantially speeded up under MW irradiation (entries 7-11, Table
2); however, the observed reactivity was only 40-75% of those
found for 8 under the same conditions.
Cyclotrimerization Reactionsa
alkyne
entry R1 R2
T
time conversionc product
catalyst
solvent (°C) (min)
(%)
ratiof a:b:c
1
2
Me Me 8
Et Et
CDCl3 27 180
CDCl3 27 30
CDCl3 62 10
99
99
99
99
99
99
99
60
99
99
8
THF
62 15
3
nPr nPr 8
CDCl3 27 40
CDCl3 62 30
THF
62 30
4
5
Ph Ph
Me Et
8
8
CDCl3 62 24 h
CDCl3 27 180
50 60
33:67:0
33:67:0
(35:65:0)
18:63:19
18:60:22
(17:61:22)
22:78:0
6
Me nPr 8
CDCl3 27 90
62 60
99
99
7
8
H
Ph
8
CDCl3 27 330
62 180
CHCl3 27 30
62 30
99
99
6
23
19:81:0
It should also be noticed that the selectivity in relation to an
b
Et Et SiO2-PdCl2
isomer ratio of 1,3,5-/1,2,4-C6R1 R23 remained about the same
3
88d
91d
NRe
NRe
NRe
NRe
99
regardless with or without MW for systems starting with
MeCt CEt or HCt CPh in either bmimPF6 or THF (entries 4,
6, 9, 11, and 14 in Table 2). However, the selectivity of 1,2,3-
27 36 h
62 16 h
CDCl3 27 30
62 10
CDCl3 27 30
62 10
9
Et Et
9
C6R1 R23 for the system starting with MeCt CnPr in bmimPF6
3
10 Et Et 10
has dramatically decreased under MW conditions. For example,
an isomer ratio of 1,3,5-/1,2,4-/1,2,3-C6R1 R23 was changed from
11 Et Et Pd(PhCN)2Cl2 CDCl3 27 30
62 10
3
99
18:60:22 without MW to 22:75:3 with MW for catalyst 8, and
the isomer ratio was changed from 18:63:19 without MW to
25:73:2 with MW for Pd(CH3CN)2Cl2, while the selectivity of
12 Et Et Pd(CH3CN)2Cl2 CDCl3 27 30
62 30
95
99
13 Ph Ph Pd(CH3CN)2Cl2 CDCl3 62 24 h
14 nPr nPr Pd(CH3CN)2Cl2 CDCl3 27 40
15 Me Et Pd(CH3CN)2Cl2 CDCl3 27 180
16 Me nPr Pd(CH3CN)2Cl2 CDCl3 27 90
62 60
56
50
47
40
70
75
1,2,3-C6R1 R23 for the same reaction was even dropped to zero
3
35:65:0
19:62:19
18:63:19
17:83:0
in THF to give a 1,3,5-/1,2,4-/1,2,3-C6R1 R2 ratio of 55:45:0.
3
3
As stated earlier it has been proposed in many publicatons that
the two-product and the three-product systems followed different
reaction pathways. The results obtained in our study implied
that the MW may suppress the formation of cyclobutadienyl
complex, which has been suggested as the active intermediate
for the three-product mechanism. Moreover the THF solvent
system probably further facilitated the formation of a pallada-
cyclopentadiene intermediate to give only two products, 1,3,5-
and 1,2,4-C6R1 R2 .
17
H
Ph Pd(CH3CN)2Cl2 CDCl3 27 330
a Reaction conditions: alkyne (0.075 mmol) in CDCl3 (1 mL), catalyst
loading ) 4 mol %. b Reaction conditions: alkyne (0.56 mmol) in
CDCl3 (3 mL), catalyst loading
) 4
mol %. c Conversions were
determined by 1H NMR spectroscopy. d Products were purified and
isolated by flash chromatography on SiO2 with hexane/ethyl acetate.
e NR ) no observable reaction. f Isomer ratios were determined by
GC-MS and numbers in parentheses were obtained by using NMR
spectroscopy.
3
3
There have been several recent reports on the use of MW
radiation at various stages of the nucleation and growth process
for synthesis of Au nanostructures.29,31 We have learned from
the literature that too much MW power fast absorbed by media
would lead to a rapid rise in temperature and could disrupt the
micelles of Au cores to cause severe size and shape changes of
the aggregates.32 In our study MW with 300 W of working
power (2.5 GHz) was introduced to the reaction systems for
1-5 min depending on the reaction conditions. The recovered
colloidal catalysts from the MW-assisted catalytic cyclotrim-
selectivity was not only highly dependent on the metal center
and the solvent polarity, but also substantially affected by
the size of substituents on both sides of unsymmetrical
alkynes.19h,i 27b The selectivities observed in both hybrid
,
system 8 and homogeneous system Pd(CH3CN)2Cl2 are similar
(entries 5-7 and 15-17, Table 1), and they are also compatible
with those found for other reported Pd systems.
MW radiation is an alternative to conventional heating for
transforming electromagnetic energy into heat, though many
molecular solvents such as water, alcohols, DMF, and ethyl-
eneglycol have high dielectric losses and are good media for
MW rapid heating. However, due to the nature of molecular
solvents the MW should be applied in a pulse mode to get better
temperature control.29 On the contrary, room temperature ionic
(28) (a) Wakatsuki, Y.; Nomura, O.; Kitaura, K.; Morokuma, K.; Yamazaki,
H. J. Am. Chem. Soc. 1983, 105, 1907–1912. (b) McAlister, D. R.; Bercaw,
J. E.; Bergman, R. G. J. Am. Chem. Soc. 1977, 99, 1666–1668.
(29) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Kubokawa, M.; Tsuji, T.
Chem. Eur. J. 2005, 11, 440–452.
(30) (a) Chen, I.-H.; Young, J.-N.; Yu, S. J. Tetrahedron 2004, 60, 11903–
11909. (b) Leadbeater, N. E.; Torenius, H. M. J. Org. Chem. 2002, 67, 3145–
3148.
(27) (a) Parshall, G. W.; Ittel, S. D. In Homogeneous Catalysis: The
Accplications and Chemistry of Catalysis by Soluble Transition Metal Complexes,
2nd ed.; John Wiley: New York, 1992; p 208. (b) Ehmann, W. J.; Whitesides,
G. M. J. Am. Chem. Soc. 1969, 91, 3800–3807.
(31) (a) Doolittle, J. W., Jr.; Dutta, P. K. Langmuir 2006, 22, 4825–4831.
(b) Shen, M.; Du, Y.; Yang, P.; Liang, L. J. Phys. Chem. Solids 2005, 66, 1628–
1634. (c) Liu, F.-K.; Ker, C.-J.; Chang, Y.-C.; Ko, F.-H.; Chu, T.-C.; Dai, B.-T.
Jpn. J. Appl. Phys. 2003, 42, 4152–4158.
J. Org. Chem. Vol. 73, No. 13, 2008 4925

Lin et al.
SCHEME 3. Syntheses of Molecular Pd(II) Complexes HO(CH2)11N(H)(O)P(2-py)2PdCl2 (9) and
Br(CH2)11N(H)(O)P(2-py)2PdCl2 (10)
SCHEME 4. The SiO2-Bound PdCl2 Catalyst
SCHEME 5. Plausible Mechanisms for the Metal-Catalyzed Alkyne Cyclotrimerization
erization reaction of 3-hexyne was subjected for electron
microscopy studies. The TEM images of the recovered 8 have
shown no sign of MW influence on the size or shape of Au
cores and the average diameter of Au NPs was comparable to
those of freshly prepared 8 (see the Supporting Information).
Most current homogeneous Pd catalysts cope with difficulties
in quantitative separation, catalyst recovery, and catalyst
deactivations by formation of inactive colloidal species at
elevated temperatures,33 whereas most heterogeneous Pd cata-
lysts suffer from low turnover frequencies and reusability34 due
to either particle aggregation or leaching.35–37 Since the Au NPs
surface bound Pd(II) complex catalysts in our study were
heterogeneously suspended in either molecular CHCl3, THF,
or ionic bmimPF6, they can be easily separated and quantita-
tively recovered from the reaction mixture through simple
(33) Beller, M.; Fisher, H.; Kuhlein, K.; Reisinger, C.-P.; Herrmann, W. A.
J. Organomet. Chem. 1996, 520, 257–259.
(32) Aslan, K.; Geddes, C. D. Anal. Chem. 2007, 79, 2131–2136.
4926 J. Org. Chem. Vol. 73, No. 13, 2008

Au Nanoparticle-Supported Pd(II) Interphase Catalysts
TABLE 2. MW-Assisted [2+2+2] Alkyne Cyclotrimerization
Conclusions
Reactions Catalyzed by RS-Au-PdCl2 NPs (8)a
In summary, we have developed a method to immobilize
controllable amounts of molecular palladium(II) complex
catalysts onto the surface of Au NPs. The resulting colloidal
particles are readily dissolvable and precipitable, and their
structures and reaction can be easily studied by solution phase
NMR spectroscopy. We have also demonstrated a greener and
highly efficient methodology for the construction of polysub-
stituted benzene rings with fairly good selectivity by using these
Au NPs-bound interphase Pd(II) catalysts in ionic liquid under
MW irradiation conditions. As compared to their corresponding
unbound free form the Au NPs-bound Pd(II) complexes were
shown to be more effective catalysts to provide significant
reactivity enhancement. In addition, these hybrid catalysts can
be easily separated and quantitatively recovered from reaction
mixtures by simple centrifugation and filtration. The recovered
catalysts can then be recycled many times without loss of
reactivity.
As demonstrated herein hybrid catalysts of this type congre-
gate the advantages from both heterogeneous and homogeneous
systems. It is expected that many sustainable and applicable
catalysts of this form can be made by tethering a wide range of
metal complexes or even organocatalysts to a variety of metal
nanosurfaces, and they may be useful for promoting many types
of chemical reactions in a greener fashion.
alkyne
entry R1 R2
time conversiond product
catalyst
solvent (min)
(%)
ratioe a:b:c
1
2
3
4
5
6
7
8
9
Et Et
8
bmimPF6 1.5
bmimPF6 2.0
bmimPF6 10
>99
>99
32
nPr nPr 8
Ph Ph
Me Et
8
8
bmimPF6 2.0
bmimPF6 3.0
bmimPF6 5.0
>99
>99
>99
53
29:71:0
22:75:3
21:79:0
Me nPr 8
H
Ph 8
Et Et Pd(CH3CN)2Cl2 bmimPF6 1.5
nPr nPr Pd(CH3CN)2Cl2 bmimPF6 2.0
Me Et Pd(CH3CN)2Cl2 bmimPF6 2.0
65
40
68
75
35:65:0
25:73:2
20:80:0
10 Me nPr Pd(CH3CN)2Cl2 bmimPF6 3.0
11
12 Et Et
13 nPr nPr 8
14 Me Et
15 Me nPr 8
H
Ph Pd(CH3CN)2Cl2 bmimPF6 5.0
8
THFb
THFb
THFc
THFb
5.0
7.0
8.5
98
99
99
99
8
29:71:0
55:45:0
10
a Reaction conditions: alkyne (0.075 mmol) in bmimPF6 or THF (1
mL), catalyst loading ) 4 mol % under 300 W MW irradiation. b Under
MW irradiation (max 300 W) and a preset temperature of 62 °C.
c Under MW irradiation (max 300 W) and a preset temperature of 50
°C. Conversions were determined by H NMR spectroscopy. e Products
d
1
were purified and isolated by flash chromatography on SiO2 with
hexane/ethyl acetate (10/1) as eluent and isomer ratios were determined
by GC-MS.
Experimental Section
A. Synthesis of Molecular Palladium(II) Complex Catalysts.
Synthesis of Palladium Complex 9. Compound 5 (0.5 g, 1.3 mmol)
was added to a solution of Pd(CH3CN)2Cl2 (0.34 g, 1.3 mmol) in
CH3CN (30 mL). The mixture was stirred for 6 h at ambient
temperature. All volatiles were removed to give 9 as a yellow solid
TABLE 3. Comparative Recycling and Reuse of Catalysts
RS-Au-L-PdCl2 8 and Pd(CH3CN)2Cl2 in the [2+2+2]
Cyclotrimerization of 3-Hexynea
cycle (% yieldb)
1
(0.70 g, 95% yield). H NMR (400 MHz, d6-DMSO, 27 °C): δ
catalyst
1
2
3
4
5
6
7
8
9
1.17-1.37 (br, 16H, -(CH2)8-), 1.66 (br, 2H, CH2CH2OH), 3.00
(br, 2H, CH2NH), 3.35 (br, 2H, CH2OH), 6.99 (br, 1H, NH), 7.76
(m, 2H, py), 8.22 (m, 4H, py), 9.11 (m, 2H, py) ppm. 13C NMR
(100 MHz, d6-DMSO, 27 °C): δ 25.7, 26.4, 28.9, 29.1, 29.2, 29.3,
31.4, 32.8, 41.7, 61.1, 128.4, 131.6 (3JP-C ) 3.0 Hz), 140.0 (3JP-C
) 9 Hz), 148.4 (1JP-C ) 148.0 Hz), 156.0 (2JP-C ) 88.0 Hz), ppm.
31P NMR (160 MHz, d6-DMSO, 27 °C): δ 19.0 ppm. Anal. Calcd
for C21H32N3PO2Cl2Pd: C, 44.50; H, 5.69; N, 7.41. Found: C, 44.12;
H, 6.02; N, 7.05.
8
>99 >99 >99 >99 90 82 75 68 57
Pd(CH3CN)2Cl2 >99 82 44 20
a Reaction conditions: alkyne (0.0375 mmol) in CDCl3(0.5 mL) at 34
°C, catalyst loading ) 23.2 mg (8 mol %), reaction time ) 10 min for
b
1
each cycle. Determined by H NMR spectroscopy analysis.
filtration. The 1H NMR spectrum of 8 recovered from the fifth
cycle of catalytic cyclotrimerization of 3-hexyne showed
negligible difference from that of freshly prepared 8 (Figure
2d,e). Also, the lack of any detectable Au or Pd species by AA
spectroscopy in the filtrate of the reaction solution after catalysis
further verified that catalyst 8 was able to resist Pd leaching
under our reaction conditions. The recovered colloids after
catalysis were also checked by TEM and their average particle
sizes were found to be comparable to those before catalysis (see
the Supporting Information). A series of catalyst recycling
experiments were readily accomplished in the system containing
0.075 mol of 3-hexyne and 23.2 mg of RS-Au-L-PdCl2 (8) (Pd-
to-substrate ratio ) 8 mol %) in CDCl3 (0.5 mL) at 34 °C for
10 min. As shown in Table 3, the catalytic system can be
effectively recycled with reactivity remaining at ∼90% yield
throughout the first five cycles before it gradually drop to ∼60%
after the ninth cycle, whereas the recycling procedure for the
molecular Pd(CH3CN)2Cl2 catalyst system was a lot more
complicated and less efficient. The recovered Pd(CH3CN)2Cl2
showed a more than 50% decrease in catalytic activity at the
third cycle.
Synthesis of Palladium Complex 10. Compound 6 (0.5 g, 1.1
mmol) was added to a solution of Pd(CH3CN)2Cl2 (0.29 g, 1.1
mmol) in CH3CN (30 mL). The mixture was stirred for 3 h at
ambient temperature. All volatiles were removed to give 10 as a
1
yellow solid (0.66 g, 94% yield). H NMR (400 MHz, d6-DMSO,
27 °C): δ 1.17-1.37 (br, 14H, -(CH2)7-), 1.67 (m, 2H,
CH2CH2NH), 1.76 (m, 2H, CH2CH2Br), 3.00 (br, 2H, CH2NH),
3.50 (br, 2H, CH2Br), 6.98 (br, 1H, NH), 7.76 (m, 2H, py), 8.21
(m, 4H, py), 9.10 (m, 2H, py) ppm. 13C NMR (100 MHz, d6-DMSO,
(34) (a) Tsuji, J. In Palladium Reagents and Catalysts; Wiley: New York,
1995. (b) Choudary, B. M.; Madhi, S.; Chowdari, N. S.; Kantam, M. L.; Sreedhar,
B. J. Am. Chem. Soc. 2002, 124, 14127–14136. (c) Reetz, M. T.; Lohmer, G.
Chem. Commun. 1996, 1921–1922. (d) Crisp, G. T. Chem. Soc. ReV. 1998, 27,
427–436. (e) Genet, J. P.; Savignac, M. J. J. Organomet. Chem. 1999, 576, 305–
317. (f) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009–3066.
(g) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750–3754.
(h) Biffis, A.; Zecca, M.; Basato, M. J. Mol. Catal. A 2001, 173, 249–274.
(35) Mehnert, P.; Weaver, D. W.; Ying, J. Y. J. Am. Chem. Soc. 1998, 120,
12289–12296.
(36) (a) Shmidt, A. F.; Mametova, L. V. Kinet. Catal. 1996, 37, 406–408.
(b) Wagner, M.; Ko¨hler, K.; Djakovitch, L.; Weinkauf, S.; Hagen, V.; Muhler,
M. Top. Catal. 2000, 13, 319–326. (c) Zhao, F.; Bhanage, B. M.; Shirai, M.;
Arai, M. Chem. Eur. J. 2000, 6, 843–848.
(37) Biffis, A.; Zecca, M.; Basato, M. Eur. J. Inorg. Chem. 2001, 1131–
1133.
J. Org. Chem. Vol. 73, No. 13, 2008 4927

Lin et al.
27 °C): δ 26.3, 26.4, 27.6, 28.2, 28.7, 29.0, 31.4, 32.4, 35.3, 41.5,
45.5, 128.2, 131.5 (3JP-C ) 16 Hz), 139.82 (3JP-C ) 9 Hz), 149.1
(1JP-C ) 147 Hz), 155.8 (2JP-C ) 88 Hz) ppm. 31P NMR (160 MHz,
d6-DMSO, 27 °C): δ 19.1 ppm.
ridylphosphinic amido n-undecanethiolate-protected Au NPs, RS-
Au-L (7) (60 mg) was dissolved in degassed CHCl3 (30 mL). To
this solution was added Pd(CH3CN)2Cl2 (6 mg, 0.034 mmol), and
the reaction mixture was stirred for 3 h at ambient temperature.
The solution was concentrated to a minimum amount and was
washed first with acetone (3 × 50 mL) then with CHCl3 (3 × 50
mL). Further precipitation from CHCl3 was performed continuously
until no more free Pd(CH3CN)2Cl2 was present. Centrifugation gave
the colloids as black solid product, RS-Au-L-PdCl2 (8) (40 mg).
1H NMR (400 MHz, d6-DMSO, 27 °C): δ 0.81 (br, CH3), 1.2-1.80
(br, methylene protons), 2.92-3.0 (m, CH2NH), 7.0 (m, NH), 7.77
(m, py), 8.25 (m, py), 9.10 (m, py) ppm. 13C NMR (125 MHz,
d6-DMSO, 27 °C): δ 14.1, 22.3, 26.3, 29.1, 30.6, 31.5, 128.3, 131.5,
139.9, 149.0 (1JP-C ) 122 Hz), 155.8 ppm. 31P NMR (160 MHz,
d6-DMSO, 27 °C): δ 18.9 ppm. TEM (4.9 ( 1 nm), the colloids
contained an average number of 4521 gold atoms/core and 287
adsorbed n-octanethiolate molecules/Au core and 207 adsorbed
Pd(II)-functionalized n-undecanethiolate molecules/Au core.
C. Catalytic [2+2+2] Alkyne Cyclotrimerization Reactions.
General Procedure. To a suspension of RS-Au-L-PdCl2 (8) (11.6
mg, containing 3 × 10-3 mmol of Pd(II)) in 1.0 mL of CDCl3 was
added 3-hexyne (6.2 mg, 0.075 mmol). The reaction mixture was
stirred at room temperature for 30 min or at 62 °C for 10 min.
After the reaction, the solids were filtered off and the filtrate was
dried under vacuum for direct use in NMR and GC-MS analysis.
The [2+2+2] alkyne cyclotrimerization reaction product was further
purified on a short silica plug (63-200 mesh; eluting with hexane)
for yield calculation. The spectroscopic characteristics of already
known products were compared with published data.
MW-Assisted [2+2+2] Alkyne Cyclotrimerization Reactions.
To a bmimPF6 (1-butyl-3-methylimidazolium hexafluorophosphate,
1.0 mL) suspension of RS-Au-L-PdCl2 (8) (11.6 mg, containing 3
× 10-3 mmol of Pd(II)) in a Teflon digestion vessel was added
3-hexyne (6.2 mg, 0.075 mmol). The reaction vessel was placed
under 300 W MW (CEM MARS 5e, continuous wave mode) for
1.5 min. The resulting gooey mixture was extracted with hexane
(3 × 5 mL). The extract was used directly for NMR and GC-MS
analysis. The cyclotrimerized products were further purified on a
short silica plug (63-200 mesh; eluting with hexane) for yield
calculation. The spectroscopic characteristics of already known
products were compared with published data.
B. Synthesis of Various Thiolate-Protected Au NPs. Octaneti-
olate-Protected Au NPs (RS-Au). To a solution of tetraoctylam-
monium bromide (500 mg, 0.1 mmol) in CHCl3 (40 mL) was added
a solution of tetrachloroauric acid (200 mg, 0.5 mmol) in water
(15 mL). The mixture was stirred vigorously for 30 min to ensure
complete transfer of AuCl4 ·3H2O, at which point the decoloration
of the aqueous phase took place while the organic phase turned
orange-brown. After this, a mixture of n-octanethiol (200 mg, 1.4
mmol) in CHCl3 (20 mL) was added to the solution. After 10 min,
a freshly prepared aqueous solution of NaBH4 (200 mg, 5.5 mmol
in 20 mL of H2O) was added dropwise to the reaction mixture.
The colloids formed instantaneously as indicated by the color
change of the solution from orange to red and then to black-brown.
After 12 h, the organic layer was collected. The colloidal solution
was then concentrated to 3 mL, and 150 mL of ethanol was added
to allow sitting at room temperature for 1 h for colloid precipitation.
Precipitation from ethanol was carried out continuously until no
more free ligand could be detected by 1H NMR spectroscopy
(CDCl3, no R-CH2 resonance at ∼2.49 ppm) and IR spectroscopy
(absence of νasym(SH) band in the region of 2569 cm-1). Centrifu-
1
gation gave the colloids as black solids (75 mg). H NMR (400
MHz, CDCl3, 27 °C): δ 0.88 (br, 3H, CH2CH3), 1.57-1.80 (br,
10H, CH2(CH2)5CH3), 2.46-2.51 (br, 2H, CH2CH2SH), 2.9-3.5
(br, 2H, CH2S-Au) ppm. 13C NMR (100 MHz, CDCl3, 27 °C): δ
14.1, 22.6, 22.8, 29.0-29.6, 32.0 ppm. TEM (2.7 ( 0.5 nm), the
colloids contained an average number of 755 gold atoms/core38
and 1470 adsorbed octanethiolate molecules per Au core.
Mixed Octanethiolate- and Dipyridylphosphinic Amido Unde-
canethiolate-Protected Au NPs (RS-Au-L, 7). Freshly prepared
octanethiolate-protected Au NPs, RS-Au (75 mg) were dissolved
in degassed CHCl3 (25 mL). To this solution was added ligand 4
(60 mg, 0.15 mmol), and the reaction mixture was stirred for 16 h
at 70 °C. The solution was concentrated to a minimum amount
(0.5 mL), then was washed with hexane (3 × 50 mL). A degassed
acetone solvent (150 mL) was added and the solution was allowed
to sit at room temperature for 1 h for gold colloid precipitation.
Further precipitation from acetone was performed continuously until
no more free ligand 4 could be detected by 1H NMR spectroscopy
(CDCl3, no R-CH2 resonance at ∼2.49 ppm) and IR spectroscopy
(absence of νasym(SH) band in the region of 2576 cm-1). Centrifu-
gation gave the colloids as black solid product, RS-Au-L (7) (60
Acknowledgment. Funding for this work was provided by
the National Science Council of Taiwan (NSC-94-2113-M-194-
008), for which the authors are grateful.
1
mg). H NMR (400 MHz, d6-DMSO, 27 °C): δ 0.78 (br, CH3),
0.87-1.90 (br, methylene protons), 2.83 (br, CH2NH), 5.32 (m,
NH), 7.42 (m, py), 7.86 (m, py), 7.95 (m, py), 8.62 (m, py) ppm.
13C NMR (125 MHz, d6-DMSO, 27 °C): δ 14.1, 22.5, 26.5, 29.5,
31.8, 117.0, 125.6, 127.8 (2JP-C ) 21 Hz), 136.4, 138.2, 150.1 (2JP-C
) 20 Hz), 156.3 (1JP-C ) 123 Hz) ppm. 31P NMR (160 MHz, d6-
DMSO, 27 °C): δ 17.4 ppm. TEM (3.9 ( 0.4 nm), the colloids
contained an average number of 2278 gold atoms/core and 607
adsorbed octanethiolate molecules per Au core and 497 adsorbed
bipyridylphosphinic amido n-undecanethiolate molecules per Au
core.
Supporting Information Available: General Experimental
Methods, detailed synthetic procedures for compounds 1-6,
TEM images of RS-Au-L NPs 7 and RS-Au-L-PdCl2 NPs 8,
UV-vis spectra of compound 4 and RS-Au-L NPs 7 and RS-Au-
L-PdCl2 NPs 8, analytical data for alkanethiolates-proctected RS-
1
Au NPs, RS-Au-L NPs 7, and RS-Au-L-PdCl2 NPs 8, H and
13C NMR spectra of compounds 1-6, 9, and 10 the alkanethi-
olates-proctected RS-Au NPs, RS-Au-L NPs 7, and RS-Au-L-
PdCl2 NPs 8, and IR spectra of compound 4 and alkanethiolates-
proctected RS-Au NPs, RS-Au-L NPs 7, and RS-Au-L-PdCl2
NPs 8. This material is available free of charge via the Internet
at http://pubs.acs.org.
Synthesis of Palladium-Functionalized Au NPs (RS-Au-L-
PdCl2, 8). A freshly prepared mixed n-octanethiolate- and bipy-
(38) Leff, D. V.; Ohara, C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem.
1995, 99, 7036–7041.
JO800524H
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