Organometallic hollow spheres bearing bis(N-Heterocyclic carbene)-palladium species: Catalytic application in three-component Strecker reactions

Article abstract of DOI:10.1002/anie.201003101

Hollow-sphere catalysts were prepared by means of 3D network formation between a tetraimidazolium building block and palladium acetate. The bis(N-heterocyclic carbene)-palladium species that were concomitantly formed during growth of the hollow spheres sh

Full text of DOI:10.1002/anie.201003101

Communications
DOI: 10.1002/anie.201003101
Organometallic Hollow Spheres
Organometallic Hollow Spheres Bearing Bis(N-Heterocyclic
Carbene)–Palladium Species: Catalytic Application in Three-
Component Strecker Reactions**
Jaewon Choi, Hye Yun Yang, Hae Jin Kim, and Seung Uk Son*
Recently, diverse micro- and submicroparticles have been
designed and prepared for tailored applications in adsorbents,
biomedical assays, and heterogeneous catalysis.[1] In addition
to conventional inorganic particles, functional spheres can be
formed by coordination-directed assembly of molecular
building blocks.[2] In this case, unique molecule-based func-
tionalities can be introduced into the spheres with a high
density of molecular functional sites. For example, we
reported formation of submicrospheres through a hapticity
change of the [(h6-hydroquinone)Rh(cod)]BF4 (COD = 1,5-
cyclooctadiene) building block, with successful application as
catalysts for polymerization of phenylacetylene.[2b]
On the basis of structure, these molecular spherical
materials can be divided into three classes: simple sphere,
core/shell, and hollow sphere. Usually, the catalytic function
of the spheres is related to the chemical properties of the
surface, and the chemical species inside the spheres usually
cannot participate. Thus, the core/shell structure is more
appropriate to save materials costs by locating relatively
inexpensive building blocks inside the spheres.[3] In this
regard, we prepared organometallic spheres having a Mn–
quinonoid core/Rh–quinonoid shell structure.[3b] Ultimately,
in view of cost, a hollow shape with an empty inner space is
more appropriate. However, direct formation of a molecular
hollow sphere by coordination-directed assembly is quite
difficult[4] and usually sequential synthetic steps are required
with a designed template.[5] For example, Prussian blue hollow
spheres were recently fabricated by using a block copolymer
as template material.[5]
imidazolium salts, show powerful coordination ability toward
a wide range of metal ions. We have studied the synthesis of
new functional materials based on NHC chemistry for diverse
applications including heterogeneous catalysts and hydrogen
storage materials.[7] Here we report the template-free syn-
thesis of organometallic hollow spheres with concomitant
formation of bis(NHC) Pd species and their catalytic appli-
cations in one-pot three-component reactions.
Usually, appropriate bases are required to generate NHC
moieties by abstracting the proton at the 2-position of 1,3-
disubstituted imidazolium salts.[6] However, some metal
reagents have been known to react directly with imidazolium
salts to form metal NHC species. For example, silver oxide[8]
and copper oxide[9] can function as base and metal source. In
addition, metal acetates such as palladium acetate can form
metal NHC species by direct reaction with imidazolium
salts.[10] Based on this chemistry, we designed a tetrahedral
imidazolium building block for generating organometallic
particles bearing metal NHC species through formation of 3D
infinite networks of molecular building blocks (Scheme 1).
One-dimensional main-chain metallopolymers have been
prepared by using predesigned bis-phosphines or bis-NHC
ligands.[11]
First, tetrakis(4-bromophenyl)methane was prepared
from tetraphenylmethane by a literature method.[12] The
four imidazolyl groups were then introduced by Ullmann-
type coupling with copper catalysis. After screening several
copper reagents, including the copper oxides, copper iodide
showed the best reactivity. The resultant neutral tetrakis[4-(1-
imidazolyl)phenyl]methane (1) was further treated with
methyl iodide to form tetrahedral building block 2 having
four imidazolium salts.
N-Heterocyclic carbene (NHC) ligands are currently very
popular in organometallics.[6] The NHCs, generated by
abstraction of the proton at the 2-position of 1,3-disubstituted
For preparation of organometallic hollow spheres
(OMHS), building block 2 (0.5 equiv relative to Pd) was
treated with palladium acetate in DMF (5 mL) at 1108C.
Gradually, a pale yellow precipitate was formed on the
bottom of the glassware. After washing with DMF and
dichloromethane and drying at 1008C for 10 h under vacuum,
the resultant solid was investigated by scanning electron
microscopy (SEM). Figure 1a shows a typical SEM image of
the spherical particles. The average sphere size was calculated
to (1.50 Æ 0.15) mm by counting 544 particles (Figure 1c). The
negligible change in the size of the spheres when the solvent
volume was changed from 5.0 to 20 mL implies that the
particle size does not result from conventional kinetic effects
in the growth process,[13] but rather from the solubility
limitation of the particles. Interestingly, careful investigation
of the SEM images of the spheres showed different contrast
[*] J. Choi, H. Y. Yang, Prof. S. U. Son
Department of Chemistry and Department of Energy Science
Sungkyunkwan University
Suwon 440-746 (Korea)
Fax: (+82)31-299-4572
E-mail: sson@skku.edu
Dr. H. J. Kim
Korea Basic Science Institute
Daejeon 350-333 (Korea)
[**] This work was supported by grant NRF-2009-0084799 and the WCU
program (R31-2008-000-10029-0) funded by MEST of Korea. H.Y.Y.
thanks NRF-2009-0094024 (Priority Research Centers Program) for
grants. H.J.K. acknowledges the Hydrogen Energy R&D Center, a
21st century Frontier R&D Program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003101.
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and iodine (Figure 1d). Again, the intensity of
the mapped images showed the characteristic
properties of a hollow structure with contrast
between the interiors and walls of the spheres.
The powder X-ray diffraction pattern (XRPD) of
OMHS showed amorphous character. The ther-
mal stability of the hollow spheres was inves-
tigated by thermogravimetric analysis (TGA),
which showed them to be stable up to at least
2508C (Figure 1e).
To confirm the chemical structure of the
hollow spheres, solid-state 13C NMR spectro-
scopic studies were conducted. As shown in
Figure 2b, all peaks were clearly assignable to
the carbon atoms in the expected structure with
help of a model compound (see below). The
13C NMR peaks from the methyl and methylene
groups, the different benzene rings, and the
imidazole ring appeared at 39, 64, and 124–
147 ppm. In particular, the 13C NMR peak for
carbene carbon appeared at 168 ppm, which
matches well with known values (167–169 ppm)
in [(nhc)2PdI2] complexes.[15] In addition, a model
compound was prepared by treating 1-methyl-3-
(4-tolyl)imidazolium iodide (Figure 2a) with pal-
ladium acetate in DMF at 1108C for one day to
Scheme 1. Formation of a 3D infinite network through NHC chemistry
involving tetrahedral imidazolium building block 2.
between the center and side parts of the spheres, which
implies a hollow structure of the materials (Figure 1a and
Figure S1 in Supporting Information). Sometimes, open
spheres showed empty inner spaces (inset of Figure 1a) In
addition, when the spheres were heated for several days,
distorted spheres were observed (see representative image in
the inset of Figure 1a and Figure S2 in the Supporting
Information) To clarify the structure, the spheres were
microtomed after being embedded into epoxy resin, and the
cut spheres showed an empty inner space. (Figure 1b) For
further investigation of the structural homogeneity of the
materials, transmission electron microscopy (TEM) studies
were conducted and showed vivid contrast difference in all
images (at least 25) of spheres with bright core and dark shell
(Figure S3 in the Supporting Information).
To acquire information on the chemical components of
the hollow spheres, elemental analysis (EA) by combustion
was conducted, which clearly supports the formation of the
[(nhc)2PdI2] species. The observed contents of carbon (36.65),
nitrogen (8.62), and hydrogen (2.85 wt%) in the hollow
spheres matched well with the calculated values of for
[{C41H36N8Pd2I4}n] (C 36.18, N 8.23, H 2.67 wt%). The iodide
counteranions of building block 2 formed part of the ligand
sphere in the {(nhc)2Pd} species, a common observation in the
synthesis of mono- and polymeric Pd NHC complexes from
imidazolium halides and palladium acetate.[11,14] Elemental
mapping by energy dispersive X-ray absorption spectroscopy
(EDS) confirmed a homogeneous distribution of palladium
Figure 1. a) Typical SEM image of organometallic hollow spheres
(OMHS) bearing {(nhc)2Pd} species (inset: selected open and dis-
torted spheres); b) SEM image of microtomed spheres; c) size distri-
bution diagram; d) EDS elemental-mapping images of hollow spheres;
e) TGA curve of OMHS.
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Figure 3. Top: OMHS-catalyzed (4.0 mol% Pd relative to acetophe-
none) three-component Strecker reaction at room temperature for 10 h
between acetophenone (1.0 mmol), aniline (1.1 mmol), and trimethyl-
silyl cyanide (TMSCN, 2.0 mmol). [a] Conversion. [b] Yield of isolated
product. Bottom: SEM images of OMHS before and after catalysis.
been reported for this reaction. Most studies on the one-pot
three-component Strecker reaction focus on aldehydes, and
efficient systems for ketones are still lacking. Hence, in this
study, we focused on the one-pot Strecker reaction of ketones.
As shown in Figure 3, OMHS (4.0 mol% Pd relative to
substrate) showed excellent activity in the one-pot Strecker
reaction of acetophenone with aniline at room temperature.
In a control experiment without catalyst, no conversion of
acetophenone occurred. The catalysts were easily isolated by
centrifugation and reused without loss of the original shape
and catalytic activity for three successive runs. We speculate
that excellent maintenance of the structure and catalytic
activities may originate from the low reaction temperature
and relatively simple Lewis acidic function of the catalysts
(see Figure S5 in Supporting Information for SEM image of
recovered OMHS).
Table 1 summarizes the catalytic activity of the hollow
spheres toward a variety of ketones. In a recent report on one-
pot three-component Strecker reactions catalyzed by homo-
geneous Pd NHC complexes, 3.0 mol% catalyst gave 86%
conversion of 4-methoxyacetophenone in 24 h.[20] In contrast,
the heterogeneous systems presented herein (4.0 mol% Pd)
showed 92% conversion of 4-methoxyacetophenone in 10 h.
As reported in the literature, the present catalytic system also
showed significant functional-group dependency, possibly due
to the coordination ability of the functional groups of the
substrates.[19,20] In the case of 4-nitroacetophenone, 2-ace-
tylthiophene, and 3-acetylpyridine, conversion of the sub-
strate was almost completely or significantly suppressed.
(Table 1, entries 4, 6, and 7) In sharp contrast to 2-acetyl-
thiophene, 2-acetylfuran showed good conversion to the
expected product (Table 1, entry 5). These observations
strongly imply that the catalytic function of the hollow
spheres originates from Lewis acidic activation of the ketone
or imine through coordination of palladium. In the case of
mono-acetyl naphthalenes, a significant steric effect was
observed (Table 1, entries 8 and 9) Compared with 2-acetyl-
naphthalene, the conversion of 1-acetylnaphthalene was
significantly suppressed.
Figure 2. A model compound (a) and solid-phase 13C NMR spectrum
of OMHS (b). Peaks indicated by asterisks correspond to DMF.
give the corresponding [(nhc)2PdI2] complex, which was fully
1
characterized by H and 13C NMR spectroscopy and high-
resolution mass spectrometry.[16] As expected, the 13C NMR
peak of the carbene carbon atom of the model compound
appeared at d = 168 ppm.
According to time-dependent TEM studies, the hollow
core was gradually formed from a nonhollow sphere during
the reaction. We speculate that formation of a more compact
network during the ripening process generates the hollow
structure, as observed in template-free synthesis of metal
oxide or metal chalcogenide hollow spheres in materials
science[17] (Figure S4 in the Supporting Information).
Recently, diverse metal NHC complexes have been
prepared and applied as catalysts in various organic trans-
formations.[6] The unique coordination abilities of the NHC-
based ligands are responsible for their unprecedented stabil-
ity and catalytic reactivity. Among them, Pd NHC complexes
À
À
attract special attention as catalysts in C C and C N bond
formation.[18] Considering the existence of Pd NHC species
and the atom-economical hollow structure, we applied
OMHS as a heterogeneous catalyst.
Among the diverse reactions, the three-component
Strecker reaction attracted our attention.[19] The Strecker
reaction, forming a-amino nitriles by reaction of an amine, a
cyanide source, and an aldehyde or ketone, is especially useful
for the synthesis of a-amino acid derivatives, even on an
industrial scale (Figure 3). Although this reaction has
attracted continuous attention, the Pd NHC catalysts were
only reported recently.[20] Moreover, as far as we are aware, no
heterogeneous systems based on metal NHC complexes have
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0.085 mmol) was added and the reaction mixture heated at 1108C for
Table 1: One-pot three component Strecker reactions of ketones cata-
lyzed by OMHS.[a]
2 d. A pale yellow solid was formed and washed with DMF and
methylene chloride. After drying at 1008C for 10 h under vacuum,
OMHS (25 mg) was isolated in 43% yield (based on Pd). The detailed
synthetic procedure for building block 2 is described in the Support-
ing Information.
Entry
Ketone
Product
Yield [%][b]
96 (85)
1
Procedure for catalytic reactions: OMHS (27 mg, 0.040 mmol
Pd), ketone (1.0 mmol), aniline (0.10 mL, 1.1 mmol), and TMSCN
(0.27 mL, 2.0 mmol) were added to 2 mL of dichloromethane. The
reaction mixture was stirred for 10 h at room temperature. After
reaction, the catalyst was recovered by centrifugation and dichloro-
methane was removed by evaporation. The crude product was
directly analyzed by 1H NMR spectroscopy and purified by flash
column chromatography.
2
3
4
5
6
7
95 (91)
92 (83)
3 (–)[c]
Received: May 12, 2010
Revised: July 15, 2010
Published online: September 10, 2010
95 (89)
56 (44)
51 (50)
Keywords: carbene ligands · heterogeneous catalysis ·
.
hollow spheres · palladium · Strecker reaction
8
9
68 (65)
91 (86)
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In conclusion, organometallic hollow spheres bearing
{(nhc)2Pd} species were prepared for the first time by a
template-free route using tetraimidazolium building blocks
and palladium acetate. Their physical structure and properties
were characterized by SEM, TEM, microtoming experiments,
and TGA, and their chemical composition by EDS, solid-
phase 13C NMR spectroscopy and EA. The organometallic
hollow spheres demonstrate excellent activities as heteroge-
neous catalysts in one-pot three-component Strecker reac-
tions of ketones. We believe that this approach can be
extended to the development of more diverse NHC-based
heterogeneous catalysts.
Experimental Section
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All SEM and EDS-mapped images were taken with an FE-SEM
(JSM6700F). The TEM images were taken with a JEOL 2100F unit
operated at 200 kV. Solid-phase 13C NMR spectra were recorded on a
400 MHz Solid State Bruker DSX NMR spectrometer at Korea Basic
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Synthesis of OMHS: Building block 2 (50 mg, 0.043 mmol) was
dissolved in DMF (5 mL) by sonication. Palladium acetate (19 mg,
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