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
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 7718 –7722