Concomitant formation of n-heterocyclic carbene-copper complexes within a supramolecular network in the self-assembly of imidazolium dicarboxylate with metal ions

Article abstract of DOI:10.1021/ic900846s

A new building block containing an imidazolium salt was synthesized and used for the construction of supramolecular networks with metal ions. We discovered the concomitant formation of the N-heterocyclic carbene-copper complex (CN = 2) in the selfassembly

Full text of DOI:10.1021/ic900846s

Inorg. Chem. 2009, 48, 6353–6355 6353
DOI: 10.1021/ic900846s
Concomitant Formation of N-Heterocyclic Carbene-Copper Complexes within a
Supramolecular Network in the Self-Assembly of Imidazolium Dicarboxylate with
Metal Ions
Jiseul Chun,† Il Gu Jung,† Hae Jin Kim,‡ Mira Park,§ Myoung Soo Lah,*,§ and Seung Uk Son*,†
†
Department of Chemistry and Energy Science, Sungkyunkwan University, Suwon 440-746, Korea, ‡Korea Basic
Science Institute, Daejeon 350-333, Korea, and §Department of Chemistry and Applied Chemistry, College of
Science Technology, Hanyang University, Ansan 426-791, Korea
Received May 1, 2009
A new building block containing an imidazolium salt was synthe-
sized and used for the construction of supramolecular networks
with metal ions. We discovered the concomitant formation of the
N-heterocyclic carbene-copper complex (CN = 2) in the self-
assembly of imidazolium dicarboxylates and copper nitrates in
N,N-dimethylformamide under heating. The proton in the 2 position
of the imidazolium salt was abstracted, and CuII was reduced to CuI
during the self-assembly process.
studies for applications of NHC-metal complexes have been
conducted.4a In this paper, we report on the synthesis and
self-assembly of imidazolium-based building blocks and the
unexpected concomitant formation of the NHC-copper
complex within the supramolecular networks. As far as the
authors are aware, this is the first example of the introduction
of an organometallic NHC-metal complex within a supra-
molecular network (Scheme 1).
Scheme 2 shows the synthetic pathway for the building
block L used in this study. First, the iodide group was
introduced at the para position of 2,6-diisopropyl-1-aniline.
Then, the diimine was prepared by reaction with 0.60 equiv of
glyoxal. Two iodide groups were then replaced with CO2Me
groups through palladium catalysis. The successive hydro-
lysis produced the new building block, L, in good isolated
yield.5 Diverse transition-metal salts such as zinc, manganese,
cobalt, and cadmium nitrates have been screened as con-
nectors in a variety of solvent systems to obtain the self-
assembled crystalline solid.
Self-assembly using the “predesigned building block” is a
powerful tool for the development of new solid materials
because it yields self-assembled architectures not only
with regular, well-defined inner structures but also with
tailored functionalities.1 In this regard, diverse functional
building blocks were designed.1,2 Recently, the introduction
of active metal sites into supramolecular networks has
been an important task because the resultant networks have
shown excellent interactions with small molecules such as
hydrogen.3
Among these trials, in the case of cadmium nitrate, we
could obtain a single-crystalline solid that formed the 1D
chain, as shown in Figure 1, where two carboxylates were
coordinated to a single cadmium ion and two chlorides
functioned as bridging ligands between the cadmium units.6
Through this, the empty space was formed between the two
building blocks (L; Figure 1a). The distances between the two
imidazole rings and two cadmium connectors were 10.2 and
N-Heterocyclic carbene (NHC) has been extensively stu-
died in the field of organometallics4 as a ligand comparable to
the conventional phosphine ligands capable of coordinating
with a wide range of transition metals. Additionally, diverse
*To whom correspondence should be addressed. E-mail: sson@skku.edu
(S.U.S.), mslah@hanyang.ac.kr (M.S.L.).
˚
17.6 A, respectively. Two N,N-dimethylformamide (DMF)
molecules were also trapped inside this space (Figure 1b). The
central imidazolium salts were intact, which indicates that
further treatment with a base and coordination of the metal
are needed for the introduction of the NHC-metal moiety
into the supramolecular networks.
The most impressive structure was obtained using copper
nitrate as a connector in DMF under heating at 110 °C. The
assembled supramolecular structure is displayed in Figure 2.6
(1) (a) Chatterjee, B.; Noveron, J. C.; Resendiz, M. J. E.; Liu, J.;
Yamamoto, T.; Parker, D.; Cinke, M.; Nguyen, C. V.; Arif, A. M.; Stang,
P. J. J. Am. Chem. Soc. 2004, 126, 10645. (b) Fujita, M.; Kwon, Y. J.;
Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151.
(2) Selected examples: (a) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.;
Jeon, Y.; Kim, K. Nature 2000, 404, 982. (b) Ma, L.; Lee, J. Y.; Li, J.; Lin, W.
Inorg. Chem. 2008, 47, 3955. (c) Wu, C.-D.; Hu, A.; Lin, Z.; Lin, W. J. Am.
Chem. Soc. 2005, 127, 8940. (d) Ma, L.; Mihalcik, D. J.; Lin, W. J. Am.
Chem. Soc. 2009, 131, 4610.
ꢀ
(3) Selected recent examples: (a) Dinca, M.; Long, J. R. Angew. Chem.,
ꢀ
Int. Ed. 2008, 47, 6766. (b) Horike, S.; Dinca, M.; Tamaki, K.; Long, J. R. J.
Am. Chem. Soc. 2008, 130, 5854.
(5) Conventional synthetic procedure for diarylimidazolium salts: Jung, I.
G.; Seo, J.; Chung, Y. K.; Shin, D. M.; Chun, S.-H.; Son, S. U. J. Polym. Sci.,
Part A: Polym. Chem. 2007, 45, 3042.
(4) (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440. (b)
Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776. (c) Herrmann, W.
A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(6) See the Supporting Information for crystallographic information.
r
2009 American Chemical Society
Published on Web 06/15/2009
pubs.acs.org/IC

6354 Inorganic Chemistry, Vol. 48, No. 14, 2009
Chun et al.
Scheme 1. Conventional Route for the Formation of NHC-Metal
Complexes from Imidazolium Salts (a) and Synthetic Pathways for
Supramolecular Networks Bearing NHC-Metal Complexes (b)
Figure 1. Self-assembled supramolecular structure between the building
block L and cadmium(II) nitrate (a) in DMF and a single-crystalline
X-ray structure (b) showing the trapping of two DMF molecules in the
cavity.
Scheme 2. Preparation of Building Block La
a (a) 1.1 equiv of I2, diethyl ether, overnight, 2.0 equiv of K2CO3; (b)
0.60 equiv of glyoxal, catalyst formic acid, methanol, 36 h, room
temperature; (c) 1.2 equiv of paraformaldehyde, 1.1 equiv of HCl (4.0
M aqueous solution), toluene, 110 °C, 2 h, room temperature, overnight;
(d) 3 mol % Pd(OAc)2, 12 mol % P(o-tolyl)3, 3 atm of CO, MeOH, 2.0
equiv of NEt3, 90 °C, 6 h; (e) excess (6 equiv) LiOH (0.10 M aqueous
solution), methanol, room temperature, 4 h.
Figure 2. Self-assembled supramolecular structure between L and cop-
per(II) nitrate in DMF: scheme of reaction (a); ORTEP drawing of the
structural unit with 30% thermal ellipsoids (b).
simple model reaction between1,3-bis(2,6-diisopropylphenyl)
imidazolium chloride and copper(II) nitrate (Scheme 3). As
expected, NHC-copper(I) chloride (2) was formed in DMF
under heating.7 Interestingly, the existence of halide as a
counteranion was critical to obtaining the NHC-copper
complexes.
Interestingly, there were two kinds of copper species within
the structure. The first was a copper paddlewheel structure
that functioned as a connector of building blockL (Figure 2a,
b). The overall structure is the two-dimensional undulating
grid structure, as shown in Figure 3a,b. The distances
between the two imidazole rings on the long and short sides
Ithas been reported that reduction of thecopper(II)species
˚
can be achieved via reaction of the amine under heating.8 To
were 23.4 and 11.3 A, respectively. The second copper species
was copper(I) chloride coordinating to the NHC coordina-
tion mode in the center of building block L. It is quite
interesting that the proton at the 2 position of the imidazo-
lium salts in L was abstracted during the self-assembly
process, concomitantly forming the organometallic NHC-
copper units. A further interesting point is the reduction of
the copper(II) species to a copper(I) species. We conducted a
(7) See the Supporting Information for details. (a) Jurkauskas, V.; Sadighi,
J. P.; Buchwald, S. L. Org. Lett. 2003, 5, 2417. (b) Liu, R.; Herron, S. R.;
Fleming, S. A. J. Org. Chem. 2007, 72, 5587.
(8) (a) Son, S. U.; Park, I. K.; Park, J.; Hyeon, T. Chem. Commun. 2004,
778. (b) Chen, X.-M.; Tong, M.-L. Acc. Chem. Res. 2007, 40, 162. (c) Hu, S.;
Chen, J.-C.; Tong, M.-L.; Wang, B.; Yan, Y.-X.; Batten, S. R. Angew.
Chem., Int. Ed. 2005, 44, 5471.

Communication
Inorganic Chemistry, Vol. 48, No. 14, 2009 6355
Scheme 3. Reaction of 1,3-Bis(2,6-diisopropylphenyl)imidazolium
Chloride with Copper(II) Nitrate in DMF under Heating
Among them, copper catalysis using the NHC-copper
complex has attracted continuous attention because cheaper
copper catalysis is expected to replace palladium with the
help of unprecedented electronic effects provided by NHC
ligands.11 In actuality, NHC-copper complexes have shown
excellent catalytic activities in hydrosilylation, cycloaddition,
enantioselective conjugated addition, and other such trans-
formations.11 On the other hand, to develop gas storage
materials, the incorporation of an unsaturated metal site
within the supramolecular network is important.3 However,
because unsaturation usually implies an instability of the
metal centers, their attainment can be capricious. To this
point, the strong donor ability of the NHC ligands can be a
good platform to incorporate and stabilize the unsaturated
metal centers. Thus, we believe that the discovery of the
concomitant formation of the NHC-copper complex (CN =
2) in the self-assembly of imidazolium dicarboxylates and
copper nitrates in DMF under heating can be applied not
only to form more diverse functional networks containing
NHC-copper units by the design of new building blocks with
diverse geometries but also to develop new heterogeneous
catalysts and gas storage materials.
Figure 3. Cartoon of the supramolecular network structure formed by
the assembly of L and copper(II) nitrate in DMF (a) and its X-ray
structure (b).
Acknowledgment. This work was supported by a Korea
Research Foundation grant funded by the Korean Gov-
ernment (MOEHRD, Basic Research Promotion Fund;
Grant KRF-2008-313-C00455). Dr. I. G. Jung thanks the
Hydrogen Energy R&D Center, a 21st century Frontier
R&D Program funded by the Ministry of Science and
Technology of Korea. The authors acknowledge PAL for
beamline use (2008-3063-10).
elucidate the chemical species playing the role of a reductant
and base, more specific evidence is required. However, the
above model reaction clearly verified that the reaction con-
ditions (heating of the imidazolium cloride and coppernitrate
in DMF) have a potential to generate the NHC-copper
complex. It is common to form the copper paddlewheel by
reaction of the carboxylate and copper ion.9 Thus, self-
assembly of dicarboxylates having imidazolium chloride with
copper nitrate in DMF is a good strategy to acquire a
supramolecular structure containing theNHC-copper units.
Recently, new catalytic organic transformations based on
NHC-metal complexes have been extensively studied.4,10
Supporting Information Available: Experimental general
procedures, detailed synthetic procedure of building block L,
characterization data of new compounds, and crystallographic
data (CCDC 719757 and 719758). This material is available free
of charge via the Internet at http://pubs.acs.org.
ꢁ
(9) (a) Agterberg, F. P. W.; Provo Kluit, H. A. J.; Driessen, W. L.;
Oevering, H.; Buijs, W.; Lakin, M. T.; Spek, A. L.; Reedijk, J. Inorg. Chem.
1997, 36, 4321. (b) The Cambridge Structural Database, release 1994: Allen,
F. H.; Kennard, O.; Taylor, R. Acc. Chem. Res. 1983, 16, 146.
(10) Selected examples: (a) Maishal, T. K.; Alauzun, J.; Basset, J.-M.;
Coperet, C.; Corriu, R. J. P.; Jeanneau, E.; Mehdi, A.; Reye, C.; Veyre, L.;
Thieuleux, C. Angew. Chem., Int. Ed. 2008, 47, 8654. (b) Hadei, N.;
Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. J. Org. Chem. 2005, 70,
8503.
(11) Selected examples: (a) Diez-Gonzalez, S.; Nolan, S. P. Angew. Chem.,
Int. Ed. 2008, 47, 8881. (b) Geng, J.; Lindqvist, J.; Mantovani, G.; Haddleton,
D. M. Angew. Chem., Int. Ed. 2008, 47, 4180. (c) Lee, J. M.; Park, E. J.; Cho,
S. H.; Chang, S. B. J. Am. Chem. Soc. 2008, 130, 7824. (d) Ntaganda, R.;
Bhartesh, D.; Macdonald, C. L. B.; Thadani, A. N. Chem. Commun. 2008, 46,
6200. (e) Matsumoto, Y.; Yamada, K.; Tomioka, K. J. Org. Chem. 2008, 73,
4578.