Cu2O: A versatile reagent for base-free direct synthesis of nhc-copper complexes and decoration of 3D-MOF with coordinatively unsaturated NHC-copper species

Article abstract of DOI:10.1021/om900768w

A new and direct synthetic route for N-heterocyclic carbene (NHC)-copper complexes was disclosed by heating imidazolium halides with Cu₂O. The related synthetic procedure was quite simple and the product easily separable from the reaction mixture by simple filtration. Ultimately, this reaction was successfully applied to decorate three-dimensional (3D)-metal organic frameworks (MOFs) with coordinatively unsaturated NHC-copper species

Full text of DOI:10.1021/om900768w

1518 Organometallics 2010, 29, 1518–1521
DOI: 10.1021/om900768w
Cu2O: A Versatile Reagent for Base-Free Direct Synthesis of
NHC-Copper Complexes and Decoration of 3D-MOF with
Coordinatively Unsaturated NHC-Copper Species
Jiseul Chun,† Han Sol Lee,† Il Gu Jung,† Soon Won Lee,† Hae Jin Kim,‡ and Seung Uk Son*,†
†Department of Chemistry and Department of Energy Science, Sungkyunkwan University, Suwon 440-746,
Korea, and ‡Korea Basic Science Institute, Daejeon 350-333, Korea
Received September 3, 2009
Scheme 1. Synthetic Approach for Active Metal Site-Containing
MOFs: (a) Assembly of the Active Metal Site-Containing Build-
ing Blocks; (b) Formation of the Active Metal Sites at Nodes in the
Self-Assembly Process; (c) Post-treatment of MOF with Metal
Reagents; (d) in Situ Formation of Active Metal Sites during
Construction of MOFs Using Two-Metal Sources
Summary: A new and direct synthetic route for N-heterocyclic
carbene (NHC)-copper complexes was disclosed by heating
imidazolium halides with Cu2O. The related synthetic proce-
dure was quite simple and the product easily separable from the
reaction mixture by simple filtration. Ultimately, this reaction
was successfully applied to decorate three-dimensional (3D)-metal
organic frameworks (MOFs) with coordinatively unsaturated
NHC-copper species.
Incorporation of unsaturated metal sites into supramole-
cular networks is an important issue because the resultant
metal-decorated networks can be applied as specific cata-
lysts1 or gas-storage materials.2 Typically, in the literature,
three approaches have been applied to achieve this goal.3-5
As illustrated in Scheme 1, the first method involves pre-
paration of unsaturated metal-containing building blocks
with additional free coordination modes and construction of
the supramolecular networks through a self-assembly pro-
cess3 (Scheme 1a). In this case, significant time is required for
synthesis of the delicately designed building blocks, and the
prepared building blocks must be stable in the self-assembly
process. In the second method, during assembly of the
supramolecular networks between the conventional building
blocks and metal connectors, active metal sites are generated
in the nodes of the supramolecular networks4 (Scheme 1b).
However, obtainment of such active metal sites within the
supramolecular networks is seldom predictable. In the third
method, after obtaining the porous networks without active
metal sites, the supramolecular networks are post-treated
with target metal sources.5 In this case, homogeneous dis-
tribution of the active metal sites requires clarification, as the
unidentified metal solids can be deposited either inside or on
the surface of the supramolecular networks. In addition, the
supramolecular networks should be stable enough during the
reaction with metal sources (Scheme 1c). An alternative and
ideal route would be in situ decoration of supramolecular
networks with metal sites during the construction process of
networks, as displayed in Scheme 1d. Usually, more than two
kinds of metal sources have been used to achieve the active
metal site-decorated networks. However, known examples
of this approach are quite rare.6
Recently, we reported the formation of a 2D-supramolecular
network via assembly of 1,3-bis(4-carboxy-2,6-diisopropylphe-
nyl)imidazolium chloride, L1, with copper(II) nitrate.7 During
the assembly process, copper ions formed a paddle wheel struc-
ture and functioned as the connector. In addition, copper(II) ions
were converted to the copper(I) species, while unexpectedly,
N-heterocyclic carbene (NHC)-copper complexes formed within
the network. The resultant basic skeleton of the network was
*Corresponding author. E-mail: sson@skku.edu. Tel: þ82-31-299-
4572. Fax: þ82-31-290-5932.
(1) (a) Wu, C.-D.; Lin, W. Angew. Chem., Int. Ed. 2007, 46, 1075.
(b) Tannenbaum, R. Chem. Mater. 1996, 6, 550.
ꢀ
(2) Dinca, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766.
(3) (a) Kitaura, R.; Onoyama, G.; Sakamoto, H.; Matsuda, R.; Noro,
S.-I.; Kitagawa, S. Angew. Chem., Int. Ed. 2004, 43, 2684. (b) Oh, M.;
Carpenter, G. B.; Sweigart, D. A. Acc. Chem. Res. 2004, 37, 1. (c) Cho, S.-
H.; Ma, B.; Nguyen, S. T.; Hupp, J. T.; Albrecht-Schmitt, T. Chem. Commun.
2006, 2563. (d) Son, S. U.; Reingold, J. A.; Kim, S. B.; Carpenter, G. B.;
Sweigart, D. A. Angew. Chem., Int. Ed. 2005, 44, 7710. (e) Suslick, K. S.;
Bhyrappa, P.; Chou, J.-H.; Kosal, M. E.; Nakagaki, S.; Smithenry, B. W.;
Wilson, S. R. Acc. Chem. Res. 2005, 38, 283. (f) Kaye, S. S.; Long, J. R. J.
Am. Chem. Soc. 2008, 130, 806.
(4) (a) Demessence, A.; D’Alessandro, D. M.; Foo, M. L.; Long, J. R.
ꢀ
J. Am. Chem. Soc. 2009, 131, 8784. (b) Dinca, M.; Dilly, A.; Liu, Y.; Brown,
G. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876.
€
(c) Yazaydn, A. O.; Benin, A. I.; Faheem, S. A.; Jakubczak, P.; Low, J. J.;
Willis, R. R.; Snurr, R. Q. Chem. Mater. 2009, 21, 1425.
(5) (a) Wu., C.-D.; Hu, A.; Zhang, L.; Lin, W. J. Am. Chem. Soc.
2005, 127, 8940. (b) Mulfort, K. L.; Hupp, J. T. J. Am. Chem. Soc. 2007,
(6) Szeto, K. C.; Prestipino, C.; Lamberti, C.; Zecchina, A.; Bordiga,
S.; Bjørgen, M.; Tilset, M.; Lillerud, K. P. Chem. Mater. 2007, 19, 211.
(7) Chun, J.; Jung, I. G.; Kim, H. J.; Park, M.; Lah, M. S.; Son, S. U.
Inorg. Chem. 2009, 48, 6353.
ꢀ
129, 9604. (c) Dinca, M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 11172.
(d) Liu, Y.; Kravtsov, V. C.; Larsen, R.; Eddaoudi, M. Chem. Commun. 2006,
1488.
r
pubs.acs.org/Organometallics
Published on Web 03/16/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 7, 2010 1519
strictly limited to a 2D-network due to the unique chemical pro-
perties of the copper(II) nitrate as connector. Hence, to achieve
more diverse structures and a more generalized synthetic route for
active metal site-containing networks, an improved and syste-
matic protocol was needed. To this end, it was rationalized that
two kinds of independent metal sources were compulsory: the
first to induce construction of the supramolecular networks by
connecting the building blocks, and the second to form the active
metal sites by reaction with the building blocks (Scheme 1d).
To realize this approach, new copper reagents having
reasonable reactivity toward imidazolium salts were requisite
to form the NHC-copper complex. Usually, in the literature,
NHC-copper complexes are prepared via two synthetic
routes:8,9 (i) treatment of imidazolium salts with an appro-
priate base and sequential reaction with copper sources to
yield the NHC-copper complexes;8 (ii) reaction of silver oxide
(Ag2O) with imidazolium salts to form the NHC-silver com-
plex, whereby silver moieties are replaced with copper sources
to form the NHC-copper complexes9 (Scheme 2).
In contrast to the literature methods, during the screening
of the reactivity of diverse copper reagents toward imidazo-
lium salts in the absence of base, we found that Cu2O reacted
directly with the imidazolium salts to form the NHC-copper
complex.10 As far as we are aware, this discovery was not
reported in the literature and can beapplied asa new and more
general synthetic route for copper-NHC complexes. Table 1
summarizes the reactivity of Cu2O toward several imidazo-
lium salts to form the corresponding NHC-copper complexes.
Cu2O showed very similar chemical properties to Ag2O
toward the imidazolium salts. It has been well established
that Ag2O possess good reactivity toward imidazolium
halides in the absence of base to form NHC-silver halide
complexes.11 As shown in entries 1-5 in Table 1, when the
counterions were halides, the NHC-copper complexes were
successfully formed in good yields, indicating that the cop-
per-halide bond formation is an important driving force in
this reaction. It is noteworthy that the same behavior was
observed for Ag2O.11 Powdery Cu2O is not soluble in con-
ventional organic solvents; thus, isolation of the product
Scheme 2. Synthetic Routes for NHC-Copper Complexes
from the reaction mixture was quite straightforward: simple
filtration to remove the unreacted Cu2O and evaporation of
the solvent to result in formation of microcrystalline solids.
Recently, 1,3-bis(2,6-diisopropylphenyl)imidazoline-copper
halides, denoted as (IPr)CuX, have shown unprecedented
reactivity in diverse organic transformations.12 Presently,
1,3-bis(2,6-diisopropylphenyl)imidazoline-copper halides
are also commercially available.13 Thus, we believe that
Cu2O routes can be effectively applied for mass production
of these complexes. In addition, in this work, two new NHC-
copper complexes (compounds 1 and 2), in entries 6 and 7 in
Table 1, were prepared and fully characterized.10 Figure S1
in the Supporting Information shows the single-crystal
X-ray structure of compound 2.14
Next, we applied the reaction between imidazolium salts and
Cu2O for the decoration of a 3D-supramolecular network with
coordinatively unsaturated NHC-copper moieties. Using Cu2O
as an NHC-Cu inducing reagent in the self-assembly process of
L1, the systematic approach for decoration of supramolecular
networks with NHC-copper species was developed; Scheme 3
summarizes the overall observations.
During screening of the diverse cationic connectors in the
self-assembly of L1, a 3D-network was formed when cerium-
(III) nitrate was used as a connector.15 As shown in Figure 1,
two cerium ions coordinate to eight carboxylate groups in the
building block. Four carboxylates were coordinated to one
cerium ion, and the other four carboxylates functioned as
bridging ligands by coordinating with the two cerium ions.
The coordination pattern of the eight building blocks resulted
in formation of four helical chains around two cerium ions.
ꢁ
(8) (a) Dıez-Gonzalez, S.; Stevens, E. D.; Nolan, S. P. Chem. Com-
mun. 2008, 4747. (b) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org.
Lett. 2003, 5, 2417.
(9) (a) Arnold, P. L.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C.
Chem. Commun. 2002, 2340. (b) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J.
Am. Chem. Soc. 2003, 125, 12237. (c) Hu, X.; Castro-Rodriguez, I.; Hu, X.;
Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755.
(d) Larsen, A. O.; Leu, W.; Oberhuber, C. N.; Campell, J. E.; Hoveyda, A. H.
J. Am. Chem. Soc. 2004, 126, 11130. (e) Alexakis, A.; Winn, C. L.; Guillen,
F.; Pytkowicz, J.; Roland, S.; Mangeney, P. Adv. Synth. Catal. 2003, 345,
345.
(12) Selected examples: 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. (c) Díez-Gonzalez, S.; Stevens, E. D.; Nolan, S. P.
(10) Typical procedure for the synthesis of NHC-copper complexes:
imidazolium salt (0.19 mmol) and Cu2O (0.14 mmol) in 4 mL of dioxane
were heated at 100 °C for 16 h. Then the reaction mixture was cooled to
room temperature. The unreacted Cu2O was removed by filtration, and
the remaining solvent was evaporated to form precipitates. The solid was
washed with water to remove any salts and then dried under vacuum.
The spectroscopic characterization data of (IPr)CuX in entries 1-3
matched well with those in the literature (ref 12). Characterization data
of compound 1: 1H NMR (300 MHz, CDCl3) δ 7.60 (s, 4H), 7.11 (s, 2H),
2.45 (septet, J = 6.9 Hz, 4H), 1.27 (d, J = 6.9 Hz, 12H), 1.20 (d, J = 6.9,
12H) ppm; 13C NMR (75 MHz, CDCl3) δ 180.6, 147.8, 134.2, 133.9,
123.4, 97.6, 28.8, 24.7, 23.8 ppm. Anal. Calcd for C27H34N2I2CuCl: C,
43.86; H, 4.63. Found: C, 43.91; H, 4.48. Characterization data of
compound 2: 1H NMR (300 MHz, CDCl3) δ 7.99 (s, 4H), 7.19 (s, 2H),
4.00 (s, 6H), 2.58 (septet, J = 6.9 Hz, 4H), 1.33 (d, J = 6.6 Hz, 12H), 1.27
(d, J = 6.9, 12H) ppm; 13C NMR (75 MHz, CDCl3) δ 180.2, 166.4,
146.2, 137.9, 132.3, 125.8, 123.3, 52.5, 29.0, 24.7, 23.8 ppm. Anal. Calcd
for C31H40O4N2CuCl: C, 61.68; H, 6.68. Found: C, 61.70; H, 6.60.
(11) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978.
Chem. Commun. 2008, 4747. (d) Liu, J.; Zhang, R.; Wang, S.; Sun, W.; Xia,
C. Org. Lett. 2009, 11, 1321.
(13) Catalogue of Aldrich Chemical Co. and STREM Chemicals Inc.
(14) The crystals of compound 1 have a plate-like morphology and
were not suitable for single-crystal X-ray analysis. Crystallographic data
for compound 2: Mr = 603.64, monoclinic, space group C2/c, a =
15.847(4) A, b = 9.503(2) A, c = 21.959(4) A, β = 106.577(6)o, V =
3170(1) A3, Z = 4, F = 1.265 g cm-3, completeness 96.3%, 13 113
collected reflections, 3818 crystallographically independent reflections
(Rint = 0.0560), R1 = 0.0557 (I > 2σ(I)), wR2 = 0.1509 (all data).
(15) L1 (0.113 g, 0.220 mmol) and cerium nitrate (0.115 g, 0.265
mmol) were dissolved in DMF (4 mL), and the reaction mixture was
heated at 110 °C for 3 days. The colorless crystal was formed on the wall
of the vial. Crystal data for (L1-Cl)2CeNO3: Mr = 1153.31, monoclinic,
space group P21/c, a = 14.868(4) A, b = 25.174(6) A, c = 22.523(6) A,
β = 98.458(4)o, V = 8338(4) A3, Z = 4, F = 0.919 g cm-3, completeness
99.1%, 30 887 collected reflections, 8508 crystallographically indepen-
dent reflections (Rint = 0.0794), R1 = 0.0500 (I > 2σ(I)), wR2 = 0.1446
(all data).

1520 Organometallics, Vol. 29, No. 7, 2010
Table 1. Direct Synthesis of the NHC-Copper Complexes by Reaction of Imidazolium Salts with Cu2Oa
Chun et al.
a Reaction conditions: 0.19 mmol of imidazolium salts, 0.75 equiv of Cu2O, 5 mL of dioxane, 100 °C, 16 h. b Isolated yield. c No reaction.
Scheme 3. Systematic Approach for Decoration of the 3D-Supramolecular Network with NHC-Copper Moieties
A more detailed structure of the helical chains is dis-
played in Figure S2 in the Supporting Information. Inter-
estingly, between the helical chains, big channels were
formed, as shown in Figure 2. The imidazolium salts in
the building blocks were intact during the self-assembly
process. The two cerium ions, eight carboxylates, and four
imidazolium salts have þ6, -8, and þ4 charges, respec-
tively. For the charge balance, two nitrates having a -2
charge per two cerium ions existed in the channels to
compensate for the positive charge of the basic network
skeleton, which was supported by elemental analysis.
Finally, an attempt was made to construct 3D-supramole-
cular networks using L1 and cerium nitrate in the presence of
Cu2O. The observed network was nearly identical to the
structure formed without Cu2O. However, NHC-copper spe-
cies were formed inside the 3D supramolecular networks.16
The incorporated copper species are displayed as blue balls in
Figure 3. Although the channel structure is not simple due to
the helical framework, the narrowest pore size was calculated
as ca. 6.5 ꢀ 8.4 A.
Interestingly, the NHC-copper moieties were formed for
every two building blocks (not all building blocks) in each
helical chain. This can be rationalized considering the charge
balance. In the original 3D-network formed by assembly of
Ce3þ with L1, the network skeleton has a positive charge,
which was compensated for by incorporation ofnitrates inside
the channels. By formation of an NHC species for every two
building blocks in the network, all positive charges of the
skeleton were compensated; two cerium ions (þ6); eight
carboxylates (-8); two imidazolium ions (þ2); two NHC-
copper moieties (0). If NHC-copper moieties were formed at
every building block, the network skeleton should have a
negative total charge, which implies that there should be
suitable countercations inside the network for compensation
of the negative charge.
(16) Cu2O (11.7 mg, 0.081 mmol), L1 (0.113 g, 0.220 mmol), and
cerium nitrate (0.115 g, 0.265 mmol) were suspended in DMF (4 mL),
and the reaction mixture was heated at 110 °C for 3 days. The pale
brown, transparent crystals were formed on the wall of the vial. Crystal
data for (L1-Cl)(L1-Cl-H)CeCuCl: Mr = 1189.28, monoclinic, space
group P21/c, a = 15.483(2) A, b = 24.962 2) A, c = 22.355(2) A,
β = 99.575(6)o, V = 8519(1) A3, Z = 4, F = 0.927 g cm-3, completeness
98.2%, 28 417 collected reflections, 5003 crystallographically indepen-
dent reflections (Rint = 0.1078), R1 = 0.0666 (I > 2σ(I)), wR2 = 0.1892
(all data).
In conclusion, we found a new and direct synthetic route
for the synthesis of NHC-copper halide complexes using

Communication
Organometallics, Vol. 29, No. 7, 2010 1521
Figure 1. 3D-network structure formed by assembly of L1 and Ce3þ (the bridging and nonbridging carboxylates were differentiated by
red and green colors, respectively).
Figure 2. Cartoon and X-ray structure of the 3D-supramole-
cular network formed by self-assembly of L1 with Ce3þ. Violet
balls indicate cerium ions. Nitrate ions were omitted for clarity.
Cu2O and successfully applied it to the incorporation of
coordinatively unsaturated NHC-copper species inside the
supramolecular structure. The self-assembly between cerium
nitrates and imidazolium dicarboxylates resulted in the
formation of a 3D porous supramolecular structure. Using
two kinds of metal sources, cerium nitrates and Cu2O, the
porous structure was in situ decorated with NHC-copper
Figure 3. Cartoon and X-ray structures of the NHC-copper deco-
species during the construction process of the network. We
rated 3D-network. Blue and violet balls indicate incorporated
believe that the strategy in this study can be extended to the
construction of more diverse supramolecular systems by
copper in building blocks and cerium connectors, respectively.
designing appropriate building blocks containing imidazo-
lium halides.
(Priority Research Centers Program). H.J.K. thanks the
Hydrogen Energy R&D Center, a 21st Century Frontier
R&D Program.
Acknowledgment. This work was supported by grants
NRF-2009-0084799 and R31-2008-000-10029-0 (WCU
program) through the National Research Foundation of
Korea funded by the Ministry of Education, Science and
Technology. I.G.J. is thankful for grants NRF-2009-0094024
Supporting Information Available: X-ray structure of com-
pound 2, cartoon of helical chain and crystallographic data
(CCDC 745333, 745334, and 745622). This material is available
free of charge via the Internet at http://pubs.acs.org.