A new route to supramolecular isomers via molecular templating: Nanosized molecular polygons of copper(I) 2-methylimidazolates

Article abstract of DOI:10.1021/ja045249l

A new facile synthetic strategy successfully leads to the isolation of two polygons of high numbers of sides constructed by simple, bent imidazolate bridging ligands and two-coordinate Cu^I ions upon templating of circular organic molecules, which were characterized by crystallography. Copyright

Full text of DOI:10.1021/ja045249l

Published on Web 09/25/2004
A New Route to Supramolecular Isomers via Molecular Templating:
Nanosized Molecular Polygons of Copper(I) 2-Methylimidazolates
Xiao-Chun Huang, Jie-Peng Zhang, and Xiao-Ming Chen*
School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, P.R. China
Received August 6, 2004; E-mail: cescxm@zsu.edu.cn
The design and preparation of supramolecular macrocyclic
architectures has attracted intense interest in the past decade.
Successful strategies have been developed for the construction of
metal macrocyclic structures with triangular and square shapes,1
but molecular polygons of higher numbers of sides are thus far
scarce; just a few cases (up to octagon) are known,2 although a
Figure 1. Perspective drawing of 1 (color code: Cu, red; C, green; H,
series of mesoscopic metallocycles without crystal structures has
also been reported.3 In principle, different molecular polygons can
be rationally constructed by proper selection of corners (with an
appropriate angle) and linear components; however, it is still a
challenge to control the formation of such supramolecular isomers
in coordination polymers.4
Simple imidazolate derivatives are bent exo-bidentate ligands,
and can be expected to show a linear or slightly bent im-CuI-im
geometry in binary copper(I) imidazolates which may have different
architectures or supramolecular isomers in the 1:1 metal/ligand ratio.
Scheme 1 illustrates some supramolecular isomers that may be
generated by utilizing an angular ligand (M-L-M ca. 135-145°)
and a linear two-coordinate metal ion (such as CuI): polygons
(octagon for 135°, nonagon for 140°, and decagon for 145°), helices,
and zigzag chains. Additional supramolecular isomers include larger
rings (nonplanar), and catenanes may also be possible. However,
only one-dimensional (1D) metal-organic polymeric architectures
rather than the discrete polygonal ones have been observed for CuI
imidazolates.5
gray; N, blue).
hydrophobic, especially circular, organic molecules such as toluene,
benzene, and cyclohexane in the reaction system. This speculation
did lead to the isolation of predesigned, organically cornered,
uniform molecular octagons ([Cu8(mim)8]‚toluene (2),7 as well as
[Cu8(mim)8]‚benzene and [Cu8(mim)8]‚cyclohexane (see Supporting
Information (SI)), which were generated with the corresponding
solvents. Furthermore, higher molecular decagons [Cu10(mim)10]‚
(p-xylene)2 (3)7 and [Cu10(mim)10]‚(naphthalene)2 (see SI) have also
been obtained by using larger organic molecules as templates.
The chainlike 1 and the above neutral molecular polygons are
isomers in a broad sense of supramolecular isomerism. In the crystal
structures of 1-3,8-10 the coordination geometries of CuI atoms
are similar, and each CuI atom is ligated in a basically linear fashion
by two N atoms from the µ-bridged mim groups (Cu-N 1.851-
(4)-1.875(4) Å, N-Cu-N 170.5(2)-176.5(3)°).
The neutral, flattened octagon of 2 consists of eight CuI ions,
eight mim anions with a toluene molecule in the cavity (Figure 2).
Such molecular octagon is the structurally characterized uniform
(metal-to-ligand ration being 1:1 in our case) polygon of the highest
number of sides, and only the structure of a nonuniform octagon is
known hitherto.2h The inner and outer diameters of the octagon are
ca. 0.99 and 1.86 nm, respectively. The octagon is bisected by a
crystallographic mirror plane through the top and bottom methyl
groups of mim ligands in Figure 2. Most of the mim rings are
basically coplanar (deviations ca. 4.3-10.4°) with the octagon plane
defined by the CuI atoms, whereas one mim (at the bottom) is
twisted by 61.6° from the plane. The octagons further stack to a
thick (ca. 5.5 Å) sheet, via short intermolecular Cu‚‚‚Cu contacts
(2.786(1)-2.879(1) Å), which has only weak van der Waals
interactions with the adjacent sheets (see SI). Both [Cu8(mim)8]‚
benzene and [Cu8(mim)8]‚cyclohexane are isostructural to 2 (see
SI), but the solvent molecules are disordered.
A pair of CuI atoms and a pair of mim groups (the top and bottom
ones) are orientated in two possible orientations in the structure of
310 which results in two configurations of decagons in 3. One is
very flattened (Figure 3) in which the mim rings have deviations
ca. 9.7-15.0° with the plane defined by the CuI atoms, whereas
the other is more distorted from an ideal decagon in which the top
and bottom mim rings are nearly perpendicular (ca. 72°) to the
decagonal plane (see SI). This larger decagon has the inner and
outer sizes of ca. 1.36 and 2.34 nm, respectively. The decagons
are stacked via short Cu‚‚‚Cu contacts (2.828(1)-2.89(1) Å) to a
3D network featuring 1D channels (diameter ca. 0.5 nm). Disordered
Scheme 1. Schematic Representation of Four Structural
Supramolecular Isomers Possible for an Angular Ligand
(135-145°) and a Linear Spacer: (a) Zigzag Chain, (b) Helix, (c)
Octagon, and (d) Decagon
Following our recent report of a hydrothermal approach of tuning
the reduction degree of CuII ions into CuI for generation of mixed-
valence CuI/CuII imidazolate polymers,6 we could now synthesize
a Cu(I) 2-methylimidazolate (mim) [Cu(mim)]n (1),7 which exhibits
a zigzag chainlike structure (Figure 1). In the structure of 1,8 the
CuI-L-CuI angles vary in the range of 145-152°, and the dihedral
angles of a pair of mim ligands bound to a CuI atom vary in the
range of 52°-86°, implying the flexibility for the formation of other
possible polymeric structures via tuning of the configuration. A
question that arises here is what can induce the formation of a
circular structure? Geometrically, the formation of a ring structure
requires the hydrophobic mim methyl groups pointing inward the
ring. It can therefore be speculated that, at a certain orientation, a
polygonal structure may possibly be formed by templating with
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10.1021/ja045249l CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have successfully isolated two different polygons
of higher numbers of sides constructed by simple, bent exo-bidentate
organic ligands and two-coordinate CuI ions in the presence of
different molecular templates. This work represents a new, facile
route for synthesis of organically cornered polygons via templates,
which may possibly be extended to different molecular polygons
with appropriate metal ions and exo-bidentate ligands, upon
templating with organic molecules in different sizes.
Acknowledgment. This work was supported by NSFC (No.
20131020) and Ministry of Education of China.
Supporting Information Available: Synthesis details and ad-
ditional plots. An X-ray crystallographic file in CIF format for the
structural determination of 1-3. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) For examples, see: (a) Romero, F. M.; Ziessel, R.; Dupontgervais, A.;
Vandorsselaer, A. Chem. Commun. 1996, 551-553. (b) Fujita, M.; Sasaki,
O.; Mitsuhashi, T.; Fujita, T.; Yazaki, J.; Yamaguchi, K.; Ogura, K. Chem.
Commun. 1996, 1535-1536. (c) Slone, R. V.; Hupp, J. T.; Stern, C. L.;
Albrechtschmitt, T. E. Inorg. Chem. 1996, 35, 4096-4097. (d) Stang, P.
J.; Cao, D. H.; Saito, S.; Arif, A. M. J. Am. Chem. Soc. 1995, 117, 6273-
6283.
(2) For examples, see: (a) Hasenknopf, B.; Lehn, J. M.; Kneisel, B. O.; Baum,
G.; Fenske, D. Angew. Chem., Int. Ed. Engl. 1996, 35, 1838-1840. (b)
Campos-Fernandez, C. S.; Clerac, R.; Koomen, J. M.; Russell, D. H.;
Dunbar, K. R. J. Am. Chem. Soc. 2001, 123, 773-774. (c) Hasenknopf,
B.; Lehn, J.-M.; Boumediene, N.; Dupont-Gervais, A.; van Dorsselaer,
A.; Kneisel, B.; Fenske, D. J. Am. Chem. Soc. 1997, 119, 10956-10962.
(d) Stang, P. J.; Persky, N.; Manna, J. J. Am. Chem. Soc. 1997, 119, 4777-
4778. (e) Newkome, G. R.; Cho, T. J.; Moorefield, C. N.; Baker, G. R.;
Cush, R.; Russo, P. S. Angew. Chem., Int. Ed. 1999, 38, 3717-3721. (f)
Ali, M. M.; MacDonnell, F. A. J. Am. Chem. Soc. 2000, 122, 11527-
11528. (g) Abourahma, H.; Moulton, B.; Kravtsov, V.; Zaworotko, M. J.
J. Am. Chem. Soc. 2002, 124, 9990-9991. (h) Jones, P. L.; Byrom, K. J.;
Jeffery, J. C.; McCleverty, J. A.; Ward, M. D. J. Chem. Soc., Chem.
Commun. 1997, 1361-1362.
Figure 2. Perspective drawings of 2: (a) top-view, (b) side-view.
(3) (a) Jiang, H.; Lin, W. J. Am. Chem. Soc. 2003, 125, 8084-8085. (b) Jiang,
H.; Lin, W. J. Am. Chem. Soc. 2004, 126, 7426-7427.
(4) (a) Masaoka, S.; Tanaka, D.; Nakanishi, Y.; Kitagawa, S. Angew. Chem.,
Int. Ed. 2004, 43, 2530-2534. (b) Moulton, B.; Zaworotko, M. J. Chem.
ReV. 2001, 101, 1629-1658. (c) Leininger, S.; Olenyuk, B.; Stang, P. J.
Chem. ReV. 2000, 100, 853-907. (d) Fujita, M. Chem. Soc. ReV. 1998,
27, 417-425.
(5) Tian, Y.-Q.; Xu, H.-J.; Weng, L.-H.; Chen, Z.-X.; Zhao, D.-Y.; You, X.-
Z. Eur. J. Inorg. Chem. 2004, 1813-1816.
(6) Huang, X.-C.; Zhang, J.-P.; Lin, Y.-Y.; Yu, X.-L.; Chen, X.-M. Chem.
Commun. 2004, 1100-1101.
(7) A mixture of Cu(NO3)2‚3H2O (0.242 g, 1.0 mmol), Hmim (0.081 g, 1.0
mmol), aqueous ammonia (25%, 5 mL), and methanol (2 mL) was stirred
for 15 min in air, then transferred and sealed in a 23-mL Teflon reactor,
which was heated at 160 °C for 80 h. Upon cooling to room temperature
at a rate of 5 °C h-1, the resulting yellow needle crystals of 1 were isolated
(yield ca. 30%). Elemental analysis calcd (%) for 1 (C4H5CuN2): C 33.22,
H 3.48, N 19.37; found: C 33.18, H 3.54, N 19.32. The pale-yellow plate
crystals of 2 (yield ca. 15%) and 3 (yield ca. 35%) were produced by a
similar process that uses toluene (2 mL) and p-xylene (2 mL) in place of
methanol, respectively. Elemental analysis calcd (%) for 2 (C39H48
-
Cu8N16): C 37.50, H 3.87, N 17.94; found: C 37.45, H 3.95, N 17.91;
for 3 (C56H70Cu10N20): C 40.55, H 4.25, N 16.89; found: C 40.42, H
4.41, N 16.78. All of 1-3 are basically insoluble in common organic
solvent; 1 and 2 are relatively stable compared to 3 that is oxidized in a
few minutes upon exposure in air. Preliminary measurements of solids
1-3 showed photoluminescence at room temperature with the emission
peaks at 495, 512, and 515 nm, respectively.
Figure 3. Perspective drawings of 3: (a) top-view, (b) side-view.
p-xylene guests are located in the cavities, and one orientation of
some solvent guests is shown in Figure 3.
(8) Crystal data for 1 (C4H5CuN2) at 293 K: Monoclinic, C2/c, a ) 17.0610-
(15) Å, b ) 11.2990(10) Å, c ) 8.3845(7) Å, â ) 110.0780(10)°, V )
1518.1(2) Å3; Z ) 12, F ) 1.899 g cm-3, µ ) 4.164 mm-1, F(000) )
864, 2θmax ) 54.0°. Final residuals (for 98 parameters) were R1 ) 0.0314
for 1638 reflections with I g 2σ(I), R1 ) 0.0349, wR2 ) 0.0829, and S )
1.061 for all 4340 data.
(9) Crystal data for 2 (C39H48Cu8N16) at 123 K: Orthorhombic, Cmc21, a )
20.7600(14) Å, b ) 10.8497(7) Å, c ) 21.0042(14) Å, V ) 4731.0(5)
Å3; Z ) 4, F ) 1.754 g cm-3, µ ) 3.571 mm-1, F(000) ) 2504, 2θmax
) 56.0°. Final residuals (for 276 parameters) were R1 ) 0.0431 for 5321
reflections with I g 2σ(I), R1 ) 0.0531, wR2 ) 0.0978, and S ) 1.035
for all 15355 data.
Discrete supramolecular isomers were suggested to be thermo-
dynamically favored compared with the chain ones. The difficulty
in isolation of the desired polygons using imidazolates may be due
to the fast formation of the chains, indicating that the polygons are
disfavored in the kinetic point of view. On the basis of our
observation, we may suggest that the use of alkyl-containing mim
bridges and hydrophobic, circular template molecules should be
responsible for the formation of uniform polygons. It should also
be noted that no crystallographically characterized examples of two
supramolecular isomers of polygons of higher numbers of sides
containing the same organic and metal components have been
documented thus far.3
(10) Crystal data for 3 (C56H70Cu10N20) at 293 K: Monoclinic, C2/m, a )
14.4446(11) Å, b ) 27.559(2) Å, c ) 11.9914(9) Å, â ) 126.7750(10)°,
V ) 3823.6(5) Å3; Z ) 2, F ) 1.441 g cm-3, µ ) 2.766 mm-1, F(000)
) 1672, 2θmax ) 52.0°. Final residuals (for 219 parameters) were R1
0.0592 for 3838 reflections with I g 2σ(I), and R1 ) 0.1020, wR2
0.1869, S ) 0.994 for all 11807 data.
)
)
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