Angewandte
Chemie
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ular dynamics, which uses a combined molecular dynamics/
molecular mechanics (MD/MM) approach. The lowest energy
4·3d assembly assisted by a combination of C Br···Cl and
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N H···Cl bonds with charge-assisted p-stacking interactions.
coconformation obtained by MD/MM (4·3d-A) was then
In conclusion, halogen bonding has been exploited for the
first time in the assembly of an interpenetrated molecular
system. Halogen bonding has been demonstrated to effect
and enhance the strength of chloride ion templated pseudo-
rotaxane formation between a 2-bromo-functionalized imi-
dazolium threading component and an isophthalamide mac-
rocycle, as compared to hydrogen-bonded pseudorotaxane
analogues. In addition, whereas a cooperative 4,5-imidazo-
lium hydrogen-bonding chloride ion chelation effect facili-
tates the formation of pseudorotaxane 4·3c, the strongly
linear nature of halogen bonding negates such a cooperative
effect taking place with 4,5-dibromoimidazolium chloride
derivative 3e, and, as a consequence, interpenetration does
not occur. Work toward the construction of interlocked
halogen-bonded host systems for anion recognition applica-
tions is currently underway.
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DFT-optimized (Figure 5). The C Br···Cl halogen-bond
Figure 5. DFT-optimized structure of 4·3d (co-conformation 4·3d-A)
Received: March 23, 2010
Published online: June 22, 2010
showing the linear C Br···ClÀ halogen bond (black dashed line) and
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the N H···ClÀ hydrogen bonds (yellow dashed lines). Br brown, C gray,
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N blue, O red. Imidazolium alkyl chains are shown in orange, and the
chloride ion is shown as a green sphere.
Keywords: anions · halogen bonding · pseudorotaxanes ·
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supramolecular chemistry · template synthesis
interaction is present with a distance of 2.92 ꢁ (RBr–Cl
0.81) and a bond angle of 174.88. As observed for other
related pseudorotaxane systems,[27] the imidazolium deriva-
tive also partakes in charge-assisted p-stacking interactions
with the hydroquinone units of macrocycle 4. The chloride
anion establishes hydrogen bonds with the amide protons of
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the isophthalamide with H···ClÀ distances 2.50 ꢁ and 2.49 ꢁ.
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The strength of the C Br···Cl interaction in 3d and
4·3d-A was evaluated through a natural bond order (NBO)
population analysis carried out for these two molecules and
3d* (Table S1 in the Supporting Information).[32] These
studies demonstrate that the magnitude of the “s-hole” in
3d*, 3d, and 4·3d is greater than that in CF3Br. The natural
charges calculated for these compounds (Table S1 in the
Supporting Information) follow the same trend. Wiberg bond
indices (WI)[33] were also calculated (Table S2 in the Support-
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ing Information). The C Br distance in 3d is longer and the
respective WI is smaller than in 3d*, thus implying that this
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bond is weakened by the C Br···Cl interaction whose WI is
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rather large (0.3401), almost half of the respective C Br value
(0.8477). Although halogen bonds are normally considered to
be noncovalent interactions, the relatively large WI indicates
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that the C Br···Cl interaction is strong, with relevant
covalent character. The covalent contribution for strong
halogen bonds was previously identified, based on ab initio
and quantum theory of atoms in molecules (QTAIM)
analysis, by Zou and co-workers.[34] On forming the pseudor-
À
otaxane 4·3d-A, the C Br bond becomes stronger (WI =
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1.0248), whereas the C Br···Cl halogen bonding is weakened
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(WI = 0.1335) because of the new N H···Cl interaction.
Nonetheless, the C Br···Cl interaction is still strong enough
to maintain the pseudorotaxane arrangement, which is
consistent with the experimental data. In summary, all
calculations support the formation of the pseudorotaxane
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[19] A. Mele, P. Metrangolo, H. Neukirch, T. Pilati, G. Resnati, J. Am.
[20] M. G. Sarwar, B. Dragisic, S. Sagoo, M. S. Taylor, Angew. Chem.
2010, 122, 1718; Angew. Chem. Int. Ed. 2010, 49, 1674.
Angew. Chem. Int. Ed. 2010, 49, 5322 –5326
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