Chemistry Reference
In-Depth Information
2.1 Simple Coordination of Cucurbit[5]urils
with Metal Ions
The unsubstituted or substituted cucurbit[5]urils (Q[5] and SQ[5]s, respectively)
are the smallest members or homologs in the Q[
n
] family. The first known Q[5] is
decamethylcucurbit[5]uril (Me
10
Q[5]), which was reported in 1992 [
6
]. The host-
guest chemistry of the Q[5]s has received less attention than their larger homologs
because of the limited size of the portals and the capacity of their cavities [
7
-
10
].
To the best of our knowledge, the first reported crystal structure is a Q[5]-based
coordination complex with a molecular bowl confirmation by covering a barium
ion on one portal of a decamethylcucurbit[5]uril molecule, and the “bowls” further
construct a honeycomb-like supramolecular assembly through hydrogen bonding
and ion-dipole interactions (Fig.
2.1
) [
11
].
Unsubstituted Q[5] and its substituted derivatives SQ[5]s are the small-
est members of their respective homologous families. Although the smaller por-
tal size of Q[5]s inhibits the entry of most organic molecules into the central
cavity, and hence reduces the possible range of Q[5]-based host-guest inclu-
sion complexes, it offers a “concentrated” set of five portal carbonyl oxygens so
that larger metal ions are able to fully cover the portal and bind to all five oxy-
gens to yield a stable metal-Q[5] complex. Recent studies reveal that Q[5]s
have a strong tendency to coordinate with metal ions and form metal-ion-lidded
molecular capsules or molecular bowls because Q[5]s have the smaller por-
tal size and more concentrated portal carbony oxygens at portals, which could
directly coordinate with metal ions, including alkali, alkaline earth, transi-
tion, and lanthanide or uranium [
12
-
16
]. Moreover, different Q[5]s, such as
the unsubstituted Q[5], and some representative alkyl-substituted Q[5]s, like
Me
10
Q[5], dimethylcucurbit[5]uril (DMeQ[5]), 1,2,4-hexamethylcucurbit[5]uril
Fig. 2.1 a
Complex cation of [Me
10
Q[5](H
2
O)
3
Ba]
2
+
.
b
Stacking of the complexes in the
crystals
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