Chemistry Reference
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Fig. 2.13
X-ray crystal structures of Q[6] with the alkali metal ion:
a
molecular capsule with
Cs
+
+
+
;
b
molecular bowl with Cs
;
c
molecular capsule with Na
; and
d
the representation of
molecular capsule and molecular bowl built by Q[
n
]s
Cs1 and the portal carbonyl oxygens is 3.305 Å on average, and can reach a maxi-
mum of 3.616 Å.
Kim and coworkers first reported a simple Q[6]/Cs
+
complex in 1998 [
32
], in
which the Cs
+
cation can effectively coordinate only four of six carbonyl oxy-
gens of a portal of a Q[6] molecule. The lengths of the Cs-O
carbonyl
bonds are in
the range 3.100-3.378 Å (Fig.
2.13
a). However, with the aid of an organic guest
(tetrahydrofuran, THF), the Cs
+
cation fully covers the portal of the Q[6] mol-
ecule, and forms a typical molecular bowl. The resulting Cs-O
carbonyl
bond
lengths are in the range 3.144-3.584 Å. In particular, the bond length of Cs-O
THF
is only 2.934 Å, suggesting that the strong bonding between the Cs
+
cation and
the included THF further draws the Cs
+
cation toward the portal of the Q[6]
molecule, resulting in the formation of the fully covered Q[6]/THF/Cs
+
molecu-
lar bowl (Fig.
2.13
b). Such phenomenon exemplifies a typical instance involving
Q[
n
]-based host-guest/coordination chemistry. An earlier case involving Q[
n
]-
based host-guest/coordination chemistry is the Q[6]/THF/Na
+
molecular cap-
sule also reported by Kim and coworkers in 1996 [
33
]. In this complex, two Na
+
cations are capped at each of the two portals of the Q[6] molecule, and a THF
molecule is included in the cavity of the Q[6] molecule; thus, a sealed molecular
capsule is formed (Fig.
2.13
c). The bonding and release of the included THF can
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