Chemistry Reference
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Fig. 2.4
X-ray structures of dinuclear capsule-like complexes of La
3
+
-Q[5]:
a
complex with an
included nitrate anion;
b
the corresponding portal sizes;
c
complex with an included chloride ion;
d
the corresponding portal sizes
Fig. 2.5
X-ray crystal structures of molecular capsules and molecular bowls of CyH
5
Q[5] with
alkaline earth metal ions:
a
Ca
2
+
;
b
Sr
2
+
; and
c
Ba
2
+
and molecular bowls depended on the ionic radius of the metal cation (Fig.
2.5
).
In the CyH
5
Q[5]/Ca
2
+
complex, the CyH
5
Q[5] portal is larger than the cal-
cium cation. Thus, the CyH
5
Q[5]-based molecular capsule is not completely
closed, and the CyH
5
Q[5] molecule even experiences some deformation. In the
CyH
5
Q[5]/Sr
2
+
complex, the radius of the strontium cation is slightly larger than
the CyH
5
Q[5] portal, and thus they form a completely closed molecular capsule.
In the CyH
5
Q[5]/Ba
2
+
complex, the barium ion can only cover one portal of the
CyH
5
Q[5] molecule and juxtaposes itself near the portal of other CyH
5
Q[5] mol-
ecule, generating an opened molecular capsule. Based on the radii of these three
metal ions, which are 0.99, 1.12, and 1.34 Å, the radius of the metal ion is the
underlying reason behind these differences.
Generally, metal ions that can cover the portals of a Q[5] molecule have a
radius larger than 0.84 Å, which is the radius of the Cd
2
+
cation. Nevertheless,
there are other factors that could lead to the formation of Q[5]s with metal
ions with a smaller radius, such as Hg
2
+
(0.69 Å) [
21
]. In recent years, there
has been a trend toward the use of third species as structure directing agents in
Q[
n
]-metal systems, and the driving forces could be the so-called outer surface
interactions of Q[
n
]s [
22
]. These species produce Q[
n
]-based supramolecular
assemblies whose properties, structural novelties, and functionalities exceed those
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