Chemistry Reference
In-Depth Information
The number of possible structures increases rapidly with increasing number of
arene units. Networks G and H are examples of structures with six and eight arene
units, respectively. In G, three units of the first set (top) are connected to all units of
the other set (bottom), and this is called a complete bipartite graph. This graph is
called utility graph
K
3,3
and features nonplanarity (see Sect.
6.2
). Each unit has
three linkers in H to form a cubic motif, called a 3-regular graph in the graph theory.
A tritopic linker can connect three arene units. Network I consists of two tritopic
linkers and three arene units. In network J, four arene units are connected by four
tritopic linkers in all possible combinations. This graph is identical to tetrahedrane
motifs J
0
and J
00
. Arene units and linkers occupy corners and faces of a tetrahedron,
respectively, in J
0
and vice versa in J
00
.
2.5 Other Structural Features
One needs to consider shape, size, flexibility, and chirality in discussing the
structural features of aromatic networks, which depend on the number, type, and
combination of arene units and linkers. Shape-persistent networks have rigid
frameworks and interesting properties, such as self-assembly or molecular recogni-
tion. In contrast, flexible networks are conformationally mobile and their observed
symmetry depends on the time scale of conformational changes and the methods of
observation. Large 3D networks tend to have a cavity within their frameworks. If
the cavity is sufficiently large to accommodate foreign molecules, the inclusion
phenomenon is expected. Chiral networks are fascinating from the viewpoint of
stereochemistry [
24
]. Such structures can be constructed from achiral building
units, where the network itself is inherently chiral. The resolution of chiral 3D
network compounds should offer valuable information on the chiroptical properties
of novel enantiomers. Another geometrical option for networks with macrocyclic
rings is topological isomers [
25
,
26
]. Catenanes and knots have nonplanar
topologies, which have afforded new aspects in structural chemistry. These
examples will be introduced in Sect.
6
.
3 Cyclophanes with Saturated Carbon Linkers
Cyclophanes are a class of cyclic compounds consisting of one or more arene units
and saturated or unsaturated linkers. These compounds have been extensively
studied in aromatic chemistry because of their unusual structure, conformation,
and transannular interactions. The chemistry of cyclophanes has been reviewed in
several topics and review articles [
27
-
29
]. When cyclophanes have multiple arene
units, especially arenes larger than benzene, molecules tend to take cage-like
structures to form 3D space. In this section we will introduce such cyclophanes
with naphthalene or anthracene units and saturated carbon linkers as prototypes of
3D networks.
