Three-Dimensional Aromatic Networks - Polyarenes - page 114

tetrahedral carbon atoms, pyramidal nitrogen atoms, or other atomic groups as
bridgehead moieties. Tetrahedral carbon atoms also function as tetratopic linkers
[ 13 ]. The structural variation of 3D networks can be dramatically enhanced by
using such highly branched linkers.
Not only strong covalent or coordination bonds but also weak interactions, such
as hydrogen bonds, can connect linker moieties. Hydrogen bonds are generally too
weak to connect two chains firmly, but complementary multi-point hydrogen bonds
strengthen the linkage, as found in carboxylic acids or base pairs in nucleic acids
[ 14 , 15 ]. In some molecular networks, weak interactions are utilized as supplemen-
tal linkers to support the main framework rather than as main linkers to connect
aromatic units. Such interactions often play important roles in controlling the
structures and properties of molecular systems.
2.3 Construction of Aromatic Networks
Aromatic networks are constructed from the aromatic units and linkers mentioned
above by chemical reactions. In terms of the position of bond formation, there are
two approaches to connect two or more aromatic units. One is the formation of
bonds between arene units and linker terminals. Cross coupling reactions are
commonly used for this purpose because a wide variety of reactions are now
available given the recent advances in synthetic methodology [ 16 ]. Arenes with
halogen substituents (Ar-X) or those with metal substituents (Ar-M) are common
synthetic precursors. Conventional aromatic substitution reactions are also practical
for the extension of the aromatic network [ 17 ]. The second approach, which is valid
for only two-atom linkers or longer linkers, is the formation of a bond between
atoms in linker chains from substituted arene units. Any bond formation reactions
are available depending on the functional groups in the pre-linker substituents. In
particular, ring-closing metathesis is a new technique to connect two terminal
alkene substituents across arene units [ 18 ]. In both connection approaches, each
bond formation process should be highly effective to construct large networks in
reasonable yields.
Some large aromatic networks are constructed by stepwise reactions and the
macrocyclization of linear precursors or the further cyclization of cyclic
compounds is often carried out in the last step [ 19 , 20 ]. In other cases, complicated
networks can be formed from simple building units in a single procedure that
involves the formation of several bonds. Such composite reactions or self-assembly
processes work surprisingly well when the reaction sites are controlled by structural
requirement and pre-organization. In particular, the direction of bonds from linkers
and arene units is very important to control the shape and size of the network to be
formed. This concept, known as “directional bonding approach,” is summarized in
the review articles by Stang et al. [ 21 , 22 ].