Chemistry Reference
In-Depth Information
Scheme 1 Typical two-atom
linkers in 3D aromatic
networks
H
CC
H
C
C
C
C
C
C
H
H
H
H
H
H
CO
H
C
N
C
N
C
O
H
H
H
O
O
elements can be used. These linkers are not linear and the bond angles at the C or O
atom are usually 110-120
. For example, several
m
-phenylene groups are
connected to methylene groups as linkers to form 3D cyclic structures in
calixarenes [
8
]. Metal atoms are also used as linkers via coordination bonds with
ligands in aromatic units, such as pyridines. For a square-planar metal,
cis
and
trans
junctions offer angles of 90
and 180
, respectively. In order to regulate the
geometry, the other two coordination sites are blocked by other ligands, e.g., a
bidentate chelate ligand for the
cis
linkage. These one-atom linkers are generally
less flexible than saturated longer linkers, as discussed below.
Two-atom linkers that connect two units via three bonds are commonly found in
3D aromatic networks. Typical two-atom linkers are shown in Scheme
1
.In
ethylene (
CH
2
CH
2
-), a common saturated carbon linker, two bonds connecting
aromatic units are separated by ca. 1.54
˚
and can extend in various directions
depending on the conformation about the central C-C bond. In contrast, unsaturated
two-atom carbon linkers have fixed directions. The two bonds extend to the same
side or the opposite side of the double bond in
cis
- and
trans-
ethenylene (vinylene,
-CH
C-) is a typical linear linker, in which the
distance between terminal arene units is ca. 4.2
˚
CH-) linkers. Ethynylene (
C
═
[
9
-
11
]. Ethenylene and
ethynylene linkers can extend the
-conjugation between arene units across the
double bond and the triple bond, respectively, depending on the conformation. For
both linkers the conjugation is maximum in the fully planar conformation and less
effective in nonplanar conformations. One carbon atom or two carbon atoms in
those two-carbon linkers can be replaced with other elements giving a variety of
choices. Some examples are ethers, imines, amides, and esters (Scheme
1
). Linkers
with more than three atoms or more than four bonds are essentially a combination of
the above linkers. Generally, the mobility and flexibility of linker moieties increase
with an increase of chain length. However, 1,3-butadiynylene (
p
C
C-C
C-) is a
typical rigid linear linker separated by 6.8
˚
.
Linkers with two or more atom chains can be distinguished by their orientation.
Regarding two-atom linkers, the -X-X- type linkers are symmetrical and have no
direction, whereas the -X-Y- type linkers are unsymmetrical and have direction.
Simple esters and amides are typical directed linkers. Therefore the use of unsym-
metrical linkers considerably increases the number of possible network structures
involving constitutional isomers and stereoisomers.
The term tritopic linkers refers to linkers with three positions that enable linkage
to arene units. Three tethers extend from a central atom (or an atomic group) in a
pyramidal or trigonal shape [
12
]. Typical pyramidal
tritopic linkers involve
