Chemistry Reference
In-Depth Information
Fig. 25 Iodide inclusion in a
macrotricyclic quaternary
ammonium receptor.
(a) Crystal structure of the
[14···I]
3+
complex [
52
].
(b)I
is placed in the centre
of the nearly regular
tetrahedron described by the
four nitrogen atoms; N···I
distance 4.54
0.04
˚
,
tetrahedron side 7.4
0.3
˚
Diprotonated katapinands and the tetraprotonated spherical cryptand 13H
4
4+
are
comfortable cages for anions, but, in order to maintain a positive charge, they must
operate under strongly acidic conditions. On the other hand, a permanent positive
charge can be imparted to the receptor by alkylating, rather than protonating,
tertiary amine groups. Such a progress in anion receptor design was achieved
by Schmidtchen, who, in 1977, reported receptor 14
4+
, containing four quaternary
nitrogen atoms linked by -(CH
2
)
6
- aliphatic chains [
51
]. In particular, each
nitrogen is linked to the other three nitrogen atoms, according to a connectivity
that is expected to generate a tetrahedral arrangement of the four quaternary
ammonium groups. The crystal and molecular structure of the iodide inclusion
complex is shown in Fig.
25a
[
52
]. I
is placed in the middle of a regular tetrahe-
dron whose vertices are the four nitrogen atoms (Fig.
25b
). In the present case, the
complex is held together only by ionic attractive interactions. In this context, a
tetrahedral arrangement of the quaternary ammonium groups maximises attractive
electrostatic attraction and minimises electrostatic repulsion between positive
charges. 14
4+
forms stable complexes in water with Br
and I
(log
K
3.0 and
2.7, respectively). Increasing the length of the linking aliphatic chains from
-(CH
2
)
6
- to -(CH
2
)
8
- inverts selectivity in favour of
¼
I
(log
K
¼
2.0 for
Br
and 2.5 for I
).
Papers by Lehn and Schmidthen were well received by the chemical community
and stimulated the development of the novel discipline of anion coordination
chemistry. The numerous anion receptors synthesised over the last three decades
are not necessarily cage shaped. However, a polycyclic arrangement of the receptor
was and is still highly required because it favours the formation of stable complexes
in solution and may allow stringent recognition selectivity based on size/shape
complementarity. Further examples of positively charged cage shaped receptors
will be presented in Sects. 5 and 6.
In the evolution of anion coordination chemistry, it was discovered that neutral
molecules can also operate as effective receptors for anions provided that they
contain polarised N-H fragments (e.g. of amides [
53
], ureas [
54
], thioureas [
55
]or
pyrroles [
56
]), which act as H-bond donors for anions. These receptors cannot
compete for hydrogen bonding with water and alcohols and must be studied in
aprotic solvents of varying polarity, e.g. CHCl
3
, MeCN, DMSO. In this vein,
