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4.5 Self-Sorting Effects in Helicate Formation
As mentioned in the introduction helicates are very interesting supramolecular aggregates
to study the principle effects associated with self-assembly processes. One example would
be self-sorting processes in mixtures of different types of ligands. In principle, one can
expect three possible scenarios in such an experiment: (a) there is no selectivity at all so
that one would obtain a statistical mixture of all possible assemblies, (b) only homo-
stranded helicates are formed that contain only one sort of ligand, or (c) only hetero-
stranded helicates are formed that contain different sorts of ligands. The latter two
scenarios would both be results of self-sorting effects - (b) would be called self-
recognition and (c) self-discrimination. So far these self-sorting phenomena have been
investigated by different approaches and we will concentrate on studies with achiral lig-
ands first.
Actually, it should be noticed that J.-M. Lehn's heterostranded helicate [Cu
II
3
(
15
)
(
17
)]
6รพ
shown in Figure 4.9 is one of the very rare examples for self-discrimination in
helicate chemistry [20,21]. The other extreme - self-recognition - is however obviously
far more common in helicate chemistry. Already in 1993 J.M. Lehn could demonstrate
that only homostranded helicates are formed from a mixture of the four different oligobi-
pyridine ligands
20
-
23
when mixed with the appropriate amount of copper(I) ions
according to two factors: (a) the maximum occupancy rule that favours those aggregates
where all coordination sites of metal ions and ligands are used and none of them stay
unsaturated, and (b) the entropically favourable formation of discrete aggregates com-
pared to polymeric ones at concentrations below the effective molarity (i.e. the concentra-
tion where the formation of oligomers and polymers starts to compete energetically with
the formation of the discrete assemblies of low nuclearity; Figure 4.12) [28].
In the same study J.-M. Lehn and co-workers also demonstrated that using different
linker groups in ligand strands that lead to a different distance between the individual
bipyridine metal binding sites also results in self-recognition. A similar observation was
also made by K.N. Raymond's group when studying mixtures of bis(catecholate) ligands
(
24
-H
4
)-(
26
-H
4
) with rigid linkers of different length (Figure 4.13) [29].
Finally, a third approach was reported by Y. Cohen in which ligand strands comprising
bipyridine or bithiophene ligand units also showed self-recognition behaviour [30].
Within chiral ligands another assumedly subtle difference between two ligands would
be the inversion of stereogenic elements (i.e. the use of stereoisomeric ligands). So far, a
number of racemic ligands have been studied in this context. An example published by
the group of P.K. Mascharak is ligand
27
(Figure 4.14). This ligand was found to undergo
selective self-recognition to give a racemic mixture of helical complexes with quadruple
planar coordinated copper(II) ions [31].
The situation gets more complicated if helicates are formed that bear stereogenic metal
centres (either in a tetrahedral or an octahedral coordination geometry). In such cases
even a completely selective self-sorting process can lead to different diastereoisomeric
helicates due to the possibility of the metal ions to adopt a L-orD-configration. Such a
system was first reported by T.D.P. Stack at the end of the 1990s. When he investigated
the chiral, racemic stereoisomers (
R
,
R
)-
3
and (
S
,
S
)-
3
of G. van Kotens
meso
-configured
bis(pyridylimine) ligand (
R
,
S
)-
3
he found that the self-assembly proceeds exclusively via
a self-recognition process, yielding a racemic pair of homochiral double-stranded
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