Chemistry Reference
In-Depth Information
N
O
O
N
OH
N
O
OH
O
H
1
Figure 4.1 Rhodotorulic acid (1) and its helical dinuclear complex (D,D)-[Fe 2 (1) 3 ] (PM3-min-
imized structure based on the proposed structure of Ref. [2b]).
where at last one ligand strand is wound around two or more metal centres in a helical
fashion.
Progress in analytical techniques, synthetic methods, and the Nobel price for
supramolecular chemistry and molecular recognition awarded to C.J. Pedersen, D.J.
Cram, and J.-M. Lehn in 1987 paved the way for the enormous development of this
field which continues today. This also holds true for metallosupramolecular aggre-
gates like helicates that have gained increasingly more attention since the late
1980s, as can be seen by the impressive list of review articles on these interesting
compounds [7]. Although helicates are often referred to as model systems that mimic
DNA, this is not really true since there are hardly any systems that carry sequential
information and there are almost no heterostranded helicates, that is, helicates that
are formed from complementary but not self-complementary strands. Nevertheless,
they are aesthetically very appealing and they can be used as almost ideal models to
study all kind of self-processes, such as self-assembly and self-sorting in terms of
self-recognition, or self-discrimination. They also allow for the investigation of sec-
ondary thermodynamic factors associated with self-assembly processes like allostery
or cooperativity. Furthermore, they can also be employed to study the kinetic effects
of the formation of self-assembled structures or regarding their dynamic behaviour
during interconversion, or to learn more about template effects. Another important
aspect is of course chirality: although by far not as well developed as the synthesis
of covalently assembled molecules, stereoselective molecular recognition or the ster-
eocontrolled formation of non-covalently assembled aggregates has also evolved into
a major issue in supramolecular chemistry [8]. In this respect, helicates are very
important since helical structures are per se chiral objects and the mechanical cou-
pling of two or more metal binding sites in a ligand strand offers an outstanding
opportunity to control the relative stereochemistry of stereogenic metal centres which
is difficult to achieve otherwise [9].
This overview will focus on the structural aspects involved in the self-assembly of heli-
cates. This comprises structural dynamics, template effects, sequence selectivity, and self-
sortingeffectswithregardtoligandsize,numberofbindingsites,andespecially
stereoselectivity.
 
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