Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 418

Figure 12.11 Ionic modulation of reversible extension/ contraction motional process upon
single/ double helix interconversion of 22. Reprinted with permission from [ref 85] Copyright
2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
between the single- and double-stranded structures could be modulated once again by the
addition of Ag þ and the cryptand to remove the metal. As shown by the crystal structures,
switching between the two forms was accompanied by a dimensional change of nearly
300% (Figure 12.11).
The structure of the py-pym oligomers was dominated by transoid conformation of the
a,a 0 -connected aza-heterocycles. Once the heterocycles are connected by a hydrozone
(hyz) unit, linear, bent, and helical structures all become possible, depending on the type
and substitution of the aromatic heterocycles [86]. When these oligomers interact with
metal ions, a variety of motions become possible. As shown in Figure 12.12, for three
meta -attached pym units connected by hydrazones, the angle defined by the three hetero-
cycles is 60 in the free ligand. Metal coordination bends the structure in opposite direc-
tions in a “flapping” motion to produce a linear structure. For three para -linked pyrazine
(pz) units, the free ligand is linear. Even though the metal coordination does not change
the overall linear structure, the chain is rotated by 60 in a “twirling” motion. Since the
information of preferred conformation is coded within the foldamer sequence, it is easy to
image that combination of these “motional codons” will lead to materials with highly
sophisticated molecular motions.
Schafmeister and colleagues prepared spiroladder oligomers based on bis-functional-
ized amino acids such as 24 - 27 [87-90]. This approach was very different from those
taken by most foldamer chemists. Instead of trying to control the conformation of
flexible foldamer, they used the covalent framework and the inherent conformational pref-
erence of the spirorings to generate rigid structures with predetermined 3-D structures.
Molecule 28 contains two rigid molecular rods made by four bis(amino acids). The two
8-hydroxyquinoline end groups are known to form a 2 : 1 complex with Cu 2þ . The two
rods are connected by a flexible ornithine, which could act as a hinge as the molecule
undergoes expansion and contraction upon reversible metal binding. The molecule indeed