Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 405

a metallofoldamer and thus endow new functions to the molecule. In the meantime, the
conformational preference of the foldamer chain can create unusual coordination motifs
around the metal and allow it to behave in unconventional ways. Such interplay between
the metal and the foldamer expands the repertoire of both and represents the most exciting
aspect of metallofoldamers. In the following sections, we will illustrate how these inter-
actions play out and benefit both partners with selected examples of synthetic
metallofoldamers.

12.2 Metallofoldamers in Molecular Recognition

Moore and coworkers were among the first to design foldamers whose conformations
depend on both the noncovalent interactions of the foldamer chain and the metal-ligand
coordination [24]. The metallofoldamers were based on their well characterized meta -
phenyleneethynylene ( m PE) foldamer 1 . The foldamer contains an aromatic backbone
that is solvated well by chloroform but poorly by acetonitrile. When the number of aro-
matic units reaches 8 to 10, the m PE oligomer adopts a helical conformation in acetoni-
trile, with the collapsed aromatic core surrounded by the solvophilic tri(ethyleneglycol) or
Tg groups [25]. The foldamer can be viewed as a conformationally tunable mimic of mac-
rocycle 2 . Because of the 120 bond angle in the meta -phenylene linkage, a folded m PE
has six aryl units per helical turn. The helix has a tubular cavity in the middle which can
bind hydrophobic molecules with complementary geometry (Figure 12.1) [26].
By equipping every other aryl group in the m PE oligomer with a cyano group, Moore
turned 1 into a metal-binding foldamer ( 3 ). Unlike a multidentate metal-binding macryo-
cycle such as an azacrown, 3 may adopt either a folded or unfolded conformation. The
authors studied the binding of Ag þ in tetrahydrofuran (THF), a solvent in which m PE
oligomers adopt random coil conformations. Upon the addition of silver triflate (AgOTf),
the control compound 1 ( n 12) showed little change in the 1 H NMR or UV/Vis spectra

¼

but 3 ( n 6) displayed the typical transition to the cisoid conformation associated with

the folded m PE [24]. The latter was found to bind two equivalents of silver ions as

¼

Figure 12.1 Relationship between the intermolecular aggregation of (a) m PE macrocycle 2
and the intramolecular folding of (b) linear m PE oligomer 1. Reprinted with permission from
[ref 25] Copyright 2006 American Chemical Society.