Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 417

Figure 12.10 Crystal structures of (a) the [44] grid complex, 20 8 -(Pb 2þ ) 16 , and (b) the dou-
ble-cross complex, 20 4 -(Pb 2þ ) 12 . Reprinted with permission from [ref 83] Copyright 2003
American Chemical Society.

The organic framework may be modified to introduce additional functions to the oligo-
heterocycles. When Lehn and colleagues replaced the pym groups with 1,8-naphthyridine
(napy), 1 H NMR spectroscopy indicated that the helical conformer was maintained in the
free ligand [84]. The napy group enlarged the inner cavity of the folded helix and also
strengthened the solvophobic and p-p interactions in the folded state. Remarkably, these
molecules could aggregate along the helical axis to form longer helices. The aggregation
was shown to be promoted by a decrease in the size of the aromatic side chain, an increase
in the ionic strength of the solution, and, most interestingly, the binding of cationic metal
ions in the interior of the helices.

Different metal ions often have different coordination motifs. The py-pym oligomers
22 and 23 both formed single-stranded helical structures in solution and in the solid state.
The addition of AgOTf brought significant changes to the 1 H NMR spectra of the com-
pounds. With less than one equivalent silver ion, complex spectra were obtained, indicat-
ing the coexistence of many slowly exchanging species. At [oligomer]/[Ag þ ] 1:1,

however, the spectra were simplified, consistent with the formation of highly symmetrical
structures. The spectra did not change upon further increase of the silver ions. A combina-
tion of NMR spectroscopy, mass spectrometry, and X-ray crystallography established that
a double-stranded helix was formed in the presence of Ag þ [85]. The interconversion

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