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Figure 12.3 Schematic representation of the concept of folding-based supramolecular chela-
tion. Reprinted with permission from [ref 27] Copyright 2005 American Chemical Society.
complex of
4
and Pd
2þ
in acetonitrile, indicative of a transition from the unfolded to the
folded conformation. The conformational change was also confirmed by
1
H NMR spec-
troscopy. Isothermal calorimetry revealed that the overall free energy for the complex-
ation had a nearly linear relationship with the chain length and became more negative
as the chain lengthened. Most interestingly, the binding enthalpy was more favorable in
4
(
n
¼
7) than either the shorter (
n
¼
5) or the longer (
n
¼
9) ligand. The enthalpic gain
from the pentamer (
4
,
n
7) was anticipated because more
aryl-aryl contacts would occur in the folded complex when the chain became longer. For
the pyridyl with nine
m
PE units (
4
,
n
¼
5) to the heptamer (
4
,
n
¼
9), the total number of aryl groups was 10 and
large enough for the ligand itself to fold in acetonitrile; thus, the net increase in aryl-aryl
interactions was not as significant during complexation with the metal. In other words, the
benefit of folding toward complexation was the largest when the metal-ligand complex
was long enough to fold but the ligand itself was not. The concept was latter applied to
the synthesis of supramolecular coordination polymers from bispyridyl-functionalized
m
PE oligomers [28].
In the above examples, metal ions were used to promote the folding. Ajayaghosh
et al.
reported the opposite effect of the metal ion; that is, unfolding a helix by metal-ligand
interactions [29]. Similar to the
m
PE oligomers,
5
was a random coil in chloroform and a
helix in acetonitrile. The folding was characterized by a number of techniques including
solvent titration, fluorescence spectroscopy, and circular dichroism (CD). Because the
folding of
5
was driven by solvophobic and p-p interactions in acetonitrile, the hydropho-
bicity of the foldamer backbone was critical to the folded conformation. For the same
reason, as the polarity of the aromatic backbone was increased by the coordination of the
bispyridyl groups to zinc cations, the helix unfolded into a random coil. The effect was
completely reversible. Once the metal ions were removed by a stronger ligand, EDTA,
the extended chain spontaneously folded back to the helix.
¼
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