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Figure 11.6 X-Ray crystal structures of Cu(1g) (left) and Cu(1r) (right). The solvent molecules
and non-amide hydrogens are removed for clarity. Atom key: open circles ¼carbon; circles
with horizontal lines ¼nitrogen; circles with vertical lines ¼oxygen; and solid circles ¼
copper. Reprinted with permission from Ref. [19]. Copyright 1996 American Chemical
Society.
nucleate a helical structure in solution and the second whether various metal ions
would result in different three-dimensional architectures [19].
The combination between Ni
2þ
and (
1
) [after its deprotonation to H
2
(
1
)] afforded the
complex Ni(
1
) which, according to its
1
H-NMR spectrum, exists in solution as a racemic
mixture of left- and right-handed helices. The hydrogen bonds within the appended arrays
are still present in the metal complex and participate in the stabilization of the metal-
lohelix in solution. Subsequently, In order to assess whether coordination changes at the
metal center could influence helicity, Cu(
1
) was synthesized. Crystallization of Cu(
1
)
from solution afforded two structurally distinct isomers: a green complex having a dis-
torted square pyramidal coordination geometry about the copper(II) ion, Cu(
1g
), and a
tetra-coordinated red complex Cu(
1r
) (Figure 11.6). The differences in coordination
numbers between these two clusters are due to the absence of the weakly coordinated
amide-oxygen donor in Cu(
1r
). Both copper complexes nucleate the formation of helical
structures; however, their different coordination geometries result in two distinct helices.
In the case of Cu(
1g
), the additional Cu-O bond causes non-symmetrical Cu-O(amide)
interactions between the arrays and the metal chelate, which results in a helical structure
with a pronounced non-symmetrical twist and a microporous crystal lattice (Figure 11.7,
left); the pores are formed via parallel aryl ring p-stacking between the axial appendages
of the complex.
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