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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 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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