Interaction of Biomimetic Oligomers with Metal Ions - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 383

Figure 11.7 Space-filling representation of Cu(1g) (left) and Cu(1r) (right). The solvent mole-
cules and non-amide hydrogens are removed for clarity. Reprinted with permission from
Ref. [19]. Copyright 1996 American Chemical Society.
In contrast, in the case of Cu( 1r ), the lack of one Cu-O bond leads to a
realignment of the appended arrays and produces a more compact helix, which does
not have a porous morphology (Figure 11.7, right); the helical molecules of Cu( 1r )
associate to form longer, extended helices in the solid state (Figure 11.3). Each
extended helix consists of alternating right- and left-handed helical monomers that
assemble through intermolecular p-stacking interactions. Although these results are
only valid in the solid state [the structural properties of Cu( 1g )andCu( 1r )areiden-
tical in solution], it is possible to conclude that metal coordination to H 2 ( 1 ) enables
the nucleation of helical structures and the generation of new helices which can be
altered upon variations in the metal ion precursor.
Studies on the complexes Cu 1 [including both Cu( 1g ) and Cu( 1r )], as described above,
suggested that the presence or lack of Cu-O amide bonds is a highly important factor for
determining the final structure of the produced helices. To further investigate possibilities
in which the helical structure could be modified, Borovik's group was interested in find-
ing ways to control the formation of these Cu-O bonds. Thus, Cu 1 was treated with two
different ligands, pyridine and lutidine, which are basic enough to substitute the oxygen
amide groups [20]. Changes in the spectroscopic properties of Cu 1 upon binding to either
pyridine, forming Cu 1 (py), or lutidine, forming Cu 1 (lut), implied that the coordination
environment about the Cu 2þ centers has been altered. An X-ray diffraction study on
Cu 1 (py) confirms that the binding of a single pyridine to copper causes a significant struc-
tural rearrangement. The two oxygen amide donors that were coordinated originally to the
copper in Cu 1 are rotated away from the copper in Cu 1 (py) and no longer interact with
the metal ion (Figure 11.8, left). Consequently, a chiral cleft about the exogenous pyridine
is formed by the appended groups in H 2 ( 1 ) 2 . An interesting characteristic of the Cu 1 (py)
crystal structure is that all the complexes within the lattice have the same helical chirality.