Interaction of Biomimetic Oligomers with Metal Ions - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 384

Figure 11.8 X-Ray crystal structures Cu1(py) (left) and Cu1(lut) (right). The solvent molecules
and non-amide hydrogens are removed for clarity. Reprinted with permission from Ref. [20].
Copyright 1996 American Chemical Society.
This is highly unexpected because H 2 ( 1 ) is achiral in solution. One possible explanation
for the helicity of the crystals could be that spontaneous resolution occurs as individual
crystals form in solution. Thus, a crystal of the other handedness has been isolated from a
solution of Cu 1 (lut). It was characterized and found similar to that described above for
Cu 1 (py), the major difference being the opposite helicity (Figure 11.8, right). This selec-
tive crystallization is dictated by the lattice architecture of the clusters in the solid state;
stable crystals can be formed only if the helices within an array are perfectly aligned. This
stabilization is only possible if individual metallohelices are of the same helicity. These
results demonstrate how weak intra- and intermolecular interactions can be used to
assemble chiral metallofoldamers.
The structural properties observed for Cu 1 (py) and Cu 1 (lut) appear to be present
only in the crystalline state. Preliminary electronic absorbance and circular dichroism
results suggest that the complexes racemize upon redissolution at room temperature.
In order to overcome this problem, the Brovik group has synthesized derivatives of
H 2 ( 1 ) in which the acetyl phenyl groups on the appended arms are replaced by enan-
tiomerically pure groups, as depicted in Figure 11.9 [21]. The spectroscopic data
obtained from the 1 H-NMR spectra of all the new ligands, namely H 2 ( 2a ), H 2 ( 2b )and
H 2 ( 2c ) indicate that the molecules maintain their hydrogen bonds in solution. These
hydrogen-bonding interactions decrease the freedom of rotation of the two appended
groups, resulting in a fairly rigid molecular structure. Moreover, the solid-state struc-
ture of crystalline H 2 ( 2a ), as determined by X-ray diffraction methods, confirmed that,
similar to H 2 ( 1 ), H 2 ( 2a ) has a helical structure which is stabilized by networks of
intramolecular hydrogen bonds.