Design of Supramolecular Materials: Liquid-Crystalline Helicates - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 262

Figure 7.15 Schematic representation of the columnar arrangement of the metal free ligand
after cyanide extraction from the duramer as deduced from X-ray diffraction on micro-slides of
the polymer.
Interestingly, the Cu(I) can be extracted from the green duramer using KCN in DMF,
leaving the duramer morphology unchanged while the materials becomes pale yellow
(Figures 7.15 and 7.12c). Unfortunately, this process is very slow probably because it is
limited by the diffusion of cyanide anions inside the polymeric matrix. X-ray diffraction
of the metal-free polymers shows a columnar organization of the ligand with cell parame-
ters close to the one measured with the helicate (Figure 7.13 right). This confirms that
immobilization of the helicate due to reticulation of the acrylate function is an interesting
method of access to well defined nano-structured netwoks.
Further developments include the use of these ligands to complex metals in a octahe-
dral environment. As a first step, Fe(II) and Ni(II) complexes have been prepared and
characterized by X-ray crystallography (Figure 7.16). In the Ni case with bipy ligand 7 ,
the six-coordinate nickel atom is surrounded by four N atoms belonging to bipy subunits
and two N atoms of imino groups belonging to each ligand. The additional two imino
functions (which are not ligated to the metal center), lies at 4.4 and 3.4 A in a trans con-
formation for the former and in a cis conformation for the latter. In the latter case some
electronic interaction wit the metal center are effective. Here the mode of coordination of
the ligand is of Type VII (Figure 7.17). In the iron(II) complex with ligand 8 , the metal is
surrounded by two imino-naphthyridine ligands in a chelating fashion. The eight N donor
atoms define a distorted square antiprism, which is composed of two sets of trapezoidal
arrangements in perpendicular orientations (Figure 17.16 right).
The structure of this complex exhibits some striking similarities with that of [Fe(1,8-
naphthyridine) 4 ] 2þ [71], which also shows a dodecahedral arrangement of the ligand and
the coordination mode of the ligand is of Type VIII shown in Figure 7.17. It displays
interesting mesomorphic behavior over a large temperature range, depending on the
nature of the counter-anion. Most of these salts display a columnar mesophase with octa-
hedral symmetry.