Chemistry Reference
In-Depth Information
Ar
Ar
N
N
N
N
N
M
M
M
M
M
M
N
N
N
N
Ar
Ar
Type I
Type II
Ar
Ar
N
N
N
N
N
N
N
M
M
Ar
Ar
Ar
Ar
R
M
M
R
O
Type III
Type IV
Ar
Ar
M
M
N
N
N
N
N
N
N
N
M
Type V
Type VI
Figure 7.1 Various coordination modes of imino-polypyridine ligands and corresponding
oligopyridine ligands.
structural variety of the synthetic protocol, (iii) the excellent donating capabilities of
the nitrogen atom of the imine function, and (iv) the strength of the metal-imine
bond. Bidentate (type III), tridentate (type IV), tetradendate (type II), and pentaden-
date (type VI) Schiff-base ligands are easily obtained by condensation of the corre-
sponding dialdehyde or diketone with aromatic amines under acidic conditions [36].
In particular, 2,6-disubstituted pyridine derivatives have been thoroughly investigated
due to the strong coordination abilities of the
N
,
N
,
N
donor towards transition metal
and lanthanide salts [37]. Several of these complexes are efficient catalysts for olefin
polymerization [38] and others have promise as molecular templates for the con-
struction of more sophisticated systems [39].
The aim of this chapter is to give an overview of developments in the construction of
supramolecular frameworks which combine the covalent bond-forming capability of
metallic centers with imino-functionalized oligopyridine ligands. We review the forma-
tion of metallohelicates and the integration of the resulting helicoidal structures into orga-
nized assemblies, notably liquid crystals. We also discuss the structure requirements for
metallohelicate assembly into mesophases and rigidification of these mesophases by
polymerization, together with their possible applications. Different aspects concerning
the mechanism of assembly and subsequent dissociation in the presence of a competing
ligand, the stabilization of the metallohelicate against oxidative dissociation and its impli-
cation for redox catalysis, as well as the internal flexibility of the emerging metalloheli-
cate, will not be discussed in detail.
Search WWH ::
Custom Search