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Figure 8.14 Sequential peptide-type ligands bearing pyridine, bipyridine and terpyridine as
metal coordination sites.
center. This enforces the formation of the hetero-stranded complex [34]. Recent studies
by Williams
et al.
transferred the Lehns concept to related peptide derivatives.
The aminoethylglycine derivatives
19-21
were prepared (Figure 8.14). These bear as
side chain functionalities mono- (pyridine), bi- (bipyridine), and tridentate (terpyridine)
units. The addition of copper(II) salts results in the formation of double-stranded helicates
with an antiparallel orientation of the two strands. The self-assembly is followed by spec-
trophotometry and the formed coordination compounds are characterized by positive ESI-
MS spectra.
In the case of
19
and
20
a dinuclear helicate is formed [35] while ligand
21
results in a
trinuclear complex with two copper(II) ions coordinating to terpyridine and pyridine and
one copper(II) binding to two bipyridines. Thus each of the copper(II) ions is tetra-coor-
dinated. Due to the high oxidation state at the metal, a square planar geometry is preferred
(Figure 8.15) [36].
This describes how helicate-type coordination complexes can self-assemble from lig-
ands, which are based on peptidic units. Here different motifs are used. The metal coordi-
nating units are either attached to the termini of the peptide or are introduced as side
chains. In addition, it has been shown that the incorporation of coligands can influence
the outcome of a coordination study and that it is possible to control the relative orienta-
tion (parallel or antiparallel) of sequential/directional ligands. The ligands themselves are
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