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analysis, a
P
-helical conformational preference was exhibited by all three of the complexes,
identical to the preference observed for the parent (metal-free) dendron. Moreover,
1
H-NMR peak analysis indicated that metal coordination increases the helical inter-
conversion barrier and thus the dynamic helicity of the dendron, making it conformation-
ally “locked”.
Nitschke and Huc reported the use of metal complexes as dynamic connection elements
between oligomeric helical segments [38]. Specifically, Cu
þ
and Fe
2þ
complexes were
applied to link and define the relative orientation of two helices mimicking a turn struc-
ture in proteins, but at an unconventional angle. This work focused on the aromatic oli-
goamides of 8-amino-2-quinoline carboxylic acid, which adopt particularly stable helical
conformations in the solid state and in a wide variety of solvents [39]. Amine-functional-
ized tetramer
9
(Figure 11.17) adopts a helical conformation that spans over one and a
half turns. The reaction of
9
with 6-methyl-2-formylpyridine and CuBF
4
produced a pseu-
dotetrahedral Cu
þ
complex
10
, as characterized by mass spectrometry,
1
H-NMR spec-
troscopy and X-ray crystallography. The conformation of
10
features several intrinsically
chiral elements that are all expected to undergo dynamic exchange: the right (
P
) or left
(
M
) handedness of the two helical segments and the L or D configuration [40] of the metal
complex.
1
H-NMR spectroscopy indicates a high degree of influence on the handedness
of each helix by the configuration of the neighboring metal. Crystallographic investiga-
tions allowed the characterization of four out of the six possible forms of
10
.
As expected, the two 2-iminopyridine moieties formed a tetrahedral complex with Cu
þ
.
This geometry dictated an unusual, perpendicular, orientation between the two helices
(Figure 11.18). The unconventional 90
angle between two helices in
10
constitutes a
unique motif that hints at the prospect of assembling large square structures comprised of
helical oligomer “edges” bearing amine functions at both extremities linked by metal
complexes at each “corner”. Mixing
1
with 2-formylpyridine and Fe(BF
4
)
2
in acetonitrile
resulted in a mixture of compounds, but only one gave crystallography suitable crystals.
X-Ray diffraction analysis provided the structure of a racemic
M
D
M
/
P
L
P
Fe
2þ
complex
i
BuO
O
CuBF
4
(0.5 equiv)
N
N
N
O
H
NH
2
4
NO
2
9
i
BuO
O
2
N
O
H
N
Cu
+
N
N
N
N
N
N
N
H
4
O
NO
2
i
BuO
10
4
Figure 11.17 Equilibrium between 9 and tetrahedral Cu
þ
complex 10. Reprinted with per-
mission from Ref. [38]. Copyright 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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