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interconversion between the two isomers is a single-step process
involving the rotation of one quinoline unit.
The inclusion complexes of the palladium metallocycles
M7a
with aromatic guests were also studied. Addition of one equivalent
of hydroquinone (
S1a
) to an equimolar 10 mM D
O solution of
2
ligand
L7
·NO
and (en)Pd(NO
)
results in the appearance of a
3
3
2
1
13
reddish-colored solution. The
H and
C NMR spectra showed
an approximate 1:1 mixture of the
syn
and
anti
S1a
⊂
M7a
·6NO
3
atropoisomers, as can be concluded from the two sets of signals
and 2D NMR data. Similar results were obtained with resorcinol,
cathecol, and phloroglucinol.
The
inclusion
complex
with
1,5-bis[2-(2-hydroxyethoxy)-
ethoxy]naphthalene (
S2b
) has also been studied. In this case, the
anti
stereoisomer is the only one present in the solution, as shown
by NMR analysis. The
atropoisomer is the only one compatible
with the presence of the inversion center evidenced by the NMR
spectrum. As expected, the X-ray structure of
anti
S2b
⊂
M7a
·6NO
3
shows the exclusive presence of the
stereoisomer in the solid
state (see Fig. 11.9). Both components lie about a common inversion
center located at the centroid defined by the guest. Four [C-H
anti
π]
interactions between the guest (at positions C-3, C-4, C-7, and C-8)
and the quinoline rings, resulting from the perfect host-guest fit,
further contribute to the structure stabilization in solution and in
solid phase. In the
isomer, only three of these interactions would
be possible. Steric hindrance between the polyether chains and the
quinoline could also contribute to the destabilization of the
syn
syn
system. An additional experiment was carried out trying to force the
formation of the
stereoisomer. This time, the template of choice,
1,3-bis[2-(2-hydroxyethoxy)ethoxy]naphthalene (1,3-DHN), is the
result of shifting the unfavorable polyether chain at the C-5 position,
which creates steric hindrance with the quinoline moiety in the
syn
syn
stereoisomer, to the C-3 position, in which a similar impediment is
introduced with the quinoline in the undesired
1
H
NMR spectrum of the resulting solution shows the presence of two
isomers in an approximate 2:1 ratio. Although a specific assignment
of the two sets of signals to a particular isomer (
anti
isomer. The
) was
not possible, these results clearly indicate that the nature of the
aromatic guest plays an important role in the proportion of the
two atropoisomers present in the solution. The existence of two
syn
or
anti
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