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S1b
) increased only very slightly the regioselectivity with respect
to the substrates without those chains. These observations clearly
support the fact that the π
-
π
stacking forces are the main source of
the regioselectivity.
11.6
Molecular Knots
Continuing our investigations on self-assembled [2] and [3]
catenanes, we reported the structure and synthesis of a doubly
interlocked [2]catenane, a new molecular Salomon link, obtained
by a five-component self-assembly process based on coordinative
bonds and π-donor/π-acceptor interactions. The self-assembly
was accomplished following the same three-component strategy
described in the [2] and [3]catenane syntheses. In this case the
topology of the new species is probably conditioned by the larger
ring size and flexibility of the π-donor component (
TPP68C20
),
which supplies four dioxoaryl units to participate in π
-
π
interactions
with the metallocycle [36].
It is well known that 2,7-diazapyrenium salts are good electron
acceptors, so we carried out the synthesis of ligand
from
2,7-diazapyrene and dibromomethane. As expected, the formation of
the square metallocycle from
L8
·2PF
6
L8
·2PF
and (en)PdOTf
proceeded at
6
2
room temperature in CD
. The spectroscopic characteristics are
very similar to those of the corresponding bipyridine metallocycles
described earlier.
NO
3
2
The diazapyrenium catenane was self-assembled from ligand
L8
·2PF
, cyclophane
TPP68C20
, and (en)PdOTf
in CD
NO
(Scheme
6
2
3
2
1
11.6). The
H NMR spectrum recorded at room temperature showed
separated resonances for the parallel and orthogonal diazapyrenium
subunits. Interestingly, each set appeared as eight resonances due to
the nonequivalence of the eight diazapyrene protons, four singlets
for the
ones. In other words, the
rotation of the diazapyrenes along its N-N axis at room temperature
is slow on the NMR time scale, resulting in the protons of both sides
of the equatorial plane of the macrocycle becoming nonequivalent.
Also, methylene protons became nonequivalent, with a geminal
coupling constant of 14.0 Hz.
α
protons, and four doublets for the
γ
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