Biology Reference
In-Depth Information
11.4.3
Stereoselective Self-Assembly of
Metallomacrocycles Induced by an Aromatic
Guest
The design of receptors capable of adapting their structure to bind
a guest in a reminiscent of Koshland's induced model for substrate
binding to enzymes has attracted much attention because such
induced fit is the basis of many allosteric systems. The incorporation
of 4-substituted quinoline moieties in the short sides of a molecular
rectangle, which can rotate around the CH
-N axis gives rise to the
2
existence of two atropoisomers (
) with different cavity
characteristics (Fig. 11.8). The advantage of this “flexible” receptor is
that it can change its shape in order to maximize the intermolecular
interactions with different guests (induced fit).
syn
and
anti
Figure 11.8
Structures of
syn
and
anti
isomeric metallocycles
M7
.
The reaction between ligand
L7
·NO
and Pt(en)(NO
)
was
3
3
2
carried out at 100
C to exploit the “molecular lock” strategy. The
reaction allowed the isolation of a 1:1 mixture of the
°
syn
and
anti
atropoisomers of
as it can be seen from the inspection of the
1D and 2D NMR spectra [28]. The
M7b
1
H NMR spectrum of the mixture
recorded at room temperature is consistent with the presence
of a 1:1 mixture of the
syn
and
anti
atropoisomers. The activation
barrier for the interconversion
syn/anti
process was calculated
‡
c
-1
by the coalescence method (
). The computed
Gibbs free energy barrier for the same interconversion process
amounts to 69.9 kJ mol
∆
G
= 75.3 kJ mol
-1
, which is in reasonably good agreement
with the experimental obtained value. These results show that the
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