Chemistry Reference
In-Depth Information
Fig. 22 Structure of the macrocyclic bisporphyrin receptors reported by Aida
actually be detrimental to the binding event [
166
]. We will now see a few examples
of macrocyclic hosts for fullerenes to illustrate these points.
In 1999, Aida, Saigo and co-workers published the first example of a
bisporphyrin macrocyclic host for C
60
(52a in Fig.
22
)[
167
], initiating what
would become one of the largest and most successful families of receptors for
fullerenes [
140
]. Through multiple structural variations they have been able to
establish some clear structure-binding affinity relationships. For instance, it has
become apparent that not only the length but also the flexibility of the links between
the porphyrins is critical to the association event. The synthetic precursors to the
alkane spacers are the corresponding alkynes, 52b. The macrocycles with these
rigid spacers do not show
any
sign of binding towards C
60
, despite theoretically
having a cavity of the right size. In contrast, the macrocycles with flexible alkane
spacers show remarkably high binding constants, including the world-record in
complex stability, 52c, with an incredibly high log
K
a
¼
8.1 toward C
60
in 1,2-
dichlorobenzene (
o
-DCB) at room temperature [
168
]. This is a dramatic example of
the price one has to pay for increased preorganization: it is often a make or break
situation.
In the last few years, examples of macrocyclic hosts for fullerenes including
more than one porphyrin recognition motif have been reported. Anderson's group
has described a rigid cyclic porphyrin trimer, 53, which associates with C
60
with
log
K
a
¼
6.2 [
169
]. The cavity of 53 is actually better suited to host higher
