Chemistry Reference
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Fig. 23 Structures of the cyclic porphyrin trimer 53 and of nanobarrel 54
fullerenes, so it shows a binding constant of log
K
a
¼
8.2 for C
70
and log
K
a
>
9 for
C
86
, all in toluene at room temperature.
Osuka and co-workers have gone one step further, linking four porphyrin units to
form what they called a “nanobarrel” (54 in Fig.
23
)[
170
]. The solid state structure
of 54 shows a rigid concave cavity of adequate size to associate with C
60
. Despite
this, the authors reported a binding constant of log
K
a
¼
5.7 in toluene at room
temperature, perhaps not as large as could be anticipated. This might be due to the
use of Ni porphyrins, since previous studies have shown a decrease in the binding
constant of approximately one order of magnitude from zinc to nickel in macrocy-
clic porphyrin dimers [
171
]. Alternatively it might be that the very rigid structure of
54 is not sufficiently flexible to optimize the C
60
-porphyrin distances, again
showing the adverse effects of an excess of preorganization.
Our group has also gone the distance from tweezers to macrocycles. We have
recently synthesized a family of nine macrocyclic exTTF-based receptors, in which
we have conserved the basic features of the tweezers design (two exTTF units
linked through an aromatic spacer) and added alkene-terminated alkyl spacers, to
perform ring-closing metathesis [
172
,
173
]. We produced systematic variations of
both the aromatic and the alkyl spacers, as shown in Fig.
24
. The structural variation
strategy proved to be successful, as among the family of hosts we found some of the
best purely organic hosts for both C
60
and C
70
. For instance,
p
-xylmac12 associates
with C
60
with log
K
a
¼
6.5 in chlorobenzene and 7.5 in benzonitrile, both at room
temperature. Perhaps more interestingly, the synthesis of such a complete family of
macrocyclic receptors showed that even very small variations in structure can lead
to huge changes in binding abilities. For example, there is a difference of three
orders of magnitude between the association constant of
p
-xylmac12 towards C
60
and that of
p
-xylmac10 (log
K
a
¼
3.5 under the same experimental conditions).
Besides changes in the stability constants, we even found variations in the stoichi-
ometry of the associates. The smaller members of the family,
p
-xylmac10 and
m
-xylmac10, associate with C
70
forming both 1:1 and 2:1 host:guest complexes,
while naphmac10, with only a slightly bigger cavity, forms exclusively 1:1
associates.
