Chemistry Reference
In-Depth Information
Fig. 16 Scheme depicting
the concave-convex
interaction vs planar
ˀˀ
interaction
that very weak, non-directional dispersion interactions (mainly van der Waals and
ˀˀ
) will account for the vast majority of the binding energy in host-guest
complexes [
139
]. Since these forces depend directly on surface area, the shape
complementarity between host and guest becomes critical. In this sense, distorted
concave recognition motifs seem ideally suited for the association of the convex
fullerenes. The importance of this shape complementarity is beautifully illustrated
by the distortion from planarity observed in the solid state structure of some
porphyrin-fullerene supramolecular complexes reported by Aida. In the associates
the porphyrins adopt a non-planar concave conformation to maximize the positive
interactions with the C
60
guest, even at the expense of some degree of conjugation
[
140
]. Besides the optimization of dispersion interactions due to shape complemen-
tarity between concave hosts and convex fullerene guests, in 2006 Kawase and
Kurata suggested that there might be an additional positive effect arising from the
unsymmetrical nature of the
orbitals of the contorted molecules with respect to
the convex (outer in the case of the fullerenes) and concave (inner in the case of the
fullerene) sides [
141
]. They termed this “concave-convex interaction” (Fig.
16
).
Despite this, most of the examples of receptors for fullerene reported to date rely
on planar recognition motifs. Curved molecules are geometrically tensioned
structures, with bond angles away from the preferred ones and consequently are
not always an easy synthetic target. However, some concave molecules are syn-
thetically accessible and have been employed as hosts for fullerenes. We will now
overview some prominent examples.
Corannulene consists of five benzene rings fused into a central five-member ring.
As could be expected considering its size - nearly identical to that of C
60
and thus
too small to associate with it - chemical derivatization is necessary to enlarge the
cavity of corannulene and observe binding. Following this strategy, several
monotopic receptors have been reported [
142
].
A ditopic receptor, “buckycatcher” 40, was synthesized by Sygula et al. joining
together two units of coronene via a rigid aromatic spacer [
143
], forming a
tweezers-like host [
144
]. It forms stable complexes with C
60
(log
K
a
¼
ˀ
3.9,
d
8
-
toluene, room temperature),
1
in which the fullerene is included between the two
coronene units, as demonstrated through X-ray diffraction studies on cocrystals of
40 and C
60
(Fig.
17
).
Another very interesting family of curved aromatic hosts for fullerenes are the
cyclic [
n
]
para
-phenyl acetylenes (CPPAs, 41-44 in Fig.
18
) reported by Kawase
1
All through the article we will report binding constants as logarithms, and without an error
interval, for simplicity. The reader can refer to the original publications for these data.
