Chemistry Reference
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Fig. 9 Crystal structures of 12 HMB and 13 tol [
17
]
yielded very strong inclusion complexes with a Gibbs activation energy for
dissociation (
0.3 kcal/mol [
18
]. This interaction is assumed to be
stronger then the calix[8]arene-C
60
complex that is used to purify C
60
from carbon
soot on a commercial scale [
19
,
20
]. The authors attribute the strength of this
interaction to polar electrostatic interactions of the CPPA
Δ
G
{
)of9.9
-system with the
electron-deficient [5:6] fusions in fullerene. Furthermore, the complex of bis
(ethoxycarbonyl)methanofullerene and [6]CPPA was crystallized and X-ray crys-
tallographic data confirmed the orientation of the CPPA phenyl rings in proximity
to the [5:6] fullerene fusions (Fig.
10
).
This structure also revealed that the fullerene is only partially encapsulated, with
an average intermolecular distance of 3.4
˚
, similar to the interlayer distance in
multi-walled carbon nanotubes and multi-layer graphene. From this discovery,
Kawase was able to investigate further the dynamics of the [6]CPPA-fullerene
interaction [
21
] and also to synthesize naphthyl- derivatives of CPPAs with deeper
cavities and stronger interactions such as 16-18 (Figs.
11
and
12
)[
22
,
23
] (Table
1
).
With experience synthesizing CPPA-fullerene complexes, Kawase went on to
design and isolate an onion complex of naphthyl substituted [9]CPPA and [6]CPPA
with C
60
(Fig.
13
)[
24
].
Similar multilayer arrangements are manufactured by “top-down” methods (multi-
walled carbon nanotubes, buckyonions) but this was the first example of the assembly
of such a structure from the bottom up [
25
,
26
]. Furthermore, at the time it represented
the first double inclusion complex of three synthetically accessible organic molecules
[
24
]. Kawase found that, like the previous CPPA-fullerene complexes, the intermo-
lecular distance in this onion complex was 3.2
˚
, closely resembling the equivalent
distance in carbon materials. This observation demonstrated further proof that
discrete, small molecule belts can be accurate models of larger carbon structures.
π
