Chemistry Reference
In-Depth Information
18
18
C
60
Scheme 7 Naphthalene macrocycle 18 and its C
60
complex
4.2 Naphthalene Macrocycles
Kawase et al. reported various cyclic arylene-ethynylene oligomers as carbon
nanorings showing novel structures and properties [
52
,
53
]. Compound 18 is an
example of six 1,4-naphthylene units (Scheme
7
). This compound forms a complex
with C
60
more tightly than the corresponding phenylene derivative. The NMR
spectra of 18
90
C and
the major isomer was assignable to the symmetric isomer with all fused benzene
groups at the same side. It is noteworthy that the flipping of one naphthylene unit
shown in Scheme
7
takes place with maintaining the complexation, as revealed by
VT
13
C NMR spectra of a complex with
13
C-enriched C
60
.
C
60
complex revealed the presence of two isomers at
4.3 Anthracene Macrocycles
An anthracene unit possesses a rectangular-like shape and several possibilities of
linkage. The mode of substitutions determines the distance and direction of the
structural extension. Typical examples are 1,8-subsituted, 1,5-subsituted, and 9,10-
subsituted anthracene (hereafter abbreviated as 1,8-A and so on) units with zigzag,
crank, and linear connectivities, respectively [
54
,
55
]. In terms of geometry and
synthesis, the use of 1,8-A units is convenient for the construction of 3D structures.
Compound 19 with four 1,8-A units and four ethynylene linkers was synthesized
by successive cross coupling reactions from 1,8-diiodoanthracene and 1,8-
diethynylanthracene (Scheme
8
)[
56
,
57
]. X-ray analysis revealed that this tetramer
had a 3D diamond-prism structure of
D
2
symmetry. Two pairs of anthracene units
take a parallel orientation at the interfacial distance of 3.38
˚
, which is comparable
to the sum of the van der Waals radii of two aromatic carbons (1.7
˚
2).
Tetramer 19 undergoes a dynamic process between the two diamond forms via
rotation about the four acetylene linkers. The barrier to this process (38 kJ/mol) was
determined by dynamic NMR spectroscopy. A relatively high barrier is attributed to
the
interactions between the anthracene units stabilizing the original state.
This basic model was modified by two methods, the introduction of substituents
on anthracene units or the incorporation of long butadiynylene linkers. Compounds
20 and 21 are such examples having chiral 3D structures [
58
,
59
]. Tetramer 20 has
p
p
