Chemistry Reference
In-Depth Information
ether with an excess of sodium hydride effected a nucleophilic aromatic substitution
to form the 2,3-bis(diethylene glycol methyl ether) derivative 35.
2.1.2 Structure and Properties of Simple Corannulene Derivatives
The curvature and rigidity of a buckybowl can be characterized by the bowl depth,
POAV (
π
-orbital axis vector) pyramidalization angle [
58
-
60
], and bowl-to-bowl
inversion barrier (
G
{
inv
). The experimental data of the first two are available by
single-crystal X-ray crystallography of the desired bowl molecule. Because the
barriers of many corannulene derivatives lie in the region of 7-20 kcal/mol, the
ʔ
ʔ
G
{
inv
can be conveniently analyzed by variable temperature NMR study of a
suitably derivatized molecule.
The C-C bonds in corannulene are conveniently classified as Rim, Flank, Hub,
and Spoke (Fig.
1
). X-Ray crystallographic analysis of 1 showed a hub-to-rim bowl
depth of 0.87
[
63
,
64
], as measured from the center of the five-membered ring to
the plane containing the peripheral aromatic carbon atoms, and this is significantly
shallower than would be expected for the polar cap within C
60
(ca. 1.5
Å
)[
65
].
The bowl depth for simple corannulene derivatives strongly depends upon the
kind, number, and position of substituent(s). Generally, they have smaller or
comparable values to corannulene. The representative examples include 28-F
(0.87
Å
)
[
41
]. 1,2,5,6-Tetrabromocorannulene 18 is an exception; its bowl depth was deter-
mined to be 0.91
)[
40
], 28-Cl (0.86
)[
40
], 21c (0.80
)[
40
], and 23c (0.82
Å
Å
Å
Å
[
39
].
The POAV pyramidalization angle of the carbon on the central five-membered
ring (hub) in corannulene is 8.2
, in contrast to 11.6
for C
60
. The rim carbons are
predicted to be only slightly pyramidalized with pyramidalization angles in the
range of 1-2
. Unlike the crystal structures of pyrene and related aromatic com-
pounds of comparable surface area, the crystal structure of 1 is void of any aromatic
face-to-face or bowl stacking. However, a mixture of corannulene and fullerene C
60
forms cocrystals [1·C
60
][
66
]. The shortest distances from C
60
to the concave
surface of 1 and the convex surface of another 1 were determined to be 3.75 and
3.21
Å
, respectively.
The bowl-to-bowl inversion process of corannulene (1) is defined as bowl form
1 converting to form 1
0
via a planar transition structure 1
{
(Fig.
1
). The difference in
energy between 1 and 1
{
represents the bowl inversion energy (
Å
G
{
inv
), which can
be measured experimentally by variable temperature
1
H NMR study of suitably-
substituted corannulenes. Values of
ʔ
G
{
inv
were determined to lie in the range of
10.2-11.3 kcal/mol, depending on the substituent R. Benzylcorannulene [
61
],
isopropylcorannulene [
61
], and bromomethylcorannulene [
61
] all exhibit a slightly
higher
ʔ
ʔ
G
{
inv
than 1-corannulenyl-1-methyl-1-ethanol [
51
]. The experimental
G
{
inv
for the non-substituted parent 1 was estimated to be 11.5 kcal/
mol although it cannot be directly measured due to symmetry [
62
]. A recent
investigation indicated that the bowl-to-inversion barrier for corannulene can be
ʔ
value of
