Synthesis, structure, and aromaticity of a hoop-shaped cyclic benzenoid [10]cyclophenacene

Article abstract of DOI:10.1021/ja029915z

The first hoop-shaped cyclic benzenoid compounds, [10]cyclophenacene derivatives that contain 40 π electrons, have been synthesized in three or four steps from [60]fullerene by rationally designed chemical modification. The compounds thus synthesized are chemically stable, yellow-colored, luminescent, and EPR-silent. X-ray crystallographic analysis provided high precision structural data sets. On the basis of these results and theoretical investigations, the new cyclic benzenoid molecules were proven to be aromatic. Copyright

Full text of DOI:10.1021/ja029915z

Published on Web 02/12/2003
Synthesis, Structure, and Aromaticity of a Hoop-Shaped Cyclic Benzenoid
[10]Cyclophenacene
Eiichi Nakamura,* Kazukuni Tahara, Yutaka Matsuo, and Masaya Sawamura†
Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received December 24, 2002 ; E-mail: nakamura@chem.s.u-tokyo.ac.jp
Having thus far remained hypothetical,1,2 hoop-shaped cyclic
benzenoid compounds have attracted the interest of chemists for
half of a century3 for their aromaticity,4 potential utilities in materials
science,1 and their structures themselves that have challenged
synthetic chemists for a number of years.2 Comprising a part of
the structure of a carbon nanotube (CNT), they are now also the
subject of interest of a broader scientific community.5 We report
herein the first synthesis of the hoop-shaped benzenoid, specifically,
carbon-capped derivatives of [10]cyclophenacene 1 (Figure 1b) and
its derivatives 4 and 5. The synthesis was achieved in three or four
steps by rationally designed chemical modification of [60]fullerene.
Single-crystal X-ray crystallographic analysis afforded a high-
precision structure ((0.02 Å) of this CNT substance. Note that the
structural information on the CNTs has been obtained largely by
scanning tunneling microscopic measurements, which are not
precise enough to discuss the C-C bond lengths.6
Figure 1. [10]Cyclophenacene and its carbon-substituted derivatives. (a)
Some representative resonance structures of the parent compound C40H20
(A: C5V symmetry). Hydrogen atoms are omitted for clarity. (b) Structure
of C60R1 R2 H2 (1 and B). The [10]cyclophenacene system is made darker
5
5
than the rest for clarity.
A 40 π-electron cyclic benzenoid A, [10]cyclophenacene (C40H20),
can be generated by rolling a polyphenantherene-type graphite
ribbon7 and hence represents the shortest armchair (5,5) CNT. The
nature of this cyclic benzenoid may be described by certain
resonance structures such as those shown in Figure 1a,8 and previous
theoretical studies predicted it to be aromatic.4 This latter point
has yet to be proven by experiments.
There are a priori two rational synthetic approaches to the hoop-
shaped benzenoid molecules,1,2 one by rolling a flat precursor9 and
the other by detraction of the conjugated system of fullerenes or
CNTs. None have thus far been successful. Our approach shown
in Figure 2 relies on an organocopper reaction that converts the
[60]fullerene exclusively and quantitatively into a cyclopentadiene
compound 2.10 Repetition of the reaction on the bottom 50
π-electron part of 2 should directly produce the desired cyclic 40
π-electron system in three steps overall from [60]fullerene.
However, it did not take place at all: The copper reagent
deprotonates 2 to generate the corresponding cyclopentadienyl
anion, which is entirely unreactive toward further addition reactions.
To circumvent this problem, we temporarily protected the acidic
hydrogen atom in 2 with a cyano group, which was later removed
after the second penta-addition was achieved. In this synthetic
sequence, the electronegative cyano protective group also acts to
increase the electrophilicity of the 50 π-electron system (the first
reduction potential of 2, -1.48 V (Fc/Fc+); that of 3, to -1.35 V
(Fc/Fc+) in tetrahydrofuran (THF)).
Figure 2. Synthesis of [10]cyclophenacene derivatives. Reaction conditions,
a: (1) MeMgBr (30 equiv), CuBr‚SMe2 (30 equiv), N,N′-dimethylimida-
t
zolidinone (DMI, 30 equiv) in THF, (2) NH4Cl/H2O, 92%. b: (1) BuOK
(1.1 equiv) in THF, (2) TsCN (1.2 equiv) in PhCN, 63%. c: (1) PhMgBr
(30 equiv), CuBr‚SMe2 (30 equiv), DMI (30 equiv) in THF, (2) NH4Cl/
H2O, 14%. d: (1) Li+[C10H10]- (30 equiv) in PhCN, (2) NH4Cl/H2O, 82%.
e: (1) KH in THF, (2) under air in THF, 42%. For 1, 4, and 5, isomers
always formed as to the relative stereochemistry of the top and the bottom
pentagons (only one isomer is shown in Figure 2).
The cyanide group in 4 was reductively removed by treatment
with lithium-naphthalenide to obtain C60Me5Ph5H2 (1) in 82% yield.
This compound was found not to give any of the EPR signals (solid,
at 4 K). In agreement with its closed shell, aromatic (vide infra)
character, the 40 π-electron system was found to be chemically
stable. Treatment of 1 with potassium hydride followed by exposure
to molecular oxygen afforded the penta-oxygenated product C60-
Me5Ph5O3(OH)2 (5) that gave single crystals suitable for X-ray
analysis. The absorption spectra of the [10]cyclophenacene mol-
ecules (1, 4, and 5 in cyclohexane) are similar to each other,
showing a maximum at ca. 260 nm with broad absorption that
extends to ca. 500 nm. The compounds 1 and 5 are strongly
luminescent, emitting bright yellow light (ca. 560 nm and ca. 620
The synthesis started with penta-methylated [60]fullerene 2,
which was synthesized in 92% isolated yield from [60]fullerene.11
t
It was treated first with BuOK and then with p-toluenesul-
fonyl cyanide (TsCN) to obtain the cyano fullerene 3 in 63%
yield. Treatment of 3 with a phenylcopper reagent afforded the
[10]cyclophenacene C60(CN)Me5Ph5H (4) in 14% isolated yield.12
† Present address: Department of Chemistry, Hokkaido University, Kita-ku,
Sapporo 060-0810, Japan.
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J. AM. CHEM. SOC. 2003, 125, 2834-2835
10.1021/ja029915z CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Experimental Data (Standard Deviation in Parentheses)
Obtained by X-ray Crystallographic Analysis and Theoretical
Optimized Structures of Model Compounds A (C40H20) and B
Acknowledgment. The present research was supported by a
Grant-in-Aid for Scientific Research (Specially Promoted Research)
and The 21st Century COE Programs in Fundamental Chemistry.
We thank Prof. J. Aihara for fruitful discussions on aromaticity.
(C60H12
)
bond length (Å)a,b
iv
compounds
i
ii
iii
v
vi
vii
Supporting Information Available: Synthetic procedure, crystal-
lographic data, and computational details of [10]cyclophenacene (PDF,
CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
5
1.37(2) 1.44(2) 1.43(1) 1.40(2) 1.43(1) 1.45(1) 1.36(1)
A
(HF)
(B3LYP) 1.37
(PM3)
1.35
1.44
1.44
1.43
1.43
1.45
1.44
1.38
1.42
1.40
1.36
B
References
(HF)
(B3LYP) 1.37
(PM3) 1.37
1.35
1.44
1.45
1.44
1.43
1.44
1.44
1.38
1.39
1.40
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a i, iii, v, vii: Average lengths of five equivalent bonds. ii, iv, vi: Average
lengths of 10 equivalent bonds. Calculated data for A and B refer to the
geometry optimized structures (C2h symmetry and D5d symmetry, respec-
tively) obtained by the hybrid density functional method (B3LYP), the
Hartree-Fock ab initio method (HF) using the 6-31G* basis set, and the
semiempirical PM3 method. b See Figure 1 for the numbering of the C-C
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nm) with quantum efficiency of 0.10 in cyclohexane (irradiation
at 366 nm, rhodamin B as standard; see Supporting Information).
X-ray crystallographic structures were obtained for 5 (Table 1).
Whereas the double bonds on the edge are short (bonds i and vii,
1.36(2) and 1.37(2) Å, respectively), bond alternation in the equator
region is very small (iii, v ) 1.43(1) and iv ) 1.40(2) Å). This
experimental structure obviously does not conform to the “ideal
graphitic structure”, which was assumed in most of the previous
theoretical studies of CNTs,13 but rather similar to the Kekule´
structure (a) in Figure 1a. The geometries of the model compounds
C40H20 (A) and C60H12 (B) were optimized with quantum mechan-
ical calculations and found to reproduce the experimental data very
well (Table 1). Note that, in contrast to the [10]cyclophenacenes,
there is distinctive bond alternation in [60]fullerene (1.36 vs 1.47
Å),14 in 1,3-butadiene (1.35 vs 1.47 Å),15,16 and in 20 π-electron
cyclic cis-polyacetylene (1.36 vs 1.46 Å, B3LYP/6-31G*-optimized,
see Supporting Information).
Nucleus independent chemical shift (NICS)17 is a useful measure
of the magnetic shielding effect of the aromatic ring current.
Analysis of the NICS values for six-membered rings of cyclophen-
acene in the model compounds A and B indicates that the hoop-
like 40 π-electron system is aromatic (NICS ) -8.62 and NICS
) -11.46 to -11.99, respectively) and the other rings in B are
nonaromatic (NICS ) -1.27 to 0.30). The center of gravity (CG;
NICS ) -7.25 and NICS ) -11.58) is predicted to be subject to
an aromatic shielding effect (a value experimentally provable by
3He NMR experiments).18
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The synthesis of the [10]cyclophenacene compounds, which
represent the shortest (5,5) CNT, provided the first information on
the structure as well as chemical and physical properties of the
hoop-shaped cyclic benzenoid. The compounds were found to be
stable and aromatic, and luminescent (1 and 5). This last property
is intriguing in view of the recent reports on luminescent CNT.19
Finally, the bifunctional nature of compounds (cf. 1) suggests their
use as linkers in a metal/fullerene alternating polymer20 through
formation of η5-metal fullerene complexes.21
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E. Nature 2002, 419, 702.
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