"Double-concave" graphene: Permethoxylated hexa-peri- hexabenzocoronene and its cocrystals with hexafluorobenzene and fullerene

Article abstract of DOI:10.1002/anie.200461445

Extremely nonplanar permethoxylated hexa-peri-hexabenzocoronene can be prepared by using a facile ferric chloride cyclodehydrogenation procedure. The combination of a rigid "double-concave" aromatic core with 18 flexible methoxy groups at the periphery ma

Full text of DOI:10.1002/anie.200461445

Angewandte
Chemie
the emergence of a few macrocyclic fullerene receptors.[6]
Porphyrins and metalloporphyrins with planar p surfaces
have also been shown to interact with the curved p surface of
fullerenes, predominantly through van der Waals dispersion
forces.[7]
The arene–perfluoroarene stacking interactions, which
have been shown to occur in numerous 1:1 complexes,[8] are of
substantial theoretical and practical importance because of
their role in solid-state polymerization,[9a] cross-linking of
hydrogels,[9b] molecular electronics[9c] and liquid-crystal stabi-
lization.[9d]
Herein, we introduce a strongly twisted graphene mole-
cule, namely, permethoxylated hexa-peri-hexabenzocoronene
(permethoxylated HBC, 4) with a remarkable “double-
concave” conformation, as revealed by single-crystal analysis.
The combination of a rigid “double-concave” aromatic core
with eighteen flexible methoxy groups at the periphery
suggests 4 is an ideal model compound for supramolecular
host–guest chemistry. Accordingly, we elucidate the single-
crystal structures of its crystalline inclusion complexes with
hexafluorobenzene (HFB) and fullerene guest molecules, the
self-assembly of which is controlled mainly by arene–
perfluroarene interactions and geometrically complementary
van der Waals interactions, respectively.
Permethoxylated HBC 4 was prepared in three simple
steps from commercially available starting materials
(Scheme 1). The synthesis of the symmetric 3,3’,4,4’,5,5’-
hexamethoxydiphenylacetylene (2) was based on a Stille-type
coupling of 5-bromo-1,2,3-trimethoxybenzene (1) with bis-
(tributylstannyl)acetylene. Subsequent cyclotrimerization of
the diphenylacetylene with [Co2(CO)8] yielded the hexaphe-
nylbenzene precursor molecule 3 bearing 18 alkoxy substitu-
ents. The remarkable oxidative cyclodehydrogenation of 3
was carried out under the established ferric chloride con-
ditions.[10] The crude products were purified by column
chromatography to yield 4 as an orange solid in 52% yield
for the last step (see Supporting Information).
Noncovalent Interactions
“Double-Concave” Graphene: Permethoxylated
Hexa-peri-hexabenzocoronene and Its Cocrystals
with Hexafluorobenzene and Fullerene**
Zhaohui Wang, Florian Dꢀtz, Volker Enkelmann, and
Klaus Mꢁllen*
Polycyclic aromatic hydrocarbons (PAHs) represent a unique
class of organic molecules since they combine the fascination
of fullerene analogues with the outstanding materials proper-
ties of graphite and conducting polymers.[1] Their planarity is
often presumed to be their most significant geometric
characteristic, however, the synthesis of molecules which
are at variance with this structural truism has drawn persistent
attention from many organic chemists, in particular in view of
the total synthesis of fullerene.[2] As a result of the remarkable
inner strain of these bent p systems, most of the synthetic
routes require extreme conditions such as flash vacuum
pyrolysis (FVP), and therefore suffer from low yields and
tedious chromatographic separation steps, although a few
solution-phase synthesis have been reported.[3]
Recently there has been an increasing interest in supra-
moleular fullerene chemistry because of its potential appli-
cations in chemistry, biology, and materials science.[4] The
design of host molecules capable of recognizing fullerenes is
mainly based on a kind of complementary principle where the
utilization of concave/convex interactions[5] has resulted in
Permethoxylated HBC 4 is the first persubstituted HBC
derivative prepared. The additive effect of 18 donor groups to
the outer phenyl rings should facilitate an oxidative ring
closure whereas the complete substitution of the outer
perimeter by alkoxy groups successfully prevents chlorination
of the core. The absorption maximum of 4 shows a significant
bathochromic shift of 37 nm with respect to the unsubstituted
parent HBC as a consequence of the 18 electron-donating
alkoxy substituents and the expected nonplanarity caused by
steric congestion[11] (see Supporting Information).
[*] Dr. Z. Wang, Dr. F. Dꢀtz,[+] Dr. V. Enkelmann, Prof. Dr. K. Mꢁllen
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-351
E-mail: muellen@mpip-mainz.mpg.de
[+] Present address: BASF PolymerResearch
To determine the three-dimensional structure of 4,
crystals suitable for single-crystal X-ray structure analysis
were obtained by slow evaporation of a solution of 4 in
dichloromethane at room temperature.[12] The crystal struc-
ture was found to be markedly nonplanar, thus reflecting the
presence of pronounced peri interactions (Figure 1a). The six
carbon atoms of the central benzene ring are coplanar to
within 0.014 ꢀ. The steric congestion in the bay region forces
the outer aromatic rings to flip up and down in an alternating
manner with respect to the inner ring. A similar regular
pattern is also observed for the orientation of the methoxy
groups which alternately point up and down. The orientation
GKS/A-B001, 67056 Ludwigshafen (Germany)
[**] This work was financially supported by the Zentrum fꢁr Multi-
funktionelle Werkstoffe und Miniaturisierte Funktionseinheiten
(BMBF 03N6500), the EU-TMR project SISITOMAS, the Deutsche
Forschungsgemeinschaft (Schwerpunkt Feldeffekttransistoren),
and the EU project DISCEL (G5RD-CT-2000–00321). We thank R.
Bauer for his help with graphical presentations.
Supporting information for this article (the synthesis of compound
4, packing arrangement of the [(hfb)2ꢀ4] complex, and UV/Vis
absorption spectra of 4) is available on the WWW under http://
www.angewandte.org or from the author.
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DOI: 10.1002/anie.200461445
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Inclusion of hexafluorobenzene should be
revealing since electron-deficient and elec-
tron-rich aromatic rings (for example, HFB
and the central coplanar benzene ring of 4,
respectively) have an overwhelming prefer-
ence for a face-to-face or p-stacked sand-
wich geometry because of favorable electro-
static quadrupole–quadrupole interactions.
Furthermore, a space-filling model shows
clearly that HFB fits perfectly into the
cavity of 4 (Figure 2).
Crystals of the 2:1 complex [(hfb)2ꢀ4]
possess an asymmetric unit with half a
molecule of 4 (on a center of symmetry)
and one HFB molecule. HFB and the
central ring in 4 are not strictly parallel.
As a consequence, the distance of the
C atoms of HFB to the plane of 4 ranges
from 3.27 to 3.37 ꢀ, and the F atoms of HFB
Scheme 1. Synthetic route to permethoxylated HBC (4).
Figure 1. a) Single-crystal structure of 4; b) herringbone packing
arrangement of 4 in the solid state.
of the two meta-methoxy groups is found to coincide with that
of the twisted phenyl ring, for example, in the case of a phenyl
ring flipping down they are oriented in the same direction.
The para-methoxy group is oriented in the opposite way.
Figure 2. Space-filling model of the complex formed between HFB
and 4.
The severe peri interactions destabilizing
a planar
D6h form result in the molecule displaying two concave
faces and adopting a centrosymmetric conformation. A
similar alternating distortion is observed in perchlorocoro-
nene.[13] In contrast to perchlorocoronene which exhibits only
a slight distortion (b = 4.88, b is the angle between the central
aromatic ring and the distorted outer rings), 4 displays a
significant deviation from the planar geometry, with a
maximum angle of b = 16.88 (for comparison, the fullerene
segment corannulene shows an angle of 23.68).[14]
from 3.15 to 3.43 ꢀ. This distance is closer than the typical
stacking separation in arene–perfluoroarene complexes
(ca. 3.5 ꢀ), which indicates a strong interaction probably
arising from the electron-donating effect of the methoxy
groups (Figure 3). Adjacent sandwich complexes are off-set
Permethoxylated HBC (4) crystallizes in the common
space group P21/c and exhibits a herringbone-type structure
with a large interplanar distance of 13.05 ꢀ. This packing is
quite different from that of parent HBC and hexa-tert-butyl-
HBC;[15] the 18 overcrowded methoxy groups dramatically
change the packing pattern from a face-to-face to an edge-to-
face arrangement, since the interplanar distance of 4 is far too
large to enable any intermolecular p–p interactions in the
crystals (Figure 1b).
The extraordinary “double-concave” conformation clas-
sifies permethoxylated HBC (4) as a new host molecule.
Figure 3. Single-crystal structure of the [(hfb)2ꢀ4] complex: a) top view
and b) side view.
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laterally from each other to give a tilted columnar arrange-
ment, in contrast to the normal packing pattern of infinite
stacks of alternating arene and perfluoroarene molecules. The
close contacts between stacks of complexes consist of
F···F interactions between the fluorine atoms at the ends of
two neighboring HFB molecules. The F···F separation of
2.79 ꢀ is comparable to a sum of the van der Waals radii of
2.90 ꢀ.[16] This packing preference reflects the combined
effect of maximized electrostatic interactions with the two
HFB molecules sandwiching the HBC molecules and com-
patible accommodation of HFB into the cavity formed by
flexible surrounding methoxy groups.
molecules on both sides, the flexible methoxy groups at the
periphery of 4 adjust their orientation to the size and shape of
the guest molecules. Indeed, 4 possesses D3d symmetry in the
crystals of the fullerene complex, in contrast to the centro-
symmetry in its dichloromethane solvates and the HFB
complexes. A comparison of the crystal structures shows that
the orientation of the 18 methoxy groups changes to provide a
cavity into which the different guest molecules may be
accommodated.
We have presented a novel approach to an extremely
nonplanar persubstituted HBC derivative by using the facile
cyclodehydrogenation procedure with ferric chloride at room
temperature. X-ray single-crystal analysis revealed that per-
methoxylated HBC (4) has an extraordinary “double-con-
cave” conformation in which the dramatic deviation from
planarity arises because of steric congestion in the bay region.
The remarkable propensity of guest molecules to change the
self-assembly of this novel host molecule and the fascinating
structures obtained in this way are intriguing in view of the
design of new host–guest systems and materials, for example,
based on functionalized fullerenes and perfluoroarenes.
Attaching long alkyl chains onto the PAH core should give
it liquid-crystalline properties.
The “double-concave” permethoxylated HBC (4) was
expected to be geometrically complementary to the globular
C
60 shape, and the inclusion of “thick” C60 will also afford
sufficient separation between HBC molecules and avoid
steric crowding of the methoxy groups. Slow evaporation of
the solvent from a solution of 4 and fullerene (1:1) in CS2
yielded black crystals. The crystal structure is shown in
Figure 4. The fullerene is positioned exactly on the central
Received: July 27, 2004
Keywords: crystal engineering · fullerenes · host–guest systems ·
.
noncovalent interactions · self-assembly
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[12] Data collections for the crystal-structure analysis were per-
formed on a Nonius KCCD diffractometer equipped with a
Cryostream cooler with graphite monochromated MoKa radia-
tion. The structures were solved by direct methods (Shelxs) and
refined on F with anisotropic temperature factors for the non-
hydrogen atoms. The H atoms were refined with fixed isotropic
temperature factors in the riding mode. 4·4CH2Cl2: C32H31Cl4O9,
monoclinic, space group P21/c, T= 150 K, a = 14.6101(7), b =
13.0471(6), c = 16.2439(7) ꢀ, b = 91.990(3)8, V= 3094.5(4) ꢀ3,
Z = 4, 1calcd = 1.505 gcmÀ3, m = 0.438 mmÀ1; 12418 reflections
measured, of which 7485 were unique (Rint = 0.026) and 3661
were observed; R = 0.086, Rw = 0.0817. 4·2C6F6: C36H27F6O9,
¯
triclinic, space group P1, T= 120 K, a = 11.1105(5), b =
11.7330(5), c = 13.0297(6) ꢀ, a = 73.425(1), b = 88.923(1), g =
75.098(1)8, V= 1570.2(2) ꢀ3, Z = 2, 1calcd = 1.517 gcmÀ3
, m =
0.131 mmÀ1; 29594 reflections measured, of which 8552 were
unique (Rint = 0.048) and 5203 were observed; R = 0.0651, Rw =
¯
0.0698. 4·C60: C20H9O3, trigonal, space group R3, T= 120 K, a =
22.1940(7), b = 22.1940(7), c = 12.8191(5) ꢀ, V= 5468.4(6) ꢀ3,
Z = 6, 1calcd = 1.624 gcmÀ3, m = 0.110 mmÀ1; 26529 reflections
measured, of which 3375 were unique (Rint = 0.041) and 1687
were observed; R = 0.0872, Rw = 0.0967. The fullerene molecule
was found to be disordered even at low temperatures such that
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