Synthesis and crystal structures of extremely crowded oligophenylenes as model precursors to 'cubic graphite'

Article abstract of DOI:10.1016/j.tet.2006.03.071

By using drastic conditions for a Diels-Alder cycloaddition reaction, it was possible to synthesize an oligophenylene with an extremely dense packing of the benzene rings. Crystallographic data could be obtained and a projection of the structure on the pl

Full text of DOI:10.1016/j.tet.2006.03.071

Tetrahedron 62 (2006) 5417–5420
Synthesis and crystal structures of extremely crowded
oligophenylenes as model precursors to ‘cubic graphite’
*
Daniel Wasserfallen, Gunter Mattersteig, Volker Enkelmann and Klaus Mullen
¨
Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Received 19 January 2006; revised 20 March 2006; accepted 22 March 2006
Available online 27 April 2006
Abstract—By using drastic conditions for a Diels–Alder cycloaddition reaction, it was possible to synthesize an oligophenylene with an
extremely dense packing of the benzene rings. Crystallographic data could be obtained and a projection of the structure on the plane of
the central phenyl ring reveals that the molecule retained its theoretical threefold symmetry with only minor deviations. Due to its dense
packing of interlocked benzene rings, this oligophenylene could be furthermore used as a suitable precursor for constructing a subunit of
‘cubic graphite’.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
crystallographic data could be obtained are based on the
Diels–Alder cycloaddition between tetraphenylcyclo-
pentadienones and arylacetylenes.5,6 This approach in com-
bination with palladium-catalyzed Hagihara–Sonogashira
coupling reactions12 leads to a great diversity of higher
generation polyphenylene dendrimers, and when applied to
a functionalizable core to a versatile synthesis and self-
assembly of star-type hexabenzocoronenes.13 A different
strategyto novel three-dimensional polyphenylenestructures
utilizes the Suzuki coupling reaction of dibromo-hexaphen-
ylbenzene units with complementary bis(boronic acid)
derivatives. This route affords a novel oligophenylene
The search for novel three-dimensional polyphenylene
structures also includes a carbon phase so-called ‘cubic
graphite’, proposed and attempted to synthesize by Gibson
et al.1 The name cubic graphite implies both, the cubic sym-
metry of such a phase and the close relationship between its
solid-state structure and that of conventional graphite. In
such a phase each benzene ring is connected to six other dif-
ferent rings and each benzene ring is part of three equivalent
polyparaphenylene chains.
Four neighboring benzene rings are arranged to a tetraphen-
ylene ring with a strong mutual twist between them (Fig. 1).
Thus, within such a novel phase, all carbon atoms become
equivalent. This material would present the perfect three-
dimensional polyphenylene structure2,3 and should possess
high thermal and mechanical stability. These features and
the presence of large diffusion channels for lithium cations
and its high predicted maximum charge storage capacity
make ‘cubic graphite’ a candidate for rechargeable batteries,
battery-like supercapacitors, and electrochromic displays.2,3
Although this particular carbon phase might never be acces-
sible due to the extremely crowded three-dimensional
arrangement of hexaphenylbenzene (HPB) units, highly
dense polycyclic aromatic hydrocarbons, and nanosized
dendritic or hyperbranched polyphenylenes have gained
even more attention as fascinating synthetic targets.3–11
The biggest oligophenylene nanostructures from which
Keywords: Cubic graphite; Oligophenylene; Dendrimer; Dense packing;
Diels–Alder; Cycloaddition.
* Corresponding author. +49 6131 379 150; fax: +49 6131 379 350; e-mail:
muellen@mpip-mainz.mpg.de
Figure 1. Three-dimensional representation of a subunit of cubic graphite.
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.071

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D. Wasserfallen et al. / Tetrahedron 62 (2006) 5417–5420
macrocycle related to ‘phenylogous cubic graphite’ with
a large central cavity and the formation of channels within
the crystal.14 A further approach to crowded polyphenylenes
is based on the thermal ring opening of biphenylene units
with subsequent dimerization to tetraphenylene and leads
to ‘cubic graphite’ related structures.15
and the resulting low yields stand in contrast to other ethynyl
precursor molecules, where the products also show a dense
packing of the benzene rings.6 However, one major differ-
ence is that the ethynyl functionalities in these examples
were situated spatially less close than for the precursor 2,
which indicates the strong influence of the steric situation
upon the Diels–Alder cycloaddition.
Herein, we present the straightforward synthesis and crystal
structures16 of the densely packed oligophenylenes 3 and
4. The 1,3,5-tris-(phenylethynyl)-2,4,6-triphenylbenzene
(2) was synthesized via a cobaltoctacarbonyl mediated
cyclotrimerization of diphenylbutadiyne 1 following a
literature procedure17 (Scheme 1). Looking at the sterical
demands of 4, a threefold Diels–Alder reaction with 2
seemed to be impossible. Indeed, reaction of both compo-
nents in refluxing diphenyl ether under argon atmosphere
proceeded very slowly. The generation of the monoadduct
was detected by thin layer chromatography (TLC) and
field-desorption mass spectrometry (FDMS) soon after the
reaction started. However, the formation of the bisadduct 3
required four days. Under these reaction conditions, target
compound 4 was not formed even after an extremely pro-
longed reaction time of one month. Instead, decomposition
became rampant and therefore a rather uncommon prepara-
tive method was chosen. A mixture of the acetylene deriva-
tive 2 and excess tetraphenylcyclopentadienone was placed
in a glass ampoule. After evacuation (10ꢀ5 mbar), the
ampoule was sealed and heated for 72 h at 265 ꢁC. A com-
plex reaction mixture was obtained, which made a full sep-
aration and characterization of all components impossible.
After column chromatography and washing, only minor
amounts of 4 could be isolated, and the amount of double
Diels–Alder product 3 was also very modest. Although the
yield of 4 was rather low, these conditions seemed to provide
an optimum for the formation of 4. While a decrease in tem-
perature prevented the formation of a melt, and therefore,
good mixing, an increase resulted in more drastic conditions,
favoring undesired side reactions. These harsh conditions
The molecular structure of both compounds could be as-
signed by MS, 1H NMR, and 13C NMR spectroscopy; how-
ever, absolute certainty provided their crystal structures.
Whereas isolation of 3 via column chromatography did not
provide any difficulties, the fraction containing 4 consisted
of several components. Field-desorption and MALDI-TOF
MS analysis showed the presence of a peak at m/z
1706 Da, most likely corresponding to the norbornadienone,
which is subsequently transferred to the final product 4 under
CO extrusion. The presence of such a norbornadienone is
substantiated by a recent publication10 demonstrating that
significant steric crowding supports the stability of that par-
ticular class of compounds. Additionally, one has to consider
that applying the above described reaction conditions, it is
not possible to estimate to what extent partial cyclodehydro-
genation will also come into play, making an unambiguous
structural assignment even more difficult.
The 1H NMR spectra of both oligophenylenes, 3 and 4, show
resonances at unusually high-field positions for aromatic
ring protons implying that they are placed above the centers
of neighboring aromatic rings. While the aromatic proton
resonances assigned to the ‘core’ of 3 resonate at d¼5.74,
those attributable to core 4 (Fig. 3 b, d, f) appear at
d¼4.83. These strong high-field shifts are caused by the
magnetic anisotropy as a result of the ring currents of the sur-
rounding ortho-phenyl rings of the pentaphenyl units. Simi-
lar effects have been reported in the literature.5,7 The 13C
NMR spectrum of 3 indicated a C2v symmetry on the time-
scale of the solution-based NMR experiment, as it displays
34 resonances. This is consistent with a fast, unrestricted
rotation of all phenyl groups including the ones directly
connected to the central phenyl ring. Due to the very poor
solubility of 4 in all organic solvents, the acquisition of
13C NMR spectral data had to be performed at 373 K using
C2D2Cl4 as solvent. The spectrum shows 22 signals corre-
sponding to a D3h symmetry of the molecule, which would
be expected within the fast exchange limit for all single
bond rotations.
Crystals of 3 and 4 were grown from tetrachloroethane and
methylenechloride, respectively, by slow evaporation at
room temperature (Fig. 2). They crystallize as solvates and
especially 3, which contains five solvent molecules per
asymmetric unit, decomposes quickly under ambient condi-
tions.18 A projection of the structure of 4 on the plane of the
central phenyl ring reveals that here the molecule retains its
theoretical threefold symmetry with only minor deviations.
This can be explained by the rigidity of the intramolecular
packing in this overcrowded molecule, which reduces the
distortions by outer forces, e.g., packing effects. Thus the
molecular shape is almost cylindrical. As a consequence
3 crystallizes in a pseudohexagonal packing (azc, bz
120ꢁ), which is expected for an array of disks. Compound
3 appears to be much less rigid owing to its reduced inner
Scheme 1. Reaction conditions: (i) Co2(CO)8, dioxane, 8 h, reflux; (ii)
tetraphenylcyclopentadienone, 265 ꢁC, 72 h, 3 (34%), 4 (2%).

D. Wasserfallen et al. / Tetrahedron 62 (2006) 5417–5420
5419
Figure 2. Crystal structures of (A) compound 3 and (B) compound 4 (projection along the central phenyl ring).
packing density and especially the angles, which are deter-
mined by the three phenyl substituents (Fig. 3 b, d, f) with
the central phenyl ring, seem to be affected considerably
by intermolecular contacts.
precursors of this extremely challenging carbon framework.
This will also promote efforts into the creation of related14,15
and new, intriguing graphite models. A promising approach
to achieve the latter goal includes spatial restricted cyclode-
hydrogenations applied to polyphenylene dendrimers. Thus,
a suitable arrangement of the dendrons, predetermined by
the core, would just allow for oxidative cyclodehydrogena-
tion within the wings and not between them. This concept
might create not only giant propeller-shaped molecules
with blades comprised of PAHs, but also large channels
and cages for storage and transport are feasible.19
In summary, the results presented herein show that, by using
rather drastic synthetic conditions, highly dense oligophenyl-
enes are accessible. Compound 4, due to its dense packing of
benzene rings, is furthermore a suitable precursor for con-
structing a subunit of ‘cubic graphite’. The key structural
motif comprises of a HPB subunit having in ortho- and
ortho0-positions each of the three alternating (Fig. 3 a, c, e)
benzene rings connected with further two benzene rings.
The latter ones would allow for the formation of six alternat-
ing tetraphenylene rings in a final cyclodehydrogenation
step involving the other three alternating (Fig. 3 b, d, f) benz-
ene rings of the central HPB unit. After the formation of the
red dotted bonds in Figure 3A, one would obtain a subunit of
‘cubic graphite’, represented schematically in Figure 3B.
2. Experimental
2.1. General
Freshly powdered 1,3,5-tris(ethynylphenyl)-2,4,6-trisphenyl-
benzene (2) (75 mg, 0.124 mmol) and tetraphenylcyclo-
pentadienone (245 mg, 0.637 mmol) were placed in a glass
ampoule. The mixture was thoroughly mixed, and the
ampoule afterward evacuated to 10ꢀ5 mbar. After sealing,
However, the extremely low yield towards 4 forces us to seek
for more efficient synthetic approaches to create suitable
Figure 3. (A) Necessary bond formations from 4 (red dotted lines) towards a subunit of ‘cubic graphite’; (B) schematic representation of the theoretically
formed product (semi-empirical AM1 calculation).

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D. Wasserfallen et al. / Tetrahedron 62 (2006) 5417–5420
it was heated in an electric oven at 2 ꢁC/min to 265 ꢁC. This
temperature was maintained for 72 h. The ampoule was
cooled to room temperature and carefully opened. The solid
red-brownish residue was removed from the ampoule and
subjected to column chromatography (silica gel, n-hexane/
dichloromethane 2:1) to yield, after removal of the
solvent by rotary evaporation and careful washings with
n-pentane, the double- and triple-Diels–Alder product 3
(55 mg, 0.042 mmol, 34%) and 4 (5 mg, 0.003 mmol, 2%),
respectively.
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12. Taskahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627–630.
2.1.1. Compound 3. Colorless solid; mp>300 ꢁC; 1H NMR
(700 MHz, C2D2Cl4, 306 K): d¼7.16–6.93 (m, 16H), 6.88–
6.73 (m, 12H), 6.72–6.55 (m, 27H), 6.54–6.41 (m, 8H), 6.28
(s, 3H), 5.74(w, 4H); 13C NMR (176 MHz, C2D2Cl4, 318 K):
d¼144.32, 142.59, 141.97, 141.52, 141.26, 141.16, 141.03,
139.98, 139.69, 138.35, 137.64, 136.54, 133.89, 133.37,
132.45, 131.92, 131.73, 131.57, 131.02, 128.23, 127.88,
127.02, 126.62, 126.49, 126.37, 126.01, 125.83, 125.41,
124.96, 124.74, 123.86, 123.74, 96.10, 92.05; FDMS
(8 kV): m/z (%): 1320 (100%) [M+].
13. Wu, J.; Watson, M. D.; Mullen, K. Angew. Chem. 2003, 115,
¨
5487–5491.
14. Shen, X.; Ho, D. M.; Pascal, R. A., Jr. Org. Lett. 2003, 5,
369–371.
15. Mindach, L.; Mullen, K. Adv. Mater. 1996, 8, 504–507.
2.1.2. Compound 4. Colorless solid; mp>300 ꢁC; 1H NMR
¨
3
16. Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publica-
tion numbers CSD 285923 and CSD 285924 and can be
obtained free of charge, on application to: CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk).
(700 MHz, C2D2Cl4, 306 K): d¼6.95 (t, J¼7.3 Hz, 6H),
3
3
6.88 (d, J¼7.7 Hz, 7H), 6.85 (t, J¼7.3 Hz, 7H), 6.78 (t,
3J¼7.7 Hz, 9H), 6.58 (d, J¼6.0 Hz, 12H), 6.52 (w, 20H),
3
3
3
6.38 (d, J¼5.1 Hz, 9H), 6.16 (d, J¼8.5 Hz, 14H), 4.83
(w, 6H); 13C NMR (176 MHz, C2D2Cl4, 373 K):
d¼142.15, 141.89, 141.72, 141.66, 141.54, 139.75, 139.59,
138.69, 137.90, 135.89, 135.79, 132.29, 131.62, 131.52,
126.49, 126.24, 126.01, 125.55, 125.15, 124.97, 124.71,
124.22; FDMS (8 kV): m/z (%): 1676 (100%) [M+].
´
17. Hubel, W.; Merenyi, R. Chem. Ber. 1963, 96, 930–943.
¨
18. Crystals were mounted in closed capillaries and the data collec-
tions were carried out at 120 K on a Nonius KappaCCD dif-
fractometer with graphite monochromated Mo Ka radiation.
19,149 and 19,089 unique reflections were collected for 4
and 3, respectively, of which 4121 and 14,881 reflections
were considered observed (I>22(I)). The structures were
solved by direct methods and refined by full-matrix least
squares analyses on F2 with anisotropic temperature factors
for all non-hydrogen atoms. The H atoms were refined with
fixed isotropic temperature factors in the riding mode. Final
R values for all reflections were 0.115 (Rw 0.194) (4) and
0.068 (Rw 0.117) (3). The R values for the observed reflections
were 0.069 (Rw 0.107) (4) and 0.055 (Rw 0.099) (3).
Acknowledgements
Support from the Sonderforschungsbereich 625 is gratefully
acknowledged.
References and notes
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¨
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