Three-dimensionally arranged cyclic p-hexaphenylbenzene: Toward a bottom-up synthesis of size-defined carbon nanotubes

Article abstract of DOI:10.1002/chem.201203227

Go to three dimensions: As a step toward a bottom-up synthesis of size-defined carbon nanotubes (CNTs), [3]cyclo-4′, 4′′′′-hexaphenylbenzenes ([3]CHPBs) were synthesized and investigated. Theoretical and experimental results revealed that [3]CHPBs possess highly twisted [9]cyclo-p-phenylene cores. [3]cyclo-2,11-(Hexa-peri- hexabenzocoronene) ([3]CHBC) was also examined for CNT synthesis (see figure). Copyright

Full text of DOI:10.1002/chem.201203227

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DOI: 10.1002/chem.201203227
Three-Dimensionally Arranged Cyclic p-Hexaphenylbenzene: Toward a
Bottom-Up Synthesis of Size-Defined Carbon Nanotubes
Tomohiko Nishiuchi,* Xinliang Feng, Volker Enkelmann, Manfred Wagner, and
Klaus Mꢀllen*[a]
Hexa-peri-hexabenzocoronene (HBC) is one of the funda-
mental polycyclic aromatic hydrocarbons (PAHs), which can
be synthesized from hexaphenylbenzene (HPB) through oxi-
dative cyclodehydrogenation involving a Lewis oxidant/acid,
such as ferric chloride (FeCl3). HBC derivatives have been
widely synthesized and studied not only for tuning the elec-
tronic properties at the molecular level, but also for control-
ling the supramolecular behavior in solution and the solid
state.[1] With the recent boom in graphene research,[2]
bottom-up approaches to access size- and/or shape-defined
graphene nanoribbons (GNRs)[3] and nanographenes
(NGs)[4] have become appealing because the electronic
properties of GNRs and NGs are largely dependent on the
size and edge structures (i.e., zigzag or armchair). Therefore,
precise synthesis of these materials is required for develop-
ing the next generation of nanoscale electronic and spintron-
ic devices.[5] Naturally, the p extension of HBCs in a one-di-
mensional or two-dimensional manner from the correspond-
Figure 1. Concept of p extensions of HBC from 1D (GNRs) and 2D
(NGs) to 3D (CHBCs and (n,n)-CNTs segments).
ing polyphenylene precursors is a convenient method to
build GNRs or NGs with defined sizes and/or shapes, of
which several have been synthesized and investigated
(Figure 1).[1–4] On the other hand, unlike GNRs and NGs,
bottom-up synthesis of three-dimensional PAHs, fragments
of fullerenes or carbon nanotubes (CNTs), remains a chal-
lenging task.[6] Therefore, we have extended this concept to
three dimensions, and for this purpose, three-dimensionally
arranged cyclic p-hexaphenylbenzenes (cyclo-4’,4’’’’-hexa-
phenylbenzenes (CHPBs), Figure 2) were designed as a step
toward CNTs with defined size and chiral indices (n,m).
The as-synthesized CHPBs could serve as fundamental pre-
properties of [3]CHPBs and the
attempts of cyclodehydrogena-
tion thereof for the synthesis of
[3]CHBCs,
ment.
a (9,9)-CNT seg-
Scheme 1 shows the synthetic
procedure for 1a and 1b.
Diiodo precursor monomers 2a
and 2b, the key intermediates
toward CHPBs, were prepared
Figure 2. Structure
CHPBs.
of
the
cursors
for
cyclo-2,11-(hexa-peri-hexabenzocoronenes)
(CHBCs), that is, segments of (n,n) CNTs with approxi-
mately 1.0 nm long side walls. Additionally, CHPBs possess
a cyclo-p-phenylene (CPP)[7] core and, therefore, investiga-
tion of the differences between CHPBs and CPPs is also an
attractive point. Herein, we report the synthesis and physical
from
2,3,5,6-tetraphenyl-1,4-
benzoquinone[8] in two steps
(see the Supporting Informa-
tion). Macrocyclization of 2a and 2b was achieved by Ya-
mamoto coupling reactions.[7h,j,9] We had anticipated that
this reaction might afford mixtures of trimers, 3a and 3b,
and tetramers as major compounds. However, only trimers
3a and 3b were obtained in moderate yields (40 and 42%,
respectively). This is due to the dihedral angle formed by
the two iodophenyl rings of 2, which is estimated to about
508 by theoretical calculations (see the Supporting Informa-
tion). For the reductive aromatizations to yield [3]CHPBs
1a and 1b, we used low-valent titanium, which can be pre-
pared from the reaction of TiCl3 or TiCl4 with, for example,
[a] Dr. T. Nishiuchi, Dr. X. Feng, Dr. V. Enkelmann, Dr. M. Wagner,
Prof. Dr. K. Mꢀllen
Max Planck Institute for Polymer Research
Ackermannweg 10, Mainz, 55127 (Germany)
Fax : (+49)6131-379-350
E-mail: nishiuch@mpip-mainz.mpg.de
muellen@mpip-mainz.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203227.
Chem. Eur. J. 2012, 18, 16621 – 16625
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positions of the CPP core. This strain energy is 28 kcalmolꢀ1
higher than that of [9]CPP (65.6 kcalmolꢀ1) and close to
that of [6]CPP (96.0 kcalmolꢀ1).[11b]
To investigate the rotation of phenyl rings of 1 in solution,
1
variable-temperature H NMR spectra of 1a were measured,
because the flexibility of each phenyl ring may influence the
efficiency of the cyclodehydrogenation. In addition, protons
a and b (see Figure 4) of the CPP core were expected to
separately appear as “inner cavity proton” and “outer cavity
proton” at low temperature. However, despite the highly
twisted structure of the CPP core, the rotational speed of
each benzene ring is still fast and only a broadening behav-
ior of all protons was observed at low temperature.
Figure 4. Variable-temperature 1H NMR spectrum of 1a (500 MHz, sol-
vent: CD2Cl2).
Scheme 1. Reagents and conditions: a) [NiACTHNUTRGNENUG(cod)2], (cod=1,5-cycloocta-
diene), 1,5-cyclooctadiene, 2,2’-bipyridine, DMF/toluene/THF, 808C;
b) TiCl4/LiAlH4 (1:4), THF, 808C, 3 days.
UV/Vis and fluorescence spectra of 1a and 1b are shown
in Figure 5. Due to the substitution of tert-butyl groups, lmax
of 1b appeared at slightly longer wavelength than that of
1a. Similarly, fluorescence spectra of 1a and 1b showed
small differences at the onsets of the emissions (1a: 365 nm,
1b: 350 nm) and the first emission maxima (1a: 405 nm, 1b:
400 nm). As expected from theoretical calculations, UV/Vis
absorption and fluorescence emission of 1a and 1b were ob-
alkali metals (Li, Na, K), zinc, a zinc–copper couple, or lithi-
um aluminum hydride (LiAlH4).[10] We used a method in-
volving TiCl4 and LiAlH4 to produce [3]CHPBs. LiAlH4 was
added to TiCl4 (LiAlH4/TiCl4 =4:1) in THF. Addition of 3a
or 3b at 808C gave 1a or 1b in good yields (73 and 81%, re-
spectively).
A structure of 1a, optimized theoretically by using DFT
calculations (B3LYP/6-31G*), showed C3 symmetry with a
highly twisted CPP core (Figure 3). The diameter of 1a is
12.2 ꢂ. Dihedral angles between rings A and B, and rings B
and B’ are 51.28 and 78.78, respectively. Based on this result,
the CPP core is not expected to possess extended p conjuga-
tion. To estimate the strain energy (DH) of 1a, we used a
homodesmotic reaction,[11] which gave a high strain energy
of 93.9 kcalmolꢀ1 due to the phenyl substitutions at the side
served at shorter wavelengths than for [9]CPP (lmax
=
340 nm, lemffi500 nm).[7a,n] In addition, the highly twisted
structure of 1a and 1b also affected the redox properties
(see the Supporting Information). Cyclic voltammograms of
1a and 1b showed similar irreversible oxidation waves at
Figure 3. Optimized structure of 1a; a) side view and b) top view. The
CPP core is represented as balls and sticks, and phenyl substitutions are
represented as wireframes for clarity.
Figure 5. UV/Vis and fluorescence spectra of 1a and 1b (CH2Cl2).
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Chem. Eur. J. 2012, 18, 16621 – 16625

Size-Defined Carbon Nanotubes
COMMUNICATION
1.00 V. These oxidation potentials are higher than those of
[9]CPP (E1/2 =0.70 V (vs. Fc/Fc+)).[7n]
Single crystals of 1a and 1b could be prepared by recrys-
tallization from a dichloromethane/acetonitrile mixture and
dichloromethane, respectively, and X-ray analysis was per-
formed (Figure 6).[12] In the crystalline state, contrary to
cyclodehydrogenated compounds. Although the cyclodehy-
drogenation of 1a afforded partially dehydrogenated and
chlorinated compounds, which were identified by MALDI-
TOF MS (see the Supporting Information), the reaction of
1b proceeded more smoothly than that of 1a, and the corre-
sponding MS signals of 4b as well as of the monochlorinated
4b were observed, because the electron-donating nature of
tert-butyl groups enhances the efficiency of cyclo-
AHCTUNGTREGdNNUN ehydrogenation. In particular, the isotopic signal pattern of
the monochlorinated 4b was in good agreement with the si-
mulated signal pattern (Figure 7). However, together with
Figure 6. X-ray crystal structures of a) 1a (ORTEP drawing) and b) 1b
(the CPP core and the dichloromethane molecules are represented by
the CPK model and phenyl substitutions are represented as capped sticks
for clarity). Protons are omitted for clarity.
what was predicted by the theoretical calculations, the six
aryl substitutions of 1a and 1b fill the cavity space. Due to
the crystal packing force, 1a and 1b show C1 symmetry and
the [9]CPP core possesses an ellipsoidal shape. Whereas the
cavity of 1a is filled by its own phenyl rings, two dichloro-
methane molecules are incorporated in the cavity of 1b to
fill an extra space, because the 4-tert-butylphenyl groups are
too large to fill the cavity. The diameters of the major and
minor axes of 1a and 1b are approximately 13.6 and 11.0 ꢂ,
and 13.0 and 11.2 ꢂ, respectively. In addition, owing to the
highly twisted structures of 1a and 1b, no quinoidal distribu-
tion in the CPP core was observed as shown in [9]CPP[7j]
(see the Supporting Information).
Finally, the cyclodehydrogenations of 1a and 1b to pro-
duce [3]CHBCs 4a and 4b were attempted by using several
oxidation conditions, such as CH3SO3H with 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ),[13] phenyliodonium
bis(trifluoroacetate) (PIFA) with BF3·Et2O,[14] or FeCl3
(Scheme 2). Through optimizations of the cyclodehydroge-
nation, we found that addition of FeCl3 and heating to
reflux in dichloromethane is the best method to yield highly
Figure 7. MALDI-TOF MS spectrum of after cyclodehydrogenation of
1b.
the corresponding MS signals, several intermediates of 4b,
which are partially dehydrogenated compounds, were also
identified. The partially dehydrogenated compounds result
from the high ring strain of the [9]CPP core, which may
cause interference with the cyclodehydrogenation reaction.
Prolonged reaction time and more rigorous conditions (with
1,2-dichloroethane at 1008C in a sealed tube) did not im-
prove the cyclodehydrogenation, but led to more chlorinat-
ed products. Separation and purification of 4b have so far
been unsuccessful. Hence, unfortunately, the exact evidence
for the formation of pure [3]CHBC 4b was not obtained,
but our numerous attempts revealed that cyclodehydrogena-
tion of a cyclic p system did
proceed. Although 1b was a
colorless solution with a blue
emission, after cyclodehydroge-
nation it exhibited an orange-
red color and a yellow-orange
emission (in dichloromethane
solution), suggesting an effec-
tive p conjugation of the ring
structures (see the Supporting
Information).
In summary, we have descri-
bed the synthesis and crystal
structure
determination
of
Scheme 2. Cyclodehydrogenation reaction of 1a and 1b.
[3]CHPBs 1a and 1b.[15] Macro-
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T. Nishiuchi, K. Mꢀllen et al.
[7] For cyclo-p-phenylene and its related derivatives, see; a) R. Jasti, J.
Bhattacharjee, J. B. Neaton, C. R. Bertozzi, J. Am. Chem. Soc. 2008,
130, 17646; b) R. Jasti, C. R. Bertozzi, Chem. Phys. Lett. 2010, 494,
1; c) T. J. Sisto, M. R. Golder, E. S. Hirst, R. Jasti, J. Am. Chem. Soc.
2011, 133, 15800; d) J. Xia, R. Jasti, Angew. Chem. Int. Ed. 2012, 51,
2474; e) T. J. Sisto, X. Tian, R. Jasti, J. Org. Chem. 2012, 77, 5857;
f) H. Takaba, H. Omachi, Y. Yamamoto, J. Bouffard, K. Itami,
Angew. Chem. 2009, 121, 6228; Angew. Chem. Int. Ed. 2009, 48,
6112; g) H. Omachi, S. Matsuura, Y. Segawa, K. Itami, Angew.
Chem. 2010, 122, 10400; Angew. Chem. Int. Ed. 2010, 49, 10202;
cyclization of 2a and 2b were achieved by Yamamoto cou-
pling reactions with moderate yield of the products, and re-
ductive aromatizations of 3a and 3b were performed by
using low-valent titanium to give good yields of the prod-
ucts. The precursor monomer 2 offers further opportunities
for the synthesis of a variety of [3]CHPBs derivatives; for
example, [3]CHPBs substituted with thiophenes, pyrroles, or
other aryl groups could also be synthesized. Compared with
nonsubstituted [9]CPP, 1a and 1b showed greater HOMO–
LUMO gaps, due to high ring strain induced by the aryl sub-
stitutions. X-ray crystallography revealed the unique twisted
structure of 1a and 1b. To overcome the problems related
to the cyclodehydrogenation, synthesis of “higher” homo-
logues of 1 with reduced ring strains as well as synthesis of
size-defined CNTs, that is, segments of (n,n) CNTs, is cur-
rently underway in our group.
ˇ
h) Y. Segawa, S. Miyamoto, H. Omachi, S. Matsuura, P. Senel, T. Sa-
samori, N, Tokitoh, K. Itami, Angew. Chem. 2011, 123, 3302;
Angew. Chem. Int. Ed. 2011, 50, 3244; i) H. Omachi, Y. Segawa, K.
ˇ
Itami, Org. Lett. 2011, 13, 2480; j) Y. Segawa, P. Senel, S. Matsuura,
H. Omachi, K. Itami, Chem. Lett. 2011, 40, 423; k) A. Yagi, Y.
Segawa, K. Itami, J. Am. Chem. Soc. 2012, 134, 2962; l) H. Omachi,
Y. Segawa, K. Itami, Acc. Chem. Res. 2012, 45, 1378; m) S. Yamago,
Y. Watanabe, T. Iwamoto, Angew. Chem. 2010, 122, 769; Angew.
Chem. Int. Ed. 2010, 49, 757; n) T. Iwamoto, Y. Watanabe, Y. Saka-
moto, T. Suzuki, S. Yamago, J. Am. Chem. Soc. 2011, 133, 8354;
o) T. Iwamoto, Y. Watanabe, T. Sadahiro, T. Haino, S. Yamago,
Angew. Chem. 2011, 123, 8492; Angew. Chem. Int. Ed. 2011, 50,
8342; p) S. Hitosugi, W. Nakatani, T. Yamasaki, H. Isobe, Nat.
Commun. 2011, 2, 492.
Acknowledgements
[8] I. Ullah, R. A. Khera, M. Hussain, A. Villinger, P. Langer, Tetrahe-
dron Lett 2009, 50, 4651.
This work was financially supported by the DFG Priority Program SPP
1459, the BMBF Graphenoid project, the ESF Project GOSPEL (Ref
Nr: 09-EuroGRAPHENE-FP-001), the EU Project GENIUS and an
ERC grant on NANOGRAPH.
[9] a) S.-H. Jung, W. Pisula, A. Rouhanipour, H. J. Rꢃder, J. Jacob, K.
Mꢀllen, Angew. Chem. 2006, 118, 4801; Angew. Chem. Int. Ed. 2006,
45, 4685; b) W. Pisula, M. Kastler, C. Yang, V. Enkelmann, K.
Mꢀllen, Chem. Asian J. 2007, 2, 51; c) S. C. Simon, B. Schmaltz, A.
Rouhanipour, H. J. Rꢃder, K. Mꢀllen, Adv. Mater. 2009, 21, 83;
d) W. Nakanishi, T. Yoshioka, H. Taka, J. Y. Xue, H. Kita, H. Isobe,
Angew. Chem. 2011, 123, 5435; Angew. Chem. Int. Ed. 2011, 50,
5323; e) M. G. Schwab, T. Qin, W. Pisula, A. Mavrinski, X. Feng, M.
Baumgarten, H. Kim, F. Laquai, S. Schuh, R. Trattnig, E. J. W. List,
K. Mꢀllen, Chem. Asian J. 2011, 6, 3001.
Keywords: carbon nanotubes
· cyclodehydrogenation ·
cyclo-p-phenylene · polycyclic compounds · hydrocarbons
[1] a) M. D. Watson, A. Fechtenkçtter, K. Mꢀllen, Chem. Rev. 2001,
101, 1267; b) J. Wu, W. Pisula, K. Mꢀllen, Chem. Rev. 2007, 107,
718; c) W. Pisula, X. Feng, K. Mꢀllen, Chem. Mater. 2011, 23, 554;
d) J. P. Hill, W. Jin, A. Kosaka, T. Fukushima, H. Ichihara, T. Shimo-
mura, K. Ito, T. Hashizume, N. Ishii, T. Aida, Science 2004, 304,
1481; e) R. Yamaguchi, S. Hiroto, H. Shinokubo, Org. Lett. 2012, 14,
2472.
[2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang,
S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666.
[3] a) M. Mꢀller, V. S. Iyer, C. Kꢀbel, V. Enkelmann, K. Mꢀllen,
Angew. Chem. 1997, 109, 1679; Angew. Chem. Int. Ed. Engl. 1997,
36, 1607; b) V. S. Iyer, K. Yoshimura, V. Enkelmann, R. Epsch, J. P.
Rabe, K. Mꢀllen, Angew. Chem. 1998, 110, 2843; Angew. Chem. Int.
Ed. 1998, 37, 2696; c) D. Wasserfallen, M. Kastler, W. Pisula, W. A.
Hofer, Y. Fogel, Z. Wang, K. Mꢀllen, J. Am. Chem. Soc. 2006, 128,
1334; d) X. Yang, X. Dou, A. Rouhanipour, L. Zhi, H. J. Rꢃder, K.
Mꢀllen, J. Am. Chem. Soc. 2008, 130, 4216; e) Y. Fogel, L. Zhi, A.
Rouhanipour, D. Andrienko, H. J. Rꢃder, K. Mꢀllen, Macromole-
cules 2009, 42, 6878; f) L. Dçssel, L. Gherghel, X. Feng, K. Mꢀllen,
Angew. Chem. 2011, 123, 2588; Angew. Chem. Int. Ed. 2011, 50,
2540.
[10] H. M. Walborsky, H. H. Wꢀst, J. Am. Chem. Soc. 1982, 104, 5807.
[11] a) S. M. Bachrach, J. Chem. Educ. 1990, 67, 907; b) Y. Segawa, H.
Omachi, K. Itami, Org. Lett. 2010, 12, 2262; c) S. M. Bachrach, D.
Stꢀck, J. Org. Chem. 2010, 75, 6595.
[12] a) Crystal data for 1a:
C126H84·3CH2Cl2, MW =1852.40, triclinic,
¯
space group P1, a=13.3974(4), b=16.6360(4), c=24.8473(5) ꢂ, a=
95.0665(13), b=96.3597(13), g=110.3886(10)8, V=5111.0(2) ꢂ3,
Z=2, 1=1.204 gcm3, T=120 K. Data were collection with MoKa ra-
diation (Nonius KCCD diffractometer): 46171 measured reflections,
22839 unique reflections, Rmerge =0.0621, 18863 observed reflections
(I>2.0s(I)). The structure was solved by direct methods (Shelxs),
refined by full matrix least squares on F with anisotropic tempera-
ture factors for the non-hydrogen atoms. Final R=0.0649, RW
0.0611, GOF=0.9208; b) Crystal data for 1b: 174H180·7CH2Cl2,
W =2864.86, orthorhombic, space group Pbca, a=25.5816(7), b=
=
C
M
33.8376(9), c=38.8518(9) ꢂ, a=b=g=908, V=33630.9(15) ꢂ3, Z=
8, 1=1.132 gcm3, T=120 K. Data were collection with MoKa radia-
tion (Nonius KCCD diffractometer): 64551 measured reflections,
30606 unique reflections, Rmerge =0.0706, 18372 observed reflections
(I>2.0s(I)). The structure was solved by direct methods (Shelxs),
refined by full matrix least squares on F with anisotropic tempera-
[4] a) C. D. Simpson, J. D. Brand, A. J. Berresheim, L. Przybilla, H. J.
ture factors for the non-hydrogen atoms. Final R=0.0501, RW
=
ˇ
´
Rꢃder, K. Mꢀllen, Chem. Eur. J. 2002, 8, 1424; b) J. Wu, Z. Tomovic,
ˇ
0.0661, GOF=0.9619. CCDC-892921 (1a) and CCDC-892922 (1b)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
V. Enkelmann, K. Mꢀllen, J. Org. Chem. 2004, 69, 5179; c) Z. To-
´
movic, M. D. Watson, K. Mꢀllen, Angew. Chem. 2004, 116, 773;
Angew. Chem. Int. Ed. 2004, 43, 755; d) X. Feng, V. Marcon, W.
Pisula, M. R. Hansen, J. Kirkpatrick, F. Grozema, D. Andrienko, K.
Kremer, K. Mꢀllen, Nat. Mater. 2009, 8, 421.
[13] L. Zhai, R. Shukla, R. Rathore, Org. Lett. 2009, 11, 3474.
[14] a) Y. Kita, M. Gyoten, M. Ohtsubo, H. Thoma, T. Takada, Chem.
Commun. 1996, 1481; b) B. T. King, J. Kroulꢄk, C. R. Robertson, P.
Rempala, C. L. Hilton, J. D. Korinek, L. M. Gortari, J. Org. Chem.
2007, 72, 2279.
[5] Y. W. Son, M. L. Cohen, S. G. Louie, Nature 2006, 444, 347.
[6] a) Y.-T. Wu, J. S. Siegel, Chem. Rev. 2006, 106, 4843; b) V. M. Tsefri-
kas, L. T. Scott, Chem. Rev. 2006, 106, 4868; c) L. T. Scott, E. A.
Jackson, Q. Zhang, B. D. Steinberg, M. Bancu, B. Li, J. Am. Chem.
Soc. 2012, 134, 107.
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Size-Defined Carbon Nanotubes
COMMUNICATION
[15] The concept, synthesis, properties, and X-ray structure of 1a were
first reported at the 14th International Symposium on Novel Aro-
matic Compounds (ISNA-14, July 2011; POSTER 12: “p-Expanded
Cycloparaphenylenes: Toward a Bottom-Up Synthesis of Size-De-
fined Carbon Nanotubes”). Shortly thereafter Jasti and co-workers
reported a tetraphenyl-substituted [12]CPP (see Ref. [7e]).
Received: September 10, 2012
Published online: December 7, 2012
Chem. Eur. J. 2012, 18, 16621 – 16625
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16625