A grossly warped nanographene and the consequences of multiple odd-membered-ring defects

Article abstract of DOI:10.1038/nchem.1704

Graphite, the most stable form of elemental carbon, consists of pure carbon sheets stacked upon one another like reams of paper. Individual sheets, known as graphene, prefer planar geometries as a consequence of the hexagonal honeycomb-like arrangements of trigonal carbon atoms that comprise their two-dimensional networks. Defects in the form of non-hexagonal rings in such networks cause distortions away from planarity. Herein we report an extreme example of this phenomenon. A 26-ring C 80 H 30 nanographene that incorporates five seven-membered rings and one five-membered ring embedded in a hexagonal lattice was synthesized by stepwise chemical methods, isolated, purified and fully characterized spectroscopically. Its grossly warped structure was revealed by single-crystal X-ray crystallography. An independent synthetic route to a freely soluble derivative of this new type of 'nanocarbon' is also reported. Experimental data reveal how the properties of such a large graphene subunit are affected by multiple odd-membered-ring defects.

Full text of DOI:10.1038/nchem.1704

ARTICLES
PUBLISHED ONLINE: 14 JULY 2013 | DOI: 10.1038/NCHEM.1704
A grossly warped nanographene and the
consequences of multiple odd-membered-
ring defects
Katsuaki Kawasumi1, Qianyan Zhang2, Yasutomo Segawa1, Lawrence T. Scott2 and Kenichiro Itami1,3
*
*
Graphite, the most stable form of elemental carbon, consists of pure carbon sheets stacked upon one another like reams of
paper. Individual sheets, known as graphene, prefer planar geometries as a consequence of the hexagonal honeycomb-like
arrangements of trigonal carbon atoms that comprise their two-dimensional networks. Defects in the form of non-hexagonal
rings in such networks cause distortions away from planarity. Herein we report an extreme example of this phenomenon.
A 26-ring C80H30 nanographene that incorporates five seven-membered rings and one five-membered ring embedded in a
hexagonal lattice was synthesized by stepwise chemical methods, isolated, purified and fully characterized spectroscopically.
Its grossly warped structure was revealed by single-crystal X-ray crystallography. An independent synthetic route to a freely
soluble derivative of this new type of ‘nanocarbon’ is also reported. Experimental data reveal how the properties of such a
large graphene subunit are affected by multiple odd-membered-ring defects.
odern nanocarbon science was born in 1985 when C60 and polycyclic aromatic hydrocarbons (PAHs) through C–H activation
the higher fullerenes were discovered1. The field expanded followed by cyclodehydrogenation32. When this reaction sequence
M
rapidly and attracted even more widespread interest with is applied to the bowl-shaped PAH corannulene (1), we found
the discovery of carbon nanotubes in 19912 and the subsequent iso- that it does not stop at the fivefold annulated product 3, but con-
lation of single-layer graphene in 20043. The hollow balls, long tubes tinues all the way to the grossly warped C80H30 nanographene 4
and flat sheets of trigonal carbon atoms that make up these structu- (Fig. 1b). The cyclization reaction conditions append five polycyclic
rally distinct families of all-carbon molecules exhibit a range of wings to the central hydrocarbon bowl and then stitch them
remarkable properties that has spawned an ever-more intensifying together to create five new seven-membered rings. Ten new
revolution in materials science4. Herein we report a grossly carbon–carbon bonds are formed in a single reaction.
warped nanographene, the first member of a new family of nanocar-
The expansion of corannulene (1) to give the C80H30 nanogra-
bons (Fig. 1a). This chiral, warped C80H30 nanographene, with five phene 4 quadruples the number of atoms in the carbon lattice
seven-membered rings and one five-membered ring embedded in a and increases the number of rings from six to 26 in just two steps.
hexagonal lattice of trigonal carbon atoms, can be synthesized The lack of regioselectivity in the fivefold C–H arylation produces
rapidly from commercially available corannulene in just two steps a mixture of products, which limits the yield in the first step of
by C–H activation reactions5,6. A number of unique structural and the synthesis, but the isomerically pure 1,3,5,7,9-pentakis(2-biphe-
physical properties were uncovered, such as facile bowl-to-bowl nylyl)corannulene (6) is accessible in high yield by a self-correcting,
inversion of the central corannulene, a unique racemization exhaustive C–H borylation of 1 (ref. 33), followed by a Suzuki–Miyaura
pathway, high solubility in common organic solvents and a coupling of the resulting 1,3,5,7,9-pentakis(Bpin)corannulene (5).
widened highest occupied molecular orbital (HOMO)–lowest unoc- Cyclodehydrogenation of intermediate 6 similarly produces 4
cupied molecular orbital (LUMO) gap.
(Fig. 1b).
A third route to this highly contorted 80-carbon ring system
Since the early 1990s, we7–10 and other chemists11–13 have set our
sights on developing new laboratory methods for the chemical syn- through C–H activation was also developed (Fig. 1b). Thus, cyclode-
thesis of small, well-defined, carbon-rich compounds and nanocar- hydrogenation of decakis(4-t-butylphenyl)corannulene (7), which
bons, motivated partly by the desire to study their unique we recently synthesized directly from corannulene (1) by tenfold
properties, but also as practice for synthesizing full-size, structurally C–H arylation34, gives the same 26-ring carbon framework in 62%
uniform carbon materials ‘from the ground up’. Large numbers of yield, decorated now by ten t-butyl groups on the perimeter (8).
bowl-shaped fullerene fragments7–14, circular nanotube sections14–26
The strategy of stitching together phenyl groups or substituted
and disc-shaped graphene substructures27–29 have now been pre- phenyl groups by cyclodehydrogenation reactions has been used
pared and studied. Even chemical syntheses that produce C60 to produce many large, planar PAHs, especially during the past
(ref. 30) and the first structurally pure, short, rigid carbon nano- 10–15 years27–29. Much of the success in this chemistry can be
tube31 are reported, both in isolable quantities.
attributed to the formation of strain-free six-membered rings
We have begun to develop methods for the rapid, controlled from crowded starting materials. The formation of strained seven-
build-up of large graphene subunits from smaller molecular plat- membered rings by cyclodehydrogenation reactions, however, is
forms. For this purpose, few methods have proved as powerful as almost unprecedented35–37. For example, we have already confirmed
our recently introduced palladium-catalysed biphenylation of that [6]helicene does not undergo cyclodehydrogenation to form
1Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan, 2Merkert Chemistry Center, Boston College,
Chestnut Hill, Massachusetts 02467-3860, USA, 3Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602,
Japan. e-mail: itami.kenichiro@a.mbox.nagoya-u.ac.jp; lawrence.scott@bc.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1704
ARTICLES
a
Unprecedented structure
Unique properties
• A grossly warped graphene subunit
• Multiple odd-membered-ring defects
• Yellow powder (m.p. >300 °C)
• Soluble in common organic solvents
• Ultraviolet absorption (λmax = 491 nm)
• Green fluorescence (λmax = 504, 535 nm)
• Relatively large HOMO–LUMO gap
Rapid synthesis
• Two or three steps from corannulene
• Three C–H activation routes established
• Late-stage introduction of seven-membered rings
b
R
R
R
R
i. Tris(o-biphenylyl)boroxin,
Pd(OAc)2,
R
R
o-chloranil
21%
1
R
R
ii. DDQ,
TfOH
R
R
2
3 (Not observed)
50%
iii. Ir cat., B2(pin)2
>95%
R
R
B(pin)
R
R
(pin)B
(pin)B
iv. 2-bromobiphenyl,
v. DDQ,
R
Pd cat.
R
TfOH
B(pin)
40%
88%
B(pin)
R
5
R
6
R
R
4 (R = H; C80H30
)
tBu
tBu
vii. FeCl3
62%
8 (R = t-Bu; C120H110
)
tBu
tBu
tBu
vi. Tris(p-(t-butyl)phenyl)boroxin,
Pd(OAc)2,
o-chloranil
O
tBu
B(pin) =
B
O
23%
tBu
tBu
tBu
tBu
7
Figure 1 | Structure of 4, its salient properties and three different synthesis methods. a, Important features of the grossly warped C80H30 nanographene.
b, Synthetic routes to C80H30 (4) and its deca-t-butyl derivative C120H110 (8) from corannulene (1). For Route 1, direct fivefold C–H biphenylation, the
reaction conditions were (i) tris(o-biphenylyl)boroxin (2.0 equiv.), Pd(OAc)2 (20 mol%), o-chloranil (5.0 equiv.), DCE, 80 8C, 16 h and (ii) DDQ (10 equiv.),
trifluoromethanesulfonic acid (TfOH)/CH2Cl2 (5:95), 0 8C, 30 minutes. For Route 2, stepwise fivefold C–H borylation–arylation, the reaction conditions were
(iii) (Ir(OMe)(cod))2 (20 mol%), B2(pin)2 (5.2 equiv.), 4,4′-dimethylbipyridyl (40 mol%), potassium t-b′ utoxide (10 mol%), tetrahydrofuran (THF), 85 8C,
′
.
four days, (iv) 2-bromobiphenyl (20 equiv.), Pd2(dba)3 CHCl3 (10 mol%), 2-dicyclohexylphosphino-2 ,6 -dimethoxybiphenyl (SPhos, 20 mol%), Cs2CO3
(10 equiv.), toluene/water (2:1), 80 8C, 24 hours and (v) DDQ (10 equiv.), TfOH/CH2Cl2 (5:95), 0 8C, 30 minutes. For Route 3, direct tenfold ‘total’ C–H
phenylation, the reaction conditions were (vi) tris(p-(t-butyl)phenyl)boroxin, Pd(OAc)2, o-chloranil, DCE, 80 8C, repeat for 3–4 cycles and (vii) FeCl3 (31 equiv.),
CH2Cl2/nitromethane (115:1), 25 8C, one hour. cod, 1,5-cyclooctadiene; pin, pinacol; dba, dibenzylideneacetone; cat., catalyst; DCE, 1,2-dichloroethane;
DDQ, 2,3-dichloro-5,6-dicyanobenzoquinone.
hexa[7]circulene38 under otherwise identical conditions. It is (Fig. 2a). Five-membered rings embedded in a hexagonal lattice
remarkable that our syntheses of this 80-carbon nanographene impart geodesic curvature, as in fullerenes7,39, but seven-membered
(Fig. 1b) introduce five new strained seven-membered rings in a rings introduce so-called ‘negative curvature’40. The dense accumu-
single chemical reaction.
lation of both kinds of odd-membered-ring defects in 4 leads to its
An X-ray crystal structure of 4 confirms the distortion caused by unique double-concave structure (Fig. 2b). The central corannulene
the cluster of odd-membered rings in this sheet of carbon atoms moiety adopts a shallow bowl-shaped geometry with a bowl depth of
740
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1704
ARTICLES
13 Å
a
b
P
P
M
M
0.37 Å
13 Å
M
P
M
P
6.3 Å
M
P
Figure 2 | ORTEP images showing the warped structure of the new C80H30 nanographene 4, taken from the X-ray crystal structure. a, Top view of
enantiomer pair of 4, PMPMP-isomer (left) and MPMPM-isomer (right). b, Side view. In the ORTEP images all of the atoms shown are carbons (hydrogen
atoms on the perimeter are omitted for clarity) and thermal ellipsoids for the carbon atoms show 50% positional probability.
0.37 Å (Fig. 2b). More interestingly, the presence of five helical temperature and send a wave-like motion around the perimeter of
hexa[7]circulene moieties38, each with M or P chirality around the the molecule. As the temperature is lowered and bond rotations
seven-membered ring, makes 4 a chiral molecule with enantiomers become slower on the NMR timescale, the coalesced signals
of MPMPM and PMPMP configuration (Fig. 2a). The strain in this broaden. The same behaviour is observed in the variable-tempera-
1
nanographene is manifest in the deformation of so many C–C–C ture H NMR spectrum of the deca-t-butyl derivative 8 (Fig. 3e).
bond angles away from the natural value of 1208; from the X-ray Two coalesced singlets are seen at high temperatures (d ¼ 8.51 and
structure of 4, the C–C–C bond angles are seen to range from d ¼ 7.44 ppm), and they broaden as the solution is cooled. The
106.0(6)8 to 138.3(7)8. To the best of our knowledge, 4 represents exceptionally good solubility of this derivative permits the recording
the largest PAH, other than fullerenes and their derivatives, whose of spectra even at 220 8C, at which temperature each of the two
structure has been determined by X-ray crystallography41.
singlets splits into five singlets. Line-shape analysis of the vari-
The low solubility of graphite, and of even moderately sized able-temperature NMR spectrum gives experimental values for
planar nanographenes27–29, stems from the strong van der Waals the activation enthalpy (DH‡) and entropy (DS‡) associated
attraction associated with large-area contacts between adjacent gra- with this dynamic process of DH‡ ¼ 13.6+1.5 kcal mol21 and
phene faces. Compounds 4 and 8, however, lack such large-area DS‡ ¼ 0.2+0.6 cal mol21 K21
,
respectively. The experimental
contacts with their neighbours in the solid state (see the X-ray (DH‡) value is somewhat smaller than that predicted by molecular
crystal packing diagrams in the Supplementary Information) and orbital calculations for unsubstituted 4 (18.9 kcal mol21 (Fig. 3a)).
are soluble in common organic solvents. The t-butyl groups on 8 The central bowl in 4 continues to invert rapidly even at 220 8C
enhance the solubility further41, and this C120H110 hydrocarbon is (calculated barrier of only 1.7 kcal mol21), which explains why
freely soluble even in hexane!
the time-averaged symmetry of the molecule drops from D5h to
Though frozen in a single conformation by crystal packing C2 but not to C1.
constraints in the solid state, this 26-ring carbon framework is
Non-hexagonal-ring defects, particularly the imposition of the
highly flexible in solution and changes its shape constantly. In the seven-membered ring negative curvature, not only cause graphene
static structure (Fig. 2), each of the ten benzo groups around the per- sheets to warp, but also are predicted to alter their electronic and
imeter of the molecule is structurally distinct from all the others and optical properties44. These changes derive not only from non-pla-
occupies a unique environment. When unconstrained in solution, narity, but also from changes in connectivity. Unlike all-hexagonal
however, the molecule can equalize all ten benzo groups around graphene, carbon lattices that contain odd-membered rings are
its perimeter, which renders them time-averaged equivalent by ‘non-alternant’, and p systems of this class are known to have elec-
rotations around the rim bonds between them and bowl-to-bowl tronic and optical properties that differ from those of their alternant
inversion of the central corannulene. Density functional theory counterparts (for example, azulene versus naphthalene)45. Figure 4a
(DFT) calculations at the B3LYP/6-31G(d) level of theory predict shows a planar, defect-free C78H30 nanographene (9) synthesized by
activation energies of 1.7 and 18.9 kcal mol21, respectively, for the Mu¨llen and co-workers46 that compares well in size to the warped
bowl-to-bowl inversion and the racemization of 4 (Fig. 3a). The C80H30 nanographene 4. DFT calculations at the B3LYP/6-31G(d)
barrier calculated for bowl-to-bowl inversion of 4 is considerably level of theory predict that the odd-membered-ring defects cause
lower than that calculated for corannulene (10.4 kcal mol21) at the energy of the valence band (HOMO) of 4 to drop below that
the same level of theory (Fig. 3b); the best estimate from exper- of 9 (EHOMO ¼ 25.13 and 24.95 eV, respectively). The conduction
iments42,43 is about 11 kcal mol21. The shallowness of the bowl in bands (LUMOs) of
4 and 9, however, remain similar
4 relative to that in the free corannulene molecule (1) (bowl (ELUMO ¼ 22.07 and 22.05 eV, respectively), which gives the
depths ¼ 0.37 and 0.87 Å, respectively) is expected to lower the warped nanographene 4 a somewhat wider band gap than that of
energetic cost for inversion in 442,43
.
9 (3.06 and 2.90 eV, respectively).
Evidence for rapid equalization of the ten benzo groups on the
Experimentally, the influence of the odd-membered-ring defects
perimeter can be seen in the 1H NMR spectrum of 4, which on optical properties can be seen in the ultraviolet–visible (UV-vis)
shows only three signals (a triplet and two doublets), as expected absorption spectra of 4 versus that of 9 (Fig. 4b). The b band in the
for ten equivalent AMX three-spin systems at 100 8C (Fig. 3d). spectrum of the planar nanographene 9 (440 nm) is located at a
Rotations around rim bonds sequentially equilibrate the M and P longer wavelength than that of the warped nanographene 4
chiralities of successive hexa[7]circulene moieties at this (418 nm), and the longest wavelength absorption maximum of 9
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1704
ARTICLES
a
TSrac
TSflip
1.7
18.9
MPMPM
MPMPM
PMPMP
0.0 (kcal mol–1
)
0.0
0.0
c
b
C2 axis
σ Plane
TS
10.4
TSflip
TSrac
0.0 (kcal mol–1
)
0.0
e
Observed
Simulated
d
T (°C)
T (°C)
k (s–1
)
100
60
40
6,000
80
3,000
880
80
20
60
40
0
27
–10
–20
25
12
9.0
8.5
8.0
ppm
7.5
7.0
7.8 7.6 7.4 7.2 7.0 7.8 7.6 7.4 7.2 7.0
ppm ppm
Figure 3 | Conformational flexibility of the grossly warped C80H30 nanographene. a, Bowl-to-bowl inversion (left, MPMPM ⇔ TSflip ⇔ MPMPM) and
racemization (right, MPMPM ⇔ TSrac ⇔ PMPMP) pathways of 4 determined by DFT calculation. Values (kcal mol21) are relative Gibbs free energies (DG)
at 298.15 K and 1 atm calculated at the B3LYP/6-31G(d) level of theory. The activation energies of bowl-to-bowl inversion and racemization of 4 are predicted
to be 1.7 and 18.9 kcal mol21, respectively. The bowl-to-bowl inversion barrier of 4 is considerably lower than that of corannulene (b). b, Pathway and
activation energy (10.4 kcal mol21) of bowl-to-bowl inversion of corannulene determined by DFT calculations at the same level of theory. Both side views and
top views of corannulene structures are shown. c, C2 axis and s plane in the transition states of bowl-to-bowl inversion (TSflip) and racemization (TSrac) of
4, respectively. d, Variable temperature 1H NMR spectrum of 4 (400 MHz, C2D2Cl4) reveals the rapid equalization of the ten benzo groups on the
perimeter. e, Variable-temperature 1H NMR spectrum of (t-Bu)10C80H20 8 (500 MHz, C2D2Cl4) gives the experimental values for the activation enthalpy
(DH‡ ¼ 13.6+1.5 kcal mol21) and entropy (DS‡ ¼ 0.2+0.6 cal mol21 K21) associated with a wave-like motion around the perimeter of the molecule. k, rate
constant (s21).
(shoulder at ꢀ525 nm) lies beyond that of 4 (491 nm). No fluorescence Ered,1 ¼ 21.59 V, Ered,2 ¼ 22.02 V and Ered,3 ¼ 22.31 V (versus
has been reported for 9, but 4 fluoresces at 504 and 535 nm with a Fc/Fcþ). Although a comparison of these cyclic voltammetry data
quantum efficiency of 0.26 (see the Supplementary Information).
with those of a planar reference system is not possible because
Moreover, the sufficient solubility of deca-t-butyl derivative 8 planar nanographenes such as 9 are extremely insoluble, a compari-
enabled cyclic voltammetry measurements (see the Supplementary son with corannulene (1) or fullerenes is interesting. Following the
Information). In CH2Cl2, 8 displays two reversible oxidations general trend that five-membered rings usually impart good elec-
with first and second oxidation potentials of Eox,1 ¼ þ0.63 V and tron-accepting ability, corannulene47,48 and C60 fullerene49 are
Eox,2 ¼ þ0.97 V versus the ferrocene/ferrocenium couple (Fc/Fcþ). reduced easily (up to four- and six-electron reductions, respect-
In the negative potential region, reversible redox waves were ively), but are somewhat difficult to oxidize. In contrast, our
observed at the first, second and third reduction potentials of warped nanographenes can be oxidized easily as well as be
742
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1704
ARTICLES
to silica-gel column chromatography (eluent, hexane/CH2Cl2 ¼ 100:0 to 80:20) to
give pentakis(o-biphenylyl)corannulene (2: 23 mg, 21% yield) as a mixture
of regioisomers.
a
Synthesis of C80H30. To a solution of pentakis(o-biphenylyl)corannulene (2, 10 mg,
10 mmol, 1.0 equiv.) in dry CH2Cl2 (1.9 ml) was added 2,3-dichloro-5,6-
dicyanobenzoquinone (DDQ, 23 mg, 0.1 mmol, 10 equiv.) at 0 8C. After stirring for
five minutes, trifluoromethanesulfonic acid (0.1 ml) was added to the mixture. The
mixture was further stirred for 30 minutes at 0 8C. The reaction mixture was
neutralized with saturated aqueous NaHCO3 and then extracted with CH2Cl2. The
combined organic phase was dried over MgSO4 and the organic solvent was removed
under reduced pressure. The residue was then dissolved into tetrachloroethane and
incubated at 100 8C for 30 minutes. The thus-obtained precipitate was collected by
filtration and the residue was dried under vacuum to afford C80H30 (4, 4.9 mg, 50%
yield) as a yellow powder. Recrystallization of 4 from hot 1,1,2,2-tetrachloroethane
9
.
yielded yellow crystals of C80H30 2C2H2Cl4 suitable for X-ray crystal structure
analysis. Details of the crystal data and a summary of the intensity data collection
parameters for C80H30 are listed in the Supplementary Information. CCDC 919707
contains 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.
b
440
Received 7 February 2013; accepted 5 June 2013;
published online 14 July 2013
323
9
4
418
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Fivefold C–H arylation of corannulene. A solution of Pd(OAc)2 (4.5 mg, 20 mmol,
20 mol%), o-chloranil (122 mg, 0.50 mmol, 5.0 equiv.), corannulene (1: 25 mg,
0.10 mmol, 1.0 equiv.) and tris(o-biphenylyl)boroxin (108 mg, 0.20 mmol, 2.0
equiv.) in dry 1,2-dichloroethane (DCE) (10 ml) was stirred at 80 8C for 16 hours
under argon. The reaction mixture was then passed through a pad of silica gel with
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1704
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Additional information
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
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L.T.S. and K.I.
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Competing financial interests
The authors declare no competing financial interests.
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