Bowl-shaped fragments of C70 or higher fullerenes: Synthesis, structural analysis, and inversion dynamics

Article abstract of DOI:10.1002/anie.201208200

Super bowl: Fragments of C₇₀ or higher fullerenes were easily prepared from 1,8-bis(arylethynyl)naphthalenes. The curved structures were identified by X-ray crystallography. These rigid bowl-shaped molecules have a deep bowl depth (approximately 2.30 A) and their bowl inversions proceed via an S-shaped transition structure with a very high inversion barrier (approximately 80 kcal mol^-1). Copyright

Full text of DOI:10.1002/anie.201208200

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DOI: 10.1002/anie.201208200
Buckybowls
Bowl-Shaped Fragments of C70 or Higher Fullerenes: Synthesis,
Structural Analysis, and Inversion Dynamics**
Tsun-Cheng Wu, Min-Kuan Chen, Yen-Wei Lee, Ming-Yu Kuo,* and Yao-Ting Wu*
Most investigations into fullerene chemistry have focused on
C60,[1] not only owing to its abundance and low cost, but also
because of its simple structure. C60 is composed of 60
fragments of C60 has been intensively investigated in the
past two decades.[4] In contrast, the bowl-shaped subunits of
C70 that contain the unique bond type VIII are almost
completely unstudied; to the best of our knowledge, only
tetraindenopyrene derivative 3 has been reported.[7] Theo-
retical analysis has revealed that 3 is not a rigid molecule, and
À
equivalent carbon atoms with two distinct types of C C
bonds.[2] In contrast, C70 contains five types of carbon atoms
[3]
À
(a–e) with eight types of C C bonds (I–VIII). The C70
molecule can be regarded as two C60-like hemispheres that
are connected by a new set of ten carbon atoms (labeled e).
Studies of the bowl-shaped subunits of fullerenes may help
give an insight into their chemistry. These buckybowls are
its bowl-to-bowl inversion barrier is only 0.33 kcalmolÀ1
.
Compounds 9 (Scheme 1) are ideal prototypes for investiga-
important because of their interesting physical properties.[4]
A
very unusual characteristic of these p bowls is their polarity,
which is generated by the non-uniform electronic distribution
within the curved surface.[5] As a result, they may form polar
crystals that exhibit the pyroelectric effect.[6] The chemistry of
corannulene (1), sumanene (2), and other bowl-shaped
[*] Dr. T.-C. Wu, M.-K. Chen, Prof. Y.-T. Wu
Department of Chemistry, National Cheng Kung University
No. 1 Ta-Hsueh Rd., 70101 Tainan (Taiwan)
E-mail: ytwuchem@mail.ncku.edu.tw
Scheme 1. Preparation of buckybowls 9. DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone,
DMF=N,N’-dimethylformamide.
Y.-W. Lee, Prof. M.-Y. Kuo
Department of Applied Chemistry, National Chi Nan University
No. 1 University Rd., 54561 Puli, Nantou (Taiwan)
E-mail: mykuo@ncnu.edu.tw
tion because they contain all the types of carbon atoms that
are in C70 and are also fragments of many higher fullerenes,[8]
including C76,[9] C78,[9] and C84.[10] This investigation presents
the synthesis, structures, and physical properties of new bowl-
shaped molecules 9.
[**] Metal-Catalyzed Reactions of Alkynes. Part XIII. This work was
supported by the National Science Council of Taiwan (NSC 101-
2628-M-006-002-MY3 and NSC 99-2113-M-260-007-MY3). We also
thank Prof. S.-L. Wang and P.-L. Chen (National Tsing Hua
University, Taiwan) for the X-ray structure analyses, and the
National Center for High-Performance Computing for providing
computational resources. The authors are indebted to Prof. J. Siegel
(University of Zurich, Switzerland) for his useful suggestions.
Based on the successful syntheses of 4 and 5,[11] bucky-
À
bowls 9 were synthesized using metal-catalyzed C C bond
formation reactions.[12] The synthetic approach is presented in
Scheme 1; 8aa was efficiently prepared by the simple Rh-
catalyzed [2+2+2] cycloaddition of diyne 6a with acenaph-
thylene (7a), and was subsequently aromatized by treatment
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208200.
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Table 1: Selected distances and the torsion angles for C70, 9aa, and 9ba
with DDQ. The Pd-catalyzed cyclization of 8aa gave 9aa in
low yield. This three-step protocol was also adopted for the
preparation of new fullerene fragments, such as more-
extended buckybowls 9ba and 9bc, both of which are also
fragments of C70,[3] C76,[9] C84,[10] and other fullerenes. It should
be emphasized that the solubility of 9aa and 9ba in common
organic solvents is low, and some of the product remained on
the column after chromatography. An attempt was made to
synthesize 9bc because we believe its two n-butyl groups
would likely improve the solubility. Unfortunately, only
a trace amount of 9bc was obtained. Presumably, the highly
curved structure was the main reason for the inefficient
conversion.
based on X-ray crystallography.[a]
b
C70
9aa
9ba
The structures of 9aa and 9ba were analyzed by X-ray
crystallography, and found to be bowl-shaped (Figure 1).[13]
The depth of the bowl 9aa was determined to be 2.28 ꢀ, by
Bond length [ꢀ]/bond type
À
C2 C3/A
1.379(1)
1.377(5)
1.368(3)
À
C3 C4/B
1.456(1)
1.456(1)
1.440(1)
1.401(1)
1.478(2)
1.416(2)
1.421(2)
1.415(3)
1.387(2)
1.443(3)
1.418(2)
1.430(2)
1.425(2)
1.397(2)
1.444(2)
À
C4 C5/B
À
C5 C5’/B
À
C5 C6/A
À
C6 C15/C
Torsion angle [8]
C5-C6-C7’-C6’
C5’-C5-C7-C7’
11.0
0
12.9
0
12.3
0.3
[a] The values obtained by averaging the symmetry-related bond
distances. [b] Taken from C70·(S8)6 (Ref. [3b]).
mately 128. This twisted conformation is slightly more stable
(5.57 kcalmolÀ1 for 9aa) than a planar conformation, based
on density functional theory (DFT) calculations (see the
Supporting Information). The twisted six-membered rings
were also observed in C70 (11.08). To further examine the
conformation of the central six-membered ring, the structures
of 2, 11, and 12 were analyzed and compared. In the solid
Figure 1. X-ray crystallographic structures and the POAV pyramidaliza-
tion angles of buckybowls 9aa and 9ba. Only the carbon atoms are
shown for clarity, and the thermal ellipsoids are set at 30% and 50%
probability for 9aa and 9ba, respectively.
measuring the perpendicular distance from the center of the
hexagonal base defined by C5, C6, C7, C7’, C6’ and C5’ to the
plane composed of the 14 rim carbon atoms (Table 1).
Buckybowl 9aa is less curved than its counterpart in C70, as
revealed by comparing the distances between the two carbon
atoms C2 and C10 (8.21 ꢀ for 9aa and 7.90 ꢀ for C70). Similar
to corannulene (1) and acecorannulene (10),[14] the peri an-
nelation in 9ba increases both the bowl depth (2.33 ꢀ) and
the curvature (d(C2–C10) = 8.03 ꢀ).
The point symmetries of buckybowls 9aa and 9ba are
quasi-C2v and quasi-Cs, respectively. The slight deviation from
the ideal geometry may be caused by intermolecular inter-
actions in the solid. Table 1 shows selected structural data for
state, the central six-membered ring in curved sumanene
(2)[15] and that in planar coronene (12)[16] are planar, whereas
that in 11 is found to have a torsion angle of approximately 58,
based on a DFT calculations (Supporting Information). These
findings suggest that the arrangement of five- and six-
membered rings on the rim affects the conformation of the
central six-membered ring in these arenes.
The POAV (p-orbital axis vector) pyramidalization
angle[17] is useful for quantifying the curvature of buckybowls.
For example, the values for planar benzene and C60 are 08 and
11.68, respectively. The maximum POAV pyramidalization
angle of 9aa was observed at the C5/C7 position, with a mean
value of 10.88 (Figure 1). The additional ethylene bridge in
9ba strongly increases the POAV pyramidalization angle at
the C3 position to 11.48. The twisted conformation of the
central six-membered ring in 9 explains the low POAV
À
9 and C70. Most of the C C bond lengths in 9aa and 9ba are
smaller than those of their counterparts in C70. Table 1
presents segments of 9 and their representative bond types A–
C (shown in bold); the bond lengths follows the order C > B >
A. Importantly, bond type C, which is bond VIII on the
À
equator of C70 and C6 C15 in buckybowls 9, is found only in
C70 or higher fullerenes. Like C60,[2] C70 and buckybowls 9
À
generally exhibit alternating C C distances, such that the 6:6
ring junctions are shorter than the 6:5 ring junctions, but bond
type C is an exception. For a detailed comparison of the
structural data of 9 and C70, see the Supporting Information.
The central six-membered ring in both 9aa and 9ba is
twisted with a torsion angle (C5-C6-C7’-C6’) of approxi-
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pyramidalization angle at atom C6. Buckybowls 9 are less
curved than 5 (12.88) or the pole carbon atoms in C70
(12.18),[3b] because they are on the longitudinal part of C70.
The molecular aggregates of 9aa and 9ba in the solid state
differ completely from each other, even though the only
structural difference between 9aa and 9ba is the ethylene
bridge. Figure 2a shows an infinite stack of bowl molecules
more densely. The core repeating structure of 9ba can be so
viewed as a tab-to-cavity cc/cc dimer (Figure 2 b).
The inversion of all previously studied buckybowls, such
as corannulene (1),[14b,19] sumanene (2),[20] and even the highly
curved 4,[11] proceeds via a planar transition state (route 1 in
Scheme 2), to the best of our knowledge. However, based on
Scheme 2. Inversion dynamics of buckybowls.
DFT calculations at the B3LYP/cc-pVDZ level, 9 should have
a different inversion route. As indicated in Scheme 2, the
inversion of 9aa by route 2 via an S-shaped transition
structure is suggested to have a lower inversion barrier
(DG2° = 79.8 kcalmolÀ1) than route 1 (DG1° = 116.3 kcal
molÀ1, Supporting Information). This unusual result is likely
due to the unique structure of 9 (see below). The pseudo
intrinsic reaction coordinate (pseudo-IRC) verifies the inver-
sion mechanism of 9aa (see the Supporting Information). To
describe the inversion process, the segments of 9aa are
classified as two pole regions and a central part along the C2–
C10 axis. The inversion process concerns the relative motion
of these segments (Figure 3). Pulling down a pole region while
simultaneously raising the interior of the central part (C5, C6,
C7, C7’, C6’, and C5’) transforms bowl 1 into the S-shaped
transition structure, which is converted into bowl 2 by
continuously raising the central part and pushing down on
the other pole region.
Figure 2. a) Bowl stacks and intermolecular interactions of 9aa and
b) a concave/concave dimer of 9ba.
9aa. The bowl directions of the molecular pairs A and B up
the stack are the same, but the bowls slip from side to side
with a slipping angle of 19.58. The slipping angle is defined as
that formed between the stacking axis and the normal to the
central six-membered ring of the molecules. Although the
separation between two molecules A is 8.375 ꢀ, this separa-
tion produces very close contacts between two pairs of
nonbonded carbons in adjacent bowls (that is, between A and
B). The shortest of these contacts are 3.202 ꢀ between C11/
C11’ in a bowl and C7/7’ in the next bowl, and 3.387 ꢀ
between C18/C18’ in one bowl and C11/C11’ in the adjacent
one. These two values are smaller than the sum of the van der
Waals radii of two carbons (3.42 ꢀ).[18] Notably, all of the bowl
stacks are aligned in one direction to generate polar crystals
of space group Cmc21. Unlike the polar columnar packing
seen for 9aa, 9ba packs in a centrosymmetric crystal form.
Why do these closely related structures pack so differently?
One possible explanation considers the nature of concave/
convex (cc/cv) orientation of nearest neighbors to the
concave/concave (cc/cc) orientation. For a bowl with no
perturbation to its rim, cc/cv stacking fills space densely,
whereas cc/cc leaves a void; the degree of “penalty” being
proportional to the size of the void. For high curvature bowls,
the cc/cc polymorph is excluded. In contrast, for a bowl with
a tab on its rim, the tab of one bowl can “fill” the cavity of
another bowl in a cc/cc dimer, thus allowing this form to pack
As expected, peri annelation in 9ba and 9bb increases
their inversion barriers (Table 2). The high inversion barrier
of 9 (approximately 80–90 kcalmolÀ1) reveals that they are
static bowls at room temperature, unlike corannulene
(approximately 11 kcalmolÀ1)[14b,19] and sumanene (approx-
Figure 3. Energy diagram of the inversion process of 9aa (side view).
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Table 2: Theoretical analysis of bowl-to-bowl inversion barriers.[a]
neutron diffraction: g) A. V. Nikolaev, T. J. S. Dennis, K. Pras-
sides, A. K. Soper, Chem. Phys. Lett. 1994, 223, 143.
Compound
DG1
DG2
DDG°
°
°
[4] Reviews for buckybowls, see: a) Fragments of Fullerenes and
Carbon Nanotubes: Designed Synthesis, Unusual Reactions, and
Coordination Chemistry (Eds.: M. A. Petrukhina, L. T. Scott),
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4
5
56.2[b]
134.3
116.3
148.4
181.4
–
–
124.3
79.8
84.3
87.4
10.0
36.5
64.1
90.4
9aa
9ba
9bb[c]
[a] Calculated in kcalmolÀ1 at B3LYP/cc-pVDZ level.
DDG° =DG1 ÀDG2 . [b] Ref. [11]. [c] Substituents for 9bb: R
R
R
°
°
1
1
2
À
=
À
2
=
R
(CH2)2.
[5] K. K. Baldridge, J. S. Siegel, Theor. Chem. Acc. 1997, 97, 67.
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imately 20 kcalmolÀ1).[20] The inversion barrier of 5 is even
higher (124.3 kcalmolÀ1) and the inversion of 5 most probably
also proceeds along route 2 (Scheme 2) via an S-shaped
transition structure. An interesting transition in the inversion
process was observed from 4 (through route 1) to 5/9 (via
route 2). In compounds 9, the preference for route 2 follows
the order 9bb > 9ba > 9aa (Table 2). The longitudinal length
of these buckybowls critically affects the inversion dynamics:
greater distance corresponds to greater preference for
route 2. The less-curved structure and the twisted central
six-membered ring of 9 cause these molecules to have a lower
inversion barrier than 5.
In conclusion, this investigation demonstrates a simple
synthetic approach for preparing buckybowls 9, which exhibit
unique structural properties and inversion dynamics, and help
to give an insight into the chemistry of the longitudinal part of
C70. Chiral derivatives of 9 should be configurationally stable
at room temperature,[21] owing to the high predicated bowl
inversion. Preparation of an enantiopure buckybowl 9 is
under way.
[10] X-ray structure of [C84(Cs)·Ag] complex: L. Epple, K. Amsharov,
K. Simeonov, I. Dix, M. Jansen, Chem. Commun. 2008, 5610.
[11] T.-C. Wu, H.-J. Hsin, M.-Y. Kuo, C.-H. Li, Y.-T. Wu, J. Am.
Chem. Soc. 2011, 133, 16319.
[12] Several buckybowls have been synthesized by Pd-catalyzed
intramolecular arylation. For recent examples, see: a) E. A.
Jackson, B. D. Steinberg, M. Bancu, A. Wakamiya, L. T. Scott, J.
Am. Chem. Soc. 2007, 129, 484; b) B. D. Steinberg, E. A.
Jackson, A. S. Filatov, A. Wakamiya, M. A. Petrukhina, L. T.
Scott, J. Am. Chem. Soc. 2009, 131, 10537; c) H.-I. Chang, H.-T.
Huang, C.-H. Huang, M.-Y. Kuo, Y.-T. Wu, Chem. Commun.
2010, 46, 7241; d) A. C. Whalley, K. N. Plunkett, A. A. Gor-
odetsky, C. L. Schenck, C.-Y. Chiu, M. L. Steigerwald, C.
Nuckolls, Chem. Sci. 2011, 2, 132; for other nonpyrolysis
coupling techniques, see: e) Y.-T. Wu, T. Hayama, K. K.
Baldridge, A. Linden, J. S. Siegel, J. Am. Chem. Soc. 2006, 128,
6870; f) K. T. Rim, M. Siaj, S. Xiao, M. Myers, V. D. Carpentier,
L. Liu, C. Su, M. L. Steigerwald, M. S. Hybertsen, P. H.
McBreen, G. W. Flynn, C. Nuckolls, Angew. Chem. 2007, 119,
8037; Angew. Chem. Int. Ed. 2007, 46, 7891; g) O. Allemann, S.
Duttwyler, P. Romanato, K. K. Baldridge, J. S. Siegel, Science
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[13] CCDC 901335 (9aa) and 901336 (9ba) contains the supplemen-
tary 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.
[14] a) A. Sygula, H. E. Folsom, R. Sygula, A. H. Abdourazak, Z.
Marcinow, F. R. Fronczek, P. W. Rabideau, J. Chem. Soc. Chem.
Commun. 1994, 2571; for the structure/energy correlation of
corannulene-based buckybowls, see: b) T. J. Seiders, K. K.
Baldridge, G. H. Grube, J. S. Siegel, J. Am. Chem. Soc. 2001,
123, 517.
Received: October 11, 2012
Published online: December 6, 2012
Keywords: alkynes · buckybowl · cycloaddition · fullerenes ·
.
inversion dynamics
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