Production, isolation and structural characterization of [92]fullerene isomers

Article abstract of DOI:10.1039/b208820g

High-resolution ¹³C NMR has been used to structurally characterize a single isomer possessing C₂ molecular sym)metry as well as an inseparable mixture of other isomers of [92]fullerene, produced from dysprosium arc-burned soot, separ

Full text of DOI:10.1039/b208820g

Production, isolation and structural characterization of [92]fullerene
isomers†
Nikos Tagmatarchis,ab Denis Arcon,c Maurizio Prato*b and Hisanori Shinohara*a
a Nagoya University, Department of Chemistry, Nagoya 464-8602, Japan. E-mail: noris@cc.nagoya-u.ac.jp;
Fax: + 81 52 7891169; Tel: + 81 52 7892482
b Università di Trieste, Dipartimento di Scienze Farmaceutiche, Piazzale Europa 1, Trieste 34127, Italy.
E-mail: prato@units.it; Fax: + 39 040 52572; Tel: + 39 040 558 7883
c
Institut Jozef Stefan, Jamova 39, 1000 Ljubljana, Slovenia. E-mail: denis.arcon@ijs.sl; Fax: + 386 1 426
3269; Tel: + 386 1 477 3492
Received (in Cambridge, UK) 10th September 2002, Accepted 14th October 2002
First published as an Advance Article on the web 11th November 2002
High-resolution 13C NMR has been used to structurally
characterize a single isomer possessing C2 molecular sym-
metry as well as an inseparable mixture of other isomers of
[92]fullerene, produced from dysprosium arc-burned soot,
separated and isolated via multi-stage recycling HPLC.
Higher fullerenes1 and endohedral metallofullerenes2 can
nowadays be obtained in reasonable amounts due to enormous
advances in high pressure liquid chromatography (HPLC).
Once isolated in pure form, the otherwise difficult structural
assignment to the new fullerene becomes possible by high-field
13C NMR spectroscopy. Theoretical calculations have proposed
possible structures of various higher fullerene isomers satisfy-
ing the isolated pentagon rule (IPR).3 Although the fullerene
formation process has yet to be revealed, several hypothetical
formation mechanisms have been proposed such as the
Fig. 1 First-stage HPLC profile on a 5PYE column of soot containing
pentagon road,4 the fullerene road,5 ring stacking,6 ring
various dysprosium endohedral metallofullerenes together with empty
fullerenes. Region A is subjected to further multi-stage recycling HPLC
treatment to separate and isolate isomers of [92]fullerene as described in the
text.
coalescence7 and annealing.
Recently we pointed out that the doped metal atoms may play
a significant role not only in the relative production yield of
isomers of higher fullerenes but also in the nature and symmetry
of the isomer formed.8
There are in total 86 different structural isomers for
[92]fullerene that obey the IPR rule.9 To the best of our
knowledge, up to now, none of them has been isolated in
isomer-pure form.10 In this communication, we report the
production and structural characterization of one single isomer
possessing C2 molecular symmetry and a mixture of some other
isomers of [92]fullerene.11
By using direct current arc discharge of Dy-graphite
composite rods (12.5 3 12.5 3 300 nm, 0.8 wt%, Toyo Tanso
Co.) the so-produced soot12 was collected under totally
anaerobic conditions to avoid any air degradation during the
collection procedure. After Soxhlet extraction by carbon
disulfide, the separation of [92]fullerene isomers was achieved
by multi-stage HPLC (LC-908-C60, Japan Analytical Industry).
The first-stage HPLC chromatogram utilizing a 5PYE column
(20 3 250 mm, Nakalai Tesque, 15 ml min21 flow rate, toluene
as eluent) is shown in Fig. 1. We focused on area A which
contained empty [90], [92] and [94]fullerenes together with
several dysprosium endohedral metallofullerenes as revealed by
laser-desorption time-of-flight mass spectrometry (LD-TOF
MS). The second-stage recycling HPLC of fraction A on
Buckyclutcher column (20 3 300 nm, Regis Chemical, 10 ml
min21 flow rate, toluene as eluent) resulted in the complete
removal of dysprosium endohedral metallofullerenes from the
latter mixture. Further recycling (HPLC treatment of the latter
fraction containing empty fullerenes) on Buckyprep (20 3 250
mm, Nakalai Tesque) and 5PYE columns eventually resulted in
separating two peaks which were confirmed to be isomers of
[92]fullerene by both positive and negative modes of LD-
TOF.
The high resolution 13C NMR spectrum of the first isolated
material (ca. 2.8 mg) consists of 46 equally-intense lines as
shown in Fig. 2.
The chemical shifts of these lines are spread over between
150 to 130 ppm.† This is the region where the sp2 hybridized
carbons of fullerenes normally appear. The number of 13C NMR
lines and their typical chemical shifts jointly indicate unambigu-
ously that the current isolated isomer of [92]fullerene is
assigned to possess C2 molecular symmetry. However, due to
the existence of 26 different structural isomers of [92]fullerene
with C2 molecular geometry, the assignment of the currently
isolated isomer to any particular single one is precluded at this
stage. Two-dimensional high resolution 13C NMR spectroscopy
will provide more information and resolve the above issue in the
near future.
In contrast to this straightforward structural assignment for
the [92-C2]fullerene, the assignment for the second isolated
[92]fullerene (ca. 3.7 mg) is more complicated. 13C NMR
measurement has revealed 135 lines integrated for 138 sp2
carbon atoms as shown in Fig. 3.†
However, out of the large number of possible structural
isomers, many of them can be excluded simply by correlating
our experimental result (i.e. 135 lines integrated for 138 sp2 C
atoms) with the number of 13C NMR lines expected for a
random mixture of [92]fullerene isomers (i.e. 92 lines for C1
geometry, 46 lines for C2 geometry, 23 lines for D2 geometry
etc.). In addition, it has already been shown that up to
[90]fullerenes so far isolated 23 fullerene isomers (out of 24
different kind of fullerenes) commonly have at least one C2
symmetry axis in their molecular structure.13 It is probable that
our second isolated material is a mixture of either (i) three C2,
†
Electronic supplementary information (ESI) available: NMR data and
UV–VIS spectra of [C92-C2] and [C92]-mixture. See http://www.rsc.org/
suppdata/cc/b2/b208820g/
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CHEM. COMMUN., 2002, 2992–2993
This journal is © The Royal Society of Chemistry 2002

The UV-Vis-NIR electronic absorption spectra of the
currently studied [92]fullerene isomers were recorded between
400 and 2000 nm in carbon disulfide solutions (See ESI†). For
the newly produced and structurally characterized [92-C2]ful-
lerene there are characteristic absorptions at 560, 622, 885 and
1040 nm. The onset of the electronic absorption spectrum
continues down to 1220 nm and, as it corresponds to the lowest
electronic transitions, we expect that this material possesses a
relatively small HOMO–LUMO energy band gap. Finally, the
characteristic absorptions for the inseparable mixture of
[92]fullerene appear at 528, 656 and 742 nm with the absorption
onset at 1440 nm.
In conclusion, we have succeeded in isolating and structur-
ally characterizing a new [92-C2]fullerene via multi-stage
recycling HPLC and high-resolution 13C NMR measurements,
respectively. A mixture of at least two other inseparable
structural isomers of [92]fullerene was also separated and
characterized.
The work in Japan was supported by JSPS and in Italy by the
European Union, Human Potential Network ‘FUNCARS’,
contract HPRN-1999-00011, MURST (PRIN 2000,
MM03198284), CNR programme ‘Materiali Innovativi (legge
95/95)’. Assistance of Dr M. Polak in high resolution 13C NMR
measurements is greatly acknowledged.
Fig. 2 High-resolution 13C NMR spectrum [600 MHz, CS2 solution,
Cr(acac)3 as relaxant and acetone-d6 for the internal lock at 25 °C] of the
purified single isomer of [92-C2]fullerene. Insets are expanded regions
between 150–145.5 and 145.5–140 ppm, respectively, for clarity.
Notes and references
1 C. Thilgen and F. Diederich, Top. Curr. Chem., 1999, 199, 135;
Fullerenes: Chemistry, Physics and Technology, Eds. K. M. Kadish and
R. S. Ruoff, Wiley Interscience, New York, 2000, pp. 283–330.
2 H. Shinohara, Rep. Prog. Phys., 2000, 63, 843.
3 H. Kroto, Nature, 1987, 329, 529.
4 R. E. Smalley, Acc. Chem. Res., 1992, 25, 98 and references therein.
5 J. R. Heath, ACS Symp. Ser., 1991, 481, 1.
6 T. Wakabayashi and Y. Achiba, Chem. Phys. Lett., 1992, 190, 465; T.
Wakabayashi, K. Kikuchi, S. Suzuki, H. Shiromaru and Y. Achiba, Z.
Phys. D., 1993, 26, 258; T. Wakabayashi, H. Shiromaru, K. Kikuchi and
Y. Achiba, Chem. Phys. Lett., 1993, 201, 470.
7 S. W. McElvany, M. M. Ross, N. S. Goroff and F. Diederich, Science,
1993, 259, 1594; J. M. Hunter, J. L. Fye and M. F. Jarrold, Science,
1993, 260, 784; G. von Helden, N. G. Cotts and M. T. Bowers, Nature,
1993, 363, 60; G. von Helden, N. G. Cotts and M. T. Bowers, J. Am.
Chem. Soc., 1993, 115, 4363.
Fig. 3 High-resolution 13C NMR spectrum [600 MHz, CS2 solution,
Cr(acac)3 as relaxant and benzene-d6 for the internal lock at 25 °C] of the
inseparable isolated mixture of [92]fullerene.
8 N. Tagmatarchis, K. Okada, T. Tomiyama, T. Yoshida, Y. Kobayashi
and H. Shinohara, Chem. Commun., 2001, 1356.
9 P. W. Fowler and D. E. Manolopoulos, An Atlas of Fullerenes,
Clarendon, Oxford, 1995, pp. 270–277.
10 Y. Achiba, K. Kikuchi, Y. Aihara, T. Wakabayashi, Y. Miyake and M.
Kainosho, in The Chemical Physics of Fullerenes 10 and 5 Years Later,
Ed. W. Andreoni, Kluwer Academic Publishers, 1996, pp. 139–147;
Although in that report there are tabulated 4 isomers for [92]fullerene
possessing molecular symmetries C2, C2, D2 and D2, respectively,
neither their 13C NMR with the appropriate peaks assignment is given
nor their production and isolation in isomer-free form is described in the
text.
11 For some recent theoretical calculations on stabilities of IPR [92]full-
erenes see: Z. Slanina, X. Zhao, P. Deota and E. Osawa, J. Mol. Mod.,
2000, 6, 312.
12 N. Tagmatarchis and H. Shinohara, Chem. Mater., 2000, 12, 3222.
13 Y. Achiba, K. Kikuchi, Y. Aihara, T. Wakabayashi, Y. Miyake and M.
Kainosho, Mat. Res. Soc. Symp. Proc., 1995, 359, 3.
(ii) one C2 and one C1, (iii) two C2 and two D2, (iv) one C1 and
two D2, or (v) six D2 symmetrical isomers of [92]fullerene. Case
(v) can be excluded as there exists only four [92-D2]fullerenes
that obey IPR.9
We tried to resolve the above mixture to its components by
cutting the corresponding HPLC fraction in half and HPLC
recycling each fraction, or by cutting it in three parts and further
recycling each part with a variety of HPLC columns and solvent
flow rates. However, in all cases, the UV-Vis-NIR electronic
absorption spectra of every part of cut-and-HPLC recycled peak
were identical to each other. Furthermore, no sign of peak
splitting was observed during the recycling procedure while the
recycled peak only became broader. Thus, it is reasonable to
assume that the second isolated fraction of [92]fullerene is an
inseparable mixture of isomers described previously at least
under the applied experimental purification procedures.
CHEM. COMMUN., 2002, 2992–2993
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