spectra the attempts to isolate one by HPLC were unsuccessful.
We regard the low fullerene yield to be probably connected to
the presence of weak bonds in the initial precursor, which are
not stable enough under FVP conditions, and/or the existence
of the fullerene in form of derivatives.16
We are optimistic that optimization of the precursors structure
by removing less stable single bonds and introducing radical
promoters will allow the pyrolysis temperature to be reduced
and increase the selectivity of fullerene formation.
Although the MS data clearly show the selective condensation
of compound 4 to C78-fullerene, the yield was not sufficient
for determination of the isomer composition. Taking into account
that the separation of individual higher-fullerene isomers
represents an almost unfeasible task, the isomerization of the
fullerene cage during synthesis can completely nullify the effort
of direct synthesis. One of the plausible processes for isomer-
ization of fullerenes under FVP conditions is the so-called Stone-
Wales (SW) or “pyracylene” rearrangement, which is a 90°
rotation of C2 unit.17 The SW rearrangement proceeds via an
antiaromatic 4-electron-4-center transition state and thus is
thermally forbidden under the Woodward-Hoffmann rules for
concerted reactions.18 However, according to quantum mechan-
ical calculations such kind of rearrangement can be realized
through a route different from the one originally suggested. The
multistep SW rearrangement can be characterized by relatively
low activation energy,19 especially in the presence of radi-
cals.20,21 The C78 cage represents the smallest fullerene in which
SW rearrangement can give stable IPR isomers: C78:5 (D3h)
T C78:3 (C2V) T C78:2 (C2V) T C78:4 (D3h).22 For other higher
fullerenes the number of IPR isomers which can be transformed
one into another by SW rearrangement grows drastically. Thus,
SW isomerization gives nine stable isomers in the case of C82,
21 isomers in the case of C84, etc.7 Although it was shown that
no SW transformation takes place in C84 (D2) at 700 °C,23 there
are no data about the stability of higher fullerenes at higher
temperatures. For verification of the possibility of SW rear-
rangement to take place under pyrolysis conditions, the C78:
2(C2V) and C78:3(C2V) were isolated and subjected to pyrolysis.
According to HPLC data of the pyrolysis products, both of the
fullerenes proved to be stable under FVP conditions at 1100
°C. No transformation of one isomer into another was observed.
Obviously, SW rearrangement does not take place in fullerene
molecules under FVP condition, at least at a temperature lower
than 1100 °C. Moreover, no decomposition processes with loss
of C2 clusters and formation of lower fulerenes (C60 and C70)
were detected during FVP. Thus, FVP technique is very
promising for the synthesis of individual isomers of higher
fullerenes.
Experimental Section
The MALDI-TOF MS spectra were obtained by using DCTB
(trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malono-
nitrile) or mixture of DCTB with silver triflate as the matrix. The
fullerene-containing samples were analyzed in the LDI mode.
Chromatographic purifications were carried out with flash grade
silica gel Kieselgel 60 (0.06-0.2 mm), Roth. The multistep
separation with recycling system (Buckyprep, toluene) was used
for isolation of C78:3 and mixture of C78:1 and C78:2. The separation
of C78:2 from the mixture and further HPLC analysis were carried
out on reverse phase column - Supelcosil LC-18-DB, size 4.6 ×
250 mm, mobile phase of toluene/methanol (1:1), UV-detection.
Truxenone (Diindeno[1,2-a;1′,2′-c]fluorene-5,10,15-trione) was pre-
pared according to the described procedure.25 Details of the
pyrolysis apparatus are reported elsewhere.16 In a typical experiment
10-20 mg of compound 4 were pyrolyzed. The resulting products
were extracted with toluene, filtered through microfilters (0.2 µm)
and analyzed by LDI-TOF MS and HPLC. In the case of fullerene
pyrolysis samples of 1 mg were used.
4-(1-Naphthyl)toluene (1). Method A.1-Bromonaphthalene (8.3
g, 40 mmol) was added to freshly prepared 4-methylphenylmag-
nesium bromide (40 mmol) in 30 mL of Et2O. After addition of 10
mg of Ni(acac)2, the mixture was stirred for 2 h at room temperature
and then heated under reflux for 1 h. The solution was cooled,
diluted with 0.05 M HCl, and extracted with ether. The extract
was dried over Na2SO4 and evaporated. The crude product was
purified by silica gel chromatography using 20:1 mixture of hexane/
DCM. Resulting product containing about 10% of 4,4′-dimethyl-
biphenyl was used in the next step without further purification
(colorless oil, 6.8 g, 78%). Method B. 4-Bromotoluene (1.7 g, 10
mmol) and 1-naphthaleneboronic acid (2.1 g, 15 mmol) were
dissolved in a mixture of 40 mL of acetone and 40 mL of water.
Pd(OAc)2 (10 mg, 0.045 mmol) and Na2CO3 (2.1 g, 20 mmol) were
added, and the resulting mixture was stirred at room temperature
for 18 h. The separation of the product was the same as in method
A: colorless oil (1.8 g, 84%) was obtained which slowly solidified;
white solid; Rf ) 0.53 (hexane/DCM 20:1); 1H NMR (CDCl3, 300
MHz) δ ) 2.44 (s, 3H), 7.27-7.29 (m, 2H), 7.37-7.52 (m, 6H),
7.81-7.93 (m, 3H); 13C NMR (CDCl3, 300 MHz) δ ) 21.21,
125.37, 125.68, 125.89, 126.09, 126.86, 127.42, 128.23, 128.95,
129.94, 131.64, 133.82, 136.88, 137.78, 140.21.
4-(1-Naphthyl)benzyl Bromide (2). Compound 1 6.5 g (30
mmol) was dissolved in 30 mL of CCl4. NBS (13.0 g) and catalytic
quantities of benzoyl peroxide were added. The resulting mixture
was refluxed for 2-3 h, cooled, filtered, and washed with water.
The solution was dried over Na2SO4 and evaporated. The crude
product was purified by silica gel chromatography using 10:1
mixture of hexane/DCM: yellowish oil (6.9 g, 78%); Rf ) 0.34
Summing up, the C78-fullerene related structure was obtained
through a 5-step synthesis and investigated as a pyrolytical
precursor. Taking into account that C78 fullerene is not being
formed by random assembling during pyrolysis of different
aromatic compounds unlike C60 fullerene,24 the results obtained
are evidence of the direct fullerene formation by FVP.
1
(hexane/DCM, 20:1); H NMR (CDCl3, 300 MHz) δ ) 4.54 (s,
2H), 7.36-7.49 (m, 8H), 7.83-7.87 (m, 3H); 13C NMR (CDCl3,
300 MHz) δ ) 33.37, 125.33, 125.81, 125.82, 126.12, 126.91,
127.86, 128.30, 128.95, 130.46, 131.41, 133.78, 136.71, 138.82,
140.96; MALDI-TOF MS (DCTB/Ag-triflate) m/z ) 402.87 [M
+ Ag]+.
(16) Amsharov, K. Yu.; Simeonov, K.; Jansen, M. Carbon 2007, 45,
337.
(17) Stone, A. J.; Wales, D. J. Chem. Phys. Lett. 1986, 128, 501.
(18) Woodward, R. B.; Hoffmann, R. The conserVation of orbital
symmetry; Verlag Chemie: Weinheim, 1970.
(19) Murry, R. L.; Strout, D. L.; Odom, G. K.; Scuseria, G. E. Nature
1993, 366, 665.
(20) Adler, R. W.; Harvey, J. N. J. Am. Chem. Soc. 2004, 126, 2490.
(21) Nimlos, M. R. J. Phys. Chem. A 2005, 109, 9896.
(22) Fowler, P. W.; Manolopoulos, D. E.; Ryan, R. P. Carbon 1992, 30,
1235.
(23) Hawkins, J. M.; Nambu, M.; Meyer, A. J. Am. Chem. Soc. 1994,
116, 7642.
5,10,15-Triol-5,10,15-tris(4-(1-naphthyl)benzyl)-10,15-dehy-
dro-5H-diindeno[1,2-a; 1′,2′-c]fluorene (3). Magnesium (0.36 g,
15 mmol) was activated by catalytic quantities of I2 in 10 mL of
dry diethyl ether. Compound 2 (3.0 g, 10 mmol) as a solution in
10 mL of Et2O was added slowly under intensive stirring. After
being refluxed for 1 h, the mixture was cooled and filtrated.
(24) Osterodt, J.; Zett, A.; Vo¨gtle, F. Tetrahedron 1996, 52, 4949.
(25) Dehmlow, E. V.; Kelle, T. Synth. Commun. 1997, 27, 2021.
J. Org. Chem, Vol. 73, No. 7, 2008 2933