Synthesis of endohedral di-and monometallofullerenes Y2@C 84, Ce2@C78, and M@C82 (M = Y, Ce)

Article abstract of DOI:10.1007/s11172-007-0338-z

Endohedral metallofullerenes Y₂@C₈₄, Ce ₂@C₇₈, and M@C₈₂ (M = Y, Ce) were synthesized by the electric arc method and isolated from the soot using extraction with o-dichlorobenzene. Pure (98%) endohedr

Full text of DOI:10.1007/s11172-007-0338-z

Russian Chemical Bulletin, International Edition, Vol. 56, No. 11, pp. 2140—2144, November, 2007
2140
Synthesis of endohedral diꢀ and monometallofullerenes
Y2@C84, Ce2@C78, and M@C82 (M = Y, Ce)
I. E. Kareev,ꢀ V. P. Bubnov, and E. B. Yagubskii
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 515 5420. Eꢀmail: kareev@icp.ac.ru; bubnov@icp.ac.ru
Endohedral metallofullerenes Y2@C84, Ce2@C78, and M@C82 (M = Y, Ce) were syntheꢀ
sized by the electric arc method and isolated from the soot using extraction with oꢀdichloꢀ
robenzene. Pure (98%) endohedral dimetallofullerenes Y2@C84 and Ce2@C78 were isolated for
the first time from oꢀdichlorobenzene extracts using HPLC and characterized by mass specꢀ
trometry and spectrophotometry.
Key words: endohedral metallofullerenes, electric arc synthesis, extraction, highꢀperforꢀ
mance liquid chromatography, mass spectrometry, spectrophotometry.
Unique structure of endohedral metallofullerenes
of rareꢀearth metalꢀcontaining graphite electrodes are disꢀ
cussed and no analysis of the influence of the electric arc
parameters and constructive features of electric arc setups
on an increase in the yield of EMF in the soot is given. In
this work, we synthesized for the first time and characterꢀ
ized endohedral dimetallofullerenes Y2@C84 and Ce2@C78
by the optimization of all steps of the EMF synthesis.
(EMF) and diversity of their properties, depending on the
nature of encapsulated metal and fullerene, excite great
interest from the viewpoint of studying their chemical and
physicochemical properties. It can be expected that sucꢀ
cess in the area of synthesis and investigation of the physiꢀ
cochemical properties of EMF would form a basis for
practical use and creation of novel materials with unique
electrical, magnetic, optical, chemical, and biological
properties, for instance, organic ferromagnetics, laser and
Seignetteꢀelectric materials, and pharmaceutical and raꢀ
diopharmaceutical preparations.1—4
However, EMF are poorly studied up to present. Pubꢀ
lished data on their chemical properties are virtually lackꢀ
ing. The main reason for this situation is restricted accesꢀ
sibility of EMF because of problems in their synthesis and
isolation in preparative amounts.
Experimental
The process of synthesis of EMF with yttrium and cerium
includes four steps, each of which plays a substantial role in the
synthesis: (1) preparation of the composite graphite electrodes
containing rareꢀearth metals Y and Ce; (2) electric arc vaporizaꢀ
tion of the composite graphite electrodes and preparation of the
EMFꢀcontaining soot; (3) isolation of fullerenes and EMF from
the soot by extraction; (4) isolation of individual EMF by
twoꢀstep HPLC.
The known methods for the synthesis of EMF (laser
and electric arc vaporization of composite graphite elecꢀ
trodes) and traditional methods of their isolation from
soot (extraction with toluene, carbon disulfide, oꢀdichloꢀ
robenzene (DCB)) make it possible to obtain extracts in
low yield (0.5—3%) and EMF content (0.1—1%).1,5,6
Multistep HPLC is used to isolate pure EMF from the
extracts. However, the chromatographic process is very
laborꢀconsuming because of the low EMF content in the
extracts. Pure EMF (96—99%) have been prepared so far
only in milligram quantities and are almost inaccessible
for a wide range of researchers.1
The composite graphite electrodes containing rareꢀearth
metals (yttrium or cerium) with the optimum ratio M/C ~1%
were prepared as follows: (1) holes 2.9 mm in diameter
were drilled at two sides in the center of a graphite rod (extra
pure graphite for spectral analysis, O.S.Ch.ꢀ7ꢀ3 trade mark,
6×160 mm); (2) a blend was prepared: metallic yttrium (or
cerium) filings were mixed with the graphite powder and graphꢀ
ite GC cement (Dylon Industries Inc.), which is used as a bindꢀ
ing, in weighed ratios of 1 : 0.57 : 1.43 and 1 : 0.56 : 1.11 for Y
and Ce, respectively; the prepared blend was thoroughly stirred,
packed in the hole of the graphite rod, and carefully molded;
(3) the rods were subjected to thermal treatment in three steps.
Treatment in a vacuum oven (~10–3 Torr) for 4—5 h at 130 °C
was carried out in the first step. At this temperature the graphite
cement solidified. Then the rod can be heated to high temperaꢀ
tures (up to 3000 °C), not be afraid of its decomposition. In the
second step, the thermal treatment continued for 4 h at 1100 °С
It should be mentioned that imperfect methods for
EMF isolation prevent the optimization of conditions for
their synthesis. As a rule, only final results are presented
in the literature, while no conditions for the preparation
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2067—2071, November, 2007.
1066ꢀ5285/07/5611ꢀ2140 © 2007 Springer Science+Business Media, Inc.

Endohedral diꢀ and monometallofullerenes
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 11, November, 2007 2141
in vacuo (~103 Torr). This step is necessary to transform the
binding, which is present in the composition of the graphite
cement, into carbon. In the third step, the thermal treatment
was carried out directly in an electric arc reactor for 30 min
at 1800—1900 °С in vacuo (10–3 Torr). The necessary temꢀ
perature was achieved by passing a constant electrical curꢀ
rent (180—200 A) through the electrode. The latter thermal
treatment resulted in the formation of metal carbides MC2
(M = Y, Ce) in the composite electrode. Their presence in the
arc as vapor along with carbon particles C2 is a necessary condiꢀ
tion for EMF formation.7 In addition, at ~1800 °С the elecꢀ
trodes are almost completely cleaned from oxygen and other
gases adsorbed in pores of the rod, favoring more stable burning
of the arc and increasing the EMF content in the syntheꢀ
sized soot.
extracts EMF M@C82 (M = Y, Ce) from the soot, which
is confirmed by the data of mass spectrometry and HPLC.
The yield of the DCB extracts reaches 5% of the soot
weight, which is 2—3 times higher than the values known
from literature.1,11 This result was obtained by the optiꢀ
mization of conditions for preparation of the composite
graphite rods and their vaporization in the electric arc.
The mass spectra and chromatograms of the DCB exꢀ
tracts containing EMF with yttrium and cerium are preꢀ
sented in Figs 1 and 2, respectively. It was found on the
basis of the mass spectrometric and HPLC data that the
content of EMF Ce@C82 and Y@C82 in the DCB extracts
was 1—2% of the extract weight.
The obtained samples of the extracts of EMF with
yttrium and cerium were used to isolate individual EMF
M@C82, Y2@C84, and Ce2@C78 by twoꢀstep HPLC. In the
first step of HPLC, the DCB extract dissolved in oꢀxylene
and containing EMF with yttrium or cerium was sepaꢀ
rated on the Cosmosil Buckyprep column using toluene
as the eluent. In this step (see Figs 1, b and 2, b), fracꢀ
tion A or B containing, according to the mass spectrometꢀ
ric data, monometallofullerenes M@C82, dimetalloꢀ
The soot containing EMF with yttrium or cerium was preꢀ
pared by the vaporization of the composite graphite electrodes
in the electric arc reactor designed and produced by us.8—10 The
optimum conditions for electrode vaporization in the arc are the
following: helium pressure 120 Torr, arc current 80—90 A, voltꢀ
age 27—28 V, arc length 5 mm, distance from the arc to the
cooled reactor surface 50 mm, and rate of electrode vaporizaꢀ
tion 4 mm min–1
.
Endohedral metallofullerenes with yttrium and cerium were
isolated from the soot by extraction with oꢀdichlorobenzene.
A weighed sample of the soot (10—15 g) was loaded in a celluꢀ
lose (Whatman Int. Ltd.) extraction thimbler, and the thimbler
was placed in a glass flask containing 1 L of DCB (≥99% (GC),
SigmaꢀAldrich). Extraction was carried out in an argon atmoꢀ
sphere for 3—4 h at the boiling point of DCB (180.5 °C). Then
a solution of fullerenes and EMF was thoroughly filtered
(0.5 µ PTFE, Phenomenex Filter Membranes), the solvent was
distilled off on a rotary evaporator, and the precipitate was dried
for ~1 h. The extraction (usually seven—nine cycles) was carried
out until a fresh portion of the solvent virtually stopped staining.
The yield of the DCB extracts was ~5% of the soot weight.
To isolate individual EMF, multistep HPLC was used
(HPꢀ1050, Hewlett Packard Co.) with successive columns
packed with sorbents of two types: Cosmosil Buckyprep
(20 mm × 250 mm) and Riges Buckyclutcher (21 mm × 250 mm).
Mass spectrometric investigation was carried out on a timeꢀ
ofꢀflight mass spectrometer with laser desorption/ionization
(MALDIꢀTOF, VoyagerꢀDETM PRO BiospectrometryTM Workꢀ
station, Applied Biosystems, USA). A pulse nitrogen laser with
the wavelength 337 nm, frequency 3 Hz, and pulse duration
0.5 ns was used. Positive and negative ions were detected in the
reflectron mode. A sulfur (S8) solution in toluene was used as
the matrix. The sample under study and the matrix (in the molar
ratio ~1 : 1000) were mixed and deposited as a microdrop on a
metallic target, and the solvent was evaporated at room temꢀ
perature to obtain a film of the substance under study.
I (arb. units)
+
C60
300
a
+
C70
200
100
+
C78
+
C84
+
Y@C82
800
1000
1200
m/z
A
C60
C70
C76
b
C84
0.10
0.05
C78
A
C90
C86
Absorption spectra of the EMF samples in toluene were
recorded in the wavelength interval from 300 to 1100 nm on a
Varian Cary 500 Scan UVꢀVisꢀNIRꢀspectrophotometer instruꢀ
ment in standard quartz cells 10 mm thick.
10
20
30
40
tr/min
Fig. 1. MALDIꢀTOF mass spectrum of positive ions (a) and the
HPLC chromatogram (Cosmosil Buckyprep column, elution
rate 18 mL min–1, toluene as eluent) of the DCB extract of the
soot containing EMF with yttrium (b). Here and in Figs 2—6
tr is the retention time.
Results and Discussion
Along with empty fullerenes C60 and C70 and higher
fullerenes (C76, C78, C82, C84, and others), DCB also

2142
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 11, November, 2007
Kareev et al.
I (arb. units)
A
–
Ce@C82
C60
150
100
50
a
Ce2@C78
C88
a
–
C70
–
Ce2@C78
C84
–
C78
–
Ce@C82
b
800
1000
1200
m/z
A
Ce@C82
0.10
0.05
C60
C70
C76
b
c
C84
C78
B
5
10
15
tr/min
Fig. 3. HPLC chromatograms (Regis Buckyclutcher column,
elution rate 12 mL min–1, toluene as eluent) of fraction B (a),
endohedral dimetallofullerene Ce2@C78 (b), and monometalloꢀ
fullerene Ce@C82 (c).
C90
C86
10
20
30
40
tr/min
A
Fig. 2. MALDIꢀTOF mass spectrum of negative ions (a) and the
HPLC chromatogram (Cosmosil Buckyprep column, elution
rate 18 mL min–1, toluene as eluent) of the DCB extract of the
soot containing EMF with cerium (b).
Y2@C84
C88
Y@C82
a
fullerenes M2@Cn (M = Y, n = 84; M = Ce, n = 78), and
fullerene С88 was separated from fullerenes С60 and С70
and higher fullerenes (С76—С110). In the second step, the
collected fractions A and B were separated on the Regis
Buckyclutcher column (toluene as eluent) (Figs 3, a
and 4, a). As a result, compounds M@C82 (Figs 3, c
and 4, c) and M2@Cn (Figs 3, b and 4, b) were isolated.
The separation process at the second step was repeated
2—3 times to obtain the highꢀpurity (~98%) EMF samples.
The purity of the isolated endohedral monometalloꢀ
fullerenes M@C82 (M = Y, Ce) and dimetallofullerenes
M2@Cn was estimated using MALDIꢀTOF mass specꢀ
troscopy and HPLC analysis. The mass spectrum of posiꢀ
tive ions and the chromatogram of pure EMF Ce@C82 are
presented in Fig. 5. The mass spectrum contains only the
peak with m/z 1124, belonging to a single compound,
namely, EMF Ce@C82. The purity of the sample obꢀ
tained is also confirmed by the chromatogram (see Fig. 5,
inset). According to the data of HPLC and mass specꢀ
trometry, the purity of the isolated EMF Ce@C82 was
~98%. Analogous results were obtained by the
estimation of the purity of the EMF sample with
yttrium (Fig. 6).
Y2@C84
b
Y@C82
c
5
10
15
tr/min
Fig. 4. HPLC chromatograms (Regis Buckyclutcher column,
elution rate 12 mL min–1, toluene as eluent) of fraction A (a),
endohedral dimetallofullerene Y2@C84 (b), and monometalloꢀ
fullerene Y@C82 (c).
In addition to the pure samples of endohedral monoꢀ
metallofullerenes with cerium and yttrium, individual

Endohedral diꢀ and monometallofullerenes
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 11, November, 2007 2143
I (arb. units)
A
I (arb. units)
4000
1.0
0.8
0.6
0.4
0.2
6000
+
+
Ce@C82
Y2@C84
A
Ce@C82
0.09
0.06
0.03
3000
2000
1000
4000
2000
1000 1200 1400
m/z
20
40
tr/min
m/z
1000
1200
1400
1600
400
600
800
1000
λ/nm
Fig. 5. MALDIꢀTOF mass spectrum of positive ions of pure
EMF Ce@C82. The HPLC chromatogram (Cosmosil Buckyprep
column, elution rate 18 mL min–1, toluene as eluent) of pure
EMF Ce@C82 is shown in inset.
Fig. 7. Absorption spectrum of pure EMF Y2@C84 in toluene.
The MALDIꢀTOF mass spectrum of positive ions of pure EMF
Y2@C84 is shown in inset.
A
I (arb. units)
1.0
+
Y@C82
I (arb. units)
+
Ce2@C78
9000
A
Y@C82
15000
10000
5000
0.8
0.6
0.4
0.2
0.08
0.06
6000
0.04
0.02
1000 1200 1400
m/z
3000
20
40
tr/min
m/z
1000
1200
1400
1600
400
600
800
1000
λ/nm
Fig. 6. MALDIꢀTOF mass spectrum of positive ions of pure
EMF Y@C82. The HPLC chromatogram (Cosmosil Buckyprep
column, elution rate 18 mL min–1, toluene as eluent) of pure
EMF Y@C82 is shown in inset.
Fig. 8. Absorption spectrum of pure EMF Ce2@C78 in toluene.
The MALDIꢀTOF mass spectrum of positive ions of pure EMF
Ce2@C78 is shown in inset.
dimetallofullerenes Y2@C84 and Ce2@C78 have been isoꢀ
lated for the first time in the second step of HPLC and
characterized (Figs 7 and 8). The MALDIꢀTOF mass
spectra of positive ions of dimetallofullerenes Y2@C84 and
Ce2@C78 are shown in Figs 7 and 8 (inset). The spectra
exhibit only the peaks with m/z 1186 and 1216 correꢀ
sponding to the Y2@C84 and Ce2@C78 ions, respecꢀ
tively. According to the HPLC and MALDIꢀTOF data,
purity of the isolated Y2@C84 and Ce2@C78 compounds
was ~98%.
The absorption spectra of EMF are known to be very
sensitive to the structure (symmetry) of the carbon cage.
Metal atoms, such as yttrium, lanthanum, cerium, gadoꢀ
linium, or holmium, inserted inside the carbon cage of
the same symmetry exert a weak effect on the general
character of the spectrum and insignificantly shift the
positions of the characteristic bands (by 10—20 nm). The
spectra of the monometallofullerenes M@C82 isolated by
us agree with those described earlier.1,12 The absorption
spectrum of dimetallofullerene Y2@C84 (see Fig. 7) conꢀ
tains a wide set of characteristic bands at 355, 482, 625,
710, and 785 nm and the absorption edge at 800 nm.
The absorption spectrum of dimetallofullerene
Ce2@C78 in toluene with the characteristic bands at 387,
515, 555, and 637 nm and the absorption edge about
1000 nm is presented in Fig. 8. Dimetallofullerene
La2@C78 has been isolated previously,13 and the 13C NMR
spectroscopic data showed that the structure of the carꢀ
bon cage of this molecule corresponded to the D3hꢀC78
symmetry (78 : 5). The absorption spectrum of La2@C78
in carbon disulfide is similar to that of Ce2@C78 in toluꢀ
ene and contains an analogous set of characteristic bands
with a slight shift (from 3 to 18 nm) at 386, 533, 561, and
647 nm and the absorption edge at 1000 nm. It can be
+
+

2144
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 11, November, 2007
Kareev et al.
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assumed that the Ce2@C78 compound isolated by us has
the same structure of the carbon cage as the La2@C78
molecule: D3hꢀC78 (78 : 5).
The optimization of the conditions of preparation of
the composite graphite electrodes and their vaporization in
the electric arc made it possible to obtain the soot enriched
in EMF. As a result, pure EMF M@C82 (M = Y, Ce)
samples were prepared and endohedral dimetallofullerenes
Y2@C84 and Ce2@C78 have been synthesized for the
first time, isolated, and characterized by MALDIꢀTOF,
HPLC, and optical spectroscopy. The structure of the
carbon cage for dimetallofullerene Ce2@C78 correspondꢀ
ing to the D3h symmetry (78 : 5) was proposed.
The authors are grateful to S. F. Lebedkin for help in
isolation of chromatographically pure samples of endoꢀ
hedral metallofullerenes.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 06ꢀ03ꢀ
33147ꢀa and 05ꢀ03ꢀ33051ꢀa) and the CRDF (Grant
RUC2ꢀ2830ꢀMOꢀ06).
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Received July 3, 2007