Endohedral gadolinium-containing metallofullerenes in the trifluoromethylation reaction

Article abstract of DOI:10.1007/s11172-008-0192-7

An efficient method for the synthesis of trifluoromethyl derivatives of endohedral gadolinium-containing metallofullerenes was proposed. High-purity (98-99%) trifluoromethyl derivatives Gd@C₈₂(CF₃) ₅ (two isomers) and Gd

Full text of DOI:10.1007/s11172-008-0192-7

Russian Chemical Bulletin, International Edition, Vol. 57, No. 7, pp. 1486—1491, July, 2008
1486
Endohedral gadoliniumꢀcontaining metallofullerenes
in the trifluoromethylation reaction
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
An efficient method for the synthesis of trifluoromethyl derivatives of endohedral gadoliniumꢀ
containing metallofullerenes was proposed. Highꢀpurity (98—99%) trifluoromethyl derivatives
Gd@C82(CF3)5 (two isomers) and Gd2@C80(CF3) have been synthesized for the first time.
They were isolated and characterized by HPLC, MALDIꢀTOF mass spectrometry, and UV—Vis
spectroscopy.
Key words: endohedral metallofullerenes, dimethylformamide, gadolinium, high performance
liquid chromatography, trifluoromethylation, mass spectrometry, UV—Vis spectroscopy.
Endohedral metallofullerenes (EMFs) are unique
the laborꢀconsuming and expensive multistage separaꢀ
tion using HPLC.1,2,4 As a result, the chromatographiꢀ
cally pure EMFs are available only in milligram amounts,
which impedes the development of the EMF chemistry.
Presently, the state of investigation in this area can be
compared with that of the studies of fullerenes before
the discovery of the efficient electric arc method for synꢀ
thesis of fullerenes C60,5 which was proposed in 1990.
Due to this method, fullerenes became accessible to
a wide range of synthetic chemists, and this was the start
of the vigorous development of the C60 chemistry. Unꢀ
like fullerenes, the problems of synthesis of the EMFs in
preparative amounts has not been solved yet, explaining
a small number of studies on EMF functionalization.
The evolution of investigations in the field of the
EMFs can be seen from several studies. The first EMF
derivatives were synthesized in 1995 using milligram
amounts of the chromatographically pure EMFs. It was
found6 that the cycloaddition of disilacyclopropane to
Gd@C82 occurred with a higher rate than the cycloꢀ
addition to analogous "empty" fullerene. Then other
EMF cycloadducts were synthesized.7,8 In addition, the
[2+2] cycloadduct of Gd@C82(C6H4) (two isomers) was
synthesized, isolated, and characterized by HPLC, mass
spectrometry, and UV—Vis spectroscopy.9 Perfluoroꢀ
alkylation at the interface was successfully used for the
synthesis of La@C82(C8F17)2.10 However, the structure
of the synthesized compounds was not developed by NMR
spectroscopy because of their paramagnetic nature. In
the recent time, the waterꢀsoluble EMF derivatives were
synthesized in order to use them as contrast agents in
NMR tomography.11—13 The waterꢀsoluble polyhydroxy
derivatives of EMFs Gd@C82(OH)n were synthesized,
nanocarbon compounds containing one or several metal
atoms inside the fullerene molecule. A wide list of eleꢀ
ments from the D. I. Mendeleev Table, whose atoms are
incorporated into a fullerene molecule, is presently
known.1,2 The EMFs containing atoms from the second
(Ca, Sr, Ba) and third (Sc, Y, La) Groups, as well as
lanthanides, are most actively studied. The most part of
the EMFs have such an electronic structure that all or
some valence electrons of the metal atom, which is situatꢀ
ed inside the fullerene cage, transfer to the fullerene
molecule. As a result, the EMF molecule gains proꢀ
perties that differ from those of "empty" fullerenes.
The presence of additional weakly bound electrons at
the external shell of the EMF molecules imparts properꢀ
ties of a strong reducing agent to these compounds and
causes their high reactivity and a possibility to participate
in various chemical reactions leading to the addition of
diverse atoms, molecules, or radicals to the EMFs. Sucꢀ
cessful functionalization of "empty" fullerenes and
the synthesis of new materials3 from these fullerenes sugꢀ
gest that one should search for routes of practical use of
the EMFs in the synthesis of their derivatives with higher
solubility and unique electric, magnetic, optical, chemiꢀ
cal, and biological properties.
At present, the chemistry of the EMFs, unlike
the properties of the wellꢀknown exohedral modificaꢀ
tions of "empty" fullerenes, is poorly studied. Such an
important question as an effect of different metal atoms
incapsulated into the fullerene shell on the reactivity of
the EMFs remains unanswered. This situation is related
to the restricted accessibility of the EMFs caused by
difficulties of isolation of pure EMFs, which requires
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1457—1462, July, 2008.
1066ꢀ5285/08/5707ꢀ1486 © 2008 Springer Science+Business Media, Inc.

Trifluoromethylated endohedral metallofullerenes Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1487
and their higher efficiency as contrast agents for NMR
tomography compared to the presently accessible comꢀ
mercial compounds was shown.11 Chromatographically
pure (<99%) Gd@C82 was used as the starting EMF. The
structure of the synthesized compound was not deterꢀ
mined, because the product represented a mixture of
the polyhydroxy derivatives Gd@C82(OH)n (n ≈ 40). An
original method for the synthesis of waterꢀsoluble EMF
derivatives Gd@C60(C(COOH)2)10 was proposed12 usꢀ
ing the sample enriched in Gd@C60. Thus, an analysis of
published data showed that the development of investigaꢀ
tion on the chemistry of EMFs based on chromatographꢀ
ically pure EMFs is a rather difficult task.
In this work, to synthesize the trifluoromethyl derivaꢀ
tives of EMFs Gd@C82(CF3)x, we propose to use stable
anionic complexes of EMFs as DMF extracts enriched in
Gd@C82 instead of the chromatographically pure neutral
EMFs. The problem of the synthesis of the anionic EMF
complexes in the form of DMF extracts in gram amounts
has successfully been solved earlier.14—16
I (arb. units)
a
+
C60
150
100
50
0
+
C70
+
Gd@C82
+
C78
+
+C84
+
C76
C82
800
I (arb. units)
900
1000
1100
m/z
b
–
Gd@C82
80
40
0
Experimental
–
Gd@C80
–
Gd2@C80
The soot containing the Gd@C2n EMFs was prepared by
the evaporation of the composite graphite electrodes compounded
by gadolinium in the electric arc reactor designed and produced by
us.14,17,18 The endohedral metallofullerene Gd@C82 in the anionic
form was isolated by twoꢀstage extraction from the soot consequentꢀ
ly with oꢀdichlorobenzene (DCB) and DMF. A weighed sample of
the soot (10—15 g) was loaded into a cellulose extraction thimbler
(Whatman Int. Ltd.), and the thimbler was placed in a glass flask
containing 1 L of the solvent. At the first stage, the soot was multiply
extracted with DCB, which well purified the soot from fullerenes
C60, C70 and higher fullerenes (С76, С78, С82, С84, and others) and
also isolated an insignificant amount of the EMFs. The extraction
with DCB (≥99%, Aldrich) was carried out under argon for 3—4 h
at the boiling temperature of the solvent (180.5 °С). Then a solution
of the fullerenes and EMFs was thoroughly filtered (0.5 μ PTFE,
Phenomenex Filter Membranes), the solvent was distilled off on
a rotary evaporator, and the residue was dried for ~1 h. The extraction
was carried out until the fresh portion of the solvent almost stopped
coloring (usually 7—9 cycles). The DCB extract (5% yield from the
soot weight), according to the MALDIꢀTOF mass spectrometric
(Fig. 1, a) and HPLC data, mainly contained fullerenes С60 and
С70, higher fullerenes (С76, С78, С82, and С84), and an insignificant
amount of the Gd@C82 EMF. After the "empty" fullerenes were
removed from the soot, at the second stage the soot residue was
multiply treated with the polar solvent DMF (≥99% (GC), Sigmaꢀ
Aldrich) to obtain the EMFs in the anionic form. The EMFs were
extracted from the soot for 3—4 h under argon at the boiling
temperature of the solvent (0.5 μ PTFE, Phenomenex Filter Memꢀ
branes), the solvent was distilled off on a rotary evaporator, and the
residue was dried for 1 h in vacuo at 90 °С. The extraction was
continued until the fresh portion of the solvent nearly stopped
coloring (usually 6—7 cycles). After the solvent was removed,
the DMF extract (5—8% yield of the soot weight) contained at most
11—13 wt.% DMF; according to the data of MALDIꢀTOF mass
spectrometry (see Fig. 1, b) and UV—Vis spectroscopy, the extract
800
1000
1200
m/z
Fig. 1. The MALDIꢀTOF mass spectra of positive ions from
the DCB extract of the soot (a) and negative ions from the DMF
extract of the soot (b).
contained almost no "empty" fullerenes and represented a mixture of
the EMFs, whose main component was Gd@C82 (~80 mol.%)
combined with small amounts of Gd@C80 and Gd2@C80.
To carry out the highꢀtemperature solidꢀphase reaction of
the DMF extract of Gd@C2n with silver trifluoroacetate, 100 mg of
the DMF extract of Gd@C2n was mixed with 500 mg of CF3COOAg
(≥98.0%, Aldrich). The mixture was loaded into a quartz reactor,
rapidly heated (heating rate 40 °C min–1) to 300—400 °C in dyꢀ
namic vacuum (10–6 Torr), and kept for 6—10 h. The depth of
the reaction was monitored by IR spectroscopy. The reaction was
carried out until the bands at 724, 805, and 840 cm–1, which are
characteristic of CF3COOAg, disappeared from the IR spectra of the
reaction product.
The pure (~98%) trifluoromethyl EMF derivatives were isolatꢀ
ed by multistage preparative HPLC (HPꢀ1050, Hewlett Packard
Co.) using consecutively columns with two types of sorbents: Cosꢀ
mosil Buckyprep (20 mm × 250 mm) and Riges Buckyclutcher
(21 mm × 250 mm) with toluene or a toluene—hexane mixꢀ
ture as eluent.
Mass spectrometry was carried out on a timeꢀofꢀflight mass
spectrometer with laser desorption/ionization (MALDIꢀTOF, Voyꢀ
agerꢀDETM PRO BiospectrometryTM Workstation, Applied Bioꢀ
systems, USA). The pulse nitrogen laser with a wavelength of 337
nm, a frequency of 3 Hz, and a pulse duration of 0.5 ns was used.
Positive and negative ions were detected in the reflectron mode.
A sulfur (S8) solution in toluene was applied as a matrix. The sample

1488
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Kareev et al.
under study and the matrix were mixed in the ~1 : 1000 ratio, the
mixture was deposited as a microdroplet on a metallic target, and the
solvent was evaporated at room temperature to obtain the film of the
studied substance.
UV—Vis spectra of the trifluoromethyl EMF derivatives were
recorded in toluene in the wavelength range from 300 to 1100 nm on
a Varian Cary 500 Scan UVꢀVisꢀNIR spectrophotometer in 10ꢀmm
standard quartz cells.
interesting that the EMFs containing yttrium and gadoꢀ
linium form derivatives containing less than five CF3
groups, whereas the usual highꢀtemperature reactions
between CF3COOAg and "empty" fullerenes afford mixꢀ
tures of the C60/70(CF3)n products with a very wide distꢀ
ribution of the functional groups (n = 2—22).20—22
The HPLC chromatogram (Cosmosil Buckyprep colꢀ
umn) of the products of the reaction between the DMF
extract of the gadoliniumꢀcontaining EMFs and
CF3COOAg is presented in Fig. 2, b. At the first stage of
the separation of the Gd@C82(CF3)x sample (see Fig. 2, b),
fractions A and B with retention times of 31 and 40.5 min,
respectively, were collected. They mainly contained difꢀ
ferent isomers of Gd@C82(CF3)5 (according to the
MALDIꢀTOF mass spectrometric data). At the second
stage (Fig. 3), the individual compound Gd@C82(CF3)5
(retention time 9 min) was isolated from fraction B
(see Fig. 3, b), and fraction A was divided into four parts
(Аꢀ1—Аꢀ4) (see Fig. 3, a). The mass spectrometric analꢀ
ysis of these fractions showed that fractions Аꢀ2 and Аꢀ4
contained Gd@C82(CF3)3 and Gd@C82(CF3), respecꢀ
tively, and fraction Аꢀ3 is the trifluoromethyl dimetalloꢀ
fullerene derivative Gd2@C80(CF3). The mass spectrum
of fraction Аꢀ3 (Fig. 4, a) contains two peaks: the main
peak (m/z 1343) corresponds to the Gd2@C80(CF3)+
molecular ion and the peak with m/z 1274 corresponds to
the Gd2@C80 fragmentation ion. The isotope distribuꢀ
tion observed for the peak with m/z 1343 (see Fig. 4, b)
agrees well with that theoretically calculated for the
Gd2@C80(CF3) molecule (see Fig. 4, c), which confirms
the molecular formula Gd2@C80(CF3) of the isolated
compound.
Results and Discussion
The MALDIꢀTOF mass spectrum of the product of
the highꢀtemperature reaction of silver trifluoroacetate
and the DMF extract of the EMFs enriched in Gd@C82 is
shown in Fig. 2, a. In addition to the major component of
the reaction product Gd@C82(CF3)5, the mass spectrum
exhibits ions of the trifluoromethyl EMF derivative
Gd2@C80 with one or three CF3 groups. We have earliꢀ
er19 shown that a similar composition of the products is
obtained due to the reaction of silver trifluoroacetate and
the DMF extract of the EMFs enriched in Y@C82. It is
I (arb. units)
a
*
120
80
40
0
According to the mass spectrometric data, fraction
Аꢀ1, which was isolated as a sole peak on the Regis Bucꢀ
kyclutcher column (toluene as eluent), contained two
components (Fig. 5, a): Gd@C82(CF3)5 (m/z 1486) and
Gd2@C80(CF3)3 (m/z 1481). These compounds have
close molecular weights, but the isotope distribution conꢀ
1200
1400
m/z
I (arb. units)
b
0.4
A
A
a
b
A
CxFy
0.3
0.2
0.1
0
0.3
0.2
0.1
0
0.3
0.2
0.1
0
Gd@C82(CF3)5
B
CxFy
Aꢀ1
Aꢀ2
Aꢀ3
Aꢀ4
10
20
30
40
tr/min
Fig. 2. The MALDIꢀTOF mass spectrum of positive ions (a) and
the HPLC chromatogram (Cosmosil Buckyprep column, toluene
as eluent, eluting rate 18 mL min–1) (b) of the trifluoromethylation
products Gd@C82(CF3)x. The peak marked by * belongs to
the Gd@C82(CF3)5 ions. Here and in Figs 3 and 6, tr is
the retention time.
4
8
12 tr/min
4
8
12 tr/min
Fig. 3. The HPLC chromatograms (Regis Buckyclutcher column,
toluene as eluent, eluting rate 12 mL min–1) of fractions А (a)
and B (b).
+

Trifluoromethylated endohedral metallofullerenes Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
1489
I (arb. units)
I (arb. units)
I (arb. units)
a
b
c
1400
1200
1000
800
600
400
200
0
Gd2@C80+(CF3)
1400
1200
1000
800
600
400
200
0
1400
1200
1000
800
600
400
200
0
1200 1400 m/z
1340 1350 m/z
1340 1350 m/z
Fig. 4. The MALDIꢀTOF mass spectrum of positive ions of the pure trifluoromethyl derivative Gd2@C80(CF3) (a), the observed isotope
distribution of the peak with m/z 1344 (b), and the theoretical isotope distribution for the Gd2@C80(CF3) molecule (c).
firms the presence of two components in the sample
(see Fig. 5, b). The subsequent separation of fraction Аꢀ1
(Fig. 6) on the Regis Buckyclutcher column (toluene—heꢀ
xane (40 : 60) mixture as eluent) made it possible
to isolate the individual trifluoromethyl derivative
Gd@C82(CF3)5 (retention time 20.4 min) (fraction
Аꢀ1ꢀ1) and a mixture of two isomers of the trifluoromethꢀ
yl derivatives of the Gd2@C80(CF3)3 dimetallofullerene
(fraction Аꢀ1ꢀ2).
The mass spectrum of fraction Аꢀ1ꢀ1 (see Fig. 5, c)
exhibits only the peak with m/z 1486, which corresponds
to the molecular ion of Gd@C82(CF3)5. The observed
isotope distribution of the peak with m/z 1486 (see Fig. 5,
c) is well consistent with the spectrum theoretically calꢀ
I
I
I
a
b
c
*
*
+
Gd@C82(CF3)5
1480
1490 m/z
1480
1490 m/z
1480
1490 m/z
d
e
f
I
I
I
+
Gd@C82(CF3)5
Gd2@C80(CF3)3
Gd2@C80(CF3)3
1480
1490 m/z
1480
1490 m/z
1480
1490 m/z
Fig. 5. The MALDIꢀTOF mass spectra of positive ions: fraction Aꢀ1 isolated on the Regis Buckyclutcher column (toluene as eluent, eluting
rate 12 mL min–1) (a); mass spectral region of fraction Aꢀ1 with m/z 1470—1500 (b); fraction Aꢀ1ꢀ1 (c); theoretical isotope distribution for
the Gd@C82(CF3)5 molecule (d); fraction Аꢀ1ꢀ2 (e) and the theoretical isotope distribution for the Gd2@C80(CF3)3 molecule (f). The peak
+
marked by * belongs to Gd@C82(CF3)5+ and Gd2@C80(CF3)3
.

1490
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 7, July, 2008
Kareev et al.
A
A
0.12
2.0
1.5
1.0
0.5
1
2
0.08
0.04
0
3
Aꢀ1ꢀ1
Aꢀ1ꢀ2
10
20
30
tr/min
400
600
800
1000 λ/nm
Fig. 6. The HPLC chromatogram of fraction Аꢀ1 on the Regis
Buckyclutcher column (toluene—hexane (40 : 60) mixture as eluꢀ
ent, eluting rate 12 mL min–1).
Fig. 7. The UV—Vis spectra of the trifluoromethyl EMF derivatives
in toluene: Gd@C82(CF3)5, isomer 1 (1); Gd@C82(CF3)5, and
isomer 2 (2); Gd2@C80(CF3) (3).
culated for the Gd@C82(CF3)5 molecule (see Fig. 5, d),
which confirms this molecular formula of the isolatꢀ
ed compound. The mass spectrum of fraction Аꢀ1ꢀ2
(see Fig. 5, e) contains only the peak with m/z 1481
corresponding to the molecular ion Gd2@C80(CF3)3, and
the isotope distribution observed for this ion (see Fig. 5, e)
agrees well with that theoretically calculated for the
Gd2@C80(CF3)3 molecule (see Fig. 5, f), confirming
the molecular formula Gd2@C80(CF3)3 for the isolated
compounds.
position of the functional groups. Therefore, the major
distinctive features of the UV—Vis spectra of molecules
of the trifluoromethyl EMF derivatives are determined
by the structure of the fullerene shell and the motif of
addition of the CF3 groups rather than by the nature of
the inserted metal atom.
Thus, the efficient method was proposed for the synꢀ
thesis of the trifluoromethyl gadoliniumꢀcontaining EMF
derivatives from the DMF extracts enriched in Gd@C82.
The Gd@C82(CF3)5 (isomers
1
and 2) and
According to the HPLC and MALDIꢀTOF mass specꢀ
trometric data, the purity of the isolated compounds
Gd@C82(CF3)5 (isomers 1 and 2) and Gd2@C80(CF3)
was ~98—99%.
Gd2@C80(CF3) trifluoromethyl derivatives of high puriꢀ
ty (98—99%) have been synthesized for the first time and
isolated by preparative HPLC. It was found that the highꢀ
temperature solidꢀphase reaction of silver trifluoroaceꢀ
tate with the DMF extract of the Gd@C2n EMFs, unlike
the reaction with fullerene C60, is selective and its major
product is the diamagnetic compound Gd@C82(CF3)5.
The absorption spectra of the trifluoromethyl EMF
derivatives Gd@C82(CF3)5 (isomers 1 and 2) and
Gd2@C80(CF3) in toluene measured in the visible and
nearꢀIR interval are presented in Fig. 7. The absorption
spectrum of Gd2@C80(CF3) has weak bands at 650 and
890 nm and the absorption edge at 1100 nm. The spectra
of isomers 1 and 2 of Gd@C82(CF3)5 (see Fig. 7) differ
substantially from the UV—Vis absorption spectrum of
the starting Gd@C82 EMF containing the characteristic
bands at 631, 966, and 1409 nm. The spectra of the
isomers of the trifluoromethyl derivative Gd@C82(CF3)5
have no common features and are presented by intrinsic
sets of the bands: at 435, 578, 645, 780, and 880 nm with
the absorption edge at 1000 nm and at 490, 590, 714,
792, and 865 nm with the absorption edge at 1000 nm
for isomers 1 and 2, respectively. At the same time,
the spectra of the EMFs Y@C82(CF3)5 (isomer 1)23 and
Gd@C82(CF3)5 (isomer 1) contain common features.
A similar pattern is also observed for Y@C82(CF3)5 (isoꢀ
mer 2)23 and Gd@C82(CF3)5 (isomer 2). Probably, this
is related to the fact that the carbon cages with the grafted
functional groups have the same structure and mutual
The authors are grateful to S. F. Lebedkin for help in
isolation of chromatographically pure samples of the triꢀ
fluoromethyl derivatives of the endohedral metalloꢀ
fullerenes.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 06ꢀ03ꢀ
33147ꢀa, 05ꢀ03ꢀ33051ꢀa, and 08ꢀ03ꢀ01004ꢀa), the CRDF
Foundation (Grant RUC2ꢀ2830ꢀMOꢀ06), and the Rusꢀ
sian Science Support Foundation.
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Received December 12, 2007