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Isolation and Characterization of the Dimetallofullerene Ce2@C80.

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Isolation and Characterization
of the Dimetallofullerene Ce2@C80
Junqi Ding and Shihe Yang*
The availability of pure metallofullerenes in milligram quantities has allowed a number of metallofullerenes to be characterized by a variety of spectroscopic means.['] Although there have
been some studies on Sc,@C2, ,[21 the main research effort has
been directed towards metallofullerenes of the type MGC,,
with a single encaged metal atom.['] The first soluble dimetallofullerene, La,@C,,, was reported soon after the method of
synthesizing fullerenes on the macroscopic scale was developed.r31Most recently, Achiba et al. described the first isolation
and electrochemical studies of La,@C,, .I4] This dimetallofullerene was found to be an even better electron acceptor than
any metallofullerene of the type M@C,, . The first derivatization of La,@C,, has also been achieved by the same group.L5'
Calculations have predicted that the Zhisomer of C,, should be
able to accommodate two La atoms to form a thermodynamically and kinetically stable endohedral f~llerene.[~*The remarkable stability of La,@&
comes from the fact that all six
valence electrons of the two La atoms are transferred to the C,,
cage; these six electrons are just enough to form a closed-shell
electronic structure for the carbon cage with a large HOMOLUMO gap. However, direct experimental confirmation for the
oxidation states of the metal atoms inside the carbon cage has
not yet been available.
We have recently reported a technique for the efficient separation of Ce@C,, and also its spectroscopic characterization.r6.1'
Ce was shown to have an oxidation state of I I I . ~ ' ~This new
method also allowed separation and characterization of the new
dimetallofullerene Ce,@C,,, which like Ce@C,, was detected
in previous laser desorption mass spectrometry experiments.[']
We report here on the separation of Ce,@C,, and on its characterization by UV-Vis-NIR absorption spectroscopy and X-ray
photoelectron spectroscopy (XPS).
The high purity of Ce,@C,, isolated by HPLC can be seen
from the negative-ion desorption chemical ionization (DCI)
mass spectrum shown in Figure 1; a purity of > 9 9 % can be
Figure 2 shows the UV-Vis-NIR absorption spectrum of
Ce,@C,, in the wavelength range of 300 to 2100 nm. The spectrum exhibits a monotonically decreasing absorption coefficient
with increasing wavelength without well-defined, sharp features. This relatively featureless spectrum is in contrast to those
of the empty fullerenes['O1as well as of the monometallofullerenes.[lclWhetten et al. proposed that in La,@C,, the C,, cage
has Z, symmetry, resulting in overall D,, symmetry for the
' 'I This is supported by a recent theoretdimetall~fullerene.[~~
ical calculation.[61In this and earlier workc5]it was demonstrated that La,@C,, can be formally represented by Lai+@C&,
forming a closed-shell electronic structure with a HOMOLUMO gap larger than that of M@C,, (M = rare earth metal).
As will be shown below, C,, in Ce,@C,, also acquires the six
valence electrons from the two Ce atoms, presumably resulting
[*] Prof. S. Yang, J. Ding
Department of Chemistry
Hong Kong University of Science and Technology
Clear Water Bay, Kowloon (Hong Kong)
Fax: Int. code +(23S8)1594
The authors would like to thank the Department of Chemistry a: HKUST for
its support for the fullerene project. This work was supported by an RGC
Grant (HKUST601i9SP) administrated by the UGC of Hong Kong.
The assistance of L:T. Weng is acknowledged.
8 VCH Verlag.~gesellrchafimhH, 0-69451
Weinheim. 1996
Fig. 1. Methane DCI negative-ion mass spectrum of the Ce,(u;C,, sample after
HPLC separation. The insets show the calculated (left) and observed (right) isotopic
distributions for Ce,(a>C,,. I = intensity (arbitrary units)
Fig. 2. UV-Vis-NIR absorption spectrum of Ce,(dC,, in toluene. A
(arbitrary units).
in the same electronic configuration as that of La,@C,, . The
closed-shell electronic structure of C& in Ce,@C,, is likely to
account for the absence of NIR absorption peaks in the spectrum of Ce,@C,,.
Figure 3 shows the XPS spectrum of Ce,@C,, in the region
of the Ce 3d3,, and 3d,,, core levels. The spectrum is quite
similar to that of Ce@C,, with two main features for the 3d3,,
and 3d,,, levels in order of decreasing binding energy.[,] Both
features show a strong peak and a weak shoulder. For the 3d,,,
feature, the stronger peak at higher binding energy (886.57 eV)
can be explained by the screening of only the surrounding
fullerene orbitals, while the weak shoulder at lower binding
energy (881.83 eV) may be accounted for by screening arising
from charge transfer to the empty 4f
It is known that Ce4+ has a characteristic XPS (3d) peak at
a binding energy of approximately 914 eV arising from a transition to the 3d94f0 final
The intensity of this peak has
been used for estimating Ce4+ c~ncentration."~]
Since we did
not observe this characteristic peak, we can rule out the presence
of Ce4+ in Ce,(@C,,. As no characteristic peak for CeZ+ is
known in the 3d core level region, we cannot rule out Ce2+ in
the same way. However, the location and shape of the Ce(3d)
0570-0833196/3S19-2234 $ 15.00 f .2SjO
Angeu. Chem. Int. Ed. En@. 1996. 35. No. 19
injection volume was 1 mL and the elution rate was 1 m l m i n - ’ . The sample of
Ce2@C,, collected from HPLC was analyzed by DCI negative-ion mass spectrometry (Finnigan TSQ7000).
The UV-Vis-NIR absorption spectrum of Ce,@ C,, in toluene was recorded with a
Perkin-Elmer spectrometer (Lambda 19). For the XPS study. we prepared a
Ce,(a C,, film on a polycrystalline Au substrate. To clean the Au surface, we heated
a small piece of gold foil with a natural gas torch, dipped i t in methanol. and dried
it in N, gas. Several drops of a concentrated solution of Ce,.n C,, in toluene were
transferred t o the Au foil. The evaporation of the solvent left a uniform film of
Ce,((( C,,, which was was dried and washed with n-hexane. An XPS measurement
was taken using monochromatized Al,, radiation (Iir = 1486.6 eV) with a n energy
resolution of roughly 0.6 eV (Perkin-Elmer PHI 5600).
Received: March 7, 1996 [Z8908IE]
German version. Angeiv. Chem. 1996, 108. 2369-2371
Keywords: cerium compounds * fullerenes * metallofullerenes
Fig. 3. XPS spectrum of Ce,(aC,, on a gold substrate in the core level region of
Ce(3d). The smooth line results from a least-square fitting of the spectral peaks
using a linear baseline. The occurrence of two features (3d,,,: signal at higher
energy; 3d,,,: signal at lower energy) is due to spin-orbital splitting of the 3d
core level. E = binding energy. N ( E ) / N = photoelectron signal intensity (arbitrary
peak in the XPS spectrum of Ce,@C,, are rather similar to
those of cerium trihalide~,[’~]
suggesting that the oxidation state
of Ce is 111. The ratio of the intensities of the peaks at higher and
lower energies is known to be very sensitive to the electronegativity of the ligands.[‘6’Our analysis shows that the electronegativity of the fullerene cage is somewhat less than that of chlorine.
This conclusion is similar to that we drew for Ce@C,,[’l and
Weaver et al. reported for La(a,C,,.I”]
In fact, the XPS spectram of Ce,@C,, is at first glance almost
identical to that of Ce@C,,; however, for Ce,@Ca0 the curvefitting can be improved by assuming more peaks. For example,
Figure 3 shows the curve obtained by assuming one extra small
peak at a lower binding energy. The approximation may be
further improved by assuming small peaks at the high binding
energy side of the two strong peaks. The presence of a few extra
small peaks in addition to the strong peak and weak shoulder
characteristic of Ce3+ may signify the interaction between the
two Ce atoms in the carbon cage. Further studies are required
to support this hypothesis.
We have reported the first successful separation of the new
endohedral dimetallofullerene Ce,@C,, and its UV-Vis-NIR
absorption and XPS spectra. The UV-Vis-NIR spectrum exhibits monotonically decreasing absorption as a function of
wavelength without any sharp absorption features. The XPS
spectrum suggests that the Ce atoms encaged in C,, are in the
form of Ce3+.This implies that in the cage one valence electron
remains on each metal atom. The nature of their spin-spin
coupling within the carbon cage constitutes a fascinating subject
for future research.
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Experimen frrl Procedure
The procedures for isolating the metallofullerenes have been described previousIy[7]. Soot containing metallofullerenes was produced by the standard arc vaporization method using a composite anode which contains graphite and cerium in its
oxide or carbide form. The rod was then subjected to a DC contact-arc discharge
as an anode under an He atmosphere of 50 Torr. The soot was collected and extracted for 8 h in a Soxhlet extractor with N,N-dimethylformamide (DMF, 99.9%,
BDH). After removal of the D M F by evaporation, a black powder (ca. 1 % of the
soot) was obtained. The soluble fraction was dissolved in toluene and injected into
an HPLC. A buckyprep column (4.6 mm x 250 mm: Cosmosil, Nacalai Tesque Inc.)
similar to a PYE column was used, and toluene served as the mobile phase. The
A n w i v . Chem. In!. Ed E n d . 1996, 35, No. 19
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isolation, ce2, characterization, c80, dimetallofullerenen
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