Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 151–154, 2002

151

Study of Metallofullerenes Encapsulating Actinides
Kazuhiko Akiyama,∗,a,b Keisuke Sueki,a Kazuaki Tsukada,b Tsuyoshi Yaita,b Yoko Miyake,aHiromitsu Haba,b Masato Asai,b Takeshi Kodama,a Koichi Kikuchi,a Tsutomu Ohtsuki,cYuichiro Nagame,b Motomi Katada,a and Hiromichi NakaharaaaGraduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japanb
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195,
Japan
c
Laboratory of Nuclear Sciences, Tohoku University, Sendai, Miyagi 980-8577, Japan
Received: March 17, 2002; In Final Form: May 1, 2002
The oxidation state of actinide elements encapsulated in fullerenes is studied. HPLC elution behavior of actinide
fullerenes is classified into two groups; the elution behavior of the first group, encapsulating U, Np, and Am, issimilar to that of the light lanthanide fullerenes, such as La, Ce, Pr, and Nd, while the behavior of the second group,
encapsulating Th and Pa, is quite different from that of any lanthanide fullerenes. The chemical species in the main
HPLC elution peak of each group were identified as M@C82 and M@C84 (M = metal atom) from the mass of the U
and Th fullerenes, respectively. The oxidation states of the U and Th atoms in the fullerenes were deduced to be 3+
and 4+, respectively, from the UV/vis/NIR absorption and XANES spectroscopy.

1. Introduction
Since the discovery of fullerenes by Kroto and Smalley,1 themolecules with the special cage structure have been attracting
interests of many scientists as a new functional material. The
size of the inner space of this cage is about 7 Å in diameter forthe smallest fullerene, C60 , and is sufficiently spacious to takeseveral atoms or a small cluster inside. In addition, it has been
pointed out2–16 that the atmosphere of this inner space is very
reductive owing to the existence of many unsaturated π orbitals
from the carbon atoms that form the cage structure. For example, the lanthanide elements of Sm, Eu, Tm, and Yb, which takecommonly 3+ oxidation state in solid compounds and in aqueous solutions, take 2+ oxidation state in the fullerene cage. This
fact indicates that the inner space of the fullerene is so reductivethat the encapsulated atom is stabilized with its lower oxidation
state for a long period.
Among the f -block elements, the actinide elements are
known to take various oxidation states depending on the surrounding environments. It is, therefore, interesting from theview point of the actinide science to investigate the oxidation
state of the actinide elements in the reductive environment suchas the inside of fullerene cages. In this paper, we discuss theoxidation state of actinide elements in fullerene cages.

tracers. For an examination of physico-chemical properties of
the actinide fullerenes, macroscopic quantities of Th and U metallofullerenes were also prepared by the carbon rods containing
a few grams of Th and U instead of the La carrier. The purification of these metallofullerenes was performed by collecting
the HPLC fractions that contained the main species of Th and U
metallofullerenes according to the result of the radiotracer experiment on the Buckyprep and 5PBB columns.
2.2. UV/vis/NIR Absorption and XANES Spectroscopy ofTh and U Metallofullerenes. The UV/vis/NIR absorption
spectroscopy and the XANES (X-ray Absorption Near Edge
Structure) spectroscopy were performed for the purified main
species of the Th and U metallofullerenes to investigate the electronic state of these species. These metallofullerenes dissolved
in toluene were put in the quartz cell (10 mm light path and
2 mm width) for the observation of the UV/vis/NIR absorption
spectra in the wavelength region of 400 nm to 1600 nm. To
take the Th LIII -edge and U LIII -edge XANES spectra, the samples were prepared by dropping the CS2 solutions on the crystal plates with a shallow dip in the center and then dried. The
absorption spectra were taken at the photon factory of KEK in
Tsukuba using a Si(111) single crystal monochromator. All the
XANES spectra were acquired in the fluorescence mode using a
Ge detector.

2. Experimental Procedures

3. Results and Discussion

2.1. Preparation and HPLC Separation of Actinide Metallofullerenes. The soot mixture including actinide metallofullerenes was produced by the arc-discharge method using
a carbon rod which was impregnated with radioactive tracers, 234 Th, 233 Pa, 237 U, 239 Np, and 240 Am, and La(NO3 )3 as
a carrier. The atom ratio for C/La was set to be 100/1.
The condition of the arc-discharge was the same as that described in Reference 17. The crude fullerene including themetallofullerenes was extracted by CS2 from the primarysoot and then re-dissolved in toluene for the injection into
HPLC columns of Buckyprep (10 mmφ × 250 mm) and 5PBB
(10 mmφ × 250 mm). The Buckyprep and 5PBB columns wereoperated with the flow rate of 3.2 and 6.0 mL/min, respectively.
The effluent from the column was collected for every 1 min andmonitored by an on-line UV absorption detector. The HPLC
elution behavior of the actinide metallofullerenes was monitoredby the off-line detection of γ and X rays from the actinide radio-

3.1. HPLC Elution Behavior of Actinide Metallofullerenes. Figures 1(a) and (b) show the HPLC elution
curves of the actinide metallofullerenes observed with the
Buckyprep and 5PBB columns, respectively. It was foundthat the HPLC elution behavior was classified into two groups
from the position of the elution peaks. The position of theelution peaks of the first group, encapsulating U, Np, and Am,
is identical to that of the light lanthanide fullerenes, such as La,
Ce, Pr, and Nd.2, 18–20 In the second group, encapsulating Th
and Pa, the elution curves are completely different from thoseof any lanthanides.
For the Buckyprep column, the elution behavior of fullerenes
is ruled by both the number of π electrons on the cage and thesymmetry of the fullerene cage, whereas in the 5PBB column
the fullerenes are mainly separated by the number of π electronsof the cage and the symmetry of the cage does not affect the separation so much. The similarity of the retention time of the main
species of U, Np, and Am metallofullerenes with that of somelanthanides indicates that these main species have the same car-

∗Corresponding author. E-mail: kakiyama@popx.tokai.jaeri.go.jp.
FAX: +81-29-282-5939.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

152 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Akiyamabon number, symmetric structure, and charge on the cage. For
the lanthanide species, these main species have been assigned
as M@C82 which encapsulates a trivalent lanthanide cation.
The main species of U metallofullerene was actually identified
as U@C82 by TOF/MS (Time of Flight / Mass Spectroscopy).21
Accordingly, this result suggests that the oxidation state of theactinides encapsulated as M@C82 is identical to that of the lanthanides in M@C82 . On the other hand, the retention time of
metallofullerenes of Th and Pa was found to be about 5 minlonger in the 5PBB column, and 30 min longer in the Buckyprep
column compared to those for the above-stated group. The main
component of the elution peak for the Pa species is considered to
be M@C84 type metallofullerene like Th@C84 which was identified by the TOF/MS measurement.22 Stevenson et al. reported
the correlation between the retention time and the number ofelectrons on the fullerene cage for 5PBB column separation.23
The retention time of M@C84 has been known only for Sm, Eu,
Tm, and Yb species whose charge on the cage has been revealed
to be 2−. These M@C84 can accept extra two electrons on the
C84 cage and the retention time of these species is nearly equal
to that of the hollow C86 . While the retention time of the M@C84
for Th and Pa is slightly longer than that of the former M@C84
species. According to the Stevenson’s concept, the number of πelectrons on the cage of M@C84 for Th and Pa is deduced to begreater than that of the former M@C84 .
3.2. Spectroscopic Properties of Th@C84 and U@C82 .
Figures 2(a) and 2(b) show the UV/vis/NIR absorption spectraof the isolated U@C82 and Th@C84 in toluene, respectively. The
U@C82 spectrum in Figure 2(a) has three characteristic absorption peaks at the wavelengths of 619 nm, 977 nm, and 1352 nm
which coincide well with those of M@C82 of the trivalent lanthanides as shown in Figure 2(c).24 This similarity strongly sugD&89
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gests that the charge of the C82 cage of the U@C82 is 3−. In
addition, it is interesting that the replacement of encapsulated
metal from lanthanide to uranium does not affect the absorption
peaks of M@C82 .
Up to now, the UV/vis/NIR spectrum of the isolated M@C84
is measured only for Sm which are shown in Figure 2(d).
Okazaki et al. reported that Sm@C84 has three topologically different isomers and they determined the oxidation state of Sm in
C84 to be divalent from the electron energy loss spectroscopy(EELS).25 The C84 fullerene cage is expected to have 24 structural isomers which satisfy IPR.26 Considering the number ofthe isomers of the air stable Sm@C84 , it can be assumed thatthe three of those C84 would be stabilized as the C2−
84 . If theUV/vis/NIR absorption spectrum of M@C84 were not so muchaffected by the encapsulated metal atom like the M@C82 case,the observed difference in the absorption spectra between Sm
and Th supports that the charge of the C84 cage of Th@C84 is
not divalent.
To confirm the oxidation state of the U and Th atoms in themetallofullerene, U LIII -edge and Th LIII -edge XANES spectra
were examined as shown in Figures 3(a) and 3(b) together withthe reference materials of UCl3 , UO2 , and Th(NO3 )4 ·2H2 O. TheU LIII -edge spectrum of U@C82 is clearly shifted to lower energy side in comparison with that of UO2 . To make clear thedifference between these spectra, the edge energy was evaluatedby fitting the spectra with a Gaussian plus arctangent function(the inflection point of the arctangent function is defined as E0 ).
The E0 of U@C82 was found to be shifted to lower energy sideby 5 eV in comparison with that of UO2 and is nearly equal
to that of UCl3 . From the results of the HPLC investigation,
UV/vis/NIR, and XANES spectroscopy, it is concluded that theoxidation state of the U atom encapsulated in the C82 cage is 3+,
and the charge coupling in U@C82 is suggested to be U3+ @C3−
82 .
The E0 of the Th LIII -edge spectrum for Th@C84 also showsa small shift to the lower energy side by about 3 eV comparedwith that of the reference material of Th(NO3 )4 ·2H2 O. To consider the magnitude of the shift for U LIII -edge, it can be said
that the oxidation state of Th is not divalent. Unfortunately, noreference materials for Th(III) being available, we cannot conclude whether the oxidation state of Th in the C84 is 3+ or 4+.

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Figure 1. HPLC elution curves of actinide metallofullerenes on (a) the
Buckyprep column and (b) the 5PBB column. For both columns, theelution curves of U, Np, and Am metallofullerenes are similar to thoseof the light lanthanide fullerenes such as Ce. On the other hand, theelution curves of Th and Pa metallofullerenes are quite different fromthose of the any lanthanide metallofullerenes.22

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Figure 2.
UV/vis/NIR absorption spectra of isolated (a) U@C82(Ref. 22) and (b) Th@C84 in toluene, and those of (c) Nd@C82 and(d) Sm@C84 in toluene.25

$EVRUEDQFH  DUE XQLWStudy of Metallofullerenes Encapsulating Actinides

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

the JAERI tandem accelerator for providing the stable ion beam
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Figure 3. LIII -edge XANES spectra of (a) U@C82 , (b) Th@C84 , andreference materials of UCl3 , UO2 , and Th(NO3 )4 2H2 O. The arrows indicate the absorption edges E0 .27

It can be only mentioned that the Th atom encapsulated in the
C84 fullerene is subjected to more reductive chemical environment than that of the nitrate. For Th@C84 , 13 C-NMR study was
also carried out in order to determine the molecular structureof Th@C84 which will be reported elsewhere. Here we only
mention that the 13 C-NMR spectra could be observed. This
fact strongly suggests that no unpaired electron exists on thefullerene cage and, that is, only even number of electrons aretransferred from the encapsulated atom to the cage. Thus, weconclude that the oxidation state of Th being 4+ with the chargeof the cage of C4−84 .
4. Conclusion
The HPLC elution behavior of the actinide metallofullerenes
was classified into two groups from the elution curves. The elution curves of the first group, encapsulating U, Np, and Am,
were similar to those of the light lanthanide fullerenes such as
La, Ce, Pr, and Nd metallofullerenes whose main component
was M@C82 . The UV/vis/NIR absorption and XANES spectroscopy for U@C82 revealed that the charge coupling of themetallofullerenes of the first group was M3+ @C3−
82 . The elution curves of the second group consisting of Th and Pa weredifferent from those of any lanthanides. The chemical speciesof the main HPLC elution peak of this group was found to be
M@C84 from the TOF/MS measurements. From the results of
the UV/vis/NIR absorption spectra and 13 C-NMR spectroscopy
for Th@C84 , it was suggested that the charge of C84 in Th@C84
was 4+ although the XANES spectroscopy indicated that the Th
atom in C84 cage was somewhat more negative than 4+.
Acknowledgement. This work was partly supported by the
REIMEI Research Resources of Japan Atomic Energy ResearchInstitute and by the Grant-in-Aid for the Ministry of Education,
Culture, Sports, and Science of Japan. We thank to the crew ofReferences(1) H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, and R.
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154 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002(26) Isolated Pentagon Rule (IPR): that is the rule to explain thestability of the cage like structure of fullerenes. Each fivemembered in the most stable cage are never adjacent to eachother.

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