Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 49 (2016)

Quantitative study on metallofullerene separationby chemical reduction
K. Akiyama,∗1,∗2 K. Chiba,∗1 H. Haba,∗2 and S. Kubuki∗1
Endohedral metallofullerene (EMF) is expected tobe applied in electronics and medicine because of its
excellent donor-acceptor properties and unique structure. Among these useful applications, the applicationin nuclear medicine using radioactive metallofullerenes
is promising. However, the production yield of EMFs
is generally low: for example, the yield of EMFs inwhole fullerene species is less than 1% with the commonly employed arc discharge method. Therefore,
the separation and puriﬁcation of EMFs from a large
amount of empty fullerene species are needed. In general, high-performance liquid chromatography (HPLC)
is employed for the separation/puriﬁcation of EMFs
from empty fullerenes. Because this method is notalways eﬃcient from the viewpoint of cost and time
consumption, a more eﬃcient method is required tobe developed.
Recently, low-cost separation methods for EMFs
using the chemical redox reaction were reported.1,2)
Considering the separation of carrier-free radioactive
EMFs, separations through the reduction of EMFs using an electron donor are expected to be suitable because these methods do not include a process using alarge amount of carriers such as the formation of theprecipitate. However, these separation methods havesome problems because the separation eﬃciency is notvery high and the number of steps is large. Currently,
we are attempting to develop a separation methodwith higher eﬃciency and smaller number of steps thanthe other methods through the chemical reduction ofEMFs. In this study, we employed the radio-tracer
method to acquire quantitative data for the separation by chemical reduction for determining the best
separation condition.
To obtain the radio tracer of 139 Ce, a dried La2 O3pellet (600 mg/cm2 ) was irradiated with 3 µA of 24
MeV deuteron for 2 h at the RIKEN AVF cyclotron.
After the irradiation, 139 Ce and the target material
La were converted into their nitrate forms, dissolvedin ethanol, and then injected into porous carbon rodsfor fullerene generation. Radioactive 139 Ce EMFs
were produced by the arc discharge method and extracted together with empty fullerenes from the sootby CS2 . Mixed solutions of polar solvents (acetonitrile: MeCN, acetone, and methanol: MeOH) andsmall amounts of electron donors (triethylamine: TEA,tripropylamine: TPA, tetrahydrofurane: THF, N,N,dimethylformanide: DMF, pyridine, and aniline) for
the reduction of EMFs were added into these CS2 so∗1∗2

Department of Chemistry, Tokyo Metropolitan UniversityRIKEN Nishina Center- 243 -lutiosn and then stirred. To separate the produced
EMF anion from the residual empty fullerene precipitate, these solutions were passed through a membraneﬁlter. A few drops of an aqueous solution of dichloroacetatic acid were added into the ﬁltrate samples for neutralizing the EMF anion, and the neutralized EMFs
were re-dissolved in chlorobenzene (F). The residualempty fullerenes on membrane ﬁlters were dissolved in
CS2 for recovery from a ﬁlter (P). The γ ray emitted
from these solutions and ﬁlter were detected using aGe detector.
Figure 1 shows the rate% of radioactivity observedin F, P, and ﬁlter for each sample. In principle, radioactivity observed in F is derived from radioactiveEMFs separated from empty fullerenes and that in P
is derived from radioactive EMFs that remained in theempty fullerene precipitate. Accordingly the rate% ofF indicates the separation eﬃciency of EMFs. Considering the donor number (DN) which is known as
an index indicating the strength of the electron donor
properties, the electron donors used in this study canbe arranged according to their performance as TEA>TPA >aniline >pyridine >DMF >THF.3) In the caseof MeCN as the polar solvent, a rate% greater than
80% in F was found with alkylamines such as TPA andTEA. These results indicate that good electron donors
such as TPA and TEA easily reduce EMFs. From theviewpoint of the polarity of the solvent, the rate% ofF for the solvents with large polarity such as MeCNis found to be larger than that for the solvents withsmall polarity such as acetone. On the other hand, therate% of F for MeCN which is a non-protic solvent issuperior to that for MeOH which is a protic solvent inspite of both polarities being comparable.

Fig. 1. Observed radioactivity in solution F (black), solution P (gray), and filter (white) for each sample.

References
1) K. Akiyama et al., J. Am. Chem. Soc. 134, 9762 (2012).
2) T. Kodama et al., Chem. Lett. 34, 464 (2005).
3) V. Gutmann: The Donor-Acceptor Approach to Molecular Interactions (Plenum, New York, 1978).