Synthesis and Stability of Actinium-225 Endohedral Fullerenes, 225Ac@C60

We report the first synthesis of 225 Ac ( t 1/2 = 10 days) endohedral fullerenes, 225 Ac@C 60 . The 225 Ac@C 60 was produced with a 12 ± 2% efficiency by applying an electrical arc discharge between a source of α-particle emitter 225 Ac (∼1 mCi, electroplated on a Pt disk) and a thin coat of "preformed" C 60 on an Al disk (C 60 thickness of ∼0.25 mg/cm 2 ). After formation by electrical arc discharge, the resulting radiofullerenes on the Al disk were dissolved in toluene under anaerobic conditions and converted to a malonate derivative using the Bingel reaction. Subsequent to repeated washings of the organic phase with dilute acidic solutions to remove exohedral 225 Ac, ∼45% of 225 Ac activity was retained in the organic phase, which resisted extraction into the aqueous phase. Failure to extract the 225 Ac from the organic phase provided definitive evidence that the 225 Ac is located inside of the fullerene. The formation of 225 Ac@C 60 was further confirmed using a classical hot-atom chemistry technique in which the organic phase containing purified endohedral 225 Ac@C 60 malonate was contacted with fresh dilute acid to repeatedly extract the ionic 4.8 m 221 Fr and 45.6 m 213 Bi activities (decay daughters of 225 Ac), which were released by molecular disruption due to nuclear recoil. The result from the extraction experiments was further supported by a series of thin-layer chromatography and high-pressure liquid chromatography analysis of the organic phase containing 225 Ac@C 60 or 225 Ac@C 60 malonate. Taken together, studies show that, like polydentate chelators, single-wall fullerenes are not capable of retaining the 225 Ac decay daughters.