Synthesis and Stability of Actinium-225 Endohedral Fullerenes, (225)Ac@C(60).

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.