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

We report the first synthesis of ²²⁵Ac (t _1/2 = 10 days) endohedral fullerenes,²²⁵Ac@C₆₀. The ²²⁵Ac@C₆₀ was produced with a 12 ± 2% efficiency by applying an electrical arc discharge between a source of α-particle emitter ²²⁵Ac (∼1 mCi, electroplated on a Pt disk) and a thin coat of "preformed" C₆₀ on an Al disk (C₆₀ thickness of ∼0.25 mg/cm²). 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 ²²⁵Ac, ∼45% of ²²⁵Ac activity was retained in the organic phase, which resisted extraction into the aqueous phase. Failure to extract the ²²⁵Ac from the organic phase provided definitive evidence that the ²²⁵Ac is located inside of the fullerene. The formation of ²²⁵Ac@C₆₀ was further confirmed using a classical hot-atom chemistry technique in which the organic phase containing purified endohedral ²²⁵Ac@C₆₀ malonate was contacted with fresh dilute acid to repeatedly extract the ionic 4.8 m ²²¹Fr and 45.6 m ²¹³Bi activities (decay daughters of ²²⁵Ac), 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 ²²⁵Ac@C₆₀ or ²²⁵Ac@C₆₀ malonate. Taken together, studies show that, like polydentate chelators, single-wall fullerenes are not capable of retaining the ²²⁵Ac decay daughters.