Endohedral fullerenes, which are carbon cages that encapsulate atoms, ions or clusters have been known since the early days immediately following the discovery of the fullerenes in 1985 by Smalley, Kroto and Curl. A wide range of cages, varying in size, shape and electronic properties have been observed which encapsulate individual atomic or ionic species or clusters that possess complementary characteristics. Of the many endohedral fullerenes that have been characterized and reported, lanthanide endofullerenes have received a lot of attention but the corresponding actinide systems have not been extensively investigated experimentally, although theoretical calculations have been reported, especially after the original report by Smalley et al in 1992 of the mass spectral observation of mono- and di-metallic Uranium endohedrals with formulas U@C_2n (n=14-36) and U₂@C_2m (m=25-30). The latter species containing two U ions inside have been the subject of multiple theoretical calculations using multiple cages, mainly C₆₀, C₇₀, C₈₀ and C₉₀. However, experimental reports of these exotic compounds are limited to three and only one involved purification using a two-stage HPLC protocol and presented the UV-Vis spectra for U@C₈₂ and U₂@C₈₀.

We have undertaken a systematic study of U endohedrals and have succeeded in identifying many different compounds using HRMS, and have succeeded in purifying and characterizing several mono-metallic endohedrals, including their x-ray single crystal structures, specifically U@C₇₄ and two different isomers of U@C₈₂.

Of particular interest for this presentation are totally unexpected Uranium endohedrals with formula U₂C_(2n+1), where n=39, 40 which correspond to carbides containing single C atoms encapsulated with two Us inside C₇₈ and C₈₀ cages. More interestingly, U₂X@C_2n compounds have also been detected, where X=N and O, besides C. These latter compounds will be described and their properties will be presented and discussed.