Small Cage Uranofullerenes: 27 Years after Their First Observation

The tetravalently stabilized fullerene cage of C₂₈ is historically the most elusive small fullerene cage observed by employing the laser vaporization synthesis methodology. Its first observation reported by Smalley et al. in 1992 suggests that C₂₈ is potentially the smallest and most stable fullerene ever observed. By using the Krätschmer?Huffman arc discharge synthesis method, we have recently succeeded in synthesizing a series of uranium‐endohedral fullerenes which differ from those reported by Smalley and co‐workers. Intrigued by this interesting mismatch, we tuned our experimental conditions to favor the formation and detection of these missing species. Experiments done using solvents of varying polarity allowed the observation of several empty and uranofullerenes. Extractions with pyridine and o‐DCB allowed for observation of small U@C_2n (2n=28, 60, 66, 68, 70) by high resolution Fourier‐Transform Ion Cyclotron Resonance Mass Spectrometry (FT‐ICR MS). This is the first time that U@C₂₈ is observed in soot produced by the Krätschmer‐Huffman arc‐discharge methodology. Carbon cage selection and spin density distribution on the endohedral metallofullerenes (EMFs) U@C₆₀, U@C₇₀, and U@C₇₂ were studied by means of density functional theory (DFT) calculations. A plausible pathway for the formation of U@D_3h‐C₇₄ from U@D_5h‐C₇₀ through two C₂ insertions and one Stone‐Wales rearrangement is proposed.