Small Cage Uranofullerenes: 27 Years after Their First Observation

The tetravalently stabilized fullerene cage of C-28 is historically the most elusive small fullerene cage observed by employing the laser vaporization synthesis methodology. Its first observation reported by Smalley etal. in 1992 suggests that C-28 is potentially the smallest and most stable fullerene ever observed. By using the Kratschmer-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@C-28 is observed in soot produced by the Kratschmer-Huffman arc-discharge methodology. Carbon cage selection and spin density distribution on the endohedral metallofullerenes (EMFs) U@C-60, U@C-70, and U@C-72 were studied by means of density functional theory (DFT) calculations. A plausible pathway for the formation of U@D-3h-C-74 from U@D-5h-C-70 through two C-2 insertions and one Stone-Wales rearrangement is proposed.