For
the first time, actinide endohedral metallofullerenes (EMFs)
with non-isolated-pentagon-rule (non-IPR) carbon cages, U@C80, Th@C80, and U@C76, have been successfully
synthesized and fully characterized by mass spectrometry, single crystal
X-ray diffractometry, UV–vis–NIR and Raman spectroscopy,
and cyclic voltammetry. Crystallographic analysis revealed that the
U@C80 and Th@C80 share the same non-IPR cage
of C
1(28324)-C80, and U@C76 was assigned to non-IPR U@C
1(17418)-C76. All of these cages are chiral and have never
been reported before. Further structural analyses show that enantiomers
of C
1(17418)-C76 and C
1(28324)-C80 share a significant
continuous portion of the cage and are topologically connected by
only two C2 insertions. DFT calculations show that the
stabilization of these unique non-IPR fullerenes originates from a
four-electron transfer, a significant degree of covalency, and the
resulting strong host–guest interactions between the actinide
ions and the fullerene cages. Moreover, because the actinide ion displays
high mobility within the fullerene, both the symmetry of the carbon
cage and the possibility of forming chiral fullerenes play important
roles to determine the isomer abundances at temperatures of fullerene
formation. This study provides what is probably one of the most complete
examples in which carbon cage selection occurs through thermodynamic
control at high temperatures, so the selected cages do not necessarily
coincide with the most stable ones at room temperature. This work
also demonstrated that the metal–cage interactions in actinide
EMFs show remarkable differences from those previously known for lanthanide
EMFs. These unique interactions not only could stabilize new carbon
cage structures, but more importantly, they lead to a new family of
metallofullerenes for which the cage selection pattern is different
to that observed so far for nonactinide EMFs. For this new family,
the simple ionic A
q+@C2n
q– model makes predictions
less reliable, and in general, unambiguously discerning the isolated
structures requires the combination of accurate computational and
experimental data.
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@C2n (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@D3h‐C74 from U@D5h‐C70 through two C2 insertions and one Stone‐Wales rearrangement is proposed.
Correction for ‘A new family of fullerene derivatives: fullerene–curcumin conjugates for biological and photovoltaic applications’ by Edison Castro et al., RSC Adv., 2018, 8, 41692–41698.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.