The nature of actinide-actinide bonds has attracted considerable attention for a long time, especially since recent theoretical studies suggest that triple and up to quintuple bonds should be possible, but little is known experimentally. Actinide-actinide bonds inside fullerene cages have also been proposed, but their existence has been debated intensively by theoreticians. Despite all the theoretical arguments, critical experimental data for a dimetallic actinide endohedral fullerene have never been obtained. Herein, we report the synthesis and isolation of a dimetallic actinide endohedral metallofullerene (EMF), U@C. This compound was fully characterized by mass spectrometry, single crystal X-ray crystallography, UV-vis-NIR spectroscopy, Raman spectroscopy, cyclic voltammetry, and X-ray absorption spectroscopy (XAS). The single crystal X-ray crystallographic analysis unambiguously assigned the molecular structure to U@ I (7)-C. In particular, the crystallographic data revealed that the U-U distance is within the range of 3.46-3.79 Å, which is shorter than the 3.9 Å previously predicted for an elongated weak U-U bond inside the C cage. The XAS results reveal that the formal charge of the U atoms trapped inside the fullerene cage is +3, which agrees with the computational and crystallographic studies that assign a hexaanionic carbon cage, ( I -C). Theoretical studies confirm the presence of a U-U bonding interaction and suggest that the weak U-U bond in U@ I (7)-C is strengthened upon reduction and weakened upon oxidation. The comprehensive characterization of U@ I (7)-C and the overall agreement between the experimental data and theoretical investigations provide experimental proof and deeper understanding for actinide metal-metal bonding interactions inside a fullerene cage.

First X-ray structures and metal oxidation state dependence on cage isomerism for U-EMFs.

Endohedral metallofullerenes (EMFs) containing lanthanides have been intensively studied in recent years. By contrast, actinide endohedral fullerenes remain largely unexplored. Herein, for the first time, we report the single crystal structure and full characterization of an actinide endohedral fullerene, Th@C, which exhibits remarkably different electronic and spectroscopic properties compared to those of lanthanide EMFs. Single crystal X-ray crystallography unambiguously established the molecular structure as Th@C(8)-C. Combined experimental and theoretical studies reveal that Th@C(8)-C is the first example of an isolated monometallofullerene with four electrons transferred from the metal to the cage, with a surprisingly large electrochemical band gap of 1.51 eV. Moreover, Th@C(8)-C displays a strong vibrationally coupled photoluminescence signal in the visible region, an extremely rare feature for both fullerenes and thorium compounds.

Clusterfullerenes Dy2O@C72 and Dy2O@C74 demonstrate a fine balance of exchange and dipolar interactions and slow relaxation of magnetization.

Endohedral mono-metallofullerenes are the prototypes to understand the fundamental nature and the unique interactions between the encapsulated metals and the fullerene cages. Herein, we report the crystallographic characterizations of four new U-based mono-metallofullerenes, namely, U@C s (6)-C 82 , U@C 2 (8)-C 84 , U@C s (15)-C 84 , and U@C 1 (12)-C 86 , among which the chiral cages C 2 (8)-C 84 and C 1 (12)-C 86 have never been previously reported for either endohedral or empty fullerenes. Symmetrical patterns, such as indacene, sumanene, and phenalene, and charge transfer are found to determine the metal positions inside the fullerene cages. In addition, a new finding concerning the metal positions inside the cages reveals that the encapsulated metal ions are always located on symmetry planes of the fullerene cages, as long as the fullerene cages possess mirror planes. DFT calculations show that the metal−fullerene motif interaction determines the stability of the metal position. In fullerenes containing symmetry planes, the metal prefers to occupy a symmetrical arrangement with respect to the interacting motifs, which share one of their symmetry planes with the fullerene. In all computationally analyzed fullerenes containing at least one symmetry plane, the actinide was found to be located on the mirror plane. This finding provides new insights into the nature of metal−cage interactions and gives new guidelines for structural determinations using crystallographic and theoretical methods.