Synthesis and Characterization of Two Isomers of Th@C82: Th@C2v(9)-C82 and Th@C2(5)-C82.

Actinide endohedral fullerenes have demonstrated remarkably different physicochemical properties compared to their lanthanide analogues. In this work, two novel isomers of Th@C82 were successfully synthesized, isolated, and fully characterized by mass spectrometry, X-ray single crystallography, UV-vis-NIR spectroscopy, Raman spectroscopy, and cyclic voltammetry. The molecular structures of the two isomers were determined unambiguously as Th@C2v(9)-C82 and Th@C2(5)-C82 by single-crystal X-ray diffraction analysis. Raman and UV-vis-NIR spectroscopies further confirm the assignment of the cage isomers. Electrochemical gaps suggest that both Th@C2v(9)-C82 and Th@C2(5)-C82 possess a stable closed-shell electronic structure. The computational results further confirm that Th@C2v(9)-C82 and Th@C2(5)-C82 exhibit a unique four-electron charge transfer from the metal to the carbon cage and are among the most abundant isomers of Th@C82.

Th@D5h(6)-C80: a highly symmetric fullerene cage stabilized by a single metal ion.

A novel endohedral metallofullerene (mono-EMF), Th@D5h(6)-C80, has been successfully synthesized and fully characterized by mass spectrometry, single crystal X-ray diffraction, UV-vis-NIR and Raman spectroscopy and cyclic voltammetry. Single crystal XRD analysis unambiguously assigned the fullerene cage as D5h(6)-C80, the first example in which the highly symmetric cage is stabilized by a single metal ion. The combined experimental and theoretical studies further reveal that the D5h(6)-C80 cage, known only for its stabilization by 6-electron transfer, is stabilized by the 4-electron transfer from the encapsulated Th ion for the first time.

Characterization of a strong covalent Th3+-Th3+ bond inside an Ih(7)-C80 fullerene cage.

The nature of the actinide-actinide bonds is of fundamental importance to understand the electronic structure of the 5f elements. It has attracted considerable theoretical attention, but little is known experimentally as the synthesis of these chemical bonds remains extremely challenging. Herein, we report a strong covalent Th-Th bond formed between two rarely accessible Th3+ ions, stabilized inside a fullerene cage nanocontainer as Th2@Ih(7)-C80. This compound is synthesized using the arc-discharge method and fully characterized using several techniques. The single-crystal X-Ray diffraction analysis determines that the two Th atoms are separated by 3.816 Å. Both experimental and quantum-chemical results show that the two Th atoms have formal charges of +3 and confirm the presence of a strong covalent Th-Th bond inside Ih(7)-C80. Moreover, density functional theory and ab initio multireference calculations suggest that the overlap between the 7s/6d hybrid thorium orbitals is so large that the bond still exists at Th-Th separations larger than 6 Å. This work demonstrates the authenticity of covalent actinide metal-metal bonds in a stable compound and deepens our fundamental understanding of f element metal bonds.

Highly selective encapsulation and purification of U-based C78-EMFs within a supramolecular nanocapsule.

The ability of the tetragonal prismatic nanocapsule 1·(BArF)8 to selectively encapsulate U-based C78 EMFs from a soot mixture is reported, showing enhanced affinity for C78-based EMFs over C80-based EMFs. Molecular recognition driven by the electrostatic interactions between the host and guest is at the basis of the high selectivity observed for ellipsoidal C78-based EMFs compared to spherical C80-based EMFs. In addition, DFT analysis points towards an enhanced breathing adaptability of nanocapsule 1·(BArF)8 to C78-based EMFs to further explain the selectivity observed when the host is used in the solid phase.

Th@C1(11)-C86: an actinide encapsulated in an unexpected C86 fullerene cage.

A novel actinide endohedral fullerene with an unexpected chiral cage, Th@C1(11)-C86, was synthesized and characterized. DFT calculations suggest that this low symmetry cage was favoured as a consequence of the strong interaction between Th and the cage, which makes the predictions by the ionic model less reliable for these endohedral mono-metallofullerenes.

Synthesis and Characterization of Non-Isolated-Pentagon-Rule Actinide Endohedral Metallofullerenes U@ C1(17418)-C76, U@ C1(28324)-C80, and Th@ C1(28324)-C80: Low-Symmetry Cage Selection Directed by a Tetravalent Ion.

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 C1(28324)-C80, and U@C76 was assigned to non-IPR U@ C1(17418)-C76. All of these cages are chiral and have never been reported before. Further structural analyses show that enantiomers of C1(17418)-C76 and C1(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+@C2 n q- model makes predictions less reliable, and in general, unambiguously discerning the isolated structures requires the combination of accurate computational and experimental data.

Purification of Uranium-based Endohedral Metallofullerenes (EMFs) by Selective Supramolecular Encapsulation and Release.

Supramolecular nanocapsule 1⋅(BArF)8 is able to sequentially and selectively entrap recently discovered U2 @C80 and unprecedented Sc2 CU@C80 , simply by soaking crystals of 1⋅(BArF)8 in a toluene solution of arc-produced soot. These species, selectively and stepwise absorbed by 1⋅(BArF)8 , are easily released, obtaining highly pure fractions of U2 @C80 and Sc2 CU@C80 in one step. Sc2 CU@C80 represents the first example of a mixed metal actinide-based endohedral metallofullerene (EMF). Remarkably, the host-guest studies revealed that 1⋅(BArF)8 is able to discriminate EMFs with the same carbon cage but with different encapsulated cluster and computational studies provide support for these observations.

U2@ I h(7)-C80: Crystallographic Characterization of a Long-Sought Dimetallic Actinide Endohedral Fullerene.

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), U2@C80. 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 U2@ I h(7)-C80. 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 C80 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 h-C80)6-. Theoretical studies confirm the presence of a U-U bonding interaction and suggest that the weak U-U bond in U2@ I h(7)-C80 is strengthened upon reduction and weakened upon oxidation. The comprehensive characterization of U2@ I h(7)-C80 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.

Single crystal structures and theoretical calculations of uranium endohedral metallofullerenes (U@C2n , 2n = 74, 82) show cage isomer dependent oxidation states for U.

Charge transfer is a general phenomenon observed for all endohedral mono-metallofullerenes. Since the detection of the first endohedral metallofullerene (EMF), La@C82, in 1991, it has always been observed that the oxidation state of a given encapsulated metal is always the same, regardless of the cage size. No crystallographic data exist for any early actinide endohedrals and little is known about the oxidation states for the few compounds that have been reported. Here we report the X-ray structures of three uranium metallofullerenes, U@D3h-C74, U@C2(5)-C82 and U@C2v(9)-C82, and provide theoretical evidence for cage isomer dependent charge transfer states for U. Results from DFT calculations show that U@D3h-C74 and U@C2(5)-C82 have tetravalent electronic configurations corresponding to U4+@D3h-C744- and U4+@C2(5)-C824-. Surprisingly, the isomeric U@C2v(9)-C82 has a trivalent electronic configuration corresponding to U3+@C2v(9)-C823-. These are the first X-ray crystallographic structures of uranium EMFs and this is first observation of metal oxidation state dependence on carbon cage isomerism for mono-EMFs.

Unique Four-Electron Metal-to-Cage Charge Transfer of Th to a C82 Fullerene Cage: Complete Structural Characterization of Th@C3v(8)-C82.

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@C82, 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@C3v(8)-C82. Combined experimental and theoretical studies reveal that Th@C3v(8)-C82 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@C3v(8)-C82 displays a strong vibrationally coupled photoluminescence signal in the visible region, an extremely rare feature for both fullerenes and thorium compounds.