Unique Four-Electron Metal-to-Cage Charge Transfer of Th to a C₈₂ Fullerene Cage: Complete Structural Characterization of Th@C_3v(8)-C₈₂

Abstract

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.

Th@C76. Computational characterization of larger actinide endohedral fullerenes

“…For comparison, we also optimized the structure of Th@C 3v (8)‐C 82 at the same level of theory. Consistent with the X‐ray structure, the metal favors to locate above a triple‐hexagon‐junction (THJ) carbon of C 3v (8)‐C 82 and the calculated Th‐C distance of 2.36 Å at the current level of theory is very close to the experimental value (2.34 Å). The short Th‐C distances in Th@C 2 n (2 n = 76, 82) suggest strong metal‐cage interactions.…”

“…The binding energy ( E b ) of Th@C 76 , defined as the difference between the total energy of the endohedral and the sum of the total energies of metal and cage, is calculated to be as large as 260.8 kcal/mol. Consistent with the shorter metal‐cage distance and higher relative yield in experiment, the E b value of Th@C 3v (8)‐C 82 is calculated to be 270.3 kcal/mol at the same level of theory.…”

“…To quickly find the parent cage of an EMF molecule, a well‐accepted and efficient strategy is to screen the low‐energy empty cages with appropriate negative charges. Since the internal Th atom formally donates four electrons to the fullerene cage, six low‐lying tetra‐anions of C 76 with the number of pentagon adjacencies (PA) less than two, including T d (19151), C 2v (19138), C 1 (17459), C s (19142), C 1 (17894), and D 2 (19150) were selected to form Th@C 76 . In addition, two low‐energy cages with PA = 2, namely C s (17490) and C 1 (17465) were also considered for comparison.…”

“…The same analysis can also be used to explain the metal position of Th@C 82 (Supporting Information Figure S1), indicating the critical role of electrostatic interactions in stabilizing the metal due to the significant amount of electron transfer (4e). In the single crystal structure, there is disorder in the Th 4+ position for Th@C 82 , whereas the thorium ion in Th@C 76 should be fixed according to our calculations (vide infra). The Coulomb force is proportional to the charge values, but inversely proportional to the distance between the point charges.…”

“…Significantly, Wang et al more recently prepared a large family of Th@C 2 n (2 n = 74–96) by a modified arc discharge method, and for the first time, they successfully isolated Th@C 82 and unambiguously characterized its structure as Th@C 3v (8)‐C 82 by X‐ray crystallographic analysis . Interestingly, compared with the conventional divalent or trivalent lanthanide EMFs, it exhibits an unprecedented four electrons transfer from a single metal atom to the cage and its electronic structure is thus described as Th 4+ @ .…”