Th-Based Endohedral Metallofullerenes: Anomalous Metal Position and Significant Metal-Cage Covalent Interactions with the Involvement of Th 5f Orbitals

Abstract

Endohedral metallofullerenes (EMFs) containing actinides are rather intriguing due to potential 5f-orbital participation in the metal-metal or metal-cage bonding. In this work, density functional theory calculations first characterized the structure of recently synthesized ThC as Th@ D(14246)-C. We found that the thorium atom adopts an unusual off-axis position inside cage due to small metal ion size and the requirement of large coordination number, which phenomenon was further extended to other Th-based EMFs. Significantly, besides the strong metal-cage electrostatic attractions, topological and orbital analysis revealed that all the investigated Th-based EMFs exhibit obvious covalent interactions between metal and cage with substantial contribution from the Th 5f orbitals. The encapsulation by fullerenes is thus proposed as a practical pathway toward the f-orbital covalency for thorium. Interestingly, the anomalous internal position of Th led to a novel three-dimensional metal trajectory at elevated temperatures in the D-C cavity, as elucidated by the static computations and molecular dynamic simulations.

Robust metal‐pentagon interactions in the Th‐based endohedral metallofullerenes revealed by DFT calculations

“…According to the mass spectra signals of Th@C 2 n (2 n = 74‐96) reported by Chen and coworkers, the possible structures of Th@D 3h (1)‐C 74 , Th@T d (2)‐C 76 , and Th@C s (10)‐C 84 are quickly proposed by our and other's theoretical calculations . Interestingly, we found that the Th atom in Th@D 3h (1)‐C 74 may undergo an unprecedented three‐dimensional motion in the cage . Besides the Th‐EMFs, Echegoyen and coworkers recently obtained three uranium metallofullerenes U@D 3h (1)‐C 74 , U@C 2 (5)‐C 82 , and U@C 2v (9)‐C 82 .…”

“…Full geometry optimizations were carried out by using the M06‐2X functional and the standard split‐valence 6‐31G* basis set for C and the Stuttgart/Dresden relativistic effective core potential and corresponding basis set for metals. The current theoretical level is denoted here as M06‐2X/6‐31G*~SDD, which has been successfully employed to study Th‐based EMFs . After optimizations, harmonic vibrational frequency analysis was then conducted at the same level of theory to identify the nature of located stationary points on the potential energy surface (PES).…”

“…Note that, although holding a formal 4+ oxidation state, the thorium cation resides close to only three pentagons of the C 3v (8)‐C 82 cage, whereas four pentagons are indispensable for thorium(IV) coordination in typical organometallic complexes such as Th( η 5 ‐C 5 H 5 ) 4 . According to the mass spectra signals of Th@C 2 n (2 n = 74‐96) reported by Chen and coworkers, the possible structures of Th@D 3h (1)‐C 74 , Th@T d (2)‐C 76 , and Th@C s (10)‐C 84 are quickly proposed by our and other's theoretical calculations . Interestingly, we found that the Th atom in Th@D 3h (1)‐C 74 may undergo an unprecedented three‐dimensional motion in the cage .…”

“…For the non‐IPR ones, the metal atom usually resides close to the pentagon adjacencies (PAs), resulting in released local strain by charge transfer and coordination interactions . For example, as a typical divalent metal, calcium tends to bind with and stabilize one PA of fullerene, as found in Ca@C 2 (10612)‐C 72 and Ca@C 1 (14109)‐C 74 . This phenomenon can be understood by considering that the transferred two electrons efficiently convert an antiaromatic 8π‐electron pentalene to a local Hückel‐aromatic 10π‐electron motif …”

“…Herein, as a part of our recent series of studies on Th‐based EMFs, we carefully investigated the Th‐cage interactions in a large number of fullerenes covering different sizes (C 64 ‐C 88 ) and types (both IPR and non‐IPR) by means of density functional theory (DFT) calculations. We found that, although formally transferring four electrons to the cage, the Th atom tends to coordinate with only three neighboring pentagons regardless the relative position and distance among them.…”