Encapsulation of Monometal Uranium into Fullerenes C_2n(2n= 70–74): Important Ionic U⁴⁺C_2n^4–Characters and Covalent U-Cage Bonding Interactions

Abstract

By using density functional theory calculations combined with statistical thermodynamic analyses, the stabilization performance of a series of fullerene cages C2n (2n = 70–74) via encapsulating monometal uranium was systematically and thoroughly investigated. Results indicate that fullerene cages D 5h (8149)-C70 and D 3h (14246)-C74 obeying the isolated pentagon rule and C 2(10612)-C72 featured with one pentalene moiety were the most promising candidates to encage uranium. Subsequent Mulliken spin density distribution and frontier molecular orbital analyses suggest that four formal electron transfer occurs from monometal U to above the carbon cages. There also exists a high degree of covalent character between the atom U and fullerenes C2n based on Mayer bond order and quantum theory of atoms in molecule (QTAIM) analyses, indicative of the cooperative stabilization by both ionic and covalent bonding interactions. In addition, investigations on the above-mentioned U@C2n isomers and other favorable candidates (U@C s (8094)-C70, U@C 1(10610)-C72, U@C 1(13393)-C74, and U@C 1(14049)-C74) reveal that these isomers could be closely linked via simple C2 addition and Stone–Wales transformation. These results will provide a systematic understanding on U-based endohedral metallofullerenes (EMFs) and also might be helpful for further exploration of EMF growth mechanisms.

Discovery of Non-Isolated-Pentagon-Rule Fullerenes from Computational Characterization of U2O@C72

“…Thus, the pure functional (BP86) yields different results from the hybrid functionals (M06-2X, B3LYP). Since the three functionals are all popular and perform reliably in the recent computational characterizations of U-containing EMFs, − , we decided to include all the three isomers in the following discussions. This prevents the possible omission of any minor U 2 O@C 72 isomer produced in the experiments .…”

“…Full geometry optimizations were conducted by employing the M06-2X, BP86, , and B3LYP , exchange-correlation functionals. The three functionals have been largely employed for characterizing the possible structures of U-containing EMFs. − , The standard 6-31G* all-electron basis set and the Stuttgart/Dresden quasi-relativistic effective core potential (ECP) and corresponding basis set (SDD) were employed for C/O and U, respectively. Harmonic vibrational frequency calculations at the same level of theory were then carried out to verify that all the obtained stationary points are true energy minima on the potential energy surface.…”

“…In this regard, the strong metal-cage interactions in actinide EMFs are very promising to stabilize the strained pentagon adjacencies (PAs) by large charge transfer and effective coordination. Indeed, mono-EMFs U@ C 2 (10612)-C 72 , U@ C 1 (17418)-C 76 , U@ C 1 (28324)-C 80 , U@ D 3 (31921)-C 80 , and U@ C s (51365)-C 84 were all demonstrated to violate IPR either experimentally or theoretically. We recently also found that the experimentally reported endohedral clusterfullerene (ECF) U 2 O@C 76 could be U 2 O@ C s (17490)-C 76 or U 2 O@ C 2 v (19138)-C 76 according to the density functional theory (DFT) calculations .…”

Overlooked Effects of La-4f Orbitals in Endohedral Metallofullerenes

“…The interactions between the encased metal atoms, metal and nonmetal atoms, and metal atoms and the outer cage are rather critical because they fundamentally affect the structural characteristics, energetic stabilities, and various properties of EMFs. , First, the ubiquitous metal-to-cage charge transfer leads to the well-accepted ionic model for EMFs, ,, which describes the purely electrostatic interaction between the metal cation and cage anion. However, obvious orbital overlaps between the metal and cage in many cases also render a substantial covalent character. ,− Second, for the EMFs containing two and more metal atoms (at most four thus far), the valence electrons remaining on the metal orbitals could be shared to form two-center or multiple-center metal–metal bonds. ,− Third, the inner nonmetal elements (C, N, O, S) in the cluster fullerenes also accept some electrons from the metal(s), resulting in the ionic or polar covalent bonds between the metal and nonmetal atoms.…”

Theoretical Insight into Thermodynamically Optimal U@C84: Three-Electron Transfer Rather Than Four-Electron Transfer

“…Very recently, researchers have paid more attention to actinide mono-EMFs, which originate from a series of U-based mono-EMFs U@C 2 n (2 n = 28–72) with spectrometric signals by Smalley et al in 1992 . So far, reported actinide mono-EMFs mainly involve U-based mono-EMFs U@C 2 n (2 n = 70, 72, 74, 76, 80, and 82) and Th-based mono-EMFs Th@C 2 n (2 n = 74, 76, 80, 82, 84, and 86). − Notably, the electronic structures are complicated for U-based mono-EMFs, among which the number of electrons transferred depends on the geometries and sizes of the cages. Concretely, four-electron transfer occurs in U@ C 2 (5)-C 82 , while three-electron transfer occurs in U@ C 2 v (9)-C 82 , showing that the number of electrons transferred depends on the isomers in U@C 82 .…”