Unraveling actinide-actinide bonding in fullerene cages: a DFT versus ab initio methodological study

Actinide-actinide bonding poses a challenge for both experimental and theoretical chemists because of both the scarcity of experimental data and the exotic nature of actinide bonding due to the involvement and mixing of actinide 7s-, 6p-, 6d-, and particularly 5f-orbitals. Only a few experimental examples of An-An bonding have been reported so far. Here, we perform a methodological study of actinide-actinide bonding on experimentally known Th2@C80 and U2@C80 systems. We compared selected GGA, meta-GGA, hybrid-GGA and range-separated hybrid-GGA functionals with the results obtained using a multireference CASPT2 method, which we consider as a reference point. We show that functionals such as BP86, PBE or TPSS perform well for predicting geometries, while range-separated hybrids are superior in the description of the chemical bonding. None of the tested functionals were deemed reliable regarding the correct electronic spin ground state. Based on the results of this methodological study, we re-evaluate selected previously studied diactinide fullerene systems using more reliable protocol.

Progress in solid state and coordination chemistry of actinides in China

In the past decade, the area of solid state chemistry of actinides has witnessed a rapid development in China, based on the significantly increased proportion of the number of actinide containing crystal structures reported by Chinese researchers from only 2% in 2010 to 36% in 2021. In this review article, we comprehensively overview the synthesis, structure, and characterizations of representative actinide solid compounds including oxo-compounds, organometallic compounds, and endohedral metallofullerenes reported by Chinese researchers. In addition, Chinese researchers pioneered several potential applications of actinide solid compounds in terms of adsorption, separation, photoelectric materials, and photo-catalysis, which are also briefly discussed. It is our hope that this contribution not only calls for further development of this area in China, but also arouses new research directions and interests in actinide chemistry and material sciences.

From π Bonds without σ Bonds to the Longest Metal-Metal Bond Ever: A Survey on Actinide-Actinide Bonding in Fullerenes

Actinide-actinide bonds are rare. Only a few experimental systems with An-An bonds have been described so far. Recent experimental characterization of the U₂@I_h(7)-C₈₀ (J. Am. Chem. Soc. 2018, 140, 3907) system with one-electron two-center (OETC) U-U bonds as was predicted by some of us (Phys. Chem. Chem. Phys. 2015, 17, 24182) encourages the search for more examples of actinide-actinide bonding in fullerene cages. Here, we investigate actinide-actinide bonding in An₂@D_5h(1)-C₇₀, An₂@I_h(7)-C₈₀, and An₂@D_5h(1)-C₉₀ (An = Ac-Cm) endohedral metallofullerenes (EMFs). Using different methods of the chemical bonding analysis, we show that most of the studied An₂@C₇₀ and An₂@C₈₀ systems feature one or more one-electron two-center actinide-actinide bonds. Unique bonding patterns are revealed in plutonium EMFs. The Pu₂@I_h(7)-C₈₀ features two OETC Pu-Pu π bonds without any evidence of a corresponding σ bond. In the Pu₂@D_5h(1)-C₉₀ with r_Pu-Pu = 5.9 Å, theory predicts the longest metal-metal bond ever described. Predicted systems are thermodynamically stable and should be, in principle, experimentally accessible, though radioactivity of studied metals may be a serious obstacle.

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

Four-electron transfer from U to the fullerene cage commonly exists in U@C_2n (2n < 82) so far, while four- and three-electron transfers, which depend on the cage isomers, simultaneously occur in U@C₈₂. Herein, detailed quantum-chemical methods combined with statistical thermodynamic analysis were applied to deeply probe into U@C₈₄, which is detected in the mass spectra without any further exploration. With triplet ground states, novel isomers including isolated-pentagon-rule U@C₂(51579)-C₈₄ and U@D₂(51573)-C₈₄ as well as nonisolated-pentagon-rule U@C_s(51365)-C₈₄ were identified as thermodynamically optimal. Surprisingly, there were unexpected three-electron transfers, which directly led to one unpaired electron on the cage, in all of the three isomers. Significant covalent interactions between the cage and U successively weakened for U@D₂(51573)-C₈₄, U@C₂(51579)-C₈₄, and U@C_s(51365)-C₈₄. Besides, the IR absorption spectra were simulated as a reference for further structural identification in the experiment. Last but not least, the potential reaction sites were predicted to facilitate further functionalization and thus achieve promising applications for U@C₈₄.

Th@C86, Th@C82, Th@C80, and Th@C76: role of thorium encapsulation in determining spherical aromatic and bonding properties on medium-sized endohedral metallofullerenes

Thorium encapsulated metallofullerenes (Th-EMFs) with external C76, C80, C82, and C86 cages have been synthesized, with the 13C-NMR spectrum recorded for Th@C82. Here, we explore computationally the chemical bonding, NMR and spherical aromaticity of Th@C82 and related thorium-encapsulated metallofullerenes. Our results show that these Th-EMFs are new examples of spherical aromatic structures, representing interesting low-symmetry exceptions to the Hirsch 2(N + 1)2 rule of spherical aromaticity. Their electronic structures are based on π-electron counts of 80, 84, 86, and 90, respectively, with a shell structure ranging from S2P6D10F14G18H22I8 to S2P6D10F14G18H22I18, where the partially filled I-shell remains as a frontier orbital. Their behavior is comparable to that of the spherical aromatic alkali-C606- phases, which in addition to the favorable endohedral Th-fullerene bonding account for their particular abundance exhibiting the ability to sustain a long-range shielding cone as a result of the favorable metal-cage bonding. This rationalization of such species as neutral spherical aromatic EMFs suggests the possibility of an extensive series of aromatic fullerenes with nuclearity larger than C60 buckminsterfullerene as stable building blocks towards nanostructured metal-organic materials.

U2C Unit in Fullerenes: Robust Multicenter Bonds with a Cluster Shape Controlled by Cage Size and Charge Transfer

Stimulated by the recent successful synthesis and crystallographic characterization of the first diuranium carbide endohedral metallofullerene (EMF) U₂C@I_h(7)-C₈₀ ( Zhang et al. Nat. Commun. ; 2018 ), density functional theory calculations were performed for a series of U₂C@C_2n (2n = 60, 68, 72, 78, 80, 88, 96, and 104) analogues. The internal UCU bond angle increases from 96.9° in I_h-C₆₀ to 180.0° in D_3d-C₁₀₄, exhibiting cage-size-dependent cluster configuration. However, further evidence suggests that the U₂C shape may be also affected by the amount of charge transferred from the cluster to the outer cage with 6e and 4e favoring bent and linear, respectively. The change of the bond angle closely correlates with the charge and hybrid state of the internal atom. Significantly, besides the covalent two-center two-electron (2c-2e) U-C bonds, the U₂C unit always features two 3c-2e bonds regardless of its size, shape, and charge state. Furthermore, for the cluster-cage interactions, besides the dominated electrostatic attractions, all these EMFs show an obvious covalent character with the substantial participation of U 5f valence orbitals.

Fullerenes as Nanocontainers That Stabilize Unique Actinide Species Inside: Structures, Formation, and Reactivity

Fullerene carbon cages can encapsulate a wide variety of atoms, ions, clusters, or small molecules inside, resulting in stable compounds with unusual structures and electronic properties. These compounds are collectively defined as endohedral fullerenes. The most studied endohedral fullerenes are those containing metal atoms or ions inside, and these are referred to as endohedral metallofullerenes (EMFs). For EMFs, the inner isolated space of the fullerene cages can lead to the stabilization of unique clusters, which are otherwise not synthetically accessible. This offers an excellent environment and opportunity for investigating the nature of previously unobserved metal-metal, metal-non-metal, and metal-fullerene interactions, which are of fundamental interest and importance. Up until now, most of the work in this field has been mainly focused on the rare-earth metals and related elements (groups II, III, and IV). The encapsulation of other elements of the periodic table could potentially lead to totally new structures and bonding motifs and to material properties beyond those of the existing EMFs. Actinides were originally explored as encapsulated elements in fullerenes when Smalley et al. ( Science 1992 , 257 , 1661 ) reported mass spectral evidence of actinide endohedral fullerenes back in 1992. However, the full characterization of these actinide endohedral fullerenes, including single crystal X-ray diffractometric analyses, was not reported until very recently, in 2017. In this Account, we highlight some recent advances made in the field of EMF compounds, focusing primarily on the molecular and electronic structures of novel actinide-based EMFs, new evidence for the formation mechanisms of EMFs, and the influence of the entrapped species on the reactivity and regiochemistry of EMF compounds. We recently reported that some monometallic actinide EMFs represent the first examples of tetravalent metals encapsulated inside fullerenes that exhibit considerably stronger host-guest interactions when compared to those observed for the lanthanide EMFs. These unusually strong metal-cage interactions, along with very high mobilities of the actinides inside the fullerene cages at high temperatures, result in the stabilization of unexpected non-IPR (isolated pentagon rule) fullerene cages encapsulating only one metal ion. Strikingly, such covalent stabilization factors had never been previously observed, although Sm@C_2v(19138)-C₇₆ was the first reported mono-EMF with a non-IPR cage, see details below. In addition, we showed that a long sought-after actinide-actinide bond was obtained upon encapsulation of U₂ inside an I_h(7)-C₈₀ fullerene cage. More interestingly, we demonstrated that actinide multiple bonds, which are very difficult to prepare by conventional synthetic methods, are stabilized when trapped inside fullerene cages. A totally unexpected and previously unreported uranium carbide cluster, U═C═U, was fully characterized inside an EMF, U₂C@I_h(7)-C₈₀, which, for the first time, clearly exhibits two unsupported axial U═C double bonds that are ∼2.03 Å long. We also discovered that synthetic bis-porphyrin nanocapsules exhibit exquisitely selective complexation of some of these uranium endohedral compounds, providing the basis for a nonchromatographic EMF purification method for actinide EMFs. Regarding EMF formation mechanisms, we suggested that novel carbide EMF structures, that is, Sc₂C₂@C_s(hept)-C₈₈, are likely key intermediates in a bottom-up fullerene growth process. Additionally, the structural correlation between chiral carbon cages during a bottom-up growth process was shown to be enantiomer-dependent. The influence of the encapsulated clusters on the chemical reactivity of EMFs is discussed at the end, which showed that the regioselectivities of multiple additions to the fullerene cages are remarkably controlled by the encapsulated metal clusters.

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

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

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@D_3h-C₇₄, U@C₂(5)-C₈₂ and U@C_2v(9)-C₈₂, and provide theoretical evidence for cage isomer dependent charge transfer states for U. Results from DFT calculations show that U@D_3h-C₇₄ and U@C₂(5)-C₈₂ have tetravalent electronic configurations corresponding to U⁴⁺@D_3h-C₇₄^4– and U⁴⁺@C₂(5)-C₈₂^4–. Surprisingly, the isomeric U@C_2v(9)-C₈₂ has a trivalent electronic configuration corresponding to U³⁺@C_2v(9)-C₈₂^3–. 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.

Unwilling U–U bonding in U2@C80: cage-driven metal–metal bonds in di-uranium fullerenes

Experimentally known U₂@C₈₀ has a double ferromagnetic U–U bond. U–U bonding in diuranium fullerenes is fine-tuned by the cage.

In situ spectroscopy and spectroelectrochemistry of uranium in high-temperature alkali chloride molten salts

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.