ThC2@C82versus Th@C84: unexpected formation of triangular thorium carbide cluster inside fullerenes

Synthesis of the first thorium-containing clusterfullerenes, ThC2@Cs(6)–C82 and ThC2@C2(5)–C82, is reported. These two novel actinide fullerene compounds were characterized by mass spectrometry, single-crystal X-ray diffraction crystallography, UV–vis–NIR spectroscopy, and theoretical calculations. Crystallographic studies reveal that the encapsulated ThC2 clusters in both Cs(6)–C82 and C2(5)–C82 feature a novel bonding structure with one thorium metal center connected by a C 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 C unit, forming an isosceles triangular configuration, which has not been hitherto observed for endohedral fullerenes or for solid phase thorium carbides. Electronic structure calculations assign a formal electronic structure of [Th4+(C2)2−]2+@[C82]2−, with pronounced donation bonding from (C2)2− to Th4+, secondary backbonding from the fullerene to thorium and Th–C double bond character in both compounds. This work presents a new family of endohedral fullerenes, MC2@C2n−2, being unexpected isomers of MC2n, and provides broader understanding of thorium bonding.


Introduction
Fullerenes are known for their ability to encapsulate clusters, which results in the formation of unique host-guest molecular compounds-endohedral clusterfullerenes. [1][2][3] The unique interaction and mutual stabilization between the metalcontaining clusters and fullerenes gave rise to fascinating electronic structures and potential applications of these compounds. [4][5][6][7] To date, most lanthanides have been encapsulated in fullerene cages. 8 Our recent research showed that novel actinide clusters can also be captured and stabilized by fullerene cages, such as U 2 C@I h (7)-C 80 , U 2 C 2 @I h (7)-C 80 , or UCN@C 82 . [9][10][11] These systems exhibit substantially different electronic structures compared to known lanthanide-based analogs. In particular, the encapsulated uranium clusters reveal bonding properties that have never been observed in conventional uranium compounds. Thus, the exploration of novel actinide cluster fullerenes will not only expand the scope of endohedral fullerenes, but also have signicance regarding the understanding of fundamental actinide chemistry. However, all of the actinide cluster fullerenes discovered thus far were based on uranium; other actinide cluster fullerenes have yet to be explored. 12 Thorium is arguably the new frontier of nuclear energy. 13 Attempts have been made to synthesize and characterize thorium compounds for use as potential fuels in advanced reactors. Recently, thorium carbides have attracted increasing attention because these compounds are suitable for highburnup and high-temperature operations with increased "margin to melting" in the framework of modern nuclear systems. 14 Many advantages of thorium carbides, such as high melting points, corrosion resistivity, low thermal expansion coefficients and high thermal conductivity, have been reported in recent research. 15, 16 Therefore, understanding the behavior and properties of thorium carbides is essential to explore their potential application as nuclear reactor fuel materials. 17,18 Thorium carbides (ThC n , n = 1-6) have been detected in vapors above solid carbides or metal alloys in graphite systems, and partial pressures of thorium carbides were measured by mass spectrometry. [19][20][21][22] Thorium dicarbide (ThC 2 ), as the main type of stoichiometric thorium carbides, exists in polymorphic modications at ambient pressure. 16,[23][24][25] However, the structural and electronic properties of ThC 2 have only been studied by theoretical calculations. 18,23,26 Thus far, the molecular structure of ThC 2 has not been observed in the condensed phase.
On the other hand, carbide cluster fullerenes (CCFs) are an important branch of endohedral cluster fullerenes and have been extensively investigated in the past two decades. 12 The rst reported CCF is Sc 2 C 2 @D 2d (23)-C 84 , initially assigned as a dimetallofullerene (EMF), Sc 2 @C 86 . 27 This discovery conrmed that the composition M 2 C 2n could exist as M 2 C 2n or as M 2 C 2 @C 2n-2 . Subsequent studies revealed a large family of CCFs with variable M 2 C 2 clusters encapsulated inside different fullerene cages, such as Sc 2 C 2 @C 2n , 28-30 Gd 2 C 2 @C 2n , 31 Lu 2 C 2 @C 2n 32 et al. 2 In addition, composition Sc 3 C 82 was also reassigned as Sc 3 C 2 @I h -C 80 . 33 Up to now, a large variety of CCFs entrapping multiple (2-4) metal atoms have been reported. However, monometallic carbide cluster fullerenes have not been yet available and whether M@C 2n can exit as as MC 2n or as MC 2 @C 2n−2 has yet to be explored.

Results and discussion
Synthesis and isolation of ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 Thorium-based endohedral metallofullerenes were synthesized by a modied Krätschmer-Huffman DC arc discharge method. Graphite rods packed with U 3 O 8 /ThO 2 and graphite powder (molar ratio of U : Th : C = 1 : 1 : 30) were vaporized in the arcing chamber under a 200 Torr He atmosphere. The resulting soot was then collected and extracted with CS 2 for 12 h. A series of thorium-containing endohedral metallofullerenes were generated from this process (Fig. S2 †) and in this work, two novel isomers(I,II) of ThC 84 (later assigned as ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 ) were isolated by a multistage highperformance liquid chromatography (HPLC) separation process (Fig. S1 †). The purity of the isolated compounds was conrmed by positive-ion-mode matrix-assisted laser/ desorption ionization time-of-ight mass spectrometry (MALDI-TOF/MS), as shown in Fig. 1. The mass spectra of ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 show peaks at m/z = 1240.196 and 1240.204. In addition, the experimental isotopic distributions of the two samples agree well with theoretical predictions. The whole molecule of ThC 2 @C s (6)-C 82 , including the fullerene cage and the encapsulated cluster, shows two equivalent orientations with the same occupancy of 0.5, which is common in many analogous metallofullerene/Ni II (OEP) cocrystal systems. 1 The encapsulated Th ion shows only slight disorder with a total occupancy of 0.5 for the three disordered sites Th1-Th3. Th1 is assigned as the major Th site, as it has a much higher occupancy of 0.418 compared to those of the other two sites (0.0489 and 0.0326 for Th2 and Th3, respectively). Furthermore, Th1A, Th2A and Th3A are also generated via their mirror-related counterparts, Th1, Th2 and Th3, due to the same crystallographic mirror plane. Further structural analysis shows that Th1 is situated on the symmetry plane of the C s (6)-C 82 cage, while Th1A is located away from the symmetry plane ( Fig. S4 †). The density functional theory (DFT) calculation results also suggest that the Th1 site is approximately 13 kcal mol −1 lower in energy for all functionals than Th1A (Table S2 †). In addition, previous studies suggest that the metal ion prefers to remain symmetrically aligned with interacting motifs that share one of the symmetry planes with the fullerene containing mirror planes. 34 Therefore, we assign Th1 as the optimal position of ThC 2 @C s (6)-C 82 (Fig. 2a).

Molecular and electronic structures of
The crystallographic results of ThC 2 @C 2 (5)-C 82 also show two orientations of the fullerene molecule with equal occupancy of 0.5. These two orientations are related by the molecular crystallographic mirror. The Th1 site is the major Th position for ThC 2 @C 2 (5)-C 82 , with a fractional occupancy of 0.281. Th1A has the same occupancy of 0.281 and is symmetrical with Th1 through a crystallographic mirror. The rest of the minor sites are displayed in Fig. S3(b). † Th1 is located beneath the corresponding hexagon, with the shortest metal-cage distances of 2.542(12) A (Th1-C3) and 2.589(13) A (Th1-C2). Its mirrorrelated counterpart, Th1A, on the other hand, has the closest Th1A can be assigned as the optimal site only by crystallographic analysis, and their metal-cage distances are very similar. Therefore, theoretical calculations were employed to further determine the optimized position of the encapsulated ThC 2 cluster relative to the selected cage orientation. The results show that the Th1 site has a lower energy than the Th1A site for ThC 2 @C 2 (5)-C 82 (Table S2 †). Thus, the optimal ThC 2 cluster orientation can be accurately determined, as shown in Fig. 2b.
The ThC 2 cluster in C s (6)-C 82 features two almost identical Th-C distances, 2.360(11) and 2.353(10) A, respectively, leading to an isosceles triangular conguration. ThC 2 in C 2 (5)-C 82 has a similar but slightly distorted isosceles triangular conguration, with a Th-C bond length difference of 0.05 A. Note that the metal-C cage distances in the two ThC 2 @C 82 isomers, as mentioned already, are also somewhat different [2.546(13)-  . This result suggests that the variable isomeric cage structure has a slight impact on the interaction between Th and cage carbon, which likely results in differences in the ThC 2 cluster congurations inside the two C 82 cage isomers. The symmetric isosceles triangular structure conguration of the ThC 2 cluster encapsulated in either C 2 (5)-C 82 or C s (6)-C 82 is similar to a previously reported theoretically optimized structure of the ThC 2 molecule: 18 Kovacs and coworkers predicted that, for neutral ThC 2 , the symmetric triangular arrangement is much more stable than alternate linear or asymmetric triangular conformations. 18,41 The Th-C distance in the symmetric triangular isolated ThC 2 molecule obtained in the previous calculations is 2.281 A, 18,41 which is shorter than the experimentally obtained Th-C bond length for ThC 2 @C 82 (2.360(11)/2.353(10) A for ThC 2 @C s (6)-C 82 and 2.334(15)/ 2.385(14) A for ThC 2 @C 2 (5)-C 82 ). The variability of the Th-C distance may be rationalized by the fact that the coordination bonding between the Th and C 2 moiety in ThC 2 @C 82 is weakened by the coordination interaction between Th and the fullerene cage, as discussed later.
A closer look at the symmetric structure of ThC 2 @C s (6)-C 82 shows that, although the encapsulated ThC 2 can have many possible orientations relative to the cage, both the metal atom, Th1, and the cluster, ThC 2 , are located right on the symmetry plane. Further analysis of the crystallographic data of other mononuclear cluster fullerene-containing symmetry planes, such as MCN@C s (6) Fig. 4 suggests that the encapsulated mononuclear clusters are all located on the mirror planes of fullerene cages (see Fig. 3). 5,11,[42][43][44][45] Previous studies of monometallic fullerenes (only one metal ion inside the cage) have found that in fullerene cages containing symmetry planes, the metal prefers to occupy a symmetric arrangement with respect to the interacting motifs, which share one of their symmetry planes with the fullerene. 34 This observation further suggests that the endohedral mononuclear cluster also prefers to share a symmetry plane with the fullerene cages. That is, in general, as long as the fullerene encapsulating a mononuclear cluster possesses mirror planes, the entire molecule tends to be symmetric.
The identication of the encapsulated ThC 2 cluster expands our understanding of endohedral fullerenes. It represents a new type of endohedral cluster MC 2 , in which a single metal ion is coordinated to a C^C unit. In previous fullerene studies, if a fullerene compound was identied by mass spectrometry as MC 2n , it can be intuitively assigned as a mono-metallofullerene, i.e., M@C 2n , in which only a single metal ion is encapsulated inside the cage. The discovery of MC 2 @C 2n , however, breaks this paradigm and suggests that MC 2n can also be the isostructural isomer of MC 2 @C 2n−2 . Moreover, it provides the rst crystallographic observation of a discrete ThC 2 , which may be benecial for the better understanding of those thorium carbide gas molecules generated in high temperature.

Theoretical investigation
DFT calculations for ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 identied spin-singlet ground states. It is known that C 82 can accept two electrons, for example, in the case of Sm@C 82 and TbCN@C 82 [C s (6) and C 2 (5) isomers]. 46,47 Based on the frontier molecular orbitals in our calculations, we veried that there is a transfer of two electrons from the encapsulated ThC 2 cluster to the C 82 cage, i.e., the system adopts a formal closed-shell [Th 4+ (C 2 ) 2− ] 2+ @[C 82 ] 2− electron conguration. Therefore, we interpret that ThC 2 @C 82 isomers have similar two-electron transfer to those of Sm@C 82 and TbCN@C 82 . In all three cases, isomeric structures of C s (6)-C 82 and C 2 (5)-C 82 are stabilized by the metal/cluster-to-cage two electron transfer. 46,47 For the spin-singlet states, the structural parameters optimized with the B3LYP hybrid functional match the experimental data  better than other tested functionals. The following discussion is based on all-electron scalar relativistic B3LYP optimizations and the corresponding electronic structures.
The metal-ligand bonding in ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 , was characterized in terms of natural localized molecular orbitals (NLMOs) and Wiberg Bond Orders (WBOs). In ThC 2 @C s (6)-C 82 , as shown in Fig. 3, there are three pairs of NLMOs, one s and two p, describing the formal triple bond of C 2 2− . There is pronounced covalency with thorium. The two p NLMOs display three-center characteristics involving Th, with 10% and 9% weights of the orbital density associated with Th 6d-5f hybrids. The carbon lone pairs are even stronger donating, with 13% weight at thorium. The WBO for C 2 2−  . 49 These data help rationalizing the aforementioned short Th-C distances. Some of the NLMOs centered in the fullerene also have density at Th; the corresponding plots are shown in Fig. S9. † Among them, the strongest Th-C(cage) interaction has 6% Th weight. Therefore, the formal transfer of two electrons from ThC 2 to C 82 is accompanied by secondary cage-metal backbonding.
The main difference between ThC 2 @C 2 (5)-C 82 and ThC 2 @C s (6)-C 82 (Fig. S8 and S10 †) is the backbonding between Th and the fullerene, with only 4% for the largest Th weight in the former, which may rationalize the slightly higher energy of ThC 2 @C 2 (5)-C 82 by 2 kcal mol −1 .

Conclusions
For the rst time, thorium clusters were encapsulated inside fullerene cages. ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 were synthesized and characterized by mass spectrometry, singlecrystal X-ray diffraction crystallography, UV-vis-NIR spectroscopy and DFT calculations. Crystallographic studies reveal that a mononuclear carbide, which has never been found in endohedral fullerenes, is stabilized inside a C 82 cage. The two Th-C bond lengths of the ThC 2 cluster encapsulated in both C s (6)-C 82 and C 2 (5)-C 82 are 2.360(11)/2.353(10) A and 2.334(15)/2.385 (14) A, presenting isosceles triangular congurations, although the latter shows slight distortion, likely affected by the different cage isomeric structures. DFT calculations for two isomers of ThC 2 @C 82 revealed that the electronic structure can be described as a spin singlet ground state, formally [Th 4+ (C 2 ) 2− ] 2+ @[C 82 ] 2−, with pronounced donation bonding from (C 2 ) 2− to Th 4+ and secondary backbonding from the fullerene to thorium. The triangular cluster [ThC 2 ] 2+ is more stable in the C s (6)-C 82 cage (1a) than in the C 2 (5)-C 82 cage (2a), which is in part rationalized by a weaker backbonding in the latter. Theoretical analysis also shows a triple bond in the C 2 2− fragment that is somewhat weakened by the donation to the metal. The calculations provide an intuitive description of the bonding of actinide and main group atoms as they are encapsulated in fullerenes. This work expands the scope of both endohedral fullerenes and actinide compounds. ThC 2 @C 82 represents a new family of endohedral fullerenes, which reveals for the rst time that MC 2n fullerenes, the most commonly observed endohedral fullerenes, may have two isomeric structures, namely, M@C 2n versus MC 2 @C 2n−2 . Furthermore, identication of the unique bonding motif of ThC 2 deepens our understanding of the chemical bonding of thorium.

Spectroscopic study
Positive-ion mode matrix-assisted laser desorption ionization time-of-ight (MALDI-TOF) (Bruker, Germany) was employed for mass characterization. The UV-vis-NIR spectra of the puri-ed ThC 2 @C 82 were measured in CS 2 solution with a Cary 5000 UV-vis-NIR spectrophotometer (Agilent, USA).

X-ray crystallographic study
The black block crystals of ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 were obtained by slow diffusion of the CS 2 solution of the corresponding metallofullerene compounds into the benzene solution of [Ni II (OEP)]. Single-crystal X-ray data of ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 were collected at 120 K on a diffractometer (Bruker D8 Venture) equipped with a CCD collector. The multiscan method was used for absorption correction. The structures were solved using direct methods 50 and rened on F 2 using full-matrix least-squares using the SHELXL2015 crystallographic soware packages. 51 Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. Crystal data for ThC 2 @C s (6)-C 82 $[Ni II (OEP)]$2C 6 H 6 and ThC 2 @C 2 (5)-C 82 $[Ni II (OEP)]$2C 6 H 6 are provided in Table  S4. †

Computational details
Kohn-Sham density functional calculations were performed for ThC 2 @C s (6)-C 82 and ThC 2 @C 2 (5)-C 82 structures with the 2017 release of the Amsterdam Density Functional (ADF) suite. 52 Different functionals, including the Perdew-Burke-Ernzerhof (PBE) and Becke-Perdew (BP86) nonhybrid functional, a global hybrid based on PBE with 25% exact exchange (PBE0), and the popular B3LYP hybrid functionals, were used in conjunction with all-electron Slater-type atomic orbital (STO) basis sets of triple-z polarized (TZP) quality for the geometry optimizations and electronic structure analyses. [53][54][55][56][57][58] Relativistic effects were considered by means of the scalar-relativistic Zeroth-Order Regular Approximation (ZORA) Hamiltonian. 59 An atompairwise correction for dispersion forces was considered via Grimme's D3 model augmented with Becke-Johnson (BJ) damping. 60 To quantify the compositions of the chemical bonds for selected optimized systems, natural localized molecular orbital (NLMO) analyses were carried out with the NBO program, version 6.0, interfaced with ADF. 61 Data availability CCDC 2183932 and 2183933 contain the supplementary crystallographic data for this paper. † These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/data_request/cif. All other data supporting the ndings of this study are available from the corresponding authors on request.

Conflicts of interest
There are no conicts to declare.