A diuranium carbide cluster stabilized inside a C₈₀ fullerene cage

Abstract

Unsupported non-bridged uranium–carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing I_h(7)-C₈₀ fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@I_h(7)-C₈₀. This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (Ni^II-OEP) to produce UCU@I_h(7)-C₈₀·[Ni^II-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium–carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@I_h(7)-C₈₀ and the covalent nature of the U(f¹)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

While metal–carbon double bonds are common in transition metal chemistry and catalysis, unsupported uranium–carbon double bonds remain highly challenging to prepare. Here, the authors stabilize and characterize a U=C=U cluster containing unusually short, unsupported double bonds inside an I_h(7)-C₈₀ fullerene cage.

Introduction

Understanding the nature of actinide-ligand multiple bonding remains a modern day challenge owing to the complex electronic structures of these elements, and as a consequence, our comprehension of their chemistries lags behind that of the more commonly studied transition metals^1–5. Importantly, recent developments in the synthesis and study of molecular uranium complexes containing a variety of U=E(L) or U≡E(L) moieties (E = O, S, Se, Te, N, NR, P, As; L = other ligands) have provided valuable insights regarding the participation of the 5f and 6d valence orbitals in chemical bonding^1,2,6–27. Conspicuously missing from this list are U=C bonds, specifically those which lack ancillary heteroatom or chelating support. While metal–carbon double bonds (i.e., Schrock carbenes) are common in transition metal chemistry and catalysis, unsupported U=C bonds have long remained a major and outstanding synthetic target. Some success towards the synthesis of An=C bonds has been realized through the use of heteroatom stabilizing chelating ligands such as U=[C(Ph₂PNSiMe₃)₂](O)Cl₂ or U=[C(Ph₂PS)] (BH₄)₂ (THF)₂, among others^4,27–33. In a few rare instances, the isolation of non-chelated U=C bonds has been achieved in complexes such as (η⁵-C₅H₅)₃U=CHPMe₂Ph and [N(SiMe₃)₂]₃U=CHPPh₃, using formally dianionic ylidic ligands^31,34. Electronically, the U=C units found in these methanediide ([κ³-CL₂]²⁻) or ylidic ([C(H)(PR₃)]²⁻) examples are best described as highly polarized, nucleophilic carbenes with σ and modest π bonding overlap between uranium and carbon. The α-carbon bonded heteroatom(s) (e.g., phosphorous) aids the delocalization of the carbon-centered charge accumulation. While compounds possessing unsupported U=C bonds are difficult to prepare under typical synthetic conditions, Andrews and co-workers have identified alkylidene and even alkylidyne species such as H₂C=UX₂ and HC≡UX₃ (X= H, F, Cl or Br) in low-temperature noble-gas matrix isolation experiments, providing evidence for such bonding motifs^35–37. However, these compounds are too reactive to be isolated under typical synthetic conditions. The knowledge gained from studying stable compounds featuring U=C multiple-bonds can be extended to uranium carbide ceramics, which offer improved thermal density and higher conductivity over current UO₂ nuclear fuels^38–40.

Based on our recent success with the isolation of new actinide endohedral fullerenes, we turned our attention towards the synthesis and isolation of carbon cages encapsulating clusters that possess U=C bonds. The internal hollow cavity of C_2n-fullerenes can encapsulate and stabilize novel metallic clusters, especially some which are highly reactive and virtually impossible to prepare independently^41–43. Recently, we reported the first crystallographically characterized examples of actinide endohedral metallofullerenes (Th@C₈₂ and U@C_2n, 2n = 74, 82) and described their unique electronic properties^43,44. We therefore hypothesized that fullerene cages would provide an ideal architecture and electronic environment to trap and stabilize unique actinide clusters with novel bonding motifs.

Herein, using the I_h(7)-C₈₀ fullerene cage as a molecular nanocontainer, we report, to the best of our knowledge, the first structurally characterized example of unsupported uranium U=C bonds found in UCU@I_h(7)-C₈₀, possessing the unprecedented dimetallic-carbide cluster U=C=U. X-ray crystallographic analysis reveals two very short U=C bonds of 2.033(5)/2.028(5) Å, with an unexpected nonlinear U=C=U bond angle of 142.8(3)°.

Results

Synthesis of U₂C@C₈₀

U₂C@C₈₀ was synthesized by the Krätschmer–Huffman arc discharge method⁴⁵. Graphite rods, packed with graphite and finely dispersed U₃O₈, were vaporized in an arcing chamber under a He atmosphere. The compound was isolated and purified using a multistage high-performance liquid-chromatography protocol (HPLC). The composition and purity of the isolated U₂C@C₈₀ was confirmed by high-resolution matrix-assisted laser desorption-ionization time-of-flight positive-ion-mode mass spectrometry (MALDI-TOF/MS), which presents a prominent molecular ion peak with a mass-to-charge ratio of m/z = 1448.103 (Supplementary Fig. 1), corresponding to the [U₂C₈₁]⁺ empirical formula. The mass spectral isotopic distribution pattern matches the theoretically predicted one, thus confirming the molecular composition. In addition, an energy dispersive spectroscopic analysis of the purified sample was employed to determine the elementary composition of the compound. The spectrum shows characteristic peaks of uranium and carbon (Supplementary Fig. 2), which confirmed the assignment of this molecule to U₂C₈₁. Moreover, the powder X-ray diffraction pattern of U₂C@C₈₀ proves that the purified sample of U₂C@C₈₀ used for the experimental characterizations below, exists as a pure phase (Supplementary Fig. 5).

Molecular structure of UCU@I_h(7)-C₈₀·[Ni^II-OEP]

The molecular structure of UCU@C₈₀ was determined by single-crystal X-ray diffraction analysis. Slow diffusion of nickel(II) octaethylporphyrin (Ni^II-OEP) in benzene into a CS₂ solution of UCU@C₈₀ yielded black UCU@I_h(7)-C₈₀·[Ni^II-OEP] cocrystals (1). In cocrystal 1, UCU@C₈₀ is observed to adopt a slightly distorted icosahedral I_h(7)-C₈₀ cage structure (Fig. 1a) that is highly ordered. Similarly, inside the fullerene cage, the central carbon atom C₀ of the endohedral UC₀U unit is fully ordered, while the U atoms are slightly disordered. Two major U positions (U1 and U2) have common dominant occupancy of 0.853(3), while the residual occupancies are located on both sides within ca. ½ to 1 Å distance from the two main positions, possibly indicating some large amplitude motions of the UC₀U cluster (Supplementary Fig. 6). The distances between the major U1,2 sites and the carbons of the adjacent aromatic rings of the cage all lie within a narrow range of around 2.50 Å (2.471(5) to 2.543(5) Å, Fig. 1b), corresponding to distances of the ring centers Cti to Ui of 2.042(1) Å and angles Cti−Ui−C₀≈159(1)°. The U–C₀ bonds are 2.03 Å (U1–C₀ = 2.033(5) Å, U2–C₀ = 2.028(5) Å), forming a nearly linear Ct1–U1—U2–Ct2 chain (Fig. 1b) bridged by a non-linear –C₀– carbide anion.

Fig. 1.

ORTEP drawing of UCU@I_h(7)-C₈₀·[Ni^II-OEP] with 40% probability ellipsoids. a UCU@I_h(7)-C₈₀·[Ni^II-OEP] structure showing the relationship between the fullerene cage and the [Ni^II-OEP] ligands. The two U1/U2 sites have common occupancy of 0.853(3). Four minor U sites (Supplementary Fig. 6) and the solvent molecules are omitted here for clarity. b Fragment view showing the interaction of the major U1–C₀–U2 cluster with the closest aromatic ring fragments of the cage with centers Ct1 and Ct2. The orange line connects Ct1–U1–U2–Ct2

For hexa-coordinated uranium, Pyykkö found that two short and strong axial and four long and weak equatorial bonds are energetically near-equivalent to two long and weak axial and four short and strong equatorial bonds^46,47. Therefore, particularly short axial bonds are expected when additional equatorial coordination is absent, as in the present case. Indeed, most U-Ct(arene) distances are reported between 2.5 and 2.9 Å, for instance 2.651(4) to 2.698(4) Å for the dinuclear U(V) inverse sandwich complex [{U^V(Ts)}₂(η⁶:η⁶-C₆H₅Me)]⁴⁸. However, by far the most salient features of 1 are the unusually short U–C₀ bond lengths, (U1–C₀ = 2.033(5) Å, U2–C₀ = 2.028(5) Å) and the unusual bond angle of 142.8(3)° for U1–C₀–U2 (Table 1). In fact, the shortest U–C bond distance in poly-coordinated U-complexes to date is 2.184(3) Å, reported by Liddle et al. for the U(VI) carbene complex U=[C(Ph₂PNSiMe₃)₂](O)Cl₂²⁷. Interestingly, the U–C bond lengths of the UC₀U unit are between the 2.067(7) Å and 1.948(7) Å, theoretically calculated distances for uranium alkylidene and alkylidyne complexes U=CH₂(H)F and U≡CH(Cl)₃^35–37, and are nearly identical to the 2.01 Å distance calculated from the sum of the covalent radii⁴⁹. Furthermore, the U–C₀ distances in 1 are longer than the triple bond of the U≡O unit (1.78 Å) in the uranyl dication UO₂²⁺, the U≡NR bonds (1.844(6) Å) of U(N^tBu)₂(I)₂(THF)₂), and the terminal U≡N bond (1.83(2) Å for the uranium nitride complex (Tren^TIPS)UN), even when accounting for differences in the atomic radii of O, N, and C of ca. 0.05 Å each^18,23,50.

Table 1.

Calculated and experimentally derived geometric parameters of UCU groups

Geometric parameter	Calc.a UCU@(3I)2 (molecule)	Calc.a UCU@(C7H7)2 (molecule)	Calc.a UCU@Ih(7)-C80 (molecule)	Exptl.b UCU@Ih(7)-C80 •[NiII-OEP] (crystal)
U–C /Å	2.024	2.074	2.022	2.033/2.028
U∙∙∙U /Å	3.918	4.118	3.917	3.849
Ligand→U /Å	2.33c	2.52–2.55	2.47–2.51	2.47–2.54
U–C–U / o	150.9	166.1	151.3	142.8
Fullerene C–C /Å	—	—	100 × 1.43 ± 0.01	110 × 1.43 ± 0.03
‘π-Donating’ C–C /Å	—	14 × 1.43 ± 0.01	20 × 1.46 ± 0.01	10 × 1.48 ± 0.01

^aFrom scalar relativistic ZORA Kohn–Sham PBE VTZp approaches

^bFrom crystal structure analysis by X-ray diffraction

^cFor comparison of I−U with C−U, we have subtracted the I–C bond-radii difference of 0.55 Å from the actual I–U distance

Altogether, the bond lengths observed for 1 strongly support an Arene≡U=C=U≡Arene structure possessing two uranium–carbon double bonds formed by a bridging carbide atom, formally C⁴⁻. The bending of the U=C=U cluster of 1 was an unexpected exception to the common linear geometries of sp_σ¹–p_π² hybridized carbon atoms found for main-group molecules of type E=C=E (E=O, NR, CR₂), though a few cases with angles between 180° and 90° have been theoretically proposed and observed in recent years⁵¹.

Computational studies of molecular structure and bonding in UCU@I_h(7)-C₈₀

Quantum-chemical calculations were performed to further verify the unique structural parameters of UCU@I_h(7)-C₈₀ and gain insight about the bonding of the encapsulated bent U=C=U cluster. The molecular structure was modeled for free UCU@I_h(7)-C₈₀ and for LUCUL (L = C₇H₇, 3I) model molecules, using quasi-relativistic density-functional (DF) approximations (Supplementary Figs. 13, 14) and ab-initio CASSCF(10e,12o)-PT2 approaches (Table 1). Additional information of the structure, orbitals and electronic states of LUCUL is available in Supplementary Figs. 15–19 and Supplementary Tables 3–5. For the three calculated cases, the formal oxidation state⁵² is identified as 5+ for each U, derived from the formal [L³⁻≡>U⁵⁺<=C⁴⁻=>U⁵⁺<≡L³⁻] Lewis structure, where L³⁻≡>stands for the cage arene units that accept 3 electrons from each uranium atom and forms 3 dative pair bonds with the nearly empty U-5f6d valence shell. In the case of UCU@I_h(7)-C₈₀, the two L³⁻ units are the ligating arene units of the encapsulating C₈₀^6– closed-shell cage. The C and I atoms of the model units (C₇H₇) and (3I) have similar electronegativity as C of (C₈₀), and the model units also become closed-shell ligands upon accepting 3 electrons from each U. The effective physical partial charges in all three cases remain small, around −1 for formal C⁴⁻, around +½ to +1 for formal U⁵⁺, and small for the ligand atoms of C₈₀, 2C₇H₇ or 6I (Supplementary Table 1).

The atomic free valence and the spin and orbital populations (Supplementary Table 1) indicate a single f-electron on each U atom. The experimental U–C₀ bond lengths, though unprecedentedly short (2.03 Å), are well reproduced in all three computed models (2.02 Å for ligands C₈₀ or (I₃)₂, 2.07 Å for (C₇H₇)₂). Further, the observed unusual U–C₀–U bond angle of 142.8° is comparable to those computed for UCU@C₈₀ (151.3°) and (3I)UCU(3I) (150.9°), while the bulkier ligands of (C₇H₇)UCU(C₇H₇) led to 166.1°, which is still far from linear (Table 1). Neither the bond lengths nor the bond angle of the UCU unit appear to be significantly distorted by the encapsulation in C₈₀.

We chose the localized equivalent molecular orbital picture of the two simpler model systems to sketch the correlated and spin–orbit coupled valence electronic structure of ligated UC₀U. (C₇H₇)UCU(C₇H₇) (Fig. 2) and (3I)UCU(3I) (Supplementary Fig. 15) gave very similar results. Each U forms a strong σ-bond to the central C₀ atom through covalent overlap of hybridized U(5 f6d) orbitals with C(2s2p) hybrids. The U–C bonds of dominant π-character can be described as three-center two-electron (3c2e) bonds that possess some σ-admixture. The large electronegativity difference of C and U (1.3 Pauling units) leads to bond-pair polarization toward C₀, especially for the π-type pairs, which are strongly localized on C₀ and lead to its effective negative charge. The U–C interactions are characterized by Mayer bond orders (BO) of 1.4 from DF calculations (comparable to 1.6 for the U=As double bond in [K(B15C5)₂] {[N(CH₂CH₂NSiⁱPr₃)₃] U=AsH})²⁶ and by a Roos effective bond order (EBO) of 1.9 (counting the formally bonding—antibonding NOs from our CASSCF calculation of (C₇H₇)UCU(C₇H₇), which substantiates the U=C double bond character).

Fig. 2.

Natural Localized Molecular Orbitals (NLMOs) of sandwiched UCU. The UCU@(C₇H₇)₂ molecule formally consists of a (UCU)⁶⁺ unit between two aromatic (C₇H₇)³⁻ rings. Spin–orbit coupled multi-reference all-electron relativistic wavefunction calculations (state-averaged CASSCF) were analyzed with the NBO program [S27]. The contour values of the NLMOs generated thereby are ±0.042√(e/ų). a The two 1-center U-5f¹-type NLMOs, each occupied by one electron. b The U–C (left) and C–U(right) 2-center 2-electron (2c2e) σ-pair NLMOs, each consisting of 67% C-2sp¹, 31% U-5f^½ 6d and 2% tail-contribution from the other U. c The vertical π-type and the in-plane dominantly π-type U–C–U 3-center 2-electron (3c2e) NLMOs of 65% C-2p_s and 17% of each of the two U-5f^½ 6d

The bonding motif is nicely illustrated by the electron localization function (ELF) map in Fig. 3, which shows the electron–density accumulation of the two two-center U–C σ-bonds and of the in-plane three-center U–C–U π-type bond, as well as the multiple maxima of the inner-valence non-bonding 5f electrons. Thus, the bending of the U=C=U unit can be interpreted in terms of the localized negative charge build-up on C₀, which generates lone electron-pair density and gives rise to an sp¹/sp²-hybridized type geometry at carbon with bond angles in the middle between 120° and 180°. This charge accumulation is partially offset by donation into the U(5f6d) orbitals. Thus the bonding model can be described as highly polarized U=C interactions with partial π-overlap strengthened by electrostatic attractive forces between the cationic U⁵⁺ atoms and the anionic C₀⁴⁻ carbide bridge.

Fig. 3.

ELF-plot of ligated UCU in the molecular plane. The left and right ligands are two (I₃) groups. ELF is the ‘electron localization function’ of Becke and Edgecombe, showing electronic accumulations in the inner atomic shells, in the atomic lone-pair regions, and in covalent and dative bond regions

Spectroscopic properties of UCU@I_h(7)-C₈₀

Nuclear Magnetic Resonance (NMR) spectroscopy was used to characterize UCU@I_h(7)-C₈₀. The ¹³C NMR spectrum of UCU@I_h(7)–C₈₀ measured at 298 K shows only two sharp signals at 138.53 and 125.14 ppm with a 3:1 intensity ratio, corresponding to the sets of 60 and 20 equivalent carbon atoms of the unperturbed I_h–C₈₀ fullerene cage (Supplementary Fig. 7). The apparent symmetry is likely due to fast and large-amplitude librations of the cage, similar to those proposed for M₂@I_h-C₈₀ (M = La, Ce) and M₃N@C₈₀ (M = Sc, Y)^42,53,54. Unfortunately, no signal was detected for the single bridging C₀ carbon atom, likely due to limited sample amount, as well as to paramagnetic broadening effects from the unpaired 5f¹ electrons on each uranium. Noteworthy is that the ¹³C NMR spectrum measured at lower temperature (i.e., 283 K) shows slightly shifted signals at 138.35 and 124.65 ppm, respectively, with a slightly larger chemical shift difference (Δδ = 13.70 ppm) than those observed at 298 K (Δδ = 13.39 ppm). Such a trend is very similar to that observed for Ce₂@I_h–C₈₀⁵⁵, consistent with the paramagnetic nature of UCU (see below) and with the presence of an unpaired f electron on the formal U⁵⁺ (5f¹6d⁰7s⁰) ions (Supplementary Table 1)⁵⁴. On the other hand, the slight temperature dependence of Δδ/ΔT = 0.02 ppm/K might also be accounted for by the libration of the UCU cluster that neutralizes the paramagnetic effects (mainly pseudocontact interactions) for all carbon atoms.

We also utilized Fourier transform infrared absorption (FTIR), Raman emission and photoluminescence (PL) spectroscopies together with quantum-chemical calculations to further characterize UCU@I_h(7)-C₈₀. An experimental and theoretical spectral overview is presented in Fig. 4. The high-wavenumber range from 1600 to 1100 cm^‒1 contains 104 carbon cage C–C stretching vibrations, resembling those of other I_h-C₈₀ based endohedral fullerenes such as Ln₃N@C₈₀^41,56. Typical characteristics are the major overlaid bands around 1380 cm⁻¹ and the featureless gap between 1100 and 900 cm^‒1. The remaining 130 collective deformations originating from bending and torsional motions of the carbon cage atoms show up between 900 and 200 cm⁻¹.

Fig. 4.

Observed and theoretically predicted vibrational features of UCU@I_h(7)-C₈₀. The left upper curve (black) presents the observed Infrared absorption (IR) spectrum vs. wavenumber from 1600 to 600 cm^‒1, with quantum-chemical density-functional simulation below (in blue). On the right is the observed Raman emission (in red) vs. wavenumber from 600 to 100 cm^‒1, with calculated wavenumbers (‘Theor. Freqs.’) in the 600–0 cm^‒1 range (below in magenta). Observed local UCU vibrations above 100 cm^‒1 are indicated by heavy dots. Observed and calculated spectral C₈₀-cage features represent either a single mode, or an overlay of several near-degenerate ones. indicates a contamination by CS₂

Quantum-chemical calculations suggest that an endohedral cluster of n atoms (here n = 3) shows 3 frustrated translational rocking modes against the cage (at 33, 59, and 115 cm^‒1), 3 frustrated torsional wagging modes (at 16, 50, and 142 cm^‒1), and 3n – 6 = 3 internal vibrations. The latter comprise the UC₀U bending mode (at 97 cm^‒1), the symmetric stretch (coupled to near-degenerate cage deformations from 250 to 300 cm^‒1), and the asymmetric stretch (at 780 cm^‒1). The observed weak features in the Raman spectrum above 100 cm^‒1 at 126, 148, and 277 cm^‒1 agree reasonably well with the predicted UCU modes given above (in bold italics, estimated as weakly Raman active). Notably, the asymmetric UCU stretch appears as a pronounced feature in the IR spectrum at 785 cm^‒1, which is consistent with the high negative charge on the C₀ atom of the endohedral UCU cluster, and with the crystallographic observation of very short axial U=C bonds.

Further, an experimental violet-blue photo-luminescence progression of U₂C@I_h(7)-C₈₀ starting at a wavelength of 430 nm (~2.95 eV) in steps of ca. 1425 cm^‒1 (Supplementary Fig. 8) may originate from a ligand-to-metal charge-transfer excitation of the highest occupied molecular orbital, which has dominant density on the fullerene cage near the U atoms. There are also C_ring-breathing vibrations around 1400 cm⁻¹ localized in the spatial C_ring–U–C–U–C_ring region.

The redox properties of UCU@I_h(7)-C₈₀, as investigated by means of cyclic voltammetry, show a surprisingly small electrochemical gap of only 0.83 eV (Table 2, Supplementary Fig. 9). Most reported transition metal cluster fullerenes of I_h(7)-C₈₀ have gaps larger than 1 eV⁴¹. Typical M₃N@I_h(7)-C₈₀ fullerenes have gaps of 1.8–2.2 V. In particular, the first reduction potential of UCU@I_h(7)-C₈₀ (−0.41 V) is much more positive than that of any other reported M₃N@I_h(7)-C₈₀ fullerene⁵⁷, e.g., −1.26 V for Sc₃N@I_h(7)-C₈₀⁵⁸ and −0.94 V for TiLu₂C@I_h(7)-C₈₀⁵⁹. UCU@I_h(7)-C₈₀ obviously exhibits a much better electron-accepting ability than other I_h(7)-C₈₀ clusterfullerenes. This significant difference between UCU@I_h(7)-C₈₀ and other reported M₃N@C₈₀ fullerenes indicates a major difference of their electronic structures, which may be caused by the encapsulation of the U=C0=U cluster with an open d–f shell electronically interacting with the cage. Interestingly, when compared to most other I_h(7)-C₈₀ cluster fullerenes, the redox potentials of UCU@I_h(7)-C₈₀ are close to those of dilanthanide-fullerene, such as La₂@I_h(7)-C₈₀, which exhibits an electrochemical gap of 0.87 eV⁶⁰. In general, the electrochemical gaps are related to the HOMO-LUMO gaps of the closed-shell fullerene subsystems. However, in the present case, the encapsulated U=C₀=U cluster has a low-lying open U-5f shell, causing an abnormally small f–f gap of the UCU@I_h(7)-C₈₀ molecule. The reversible reduction of UCU@I_h(7)-C₈₀ is also quite remarkable, since M₃N@I_h(7)-C₈₀ fullerenes with group 3 metals exhibit irreversible reductions⁵⁷.

Table 2.

Redox potentials and electrochemical gaps of UCU@I_h(7)–C₈₀, La₂@I_h(7)–C₈₀, and Sc₃N@I_h(7)–C₈₀

Fullerenes	E 2+/+ a	E +/0 a	E 0/− a	E -/2− a	E 2−/3− a	E gap b	Ref.
UCU@Ih(7)-C80	+1.05d	+0.42c	−0.41c	−1.34c		0.83	This work
La2@Ih(7)-C80	+0.95c	+0.56c	−0.31c	−1.72c	−2.13d	0.87	60
Sc3N@Ih(7)-C80		+0.97c	−1.26d	−1.62d	−1.82d	2.23	58

^aRedox potentials in V vs. ferrocene couple

^bElectrochemical gaps in eV

^cHalf-wave potential (reversible redox process)

^dPeak potential (irreversible redox process)

The XPS spectrum of UCU@I_h(7)-C₈₀ was also recorded (Supplementary Fig. 10). The U-4f_7/2 ionization appears at 378.6 eV. In general, ionization energies increase with the formal oxidation state, e.g., from the U-4f_7/2 value of ca. 377_1/3 eV for the neutral uranium metal to ca. 381_3/4 eV for some uranyl(VI) salts. The present value of 378.6 eV is consistent with the very low effective charges on U as computed for UC₀U@C₈₀ (see below, U ~ +1.0e; C_o ~ −1.0e, C_fullerene ~ ± 0.0e), caused by the strong electron pair donation of formal C₈₀⁶⁻ and C₀⁴⁻ to the formal U⁵⁺ ions.

To gain further insight into the unique electronic characteristics of the U=C=U cluster in UCU@C₈₀, the magnetic susceptibility was measured (see Supplementary Fig.11 and Methods). The magnetic curves exhibit two regimes, (i) one at “high temperatures” from 300 K down to ca. 60 K, and (ii) another one at “low temperatures” from ca. 40 K down to 2 K. The “high temperature” region exhibits a huge, basically temperature-independent paramagnetism (TIP) of yet unknown origin. It is unknown for uranium complexes with common ligands^{21,22,48,61–65} and may possibly be due to the cooperative coupling of UCU and C₈₀ in the solid and related to the low band-gap calculated for UCU@C₈₀ (see Supplementary “Quantum Computational Methods”).

At “low temperatures”, we observed a Curie type paramagnetism ~μ²/T on top of the TIP, with net μ-values increasing with temperature from below to above a μ_Bohr, which appears compatible with hardly coupled unique units of type η³L≡U(V)=C=U(V)≡Lη³, i.e., the endohedral di-uranium carbide fullerene molecules. The “low-temperature” literature values of μ_eff^{21,22,48,61–65} for mono- and bridged di-metallic U(V) and U(IV) complexes, typically increase, too, from low values up to 3 μ_Bohr. It shows the expected temperature dependent weak coupling of two spins and with the two partially quenched U-f±3 orbital angular momenta, the experimental curves being reasonably reproduced by numerical simulations (see Supplementary Method “Quasi-relativistic correlated ab initio approaches” on the (C₇H₇)UCU(C₇H₇) model).

Attempts to further resolve the electronic structure using EPR spectroscopy were unsuccessful, as no clearly defined signal was observed at 4 K (Supplementary Fig. 12). Factors such as near degenerate electronic states and cage shielding effects as well as the huge TIP complicate the analysis. Thus, additional studies of the electronic properties of this unique and unprecedented system are warranted and will be communicated in due time.

Discussion

In summary, the overall agreement between the crystallographic, luminescence, Raman, IR, core-electronic, magnetic and voltammetric results, and the quantum-computational findings, conclusively show that a pure UCU@I_h(7)-C₈₀ compound was synthesized, which consists of a weakly perturbed I_h-C₈₀ cage and a sandwiched, bent and strongly bound polar endohedral near-symmetric U=C=U unit. The quantum-chemical results suggest that both U atoms in the UCU unit have a formal +5 oxidation state. The encapsulation causes little bond length or angle deformation of the UCU fragment. The encapsulation protects the axially ligated U atoms from binding with harder or more electronegative ligands like O or N (as compared to carbon in the forms of C₀⁴⁻ and C₈₀⁶⁻) so that C₀ can participate in primarily axial L≡>U=C₀ bonding at unusually short U–C₀ distances of 2.03 Å, and is not pushed into the weaker bonding equatorial plane of an E=U=E building block. The discovery of unsupported U=C bonding in a molecular compound confirms the distinction between “diagonal” and “poly-coordinated” uranium, and between “axial” covalent and “equatorial” dative bonding mechanisms. The work reported here offers a deeper understanding of the fundamentals of uranium bonding properties. This study also demonstrates that fullerene cages can be utilized as effective nanocontainers to stabilize and study rare and reactive clusters which contain actinide metal–ligand bonds.

Methods

Synthesis, separation and purification of U₂C@I_h(7)-C₈₀

A soot containing uranium fullerenes was synthesized by a direct-current arc discharge method. Graphite rods of U₃O₈ and graphite powder were annealed in a tube furnace at 1000 °C for 20 h under an Ar atmosphere and then burned in the arcing chamber under a 300 Torr He atmosphere. In total 1.87 g of graphite powder and 1.83 g of U₃O₈ (24:1 atom number ratio) were packed in each rod (6.7 g, without filling). The collected raw soot was refluxed in chlorobenzene under Ar atmosphere for 12 h and the solution was filtered. After solvent removal, the fullerenes were extracted by dissolving in toluene. On average ca. 140 mg of crude fullerene mixture per rod was obtained. In total, 60 carbon rods were burned in this work. Separation and purification of U₂C@I_h(7)-C₈₀ was achieved by a three-stage HPLC procedure (Supplementary Fig. 3, using columns including a Buckyprep M column (25 × 250 mm², Cosmosil, Nacalai Tesque, Japan), a Buckprep D column (10 × 250 mm², Cosmosil, Nacalai Tesque, Japan) and a Buckprep column (10 × 250 mm², Cosmosil, Nacalai Tesque, Japan). Toluene was used as the mobile phase and the UV detector was adjusted to 310 nm for fullerene detection. The HPLC procedures and corresponding MALDI-TOF spectra for the isolated fractions are shown in Supplementary Figs. 3, 4. In total, ca. 2 mg of highly purified U₂C@C₈₀ was obtained for characterization.

Single crystal and powder X-ray diffraction analysis

Black co-crystals of UCU@I_h-C₈₀·[Ni^II-OEP] were obtained by allowing the benzene solution of [Ni^II-OEP](1.2 mg/mL, 0.8 mL) and the CS₂ solution of UCU@I_h-C₈₀(1 mg/mL, 0.5 mL) to diffuse together. X-ray diffraction data were collected at 120 K using a diffractometer (APEX II; Bruker Analytik GmbH) equipped with a CCD collector. The Multiscan method was used for the absorption correction. The structure was resolved using direct methods (SIR2004) and refined on F² with the full-matrix least-squares approach using SHELXL2014^66,67 within the WinGX package⁶⁸.

Crystal Data for UCU@I_h-C₈₀·[Ni^IIOEP]·1.5C₆H₆·CS₂: C₁₂₇H₅₃N₄NiS₂U₂, M = 2159.70, 0.05 × 0.05 × 0.03 mm³, monoclinic, P 21/c (No. 14), a = 17.678(4) Å, b = 16.970(3) Å, c = 26.695(5) Å, β = 106.65(3)^°, V = 7673(3) ų, Z = 4, ρ_calcd = 1.934 g cm⁻³, μ = 4.578 mm⁻¹, θ = 2.688–27.092°, T = 120 K, R₁ = 0.0610, wR₂ = 0.1045 for all data; R₁ = 0.0388, wR₂ = 0.0610 for 16,771 reflections (I > 2.0σ(I)) with 1278 parameters and 784 restraints. Goodness of fit indicator 1.007. Maximum residual electron density 2.096 e Å⁻³. For further details see the Supplementary Data 1, 2.

In addition, a powder X-ray diffraction analysis of UCU@C₈₀ was conducted at the Shanghai Synchrotron Radiation Facility (SSRF) with wavelength λ = 0.6199 Å, and the intensity vs. 2θ pattern was recorded, see Supplementary Fig. 5.

Computational methods

Open-shell Kohn–Sham single-reference calculations were carried out with the ADF-2016, Gaussian-09 and ORCA-4.0 software, applying scalar and spin–orbit-coupled quasi-relativistic approaches (RECP or ZORA) with triple-ζ double-polarized basis sets. Bonding analyses were then performed using the NBO5.0 and Multiwfn codes. Multi-reference CASSCF, CASPT2, and RAS-SI/SO wave-function calculations were performed using the MOLCAS-V8.1 software applying the quasi-relativistic DKH Hamiltonian. Further details are given in the Supplementary “Quantum Computational Methods”.

Data availability

The coordinates for the X-ray structure of 1 are available free of charge from the Cambridge Crystallographic Data Centre under deposition no. CCDC 1572891. All other data supporting the findings of this study are available from the corresponding authors on request.

Author contributions

N.C., L.E., and X.Z. conceived and designed the experiments. X.Z. and Y.W. synthesized and isolated all the compounds. W.L., J.L., W.H.E.S., X.C., A.H., S.G., D.C.S., T.J.D., and J.A. performed the computations and theoretical analyses. L.F. performed the crystallographic analysis. N.A. and S.D. provided the XPS analysis. X.L. carried out the NMR test. G.V. and A.A.P. measured and analyzed the magnetic data. N.C., L.E., W.H.E.S., J.L., L.F., S.F., X.Z, W.L., S.G., J.A., and S.W. co-wrote the manuscript.

Competing interests

The authors declare no competing interests.