A diuranium carbide cluster stabilized inside a C80 fullerene cage

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 Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-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@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

(II) right. The corresponding MALDI-TOF mass spectrum. (III) left. The third-step HPLC isolation profile of fraction 12-5, collected from the second-step (10250 mm Buckyprep column; toluene as eluent; flow rate 4.0 mL/min; injection volume 4 mL). (III) right. The corresponding MALDI-TOF mass spectra. It shows that the purified U2C@Ih(7)-C80 was obtained in fraction 12-5-7 (III left).   Left: Measured. From the observed magnetization, an assumed temperature independent susceptibility background was subtracted, aimed at isolating the low temperature dependent paramagnetic contribution. By subtracting the feature just below 50 K (probably a contamination by solid O2), smooth curves could be obtained. Due to the hypothetical background-correction and the small amount of substance, the absolute values for χT have large uncertainties, therefore the cm 3 mole −1 K units are named 'arbitrary units' as a precaution.
Right: Simulated. One of our many magnetic example simulations is shown, using a single-ion crystal (each U(f 1 ) with an mJ=5/2 ground state), the crystal field modelled by a single B20 term of 4 cm −1 (inducing easy-axis anisotropy) and an anti-ferromagnetic exchange coupling constant between the two U spins of −0.5 cm −1 . The experimental curves appear consistent with weak anisotropic antiferromagnetic coupling of two spins per molecule, somewhat coupled to orbital angular momenta. The experimentally deduced T· χ (given in arbitrary units) appears to correspond to an effective molecular moment of the UCU@C80 molecules of a fraction of a μBohr at the lowest temperatures, increasing at higher temperatures up to not more than 2 μBohr.

Mass, optical (UV-Vis-NIR, PL, Raman, FTIR) and NMR spectroscopies
A positive-ion mode matrix-assisted laser desorption/ionization time-of-flight facility (Bruker, Germany) was employed for the mass characterization. UV-Vis-NIR spectrum of purified UCU@Ih(7)-C80 was measured in CS2 solution with a Cary 5000 spectrophotometer (Agilent, USA). The steady-state photoluminescence (PL) spectrum was recorded with an FLS980 spectrometer (Edinburgh Instrument, UK) by excitation at 406 nm at room temperature. The Raman spectrum was recorded on a Horiba Lab RAM HR Evolution Raman spectrometer using a laser at 633 nm. The Micro Fourier transform infrared spectrum was recorded at room temperature by a Vertex 70 spectrometer (Bruker, Germany) with a resolution of 4 cm −1 . For the IR and Raman measurements, the sample was drop-coated on aluminized paper and a quartz plate, respectively. The residual CS2 was removed in a drying chamber in vacuum at 100 ℃ . For the 13 C NMR spectroscopic measurements, the UCU@Ih(7)-C80 sample (ca.1.5 mg) was dissolved in CS2 (0.8 mL) and placed into the NMR tube. A capillary containing acetone-D6 was used as an internal lock. The 13 C NMR spectroscopic measurements were performed at 150 MHz (chemical shift measured in the range of -15 to 250 ppm) with an Agilent Direct-Drive II 600 MHz spectrometer (Agilent, USA) at 298 K.

Energy dispersive spectroscopy (EDS)
For the EDS analysis with a Transmission Electron Microscope (TEM), the purified sample was dispersed in an alcohol solution and then deposited on a TEM grid. The EDS spectrum was recorded on the TEM (FEI TECNAI G2 F20 200 kV) equipped with an EDS system (PV97-61700ME). The EDS spectrum shows characteristic peaks of uranium and carbon elements (Supplementary Figure  1a).

X-ray Photo-Electron Spectroscopy (XPS)
The XPS experiments were carried out in an ultra-high vacuum (UHV) chamber with a base pressure of 1·10 -10 mbar. The associated XPS set-up consists of a monochromatic X-ray source providing Al-Kα radiation (ћω = 1486.6 eV) and a Phoibos 150 high resolution hemispherical electron analyzer (Specs, Germany) for photoelectron detection. UCU@Ih(7)-C80 droplets were deposited on Si supporting substrates. The angle between the substrate surface-normal and the detector was set to 35°. After subtraction of a Shirley background, the core levels of the main U-4f doublet were fitted with asymmetric approximated Voigt profiles. whereas the associated shake-up satellites were deconvoluted with symmetric approximated Voigt profiles. The spin-orbit splitting (U-4f 7/2 -5/2) was set to 10.8 eV with an intensity ratio of 4:3 for both core levels and satellites.

Electrochemical Studies
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out in odichlorobenzene using a CHI-660E instrument. A conventional three-electrode cell consisting of a platinum counter electrode, a glassy carbon working electrode, and a silver reference electrode were used for both measurements. (n-Bu)4NPF6 (0.05 M) was used as supporting electrolyte. The CV and DPV curves were measured at scan rates of 100 and 20 mV/s, respectively.

Magnetometric Measurements
A sample of pristine solid UCU@Ih(7)-C80 (MW = 1448.8g) was prepared by drop-casting a carbon disulfide solution of ca. 0.1 mg, i.e. ca. 10 -7 mol of UCU@C80, mixed with polystyrene. Orientational motion of microcrystalline fullerene particles in strong magnetic fields may result in artifacts of the magnetic response, and encapsulating of the fullerene in the polymer matrix ensures that this is not happening. The magnetization was determined in magnetic fields of flux densities from 0.5 up to 7 Tesla, at temperatures T from 1.8 to 300 Kelvin, using an MPMS3 Vibrating Sample Magnetometer (VSM) with a sweeping rate of 2 K/min. We corrected for the expected diamagnetism of the fullerene and encapsulation-polymer. In addition, the significant temperature-independent background magnetism was tentatively subtracted, so that the low-temperature Curie-Weiss type temperature-dependent contribution to susceptibility, χCW, was obtained. The resulting product T·χCW vs. T curves are plotted in Supplementary Figure 9, together with theoretical simulations.

EPR Measurements
Attempts to further resolve the electronic structure of UCU@Ih(7)-C80 using EPR spectroscopy. Samples was dissolved in a toluene solution with about 0.5 mg/ml and then added into a quartz tube for EPR experiments at 5 K. The EPR spectrum was measured on a Bruker Elexsys E580 spectrometer. No clearly defined signal was observed, see Supplementary Figure 8. Factors such as nuclear quadrupole couplings, near degenerate electronic states and cage shielding effects complicate the analysis. Thus, additional studies of the electronic properties of this unique and unprecedented system is warranted and will be communicated in due time.

Quantum Computational Methods
Quantum chemical calculations were performed with theories at different levels of sophistication.
(a) Quasi-relativistic density-functional approaches The program codes ADF2016.101, Gaussian09, and ORCA4.0 were applied for the electronic SCF and nuclear geometry optimizations. Bonding analyses were then performed using the NBO5.0 and Multiwfn codes [1][2][3][4][5][6] . In order to improve the convergence of the two-open-shells singlet state, the two-orbital-mixing approach was applied at the spin-averaged spin-unrestricted Kohn-Sham level of approximation. Relativity in the U atoms was accounted for either by scalar-relativistic or by spinorbit coupled effective small-core pseudopotentials or by the correspondingly frozen atomic-core zero-order regular approach 7,8 . Perdew-Burke-Ernzerhoff's density functional was applied, either the simple gradient correct one (PBE) or the exchange-hybrid one (PBE0) without or with the dispersioncorrection by the Becke-Johnson damping scheme (D3BJ) [9][10][11][12] . Basis sets of Gaussian cc-pVTZ or of Slater triple-valence-zeta polarized type were used as supplied in the codes. The geometry optimizations and frequency calculations conducted with ORCA were performed with the ZORA-def2-SVP for the carbon atoms (exponents from the def2-SVP basis set recontracted for ZORA and the SARC-ZORA-TZVP basis for the uranium atoms [13][14][15] . The single point energy calculations conducted with ORCA were performed with either the ZORA-SV(P) (for C) and SARC-ZORA-TZVP (for U) basis sets (bond order analyses and generation of the wfn file) or ZORA-def2-TZVP (for C) and SARC-ZORA-TZVPP (for U) basis sets (orbital energies and gaps).
16 states of 8 electronic scalar (SR) levels were calculated, 4 formally degenerate spin-triplet ones: 2× 3 B1, 1× 3 A2, 1× 3 B2, and 4 formally degenerate spin-singlet ones: 2× 1 A1, 1× 1 A2, 1× 1 B2. The same active space was used, and the same electronic states were calculated, in CASSCF calculations with C2v and C1 symmetry. Additional dynamic correlation was introduced with the complete active space perturbation theory at second order (CASPT2) method 18 . In CASPT2 calculations, the default imaginary shift (0.20) and ionization potential electron affinity shift (0.25) were used. The SR wave functions were then used to build a state-interaction (SI) matrix for the spin orbit coupling (SOC) operator, whose diagonal was dressed with either CASSCF or CASPT2 correlated energies. The diagonallization of this matrix yielded SOC corrected electronic states and energies. The electronic structure and bonding in the ( 7 -C7H7)UCU( 7 -C7H7) complex was further scrutinized from the view point of localized orbitals. Natural bond orbital (NBO) calculations were performed, using the CASSCF and SO-CASSCF densities, with a locally developed program 26 interfacing Molcas with NBO6. 27 Plots of NBOs and natural localized molecular orbitals (NLMOs) were also provided.
Discussion: Calculated low-lying electronic states for the ( 7 -C7H7)UCU( 7 -C7H7) complex are listed in Supplementary Tables 3 and 4 while converged sets of active space NOs, and their occupation numbers before and after the treatment of SOC, are shown in Supplementary Figure 14.
The bonding (and antibonding) NOs involve the 2s and 2p valence shells of the C atom and valence U 5f and 6d shells. The two unpaired electrons are distributed among the four, U1 and U2 5f-based, nearly degenerate nonbonding NOs giving rise to a pool of low-lying and closely spaced electronic states. The SR-CASSCF wave-function compositions and relative energies listed in Supplementary  Table 3 stand for a nearly eightfold degenerate electronic ground state. In both C2v and C1 symmetry groups, the SR ground state is spin-triplet ( 3 B1 and 3 A1 in C2v), separated by only a few meV from the remaining computed states. As imposed by the point group, the most noticeable energy gaps in C2v occur between the two SA states in 3 B1 symmetry and between the lowest 3 B1 state and the two SA states in 1 A1 symmetry. A similar qualitative and quantitative trend is present for the corresponding states in the C1 point group (Supplementary Table 3). The additional dynamic correlation, introduced through CASPT2 calculations with C2v symmetry constraints, favors a nearly degenerate spin-singlet ground state, 1 B2 and 1 A2, separated to a slightly larger extent from the remaining computed states, by 0.02 eV from the 3 A2 and 3 B2 states, and by 0.13 eV from the sets of 3 B1 and 1 A2 states. These energy gaps may however be slightly overestimated due to slight artificial symmetry breaking induced by CASPT2. Note that in an ideal linear symmetry, the calculated eight electronic states should be exactly degenerate.
When SOC is accounted for (Supplementary Table 4), CASSCF predicts an essentially spin-triplet SOC electronic ground state, parented in the spin components of the 3 B1 and 3 A2 SR states when C2v symmetry constraints are used. Relaxing these constraints, a similar SOC ground state is obtained although the wave function has admixtures from spin components of all the coupled SR spin-triplet states. The additional dynamic correlation, projected on the diagonal of the SOC matrix in C2v calculations (see CASPT2 values in Supplementary Table 4), favors a large admixture (53%) of the 1 A2 spin component in the SOC ground state. That is, the ground state in SO-CASPT2 calculations is predicted to be a nearly equal spin-singletspin-triplet mixture.
Irrespective of the nature of the electronic ground state, point group symmetry or formalism (SR or SOC), the bonding picture in ( 7 -C7H7)UCU( 7 -C7H7) is unchanged. That is, since all the Slater determinants that build up the eight nearly-degenerate states only differ in the occupations of the nonbonding U(5f) NOs (i.e. the occupations of the remaining NOs are nearly identical for all calculated electronic states). For the lowest-lying SR state, active space NOs and natural occupation numbers obtained in SR-CASSCF calculations with C2v and C1 symmetry constraints, respectively, are shown in Supplementary Figure 14. Notably, the NOs show that the U-C-U backbone is maintained by two three-center bonds, each of them bearing a formal bond order of two, considering a single determinant formalism (Hartree-Fock or density DFT). In a multi-determinant CASSCF one, from the occupations of the bonding and nonbonding NOs (Supplementary Figure 14) an effective bond order (EBO) of 1.94 is obtained. Obviously, since SOC leads to admixtures of spin-components of SR states that exhibit similar EBOs, the SOC-EBO of any calculated SOC state is 1.94. The example of the ground SOC state (obtained through SOC-CASSCF) is shown in Supplementary  Figure 14.
In terms of localized orbitals, the bonding picture in the ( 7 -C7H7)UCU( 7 -C7H7) complex predicted by NBO calculations is in full agreement with the one described above (in terms of (SOC-)CASSCF NOs). Plots of NBOs and NLMOs corresponding to the active-space NOs (reported in Supplementary Figure 14) are displayed in Supplementary Figure 15, for the two lowest-lying spinsinglet and spin-triplet states obtained in CASSCF calculations (C1 symmetry), and in Supplementary Figure 16, for the lowest-lying SOC state displayed in Supplementary Table 4 (C1 symmetry). The results strengthen the fact that the U-C-U back-bone is maintained by two three-center bonds. The calculated (SOC-)EBO, for each bond, using the natural occupations of the NLMOs, is 1.95, in full agreement with the EBO predicted by (SOC-CASSCF), 1.94.