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.

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

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

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

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