A charged diatomic triple-bonded U≡N species trapped in C82 fullerene cages

Abstract

Actinide diatomic molecules are ideal models to study elusive actinide multiple bonds, but most of these diatomic molecules have so far only been studied in solid inert gas matrices. Herein, we report a charged U≡N diatomic species captured in fullerene cages and stabilized by the U-fullerene coordination interaction. Two diatomic clusterfullerenes, viz. UN@C_s(6)-C₈₂ and UN@C₂(5)-C₈₂, were successfully synthesized and characterized. Crystallographic analysis reveals U-N bond lengths of 1.760(7) and 1.760(20) Å in UN@C_s(6)-C₈₂ and UN@C₂(5)-C₈₂. Moreover, U≡N was found to be immobilized and coordinated to the fullerene cages at 100 K but it rotates inside the cage at 273 K. Quantum-chemical calculations show a (UN)²⁺@(C₈₂)^2- electronic structure with formal +5 oxidation state (f¹) of U and unambiguously demonstrate the presence of a U≡N bond in the clusterfullerenes. This study constitutes an approach to stabilize fundamentally important actinide multiply bonded species.

Fig. 1. ORTEP drawing of UN@C₂(5)-C₈₂·[Ni^II(OEP)] and UN@C_s(6)-C₈₂·[Ni^II(OEP)] with 20% thermal ellipsoids.

a UN@C₂(5)-C₈₂·[Ni^II(OEP)]. b UN@C_s(6)-C₈₂·[Ni^II(OEP)]. Only the major U site is shown. For clarity, the solvent molecules and minor metal sites are omitted.

Fig. 2. Structures and distances between U and the fullerene cage.

a, d U@C_s(6)-C₈₂, b, e UCN@C_s(6)-C₈₂, and c, f UN@C_s(6)-C₈₂.

Fig. 3. Molecular structure of UN@C_s(6)-C₈₂ measured with single-crystal X-ray diffraction at variable temperatures from 100 K to 273 K.

The displacement parameters are shown at the 20% probability level for the encapsulated UN cluster. The structures are drawn from the chosen specific direction of the crystal to compare the dynamics of the UN@C_s(6)-C₈₂. Color code: blue for N, and red for U.

Fig. 4. Spectroscopic characterization of UN@C_s(6)-C₈₂ and UN@C₂(5)-C₈₂.

a UV−vis−NIR spectra of UN@C_s(6)-C₈₂ (I) and UN@C₂(5)-C₈₂ (II). b FTIR spectra of UN@C₂(5)-C₈₂. c, d Experimental Raman spectra of UN@C₂(5)-C₈₂ and theoretical simulations. e, f Experimental Raman spectra of UN@C_s(6)-C₈₂ and theoretical simulations. Source data are provided as a Source Data file.

Fig. 5. NLMOs and atomic orbital %-weights for the U≡N triple bond.

NLMOs (±0.03 a.u. isosurfaces) and atomic orbital %-weights for the spin-doublet ground states of UN@C₂(5)-C₈₂ (left) and UN@C_s(6)-C₈₂ (right). Alpha ($\alpha$)- and beta ($\beta$)-spin orbitals are plotted separately.

Fig. 6. Natural orbitals and populations from wavefunction calculations for UN and UN²⁺.

Natural Orbital (NO) isosurfaces (±0.04 a.u.) and populations calculated for the Λ = 3 ⁴H ground state of UN without (a) and with PT2 treatment of dynamic correlation (c). NO isosurfaces (±0.04 a.u.) and populations calculated for the Λ = 5 ²Φ spin-free GS of UN²⁺ without (b) and with the treatment of dynamic correlation (d). For degenerate NOs, one representative isosurface plot and the combined populations are shown.