U2N@Ih(7)-C80: fullerene cage encapsulating an unsymmetrical U(iv)NU(v) cluster

For the first time, an actinide nitride clusterfullerene, U2N@I-h(7)-C-80, is synthesized and fully characterized by X-ray single crystallography and multiple spectroscopic methods. U2N@I-h(7)-C-80 is by far the first endohedral fullerene that violates the well-established tri-metallic nitride template for nitride clusterfullerenes. The novel U=N=U cluster features two U=N bonds with uneven bond distances of 2.058(3) angstrom and 1.943(3) angstrom, leading to a rare unsymmetrical structure for the dinuclear nitride motif. The combined experimental and theoretical investigations suggest that the two uranium ions show different oxidation states of +4 and +5. Quantum-chemical investigation further reveals that the f(1)/f(2) population dominantly induces a distortion of the U=N=U cluster, which leads to the unsymmetrical structure. A comparative study of U2X@C-80 (X = C, N and O) reveals that the U-X interaction in U=X=U clusters can hardly be seen as being formed by classical multiple bonds, but is more like an anionic central ion Xq- with biased overlaps with the two metal ions, which decrease as the electronegativity of X increases. This study not only demonstrates the unique bonding variety of actinide clusters stabilized by fullerene cages, showing different bonding from that observed for the lanthanide analogs, it also reveals the electronic structure of the U=X=U clusters (X = C, N and O), which are of fundamental significance to understanding these actinide bonding motifs.