UCN@C_s(6)-C₈₂: An Encapsulated Triangular UCN Cluster with Ambiguous U Oxidation State [U(III) versus U(I)]

Understanding the chemical behavior of actinide elements is essential for the effective management and use of actinide materials. In this study, we report an unprecedented η²(side-on) coordination of U by a cyanide in a UCN cluster, which was stabilized inside a C₈₂fullerene cage. UCN@C_s(6)-C₈₂was successfully synthesized and fully characterized by mass spectrometry, single crystal X-ray crystallography, cyclic voltammetry, spectroscopy, and theoretical calculations. The bonding analysis demonstrates significant donation bonding between CN^-and uranium, and covalent interactions between uranium and the carbon cage. These effects correlate with an observed elongated cyanide C-N bond, resulting in a rare case where the oxidation state of uranium shows ambiguity between U(III) and U(I). The discovery of this unprecedented triangular configuration of the uranium cyanide cluster provides a new insight in coordination chemistry and highlights the large variety of bonding situations that uranium can have.