USc₂C₂ and USc₂NC Clusters with U-C Triple Bond Character Stabilized Inside Fullerene Cages

The chemistry of f-block metal-carbon multiple bonds is underdeveloped compared to well-established carbene complexes of the d-block transition metals. Herein, we report two new actinide-rare earth mixed metal carbides and nitrogen carbide cluster fullerenes, USc₂C₂@D_5h(6)-C₈₀ and USc₂NC@D_5h(6)-C₈₀, which contain U-C bonds with triple bond character and were successfully synthesized and characterized by mass spectrometry, UV-vis-NIR spectroscopy, Fourier transform infrared spectroscopy, single crystal X-ray diffraction, and DFT calculations. Crystallographic studies show that the two previously unreported clusters, USc₂C₂ and USc₂NC, are stabilized in the D_5h(6)-C₈₀ carbon cage and adopt unique trifoliate configurations, in which C₂/NC units are almost vertically inserted into the plane defined by the U and two Sc atoms. Combined experimental and theoretical studies further reveal the bonding structure of USc₂C₂ and USc₂NC, which contain C═U(VI)═C and C═U(V)═N bonding motifs. The electronic structures of the two compounds are determined as U⁶⁺(Sc₂)⁶⁺(C^4-)₂@D_5h(6)-C₈₀^4- and U⁵⁺(Sc₂)⁶⁺(N)^3-(C)^4-@D_5h(6)-C₈₀^4-, respectively. Quantum-chemical studies confirm that the U-C bonds in both molecules show unprecedented multicenter triple-bond character. The discovery of this unique U-C multiple bond offers a deeper understanding of the fundamentals of uranium chemistry.