X-ray crystallographic analysis reveals two very short U=C bonds of 2.033(5)/2.028(5)вЂ‰Г…, with an unexpected nonlinear U=C=U bond angle of 142.8(3)В°. Herein, using the I h(7)-C 80 fullerene cage as a molecular nanocontainer, we report, to the best of our knowledge, the first structurally characterized example of unsupported uranium U=C bonds found in I h(7)-C 80, possessing the unprecedented dimetallic-carbide cluster U=C=U. We therefore hypothesized that fullerene cages would provide an ideal architecture and electronic environment to trap and stabilize unique actinide clusters with novel bonding motifs. Recently, we reported the first crystallographically characterized examples of actinide endohedral metallofullerenes 82 and 2 n, 2 nвЂ‰=вЂ‰74, 82) and described their unique electronic properties 43, 44. The internal hollow cavity of C 2 n-fullerenes can encapsulate and stabilize novel metallic clusters, especially some which are highly reactive and virtually impossible to prepare independently 41, 42, 43. The knowledge gained from studying stable compounds featuring U=C multiple-bonds can be extended to uranium carbide ceramics, which offer improved thermal density and higher conductivity over current UO 2 nuclear fuels 38, 39, 40.Д«ased on our recent success with the isolation of new actinide endohedral fullerenes, we turned our attention towards the synthesis and isolation of carbon cages encapsulating clusters that possess U=C bonds. However, these compounds are too reactive to be isolated under typical synthetic conditions. While compounds possessing unsupported U=C bonds are difficult to prepare under typical synthetic conditions, Andrews and co-workers have identified alkylidene and even alkylidyne species such as H 2C=UX 2 and HCв‰ЎUX 3 (X= H, F, Cl or Br) in low-temperature noble-gas matrix isolation experiments, providing evidence for such bonding motifs 35, 36, 37. The О±-carbon bonded heteroatom(s) (e.g., phosphorous) aids the delocalization of the carbon-centered charge accumulation. Electronically, the U=C units found in these methanediide ( 2в€’) or ylidic ( 2в€’) examples are best described as highly polarized, nucleophilic carbenes with Пѓ and modest ПЂ bonding overlap between uranium and carbon. In a few rare instances, the isolation of non-chelated U=C bonds has been achieved in complexes such as ( О· 5-C 5H 5) 3U=CHPMe 2Ph and 3U=CHPPh 3, using formally dianionic ylidic ligands 31, 34. Some success towards the synthesis of An=C bonds has been realized through the use of heteroatom stabilizing chelating ligands such as U=(O)Cl 2 or U= (BH 4) 2 (THF) 2, among others 4, 27, 28, 29, 30, 31, 32, 33. While metalвЂ“carbon double bonds (i.e., Schrock carbenes) are common in transition metal chemistry and catalysis, unsupported U=C bonds have long remained a major and outstanding synthetic target. Conspicuously missing from this list are U=C bonds, specifically those which lack ancillary heteroatom or chelating support. Importantly, recent developments in the synthesis and study of molecular uranium complexes containing a variety of U=E(L) or Uв‰ЎE(L) moieties (E = O, S, Se, Te, N, NR, P, As L = other ligands) have provided valuable insights regarding the participation of the 5f and 6d valence orbitals in chemical bonding 1, 2, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27. Understanding the nature of actinide-ligand multiple bonding remains a modern day challenge owing to the complex electronic structures of these elements, and as a consequence, our comprehension of their chemistries lags behind that of the more commonly studied transition metals 1, 2, 3, 4, 5. The structural features of I h(7)-C 80 and the covalent nature of the U(f 1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uraniumвЂ“carbon distances (2.03вЂ‰Г…) indicative of covalent U=C double-bond character. This endohedral fullerene was prepared utilizing the KrГ¤tschmerвЂ“Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (Ni II-OEP) to produce I h(7)-C 80 Here we report that, utilizing I h(7)-C 80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of I h(7)-C 80. Unsupported non-bridged uraniumвЂ“carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding.

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