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A Diuranium Carbide Cluster Stabilized Inside a C80 Fullerene Cage

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A Diuranium Carbide Cluster Stabilized Inside a C80 Fullerene Cage ARTICLE DOI: 10.1038/s41467-018-05210-8 OPEN A diuranium carbide cluster stabilized inside a C80 fullerene cage Xingxing Zhang1, Wanlu Li2, Lai Feng3, Xin Chen2, Andreas Hansen4, Stefan Grimme4, Skye Fortier5, Dumitru-Claudiu Sergentu6, Thomas J. Duignan6, Jochen Autschbach 6, Shuao Wang7, Yaofeng Wang1, Giorgios Velkos8, Alexey A. Popov 8, Nabi Aghdassi9, Steffen Duhm 9, Xiaohong Li1, Jun Li2, Luis Echegoyen4, W.H.Eugen Schwarz 2,10 & Ning Chen1 1234567890():,; Unsupported non-bridged uranium–carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)- C80. This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce II UCU@Ih(7)-C80·[Ni -OEP] as single crystals. X-ray diffraction analysis reveals a cage-sta- bilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium–carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural 1 features of UCU@Ih(7)-C80 and the covalent nature of the U(f )=C double bonds were further affirmed through various spectroscopic and theoretical analyses. 1 Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China. 2 Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China. 3 Soochow Institute for Energy and Materials InnovationS (SIEMIS), College of Physics, Optoelectronics and Energy & Collaborative, Soochow University, Suzhou, Jiangsu 215006, China. 4 Mulliken Center for Theoretical Chemistry, Universität Bonn, 53115 Bonn, Germany. 5 Department of Chemistry, University of Texas at El Paso, 500 West University Avenue, El Paso, TX, 79968, USA. 6 Department of Chemistry, University at Buffalo, State University of New York, Buffalo, NY 14260-3000, USA. 7 School of Radiological and Interdisciplinary Sciences & Collaborative Innovation Center of Radiation Medicine of Jiangsu, Higher Education Institutions, Soochow University, Suzhou, Jiangsu 215123, China. 8 Nanoscale Chemistry, Leibniz Institute for Solid State and Materials Research, 01069 Dresden, Germany. 9 Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China. 10 Physikalische und Theoretische Chemie, Universität Siegen, 57068 Siegen, Germany. These authors contributed equally: Xingxing Zhang, Wan-lu Li, Lai Feng. Correspondence and requests for materials should be addressed to J.L. (email: [email protected]) or to L.E. (email: [email protected]) or to N.C. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:2753 | DOI: 10.1038/s41467-018-05210-8 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05210-8 nderstanding the nature of actinide-ligand multiple was isolated and purified using a multistage high-performance bonding remains a modern day challenge owing to the liquid-chromatography protocol (HPLC). The composition and U fi complex electronic structures of these elements, and as a purity of the isolated U2C@C80 was con rmed by high-resolution consequence, our comprehension of their chemistries lags behind matrix-assisted laser desorption-ionization time-of-flight posi- that of the more commonly studied transition metals1–5. tive-ion-mode mass spectrometry (MALDI-TOF/MS), which Importantly, recent developments in the synthesis and study of presents a prominent molecular ion peak with a mass-to-charge molecular uranium complexes containing a variety of U=E(L) or ratio of m/z = 1448.103 (Supplementary Fig. 1), corresponding to ≡ = = + U E(L) moieties (E O, S, Se, Te, N, NR, P, As; L other the [U2C81] empirical formula. The mass spectral isotopic dis- ligands) have provided valuable insights regarding the participa- tribution pattern matches the theoretically predicted one, thus tion of the 5f and 6d valence orbitals in chemical bonding1,2,6–27. confirming the molecular composition. In addition, an energy Conspicuously missing from this list are U=C bonds, specifically dispersive spectroscopic analysis of the purified sample was those which lack ancillary heteroatom or chelating support. While employed to determine the elementary composition of the com- metal–carbon double bonds (i.e., Schrock carbenes) are common pound. The spectrum shows characteristic peaks of uranium and in transition metal chemistry and catalysis, unsupported U=C carbon (Supplementary Fig. 2), which confirmed the assignment bonds have long remained a major and outstanding synthetic of this molecule to U2C81. Moreover, the powder X-ray diffrac- = fi target. Some success towards the synthesis of An C bonds has tion pattern of U2C@C80 proves that the puri ed sample of been realized through the use of heteroatom stabilizing chelating U2C@C80 used for the experimental characterizations below, = = ligands such as U [C(Ph2PNSiMe3)2](O)Cl2 or U [C(Ph2PS)] exists as a pure phase (Supplementary Fig. 5). 4,27–33 (BH4)2 (THF)2, among others . In a few rare instances, the isolation of non-chelated U=C bonds has been achieved η5 = II in complexes such as ( -C5H5)3U CHPMe2Ph and [N Molecular structure of UCU@I (7)-C ·[Ni -OEP]. The mole- = h 80 (SiMe3)2]3U CHPPh3, using formally dianionic ylidic cular structure of UCU@C80 was determined by single-crystal X- ligands31,34. Electronically, the U=C units found in these ray diffraction analysis. Slow diffusion of nickel(II) octaethyl- κ3 2− 2− II methanediide ([ -CL2] ) or ylidic ([C(H)(PR3)] ) examples porphyrin (Ni -OEP) in benzene into a CS2 solution of II are best described as highly polarized, nucleophilic carbenes with UCU@C80 yielded black UCU@Ih(7)-C80·[Ni -OEP] cocrystals σ and modest π bonding overlap between uranium and carbon. (1). In cocrystal 1, UCU@C is observed to adopt a slightly α 80 The -carbon bonded heteroatom(s) (e.g., phosphorous) aids the distorted icosahedral Ih(7)-C80 cage structure (Fig. 1a) that is delocalization of the carbon-centered charge accumulation. While highly ordered. Similarly, inside the fullerene cage, the central = fi compounds possessing unsupported U C bonds are dif cult to carbon atom C0 of the endohedral UC0U unit is fully ordered, prepare under typical synthetic conditions, Andrews and co- while the U atoms are slightly disordered. Two major U positions workers have identified alkylidene and even alkylidyne species (U1 and U2) have common dominant occupancy of 0.853(3), = ≡ = such as H2C UX2 and HC UX3 (X H, F, Cl or Br) in low- while the residual occupancies are located on both sides within ca. temperature noble-gas matrix isolation experiments, providing ½ to 1 Å distance from the two main positions, possibly indi- 35–37 evidence for such bonding motifs . However, these com- cating some large amplitude motions of the UC0U cluster (Sup- pounds are too reactive to be isolated under typical synthetic plementary Fig. 6). The distances between the major U1,2 sites conditions. The knowledge gained from studying stable com- and the carbons of the adjacent aromatic rings of the cage all lie pounds featuring U=C multiple-bonds can be extended to ura- within a narrow range of around 2.50 Å (2.471(5) to 2.543(5) Å, nium carbide ceramics, which offer improved thermal density Fig. 1b), corresponding to distances of the ring centers Cti to Ui 38–40 − − ≈ – and higher conductivity over current UO2 nuclear fuels . of 2.042(1) Å and angles Cti Ui C0 159(1)°. The U C0 bonds – = – = Based on our recent success with the isolation of new actinide are 2.03 Å (U1 C0 2.033(5) Å, U2 C0 2.028(5) Å), forming a endohedral fullerenes, we turned our attention towards the nearly linear Ct1–U1—U2–Ct2 chain (Fig. 1b) bridged by a non- synthesis and isolation of carbon cages encapsulating clusters that linear –C – carbide anion. = 0 possess U C bonds. The internal hollow cavity of C2n-fullerenes can encapsulate and stabilize novel metallic clusters, especially some which are highly reactive and virtually impossible to pre- a b 41–43 fi 2.465Å pare independently . Recently, we reported the rst crystal- 2.417Å C01A 2.506Å C02A C06A lographically characterized examples of actinide endohedral Ct1 2.524Å C05A C03A metallofullerenes (Th@C and U@C ,2n = 74, 82) and descri- 2.526Å 82 2n C04A bed their unique electronic properties43,44. We therefore hypo- 2.539Å thesized that fullerene cages would provide an ideal architecture U1 and electronic environment to trap and stabilize unique actinide U1 C clusters with novel bonding motifs. 0 Herein, using the Ih(7)-C80 fullerene cage as a molecular U2 C nanocontainer, we report, to the best of our knowledge, the first 0 2.501Å C70A 2.543Å structurally characterized example of unsupported uranium U=C C71A C80A bonds found in UCU@Ih(7)-C80, possessing the unprecedented U2 2.465Å Ct2 C78A C73A = = C77A dimetallic-carbide cluster U C U. X-ray crystallographic ana- 2.500Å 2.535Å lysis reveals two very short U=C bonds of 2.033(5)/2.028(5) Å, 2.498Å II with an unexpected nonlinear U=C=U bond angle of 142.8(3)°. Fig. 1 ORTEP drawing of UCU@Ih(7)-C80·[Ni -OEP] with 40% probability II ellipsoids. a UCU@Ih(7)-C80·[Ni -OEP] structure showing the relationship between the fullerene cage and the [NiII-OEP] ligands. The two U1/U2 sites Results have common occupancy of 0.853(3).
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